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.

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.

While Au electro-oxidation in acidic aqueous media on a phenomenological level proceeds directly from Au(0) to Au(III), it has previously been suggested that Au(I) states are intermediate species of the oxidation mechanism. Here, additional evidence for the transient Au(I) is provided by the probing the electro-oxidation of Au electrode operando in a pH = 3 perchloric acid (HClO4) electrolyte by high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD–XANES) at potentials up to 1.8 V versus the reversible hydrogen electrode (RHE). The perchlorate ions (ClO4−) in the electrolyte are used as sacrificial oxidizing agents. The reduced perchlorate compounds in turn produce chloride ions, which react with Au ions to form Au–Cl compounds. The operando HERFD–XANES detects and identifies the chlorinated compounds as surficial Au(I), present during the early stages of Au oxidation. It is further inferred that Au(I) is accessed by the electrolyte. These observations are consistent with the previously hypothesized route for Au electro-oxidation involving charge transfer after a dipole-induced place-exchange step.

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.

Hard x-ray angle-resolved photoemission spectroscopy reveals the momentum-resolved band structure in an epitaxial Mn₂Au(001) film capped by a 2-nm-thick ferromagnetic permalloy layer. By magnetizing the permalloy capping layer, the exceptionally strong exchange bias aligns the Néel vector in the Mn₂Au(001) film accordingly. Uncompensated interface Mn magnetic moments in Mn₂Au were identified as the origin of the exchange bias using x-ray magnetic circular dichroism in combination with photoelectron emission microscopy. Using time-of-flight momentum microscopy, we measure the asymmetry of the band structure, E(k)≠E(-k), in Mn₂Au resulting from the homogeneous orientation of the Néel vector. Comparison with theory shows that the Néel vector, determined by the magnetic moment of the top Mn layer, is antiparallel to the permalloy magnetization. The experimental results demonstrate that hard x-ray photoemission spectroscopy can measure the band structure of epitaxial layers beneath a metallic capping layer and corroborate the asymmetric band structure in Mn₂Au that was previously inferred only indirectly.