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.

A successful energy transition hinges on developing new, efficient, and sustainable catalysts. To develop such catalysts, we must improve our understanding of heterogeneous catalytic mechanisms. Real catalytic interfaces are not static systems, but are complex and highly dynamic, and to gain truly applicable information it is imperative to study catalytic interfaces under realistic conditions, while the reaction occurs. In this thesis, a range of methods – from relatively simple product quantification with mass spectrometry to state-of-the-art X-ray spectroscopic measurements of high-pressure and electrocatalytic reactions – are used to gain mechanistic information about a series of heterogeneous catalytic reactions.

First, the mechanism of water-promoted CO oxidation is studied over two supported Au-nanoparticle catalysts, Au-TiO₂ and Au-Fe₂O₃. It is observed that CO₂ production occurs over the Au-Fe₂O₃ catalyst under O₂-free conditions, which is inconsistent with previously proposed mechanisms. Guided by isotope exchange measurements and density functional theory, we propose a new Mars-van Krevelen mechanism over Au-Fe₂O₃.

Next, the Haber-Bosch reaction over Fe and Ru single crystals is investigated using high-pressure X-ray photoelectron spectroscopy (XPS). We find that all surfaces are fully metallic under reaction conditions. Additionally, we show that the rate determining step over Ru is the dissociation of adsorbed N₂. On the Fe surfaces, the rate determining step is also N₂ dissociation at high temperatures, but at lower temperatures the hydrogenation steps become rate limiting.

Using the same instrument, a method is developed for studying electrode-electrolyte interfaces using XPS and is used to probe a Cu(111) electrode surface during the CO reduction reaction. The results suggest that, firstly, the mechanism for methane formation on this surface occurs via an atomic carbon intermediate, and that the mechanism for acetate formation on Cu(111) must proceed, at least in part, via a surface-based carboxylation step.

Finally, a new high-performance catalyst for the alkaline hydrogen evolution reaction (HER) is presented, NiCo-Co₂Mo₃O₈, and the reaction mechanism is investigated using K-edge X-ray absorption spectroscopy and Kβ X-ray emission spectroscopy. The results point towards in-situ formed Mo(III) sites within the oxide phase as being the active sites for hydrogen evolution, while Co(II) sites aid H₂O adsorption.

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.

We provide experimental evidence that is inconsistent with often proposed Langmuir−Hinshelwood (LH) mechanistic hypotheses for water-promoted CO oxidation over Au–Fe₂O₃. Passing CO and H₂O, but no O₂, over Au-γ-Fe₂O₃ at 25 °C, we observe significant CO₂ production, inconsistent with LH mechanistic hypotheses. Experiments with H₂¹⁸O further show that previous LH mechanistic proposals cannot account for water-promoted CO oxidation over Au-γ-Fe₂O₃. Guided by density functional theory, we instead postulate a water-promoted Mars–van Krevelen (w-MvK) reaction. Our proposed w-MvK mechanism is consistent both with observed CO₂ production in the absence of O₂ and with CO oxidation in the presence of H₂¹⁸O and ¹⁶O₂. In contrast, for Au-TiO₂, our data is consistent with previous LH mechanistic hypotheses. 

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.