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. 

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.

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.