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.