Role of Oxygen in Enhancing Catalytic Activity for Ammonia Synthesis: The Case of Ru/CaH2–2xOx

This study investigated how oxygen content influences the performance of Ru/CaH2–2xOx (x = 0.00–0.44) catalysts for ammonia synthesis. The reaction rate shows a volcano-type dependence on x: the catalytic activity increases and the apparent activation energy decreases up to x ≈ 0.29, after which both metrics deteriorate. X-ray diffraction revealed that the most active component possessed the smallest unit-cell volume, consistent with the lattice contraction driven by oxygen-induced hydrogen vacancies. Density functional theory calculations and kinetic modeling indicated that these vacancies enhance electron donation from the CaH2–2xOx support to the Ru nanoparticles, mitigating hydrogen poisoning and shifting to the rate-determining step from N2 dissociation to NH* or NH2* formation. By correlating the lattice volume, electron transfer, and reaction kinetics, this work highlights how the controlled oxygen content and the associated vacancy chemistry can be used to tune the catalyst structure and maximize ammonia synthesis rates.