Anion vacancies activate N2 to ammonia on Ba–Si orthosilicate oxynitride-hydride

Abstract

Anion vacancies on metal oxide surfaces have been studied as either active sites or promoting sites in various chemical reactions involving oxidation/reduction processes. However, oxide materials rarely work effectively as catalysts in the absence of transition metal sites. Here we report a Ba–Si orthosilicate oxynitride–hydride as a transition-metal-free catalyst for efficient ammonia synthesis via an anion-vacancy-mediated mechanism. The facile desorption of H⁻ and N³⁻ anions plus the flexibility of the crystal structure can accommodate a high density of electrons at vacancy sites, where N₂ can be captured and directly activated to ammonia through hydrogenation processes. The ammonia synthesis rates reach 40.1 mmol g⁻¹ h⁻¹ at 300 °C by loading ruthenium nanoparticles. Although not found to dissociate N₂, Ru instead facilitates the formation of anion vacancies at the Ru–support interface. This demonstrates a new route for anion-vacancy-mediated heterogeneous catalysis.