Unraveling the Determining Factor of Titanium Oxynitride and Nitride for Ammonia Synthesis.

Nitride-based support materials have attracted significant attention in heterogeneous catalytic ammonia synthesis. However, the correlation between the formation energy of nitrogen vacancies and the electronic work function remains unclear. In this work, we demonstrate that nitrogen vacancies in TiN and TiO0.28N0.72 support play a crucial role in enhancing the catalytic generation of ammonia. These vacancies act as active sites for N2 activation and hydrogen transfer, effectively mitigating hydrogen poisoning via the Mars-van Krevelen (MvK) mechanism. Through a combination of experimental analyses (XRD, XANES, and TG-DSC) and DFT calculations, we reveal that TiN exhibits a superior catalytic performance due to its lower nitrogen vacancy formation energy. This lower energy facilitates lattice nitrogen exchange and generates nitrogen vacancies. In contrast, despite having a lower work function, TiO0.28N0.72 is limited in performance by its higher nitrogen vacancy formation energy. This work establishes nitrogen vacancy engineering as a universal strategy for designing high-efficiency metal nitride catalysts, bridging the gap between the electronic properties and defect thermodynamics in heterogeneous catalysis.