Insight into the role of tunable nitrogen vacancies in transition metal nitrides for ammonia synthesis

The thermally catalyzed nitrogen reduction reaction (NRR) is significant in the fertilizer industry and basic catalytic science. This study employs density-functional theory (DFT) calculations to explore the performance of 12 metal nitrides in thermocatalytic NRR by focusing on them. The surface incompleteness in the catalytic environment is simulated by constructing nitrogen vacancies on the metal nitride surfaces. Then the catalytic activity of these surfaces is evaluated in the thermally catalyzed ammonia synthesis process under specific experimental conditions (temperatures of 573, 673, and 773 K, and a pressure of 1 bar), as well as the effect on the activation of N₂ and H₂ molecules. It was found that the NRR performance can be optimized by considering the relationship between the N coordination structure on the catalyst surface and the NRR activity, thus identifying LaN(110)-V(N) and NbN(110) as two highly promising catalysts with notable stability and kinetic activity. It is also found that for metal nitride catalysts, a surface with lattice nitrogen as tetra-coordinated exhibits better NRR activity with lower reaction energy, and the distal pathway is more favorable for all catalyst surfaces studied. The present results provide new ideas for developing efficient metal nitride catalysts for ammonia synthesis and enrich the basic knowledge of metal nitride-catalyzed NRR.