Mechanism of ammonia synthesis on Fe₃Mo₃N

Ammonia (NH₃) synthesis is an essential yet energy-demanding industrial process. Hence, there is a need to develop NH₃ synthesis catalysts that are highly active under milder conditions. Metal nitrides are promising candidates, with the η-carbide Co₃Mo₃N having been found to be more active than the industrial Fe-based catalyst. The isostructural Fe₃Mo₃N catalyst has also been identified as highly active for NH₃ synthesis. In the present work, we investigate the catalytic ammonia synthesis mechanisms in Fe₃Mo₃N, which we compare and contrast with the previously studied Co₃Mo₃N. We apply plane-wave density functional theory (DFT) to investigate surface N vacancy formation in Fe₃Mo₃N, and two distinct ammonia synthesis mechanisms. The calculations reveal that whilst N vacancy formation on Fe₃Mo₃N is more thermodynamically demanding than for Co₃Mo₃N, the formation energies are comparable, suggesting that surface lattice N vacancies in Fe₃Mo₃N could facilitate NH₃ synthesis. N₂ activation was found to be enhanced on Fe₃Mo₃N compared to Co₃Mo₃N, for adsorption both at and adjacent to the vacancy. The calculated activation barriers suggest that, as for Co₃Mo₃N, the associative Mars van Krevelen mechanism affords a much less energy-demanding pathway for ammonia synthesis, especially for initial hydrogenation processes.