Mechanism of ammonia synthesis on Fe 3 Mo 3 N.

Ammonia (NH 3 ) synthesis is an essential yet energy-demanding industrial process. Hence, there is a need to develop NH 3 synthesis catalysts that are highly active under milder conditions. Metal nitrides are promising candidates, with the η-carbide Co 3 Mo 3 N having been found to be more active than the industrial Fe-based catalyst. The isostructural Fe 3 Mo 3 N catalyst has also been identified as highly active for NH 3 synthesis. In the present work, we investigate the catalytic ammonia synthesis mechanisms in Fe 3 Mo 3 N, which we compare and contrast with the previously studied Co 3 Mo 3 N. We apply plane-wave density functional theory (DFT) to investigate surface N vacancy formation in Fe 3 Mo 3 N, and two distinct ammonia synthesis mechanisms. The calculations reveal that whilst N vacancy formation on Fe 3 Mo 3 N is more thermodynamically demanding than for Co 3 Mo 3 N, the formation energies are comparable, suggesting that surface lattice N vacancies in Fe 3 Mo 3 N could facilitate NH 3 synthesis. N 2 activation was found to be enhanced on Fe 3 Mo 3 N compared to Co 3 Mo 3 N, for adsorption both at and adjacent to the vacancy. The calculated activation barriers suggest that, as for Co 3 Mo 3 N, the associative Mars van Krevelen mechanism affords a much less energy-demanding pathway for ammonia synthesis, especially for initial hydrogenation processes.