On the possibility of an Eley–Rideal mechanism for ammonia synthesis on Mn6N5+x (x = 1)-(111) surfaces

Abstract

Recently we reported an Eley–Rideal/Mars–van Krevelen mechanism for ammonia synthesis on cobalt molybdenum nitride (Co₃Mo₃N). In this mechanism hydrogenation of activated dinitrogen occurs directly from the gas phase in a low barrier step forming a hydrazinylidene intermediate NNH₂. In this paper we study whether such a mechanism of ammonia synthesis could occur on the (111) surface of another metal nitride, Mn₆N_5+x (x = 1), as this would explain the low-T ammonia synthesis activity of Co₃Mo₃N. We find that although N₂ adsorbs more strongly than H₂ on the (111) surface, having also examined the (110) and the (100) surface, N₂ is not significantly activated when adsorbed in an end-on configuration. The hydrogenation reactions via an Eley–Rideal mechanism are all high barrier processes (>182 kJ mol⁻¹) and therefore an Eley–Rideal mechanism for ammonia synthesis is predicted to not occur on this material unless there are high temperatures. Our study indicates that the fact that an Eley–Rideal/Mars–van Krevelen mechanism occurs on Co₃Mo₃N is a result of the stronger activation of dinitrogen at nitrogen vacancies when dinitrogen is adsorbed in an end-on configuration.