Electrochemical synthesis of ammonia via Mars-van Krevelen mechanism on the (111) facets of group III–VII transition metal mononitrides

Abstract

Density functional theory (DFT) calculations were carried out on a new class of materials in pursuit of nitrogen activation and electrochemical ammonia formation at ambient conditions. The source of proton provided by the anode could be either water splitting or H2. But we focused only on the cathode reaction here where nitrogen is reduced to ammonia. The Mars-van Krevelen mechanism was studied on the (111) facets of the NaCl-type structure of earlier transition metal mononitrides of Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Hf, Ta, W, and Re. The catalytic activity was investigated, free energy of all intermediates was calculated along the reaction path and free energy diagrams were constructed to explore the potential-determining steps of the reaction and accordingly estimate onset potential necessary for nitrogen activation on each different metal nitrides. The possibility of catalyst poisoning in electrochemical environment was also scrutinized at the bias needed for running the reaction. In addition, hydrogen production on all these nitride candidates was explicitly considered within our mechanistic model by removing the constraint of proton adsorption occurring only on surface nitrogen atoms and accordingly most of these candidates show capability for suppressing competitive hydrogen production, in contrast to metallic surfaces that almost exclusively evolve hydrogen gas. The likelihood of catalyst decomposition and catalyst regeneration was assessed for the most interesting nitrides. It was found that the only active and stable nitride catalyst that can regenerate itself and activate nitrogen to ammonia is NbN, and others should decompose to their parental metals under operational conditions.