Biomimetic Nitrogen Fixation Catalyzed by Transition Metal Sulfide Surfaces in an Electrolytic Cell

The nitrogen reduction reaction was investigated on the surfaces of 18 different stable transition metal sulfides using density functional theory calculations. YS, ScS, and ZrS were modeled in the rocksalt structure with the (1 0 0) facet; TiS, VS, CrS, NbS, NiS, and FeS in NiAs-type structure with the (1 1 1) facet; and MnS2, CoS2, IrS2, CuS2, OsS2, FeS2, RuS2, RhS2, and NiS2 in pyrite structure for both the (1 0 0) and (1 1 1) orientations. As the first step towards determination of sulfides that are less prone to hydrogen evolution, the competition between adsorption of NNH and H (for the associative mechanism), and between adsorption of N and H (for the dissociative mechanism) on these surfaces was considered. The catalytic activity through both the associative and dissociative mechanisms was explored and the overpotential required for electrochemical ammonia formation is reported. The scaling relations and volcano plots were constructed with free energy of adsorption of NNH or N on the surface as the descriptor. RuS2 was observed as the most active sulfide that could catalyze nitrogen reduction to ammonia at potentials around -0.3 V through the associative mechanism. NbS, CrS, TiS, and VS are also promising candidates for both the associative and dissociative mechanisms with overpotentials for nitrogen reduction around 0.7-1.1 V.