Nitrogen Reduction Reaction to Ammonia on Transition Metal Carbide Catalysts.

Abstract

The development of a low-cost, energy-efficient, and environmentally friendly alternative to the currently utilized Haber-Bosch process to produce ammonia is of great importance. Ammonia is an essential chemical used in fertilizers and a promising high-density fuel source. The nitrogen reduction reaction (NRR) has been explored intensively as a potential avenue for ammonia production using water as proton source, but to this day a catalyst capable of producing this chemical at high Faradaic efficiency (FE) and commercial yield and rates has not been reported. Here, we investigate the activity of transition metal carbide (TMC) surfaces in the (100) facets of the rocksalt (RS) structure as potential catalysts for the NRR. In this study, we use density functional theory (DFT) to model reaction pathways, estimate stability, assess kinetic barriers, and compare adsorbate energies to determine the overall performance of each TMC surface. For pristine TMC surfaces (with no defects) we find that none of the studied TMCs possess both exergonic adsorption of nitrogen and the capability to selectively protonate nitrogen to form ammonia in the desired aqueous solution. ZrC, however, is shown to be a potential catalyst if used in a non-aqueous electrolyte. To circumvent the endergonic adsorption of nitrogen onto the surface, a carbon vacancy was introduced. This provides a well-defined high coordination active site on the surface. In the presence of a vacancy VC, NbC, and WC showed efficient nitrogen adsorption, selectivity towards ammonia, and a low overpotential (OP). NbC did, however, display an unfeasible kinetic barrier to nitrogen dissociation for ambient-condition purposes, and thus it is suggested for high tempearture/pressure ammonia synthesis. Both WC and VC in their RS (100) structure are promising materials for experimental investigations in aqueous electrolytes, and ZrC could potentially be interesting for non-aqueous electrolytic systems.