Impact of H-termination on the nitrogen reduction reaction of molybdenum carbide as an electrochemical catalyst

Abstract

Transition metal molybdenum (Mo) exhibits a strong capacity to adsorb nitrogen (N₂), but the Mo–N₂ interaction is too strong and thus it is difficult for ammonia (NH₃) to be released from the catalyst surface. Bonding with nonmetals with strong electronegativity is helpful to weaken the Mo–N₂ interaction, while the effect of hydrogen termination on catalyst surfaces needs to be evaluated given that the hydrogen evolution reaction (HER) is a key side reaction. This computational work aims to explore α-molybdenum carbide (Mo₂C, orthorhombic phase) as an electrochemical catalyst for the full nitrogen reduction reaction (NRR). Our density functional theory (DFT) calculations focus on a (100) surface and demonstrate that (i) surface molybdenum and carbon can be terminated by hydrogen via the Volmer step and (ii) the NRR can occur on H-terminated Mo₂C(100) with an energy requirement of 1.0–1.4 eV, depending on H-coverage. Although C–Mo bonding can remarkably reduce difficulty in NH₃ release from a Mo-site, H-terminals result in performance deterioration. These results provide new insights into the development of NRR catalysts.