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

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.