Density Functional Theory Study of Triple Transition Metal Cluster Anchored on the C(2)N Monolayer for Nitrogen Reduction Reactions.

The electrochemical nitrogen reduction reaction (NRR) is an attractive pathway for producing ammonia under ambient conditions. The development of efficient catalysts for nitrogen fixation in electrochemical NRRs has become increasingly important, but it remains challenging due to the need to address the issues of activity and selectivity. Herein, using density functional theory (DFT), we explore ten kinds of triple transition metal atoms (M(3) = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) anchored on the C(2)N monolayer (M(3)-C(2)N) as NRR electrocatalysts. The negative binding energies of M(3) clusters on C(2)N mean that the triple transition metal clusters can be stably anchored on the N6 cavity of the C(2)N structure. As the first step of the NRR, the adsorption configurations of N(2) show that the N(2) on M(3)-C(2)N catalysts can be stably adsorbed in a side-on mode, except for Zn(3)-C(2)N. Moreover, the extended N-N bond length and electronic structure indicate that the N(2) molecule has been fully activated on the M(3)-C(2)N surface. The results of limiting potential screen out the four M(3)-C(2)N catalysts (Co(3)-C(2)N, Cr(3)-C(2)N, Fe(3)-C(2)N, and Ni(3)-C(2)N) that have a superior electrochemical NRR performance, and the corresponding values are -0.61 V, -0.67 V, -0.63 V, and -0.66 V, respectively, which are smaller than those on Ru(0001). In addition, the detailed NRR mechanism studied shows that the alternating and enzymatic mechanisms of association pathways on Co(3)-C(2)N, Cr(3)-C(2)N, Fe(3)-C(2)N, and Ni(3)-C(2)N are more energetically favorable. In the end, the catalytic selectivity for NRR on M(3)-C(2)N is investigated through the performance of a hydrogen evolution reaction (HER) on them. We find that Co(3)-C(2)N, Cr(3)-C(2)N, Fe(3)-C(2)N, and Ni(3)-C(2)N catalysts possess a high catalytic activity for NRR and exhibit a strong capability of suppressing the competitive HER. Our findings provide a new strategy for designing NRR catalysts with high catalytic activity and selectivity.