A theoretical study of electrocatalytic ammonia synthesis on single metal atom/MXene

Abstract Electrocatalytic ammonia synthesis under mild conditions is an attractive and challenging process in the earth's nitrogen cycle, which requires efficient and stable catalysts to reduce the overpotential. The N 2 activation and reduction overpotential of different Ti 3 C 2 O 2 -supported transition metal (TM) (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Cd, and Au) single-atom catalysts have been analyzed in terms of the Gibbs free energies calculated using the density functional theory (DFT). The end-on N 2 adsorption was more energetically favorable, and the negative free energies represented good N 2 activation performance, especially in the presence Fe/Ti 3 C 2 O 2 (−0.75 eV). The overpotentials of Fe/Ti 3 C 2 O 2 , Co/Ti 3 C 2 O 2 , Ru/Ti 3 C 2 O 2 , and Rh/Ti 3 C 2 O 2 were 0.92, 0.89, 1.16, and 0.84 eV, respectively. The potential required for ammonia synthesis was different for different TMs and ranged from 0.68 to 2.33 eV. Two possible potential-limiting steps may be involved in the process: (i) hydrogenation of N 2 to *NNH and (ii) hydrogenation of *NH 2 to ammonia. These catalysts can change the reaction pathway and avoid the traditional N–N bond-breaking barrier. It also simplifies the understanding of the relationship between the Gibbs free energy and overpotential, which is a significant factor in the rational designing and large-scale screening of catalysts for the electrocatalytic ammonia synthesis.