Mechanistic insights into N₂ activation by RhCo₃ via d-d orbital coupling

The synthesis of ammonia (NH₃) from nitrogen (N₂) under mild conditions is a great challenge, in which the electron-donating ability of the catalyst is the key for N₂ activation. In this work, the above process was studied using quantum chemical calculations using the density functional method. The results show that Rh and Co exhibit unsaturated d-electron configurations, with d-d orbital coupling occurring within the RhCo₃ metal cluster, resulting in bimetallic synergy and spin effects. The Rh atom serves as an electron modulation center and active site for reactant activation, while the Co metal synergistically enhances electron back-donation effects. The RhCo₃ cluster exhibits different adsorption and kinetic behaviors across the triplet, quintet, and septet potential energy surfaces, among which the septet state shows the most favorable catalytic performance. Energy span (δE) model analysis further indicates that, at low and moderate temperatures, the N₂ adsorption is the key factor governing the catalytic activity of RhCo₃. At 298 K, the reaction displays a δE value of 2.03 eV, and the catalytic activity increases with the temperature. However, at higher temperatures, the NH₃ desorption becomes the rate-determining process, shifting the turnover-determining transition state beyond the turnover-determining intermediate, and the energy span increases to 2.95 eV. These findings elucidate the temperature-dependent catalytic mechanism of RhCo₃ and provide theoretical insights for the rational design of efficient bimetallic catalysts for ammonia synthesis.