Theoretical study of ammonia synthesis catalysed by trimetallic clusters with or without a sumanene support

DFT calculations were utilized to explore the electrocatalytic nitrogen reduction reaction (NRR) mechanisms catalyzed by trimetallic clusters M₃ (M = Ti, Zr, V, and Nb), both unsupported and supported by bowl-shaped sumanene. The substrate enhanced N₂ adsorption and activation but hindered hydrogenation due to more negative adsorption energies. The substrate promoted hydrogenation of nitrogen, reducing the interference of the hydrogen evolution reaction (HER) and enhancing the NRR selectivity. Three fundamental and three mixed pathways were investigated, and the rate-determining step (RDS) was identified for each pathway. Through a consecutive pathway, V₃ exhibits the best catalytic performance with the free energy change of the RDS (ΔG_RDS) as 0.82 eV, while the optimal supported catalyst, Nb₃ supported on sumanene, has a ΔG_RDS of 1.43 eV. The introduction of the substrate generally increased ΔG_RDS by 0.3-0.8 eV. The substrate can effectively regulate the distance between metal atoms and reduce the change in geometric structures of M₃ clusters during the reaction process, thereby enhancing the structural stability of the active sites in the NRR process. The substrate can reduce the reactivity differences among catalysts with different metal types. This so-called blurring effect allows cheap metals to partially replace noble metals while maintaining catalyst performance. A linear correlation between charge changes on M₃ or M₃ together with the substrate and ΔG was observed, providing a potential method for optimizing the catalyst performance and designing new catalysts.