Valence Electron Configuration: A Missed Key Descriptor for Ammonia Synthesis Catalysis

Despite the historical significance of ammonia synthesis, conventional models (which identify multi-atom ensembles as active sites) fail to rationalize periodic trends in catalytic activity. In this work, we propose a novel mechanistic framework governed by the valence electronic configuration of metal atoms, specifically the availability of exposed half-filled orbitals (HFOs). It is assumed that catalytic efficiency relies on single active site to simultaneously cleave H-H and N-N bonds while serving as templates to facilitate N-H bond formation, which is suggested here as the rate-determining step. The model suggests that Group 8 metals (Fe, Ru, Os), possessing four HFOs, may mediate the formation of two N-H bonds per N-H bond formation step. Conversely, Group 9 metals, with three HFOs, are proposed to be kinetically limited to forming a single N-H bond, while Groups 10-12 lack sufficient HFOs for supporting N-H bond formation. Furthermore, the inactivity of Groups 4-7 might be attributed to stable nitride formation occupying available orbitals. By correlating catalytic performance directly with intrinsic electronic configurations rather than empirical macroscopic descriptors, and by framing the catalyst as an orbital template that directs bond reorganization rather than merely a substrate for intermediate adsorption/desorption, this framework may explain the necessity of catalysts and offer a theoretical basis for a priori catalyst design tailored to specific atomic properties.