Inverted Trends of the Brønsted–Evans–Polanyi Relation in N2 Dissociation Originated from a Bonding-Dependent Adsorption Mechanism

Abstract

The search for efficient Haber–Bosch catalysts toward ammonia production under mild conditions is never-ending, which is greatly limited by the Brønsted–Evans–Polanyi (BEP) relationship. Great efforts have been put into optimizing the BEP relations and achieving the Sabatier optimum, which requires a balance between the dissociation and hydrogenation of nitrogen. However, challenges in this field inspire us to believe that completely breaking the linear BEP relations is indeed the final target although out of sight in such a holy grail reaction. Here, based on the first-principles calculations, we discover inverted trends of BEP relation of N₂ dissociation to approach the kinetic optimum of ammonia synthesis on Fe-based single-atom alloys. It is found that the adsorption characteristic of N–N transition states follows the 10-electron count rule, while that of the final states mimics the d-band model, which accounts for the inversion. Crystal orbital Hamiltonian populations (COHP) and Bader charge analysis further corroborate that a bonding-dependent adsorption mechanism lies at the root of the inverted trends of the BEP relation. Our finding not only paves the way for the milder Haber–Bosch process but also promotes explorations of breaking the linear BEP relations of the critical steps in various chemical reactions.