Fluctuation-Mediated Model for Hydrogen-Inhibited N₂ Dissociation on Iron─Implications for Ambient Ammonia...

By virtue of its ability to dissociate the inert N₂ molecule under ambient conditions, iron has been pursued as a candidate for prospective near-ambient ammonia electrosynthesis. The competition between adsorbate species in an electrochemical setting, however, leads to kinetic adsorption behavior that deviates significantly from the zero-coverage limit, and here, we present a theoretical framework that captures the possibility of dissociation mediated by fluctuations in otherwise site-blocking adsorbate configurations. While the (111) and (211) facets of iron are known to facilitate N₂ dissociation, systematic density functional computations reveal a restrictive increase in activation energy for realistic hydrogen coverages encountered under electrochemical conditions. A half-monolayer coverage of surface-bound hydrogen reduces the dissociation rate by a factor of ca. 10⁶ and 10³ on, respectively, Fe(111) and Fe(211). Low coverages are therefore required to maintain a significant rate of N₂ dissociation, establishing precise control of surface hydrogenation as another fundamental challenge for nitrogen reduction in proton-rich environments.