Origins of Nitrite and Nitrate Selectivity in Aqueous Electrocatalytic Ammonia Oxidation by a Mononuclear Copper...

The electrocatalytic oxidation of ammonia to nitrite and nitrate catalyzed by the mononuclear copper-based complex [Cu(pyalk)₂] (pyalk = 2-(pyridin-2-yl)propan-2-oate) has recently demonstrated remarkable electroselectivity (preferential formation of nitrate or nitrite as a function of the applied potential) and chemoselectivity (water vs ammonia oxidation as a function of pH). However, the mechanistic origin of these selectivities remains unclear. Herein, we provide new insights into the complete electrochemical catalytic cycle of ammonia oxidation (AO) in aqueous media from quantum mechanical calculations. Our results show that the transformation does not proceed through a single oxyl-centered pathway, but through competing oxidation-driven sequences that activate Cu complexes to enable successive N-O bond formation. New mechanistic scenarios are identified in which water participates as a nucleophile, while the ammonia buffer plays a crucial role throughout the catalytic cycle. In addition, ammonia saturation enables participation in the secondary coordination sphere, promoting key intermolecular interactions favorable to N-O bond coupling. Finally, we demonstrate that subsequent electrochemical oxidation steps act as the primary driving forces governing the experimentally observed selectivity. Overall, this work establishes an unprecedented mechanistic framework for copper-mediated ammonia oxidation and provides molecular-level insights into the factors that control selectivity in intricate electrocatalytic nitrogen transformations.