(710g) Elucidating the Role of Oxygen and Hydrogen in the Early Elementary Steps of Nitrate Reduction

The nitrate reduction reaction provides a way to cleanse nitrate-leached wastewater, or regenerate ammonia with renewable electricity. Recent literature has suggested that the rate determining step for nitrate reduction is the initial step of dissociating nitrate, and unusually consider it as a thermochemical step. In this study, we dive into the early elementary steps from NO₃^- to NO* across different transition metal surfaces to determine whether protonic hydrogen play any role in electrochemically catalyzing the reaction. We use density functional theory to determine the adsorption and kinetic mechanisms of nitrate reduction within an explicit aqueous electrochemical double layer. We find that hydrogen coverage does play a role in weakening the binding energetics of nitrogen intermediates. Although in contrast to earlier studies, our microkinetic model shows that nitrate dissociation is generally facile on most transition metals, with the exception of precious metals where the protonation of nitrate is a necessary step. Notably, further reduction of nitrite towards ammonia may require competitive removal of the oxygen specie. We present our findings by modeling nitrate reduction on a TiH₂(111)surface.