Theoretical Insights into Metal-Catalyzed Associative Electrochemical Conversion of NO_<i>x</i> in Aqueous Solution

The present study investigated a mechanistic analysis of the electrocatalytic reduction of nitrogen oxides (NOx; x = 1, 2) into ammonia (NH3) on flat surfaces of selected middle-to-late d-block metals, viz., Fe, Co, Ni, Cu, Zn, Mo, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Our DFT calculations were carried out in aqueous solution via an implicit solvation model, also providing insights into the impact of the pH on the reduction process. At a mechanistic level, the first four reductive steps entail the O-hydrogenation from NOx via the path *NO2 → *NO2H → *NO (+H2O) → *NOH → *N (+H2O). Subsequently, the as-generated N adatoms are hydrogenated to form ammonia following the path *N → *NH → *NH2 → *NH3. In this mechanism, the conversions of *NO2 into *NO2H and *NO into *NOH were generally identified as the rate-determining steps. In addition, binding of NO2 and NO has been computed to be stronger than NH3 adsorption in most cases, which prevents ammonia poisoning on the metal surfaces. Finally, our results show the existence of periodic behaviors in all of the modeled stages of the NOx reduction mechanism, revealing strong correlation among the free energies involved in the steps *NO to *NOH and *NOH or *N to *NHx at different levels of hydrogenation.