Understanding Activity Trends in Electrochemical Dinitrogen Oxidation over Transition Metal Oxides

Abstract

Nitric acid (HNO₃) is a critical commodity chemical produced on an enormous scale via oxidation of ammonia NH₃ in the Ostwald process and, as such, is responsible for a significant fraction of global greenhouse gas emissions. Formation of nitric acid by direct oxidation of dinitrogen via the electrochemical nitrogen oxidation reaction (N2OR) is an attractive alternative but has so far largely remained elusive. Toward advancing our fundamental understanding of the limitations of the N2OR, in this article, we investigated the competitive adsorption dynamics of nitrogen (N₂) and water oxidation intermediates such as hydroxide (OH) on a range of transition metal oxides. Using density functional theory (DFT) calculations, we explore three possible N2OR mechanisms: direct adsorption and dissociative adsorption of N₂, and a Mars-van Krevelen (MvK)-type mechanism involving the adsorption of N₂ on a surface-bound atomic oxygen. We observed a strong linear scaling relation between the adsorption energy of N₂ and OH on the metal-terminated transition metal oxide, suggesting that under typical highly oxidizing operating conditions for the N2OR (U_RHE > 1.24 V), water oxidation intermediates such as OH are likely to dominate the surface, leading to vanishingly small coverage of adsorbed N₂. From this result, we find that direct or dissociative adsorption of N₂ is unlikely, suggesting an MvK-type mechanism for the N2OR. Probing this mechanism further using DFT, we find that the reaction energetics are largely less favorable than water oxidation due to the high activation barrier for N₂ adsorption, which we find to be the rate-determining step for the process. Our experimental results corroborate these findings with the majority of tested catalysts exhibiting poor N2OR selectivity and a rate-determining step involving N₂(g). However, dynamic potential control emerged as a possible strategy to enhance N2OR activity as it may limit the oxygen evolution reaction (OER) and promote N₂ adsorption. This work underscores the challenges in achieving efficient N2OR, highlighting the need for unconventional catalyst designs and operational strategies, such as electrolyte engineering and dynamic potential control, to overcome the inherent kinetic and thermodynamic barriers.