Understanding activity trends in electrochemical dinitrogen oxidation over transition metal oxides

Nitric acid (HNO3) is a critical commodity chemical produced on enormous scale via oxidation of ammonia (NH3) in the Ostwald process and, as such, is responsible for a significant fraction of global greenhouse gas emission. Formation of nitric acid by directly oxidizing dinitrogen via the electrochemical nitrogen oxidation reaction (N2OR) is an attractive alternative, but has so far largely remained elusive. Towards advancing our fundamental understanding of the limitations of the N2OR, in this work 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 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 a 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 findings corroborate these findings, demonstrating that the majority of tested catalysts exhibited poor N2OR selectivity with a rate-determining step involving N2 (g), primarily facilitating the oxygen evolution reaction (OER). However, dynamic potential control emerged as a possible strategy to enhance N2OR activity, as it may limit OER and promote N2 adsorption. This work underscores the challenges in achieving efficient N2OR, highlighting the need for novel catalyst designs and operational strategies, such as electrolyte engineering and dynamic potential control, to overcome the inherent kinetic and thermodynamic barriers.