Understanding Activity Trends in Electrochemical Dinitrogen Oxidation over Transition Metal Oxides

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 (URHE > 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.