Computational insights and strategic choices of nitrate and nitric oxide electroreduction to ammonia

Abstract

Electrochemical nitrate reduction (eNO₃RR) and nitric oxide reduction (eNORR) to ammonia have emerged as promising and sustainable alternatives to the traditional Haber-Bosch method for ammonia production, particularly within the recently proposed reverse artificial nitrogen cycle route: N₂ → NO_x → NH₃. Notably, experimental studies have demonstrated that eNORR exhibits superior performance over eNO₃RR on Cu₆Sn₅ catalysts. However, the fundamental mechanisms underlying this difference remain poorly understood. Herein, we performed systematic theoretical calculations to explore the reaction pathways, electronic structure effects, and potential-dependent Faradic efficiency associated with ammonia production via these two distinct electrochemical pathways (eNORR and eNO₃RR) on Cu₆Sn₅. By implementing an advanced ‘adaptive electric field controlled constant potential (EFC-CP)’ methodology combined with microkinetic modeling, we successfully reproduced the experimental observations and identified the key factors affecting ammonia production in both reaction pathways. It was found that eNORR outperforms eNO₃RR because it circumvents the *NO₂ dissociation and *NO₂ desorption steps, leading to distinct surface coverage of key intermediates between the two pathways. Furthermore, the reaction rates were found to exhibit a pronounced dependence on the surface coverage of *NO in eNORR and *NO₂ in eNO₃RR. Specifically, the facile desorption of *NO₂ on the Cu₆Sn₅ surface in eNO₃RR limits the attainable surface coverage of *NO, thereby impeding its performance. In contrast, the eNORR can maintain a high surface coverage of adsorbed *NO species, contributing to its enhanced ammonia production performance. These fundamental insights provide valuable guidance for the rational design of catalysts and the optimization of reaction routes, facilitating the development of more efficient, sustainable, and scalable techniques for ammonia production.