Revealing the origin of activity and selectivity in nitrate to ammonia conversion on single transition metal atom catalysts supported by a Ti2NO2 monolayer

In recent years, the issue of nitrate (NO₃^-) contamination has become an increasingly severe problem for human life. Electrocatalytic nitrate reduction to ammonia (NH₃) is one of the promising strategies to eliminate nitrate contamination. However, the reduction of NO₃^- to NH₃ is a multi-electron process and is susceptible to interference from by-products and competing hydrogen evolution reactions (HER). Introducing a single transition metal (TM) atom onto MXene-based surfaces can alter MXenes' electronic configuration, enhancing their catalytic performance and the Faraday efficiency of the nitrate reduction reaction (NO₃RR). In this work, we investigated the initial activation mechanisms of nitrate on various TM-modified Ti₂NO₂ (TM@Ti₂NO₂) catalysts and their NO₃RR performance using first-principles calculations, aiming to select effective NO₃RR electrocatalysts. The results indicated that both V@Ti₂NO₂ and Cr@Ti₂NO₂ were viable catalysts for NO₃RR, showing particular promise for the efficient conversion of NO₃^- to NH₃ at the most favorable limiting potentials of -0.41 V and -0.52 V, respectively. Further electronic structure analysis (density of states, COHP, and the descriptor ψ) confirmed that the single TM atom supported the boost in product selectivity and efficiency of NH₃ by acting as an electron "bridge" to strengthen the interaction between NO₃^- and MXenes. AIMD simulations indicated that V@Ti₂NO₂ and Cr@Ti₂NO₂ maintained dynamical stability at the reaction temperature. These findings lay the foundation for a deeper understanding of the initial activation mechanisms and provide fresh theoretical insights into the design of MXene-based electrocatalysts with high NO₃RR performance.