DFT Study on Efficient Electrocatalytic Nitric Oxide Reduction to Hydroxylamine over TM/Ti3C2O2 in Aqueous Solution

Abstract Electrocatalytic selective reduction of nitric oxide (NORR) provides a feasible pathway for the green synthesis of hydroxylamine (NH2OH) under mild conditions. Maintaining N–O bond integrity is critical for NH2OH selectivity, as it prevents the formation of ammonia. Single-atom catalysts (SACs) have garnered widespread attention in regulating hydroxylamine selectivity due to their ability to modulate the adsorption configuration of NO. Herein, 27 transition metal TM/Ti3C2O2 SACs were designed, and their NORR reaction mechanism was explored via density functional theory (DFT) calculations. The results confirm that Cu/Ti3C2O2 is the most promising SAC, with the first NO hydrogenation step serving as the potential-determining step (PDS) of NORR. Based on implicit and explicit solvation models, Gibbs free energy calculations demonstrate that solvation effects strengthen N–O bond and boost NORR activity. Simultaneously, these effects elevate the barrier for NH2OH dehydration to ammonia, thereby improving product selectivity. Analysis of pH and applied potential effects indicate that an acidic environment is more favorable for the NORR process, and hydroxylamine selectivity increases with the elevation of reduction potential. Moreover, in the explicit solvation environment containing 25 H2O molecules, the structure of the *H2NO intermediate is altered due to hydrogen bonding, subsequently affecting the reaction pathway and corresponding PDS. This work not only predicts highly efficient NORR catalysts but also reveals the mechanism of solvation effects in near-realistic solutions, providing theoretical guidance for the design of single-atom catalysts and the experimental synthesis of hydroxylamine.