Role of transition metal d-orbitals in single-atom catalysts for nitric oxide electroreduction to ammonia

Recently, surging interests exist in direct electrochemical ammonia (NH3) synthesis from nitric oxide (NO) due to the dual benefit of NH3 synthesis and NO removal. However, designing highly efficient catalysts is still challenging. Based on density functional theory, the best ten candidates of transition-metal atoms (TMs) embedded in phosphorus carbide (PC) monolayer is screened out as highly active catalysts for direct NO-to-NH3 electroreduction. The employment of machine learning-aided theoretical calculations helps to identify the critical role of TM-d orbitals in regulating NO activation. A V-shape tuning rule of TM-d orbitals for the Gibbs free energy change of NO or limiting potentials is further revealed as the design principle of TM embedded PC (TM-PC) for NO-to-NH3 electroreduction. Moreover, after employing effective screening strategies including surface stability, selectivity, the kinetic barrier of potential-determining step, and thermal stability comprehensively studied for the ten TM-PC candidates, only Pt embedded PC monolayer has been identified as the most promising direct NO-to-NH3 electroreduction with high feasibility and catalytic performance. This work not only offers a promising catalyst but also sheds light on the active origin and design principle of PC-based single-atom catalysts for NO-to-NH3 conversion.