Theoretical Insights into Designing Single-Atom Catalysts on Defective MXenes for Efficient Reduction of Nitrate into Ammonia

Electrocatalytic nitrate reduction reaction (NO3RR) is a promising approach for converting nitrate into environmentally benign or even value-added products such as ammonia (NH3) using renewable electricity. However, the poor understanding of the catalytic mechanism on metal-based surface catalysts hinders the development of high-performance NO3RR catalysts. In this study, we have systematically explored the NO3RR mechanism of single-atom catalysts (SACs) by constructing single transition metal atoms supported on MXene with oxygen vacancies (Ov-MXene) using density functional theory (DFT) calculations. Our results indicate that Ag/Ov-MXene (for precious metal) and Cu/Ov-MXene (for non-precious metal) are highly efficient SACs for NO3RR toward NH3, with low limiting potentials of −0.24 and −0.34 V, respectively. Furthermore, these catalysts show excellent selectivity towards ammonia due to the high energy barriers associated to the formation of byproducts such as NO2, NO, N2O, and N2 on Ag/Ov-MXene and Cu/Ov-MXene, effectively suppressing the competitive hydrogen evolution reaction (HER). Our findings not only offer new strategies for promoting NH3 production by MXene-based SACs electrocatalysts under ambient conditions but also provide insights for the development of next-generation NO3RR electrocatalysts.