Surface engineering of nitride-based MXenes through oxygen vacancies and single-atom catalysts for enhanced nitrate reduction

The electrochemical nitrate reduction reaction (NO   3RR) offers a sustainable route for ammonia production and water remediation, but designing catalysts with high activity and selectivity remains a challenge. This study provides a comprehensive DFT-based investigation of transition metal (TM) single-atom catalysts (SACs) supported on oxygen-deficient nitride-based MXenes (O   v-M   2NO   2, M = Mo, Ti, V, W). Fourteen late TMs from Groups VIII, IB, and IIB were systematically screened by evaluating structural stability, nitrate adsorption affinity, and full reaction energetics. Among the candidates, Rh/O   v-Mo   2NO   2 exhibits exceptional catalytic performance, with a remarkably low limiting potential of –0.03 V and strong suppression of competing hydrogen evolution. Electronic structure analyses, including PDOS, COHP,    d-band center, and charge transfer, reveal that the superior performance stems from a synergistic interplay between the Rh single atom, the oxygen vacancy, and the Mo   2NO   2 substrate, optimizing the    d-band center for balanced adsorption and facilitating crucial reaction steps. This study presents Rh/O   v-Mo   2NO   2 as a promising catalyst and validates nitride MXenes as a platform for designing advanced NO   3RR catalysts.