Atomic catalyst supported on oxygen defective MXenes for synergetic electrocatalytic nitrate reduction to ammonia: A first principles study

The constant downscaling of catalysts to atomic catalyst with maximized atomic utilization efficiency provides a way for cost-efficient catalysis using a minimum amount of metals. However, the current studies are insufficient in the understanding of feasibility and mechanism of atomic catalyst on nitrate electroreduction toward ammonia (NRA). Herein, using first-principles calculations, we systematically explore the NRA mechanism of MXenes-based atomic catalysts TMn@M3C2O2 (TM = non-noble transition metal; M = Ti, V, Cr, Nb and Mo; n = 1, 2, and 3, which represents single-, double-, and triple-atom catalysts on MXenes). Nb3- and Nb2-modified V3C2O2 (Nb3@V3C2O2 and Nb2@V3C2O2) shows high NRA activity with low potential determination steps (−0.22 and −0.26 eV, respectively) and suppression to hydrogen evolution reaction (HER). The doping of Nb atoms causes a negatively upshift of the MXenes d-band center and creates more electron-deficient sites, which regulates the strong adsorption of nitrate and weaken the adsorption of *NHx (x = 1–3), thereby facilitating easy deoxidation and hydrogenation. This study not only theoretically verified the feasibility of atomic catalyst for NRA, but also provided a new idea for rational design of MXenes-based non-noble metal atomic catalyst.