Functionalized MXenes for efficient electrocatalytic nitrate reduction to ammonia

The electrocatalytic reduction of nitrate to ammonia is of scientific and practical significance. The feasibility of using two-dimensional transition metal carbides, M3C2 MXenes, as an electrocatalyst for converting NO3- to NH3, is demonstrated by using density functional theory calculations. Reaction active center analysis reveals that the reaction prefers to occur on the basal plane rather than the edge plane. By systematically analyzing thermodynamic and kinetic aspects, the most probable reaction pathway was determined to be consecutive deoxygenation followed by hydrogenation: NO3- -> *NO3 -> *NO2 -> *NO -> *N -> *NH -> *NH2 -> *NH3 -> NH3(g). Furthermore, it's found that the deoxygenation processes are exothermic while the hydrogenation processes are endothermic. Both catalytic deoxygenation and hydrogenation processes in NRA are substantially affected by pH. Thus, the rate-determining step and overpotential exhibit pH dependent characteristics. For unfunctionalized MXenes, the NRA is suppressed due to the strong hydrogen evolution reaction (HER). By functionalization, the NRA catalytic activity of Ti3C2 and transition metal doped Ti3C2 MXenes increases effectively. This improvement is attributed to the high oxidation states of Ti atoms at catalytic centers and weakening of *NHx adsorption on the MXene surfaces, thereby facilitating easy hydrogenation. In particular, partially O-vacant Ti3C2O2 is recognized as one promising NRA electrocatalyst, with the free energy change of every reaction step being negative. The findings of this work provide new strategies for the rational design of MXene-based NRA electrocatalysts with universal significance.

Automated summary

This study demonstrates the feasibility of using two-dimensional transition metal carbides, M3C2 MXenes, as an electrocatalyst for the reduction of nitrate to ammonia. Through density functional theory calculations, it is found that the reaction prefers to occur on the basal plane. By analyzing the thermodynamics and kinetics, the most probable reaction pathway is determined to be consecutive deoxygenation and hydrogenation. Functionalization of MXenes enhances the catalytic activity, with partially O-vacant Ti3C2O2 identified as a promising NRA electrocatalyst. These findings provide new strategies for the rational design of MXene-based NRA electrocatalysts with universal significance.