Screening MXene-based single-atom catalysts for selective nitrate-to-ammonia electroreduction

Abstract

Electrocatalytic nitrate reduction to ammonia (NRA) is a promising way to regulate the global nitrogen cycle and synthesize ammonia. The key factor to achieve efficient NRA is to develop desired catalysts with high activity, selectivity and stability. Herein, a new family of single-atom catalysts (SACs) for NRA are screened by anchoring transition metal (TM: 3d = Sc–Zn, 4d = Y–Cd, 5d = La–Hg) atoms on two-dimensional Ti3C2O2 MXene (Ti3C2O2-TM). Using density functional theory calculations, the surface O functional group on Ti3C2O2 is found to deliver high anchoring ability for TM, resulting in excellent anti-dissolution stability. Besides, strong p-d coupling between the anchored TM and the nitrate facilitates nitrate activation. Ten Ti3C2O2-TMs with orbitals around d5 (TM = Cr, Mn, Fe, Tc, Ru, W, Re, Os, Ir, and Pt) are first screened out based on the relatively high unsaturated anchored TM d orbitals at Fermi level to achieve good stability and nitrate activation. The most favorable reaction pathway is determined being the continuous deoxygenation, followed by hydrogenation: NO 3 − → *NO3 → *NO2 → *NO → *N → *NH → *NH2 → *NH3 → NH3(g). Three Ti3C2O2-TMs including Ti3C2O2-CrSA, Ti3C2O2-ReSA, and Ti3C2O2-OsSA with low overpotentials are regarded as the outstanding catalysts for NRA. These findings could open up new strategies for an advanced ammonia synthesis route with low energy consumption and low carbon emission.