Single molybdenum atom anchored on 2D Ti2NO2 MXene as a promising electrocatalyst for N2 fixation

Abstract

The electrocatalytic synthesis of ammonia (NH₃) at ambient temperature is an attractive and challenging subject in the chemical industry. The synthesis of NH₃ under ambient conditions requires efficient and stable electrocatalysts with ultralow overpotential to ensure low energy consumption and high NH₃ yield. Herein, electrocatalysts consisting of a single transition metal (TM) atom (TM = Mo, Mn, Fe, Co, Ni, or Cu) anchored on 2D M₂NO₂ MXene (M = Ti, V, and Cr), designated as TM/M₂NO₂, are designed for N₂ reduction reaction (NRR) by density functional theory calculations. The results show that the bonding strength between Mo and Ti₂NO₂ is strong. The overpotential (η_NRR) of Mo/Ti₂NO₂ surface-catalyzed NRR is estimated to be as low as 0.16 V via an enzymatic mechanism, which is lower than those reported to date. For Mo/V₂NO₂ and Mo/Cr₂NO₂ catalysts, the NRR occurs through the consecutive mechanism and enzymatic mechanism, with corresponding η_NRR values of 0.38 V and 0.22 V, respectively. In addition, the reaction Gibbs free energy of NH₃ desorption from the Mo/Ti₂NO₂ surface is only 0.12 eV. Electronic structure analysis indicates that Mo/Ti₂NO₂ shows metallic characteristics, which ensures the efficient transfer of electrons between Mo and Ti₂NO₂. Ab initio molecular dynamics simulations indicate that the Mo atom can be stably immobilized on the Ti₂NO₂ substrate to prevent its aggregation into Mo clusters. Further analysis illustrates that hydrogen adsorption is not favored on the Mo/Ti₂NO₂ surface. Mixing the N₂ source with extra gases, such as NO₂, NO, SO₂, SO, and O₂, should be avoided for NRR on Mo/Ti₂NO₂ surface. These predictions offer a new opportunity for the electrocatalytic synthesis of NH₃ by N₂ reduction in the future.