Increasing Mo5+ in M-doped La2(MoO4)3 (M = Fe, Co, Ni, Cu, and Zn) toward efficient electrocatalytic nitrogen fixation

Abstract

The efficiency of the nitrogen reduction reaction (NRR) is limited by the stability of N₂ and the sluggish reaction. In this work, a method to improve the NRR performance by modulating the Mo⁵⁺ content through A-site M doping is proposed. We synthesized A-site M (M = Fe, Co, Ni, Cu, and Zn) doped La₂(MoO₄)₃ by a hydrothermal method. The enhanced electron-donating capacity of M (Fe > Co > Ni > Cu > Zn) facilitates the conversion of Mo⁶⁺ to Mo⁵⁺. The obtained order of Mo⁵⁺ percentage is (Fe: 52% > Co: 39% > Ni: 32% > Cu: 21% > Zn: 18%), which was proved by X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Importantly, the percentage of Mo⁵⁺ is positively correlated with the NH₃ yield rate, faradaic efficiency (FE), and N₂ adsorption energy. This correlation is because Mo⁵⁺ activates the N₂ and promotes the hydrogenation reaction. Accordingly, the Fe-doped La₂(MoO₄)₃ (Fe–LaMo) exhibits the highest Mo⁵⁺ content and presents advanced NRR performance (30.4 μg h⁻¹ mg_cat⁻¹, 3.6%). The effort of Mo⁵⁺ is discussed. Meanwhile, the percentage of Mo⁵⁺ can be controlled by transition metal doping, which enables the modulation of the catalytic performance.