A Metal Coordination Number Determined Catalytic Performance in Manganese Borides for Ambient Electrolysis of Nitrogen to Ammonia

Abstract

A new strategy that can effectively increase the nitrogen reduction reaction performance of catalysts is proposed and verified by tuning the coordination number of metal atoms. It is found that the intrinsic activity of Mn atoms in the manganese borides (MnB_x) increases in tandem with their coordination number with B atoms. Electron-deficient boron atoms are capable of accepting electrons from Mn atoms, which enhances the adsorption of N₂ on the Mn catalytic sites (*) and the hydrogenation of N₂ to form *NNH intermediates. Furthermore, the increase in coordination number reduces the charge density of Mn atoms at the Fermi level, which facilitates the desorption of ammonia from the catalyst surface. Notably, the MnB₄ compound with a Mn coordination number of up to 12 exhibits a high ammonia yield rate (74.9 ± 2.1 µg h^-1 mg_cat ^-1) and Faradaic efficiency (38.5 ± 2.7%) at -0.3 V versus reversible hydrogen electrode (RHE) in a 0.1 m Li₂SO₄ electrolyte, exceeding those reported for other boron-related catalysts.