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

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 (MnBx) 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 mu g h-1 mgcat-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. Multivalent Mn-atom possesses intrinsic capability to synthesize a series of compounds of varied Mn coordination numbers upon reacting with numerous p-block elements (electron-deficient B-atom). Ambient electrocatalytic activity for N2 reduction to NH3 of Mn2B, MnB, MnB2, and MnB4 enhances as Mn coordination numbers increase for MnBx in the neutral electrolyte (0.1 m Li2SO4) as confirmed both experimentally and theoretically. image