The Importance of Molybdenum(IV) Active Sites in Promoting Electrochemical Reduction of N2 to NH3 with MoFe Bimetallic Catalysts

Abstract

To overcome the low faradaic efficiency (FE) of single Mo or Fe based electrocatalysts in nitrogen reduction reactions (NRR) due to the competition from the hydrogen evolution reaction (HER), a series of bimetallic MoFe compound catalysts were prepared under an NH3 atmosphere through a facile precipitation-pyrolysis method. The formed tetravalent Mo was found to be capable of inducing better electronic interactions between the surface nitrogen species and the Fe metal groups, thus improving the FE. It was demonstrated that the prepared ternary MoFe-N catalyst exhibited a remarkable FE of 33.26 % and a high NH3 yield rate of 33.31 μg h−1 mg−1 cat. for NRR, which was believed to have been caused by an obvious change in the valence of Mo that resulted in a lower HER activity. X-ray photoelectron spectroscopy analysis further revealed that thermal processing under an NH3 atmosphere formed the Mo(IV) active sites in Mo–N bond, which led to a significant suppression in HER activity. Finally, through the study of the surface hydrogenation mechanism, it was concluded that the synergistic effect of the adsorbed H* and Mo active sites was the main reason for the improved performance of NRR.