Understanding the Synergy between Fe and Mo Sites in the Nitrate Reduction Reaction on a Bio-Inspired Bimetallic MXene Electrocatalyst

Mo- and Fe-containing enzymes catalyze the reduction of nitrate and nitrite ions in nature. Inspired by this activity, we study here the nitrate reduction reaction (NO3RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo2CTx : Fe (T-x are oxo, hydroxy and fluoro surface termination groups). Mo2CTx : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo2CTx) and features a Faradaic efficiency (FE) and an NH3 yield rate of 41 % and 3.2 mu mol h(-1) mg(-1), respectively, for the reduction of NO3- to NH4+ in acidic media and 70 % and 12.9 mu mol h(-1) mg(-1) in neutral media. Regardless of the media, Mo2CTx : Fe outperforms monometallic Mo2CTx owing to a more facile reductive defunctionalization of T-x groups, as evidenced by in situ X-ray absorption spectroscopy (Mo K-edge). After surface reduction, a T-x vacancy site binds a nitrate ion that subsequently fills the vacancy site with O* via oxygen transfer. Density function theory calculations provide further evidence that Fe sites promote the formation of surface O vacancies, which are identified as active sites and that function in NO3RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.