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

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 (NO₃ RR) catalyzed by an Fe-substituted two-dimensional molybdenum carbide of the MXene family, viz., Mo₂ CT_x  : Fe (T_x are oxo, hydroxy and fluoro surface termination groups). Mo₂ CT_x  : Fe contains isolated Fe sites in Mo positions of the host MXene (Mo₂ CT_x ) and features a Faradaic efficiency (FE) and an NH₃ yield rate of 41 % and 3.2 μmol h^-1  mg^-1 , respectively, for the reduction of NO₃ ^- to NH₄ ⁺ in acidic media and 70 % and 12.9 μmol h^-1  mg^-1 in neutral media. Regardless of the media, Mo₂ CT_x  : Fe outperforms monometallic Mo₂ CT_x 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 NO₃ RR in close analogy to the prevailing mechanism of the natural Mo-based nitrate reductase enzymes.