Design of 3d transition metal-embedded asymmetric HMo₂CF for electrocatalytic conversion of N₂ to NH₃

The electrochemical reduction of N₂ to NH₃ (NRR) is challenging due to the lack of efficient catalysts under mild conditions. We constructed a series of 3d-transition-metal (3d-TM)-embedded asymmetric 2D MXene HMo₂CF with one H or F vacancy (H_v or F_v) based on first-principles calculation. Due to the strong steric effect of the surface-covered H or F terminals, N₂ is favored to be adsorbed on H_v or F_v through the end-on mode rather than the side-on mode. Compared to NRR on the exposed Mo at F-vacancy (denoted as Mo^Fv) of HMo₂CF_v, TM-substituted (TM = V, Cr, Mn, and Fe) Mo^Fv improved NRR activities by reducing the barriers to 0.70, 0.59, 0.58, and 0.72 eV, respectively, from the original 0.81 eV. Particularly, Mn- or Cr-embedded HMo₂CF_v exhibited the best catalytic performances among 3d-TMs (Ti-Ni) undergoing alternating or distal mechanism, where the potential determining step (PDS) occurs at the first hydrogenation of N₂ to NNH with a barrier of 0.58 or 0.59 eV. For TM-substituted (TM = Ti to Ni) Mo adjacent to F-vacancy, the catalytic barriers varied slightly in the range of 0.72 to 0.82 eV. The adsorption energy comparison indicated that TM-embedded HMo₂CF exhibited higher selectivity toward N₂ end-on adsorption than H₂ to initialize the following NRR process. Greater electron transfer between TM-N₂ was ascribed to the moderate N₂ adsorption. Our work is expected to provide an insightful method for designing efficient NRR catalysts.