Regulation of transition metal atoms supported on defective h-BN by adjacent monovacancies for electrochemical CO2 reduction: mechanism and d-band spin-polarization effect

Abstract

Transition metal (TM)-doped defective hexagonal boron nitride (h-BN) single-atom catalysts (SACs) show significant promise for the electrochemical carbon dioxide reduction reaction (CO₂RR). Given that defect engineering has emerged as a potent strategy for enhancing the catalytic performance of two-dimensional SACs and extensive experimental studies have observed that doping transition metals into defective two-dimensional substrates promotes the formation of adjacent vacancies, a comprehensive theoretical investigation is essential to elucidate the impact of different adjacent vacancies on the catalytic properties of TM-doped h-BN SACs. This study employs density functional theory calculations to investigate the regulatory effects and mechanisms of five types of adjacent boron and nitrogen monovacancies on the CO₂RR catalytic performance of Fe, Co, and Mo atoms anchored on defective h-BN (denoted as M-vac@BN, where M = Fe, Mo, Co and vac = B₁, B₂, B₃, N₁, N₂). Stability analysis reveals that the position and type of adjacent monovacancies significantly impact the stability of the supported metal atoms. Volcano plot and linear relationship analysis demonstrate that the CO adsorption energy (E_B(CO)) serves as a reliable descriptor for predicting the overpotential for CO₂RR on M-vac@BN. Strategic introduction of specific adjacent monovacancies can effectively tune the CO adsorption strength, thereby influencing the catalytic activity. More interestingly, a strong linear relationship is observed between the magnetic moments of transition metal atoms M in M-vac@BN (M = Co, Mo) and the integrated projected crystal orbital Hamiltonian population (IpCOHP) of the M–C bonds in CO adsorption intermediates, which arises from the linear relationship between the M–C bond strength in CO adsorption intermediate and the d-band spin polarization of the M atom in M-vac@BN. Specifically, enhanced d-band spin polarization strengthens the M–C bond by broadening the bonding peak in the projected crystal orbital Hamiltonian population (pCOHP) of the M–C bond.