Understanding the Dynamics of C-OH Bond Activation over Mo 2 C under Hydrodeoxygenation Reaction Conditions

Hydrodeoxygenation (HDO) of biomass to value-added chemicals using transition metal carbides (TMCs) serves as a two-fold solution to overreliance on fossil-based feedstocks and excessive usage of noble metal catalysts. Specifically, Mo 2 C and W 2 C have emerged as viable alternatives to noble metal catalysts for HDO reactions. However, the underlying reaction mechanisms and structure-activity relationships for TMC catalysts are still not fully understood. Our previous mechanistic studies (Meena et al. ACS Catalysis, 2023, 13, 13446–13455), based on static DFT calculations and microkinetic modeling, indicated that C-OH bond dissociation (C 4 H 9 OH⋆ →C 4 H 9 ⋆ + OH⋆) is the rate-determining step (RDS) of HDO reaction over Mo 2 C. Nevertheless, these static calculations, which neglect thermal fluctuations, cannot fully capture the catalysts’ behavior under working conditions. To address this limitation, here, we present a comprehensive computational investigation of the RDS under more realistic reaction conditions using ab initio Molecular Dynamics simulations combined with the path metadynamics enhanced sampling technique. Path metadynamics runs reveal that the Mo 2 C surface is stable at the reaction temperature and no significant surface reconstructions take place. Further, the C-OH activation barrier obtained at 623 K using path metadynamics (38 kcal/mol) is higher than that from static DFT calculation (30 kcal/mol), likely emphasizing the significance of entropic contributions. The comparison of the barriers of C-OH activation over Mo 2 C and Zr- and Co-doped Mo 2 C surfaces revealed similar activity trends compared to that from static DFT, i.e., Zr doping improves the activity and Co doping inhibits the activity