Predicting active, selective and stable Mo2C–based bimetallic carbides for direct deoxygenation and hydrogenation reactions: A computational screening

Abstract

Transition metal carbides (TMCs), especially molybdenum carbide (Mo₂C), represent an economical and attractive alternative to precious noble metal catalysts for processes involving hydrogenation (HYD) and direct deoxygenation (DDO) reactions such as hydrodeoxygenation (HDO). Although Mo₂C has good activity for DDO, it is not selective as it displays similar activity towards HYD as well. Moreover, stability against oxygen poisoning is a critical issue as it is the major reason of catalyst deactivation. Herein, using model compounds with different oxygen functionalities (phenol, guaiacol, and 5-HMF), we employed density functional theory (DFT) calculations to investigate three key reactions (C-O dissociation, oxygen removal and ring hydrogenation) representing activity, stability and selectivity which are critical to the HDO process on 45 Mo₂C-based bimetallic carbide catalysts (Co, Cr, Fe, Mn, Nb, Ni, Ti and Zr) obtained from the Materials Project database. Among these catalysts, different dopants, stoichiometric ratios and terminations were considered. First, reaction energies of oxygen removal were used to identify stable catalysts. Subsequently, with the energy barriers of DDO and HYD for activity and their differences for selectivity, best candidates for DDO (Mo-terminated Fe₃Mo₃C, Mn-terminated Mn₃Mo₃C and Ni-terminated Ni₃Mo₃C) and HYD (triclinic Mo-terminated Fe₁₁MoC₄ and Ni-terminated Ni₆Mo₆C) were identified. Since strong oxygen binding favors DDO but hinders oxygen removal, an optimal range of oxygen binding energy (OBE) between −100 kJ/mol to −200 kJ/mol has been proposed. In addition, a parameter encompassing the three critical aspects (activity, stability, and selectivity) of a catalyst was proposed. Using OBEas a descriptor and the proposed parameter, a qualitative trend was obtained which can be used for preliminary screening of large databases of catalysts, including TMCs other than Mo₂C-based, for processes involving DDO and HYD reactions.