Comparative DFT study of methanol decomposition on Mo₂C(001) and Mo₂C(101) surfaces

In this study, the complete reaction mechanism of methanol decomposition on metallic Mo₂C(001) and Mo/C-mixed Mo₂C(101) hexagonal Mo₂C crystalline phases was systematically investigated using plane-wave-based periodic density functional theory (DFT). The main reaction route for Mo₂C(001) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CHO + 3H → CO + 4H → C + O + 4H. Hence, C, O, and H are the main products. It was found that the energy barrier for CO dissociation was low. Therefore, it was concluded that the Mo₂C(001) surface was too active to be easily oxidized or carburized. The optimal reaction pathway for Mo₂C(101) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH₃ + O + H → CH₄ + O. Therefore, CH₄ is the major product. The hydrogenation of CH₃ leading to CH₄ showed the highest energy barrier and the lowest rate constant and should be the rate-determining step. In addition, the formation of CO + 2H₂ was competitive on Mo₂C(101), and the optimal path was CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH + O + 3H → C + O + 4H → CO + 2H₂. The computed energy barrier and rate constant indicate that the rate-determining step is the last step in CO formation. In agreement with the experimental observations, the results provide insights into the Mo₂C-catalyzed decomposition of methanol and other side reactions.

All calculations were performed by using the plane-wave based periodic method implemented in Vienna ab initio simulation package (VASP, version 5.3.5), where the ionic cores are described by the projector augmented wave (PAW) method. The exchange and correlation energies were computed using the Perdew, Burke and Ernzerhof functional with the latest dispersion correction (PBE-D3).