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

CONTEXT

In this study, the complete reaction mechanism of methanol decomposition on metallic MoC(001) and Mo/C-mixed MoC(101) hexagonal MoC crystalline phases was systematically investigated using plane-wave-based periodic density functional theory (DFT). The main reaction route for MoC(001) is as follows: CHOH → CHO + H → CHO + 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 MoC(001) surface was too active to be easily oxidized or carburized. The optimal reaction pathway for MoC(101) is as follows: CHOH → CHO + H → CHO + 2H → CH + O + 2H → CH + O + H → CH + O. Therefore, CH is the major product. The hydrogenation of CH leading to CH showed the highest energy barrier and the lowest rate constant and should be the rate-determining step. In addition, the formation of CO + 2H was competitive on MoC(101), and the optimal path was CHOH → CHO + H → CHO + 2H → CH + O + 2H → CH + O + 3H → C + O + 4H → CO + 2H. 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 MoC-catalyzed decomposition of methanol and other side reactions.

METHODS

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).