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

CONTEXT

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.

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).

Mechanistic insights into the dehydrogenation of formaldehyde, formic acid and methanol using the Pt4 cluster as a promising catalyst

Reaction mechanisms of the dehydrogenation of formaldehyde, formic acid and methanol on the Pt₄ cluster were computationally investigated using density functional theory (DFT) with the B3LYP functional in the conjunction with the aug-cc-pVTZ basis sets for H, C and O atoms, and the cc-pVDZ-PP basis set for Pt. Herein, the key mechanistic aspects of three possible pathways of the dehydrogenation of these compounds are summarized. The results indicate that the formation of H₂ and CO or CO₂ molecules is more energetically favorable than the generation of H and H₂O, HCHO products. Generally, the formation of H₂ molecule in the presence of catalysts is more favorable than the direct decomposition of either HCHO, HCOOH or CH₃OH molecule. The use of Pt₄ catalyst significantly reduces the energy barriers for C-H and O-H bond cleavage of all three compounds to 14, 9 and 12 kcal/mol, respectively. The decomposition of HCOOH is found to be the most energetically favorable. In addition, the mechanistic insights of the reactions confirm the reduction of the energy barriers of the gas-phase dehydrogenation by 67-82 kcal/mol and bring it to the values smaller than 14 kcal/mol in the presence of the Pt₄ catalysts.