Dissociation mechanisms of CH₄ on pristine, N-doped and vacancy graphene by DFT study

Methane is used as the carbon precursor for the fabrication of C/C composites by using chemical vapor infiltration. Demystify the heterogeneous reactions on the carbon surface during the pyrolysis of methane is important for us to understand the formation and growth of the pyrocarbon. In this paper, the dissociation mechanisms from CH₄ to C₂H₂ on pristine graphene, N-doped graphene, and vacancy graphene during the chemical vapor infiltration process are systematically studied using the density functional theory. After the optimized adsorption and co-adsorption configurations are obtained, the transition state calculations are performed to identify the reaction pathways. The adsorption energy, energy barrier, reaction energy, Mulliken atomic charges, and partial density of states of the dissociation processes are analyzed. We revealed that the adsorption of molecules CH₄, C₂H₆, C₂H₄, and C₂H₂ on the substrates are physisorption, and the adsorption of CH₃/H, 2CH₃, C₂H₅/H, C₂H₅, C₂H₃/H, and C₂H₃ on these substrates are strong chemisorption. In addition, these reactions are easier to take place on the vacancy graphene, yet much harder on the N-doped graphene.