  We study the dissociative adsorption of methane at the surface of graphene.
Free energy profiles, which include activation energies for different steps of
the reaction, are computed from constrained ab initio molecular dynamics. At
300 K, the reaction barriers are much lower than experimental bond dissociation
energies of gaseous methane, strongly indicating that graphene surface acts as
a catalyst of methane decomposition. On the other hand, the barriers are still
much higher than on nickel surface. Methane dissociation therefore occurs at a
higher rate on nickel than on graphene. This reaction is a prerequisite for
graphene growth from precursor gas. Thus, the growth of the first monolayer
should be a fast and efficient process while subsequent layers grow at
diminished rate and in a more controllable manner. Defects may also influence
reaction energetics. This is evident from our results, in which simple defects
(Stone-Wales defect and nitrogen substitution) lead to different free energy
landscapes at both dissociation and adsorption steps of the process.
