Initial Steps in CH4 Pyrolysis on Cu and Ni

Methane pyrolysis on solid catalysts begins with C–H bond dissociation. In the formation of solid carbon, the initial C–C bond formation is thought to occur on/in the surface of most transition-metal catalysts and not in the gas phase. This has been shown to be true for Ni surfaces. In CH4–D2 exchange reactions on polycrystalline Ni, CD4 is the major product observed, which is consistent with surface-mediated dehydrogenation. However, on Cu surfaces, CH3D is the dominant exchange product observed, suggesting that deep dehydrogenation is unfavorable. Further, in a methane flow cell reactor, the carbon density and crystal size of the graphene formed from pyrolysis on Cu surfaces depend on the distance downstream in the flow field, whereas no such dependence is observed for Ni. Density functional theory calculations support an entropically favorable pathway, whereby on Cu after dissociative chemisorption of methane, the surface adsorbed CH3° desorbs rather than undergoing further dehydrogenation; on Ni, complete dehydrogenation is favorable. Reactive methyl radicals from Cu surfaces would participate in gas-phase pyrolysis upon desorption, forming the initial C–C bond in the gas phase with subsequent re-adsorption and surface modification of gas-phase generated oligomers.