Carbon deposition and hydrocarbon formation on group VIII metal catalysts

Carbon formed from CO in CO2 at around 500 degrees C deposited as nanotubes and encapsulating carbons on a supported Ni catalyst without H-2 or as filaments if H-2 was present. A thermodynamic model explains how hydrogen in low concentrations controls filament morphology and why equilibrium is shifted from that for graphite during carbon deposition. Carbon deposition reaction rates at low carbon activity in the absence of hydrogen are reported. A new concept of the rate limiting step for metal-catalyzed carbon formation is proposed, where conditions inside the bulk metal are at least as important as the surface chemistry. A mechanism with an unusual type of active site can qualitatively explain carbon formation rates through a broad range of reaction conditions. When hydrogen is present, a series of hydrocarbons are believed to form, as in fuel synthesis (Fischer-Tropsch) chemistry. Surface vinyl species that have been recently shown to be intermediates in Fischer-Tropsch chemistry also may polymerize to form graphene. The formation of vinyls from CO and H via surface alkyls occurs at a greater rate than methane formation when the supply of hydrogen is limited. Hydrogen from the bulk catalyst metal (not surface adsorbed) hydrogenates the surface alkyls, indicating that hydrogen solubility may control the metal-catalyzed formation of various hydrocarbons and eventually solid graphitic carbon.