The accelerating and retarding effects of hydrogen on carbon deposition on metal surfaces

Carbon deposition of benzene on iron was studied at 550-700°C with 0-1 atm hydrogen in the carrier gas. At least three types of carbon are formed: amorphous, graphitic and carbidic (Fe₃C). The surface of Fe₃C is essentially inactive for benzene decomposition. In the presence of H₂, a metallic surface is maintained resulting in a high activity and hence an accelerating effect by H₂. In the reaction system five competing reactions are involved and the net rate of carbon deposition is the sum of the individual rates. Based on the results in this study, the retarding effects of H₂ on carbon deposition reported in the literature can also be explained. The methanation reaction of surface carbon by H₂ becomes important under conditions when the surface is relatively inactive for hydrocarbon decomposition, and under such conditions, H₂ has a retarding effect on carbon deposition.