The role of carbon deposits in metal-catalysed reactions of hydrocarbons

The interaction of hydrocarbons with metal surfaces usually results in the formation of strongly held residues either by dehydrogenation or by polymerization or by fragmentation into monocarbon species. The extent and structure of this deposit, which may be characterized in particular by AES, SIMS and LRS, depends on the nature of the metal or the composition of the alloy used: large ensembles of the active metal atoms favour disruption of hydrocarbon molecules, which is also helped by higher temperatures and low H-2 pressures. Excessive multiple bond formation with the surface is inhibited by chemisorbed H atoms and by alloying the active metal either with an inert component that interferes with the sites at which disruption occurs, or with an element that is more active in hydrogenolysis of carbon-metal bonds. Monocarbon species can homologate; species such as ethylidyne (=C-CH3) may participate in alkene hydrogenation in the steady state; and excessively dehydrogenated forms of alkanes act as poisons for their hydrogenolysis. Partial deactivation of Pt catalysts by these species alters product selectivities for alkane reactions in useful and informative ways.