Carbonaceous deposition associated with the catalytic steam-reforming of hydrocarbons over nickel alumina catalysts

Abstract

The reactions of n-pentane, n-hexane, n-heptane, 1-heptene, cyclohexane, benzene, toluene, ethylbenzene, and the xylenes with steam have been investigated using 75% w/w nickel/alumina catalysts at 475 °C, with particular reference to the formation of surface carbonaceous deposits. Using steam/hydrocarbon ratios of between 0 and 15, up to four distinct types of surface carbonaceous deposit have been identified. Extensive carbon filament formation is observed when no steam or low steam concentrations are used. A hydrocarbonaceous polymeric species, extractable from the used catalysts with tetrachloromethane, is formed with each hydrocarbon under all reaction conditions. The chemical identity of the polymer, established by infrared spectroscopy and mass spectrometry, is apparently independent of the reactant hydrocarbon. Evidence for a reactive and a nonreactive surface carbon has also been obtained. Admission of [ 14C]CO or [ 14C]CO 2 with the reactant hydrocarbon leads to scrambling of the [ 14C]label among the reaction products and the surface deposit. A mechanism for the formation of both the gaseous reaction products and the various surface residues is discussed in terms of the adsorbed intermediates. The use of carbon monoxide adsorption as a probe for exposed surface nickel shows that, although the exposure of the nickel surface is markedly reduced by the formation of surface deposits, the activity of the catalyst for the gasification of the hydrocarbons is virtually unaffected. The possibility of reaction occurring on a carbonaceous overlayer on the nickel surface is discussed.