The mechanism of the carbon dioxide reforming of methane was investigated over a nickel-on-silica catalyst. Non-steady-state and steady-state isotopic transient experiments combined with in situ DRIFT spectroscopy investigations were used to quantify the amount of the various adspecies present on the working catalyst surface. It was found that as soon as the catalyst is contacted with the reacting mixture, dehydrogenated carbon adspecies originating from the initial adsorption of methane and carbon dioxide are deposited on the nickel particles, Under steady-state reaction conditions, a permanent pool of adspecies equivalent to one monolayer of carbide-like species is continuously fed by the dissociative activation of gaseous methane. This initial activation step of methane is shown to be reversible, since it allows a fast CH4/CD4 exchange characterised by a marked isotopic effect. This pool of adspecies constitutes a reservoir of active carbon able to be oxidised into CO by oxygen atoms arising from the simultaneous carbon dioxide dissociation. This oxidation step which does not involve any C-H bond activation is assumed to be rate limiting since no kinetic isotopic effect is found for the formation of CO under the stoichiometric reforming conditions. Gaseous CO is also directly produced from the latter CO2 dissociation. Adsorption/desorption equilibria ensure a fast interconversion between gaseous CO2 and CO, as attested by their isotopic scrambling. A similar adsorption/desorption equilibrium is proposed for H2O which, combined with the reversible activation of CO2 and CO, leads to the achieved water-gas-shift equilibrium, Besides the rapid steps which constitute the catalytic cycle, slow side reactions involving the migration of the carbide-like species through the nickel particle and their transformation into structured coke or graphite growing as hollow whiskers are proposed to account for the ageing phenomena described in Part I of this work, A particular configuration of active sites is proposed on the basis of the main mechanistic statements. (C) 1996 Academic Press, Inc.
