Characterization of carbonaceous species formed during reforming of CH4 with CO2 over Ni/CaO-Al2O3 catalysts studied by various transient techniques

Carbon dioxide reforming of methane to synthesis gas at 750 degrees C over 5 wt% Ni/CaO-Al2O3 catalysts has been investigated with respect to effects of support composition (CaO to Al2O3 ratio) on catalyst stability, amount and reactivity of carbon species formed during reaction, and relative proportion of reaction routes that lead to carbon formation (CH4 vs CO2 molecule). Temperature-programmed oxidation (TPO) and hydrogenation (TPH) experiments, following reforming reaction with 20% CH4/20% CO2/He and 20% (CH4)-C-13/20% CO2/He mixtures, have been conducted for the aforementioned carbon characterization studies. Two kinds of carbon species (free of chemically bound hydrogen) were mainly found to accumulate on the catalyst surface, where the amount and reactivity of them are influenced by the CaO/Al2O3 ratio used to deposit the nickel metal. Transient isothermal hydrogenation experiments of the carbon species formed during reforming reaction resulted in CH4 responses, where the time of appearance of the CH4 peak maximum in hydrogen stream as a function of hydrogenation temperature was used to obtain the intrinsic activation energy of the hydrogenation process. It was found that this activation energy is influenced by the support composition. TPO experiments conducted following reforming reaction with (CH4)-C-13/CO2/He mixture have demonstrated that the relative amount of adsorbed carbon species formed via the CH4 and CO2 molecular routes was strongly dependent on support composition. H-2 temperature-programmed desorption, temperature-programmed reduction, and X-ray photoelectron spectroscopic measurements conducted over the present catalysts suggest that the nickel particle morphology and its size distribution must be influenced by the support composition, which in turn controls the origin, the kinetics, and the reactivity of carbon deposition under reforming reaction conditions. (C) 1996 Academic Press, Inc.