Low temperature oxidation of methane over Pd-based catalysts - effect of support oxide on the combustion activity

Pd catalysts supported on metal oxides were investigated for low temperature catalytic combustion of methane. Of the catalysts examined, Pd/Al2O3-36NiO and Pd/SnO2 catalysts demonstrated excellent activities, despite its low specific surface area as compared with the conventional Pd/gamma-Al2O3 catalyst. It was observed by a transmission electron microscope and an X-ray line broadening analysis that the PdO phase was well-dispersed on the surface of the SnO2 and Al2O3-36NiO supports, respectively, despite their low surface areas. Temperature-programmed desorption (TPD) of oxygen has revealed that the catalytic activity depends on the adsorption state of oxygen on palladium. It was suggested from TPD of O-2, in situ XPS, and in situ XRD that the dissociation of oxygen from the PdO bulk, as well as from its surface proceeded at higher temperatures for the Pd/Al2O3-36NiO and Pd/SnO2 catalysts than for Pd/Al2O3 because of the strong palladium-support interaction. The catalytic activity on Pd/SnO2 was zero-order with respect to the oxygen concentration, whereas an increase in water concentration significantly deteriorated the catalytic activity. This suggests that adsorption of both oxygen and water is strong on the Pd surface. The support oxides appear to modify relative adsorption strength oxygen and water, and then combustion activity of palladium. (C) 2001 Elsevier Science B.V. All rights reserved.