Methane combustion over supported palladium catalysts I. Reactivity and active phase

The performance of Al2O3, ZrO2 and ZrO2 stabilized with SiO2 (ZrO2-s) supported palladium catalysts for the methane combustion was studied between 473 and 873 K. The nature of the surface species of palladium catalysts under reaction conditions were detected by FT-IR and microcalorimetry of CO adsorbed. The different behavior of palladium catalysts under reaction conditions is attributed to support effects associated to differences in thermal conductivity and oxygen mobility of supports. Prereduction of the catalysts enhances their activity. Under reaction conditions, the prereduced sample becomes partially oxidized by preferential adsorption/reaction of oxygen both on Pd ( 1 1 1) planes and on the sites that can multibondedly adsorb CO. The reconstruction of the metallic particles and the formation of PdOx (0<x<less than or equal to>1) phase were directly observed by FT-IR and microcalorimetry of adsorbed CO. Combination of different characterization techniques with reaction results suggests that a mixed phase, Pd-0/PdOx, is the most active phase for methane combustion, and that a redox mechanism may occur on this phase. (C) 2000 Elsevier Science B.V. All rights reserved.