Abstract

Surface properties and catalytic activity changes of Pd/SiO2 catalysts prepared by sol-gel processing have been investigated by evaluating the specific surface area, porosity, metal dispersion, surface acidity, and surface composition. As a comparison, Pd/SiO2 catalysts prepared by impregnation of SiO2 obtained by the sol-gel method were also studied. Both series of catalysts were tested in cycles of methane combustion. It has been found that the pH of gelation modifies strongly the porosity and surface area of the catalysts. However, no significant changes were observed in the metal dispersion. XPS results showed that for the catalysts obtained by the sol-gel procedure the Pd/Si surface atomic ratio does not change before or after the combustion cycle. However, in those catalysts prepared by the impregnation method, a significant decrease in the Pd/Si ratio was observed. The catalytic activity in the methane combustion under stoichiometric mixture of diluted CH4/O2 was studied at different temperatures from 473 K up to the temperature required for a total conversion of all the catalysts. It was found that the activity is higher after their use in the catalytic reaction. This behaviour is attributed to the presence of Pd?+ species which improve the catalytic activity. © 1998 Academic Press.