Thermal stability of silica supported palladium catalysts prepared by the sol-gel method

Abstract

The thermal stability of silica supported Pd catalysts prepared by the sol-gel method was studied. When the average particle size coincided with the average pore diameter, the Pd particles were found to be stable at temperatures of up to 650°C for 22 h in an oxygen atmosphere. Treatment in hydrogen at the same temperature resulted in considerable Pd sintering. A possibility for this sintering in hydrogen is the formation of a thermally unstable Pd β-hydride. For catalysts prepared under conditions which lead to a mismatch between particle size and average pore diameter, sintering in both oxygen and hydrogen was observed to occur. Sintering rates in oxygen were found to depend on initial particle size. Small Pd particles were found to sinter in oxygen to a greater extent than large particles. Reaction rates for the disappearance of n-hexane on both Pd/sol-gel catalysts and on catalysts prepared by traditional ion-exchange methods were comparable. For this reason, metal occlusion as a result of pore plugging does not occur.