Pd/SiO2-Cogelled Aerogel Catalysts and Impregnated Aerogel and Xerogel Catalysts: Synthesis and Characterization

Abstract

Pd/SiO 2 aerogel catalysts have been synthesized by cogelation of tetraethoxysilane with complex Pd 2+ [NH 2 -CH 2 -CH 2 -NH-(CH 2 ) 3 -Si(OCH 3 ) 3 ] 2 containing a hydrolysable silyl group. Pd catalysts have also been prepared by impregnation of silica aerogels and xerogels with solutions of this complex or Pd 2+ (NH 3 ) 4 followed by drying under hypercritical conditions or under vacuum. It appears that, in cogelled samples, the Pd 2+ complex acts as a nucleation agent in the formation of silica particles. The resulting catalysts are then composed of completely accessible palladium crystallites with a diameter of about 2.3 nm located inside silica particles with a monodisperse microporous distribution centered on a hydraulic radius of 0.43 nm. The continuous meso- and macropore distribution is located in voids between particles and between aggregates constituted of those particles. In impregnated samples, it appears that Pd particles are located outside SiO 2 particles. The relation between the texture of cogelled and impregnated catalysts and their behavior toward sintering has been studied in detail. In cogelled samples, it has been shown that Pd crystallites cannot migrate because they are trapped in the pores of SiO 2 particles. These catalysts are sinterproof during hypercritical drying. When Pd crystallites are not trapped inside SiO 2 particles, hypercritical drying leads to an extensive sintering of metal particles. During calcination of impregnated catalysts dried under vacuum, sintering occurs to a various extent depending on the support texture. The Pd crystallites size decreases when the pore size distribution is shifted toward the small sizes.