[en] Pd/SiO2, Ag/SiO2 and CU/SiO2 xerogel catalysts have been synthesized by cogelation of tetraethoxysilane (TEOS) and chelates of Pd, Ag and Cu with 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS). The resulting materials are composed of completely accessible metallic crystallites with a diameter of about 3 nm, located inside silica particles exhibiting a monodisperse microporous distribution centered on a pore width of about 0.8 nm. Voids between silica particles and between aggregates of silica particles constitute the material meso- and macroporous texture. The EDAS/TEOS ratio is the main operating variable to tailor the material morphology. When the EDAS/TEOS ratio decreases, metallic particle size decreases, whereas silica particle size increases. Thus the meso- and macropores between silica particles are larger. So the total pore volume can reach 5 cm(3)/g when the EDAS/TEOS ratio decreases. The remarkable morphology of these materials can be explained by the rule in which the EDAS metal complex acts as a nucleation agent in the formation of silica particles. A nucleation mechanism by EDAS-metal complex based on mass equations and the nuclei formation kinetics allows fitting the entire results. (C) 2004 Elsevier B.V. All rights reserved.
Fonds de la Recherche Scientifique (Communauté française de Belgique) - F.R.S.-FNRS