Rh/SiO2 catalysts prepared by the sol-gel method

Abstract

A series of Rh/SiO₂ catalysts were prepared by the sol-gel method. The precursors used were tetraethoxysilane (TEOS) dissolved in ethanol and rhodium(III) acetylacetonate dissolved in acetone. Preparative variables include the amount of water added to the sol and the gelation temperature. Typical BET surface areas were 550–700 m² g⁻¹ with very narrow pore size distributions centered at approximately 4 nm. Sample characterization studies were performed using TGA-DTA-MS to determine the effect of O₂ pretreatment and ²⁹Si solid-state NMR to determine the extent to which silica was condensed. Sintering studies of Rh/SiO₂ catalysts for which there was a reasonable match between metal particle size and pore diameter were performed. Catalysts prepared by ion-exchange having comparable dispersions to those prepared by the sol-gel method also showed considerable resistance to sintering in O₂. Following heat treatment in O₂ at 650°C, the particle size distribution of catalysts prepared by ion-exchange appear to broaden to a greater extent than catalysts prepared by the sol-gel method. This suggests that the sol-gel preparation is more effective in retaining the metal particles within the narrow pores of the support.