Study on Hydrothermal Stability and Catalytic Activity of Nanosphere-Walled Mesoporous Silica

Abstract

The Rh-embedded mesoporous silica (RMS) with a silica-nanosphere pore wall was successfully synthesized using silica nanospheres as a framework and citric acid as a nonsurfactant template. The RMS had a three-dimensionally interconnected and disordered wormhole-like mesostructure, and its pore size was easily controlled by simply changing the citric acid concentration or carrying out the aging treatment. In particular, we have first reported high hydrothermal stability and catalytic activity of the nanosphere-walled mesoporous silica in an ethanol steam reforming reactor at high temperature. As a result, the RMS showed 1.8% and 1.3% reduction of BET surface area and total pore volume after the ethanol steam reforming test for 5 h in the temperature range of 400 °C to 600 °C, whereas Rh-impregnated SBA-15 (RSBA15) showed 31.3% and 14.0% reduction of those. It is well-known that mesoporous materials have not been widely used as catalysts or catalyst supports in industry yet because of their poor hydrothermal stability. Accordingly, we have improved the hydrothermal stability of mesoporous silica through using silica nanospheres as a framework and have confirmed that the high hydrothermal stability of the RMS was attributed to the thermodynamically stable sphere-shaped pore wall, large intra-micropore volume of the silica-nanosphere wall, the thicker pore wall, and the highly branched structure of colloidal silica synthesized under base-catalyzed conditions. Moreover, compared to the RSBA15, the RMS gave a higher catalytic activity for the ethanol steam reforming because of its larger pore size and three-dimensionally interconnected pore structure, which lead to improvement in accessibility of reactants to the catalytic active sites.