Characterization of Porous Catalysts and Catalyst-Support Interactions

摘要

A commercial pelleted catalyst (6 percent SiO sub 2 on Al sub 2 O sub 3 ) was used. Nitrogen adsorption increased after extensive outgassing at successively higher temperatures (25 to 400 exp 0 C). The apparent Brunauer-Emmett-Teller (BET) specific surface area increased from 175 to 205 m exp 2 /g for greater degrees of degassing. This increase was due to flexing of the matrix with little or no micropore generation. Hysteretic capillary condensation of the nitrogen in the mesoporous intracrystalline voids was noted to be little affected by the residual adsorbed species. Mass-spectrometric monitoring of the thermal desorption revealed that only H sub 2 O (95 percent), CO sub 2 (5 percent), and CO, N sub 2 (0.75 percent) were evolved. Kinetic evaluation of the desorption process indicates two mechanisms of release of these chemisorbed species, one in play below 200 exp 0 C and the other predominating above 200 exp 0 C. Heats of immersion of these outgassed samples in water further aid in understanding the dehydration-rehydration processes involved. Comparisons with results of other work are presented. The surface of the alumina seems to be altered appreciably by the silica component. The energetic analyses indicate a drastic reduction of the surface hydroxyl component so prevalent on pure alumina surfaces. Extensive hydration via hydrogen bonding does occur, and the nature and amount of surface available are markedly altered. (ERA citation 03:038784)