Thermally stable supported metal catalysts can be prepared by matching the metal particle size to the average pore diameter. Preparative variables aimed at achieving this match are presented. In the 3-8 pH range, blank silica gels prepared using tetraethoxysilane (TEOS) were observed to have BET surface areas of 350-800 m$\sp2$/g and mesoporous structures with an average pore diameter of between 4 and 25 nm at a constant H$\sb2$O/TEOS ratio of 10. Silica aerogels have comparable BET surface areas, larger pore diameters, and pore volumes than xerogels. In the case of supported Pt catalysts prepared by sol-gel synthesis, the effect of preparative variables, including metal precursor, metal loading, reactant stoichiometries, and solvent removal, have been studied. The use of Pt(AcAc)$\sb2$ resulted in average pore diameters which could be varied in the 4-20 nm range by adjusting the H$\sb2$O/TEOS ratio during synthesis. BET surface areas in the 500-800 m$\sp2$/g and metal dispersions of 50-85% can be achieved. The H$\sb2$/n-hexane reaction was studied on these catalysts. Catalytic activities were found to be comparable or slightly superior to supported metal catalysts prepared by traditional methods. In the low conversion region ($<$1.5%), the hydrogenolysis selectivity C$\sb2$/C$\sb1$ was observed to pass through a maximum as a function of n-hexane conversion. This maximum was observed to depend on particle size Pt/SiO$\sb2$ catalysts with well defined pore size distributions and metal particle sizes were prepared by the sol-gel method. Tetraethoxysilane (TEOS) and Pt(AcAc)$\sb2$ were used as precursors. Pore size distributions were controlled by varying the H$\sb2$O/TEOS ratio during synthesis. It was found that when the particle size was matched to coincide with the average pore diameter the resulting Pt/SiO$\sb2$ catalysts were resistant to sintering in O$\sb2$ at temperatures up to 675$\sp\circ$C. In order to insure that sintering did not occur it was found necessary to reduce the metal loading to between 0.2 to 0.3 wt%. In addition to a high resistance to sintering these catalysts were observed to be structurally stable. BET surface areas in excess of 800 m$\sp2$/g were obtained and were found to decrease only slightly following thermal treatment in O$\sb2$ at 675$\sp\circ$C. Additionally, pore structure and pore volume were observed to decrease only slightly following thermal treatment in oxygen
