Genesis and structural properties of alkoxide-prepared titania–silica xerogels

Abstract

Titania–silica mixed oxide gels with a 1 : 4 Ti : Si molar composition were prepared from the hydrolysis of tetraethoxy orthosilicate (TEOS) and tetraisopropoxy orthotitanate (TIOT), using different amounts of water, acid or base, and hydrolysis procedure. Surface area, pore structure, and crystallinity were compared in relation to sol-gel process conditions. Gels in which the TEOS was hydrolysed using a basic medium possessed larger mesopores and correspondingly lower surface area than gels prepared with acid-hydrolysed TEOS, but were more resistant to mesopore collapse upon drying, owing to the spherically shaped silica subunits in these gels. Gels hydrolysed by a two-stage procedure, involving partial hydrolysis of TEOS in acid, followed by mixing with pure TIOT and completion of hydrolysis, are most susceptible to pore collapse upon drying, producing a completely microporous xerogel. Pore collapse did not occur when the wet-gel material was aged in slightly basic medium for several days. All gels prepared by the two-stage hydrolysis procedure remain X-ray amorphous after treatment in air at 870 K, whereas gels produced by mixing completely hydrolysed component sols show crystallization of the titania. Thermal analysis runs show water loss primarily results from weakly bound water. In general, a residual amount of unreacted alkoxy groups is present in gels prepared without a large excess of hydrolysis water. Highest residual organic amounts of 2–8 wt. % were found with the two-stage hydrolysed gels. Rutile was only detected in gels prepared in excess water, but the extent of rutile formation, as well as crystallite growth, is considerably less than in similarly prepared pure titania gels.