Inhomogeneities in acid-catalyzed titania-silica and zirconia-silica xerogels as revealed by small-angle x-ray scattering

The small-angle x-ray scattering (SAXS) technique was used to investigate inhomogeneities on the scale of 10 to 600 Angstrom in acid-catalyzed titania-silica and zirconia-silica xerogels. SAXS of (TiO2)(SiO2)(1-x) and (ZrO2)(x)(SiO2)(1-x) xerogels with x < 0.1, in which there was no phase separation, showed the presence of two types of inhomogeneity. For Q < 0.05 Angstrom(-1) there was a clear departure from Pored scattering which showed that xerogel powder particle surfaces were rough. For 0.1 < Q < 0.4 Angstrom(-1) there was a plateau feature corresponding to micropores within the silica-based network, and this feature changes with heat treatment. SAXS of xerogels with x > 0.3 showed the presence of phase-separated regions of metal oxide, which were initially amorphous and crystallized at higher temperatures. A (TiO2)(0.18)(SiO2)(0.82) xerogel that was not initially phase separated became phase separated after heat treatment at 750 degrees C due to reduced solubility of Ti in the silica network.