XANES study of Ti coordination in heat-treated (TiO2)(x)(SiO2)(1-x) xerogels

Accurate measurements of the height and position of the preedge peak in Ti K-edge XANES (X-ray absorption near-edge structure) can distinguish 4-, 5-, and 6-fold coordination of Ti by O (Ti-[4], Ti-[5], and Ti-[6], respectively), and mixtures thereof. This approach has been applied to titania-silica xerogels, (TiO2)(x)(SiO2)(1-x), prepared from Si and Ti alkoxides using a prehydrolysis step and acid catalyst, with 0.18 < x < 0.75. As expected, samples with x = 0.75 contain phase-segregated, amorphous, pre-anatase Ti-[6], which is converted to anatase after heating at 500 degrees C. Samples with x = 0.18 contain a majority of isolated, distorted Ti-[6] before heating, which is converted to Ti-[4] substituted for Si after heating at 750 degrees C. This is the first time that the changes in the coordination of isolated Ti have been unambiguously described. Xerogels with 0.25 < x < 0.35 and heat treatment at 750 degrees C showed concentration limits of Ti-[4] substitution for Si of 10-15mol %. Slightly higher values were obtained for xerogels prepared using acetylacetone, but the main difference was that the latter had a significantly larger content of distorted Ti-[6] before heat treatment. There is no evidence of significant amounts of Ti-[5].