XANES Study of Ti Coordination in Heat-Treated (TiO₂)<i>_x</i>(SiO₂)_1-<i>_x</i> Xerogels

Abstract

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 ( [4] Ti, [5] Ti, and [6] Ti, respectively), and mixtures thereof. This approach has been applied to titania−silica xerogels, (TiO 2 ) x (SiO 2 ) 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 [6] Ti, which is converted to anatase after heating at 500 °C. Samples with x = 0.18 contain a majority of isolated, distorted [6] Ti before heating, which is converted to [4] Ti substituted for Si after heating at 750 °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 °C showed concentration limits of [4] Ti 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 [6] Ti before heat treatment. There is no evidence of significant amounts of [5] Ti.