In-situ high-temperature XANES observations of rapid and reversible changes in Ti coordination in titania-silica xerogels

The height and position of the pre-edge peak in Ti K-edge X-ray absorption near-edge structure (XANES) is a sensitive indicator of the coordination of Ti. This method is used in situ to investigate Ti coordination in titania- silica xerogels with low TiO2 content. Unheated xerogels contain Ti with is olated, distorted 6-fold coordination (Ti-[6]). Initial heating causes [Ti- [6] to be rapidly converted into 4-fold coordinated Ti(Ti-[4]), which upon cooling reverts to Ti-[6]. Increased heat treatment creates more stable Ti- [4], which remains after cooling. Thus, the coordination of Ti depends on a mbient conditions in addition to heat treatment. In-situ XANES is important for distinguishing different kinds of Ti-[4], and hence for understanding catalytic properties. (C) 1999 Elsevier Science B.V. All rights reserved.