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 isolated, distorted 6-fold coordination ([6]Ti). Initial heating causes [6]Ti to be rapidly converted into 4-fold coordinated Ti([4]Ti), which upon cooling reverts to [6]Ti. Increased heat treatment creates more stable [4]Ti, which remains after cooling. Thus, the coordination of Ti depends on ambient conditions in addition to heat treatment. In-situ XANES is important for distinguishing different kinds of [4]Ti, and hence for understanding catalytic properties.