Effect of the composition of titania–silica on its physical and catalytic properties

Abstract

The effect of the composition of titania–silica on its physical and catalytic properties has been investigated. There were two types of Ti species: segregated TiO₂ and isolated Ti species which interacted with Si—O groups. As the Ti content decreased, the grain size of TiO₂ in it decreased and the proportion of the isolated Ti species seemed to increase. The Ti content affected the oxidation–reduction properties of the titania–silica. The electron-transfer steps in the oxidation of CO and to form the 7,7,8,8-tetracyanoquinodimethane radical anion were suppressed on the titania–silica with low Ti content. This phenomenon was explained by a size quantization effect of TiO₂. The bandgap between the valence and conduction bands of the TiO₂ was widened as its grain size decreased, and its electron-accepting and -donating abilities decreased. Titania–silica with a low Ti content (with small TiO₂ grains) had low redox ability. Acid sites were produced by the combination of Ti and Si. The Ti content also affected the strength of these acid sites. It was assumed that titania–silica with high Ti content could easily accommodate electrons donated by bases or proton sources into its large TiO₂ grain and exhibited strong acidity. Titania–silica with low Ti content had mainly weak acid sites due to the low electron-withdrawing ability of its small TiO₂ grains. It was also speculated that the isolated Ti species decomposed hydrogen peroxide via a non-radical path. Hydrogen peroxide decomposed more effectively on the titania–silica with lower Ti content which had higher proportion of isolated Ti species.