Supporting titanium dioxide photocatalysts on silica gel and hydrophobically modified silica gel

This report describes preparation and characterization of TiO2 supported on silica gels and modified silica gels. The objective is manipulation of the photocatalytic TiO2 phase on the support, separately from manipulation of the organic solute adsorption and mass transport characteristics of the support. If the two features can be controlled separately, these systems should become good models for elucidation of mechanistic behavior of supported photocatalysts. TiO2 characteristics and adsorption of substrates are key variables. TiO2 may be anchored on silica gel 100 by hydrolysis of Ti(OR)(4) (R = isoprophyl) or heating mixtures of silica gel and TiO2 crystals (p25). In the first case, TiO2 penetrates pores of silica gel and forms a phase which is either amorphous or of particle size small enough to give broad lines in powder XRD at angles suggesting anatase. In the second case, anatase particles too large to penetrate pores are anchored on the outer surface. The former method produces a superior photocatalyst. The silica surface can be modified by esterification with octanol to produce a photocatalyst which adsorbs solutes from an aqueous medium in a manner describable by an octanol/water partition coefficient. This modification has little effect on photocatalytic reactivity. A larger particle size silica gel (silica gel 58) produces a photocatalyst with TiO2 in the interior of the pores which is less active for oxidation of 2,4-dichlorophenol but produces long-lived trapped electron colors in the presence of oxidizable substrates (e.g. propanol). This indicates a way to explore the role of intraparticle diffusion in the larger particles which are attractive for large scale applications.