demonstrated a more efficient use of the TiO2 sites than for TiO2 alone. The highest

initial degradation rate was found with a 30/70 wt% TiO2-SiO2 composite.

 Jung and Park (2000) also found 30 wt% titania (with gels prepared in a similar

process to Anderson and Bard) to be the optimum in their studies for the mineralization

of trichloroethylene. In addition, they concluded that the high porosity and large pore size

of the silica facilitated the mass transfer of reactants, resulting in a higher rate of

degradation than a plain slurry of Degussa P25.

 Chun et al. (2001) used a different preparation method than Jung and Park (2000)

and Anderson and Bard (1995) and still found 30 wt% TiO2 to be an optimum. Chun et

al. additionally showed, using two organic compounds with differing characteristics, that

there is a strong correlation between adsorption of the compound on the mixed oxide

surface and the destruction rate of that compound. In conclusion, silica-titania composites

can be efficiently utilized in heterogeneous photocatalysis systems for the adsorption and

subsequent destruction of unwanted compounds in an aqueous solution. In addition, a

dosage of 30 wt% (equal to 12% on a weight per volume of silica precursor) was also

found to be an optimum loading in this research (see Chapter 4) for the silica-titanium

composites created using a sol-gel doping method (see Chapter 3).

 2.5 Silica-Titania Composites

 Silica is the most abundant oxide on the earth, yet despite this abundance, silica is

predominantly made by synthetic means for its use in technological applications. Since

synthetic silicas have a higher surface area than naturally occurring forms of silica, the

synthetic silicas provide the best adsorption and catalyst support structure for

heterogeneous photocatalysis. Although silica has a simple chemical formula (SiO2), it

can exist in a variety of forms, each with its own structural characteristics, as well as