APPENDIX A
COMMON PREPARATION METHODS OF MIXED AND SUPPORTED OXIDES AS
 DISCUSSED BY GAO AND WACHS (1999).

 A.1 Mixed Oxides

A.1.1 Sol-Gel Hydrolysis

 Sol-gel hydrolysis involves the acid hydrolysis and condensation of chemically

mixed Ti- and Si- alkoxides. In summary, a Ti atom acts as a substitute for a Si atom in

the silica network. It is the most widely used method due to its capability in controlling

the textural and surface properties of the mixed oxides, but the synthesis conditions can

greatly affect the homogeneity of the final product. There is a recently developed two-

stage hydrolysis procedure that solves this problem and results in the highest

homogeneity. Yet, these mixed oxides can only be obtained at low TiO2 content, less than

15 wt%. At higher Ti content, due to the larger size of the Ti atom versus Si, atom, there

is distortion of the SiO2 network. This increases the OH content on the silica surface and

affects the crystal phase of the TiO2.

A.1.2 Coprecipitation (Not Used In This Study)

 Coprecipitation involves the simultaneous precipitation of Si02 and TiO2 (at high

pH) that also includes the formation of linkages with each other. This method also has

similar problems to sol-gel hydrolysis.