TITANIA COATINGS ON MONODISPERSE SILICA SPHERES (CHARACTERIZATION USING 2-PROPANOL DEHYDRATION AND TEM)

Titania coatings ranging from sub-monolayer to approximate to 7 nm thi ck have been deposited on silica by hydrolysis of titanium alkoxide pr ecursors. Nonporous model silica particles of spherical shape were use d as the support for titania. The ratio of titanium alkoxide to water and the dilution of the reactant mixture in ethanol control the nature of the coating. If the alkoxide concentration is too high, precipitat ion of second-phase titania particles occurs rather than a uniform coa ting. On the other hand, higher water concentrations led to aggregated spheres being connected with titania necks. Isolated silica spheres w ith uniform coatings of titania are obtained when the alkoxide concent ration is kept low. The samples were studied using transmission and sc anning electron microscopy. 2-Propanol dehydration was used as a probe reaction, since it was found that reactivity for propene formation is directly correlated with anatase surface area. Using this correlation , the effective titania surface area of each sample could be derived. However, it was found that the correlation does not work for titania/s ilica samples heated to temperatures less than 673 K, since the titani a remains amorphous. Hence a 773 K calcination was used to convert the titania to anatase before deriving effective titania surface areas. A greement between the BET surface area and the effective titania surfac e area implies that complete coverage of the silica has been achieved in our study.