Titania Coatings on Monodisperse Silica Spheres (Characterization Using 2-Propanol Dehydration and TEM)

Abstract

Titania coatings ranging from sub-monolayer to ≈7 nm thick have been deposited on silica by hydrolysis of titanium alkoxide precursors. Nonporous model silica particles of spherical shape were used 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, precipitation of second-phase titania particles occurs rather than a uniform coating. On the other hand, higher water concentrations led to aggregated spheres being connected with titania necks. Isolated silica spheres with uniform coatings of titania are obtained when the alkoxide concentration is kept low. The samples were studied using transmission and scanning 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/silica samples heated to temperatures less than 673 K, since the titania remains amorphous. Hence a 773 K calcination was used to convert the titania to anatase before deriving effective titania surface areas. Agreement between the BET surface area and the effective titania surface area implies that complete coverage of the silica has been achieved in our study.