CVD-TITANIA ON FUMED SILICA SUBSTRATE

A titania (anatase) was synthesized on a fumed silica substrate with t he concentration of TiO2 (C-TiO2) from 0.6 to 32.4 wt% using a chemica l vapor deposition (CVD) technique. All studied characteristics of CVD -titania/silica (CVD-TS), such as abundance of active surface sites, a mount of adsorbed water, particle size distribution, electrophoretic m obility, and free energy changes of interfacial water, depend nonlinea rly on C-TiO2 due to alterations in the structure and distribution of the titania phase in TS with increasing C-TiO2. The separated anatase phase is well defined in CVD-TS at C-TiO2 approximate to 5 wt%, Which is lower than that for fumed TS (about 9 wt%). The anatase phase trans ition to rutile is inhibited by the silica matrix and the correspondin g temperature increases. At relatively low temperatures (below 400 K) of CVD-TS treatment, the amount of Bronsted acid sites is higher than that for fumed TS with analogous concentration of titania, but for T a bove 600 K, the opposite relationship is observed. The effective parti cle diameter for CVD-TS suspended in water decreases faster with incre asing C-TiO2 than for fumed TS. A particle size distribution in an aqu eous suspension of TS differs from that for an original fumed silica s uspension and the main contribution is given by the particles with siz es closely related to aggregates of primary silica particles. (C) 1998 Academic Press.