CVD-titania on fumed silica substrate

A titania (anatase) was synthesized on a fumed silica substrate with the concentration of TiO2 (C-TiO2) from 0.6 to 32.4 wt% using a chemical vapor deposition (CVD) technique. All studied characteristics of CVD-titania/silica (CVD-TS), such as abundance of active surface sites, amount of adsorbed water, particle size distribution, electrophoretic mobility, and free energy changes of interfacial water, depend nonlinearly 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 transition to rutile is inhibited by the silica matrix and the corresponding 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 above 600 K, the opposite relationship is observed. The effective particle diameter for CVD-TS suspended in water decreases faster with increasing C-TiO2 than for fumed TS. A particle size distribution in an aqueous suspension of TS differs from that for an original fumed silica suspension and the main contribution is given by the particles with sizes closely related to aggregates of primary silica particles. (C) 1998 Academic Press.