are possible: the second phase forms (a) a continuous layer A titania (anatase) was synthesized on a fumed silica substrate or (b) separated clusters and individual particles. Earlier we with the concentration of TiO 2 (C TiO 2 ) from 0.6 to 32.4 wt% using studied fumed TS (8, 10, 11,(20)(21)(22). The properties of a chemical vapor deposition (CVD) technique. All studied charac-CVD-TS and fumed TS differ due to alterations in the titania teristics of CVD-titania/silica (CVD-TS), such as abundance of phase distribution and in the structure of the interfaces, active surface sites, amount of adsorbed water, particle size distriwhere GSi{O{TiG or GSi{O(H){TiG connecbution, electrophoretic mobility, and free energy changes of intions are formed. GSi{O(H){TiG (Brønsted acid sites terfacial water, depend nonlinearly on C TiO 2 due to alterations in (B-sites) possessing catalytic activity (12, 14)) and GSi{the structure and distribution of the titania phase in TS with O{TiG bonds were observed in fumed TS obtained by increasing C TiO 2 . The separated anatase phase is well defined in combustion of SiCl 4 and TiCl 4 in an oxygen-hydrogen flame
CVD-TS at C TiO 2 É 5 wt%, which is lower than that for fumed (above 1300 K) (8, 19) as well as after TiCl 4 interaction TS (about 9 wt%). The anatase phase transition to rutile is inhibwith a silica surface in vacuum (9). Titania (anatase) is ited by the silica matrix and the corresponding temperature inused as a durable catalyst for decomposition of chemicals creases. At relatively low temperatures (below 400 K) of CVDin wastewater under UV irradiation because of its sensitizer TS treatment, the amount of Brønsted acid sites is higher than properties (14b). TS possessing this feature of titania can, that for fumed TS with analogous concentration of titania, but for however, possess stronger B-sites than original anatase.
T above 600 K, the opposite relationship is observed. The effective particle diameter for CVD-TS suspended in water decreases faster Also, TS and TS/Al 2 O 3 are used as pigments in which the with increasing C TiO 2 than for fumed TS. A particle size distribudiameter (d ef ) of particles is approximately 0.3 mm (7, 23) tion in an aqueous suspension of TS differs from that for an origior as additives for improvement of the polymer film propernal fumed silica suspension and the main contribution is given by ties (22). Silica and alumina applied for the coverage of the the particles with sizes closely related to aggregates of primary titania pigment surface can be deposited by the sol-gel silica particles. ᭧ 1998 Academic Press method or CVD technique (24). The sol-gel technique Key Words: CVD-titania/silica; particle structure; anatase on allows to obtain relatively ''smooth'' coverage (25), but silica; electrophoretic mobility; particle size distribution; 1 H NMR, this route is expensive and give much wastewater. Some interface modeling.
authors assumed that the CVD technique can give a continuous film with titania on oxide supports (26). However, features of formed films depend on the reactivity of the substrate EXPERIMENTAL of these oxides is formed (using, e.g., CVD technique) on another which is a substrate. In the latter case two variants Materials. Titania synthesis was performed by the CVD method using fumed silica (Aerosil, 99.9%, ''Chlorovinyl,''