fumed titania structure depends strongly on dopants (SiCl 4 , Fumed titania and silica/titania (ST) were studied by optical AlCl 3 , SnCl 4 ) changing the rutile/anatase composition and spectroscopy of an adsorbed color indicator, X-ray, high-energy the particle size distribution. In the case of silica gel involvelectron diffraction, electrophoresis, and quantum chemical mething titania or alumina impurities, Ti or Al atoms can isomorods. Titania in ST with the TiO 2 concentration (C TiO 2 ) above 5 wt phously substitute Si in the silica matrix and Ti or Al as % as well as original fumed titania has blended crystalline strucwell as Si are 4-fold O-coordinated (13-16). An individual ture with rutile and anatase; therewith particle cores consist of titania phase, in which Ti atoms are 6-fold coordinated, is rutile, and particle shells are mainly anatase. The ST surface posformed upon pyrogenic synthesis of ST with C TiO 2 above 5 sesses higher Brønsted acidity, caused by bridging hydroxy groups wt % (17, 18). Hydrated ST possesses Brønsted acid sites Ti{O(H){Si, than that for original titania (Ti{O(H){Ti) or silica (SiOH), and its maximum is for C TiO 2 É 20 wt %. Electro-(B-sites) (19-21), and heated ST has Lewis acid sites (Lsites) with incompletely coordinated Ti 3/ (15, 22). There phoretic mobility of the ST particles depends on the titania concentration nonlinearly and passes though a maximum for C TiO 2 Å 9 are different views regarding these B-site structure, e.g., Tiwt % if pH § 5. A C TiO 2 increase leads to reduction of a ST particle {O{Si(OH), Si{O{Ti(OH), or Si{O(H){Ti for ST aerogel or fumed ST and Ti{O(H){Ti for titania size in aqueous suspensions from effective diameter of 15 mm for agglomerates of pure silica to 0.4 mm for ST with C TiO 2 Å 36 wt (19, 21). An extreme dependence of the Si{O{Ti bond %. According to theoretical simulation, the Ti atoms in the bound-amount in ST on C TiO 2 was found using IR spectroscopy ary cells on the steps of the titania or ST particles can easily giving a maximum of optical density of a band at 950 cm 01 change the coordination number upon water adsorption and defor SiO-stretching vibrations in Si{O{Ti for C TiO 2 É 20 sorption.