Structure of fumed titania and silica/titania and influence of the nature of surface sites on interaction with water

Fumed titania and silica/titania (ST) were studied by optical spectroscopy of an adsorbed color indicator, X-ray, high-energy electron diffraction, electrophoresis, and quantum chemical methods. Titania in ST with the TiO2 concentration (C-TiO2) above 5 wt % as well as original fumed titania has blended crystalline structure with rutile and anatase; therewith particle cores consist of rutile, and particle shells are mainly anatase. The ST surface possesses higher Bronsted acidity, caused by bridging hydroxy groups Ti-O(H)-Si, than that for original titania (Ti-O(H)-Ti) or silica (SiOH), and its maximum is for C-TiO2 approximate to 20 wt %. Electrophoretic mobility of the ST particles depends on the titania concentration nonlinearly and passes though a maximum for C-TiO2 = 9 wt % if pH greater than or equal to 5. A C-TiO2 increase leads to reduction of a ST particle size in aqueous suspensions from effective diameter of 15 mu m for agglomerates of pure silica to 0.4 mu m for ST with C-TiO2 = 36 wt %. According to theoretical simulation, the Ti atoms in the boundary cells on the steps of the titania or ST particles can easily change the coordination number upon water adsorption and desorption. (C) 1997 Academic Press.