Dispersion stability and photo-activity of rutile (TiO2) powders

The dispersion stability and photo-activity of rutile (TiO2) powders are reported. Factors influencing these properties, such as the pH values of the solutions used to treat the surfaces and the concentration of inorganic solids such as SiO2 and Al2O3 deposited onto the surfaces, are investigated. Si only on the surface of TiO2 gave rise to a poor coating efficiency, but a mixture of Al and Si improved it considerably. SiO2 precipitates onto the surface of rutile at a higher pH (9.04) than Al2O3 (7.9), and it can be assumed that, in cases where both are precipitated from one mixture, a layer of silica is first formed. Temperature seems to have a negligible effect on the coating efficiency of Al2O3 if Al-2(SO4)(3) is used as a surface agent, but when NaAlO2 is used a non-linear correlation exists between temperature and coating efficiency, which is higher at 291 K than at higher temperatures. TiO2 surfaces treated with SiO2 show a greater dispersion stability than those treated with Al2O3. In the cases of SiO2/Al2O3 mixtures that were deposited onto the surfaces, the dispersion stability increased when the Si/Al ratios exceeded 0.6, although the absolute mass of Al2O3 deposited also affects this stability. The influence of various organic reagents on the dispersion stability is reported. The photoactivity of the samples was determined by means of the degradation of methylene blue on the surface. (C) 1998 Academic Press.