Film-Coating Process of Hydrated Alumina on TiO2 Particles

The mechanism of the film-coating process of hydrated alumina on TiO2 particles in aqueous solution was experimentally studied. pH and (zeta)-potential changes during the mixing process of the TiO2 suspension and Al2(SO4)3 solution were investigated. TiO2 particles obviously promote the hydrolysis of Al2(SO4)3 in both acidic and basic solutions and adsorb positively charged OH-Al species in slurries. A static repulsion dominates the suspension system of OH-Al species and TiO2 particles, in which they randomly collide with each other because they carry the same sign of charge. When the OH-Al species or TiO2 particles have enough energy to cross the repulsion threshold, the -OH groups on the surface of the TiO2 particles will condense with the OH-Al species, leading to the coating of OH-Al species on the surface of the TiO2 particles. However, completely inelastic collisions also occur between the OH-Al species, and the formation of hydrous alumina is unavoidable unless the local concentration of the coating reagent in the suspension is well controlled. The morphology and surface chemical states of coated TiO2 particles were also analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). TiO2 particles coated with hydrous alumina exhibit electrokinectic behavior similar to that of pure Al(OH)3 gel.