Characterization of fumed alumina/silica/titania in the gas phase and in aqueous suspension

Fumed oxide alumina/silica/titania was studied in comparison with fumed alumina, silica, titania, alumina/silica, and titania/ silica by means of XRD, H-1 NMR, IR, optical, dielectric relaxation, and photon correlation spectroscopies, electrophoresis, and quantum chemical methods. The explored Al2O3/SiO2/TiO2 consists of amorphous alumina (similar to 22 wt%), amorphous silica (similar to 28 wt%), and crystalline titania (similar to 50 wt%, with a blend of anatase (88%) and rutile (12%)) and has a wide assortment of Bronsted and Lewis acid sites, which provide a greater acidity than that of individual fumed alumina, silica, or titania and an acidity close to that of fumed alumina/silica or titania/silica. The changes in the Gibbs free energy (BG) of interfacial water in an aqueous suspension of Al2O3/SiO2/TiO2 are close to the Delta G values of the dispersions of pure rutile but markedly lower than those of alumina, anatase, or rutile covered by alumina and silica. The zeta potential of Al2O3/SiO2/TiO2 (pH of the isoelectric point (IEP) equals approximate to 3.3) is akin to that of fumed titania (pH(IEPTiO2) approximate to 6) at pH > 6, but it significantly differs from the zeta of fumed alumina (pH(IEPAl2O3) approximate to 9.8) at any pH value as well as those of fumed silica, titania/silica, and alumina/silica at pH < 6. The particle size distribution in the diluted aqueous suspensions of Al2O3/SiO2/TiO2 studied by means of photon correlation spectroscopy depends relatively slightly on pH in contrast to the titania/silica or alumina/silica dispersions. Theoretical calculations of oxide cluster interaction with water show a high probability of hydrolysis of Al-O-Ti and Si-O-Ti bonds strained at the interface of alumina/titania or silica/titania due to structural differences in the lattices of the corresponding individual oxides. Ab initio calculated chemical shift 6, values of H atoms in different hydroxyl groups at the oxide clusters and in bound water molecules are in agreement with the H-1 NMR data and show a significant impact of charged particles (H3O+ or OH-) on the average delta(H) values of water droplets with (H2O)(n) at n between 2 and 48. (C) 1999 Academic Press.