ACTIVE-SITE NATURE OF PYROGENIC ALUMINA SILICA AND WATER BOUND TO SURFACES/

The natures of the pyrogenic alumina/silica (AS) surface and water ads orbed on AS were studied by H-1 NMR, infrared (IR), dielectric relaxat ion (DRS), and optical spectroscopies, thermally stimulated depolariza tion (TSD), and quantum chemical methods. An influence of the alumina content on the AS characteristics is nonlinear and stronger for the bo undary of air/adsorbed water/oxide than for the liquid water/oxide int erface. The H-1 NMR study shows that the AS particles in aqueous suspe nsion can strongly disturb 6-14 monolayers of interfacial water unfroz en at 200 K < T < 273 K. The alumina content does not have potent effe cts on the IR spectra independently of pretreatment temperatures, but other experiments exhibit the marked influence of the AS constitution on the oxide properties that is caused by both the alumina phase and t he phase boundary of Al2O3/SiO2. The theoretical simulation demonstrat es that adsorbed water can promote dissociative adsorption and associa tive desorption of water molecules especially in the regions of contac t between the alumina and silica fragments in the AS particles, where the Bronsted acid sites are located and the adsorbed water clusters ha ve a maximum size according to DRS and TSD data.