Active site nature of pyrogenic alumina/silica and water bound to surfaces

The natures of the pyrogenic alumina/silica (AS) surface and water adsorbed on AS were studied by H-1 NMR, infrared (IR), dielectric relaxation (DRS), and optical spectroscopies, thermally stimulated depolarization (TSD), and quantum chemical methods. An influence of the alumina content on the AS characteristics is nonlinear and stronger for the boundary of air/adsorbed water/oxide than for the liquid water/oxide interface. The H-1 NMR study shows that the AS particles in aqueous suspension can strongly disturb 6-14 monolayers of interfacial water unfrozen at 200 K < T < 273 K. The alumina content does not have potent effects 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 the phase boundary of Al2O3/SiO2. The theoretical simulation demonstrates that adsorbed water can promote dissociative adsorption and associative desorption of water molecules especially in the regions of contact between the alumina and silica fragments in the AS particles, where the Bronsted acid sites are located and the adsorbed water clusters have a maximum size according to DRS and TSD data.