ABSTRACT
Fumed oxide alumina/silica/titania was studied in comparison with fumed alumina, silica, titania, alumina/silica, and titania/silica by means of XRD, (1)H NMR, IR, optical, dielectric relaxation, and photon correlation spectroscopies, electrophoresis, and quantum chemical methods. The explored Al(2)O(3)/SiO(2)/TiO(2) consists of amorphous alumina ( approximately 22 wt%), amorphous silica ( approximately 28 wt%), and crystalline titania ( approximately 50 wt%, with a blend of anatase (88%) and rutile (12%)) and has a wide assortment of Brønsted 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 (DeltaG) of interfacial water in an aqueous suspension of Al(2)O(3)/SiO(2)/TiO(2) are close to the DeltaG 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 Al(2)O(3)/SiO(2)/TiO(2) (pH of the isoelectric point (IEP) equals approximately 3.3) is akin to that of fumed titania (pH(IEP(TiO2)) approximately 6) at pH > 6, but it significantly differs from the zeta of fumed alumina (pH(IEP(Al2O3)) approximately 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 Al(2)O(3)/SiO(2)/TiO(2) 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 delta(H) values of H atoms in different hydroxyl groups at the oxide clusters and in bound water molecules are in agreement with the (1)H NMR data and show a significant impact of charged particles (H(3)O(+) or OH(-)) on the average delta(H) values of water droplets with (H(2)O)(n) at n between 2 and 48. Copyright 1999 Academic Press.
ABSTRACT
The structure of porous silica gel/titania synthesized using chemical vapor deposition (CVD) of titania via repeated reactions of TiCl(4) with the surface and subsequent hydrolysis of residual Ti-Cl bonds at different temperatures was investigated by means of low-temperature nitrogen adsorption-desorption, X-ray diffraction (XRD), IR spectroscopy, and theoretical methods. A globular model of porous solids with corpuscular structure was applied to estimate the porosity parameters of titania/silica gel adsorbents. The utilization of this model is useful, for example, to predict conditions for synthesis of titania/silica with a specified structure. Analysis of pore parameters and fractal dimension suggests that the porosity and fractality of samples decrease with increasing amount of TiO(2) covering the silica gel surface in a nonuniform layer, which represents small particles embedded in pores and larger particles formed at the outer surface of silica globules. Theoretical simulation shows that the Si-O-Ti linkages between the cover and the substrate can be easily hydrolyzed, which is in agreement with the IR data corresponding to the absence of a band at 950 cm(-1) (characteristic of Si-O-Ti bridges) independent of the concentration of CVD-titania. Copyright 1999 Academic Press.
