ABSTRACT
Fumed oxides, such as silica, alumina, titania, and mixed X/silicas (X=Al(2)O(3) (AS), TiO(2) (TS), CVD-TiO(2), Al(2)O(3)/TiO(2) (AST)), pristine or covered by carbon deposits formed due to pyrolysis of cyclohexene, were studied using nitrogen adsorption-desorption, photon correlation spectroscopy particle sizing, and electrophoresis. A significant influence of the nature of surface-active sites and structural features of oxides (individual silica, mixed fumed, or prepared using chemical vapor deposition (CVD)) on the pyrolysis of cyclohexene is observed with respect to the pore size distributions due to differences between primary particles in aggregates and on their outer surfaces in the filling of channels by pyrocarbon, resulting also in a decrease in fractal dimension. Structural characteristics and dependences of the particle size distribution and electrokinetic potential of X/SiO(2) and C/X/SiO(2) on the pH of aqueous suspensions suggest that the carbon deposit covers mainly acidic sites at the X/SiO(2) interfaces and X phase patches possessing catalytic activity in pyrolysis, as the negative charge of particles is reduced by pyrocarbon grafting. Copyright 2000 Academic Press.
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
Highly disperse germania has been synthesized on the surface of fumed silica by the chemical vapor deposition (CVD) technique. Aqueous suspensions both of unmodified fumed silica and of the obtained germania/silica (GS) samples have been studied by electrophoresis, photon correlation spectroscopy, nuclear magnetic resonance (1H NMR), and quantum chemical semiempirical AM1-SM1 methods. For suspensions of GS samples, electrophoretic mobility, isoelectric point (IEP), particle size distribution, average effective diameter (Def), and free energy changes (DeltaG) of interfacial water were found to depend nonlinearly on the concentration of germania (CGeO2). At small concentrations, the structure of the synthesized germania is clustered rather than layered. For the last samples, maximum reduction of the free energy for interfacial water and larger values of IEP and Def in comparison with GS at high CGeO2 or unmodified silica have been observed. In suspensions, the particle size distributions for silica and GS are uni-, bi-, and trimodal and depend on pH and CGeO2. Values of polydispersity (P) are very rarely smaller than 0.05 and are usually between 0.25 and 0.70, which points to the wide size distribution. The solvation energy of the charged and uncharged GS clusters calculated by AM1-SM1 method decreases with increasing germania concentration. Copyright 1998 Academic Press.
ABSTRACT
Thermally stimulated depolarization spectra of frozen aqueous suspensions of fumed silica show a dependence of relaxation processes on the concentration of silica (CSiO2) due to a change in the particle-particle interaction (their electrical double layers) and the concentration of bulk water (ice) with increasing CSiO2. A significant fraction of agglomerates of primary particle aggregates are not decomposed in aqueous suspensions of fumed silica. The thickness of an interfacial water layer perturbed by the silica surface or protein molecules was estimated from the dependence of 1H NMR signal intensity of unfrozen water on temperature below 273 K, separating the signals of water molecules weakly and strongly bound to the surface. Protein molecules adsorbed on fumed silica aggregates form hydrogen bonds with identical withSiOH or identical withSiO- groups by amino groups, e.g., H+NR from the zwitterion fragments. Aqueous suspensions of mechanochemically activated fumed silica do not lose protein adsorption ability during a long period in which the particles remain as microscaled agglomerates. According to AM1-SM1 calculations, complexes such as identical withSiO(H)···H+NRCOO- are stabilized in aqueous suspensions of silica and the protein desorption rate decreases with increasing molecular weight of proteins.
