Silica/titania particles

Electron micrographs (scanning and transmission) showed that tungsten carbide is well dispersed on the surface of each support as nanosized particles (20 - 50 nm) as typified by the images in Figs. 3 (a b). However, BET surface area decreased in the order alumina > silica > titania > zirconia. With highest surface area obtained for each support being 240,133,18 and 9 m g respectively. 

Li, Y. and Kim, S.J. (2005) Synthesis and characterization of nano titania particles embedded in mesoporous silica with both high photocatalytic activity and adsorption capability. Journal of Physical Chemistry B, 109, 12309-12315. 

We also include in this class of quasi-2D nanostructured materials Titania deposited inside ordered mesoporous silica (because an inner coating of mesoporous silica may be realized), or nano-dot type Titania particles well dispersed in the ordered porous matrix. We do not consider here solids which contain linear or zig-zag type TiOTiO-nanowires in a microcrystalline porous framework, such as ETS-4 and ETS-10, notwithstanding the interest of these materials also as photocatalysts,146-151 because these nanowires are located inside the host matrix, and not fully accessible from the gas reactants (the reactivity is essentially at pore mouth). 

Jung, K. Y. and Park, S. B. (2000). Enhanced photoactivity of silica-embedded titania particles prepared by sol-gel process for the decomposition of trichloroethylene. Appl. Catal. B-Environ. 

Bansal, V., Rautaray, D., Bharde, A., Ahire, K., Sanyal, A., Ahmad, A., and Sastry, M. 2005. Fungus-mediated biosynthesis of silica and titania particles. Journal of Material Chemistry, 15 2583-9. 

High quality microporous membranes are almost exclusively reported for silica or for binary silica-titania or silica-zirconia systems [42,46]. This is due to the very fast hydrolysis and condensation rates of the metal organic precursor of the metals relevant for membrane synthesis (Ti, Zr, Sn, Al). This usually results in too large particles in the precursor solution. Though many authors claim to have produced microporous materials by sol-gel methods (see e.g. Section 8.2.3), only a few have shown the synthesis of membranes of these materials and a still smaller number has characterised them with appropriate separation properties to be reasonably defect free. Therefore in the remainder of Section 8.2.1 a focus will be given to silica-based membranes. 

This simple picfure can only be partly correct, as it has been shown that both unsupported ultrafine titania nanoparticles [10] and silica-supported monodis-persed subnanometric titania particles [11] are active catalysts for olefin epoxidation with hydroperoxides. In these small titania particles, the titanium atoms possess coordination states between four and six. Quanfum chemical calculations derived similar activation energies for oxygen transfer on mononuclear four- and five-coordinafed fifanium sifes [12,13]. 

FIGURE 13.7 Representation of titanium-on-silica catalyst in SMPO as a range of silica-supported—and therefore stabilized—ultrafine titania particles containing catalytically active, coordinatively unsaturated titanate sites. 

PZC/IEP of Titania Coating on Silica Deposited from Pre-Existing Titania Particles... 

When the polymer becomes partially swollen, the catalyst particle size can also sometimes influence comonomer incorporation [493]. The catalyst particle size determines the polymer particle size, and when the polymer is in a partially swollen state, monomer diffusion can become more important. A typical example is illustrated by the data of Figure 46. A Cr/silica-titania was sized by screening into three narrow portions centered at about 25, 110, and 220 pm in diameter. Each portion... 

FIGURE 46 Densities of polymers made with Cr/silica-titania (650 °C) of varying particle diameters. Under conditions of swelling, larger particle size sometimes tends to encourage 1-hexene incorporation, but lower the activity. 

The titania particles precipitate under reaction conditions very similar to those of the silica systems discussed earlier. A critical nucleation concentration of 1.5-3 times [C]eq is measured. This low supersaturation level is not reached until very late in the precipitation reaction (Figure 3). The rate of loss of soluble titania is also independent of the presence of solid surface area. Finally, on the basis of measures of particle surface potentials, nuclei of sizes less than about 20 nm are expected to be unstable and to rapidly aggregate. These results again indicate that during the precipitation of titania, nucleation may occur over much of the reaction period and final particle sizes may be determined by the aggregation of primary particles. These conclusions are supported by the transmission electron microscopy work of Diaz-Gomaz et al. (30). 

The dense silica (DS) process involves the exposure of titania particles to aqueous silica solutions of increasing silica concentration. The process is examined in this chapter by relating silica adsorption on titania surfaces to solution pH and concentration and to the various monomeric, multimeric, and polymeric species present in aqueous solutions of silica. Microelectrophoresis and gas adsorption studies reveal that adsorption of monomeric silica occurs via hydrated cation sites that constitute only approximately 40% of titania surfaces. These anchoring sites provide a base for complete surface coverage and buildup of silica multilayers (coatings), a buildup that occurs when the silica concentration is increased sufficiently at the chosen pH (around 10 in the DS process) to induce polymerization. 

Figure 2. Transmission electron micrograph of dense-silica-coated titania particles magnification, 200,000 5.0 wt % silica loading. (Reproduced with permission from reference 23. Copyright 1979.)...

Figure 3. Dependence of the amount of Ag photodeposited on silica-coated titania particles on the thickness of silica coating. (Reproduced with permission from reference 13. Copyright 1989.)...

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