Dense silica coatings

Figure 2. Schematic representations (1) of dense silica coatings formed i...

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.)...

Dense Silica Coatings on Micro- and Nanoparticles by Deposition of Monosilicic Acid... 

An upper limit to the thickness of dense silica coatings on hydroxyl-ated surfaces can be obtained with some of the conventional coating techniques (I). We have found that this limit can be extended significantly by coating with monosilicic acid (MSA). 

In this chapter processes for coating a-alumina particles with MSA and for characterizing the coated particles to show the MSA is deposited as monomeric units to form a dense silica coating are described. The same procedures can be applied to coating titania and p-zeolite particles (4). 

The economic impact of Iler s silica coating process will continue to play a major role in the coatings industry, from the perspective of the pigment manufacturers as well as the coatings producer. A conservative estimate of 300 mUlion is placed on the value of dense silica coated titanium dioxide pigments that were sold in 1990 in the world pigment marketplace. The total value of manufactured coatings products is estimated to be in excess of 4 billion. 

One of the first examples of mesoscopic-macroscopic two-dimensional ordering within a structure involved a bacterial superstructure formed from the co-aligned multicellular filaments of Bacillus subtilis that was used to template macroporous fibers of either amorphous or ordered mesoporous silica [82], The interfilament space was mineralized with mesoporous silica and, following removal of the organic, a macroporous framework with 0.5 pm wide channels remained. Mesoporous silica channel walls in this hierarchical structure were curved and approximately 100 nm in thickness. Dense, amorphous walls were obtained by replacing the surfactant-silicate synthesis mixture with a silica sol solution. The difference in the mode of formation between porous and non-porous wall structures was explained in terms of assembly from close-packed mesoporous silica coated bacterial filaments in the former compared to consolidation of silica nanoparticles within interfilament voids in the latter. 

Values of A, 4 for nitrogen adsorption at 0 = 0.1 are recorded in Table 10.13, which also contains the corresponding adsorption energy data for silica-coated rutile. It was independently confirmed that the surface properties of the latter sample, which had a coating of 2.6% dense silica, were very similar to those of pure silica (Furlong etal., 1980). 

Thus, silica coating appears to retard both the formation of the hard carbon skin and the growth of the secondary (less dense) coke deposit at cracks in the skin, with the overall result that in cracking runs up to three days in length in the laboratory ESC reactor the coke formation is inhibited by a factor of 3-4 compared to uncoated tubes. 

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 8. Silica layer thickness versus silica loading determined by TEM and XPS (ESCA) methods for silica-coated a-alumina particles. The theoretical curve was calculated for uniform silica coating of dense spherical a-alumina particles with equivalent 150-nm diameters. (Reprinted with permission from reference 7. Copyright 1989.)...

In this work we tried to create conditions to deposit MSA on the hydroxylated surface of a-alumina in dense silica layers. The surface-sensitive electrokinetic measurements, DRIFT, SIMS, and XPS, show that the coating grows by deposition of molecular units on the surface of the alumina, whereas TEM, XPS, and surface area measurements show that thick, nonporous silica coatings can be grown. Characterization of particles coated with submonolayer or thicker MSA give insights into the nature of the coatings and the deposition mechanism. 

TABLE 3 Stability as Measured by Amount of Iron Leached Out in Acid (pH 2.5) and Saturation Magnetization of Silica-Coated y-Fe203 Dense Liquid, Sol-Gel, and Two-step Coating Processes at 11% Si02... 

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