Silica/iron composite gels

The key property required of the inorganic species is ability to build up (polymerize) around the template molecules into a stable framework. As is already evident in this article, the most commonly used inorganic species are silicate ions, which yield a silica framework. The silica can be doped with a wide variety of other elements (heteroatoms), which are able to occupy positions within the framework. For example, addition of an aluminium source to the synthesis gel provides aluminosilicate ions and ultimately an aluminosilicate mesoporous molecular sieve. Other nonsilica metal oxides can also be used to construct stable mesoporous materials. These include alumina, zirconia, and titania. Metal oxide mesophases, of varying stability, have also been obtained from metals such as antimony (Sb), iron (Fe), zinc (Zn), lead (Pb), tungsten (W), molybdenum (M), niobium (Nb), tantalum (Ta), and manganese (Mn). The thermal stability, after template removal, and structural ordering of these mesostructured metal oxides, is far lower, however, than that of mesoporous silica. Other compositions that are possible include mesostructured metal sulfides (though these are unstable to template removal) and mesoporous metals (e.g., platinum, Pt). 

Iron oxide silica aerogel composites have been prepared by the sol-gel method followed by supercritical drying [6] and are found to be two or three orders of magnitude more reactive than the conventional iron oxide. The increase in reactivity was attributed to the large surface area of iron oxide nanoparticles supported on the silica aerogel. 

Chastellain et al. confirmed the sensitivity of the power loss with respect to the size distribution, comparing power losses of silica-iron oxide nanocomposites synthesized by different routes [92]. Although for biomedical applications iron oxides are the main materials being studied because of their low toxicity, several studies have also evaluated the role of other materials for hyperthermia such as manganese perovskite nanoparticles [33]. The heating properties of several ferrites synthesized by sol-gel (Fe, Li, Ni/Zn/Cu, Co, Co/Ni, Ba, and Sr ferrites) have also been characterized [93]. However, in this kind of studies, it is complicated to isolate the effect of the composition on the heating properties of the material from other factors that may influence as well, such as the particle size or size distribution. 

The Mossbauer spectrum of ferrous Y-zeolite is somewhat similar to that of the reduced silica gel samples (103). The spectrum consists of two overlapping and partially resolved doublets with the inner doublet, 3 = 0.89 mm sec-1 and A = 0.62 mm sec-1, being attributed to the ferrous ion on the surface. In both the Y-zeolite and the reduced iron oxide on silica samples, the inner doublets representing surface ferrous states are the first to be affected by adsorption of polar molecules, but in the case of Y-zeolite the addition of excess amounts of water or ammonia causes the disappearance of the spectrum, and this has been interpreted in terms of "solvation of the ferrous ions by absorbate causing weakening of the bonding to the crystalline lattice. It is also possible that the spectrum is a composite representing a multiplicity of parameters. 

As in the case of iron, the hydrodynamic regime of the water basin has a considerable effect on the processes of accumulation of siUca. In the case of simultaneous deposition of silica and iron, the sols of their hydroxides are usually converted to gels having the composition of ferruginous chlorites. In the case of coagulation of silica sol around submarine active springs with constant discharge, only small lenticular intercalations of spilite are formed. 

Other type of complexes have also been used for the oxidation of hydrocarbons. For instance, Fujiwara and coworkers employ a coordinated complex of palladium with o-phenanthroline as an efficient catalyst for the direct conversion of benzene into phenol. Moro-oka and coworkers use an oxo-binuclear iron complex, whereas Machida and Kimura work with macrocyclic polyamines. Sasaki and coworkers employ Pd-Cu composite catalysts, which are prepared by impregnating the respective metal salts on silica gel. 

Noncrystalline materials predominate in the clay fractions of many soils derived from volcanic ash (Tamura et al [1953] SuDO [1954, 1956] Fieldes [1955] Fieldes and Taylor [1961]), and appreciable amounts may also be found in many other soils (Kirkman et al [1966]), but their classification presents many problems. Thus, they are extremely variable in composition and consist essentially of mixed gels of silica, alumina, iron oxide, etc., in a very wide range of proportions. It seems impossible, therefore, to give a mineralogical nomenclature that covers the range of composition of minerals in soils, although the general classification scheme in Table 6 (Mackenzie [1957]) may be useful on occasion. 

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