Titania polymorphism

Fig. (15). Production of CO and CH4 on the untreated and the treated titania polymorphs Reproduced with permission from ref [123]. 2012 American Chemical Society.

Fig. 7 Corresponding Raman spectra of the titania polymorphs found in the sample, and the aluminium association.

Titanium Dioxide. The recrystallization of titanium dioxide in a cover-coat glass is very important to the development of thin, highly opaque finish coats. Titania, Ti02, is the primary opacifying agent for white finish coats. Two polymorphic forms of titania, anatase and mtile, may be present in... 

Another polymorph of titania with a tetragonal structure a = 3.73 A and c = 9.37 A. Anatase has edge-sharing [Ti06] octahedra joined at the corners so that the oxygen atoms are in a slightly distorted cubic arrangement. At... 

A related problem is the reduction in support surface area. This is especially a problem in the case of titania, where the anatase polymorph is only stable under oxidative regeneration conditions from about 400°C to 750°C. The addition of Si, Zr and Ta as promoter elements may avoid or diminish surface collapse of the support oxide. 

Titania most often appears in the form of one of two predominant crystallographic polymorphs, rutile and anatase. At least seven other distinct phases have been characterized by x-ray diffraction [3], but none are sufficiently commonplace to play a role in pigmentation using titanias. Rutile possesses a higher refractive index than anatase. It is also inherently considerably less photoactive than anatase. For these reasons, rutile has become the predominant polymorph of titania for pigmentary uses in polymers. 

Figure 6.11. A. O natural abundance MAS NMR spectra of zirconia polymorphs. The peak marked A is from oxygen in the alumina rotor. The asterisks denote spinning side bands. From Bastow and Stuart (1990) by permission of Elsevier Science. B. MAS NMR spectra of titania gel heated to various temperatures, showing the evolution of rutile at the expense of anatase. From Bastow et at. (1993) by permission of the Royal Society of Chemistry. C. Static and MAS O NMR spectra of cubic Y2O3. Adapted from Florian et at. (1995).

Titania Titania has three common polymorphs, anatase, rutile, and brookite, and is widely used as a white pigment The technological applications are as a photocatalyst [293], the removal of organic pollutants from water, and in photovoltaic devices [294] which convert light to electricity. Bulk titania is a n-type semiconductor, the conductivity of which can be enhanced by either reduction or doping. 

Titania has three naturally occurring polymorphs anatase, brookite, and rutile. Anatase and brookite are considered to be kinetic products despite the fact that depending on the particle size, anatase becomes more stable than rutile. The anatase-rutile transition is exothermic and irreversible, occurring in the range 400-1200°C [25,26]. 

TiOz Polymorph The Interest of Anatase and Amorphous Titania... 

Table 1.S Physical properties of various polymorphs of titania single crystals.

Some physical properties of the three polymorphs of Ti02 are hsted in Table 1.5. Systematic data on titania single crystals are relatively scarce, and this applies especially to brookite, most likely due to difficulties with its preparation. The presence of impurities also results in a significant scatter of the property values. 

Also, titania is a polymorphic material the most usual phases are anatase (SG = I4i/amd, Z = 4) and rutile (S.G. P42/mnm, Z = 2), the latter being always thermodynamically stable. In addition, titanias are highly ionic oxides with medium-high Lewis acidity, significant basicity and weak Brpnsted acidity if at all. Characterization data show that on anatase stronger Lewis acid sites are usually detectable than on rutile [261,262]. 

Borothermic reduction was confirmed at a lower temperature, and the carbothermic reduction occurred at a higher temperature (33), which favors the production of high-purity powders. TiB2 and ZrBj powders were also synthesized by mechanochemical treatment of titania and zirconia powders with amorphous boron followed by a relatively low-temperature annealing (1100°C) (34). The occurrence of polymorphic transformations has been observed with milling time for both systems, together with a decrease upon annealing of the temperature of the boro-... 

In the present study we attempt to overcome this difficulty by decreasing the pzc of the titania support. This would increase the number of the receptor sites (negatively charged hydroxyls) on its surface and thus would allow an increase of the deposited Ni [3]. The change of suspension temperature is perhaps an effective tool for regulating the pzc and the concentration of the aforementioned receptor sites, at constant pH. Since controversial studies dealing with the influence of temperature on the pzc of titania had appeared in the literature [6-9] (most of them concern titania in the rutile form), we first studied systematically the variation of pzc, with temperature, of various titanias in the anatase polymorph used as catalytic supports. Then, we studied systematically the deposition of Ni species on titania surface at various temperatures and constant pH values at each temperature. The ultimate goal of our study is the maximization of the extent of interfacial deposition of the [Ni(H20)6] species on the titania surface. 

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