Processing of Titania

Pure primary titanium dioxide does not occur naturally, but is derived by weathering from ilmenite (FeTiOs), perovskite (CaTiOs), and titanite (sphene) (CaTiSiO ). The weathering products form leuxocene ores with up to 68% mtile content. The 

These ores are the principal raw materials used in the manufacture of titanium dioxide pigment The first step to purify the ore is basically a refinement, using either sulfuric acid (sulfate process Gesenhues et al., 2003) or chlorine (chloride process) as an extraction agent 

To produce the rutile form of titanium dioxide, the clarified Uquor is hydrolyzed in the presence of a rutile seeding agent, obtained by neutralizing a small portion of the mother liquor in the presence of hydrochloric acid. The resultant crystals are filtered, washed, and calcined at temperatures between 900 and 930 °C. 

Chloride process. The chloride process yields the rutile form of titanium dioxide. At temperatures between 800 and 1200 C, chlorine is reacted in a fluidized-bed reactor with a titanium-containing mineral under reducing conditions (in the presence of coke) to form anhydrous TiCh- Further purification requires separa-hon by fractional condensation. The conversion of TiCl, to Ti02 may be accomplished by either direct thermal oxidation, or by reaction with steam in the vapor phase at temperatures in the range of 900—1400 °C. A smaU amount of AICI3 is generaUy added to promote formation of the rutile form. The titanium dioxide product is then washed, calcined, and packaged (Kuznetsof, 2006). 

Alternatively, titanium-containing minerals can be reacted with concentrated hydrochloric acid to form solutions of TiCh which are then further purified. Hydrolysis of the tetrachloride wiU yield the dioxide which is filtered off, washed, calcined, and packaged. 

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Figure 20.16 schematically illustrates the process of titania film growth by ALD. The substrate is hydroxylated first, prior to the introduction of titanium precursor, titanium tetrachloride. Titanium tetrachloride reacts with the surface hydroxyl groups through a surface condensation reaction ... 

Peres-Durand S, Rouviere J and Guizard C 1995 Sol-gel processing of titania using reverse micellar systems as reaction media Coll. Surf. 98 270... 

Nishide, T., T. Yabe, N. Miyabayashi, M. Sano (2004) Analysis of Firing Processes of Titania Gel Films Fabricated by Sol-Gel Processes. Thin Solid Films, 467 43-49. 

The present study revealed effects of various rutile/anatase ratios in titania on the reduction behaviors of titania-supported cobalt catalysts. It was found that the presence of rutile phase in titania could facilitate the reduction process of the orbalt catalyst. As a matter of fact, the number of reduced cobalt metal surface atoms, which is related to the overall activity during CO hydrogenation increased. 

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

The measured BET surface areas of titania samples were in the range of 99-116 m /g. It was found that surface area of titania decreased (as shown in Table 1) with increasing %02 during calcinations process whereas the crystallite size was apparently constant. 

Various methods are applied to the synthesis of titania particles including sol-gel method, hydrothermal method [2], citrate gel method, flame processing and spray pyrolysis [1]. To utilize titania as a photocatalyst, the formation of ultrafme anatase titania particles with large crystallite size and large surface area by various ways has been studied [4]. 

Another distinguishing feature of titania prepared by flame spray pyrolysis is the draar e of anatase crystallite size with the increase of flame temperature. Generally, the increase of preparation temperature increases the crystallite size in other processes such as sol-gel method, hydrothermal method [2, 3], flame processing and conventional spray pyrolysis. The decrease of crystallite size was directly related to the decrease of particle size. Fig. 5 shows SEM and TEM images of titania particles prepared by flame spray pyrolysis. 

An interesting recent example of successful application of the SSG process combined with ensuing supercritical drying is the design of titania-silica mixed oxides for the epoxidation of bulky olefins [16-18]. This example will be used to illustrate the opportunities the combined use of SSG and SCD provide for tailoring the chemical and structural properties of mixed oxides. 

The technology for the production of titania slag by direct reduction smelting in electric arc furnaces (particularly for smelting of iron titanates and ilmenites having lower amounts of titania) is very old and the process has been in vogue in Canada, Norway, South Africa,... 

Some non-silica sol-gel materials have also been developed to immobilize bioactive molecules for the construction of biosensors and to synthesize new catalysts for the functional devices. Liu et al. [33] proved that alumina sol-gel was a suitable matrix to improve the immobilization of tyrosinase for detection of trace phenols. Titania is another kind of non-silica material easily obtained from the sol-gel process [34, 35], Luckarift et al. [36] introduced a new method for enzyme immobilization in a bio-mimetic silica support. In this biosilicification process precipitation was catalyzed by the R5 peptide, the repeat unit of the silaffin, which was identified from the diatom Cylindrotheca fusiformis. During the enzyme immobilization in biosilicification the reaction mixture consisted of silicic acid (hydrolyzed tetramethyl orthosilicate) and R5 peptide and enzyme. In the process of precipitation the reaction enzyme was entrapped and nm-sized biosilica-immobilized spheres were formed. Carturan et al. [11] developed a biosil method for the encapsulation of plant and animal cells. 

Blumenfeld An early version of the Sulfate process for making titanium dioxide pigment, in which the nucleation of the precipitation of titania hydrate is accomplished by dilution under controlled conditions. Invented by J. Blumenfeld, a Russian working in London in the... 

The preparation of Titania nanocoils has been yet not investigated in literature. However, quite recent results258 show that the effective structure of Titania nanotube likely produced by controlled anodization process is that of a helical (compressed) nanocoil. Fig. 11 shows this concept. It was also demonstrated that the formation of these helical nanocoils improves the photo-generated current compared to samples after short anodization where only a Titania layer is formed. 

Different from the formation mechanism of titania nanotubes, Fe203 nanotubes are formed by a coordination-assisted dissolution process [95]. The presence of phosphate ions is the crucial factor that induces the formation of a tubular structure, which results from the selective adsorption of phosphate ions on the surfaces of hematite particles and their ability to coordinate with ferric ions. 

Oguri Y, Riman RE, Bowen HK (1988) Processing of anatase prepared from hydrothermally treated alkoxy derived hydrous titania. J Mater Sci 23 2897-2904... 

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