Titania

4 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. 

The titania used for catalysis is commonly prepared by the aqueous hydrolysis of titanium salts or by the flame hydrolysis of TiCl4. 48 jt jg found in two crystalline forms, rutile and anatase, with the form produced depending on the temperature and preparation procedure used. Anatase is more stable at the temperatures normally used for catalytic processes and, thus, is the more common support. The anatase prepared by flame hydrolysis of TiCl4 has surface areas near 40-80 m2/g and mean pore diameters of about 50 nm. That prepared by aqueous 

The titania obtained by the acid hydrolysis of tetrabutoxytitanium has surface areas of 190-200 m /g and average pore diameters of 15-20 nm. Even higher surface areas can be obtained using supercritical drying.20 

There are about 4-5 surface hydroxy groups per square nanometer on anatase and these are of two different types. One type is basic in nature and can be exchanged by F . This type of site is responsible for the adsorption of acidic species onto the surface and initiates the adsorption of base sensitive compounds such as Mo02(acac)2. The second type of surface hydroxide is acidic. It interacts strongly with base and initiates the adsorption of acid sensitive compounds such as Fe(acac)3.2 This amphoteric nature of titania is illustrated by the zeta potential versus pH curve shown in Fig. 9.9. Cationic catalyst precursers are adsorbed at pH values greater than 7-8 and anionic species from solutions having a pH less than about 5. 

Surface areas of different types of carbon supports.22 

The functional groups present in charcoal are phenols, carboxylic acids, quinones, ketones and lactones. They are essentially acidic supports. The nature and extent of the functionalities on the charcoal particle surface are a function of the material used in the carbonization and the type and duration of the activation procedure. In addition, treatment of these charcoals with oxidizing agents such as nitric acid or hydrogen peroxide increases the number of acid species present. A similar treatment will also functionalize the non-porous carbon blacks.25 Because of this it is difficult to draw any general conclusions concerning the adsorption capabilities of these charcoals other than to say that being acidic they will most readily adsorb cationic species. 

The mineral for titania (Ti02) is rutile. Rutile is a constituent of igneous rocks. It is an important constituent of beach sands. 

Titanium dioxide is also produced from ilmenite. Ilmenite is also termed ferrous titanate (FeTiOg). It is reacted with sulfuric acid. The temperature of the reaction is maintained in the range of 150°C-180°C. The result of this 

The solution of Ti0S04 is separated from the precipitated hydrated ferrous sulfate by filtration. 

The solution is diluted with water at 90°C. Then, titanyl hydroxide gets precipitated, as shown in Equation 12.9. The precipitate is filtered out and washed. It is then calcined at about 1000°C. 

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Fig. X-4. Water contact angle of titania-coated glass after treatment with trimethyloc-tadecylammonium chloride as a function of the number of coating treatments with 1.1% polydibutyl titanate. (From Ref. 51.)...

Valden M, Lai X and Goodman D W 1998 Onset of catalytic activity of gold clusters on titania with the appearance of nonmetallic properties Science 281 1647... 

Kurrat R, Prenosil J E and Ramsden J J 1997 Kinetios of human and bovine serum albumin adsorption at silioa-titania surfaoes J. Colloid Interface Sol. 185 1-8... 

Figure C2.17.1. Transmission electron micrograph of a Ti02 (anatase) nanocrystal. The mottled and unstmctured background is an amorjihous carbon support film. The nanocrystal is centred in die middle of die image. This microscopy allows for die direct imaging of die crystal stmcture, as well as the overall nanocrystal shape. This titania nanocrystal was syndiesized using die nonhydrolytic niediod outlined in [79].

Figure C2.17.8. Powder x-ray diffraction (PXRD) from amoriDhous and nanocry stalline Ti02 nanocrystals. Powder x-ray diffraction is an important test for nanocrystal quality. In the top panel, nanoparticles of titania provide no crystalline reflections. These samples, while showing some evidence of crystallinity in TEM, have a major amoriDhous component. A similar reaction, perfonned with a crystallizing agent at high temperature, provides well defined reflections which allow the anatase phase to be clearly identified.

Figure C2.17.9. Size-dependent changes in PXRD linewidtlis. PXRD can be used to evaluate tire average size of a sample. In tliese cases, different samples of nanocrystalline titania were analysed for tlieir grain size using tire Debye-Scherr fonnula. As tire domain size increases, tire widtlis of tire diffraction peaks decrease.

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

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

Kumar K P, Keizer K and Burggraaf A J 1994 Stabiiization of the porous texture of nanostructured titania by avoiding a phase transformation J. Mater. Sc/. Lett. 59... 

Edeison L H and Giaeser A M 1988 Roie of particie substructure in the sintering of monosized titania J. Am. Ceram. See. 71 225... 

Penn R L and Banfieid J F 1999 Formation of rutiie nuciei at anatase (112) twin interfaces and the phase transformation mechanism in nanocrystaiiine titania Am. Miner. 84 871... 

For other adsorptives the experimental evidence, though less plentiful than with nitrogen, supports the view that at a given temperature the lower closure point is never situated below a critical relative pressure which is characteristic of the adsorptive. Thus, for benzene at 298 K Dubinin noted a value of 017 on active carbons, and on active charcoals Everett and Whitton found 0-19 other values, at 298 K, are 0-20 on alumina xerogel, 0-20-0-22 on titania xerogel and 017-0-20 on ammonium silicomolybdate. Carbon tetrachloride at 298 K gives indication of a minimum closure point at 0-20-0-25 on a number of solids including... 

Type I isotherms are characterized by a plateau which is nearly or quite horizontal, and which may cut the p/p° = 1 axis sharply or may show a tail as saturation pressure is approached (Fig. 4.1). The incidence of hysteresis varies many Type I isotherms exhibit no hysteresis at all (Fig. 4.1), others display a definite loop, and in others there is hysteresis which may or may not persist to the lowest pressures ( low-pressure hysteresis ) (Fig. 4.2). Type 1 isotherms are quite common, and are no longer restricted, as seemed at one time to be the case, to charcoals. Many solids, if suitably prepared, will yield Type 1 isotherms the xerogcls of silica, titania, alumina... 

The experimental material was a sample of rutile on which a layer of tnicrocrystalline titania had been deposited. Isotherms of nitrogen were determined on the original material outgassed at 1S0°C and on samples that had been outgassed at 25°, 150° or 250°C respectively after being charged with n-nonane. 

Fig. 4.15 a,-plots for the adsorption of nitrogen on a sample of microporous titania, before and after nonane treatment. Curve (A), before nonane pre-adsorption curves (B), (C), (D) after nonane pre-adsorption, followed by outgassing at (B) 250° (C) 150°C (D) 25°C. The a,-plots were based on standard nitrogen isotherms having the same c-values as the isotherms of... 

A detailed study of the physical and chemical adsorption of water on three xerogels, ferric oxide, alumina and titania, as well as on silica (cf. p. 272) has been carried out by Morimoto and his co-workers. Each sample was outgassed at 600°C for 4 hours, the water isotherm determined at or near 20°C, and a repeat isotherm measured after an outgassing at 30 C. The procedure was repeated on the same sample after it had been evacuated at a... 

Physisorption and chemisorption of water on alumina, titania and ferric oxide selection of results (Morimoto ef a/. )... 

The isotherms of water on titania sometimes, but not always, exhibit an unusual feature (Dawson, Parfitt and co-workers ) in the form of a second knee X in the pjp° range 0-2 to 0-3, in addition to a rather rounded knee (cf. D in Figs. 5.21 and 5.22) at a lower relative pressure p/p° 0-05. 

It is less well known, but certainly no less important, that even with carbon dioxide as a drying agent, the supercritical drying conditions can also affect the properties of a product. Eor example, in the preparation of titania aerogels, temperature, pressure, the use of either Hquid or supercritical CO2, and the drying duration have all been shown to affect the surface area, pore volume, and pore size distributions of both the as-dried and calcined materials (34,35). The specific effect of using either Hquid or supercritical CO2 is shown in Eigure 3 as an iHustration (36). 

Fig. 3. Effect of using either liquid or supercritical carbon dioxide on the textural properties of titania aerogels calcined at the temperatures shown. (—), dried with Hquid carbon dioxide at 6 MPa and 283 K (-------), dried with supercritical carbon dioxide at 30 MPa and 323 K. Reproduced from Ref. 36.

Fig. 7. The effect of preparation on the pore size distribution (a), titanium dispersion (b), and the activity for epoxidation of cyclohexene (c) of titania—siUca containing 10 wt % titania and calcined in air at 673 K. Sample A, low-temperature aerogel Sample B, high-temperature aerogel Sample C, aerogel.

Acetylation of acetaldehyde to ethyUdene diacetate [542-10-9], a precursor of vinyl acetate, has long been known (7), but the condensation of formaldehyde [50-00-0] and acetic acid vapors to furnish acryflc acid [97-10-7] is more recent (30). These reactions consume relatively more energy than other routes for manufacturing vinyl acetate or acryflc acid, and thus are not likely to be further developed. Vapor-phase methanol—methyl acetate oxidation using simultaneous condensation to yield methyl acrylate is still being developed (28). A vanadium—titania phosphate catalyst is employed in that process. 

The process known as transimidization has been employed to functionalize polyimide oligomers, which were subsequentiy used to produce polyimide—titania hybrids (59). This technique resulted in the successhil synthesis of transparent hybrids composed of 18, 37, and 54% titania. The effect of metal alkoxide quantity, as well as the oligomer molecular weight and cure temperature, were evaluated using differential scanning calorimetry (dsc), thermogravimetric analysis (tga) and saxs. 

The abihty of organically modified ceramics based on alumina, zkconia, titania, or siUca (and mixtures of each) to function as abrasion-resistant coatings has also been studied (62). Eor example, polycarbonate, when coated with an epoxy—aluminosihcate system, experiences a significant reduction in the degree of hazing induced by an abrader, as compared to uncoated polycarbonate. 

Minerals. Iron-bearing minerals are numerous and are present in most soils and rocks. However only a few minerals are important sources of iron and thus called ores. Table 2 shows the principle iron-bearing minerals. Hematite is the most plentiful iron mineral mined, followed by magnetite, goethite, siderite, ilmenite, and pyrite. Siderite is unimportant in the United States, but is an important source of iron in Europe. Tlmenite is normally mined for titania with iron as a by-product. Pyrite is roasted to recover sulfur in the form of sulfur dioxide, leaving iron oxide as a by-product. 

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