Anatase structures, titanium oxides

There are two forms of titanium dioxide used in rubber anatas and rutile types that differ in crystalhne structure. An anatase-type titanium oxide pigmented vulcanizate can have an outstanding (bluish) white color, while most rutile titanium dioxides give a cream-colored white rubber vulcanizate. However, rutile types have 20 percent more covering power than do anatas types. Also, rutile types give the more fight- and weather-resistant vulcanizates. Nevertheless, anatas types are used where a more nearly pure white material is required. 

The lithium-titanium oxides are prepared by heating a mixture of anatase (Ti02) and LiOH at a high temperature. The product heated at 800-900 °C has a spinel structure of Li4/3Ti5y304. When the charge and discharge cycles are performed... 

The process has been commercially implemented in Japan since 1977 [1] and a decade later in the U.S., Germany and Austria. The catalysts are based on a support material (titanium oxide in the anatase form), the active components (oxides of vanadium, tungsten and, in some cases, of molybdenum) and modifiers, dopants and additives to improve the performance, especially stability. The catalyst is then deposited over a structured support based on a ceramic or metallic honeycomb and plate-type structure on which a washcoat is then deposited. The honeycomb form usually is an extruded ceramic with the catalyst either incorporated throughout the stmcture (homogeneous) or coated on the substrate. In the plate geometry, the support material is generally coated with the catalyst. 

The rutile structure. Titanium dioxide crystallizes tn three crystal forms at utmospheric pressure anatase, brookite, and rutile (Fig. 4.4a). Only the last (tetragonal P42/mnin) will be considered here. The coordination numbers are 6 for the cation (six oxide anions arranged approximately octahedrally about the titanium ions) and 3 for the anion (three tiianium ions trigonally about the oxide ions). The rutile structure is also found in the dioxides of Cr, Mn, Ge, Ru, Rh, Sn, Os. Ir, Pt. and Pb. 

TP-Raman spectroscopy was used to evaluate the thermal stability of titanium oxide nanotubes (Blume, 2001). Titania nanotubes are more stable in oxidizing than in inert atmospheres. Titania nanotubes were transformed into anatase at 250 °C in an inert atmosphere, whereas the nanotube structure remained stable up to a temperature of 400 °C in the presence of air. It was proposed that oxygen prevents segregation of hydroxyl and Ti4+ ions and the ensuing reduction to Ti3+, which would otherwise lead to the generation of a nonstoichio metric anatase phase (Blume, 2001). 

Titanium Oxide 6 Complex Oxides Sulfide. The naturally occurring dioxide Ti02 has three crystal modifications, rutile, anatase, and brookite. In rutile, the commonest, the Ti is octahedral and this structure is a common one for MX2 compounds. In anatase and brookite there are very distorted octahedra of oxygen atoms about each titanium, two being relatively close. Although rutile has been assumed to be the most stable form because of its common occurrence, anatase is 8 to 12 kJ mol-1 more stable than rutile. 

Incorporation of titanium oxide species within the framework of mesoporous silicas has been shown to produce highly efficient photocatalytic materials. Extremely careful preparation conditions [84] leads to highly structured materials comprising anatase nanoparticles of dimension between 5 and 10 run. The channeled structure, together with the hydrophobic/hydrophilic character, are also key features controUing their enhanced photoreactivity. The photocatalytic activity of such mesoporous catalysts has been studied for the degradation of phenol in aqueous solutions [85]. It was observed that for structured mesoporous materials with low Ti content, the turnover frequency was four times greater than that for standard P25. 

Change in the selectivity patterns of transition metal ion/H+ systems has been encountered with the amorphous and anatase types of hydrous titanium oxides with different crystallinities [24]. Potassium titanate, KjO nXi02 (n = 2-4), in particular, exhibits a layered structure. Fibrous titanic acid, H2Ti409 nHjO, is obtained by acid treatment of fibrous K2Ti409 nH20 and shows higher selectivity for K, Rb and Cs than the amorphous titanic acid [206]. 

Figure 7 shows the HREELS spectrum acquired at room temperature and at an incidence angle of 60° from the surface normal for a 30 MLE titanium oxide film prepared as described above. Fundamental phonons of 54meV(v2) as well as overtone losses and combinations are seen. The loss at 149 meV is due to v + V2, and the loss at 245 meV is due to v + 2v2. That Ti02 films prepared in this manner are anatase and can be ruled out because the anatase phase exhibits fundamental surface phonons at 44 and 98meV for the (100) face and 48 and 92meV for the (001) face. The combined use of FEED, AES, XPS, and HREELS has revealed that the thin Ti02 films prepared via the above procedure are rutile with a surface structural orientation primarily (100). 

Let us finally turn to a brief discussion of the third term, Ti02 (i.e., titanium dioxide). Ti02 has three different crystal structures [18] rutile, anatase, and brookite only the former two of them are commonly used in photocatalysis. Like for many other metal oxides (also for titanium oxide) have the respective structural, optical, and electronic properties... 

The Raman spectra for the Ti02 sample showed bands at 145, 197, 397, 516, 638 cm characteristic of anatase, and a shoulder at 448 cm", indicating a small portion of rutile. The y-alumina support showed no Raman active modes [5]. The Raman spectra of 7TA, IOTA and 13TA support samples exhibited weak bands due to anatase and the feature of y-alumina. The rutile form was not detected for any mixed support. The Raman spectra also showed a weak band in the 800-900 cm region which is indicative of Ti-O-Ti bonds [7]. Bands in the 950-1000 cm" region were not observed and this suggests that surface titanium oxide species do not contain Ti=0 moieties. Thus it is concluded that Ti02 forms a surface layer on alumina which has a polymeric structure. 

The peculiarities of the defect structure of TiC and TiN allow for oxygen dissolution on the first stage of oxidation. Titanium oxides appear later, when the material structure is saturated with oxygen. Anatase was observed at lower temperatures and/or short oxidation times (5 min). With increasing time and temperature, rutile is formed as the only oxidation product [108]. 

The diffraction patterns of the decomposed aerosol products of TiO and Fe-TiOj obtained by spray pyrolysis showed poor crystallinity. Specimens were therefore further calcined at temperatures of 823K, 873IG 973K and 1073K in air for a period of 2 hours in order to improve their crystalline structure. Fig.1. shows the x-ray diffraction patterns of titanium dioxide (anatase), and a series of iron-titanium oxide specimens of nominal Fe(IlI) concentration equivalent to 0.1, 1, 2, 5 and 10 atom%, prepared by the spray pyrolysis method and subsequently calcined at 823K in air for 2 hours. 

As demonstrated above by the results of semiempirical calculations of metal oxides, the full valence of the metal atom, defined according to (9.13), correlates with the metal oxidation state. Such a conclusion was confirmed later in ab-initio HF calculations [574] of titanium oxides. Although the quadrivalent state (oxidation state IV) of the titanium atom is the most stable, the existence of oxygen compoimds of titanium in formal oxidation states of III and II, as well as of a series of nonstoichiometric compounds was established. Table 9.4 presents the space-group symbol, the number of formula units in the cell, and the shortest Ti-Ti and Ti-0 experimental distances for TiO in hexagonal structure, Ti2 03 in corundum structure and Ti02 (in the rutile (r), anatase (a), and brookite (b) modifications). 

Titanium dioxide (in rutile and anatase structures) is the most investigated crystalline system in the surface science of metal oxides. The review article [783] summarizes the results of experimental and theoretical studies of titanium dioxide (bulk and surface) made up to 2002 inclusive. The information about calculations of the surface reconstruction, surface defects and growth of metals on Ti02 is also included. The results of the later theoretical studies of rntHe surfaces can be found in [784-795] and references therein. In the majority of the calculations the slab model was used for the study of periodic surface structures. 

---