Anatase structure

The theoretical result is derived that ionic compounds MXS will crystallize with the fluorite structure if the radius ratio Rm/Rx is greater than 0.65, and with the rutile (or anatase) structure if it is less. This result is experimentally substantiated. 

The band gap energy of modified catalysts decreased down to 1.6 eV, and the basic structure and physical properties of the catalysts were not changed during modification process. All of the synthesized Ti02 were anatase structure but commercial Ti02 were contained 30% rutile structure. However, the catalytic activity of modified catalysts using two different Ti02 were almost the same in this reaction conditions. 

According to this rule, rutile and, at high pressures, the modification with the oc-Pb02 structure are the most stable forms of Ti02. Numerous compounds crystallize in the rutile type and some in the oc-Pb02 type, whereas scarcely any examples are known for the brookite and the anatase structures. 

Mixed Ti-W oxides, after deposition of Pt, are active in the mineralization of toluene with sunlight excitation due to the low bandgap (2.7 eV) caused by the presence of both W and Pt in the anatase structure [87]. Vanadium was also used to prepare visible light active photocatalysts in dye degradation [88]. 

The comparison of the structure of Ti02 with that of MgH2 shows that the anatase-type structure has not yet been observed for MgH2 [97]. However, theoretical calculations of the assumed anatase structure exhibit a destabilized hydride compared with a-MgH2 (Figure 5.58). 

Titanium Silicates. XRD data show that all the TS-1 samples are very crystalline and have the MFI structure. The Ti02 obtained after calcination has the anatase structure. 

TiOj, both anatase and rutile structures were found. On the contrary, the TiO (ST) sample showed only anatase structure. The average crystallite size was 9.3 nm for TiOj (ST) samples by using Scherrer s equation [8], which was significantly smaller compared to the P25 powder, with an average crystallite size between 15 and 25 nm [9]. 

Crystallization in the anatase structure was reported to occur in the sol-gel processed TiO after calcination between 350°C and 500°C [5]. However, crystallized TiO particles can be produced by using the solvothermal method without any post-treatment. 

Annealing in air increased the crystal size (up to 25 mn at 600°C) without damage to the film while retaining the anatase structure (the rutile structure was barely noticeable after 2 hr at 600°C). 

It is interesting that rutile, with only two shared edges per octahedron, is reported to be more stable than brookite and anatase, and, moreover, that many substances MXa have the rutile structure, whereas only titanium dioxide has been reported to have the brookite and anatase structures. 

Microfine TiOz with predominant anatase structure can also be manufactured by reductive flame hydrolysis of TiCl4 at < 700°C [5.287] ... 

In our previous ab initio computations it was assumed that the anatase structure maintained its tetragonal structure on intercalation of the alkali metal atoms. (This was necessitated by the at that time considered heavy computational requirements and the lack of efficient automated procedures for geometry optimizations.) Experimentally, it is known that the cell symmetry decreases from tetragonal for Ti02 to orthorhombic when Li0.5TiO2 is formed [129, 133]. 

Osterwalder, J., T. Droubay, T. Kaspar, J. Williams, C.M. Wang and S.A. Chambers (2005). Growth of Cr-doped Ti02 films in the rutile and anatase structures by oxygen plasma assisted molecular beam epitaxy. Thin Solid Films, 484(1-2), 289-298. 

It is thus important to realise that the candidate structure with the lowest cost function is not necessarily the crystal structure that is observed. Furthermore, there is a need to save (for further analysis in stage 2) the best candidate after every 50 cycles, say, rather than just the best candidate structure at the end of stage 1. Ultimately, a comparison of the simulated and observed powder diffrac-tograms is required in a final stage . It is also noted that, after stage 2, the anatase structure had a lower lattice energy (a better measure of quality) than the stretched Cu20 polymorph. 

The structure of titanium complexes affects the formation of hydrated titanium dioxide structure, since rutile and anatase lattices are composed of TiO octahedrons connected in definite manner. The formation of anatase structure occurs when two octahedral complexes form a common vertex. When two octahedrons are united via their edges, rutile structure is formed. Based on this assumption, it is considered that if titanium (IV) complexes with one reactive centre are formed during hydrolysis, anatase structure is formed if there are two reactive centres, then rutile structure is formed. 

Figure 20. Anatase structure of Ti02 where filled balls are titanium and open circles are oxygen.

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