Activity compared with anatase

The three TS-1 catalysts with similar Ti contents have cuboidal morphology with comparable particle sizes of 0.2-0.3 pum (as shown in SEM pictures, Fig. 53). The EPR spectra of the samples in contact with aqueous H202 (46%) (Fig. 54) indicate that the ratio of the A to B superoxo species in various TS-1 samples increases in the order TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase). Catalytic activity for phenol hydroxylation and allyl alcohol epoxi-dation (Table LIII) was found to parallel the A/B ratio of the oxo-Ti species (TS-1 (fluoride) < TS-1 (with anatase) < TS-1 (without anatase)). 

Both anatase and rutile have tetragonal crystal structures, but in anatase the elongated unit cell contains four molecules compared with two in the unit cell of rutile. Anatase transforms exothermally and irreversibly to rutile at temperatures above <1200 K (7-9). The rate of transformation is relatively slow and is governed by the nature and amount of impurities (]7) Certain impurities markedly decrease the transformation temperature which can be as low as 700 K in the presence of an alkali flux (10). These observations led to the conclusion that anatase is a metastable phase which persists up to a temperature sufficient to activate the crystal rearrangement. This was confirmed by the calorimetric data of Navrotsky and Kleppa (l ), who concluded that anatase is metastable with respect to rutile at all temperatures. 

The catalysts prepared from V2O5 mixtures with anatase by UHIG, as reported by Sobalik et al., also exhibited strong interaction between a vanadium oxide overlayer and the anatase support. The resulting materials were structurally similar (if not identical) to catalysts that were conventionally prepared by impregnation methods. The catalytic activity of the UHIG catalysts in the SCR reaction was also comparable to that of conventional catalysts. A most remarkable result of the work of Sobalik et al. 

The last example that I would like to present refers to the V(vi)/Ti02 (anatase) catalysts. From the ratio of the XPS intensities of zpziz Ti2p photoelectrons and from the AEM results (table 10. Fig. 5) it may be inferred that the active surface of the catalyst prepared by EDF is higher compared with that obtained for the catalyst prepared by NDl. Moreover, the ratio of the XPS intensities of V(v) to V( v) photoeiectrons indicate that relatively stronger "supported phase-support" interactions are exerted in the EDF catalyst. Both observations explain why in the EDF sample is not formed crystalline V2O5 (FT-IR). 

Another study reported that to obtain carbon-doped 1102, oxidation at 350°C for more than 50 h was recommended without any annealing in O2 atmosphere [159]. The substituted carbon atoms were confirmed by the Cls spectrum of XPS at 282 eV, which was due to the Ti-C bond (281.9 eV). Ti2p peaks in its XPS spectrum were not different from those of pure Ti02. It was also reported that carbon-doped Ti02 with high photocatalytic activity can be obtained from TiC by oxidation at 350°C for 8 h [160] in this case the amount of substituted carbon was about 0.7 mass%. This material was gray-white in color and its diffuse reflectance spectrum showed a red shift of 36 nm compared with pure anatase-type 1102. Its photoactivity under visible light irradiation was confirmed by the decomposition of trichloroacetone. 

Fig. 24 Rod-like anatase mesocrystals and their photocatalytic activity, (a-c) TEM images with the SAED pattern of the anatase Ti02 rod-like mesocrystals, (d) photocatalytic activities of the mesocrystal compared with the commercial standard Xi02 (P25). Reprinted with permission from the Royal Society of Chemistry...

The photocatalytic activity of the samples, as well their composition, depended on the hydrolysis solutiorr. Table 1 reports the values of the initial degradation rate of 4-NP, ro, determirred in the presertce of the most active Ti02 samples. The mixed systems revealed an erthanced photoactivity compared with that of the pure Ti02 polymorphic phases and some sarrrples were more active than Degussa P25. The most efficient samples consisted of a ternary rrrixture of anatase, brookite and ratile. 

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