Rutile crystalline phases

The photocatalyst (Ti02) used in this work was Degussa P25. Based on the manufacturer s information, the titania particles are a mixture of anatase and rutile crystalline phases (mostly anatase). Ti02 has an average particle size of 30nm and surface area 50 mVg. The other photocatalyst used was ZnO powder (Merck), about 99% pure, possessing BET surface area 4.5 m /g. The reactive red MSB dye (Chika Ltd) was used as such in the present study. 

Structurally, both Fe-Sb-0 and Sn-Sb-0 catalysts contain the rutile crystalline phase as the majority solid-state component. For the former, the majority phase is FeSb04 (44), whereas in the case of the latter, the dominate phase is cassiterite Sn02 containing Sb +, and possibly Sb + cations in solid solution with the rutile tin oxide phase (33,34). 

Ohno, T., K. Sarukawa, K. Tokieda, and M. Matsumura, Morpholotogy of a TiOj photocatalyst (Degussa, P25) consisting of anatase and rutile crystalline phases. 

Powder diffraction patterns have three main features that can be measured spacings, peak intensities, and peak shapes. Because these patterns are a characteristic fingerprint for each crystalline phase, a computer can quickly compare the measured pattern with a standard pattern from its database and recommend the best match. Whereas the measurement of spacings is quite straightforward, the determination of peak intensities can be influenced by sample preparation. Any preferred orientation, or presence of several larger crystals in the sample, makes the interpretation of the intensity data difficult. The most common structures of inorganic pigments are rutile, anatase, and spinel. 

On heating to higher temperatures, no crystalline phases are observed until anatase crystallizes at 1000 °C. At 1400 °C, anatase, rutile and crystobalite are the only products. No single-phase material is obtained. The lack of correspondence between the TGA ceramic yield and the theoretical ceramic yield calculated for TiSi04 presages this problem. The exact reasons for the formation of a mixed-oxide phase are unknown at the moment, but they clearly contrast with the behavior of the Zr and Hf analogs. 

The multifunctionality is achieved through either the combination of two different compounds (phase-cooperation) or the presence of different elements inside a single crystalline structure. In antimonates-based systems, cooperation between the metal antimonate (having a rutile crystalline structure), employed for propane oxidative dehydrogenation and propene activation, and the dispersed antimony oxide, active in allylic ammoxidation, is made more efficient through the dispersion of the latter compound over the former. In metal molybdates, one single crystalline structure contains both the element active in the oxidative dehydrogenation of the hydrocarbon (vanadium) and those active in the transformation of the olefin and in the allylic insertion of the N H2 species (tellurium and molybdenum). 

X-Ray Diffraction. Diffractograms of the catalysts show that crystalline phases containing iron are formed upon calcination. In catalysts supported on rutile hematite (a-Fe202) is formed. When the iron phase is applied onto anatase, pseudo-brookite Fe2TiOs) is observed. The formation of pseudobrookite has previously been observed to take place only at 800°C for P-25 titania impregnated with iron solutions [71, but also at lower temperatures (550°C) when samples were prepared by co-precipitation or impregnation of freshly precipitated 100% anatase. In those samples, iron was in intimate contact with titania [8]. The results obtained with XRD indicate that iron is in intimate contact with the support indeed. 

The observed phase selectivity phenomenon vividly shows that the crystalline phase of titania nanoparticles has a major effect on their reactivity. A similar phenomenon had been recently described for dissolution of a similar Ti02 sample in HF.16 This reaction can be used for the preparation of a pure nanoscale rutile phase for use as a photocatalyst. 

Titania membranes prepared at a temperature lower than about 350 C are essentially amorphous. At 350 or so, phase transition to a crystalline phase of anatase begins to occur. The transformation to anatase (tetragonal in crystallinity) is complete and the new phase of rutile (also tetragonal) begins at a temperature close to 450 0. Transformation to rutile is complete at about 600 C [Xu and Anderson, 1989]. Thus, at a temperature between 450 and bOO C, both anatase and rutile phases are present. It has been suggested that this temperature range may be lower at 350-550 C [Larbot et al., 1986 Burggraaf etal., 1989]. 

The surface crystal structure and particle size can also influence photoelectro-chemical activity. The mode of pretreatment, for example, dictates whether titanium dioxide exists in the anatase phase (as is likely in samples which have been calcined at temperatures below 500 °C) or in the rutile phase (from calcination temperatures above 600 °C) or as a mixture of the two phases for pretreatments at intermediate temperature ranges. The effect of crystalline phase could be easily demonstrated in the photocatalytic oxidation of 2-propanol and reduction of silver sulfate, where anatase is active for both systems. But when the catalyst was partially covered with platinum black, alcohol oxidation was easy, but silver ion reduction was suppressed. On rutile, redox activity was observed for Ag+, alcohol oxidation was negligible [85]. 

Crystallinity due to V2O5 starts appearing above 3,4 mole% V2O5 loading onwards, Andersson [16] and Andersson and Lundin [17] have observed a crystalline phase due to rutile only in 10 mole% V20j/Ti02 catalyst calcined at 1150°C... 

TiCLj was hydrolysed in NaN3 solutions with the aim to investigate the role played by Ns ions both on the crystalline phase and on the size of the particles. As shown in Table 1, mtUe was formed at pH 0.7 when the Ti N molar ratio was 1 4 (sample ANl), whilst anatase was obtained if the ratio was 1 40 (sample AN4). For Ti N molar ratio equal to 1 4, only anatase was formed at pH 3.0 (sample AN2), whilst a mixture of anatase and rutile was produced at pH... 

The extensive use of promoter elements has been employed with antimonate catalysts just as it has been with molybdate catalysts. This has been used to good effect on the Fe-Sb-0 catalyst in the development of several generations of commercial propylene ammoxidation catalysts (47). As seen from structural analyses, the major crystalline phase present in the catalyst is FeSb04, having the statistical rutile (Ti02) structure (44). A second antimony oxide phase, Q -Sb204, is also present in smaller amounts relative to the rutile phase. The presence of the antimony oxide phase is necessary for optimal activity and selectivity (48). 

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