Anatase—rutile transition

XRD analysis of the xerogels obtained by drying pure titanium dioxide sol at 70°C showed the presence of the nanocrystalline anatase phase [109]. Thermal treatment of this xerogel resulted in the growth of anatase crystallites up to 400°C. The anatase-to-rutile transformation began to occur at 450-500°C. This process was practically completed at 700°C, and only rutile phase existed at rcaic > 700°C. This feature of Ti02 xerogels is typical and well known (see, for example, [109]). Thus, it can be concluded that anatase-rutile transition temperature of nanosized particles is considerably lower than that of the... 

Deactivation of catalysts is a major problem in o-xylene oxidation [6,7]. For this reaction, deactivation has been mainly attributed to the irreversible anatase - rutile transformation [2,3]. In fact, anatase was found to be the best support for vanadium pentoxide catalysts leading the presence of rutile to lower activities and selectivities [8,9], The anatase-rutile transition can take place at temperatures above 973 K [10] but it is known that the presence of vanadia promotes such transformation [11-14] which, in these conditions, can start at 773 K [14], Such temperatures are easily attained in industrial reactors due to the high exothermicity of o-xylene oxidation that can lead to the formation of temperature profiles lengthwise with pronounced maxima (hot spot) [1]. 

Effects of metal salts, DMF, and DMSO on the anatase-rutile transition... 

Titania has three naturally occurring polymorphs anatase, brookite, and rutile. Anatase and brookite are considered to be kinetic products despite the fact that depending on the particle size, anatase becomes more stable than rutile. The anatase-rutile transition is exothermic and irreversible, occurring in the range 400-1200°C [25,26]. 

The anatase-rutile transition is not observed in supported samples. This transition is generally catalyzed by oxide mixture we conclude that y-alumina support hinders the crystallization of the TiOj anatase structure, in agreement with DRX data. 

A simple approach to prepare doped Ti02 powders relies on mixing a titanium alkoxide with a solution of the dopant precursor (namely, vanadium oxychloride) in dichloromethane. Then the solvent is evaporated, and either Ti isopropoxide or Ti tetrachloride is added. After reacting the final mixture with water and acetone, the powders are dried and calcined. Thick film sensors are fabricated by screen printing a paste of annealed powders surprisingly, the vanadium doping facilitates the anatase-rutile transition, in contrast to what has... 

The aim of this chapter is to report thermal studies of anatase-rutile structural transitions of anatase powders obtained by sol- el process. The titania syntheses were performed in metal chloride saturated aqueous solutions, as well as DMF and DMSO solutions. 

For the titania powder synthesized in CaCl2 solution, the anatase-rutile structural transition, associated with a weU-defined exothermic peak, occurs at 512°C, as verified by DTA analysis. For the titania powders synthesized in solutions of K, Ni, Co, and Mn chlorides, the transition occurs at 460°C, 455°C, 510°C, and 570°C, respectively. So, variations of 55°C can be observed for such structural transitions, depending on the metal chloride used. 

For both samples, the anatase—rutile structural transition, associated with a well-defined exothermic peak, occurs at 485°C. Such fact suggests that the temperature of the structural transition is associated with the mean crystaUite sizes, which have, for both samples, the same value. From 600°C to 800°C, an endothermic peak is observed for both samples, associated with a minor mass loss step, probably due to the sublimation of a minor amount of powder or the thermal degradation of the last alkoxide molecules, trapped into the three dimensional oxide structure. As an example, the TG—DTA curve for the DMF-synthesized powder is shown in Fig. 4.11. 

The experimental results show that organic species (DMF and DMSO, in this case) induce the formation of titania crystallites of low dimensions (6nm). Furthermore, the temperatures needed to promote the anatase—rutile structural transition can be decreased by 200°C, when compared with other sol—gel-prepared titania powders [27], using the experimental procedures reported in this work. 

Polymorphs which do not differ in the coordination number, have similar band gaps, viz. for anatase, rutile and amorphous TiOa these are, respectively, 3.5,3.2 and 3.8 eV for direct, or 3.2,2.9 and 3.0 eV for indirect transitions. Crystals of ZnS, CdS and CdSe, on transition from wurtzite to cubic forms change band gaps from 3.9 to 3.7 eV, from 2.50 to 2.41 eV, and from 1.70 to 1.74 eV, respectively. At the same time, a transformation of diamond into graphite decreases g from 5.5 eV to zero. 

Among other oxides, it has already been reported that the addition of zirconia to titania can contribute to remarkably increase the final surface area of the catalyst [192-197], the anatase-to-rutile transition temperature, [193, 195, 198] the surface acidity and the overall adsorption and hydrophilic properties. All these changes are expected to enhance also the photocatalytic activity. However, the fact that Zr02 has a bandgap much larger ( 5 eV) than Ti02 (3.0-3.2 eV) prevents its use as a photocatalyst under UVA illumination. 

Figure 1. TGA curve of the oxidation of TiS2 to Ti02. The unusual inflection at 620 6° is apparently associated with the anatase-to-rutile (Ti02) phase transition. XPD combined with TGA suggests the rapid oxidation at about 350° results in the presence of similar amounts of anatase and rutile, with the residual sulfur trapped in amorphous grain boundaries. The enhanced oxidation of this residual sulfur at about 620° may be associated with interfacial changes initiated by the onset of the anatase-to-rutile transition, which is complete by the end of the analysis. ...

Pressure-induced phase transitions in the titanium dioxide system provide an understanding of crystal structure and mineral stability in planets interior and thus are of major geophysical interest. Moderate pressures transform either of the three stable polymorphs into the a-Pb02 (columbite)-type structure, while further pressure increase creates the monoclinic baddeleyite-type structure. Recent high-pressure studies indicate that columbite can be formed only within a limited range of pressures/temperatures, although it is a metastable phase that can be preserved unchanged for years after pressure release Combined Raman spectroscopy and X-ray diffraction studies 6-8,10 ave established that rutile transforms to columbite structure at 10 GPa, while anatase and brookite transform to columbite at approximately 4-5 GPa. 

This paper presents the results of ab initio calculation investigating the pressure dependence of properties of rutile, anatase and brookite, as well as of columbite and hypothetical fluorite phases. The main emphasis is on lattice properties since it was possible to locate transitions and investigate transformation precursors by using constant-pressure optimization algorithm. 

Present in the next sections are the LDA results for equilibrium structure, pressure-induced transitions and electronic properties of various polymorphs, and the comparative analysis of the results for rutile and anatase that were obtained using LDA and GGA forms of the exchange-correlation potential. 

L.-G. Liu and T.P. Memagh, Phase transitions and Raman spectra of anatase and rutile at high pressures... 

XANES spectroscopy shows that a narrow and intense pre-edge peak at 4967 eV, due to the Is 3pd electronic transition involving Ti atoms in tetrahedral coordination, is present in well-manufactured TS-1 (Fig. 2c). Conversely this electronic transition of Ti(IV) species in Ti02 (anatase or rutile) is characterized by a very low intensity due to the small pd hybridization in octahedral symmetry. Indeed the transitions l2g are symmetrically forbidden in the case of octahedral coordination of Ti (IV), but the transition Ai T2 is allowed in the case of tetrahedral coordination of Ti(IV), as in the case of [Ti04] units [52,58-61,63,68]. 

The photolytic reduction of N2 at TiO -suspensions was at first reported by Schrauzer et al. Small amounts of NH3 and N2H4 were obtained as products. The highest activity was found with anatase containing 20-30 % rutile. Very low yields were also obtained with p-GaP electrodes under illumination It is much easier to produce NH3 from NO -solutions at CdS- and Ti02-particles using S -ions as hole scavengers . Efficiencies are not reported yet. Recently the formation of NH3 from NO was observed at p-GaAs electrodes under illumination. In this case NH3-formation was only found in the presence of transition metal ions or their complex with EDTA. 

Pilling-Bedworth ratio of 1 96, anatase phase films can show cracks and fissures with, consequendy, a loss of mechanical stability, however a hydrogen reduction treatment above 600°C leads to phase transition from anatase (101) to rutile (110) [43] with XRD detecting TiH2 upon prolonged hydrogen treatment of titania. As shown in Fig. 4.4, introduction of vanadium increases the intensity of the anatase Ti02 peak above 700°C disappearance of the vanadium (001) peak and the simultaneous appearance of the rutile (110) peak are observed, but anatase continues to dominate even after heat treatment at 800° C. A sharp vanadium (001) peak is observed for heat treatments carried out in air, while no vanadium peak has been seen in the case of heat treatment at 600°C in presence of Ar/H2. 

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