Pure titania pulp containing amorphous titania was heated at different temperatures and times. Above 650uC anatase phase was evolved and between 900 and 1000uC, anatase-rutile transformation occurred. The anatase-rutile transformation in TiO 2 in the presence of different transition metal oxides, namely Fe 2 O 3 , Cr 2 O 3 , NiO, CuO and MnO 2 under argon and hydrogen atmospheres was investigated. The different phases of TiO 2 were determined using powder X-ray diffraction (XRD). The anatase-rutile transformation temperature was found to be lowered in the presence of transition metal oxides. The transformation temperature was found to vary much in argon and hydrogen atmospheres compared to air in the presence of the metal oxides. Also the method of preparation of metal oxide doped TiO 2 influences rutilation. Other methods such as chemical analysis, surface area measurements and crystallite size calculation were used for the characterisation of the samples. The surface area of heated samples was found to be decreased while crystallite size increased due to rutilation on heating. The samples were also observed under a scanning electron microscope to characterise the microstructural changes associated with each thermal treatment and atmosphere. The morphology of doped titania changes much on heating due to phase modification. The atmosphere of heating also has important effect on deciding the morphology of rutilated titania.
TiO 2 is produced and marketed in two main grades viz. anatase and rutile. Both anatase and rutile have their own pigmentry properties and hence cannot be substituted by each other. Pure anatase on heating at higher temperatures undergoes crystallographic rearrangement to form rutile. This transformation in presence of NiO and Fe 2 O 3 under air atmosphere was studied using XRD and SEM. The transformation temperature was found to be reduced much in presence of NiO and Fe 2 O 3 and the extent of lowering was higher for NiO than Fe 2 O 3. The activation energy for this transformation was also calculated. The method of preparation had major influence on the transformation.
heating. High pressure modi cations of TiO 2 have been reported by Dachille and Roy,18 Bendeliani et al.,19,20 and Metal oxides supported on T iO 2 are gaining in Anatase transforms irreversibly and exoance as a result of their applications in catalysis.thermically to rutile in the range 880-1200 K depending on Reactions carried out over T iO 2 supported catalysts the method of sample preparation, and at atmospheric include hydrodesulphurisation (HDS), partial or selecpressure the transformation depends on time and tempertive oxidation, hydrogenation of carbon monoxide, ature as well as on impurity concentration.22 The rate of selective catalytic reduction of NO x , coal liquefaction, transformation of pure anatase has been found to be and ammonia synthesis. Catalytic activity depends on immeasurably slow below 885 K and extremely rapid above the phase of the T iO 2 skeleton (anatase, rutile, or 1000 K.23,24 The transformation is a nucleation growth brookite). Catalytic activity is high for anatase, but process and follows a rst order rate law, with an activation decreases when it transforms to rutile on heating at energy of around 38 kJ molÕ 1 (Ref. 22). Impurities such as high temperature for extended periods. In the present CuO, CoO, and Li 2 O and reducing atmospheres accelerate study, the eVect of doping with Fe 2 O 3 or Cr 2 O 3 on the transformation, while sulphate, phosphate, and uoride the anatase to rutile transformation was investigated. Samples prepared by coprecipitation and wet impreganions, P5+ and S6+ ions, and vacuum stabilise the anatase nation were heated in air at various temperatures. T he form and inhibit the transformation process.25,26 It has calcined samples were characterised by chemical analybeen reported that the particle and crystallite size of anatase sis, XRD, surface area measurements, SEM, and grainincrease markedly in the region of crystal structure transsize measurements. It was observed that the degree of formation. Smaller particle size and larger surface area of anatase transformation was higher on doping with 15% anatase favour the transition.27 Detailed X-ray studies Fe 2 O 3 or Cr 2 O 3 than with 5%. T he method of prepindicate that the unit cell expands prior to the transformaration plays a major role in the transformation, ation to rutile.28 TiO 2 in the rutile modi cation forms solid coprecipitated samples always giving higher rutile persolutions with other transition metal oxides, and the propcentages than wet impregnated ones. T he activation erties of transition metal oxides are modi ed by the presence energy for the transformation is lower for 15% than of TiO 2 . Crystalline anatase generally exhibits higher photofor 5% doped samples and for coprecipitated than for catalytic activity than rutile.29 Decreasing the particle size wet impregnated ones.BCT /635 below 10 nm leads to a signi cant increase in activity. For sensor applications, high surface area is an important
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