Phase transformation titania

Kumar K P, Keizer K and Burggraaf A J 1994 Stabiiization of the porous texture of nanostructured titania by avoiding a phase transformation J. Mater. Sc/. Lett. 59... 

Penn R L and Banfieid J F 1999 Formation of rutiie nuciei at anatase (112) twin interfaces and the phase transformation mechanism in nanocrystaiiine titania Am. Miner. 84 871... 

The various ratios of rutileranatase in titania support were obtained by calcination of pure anatase titania (obtained fi om Ishihara Sangyo, Japan) in air at temperatures between 800-1000°C for 4 h. The high space velocity of air flow (16,000 h" ) insured the gradual phase transformation to avoid rapid sintering of samples. The ratios of rutile anatase were determined by XRD according to the method described by Jung et al. [5] as follows ... 

The present research showed a dependence of various ratios of rutile anatase in titania as a catalyst support for Co/Ti02 on characteristics, especially the reduction behaviors of this catalyst. The study revealed that the presence of 19% rutile phase in titania for CoATi02 (C0/RI9) exhibited the highest number of reduced Co metal surface atoms which is related the number of active sites present. It appeared that the increase in the number of active sites was due to two reasons i) the presence of ratile phase in titania can fadlitrate the reduction process of cobalt oxide species into reduced cobalt metal, and ii) the presence of rutile phase resulted in a larger number of reduced cobalt metal surface atoms. No phase transformation of the supports further occurred during calcination of catalyst samples. However, if the ratios of rutile anatase were over 19%, the number of active sites dramatically decreased. 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

In the contradistinction to the titania, silica or iron oxide the zirconium hydroxide is more stable than its oxide. From this reason, an important is question, how deep may the hydration process change the surface area and dispersion of the sample. Also, how can it lead to the phase transformations. 

Hydrothermal conditions are frequently used to synthesize (e g., Yang et al. 2000 Wang and Ying 1999 Yanagisawa and Ovenstone 1999) and treat (Penn and Banfield 1998, 1999a,b) nanocrystalline titania samples. The surrounding phase now is water or an aqueous solution. Experiments are normally conducted between 100-300° C and at relatively low pressures (at 300°C the saturated vapor pressure of water is only 8.5 MPa (CPC book), far less than pressures applied in many solid state phase transformation experiments). 

We have studied the phase transformation from nanometer-sized amorphous titania (Ti02) to nanocrystalline anatase at 300 - 400° C (unpublished). Amorphous titania samples were prepared by fast hydrolysis of titanium ethoxide in water at 0° C (Zhang et al. 2001). The extent of transformation was monitored using XRD determination of the phase mixture as a function of time. We also found that the transformation kinetics do not follow the widely employed JMAK equation. 

For reasons noted above, titania (Ti02) has been a popular experimental model system for investigating the fundamental ways in which crystal size alters thermally-driven reactions. Also, temperature leads to rapid coarsening and phase transformations that modify the utility of nanoparticle titania for commercial applications. 

Pressure-induced phase transformations for anatase-Ti02 were monitored by Raman spectroscopy.40 Raman spectroscopy was used to characterise rutile titania nanocrystalline particles with high specific surface areas.41 Micro-Raman spectra were used to follow surface transformations induced by excimer laser irradiation of Ti02.42 There was Raman spectroscopic evidence for modification of a titania surface by attached gold nanoparticles.43... 

K.-N.P. Kumar, K. Keizer and A.J. Burggraaf, Stabilisation of the porous structure of nanostructured titania avoiding the phase transformation. /. Mater. Sci. Lett., 13 (1994) 59. 

Calcination of powders in the presence of different gases may induce solid phase transformation, which in turn affects the PZC/IEP. Hydrogen-treated and untreated zirconia were studied in [160], but no substantial shift in CIP was detected. Two titanias were heated in O2 or in H2 at 530 or 6OO C, but no substantial change in lEP or CIP was observed in one sample [161], Dehydration of titania (rutile) as a function of temperature was studied in [162]. The Og of silica was depressed by a factor of 10 by heating at 800°C for 3 hours, further heating (up to 36 hours) did not affect CTq. Rehydration of heated powders for 3-56 days brought about a gradual increase in Og [163], A few examples of different phase transformations induced in the same initial material by calcination at various temperatures are presented in Chapter 3. 

In the case of titania, the stabilising effect of yttria was not observed below a heat treatment of 600 C as shown in Table 6. Pure titania shifted from anatase to rutile at 900 °C and the phase transformation was accompanied by a very strong decrease of the BET surface area. The doped sample, although it kept the anatase structure until reaching a treatment temperature of 1200 °C, was also characterised by a very strong diminution of its surface area at 900 °C which, however, could not be explained by a sole crystalline change as shown in Table 6. 

The XRD patterns as shown in Fig 1 obtained for all the samples after their reduction in hydrogen indicate the presence of unreduced platinum oxides even after HTR in the supported titania gel and mixed oxide sample. However, complete reduction to metallic platinum was observed on supported commercial titania system. It can be observed that the phase transformation of titania from anatase to rutile does not occur under the experimental... 

Hydroxyapatite/titania layers were spin-coated on the surface of TiZr alloy at a speed of 3000 r.p.m. for 15 s, followed by a heat treatment at 600 °C for 20 min in an argon atmosphere (Wen et al., 2007). The coating displayed excellent bioactivity when soaked in a SBF for an appropriate period. Differential scanning calorimetry, TGA, XRD and SEM in conjunction with energy dispersive spectroscopy were used to characterise the phase transformations and the surface structures and to assess the in vitro tests. The titania (anatase) layer exhibited a cracked surface and the HAp layer showed a uniform dense structure. Both layers were about 25 im thick. 

Xing-Zhao Ding, Xiang-huai Liu, Grain growth enhanced by anatase-rutile phase transformation in gel-derived nanocrystalline titania powders. Journal of Alloys and Compounds, )91, 248 p. 143-145. 

Titania exists in two tetragonal forms, a metastable phase, anatase, and a stable form, rutile. The volume free energy of the rutile phase is always lower than that of anatase. Therefore, upon heat treatment, the anatase phase transforms to the stable rutile form. The transformation is non-reversible, and is from metastable to stable the transition temperature ranges from 450 to 1200 °C. Factors that aflect the physical and chemical properties of Ti02 crystals include intrinsic electronic structure, size, shape, organization, and surface properties of particles."... 

Characteristics of phase transformation of sol-gel derived alumina, zirconia and titania... 

Alumina, titania and zirconia in their metastable phases will transform to their stable phases. Such phase transformation usually occurs via a nucleation and crystal growth process [34, 37]. Kinetically, however, the phase transformation will not occur or be observed at low temperatures. For alumina, the 7-AI2O3 to a-AhOa (via 5- and 6-aluminas) phase transformation is found to start at temperature around 900°C. This is accompanied with a sharp decrease in the surface area and increase in the pore size of the alumina adsorbent. The surface area of y-AhOs adsorbent will decrease with time at temperatures lower than 900°C due primarily to sintering [34]. 

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