Anatase-rutile Transformation Research Articles

TiO 2 accessory minerals: coarsening, and transformation kinetics in pure and doped synthetic nanocrystalline materials

In this paper we review the distribution, composition and relationships among the four TiO 2 minerals [anatase, brookite, rutile and TiO 2 (B)] and discuss the possible importance of a fifth phase, TiO 2 II, which has a stability field at high pressure. We also consider the geological application of kinetic data for the anatase to rutile transformation. Previous experimental studies of this reaction were carried out between 610° and 1000°C. Extrapolation using reported rate laws over geologic time results in predicted and natural distribution patterns that are broadly consistent. Experimental data reported here show enhanced anatase-rutile transformation rates if the anatase is very finely crystalline. We attribute the rapid transformation in these materials to a lower activation energy (∼ 60 kcal mol −1 compared to 80–148 kcal mol −1 reported previously). Rapid coarsening accompanies early, rapid transformation. Declining coarsening rates correlate strongly with declining transformation rates and extrapolate to those calculated from published data for coarsely crystalline material. The first-formed crystals of rutile have average volumes approximately eight times greater than those of coexisting anatase, suggesting that anatase must achieve a critical particle size before it transforms to rutile. Dopants can affect both coarsening and transformation rates. Y (1%) retards both coarsening of anatase and its transformation to rutile. However, Cr and Ta have very little effect on coarsening rates but significantly reduce transformation rates. Cr in anatase lowers both the transformation activation energy (∼ 47 kcal mol −1) and the frequency factor (∼ 10 11 hr −1). Together these result in comparable transformation rates at temperatures ∼ 50°C higher than in pure anatase; Ta exerts a similar effect. The distribution of the impurities (within particles vs. on the surface) may largely explain differences between Y and Ta or Cr. Results suggest that quantification of the dependence of the transformation kinetics on impurities may allow the interpretation of anatase-rutile distribution patterns to provide time-temperature information in low-grade metasediments.

Study of the anatase-rutile transformation in TiO2 powders obtained by laser-induced synthesis

Powder samples of pure anatase were produced using laser-induced pyrolysis of titanium alkoxides, and the catalysts were prepared using conventional wet impregnation methods. The diffraction patterns were interpreted in microstructural terms by Fourier analysis of their peak profiles. The transition temperature for the anatase-rutile transition in these catalysts was found between 500° and 550 °C. For the reflections of the anatase phase, a decrease of their Bragg (2θ) positions was observed up to 550 °C when the presence of the rutile phase becomes important. The response of the anatase structure to the thermal treatment is anisotropic with the c-axis showing the highest sensitivity to the observed expansion of the lattice. The rutile Bragg reflections are sharper than those of the anatase phase. The corresponding microstructural parameters indicate that, in all cases, the transformation is accompanied by an increase of the crystallites and/or of the lattice perfection. The evolution of these parameters is influenced by the presence of vanadium. The V-treated surface layer must be particularly distorted and apparently act as a restraint to perfecting by thermal treatments. Only the transition to rutile is capable of overcoming that restraint by allowing crystallite growth at the expense of the smaller and distorted anatase crystallites.

Effect of Hydrolysis Conditions on Thermal Transformation of Alkoxide-Derived Titanium Dioxide

The relationship between the hydrolysis conditions of titanium tetraisopropoxide (TTIP) and the thermal transformation of the hydrolysis products was investigated as a part of study on the properties of TiO2 formed by the hydrolysis of titanium alkoxide. The amorphous products formed under the HNO3 and HCl acidic conditions transformed to anatase at about 200°C. This temperature was about 100°C higher than that of the amorphous products formed with neutral water, and was not correlated with the H2O/TTIP molar ratio. On the other hand, the anatase-rutile transformation was significantly accelerated under the acidic condition. In particular, the single-phase rutile particles which were needle-shaped and about 0.1μm in length were formed at room temperature when the concentrations of acids were more than 6wt% for HNO3 and more than 4wt% for HCl at the molar ratio of H2O/TTIP=20. In the case of the hydrolysis product formed under H2SO4 acidic condition, on the contrary, the temperatures of the amorphous-anatase and anatase-rutile transformations were higher than those of hydrolysis products with neutral water, and its values were 300 and 700°C, respectively.

Crystalline transformation of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O by thermal treatments

The thermal transformations of Ti-B-O, Al-Ti-B-O, Si-Ti-B-O and Al-Si-Ti-B-O have been investigated using the methods of thermal analysis and X-ray powder diffraction. The materials are a crystalline series of TiO2 with partial replacement of titanium by the elements, aluminium, boron and silicon. The anatase form of the materials was transformed to the rutile form at 520∼680 °C for Ti-B-O, 880∼950 °C for Al-Ti-B-O, 1080∼1280 °C for Si-Ti-B-O and 1180∼1330 °C for Al-Si-Ti-B-O. The rate constant for the anatase-rutile transformation of Ti-B-O was 6.908×10−3 min−1 under isothermal conditions at 680 °C. Analysis of the kinetic data obtained by differential thermal analysis (DTA) gave the activation energy for transformation of anatase into rutile as 663.7 Kcal mol−1 for Al-Ti-B-O. The lattice parameters for the compounds studied at various temperatures were calculated by least-squares fitting of the X-ray powder diffraction data.

Strong metal-support interaction in Ni/TiO2 catalysts: in situ EXAFS and related studies

In situ EXAFS and X-ray diffraction investigations of Ni/TiO2 catalysts show that NiTiO3 is formed as an intermediate during calcination of catalyst precursors prepared by the wet-impregnation method; the intermediate is not formed when ion-exchange method is used for the preparation. On hydrogen reduction, NiTiO3 gives rise to Ni particles dispersed in the TiO2(rutile) matrix. The occurrence of the anatase-rutile transformation of the TiO2 support, the formation and subsequent decomposition/reduction of NiTiO3 as well as the unique interface properties of the Ni particles are all factors of importance in giving rise to metal-support interaction. Active TiO2(anatase) prepared from gel route gives an additional species involving Ni3+.

Process effects on structural properties of TiO2 thin films by reactive sputtering

We have been investigating the growth of crystalline titanium dioxide films at deposition temperature between room temperature and about 400 °C. The films were prepared by a dc magnetron reactive sputtering on glass and Si(100) substrates. The structural properties of the films were analyzed by x-ray diffractometry. The influence of total pressure, oxygen mole fraction in the mixture of Ar–O2, discharge current, and substrate temperature on the structural properties were studied. In addition to higher substrate temperature, low total pressure, high discharge current, and small oxygen mole fraction were shown to be preferable for growing anatase–rutile mixture films. Annealing of the films in air at 850 °C showed that anatase–rutile transformation strongly depends on the deposition temperature; the films deposited at temperature below 400 °C were converted to the anatase–rutile mixture films, and the films deposited at 400 °C to complete rutile films.

Some optical properties of enamels

Optical properties of enamels (gloss, colour, opacity) are closely related to structural parameters. Investigations on the opacification of TiO2-opacified enamels using the Kubelka-Munk theory gave the scattering and absorption coefficients reported in this paper. The relative stability of reflectance behaviour with increasing firing time is due to a compensation of the effects of the decrease both in the scattering and the absorption coefficients. Up to an optimum TiO2 concentration, the scattering coefficients increase, coupled with a decreasing crystal size. Above the optimum concentration, an agglomeration effect leads to decreasing scattering coefficients. The relatively strong absorption in titania enamels is coupled with TiO2 crystals (lattice defects, contaminants). The presence of 3d-element oxides influences the anatase-rutile transformation in a very different manner. Whereas some ions show only a small influence (Mn2+, Cu2+), Cr3+ and Ni2+, in particular, greatly accelerate the transformation, yielding strongly coloured rutile mixed crystals.

Anatase-rutile transformation in Fe/TiO2, Th/TiO2 and Cu/TiO2 catalysts and its possible role in metal-support interaction

Based onin-situ Mossbauer and X-ray diffraction studies, it is shown that in the Fe/TiO2 catalyst, the anatase-rutile transformation of the TiO2 support is facilitated by the Fe2+ ions formed during the reduction. The transformation occurs at lower temperatures in Th/TiO2 and Cu/TiO2 compared to pure TiO2. In general, the transformation of anatase to rutile seems to occur at or below the temperature (∼770 K) at which strong-metal-support-interaction manifests itself.

Untersuchungen zur Kinetik der Anatas-Rutil-Umwandlung an DeNOx-Katalysatoren für V2O5-WO3-TiO2 (Anatas) und V2O5-MoO3-TiO2 (Anatas)

The effect of preparation — specific parameters such as energy input, thermal treatment, V2O5-content — on the kinetics of the anatase-rutile transformation, were investigated and their relative contributions analysed. The various degrees of transformation were determined by quantitative X-ray diffraction measurements and checked for various rate laws. The rate constants evaluated according to Avrami for the V2O5—WO3—TiO2 (anatase)-system were extrapolated from Arrhenius plots for temperature values corresponding to conditions in power plants. The experimental results yield information on the stabilities (exposition periods) of the anatase-modification which react sensitively towards changes in phase composition at the anatase phase boundary, thus reflecting activation and deactivation processes which have an effect on the kinetics of the anatase-rutile phase transformation. The kinetic behaviour determined according to Avrami for the V2O5—WO3—TiO2 (anatase)-system has not been found for the V2O5—MoO3—TiO2 (anatase)-system. A unique feature of MoO3-systems consists in the following: after passing through a kinetically controlled region they form stable phases which can no longer be influenced kinetically. The anatase content of such phases can be adjusted by maintaining specific preparation parameters.

Structure and selectivity changes in vanadia—titania catalysts used to promote the reduction of nitric oxide with ammonia

Heat treatment of vanadia—titania (anatase) catalysts at temperatures lower than those required for the anatase—rutile transformation has been found to cause changes in the structure of the catalyst and the selectivity for the reaction between ammonia and nitric oxide. As the temperature increases from 350 to 440°C, and as the time of heat treatment increases, the surface area decreases, the amount of vanadia (V) species weakly interacting with titania increases and needle-like crystals are formed. At the same time, the reduction of nitric oxide to nitrogen decreases while the formation of nitrous oxide is promoted. Washing the catalyst with ammonia solution removes the crystal deposits and avoids the production of nitrous oxide.

AlPO4/TiO2 catalysts. Part 2.—Structure, texture and catalytic activity of systems precipitated with ammonia or ethene oxide

Two series of AlPO4/TiO2(APTi) catalysts containing different weight compositions have been prepared by the precipitation of AlPO4(Al/P = 1) on commercial TiO2( > 99.9 % Anatase, Aldrich) using ammonia (APTi-A series) or ethene oxide (APTi-E series) as the precipitation agents. Physicochemical characterization of these catalysts was carried out by nitrogen adsorption, TG, X.r.d., i.r. spectroscopy and surface-acid measurements. The surface area, pore volume, crystal structure and surface-acid character were found to be dependent on the precipitation agent, chemical composition and calcination temperature. Changes in surface-acid character were determined by application of a dynamic method that consists in determining both the AlPO4/TiO2's cataytic activity and selectivity in cyclohexene skeletal isomerization using the Bassett–Habgood kinetic model for first-order processes. X-ray diffraction studies show that the incorporated AlPO4 inhibits the polymorphic anatase–rutile transformation in the calcination process of the APTi-A system. Also, APTi-A systems are less affected by thermal treatment than APTi-E ones.It was found that both the apparent rate constant and the selectivity are dependent on the calcination temperature although the most important influence is that of catalyst composition i.e. the AlPO4/TiO2 weight ratio. Thus, a decrease in catalytic activity is observed when TiO2 content increases. Selectivity studies carried out by the OPE curves and the Wheeler criterion show that isomerization products, 1- and 3-methylcyclopentene, are competitive stable primary reaction products coming from cyclohexene across a parallel reaction network.

Structure and selectivity changes in vanadia-titania catalysts used to promote the reduction of nitric oxide with ammonia

Heat treatment of vanadia-titania (anatase) catalysts at temperatures lower than those required for the anatase-rutile transformation has been found to cause changes in the structure of the catalyst and the selectivity for the reaction between ammonia and nitric oxide. As the temperature increases from 350 to 440°C, and as the time of heat treatment increases, the surface area decreases, the amount of vanadia (V) species weakly interacting with titania increases and needle-like crystals are formed. At the same time, the reduction of nitric oxide to nitrogen decreases while the formation of nitrous oxide is promoted. Washing the catalyst with ammonia solution removes the crystal deposits and avoids the production of nitrous oxide.

High temperature dilatometer study of special ceramics and their sintering kinetics

Many properties of special ceramic materials, often closely related, such as sintering temperature, shrinkage in firing, mineral reactions and strength can be studied by thermal analysis. Also the influence of the type, structure and preparation of raw materials, plasticizers and binding materials for forming and compressing, as well as their compatibility with protective coatings (glazes, varnishes, metal films) are investigated by thermal analysis. Computerization of the routine application of important TA methods has opened up new horizons for exact quantitative evaluation due to considerable savings in time and man-power. During the recent development of rutile ceramics the sintering process at approximately 1100°C was studied with dilatometry. Using a rapid, high temperature dilatometer the sintering kinetics at different heating rates (5–50 K min −1) were determined and the activation energy for the sintering process computed. From expansion of the curve section up to 1000 °C in the computer version, the anatase-rutile transformation can be clearly seen between 800 and 900 °C, i.e. a small change can be evaluated as well as the larger sintering process. Sintering studies on lead zirconate titanate (PZT) ceramics are also described.

Thermally induced crystallization of amorphous-titania films

Three types of amorphous-titania films prepared by ion- and electron-beam techniques have been annealed thermally. An amorphous-crystalline transformation is found in each of these film types at around 350 °C. Its resulting microcrystalline structure and the exact transition temperature appear to be dictated by the rutile microcrystalline seed present in the as-deposited films under different deposition conditions. An amorphous film with a weak rutile seed crystallizes at a lower temperature into the anatase structure, while a film with a relatively strong rutile base crystallizes into the rutile structure at a somewhat higher temperature. It is demonstrated that Raman spectroscopy is a simple and effective tool for characterization of these submicron-thick amorphous films and for the dynamical study of such a phase transformation. Accompanying this amorphous-crystalline transformation, a two-order increase in elastic light scattering is noted implying optical degradation associated with microcrystalline boundaries. In addition, results of the anatase–rutile transformation at a temperature near 900 °C are presented.

Inhibition mechanism of the anatase-rutile phase transformation by rare earth oxides

Inhibition mechanism of the anatase-rutile phase transformation by rare earth oxides

Structure, activity and selectivity of V2O5-TiO2 catalysts for o-xylene conversion

The formation of needle shaped crystallites of V6O13 at low V concentrations on V2O5-anatase coated catalysts explains the low selectivity for phthalic anhydride during o-xylene oxidation. The (010) plane of V-oxide, most active for selective oxidation of o-xylene, is not accessible and the contact of this plane with the anatase faces promotes the anatase-rutile transformation and the incorporation and blocking of V4+ ions.

Alkoxide Hydrolysis and Preparation of TiO2Powders

Hydrolysis of titanium isopropoxide was studied under controlled conditions in presence of dry isopropanol. The alkoxide-water interaction presumably proceeded via several intermediate reactions with soluble products; at an advanced stage of reaction, the products of hydrolysis became insoluble, marked by a sudden appearance of uniform turbidity. The rate of hydrolysis, leading to the ultimate formation of an insoluble species, was found to be dependent on the concentration of water and Ti(OPri)4 in isopropanol, as well as the molar ratios of the reactants. Weight loss experiments showed the precipitated product to be metatitanic acid, TiO(OH)2. Thermal analysis showed a broad endotherm at around 135°C, corresponding to the loss of water, to be the only thermal effect; separate calcination and X-ray diffraction studies showed that formation of anatase from the amorphous precursor took place around the same temperature. No thermal effect was obtained for the anatase-rutile transformation. However, on treatment of the precipitated powder with NH4OH (used as a dispersant of the particles), an exothermic effect was noted at about 355°C; this was explained to be due to the breakdown of surface-exchanged NH4 ion and partial oxidation of NH3. Electron microscopic studies showed that the initial amorphous TiO(OH)2 was extremely fine (at least 0.1–0.2 micron), which increased in size on calcination and crystallization to a maximum of 0.5 micron, though agglomeration was a major problem in determining the particle size distribution.

Mechanism and Kinetics of Reactions in the System Ni‐Ti‐O

Reactions at T=600° to 1250°C between nickel oxide and titanium dioxide, metallic nickel and titanium dioxide, and the oxidation of Ni‐TiO2 cermet were studied. The formation of compouuds was observed by temperature‐programmed reduction in hydrogen flow. At T > 600°C, NiTiO3 forms. During the initial stages of the reaction, a solid solution of TiO2 in NiO is formed. The reaction is influenced by anatase‐rutile transformation. The apparent activation energy of the formation of NiTiO3 is considerably lower during the reaction in the mixture of metallic nickel with titanium dioxide (218 kJ°mol‐1 for powder and 255 kJ.mo1‐1 for cermet oxidation) than it is for the reaction between NiO and rutile (385 kJ.mol‐1).

Inhibition mechanism of the anatase-rutile phase transformation by rare earth oxides

The effect of rare earth oxide additions on the anatase-rutile transformation in doped TiO 2 was investigated. Y 2O 3, La 2O 3, CeO 2, Nd 2O 3, Sm 2O 3, Gd 2O 3, Tb 4O 7, Ho 2O 3, Dy 2O 3, Tm 2O 3 and Yb 2O 3, were found to suppress the transformation. The transformation process occurs in three stages. In the first stage, rare earth metal ions dissolve into interstitial sites in anatase. The resulting decrease in oxygen vacancies caused by the solid solution suppresses the nucleation of the transformation. In the second stage, the activation energy of the transformation was found to be 124 ± 3 Kcal/mol and 138 ± 4 Kcal/mol for pure anatase and anatase with 1 mol% Dy 2O 3, respectively. The transformation mechanism of this stage is not necessarily affected by additives. The final stage is affected by the amount of rare earth oxide-titania compound phase.

Impurity effects on anatase-rutile transformation.

Impurity effects on the phase transformation from anatase to rutile were studied under normal atmospheric conditions. The rate equation of this phase transformation is explained by the Avrami model. Impurities of metal oxides having MO6 octahedra in a structural unit plays a very important role on anatase-rutile transformation, and V2O5 reacts as an accelerator, while WO3, Nb2O5, Ta2O5 and Cr2O3 are of inhibitors. The activation energy of this phase transformation was given to be 50 kcal/mol from pure anatase, 35 kcal/mol from antase stabilized by H3PO4, and 125 kcal/mol from anatase added W03 as an impurity. The value of the activation energy from pure antase was about half of the smallest one reported previously. The variation of the activation energy reflects the difference in the inhibiting mechanism of anatase stabilized by H3PO4 or WO3.