Titania sintering

Silica sinters by a viscous flow sintering mechanism while titania sinters by grain boundary diffusion. Kingery et al. (28) and Kobata et al. (29) have derived the... 

In order to exploit the application of titania in electronic ceramics, Nair et al. (1999) studied sintering of nanostructured pure and doped titania samples. An aqueous sol containing titania particles of 10 nm or less size was used for the preparation ofpure and Cu, Ni, La-doped gel samples. The densification temperature and properties of some selected titania compositions are also shown in Table 6-2. Figures 6-2(a) and (b) show the field emission scanning electron micrographs (FE-SEM) ofpure titania sintered at 750°C and 800°C for 8 h. It may be noted that the 750°C calcined gel samples, though porous. 

Edeison L H and Giaeser A M 1988 Roie of particie substructure in the sintering of monosized titania J. Am. Ceram. See. 71 225... 

Barium carbonate also reacts with titania to form barium titanate [12047-27-7] BaTiO, a ferroelectric material with a very high dielectric constant (see Ferroelectrics). Barium titanate is best manufactured as a single-phase composition by a soHd-state sintering technique. The asymmetrical perovskite stmcture of the titanate develops a potential difference when compressed in specific crystallographic directions, and vice versa. This material is most widely used for its strong piezoelectric characteristics in transducers for ultrasonic technical appHcations such as the emulsification of Hquids, mixing of powders and paints, and homogenization of milk, or in sonar devices (see Piezoelectrics Ultrasonics). 

In another example, Ti02 can be deposited on a siHca support body in order to obtain a stable high surface titania. This is necessary because Ti02 sinters badly on heating in the bulk oxide and loses surface area. The Ti02 Si02 combination is useful as a catalyst for the oxidation of o-xylene to phthaHc anhydride. 

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 ... 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

This is apparent in Fig. 15, where the surface area of silica-titania cogels is plotted against calcining temperature. Pure silica catalyst exhibits very little drop in surface area, but when titania is added the surface area becomes unstable. The more titania added, the lower the temperature needed to cause sintering. 

This explains the melt index behavior of coprecipitated silica-titania catalysts which is shown in Fig. 16. With each catalyst, the MI rises with increasing calcining temperature until sintering begins, then it drops. The... 

Fig. IS. A drop in surface area marks the onset of sintering in a series of cogelled Cr/ siiica-titania catalysts. Titania decreases the thermal stability of the catalyst.

This sintering is associated with a tendency toward phase separation between silica and the titania. X-ray photoelectron spectroscopy (XPS) indicates that the titania tends to migrate to the surface. This is shown in Table VII, where the XPS intensity ratio Ti/Si is listed for a coprecipitated siiica-titania sample calcined at various temperatures. As the temperature increases, the intensity ratio also increases. Since XPS is a surface technique, this indicates more titania near the surface. 

Notice also in Table VII that this is not true of the first method of titania incorporation. When the titania is applied as a surface layer it does not promote sintering, and according to the XPS intensity it does not migrate. 

Fig. 16. The relative melt index potential (RMIP) of a series of cogelled Cr/silica titania catalysts rises and then falls with calcining temperature, indicating first dehydroxylation then sintering. However, the more titania in the catalyst, the more easily it sinters and therefore the lower the temperature at which peak RMIP develops.

Calcining in CO Protects High-Titania Catalysts from Sintering... 

Cr/alumina can be modified like Cr/silica. Adding titania is not particularly useful, but replacing the hydroxyls with fluoride does boost the activity by as much as 10-fold (62). An example is shown in Fig. 22, where activity is plotted versus the amount of fluoride impregnated onto a highly porous alumina. Too much fluoride tends to sinter the alumina and destroy the activity. Other modifications which improve the activity of Cr/alumina include adding chloride, sulfate, boria, phosphate, or 1-5% silica (62, 78). 

Trillo et al. (47,137) have reported compensation behavior in oxide-catalyzed decomposition of formic acid and the Arrhenius parameters for the same reactions on cobalt and nickel metals are close to the same line, Table V, K. Since the values of E for the dehydration of this reactant on titania and on chromia were not influenced by doping or sintering, it was concluded (47) that the rate-limiting step here was not controlled by the semiconducting properties of the oxide. In contrast, the compensation effect found for the dehydrogenation reaction was ascribed to a dependence of the Arrhenius parameters on the ease of transfer of the electrons to the solid. The possibility that the compensation behavior arises through changes in the mobility of surface intermediates is also mentioned (137). 

The decrease in activity of heterogeneous Wacker catalysts in the oxidation of 1-butene is caused by two processes. The catalyst, based on PdS04 deposited on a vanadium oxide redox layer on a high surface area support material, is reduced under reaction conditions, which leads to an initial drop in activity. When the steady-state activity is reached a further deactivation is observed which is caused by sintering of the vanadium oxide layer. This sintering is very pronounced for 7-alumina-supported catalysts. In titania (anatase)-supported catalysts deactivation is less due to the fact that the vanadium oxide layer is stabilized by the titania support. After the initial decrease, the activity remains stable for more than 700 h. 

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