Alumina phase transition

Pyrene has been used to investigate the extent of water penetration into micelles and to accurately determine critical micellar concentrations (Kalyanasundaram, 1987). Polarity studies of silica or alumina surfaces have also been reported. In lipid vesicles, measurement of the ratio Ii/Iui provides a simple tool for determination of phase transition temperatures and also the effect of cholesterol addition. 

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

Among the several transition alumina phases, y-Al203 is the most important and most studied phase for catalysis [57, 58]. However, even nowadays, several aspects of its structural and surface chemistry are still not well understood, since y-Al203 is a poorly crystalline solid, showing some variation in its structural stoichiometry and a wide range of defects. In the last 50 years, several empirical models for y-AI2O3 surface have been reported, trying to explain the complexity of this surface... 

Also in Fe-containing materials, promoting effect on the formation of the final layered-alumina phase was observed.24 In the completely substituted material BaFe120i9 a monophasic sample with magnetoplumbite structure was obtained at 700 °C. This low formation temperature was related to the greater mobility of oxygen and Ba ions in the lattice of Fe oxides than in A1 oxides. Indeed, the transition from y-+a alumina occurs in Fe oxides at temperatures hundreds of degrees lower than those required for phase transitions in A1 oxides. 

The crystal structure and the sintering behavior of hexaaluminates was widely investigated. The relation of sintering resistance to anisotropic ion diffusion in the layered alumina phase was clarified to a large extent. Other evidence suggests that combustion activity is obtained through a redox mechanism associated with reversible variation of oxidation state of the transition metal ions in the structure. Mn was the best and most stable active component. However, fundamental and applied studies are needed to better clarify the redox mechanism of the reaction and how it is related to the chemical and structural features of the Mn-containing layered-alumina phase. This could also provide useful information for the development of an optimum catalyst composition,... 

The molecular state ot vanadium oxide supported on ditterent alumina phases (7, S-0, and a) was investigated with Raman spectroscopy. The supported vanadium oxide was "found to "form a molecularly dispersed overlayer on the di-f-ferent alumina phases. The molecular state oT the surTace vanadium oxide phase, however, was dependent on the nature oT the alumina support. This variation was primarily due to the presence oT surTace impurities, in particular sodium oxide. The surface sodium oxide content was found to increase with the calcination temperature required to form the different transitional alumina phases (a, 6-0, 7). The... 

For the ACs the data are representative of the samples after heat-treatment at all three temperatures since during their fabrication these materials have already been treated at temperatures in excess of 850°C. However, for the alumina and clay samples the surface areas and pore volumes are shown after treatment at each temperature as these materials undergo various phase transitions that lead to sintering of the samples and shifts in their relative pore size distributions with heat-treatment. The particle size was determined from the corresponding MIP curve for the powder raw material. The Sbet in the case of microporous ACs should be considered as an apparent surface area due to the micropore filling mechanism associated with these materials [15]. The external area and micropore volumes were calculated from the slope and intercept of the t-plots of the corresponding isotherms. The total pore volume was taken as the amount of gas adsorbed at a relative pressure of 0.96 on the desorption isotherm, equivalent to a pore diameter of 50 nm. The mesopore volume was calculated from the difference in the total pore volume and the micropore volume. 

Alumina membranes. It has been established that several phases of alumina exist and a particular phase of alumina is determined not only by the temperature it has experienced but also by the chemical path it has taken. For commercial membrane applications, the alpha- and gamma-phases of alumina are the most common. Alpha-alumina membranes are well known for their thermal and hydrothermal stabilities beyond 1,000 C. In fact, other transitional forms of alumina will undergo transformation towards the thermodynamically stable alpha-alumina at elevated tcmjxratures beyond 900 C. On the contrary, commercial gamma-alumina membranes are typically calcined at 400-600 C during production and are, therefore, subject to potential structural changes beyond 600°C. Moreover, alumina chemistry reveals that phase transition also occurs beyond that temperature [Wefers and Misra, 1987]. 

The presence of water at elevated temperatures is also a critical factor in the performance of gamma-alumina membranes. Water has been known to promote the aforementioned phase transition of alumina at a lower tempeiauire. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Figure 9.4 Phase transition of alumina [Wefers and Misra, 1987]...

Abbattista et al. (26) found that phosphorus addition prevents crystallization of the y-alumina phase and the transformation from y- to a-alumina in the system AI2O3 —AIPO4 (Fig. 23). More precisely, Morterra et al. (77) reported that phosphates do not affect the phase transition from low-temperature spinel alumina (y-alumina) to high-temperature spinel aluminas 8 and 6 phases) but delay the transition of 8 and 9 to a-alumina (corundum). Stanislaus et al 46) also reported that phosphorus significantly improves the thermal stabihty of the y-alumina phase in P/Al catalysts. However, the same authors found that the positive effect of phosphorus seems to be canceled in the presence of molybdenum due to the formation of aluminum molybdate. Thermal treatments of MoP/Al catalysts at temperatures >700°C result in a considerable reduction of SSA and mechanical strength. The presence of phosphorus does not prevent the reaction between the molybdenum oxo-species and alumina since the interaction between molybdates and phosphates is weak. The presence of nickel does not obviously affect the positive effect of phosphorus in terms of thermal stability 46). On the other hand, Hopkins and Meyers 78) reported that the thermal stability of commercial CoMo/Al and NiMo/Al catalysts is improved by the addition of phosphorus. 

Martin ST (2000) Phase transitions of aqueous atmospheric particles. Chem Rev 100 3403-3453 Martin ST, Han JH, Hung HM (2001) The size effect of hematite and alumina inclusions on the efflorescence relative humidities of aqueous ammonium sulfate particles. Geophys Res Lett 28 ... 

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