Alumina transitional phases

The internal surface area of a porous inorganic membrane is often significantly affected by the heat treatment temperature. Leenaars, Keizer and Burggraaf (1984) have shown that, even if the crystallite size of the membrane precursor particles remains essentially the same (from the X-ray line-broadening measurements), the surfaee area of a transition-phase alumina membrane decreases with increasing calcination temperature. Con-... 

Transition Metal Salts and Oxides on Alumina. Transition metal salts, particularly chlorides and nitrates, are frequently used as starting materials for the preparation of supported transition metal oxides or supported precursors for supported metal catalysts. Also, many catalytic materials, particularly supported molybdenum and tungsten oxide and sulfide catalysts, contain transition metal ions, namely Co, Ni , and Fe " as promoters. Thus, it is interesting to study the spreading and wetting behavior of salts of these transition metals and of their oxides. This is of particular importance for promoted catalyst materials, since in practice the incorporation of the active phase and the promoter should be possible in one step for economic reasons. 

TEM and XRD analysis give consitent results the phase transitions and the bulk features of aluminas are not appreciably irrfluenced by the presence of Ce03, and the pure oxides seem to coexist separately, at least as long as alumina is in the spinel (transition) phases. 

Even if the methods of structural and morphological analysis suggest that the gross features of the ACE preparations are those expected of a plain mixed system (at least up to the transition to the a-Al203 phase, catalytically inactive), surface physico-chemical analysis methods indicate that, on a microscopic scale, the situation is quite different. In fact, the two oxidic systems do affect one another appreciably, (i) The presence of the transition-phase alumina induces in the Ce02 network smaller crystallites size and enhanced acidity of the surface... 

The alumina based fibers discussed in section 3.2 possess a range of compositions. They can be short, as with the S affil fibers or continuous, as with the others described. Their properties at room temperature depend on the ct-alumina content and at high temperature, the presence of any second phase (31). The Saffil fiber contains a few percent of silica with the remainder of the composition being alumina in one of its transition phases or as a mixture of transition phases and a-alumina. Table 7 shows the changes in processing of fibers of this type (32).The properties of alumina based fibers areshown in Table 8. Figure 3 shows the tensile curves of a pure a-alumina fiber, the Fiber FP, which had a grain size of 0.5 p,m (23). 

The microstructural features of transition alumina fibers virhich can be formed by calcining dry spun alumina-silica gel fibers are shown in Table III. The first phase that crystallizes from the amorphous gel is r -alumina. This phase is a spinel with vacancies distributed in the octahedral sites [10]. The calcination temperatures corresponding to each transition alumina can be estimated from Figure 2b. 

Transition, glass. See Glass transition Transitional alumina, 600-607. See also Aluminum oxide Transitional phase, 599 Transition metal alkoxide, 42-59 chemical modification, 52-59 condensation, catalyst effect, 47-49 condensation, kinetics, 45, 47 condensation, 43-59 hydrolysis, catalyst effect, 47-49 hydrolysis, 43, 45, 47 Transition metal alkoxide sols solvent effect, 47 structure, 49-59... 

Compared to amorphous systems, structural evolution in crystalline systems may be dominated by the effects of phase transformations [190] which often occur in conjunction with dehydration or when metastable transitional phases are involved. The most thoroughly studied crystalline gel system is alumina prepared by the Yoldas process [47,191,192]. (See Chapter 2.) Xerogels, which are often completely transparent, are typically composed of... 

As an additional probe of metal activity, we monitored benzene hydrogenation activity. As seen in Figure 9, Pt-containing rare earth catalysts have lower hydrogenation activity than chlorided alumina catalysts this result reflects inhibition of metal activity on these supports relative to conventional transitional alumina supports. Whereas the acid strength can be adjusted close to that of chlorided and flourided aluminas, metal activity is somewhat inhibited on these catalysts relative to halided aluminas. This inhibition is not due to dispersion, and perhaps indicates a SMSI interaction between Pt and the dispersed Nd203 phase. 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

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