Y-aluminium oxide

The development of stress during calcination is shown in Fig. 8.20 for boehmite membranes calcined at 600°C (thickness after calcination is 5 pm). Curve c in Fig. 8.20 represents the curve which is corrected for support effects (see the preceding section on this subject). Three heating and cooling cycles are shown. During the first heating the Al-hydroxide particles of the gel are transformed to boehmite and subsequently to (hydrated) y-aluminium oxide particles and the shape of the first peak of curve c differs from the subsequent peaks. The maximum tensile stress calculated from the deflection amounts about 30 MPa. 

Vitamin E being the fastest growing vitamin, 2,3,6-TMP constitutes the fastest growing m-cresol derivative. One critical factor for fast growth will, however, depend on availability of m-cresol. Currently, m-cresol is available mostly in Japan, USA, and W. Europe mostly as a co-product during manufacture of BHT from meta-para-cresols. Can also be obtained by methylation of 2,6-xylenol on y-aluminium oxide at about 355°C using a 2,6 xylenohmethanol ratio of 2 1. 

The results of the characterization of extrusion behaviour are explained using ceramic catalyst carrier compounds based on y-aluminium oxide. The solid phase of all compounds was the same, consisting of y-aluminium oxide and pseudobohmite in a ratio of 2 1. 

Radiochemical studies indicate that the pore base is the actual site of formation of aluminium oxide, presumably by transport of aluminium ions across the barrier-layer, although transport of oxygen ions in the opposite direction has been postulated by some authorities. The downward extension of the pore takes place by chemical solution, which may be enhanced by the heating effect of the current and the greater solution rate of the freshly formed oxide, but will also be limited by diffusion. It has been shown that the freshly formed oxide, y -AljOj, is amorphous and becomes slowly converted into a more nearly crystalline modifipation of y-AljO . 

Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crys-tallographically ordered a-Al203, and the porous amorphous t]- and y-Al203. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of phenol or the dehydration of formic acid. 

From 30 at % Ca content in y-Al203 new peaks appear in the spectra which can be attributed to Cai2AIu033 (Mayenite) [13] (see Fig.5c ). This mixed calcium-aluminium oxide has a cubic structure with the cell parameter a = 11.982 A... 

Arai Y, Elzinga EJ, Sparks DL (2001) X-ray absorption spettroscopy investigation of arsenite and arsenate adsorption on the aluminium oxide-water interface J Colloid Interf Sci 235 80-88... 

Lastly, the positions of hydrogen atoms have been located in a series of low-surface-area aluminium oxide-hydroxides. Hence, the long known solid-state structures of diaspore (a-AlOOH) and boehmite (y-AlOOH) have been recently probed using H CRAMPS [45] along with X-ray and neutron diffraction techniques [46]. For diaspore results point to the presence of Alaf/Xa-OH) groups with 6-fold and 4-fold coordination at aluminium and oxygen, respectively, while Al2(/x-OH) moieties with 6-coordinate aluminium and 3-coordinate oxygen are revealed in boehmite [46]. 

Tsai, R.Y., C.N. Wang, and H.L. Chan. 1995. Aluminium oxide crystal microdermabrasion. A new technique for treating facial scarring. Dermatol Surg 21 539. 

Fig. 7. Chemical state plot for Al in several aluminium oxide reference samples and in CHCIF, activated y-alumina (circles give uncertainties of measurements) BE and KE data are referenced to Au 4f7,j (84.0 eV).

The -Alumina-related Structures.—Originally the compound )3-alumina was taken to be a binary aluminium oxide, but early Y-ray structure determinations and associated chemical analysis showed that the formula was approximately NaAlnOi7. Since then a number of isostructural compounds have been characterized in which sodium is replaced by other monovalent ions, particularly silver, and aluminium by other trivalent ions, notably gallium and iron. In addition, a number of other phases have been prepared which are structurally closely related to )8-alumina. Four principal structures are known, which are labelled / ", and P"". These can also be prepared with other monovalent cations replacing sodium, and some seem only to be formed when a few per cent of divalent cations, particularly magnesium, are present, so that they are, in fact, quaternary phases. The structure and stoicheiometry of these compounds has been summarized recently and we will only consider here those aspects relevant to the present topic. 

Chen, Y.R., Butler, J.N. Stumm, W. (1973) Kinetic study of phosphate reaction with aluminium oxide and kaolinite. Environ. Sci. Tech-nol. 1, 327-332. 

Supporting Silogel Pt and Silogel Pd nano-composites onto conventional y-alumina was performed as follows. 2g of aluminium oxide (activated, acidic, std grade, ca. 150... 

XRD-results have shown that aluminium oxide was formed at 600-700 °C. It consists of nano-dimensional grains and have the y-structure providing the high reactivity of fibrous material. When fibres of aluminium oxide were heated to 900 °C, its crystal structure varied from y- to 0-phase, and above 1200 °C it changed to a-corundum. It is illustrated by the patterns in Fig. 1. 

Aluminium oxide or alumina (AlgOg) exists in two principal forms— a- and y-AlgOg (the so-called P-AlgOg is not the pure oxide but an aluminate having the formula NagO.l 1 AlgOa). 

Some oxides and hydroxides are able to react with both acids and bases, thereby functioning as both bases and acids, respectively. Water is probably the most common example, but in this section we consider the amphoteric nature of metal oxides and hydroxides. The y-form of aluminium oxide, y-Al203, reacts with acids (equation 7.39) and with hydroxide ion (equation 7.40). ... 

Aluminium oxide occurs in two main forms a-alumina corundum) and Y-AI2O3 activated alumina). The solid state structure of 0t-Al2O3 consists of an hep array of ions with cations occupying two-thirds of the octahedral interstitial sites. ot-Alumina is extremely hard and is relatively unreactive (e.g. it is resistant to attack by acids). Its density (4.0gcm ) exceeds that of Y-AI2O3 (3.5gcm ) which has a defect spinel structure (see Box 13.6 and Section 21.10). The a-form is made by dehydrating... 

Composition The main constituent of the film is aluminium oxide, in a form which varies in constitution between amorphous AljO, and y-AI Oj, together with some monohydrate, AljOj.HjO. In the presence of moisture, both the anhydrous forms are gradually transformed into the monohydrate, and the water content of as-formed films is, therefore, somewhat variable. 

The same considerations apply to the surfaces of the many crystalline modifications of aluminium oxide (Boehm 1966). Besides a-A Os (corundum) there are at least eight oxides mostly classed under the term of Y-AI2O3 (cf. Ginsberg et al. 1957). Some of these forms may be non-stoechiometric oxide-hydroxides. 

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