Oxides of B, Al, Ga

Aluminum oxide is widely used as adsorbents and catalysts. As industrial catalysts, it is mostly used as catalyst supports for metals (Pt, Pd, etc.) and metal oxides (Cr, Mo, etc.). 

Aluminum oxide (alumina or AI2O3) occurs in various crystallographic modifications. Completely anhydrous aluminum oxide is a-Al203 (corrundum), which is the most stable. It has a structure of hexagonal closest packing of oxide ion, with Al ion occupying two-thirds of the octahedral sites, a-Alumina is prepared by heat treatment above ca. 1470 K of aluminum hydroxides, oxyhydroxides, or alumina hydrates, which are obtained by neutralization of solution of aluminum salts or hydrolysis of aluminum alkoxides. Heat treatments at lower temperatures form various kinds of transi- 

Surface areas of aluminas obtained by calcination of alumina hydrates at 550— 1100 K are usually 100— 300 m g The surface area of 7- and r -alumina is and that of ct-alumina a few m g  

Activity for adsorption and catalytic function of alumina is revealed by partial de-hydroxylation of its surface. The variation of the density of surface hydroxyl groups is shown inf Fig. 3.34. The isoelectric point has been reported to be about 7. Proton exchange was hardly observed in the reaction of alumina in aqueous solution of sodium acetate.  

Adsorption properties have been widely studied. Water adsorbs physically (desorbing at 373 - 393 K), and chemically (ca. 10 molecules cm desorbing at about 573 K) and by surface hydroxylation. Complete dehydroxylation requires heat treatment at about 1300 K. NH3 molecules adsorb very strongly and extensively, the strength 

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