Aluminum oxide defect sites

As described above, the presence of the aluminum in the aluminosilicates imparts an increased acidity to the solid because of the replacement of a tetracoordinate silicon with a tricoordinate aluminum. The catalytic activity of many mixed oxide catalysts is also related to the presence of similar defect sites in... 

A small amount of copper in the mixed oxide facilitates the hydrogenation of the nickel oxide. As discussed in Chapter 12, reduction of bulk nickel oxide was facilitated by the presence not only of copper but also of silver. It would seem, then, that if silver were present in the coprecipitated nickel-aluminum oxide, it should also increase the rate of nickel oxide hydrogenation. High nickel areas were obtained when the partial pressure of water in the reducing gas was kept low.3>9 The presence of a small amount of aluminum oxide in the reduced nickel particles distorts the crystallites and produces more of the catalytically active surface defect sites.5... 

According to FTIR spectroscopy of adsorbed CO (Fig. 3, Table 3), the anchoring of aluminum oxide compounds on the surface of initial Y-AI2Q3 support decreased the concentration of weak Lewis acid sites with ab. Vco = 2191 cm-i (regular defects of the alumina surface including the octahedral aluminum ion) without changes in the concentration of other types of Lewis acid sites. 

Serebrennikova, L, and H.S. White. 2001. Scanning electrochemical microscopy of electroactive defect sites in the native oxide film on aluminum. Electrochem Solid-State Lett 4 (1) B4. 

At high temperatures the spinel MgAl204 can take in excess alumina to a composition of approximately 70 mol% A1203 (Fig. 4.5). (a) What are the possible formulas that fit the composition of this spinel Write the defect formation equation for the reaction if the excess A1 is (b) distributed over both magnesium and aluminum sites and (c) only over aluminum sites. Assume that there is no electronic compensation in the insulating oxide. 

Alumina, silica and many other metal oxides are insulators. However, recent experiments indicate that the surfaces of these insulators are mainly ionic (Masel, 1996). The pristine or freshly cleaved surfaces of single crystals of these oxides (cleaved under ultrahigh vacuum) are fairly inert and do not have significant adsorption capacities for even polar molecules such as CO and S02 (Masel, 1996 Henrich and Cox, 1994). However, the surface chemistry and adsorption properties are dominated by defects on real surfaces. For example, oxide vacancies on alumina expose the unsaturated aluminum atoms, which are electron acceptors, or Lewis acid sites. 

Three types of solvent or solute delocalization have now been examined, as summarized in Table III for three different adsorbent types (four, if we distinguish Cig-deactivated silica from silica). The theoretical requirements on the configuration and density of adsorption sites were discussed earlier (Section II,B) for a given type of localization/delocalization to be possible. In each case the nature of adsorption sites is fairly well understood for the four adsorbents of Table III, as disucssed in Ref. / and 17 and shown in Fig. 14. Thus, in the case of alumina, surface hydroxyls do not function as adsorption sites. Although surface oxide atoms are capable of interacting with acidic adsorbate molecules (see below), in most cases the adsorbate will interact with a cationic center (either aluminum atom or lattice defect) in the next layer. As a result, we can say that in most cases adsorption sites on alumina are buried within the surface, rather than being exposed for covalent site-adsorbate interaction. These sites are also rigidly positioned within the surface. Finally, the... 

In the above equation, Al i represents the normal aluminum ion in the regular site of the oxide film and Vai" represents the negatively charged aluminum vacancy in the oxide film. Here, the Krbger-Vink notation for representing point defects in solids is used (for a more detailed view, see Chapter 9). In alkaline media, generation of aluminum vacancies can be explained by the occurrence of a process involving water molecules adsorbed on the oxide film ... 

Note the Y-AI2O3 crystal structure is best described as a defect spinel structure comprised of a 2 X 2 X 2 fee array of oxide ions with 21/3 aluminum ions divided over the octahedral and tetrahedral interstices. By contrast, a-Al203 is an HCP array of 0 , with AP in 2/3 of the octahedral sites. For thermal transformatiems of alumina, see Stumpf, H. C. Russell, A. S. Newstane, J. W. Tucker, C. M. Ind. Eng. Chem. 1950,42, 1398. 

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