Basic sites, aluminum oxide

The surface was actually a film of native aluminum oxide it did not adsorb pyridine but did adsorb chloroform showing the oxide to have no acid sites, but basic sites. Treatment of the aluminum oxide with aqueous carbonate solutions clearly enhanced the basicity, as evidenced by stronger adsorption of chloroform. By observing the temperature coefficient of adsorption isotherms with ellipsometry one can actually determine heats of adsorption on a square centimeter of flat surface. 

J. Bolger proposed that the isoelectric pH in water could be used to rank the acidic or basic surface properties of oxides (37) and there is much merit in the suggestion. Certainly the basicity of aluminum oxide is predicted by its isoelectric pH of 9.0 and the acidity of silica is predicted by its isoelectric pH of 3.0. However when a surface has both basic and acidic sites like many iron oxides, the isoelectric pH s near 7 do not really tell the whole story. 

It seems plausible that amphoteric aluminum oxide plays the role of catalase amino acid fragments the acidic-basic sites. 

Aluminum oxide is a widely used catalyst, mainly as a support. The surface of AI2O3 possesses both acidic and basic sites after heat treatment above 670 K. The acid sites are mainly Lewis acid sites strong Bronsted sites are absent. The Lewis acid site is visualized as a coordinatively unsaturated A1 atom formed by dehydroxylation, and the weak Broonsted sites act as acidic surface hydroxyls, while the basic sites are considered to be basic hydroxyls and oxide ions. Three OH groups are... 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

Various hydrated aluminum hydroxides serve as starting material for TLC alumina. By a series of nonuniform thermal dehydration processes, a variety of aluminas are obtained, and the ones most suitable for TLC are the crystalline modifications of x-ALOs and y-ALO (Rossler, 1969 Snyder, 1975). The physicochemical properties or the exact nature of adsorption sites of alumina are not well understood. Snyder (1975) has suggested that exposed A1 atoms, strained Al-O bonds, and perhaps other cationic sites serve as adsorption sites, whereas, unlike silica, surface hydroxyl groups are probably not important. Acids are probably retained by interaction with basic sites such as surface oxide ions. Gasparic and Churacek (1978) report that every A1 atom is surrounded by six atoms of... 

Oxygen atoms in a zeolite lattice have an intrinsic Lewis base character due to the electron pairs they can donate and to the partial negative charges they bear. Silicon or aluminum atoms provide partial positive charges, which reduce the strength of such a donation [8,54]. However, every zeolite framework possesses an intrinsic Lewis base character. Additional basic sites can be created by metal or metal oxide clusters within the micropores of the zeolite [54]. The different basic sites will be discussed in the following paragraphs. 

Oxide ions in the dehydrated aluminum phosphorous oxide are held primarily by the P atoms, and the P-O bond is of covalent nature. Therefore, the oxide ions are inadequate to act as base sites. Two types of Lewis acid sites are demonstrated from IR measurement of coadsorption of ammonia and pyridine one is isolated and the other is the acid site adjacent to basic site. ... 

Aluminum phosphorous oxide shows characteristic features in butene isomerization. At low pretreatment temperatures (473 — 773 K), the catalyst behaves like a typical acid catalyst the reaction proceeds via r-butyl cation intermediate and gives cisitrans ratios close to unity. At high pretreatment temperatures, however, cis-tmas interconversion predominates to an overwhelming deg ree double bond migration becomes very slow. The activity maximum is observed when aluminum phosphorous oxide is outgassed at 1173 K. The specific catalytic activity is considered to be due to a pair of strong acidic and weak basic sites favoring cis-trans isomerization... 

Considerable evidence exists indicating that the acidity of an oxide surface can vary according to the pretreatment. For example, Finlayson and Shah [12] used flow microcalorimetry to characterize the oxidized surfaces of three aluminum specimens that had received different pretreatments. They found that the surface chemistry of the three samples was considerably different but was dominated by Lewis base sites in all cases. The peel strength of ethylene/acrylic acid copolymers laminated against the substrates increased as the basicity of the substrate and the acrylic acid content of the co-polymer increased. 

Metal oxides have surface sites which are acidic, basic, or both and these characteristics control important properties such as lubrication, adhesion, and corrosion. Some of the newer infrared techniques such as lazer-Raman and Fourier transform infrared reflection spectroscopy are important tools for assessing just how organic acids and bases interact with the oxide films on metal surfaces. Illustrations are given for the adsorption of acidic organic species onto aluminum or iron surfaces, using Fourier transform infrared reflection spectroscopy. 

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