The Adsorption of Water on SiO2, Al2O3, and SiO2·Al2O3. The Relation between the Amounts of Physisorbed and Chemisorbed Water

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The specific surface area, the water content, and the water adsorption isotherm have been measured in order to investigate the interaction between the surfaces of SiO2, Al2O3, and SiO2·Al2O3 and water molecules. Al2O3 gives II-type adsorption isotherms and the amount of adsorbed water on it increases with a rise in the temperature of pretreatment, whereas SiO2 gives isotherms approximate to the III-type and the amount of adsorbed water is larger when treated at lower temperatures. With SiO2·Al2O3, it was found that the type of isotherms and the effect of the pretreatment temperature are similar to those of Al2O3, and that the amount of adsorption increases with an increase in the Al2O3 content. By analysing the adsorption data, the ratio of the amount of physisorbed water in the first layer to the amount of the underlying chemisorbed water was calculated. As a result, it was found that the H2O:OH ratio is 1:2 on the surface of Al2O3, whereas it is 1:1 on the surface of SiO2. The analysis of the isotherms on SiO2·Al2O3 showed that the adsorption property of water on them is not additive: e.g., the amount of adsorbed water on SiO2·Al2O3 is larger than the sum of the water adsorbed on each component. Thus, it has been concluded that, on the surface of SiO2 in contact with Al2O3, the surface hydroxylation proceeds more rapidly than on the surface of pure SiO2.
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