Chemisorption sites on porous silica glass and on mixed-oxide catalysts

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Abstract
It has been shown previously that two types of adsorption sites exist on the surface of porous silica glass (Corning code no. 7930) for the adsorption of polar molecules, such as ammonia. One site is the surface hydroxyl group which forms a hydrogen bond to the adsorbate molecule. The second type of site is provided by traces of foreign oxides on the glass surface.In the present study, the infrared spectrum of ammonia adsorbed on the silica glass has been compared with that of ammonia adsorbed on samples prepared by adding B2O3, ZrO2, and Al2O3 to a pure silica powder. This comparison suggests that surface boron atoms (a Lewis acid) are the adsorption sites for ammonia on the porous silica glass. An additional type of site (a Bronsted acid or a different type of Lewis acid site, such as aluminium atoms) may exist in small concentrations on the glass and be responsible for isomerization and polymerization reactions with olefins.

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