Abstract
The adsorption of CO 2 on SiO 2 was studied by means of infrared spectroscopy. The desorption of CO 2 is accounted for by the assumption of two kinds of adsorbed species of CO 2 on SiO 2. By adsorption measurements after modification of the surface OH groups on SiO 2 by heat treatments, it was deduced that these two adsorbed CO 2 species are related to the isolated surface OH groups (which have an absorption band at 3750 cm −1) and the bonded OH groups (which have a broad band centered at 3550 cm −1), respectively. The isosteric heats of adsorption and the activation energies for desorption were calculated for both CO 2 adsorbed species. They are 5.0 and 5.2 kcal/mol, respectively, for CO 2 adsorbed on isolated OH groups, and 5.1 and 5.0 kcal/mol for bonded OH groups. The changes in the standard entropies of adsorption are also discussed in relation to the structure of CO 2 hydrogen-bonded to the OH groups; the CO 2 adsorbed on the bonded OH groups has a higher entropy than that adsorbed on the isolated OH groups.
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