Infrared study of CO2 adsorption on SiO2

The adsorption of CO2 on SiO2 was studied by means of infrared spectroscopy. The desorption of CO2 is accounted for by the assumption of two kinds of adsorbed species of CO2 on SiO2. By adsorption measurements after modification of the surface OH groups on SiO2 by heat treatments, it was deduced that these two adsorbed CO2 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 CO2 adsorbed species. They are 5.0 and 5.2 kcal/mol, respectively, for CO2 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 CO2 hydrogen-bonded to the OH groups; the CO2 adsorbed on the bonded OH groups has a higher entropy than that adsorbed on the isolated OH groups.