Abstract
Adsorption isotherms of carbon dioxide on the Na-A zeolite were measured in the temperature range from 273 to 333 K. Henry’s constants and isosteric adsorption heats calculated from the temperature dependence of CO 2 isotherms were combined with results of the variable temperature IR spectroscopy to obtain a spectroscopic signature of adsorption complexes and corresponding thermodynamic characteristics. The adsorption of CO 2 in Na-A zeolite was also investigated computationally, using a periodic density functional model improved for the proper description of dispersion interactions. Based on a good agreement between experimental and calculated adsorption heats and shifts of asymmetric CO 2 stretching frequency due to adsorption the following model of CO 2/Na-A system is proposed: (i) CO 2 adsorption complexes formed at a low coverage involves interaction of CO 2 with three Na + cations in the LTA supercage; they are characterized by the IR band at 2349 cm −1. (ii) CO 2 adsorption complexes at higher coverages are formed between a pair of Na + cations; they are characterized by the IR band centered at 2360 cm −1. Almost half of the interaction energy between CO 2 and Na-A is due to the dispersion interaction.
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