Combined volumetric, infrared spectroscopic and theoretical investigation of CO2 adsorption on Na-A zeolite
Abstrakt
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 CO2 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 CO2 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 CO2 stretching frequency due to adsorption the following model of CO2/Na-A system is proposed: (i) CO2 adsorption complexes formed at a low coverage involves interaction of CO2 with three Na(+) cations in the LTA supercage; they are characterized by the IR band at 2349 cm(-1). (ii) CO2 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).