Thermodynamics of CO adsorption on the zeolite Na-ZSM-5 - A combined microcalorimetric and FTIR spectroscopic study

Carbon monoxide was found to adsorb, at room temperature, on Na-ZSM-5. IR s pectra of adsorbed CO showed two main bands at 2176 and 2112 cm(-1), which were assigned to the C-O stretching mode of Na+ ... CO and Na+ ... OC adduc ts, respectively. The complex structure of these bands suggested that catio n sites in Na-ZSM-5 are not all identical. Further proof of the presence of slightly different cation sites was obtained by IR spectroscopy of adsorbe d carbon dioxide. Microcalorimetry of adsorbed CO, in conjunction with IR s pectroscopy and volumetric adsorption data, allowed a detailed thermodynami c study to be carried out on the CO/Na-ZSM-5 system. CO adsorption was foun d to follow a Langmuir-type isotherm. The (exothermic) adsorption process s howed a differential heat of adsorption of ca. 27 kJ mol(-1). The enthalpy change in the formation of Na+ ... CO and Na+ ... OC species was found to b e Delta H degrees = -28 and Delta H degrees = -24 kJ mol(-1), respectively, while the value of T Delta S degrees = -41 kJ mol(-1) was derived for both adsorption modes.