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

Abstract

Carbon monoxide was found to adsorb, at room temperature, on Na-ZSM-5. IR spectra 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 adducts, respectively. The complex structure of these bands suggested that cation 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 adsorbed carbon dioxide. Microcalorimetry of adsorbed CO, in conjunction with IR spectroscopy and volumetric adsorption data, allowed a detailed thermodynamic study to be carried out on the CO/Na-ZSM-5 system. CO adsorption was found to follow a Langmuir-type isotherm. The (exothermic) adsorption process showed 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 be ΔH°=-28 and ΔH°=-24 kJ mol^-1, respectively, while the value of TΔS°=-41 kJ mol^-1 was derived for both adsorption modes.