Combined Experimental and Theoretical Investigations of Heterogeneous Dual Cation Sites in Cu,M-FER Zeolites

Carbon monoxide is frequently used as a probe molecule for characterization of adsorption sites in zeolites by means of infrared (IR) spectroscopy. IR spectra of carbonyl species in zeolites are usually understood within a concept of CO adsorption on a single cation site. This concept, however, is not sufficient in microporous materials when the concentration of cationic sites increases. Adsorption complexes formed on homogeneous dual cation sites were recently described (J. Phys. Chem. B 2006, 110, 22542) based on a combination of experimental and theoretical investigations. The concept of dual cation sites is extended for the situation where CO is strongly bound on the Cu+ cation and it also interacts with the,extra-framework alkali-metal cation. The existence and properties of the CO adsorption complexes on such heterogeneous dual cation sites are discussed for Cu,M-FER zeolites (M = H, Na, K, Cs) having various compositions. Based on a good agreement between theoretical (periodic density functional theory) and experimental (IF., microcalorimetry, TPD) results, the interpretation of some IR features is offered.