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 (IR, microcalorimetry, TPD) results, the interpretation of some IR features is offered.