CO interaction with alkali metal cations in zeolites: A density functional model cluster study

The interaction of CO molecules with cations of alkali exchanged zeolites (Li, Na, K, Rb, and Cs forms) has been studied theoretically using a density functional (DF) method. The modification of the alkali action due to the presence of the zeolite framework has explicitly been taken into account by employing various model clusters. Inclusion of zeolite framework effetcs is shown to be important for obtaining quantitative agreement with experiment. Carbon monoxide bound to a cationic site via the oxygen center is found to exhibit an adsorption-induced red shift of the CO stretching frequency in contrast to the well-established blue shift of the common C-bound species. According to our DF results there is a certain probability (increasing with the size of the cation up to 15% at 100 K) for the presence of O-bound CO species in alkali-exchanged zeolites. The experimentally observed red-shifted satellite of the IR band of the CO stretching mode is assigned to such unusually coordinated probe molecules.