Comparative IR Spectroscopy Study of Low-Temperature H2 and CO adsorption on Na-Zeolites

Extraframework cation sites in the sodium forms of the zeolites ZSM-5, mordenite, Linde-4A and faujasite-type X and Y have been investigated by using low-temperature adsorption of dihydrogen and carbon monoxide as IR spectroscopic probes. The extent of H-H and C-O bond polarization was found to be dependent not only on the cation electrostatic field, but also on the neighbouring oxygen atoms of the zeolite framework. The influence of these oxygen atoms is most keenly felt by adsorbed molecular hydrogen, but they also affect the IR frequency shift of the stretching vibration of adsorbed carbon monoxide. The Si : Al ratio of the zeolite framework modulates the basic strength of the oxygen atoms, and this was found to be reflected in the IR stretching frequency of both adsorbed molecules, H2 and CO.