The perturbation of surface species of zeolite HY by Xe has been studied with the use of in situ FT-IR spectroscopy. The Bronsted acid sites, characterized by a band at 3638 cm-1, are located in the zeolite supercages, as suggested previously, while Bronsted acid sites attributed to a 3545-cm-1 band are located in the small zeolite cages and are inaccessible to the Xe molecules. The Bronsted acid sites in the supercages are found to have a weaker acidity than those in the small cages. The data also indicate that pyridine molecules are more strongly bonded to the supercage Bronsted acid sites than are the NH3 molecules. This work is an example of the usefulness of liquid-nitrogen-temperature FT-IR techniques, coupled with multiple-scan data acquisition methods, for studying weak interactions between Xe and zeolite surface species in detail. This work also shows how FT-IR spectroscopy can complement Xe-129 NMR spectroscopy in studies of the nature of zeolite surface species using Xe as a probe molecule.
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