ADSORPTION OF HYDROGEN IN CA-EXCHANGED NA-A ZEOLITES PROBED BY INELASTIC NEUTRON-SCATTERING SPECTROSCOPY

The hindered rotations and vibrations of molecular hydrogen, deuterium , and deuterium hydride adsorbed into partially and fully Ca ion excha nged Na-A zeolites have been studied at low temperatures with the use of inelastic neutron scattering (INS) techniques. A loading of one mol ecule per supercage was used in an attempt to avoid multiple site occu pancies. Nonetheless, the INS spectra of the rotational transitions of H-2, HD, and D-2 clearly show that physical adsorption occurs on seve ral different sites. Both the CaNa-A and Ca-A samples have strong rota tional bands near 6 meV. On this basis, we conclude that the principal adsorption site in CaNa-A is in the vicinity of the Ca2+ cation where H-2 experiences a rotational barrier of about 4.3 kJ/mol.