Adsorption of Hydrogen in Ca-Exchanged Na-A Zeolites Probed by Inelastic Neutron Scattering Spectroscopy

Abstract

The hindered rotations and vibrations of molecular hydrogen, deuterium, and deuterium hydride adsorbed into partially and fully Ca ion exchanged Na-A zeolites have been studied at low temperatures with the use of inelastic neutron scattering (INS) techniques. A loading of one molecule per supercage was used in an attempt to avoid multiple site occupancies. Nonetheless, the INS spectra of the rotational transitions of H2, HD, and D2 clearly show that physical adsorption occurs on several different sites. Both the CaNa-A and Ca-A samples have strong rotational 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 H2 experiences a rotational barrier of about 4.3 kJ/mol.