Journal of Physical Chemistry, Vol.100, No.25, 10646-10651, 1996
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 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 H-2, HD, and D-2 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 H-2 experiences a rotational barrier of about 4.3 kJ/mol.
Keywords:MOLECULAR-HYDROGEN;CRYSTAL-STRUCTURE;ROTATION;COMPLEXES;GRAPHITE;BARRIER;H2;HD;DIFFUSIVITY;DIHYDROGEN