A quasi-elastic and inelastic neutron scattering study of H-2 in zeolite

The diffusion of molecular hydrogen adsorbed in zeolite 13X at high coverag e's has been studied by quasi-elastic neutron scattering Sor temperatures r anging from 0 to 60 K, No diffusive morion was observed, within instrumenta l resolution, at temperatures below 20 K, well above the bulk melting point of H-2, in contrast to recent NMR studies. The diffusive motion of the ads orbed H-2 could be described by a liquid like jump diffusion model above 35 K. The diffusion coefficient demonstrates an Arrhenius behavior. D = D-0 e xp(E/kT), with D-0 = 1.2 x 10(-8) m(2)/s and an activation energy of E = 62 K. The inelastic scattering spectrum has also been studied. The scattering consisted of several peaks superimposed on a broad background extending fr om 1 meV to above 100 meV mid is consistent with rotational transitions for a hindered rotor in a strong orientational potential with a broad distribu tion of barrier heights.