A quasi-elastic and inelastic neutron scattering study of H2 in zeolite

The diffusion of molecular hydrogen adsorbed in zeolite 13X at high coverage's has been studied by quasi-elastic neutron scattering for temperatures ranging from 0 to 60 K. No diffusive motion was observed, within instrumental resolution, at temperatures below 20 K, well above the bulk melting point of H2, in contrast to recent NMR studies. The diffusive motion of the adsorbed H2 could be described by a liquid like jump diffusion model above 35 K. The diffusion coefficient demonstrates an Arrhenius behavior: D = D0 exp(E/kT), with D0 = 1.2 x 10-8 m2/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 from 1 meV to above 100 meV and is consistent with rotational transitions for a hindered rotor in a strong orientational potential with a broad distribution of barrier heights