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 H ₂ , in contrast to recent NMR studies. The diffusive motion of the adsorbed H ₂ 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 × 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 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.