Combined quasi-elastic neutron scattering and NMR study of hydrogen diffusion in zeolites

Results of the first pulsed field gradient (PFG) NMR studies of hydrogen diffusion in zeolites are presented and compared with corresponding measurements by quasielastic neutron scattering (QENS). The results of the two techniques were in satisfactory agreement. Similarly as with other probe molecules, also the hydrogen diffusion is found to depend decisively on the zeolite type under study. The hydrogen diffusivity is generally found to decrease with decreasing free apertures of the zeolite pore structure. Only in the case of silicalite, where the diffusivities are found to be exceptionally small, is this trend violated. This finding is correlated with a partial capturing of the hydrogen molecules within the pentasil chains. Comparison between the 1H MAS NMR and QENS data suggests mean trapping times of the order of 200 µs.