Quasi-elastic neutron scattering was used to determine the self- and transport diffusion coefficients of H2 which was adsorbed in NaX zeolite. In the case of H2, incoherent scattering predominated and self-diffusion was therefore measured. In the case of D2, the coherent and incoherent contributions were of the same order of magnitude, so that both the collective and individual motions could be characterized. At low D2 concentrations, the self- and transport diffusivities had similar values. At higher loadings, the transport diffusivity increased rapidly and exceeded the self-diffusivity (figure 13). Only when close to the saturation point of the zeolite did the transport diffusivity begin to decrease. This was thought to be the first time that transport and self-diffusion coefficients had been measured simultaneously for an adsorbed molecule.

Simultaneous Measurement of Self- and Transport Diffusivities in Zeolites. H.Jobic, J.Kärger, M.Bée: Physical Review Letters, 1999, 82[21], 4260-3

Figure 13

Various Diffusivities of D2 in Zeolite

(a: transport, b: corrected, c: self)