Simultaneous Measurement of Self- and Transport Diffusivities in Zeolites

We have used quasielastic neutron scattering to derive the self- and transport diffusion coefficients of hydrogen molecules adsorbed in NaX zeolite. For H₂, incoherent scattering is dominant so that self-diffusion is measured. For D₂, the coherent and incoherent contributions are of the same order of magnitude so that both collective and individual motions can be characterized. At low D₂ concentration, the self- and transport diffusivities have similar values. For higher loadings, the transport diffusivity increases rapidly and exceeds the self-diffusivity. Only close to the saturation of the zeolite does the transport diffusivity start to decrease. To our knowledge this is the first time that transport and self-diffusion coefficients have been measured simultaneously for an adsorbed molecule. In contrast to numerous examples where discrepancies have been stated, these results are in agreement with each other.