Large hysteretic hydrogen adsorption in Double Wall Carbon Nanotubes bundles

We have observed isothermal large hysteretic hydrogen adsorption in samples made out of Double Wall Carbon Nanotubes (DWCNTs) bundles at 50 K, 77 K and 150 K and up to 1.3 MPa of pressure. In order to gain information about the microscopic mechanisms responsible for this remarkable phenomena we made a series of inelastic neutron scattering measurements at various temperatures and H2 loads. The elastic component of the signals (diffraction) observed at different loads are consistent with the uptake of the H2 molecules within the interstitials of the bundles. The quasielastic part, i.e., that centered around the null energy transfer, displays a dependence on momentum transfer which is reproduced by a 1D dimensional diffusion model as expected for interstitial confinement of the H2 fluid. Finally, the inelastic signals reveal rotational spectra that preclude the possibility of chemisorption onto the sample metallic impurities. To the best of our knowledge this is the first observation of supercritical H2 hysteretic adsorption in nanostructured carbons not ascribed to chemisorption onto metallic centers. Instead, the results point to the hierarchy of energy and entropic barriers associated with the adsorption into the bundle interstitials through the external grooves as the microscopic origin of the observed metastable adsorption.