Krypton adsorption has been investigated on multi-walled carbon nanotubes prepared by catalytic decomposition of acetylene. Isotherms measured between 77.5 and 83.7K from the first stages of adsorption to the adsorbate saturation vapor pressure are compared to those obtained in the same conditions on graphite. The results are discussed in the light of the nanotube morphology, as determined by transmission electron microscopy.
Krypton adsorption proceeds, as on graphite, by successive monomolecular layer condensations on a same surface. From the morphology of the tubes, adsorption probably occurs only on their external surfaces. The first adsorbed layer is commensurate with the substrate. At its completion, it undergoes a transition into an incommensurate solid of higher density. The curvature of the graphene sheets, with respect to those of graphite, produces a stabilization of the commensurate film. The isosteric heat of adsorption Qst was measured to be 11.6 kJ/mol (ΔQst/Qst = 6%) and is lower than that of krypton on graphite.
Moreover, the monolayer condensation pressures are higher than those observed with graphite, which probably leads to an incomplete wetting of the surface by limiting the number of krypton monomolecular layers adsorbed before its bulk condensation.