Gas adsorption on single-wall carbon nanotube bundles and charcoal samples

We have studied the adsorption properties of He-4 on single-wall carbon nanotube (SWNT) bundles and activated carbon (charcoal) samples using a temperature-programmed desorption (TPD) technique. The He-4 binding energy, the dose temperature dependence of adsorption, and the competitive adsorption of binary mixture gases among He-4, He-3, H-2, and Xe were measured. The energy on SWNTs was as high as 910 K at similar to 1 x 10(17) atom/mg coverage, decreased as the coverage increased, and agrees with the previously reported energy values where the coverages overlap. On charcoal the He-4 binding energy was constant at 403 +/- 11 K. He-4 adsorption showed an activated adsorption behavior when He-4 was dosed below similar to 30 K with activation energy similar to 20 K to some sites of the SWNT bundles. We argue that these sites are interstitial channel (IC) sites, and the activated adsorption was the reason why some of the previously reported gas adsorption studies where He-4 was dosed at low temperature could not detect IC adsorption. On charcoal He-4 did not show activated adsorption behavior when dosed at similar to 15 K. Mixture gas adsorption measurements on SWNT samples showed that the relative binding strengths were He-3 < He-4 < H-2 < Xe. Results showing He-3 < He-4 coincide with the predictions of quantum sieving on SWNT bundles. When He-4 was dosed at 275 K and H-2 was dosed at 19 K, He-4 adsorption was stable against H-2, which indicates limited access of H-2 at low temperature to some of the sites where He-4 preadsorbed. Again we argue that the likely candidates for such sites are IC sites on SWNT bundles.