  We present a study on the kinetics of oxygen adsorption and desorption from
single-wall carbon nanotube (SWNT) and highly oriented pyrolytic graphite
(HOPG) samples. Thermal desorption spectra for SWNT samples show a broad
desorption feature peaked at 62 K which is shifted to significantly higher
temperature than the low-coverage desorption feature on HOPG. The low-coverage
O2 binding energy on SWNT bundles, 18.5 kJ/mol, is 55% higher than that for
adsorption on HOPG, 12.0 kJ/mol. In combination with molecular mechanics
calculations we show that the observed binding energies for both systems can be
attributed to van der Waals interactions, i.e. physisorption. The experiments
provide no evidence for a more strongly bound chemisorbed species or for
dissociative oxygen adsorption.
