Adsorption on sp2-Hybridized Carbon Surfaces

The structural heterogeneity of industrial carbon sorbents prohibits detailed molecular understanding of their active sites for physical and chemical adsorption. Therefore, careful studies of structurally well-defined carbon surfaces under wellcontrolled experimental conditions are of interest. The adsorption of molecules on single wall carbon nanotubes (SWNTs) and highly oriented pyrolytic graphite (HOPG) has been studied under ultrahigh vacuum using temperature programmed desorption (TPD) and Auger electron spectroscopy (AES). Modification of SWNTs by Li atoms was found to be effective for increasing the binding energy of n-heptane, as determined by TPD and confirmed by density functional theory (DFT). Lithium ionizes on the surface, producing a localized charge center to interact attractively with adsorbed n-heptane. The resulting induced dipole produces a stronger alkane interaction with Li/SWNTs compared to clean SWNTs. In the case of a more reactive polar molecule, CH 3Cl, Li/SWNTs were found to readily dissociate CH 3Cl at low temperature. The chemical bonding of methyl groups to SWNT defect sites was discovered and the bonding structures proposed are supported by DFT calculations. A low yield of volatile organic products from the reaction was also detected. The role of carbon defect sites in diffusion and in the covalent chemistry of sp 2 carbon surfaces was further explored utilizing high quality HOPG with a low density of defects (< 1 ppm). The low temperature diffusion of lithium across the surface into the bulk of graphite was studied with Auger electron spectroscopy. The measured activation energy for Li diffusion is 0.16 eV, corresponding to a very high diffusion coefficient, D = 5 x 10 -6 cm 2 s -1 at 300 K. When CH 3Cl is exposed to the Li-covered graphite surface, the C-Cl bond is broken, resulting in the formation of CH 3 radicals. The attachment of CH 3 to the basal plane via a puckered surface structure is confirmed by DFT calculations, iiwhich show the conversion of the surface sp 2 -C to sp 3 -C in producing the surface C-CH 3 bond. Upon heating, a low yield of hydrocarbon products was also found to desorb. These studies comparing the surfaces of SWNTs and graphite have yielded new insights into CH 3 surface chemistry and Li surface mobility, as well as the means to use Liinduced chemistry to enhance chemical reactivity at carbon surfaces.

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