Adsorption equilibrium of sulfur hexafluoride on multi-walled carbon nanotubes.

The adsorption of sulfur hexafluoride (SF(6)) on multi-walled carbon nanotubes (MWNTs) was investigated. The properties of MWNTs were characterized and the adsorption capacities of SF(6) on MWNTs at different concentrations and temperatures were collected. H(2)SO(4)/H(2)O(2) oxidation or KOH activation of MWNTs has effectively introduced the surface oxides and modified the microstructure without destruction of their graphitic crystalline structure. The MWNTs, especially the modified samples, are expected to be promising adsorbents for SF(6) removals from air. The saturated capacities of SF(6) with a concentration of 518 ppmv on the MWNTs ranged from 278 to 497 mg/g at 25 degrees C. The Toth equation has been reported to fit the adsorption data better than the Freundlich or Langmuir equation. The pi-pi dispersion interaction followed by the multi-layer adsorption and the electron donor-acceptor interaction were proposed to be the major adsorption mechanisms, depending on the adsorption temperature. The isosteric heat of adsorption, ranging from 51 to 124 kJ/mol with a loading of 30-300 mg/g, decreased with increasing SF(6) loading, reflecting the energetic heterogeneity of the MWNTs. These results suggest that the adsorption of SF(6) on MWNTs could be associated with binding to defect sites.