Hydrogen sorption by single-walled carbon nanotubes prepared by a torch arc method

Single-walled carbon nanotubes (SWCNTs) were prepared by a novel torch arc method operated in open air and isotherms of hydrogen sorption were volumetrically measured at 295 and 77 K under pressures below 110 kPa. The averaged diameter of SWCNTs was 1.32 nm, as measured by high-resolution transmission electron microscopy, and the dominant presence of the (9,9) armchair Danotubes accompanied by the (10,10) armchair nanotubes was suggested by the breathing mode Raman scattering. The maximum hydrogen concentrations reached 0.932 wt.% at 295 K under 106.7 kPa and 2.37 wt.% at 77 K under 107.9 kPa with the hydrogen uptakes not yet saturated. It is likely that the sorbed hydrogen molecules resided on the exterior surfaces of SWCNTs. The higher surface area of SWCNTs by a factor of 10 as compared to the graphitic raw material would play a dominant role for hydrogen sorption to bring about the higher hydrogen wt.% by a factor of 10. The high graphitic nature of SWCNTs would also play an important role, but the observed surface H/C ratio, 0.680 at 77 K under 100 kPa, exceeded the expected value, 0.333, for the commensurate adsorption structure, and the uppermost value, 0.49, for the densest monolayer, which suggested the presence of unknown sorption mechanism(s) other than physisorption on carbon hexagons. (C) 2002 Elsevier Science B.V. All rights reserved.