Hydrogen uptake in single-walled carbon nanotubes synthesized by the hydrogen arc plasma jet method.

Carbon nanotubes were synthesized by a modified electric arc discharge method under hydrogen atmosphere using a catalytic mixture of powders with composition C/Ni/Co/Fe/FeS. The samples were characterized with transmission and scanning electron microscopy, Raman spectroscopy and thermo-gravimetric analysis. Unpurified samples contain 20 wt{\%} of carbon nanotubes, 2 wt{\%} of other forms of carbon and 78 wt{\%} of catalytic metals. Adsorption/desorption of hydrogen in unpurified samples at atmospheric pressure measured with gravimetric methods was of the order of 2 wt{\%}. Samples for these measurements were prepared with a three-step treatment: (i) oxidation in air for 30 min at 500 $^\circ$C, (ii) ball- milling for 1 hr, and (iii) second oxidation in air for 30 min at 500 $^\circ$C. This treatment was applied to eliminate other forms of carbon as well as to shorten the tubes and open their caps. Hydrogen desorption was also measured in degassed samples (400 $^\circ$C for 20 min in vacuum) which adsorbed H$_ {2} $ while cooling to room temperature. Desorption in degassed/hydrogenated samples is a two-step process which we attribute to distinct desorption characteristics of nanotubes and of residual oxidized metal catalysts.