Page 327 - Adsorbents fundamentals and applications

312 SORBENTS FOR APPLICATIONS

The isotherm at 77 K exhibits a peak. This occurs when the absolute adsorbed
density approaches saturation and the bulk density of the supercritical fluid begins
to rise faster with increasing pressure (Menon, 1968; Benard and Chahine, 2001).

Single-Wall Carbon Nanotubes. The first report on hydrogen storage in
SWNTs was by Dillon et al. (1997). A summary of the subsequent reports on
experimental data on hydrogen storage in SWNTs, MWNTs and GNF is given
in Table 10.6.
The storage capacity of Dillon et al. (1997) was estimated from TPD exper-
iment. A small sample (of ∼1 mg) of soot from arc discharge of Co/graphite
was the sorbent. The sample was placed in a Pt foil and exposed to 300 torr
H 2 for 10 min at 273 K, followed by 3 min at 133 K (in H 2 ). The sample was
then cooled to 90 K while being evacuated prior to TPD. The evolved H 2 was
detected by a mass spectrometer. For SWNT soot samples pretreated at 970 K in
vacuum, broad H 2 desorption peaks were detected in the temperature range from
170 to 450 K, with peaks near 300 K. Using TPD curves from different heating
rates, the activation energy for desorption (assuming first-order desorption kinet-
ics) was calculated at 19.6 kJ/mol. From TEM images of the soot, the content of
SWNTs was estimated as 0.1–0.2%. From these data, an estimate for H 2 storage
capacity of 5–10% was obtained. The possible chemisorption of hydrogen by
Pt was ruled out by calibration runs. The soot samples contained approximately
20% Co nanoparticles. The possible chemisorption on Co was also ruled out
(Dillon et al., 1997; Dillon et al., 1999).
Ye et al. (1999) measured the isotherms of SWNTs (purified samples pro-
duced by laser-vaporization) by using volumetric technique. The samples were
◦
pretreated by heating in vacuo at 220 C for 1 h. The highest adsorption was
obtained on purified crystalline ropes, that is, 8 wt % at ∼40 atm and 80 K. How-
ever, in contrast to the results of Dillon et al. (1997), high adsorption capacities
were not observed at 300 K and pressures below 1 atm.
High adsorption capacities on SWNTs at 298 K were reported by Liu et al.
(1999). Volumetric technique was used. The samples contained ∼50–60% SWNTs
with diameters near 1.8 nm. Mixed Ni/Co/Fe was used as the catalyst to produce
the SWNTs by arc discharge. The samples were pretreated at 773 K in vacuo for
2 h. The uptake was very slow (∼70% completion in 1 h). The highest capac-
ity was 4.2 wt % at 10 MPa and 298 K. Approximately 80% of the adsorbed
hydrogen could be desorbed at room temperature.
The SWNTs produced by laser-vaporization and arc discharge are usually
capped by fullerene-like structures. Those from catalytic decomposition are usu-
ally capped by metal particles. Ye et al. (1999) and Dillon et al. (2000) both
used ultrasonication to open the tubes. In the work of Dillon et al. (2000), it was
reported that by using the ultrasonic probe of Ti-alloy (with 9 wt % Ti, 6 wt %
Al, and 4 wt % V), the alloy (TiAl 0.1 V 0.04 ) was incorporated into the SWNTs as
contamination. The maximum adsorption capacity was ∼7 wt %, and upon TPD,
two desorption peaks occurred: approximately 2.5% evolved at 300 K, while the
remainder evolved between 475–850 K. It was suspected that the alloy con-
taminant acted as a catalyst that stimulated the adsorption and desorption. This