Page 331 - Adsorbents fundamentals and applications

316 SORBENTS FOR APPLICATIONS

Monte Carlo Simulations. Following the initial reports of hydrogen storage
in carbon nanotubes, a large number of rigorous molecular simulations were
published (Stan and Cole, 1998; Darkrim et al., 1998; Rzepka et al., 1998;
Darkrim and Levesque, 1998; Wang and Johnson, 1999a and 1999b; Simonyan
et al., 1999; Gordon and Saeger, 1999; Williams and Eklund, 2000; Darkrim and
Levesque, 2000; Meregalli and Parrinello, 2001; Simonyan and Johnson, 2002).
Reviews and summaries of these publications are available elsewhere (Dillon and
Heben, 2001; Meregalli and Parrinello, 2001; Simonyan and Johnson, 2002) and
are not repeated here.
Grand canonical Monte Carlo simulations using empirical and reliable poten-
tials for hydrogen–carbon and hydrogen–hydrogen interactions were used. Ad-
sorption of H 2 molecules is allowed to take place both inside and outside the
tubes, that is, in a bundle. All possible tube diameters and inter-tube spacings
have been assumed. As expected, adsorption potentials are enhanced within a
SWNT over that in a slit pore of the same size. However, all simulations pre-
dicted less than 2.4 wt % storage at 298 K and 100 atm. The highest amount
of 2.4 wt % was obtained with bundles having a tube diameter of 2 nm (Wang
and Johnson, 1999a). Wang and Johnson (1999b) showed that the DOE target
of 6.5% cannot be reached even by tripling the fluid-wall potential at ambient
temperature.
Charging the SWNT can in increase the potential between SWNT and H 2 .
Thus, H 2 adsorption on Li-doped SWNTs (bundles) have been investigated by
Simonyan and Johnson (2002) and Meregalli and Parrinello (2001). In the work
of Simonyan and Johnson, both H 2 -Li 2 interactions and H 2 -charge interactions
were included. However the predicted enhancement of adsorption was only about
40%, for example, at 298 K and 120 atm, the hydrogen capacity was increased
from 1.25 wt % for a pure SWNT bundle to 1.9 wt % for Li-doped bundle
(Li/C = 0.1).

Molecular Orbital Calculations. A common feature that exists in all reported
experimental results, whether at ambient temperature or at cryogenic tempera-
tures, is the slow uptake of H 2 . In all cases, the uptakes were slow and equilibria
were reached after a time span of the order of hours. Meanwhile, the adsorbed
hydrogen could not be desorbed completely in vacuo. For example, 21–25% of
the adsorbed hydrogen remained and had to be desorbed at 473 K in vacuo (Liu
et al., 1999). Both results are clear indications of chemisorption. In all processes
for synthesis of nanotubes and GNF, one or more of the metals from Co, Ni,
and Fe must be used. Regardless of the processes that are used to purify the
products, significant amounts of metals remain. Since metals were contained in
the nanotubes in all cases, dissociation of hydrogen to atomic H was likely to
have occurred. In addition, hydrogen dissociation is known to occur on the edge
2
sites of graphite with a free sp electron per carbon site (e.g., Ishikawa et al.,
1975), although such a process would be much slower than that on metals at
ambient temperature. Consequently, hydrogen dissociation followed by spillover
and chemisorption on the surface of nanotubes is an entire possibility.