Page 333 - Adsorbents fundamentals and applications

318 SORBENTS FOR APPLICATIONS

Table 10.7. Bond energies (expressed by energies of ad-
sorption) of H atoms on surface sites on graphite (see
Figure 10 for models used) from ab initio molecular orbital
calculations

Model Bond Energy (kcal/Mol)
C (H on armchair edge) −85.27
D(Honzigzagedge) −90.24
E (H adjacent basal sites) −27.04
F (H on alternating basal sites) −46.47
Yang and Yang, 2002.

Model E, the energy drops from 46.47 kcal/mol (two H on alternate sites) to
27.04 kcal/mol (two H on adjacent sites).
Molecular orbital calculations have been published recently on the bonding
of F atoms (Seifert et al., 2000; Kudin et al., 2001) and H atoms (Bauschlicher,
2000 and 2001; Froudakis, 2001) on SWNT. Both Bauschlicher and Froudakis
studied the bonding of H atoms on the exterior wall of the SWNT (both con-
sisting of 200 C atoms, but with different diameters). Bauschlicher calculated
the bond energies for the tube with 1H and 2H and 24%, 50%, and 100% cov-
erages (assuming 1H/1C). The average C–H bond energy for the first H was
21.6 kcal/mol, and 40.6 kcal/mol for the first two H atoms. The average bond
energy for 50% coverage was 57.3 kcal/mol, decreasing to 38.6 kcal/mol for
100% coverage. Froudakis studied the bonding of 1H with the tube where the
H atom approached the tube wall in two ways: direct approach to the top of a
carbon atom, and approach along the centerline of a hexagon (Froudakis, 2001).
The energy minima were, respectively, 21 kcal/mol and 56 kcal/mol. Froudakis
also reported the C–C bond lengths in the nanotube after H bonding. With 16 H
bonded to 64 C on the 200-atom tube, the C–C bond length increased from
143 to 159 pm.
The C–H bond energies on the tube are in general agreement with that on the
basal plane of graphite. The energies on the basal plane are 46.47 kcal/mol for two
alternating or separated H atoms, and 27.04 kcal/mol for two adjacent H atoms. A
similar crowding effect was also seen on the curved tube, that is, 57.3 kcal/mol
for 50% coverage and 38.6 kcal/mol for 100% coverage. The increase in the
C–C bond lengths from 143 pm to 155–159 pm by adsorption of H was also
seen on the basal plane of graphite, and are, in fact, in excellent agreement with
the results of Yang and Yang (2002).
From the above comparison, it is unlikely that the adsorption of H atoms
would differ significantly on the basal plane of graphite and on the exterior wall
of the nanotube. Based on the results above, it is possible that the binding energies
could be lowered further for adsorption inside the tube. Also, no calculations have
been made on H atoms adsorbed between two layers of graphite at a d-spacing of
3.35 ˚ A. This could be the situation for hydrogen storage on GNF and MWNTs.