Page 266 - Adsorbents fundamentals and applications

CARBON NANOTUBES 251

10 5
(3,6)
10 4 (10,10) interstice
S 0 (T 2 /H 2 ) 10 3

10 2
(2,8)
10 1
(6,6) (10,10) (18,18)
10 0
5 10 15 20 25
Tube diameter (Å)
Figure 9.12. S 0 (equilibrium selectivity at pressure approaching zero) for T 2 /H 2 as a function
to tube diameter, from Eq. 9.3 (diamonds) and path integral Monte Carlo calculations (circles),
(n, m) are tube indices, which are related to the tube diameter via Eq. 9.1, from Challa et al.
(2001) with permission.

10000

1000

S 0 100

10

1
20 30 40 50 60 70 80
Temperature (K)

Figure 9.13. Selectivity S 0 in the SWNT interstices as a function of temperature for T 2 /H 2
(circles), HT/H 2 (triangles), and T 2 /HT (squares). From Challa et al. (2001) with permission.

The quantum sieving effects drop rapidly as the temperature increases, as
shown in Figure 9.13. Using Eq. 9.3, Challa et al. (2001) calculated the selec-
tivities for three different hydrogen isotope mixtures in the interstices of (10,10)
SWNT bundles at different temperatures. The drop was due to the increased con-
tribution from the excited states. Thirty energy states (in Eq. 9.3.) were used in
their calculations. However, a selectivity of 5.2 for T 2 /H 2 still remained at 77 K.
It should be noted that the quantum sieving effects are not limited to the carbon
nanotubes. Small-pore molecular sieves such as ALPO 4 -22 also could have such
effects (Wang et al., 1999). However, carbon nanotubes have the advantage of
being the most smooth and uniform pores. The interesting quantum sieving effects
remain to be proven experimentally.