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Number of H

Figure 7. Binding energy per hydrogen molecule with respect to the number of the H2 molecules. The hexagons represent the calculated values where the line was fitted. Modified from [41], G. E. Froudakis, Nano Lett. 1, 531 (2001). © 2001, American Chemical Society.

Number of H

Figure 6. Three of the alkali metal doped (5,5) SWNTs used by Froudakis [41] to study the interaction with molecular hydrogen. The first has one H2 per K, the second two, and the third three. A magnifying part of all pictures is also presented.

Figure 7. Binding energy per hydrogen molecule with respect to the number of the H2 molecules. The hexagons represent the calculated values where the line was fitted. Modified from [41], G. E. Froudakis, Nano Lett. 1, 531 (2001). © 2001, American Chemical Society.

geometrical constrains (i.e., the space around the K atoms has a maximum number of H2 molecules that can be introduced without having steric interactions). From this graph we can estimate the amount of H2 molecules that can be attached to a doped tube according to the temperature that plays the role of the energetic cutoff.

For the second question (why the doped tubes have a larger hydrogen uptake) we have to understand the nature of the H2 interaction with the pure carbon and the alkali-doped nanotubes. In the case of the doped tube there is a charge transfer from the alkali metal to the tube. This charge was calculated by Mulliken population analysis to be 0.6 \e\ for the K doped tube [41]. The positively charged K atoms polarize the H2 molecules. Even though there is no charge transfer from the H2 to the K the charge induced dipole interaction gives the character of the bonding (Fig. 8). In the case of the pure tube, where the H2 interaction was calculated for comparison, there is neither charge transfer nor polarization of the H2 molecule and this results in an extremely weak interaction, under the accuracy of our theoretical level.

Comparing these results [41] with previous work for atomic hydrogen we can see physisorption of the molecular hydrogen to doped or undoped SWNTs while for the atomic hydrogen we have chemisorption. There is in agreement of our QM/MM results [41] and the semiempirical results of Dubot and Cenedese [29] indicating that the alkali metal is responsible for the adsorption of molecular hydrogen to doped tubes. Nevertheless they predict an adsorption energy of 11.5 kcal/mol [29], while we found 3.4 kcal/mol [41]. The fact that we use K while they use Li for doping the tube does not explain this large difference, which is probably due to the empirical nature of their calculations [29]. In addition, the explanation that the alkali metal act as a catalytic active center for the H2 dissociative adsorption proposed by Chen et al. [9] does not seem very possible since the alkali metal-H2 interaction is too weak to cause a H2 dissociation.

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