Ab initio molecular orbital study of adsorption of atomic hydrogen on graphite: Insight into hydrogen storage in carbon nanotubes

Ab initio molecular orbital (MO) calculations are performed to study the adsorption of H atoms on three faces of graphite: (0001) basal plane, (1010) zigzag edge and (1121) armchair edge. The relative energies of adsorption (or C-H bond energies) follow the order: zigzag edge > armchair edge > basal-plane edge, in agreement with previous semi-empirical MO results. However, it is found that adsorption on the basal plane sites is exothermic and stable, in contrast to previous semi-empirical results. On the edge sites, the C-H bond energy decreases by nearly 30 kcal/mol when two H atoms are adsorbed on the same site. On the basal plane, the C-H bond energy decreases from 46 kcal/mol when two H are adsorbed on alternating sites to 27 kcal/mol when they are adsorbed on two adjacent sites. Literature MO results of H adsorption on the exterior wall of SWNT are in fair agreement with that on the basal plane of graphite. The value 27 kcal/mol agrees well with experiment (23 kcal/mol) of TPD of hydrogen from MWNT. Three common features exist in the reported experiments on hydrogen storage in carbon nanotubes: slow uptake, irreversibly adsorbed species, and the presence of reduced transition metals (Fe, Co or Ni). Combined with the MO results, a mechanism that involves H ₂dissociation (on metal catalyst) followed by H spillover and adsorption (on nanotubes) is proposed for hydrogen storage in carbon nanotubes.