Journal
CARBON
Volume 40, Issue 3, Pages 437-444Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/S0008-6223(01)00199-3
Keywords
graphite; carbon nanotubes; adsorption; modeling; adsorption properties
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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, dissociation (on metal catalyst) followed by H spillover and adsorption (on nanotubes) is proposed for hydrogen storage in carbon nanotubes. (C) 2002 Elsevier Science Ltd. All rights reserved.
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