Interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes (SWNT) is investigated using first-principles plane wave method. It is found that the binding energy of the ${\mathrm{H}}_2$ physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic. Although the bonding is strengthened upon adsorption directly to the Li atom, its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of a Pt atom. A single ${\mathrm{H}}_2$ is chemisorbed to the Pt atom on the SWNT either dissociatively or molecularly. The dissociative adsorption is favorable energetically and is followed by the weakening of the Pt-SWNT bond. Out of two adsorbed ${\mathrm{H}}_2$, the first one can be adsorbed dissociatively and the second one is chemisorbed molecularly. The nature of bonding is a very weak physisorption for the third adsorbed ${\mathrm{H}}_2$. Palladium also promotes the chemisorption of ${\mathrm{H}}_2$ with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and these results advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.

• Received 12 March 2005

DOI:https://doi.org/10.1103/PhysRevB.72.155404