  We investigated interaction between hydrogen molecules and bare as well as
functionalized single-wall carbon nanotubes (SWNT) using first-principles plane
wave method. We found that the binding energy of the H$_{2}$ physisorbed on the
bare SWNT is very weak, and can be enhanced neither by increasing the curvature
of the surface through radial deformation, nor by the coadsorption of Li atom
that makes the semiconducting tube metallic. Though the bonding is strengthened
upon adsorption directly to Li atom, yet its nature continues to be
physisorption. However, the character of the bonding changes dramatically when
SWNT is functionalized by the adsorption of Pt atom. Single H$_{2}$ is
chemisorbed to Pt atom on the SWNT either dissociatively or molecularly. If
Pt-SWNT bond is weakened either by displacing Pt from bridge site to a specific
position or by increasing number of the adsorbed H$_{2}$, the dissociative
adsorption of H$_{2}$ is favored. For example, out of two adsorbed H$_{2}$,
first one can be adsorbed dissociatively, second one is chemisorbed
molecularly. The nature of bonding is weak physisorption for the third adsorbed
H$_{2}$. Palladium also promotes the chemisorption of H$_{2}$ with relatively
smaller binding energy. Present results reveal the important effect of
transition metal atom adsorbed on SWNT and advance our understanding of the
molecular and dissociative adsorption of hydrogen for efficient hydrogen
storage.
