  We report a first-principles study, which demonstrates that a single Ti atom
coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules.
The first H$_2$ adsorption is dissociative with no energy barrier while other
three adsorptions are molecular with significantly elongated H-H bonds. At high
Ti coverage we show that a SWNT can strongly adsorb up to 8-wt% hydrogen. The
system is quite stable and exhibits associative desorption upon heating, a
requirement for reversible storage. These results advance our fundamental
understanding of dissociative adsorption of hydrogen in nanostructures and
suggest new routes to better storage and catalyst materials.
