Density-functional calculations of the adsorption of molecular hydrogen on a planar graphene layer and on the external surface of a (4,4) carbon nanotube, undoped and doped with lithium, have been carried out. Hydrogen molecules are physisorbed on pure graphene and on the nanotube with binding energies about 80–90meV∕molecule. However, the binding energies increase to 160–180meV∕molecule for many adsorption configurations of the molecule near a Li atom in the doped systems. A charge-density analysis shows that the origin of the increase in binding energy is the electronic charge transfer from the Li atom to graphene and the nanotube. The results support and explain qualitatively the enhancement of the hydrogen storage capacity observed in some experiments of hydrogen adsorption on carbon nanotubes doped with alkali atoms.

Density functional theory has been used to study the interaction of molecular and atomic hydrogen with (5,5) and (6,6) single-wall carbon nanotubes. Static calculations allowing for different degrees of structural relaxation are performed, in addition to dynamical simulations. Molecular physisorption inside and outside the nanotube walls is predicted to be the most stable state of those systems. The binding energies for physisorption of the ${\mathrm{H}}_2$ molecule outside the nanotube are in the range 0.04–0.07 eV. This means that uptake and release of molecular hydrogen from nanotubes is a relatively easy process, as many experiments have proved. A chemisorption state, with the molecule dissociated and the two hydrogen atoms bonded to neighbor carbon atoms, has also been found. However, reaching this dissociative chemisorption state for an incoming molecule, or starting from the physisorbed molecule, is difficult because of the existence of a substantial activation barrier. The dissociative chemisorption deforms the tube and weakens the $\mathrm{C}–\mathrm{C}$ bond. This effect can catalyze the shattering and scission of the tube by incoming hydrogen molecules with sufficient kinetic energy.

Density functional calculations of palladium absorbed on graphene have been performed to study the first stages of Pd coating and/or Pd cluster formation on the graphene surface, a question of great relevance to many experiments. We have found that palladium atoms deposited on graphene have a strong tendency to form clusters. Three-dimensional clusters are more stable than planar clusters and the transition from planar to three-dimensional Pd clusters adsorbed on graphene occurs very early as a function of cluster size, at Pd4, as a consequence of the strong Pd-Pd interaction. Palladium might enhance hydrogen storage in porous materials by surface reactions. However, it is a heavy element and the formation of three-dimensional Pd clusters decreases the cluster surface and increases the cluster weight. Hence, a way should be found to prevent clusterization or to disperse finely the palladium atoms deposited on carbon materials. The adsorbed clusters are weakly magnetic, which may be of interest for some applications.