An investigation was made of the interaction of the H2 molecule with a graphene layer and with a small-radius carbon nanotube using ab initio density functional methods. The H2 could interact with materials such as graphene, graphite and nanotubes via either physisorption or chemisorption. The physisorption mechanism involved the binding of the hydrogen molecule on the material as a result of weak van der Waals forces, while the chemisorption mechanism involved the dissociation of the hydrogen molecule and the reaction of both hydrogen atoms with the unsaturated C-C bonds to form C-H bonds. In these calculations, account was taken of van der Waals interactions by using a recently developed method based upon the concept of maximally localized Wannier functions. Several adsorption sites and orientations of the hydrogen molecule relative to the carbon surface were explored and the associated binding energies and adsorption potentials were computed. The most stable physisorbed state on graphene was found to be the hollow site in the center of a carbon hexagon, with a binding energy of -48meV, in good agreement with experimental results. The analysis of diffusion pathways between different physisorbed states on graphene showed that molecular hydrogen could easily diffuse at room temperature between configurations which were separated by energy barriers as low as 10meV. Also computed were the potential energy surfaces for the dissociative chemisorption of H2 on highly symmetrical sites of graphene; the lowest activation barrier found being 2.67eV. Much weaker adsorption characterized the physisorption interaction of the H2 molecule with the small radius (2,2) carbon nanotube. The barriers to H2 dissociation on the nanotube external surface were significantly lowered with respect to the graphene case, reflecting the marked effect of the substrate curvature in promoting hydrogen dissociation.

Physisorption, Diffusion, and Chemisorption Pathways of H2 Molecule on Graphene and on (2,2) Carbon Nanotube by First Principles Calculations. Costanzo, F., Silvestrelli, P.L., Ancilotto, F.: Journal of Chemical Theory and Computation, 2012, 8[4], 1288-94