Physisorption, Diffusion, and Chemisorption Pathways of H2 Molecule on Graphene and on (2,2) Carbon Nanotube by First Principles Calculations.

We investigate the interaction of the H2 molecule with a graphene layer and with a small-radius carbon nanotube using ab initio density functional methods. H2 can interact with carbon materials like graphene, graphite, and nanotubes either through physisorption or chemisorption. The physisorption mechanism involves the binding of the hydrogen molecule on the material as a result of weak van der Waals forces, while the chemisorption mechanism involves the dissociation of the hydrogen molecule and the ensuing reaction of both hydrogen atoms with the unsatured C-C bonds to form C-H bonds. In our calculations, we take into account van der Waals interactions using a recently developed method based on the concept of maximally localized Wannier functions. We explore several adsorption sites and orientations of the hydrogen molecule relative to the carbon surface and compute the associated binding energies and adsorption potentials. The most stable physisorbed state on graphene is found to be the hollow site in the center of a carbon hexagon, with a binding energy of -48 meV, in good agreement with experimental results. The analysis of diffusion pathways between different physisorbed states on graphene shows that molecular hydrogen can easily diffuse at room temperature from one configuration to another, which are separated by energy barriers as small as 10 meV. We also compute the potential energy surfaces for the dissociative chemisorption of H2 on highly symmetric sites of graphene, the lowest activation barrier found being 2.67 eV. Much weaker adsorption characterizes instead the physisorption interaction of the H2 molecule with the small radius (2,2) CNT. The barriers for H2 dissociation on the nanotube external surface are significantly lowered with respect to the graphene case, showing the remarkable effect of the substrate curvature in promoting hydrogen dissociation.