Mechanistic study on hydrogen spillover onto graphitic carbon materials

We present a systematic study on the possible mechanisms of hydrogen spillover onto several carbon-based materials using density functional theory (DFT). Adsorption and diffusion of atomic hydrogen on a graphene sheet, single-walled carbon nanotubes, and a polyaromatic compound, hexabenzocoronene, were calculated, and the potential energies along the selected adsorption and diffusion minimum energy pathways were mapped out. We show that the migration of H atoms from a Pt cluster catalyst to the substrates is facile at ambient conditions with a small energy barrier, although the process is slightly endothermic, and that the H atoms can be either physisorbed or chemisorbed on carbon surfaces. Our results indicate that diffusion of H atoms in a chemisorbed state is energetically difficult since it requires C-H bond breaking and hydrogen spillover would occur likely via physisorption of H atoms. The curvature of the carbon materials is found to have a pronounced influence on the mobility of H atoms. The role of the "bridge" materials used in experiments is also discussed.