Water-mediated cooperative migration of chemisorbed hydrogen on graphene

The kinetics associated with the migration of chemisorbed hydrogen on a graphene sheet is studied using density-functional theory. Chemisorbed H atoms interact strongly through the carbon sheet and each chemisorbed H atom must form a pair with a H atom bound on the opposite side of the sheet in order to lower the energy with respect to the free H2 state. The two H atoms in a pair are correlated and migrate cooperatively. Because of the strong C-H bonds, the barrier to H cooperative migration is higher than 2.0 eV. However, when mediated by H2O molecules, the barrier can be reduced to less than 0.8 eV. The H pairing up leads to distinctive behavior of graphene hydrogenation, different from H chemisorption on a graphite surface. This study also demonstrates the superior effectiveness of water activation of C-H bonds and uncovers the mystery of fast kinetics of H spillover [Y. W. Li and R. T. Yang, J. Am. Chem. Soc. 128, 8136 (2006).