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 ${\mathrm{H}}_2$ 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 ${\mathrm{H}}_2\mathrm{O}$ 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)].

Figure 1

Top and side views of a graphene sheet with an embedded Pd particle, which is saturated with hydrogen. The spillover process is initiated when the H atoms migrate from the Pd particle to the carbon sheet.

Figure 2

Different mechanisms of H migration: (a) independent migration, (b) cooperative migration without water, (c) and (d), cooperative migration mediated by one and two ${\mathrm{H}}_2\mathrm{O}$ molecules. All values are numbered in eV, referenced to the free ${\mathrm{H}}_2$ state [dashed line in (d)]. See the text for a detailed description.

Figure 3

Migration pathways of an H pair along the rim of an H16 island. The fixed H atoms in the island are shown by solid and open circles, whereas the trajectory of the H pair is shown by the arrow-headed broken lines with their positions and positive (negative) sign marked by integers in black (gray). (a) Cooperative migration in a step-by-step mode, mediated by two ${\mathrm{H}}_2\mathrm{O}$ olecules. (b) Striding mediated by multiple water molecules (indicated below the dashed line). (c) and (d), the formation energy of the H pair and the migration barriers, in correspondence to, respectively, (a) and (b). All values are numbered in eV, referenced to the energy level of free ${\mathrm{H}}_2$ (dashed lines).

Figure 4

Partial charge density of the transition states in H migration: (a),(b), and (c) are the transition states for the steps shown, respectively, in Fig. 2, Fig. 2, and Fig. 3 $(1,4)\to (1,5)$. Red, white, and dark cyan balls denote, respectively, O, H, and C atoms. Contours are plotted in the energy range $[-0.8,0]\mathrm{eV}$ referenced to the Fermi levels. The numbers (in Å) show the OH distance relative to the standard O-H bond length in a ${\mathrm{H}}_2\mathrm{O}$ molecule.