Mechanistic Study on Hydrogen Spillover onto Graphitic Carbon Materials

Abstract

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.

Hydrogen transport on graphene: Competition of mobility and desorption

“…4). It clearly shows that, in agreement with earlier papers, 22,42,45 the hydrogen jump between NN carbon atoms requires the lowest energy. The energy profiles for hydrogen jumps according to PES are shown in Fig.…”

“…3. A similar double-hump profile was obtained earlier 42 by the NEB method, although the predicted energy maximum was lower (0.95 eV 42 ) than in the current study. This energy maximum was interpreted in Ref.…”

“…It is also seen that a direct hydrogen jump to the 2NN carbon requires overcoming a higher barrier than a sequence of two 1NN jumps (as suggested earlier in Ref. 42). …”

“…In some cases, the predicted barriers (1.035, 40 1.01, 16 0.98, 22 or 0.94 eV 41 ) are only slightly lower than the detrapping energy; in others, the difference is more pronounced. For example, the diffusion barrier of 0.78 eV has been predicted by DFT calculations 42 and tight-binding molecular dynamics simulations. 43 Very recently, it has been claimed that the hydrogen diffusion barrier can be as low as 0.29 eV.…”

Stability of hydrogenation states of graphene and conditions for hydrogen spillover

“…[8][9][10] In parallel, researchers have discussed the hydrogen spillover phenomena in which hydrogen atoms are claimed to be stored as chemisorption states after migration from catalytic metal sites. [11][12][13][14] …”