Adsorption of hydrogen atoms on a single graphite sheet ͑graphene͒ has been investigated by first-principles electronic structure means, employing plane-wave based periodic density functional theory. A 5 ϫ 5 surface unit cell has been employed to study single and multiple adsorptions of H atoms. Binding and barrier energies for sequential sticking have been computed for a number of configurations involving adsorption on top of carbon atoms. We find that binding energies per atom range from ϳ0.8 to ϳ1.9 eV, with barriers to sticking in the range 0.0-0.15 eV. In addition, depending on the number and location of adsorbed hydrogen atoms, we find that magnetic structures may form in which spin density localizes on a ͱ 3 ϫ ͱ 3R30°sublattice and that binding ͑barrier͒ energies for sequential adsorption increase ͑decrease͒ linearly with the site-integrated magnetization. These results can be rationalized with the help of the valence-bond resonance theory of planar conjugated systems and suggest that preferential sticking due to barrierless adsorption is limited to formation of hydrogen pairs.

No abstract

The crystal structure and thermodynamic stability of a number of proposed models for the inclusion of Ti in NaAlH 4 have been calculated by employing density functional theory in the generalized gradient approximation. It was shown that the least unfavorable location of Ti is close to the surface, replacing Al in the host lattice. Intricate complexation is simulated around the included Ti atom, and the preferred coordination number of H around Ti is eight. The Ti content was varied by the supercell approach, and even at 3 mol % Ti the resulting cell parameters were predicted to be significantly changed from the pure alanate. In addition, the most stable configurations were found to be thermodynamically metastable compared to the pure alanate and Ti standard state phases, and an ordered doped phase with significant bulk Ti content is thus ruled out by this study. It is proposed that Ti most probably works as a catalyst, situated at the surface of the alanate.