Molecular and dissociative adsorption of multiple hydrogen molecules on transition metal decoratedC60

Abstract

Recently we have predicted [Phys. Rev. Lett. May 2005] that Ti-decorated carbon nanotubes can absorb up to 8-wt% hydrogen at ambient conditions. Here we show that similar phenomena occurs in light transition-metal decorated C60. While Sc and Ti prefers the hexagon (H) sites with a binding energy of 2.1 eV, V and Cr prefers double-bond (D) sites with binding energies of 1.3 and 0.8 eV, respectively. Heavier metals such as Mn, Fe, and Co do not bond on C60. Once the metals are absorbed on C60, each can bind up to four hydrogen molecules with an average binding energy of 0.3-0.5 eV/H2. At high metal-coverage, we show that a C60 can accommodate six D-site and eight H-site metals, which can reversible absorb up to 56 H2 molecules, corresponding to 7.5 wt%.PACS numbers: 61.46.+w,84.60.Ve,81.07.De An efficient storage media for hydrogen is crucial for the advancement of hydrogen and fuel-cell technologies 1 . There have been a great number of reports on the search for new routes to engineer nanomaterials so that (a) they dissociate H 2 molecules into H atoms and (b) reversibly adsorb hydrogen molecules at ambient conditions 1,2,3,4,5,6,7,8,9 . Much effort has been focused on the engineering of carbon-based materials such as nanotubes 10,11 and metal hydrides such as alanates 12 . It is found that while hydrogen-carbon interaction is too weak 10 , the metal-hydrogen interaction is too strong for hydrogen storage at ambient conditions. Very recently we have shown 13 a novel way to overcome this difficulty by forming artificial metal-carbide like structures on carbon singled-wall nanotubes (SWNT). From accurate first-principles calculations, we show that a single Ti-atom adsorbed on a SWNT can strongly bind up to four hydrogen molecules 13 . At large Ti coverage we find that a (8,0) SWNT can store hydrogen molecules up to 8-wt%, exceeding the minimum requirement of 6-wt% for practical applications. Even though these results were totally unexpected, we explained them by a simple Dewar, Chatt, and Duncanson (DCD) model 14,15 , where the interaction is due to donation of charge from the highest occupied orbital of the ligand to the metal empty states and a subsequent back donation from filled d-orbitals into the lowest unoccupied orbital of the ligand.Here we show that similar phenomena also occurs in light-transition metal decorated C 60 molecules. Below we first discuss several possible absorption sites for a single Ti atom on a C 60 molecule. We then show how a single Ti atom on a C 60 can bind up to four hydrogen molecules via Kubas interaction 14,15 . Multiple metal coverage cases, yielding up to 8 wt% hydrogen absorption, are discussed next. Finally, we briefly discuss the results for other transition metals from Sc to Co.The energy calculations were performed within the plane-wave implementation 16 of the generalized gradient approximation 17 to DFT. We used Vanderbilt ultra- soft pseudopotentials 18 treating the following electronic states as valence: Ti: 3s 2 3p 6 3d 2 4s 2 , C: 2s 2 2p 2 and H: 1s. The cutoff e...

On the structure and normal modes of hydrogenated Ti-fullerene compounds

“…We discarded the case of the bond with a pentagonal ring, since the calculation predicts one imaginary frequency. These results disagree with those obtained by Yildirim et al (2005) that using the supercell and a pseudopotential approximation, reported that the Ti adsorbed on the hexagonal ring is the most stable one. Nevertheless, our results are consistent with the known capacity of transition metals to bind to the (6,6) bond (Mathur et al 1998;Shin et al 2006).…”

“…The number of studies of new nanostructures (Zhao et al 2005;Iñiguez et al 2007;Yildirim et al 2005;Bauschlicher et al 2002;Zope et al 2009;Zhang et al 2000;Yoon et al 2008;Yang et al 2009;Lee et al 2008;Durgun et al 2008;Koh et al 2011, Kiran et al 2006 able to absorb H 2 in at least 9 wt% have increased in the last years, following the goal proposed by the US Department of Energy of developing a system with this net gravimetric capacity for hydrogen storage. Among the different materials under consideration, systems composed by C 60 compounds coated with transition metals are between the most promising ones.…”

Titanium-Decorated Carbon Nanotubes as a Potential High-Capacity Hydrogen Storage Medium

“…Such metals are known to interact with the σ * -antibonding of the hydrogen molecules strongly, destabilizing dihydrogen structure against classical hydride formation. Our preliminary results based on a structural optimization starting from t80TiH 2 -3H 2 with Ti replaced by Pt/Pd indeed indicate that the side hydrogen molecules are not bonded and leave the system immediately [24]. We did observe that two H 2 do indeed bind to Pt/Pd forming a PtH 4 (or PdH 4 ) classical hydride cluster, which was not bonded to the nanotube.…”

“…However we expect the light transition metals like Sc and V to show similar behavior since they have both occupied and empty d-orbitals with a similar IP to Ti. We are currently investigating a large number of transition metals and the results will be published elsewhere [24].…”

Transition-Metal-Ethylene Complexes as High-Capacity Hydrogen-Storage Media

“…Recently, a novel concept to overcome these obstacles has been suggested [7][8][9][10][11][12][13][14]. It was predicted that a single Ti atom affixed to carbon nanostructures, such as C 60 or nanotubes, strongly adsorbs up to four hydrogen molecules [7,8,10].…”