Hubert Valencia - Trends in the Hydrogen Activation and Storage by Adsorbed 3d Transition Metal Atoms onto Graphene and Nanotube Surfaces: A DFT Study and Molecular Orbital Analysis

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      Hubert Valencia, Adria Gil, and Gilles Frapper, J. Phys.   Chem. C, 2015,   119 (10), pp 5506–5522   (2015).


      The hydrogen activation by functionalized graphene and (8,0)   single-walled carbon nanotubes (SWCNTs) with individual 3d   transition metal atoms was modeled using density functional   theory calculations. The metal center saturation by hydridic   atoms and/or activated H2 molecules was evaluated   along the 3d series (M = Sc–Ni). The structural geometry,   magnetism, and binding energies were analyzed in terms of the   density of states, Bader charges, and organometallic   (H2)y(H)xM(η6-C6H6)   orbital molecular models. Two expected coordination modes of   H2 were localized, the dissociated dihydride (D) and   the molecular Kubas coordination (K), on metal centers adsorbed   onto two graphitic-based supports, graphene and (8,0) SWCNT.   Their corresponding binding energies (Eb)   were computed and compared at the PW91/PAW-plane wave level of   theory in periodical conditions. For graphene,   Eb in the D mode increases on the left and on   the right of Cr. This D mode is the preferred mode for the most   electropositive atoms (Sc and Ti). Eb for K   mode increases from Sc to Ni, except for Cr. These trends within   the row are explained by the shape, the size, and the energy of   metal atomic orbitals and are related to the stabilization of the   1b1 MO because of the   dyzu* interaction. Moreover, in   the case of the K mode, π back-donation plays also an important   role to explain this behavior. No significant variation of trends   was observed when going from graphene to SWCNT. In some cases, a   new kind of coordination mode appears: the so-called lengthened   mode with the H–H distance longer than K mode but shorter than D   mode. Finally, maximum capacity for hydrogen storage at the metal   center was studied, considering metal diffusion from   η6 to η2 positions. Systems containing Sc,   Ti, Co, and Fe are good candidates for hydrogen storage.

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