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
Résumé
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 H-2 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 (H-2)(y)(H)(x)M(eta(6)-C6H6) orbital molecular models. Two expected coordination modes of H-2 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). E-b 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 d(yz)/sigma(u)* interaction. Moreover, in the case of the K mode, pi 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 eta(6) to eta(2) positions. Systems containing Sc, Ti, Co, and Fe are good candidates for hydrogen storage.