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Rioual S.,CNRS Brittany Magnetism Laboratory | Lescop B.,CNRS Brittany Magnetism Laboratory | Quentel F.,University of Western Brittany | Gloaguen F.,University of Western Brittany
Physical Chemistry Chemical Physics | Year: 2015

An electrocatalytic material for the H2 evolution reaction (HER) in acidic aqueous solution has been prepared by electropolymerization of Co(ii) dibenzotetraaza[14] annulene (CoTAA). Chemical analysis by X-ray photoelectron spectroscopy (XPS) confirms that the structural integrity of the [CoII-N4] motif is preserved in the poly-CoTAA film. In acetate buffer solution at pH 4.6, an overpotential η = -0.57 V is required to attain a catalytic current density -ik = 1 mA cmgeom-2. The faradaic efficiency of poly-CoTAA for the HER is 90% over a period of one hour of electrolysis, but there is a decrease of the apparent concentration of Co sites after prolonged H2 production, which we ascribe to partial demetallation of the poly-CoTAA film at negative potentials. This journal is © the Owner Societies 2015.

Lebon A.,CNRS Brittany Magnetism Laboratory | Garcia-Fuente A.,University of Valladolid | Vega A.,University of Valladolid | Aguilera-Granja F.,Autonomous University of San Luis Potosi
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

We report a theoretical density functional theory study of the absorption and adsorption of hydrogen in Pd core/Pt shell cubo-octahedral nanoparticles of 55 and 147 atoms. We have explored all the possible sites available for hydrogen loading. The relative stability of H in the different sites is calculated and discussed in terms of the local geometrical and chemical environments. As a general trend, the most stable absorption sites are pyramidal interstices at the interface offering a volume for H around 2×2.60 3, with a moderate atomic rearrangement upon H insertion and in which the H atom maximizes the number of Pd neighbors. Our theoretical findings give support to the recent hydrogen pressure-composition isotherms and NMR measurements carried out in this type of bimetallic nanoparticle. © 2011 American Physical Society.

Lebon A.,CNRS Brittany Magnetism Laboratory | Aguado A.,University of Valladolid | Vega A.,University of Valladolid
Physical Chemistry Chemical Physics | Year: 2015

We demonstrate, by means of fully unconstrained density functional theory calculations, that cluster Zn17 endohedrally doped with a Cr impurity can be qualified as a magnetic superalkali cluster. We explain the origin of its high stability, its low vertical ionization potential and its high total spin magnetic moment which amounts to 6 μB, exactly the same value as that of the isolated Cr atom. With the aim of exploring the possibility of designing a bistable magnetic nanoparticle, with a corresponding inter-unit exchange coupling, we also consider the assembling of two such units through different contact regions and in different magnetic configurations. Furthermore, we analyze up to which extent is the Zn shell able to preserve the electronic properties of the embedded Cr atom, both against coalescence of the two superatoms forming the magnetically bistable nanoparticle, and upon the adsorption of an O2 molecule or even under an oversaturated O2 atmosphere. Our results are discussed not only emphasizing the fundamental physical and chemical aspects, but also with an eye on the new prospects that those Cr@Zn17 magnetic superalkali clusters (and others of similar kind) may open in spintronics-, molecular electronics- or biomedical-applications. © 2015 Owner Societies.

Lebon A.,CNRS Brittany Magnetism Laboratory | Aguado A.,University of Valladolid | Vega A.,University of Valladolid
Journal of Physical Chemistry C | Year: 2015

A systematic theoretical study of the doping of Zn17 nanoparticle with 3d transition-metal (TM) impurities is presented. Calculations are conducted within the density functional framework as implemented in the VASP code at the generalized gradient approximation for the exchange and correlation effects. The ground state of Zn17 has been recently shown to be a dihedral superatom hollow cage in a singlet state [ Angew. Chem., Int. Ed. 2015, 54, 2111 ]. We show that, among the 3d elements, endohedral doping of a quasi-undeformed cage results favorable only for Cr and Cu, which as free atoms display a very stable (half-filled and fully occupied, respectively) d shell electron configuration. This structural configuration allows to maximize the spin magnetic moment of the chromium-doped cage. Co and Ni also adopt an endohedral configuration, but with a strong deformation of the cage and a rather compact structure due to the large electron hybridization between the TM states and those of the Zn host. Doping with the other TM impurities results in an exohedral arrangement, also concomitant with a marked electron hybridization. Most of the TM impurities retain a large part of the spin magnetic moment that they have in a vacuum due to localization of part of their 3d states. The Zn atomic environment is found to be spin-polarized and modulates the total spin moment of these molecular magnets depending on the induced moments and magnetic couplings. © 2015 American Chemical Society.

Lebon A.,CNRS Brittany Magnetism Laboratory | Garcia-Fuente A.,University of Valladolid | Vega A.,CNRS Brittany Magnetism Laboratory | Vega A.,University of Valladolid | Aguilera-Granja F.,Autonomous University of San Luis Potosi
Journal of Physical Chemistry C | Year: 2012

A theoretical study of hydrogen deposition and insertion in bimetallic Pd-Pt nanoalloys has been carried out in the framework of the density functional theory. Our model systems are 147-atom clusters of cuboctahedral shape and stoichiometries of Pd 1.00, Pd 0.91Pt 0.09, Pd 0.82Pt 0.18, and Pd 0.54Pt 0.46, resembling those recently produced and characterized in the context of H insertion (Kobayashi, H. et al. J. Am. Chem. Soc.2010, 132, 5576.). Adsorption and absorption energies have been computed for H in all stable positions for the different compositions. Absorption is found to improve for nanoalloys with a small Pt concentration of 8-20%. However, when the Pt content approaches 50%, the absorption capability worsens, even as compared with the pure Pd nanoparticles. These trends are fully consistent with the experimental data. The local geometrical and electronic environments of hydrogen in these alloy nanoparticles are explored in detail so as to understand the observed behavior. Concerning the H deposition, our results indicate that Pd-Pt nanoalloys with high Pt content might be efficient in regard to preferential oxidation reaction mediated by hydrogen. © 2011 American Chemical Society.

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