Eleonora Spanó

Università degli Studi di Milano-Bicocca, Milano, Lombardy, Italy

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Publications (11)38.17 Total impact

  • Eleonora Spanó · Gloria Tabacchi · and Aldo Gamba · Ettore Fois
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    ABSTRACT: The ethylene epoxidation cycle in a H2O2/H2O-loaded Ti zeolite has been simulated by a Car-Parrinello approach. Results indicate a process where the zeolitic framework is the active oxygen mediator. The dissociative chemisorption of H2O2 leads, via a transient Ti-hydroperoxo species, to H2O and a Ti-peroxo zeolite intermediate. Transfer of active oxygen to ethylene follows, giving the epoxide and recovering the catalyst. A thorough theoretical characterization indicates that the active oxidizing species is an asymmetric eta2-Ti-peroxo, absorbing in the visible range. The lability of the intermediate is found related to eta2 <--> eta1 interconversions of the Ti-peroxo structure. The interconversions, triggered by water molecules, could account for the experimentally found reduced catalytic activity in aged TS-1 catalysts. The results provide a microscopic picture of the reactivity and dehydration/aging processes of the catalyst fully consistent with experiments and highlight the fundamental role of the Lewis acid character of Ti in the formation, reactivity, and degradation of the active oxidizing species.
    No preview · Article · Nov 2006 · The Journal of Physical Chemistry B
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    ABSTRACT: The excitation energies of small ZnS nanoclusters characterized in previous studies have been calculated using TDDFT. The relativistic pseudopotentials of Stevens et al have been used, including Zn 4s2 electrons and S 3s2 and 3p4 electrons as valence electrons. Results obtained with these pseudopotentials are compared to those obtained considering also Zn 3s23p63d10 electrons in the valence part, and demonstrated to be consistent. The results show that spheroid-like bubble structures have absorption energies in the range of 5–5.3 eV for small sizes, which decreases to 5 eV with increasing particle size.
    No preview · Article · Aug 2006 · Nanotechnology
  • E. Spanò · M. Bernasconi · E. Kopnin
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    ABSTRACT: Based on density functional perturbation theory, we predict that hole-doped MgB2C2, which is isoelectronic and structurally similar to MgB2, has a strong electron-phonon coupling constant. By substituting Mg atoms with alkali metals (Li,Na), pristine insulating MgB2 turns metallic with holes in the σ bands at the Fermi level. Calculation of the formation enthalphies show that hole-doped LixMg(1−x)B2C2 or NaxMg(1−x)B2C2 for x=0.125–0.25 might be synthesized experimentally under conditions of Mg deficiencies. We find that the contribution of the σ bands to the electron-phonon coupling constant of Li0.125Mg0.875B2C2 is Λσσ=0.91, due to the modulation of the σ bands produced by stretching modes of the borocarbide hexagonal planes. Based on ab initio phonons and electron-phonon coupling constants, we estimate from the McMillan formula a value for superconductive Tc of 67 K (for μ*=0.1), higher than that of MgB2 mainly because of larger phonon frequencies (ωln=777 cm−1).
    No preview · Article · Jul 2005 · Physical Review B
  • E. Spanò · M. Bernasconi
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    ABSTRACT: Based on density-functional theory and density-functional perturbation theory we have studied the thermodynamical and vibrational properties of Mg(AlH4)2. The crystal structure recently proposed on the basis of x-ray powder-diffraction data has been confirmed theoretically by the comparison of the experimental and theoretical IR and Raman spectra. The main discrepancy regards the position of the hydrogen atoms which makes the theoretical AlH4 tetrahedra more symmetric than that experimental inferred from data. The calculated thermodynamical decomposition temperature is also in good agreement with the experimental result.
    No preview · Article · May 2005 · Physical Review B
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    ABSTRACT: Earlier studies have shown that the most stable structures for (ZnS)n clusters with n = 10-47 are hollow polyhedral clusters ("bubbles"). We report a detailed study of larger clusters, where n = 50, 60, 70, and 80, for which onionlike or "double bubble" structures are predicted. We report calculations of the vibrational spectra and the electronic structure of bubble and double bubble clusters, which may assist in their experimental identification.
    No preview · Article · Mar 2005 · The Journal of Physical Chemistry B
  • Ettore Fois · and Aldo Gamba · Eleonora Spanó
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    ABSTRACT: A combined Car−Parrinello molecular dynamics blue moon sampling approach has been adopted to study the competitive attack of H2O and H2O2 at a tetracoordinated Titanium site in a Ti−zeolite. The results indicate that, although the attack of water to form a trigonal bipyramidal center is thermodynamically more stable, the attack of hydrogen peroxide to form a similar adduct is kinetically favored. In both cases, solvent cooperation is effective in the formation of the adducts. The relevance of such a result in relation to the catalytic properties of Ti−zeolites is discussed.
    No preview · Article · Jun 2004 · ChemInform
  • Eleonora Spanó · Said Hamad · C Richard A Catlow
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    ABSTRACT: Following recent studies which showed that the most stable structures for (ZnS)(n) clusters (n= 10-47) are the so-called "bubble clusters", in which all the atoms are three-coordinated, we have used simulated annealing techniques to find the most stable structure for a larger cluster, (ZnS)(60). We find an onion-like structure, with one small cluster enclosed inside a bigger one. The inner cluster has the structure of a sodalite cage. Bonding between the inner and the outer clusters creates a network of four-coordinated atoms.
    No preview · Article · May 2004 · Chemical Communications
  • Ettore Fois · and Aldo Gamba · Eleonora Spanò
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    ABSTRACT: We present the results of ab initio molecular dynamics simulations on titanium offretite. The aim of this work is to investigate at a microscopic level the room temperature behavior of titanium inserted into a fully periodic zeolitic framework in dry and hydrated conditions. Structural analysis indicates that titanium, at low water loading, is tetrahedrally coordinated by framework oxygens only. On the other hand, at higher loading, when water molecules are activated via hydrogen bonding, they interact with the Ti site, leading to an increase of the Ti coordination number up to 5. The insertion of a Ti center causes only moderate and local distortions to the silicate framework. Features due to Ti in the calculated vibrational spectra are in line with IR and Raman experiments.
    No preview · Article · Nov 2003 · The Journal of Physical Chemistry B
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    Eleonora Spanó · Said Hamad · C. Richard A. Catlow
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    ABSTRACT: Energy minimized structures of (ZnS)n clusters, with n ranging from 10 to 47, have been generated using simulated annealing techniques based on interatomic potentials. The clusters have “bubble” like polyhedral structures, in which all of the atoms are three coordinated, rather than the densely packed bulklike structures. Density functional theory calculations also show that the bubble clusters are more stable than the bulklike clusters.
    Full-text · Article · Aug 2003 · The Journal of Physical Chemistry B
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    Ettore Fois · Aldo Gamba · Eleonora Spano · Gloria Tabacchi
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    ABSTRACT: A simulation study on the rotational behavior of neutral and charged molecular systems hosted in zeolitic cages is presented here. Synthetic sodalites containing both molecular (O2) and anionic (NO2−, NO3−) species and an hydrated Ti-offretite have been simulated by first-principles molecular dynamics. Our results show that both host–guest and guest–guest interactions directly influence the rotational motion of the caged species.
    Full-text · Article · Jan 2003 · Journal of Molecular Structure
  • Ettore Fois · Aldo Gamba · Eleonora Spanò
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    ABSTRACT: We present the results of ab initio molecular dynamics simulations applied to ‘all-silica sodalite’ [Si12O24], modified by doping with P or As atoms, to investigate the possible technological interest of the new systems. The doping represents an alternative technique to insert electrons into a zeolitic cavity. The simulation of radial distribution functions, coordination numbers, bond distances and bond angles variations, together with electron densities, electronic density of states and vibrational spectra has revealed that these systems could be of possible interest as new optical materials.
    No preview · Article · Jan 2001 · Physical Chemistry Chemical Physics