1,720,975 research outputs found
A Theoretical Study of the Electronic Structure of O2 Interstitial Impurities in Silica
No full textThe electronic and molecular structure of interstitial O2 in silica (SiO2) has been theoretically investigated by coupling the molecular cluster model to density functional theory. Calculations include the evaluation of optimised geometrical parameters, guest–host interaction energies and impurity vibrational frequencies. Results indicate that the inclusion of O2 in SiO2 is an endothermic process scarcely affecting the impurity electronic and structural properties
A Theoretical Study of the Interaction of CO2 with Hydroxylated α-Alumina
No full textDensity functional molecular cluster calculations have been used to investigate the reaction of carbon dioxide (CO2) with hydroxylated α-alumina. The substrate has been modeled by considering the α-Al2O3(0001) surface, and the formation of bidentate chelating carbonate (BCC) and monodentate hydrogencarbonate (MHC) species has been considered. Adsorbate geometries, chemisorption enthalpies, and adsorbate vibrational parameters are computed and discussed. In agreement with experimental evidences [M. Casarin, D. Falcomer, A. Glisenti, A. Vittadini, Inorg. Chem. 42 (2003) 436], the most stable product is found to be a BCC surface complex
Interstitial O3 in Silica: a Molecular Cluster Density Functional Study
No full textThe electronic and molecular properties of interstitial O3 in SiO2 have been theoretically studied by coupling the molecular cluster model to the density functional theory. We find that, on passing from the free species to the interstitial one, electronic and molecular structures of O3 are only slightly perturbed. Moreover, in agreement with the experimental assignment of Skuja et al., it is confirmed that the ubiquitous absorption band at 4.8 eV characterizing 7.9 eV photon-irradiated SiO2 samples includes a contribution due to excitations between O3 based occupied and unoccupied MOs
Experimental and Theoretical Study of the Interaction of CO2 with α-Al2O3
No full textDensity functional molecular cluster calculations are combined with X-ray photoelectron spectroscopy (XPS), quadrupolar mass spectrometry (QMS), and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy to investigate the interaction of CO2 with α-Al2O3 and partially reduced α-Al2O3. The electronic structure of the stoichiometric and partially reduced substrate, adsorbate geometries, chemisorption enthalpies, and adsorbate vibrational parameters are computed and discussed. Theoretical results agree quite well with experimental data and previous theoretical investigations. As far as the adsorbate−substrate interaction is concerned, the results of our calculations indicate that CO2 forms bidentate-chelating carbonate species. The bonding scheme of this surface complex implies a significant substrate → adsorbate transfer of charge (from the occupied dangling bond of a surface Lewis base site into one component of the CO2 2πu LUMO) assisted by a definitely weaker adsorbate → substrate donation (from one component of the CO2 1πg HOMO into an empty dangling bond of a surface Lewis acid site). Our estimate of the chemisorption enthalpy (−15 kcal/mol) agrees quantitatively with calorimetric data reported for CO2 adsorbed on high surface area α-alumina (−16.0 kcal/mol). [Mao, C.-F.; Vannice, M. A. Appl. Catal. A 1994, 111, 151.] According to XPS and QMS outcomes, theoretical results predict that the interaction of CO2 with partially reduced α-Al2O3 gives rise to the reduction of the adsorbate to CO and to the concomitant substrate reoxidation
Photocatalytic activity of TiO2 doped with boron and vanadium
No full textBoron (B)- and vanadium (V)-doped TiO2 photocatalysts were synthesized using modified sol-gel reaction processes and characterized by X-ray diffraction (XRD), Raman spectroscopy and N-2 physisorption (BET). The photocatalytic activities were evaluated by monitoring the degradation of methylene blue (MB). The results showed that the materials possess high surface area. The addition of B favored the transformation of anatase to rutile, while in the presence of V, anatase was the only phase detected. The MB degradation on V-doped TiO2 was significantly affected by the preparation method. In fact while the presence of V in the bulk did not influence strongly the photoreactivity under visible irradiation, an increase of surface V doping lead to improved photodegradation of MB. The degradation of MB dye indicated that the photocatalytic activities of TiO2 increased as the boron doping increased, with high conversion efficiency for 9 mol% B doping
The Different Supramolecular Arrangements of the Triangular [Cu3(μ3-OH)(μ-pz)3]2+ (pz = Pyrazolate) Secondary Building Units. Synthesis of a Coordination Polymer with Permanent Hexagonal Channels
No full textBy reaction of the trinuclear triangular copper(II) complex [Cu3(μ3-OH)(μ-pz)3(MeCOO)2(Hpz)] (Hpz = pyrazole), 1b, with aqueous HCl, four different crystalline species were formed and recovered through fractional crystallization. In order, the hexanuclear dicationic [{Cu3(μ3-OH)(μ-pz)3(Hpz)2}2(μ-MeCOO)2](Cl)2·2H2O, 2, the mononuclear [CuCl2(Hpz)4], 3, the heptanuclear neutral [{Cu3(μ3-OH)(μ-pz)3(Cl)2(Hpz)2(H2O)}2{CuCl2(Hpz)2}], 4, and the hexanuclear neutral [{Cu3(μ3-OH)(μ-pz)3(Cl)(Hpz)3}2(μ-Cl)2]·H2O, 5, complexes were obtained. Compounds 2, 4, and 5 all maintain the [Cu3(μ3-OH)(μ-pz)3]2+ core; nevertheless, they exhibit relevant differences in their molecular structures and supramolecular arrangements. In compound 2 a hexanuclear cluster, based on two monodentate acetate groups bridging two [Cu3(μ3-OH)(μ-pz)3(Hpz)2] units and clearly reminiscent of the structure of 1b, was observed, while the sequential replacement of the acetates by chloride ions generated 4 and 5. Although these two compounds were formed according to the same stoichiometry, they are characterized by very different molecular and supramolecular structures. The hexanuclear species 5 arranged, through hydrogen bonds, into a 3D, nonporous metal−organic framework (MOF), while the heptanuclear species 4 self-assembled through Cu−Cl bridges, giving rise to a 3D MOF having permanent hexagonal, star-shaped, parallel channels. The internal free diameter of these channels is about 4 Å, leading to a free space corresponding to ca. 9% of the total crystal volume
Lanthanide-doped strontium barium niobate (SBN) materials: A vibrational investigation
No full textVibrational Raman spectra have been measured for strontium barium niobate (Sr0.5Ba0.5Nb2O6, SBN50) single crystals and nanopowders doped with 1 mol% of the luminescent ions Eu3+ and Er3+. The nanocrystalline materials show slightly broader spectra with respect to the single crystals with the same composition. The presence of the Eu3+ and Er3+ ions at the 1 mol% doping level, and the present particle size (200 nm) do not appear to affect the ferro-to-paraelectric phase transition temperature typical of undoped SBN50 single crystals, as detected from anomalies in the dependence of the position of the Raman peak around 635 cm(-1) as a function of the temperature
Photocatalytic, spectroscopic and transport properties of lanthanide-doped TiO2 nanocrystals
No full textThe photocatalytic properties of the anatase form of nanocrystalline TiO2 doped with lanthanide ions (Eu3+,Sm3+ and Er3+), obtained by a sol–gel preparation method, are studied by means of the degradation of methylene blue. It is observed that the presence of Sm3+ ion as a dopant significantly improves the photocatalytic activity of anatase TiO2 with respect to the Eu3+ or Er3+ ions.For the understanding of such a catalytic process the charge transport in these systems is studied by an appropriate Monte Carlo simulation which includes phonons, charged impurities and traps. The obtained results evidence the important role of the traps for the transit of the electrons in the systems investigated here. They decrease the diffusion coefficient by about three orders of magnitude with respect to the bulk, from which it is possible to obtain a simple estimate of the reaction rate in agreement with experimental results. All the lanthanide-doped samples show a strong luminescence in the visible region. The emission bands are affected by a notable inhomogeneous broadening, indicating a high disorder of the crystalline environment of the lanthanide ions in the titania host
Synthesis, Structural Characterization and Optical Spectroscopy of Nanocrystalline Lanthanide doped Gd3Ga5O12
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