1,720,985 research outputs found
ZEOLITI
No full textZEOLITI. Definizione e proprietà delle zeoliti/Zeoliti naturali e sintetiche: struttura chimica e cristallografica/Sintesi e caratterizzazione mediante metodi chimico-fisici/Applicazioni tecnologiche/Conclusioni/Bibliografia
Charge distribution and local and non-local screening effects studied by means of the Auger parameter and chemical state plots
No full textWe show that the Auger parameter can be helpful in the analysis of the screening mechanism in transition metal compounds. The position in the chemical state plots of different classes of compounds is discussed. The constancy of the Auger parameter for Cu(II) compounds is interpreted as further evidence of the electronic nature of the 2p3/2 final state. For Cu(I) compounds, as in the case of Zn(II) compounds (d10 electronic configuration), a large dependence of the Auger parameter on the chemical nature of the ligands suggests a non-local screening mechanism in the final state, i.e. a strong influence of electronic polarizability of the ligands on the relaxation energy. In this case, the extended s-p cation orbitals are involved in the screening process. The charge distribution in the initial state can be studied analytically or by using chemical state plots. It appears that the Auger parameter and the chemical state plots permit an accurate characterization of the ground-state electronic structure of cations in transition element compounds and of the screening mechanism in the final state
Auger parameter and chemical state plots for copper-containing and zinc containing compounds-charge distribution and screening effects
No full textCu- and Zn-containing compounds are characterised making use of the Wagner Auger parameter and of the so-called chemical state plots to obtain information on the relative initial-state charge distributions and on the relative extra-atomic relaxation energy. The position in the plots of different classes of compounds is discussed. To explain the constancy of the Auger parameter for Cu(II) compounds and metallic Cu, it is suggested that the energy involved in the electron transfer 2p53d10L is similar to the screening energy in the metallic state
APPLICATION OF THE AUGER PARAMETER IN THE CHARACTERIZATION OF SMALL COPPER PARTICLES SUPPORTED ON INSULATORS
No full textA simple electrostatic model permits the estimation of Auger parameter shifts of core-ionized atoms in compounds with respect to the free atom in the gas phase. It is shown that the Auger parameter shift is a function of the number, distance, electronic polarizability and local geometry of the nearest-neighbour ligands around the core-ionized atom. We have investigated the relationship between the Cu Auger parameter shifts and the nuclearity of Cu clusters entrapped in A zeolites. It is shown that our model is useful to estimate this dependence. The model has also been applied with success to rationalize Auger parameter data on small copper clusters supported on alpha-Al2O3 (0001) surfaces and polycrystalline ZnO and graphite. We demonstrate that the screening properties of metal bulk and clusters of very low nuclearity are very similar
New advancements in the theory of the Auger parameter: Applications to the characterization of small metallic particles
No full textA simple electrostatic model permits the calculation of the Auger parameter shifts of core-ionized atoms in compounds with respect to the free atom in the gas phase. It is shown that the Auger parameter shift is a function of the number, distance, electronic polarizability and local geometry of the nearest-neighbour ligands around the core-ionized atom. We have investigated the relationship between the Pd Auger parameter shifts and the nuclearity of Pd clusters entrapped in Y zeolites. It is shown that our model is useful to estimate this dependence. It is demonstrated that the Auger parameter shift depends not only on the nuclearity and geometry of the cluster but also on the nature of the heteroatoms in its close vicinity. This result could be explained by the influence of these heteroatoms on the electronic polarizability of the surface Pd atoms of the cluster. The model has been applied also to the characterization of small copper clusters entrapped in A zeolite and supported on (0001) α-alumina surfaces. In any case, we demonstrate that the screening properties of both metal bulk and clusters of very low nuclearity are very similar
Characterization by x-ray absorption, x-ray powder diffraction, and magnetic susceptibility of Cu-Zn-Co-Al-containing hydroxycarbonates, oxycarbonates, oxides, and their products of reduction
No full textCopper-zinc-cobalt-aluminium-containing crystalline hydroxycarbonates having hydrotalcite structure have been prepared by coprecipitation. X-ray powder diffraction (XRPD), magnetic susceptibility, and extended X-ray absorption fine structure (EXAFS) indicate that Cu2+, Zn2+, and Co2+ are present in an octahedral environment. Calcination of the hydroxycarbonates at 723 K produces quasi-amorphous oxycarbonates where Cu2+ and Co2+ still retain octahedral coordination and cobalt is almost completely oxidized to Co3+. The coordination of Zn2+, at this stage, is intermediate between the octahedral one of the precursors and the tetrahedral one of ZnO and Zn-based spinels. Further calcination at 973 K produces a mixture of crystalline oxides such as CuO, ZnO, CuAl2O4, ZnAl2O4, and ZnCo2O4. EXAFS analysis of these samples indicates that copper is mainly in a fourfold coordination (although two longer Cu-O distances are also detected), zinc is tetrahedral, and cobalt (as Co3+) is essentially octahedral. EXAFS and XANES investigations performed after in situ reduction (10% H2/N2, at 523 and 623 K) on the oxycarbonates and oxides reveal that the total Cu2+ → Cu0 reduction occurs only at 623 K in both series of samples, Co3+ is reduced to Co2+ only at 623 K in the oxycarbonates, and Zn2+ is never reduced. © 1996 Academic Press, Inc
THE REDUCTION PROCESS OF COPPER-ZINC OXIDE-(ALUMINA) METHANOL CATALYSTS
No full textCuO-ZnO-Al2O3 catalysts at different copper and zinc contents were obtained by thermal decomposition of hydroxycarbonate precursors at 350°C in air. They were characterized by using X-ray diffraction, diffuse reflectance spectroscopy and surface area determination. We find disticnt CuO and ZnO phases whose particle sizes, determined by XRD line broadening, are in the range 65-90 Å for CuO and around 100 Å for ZnO in the binary CuO/ZnO systems. For pure CuO the particle dimensions are higher (D~140 A ̊). For the three component oxide, CuO|ZnO|Al2O3=60|30|10, the particle sizes of both CuO and ZnO phases are around 60 Å. The BET surface areas are around 70 m2/g for the two components oxides, ~40 for pure CuO and for the three component systems. The reduction of the copper oxide (CuO) has been studied by in situ XRD. The reduction to metallic copper proceeds through the formation of intermediate Cu2O which is present together with metallic copper even at relatively high extents of reduction. It has also been observed that the reduction process is much slower in the three component system than for pure CuO or the two component, CuO-ZnO, system. The presence of alumina in intimate contact with CuO and ZnO (due to the preparation conditions of the precursors) thus seems to hinder the reduction of the Cu2+ species (to Cu+ or Cuo) and to prevent the nucleation of both cuprous oxide and metallic copper partcles. © 1989
Copper-manganese mixed oxides: formation, characterization and reactivity under different conditions
No full textCu-Mn hydroxycarbonate-carbonate precursors, prepared by coprecipitation of nitrates, consist of Mn-containing malachite and Cu-containing rhodochrosite. Cu-Mn mixed oxides were obtained by heating the precursors at 723 K for 24 h in N2 and in air. The decomposition of the precursors depends on the atmosphere conditions (air, N2 or CO2) used during the thermal treatment and leads to several Cu-Mn oxides. For the Cu(I)xCu(II)1-xMn(III)2-xMn(IV)xO4 cubic spinel the redox equilibrium Cu(II)+Mn(III)⇆Cu(I)+Mn(IV) is shifted towards right or left for samples decomposed in air or in N2, respectively. In situ XRD and XPS reduction studies have shown that the final products of reduction are metallic copper and MnO, and that the N2-treated samples are more or less reducible than the air-treated ones depending on the initial CuO/CuMn2O4 compositional ratios
Structural and Electronic Properties of Sodalite: An ab Initio Molecular Dynamics Study
No full textStructural, vibrational, and electronic properties of crystalline sodalite are analyzed using ab initio molecular dynamics based on density functional theory (Car-Parrinello method). This is the first theoretical study of sodalite based on first principles, in which the full periodicity of the lattice is explicitely taken into account. The computed structural and vibrational properties are in good agreement with available experimental data. The dependence of such properties upon different ions inside the sodalite framework show the correct trend. We have also analyzed the properties of the neutral sodalite framework with no interframework atoms: this ideal structure is found to be stable and metallic; its electronic structure elucidates the origin of the active sites of this material. Moreover we have synthesized and characterized two sodalites: hydroxysodalite and hydrosodalite. Photoelectron spectroscopy and fouler transform infrared spectroscopy data for these materials are shown
XPS investigation on Co-Cu mixed oxide catalysts
No full textMetastable and isomorphous cobalt-copper hydroxysalts with Co:Cu = 100:0, 70:30 and 50:50 atomic ratios have been prepared as precursors by coprecipitation at constant pH from solutions of cobalt and copper nitrates added to a solution of NaHCO3. Mixed oxides were obtained by decomposition of the precursors at 723 K for 24 h in air and in N2. The surface of both precursors and mixed oxides was investigated by XPS. Only Co2+ and Cu2+ species are revealed for the precursors. The mixed oxides present CO2+, CO3+, Cu+ and Cu2+ species depending on the composition and on the calcination atmosphere. Quantitative analysis has evidenced a cobalt enrichment at the surface for the Co : Cu = 70:30 and 50:50 precursors. With respect to mixed oxides, the samples calcined in N2 are characterized by a homogeneous distribution of cobalt and copper both in the bulk and on the surface; in contrast, for the corresponding samples calcined in air the original cobalt enrichment of the precursors is preserved
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