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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    ISSN: 1573-4951
    Keywords: Catalyst design ; Substrates, Substrate-sorbant interaction ; Reaction pathways ; Zeolites
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary and conclusions The strategy summarised above in which modelling calculations are combined with QM cluster calculations is a viable procedure for the study of catalysis. Our discussion has emphasised studies of zeolite catalysts but the same approach could be used in modelling reactions catalysed on metal and metal oxide surfaces. Moreover, the techniques and potentials are available for these methods to have a wide range of applications.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The fluorite structured mixed metal fluorides Rb1−x Bi x F1+2x (0.5≤x≥0.75) are exceptionally good F− ion conductors, a property which is clearly related to the mixed nature of the cation sub-lattice. Extended X-ray absorption fine structure (EXAFS) has been used to study the local structure of the two types of cation in Rb1−x Bi x F1+2x as a function of x. The results reveal marked differences for the local environments of Rb+ and Bi3+. Considerable short-range order develops as x deviates from 0.5, and this can be correlated with the relative conductivities of these materials. Molecular dynamics (MD) has been used to yield complementary information on structural properties. Simulations were performed for a series of temperatures between 80 K and 750 K. Excellent agreement between EXAFS and MD is obtained. A non-collinear interstitialcy mechanism is proposed for anion diffusion, which is seen to be the basis for more complex concerted processes.
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  • 3
    ISSN: 1573-4803
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Abstract The average in-plane stress in a number of polycrystalline NiO thermal oxidation scales has been measured by the sin2ψ technique at the Synchrotron Radiation Source, Daresbury, using the high-resolution diffractometer 8.3, which exploits parallel-beam X-ray optics. There are a number of distinct advantages for stress measurements using this arrangement. Shifts in peak positions of much less than 0.005
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1573-4846
    Keywords: Molecular Dynamics ; Density Functional Theory ; sol-gel solutions ; silica clusters
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Density Functional Theory is used to study water, methanol, ethanol, TMOS, and TEOS molecules and the most important silica clusters participating in sol-gel processes. Calculated bond lengths, bond angles and electric dipole moments compare well with experimental data. The energy of these molecules is reported and used to discuss the energetics of the hydrolysis and condensation reactions. Molecular Dynamics is employed to simulate liquid water, methanol, ethanol, TMOS, TEOS and experimental sol-gel solutions. Calculated densities and enthalpies of vaporisation compare well with experimental data. Preliminary results are presented for MD simulations of sol-gel solutions.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1573-4900
    Keywords: Zeolites ; Catalysis ; Lattice simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Summary Lattice simulations and quantum mechanical techniques are used to study the energetics involved in the activation of the Ni-zeolite-Y catalyst, which requires migration of the Ni2+ cation from the S1 (hexagonal prism) to the supercage. We show that the barrier to migration of the nickel ions may be overcome by interaction of the migrating ion with molecules such as H2O, NH3 and C2H2 in the supercage, thereby explaining the role of the latter species in the activation process.
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  • 6
    ISSN: 1572-879X
    Keywords: selective oxidation ; V2O5 catalysts ; computation ; simulation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Simulation techniques have been employed to investigate the differences in the low energy adsorption configurations of ethene and ethane on the TiO2 supported and unsupported V2O5(001) surface. We find that the ethene molecule approaches much closer to thesupported V2O5(001) surface which is reflected in the 40 kj mol−1 higher adsorption energy. The low energy adsorption configuration located for ethane on the supported V2O5 shows that the molecule does not approach as close to the supported V2O5 surface as does ethene, resulting in the adsorption energy of ethane being 52 kJ mol−1 lower than that of ethene on the supported V2O5 surface.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1572-879X
    Keywords: Zeolitic catalysts ; transition metal ions ; nickel ; computation ; environment ; EXAFS ; modelling catalysts
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Computer modelling techniques are used to investigate the local structure of the zeolite framework around Ni2+ ions in the SI sites of Ni exchanged zeolite-Y. Our calculations show that there are pronounced inward relaxations (0.4 Å–0.6 Å) of the surrounding oxygen ions. The results allow a detailed rationalisation of recent EXAFS and diffraction studies on this zeolite.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1572-879X
    Keywords: Computer modelling ; non-stoichiometry ; oxide catalysis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Computer simulation techniques are used to develop defect models for the non-stoichiometric spinel with composition Zn1+xCr2−xO4 system. The results are used to provide an interpretation of the variation with composition of the catalytic activity of the monophasic material. We suggest that the increase in the catalytic activity with the Zn/Cr ratio is associated with an increased concentration of oxygen vacancies.
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  • 9
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: We describe a method for minimizing the lattice energy of molecular crystal structures, using a realistic anisotropic atom-atom model for the intermolecular forces. Molecules are assumed to be rigid, and the structure is described by the center of mass positions and orientational parameters for each molecule in the unit cell, as well as external strain parameters used to optimize the cell geometry. The resulting program uses a distributed multipole description of the electrostatic forces, which consists of sets of atomic multipoles (charge, dipole, quadrupole, etc.) to represent the lone pair, π electron density, and other nonspherical features in the atomic charge distribution. Such ab initio based, electrostatic models are essential for describing the orientation dependence of the intermolecular forces, including hydrogen bonding, between polar molecules. Studies on a range of organic crystals containing hydrogen bonds are used to illustrate the use of this new crystal structure relaxation program, DMAREL, and show that it provides a promising new approach to studying the crystal packing of polar molecules. © 1995 by John Wiley & Sons, Inc.
    Additional Material: 3 Ill.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1572-9028
    Keywords: computer simulation ; surface structures ; oxides ; silicates ; microporous materials
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract We review the application of computer simulation techniques to modelling the bulk and surface structures of complex materials. We show how recent developments have led to an increasingly predictive rôle for such calculations. We describe the use of simulations in modelling the adsorption of molecules on surfaces in problems of direct relevance to current catalytic studies.
    Type of Medium: Electronic Resource
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