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  • 1990-1994  (7)
  • 1
    Electronic Resource
    Electronic Resource
    The effects of phase transitions, surface diffusion, and defects on surface catalyzed reactions: Fluctuations and oscillations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8306-8313 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Kinetic oscillations in catalytic reactions are often attributed either to attractive absorbate–absorbate interactions between adatoms or to required vacant sites for chemical reaction and are usually described by ordinary differential equations. These mechanisms are examined as possible causes for oscillations in surface reactions using Monte Carlo simulations. Self-sustained oscillations driven by a phase transition on the catalyst surface are found. It is shown that spatial inhomogeneity of the surface controls the existence of oscillations which therefore cannot be modeled using ordinary differential equations. The problem of synchronization of numerous oscillators on the surface is investigated. In particular, the influence of adatom migration and surface defects on Hopf bifurcation is examined. It is demonstrated that sites on surface with strong binding energy result in more chaotic oscillations whose amplitude decreases. The influence of concentration and distribution of defects on the kinetic oscillations is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459313
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Structures of small metal clusters. II. Phase transitions and isomerization (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6891-6901 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The binding energy, atom coordination numbers, bond lengths, surface restructuring, and bulk melting behavior of small clusters versus temperature are compared for the Lennard-Jones (LJ) potential and embedded atom (EA) potential using the Monte Carlo method with parameters fitted to Ni. We find that EA clusters are more thermally stable than LJ clusters with regard to evaporation. For small clusters whose minimum energy structure is polyicosahedral, a smooth change of physical properties with temperature is observed for both potentials. However, for clusters whose minimum energy structure is quasicrystalline, a structural phase transition analogous to a first order transition of bulk materials can be found for both potentials. This structural phase transition is manifested by discontinuous changes of atom coordination numbers and bond length, and in some cases, of energy. Implications of this transition in catalysis are discussed. Isomerization between minima of each one of the two potential hypersurfaces along with its dependence on temperature are examined. The many body nature of the EA potential results in lower isomerization probabilities between EA isomers as contrasted to LJ isomers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462583
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Structures of small metal clusters. I. Low temperature behavior (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 6880-6890 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The binding energy, surface energy, surface atom coordination numbers, and bond lengths and angles of small clusters at low temperatures are calculated with parameters fitted to Ni using both the Lennard-Jones (LJ) potential and the embedded atom (EA) potential to assess the sensitivity of interatomic potential on cluster structure. Simulations are performed by implementation of the simulated annealing method in a canonical ensemble Monte Carlo technique. We examine clusters with n≤34 atoms and we find that they are noncrystalline (with the exception of n=6). The most stable structure of clusters consisting of n≤15 atoms and n=19 atoms is the same for both potentials (with the exception of n=8). However, the most stable structure of clusters with n≥16 atoms is different for the two potentials (with the exception of n=19). Smeared angular distribution and pair distribution functions are found for many EA clusters whereas sharp, well defined peaks exist for LJ clusters. A discontinuous transition from polyicosahedral to quasicrystalline structure is found from n=30 to n=31 atoms for the LJ potential. This transition occurs at smaller n for the EA potential. Surface atom coordination numbers are found to be nonmonotonic functions of cluster size. The existence of multiple structures of small clusters and the effect of quenching rate during crystallization on the final shape of clusters are also examined.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.462582
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  • 4
    Electronic Resource
    Electronic Resource
    Ignition and extinction of flames near surfaces: Combustion of CH4 in air (1994)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 40 (1994), S. 1005-1017 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Ignition and extinction characteristics of homogeneous combustion of methane in air near inert surfaces are studied by numerical bifurcation theory for premixed methane/air gases impinging on planar surfaces with detailed chemistry involving 46 reversible reactions and 16 species. One-parameter bifuraction diagrams as functions of surface temperature and two-parameter bifurcation diagrams as functions of equivalence ratio and strain rate are constructed for both isothermal and adiabatic walls. Lean and rich composition limits for ignition and extinction, and energy production are determined from two parameter bifurcation diagrams. For a strain rate of 500 s-1, CH4/air mixtures exhibit hysteresis from ∼ 0.5% up to ∼ 12.5% and from ∼ 5.5% up to ∼ 13.5% near isothermal surfaces and adiabatic walls, respectively. Ignition temperature rises with composition from 1,700 to 1,950 K, without a maximum around the stoichiometric ratio. Under some conditions multiple ignitions and extinctions can occur with up to five multiple solutions, and wall quenching, kinetic limitations, and transport can strongly affect flame stability. Flames near the stoichiometric ratio cannot be extinguished by room temperature surfaces for sufficiently low strain rates. The role of intermediates in enhancing or retarding ignition and extinction is studied, and implications of the effect of catalytic surfaces on homogeneous ignition and extinction are discussed. Removal of H atoms and CH3 radicals by wall adsorption can increase extinction and ignition temperature of 6% CH4 in air by up to 300 K for a strain rate of 500 s-1.
    Additional Material: 9 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690400611
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  • 5
    Electronic Resource
    Electronic Resource
    Products in methane combustion near surfaces (1994)
    Hoboken, NJ : Wiley-Blackwell
    AIChE Journal 40 (1994), S. 1018-1025 
    Details
    ISSN: 0001-1541
    Keywords: Chemistry ; Chemical Engineering
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Emission of carbon monoxide (CO), formaldehyde (CH2O), and unburned methane (CH4) are calculated for premixed methane/air mixtures impinging on a flat surface as functions of surface temperature, equivalence ratio, and strain rate with detailed chemistry involving 46 reversible reactions and 16 species using numerical bifurcation theory. Multiple solutions with different selectivities to stable products are found. On the extinguished branch unburned CH4, molecular hydrogen (H2), CO, and CH2O dominate, whereas on the ignited branch carbon dioxide (CO2) predominates near the surface. Cold walls can promote the selectivity to CO and CH2O near extinction, and high flow rates can increase considerably the formation of CO, CH2O, and unburned CH4. For example, an ignited stoichiometric methane/air mixture (9.5% CH4 in air) impinging on a surface of 1,000 K is calculated to produce 2% CO, 150 ppm CH2O, and 3% unburned CH4 for a strain rate of 500 s-1. Maximum efficiency of CH4 and minimum selectivity to CH2O occur near the stoichiometric ratio, whereas minimum selectivity to CO occurs for fuel lean mixtures. Comparison of combustion near surfaces with freely propagating flames is also shown.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/aic.690400612
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  • 6
    Electronic Resource
    Electronic Resource
    Comparison of small metal clusters: Ni, Pd, Pt, Cu, Ag, Au (1993)
    Springer
    The European physical journal 26 (1993), S. 156-158 
    Details
    ISSN: 1434-6079
    Keywords: 36.40
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Structures of small clusters of Ni, Pd, Pt, Cu, Ag, and Au with n=2−34 and n=55 atoms are calculated as functions of number of atoms and temperature by the simulated annealing Monte Carlo method using an embedded atom potential.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01425649
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  • 7
    Electronic Resource
    Electronic Resource
    Finite element formulations for large-scale, coupled flows in adjacent porous and open fluid domains (1994)
    Chichester : Wiley-Blackwell
    International Journal for Numerical Methods in Fluids 18 (1994), S. 1185-1209 
    Details
    ISSN: 0271-2091
    Keywords: Engineering ; Engineering General
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Notes: Two approaches which employ the finite element method to solve for large-scale, coupled, incompressible flows through adjacent porous and open domains are developed and evaluated in a model for the spontaneous ignition of coal stockpiles. Both formulations employ the Navier-Stokes equations do describe flow in the open region; two different descriptions, Darcy's law and the Brinkman equation, are employed to model flows within the porous region. The formulation which uses Darcy's law employs the Beavers-Joseph slip condition and a novel implementation of the interfacial conditions. The other approach invokes the Brinkman equation: this considerably simplifies the implementation of matching conditions at the interface between the porous and open fluid domains, but also results in velocity boundary layers in the porous region adjacent to this interface which can be difficult to resolve numerically. A direct comparison of model results shows that the Darcy-slip formulation produces solutions which are more accurate and more economical to compute than those obtained using the Brinkman formulation.
    Additional Material: 11 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/fld.1650181205
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