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  • 1
    Electronic Resource
    Electronic Resource
    Effective medium potentials for molecule–surface interactions: H2 on Cu and Ni surfaces (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 7461-7471 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new approximate method is developed for the calculation of the adiabatic potential energy surface for a molecule outside a metal surface. It is computationally fast enough to be useful in simulations of the dynamics of adsorbing and desorbing molecules. The method is characterized by the fact that the functional form of the total energy expression is derived from density functional theory, that each of the terms entering can be given a precise physical interpretation, and that most of the parameters entering can be calculated, within the local density approximation. The method is explicitly derived for H2 outside metal surfaces and the applicability is illustrated for H2 adsorbing on various Cu and Ni surfaces. Although very approximate, the calculated potentials seem to include a number of features observed experimentally: Ni is more active in dissociating H2 than Cu, and open surfaces are more active than close-packed ones. Moreover, the method is simple enough that one can contemplate studying variations in dissociation pathways over the surface unit cell. For the Cu surfaces these variations are substantial accounting for at least part of the variation of the sticking coefficient with the kinetic energy of the incoming molecule. Because of the transparent nature of the energy expression, all these trends can be given a simple physical interpretation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456679
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  • 2
    Electronic Resource
    Electronic Resource
    The dissociative adsorption of hydrogen: Two-, three-, and four-dimensional quantum simulations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2879-2884 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A quantum wave packet calculation for the activated dissociative adsorption of H2 is presented. Restricting the motion of the molecule to lie within a plane normal to the surface we have treated all four molecular degrees of freedom exactly. We compare results obtained using two-, three-, and four-dimensional simulations on the same potential and show that by restricting the molecular orientation, important dynamical effects are lost. The potential employed in the calculations has been obtained using the effective medium approximation. In the simulations it has been possible to treat dissociation, rotations and diffraction on an equal footing. By including a rotational degree of freedom, it is seen that strong orientational effects occur near to the transition state and result in an anisotropic selectivity in the dissociation. By examining the state-to-state scattering probabilities, it is possible to use the nonreacting (scattered) fraction to provide information on the reactive potential energy surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458873
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  • 3
    Electronic Resource
    Electronic Resource
    Ammonia synthesis at low temperatures (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5343-5347 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional theory (DFT) calculations of reaction paths and energies for the industrial and the biological catalytic ammonia synthesis processes are compared. The industrial catalyst is modeled by a ruthenium surface, while the active part of the enzyme is modeled by a MoFe6S9 complex. In contrast to the biological process, the industrial process requires high temperatures and pressures to proceed, and an explanation of this important difference is discussed. The possibility of a metal surface catalyzed process running at low temperatures and pressures is addressed, and DFT calculations have been carried out to evaluate its feasibility. The calculations suggest that it might be possible to catalytically produce ammonia from molecular nitrogen at low temperatures and pressures, in particular if energy is fed into the process electrochemically. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481103
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  • 4
    Electronic Resource
    Electronic Resource
    Surface chemistry Catalysis frozen in time (2001)
    [s.l.] : Nature Publishing Group
    Nature 414 (2001), S. 405-406 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Solid surfaces act as catalysts for a large number of reactions: it is estimated that 20–30% of the gross national product in developed countries is dependent one way or another on this sort of catalysis. The surface acts by adsorbing the reactants and encouraging them to react until the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/35106674
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  • 5
    Electronic Resource
    Electronic Resource
    A theoretical study of CH4 dissociation on pure and gold-alloyed Ni(111) surfaces (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 5595-5604 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a density functional theory study of the first step of CH4 adsorption on the Ni(111) surface, dissociation into adsorbed CH3 and H. The rupture of the C–H bond occurs preferentially on top of a Ni atom, with a dissociation barrier of about 100 kJ/mol (including zero point corrections). The transition state involves considerable internal excitation of the molecule. The active C–H bond is both stretched to 1.6 A(ring) and tilted relative to the methyl group. A normal mode analysis shows that the reaction coordinate is mainly a C–H stretch, while the orientation of the C–H bond relative to the surface is responsible for the highest real mode. Alloying the surface with gold also affects the reactivity of the Ni atoms on adjacent surface sites. The dissociation barrier is increased by 16 and 38 kJ/mol for a Ni atom with one or two gold neighbors, respectively. We attribute these changes to a shift in the local density of d states at the nickel atoms in the neighborhood of gold. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472399
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  • 6
    Electronic Resource
    Electronic Resource
    Immobilization mechanisms for ion-implanted deuterium in aluminum (1985)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 58 (1985), S. 1841-1850 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Aluminum was ion implanted with deuterium (D) and then subjected to linear temperature ramping, and the resulting D redistributions were monitored using the ion-induced nuclear reaction D(3He, p)4He. Data from such experiments were analyzed in terms of various immobilization processes, utilizing numerical solutions of the appropriate diffusion formalism. The identification of mechanisms was augmented by transmission electron microscopy. Irradiation defects believed to be of vacancy type were shown to trap the D with a binding enthalpy of 0.52±0.10 eV relative to solution sites, in excellent agreement with calculations based on effective medium theory. Stronger binding at the surface oxide was quantitatively described by assuming the formation of D2 molecules at the metal-oxide interface. At higher implanted concentrations the immobilization of D by precipitation of D2 bubbles was observed, and the subsequent release from these bubbles at more elevated temperatures was described by diffusion theory. Small, high-pressure He bubbles formed by ion implantation of He did not trap the D more strongly than the irradiation defects, in contrast to observations in a number of other metals, but consistent with predictions of effective medium theory for Al.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.336037
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  • 7
    Electronic Resource
    Electronic Resource
    Multiple hydrogen occupancy of vacancies in Fe (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 1788-1794 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The binding of multiple deuterium (D) atoms to vacancies in iron (Fe) is investigated both experimentally, by use of ion-beam techniques, and theoretically, by use of the effective-medium scheme. The experimental D-release stages are accounted for by trapping at vacancies with a binding enthalpy that depends on occupancy. It is found that for 1–2 D in a vacancy, the trap strength is 0.63 eV, whereas the binding enthalpy for 3–6 D in a vacancy is 0.43 eV. These results are in good agreement with predictions from the effective-medium theory.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.338020
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  • 8
    Electronic Resource
    Electronic Resource
    Why gold is the noblest of all the metals (1995)
    [s.l.] : Nature Publishing Group
    Nature 376 (1995), S. 238-240 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] In discussing nobleness, we distinguish between the ability of the metal to form and break bonds at the surface, and its ability to form new compounds (such as oxides and carbides) or be dissolved. The latter ability is closely related to the former: to form a compound it is necessary that the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/376238a0
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  • 9
    Electronic Resource
    Electronic Resource
    Enhancement of surface self-diffusion of platinum atoms by adsorbed hydrogen (1999)
    [s.l.] : Macmillan Magazines Ltd.
    Nature 398 (1999), S. 134-136 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Surface diffusion of atoms is an important phenomenon in areas of materials processing such as thin-film growth and sintering. Self-diffusion (that is, diffusion of the atoms of which the surface is comprised) has been much studied on clean metal and semiconductor surfaces,. But in most cases ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/18185
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  • 10
    Electronic Resource
    Electronic Resource
    Interaction of hydrogen isotopes with metals: Deuterium trapped at lattice defects in palladium (1990)
    Springer
    Journal of fusion energy 9 (1990), S. 257-261 
    Details
    ISSN: 1572-9591
    Keywords: Trapping enthalpies ; lattice location ; hydrogen isotopes ; metals ; defects ; vacancies ; voids ; multiple occupancy of vacancies
    Source: Springer Online Journal Archives 1860-2000
    Topics: Energy, Environment Protection, Nuclear Power Engineering
    Notes: Abstract From an interplay between theory based on the effective-medium scheme and experiments, an extremely simple picture has evolved which is capable of describing a vast number of experimental quantities related to interaction of hydrogen with metals, especially the trapping of hydrogen at defects. It is shown that the trap strengths are determined mainly by the interstitial electron density, and any open structures in the lattice leads to a trap, with the vacancies and voids being the strongest traps. It is also found theoretically and experimentally that up to six hydrogen atoms can be accomodated in a vacancy, and the change in trap strengths with occupancy has been determined. Recent results for the trapping of deuterium to defects in Pd are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01059239
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