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  • 1
    Electronic Resource
    Electronic Resource
    Dynamics of the ultraviolet photochemistry of water adsorbed on Pd(111) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4609-4619 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: UV-laser irradiation (hν=6.4 eV and 5.0 eV) of the water bilayer adsorbed on a Pd(111) surface leads to molecular desorption and to conversion of the adsorbed state as manifested in thermal desorption spectra. The latter effect is attributed to photodissociation of water on the surface. Time-of-flight measurements show that water molecules desorb with a translational energy of about 600 K for both photon energies indicating a nonthermal process. While desorption is largely suppressed with adsorbed multilayers, conversion within the first layer still proceeds. The dependence of the desorption yield on angle of incidence and polarization of the light reveals substrate excitations as the dominant primary step. A strong variation of cross sections with isotopic substitution is observed. This is interpreted as evidence for the operation of a mechanism involving excitation onto an isotope-independent excited potential energy surface followed by rapid deexcitation to the ground state so that, of the total number of species excited, only a small mass dependent fraction actually fragments or desorbs.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460589
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  • 2
    Electronic Resource
    Electronic Resource
    Kinetic oscillations in the NO + CO reaction on Pt(100): Experiments and mathematical modeling (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 2109-2126 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction of NO and CO on Pt(100) exhibits two branches of steady state production of N2 and CO2 and the occurrence of kinetic oscillations. This system was studied under steady flow conditions in the 10−6 mbar total pressure range using low-energy electron diffraction-(LEED), work function measurement, and mass spectrometry for determination of the reaction rate. These studies revealed that kinetic oscillations can only be initiated from one of the two stable reaction branches. Two separate existence regions were detected in which the oscillations are always damped. Oscillations can be very reproducibly excited by slight decreases in temperature. The 1×1(large-closed-square)hex phase transition of the surface structure was observed to take place only in one of the two regions of reaction rate oscillations. Its influence seems to be of minor relevance to the mechanism of oscillations as oscillations in one region occur on the surface that maintains a 1×1 structure. The experiments were modeled by a set of coupled differential equations based on knowledge about the elementary reaction steps. The model calculations reproduced the steady states of the reaction as well as the occurrence of kinetic oscillations in different ranges in excellent agreement with experimental observation. In the model, the phase transition also has no relevance for the oscillation mechanism. The occurrence of oscillations can be rationalized in terms of a periodic sequence of autocatalytic "surface explosions'' and the restoration of an adsorbate-covered surface. The damping, experimentally observed, is attributed to insufficient spatial coupling between different regions of the surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461010
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  • 3
    Electronic Resource
    Electronic Resource
    Photochemical pathways of water on palladium (111) at 6.4 eV (1991)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 95 (1991), S. 8393-8402 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100174a065
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  • 4
    Electronic Resource
    Electronic Resource
    The mechanism of kinetic oscillations in catalytic oxidation of CO on Pt(210) (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 6162-6170 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Under isothermal conditions at low pressure (10−4 Torr), the catalytic oxidation of CO on a Pt(210) surface exhibits kinetic oscillations which have been investigated using Video-LEED, measurement of the CO2 production rate and the variation of work function. An induction period of ∼30 to 60 min, which has been shown to be due to a facetting of the surface exists before the appearance of kinetic oscillations. If reaction conditions are chosen which correspond to the high rate branch of Langmuir Hinshelwood kinetics, the Pt(210) surface facets into (310) and (110) orientations. The facetting process is associated with a decrease in catalytic activity caused by a lowering of the oxygen sticking coefficient. In situ LEED experiments demonstrated that the oscillations in the reaction rate are associated with periodic intensity variations of the half-order LEED beams belonging to (110) facets. Thus, the oscillations appear to be driven by the CO-induced 1×1(arrow-right-and-left)1×2 phase transition on (110) facets in the same way as has been verified for the system Pt(110)/CO+O2. The involvement of a facetting process explains the characteristic properties of kinetic oscillations on Pt(210) such as the relatively low high-temperature limit of ≈500 K, the existence of an induction period and the period length which is on the order of minutes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.461584
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  • 5
    Electronic Resource
    Electronic Resource
    Exoelectron emission during oxidation of Cs films (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 3756-3766 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: During oxidation of thin Cs films, a nonadiabatic surface reaction manifests itself in the emission of electrons. This effect was investigated in detail by combining measurements of the current and of energy distributions of these exoelectrons with studies on the electronic properties of the surface by means of ultraviolet photoelectron spectroscopy and metastable deexcitation spectroscopy. Exoelectron emission occurs via Auger deexcitation of the empty state derived from the O2 affinity level. This process is confined to the stage Cs2O2→CsO2 in which resonance ionization of the affinity level of the impinging O2 molecule upon crossing the Fermi level EF is efficiently suppressed due to the absence of metallic states near EF. A kinetic model based on the successive steps involved in the oxidation of Cs is developed which describes qualitatively well all the experimental findings.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460826
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  • 6
    Electronic Resource
    Electronic Resource
    Temporal and spatial self-organisation in catalysis at single crystal surfaces (1991)
    Springer
    Catalysis letters 9 (1991), S. 219-230 
    Details
    ISSN: 1572-879X
    Keywords: Oscillatory catalytic reactions ; spatial differences in surface concentrations of reactants ; carbon monoxide oxidation on Pt(110)
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract The elementary steps underlying the mechanism of a catalytic reaction are also responsible for its rate under steady-state conditions. In special cases the rate will no longer be stationary, but may become oscillatory or even chaotic. These phenomena have to be ascribed to the nonlinear character of the coupled differential equations modelling the temporal behavior of the surface concentrations of the reacting species. As a consequence, these concentrations may also exhibit spatial differences, even on ana priori uniform single crystal surface, leading to spatiotemporal patterns such as propagating and standing waves as well as ‘chemical’ turbulence. Experimental evidence for these effects is presented for a particular system, the oxidation of CO on a Pt(110) surface.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00773180
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  • 7
    Electronic Resource
    Electronic Resource
    Microstructure of the activated industrial ammonia synthesis catalyst (1991)
    Springer
    Catalysis letters 11 (1991), S. 19-31 
    Details
    ISSN: 1572-879X
    Keywords: Ammonia synthesis ; iron catalyst ; surface characterization ; promoter effects ; nitrides ; ion scattering spectroscopy ; XPS ; HRTEM
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Industrial doubly-promoted iron catalysts and model systems of singly-promoted K- and Al-iron catalysts were characterised by their catalytic performance at 1 bar pressure. The relevance of bulk nitrogen for catalytic performance is shown. The catalysts were also activated in an in-situ reaction chamber of a He-ion scattering spectrometer (ISS) and their top atomic layer elemental composition was determined after they had reached similar performance as in the microreactor tests. The bulk microstructure of these samples was investigated by high resolution transmission electron microscopy (TEM) and microdiffraction. All evidence indicates that small highly crystalline α-Fe platelets act as active phase. Their surfaces are covered to a large extent by promotor compounds which are partly present as poorly crystalline aggregates with iron oxide leaving only a small fraction of elemental iron directly exposed to the gas phase. The intimate contact between iron crystals and promoters particles prevents recrystallisation and is the key to the understanding of the structural stability of the catalyst system.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00866897
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  • 8
    Electronic Resource
    Electronic Resource
    Surface and subsurface products of the interaction of O2 with Ag under catalytic conditions (1991)
    Springer
    Catalysis letters 11 (1991), S. 253-265 
    Details
    ISSN: 1572-879X
    Keywords: Silver ; oxygen ; selective oxidation ; thermal desorption ; subsurface compounds
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Polycrystalline Ag treated with O2 at pressures up to 1 bar and at temperatures up to 920 K was examined by TDS. It was found that the peak commonly observed at 595±25 K and assigned to adsorbed atomic oxygen (Oa) is shifted to higher desorption temperatures with rising dosing temperatures. Since saturation of the uptake was not possible either, this peak was identified as dissolved atomic oxygen (Od). The existence of a maximum for the O2 uptake after exposure at 820 K was found. Hence, under conditions of formaldehyde synthesis (≈ 920 K) the ability of the catalyst to dissolve oxygen is near its maximum.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00764316
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