Recent Publications and Associated Movies

  1. M. Dornhege, C. Punckt, J. L. Hudson, H. H. Rotermund
    Spreading of Corrosion on Stainless Steel
    J. Electrochem. Soc. 154 (2007) C24-C27.
    FULL TEXT
     
  2. C. Beta, A. S. Mikhailov, H. H. Rotermund, G. Ertl
    Defect-mediated turbulence in a catalytic surface reaction
    Europhys. Lett. 75, 868-874 (2006).
    FULL TEXT
     
  3. C. Punckt, P. Sanchez Bodega, H. H. Rotermund
    Quantitative Measurement of the Deformation of Ultra-thin Platinum Foils During
    Adsorption and Reaction of CO and O2
    Surf. Sci. 600, 3101-3109 (2006).
    FULL TEXT
     
  4. Qiao Liang, I. G. Kevrekidis, C. Punckt, H. H. Rotermund
    Guiding chemical pulses through geometry
    Phys. Rev. E 73, 036219 (2006).
    FULL TEXT
     
  5. Qiao Liang, I. G. Kevrekidis, C. Punckt, H. H. Rotermund
    Geometry-induced pulse instability in microdesigned catalysis: The effect of boundary curvature
    Phys. Rev. E 73, 036217 (2006).
    FULL TEXT
     
  6. J. Starke, C. Reichert, M. Eiswirth, H. H. Rotermund, G. Ertl
    Fluctuation-induced pattern formation in a surface reaction
    Europhys. Lett. 73 (6), 820-825 (2006).
    FULL TEXT
     
  7. Harm H. Rotermund
    Chaos und Ordnung auf Oberflächen
    Verhandlungen der Gesellschaft Deutscher Naturforscher und Ärzte
    der 123. Versammlung in Passau, Sept. 2004, S. Hirzel Verlag, Stuttgart, 69-88 (2005)
     
  8. Beta, C.; Mikhailov, A. S.
    Controlling spatiotemporal chaos in oscillatory reaction-diffusion systems by time-delay autosynchronisation
    Physica D 199, 173-184 (2004).
    FULL TEXT
     
  9. Beta, C.; Moula, M. G.; Mikhailov, A. S.; Rotermund, H. H.; Ertl, G.
    Excitable CO Oxidation on Pt(110) under Nonuniform Coupling
    Phys. Rev. Lett. 93, 188302 (2004).
    FULL TEXT
     
  10. Wolff, J.; Stich, M.; Beta, C.; Rotermund, H. H.
    Laser-induced target patterns in the oscillatory CO oxidation on Pt(110)
    J. Phys. Chem. B 108, 14282-14291 (2004).
    FULL TEXT
     
  11. Punckt, C.; Bölscher, M.; Rotermund, H.H.; Mikhailov, A.S.; Organ, L.; Budiansky, N.; Scully, J.R.; Hudson, J.L.
    Sudden Onset of Pitting Corrosion on Stainless Steel as a Critical Phenomenon
    Science 305, 1133-1136 (2004).
    FULL TEXT
    for supporting online material click here
    see also the press release "Dem Lochfraß bei Edelstahl auf der Spur"
     
  12. Bertram, M.; Beta, C.; Rotermund, H. H.; Ertl, G.
    Complex Patterns in a Periodically Forced Surface Reaction
    J. Phys. Chem. B 107, 9610-9615 (2003).
    FULL TEXT
     
  13. Cirak, F.; Cisternas, J. E.; Cuitio, A. M.; Ertl, G.; Holmes, P.; Kevrekidis, I. G.; Ortiz. M.; Rotermund, H.H.; Schunack, M.; Wolff, J.
    Oscillatory Thermo-Mechanical Instability of an Ultrathin Catalyst
    Science 300, 1932-1936 (2003).
    FULL TEXT
    for supporting online material click here
    see also press release on Pulsating Chemistry
     
  14. Wolff, J.; Rotermund, H.H.
    Local periodic forcing of CO oxidation on a Pt(110) surface
    NJP 5, 60.1-60.12 (2003).
    FULL TEXT
     
  15. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Li, X.; Kevrekidis, I. G.
    Gentle dragging of reaction waves
    Phys. Rev. Lett. 90, 018302 1-4 (2003).
    FULL TEXT
     
  16. Bertram, M.; Beta, C.; Pollmann, M.; Mikhailov, A.S.; Rotermund, H. H.; Ertl, G.
    Pattern formation on the edge of chaos: experiments with CO oxidation
    on a Pt(110) surface under global delayed feedback
    Phys. Rev. E 67, 036208 (2003).
     
  17. Beta, C.; Bertram, M.; Mikhailov, A.S.; Rotermund, H. H.; Ertl, G.
    Controlling turbulence in a surface chemical reaction by time-delay
    autosychronization
    Phys. Rev. E 67, 046224 (2003).
     
  18. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Li, X.; Kevrekidis, I. G.
    Local manipulation of catalytic surface reactivity
    J. Catalysis 216, 246-256 (2003).
     
  19. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Katsoulakis, M.; Li, X.; Kevrekidis, I. G.
    Wave initiation through spatiotemporally controllable perturbations
    Phys. Rev. Lett. 90, 148301 (2003).
     
  20. X. Li, A. G. Papathanasiou, I. G. Kevrekidis, M. Pollmann and H. H. Rotermund.
    Front Initiation on Microdesigned Composite Catalysts
    Chaos 12, 190-203 (2002).
     
  21. A. G. Papathanasiou, J. Wolff, I. G. Kevrekidis, H. H. Rotermund and G. Ertl
    Some Twists and Turns in the Path of Improving Surface Activity
    Chem. Phys. Lett. 358, 407-412 (2002).
     
  22. H. H. Rotermund, M. Pollmann and I.G. Kevrekidis
    Pattern Formation during the CO-oxidation involving Subsurface Oxygen
    Chaos 12, 157-163 (2002).
     
  23. H. H. Rotermund:
    Real Time Imaging of Surface Catalytic Reactions
    Phys. Stat. Sol. 118, 1537-1548 (2001).
     
  24. Wolff, J.; Papathanasiou, A. G.; Kevrekidis, I. G.; Rotermund, H. H.; Ertl, G.
    Spatio-Temporal Addressing of Surface Activity
    Science 294, 134-137 (2001).
    for playing movies click here
     
  25. Pollmann, M.; Bertram, M.; Rotermund, H.H.
    Influence of time delayed global feedback on pattern formation in oscillatory CO oxidation on Pt(110)
    Chem. Phys. Lett. 346, 123-128 (2001).
     
  26. Kim, M.; Bertram, M.; Pollmann, M.; von Oertzen, A.; Mikhailov, A. S.; Rotermund, H. H.; Ertl, G.:
    Controlling chemical turbulence by global delayed feedback:
    Pattern formation in catalytic CO oxidation on Pt(110)
    Science 292, 1357-1360 (2001).
    for playing movies click here
     
  27. Pollmann, Michael; Rotermund, Harm Hinrich; Ertl, Gerhard; Li, Xiujiang; Kevrekidis, Ioannis G.:
    Formation of 2-d concentration pulses on microdesigned composite catalyst surfaces
    Phys. Rev. Lett. 86, 6038-6041 (2001).
     
  28. Oertzen, A. von; Rotermund, H. H.; Mikhailov, A.S.; Ertl, G.:
    Standing wave patterns in the CO oxidation reaction on a Pt(110) surface: experiments and modeling
    J. Phys. Chem. B 104, 3155-3178 (2000).
     
  29. Dicke, J.; Erichsen, P.; Wolff, J.; Rotermund, H.H.:
    Reflection anisotropy microscopy: Improved setup and applications to CO oxidation on platinum
    Journal of Electron Spectroscopy and Related Phenomena 98-99, 41-54 (1999).
     
  30. Dicke, J.; Rotermund, H. H.; Lauterbach, J.:
    Formation of surface oxides on Pt(100) during CO oxidation in the mbar pressure range
    Surf. Sci. 454, 352-357 (2000).
     
  31. H. H. Rotermund, K. Krischer and B. Pettinger:
    Imaging of reaction fronts at surfaces and interfaces. Chapt. 4 in "Imaging of surfaces and interfaces". Frontiers of Electrochemistry,
    Editors: J. Lipkowski and P. N. Ross, Vol. 5, Wiley-VCH, New York, 139-209 (1999).
     
  32. H. H. Rotermund:
    Pattern formation in a surface reaction with global coupling
    The IMA Volumes in Mathematic and its Applications, Springer Verlag, Vol. 115, 231-248 (1999).
     
  33. Harm Hinrich Rotermund:
    Imaging Surface Reactions with a Photoemission Electron Microscope
    Journal of Electron Spectroscopy and Related Phenomena 98-99, 41-54 (1999).
     
  34. A. von Oertzen, A. Mikhailov, H. H. Rotermund and G. Ertl:
    Subsurface oxygen in the CO oxidation reaction on Pt(110): Experiments and mathematical modelling.
    J. Phys. Chem. B 102, 4966 - 4981 (1998).
     
  35. S. Kelling, S. Cerasari, H.H. Rotermund, G. Ertl and D.A. King:
    A photoemission electron microscopy (PEEM) study of the effect of surface acoustic waves on catalytic CO oxidation over Pt{110}
    Chem. Phys. Lett. 293, 325 - 330 (1998).
     
  36. J. Lauterbach, K. Asakura, P.B. Rasmussen, H.H. Rotermund, M. Bär, M.D. Graham, I.G. Kevrekidis, G. Ertl:
    Catalysis on mesoscopic composite surfaces: Influence of palladium boundaries on pattern formation during CO oxidation on Pt(110)
    Physica D 123, 493-501 (1998).
     
  37. K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl:
    Spatio-temporal pattern formation during the catalytic oxidation of CO on a Pt(100) surface modified with submonolayers of Au.
    Surf. Sci. 374, 125 - 141 (1997).
     
  38. H. H. Rotermund:
    Imaging of dynamic processes on surfaces by light.
    Surf.Sci.Reports 29, 265 - 366 (1997).
     
  39. A. von Oertzen, A. Mikhailov, H. H. Rotermund and G. Ertl:
    Subsurface oxygen formation on the Pt(110) surface: Experiments and mathematical modelling,
    Surf. Sci. 350, 259-270 (1996).
     
  40. H. H. Rotermund, G. Haas, R. U. Franz, R. M. Tromp and G. Ertl:
    Imaging pattern formation in surface reactions from ultra-high vacuum to atmosperic pressures
    Science 270, 608 - 610 (1995).
     
  41. G. Haas, M. Bär, I. G. Kevrekidis, P. B. Rasmussen, H. H. Rotermund, and G. Ertl:
    Observation of front bifurcations in contrilled geometries: From one to two dimensions
    Phys. Rev. Lett. 75, 3560 - 3563 (1995).
     
  42. K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl:
    Spatio-temporal concentration patterns associated with the catalytic oxidation of CO on Au covered Pt(110) surfaces
    J. Chem. Phys. 102, 8175 - 8184 (1995).
     
  43. M. D. Graham, M. Bär, I. G. Kevrekidis, K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl:
    Catalysis on Microstructured Surfaces: Effects of Boundaries on Pattern Formation during the CO-oxidation in complex Pt domains
    Phys. Rev. E 52, 76-93 (1995).
     
  44. M. Bär, S. Nettesheim, H. H. Rotermund, M. Eiswirth and G. Ertl:
    Transition between fronts and spiral waves in a bistable surface reaction
    Phys. Rev. Lett. 74, 1246 (1995).
     
  45. V. Gorodetski, J. Lauterbach, H. H. Rotermund,J.H. Block and G. Ertl:
    Reaction-diffusion waves from coupling between adjacent crystal planes in heterogeneous catalysis
    Nature 370, 276-279 (1994).
     
  46. J. Lauterbach, K. Asakura and H. H. Rotermund:
    Subsurface oxygen on Pt(100): kinetics of the transition from the chemisorbed to the subsurface state and its reaction with CO, H2 and O2
    Surf. Sci. 313, 52-63 (1994).
     
  47. M. D. Graham, I. G. Kevrekidis, K. Asakura, J. Lauterbach, K. Krischer, H. H. Rotermund and G. Ertl:
    Effects of Boundaries on Pattern Formation: Catalytic Oxidation of CO on Platinum
    Science 264, 80-82 (1994).


  1. Spreading of Corrosion on Stainless Steel: Simultaneous Observation of Metastable Pits and Oxide Film

  2. M. Dornhege, C. Punckt, J. L. Hudson, H. H. Rotermund

    Abstract

    In situ observations of pitting corrosion as it spreads across the surface of stainless steel are carried out. We apply two different imaging methods simultaneously, viz., ellipsomicroscopy for visualizing changes of surface film properties, and contrast enhanced microscopy for monitoring nucleation and reactivation of metastable corrosion pits. A correlation between oxide film weakening caused by individual pits and the nucleation of subsequent pits is found. The existence of front propagation as a component of the transition to pitting corrosion shows that characteristics of this process are consistent with the behaviour of stochastic reaction-diffusion systems.
    [References: 25]

  3. Defect-mediated turbulence in a catalytic surface reaction

  4. C. Beta, A. S. Mikhailov, H. H. Rotermund, G: Ertl

    Abstract

    We analyze the statistical properties of topological defects in an experimental reaction-diffusion system - the catalytic CO oxidation on Pt(110). Based on the gain and loss rates of defects, a probabilistic description of defect-mediated turbulence in this system is derived. We observe deviations from theoretical predictions for the statistics of topological defects that can be explained by the presence of short-range correlations between defects and underline the importance of correlations for defect dynamics in such systems.
    [References: 24]

  5. Quantitative Measurement of the Deformation of Ultra-thin Platinum Foils During Adsorption and Reaction of CO and O2

  6. C. Punckt, P. Sanchez Bodega, H. H. Rotermund

    Abstract

    An ultra-thin self-supporting Pt-foil with a thickness of 300 nm and 4 mm diameter has a heat capacity of only 10 µJ/K. Thus, even small amounts of heat deposited within the thin metal foil cause a significant temperature increase. During the adsorption of, for example, CO heat in the order of 30 µJ/ML is released into the metal. If the rim of a Pt(110) single crystal foil is rigidly mounted on a substrate, a temperature increase of the foil induced by adsorption and reaction of CO and O2 causes thermo-elastic stress. This will lead to macroscopic deformations of the metal foil. To quantify these deformations an imaging Michelson-interferometer was set up, capable to detect small deflections of the ultra-thin Pt catalyst down to 10-20 nm. Adsorption of CO and O2 causes a clear mechanical response of the Pt foil. Depending on sample temperature and partial pressures of the reactants fronts and pulses of deformation were found. The system can be calibrated by using continuous and chopped laser light. The optical properties of the catalytic surface change in the presence of adsorbates. This complicates the analysis, but can be avoided in an improved setup.
    [References: 27]

  7. Guiding chemical pulses through geometry

  8. Qiao Liang, I. G. Kevrekidis, C. Punckt, H. H. Rotermund

    Abstract

    We study computationally and experimentally the propagation of chemical pulses in complex geometries. The reaction of interest, CO oxidation, takes place on single crystal Pt(110) surfaces that are microlithographically patterned; they are also addressable through a focused laser beam, manipulated through galvanometer mirrors, capable of locally altering the crystal temperature and thus affecting pulse propagation. We focus on sudden changes in the domain shape (corners in a Y-junction geometry that can affect the pulse dynamics); we also show how brief, localized temperature perturbations can be used to control reactive pulse propagation. The computational results are corroborated through experimental studies in which the pulses are visualized using reflection anisotropy microscopy.
    [References: 42]

  9. Geometry-induced pulse instability in microdesigned catalysis: The effect of boundary curvature

  10. Qiao Liang, I. G. Kevrekidis, C. Punckt, H. H. Rotermund

    Abstract

    We explore the effect of boundary curvature on the instability of reactive pulses in the catalytic oxidation of CO on microdesigned Pt catalysts. Using ring-shaped domains of various radii, we find that the pulses disappear (decollate from the inert boundary) at a turning point bifurcation, and we trace this boundary in both physical and geometrical parameter space. These computations corroborate experimental observations of pulse decollation.
    [References: 40]

  11. Fluctuation-induced pattern formation in a surface reaction

  12. J. Starke, C. Reichert, M. Eiswirth, H. H. Rotermund, G. Ertl

    Abstract

    Spontaneous nucleation, pulse formation and propagation failure have been observed experimentally in CO oxidation on Pt(110) at intermediate pressures (approximate to 10(-2) mbar). This phenomenon can be reproduced with a stochastic model that includes temperature effects. Nucleation occurs randomly due to fluctuations in the reaction processes, whereas the subsequent damping out essentially follows the deterministic path. Conditions for the occurence of stochastic effects in the pattern formation during CO oxidation on Pt are discussed.
    [References: 15]

  13. Chaos und Ordnung auf Oberflächen

  14. Harm H. Rotermund

    Abstract

    Unsere Welt unterliegt in weitern Bereichen den Gesetzen der nichtlinearen Dynamik. Dies beginnt auf kleinster Skala bei selbstorganisierten Molekülen, zeigt sich in größerer Dimension beim Kammerflimmern des Herzens oder am Wetter und lässt sich selbst in den Größenordnungen des Universums bei Galaxien wiederfinden. Um diese Vielfalt zu erklären, sind einfache Modellsysteme notwendig, die dennoch die ganze Komplexität der nichtlinearen Dynamik aufweisen. Dafür eignen sich besonders chemische Systeme, die sich meist durch Reaktions-Diffusions-Modelle theoretisch charakterisieren lassen. In ihrer einfachsten Form sind es zweidimensionale Oberflächenreaktionen bei der heterogenen Katalyse mit einer Fülle von Mustern. Daher liegt der Schwerpunkt dieser Darstellung bei der CO-Oxidation auf Platin-Oberflächen, wie sie in jedem Autokatalysator abläuft. Es wird gezeigt, wie sich das "Chaos" kontrollieren lässt und eine neue Ordnung entsteht.
    [References: 42]

  15. Controlling spatiotemporal chaos in oscillatory reaction-diffusion systems by time-delay autosynchronisation

  16. Beta, C.; Mikhailov, A. S.

    Abstract

    Diffusion induced turbulence in spatially extended oscillatory media near a supercritical Hopf bifurcation can be controlled by applying global time-delay autosynchronisation. We consider the complex Ginzburg-Landau equation in the Benjamin-Feir unstable regime and analytically investigate the stability of uniform oscillations depending on the feedback parameters. We show that a noninvasive stabilization of uniform oscillations is not possible in this type of system. Th synchronization diagram in the plane spanned by the feedback parameters is derived. Numerical simulations confirm the analytical results and give additional information on the spatiotemporal dynamics of the system close to complete synchronization.
    [References: 19]

  17. Excitable CO Oxidation on Pt(110) under Nonuniform Coupling

  18. Beta, C.; Moula, M. G.; Mikhailov, A. S.; Rotermund, H. H.; Ertl, G.

    Abstract

    A new feedback scheme for guided spatiotemporal pattern formation in reaction-diffusion systems is introduced. In contrast to previously established control methods, we present a coupling protocol that is sensitive to the presence of coherent structures in the medium. Applying this feedback to the catalytic CO oxidation on Pt(110) in both experiments and numerical simulations, we show that temporal evolution and spatial extension of self-organizing objects can be efficiently controlled.
    [References: 16]

  19. Laser-induced target patterns in the oscillatory CO oxidation on Pt(110)

  20. Wolff, J.; Stich, M.; Beta, C.; Rotermund, H. H.

    Abstract

    The formation of target patterns in oscillatory CO oxidation on Pt(110) is studied using the controlled creation of temperature heterogeneities on the catalyst surface induced by a focused laser beam. Outward and, for the first time, inward traveling target waves are observed as well as target patterns that are spatially confined to the immediate vicinity of the temperature inhomogeneity. The experimental results are compared to theoretical studies of the complex Ginzburg-Landau equation with a nonuniform frequency distribution and to numerical simulations of the Krischer-Eiswirth-Ertl model for the catalytic CO oxidation on Pt(110). Both the complex Ginzburg-Landau equation and the kinetic model need to be extended by a global coupling term to obtain good agreement with the whole range of experimental observations.
    [References: 40]

  21. Sudden Onset of Pitting Corrosion on Stainless Steel as a Critical Phenomenon

  22. Punckt, C.; Bölscher, M.; Rotermund, H.H.; Mikhailov, A.S.; Organ, L.; Budiansky, N.; Scully, J.R.; Hudson, J.L.

    Abstract

    Stainless steels undergo a sharp rise in pitting corrosion rate as the potential, solution concentration or temperature is changed only slightly. We report experiments using real-time microscopic in situ visualizations that resolve the nucleation and evolution of individual pits during the transition. They suggest, that the sudden onset of corrosion is explained by an explosive autocatalytic growth in the number of metastable pits and that stabilization of individual pits takes place only later. This finding agrees with a theoretical approach treating the onset of pitting corrosion as a cooperative critical phenomenon resulting from interactions among metastable pits, and it extends perspectives on the control and prevention of corrosion onset.
    [References: 27]

  23. Complex Patterns in a Periodically Forced Surface Reaction

  24. Bertram, M.; Beta, C.; Rotermund, H. H.; Ertl, G.

    Abstract

    Effects of time-periodic, spatially uniform forcing on oscillatory chemical turbulence are studied in experiments with catalytic CO oxidation on a platinum(110) single-crystal surface. A variety of complex spatiotemporal reaction-diffusion patterns is investigated under variation of the forcing amplitude and frequency. Near harmonic resonance with the forcing, intermittent turbulence characterized by the presence of localized turbulent bubbles on a homogeneously oscillating background and disordered cellular structures are observed. In the parameter region where the system exhibits 2:1 subharmonic resonance, irregular oscillatory stripes and cluster patterns are found. For such resonant patterns, a frequency demodulation technique is used to reconstruct phase and amplitude variables from the experimental data.
    [References: 37]

  25. Oscillatory Thermo-Mechanical Instability of an Ultrathin Catalyst

  26. Cirak, F.; Cisternas, J. E.; Cuitio, A. M.; Ertl, G.; Holmes, P.; Kevrekidis, I. G.; Ortiz. M.; Rotermund, H.H.; Schunack, M.; Wolff, J.

    Abstract

    Because of the small thermal capacity of ultrathin ( 200 nanometers) metal single crystals, it is possible to explore the coupling of catalytic and thermal action at low pressures. We analyzed a chemothermomechanical instability inthis regime, in which catalytic reaction kinetics interact with heat transfer and mechanical buckling to create oscillations. These interacting components are separated and explored through experimentation, mathematical modeling, and scientific computation, and an explanation of the phenomenon emerges from their synthesis.
    [References: 19]

  27. Local periodic forcing of CO oxidation on a Pt(110) surface

  28. Wolff, J.; Rotermund, H.H.

    Abstract

    The effect of local periodic forcing on uniformly oscillating CO oxidation on a Pt(110) surface is investigated utilizing our recently developed addressable catalyst approach. The heating, provided by a focused laser beam, serves as the locally applied external force. The observations are ascribed to the interaction between local actuation and global coupling through the gas phase leading to the suppression of the locally developed patterns.
    [References: 29]

  29. Gentle dragging of reaction waves

  30. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Li, X.; Kevrekidis, I. G.

    Abstract

    Using a recently realized "addressable catalyst surface" we study the interaction of chemical reaction waves with prescribed spatiotemporal fields. In particular, we study how a traveling chemical pulse is "dragged" by a localized, moving temperature heterogeneity as a function of its intensity and speed. The acceleration and eventual "detachment" of the wave from the heterogeneity is also explored through simulation and stability analysis.
    [References: 29]

  31. Pattern formation on the edge of chaos: experiments with CO oxidation on a Pt(110) surface under global delayed feedback

  32. Bertram, M., Beta, C.; Pollmann, M.; Mikhailov, A.S.; Rotermund, H. H.; Ertl, G.

    Abstract

    Experiments with catalytic oxidation of carbon monoxide on Pt(110) show that chemical turbulence in this system can be suppressed by application of appropriate global delayed feedback. Different spatiotemporal patterns, seen near the transition from turbulence to uniform oscillations, are investigated. Such patterns include intermittent turbulence, oscillatory standing waves, cellular structures, and phase clusters. Using a method based on the Hilbert transform, spatial distributions of local phase and amplitude in these patterns are reconstructed from the experimental data.
    [References: 39]

  33. Controlling turbulence in a surface chemical reaction by time-delay autosychronization

  34. Beta, C.; Bertram, M.; Mikhailov, A.S.; Rotermund, H. H.; Ertl, G.

    Abstract

    A global time-delay feedback scheme is implemented experimentally to control chemical turbulence in the catalytic CO oxidation on a Pt(110) single crystal surface. The reaction is investigated under UHV conditions by means of photoemission electron microscopy (PEEM). We present results showing that turbulence can be efficiently suppressed by applying time-delay autosynchronization (TDAS). Hysteresis effects are found in the transition regime from turbulence to homogeneous oscillations. At optimal delay time, we find a discontinuity in the oscillation period that can be understood in terms of an analytical investigation of a phase equation with time-delay autosynchronization.
    [References: 47]

  35. Local manipulation of catalytic surface reactivity

  36. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Li, X.; Kevrekidis, I. G.

    Abstract

    Utilizing a focused laser beam manipulated through computer-controlled mirrors, and capable of 'writing' spatiotemporal temperature fields on a surface, we explore here the fundamental impact of localized spatiotemporal perturbations on a simple reaction diffusion system. Our two-dimensional model system is the low-pressure catalytic oxidation of CO on Pt(110), a reaction exhibiting well-understood spatiotemporal patterns. In the simplest case the laser spot causes the ignition of a reaction wave by a single critical "kick" at a selected surface location. The cooperativeness between two, separated in time and/or space, local subcritical perturbations is then explored. A temperature heterogeneity moving along a line may ignite waves along its path, or can drag preexisting pulses. In the oscillatory region we find localized beat patterns when the laser spot moves along a circle. The ratio between the underlying natural oscillation frequency and the forcing (circle-writing) frequency is important here. Finally we demonstrate how pulses, the basic building blocks of chemical patterns, can be modified, guided and erased and how the overall reaction rate can be increased through localized actuation. Computational studies supplement and rationalize the experimental findings.
    [References: 16]

  37. Wave initiation through spatiotemporally controllable perturbations

  38. Wolff, J.; Papathanasiou, A. G.; Rotermund, H. H.; Ertl, G.; Katsoulakis, M.; Li, X.; Kevrekidis, I. G.

    Abstract

    We study the initiation of pulses and fronts in a two-dimensional catalytic reaction diffusion system: CO oxidation on Pt(110). Using a computer controlled mobile focused laser beam, we impart various patterns (in space and time) of localized temperature ``kicks" to the surface. We explore, and also rationalize through modeling, the cooperativity of such subcritical perturbations in both the excitable and the bistable regime. By relating these experiments to rare events in stochastic dynamics we suggest a new perspective for application of our system.
    [References: 31]

  39. Front Initiation on Microdesigned Composite Catalysts

  40. Li, X.; Papathanasiou, A. G.; Kevrekidis, I. G.; Rotermund, H. H.; Ertl, G.

    Abstract

    We first briefly review the subject of spatiotemporal pattern formation on microdesigned composite catalysts. One of the most significant interaction mechanisms between different reacting domains consisting of different metal catalysts such as Pt and Rh, coupled through surface diffusion! is the initiation of reaction fronts at the interface between them. We then explore in some detail the effect of two-dimensional composite geometry on this basic building block of composite catalyst dynamics. [References: 76]

  41. Some Twists and Turns in the Path of Improving Surface Activity

  42. Papathanasiou, A. G.; Wolff, J.; Kevrekidis, I. G.; Rotermund, H. H.; Ertl, G.

    Abstract

    Determining favourable operating conditions is a major issue in the design of chemical processes. ìFavourableî here implies a compromise between economics (productivity, selectivity), flexibility, and constraints imposed by considerations such as safety or the environment:  this is the realm of optimisation. Usually chemical processes are designed to operate under optimal steady state conditions, possibly stabilized through feedback. Strategies to operate the entire process under non-steady-state (e.g. periodic)1-3 conditions are also being developed, leading to the emergence of commercially successful processes such as pressure swing operation4 or reverse flow reactors5. In loose analogy with resonance in linear systems, the benefits emerge when the characteristic times (periods) of the non-steady operation are close to important intrinsic time constants of the system itself. The present contribution goes beyond this approach to explore paths towards the optimisation of reaction rates by interacting simultaneously with intrinsic system time and space scales. These policies are implemented on a catalytic surface addressable through a moving focussed laser beam. The surface is thereby differentially heated in a locally resolved manner in space and time. The ability to dictate reaction conditions in space and time, whether in open or in closed loop6-8, opens new directions for reaction control through the integration of chemistry and systems theory.[References: 24]

    Keywords: CO-oxidation, Pt(110), Optical feedback, Surface-reaction,

  43. Pattern Formation during the CO-oxidation involving Subsurface Oxygen

  44. Rotermund, H.H.; Pollmann, M.; Kevrekidis, I. G.

    Abstract

    This review focuses on subsurface oxygen and its influence on pattern formation during CO-oxidation on Platinum surfaces. For the observation of spatiotemporal pattern formation during catalytic reactions the photoelectron emission microscope (PEEM) has proven to be an excellent real-time imaging technique, capable of tracking local work function changes. The existence of subsurface oxygen on platinum-like surfaces has been extensively discussed and for Palladium its presence has been clearly established during rate oscillations. Subsurface oxygen is defined at this point as an atomic O species located directly underneath the uppermost metal crystal layer; its dipole moment therefore considerably lowers the work function of the surface.
    Here we review some of the investigations involving subsurface oxygen, focusing on the role subsurface oxygen might play in pattern formation during CO-oxidation on Platinum. We will also present some new results, where this species clearly interacts with chemisorbed oxygen under restrictions by boundary conditions on the Pt(110) single crystal. These previously constructed (through microlithography) domain boundaries on the surface are made out of Rh or Pd, and they are acting as an additional source of CO molecules for the Pt surface. [References: 44]

  45. Real Time Imaging of Surface Catalytic Reactions

  46. Rotermund,H. H.

    Abstract

    This review focuses on various methods to image surface reactions in real time. Development of the photoelectron emission microscope (PEEM), which is capable of following local work function changes in real time, was of fundamental importance in work dedicated to understanding pattern formation during catalytic reactions. The focus of this review is on optical methods using polarized light capable of imaging adsorbates on top of the surface, which are typically less than one monolayer (ML) thick. These imaging methods include Ellipso Microscopy for Surface Imaging (EMSI) and Reflection Anisotropy Microscope (RAM), and the commercially available infrared (IR) technique, which nowadays can follow reaction fronts due to the temperature change even at low pressures. Some new results will be presented where in addition to the optical imaging a focused laser was used to change pattern formation during the CO-oxidation on a Pt(110) surface in various ways.

  47. Spatio-Temporal Addressing of Surface Activity

  48. Wolff, J.; Papathanasiou, A. G.; Kevrekidis, I. G.; Rotermund, H. H.; Ertl, G.

    Abstract
    We have been able to modify the surface catalytic activity in real time and space by focussing an addressable laser beam to differentially heat a Pt(110) single crystal surface.  Ellipsomicroscopy imaging of local conditions (such as reactant and product local coverages) enables us to close the loop between sensing and actuation (both spatio-temporally resolved). Pulses and fronts, the basic building blocks of patterns, can be formed, accelerated, modified, guided and destroyed at will. Image processing and feedback allow the design and implementation of new classes of non-local evolution rules. [References: 38]

    Keywords: CO-oxidation, Pt(110), Optical feedback, Surface-reaction,

  49. Influence of time delayed global feedback on pattern formation in oscillatory CO oxidation on Pt(110)

  50. Pollmann, M.; Bertram, M.; Rotermund, H.H.

    Abstract

    Spatiotemporal pattern formation in the catalytic CO oxidation on Pt(110) is controlled by means of global delayed feedback applied through the gas phase. Using photoemission electron microscopy we have investigated the dynamical response of the system to a change of feedback intensity and time delay. Well defined synchronization and desynchronization regimes alternate when the delay is varied. In the case of synchronization the surface shows spatial homogeneous oscillations whereas in the case of desynchronization nonuniform patterns are observed. Furthermore two-phase clusters with period doubled local oscillations are studied. Phase balance between single cluster areas and breathing cluster modes are observed. [References: 22]

    Keywords: Ginzburg-Landau equation, Optical feedback, Surface-reaction, Stabilization System, Spirals, Waves, Chaos

  51. Controlling chemical turbulence by global delayed feedback: Pattern formation in catalytic CO oxidation on Pt(110)

  52. Kim, M.; Bertram, M.; Pollmann, M.; von Oertzen, A.; Mikhailov, A. S.; Rotermund, H. H.; Ertl, G.

    Abstract

    Control of spatiotemporal chaos is one of the central problems of nonlinear dynamics. We report on suppression of chemical turbulence by global delayed feedback using, as an example, catalytic carbon monoxide oxidation on a platinum (110) single-crystal surface and carbon monoxide partial pressure as the controlled feedback variable. When feedback intensity was increased, spiral-wave turbulence was transformed into new intermittent chaotic regimes with cascades of reproducing and annihilating local structures on the background of uniform oscillations. The global feedback further led to the development of cluster patterns and standing waves and to the stabilization of uniform oscillations. These findings are reproduced by theoretical simulations. [References: 29]

    Keywords: Ginzburg-Landau equation, Optical feedback, Surface-reaction, Stabilization System, Spirals, Waves, Chaos

  53. Formation of 2-d concentration pulses on microdesigned composite catalyst surfaces

  54. Pollmann, Michael; Rotermund, Harm Hinrich; Ertl, Gerhard; Li, Xiujiang; Kevrekidis, Ioannis G.

    Abstract

    We study the effect of microdesigned composite geometries on pattern formation during the catalytic oxidation of CO on Pt-Ti, Pt-Rh, and Pt-Pd composite catalysts. In particular, we find experimentally (and rationalize through modeling) that adsorbate surface transport through the second (active) component drastically affects the shapes and interactions of concentration patterns (traveling pulses) observed on pure Pt.

  55. Standing wave patterns in the CO oxidation reaction on a Pt(110) surface: experiments and modeling

  56. Oertzen, A. von; Rotermund, H. H.; Mikhailov, A.S.; Ertl, G.

    Abstract

    Standing waves are a special type of spatio-temporal pattern observed in the CO oxidation reaction on Pt(110). We present new experimental data that indicate the important role played by the formation of subsurface oxygen. The formation of these patterns is correlated with reflective collisions of traveling waves that have previously been found in the same reaction. We also show that global coupling through the gas phase and external forcing are essential for stabilizing and synchronizing the standing wave patterns. The principal properties of the observed patterns are reproduced by numerical simulations based on an extended reaction-diffusion model. [References: 58]

    Keywords: Reaction-diffusion systems. Ginzburg-landau equation. Single-crystal surfaces. Subsurface oxygen. Kinetic oscillations. Spiral waves. Structural transformation. Heterogeneous catalysis. Controlling turbulence. Thermal-desorption.

  57. Reflection anisotropy microscopy: Improved setup and applications to CO oxidation on platinum

  58. Dicke, J.; Erichsen, P.; Wolff, J.; Rotermund, H.H.

    Abstract

    Reflection anisotropy microscopy ( RAM) is a novel technique to spatially resolve the optical anisotropy of surfaces. We describe a substantially improved set-up, utilizing a Foster prism. As a result the image contrast was significantly enhanced and the light source now can be polychromatic. We also present applications to CO oxidation on platinum. For CO adsorption on Pt(110), the RAM signal was correlated to the surface structure. For CO oxidation on Pt(110) under the conditions of pattern formation, the surface reconstruction was spatially resolved. For a polycrystalline platinum foil individual crystal grains were imaged and their crystallographic structure was identified.

    Keywords:Adsorption kinetics; Catalysis; Ellipsometry; Low energy electron diffraction ( LEED); Low index single crystal surfaces; Platinum; Polycrystalline surfaces; Surface chemical reaction

  59. Formation of surface oxides on Pt(100) during CO oxidation in the mbar pressure range

  60. Dicke, J.; Rotermund, H. H.; Lauterbach, J.

    Abstract

    CO oxidation on Pt(100) at oxygen pressures of 9.0x10-2 mbar was studied. An unusual decrease in catalytic activity with time was observed, which has not been observed at lower pressures. Post-reaction Auger and TDS measurements showed the formation of a surface oxide species, which deactivated the catalyst. The surface oxide is compared to subsurface oxygen and to surface oxide studied previously on low index Pt single crystals. An adsorbate induced surface reconstruction is proposed as a possible driving force for oxide formation.

    Keywords: Auger electron spectroscopy; Carbon monoxide; Catalysis; Oxidation; Platinum; Solid?gas interfaces; Surface chemical reaction; Thermal desorption spectroscopy

  61. Imaging of reaction fronts at surfaces and interfaces.

  62. Chapt. 4 in "Imaging of surfaces and interfaces".
    Frontiers of Electrochemistry
    H. H. Rotermund, K. Krischer and B. Pettinger:

    Abstract

    book section, review article, abstract omitted

  63. Pattern formation in a surface reaction with global coupling

  64. H. H. Rotermund:

    Abstract

    Surface reactions exhibit unique features as model systems for non-linear effects in chemical reactions. In addition they have an immense importance in heterogeneous catalysis in the chemical industry. Dynamic processes on surfaces, like the Pt - catalyzed CO-oxidation, can be described by a set of reaction-diffusion equations. For a certain range of reactants partial pressures and temperature of the sample, pattern formation like spiral waves, target patterns or solitary waves can be observed. When global coupling via the gas phase is introduced strong temporal oscillations may occur, sometimes exhibiting spatio-temporal patterns like standing waves, period doubling and chaotic behavior. The patterns mentioned were found under isothermal conditions. Of course, when increasing the reaction pressure, due to the exothermic nature of the CO-oxidation, temperature variations can be explored, observable with a sensitive InfraRed (IR) camera.

  65. Imaging Surface Reactions with a Photoemission Electron Microscope

  66. H.H. Rotermund

    Abstract

    The photoemission electron microscope (PEEM), invented more than 60 years ago, began being utilized for imaging surface reactions less than 10 years ago making observations of pattern formation during heterogeneous catalytic CO-oxidation on platinum. First investigations of adsorption and reaction of oxygen on a beforehand CO covered Pt surface are discussed, showing the growth of isolated oxygen island up to millimeter sizes. Features like plane wave-front propagation and a "memory effect" of islands have been observed. Then pattern formation during CO-oxidation on a Pt(110) surface, with both reactants supplied out of the gas phase, is studied. By creating boundaries on the surface utilizing microlithography single spiral waves can be isolated. Here experiments with reactive boundaries of Pd are shown. Subsurface oxygen plays a role for the oscillation cycles.

  67. Subsurface oxygen in the CO oxidation reaction on Pt(110): Experiments and mathematical modelling.

  68. A. von Oertzen, A. Mikhailov, H. H. Rotermund and G. Ertl:

    Abstract

    New experimental evidence on the formation of spatio-temporal concentration patterns as imaged by photoemission electron microscopy (PEEM) supports our earlier kinetic model for subsurface oxygen formation in the CO oxidation reaction on Pt(110). Extensive experimental data on oxygen island growth and conversion is used to derive the activation energy and the pre-exponential factor for the process of subsurface oxygen formation. We find that under reaction conditions formation of subsurface oxygen may significantly influence properties of traveling con-centration waves. Instead of annihilation of two colliding waves, their reflection and two-into-one collision events are observed in the experiments. This behavior is correctly reproduced by the model.

  69. A photoemission electron microscopy (PEEM) study of the effect of surface acoustic waves on catalytic CO oxidation over Pt{110}

  70. S. Kelling, S. Cerasari, H.H. Rotermund, G. Ertl and D.A. King:

    Abstract

    A photoemission electron microscope (PEEM) was used to study the influence of surface acoustic waves (SAWs) on the reactive behaviour of adsorbate structures during CO oxidation over Pt{110} at temperatures around 485 K and CO and O-2 pressures around 10(-5) and 10(-4) mbar, respectively. It was found that surface acoustic wave excitation can suppress and reverse the process of CO poisoning of the catalyst, and thus bring the system back into the highly reactive regime. From work function measurements of the CO-covered surface during acoustic excitation it is concluded that the observed effect is due to acoustically induced CO desorption.

  71. Catalysis on mesoscopic composite surfaces: Influence of palladium boundaries on pattern formation during CO oxidation on Pt(110)

  72. J. Lauterbach, K. Asakura, P.B. Rasmussen, H.H. Rotermund, M. Bär, M.D. Graham, I.G. Kevrekidis, G. Ertl

    Abstract

    The effect of well defined palladium boundaries on the formation of reaction diffusion patterns during the oxidation of CO on Pt(110) has been investigated. The Pd boundaries were created by a microlithography technique. Reaction diffusion patterns formed on this surface were observed by photoemission electron microscopy (PEEM). On a multilayer Pd-covered Pt(110) surface, depending on the partial pressures, Pd areas may act as a reservoir for CO for adjacent Pt regions. Therefore, patterns forming on the bare Pt-areas exhibit a pronounced interaction with the artificial Pt/Pd-boundaries.

  73. Spatio-temporal pattern formation during the catalytic oxidation of CO on a Pt(100) surface modified with submonolayers of Au

  74. K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl

    Abstract

    The formation and development of spatio-temporal concentration patterns associated with the catalytic oxidation of carbon monoxide on a Pt(100) surface modified by deposition of submonolayers of Au was investigated by means of photoemission electron microscopy (PEEM). The presence of Au atoms essentially reduces the adsorption probability for oxygen and the diffusion coefficient for adsorbed CO leading to alterations of the velocities for propagation of O- and CO-waves. At the boundaries between domains of differing Au content phenomena of refraction and reflection occurred. While refraction follow Snell's law, no total reflection was found. The observed effects as well as characteristic differences to the previously studied Pt(110) surfaces were qualitatively explained on the basis of the elementary steps involved.

  75. Imaging of dynamic processes on surfaces by light

  76. H. H. Rotermund

    Abstract

    This work focuses on imaging of dynamic processes on surfaces, using light to illuminate the area of interest. The methods discussed here are those in which the photo-electrons emitted from or the light reflected off the surface are measured. While the first approach is well known since electron microscopy was invented and has been used in surface science applications for a decade, genuine optical microscope methods using polarized light were first developed in 1995 for imaging surface reactions. The results discussed here are from different fields of surface research. These include the imaging of adsorption phenomena, surface diffusion and growth processes. The main emphasis will be on pattern formation of surface reactions under strictly controlled parameters. The most recent techniques expand the range of observable pressure conditions by many orders of magnitude, thus bridging the pressure gap in imaging surface reactions.

  77. Subsurface oxygen formation on the Pt(110) surface: Experiments and mathematical modelling

  78. A. von Oertzen, A. Mikhailov, H. H. Rotermund and G. Ertl

    Abstract

    Experiments using of photoemission electron microscopy (PEEM) reveal that regions on a Pt(110) surface covered by chemisorbed O atoms may be converted into a subsurface O-phase, provided that it is preceded by the interaction of CO initiating the 1 x 2 to 1 x 1 transformation of the surface structure. The presence of subsurface oxygen favors, on the other hand, also lifting of the surface reconstruction. A mathematical model of this process is developed using parameters derived from previous independent experiments and numerical simulations fitting new data to experimental findings. Experiments by means of photoemission electron microscopy (PEEM) reveal that regions on a Pt(110) surface covered by chemisorbed O atoms may be converted into a subsurface O-phase, provided that it is preceded by the interaction of CO initiating the 1 x 2 to 1 x 1 transformation of the surface structure. The presence of subsurface oxygen favors, on the other hand, also lifting of the surface reconstruction. A mathematical model of this process is developed using parameters derived from previous independent experiments and numerical simulations fitting new data to experimental findings.

  79. Imaging pattern formation in surface reactions from ultra-high vacuum to atmosperic pressures

  80. H. H. Rotermund, G. Haas, R. U. Franz, R. M. Tromp and G. Ertl

    Abstract

    Two optical methods that allow pattern formation to be studied at an arbitrary pressure are applied to image concn. patterns of adsorbed species assocd. with heterogeneous catalytic reactions. In contrast to most surface phys. techniques, these methods are not restricted to high vacuum conditions and thus bridge the "pressure gap". With CO oxidn. on a (110) surface of Pt as an example, the coupling mechanisms responsible for spatiotemporal self-organization in surface reactions are followed from reaction-diffusion control to the thermokinetic region assocd. with phenomena not previously obsd. in pattern formation.

  81. Observation of front bifurcations in controlled geometries: From one to two dimensions

  82. G. Haas, M. Bär, I. G. Kevrekidis, P. B. Rasmussen, H. H. Rotermund, and G. Ertl

    Abstract

    The introduction of microstructures in the form of quasi-one-dimensional channels on a Pt(100) surface allows the detection of front bifurcations for concentration waves from adsorbed O and CO species accompanying their catalytic reaction. At the exits of these channels into larger circular domains, front reflection is observed depending on the width of the channels and the external parameters. The findings are reproduced within a reaction-diffusion model.

  83. Spatio-temporal concentration patterns associated with the catalytic oxidation of CO on Au covered Pt(110) surfaces

  84. K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl

    Abstract

    Submonolayers of gold were evaporated onto a Pt(110) surface in order to study the influence of surface modification on the formation of spatio-temporal concentration patterns during catalytic oxidation of carbon monoxide as imaged by means of photoemission electron microscopy (PEEM). Already the presence of 0.05 monolayers of uniformly distributed Au atoms affects markedly the kinetic parameters and thereby the properties of spatio-temporal patterns. The excitable regime is reached at lower CO pressures than with bare Pt(110), the width of the oxygen waves and the wavelength of spiral patterns decrease, the velocity of front propagation is reduced, and the density of nucleation centers for waves increases. The observed effects can be rationalized on the basis of the well-established underlying mechanism in terms of modification of the sticking coefficients for oxygen and carbon monoxide and of the diffusion coefficient for adsorbed CO. Additional effects such as penetration of waves into a monostable region, refraction etc. come into play if waves pass the border between surface areas with differing chemical properties.

  85. Catalysis on Microstructured Surfaces: Effects of Boundaries on Pattern Formation during the CO-oxidation in complex Pt domains

  86. M. D. Graham, M. Bär, I. G. Kevrekidis, K. Asakura, J. Lauterbach, H. H. Rotermund and G. Ertl

    Abstract

    The exploration of pattern formation by reaction-diffusion systems in complex bounded domains has begun only recently. While theoretical and numerical information points to a strong interaction between patterns and boundaries, experiments are rare and for heterogeneous catalytic reactions practically nonexistent. By constructing (using microlithography) catalytic surfaces of arbitrary shape and size, we are able to study this interaction for the catalytic oxidation of CO on Pt(110). Experiments along these lines shed light on issues such as anisotropic diffusion and the behavior of individual defects. In addition, certain geometries give rise to patterns that have not been observed on the untreated catalyst and bring to light surface mechanisms that have no analog in homogeneous reaction-diffusion systems. Simple domains of controlled size constitute paradigms that make the comparisons between theory and experiment more fruitful, as we demonstrate through modeling and simulation of such examples. This approach opens the way for systematically probing certain aspects of pattern formation unique to heterogeneous catalysis.

  87. Transition between fronts and spiral waves in a bistable surface reaction

  88. M. Bär, S. Nettesheim, H. H. Rotermund, M. Eiswirth and G. Ertl

    Abstract

    The transition between wave fronts and spiral waves in the CO oxidn. on a Pt (110) surface was studied by photoemission electron microscopy and treated theor. by simulations with a reaction-diffusion model under bistable conditions. Periodic external forcing of spirals near the transition to front behavior led to a complex pattern best characterized as a pulsating spiral-front hybrid, thus confirming the bistable nature of the underlying reaction kinetics.

  89. Reaction-diffusion waves from coupling between adjacent crystal planes in heterogeneous catalysis

  90. V. Gorodetski, J. Lauterbach, H.-H. Rotermund,J.H. Block and G. Ertl

    Abstract

    Understanding of the mechanisms and kinetics of heterogeneous catalytic reactions has come largely from the study of gas-solid interactions on well defined single-crystal surfaces. But real catalysts usually consist of nanometer-sized particles on which several different crystal planes are exposed. In general, it has been assumed that their properties can be regarded as a superposition of the contributions from each individual structural element. Here the authors show that this assumption may be invalid, even qual., in certain cases. The authors have studied the oxidn. of hydrogen on platinum surfaces at low pressure and room temp. On a macroscopic Pt(100) single crystal the reaction reaches a steady state with a uniform distribution of adsorbates. But on the platinum tip of a field ion microscope , on which several different crystal planes are exposed, the reaction has a very different character. The tip contains a region of the (100) plane just 40 nm in diam., on which the reaction rate displays sustained temporal oscillations . This effect is assocd. with continuously changing distributions of the adsorbed species in the form of propagating waves which are generated by coupling of reactions occurring on adjacent crystal planes. This kind of interaction between different crystal planes may exert a profound influence on the kinetics of heterogeneous catalysis.

  91. Subsurface oxygen on Pt(100): kinetics of the transition from chemisorbed to subsurface state and its reaction with CO, H2 and O2

  92. J. Lauterbach, K. Asakura and H.H. Rotermund

    Abstract

    By photoemission electron microscopy (PEEM) it is possible to image a surface via its work function. On a CO covered Pt(100) surface we can prepare oxygen islands of several 100 µm extension and irregular shape, which appear dark in the PEEM image due to their higher work function. As the surface is heated to temperatures above 650 K we observe the conversion of these dark islands into very bright ones with work functions much lower than even that of the clean surface arising from an inverted dipole moment of O atoms beneath the surface. By investigating the speed of the conversion into the subsurface state versus the temperature we found an activation energy of about 15 kcal/mol for the process.
    Only partially transformed oxygen islands were used to study the reactivity and the formation of this species further: CO and H2 both react with subsurface oxygen rather slowly, while at the interface between the subsurface and the chemisorbed phase both adsorbents accelerate the conversion of parts of the remaining oxygen atoms towards the subsurface state. Additional oxygen adsorption in contrast does not contribute to a further transformation, due to a very low sticking probability for oxygen a slow increase of the work function of the areas showing the subsurface state can be observed. Even at saturation doses the work function clearly stays below the values for the clean surface.

  93. Effects of Boundaries on Pattern Formation: Catalytic Oxidation of CO on Platinum

  94. M. D. Graham, I. G. Kevrekidis, K. Asakura, J. Lauterbach, K. Krischer, H.-H. Rotermund and G. Ertl

    Abstract

    The effect of boundaries on pattern formation was studied for the catalytic oxidation of CO on Pt surfaces. Photolithography was used to create microscopic reacting domains on polycrystaline and sigle crystaline Pt (110) surfaces with inert Ti overlayers. Certain domain geometries give rise to pattern that have not been observed on the untreated catalyst and bring to light surface mechamism that have no analog in homogeneous reactio-diffusion systems.


    last modified 10.11.2006 (C. Punckt)