Laterally varying surface concentrations associated with the oscillatory oxidation of carbon monoxide on a Pt(l 10) surface were imaged by photoemission electron microscopy. Depending on the applied conditions, a large variety of spatiotemporal patterns were observed that are characteristic for the nonlinear dynamics of reaction-diffusion systems.PACS numbers: 68.35.-p, 82.65.-i Chemical reactions far from equilibrium may exhibit, even if operated under stationary continuous-flow conditions, various phenomena of temporal and spatial selforganization. Effects belonging in these categories have already been widely investigated, both experimentally and theoretically, with homogeneous reactions in solution, in particular, with the famous Belousov-Zhabotinskii (BZ) reaction. 1 Corresponding studies with heterogeneous reactions occurring at well-defined single-crystal surfaces were initiated only more recently. 2 Among the latter, the catalytic oxidation of CO on a Pt(llO) surface was found to exhibit a very rich variety of oscillatory kinetics. The underlying microscopic mechanism has been explored in great detail, enabling satisfactory theoretical modeling of the temporal behavior. 2,3 Generally, the occurrence of temporal variations of the concentrations of the species involved in the reaction implies that these also vary spatially, unless local differences are eliminated by "stirring," i.e., rapid convection. With the BZ reaction (and other homogeneous reactions in solution), the resulting spatiotemporal concentration patterns can easily be made visible by the associated color differences. More refined techniques are, however, required to image local variations of surface properties. For example, recent attempts to image patterns associated with the CO oxidation reaction include the application of scanning low-energy electron diffraction, 4 as well as scanning photoemission microscopy 5 (SPM) and photoemission electron microscopy (PEEM). 6 The latter technique is particularly versatile due to its high temporal (-10 ms) and lateral (-0.1 j/m) resolution, and a newly designed instrument of this type 7 was applied in the present work to image an unprecedented rich variety of spatiotemporal patterns associated with the oscillatory reaction of catalytic CO oxidation on Pt( 110).The principle of the PEEM technique consists in illuminating the sample surface with UV light from a deuterium lamp (with its cutoff near 6.9-eV photon energy) and to image the lateral intensity distribution of the photoemitted electrons through a system of electrostatic lenses after amplification by a channel plate onto a "back-view" fluorescence screen. The intensity of photoemitted electrons depends on the (local) work function O which in turn is affected by the dipole moments of the adsorbate complexes.In the current investigation, Oof a clean Pt(l 10) surface increases 0.3 eV when saturated with CO, and 0.5 eV through oxygen chemisorption. Therefore, areas covered by O a d appear dark in the images, while those covered by CO a d are brigh...

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.

The dynamic behavior of elliptical front propagation and spiral-shaped excitation concentration waves associated with the catalytic oxidation of CO on oa Pt ( 110) -surface was investigated by means of photoemission electron microscopy (PEEM). The properties of these patterns can be tuned through the control parameters, viz., the partial pressures of CO and O 2 and the sample temperature. Over a wide range of control parameters the transition between two metastable states (CO ad and Oad covered surface) proceeds via nucleation and growth of elliptical reactiondiffusion (RD)-fronts. Front velocities and critical radii for nucleation are determined by the diffusion of adsorbed CO under reaction conditions. If at constant Po 2 , T the CO partial pressure is increased beyond a critical value a transition to qualitatively different dynamic behavior takes place. The elliptical fronts give way to oxygen spiral waves of excitation spreading across the CO-covered areas. For fixed experimental conditions a broad distribution of spatial wavelengths and temporal rotation periods was found. This effect has to be attributed to the existence of surface defects of /Lm-size to which the spiral tip is pinned. These data lead to a dispersion relation between the front propagation velocity and the wavelength, respectively, period. In addition, the dynamics of free spiral-shaped excitation waves was investigated under the influence of externally modulated temperature. Now the spiral starts to drift, resulting in distortion of the Archimedian shape and a pronounced Doppler effect.

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