Inhaltszusammenfassung

The catalytic CO oxidation on Iridium(lll) surfaces was investigated experimentally under ultrahigh vacuum (UHV) conditions using mass spectroscopy to detect gaseous products and photoelectron emission microscopy (PEEM) to visualize surface species. The underlying reaction-diffusion system based on the Langmuir-Hinshelwood mechanism was analyzed numerically. The effect of noise on this bistable surface reaction was examined. In a surface science experiment the effects on product formation and...The catalytic CO oxidation on Iridium(lll) surfaces was investigated experimentally under ultrahigh vacuum (UHV) conditions using mass spectroscopy to detect gaseous products and photoelectron emission microscopy (PEEM) to visualize surface species. The underlying reaction-diffusion system based on the Langmuir-Hinshelwood mechanism was analyzed numerically. The effect of noise on this bistable surface reaction was examined. In a surface science experiment the effects on product formation and the development of spatio-temporal patterns on the surface were explored. The influence of noise on the reaction rates and the formation of spatio-temporal patterns on the surface was analyzed by superposing noise of Gaussian white type on the feed gas composition, characterized by the molar fraction of CO Y (deviation AY), which represents multiplicative and additive noise. PEEM was used to visualize spatio-temporal adsorbate patterns on the surface as expected from the observations in the CO2 rate measurements. CO- and oxygen-covered regions on the Ir(lll) surface are visible in PEEM images as gray and black areas as a consequence of their work function contrast. Islands of the adsorbate, corresponding to the globally stable branch, are formed in a background of the other one. The long transient times are the result of the extremely slow domain wall motion of these islands (around 0.05 /iras-1). For small noise only a few islands nucleate and grow until they merge and finally fill the whole surface. With increasing noise the number of islands increases and their maximum size decreases. At constant AY the island wall velocity and the number of islands increases when Y approaches the hysteresis boundary. In the hysteresis region CO oxidation on Iridium(l11) surfaces is dominated by domain formation and wall motion for small to moderate noise strength. The island density increases with noise, but the wall velocity is independent of applied AY. The recorded islands show an elliptic shape and not a circular one as expected from the symmetries of the Ir(lll) surface. Experiments on CO oxidation on an intentionally stepped surface (Ir(977)) revealed that this observation is the result of an anisotropy in surface diffusion due to the limited terrace width on a real surface. The elliptic islands are aligned with the terraces and their aspect ratio is given by the corresponding wall velocities parallel and normal to the terraces.» weiterlesen» einklappen

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