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Catalysis; surface chemical reaction; ellipsometry
Abstract:
CO oxidation on platinum has become a model reaction in surface science due to its simple reaction mechanism. Studies carried out under ultrahigh vacuum conditions with single crystals had shown that for a constant feed of the reactants the reaction rate can be oscillatory in time. With the invention of spatially-resolved adsorbate sensitive methods, as e.g. "Photoemission Electron Microscopy” (PEEM), the pattern formation of adsorbates was imaged for the first time. Utilizing the results gained under ultrahigh vacuum conditions in the industrial process is often not possible, due a pressure difference of approximately six orders of magnitudes between the two systems. Bridging this "pressure gap” turns out to difficult, because many of the surface scientist’s tools, which rely on the detection of electrons, cannot be employed due to the electron’s pressure dependent free mean path. The use of optical methods offers an alternative, since they are almost pressure independent. The methods, which in part were developed and employed in this thesis, are "Ellipsomicroscopy for Surface Imaging” (EMSI), "Reflection Anisotropy Microscopy” (RAM), "Fourier Transform Infrared Imaging” (FTIR Imaging), and "Infrared Thermography” (IRT). For EMSI the improvement of the image contrast allowed the surface reconstruction to be imaged indirectly for the first time. The observation of a so-called "memory effect” on Pt(110) gained new insight into the mechanisms of CO oxidation. Due to the thermal activated lifting of the surface reconstruction reconstructed patches on a non-reconstructed surface could be observed over an extended period of time. Detailed studies of the contrast mechanism of EMSI and RAM accomplished a more thorough understanding of the two methods. With the achieved knowledge a temperature regime was found, in which the adsorbate coverage and the surface reconstruction could be imaged independently for the first time. Studies of CO oxidation on Pt(100) in the mbar pressure range showed the formation of a thermal and chemical stable surface oxide, which was promoted by the surface reconstruction. Global oscillations in surface temperature and the formation of temperature patterns were observed for the first time for CO oxidation on polycrystalline platinum in the mbar pressure range. These temperature changes could be explained by the periodic formation and reduction of an surface oxide. Furthermore with RAM individual crystallites of a polycrystalline surface were imaged and identified as (110)-patches for the first time. Besides CO oxidation on platinum EMSI was also successfully employed to image microstructured xylyl dithiol monolayers on GaAs. Employing the optical techniques gained new insight into CO oxidation on platinum. The measurements on Pt(100) and polycrystalline platinum are a first step towards bridging the pressure gap and at the same time prove the pressure independence of the optical methods. The observed oxide formation demonstrates the pressure dependence of catalytic reactions.
