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Imaging Pattern Formation in Surface Reactions from Ultrahigh Vacuum up to Atmospheric Pressures
(1995)
Two optical methods that allow pattern formation to be investigated at an arbitrary pressure are here applied to image concentration patterns of adsorbed species associated with heterogeneous catalytic reactions. In contrast to most surface physical techniques, these methods are not restricted to high vacuum conditions and thus bridge the “pressure gap.” With carbon monoxide oxidation on a (110) surface of platinum as an example, the coupling mechanisms responsible for spatiotemporal self-organization in surface reactions were followed from reaction-diffusion control to the thermokinetic region, associated with phenomena not previously observed in pattern formation.
Two surface-sensitive optical imaging methods, Ellipso-Microscopy for surface Imaging (EMSI) and Reflection Anisotropy Microscopy (RAM) are introduced. They allow imaging of pattern formation on surfaces, e.g., due to submonolayer coverages of adsorbates, at any arbitrary pressure.
In spatio-temporal pattern formation during heterogeneously catalysed reactions this bridges the ‘pressure gap’ between well-defined UHV experiments and ‘real catalysis’. For the CO oxidation on Pt(110), the parameter space for pattern formation was extended up to 100 mbar, i.e., by 5 orders of magnitude compared to earlier investigations by Photo-Emission Electron Microscopy (PEEM) which had to be conducted below 10−3 mbar. With increasing pressure, the synchronisation mechanisms responsible for the observed pattern showed a gradual shift from reaction-diffusion to thermal-kinetic coupling unveiling previously unseen features of pattern formation in catalysis.