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Pattern formation; CO oxidation; Pt(110); control; catalysis
Abstract:
This thesis investigates the effects localized spatiotemporal perturbations have on a simple reaction diffusion system, the CO oxidaton on Pt(110). This reaction exhibits well-understood spatiotemporal patterns and is therefore well suited as a model system. The localized spatiotemporal perturbations are realized using a focused laser beam and two computer-controlled galvanic mirrors. It is shown that this setup allows precise ''writing'' of spatiotemporal temperature fields onto the surface of the Pt(110) single crystal.
Starting with simple perturbations -- the stationary laser spot -- this work analyzes the effects of progressively more complex spatiotemporal temperature fields. Already the stationary laser spot -- one point in time and space -- is found to show a considerable variety of phenomena: In the excitable regime CO waves are decelerated or deleted, while oxygen waves are accelerated or even created. For the creation of oxygen waves thresholds with respect to laser power and heating duration are determined. In the oscillatory regime target patterns with outwardly (both extended and localized) and inwardly propagating waves are observed and compared to simulations1.
In a second step combinations of two subcritical heating events are investigated in the excitable regime. The locally increased CO desorption at the site(s) of the perturbations is the key to understanding the observed temporal, spatial or spatiotemporal cooperation.
These findings are extended to laser spot movements along a line, both with supercritical laser power -- the stationary laser spot causes the emission of oxygen waves -- and with subcritical laser powers. Depending on its speed the supercritical laser either produces Mach-cone like waves, produces waves only on parts of its path or doesn't seem to affect the medium. The subcritical laser is used to ''drag'' a preexisting wave. For higher speeds the laser spot loses the wave. This loss is analyzed in detail by simulations2.
Further increasing the complexity of the perturbations the effect of locally periodic temperature field inhomogeneities on the homogeneously oscillating medium is investigated by moving the focused laser beam around a circle on the sample. Contrary to observations made earlier when forcing a system globally, no entrainment could be found. The observations are explained by extending the results found for the stationary laser spot.
Increasing the complexity even more, some examples for feedback experiments with spatiotemporally resolved detection and actuation are studied. Here, small changes in the feedback algorithm lead to significant changes in the combined system consisting of the reaction and the computer, e.g. the emergence of stable states.
This work also takes a look at the potential applicability of the newly acquired knowledge: The guiding of CO waves along a predetermined path thus ''delivering'' a chemical species to a certain location is demonstrated. Also, strategies for spatiotemporally variable operation of the reaction with the aim to increase CO2 production are explored. This goal is reached, however, only in a low CO2 production regime.
1M. Stich, Berlin.
2X. Li, Princeton and A. G. Papathanasiou, Berlin.
