Abstracts Biological Sciences

Kinetic Oscillations and Spatiotemporal Self-Organization in Electrocatalytic Reactions

by Klaus-Peter Strasser

Numerous electrochemical systems exhibit spontaneous, dynamical instabilities upon sufficient displacement from chemical equilibrium by means of an overvoltage (spontaneous self-organization). The most inportant regimes emerging from a single, stable state are bistability, kinetic oscillations or deterministic chaos. Also, in the presence of an appropriate spatial coupling allows the occurrence of complex spatial regimes such as propagating or stationary waves. In the present thesis, spontaneous current oscillations during the electrocatalytic oxidation of formic acid on platinum was investigated both experimentally and theoretically. A kinetic model allowed the simulation of relevant dynamical features and resulted in a detailed mechanistic understanding of the underlying reaction processes. Spatially resolved measurements of the local electrode potential along a ring electrode (the reference electrode being in the center) revealed intriguing spatially inhomogeneous behavior of the interfacial potential: ´Remote triggering´ of activation fronts under bistable conditions and ´Standing Potential Waves´ indicated a negative non-local migration coupling across the electrolyte. Another chapter deals with the mechanistic basis of current and potential oscillations during the electroctalytic reduction of iodate on noble metal electrodes. A simple kinetic model again helped elucidate the underlying destabilizing electrochemical mechanism. Finally, experimental feedback control methods - applied to electrochemical oscillators - were shown to provide valuable information for the assignment of mechanistic roles to individual chemical species. Based on information known from literature and all information gathered from the present thesis, the last chapter suggests a mechanistic classification scheme of oscillatory, electrochemical systems. Moreover, an experimental, operational method was proposed for the purpose of a systematic, stepwise classification of unknown electrochemical oscillators. </BODY> </HTML> Zahlreiche elektrochemische Reaktionssysteme zeigen jenseits einer kritischen Überspannung spontane dynamische Instabilitäten (dissipative Strukturen) wie z.B. die Bistabilität zweier stationäre Zustände, kinetische Ratenoszillationen oder deterministisches Chaos. In der Gegenwart von räumlichen Kopplungen aufgrund von Diffusion oder Migration können zusätzlich spontane, räumliche Inhomogenitäten von Systemvariablen (stationäre oder laufende Wellen oder Domänen) auftreten. Die experimentelle Untersuchung, die kinetische Modellierung sowie das mechanistische Verständnis solcher dynamischen Muster anhand von ausgewählten, elektrokatalytischen Modellreaktionen ist der Gegenstand der vorliegenden Arbeit. Die elektrokatalytische Oxidation von Ameisensäure an Platinelektroden bildet einen ersten thematischen Schwerpunkt. Spontane Stromoszillationen auf niederindizierten Einkristallflächen wurden mit elektrochemischen Methoden charakterisiert. Hierauf aufbauend…