P. J. Plath

Accelerating fronts during the electrodissolution of cobalt

Abstract Experimental studies have been carried out with ribbon cobalt electrodes in buffered phosphate solutions of different concentration and pH under potentiostatic control. Bistable as well as oscillatory behavior have been observed in the system. The visible fronts which occur in both bistable and oscillatory conditions undergo acceleration. The Flade potential and the dissolution time of the passive layer as a function of potential and pH have been determined and have been related to the spatiotemporal behavior of the system.

Rotating waves on disk and ring electrodes

Experimental studies have been carried out with cobalt disk and ring electrodes in buffered phosphate solutions under potentiostatic control with the Haber–Luggin capillary of the reference electrode placed in the centre of the electrode close to the surface.The applied potential is adjusted to be cathodic with respect to the Flade potential. Visible fronts outlining a propagating wave and characterizing the transition from the passive to the active state of the metal rotate around the tip of the reference electrode while the overall measured current remains almost constant. At applied potentials close to the Flade potential a modulation of the rotating wave and, correspondingly, of the current time curves, is observed.

Cooperative effects in heterogeneous catalysis. Part 1.—Phenomenology of the dynamics of carbon monoxide oxidation on palladium embedded in a zeolite matrix

Hysteresis behaviour, simple and complex oscillations showing a fractal pattern, long-term changes of the oscillations as well as periodic multistability have been uncovered during the oxidation of CO. Pd zeolites with different amounts of Pd particles per unit volume were used as a catalyst in order to study the synchronization process during the reaction. Zeolites with the highest Pd content showed the largest capability to oscillate, with amplitudes up to 95% of the conversion. Examination of the catalyst before and after the reaction leads to the assumption of a cyclic oxidation–reduction mechanism. Coupling of the individual particles is suggested by a concentration wave travelling across the surface.

Global coupling effects on spatiotemporal patterns on a ring electrode

Experimental results on spatiotemporal pattern formation on a ring electrode during the oscillatory electrodissolution of cobalt in 1.0 M phosphoric acid are reported; the ring is the area between two non-concentric circles and its width thus varies continuously with angle. The experiments were controlled potentiostatically with the tip of the capillary of the reference electrode placed in the center of the ring 1 mm above the surface. Under these conditions rotating waves, with and without modulation, occur. The influence of positive global coupling (in the form of an external resistor of variable size in series with the working electrode) on the pattern formation is investigated. A small added global coupling smoothes variations in speed and size of a rotating active area which are caused by the deformation of the ring and surface heterogeneities. A larger global coupling leads to non-traveling patterns including complete synchronization of the ring and also synchronization within subsections or smaller areas of the ring; these subsections themselves oscillate in a phase-locked or quasiperiodic fashion.

Cooperative effects in heterogeneous catalysis. Part 2.—Analysis and modelling of the temperature dependence of the oscillating catalytic oxidation of CO on a palladium Al2O3-supported catalyst

The temperature dependence of the dynamics of the heterogeneous oxidation of CO on palladium supported by an amorphous Al2O3 carrier has been studied. The complex structure of the observed time series has been investigated by fast-Fourier-transform analysis. The resulting spectra are characterized by at most three frequencies, the ratio of which is nearly an integer, and by frequency locking. For lower temperatures sub-harmonics of these frequencies become important. The fractal character of the time series could be explained qualitatively by a one-dimensional automaton representing a concentration wave travelling on a fractal network of Pd particles in the catalyst bed.

Dynamics of the Heterogeneous Catalytic Oxidation of Carbon Monoxide on Zeolite Supported Palladium

The complex temporal behavior of the heterogeneous catalytic oxidation of carbon monoxide on palladium and platinum catalysts has been studied intensively in recent years [1–12].