REACTION-DIFFUSION PATTERNS IN THE CATALYTIC COOXIDATION ON PT(110) -FRONT PROPAGATION AND SPIRAL WAVES

The dymanic behavior of elliptical front propagation and spiral-shaped excitation concentration waves associated with the catalytic oxidatio n of CO on a Pt(110)-surface was investigated by means of photoemissio n electron microscopy (PEEM). The properties of these patterns can be tuned through the control parameters, viz., the partial pressures of C O and O2 and the sample temperature. Over a wide range of control para meters the transition between two metastable states (COad and O(ad) co vered surface) proceeds via nucleation and growth of elliptical reacti on-diffusion (RD)-fronts. Front velocities and critical radii for nucl eation are determined by the diffusion of adsorbed CO under reaction c onditions. If at constant p(O2), T the CO partial pressure is increase d beyond a critical value a transition to qualitatively different dyna mic behavior takes place. The elliptical fronts give way to oxygen spi ral waves of excitation spreading across the CO-covered areas. For fix ed experimental conditions a broad distribution of spatial wavelengths and temporal rotation periods was found. This effect has to be attrib uted to the existence of surface defects of mum-size to which the spir al tip is pinned. These data lead to a dispersion relation between the front propagation velocity and the wavelength, respectively, period. In addition, the dynamics of free spiral-shaped excitation waves was i nvestigated under the influence of externally modulated temperature. N ow the spiral starts to drift, resulting in distortion of the Archimed ian shape and a pronounced Doppler effect.