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 oxidation of CO on a Pt(110)-surface was investigated by means of photoemission electron microscopy (PEEM). The properties of these patterns can be tuned through the control parameters, viz., the partial pressures of CO and O2 and the sample temperature. Over a wide range of control parameters the transition between two metastable states (COad and O(ad) covered surface) proceeds via nucleation and growth of elliptical reaction-diffusion (RD)-fronts. Front velocities and critical radii for nucleation are determined by the diffusion of adsorbed CO under reaction conditions. If at constant p(O2), T the CO partial pressure is increased beyond a critical value a transition to qualitatively different dynamic behavior takes place. The elliptical fronts give way to oxygen spiral waves of excitation spreading across the CO-covered areas. For fixed experimental conditions a broad distribution of spatial wavelengths and temporal rotation periods was found. This effect has to be attributed to the existence of surface defects of mum-size to which the spiral 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 investigated under the influence of externally modulated temperature. Now the spiral starts to drift, resulting in distortion of the Archimedian shape and a pronounced Doppler effect.