UV-PHOTOEMISSION ELECTRON-MICROSCOPY INVESTIGATION OF PATTERN-FORMATION DURING OXIDATION OF CO ON A PLATINUM(210) SURFACE

The formation of spatiotemporal patterns during catalytic CO oxidation on a Pt(210) surface was followed using photoemission electron microscopy (PEEM). Depending on the choice of reaction parameters (flow rate, P(CO), P(O2), sample temperature) the reaction exhibited both steady and oscillatory rates. In the steady state, the surface was covered by either oxygen or carbon monoxide. Oscillatory behaviour occurred over a narrow range of parameters and 02 pressures > 10(-4) Torr in the transition region between the two steady states. The appearance of oscillations was preceded by the nucleation of small oxygen islands and the formation of reactive wavefronts which frequently led to target or spiral patterns. Interestingly, the formation of spirals is often preceded by the rupture of one of the inner target pattern rings. Preferential nucleation of oxygen islands was observed close to surface defects on both a microscopic and a macroscopic scale (scratches etc.). For most cases the velocity of the reactive fronts was isotropic and was not correlated with any particular azimuthal symmetry direction of the (210) surface. Our results underline the usefulness of PEEM in the study of pattern formation and reactive diffusion processes on surfaces, and can thus help to clarify the microscopic reaction mechanisms.