SPIRALS AND PROPAGATING REACTION FRONTS DURING CATALYTIC CO OXIDATION ON A CYLINDRICAL PT SINGLE-CRYSTAL

The spatiotemporal patterns that are associated with kinetic oscillations in the catalytic CO oxidation have been investigated on a Pt single crystal of cylindrical shape whose axis is oriented in the [001] direction. The experiments were conducted in the 10(-5) and 10(-4) Torr range between 450 and 550 K using a photoemission electron microscope for imaging the spatial adsorbate distribution during the reaction. The cylinder surface exhibits all orientations of the [001] zone including (100), (110), and (210) whose oscillatory behavior has been studied before. On Pt (110) rotating spirals, target patterns, and ordered arrays of reaction fronts are observed, but these regular patterns were confined to a narrow orientational range of +/- 2-degrees around (110). Further away fluctuating irregular patterns are found which finally merge into the oxygen-covered surface characteristic of the more active orientations around (210). Under appropriate conditions kinetic oscillations can be found on (210) which are again associated with spatial pattern formation. The transition from a CO-covered to an oxygen-covered surface proceeds via a propagating reaction front, while the transition in the reverse direction takes place through the growth of small CO islands distributed randomly across the surface. Finally, an oscillation mode has been found in which the phase boundaries separating the oxygen-covered active zone from the inactive CO-covered zone on the cylinder surface periodically shifts thus causing an expansion/contraction cycle of the active zone. The phase boundary in between (210) and (100) can also act as a source for oxygen islands which are detached from the phase boundary and then shrink while traveling as a whole towards (100).