FIELD-ION MICROSCOPIC STUDIES OF THE CO OXIDATION ON PLATINUM - BISTABILITY AND OSCILLATIONS

The oscillating CO oxidation is investigated on a Pt-field emitter tip by using the field ion mode of surface imaging of O-ad sites With O-2 as imaging gas. Based on data of the titration reactions [V. Gorodets kii, W. Drachsel, and J. H. Block, J. Chem. Phys. 100, 6907 (1994)], e xternal control parameters for the regions of bistability and of self- sustained isothermal oscillations could be found. On a field emitter t ip, oscillations can be generated in a rather large parameter space. T he anticlockwise hysteresis of O-2(+) ion currents in temperature cycl es occurs in agreement with results on single crystal planes. Unexpect ed regular oscillation sequences could occasionally be obtained on the small surface areas of a field emitter tip and measured as function o f the CO partial pressure and of the temperature. Different stages wit hin oscillating cycles were documented by field ion images. Oscillatio ns of total ion currents are correlated with variations in the spatial brightness of field ion images. In the manifold of single crystal pla nes of a field emitter {331} planes around the {011} regions are start ing points for oscillations which mainly proceed along [100] vicinals. This excludes the {111} regions from autonomous oscillations. With sl ightly increased CO partial pressures fast local oscillations at a few hundred surface sites of the Pt(001) plane display short-living CO is lands of 40 to 50 Angstrom diameter. Temporal oscillations of the tota l O-2(+) ion current are mainly caused by surface plane specific spati al oscillations. The synchronization is achieved by diffusion reaction fronts rather than by gas phase synchronization.