THE SURFACE SPECIFICITY OF THE OSCILLATING CO OXIDATION ON PLATINUM INVESTIGATED BY FIELD-ION MICROSCOPY

The in-situ observation of the CO oxidation in the field ion microscop e is possible due to the preferential field ionization of molecular ox ygen which acts as imaging gas and displays O(ad) sites with atomic re solution. A dynamic bistability of the oxidation reaction is caused by catalytically inactive CO(ad)-covered Pt surfaces and active O(ad)-co vered ones. Self-sustained long-lasting oscillations of the CO + 1/2 O 2 --> CO2 reaction are investigated at p(CO) almost-equal-to 10(-6) to 10(-5) mbar, p(O2) almost-equal-to 1 X 10(-4) to 5 X 10(-4) mbar, and temperatures of T = 330 K to 550 K. Oscillation frequencies increase from 10(-3) s-1 at 330 K to 1 s-1 at 550 K. A pronounced surface selec tivity is found, surface regions near the (110) planes, i.e. {331} ter races, act as pacemakers. The Pt{111} planes are always CO(ad)-covered and do not participate in oscillations. Some general conclusions are drawn on the surface specificity of catalytic surface reactions and on still open questions of oscillating surface reactions.