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 microscope is possible due to the preferential field ionization of molecular oxygen which acts as imaging gas and displays O(ad) sites with atomic resolution. A dynamic bistability of the oxidation reaction is caused by catalytically inactive CO(ad)-covered Pt surfaces and active O(ad)-covered ones. Self-sustained long-lasting oscillations of the CO + 1/2 O2 --> 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 selectivity is found, surface regions near the (110) planes, i.e. {331} terraces, 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.