Item

Abstract

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 + 12 O₂ → CO₂ reaction are investigated at pCO ≈ 10^-6 to 10^-5 mbar, pO₂ ≈ 1 × 10^-4 to 5 × 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.