OSCILLATIONS IN THE CARBON-MONOXIDE OXIDATION ON PLATINUM SURFACES OBSERVED BY FIELD ELECTRON-MICROSCOPY

Platinum field emitter tips ((111)-, (001)-, and (012)-oriented) are used to investigate local surface properties during the oscillatory regime of the CO-oxidation at T = 430-480 K and partial pressures p(O2) almost-equal-to 5 x 10(-4) mbar and p(CO) almost-equal-to 5 x 10(-5) mbar. Changes in surface coverage - and catalytic activity - are indicated by variations of the local work function and field emission current. Self-sustaining isothermal oscillations could be observed on different crystal planes. (i) A fast ''switching on'' of emission currents is initially noticed when the oxygen adsorption layer is transferred into a CO-layer. Starting - after a certain induction period - from the (011)-region the oxygen side is quickly switched into the CO-side, which slowly returns back to the catalytically active oxygen side. (ii) The formation of reactive wave fronts starts in the (011)-region and propagates in the direction of (111)- and (100)-planes. The CO-wavefront and the O-wavefront move alternately in opposite directions and display different propagation velocities, which could be measured. The frequencies and amplitudes of oscillations critically depends on temperatures and partial pressures. Compared to single crystal planes a broadening and shift of oscillatory regimes in parameter space is noticed. In certain cases period doubling could be measured.