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

Platinum field emitter tips ((111)-, (001)-, and (012)-oriented) are u sed to investigate local surface properties during the oscillatory reg ime of the CO-oxidation at T = 430-480 K and partial pressures p(O2) a lmost-equal-to 5 x 10(-4) mbar and p(CO) almost-equal-to 5 x 10(-5) mb ar. Changes in surface coverage - and catalytic activity - are indicat ed by variations of the local work function and field emission current . Self-sustaining isothermal oscillations could be observed on differe nt crystal planes. (i) A fast ''switching on'' of emission currents is initially noticed when the oxygen adsorption layer is transferred int o a CO-layer. Starting - after a certain induction period - from the ( 011)-region the oxygen side is quickly switched into the CO-side, whic h slowly returns back to the catalytically active oxygen side. (ii) Th e formation of reactive wave fronts starts in the (011)-region and pro pagates in the direction of (111)- and (100)-planes. The CO-wavefront and the O-wavefront move alternately in opposite directions and displa y different propagation velocities, which could be measured. The frequ encies and amplitudes of oscillations critically depends on temperatur es and partial pressures. Compared to single crystal planes a broadeni ng and shift of oscillatory regimes in parameter space is noticed. In certain cases period doubling could be measured.