THE COOXIDATION ON PD-RICH SURFACES OF PDCU(110) - HYSTERESIS IN REACTION-RATES

The oxidation of carbon monoxide on PdCu(110) single crystal alloy sur faces was investigated. During temperature variations, at constant CO and O-2 pressures, regions of bistability could be detected by mass sp ectrometric reaction rate measurements, and a hysteresis was recorded. Initially, the top layer (TL) contained approximate to 100% Pd atoms. After CO-oxidation experiments Cu/Pd ratio was increased up to approx imate to 0.7, as recorded by AES with surface region (SR) analysis of approximate to 4 layers. This increase was due to a chemisorption indu ced segregation of Cu atoms from the bulk into the three subsurface la yers (3SSL), that lie beneath the TL. After prolonged CO-oxidation exp eriments, with CO pressures up to approximate to 10(-3) mbar, the LEED pattern changed from a(1 X 1) pattern, for the clean surface, to a c( 2 X 2) pattern. The CO-TDS measurements showed that after CO-oxidation measurements all the TDS maxima obtained shifted to lower temperature s. Observation of hysteresis with this alloy surface indicates that it may be possible to observe kinetic oscillations during this reaction.