Reaction between CO and a pre-adsorbed oxygen layer on supported palladiumclusters

The transient kinetics of reaction between carbon monoxide and an oxygen mo nolayer pre-adsorbed on palladium clusters supported on MgO(100), has been studied for various cluster sizes (4-15 nm), in the temperature range 120-4 00 degrees C, using molecular beam and mass spectrometry under ultrahigh va cuum. When the CO beam is opened, the CO2 production rate first increases i nstantaneously, and then increases slowly to its maximum, before decreasing to zero due to the lack of oxygen. The period of slow increase of the reac tion rate, namely the induction period, appears at about 200 degrees C and becomes longer when temperature increases. Although it is not observed on P d(111), this peculiar reaction kinetics does not depend on cluster size, an d is attributed to a precursor state of CO chemisorption. The oxygen covera ge at saturation is found equal to 0.4. At this high oxygen coverage, gaseo us CO physisorbs above the oxygen adlayer. At high temperature, the precurs or is more likely to desorb, which reduces its chemisorption probability, a nd thus the CO2 production rate. The temperature-dependant kinetics of CO a dsorption and reaction with oxygen has been simulated thanks to a simple ki netic model accounting for the precursor mechanism. (C) 2000 Elsevier Scien ce B.V. All rights reserved.