CLUSTER-SIZE DEPENDENT KINETICS FOR THE OXIDATION OF CO ON A PD MGO(100) MODEL CATALYST/

A kinetic study of the dependence of the oxidation of CO on the size o f Pd clusters supported on MgO(100) has been carried out using molecul ar beam relaxation spectroscopy (MBRS) and transmission electron micro scopy (TEM). The reaction was studied in the temperature range from 40 0 to 600 K and in the cluster size range from 2.8 nm to 13 nm at O-2 a nd CO partial pressures of about 2 X 10(-5) Pa. It has been found that the steady-state reaction rate displays a strong dependence on the cl usters size and cluster density. This effect is mainly due to spillove r of CO from the support to the clusters. Even though this suggests th at the reaction proceeds quasi-identical for the different cluster siz es large differences in the shape of the MBRS signals, that is the kin etics of the reaction, have been found as a function of cluster size. The simulation of the reaction rate as a function of temperature for d ifferent cluster sizes shows that the spillover cannot explain the dat a for small cluster sizes. Therefore, we conclude that a size effect e xists for this particular reaction. Furthermore, we have been able to show that for temperatures below 500 K a second strongly bound CO adso rption state exists with a residence time that is about 10 times large r than that of the known CO state on single crystal surfaces.