ABSTRACT
Figure 6. 8c illustrates the situation which is reached if at 200 K CO exposure of an oxygen-saturated surface is continued. LEED investigations exhibit, in this case, the formation of a new (2 xl) overstructure which never was observed after chemisorption of oxygen or CO alone, and hence must be assumed to be a Inixed overstructure containing both species 0ads and COads in the unit cell. This conclusion is also supported by the XPS measurements exhibiting a shift in the CO-derived valence levels which was not observed when separate islands of 0ads and COads were present. The observed lowering of the ionization energy of the CO 20 level was explained in terms of partial electron transfer from CO to the neighboring atoms [23]. The local coverages within the domains of this mixed-surface phase are 80 = 8CO = 0.5. This means that the adsorbed ° atoms are further compressed. As the consequence nearly half of the surface becomes accessible for additional chemisorption of CO and in fact the appearance of a (2 xl) structure was always accompanied by the simultaneous appearance of a (/3 x /3) R 30° structure of COads'
Thermal desorption curves obtained after successive exposure of the Pd (111) surface to oxygen and subsequently to carbon monoxide are shown in Fig. 6.9. It is seen that with the surface covered with preadsorbed oxygen after a short exposure to CO the maximum rate of C02 formation was observed at about 480 K but is continuously shifted to lower temperatures with increasing exposure to CO. The simplest explanation of this effect is to assume that squeezing of the initial (2 x 2) overstructure of oxygen by increasing amounts of chemisorbed CO into the (/3 x /3) R 30° structure is accompanied by a decrease of oxygen bonding energy which also results in the lowering of activation energy of C02 formation. At higher exposures, e. g., 1 L of oxygen and more than 2 L of CO, a new peak at about 290 K is the result of formation of mixed-surface phase in which both 0ads and COads species are in intimate contact. In such a situation the activation energy of the reaction between both adsorbed species must be appreciably lower and the temperature of the new TDS peak must also be lower.
