Oxidation of CO on Palladium

Aside from a series of earlier papers (163-173), during the past decade numerous studies on the oxidation of CO were performed under well-defined conditions with single crystals (2, 88, 155, 172-176) as well as with poly- 

Surface Interactions between Oxygen and Carbon Monoxide 

Co-adsorption and mutual interactions between the reactants on the surface form the basis for understanding the microscopic steps of the reaction. Since product formation takes place rather rapidly above room temperature, this information mainly became available from low-temperature studies. As a result, these processes are much more complicated than can be described by a Langmuir-type adsorption model (i.e., simple competition for free adsorption sites) and, moreover, an asymmetric behavior is found which means that pre-adsorbed CO inhibits the adsorption of oxygen, whereas the reverse is not the case. At very low surface concentrations of CO and Oad these will be randomly distributed over the surface as illustrated schematically by Fig. 32a (88). 

With 0CO 1/3 (i.e., for coverages beyond the completion of the y/3 x y/3 R 30° structure) a Pd(lll) surface is no longer able to dissociatively adsorb oxygen. Since this is a necessary prerequisite for C02 formation, the reaction is inhibited by CO if its coverage is too high. At lower CO concentrations on the surface oxygen can be co-adsorbed. Both components then form separate domains on the surface [competitive adsorption (182)] as becomes evident from LEED observations (172). The mean domain diameter is at least of the order of 100 A i.e., the coherence width of the electrons used with this technique. This indicates the existence of repulsive interactions between Oad and COad. As can be seen from the schematic sketch of Fig. 32b, eventual product formation can then only occur along the boundaries of these islands. 

The situation becomes more complicated if oxygen is adsorbed first. Even if the surface is saturated with Oad (forming a 2 x 2 structure with 0O = 0.25 as illustrated by Fig. 33a), it is able to adsorb considerable amounts of CO. The first molecules compress their environment of Oad into domains of a y/i x 3 R 30° structure with a local coverage of 0O, oc = 0-33 (which can never be reached by exposing a clean Pd(l 11) surface to 02 alone). Completion of this stage is illustrated by Fig. 33b and is still characterized by a 

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