CHEMICAL-PROPERTIES OF BIMETALLIC SURFACES - THE REACTION OF O2 AND NO2 WITH ZN ON RU(001)

The dissociation Of O2 and NO2 on Zn/Ru(001) surfaces and the properties of the resulting ZnO(x) adlayers have been investigated using TDS, XPS, and XAES. For submonolayer coverages of O and Zn on Ru(001), there is no lateral bonding between the adatoms. In these cases, strong Ru <-> O and Ru <-> Zn interactions prevent the formation of Zn-O bonds. ZnO(x) films were observed after increasing the Zn and O coverage above 1 monolayer. Some of these films displayed the typical O(1s) XPS and Zn L3M45M45 Auger spectra of polycrystalline ZnO, decomposing at temperatures between 800 and 1050 K. At 80 K, the initial dissociation probability of O2 on Zn1.0/Ru(001) is almost 2 orders of magnitude smaller than that on polycrystalline Zn. This reduction in the rate Of O2 dissociation is accompanied by a change in the type of oxygen formed: for the supported Zn monolayer, surface oxygen predominates; whereas for pure Zn, oxygen incorporated into the bulk of the film is the dominant species. The dissociation of NO2 is similar on Zn1.0/Ru(001) and polycrystalline Zn. At 80 K, the initial NO2 adsorption is dissociative: NO2,g --> NO(g) + O(a). The oxygen adatoms passivate the surface, and the subsequent adsorption of NO2 is molecular. Chemisorbed NO2 decomposes at temperatures between 160 and 250 K to yield gaseous NO and oxygen atoms that remain on the surface (Zn1.0/Ru(001) case) or migrate in to the bulk (pure Zn case). At 300 K, the probability for dissociation of NO2 on Zn surfaces is 2 orders of magnitude larger than the probability for dissociation of O2.