Oxidation of palladium on Au(111) and ZnO(0001) supports.

The oxidation behavior of supported Pd-deposits on Au(111) and ZnO(0001) single crystals has been studied by x-ray photoemission spectroscopy (XPS). Oxidation has been carried out ex situ in a high-pressure cell and subsequent vacuum-transfer and characterization by ultra-high vacuum XPS, as well as in situ characterization by synchrotron based near ambient pressure XPS. On Au(111) alloying of Pd with the substrate competes with oxidation and only for sufficiently thick Pd films oxidation is obtained. For Pd deposits on ZnO the oxidation condition depends on the amount of deposited Pd. Thicker Pd-deposits behave similar to bulk Pd-samples, while for thinner films the oxidation temperatures may be lowered. Interestingly, for very small amounts of Pd, in situ XPS shows full oxidation at room temperature and at less than 0.6 mbar O2 pressure. This indicates lowering of the kinetic barriers for oxidation of very small supported Pd-clusters. The formed oxide is, however, not stable in ultra high vacuum and a slow reduction is observed. The instability of this oxide in UHV indicates that the formed Pd-oxide at the interface to ZnO may have different chemical properties compared to bulk PdO or surface oxides on Pd.