Abstract
Abstract Photoemission technique was used to study the surface composition of PtPd, PtRu and PtIr alloys in a wide range of bulk compositions. The samples were submitted to in-situ oxygen exposures at atmospheric pressure and various temperatures in the 200 to 600°C range. Oxygen induced changes of surface composition were followed. Special attention was paid to the various oxidation states of the metals characterized by high-binding energies shifts of the core-levels. Oxidation properties of platinum but also of Pd, Ru and Ir drastically changed by alloying. The Pt-based alloys could be oxidized provided the impurity (i.e. Pd, Ru or Ir) surface concentration was higher than a limit situated between 13% and 20% for all three systems. Above this limit, the impurity and the platinum atoms were both oxidized. Three different Pt oxidation states were detected and identified with PtO, α-PtO 2 and β-PtO 2 , the occurrence of which depended on the composition and the temperature of oxidation treatment. In the low impurity content range, neither the impurity nor platinum could be oxidized. Several hypotheses (kinetics of oxidation, oxygen spill-over, percolation effects, changes in local electronic states) are discussed to elucidate this phenomenon.
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