The structure and stability of surface platinum oxide and of oxides of other noble metals

The formation and stability of surface layers of platinum oxides in platinum single crystals has been studied in ultrahigh vacuum. Low energy electron diffraction (LEED) was used to identify the ordered structures that formed on the surface of Pt(111), Pt(332), and Pt(110). It appears that these structures can be related to hexagonal planes of PtO₂. The cleanliness of the surface was monitored by Auger electron spectroscopy (AES). The presence of impurities like Ca and Si must be avoided as they oxidize preferentially to the Pt. It is shown that the Pt oxide layers are stabilized by the very slow kinetics of oxygen diffusion to the surface which is responsible for the observed long life of the oxide layers under most catalytic reactions that are carried out at temperatures below 500°C. The stability of other oxides of noble metals that have been observed in UHV studies is also reviewed.