Atmospheric pressure oxidation of Pt(111)

The oxidation of the platinum (111) single crystal surface and the formation of platinum dioxide have been studied by in situ surface x-ray diffraction and x-ray reflectivity. At an oxygen partial pressure of 500 mbar and at temperatures from 520 to 910 K, the experiments disclose the growth of two atomic layers of a bulk-like, strongly distorted alpha-PtO2. The epitaxial Pt oxide layer is oriented hexagon on hexagon with respect to the Pt(111) surface leading to a (8 x 8) superstructure surface unit cell. In a second set of experiments, a 100 A thick epitaxial Pt(111) film on sapphire was exposed to oxygen. At 670 K and near atmospheric oxygen pressures we find the formation of a several angstrom thick oxide layer. After annealing the sample at 720 K in a vacuum the oxide layer desorbs, recovering the 100 A thick Pt film. Subsequent oxidation at 720 K and reduction cycles lead to a slight increase in surface roughness and the formation of macroscopically visible holes in the Pt film. These results point to a Pt re-dispersion which sets in during chemical reactions at atmospheric pressures.