Surface oxidation of a Pt–20% CO alloy: An x-ray photoelectron spectroscopy and low-energy electron diffraction study on the [100] and [111] oriented single-crystal surfaces

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Abstract
The interaction of oxygen with the surface of a Pt alloy containing ∼20 at. % of cobalt was studied by low-energy electron diffraction (LEED) and x-ray photoelectron spectroscopy (XPS). At this composition, cobalt was found to be randomly substituted in the Pt fcc lattice both in the bulk and at the surface. [111] and 100] oriented single-crystal surfaces as well as polycrystalline surfaces were studied. Dosing these surfaces with oxygen at room temperature at pressures lower than ∼1×10−5 Torr resulted in the formation of a chemisorbed oxygen layer. At temperatures over ∼700 K dosing with oxygen in the same range of pressures led to the formation of a sequence of ordered oxide overlayers. XPS and LEED data showed that the first layer to form was an epitaxial monolayer structurally equivalent to either the (111) or (100) plane of cobalt monoxide (CoO). Under conditions of controlled oxygen dosing, CoO islands of monatomic thickness nucleated and randomly decorated a nearly pure Pt surface. This surface-decoration structure was metastable over a narrow range of oxygen partial pressures and surface temperature. At higher temperatures and pressures, the oxide islands first coalesced, forming a compact monolayer that completely blocked the surface for chemisorption, then changed in stoichiometry and structure to form a structural precursor to Co3 O4.

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