Coverage and Substrate Effects on the Structural Change of FeOx Nanostructures Supported on Pt

Structural changes of FeOx nanostructures supported on Pt(111) and Pt foil with response to oxidation and reduction treatments in O-2 and H-2 atmospheres upto 1.0 bar have been investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. We show that submonolayer O-Fe bilayer (FeO) structure on Pt(111) can be transformed to O-Fe-O trilayer (FeO2) upon oxidation in 5.0 x 10(-6) mbar O-2, while the FeO to FeO2 transformation happens over the full FeO film only with the O-2 partial pressure above 1.0 x 10(-3) mbar. Reduction of the submonolayer FeO2 structure back to the FeO structure occurs when exposed to 1.0 mbar H-2 at room temperature (RT). In contrast, the full FeO2 structure can be kept even under 1.0 bar H-2 exposure condition. The FeOx coverage and FeOx/Pt boundary play a critical role in the redox behavior of the supported FeOx nanostructures. Furthermore, we show that the FeOx nanostructures supported on Pt foil can be oxidized in a similar way as those on the Pt(111) surface. However, the Pt foil supported FeO2 nanostructures can be more deeply reduced to the state close to metallic Fe in 1.0 mbar H-2 at RT. The close-packed Pt(111) surface exhibits a stronger confinement effect on the FeO overlayer than the open polycrystalline Pt surface.