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

Structural changes of FeO_x nanostructures supported on Pt(111) and Pt foil with response to oxidation and reduction treatments in O₂ and H₂ 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 × 10⁻⁶ mbar O₂, while the FeO to FeO₂ transformation happens over the full FeO film only with the O₂ partial pressure above 1.0 × 10⁻³ mbar. Reduction of the submonolayer FeO₂ structure back to the FeO structure occurs when exposed to 1.0 mbar H₂ at room temperature (RT). In contrast, the full FeO₂ structure can be kept even under 1.0 bar H₂ exposure condition. The FeO_x coverage and FeO_x/Pt boundary play a critical role in the redox behavior of the supported FeO_x nanostructures. Furthermore, we show that the FeO_x 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 H₂ at RT. The close-packed Pt(111) surface exhibits a stronger confinement effect on the FeO overlayer than the open polycrystalline Pt surface.