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Abstract:
FeO(1 1 1) films grown on a Au(1 1 1) substrate were studied in the low temperature CO oxidation reaction at near-atmospheric pressure. Enhanced reactivity over the otherwise inert Au(1 1 1) surface was only observed if the iron oxide films possessed so-called “weakly bound oxygen” (WBO) species upon oxidation at elevated pressures. The reaction rate measured under O-rich conditions (CO/O2=1/5, totally 60 mbar, He balance to 1 bar) was found to correlate with the total amount of WBO measured in the “oxidized” films by temperature programmed desorption. The initial reaction rate measured as a function of the film coverage showed a maximum at about one monolayer (ML), in contrast to ≈0.4 ML obtained for the Pt(1 1 1)-supported FeO(1 1 1) films measured with the same setup. When compared to FeO(1 1 1)/Pt(1 1 1), WBO species on FeO(1 1 1)/Au(1 1 1) desorb at a much lower (i.e., by ≈200 K) temperature, but also in much smaller amounts. Scanning tunneling microscopy studies showed that the FeO(1 1 1) layer on Au(1 1 1) is fairly stable towards high pressure oxidation in the low coverage regime, but undergoes substantial reconstruction at near-monolayer coverages, thus resulting in poorly-defined structures. Comparison of structure–reactivity relationships observed for Au(1 1 1) and Pt(1 1 1) supported FeO(1 1 1) films revealed the complex role of a metal support on reactivity. Although a strong interaction with the Pt(1 1 1) surface stabilizes a planar FeO(1 1 1)-derived structure for the active oxide phase, in the case of a more weakly interacting Au(1 1 1) surface, the reaction atmosphere induces structural transformations governed by the thermodynamic phase diagram of the iron oxide, albeit it seems crucial to have a dense FeO(1 1 1) film as the precursor.
