Support Effects on CO Oxidation on Metal-supported Ultrathin FeO(111) Films

FeO(111) films grown on a Au(111) substrate were studied in the low temperature CO oxidation reaction at near-atmospheric pressure. Enhanced reactivity over the otherwise inert Au(111) 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/O-2=1/5, totally 60mbar, He balance to 1bar) 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 approximate to 0.4ML obtained for the Pt(111)-supported FeO(111) films measured with the same setup. When compared to FeO(111)/Pt(111), WBO species on FeO(111)/Au(111) desorb at a much lower (i.e., by approximate to 200K) temperature, but also in much smaller amounts. Scanning tunneling microscopy studies showed that the FeO(111) layer on Au(111) 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(111) and Pt(111) supported FeO(111) films revealed the complex role of a metal support on reactivity. Although a strong interaction with the Pt(111) surface stabilizes a planar FeO(111)-derived structure for the active oxide phase, in the case of a more weakly interacting Au(111) 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(111) film as the precursor.