Nanosized 2D ceria islands randomly distributed over the Pt(111) surface have been prepared by oxidation of the nucleating Ce submonolayer and characterized using XPS and STM for coverages of 0.3 and 0.7 ML. Catalytic CO oxidation over the resulting, well-defined CeO x /Pt(111) model catalytic system of the "inverse supported catalyst" type has been studied using the UHV chamber as a flow reactor. The CO 2 production rate was monitored mass spectrometrically in the temperature range of 413 to 553 K at variable CO/O 2 compositions in the 10 -5 mbar pressure range. The behavior of the present CeO x /Pt(111) system in the CO oxidation reaction is summarized in kinetic phase diagrams separating regions of high and low reactivity (both monostable) and that of bistability. A significantly enhanced reactivity and a remarkable shift of the bistable region of the reaction toward higher CO pressures were observed when compared to a clean Pt(111) surface. An "active border" concept is proposed to explain the strong local enhancement of catalytic activity.

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