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Abstract

CO oxidation is studied on inverse clean metallic Cu⁰ and ZnO_x-modified Cu⁰ model catalysts. ZnO_x films with a coverage of 1.8 monolayers exhibit a pronounced low-temperature CO oxidation rate maximum at ˜500 K, followed by an intermediate rate minimum. Initially, a fully Cu°-shielding ZnO_x layer, consisting of layer-by layer grown Zn° and Wurtzite-like ZnO domains at the Zn° island perimeter, shields the metallic Cu substrate from the reaction mixture and protects it from oxidative deactivation by oxygen up to ≈450 K. Above this temperature, thermal Zn desorption from Zn⁰ patches sets in, which leads to the in-situ formation of an active ZnO_x-Cu⁰ phase boundary. Once formed, this boundary strongly speeds up the delivery of oxygen to the bare Cu⁰ surface and, thus, the reaction rate to CO₂. In due course, also the oxidation of Cu⁰ to Cu₂O is enhanced, leading to the observed deactivation. Structurally, the overall process resembles the breakdown of a passivating layer, leading to localized corrosion and fast oxidation to Cu₂O, together with corresponding dewetting of Zn⁰ by desorption, ZnO formation and Zn⁰ alloying into the copper bulk at higher temperatures.