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Zusammenfassung:
CO oxidation is studied on inverse clean metallic Cu0 and ZnOx-modified Cu0 model catalysts. ZnOx 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 ZnOx 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 ZnOx-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.
