Structural and kinetic aspects of CO oxidation on ZnOx-modified Cu surfaces

CO oxidation is studied on inverse clean metallic Cu-0 and ZnOx-modified Cu-0 model catalysts. ZnOx films with a coverage of 1.8 monolayers exhibit a pronounced low-temperature CO oxidation rate maximum at (similar to)500 K, followed by an intermediate rate minimum. Initially, a fully Cu degrees-shielding ZnOx layer, consisting of layer-by layer grown Zn degrees and Wurtzite-like ZnO domains at the Zn degrees island perimeter, shields the metallic Cu substrate from the reaction mixture and protects it from oxidative deactivation by oxygen up to approximate to 450 K. Above this temperature, thermal Zn desorption from Zn-0 patches sets in, which leads to the in-situ formation of an active ZnOx-Cu-0 phase boundary. Once formed, this boundary strongly speeds up the delivery of oxygen to the bare Cu-0 surface and, thus, the reaction rate to CO2. In due course, also the oxidation of Cu-0 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 Zn-0 by desorption, ZnO formation and Zn-0 alloying into the copper bulk at higher temperatures.