Interaction of bimetallic Zn/Au(111) surfaces with O2 or NO2 and formation of ZnOx/Au(111)

Abstract Continuing our efforts to elucidate the working principle of Au/ZnO catalysts for the synthesis of green methanol from CO2 and H2, we present a systematic X-ray photoelectron spectroscopy (XPS) study on the interaction of bimetallic Zn/Au(111) surfaces with O2 and NO2 (O*), on the oxidation of Zn atoms in these bimetallic model systems and on the formation ZnOx species or thin films on these surfaces. At room temperature, we find little adsorption and essentially no oxidation of Zn atoms alloyed into the Au(111) near-surface region upon exposure to O2 or NO2, and accordingly no Zn de-alloying. LT interaction of O2 or NO2 with Zn submonolayer structures leads to higher oxygen and/or NO2 coverages. Formation of oxidic ZnOx species / structures occurs, however, only after annealing above RT, both after O2 and NO2 exposure. This involves partial conversion of adsorbed species into ZnOx. ZnO/Au(111) thin films, especially thicker ones, can be efficiently prepared by reactive deposition of Zn in O2 or NO2, followed by annealing. RT deposition in NO2 gives a higher fraction of (incompletely) oxidized ZnOx compared to O2, Fully oxidized ZnO is formed after subsequent annealing, upon thermally activated conversion of adsorbed Oad/OH species into oxidic oxygen and dissolution of remaining metallic Zn atoms into the Au bulk. Overall, the study provides detailed insights into the formation of ZnOx thin films on Au(111).