The combination of a reducible transition metal oxide and a noble metal such as Pt are often active low-temperature catalysts for the preferential oxidation of CO in excess H2 gas...

Ultrathin cobalt oxide films supported on noble-metal surfaces have received much attention as interesting examples of low-temperature CO oxidation catalysts. It is expected that the activity of the cobalt oxides is closely linked with the structure and morphology of the film, but a direct operando correlation between CO oxidation activity, nanoscale structure, oxidation state, and surface composition has been missing. Here, we use a combination of operando ambient pressure scanning tunneling microscopy and ambient pressure X-ray photoelectron spectroscopy to investigate varying submonolayer coverages of CoO supported on Pt(111) under CO oxidation conditions. The goal is to compare the structural and spectroscopic features as the samples are exposed to O-rich CO/O 2 gas mixtures at millibar pressure and brought to temperatures where CO oxidation occurs. Upon first exposure to millibar gas mixtures, the initial bilayer CoO film is oxidized to trilayer CoO 2 , characterized by a preserved film morphology and Co in a predominant 3+ oxidation state. However, upon temperature increase during the CO oxidation reaction, the cobalt oxide ultrathin film undergoes dewetting into nanoparticles. On the basis of the XPS signature, we conclude that these nanoparticles have a Co 3 O 4 -like structure. The results underline the importance of operando observations of surface structures. This new insight into the Co oxide/metal interface may aid in our understanding of reactivity of metal oxide coated noble-metal particles in general.