Adsorption and Activation of CO on Co3O4(111) Thin Films

To explore the catalytic properties of cobalt oxide at the atomic level, we have studied the interaction of CO and O2 with well-ordered Co3O4(111) thin films using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HR-XPS), infrared reflection absorption spectroscopy (IRAS), and temperature-programmed desorption spectroscopy (TPD) under ultrahigh vacuum (UHV) conditions. At low coverage and temperature, CO binds to surface Co2+ ions on the (111) facets. At larger exposure, a compressed phase is formed in which additional CO is located at sites in between the Co2+ ions. In addition, a bridging carbonate species forms that is associated with defects such as step edges of Co3O4(111) terraces or the side facets of the (111) oriented grains. Preadsorbed oxygen neither affects CO adsorption at low coverage nor the formation of the surface carbonate, but it blocks formation of the high coverage CO phase. Desorption of the molecularly bound CO occurs up to 180 K, whereas the surface carbonate decomposes in a broad temperature range up to 400 K under the release of CO and, to a lesser extent, of CO2. Upon strong loss of crystalline oxygen, the Co3O4 grains eventually switch to the CoO rocksalt structure.