Adsorption and Oxidation of CO on Co3O4/Ir(100) Thin Films

The combination of noble metals and reducible metal oxides often displays superior catalytic properties in low-temperature CO oxidation reactions. In the present study, we investigated the adsorption and activation of CO on submonolayer and monolayer CoQ(x) films deposited on Ir(100) at low temperature under ultra-high vacuum (UHV) conditions. Using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED), we found that the CoOx films have a (3 x 1) structure and consist of a Co-O bilayer and an O-Co-O trilayer associated with Co2+ and Co3+ states, respectively. A combination of XPS, temperature-programmed desorption (TPD), and infrared reflection absorption spectrum (IRAS) reveals that the presence of the interface of the submonolayer CoOx film and Ir significantly enhances the catalytic activity of cobalt oxide for CO oxidation. In contrast, a monolayer film of CoOx without exposing CoOx-Ir interface sites is hardly active for CO oxidation. CO adsorbs on the bridge sites of Ir atoms on the submonolayer CoOx/Ir(100) surface at 100 K. Upon heating, CO partially reacts with oxygen on the surface to produce CO2, which desorbs at 215 K, while unreacted CO molecules adsorbed on Ir bridge sites migrate to top sites. Increasing the temperature leads to CO2 desorption at 295 and 325 K, arising from the reaction of CO with chemisorbed oxygen on the Ir surface and oxygen of cobalt oxide, respectively. These results reveal the importance of the interface between Ir and cobalt oxide in the oxidation of CO.