Electronic Structure of Magnesia-Ceria Model Catalysts, CO2 Adsorption, and CO2 Activation: A Synchrotron Radiation Photoelectron Spectroscopy Study

We have studied the electronic properties of single crystal based ceria and magnesia-ceria model catalysts, the CO2, adsorption, and the CO2-induced reoxidation of these systems by synchrotron radiation photoelectron spectroscopy (SR-PES). All model systems were prepared starting from a fully stoichiometric and well-ordered CeO2(111) film grown on Cu(111). Different magnesia-ceria mixed oxide films were prepared by physical vapor deposition (PVD) of magnesium, oxygen treatment, and subsequent annealing. The preparation procedure was varied to obtain samples with different oxidation state, structure, and surface composition. Different carbon-containing species were identified, including surface carbonates formed in the vicinity of Mg2+ and Ce3+/4+ and surface carboxylates. The presence of Mg2+ was observed to strongly enhance carbonate formation but suppress the formation of carboxylates. Changes in the oxidation state of ceria upon CO2 exposure were monitored with highest sensitivity by resonant photoelectron spectroscopy (RPES). Reoxidation of Ce3+ was observed to be suppressed on magnesia-containing samples, even in the presence of a high surface concentration of Ce3+. The findings suggest that carboxylates are an intermediate step of reoxidation, whereas the generation of stable surface carbonates inhibits formation of this intermediate and, therefore, CO2-induced reoxidation