CO2 activation on single crystal based ceria and magnesia/ceria model catalysts

Novel multifunctional ceria based materials may show an improved performance in catalytic processes involving CO2 activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO2 activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO2(111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO2-x/Cu(111) (reduced), MgO/CeO2-x/Cu(111) (reduced), mixed MgO-CeO2/Cu(111) (stoichiometric), and mixed MgO-CeO2-x/Cu(111) (reduced). The morphology of the model surfaces is characterized by means of scanning tunneling microscopy (STM), whereas the electronic structure and reactivity is probed by X-ray photoelectron spectroscopy (XPS). The experimental approach allows us to compare the reactivity of samples containing different types of Ce3+, Ce4+, and Mg2+ ions towards CO2 at a sample temperature of 300 K. Briefly, we detect the formation of two CO2-derived species, namely carbonate (CO3 (2-)) and carboxylate (CO2 (-)) groups, on the surfaces of all investigated samples after exposure to CO2 at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO2-x/Cu(111) occurs at large doses of CO2. The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce3+ to Ce4+ by CO2 decreases with increasing degree of intermixing between MgO and CeO2-x. Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.