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 CO 2 activation and reforming of hydrocarbons. Towards a more detailed understanding of the underlying surface chemistry, we have investigated CO 2 activation on single crystal based ceria and magnesia/ceria model catalysts. All model systems are prepared starting from well-ordered and fully stoichiometric CeO 2 (111) films on a Cu(111) substrate. Samples with different structure, oxidation state and compositions are generated, including CeO 2-x /Cu(111) (reduced), MgO/CeO 2-x /Cu(111) (reduced), mixed MgO-CeO 2 /Cu(111) (stoichiometric), and mixed MgO-CeO 2-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 Ce 3+ , Ce 4+ , and Mg 2+ ions towards CO 2 at a sample temperature of 300 K. Briefly, we detect the formation of two CO 2 -derived species, namely carbonate (CO 3 2- ) and carboxylate (CO 2 - ) groups, on the surfaces of all investigated samples after exposure to CO 2 at 300 K. In parallel to formation of the carbonate species, slow partial reoxidation of reduced CeO 2-x /Cu(111) occurs at large doses of CO 2 . The reoxidation of the reduced ceria is largely suppressed on MgO-containing samples. The tendency for reoxidation of Ce 3+ to Ce 4+ by CO 2 decreases with increasing degree of intermixing between MgO and CeO 2-x . Additionally, we have studied the stability of the formed carbonate species as a function of annealing temperature.