Abstract:

CeO₂-based catalysts are promising for use in various important chemical reactions involving CO₂, such as the dry reforming of methane to produce synthesis gas and methanol. CeO₂ has a superior ability to store and release oxygen, which can improve the catalytic performance by suppressing the formation of coke. Although the adsorption and activation behavior of CO₂ on the CeO₂ surface has been extensively investigated in recent years, the intermediate species formed from CO₂ on ceria has not been clearly identified. The reactivity of the ceria surface to CO₂ has been reported to be tuned by introducing CaO, which increases the number of basic sites for the ceria-based catalysts. However, the mechanism by which Ca²⁺ ions affect CO₂ decomposition is still debated. In this study, the morphologies and electronic properties of stoichiometric CeO₂(111), partially reduced CeO_2-x(111) (0 < x < 0.5), and calcium-doped ceria model catalysts, as well as their interactions with CO₂, were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and synchrotron radiation photoemission spectroscopy. Stoichiometric CeO₂(111) and partially reduced CeO_2-x(111) films were epitaxially grown on a Cu(111) surface. STM images show that the stoichiometric CeO₂ film exhibits large, flat terraces that completely cover the Cu(111) surface. The reduced CeO_2-x film also has a flat surface and an ordered structure, but dark spaces are observed on the film. Different Ca-doped ceria films were prepared by physical vapor deposition of metallic Ca on CeO₂(111) at room temperature and subsequent annealing to 600 or 800 K in ultrahigh vacuum. The different preparation procedures produce samples with various surface components, oxidation states, and structures. Our results indicate that the deposition of metallic Ca on CeO₂ at room temperature leads to a partial reduction of Ce from the +4 to the +3 state, accompanied by the oxidation of Ca to Ca²⁺. Large CaO nanofilms are observed on CeO₂ upon annealing to 600 K. However, small CaO nanoislands appear near the step edges and more Ca²⁺ ions migrate into the subsurface of CeO₂ upon annealing to 800 K. In addition, different surface-adsorbed species are identified after CO₂ adsorption on ceria (CeO₂ and reduced CeO_2-x) and Ca-doped ceria films. CO₂ adsorption on the stoichiometric CeO₂ and partially reduced CeO_2-x surfaces leads to the formation of surface carboxylate. Moreover, the surface carboxylate species is more easily formed on reduced CeO_2-x with enhanced thermal stability than on stoichiometric CeO₂. On Ca-doped ceria films, the presence of Ca²⁺ ions is observed to be beneficial for CO₂ adsorption; further, the carbonate species is identified.

Key words: Calcium, Ceria, Scanning tunneling microscopy, X-ray photoelectron spectroscopy, Synchrotron radiation photoemission spectroscopy