Photoemission Spectroscopy Study of Cu/CeO 2 Systems: Cu/CeO 2 Nanosized Catalyst and CeO 2 (111)/Cu(111) Inverse Model Catalyst Vladimı ´r Matolı ´n,* ,† Libor Sedla ´ c ˇ ek, † Iva Matolı ´nova ´ , † Frantis ˇek S ˇ utara, † Toma ´ s ˇ Ska ´ la, ‡ Br ˇ etislav S ˇ mı ´d, † Jir ˇ ı ´ Libra, † Va ´ clav Nehasil, † and Kevin C. Prince ‡ Charles UniVersity, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, V Holes ˇoVic ˇ ka ´ ch 2, 18000 Prague 8, Czech Republic, and Sincrotrone Trieste, Strada Statale 14, km 163.5, 34012 BasoVizza-Trieste, Italy ReceiVed: September 26, 2007; In Final Form: December 17, 2007 Cerium oxide films equivalent to 2 ML of CeO 2 were grown at 520 K in an oxygen atmosphere on a clean Cu(111) substrate in order to prepare a model catalytic system. This “inverse model catalyst” was characterized by low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS) of core levels, and resonant photoelectron spectroscopy (RPES) of the valence band. Samples annealed at 770 K exhibited a LEED pattern corresponding to the (1.5 × 1.5) CeO 2 (111)/Cu(111) structure that can be interpreted as formation of a flat, well-ordered cerium oxide overlayer aligned with the principal crystallographic axes of the substrate. The model catalytic system corresponds well to a copper-loaded ceria nanopowder catalyst that exhibits growth of Cu(111) film structure on CeO 2 (111) planes. Lowering of the CO oxidation temperature due to the Cu loading is explained by CO adsorption on copper in the vicinity of highly active ceria planes providing oxygen for the reaction. 1. Introduction It is known that cerium dioxide (CeO 2 ) is an important catalyst in many chemical reactions, for example, NO reduction under oxidizing conditions or CO oxidation under reducing conditions in automotive exhaust catalysts. 1 Several studies indicate that the chemical state of ceria is a critical factor dominating the catalytic behavior. Cu-CeO 2 is a highly active catalyst for CO oxidation by oxygen and for water gas shift. 2-5 The remarkable redox ability of CuO-CeO 2 at lower temper- ature was found to play an essential role in CO oxidation reactions. It was found that only a small amount of Cu promotes CeO 2 catalytic activity by several orders of magnitude, 6-12 but the reaction mechanism is not fully understood. Model studies of the metal-cerium oxide catalysts were the principal motivation of cerium oxide growth studies on single- crystalline transition-metal substrates, Pt(111), 13,14 Rh(111), 15 Re(0001), 16 Au(111). 17 Siokou and Nix 18 grew cerium oxide on Cu(111) by depositing 10 ML of Ce at room temperature. Freshly deposited cerium was oxidized at room temperature by oxygen exposure giving Ce 2 O 3 ; annealing in oxygen to 930 K gave Ce 4+ oxide and discontinuous layers. In previous studies, it was shown that the reduction of CeO 2 resulted in the formation of oxygen vacancies on the cerium oxide surface and a CeO 2 (111) f Ce 2 O 3 (0001) phase transition, which corresponds in general to the crystal structure transition from the cubic fluorite lattice (Fm3m space group) of CeO 2 with the layer sequence -O 2- -Ce 4- -O 2- - to Ce 2 O 3 . 14-16 Bulk Ce 2 O 3 has the hexagonal crystal structure (P-3m1) characterized by stacking of complete Ce and O layers with -Ce 3+ -O 2- - Ce 3+ -O 2- -O 2- - repeated in the [0001] direction. 19 The electronic structure of the CeO 2 oxide is characterized by unoccupied 4f states of Ce 4+ (4f 0 ) while the Ce 2 O 3 oxide has a Ce 3+ (4f 1 ) configuration. 20 Different 4f configurations for Ce 4+ and Ce 3+ result in different core-level and valence-band (VB) structures. 21,22 Photoelectron spectroscopy is a powerful tool for Ce 4f state investigation. There are many spectroscopic data showing different 4f configurations using Ce 3d and Ce 4d core- level and Ce VB spectra 1,13,14,16,23,24 including resonant tech- niques in the Ce 4d-4f photoabsorption region. 4,25-31 One of the important properties of ceria is its oxygen storage capacity, which can provide oxygen to the gas mixture in catalytic contexts. The key factor for this property is the reversible transformation from Ce 4+ to Ce 3+ . The interaction at the interface between the ceria and the added metal may promote this behavior and consequently the catalytic activity for CO oxidation, perhaps via the creation of active sites at the oxide-metal boundary. 32,33 This is conventionally described as strong metal-support interaction (SMSI). In order to understand these interactions, we investigated the valence-band states of nanosized ceria powder doped with copper by means of resonant photoelectron spectroscopy in the Ce 4d-4f photoabsorption region. The obtained results were compared with those of a similar study on the model inverse catalyst prepared by growing CeO 2 (111) islands on a Cu(111) substrate. 2. Experimental Details Metal-loaded ceria powder was prepared by a conventional impregnation technique. The CeO 2 powder of submicrometer particles (Alfa Aesar, 99.5% purity) was added to a toluene solution of Cu(O 2 C 2 H 3 ) 2 ‚H 2 O, containing 8 wt % of metal with respect to ceria. The mixture was stirred and then evaporated under vacuum at 333 K to remove toluene and dry the sample. In order to decompose copper acetate, we reduced the powder * Corresponding author. E-mail: matolin@mbox.troja.mff.cuni.cz; tel: +420 221 912 323; fax: +420 283 072 297. † Charles University. ‡ Sincrotrone Trieste. 3751 J. Phys. Chem. C 2008, 112, 3751-3758 10.1021/jp077739g CCC: $40.75 © 2008 American Chemical Society Published on Web 02/14/2008