Study of CO Oxidation Activity and Chemical Characterisation of Cu/CeO2 Powder

The catalytic activity for CO oxidation over cerium dioxide (CeO2) and Cu-modified ceria (Cu/CeO2) powders prepared by wetness impregnation method was investigated in flow tube reactor system under atmospheric pressure. Reaction curves of powder samples pre-treated by calcination in mixture of argon and oxygen and by reduction in hydrogen, all at 300 °C were measured. X-ray photoelectron spectroscopy (XPS) was used for chemical analysis of the powders. The photoemission spectra obtained for as received and calcined sample before and after reaction are discussed in terms of ceria reducibility and Cu-Ce bond formation. Introduction It is known that cerium dioxide (CeO2) is an important catalyst in many chemical reactions, e.g. the NO reduction under oxidation conditions or CO oxidation under reducing conditions in automotive exhaust catalysts [Mullins 1998]. Several studies indicate that the chemical state of ceria is a critical factor dominating the catalytic behaviour. Noble metal catalysts have been found to be active for number of chemical reactions in the lower temperature range (170 – 300 °C). CO oxidation over Cu/CeO2 has been demonstrated by FlytzaniStephanopoulos [Liu 1994, Liu 1995]. The remarkable redox ability of CuO/CeO2 at lower temperatures was found to play an essential role in CO oxidation reaction. It was shown the catalytic behavior of CuO/CeO2 greatly depends on the preparation routes [Tang 2004]. The catalysts for fundamental studies are prepared by various conventional techniques such as impregnation, ion exchange, anchoring/grafting, spreading and wetting, hydrolysis, and homogeneous deposition– precipitation [Janssen 1997]. Presently, it is a new trend regarding novel chemical techniques of synthesis that can lead to ultra fine, high-surface-area catalysts for heterogeneous catalysis [Bera 1999]. A number of spectroscopic techniques have been used to assign the oxidation state of Ce in cerium oxides. Very common method is XPS of the Ce 3d core level [Pfau 1994, Burroughs 1976, Fallah 1995]. These spectra are characterized by complex but distinct features that are related to the final state occupation of the Ce 4f level [Pfau 1994, Burroughs 1976]. Deviations from the ideal bulk stoichiometry CeO2 lead to the reduction of Ce ions to Ce and to occupied electronic Ce 4 f states in the band gap. In the context of heterogeneous catalysis over CeO2-x and noble-metal-doped CeO2-x electronic states attributed to Ce ions at surface sites are of particular interest, e.g., for the CO oxidation. Experimental Preparation of Catalysts Wetness impregnation is the method we used for preparation of our samples. The cerium (IV) oxide powder (99.5 % min, Alfa Aesar, John Matthey Company) and copper (II) acetate (99.9 % metals basis, Cu(OOCCH3)2.H2O, Alfa Aesar, John Matthey Company) were used as the sources of cerium and copper, respectively. The powders were mixed with toluene (C7H8) in a magnetic stirrer for 16 hours and then dried. Sample calcination proceeded 2 hours in a flow of argon and oxygen. The temperature during calcination was 300 °C and the flow rates 200 ml.min for Ar and 50 ml.min for O2. For powder reduction we used a flow of hydrogen and the temperature 300 °C. WDS'05 Proceedings of Contributed Papers, Part III, 574–579, 2005. ISBN 80-86732-59-2 © MATFYZPRESS