The characterisation and reactivity of nanostructured cerium-copper-oxide composites for environmental catalysis
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The characterisation and reactivity of nanostructured cerium-copper-oxide composites for environmental catalysis Othon Adamopoulos1, Yu Zhang1, Mark Croft2, Igor Zakharchenko3, Thomas Tsakalakos3, Mamoun Muhammed1 1 Dept of Materials Science and Engineering, The Royal Institute of Technology, Stockholm S-100 44, Sweden 2 Dept of Physics, and 3 Dept of Ceramics and Materials Engineering, Rutgers University, Piscataway, NJ, U.S.A ABSTRACT Nanophase Ce-Cu-O powder has been synthesised via co-precipitation approach, whereas computer-assisted thermodynamic modelling was used to predict the optimum experimental criteria. The properties of Ce-Cu-O composite (10 mol% CuO) have been extensively investigated by a variety of techniques. The obtained materials show different morphology compared to other doped ceria salts, with uniform distribution of Ce and Cu throughout the powder. Detailed XRD study does not show the formation of solid solution and the composites consist of two separate metal oxides. X-ray absorption spectroscopy study shows a variation of the existence of Cu and Ce in several oxidation states. Copper oxide seems to exist evenly distributed along the surfaces and boundaries of the crystal grains of CeO2 on the nanoscale. The temperature-programmed reduction indicated that Ce-Cu-O composite has an enhanced oxygen storage capacity compared to CeO2 or pure copper oxide. INTRODUCTION The catalytic properties of some oxide ceramics and their solid solution derivatives are greatly dependent upon the particle size, the structural distortion and the chemical nonstoichiometry. When the grain size decreases below 100 nm, the density of the defects increases. Stoichiometric defects such as oxygen deficiency (vacancy) and its mobility on the catalyst surface are of great importance for redox gas reactions. Therefore, by doping ceria, CeO2, with several isovalent or noniso-valent cations, its catalytic behaviour becomes more intense [1]. CeO2 plays significant roles in several catalytic processes [2]. In recent years, it has been extensively used as a promoter in three-way catalysts (TWCs) for automotive emission control. Under various redox conditions, the oxidation state of cerium may vary between +3 and +4. Owing to its nonstoichiometric behaviour, CeO2 has proved to be a good promoter for oxygen storage, while some noble metals are used as major catalysts. Related to the system of Ce-Cu, it has been reported that 10% of Cu gets doped in ceria lattice [3]. Herein it is reported, hardly is Cu fully doped in CeO2 face centred crystal structure, and the physicochemical properties are investigated. EXPERIMENTAL DETAILS Computer modelling of chemical equilibrium was used to design the optimum coprecipitation conditions. The synthesis took place at specially constructed laboratorial synthesis Y8.11.1
reactor, where the addition of the solutions was accomplished simultaneously and automatically. Afterwards, the precipitate was washed and dried at 105 oC overnight. The precursor was calcined at 400oC, 500oC and 600oC for two
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