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Controlled Hydrothermal Synthesis of Complex Mixed Oxides Using Solution Redox Chemistry Richard I. Walton,*1 Kripasindhu Sardar,1 Helen Y. Playford,1 Deena R. Modeshia,1 Richard J. Darton,1 Janet Fisher2 and David Thompsett2 1

Department of Chemistry, University of Warwick, Coventry, CV4 7AL, United Kingdom.

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Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading, RG4 9NH

United Kingdom. ABSTRACT We present the results of a study of the solvothermal synthesis of mixed-metal ceriumcontaining oxides all prepared from CeCl3.7H2O at less than 250 oC in single step reactions. The use of NaBiO3 in the presence of aqueous NaOH yields fluorite solid solutions Ce1-xBixO2-x/2 (x ≤ 0.6), whereas the use of either H2O2 or NaBrO3 as oxidant in the presence of TiF3 yields a Ce(IV) pyrochlore (Na0.33Ce0.67)2Ti2O7. With replacement of a fraction of the Ti reagent by Sn(IV) acetate, tin doping is possible in the pyrochlore. The materials have all been assessed for their use in catalysis by performing temperature programmed reduction (TPR) experiments under dilute hydrogen flow. The cerium-bismuth oxides show large and apparently reversible hydrogen uptake, but in situ powder X-ray diffraction shows that this is accompanied by phase separation into bismuth metal and CeO2 that occurs over 5 or more TPR cycles. In contrast, the cerium (IV) titanate pyrochlore shows reversible reduction at low temperature (150 oC, after an activation step), which gives the material potential use as a precious metal support for catalysis: such as in the water-gas-shift reaction. Although Sn doping lowers the onset of reduction of the pyrochlore, consistent with an expanded lattice, the materials suffer from collapse to give SnO. INTRODUCTION Cerium dioxide, CeO2, is well-known for its uses in catalysis [1,2], where it has important applications in areas of environmental concern, such as in automotive catalytic converters, and in water gas shift (WGS, the conversion of CO and water to CO2 and hydrogen). In these situations CeO2 is used as a redox-active solid support for precious metals: this relies on the oxide storage properties of the materials, that derive from the open fluorite lattice and the ability of cerium to convert reversibly between the +4 and +3 oxidation states [1,2]. There is a considerable drive to produce catalysts that operate at low temperatures with the desire to reduce burning temperatures of fuels, for example, and in the case of WGS the need to improve efficiency in equilibrium where the forward reaction is exothermic. Doping of ceria is an established strategy to tune its redox properties, and a large variety of dopants have been studied, from aliovalent metal ions such as Y3+, Gd3+, Sm3+ and Sr2+ [3-5], to isovalent metals such as Zr4+ [6] and Sn4+ [7]. In the latter situation a distortion of the ceria lattice may aid oxide ion migration, and in the former case

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this may be coupled with the introduction of a greater concentration of oxide-ion vacancies that enhance oxygen storage. The case of Zr-doped