Avenue Albert Einstein, 69626 Vi/leurbanne, France A copper-cerium oxide sample prepared with an atomic ratio, Cu : Ce, of 0.01 has been studied by electron paramagnetic resonance (EPR) spectroscopy. A well resolved spectrum of copper(11) ion pairs has been evi denced and the hyperfine structure of the perpendicular components clearly appears. The high resolution can be explained by the presence of two nearly equivalent Cu2+ ions separated by an oxygen ion. A correlation in EPR parameters has been found between the signal corresponding to a single Cu2+ ion, a precursor of the dimer, and the copper(11) ion pair spectrum.

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Conditions for preparing copper-cerium mixed oxides by coprecipitation have been examined. X-Ray diffraction (XRD), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the oxides calcined at different temperatures up to 1073 IS. In all cases, a solid solution is formed by the insertion of Cu2+ ions in substitutional Paper 5/04920B;

MeCeO oxides (Me = Cu2+, Mn2+, V4+, Mo5+), with an atomic ratio Me/Ce = 0.1, have been prepared by impregnation of ceria (CeO,) with the corresponding Me salt. Subsequently, the solids were oxidized or reduced at given temperatures. An EPR study evidenced the presence of dimers in the solids. A fine structure was obtained for CuCeO and MoCeO oxides. The signal at half magnetic field intensity (Am, = f2), characteristic of dimers, was present in all the MeCeO oxides with a hyperfine structure for Cu2+, Mn2+ or V4+ ions. From the relative intensity of the half-field and the normal-field signals characterizing the dimer spectrum, the distance between the Me ions has been calculated. For CuCeO and MoCeO oxides, monomer ions have been evidenced as precursors of the dimers. The dimer spectrum of Mo5+ ions has been observed only at 77 K and not at room temperature. In all cases, the dimers are located in oxygen vacancies near Ce3+ ions, present in the solid before the introduction of transition-metal ions.

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