Transition-metal ion dimers formed in CeO2: an EPR study

Abstract

MeCeO oxides (Me = Cu²⁺, Mn²⁺, V⁴⁺, Mo⁵⁺), with an atomic ratio Me/Ce = 0.1, have been prepared by impregnation of ceria (CeO₂) 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 (Δm_s=±2), 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.