Investigation of Binary and Ternary Cu−V−Ce Oxides by X-ray Diffraction, Thermal Analysis, and Electron Paramagnetic Resonance

Abstract

The influence of vanadyl and copper precursors, impregnated on ceria, was evaluated by X-ray diffraction (XRD), thermal analysis (TG-DSC), and electron paramagnetic resonance (EPR) of binary and ternary oxides. The formation of a copper oxalate phase from copper nitrate and vanadyl oxalate was revealed during the preparation of the ternary oxide (1Cu1V10Ce). Three types of Cu(II) species in the dried copper containing solids were evidenced: (i) Cu(II) cations with an elongated octahedral symmetry attributed to the copper nitrate precursor, (ii) a copper oxalate phase with a compressed octahedral symmetry and (iii) well-dispersed Cu2+ ions on the ceria surface, located in a tetragonally distorted octahedral crystal field and surrounded by less than six ligands. The dispersion of the copper(II) cations over the ceria support surface was facilitated by the copper nitrate precursor. The EPR intensities clearly show that the increase of the oxalate precursor content induces a large fraction of copper that escapes detection by EPR and could be consistent with the presence of large Cu(II) agglomerates. After the calcination of the solids at 300 °C, only one copper species was evidenced and assigned to Cu2+ ions located in octahedral sites tetragonally distorted. The distortion was more pronounced in the presence of vanadium than in the case of the copper cerium oxide samples. The high dispersion of the copper(II) cations over the ceria support surface, owing to the copper nitrate precursor, was confirmed even after its thermal decomposition.