Thermal analysis and temperature-programmed reduction studies of copper–zirconium and copper–zirconium–yttrium compounds

Differential thermal analysis evidenced that introduction of copper or yttrium into zirconia matrix delays its crystallization temperature. This result was connected to the stabilization of the zirconia tetragonal phase observed by X-ray diffraction. In addition, it was observed a decomposition delay of supported copper nitrate with yttrium content in a zirconia support. Temperature-programmed reduction revealed the existence of different copper species: isolated copper ions, copper clusters, small particles of copper oxide and bulk copper oxide. The reduction temperature of these species was strongly influenced by the preparation method of Cu–Zr systems and by the presence of yttrium in ZrO2. But in all the samples, isolated copper ions and clusters were postulated to be reduced at the same temperature since these species have the same electron paramagnetic resonance parameters and then occupy the same sites. A correlation was done between the reducibility of these copper species and their activity in propene and toluene oxidation reactions. In the absence of yttrium, the best activity measured was correlated to an easier reduction but also to a higher quantity of isolated copper ions and clusters. Addition of yttrium to the support modified the copper interaction. At low copper/zirconium atomic ratio (0.01), the reduction temperature of small CuO particles increased with the yttrium content in the support. At high copper content (Cu/Zr = 0.1), the easier copper species reduction was obtained for Y2O3 content of 5 wt.%, the corresponding solid providing the best activity in the propene oxidation.