Physico-chemical investigation of catalytic oxidation sites in 4%Rh/CeO₂ catalysts prepared by impregnation and deposition–precipitation methods

4 wt% Rh/CeO₂ catalysts were prepared by two different methods: deposition–precipitation (DP) and impregnation (Imp). X-ray diffraction (XRD), differential scanning calorimetry and thermogravimetry (DSC-TG), temperature-programmed reduction (TPR), and electron paramagnetic resonance (EPR) were used for physicochemical characterization. The solids were tested in propylene and carbon black oxidation reactions. The 4%Rh/CeO₂ (DP) showed better catalytic performance in both reactions compared to the catalyst prepared by the impregnation method. The XRD technique evidenced the formation of Rh₂O₃ phase in the DP-solid after calcination of this latter at 400 °C for 4 h. EPR evidenced, only in the DP-solid, the presence of O₂ ⁻ species in interaction with the CeO₂ surface whereas Rh⁴⁺ ions in the form of clusters were identified in both solids. The TPR technique showed that the DP-solid was reduced by hydrogen at lower temperature compared to the impregnated one. The higher catalytic performance of the DP-solid was attributed to the presence of O₂ ⁻ species along with the presence of Rh₂O₃ phase in ceria and to the better reducibility and lower particle size of the rhodium species.