The Role of Different Types of CuO in CuO-CeO2/Al2O3 for Total Oxidation

Different types of copper oxide in CuO-CeO2/Al2O3 were investigated structurally and catalytically. Standard preparation lead to the presence of CuO monocrystals and Ce1-xCuxO2-x as evidenced by XRD, HRTEM and EDX. Washing with nitric acid removed the monocrystals, while leaving the Ce1-xCuxO2-x solid solution. The Fourier transformed Cu K patterns for the HNO3-washed catalyst showed a diminished second Cu shell, characteristic for Ce1-xCuxO2-x as well as for X-ray amorphous nano-sized CuAl2O4 spinel phase. XANES analysis and first shell Cu fitting confirmed this CuAl2O4 like structure outside Ce1-xCuxO2-x. By modeling the Ce K EXAFS signal in the washed sample, the amount of Cu in Ce1-xCuxO2-x was determined to be 18 +/- 6 %. Total oxidation of CO and C3H8 as model reactions showed little difference in activity between non-washed and washed CuO-CeO2/Al2O3, indicating that CuO monocrystals were hardly active at temperatures below 450 degrees C. XRD performed in situ during TPR and TPO showed that CeO2 in Ce1-xCuxO2-x added to the redox capacity of both CuO-CeO2/Al2O3 samples as it was partially reduced at 300 degrees C. At higher temperatures, the CuO monocrystals also participated in reaction, leading to a higher activity for the oxidation of CH4.