Nature of Active Sites on Cu-CeO2 Catalysts Activated by High-Temperature Thermal Aging

Cu clusters supported on CeO2 nanorods (NRs) were found to exhibit significantly enhanced activity and thermal/hydrothermal durability for low-temperature CO oxidation when the catalyst was calcined at 800 degrees C in air prior to the catalytic measurements. The effect of calcination temperature and Cu loading on Cu-CeO2 catalysts was studied by the combination of powder X-ray diffraction (PXRD), Raman spectroscopy, H-2-temperature program reduction (H-2-TPR), ambient-pressure X-ray photoelectron spectroscopy (APXPS), synchrotron radiation photoelectron spectroscopy (SRPES), and X-ray absorption spectroscopy (XAS). Cu atoms were found to incorporate into the ceria lattice in air at below 500 degrees C, leading to the formation of a bulk CuyCe1-yO2-x solid solution. As the calcination temperature was raised to 800 degrees C, the solubility of Cu ions in bulk ceria was reduced sharply. Surface segregation of Cu atoms led to the formation of CuOx and a surface Cu-doped ceria thin layer, resulting in a much enhanced activity for CO oxidation. Moreover, the activities of calcined Cu-CeO2 catalysts did not increase with the Cu loading but exhibited an optimal activity at similar to 2 wt % of Cu loading, where segregated CuOx species gave an average size in the sub-nanometer (sub-nm) range. Larger CuOx nanoparticles (NPs) on the Cu-doped ceria thin layer exhibited a lower activity toward CO oxidation. We believe the enhanced catalytic properties of the thermally aged Cu-CeO2 catalysts are attributed to the formation of an interface between sub-nm CuOx and the Cu-doped ceria thin layer. Our study thus sheds light on the nature of most active sites on Cu/CeO2 catalysts and facilitates the rational design of ceria-supported metal catalysts for oxidation reactions.