N2O temperature-programmed oxidation and EXAFS studies on the dispersion of copper in ceria-supported nanocatalysts

The dispersion of copper in ceria-supported catalysts was investigated. By variation of the copper loading in a series of CuxCe1-xO2-x samples, it was possible to separate the oxidation of the copper component and the cerium oxide support during N2O temperature-programmed oxidation (TPO). Quantitative analysis confirmed that the copper component was completely reduced by hydrogen and completely oxidized by N2O at 300 degrees C. Extended X-ray absorption fine structure (EXAFS) studies on as-prepared powders as well as on annealed samples were performed under reducing/oxidizing conditions at elevated temperatures. Structural refinement during simulation suggested that in the oxidized state, copper was mostly present as isolated ions, whereas small clusters of metallic copper were found under reducing conditions. A wetting/de-wetting transition of the copper component on the ceria surface was suggested to cause this change in morphology. Annealing at high temperatures, which resulted in a substantial decrease in the specific surface area of the catalyst, also induced the precipitation of CuO as a second phase, as confirmed by X-ray diffraction. The enhanced redox activity of dispersed copper in the as-prepared samples as compared to that of the copper bulk phase in annealed samples was evident from in situ EXAFS measurements.