Redox properties of nanocrystalline Cu-doped cerium oxide studied by isothermal gravimetric analysis and X-ray photoelectron spectroscopy

Nanostructured CeO2-x and Cu0.15Ce0.85O2-x were synthesized by high-pressur e magnetron sputtering. X-ray diffraction was used for phase analysis and g rain size characterization. The structural analysis indicated that Cu was h ighly dispersed, which could be explained by surface segregation on cerium oxide nanocrystals. The reduction and oxidation of dispersed copper in nano structured Cu0.15Ce0.85O2-x was investigated by isothermal gravimetric anal ysis (IGA) and X-ray photoelectron spectroscopy (XPS). In IGA, the sample w eight was measured at constant temperature during in situ reduction and oxi dation in 2% CO/He or 5% H-2/He and 15% O-2/He, respectively. The weight ch anges of microcrystalline CeO2 and nanocrystalline CeO2-x, and Cu0.15Ce0.85 O2-x samples were measured at temperatures between 200 and 500 degrees C. T he analysis revealed that more than one oxygen atom was extracted per Cu at om in Cu0.15Ce0.85O2-x during reduction, which was reversibly restored duri ng oxidation. The Cu valence state in Cu0.15Ce0.85O2-x after oxidation or r eduction was independently determined by XPS. The Cu 2p core-level spectra and LMM Auger spectra of Cu0.15Ce0.85O2-x were compared with those of a pol ycrystalline copper reference. The analysis showed that all three oxidation stares of copper (i.e., 0, 1+ and 2+) could be present depending on the re duction/oxidation specifications. Furthermore, Cu-0 and Cu1+ were found to coexist in a stable state under certain conditions.