Redox Properties of Nanocrystalline Cu-Doped Cerium Oxide Studied by Isothermal Gravimetric Analysis and X-ray Photoelectron Spectroscopy

Nanostructured CeO 2 - x and Cu 0.15 Ce 0.85 O 2 - x were synthesized by high-pressure magnetron sputtering. X-ray diffraction was used for phase analysis and grain size characterization. The structural analysis indicated that Cu was highly dispersed, which could be explained by surface segregation on cerium oxide nanocrystals. The reduction and oxidation of dispersed copper in nanostructured Cu 0.15 Ce 0.85 O 2 - x was investigated by isothermal gravimetric analysis (IGA) and X-ray photoelectron spectroscopy (XPS). In IGA, the sample weight was measured at constant temperature during in situ reduction and oxidation in 2% CO/He or 5% H 2 /He and 15% O 2 /He, respectively. The weight changes of microcrystalline CeO 2 and nanocrystalline CeO 2 - x and Cu 0.15 Ce 0.85 O 2 - x samples were measured at temperatures between 200 and 500 °C. The analysis revealed that more than one oxygen atom was extracted per Cu atom in Cu 0.15 Ce 0.85 O 2 - x during reduction, which was reversibly restored during oxidation. The Cu valence state in Cu 0.15 Ce 0.85 O 2 - x after oxidation or reduction was independently determined by XPS. The Cu 2p core-level spectra and LMM Auger spectra of Cu 0.15 Ce 0.85 O 2 - x were compared with those of a polycrystalline copper reference. The analysis showed that all three oxidation states of copper (i.e., 0, 1+, and 2+) could be present depending on the reduction/oxidation specifications. Furthermore, Cu 0 and Cu 1+ were found to coexist in a stable state under certain conditions.