Morphological and Structural Changes during the Reduction and Reoxidation of CuO/CeO2and Ce1–xCuxO2Nanocatalysts:In SituStudies with Environmental TEM, XRD, and XAS

We have studied the structural, morphological, and electronic properties of CuO/CeO 2 and Ce 1– x Cu x O 2 nanocatalysts during reduction/oxidation cycles using H 2 and O 2 as chemical probes. Time-resolved in situ characterization was performed by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) as well as aberration-corrected environmental transmission electron microscopy (ETEM). We have found that both types of nanocatalysts reduce to a Cu/CeO 2 biphase system with significant oxygen vacancies in CeO 2 . Important variations are seen in the Cu particle size and metal dispersion depending on the initial state of the copper oxide–ceria systems. During subsequent in situ oxygen annealing, the Cu precipitated from the CuO/CeO 2 system reoxidized to form CuO through a Cu 2 O intermediate phase as expected. However, the Cu precipitated from the Ce 0.8 Cu 0.2 O 2 solid solution behaved rather differently under oxidizing conditions, and neither oxidized to form CuO nor fully returned to a bulk Ce 0.8 Cu 0.2 O 2 phase in solid solution. We found that ∼50% of the Cu returned to a Ce 1– x Cu x O 2 solid solution, while the remainder was observed by in situ ETEM to form an amorphous copper oxide phase with a Cu oxidation state similar to Ce 1– x Cu x O 2, but with a local bonding environment similar to CuO. The behavior of the reduced Ce 0.8 Cu 0.2 O 2 reflects strong interactions between Cu and the ceria matrix and illustrates the advantages of working with solid solutions of mixed oxides.