Morphological and Structural Changes during the Reduction and Reoxidation of CuO/CeO2 and Ce_(1-x)Cu_xO2 Nanocatalysts: In Situ Studies with Environmental TEM, XRD, and XAS

摘要

We have studied the structural, morphological, and electronic properties of CuO/CeO2 and Ce_(1-x)Cu_xO2 nanocatalysts during reduction/oxidation cycles using H2 and O2 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/CeO2 biphase system with significant oxygen vacancies in CeO2. 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/CeO2 system reoxidized to form CuO through a Cu2O intermediate phase as expected. However, the Cu precipitated from the Ce_(0.8)Cu_(0.2)O2 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)O2 phase in solid solution. We found that ~50% of the Cu returned to a Ce_(1-x)Cu_xO2 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_xO2 but with a local bonding environment similar to CuO. The behavior of the reduced Ce_(0.8)Cu_(0.2)O2 reflects strong interactions between Cu and the ceria matrix and illustrates the advantages of working with solid solutions of mixed oxides.