Structural Characteristics and Redox Behaviors of Ce₁_-<i>_x</i>Cu<i>_x</i>O<i>_y</i> Solid Solutions

Abstract

Nanosized Ce1-xCuxOy materials were prepared by complexation−combustion method. The structural characteristics and redox behaviors were investigated using X-ray diffraction (XRD), temperature programmed reduction (H2-TPR), UV−Vis, and Raman spectroscopies. In XRD patterns, no evidence of CuO diffraction peaks are observed for the Ce1-xCuxOy samples calcinated at 650 °C for 5 h, until the Cu/(Ce + Cu) ratio is higher than 0.4. The stepwise decrease of the 2θ value of CeO2 in Ce1-xCuxOy with the increasing of Cu concentration suggests that the Cu2+ ions incorporate into the CeO2 lattice to form Ce1-xCuxOy solid solutions for low Cu/(Ce + Cu) ratios (x ≤ 0.1). The CuO phase begins to segregate from the solid solutions with the further increasing of Cu/(Ce+Cu) ratio. The Raman mode at 1176 cm-1 ascribed to the enhanced defects appears for CeO2 and the Ce0.9Cu0.1Oy solid solution. Compared with CeO2 alone, the Raman mode of cubic CeO2 shifts from 462 to 443 cm-1 for the Ce0.9Cu0.1Oy solid solution. The H2 consumption of the fresh Ce0.95Cu0.05Oy is 1.65 times higher than that needed to reduce CuO to Cu, and it increases to 2.4 after a reoxidation of the partially reduced Ce0.95Cu0.05Oy at 300 °C, which indicates that the CeO2 phase is also extensively reduced. Compared with the high Cu/(Ce+Cu) ratio sample Ce0.7Cu0.3Oy, the Ce0.9Cu0.1Oy solid solution shows high and stable redox property even after different reoxidation temperatures. When the reoxidation temperature exceeds 200 °C, the α peak (∼170 °C) ascribed to the reduction of surface oxygen disappears, and the β peak (∼190 °C) ascribed to the reduction of Cu2+ species and the partial reduction of bulk CeO2 shifts to higher temperatures with the H2 consumption 1.16 times higher than that for fresh sample. The result demonstrates that the redox property of the CeO2 is significantly improved by forming the Ce1-xCuxOy solid solutions.