Structural Characterization and Catalytic Evaluation of Supported Copper-Ceria Catalysts for Soot Oxidation

The present investigation was undertaken with the aim of designing a noble-metal-free diesel soot oxidation catalyst that can work at relatively low temperatures. Accordingly, a series CeO2-Al2O3, CeO2-ZrO2, and CeO2-SiO2 mixed-oxide-supported copper catalysts were prepared by a modified deposition precipitation method from ultradilute aqueous solutions and evaluated for soot oxidation at normal atmospheric pressure. The structural and electronic properties were investigated by various physicochemical techniques, namely, X-ray diffraction (XRD), Raman spectroscopy, temperature-programmed desorption (TPD), temperature-programmed reduction-oxidation (TPR-TPO), temperature-programmed mass spectroscopy (TP-MS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Formation of a Ce1-xCuxO2 solid solution was inferred from XRD, TPR, and Raman spectroscopy studies. The TEM results revealed the presence of copper ceria nano-oxides of similar to 5-7-nm size in highly dispersed form. The TPR-TPO results suggested a profound influence of mixed-oxide supports on the reduction behavior of copper oxide. In particular, the TP-MS results provided direct evidence for labile oxygen generation. Among the investigated catalysts, the CuO/CeO2-ZrO2 combination exhibited excellent catalytic activity for soot oxidation, with T-1/2 = 611 K. Incorporation of Cu2+ into the ceria-zirconia solid solution favored the creation of more structural defects, which accelerate the oxygen diffusion and induce more of the surface-active oxygen species that are responsible for the enhanced soot oxidation activity at lower temperatures.