Nanostructured Ceria-Zirconia as an Oxygen Storage Component in Three-Way Catalytic Converters-Thermal Stability

摘要

Nanostructured CeO_2 and CeO_2-ZrO_2 solid solutions with Ce/Zr ratios of 0.75/0.25 and 0.50/0.50 were produced from aqueous solutions by precipitation/coprecipitation and hy-drothermal crystallization in an autoclave at 180℃ for times up to 10 h. The thermal stability of these systems was studied by X-ray diffraction (XRD), differential thermal analysis (DTA) thermogravimetric analysis (TG) and transmission electron microscopy (TEM). Initial crystallite size was about 3nm for all powder compositions as measured by XRD peak broadening. Powders were calcined in air at 300℃, 500℃, 800℃ and 1000℃ for periods of up to 600 minutes. The crystallite sizes in pure Ceria, Ce75/Zr25 and Ce50/Zr50 powders were 137 nm, 23 nm and 37 nm respectively after 600 minutes at 1000℃ indicating a significantly higher thermal stability, i.e. greater resistance to coarsening, in nanostructured ceria-zirconia solid solutions. TEM images of the pure ceria powder showed clear evidence of sintering after calcination at 1000℃. In contrast, TEM images of the ceria-zirconia solid solution powder calcined under similar conditions showed it to be still loosely agglomerated with significantly smaller crystallite size. This finding suggests that ceria-zirconia solid solution have higher effectiveness in service compared with pure ceria due to higher thermal stability.