YCeZrO Ternary Oxide Solid Solution Supported Nonplatinic Lean-Burn NOx Trap Catalysts Using LaCoO₃ Perovskite as Active Phase

A series of nonplatinic ceria-based oxides supported catalysts LaCoO 3 /K 2 CO 3 /S (S=CeO 2, Ce 0.75 Zr 0.25 O 2 or 5%Y/Ce 0.75 Zr 0.25 O 2 ) were prepared by successive impregnation and employed for lean-burn NOx trapping. It is found that the doping of Zr or YZr into CeO 2 facilitates the formation of CeZrO binary or YCeZrO ternary solid solutions, increasing the specific surface area and improving the redox property of ceria. The results of EXAFS and H 2 -TPR reveal that the LaCoO 3 in the solid solution supported catalysts possesses higher dispersion and better reducibility than that in CeO 2 supported one. The result of O 2 -TPD shows that the surface active oxygen species are remarkably increased after loading LaCoO 3 on the supports. The YCeZrO ternary solid solution supported catalyst containing 5 wt % K 2 CO 3 exhibits the best performance for NO oxidation and reduction at 350 °C, showing a high NO-to-NO 2 conversion (66.5%) at lean condition, and a very high NOx reduction percentage (98.2%) and an extremely high NOx-to-N 2 selectivity (98.8%) at rich condition in the absence of CO 2 . After addition of 5 vol % CO 2 in the atmosphere, the NOx reduction percentage during NOx storage and reduction tests can still be maintained at high level (above 90%) in the temperature region of 350–400 °C. The results of FT-IR, CO 2 -TPD and in situ DRIFTS indicate that the potassium in catalysts exists as −OK groups, K 2 O, bulk or bulk-like K 2 CO 3 . At low K 2 CO 3 loading (≤5 wt %), NOx is stored as nitrates with diverse coordination structures, while at higher K 2 CO 3 loading (8 wt %) it is mainly stored as bulk nitrates without forming nitrites. Based upon all characterizations, a carbonate-involved NOx storage and reduction mechanism is revealed in molecular level.