Probing BaO Doping Effect on the Structure and Catalytic Performance of Pd/CexZr1–xO2 (x = 0.2–0.8) Catalysts for Automobile Emission Control

Abstract

Pd/CeO2-ZrO2-BaO (Pd/CZB) catalysts with 5.0 wt % BaO and different CeO2/ZrO2 molar ratio were synthesized. The modification with BaO significantly promotes the catalytic activity of HC and NOx conversions when the molar ratio of CeO2 to ZrO2 is 2/1, because the substitution of Cex+/Zr4+ ions by Ba2+ promotes the formation of oxygen vacancy as revealed by XRD refinement analysis results and improves the mobility of active oxygen as a result of electronic and structural modifications. Among the Ba-doped catalysts, the crystalline form of the CZB supports gradually transform from cubic and pseudocubic to stable tetragonal phase as the increase of ZrO2 content. Meanwhile, the Raman results show that addition of moderate ZrO2 promotes the formation of structural defects, resulting in higher OSC and NOx storage efficiency. On the other hand, the introduction of moderate ZrO2 facilitates the dissociative adsorption of NO and promotes the deep oxidation of HC. Therefore, Pd/CZB catalysts with CeO2/ZrO2 molar ratio of 3/1–1/1, especially Pd/CZB21, show a much better catalytic activity of HC and NOx eliminations and a wider dynamic operation window. Zr-rich catalysts present worse catalytic activity of CO oxidation compared with Ce-rich catalysts, which is mainly arising from the decreased amount of active sites. After thermal aging treatment, Zr-rich catalysts (CeO2/ZrO2 < 1) undergo less severe deterioration of the catalytic activity compared with Ce-rich catalysts (CeO2/ZrO2 ≥ 1) as a result of better thermostability, and Pd/CZB11-a presents the best catalytic performance and widest dynamic operation window.