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

Pd/CeO 2 -ZrO 2 -BaO (Pd/CZB) catalysts with 5.0 wt % BaO and different CeO 2 /ZrO 2 molar ratio were synthesized. The modification with BaO significantly promotes the catalytic activity of HC and NO x conversions when the molar ratio of CeO 2 to ZrO 2 is 2/1, because the substitution of Ce x + /Zr 4+ ions by Ba 2+ 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 ZrO 2 content. Meanwhile, the Raman results show that addition of moderate ZrO 2 promotes the formation of structural defects, resulting in higher OSC and NO x storage efficiency. On the other hand, the introduction of moderate ZrO 2 facilitates the dissociative adsorption of NO and promotes the deep oxidation of HC. Therefore, Pd/CZB catalysts with CeO 2 /ZrO 2 molar ratio of 3/1–1/1, especially Pd/CZB21, show a much better catalytic activity of HC and NO x 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 (CeO 2 /ZrO 2 < 1) undergo less severe deterioration of the catalytic activity compared with Ce-rich catalysts (CeO 2 /ZrO 2 ≥ 1) as a result of better thermostability, and Pd/CZB11-a presents the best catalytic performance and widest dynamic operation window.