(426f) Effect of the Catalyst Mileage on the Oxidation Properties of Pd-Based Twc Contained in Wcc near Engine

The latest US emission levels for gasoline engine vehicle require the application of the WCC (warm-up catalytic converter) in order to meet the legislation limits. Especially, palladium based TWC is one of the most efficient catalysts for reducing hydrocarbon (HC) emissions in the cold-start of the automobile as well as less expensive than Pt and/or Rh containing TWC. However, thermal sintering and poisoning of the catalysts have always been an issue for TWCs near engine. In the present study, the effect of the catalyst deactivation on CO and hydrocarbon oxidation activities of commercial Pd based TWCs in WCC including Pd only and Pd/Rh was investigated with respect to the mileage of the field-aged catalysts, fresh, 21k and 55k miles. The catalytic activity of CO and HC oxidations for the catalysts examined reveals a serious deactivation with respect to the catalyst mileage, particularly under the rich feed condition. The cause of the alteration of the TWC activity by the mileage has been elucidated on the basis of the change of the catalyst physicochemical characteristics with SR-XRD, XANES, and XPS. The phase separation of CeO₂ from Ce_1-xZr_xO₂ as an OSC component has been observed for the 21 k catalyst. CeO₂ is further transferred to thermally stable Ce(III)₂(SO₄)₃ by sulfur and to Ce(III)PO₄ by phosphorous contained in engine oil as identified with H2 TPR. The initial composition of Ce_1-xZr_xO₂ and its interaction to noble metals are important for improving the deterioration of the oxidation activities of TWCs near engine, since the degree of CeO₂ separated from Ce-Zr mixed oxide directly correlates to the amount of the deactivation precursors, particularly Ce(III)₂(SO₄)₃ deposited on the catalyst surface. It is a primary cause for sintering the metals on the surface of TWC, the most commonly insisted catalyst deactivation for automotive catalysis.

Keywords: Three-way catalyst; Pd-based catalyst; CO and HC oxidation; Deactivation; OSC; Ce₂(SO₄)₃; CePO₄