Changes in Ba Phases in BaO/Al 2 O 3 upon Thermal Aging and H 2 O Treatment
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Catalysis Letters Vol. 105, Nos. 3–4, December 2005 ( 2005) DOI: 10.1007/s10562-005-8700-y
Changes in Ba phases in BaO/Al2O3 upon thermal aging and H2O treatment Do Heui Kim*, Ya-Huei Chin, Ja Hun Kwak, Ja´nos Szanyi, and Charles H.F. Peden* Pacific Northwest National Laboratory, Institute for Interfacial Catalysis, 902 Battelle Boulevard MSIN K8-93, P.O. Box 999, Richland, WA 99352, USA
Received 23 July 2005; accepted 12 September 2005
The effects of thermal aging and H2O treatment on the physicochemical properties of BaO/Al2O3 (the NOx storage component in the lean NOx trap systems) were investigated by means of X-ray diffraction (XRD), BET, TEM/EDX and NO2 TPD. Thermal aging at 1000 C for 10 h converted dispersed BaO/BaCO3 on Al2O3 into low surface area crystalline BaAl2O4. TEM/EDX and XRD analysis showed that H2O treatment at room temperature facilitated a dissolution/reprecipitation process, resulting in the formation of a highly crystalline BaCO3 phase segregated from the Al2O3 support. Crystalline BaCO3 was formed from conversion of both BaAl2O4 and a dispersed BaO/BaCO3 phase, initially present on the Al2O3 support material after calcinations at 1000 and 500 C, respectively. Such a phase change proceeded rapidly for dispersed BaO/BaCO3/Al2O3 samples calcined at relatively low temperatures with large BaCO3 crystallites observed in XRD within 10 min after contacting the sample with water. Significantly, we also find that the change in barium phase occurs even at room temperature in an ambient atmosphere by contact of the sample with moisture in the air, although the rate is relatively slow. These phenomena imply that special care to prevent the water contact must be taken during catalyst synthesis/storage, and during realistic operation of Pt/BaO/Al2O3 NOx trap catalysts since both processes involve potential exposure of the material to CO2 and liquid and/or vapor H2O. Based on the results, a model that describes the behavior of Ba-containing species upon thermal aging and H2O treatment is proposed. KEY WORDS: BaO/Al2O3; BaAl2O4; lean NOx trap; NO2 TPD; NOx storage; nitric oxide.
1. Introduction During recent years, lean NOx traps (LNTs) are considered as one of the most promising solutions for gasoline lean burn and diesel engine exhaust emission control in order to meet stringent requirements on emission levels being implemented in the immediate future [1]. This technology, introduced in 1994 by researchers at Toyota [2], is based on the switching of the engine operating conditions between lean and rich cycles. Typical LNT catalysts consist of 1–2 wt% of precious metal (Pt and Rh) and 10–20 wt% of a storage material (Ba or K) dispersed on an Al2O3 support. During the lean cycle, excess NOx is oxidized over Pt and stored as nitrate on the storage material. When the engine is subsequently switched to the oxygen-deficient stage, i.e. the rich cycle, stored nitrates are released and/ or reduced to N2. It has been recognized that deactivation on LNTs is related to poisoning of Ba storage sites by sulfur oxides
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