Effect of Hydrothermal Aging on Physical and Chemical Properties of a Commercial NOx-Storage Catalyst
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Fuel consumption for NOx trap regeneration depends on the regeneration frequency, which is a function of NOx trap performance parameters. Hence, the impact of thermal aging must be considered in the operation strategy. For this purpose, the effect of thermal aging on the physical and chemical properties of a commercial NOx storage/reduction catalyst containing Pt/Pd/Rh has been investigated. After thermal treatment under various conditions, the samples were characterized using CO chemisorption measurements, X-ray diffraction (XRD), N2 physisorption, and electron microscopy. It is shown that the major impact of lean hydrothermal treatment on the catalyst activity is the continuous decrease of the specific surface area with aging temperature due to collapse of smaller pores. The noble metal nanoparticles remained highly dispersed, even after lean aging at 800 °C. In contrast, noble metal sintering represents the main aging effect of lean/rich cycling at elevated temperatures. Furthermore, XRD and selected area electron diffraction revealed a significant loss of crystallinity for the barium component in consequence of lean/rich aging treatment. However, independent from the aging treatment, no formation of mixed oxides could be observed.
KeywordsCommercial NOx storage/reduction catalyst Aging XRD BET HR-TEM/EDX CO chemisorption
The authors gratefully acknowledge the Forschungsvereinigung Verbrennungskraftmaschinen e.V. (FVV) for the financial support, Umicore AG & Co. KG for providing the catalyst and K. Hauff and U. Nieken (ICVT, University of Stuttgart) for catalyst pretreatment and the good cooperation within the FVV project. This work was partly carried out with the support of the Karlsruhe Nano Micro Facility (KNMF, www.kit.edu/knmf), a Helmholtz Research Infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu). The authors thank D. Wang for support during TEM/STEM measurements.