(111a) Keynote Speaker Abstract: Twin Flame Spray Pyrolysis Production of Pt/K/Al2O3 NOx Storage-Reduction Catalysts

New catalysts for NO_x removal are development to meet upcoming stricter emission limits. To reduce NO_x under oxygen rich conditions is a major challenge as encountered in lean burn and direct injection engines [1]. NO_x storage-reduction (NSR) catalyst can trap exhaust NO_x under fuel lean conditions on alkali- or alkaline earth metals in the form of metal-nitrates [2] whereby K and Ba are reported to be the best [3]. The NO_x trap is regenerated during a short fuel rich period, where the metal-nitrates are decomposed and the released NO_x are reduced to nitrogen [4]. Potassium and barium have been studied extensively for their NO_x storage and regeneration behavior whereby K showed higher performance especially at elevated temperatures [5]. Furthermore K₂CO₃ is less toxic and cheaper than BaCO₃. The main drawback of K is its low resistance to sulfur poisoning, whereby regeneration is possible but only above 700°C [6]. Although NSR catalysts are typically used at high temperatures (>350°C), their performance below 350°C is also important [7].

Here, we prepared Pt/K/Al₂O₃ catalysts using a twin flame spray pyrolysis (FSP) setup [8] allowing to separate synthesis of the support and storage components of the NSR catalyst [9]. This setup also allows controlling the location of Pt-clusters on the support or the storage sites of the NSR catalysts [10]. Raman investigations showed amorphous K₂CO₃ to bepresent, exhibiting a higher NO_x conversion compared to Ba at high as well as low temperatures. The K concentration is a crucial factor for the steady-state NO_x conversion efficiency of a given fuel lean/rich cycle length. An advantage of the K based catalysts was that during the switch from fuel lean to fuel rich the typical undesired overshooting of the NO_x was decreased for higher temperatures. The superior performance was attributed to good K distribution in the sample as evidenced by STEM combined with EDX analysis. Preferential Pt deposition on K increased the catalyst efficiency, especially during the reduction phase. In contrast to flame made BaCO₃ which transforms from orthorhombic to monoclinic structure, amorphous K₂CO₃ showed no measurable crystal change and stayed stable even during TGA measurements up to 1000°C.

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