(610e) Preferential Deposition of Noble Metals Pt and Pd in K/Al2O3 NOx Storage-Reduction Catalysts

To reduce automobile emission, NOx storage-reduction (NSR) catalysts can be used to trap NOx under fuel lean conditions on an alkali or alkaline-earth metal (typically Ba or K) in the form of metal-nitrates [1]. The NOx trap is regenerated under fuel rich conditions by decomposing the nitrates and reducing all NOx to nitrogen [2]. As noble metal, typically Pt is used [3] which is only surpassed by Pd at temperatures below 400 °C [4]. With a two flame spray pyrolysis (FSP) unit [5], Pt/Pd/K/Al2O3-catalysts were prepared with Pd and Pt clusters selectively deposited on the K-component or the alumina support and the effect of the noble metal was analyzed. Both noble metals were selectively deposited [6] on the K-storage component or the Al-support in various combinations to investigate the best location of the noble metal and the most favorable Pt/Pd weight ratio. At low temperatures [7], where NOx reduction is most challenging, the noble metal catalysts activity changes significantly. The catalysts were characterized by XRD, BET, CO chemisorption and their NOx storage-reduction behavior was investigated during fuel lean/rich cycling. At 300 °C all catalysts showed NOx conversion above 90%. Among the monometallic catalysts, Pt showed the best performance at 250 °C when it was deposited on the K component while Pd was most effective when deposited on the alumina constituent. At 250 °C the NOx reduction activity of Pd was low and its presence in some cases even lowered the overall NOx conversion rate in bimetallic systems.

References:

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