(136h) The Importance of Pt Dispersion In DeNOx Catalysts

Authors: 
Büchel, R., ETH Zurich
Pratsinis, S. E., Swiss Federal Institute of Technology, Particle Technology Laboratory, ETH Zurich
Baiker, A., ETH Zurich


NOx storage-reduction (NSR) is successfully applied for exhaust gas treatment of lean fuel engines. Under fuel lean conditions, exhaust NOx is oxidized over a noble metal (Pt) and then trapped on an alkali- or alkaline earth metal (Ba) in the form of metal-nitrates. During the subsequent short fuel rich period the NOx originating from decomposition of the metal-nitrates is reduced on the metal component to nitrogen and the cycle restarts. [1]

The noble metal (Pt) was found to play an important role in NSR systems [2] in order to oxidize NO under fuel lean conditions and to reduce NOx species to N2 under fuel rich condition. To use the Pt in the most efficient way, the available Pt is finely dispersed over the support, increasing the NO oxidation reaction [3]. But higher Pt dispersions, and therefore smaller Pt clusters, do not always lead to the highest NSR conversion rates, as the catalytic activity of Pt is also a function of the coordination and geometry of the Pt and smaller Pt clusters tend to form PtOx [4].

            With a flame spray pyrolysis (FSP) setup [5, 6] the production to cooling rate was varied changing the catalyst specific surface areas from 80 to 220 m2/g. For these materials a Pt dispersion of 5 to 25% was measured: With higher SSA the Pt dispersion decreased, what is unexpected but was confirmed by counting Pt clusters on high resolution scanning transmission electron microscopy (STEM) pictures. The Pt was preferentially located on Ba and for small BaCO3 particles a bimodal size distribution of the Pt was seen, whereas for bigger BaCO3 particles the size distribution of the Pt clusters was becoming narrower. The measured NSR activity changes with particle size and can be correlated with the resistance of Pt to be oxidation under fuel lean conditions.

References:

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