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

NO_x storage-reduction (NSR) is successfully applied for exhaust gas treatment of lean fuel engines. Under fuel lean conditions, exhaust NO_x 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 NO_x 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 NO_x 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 PtO_x [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:

[1]        W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, J.E. Parks, Cat. Rev. - Sci. Eng. 46 (2004) 163-245.

[2]        M.O. Symalla, A. Drochner, H. Vogel, R. Büchel, S.E. Pratsinis, A. Baiker, Appl. Catal., B 89 (2009) 41-48.

[3]        J.H. Lee, H.H. Kung, Catal. Lett. 51 (1998) 1-4.

[4]        L. Olsson, E. Fridell, J. Catal. 210 (2002) 340-353.

[5]        R. Strobel, L. Madler, M. Piacentini, M. Maciejewski, A. Baiker, S.E. Pratsinis, Chem. Mater. 18 (2006) 2532-2537.

[6]        R. Büchel, R. Strobel, F. Krumeich, A. Baiker, S.E. Pratsinis, J. Catal. 261 (2009) 201-207.