(701d) The Rational Synthesis of Bimetallic Pt/Pd Diesel Exhaust Catalysts

Many bimetallic catalysts are today prepared by co-impregnation (also called incipient wetness, pore filling, or dry impregnation) in which nominal amounts of metals are placed into the amount of solution just necessary to fill the pore volume of support.  While dry impregnation (DI) is simple and the amounts of metals easily controlled, no attention is paid to metal-support or metal-metal interaction.  When DI-deposited metal precursors are reduced, the particles tend to be large in size and the two metals poorly contacted.

In this work we demonstrate the application of “strong electrostatic adsorption” (SEA) for the rational synthesis of bimetallic catalysts with high metal dispersion and intimate metal1-metal2 interaction.  Two variations will be demonstrated:  with simultaneous or co-SEA, two metal precursors are simultaneously placed in solution and electrostatically adsorbed onto support in a mixed monolayer of precursors.  This produces homogeneously alloyed bimetallic particles when the precursor layer is reduced.  On the other hand, SEA can be applied in a two step sequence (seq-SEA); the core metal precursor is electrostatically adsorbed onto support and oxidized, and then the shell metal precursor is selectively adsorbed onto the core metal oxide and not onto the support.  A core-shell morphology persists after the reduction step.  

Pt/Pd bimetallic catalysts supported by oxidized and unoxidized carbon, alumina and silica have been synthesized by co-SEA.  Seq-SEA has been used to produce Pt shell/Pd cores on silica and alumina, and Pt shell/Pt cores on alumina.  These are compared with DI-prepared catalysts of the same nominal composition.  The materials were characterized by powder XRD, STEM, EDXS nanoparticle mapping, and TPR.  Characterization will be correlated to oxidation activity for a model diesel exhaust stream (CO and propane) at the Johnson-Matthey Technical Centre in Sonning Common, England.