The selective catalytic reduction  (SCR) of NOx from lean burn gasoline and diesel vehicles utilizes metal (Cu, Fe) - exchanged zeolite (ZSM-5, BEA, SSZ-13) catalysts  and NH3 as the reductant.  The challenge of  achieving high NOx conversion and N2 selectivity along with minimal release  of NH3 over a wide range of transient operating conditions calls for catalysts with multiple functions.  We describe three successful applications of multi-layer monolithic catalysts containing one or more layers with a zeolitic function. The first application combines Fe- and Cu-exchanged zeolitic layers to expand the temperature range to give high NOx conversion.  The second application combines NOx storage and reduction (NSR) with selective catalytic reduction (SCR) in a dual-layer structure.  The role of the NSR catalyst is to generate NH3 in situ, to react with NO to form N2 in this �LNT/SCR� catalytic system.  The third application combines a Pt/Al2O3 bottom layer and a zeolitic top layer to oxidize NH3 in high selectivity to N2.  This dual-layer �ammonia slip catalyst� relies on the zeolite layer to carry out SCR chemistry to convert the NO product from the ammonia oxidation.  In each of these cases the synergistic coupling between the two layers is paramount.  We describe the essential role of the zeolitic layer in achieving the desired high conversion and selectivity and how the dual layer design out-performs single-layer zoned configurations.

Selective catalytic reduction in dual layer monolithic catalysts

247th National Meeting (2014)

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