Browse Publications Technical Papers 2000-01-0860
2000-03-06

Palladium and Platinum/Rhodium Dual-Catalyst Emission Solutions for Close-Coupled or Underfloor Applications 2000-01-0860

Dual-brick catalyst systems containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) are effective emission solutions for both close-coupled and underfloor LEV/ULEV applications due to optimal hydrocarbon light-off, NOx control, and balance of precious metal (PGM) usage. Dual-brick [Pd +Pt/Rh] systems on 3.8L V-6 LEV-calibrated vehicles were characterized as a function of PGM loading, catalyst technology, converter volumes, and substrate cell density.
While hydrocarbon emissions improve with increasing Pd loading, decreasing the front catalyst volume at constant Pd content (resulting in higher Pd density) improved light-off emissions. Use of 600cpsi substrates improved underfloor NMHC emissions on a 3.8L vehicle by ∼ 6-10mg/mi compared to 400cpsi catalysts, and thus allowing reduction of catalyst volume while achieving ULEV emission levels without air addition. Non-optimized air addition to the underfloor system further reduced NMHC emissions by 8-14mg/mi, although increasing NOx emissions by 7-20mg/mi, and higher over smaller converter volumes.
For dual close-coupled two-brick converter systems, hydrocarbon emissions were not greatly affected by catalyst volume, but lower NOx emissions were favored by increased converter volume. Larger converters with ceria-containing washcoats in the front Pd catalyst had more robust NOx performance during transient A/F excursions on the prototype 3.8L vehicle. This dual close-coupled converter system is capable of ULEV emissions on this prototype vehicle with additional bank-to-bank A/F control improvements.

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