Dual-Catalyst Underfloor LEV/ULEV Strategies for Effective Precious Metal Management 1999-01-0776
Dual-brick catalyst systems containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) provide an effective strategy for managing Pt, Pd and Rh precious metal inventories while achieving LEV/ULEV emission standards. Engine aged dual-brick converters containing front Pd catalysts followed by rear Pd/Rh or Pt/Rh TWCs demonstrated LEV emission levels in an underfloor location on a TLEV calibrated 3.8L vehicle, and achieved ULEV emissions with air addition. Using identical advanced washcoat formulations stabilized with ceria-zirconia promoters, single-brick Pt/Rh TWCs demonstrated equivalent performance to Pd/Rh TWCs after thermally severe aging, and dual-brick [Pd + Pt/Rh] systems also had equivalent performance to [Pd + Pd/Rh] catalyst systems. While a Pd-only system also achieved 100K mi equivalent LEV emissions, both dual-brick options lowered emissions further using substantially lower loadings and more balanced precious metal usage.
The front Pd-only catalyst effectively controls HC emissions with loading dependent on converter inlet temperatures, but can be formulated to also help control NOx emissions. Cerium-free Pd formulations demonstrated 50°C lower aged light-off temperatures than Pd TWCs during modulated, non-modulated, and lean light-off conditions that were confirmed in vehicle testing. Higher Pd loadings may require catalyst washcoat optimizations, as aged catalyst light-off improved with increased Pd dispersion. Additional HC rapid temperature ramp light-off benefits and vehicle HC performance are demonstrated with higher cell density and thinner-wall substrates. When the front Pd catalyst formulation also contained stabilized ceria-zirconia promoters, NOx control of the dual-brick systems was further improved.