Palladium/Rhodium Dual-Catalyst LEV 2 and Bin 4 Close-Coupled Emission Solutions 2007-01-1263
Dual-monolith catalyst systems containing Pd/Rh three-way catalysts (TWCs) provide effective emission solutions for LEV2/Bin 5 and Bin 4 close-coupled applications at low PGM loadings. These systems combine washcoat technology and PGM distribution for front and rear catalysts resulting in optimal hydrocarbon and NOx light-off and transient NOx control. The dual-catalyst [Pd/Rh + Pd/Rh] systems are characterized as a function of Pd-Rh content, PGM location, and catalyst technology for 4-cyl [close-coupled + underfloor] systems and 6-cyl close-coupled applications.
The current Pd/Rh dual-catalyst converters significantly reduce NOx emissions compared to earlier [Pd + Pt/Rh] or [Pd + Pd/Rh] LEV/ULEV systems by utilizing uniform Rh distribution and new OSC materials. These new design strategies particularly impact NOx performance, especially during transient A/F excursions. NOx and NMHC emissions were further improved by Pd distribution and Pd/Rh ratio optimization, and reducing the thermal inertia of the front Pd/Rh catalyst.
Additional NOx benefits of 12mg/mi were achieved with a 1L under-floor Pd/Rh catalyst following a 1.3L close-coupled Pd/Rh system on a 2.2L vehicle. This configuration achieved 20mg/mi NOx and 35mg/mi NMHC (<50% 50K Bin 5 standard) using only 2g Pd and 0.2g Rh per vehicle on 400/4.5 substrates. The more stringent Bin 4 NOx emission levels for a 6-cyl close-coupled converter were achieved with higher Pd-Rh content on a smaller front 600cpsi/4mil catalyst coupled with an improved oxygen-storage component in the washcoat.