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Dual-monolith converters containing Pd-only catalysts followed by Pt/Rh three-way catalysts (TWCs) provide effective emission solutions for NLEV and Tier IIa close-coupled dual-bank V-8 applications due to optimal hydrocarbon and NOx light-off, transient NOx control, and balance of precious metal (PGM) usage. Dual-catalyst [Pd +Pt/Rh] systems on a 5.3L V-8 LEV light truck vehicle were characterized as a function of PGM loading, catalyst technology, and substrate cell density. NLEV hydrocarbon emission control of the 6500 lb vehicle was optimal using dual 1.2L converters with each containing front ceria-free Pd catalysts coupled with rear Pt/Rh TWCs. Advanced non-air prototype calibrations coupled with reduced catalyst washcoat mass on 600cpsi/4mil substrate resulted in minimal Pd usage of ∼0.02 toz/vehicle due to achieving catalyst inlet temperatures of 350-400°C in <10 sec on both banks of the V-8 engine.

Palladium-rhodium catalyst technologies have been investigated to establish the relationship between emission performance and their oxygen storage capacity (OSC) or other physical properties. Catalyst performance was evaluated using stand dynamometer and FTP testing after both oven air aging and engine aging. Monolith catalysts were characterized for aged surface area and precious metal dispersion. Various components of the washcoat supports were characterized by surface area and X-ray diffraction (XRD) analysis for phase composition and CeO2-ZrO2 solid solution crystallite size. The correlation between OSC delay times and tailpipe emissions for NMHC, CO and NOx was highly nonlinear in these studies. Addition of CeO2-ZrO2 solid solution components to the washcoat significantly improved steady state activity after aging, but did not significantly affect the correlation between emissions and OSC.

The beneficial effects of CeO2/ZrO2 solid solutions on the performance of fully formulated Pt, Rh TWC (three-way-conversion) catalysts were measured using both stand dynamometer and FTP testing after severe engine aging. The performance advantages were consistent with an enhancement of the chemical promotional effects of CeO2. These included increased effectiveness for CO and NOx conversion and to a lesser extent for HC compared to catalysts prepared with the same loading of Ce and Zr but no solid solution formation. Higher performance could be achieved with the CeO2/ZrO2 solid solution catalysts having half the Ce loading of conventional catalysts prepared with pure CeO2. The physico-chemical properties of the catalysts were characterized using both XRD and TPR. XRD was used to determine the degree of solid solution formation between CeO2 and ZrO2 and TPR was used to characterize the redox properties/oxygen storage of the catalysts before and after aging.

Both laboratory and engine dynamometer testing were used to characterize the relative activity of Pt, Pd and Rh supported on Ce and/or La stabilized supports. In the laboratory studies performance was measured after laboratory aging under conditions designed to simulate severe engine aging. The impact of Pt-Rh and Pd-Rh alloying on performance was examined as well as the cumulative effect of both metals on overall activity. The performance of laboratory aged non-alloyed Pt-Rh and Pd-Rh catalysts was dominated by the Rh function. For Pt-Rh the overall performance features for CO and NOx conversion were very similar over the Rh-only, Pt + Rh (separated metals) and alloyed Pt-Rh catalysts. Pt-Rh alloying was found to have a detrimental impact on high temperature HC performance.