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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.

Palladium-rhodium three-way catalysts (Pd/Rh TWCs) demonstrate the most versatile capability in achieving California LEV/ULEV emission standards for various applications ranging from thermally severe high-temperature close-coupled locations to thermally limited cooler underfloor environments. Engine aged dual-brick converters consisting of front Pd catalysts followed by rear Pd/Rh TWCs met ULEV emission standards for a high-temperature close-coupled 4.3L medium-duty truck and LEV standards for a much cooler underfloor 3. IL passenger car location, as well as intermediate environments experienced by a 3.8L vehicle. The Pd + Pd/Rh TWC systems have performance advantages and are more cost effective compared to Pd-only or trimetal (Pt/Pd/Rh) systems. For thermally limited systems such as the 3. IL system, addition of air to the Pd + Pd/Rh system was necessary.

An H2S odor problem has appeared for certain vehicles fitted with modern three-way catalysts. A sulfur storage/H2s release mechanism is proposed as a source of the odor problem. The effects of various operating parameters on the release of H2S are presented. Two methods of modifying three-way catalysts to minimize H2S release while maintaining good catalyst performance and high temperature durability are demonstrated.