Characterization of OBD-II Features of Advanced Pd-Rh Catalysts and Relation to Catalyst Composition and Design

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. However, when catalyst systems consisted of close-coupled plus underfloor catalysts, a forced linear correlation between OSC and tailpipe NMHC emissions could be generated. This was done by monitoring the OSC characteristics of only one of the catalysts rather than the catalyst system as a whole.

Detailed analysis of performance coupled with other aged catalyst features such as surface area and precious metal dispersion demonstrated that NMHC, NOx and CO emissions could be directly related to changes in precious metal dispersion (or availability) or Pd support surface area for the aged catalysts. Hence, the dominant precious metal controlling performance was Pd. The time required for catalyst light-off is the most determining factor especially for HC emissions, which accounts for the poor correlation between OSC and tailpipe emissions since the OSC does not contribute to the light-off performance of the catalysts.