Reactor Evaluation of Ceria-Zirconia as an Oxygen Storage Material for Automotive Catalysts 970462

We have prepared and tested laboratory scale monoliths wash-coated with 10, 20 and 30 wt% of either CeO₂ or Ce_.75Zr_.25O₂ (remainder is alumina). Wet impregnation was used to load the wash-coated monoliths with 50g/ft Pt:Rh at a 5:1 ratio. The catalyst were aged at temperatures between 825°C and 950°C using a cycled redox aging. The catalysts were then tested in a full-feed simulated exhaust laboratory reactor with air-to-fuel ratio (A/F) perturbations (frequencies at 1 and 3 Hz and amplitudes up to +/- 0.8 A/F). Even the lowest loading of Ce_.75Zr_.25O₂ outperformed all three loadings of CeO₂ over a full range of reaction temperatures, A/F perturbations, and catalyst space velocity (SV). Our data indicates that the ceria-zirconia catalysts can tolerate cycled redox aging at sustained bed temperatures at least 25°C higher (∼925°C vs. < 900°C) than can ceria. For the CeO₂ catalysts aged at or above 900°C we observed an inverse correlation of catalyst activity to CeO₂ loading. Using activity measurements, we attributed this inverse correlation to excessive sintering of the precious metals (PM) in the highly CeO₂ loaded parts. For the Ce_.75Zr_.25O₂ catalysts we found the performance to be insensitive to catalysts aging up to aging temperature of 925°C (for 8 hours). For the Ce_.75Zr_.25O₂ catalysts we found the performance to be independent of Ce_.75Zr_.25O₂ loading between 10 and 30wt%. Reactor measurements of OSC show that increasing the Ce_.75Zr_.25O₂ loading was not increasing OSC in the aged parts even though the higher loaded fresh catalysts had higher OSC. Taken together our data suggest that poor PM distribution to the Ce_.75Zr_.25O₂ resulted in an “under-promotion” of this oxygen storage materials.