Innovative Materials with High Stability, High OSC, and Low Light-Off for Low PGM Technology 2004-01-1274
To meet severe legislation limits, car manufacturers use close-coupled three-way catalysts (TWC) to reduce cold start emissions. These catalysts show long term durability and thermal stability at temperatures higher than 1000 °C. Mixed ceria-zirconia oxides are an important component of these TWCs. Ceria-rich, mixed oxides have a high crystalline structure defects density, and thus show high oxygen storage capacity (OSC). They are appropriate precious metal supports up to 1000°C. For applications which require higher thermal durability, zirconia-rich mixed oxides are used to make active catalysts because of their good thermal stability.
Rhodia has developed a new generation of mixed oxides with high OSC and high thermal stability, covering a wide range of compositions. These materials show phase stability and thermal stability at temperatures greater than 1100°C for Zr-rich as well as Ce-rich compositions. They can be used advantageously as precious metal carriers in close-coupled or underfloor catalysts demonstrating improved catalytic performance.
Regardless of composition (ceria-rich as well zirconia-rich mixed oxides), these materials show surface areas higher than 20 m2/g and 5 to 7 m2/g respectively after air aging at 1100°C and 1200°C.
Aged Rh model powder catalysts based on these new materials show improved hydrocarbon and CO light-off temperature compared to existing materials. Typically, the T20%-CO light-off temperature is 30°C less for a Ce-Zr (50/50 at%) based catalyst than for a zirconia-rich based catalyst. Pt based powder catalysts show the same trend. In addition, with new ceria-rich mixed oxide based catalysts, the cross-over point (COP) conversion is less affected by a decrease of the precious metal loading than with a catalyst based on a zirconia-rich mixed oxide.
Due to their high OSC, thermal stability and low light off performance, such materials are preferred precious metal carriers for cost effective catalysts with low precious metal loadings.