From Zr-rich to Ce-rich: thermal stability of OSC materials on the whole range of composition

Abstract

Car manufacturers use close coupled three way catalysts to reduce cold start emissions. These close coupled catalysts must show long term durability and thermal stability at temperatures higher than 1000 °C. Thus, during the past 10 years, a significant amount of attention has been paid to the design of CeO₂-ZrO₂ OSC materials with improved surface area, phase and OSC stability. Solid solutions of Ce/Zr mixed oxides are now commonly available on industrial scale. Recent studies have demonstrated that the introduction of rare earth oxides promoters such as La, Pr, Nd, Y, Sm [ENGELHARD WO patent o2 30546; OMG-DMC² EP Patent 1 181 970; DAIHATSU EP Patent 1 174 174; DELPHI US Patent 6 387 338; ENGELHARD WO Patent 02 22242] or aluminium oxide [TOYOTA EP Patent 1 172 139; Kanzawa et al. SAE Technical Paper 2003-01- 0811] in the Ce/Zr matrix is a very good way to improve the thermal stability and the redox properties of these materials. Positive interactions between PGM and these ‘promoters’ are also claimed. Nevertheless, not only the composition but the optimization of the manufacturing processes is a key factor to obtain both high thermal stability (expressed as surface area and XRD phase purity) and OSC. To meet the increasingly stringent emission limits, Rhodia has developed a new generation of CeZr based mixed oxides. These materials show phase stability and thermal stability at temperatures higher than 1100 °C in a wide composition range: from Zr-rich to Ce-rich mixed oxides. These characteristics have been obtained with a new manufacturing process which leads to highly porous materials. The controlled morphology at microscopic and macroscopic scale prevents particle sintering under severe ageing conditions. Such materials are preferred PGM supports for cost effective catalysts with low PGM loadings.