Abstract
Keywords - Automotive Depollution Catalyst Oxygen Storage Capacity Doped-Cerium- Zirconium mixed oxides Thermal stability
Abstract - Rare earth compounds, especially CeO2, are widely used in automotive catalysis. Cerium dioxide contributes to precious metals stabilisation and promotion, oxygen storage and release over a wide range of temperature, and promotion of water-gas shift and steam reforming reactions. Increasing variety of applications in automotive pollution control leads to request materials with a wide range of properties. Not only high surface area materials, but also high thermal stability and very different range of reactivity materials are needed to cover the different applications. Hence Rhodia has developped different type of Cerium containing materials:
1st generation of Ce-Zr mixed oxides with more than 200 m²/g as is and keeping more than 50 m²/g after aging at 800°C Pure ceria shows initial surface area higher than 250 m²/g and 60m²/g and 45 m²/g after aging respectively at 800°C and 900°C. This can be of great use in TWC and Diesel Oxicat applications as Pt support.
2nd generation of Ce-Zr mixed oxides with higher thermal stability up to 1200°C: Whatever the composition, ceria rich as well zirconia rich mixed oxides show surface area higher than 45 m²/g at 1000°C, more than 20 m²/g at 1100°C in air and higher than 5 m²/g after aging at 1200°C in air. Catalyst manufacturers can tune OSC function independently of the thermal stability. These materials can be integrated in Ultra Low PGM TWC Technology.
Next generation of Ce/Zr mixed oxides with outstanding OSC behaviour and thermal stability
Very Highly Reducible (VHR) Ce/Zr mixed oxides. For commercially available CeZr (50/50 at) mixed oxides, the percentage of Ce(IV) converted in Ce(III) by H2 or CO in Temperature Programmed Reduction measurements is less than 50%. Hence Rhodia has developped new process to prepare materials with the same type of composition but with Ce(IV) reducibility exceeding 80%. These materials show initial surface area higher than 50 m²/g. They are preferred PGM carriers for underfloor catalysts.
Completely stable Ce/Zr mixed oxides with the same surface area (about 20 m²/g) from 900 to 1150 °C. The sintering of such materials is delayed. The surface area is still 15 m²/g at 1200°C in air. The extremely high thermal stability may prevent against PGM sintering in Very close-coupled catalysts. Moreover, the complete stability of surface area and OSC performance on a wide range of temperature may be used to tune the catalyst response for OBD function.
For each of the oxides mentioned before, this paper will focus on the key material characteristics : Stability expressed as BET surface area, XRD phase purity, Pore size distribution, H2 TPR reducibility and catalytic behaviour.