New CeO2-ZrO2 Mixed Oxides with Improved Redox Properties for Advanced TWC Catalysts 2006-01-0852
Materials with Oxygen Storage Capacity (OSC) are key components in the formulation of Three Way Catalysts (TWC) for more than 2 decades. To get advanced TWC catalysts complying with the coming more severe regulations, one needs among others to increase both the thermal stability and the redox properties of OSC materials.
In terms of thermal stability, Rhodia obtained in the recent years significant improvements by developing a new wet process yielding pure phase and high surface area OSC materials even after harsh ageing conditions at temperatures higher than 1100°C [1, 2]. These materials are preferred precious metals carriers .
This paper deals with recent progress achieved in terms of redox properties of CeO2/ZrO2 mixed oxides. Increasing the bulk oxygen mobility of the CeZr mixed oxides is of great interest specifically to increase the conversion of the pollutants under transient modes.
Already described materials [4, 5 and 6] show very high oxygen availability and mobility. Generally speaking, these materials show a very specific structural arrangement of the Ce and Zr cations in a so-called ‘pyrochlore’ structure. Unfortunately the way of making these oxides consists in submitting an existing CeZr mixed oxide to successive reductive and oxidative thermal treatments at high temperature which yields to very poor surface area - ie below 5 m2/g - and thus to a very limited interest in heterogeneous catalysis.
Thanks to a new process, with mild conditions, Rhodia developed Very Highly Reducible (CeZr Hi-OSC) materials on the whole composition range - From Ce-rich to Zr-rich. These materials show a surface area higher than 20 m2/g even after air treatment at 1000°C/4h. This paper deals with the main properties of such materials and their interest for mild temperature applications.