The Role of Zirconium in Novel Three-Way Catalysts

Zirconium dioxide (zirconia) is a well-known material often being a major component in the washcoat systems of three-way catalysts (TWC) and diesel oxidation catalysts. One important characteristic of zirconia containing washcoats is an improved aging stability which is required to meet the more and more stringent emission standards. In the last few years the utilization of zirconia became even more important - especially for high sophisticated three-way washcoat systems. This was due to the development of high temperature stable oxygen storage components, containing cerium dioxide (ceria) in combination with different other oxides - one very promising candidate being zirconia.

In the present work the results of a research program are discussed, focusing on the influence of zirconia in combination with ceria and additional rare earth promoters on the stability of the oxygen storage characteristics. The performance of these materials was tested in powder experiments as well as in completely formulated Pd-containing TWC. Special emphasis is put on a new developed dynamic oxygen storage model gas test, on activity tests in a model gas and on real engine evaluation for the most promising systems.

In summary the best catalyst performance could be achieved with zirconia rich and ceria poor oxygen storage components in combination with small amounts of extra stabilizers. These new developed zirconia rich materials enhance significantly the stability of the oxygen storage component per mass unit ceria. By this, the amount of expensive ceria in the washcoat could be decreased dramatically upon maintaining improved activity of Pd-based catalysts, even after high temperature engine aging.