Oxygen storage materials for automotive catalysts: ceria-zirconia solid solutions

摘要

This paper reviews progress in the development of oxygen storage materials for automotive exhaust catalysts. The research was mainly conducted as a study and development exercise in the author's laboratory in Japan. Ccria-lanthana solid solutions (CL) and the first generation of ceria-zirconia solid solutions (CZ) were developed as excellent oxygen storage materials for automotive catalysts in the 1980s. These materials consist of ceria doped with less than 20 mol% of La~(4+) or Zr~(4+). An increase in oxygen defects in CL and CZ under reductive conditions is responsible for an enhanced oxygen storage capability on the cerium atoms. An accurate measure of the oxygen storage capacity (OSC) per cerium is very important for theoretical and practical treatments of the catalyst. The term "partial OSC" was introduced to describe this capacity and to differentiate it from the usual definition of the OSC, known also as the "total OSC". After the development of CL and CZ, a new technology was developed to dissolve more than 20 mol% of zirconia in the ceria, allowing second generation CZ and third generation CZ (known as ACZ, which is doped with alumina) to be successfully developed in the 1990s. The partial OSC of these materials increases with increasing amounts of zirconia dissolved in the ceria, and also with decreasing material particle size after an engine durability test. In the case of ACZ, alumina was added to CZ based on the "diffusion barrier concept", in which a diffusion barrier layer inhibits the coagulation of CZ and A when the material is required for duty at high temperature in air. Furthermore, the relationship between the total or partial OSC and the structure of the ceria-zirconia solid solutions is explained in this paper. For ceria zirconia solid solutions composed of equimolar CeO_2 and ZrO_2(Ce/Zr = 1), the total or partial OSC of the κ-phase CeZrO_4, in which the cerium and zirconium ions are regularly distributed, was about twice as large as that of a ceria-zirconia solid solution with a relatively irregular distribution of cerium and zirconium ions, and about five times larger than that of a mixture of ceria powder and zirconia containing only a small amount of ceria-zirconia solid solution. It corresponds to about 89% of the theoretical maximum value. For a ceria-zirconia solid solution composed of non-equimolar CeO_2 and ZrO_2(Ce/Zr ≠ 1), the partial OSC of a ceria-κ-phase solid solution with a zirconia content of between 30 and 50 mol% is much higher than that of a ceria-zirconia solid solution of the same zirconia content. The partial OSC of a κ-phase and zirconia mixed oxide, which is formed by reducing the material at 1200℃, reaches a value above 0.20 mol-O_2/mol-Ce (about 80% of the theoretical maximum value of the partial OSC), when the zirconia content is between 50 and 80 mol%. The Toyota Motor Corp. has put automotive three-way catalysts containing the first, second and third generations of CZ into practical use on a global basis.