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Automotive catalysts with a good thermal durability have been developed by modifying the composition of additives. The stability of alumina supports against the loss of the surface area depends on the ionic radius and amount of additives. Some lanthanides and alkaline earthes with large ionic radii of 0.11-0. 15nm are most effective. Among these elements, lanthanum improves not only the alumina stability but the catalytic activities of rhodium and cerium oxide. Infrared spectroscopic studies show that lanthanum oxide affects NO adsorbed on Rh to improve the activity for NO reduction. Moreover, lanthanum forms the complex oxide with cerium to improve the activity of cerium oxide. CO pulse reactions on the complex oxide (Ce, La)O2 -x have proved that the oxygen defect in the lattice promotes the diffusion of oxygen atoms to improve the oxide activity.