Abstract: Catalyst compositions for the treatment of vehicular exhaust gases are based on zirconium and cerium oxides, have a cerium oxide content of at most 50% by weight, a level of reducibility of at least 95% after calcination in air at 600°, and a specific surface area after calcination for 4 hours at 1100° of at least 15 m2/g; such compositions are prepared by forming an aqueous mixture containing zirconium and cerium compounds, by heating this mixture to at least 100° and, after the heating, adjusting it to a basic pH, by adding a surfactant additive to the precipitate obtained from this mixture and by calcinating the precipitate in an inert gas or under vacuum at a temperature of at least 900° and then in an oxidizing atmosphere at a temperature of at least 600°.

Abstract: Catalyst compositions for the treatment of vehicular exhaust gases are based on zirconium and cerium oxides, have a cerium oxide content of at most 50% by weight, a level of reducibility of at least 95% after calcination in air at 600°, and a specific surface area after calcination for 4 hours at 1100° of at least 15 m2/g; such compositions are prepared by forming an aqueous mixture containing zirconium and cerium compounds, by heating this mixture to at least 100° and, after the heating, adjusting it to a basic pH, by adding a surfactant additive to the precipitate obtained from this mixture and by calcinating the precipitate in an inert gas or under vacuum at a temperature of at least 900° and then in an oxidizing atmosphere at a temperature of at least 600°.

Abstract: A composite oxide having oxygen absorbing and desorbing capability containing 90 to 100 weight % total of cerium, zirconium, and hafnium based on a total amount of metals contained in the composite oxide, the total amount of cerium, zirconium, and hafnium consisting of 14.0 to 70.5 at % cerium, 29.49 to 72.5 at % zirconium, and 0.01 to 13.5 at % hafnium, wherein degree of solid solution of the composite oxide is not lower than 70%, and wherein the composite oxide has reducing property that not less than 90% of tetravalent cerium contained in the composite oxide is reduced into trivalent cerium when the composite oxide is held at 600.degree. C. under a reducing atmosphere; and a method for preparing the composite oxide including the steps of preparing a composite salt from a starting material solution wherein 85 weight % or more of the cerium ions are Ce.sub.4+ by coprecipitation, and subjecting the resulting precipitate to a particular calcining steps including reducing calcination.