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A cerium-zirconium composite metal oxide having improved durability at high temperature and a stable oxygen storage capacity is provided. The cerium-zirconium composite metal oxide is characterized in that the total mole number of Ce and Zr is at least 85% based on the total mole number of metal in

A cerium-zirconium composite metal oxide having improved durability at high temperature and a stable oxygen storage capacity is provided. The cerium-zirconium composite metal oxide is characterized in that the total mole number of Ce and Zr is at least 85% based on the total mole number of metal in the composite metal oxide, a molar ratio Ce/Zr is within a range from 1/9 to 9/1, and an isoelectric point of the composite metal oxide is more than 3.5. Preferably, the molar ratio Ce/Zr is within a range from 3/7 to 7/3 and the isoelectric point is within a range from 3.8 to 5.0, and the cerium-zirconium composite metal oxide contains a rare earth metal (excluding Ce) in a concentration of less than 15% by mole based on the total mole number of metal in the composite metal oxide. Also the present invention provides a cerium-zirconium composite metal oxide, characterized in that CeO2 forms a core surrounded by ZrO2.

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What is claimed is: 1. A cerium-zirconium composite metal oxide, wherein a total mole number of Ce and Zr is at least 85% based on the total mole number of metal in the composite metal oxide, wherein a molar ratio Ce/Zr is within a range from 1/9 to 9/1, and wherein an isoelectric point of the comp

What is claimed is: 1. A cerium-zirconium composite metal oxide, wherein a total mole number of Ce and Zr is at least 85% based on the total mole number of metal in the composite metal oxide, wherein a molar ratio Ce/Zr is within a range from 1/9 to 9/1, and wherein an isoelectric point of the composite metal oxide is more than 3.5. 2. The cerium-zirconium composite metal oxide according to claim 1, wherein the molar ratio Ce/Zr is within a range from 3/7 to 7/3 and the isoelectric point is within a range from 3.8 to 5.0. 3. The cerium-zirconium composite metal oxide according to claim 2, which contains rare earth metal (excluding Ce) in a concentration of less than 15% by mole based on the total mole number of metal in the composite metal oxide. 4. An exhaust gas purifying catalyst comprising the cerium-zirconium composite metal oxide of claim 2 and a noble metal supported on the cerium-zirconium composite metal oxide. 5. A method for synthesizing the cerium-zirconium composite metal oxide of claim 2, which comprises mixing a ceria sol and a zirconium compound solution or a zirconia sol to prepare a suspension, and drying and firing the mixture. 6. The cerium-zirconium composite metal oxide according to claim 1, which contains rare earth metal (excluding Ce) in a concentration of less than 15% by mole based on the total mole number of metal in the composite metal oxide. 7. An exhaust gas purifying catalyst comprising the cerium-zirconium composite metal oxide of claim 6 and a noble metal supported on the cerium-zirconium composite metal oxide. 8. A method for synthesizing the cerium-zirconium composite metal oxide of claim 6, which comprises mixing a ceria sol and a zirconium compound solution or a zirconia sol to prepare a suspension, and drying and firing the mixture. 9. An exhaust gas purifying catalyst comprising the cerium-zirconium composite metal oxide of claim 1 and a noble metal supported on the cerium-zirconium composite metal oxide. 10. A method for synthesizing the cerium-zirconium composite metal oxide of claim 1, which comprises mixing a ceria sol and a zirconium compound solution or a zirconia sol to prepare a suspension, and drying and firing the mixture. 11. A cerium-zirconium composite metal oxide, wherein a total mole number of Ce and Zr is at least 85% based on the total mole number of metal in the composite metal oxide and wherein CeO2 forms a core surrounded by ZrO2, and wherein an isoelectric point of the composite metal oxide is more than 3.5. 12. The cerium-zirconium composite metal oxide according to claim 11, wherein the CeO2 core has a diameter within a range from 5 to 20 nm. 13. An exhaust gas purifying catalyst comprising the cerium-zirconium composite metal oxide of claim 12 and a noble metal supported on the cerium-zirconium composite metal oxide. 14. An exhaust gas purifying catalyst comprising the cerium-zirconium composite metal oxide of claim 11 and a noble metal supported on the cerium-zirconium composite metal oxide. 15. A method for synthesizing the cerium-zirconium composite metal oxide of claim 11, which comprises mixing a ceria sol and a zirconium compound solution or a zirconia sol to prepare a suspension, and drying and firing the mixture.