Production process of composite metal oxide

A process for producing a composite metal oxide of an acidic metal oxide and a basic metal oxide, the acidic metal oxide having an electronegativity as a metal oxide of more than 2.80, and the basic metal oxide being an oxide of a basic metal selected from the group consisting of a rare earth and an alkaline earth metal, wherein the process comprises:

• (a) providing an aqueous solution containing a colloidal particle of said acidic metal oxide and a salt of said basic metal,
• (b) adjusting the pH of said aqueous solution to a pH at which a part of said basic metal dissolves in said aqueous solution, the remaining of said basic metal precipitates as a hydroxide and has a positive zeta potential, and the colloidal particle of said acidic metal oxide is not dissolved and has a negative zeta potential, and then maintaining this pH over a predetermined time to obtain a precursor of said composite metal oxide, and
• (c) drying and firing the precursor of said composite metal oxide.