Complex oxide, method for producing same, and exhaust gas purifying catalyst
Disclosed are a composite oxide which is capable of maintaining a large volume of pores even used in a high temperature environment, and which has excellent heat resistance and catalytic activity, as well as a method for producing the composite oxide and a catalyst for exhaust gas purification employing the composite oxide. The composite oxide contains cerium and at least one element selected from aluminum, silicon, or rare earth metals other than cerium and including yttrium, at a mass ratio of 85:15 to 99:1 in terms oxides, and has a property of exhibiting a not less than 0.30 cm 3 /g, preferably not less than 0.40 cm 3 /g volume of pores with a diameter of not larger than 200 nm, after calcination at 900° C. for 5 hours, and is suitable for a co-catalyst in a catalyst for vehicle exhaust gas purification.
1. A method for producing a composite oxide having a property of exhibiting a not less than 0.30 cm 3 /g volume of pores with a diameter of not larger than 200 nm after calcination at 900° C. for 5 hours, said method comprising the steps of:
(a) providing a cerium solution not less than 90 mol % of which cerium ions are tetravalent,
(b) heating and maintaining said cerium solution obtained from step (a) up to and at not lower than 60° C.,
(c) adding an oxide precursor of at least one element selected from aluminum, silicon, or rare earth metals other than cerium and including yttrium, to a cerium suspension obtained through said heating and maintaining,
(d) heating and maintaining said cerium suspension containing said oxide precursor of at least one element selected from aluminum, silicon, or rare earth metals other than cerium and including yttrium, up to and at not lower than 100° C.,
(e) neutralizing said suspension obtained from step (d),
(f) adding a surfactant to said suspension neutralized in step (e), followed by a retention time of 10 minutes to 6 hours to obtain a precipitate, without subjecting said suspension to washing between step (e) and step (f), and
(g) calcining said precipitate,
to thereby obtain a composite oxide having a property of exhibiting a not less than 0.30 cm 3 /g volume of pores with a diameter of not larger than 200 nm after calcination at 900° C. for 5 hours.
2. The method according to claim 1 , wherein a cerium content of said cerium solution in step (a) is 5 to 80 g/L in terms of CeO 2 .