Complex oxide, method for producing same and exhaust gas purifying catalyst

Abstract

Disclosed are a composite oxide which is capable of maintaining a large specific surface area even used in a high temperature environment, and which has excellent heat resistance and reducibility, 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 of rare earth metal elements other than cerium and including yttrium, at a mass ratio of 85:15 to 99:1 in terms oxides, and further containing silicon at more than 0 parts by mass and not more than 20 parts by mass in terms of SiO.sub.2 with respect to 100 parts by mass of the total of the cerium and the at least one of rare earth metal elements other than cerium and including yttrium, wherein the composite oxide has a specific surface area of not less than 40 m.sup.2/g as measured by the BET method after calcination at 900? C. for 5 hours, and a reducibility of not lower than 30% as calculated from measurement of temperature-programmed reduction from 50? C. to 900? C. after calcination at 1000? C. for 5 hours, and is particularly suitable for a co-catalyst for a catalyst for exhaust gas purification.

Claims

1. A composite oxide consisting of (a) cerium; (b) at least one of rare earth metal elements other than cerium, at a mass ratio of 85:15 to 99:1 in terms oxides; and (c) silicon at more than 0 parts by mass and not more than 20 parts by mass in terms of SiO.sub.2 with respect to 100 parts by mass of a total of said cerium (a) and said at least one of rare earth metal elements other than cerium (b), wherein said composite oxide has properties of exhibiting a specific surface area of not less than 40 m.sup.2/g as measured by BET method after calcination at 900? C. for 5 hours, and a reducibility of not lower than 30% as calculated from measurement of temperature-programmed reduction from 50? C. to 900? C. after calcination at 1000? C. for 5 hours.

2. The composite oxide according to claim 1 having a property of exhibiting a specific surface area of not less than 60 m.sup.2/g as measured by BET method after calcination at 900? C. for 5 hours.

3. The composite oxide according to claim 1 having a property of exhibiting a specific surface area of not less than 25 m.sup.2/g as measured by BET method after calcination at 1000? C. for 5 hours.

4. The composite oxide according to claim 1 having a property of exhibiting a reducibility of not lower than 40% as calculated from measurement of temperature-programmed reduction from 50? C. to 900? C. after calcination at 1000? C. for 5 hours.

5. The composite oxide according to claim 1, wherein a content of silicon (c) is 5 to 20 parts by mass in terms of SiO.sub.2 with respect to 100 parts by mass in total of said cerium (a) and said at least one of rare earth metal elements other than cerium (b) in terms of oxides.

6. A method for producing a composite oxide of claim 1 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 of rare earth metal elements other than cerium, 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 of rare earth metal elements other than cerium up to and at not lower than 100? C., (e) adding a precipitant to the suspension obtained from step (d) to obtain a precipitate, (f) calcining said precipitate, (g) impregnating an oxide obtained through said calcining, with a solution of a silicon oxide precursor, and (h) calcining said oxide impregnated with the solution of a silicon oxide precursor.

7. A method for producing a composite oxide of claim 1 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 a silicon oxide precursor and an oxide precursor of at least one of rare earth metal elements other than cerium, to a cerium suspension obtained through said heating and maintaining, (D) heating and maintaining said cerium suspension containing said silicon oxide precursor and said oxide precursor of at least one of rare earth metal elements other than cerium, up to and at not less than 100? C., (E) adding a precipitant to the suspension obtained from step (D) to obtain a precipitate, and (F) calcining said precipitate thus obtained.

8. The method according to claim 6 wherein a cerium content of said cerium solution in step (a) or (A) is 5 to 100 g/L in terms of CeO.sub.2.

9. A catalyst for exhaust gas purification comprising the composite oxide according to claim 1.

Description

EXAMPLES

(1) The present invention will now be explained in more detail with reference to Examples and Comparative Examples, which are not intended to limit the present invention.

Example 1

(2) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 1 part by mass of silicon oxide was added.

(3) 50 g of a ceric nitrate solution in terms of CeO.sub.2 containing not less than 90 mol % tetravalent cerium ions was measured out, and adjusted to a total amount of 1 L with pure water. The obtained solution was heated to 100? C., maintained at this temperature for 30 minutes, and allowed to cool down to the room temperature, to thereby obtain a cerium suspension.

(4) After the mother liquor was removed from the cerium suspension thus obtained, 10.4 ml of a lanthanum nitrate solution (2.6 g in terms of La.sub.2O.sub.3), 10.3 ml of a praseodymium nitrate solution (2.6 g in terms of Pr.sub.6O.sub.11), 2.5 g of colloidal silica (0.5 g in terms of SiO.sub.2) were added, and the total volume was adjusted to 1 L with pure water.

(5) Then the cerium suspension containing precursors of lanthanum oxide, praseodymium oxide, and silicon oxide was maintained at 120? C. for 2 hours, allowed to cool, and neutralized to pH 8.5 with aqueous ammonia.

(6) A slurry resulting from the neutralization was subjected to solid-liquid separation by Nutsche filtering to obtain a filter cake, which was calcined at 500? C. for 10 hours in the atmosphere to obtain a composite oxide powder mainly composed of cerium oxide with 1 part by mass of silicon oxide with respect to 100 parts by mass in total of cerium oxide, lanthanum oxide, and praseodymium oxide contained at 90:5:5 by mass.

(7) The obtained composite oxide powder was measured of the specific surface areas by the BET method after calcination at 900? C. for 5 hours and at 1000? C. for 5 hours, in the atmosphere. Further, the cerium oxide reducibility was calculated from the measurement of temperature-programmed reduction (TPR) from 50? C. to 900? C. after calcination at 1000? C. for 5 hours. The results are shown in Table 1.

Example 2

(8) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 2 parts by mass of silicon oxide was added.

(9) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 2 parts by mass of silicon oxide with respect 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 1 except that the amount of the colloidal silica was 4.9 g (1.0 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 3

(10) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(11) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 1 except that the amount of the colloidal silica was 12.7 g (2.6 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 4

(12) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 10 parts by mass of silicon oxide was added.

(13) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 10 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 1 except that the amount of the colloidal silica was 25.4 g (5.2 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 5

(14) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 20 parts by mass of silicon oxide was added.

(15) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 20 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 1 except that the amount of the colloidal silica was 50.8 g (10.4 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 6

(16) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 2 parts by mass of silicon oxide was added, and prepared by a method different from Example 2.

(17) 50 g of a ceric nitrate solution in terms of CeO.sub.2 containing not less than 90 mol % tetravalent cerium ions was measured out, and adjusted to a total amount of 1 L with pure water. The obtained solution was heated to 100? C., maintained at this temperature for 30 minutes, and allowed to cool down to the room temperature, to thereby obtain a cerium suspension.

(18) After the mother liquor was removed from the cerium suspension thus obtained, 10.4 ml of a lanthanum nitrate solution (2.6 g in terms of La.sub.2O.sub.3) and 10.3 ml of a praseodymium nitrate solution (2.6 g in terms of Pr.sub.6O.sub.11) were added, and the total volume was adjusted to 1 L with pure water.

(19) Then the cerium suspension containing precursors of lanthanum oxide and praseodymium oxide was maintained at 120? C. for 2 hours, allowed to cool, and neutralized to pH 8.5 with aqueous ammonia.

(20) A slurry resulting from the neutralization was subjected to solid-liquid separation by Nutsche filtering to obtain a filter cake, which was calcined at 300? C. for 10 hours in the atmosphere to obtain a rare earth composite oxide mainly composed of cerium oxide with 5% by mass each of lanthanum oxide and praseodymium oxide.

(21) Then 16.1 g of the rare earth composite oxide thus obtained was placed in a beaker, to which an ethanol solution of 1.04 g tetraethyl orthosilicate (0.30 g in terms of SiO.sub.2) in a total amount of 9 ml was added to impregnate the rare earth composite oxide with a solution of a silicon oxide precursor by pore-filling.

(22) The rare earth composite oxide impregnated with the solution of a silicon oxide precursor was dried at 120? C. for 10 hours, and calcined at 500? C. for 10 hours in the atmosphere to obtain a composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 2 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide. The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 7

(23) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(24) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 6 except that the amount of the tetraethyl orthosilicate was 2.60 g (0.75 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 8

(25) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 10 parts by mass of silicon oxide was added.

(26) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 10 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 6 except that the amount of the tetraethyl orthosilicate was 5.20 g (1.5 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 9

(27) This example relates to a composite oxide containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass in total of 100 parts by mass, with respect to which 20 parts by mass of silicon oxide was added.

(28) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide, lanthanum oxide, and praseodymium oxide at 90:5:5 by mass and 20 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide, lanthanum oxide, and praseodymium oxide, was prepared in the same way as in Example 6 except that the amount of the tetraethyl orthosilicate was 10.4 g (3.0 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 10

(29) This example relates to a composite oxide containing cerium oxide and lanthanum oxide at 90:10 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(30) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide and lanthanum oxide at 90:10 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide and lanthanum oxide, was prepared in the same way as in Example 6 except that instead of adding the lanthanum nitrate solution and the praseodymium nitrate solution, only 20.8 ml of the lanthanum nitrate solution (5.2 g in terms of La.sub.2O.sub.3) was added, and the amount of tetraethyl orthosilicate was 2.60 g (0.75 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 11

(31) This example relates to a composite oxide containing cerium oxide and lanthanum oxide at 85:15 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(32) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide and lanthanum oxide at 85:15 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide and lanthanum oxide, was prepared in the same way as in Example 10 except that instead of adding the lanthanum nitrate solution and the praseodymium nitrate solution, only 33.1 ml of the lanthanum nitrate solution (8.3 g in terms of La.sub.2O.sub.3) was added. The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 12

(33) This example relates to a composite oxide containing cerium oxide and praseodymium oxide at 90:10 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(34) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide and praseodymium oxide at 90:10 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide and praseodymium oxide, was prepared in the same way as in Example 6 except that instead of adding the lanthanum nitrate solution and the praseodymium nitrate solution, only 20.5 ml of the praseodymium nitrate solution (5.2 g in terms of Pr.sub.6O.sub.11) was added, and the amount of tetraethyl orthosilicate was 2.60 g (0.75 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 13

(35) This example relates to a composite oxide containing cerium oxide and neodymium oxide at 90:10 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(36) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide and neodymium oxide at 90:10 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide and neodymium oxide, was prepared in the same way as in Example 6 except that instead of adding the lanthanum nitrate solution and the praseodymium nitrate solution, 23.5 ml of a neodymium nitrate solution (5.2 g in terms of Nd.sub.2O.sub.3) was added, and the amount of tetraethyl orthosilicate was 2.60 g (0.75 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Example 14

(37) This example relates to a composite oxide containing cerium oxide and yttrium oxide at 90:10 by mass in total of 100 parts by mass, with respect to which 5 parts by mass of silicon oxide was added.

(38) A composite oxide powder mainly composed of cerium oxide and containing cerium oxide and yttrium oxide at 90:10 by mass and 5 parts by mass of silicon oxide with respect to 100 parts by mass in total of the cerium oxide and yttrium oxide, was prepared in the same way as in Example 6 except that instead of adding the lanthanum nitrate solution and the praseodymium nitrate solution, 22.9 ml of a yttrium nitrate solution (5.2 g in terms of Y.sub.2O.sub.3) was added, and the amount of tetraethyl orthosilicate was 2.60 g (0.75 g in terms of SiO.sub.2). The properties of the composite oxide powder thus obtained were evaluated in the same way as in Example 1. The results are shown in Table 1.

Comparative Examples 1 to 4

(39) These examples relate to composite oxides without silicon oxide, which were obtained before the impregnation with the solution of a silicon oxide precursor in Examples 10, 11, 12, and 6, respectively. The properties of the obtained oxide powders were evaluated in the same way as in Example 1. The results are shown in Table 1.

(40) TABLE-US-00001 TABLE 1 Si content in terms of Composition of SiO.sub.2 with respect to Specific Ce + RE in 100 parts by mass in surface area Reducibility terms of oxides total of oxides of Ce + RE (m.sup.2/g) (%) (mass %) (parts by mass) 900? C./5 h 1000? C./5 h 1000? C./5 h Example 1 Ce/La/Pr = 90/5/5 1 41 26 33 Example 2 Ce/La/Pr = 90/5/5 2 53 33 41 Example 3 Ce/La/Pr = 90/5/5 5 83 55 51 Example 4 Ce/La/Pr = 90/5/5 10 101 70 61 Example 5 Ce/La/Pr = 90/5/5 20 108 68 82 Example 6 Ce/La/Pr = 90/5/5 2 52 33 36 Example 7 Ce/La/Pr = 90/5/5 5 83 55 43 Example 8 Ce/La/Pr = 90/5/5 10 103 74 66 Example 9 Ce/La/Pr = 90/5/5 20 105 70 76 Example 10 Ce/La = 90/10 5 80 55 49 Example 11 Ce/La = 85/15 5 63 41 42 Example 12 Ce/Pr = 90/10 5 80 56 51 Example 13 Ce/Nd = 90/10 5 68 49 43 Example 14 Ce/Y = 90/10 5 64 45 42 Comp. Ex. 1 Ce/La = 90/10 0 33 22 27 Comp. Ex. 2 Ce/La = 85/15 0 31 21 26 Comp. Ex. 3 Ce/Pr = 90/10 0 32 19 35 Comp. Ex. 4 Ce/La/Pr = 90/5/5 0 34 21 26 RE stands for rare earth metal elements other than cerium and including yttrium.