DOUBLE-LAYER THREE-WAY CATALYST WITH IMPROVED AGING STABILITY

Abstract

The present invention relates to a catalyst comprising two layers on an inert catalyst carrier, wherein a layer A lying directly on the catalyst carrier contains at least one platinum group metal and one cerium/zirconium/SE mixed oxide, and a layer B, applied on layer A and in direct contact with the flow of exhaust gas, contains at least one platinum group metal and a cerium/zirconium/SE mixed oxide, wherein SE stands for a rare earth metal other than from cerium, characterized in that the fraction of SE oxide in the cerium/zirconium/SE mixed oxide of layer A is less than the fraction of SE oxide in the cerium/zirconium/SE mixed oxide of layer B.

Claims

1. Catalyst comprising two layers on an inert catalyst support, wherein a layer A contains at least one platinum group metal, as well as a cerium/zirconium/SE mixed oxide, and a layer B applied to layer A contains at least one platinum group metal, as well as a cerium/zirconium/SE mixed oxide, wherein SE stands for a rare-earth metal other than cerium, characterized in that the proportion of the SE oxide in the cerium/zirconium/SE mixed oxide of layer A is less than the proportion of the SE oxide in the cerium/zirconium/SE mixed oxide of layer B, calculated respectively in wt % and relative to the cerium/zirconium/SE mixed oxide.

2. Catalyst according to claim 1, characterized in that layer A and layer B, independently of each other, contain, as platinum group metal, platinum, palladium, rhodium, or mixtures of at least two of these platinum group metals.

3. Catalyst according to claim 1, characterized in that, as platinum group metal, layer A contains platinum, palladium, or platinum and palladium, and layer B contains palladium, rhodium, or palladium and rhodium.

4. Catalyst according to claim 1, characterized in that, as platinum group metal, layer A contains palladium, and layer B contains rhodium, or palladium and rhodium.

5. Catalyst according to claim 1, characterized in that layer A and layer B contain active aluminum oxide.

6. Catalyst according to claim 5, characterized in that the platinum group metal in layer A and/or in layer B is supported wholly or in part on active aluminum oxide.

7. Catalyst according to claim 1, characterized in that the SE oxide in the cerium/zirconium/SE mixed oxide is lanthanum oxide, yttrium oxide, praseodymium oxide, neodymium oxide, samarium oxide, or mixtures of one or more of these metal oxides.

8. Catalyst according to claim 1, characterized in that the SE oxide in the cerium/zirconium/SE mixed oxide is a mixture of lanthanum oxide and yttrium oxide.

9. Catalyst according to claim 1, characterized in that the proportion of the SE oxide in the cerium/zirconium/SE mixed oxide in layer A is 1 to 12 wt %—preferably, 3 to 10 wt %, and even more preferably, 6 to 9 wt %—relative to the cerium/zirconium/SE mixed oxide in each case.

10. Catalyst according to claim 1, characterized in that the proportion of the SE oxide in the cerium/zirconium/SE mixed oxide in layer B is 2 to 25 wt %—preferably, 10 to 20 wt %, and even more preferably, 14 to 18 wt %—relative to the cerium/zirconium/SE mixed oxide in each case.

11. Catalyst according to claim 1, characterized in that the weight ratio of cerium oxide to zirconium oxide in the cerium/zirconium/SE mixed oxide in layer A is 0.1 to 1.0—preferably, 0.2 to 0.7, and even more preferably, 0.3 to 0.5.

12. Catalyst according to claim 1, characterized in that the weight ratio of cerium oxide to zirconium oxide in the cerium/zirconium/SE mixed oxide in layer B is 0.1 to 1.0—preferably, 0.2 to 0.7, and even more preferably, 0.3 to 0.5.

13. Catalyst according to claim 1, characterized in that it comprises two layers on an inert catalyst support, wherein a layer A contains palladium, active aluminum oxide, as well as a cerium/zirconium/lanthanum/yttrium mixed oxide, and a layer B applied to layer A contains rhodium, or palladium and rhodium, active aluminum oxide, as well as a cerium/zirconium/lanthanum/yttrium mixed oxide, characterized in that the proportion of the sum of lanthanum oxide and yttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide of layer A is less than the proportion of the sum of lanthanum oxide and yttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide of layer B, calculated respectively in wt % and relative to the cerium/zirconium/lanthanum/yttrium oxide.

14. Catalyst according to claim 13, characterized in that the proportion of the sum of lanthanum oxide and yttrium oxide in the cerium/zirconium/lanthanum/yttrium mixed oxide of layer A is 6 to 9 wt % relative to the cerium/zirconium/lanthanum/yttrium mixed oxide of layer A, and 14 to 18 wt % in the cerium/zirconium/lanthanum/yttrium mixed oxide of layer B relative to the cerium/zirconium/lanthanum/yttrium mixed oxide of layer B, calculated in each case in wt % and relative to the cerium/zirconium/lanthanum/yttrium mixed oxide.

15. Catalyst according to claim 1, characterized in that layer A lies directly on the inert catalyst support.

Description

EXAMPLE 1

[0046] A double-layer catalyst was produced by first producing two suspensions. The composition of the first suspension for layer A (relative to the volume of the catalyst support) was:

[0047] 40 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0048] 40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt % CeO.sub.2, 67.5 wt % ZrO.sub.2; 3.5 wt % La.sub.2O.sub.3, and 4 wt % Y.sub.2O.sub.3

[0049] 5 g/L of BaSO.sub.4

[0050] 3.178 g/L of Pd

[0051] The composition of the second suspension for layer B (relative to the volume of the catalyst support) was:

[0052] 60 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0053] 47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt % CeO.sub.2, 60 wt % ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 12.5 wt % Y.sub.2O.sub.3

[0054] 0.177 g/L of Pd

[0055] 0.177 g/L of Rh

EXAMPLE 2

[0056] A double-layer catalyst was produced analogously to example 1. The composition of the first suspension for layer A was: [0057] 40 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3 [0058] 40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20.5 wt % CeO.sub.2, 67.5 wt % ZrO.sub.2, 4.5 wt % La.sub.2O.sub.3, and 7.5 wt % Y.sub.2O.sub.3

[0059] 5 g/L of BaSO.sub.4

[0060] 3.178 g/L of Pd

[0061] The composition of the second suspension for layer B was:

[0062] 60 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0063] 47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20 wt % CeO.sub.2, 60 wt % ZrO.sub.2, 5 wt % La.sub.2O.sub.3 and 15 wt % Y.sub.2O.sub.3

[0064] 0.177 g/L of Pd

[0065] 0.177 g/L of Rh

EXAMPLE 3

[0066] A double-layer catalyst was produced analogously to example 1. The composition of the first suspension for layer A was:

[0067] 40 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0068] 40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 20.5 wt % CeO.sub.2, 67.5 wt % ZrO.sub.2, 4.5 wt % La.sub.2O.sub.3, and 7.5 wt % Y.sub.2O.sub.3

[0069] 5 g/L of BaSO.sub.4

[0070] 3.178 g/L of Pd

[0071] The composition of the second suspension for layer B was:

[0072] 60 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0073] 47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 15 wt % CeO.sub.2, 60 wt %

[0074] ZrO.sub.2, 7 wt % La.sub.2O.sub.3, and 18 wt % Y.sub.2O.sub.3

[0075] 0.177 g/L of Pd

[0076] 0.177 g/L of Rh

COMPARATIVE EXAMPLE 1

[0077] A double-layer catalyst was produced analogously to example 1. The composition of the first suspension for layer A was:

[0078] 40 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0079] 40 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt % CeO.sub.2, 67.5 wt %

[0080] ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 4 wt % Y.sub.2O.sub.3

[0081] 5 g/L of BaSO.sub.4

[0082] 3.178 g/L of Pd

[0083] The composition of the second suspension for layer B was:

[0084] 60 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0085] 47 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt % CeO.sub.2, 67.5 wt %

[0086] ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 4 wt % Y.sub.2O.sub.3

[0087] 0.177 g/L of Pd

[0088] 0.177 g/L of Rh

[0089] Example 1 and comparative example 1 were aged in an engine test bench aging process. The aging process consists of an overrun fuel cut-off aging with an exhaust gas temperature of 950° C. in front of the catalyst inlet (1,030° C. maximum bed temperature). The aging time was 76 hours.

[0090] The start-up performance was tested on an engine test bench at a constant average air/fuel ratio λ, and the dynamic conversion was tested with changes of A. Table 1 contains the temperatures T.sub.50 at which 50% respectively of the considered components are converted. In so doing, the start-up performance was determined with a stoichiometric exhaust gas composition (λ=0.999 with ±3.4% amplitude).

TABLE-US-00001 TABLE 1 Results of the start-up performance after aging for example 1 and comparative example 1 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichiometric stoichiometric stoichiometric Comparative 391 402 398 example 1 Example 1 381 391 388

[0091] The dynamic conversion performance was determined in a range for A of 0.99 to 1.01 at a constant temperature of 510° C. In so doing, the amplitude of A was ±3.4%. Table 2 contains the conversion at the point of intersection of the CO and NOx conversion curves, as well as the associated HC conversion.

TABLE-US-00002 TABLE 2 Results of the dynamic conversion performance after aging for example 1 and comparative example 1 CO/NOx conversion at the HC conversion at λ of the point of intersection CO/NOx point of intersection Comparative 73.5% 92 example 1 Example 1 .sup. 79% 93

[0092] Example 1 according to the invention shows a significant improvement in the start-up performance and in the dynamic CO/NOx conversion after aging.

[0093] In the following examples 4 and 5, and in comparative example 2, double-layer catalysts were produced by twice coating flow-through honeycomb bodies made from ceramic with 93 cells per cm.sup.2 and with a wall thickness of 0.1 mm, as well as dimensions of 10.2 cm in diameter and 15.2 cm in length, To this end, two different suspensions were produced respectively for layer A and B. The support was then first coated with the suspension for layer A and then calcined in air for 4 hours at 500° C. Subsequently, the support coated with layer A was coated with the suspension for layer B and then calcined under the same conditions as for layer A.

EXAMPLE 4

[0094] A double-layer catalyst was produced by first producing two suspensions. The composition of the first suspension for layer A (relative to the volume of the catalyst support) was:

[0095] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0096] 50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 39 wt % CeO.sub.2, 51 wt %

[0097] ZrO.sub.2, 3 wt % La.sub.2O.sub.3, and 7 wt % Y.sub.2O.sub.3

[0098] 5 g/L of BaSO.sub.4

[0099] 1.483 g/L of Pd

[0100] The composition of the second suspension for layer B (relative to the volume of the catalyst support) was:

[0101] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0102] 65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt % CeO.sub.2, 60 wt %

[0103] ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 12.5 wt % Y.sub.2O.sub.3

[0104] 0.177 g/L of Pd

[0105] 0.177 g/L of Rh

EXAMPLE 5

[0106] A double-layer catalyst was produced analogously to example 4. The composition of the first suspension for layer A was:

[0107] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0108] 50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 25 wt % CeO.sub.2, 67.5 wt %

[0109] ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 4 wt % Y.sub.2O.sub.3

[0110] 5 g/L of BaSO.sub.4

[0111] 1.483 g/L of Pd

[0112] The composition of the second suspension for layer B was:

[0113] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0114] 65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 24 wt % CeO.sub.2, 60 wt %

[0115] ZrO.sub.2, 3.5 wt % La.sub.2O.sub.3, and 12.5 wt % Y.sub.2O.sub.3

[0116] 0.177 g/L of Pd

[0117] 0.177 g/L of Rh

COMPARATIVE EXAMPLE 2

[0118] A double-layer catalyst was produced analogously to example 4. The composition of the first suspension for layer A was:

[0119] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0120] 50 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 39 wt % CeO.sub.2, 51 wt %

[0121] ZrO.sub.2, 3 wt % La.sub.2O.sub.3, and 7 wt % Y.sub.2O.sub.3

[0122] 5 g/L of BaSO.sub.4

[0123] 1.483 g/L of Pd

[0124] The composition of the second suspension for layer B was:

[0125] 70 g/L of activated aluminum oxide stabilized with 4 wt % of La.sub.2O.sub.3

[0126] 65 g/L of cerium/zirconium/lanthanum/yttrium mixed oxide with 22 wt % CeO.sub.2, 68 wt % ZrO.sub.2, 2 wt % La.sub.2O.sub.3, 5 wt % Nd.sub.2O.sub.3, and 3 wt % Y.sub.2O.sub.3

[0127] 0.177 g/L of Pd

[0128] 0.177 g/L of Rh

[0129] Examples 4 and 5, as well as comparative example 2, were aged in an engine test bench aging process. The aging process consists of an overrun fuel cut-off aging with an exhaust gas temperature of 950° C. in front of the catalyst inlet (1,030° C. maximum bed temperature). The aging time was 76 hours.

[0130] The start-up performance was tested on an engine test bench at a constant average air/fuel ratio λ, and the dynamic conversion was tested with changes in A.

[0131] Table 3 contains the temperatures T50 at which 50% respectively of the considered components are converted. In so doing, the start-up performance was determined with stoichiometric exhaust gas composition (λ=0.999 with ±3.4% amplitude) and with slightly lean exhaust gas composition (λ=1.05 without amplitude).

TABLE-US-00003 TABLE 3 Results of the start-up performance after aging for examples 4 and 5 and comparative example 2 T50 HC T50 CO T50 NOx stoichio- stoichio- stoichio- T50 HC T50 CO metric metric metric lean lean Comparative 403 420 416 383 382 example 2 Example 4 391 411 401 371 369 Example 5 384 397 392 370 369

[0132] The dynamic conversion performance was determined in a range for λ of 0.99 to 1.01 at a constant temperature of 510° C. In so doing, the amplitude of A was ±3.4%. Table 4 contains the conversion at the point of intersection of the CO and NOx conversion curves, as well as the associated HC conversion.

TABLE-US-00004 TABLE 4 Results of the dynamic conversion performance after aging for examples 4 and 5 and comparative example 2 CO/NOx point HC conversion at λ of the of intersection CO/NOx point of intersection Comparative 81.5% .sup. 95% example 2 Example 4 86.5% 95.5% Example 5 .sup. 95% 96.5%

[0133] Examples 4 and 5 according to the invention show a significant improvement in the startup performance and in the dynamic CO/NOx conversion after aging, wherein example 5 shows the greatest activity,

[0134] Further examples were prepared analogously to example 5, with the difference being that rare-earth metal oxides (SE.sub.xO.sub.y), as specified in Table 5, were used in the cerium/zirconium/rare-earth metal mixed oxides,

TABLE-US-00005 TABLE 5 wt % wt % SE.sub.xO.sub.y 1 SE.sub.xO.sub.y 2 Example Layer of CeO.sub.2 of ZrO.sub.2 wt % wt % 6 A 40 50 La.sub.2O.sub.3 5 — — B 30 55 La.sub.2O.sub.3 12 — — 7 A 40 50 Y.sub.2O.sub.3 7.5 — — B 30 55 Y.sub.2O.sub.3 15 — — 8 A 40 50 La.sub.2O.sub.3 5 Pr.sub.6O.sub.11 5 B 30 55 La.sub.2O.sub.3 5 Pr.sub.6O.sub.11 10 9 A 30 63 La.sub.2O.sub.3 2 Nd.sub.2O.sub.3 5 B 25 60 La.sub.2O.sub.3 5 Nd.sub.2O.sub.3 10 10 A 30 62 Nd.sub.2O.sub.3 3 Pr.sub.6O.sub.11 5 B 30 57 Nd.sub.2O.sub.3 5 Pr.sub.6O.sub.11 8 11 A 40 54 La.sub.2O.sub.3 3 Sm.sub.2O.sub.3 3 B 30 55 La.sub.2O.sub.3 5 Sm.sub.2O.sub.3 10 12 A 40 51.5 Nd.sub.2O.sub.3 3.5 Y.sub.2O.sub.3 5 B 30 55 Nd.sub.2O.sub.3 5 Y.sub.2O.sub.3 10