SINGLE-LAYER 3-WAY CATALYTIC CONVERTER

Abstract

The invention relates to a catalytic converter for removing carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of internal combustion engines operated with stoichiometric air-fuel mixture, which catalytic converter comprises a substrate of the length L and a catalytic coating, characterized in that the coating is located on the walls of the substrate and extends, proceeding from one end of the substrate, over a length corresponding to at least 50% of L and comprises active aluminum oxide, two cerium/zirconium/rare-earth-metal mixed oxides different from each other, and at least one platinum group metal.

Claims

1. Catalytic converter for removing carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of internal combustion engines operated with stoichiometric air-fuel mixture, comprising a ceramic through-flow substrate of length L and a catalytic coating, characterized in that the coating is located on the walls of the substrate and extends from one end of the substrate over a length of at least 50% of L and has active aluminum oxide, at least two cerium/zirconium/rare earth metal mixed oxides that differ from one another and at least one platinum group metal, and the first cerium/zirconium/rare earth metal mixed oxide has a weight ratio of cerium oxide to zirconium oxide of 0.7 to 0.1, which is smaller than in the second cerium/zirconium/rare earth metal mixed oxide, which has a cerium oxide to zirconium oxide weight ratio of 0.5 to 1.5.

2. Catalytic converter according to claim 1, characterized in that the weight ratio of aluminum oxide to the sum of the two cerium/zirconium/rare earth metal mixed oxides is in the range from 10:90 to 60:40.

3. Catalytic converter according to claim 1, characterized in that the weight ratio of the first cerium/zirconium/rare earth metal mixed oxide to the second cerium/zirconium/rare earth metal mixed oxide is in the range from 4:1 to 1:4.

4. Catalytic converter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a higher zirconium oxide content than the second cerium/zirconium/rare earth metal mixed oxide.

5. Catalytic converter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a cerium oxide content of 10% to 40% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

6. Catalytic converter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide has a zirconium oxide content of 40% to 90% based on the weight of the first cerium/zirconium/rare earth metal mixed oxide.

7. Catalytic converter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide has a cerium oxide content of 25% to 60% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

8. Catalytic converter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide has a zirconium oxide content of 20% to 70% based on the weight of the second cerium/zirconium/rare earth metal mixed oxide.

9. Catalytic converter according to claim 1, characterized in that both cerium/zirconium/rare earth metal mixed oxides are doped with lanthanum oxide.

10. Catalytic converter according to claim 1, characterized in that the lanthanum oxide content is >0% to 10% based on the weight of the respective cerium/zirconium/rare earth metal mixed oxide.

11. Catalytic converter according to claim 1, characterized in that the first cerium/zirconium/rare earth metal mixed oxide is doped with yttrium oxide in addition to lanthanum oxide.

12. Catalytic converter according to claim 1, characterized in that the second cerium/zirconium/rare earth metal mixed oxide is doped not only with lanthanum oxide but also with a further metal oxide from the group of rare earth metal oxides, preferably with praseodymium.

13. Catalytic converter according to claim 1, characterized in that the catalytically active coating contains the precious metals platinum, palladium, rhodium or mixtures thereof.

14. Method for removing carbon monoxide, hydrocarbons and nitrogen oxides from the exhaust gas of combustion engines operated with stoichiometric air-fuel mixture, characterized in that the exhaust gas is conducted through a catalytic converter in accordance with claim 1.

Description

[0051] FIG. 1 shows a catalytic converter according to the invention comprising a ceramic substrate (1) of length L and channels which extend in parallel between a first end (2) and a second end (3) of the substrate and are separated by walls (4). The coating (5) is located on the walls of the ceramic substrate (1).

[0052] The invention is explained in more detail in the following examples.

EXAMPLES

[0053] Three catalytic converters each were provided with different catalytically active coatings. Ceramic through-flow substrates of cordierite having a diameter of 4 inches and a length of 6 inches and a cell density of 600 cpsi and a wall thickness of 4.3 mil were used as substrates. Each catalytic converter was provided with a coating of 161 g/l based on the volume of the ceramic honeycomb body.

Comparative Example 1

[0054] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an oxygen storage component containing 25% by weight cerium oxide, 68% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 4% by weight yttrium oxide. The weight ratio of aluminum oxide to oxygen storage component was 50:50. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 161 g/l, the precious metal load amounted to 1.509 g/l having a palladium to rhodium ratio of 9:1. The coated catalytic converter thus obtained was dried and then calcined.

Comparative Example 2

[0055] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an oxygen storage component containing 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide.

[0056] The weight ratio of aluminum oxide to oxygen storage component was 50:50. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 161 g/l, the precious metal load amounted to 1.509 g/l having a palladium to rhodium ratio of 9:1. The coated catalytic converter thus obtained was dried and then calcined.

Example 1 According to the Invention

[0057] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component comprising 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide, and a second oxygen storage component comprising 24% cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide to oxygen storage components was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 161 g/l, the precious metal load amounted to 1.509 g/l having a palladium to rhodium ratio of 9:1. The coated catalytic converter thus obtained was dried and then calcined.

[0058] In order to determine the catalytic properties of the catalytic converter according to the invention, a catalytic converter each of Comparative Example 1, Comparative Example 2 and Example 1 was aged in an engine test bench aging. The aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950 C. before the catalyst input (maximum bed temperature of 1030 C.). The aging time was 38 hours.

[0059] Subsequently, an engine test bench was used to test the light-off performance at a constant average air ratio , and the dynamic conversion with changes of was tested. Table 1 contains the temperatures T.sub.50 at which 50% of the considered components are respectively converted. In this case, the light-off performance was determined with a stoichiometric exhaust gas composition (=0.999 with 3.4% amplitude).

TABLE-US-00001 TABLE 1 Results of the light-off performance after aging for Example 1 and Comparative Examples 1 and 2 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichio- stoichio- stoichio- metric metric metric Comparative Example 1 421 433 432 Comparative Example 2 427 436 437 Example 1 418 427 427

[0060] Example 1 according to the invention shows a marked improvement in light-off performance compared to the two comparative examples.

[0061] The dynamic conversion performance was determined in a range for of 0.99 to 1.01 at a constant temperature of 510 C. The amplitude of in this case amounted to 6.8%. 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 Examples 1 and 2 HC conversion at CO/NOx conversion at of the CO/NOx the point of intersection point of intersection Comparative Example 1 73% 95% Comparative Example 2 88% 97% Example 1 95% 97%

[0062] Example 1 according to the invention shows a marked improvement in the dynamic CO/NOx conversion after aging than the two comparative examples.

[0063] OSC Properties:

[0064] The oxygen storage capacity was determined in two different experiments. Table 3 shows the values for the lambda step test which characterizes the static oxygen storage capacity. The air/fuel ratio before the catalytic converter is changed from rich (=0.96) to lean (=1.04). The stored oxygen quantity is calculated from the delay time of the post-cat lambda probe in comparison to the pre-cat lambda probe.

TABLE-US-00003 TABLE 3 Static oxygen storage capacity after aging for Example 1 and Comparative Examples 1 and 2 Oxygen storage capacity (mg/l) Comparative Example 1 132 Comparative Example 2 252 Example 1 277

FURTHER EXAMPLES

Comparative Example 1

[0065] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an oxygen storage component containing 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide. The weight ratio of aluminum oxide to oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 76.27 g/l, the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated catalytic converter thus obtained was dried and then calcined.

Comparative Example 2

[0066] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with an oxygen storage component containing 24% by weight cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. The weight ratio of aluminum oxide to oxygen storage component was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 76.27 g/l, the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated catalytic converter thus obtained was dried and then calcined.

Example 1 According to the Invention

[0067] Aluminum oxide stabilized with lanthanum oxide was suspended in water together with a first oxygen storage component comprising 40% by weight cerium oxide, 50% by weight zirconium oxide, 5% by weight lanthanum oxide and 5% by weight praseodymium oxide, and a second oxygen storage component comprising 24% cerium oxide, 60% by weight zirconium oxide, 3.5% by weight lanthanum oxide and 12.5% by weight yttrium oxide. Both oxygen storage components were used in equal parts. The weight ratio of aluminum oxide to oxygen storage components was 30:70. The suspension thus obtained was then mixed with a palladium nitrate solution and a rhodium nitrate solution under constant stirring. The resulting coating suspension was used directly for coating a commercially available substrate, the coating being applied over 100% of the substrate length. The total load of this catalytic converter amounted to 76.27 g/l, the precious metal load amounted to 1.271 g/l having a palladium to rhodium ratio of 5:1. The coated catalytic converter thus obtained was dried and then calcined.

[0068] Dynamic Pressure:

TABLE-US-00004 600 m.sup.3/h 900 m.sup.3/h Comparative Example 1 52.9 mbar 0.2 mbar 107.4 mbar 0.3 mbar Comparative Example 2 53.3 mbar 0.4 mbar 107.2 mbar 0.5 mbar Example 1 53.0 mbar 0.6 mbar 105.9 mbar 0.6 mbar

[0069] In order to determine the catalytic properties of the catalytic converter according to the invention, a catalytic converter each of Comparative Example 1, Comparative Example 2 and Example 1 was aged in an engine test bench aging. The aging process consists of an overrun cut-off aging process with an exhaust gas temperature of 950 C. before the catalyst input (maximum bed temperature of 1030 C.). The aging time was 19 hours.

[0070] Subsequently, an engine test bench was used to test the light-off performance at a constant average air ratio , and the dynamic conversion with changes of was tested. Table 1 contains the temperatures T.sub.50 at which 50% of the considered components are respectively converted. In this case, the light-off performance with stoichiometric exhaust gas composition (=0.999 with 3.4% amplitude) was determined.

TABLE-US-00005 TABLE 1 Results of the light-off performance after aging for Example 1 and Comparative Examples 1 and 2 T.sub.50 HC T.sub.50 CO T.sub.50 NOx stoichio- stoichio- stoichio- metric metric metric Comparative Example 1 391 399 406 Comparative Example 2 370 377 377 Example 1 374 379 379

[0071] The dynamic conversion performance was determined in a range for of 0.99 to 1.01 at a constant temperature of 510 C. In so doing, the amplitude of was 6.8%. 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-00006 TABLE 2 Results of the dynamic conversion performance after aging for Example 1 and Comparative Examples 1 and 2 HC conversion at CO/NOx conversion at of the CO/NOx the point of intersection point of intersection Comparative Example 1 82% 96% Comparative Example 2 81.5% 97% Example 1 90% 97%

[0072] Example 1 according to the invention shows a marked improvement in the dynamic CO/NOx conversion after aging, while the light-off performance is similarly good as in Comparative Example 2 but better than in Comparative Example 1.

[0073] OSC Properties:

[0074] The oxygen storage capacity was determined in two different experiments. Table 3 shows the values for the lambda step test which characterizes the static oxygen storage capacity. The air/fuel ratio before the catalytic converter is changed from rich (=0.96) to lean (=1.04). The stored oxygen quantity is calculated from the delay time of the post-cat lambda probe in comparison to the pre-cat lambda probe.

TABLE-US-00007 TABLE 3 Static oxygen storage capacity after aging for Example 1 and Comparative Examples 1 and 2 Oxygen storage capacity (mg/l) Comparative Example 1 182 Comparative Example 2 132 Example 1 194

[0075] In another test, dynamic oxygen storage capacity is determined. At an average value of =1, the exhaust gas is subjected to various amplitudes with a frequency of 1 Hz. The amplitude signal of the post-cat lambda probe is divided by the amplitude signal of the pre-cat lambda probe. The smaller the value, the better the dynamic oxygen storage capacity. The results are shown in Table 4.

TABLE-US-00008 TABLE 4 Dynamic oxygen storage capacity after aging for Example 1 and Comparative Examples 1 and 2 2% 3.4% 6.8% amplitude amplitude amplitude Comparative Example 1 0.24 0.37 0.41 Comparative Example 2 0.08 0.13 0.28 Example 1 0.09 0.14 0.23

[0076] The example according to the invention shows both a high static and a very good dynamic oxygen storage capacity after aging.