THREE-WAY CATALYST

Abstract

The present invention relates to a catalyst comprising a carrier substrate of the length L extending between substrate ends a and b and three washcoat zones A, B and C wherein washcoat zone A comprises one or more first platinum group metals and extends starting from substrate end a over a part of the length L, washcoat zone C comprises one or more first platinum group metals and extends starting from substrate end b over a part of the length L, and washcoat zone B comprises the same components as washcoat zone A and in addition, one or more second platinum group metals and extends between washcoat zones A and C, wherein L=L.sub.A+L.sub.B+L.sub.C, wherein L.sub.A is the length of washcoat zone A, L.sub.B is the length of substrate length B and L.sub.C is the length of substrate length C.

Claims

1. Catalyst comprising a carrier substrate of the length L extending between substrate ends a and b and three washcoat zones A, B and C wherein washcoat zone A comprises one or more first platinum group metals and extends starting from substrate end a over a part of the length L, washcoat zone C comprises one or more first platinum group metals and extends starting from substrate end b over a part of the length L, and washcoat zone B comprises the same components as washcoat zone A and in addition one or more second platinum group metals and extends between washcoat zones A and C, wherein L=L.sub.A+L.sub.B+L.sub.C, wherein L.sub.A is the length of washcoat zone A, L.sub.B is the length of washcoat zone B and L.sub.C is the length of washcoat zone C.

2. Catalyst according to claim 1, wherein first and second platinum group metals are the same or are different.

3. Catalyst according to claim 1, wherein the first platinum group metal is platinum, palladium and/or rhodium and the second platinum group metal is platinum, palladium or rhodium.

4. Catalyst according to claim 1, wherein the first platinum group metal is palladium and rhodium and the second platinum group metal is palladium.

5. Catalyst according to one claim 1, wherein the first platinum group metal is palladium and rhodium and the second platinum group metal is rhodium.

6. Catalyst according to claim 1, wherein the first platinum group metal is palladium and rhodium and the second platinum group metal is platinum.

7. Catalyst according to claim 1, wherein washcoat zone A comprises two washcoat zones A1 and A2, which both extend over the length L.sub.A, wherein washcoat zone A1 comprises one or more first platinum group metals and washcoat zone A2 comprises one or more first platinum group metals different from the one or more first platinum group metals of washcoat zone A1, washcoat zone C comprises two washcoat zones C1 and C2, which both extend over the length L.sub.C, wherein washcoat zone C1 comprises one or more first platinum group metals and washcoat zone C2 comprises one or more first platinum group metals different from the first one or more platinum group metals of washcoat zone C1, and washcoat zone B comprises two washcoat zones B1 and B2, which both extend over the length L.sub.B, wherein washcoat zone B1 comprises the same components as washcoat zone A1 and washcoat zone B2 comprises the same components as washcoat zone A2 and wherein washcoat zones B1 and B2 comprise in addition one or more second platinum group metals.

8. Catalyst according to claim 7, wherein the first platinum group metal in washcoat zone A1 is palladium and/or rhodium and the first platinum group metal in washcoat zone A2 is rhodium

9. Catalyst according to claim 7, wherein the first platinum group metal in washcoat zone A1 is palladium and rhodium and the weight ratio Pd: Rh is from 10:1 to 1:10.

10. Catalyst according to claim 1, wherein the first and second platinum group metals are independently from each other supported on a carrier material.

11. Catalyst according to claim 10, wherein the carrier material is selected from the group consisting of alumina, silica, magnesia, titania, zirconia, ceria, mixtures comprising at least one of these materials and mixed oxides comprising at least one of these materials.

12. Catalyst according to claim 1, wherein washcoat zone A extends over 15 to 50% of the length L of the carrier substrate, washcoat zone B extends over 7 to 30% of the length L of the carrier substrate and washcoat zone C extends over 20 to 78% of the length L of the carrier substrate.

13. Catalyst according to claim 1, wherein the carrier substrate of the length L is a flow-through or filter substrate.

14. Method for the manufacturing of a catalyst according to claim 1 by a four-step process which comprises: coating of the carrier substrate with a coating suspension (washcoat) which contains the components of washcoat zone A over its entire length L, applying a hydrophobic masking zone extending from substrate end a over the length L.sub.A, dipping the coated carrier substrate in an aqueous solution containing a water-soluble compound of the second platinum group metal starting from substrate end a until the length L.sub.A+L.sub.B, so as to form washcoat zone B and drying and heating the coated carrier substrate so as to remove the masking zone.

15. Catalyst system comprising a catalyst according to claim 1 and another three-way catalyst, a gasoline particulate filter, a HC trap and/or a NO.sub.x trap.

16. Catalyst system according to claim 15, wherein substrate end b of the catalyst is followed by a conventional three-way catalyst.

17. Catalyst system according to claim 15, wherein substrate end b of the catalyst follows a conventional three-way catalyst.

18. Method for treating exhaust gases of a combustion engine, wherein the exhaust gas is passed over the catalyst of claim 1, wherein it enters the catalyst at substrate end a and exits it at substrate end b.

19. Method according to claim 18, wherein the catalyst is arranged in close coupled position.

20. Method for treating the exhaust gas of a lean-burn engine, wherein the exhaust gas is passed over the catalyst of claim 1, wherein it enters the catalyst at substrate end a and exits it at substrate end b.

Description

COMPARISON EXAMPLE 1

[0076] Comparison Example 1 is a 1-Layer Pd/Rh technology that was prepared as follows:

[0077] The required amount of water was weighed out and HNO.sub.3 added at 0.5 wt % of the final solids content of the slurry to be prepared. A stabilized alumina was then added containing 3 wt % La.sub.2O.sub.3 for stabilization followed by an equal amount of a second high porous stabilized alumina containing 4 wt % La.sub.2O.sub.3, BaSO.sub.4 was then added with stirring followed by lanthanum acetate and finally the OSC material. The OSC material consisted of CeO.sub.2=40 wt %, ZrO.sub.2+HfO.sub.2=50 wt %, La.sub.2O.sub.3=5.0 wt % and Pr.sub.6O.sub.11=5.0 wt %. The slurry was then milled using a Sweco type mill to a mean particle size of 5-7 micrometers, 90% of the diameter distribution was 18-20 micrometers and a 100% pass of less than 45 micrometers (i.e., 100% of the particles had a particle size less than 45 micrometers). The slurry was then weighed and the LOI (loss on ignition) measured at 540 C. to determine the total calcined solids content. Based on this value the Pd and Rh quantities were calculated to give the target loadings based on a final calcined washcoat loading of 2.85 g/in.sup.3 or 174 g/L. Rh nitrate was added first dropwise followed by stirring for 15 minutes and then the Pd was added dropwise as Pd nitrate with stirring. After the Pd addition, the slurry specific gravity was adjusted to a range of 1.4 to 1.6 dependent on the washcoat loading target and substrate type to be coated.

[0078] Coating was performed by dipping one end of a honeycomb ceramic monolith (commercially available flow through substrate made of cordierite, 3.66 Round6.0 Long, 400 cpsi, 6.5 mill wall thickness) into the washcoat slurry, followed by drawing the slurry up into the channels using a vacuum. The monolith was then removed from the slurry and the channels cleared by applying a vacuum to its other end. Washcoat loading was controlled by varying specific gravity, and other coating parameters such as vacuum time and the amount of slurry drawn into the honeycomb channels. After applying the washcoat, the monolith was calcined at 540 C. for 2 hours. The final WC loading on a dry calcined basis was stabilized alumina=80 g/L, BaSO.sub.4=13 g/L, OSC=80 g/L and La.sub.2O.sub.3=1.6 g/L giving a total loading 174 g/L. The washcoat layer was coated over the total length of the monolith. Two samples were built at different PGM loadings, one at a Pd loading of 50 g/ft.sup.3 and Rh=3.0 g/ft.sup.3 (CC1) and the second at Pd=14 g/ft.sup.3 and Rh=1.0 g/ft.sup.3 (CC2).

Example 1 (C1)

[0079] The PGM banded or zoned catalyst of the current invention was prepared as follows. In the current example the high Pd loaded monolith CC1 of Comparative Example 1 was used with a WC loading of 174 g/L. and a Pd loading of 50 g/ft.sup.3. The masking band was applied by dipping one end of the monolith in pure cocoa butter that was heated in a water bath to 50 C. to give a low viscosity fluid that could easily flow into the monolith channels and was injected to a length of 16 mm. Cocoa butter melts at a temperature of 38 C. The excess masking agent in the channels was removed by blowing forced air through the monolith channels from the opposite end of the monolith. The banded part was allowed to sit in air at room temperature for 12 Hrs so the masking agent cooled to room temperature and formed a solid uniform water-impervious layer over the washcoat at the inlet of the part.

[0080] The application of the Pd band or zone was carried out as follows. An aqueous solution consisting of a thickening agent in water was prepared where the thickening agent used was a commercial polysaccharide. This was added to control and limit wicking of the aqueous Pd solution when applied to give the banded zone. The thickening agent was added at 0.5 wt % based on the total weight of solution. Different surfactants can also be used to lower the surface tension of the Pd solution and minimize wicking thus improving control of the Pd band length. To this solution was added Pd tetra-amine acetate at a concentration that was determined based on the Pd loading target in the banded zone, the band/zone length and the amount of solution need to reach the end of the banded zone when injected over the masked zone assuming no solution or Pd uptake. To determine the Pd solution concentration an initial wet weight uptake for the monolith was measured using a solution of the polysaccharide in water without the Pd salt present. In the current example the masked zone length was 16 mm and the target Pd zone/band length was 49 mm. The weight of solution uptake calculated was 73.5 grams with a Pd concentration of 28.5 mg/g of solution. After application of the Pd band, the excess solution was removed by vacuuming from the injection end of the monolith. The banded/zoned part was then calcined in an up-flow forced air oven with the masking band located at the top of the monolith. The calcination temperature was 550 C. for 2 Hrs. After calcination, the part was split open longitudinally to confirm that the bands/zones had been applied as intended. The pictures shown in FIG. 3 confirm that the band/zone was applied as intended. Further, monolith slices were removed from zones A, B and C and the Pd content measured using ICP (Inductively Coupled Plasma) analysis and the data is presented in FIG. 4 as g/ft.sup.3 Pd for each zone. From the Pd analysis shown in FIG. 4 it is evident that the high Pd concentration band/zone was successfully applied.

Example 2 (C2)

[0081] In a second banding experiment the monolith CC2 of Comparative Example 1 with the lower loading of Pd (Pd=14 g/ft.sup.3) was banded as described in Example 1. In this case the target Pd zone length was 74 mm with a front masked zone of 16 mm as in Example 1. The calculated solution uptake was 112.9 grams with a Pd concentration of 28.5 mg/g. After calcination, the part was split in half longitudinally and the resultant visualization of the bands are shown in FIG. 5 confirming that the intended banding was achieved. In FIG. 6 is shown the results for Pd loading after ICP analysis of a monolith slice taken from zones A, B and C. It is evident that the Pd band was successfully applied.