LAYERED TWC

Abstract

The present invention relates to a three-way catalyst (TWC) for treatment of exhaust gases from internal combustion engines operated with a predominantly stoichiometric air/fuel ratio, so called spark ignited engines.

Claims

1. Catalyst for the mitigation of noxious pollutants emitted from predominately stoichiometrically combusting engines comprising a carrier substrate of the length L extending between substrate ends (a) and (b) and at least three washcoat layers A, B, and C, wherein washcoat layer A comprises Rh and a supporting oxide and extends starting from substrate end (b) over a part of the length L, and washcoat layer C comprises one or more platinum group metals, a supporting oxide and extends over part or all of the length L, and washcoat layer B comprises Pd and a supporting oxide of alumina, and extends starting from substrate end (a) over a part of the length L, while washcoat layers A and B are coated directly onto washcoat layer C, wherein L.sub.A is the length of washcoat layer A, L.sub.B is the length of washcoat layer B, and L.sub.C is the length of washcoat layer C, and wherein washcoat layer B has a total loading of not more than 100 g/L while having a Pd content of between 0.4-20 g/L and a length L.sub.B which is not more than 40% of the length L, and wherein washcoat layer B comprises a wt.-ratio of OSC-material to alumina of OSC/Al.sub.2O.sub.3 of 0.4-0.6:0.6-0.4 relative to the total of OSC and alumina.

2. Catalyst according to claim 1, wherein washcoat layer C is coated directly onto the carrier substrate.

3. Catalyst according to claim 1, wherein layer A comprises only Rh as PGM.

4. Catalyst according to claim 3, wherein layer A comprises Rh in an amount of from 0.05 g/L to 4.0 g/L.

5. Catalyst according to claim 1, wherein layer B comprises only Pd as the PGM.

6. Catalyst according to claim 5, wherein layer B comprises Pd in an amount of from 0.4 g/L to 20 g/L.

7. Catalyst according to claim 1, wherein the supporting oxide of layers B and C is selected from the group consisting of alumina, silica, magnesia, titania, zirconia, ceria, rare earths such as lanthanum, neodymium, praseodymium, yttrium, mixtures comprising at least one of these materials and mixed oxides comprising at least one of these materials.

8. (canceled)

9. Catalyst according to claim 1, wherein washcoat layer A extends over 30 to 75% of the length L of the carrier substrate, washcoat layer B extends over 7 to 30% of the length L of the carrier substrate and washcoat layer C extends over 70 to 100% of the length L of the carrier substrate.

10. Catalyst according to claim 1, wherein L>L.sub.A+L.sub.B and wherein L.sub.C is equal to L.

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

12. Catalyst system comprising a first catalyst according to claim 1 and another three-way catalyst, a gasoline particulate filter, a HC trap and/or a NOx trap.

13. Catalyst system according to claim 12, wherein substrate end (b) of the first catalyst is followed by a conventional three-way catalyst.

14. Catalyst system according to claim 12, wherein substrate end (a) of the first catalyst follows a conventional three-way catalyst.

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

16. Method according to claim 15, wherein the catalyst is arranged in close coupled position.

17. Method for treating the exhaust gas of a spark ignition engine, wherein the exhaust gas is passed over the catalyst of claim 1, and wherein the exhaust gas enters the catalyst at substrate end (a) and exits the catalyst at substrate end (b).

18. Catalyst according to claim 1, wherein layers A and B are spaced by a gap or overlap for a length of at or less than 10% of L.sub.B.

19. Catalyst according to claim 1, wherein layers A and B are directly coated onto layer C.

20. Catalyst according to claim 1, wherein the washcoat loading of layer B is 40 to 65 g/L.

21. Method according to claim 17 wherein layer B is contacted by the exhaust gas before layers A and C.

Description

FIGURES

[0034] FIG. 1: Catalyst of the invention having a high Pd-loaded low WC-loaded layer (1) followed immediately by a Rh-containing layer (2) on a PGM-containing WC layer (3) on a carrier substrate (4) having ends (a) and (b).

[0035] FIGS. 2a, b, c, and d: Displayed are 4 inventive catalyst concepts (TWC_1 to TWC_4) which have been tested.

[0036] FIG. 3 a, b and c: Shows the results in light-off and fast-light-off experiments of inventive catalysts concepts TWC_1 to TWC_4.

[0037] FIG. 4: Results of tests for performance in fast-light-off and light-off on changing WC-load in layer B.

[0038] FIG. 5: Results of tests for performance in fast-light-off and light-off on changing OSC/Al2O3-ratios in layer B.

EXAMPLES

[0039] A large number of architectures and zoning studies led to the proposed WC structure and PGM placement as shown in the below FIG. 1. In this study, all samples were built using a precision piston coater and have the same total PGM. The WCs also contain the same material type and total material content.

[0040] The only difference is the PGM placement and the configuration of the WC layers. TWC_3 is the reference experimental part with the a zoned washcoat layer. The four experimental parts TWC_1, TWC_2, TWC_3 and TWC_4 are drawn in FIG. 2. These four parts have the same total PGMs and similar washcoat loadings. Detailed specifications are shown in the table1. The total Pd loading was 3.7 g/L, the total Rh loading was 0.3 g/L, and the total washcoat loading was in the range of 140-155 g/L. The substrates utilized were of identical dimensions and cell density and consisted of ceramic substrates that were φ118.4 mm×L91 mm, 600 cell/3.5 mill cell structure.

TABLE-US-00001 TABLE 1 TWC_1 TWC_2 TWC_3 TWC_4 Layer A B C A B C A B C A B C Length mm 51 30 91 51 30 91 91 30 51 61 30 51 WC g/pc 88 40 27 88 25 27 88 40 27 88 40 27 Pd g/pc 0.1 2.4 1.2 0.1 2.4 1.2 0.1 2.4 1.2 0.1 2.4 1.2 Rh g/pc 0.3 0 0 0.3 0 0 0.3 0 0 0.3 0 0 Total WC g/pc 155 140 155 155 Total Pd g/pc 3.7 3.7 3.7 3.7 Total Rh g/pc 0.3 0.3 0.3 0.3 *All tested parts had the gap between front layer and rear layer to avoid the contamination.

[0041] After several modifications of the WC loading in the front top WC-layer B, a low WC-content around 40-65 g/L gave the FLO performance and improved LO performance which probably resulted from enhanced mass-transfer and lower thermal inertia as shown in the FIG. 4. When the OSC/Al2O3 ratio was varied in the top-front WC layer (high Pd WC) in the above architecture, an OSC/Al2O3=1/1 was found to be the best for both FLO and LO performance as shown in the FIG. 5.

[0042] Comparison testing was carried out using TWC_1 to TWC_4.

Evaluation on Engine Dyno Bench

[0043] Four parts of TWC_1, TWC_2, TWC_3 and TWC_4 were engine aged to full useful life equivalent to 100,000 miles of road aging using a specific accelerated aging cycle. The cycle consisted of repetitive stoichiometry/fuel cut/rich phases and lasts for 50 hours. The peak temperature during air injection measured one inch from the catalyst inlet face was 965° C.

[0044] After the above aging, poison aging was carried out on the same engine using a fuel that was doped with 0.1 wt % of a phosphorous compound. The doping level was such that after 50 hours of stoichiometric aging at 700° C. the catalysts was were loaded with 6.6 g of P.sub.2O.sub.5 assuming all the phosphorous was adsorbed by the catalyst.

[0045] The aged catalysts were evaluated on a stand dyno using a 6.0 L GM engine before/after poisoning aging. The catalysts were connected to the exhaust manifold using a stainless-steel pipe. The test results are shown in FIGS. 3, 4 and 5, respectively.

[0046] The FLO testing was carried out using a 21.4 g/sec exhaust gas flow. The mean lambda of the exhaust gas was 1.000 with a lambda modulation of ±0.045 at 1 Hz. Data was collected at 1 Hz. Initially the catalyst was heated by the exhaust gas to 500° C. or close to 500° C. after which it was cooled down. During cool-down the exhaust was switched to a bypass line so that it did not pass through the catalyst. When the bed temperature of the catalyst was cooled to 50° C. the exhaust was switched from the by-pass line to the on-line position, so exhaust now passed through the catalyst resulting in the catalyst temperature increasing rapidly at a rate of 1350° C./minute in the initial 20 seconds. The time needed to reach 50% HC-conversion (T.sub.50) was measured and compared for the four catalysts.

[0047] The LO testing was carried out using a 25 g/sec exhaust gas flow. The temperature was ramped from 135° C., inlet gas temperature to the catalyst sample to 500° C. at a rate of 51° C./minute. The mean lambda of the exhaust gas was 1.000 with a lambda modulation of ±0.045 at 1 Hz. Data was collected at 1 Hz. The inlet gas temperature needed to reach 50% HC-conversion (T50) was measured and compared for the four catalysts.

[0048] The results are shown in FIG. 3-a, 3-b. The catalyst having the lowest T50 number is the preferred one. FIG. 3-b shows the comparisons for the P.sub.2O.sub.5 poisoned parts. FIG. 3-a shows the comparisons after thermal aging which means before poisoning. It is observed that TWC_1 of the current invention showed the best performance as it had the lowest T50 time of FLO and the T50 temperature of LO. Even after poisoning aging it found the architecture of TWC_1 part did not have disadvantage.