HYDROCARBON TRAP 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 a first washcoat zone, which comprises a) a zeolite, b) a redox active base metal compound and c) palladium in oxidic or metallic state which is fixed to the surface of a support oxide.

Claims

1. Catalyst comprising a carrier substrate of the length L extending between substrate ends a and b and a first washcoat zone which comprises a) a zeolite b) a compound of a redox active base metal selected from the group consisting of Cu, Ni, Co, Mn, Fe, Cr, Ce, Pr, Tb, Sn and In, and c) palladium in oxidic or metallic state fixed to the surface of a support oxide.

2. Catalyst according to claim 1, wherein the zeolite belongs to the structure type code BEA, FAU, FER, MFI or MOR.

3. Catalyst according to claim 1 and/or 2, wherein the zeolite is beta zeolite.

4. Catalyst according to claim 1 wherein it comprises zeolite in an amount of 20 to 90% by weight based on the weight of the catalyst.

5. Catalyst according to claims 1, wherein the redox active base metal is iron.

6. Catalyst according to claim 5, wherein the iron is in form of an iron oxide.

7. Catalyst according to claim 6, wherein it comprises the iron oxide in an amount of 1.0 to 10.0%by weight, based on the weight of the catalyst and calculated as Fe.sub.2O.sub.3.

8. Catalyst according to claim 1, wherein it comprises palladium in an amount of 0.1 to 5% by weight, based on the weight of the catalyst and calculated as palladium metal.

9. Catalyst according to claim 1, wherein it comprises the support oxide in an amount of 1.0 to 50.0% by weight, based on the weight of the first washcoat zone.

10. Catalyst according to claim 1, wherein the support oxide is selected from the group consisting of alumina, alumina/silica mixed oxides, magnesia/alumina mixed oxides, ceria, ceria-zirconia mixed oxides and alumina-ceria mixed oxides.

11. Catalyst according to claim 1, wherein the redox active base metal compound is an oxide and present within and/or on the surface of the zeolite.

12. Catalyst according to claim 1, wherein the carrier substrate is a flow through or filter substrate.

13. Catalyst according to claim 1, wherein the carrier substrate comprises a second washcoat zone which comprises platinum, palladium and/or rhodium.

14. Catalyst according to claim 1, wherein the carrier substrate comprises a third washcoat zone which comprises a zeolite and is free of palladium.

15. Method for the adsorption of hydrocarbons contained in the exhaust gas of a combustion engine comprising passing the exhaust gas over a catalyst according to claim 1.

Description

Comparison Example 1

[0056] Slurry preparation begins with addition of a commercially available alumina stabilized silica sol to water and mixing. This material represents 4.5 wt. % of the final calcined washcoat loading. This step was followed by the addition of a commercially available boehmite and iron nitrate at contents of 1.0 and 4.5 wt, % respectively of the final calcined washcoat. Finally, a beta zeolite in the ammonium form and having a SAR value of 25 was added and the slurry aged for two days. This slurry was then coated onto a ceramic substrate have 400 cpsi/6.5 mill cell structure and 4 round by 6 long giving a total volume of 1.2 liters and a washcoat load of 3.64 g/in.sup.3 or 222 g/L. The hydrocarbon trap this obtained is subsequently called CC1.

Comparison Example 2

[0057] To CC1 according to Comparison Example 1 was added a three-way-catalyst (TWC) overcoat comprising platinum, palladium and rhodium on a commercially available oxygen storage component (OSC) carrier. The noble metals were fixed to the surface of the OSC by generating a slurry of the PGM nitrate salts and the OSC component with subsequent drying and calcination. The composition of the OSC was 70% CeO.sub.2 and 30% ZrO.sub.2. Calcination was carried out in air at 600 C. for 6 hours so the PGMs would be in either their stable metal or oxide form. The loading was Pt=0.4 wt. %, Pd=9.2 wt. % and Rh=0.2 wt. %.

[0058] The PGM loaded OSC consisted of 73% of the TWC washcoat layer. For slurry preparation and coating this PGM doped OSC was mixed with a non-stabilized theta alumina, milled and then coated onto the trap layer (CC1) giving a total TWC washcoat loading of 0.816 g/in.sup.3 or 50 g/L. The total washcoat loading for obtained catalyst CC2 was 4.45 g/in.sup.3 or 272 g/L.

EXAMPLE 1

[0059] a) The process described in Comparative Example 1 was repeated. [0060] b) For the preparation of a second trap layer containing precious metal doped oxygen storage component on top of the layer obtained in step a) a washcoat was prepared in line with Comparison Example 1 with the difference that after addition of the beta zeolite a the palladium doped OSC (The composition of the OSC was 70% CeO.sub.2 and 30% ZrO.sub.2, same as in CC2) was now added as the final step in washcoat preparation for this layer. The palladium doped OSC consisted of 21.1 wt. % of this layer on a dry calcined basis. After coating the 2.sup.nd layer the washcoat loading was 1.39 g/in.sup.3 or 85 g/L. This gave a total loading of 5.03 g/in.sup.3 or 307 g/L. The catalyst this obtained is called C1.

EXAMPLE 2

[0061] The process described in Example 1 was repeated except that only half of the palladium/OSC was included in the top or 2.sup.nd trap layer at 12.8 wt. %. The catalyst thus obtained is called C2.

EXAMPLE 3

[0062] The process described in Example 1 was repeated except that the palladium in the top layer was impregnated onto a commercially available La stabilized alumina. The catalyst thus obtained is called C3.

[0063] Evaluation of catalysts CC1, CC2, C1 and C2

[0064] a) Aging Protocol

[0065] The catalysts were aged simultaneously on a stand dyno. Engine Dynamo-meter aging was carried out using an 8.1 L V-8 engine with MPFI. Up to four exhaust systems could be aged in parallel, one per channel. The catalyst inlet bed T was maintained at 700 C. 10 C. over each trap with an exhaust gas flow 202.5 g/s over each catalyst converter. The aging was run at stoichiometry without AP: oscillation and lasted 50 hours.

[0066] b) Testing.

[0067] The testing procedure as described in Nunan et. al. SAE 2013-01-4297 was used.

[0068] c) The results are given in FIGS. 1 to 16

[0069] The total amount of hydrocarbons (HCs) going into the respective trap (trap in) and leaving the trap (trap out) as measured by FID (Flame ionization detector) are shown in FIGS. 1 and 2. The total trap in to the four traps C1, C2, CC1 and CC2 are identical but the trap out amounts are quite different. It is clearly seen that C1 and C2 have significantly reduced trap out emissions. Further, the trap out emissions for CC1 and CC2 are identical.

[0070] Thus placing the PGM/OSC as a separate layer over the HC trap has no impact on the trapping efficiency. On the other hand, when the PGM/OSC is located within the trap layer a significant reduction in trap out hydrocarbon emissions is seen.

[0071] In FIGS. 3 and 4 these features are further emphasized when the trapping efficiency over the 1.sup.st 130 C. of the temperature ramp is considered (FIG. 3) and also when the total HC trapping efficiency as mg of HC over the full test is compared (FIG. 4). Again, it is apparent that the two reference catalysts CC1 and CC2 are identical and clearly weaker than the two inventive catalysts C1 and C2.

[0072] In FIGS. 5 and 6 are shown toluene trap in (FIG. 5) and trap out (FIG. 6). All the toluene going into the trap is trapped but no toluene is exiting the trap up to 330 C.

[0073] In FIGS. 7 to 10 are compared the ethene and propene concentrations trap in and trap out of CC1, CC2, C1 and C2. It is apparent that the trapping of both these alkenes over C1 and C2 is significantly greater than CC1 and CC2.

[0074] The advantages of C1 and C2 are further illustrated in FIGS. 11 and 12. Here the mg of ethene (FIG. 11) and propene (FIG. 12) stored is compared over the full test. Again, it is dearly evident that C1 and C2 with the PGM/OSC in the trap layer store ethene and propene more effectively than CC1 and CC2.

[0075] FIGS. 13 to 16 show advantages of C3 when compared to CC1 as regards total HC and ethene trapping.