PASSIVE NITROGEN OXIDE ADSORBER

Abstract

The present invention relates to a catalyst, comprising a carrier substrate of the length (L) which extends between two carrier substrate ends (a and b) and has two coating zones (A and B), wherein the coating zone (A) comprises a zeolite and palladium and, proceeding from the carrier substrate end (a), extends on a part of the length (L), the coating zone (B) comprises the same components as coating zone (A) and platinum and, proceeding from the carrier substrate end (b), extends on a part of the length (L), wherein L=L.sub.A+L.sub.B, wherein LA denotes the length of the coating zone (A) and L.sub.B denotes the length of the coating zone (B). The invention also relates to an exhaust system containing said catalyst.

Claims

1. Catalyst comprising a carrier substrate of length L, which extends between two carrier substrate ends a and b and comprises two coating zones A and B, wherein coating zone A comprises a zeolite and palladium and extends from carrier substrate end a along a part of length L, coating zone B comprises the same components as coating zone A and platinum and extends starting from carrier substrate end b along a part of length L, wherein L=L.sub.A+L.sub.B applies, wherein L.sub.A is the length of the coating zone A and L.sub.B is the length of the coating zone B.

2. Catalyst according to claim 1, characterized in that the largest channels of the zeolite are formed by 6 tetrahedrally coordinated atoms and the zeolite belongs to structure types AFG, AST, DOH, FAR, FRA, GIU, LIO, LOS, MAR, MEP, MSO, MTN, NON, RUT, SGT, SOD, SVV, TOL or UOZ.

3. Catalyst according to claim 1, characterized in that the largest channels of the zeolite are formed by 8 tetrahedrally coordinated atoms and the zeolite belongs to structure types ABW, ACO, AEI, AEN, AFN, AFT, AFV, AFX, ANA, APC, APD, ATN, ATT, ATV, AVL, AWO, AW, BCT, BIK, BRE, CAS, CDO, CHA, DDR, DFT, EAB, EDI, EEI, EPI, ERI, ESV, ETL, GIS, GOO, IFY, IHW, IRN, ITE, ITW, JBW, JNT, JOZ, JSN, JSW, KFI, LEV, -LIT, LTA, LTJ, LTN, MER, MON, MTF, MWF, NPT, NSI, OWE, PAU, PHI, RHO, RTH, RWR, SAS, SAT, SAV, SBN, SIV, THO, TSC, UEI, UFI, VNI, YUG or ZON.

4. Catalyst according to claim 1, characterized in that the largest channels of the zeolite are formed by 9 tetrahedrally coordinated atoms and the zeolite belongs to structure types -CHI, LOV, NAB, NAT, RSN, STT or VSV.

5. Catalyst according to claim 1, characterized in that the largest channels of the zeolite are formed by 10 tetrahedrally coordinated atoms and the zeolite belongs to structure types FER, MEL, MFI, MTT, MWW or SZR.

6. Catalyst according to claim 1, characterized in that the largest channels of the zeolite are formed by 12 tetrahedrally coordinated atoms and the zeolite belongs to structure types AFI, AFR, AFS, AFY, ASV, ATO, ATS, BEA, BEC, BOG, BPH, CAN, CON, CZP, DFO, EMT, EON, EZT, FAU, GME, GON, IFR, ISV, IWR, IWV, IWW, LTL, MAZ, MEI, MOR, MOZ, MSE, MTW, NPO, OFF, OSI, -RON, RWY, SAO, SBE, SBS, SBT, SFE, SFO, SOS, SSY, USI or VET.

7. Catalyst according to claim 1, characterized in that the zeolite belongs to structure types ABW, AEI, AFX, BEA, CHA, ERI, ESV, FAU, FER, KFI, LEV, LTA, MFI, SOD or STT.

8. Catalyst according to claim 1, characterized in that the palladium and the platinum are present as a cation in the zeolite structure.

9. Catalyst according to claim 1, characterized in that coating zones A and B have a weight of 0.5 to 3% by weight of palladium, based on the sum of the weights of zeolite and palladium and calculated as palladium metal, comprise coated, ionically exchanged zeolites of the structure type ABW, AEI, AFX, BEA, CHA, ERI, ESV, FAU, fer, KFI, LEV, LTA, MFI, SOD or STT, and coating zone B additionally comprises 5 to 10% by weight of platinum, based on the weight of the palladium in coating zone B and calculated as platinum metal.

10. Catalyst according to claim 1, characterized in that coating zone B contains the same components in the same amounts as coating zone A, and platinum.

11. Catalyst according to claim 1, characterized in that coating zone A contains no platinum.

12. Catalyst according to claim 1, characterized in that coating zones A and B are not identical.

13. Exhaust gas system comprising a) a catalyst which comprises a carrier substrate of length L, which extends between two carrier substrate ends a and b and comprises two coating zones A and B, wherein coating zone A comprises a zeolite and palladium and extends from carrier substrate end a along a part of length L, coating zone B comprises the same components as coating zone A and platinum and extends starting from carrier substrate end b along a part of length L, wherein L=L.sub.A+L.sub.B applies, wherein L.sub.A is the length of the coating zone A and L.sub.B is the length of the coating zone B. and b) an SCR catalyst

14. Exhaust gas system according to claim 13, characterized in that the SCR catalyst is a zeolite belonging to the scaffold type BEA, AEI, CHA, KFI, ERI, LEV, MER or DDR and is exchanged with copper, iron or copper and iron.

15. Method for purifying the exhaust gases of motor vehicles operated with lean-burn engines, characterized in that the exhaust gas is passed through an exhaust gas system according to claim 14.

Description

COMPARATIVE EXAMPLE 1

[0052] a) A zeolite of structure type FER is impregnated with 3% by weight of palladium (from commercially available palladium nitrate) (incipient wetness). The powder thus obtained is then dried in stages at 120 and 350 C. and calcined at 500 C.

[0053] b) The resulting calcined powder containing Pd is suspended in demineralized water, mixed with 8% of a commercially available binder based on boehmite and ground in a ball mill. Subsequently, according to a conventional method, a commercially available honeycomb ceramic substrate (flow-through substrate) is coated along its entire length with the washcoat thus obtained. The washcoat load is 100 g/L, based on the Pd-containing zeolites (corresponding to 108 g/L incl. binder), which corresponds to a palladium load of 85 g/ft.sup.3 Pd. Finally, calcination takes place at 550 C. The catalyst is referred to below as VK1.

COMPARATIVE EXAMPLE 2

[0054] The catalyst obtained from comparative example 1 is impregnated with a Pt-nitrate solution over the entire length L in such a way that the quantity of platinum applied corresponds to 10% by weight of the quantity of palladium applied in comparative example 1. The platinum load is thus 8.5 g/ft.sup.3 Pt. Finally, calcination takes place at 550 C. The catalyst is referred to below as VK2.

EXAMPLE 1

[0055] Comparative example 2 is repeated with the difference that the amount of platinum applied, which in this case is only 8.8% by weight of the amount of palladium applied in comparative example 1, is only impregnated over 50% of the length L from the entrance. The platinum load is thus 7.48 g/ft.sup.3. Finally, the mixture is calcined at 550 C. The catalyst is referred to below as K1.

Testing

[0056] a) The catalysts VK1, VK2 and K1 were hydrothermally aged for 16 hours at a temperature of 650 C. b) They were then subjected to a NOx storage test with a temperature-programmed desorption (TPD). This took place in a suitable model gas reactor using a so-called drill core with the dimensions 13 (diameter x length) and a cell size of 400 cpsi as well as a wall thickness of 4.3 mil. Two different gas compositions are used in the course of the test: At a space velocity of 30 000 1/h, 200 ppm nitrogen oxide, 200 ppm carbon monoxide and 50 ppm n-decane (as C10, corresponding to 500 ppm as C1) are present, as well as the gases oxygen in 12% by volume and water in 10% by volume. At the beginning of the measurement, the aforementioned gas mixture is switched to bypass for a period of 2 minutes at a temperature of 100 C. After the 2 minutes have elapsed, the aforementioned gas mixture is passed over the drill core, wherein the temperature is kept constant at 100 C. for 10 minutes, before the exhaust gas is then heated with a heating ramp of 15 C./min. Once the desired final temperature of 600 C. has been reached, this is maintained for a further 10 minutes, in order to ensure the complete emptying of the drilling core.

[0057] The results are shown in FIG. 1. This shows the NOx emissions after the catalyst. According to FIG. 1, catalysts VK1 and VK2 store nitrogen oxide almost identically at 100 C. (storage phase), whereas catalyst K1 has the highest storage capacity by far. The stored amount of nitrogen oxide is described by the area enclosed by the y axis, a line parallel to the y axis with y=200, and the measured curve. In the desorption phase, it is found that all catalysts desorb the full amount of the adsorbed nitrogen oxide after 1500 seconds at the latest.

[0058] FIG. 2 shows the repetition of the above-described experiment with catalyst K1 in different installation directions, Catalyst K1 was once introduced with the Pt-containing zone upstream in the model gas reactor (K1 in FIG. 2); at another time with the Pt-containing zone downstream (K1 inv in FIG. 2). In the upstream case (K1), the nitrogen oxide storage capacity is higher and its desorption is later (that is, at higher temperatures). In the downstream case (K1 inv), the nitrogen oxide storage capacity is lower and its desorption takes place earlier (that is, at lower temperatures).