USE OF A PALLADIUM/PLATINUM/ZEOLITE-BASED CATALYST AS PASSIVE NITROGEN OXIDE ADSORBER FOR PURIFYING EXHAUST GAS

Abstract

The invention relates to the use of a catalyst as a passive nitrogen oxide adsorber, which has a carrier substrate, a zeolite, palladium, and platinum, wherein the palladium is provided in a quantity of 0.01 to 10 wt. %, based on the sum of the weights of zeolite, platinum, and palladium and calculated as a palladium metal, and platinum in a quantity of 0.1 to 10 wt. %, based on the weight of the palladium and calculated as a platinum metal. The invention also relates to the use of said catalyst in connection with a SCR catalyst in an exhaust gas system.

Claims

1. Use of a catalyst comprising a carrier substrate of length L and a coating A comprising a zeolite, palladium and platinum, wherein palladium is present in amounts of 0.01 to 10 wt. %, based on the sum of the weights of zeolite, platinum and palladium, and is calculated as palladium metal, and platinum is present in amounts of 0.1 to 10 wt. %, based on the weight of palladium, and is calculated as platinum metal, as a passive nitrogen oxide adsorber that stores nitrogen oxides in a first temperature range and releases them again in a second temperature range, wherein the second temperature range is at higher temperatures than the first temperature range.

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

9. Use according to claim 1, characterized in that it comprises a zeolite of structure type ABW, AEI, AFX, BEA, CHA, ERI, ESV, FAU, FER, KFI, LEV, LTA, MFI, SOD or STT with 0.5 to 3 wt. % of palladium, based on the sum of the weights of zeolite, platinum and palladium, and calculated as palladium metal, and 0.5 to 5 wt. % of platinum, based on the weight of palladium, and calculated as platinum metal.

10. Use according to claim 1, characterized in that the carrier substrate carries another catalytically active coating B, which Is a coating that is active in terms of catalytic oxidation and comprises platinum, palladium or platinum and palladium on a carrier material.

11. Use according to claim 1, characterized in that a zeolite of structure type ABW, AEI, AFX, BEA, CHA, ERI, ESV, FAU, FER, KFI, LEV, LTA, MFI, SOD or STT coated with 0.5 to 3 wt. % of palladium, based on the sum of the weights of zeolite, platinum and palladium, and calculated as palladium metal, and 0.5 to 5 wt. % of platinum, based on the weight of the palladium, and calculated as platinum metal, extends directly on the carrier substrate over its entire length L, and on this coating there is a coating containing platinum or platinum and palladium in a mass ratio of 2:1 to 14:1 over the entire length L.

12. Use according to claim 1, characterized in that the catalyst is a component of an exhaust gas system comprising an SCR catalyst.

13. Use according to claim 12, 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.

Description

EXAMPLE 1

[0094] a) A zeolite of type SSZ-13 (structure type CHA, SAR=14) is impregnated with 2 wt. % 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.

[0095] 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 150 g/L, based on the Pd-containing zeolites (corresponding to 162 g/L incl. binder), which corresponds to a palladium load of 85 g/ft.sup.3 Pd.

[0096] c) The catalyst obtained according to step b) is impregnated with a Pt nitrate solution, such that the amount of platinum applied corresponds to 1 wt. % of the amount of palladium applied in step b). The platinum load is thus 0.85 g/ft.sup.3 Pt. Finally, calcination takes place at 550 C.

EXAMPLE 2

[0097] Example 1 is repeated with the difference that, in step c), the amount of platinum applied is 0.1 wt. % of the amount of palladium applied in step b). The platinum load is thus 0.085 g/ft.sup.3.

COMPARATIVE EXAMPLE 1

[0098] Example 1 is repeated with the difference that step c) has been omitted.

EXAMPLE 3

[0099] Example 1 is repeated with the difference that a zeolite of structure type BEA (SAR=10) is used.

COMPARATIVE EXAMPLE 2

[0100] Example 3 is repeated with the difference that step c) has been omitted.

EXAMPLE 4

[0101] Example 1 is repeated with the difference that a zeolite of structure type AEI is used.

EXAMPLE 5

[0102] In a further step, the catalyst obtained in Example 1 is likewise coated over its entire length with a washcoat containing platinum supported on aluminum oxide, using a conventional process. The washcoat load of the second layer is 75 g/L, the platinum load is 20 g/ft.sup.3.

EXAMPLE 6

[0103] The catalyst according to Example 5 is combined with a second coated flow-through substrate to form an exhaust system. The second flow substrate is thereby exchanged with a zeolite of the chabazite structure type exchanged with 3 wt. % of copper (calculated as CuO). The washcoat load of the second flow-through substrate is 150 g/L.

[0104] Comparative Experiments

[0105] The catalysts according to Examples 1, 2 and Comparative Example 1, along with Example 3 and Comparative Example 2, are subjected to an NOx storage test with subsequent temperature-programmed desorption (TPD).

[0106] This occurs in a suitable model gas reactor using a so-called drill core with the dimensions 13 (diameterlength) and a cell size of 400 cpsi along with a wall thickness of 4.3 mil.

[0107] Two different gas compositions are used in the course of the test:

[0108] a) Lean phase without NO; and

[0109] b) Storage phase with NO.

[0110] Lean phase a) is characterized in that, at a space velocity of 50,000 1/h, the gases of oxygen are present in 8% by volume, carbon dioxide in 10% by volume, and water in 10% by volume. Storage phase b) differs from lean phase a) in that, at a space velocity of 30,000 1/h, 500 ppm of nitrogen oxide is present in addition to the first three gases. At the beginning of the measurement, the core is baked for a period of 15 minutes at a temperature of 550 C. under gas condition a), in order to start with an empty level of the catalyst, then cooled to a temperature of 100 C. At that point, gas condition b) is switched to for a duration of 40 minutes at a temperature of 100 C. At the end of this 40 min, gas condition a) is set again and the temperature is simultaneously increased at a rate of 60 K/min (temperature-programmed desorption) until a final temperature of 550 C. has been reached. This final temperature is maintained for a further 15 minutes.

[0111] The results are shown in FIGS. 1 to 2.

[0112] According to FIG. 1, the catalysts in Examples 1, 2 and Comparative Example 1 store nitrogen oxide almost identically at 100 C. (storage phase). The desorption phase shows that the catalyst of Comparative Example 1 desorbs a part of the nitrogen oxide at approximately 200 C. and a further considerable part only between 400 and 500 C. Since this temperature range is hardly reached in modern exhaust systems, this means that the catalyst in Comparative Example 1 no longer completely desorbs the stored nitrogen oxide and thus has less storage capacity available in a new cycle.

[0113] In contrast, the catalysts in Examples 1 and 2 desorb the stored nitrogen oxide at lower temperatures. More storage capacity is thus available in a subsequent cycle.

[0114] FIG. 2 shows an analogous image. The catalysts in Example 3 and Comparative Example 2 store nitrogen oxide almost identically at 100 C. (storage phase). In contrast, in the desorption phase, the catalyst in Example 3 desorbs most of the nitrogen oxide at a temperature of approximately 150 C., while the catalyst of Comparative Example 2 desorbs a considerable proportion of the stored nitrogen oxide only at approximately 400 C. In a subsequent cycle, in the case of the catalyst of Comparative Example 2, less storage capacity is available.