EXHAUST SYSTEM WITH A MODIFIED LEAN NOx TRAP

Abstract

An exhaust system for treating an exhaust gas from an internal combustion engine is disclosed. The system comprises a modified lean NO.sub.x trap (LNT), a urea injection system, and an ammonia-selective catalytic reduction (NH.sub.3-SCR) catalyst. The modified LNT comprises platinum, palladium, barium, and a ceria-containing material, and has a platinum:palladium molar ratio of at least 3:1. The modified LNT stores NO.sub.x at temperatures below about 200? C. and releases the stored NO.sub.x at temperatures above about 200? C. The urea injection system injects urea at temperatures above about 180? C.

Claims

1. An exhaust system for treating an exhaust gas from an internal combustion engine, comprising: (a) a lean NO.sub.x trap (LNT), wherein the LNT comprises platinum, palladium, barium, and a ceria-containing material and the LNT has a platinum:palladium molar ratio of at least 3:1; (b) a urea injection system; and (c) an ammonia-selective catalytic reduction (NH.sub.3-SCR) catalyst, wherein there is no intervening catalysts between the LNT and the NH.sub.3-SCR catalyst.

2. (canceled)

3. The exhaust system of claim 1 wherein the LNT has a platinum:palladium molar ratio of at least 4:1.

4. The exhaust system of claim 1 wherein the LNT has a barium loading of greater than 150 g/ft.sup.3.

5. The method of claim 1 wherein the LNT has a barium loading of greater than 400 g/ft.sup.3.

6. The exhaust system of claim 1 wherein the NH.sub.3-SCR catalyst is selected from the group consisting of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, and a metal/zeolite.

7. The exhaust system of claim 6 wherein the metal/zeolite comprises a metal selected from the group consisting of iron or copper and a zeolite selected from the group consisting of a beta zeolite, a faujasite, an L-zeolite, a ZSM zeolite, an SSZ-zeolite, a ferrierite, a mordenite, a chabazite, an offretite, an erionite, a clinoptilolite, a silicalite, an aluminum phosphate zeolite, and a mesoporous zeolite.

8. The exhaust system of claim 1 wherein the NH.sub.3-SCR catalyst is an ammonia-selective catalytic reduction filter (NH.sub.3-SCRF).

9. The exhaust system of claim 8 wherein the NH.sub.3-SCRF comprises a filter having a NH.sub.3-SCR catalyst selected from the group consisting of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, and a metal/zeolite.

10. A method for treating exhaust gas from an internal combustion engine of a vehicle, comprising: (a) passing the exhaust gas over a lean NO.sub.x trap (LNT) to remove oxides of nitrogen (NO.sub.x) from the exhaust gas at temperatures below about 200? C. and release the NO.sub.x at temperatures above about 200? C., wherein the modified LNT comprises platinum, palladium, barium, and a ceria-containing material and the LNT has a platinum:palladium molar ratio of at least 3:1; (b) injecting urea into the exhaust gas downstream of the LNT at temperatures above about 180? C.; and (c) passing an exhaust gas containing released NO.sub.x from the LNT and urea over a NH.sub.3-SCR catalyst to convert the NO.sub.x to nitrogen; wherein there is no intervening catalysts between the LNT and the NH.sub.3-SCR catalyst.

11. The method of claim 10 further comprising periodically subjecting the LNT to a temperature above about 500? C. in a rich air:fuel ratio environment to remove sulfur that has accumulated on the modified LNT.

12. The method of claim 10 wherein the LNT has a platinum:palladium molar ratio of at least 4:1.

13. The method of claim 10 wherein the LNT has a barium loading of greater than 150 g/ft.sup.3.

14. The method of claim 10 wherein the LNT has a barium loading of greater than 400 g/ft.sup.3.

15. An exhaust system for treating an exhaust gas from an internal combustion engine, comprising: a lean NOx trap (LNT), wherein the LNT comprises platinum, palladium, barium, and a ceria-containing material and the modified LNT has a platinum:palladium molar ratio of greater than 4:1; and an ammonia-selective catalytic reduction (NH.sub.3-SCR) catalyst.

16. The exhaust system of claim 15 wherein there is no intervening catalysts between the LNT and the NH.sub.3-SCR catalyst.

17. The exhaust system of claim 15, further comprising a urea injection system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0010] The invention is an exhaust system for treating an exhaust gas from an internal combustion engine. The system comprises a modified lean NO.sub.x trap (LNT). Lean NO.sub.x traps are well known in the art. Lean NO.sub.x trap are typically designed to adsorb NO.sub.x under lean exhaust conditions, release the adsorbed NO.sub.x under rich conditions, and reduce the released NO.sub.x to form N.sub.2.

[0011] LNTs typically include a NO.sub.x-storage component, an oxidation component, and a reduction component. The NO.sub.x-storage component preferably comprises alkaline earth metals (such as barium, calcium, strontium, and magnesium), alkali metals (such as potassium, sodium, lithium, and cesium), rare earth metals (such as lanthanum, yttrium, praseodymium and neodymium), or combinations thereof. These metals are typically found in the form of oxides. Typically, platinum is included to perform the oxidation function and rhodium is included to perform the reduction function. These components are contained on one or more supports.

[0012] The oxidation/reduction catalyst and the NO.sub.x-storage component are preferably loaded on a support material such as an inorganic oxide to form an LNT for use in the exhaust system.

[0013] The modified LNT of the invention is designed to have a different function than known LNTs, in that they are designed to store NO.sub.x at temperatures below about 200? C. and release the stored NO.sub.x at temperatures above about 200? C. The modified LNT comprises platinum, palladium, barium, and a ceria-containing material. The ceria-containing material is preferably ceria, ceria-zirconia, ceria-zirconia-alumina, or mixtures thereof. More preferably, the ceria-containing material is ceria.

[0014] The modified LNT has a platinum:palladium molar ratio of at least 3:1, more preferably greater than 4:1.

[0015] The modified LNT also comprises barium. The barium may be added to the modified LNT by any known means. For instance, a barium compound may be loaded onto the ceria-containing material or the support material by any known means, the manner of addition is not considered to be particularly critical. For example, a barium compound (such as barium acetate) may be added to the ceria-containing material or the support material by impregnation, adsorption, ion-exchange, incipient wetness, precipitation, or the like. Preferably, the modified LNT comprises at least 150 g/ft.sup.3 loading of barium, and in some instances the modified LNT may comprise at least 400 g/ft.sup.3 loading of barium.

[0016] Preferably, the modified LNT also comprises a support. The support is preferably an inorganic oxide, and more preferably includes oxides of Groups 2, 3, 4, 5, 13 and 14 elements. Most preferably, the support is an alumina, silica, titania, zirconia, magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide, a mixed oxide or composite oxide of any two or more thereof (e.g. silica-alumina, magnesia-alumina), and mixtures thereof. The support may also preferably contain cerium. Useful supports preferably have surface areas in the range 10 to 1500 m.sup.2/g, pore volumes in the range 0.1 to 4 mL/g, and pore diameters from about 10 to 1000 Angstroms. High surface area supports having a surface area greater than 80 m.sup.2/g are particularly preferred.

[0017] The modified LNT of the invention stores NO.sub.x at temperatures below about 200? C. and releases the stored NO.sub.x at temperatures above about 200? C.

[0018] The modified LNT is preferably coated on a substrate. The substrate is preferably a ceramic substrate or a metallic substrate. The ceramic substrate may be made of any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates and metallo aluminosilicates (such as cordierite and spodumene), or a mixture or mixed oxide of any two or more thereof. Cordierite, a magnesium aluminosilicate, and silicon carbide are particularly preferred.

[0019] The metallic substrate may be made of any suitable metal, and in particular heat-resistant metals and metal alloys such as titanium and stainless steel as well as ferritic alloys containing iron, nickel, chromium, and/or aluminum in addition to other trace metals.

[0020] The substrate may be a filter substrate or a flow-through substrate, and is most preferably a flow-through substrate, especially a honeycomb monolith. The substrate is typically designed to provide a number of channels through which vehicle exhaust passes. The surface of the channels is loaded with the three-way catalyst.

[0021] The modified LNT may be added to the substrate by any known means. For example, the modified LNT material may preferably applied and bonded to the substrate as a washcoat, a porous, high surface area layer bonded to the surface of the substrate. The washcoat is typically applied to the substrate from a water-based slurry, then dried and calcined at high temperature. Preferably, the modified LNT is added to the substrate as a single layer, although additional layers may be used.

[0022] The exhaust system of the invention also comprises an ammonia-selective catalytic reduction (NH.sub.3-SCR) catalyst. The NH.sub.3-SCR catalyst may comprise any known NH.sub.3-SCR catalysts, which are well-known in the art. A NH.sub.3-SCR catalyst is a catalyst that reduces NO.sub.x to N.sub.2 by reaction with nitrogen compounds (such as ammonia or urea).

[0023] Preferably, the NH.sub.3-SCR catalyst is comprised of a vanadia-titania catalyst, a vanadia-tungsta-titania catalyst, or a metal/zeolite. The metal/zeolite catalyst comprises a metal and a zeolite. Preferred metals include iron and copper. The zeolite is preferably a beta zeolite, a faujasite (such as an X-zeolite or a Y-zeolite, including NaY and USY), an L-zeolite, a ZSM zeolite (e.g., ZSM-5, ZSM-48), an SSZ-zeolite (e.g., SSZ-13, SSZ-41, SSZ-33), a ferrierite, a mordenite, a chabazite, an offretite, an erionite, a clinoptilolite, a silicalite, an aluminum phosphate zeolite (including metalloaluminophosphates such as SAPO-34), a mesoporous zeolite (e.g., MCM-41, MCM-49, SBA-15), or mixtures thereof; more preferably, the zeolite is a beta zeolite, a ferrierite, or a chabazite.

[0024] The NH.sub.3-SCR catalyst is preferably coated on a ceramic or a metallic substrate, as described above. The substrate is typically designed to provide a number of channels through which vehicle exhaust passes, and the surface of the channels will be preferably be coated with the NH.sub.3-SCR catalyst.

[0025] The substrate for the NH.sub.3-SCR catalyst may be a filter substrate or a flow-through substrate. Preferably, the NH.sub.3-SCR catalyst is coated onto a filter, which is known as an ammonia-selective catalytic reduction filter (NH.sub.3-SCRF). SCRFs are single-substrate devices that combine the functionality of an NH.sub.3-SCR and particulate filter. They are used to reduce NO.sub.x and particulate emissions from internal combustion engines.

[0026] The system of the invention further comprises a urea injection system. The urea injection system preferably comprises a urea injector that injects urea into the exhaust gas stream upstream of the NH.sub.3-SCR catalyst and downstream of the modified LNT. The urea injection system will preferably consist of a nozzle to produce well defined droplets of urea solution. The droplet size is preferably less than 500 microns to allow rapid evaporation and urea decomposition. The injector pressure and pump rate will be such to allow effective mixing in the exhaust gas stream.

[0027] The urea injection system will also preferably consist of a urea tank, transfer lines and possibly a heating system to avoid freezing of the urea solution.

[0028] Preferably, the urea injection system injects urea at temperatures above about 180? C.

[0029] The invention also includes a method for treating an exhaust gas from an internal combustion engine. The method comprises passing the exhaust gas over the modified LNT described above. The modified LNT removes oxides of nitrogen (NO.sub.x) from the exhaust gas at temperatures below about 200? C., and releases the NO.sub.x at temperatures above about 200? C. At temperatures above about 180? C., urea is injected into the exhaust gas downstream of the modified LNT, and the exhaust gas containing released NO.sub.x from the modified LNT and urea is passed over a NH.sub.3-SCR catalyst. The released NO.sub.x is converted to nitrogen by the reaction of ammonia (generated from urea) with NO.sub.x over the NH.sub.3-SCR catalyst. The released NO.sub.x is the NO.sub.x that is stored on the modified LNT at low temperatures and is then released at the higher temperatures, and also includes NO.sub.x that is passes over the NH.sub.3-SCR NH.sub.3-SCR catalyst without being stored.

[0030] Preferably, the modified LNT is periodically subjected to a rich desulfation step. The presence of sulfur compounds in fuel may be detrimental to the modified LNT since the oxidation of sulfur compounds leads to sulfur oxides in the exhaust gas. In the LNT, sulfur dioxide can be oxidized to sulfur trioxide over the platinum group metals and form surface sulfates on the LNT surface (e.g., barium oxide or barium carbonate reacts with sulfur trioxide to form barium sulfate). These sulfates are more stable than the nitrates and require higher temperatures (>500? C.) to desulfate.

[0031] In rich desulfation, the modified LNT is typically subjected to a temperature above about 500? C. in rich air:fuel ratio environment to accomplish sulfur removal. The desulfation is preferably performed by increasing exhaust temperatures through a post-injection of fuel. Desulfation strategies may include a single, continuous rich period, or a series of short rich air-to-fuel ratio pulses.

[0032] The following examples merely illustrate the invention. Those skilled in the art will recognize many variations that are within the spirit of the invention and scope of the claims.

Example 1: Preparation of Modified LNTs

[0033] Modified LNT 1A (Pt:Pd Ratio=5:1):

[0034] A 400 cells per square inch (cpsi) flow-through cordierite substrate monolith is coated with a NO.sub.x absorber catalyst formulation comprising a single layer comprising 1.5 g/in.sup.3 Ce/magnesium-aluminate spinel, 3 g/in.sup.3 particulate ceria, 94 g/ft.sup.3 Pt, and 19 g/ft.sup.3 Pd. The washcoat is coated on the virgin substrate monolith using the method disclosed in WO 99/47260, followed by drying for 30 minutes in a forced air drier at 100? C. and then firing at 500? C. for 2 hours.

[0035] Comparative LNT 1B: (Pt:Pd Ratio=2:1)

[0036] Comparative LNT 1B is prepared according to the procedure for Modified LNT 1A, with the exception that the NO.sub.x absorber catalyst formulation comprising a single layer contains 53.3 g/ft.sup.3 Pt, and 26.6 g/ft.sup.3 Pd.

Example 2: No.SUB.x .Storage Testing

[0037] LNT 1A (1.6 L catalyst volume) is hydrothermally aged at 800? C. for 5 hours, and is then tested over the NEDC drive cycles on a 1.6 liter engine employing low pressure exhaust gas recirculation (EGR). No rich purging is employed during the testing.

[0038] The results show that the modified LNT 1A stores about 0.5 g NO.sub.x up to about 200? C., followed by near complete thermal release of the stored NO.sub.x from 200 to 300? C., showing that the modified LNT of the invention are capable of use with a NH.sub.3-SCR system.

Example 3: Desulfation Testing

[0039] LNT 1A and Comparative LNT 1B are subjected to a rich desulfation regeneration to test the effect on CO conversions. Comparative LNT 1B (1.6 liter catalyst volume), hydrothermally aged at 800? C. for 5 hours, is tested over repeated lean NEDC drive cycles on a 1.6 liter engine employing low pressure exhaust gas recirculation (EGR). Repeated lean NEDC cycles are completed to assess the deterioration in cycle CO conversion with extended lean operation. The results are shown in Table 1. Over the course of ten NEDC cycles, the tailpipe CO emission increase from 1.3 g CO to 4.7 g CO. Following the tenth lean cycle, a series of three NEDC cycles with a 3-second rich purge on the 100 kph cruise are performed to reactivate the catalyst. Analysis shows that one 3-second rich purge is sufficient to re-activate the catalyst, as the tailpipe CO emission is reduced to 1.8 g CO. A further 10 lean only NEDC cycles are completed to follow the CO deactivation again, and on the tenth cycle the tailpipe CO emission increases to 5.6 g CO. Following the second NEDC cyle, a 500-550? C. inlet temperature desulfation is performed for 10 minutes at lambda 0.95, and a third 10-cycle NEDC testing is performed. The results show that high temperature desulfation deactivates that LNT toward CO conversion. Following the third NEDC cycle, a 10 minute lean DPF regeneration (600? C. in an oxygen-containing atmosphere) is completed. Results show that the original CO conversion is not recovered upon this regeneration.

[0040] In contrast, modified LNT 1A of the invention shows good CO conversion following rich desulfation, where comparison LNT 1B loses CO conversion following rich desulfation.

TABLE-US-00001 TABLE 1 Tailpipe CO emissions CO tailpipe emissions (g) Test Run 1.sup.st cycle 10.sup.th cycle NEDC # 1 1.3 4.7 NEDC # 2 - 1.8 5.6 after rich purge NEDC # 3 - 5.0 6.2 after high temperature desulfation NEDC # 4 - 4.4 after hot lean regeneration