GB2376903A

GB2376903A - Nitrogen oxides emission control under lean-burn conditions

Info

Publication number: GB2376903A
Application number: GB0205805A
Authority: GB
Inventors: Paul Joseph Andersen
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2001-04-05
Filing date: 2002-03-13
Publication date: 2002-12-31
Also published as: GB0205805D0

Abstract

A nitrogen oxide trap/catalyst system which provides an NOx reducing catalyst, (LNC) upstream from a lean NOx trap/catalyst (LNT) for use in a lean-burn engine exhaust system. The LNC catalyzes the reduction of NOx compounds through a reaction with hydrocarbons in the exhaust gas at temperatures of 300 {C to 600 {C. Also disclosed is a three-way catalyst (TWC) positioned between a NOx reducing catalyst (LNC) and a lean NOx trap/catalyst (LNT).

Description

NOx REDUCTION SYSTEM FOR LEAN-BURN ENGINES TECHNICAL FIELD OF THE INVENTION The present invention relates to improvements in emissions control for lean-bum vehicle engines. In particular, the present invention is directed to emission control of nitrogen oxides under lean conditions.

BACKGROUND Regulatory agencies have promulgated strict controls on the amounts of carbon monoxide, hydrocarbons and nitrogen oxides which automotive vehicles are permitted to emit. The implementation of these controls has resulted in the use of catalytic converters to reduce the amount of pollutants emitted from automobiles.

The problems associated with the control of regulated emissions from internal combustion engines are well known. For example, air pollution and acid rain seriously affect terrestrial and aquatic ecosystems. The exhaust gases from vehicle engines primarily contain carbon oxides (CO and CO2), nitrogen oxides (NOx), hydrocarbons, sulfur dioxide, and soot. At present, one of the most significant problems is removal of nitrogen oxides, NOx, which are produced during high temperature combustion. In the case of"lean-bum engines,"in which there is an excess of oxygen in the exhaust gases, the reduction of NOx to N2 is particularly difficult because reducing components in the exhaust are often completely consumed by the oxygen that is present in large excess. Catalytic conversion is also impaired in such systems due to the fact that the exhaust gases are cooled by excess air. Thus,

European Stage m emissions requirements, as well as United States legislated emissions requirements, are difficult to meet for vehicles utilizing lean-bum engines.

To improve the emissions performance achievable from lean-bum engines, it has been proposed to use an adsorbent material to adsorb the nitrogen oxides produced during lean vehicle operation and to desorb the adsorbed NOx periodically under rich conditions and convert this desorbed NOx to N2. Typically, the adsorbent is combined with a catalyst which, under lean conditions, catalyzes the oxidization of NO to N02 then further oxidizes the N02 to stored nitrate. Periodically the adsorbent is regenerated under rich (oxygen deficient) conditions, in which the stored nitrate is desorbed and catalytically reduced to N02 and N2. Such a combination, referred to as a lean NOx trap/catalyst or LNT, is disclosed in a number of references. International Patent Publication Number WO 93/12863 generally discloses the use of an LNT for controlling NOx emissions from lean bum vehicles.

During engine operation under lean conditions, the LNT oxidizes NO to N02 over a platinum catalyst according to this reference and then stores the N02 in a solid form, usually a nitrate such as Ba (N03) 2. Periodically, the engine is adjusted to run rich (i e oxygen deficient) for a short period, thereby delivering a regeneration pulse to the LNT.

A lean NOx catalyst (LNC) that selectively catalyzes the reduction of NOx by hydrocarbons has been disclosed by Bethke et al. (K. A. Bethke, M. C. Kung, B. Yang, M. Shah, D. Alt, C. Li and H. H. Kung, Metal Oxide Catalyst Lean NOx Reduction, Catalysis Today, vol. 26, pgs. 169-183 (1997) ). According to that publication, the hydrocarbon-NOx reduction reaction converted only about 30 to 50% of the NOx under conditions of interest. The catalysts disclosed in the publication are either materials containing platinum group metals or base metal materials. Another publication by Adams et al. , (K. Adams, J. Cavataio and R. Hammerle, Lean NOx Catalysis for Diesel Passenger Cars : Investigating Effects of Sulfur Dioxide and

Space Velocity, Applied Catalysis B : Environmental, vol. 10, pgs. 157-181 (1996)), also describes lean NOx catalysis as it applies to diesel engines.

Other references also generally disclose catalytic support materials and catalytic materials that when combined form an LNC. For example, the publication by Tabata et al. , (T. Tabata, M. Kokitsu and O. Okada, Study on Patent Literature of Catalysis a New NOx Removal Process, Catalysis Today, vol. 22, pgs. 147-169 (1994) ), summarizes metal and metal oxide catalytic materials in combination with support materials that function to reduce NOx compounds. Although the reference does not provide guidance as to which combinations are superior to others, the reference does provide over two hundred different catalyst/support material combinations that function as an LNC.

Another reference that discloses catalytic support materials and catalytic materials that when combined selectively catalyze the reduction of NOx in the presence of hydrocarbon (referred to herein as LNC catalysts) is the reference by Parvulescu et al. (V I. Parvulescu, P Grange, B. Delmon, Catalytic Removal of NO, Catalysis Today, vol. 46, pgs. 233-316 (1998)).

One system including an LNT with a separate catalyst upstream is disclosed in European Patent EP 971 104, but the upstream catalyst in that system is not believed to function as an LNC.

Notwithstanding the foregoing, there remains a need for an improved NOx removal system for automotive lean-bum operation emissions.

SUMMARY OF THE INVENTION The present invention pertains to a nitrogen oxide trap/catalyst system.

The present invention differs from prior nitrogen oxide trap/catalysts (LNT) systems by providing an NOx reducing catalyst (LNC) upstream from an LNT in a lean-bum engine exhaust system. The LNC directly catalyzes the reduction of NOx compounds through a reaction with hydrocarbons in the exhaust gas at temperatures typically in the range of about 300 C to 600 C Optionally, a three-way catalyst (TWC) may also be included in the system. Preferably the TWC is placed downstream of the LNC and upstream of the LNT.

DETAILED DESCRIPTION OF THE INVENTION The present invention comprises an exhaust emission control system for an engine producing an oxygen rich exhaust gas (i. e. a lean exhaust) comprising a trap/catalyst for NOx compounds (an LNT) and a NOx reduction catalyst (an LNC) positioned to contact the exhaust gas before the exhaust gas enters the LNT.

1. The Lean NOx Catalyst (LNC) The LNC comprises a catalytic material that is supported on a high surface area oxide support material. When combined, the catalytic material and the support material form a catalyst, which is effective to enhance the reduction of NOx compounds by reaction with hydrocarbons ordinarily at 300 OC to 600 C. Both natural and synthetic zeolites as well as acidic, basic or neutral zeolites may be used as a support material. Natural zeolites include faujasites, clinoptilolites, mordenites, and chabazites. Synthetic zeolites include ZSM-5, beta, Y, ultrastable-Y, mordenite, and

ferrierite, with ZSM-5 preferred. The Si02 : Ah03 ratio for these support materials is typically in the range of 2-1000, with a preferred Si02 : Ah03 ratio of 30-300. The

support material may also comprise metal oxides such as, but not limited to, zirconium oxide (zur02) and aluminum oxide (ail203).

The support material is impregnated with the catalytic material.

Impregnation may be effected by contacting the support material with a soluble salt solution, such as an acetate or a nitrate of the catalytic material, and drying the wet support material with heating to remove water, leaving the catalytic material in intimate contact with the support material. A similar effect may be produced by slurrying a powdered form of the support material in water and depositing the slurry on a monolithic substrate, drying the slurry to leave the support material in intimate contact with the substrate and then dipping, pouring or spraying a solution of the catalytic material to be coated over the coated substrate and drying that solution, with heat, as above, to leave the catalytic material in intimate contact with the support material on the substrate. Generally, these are referred to as incipient wetness techniques.

Because the LNC is positioned upstream from the LNT (and thus closer to the engine) the LNC must ordinarily function at temperatures of about 300 C to 600 C. Therefore, the preferred catalytic material for use in the LNC of the present system is one or more metals from the group consisting of base metals (such as copper, chromium, iron, zinc, nickel, manganese and cobalt) and transition metals (such as gallium, lanthanum and vanadium). Of these copper is preferred. ll. The Lean NOx Trap/Catalyst (LNT) Typically, the LNT of the present invention comprises a catalyst support material and a supported catalytic material, which when combined form a catalyst. The LNT also further comprises an adsorbent. Both natural and synthetic zeolites as well as acidic, basic or neutral zeolites may be used as catalyst support

material, as described above for the LNC. Further, the support material may also comprise metal oxides such as, but not limited to, zirconium oxide (Zr02) and aluminum oxide (Ah03).

When the exhaust flowing into the LNT is lean (i. e. high oxygen content), NOx compounds in the exhaust are catalytically oxidized to N02 and then to nitrate, which is adsorbed into the adsorbent.

The adsorbent is comprised of at least one of the following: 1) alkali metals, for example potassium, sodium, lithium and cesium; 2) alkali-earth metals, for example barium, magnesium and calcium; 3) rare-earth metals, for example lanthanum, cerium, and neodymium; and 4) oxides of any of the above. Typically barium oxide (BaO) is employed. The operating temperature of the LNT may be limited by the adsorbent selected. For example, with BaO as the adsorbent, the operating temperature should be below about 400 C.

When the air-fuel ratio of the air-fuel mixture fed into the combustion chamber is rich (i. e. oxygen deficient), the exhaust gas is also rich and in that circumstance the adsorbent releases the adsorbed nitrate typically as NOx compounds.

Such a rich air-fuel ratio is used periodically as a regeneration pulse to assist in the regeneration of the LNT adsorbent. In the oxygen deficient environment present during the regeneration pulse, NOx compounds released from the adsorbent are catalytically reduced to N2 upon contact with the catalyst in the presence of carbon monoxide and residual hydrocarbons in the exhaust gas.

Therefore, catalysts useful in the LNT must catalyze the oxidation of NO to N02 and nitrates in a lean environment and also catalyze the reduction of nitrates and NOx compounds to N2 in a rich environment (oxygen deficient). With most suitable catalysts, both oxidation and reduction reactions of NOx occur at about

250 -500 C. A suitable catalyst for the above-described reactions typically comprises a catalytic material selected from the group of platinum group metals such as platinum, palladium and rhodium or a combination thereof and a support material, as described above.

Because the LNC is positioned upstream of the LNT, loading in the LNT is reduced. This diminishes the amount of NOx compounds that require adsorption, de-sorption and reduction in the LNT and enhances the overall effectiveness of the system in reducing NOx in the exhaust gas. This permits either lengthening of the period of time between regeneration pulses supplied to the LNT, as compared to prior systems, or shortening the duration of the regeneration pulses, With a resultant increase in fuel economy.

Because the present invention requires a catalytic system which requires reducing NOx upstream from the LNT, it is characteristically different from systems in which NOx is oxidized ahead of the LNT, even though such systems may utilize similar catalysts.

While a TWC may also optionally be placed before the LNC or between the LNC and the LNT, the latter is preferred.

The distinct catalysts of the LNC and LNT and that of the optional TWC, as used in the present invention, may be located on one or more distinct catalyst substrates or carriers. Alternatively, they may be located on a single substrate in distinct catalyst zones. ill. Testing A The LNC

A LNC comprising copper (Cu) supported on ZSM-5 was prepared by impregnating an aqueous Cu (N03) 2 solution into ZSM-5 that had a SiOs/AbO ratio = 55. The impregnation was done according to the well-known incipient wetness technique.

After the solution was impregnated, the Cu/ZSM-5 was dried for 12 hours overnight at 60 C in flowing air. After drying, the Cu/ZSM-5 was fired in static air at 500 C for 2 hours. The final Cu/ZSM-5 material had a composition of about 6 weight percent Cu. The Cu/ZSM-5 material was then slurried in water, milled with a ball mill, and coated on a ceramic cylindrical monolith substrate having a cell density of 0. 75" diameter x 1"long, 400 cells/in2. The Cu/ZSM-5 loading was about 3. 56g/in.

B. The LNT A two layer coated LNT was prepared in the manner described below.

Specifically, slurry A was formed by co-milling an aqueous slurry of zirconyl acetate, alumina, and cerium hydrate in a ball mill to a particle size of approximately 5 microns Tetraamine platinum chloride and barium acetate were dissolved in water to form Solution A.

Slurry A was coated on a ceramic cylindrical monolith,. 75"diameter x 1"long, having about 400 cells/in. The coated monolith was then dried in flowing air at 60 C and fired in flowing air at 500 C. The coated monolith was then dipped in Solution A, and subsequently dried in flowing air at 60 C. The coated monolith was then fired in flowing air at 500 C to complete the first catalyst layer. The first layer as loaded on the substrate had a density of about 3.08 g/in3 and a composition by weight, of about 56. 9% A1203, 17.9% Ce02, 6.5% Zr02, 16.9% BaO, and 1.8% Pt.

Next, a preformed Ce/Nd/La stabilized ZrC was dispersed in water.

The stabilized ZrO : had a composition of about 73. 11% ZrO2, 20.00% Ce02, 5.25% Nd203, and 1.64% La203. An aqueous rhodium nitrate solution was added to the stabilized Zr02 slurry and then mixed for 12 hours to complete Slurry B. Separately, gamma alumina was added to water in a ball mill and milled to an average particle size of 5 microns to form Slurry C. Slurry B and Slurry C were blended in a 5: 1 solids ratio to form Slurry D. Cerium nitrate was dissolved in water to form Solution B.

Cesium nitrate was dissolved in water to form Solution C. Slurry D was coated as a second layer on the coated monolith. The twice-coated monolith was then dried in flowing air at about 60 C and then fired in flowing air at 500 C. The coated monolith was then dipped in Solution B and dried in flowing air at 600 C and then fired in flowing air at 500 C. The coated monolith was then dipped in Solution C then dried in flowing air at 600 C and then fired in flowing air at 500 C to complete the second layer. The second layer was loaded on the substrate at 2.42 g/in3 and had a composition of 13.7% A, 25.5% Ce02, 50.5% Zur02, 1.1% La203, 3.6% Nd203, 0.6% Rh203, and 5.0% Cs2O, thus forming the LNT.

C. Results The LNC and the LNT described above (the LNT having a platinum group metal ratio of platinum, palladium and rhodium of about: Pt: Pd: Rh = 5: 0: 1 at a total loading of 120 gift\ were tested for hydrocarbon, CO, and NOx conversion activities according to standard laboratory evaluation procedures, described below.

The LNC and the LNT were tested both separately, for comparison purposes, and in a system configuration in accordance with the present invention.

The laboratory evaluation procedure involved exposing the catalyst coated ceramic monoliths described above to a gas stream with a total flow rate of 3 standard liters/min. The gas composition was cycled between lean and rich conditions.

Three different lean/rich conditions were examined in which the duration of the lean and rich portions of the cycles were altered. The three variations were 30 seconds (s) lean/2s rich, 60s lean/5s rich, or 120s lean/5s rich. At a given temperature, 10 cycles were run and HC, CO, and NOx conversions were calculated for each of the last 3 cycles and averaged. Cycles were run for all three timing combinations at increasingly higher temperatures, namely 220 C, 350 C and 500 C as set forth in Table 3. These lean/rich gas compositions are listed below in Table 1.

LNT: The LNT was tested under conditions simulating the exhaust that would be discharged from an upstream TWC mounted on an engine designed to operate with a NOx storage and release catalyst (i. e. cycling between a lean and rich exhaust). The LNT was first heated in air at 850 for 24 hours.

After heat treatment, the sample was loaded in a laboratory reactor and tested according to the lean/rich cycled condition test procedure detailed above with a gas composition as shown in Table 1 (the composition set forth in Table 1 is referred to herein as"baseline conditions"). The baseline conditions simulate an exhaust composition typically discharged from an upstream TWC in a lean-bum engine.

Table 1

Gas Rich Lean NOx 500 ppm 500 ppm C3H6(as C1) 4000 CA (as C1) 2000 H2 2. 5% CO 7. 5% 02-12% H2O 10% 10% CO2 10% 10% S02 5ppm 5ppm

The total gas flow rate was 3 liters/minute. As described above, three different lean/rich conditions were examined in which the duration of the lean and rich portions of the cycles was altered. As described above, the three variations were (1) 30s lean/2s rich, (2) 60s lean/5s rich, and (3) 120s lean/5s rich. At a given temperature, 10 cycles were run and hydrocarbon, CO, and NOx conversions were calculated for each of the last three cycles and averaged. Cycles were run for all three timing combinations at increasingly higher temperatures. For comparative purposes, the results of the tests of the LNT tested under baseline conditions are disclosed in Table 3 and are designated as"LNT Baseline." LNC: The LNC material was tested under alternate conditions, referred to hereafter as"constant hydrocarbon"or"Constant HC."This gas composition is meant to simulate exhaust directly emitted from a lean bum engine and not treated with a TWC.

For the test, a cylindrical sample of the LNC material described above was loaded in a laboratory reactor and tested according to the lean/rich cycled gas condition test procedure also described above.

With constant hydrocarbon testing, the hydrocarbon concentration was 4000 ppm CsHa and 2000 ppm C3Hs under both lean and rich conditions. The complete gas concentration for the constant hydrocarbon conditions is set forth in Table 2 below.

Table 2

Gas Rich Lean NOx 500 ppm 500 ppm Cas 4000 4000 C3H8 2000 2000 H2 2. 5% CO 7. 5% O2 - 12% H2O 10% 10% CO2 10% 10% 502 5ppm 5ppm

The total gas flow rate was 3 liters/minute. As described above, three different lean/rich conditions were examined in which the duration of the lean and rich portions of the cycles was altered. The three variations were (1) 30s lean/2s rich, (2) 60s lean/5s rich, and (3) 120s lean/5s rich. At a given temperature, 10 cycles were run and hydrocarbon, CO, and NOx conversions were calculated for each of the last three cycles and averaged. Cycles were also run for all three timing combinations at increasingly higher test temperatures, as set forth in Table 3. For comparative purposes, the results of the tests of the LNC material (described in Table 3 as

Cu/ZSM5) tested under constant hydrocarbon conditions are designated in Table 3 as (A).

Next, the sample of the LNT composition that was tested, as previously described, under conditions to simulate the exhaust which would be discharged from an upstream TWC (i. e. baseline conditions), was re-tested under constant hydrocarbon conditions. The hydrocarbon, carbon monoxide, and nitrogen oxide conversions at each test temperature under each lean/rich cycle condition in this re-test are shown in Table 3. For comparative purposes, the results of testing the LNT under these conditions are designated in Table 3 as (B).

Combination : LNC and LNT The sample of the LNC composition that had been tested under constant hydrocarbon conditions, as described above, was loaded in the laboratory reactor. The sample of the LNT composition that had been tested under constant hydrocarbon conditions, as described above, and designated in Table 3 as" (B)" was loaded into the laboratory reactor immediately downstream of the LNC sample. This two-component system, which is exemplary of the present invention, was then tested according to the lean/rich cycled gas condition test procedure. The hydrocarbon, CO and NOx conversions at each test temperature under each lean/rich cycle condition are shown in Table 3. For comparative purposes, the results of testing this twocomponent system are designated in Table 3 as (C).

Table 3. Catalyst Efficiencies

220 oC 350 oC 500 oC Position HC CO NOx HC CO NOx HC CO NOx 30s Lean/2s Rich LNT - Baseline 0 14 34 29 42 86 64 65 68 (A) LNC - Constant HC 0 0 0 29 1 8 74 55 30 (B) LNT - Constant HC 57 32 68 76 61 84 94 69 33 (C) LNC-LNT - Constant HC 51 33 75 79 88 98 98 94 72 60s Lean/Ss Rich LNT-Baseline 0 6 29 15 21 81 51 34 62 (A) LNC-ConstantHC 0 0 0 29 5 11 72 27 31 (B) LNT - Constant HC 54 13 61 71 27 76 91 37 35 (C) LNC-LNT-Constant HC 47 14 65 76 51 97 97 94 70 120s Lean/5s Rich LNT-Baseline 0 6 22 18 26 69 49 33 51 (A) LNC - Constant HC 0 0 0 26 0 10 19 72 34 (B) LNT - Constant HC 55 12 52 70 30 63 92 36 32 (C) LNC-LNT - Constant HC 48 13 53 74 50 91 96 89 68

Table 3 discloses the hydrocarbon, carbon monoxide and nitrogen oxide percent conversions at each temperature under each lean/rich cycle condition described in the Table.

The data in Table 3 indicates that (C), the two component system of a LNC and LNT combined, had higher NOx conversion compared to the hypothetical combination of (A) and (B). The improvement is particularly large for cycling conditions in which the regeneration frequency is reduced. For example, at 3500 C and a regeneration frequency of 120s lean/5s rich, the NOx conversion of (A), the LNC at constant hydrocarbon, was 10%. The NOx conversion of (B), the LNT at constant hydrocarbon, was 63%. However, the NOx conversion of (C), the LNCLNT system at constant hydrocarbon, was 91%-converting about 24% more NOx than (A) and (B) hypothetically combined in series.

While this invention has been disclosed with respect to specific embodiments thereof, it is not limited thereto. The subjoined claims are intended to be construed to encompass the present invention in its full spirit and scope including such other variants and modifications as may be made by those skilled in the art without departing from that true spirit and scope thereof

Claims

What is Claimed is:

1. An exhaust emission control system for an engine producing an oxygen rich exhaust gas under ordinary operating conditions and a reduced oxygen containing exhaust gas under periodic oxygen lean conditions, said oxygen rich exhaust gas and said reduced oxygen containing exhaust gas further including unburned hydrocarbons and NOx compounds, said system comprising: a first catalyst, said first catalyst comprising a catalyst support material and a catalytic material; wherein said first catalyst is a lean NOx catalyst adapted to reduce NOx by reaction with hydrocarbons when said system operates with said first catalyst at a temperature of 300 C to 600 C ; a second catalyst positioned to contact said exhaust gas after said first catalyst, said second catalyst comprising a catalyst support material, a catalytic material and an NOx adsorbent in intimate contact with said support material and said catalytic material; wherein under ordinary operating conditions said second catalyst is adapted to oxidize NOx compounds and said adsorbent is adapted to adsorb oxidized NOx compounds as nitrates; and wherein under periodic oxygen lean operating conditions said adsorbent is adapted to desorb said adsorbed nitrates as NOx compounds and said second catalyst is adapted to reduce said desorbed NOx compounds by reaction with said unburned hydrocarbons.

2. The exhaust emission control system as set forth in claim 1, further comprising a three-way catalyst.

3. The exhaust emission control system as set forth in claim 2, wherein said three-way catalyst is positioned between said first and second catalysts.

4. The exhaust emission control system as set forth in claim 1, wherein said catalytic material of said second catalyst comprises at least one metal selected from the group consisting of platinum, palladium and rhodium.

5. The exhaust emission control system as set forth in claim 4, wherein said catalytic material of said second catalyst comprises platinum and rhodium in a weight ratio of 5: 1.

6. The exhaust emission control system as set forth in claim 4, wherein said system is adapted to operate with said second catalyst at a temperature of 250 C to 500 C.

7. The exhaust emission control system as set forth in claim 1, wherein said catalyst support material of one or both of said first and second catalysts comprises a catalyst support material selected from the group consisting of natural zeolites, synthetic zeolites, zinc oxide and aluminum oxide.

8. The exhaust emission control system as set forth in claim 1, wherein said catalyst support material of one or both of said first and second catalysts comprises at least one natural zeolite selected from the group consisting offaujasites, clinoptilolites, mordenites, and chabazites.

9. The exhaust emission control system as set forth in claim 1, wherein said catalyst support material of one or both of said first and second catalysts comprises at least one synthetic zeolite selected from the group consisting ofZSM-5, beta, Y, ultrastable-Y, mordenite, and ferrierite.

10. The exhaust emission control system as set forth in claim 1 wherein said NOx adsorbent comprises at least one metal or metal oxide selected from the group consisting of potassium, sodium, lithium, cesium, barium, magnesium, calcium, lanthanum, cerium, neodymium, potassium oxide, sodium oxide, lithium oxide, cesium oxide, barium oxide, magnesium oxide, calcium oxide, lanthanum oxide, cerium oxide, and neodymium oxide.

11. The exhaust emission control system as set forth in claim 10 wherein said NOx adsorbent comprises barium oxide.

12. The exhaust emission control system as set forth in claim 1, wherein said catalytic material of said first catalyst comprises at least one metal selected from the group consisting of copper, chromium, iron, zinc, nickel, manganese, cobalt, gallium, lanthanum and vanadium.

13. The exhaust emission control system as set forth in claim 12, wherein said catalytic material of said first catalyst comprises copper.

14. The exhaust emission control system as set forth in claim 13,

wherein said system is adapted to operate with said second catalyst at a temperature of 250 C to 500 C.

15. The exhaust emission control system as set forth in claim 14, wherein said catalytic material of said second catalyst comprises at least one metal selected from the group consisting of platinum, palladium and rhodium.

16. The exhaust emission control system as set forth in claim 15, wherein said NOx adsorbent comprises barium oxide.

17. The exhaust emission control system as set forth in claim 16, wherein said catalyst support material of both of said first and second catalysts comprises aluminum oxide.

18. The exhaust emission control system as set forth in claim 16, further comprising a three-way catalyst.

19. The exhaust emission control system as set forth in claim 18, wherein said three-way catalyst is positioned between said first and second catalysts.

20. The exhaust emission control system as set forth in claim 1, wherein said first catalytic material and said second catalytic material are coated on a single catalyst support material with distinct zones.

21. The exhaust emission control system as set forth in claim 1, wherein said first catalytic material and said second catalytic material are coated on separate catalyst support materials.

22. A method for controlling the exhaust emission for an engine producing an oxygen rich exhaust gas under ordinary operating conditions and a reduced oxygen containing exhaust gas under periodic oxygen lean conditions, said oxygen rich exhaust gas and said reduced oxygen containing exhaust gas further including unburned hydrocarbons and NOx compounds, said method comprising: A. under ordinary operating conditions: first contacting said exhaust gas with a lean NOx catalyst and thereby reducing NOx compounds by reaction with hydrocarbons; and then contacting said exhaust gas with (1) a second catalyst adapted to oxidize NOx compounds by reaction with oxygen and (2) an adsorbent adapted to adsorb oxidized NOx compounds as nitrates and thereby adsorbing said oxidized NOx compounds as nitrates; and B under periodic oxygen lean conditions first contacting said exhaust gas with said lean NOx catalyst and thereby reducing NOx compounds by reaction with hydrocarbons; and then contacting said exhaust gas with said second catalyst and said adsorbent and thereby desorbing said adsorbed nitrates.

23. The method as set forth in claim 22, further comprising the step of oxidizing a portion of said hydrocarbons by contacting said exhaust gases with a three-way catalyst after said exhaust gases have contacted said first catalyst and before said exhaust gases have contacted said second catalyst.

24. The method of claim 23, wherein said second catalyst comprises at least one metal selected from the group consisting of platinum, palladium and rhodium.

25. The method of claim 24, wherein said first catalyst comprises copper.

26. The method of claim 25, wherein said adsorbent comprises barium oxide.