Exhaust purification device of internal combustion engine

Abstract

An SO.sub.X trap catalyst 12 and NO.sub.X purification catalyst 13 are arranged in an engine exhaust passage. A substrate 50 of the NO.sub.X purification catalyst 13 is formed with a coat layer comprised of at least the two layers of an upper coat layer 51 and a lower coat layer 52. The lower coat layer 52 is formed from an NO.sub.X storage catalyst storing the NO.sub.X contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releasing the stored NO.sub.X when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich. The upper coat layer 51 is formed from a material of a weaker basicity than this NO.sub.X storage catalyst.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an exhaust purification device of an internal combustion engine.

[0003] 2. Description of the Related Art

[0004] Known in the art is an internal combustion engine arranging in an engine exhaust passage an NO.sub.X storage catalyst storing NO.sub.X contained in exhaust gas when the air-fuel ratio of the inflowing exhaust gas is lean and releasing the stored NO.sub.X when the air-fuel ratio of the inflowing exhaust gas becomes a stoichiometric air-fuel ratio or rich. In this internal combustion engine, NO.sub.X formed when burning fuel under a lean air-fuel ratio is stored in the NO.sub.X storage catalyst. On the other hand, as the NO.sub.X storage catalyst approaches saturation of the NO.sub.X storage ability, the air-fuel ratio of the exhaust gas is temporarily made rich, whereby NO.sub.X is released from the NO.sub.X storage catalyst and reduced.

[0005] However, fuel and lubrication oil contain sulfur. Therefore, the exhaust gas also contains SO.sub.X. This SO.sub.X is stored together with the NO.sub.X in the NO.sub.X storage catalyst. This SO.sub.X is not released from the NO.sub.X storage catalyst by just making the air-fuel ratio of the exhaust gas rich. Therefore, the amount of SO.sub.X stored in the NO.sub.X storage catalyst gradually increases. As a result, the storable NO.sub.X amount ends up gradually decreasing.

[0006] Therefore, to inhibit SO.sub.X from being sent into the NO.sub.X storage catalyst, there is known an internal combustion engine arranging an SO.sub.X trap catalyst in the engine exhaust passage upstream of the NO.sub.X storage catalyst (see Japanese Patent Publication (A) No. 2005-133610). In this internal combustion engine, the SO.sub.X contained in the exhaust gas is trapped by the SO.sub.X trap catalyst, therefore the flow of SO.sub.X into the NO.sub.X storage catalyst is inhibited. As a result, it is possible to prevent the storage of SO.sub.X from causing the storage ability of the NO.sub.X to drop.

[0007] Note that when using this SO.sub.X trap catalyst, if the SO.sub.X trap catalyst falls in SO.sub.X trap ability, SO.sub.X ends up flowing into the NO.sub.X storage catalyst. In this SO.sub.X trap catalyst, however, if raising the SO.sub.X trap catalyst in temperature and making the exhaust gas flowing into the SO.sub.X trap catalyst a rich air-fuel ratio, it is possible to make the SO.sub.X trap catalyst release the absorbed SO.sub.X, therefore it is possible to regenerate the SO.sub.X trap catalyst. However, if making the SO.sub.X trap catalyst release SO.sub.X in this way, since the NO.sub.X storage catalyst has a strong basicity, even if the air-fuel ratio of the exhaust gas is rich, the problem arises that the released SO.sub.X ends up being stored in the NO.sub.X storage catalyst.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide an exhaust purification device of an internal combustion engine capable of suppressing the SOx released from an SOx trap catalyst to be stored in an NOx storage catalyst.

[0009] According to the present invention, there is provided an exhaust purification device of an internal combustion engine arranging an SO.sub.X trap catalyst able to trap SO.sub.X contained in exhaust gas in an engine exhaust passage and arranging NO.sub.X purification catalyst having a function of storing and releasing NO.sub.X contained in exhaust gas in the exhaust passage downstream of the SO.sub.X trap catalyst, wherein a coat layer comprised of at least two layers of an upper coat layer and a lower coat layer is formed on a substrate of the NO.sub.X purification catalyst, the lower coat layer is formed from an NO.sub.X storage catalyst storing the NO.sub.X contained in exhaust gas when the air-fuel ratio of the exhaust gas is lean and releasing the stored NO.sub.X when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich, and the upper coat layer is formed from a material weaker in basicity than said NO.sub.X storage catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] These and other objects and features of the present invention will become more apparent from the following description of the preferred embodiments given with reference to the attached drawings, in which:

[0011] FIG. 1 is an overview of a compression ignition type internal combustion engine;

[0012] FIG. 2 is a cross-sectional view of the surface part of a substrate of an NO.sub.X purification catalyst;

[0013] FIG. 3 is a cross-sectional view of the surface part of a substrate of an SO.sub.X trap catalyst;

[0014] FIG. 4 is a flow chart for an exhaust purification processing; and

[0015] FIGS. 5A and 5B are views showing maps of a stored NO.sub.X amount NOXA etc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] FIG. 1 is an overview of a compression ignition type internal combustion engine.

[0017] Referring to FIG. 1, 1 indicates an engine body, 2 a combustion chamber of each cylinder, 3 an electronically controlled fuel injector injecting fuel into each combustion chamber 2, 4 an intake manifold, and 5 an exhaust manifold. The intake manifold 4 is connected through an intake duct 6 to a compressor 7a of an exhaust turbocharger 7, while an inlet of the compressor 7a is connected through an intake air detector 8 to an air cleaner 9. Inside the intake duct 6, a throttle valve 10 driven by the step motor is arranged. Further, around the intake duct 6, a cooling device 11 for cooling the intake air flowing through the intake duct 6 is arranged. In the embodiment shown in FIG. 1, the engine cooling water is led into the cooling device 11 where the engine cooling water is used to cool the intake air.

[0018] On the other hand, the exhaust manifold 5 is connected to an inlet of an exhaust turbine 7b of the exhaust turbocharger 7. The outlet of the exhaust turbine 7b is connected to an inlet of an SO.sub.X trap catalyst 12 able to trap SO.sub.X contained in the exhaust gas. Further, the outlet of the SO.sub.X trap catalyst 12 is connected to an NO.sub.X purification catalyst 13 having a function of storing and releasing NO.sub.X contained in the exhaust gas. On the other hand, inside the exhaust manifold 5, a reducing agent feed valve 14 for feeding reducing agent comprised of for example a hydrocarbon into the exhaust gas flowing through the exhaust manifold 5 is attached.

[0019] The exhaust manifold 5 and intake manifold 4 are connected to each other through an exhaust gas recirculation (hereinafter referred to as "EGR") passage 15. Inside the EGR passage 15, an electronic control type EGR control valve 16 is arranged. Further, around the EGR passage 15, a cooling device 17 for cooling the EGR gas flowing through the EGR passage 15 is arranged. In the embodiment shown in FIG. 1, engine cooling water is led to the cooling device 17 where the engine cooling water cools the EGR gas. On the other hand, each fuel injector 3 is connected through a fuel tube 18 to a common rail 19. This common rail 19 is fed with fuel from an electronically controlled variable discharge fuel pump 20. The fuel fed into the common rail 19 is fed through each fuel tube 18 into the fuel injector 3.

[0020] The electronic control unit 30 is comprised of a digital computer and is provided with a ROM (read only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 34, input port 35, and output port 36 which are interconnected to each other by a bi-directional bus 31. As shown in FIG. 1, the output signal of the intake air detector 8 is input through a corresponding AD converter 37 to the input port 35. Further, the accelerator pedal 40 is connected to a load sensor 41 generating an output voltage proportional to the amount of depression L of an accelerator pedal 40. The output voltage of the load sensor 41 is input through the corresponding AD converter 37 to the input port 35. Further, the input port 35 is connected to a crank angle sensor 42 generating an output pulse each time the crankshaft rotates by for example 15.degree.. On the other hand, the output port 36 is connected through corresponding drive circuits 38 to the fuel injector 3, the step motor for driving the throttle valve 10, the reducing agent feed valve 14, the EGR control valve 16, and the fuel pump 20.

[0021] First, the NO.sub.X purification catalyst 13 shown in FIG. 1 will be explained. The substrate of this NO.sub.X purification catalyst 13 is for example formed from cordierite. FIG. 2 illustrates the cross-section of the surface part of this substrate 50. As shown in FIG. 2, the substrate 50 is formed with a coat layer comprised of at least the two layers of an upper coat layer 51 and lower coat layer 52.

[0022] The lower coat layer 52 is comprised of a layer of an NO.sub.X absorbent 53 formed on the surface of the substrate 50 and a precious metal catalyst 54 carried and diffused on the layer of this NO.sub.X absorbent 53. In this embodiment of the present invention, platinum Pt is used as this precious metal catalyst 54. As the ingredient forming the NO.sub.X absorbent 53, for example, at least one element selected from potassium K, sodium Na, cesium Cs, and other such alkali metals, barium Ba, calcium Ca, and other such alkali earths, lanthanum La, yttrium Y, and other rare earths is used. Note that in FIG. 2, platinum Pt 54 is drawn greatly exaggerated. The actual dimensions are considerably small.

[0023] Now, if the ratio of the air and fuel (hydrocarbons) fed into the engine intake passage, combustion chamber 2, and exhaust passage upstream of the NO.sub.X purification catalyst 13 is called the "air-fuel ratio of the exhaust gas", an NO.sub.X absorption and release action such that the NO.sub.X absorbent 53 absorbs the NO.sub.X when the air-fuel ratio of the exhaust gas is lean and releases the absorbed NO.sub.X when the oxygen concentration in the exhaust gas falls is performed.

[0024] That is, explaining this taking as an example the case of using barium Ba as the ingredient forming the NO.sub.X absorbent 53, when the air-fuel ratio of the exhaust gas is lean, that is, the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas diffuses in the upper coat layer 51 as shown in FIG. 2, reaches the deep part of the upper coat layer 51, then is oxidized on the platinum Pt 54 to become NO.sub.2, next is absorbed in the NO.sub.X absorbent 53 and bonds with the barium carbonate BaCO.sub.3 to diffuse in the form of nitrate ions NO.sub.3.sup.- into the NO.sub.X absorbent 53. In this way, NO.sub.X is absorbed in the NO.sub.X absorbent 53. So long as the oxygen concentration in the exhaust gas is high, NO.sub.2 is formed on the platinum Pt 54. So long as the NO.sub.X absorbent 53 is not saturated in NO.sub.X absorption ability, NO.sub.2 is absorbed in the NO.sub.X absorbent 53 and nitrate ions NO.sub.3.sup.- are formed.

[0025] As opposed to this, for example if the reducing agent feed valve 14 feeds the reducing agent to make the exhaust gas a rich air-fuel ratio or stoichiometric air-fuel ratio, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the reverse direction (NO.sub.3.sup.-.fwdarw.NO.sub.2), therefore the nitrate ions NO.sub.3.sup.- in the NO.sub.X absorbent 53 are released in the form of NO.sub.2 from the NO.sub.X absorbent 53. Next, the released NO.sub.X diffuses in the upper coat layer 51, then is reduced by the unburned HC and CO contained in the exhaust gas. In this way, the lower coat layer 52 is formed from an NO.sub.X storage catalyst storing the NO.sub.X contained in the exhaust gas when the air-fuel ratio of the exhaust gas is lean and releases the stored NO.sub.X when the exhaust gas is a stoichiometric air-fuel ratio or rich.

[0026] However, as explained above, when the air-fuel ratio of the exhaust gas is lean, that is, when burning the fuel under a lean air-fuel ratio, the NO.sub.X in the exhaust gas is absorbed in the NO.sub.X absorbent 53. However, when the fuel continues to be burned under a lean air-fuel ratio, the NO.sub.X absorbent 53 eventually ends up becoming saturated in NO.sub.X absorption ability, therefore the NO.sub.X absorbent 53 ends up becoming unable to absorb the NO.sub.X. Therefore, in this embodiment of the present invention, before the NO.sub.X absorbent 53 becomes saturated in absorption ability, the reducing agent feed valve 14 feeds the reducing agent to make the exhaust gas temporarily rich in air-fuel ratio and thereby make the NO.sub.X absorbent 53 release the NO.sub.X.

[0027] On the other hand, the exhaust gas contains SO.sub.X, that is, SO.sub.2. If this SO.sub.2 flows into the lower coat layer 52, this SO.sub.2 is oxidized on the platinum Pt 54 and becomes SO.sub.3. Next, this SO.sub.3 is absorbed in the NO.sub.X absorbent 53, bonds with the barium carbonate BaCO.sub.3, is diffused in the form of sulfate ions SO.sub.4.sup.2- in the NO.sub.X absorbent 53, and forms stable sulfate BaSO.sub.4. However, the NO.sub.X absorbent 53 has a strong basicity, so this sulfate BaSO.sub.4 is stable and hard to break down. If just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO.sub.4 remains as it is without breaking down. Therefore, in the NO.sub.X absorbent 53, the sulfate BaSO.sub.4 increases along with the elapse of time, therefore the NO.sub.X amount which the NO.sub.X absorbent 53 can absorb falls along with the elapse of time.

[0028] Therefore, in an embodiment of the present invention, an SO.sub.X trap catalyst 12 is arranged upstream of the NO.sub.X purification catalyst 13 to trap the SO.sub.X contained in the exhaust gas by this SO.sub.X trap catalyst 12 and thereby prevent SO.sub.X from flowing into the NO.sub.X absorbent 53. Next this SO.sub.X trap catalyst 12 will be explained.

[0029] This SO.sub.X trap catalyst 12 is for example comprised of a honeycomb structure monolithic catalyst. FIG. 3 illustrates the cross-section of the surface part of the substrate 55 of the SO.sub.X trap catalyst 12. As shown in FIG. 3, a coat layer 56 is formed on the surface of the substrate 55. A precious metal catalyst 57 is carried and diffused on the surface of this coat layer 56.

[0030] In this embodiment of the present invention, platinum is used as the precious metal catalyst 57. As the part forming the coat layer 56, for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Cs, or other alkali earth, lanthanum La, yttrium Y, or other rare earth may be used. That is, the coat layer 56 of the SO.sub.X trap catalyst 12 exhibits a strong basicity.

[0031] Now, SO.sub.X contained in the exhaust gas, that is, SO.sub.2, is, as shown in FIG. 3, oxidized on the platinum Pt 57, then is trapped by the coat layer 56. That is, SO.sub.2 diffuses in the form of sulfate ions SO.sub.4.sup.2- into the coat layer 56 and forms a sulfate. Note that as explained above, the coat layer 56 exhibits a strong basicity. Therefore, as shown in FIG. 3, part of the SO.sub.2 contained in the exhaust gas is directly trapped in the coat layer 56.

[0032] In FIG. 3, the shading in the coat layer 56 shows the concentration of the trapped SO.sub.X. As will be understood from FIG. 3, the SO.sub.X concentration in the coat layer 56 is highest near the surface of the coat layer 56. The further in, the lower it becomes. If the SO.sub.X concentration near the surface of the coat layer 56 increases, the surface of the coat layer 56 weakens in basicity and the SO.sub.X trap ability weakens. That is, the SO.sub.X trap rate drops. In this case, at this SO.sub.X trap catalyst 12, if raising the SO.sub.X trap catalyst 12 in temperature under a lean air-fuel ratio of the exhaust gas, the SO.sub.X trap rate can be restored.

[0033] That is, if making the SO.sub.X trap catalyst 12 rise in temperature under a lean air-fuel ratio of the exhaust gas, the SO.sub.X present concentrated near the surface of the coat layer 56 diffuses toward the deep part of the coat layer 56 so that the SO.sub.X concentration in the coat layer 56 becomes uniform. That is, the sulfate produced in the coat layer 56 changes from an unstable state where it concentrates near the surface of the coat layer 56 to the stable state where it diffuses uniformly in the coat layer 56 as a whole. If the SO.sub.X present near the surface of the coat layer 56 diffuses toward the deep part of the coat layer 56, the SO.sub.X concentration near the surface of the coat layer 56 falls, therefore if the SO.sub.X trap catalyst 12 is raised in temperature, the SO.sub.X trap rate is restored.

[0034] On the other hand, if the SO.sub.X trap catalyst 12 is further increased in SO.sub.X trap amount, even if the SO.sub.X trap catalyst 12 is raised in temperature, the SO.sub.X trap rate will no longer be restored. However, this SO.sub.X trap catalyst 12 has the property of releasing the trapped SO.sub.X in the form of SO.sub.2 if making the exhaust gas flowing into the SO.sub.X trap catalyst 12 rich in the state of raising the SO.sub.X trap catalyst 12 in temperature to about 600.degree. C. or more.

[0035] Therefore, in an embodiment in the present invention, when SO.sub.X should be released from the SO.sub.X trap catalyst 12, the reducing agent is fed from the reducing agent feed valve 14 to make the SO.sub.X trap catalyst 12 rise in temperature to about 600.degree. C. or more and make the exhaust gas flowing into the SO.sub.X trap catalyst 12 a rich air-fuel ratio to thereby restore the SO.sub.X trap rate of the SO.sub.X trap catalyst 12.

[0036] Note that by making the exhaust gas a rich air-fuel ratio in this way, if the SO.sub.X trap catalyst 12 releases SO.sub.X in the form of SO.sub.2, this SO.sub.2 flows into the NO.sub.X purification catalyst 13. On the other hand, if the exhaust gas is made a rich air-fuel ratio in this way, the exhaust gas flowing into the NO.sub.X purification catalyst 13 will not contain almost any oxygen O.sub.2. In this case, in the same way as the case of the SO.sub.X trap catalyst 12 shown in FIG. 3, SO.sub.2 is oxidized in the NO.sub.X purification catalyst 13 as well, so is trapped in the NO.sub.X purification catalyst 13. Therefore, as explained above, when the exhaust gas has almost no oxygen O.sub.2, SO.sub.2 would appear not to be trapped in the NO.sub.X purification catalyst 13.

[0037] However, even if the exhaust gas contains almost no oxygen O.sub.2, when the surface of the NO.sub.X purification catalyst 13 or the vicinity of the surface is strong in basicity, the SO.sub.2 in the exhaust gas ends up being trapped in the NO.sub.X purification catalyst 13. That is, as explained above, the NO.sub.X storage catalyst forming the lower coat layer 52 is strong in basicity, so when forming only a layer of this NO.sub.X storage catalyst on the substrate 50, even when the exhaust gas contains almost no oxygen O.sub.2, the NO.sub.X storage catalyst traps SO.sub.X. As a result, the NO.sub.X storage catalyst falls in NO.sub.X storage ability.

[0038] Therefore, in the present invention, as shown in FIG. 2, the upper coat layer 51 comprised of a material with a weaker basicity than the NO.sub.X storage catalyst is formed on the lower coat layer 52 comprised of the NO.sub.X storage catalyst. In this way, if forming the upper coat layer 51 from a material weaker in basicity from the NO.sub.X storage catalyst, when the exhaust gas is made a rich air-fuel ratio to make the SO.sub.X trap catalyst 12 release SO.sub.X, the SO.sub.2 becomes hard to trap in the upper coat layer 51. As a result, it is possible to keep SO.sub.X from being stored in the NO.sub.X storage catalyst forming the lower coat layer 52.

[0039] Note that when the SO.sub.2 rides the exhaust gas and diffuses inside the upper coat layer 51 toward the lower coat layer 52, if holding this SO.sub.2 in the upper coat layer 51, it is possible to inhibit SO.sub.2 being trapped by the NO.sub.X storage catalyst. Therefore, the upper coat layer 51 is preferably formed from a material able to hold SO.sub.2. In this case, as the material of the upper coat layer 51, it is possible to use various types of zeolite or alumina able to adsorb SO.sub.2.

[0040] Further, if forming the upper coat layer 51 from an acid material with less ability to attract SO.sub.2 compared with a material weak in basicity, the trapping action of SO.sub.2 on the upper coat layer 51 is further weakened. Therefore, the upper coat layer 51 is preferably formed from an acid material. As this acid material, various types of zeolite, alumina, titanium composite oxides, and tungsten composite oxides may be used.

[0041] Further, if the upper coat layer 51 has a function of oxidizing the SO.sub.2, the SO.sub.2 in the exhaust gas ends up being trapped in the upper coat layer 51. Therefore, as the upper coat layer 51, it is preferable to use a material not having an oxidizing function, that is, a material not carrying a precious metal catalyst. Note that if considering the above various conditions, it is possible to form the upper coat layer 51 from Fe-zeolite, titania-vanadium, or another NO.sub.X selective reducing catalyst able to selectively reduce NO.sub.X in the presence of for example ammonia.

[0042] Further, the faster the exhaust gas flowing through the SO.sub.X trap catalyst 12 in spatial velocity, the harder it is for the SO.sub.2 to diffuse in the upper coat layer 51. Therefore, in this embodiment of the present invention, when SO.sub.X should be released from the SO.sub.X trap catalyst 12, the exhaust gas flowing into the SO.sub.X trap catalyst 12 is made a rich air-fuel ratio when the exhaust gas flowing through the SO.sub.X trap catalyst 12 has more than a predetermined spatial velocity.

[0043] Next, referring to FIG. 4, an example of the exhaust purification processing method will be explained. Note that the routine shown in FIG. 4 is executed by interruption every predetermined time.

[0044] Referring to FIG. 4, first, at step 60, the NO.sub.X amount NOXA stored in the lower coat layer 52, that is, in the NO.sub.X storage catalyst, per unit time is calculated. This NO.sub.X amount NOXA is stored as a function of the required torque TQ and engine speed N in the form of the map shown in FIG. 5A in advance in the ROM 32.

[0045] Next, at step 61, this NOXA is added to the NO.sub.X amount .SIGMA.NOX stored in the NO.sub.X storage catalyst. Next, at step 62, it is judged if the stored NO.sub.X amount .SIGMA.NOX exceeds the allowable value NX. When .SIGMA.NOX>NX, the routine proceeds to step 63 where a rich processing of changing the air-fuel ratio of the exhaust gas from lean to rich by feeding the reducing agent from the reducing agent feed value 14 is performed and .SIGMA.NOX is cleared. At this time, NO.sub.X is released from the NO.sub.X storage catalyst.

[0046] Next, at step 64, the SO.sub.X amount exhausted from the engine per unit time, that is, the SO.sub.X amount SOXA trapped in the SO.sub.X trap catalyst 12 per unit time, is calculated. This SO.sub.X amount SOXA is stored as a function of the required torque TQ and engine speed N in the form of a map as shown in FIG. 5B in advance in the ROM 32. Next, at step 65, this SOXA is added to the SO.sub.X amount .SIGMA.SOX trapped in the SO.sub.X trap catalyst 12. Next, at step 66, it is judged if the trapped SO.sub.X amount .SIGMA.SOX exceeds an allowable value SX.

[0047] When .SIGMA.SOX>SX, the routine proceeds to step 67 where it is judged if the conditions for release of SO.sub.X stand or not, for example, if the exhaust gas flowing through the SO.sub.X trap catalyst 12 has more than a predetermined spatial velocity, that is, if the amount of intake air is a set value or more. When the amount of intake air is a set value or more, the routine proceeds to step 68 where a temperature raising control is performed. Namely, the exhaust gas is maintained at a lean air-fuel ratio and, reducing agent is fed from the reducing agent feed valve 14 so as to make the SO.sub.X trap catalyst 12 rise in temperature to the SO.sub.X release temperature. Next, at step 69, a rich processing of maintaining the exhaust gas flowing into the SO.sub.X trap catalyst 12 at a rich air-fuel ratio by the reducing agent fed from the reducing agent feed valve 14 is performed and .SIGMA.SOX is cleared. At this time, SO.sub.X is released from the SO.sub.X trap catalyst 12.

[0048] While the invention has been described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.

Claims

1. An exhaust purification device of an internal combustion engine arranging an SO.sub.X trap catalyst able to trap SO.sub.X contained in exhaust gas in an engine exhaust passage and arranging NO.sub.X purification catalyst having a function of storing and releasing NO.sub.X contained in exhaust gas in the exhaust passage downstream of the SO.sub.X trap catalyst, wherein a coat layer comprised of at least two layers of an upper coat layer and a lower coat layer is formed on a substrate of said NO.sub.X purification catalyst, the lower coat layer is formed from an NO.sub.X storage catalyst storing the NO.sub.X contained in exhaust gas when an air-fuel ratio of the exhaust gas is lean and releasing the stored NO.sub.X when the air-fuel ratio of the exhaust gas is a stoichiometric air-fuel ratio or rich, and the upper coat layer is formed from a material weaker in basicity than said NO.sub.X storage catalyst.

2. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from a material able to hold SO.sub.2.

3. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from an acid material.

4. An exhaust purification system of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from a material not having an oxidizing function.

5. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein said upper coat layer is formed from an NO.sub.X selective reducing catalyst able to selectively reduce NO.sub.X.

6. An exhaust purification device of an internal combustion engine as set forth in claim 1, wherein when SO.sub.X should be released from the SO.sub.X trap catalyst, the air-fuel ratio of the exhaust gas flowing into the SO.sub.X trap catalyst is made rich.

7. An exhaust purification device of an internal combustion engine as set forth in claim 6, wherein when SO.sub.X should be released from the SO.sub.X trap catalyst, the air-fuel ratio of the exhaust gas flowing into the SO.sub.X trap catalyst is made rich when the exhaust gas flowing into the SO.sub.X trap catalyst has more than a predetermined spatial velocity.