JP6538246B2 - Exhaust gas purification system for internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification system for an internal combustion engine.

  Conventionally, direct injection gasoline engines have been adopted as gasoline engines mounted on automobiles and the like from the viewpoint of improving combustion efficiency and the like. However, since particulate matter (hereinafter referred to as "PM") is generated in this direct injection gasoline engine, an exhaust purification filter for trapping PM in the exhaust passage of the gasoline engine with the recent tightening of emission regulations. The examination of the technique which provides (Gasoline Particulate Filter (Hereafter, the abbreviation "GPF" may be used.) Is provided) is advanced.

Further, in an exhaust passage of the gasoline engine, CO contained in the exhaust, for purifying HC and NO _x, the three-way catalyst (hereinafter may be used abbreviations as "TWC".) To the honeycomb support A supported catalytic converter is provided. Particularly in recent years, a plurality of catalytic converters are arranged in series in the exhaust passage to satisfy the required purification performance. Therefore, it is not preferable from the viewpoint of pressure loss and cost to newly provide GPF in the exhaust passage in addition to the plurality of catalytic converters.

  Therefore, in addition to the PM capture function possessed by the filter substrate, there is proposed a technology for imparting a three-way purification function by the TWC by supporting the TWC on the filter substrate of GPF (see, for example, Patent Document 1) . According to this technology, it is expected that pressure loss and cost problems can be solved by integrating GPF and TWC. That is, by giving the function of TWC to the GPF, the number of catalytic converters to be provided in the exhaust passage can be reduced accordingly, so that the increase in pressure loss and cost in the entire exhaust purification system can be suppressed, which is rational. It is.

Japanese Patent Application Publication No. 2013-500857

  Here, the problem of the exhaust gas purification system in which the catalytic converter and the GPF are disposed in series in the exhaust passage will be examined in detail. As mentioned above, by giving the function of TWC to GPF, it is expected that the number of TWCs required to achieve the exhaust purification performance required accordingly can be reduced. However, in order to secure the PM trapping function, the filter base used for GPF is provided with a plug unlike the honeycomb support used for the catalytic converter. For this reason, the filter base material of GPF has a high pressure loss without supporting TWC, and can not support as many TWC as a honeycomb support. That is, the GPF provided with the TWC function can not simply replace the catalytic converter. Therefore, in an exhaust gas purification system in which a catalytic converter and a GPF are arranged in series, when trying to ensure the exhaust gas purification performance of the entire system, the filter base material of the GPF can not but be forced to carry a large amount of TWC. An increase in pressure loss across the system is a concern.

As described above, when the filter base material of GPF is made to support TWC, it becomes difficult to achieve both pressure loss and exhaust gas purification performance. For this reason, development of TWC suitable for a filter base material is desired. For example, in the technology of Patent Document 1, when using a TWC containing Rh as a catalyst metal, in particular, the NO _x purification performance may be greatly reduced. For example, a TWC having a two-layer structure of an Rh layer and a Pd layer is known as a TWC having an excellent three-way purification function, but when a filter base material of GPF is loaded with such a TWC having a two-layer structure Induces pressure loss. For this reason, it is conceivable to support TWC having a single layer structure obtained by mixing Rh and Pd on the filter substrate. However, in this case, Ba, which is usually added to the Pd layer, is brought into contact with or in proximity to Rh in order to suppress the deterioration of Pd and to improve the NO _x adsorption property. Then, Rh is oxidized to be oxidized by the electron donating action of Ba, and as a result, the NO _x reducing ability of Rh decreases, so there is a problem that GPF can not achieve much large NO _x purification performance.

  The present invention has been made in view of the above, and its object is to provide an exhaust purification system in which a catalytic converter and a GPF are arranged in series in an exhaust passage, while suppressing an increase in pressure loss in the entire system. An object of the present invention is to provide an exhaust gas purification system of an internal combustion engine capable of exhibiting an exhaust gas purification performance.

(1) An exhaust gas purification system (for example, an exhaust gas purification system 2 described later) of an internal combustion engine (for example, an engine 1 described later) is provided in an exhaust passage (for example, an exhaust pipe 3 described later) of the internal combustion engine A first air-fuel ratio sensor (for example, a LAF sensor 51 described later) that generates a signal according to the air-fuel ratio, and a downstream side of a detection location of the first air-fuel ratio sensor in the exhaust passage An upstream catalytic converter (for example, an upstream catalytic converter 31 described later) having a catalyst, and a second air-fuel ratio sensor provided downstream of the upstream catalytic converter in the exhaust passage and generating a signal according to the air fuel ratio of the exhaust A downstream filter (for example, an O ₂ sensor 52 described later) and downstream of the exhaust passage detected by the second air-fuel ratio sensor and capturing and purifying particulate matter in the exhaust gas The air-fuel ratio of the exhaust flowing into the downstream filter is stoichiometric (ie, the complete combustion reaction is performed using the GPF 32 described later, the output signal of the first air-fuel ratio sensor, and the output signal of the second air-fuel ratio sensor). And an air-fuel ratio controller (e.g., an ECU 6 described later) that operates the air-fuel ratio of the air-fuel mixture burned by the internal combustion engine so as to converge to a subsequent target value set near the stoichiometric ratio). In the downstream filter, a plurality of cells extending from the inflow side end face to the outflow side end face of the exhaust gas are partitioned by porous partition walls, and the openings at the inflow side end face of these cells and the openings at the outflow side end face are alternately sealed. And a downstream three-way catalyst (for example, TWC 33, 33a, 33b described later) supported on the partition walls, the downstream three-way catalyst comprising a catalyst metal containing at least Rh, and oxygen storage and release And the OSC material of the downstream three-way catalyst includes a composite oxide having Nd and Pr in its crystal structure, and the upstream catalytic converter is formed from the inflow end face of the exhaust to the outflow side A honeycomb base material in which a plurality of cells extending to an end face are partitioned and formed by porous partition walls; and an upstream three-way catalyst supported on the partition walls of the honeycomb base material; The catalyst contains a catalyst metal and an OSC material having an oxygen storage / release capacity, and the content of the OSC material per unit volume in the filter substrate is based on the content of the OSC material per unit volume in the honeycomb substrate. There are also few.

  (2) In this case, the air-fuel ratio controller sets the former stage target value for the output signal of the first air-fuel ratio sensor such that the output signal of the second air-fuel ratio sensor converges to the latter-stage target value Setting means; and operation amount determining means for determining an operation amount for operating the air-fuel ratio of the air-fuel mixture burned by the internal combustion engine such that the output signal of the first air-fuel ratio sensor becomes the front target value. It is preferable to have.

  (3) In this case, the thickness of the partition of the filter base is larger than the thickness of the partition of the honeycomb base, and the porosity of the partition of the filter base is greater than the porosity of the partition of the honeycomb base The total number of cells formed in the filter substrate is preferably smaller than the total number of cells formed in the honeycomb substrate.

(4) In this case, the partition walls of the filter substrate have an average pore diameter of 15 μm or more,
The downstream three-way catalyst preferably has a particle diameter D90 of 5 μm or less when the cumulative distribution from the small particle diameter side in the particle size distribution is 90%.

  (5) In this case, the downstream three-way catalyst is supported on the inner surface of pores in the partition walls of the filter substrate in a state where Rh and Pd are contained as the catalyst metal and Rh and Pd are mixed. Is preferred.

  (6) In this case, the downstream three-way catalyst is preferably configured without containing Ba.

  (7) In this case, the total content of Nd and Pr contained in the composite oxide of the downstream three-way catalyst is preferably 10% by mass or more.

(1) In the present invention, in a so-called wall flow type GPF that captures particulate matter in exhaust gas by passing the exhaust gas through porous partition walls, a catalyst containing at least Rh the downstream three-way catalyst supported on the partition walls A metal and an OSC material having an oxygen storage and release capacity (Oxygen Storage Capacity) are comprised including those containing a complex oxide having Nd and Pr in its crystal structure. Here, among the elements that can be incorporated into the crystal structure of the composite oxide having oxygen storage / release ability, Nd and Pr have the property of having a large amount of acid points, as described in detail later. Therefore, the complex oxide having Nd and Pr in the crystal structure has a high amount of acid sites, has a high HC adsorption capacity, and a steam reforming reaction that proceeds in the presence of HC and water efficiently proceeds. Then, hydrogen is generated by the progress of the steam reforming reaction, and the generated hydrogen suppresses the oxidization of Rh constituting the downstream three-way catalyst. That is, since a reduction in the NO _x reduction ability of Rh can be avoided, high NO _x purification performance can be exhibited. Therefore, in the present invention, by using the downstream three-way catalyst capable of exerting an excellent three-way purification function as the filter base of the downstream filter, the increase in pressure loss of GPF 32 is suppressed while exhibiting a sufficient three-way purification function. be able to.

  Here, in general, the three-way catalyst includes an OSC material in order to suppress the fluctuation of the air-fuel ratio, in addition to the catalyst metal for exhibiting the three-way purification function. For this reason, in order to minimize the increase in pressure loss in the downstream filter, the content of the OSC material per unit volume in the filter substrate of the downstream filter is set to the OSC material per unit volume in the honeycomb substrate of the upstream catalytic converter It is preferable to make it less than the content of In this respect, as described later, although the amount of hydrogen generated by the steam reforming reaction is higher for Nd than for Pr, Pr has an effect of absorbing the fluctuation of the air-fuel ratio. Therefore, by using the one containing the complex oxide having Nd and Pr in the crystal structure as the OSC material contained in the downstream three-way catalyst used for the downstream filter, the high three-way purification function while suppressing the fluctuation of the air fuel ratio While reducing the content of the OSC material in the filter substrate. As described above, in the present invention, the pressure drop across the entire system is achieved by using the downstream three-way catalyst suitable for the filter substrate and further reducing the OSC material content of the downstream filter compared to the upstream catalytic converter. Excellent exhaust gas purification performance can be exhibited while suppressing the increase of

  In addition to this, in the present invention, in order to avoid the decrease of the three-way purification function in the downstream filter caused by reducing the content of the OSC material used for the downstream filter, it is provided on the upstream side of each of the upstream catalytic converter and the downstream filter. An air-fuel mixture is burned in the internal combustion engine so that the air-fuel ratio of the exhaust flowing into the downstream filter converges to a subsequent target value set in the vicinity of stoichiometry using the output signals of the first and second air-fuel ratio sensors The air-fuel ratio of First, in the present invention, the upstream catalytic converter is provided with a larger amount of OSC material than the downstream filter. For this reason, since the fluctuation of the air-fuel ratio is suppressed in the process of passing through the upstream catalytic converter, the air-fuel ratio of the exhaust flowing into the downstream filter can be stabilized. As described above, according to the present invention, the air-fuel ratio of the exhaust flowing into the downstream filter is stabilized by appropriately distributing the content of the OSC material between the upstream catalytic converter and the downstream filter, and two more air-fuel ratio sensors Reduce the content of OSC material in the downstream filter by operating the air-fuel ratio of the air-fuel mixture so that the air-fuel ratio of the exhaust gas converges to the downstream target value set in the vicinity of stoichiometry It is possible to simultaneously suppress the increase in pressure loss due to the increase in the exhaust gas purification performance.

  (2) In the present invention, the pre-stage target value for the output signal of the first air-fuel ratio sensor upstream of the upstream catalytic converter so that the output signal of the second air-fuel ratio sensor converges to the post-stage target value set near the stoichiometry Set Then, an operation amount for operating the air-fuel ratio of the air-fuel mixture burned by the internal combustion engine is determined so that the output signal of the first air-fuel ratio sensor becomes the front target value. Thereby, in the upstream catalytic converter, the air-fuel ratio of the exhaust flowing into the downstream filter converges to the subsequent target value set in the vicinity of the stoichiometry, that is, the three-way purification function in the downstream three-way catalyst continues to be exhibited. The former stage target value can be set in consideration of the response delay of the control system from the output signal of the first air-fuel ratio sensor to the output signal of the second air-fuel ratio sensor, the dead time, and the like.

  (3) FIG. 17 is a diagram showing the relationship between the wall thickness, the porosity, and the number of cells, which are parameters characterizing the filter base material of the downstream filter, and the PM trapping function or pressure loss performance by the downstream filter. First, in the present invention, the thickness of the partition walls of the filter substrate is made larger than the thickness of the partition walls of the honeycomb substrate. This makes it possible to achieve the required PM capture function for the downstream filter. However, as shown in FIG. 17, when the partition wall of the filter base is thickened, although the PM trapping function is improved, the pressure loss is increased. Therefore, in the present invention, the porosity of the partition walls of the filter substrate is made higher than the porosity of the partition walls of the honeycomb substrate. Thereby, an increase in pressure loss in the downstream filter can be suppressed. However, when the porosity is increased, the mechanical strength of the filter substrate is reduced as shown in FIG. Therefore, in order to ensure sufficient mechanical strength of the filter substrate, the porosity can not be increased excessively and the pressure loss can not be reduced sufficiently. Therefore, in the present invention, the total number of cells formed in the partition walls of the filter substrate is made smaller than the total number of cells formed in the honeycomb substrate. In the present invention, by setting the wall thickness, the porosity and the cell number as described above, sufficient PM capturing function, mechanical strength and pressure loss performance can be achieved.

  (4) In the present invention, the average pore diameter of the partition walls of the filter substrate is 15 μm or more, and the particle diameter D 90 of the downstream three-way catalyst is 5 μm or less. As a result, the particulated downstream three-way catalyst can be introduced into the pores of the partition walls, and the downstream three-way catalyst can be supported on the inner surface of such pores. Therefore, according to the present invention, it is possible to avoid an increase in pressure loss of the downstream filter caused by supporting the downstream three-way catalyst only on the surface layer of the partition wall, and it is possible to exhibit a higher three-way purification function.

(5) In the present invention, the downstream three-way catalyst is composed of Rh and Pd, and is supported on the inner surface of the pores in the partition of the filter substrate in the mixed state of Rh and Pd. As described above, when Rh and Pd are conventionally mixed and supported on the downstream filter, Ba added to the conventional Pd layer comes in contact with or in proximity to Rh, so Rh is oxidized by the electron donating action of Ba. and oxides of Te, NO _x purifying performance was reduced significantly. On the other hand, according to the present invention, as a result of the effects of the invention of (1) being exhibited remarkably, it is possible to avoid a reduction in the NO _x purification performance of Rh and to exhibit a three-way purification function superior to the conventional one. A downstream filter can be provided.
Also, while it is difficult to support the conventional downstream three-way catalyst having a two-layer structure of Rh layer and Pd layer on the inner surface of the pores of the partition walls, according to the present invention, even in the mixed Rh and Pd state Since a high three-way purification function is exhibited, the catalyst composition is preferable for supporting the inner surfaces of the partition walls.

(6) In the present invention, the downstream three-way catalyst is configured without containing Ba. Thus, because it contains no Ba in the downstream three-way catalyst, oxides of Rh is promoted by Ba NO _x purification performance as described above can be avoided from being reduced.

  (7) In the present invention, Nd and Pr are contained in an amount of 10% by mass or more in the crystal structure of the composite oxide. Thereby, more excellent three-way purification performance is exhibited.

It is a figure showing the composition of the exhaust gas purification system of the engine concerning one embodiment of the present invention. It is a cross-sectional schematic diagram of GPF which concerns on the said embodiment. It is an expansion mimetic diagram of a partition of GPF concerning the above-mentioned embodiment. It is a figure which shows the easiness of reduction | restoration of Rh by CO-TPR. It is a diagram showing the amount of acid sites of the composite oxide by NH ₃ -TPD. It is a figure which shows the hydrogen production amount by the steam reforming reaction of each complex oxide. FIG. 6 is a view showing the relationship between the temperature and the NO _x purification rate in Example 1 and Comparative Example 1; It is a figure which shows the relationship between the temperature in Example 1 and Comparative Examples 2 and 3, and an air fuel ratio absorption rate. FIG. 16 is a view showing a particle size distribution of TWC of Example 6. FIG. 6 is a view showing a supported state of TWC in partition walls of GPF in Example 1. It is a figure which shows the relationship between D90 of TWC and pressure loss in Examples 1-7. It is a figure which shows the relationship of the average pore diameter of the partition of GPF and pressure loss in Example 1 and Examples 8, 9. It is a figure which shows the relationship of the washcoat amount and pressure loss of TWC in Example 1 and Examples 10-13. It is a figure which shows the relationship between the wall thickness of GPF and pressure loss in Example 1 and Examples 17 and 18. FIG. 6 is a view showing the relationship between the air fuel ratio and the purification rate in the GPF of the first embodiment. FIG. 20 is a view showing the relationship between the air fuel ratio and the purification rate in the GPF of Example 19. FIG. 20 is a view showing the relationship between the air fuel ratio and the purification rate in the GPF of the twentieth embodiment. FIG. 21 is a view showing the relationship between the air fuel ratio and the purification rate in the GPF of Example 21. It is a diagram showing the relationship between the total content and NO _x _T50 of Nd and Pr. It is a figure which shows the relationship between the total content of Nd and Pr, and CO_T50. It is a figure which shows the relationship between the sum total content of Nd and Pr, and HC_T50. It is a figure which shows the relationship between the wall thickness which is a parameter characterizing the filter base material of GPF, the porosity, and the number of cells, the PM capture function by GPF, pressure loss performance, etc.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a view showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and an exhaust gas purification system 2 according to the present embodiment.

  The engine 1 is a direct injection gasoline engine in which fuel is directly injected into each cylinder by a fuel injection valve 11 provided for each of a plurality of cylinders. These fuel injection valves 11 operate in response to a signal from the ECU 6 described later. The ECU 6 determines the fuel injection mode such as the fuel injection amount and the fuel injection timing of these fuel injection valves 11 by an air-fuel ratio control program described later, and opens and closes the fuel injection valve 11 so that the determined fuel injection mode is realized. To drive.

The exhaust purification system 2 includes a LAF sensor 51 as a first air-fuel ratio sensor, an upstream catalytic converter 31 provided in an exhaust pipe 3 of the engine 1, an O ₂ sensor 52 as a second air-fuel ratio sensor, and an exhaust pipe 3. And the ECU 6 as an air-fuel ratio controller for operating the air-fuel ratio of the air-fuel mixture to be burned by the engine 1 using the output signals of the LAF sensor 51 and the O ₂ sensor 52. Thus, the exhaust gas of the engine 1 flowing through the exhaust pipe 3 is purified. In the following, first, the configurations of the upstream catalytic converter 31 and the GPF 32 will be described, and then the functions of the two sensors 51 and 52 and the ECU 6 will be described.

The upstream catalytic converter 31 includes a honeycomb base in which a plurality of cells extending from the inflow side end face to the outflow side end face of the exhaust gas are partitioned by porous partition walls, and the TWC supported by the partition walls of the honeycomb base material. . The TWC used in the upstream catalytic converter 31 has a function of purifying HC in the exhaust gas by oxidizing or reducing it to H ₂ O and CO ₂ , CO to CO ₂ and NO _x to N ₂ (that is, three-way conversion) Have a purification function). For this TWC, for example, a carrier obtained by supporting a noble metal such as Pd or Rh as a catalyst metal on a carrier made of an oxide such as alumina, silica, zirconia, titania, ceria or zeolite is used.

The TWC of the upstream catalytic converter 31 includes an OSC material having an oxygen storage / release capacity. The OSC material, other CeO _2, a composite oxide of CeO ₂ and ZrO ₂ (hereinafter, referred to as "CeZr composite oxide".) And the like. Among them, the CeZr composite oxide is preferably used because it has high durability. In addition, the said catalyst metal may be carry | supported by these OSC materials.

  The method of manufacturing the upstream catalytic converter 31 is not particularly limited, and is prepared by a conventionally known slurry method or the like. For example, after preparing a slurry containing the above-mentioned oxide, noble metal, OSC material and the like, the prepared slurry is coated on a honeycomb substrate made of cordierite and fired.

  The GPF 32 is provided downstream of the upstream catalytic converter 31 in the exhaust pipe 3. The GPF 32 captures and purifies PM in exhaust gas. Specifically, when exhaust gas passes through fine pores in the partition walls described later, PM is captured by depositing PM on the surface of the partition walls.

FIG. 2 is a schematic cross-sectional view of the GPF 32 according to the present embodiment.
As shown in FIG. 2, the GPF 32 comprises a filter substrate 320. The filter base 320 has, for example, a cylindrical shape elongated in the axial direction, and is formed of a porous body such as cordierite, mullite, silicon carbide (SiC) or the like. The filter base 320 is provided with a plurality of cells extending from the inflow side end face 32 a to the outflow side end face 32 b, and these cells are partitioned by the partition wall 323.

The filter base 320 includes an inflow side plugging portion 324 that seals the inflow side end face 32a. The cell in which the inflow side end face 32a is sealed by the inflow side plugging portion 324 is closed at the inflow side end while the outflow side end is opened, and the outflow side exhausts the exhaust gas having passed through the partition 323 to the downstream side The cell 322 is configured.
The inflow side plugging portion 324 is formed by sealing the plugging cement from the inflow side end face 32 a of the filter substrate 320.

The filter base 320 includes an outlet-side plugging portion 325 that seals the outlet-side end surface 32 b. The cell in which the outflow side end face 32b is sealed by the outflow side plugging portion 325 forms the inflow side cell 321 in which the outflow side end is closed while the inflow side end is opened and the exhaust flows from the exhaust pipe 3 Do.
The outflow side plugging portion 325 is formed by sealing the plugging cement from the outflow side end face 32 b of the filter base 320.

  In addition, the opening in the inflow side end face 32a of the cell and the opening in the outflow side end face 32b are alternately sealed, so that the inflow side cell 321 and the outflow side cell 322 are adjacent to each other in a lattice shape (checked shape). Is to be placed.

FIG. 3 is an enlarged schematic view of the partition wall 323 of the GPF 32 according to the present embodiment.
As shown in FIG. 3, the TWC 33 is supported on the inner surface of the pore in the partition wall 323. The TWC 33 includes a TWC 33 a containing Rh and a TWC 33 b containing Pd. These TWCs 33 are supported on the inner surface of the pore in a micronized state. The pores of the partition walls 323 are not blocked by the TWCs 33 so that a large pressure loss does not occur.

  The partition wall 323 preferably has an average pore diameter of 15 μm or more. If the average pore diameter is 15 μm or more, the TWC 33 can enter into the pore diameter due to the relationship with the particle diameter of TWC 33 described later, and the TWC 33 can be supported on the inner surface of the pore. A more preferable average pore diameter is 20 μm or more.

  The thickness of the partition wall 323 is not particularly limited, but is preferably 10 mil or less. When the thickness of the partition exceeds 10 mils, the pressure loss may increase due to the relationship between the amount of TWC supported, the average pore diameter of the partition, and the like.

  The TWC 33 is micronized with a particle diameter D90 of 5 μm or less when the cumulative distribution from the small particle size side in the particle size distribution is 90%. If D90 of the TWC 33 is 5 μm or less, the TWC 33 can enter into the pore diameter in relation to the average pore diameter of the partition wall 323 described above, and the TWC 33 can be supported on the inner surface of the pore. More preferable D90 is 3 μm or less.

  The TWC 33 contains at least Rh as a catalytic metal, and preferably contains Rh and Pd as catalytic metals as shown in FIG. These Rh and Pd may be supported by an OSC material having an oxygen storage / release capacity, which will be described later, and may be supported by a conventionally known support made of an oxide such as alumina, silica, zirconia, titania, ceria or zeolite. It is also good.

  As described above, the TWC 33 is configured to include the TWC 33a containing Rh and the TWC 33b containing Pd. As shown in FIG. 3, the TWC 33a containing Rh and the TWC 33b containing Pd are supported on the inner surface of the pore in the partition wall 323 in a mixed state.

In addition to the above-mentioned catalyst metals, the TWC 33 contains an OSC material having an oxygen storage / release capacity. As the OSC material used for this TWC 33, one containing a composite oxide having Nd and Pr in its crystal structure is used. In addition to the complex oxide having such Nd and Pr, the known OSC material used for the TWC 33 uses CeO ₂ , ZrO ₂ and their complex oxides such as known materials having the ability to absorb and release oxygen. It is also good.

In the present embodiment, the composite oxide used as the OSC material is supported in the partition wall 323 together with the catalyst metal.
The TWCs used for the upstream catalytic converter 31 and the GPF 32 oxidize HC in the exhaust gas to convert it to CO ₂ and H ₂ O, oxidize CO to convert it to CO ₂ while reducing NO _x to N ₂ Have a function to It is preferable to keep the ratio of fuel to air (hereinafter referred to as "air-fuel ratio") in the vicinity of stoichiometry in order to effectively produce catalytic action for both reactions simultaneously.

The air-fuel ratio in an internal combustion engine such as a car changes largely depending on the operating condition. Therefore, the ECU 6 performs control to keep the air-fuel ratio of the exhaust flowing into the upstream catalytic converter 31 and the GPF 32 close to the stoichiometry by performing air-fuel ratio control described later. However, controlling the air-fuel ratio by such a method is not sufficient for the catalyst to exhibit purification performance.
Therefore, an OSC material having an oxygen storage and release capacity that stores oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere is used together with a catalytic metal as a cocatalyst. For example, CeO ₂ and composite oxides of Ce and Zr are known as OSC materials.

The complex oxide used as the OSC material in the present embodiment has a structure in which part of Ce or Zr in the crystal structure of CeO ₂ or ZrO ₂ is replaced with Nd or Pr.
Nd and Pr have high HC adsorption ability, and a large amount of hydrogen is generated by a steam reforming reaction described later. Hydrogen promotes the reduction of Rh and improves the NO _x purification performance of Rh.

  In the present embodiment, in addition to Nd, Pr having a smaller amount of hydrogen generation by the steam reforming reaction than Nd is contained in the structure of the composite oxide. Since Pr has a function of absorbing the fluctuation of the air-fuel ratio to the stoichiometry, the inclusion of Pr makes it easy to maintain the air-fuel ratio in the vicinity of the stoichiometry.

The CeZrNdPr composite oxide according to the present embodiment can be prepared, for example, by the following method.
First, cerium nitrate, zirconium nitrate, neodymium nitrate and praseodymium nitrate are dissolved in pure water so as to have a desired ratio. Thereafter, an aqueous solution of sodium hydroxide is added dropwise to bring the pH of the solvent to 10, for example, to obtain a precipitate. Then, the solvent is evaporated by filtering under reduced pressure while heating the solution containing the precipitate to, for example, 60 ° C. Next, the residue is extracted and then calcined at, for example, 500 ° C. for 2 hours in a muffle furnace to obtain a CeZrNdPr composite oxide.

Further, the TWC 33 of the present embodiment is configured without containing Ba, which has been added from the viewpoint of the conventional deterioration suppression of Pd and the improvement of NO _x adsorption property.

  In the TWC 33 of the present embodiment, the total content of Nd and Pr contained in the composite oxide is preferably 10% by mass or more. If the total content of Nd and Pr contained in the composite oxide is within this range, a higher three-way purification function is exhibited. The upper limit value of the total content is preferably 20% by mass, and a more preferable range is 12% by mass to 16% by mass.

  The content ratio of Rh to Pd in the TWC 33 is not particularly limited, but preferably Rh: Pd = 1: 10 to 1: 5 on a mass basis.

  Further, the amount of supported TWC 33 per unit volume (hereinafter also referred to as “wash coat amount”) in the filter substrate 320 of the GPF 32 is not particularly limited, but is preferably 40 to 80 g / L. When the washcoat amount is less than 40 g / L, sufficient purification performance can not be obtained, and when it exceeds 80 g / L, pressure loss increases.

In the present embodiment, the TWC 33 may contain another noble metal such as Pt as a catalyst metal. In addition, GPF 32 carries a catalyst having a function other than the three-way purification function in the partition wall or on the surface of the partition wall, such as an NO _x catalyst, an oxidation catalyst, or an Ag-based catalyst for burning and removing PM deposited in GPF. It may be

Next, a method of manufacturing the GPF 32 according to the present embodiment will be described.
The GPF 32 according to the present embodiment is manufactured, for example, by dipping. In the dipping method, for example, a slurry containing a predetermined amount of constituent materials of TWC 33 is prepared by wet grinding or the like, GPF 32 is immersed in the prepared slurry, and then GPF 32 is pulled up and fired at a predetermined temperature condition, The GPF 32 can carry the TWC 33.

  In the present embodiment, it is preferable that a slurry prepared by mixing catalysts such as Rh and Pd with a ball mill or the like is pulverized to a particle size of 5 μm or less and dipped in GPF 32 once. Thereby, Rh and Pd can be supported on the inner surface of the pores in the partition wall 323 in a state of being randomly mixed.

Next, the reason why it is preferable that the TWC according to this embodiment does not contain Ba will be described with reference to FIG.
FIG. 4 is a view showing the ease of reduction of Rh by CO-TPR. Specifically, it is a figure which shows the result of having measured the ease of reduction of Rh by the presence or absence of Ba added to TWC according to the following procedure by CO-TPR (temperature rising reduction method).
In TWC, Rh was supported on Zr oxide at a ratio of 0.3% by mass and 3% by mass, respectively, and those with and without 10% by mass of Ba were respectively prepared and measured.

[CO-TPR measurement procedure]
(1) The temperature was raised in He and held at 600 ° C. for 10 minutes.
(2) The temperature was lowered to 100 ° C.
(3) Rh was reduced by heating to 800 ° C. at 10 ° C./min in 1% CO / N ₂ .
(4) The temperature was lowered to 600 ° C.
(5) in _10% O ₂ / _N 2, and held at 600 ° C. 10 min.
(6) The temperature was lowered to 100 ° C., held in He for 10 minutes, and then held in 1% CO / N ₂ for 10 minutes.
(7) In 1% CO / N ₂ , the temperature was raised to 800 ° C. at 10 ° C./min, and the change of CO ₂ emission with temperature was measured.

As shown in FIG. 4, it can be seen that the TWC containing Ba has a lower amount of CO ₂ emission at a lower temperature as compared to the TWC containing no Ba. This means that Rh is difficult to be reduced, and Ba is considered to inhibit the reduction of Rh. Therefore, the TWC in the present embodiment maintains the reduction state of Rh by not containing Ba, and exhibits high NO _x purification performance.

Next, the action of the CeZrNdPr composite oxide will be described.
Rh used as the catalyst metal is reduced in the presence of hydrogen state is maintained, thereby improving _the NO _x purification performance. Therefore, in the present embodiment, a steam reforming reaction is used. The steam reforming reaction is a reaction represented by the following formula in which steam and HC react at high temperature in the presence of a catalyst to generate hydrogen.

C _n H _m + _n H ₂ O → n CO + (n + 1/2 m) H ₂

In order to improve the amount of hydrogen produced by the steam reforming reaction, the HC adsorption capacity is important, and the HC adsorption capacity is considered to be dependent on the acid point.
Figure 5 _is a diagram showing the amount of acid sites of the composite oxide by NH 3 -TPD. Specific examples include as an element that may be present in the crystal structure of the complex oxide of Ce and Zr, Y, La, the amount of each acid point Pr and _Nd, NH 3 -TPD (Atsushi Nobori reduction Method according to the following procedure.

[NH ₃ -TPD Measurement Procedure]
(1) The temperature was raised in He and held at 600 ° C. for 60 minutes.
(2) The temperature was lowered to 100 ° C.
(3) After holding for 60 minutes in 0.1% NH ₃ / He, for 60 minutes in He.
(4) The temperature was raised to 600 ° C. at 10 ° C./min in He.

  As shown in FIG. 5, it can be seen that Nd and Pr have more acid points than Y and La. Therefore, from this result, it can be said that Nd and Pr have high HC adsorption ability.

  FIG. 6 is a graph comparing the amount of hydrogen produced by the steam reforming reaction at 500 ° C. when each element of Y, La, Pr and Nd is contained in the crystal structure of the CeZr composite oxide. In this case, the content of each element of Y, La, Pr and Nd is 7% by mass, the content of Ce is 41% by mass, and the content of Zr is 52% by mass. As shown in FIG. 6, it can be seen that Pr and Nd generate more hydrogen than Y and La.

Returning to FIG. 1, a preferable combination of the upstream catalytic converter 31 and the GPF 32 provided in the exhaust pipe 3 will be described. As described above, the upstream catalytic converter 31 and the GPF 32 are configured by supporting the TWC on the porous base material, the catalyst metal generating the three-way purification function and the OSC material having the oxygen storage / release capacity. Are common to all Here, the base and TWC used for the upstream catalytic converter 31 are not limited to those described above, and the same ones used for the GPF 32 may be used. However, considering that the air-fuel ratio in the GPF 32 on the downstream side of the O ₂ sensor 52 is stably controlled in the vicinity of the stoichiometry under the air-fuel ratio control by the ECU 6, the content of the OSC material in the upstream catalytic converter 31 and the GPF 32 (More specifically, the content of OSC material per unit volume in the substrate [g / L]), the supported amount of the three-way catalyst (more specifically, of the three-way catalyst per unit volume in the substrate The loading amount [g / L]), the cell structure of the base material, and the porosity of the base material are preferably combined so as to satisfy the following table.

  For example, as shown in the above table, the content of the OSC material per unit volume in the filter substrate of GPF 32 is preferably smaller than the content of the OSC material per unit volume in the honeycomb substrate of the upstream catalytic converter 31 . More specifically, when the content of the OSC material in the upstream catalytic converter 31 is 1, the content of the OSC material in the GPF 32 is in the range of 1 to 0.3, and more preferably about 0.35.

  Further, it is preferable that the carrying amount [g / L] of TWC 33 per unit volume in the filter substrate of GPF 32 be smaller than the carrying amount [g / L] of TWC per unit volume in the honeycomb substrate of the upstream catalytic converter 31 . More specifically, when the loading amount of TWC in the upstream catalytic converter 31 is 200 g / L, the loading amount of TWC 33 in the GPF 32 is in the range of 50 to 100 g / L, more preferably described later. As shown in the examples, it is about 60 [g / L].

  The thickness of the partition wall of the filter substrate of GPF 32 is preferably larger than the thickness of the partition wall of the honeycomb substrate of the upstream catalytic converter 31. More specifically, when the wall thickness of the GPF 32 is 8 mils, the wall thickness of the upstream catalytic converter 31 is preferably 3.5 mils.

  The porosity of the partition walls of the filter substrate of GPF 32 is preferably higher than the porosity of the partition walls of the honeycomb substrate of the upstream catalytic converter 31. More specifically, when the porosity of GPF 32 is 65%, the porosity of upstream catalytic converter 31 is preferably 35%.

  The total number of cells formed in the filter substrate of the GPF 32 is preferably smaller than the total number of cells formed in the honeycomb substrate of the upstream catalytic converter 31. More specifically, when the total number of cells of GPF 32 is 300, the total number of cells of upstream catalytic converter 31 is preferably 600. As described above, by configuring the wall thickness, the porosity, and the total number of cells, sufficient PM capturing function, mechanical strength, and pressure loss performance in the GPF 32 can be achieved.

Next, the functions of the LAF sensor 51, the O ₂ sensor 52, and the ECU 6 will be described.

The LAF sensor 51 detects the air-fuel ratio (the ratio of the fuel component to the oxygen in the exhaust gas) of the exhaust gas flowing upstream of the upstream catalytic converter 31 in the exhaust pipe 3 and transmits a signal substantially proportional to the detected value to the ECU 6 . The O ₂ sensor 52 detects the oxygen concentration (that is, the air-fuel ratio) of the exhaust gas flowing between the upstream catalytic converter 31 and the GPF 32 in the exhaust pipe 3 and transmits a signal corresponding to the detected value to the ECU 6.

Here, characteristics of output signals of the LAF sensor 51 and the O ₂ sensor 52 are compared. The LAF sensor 51 generates a signal at a level substantially proportional to the air-fuel ratio over a wider range of air-fuel ratio than the O ₂ sensor 52. That is, the signal level of the LAF sensor 51 has a linear characteristic from the rich area to the lean area, and the air-fuel ratio can be detected in a wider range than the O ₂ sensor 52 . The O ₂ sensor 52 generates a signal substantially in proportion to the oxygen concentration of the exhaust when the oxygen concentration of the exhaust is within the range Δ near the stoichiometry. That is, the level of the signal output from the O ₂ sensor 52 has a substantially binary characteristic of inverting from low to high in the vicinity of stoichiometry. Therefore, the O ₂ sensor 52 can detect the air-fuel ratio with higher sensitivity than the LAF sensor 51 within a limited range in the vicinity of the stoichiometry.

  The ECU 6 shapes input signal waveforms from various sensors such as the sensors 51 and 52, corrects the voltage level to a predetermined level, and converts an analog signal value to a digital signal value. A central processing unit executes various control programs such as air-fuel ratio control described in the above, and a drive circuit which drives various devices such as the fuel injection valve 11 of the engine 1 in a mode determined by the control program.

FIG. 1 schematically shows the procedure of air-fuel ratio control in the ECU 6. In ECU 6, by executing the air-fuel ratio control program comprising a target air-fuel ratio operation and the fuel injection amount calculation using the output signal VO2 of the output signal KACT and the O ₂ sensor 52 from the LAF sensor 51, the air-fuel mixture to be burned in the engine 1 The fuel injection amount from the fuel injection valve 11, which is the operation amount of the air-fuel ratio, is determined.

  In the fuel injection amount calculation, the ECU 6 uses a known feedback control law such as sliding mode control so that the output signal KACT of the LAF sensor 51 converges to the target air-fuel ratio KCMD calculated by the target air-fuel ratio calculation described later. Thus, the fuel injection amount from the fuel injection valve 11 is determined.

In the target air-fuel ratio calculation, the ECU 6 uses the output signal KACT of the LAF sensor 51 and the output signal VO2 of the O ₂ sensor 52 so that the TWC of the upstream catalytic converter 31 and the TWC of the GPF 32 can exhibit high three-way purification functions. Thus, the target air-fuel ratio KCMD is determined. More specifically, the target air-fuel ratio operation, ECU 6 makes the control system P from the output signal KACT of the LAF sensor 51 until the output signal VO2 of the _{O 2} sensor 52, including at least a response delay element and a dead time element model Is determined, and the target air-fuel ratio KCMD that achieves the above-mentioned purpose is determined by using operations in an adaptive sliding mode controller, a real time identifier, a state predictor and the like described below.

First, the real-time identifier, by using the output signal VO2 of the output signal KACT and the O ₂ sensor 52 from the LAF sensor 51, sequentially generates the identification value of the plurality of model parameters defined in the above model. Further, the state predictor sequentially generates the output after the dead time of the control system P, that is, the estimated value after the dead time of the output signal VO2 of the O ₂ sensor 52. The adaptive sliding mode controller is configured such that the output signal VO2 of the O ₂ sensor 52 converges on a predetermined subsequent target value set in the vicinity of the stoichiometry so that a high ternary purification function is exhibited in the TWC 33 of the GPF 32. The target air-fuel ratio KCMD is determined using the identification value generated by the real-time identifier and the estimated value generated by the state predictor.

  The details of the air-fuel ratio control algorithm in the ECU 6 as described above are described in, for example, Japanese Patent Application Laid-Open Nos. 2000-230451 and 2001-182528 by the applicant of the present application, and the like. Detailed explanation is omitted.

According to the present embodiment, the following effects are achieved.
In this embodiment, in the so-called wall-flow type GPF 32, a complex oxide having Nd and Pr in the crystal structure as a catalyst metal containing at least Rh and an oxygen storage / release capacity of the TWC 33 supported on the partition wall 323 And including.
Here, among the elements that can be incorporated into the crystal structure of the complex oxide having oxygen storage / release ability, Nd and Pr have the property of having a large amount of acid sites. Therefore, the complex oxide having Nd and Pr in the crystal structure has a high amount of acid sites, has a high HC adsorption capacity, and a steam reforming reaction that proceeds in the presence of HC and water efficiently proceeds. Then, hydrogen is generated by the progress of the steam reforming reaction, and the generated hydrogen suppresses the oxidization of Rh that constitutes the TWC 33. That is, since a reduction in the NO _x reduction ability of Rh can be avoided, high NO _x purification performance can be exhibited. Therefore, in the present invention, it is possible to suppress an increase in pressure loss of the GPF 32 while exhibiting a sufficient three-way purification function by using the TWC 33 capable of exhibiting the excellent three-way purification function as the filter base of the GPF 32.

  As mentioned above, the TWC includes the OSC material. Therefore, in order to minimize the increase in pressure loss in the GPF 32, the content of the OSC material per unit volume in the filter substrate of the GPF 32 is set to the content of the OSC material per unit volume in the honeycomb substrate of the upstream catalytic converter 31. It is preferable to make it less than content. In this respect, as described above, although the amount of hydrogen produced by the steam reforming reaction is higher for Nd than for Pr, Pr has the effect of absorbing fluctuations in the air-fuel ratio. Therefore, by using a material containing a complex oxide having Nd and Pr in the crystal structure as the OSC material contained in the TWC 33 used for the GPF 32, a high three-way purification function is exhibited while suppressing the fluctuation of the air fuel ratio. The content of the OSC material in the filter substrate can be reduced. As described above, in the present embodiment, the pressure loss in the entire exhaust gas purification system is achieved by using the TWC 33 suitable for the filter substrate and further reducing the content of the OSC material of the GPF 33 compared to the upstream catalytic converter 31. Excellent exhaust gas purification performance can be exhibited while suppressing the increase of

In addition to this, in the present embodiment, the upstream catalytic converter 31 and the GPF 32 are provided on the upstream side of the upstream catalytic converter 31 and the GPF 32 in order to avoid a decrease in the three-way purification function in the GPF 32 caused by reducing the content of OSC material used for the GPF 32 By using the output signals of the LAF sensor 51 and the O ₂ sensor 52, the air-fuel ratio of the air-fuel mixture in the engine 1 is adjusted so that the air-fuel ratio of the exhaust flowing into the GPF 32 converges to the subsequent target value set near the stoichiometry. Manipulate. First, in the present embodiment, the upstream catalytic converter 31 is provided with a larger amount of OSC material than the GPF 32. For this reason, since the fluctuation of the air-fuel ratio is suppressed in the process of passing through the upstream catalytic converter 31, the air-fuel ratio of the exhaust flowing into the GPF 32 can be stabilized. As described above, in the present embodiment, by appropriately distributing the content of the OSC material between the upstream catalytic converter 31 and the GPF 32, the air-fuel ratio of the exhaust flowing into the GPF 32 is stabilized, and then the LAF sensor 51 and O are further reduced. By operating the air-fuel ratio of the air-fuel mixture so that the air-fuel ratio of the exhaust gas converges to the post-stage target value set in the vicinity of the stoichiometry using the ₂ sensor 52, the content of the OSC material in the GPF 32 is reduced. Therefore, it is possible to simultaneously suppress the increase in pressure loss and the improvement of the exhaust gas purification performance.

Further, in the present embodiment, the target air-fuel ratio KCMD with respect to the output signal KACT of the LAF sensor 51 on the upstream side of the upstream catalytic converter 31 so that the output signal VO2 of the O ₂ sensor 52 converges to a subsequent target value set near the stoichiometry. Set Then, a fuel injection amount, which is an operation amount for operating the air-fuel ratio of the air-fuel mixture burned by the engine 1 such that the output signal KACT of the LAF sensor 51 becomes the target air-fuel ratio KCMD, is determined. As a result, the LAF sensor 51 in the upstream catalytic converter 31 is set so that the air-fuel ratio of the exhaust flowing into the GPF 32 converges to the subsequent target value set in the vicinity of the stoichiometry, ie, the three-way purification function in the TWC 33 continues to be exhibited. The target air-fuel ratio KCMD can be set in consideration of the response delay of the control system from the output signal of the sensor to the output signal of the O ₂ sensor 52, the dead time, and the like.

  Further, in the present embodiment, the thickness of the partition wall of the filter substrate of GPF 32 is made larger than the thickness of the partition wall of the honeycomb substrate of upstream catalyst converter 31. This makes it possible to achieve the PM capture function of the degree required for the GPF 32. However, as shown in FIG. 17, when the partition wall of the filter base is thickened, although the PM trapping function is improved, the pressure loss is increased. So, in this embodiment, the porosity of the partition of a filter base material is made higher than the porosity of the partition of a honey-comb base material. Thereby, an increase in pressure loss in the GPF 32 can be suppressed. However, when the porosity is increased, the mechanical strength of the filter substrate is reduced as shown in FIG. Therefore, in order to ensure sufficient mechanical strength of the filter substrate, the porosity can not be increased excessively and the pressure loss can not be reduced sufficiently. Therefore, in the present embodiment, the total number of cells formed in the partition walls of the filter substrate is made smaller than the total number of cells formed in the honeycomb substrate. In the present embodiment, by setting the wall thickness, the porosity, and the total number of cells of the GPF 32 as described above, sufficient PM capture function, mechanical strength, and pressure loss performance can be achieved.

  In the present embodiment, the average pore diameter of the partition walls 323 is 15 μm or more, and the particle diameter D90 of the TWC 33 is 5 μm or less. Thus, the micronized TWC 33 can be introduced into the pores of the partition wall 323, and the TWC 33 can be supported on the inner surface of the pore. Therefore, according to the present embodiment, it is possible to avoid an increase in pressure loss of the GPF 32 caused by the TWC 33 being supported only on the surface layer of the partition wall 323, and it is possible to exhibit a higher three-way purification function.

Further, in the present embodiment, the TWC 33 is configured to include Rh and Pd, and is supported on the inner surface of the pore in the partition wall 323 in a state where Rh and Pd are mixed. As described above, conventionally, when Rh and Pd are mixed and supported on GPF 32, the Ba added to the Pd layer contacts or approaches Rh so that Rh is oxidized and oxidized by the electron donating action of Ba. and Monoka, NO _x purifying performance was reduced significantly. On the other hand, according to the present embodiment, it is possible to provide the GPF 32 capable of avoiding the reduction of the NO _x purification performance of Rh by the above-described steam reforming reaction and exhibiting the three-way purification function superior to the conventional one.
In addition, while it is difficult to support the conventional TWC having a two-layer structure of Rh layer and Pd layer on the inner surface of the pores of the partition wall, according to this embodiment, it is also high in the mixed state of Rh and Pd. Since the original purification function is exhibited, the catalyst composition has a preferable composition for supporting the inner surfaces of the partition walls 323 with pores.

Further, in the present embodiment, the TWC 33 is configured without containing Ba. According to this embodiment, because it contains no Ba in TWC33, it is promoted oxides of Rh is _the NO _x purification performance by Ba as described above can be avoided from being lowered.

  In the present embodiment, Nd and Pr are contained in an amount of 10% by mass or more in the crystal structure of the composite oxide. Thereby, more excellent three-way purification performance is exhibited.

  The present invention is not limited to the above-described embodiment, and modifications, improvements, and the like as long as the object of the present invention can be achieved are included in the present invention.

  Next, although the Example of GPF 32 of the exhaust gas purification system 2 which concerns on the said embodiment is demonstrated, GPF 32 is not limited to the following examples.

<Examples 1 to 21, Comparative Examples 1 to 4>
The TWC and the carrier, the composite oxide, etc. were prepared according to the following procedure in the proportions shown in Table 1.
First, an aqueous medium and an additive were added, and then mixed and slurried in a ball mill. Next, the slurry was pulverized by wet pulverization or the like to adjust the particle size. Next, the mixed slurry was immersed once in GPF by the dipping method. The loading amount (wash coat amount) was 60 g / L (except for Examples 10 to 13). Thereafter, baking was performed at 700 ° C. for 2 hours to obtain GPF on which TWC is supported.
As GPF, a honeycomb structure made of NGK (inner diameter 25.4 (φ 1 inch) mm, average pore diameter 20 μm (except for Examples 8 and 9), wall thickness 8 mil (except for Examples 17 and 18), A cell number of 300, a material of cordierite, a capacity of 15 cc) was used.

<NO _x purification performance>
FIG. 7 is a view showing the relationship between the temperature and the NO _x purification rate in Example 1 and Comparative Example 1. Specifically, Nd in the OSC material, as in Example 1 with the addition of Pr, Y, Comparative Example 1 with the addition of La, is a diagram showing results of evaluating the GPF of the NO _x purifying performance under the following conditions. As shown in FIG. 7, it was found that the purification of NO _x was progressing at a temperature lower than that of Comparative Example 1 in Example 1. From this result, in Example 1 was added Nd, Pr, the OSC material in the GPF is, Y, as compared with Comparative Example 1 with the addition of La has _the NO _x purification performance was confirmed to be improved.
[NO _x purification performance evaluation conditions]
The NO _x purification performance was evaluated by measuring the NO _x concentration when the GPF was heated to 500 ° C. at 20 ° C./min in a stoichiometric gas.

<Air-fuel ratio absorption rate>
FIG. 8 is a graph showing the relationship between the temperature and the air-fuel ratio absorption rate in Example 1 and Comparative Examples 2 and 3. Specifically, for each of Comparative Example 2 in which only Nd was added to the OSC material, Comparative Example 3 in which only Pr was added, and Example 1 in which both Nd and Pr were used, the air-fuel ratio absorption ratio of GPF was measured. It is a figure which shows a result. The air-fuel ratio absorption rate was calculated by equation (1) according to the following conditions.

Air-fuel ratio absorption ratio (%) = ((air-fuel ratio amplitude (IN)-air-fuel ratio amplitude (OUT)) / air-fuel ratio amplitude (IN)) x 100
... Formula (1)

(In the formula (1), “air-fuel ratio amplitude (IN)” indicates the air-fuel ratio amplitude before passing the OSC material, and “air-fuel ratio amplitude (OUT)” indicates the air-fuel ratio amplitude after passing the OSC material).
[Conditions for measuring air-fuel ratio absorption rate]
Using an actual engine, the air-fuel ratio is oscillated at 14.5 ± 1.0 (1 Hz), and the air-fuel ratio absorption rate is measured when the temperature is raised at 30 ° C./min.

  As shown in FIG. 8, it was found that the air-fuel ratio absorption rate was high in Example 1 and Comparative Example 3 as compared to Comparative Example 2. From this result, it was confirmed that GPF in which Pr is added to the OSC material can suppress the fluctuation of the air-fuel ratio, and the air-fuel ratio can be easily kept stoichiometric.

<D90>
FIG. 9 is a graph showing the particle size distribution of TWC of Example 6. As shown in FIG. 9, it was confirmed that D90 of the TWC particles is 5 μm or less. In addition, the particle size distribution was similarly measured according to the following measurement conditions also about the other Example and the comparative example. The obtained D90 was as shown in Table 1.
[Particle size distribution measurement conditions]
Device: Laser diffraction particle size distribution measuring device (manufactured by SHIMADZU, SALD-3100)
Measurement method: Laser scattering method

<Supported state>
FIG. 10 is a view showing a supported state of TWC in the partition wall of the GPF of Example 1. Specifically, it is a mapping diagram obtained by carrying out cross-sectional SEM observation and elemental analysis by EPMA according to the following conditions for the supported state of TWC in the partition walls of GPF according to Example 1. From this result, it was confirmed that when the average pore diameter of the partition wall is 15 μm or more and the D90 of the TWC is 5 μm or less, the TWC is uniformly supported in the partition wall.
In addition, it was confirmed that the TWC is uniformly supported in the partition similarly in the other examples in which the particle diameter of the TWC is 5 μm or less.
[EPMA measurement conditions]
Device: Electron probe micro analyzer (JE0L, JXA-8100)
Measurement conditions: acceleration voltage 15 KV, irradiation current 0.04 μA, pixel size 1 μm, data collection time per cell 38 ms, beam diameter 0.7 μm

<Relationship between D90 and pressure loss>
FIG. 11 is a view showing a relationship between D90 of TWC supported by GPFs of Examples 1 to 7 and pressure loss. As shown in FIG. 11, in Examples 1 to 6 in which D90 is 5 μm or less, while the pressure drop remains at a substantially constant low level, when D90 exceeds 5 μm as in GPF of Example 7 in which D90 is 8 μm. It was found that the pressure loss increased. From this result, it was confirmed that the D90 of TWC supported on GPF is preferably 5 μm or less.

<Relationship between mean pore size and pressure loss>
FIG. 12 is a graph showing the relationship between the pressure loss and the average pore diameter of the partition walls of GPF in Example 1 and Examples 8 and 9. As shown in FIG. 12, it was found that the pressure loss slightly increased as the average pore size decreased, but the pressure loss of the GPF of Example 8 having an average pore size of 16 μm remained at a low level. From this result, it was confirmed that the average pore diameter of GPF is preferably 15 μm or more.

<Relationship between washcoat (WC) amount and pressure loss>
FIG. 13 is a view showing the relationship between the washcoat amount of TWC and the pressure loss in Example 1 and Examples 10-13. As shown in FIG. 13, it was found that the pressure drop increased as the washcoat amount increased, but the pressure loss of the GPF of Example 13 having a washcoat amount of 80 g / L remained at a low level. From this result, it was confirmed that the washcoat amount of TWC is preferably 80 g / L or less.

<Relationship between wall thickness and pressure loss>
FIG. 14 is a view showing the relationship between GPF wall thickness and pressure loss in Example 1 and Examples 17 and 18. As shown in FIG. 14, it was found that the pressure loss increased as the wall thickness increased, but the pressure loss of the GPF of Example 18 having a wall thickness of 10 mils remained at a low level. From this result, it was confirmed that the wall thickness of GPF is preferably 10 mil or less.

<Purification performance with or without Pd>
FIG. 15A and FIG. 15B are diagrams showing the relationship between the air-fuel ratio and the purification rates of CO, HC, and NO _x in the GPFs of Example 1 and Example 19, respectively. In the figure, the vertical axis represents CO, HC, the purification rate of the NO _x, respectively, the horizontal axis represents the air-fuel ratio which is the ratio of fuel and air. The term "stoichiometric" refers to a region where the air-fuel ratio is approximately 14.5.
The TWC supported by the GPF of Example 1 contains Rh and Pd, and the TWC supported by the GPF of Example 19 contains only Rh. Evaluation conditions were performed according to the following conditions.
From the evaluation results of FIG. 15A and FIG. 15B, compared with GPF of Example 1 in which Rh and Pd are included, GPF of Example 19 in which only Rh is included has a low HC purification rate in a region where the air fuel ratio is higher than stoichiometry. I found that. From these results, it was confirmed that Example 1 in which Rh and Pd were used in combination had higher three-way purification performance than Example 19 in which Rh alone was used as the TWC supported on GPF.
[HC, CO, NO _x purifying performance evaluation condition]
By using an actual engine, the catalyst inlet temperature 500 ° C. continuously changed in 20 minutes air-fuel ratio from 13.5 to 15.5, the measured HC, CO, the purification rate of NO _x.

<Purification performance with or without Ba>
FIGS. 15C and 15D are diagrams showing the relationship between the air-fuel ratio and the purification rates of CO, HC, and NO _x in the GPFs of Examples 20 and 21, respectively.
The TWC supported by GPF of Example 20 contains solid Ba (Ba sulfate) together with Rh and Pd, and the TWC supported by GPF of Example 21 contains liquid Ba (Ba acetate and Ba together with Rh and Pd). Nitric acid Ba) is included. In addition, the TWC carried by the GPF of the above-mentioned Example 1 (FIG. 15A) contains Rh and Pd but does not contain Ba. This is referred to for comparison. Evaluation conditions were evaluated by the HC, CO, the same conditions as _the NO _x purification performance evaluation criteria.
From the evaluation results of FIGS. 15A, 15C and 15D, the GPFs of Examples 20 and 21 containing solid Ba and liquid Ba are in a region where the air-fuel ratio is lower than the stoichiometry as compared with the GPF of Example 1 containing no Ba. It was found that the NO _x purification rate was low. From this result, it was confirmed that Example 1 in which Ba is not contained in the TWC carried by GPF has high exhaust gas purification performance as compared with Examples 20 and 21 in which Ba is contained.

<Purification performance due to difference in total content of Nd and Pd>
16A to 16C show the total content of Nd and Pr contained in GPFs of Example 1, Example 14, Example 15, Example 16 and Comparative Example 4, respectively, and NO _x _T50, CO_T50, HC_T50. It is a figure which shows a relation. NO _x _T50, CO_T50, the HC_T50, respectively CO, HC, represents the temperature at which 50% of the NO _x is purified, shown on the vertical axis in FIG. The horizontal axis indicates the total content (mass%) of Nd and Pr in the composite oxide. The total content of Nd and Pr is 0, 6, 12, 14, 16% by mass in the order of Comparative Example 4, Example 14, Example 15, Example 1, and Example 16, respectively.
As shown in FIGS. 16A to 16C, GPFs of Example 1, Example 14, Example 15, and Example 16 were found to be that NO _x , CO, and HC were purified at a lower temperature than Comparative Example 4. The Therefore, in order to cause GPF to exhibit a three-way purification function in the present embodiment, the total content of Nd and Pr is preferably 10% by mass to 20% by mass, and more preferably 12% by mass to 16% by mass. That was confirmed.

1 ... Engine (internal combustion engine)
2 ... Exhaust gas purification system 3 ... Exhaust pipe (exhaust passage)
31 ... upstream catalytic converter (upstream three-way catalyst)
32 ... GPF (downstream filter)
33, 33a, 33b ... TWC (downstream three-way catalyst)
320 ... filter base material 323 ... partition wall 321 ... inflow side cell (cell)
322: Outgoing cell (cell)
324: Inlet side plugging portion 325: Outflow side plugging portion 51: LAF sensor (first air-fuel ratio sensor)
52 ... O ₂ sensor (second air-fuel ratio sensor)
6 ... ECU (air-fuel ratio controller, pre-stage air-fuel ratio setting means, operation amount determination means)

Claims (9)

An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine and generating a signal according to the air-fuel ratio of the exhaust;
An upstream catalytic converter provided downstream of a detection point of the air-fuel ratio sensor in the exhaust passage and having a catalyst for purifying exhaust gas;
A downstream filter provided on the downstream side of the upstream catalytic converter in the exhaust passage for capturing and purifying particulate matter in the exhaust;
The output signal of the air-fuel ratio sensor is used to control the air-fuel ratio of the mixture burned by the internal combustion engine so that the air-fuel ratio of the exhaust flowing into the downstream filter converges to a target value set near the stoichiometry An exhaust gas purification system for an internal combustion engine comprising a controller;
In the downstream filter, a plurality of cells extending from the inflow side end face to the outflow side end face of the exhaust gas are partitioned by porous partition walls, and the openings at the inflow side end face of these cells and the openings at the outflow side end face are alternately sealed. And a downstream three-way catalyst supported on the partition walls,
The downstream three-way catalyst includes a catalyst metal containing at least Rh and an OSC material having an oxygen storage / release capacity,
The OSC material of the downstream three-way catalyst contains a complex oxide having Nd and Pr in its crystal structure,
The upstream catalytic converter comprises: a honeycomb base material in which a plurality of cells extending from an inflow side end face to an outflow side end face of exhaust gas are partitioned by porous partition walls; and an upstream three-way catalyst supported on the partition walls of the honeycomb base material , And
The upstream three-way catalyst includes a catalyst metal and an OSC material having an oxygen storage / release capacity,
An exhaust gas purification system for an internal combustion engine, wherein the content of the OSC material per unit volume in the filter substrate is smaller than the content of the OSC material per unit volume in the honeycomb substrate. An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine and generating a signal according to the air-fuel ratio of the exhaust;
An upstream catalytic converter provided downstream of a detection point of the air-fuel ratio sensor in the exhaust passage and having a catalyst for purifying exhaust gas;
A downstream filter provided on the downstream side of the upstream catalytic converter in the exhaust passage for capturing and purifying particulate matter in the exhaust;
And a controller for driving a fuel injection valve provided in the internal combustion engine such that an output signal of the air-fuel ratio sensor converges to a predetermined target value.
In the downstream filter, a plurality of cells extending from the inflow side end face to the outflow side end face of the exhaust gas are partitioned by porous partition walls, and the openings at the inflow side end face of these cells and the openings at the outflow side end face are alternately sealed. And a downstream three-way catalyst supported on the partition walls,
The downstream three-way catalyst includes a catalyst metal containing at least Rh and an OSC material having an oxygen storage / release capacity,
The OSC material of the downstream three-way catalyst contains a complex oxide having Nd and Pr in its crystal structure,
The upstream catalytic converter comprises: a honeycomb base material in which a plurality of cells extending from an inflow side end face to an outflow side end face of exhaust gas are partitioned by porous partition walls; and an upstream three-way catalyst supported on the partition walls of the honeycomb base material , And
The upstream three-way catalyst includes a catalyst metal and an OSC material having an oxygen storage / release capacity,
An exhaust gas purification system for an internal combustion engine, wherein the content of the OSC material per unit volume in the filter substrate is smaller than the content of the OSC material per unit volume in the honeycomb substrate.   The exhaust gas purification of an internal combustion engine according to claim 1, wherein the controller drives a fuel injection valve provided in the internal combustion engine such that an output signal of the air-fuel ratio sensor converges to a predetermined target value. system.   The controller operates the air-fuel ratio of the mixture burned by the internal combustion engine such that the air-fuel ratio of the exhaust flowing into the downstream filter converges in the vicinity of the stoichiometry by using the output signal of the air-fuel ratio sensor. The exhaust gas purification system for an internal combustion engine according to claim 2, characterized by The thickness of the partition wall of the filter substrate is larger than the thickness of the partition wall of the honeycomb substrate,
The porosity of the partition walls of the filter substrate is higher than the porosity of the partition walls of the honeycomb substrate,
The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 4, wherein a total number of cells formed in the filter substrate is smaller than a total number of cells formed in the honeycomb substrate. The partition walls of the filter substrate have an average pore diameter of 15 μm or more,
The internal combustion according to any one of claims 1 to 5, wherein the downstream three-way catalyst has a particle diameter D90 of 5 μm or less when the cumulative distribution from the small particle diameter side in the particle size distribution is 90%. Engine exhaust purification system.   The downstream three-way catalyst is supported on the inner surface of pores in the partition walls of the filter substrate in a state where Rh and Pd are contained as the catalytic metal and the Rh and Pd are mixed. An exhaust gas purification system for an internal combustion engine according to any one of 1 to 6.   The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 7, wherein the downstream three-way catalyst is configured without containing Ba.   The exhaust gas purification system for an internal combustion engine according to any one of claims 1 to 8, wherein the total content of Nd and Pr contained in the composite oxide of the downstream three-way catalyst is 10% by mass or more. .

Images