JP2019194478A - Exhaust emission control system for internal combustion engine - Google Patents

Abstract

To provide an exhaust emission control system for an internal combustion engine capable of exhibiting excellent exhaust emission control performance while inhibiting increase in pressure loss in the entire system in the exhaust emission control system in which a catalytic converter and a GPF are disposed in an exhaust passage in series.SOLUTION: An exhaust emission control system 2 includes: an LAF sensor 51; an upstream catalytic converter 31 provided downstream of a detection point of the LAF sensor 51; a GPF 32 provided downstream of the upstream catalytic converter 31; and an ECU 6 for controlling an air-fuel ratio of an air fuel mixture in an engine 1 by using an output signal KACT from the LAF sensor 51, so that an air-fuel ratio of exhaust gas flowing into the GPF 32 converges to a rear stage target value set in the vicinity of a stoichiometric air-fuel ratio. The GPF 32 includes a filter base material and a downstream TWC supported on a partition wall of the filter base material. The downstream TWC includes: a catalyst metal at least including Rh; and composite oxide having oxygen occlusion/emission capacity and Nd and Pr in a crystal structure.SELECTED DRAWING: Figure 1

Description

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

  Conventionally, in a gasoline engine mounted on an automobile or the like, a direct injection gasoline engine is employed from the viewpoint of improving combustion efficiency. However, in this direct injection gasoline engine, particulate matter (hereinafter referred to as “PM”) is generated, and as a result of recent tightening of emission regulations, an exhaust purification filter that traps PM in the exhaust passage of the gasoline engine. A technique for providing (Gasoline Particulate Filter (hereinafter, abbreviated as “GPF” in some cases)) is being studied.

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

  Thus, a technique has been proposed in which TWC is carried on a GPF filter base material to provide a three-way purification function by TWC in addition to the PM trapping function of the filter base material (see, for example, Patent Document 1). . According to this technology, it is expected that the problems of pressure loss and cost can be solved by integrating the GPF and the TWC. That is, by adding the TWC function to the GPF, the number of catalytic converters to be provided in the exhaust passage can be reduced by that amount, so that the pressure loss and cost increase in the entire exhaust purification system can be suppressed. It is.

Special table 2013-500857 gazette

  Here, the problem of the exhaust purification system in which the catalytic converter and the GPF are arranged in series in the exhaust passage will be examined in detail. If the GPF function is added to the GPF as described above, it is expected that the number of TWCs necessary for achieving the required exhaust purification performance can be reduced. However, the filter base material used for GPF is provided with a seal unlike the honeycomb support used for the catalytic converter in order to ensure the PM capturing function. For this reason, the GPF filter base material has a high pressure loss without supporting TWC, and cannot support as much TWC as the honeycomb support. That is, a GPF with a TWC function cannot simply be a substitute for a catalytic converter. Therefore, in an exhaust purification system in which a catalytic converter and a GPF are arranged in series, in order to ensure exhaust purification performance of the entire system, a large amount of TWC must be carried on the GPF filter base material. There is concern about an increase in pressure loss throughout the system.

In this way, if the GPF filter base material is made to carry TWC, it is difficult to achieve both pressure loss and exhaust purification performance. For this reason, development of TWC suitable for a filter base material is desired. For example, in the technique of Patent Document 1, in the case of using the TWC containing the Rh as a catalytic metal, in particular there is a case where _the NO _x purification performance is 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 ternary purification function. However, if a TWC having such a two-layer structure is supported on a GPF filter base, Incurs pressure loss. For this reason, it is conceivable that the filter base material carries a TWC having a single layer structure obtained by mixing Rh and Pd. However, in this case, Ba that is normally added to the Pd layer in order to suppress the deterioration of Pd and improve the NO _x adsorption property comes into contact with or close to Rh. Then, Rh is oxidized to an oxide by electron donating effect of Ba, results _the NO _x reduction ability of Rh is reduced, not much can be achieved larger _the NO _x purification performance in GPF, there is a problem that.

  The present invention has been made in view of the above, and an object thereof is excellent in 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 purification system for an internal combustion engine that can exhibit exhaust purification performance.

(1) An exhaust purification system (for example, an exhaust purification system 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later) is provided in an exhaust passage (for example, an exhaust pipe 3 to be 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 corresponding to the air-fuel ratio, and a downstream side of the detection location of the first air-fuel ratio sensor in the exhaust passage, purify exhaust. An upstream catalytic converter having a catalyst (for example, an upstream catalytic converter 31 described later), and a second air-fuel ratio sensor provided on the downstream side of the upstream catalytic converter in the exhaust passage and generating a signal corresponding to the air-fuel ratio of the exhaust (For example, an O ₂ sensor 52 to be described later) and a downstream filter that is provided on the downstream side of the detection location of the second air-fuel ratio sensor in the exhaust passage and captures and purifies particulate matter in the exhaust. (For example, GPF 32 described later) and the output signal of the first air-fuel ratio sensor and the output signal of the second air-fuel ratio sensor, the air-fuel ratio of the exhaust gas flowing into the downstream filter is stoichiometric (that is, complete combustion reaction) An air-fuel ratio controller (for example, ECU 6 to be described later) that operates the air-fuel ratio of the air-fuel mixture burned in 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 exhaust inflow end surface to the outflow end surface are partitioned by porous partition walls, and the openings at the inflow end surface and the outflow end surface of these cells are alternately sealed. And a downstream three-way catalyst (for example, TWC33, 33a, 33b, which will be described later) supported on the partition wall, and the downstream three-way catalyst includes a catalytic metal containing at least Rh, and oxygen storage / release. And the OSC material of the downstream three-way catalyst includes a composite oxide having Nd and Pr in the crystal structure thereof, and the upstream catalytic converter is configured so that the exhaust catalytic converter A plurality of cells extending to the end face, each of which is formed by a porous partition wall; and an upstream three-way catalyst supported on the partition wall of the honeycomb substrate. The catalyst includes a catalytic metal and an OSC material having oxygen storage / release capability, and the content of the OSC material per unit volume in the filter base material is greater than the content of the OSC material per unit volume in the honeycomb base material. There are few.

  (2) In this case, the air-fuel ratio controller sets a preceding target value for the output signal of the first air-fuel ratio sensor so that the output signal of the second air-fuel ratio sensor converges to the subsequent target value. Setting means; and an operation amount determining means for determining an operation amount for operating the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine so that the output signal of the first air-fuel ratio sensor becomes the preceding target value. It is preferable to provide.

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

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

  (5) In this case, the downstream three-way catalyst includes Rh and Pd as the catalyst metal, and is supported on the pore inner surface in the partition wall of the filter base material in a state where these Rh and Pd are mixed. Is preferred.

  (6) In this case, it is preferable that the downstream three-way catalyst is 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 exhaust gas through a porous partition wall, a downstream three-way catalyst supported on the partition wall is a catalyst containing at least Rh. A metal and an OSC material having an oxygen storage capacity (Oxygen Storage Capacity) including a composite oxide having Nd and Pr in its crystal structure are included. 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 a characteristic that the amount of acid sites is large as will be described in detail later. Therefore, the complex oxide having Nd and Pr in the crystal structure has a high amount of acid sites, and thus has a high HC adsorption capacity, and the 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 oxidation of Rh constituting the downstream three-way catalyst is suppressed by the generated hydrogen. That is, since it is possible to avoid deterioration of the NO _x reducing ability of Rh, can exhibit high _the NO _x purification performance. Therefore, in the present invention, by using a downstream three-way catalyst that can exhibit an excellent three-way purification function as the filter base material of the downstream filter, an increase in the pressure loss of the GPF 32 is suppressed while exhibiting a sufficient three-way purification function. be able to.

  Here, the three-way catalyst generally includes an OSC material in order to suppress fluctuations in the air-fuel ratio, in addition to the catalyst metal for exhibiting the three-way purification function. For this reason, in order to suppress the increase in the pressure loss in the downstream filter as much as possible, the content of the OSC material per unit volume in the filter base material of the downstream filter is set to the OSC material per unit volume in the honeycomb base material of the upstream catalytic converter. It is preferable to make it less than the content of. In this regard, as will be described later, the amount of hydrogen generated by the steam reforming reaction is higher in Nd than in Pr, but Pr has an effect of absorbing the change in the air-fuel ratio. Therefore, as the OSC material contained in the downstream three-way catalyst used for the downstream filter, a high three-way purification function while suppressing fluctuations in the air-fuel ratio by using a material containing a complex oxide having Nd and Pr in the crystal structure. Thus, the content of the OSC material in the filter base material can be reduced. As described above, in the present invention, the pressure loss in the entire system is achieved by using the downstream three-way catalyst suitable for the filter base material and further reducing the content of the OSC material in the downstream filter as compared with the upstream catalytic converter. Exhaust purification performance can be demonstrated while suppressing the increase of

  In addition to this, the present invention is provided on the upstream side of each of the upstream catalytic converter and the downstream filter in order to avoid the deterioration of the three-way purification function in the downstream filter caused by reducing the content of the OSC material used for the downstream filter. Using the output signals of the first and second air-fuel ratio sensors, the air-fuel mixture burned in the internal combustion engine is converged so that the air-fuel ratio of the exhaust gas flowing into the downstream filter converges to the target value set later in the vicinity of the stoichiometric ratio. Manipulate the air / fuel ratio. 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, in the present invention, the content of the OSC material is appropriately distributed between the upstream catalytic converter and the downstream filter, so that the air-fuel ratio of the exhaust gas flowing into the downstream filter is stabilized, and further, two air-fuel ratio sensors. To reduce the content of the OSC material in the downstream filter by manipulating 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 the stoichiometry. Therefore, it is possible to achieve both suppression of an increase in pressure loss due to and improvement of exhaust 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 in the vicinity of the stoichiometry. Set. Then, an operation amount for operating the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine is determined so that the output signal of the first air-fuel ratio sensor becomes the previous target value. As a result, in the upstream catalytic converter, the air-fuel ratio of the exhaust gas flowing into the downstream filter converges to the subsequent target value set in the vicinity of the stoichiometry, that is, so that the three-way purification function in the downstream three-way catalyst continues to be exhibited. The pre-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, dead time, and the like.

  (3) FIG. 17 is a diagram showing the relationship between the wall thickness, porosity, and cell number, which are parameters characterizing the filter base material of the downstream filter, and the PM trapping function and pressure loss performance by the downstream filter. First, in the present invention, the thickness of the partition walls of the filter base material is made larger than the thickness of the partition walls of the honeycomb base material. Thereby, the PM trapping function to the extent required for the downstream filter can be achieved. However, as shown in FIG. 17, increasing the partition wall of the filter base material improves the PM trapping function, but increases the pressure loss. Therefore, in the present invention, the porosity of the partition walls of the filter base material is set higher than the porosity of the partition walls of the honeycomb base material. Thereby, the increase in the pressure loss in a downstream filter can be suppressed. However, when the porosity is increased, the mechanical strength of the filter substrate decreases as shown in FIG. Therefore, in order to ensure sufficient mechanical strength of the filter substrate, the porosity cannot be increased excessively, and the pressure loss cannot be sufficiently reduced. Therefore, in the present invention, the total number of cells formed on the partition walls of the filter base material is made smaller than the total number of cells formed on the honeycomb base material. In the present invention, a sufficient PM trapping function, mechanical strength, and pressure loss performance can be achieved by setting the wall thickness, porosity, and number of cells as described above.

  (4) In the present invention, the average pore diameter of the partition walls of the filter base material is set to 15 μm or more, and the particle diameter D90 of the downstream three-way catalyst is set to 5 μm or less to make fine particles. As a result, the atomized 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 surfaces of the pores. Therefore, according to the present invention, it is possible to avoid an increase in the pressure loss of the downstream filter caused by the downstream three-way catalyst being supported only on the surface layer of the partition wall, and thus to exhibit a higher three-way purification function.

(5) In the present invention, the downstream three-way catalyst includes Rh and Pd, and is supported on the inner surface of the pores in the partition walls of the filter base material in a state where these Rh and Pd are mixed. As described above, conventionally, when Rh and Pd are mixed and supported on the downstream filter, Ba added to the conventional Pd layer comes into contact with or comes close to Rh. As a result, Rh is oxidized by the electron donating action of Ba. and oxides of Te, NO _x purifying performance was reduced significantly. According to the present invention, on the other hand, (1) results the effect of the invention is remarkably exhibited, it is possible to avoid the deterioration of the NO _x purification performance of Rh, it can exhibit an excellent three-way purification function than conventional A downstream filter can be provided.
In addition, it is difficult to support the conventional downstream three-way catalyst having the two-layer structure of the Rh layer and the Pd layer on the inner surface of the pores of the partition walls. According to the present invention, even in a state where Rh and Pd are mixed. Since a high ternary purification function is exhibited, the catalyst composition is preferable for supporting the partition walls on the pore inner surfaces.

(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 included in the complex oxide crystal structure in an amount of 10% by mass or more. Thereby, the more superior ternary purification performance is exhibited.

It is a figure showing composition of an engine exhaust gas purification system 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 schematic diagram of the partition of GPF which concerns on the said embodiment. It is a figure which shows the ease of the reduction | restoration of Rh by CO-TPR. 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. Is a diagram 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 factor. 6 is a graph showing a particle size distribution of TWC of Example 6. FIG. It is a figure which shows the support state of TWC in the partition of GPF of Example 1. FIG. 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 between the average pore diameter of the partition of GPF in Example 1 and Examples 8 and 9, and a pressure loss. It is a figure which shows the relationship between the washcoat amount of TWC and pressure loss in Example 1 and Examples 10-13. It is a figure which shows the relationship between the wall thickness of GPF in Example 1 and Examples 17 and 18, and a pressure loss. It is a figure which shows the relationship between the air fuel ratio in the GPF of Example 1, and a purification rate. It is a figure which shows the relationship between the air fuel ratio in GPF of Example 19, and a purification rate. It is a figure which shows the relationship between the air fuel ratio in the GPF of Example 20, and a purification rate. It is a figure which shows the relationship between the air fuel ratio in the GPF of Example 21, and a purification rate. 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 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, and the PM trapping 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 diagram showing a configuration of an internal combustion engine (hereinafter referred to as “engine”) 1 and its exhaust 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 are operated by signals from an ECU 6 described later. The ECU 6 determines a fuel injection mode such as a fuel injection amount and fuel injection timing in the fuel injection valve 11 by an air-fuel ratio control program, which will be 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 the exhaust pipe 3 of the engine 1, an O ₂ sensor 52 as a second air-fuel ratio sensor, and an exhaust pipe 3 GPF 32 as a downstream filter provided in the engine, and ECU 6 as an air-fuel ratio controller for operating the air-fuel ratio of the air-fuel mixture combusted in the engine 1 using the output signals of the LAF sensor 51 and the O ₂ sensor 52, and The exhaust of the engine 1 flowing through the exhaust pipe 3 is purified by the above. Below, after explaining the structure of the upstream catalytic converter 31 and GPF32 first, the function of the two sensors 51 and 52 and ECU6 is demonstrated.

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

The TWC of the upstream catalytic converter 31 includes an OSC material having an oxygen storage / release capability. As the OSC material, CeO ₂ , CeO ₂ and ZrO ₂ composite oxide (hereinafter referred to as “CeZr composite oxide”), and the like are used. Among these, CeZr composite oxide is preferably used because it has high durability. Note that the catalyst metal may be supported on these OSC materials.

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

  The GPF 32 is provided on the downstream side of the upstream catalytic converter 31 in the exhaust pipe 3. The GPF 32 captures and purifies PM in the exhaust. Specifically, when exhaust passes through fine pores in the partition walls, which will be described later, PM is trapped by PM being deposited 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 includes a filter base material 320. The filter base material 320 has, for example, a cylindrical shape that is long in the axial direction, and is formed of a porous material such as cordierite, mullite, or silicon carbide (SiC). The filter substrate 320 is provided with a plurality of cells extending from the inflow side end surface 32 a to the outflow side end surface 32 b, and these cells are partitioned by partition walls 323.

The filter base material 320 includes an inflow side sealing portion 324 that seals the inflow side end surface 32a. The cell in which the inflow side end surface 32a is sealed by the inflow side plugging portion 324 has an outflow side end portion that is closed while the outflow side end portion is open, and the exhaust gas that has passed through the partition wall 323 flows out downstream. A cell 322 is formed.
The inflow side sealing portion 324 is formed by enclosing a sealing cement from the inflow side end surface 32 a of the filter base material 320.

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

  In addition, the opening in the inflow side end surface 32a and the opening in the outflow side end surface 32b of the cell 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 (checkered pattern). Are arranged.

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, TWC 33 is supported on the inner surface of the pores in the partition wall 323. The TWC 33 includes a TWC 33a including Rh and a TWC 33b including Pd. These TWCs 33 are supported on the inner surfaces of the pores in the form of fine particles. Note that the pores of the partition wall 323 are not blocked by these 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. When the average pore diameter is 15 μm or more, the TWC 33 can enter the pore diameter in relation to the particle diameter of the TWC 33 described later, and the TWC 33 can be supported on the pore inner surface. 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 partition wall thickness exceeds 10 mil, the pressure loss may increase depending on the amount of TWC supported, the average pore diameter of the partition wall, and the like.

  The TWC 33 is atomized with 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%. When the D90 of the TWC 33 is 5 μm or less, the TWC 33 can enter the pore diameter in relation to the average pore diameter of the partition wall 323, and the TWC 33 can be supported on the pore inner surface. More preferable D90 is 3 μm or less.

  The TWC 33 includes at least Rh as a catalyst metal, and preferably includes Rh and Pd as catalyst metals as shown in FIG. These Rh and Pd may be supported on an OSC material having an oxygen storage / release capability described later, and are supported on a conventionally known carrier made of oxides such as alumina, silica, zirconia, titania, ceria, and zeolite. Also good.

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

The TWC 33 includes an OSC material having oxygen storage / release capability in addition to the catalyst metal as described above. As the OSC material used for the TWC 33, a material containing a complex oxide having Nd and Pr in its crystal structure is used. The OSC material used for the TWC 33 is made of a known material having oxygen storage / release capability such as CeO ₂ , ZrO _2, and these composite oxides in addition to such a composite oxide containing Nd and Pr. Also good.

In this embodiment, the composite oxide used as the OSC material together with the catalyst metal is supported in the partition wall 323.
The TWC used for the upstream catalytic converter 31 and the GPF 32 oxidizes HC in the exhaust gas to convert it to CO ₂ and H ₂ O, and oxidizes CO to CO ₂ while reducing NO _x to N _2. It has a function to do. In order to effectively produce the catalytic action for both reactions at the same time, it is preferable to keep the ratio of fuel to air (hereinafter referred to as “air-fuel ratio”) in the vicinity of stoichiometry.

The air-fuel ratio in an internal combustion engine such as an automobile varies greatly depending on the driving situation. For this reason, the ECU 6 controls the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter 31 and the GPF 32 so as to keep it close to the stoichiometry by performing air-fuel ratio control described later. However, simply controlling the air-fuel ratio in this way is not sufficient for the catalyst to exhibit purification performance.
Therefore, an OSC material having an oxygen storage / release capability of storing oxygen in an oxidizing atmosphere and releasing oxygen in a reducing atmosphere is used as a catalyst together with a catalyst metal. For example, CeO ₂ or a complex oxide of Ce and Zr is known as an OSC material.

The composite oxide used as the OSC material in the present embodiment has a structure in which part of Ce and Zr in the crystal structure of CeO ₂ and ZrO ₂ is substituted with Nd and Pr.
Nd and Pr have a high HC adsorption capacity, and a large amount of hydrogen is generated by a steam reforming reaction described later. Hydrogen to promote the reduction of Rh, improve _the NO _x purification performance of Rh.

  In the present embodiment, in addition to Nd, Pr, in which the amount of hydrogen generated by the steam reforming reaction is smaller than Nd, is also contained in the composite oxide structure. Since Pr has a function of absorbing fluctuations in the air-fuel ratio with respect to stoichiometry, the inclusion of Pr makes it easy to keep the air-fuel ratio in the vicinity of stoichiometry.

The CeZrNdPr composite oxide according to this 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. Then, a sodium hydroxide aqueous solution is dripped and pH of a solvent is set to 10, for example, and a precipitate is obtained. Thereafter, the solvent is evaporated by filtering the solution containing the precipitate under reduced pressure while being heated to 60 ° C., for example. Next, after the residue is extracted, calcination is performed, for example, at 500 ° C. for 2 hours in a muffle furnace to obtain a CeZrNdPr composite oxide.

Further, TWC33 of this embodiment has been added in view of the prior Pd degradation control and NO _x adsorbing improve, and is configured without the Ba.

  In TWC33 of this embodiment, it is preferable that the total content of Nd and Pr contained in the composite oxide is 10% by mass or more. When the total content of Nd and Pr contained in the composite oxide is within this range, a higher ternary purification function is exhibited. The upper limit 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 and Pd in TWC33 is not particularly limited, but is preferably Rh: Pd = 1: 10 to 1: 5 on a mass basis.

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

In the present embodiment, the TWC 33 may contain another noble metal such as Pt as the catalyst metal. Further, the GPF32, catalysts, for example the NO _x catalyst or oxidation catalyst, the Ag-based catalyst or the like to burn and remove the PM deposited in the GPF is carried having a function other than the three-way purification function to bulkhead or partition wall surface It may be.

Next, a method for manufacturing the GPF 32 according to this embodiment will be described.
The GPF 32 according to the present embodiment is manufactured by, for example, a dipping method. In the dipping method, for example, a slurry containing a predetermined amount of the constituent material of TWC33 is prepared by wet pulverization or the like, and after GPF32 is immersed in the prepared slurry, the GPF32 is pulled up and fired at a predetermined temperature condition. The TPF 33 can be supported on the GPF 32.

  In the present embodiment, it is preferable that a slurry prepared by mixing a catalyst such as Rh or Pd with a ball mill or the like is pulverized until the particle size becomes 5 μm or less and immersed in GPF 32 once. As a result, Rh and Pd can be carried in a state of being randomly mixed on the surface of the pores in the partition wall 323.

Next, the reason why the TWC according to the present embodiment preferably does not include Ba will be described with reference to FIG.
FIG. 4 is a diagram showing the ease of reduction of Rh by CO-TPR. Specifically, it is a diagram showing the results of measuring the ease of reduction of Rh according to the following procedure by CO-TPR (temperature rising reduction method) depending on the presence or absence of Ba added to TWC.
TWC was measured by preparing Rh on a Zr oxide at a ratio of 0.3% by mass and 3% by mass, respectively, and adding and not adding 10% by mass of Ba.

[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 raising the temperature to 800 ° C. at 10 ° C./min in 1% CO / N ₂ .
(4) The temperature was lowered to 600 ° C.
(5) Hold in 10% O ₂ / N ₂ at 600 ° C. for 10 minutes.
(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 due to the temperature of CO ₂ release was measured.

As shown in FIG. 4, it can be seen that the TWC containing Ba has a lower CO ₂ emission amount at a low temperature than the TWC not containing Ba. This means that Rh is difficult to be reduced, and Ba is considered to inhibit the reduction of Rh. Therefore, the TWC in this embodiment maintains the reduced 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 a catalyst metal is maintained in a reduced state in the presence of hydrogen, and the NO _x purification performance is improved. Therefore, in this embodiment, a steam reforming reaction is used. The steam reforming reaction is a reaction represented by the following formula in which water vapor and HC react with each other in the presence of a catalyst at a high temperature to generate hydrogen.

_{C n H m + nH 2 O} → nCO + (n + 1 / 2m) H 2

In order to improve the amount of hydrogen generated by the steam reforming reaction, the HC adsorption capacity is important, and it is considered that the HC adsorption capacity depends on the acid point.
FIG. 5 is a diagram showing the amount of acid sites of each composite oxide by NH ₃ -TPD. Specifically, the amount of acid sites of Y, La, Pr, and Nd, which can be included in the crystal structure of a complex oxide of Ce or Zr, is NH ₃ -TPD (temperature reduction). It is a figure which shows the result measured by the following procedure by method.

_[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 in 0.1% NH ₃ / He for 60 minutes, it was held in He for 60 minutes.
(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 sites than Y and La. Therefore, it can be said from this result that Nd and Pr have high HC adsorption ability.

  FIG. 6 is a graph comparing the amount of hydrogen generated 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 addition, content of each element of Y, La, Pr, and Nd at this time is 7 mass%, Ce content is 41 mass%, Zr content is 52 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 such that a porous base material carries a TWC that includes a catalytic metal that generates a three-way purification function and an OSC material that has an oxygen storage / release capability. It is common in the point to be done. Here, the base material and TWC used for the upstream catalytic converter 31 are not limited to those described above, and the same materials as those 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 near the stoichiometric control 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 [g / L] in the base material), the supported amount of the three way catalyst (more specifically, the three way catalyst per unit volume in the base material The supported amount [g / L]), the cell structure of the substrate, and the porosity of the substrate 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 base material of the GPF 32 is preferably smaller than the content of the OSC material per unit volume in the honeycomb base material 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, more preferably about 0.35.

  Further, the amount of TWC 33 supported per unit volume [g / L] on the filter base material of GPF 32 is preferably smaller than the amount of TWC supported per unit volume [g / L] on the honeycomb base material of upstream catalytic converter 31. . More specifically, when the amount of TWC supported by the upstream catalytic converter 31 is 200 [g / L], the amount of TWC 33 supported by 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 base material of the GPF 32 is preferably larger than the thickness of the partition wall of the honeycomb base material of the upstream catalytic converter 31. More specifically, when the wall thickness of the GPF 32 is 8 mil, the wall thickness of the upstream catalytic converter 31 is preferably 3.5 mil.

  The porosity of the partition walls of the filter base material of the GPF 32 is preferably higher than the porosity of the partition walls of the honeycomb base material 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 on the filter base material of the GPF 32 is preferably smaller than the total number of cells formed on the honeycomb base material 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. By configuring the wall thickness, porosity, and total number of cells as described above, a sufficient PM trapping function, mechanical strength, and pressure loss performance in the GPF 32 can be achieved.

Next, 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 (ratio of fuel component to 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, the characteristics of the output signals of the LAF sensor 51 and the O ₂ sensor 52 will be compared. The LAF sensor 51 generates a signal having 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 between a rich region and a lean region, 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 proportional to the oxygen concentration of the exhaust when the oxygen concentration of the exhaust is within the range Δ near the stoichiometric range. That is, the level of the signal output from the O ₂ sensor 52 has a substantially binary characteristic that reverses from low to high in the vicinity of the 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 near the stoichiometric range.

  The ECU 6 forms an input signal waveform from various sensors such as the sensors 51 and 52, corrects the voltage level to a predetermined level, and converts an analog signal value into a digital signal value. A central processing unit that executes various control programs such as air-fuel ratio control described in the above, and a drive circuit that 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. The ECU 6 executes an air-fuel ratio control program including a target air-fuel ratio calculation and a fuel injection amount calculation using the output signal KACT of the LAF sensor 51 and the output signal VO2 of the O ₂ sensor 52, thereby causing the air-fuel mixture to be burned in the engine 1. The fuel injection amount from the fuel injection valve 11 that 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 a high three-way purification function can be exhibited in each of the TWC of the upstream catalytic converter 31 and the TWC of the GPF 32. Thus, the target air-fuel ratio KCMD is determined. More specifically, in the target air-fuel ratio calculation, the ECU 6 is a model that includes at least a response delay element and a dead time element for the control system P from the output signal KACT of the LAF sensor 51 to the output signal VO2 of the O ₂ sensor 52. And a target air-fuel ratio KCMD that achieves the above-described object is determined by using operations in an adaptive sliding mode controller, a real-time identifier, and a state predictor described below.

First, by using the output signal KACT of the LAF sensor 51 and the output signal VO2 of the O ₂ sensor 52, the real-time identifier sequentially generates identification values of a plurality of model parameters defined by the above model. The state predictor sequentially generates an output after the dead time of the control system P, that is, an estimated value of the output signal VO2 of the O ₂ sensor 52 after the dead time. The adaptive sliding mode controller is configured so that the output signal VO2 of the O ₂ sensor 52 converges to a predetermined subsequent target value set in the vicinity of the stoichiometry so that a high three-way purification function is exhibited in the TWC 33 of the GPF 32. A 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 No. 2000-230451 and Japanese Patent Application Laid-Open No. 2001-182528 by the applicant of the present application. Detailed description is omitted.

According to this embodiment, the following effects are produced.
In the present embodiment, in the so-called wall flow type GPF 32, the TWC 33 supported on the partition wall 323 is a composite oxide having Nd and Pr in the crystal structure as a catalytic metal containing at least Rh and an OSC material having oxygen storage / release capability. 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 a characteristic that the amount of acid sites is large. Therefore, the complex oxide having Nd and Pr in the crystal structure has a high amount of acid sites, and thus has a high HC adsorption capacity, and the 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 oxidation of Rh constituting the TWC 33 is suppressed by the generated hydrogen. That is, since it is possible to avoid deterioration of the NO _x reducing ability of Rh, can exhibit high _the NO _x purification performance. Therefore, in the present invention, by using TWC33 capable of exhibiting an excellent ternary purification function as a filter base material for GPF 32, an increase in the pressure loss of GPF 32 can be suppressed while exhibiting a sufficient ternary purification function.

  As described above, the TWC includes the OSC material. For this reason, in order to suppress the increase in pressure loss in the GPF 32 as much as possible, the content of the OSC material per unit volume in the filter base material of the GPF 32 is set to the amount of the OSC material per unit volume in the honeycomb base material of the upstream catalytic converter 31. It is preferable to make it less than the content. In this regard, as described above, the amount of hydrogen generated by the steam reforming reaction is higher in Nd than in Pr, but Pr has an effect of absorbing the change in the air-fuel ratio. Therefore, as the OSC material contained in the TWC 33 used for the GPF 32, a material containing a complex oxide having Nd and Pr in the crystal structure is used, while exhibiting a high three-way purification function while suppressing fluctuations in the air-fuel ratio. The content of the OSC material in the filter substrate can be reduced. As described above, in this embodiment, the TWC 33 suitable for the filter base material is used, and further, the pressure loss in the entire exhaust purification system is reduced by making the content of the OSC material of the GPF 33 smaller than that of the upstream catalytic converter 31. Exhaust purification performance can be demonstrated while suppressing the increase of

In addition to this, in the present embodiment, in order to avoid a decrease in the three-way purification function in the GPF 32 resulting from reducing the content of the OSC material used for the GPF 32, the upstream catalytic converter 31 and the GPF 32 are provided on the upstream side. 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 gas flowing into the GPF 32 converges to the target value at the rear stage set near the stoichiometry. Manipulate. First, in this 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 gas flowing into the GPF 32 can be stabilized. As described above, in the present embodiment, the OSC material content is appropriately distributed between the upstream catalytic converter 31 and the GPF 32 to stabilize the air-fuel ratio of the exhaust gas flowing into the GPF 32, and further to the LAF sensor 51 and the OF. By operating the air / fuel ratio of the air-fuel mixture so that the air / fuel ratio of the exhaust gas is converged to the target value set in the vicinity of the stoichiometric value in the GPF 32 using the _two sensors 52, the content of the OSC material in the GPF 32 is reduced. Therefore, it is possible to achieve both suppression of an increase in pressure loss and improvement of exhaust purification performance.

Further, in the present embodiment, the target air-fuel ratio KCMD with respect to the output signal KACT of the upstream LAF sensor 51 of the upstream catalytic converter 31 so that the output signal VO2 of the O ₂ sensor 52 converges to the target value of the subsequent stage set in the vicinity of the stoichiometry. Set. Then, a fuel injection amount that is an operation amount for operating the air-fuel ratio of the air-fuel mixture burned by the engine 1 is determined so that the output signal KACT of the LAF sensor 51 becomes the target air-fuel ratio KCMD. As a result, the LAF sensor 51 in the upstream catalytic converter 31 is set so that the air-fuel ratio of the exhaust gas flowing into the GPF 32 converges to the subsequent target value set in the vicinity of the stoichiometric condition, that is, so that 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 to the output signal of the O ₂ sensor 52, dead time, and the like.

  Further, in this embodiment, the thickness of the partition wall of the filter base material of the GPF 32 is made larger than the thickness of the partition wall of the honeycomb base material of the upstream catalytic converter 31. Thereby, it is possible to achieve the PM capturing function to the extent required by the GPF 32. However, as shown in FIG. 17, increasing the partition wall of the filter base material improves the PM trapping function, but increases the pressure loss. Therefore, in this embodiment, the porosity of the partition walls of the filter base material is made higher than the porosity of the partition walls of the honeycomb base material. Thereby, the increase in the pressure loss in GPF32 can be suppressed. However, when the porosity is increased, the mechanical strength of the filter substrate decreases as shown in FIG. Therefore, in order to ensure sufficient mechanical strength of the filter substrate, the porosity cannot be increased excessively, and the pressure loss cannot be sufficiently reduced. Therefore, in this embodiment, the total number of cells formed on the partition walls of the filter base material is made smaller than the total number of cells formed on the honeycomb base material. In the present embodiment, sufficient PM trapping function, mechanical strength, and pressure loss performance can be achieved by setting the wall thickness, porosity, and total number of cells of GPF 32 as described above.

  In this embodiment, the average pore diameter of the partition walls 323 is set to 15 μm or more, and the particle diameter D90 of the TWC 33 is set to 5 μm or less to form fine particles. Thereby, the finely divided TWC 33 can be introduced into the pores of the partition walls 323, and the TWC 33 can be supported on the inner surfaces of the pores. Therefore, according to the present embodiment, it is possible to avoid an increase in the pressure loss of the GPF 32 caused by the TWC 33 being supported only on the surface layer of the partition wall 323, and thus to exhibit a higher ternary purification function.

In the present embodiment, the TWC 33 is configured to include Rh and Pd, and is supported on the inner surface of the pores in the partition wall 323 in a state where these Rh and Pd are mixed. As described above, conventionally, when Rh and Pd are mixed and supported on the GPF 32, Ba added to the Pd layer comes into contact with or close to Rh, and as a result, Rh is oxidized and oxidized by Ba's electron donating action. and Monoka, NO _x purifying performance was reduced significantly. According to the present embodiment, on the other hand, the steam reforming reaction described above can avoid deterioration of the NO _x purification performance of Rh, can provide GPF32 capable of exhibiting excellent three-way purification function than before.
In addition, it is difficult to support the conventional TWC having the two-layer structure of the Rh layer and the Pd layer on the inner surface of the pores of the partition wall. Since the original purification function is exhibited, the catalyst composition is preferable for supporting the partition walls 323 on the inner surfaces of the pores.

In the present embodiment, the TWC 33 is configured without including 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 included in the composite oxide crystal structure in an amount of 10% by mass or more. Thereby, the more superior ternary purification performance is exhibited.

  It should be noted that the present invention is not limited to the above-described embodiment, and modifications, improvements, etc. within a scope that can achieve the object of the present invention are included in the present invention.

  Next, examples of the GPF 32 of the exhaust purification system 2 according to the above embodiment will be described, but the GPF 32 is not limited to the following examples.

<Examples 1-21, Comparative Examples 1-4>
TWC, carrier, composite oxide and the like were prepared by the following procedure at the ratio shown in Table 1.
First, an aqueous medium and an additive were added and then mixed in a ball mill to form a slurry. 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 dipping. The carrying amount (wash coat amount) was 60 g / L (excluding Examples 10 to 13). Then, GPF carrying TWC was obtained by baking at 700 ° C. for 2 hours.
In addition, as GPF, a honeycomb structure made of NGK (inner diameter 25.4 (φ1 inch) mm, average pore diameter 20 μm (excluding Examples 8 and 9), wall thickness 8 mil (excluding Examples 17 and 18), 300 cells, material cordierite, capacity 15 cc) were used.

<NO _x purification performance>
FIG. 7 is a graph 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 Example 1 was purifying NO _{x at} a lower temperature than Comparative 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 a rate of 20 ° C./min in 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, the air-fuel ratio absorption rate of GPF was measured 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. 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 rate (%) = ((air-fuel ratio amplitude (IN) −air-fuel ratio amplitude (OUT)) ÷ air-fuel ratio amplitude (IN)) × 100
... Formula (1)

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

  As shown in FIG. 8, Example 1 and Comparative Example 3 were found to have a higher air-fuel ratio absorption rate than Comparative Example 2. From this result, it was confirmed that the GPF obtained by adding Pr to the OSC material can suppress the fluctuation of the air-fuel ratio and easily keep the air-fuel ratio at stoichiometry.

<D90>
FIG. 9 is a graph showing the particle size distribution of the TWC of Example 6. As shown in FIG. 9, it was confirmed that D90 of the TWC particles is 5 μm or less. The particle size distribution was measured in the same manner for other examples and comparative examples according to the following measurement conditions. The obtained D90 was as shown in Table 1.
[Particle size distribution measurement conditions]
Apparatus: Laser diffraction type particle size distribution measuring apparatus (manufactured by SHIMADZU, SALD-3100)
Measuring method: Laser scattering method

<Supported state>
FIG. 10 is a diagram illustrating a state in which the TWC is supported in the partition wall of the GPF of the first embodiment. Specifically, it is a mapping diagram obtained by carrying out cross-sectional SEM observation and elemental analysis with EPMA according to the following conditions for the state of TWC support in the partition walls of the GPF according to Example 1. From this result, it was confirmed that when the average pore diameter of the partition walls is 15 μm or more and T90 D90 has a particle diameter of 5 μm or less, the TWC is uniformly supported in the partition walls.
In addition, it was confirmed that TWC was uniformly supported in the partition wall in other examples in which the particle diameter of TWC was 5 μm or less.
[EPMA measurement conditions]
Apparatus: Electronic probe microanalyzer (JELA, 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 diagram showing a relationship between T90 D90 carried by the 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, the pressure loss remains at a substantially constant low level, whereas when D90 exceeds 5 μm as in the 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 D90 of TWC carried on GPF is preferably 5 μm or less.

<Relationship between average pore diameter and pressure loss>
FIG. 12 is a graph showing the relationship between the average pore diameter of the GPF partition walls of Example 1 and Examples 8 and 9, and the pressure loss. As shown in FIG. 12, the pressure loss slightly increased as the average pore diameter decreased, but the pressure loss of the GPF of Example 8 having an average pore diameter 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 amount of washcoat (WC) and pressure loss>
FIG. 13 is a graph showing the relationship between the TWC washcoat amount and the pressure loss in Examples 1 and 10-13. As shown in FIG. 13, the pressure loss increased as the washcoat amount increased, but it was found that the pressure loss of the GPF of Example 13 in which the washcoat amount was 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 diagram showing the relationship between the wall thickness of the GPF of Example 1 and Examples 17 and 18, and the pressure loss. 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 and without Pd>
15A and 15B are diagrams showing the relationship between the purification rate of the air-fuel ratio and the respective CO, HC, NO _x in the GPF of Example 1 and Example 19. In the figure, the vertical axis indicates the CO, HC, and NO _x purification rates, and the horizontal axis indicates the air-fuel ratio that is the ratio of fuel to air. Note that stoichiometry indicates a region where the air-fuel ratio is about 14.5.
The TWC supported by the GPF of Example 1 includes Rh and Pd, and the TWC supported by the GPF of Example 19 includes only Rh. Evaluation conditions were performed according to the following conditions.
From the evaluation results of FIGS. 15A and 15B, compared with the GPF of Example 1 including Rh and Pd, the GPF of Example 19 including only Rh has a low HC purification rate in a region where the air-fuel ratio is higher than stoichiometric. I understood that. From this result, it was confirmed that Example 1 combined with Rh and Pd had higher ternary purification performance than Example 19 using Rh alone as the TWC supported by GPF.
[HC, CO, NO _x purification performance evaluation conditions]
Using an actual engine, the purification ratio of HC, CO, and NO _x was measured by continuously changing the air-fuel ratio from 13.5 to 15.5 in 20 minutes at a catalyst inlet temperature of 500 ° C.

<Purification performance with or without Ba>
FIGS. 15C and 15D are diagrams showing the relationship between the purification rate of the air-fuel ratio and the respective CO, HC, NO _x in the GPF of Examples 20 and 21.
The TWC supported on the GPF of Example 20 includes solid Ba (sulfuric acid Ba) together with Rh and Pd, and the TWC supported on the GPF of Example 21 includes liquid Ba (acetic acid Ba and Nitric acid Ba) is included. Further, the TWC carried by the GPF of the first embodiment (FIG. 15A) includes Rh and Pd but does not include Ba. This is referred to for comparison. Evaluation conditions were evaluated under the same conditions as the HC, CO, and NO _x purification performance evaluation conditions.
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 stoichiometric value compared with the GPF of Example 1 not containing Ba. _the NO _x purification rate is found to be low. From this result, it was confirmed that Example 1 in which Ba is not included in the TWC supported by the GPF has higher exhaust purification performance than Examples 20 and 21 in which Ba is included.

<Purification performance due to difference in total content of Nd and Pd>
16A to 16C show the total contents of Nd and Pr contained in the GPFs of Example 1, Example 14, Example 15, Example 16, and Comparative Example 4, respectively, and NO _x _T50, CO_T50, and HC_T50. It is a figure which shows a relationship. NO _x _T50, CO_T50, and HC_T50 indicate temperatures at which 50% of CO, HC, and NO _x are purified, and are indicated on the vertical axis in the figure. The horizontal axis represents the total content (% by mass) of Nd and Pr in the composite oxide. The total contents of Nd and Pr are 0, 6, 12, 14, and 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, it can be seen that the GPFs of Example 1, Example 14, Example 15, and Example 16 purify NO _x , CO, and HC at a lower temperature than Comparative Example 4. It was. Therefore, in order for GPF to exhibit the ternary purification function in this 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. It was confirmed.

1. Engine (internal combustion engine)
2. Exhaust 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 ... Outflow side cell (cell)
324: Inflow side sealing portion 325 ... Outflow side sealing 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 (6)

An air-fuel ratio sensor provided in an exhaust passage of the internal combustion engine and generating a signal corresponding to the air-fuel ratio of the exhaust;
An upstream catalytic converter having a catalyst for purifying exhaust gas provided downstream of the detection point of the air-fuel ratio sensor in the exhaust passage;
A downstream filter that is provided downstream of the upstream catalytic converter in the exhaust passage and captures and purifies particulate matter in the exhaust; and
An exhaust gas purification system for an internal combustion engine, comprising: an air-fuel ratio controller for controlling the air-fuel ratio of the exhaust gas flowing into the upstream catalytic converter so as to keep it in the vicinity of stoichiometry,
In the downstream filter, a plurality of cells extending from the exhaust inflow end surface to the outflow end surface are partitioned by porous partition walls, and the openings at the inflow end surface and the outflow end surface of these cells are alternately sealed. A filter base material, and a downstream three-way catalyst supported on the partition wall,
The downstream three-way catalyst includes a catalytic metal containing at least Rh, and an OSC material having an oxygen storage / release capability,
The OSC material of the downstream three-way catalyst includes a composite oxide having Nd and Pr in its crystal structure,
The upstream catalytic converter includes a honeycomb base material in which a plurality of cells extending from an exhaust inflow end surface to an outflow end surface are defined by porous partition walls, and an upstream three-way catalyst supported on the partition walls of the honeycomb base material. With
The upstream three-way catalyst includes a catalytic metal and an OSC material having oxygen storage / release capability,
The exhaust purification system of an internal combustion engine, wherein the content of the OSC material per unit volume in the filter base material is less than the content of the OSC material per unit volume in the honeycomb base material. The partition wall thickness of the filter substrate is greater than the partition wall thickness 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,
2. The exhaust gas purification system for an internal combustion engine according to claim 1, wherein the total number of cells formed on the filter base material is smaller than the total number of cells formed on the honeycomb base material. The partition wall of the filter base material has an average pore diameter of 15 μm or more,
The exhaust gas of an internal combustion engine according to claim 1 or 2, wherein the downstream three-way catalyst has a particle size D90 of 5 µm or less when the cumulative distribution from the small particle size side in the particle size distribution is 90%. Purification system.   The downstream three-way catalyst includes Rh and Pd as the catalyst metal, and is supported on an inner surface of a pore in a partition wall of the filter base material in a state where these Rh and Pd are mixed. The exhaust gas purification system for an internal combustion engine according to any one of 1 to 3.   The exhaust purification system for an internal combustion engine according to any one of claims 1 to 4, wherein the downstream three-way catalyst is configured without containing Ba. 6. The exhaust gas purification system for an internal combustion engine according to claim 1, 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. .

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