WO2017051458A1 - 内燃機関の排気浄化システム - Google Patents
内燃機関の排気浄化システム Download PDFInfo
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- WO2017051458A1 WO2017051458A1 PCT/JP2015/076947 JP2015076947W WO2017051458A1 WO 2017051458 A1 WO2017051458 A1 WO 2017051458A1 JP 2015076947 W JP2015076947 W JP 2015076947W WO 2017051458 A1 WO2017051458 A1 WO 2017051458A1
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- air
- fuel ratio
- exhaust
- gpf
- downstream
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D46/00—Filters or filtering processes specially modified for separating dispersed particles from gases or vapours
- B01D46/24—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies
- B01D46/2403—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies characterised by the physical shape or structure of the filtering element
- B01D46/2418—Honeycomb filters
- B01D46/2425—Honeycomb filters characterized by parameters related to the physical properties of the honeycomb structure material
- B01D46/2429—Honeycomb filters characterized by parameters related to the physical properties of the honeycomb structure material of the honeycomb walls or cells
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
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- B01D46/00—Filters or filtering processes specially modified for separating dispersed particles from gases or vapours
- B01D46/24—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies
- B01D46/2403—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies characterised by the physical shape or structure of the filtering element
- B01D46/2418—Honeycomb filters
- B01D46/2425—Honeycomb filters characterized by parameters related to the physical properties of the honeycomb structure material
- B01D46/24491—Porosity
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- B01D46/2403—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies characterised by the physical shape or structure of the filtering element
- B01D46/2418—Honeycomb filters
- B01D46/2425—Honeycomb filters characterized by parameters related to the physical properties of the honeycomb structure material
- B01D46/24492—Pore diameter
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- B01D46/2403—Particle separators, e.g. dust precipitators, using rigid hollow filter bodies characterised by the physical shape or structure of the filtering element
- B01D46/2418—Honeycomb filters
- B01D46/2451—Honeycomb filters characterized by the geometrical structure, shape, pattern or configuration or parameters related to the geometry of the structure
- B01D46/2474—Honeycomb filters characterized by the geometrical structure, shape, pattern or configuration or parameters related to the geometry of the structure of the walls along the length of the honeycomb
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- B01D53/34—Chemical or biological purification of waste gases
- B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
- B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
- B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
- B01D53/945—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC] characterised by a specific catalyst
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- B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
- B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
- B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
- B01D53/9477—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on separate bricks, e.g. exhaust systems
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- F01N3/033—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices
- F01N3/035—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters in combination with other devices with catalytic reactors, e.g. catalysed diesel particulate filters
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- F02B2075/125—Direct injection in the combustion chamber for spark ignition engines, i.e. not in pre-combustion chamber
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02T10/12—Improving ICE efficiencies
Definitions
- 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.
- 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.
- 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.
- 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.
- TWC suitable for a filter base material.
- TWC containing the Rh as a catalytic metal
- NO x purification performance is greatly reduced.
- 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.
- a TWC having such a two-layer structure is supported on a GPF filter base, Incurs pressure loss.
- the filter base material carries a TWC having a single layer structure obtained by mixing Rh and Pd.
- 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.
- 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.
- 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
- 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.
- 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).
- 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 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.
- 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.
- 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.
- 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.
- the partition wall of the filter base material has an average pore diameter of 15 ⁇ m or more
- the downstream three-way catalyst preferably has a particle size D90 of 5 ⁇ m or less when the cumulative distribution from the small particle size side in the particle size distribution is 90%.
- 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.
- downstream three-way catalyst is configured without containing Ba.
- 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.
- 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.
- Nd and Pr have a characteristic that the amount of acid sites is large as will be described in detail later.
- 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.
- the three-way catalyst 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.
- 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.
- the amount of hydrogen produced by the steam reforming reaction is higher in Nd than in Pr, but Pr has the effect of absorbing fluctuations in the air-fuel ratio.
- 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.
- the content of the OSC material in the filter base material can be reduced.
- the pressure loss in the entire system is obtained 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
- 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.
- 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.
- the upstream catalytic converter is provided with a larger amount of OSC material than the downstream filter.
- the air-fuel ratio of the exhaust flowing into the downstream filter can be stabilized.
- 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.
- 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.
- 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.
- FIG. 17 is a diagram showing the relationship between the wall thickness, porosity, and number of cells, 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.
- 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.
- the PM trapping function to the extent required for the downstream filter can be achieved.
- 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.
- the increase in the pressure loss in a downstream filter can be suppressed.
- 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.
- the average pore diameter of the partition walls of the filter base material is set to 15 ⁇ m or more, and the particle diameter D90 of the downstream three-way catalyst is set to 5 ⁇ m or less to make fine particles.
- 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.
- 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.
- 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.
- Rh is oxidized by the electron donating action of Ba. and oxides of Te, NO x purifying performance was reduced significantly.
- the catalyst composition is preferable for supporting the partition walls on the pore inner surfaces.
- the downstream three-way catalyst is configured without containing Ba.
- oxides of Rh is promoted by Ba NO x purification performance as described above can be avoided from being reduced.
- Nd and Pr are included in the composite oxide in a crystal structure of 10% by mass or more. Thereby, the more superior ternary purification performance is exhibited.
- FIG. 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.
- FIG. 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.
- 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.
- engine an internal combustion engine
- 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
- 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.
- 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).
- 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.
- OSC material CeO 2 , CeO 2 and ZrO 2 composite oxide (hereinafter referred to as “CeZr composite oxide”), and the like are used.
- 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.
- 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.
- 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.
- 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 ⁇ m 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 ⁇ m 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.
- TWC33 is micronized with a particle size D90 of 5 ⁇ m or less when the cumulative distribution from the small particle size side in the particle size distribution is 90%.
- D90 of the TWC 33 is 5 ⁇ m 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 ⁇ m or less.
- the TWC 33 contains at least Rh as a catalyst metal, and preferably contains 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.
- the TWC 33 is configured to include 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.
- OSC material used for the TWC 33 a material containing a complex oxide having Nd and Pr in its crystal structure is used. Note that the OSC material used in TWC33, in addition to the complex oxide having such a Nd and Pr, CeO 2, ZrO 2 and composite oxides thereof, etc., using the known materials having an oxygen storage and release capacity Also good.
- 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.
- air-fuel ratio the ratio of fuel to air
- the air-fuel ratio in an internal combustion engine such as an automobile varies greatly depending on the driving situation.
- 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.
- 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.
- 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.
- Pr which has a smaller amount of hydrogen produced by the steam reforming reaction than Nd, is contained in the structure of the composite oxide. 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.
- 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.
- the total content of Nd and Pr contained in the composite oxide is preferably 10% by mass or more.
- 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 amount of TWC 33 supported per unit volume (hereinafter, also referred to as “wash coat amount”) in the filter base material 320 of the GPF 32 is not particularly limited, but is preferably 40 to 80 g / L.
- wash coat amount 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.
- the TWC 33 may contain another noble metal such as Pt as the catalyst metal.
- 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.
- the GPF 32 according to the present embodiment is manufactured by, for example, a dipping method.
- 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.
- 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 is 5 ⁇ m or less and is immersed in GPF 32 once.
- Rh and Pd can be carried in a state of being randomly mixed on the surface of the pores in the partition wall 323.
- 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 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.
- 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.
- 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.
- 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.
- 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.
- 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%.
- Pr and Nd generate more hydrogen than Y and La.
- 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.
- 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.
- 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]) is preferably combined so as to satisfy the following 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.
- 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.
- 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.
- 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.
- 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 ⁇ near the stoichiometric range.
- 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.
- 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.
- the fuel injection amount from the fuel injection valve 11 is determined.
- 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.
- 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.
- 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.
- 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.
- 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 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.
- 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.
- the TWC includes the OSC material.
- 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.
- the amount of hydrogen generated by the steam reforming reaction is higher in Nd than in Pr, but Pr has an effect of absorbing fluctuations in the air-fuel ratio.
- 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.
- 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
- the upstream catalytic converter 31 and the GPF 32 are provided on the upstream side.
- 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.
- the upstream catalytic converter 31 is provided with a larger amount of OSC material than the GPF 32.
- the air-fuel ratio of the exhaust gas flowing into the GPF 32 can be stabilized.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- the average pore diameter of the partition walls 323 is set to 15 ⁇ m or more, and the particle diameter D90 of the TWC 33 is set to 5 ⁇ m or less to form fine particles.
- 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.
- 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.
- 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.
- 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.
- the catalyst composition is preferable for supporting the partition walls 323 on the inner surfaces of the pores.
- 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.
- 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.
- TWC, carrier, composite oxide and the like were prepared by the following procedure at the ratio shown in Table 1.
- an aqueous medium and an additive were added and then mixed in a ball mill to form a slurry.
- the slurry was pulverized by wet pulverization or the like to adjust the particle size.
- the mixed slurry was immersed once in GPF by dipping.
- the carrying amount (wash coat amount) was 60 g / L (except Examples 10 to 13).
- GPF carrying TWC was obtained by baking at 700 ° C. for 2 hours.
- GPF honeycomb structure made of NGK (inner diameter 25.4 ( ⁇ 1 inch) mm, average pore diameter 20 ⁇ m (excluding Examples 8 and 9), wall thickness 8 mil (excluding Examples 17 and 18), 300 cells, material cordierite, capacity 15 cc) were used.
- Figure 7 is a diagram showing the relationship between the temperature and the NO x purification rate in Example 1 and Comparative Example 1.
- 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 up to 500 ° C. at 20 ° C./min in stoichiometric gas.
- 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.
- Example 1 and Comparative Example 3 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.
- 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 ⁇ m 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.
- Apparatus Laser diffraction particle size distribution measuring apparatus (manufactured by SHIMADZU, SALD-3100) Measuring method: Laser scattering method
- 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 ⁇ m or more and T90 D90 has a particle diameter of 5 ⁇ m 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 ⁇ m or less.
- Apparatus Electronic probe microanalyzer (JELA, JXA-8100) Measurement conditions: acceleration voltage 15 KV, irradiation current 0.04 ⁇ A, pixel size 1 ⁇ m, data collection time per cell 38 ms, beam diameter 0.7 ⁇ m
- FIG. 11 is a graph showing the 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 ⁇ m or less, the pressure loss remains at a substantially constant low level, whereas when D90 exceeds 5 ⁇ m as in the GPF of Example 7 in which D90 is 8 ⁇ m. It was found that the pressure loss increased. From this result, it was confirmed that D90 of TWC carried on GPF is preferably 5 ⁇ m or less.
- 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 ⁇ m remained at a low level. From this result, it was confirmed that the average pore diameter of GPF is preferably 15 ⁇ m or more.
- FIG. 13 is a graph showing the relationship between the amount of TWC washcoat and the pressure loss in Examples 1 and 10 to 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.
- 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.
- 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.
- the vertical axis indicates the CO, HC, and NO x purification rates
- 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
- 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.
- Example 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.
- 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 on the GPF of Example 20 includes solid Ba (sulfuric acid Ba) together with Rh and Pd
- the TWC supported on the GPF of Example 21 includes liquid Ba (acetate Ba and Acetate) together with Rh and Pd.
- Nitric acid Ba) is included.
- the TWC carried by the GPF of the first embodiment 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.
- ⁇ 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.
- 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 the GPF to exert 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.
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Abstract
Description
前記下流三元触媒は、粒度分布における小粒径側からの累積分布が90%となるときの粒子径D90が5μm以下であることが好ましい。
また、Rh層とPd層の2層構造を有する従来の下流三元触媒を、隔壁の細孔内表面に担持させるのは困難であるところ、この発明によればRhとPdを混合した状態でも高い三元浄化機能が発揮されるため、隔壁の細孔内表面への担持に好ましい触媒組成となっている。
図1は、本実施形態に係る内燃機関(以下、「エンジン」という)1とその排気浄化システム2の構成を示す図である。
図2に示すように、GPF32は、フィルタ基材320を備える。フィルタ基材320は、例えば軸方向に長い円柱形状であり、コージェライト、ムライト、シリコンカーバイド(SiC)等の多孔質体により形成される。フィルタ基材320には、流入側端面32aから流出側端面32bまで延びる複数のセルが設けられ、これらセルは隔壁323により区画形成される。
流入側目封じ部324は、フィルタ基材320の流入側端面32aから目封じ用セメントを封入することで形成される。
流出側目封じ部325は、フィルタ基材320の流出側端面32bから目封じ用セメントを封入することで形成される。
図3に示すように、隔壁323内の細孔内表面には、TWC33が担持される。TWC33は、Rhを含むTWC33aと、Pdを含むTWC33bを含んで構成される。これらTWC33は、微粒子化された状態で細孔内表面に担持されている。なお、隔壁323の細孔は、これらTWC33により閉塞されてはおらず、大きな圧力損失が生じないようになっている。
上流触媒コンバータ31及びGPF32に用いられるTWCは、それぞれ排気中のHCを酸化してCO2とH2Oに変換し、COを酸化してCO2に変換する一方、NOxをN2まで還元する機能を有している。この両反応に対する触媒作用を同時に有効に生じさせるためには、燃料と空気の比(以下「空燃比」という。)をストイキ近傍に保つことが好ましい。
従って、酸化雰囲気下で酸素を吸蔵し、還元雰囲気下で酸素を放出する酸素吸蔵放出能を有するOSC材が助触媒として触媒金属と共に用いられている。例えばCeO2や、CeとZrの複合酸化物等がOSC材として知られている。
Nd、PrはHC吸着能が高く、後述するスチームリフォーミング反応による水素の発生量が多い。水素はRhの還元を促進させ、RhのNOx浄化性能を向上させる。
先ず、硝酸セリウム、硝酸ジルコニウム、硝酸ネオジウム及び硝酸プラセオジウムを、所望の比率になるように、純水に溶解する。その後、水酸化ナトリウム水溶液を滴下して、溶媒のpHを例えば10にすることで、沈殿物を得る。その後、沈殿物を含む溶液を例えば60℃に加熱した状態で減圧濾過することで、溶媒を蒸発させる。次いで、残留物を抽出後、マッフル炉内において例えば500℃で2時間の仮焼を行うことで、CeZrNdPr複合酸化物を得る。
本実施形態に係るGPF32は、例えばディッピング法により製造される。ディッピング法では、例えば、TWC33の構成材料を所定量含むスラリーを湿式粉砕等により作製し、作製したスラリー中にGPF32を浸漬させた後、GPF32を引き上げて所定の温度条件で焼成を行うことにより、GPF32にTWC33を担持させることができる。
図4は、CO-TPRによるRhの還元のし易さを示す図である。具体的には、TWCに添加されるBaの有無による、Rhの還元のし易さをCO-TPR(昇温還元法)により下記手順に従って測定した結果を示す図である。
TWCはRhをそれぞれ0.3質量%、3質量%の割合でZr酸化物に担持させ、10質量%のBaを添加したものと添加しないものをそれぞれ作成して測定したものである。
(1)He中で昇温させ、600℃で10分間保持した。
(2)100℃まで降温させた。
(3)1%CO/N2中で、10℃/分で800℃まで昇温させRhを還元させた。
(4)600℃まで降温させた。
(5)10%O2/N2中、600℃で10分間保持した。
(6)100℃まで降温させ、He中で10分間保持後、1%CO/N2中で10分間保持した。
(7)1%CO/N2中で、10℃/分で800℃まで昇温させCO2放出の温度による変化を計測した。
触媒金属として用いられるRhは、水素の存在下で還元状態が維持され、NOx浄化性能が向上する。そのため本実施形態においては、スチームリフォーミング反応を利用している。スチームリフォーミング反応は、高温、触媒存在下で水蒸気とHCが反応して水素が発生する次式のような反応である。
CnHm+nH2O→nCO+(n+1/2m)H2
図5は、NH3-TPDによる各複合酸化物の酸点の量を示す図である。具体的には、CeやZrの複合酸化物の結晶構造中に含有させることのできる元素として挙げられる、Y、La、Pr及びNdそれぞれの酸点の量を、NH3-TPD(昇温還元法)で下記手順により測定した結果を示す図である。
(1)He中で昇温させ、600℃で60分間保持した。
(2)100℃まで降温させた。
(3)0.1%NH3/He中で60分間保持した後、He中で60分間保持した。
(4)He中で、10℃/分で600℃まで昇温させた。
本実施形態では、所謂ウォールフロー型のGPF32において、隔壁323に担持するTWC33を、少なくともRhを含む触媒金属と、酸素吸蔵放出能を有するOSC材として結晶構造中にNd及びPrを有する複合酸化物を含むものと、を含んで構成した。
ここで、酸素吸蔵放出能を有する複合酸化物の結晶構造中に組み込むことが可能な元素のうち、Nd及びPrは酸点の量が多い特性を有する。そのため、結晶構造中にNd及びPrを有する複合酸化物は、酸点の量が多いためHC吸着能が高く、HCと水の存在下で進行するスチームリフォーミング反応が効率良く進行する。すると、このスチームリフォーミング反応の進行により水素が生成し、生成した水素によってTWC33を構成するRhの酸化物化が抑制される。即ち、RhのNOx還元能の低下を回避できるため、高いNOx浄化性能を発揮できる。従って本発明では、優れた三元浄化機能を発揮し得るTWC33をGPF32のフィルタ基材に用いることにより、十分な三元浄化機能を発揮しつつGPF32の圧力損失の増加を抑制することができる。
また、Rh層とPd層の2層構造を有する従来のTWCを、隔壁の細孔内表面に担持させるのは困難であるところ、本実施形態によればRhとPdを混合した状態でも高い三元浄化機能が発揮されるため、隔壁323の細孔内表面への担持に好ましい触媒組成となっている。
TWC及び担体、複合酸化物等を、表1に示す割合で、以下の手順により調製した。
まず、水系媒体、添加材を添加した後ボールミルにて混合してスラリー化した。次に、スラリーを湿式粉砕等により粉砕し、粒子径を調整した。次に、ディッピング法にてGPFに、混合したスラリーを1回浸漬させた。担持量(ウォッシュコート量)は60g/Lにて行った(実施例10~13を除く)。その後、700℃×2時間焼成することで、TWCが担持されたGPFを得た。
なお、GPFとしては、NGK製のハニカム構造体(内径25.4(φ1インチ)mm、平均細孔径20μm(実施例8、9を除く)、壁厚8mil(実施例17、18を除く)、セル数300、材質コージェライト、容量15cc)を用いた。
図7は、実施例1及び比較例1における温度とNOx浄化率との関係を示す図である。具体的には、OSC材にNd、Prを添加した実施例1と、Y、Laを添加した比較例1について、以下の条件に従ってGPFのNOx浄化性能を評価した結果を示す図である。図7に示す通り、実施例1は比較例1よりも低い温度でNOxの浄化が進行していることが分かった。この結果から、GPF中のOSC材にNd、Prを添加した実施例1は、Y、Laを添加した比較例1と比較してNOx浄化性能が向上することが確認された。
[NOx浄化性能評価条件]
ストイキガス中でGPFを500℃まで20℃/分で昇温したときのNOx濃度を計測することにより、NOx浄化性能を評価した。
図8は、実施例1及び比較例2、3における温度と空燃比吸収率との関係を示す図である。具体的には、OSC材にNdのみを添加した比較例2、Prのみを添加した比較例3、NdとPrの双方を使用した実施例1のそれぞれについて、GPFの空燃比吸収率を測定した結果を示す図である。空燃比吸収率は、以下の条件に従って式(1)により算出した。
空燃比吸収率(%)=((空燃比振幅(IN)-空燃比振幅(OUT))÷空燃比振幅(IN))×100
・・・式(1)
(式(1)中、「空燃比振幅(IN)」はOSC材通過前の空燃比振幅を示し、「空燃比振幅(OUT)」はOSC材通過後の空燃比振幅を示す。)
[空燃比吸収率測定条件]
実機エンジンを用いて、空燃比を14.5±1.0(1Hz)で振幅させ、30℃/分で昇温しているときの空燃比吸収率を測定する。
図9は、実施例6のTWCの粒子径分布を示す図である。図9に示す通り、TWC粒子のD90は、5μm以下となっていることが確認された。なお、他の実施例及び比較例についても同様にして以下の測定条件に従って粒子径分布を測定した。得られたD90は表1に示す通りであった。
[粒子径分布測定条件]
装置:レーザ回折式粒子径分布測定装置(SHIMADZU社製、SALD-3100)
測定方法:レーザ散乱法
図10は、実施例1のGPFの隔壁内におけるTWCの担持状態を示す図である。具体的には、実施例1に係るGPFの隔壁内のTWCの担持状態を、以下の条件に従ってEPMAによる断面SEM観察及び元素分析を実施して得たマッピング図である。この結果から、隔壁の平均細孔径は15μm以上であり、TWCのD90が粒子径5μm以下である場合、TWCは隔壁内に均一に担持されることが確認された。
なお、TWCの粒子径が5μm以下である他の実施例についても、同様にTWCは隔壁内に均一に担持されることが確認された。
[EPMA測定条件]
装置:電子プローブマイクロアナライザ(JE0L社製、JXA-8100)
測定条件:加速電圧15KV、照射電流0.04μA、ピクセルサイズ1μm、1セルあたりのデータ採取時間38m秒、ビーム径0.7μm
図11は、実施例1~7のGPFに担持されるTWCのD90と、圧力損失との関係を示す図である。図11に示す通り、D90が5μm以下である実施例1~6は圧力損失が略一定の低いレベルに留まるのに対し、D90が8μmの実施例7のGPFのようにD90が5μmを超えると圧力損失が上昇することが分かった。この結果から、GPFに担持されるTWCのD90は5μm以下であることが好ましいことが確認された。
図12は、実施例1及び実施例8、9のGPFの隔壁の平均細孔径と、圧力損失との関係を示す図である。図12に示す通り、平均細孔径が小さくなるにつれ圧力損失がやや増大したが、平均細孔径が16μmである実施例8のGPFの圧力損失は低いレベルに留まることが分かった。この結果から、GPFの平均細孔径は15μm以上であることが好ましいことが確認された。
図13は、実施例1及び実施例10~13のTWCのウォッシュコート量と、圧力損失との関係を示す図である。図13に示す通り、ウォッシュコート量が増大するにつれ圧力損失が増大したが、ウォッシュコート量が80g/Lである実施例13のGPFの圧力損失は低いレベルに留まることが分かった。この結果から、TWCのウォッシュコート量は80g/L以下であることが好ましいことが確認された。
図14は、実施例1及び実施例17、18のGPFの壁厚と、圧力損失との関係を示す図である。図14に示す通り、壁厚が増大するにつれ圧力損失が増大したが、壁厚が10milである実施例18のGPFの圧力損失は低いレベルに留まることが分かった。この結果から、GPFの壁厚は10mil以下であることが好ましいことが確認された。
図15A及び図15Bは、実施例1及び実施例19のGPFにおける空燃比とそれぞれCO、HC、NOxの浄化率との関係を示す図である。図中、縦軸はそれぞれCO、HC、NOxの浄化率を示し、横軸は燃料と空気の比である空燃比を示す。なお、ストイキとは空燃比が約14.5である領域を示す。
実施例1のGPFに担持されるTWCには、Rh及びPdが含まれ、実施例19のGPFに担持されるTWCにはRhのみが含まれる。評価条件は以下の条件に従って行った。
図15A及び図15Bの評価結果から、Rh及びPdが含まれる実施例1のGPFと比較し、Rhのみが含まれる実施例19のGPFは、空燃比がストイキより高い領域でHC浄化率が低いことが分かった。この結果から、GPFに担持されるTWCとしてRhを単独で用いた実施例19と比較し、RhとPdを併用した実施例1の方が高い三元浄化性能を有することが確認された。
[HC、CO、NOx浄化性能評価条件]
実機エンジンを用いて、触媒入口温度500℃で空燃比を13.5から15.5まで20分間で連続的に変化させ、HC、CO、NOxの浄化率を測定した。
図15C及び図15Dは、実施例20及び実施例21のGPFにおける空燃比とそれぞれCO、HC、NOxの浄化率との関係を示す図である。
実施例20のGPFに担持されるTWCには、Rh及びPdと共に固体Ba(硫酸Ba)が含まれ、実施例21のGPFに担持されるTWCには、Rh及びPdと共に液体Ba(酢酸Ba及び硝酸Ba)が含まれる。また、前述の実施例1(図15A)のGPFに担持されるTWCにはRh及びPdが含まれるが、Baは含まれない。これを比較用として参照する。評価条件は上記HC、CO、NOx浄化性能評価条件と同様の条件で評価を行った。
図15A、図15C及び図15Dの評価結果から、固体Baや液体Baを含む実施例20及び21のGPFは、Baを含まない実施例1のGPFと比較し、空燃比がストイキより低い領域でNOx浄化率が低いことが分かった。この結果から、GPFに担持されるTWCにBaが含まれない実施例1は、Baが含まれる実施例20及び実施例21と比較して高い排気浄化性能を有することが確認された。
図16Aから図16Cは、それぞれ実施例1、実施例14、実施例15、実施例16及び比較例4のGPFに含まれるNd及びPrの合計含有量と、NOx_T50、CO_T50、HC_T50との関係を示す図である。NOx_T50、CO_T50、HC_T50とは、それぞれCO、HC、NOxの50%が浄化される温度を示し、図中の縦軸に示される。横軸は複合酸化物中におけるNdとPrの合計含有量(質量%)を示す。Nd及びPrの合計含有量は比較例4、実施例14、実施例15、実施例1、実施例16の順にそれぞれ0、6、12、14、16質量%である。
図16Aから図16Cに示す通り、実施例1、実施例14、実施例15、実施例16のGPFは比較例4に対し、低い温度でNOx、CO、HCが浄化されていることが分かった。従って本実施形態においてGPFに三元浄化機能を発揮させるには、NdとPrの合計含有量が10質量%~20質量%であることが好ましく、12質量%~16質量%であれば更に好ましいことが確認された。
2…排気浄化システム
3…排気管(排気通路)
31…上流触媒コンバータ(上流三元触媒)
32…GPF(下流フィルタ)
33,33a,33b…TWC(下流三元触媒)
320…フィルタ基材
323…隔壁
321…流入側セル(セル)
322…流出側セル(セル)
324…流入側目封じ部
325…流出側目封じ部
51…LAFセンサ(第1空燃比センサ)
52…O2センサ(第2空燃比センサ)
6…ECU(空燃比コントローラ、前段空燃比設定手段、操作量決定手段)
Claims (7)
- 内燃機関の排気通路に設けられ、排気の空燃比に応じた信号を生成する第1空燃比センサと、
前記排気通路のうち前記第1空燃比センサの検出箇所の下流側に設けられ、排気を浄化する触媒を有する上流触媒コンバータと、
前記排気通路のうち前記上流触媒コンバータの下流側に設けられ、排気の空燃比に応じた信号を生成する第2空燃比センサと、
前記排気通路のうち前記第2空燃比センサの検出箇所の下流側に設けられ、排気中の粒子状物質を捕捉して浄化する下流フィルタと、
前記第1空燃比センサの出力信号及び前記第2空燃比センサの出力信号を用いて、前記下流フィルタに流入する排気の空燃比がストイキの近傍に設定された後段目標値に収束するように前記内燃機関で燃焼させる混合気の空燃比を操作する空燃比コントローラと、を備える内燃機関の排気浄化システムであって、
前記下流フィルタは、排気の流入側端面から流出側端面まで延びる複数のセルが多孔質の隔壁により区画形成されかつこれらセルの流入側端面における開口と流出側端面における開口とが互い違いに目封じされたフィルタ基材と、前記隔壁に担持された下流三元触媒と、を備え、
前記下流三元触媒は、少なくともRhを含む触媒金属と、酸素吸蔵放出能を有するOSC材と、を含み、
前記下流三元触媒のOSC材は、その結晶構造中にNd及びPrを有する複合酸化物を含み、
前記上流触媒コンバータは、排気の流入側端面から流出側端面まで延びる複数のセルが多孔質の隔壁により区画形成されたハニカム基材と、前記ハニカム基材の隔壁に担持された上流三元触媒と、を備え、
前記上流三元触媒は、触媒金属と酸素吸蔵放出能を有するOSC材と、を含み、
前記フィルタ基材における単位体積当たりのOSC材の含有量は、前記ハニカム基材における単位体積当たりのOSC材の含有量よりも少ないことを特徴とする内燃機関の排気浄化システム。 - 前記空燃比コントローラは、前記第2空燃比センサの出力信号が前記後段目標値に収束するように前記第1空燃比センサの出力信号に対する前段目標値を設定する前段空燃比設定手段と、前記第1空燃比センサの出力信号が前記前段目標値になるように前記内燃機関で燃焼させる混合気の空燃比を操作するための操作量を決定する操作量決定手段と、を備えることを特徴とする請求項1に記載の内燃機関の排気浄化システム。
- 前記フィルタ基材の隔壁の厚さは、前記ハニカム基材の隔壁の厚さよりも大きく、
前記フィルタ基材の隔壁の気孔率は、前記ハニカム基材の隔壁の気孔率よりも高く、
前記フィルタ基材に形成されるセルの総数は、前記ハニカム基材に形成されるセルの総数よりも少ないことを特徴とする請求項1又は2に記載の内燃機関の排気浄化システム。 - 前記フィルタ基材の隔壁は、平均細孔径が15μm以上であり、
前記下流三元触媒は、粒度分布における小粒径側からの累積分布が90%となるときの粒子径D90が5μm以下であることを特徴とする請求項1から3の何れかに記載の内燃機関の排気浄化システム。 - 前記下流三元触媒は、前記触媒金属としてRh及びPdを含み且つこれらRh及びPdが混合された状態で前記フィルタ基材の隔壁内の細孔内表面に担持されることを特徴とする請求項1から4の何れかに記載の内燃機関の排気浄化システム。
- 前記下流三元触媒は、Baを含まずに構成されることを特徴とする請求項1から5の何れかに記載の内燃機関の排気浄化システム。
- 前記下流三元触媒の複合酸化物中に含まれるNd及びPrの合計含有量は、10質量%以上であることを特徴とする請求項1から6の何れかに記載の内燃機関の排気浄化システム。
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WO2021145326A1 (ja) * | 2020-01-14 | 2021-07-22 | 三井金属鉱業株式会社 | 排ガス浄化システム |
JPWO2021145326A1 (ja) * | 2020-01-14 | 2021-07-22 | ||
JP7436522B2 (ja) | 2020-01-14 | 2024-02-21 | 三井金属鉱業株式会社 | 排ガス浄化システム |
CN112834378A (zh) * | 2020-12-31 | 2021-05-25 | 清华大学苏州汽车研究院(吴江) | 一种测定排气系统中不同来源灰分质量的方法 |
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US20180347425A1 (en) | 2018-12-06 |
US10704440B2 (en) | 2020-07-07 |
US20190271246A1 (en) | 2019-09-05 |
DE112015006968T5 (de) | 2018-06-28 |
JPWO2017051458A1 (ja) | 2018-08-30 |
CN108138618A (zh) | 2018-06-08 |
JP6458159B2 (ja) | 2019-01-23 |
US10344643B2 (en) | 2019-07-09 |
CN108138618B (zh) | 2020-06-16 |
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