JP5635488B2 - Exhaust purification catalyst - Google Patents

Description

  The present invention relates to an exhaust purification catalyst that captures NOx in exhaust when the air-fuel ratio of the exhaust gas of the internal combustion engine is lean and releases and reduces the trapped NOx when the air-fuel ratio of the exhaust is stoichiometric or rich.

  In recent years, NOx in exhaust gas discharged into the atmosphere from internal combustion engines such as generators and automobiles has been regarded as a problem from the viewpoint of controlling harmful emissions. NOx is a causative substance of acid rain and photochemical smog, and its emission is regulated worldwide.

By the way, in an internal combustion engine such as a diesel engine or a gasoline lean burn engine, lean combustion is performed, so that a lot of O ₂ is contained in the exhaust gas. Of the harmful components contained in the exhaust, NOx is purified by a reduction reaction, but it is not easy to purify NOx in the exhaust containing a large amount of O ₂ .

As a technique for purifying NOx in exhaust containing a large amount of O ₂ , a technique using an exhaust purification catalyst called a lean NOx catalyst is known. According to this technique, when the air-fuel ratio of the exhaust is lean, NOx in the exhaust is captured by the exhaust purification catalyst, and NOx captured by the catalyst is released from the catalyst when the air-fuel ratio of the exhaust is made rich. , Reduced and purified. Further, NH ₃ is generated from NOx released when rich, and this is captured by the catalyst. Further, when the air-fuel ratio is returned to lean, NOx reduction and purification proceed with the trapped NH ₃ as well as trapping NOx.

Examples of the exhaust purification catalyst include a catalyst formed by combining CeO ₂ as a NOx trap, Pt, and a solid acid such as zeolite as an NH ₃ trap. In this exhaust gas purifying catalyst, first, the air-fuel ratio of the exhaust gas is at the lean, by using the O ₂ oxidizes NO occupying most of the NOx in the exhaust gas NO _2, is captured in the form of NO ₂ (below (Refer Formula (1)-(3)). Next, after making the air-fuel ratio of the exhaust rich and reducing the amount of O _{2 in} the exhaust, the CO and H ₂ O contained in the exhaust are reacted to generate H ₂ (see the following formula (4)). . Further, the produced H ₂ and NOx are reacted, and the trapped NOx is converted to NH ₃ and stored on the catalyst (see the following formula (5)). When the air-fuel ratio of the exhaust gas is made lean again, the stored NH ₃ is released and reacted with NOx in the exhaust gas, thereby efficiently reducing and purifying NOx (the following formulas (6) to (8) (See, for example, Patent Document 1 and Non-Patent Document 1).
[Chemical 1]
NO → NO (ad) ・ ・ ・ Formula (1)
2NO + O ₂ → 2NO ₂ (ad) (2)
NO ₂ → NO ₂ (ad) (3)
CO + H ₂ O → H ₂ + CO ₂ Formula (4)
5H ₂ + 2NO → 2NH ₃ (ad) + 2H ₂ O (5)
4NH ₃ + 4NO + O ₂ → 4N ₂ + 6H ₂ O (6)
2NH ₃ + NO ₂ + NO → 2N ₂ + 3H ₂ O (7)
8NH ₃ + 6NO ₂ → 7N ₂ + 12H ₂ O (8)
[In the formula, (ad) represents being trapped by the catalyst. The reactions represented by the equations (1) to (3) and the equations (6) to (8) are reactions that proceed when the air-fuel ratio of the exhaust gas is in a lean state, and in the equations (4) to (5) The reaction represented is a reaction that proceeds when the air-fuel ratio of the exhaust gas is rich. The reaction represented by the formula (4) is a water gas shift reaction, and the reaction represented by the formula (7) is more reactive than the reaction represented by the formula (6). ]

HC adsorption purification layer that adsorbs and purifies HC by containing Pt, zeolite, CeO _{2 and the} like, and Pt, alkali (earth) metal, Al ₂ O ₃ , CeO _{2 and the} like, adsorbs NOx An adsorption purification inner layer that purifies and purifies HC, and an adsorption purification outer layer that contains Rh, alkali (earth) metal, Al ₂ O ₃ , CeO _{2 and the} like, adsorbs and purifies NOx, and purifies HC. An exhaust purification catalyst that is laminated in this order has been proposed (see Patent Document 2). According to this exhaust purification catalyst, NOx can be efficiently purified under low temperature conditions.

However, when Pt, CeO ₂ and zeolite coexist in the same layer as in the above exhaust purification catalyst, they interact with each other under high temperature conditions, thereby reducing the NOx purification rate. There was a problem to do. For example, if the temperature of the catalyst is raised to a predetermined temperature of about 650 ° C. or higher in order to regenerate the catalyst that has been poisoned with SOx, the NOx purification rate is greatly reduced.

Therefore, an intermediate that contains Rh, CeO ₂ , ZrO _2, etc. and reduces NOx between the first catalyst layer that contains Pt and captures NOx and the second catalyst layer that contains zeolite and captures NH ₃ and HC. An exhaust purification catalyst in which a layer is arranged has been proposed (see Patent Document 3). According to this exhaust purification catalyst, the above problems do not occur, and high NOx purification performance and high durability can be obtained.

International Publication No. 2005/044426 JP 2004-321894 A International Publication No. 2011/049064

A NOx Reduction System Using Ammonia Storage-Selective Catalytic Reduction in Rich and Lean Positions, 15. Aachener Koloquim Fahrzeug-und Motorentennik 2006 p. 259-270

By the way, the exhaust purification catalyst is usually used by being coated on the cell inner wall of the honeycomb-shaped carrier. Therefore, in order for the exhaust purification catalyst to keep high purification performance throughout the life of the vehicle, high adhesion to the carrier is required. However, the exhaust gas purification catalyst proposed in Patent Document 3 has insufficient adhesion.
Specifically, the first catalyst layer of the exhaust purification catalyst proposed in Patent Document 3 is configured using a large amount of CeO ₂ having low adhesion in order to efficiently capture NOx. Further, the intermediate layer is made of CeO ₂ , ZrO _2, or the like with low adhesion. Further, the second catalyst layer is mainly composed of a solid acid zeolite, and a hydrophobic material is generally used for the solid acid to provide heat resistance, and these are difficult to adhere to. is there. Thus, each catalyst layer of the exhaust purification catalyst proposed in Patent Document 3 has a problem in adhesion.

  This invention is made | formed in view of the above, The objective is to provide the exhaust purification catalyst which has high adhesiveness and can suppress peeling at the time of manufacture and use.

In order to achieve the above object, the present invention is provided in an exhaust passage (for example, an exhaust pipe 2 to be described later) of an internal combustion engine (for example, an engine 1 to be described later), and converts NOx in the exhaust when the air-fuel ratio of the exhaust is lean. An exhaust purification catalyst (for example, an exhaust purification catalyst 11 described later) is provided that captures and releases and reduces the captured NOx when the air-fuel ratio of exhaust is stoichiometric or rich. The exhaust purification catalyst according to the present invention is provided on a support and is at least one selected from the group consisting of a noble metal containing at least Pt, Al ₂ O ₃ , CeO ₂ , ZrO ₂ , and a complex oxide containing Ce and Zr. and seeds, and a first catalyst layer formed of, provided on the first catalyst layer, and a noble metal including at least Rh, a composite oxide containing C e and Zr, and a second catalyst layer made of the second A third catalyst layer made of zeolite and provided on the catalyst layer, wherein a center particle diameter of catalyst particles of the first catalyst layer, the second catalyst layer, and the third catalyst layer is 4 to 9 μm. In other words, the first catalyst layer contains the most Al ₂ O ₃ and is prepared using a binder composed of Al ₂ O ₃ , and the second catalyst layer is prepared using a binder composed of ZrO _2. the third catalyst layer, SiO Characterized Rukoto be prepared using binder consisting of.

In the present invention, at least one selected from the group consisting of a noble metal containing at least Pt, Al ₂ O ₃ , CeO ₂ , ZrO ₂ and a complex oxide containing Ce and Zr, and a first catalyst layer, A second catalyst layer consisting of a noble metal containing at least Rh, at least one selected from the group consisting of CeO ₂ , ZrO ₂ and a complex oxide containing Ce and Zr, and a third catalyst layer consisting of zeolite. By stacking in this order, an exhaust purification catalyst is formed. That is, the first catalyst layer having a NOx trapping ability, since the second catalyst layer having a NOx reduction ability, a third catalyst layer having a NH ₃ and HC trapping ability, which but are stacked in this order, efficiently NOx Reduce and purify.
Moreover, in this invention, the center particle diameter of the catalyst particle which comprises each catalyst layer shall be 4-9 micrometers in any of a 1st catalyst layer, a 2nd catalyst layer, and a 3rd catalyst layer. As a result, the catalyst particles constituting each catalyst layer are not too small and the catalyst layer is not too dense. It is possible to suppress the deterioration of the adhesiveness, and it is possible to suppress the occurrence of cracks in the catalyst layer and the deterioration of the adhesiveness when exposed to a high temperature. Moreover, since the catalyst particle which comprises each catalyst layer is not too large, it can suppress that the contact of catalyst particles reduces and adhesiveness falls. Therefore, according to the present invention, it is possible to provide an exhaust purification catalyst that has high adhesion and can suppress separation during manufacture and use.

  In the present invention, all of the first catalyst layer, the second catalyst layer, and the third catalyst layer are prepared using a binder made of an oxide of a metal element contained in a material constituting each catalyst layer. The first catalyst layer is made of Al ₂ O ₃ Is the most abundant, so Al ₂ O ₃ Al consisting of oxide of Al element contained in ₂ O ₃ Prepare with a binder. In addition, since the second catalyst layer includes a composite oxide containing Ce and Zr, ZrO composed of an oxide of a Zr element generally more contained in the composite oxide containing Ce and Zr. ₂ Prepare with a binder. Further, the third catalyst layer is made of SiO composed of an oxide of Si element contained abundantly in the zeolite. ₂ Prepare with a binder.

  Where Al ₂ O ₃ Binder, ZrO ₂ Binder and SiO ₂ Each of the binders has a large number of hydroxyl groups on the surface of each particle dispersed as a sol, and this hydroxyl group enters the voids between the catalyst particles constituting each catalyst layer and reacts by heat, thereby causing the catalyst particles to interact with each other. Are combined. At this time, since each binder is made of an oxide of a metal element contained in the material constituting each catalyst layer, the reaction with the catalyst particles in each catalyst layer proceeds smoothly. Therefore, according to the present invention, in each catalyst layer, the adhesion between the catalyst particles is improved, so that the occurrence of cracks and the like is suppressed.

  In this case, the center particle diameter of the catalyst particles of the first catalyst layer is equal to or larger than the center particle diameter of the catalyst particles of the second catalyst layer, and the center particle diameter of the catalyst particles of the second catalyst layer is larger. The center particle diameter of the catalyst particles of the third catalyst layer is preferably equal to or larger than that.

  In this invention, the center particle diameter of the catalyst particles of the first catalyst layer is made equal to or larger than the center particle diameter of the catalyst particles of the second catalyst layer located in the upper layer of the first catalyst layer. Further, the center particle diameter of the catalyst particles of the second catalyst layer is made equal to or larger than the center particle diameter of the catalyst particles of the third catalyst layer located in the upper layer of the second catalyst layer. This makes it easier for the particles constituting the upper catalyst layer to enter the lower space between the layers, so-called anchor effect occurs. Therefore, according to the present invention, adhesion between layers can be further improved, and delamination can be further suppressed.

  In this case, it is preferable that the first catalyst layer is prepared by reducing the amount of the binder used relative to the material constituting the catalyst layer as compared with the second catalyst layer and the third catalyst layer.

In the present invention, the first catalyst layer is prepared by reducing the amount of binder used with respect to the material constituting the catalyst layer as compared with the second catalyst layer and the third catalyst layer. As described above, the second catalyst layer is composed of a complex oxide containing CeO ₂ , ZrO ₂ , Ce, and Zr with low adhesion, and the third catalyst layer is composed of zeolite with low adhesion. On the other hand, the first catalyst layer includes Al ₂ O ₃ having high adhesion. Therefore, according to the present invention, by preparing the first catalyst layer with a smaller amount of binder than the second catalyst layer and the third catalyst layer, an exhaust purification catalyst having high adhesion can be obtained while suppressing material costs. It is done.

  ADVANTAGE OF THE INVENTION According to this invention, it can provide the exhaust gas purification catalyst which has high adhesiveness and can suppress peeling at the time of manufacture or use.

1 is a schematic configuration diagram of an exhaust purification device including an exhaust purification catalyst according to an embodiment of the present invention. It is a figure which shows the relationship between the center particle diameter of a catalyst particle, and the weight decreasing rate after a peeling test. It is a figure which shows the relationship between the center particle diameter of an upper layer and a lower layer, and the weight decreasing rate after a peeling test. It is a figure which shows the cross-sectional observation result before and behind a peeling test when the center particle diameter of an upper layer is 9 micrometers and the center particle diameter of a lower layer is 4 micrometers. It is a figure which shows the cross-sectional observation result before and behind a peeling test when the center particle diameter of an upper layer is 4 micrometers, and the center particle diameter of a lower layer is 4 micrometers. It is a figure which shows the cross-sectional observation result before and behind a peeling test when the center particle diameter of an upper layer is 6 micrometers, and the center particle diameter of a lower layer is 6 micrometers. It is a figure which shows the cross-sectional observation result before and behind a peeling test when the center particle diameter of an upper layer is 9 micrometers, and the center particle diameter of a lower layer is 9 micrometers. It is a figure which shows the cross-sectional observation result before and behind a peeling test when the center particle diameter of an upper layer is 4 micrometers and the center particle diameter of a lower layer is 9 micrometers. It is a figure which shows the relationship between the binder kind in a 1st catalyst layer, and the weight decreasing rate after a peeling test. It is a figure which shows the relationship between the binder kind in a 2nd catalyst layer, and the weight decreasing rate after a peeling test. It is a figure which shows the relationship between the binder kind in a 3rd catalyst layer, and the weight decreasing rate after a peeling test. It is a figure which shows the relationship between the binder amount in each catalyst layer, and the weight decreasing rate after a peeling test.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a schematic configuration diagram of an exhaust purification device 10 including an exhaust purification catalyst 11 according to an embodiment of the present invention. As shown in FIG. 1, an exhaust purification catalyst 11 according to the present embodiment is provided in the middle of an exhaust pipe 2 of an engine 1 such as a diesel engine, and an exhaust air / fuel ratio control unit provided in an ECU (not shown) controls the air / fuel ratio of exhaust gas. By being controlled, NOx in the exhaust gas is reduced and purified. Specifically, the exhaust purification catalyst 11 captures NOx in the exhaust when the air-fuel ratio of the exhaust is lean, and releases and captures the captured NOx when the air-fuel ratio of the exhaust is stoichiometric or rich.
Here, “capturing” NOx in this specification includes any case where NOx is adsorbed, absorbed, or occluded.

The exhaust purification catalyst 11 according to this embodiment is configured by laminating a first catalyst layer, a second catalyst layer, and a third catalyst layer in this order on a carrier. That is, the exhaust purification catalyst 11 according to the present embodiment is an exhaust purification catalyst having a three-layer structure.
The carrier is not particularly limited, and a conventionally known carrier can be used. As the material, cordierite, alumina, aluminum titanate, stainless steel, SiC, or the like can be used, and as the shape, a honeycomb-shaped one can be used. Preferably, a cordierite honeycomb carrier (for example, 400 cells / 4 mil) is used.

The first catalyst layer is formed on the support and is disposed as the lowermost layer of a three-layer structure. The first catalyst layer has NOx trapping ability, and includes at least one selected from the group consisting of a noble metal containing at least Pt, Al ₂ O ₃ , CeO ₂ , ZrO ₂ , and a complex oxide containing Ce and Zr. It consists of. Preferably, the first catalyst layer is composed of Pt and Pd as noble metals, Al ₂ O _3, and a complex oxide containing Ce and Zr.

The noble metal in the first catalyst layer functions as an oxidant that oxidizes NO, which occupies most of the exhaust gas, to NO ₂ when lean. As the noble metal, Pt is essential, and in addition, Pd, Rh, and the like may be included. Preferably, Pt and Pd are used as noble metals. The amount of the noble metal is preferably 0.1 g / L to 20 g / L, more preferably 0.3 g / L to 10 g / L per unit volume of the support. When the amount of noble metal is less than 0.1 g / L, sufficient NOx trapping ability cannot be obtained, and when the amount of noble metal exceeds 20 g / L, further improvement of NOx trapping ability cannot be obtained. It is disadvantageous in terms of cost.

In addition, at least one selected from the group consisting of Al ₂ O ₃ constituting the first catalyst layer, CeO ₂ , ZrO ₂ , and a complex oxide containing Ce and Zr all functions as a support material for the noble metal. The noble metal is supported on these oxides.
Al ₂ O ₃ is a highly adhesive material and also has a function as an adhesion improver. Specifically, Al ₂ O ₃ improves the adhesion with the support and also the adhesion with the upper second catalyst layer. The first catalyst layer preferably contains the most Al ₂ O ₃ .

Further, the composite oxide containing CeO ₂ , Ce and Zr functions as a NOx trap. Specifically, these CeO ₂ -based materials capture NO ₂ generated by being oxidized by NO or noble metals. The complex oxide containing Ce and Zr contains Zr, and as a result, the structural stability is improved and, as a result, the heat resistance is improved.
Further, as the composite oxide containing Ce and Zr, a composite oxide containing at least one rare earth element in addition to Ce and Zr is preferably used. This is because the inclusion of the rare earth element changes the chemical state of the composite oxide, so that the temperature range in which NOx can be captured can be expanded. Examples of rare earth elements include La, Nd, Pr, and Sm.

  The first catalyst layer may contain an alkali metal such as Na, K, or Cs, or an alkaline earth metal such as Mg, Sr, or Ba in addition to the above-described noble metal. These alkali metals and alkaline earth metals are supported on the above oxides.

  The supported amount of the first catalyst layer is preferably 50 g / L to 400 g / L per unit volume of the carrier. When the loading amount of the first catalyst layer is less than 50 g / L, the NOx trapping agent is insufficient and sufficient NOx trapping ability cannot be obtained. In addition, when the loading amount of the first catalyst layer exceeds 400 g / L, the exhaust flowability is reduced, for example, the volume in which the exhaust can flow is reduced and the exhaust flow rate is increased. It cannot be demonstrated.

The second catalyst layer is formed on the first catalyst layer and disposed as an intermediate layer having a three-layer structure. The second catalyst layer has NOx reducing ability and is composed of a noble metal containing at least Rh and at least one selected from the group consisting of CeO ₂ , ZrO ₂ and a complex oxide containing Ce and Zr. Preferably, the second catalyst layer is made of Rh as a noble metal and a composite oxide containing Ce and Zr.

The precious metal in the second catalyst layer has a function of reacting NOx trapped in the first catalyst layer with reducing agents such as CO, HC and H ₂ when it is rich and directly reducing it to N ₂ , and reducing NOx. It also has a function of converting to NH ₃ by an agent. As the noble metal, Rh is essential, and in addition, Pt, Pd, or the like may be included. The amount of the noble metal is preferably 0.1 g / L to 20 g / L, more preferably 0.3 g / L to 10 g / L per unit volume of the support. When the amount of noble metal is less than 0.1 g / L, sufficient NOx reduction ability cannot be obtained, and when the amount of noble metal exceeds 20 g / L, further improvement of NOx reduction ability cannot be obtained. It is disadvantageous in terms of cost.

Further, at least one selected from the group consisting of CeO ₂ , ZrO ₂ , and complex oxides containing Ce and Zr constituting the second catalyst layer functions as a support material for the noble metal, and the noble metal is oxidized by these It is carried on the object.
Similar to the first catalyst layer, CeO ₂ , ZrO ₂ and a composite oxide containing Ce and Zr function as a NOx trapping agent. Examples of the composite oxide containing Ce and Zr include the same ones as exemplified above.

The supported amount of the second catalyst layer is preferably 100 g / L or less per unit volume of the support. When the amount of the second catalyst layer supported exceeds 100 g / L, the exhaust gas is not sufficiently circulated to the first catalyst layer, and the function of the catalyst cannot be exhibited sufficiently.
Further, the supported amount of the second catalyst layer is more preferably 10 g / L to 100 g / L per unit volume of the carrier. When the supported amount of the second catalyst layer is less than 10 g / L, the first catalyst layer containing Pt and the third catalyst layer containing zeolite described later cannot be sufficiently separated, and Pt becomes a zeolite. The NOx purification rate decreases due to the interaction.

The third catalyst layer is formed on the second catalyst layer and is disposed as the uppermost layer of the three-layer structure. The third catalyst layer is made of zeolite, which is a solid material having an HC capturing ability and an NH ₃ capturing ability, and having acid sites.
As the zeolite, it is possible to use at least one zeolite selected from the group consisting of β-type zeolite, MFI-type zeolite, Y-type zeolite, SZR-type zeolite, FER-type zeolite and mordenite-type zeolite. With respect to these various zeolites, those obtained by mixing, supporting or ion-exchange at least one selected from the group consisting of H, Fe, Cu, V, Cs and Ag are preferably used. Among these, β zeolite in which at least one of Fe and Cu is mixed, supported, or ion exchanged is preferably used.

The supported amount of the third catalyst layer is preferably 100 g / L or less per unit volume of the carrier. When the supported amount of the third catalyst layer exceeds 100 g / L, the flow of exhaust gas to the first catalyst layer having NOx trapping ability and the second catalyst layer having NOx reducing ability becomes insufficient, and the NOx trapping ability In addition, the NOx reducing ability cannot be fully exhibited.
Further, the supported amount of the third catalyst layer is more preferably 10 g / L to 100 g / L per unit volume of the carrier. When the supported amount of the third catalyst layer is less than 10 g / L, the NH ₃ trapping ability of the zeolite cannot be sufficiently exhibited, and a high NOx purification rate cannot be obtained.

  The total supported amount of the first catalyst layer, the second catalyst layer, and the third catalyst layer is preferably 450 g / L or less per unit volume of the carrier. When the total supported amount exceeds 450 g / L, the volume through which the exhaust gas can flow is reduced, the flow rate of the exhaust gas is increased, and the flowability of the exhaust gas is reduced, so that the catalyst function cannot be fully exhibited.

Next, the center particle diameters of the first catalyst layer, the second catalyst layer, and the third catalyst layer that constitute the exhaust purification catalyst 11 according to the present embodiment will be described.
Here, in this specification, the “particle diameter” means the diameter of the particles in the slurry at the time of producing the catalyst and the diameter of the catalyst particles in a state where the catalyst is supported. In the present specification, the “center particle diameter” means a particle diameter in which the particle diameter distribution is measured by using any method in these two viewpoints and the integration from the smaller distribution is 50%. To do. Examples of the method for measuring the particle size in the slurry include measurement by a laser diffraction particle size distribution measuring device, and the particle size in a state where the catalyst is supported includes measurement by observation with an SEM or the like.

  In the exhaust purification catalyst 11 according to the present embodiment, in any of the first catalyst layer, the second catalyst layer, and the third catalyst layer, central particles of particles (hereinafter referred to as “catalyst particles”) constituting each catalyst layer. The diameter is in the range of 4-9 μm. If the center particle diameter of the catalyst particles is within this range, high adhesion can be obtained. If the center particle diameter of the catalyst particles is less than 4 μm, the catalyst particles are too small and the catalyst layer becomes too dense, so cracks occur in the catalyst layer during catalyst preparation involving high-temperature treatment such as drying and firing. As a result, the adhesion decreases. Moreover, when exposed to high temperature, it can suppress that a crack generate | occur | produces in a catalyst layer and adhesiveness falls. Further, when the center particle diameter of the catalyst particles exceeds 9 μm, the catalyst particles are too large, so that the contacts between the catalyst particles are reduced and the adhesion is deteriorated.

  In the exhaust purification catalyst 11 according to the present embodiment, the center particle diameter of the catalyst particles of the first catalyst layer is equal to or more than the center particle diameter of the catalyst particles of the second catalyst layer located in the upper layer of the first catalyst layer. Is also preferably large. Furthermore, it is preferable that the center particle diameter of the catalyst particles of the second catalyst layer is equal to or larger than the center particle diameter of the catalyst particles of the third catalyst layer located in the upper layer of the second catalyst layer. Thereby, what is called an anchor effect arises between layers, and adhesiveness improves.

The center particle diameter of the catalyst particles in the slurry is measured as follows.
First, particle size distribution measurement is performed on the slurry prepared with the material constituting each catalyst layer to obtain a particle size distribution profile. For example, a particle size distribution profile is obtained using a laser diffraction scattering type particle size distribution measuring device “LA-950” manufactured by HORIBA. In the obtained particle size distribution profile, the particle size that is 50% integrated from the smaller one is defined as the central particle size. Note that the particle size distribution of the catalyst particles is maintained in the slurry state even when the catalyst particles are supported on the carrier.

The center particle diameter of each catalyst layer is controlled as follows.
As described above, the particle size distribution in the slurry state is maintained even when the particle size distribution is supported on the carrier. Therefore, the central particle diameter of the catalyst layer can be controlled by controlling the central particle diameter of the catalyst particles in the slurry state. Specifically, when preparing the catalyst slurry by mixing the material constituting each catalyst layer (the constituent material of each catalyst layer excluding the binder), the binder, and water, and pulverizing this with a ball mill, Adjust the ball diameter and grinding time of the ball mill. More specifically, the relationship between the ball diameter and pulverization time and the central particle diameter of the catalyst particles in the slurry is obtained by conducting an experiment in advance, and the ball diameter and pulverization time are set so that the desired central particle diameter is obtained. By doing so, the center particle diameter of the catalyst particles in the resulting slurry can be controlled.

The adhesion of the exhaust purification catalyst 11 is evaluated as follows.
First, the exhaust purification catalyst 11 is subjected to a peeling test that simulates a use environment in an actual vehicle. Specifically, a peeling test is performed in which a watering process, a heat treatment process, a rapid cooling process, an air blowing process, and a vibration process are successively performed. As a test piece used for the peeling test, an exhaust purification catalyst 11 supported on a carrier having a size of 1 inch φ × 30 mm is used. The weight of the exhaust purification catalyst 11 carried on the test piece is measured before and after the peel test, and the adhesion of the exhaust purification catalyst 11 is evaluated by the weight reduction rate of the exhaust purification catalyst 11 calculated according to the following formula (1). The Specifically, the larger the calculated weight reduction rate, the lower the adhesion, and the lower the calculated weight reduction rate, the higher the adhesion.

Weight reduction rate (%) = (weight before test−weight after test) / weight before test × 100 Formula (1)

The exhaust purification catalyst 11 according to the present embodiment having the above-described configuration is manufactured using a conventionally known preparation method. Preferably, it is prepared by a wash coat method.
For example, first, a slurry containing the constituent material of the first catalyst layer is prepared, and the first catalyst layer is formed on the support by a wash coat method using this slurry. Next, a slurry containing the constituent material of the second catalyst layer is prepared, and the second catalyst layer is formed on the first catalyst layer by a wash coat method using this slurry. Next, a slurry containing the constituent material of the third medium layer is prepared, and the third catalyst layer is formed on the second catalyst layer by a wash coat method using this slurry. Thereby, the exhaust purification catalyst 11 having a three-layer structure according to the present embodiment is obtained.
The amount of the catalyst supported can be adjusted by adjusting the washcoat amount after adjusting the content of the constituent materials in each slurry.

Here, each of the first catalyst layer, the second catalyst layer, and the third catalyst layer is preferably prepared using a binder made of an oxide of a metal element contained in the material constituting each catalyst layer. For example, the first catalyst layer, preferably from the highest concentration of _Al 2 _{O 3,} is preferably prepared using an _Al 2 _{O 3} binder consisting of oxides of Al element contained in the _Al 2 _{O 3.} Further, for example, since the second catalyst layer preferably includes a complex oxide containing Ce and Zr, a ZrO ₂ binder composed of an oxide of a Zr element generally contained more in the complex oxide containing Ce and Zr. It is preferable to prepare using In addition, for example, the third catalyst layer is preferably prepared using a SiO ₂ binder made of an oxide of Si element that is generally contained more in the zeolite. Thereby, in each catalyst layer, the adhesiveness of catalyst particles improves, generation | occurrence | production of a crack etc. is suppressed and adhesiveness improves.

In the present embodiment, it is preferable that the first catalyst layer is prepared with a smaller amount of binder used with respect to the material constituting the catalyst layer than the second catalyst layer and the third catalyst layer. As described above, the second catalyst layer is composed of a complex oxide containing CeO ₂ , ZrO ₂ , Ce, and Zr with low adhesion, and the third catalyst layer is composed of zeolite with low adhesion. In contrast, the first catalyst layer includes Al ₂ O ₃ having high adhesion. Specifically, in the first catalyst layer, the amount of binder used relative to the material constituting the catalyst layer is preferably 3% or more, and in the second catalyst layer, the amount of binder used relative to the material constituting the catalyst layer is set to be 3% or more. It is preferable to set it to 6% or more.

Next, the NOx purification action of the exhaust purification catalyst 11 according to this embodiment will be described.
First, when the air-fuel ratio of the exhaust gas is lean, O _{2 in} the exhaust gas and NO occupying most of the NOx pass through the third catalyst layer and the second catalyst layer and reach the first catalyst layer. It reacts by the action of a noble metal such as Pt contained in one catalyst layer to generate NO ₂ . The produced NO ₂ is captured by the complex oxide containing CeO ₂ , Ce and Zr contained in the first catalyst layer.

Next, when the air-fuel ratio of the exhaust is stoichiometric or rich, CO and H ₂ O in the exhaust undergo a water gas shift reaction to generate CO ₂ and H ₂ . In addition, HC in the exhaust gas reacts with H ₂ O, and H ₂ is generated together with CO and CO ₂ . Further, NOx in the exhaust gas, NOx trapped in the first catalyst layer (NO ₂ and NO), and the generated H ₂ and HC are functions of noble metals such as Pt contained in the first catalyst layer. To produce NH ₃ and H ₂ O. The produced NH ₃ is trapped in the zeolite contained in the third catalyst layer in the form of NH ₄ ⁺ .
At this time, the NOx trapped in the lean state reacts with the reducing agents such as CO, HC and H ₂ by the action of noble metals such as Rh contained in the second catalyst layer, and is directly reduced to N _2. Is done. As this direct reduction proceeds, the NOx purification rate increases dramatically.

Next, when the exhaust air-fuel ratio is made lean again, as described above, NOx in the exhaust is trapped in the first catalyst layer, NH ₃ trapped in the zeolite in the third catalyst layer, and in the exhaust NOx and O ₂ react to form N ₂ and H ₂ O. Thus, NOx is reduced and purified to _{N 2.} At this time, since the third catalyst layer is disposed in the uppermost layer, N ₂ generated by the reduction and purification of NOx is released efficiently.

  Note that the exhaust purification catalyst 11 according to the present embodiment has a characteristic of exhibiting high NOx purification performance at 150 to 400 ° C. Further, in the conventional exhaust purification catalyst mainly composed of alkali metal or alkaline earth metal, a temperature increase of about 700 ° C. is necessary for the regeneration treatment of the catalyst when it is poisoned with SOx. The exhaust purification catalyst 11 can regenerate the catalyst at a temperature rise of about 600 ° C. Therefore, it is possible to suppress the temperature rising energy necessary for the regeneration process of the catalyst when it is poisoned with SOx, thereby improving the fuel consumption.

According to this embodiment, the following effects are produced.
In the present embodiment, a first catalyst layer comprising a noble metal containing at least Pt, Al ₂ O _3, and at least one selected from the group consisting of CeO ₂ , ZrO ₂ and a composite oxide containing Ce and Zr, A second catalyst layer consisting of a noble metal containing at least Rh and at least one selected from the group consisting of CeO ₂ , ZrO ₂ and a complex oxide containing Ce and Zr, and a third catalyst layer consisting of zeolite, Are stacked in this order to form the exhaust purification catalyst 11. That is, the first catalyst layer having a NOx trapping ability, since the second catalyst layer having a NOx reduction ability, a third catalyst layer having a NH ₃ and HC trapping ability, which but are stacked in this order, efficiently NOx Reduce and purify.
Moreover, in this embodiment, the center particle diameter of the catalyst particle which comprises each catalyst layer shall be 4-9 micrometers in any of a 1st catalyst layer, a 2nd catalyst layer, and a 3rd catalyst layer. As a result, the catalyst particles constituting each catalyst layer are not too small and the catalyst layer is not too dense. It is possible to suppress the deterioration of the property. Moreover, when exposed to high temperature, it can suppress that a crack generate | occur | produces in a catalyst layer and adhesiveness falls. Moreover, since the catalyst particle which comprises each catalyst layer is not too large, it can suppress that the contact of catalyst particles reduces and adhesiveness falls. Therefore, according to the present embodiment, it is possible to provide an exhaust purification catalyst that has high adhesion and can suppress separation at the time of manufacture and use.

  In the present embodiment, the center particle diameter of the catalyst particles of the first catalyst layer is made equal to or larger than the center particle diameter of the catalyst particles of the second catalyst layer located in the upper layer of the first catalyst layer. Further, the center particle diameter of the catalyst particles of the second catalyst layer is made equal to or larger than the center particle diameter of the catalyst particles of the third catalyst layer located in the upper layer of the second catalyst layer. This makes it easier for the particles constituting the upper catalyst layer to enter the lower space between the layers, so-called anchor effect occurs. Therefore, according to the present embodiment, adhesion between layers can be further improved, and delamination can be further suppressed.

In the present embodiment, all of the first catalyst layer, the second catalyst layer, and the third catalyst layer are prepared using a binder made of an oxide of a metal element contained in a material constituting each catalyst layer. For example, the first catalyst layer is preferably prepared using an Al ₂ O ₃ binder made of an oxide of an Al element contained in Al ₂ O ₃ because Al ₂ O ₃ is most often contained. Further, for example, since the second catalyst layer preferably includes a complex oxide containing Ce and Zr, ZrO ₂ composed of an oxide of a Zr element that is generally more contained in the complex oxide containing Ce and Zr. Prepare with a binder. Further, for example, the third catalyst layer is prepared by using a SiO ₂ binder made of an oxide of Si element contained in a large amount in zeolite.
Here, the Al ₂ O ₃ binder, the ZrO ₂ binder, and the SiO ₂ binder all have a large amount of hydroxyl groups on the surface of each particle dispersed as a sol, and the hydroxyl groups constitute the catalyst particles. It enters into the space between them and reacts by heat to bond the catalyst particles together. At this time, since each binder is made of an oxide of a metal element contained in the material constituting each catalyst layer, the reaction with the catalyst particles in each catalyst layer proceeds smoothly. Therefore, according to this embodiment, in each catalyst layer, the adhesion between the catalyst particles is improved, so that the occurrence of cracks and the like is suppressed. As a result, the adhesion can be further improved and the peeling can be further suppressed.

In the present embodiment, the first catalyst layer is prepared by reducing the amount of binder used with respect to the material constituting the catalyst layer as compared with the second catalyst layer and the third catalyst layer. As described above, the second catalyst layer is composed of a complex oxide containing CeO ₂ , ZrO ₂ , Ce, and Zr with low adhesion, and the third catalyst layer is composed of zeolite with low adhesion. On the other hand, the first catalyst layer includes Al ₂ O ₃ having high adhesion. Therefore, according to the present embodiment, by preparing the first catalyst layer with a smaller amount of binder than the second catalyst layer and the third catalyst layer, the exhaust purification catalyst 11 having high adhesion while suppressing the material cost. Is obtained.

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

  Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

  In this example, various verifications were performed based on an exhaust purification catalyst obtained by sequentially laminating a first catalyst layer, a second catalyst layer, and a third catalyst layer shown in Table 1 below. That is, in each of the following verifications, the produced exhaust purification catalyst has the configuration shown in Table 1 unless otherwise specified.

[Relationship between ball diameter and grinding time and center particle diameter]
Taking the first catalyst layer in Table 1 as an example, the relationship between the ball diameter and pulverization time and the center particle diameter was verified according to the method for controlling the center particle diameter described above. Specifically, the center particle diameter of the catalyst particles in the slurry when the ball diameter and the grinding time were changed was measured. The measurement of the center particle diameter was performed according to the measurement method described above. The results are shown in Table 2.

  From the results in Table 2, it was found that the larger the ball diameter, the larger the central particle diameter, and the longer the grinding time, the smaller the central particle diameter. For example, it was found that the center particle diameter can be controlled to 4 to 9 μm by setting the ball diameter to 5 to 20 mm and the pulverization time to 6 to 10 hours. It has been confirmed that the same can be said for the second catalyst layer and the third catalyst layer as well as the first catalyst layer.

[Relationship between center particle size and adhesion]
First, the relationship between the center particle diameter of the catalyst particles and the adhesion was verified. Specifically, for each of four types of catalyst layers (containing noble metals) having different weight ratios between Al ₂ O ₃ and a composite oxide containing Ce and Zr (hereinafter referred to as “CeO ₂ —ZrO ₂ ”), A catalyst layer having a changed center particle diameter was prepared as a single layer. For the preparation of the catalyst layer, the above-described conventionally known preparation method was used. About each produced catalyst layer, the above-mentioned peeling test was implemented and the weight reduction rate was computed according to the above-mentioned Numerical formula (1).

FIG. 2 is a graph showing the relationship between the center particle diameter of the catalyst particles and the weight reduction rate after the peel test. As is clear from FIG. 2, in any catalyst layer, it was found that the weight reduction rate was small when the central particle diameter of the catalyst particles was 4 to 9 μm. More specifically, in a catalyst layer containing both Al ₂ O ₃ and CeO ₂ —ZrO ₂ (that is, corresponding to the first catalyst layer of the present invention), the weight reduction rate is when the center particle diameter is 4 to 9 μm. When the weight ratio of Al ₂ O ₃ is 0, that is, when the center particle diameter is 4 to 9 μm in a catalyst layer containing only CeO ₂ —ZrO ₂ (ie, corresponding to the second catalyst layer of the present invention) It was found that the weight reduction rate was 15% or less.

The results, regardless of the weight ratio of _Al 2 _{O 3} and _CeO 2 -ZrO _2, when the central particle size of the catalyst particles in the first catalyst layer and the second catalyst layer of the present invention is 4~9μm is It was confirmed that high adhesion was obtained. In addition, since the second catalyst layer having a weight ratio of Al ₂ O ₃ of 0, that is, only CeO ₂ —ZrO ₂ does not contain Al ₂ O ₃ having high adhesion, Al ₂ O ₃ and CeO ₂ —ZrO ₂ It was also confirmed that the weight reduction rate was larger than that of the first catalyst layer containing both of the above.
In addition, it is confirmed that the same is true for the third catalyst layer of the present invention made of zeolite, in that high adhesion can be obtained when the center particle diameter of the catalyst particles is 4 to 9 μm.

[Relationship between center particle size and adhesion of upper layer and lower layer]
Next, the relationship between the center particle diameter of the catalyst layers (upper layer and lower layer) adjacent to each other and the adhesion was verified. Specifically, a catalyst having a particle diameter different from that of the catalyst particles constituting the upper layer and the catalyst particles constituting the lower layer was produced. Specifically, a catalyst layer made of Pt, Pd, Rh, Al ₂ O ₃ and CeO ₂ —ZrO ₂ is prepared as the lower catalyst layer, and Pd, Al ₂ O ₃ and CeO ₂ are used as the upper catalyst layer. A catalyst layer made of -ZrO ₂ was prepared. For the preparation of the catalyst layer, the above-described conventionally known preparation method was used. About each produced catalyst layer, the above-mentioned peeling test was implemented and the weight reduction rate was computed according to the above-mentioned Numerical formula (1).

  FIG. 3 is a diagram showing the relationship between the center particle diameters of the upper layer and the lower layer and the weight reduction rate after the peel test. As is clear from FIG. 3, when both the upper layer and the lower layer have a center particle size of 4 μm, and when the upper layer has a center particle size of 4 μm and the lower layer has a center particle size of 9 μm, The weight reduction rate was found to be 1% or less. On the other hand, when the center particle diameter of the upper layer was 9 μm and the center particle diameter of the lower layer was small, 4 μm, it was found that the weight reduction rate exceeded 1%.

  From this result, it was confirmed that high adhesion can be obtained when the center particle diameter of the lower layer is equal to or larger than the center particle diameter of the upper layer. That is, the center particle diameter of the first catalyst layer is equal to or greater than the center particle diameter of the second catalyst layer, and the center particle diameter of the second catalyst layer is equal to or greater than the center particle diameter of the third catalyst layer. It was confirmed that high adhesion can be obtained by increasing the thickness. This is because when the center particle size of the lower layer is equal to or larger than the center particle size of the upper layer, the upper layer catalyst particles are lower than the lower layer, as is apparent from the catalyst particles schematically shown in FIG. It was thought that this was because the so-called anchor effect occurred because it easily entered the voids between the catalyst particles.

Cross-sectional observations before and after the peel test were performed on the upper layer and the lower layer with the center particle diameters changed as described above. SEM was used for cross-sectional observation. The results are shown in FIGS.
FIG. 4 is a diagram showing cross-sectional observation results before and after the peel test when the center particle diameter of the upper layer is 4 μm and the center particle diameter of the lower layer is 9 μm. FIG. 5 is a diagram showing cross-sectional observation results before and after the peel test when the center particle diameter of the upper layer is 4 μm and the center particle diameter of the lower layer is 4 μm. FIG. 6 is a diagram showing cross-sectional observation results before and after the peel test when the center particle diameter of the upper layer is 6 μm and the center particle diameter of the lower layer is 6 μm. FIG. 7 is a diagram showing cross-sectional observation results before and after the peel test when the center particle diameter of the upper layer is 9 μm and the center particle diameter of the lower layer is 9 μm. FIG. 8 is a diagram showing cross-sectional observation results before and after the peel test when the center particle diameter of the upper layer is 9 μm and the center particle diameter of the lower layer is 4 μm.

As shown in FIGS. 4 to 7, when the center particle diameter of the lower layer is equal to or larger than the center particle diameter of the upper layer, the upper catalyst layer remains without peeling even after the peeling test. It was confirmed that it has high adhesion.
On the other hand, as shown in FIG. 8, when the center particle diameter of the lower layer is smaller than the center particle diameter of the upper layer, the upper catalyst layer is completely peeled by the peel test, and the adhesion is not sufficient. It was confirmed.

[Relationship between binder type and adhesion]
Next, it verified about the relationship between the kind of binder used for each catalyst layer of the 1st catalyst layer of this invention, the 2nd catalyst layer, and the 3rd catalyst layer, and adhesiveness. Specifically, the amount of binder in each catalyst layer was 3%, and different types of binder were produced for each catalyst layer. For the preparation of the catalyst layer, the above-described conventionally known preparation method was used. About each produced catalyst layer, the above-mentioned peeling test was implemented and the weight reduction rate was computed according to the above-mentioned Numerical formula (1).

FIG. 9 is a diagram showing the relationship between the binder type in the first catalyst layer and the weight reduction rate after the peel test. As shown in FIG. 9, it was found that the weight reduction rate was smaller and the adhesiveness was higher when the Al ₂ O ₃ binder was used than when the ZrO ₂ binder was used. From this result, the adhesiveness can be further improved by preparing the first catalyst layer using an Al ₂ O ₃ binder made of an oxide of Al element contained in Al ₂ O ₃ constituting the first catalyst layer. Was confirmed.

FIG. 10 is a diagram showing the relationship between the binder type in the second catalyst layer and the weight reduction rate after the peel test. As shown in FIG. 10, it was found that the weight reduction rate was smaller and the adhesiveness was higher when the ZrO ₂ binder was used than when the Al ₂ O ₃ binder was used. From this result, by preparing the second catalyst layer using a ZrO ₂ binder composed of an oxide of a Zr element that is generally contained more in the complex oxide containing Ce and Zr constituting the second catalyst layer, It was confirmed that the adhesion could be improved.

FIG. 11 is a diagram showing the relationship between the binder type in the third catalyst layer and the weight reduction rate after the peel test. As shown in FIG. 11, it was found that the weight reduction rate was smaller and the adhesiveness was higher when the SiO ₂ binder was used than when the Al ₂ O ₃ binder was used. From this result, it was confirmed that the adhesion could be improved by preparing the third catalyst layer using a SiO ₂ binder composed of an oxide of Si element contained in a large amount in the zeolite constituting the third catalyst layer.

[Relationship between binder amount and adhesion]
Next, it verified about the relationship between the amount of binders used for each catalyst layer of the 1st catalyst layer of the present invention, the 2nd catalyst layer, and the 3rd catalyst layer, and adhesiveness. Specifically, for each catalyst layer, the binder type that is considered preferable from the viewpoint of improving adhesion in the above verification (that is, Al ₂ O ₃ binder in the first catalyst layer, ZrO ₂ binder in the second catalyst layer, The three catalyst layers were prepared by changing the amount of the binder (SiO ₂ binder). For the preparation of the catalyst layer, the above-described conventionally known preparation method was used. About each produced catalyst layer, the above-mentioned peeling test was implemented and the weight reduction rate was computed according to the above-mentioned Numerical formula (1).

FIG. 12 is a diagram showing the relationship between the binder amount in each catalyst layer and the weight reduction rate after the peel test. As shown in FIG. 12, in the first catalyst layer containing both Al ₂ O ₃ and CeO ₂ —ZrO ₂ , since Al ₂ O ₃ functions as an adhesion improver, the weight percentage of the binder with respect to the catalyst layer is 3 % Or more, it was confirmed that the weight reduction rate was sufficiently small and high adhesion was obtained.
In contrast, a second catalyst layer formed of _CeO 2 -ZrO ₂ free of _Al 2 _{O 3,} in the same third catalyst layer comprising a zeolite free of _Al 2 _{O 3,} to obtain a sufficiently small weight loss In order to achieve this, it has been found that the weight percentage of the binder with respect to the catalyst layer must be 6% or more.

1. Engine (internal combustion engine)
2 ... Exhaust pipe (exhaust passage)
10 ... Exhaust purification device 11 ... Exhaust purification catalyst

Claims (3)

An exhaust purification catalyst that is provided in an exhaust passage of an internal combustion engine, captures NOx in exhaust when the exhaust air-fuel ratio is lean, and releases and reduces the captured NOx when the exhaust air-fuel ratio is stoichiometric or rich Because
A first catalyst layer provided on a support and comprising a noble metal containing at least Pt, Al ₂ O _3, and at least one selected from the group consisting of CeO ₂ , ZrO ₂ and a composite oxide containing Ce and Zr. When,
Provided on the first catalyst layer, and a noble metal including at least Rh, a composite oxide containing C e and Zr, and a second catalyst layer formed of,
A third catalyst layer provided on the second catalyst layer and made of zeolite,
The first catalyst layer, the center particle size of the catalyst particles of the second catalyst layer and the third catalyst layer, Ri 4~9μm der,
The first catalyst layer contains the most Al ₂ O ₃ and is prepared using a binder made of Al ₂ O ₃ .
The second catalyst layer is prepared using a binder made of ZrO ₂ ,
The third catalyst layer, the exhaust gas purifying catalyst, wherein Rukoto be prepared using binder consisting of SiO _2. The center particle diameter of the catalyst particles of the first catalyst layer is equal to or larger than the center particle diameter of the catalyst particles of the second catalyst layer,
The exhaust purification catalyst according to claim 1, wherein the center particle diameter of the catalyst particles of the second catalyst layer is equal to or larger than the center particle diameter of the catalyst particles of the third catalyst layer. Wherein the first catalyst layer, the second as compared with the catalyst layer and the third catalyst layer, according to claim 1 or, characterized in that for the material constituting the catalyst layer is prepared by reducing the amount of the binder 2. The exhaust purification catalyst according to 2.