Catalyst for purification of exhaust gas

Abstract

The present application provides an exhaust gas purification catalyst which can suppress an increase in pressure loss and has excellent exhaust gas purification performance. An exhaust gas purification catalyst (10) includes: a wall flow type substrate having a partition wall (26); a first catalyst layer (261) formed in a region inside the partition wall (26) that is in contact with an inlet side chamber (24) from an end portion (24a) on the exhaust gas inflow side along the extension direction of the partition wall (26) at a length shorter than the entire length (L w ) of the partition wall (26); and a second catalyst layer (262) formed in a region inside the partition wall (26) that is in contact with an outlet side chamber (25) from an end portion (25a) on the exhaust gas outflow side along the extension direction of the partition wall (26) at a length shorter than the entire length (L w ) of the partition wall (26). In the above extension direction, the first catalyst layer (261) and the second catalyst layer (262) are partially overlapped with each other.

Description

[0001] This application is a divisional application of the same name filed on October 7, 2015, Application No. 201580056002.5. TECHNICAL FIELD

[0002] The present application relates to an exhaust gas purification catalyst provided in an exhaust system of an internal combustion engine. In detail, it relates to a wall flow type exhaust gas purification catalyst.

[0003] This international application claims priority based on Japanese Patent Application No. 2014-211379 filed on October 16, 2014, the entire contents of which are incorporated herein by reference. BACKGROUND

[0004] Exhaust gas emitted from an internal combustion engine such as an automobile engine contains particulate matter (PM), hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NOx), and the like, which are harmful components. In the past, a wall flow type exhaust gas purification catalyst has been used in order to efficiently remove these exhaust gas components. x

[0005] The wall flow type exhaust gas purification catalyst has an inlet side cell whose end portion is open to the exhaust gas inflow side, an outlet side cell whose end portion is open to the exhaust gas outflow side, and a porous partition wall (rib wall) that separates the two cells. Exhaust gas emitted from the internal combustion engine flows into the inlet side cell from the exhaust gas inflow side end portion, passes through the fine pores of the porous partition wall, and flows out from the exhaust gas outflow side end portion of the outlet side cell. By the exhaust gas contacting a catalyst layer (catalyst metal), the above-mentioned exhaust gas components are purified (harmless).

[0006] As prior art documents related to this, Patent Documents 1 and 2 can be cited. For example, in Patent Document 1, an exhaust gas purification catalyst having a catalyst layer of a double layer structure is disclosed. Specifically, an exhaust gas purification catalyst is disclosed which has a first catalyst layer containing Pd inside the partition wall as a whole, and a second catalyst layer containing Rh on the surface of the partition wall on the side in contact with the inlet side cell in such a manner as to completely cover the first catalyst layer.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT DOCUMENTS

[0009] Patent Document 1: Japanese Patent Application Publication No. 2009-82915

[0010] Patent Document 2: Japanese Patent Application Publication No. 2007-185571 SUMMARY

[0011] ​However, according to the research by the inventors of the present application, it is considered that the above-described exhaust gas purification catalyst has room for improvement in terms of the arrangement of the catalyst metal. That is, in such an exhaust gas purification catalyst, a first catalyst layer is provided in the entire inside of the partition wall, and a second catalyst layer is formed so as to fill the first catalyst layer. When the surface of the inlet chamber is covered with the catalyst layers as described above, the pressure loss sometimes excessively increases.

[0012] The present application has been achieved in order to solve such a problem, and has an object to provide an exhaust gas purification catalyst which suppresses an increase in pressure loss and has excellent exhaust gas purification performance.

[0013] The inventors of the present application have conducted research from various angles, and have found that, compared to a case where a catalyst layer is formed over the entire partition wall, when a catalyst layer is formed in the partition wall near the end portion on the exhaust gas inflow side (for example, the partition wall near the end portion on the exhaust gas inflow side) and the partition wall near the end portion on the exhaust gas outflow side (for example, the partition wall near the end portion on the exhaust gas outflow side), high purification performance can be obtained.

[0014] On the other hand, according to the research by the inventors of the present application, it has been found that, if a portion where no catalyst layer is formed is present in the extension direction of the partition wall, the flow rate of exhaust gas to the portion increases due to a balance with the pressure loss. Therefore, harmful components of the exhaust gas pass through the portion where no catalyst layer is formed, and the emission of the exhaust gas deteriorates.

[0015] Based on these insights, the inventors of the present application have further conducted intensive research repeatedly, and have completed the present application which can achieve the above-described object.

[0016] The exhaust gas purification catalyst according to the present application is a wall flow type exhaust gas purification catalyst which is arranged in an exhaust pipe of an internal combustion engine such as an automobile engine, and purifies exhaust gas discharged from the internal combustion engine. The exhaust gas purification catalyst disclosed herein has a substrate of a wall flow structure, a first catalyst layer, and a second catalyst layer. The substrate has an inlet chamber whose end portion on the exhaust gas inflow side is open, an outlet chamber whose end portion on the exhaust gas outflow side is open and which is adjacent to the inlet chamber, and a porous partition wall which separates the inlet chamber and the outlet chamber. The first catalyst layer is formed in the inside of the partition wall which is in contact with the inlet chamber, from the end portion on the exhaust gas inflow side in the extension direction of the partition wall, at a length shorter than the full length L w of the partition wall. The second catalyst layer is formed in the inside of the partition wall which is in contact with the outlet chamber, from the end portion on the exhaust gas outflow side in the extension direction of the partition wall, at a length shorter than the full length L w of the partition wall. Furthermore, when the length of the first catalyst layer is set as L1 and the length of the second catalyst layer is set as L2 in the extension direction, the first catalyst layer and the second catalyst layer satisfy the following formula: Lw (L1+L2) < 2L w in the above-mentioned extending direction.

[0017] By concentrating the catalyst layers in the regions that greatly contribute to exhaust gas purification performance, i.e., the vicinity of the end portion on the exhaust gas inflow side and the vicinity of the end portion on the exhaust gas outflow side, the catalyst metal can be efficiently utilized. Therefore, high purification performance can be achieved. In addition, by partially overlapping the first catalyst layer and the second catalyst layer with each other in the extending direction of the partition wall, the "passing through" of the exhaust gas can be more reliably prevented, and the exhaust gas components can be purified (harmless) more reliably. Therefore, the exhaust gas emission can be effectively reduced. In addition, by forming the two catalyst layers so as to be shorter than the full length L w of the partition wall inside the partition wall, respectively, the increase in pressure loss can be suppressed.

[0018] In the present specification, the catalyst layer "formed inside the partition wall" means that a majority of the catalyst layer exists inside the partition wall (offset). For example, it means that, when the cross section of the first catalyst layer is observed with an electron microscope, the total amount of catalyst metal in the range of 0.1 Lw in length from the end portion on the exhaust gas inflow side toward the extending direction is set to 100 mass%. Although not particularly limited, the catalyst metal existing inside the partition wall side is typically 80 mass% or more, for example, 90 mass% or more, and preferably 95 mass% or more. Therefore, it can be clearly distinguished from the case where, for example, the catalyst layer is formed outside (typically, the surface) of the partition wall, and as a result, a part of the catalyst layer inadvertently erodes into the inside of the partition wall.

[0019] In a preferred one of the exhaust gas purification catalysts disclosed herein, the length by which the above-mentioned first catalyst layer overlaps the above-mentioned second catalyst layer is 2% or more and 60% or less (preferably, 10% or more and 40% or less) of the above-mentioned L w . Thereby, the effects of the present application can be exerted at a higher level.

[0020] In another preferred one of the exhaust gas purification catalysts disclosed herein, the length (average length) LI of the above-mentioned first catalyst layer is 20% or more and 90% or less of the above-mentioned L w . By providing the catalyst metal in a portion of at least 20% from the end portion on the exhaust gas inflow side with respect to the full length L w of the extending direction of the partition wall, the exhaust gas purification ability can be better exerted. In addition, by making LI be 90% or less of L w , the increase in pressure loss can be more reliably suppressed.

[0021] In another preferred one of the exhaust gas purification catalysts disclosed herein, the length (average length) L2 of the above-mentioned second catalyst layer is 20% or more and 90% or less of the above-mentioned Lw 20% or more and 90% or less. By providing the catalyst metal to the portion of 20% or more of the entire length L2 of the partition wall from the end of the exhaust gas outflow side in the extending direction of the partition wall w 20% or more, the exhaust gas purification ability can be more effectively exerted. Further, by making L2 be L w 90% or less, the increase in pressure loss can be more effectively suppressed.

[0022] In another preferred aspect of the exhaust gas purification catalyst disclosed herein, in a thickness direction orthogonal to the above-mentioned extending direction, the thickness of the above-mentioned partition wall is set to T w , the thickness of the above-mentioned first catalyst layer is set to T1, and the thickness of the above-mentioned second catalyst layer is set to T2, the following equation is satisfied: 0.2T w ≤ (T w - T1 - T2) ≤ 0.4T w . Thus, the catalyst metal can be more effectively utilized, and the amount of catalyst metal used can be reduced. Further, by making the catalyst layers not overlap each other in the thickness direction, movement of the catalyst metal is less likely to occur. Thus, deterioration of the catalyst due to sintering or alloying can be suppressed. Therefore, the catalyst activity can be stably exerted for a long period of time. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a perspective view schematically showing a substrate of an exhaust gas purification catalyst according to an embodiment of the present application.

[0024] Figure 2 is a cross-sectional view schematically showing an end portion of a honeycomb substrate of Figure 1

[0025] Figure 3 is an enlarged cross-sectional view schematically showing a structure in the vicinity of a partition wall of an exhaust gas purification catalyst according to an embodiment of the present application.

[0026] Figure 4 is a graph comparing purification performance of exhaust gas purification catalysts. DETAILED DESCRIPTION

[0027] Hereinafter, preferred embodiments of the present application will be described with reference to the accompanying drawings. In the following drawings, the same symbols are assigned to the components, portions that achieve the same function, and sometimes repeated description is omitted or simplified. Further, the dimensional relationship (length, width, thickness, etc.) in each drawing does not necessarily reflect the actual dimensional relationship. Among them, matters other than those specifically mentioned in the present specification and matters necessary for carrying out the present application can be understood as matters based on the design of a person skilled in the art based on the existing technology in the art. The present application can be carried out based on the content disclosed in the present specification and the technical knowledge in the art.​

[0028] The exhaust gas purification catalyst disclosed herein has a substrate of a wall flow structure, and two catalyst layers provided to the partition walls of the substrate. Also, it has a feature that a part of the two catalyst layers in the extension direction of the partition walls overlap each other. Therefore, other configurations are not particularly limited. The exhaust gas purification catalyst of the present application can appropriately select the substrate, the carrier, and the catalyst metal described later, and is shaped into a desired shape according to the use.

[0029] First, the substrate of a wall flow structure is described. The substrate constitutes the skeleton of the exhaust gas purification catalyst disclosed herein. As the substrate, it is appropriate to adopt the substrate that has been used in such a use until now. Figure 1 is a schematic view showing an example of the substrate. Figure 1 The substrate shown is a honeycomb-shaped substrate (honeycomb-shaped structure body) 1 of a cylindrical shape. The honeycomb-shaped substrate 1 has a plurality of cells regularly arranged in the extension direction of the honeycomb-shaped substrate 1 (the direction of the axis of the cylindrical shape), and partition walls that separate the cells. One open end and the other open end of the extension direction between adjacent cells are alternately sealed. Figure 2 is a schematic view showing the cross section of the end portion la of the honeycomb-shaped substrate 1. In this mode, the end portion la is substantially circular. In the end portion la, the sealing portion 2 and the opening portion 4 are arranged in a checkered pattern. The porous partition wall 6 is arranged between the sealing portion 2 and the opening portion 4.

[0030] The honeycomb-shaped substrate 1 can be formed of a heat-resistant material, for example, so that it can cope with the case where the exhaust gas generated when the internal combustion engine is operated under high load conditions is exposed to high temperatures (for example, 400°C or higher), or the case where PM is removed by high-temperature combustion, and the like. As the heat-resistant material, for example, ceramics such as cordierite, aluminum titanate, silicon carbide (SiC), or alloys such as stainless steel can be listed. The capacity (total volume of the cells) of the honeycomb-shaped substrate 1 is usually 0.1 L or more, and preferably 0.5 L or more, and for example, 5 L or less, and preferably 3 L or less, and more preferably 2 L or less. The total length in the extension direction of the honeycomb-shaped substrate 1 (in other words, the total length L w in the extension direction of the partition wall 6) is usually 10 to 500 mm, and for example, about 50 to 300 mm. From the viewpoint of improving the exhaust gas purification performance and the mechanical strength, and suppressing the pressure loss, the thickness (length in the direction orthogonal to the extension direction) of the partition wall 6 is, for example, about 0.05 to 2 mm. From the viewpoint of improving the mechanical strength and suppressing the pressure loss, the porosity of the partition wall 6 is usually about 40 to 70%. From the viewpoint of improving the PM trapping performance and suppressing the pressure loss, the average pore diameter of the partition wall 6 is usually about 10 to 40 μm. In addition, the outer shape of the entire honeycomb-shaped substrate 1 can be formed into, for example, an elliptical cylindrical shape, a polygonal cylindrical shape, or the like, in addition to the cylindrical shape shown in Figure 1 is a schematic view showing an example of the substrate.

[0031] Next, a catalyst for exhaust gas purification formed using the honeycomb substrate 1 will be described.

[0032] Figure 3 is an enlarged sectional view schematically showing the structure in the vicinity of the separation wall of the catalyst for exhaust gas purification 10 according to one embodiment of the present application. In this drawing, the direction of exhaust gas flow is indicated by the arrow direction. That is, the left side of Figure 3 is the upstream of the exhaust gas flow path (exhaust pipe), and the right side of Figure 3 is the downstream of the exhaust gas flow path. The catalyst for exhaust gas purification 10 is of a so-called wall flow structure. The catalyst for exhaust gas purification 10 has an inlet-side chamber 24 (of a "C" shape) having an open end 24a on the exhaust gas inflow side, an outlet-side chamber 25 (of a "C" shape) having an open end 25a on the exhaust gas outflow side, and a porous separation wall 26 separating the two chambers. The open end 25a on the exhaust gas outflow side of the inlet-side chamber 24 and the open end 24a on the exhaust gas inflow side of the outlet-side chamber 25 are plugged with a seal 22. Two catalyst layers (i.e., a first catalyst layer 261 and a second catalyst layer 262) of prescribed properties (e.g., length and thickness, noble metal support amount) are formed inside the separation wall 26 (specifically, inside the pores of the separation wall 26).

[0033] In the catalyst for exhaust gas purification 10 thus configured, exhaust gas discharged from an internal combustion engine flows into the inlet-side chamber 24 from the open end 24a on the exhaust gas inflow side, passes through the pores of the porous separation wall 26, and flows out from the open end 25a on the exhaust gas outflow side of the adjacent outlet-side chamber 25. Harmful components in the exhaust gas are purified (harmless) by contact with the catalyst layers during passage through the catalyst for exhaust gas purification 10. For example, HC components or CO components contained in the exhaust gas are oxidized by the catalytic function of the catalyst layers, and converted (purified) into water (H2O) or carbon dioxide (CO2), etc. NO x components are reduced by the catalytic function of the catalyst layers, and converted (purified) into nitrogen (N2). Since PM components are difficult to pass through the pores of the separation wall 26, they are generally accumulated on the separation wall 26 in the inlet-side chamber 24. The accumulated PM is decomposed and removed by the catalytic function of the catalyst layers or by combustion at a prescribed temperature (e.g., around 500 to 700°C).

[0034] The two catalyst layers (the first catalyst layer 261 and the second catalyst layer 262) constitute the main body of the catalyst for exhaust gas purification 10 as sites for purifying exhaust gas. Each of the two catalyst layers has catalyst metal particles that function as oxidation and / or reduction catalysts and a carrier that supports the catalyst metal particles.

[0035] As the catalyst metal, a metal species that can function as various oxidation catalysts or reduction catalysts can be considered. As typical examples, there can be mentioned noble metals such as rhodium (Rh), palladium (Pd), platinum (Pt), and the like. Alternatively, ruthenium (Ru), osmium (Os), iridium (Ir), silver (Ag), gold (Au), and the like can also be used. In addition, an alloy formed by alloying two or more of these metals can also be used. In addition, other metal species such as alkali metals or alkaline earth metals, transition metals, and the like can also be used. From the viewpoint of improving the contact area with the exhaust gas, the catalyst metal is preferably used in the form of fine particles having a sufficiently small particle diameter. The average particle diameter of the above-mentioned catalyst metal particles (average value of the particle diameters observed by transmission electron microscopy, the same applies hereinafter) is usually about 1 to 15 nm, 10 nm or less, 7 nm or less, and further 5 nm or less.

[0036] The metal species contained in the first catalyst layer 261 and the second catalyst layer 262 can be the same or different. As an example, a metal species having a high reduction activity (e.g., rhodium) can be used in one catalyst layer (e.g., the first catalyst layer 261), and a metal species having a high oxidation activity (e.g., palladium and / or platinum) can be used in the other catalyst layer (e.g., the second catalyst layer 262). As another example, the same metal (e.g., rhodium) can be used in both catalyst layers (the first catalyst layer 261 and the second catalyst layer 262).

[0037] In a preferred embodiment, at least Rh or an alloy of Rh is contained in the first catalyst layer 261 near the exhaust gas inflow side, and at least Rh, Pd, Pt, or an alloy of these metals is contained in the second catalyst layer 262 near the exhaust gas outflow side. Thereby, the purification activity of the catalyst metal can be exerted at a high level.

[0038] The catalyst metal support ratio (the content ratio of the catalyst metal when the support is taken as 100 mass%) of the first catalyst layer 261 and the second catalyst layer 262 can be the same or different. The support ratio of the catalyst metal of each catalyst layer can differ depending on the length and thickness of the catalyst layer, and the like, and thus is not particularly limited, and can be about 1.5 mass% or less, preferably 0.05 to 1.5 mass%, and more preferably 0.2 to 1 mass% for each. By making the support ratio be the prescribed value or more, the exhaust gas purification effect by the catalyst metal is easily obtained. In addition, by making the support ratio be the prescribed value or less, the progress of the particle growth (sintering) of the metal and the increase in the pressure loss can be suppressed. In addition, it is also advantageous in terms of cost.

[0039] As a carrier for supporting the catalytic metal, inorganic compounds that have been used in this type of exhaust gas purification catalyst can be considered. Among them, it is preferred to use a porous carrier with a relatively large specific surface area (here refers to the specific surface area measured by the BET method, the same below). As preferred examples, aluminum oxide (Al2O3), cerium oxide (CeO2), zirconium oxide (ZrO2), silicon oxide (SiO2), titanium oxide (TiO2) and their solid solutions (for example, cerium oxide-zirconium oxide composite oxide (CZ composite oxide)) or their combinations can be listed. From the viewpoint of heat resistance and structural stability, the specific surface area of ​​the carrier particles (for example, aluminum oxide powder or CZ composite oxide powder) can be 10 to 500 m 2 / g, for example, 200 to 400 m 2 The average particle size of the carrier particles can typically be 1 to 500 nm, for example, 10 to 200 nm. Furthermore, the types of carriers contained in the first catalyst layer 261 and the second catalyst layer 262 can be the same or different.

[0040] The first catalyst layer 261 extends from the end 24a on the exhaust gas inflow side in the region of the partition wall 26 that contacts the inlet chamber 24, and extends for a length greater than the total length L of the partition wall 26. w The exhaust gas flowing into the inlet chamber 24 passes through the partition wall 26. Therefore, by disposing the first catalyst layer 261 inside the partition wall 26, the exhaust gas purification performance when passing through the partition wall 26 can be effectively improved. Furthermore, according to the research of the inventors of the present invention, this structure is particularly effective in reducing pressure loss when the exhaust gas flows in.

[0041] The length (average length) L1 of the first catalyst layer 261 in the extension direction may be the above L w The content of the raw material is about 20% or more, typically 25% or more, preferably 30% or more, for example 50% or more, and about 90% or less, typically 85% or less, preferably 80% or less, for example 70% or less. Figure 3 In the embodiment shown, the length L1 of the first catalyst layer 261 is the length L w The inventors' research has shown that ash (ASH), composed of incombustible components, tends to accumulate near the seal portion 22 of the inlet chamber 24. Therefore, by setting L1 below a specified value, increases in pressure loss can be appropriately suppressed. Furthermore, by setting L1 above a specified value, exhaust gas purification capabilities can be more effectively utilized.

[0042] The second catalyst layer 262 extends from the end portion 25a on the exhaust gas outflow side in the region of the partition wall 26 that is in contact with the outlet chamber 25, and extends for a length greater than the total length L of the partition wall 26. w Short length formation.

[0043] The length (average length) L2 of the extension direction of the second catalyst layer 262 can be about 20% or more, typically 25% or more, for example 30% or more, preferably 50% or more, and about 90% or less, typically 85% or less, preferably 80% or less, for example 70% or less, of the above L w Figure 3 In the illustrated manner, the length L2 of the second catalyst layer 262 is about 60% of the above L w . Thereby, an increase in pressure loss can be suppressed and high purification performance can be achieved.

[0044] wherein, in the extension direction of the partition wall 26, the length of the first catalyst layer 261 and the length of the second catalyst layer 262 are substantially equal. However, the present application is not limited thereto. For example, one catalyst layer can be relatively long and the other catalyst layer can be relatively short. Figure 3

[0045] In the exhaust gas purification catalyst 10, the total length L w of the partition wall 26, the length Ll of the first catalyst layer 261, and the length L2 of the second catalyst layer 262 satisfy the following formula: L w < (Ll + L2) < 2L w . In other words, in the extension direction of the partition wall 26, a portion of the first catalyst layer 261 and a portion of the second catalyst layer 262 overlap each other. By boldly overlapping the first catalyst layer 261 and the second catalyst layer 262 in the extension direction, it is possible to prevent the exhaust gas from being directly discharged without being purified through the portion where no catalyst layer is formed. Thereby, the exhaust gas components reliably come into contact with the catalyst layers, and it is possible to effectively reduce emissions.

[0046] The length by which the first catalyst layer 261 and the second catalyst layer 262 overlap in the extension direction differs, for example, depending on the thickness of each catalyst layer, and the like, and thus is not particularly limited. It can be typically about 2% or more, typically 5% or more, preferably 10% or more, for example 20% or more, of the above L w , and about 60% or less, typically 50% or less, preferably 40% or less. Of these, from the viewpoint of highly coexisting low cost and high performance, it is preferable to be about 10 to 25% of the above L w .

[0047] The thickness (average thickness) of the first catalyst layer 261 and the second catalyst layer 262 can differ, for example, depending on the overall thickness T w of the partition wall 26 and the length of the extension direction of the catalyst layer, and the like, and thus is not particularly limited. Typically, the first catalyst layer 261 and the second catalyst layer 262 are each about 10 to 50% of the overall thickness T​​w For example, the thickness T1 of the first catalyst layer 261 and the thickness T2 of the second catalyst layer 262 can be respectively the same as the above T w The content of the carbon dioxide is 20% or more, typically 25% or more, preferably 30% or more, for example 35% or more, and is 90% or less, typically 80% or less, for example 70% or less.

[0048] In a preferred embodiment, the overall thickness T of the partition wall is w The thickness T1 of the first catalyst layer 261 and the thickness T2 of the second catalyst layer 262 satisfy the following formula: 0.2T w ≤(T w -T1-T2)≤0.4T w In other words, a gap is left so that the first catalyst layer 261 and the second catalyst layer 262 do not contact each other in the thickness direction. That is, in the thickness direction, a gap can be left between the first catalyst layer 261 and the second catalyst layer 262 with the T w The portion formed solely by the substrate is approximately 20-40% (e.g., 25-35%) of the thickness of the catalyst. This allows for stable catalytic performance. Furthermore, it is possible to suppress the migration of the catalyst metal and the degradation of the catalyst metal due to sintering or alloying.

[0049] The catalyst layer described above can be formed by the same method as in the conventional art.

[0050] For example, Figure 3 The exhaust gas-purifying catalyst 10 of the embodiment shown can be formed as follows.

[0051] First, prepare Figure 1 、 2 The honeycomb substrate 1 shown is used to form a first catalyst layer 261 inside the partition wall of the honeycomb substrate 1. Specifically, a slurry for forming the first catalyst layer containing the desired catalyst metal component (typically a solution containing the catalyst metal in ionic form) and the desired carrier powder is prepared. The properties of the slurry (viscosity and solid content, etc.) can be adjusted by taking into account the size of the honeycomb substrate 1 used and the porosity of the partition wall 26. Next, the slurry is supplied from the end 24a of the exhaust gas inflow side of the honeycomb substrate 1 to the inlet chamber 24, and a first catalyst layer 261 of the desired properties is formed in the pores of the partition wall 26 by an internal coating method. The properties of the first catalyst layer 261 (such as thickness or porosity) can be adjusted according to the properties of the slurry and the supply amount of the slurry. Alternatively, the outlet chamber 25 can be pressurized when the above-mentioned slurry is supplied, so that a pressure difference is generated between the inlet chamber 24 and the outlet chamber 25, and the slurry is adjusted so that the above-mentioned slurry does not excessively penetrate into the partition wall 26.

[0052] Next, a slurry for forming the second catalyst layer is prepared in the same manner as when forming the first catalyst layer 261. This slurry is supplied from the exhaust gas outlet end 25a of the honeycomb substrate 1 into the outlet chamber 25, where it is coated internally to form the second catalyst layer 262 having the desired properties within the pores of the partition walls 26.

[0053] The honeycomb base material 1 after applying the slurry is dried and fired at a predetermined temperature and time. Figure 3 An exhaust gas purifying catalyst 10 is shown.

[0054] The catalyst layer forming slurry may contain, in addition to the catalyst metal and the carrier, any additional components such as conventionally known oxygen absorbing and releasing materials, binders, and additives. Examples of oxygen absorbing and releasing materials include CZ composite oxides as carriers or non-supporting materials. Examples of binders include alumina sols and silica sols.

[0055] The exhaust gas purification catalyst disclosed herein can suppress increases in pressure loss while exhibiting excellent exhaust gas purification performance. Therefore, it is suitable for use in the exhaust systems (exhaust pipes) of various internal combustion engines, such as gasoline and diesel engines in automobiles. Gasoline engines are typically controlled at a stoichiometric air-fuel ratio, so exhaust gas readily flows between the partition wall portion near the inflow end and the partition wall portion near the outflow end. Therefore, the present invention is particularly effective.

[0056] Several embodiments of the present invention will be described below, but the present invention is not limited to these specific examples.

[0057] I. Study of Catalyst Layer Dimensions (Length, Thickness)

[0058] <Example 1>

[0059] As a substrate, a cordierite honeycomb substrate with a cell number of 300 cpsi (cells per square inch), a volume (total volume including the volume of the cell passage) of 0.9 L, a total length of 105 mm, an outer diameter of 103 mm, a partition wall thickness of 0.3 mm, and a porosity of 59% was prepared.

[0060] Next, Al2O3 powder (γ-Al2O3) 40 g as a carrier, an appropriate amount of rhodium aqueous solution having a content of Rh as a catalyst metal of 0.2 g, and an appropriate amount of pure water were mixed. The resulting mixture was subjected to stirring mixing, followed by drying and firing (500°C, 1 hour), whereby a catalyst metal-supporting powder in a form in which Rh was supported on the Al2O3 powder was obtained. This catalyst metal-supporting powder, a ceria-zirconia composite oxide solution in an amount of 60 g of the ceria-zirconia composite oxide after firing, and an appropriate amount of pure water were mixed, to prepare a slurry for forming a catalyst layer.

[0061] Next, the above slurry was supplied from the end portion of the honeycomb-shaped substrate on the exhaust gas inflow side into the inside of the inflow chamber in an amount of 100 g per 1 L of the substrate after supporting of the catalyst metal, to form a first catalyst layer in the pores of the partition wall in contact with the inflow chamber (length Ll in the extending direction: 30% of the entire length of the partition wall, thickness Tl: 35% of the thickness of the partition wall). At this time, a gas was supplied from the end portion of the exhaust gas outflow side of the outflow chamber, to generate a relative pressure difference between the inflow chamber and the outflow chamber, and the depth of penetration of the slurry into the partition wall was adjusted.

[0062] Next, the above slurry was supplied from the end portion of the honeycomb-shaped substrate on the exhaust gas outflow side into the inside of the outflow chamber in an amount of 100 g per 1 L of the substrate after supporting of the catalyst metal, to form a second catalyst layer in the pores of the partition wall in contact with the outflow chamber (length L2 in the extending direction: 30% of the entire length of the partition wall, thickness T2: 35% of the thickness of the partition wall). At this time, a gas was supplied from the end portion of the exhaust gas inflow side of the inflow chamber, to generate a relative pressure difference between the inflow chamber and the outflow chamber, and the depth of penetration of the slurry into the partition wall was adjusted.

[0063] Next, drying was performed at 150°C for 1 hour, followed by firing at 500°C for 1 hour, whereby an exhaust gas purifying catalyst (Example 1) was obtained. In Example 1, a portion in which no catalyst layer was formed existed in the central portion in the extending direction of the partition wall, and the length of this portion was 40% of the entire length L w of the partition wall.

[0064] Example 2

[0065] An exhaust gas purifying catalyst (Example 2) was produced in the same manner as in Example 1, except that the length in the extending direction of the first catalyst layer and the second catalyst layer was made 50% of the entire length L w of the partition wall.

[0066] Example 3

[0067] An exhaust gas purifying catalyst (Example 3) was produced in the same manner as in Example 1, except that the length in the extending direction of the first catalyst layer and the second catalyst layer was made 60% of the entire length L wExample 3) was produced in the same manner as Example 1. In this example, the first catalyst layer and the second catalyst layer overlap each other in the extension direction by a length of 10% of the length L of the separation wall. w In other words, in the central portion of the extension direction of the separation wall, the first catalyst layer and the second catalyst layer are stacked in the thickness direction (with the portion where no catalyst layer is formed interposed therebetween) to form a multilayer structure.

[0068] Examples 4 to 9

[0069] Examples 4 to 9

[0070] The specifications of the catalyst layers are summarized in Table 1 below.

[0071] [Table 1]

[0072]

[0073] Evaluation of Exhaust Purification Performance

[0074] The exhaust purification catalysts (Examples 1 to 9) obtained above were mounted to the exhaust pipe of a gasoline engine, and the exhaust purification performance was compared. Specifically, the exhaust purification catalyst was disposed in the exhaust system of an engine test stand, the evaluation temperature (input gas temperature) of the exhaust gas was adjusted to 400°C, and the purification rates of the HC component and the NOx component were measured. The results are shown in the column of Table 1. In addition, the graph showing the comparison of the purification performance of the exhaust purification catalysts relating to Examples 1 to 5 is shown in FIG. 1. x Figure 4

[0075] First, the preferable range of the extension direction overlap was investigated. As a result, it was found that the purification performance of Example 1 was the worst. As a reason therefor, it can be considered that since there is a portion where no catalyst metal is carried in the extension direction of the separation wall, the harmful components that are not purified pass through this portion. In addition, although the purification performance of Example 2 was improved compared to Example 1, about 15% of the harmful components were still discharged without being purified. Figure 4

[0076] In contrast to this, Examples 3 to 5 in which the two catalyst layers overlap each other in the extension direction showed relatively high purification performance. In particular, the extension direction overlap was the entire length L of the separation wall.​​​w Example 3 and Example 4, which have an overlap of 10 to 40% in the extension direction, showed the most excellent purification performance.

[0077] Further, the overlap in the extension direction can be the entire length L of the separation wall w Example 5, which has an overlap of 60% in the extension direction, showed a slightly lower purification performance than Example 3 and Example 4. As a reason for this, it can be considered that the pressure loss difference between the inlet chamber and the outlet chamber is due to the fact that a large amount of catalyst metal is supported in Example 5. That is, in Example 5, the pressure loss difference between the inlet chamber and the outlet chamber is increased because a large amount of catalyst metal is supported. As a result, the exhaust gas passes through the catalyst layer (particularly, the separation wall) more quickly, and the purification performance is reduced compared to Example 3 and Example 4.

[0078] As described above, by partially overlapping the first catalyst layer and the second catalyst layer in the extension direction of the separation wall, relatively high exhaust gas purification performance can be achieved. In addition, the overlap in the extension direction can be 2% or more and 60% or less (particularly, 10% or more and 40% or less) of the entire length L of the separation wall w of the separation wall. Such results show the technical significance of the present application.

[0079] Next, comparative test examples (Example 3, Example 8) having the same overlap in the extension direction were compared, and the preferred range in the thickness direction was investigated. The purification rate of Example 3 was substantially the same as that of Example 8, which was a reference example. As a result, in the thickness direction, a gap of about 20 to 40% (typically, 25 to 35%) of T w may be provided between the first catalyst layer and the second catalyst layer. In other words, T w -T1-T2 can be 0.2T w or more and 0.4T w or less. As a result, productivity and ease of handling can be improved. Furthermore, the desired catalyst performance can be exhibited, and movement of the catalyst metal can be suppressed, and deterioration of the catalyst metal due to sintering or alloying can be suppressed.

[0080] In addition, according to the comparison of Example 2 with Example 6 and Example 7, which were reference examples, when the overlap is 0 (i.e., L w =(L1+L2)), in the thickness direction, the catalyst layer can exist everywhere, or the first catalyst layer and the second catalyst layer can be partially overlapped. In other words, the following equation can be satisfied: T w ≤(T1+T2)<2T w . As a result, relatively high exhaust gas purification performance can be achieved.

[0081] II. Study of Catalyst Metal Type

[0082] <Example 10 to Example 12>

[0083] A catalyst for purifying exhaust gas (Examples 10 to 12) was produced in the same manner as in the above Example 8, except that the kind of catalyst metal was changed to that shown in Table 2. Next, the exhaust gas purification performance was evaluated in the same manner as in the above I. The results are shown in this column of Table 2.

[0084] [Table 2]

[0085] Table 2 Study of the kind of catalyst metal

[0086]

[0087] As is clear from Table 2, the purification rate is particularly high when rhodium is used in the first catalyst layer and rhodium or palladium is used in the second catalyst layer. Therefore, as the kind of catalyst metal, it is preferable that rhodium be used in both the first catalyst layer and the second catalyst layer. Alternatively, as another preferable example, it is preferable that rhodium, which has a high reduction activity, be used in the first catalyst layer and palladium, which has a high oxidation activity, be used in the second catalyst layer.

[0088] III. Detailed study on the overlap of the two catalyst layers

[0089] Examples 13 to 16

[0090] A catalyst for purifying exhaust gas (Examples 13 to 16) was produced in the same manner as in the above Example 4, except that the length L2 of the extension direction of the second catalyst layer was formed as shown in Table 3. Next, the exhaust gas purification performance was evaluated in the same manner as in the above I. The results are shown in this column of Table 3.

[0091] [Table 3]

[0092]

[0093] As is clear from Tables 1 and 3, when the overlap of the extension directions of the two catalyst layers is made to be 10 to 25% of the total length L of the partition wall, the purification performance is particularly excellent. Such a result shows the technical significance of the present application. w

[0094] The above detailed description of the present application is based on specific examples, but these are merely illustrative and do not limit the scope of the claims. The technology recited in the claims also includes various modifications and changes of the above-described specific examples.

[0095] Explanation of symbols

[0096] 1: honeycomb-like substrate; la: end portion; 2: seal portion; 4: opening portion; 6, 26: partition wall; 10: catalyst for purifying exhaust gas; 22: seal portion; 24: inlet-side chamber; 24a: end portion on the exhaust gas inflow side; 25: outlet-side chamber; 25a: end portion on the exhaust gas outflow side; 261: first catalyst layer; 262: second catalyst layer.​

Claims

1. A wall-flow type exhaust gas purification catalyst which is arranged in an exhaust pipe of a gasoline engine and purifies exhaust gas discharged from the gasoline engine, the exhaust gas purification catalyst characterized by comprising: an inlet-side chamber whose end portion on the exhaust gas inflow side is open and an outlet-side chamber whose end portion on the exhaust gas outflow side is open, which are separated by a porous separation wall; and a first catalyst layer and a second catalyst layer which are arranged in the inside of the separation wall, wherein the first catalyst layer and the second catalyst layer each contain a catalyst metal and ceria-zirconia composite oxide, the catalyst metal of the first catalyst layer is composed of rhodium, the catalyst metal of the second catalyst layer is composed of rhodium or palladium, the first catalyst layer and the second catalyst layer each exist in the inside of the separation wall in a state of being offset from the surface of the separation wall, and the second catalyst layer is arranged in the inside of the separation wall in a state of being offset from the first catalyst layer. A wall-flow-structured substrate, wherein 2. The exhaust gas purification catalyst according to claim 1, characterized in that: the catalyst metal of the second catalyst layer is composed of rhodium. a first catalyst layer which is provided in a region of the inside of the separation wall which is in contact with the entry side chamber, from the end portion of the exhaust gas inflow side in the extension direction of the separation wall at a shorter length than the full length L of the separation wall w short length formation; 3. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: in each of the first catalyst layer and the second catalyst layer, 90 mass% or more of the total amount of the catalyst metal exists in the inside of the separation wall. a second catalyst layer which is provided in a region of the separation wall inside which the out-side chamber is present, from the end of the exhaust gas outflow side in the extension direction of the separation wall at a position which is closer to the exhaust gas inflow side than the full length L of the separation wall w a short length is formed, 4. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: the first catalyst layer and the second catalyst layer each contain ceria-zirconia composite oxide in an amount of 10 mass% or more.

5. The exhaust gas purification catalyst according to claim 4, characterized in that: the length LI of the first catalyst layer is longer than the length L2 of the second catalyst layer.

6. The exhaust gas purification catalyst according to claim 5, characterized in that: the length LI of the first catalyst layer is 1.1 to 2.0 times the length L2 of the second catalyst layer.

7. The exhaust gas purification catalyst according to claim 1 or 2, characterized in that: in the inside of the separation wall, the first catalyst layer and the second catalyst layer do not contact in the thickness direction. In the extension direction, the length of the first catalyst layer is set as LI, the length of the second catalyst layer is set as L2, and the LI w , the LI and the L2 satisfy the following formula: L w < (LI + L2) < 2L w , the first catalyst layer and the second catalyst layer partially overlap in the extension direction. ​ ​ ​ ​ ​ The length in which the first catalyst layer overlaps with the second catalyst layer is 10% or more and 25% or less of the L w . ​ ​ ​ The length LI of the first catalyst layer is 50% or more and 70% or less of the L w of the L ​ ​

Images