Catalyst for exhaust gas purification

Abstract

To provide a wall-flow-type exhaust gas purification catalyst in which there is a small pressure drop caused by deposition of PM.SOLUTION: There is provided an exhaust gas purification catalyst 1 comprising a catalyst coated filter 2. The catalyst coated filter 2 comprises a filter base material 21 and a catalyst layer 22 provided on a pore wall of the filter base material 21. The exhaust gas purification catalyst 1 has a first end, a second end, a porous partition wall W, a first cell and a second cell. The first cell is closed by the second end, the second cell is closed by the first end and the first cell and the second cell are adjacent through the porous partition wall W. In the porous partition wall W on the surface of the side of the first cell, the ratio SS / S of the total area SS of pores having an opening diameter of less than 40 μm to the total area S of the all pores is 65% or more.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a catalyst for purifying exhaust gas.

Background Art

[0002] Exhaust gas discharged from an internal combustion engine contains harmful substances such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO x x). For purifying such exhaust gas, for example, a straight-flow type exhaust gas purification catalyst containing a platinum group metal as a catalyst metal is used.

[0003] In addition, exhaust gas discharged from a diesel engine contains particulate matter (PM) at a relatively high concentration in addition to the above harmful substances. Therefore, a diesel particulate filter (DPF) is further used to remove PM from the exhaust gas for purifying the exhaust gas discharged from the diesel engine.

[0004] In recent years, regulations on PM emissions have been tightened, and there has been a need to remove PM not only from the exhaust gas discharged from diesel engines but also from the exhaust gas discharged from gasoline engines. Therefore, a gasoline particulate filter (GPF) is being used for purifying the exhaust gas discharged from gasoline engines.

[0005] As such particulate filters, for example, there is a wall-flow type exhaust gas purification catalyst in which a supported catalyst is supported on the partition walls of the filter. When such an exhaust gas purification catalyst is used, it is possible to reduce the installation space of the exhaust gas purification system and to reduce the cost of the exhaust gas purification system.

[0006] The wall-flow type exhaust gas purification catalyst is described in, for example, Patent Document 1. Patent Document 1 describes that, for example, a powder made of a metal oxide is deposited only in the pores of a porous partition wall, which is a filter wall of a wall-flow type exhaust gas purification catalyst, and the powder fills up to 50% of the total pore volume at most.

Prior Art Documents

Patent Document

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] An object of the present invention is to provide a wall flow type exhaust gas purification catalyst with a small pressure loss caused by the deposition of particulate matter.

Means for Solving the Problems

[0009] According to one aspect of the present invention, there is provided an exhaust gas purification catalyst including a catalyst-coated filter, the catalyst-coated filter including a filter substrate and a catalyst layer provided on the pore walls of the filter substrate, having a first end, a second end, a porous partition wall, a first cell, and a second cell, the first cell extending from the first end toward the second end, opening at the first end and being closed at the second end, the second cell extending from the second end toward the first end, opening at the second end and being closed at the first end, the first cell and the second cell being adjacent to each other with the porous partition wall therebetween, and the porous partition wall having, on the surface on the first cell side, a ratio S S of the total area S S of pores having an opening diameter of less than 40 μm to the total area S of all pores of 65% or more. An exhaust gas purification catalyst is provided.

[0010] Here, the “opening diameter” is a value obtained by the following method. First, image the surface of the first cell side of the porous partition wall at a magnification of 200 times to obtain a grayscale image. An optical microscope is used for this imaging. Also, this imaging is performed on a region near the center in the length direction of the catalyst-coated filter among the surfaces of the first cell side of the porous partition wall. Note that this "length direction" is the same as the length direction of the first and second cells. Next, binarize the obtained grayscale image to obtain a binary image. Then, obtain the area of each black portion on the surface of the first cell side in the binary image, and set the diameter of a circle having an area equal to this area, that is, the equivalent circle diameter, as the "aperture diameter" of the pore corresponding to that black portion. Here, as long as a certain black portion is separated from other black portions, regardless of its shape, that black portion is considered to correspond to one pore. Note that image processing software "ImageJ" can be used for this image processing.

[0011] This exhaust gas purification catalyst is a particulate filter including a catalyst layer. More specifically, this exhaust gas purification catalyst is a wall flow type exhaust gas purification catalyst in which exhaust gas sequentially passes through the first cell, the pores of the porous partition wall, and the second cell, and in this process, PM in the exhaust gas is collected by the porous partition wall.

[0012] Generally, a wall flow type exhaust gas purification catalyst is used to remove PM from exhaust gas discharged from internal combustion engines such as gasoline engines and diesel engines. For example, a wall flow type exhaust gas purification catalyst is used in a self-propelled vehicle including a gasoline engine or a diesel engine as at least a part of the power source.

[0013] In a wall flow type exhaust gas purification catalyst, the catalyst layer is provided to promote the combustion of PM collected by the porous partition wall.

[0014] For example, in a gasoline vehicle that uses the power generated in a gasoline engine as driving force and is equipped with a wall-flow type exhaust gas purification catalyst, the catalyst layer promotes the combustion of the collected PM, particularly during high-load operation periods when the gasoline engine discharges high-temperature exhaust gas, such as when driving in the suburbs or on highways.

[0015] In addition, the exhaust gas discharged from a diesel engine is at a lower temperature compared to the exhaust gas discharged from a gasoline engine. Therefore, in a diesel vehicle that uses the power generated in a diesel engine as driving force and is equipped with a wall-flow type exhaust gas purification catalyst, the temperature of the exhaust gas is increased by injecting fuel into the exhaust gas to burn the collected PM. The catalyst layer promotes this combustion and, therefore, contributes to a reduction in the fuel injected into the exhaust gas.

[0016] As described above, the catalyst layer can promote the combustion of PM, but the PM combustion by the catalyst layer is not always sufficiently promoted. For example, in a gasoline vehicle equipped with a wall-flow type exhaust gas purification catalyst, when driving a short distance while repeatedly stopping and going, the amount of PM collected exceeds the amount of PM that burns, and as a result, PM accumulates in the wall-flow type exhaust gas purification catalyst. Also, in a diesel vehicle equipped with a wall-flow type exhaust gas purification catalyst, PM accumulates in the wall-flow type exhaust gas purification catalyst during the period before fuel injection into the exhaust gas.

[0017] When PM accumulates, the pressure loss that occurs in the wall-flow type exhaust gas purification catalyst increases. When the pressure loss increases, the fuel consumption performance deteriorates. Therefore, it is desirable that the increase in pressure loss associated with the accumulation of PM be small.

[0018] In a general wall-flow type exhaust gas purification catalyst, the above ratio S S / S is much smaller than 65%. In such an exhaust gas purification catalyst, at the initial stage of PM deposition, PM accumulates in pores located within the surface area on the first cell side of the porous partition wall, that is, pores where the distance from the surface of the first cell side of the porous partition wall is, for example, 30% or less of the thickness of the porous partition wall, and the amount of PM deposited in these pores increases. When the amount of PM deposited in these pores increases, the gas flow path in the porous partition wall narrows or becomes blocked, and as a result, the pressure loss increases significantly. Therefore, at the initial stage of PM deposition, as the PM collection amount increases, the pressure loss increases rapidly.

[0019] When the deposition of PM into the pores located within the above surface area progresses to a certain extent, PM starts to deposit on the surface of the first cell side of the porous partition wall. The deposited layer composed of PM on this surface is a granular layer with a low apparent density. In this granular layer, it is difficult for the gas flow path to narrow or become blocked as the amount of PM deposited increases. Therefore, during this period, the increase in pressure loss as the PM collection amount increases is gentle.

[0020] Thus, in a general wall flow type exhaust gas purification catalyst, at the initial stage of PM deposition, the pressure loss increases rapidly and significantly. Therefore, in an automobile equipped with such an exhaust gas purification catalyst, the ratio of the period with a large pressure loss during the period from when PM starts to accumulate until the amount of its accumulation becomes sufficiently small due to combustion of PM is high.

[0021] On the other hand, in an exhaust gas purification catalyst in which the ratio S of the total area S of all pores to the total area of pores with an opening diameter of less than 40 μm on the surface of the first cell side of the porous partition wall S of S S / S is 65% or more, it is difficult for PM to reach pores located far from the surface of the first cell side of the porous partition wall. Therefore, the amount of PM deposited in the pores of the porous partition wall is small, and it is difficult for the gas flow path in the porous partition wall to narrow or become blocked. Therefore, this exhaust gas purification catalyst has a small pressure loss caused by the deposition of PM.

[0022] Ratio S S / S is preferably 70% or more. Ratio S S / S has no upper limit, but according to one example, ratio S S / S is 95% or less.

[0023] According to another aspect of the present invention, the porous partition wall has, on the surface, the total area S of the pores having an aperture diameter of 40 μm or more and less than 60 μm, which accounts for the total area S of all the pores M Ratio S M A catalyst for purifying exhaust gas according to the above aspect, in which ratio S

[0024] Ratio S S When ratio S M / S increases, ratio S M / S tends to decrease. Ratio S S / S is preferably 20% or less. Ratio S M / S has no lower limit, but according to one example, ratio S L

[0025] According to still another aspect of the present invention, the porous partition wall has, on the surface, the total area S of the pores having an aperture diameter of 60 μm or more, which accounts for the total area S of all the pores L Ratio S L A catalyst for purifying exhaust gas according to any of the above aspects, in which ratio S

[0026] Ratio S S When ratio S L / S increases, ratio S L / S tends to decrease. Ratio S L / S is preferably 10% or less. Ratio S L / S has no lower limit, but according to one example, ratio S SS

[0027] According to still another aspect of the present invention, the porous partition wall has, on the surface, the total area S of the pores having an aperture diameter of less than 20 μm, which accounts for the total area S of all the pores SS Ratio S SS A catalyst for purifying exhaust gas according to any of the above aspects, in which ratio S

[0028] Ratio S SS An exhaust gas purification catalyst with a large / S has a relatively large pressure loss in a state where PM is not deposited compared to an exhaust gas purification catalyst with a small / S. Ratio S SS / S has no lower limit, but according to one example, ratio S SS / S is 20% or more. SS / S is 20% or more.

[0029] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any of the above aspects including a honeycomb structure and plugs.

[0030] The honeycomb structure is a columnar body having a pair of opposing bottom surfaces, and a plurality of through-holes extending from one bottom surface to the other bottom surface are provided. Here, one bottom surface corresponds to the first end, and the other bottom surface corresponds to the second end. The shape of the pair of opposing bottom surfaces is, for example, circular, elliptical, oval, or polygonal.

[0031] The honeycomb structure includes partition walls that form the side walls of these through-holes. These partition walls are porous and partition adjacent through-holes.

[0032] As the material of the honeycomb structure, for example, ceramics such as cordierite, aluminum titanate, and silicon carbide can be used. A non-woven fabric made of metal or alloy may be woven into such a honeycomb structure. Alternatively, a metal or alloy such as stainless steel may be used as the material of the honeycomb structure.

[0033] Each plug closes the through-hole of the honeycomb structure at one end side. Half of the plurality of through-holes are closed at the second end side by plugs. The first cell is a space surrounded by a plug that closes the through-hole at the second end side and a partition wall that forms the side wall of this through-hole.

[0034] The remaining through-holes of the honeycomb structure not blocked at the second end side are blocked at the first end side by plugs. The second cell is a space surrounded by a plug blocking the through-hole at the first end side and a partition wall forming the side wall of this hole.

[0035] The first cell and the second cell are adjacent to each other with a partition wall of the filter substrate and a catalyst layer formed in the pores of the partition wall interposed therebetween.

[0036] As the material of the plug, for example, ceramics such as cordierite, aluminum titanate, and silicon carbide can be used.

[0037] According to still another aspect of the present invention, the filter substrate is provided with an exhaust gas purification catalyst according to any of the above aspects, wherein the volume V is in the range of 0.1 to 5 L. Here, the "volume" of the filter substrate is the volume including the spaces corresponding to the first and second cells of the filter substrate and the partition walls, and is calculated by multiplying the area of the bottom surface of the filter substrate by the height of the filter substrate. The volume V of the filter substrate is preferably 0.5 L or more. Also, the volume V of the filter substrate is preferably 3 L or less, and more preferably 2 L or less.

[0038] According to still another aspect of the present invention, the filter substrate is provided with an exhaust gas purification catalyst according to any of the above aspects, wherein the dimensions of the filter substrate in the length direction of the first cell and the second cell are in the range of 10 to 500 mm. This dimension is preferably in the range of 50 to 300 mm.

[0039] According to still another aspect of the present invention, the filter substrate is provided with an exhaust gas purification catalyst according to any of the above aspects, wherein the portion corresponding to the porous partition wall of the filter substrate, that is, the thickness of the partition wall of the filter substrate is in the range of 0.05 to 2 mm. If this thickness is reduced, the mechanical strength of the filter substrate decreases. If this thickness is increased, the porous partition wall becomes thicker, and as a result, the pressure loss in a state where PM is not deposited increases. This thickness is preferably in the range of 0.1 to 1 mm.

[0040] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any of the above aspects, wherein the porosity of the partition walls of the filter substrate corresponding to the porous partition walls, i.e., the porosity of the partition walls of the filter substrate, is in the range of 30 to 90%. Note that this "porosity" is a value obtained by the mercury intrusion method. When the porosity is increased, the mechanical strength of the filter substrate decreases. When the porosity is decreased, the porosity of the porous partition walls also decreases, and as a result, the pressure loss in a state where PM is not deposited increases. This porosity is preferably in the range of 40 to 80%.

[0041] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any of the above aspects, wherein the average pore diameter of the partition walls of the filter substrate corresponding to the porous partition walls, i.e., the average pore diameter of the partition walls of the filter substrate, is in the range of 5 to 50 μm. Note that this "average pore diameter" is a value obtained by the mercury intrusion method. When the average pore diameter is increased, the mechanical strength of the filter substrate decreases. When the average pore diameter is decreased, the pressure loss in a state where PM is not deposited increases. This average pore diameter is preferably in the range of 10 to 40 μm.

[0042] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any of the above aspects, wherein the catalyst layer contains a noble metal. The noble metal is an example of a catalyst metal. The noble metal is, for example, a platinum group element. The catalyst layer can contain, for example, one or more of platinum, palladium, and rhodium as the noble metal. These noble metals have a high ability to promote the combustion of PM.

[0043] According to still another aspect of the present invention, the mass M of the noble metal M and the volume V of the filter substrate, the ratio M M / V is in the range of 0.01 to 10 g / L, and there is provided an exhaust gas purification catalyst according to the above aspect. When the ratio M M / V is small, the effect of the noble metal in promoting the combustion of PM is small. When the ratio M M / V is increased, the cost becomes high. The ratio M M / V preferably ranges from 0.1 to 5 g / L.

[0044] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any of the above aspects, wherein the catalyst layer further includes at least one of a porous carrier carrying the noble metal and a promoter. When a porous carrier is used, it is easy to increase the specific surface area of the noble metal. When a promoter such as an oxygen storage material is used, for example, changes in the performance of the catalyst due to fluctuations in the composition of the exhaust gas can be reduced.

[0045] The porous carrier and the promoter are, for example, alumina; a composite oxide of ceria and zirconia; a polycrystal or single crystal containing this composite oxide as a main component and further containing one or more selected from the group consisting of oxides of rare earth elements other than cerium, oxides of alkaline earth metal elements, oxides of transition metal elements other than zirconium, alumina, and silica; or a combination of two or more thereof.

[0046] The average particle diameter of each of the porous carrier and the promoter is preferably in the range of 0.05 to 5 μm, and more preferably in the range of 0.1 to 3 μm. This "average particle diameter" is the median diameter obtained by the laser diffraction / scattering method.

[0047] According to still another aspect of the present invention, the mass M of the catalyst layer C and the volume V of the filter base material, the ratio M C / V provides an exhaust gas purification catalyst according to the above aspect, which is 300 g / L or less. The ratio M C / V is preferably 250 g / L or less, more preferably 150 g / L or less, still more preferably 120 g / L or less, and still more preferably 100 g / L or less.

[0048] According to still another aspect of the present invention, the mass M of the catalyst layer C and the volume V of the filter base material, the ratio M C / V provides an exhaust gas purification catalyst according to the above aspect, which is in the range of 10 to 300 g / L. The ratio M CWhen / V is small, the contribution of the catalyst layer to reducing the number of pores with a large opening diameter is small. Specific ratio M C When / V is increased, the pressure loss in a state where PM is not deposited increases. Specific ratio M C / V is preferably in the range of 20 to 250 g / L, more preferably in the range of 20 to 200 g / L, and even more preferably in the range of 30 to 100 g / L. Specific ratio M C The lower limit value of / V may be 25 g / L. Also, specific ratio M C The upper limit value of / V may be 150 g / L.

[0049] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any one of the above aspects, wherein at least a part of the catalyst layer is located in a portion of the porous partition wall on the first cell side. Here, the "portion of the porous partition wall on the first cell side" is a portion of the porous partition wall where the distance from the surface on the first cell side is 80% or less of the thickness of the porous partition wall. The entire catalyst layer may be located in a portion of the porous partition wall on the first cell side. Alternatively, the catalyst layer may be provided over the entire thickness of the porous partition wall. That is, the catalyst layer preferably extends from the surface on the first cell side of the partition wall of the filter substrate to the vicinity of the surface on the second cell side of this partition wall, for example, extends from the surface on the first cell side of the partition wall of the filter substrate to the surface on the second cell side of this partition wall. Such a structure is particularly advantageous in reducing the pressure loss in a state where PM is not deposited. Further, such a structure is also particularly advantageous in reliably burning PM and purifying other harmful substances.

[0050] According to still another aspect of the present invention, when the pores inside the filter substrate are divided into first pores having a pore diameter of 5 μm or more and less than 10 μm, second pores having a pore diameter of 10 μm or more and less than 20 μm, and third pores having a pore diameter of 20 μm or more in a cross section perpendicular to the surface of the portion of the porous partition wall on the first cell side, that is, the portion of the filter partition wall of the catalyst-coated filter on the first cell side, the filling rate R of the first pores by the catalyst layer F1 and the filling rate R of the second pores by the catalyst layerF2 and the filling rate R of the catalyst layer in the third pores F3 satisfies the inequality: R F1 < R F2 < R F3 There is provided an exhaust gas purification catalyst according to any of the above aspects that satisfies the relationship shown in the formula.

[0051] Here, the filling rate R F1 is the ratio of the total area S F1 of the first pores in the cross section to the total area S C1 of the portion of the catalyst layer located within the first pores. The filling rate R F2 is the ratio of the total area S F2 of the second pores in the cross section to the total area S C2 of the portion of the catalyst layer located within the second pores. The filling rate R F3 is the ratio of the total area S F3 of the third pores in the cross section to the total area S C3 of the portion of the catalyst layer located within the third pores.

[0052] Here, the boundaries between the continuous pores and the pore diameters of each pore are determined by a method to be described later with reference to the drawings.

[0053] The configuration specified by the above inequality can be used to reduce the number of pores with a large opening diameter while suppressing an increase in the pressure loss in a state where PM is not deposited.

[0054] According to still another aspect of the present invention, the filling rate R F1 is 40% or less, the filling rate R F2 is 40% or less, and there is provided an exhaust gas purification catalyst according to the above aspect where the filling rate R F3 is 45% or less.

[0055] Increasing these filling rates increases the pressure loss in a state where PM is not deposited.

[0056] According to still another aspect of the present invention, the filling rate RF3 There is provided an exhaust gas purification catalyst according to any one of the above aspects, wherein the filling rate R is 20% or more. F3 When it is small, a sufficient amount of catalyst cannot be arranged in the exhaust gas flow path, which is disadvantageous for purifying harmful substances.

[0057] According to still another aspect of the present invention, the filling rate R F1 is 10% or more, and there is provided an exhaust gas purification catalyst according to any one of the above aspects, wherein the filling rate R F2 is 15% or more.

[0058] The filling rate R F1 and R F2 are preferably small. Since most of the exhaust gas flows through the third pores, from the viewpoints of PM combustion and purification of other harmful substances, it is preferable to decrease the filling rate R F1 and R F2 and increase the filling rate R F3 .

[0059] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to any one of the above aspects, further including powdery inorganic particles supported by the catalyst-coated filter. Hereinafter, the above exhaust gas purification catalyst further including powdery inorganic particles is referred to as a "powder-added catalyst-coated filter".

[0060] Here, the term "powdery inorganic particles" may be in a state where the particles are not adhered to each other or to other articles, or may be in a state where such particles are immobilized on other articles by heat treatment or chemical solution treatment.

[0061] The powdery inorganic particles can be used, together with the catalyst layer, to reduce the number of pores having a large opening diameter. The inorganic particles may be primary particles or secondary particles.

[0062] According to still another aspect of the present invention, there is provided an exhaust gas purification catalyst according to the above-described aspect in which the inorganic particles are unevenly distributed on the first cell side of the porous partition wall. Alternatively, according to still another aspect of the present invention, the total amount A of the inorganic particles, the amount A1 of the inorganic particles located on the surface on the first cell side of the catalyst-coated filter, and the pores of the catalyst-coated filter, where the distance from the surface on the first cell side of the catalyst-coated filter is 20% or less of the thickness of the filter partition wall of the catalyst-coated filter corresponding to the porous partition wall, i.e., the amount A2 of the inorganic particles, satisfy the relationship represented by the inequality (A1 + A2) / A ≧ 90%. There is provided an exhaust gas purification catalyst according to any of the above aspects.

[0063] Here, the uneven distribution of the inorganic particles as described above can be confirmed by the following method. First, a cross-section of the porous partition wall is imaged with a scanning electron microscope to obtain a grayscale image. This imaging is performed on a cross-section of a portion of the porous partition wall where the distance from the first end and the distance from the second end are equal. Next, the analysis position by an energy dispersive X-ray analyzer is specified in the previous grayscale image, and the intensity of characteristic X-rays derived from elements contained only in the inorganic particles is measured. Here, line analysis along the thickness direction of the porous partition wall will be performed. From this analysis result, it can be confirmed that the inorganic particles are unevenly distributed as described above. In addition, the uneven distribution of the inorganic particles as described above can also be confirmed from a composite image obtained by superimposing points having brightness corresponding to the intensity of characteristic X-rays and colored on the previous grayscale image.

[0064] The configuration in which the inorganic particles are unevenly distributed as described above can be used to reduce the number of pores with a large opening diameter on the surface of the first cell side.

[0065] The ratio (A1 + A2) / A representing the degree to which the inorganic particles are unevenly distributed on the first cell side of the porous partition wall is preferably 90% or more. There is no limit to the upper limit value of the ratio (A1 + A2) / A. The ratio (A1 + A2) / A may be 100%.

[0066] According to still another aspect of the present invention, on the surface, the pores of the catalyst-coated filter are divided into first small pores having an opening diameter of less than 40 μm and first large pores having an opening diameter of 40 μm or more. When the portion of the powder-added catalyst-coated filter corresponding to the catalyst-coated filter, that is, the pores of the filter partition on the surface of the first cell side of the filter partition are divided into second small pores having an opening diameter of less than 40 μm and second large pores having an opening diameter of 40 μm or more, the total area S S2 of the second small pores with respect to the total area S S2 and the total area S S1 of the first small pores, the difference S S2 -S S1 ratio (S S2 -S S1 ) / S S2 is 40% or less, and the total area S L2 of the second large pores with respect to the total area S L2 and the total area S L1 of the first large pores, the difference S L2 -S L1 ratio (S L2 -S L1 ) / S L2 is 60% or more, and an exhaust gas purification catalyst according to any of the above aspects is provided.

[0067] Ratio (S L2 -S L1 ) / S L2 and ratio (S S2 -S S1 ) / S S2 satisfy the above requirements, the large pores have a greater degree of reduction in the opening diameter due to the application of inorganic particles compared to the small pores. Such a configuration is advantageous, for example, in obtaining a structure in which the ratio S S / S of the total area S S of the pores having an opening diameter of less than 40 μm in the total area S of all the pores is large, and the ratio S SS / S of the total area S SS of the pores having an opening diameter of less than 20 μm in the total area S of all the pores is small.

[0068] According to still another aspect of the present invention, there is provided a catalyst for purifying exhaust gas according to any of the above aspects, wherein the inorganic particles have an average particle diameter in the range of 1 to 50 μm.

[0069] Here, the "average particle diameter" is the median diameter obtained by the laser diffraction / scattering method. Inorganic particles having an average particle diameter within the above range are suitable, for example, for obtaining a structure in which the ratio S S / S is large and the ratio S SS / S is small. This average particle diameter is preferably in the range of 5 to 10 μm.

[0070] According to still another aspect of the present invention, there is provided a catalyst for purifying exhaust gas according to any of the above aspects, wherein the inorganic particles contain one or more selected from the group consisting of metal oxides, metal hydroxides, metal carbonates, metal phosphates, metal nitrates, metal sulfates, clay minerals, and porous inorganic substances.

[0071] Preferably, the inorganic particles consist of one or more selected from the group consisting of metal oxides, metal hydroxides, metal carbonates, metal phosphates, metal nitrates, metal sulfates, and porous inorganic substances.

[0072] The metal element contained in the inorganic particles is, for example, one or more selected from the group consisting of alkali metal elements, alkaline earth metal elements, rare earth elements, and transition metal elements. This metal element is preferably one or more selected from the group consisting of calcium, magnesium, strontium, barium, aluminum, silicon, titanium, zirconium, and cerium.

[0073] Specific examples of the inorganic particles include, for example, for metal oxides, calcium oxide, cerium oxide, titanium dioxide, zirconium dioxide, silicon dioxide, aluminum oxide, mixtures thereof, and mixed oxides. The clay mineral may be an artificial clay mineral or a natural clay mineral. As the porous inorganic substance, for example, one or more of zeolite and sepiolite can be used. The inorganic particles preferably contain calcium oxide, and more preferably consist of calcium oxide.

[0074] According to still another aspect of the present invention, in the powder-added catalyst-coated filter, the mass M of the inorganic particles with respect to the volume V of the filter substrate P of the ratio M P / V is 3 g / L or more, and an exhaust gas purification catalyst according to any of the above aspects is provided.

[0075] When the ratio M P / V is small, the effect of using inorganic particles does not appear significantly. The ratio M P / V is preferably 5 g / L or more.

[0076] According to still another aspect of the present invention, the mass M of the inorganic particles with respect to the volume V of the filter substrate P of the ratio M P / V is 50 g / L or less, and an exhaust gas purification catalyst according to any of the above aspects is provided.

[0077] When the ratio M P / V is increased, the pressure loss in the state where PM is not deposited increases. The ratio M P / V is preferably 15 g / L or less, and more preferably 10 g / L or less.

[0078] According to still another aspect of the present invention, there is provided a method for manufacturing an exhaust gas purification catalyst according to any of the above aspects, including forming the catalyst layer provided on the pore walls of the filter substrate.

[0079] The catalyst layer can be formed, for example, by the following method.

[0080] First, a slurry containing a raw material for the catalyst layer and a dispersion medium is prepared. The raw material for the catalyst layer includes a catalyst metal and, optionally, at least one of a porous carrier and a promoter. The catalyst metal can be included in the slurry in the form of a metal compound soluble in the dispersion medium, or in the form of a supported catalyst in which the catalyst metal is supported on a porous carrier. The dispersion medium is, for example, an aqueous solvent such as water.

[0081] The slurry is prepared to have an appropriate viscosity. For example, the slurry is prepared such that the viscosity at a shear rate of 400 s -1 is greater than 50 mPa·s and within the range of 150 mPa·s or less, preferably within the range of 60 to 110 mPa·s.

[0082] Here, the viscosity of the slurry is the viscosity that can be measured at room temperature by a commercially available shear viscometer. For example, by using a standard dynamic viscoelasticity measuring device (rheometer) in the art, the viscosity at the above shear rate can be easily measured. Here, "room temperature" refers to a temperature within the range of 15 to 35°C, typically a temperature within the range of 20 to 30°C, for example, 25°C.

[0083] The slurry can further contain a thickener. As the thickener, for example, cellulose-based polymers such as carboxymethyl cellulose (CMC), methyl cellulose (MC), hydroxypropyl methyl cellulose (HPMC), and hydroxyethyl methyl cellulose (HEMC) can be used. The ratio of the thickener in the total solid content of the slurry is not particularly limited as long as the viscosity of the slurry satisfies the above range, but is preferably within the range of 0.1 to 5% by mass, more preferably within the range of 0.3 to 4% by mass, and still more preferably within the range of 0.5 to 3% by mass.

[0084] Next, the above slurry is supplied from the first end side of the filter substrate, and the gas in the filter substrate is sucked from the second end side of the filter substrate. Thereby, the slurry flows into the cells with the first end of the filter substrate open, and the slurry also flows into the pores of the partition walls from the surface of the partition walls forming these cells. Thereafter, if necessary, the filter substrate supplied with the slurry is dried.

[0085] Next, the above slurry is supplied from the second end side of the filter substrate, and the gas in the filter substrate is sucked from the first end side of the filter substrate. As a result, the slurry flows into the cells opened in the portion corresponding to the second end of the filter substrate, and the slurry flows into the pores of the partition walls from the surfaces of these partition walls. Note that this step can be omitted.

[0086] The above suction conditions may vary depending on the cross-sectional diameter of the filter substrate and the like. For example, in the case of a cylindrical filter substrate having a diameter in the range of 80 to 250 mm, suction is preferably performed under the condition that the linear velocity (wind speed) of the gas flow in the vicinity of the end of the filter substrate when the filter substrate is installed in the apparatus and no slurry is supplied is in the range of 10 to 80 m / s. The suction time is not particularly limited, but is preferably in the range of 0.1 to 30 seconds. Preferred combinations of the linear velocity and the suction time are 20 m to 70 m / s and 0.5 to 25 seconds; and 40 to 60 m / s and 2 to 15 seconds.

[0087] Thereafter, the filter substrate to which the slurry has been supplied is dried and subjected to firing. In this way, a catalyst-coated filter is obtained. When a high-viscosity slurry is used and suction is performed under the above conditions, the ratio R F1 , R F2 and R F3 a catalyst-coated filter that satisfies the above-described conditions can be obtained.

[0088] Next, preferably, inorganic particles are supplied to the catalyst-coated filter. Specifically, an aerosol containing the inorganic particles as aerosol particles is supplied to the first end portion corresponding to the first end of the catalyst-coated filter. At the same time, the gas in the catalyst-coated filter is sucked from the second end portion corresponding to the second end of the catalyst-coated filter. This suction is preferably performed with the first end portion of the catalyst-coated filter installed so as to face downward.

[0089] By this operation, the aerosol flows into the cells opened on the first end side of the catalyst-coated filter and is separated into gas and inorganic particles by the filter partition walls of the catalyst-coated filter. The gas flow path composed of pores with a large pore diameter has a lower ventilation resistance compared to the gas flow path composed of pores with a small pore diameter. Therefore, more inorganic particles accumulate in the pores with a large pore diameter than in the pores with a small pore diameter.

[0090] Also, in the catalyst-coated filter with the catalyst layer formed as described above, the pores opened on the surface of the filter partition wall where the aerosol flows in are not excessively large in pore diameter even if they are pores with a large pore diameter. Therefore, the inorganic particles can be localized in the vicinity of the surface of the filter partition wall where the aerosol flows in. As described above, an exhaust gas purification catalyst according to any of the above aspects, particularly a powder-added catalyst-coated filter, can be obtained.

Brief Description of the Drawings

[0091]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Mode for Carrying Out the Invention

[0092] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments described below are more specific forms of any of the above aspects.

[0093] Each of the features described below can be combined with each of the above aspects. Also, combinations of two or more of the features described below can be combined with each of the above aspects.

[0094] In the drawings referred to below, elements having the same or similar functions are denoted by the same reference numerals, and redundant descriptions are omitted. Also, in each drawing, the dimensional ratios and shapes may be different from the actual ones.

[0095] FIG. 1 is a cross-sectional view schematically showing an exhaust gas purification catalyst according to an embodiment of the present invention. FIG. 2 is an enlarged cross-sectional view showing the porous partition wall of the exhaust gas purification catalyst shown in FIG. 1. FIG. 3 is an enlarged cross-sectional view further showing the porous partition wall of the exhaust gas purification catalyst shown in FIG. 1. In FIGS. 1 and 2, the white arrows indicate the flow direction of the exhaust gas.

[0096] The particulate filter 1 for purifying exhaust gas shown in FIGS. 1 to 3 includes the catalyst layer 22 shown in FIGS. 2 and 3. This particulate filter 1 for purifying exhaust gas has a substantially cylindrical shape. As shown in FIG. 1, the particulate filter 1 for purifying exhaust gas has a first end E1, a second end E2, a porous partition wall W, a first cell C1, and a second cell C2. The first end E1 and the second end E2 are the bottom surfaces of the cylinder.

[0097] The first cell C1 extends from the first end E1 toward the second end E2. The first cell C1 is open at the first end E1 and closed at the second end E2.

[0098] The second cell C2 extends from the second end E2 toward the first end E1. The second cell C2 is open at the second end E2 and closed at the first end E1.

[0099] The first cell C1 and the second cell C2 are adjacent to each other with the porous partition wall W interposed therebetween. The first cell C1 and the second cell C2 are arranged so as to form a checkerboard pattern at the first end E1 and the second end E2.

[0100] As shown in FIGS. 1 to 3, the particulate filter 1 for purifying exhaust gas includes a catalyst-coated filter 2. As shown in FIGS. 2 and 3, the catalyst-coated filter 2 includes a filter base material 21 and a catalyst layer 22.

[0101] As shown in FIG. 1, the filter base material 21 includes a honeycomb structure 211 and plugs 212a and 212b.

[0102] The honeycomb structure 211 is a cylinder provided with a plurality of through holes each extending from one bottom surface to the other bottom surface. One of these bottom surfaces corresponds to the first end E1, and the other bottom surface corresponds to the second end E2. The honeycomb structure 211 includes partition walls 211W that constitute the side walls of these through holes. These partition walls 211W are porous and partition adjacent through holes.

[0103] The plug 212a closes a part of the holes of the honeycomb structure 211 on the second end E2 side. The first cell C1 is located in a space surrounded by the plug 212a closing the hole on the second end E2 side and the partition wall 211W constituting the side wall of this hole.

[0104] The plug 212b closes the remaining holes of the honeycomb structure 211 on the first end E1 side. The second cell C2 is located in a space surrounded by the plug 212b closing the hole on the first end E1 side and the partition wall 211W constituting the side wall of this hole.

[0105] These plugs 212a and 212b are arranged such that the hole whose second end E2 side is closed by the plug 212a and the hole whose first end E1 side is closed by the plug 212b are adjacent to each other with the partition wall 211W in between. The first cell C1 and the second cell C2 are adjacent to each other with the partition wall 211W of the filter base material 21 and the catalyst layer 22 provided on the pore wall thereof in between.

[0106] As shown in FIGS. 2 and 3, the catalyst layer 22 is supported by the filter base material 21. Specifically, the catalyst layer 22 is provided on the pore wall of the filter base material 21. That is, the catalyst layer 22 covers the inner pore wall of the partition wall 211W.

[0107] In this structure, the catalyst layer 22 is provided over the entire thickness of the porous partition wall W or the partition wall 211W. Of the catalyst layer 22, the portion that covers the inner pore wall of the partition wall 211W and is at a distance of a predetermined value or more from the surface of the partition wall 211W on the first cell C1 side can be omitted. That is, the entire catalyst layer 22 may be located in the portion of the porous partition wall W or the partition wall 211W on the first cell C1 side.

[0108] The partition wall 211W and the portion of the catalyst layer 22 supported by the partition wall 211W constitute the filter partition wall 21W. The filter partition wall 21W is porous.

[0109] As shown in FIGS. 2 and 3, the exhaust gas purification catalyst 1 further includes inorganic particles 3. The inorganic particles 3 are located on the surface of the porous partition wall W or the filter partition wall 21W on the first cell C1 side or in the vicinity thereof.

[0110] The inorganic particles 3 are in powder form. At least a part of the inorganic particles 3 is attached to the catalyst-coated filter 2, but not fixed to the catalyst-coated filter 2. Further, the inorganic particles 3 are not fixed to each other, but can be fixed by heat treatment or chemical treatment.

[0111] The inorganic particles 3 reduce the pore diameter of the pores located near the surface of the porous partition wall W on the first cell C1 side. Near the surface of the porous partition wall W on the first cell C1 side, the filling rate of the pores of the filter partition wall 21W with the inorganic particles 3 is low for pores with a small opening diameter and high for pores with a large opening diameter.

[0112] In this exhaust gas purification catalyst 1, on the surface of the porous partition wall W on the first cell C1 side, the ratio S of the total area S of the pores with an opening diameter of less than 40 μm to the total area S of all the pores is 65% or more. As described above, in such an exhaust gas purification catalyst 1, it is difficult for PM to reach the pores located far from the surface of the porous partition wall W on the first cell C1 side. Therefore, the amount of PM deposited in the pores P of the porous partition wall W is small, and narrowing or blockage of the gas flow path in the porous partition wall W hardly occurs. Therefore, this exhaust gas purification catalyst 1 has a small pressure loss caused by the deposition of PM. S of S S / S is 65% or more. As described above, in such an exhaust gas purification catalyst 1, it is difficult for PM to reach the pores located far from the surface of the porous partition wall W on the first cell C1 side. Therefore, the amount of PM deposited in the pores P of the porous partition wall W is small, and narrowing or blockage of the gas flow path in the porous partition wall W hardly occurs. Therefore, this exhaust gas purification catalyst 1 has a small pressure loss caused by the deposition of PM.

[0113] The catalyst layer 22 is preferably configured such that the porous partition wall W has the structure described below.

[0114] That is, when the portion of the porous partition wall W on the first cell C1 side is divided into first pores with a pore diameter of 5 μm or more and less than 10 μm, second pores with a pore diameter of 10 μm or more and less than 20 μm, and third pores with a pore diameter of 20 μm or more in a cross section perpendicular to the surface of the porous partition wall W on the first cell C1 side, the total area S of the first pores F1The total area S of the portion of the catalyst layer 22 located within the first pores with respect to C1 ratio R F1 of, the total area S F2 of the second pores, the total area S of the portion of the catalyst layer 22 located within the second pores C2 ratio R F2 , and the total area S F3 of the third pores, the total area S of the portion of the catalyst layer 22 located within the third pores C3 ratio R F3 wherein, the inequality: R F1 < R F2 < R F3 is preferably satisfied. Here, the boundaries between adjacent pores and the pore diameters of each pore are determined by the method described later with reference to FIG. 4.

[0115] In such a configuration, for example, the portion near the surface on the first cell C1 side of the filter partition wall 21W has a narrower pore diameter distribution width and a smaller average pore diameter compared to the portion near the surface on the first cell C1 side of the partition wall 211W.

[0116] FIG. 4 is a cross-sectional view showing a method of separating connected pores in a cross-sectional image of a porous partition wall. FIG. 4 corresponds to an image of a cross-section of the porous partition wall W. In FIG. 4, the catalyst layer 22 and the inorganic particles 3 described later are omitted.

[0117] (Step S1) In this method, first, a cross-section of the porous partition wall W is imaged using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). This cross-section is a cross-section perpendicular to the surface on the first cell C1 side of the porous partition wall W, that is, a cross-section parallel to the thickness direction of the porous partition wall W.

[0118] (Step S2) Next, in the image thus obtained, the partition wall 211W of the filter base material 21 (hereinafter referred to as the partition wall portion) is specified. Then, in the partition wall portion, the space portion is specified. Here, not only the one spaced apart from both surfaces of the partition wall 211W like the space portion CV1, but also the one opened at at least one surface of the partition wall 211W like the space portion CV2 is specified. A part of the catalyst layer 22 or the inorganic particles 3 may be located in the space portion. Then, one of these space portions is selected.

[0119] (Step S3) Next, the area of the selected space portion is obtained, and the diameter of a circle having the same area as this area, that is, the equivalent circle diameter is calculated. Then, it is determined whether or not the equivalent circle diameter is 5 μm or less.

[0120] (Step S4) When this equivalent circle diameter is 5 μm or less, it is determined that the previous space portion corresponds to one pore, and this equivalent circle diameter is defined as the pore diameter of this pore. And when there is an unselected space portion, one of the unselected space portions is selected, and the process returns to Step S3. When there is no unselected space portion, the process ends.

[0121] In the example of FIG. 4, when the space portion CV1 is selected, since the equivalent circle diameter of the space portion CV1 is 5 μm or less, it is determined that it corresponds to one pore P1. And this equivalent circle diameter is defined as the pore diameter of this pore P1. Then, the unselected space portion CV2 is selected, and the process returns to Step S3.

[0122] (Step S5) When the equivalent circle diameter is more than 5 μm, it is determined that the above space portion corresponds to a combination of two or more connected pores. Then, the space portion is divided at a position where the equivalent circle diameter decreases up to 50% of this equivalent circle diameter, and the boundary between the plurality of regions generated thereby is defined as the pore boundary.

[0123] In the example of FIG. 4, when the space portion CV2 is selected, since the equivalent circle diameter of the space portion CV2 is more than 5 μm, it is determined that it corresponds to a connection of two or more pores. Then, the space portion CV2 is divided at a position where the equivalent circle diameter decreases to 50% of this equivalent circle diameter, and the boundary between the plurality of regions generated thereby is defined as the pore boundary.

[0124] (Sub-step SS1) Specifically, first, a large number of circles that are inscribed in the space portion and in contact with both of a pair of wall surface portions facing each other with the space portion in between are generated. Here, the wall surface portion is a portion corresponding to the boundary between the space portion and the partition portion. Here, only a circle whose center is located between a pair of main surfaces of the partition 211W is generated. Then, a reference line is generated by connecting the centers of these circles. In the example shown in FIG. 4, the reference line obtained by connecting the centers of the circles is the branched broken line CL.

[0125] (Sub-step SS2) Next, among the above circles, the circle with the largest diameter is specified (hereinafter referred to as the reference circle). In the example of FIG. 4, the circle IC1 is specified.

[0126] (Sub-step SS3) Subsequently, for the circles whose centers are arranged in one direction (hereinafter referred to as the first direction) along the reference line from the center of the reference circle, the diameters are confirmed in order from the circle with the shortest center-to-center distance from the reference circle. This confirmation is performed until a circle whose diameter is 50% of the diameter of the reference circle is found.

[0127] When such a circle (hereinafter referred to as the first circle) is found, the line segment connecting the two contact points between the first circle and the wall surface portion is defined as the boundary for dividing the space portion. When the first circle is not found, for the portion of the reference line on the first direction side with respect to the center of the reference circle, the boundary for dividing the space portion is not defined.

[0128] In the example of FIG. 4, for the circles whose centers are arranged downward along the dashed line CL from the center of the circular IC1, when the diameters are confirmed in order from the circle with the shortest center-to-center distance from the circular IC1, the circular IC2 is found as the circle with a diameter of 50% of the diameter of the circular IC1. Therefore, a line segment B1 connecting two contact points between the circular IC2 and the wall surface portion is defined as a boundary that divides the space portion CV2.

[0129] (Sub-step SS4) Next, for the circles whose centers are arranged in the opposite direction (hereinafter referred to as the second direction) along the reference line from the center of the reference circle, the diameters are confirmed in order from the circle with the shortest center-to-center distance from the reference circle. This confirmation is performed until a circle with a diameter of 50% of the diameter of the reference circle is found.

[0130] When such a circle (hereinafter referred to as the second circle) is found, a line segment connecting two contact points between the second circle and the wall surface portion is defined as a boundary that divides the space portion. When the second circle is not found, for the portion of the reference line on the second direction side with respect to the center of the reference circle, the boundary that divides the space portion is not defined.

[0131] In the example of FIG. 4, for the circles whose centers are arranged upward along the dashed line CL from the center of the circular IC1, even when the diameters are confirmed in order from the circle with the shortest center-to-center distance from the circular IC1, a circle with a diameter of 50% of the diameter of the circular IC1 is not found. Therefore, for the portion of the dashed line CL on the upward side with respect to the center of the circular IC1, the boundary that divides the space portion CV2 is not defined.

[0132] (Sub-step SS5) It is determined whether the reference line branches within the range of the portion corresponding to the line connecting the centers of the circles whose diameters were confirmed in sub-step SS4 or SS5.

[0133] When the reference line branches within the above range, for each branch destination, the same processing as in sub-step SS4 is performed.

[0134] That is, for circles centered along the reference line from the branch point and arranged in the branch direction (hereinafter referred to as the third direction), the diameters are confirmed in order from the circle with the shortest distance from the branch point to the center. This confirmation is carried out until a circle with a diameter that is 50% of the diameter of the reference circle is found.

[0135] When such a circle (hereinafter referred to as the third circle) is found, the line segment connecting the two contact points between the third circle and the wall surface portion is defined as the boundary that divides the space portion. When the third circle is not found, for the portion of the reference line on the third direction side with respect to the branch point, the boundary that divides the space portion is not defined.

[0136] When the above processing is completed or when the reference line does not branch within the above range, proceed to the next step S6.

[0137] Note that in the example of FIG. 4, the dashed line CL does not branch within the range of the portion corresponding to the line connecting the centers of the circles whose diameters were confirmed using the circle IC1 as the reference circle in sub-step SS4 or SS5. Therefore, in sub-step SS5, without defining an additional boundary, proceed to the next step S6.

[0138] (Step S6) In step S6, it is determined whether a boundary was defined in step S5.

[0139] (Step S7) If no boundary was defined in step S5, it is determined that the previous space portion is one pore, and the equivalent circle diameter is defined as the pore diameter of this pore. Then, if there is an unselected space portion, select one of the unselected space portions and return to step S3. If there is no unselected space portion, end the process.

[0140] (Step S8) If one or more boundaries were defined in step S5, among the plurality of regions obtained by dividing the previous space portion by the boundary, the one in which the center of the reference circle is located is determined to be one pore, and the equivalent circle diameter is defined as the pore diameter of this pore.

[0141] Subsequently, a portion excluding the region where the center of the reference circle is located from the above-described space portion is defined as a new space portion. Then, this space portion is selected, and the process returns to step S3.

[0142] In the example of FIG. 4, among the plurality of regions formed by dividing the space portion CV2 by the boundary B1, the region where the center of the circle IC1 is located is determined to be one pore P2, and the equivalent diameter of the circle is defined as the pore diameter of the pore P2. Then, a portion (hereinafter referred to as the first remaining portion) obtained by excluding the region corresponding to the pore P2 where the center of the circle IC1 is located from the space portion CV2 is defined as a new space portion. Then, this space portion is selected, and the process returns to step S3.

[0143] Since the equivalent diameter of the above-described first remaining portion is more than 5 μm, in step S5, the circle IC3 is specified as the reference circle. Note that a new reference line may not be generated. In step S5, circles IC4a and IC4b having a diameter of 50% of the diameter of the circle IC3 and the boundaries B1 and B2 are further specified. Next, after passing through step S6, in step S8, among the plurality of regions formed by dividing the first remaining portion by the boundaries B1 and B2, the region where the center of the circle IC2 is located is determined to be one pore P3, and the equivalent diameter of the circle is defined as the pore diameter of the pore P3. Then, a portion (hereinafter referred to as the second remaining portion) obtained by excluding the region corresponding to the pore P3 where the center of the circle IC2 is located from the first remaining portion is defined as a new space portion. Then, this space portion is selected, and the process returns to step S3.

[0144] Since the equivalent diameter of the above-described second remaining portion is 5 μm or less, in step S4, it is determined that the second remaining portion corresponds to one pore P4, and this equivalent diameter is defined as the pore diameter of this pore P4. Then, if there is an unselected space portion, one of the unselected space portions is selected, and the process returns to step S3. If there is no unselected space portion, the process ends.

[0145] The porous partition wall W preferably has a structure described below with reference to FIG. 5. FIG. 5 is a plan view schematically showing the surface of the porous partition wall on the first cell side. FIG. 5 depicts the surface of the porous partition wall W on the first cell C1 side.

[0146] On this surface, the pores P of the porous partition wall W are classified into first small pores with an opening diameter of less than 40 μm and first large pores with an opening diameter of 40 μm or more. For example, in FIG. 5, the pores P located in the lower right and upper left are first large pores, and the remaining pores P are first small pores. The circle formed by the broken line LL2 has an area equal to the opening of the pore P. Therefore, the opening diameter of the pore P is the diameter of the circle formed by the broken line LL2.

[0147] Also, in the portion of the catalyst-coated filter corresponding to the porous partition wall W, that is, on the surface of the filter partition wall 21W on the first cell side, the pores of the filter partition wall 21W are classified into second small pores with an opening diameter of less than 40 μm and second large pores with an opening diameter of 40 μm or more. For example, in FIG. 5, among the pores of the filter partition wall 21W, the pores located in the lower right and upper left are second large pores, and the remaining pores are second small pores. In FIG. 5, the pores of the filter partition wall 21W are the area surrounded by the solid line LL1. Therefore, the opening diameter of the pores of the filter partition wall 21W is the diameter of the circle having an area equal to the area surrounded by the solid line LL1.

[0148] The ratio of the difference S S2 between this total area S S2 and the total area S S1 of the first small pores S2 -S S1 to the total area S S2 -S S1 ) / S S2 is 40% or less. Also, the ratio of the difference S L2 between this total area S L2 and the total area S L1 of the first large pores L2 -S L1 to the total area S L2 -S L1 ) / S L2 is 60% or more.

[0149] In this structure, the second largest pores have a greater degree of reduction in the opening diameter due to the application of the inorganic particles 3 compared to the second smallest pores. Such a configuration is advantageous, for example, in reducing pores P with an overly large or overly small opening diameter on the surface of the first cell side of the porous partition wall.

Example

[0150] Specific examples of the present invention are described below.

[0151] <1>Production of Catalyst for Exhaust Gas Purification (Example 1) The catalyst for exhaust gas purification described with reference to FIGS. 1 to 3 was produced by the following method. First, 3 parts by mass of a palladium nitrate solution, 35 parts by mass of alumina powder, 32 parts by mass of a ceria-containing oxide, and ion-exchanged water were mixed. To this mixed solution, 1 part by mass of a polycarboxylic acid was mixed to prepare a slurry. This slurry had a viscosity η at a temperature of 25°C and a shear rate of 400 s -1 of 100 mPa·s. 400

[0152] Next, a filter substrate was prepared. Here, a filter substrate having a cylindrical shape with a volume of 1.3 L and a height of 114.3 mm was used.

[0153] Next, the above slurry was supplied to one end face (the first end face) of the filter substrate, and the gas inside the filter substrate was sucked from the other end face (the second end face) of the filter substrate. This suction was performed under the condition that the linear velocity (wind speed) of the gas flow near the end of the filter substrate when the filter substrate was installed at a temperature of 25°C and the slurry was not supplied was 50 m / S. In this way, the partition wall of the filter substrate was coated with the slurry. Note that the supply of the slurry was performed such that the amount of the catalyst layer with respect to the volume of the filter substrate in the catalyst-coated filter was 75 g / L.

[0154] Thereafter, the filter substrate coated with the slurry was dried and fired. ​In this way, a catalyst-coated filter was obtained.

[0155] Next, inorganic particles were supplied to one surface of each filter partition of the catalyst-coated filter. Specifically, an aerosol containing the inorganic particles as aerosol particles was supplied to the first end corresponding to the first end face of the catalyst-coated filter. At the same time, the gas inside the catalyst-coated filter was sucked from the second end corresponding to the second end face of the catalyst-coated filter. This suction was performed with the catalyst-coated filter installed so that the first end faced downward.

[0156] The amount of the inorganic particles with respect to the volume of the filter base material was 5 g / L. As the inorganic particles, sepiolite having an average particle diameter of 6 μm, which is a porous inorganic substance, was used. As described above, an exhaust gas purification catalyst was obtained.

[0157] (Comparative Example 1) An exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the supply of inorganic particles to the catalyst-coated filter was omitted. That is, a catalyst-coated filter was produced in the same manner as in Example 1 and used as the exhaust gas purification catalyst according to Comparative Example 1.

[0158] (Comparative Example 2) An exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the ratio of the amount of the catalyst layer with respect to the volume of the filter base material was changed from 75 g / L to 50 g / L and the supply of inorganic particles to the catalyst-coated filter was omitted.

[0159] (Comparative Example 3) An exhaust gas purification catalyst was produced in the same manner as in Example 1 except that the ratio of the amount of the catalyst layer with respect to the volume of the filter base material was changed from 75 g / L to 100 g / L and the supply of inorganic particles to the catalyst-coated filter was omitted.

[0160] <2>Measurement of the opening diameter For each of the exhaust gas purification catalysts according to Example 1 and Comparative Examples 1 to 3, the opening diameter on the surface of the first cell side of the porous partition was measured by the method described above. The results are shown in FIGS. 6 to 8 and Table 1.

[0161]

Table 1

[0162] FIG. 6 is an image obtained by binarizing a micrograph of the porous partition wall of the exhaust gas purification catalyst according to Example 1. FIG. 7 is an image obtained by binarizing a micrograph of the porous partition wall of the exhaust gas purification catalyst according to Comparative Example 1. FIG. 8 is a graph showing the open pore diameter distribution obtained for the porous partition walls of the exhaust gas purification catalysts according to Example 1 and Comparative Examples 1 to 3.

[0163] In Table 1, "S <20 / S" is the ratio of the total area S <20 of the pores having an open pore diameter of less than 20 μm to the total area S of all the pores in the micrograph of the surface of the porous partition wall on the first cell side. "S 20-40 / S" is the ratio of the total area S 20-40 of the pores having an open pore diameter in the range of 20 μm or more and less than 40 μm to the total area S of all the pores in the above micrograph. "S 40-60 / S" is the ratio of the total area S 40-60 of the pores having an open pore diameter in the range of 40 μm or more and less than 60 μm to the total area S of all the pores in the above micrograph. "S 60-80 / S" is the ratio of the total area S 60-80 of the pores having an open pore diameter in the range of 60 μm or more and less than 80 μm to the total area S of all the pores in the above micrograph. "S 80-100 / S" is the ratio of the total area S 80-100 of the pores having an open pore diameter in the range of 80 μm or more and less than 100 μm to the total area S of all the pores in the above micrograph. "S 100< / S" is the ratio of the total area S 100< of the pores having an open pore diameter of 100 μm or more to the total area S of all the pores in the above micrograph.

[0164] As shown in FIGS. 6 to 8 and Table 1, in the exhaust gas purification catalyst according to Example 1, the ratio of pores with a large opening diameter on the surface of the first cell side of the porous partition wall was small compared to the exhaust gas purification catalysts according to Comparative Examples 1 to 3.

[0165] <3> Measurement of filling rate Regarding the exhaust gas purification catalyst according to Example 1, the above-mentioned ratio (S S2 -S S1 ) / S S2 and the ratio (S L2 -S L1 ) / S L2 were calculated.

[0166] Specifically, regarding the exhaust gas purification catalyst according to Example 1, the sum of the total areas S <20 and S 20-40 obtained when calculating the above-mentioned opening diameter was taken as the total area S S1 of the exhaust gas purification catalyst according to Example 1. Also, regarding the exhaust gas purification catalyst according to Example 1, the sum of the total areas S 40-60 , S 60-80 , S 80-100 and S 100< obtained when calculating the above-mentioned opening diameter was taken as the total area S L1 of the exhaust gas purification catalyst according to Example 1. Further, regarding the exhaust gas purification catalyst according to Comparative Example 1, the sum of the total areas S <20 and S 20-40 obtained when calculating the above-mentioned opening diameter was taken as the total area S S2 of the exhaust gas purification catalyst according to Comparative Example 1. And, regarding the exhaust gas purification catalyst according to Comparative Example 1, the sum of the total areas S 40-60 , S 60-80 , S 80-100 and S 100< obtained when calculating the above-mentioned opening diameter was taken as the total area S L2 of the exhaust gas purification catalyst according to Comparative Example 1. The calculation results are shown in Table 2 below.

[0167]

Table 2

[0168] As shown in Table 2, in the exhaust gas purification catalyst according to Example 1, the ratio (S L2 -S L1 ) / S L2 was larger than the ratio (S S2 -S S1 ) / S S2 . That is, in the exhaust gas purification catalyst according to Example 1, among the pores opening on the surface of the first cell side of the porous partition wall, the pores with a larger opening diameter had a higher filling rate with inorganic particles compared to those with a smaller opening diameter.

[0169] <4>Measurement of Inorganic Particle Distribution For the exhaust gas purification catalyst according to Example 1, the distribution of inorganic particles in the thickness direction of the porous partition wall was measured. Specifically, for the exhaust gas purification catalyst according to Example 1, a cross-section of the porous partition wall was imaged with a scanning electron microscope to obtain a grayscale image. This imaging was performed on a cross-section of the part of the porous partition wall where the distance from the first end and the distance from the second end were equal. Next, the analysis position by an energy dispersive X-ray analyzer was specified in the previous grayscale image, and the intensity of the characteristic X-ray derived from calcium was measured. Here, line analysis along the thickness direction of the porous partition wall was performed. Then, a composite image was generated by superimposing points with brightness (shading value) corresponding to the intensity of the characteristic X-ray and colored on the previous grayscale image. From this composite image, the relationship between the distance from the surface of the first cell side of the catalyst-coated filter and the shading value was obtained.

[0170] FIG. 9 is a graph showing the distribution of powdery inorganic particles in the thickness direction obtained for the porous partition wall of the exhaust gas purification catalyst according to Example 1. In FIG. 9, the horizontal axis represents the distance from the surface of the first cell side of the catalyst-coated filter, and the vertical axis represents the above-mentioned shading value.

[0171] As shown in FIG. 9, in the exhaust gas purification catalyst according to Example 1, the inorganic particles were unevenly distributed on the first cell side of the porous partition wall. And in the exhaust gas purification catalyst according to Example 1, the above-mentioned amounts A, A1, and A2 satisfied the relationship represented by the inequality (A1 + A2) / A ≧ 90%. Specifically, the ratio (A1 + A2) / A was 98.7%.

[0172] <5>Evaluation of Pressure Loss For each of the exhaust gas purification catalysts according to Example 1 and Comparative Examples 1 to 3, the pressure loss was evaluated. Specifically, light oil was burned in a soot generator to generate PM, and PM was accumulated on each of these exhaust gas purification catalysts. When the accumulated amount of PM reached 1 g / L, the pressure loss of each of the exhaust gas purification catalysts was measured. The pressure loss was measured with the gas temperature at 240 °C and the gas flow rate at 250 kg / h. The results are shown in Table 3 and Figure 10.

[0173]

Table 3

[0174] In Figure 10, the horizontal axis represents the above ratio S S / S, and the vertical axis represents the pressure loss. In Table 3 and Figure 10, the ratio S S / S is the sum of the above ratio S <20 / S and S 20-40 / S. Also, in Table 3, the ratio S M+L / S is the sum of the above ratio S 40-60 , S 60-80 , S 80-100 and S 100< .

[0175] As shown in Table 3 and Figure 10, the exhaust gas purification catalyst according to Example 1 had a larger ratio S S / S and a smaller pressure loss after PM deposition compared to the exhaust gas purification catalysts according to Comparative Examples 1 to 3.

[0176] <6>Manufacture of Exhaust Gas Purification Catalyst (Example 2) An exhaust gas purification catalyst was manufactured in the same manner as in Example 1, except that the ratio of the amount of the catalyst layer to the volume of the filter substrate was changed from 75 g / L to 50 g / L.

[0177] (Example 3) An exhaust gas purification catalyst was produced in the same manner as in Example 1, except that the ratio of the amount of the catalyst layer to the volume of the filter substrate was changed from 75 g / L to 100 g / L.

[0178] (Example 4) An exhaust gas purification catalyst was produced in the same manner as in Example 1, except that the ratio of the amount of the catalyst layer to the volume of the filter substrate was changed from 75 g / L to 125 g / L.

[0179] (Example 5) An exhaust gas purification catalyst was produced in the same manner as in Example 1, except that the amount of inorganic particles per unit volume of the filter substrate was changed from 5 g / L to 20 g / L.

[0180] (Comparative Example 4) An exhaust gas purification catalyst was produced in the same manner as in Example 1, except that the amount of inorganic particles per unit volume of the filter substrate was changed from 5 g / L to 1 g / L.

[0181] <7>Influence of the amount of the catalyst layer on the open pore diameter For the exhaust gas purification catalysts according to Examples 2 to 4, the open pore diameter on the surface of the first cell side of the porous partition was measured in the same manner as described above. The results are shown in Table 4 and FIG. 11.

[0182]

Table 4

[0183] As shown in Table 4, when the amount of the catalyst layer was sufficiently large, by producing the exhaust gas purification catalyst by the above method, the ratio of pores with a large open pore diameter on the surface of the first cell side of the porous partition could be reduced. However, when the amount of the catalyst layer was increased, the total area S of all the pores decreased. Specifically, the total area S of all the pores obtained for the exhaust gas purification catalysts according to Examples 2, 3, and 4 was more than 1.3 times, more than 0.7 times, and less than 0.4 times the total area S of all the pores obtained for the exhaust gas purification catalyst according to Example 1, respectively.

[0184] <8>Influence of the amount of inorganic particles on the open pore diameter Regarding the exhaust gas purification catalysts according to Example 5 and Comparative Example 4, the open pore diameter on the surface of the first cell side of the porous partition wall was measured by the same method as described above. The results are shown in Table 5 and FIG. 12.

[0185]

Table 5

[0186] As shown in Table 5, when the amount of inorganic particles was increased, the ratio of pores with a large open pore diameter on the surface of the first cell side of the porous partition wall decreased.

[0187] <9>Measurement of the filling rate Regarding the exhaust gas purification catalysts according to Examples 1 to 5 and Comparative Examples 1 to 4, the filling rates R F1 、R F2 and R F3 were determined by the method described with reference to FIG. 4. As an example, the results obtained for the exhaust gas purification catalyst according to Example 3 are shown in Table 6 below.

[0188]

Table 6

[0189] For any of the exhaust gas purification catalysts according to Examples 1 to 5 and Comparative Examples 1 to 4, the filling rates R F1 、R F2 and R F3 satisfy the relationship shown in the inequality: R F1 <R F2 <R F3 and the filling rate R F1 is in the range of 10 to 40%, the filling rate R F2 is in the range of 15 to 40%, and the filling rate R F3 was in the range of 20 to 45%. The invention originally described in the claims is appended below. [1] A catalyst-coated filter, wherein the catalyst-coated filter is a catalyst for exhaust gas purification including a filter substrate and a catalyst layer provided on the pore walls of the filter substrate, having a first end, a second end, a porous partition wall, a first cell, and a second cell, wherein the first cell extends from the first end toward the second end, is open at the first end, and is closed at the second end, and the second cell extends from the second end toward the first end, is open at the second end, and is closed at the first end, and the first cell and the second cell are adjacent to each other with the porous partition wall therebetween, wherein the porous partition wall has, on the surface on the first cell side, a ratio S S / S of the total area S S of pores having an opening diameter of less than 40 μm to the total area S of all pores, which is 65% or more, and is a catalyst for exhaust gas purification. [2] The porous partition wall has, on the surface, a ratio S M / S of the total area S M of pores having an opening diameter of 40 μm or more and less than 60 μm to the total area S of all pores, which is 30% or less, and the catalyst for exhaust gas purification according to claim 1. [3] The porous partition wall has, on the surface, a ratio S L / S of the total area S L of pores having an opening diameter of 60 μm or more to the total area S of all pores, which is 15% or less, and the catalyst for exhaust gas purification according to claim 1 or 2. [4] The porous partition wall has, on the surface, a ratio S SS / S of the total area S SS of pores having an opening diameter of less than 20 μm to the total area S of all pores, which is 50% or less, and the catalyst for exhaust gas purification according to any one of claims 1 to 3. [5] When a portion of the porous partition wall on the first cell side divides the pores of the filter substrate into a first pore having a pore diameter of 5 μm or more and less than 10 μm, a second pore having a pore diameter of 10 μm or more and less than 20 μm, and a third pore having a pore diameter of 20 μm or more in a cross section perpendicular to the surface, the filling rate R F1 of the first pore by the catalyst layer, the filling rate R F2 of the second pore by the catalyst layer, and the filling rate R F3 of the third pore by the catalyst layer satisfy the relationship shown in the inequality: R F1 <R F2 <R F3 , and the catalyst for exhaust gas purification according to any one of claims 1 to 4. [6] The filling rate R F1 is 40% or less, the filling rate R F2 is 40% or less, and the filling rate R F3 is 45% or less, and the catalyst for exhaust gas purification according to claim 5. [7] The filling rate R F3 is 20% or more, and the catalyst for exhaust gas purification according to claim 5 or 6. [8] The exhaust gas purification catalyst according to any one of claims 1 to 7, wherein the ratio of the mass of the catalyst layer to the volume of the filter substrate is in the range of 10 to 300 g / L. [9] The exhaust gas purification catalyst according to any one of claims 1 to 8, further comprising powdery inorganic particles supported by the catalyst-coated filter.

[10] The exhaust gas purification catalyst according to claim 9, wherein the inorganic particles are unevenly distributed on the first cell side of the porous partition wall.

[11] The exhaust gas purification catalyst according to claim 9, wherein the total amount A of the inorganic particles, the amount A1 of the inorganic particles located on the surface on the first cell side of the catalyst-coated filter, and the amount A2 of the inorganic particles in the pores of the catalyst-coated filter, where the distance from the surface on the first cell side of the catalyst-coated filter is 20% or less of the thickness of the portion of the catalyst-coated filter corresponding to the porous partition wall, satisfy the relationship represented by the inequality (A1 + A2) / A ≧ 90%.

[12] On the surface, the pores of the porous partition wall are divided into first small pores with an opening diameter of less than 40 μm and first large pores with an opening diameter of 40 μm or more. When the pores of the portion corresponding to the porous partition wall of the catalyst-coated filter on the surface on the first cell side are divided into second small pores with an opening diameter of less than 40 μm and second large pores with an opening diameter of 40 μm or more, the total area S S2 of the second small pores, with respect to the total area S S2 and the total area S S1 of the first small pores, the difference S S2 -S S1 has a ratio (S S2 -S S1 ) / S S2 of 40% or less, and with respect to the total area S L2 of the second large pores, the difference S L2 between the total area S L1 and the total area S L2 -S L1 of the first large pores has a ratio (S L2 -S L1 ) / S L2 of 60% or more. The exhaust gas purification catalyst according to any one of claims 9 to 11.

[13] The exhaust gas purification catalyst according to any one of claims 9 to 12, wherein the inorganic particles have an average particle diameter in the range of 1 to 50 μm.

[14] The exhaust gas purification catalyst according to any one of claims 9 to 13, wherein the inorganic particles contain one or more selected from the group consisting of metal oxides, metal hydroxides, metal carbonates, metal phosphates, metal nitrates, metal sulfates, and clay minerals.

[15] The exhaust gas purification catalyst according to any one of claims 9 to 14, wherein the ratio of the mass of the inorganic particles to the volume of the filter substrate is in the range of 3 to 50 g / L.

Description of Symbols

[0190] 1... Catalyst for exhaust gas purification, 2... Catalyst-coated filter, 3... Inorganic particles, 21... Filter substrate, 21W... Filter partition, 22... Catalyst layer, 211... Honeycomb structure, 211W... Partition, 212a... Plug, 212b... Plug, B1... Boundary, B1... Boundary, B2... Boundary, C1... First cell, C2... Second cell, CV1... Space part, CV2... Space part, CL... Dashed line, E1... First end, E2... Second end, IC1... Circle, IC2... Circle, IC3... Circle, IC4a... Circle, IC4b... Circle, P1... Pore, P2... Pore, P4... Pore, W... Porous partition.

Claims

1. A catalyst-coated filter and powdery inorganic particles supported by the catalyst-coated filter, wherein the catalyst-coated filter is a catalyst for purifying exhaust gas including a filter base material and a catalyst layer provided on the pore walls of the filter base material, the exhaust gas purification catalyst has a first end, a second end, a porous partition wall, a first cell, and a second cell, the first cell extends from the first end toward the second end, opens at the first end, and is closed at the second end, the second cell extends from the second end toward the first end, opens at the second end, and is closed at the first end, and the first cell and the second cell are adjacent to each other with the porous partition wall interposed therebetween, On the surface on the first cell side, the ratio S Ｓ / S of the total area S Ｓ of pores having a pore diameter of less than 40 μm to the total area S of all pores in the porous partition wall is 65% or more. Ｓ is Ｓ 65% or more, and the total amount A of the inorganic particles, the amount A1 of the inorganic particles located on the surface of the catalyst-coated filter on the first cell side, and the amount A2 of the inorganic particles within the pores of the catalyst-coated filter, where the distance from the surface on the first cell side of the catalyst-coated filter is 20% or less of the thickness of the portion of the catalyst-coated filter corresponding to the porous partition wall, satisfy the relationship represented by the inequality (A1 + A2) / A ≥ 90%, on the surface, the pores of the porous partition wall are divided into first small pores having an opening diameter of less than 40 μm and first large pores having an opening diameter of 40 μm or more, and on the surface of the portion of the catalyst-coated filter corresponding to the porous partition wall on the first cell side, when the pores of the portion are divided into second small pores having an opening diameter of less than 40 μm and second large pores having an opening diameter of 40 μm or more, the ratio (SS2 - SS1) / SS2 of the difference SS2 - SS1 between the total area SS2 and the total area SS1 of the first small pores to the total area SS2 of the second small pores is 40% or less, and the ratio (SL2 - SL1) / SL2 of the difference SL2 - SL1 between the total area SL2 and the total area SL1 of the first large pores to the total area SL2 of the second large pores is 60% or more. An exhaust gas purification catalyst.

2. The porous partition wall has, on the surface, a proportion S of the total area S of all the pores, which is the total area of pores having an opening diameter of 40 μm or more and less than 60 μm Ｍ to the total area S Ｍ of the exhaust gas purifying catalyst according to claim 1, wherein S / S is 30% or less.

3. The porous partition wall has, on the surface, a ratio S of the total area S of pores having a pore diameter of 60 µm or more to the total area S of all the pores Ｌ which is 15% or less. The exhaust gas purification catalyst according to claim 1 or 2 Ｌ wherein S / S is 15% or less

4. The porous partition wall has, on the surface, a proportion S of the total area S of pores having a pore diameter of less than 20 μm in the total area S of all the pores. ＳＳ The proportion S ＳＳ / S is 50% or less. The exhaust gas purifying catalyst according to any one of claims 1 to 3.

5. Among the porous partition walls, the portion on the first cell side, in a cross-section perpendicular to the surface, when the pores of the filter substrate are divided into first pores having a pore diameter of 5 μm or more and less than 10 μm, second pores having a pore diameter of 10 μm or more and less than 20 μm, and third pores having a pore diameter of 20 μm or more, the filling rate R of the first pores with the catalyst layer Ｆ１ , the filling rate R of the second pores with the catalyst layer Ｆ２ , and the filling rate R of the third pores with the catalyst layer Ｆ３ satisfy the inequality: R Ｆ１ < R Ｆ２ < R Ｆ３ The exhaust gas purification catalyst according to any one of claims 1 to 4, which satisfies the relationship shown.

6. The filling rate R Ｆ１ is 40% or less, and the filling rate R Ｆ２ is 40% or less, and the filling rate R Ｆ３ is 45% or less. The exhaust gas purification catalyst according to claim 5

7. The filling rate R Ｆ３ The exhaust gas purification catalyst according to claim 5 or 6, wherein the filling rate R is 20% or more.

8. The ratio of the mass of the catalyst layer to the volume of the filter base material is in the range of 10 to 300 g / L. The exhaust gas purification catalyst according to any one of Claims 1 to 7.

9. The inorganic particles have an average particle diameter in the range of 1 to 50 μm. The exhaust gas purification catalyst according to any one of Claims 1 to 8.

10. The exhaust gas purification catalyst according to any one of claims 1 to 9, wherein the inorganic particles contain one or more selected from the group consisting of metal oxides, metal hydroxides, metal carbonates, metal phosphates, metal nitrates, metal sulfates, and clay minerals.

11. The exhaust gas purification catalyst according to any one of claims 1 to 10, wherein the ratio of the mass of the inorganic particles to the volume of the filter substrate is in the range of 3 to 50 g / L.

Images