JP5446457B2 - Porous film-forming coating material and porous film - Google Patents

Description

  The present invention relates to a filter body having a honeycomb structure, and more particularly to an exhaust gas purification filter suitable for removing particulate matter from exhaust gas discharged from an automobile diesel engine or the like. Exhaust gas purification in which a porous membrane having oxidation catalytic properties for particulate matter (PM), carbon monoxide (CO) and hydrocarbon (HC) is formed on the surface of a porous support made of ceramics constituting a purification filter It relates to filters.

Various substances contained in exhaust gas discharged from engines such as automobiles, particularly diesel engines, cause air pollution and have caused various environmental problems. In particular, particulate matter (PM) contained in exhaust gas is also said to cause allergic diseases such as asthma and hay fever. Diesel engine exhaust gas contains a lot of fine particulate matter with a particle size of 10 μm or less, and since these fine particulate matter floats in the air for a long time, it is easy to cause health damage. Various measures for suppressing the problem are being studied.
In general, in an automobile diesel engine, an exhaust gas purification filter is provided in order to collect these particulate substances and suppress emission. For example, a DPF (Diesel Particulate Filter) having a plugged ceramic honeycomb structure is used (for example, Patent Documents 1 and 2).
This plug-type honeycomb structure is a ceramic honeycomb structure in which both ends of a cell (gas flow path) are plugged in a checkered pattern, and is taken into the cell from one end face of the honeycomb structure. When the exhaust gas containing particulate matter passes through the pores in the partition walls between the cells, the particulate matter is collected and becomes purified gas, and this purified gas is discharged from the other end face of the honeycomb structure.

In DPF, it is required to improve the collection characteristics of particulate matter having a submicron diameter that is likely to cause health damage. And in DPF currently used, it is known that a collection characteristic will be influenced by the quantity of the particulate matter which adheres and deposits on a partition with use.
That is, in the conventional DPF, the average pore diameter of the partition walls is about 5 μm to 50 μm, and the collection efficiency (particulate matter mass basis) in the DPF in a state where the particulate materials are not deposited on the partition walls at the beginning of use is It has not reached 90%. However, when the DPF is continuously used, particulate matter accumulates on the exhaust gas inflow surface of the partition wall, and a layer of particulate matter is gradually formed on the exhaust gas inflow surface of the partition wall. Then, the phenomenon that the collection efficiency in the DPF is improved by collecting new particulate matter in the particulate matter layer and the collection efficiency approaches 100% is observed.
Since the conventional DPF achieves high collection efficiency by the mechanism as described above, the collection efficiency is high after the particulate matter layer is formed, but the amount of particulate matter deposited is small. The collection efficiency is not always satisfactory (Non-Patent Document 1).

Here, it is known that it is effective to reduce the pore diameter of the partition walls in the DPF in order to increase the collection efficiency in a state where the amount of particulate matter deposited is small. However, if the pore diameter is reduced, the gas permeability in the DPF is lowered, so that the pressure loss increases, and a problem that a sufficient exhaust gas flow rate cannot be obtained occurs.
As described above, in the conventional technique, it is impossible to achieve both high collection efficiency and low pressure loss in a state where the amount of particulate matter deposited is small, and a technique that satisfies both of these performances is required.

Further, since particulate matter is always discharged from the engine when the automobile is running, the particulate matter is gradually accumulated in the cells of the DPF honeycomb structure. If this accumulation progresses and the amount of accumulated particulate matter becomes excessive, a so-called “clogging” state occurs and the pressure loss in the DPF increases, so this particulate matter is periodically removed by some method. Therefore, it is necessary to reduce the pressure loss of the DPF.
For this reason, in a conventional DPF, when a predetermined amount of particulate matter is deposited, an operation called “regeneration” in which the exhaust gas temperature is raised and the particulate matter is burned is performed to reduce the pressure loss of the DPF. . However, in this method, in order to raise the temperature of the exhaust gas, it is necessary to inject fuel into the exhaust gas before the DPF. Since the fuel used for this regeneration does not contribute at all to the driving of the automobile, the time required for the regeneration is short and the fuel used during the regeneration can be reduced from the viewpoint of effectively using the energy of the fuel and improving the fuel consumption rate. Therefore, there has been a demand for an exhaust gas purification filter having a high so-called regeneration efficiency.

Up to now, as a method for improving the regeneration efficiency in DPF, a method of promoting oxidation of particulate matter by supporting noble metal fine particles such as platinum and silver or oxide fine particles such as cerium oxide on the partition walls of DPF as an oxidation catalyst. Has been proposed (for example, Patent Documents 3 and 4). When these methods are used, the action of the oxidation catalyst supported on the partition walls can reduce the temperature required for DPF regeneration, or shorten the high temperature holding time for regeneration, and reduce the thermal degradation of the DPF itself. Can do.
As a method for supporting the oxidation catalyst on the partition walls of the DPF, a method of impregnating the DPF honeycomb structure itself in a slurry containing fine particles of the oxidation catalyst and attaching the oxidation catalyst to the partition walls of the DPF (for example, Patent Documents 3 and 4). Alternatively, after impregnating the DPF honeycomb structure in a solution containing an oxidation catalyst metal compound, the component adhering to the partition walls of the DPF is reduced to form metal fine particles, thereby attaching the oxidation catalyst to the partition walls of the DPF. (For example, patent document 4) etc. are proposed.

Here, as a filter having a low pressure loss during use and a high particulate collection efficiency, the surface of the porous support has pores having a diameter smaller than the pores of the porous support and is thin. A filter having a structure in which a porous film is laminated is known. Examples of such a filter include a ceramic filter using porous ceramics as a material for the support and the porous membrane.
In such a ceramic filter, ceramic particles are laminated on the surface of a support made of porous ceramics to form a laminate, and the laminate is heat treated to form a porous film. In such a ceramic filter, the pore size of the porous membrane can be controlled by the particle size of the ceramic particles as the forming material, and by controlling the pore size of the porous membrane according to the size of the particles to be collected It is said that suitable collection of particulate matter can be realized.

However, when a porous membrane thin film having pores smaller than the pores of the porous ceramic is formed on the surface of the support made of the porous ceramic, the particle size of the ceramic particles constituting the porous membrane is set to be porous. It was necessary to make it smaller than the pore diameter of ceramics. Therefore, when the porous film is formed, there is a possibility that the ceramic particles enter the pores of the porous ceramic and the formation of the porous thin film becomes difficult.
Therefore, various methods for solving such problems have been proposed.

For example, a porous support made of ceramic is hydrophobized and ceramic particles having a small particle diameter are used as an aqueous slurry so that the aqueous slurry (ie ceramic particles) does not enter the pores of the support. A method has been proposed (for example, Patent Document 5).
In addition, ceramic particles (primary particles) having a small particle size are previously set as secondary particles having a size equal to or larger than the pore size of the porous support, and the slurry containing the secondary particles is used to make the porous particles. There has been proposed a method for preventing ceramic particles from entering the pores of the support by forming a membrane. As a method for producing the secondary particles, a method of pre-calcining ceramic particles (for example, Patent Document 6) and a method of adding a flocculant to a slurry to aggregate the ceramic particles (for example, Patent Document 7) have been proposed. Yes.
Further, the pores of the porous support are filled with a removable substance, the pores are closed, and then the ceramic particles are coated by applying a slurry containing ceramic particles having a small particle diameter on the surface of the porous support. Methods have been proposed to prevent entry into the pores of the support. As a method for closing the pores, there has been proposed a method (for example, Patent Document 8) in which a combustible material is used as a removable material, and the combustible material is burned and removed by a subsequent firing step.
In addition, after filling the pores of the porous support with a solvent such as water or alcohol, a slurry containing ceramic fine particles having a small particle diameter is applied to the surface of the porous support, and then the solvent is removed to obtain a homogeneous A method of forming a film (for example, Patent Documents 9 and 10) has also been proposed.

Japanese Patent Laid-Open No. 5-23512 JP-A-9-77573 JP 2005-7259 A JP 2001-73748 A JP 2000-218114 A JP-A-11-33322 JP-A-11-188217 JP-A-1-274815 Japanese Patent Publication No. 63-66566 JP 2000-288324 A

SAE Technical Paper 980545 American Society of Automotive Engineers Published in 1998 (SAE Technical Paper 980545, Society of Automotive Engineers (1998))

  As described above, in the conventional DPF, the average pore diameter of the partition wall is about 5 μm to 50 μm, which is in the order of micron diameter. Therefore, the particulate matter having a submicron diameter smaller than the average pore diameter is collected. There was a problem that was not easy.

  In order to improve the collection characteristics of particulate matter having a submicron diameter, it is one method to reduce the average pore diameter of the partition walls in the DPF. However, if the average pore diameter of the partition walls is reduced, the submicron diameter Although the collection property of the particulate matter is improved, there is a problem that the air permeability as the DPF is lowered and the pressure loss is increased. That is, the conventional DPF cannot achieve both high collection efficiency and low pressure loss (sufficient exhaust gas flow rate) particularly in a state where the amount of accumulated PM is small, and a material satisfying both of these performances has been demanded.

  Therefore, it is considered that the average pore diameter of the partition walls in the DPF is maintained at 5 μm or more and 50 μm or less, and a porous film having an average pore diameter of several tens nm to 5 μm is formed on the surface of the partition walls. In forming this porous film, it is conceivable to apply the above-described conventional technique.

However, the DPF having such a porous membrane also has the following problems. That is, in order to suitably collect particulate matter having a submicron diameter, for example, in order to form a porous film having an average pore diameter of 100 nm, the primary particle diameter of particles constituting the porous film is several tens of nm. It needs to be about. This particle size is about one-hundredth of the average pore size of DPF and is very small. Therefore, even when the conventional technique is applied as it is when forming the porous film on the partition walls of the DPF, it is difficult to avoid a part of the particles constituting the porous film from flowing into the pores of the partition walls.
Furthermore, in the honeycomb structure which is the main part of the DPF, each cell has, for example, an elongated cylindrical shape with a cross section of 1 mm square and a length of 150 mm sealed at one end, and these cells are adjacent to each other. The sealing end portions are alternately provided so that the positions of the sealing end portions of the cells are opposite to each other, and the cells have a special shape that is overlapped and formed into a honeycomb shape. On the other hand, the partition wall in the above-described prior art is plate-shaped or cylindrical with a diameter of the order of centimeters. Therefore, when forming the porous film on the partition walls of the honeycomb structure which is the main part of the DPF, even if trying to apply the above prior art, the shape is greatly different due to the special shape of the honeycomb structure. Is difficult. Moreover, even if the prior art is applicable, the process becomes complicated, and it is necessary to devise materials to be used, which may increase the manufacturing cost.
As described above, the technology for forming a porous film on the surface of the partition walls of the honeycomb structure, which is the main part of the DPF, has not yet been established.

In addition, when the oxidation catalyst fine particles are supported on the partition walls of the DPF, the oxidation catalyst fine particles exist not only on the surface of the partition walls but also inside the pores of the partition walls in the conventional method, so that the oxidation catalyst fine particles exist over the entire partition walls. . On the other hand, when the particulate matter in the exhaust gas is collected using this DPF, the particulate matter is layered on the surface of the partition wall of the DPF, and does not penetrate so much into the pores. In particular, this tendency becomes stronger after particulate matter is deposited to some extent (see Non-Patent Document 1).
Then, the oxidation catalyst fine particles present in the pores do not come into contact with the particulate matter localized on the partition wall surface of the DPF, and hardly contribute to the oxidation removal of the particulate matter. Therefore, if the oxidation catalyst fine particles can be selectively supported on the surface of the partition wall, most of the fixed oxidation catalyst contributes to the oxidation removal of the particulate matter, so that the performance of the oxidation catalyst can be effectively extracted. Such a method has not yet been disclosed.

Furthermore, the DPF may be required to have a function of oxidizing and purifying soluble organic components (SOF), hydrocarbons, carbon monoxide and the like in the particulate matter discharged from the diesel engine. For example, when the oxidation catalyst (Diesel Oxidation Catalyst: DOC) installed at the front stage (engine side) of the DPF cannot oxidize soluble organic components, carbon monoxide, and hydrocarbons, by oxidizing them with the DPF This is because soluble organic components, carbon monoxide, and hydrocarbons contained in the exhaust gas can be reduced. It is known that noble metals such as platinum (Pt) and palladium (Pd) can be suitably used as catalysts for these oxidations.
However, these noble metal catalysts have a reduced surface area and reduced catalytic activity due to a phenomenon called sintering in which the noble metals join together with heat when the DPF becomes hot due to engine operating conditions. It is common to end up.

The present invention has been made to solve the above-described problems, and the particulate matter is formed on the surface of the partition wall of the honeycomb structure type filter constituting the exhaust gas purification filter such as DPF even in a state where the amount of particulate matter deposited is small. An object of the present invention is to provide a coating material for forming a porous film capable of forming a porous film with high material collection efficiency and low pressure loss.
In addition, by improving the method of supporting the oxidation catalyst in the partition walls, the combustion time of particulate matter deposited on the surface of the partition walls during the regeneration process is shortened, and the use of fuel necessary for raising the exhaust gas temperature during the regeneration process is reduced. Another object of the present invention is to provide a porous membrane for a honeycomb structure type filter such as a DPF that can reduce the fuel consumption rate and prevent the filter function from being deteriorated.
Furthermore, it is also an object to provide a porous membrane for a honeycomb structure type filter that purifies these harmful gases by making the oxidation catalyst have an oxidation catalyst performance for carbon monoxide and hydrocarbons. And
A further object of the present invention is to provide a honeycomb structure type filter in which the catalytic activity of a catalyst made of a noble metal such as platinum or palladium is not lowered even in a high temperature environment.

  As a result of intensive studies to solve the above-mentioned problems, the inventors of the present invention have one or two kinds selected from the group consisting of at least platinum, palladium, and rhodium (Rh) in the partition walls of the DPF honeycomb structure. Noble metals and aluminum (Al), zirconium (Zr), titanium (Ti), cerium (Ce), lanthanum (La), iron (Fe), silicon (Si), zinc (Zn), magnesium (Mg), manganese (Mn), fine particles containing an oxide containing one or more elements of cobalt (Co), or a composite oxide containing at least one of these elements, and a predetermined By providing a porous membrane having a film thickness, an average pore diameter and an average porosity, high collection efficiency can be obtained even when the amount of particulate matter deposited is small, and at the same time pressure loss can be reduced. In addition, when the DPF is regenerated, the combustion time of the particulate matter deposited on the partition walls can be shortened compared to the conventional DPF, and carbon monoxide and carbonization can be achieved even when a high temperature is applied. It has been found that the hydrogen purification performance is unlikely to deteriorate.

  When a porous film is formed on the partition walls of the DPF honeycomb structure, the specific surface area, tap bulk density and average secondary particle size in the paint are controlled, and at least selected from the group of platinum, palladium and rhodium One or more kinds of noble metals and oxides containing one or more elements of aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium, manganese, cobalt, or two kinds For forming a porous film comprising fine particles containing fine oxide particles comprising a mixture of the above oxides as a forming material and a dispersion medium, the viscosity of which is controlled to 2 mPa · s or more and 1000 mPa · s or less. By using a paint, it is possible to form a porous film containing fine particles containing noble metal and oxide as components on the surface of a porous support that is a partition wall. It found that it is a performance, which resulted in the completion of the present invention.

That is, the coating material for forming a porous film of the present invention forms a porous film having an average pore diameter smaller than the average pore diameter of the porous support on the surface of the porous support of the honeycomb structure type filter. The coating material comprises a fine particle containing at least one kind of noble metal selected from the group consisting of platinum, palladium and rhodium and an oxide, and a dispersion medium in which the fine particle is dispersed. In the fine particles, the noble metal fine particles containing the noble metal and the oxide fine particles containing the oxide form metal / oxide composite fine particles, and the fine particles have a specific surface area of 1 m. ² / g or more and 125 m ² / g or less, the tap bulk density is 0.1 g / cm ³ or more and 2.0 g / cm ³ or less, and the average secondary particle diameter in the dispersion medium is 0.1 μm or more and 10 μm or less. The viscosity of the paint is 2 mPa · s or more and 1000 mPa · s or less.

The oxide is an oxide or a composite oxide containing one or more elements selected from the group consisting of aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium, manganese, and cobalt. It is desirable to be.
In the fine particles, the ratio of the noble metal to the fine oxide particles is preferably 0.1% by mass or more and 30% by mass or less.

  The porous film of the present invention is obtained by applying the porous film-forming paint of the present invention and then performing a heat treatment.

  According to the porous film-forming coating material of the present invention, the exhaust gas purification filter is controlled by controlling the specific surface area, tap bulk density, average secondary particle size, and coating viscosity of the fine particles dispersed in the dispersion medium to the above values. When applied to the surface of the porous support of the honeycomb structure type filter, the particles can be prevented from entering the pores of the porous support. Therefore, a porous membrane having noble metal and oxide, having an average pore diameter smaller than the average pore diameter of the porous support and excellent in homogeneity on the surface of the porous support of the honeycomb structure type filter. It can be formed easily.

  In addition, since the porous film can be formed by simply applying the coating for forming a porous film on the surface of the porous support and heat-treating it, the porous support can be formed in any shape. A porous film having excellent homogeneity can be easily formed on the surface thereof without being restricted by the shape or the like.

  According to the porous membrane of the present invention, since the coating film obtained by applying the coating material for forming a porous membrane of the present invention was heat-treated, a fine diameter smaller than the average pore diameter of the porous support of the honeycomb structure type filter It is possible to obtain a porous film having the pores of FIG.

It is a partially broken perspective view which shows DPF which can apply the coating material for porous film formation of this invention. It is sectional drawing which shows the partition of DPF which can apply the coating material for porous film formation of this invention.

Hereinafter, the honeycomb structure type filter of the present invention will be described with reference to FIGS. In the following description, a DPF, which is an exhaust gas purification filter used in an automobile diesel engine, will be described as an example of a honeycomb structure type filter. This embodiment will be specifically described for better understanding of the gist of the invention. Therefore, the present invention is not limited unless otherwise specified. In the following description, the honeycomb structure type filter is referred to as an exhaust gas purification filter because of the DPF as an example. In all of the following drawings, the film thicknesses and dimensional ratios of the constituent elements are appropriately changed in order to make the drawings easy to see.
First, in order to make the content of the present invention easier to understand, an exhaust gas purification filter that can be formed by using the porous film-forming paint of the present invention will be described.

[Exhaust gas purification filter (honeycomb structure type filter)]
FIG. 1 is an embodiment of a honeycomb structure type filter of the present invention, and is a partially broken perspective view showing a DPF which is an exhaust gas purification filter, and FIG. 2 is a cross-sectional view showing a partition structure of the DPF. It is the figure which expanded the surface shown with the code | symbol (beta).

  The DPF 10 is provided on a filter base 11 made of a cylindrical porous ceramic having a large number of pores (pores), a gas flow path 12 formed in the filter base 11, and an inner wall surface of the gas flow path 12. And a porous membrane 13.

The filter base 11 is a honeycomb structure made of heat-resistant porous ceramics such as silicon carbide, cordierite, aluminum titanate, silicon nitride, and the like, along the axial direction that is the flow direction of the exhaust gas G, and from the porous ceramics. A honeycomb structure is formed by the partition walls 14, and a hollow area in the axial direction surrounded by the partition walls 14 forms a large number of cellular gas flow paths 12.
One end surface α of both end surfaces in the axial direction of the filter base 11 is an inflow surface into which the exhaust gas G containing particulate matter flows, and the other end surface γ has removed the particulate matter from the exhaust gas G. The discharge surface is for discharging the purified gas C.

  Here, the “honeycomb structure” in the present embodiment is a structure in which a plurality of gas flow paths 12 are formed in the filter base 11 so as to be parallel to each other. In the figure, the shape of the cross section orthogonal to the axial direction of the gas flow path 12 is shown as a quadrangle, but is not limited thereto, and various cross sectional shapes such as a polygon, a circle, and an ellipse can be used. In addition, the gas flow path 12 disposed near the outer periphery of the filter base 11 has a part of the cross-sectional shape in an arc shape, but this is for convenience in accordance with the shape of the filter base 11, and By adopting the shape, a honeycomb structure in which the gas flow path 12 is arranged without a gap to the vicinity of the outer periphery of the filter base 11 is obtained.

The average pore diameter of the partition wall 14 is preferably 5 μm or more and 50 μm or less.
If the average pore diameter is less than 5 μm, the pressure loss due to the partition wall 14 itself is increased, which is not preferable. On the contrary, if the average pore diameter exceeds 50 μm, the strength of the partition wall 14 becomes insufficient, and it becomes difficult to form the porous film 13 on the partition wall 14, which is not preferable.

The gas flow path 12 has a structure in which the upstream end and the downstream end are alternately closed when viewed from the flow direction (longitudinal direction) of the exhaust gas G, that is, the upstream side that is the inflow side of the exhaust gas G. An inflow cell 12A having an open end and an outflow cell 12B having a downstream end that is the side from which the purified gas C is discharged are configured.
A porous film 13 is formed on the inner wall surface 12a of the inflow cell 12A (surface on the inflow cell 12A side of the partition wall 14) 12a.

  The porous film 13 may be provided not only on the inner wall surface of the inflow cell 12A but also on the inner wall surface of the outflow cell 12B (surface on the outflow cell 12B side of the partition wall 14). However, in the following description, it demonstrates as what was provided in the inner wall face of 12 A of inflow cells.

  FIG. 2 is an explanatory diagram showing the flow of exhaust gas in the DPF 10. In the DPF 10, one end surface α of both end surfaces of the filter base 11 in the axial direction is an inflow surface into which the exhaust gas G containing particulate matter flows, and the other end surface γ removes the particulate matter from the exhaust gas G. The exhaust surface from which the purified gas C is discharged.

  As shown in the drawing, the exhaust gas G containing the particulate matter 30 flowing in from the end surface α side passes through the inflow cell 12A from the end surface α side to the end surface γ side, and the pores of the partition wall 14 of the filter base 11 are included. Pass through. At this time, the particulate matter 30 contained in the exhaust gas G is collected and removed by the porous film 13 formed on the partition wall 14, particularly the inner wall surface 12 a on the inflow cell 12 A side of the partition wall 14, and the particulate matter 30. The purified gas C is removed. The purified gas C flows through the outflow cell 12B from the end surface α side to the end surface γ side, and is discharged from the end surface γ side.

Thus, since the pores of the partition wall 14 form a part of the gas flow path 12, the average pore diameter of the partition wall 14 is preferably 5 μm or more and 50 μm or less. If the average pore diameter is less than 5 μm, the pressure loss due to the partition wall 14 itself is increased, which is not preferable. On the other hand, if the average pore diameter exceeds 50 μm, the partition wall 14 tends to have insufficient strength, and the porous film 13 is formed on the partition wall 14. This is not preferable because it causes problems such as difficulty in forming the film.
The schematic configuration of the DPF 10 having the porous membrane 13 of the present embodiment is as described above.

[Porous membrane]
The porous film 13 is an independent film on the inner wall surface 12a without substantially entering the pores of the porous ceramics constituting the partition wall 14 of the filter base 11. That is, the porous film 13 is formed on the inner wall surface 12a of the inflow cell 12A in a state in which the intrusion into the partition wall 14 is suppressed, and does not block the pores formed in the partition wall 14. The porous film 13 has a large number of pores communicating with each other, and as a result, is a filter-like porous material having through holes. The particulate matter is collected by the porous film 13 before entering the partition wall 14.

  The porous film 13 is a film obtained by applying the porous film-forming coating material of the present invention to the inner wall surface 12a of the inflow cell 12A (the surface of the partition wall 14 on the inflow cell 12A side) and then heat-treating it. is there. The porous film 13 of the present invention includes at least one or more kinds of noble metals selected from the group consisting of platinum, palladium, and rhodium, and aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium And oxide fine particles having, as a component, an oxide containing one or two or more elements selected from the group consisting of manganese and cobalt, or a mixture of two or more of the oxides.

  Further, the porous membrane 13 has an average pore diameter of 0.05 μm to 5 μm, an average porosity of 35% to 90%, and an average film thickness of 40 μm or less.

[Porous membrane structure]
First, the structure of the porous membrane 13 will be described in detail. In order for the porous membrane 13 to achieve both high collection efficiency and low pressure loss, it is required that the average pore diameter, average porosity, and average film thickness of the porous membrane 13 are within a predetermined range. .

  The average pore diameter of a large number of pores of the porous membrane 13 is preferably 0.05 μm or more and 5 μm or less, more preferably 0.05 μm or more and 3 μm or less, further preferably 0.1 μm or more and 2 μm or less. is there. This pore diameter is smaller than the pore diameter of the partition wall 14, that is, 5 μm to 50 μm, which is the average pore diameter of the conventional DPF. For this reason, since the particulate matter 30 is collected by the porous film 13 and does not enter the partition wall 14, high collection efficiency can be obtained.

  Here, when the average pore diameter of the porous membrane 13 is less than 0.05 μm, the pressure loss generated by the porous membrane 13 increases, whereas when the average pore diameter exceeds 5 μm, the porous membrane 13 and the partition walls 14 are increased. Since there is no substantial difference in the pore diameter and the trapping rate of the particulate matter 30 is lowered, both are not preferable as the porous film 13. In addition, when the average pore diameter exceeds 5 μm, it is difficult to obtain high collection efficiency, particularly when the amount of particulate matter deposited is small, and when the DPF 10 is regenerated, the combustion efficiency of the particulate matter is improved. Since it is not possible, it is not preferable.

  The average porosity of the porous membrane 13 is preferably 35% or more and 90% or less, more preferably 50% or more and 90% or less, and further preferably 60% or more and 90% or less. . If the average porosity of the porous membrane is less than 35%, the pressure loss generated by the porous membrane becomes large. On the other hand, if the average porosity exceeds 90%, the strength of the porous membrane may decrease, which is not preferable. .

The average film thickness of the porous film 13 is preferably 40 μm or less.
This is because when the average film thickness of the porous film 13 exceeds 40 μm, the pressure loss generated by the porous film 13 increases. The porous film 13 can collect the particulate matter 30 and substantially does not have to enter the pores of the partition wall 14 as long as this condition is satisfied. There is no particular lower limit.

The porous membrane 13 having the above structure has an average pore diameter smaller than the pore diameter of the partition wall 14, while its film thickness is thinner than 200 to 400 μm, which is a typical thickness of a filter substrate, and has a porosity. Therefore, it is possible to obtain a sufficient exhaust gas flow with high collection efficiency and low pressure loss.
Furthermore, the provision of the porous film 13 makes it difficult for the particulate matter 30 to enter the pores of the partition wall 14 when the particulate matter 30 is deposited. For this reason, compared with the case where the porous film is not provided, the particulate matter 30 is less likely to block the pores of the partition wall 14, so that an increase in pressure loss after the particulate matter 40 is deposited can be suppressed. .

By these, in the exhaust gas purification filter in which the porous membrane of the present invention is formed, it is possible to achieve both high collection efficiency and low pressure loss from a state where the amount of particulate matter 30 deposited is small.
Further, due to the presence of the porous membrane 13, oxygen for burning the particulate matter 30 is evenly distributed in the particulate matter layer when the filter is regenerated. As a result, oxygen is reliably supplied to the entire particulate matter 30 and the particulate matter 30 to which oxygen is difficult to be supplied is reduced, so that the oxidation of the particulate matter 30 proceeds evenly. As a result, the particulate matter 30 is burned. Time can be shortened.

[Configuration and materials of porous membrane]
Next, the material constituting the porous film 13 will be described in detail. The porous film 13 has one or more kinds of noble metals selected from the group consisting of platinum, palladium, and rhodium. Since these noble metals have a catalytic action for promoting the combustion of particulate matter, inclusion of the above-mentioned noble metals in the porous film can provide an effect of the catalytic action in addition to the effect of the porous film. It becomes possible. For example, platinum is widely used as an oxidation catalyst for particulate matter because it has the action of oxidizing nitrogen monoxide contained in exhaust gas to nitrogen dioxide, and nitrogen dioxide strongly oxidizes the particulate matter.

The above-mentioned noble metal also has a catalytic action for an oxidative decomposition reaction that oxidizes soluble organic components, hydrocarbons or carbon monoxide adhering to the particulate matter and decomposes them into carbon dioxide and water. Therefore, the porous film having the above-mentioned noble metal has an effect of promoting the combustion of soluble organic components, carbon monoxide, and hydrocarbons in addition to the good collection performance of the particulate matter and the effect of the combustion catalyst of the particulate matter. Can also be obtained.
Individual oxidation reactions of particulate matter, soluble organic components, carbon monoxide, and hydrocarbons differ in the rate of catalytic reaction due to the type of noble metal, so they are contained in the porous membrane according to the exhaust gas composition to be purified. It is advisable to select the type and amount of the noble metal as appropriate.

  Further, these noble metals are not only present alone, but may be alloys composed of two or more kinds of noble metals. By alloying, for example, sintering by heat can be suppressed, a high specific surface area can be maintained, and the catalytic activity can be maintained. Further, these noble metals may include not only those having a valence of 0 but also those having a valence other than 0, such as an oxidized surface of the noble metal. Among the above-mentioned noble metals, it is more preferable to use one or both of platinum and palladium.

  On the other hand, the porous film 13 includes oxide fine particles containing one or more elements selected from the group consisting of aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium, manganese, and cobalt. Contains complex oxide fine particles. Among the metals constituting these oxides, aluminum, zirconium, titanium, cerium, lanthanum, and iron are more preferable, and aluminum, zirconium, cerium, and lanthanum are more preferable.

  Here, the oxide fine particles of the present invention may be an oxide containing one kind of metal ion or a complex oxide containing two or more kinds of metal ions.

Here, the reason why the fine particle component is the oxide or composite oxide is that the heat resistance of the porous film can be sufficiently secured. For example, in the case of a ceramic filter such as DPF, the temperature of the exhaust gas may rise to 1000 ° C. or higher, so that the heat resistance up to 1000 ° C. or higher is required even for the material of the porous membrane. By constituting the porous membrane with these oxide fine particles, the heat resistance of the porous membrane and the exhaust gas purification filter with a porous membrane can be made sufficient.
In addition, since these oxides or composite oxides are excellent in chemical stability, it is possible to attach a porous film and a porous film by configuring the porous film with fine particles of these oxides or composite oxides. The chemical stability of the exhaust gas purification filter can be made sufficient.
Examples of particularly excellent heat resistance and chemical stability include aluminum oxide and zirconium oxide.

[Precious metal retention method]
In order for the above-mentioned noble metal to exhibit an effective catalytic action in the porous membrane 13, it is preferable that the surface area of the noble metal is large and that it is in effective contact with the substance to be oxidized. That is, it is preferable that the noble metal fine particles containing the noble metal are dispersed on the surface of the oxide fine particles or composite oxide fine particles in the state of ultrafine particles. On the other hand, it is known that the catalytic activity decreases when the noble metal fine particles are aggregated and sintered by sintering to become coarse particles. Therefore, it is preferable that the noble metal ultrafine particles are metal / oxide composite fine particles integrated with oxide fine particles or composite oxide fine particles. By immobilizing these noble metal ultrafine particles on the surface of the oxide fine particles, it is possible to suppress sintering of the noble metal and suppress a decrease in catalytic action. In particular, cerium oxide is preferable because it suppresses movement of noble metal fine particles integrated (supported) on the surface on the oxide surface and suppresses sintering of the noble metal.
From this point, it is not preferable to form the porous film 13 by simply mixing noble metal fine particles and oxide fine particles or composite oxide fine particles.
It is possible to reduce the particle diameter of the oxide fine particles or composite oxide fine particles to the same level as that of the noble metal fine particles. In the case of such metal / oxide composite fine particles, the noble metal ultrafine particles are oxides. Since the movement of the noble metal is suppressed by integrating with the fine particles or the composite oxide fine particles, the sintering suppressing effect can be obtained similarly.

  As a method for integrating the noble metal fine particles into the oxide fine particles constituting the porous film, the metal / oxide composite fine particles in which the noble metal is previously integrated with the fine particles and the oxide fine particles are prepared, and then the metal / oxide composite fine particles are prepared. A porous film made of metal / oxide composite fine particles may be formed by applying a coating liquid in which is dispersed in a filter substrate. Alternatively, a coating liquid in which oxide fine particles are dispersed is applied to a filter substrate to form a porous film made of oxide fine particles, and then a coating liquid in which noble metal fine particles are dispersed or a coating liquid in which noble metal salts are dissolved is oxidized. It may be applied to a filter substrate with a porous film to form a porous film in which noble metal fine particles and oxide fine particles are integrated. In this embodiment, a metal / oxide composite fine particle in which noble metal fine particles and oxide fine particles are integrated in advance is prepared, and then a porous film made of the metal / oxide composite fine particles is formed.

  Here, in order for the noble metal in the metal / oxide composite fine particles to express the oxidation catalytic activity well, the ratio of the noble metal to the oxide in the metal / oxide composite fine particles is within a predetermined range. Is required.

The ratio of the noble metal to the oxide in the metal / oxide composite fine particles, that is, the ratio of the noble metal to 100 parts by mass of the oxide is preferably 0.1 parts by mass or more and 30 parts by mass or less, and 0.1 parts by mass or more and 20 parts by mass or less. Less than or equal to 1 part by mass is more preferred, and 0.1 to 10 parts by mass is most preferred.
When the ratio of the noble metal to the oxide is less than 0.1 parts by mass, the amount of particulate matter, soluble organic components, carbon monoxide, and hydrocarbons having noble metal oxidation catalyst performance decreases, and the noble metal is reduced. This is not preferable because it does not substantially contribute to the improvement of the combustibility of the particulate matter. On the other hand, when the ratio of the noble metal to the oxide exceeds 30 parts by mass, the fusion between the noble metals is facilitated, and the particulate matter containing the noble metal and the oxide, the soluble organic component, the carbon monoxide, the hydrocarbon Since oxidation catalyst performance tends to deteriorate, it is not preferable.

The porous membrane 13 having the above configuration has the following effects.
First, the porous film 13 contains one or more kinds of noble metals selected from the group of platinum, palladium, and rhodium as a forming material, and the noble metals that are components of the porous film 13 have oxidation catalyst characteristics. That is, it has a combustion catalytic action of the particulate matter 30. Therefore, by containing a noble metal, during the regeneration process of the exhaust gas purification filter, the combustion time of the particulate matter deposited on the partition wall is shortened, and the use of fuel necessary for raising the exhaust gas temperature in the regeneration process is reduced. The consumption rate can be reduced, and further the deterioration of the exhaust gas purification filter itself can be suppressed.
In addition, the noble metal contained in the porous membrane 13 also has a catalytic action for a reaction of oxidizing carbon monoxide and hydrocarbons to convert them into carbon dioxide and water. it can.

Next, the noble metal of the present invention is present on the surface of the partition wall 14 in the form of a porous film 13 supported on oxide fine particles. The porous membrane 13 in the present invention has an average pore size of 0.05 μm or more and 5 μm or less, and the particulate matter 30 is mainly collected by the porous membrane 13 as described above. Accordingly, the particulate matter 30 is collected in a state in which the noble metal included in the porous film 13 is in close proximity. For this reason, if it is the structure of this invention, the catalyst performance of a noble metal can be exhibited more rapidly.
Further, since the porous film 13 does not substantially enter the pores of the porous ceramics constituting the support 11, almost all of it can contribute to the oxidation removal of the particulate matter 30. That is, the noble metal does not enter the pores of the filter, and as a result, the noble metal hardly contributes to the oxidation removal of the particulate matter 30 (is wasted).

  And the porous membrane 13 resulting from the sintering of the noble metal when a high temperature is applied to the filter as compared with the case where the noble metal is directly fixed to the partition wall of the filter base by containing the oxide fine particles. And the pore size distribution change of the exhaust gas purification filter and the deterioration of the oxidation catalyst component are suppressed. This is thought to be because oxides, particularly cerium oxide, have a migration-inhibiting effect on the supported noble metal, and because there are many pores in the porous film, there are many gaps between the noble metal-supported oxide fine particles. As a result, a change in combustion characteristics of the particulate matter 30 at a high temperature can be suppressed.

  By combining these effects, the exhaust gas purification filter (honeycomb structure type filter) of the present invention has excellent trapping characteristics of the particulate matter 30 and reduced pressure loss, and oxygen supply by a porous membrane. Necessary at the time of regeneration treatment by further shortening the regeneration processing time by promoting combustion of particulate matter 30 at the time of regeneration due to equalization action and combustion catalytic action of noble metal, lowering combustion temperature, and suppressing changes in combustion characteristics. It is possible to further reduce the amount of fuel used and to further suppress the deterioration of the exhaust gas purification filter itself. Furthermore, the deterioration of the combustion performance of particulate matter, soluble organic components, carbon monoxide, and hydrocarbons due to the heat of the exhaust gas purification filter containing a noble metal can be suppressed, and the performance of the filter itself can be maintained.

[Porous film forming paint]
Next, the porous film 13 included in the above-described DPF 10 and the porous film coating material for forming the porous film 13 will be described. Here, after first describing the porous film-forming paint, the porous film 13 formed using the paint will be described.
The coating material for forming a porous film according to the present embodiment contains fine particles containing at least one noble metal selected from the group consisting of platinum, palladium, and rhodium and an oxide, and a dispersion medium. The fine particles have a specific surface area of 1 m ² / g or more and 125 m ² / g or less, a tap bulk density of 0.1 g / cm ³ or more and 2.0 g / cm ³ or less, an average secondary in the dispersion medium The particle diameter is 0.1 μm or more and 10 μm or less, and the viscosity of the paint is 2 mPa · s or more and 1000 mPa · s or less.

  Here, in order to obtain a fine porous film using fine particles containing a noble metal and an oxide as a coating material, the specific surface area and tap bulk density of the fine particles containing the noble metal and the oxide are within a predetermined range. It is required to be within.

Here, “tap bulk density” refers to “tap bulk density” defined in JIS R1628-1997 “Method of measuring bulk density of fine ceramic powder” of Japanese Industrial Standards. In addition, the standard also defines a method for measuring tap bulk density.
The reason why the tap bulk density of the fine particles is smaller than the true density (true specific gravity) is that voids are formed between the fine particles. That is, as the tap bulk density (ρt) decreases, the porosity between the fine particles increases.

Here, the main factors that determine the tap bulk density are the primary particle size and particle shape of the fine particles. When there is no difference in the particle shape, the smaller the average primary particle diameter of the fine particles, the smaller the tap bulk density and the higher the porosity of the resulting porous film, but the smaller the pore diameter. Further, when the primary particle diameters of the fine particles are equal, the higher the shape anisotropy of the fine particles, the smaller the tap bulk density, and the higher the porosity of the resulting porous film and the larger the pore diameter.
Therefore, by defining the primary particle diameter and tap bulk density of the fine particles contained in the porous film-forming paint of this embodiment, the pore diameter and pores of the porous film formed using the paint of this embodiment The rate can be controlled.

Considering the above points, the tap bulk density of the fine particles is preferably 0.1 g / cm ³ or more and 2.0 g / cm ³ or less, more preferably 0.3 g / cm ³ or more and 1.8 g / cm ^3. Hereinafter, it is more preferably 0.5 g / cm ³ or more and 1.5 g / cm ³ or less.

In order to obtain the desired pore diameter and porosity of the porous film, it is desirable to adjust the specific surface area of the fine particles contained in the porous film-forming coating material together with the tap bulk density. Is preferably 1 m ² / g or more and 125 m ² / g or less, more preferably 3 m ² / g or more and 100 m ² / g or less, and further preferably 3 m ² / g or more and 80 m ² / g or less.

  This is because if the specific surface area and tap bulk density of the fine particles contained in the coating material for forming a porous film are limited to the above ranges, the average pore diameter targeted by the present invention is 0.05 μm or more and 5 μm or less. Shows that a porous film of 35% or more and 90% or less can be formed. Further, if necessary, the pore diameter of the porous membrane can be controlled to a desired value by mixing and using a plurality of types of fine particles having different specific surface areas and tap bulk densities within the above range.

That is, if the specific surface area of the fine particles contained in the coating material for forming a porous film exceeds 125 m ² / g, or the tap bulk density is less than 0.1 g / cm ³ , the particle diameter becomes too small to obtain a porous film. It becomes difficult, and the average pore diameter of the porous material becomes small regardless of the shape of the fine particles, so that it becomes difficult to keep the average pore diameter at 0.05 μm or more, and the pressure loss caused by the porous film becomes large. Further, such particles are not preferable because it is difficult to produce them with high productivity.
On the other hand, if the specific surface area is less than 1 m ² / g or the tap bulk density is more than 2.0 g / cm ³ , the particle size of the fine particles contained in the coating material for forming a porous film becomes excessive, and thus This is not preferable because it is difficult to obtain a porous film having a pore size of 5 μm or less, and furthermore, it becomes difficult to obtain a good paint by lowering the stability of the paint.

As the noble metal contained in the coating material for forming a porous film, one or more kinds of noble metals selected from the group consisting of platinum, palladium and rhodium are used. Among these, it is more preferable to use one or both of platinum and palladium.
The oxide contained in the porous film-forming coating material is one or more selected from the group consisting of aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium, manganese, and cobalt. Fine particles containing oxide fine particles or complex oxide fine particles containing these elements are used. Among the metals constituting these oxides, aluminum, zirconium, titanium, cerium, lanthanum, and iron are more preferable, and aluminum, zirconium, cerium, and lanthanum are more preferable. In particular, cerium oxide is preferable because it suppresses movement of noble metal fine particles integrated (supported) on the surface on the oxide surface and suppresses sintering of the noble metal.

  The ratio of the noble metal to the oxide in the fine particles contained in the porous film-forming coating material is preferably 0.1% by mass or more and 30% by mass or less, more preferably 0.1% by mass or more and 20% by mass or less. From 0.1% by weight to 10% by weight is most preferable. If the mass ratio of the noble metal to the oxide is less than 0.1%, the amount of particulate matter, soluble organic components, carbon monoxide, and noble metal fine particles having an oxidation catalyst performance of hydrocarbons is reduced, and the noble metal is reduced. This is not preferable because it does not substantially contribute to the improvement of the combustibility of the particulate matter. On the other hand, if the mass ratio of the noble metal to the oxide exceeds 30%, the fusion between the noble metal fine particles is facilitated, and the particulate matter containing the noble metal and the oxide, the soluble organic component, carbon monoxide, hydrocarbon This is not preferable because the performance of the oxidation catalyst tends to deteriorate.

  Such fine particles are dispersed in a dispersion medium to form a coating material for forming a porous film. This dispersing step is preferably performed by a wet method. In addition, as a disperser used in this wet method, any of an open type and a closed type can be used. For example, a ball mill, a stirring mill, a jet mill, a vibration mill, an attritor, a high speed mill, a hammer mill, etc. Preferably used. Examples of the ball mill include a rolling mill, a vibration mill, and a planetary mill. Examples of the stirring mill include a tower mill, a stirring tank mill, a flow pipe mill, and a tubular mill.

As this dispersion medium, water or an organic solvent is preferably used. In addition, polymer monomers and oligomers alone or a mixture thereof can be used as necessary.
Examples of the organic solvent include alcohols such as methanol, ethanol, 1-propanol, 2-propanol, diacetone alcohol, furfuryl alcohol, ethylene glycol and hexylene glycol, and esters such as methyl acetate and ethyl acetate. Ether ether such as diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, ethylene glycol monoethyl ether , Ethers such as dioxane, tetrahydrofuran, acetone, methyl ethyl ketone, acetyl Preferably used are ketones such as seton and acetoacetate, acid amides such as N, N-dimethylformamide, aromatic hydrocarbons such as toluene and xylene, and only one or two of these solvents. The above can be mixed and used.
In addition, as the polymer monomer, acrylic or methacrylic monomers such as methyl acrylate and methyl methacrylate, and epoxy monomers are preferably used. Moreover, as said oligomer, a urethane acrylate oligomer, an epoxy acrylate oligomer, an acrylate oligomer etc. are used suitably.
Among these dispersion media, water, alcohols, and ketones are preferable for coating materials, and among these, water and alcohols are more preferable, and water is most preferable.

  In this coating material, surface modification of the fine particles may be performed in order to increase the affinity between the fine particles and the dispersion medium. Since these fine particles contain an oxide as a component, examples of the surface modifier include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, cysteamine, tetramethylammonium hydroxide, and aminoethanediol. However, the present invention is not limited to these, and any surface modifier may be used as long as it has a functional group adsorbed on the surface of the fine particles to be used and has a terminal group having an affinity for the dispersion medium.

  Here, in order to make fine particles containing a noble metal and an oxide good paint, the average secondary particle diameter in the paint of the fine particles containing the noble metal and the oxide, and the viscosity of the paint are predetermined. It is required to be within the range.

First, the average secondary particle diameter of the fine particles in the paint is preferably 0.1 μm or more and 10 μm or less, more preferably 0.1 μm or more and 8 μm or less, and further preferably 0.1 μm or more and 6 μm or less.
The reason why the average secondary particle diameter of the fine particles in the paint is limited to the above range is that when the average secondary particle diameter of the fine particles in the paint is less than 0.1 μm, the average pore diameter of the paint is 5 μm or more and 50 μm or less. When the coating is applied to the surface of the porous support of the exhaust gas purification filter having the above, it becomes easy for this paint to enter the inside of the porous support and it is difficult to form a porous film on the surface of the porous support. This is because it is not preferable. Also, if the average secondary particle diameter of the fine particles exceeds 10 μm, it is difficult to ensure the dispersion stability of the paint and it is difficult to obtain a homogeneous porous film, which is not preferable.

Next, the viscosity of the coating is preferably 2 mPa · s or more and 1000 mPa · s or less, more preferably 2 mPa · s or more and 500 mPa · s or less, and further preferably 2 mPa · s or more and 300 mPa · s or less.
The reason why the viscosity of the paint is limited to the above range is that when the viscosity is less than 2 mPa · s, the paint is applied to the surface of the porous support of the exhaust gas purification filter having an average pore diameter of 5 μm or more and 50 μm or less. In this case, this paint is not preferable because it easily enters the pores of the porous support and it becomes difficult to form a porous film on the surface of the porous support. In addition, when the viscosity exceeds 1000 mPa · s, it becomes impossible to sufficiently infiltrate the paint into the cell of the exhaust gas purification filter, or it becomes difficult to remove excess paint in the cell of the exhaust gas purification filter. For this reason, it is not preferable because a desired porous film is not formed or a porous film having a uniform thickness is difficult to form.

The content of the fine particles in the paint can be appropriately selected so that the viscosity of the paint and the average secondary particle diameter of the fine particles in the paint are within the range of the present invention, but preferably 2% by mass or more and 60%. It is not more than mass, more preferably not less than 3 mass% and not more than 50 mass, further preferably not less than 5 mass% and not more than 40 mass.
Here, if the content of the fine particles in the paint is less than 2% by mass, the viscosity of the paint tends to be less than 2 mPa · s, and it is necessary to increase the number of coatings to obtain a desired film thickness. This is not preferable because the property may be inferior. On the other hand, if the content of fine particles exceeds 60% by mass, it is difficult to ensure the dispersion stability of the paint, and it becomes difficult to obtain a uniform porous film, which is not preferable.

  This coating material may appropriately contain a hydrophilic or hydrophobic polymer or the like in order to give a binder function between the fine particles and a porous support such as a partition wall of DPF, for example. The polymer and the like can be appropriately selected within the range in which the average secondary particle diameter of the fine particles in the coating material and the viscosity of the coating material are in desired values while being dissolved in the dispersion medium.

Here, when water is used as a dispersion medium, examples of hydrophilic polymers include synthetic alcohols such as polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene glycol, polyacrylamide, polystyrene sulfonic acid, polyvinyl pyrrolidone polyacrylate, and polyallylamine. Molecules: Natural polymers such as polysaccharides and polysaccharide-derived substances such as cellulose, dextrin, dextran, starch, chitosan, pectin, agarose, carrageenan, chitin, mannan; gelatin, casein, collagen, laminin, fibronectin, elastin, etc. Proteins and protein-derived substances can be used.
Further, substances such as gels and sols derived from these synthetic polymers, polysaccharides, proteins and the like can also be used.

In addition, the ratio of the mass of the hydrophilic polymer to the mass of the fine particles in this paint (the mass of the polymer / the mass of the fine particles) is such that the average secondary particle diameter of the fine particles in the paint and the viscosity of the paint are as desired. However, the range is preferably 1 or less, more preferably 0.8 or less, and most preferably 0.5 or less.
The hydrophilic polymer to be used is a component that is eventually burned out by heat treatment and does not remain in the porous membrane. Therefore, if the ratio exceeds 1, the polymer content is too high and the cost increases. This is because it is undesirable. In addition, since the hydrophilic polymer is not necessarily used, the lower limit value of the range is 0.

In order to ensure the dispersion stability of the paint or to improve the coating property, a surfactant, preservative, stabilizer, antifoaming agent, leveling agent and the like may be appropriately added. These can be appropriately selected so that the average secondary particle diameter of the fine particles in the coating material and the viscosity of the coating material are in a desired range. There are no particular restrictions on the amount of these surfactants, preservatives, stabilizers, antifoaming agents, leveling agents and the like, and the viscosity of the paint and the average secondary particle size of the fine particles in the paint are within the scope of the present invention. Thus, what is necessary is just to add according to the objective to add.
In this way, fine particles containing noble metals and oxides are dispersed in a dispersion medium, and hydrophilic or hydrophobic polymers, surfactants, preservatives, stabilizers, antifoaming agents, and leveling as required. An agent or the like is added and mixed to form a coating for forming a porous film.

  According to the coating material for forming a porous film having the above-described configuration, fine particles containing a noble metal and an oxide, which are preferably used in an exhaust gas purification filter as an object of the present invention, are used, and the average pore diameter is 0. A porous film excellent in homogeneity can be easily formed with a porosity of 05 μm or more and 5 μm or less and a porosity of 35% or more and 90% or less. This coating material for forming a porous film can be formed by simply applying it, so it is easy to form a porous film with excellent homogeneity on the surface without being restricted by the shape of the object. can do.

As already described, in order for the above-described noble metal to exhibit an effective catalytic action in the porous film 13, the noble metal fine particles containing the noble metal are in the form of extremely fine particles on the surface of the oxide fine particles or the composite oxide fine particles. Dispersed and integrated metal / oxide composite fine particles are preferable. In order to form the porous film 13 with such metal / oxide composite fine particles, it is preferable that the fine particles in the coating are metal / oxide composite fine particles in advance.
That is, the fine particles containing the noble metal and the oxide in the coating are mixed particles obtained by simply mixing the noble metal fine particles and the oxide fine particles, or the noble metal fine particles and the oxide fine particles are integrated, but the noble metal fine particles are coarse. It is preferable that the metal / oxide composite fine particles have noble metal ultrafine particles integrated and fixed on the surface of the oxide fine particles.
As a method for producing the metal / oxide composite fine particles, a noble metal and an oxide are contained by a method appropriately selected from an impregnation method, a coprecipitation method, a deposition method, a kneading method, an ion exchange method, a melting method, and the like. Examples thereof include a method in which an intermediate is prepared, and then washed with water or the like as necessary, followed by drying and baking.

[Method for producing porous membrane]
Next, a method for producing a porous film using the above-described porous film forming paint will be described. The porous film of the present embodiment can be obtained by applying the above-described porous film-forming coating material to the surface of the porous support and heat-treating it.

For example, in the DPF 10, the porous film forming paint is formed on the inner wall surface (surface of the partition wall 14 on the side of the inflow cell 12 </ b> A) 12 a of the gas flow path 12 where the exhaust upstream end is opened. Is applied to form a coating film containing a large amount of solid components such as fine particles and a liquid component such as a solvent, and the resulting coating film is dried and then heat-treated to form the porous film 13.
The coating method may be appropriately selected according to the shape and material of the porous support, and is not particularly limited, but usual wet coating methods such as wash coating and dip coating can be used. In addition, after the application, a process such as removal of an excess coating liquid more than an amount necessary for obtaining a desired film thickness may be performed using compressed air or the like.
At the time of application, the porous support may be in a dry state, but the porous support is immersed in a solvent, and the air in the pores of the porous support is replaced with a solvent in advance. Those are preferred. The reason for this is that air remaining in the pores of the porous support is released as bubbles from the porous support during or after the coating process, and the porous film is not partially formed. This is because there is an effect that a uniform porous film can be obtained.

Thus, since many solvent is contained in a coating film or a porous support body, it is preferable to dry before heat processing. Although drying conditions depend on the type and amount of solvent used, they cannot be defined unconditionally. For example, in the case of water, it is preferably about 15 minutes to 10 hours at 50 ° C. or more and 200 ° C. or less.
This drying step may be performed in combination with the heat treatment step described below. For example, the heat treatment step may be performed by raising the temperature as it is after the drying step. Further, the drying step can be substantially omitted by adjusting the temperature rising condition in the heat treatment step so that the temperature raising step in the heat treatment step is combined with the drying step.

In addition to the dispersant, the coating film contains the above-described polymer, surfactant, preservative, stabilizer, antifoaming agent, leveling agent, and the like as necessary. In addition, heat treatment is performed to form a fine pore structure in the coating film.
The heat treatment temperature is preferably 200 ° C. or higher and 2000 ° C. or lower, more preferably 300 ° C. or higher and 1700 ° C. or lower. The heat treatment time is preferably 0.25 hours or more and 10 hours or less, more preferably 0.5 hours or more and 5 hours or less. The atmosphere during this heat treatment is not particularly limited, and in an atmosphere such as an oxidizing atmosphere such as air, an inert atmosphere such as nitrogen, argon, neon, or xenon, or a reducing atmosphere such as hydrogen or carbon monoxide. It can be carried out.
In addition, it is not preferable to increase the heat treatment temperature or extend the heat treatment time beyond sufficiently obtaining the characteristics of the porous film to be formed, because this promotes sintering of the noble metal fine particles contained in the coating film. Careful attention must be paid to the selection of heat treatment temperature, time, and atmosphere.

And the exhaust gas purification filter mentioned above can be obtained by forming the porous membrane of this embodiment on the surface of the porous support body which consists of porous ceramics whose average pore diameter is 5 micrometers or more and 50 micrometers or less.
The porous film-forming coating material of this embodiment has a viscosity of 2 mPa · s or more and 1000 mPa · s or less, and the average secondary particle diameter of the contained fine particles is 0.1 μm or more and 10 μm or less. Even when the coating material for film formation is directly applied to the surface of the porous support, a uniform coating film is formed on the surface of the porous support without substantially penetrating the pores of the support. Can do.

  As described above, a coating film was directly formed on the surface of the porous support using the porous film-forming coating material of the present embodiment, and the coating film was heat-treated to provide the above-described porous film. An exhaust gas purification filter (honeycomb structure type filter) can be obtained.

  As described above, the porous membrane produced as described above is superior in trapping characteristics of particulate matter as compared with conventional exhaust gas purification filters, combustion characteristics of particulate matter during regeneration, and solubility It has excellent oxidative purification performance for organic components, carbon monoxide, and hydrocarbons, and has very excellent characteristics.

  The preferred embodiments of the present invention have been described above with reference to the accompanying drawings, but it goes without saying that the present invention is not limited to such examples. Various shapes, combinations, and the like of the constituent members shown in the above-described examples are examples, and various modifications can be made based on design requirements and the like without departing from the gist of the present invention.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention concretely, this invention is not limited by these Examples.

[Example 1]
[Production of CZ (CeO ₂ —ZrO ₂ ) / Pt Composite Fine Particles]
First, 100 g of composite oxide (Ce _0.5 Zr _0.5 O ₂ ) fine particles (hereinafter referred to as CZ fine particles) in which the composition ratio of cerium and zirconium is 1: 1 is 1% by mass in terms of platinum. It was impregnated with 300 g of dinitrodiammineplatinum (II) nitric acid solution. Thereafter, the CZ fine particles and the nitric acid solution were heated to 150 ° C. to evaporate to dryness, and further fired in the atmosphere at 500 ° C. for 2 hours to prepare CZ / Pt composite fine particles-1.
The specific surface area of this CZ / Pt composite fine particle-1 was 50 m ² / g, and the tap bulk density was 1.1 g / cm ³ .
Here, what is indicated as Ce _0.5 Zr _0.5 O ₂ is an oxide having a composition ratio of cerium and zirconium of 1: 1 as a whole, and cerium and zirconium are 1: 1. In addition to CeZrO ₄ , which is a complex oxide completely dissolved in the composition ratio, there may be included complex oxides having different composition ratios of cerium and zirconium, and those containing cerium oxide alone.

[Preparation of porous film forming paint]
90 g of CZ / Pt composite fine particles-1, 9 g of a polycarboxylic acid-based dispersant, and 201 g of water were mixed for 2 hours by a ball mill to obtain CZ / Pt dispersion A having a solid content of 30% by mass. To the prepared CZ / Pt dispersion A 167 g, 67 g of an aqueous solution containing 15% by mass of methylcellulose A (2% aqueous solution at 25 ° C. has a viscosity of 5 mPa · s) and 16 g of water were added and stirred for 1 hour using a magnetic stirrer. A coating material A of Example 1 in which CZ / Pt composite fine particles were dispersed in water was obtained.
In the following examples and comparative examples, in principle, “dispersion liquid” means a dispersion medium (solvent) in which noble metal and oxide part particles or composite oxide fine particles are simply dispersed, “paint”. Indicates a dispersion obtained by adding various additives to the dispersion in consideration of the stability of the liquid and the coating property. However, if the dispersion itself is stable and excellent in coating properties, the dispersion may be used as it is as a paint.

[Measurement of average secondary particle size of paint]
The viscosity of the paint A at 25 ° C. was measured using a B-type viscometer (manufactured by Toki Sangyo Co., Ltd.). Moreover, the average secondary particle diameter of the CZ / Pt composite fine particles in the coating material A was measured by a dynamic light scattering method using a measuring device HPPS (manufactured by Malvern Instruments).

[Evaluation 1: Production of porous film using paint]
Dip coat that is pulled up after immersing a silicon carbide (SiC) honeycomb structure (DPF, average pore diameter: 10 μm, average porosity: 42%) in which the partition wall is a porous support in the paint A Was repeated three times to form a coating film made of CZ / Pt composite fine particles on the partition wall surface of the honeycomb structure. The honeycomb structure was previously immersed in pure water, and the pores were filled and held with water.
Next, the honeycomb structure after dip coating was dried at 150 ° C. for 1 hour and further heat-treated in the atmosphere at 500 ° C. for 2 hours to form a porous film composed of CZ / Pt composite fine particles on the partition wall surface of the honeycomb structure. .

[Evaluation 2: Evaluation of the produced porous film]
(2-1) Appearance Observation by Electron Microscope The honeycomb structure in which the porous film is formed as described above is broken into small portions for each partition wall of the honeycomb structure, and the partition wall on which the surface of the porous film and the porous film are formed. Was observed using a field emission scanning electron microscope (FE-SEM) S-4000 (manufactured by Hitachi Instrument Service Co., Ltd.).

(2-2) Measurement of average pore diameter and porosity of porous membrane Mercury porosimeter AutoPoreIV 9505 (manufactured by Shimadzu Corporation) was used to measure the average pore diameter and porosity of the porous membrane formed as described above. It measured by the press-fitting method. Since the measurement result is a combination of the porous membrane and the partition wall (porous support), the SiC honeycomb structure without the porous membrane was measured in the same manner as a control, From the results, the average pore diameter and porosity of the porous membrane were calculated.

(2-3) Thickness measurement of porous membrane The partition of the exhaust gas purification filter is broken, and the partition cross section is observed with a field emission scanning electron microscope (FE-SEM) S-4000 (manufactured by Hitachi Instrument Service Co., Ltd.). Thus, the thickness of the porous membrane was measured.

[Example 2]
167 g of the CZ / Pt dispersion A prepared in Example 1, 18 g of an aqueous solution containing 15% by mass of methyl cellulose B (2% aqueous solution at 25 ° C. has a viscosity of 20 mPa · s), and 65 g of water were stirred for 1 hour using a magnetic stirrer. Thus, the paint B of Example 2 in which CZ / Pt composite fine particles were dispersed in water was obtained. For this paint B, in the same manner as in Example 1, formation of a porous film, evaluation of the paint, and evaluation of the porous film were performed.

[Example 3]
167 g of CZ / Pt dispersion A prepared in Example 1, 67 g of an aqueous solution containing 15% by mass of methylcellulose B, and 16 g of water were mixed by stirring for 1 hour using a magnetic stirrer, and CZ / Pt composite fine particles were mixed in water. A dispersed paint C of Example 3 was obtained. With respect to this paint C, the porous film was formed, the paint was evaluated, and the porous film was evaluated in the same manner as in Example 1.

[Example 4]
167 g of the CZ / Pt dispersion A prepared in Example 1, 67 g of an aqueous solution containing 15% by mass of methylcellulose C (2% aqueous solution at 25 ° C. has a viscosity of 100 mPa · s), and 16 g of water were stirred for 1 hour using a magnetic stirrer. Thus, the paint D of Example 4 in which CZ / Pt composite fine particles were dispersed in water was obtained. About this coating material D, in the same manner as in Example 1, formation of a porous film, evaluation of the coating material, and evaluation of the porous film were performed.

[Example 5]
100 g of CZ fine particles used in Example 1 were impregnated with 100 g of a dinitrodiammine platinum nitric acid solution having a concentration of 0.5% by mass in terms of platinum, then evaporated to dryness at 150 ° C., and further fired at 500 ° C. in the atmosphere for 2 hours. And CZ / Pt composite fine particles-2 were prepared.
The CZ / Pt composite fine particle-2 had a specific surface area of 50 m ² / g and a tap bulk density of 1.1 g / cm ³ .
Example in which CZ / Pt composite fine particles were dispersed in water and CZ / Pt dispersion E having a solid content of 30% by mass in the same manner as in Example 1 except that this CZ / Pt composite fine particle-2 was used as the fine particles. 5 paint E was obtained. For this paint E, in the same manner as in Example 1, formation of a porous film, evaluation of the paint, and evaluation of the porous film were performed.

[Example 6]
100 g of the CZ fine particles used in Example 1 were impregnated with 500 g of a dinitrodiammine platinum nitric acid solution having a concentration of 2% by mass in terms of platinum, and then evaporated to dryness at 150 ° C., followed by baking at 500 ° C. for 2 hours in the atmosphere. CZ / Pt fine particles-3 were prepared.
The specific surface area of this CZ / Pt composite fine particle-3 was 50 m ² / g, and the tap bulk density was 1.2 g / cm ³ .
Example in which CZ / Pt composite fine particles were dispersed in CZ / Pt dispersion F having a solid content of 30% by mass and water, except that this CZ / Pt composite fine particle-3 was used as the fine particles. 6 paint F was obtained. With respect to this paint F, in the same manner as in Example 1, formation of a porous film, evaluation of the paint, and evaluation of the porous film were performed.

[Example 7]
100 g of the CZ fine particles used in Example 1 were impregnated with 150 g of a dinitrodiammine platinum nitric acid solution having a concentration of 1% by mass in terms of platinum and 150 g of an aqueous palladium nitrate solution having a concentration of 1% by mass in terms of palladium, and then evaporated to dryness at 150 ° C. Further, baking was performed at 500 ° C. in the atmosphere for 2 hours to prepare CZ / Pt—Pd composite fine particles-4.
The specific surface area of the CZ / Pt—Pd composite fine particles-4 was 50 m ² / g, and the tap bulk density was 1.1 g / cm ³ .
The CZ / Pt-Pd composite fine particles-4 were used as fine particles in the same manner as in Example 1, except that the CZ / Pt-Pd dispersion G having a solid content of 30% by mass and the CZ / Pt-Pd fine particles in water were used. A paint G of Example 7 in which was dispersed was obtained. For this paint G, the porous film was formed, the paint was evaluated, and the porous film was evaluated in the same manner as in Example 1.

[Example 8]
100 g of the CZ fine particles used in Example 1 were impregnated with 300 g of a palladium nitrate aqueous solution having a concentration of 1% by mass in terms of palladium, evaporated to dryness at 150 ° C., and further baked at 500 ° C. for 2 hours in the atmosphere. Pd composite fine particles-5 were prepared.
The CZ / Pd composite fine particles-5 had a specific surface area of 50 m ² / g and a tap bulk density of 1.1 g / cm ³ .
Example in which CZ / Pd composite fine particles were dispersed in CZ / Pd dispersion H having a solid content of 30% by mass and water, except that this CZ / Pd composite fine particle-5 was used as the fine particles. 8 paints H were obtained. With respect to this paint H, the porous film was formed, the paint was evaluated, and the porous film was evaluated in the same manner as in Example 1.

[Example 9]
Example 1 was used except that lanthanum-doped cerium oxide (La—CeO ₂ ) fine particles (the composition ratio of cerium and lanthanum was 9: 1 (Ce _0.9 La _0.1 O _{2 -X} )) were used as the fine particles. As a result, Ce _0.9 La _0.1 O _{2 -X} / Pt composite fine particles-6 were obtained.
The Ce _0.9 La _0.1 O _{2 -X} / Pt composite fine particles-6 had a specific surface area of 50 m ² / g and a tap bulk density of 1.1 g / cm ³ .
In addition, what is indicated as Ce _0.9 La _0.1 O _2-X here is an oxide having a composition ratio of cerium and lanthanum of 9: 1 as a whole, and is oxidized in cerium oxide. In addition to complex oxides in which lanthanum is dissolved, complex oxides having different composition ratios of cerium and zirconium, and those containing cerium oxide alone may be included.
The _{Ce 0.9 La 0.1 O 2-X} / Pt solid content of 30 mass% except for using the composite fine particles -6 in the same manner as in Example _{1 Ce 0.9 La 0.1 O 2-} X / Pt dispersion I was obtained. Ce _0.9 La _0.1 O _2-X / Pt composite fine particles were obtained in water in the same manner as in Example 1 except that this Ce _0.9 La _0.1 O _2-X / Pt dispersion I was used. A dispersed paint I of Example 9 was obtained. About this coating material I, the formation of the porous film, the evaluation of the coating material, and the evaluation of the porous film were performed in the same manner as in Example 1.

[Example 10]
CeO ₂ / Pt composite fine particles-7 were obtained in the same manner as in Example 1 except that cerium oxide fine particles-7 were used as the fine particles.
The CeO ₂ / Pt composite fine particles-7 had a specific surface area of 3 m ² / g and a tap bulk density of 2.0 g / cm ³ .
A CeO ₂ / Pt dispersion liquid J having a solid content of 30% by mass was obtained in the same manner as in Example 1 except that this CeO ₂ / Pt composite fine particle-7 was used. A paint J of Example 10 in which CeO ₂ / Pt composite fine particles were dispersed in water was obtained in the same manner as in Example 1 except that this CeO ₂ / Pt dispersion J was used. With respect to this paint J, in the same manner as in Example 1, formation of a porous film, evaluation of the paint, and evaluation of the porous film were performed.

[Example 11]
Al ₂ O ₃ / Pt composite fine particles-8 were obtained in the same manner as in Example 1 except that the aluminum oxide fine particles-8 were used as the fine particles.
The specific surface area of this Al ₂ O ₃ / Pt composite fine particle-8 was 125 m ² / g, and the tap bulk density was 0.1 g / cm ³ .
45 g of this Al ₂ O ₃ / Pt composite fine particle-8 was weighed, and this was mixed with 4.5 g of a polycarboxylic acid-based dispersant and 250 g of water with a ball mill for 2 hours, and the solid content was 15% by mass of Al ₂ O ₃ / Pt dispersion K was obtained. 30 g of an aqueous solution containing 15% by mass of methylcellulose A and 53 g of water are added to 167 g of this Al ₂ O ₃ / Pt dispersion K and stirred for 1 hour using a magnetic stirrer to disperse the Al ₂ O ₃ / Pt composite fine particles in water. The paint K of Example 11 was obtained. The paint J was evaluated in the same manner as in Example 1.
As the production of the porous film, dip coating in which the SiC honeycomb structure was dipped in the paint K and then pulled up in the paint J was repeated three times. Next, the dip-coated honeycomb structure was dried at 150 ° C. for 1 hour and further heat-treated in the atmosphere at 500 ° C. for 2 hours. This dip coating 3 times + 150 ° C. drying + 500 ° C. heat treatment process was repeated once more to form a porous film composed of Al ₂ O ₃ / Pt composite fine particles on the partition wall surface of the honeycomb structure. The obtained porous membrane was evaluated in the same manner as in Example 1.

[Example 12]
Al ₂ O ₃ / Pt composite fine particles-9 were obtained in the same manner as in Example 1 except that the aluminum oxide fine particles-9 were used as the fine particles.
The specific surface area of this Al ₂ O ₃ / Pt composite fine particle-9 was 15 m ² / g, and the tap bulk density was 0.8 g / cm ³ .
The _Al 2 _O 3 / Pt solid in the same manner except for using the composite fine particles -9 Example 1 to obtain a _Al 2 _O 3 / Pt dispersion L of 30 wt%. A coating material L of Example 12 in which Al ₂ O ₃ / Pt composite fine particles were dispersed in water was obtained in the same manner as in Example 1 except that this Al ₂ O ₃ / Pt dispersion L was used. With respect to this paint L, the formation of a porous film, the evaluation of the paint, and the evaluation of the porous film were carried out in the same manner as in Example 1.

[Example 13]
Al ₂ O ₃ / Pt composite fine particles-10 were obtained in the same manner as in Example 1 except that aluminum oxide fine particles-10 were used as the fine particles.
The specific surface area of this Al ₂ O ₃ / Pt composite fine particle-10 was 1 m ² / g, and the tap bulk density was 1.3 g / cm ³ .
The _Al 2 _O 3 / Pt solid in the same manner except for using the composite fine particles -10 from Example 1 was obtained _Al 2 _O 3 / Pt dispersion M of 30 mass%. A paint M of Example 13 in which Al ₂ O ₃ / Pt composite fine particles were dispersed in water was obtained in the same manner as in Example 1 except that this Al ₂ O ₃ / Pt dispersion M was used. About this coating material M, in the same manner as in Example 1, formation of a porous film, evaluation of the coating material, and evaluation of the porous film were performed.

[Comparative Example 1]
83 g of CZ / Pt dispersion A prepared in Example 1, 8 g of an aqueous solution containing 15% by mass of methylcellulose D (2% aqueous solution at 25 ° C. of 2 mPa · s), and 159 g of water were stirred for 1 hour using a magnetic stirrer. Thus, the coating material O of Comparative Example 1 in which CZ / Pt fine particles were dispersed in water was obtained. For this coating material O, the porous film was formed and the coating material was evaluated in the same manner as in Example 1.

[Comparative Example 2]
167 g of CZ / Pt dispersion A prepared in Example 1, 67 g of aqueous solution containing 15% by mass of methylcellulose E (2% aqueous solution at 25 ° C. has a viscosity of 200 mPa · s), and 16 g of water were mixed by stirring by hand. A paint P of Comparative Example 2 in which CZ / Pt fine particles were dispersed in water was obtained. For this coating material P, the porous film was formed and the coating material was evaluated in the same manner as in Example 1.

[Comparative Example 3]
Dispersion treatment of 45 g of Al ₂ O ₃ / Pt composite fine particles-8 (specific surface area 125 m ² / g) used in Example 11, 250 g of water, and 4.5 g of a polycarboxylic acid dispersant in a bead mill using zirconia beads for 4 hours. As a result, an Al ₂ O ₃ / Pt dispersion Q having a solid content of 15% by mass was obtained. To 167 g of this Al ₂ O ₃ / Pt dispersion Q, 30 g of an aqueous solution containing 15% by mass of methylcellulose A and 53 g of water are added and stirred for 1 hour using a magnetic stirrer to disperse the Al ₂ O ₃ / Pt composite fine particles in water. The paint Q of Comparative Example 3 was obtained. For this paint Q, a porous film was formed in the same manner as in Example 11, and the paint was evaluated in the same manner as in Example 1.

[Comparative Example 4]
A CeO ₂ / Pt composite fine particle-11 was obtained in the same manner as in Example 1 except that the cerium oxide fine particle-11 was used as the fine particle.
The CeO ₂ / Pt composite fine particles-11 had a specific surface area of 0.5 m ² / g and a tap bulk density of 2.4 g / cm ³ .
90 g of this CeO ₂ / Pt composite fine particle-11, 201 g of water and 9 g of a polycarboxylic acid-based dispersant were mixed for 2 hours using a ball mill to obtain a CeO ₂ / Pt dispersion R having a solid content of 30% by mass. . Comparative Example 4 in which 67 g of an aqueous solution containing 15% by mass of methylcellulose A and 16 g of water were added to 167 g of this CeO ₂ / Pt dispersion R and stirred for 1 hour using a magnetic stirrer to disperse CeO ₂ / Pt composite fine particles in water. Paint R was obtained.
When the stirring of the coating material R was stopped, the sedimentation of CeO ₂ / Pt composite microparticles immediately started and separated into two layers of a precipitate of CeO ₂ / Pt composite microparticles and a supernatant liquid in a short time. Thereby, it turned out that this coating material R is not suitable as a coating material for porous film formation.

[Comparative Example 5]
Al ₂ O ₃ / Pt composite particles 11 were obtained in the same manner as in Example 11 except that the aluminum oxide particles 11 were used as the particles.
The Al ₂ O ₃ / Pt fine particles-11 had a specific surface area of 250 m ² / g and a tap bulk density of 0.07 g / cm ³ .
The _Al 2 _O 3 / Pt except for using the composite fine particles -11 in the same manner as in Example 11 solids to obtain a _Al 2 _O 3 / Pt dispersion S of 15 mass%. A coating material S of Comparative Example 5 in which Al ₂ O ₃ / Pt composite fine particles were dispersed in water was obtained in the same manner as Example 11 except that this Al ₂ O ₃ / Pt dispersion S was used. For this coating material S, a porous film was formed in the same manner as in Example 11, and the coating material was evaluated in the same manner as in Example 1.

  Table 1 shows the preparation conditions of the paint and the porous film, and Table 2 shows the evaluation results of the obtained paint and the porous film for the above Examples and Comparative Examples.

First, according to the result of observation with an electron microscope, a porous film made of fine particles dispersed in a paint is formed on the surfaces of the partition walls of Examples 1 to 13, and fine particles which are main components of the porous film. Was hardly present in the pores of the partition walls.
In addition, as shown in the evaluation results of Tables 1 and 2, the coating materials of Examples 1 to 13 contain fine particles containing a noble metal and an oxide and a dispersion medium. Since the density, the average secondary particle diameter in the paint, and the viscosity of the paint are appropriately controlled, the average pore diameter of the porous support on the surface of the partition wall (porous support) of the SiC honeycomb structure It was possible to form a porous film having a smaller average pore diameter.

On the other hand, in Comparative Example 1, the partition walls of the honeycomb structure after the porous film was prepared were observed with a field emission scanning electron microscope (FE-SEM) S-4000 (manufactured by Hitachi Instrument Service Co., Ltd.). It was confirmed that a porous film was not formed on the surface of the film. This is presumably because the viscosity of the paint was too low, so that the paint entered the pores of the porous support and could not form a porous film.
In Comparative Example 2, the partition walls of the honeycomb structure after the porous membrane was prepared were observed in the same manner as in Comparative Example 1. As a result, the coating P did not penetrate into the honeycomb structure (inside the cells of the DPF) and the surface of the partition walls It was confirmed that a porous film was not formed. This is because the viscosity of the paint is too high, so that the paint cannot be spread inside the SiC honeycomb structure, and a coating film can be formed on the surface of the internal partition wall (porous support). It is thought that there was not.
In Comparative Example 3, the partition walls of the honeycomb structure after the porous film was prepared were observed in the same manner as in Comparative Example 1. As a result, Al ₂ O ₃ / Pt fine particles entered the pores of the partition walls, and the surface of the partition walls was porous. A film was not formed. This is because the secondary particle diameter of the fine particles containing the noble metal and the oxide is too small, so that the paint enters the pores of the partition walls (porous support), and a porous film cannot be formed. It is thought.
In Comparative Example 4, since the specific surface area of the fine particles containing the noble metal and the oxide was too small and the tap bulk density was too large, the paint itself could not be produced.
In Comparative Example 5, since the specific surface area of the fine particles containing the noble metal and the oxide was large and the secondary particle diameter of the fine particles was too small, the pores of the partition walls (porous support) of the honeycomb structure made of SiC were used. The porous film could not be formed.

  From the above results, it was confirmed that the porous film can be satisfactorily formed by using the porous film-forming paint of the present invention, and the usefulness of the present invention was confirmed.

10 DPF
DESCRIPTION OF SYMBOLS 11 Filter base | substrate 12 Gas flow path 12A Inflow cell 12B Outflow cell 13 Porous membrane 14 Partition 30 Particulate matter (alpha), (gamma) End face G Exhaust gas C Purified gas

Claims (4)

A paint for forming a porous film having an average pore diameter smaller than the average pore diameter of the porous support on the surface of the porous support of the honeycomb structure type filter,
The paint contains at least fine particles containing a noble metal selected from the group consisting of platinum, palladium, and rhodium and an oxide, and a dispersion medium in which the fine particles are dispersed. And
In the fine particles, the noble metal fine particles containing the noble metal and the oxide fine particles containing the oxide form metal / oxide composite fine particles,
The fine particles have a specific surface area of 1 m ² / g or more and 125 m ² / g or less, a tap bulk density of 0.1 g / cm ³ or more and 2.0 g / cm ³ or less, and an average secondary particle size in the dispersion medium. 0.1 μm or more and 10 μm or less,
The coating material for forming a porous film, wherein the viscosity of the coating material is 2 mPa · s or more and 1000 mPa · s or less. 2. The coating material for forming a porous film according to claim 1, wherein a ratio of the noble metal to the oxide fine particles is 0.1% by mass or more and 30% by mass or less. The oxide is an oxide or a composite oxide containing one or more elements selected from the group consisting of aluminum, zirconium, titanium, cerium, lanthanum, iron, silicon, zinc, magnesium, manganese, and cobalt. for forming a porous film coating according to claim 1 or 2 further characterized in that there.   A porous film formed by applying heat treatment after applying the coating material for forming a porous film according to any one of claims 1 to 3.

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