JP2015009212A - Exhaust gas cleaning filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning filter which can suppress an increase of pressure loss thereof while improving particulate matter collectability thereof and can also improve durability thereof and lower combustion temperature of particulate matter.SOLUTION: A filter base body 2 includes: partition walls 3 each comprising a porous material; an inflow cell 4A which is formed by the partition walls 3 and the inflow-side end for exhaust gas of which is opened; and an outflow cell 4B which is arranged at a position different from that of the inflow cell 4A of the filter base body 2, and formed by the partition walls 3 and the outflow-side end for exhaust gas of which is opened. A porous film 5 containing oxidation catalyst particles is formed on the surface of the inflow cell 4A. The oxidation catalyst particle is obtained by depositing a noble metal particle of such a state on the surface of a silicon carbide particle that the noble metal particle is covered with an oxide layer thereof. A ratio (Mn/Ms) of the mass (Mn) of the noble metal particle to that (Ms) of the silicon carbide particle is 0.005 or larger and 0.15 or lower.

Description

  TECHNICAL FIELD The present invention relates to an exhaust gas purification filter, and more specifically, is used when removing particulate matter (PM) contained in exhaust gas discharged from a diesel engine such as an automobile. Exhaust gas purification that improves the filter's durability by suppressing the increase in pressure loss while lowering the combustion temperature of particulate matter when regenerating the filter It relates to filters.

  Substances such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) contained in exhaust gas discharged from diesel engines such as automobiles cause air pollution. So far, it has 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. Therefore, an exhaust gas purification apparatus is used to purify substances that cause air pollution contained in these exhaust gases.

In general, a diesel particulate filter (DPF: Diesel Particulate Filter) having a plugged ceramic honeycomb structure is used as an exhaust gas purification filter for collecting PM in an automobile diesel engine. . This DPF is a ceramic structure made of a metal oxide such as alumina, ceria, zirconia, or a honeycomb structure made of a metal composite oxide having a perovskite structure represented by the general formula ABO ₃ (where A and B are metal elements). Both ends of the cell (gas flow path) are sealed in a checkered pattern, and a noble metal element having an oxidation catalyst function is supported on at least the surface of the cell on the inflow side gas flow path. When exhaust gas passes, PM is collected.

In this DPF, the collected PM is removed by combustion. When this PM is combusted, a noble metal element having an oxidation catalyst function carried by the DPF acts as a catalyst for combustion removal of PM, and combustion The temperature is reduced and the combustion time is shortened (see, for example, Patent Documents 1 and 2).
By the way, in the conventional catalyst for purifying exhaust gas described above, noble metal elements are essential, but on the other hand, the number of automobiles owned in emerging countries is increasing rapidly, and exhaust gas regulations in developed countries are further strengthened. Therefore, the demand for precious metal elements used in automobile exhaust gas purification devices is expected to increase further.
Therefore, as a catalyst for exhaust gas purification that can reduce the amount of noble metal element used, the surface area of the noble metal element is reduced by supporting the noble metal element on the surface of fine particles (supported particles) such as γ-alumina having a high specific surface area. A configuration has been proposed in which composite particles that have been expanded and have increased catalytic activity are used as a catalyst (see, for example, Patent Document 3).

By the way, the conventional catalyst for purifying exhaust gas described above has a problem that the specific surface area is remarkably reduced by causing crystal transition of γ-alumina to α-alumina at a high temperature of 1000 ° C. or higher.
Furthermore, the structural change due to the crystal transition from γ-alumina to α-alumina becomes a cause of promoting the sintering (grain growth) of the catalyst component. Further, when these catalysts are formed in layers on a substrate, structural changes due to the crystal transition of the catalyst may cause peeling or dropping of the formed layers.
From the above, there is a problem that a catalyst in which a noble metal element is supported on the surface of fine particles such as γ-alumina cannot maintain catalytic activity at high temperatures.
In addition, direct injection for the purpose of improving fuel efficiency and high output has been performed in gasoline engines, but in this direct injection gasoline engine, the generation of PM is unavoidable. There is an increasing need for a PM collection filter corresponding to the above.

  The present inventors have conducted intensive research to solve the above problems, and in the exhaust gas purification filter, a porous film made of silicon carbide particles is formed on the surface of at least the inflow side gas flow path side of the partition wall constituting the filter base. If the noble metal particles are supported in a state of being covered with the oxide layer formed on the surface of the silicon carbide particles, the increase in pressure loss is suppressed while improving the trapping property of the particulate matter, and Has found and proposed that the combustion temperature of particulate matter can be lowered, and as a result, not only the durability of the filter can be improved, but also the filter can be continuously regenerated (Patent Document 4). ).

Japanese Patent Laid-Open No. 5-23512 JP-A-9-77573 JP 2006-7117 A JP2013-63420A

  However, in the exhaust gas purification filter described in Patent Document 4, a suitable loading amount of the noble metal particles with respect to the silicon carbide particles has not been studied, and thus there is a risk that insufficient expression of catalyst characteristics may occur due to an insufficient amount of the noble metal particles. There was a problem that there was. In addition, there is a problem in that the production cost is increased by supporting an excessive amount of noble metal particles exceeding the value at which the catalyst characteristics are saturated.

  The present invention has been made in view of the above circumstances, and by supporting an optimal amount of noble metal particles with respect to silicon carbide particles, it is possible to increase the pressure loss while improving the collection of particulate matter. Exhaust gas purification filter that can suppress, improve the durability and lower the combustion temperature of particulate matter, and is also excellent in manufacturing cost by optimizing the amount of noble metal particles supported on silicon carbide particles The purpose is to provide.

  The present inventors have added substances such as carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), and particulate matter (PM) contained in exhaust gas discharged from diesel engines such as automobiles. As a result of intensive studies on an exhaust gas purification filter that efficiently purifies, the oxidation catalyst particles contained in the porous film on the surface of the filter base constituting the exhaust gas purification filter are supported on the surface of the silicon carbide particles, and noble metal particles are supported on the surface of the silicon carbide particles. The noble metal particles are supported in a state covered with an oxide layer, and the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to the mass (Ms) of the silicon carbide particles is 0.005 or more and 0.15 or less. If it is within the range, the combustion temperature of PM can be lowered by the interaction between the noble metal particles and the oxide layer, the durability of the filter can be improved, and further the silicon carbide particles It found that excellent exhaust gas purification filter in cost by carrying obtain a noble metal particles of the optimum supported amount relative to, and have completed the present invention.

  That is, the exhaust gas purification filter of the present invention is an exhaust gas purification filter that traps particulate matter contained in exhaust gas by passing through a filter base made of a porous body and purifies the exhaust gas, wherein the filter base is A partition wall made of a porous body, an inflow side gas passage formed by the partition wall and having an inflow end portion of an exhaust gas containing particulate matter open, and a position different from the inflow side gas passage of the filter base And an outflow side gas passage formed by the partition wall and having an exhaust gas outflow side end portion opened, and containing oxidation catalyst particles on at least the surface of the partition wall on the inflow side gas channel side And a porous membrane having a pore size smaller than that of the partition wall is formed, and the oxidation catalyst particles have noble metal particles supported on the surface of the silicon carbide particles, and the noble metal particles are oxidized. The ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to the mass (Ms) of the silicon carbide particles supported in a state covered with a layer is in the range of 0.005 or more and 0.15 or less. It is characterized by.

The oxide layer is preferably made of amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3).
The oxide layer preferably contains amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3).
The oxide layer may further contain one or more crystalline materials selected from the group consisting of SiO ₂ , SiO, SiOC ₃ , SiO ₂ C ₂ and SiO ₃ C.
The average primary particle diameter of the silicon carbide particles is preferably 0.01 μm or more and 10 μm or less.
The average primary particle diameter of the noble metal particles is preferably 1 nm or more and 50 nm or less.

According to the exhaust gas purification filter of the present invention, the oxidation catalyst particles contained in the porous membrane on the surface of the filter base are supported by the noble metal particles on the surface of the silicon carbide particles, and the noble metal particles are covered with the oxide layer. Since the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to the mass (Ms) of the silicon carbide particles is in the range of 0.005 or more and 0.15 or less, PM collection by the porous film The combustion temperature of PM can be lowered by not only the improvement of the property and suppression of the increase in pressure loss but also the interaction at the interface between the noble metal particles and the oxide layer. Therefore, it is possible to achieve both improvement in filter characteristics and improvement in durability in the exhaust gas purification filter.
As described above, an exhaust gas purification filter having good characteristics can be provided without applying an excessive load to the engine and without deteriorating fuel consumption.

It is a partially broken perspective view which shows DPF which is an example of the exhaust gas purification filter of one Embodiment of this invention. It is sectional drawing which shows the partition structure of DPF which is an example of the exhaust gas purification filter of one Embodiment of this invention. It is sectional drawing which shows the oxidation catalyst particle | grains contained in the porous membrane of the exhaust gas purification filter of one Embodiment of this invention. It is a figure which shows the relationship between each ratio (Ms / Mn) of Examples 1-6 of this invention, and Comparative Examples 1-3, CO purification temperature, and HC purification temperature.

The form for implementing the exhaust gas purification filter of this invention is demonstrated.
This embodiment is specifically described for better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[Exhaust gas purification filter]
An exhaust gas purification filter according to an embodiment of the present invention will be described. Here, a description will be given by taking as an example a DPF having a plugged ceramic honeycomb structure, which is an exhaust gas purification filter used in an automobile diesel engine.

FIG. 1 is a partially broken perspective view showing a DPF as an example of an exhaust gas purifying filter according to an embodiment of the present invention, and FIG. 2 is a cross-sectional view showing a partition structure of the DPF in a plane indicated by β in FIG. It is.
The DPF 1 allows particulate matter (PM) contained in the exhaust gas G to pass through by passing the exhaust gas G through a filter base 2 made of a cylindrical porous ceramic (porous body) having a large number of pores (pores). An exhaust gas purification filter that collects and purifies the exhaust gas G. Note that the DPF does not have to be cylindrical, and may have a shape such as a prismatic shape or an elliptical column shape.

  In the filter base 2, a gas flow path 4 is formed by a partition wall 3 made of a porous body, and the gas flow path 4 has an upstream end and a downstream when viewed from the flow direction (longitudinal direction) of the exhaust gas G. A structure in which side end portions are alternately closed, that is, an inflow cell (inflow side gas flow path) 4A in which an inflow side end portion of exhaust gas G containing particulate matter on the inflow side of exhaust gas G is opened, and this An outflow cell (outflow side gas flow path) 4B formed at a position different from the inflow cell 4A and having an outflow side end portion of the exhaust gas G opened, and an upstream end portion of the exhaust gas G in the gas flow path 4 ( A porous membrane 5 containing oxidation catalyst particles and having a pore diameter smaller than that of the filter base 2 is formed on the inner wall surface 4a of the inflow cell 4A whose inflow side end portion is opened.

The flow of exhaust gas in the DPF 1 is shown in FIG.
The exhaust gas G containing PM20 that has flowed in from the inflow surface side, that is, the end surface α side, flows into the DPF 1 from the inflow cell 4A that opens to the inflow surface, and flows in the inflow cell 4A from the end surface α side to the end surface γ side. In the process, it passes through the partition wall 3 of the filter base 2. At this time, the PM 20 contained in the exhaust gas G is collected and removed by the porous film 5 provided on the inner wall surface 4a of the inflow cell 4A (the surface of the partition wall 3 constituting the inflow cell 4A). The purified gas C from which 20 has been removed flows from the end surface α side to the end surface γ side in the outflow cell 4B, and is discharged out of the filter from the open end (end surface γ) of the outflow cell 4B.

The filter substrate 2 is a honeycomb structure made of heat-resistant porous ceramics such as silicon carbide, cordierite, aluminum titanate, silicon nitride. The filter base 2 is formed with a partition wall 3 extending along the axial direction, which is the flow direction of the exhaust gas G, and a hollow area in the axial direction surrounded by the partition wall 3 is formed from a large number of inflow cells 4A and outflow cells 4B. A cellular gas flow path 4 is formed.
Here, the “honeycomb structure” in the present embodiment means that the plurality of inflow cells 4A and the outflow cells 4B are parallel to the filter base 2, and one of the inflow cells 4A and the outflow cells 4B surrounds the other. In addition, one opening end and the other opening end are formed in opposite directions.

  The cross-sectional shape of the inflow cell 4A and the outflow cell 4B, that is, the direction orthogonal to the axial direction of the gas flow path 4 is a quadrangular shape here, but is not limited to a quadrangular shape, but a polygonal shape such as a hexagonal shape, a circular shape, an elliptical shape. Various cross-sectional shapes such as a shape can be used. In addition, the gas flow path 4 formed near the outer periphery of the filter base 2 has an arc shape in a part of the cross-sectional shape. However, the gas flow path 4 is arranged without a gap to the vicinity of the outer periphery of the filter base 2. Therefore, the gas flow path 4 having a cross-sectional shape that follows the outer shape of the filter base 2 is used.

  The average pore size of the partition walls 3 made of porous ceramics is preferably 5 μm or more and 50 μm. When the average pore diameter is less than 5 μm, the pressure loss due to the partition walls 3 itself is increased, which is not preferable. On the other hand, if the average pore diameter exceeds 50 μm, the strength of the partition walls 3 may not be sufficient, and it becomes difficult to form the porous film 5 on the partition walls 3.

  The porous film 5 is a porous film formed on the inner wall surface 4a of the inflow cell 4A, and does not enter the pores of the porous ceramics constituting the partition wall 3 of the filter base 2 so much. Is formed as an independent film on the inner wall surface 4a. That is, the porous membrane 5 is formed on the inner wall surface 4a of the inflow cell 4A in a state where it only penetrates to the entrance of the pores formed in the partition wall 3. And the porous membrane 5 has many pores, these pores communicate, and as a result, it becomes a filter-like porous which has a through-hole.

The porosity of the porous membrane 5 is preferably 50% or more and 90% or less, more preferably 60% or more and 85% or less.
If the porosity is less than 50%, the average porosity of the porous membrane 5 is equal to or lower than the porosity of the partition walls 3, which may cause pressure loss. Further, when the porosity exceeds 90%, the mechanical strength of the porous membrane 5 is lowered, so that the porous membrane 5 itself is deformed or peeled off from the inner wall surface 4a, and the oxidation catalytic action may be lowered. There is.

The thickness (film thickness) of the porous film 5 is 60 μm or less at a location where the inner wall surface 4a overlaps with the pores of the partition walls 3 on the inner wall surface 4a, and the inner wall surface 4a is flat with the solid portion of the partition walls 3. In the overlapping place, it is preferably 5 μm or more and 60 μm or less.
Here, the “hole portion” refers to an opening portion in which pores in the partition wall 3 are opened on the inner wall surface 4 a, and is a portion where there is no base material for holding the membrane under the porous membrane 13. The “solid portion” indicates a portion excluding the void portion, that is, a portion where the ceramic material constituting the partition wall 3 is present on the inner wall surface 4a.

  If the film thickness of the porous film 5 is within the above range, the air flow on the surface of the porous film 5 becomes almost uniform regardless of the state of the partition walls 3, so that the pressure loss is reduced and the PM 20 is porous. Since it is uniformly collected on the film 5, good filter characteristics are maintained. In addition, even during the regeneration process of the filter, the combustion gas for burning the collected PM 20 can flow uniformly with respect to the accumulated PM 20, so that the combustion efficiency can be improved.

  On the other hand, when the thickness of the porous membrane 5 is as thin as less than 5 μm, the air flow rate is reduced at a location where the solid portion on the surface of the porous membrane 5 overlaps with the plane, and the airflow on the surface of the porous membrane 5 is not uniform. As a result, the filter characteristics deteriorate. On the other hand, when the thickness of the porous membrane 5 exceeds 60 μm, the pressure loss due to the porous membrane 5 becomes excessive, which is not preferable.

  In the exhaust gas purification filter (DPF1) of the present embodiment, it is preferable that the porous membrane 5 has both the trapping action of PM20 and the combustion catalytic action of the trapped PM20. This is because when the porous film 5 has both the PM trapping action and the PM combustion catalytic action, the trapped PM can be easily burned and removed on the spot, so that the catalytic action can be effectively expressed.

Here, in order for the porous film 5 to have a PM trapping action, it is necessary to make the average pore diameter of the porous film 5 smaller than the average pore diameter of the partition walls 3. Specifically, it is preferably 0.05 μm or more and 3 μm or less, and more preferably 0.07 μm or more and 2.5 μm or less.
Here, if the average pore diameter is less than 0.05 μm, the pressure loss when the exhaust gas G containing PM20 passes through the porous membrane 5 becomes large, which is not preferable. On the other hand, when the average pore diameter exceeds 3 μm, the pore diameter of the porous film 5 and the pore diameter of the partition wall 3 are substantially equal, and the PM trapping property in the porous film 5 is reduced. Since the collected PM 20 is not in contact with the porous membrane 5, the catalytic effect cannot be obtained during the regeneration process of the DPF 1, and the PM combustion efficiency cannot be improved.

  In order to set the average pore diameter of the porous film 5 within the above range, the average primary particle diameter range of the silicon carbide particles 21 forming the porous film 5 is preferably 0.01 μm or more and 10 μm. Preferably they are 0.02 micrometer or more and 7 micrometers or less, More preferably, they are 0.035 micrometer or more and 5 micrometers. Here, when the average primary particle diameter of the silicon carbide particles 21 is less than 0.01 μm, the pore diameter of the obtained porous film 13 is too small, which is not preferable. On the other hand, when the average primary particle diameter exceeds 10 μm, it is obtained. Since the pore diameter of the porous membrane 13 becomes too large, it is not preferable.

  In addition, it is preferable that the amorphous oxide layer 22 is formed on the surface of the silicon carbide particles 21 like the oxidation catalyst particles 11 described later. The reason is that since the oxide layer 22 on the surface of the silicon carbide particles 21 has the ability to adsorb and desorb oxygen, the surface oxide layer-forming silicon carbide particles have the ability to oxidize and decompose. This is because it itself has a PM combustion catalyst effect and an oxidation / decomposition action of CO, HC and NOx, and can produce an auxiliary action on the oxidation catalyst particles 11.

  As a method for obtaining such silicon carbide particles 21 having an average primary particle diameter of 0.01 μm or more and 10 μm or less, it has high temperature and high activity in a non-oxidizing atmosphere, and it is easy to introduce a high-speed cooling process. There is a thermal plasma method using a certain thermal plasma. This production method is useful as a method for producing silicon carbide nanoparticles having an average primary particle diameter of 0.01 μm or more and 10 μm or less and having excellent crystallinity. By selecting a high-purity raw material, the content of impurities It is possible to obtain silicon carbide nanoparticles with very little.

Moreover, the silica precursor baking method can also be mentioned. In this method, a mixture containing a silicon-containing substance such as an organosilicon compound, silicon sol, or silicic acid hydrogel, a carbon-containing substance such as a phenol resin, and a metal compound such as lithium that suppresses grain growth of silicon carbide is obtained. In this method, silicon carbide particles are obtained by firing in a non-oxidizing atmosphere.
If necessary, it can also be obtained by using the Atchison method, the silica reduction method, the silicon carbonization method or the like.
In addition, the silicon carbide particles 12 used for the oxidation catalyst particles 11 described later can also be obtained using the same method.

Further, in order to give the porous membrane 5 a PM combustion catalytic action, the porous membrane 5 contains the oxidation catalyst particles 11. FIG. 3 is a cross-sectional view showing the oxidation catalyst particles 11 included in the porous film 5. The oxidation catalyst particles 11 have noble metal particles 13 supported on the surfaces of the silicon carbide particles 12. It is supported in a state of being covered with the crystalline oxide layer 14, and the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles 13 to the mass (Ms) of the silicon carbide particles 12 is 0.005 or more and 0 .15 or less.
In FIG. 3, the number of the noble metal particles 13 supported on the surface of the silicon carbide particles 12 is one, but the number of the noble metal particles 13 is not limited, and a plurality of noble metal particles 13 are supported. Also good.

The silicon carbide particles 12 preferably have an average primary particle diameter of 0.01 μm or more and 5 μm or less, more preferably 0.02 μm or more and 3 μm or less, and further preferably 0.035 μm or more and 1 μm or less.
Here, if the average primary particle diameter of the silicon carbide particles 12 is less than 0.01 μm, the surface activity of the silicon carbide particles 12 becomes too high, and when the oxidation catalyst particles 11 are used at a high temperature, the oxidation catalyst Sintering (growth) between the particles 11 progresses and the particle diameter becomes coarse, which is not preferable because the catalytic activity is lowered. On the other hand, when the average primary particle diameter exceeds 5 μm, the specific surface area of the silicon carbide particles 12 decreases, so that when the noble metal particles are supported on the silicon carbide particles 12, the distance between the noble metal particles 13 is shortened. Further, sintering of the noble metal particles 13 progresses during use of the exhaust gas purification filter at a high temperature, which is not preferable because the catalytic activity decreases.

The noble metal particles 13 are selected from the group consisting of platinum (Pt), gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir). It is preferable to contain 1 type or 2 types or more.
Among these, platinum, gold, palladium, and iridium are particularly preferable because they have high durability in a high temperature region of 700 ° C. or higher and high catalytic function.

The average primary particle diameter of the noble metal particles 13 is preferably 1 nm or more and 50 nm or less, more preferably 1 nm or more and 30 nm or less, and further preferably 1 nm or more and 10 nm or less.
Here, the reason why the average primary particle size is limited to 1 nm or more and 50 nm or less is preferable because the average primary particle size is less than 1 nm because the particle size is too small and the surface activity becomes too strong, and the particles easily aggregate. Because there is no. On the other hand, when the average primary particle size exceeds 50 nm, the noble metal particles cannot be covered with the oxide layer on the surface of the silicon carbide particles and are exposed to protrude from the oxide layer, so that the catalyst characteristics may be deteriorated. In addition, the catalytic activity at low temperatures is lowered, which is not preferable.

The oxide layer 4 is preferably an amorphous silicon carbide oxide or silicon oxide represented by SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3). The composition and structure of the oxide layer 4 do not need to be single, and the values of x and y in each part of the oxide layer 4 can take arbitrary values within the above range.
The oxide layer 4 may contain one or more selected from the group of crystalline SiO ₂ , SiO, SiOC ₃ , SiO ₂ C ₂ , and SiO ₃ C. By containing such a crystalline material, stability at a high temperature of 1000 ° C. or more can be improved. However, since the oxide layer 4 is very small in quantity, it is difficult to confirm that the oxide layer 4 contains a crystalline substance.

Among these amorphous and crystalline silicon carbide oxides and silicon oxides, silicon carbide oxides (compounds containing both silicon, carbon, and oxygen) are particularly preferable because they are easily bonded to the noble metal particles 3. Examples of the silicon carbide oxide include amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3), SiOC ₃ , SiO ₂ C ₂ , and SiO ₃ C. It is not limited to these.

The oxide layer 4 has the above-described amorphous silicon carbide oxide or oxide other than silicon oxide, for example, yttrium oxide (Y ₂ O ₃ ), zirconium oxide (ZrO ₂ ), as long as the characteristics are not impaired. ), Cerium oxide (CeO ₂ ), neodymium oxide (Nd ₂ O ₃ ), titanium oxide (TiO ₂ ), and the like.

In this oxidation catalyst particle 11, the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles 13 to the mass (Ms) of the silicon carbide particles 12 is in the range of 0.005 or more and 0.15 or less. is necessary. A more preferable range of this ratio (Mn / Ms) is a range of 0.03 or more and 0.05 or less.
Here, the reason why the mass ratio is limited to the above range is that, in this range, the catalyst 1 exhibits catalytic characteristics according to the amount of the noble metal particles 13 supported, and therefore a high catalyst from the low temperature region to the high temperature region. This is because it is possible to obtain a catalyst 1 that has sufficient activity and high durability and can sufficiently obtain the effect of using expensive noble metal particles.

Here, when the mass ratio (Mn / Ms) is less than 0.005, the amorphous SiO _x C _y contained in the noble metal particles 13 and the oxide layer 14 (where 0 <x ≦ 3, 0 ≦ The number of interfaces where y ≦ 3) is small, and therefore the effect of interfacial interaction is small, and good catalytic properties cannot be obtained.
On the other hand, when the mass ratio (Mn / Ms) exceeds 0.15, the number of the noble metal particles 13 is limited because the number of the noble metal particles 13 is too large. The number of interfaces of amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3) contained in the oxide layer 14 reaches saturation. Furthermore, since there are noble metal particles 13 that are not covered with the oxide layer 14, these noble metal particles 13 are likely to be sintered together (grain growth), but the noble metal particles 13 that have undergone grain growth have a reduced specific surface area. As a result, the catalyst activity is reduced. Thus, even if the loading amount of the noble metal particles 13 is increased, the catalyst characteristics are not improved, resulting in an increase in cost, which is not preferable.

The oxidation catalyst particles 11 have high catalytic ability due to the interaction at the interface between the noble metal particles 13 and the amorphous oxide layer 14.
The oxidation catalyst particles 11 are suitable for purifying PM contained in exhaust gas from internal combustion engines such as gasoline engines and diesel engines, and can be used for automobiles such as gasoline cars and diesel cars, for example.

  Here, when the PM trapping action and the PM combustion catalytic action in the porous membrane 5 are compared, in the case of the PM trapping action, the constituent particles may be a simple substance of the silicon carbide particles 21, but preferably the oxide layer 22 on the surface. Are formed, and the preferred range of the average primary particle diameter is 0.01 μm or more and 10 μm or less. On the other hand, in the case of PM combustion catalytic action, the constituent particles are oxidation catalyst particles 11, and these particles are supported by noble metal fine particles 13 on the surface, and the noble metal fine particles 13 are silicon carbide particles 12 covered with an oxide layer 14. The preferred range of the average primary particle diameter is 0.01 μm or more and 5 μm or less. That is, there is a difference between the preferred configuration of both particles in whether or not noble metal fine particles are supported and the upper limit of the average primary particle diameter.

Therefore, when the porous film 5 has both the PM trapping action and the PM combustion catalytic action, the silicon carbide particles 21 (including the oxide layer 22) and the oxidation catalyst particles 11 have the intended characteristics. What is necessary is just to combine suitably suitably.
For example, if importance is attached to the PM trapping action, the oxidation catalyst particles 11 may be included in a part mainly including the silicon carbide particles 21 (including the oxide layer 22), and the PM combustion catalytic action may be emphasized. For example, the oxidation catalyst particles 11 may be the main component, or the entire porous film 5 may be constituted by the oxidation catalyst particles 11.
Further, when the pore diameter in the porous membrane 5 needs to be expanded and PM combustion catalytic action is required, the oxidation catalyst particles 11 having an average primary particle diameter of the silicon carbide particles 12 of 10 μm or less may be used. Good. This is because even if the average primary particle diameter of the silicon carbide particles 12 exceeds 5 μm, the catalytic characteristics of the oxidation catalyst particles 11 do not rapidly decrease.

In the exhaust gas purification filter (DPF 1) of the present embodiment, when the exhaust gas G flows into the porous film 5, PM20 contained in the exhaust gas at a stage where the exhaust gas G passes through the pores of the porous film 5. The purified gas C collected and deposited on the oxidation catalyst particles 11 contained in the catalyst and from which PM20 has been removed is discharged into the atmosphere.
Next, the PM 20 deposited on the porous film 5 is burned, for example, by raising the temperature of the DPF 1, and the carbon constituting the PM is oxidized and removed as CO 2. Here, since the collected PM 20 is in contact with or close to the oxidation catalyst particles 11 included in the porous film 5, it is burned and removed with high efficiency by the catalytic effect of the oxidation catalyst particles 11.

  In this embodiment, since the catalytic characteristics of the oxidation catalyst particles 11, that is, the oxidation catalyst activity is high, the combustion temperature of the PM 20 can be lowered. Here, if the combustion start temperature of PM20 can be made equal to or lower than the temperature of the exhaust gas that passes through DPF1, PM20 collected by porous membrane 5 is quickly removed by combustion (CO2 of carbon constituting PM). This DPF1 can be a continuous regeneration system that simultaneously collects and burns PM. In such a continuous regeneration type DPF 1, it is not necessary to increase the temperature for burning PM, so that the characteristic deterioration of the DPF 1 can be prevented and the durability can be improved.

The exhaust gas G also contains CO, HC, NOx, etc., but these substances also come into contact with the oxidation catalyst particles 11 in the process in which the exhaust gas G flows through the porous membrane 5. Therefore, by controlling the conditions under which PM is collected and deposited on the porous film 5, the oxidation catalyst particles 11 are used to convert CO to CO ₂ and HC to H while collecting and depositing PM. It can be oxidized and decomposed into ₂ O and CO ₂ and NOx into NO ₂ , respectively.
As described above, in addition to the PM treatment effect, the DPF can also have a treatment effect for harmful components such as CO, HC, NOx.

Furthermore, exhaust gas can be obtained by simultaneously oxidizing and decomposing CO, HC, NOx, etc. under the conditions of a continuous regeneration system in which PM is simultaneously collected and burned (oxidation of carbon constituting the PM to CO ₂ ). Detoxification treatment of harmful components and particulate matter contained therein can be performed as a continuous treatment.
Of CO ₂ , H ₂ O, and NO ₂ produced by combustion removal of PM and oxidation / decomposition of CO, HC, NOx, etc., CO ₂ and H ₂ O are directly discharged into the atmosphere, while NO 2 ₂ is reduced to harmless N ₂ and then discharged into the atmosphere.
As described above, the purified gas from which particulate matter (PM) contained in the exhaust gas and harmful components such as CO, HC, and NOx are decomposed and removed can be discharged into the atmosphere.

[Manufacturing method of exhaust gas purification filter]
The above exhaust gas purification filter manufacturing method includes a base material preparation step ([A] step) for preparing silicon carbide particles as a base material, and a noble metal particle support step ([[ B] step), and a porous membrane forming step ([C] step) in which the silicon carbide particles carrying the noble metal particles are carried on the partition walls made of a porous body constituting the filter base to form a porous membrane (step [C]). And an oxide layer forming step ([D] step) of forming an amorphous oxide layer so as to cover the noble metal particles on the surface of the silicon carbide particles carrying the noble metal particles.
In the exhaust gas purification filter of the present embodiment, there are cases where silicon carbide particles 21 are included as particles constituting the porous membrane 5 in addition to the oxidation catalyst particles 11, but the silicon carbide particles 21 have an active catalytic action. Since it does not have, description about the silicon carbide particle 21 is abbreviate | omitted.

Of course, the order of these [A] to [D] steps is that the base material preparation step ([A] step) must be first, and the oxide layer forming step ([D] step) is supported by noble metal particles. Any order can be selected as long as the condition that it is necessary to be performed on the silicon carbide particles thus formed (which is not possible with silicon carbide particles alone) is satisfied. Here, the reason that the oxide layer forming step ([D] step) needs to be performed on the silicon carbide particles on which the noble metal particles are supported is that the noble metal particles are formed after the oxide layer is formed on the silicon carbide particles. This is because sufficient oxidation catalyst characteristics cannot be obtained even when supported.
In addition, the noble metal particle supporting step ([B] step) and the porous film forming step ([C] step) are both dissolved or dispersed in a solvent (dispersion medium), dried and removed of the solvent (dispersion medium), and heat treatment. , So that both processes can be performed simultaneously in parallel.

  From the above, in the exhaust gas purification filter manufacturing method, the following four kinds of process orders can be selected. It should be noted that “→ (right arrow)” indicates that the next step is performed after a certain step, and “=” indicates that the two steps are performed simultaneously.

(1): [A] → [B] → [C] → [D]
A partition comprising a porous body that prepares silicon carbide particles as a base material, and then supports noble metal particles on the surface of the silicon carbide particles, and then includes the silicon carbide particles supported by the noble metal particles. To form a porous film, and then an amorphous oxide layer is formed so as to cover the noble metal particles on the surface of the silicon carbide particles on which the noble metal particles are supported.

(2): [A] → [B] → [D] → [C]
Preparing silicon carbide particles as a base material, then supporting noble metal particles on the surface of the silicon carbide particles, and then amorphous so as to cover the noble metal particles on the surface of the silicon carbide particles on which the noble metal particles are supported And then forming a porous film on the filter substrate using the surface-coated noble metal particle-supported silicon carbide particles.

(3): [A] → [C] → [B] → [D]
Preparing silicon carbide particles as a base material, and then forming a porous film on the filter substrate using the silicon carbide particles, and then supporting the noble metal particles on the porous film, and then supporting the noble metal particles In this method, an amorphous oxide layer is formed so as to cover the noble metal particles on the porous film formed.

(4): [A] → [B] = [C] → [D]
After preparing silicon carbide particles as a base material, and then simultaneously carrying the support of the noble metal particles on the surface of the silicon carbide particles and the formation of a porous film on the partition walls of the filter base by the silicon carbide particles, In this method, an oxide layer is formed on the surface of a porous film made of silicon carbide carrying precious metal particles.

Next, each step will be described in detail.
"Base material preparation process"
This is a step of preparing silicon carbide particles as a base material.
What is necessary is just to select the average primary particle diameter of a silicon carbide particle in the range of 0.01 micrometer or more and 10 micrometers or less according to the characteristic of the waste gas purification filter requested | required. The silicon carbide particles are sub-micron to micron (micrometer) size silicon carbide particles such as the thermal plasma method or the silica precursor firing method if the particles are nanometer-sized particles. It can be obtained using a method, a silica reduction method, a silicon carbonization method or the like.

"Precious metal particle loading process"
Noble metal particles are supported on the surface of silicon carbide particles as a base material, or on the surface of silicon carbide particles in a porous film formed of silicon carbide particles, and porous composed of noble metal-supported silicon carbide particles or noble metal-supported silicon carbide particles. This is a process for forming a membrane.

First, a solution in which a noble metal source is dissolved in a solvent or a dispersion in which noble metal fine particles are dispersed in a dispersion medium is prepared. The solvent or dispersion medium is preferably water, but an organic solvent may be used when the noble metal source is decomposed and precipitated with water.
As a noble metal source used in this step, platinum (Pt), gold (Au), silver (Ag), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) Examples thereof include salts and compounds containing one or more kinds of noble metal elements selected from the group, such as chlorides, sulfates, nitrates, organic acid salts, complexes (complex salts), and hydroxides.
Examples of the noble metal fine particles include fine particles mainly composed of the noble metal element described above.
Moreover, as an organic solvent, a polar solvent is preferable and alcohol, ketones, etc. are used suitably.

  Next, the silicon carbide particles are immersed / dispersed in this solution or dispersion, or a porous film formed of silicon carbide particles is immersed, and then dried at a temperature of about 60 ° C. to 250 ° C. Remove the medium. Thereby, the noble metal source and the noble metal fine particles can be attached to the surface of the silicon carbide particles (including those forming the porous layer).

Next, the silicon carbide particles or porous film to which the noble metal source and the noble metal fine particles are attached are heat-treated in a reducing atmosphere containing hydrogen, carbon monoxide, or the like, or in an inert atmosphere such as nitrogen, argon, neon, or xenon.
The temperature and time during this heat treatment may be appropriately selected depending on the type of noble metal source and the noble metal fine particles, the atmospheric conditions, and the like. Usually, the heat treatment temperature ranges from 500 ° C. to 1500 ° C., the heat treatment time ranges from 10 minutes to The range is 24 hours or less. Further, the heat treatment may be performed in one stage, or may be performed in a plurality of stages with different temperatures and atmospheres.
In addition, since the temperature and time more than necessary may induce sintering of the silicon carbide particles and sintering (grain growth) of the generated noble metal particles, it is not preferable.

In this step, the noble metal source is reduced and decomposed to become noble metal, and the produced noble metal fine particles are fixed to the silicon carbide particle surface or the silicon carbide particle surface forming the porous layer. On the other hand, noble metal fine particles, even when heat-treated in a reducing atmosphere or inert atmosphere, do not change in composition, and form the silicon carbide particle surface or porous layer while maintaining the original fine particle state. It adheres to the surface of silicon carbide particles.
Through the above steps, a porous film made of silicon carbide particles having noble metal particles supported on the surface or silicon carbide particles having noble metal particles supported on the surface can be obtained.

"Porous membrane formation process"
This porous film forming step includes either silicon carbide particles as a base material, silicon carbide particles supporting noble metal particles, or silicon carbide particles supporting noble metal particles and having an oxide layer formed on the surface. A step of producing a silicon carbide particle dispersion by dispersing in a dispersion medium, and applying the resulting silicon carbide particle dispersion to a partition wall made of a porous body constituting a filter substrate, followed by drying and applying the resulting coating A step of partially sintering silicon carbide particles to form a porous film by heat-treating the film.

First, in the dispersion medium, any one of the above-described silicon carbide particles serving as a base material, silicon carbide particles supporting noble metal particles, and silicon carbide particles supporting noble metal particles and having an oxide layer formed on the surface thereof is used. To prepare a silicon carbide particle dispersion.
Any dispersion medium may be used as long as it can uniformly disperse the silicon carbide particles, and water or an organic solvent is preferably used. Moreover, you may use the polymer monomer, the single-piece | unit of an oligomer, or these mixtures as needed.

  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. , Diethyl ether, ethylene glycol monomethyl ether (methyl cellosolve), ethylene glycol monoethyl ether (ethyl cellosolve), ethylene glycol monobutyl ether (butyl cellosolve), ethers such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, dioxane, tetrahydrofuran, acetone , Ketones such as methyl ethyl ketone, acetylacetone, acetoacetate, N, N-dimethylform Acid amides such as amide, toluene, aromatic hydrocarbons such as xylene are preferably used, it is possible to use a mixture of one only or two or more of these solvents.

  Examples of the polymer monomer include acrylic or methacrylic monomers such as methyl acrylate and methyl methacrylate, and epoxy monomers. Moreover, as said oligomer, a urethane acrylate oligomer, an epoxy acrylate oligomer, an acrylate oligomer, etc. can be used.

  In order to ensure dispersion stability or improve coating properties, this dispersion liquid may be used as a dispersant (surface treatment agent), surfactant, preservative, stabilizer, antifoaming agent, leveling agent, etc. May be added as appropriate. There is no restriction | limiting in particular in addition amount, such as these dispersing agents, surfactant, antiseptic | preservative, a stabilizer, an antifoamer, and a leveling agent, What is necessary is just to add according to the objective to add.

As an apparatus applicable to the above dispersion step, there are a kneader, a roll mill, a pin mill, a sand mill, a ball mill, a planetary ball mill, etc., but in order to disperse silicon carbide particles in a dispersion medium using a dispersion medium (media). A sand mill, a ball mill, a planetary ball mill and the like are suitable.
In addition, as a dispersion medium (media), a resin-coated metal ball in which a core material made of metals such as steel and lead is coated with a resin, an inorganic oxide sintered body such as alumina, zirconia, silica, and titania, silicon nitride Nitride sintered bodies such as silicon carbide sintered bodies such as silicon carbide, glass balls such as soda glass, lead glass, high specific gravity glass, etc. are mentioned, but the dispersion medium used in this embodiment is carbonized. From the viewpoint of mixing and dispersion efficiency when silicon particles are dispersed in a dispersion, a metal ball or sintered body having a specific gravity of 6 or more, for example, a resin-coated metal ball or a resin-coated inorganic oxide having zirconia or steel as a core material Balls and the like are preferred.

Next, the above-mentioned silicon carbide particle dispersion is applied to the surface of the partition wall made of a porous material and dried to form a coating film on the surface of the partition wall.
Alternatively, a method of forming a coating film by immersing a partition wall made of a porous material in a silicon carbide particle dispersion and pulling it up and then drying it may be used.

As a coating method of the silicon carbide particle dispersion liquid, a normal wet coat method for applying the dispersion liquid to the surface of the object to be processed, such as a bar coat method, a slip cast method, a wash coat method, a dip coat method, or the like can be used. .
At this time, by controlling the viscosity of the dispersion, the wettability with respect to the partition walls, the agglomeration state of the silicon carbide particles, and the like so that the dispersion liquid does not enter the pores of the partition walls, the surface of the partition walls is carbonized. An independent porous film made of silicon particles can be formed. In addition, if the dispersion liquid enters the pores of the partition walls, the pore diameter and porosity in the partition walls are decreased, and problems such as an increase in pressure loss occur.
Drying is sufficient if water or an organic solvent contained in the silicon carbide particle dispersion can be removed, and the drying is usually performed at a temperature of about 100 ° C. to 250 ° C. in an air atmosphere.

Next, the partition wall made of the porous body on which the coating film is formed is subjected to an inert atmosphere such as nitrogen, argon, neon, or xenon, or a reducing atmosphere containing a reducing gas such as hydrogen or carbon monoxide. The heat treatment is performed at a temperature of 500 ° C. or higher and 1500 ° C. or lower, preferably 600 ° C. or higher and 1100 ° C. or lower.
By this heat treatment, the dispersion medium in the coating film, that is, the remaining water or organic solvent, or the polymer monomer, oligomer, dispersant, surfactant, preservative, stabilizer, antifoam in the dried coating film Agents, leveling agents and the like are dissipated, and silicon carbide particles are partially sintered to form a neck portion in which these particles are bonded to each other, thereby forming a porous film bonded in a porous shape.

In addition, you may integrate this drying process with this heat processing process by making it the pre-processing process of a heat processing process.
This heat treatment step is omitted if the oxide layer is formed on the surface of the silicon carbide particles and the silicon carbide particles can be partially sintered by adjusting the heat treatment conditions in the oxide layer forming step. Can do.

(When performing the precious metal particle supporting process and the porous film forming process simultaneously)
Since the above precious metal particle supporting step and the porous film forming step have similar work contents, both steps can be performed simultaneously.

First, a step of preparing a solution in which a salt of a noble metal serving as a noble metal source in the noble metal particle supporting step is dissolved or a dispersion in which noble metal compound fine particles are dispersed, and a step of preparing a dispersion medium in the porous film forming step Similar. Further, the step of immersing and dispersing silicon carbide particles in the solution or dispersion in the noble metal particle supporting step is similar to the step of producing the silicon carbide particle dispersion in the porous film forming step.
Therefore, by dissolving or dispersing the noble metal source in the silicon carbide particle dispersion medium in the porous film forming step, these steps can be combined into a single step.
As this dispersion medium, it is necessary to select a substance in which the noble metal source can be easily dissolved or dispersed, and water is particularly preferable. On the other hand, when an organic solvent is used, a noble metal source that is easily dispersed in the organic solvent may be selected.

Next, in the porous film forming step, there is a step of coating (coating) the silicon carbide particle dispersion liquid on the surface of the partition wall made of a porous body constituting the filter base. In the noble metal particle supporting step, There is no corresponding process.
Next, a step of removing water or an organic solvent as a dispersion medium by drying in the noble metal particle supporting step, a step of drying the partition wall made of a porous body coated with the silicon carbide particle dispersion in the porous film forming step, Are similar.
Since these two steps are drying steps and are substantially the same, they can be a single step. The drying temperature may be a range common to both steps, that is, a temperature of about 100 ° C. to 250 ° C.

Next, a step of forming noble metal fine particles by heat treatment in a reducing atmosphere or an inert atmosphere in the noble metal particle supporting step, and a partition made of a porous body on which a dry coating film in the porous film forming step is formed This is similar to the step of forming a porous film by heat-treating in a reducing atmosphere or in an inert atmosphere and partially sintering.
Both of these steps are substantially the same in that heat treatment is performed in a reducing atmosphere or an inert atmosphere, and thus can be a single step. The heat treatment temperature and time may be selected from the range common to both steps in consideration of the noble metal particle generation conditions and the porous film formation conditions.

  The above steps, that is, the step of dissolving or dispersing the noble metal source in the dispersion medium, the step of dispersing the silicon carbide particles in the dispersion medium, and the partition wall comprising the porous body constituting the filter substrate using the obtained silicon carbide particle dispersion liquid Noble metal particle supporting step is incorporated into the porous film forming step by performing the step of coating on the surface of the substrate, the step of drying the obtained coating film, and the step of heat treating the obtained dry coating film. The process and the porous film forming process can be performed as a simultaneous and parallel process.

"Oxide layer formation process"
This is a step of forming an oxide layer on the surface of the noble metal particle-supported silicon carbide particles by oxidizing the silicon carbide particles supporting the noble metal particles or the porous film formed of the noble metal particle-supported silicon carbide particles.

  As the oxidation treatment, the above-mentioned noble metal-supported silicon carbide particles themselves or a porous film formed of noble metal particle-supported silicon carbide particles, that is, a porous body having a layer in which the noble metal-supported silicon carbide particles are partially sintered are used. The partition wall is formed at a temperature of 600 ° C. or higher and 1000 ° C. or lower, preferably 650 ° C. or higher and 800 ° C. or lower, in an oxidizing atmosphere such as air or oxygen, for 0.5 hours or longer and 36 hours or shorter, preferably 4 hours. The oxidation treatment is performed for 12 hours or less to form an oxide layer on the surface of the silicon carbide particles.

As a result, the oxidation catalyst particles in which the noble metal particles supported on the surface of the silicon carbide particles are covered with the oxide layer, or the noble metal particles are supported on the surface of the silicon carbide particles, and the noble metal particles are oxidized. A porous film made of silicon carbide particles covered with a physical layer can be formed.
That is, the oxide layer is formed under these conditions, and if the average primary particle diameter of the noble metal particles is 50 nm or less, the noble metal particles are completely covered without protruding from the oxide layer and have high catalytic activity. Catalyst particles can be formed.

In the porous film formed in this manner, noble metal particles are present as fine particles with high dispersion (large specific surface area), and the noble metal particles can exhibit the catalytic activity peculiar to the fine particles.
In general, fine noble metal particles have a high surface activity, so that grain growth usually proceeds from a lower temperature. By the way, the noble metal particles in the present embodiment are supported on the surface of the silicon carbide particles in a fine state and are supported in a state covered with an oxide layer. Therefore, the oxide layer can be bonded to the noble metal particles, and in particular, amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3), sintering of noble metal particles can be suppressed.

In addition, although it is known that the melting point of fine noble metal particles is lowered, the noble metal particles in this embodiment are oxide layers, particularly amorphous SiO _x C _y (where 0 <x ≦ 3 , 0 ≦ y ≦ 3), the melting of itself can be suppressed.
Further, since oxygen is dissociated and adsorbed on the surface of the oxide layer containing amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3), the supported noble metal particles and amorphous Oxidation catalytic activity can be enhanced by the interaction at the interface with the high quality SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3). Due to this effect, high catalytic activity can be expressed and maintained even in a low temperature region.

Furthermore, silicon carbide particles are stable even at high temperatures, and grain growth is less likely to occur compared to oxide particles. Thereby, the surface area of amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3) is easily maintained, and the oxygen desorption amount does not decrease.
As a result, the exhaust gas purification filter of the present embodiment can easily maintain its catalytic activity even after being used at a high temperature for a long time.
As described above, the exhaust gas purification filter of the present embodiment has a high catalytic effect for PM combustion and can maintain the combustion catalytic effect for a long period of time.

  As described above, according to the exhaust gas purification filter 1 of the present embodiment, the oxidation catalyst particles 11 included in the porous film 5 on the surface of the filter base 2 are supported, and the noble metal particles 13 are supported on the surfaces of the silicon carbide particles 12. The noble metal particles 13 are supported in a state covered with the oxide layer 14, and the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles 13 to the mass (Ms) of the silicon carbide particles 12 is 0.005 or more and Since it was made into the range of 0.15 or less, the combustion temperature of PM can be lowered by the interaction at the interface between the noble metal particles 13 and the oxide layer 14. Therefore, durability can be improved.

In addition, the oxide layer 14 made of amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3) is stable even at a high temperature of 1000 ° C. or higher. High heat resistance can be maintained while maintaining high catalytic activity over a region.
As described above, an exhaust gas purification filter having good characteristics can be provided without applying an excessive load to the engine and without deteriorating fuel consumption.

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

[Example 1]
15 g of silicon carbide particles having an average primary particle size of 0.035 μm are put into a dispersion medium in which ammonium polyacrylate and an antifoaming agent are dissolved in 80 g of pure water as a surfactant. In this state, zirconia balls are used as the dispersion medium. The used dispersion treatment was performed for 180 minutes.

To the slurry thus obtained, a dinitrodiammine platinum nitric acid solution is added so that platinum is 0.066 g with respect to 1 g of silicon carbide particles, and dispersion treatment using zirconia balls as a dispersion medium is again performed for 30 minutes. And then evaporated to dryness to remove the dispersion medium. Thereafter, the following first to third heat treatments were sequentially performed.
First stage; temperature = 650 ° C., holding time = 240 minutes, atmosphere = N ₂
Second stage; temperature = 1000 ° C., holding time = 360 minutes, atmosphere = Ar
Third stage: Temperature = 700 ° C., holding time = 1800 minutes, atmosphere = atmosphere The oxidation catalyst particles of Example 1 were produced as described above.

[Example 2]
The oxidation catalyst particles of Example 2 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.100 g with respect to 1 g of silicon carbide particles.

[Example 3]
The oxidation catalyst particles of Example 3 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.150 g with respect to 1 g of silicon carbide particles.

[Example 4]
Oxidation catalyst particles of Example 4 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.050 g with respect to 1 g of silicon carbide particles.

[Example 5]
Oxidation catalyst particles of Example 5 were produced in the same manner as in Example 1 except that a dinitrodiammine platinum nitric acid solution was added so that platinum was 0.030 g with respect to 1 g of silicon carbide particles.

[Example 6]
Oxidation catalyst particles of Example 6 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.010 g with respect to 1 g of silicon carbide particles.

[Example 7]
Instead of silicon carbide particles having an average primary particle diameter of 0.035 μm, silicon carbide particles having an average primary particle diameter of 0.015 μm are used, and further, dinitro so that platinum is 0.066 g with respect to 1 g of silicon carbide particles. Instead of adding the diammine platinum nitric acid solution, the oxidation catalyst particles of Example 7 were prepared in the same manner as in Example 1 except that the silver nitrate solution was added so that the amount of silver was 0.066 g per 1 g of silicon carbide particles. Produced.

[Example 8]
Instead of silicon carbide particles having an average primary particle size of 0.035 μm, silicon carbide particles having an average primary particle size of 8 μm are used, and dinitrodiammine platinum nitrate solution is used so that platinum is 0.066 g with respect to 1 g of silicon carbide particles. In the same manner as in Example 1 except that the palladium nitrate solution was added so that palladium (Pd) was 0.066 g per 1 g of silicon carbide particles, the oxidation catalyst particles of Example 8 were added. Produced.

[Example 9]
13.5 g of silicon carbide particles having an average primary particle size of 0.8 μm and 0.15 g of silicon carbide particles having an average primary particle size of 0.03 μm, and 80 g of pure water containing ammonium polyacrylate and an antifoaming agent as a surfactant. In this state, a dispersion treatment using zirconia balls as a dispersion medium was performed for 180 minutes.
The slurry thus obtained was treated in the same manner as in Example 1 to produce oxidation catalyst particles of Example 9.

[Example 10]
An oxidation catalyst particle of Example 10 was produced in the same manner as in Example 1 except that silicon carbide particles having an average primary particle diameter of 12 μm were used instead of silicon carbide particles having an average primary particle diameter of 0.035 μm. .

[Example 11]
Instead of sequentially performing the first-stage to third-stage heat treatments, oxidation catalyst particles of Example 11 were produced in the same manner as in Example 1 except that the following first-stage heat treatment was performed. .
First stage: temperature = 1500 ° C., holding time = 1800 minutes, atmosphere = air

[Comparative Example 1]
Oxidation catalyst particles of Comparative Example 1 were prepared in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.2 g per 1 g of silicon carbide particles.

[Comparative Example 2]
Oxidation catalyst particles of Comparative Example 2 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was added so that platinum was 0.001 g with respect to 1 g of silicon carbide particles.

[Comparative Example 3]
The oxidation catalyst particles of Comparative Example 3 were produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution was not added and platinum was not supported.

[Evaluation]
For each of the oxidation catalyst particles of Examples 1 to 11 and Comparative Examples 1 to 3, the composition of the oxide layer, the supported state of the noble metal particles, the average primary particle diameter, and the carbon combustion test were evaluated.
Further, the carbon combustion test was evaluated for the evaluation sample of Comparative Example 4.

These evaluation methods are as follows.
(1) Composition of oxide layer The composition analysis of the surface of the silicon carbide (SiC) particles in the oxidation catalyst particles was performed using an X-ray photoelectron analyzer (XPS / ESCA) Sigma Probe (manufactured by VG-Scientific). .
(2) Supported state of noble metal particles and average primary particle diameter The observed state of noble metal particles was evaluated by observation with a field emission transmission electron microscope (FE-TEM) JEM-2100F (manufactured by JEOL Ltd.).
In addition, the average primary particle diameter of the noble metal particles was determined by measuring the primary particle diameters of 500 noble metal particles randomly selected from the field emission transmission electron microscope (FE-TEM) images obtained in the same manner. The average value was defined as the average primary particle diameter of the noble metal particles.

(3) Carbon combustion test An oxidation test of carbon black was performed to evaluate the combustion characteristics of the oxidation catalyst particles. The combustion characteristic evaluation by the oxidation test of this carbon black was performed as follows.
First, oxidation catalyst particles and carbon black (trade name: Acetylene carbonblack) were mixed at a ratio of particles: carbon black = 5: 1 (mass ratio), and then mixed thoroughly in a mortar to obtain a sample for evaluation. . Next, using a temperature programmed desorption spectrum apparatus (TPD apparatus), the sample is heated in the atmosphere at a temperature rising rate of 10 ° C./min, and the concentration of each of the generated CO and CO ₂ is quadruplicated. Measured by polar mass spectrometry (Q-MASS).
The temperature (peak temperature) at which the generated concentrations of CO and CO ₂ were maximized was measured to obtain the carbon combustion temperature, and the combustion characteristics of the oxidation catalyst particles or the evaluation sample were evaluated based on the carbon combustion temperature.
Table 1 shows the evaluation results of Examples 1 to 11 and Comparative Examples 1 to 4.

According to Table 1, in the oxidation catalyst particles of Examples 1 to 9, nanometer-sized noble metal particles are supported on the surface of the silicon carbide particles, and an oxide layer is formed on the surface of the silicon carbide particles. The oxide layer covered the noble metal particles.
Moreover, in the oxidation catalyst particles of Examples 1 to 5 and 7 to 9, the carbon combustion temperature is lower by 100 ° C. or more than the sample for evaluation of Comparative Example 5, and the oxidation catalyst particles of Example 6 are also compared. Compared with the sample for evaluation of Example 5, the carbon combustion temperature was nearly 90 ° C., and thus it was confirmed that a sufficient catalytic effect for PM combustion was obtained.

Next, in the oxidation catalyst particles of Examples 10 and 11, noble metal particles of about 60 nm are supported on the surface of the silicon carbide particles, and an oxide layer is formed on the surface of the silicon carbide particles. It covered at least some of the noble metal particles.
In addition, in the oxidation catalyst particles of Examples 10 and 11, the carbon combustion temperature is about 90 ° C. lower than that of the evaluation sample of Comparative Example 5, and thus has a catalytic effect on PM combustion. It has confirmed that it was low compared with 1-5 and 7-9.

  Thus, the catalytic effect of the oxidation catalyst particles of Examples 10 and 11 is lower than that of Examples 1 to 5 and 7 to 9. The amount of noble metal added is 6.6% ((Mn / Ms) is 0.00. 066) is sufficient, but the particle diameter of the noble metal particles is large, and at least a part of the noble metal particles is not covered with the oxide film on the surface of the silicon carbide particles, so that it is considered that sufficient catalytic activity is not exhibited. The reason is that in Example 10, the average primary particle diameter of the silicon carbide particles is as large as 12 μm, so that the specific surface area of the silicon carbide particles is low and the sintering (grain growth) between the noble metal particles proceeds. In addition, in Example 11, since the heat treatment temperature is high, the holding time is long, and the treatment is performed in the air atmosphere, it is considered that the sintering (grain growth) of the noble metal particles also proceeds.

On the other hand, the oxidation catalyst particle of Comparative Example 2 has too little noble metal particle amount, and the oxidation catalyst particle of Comparative Example 3 has no noble metal particle amount, so the carbon combustion temperature is as high as 750 ° C. or higher. It was confirmed that the catalytic effect on combustion was low. In Comparative Example 3 where no precious metal fine particles are present, the carbon combustion temperature is lower than that in the evaluation sample of Comparative Example 5, which is considered to be a catalytic effect of the oxide layer on the surface of the silicon carbide particles. .
The oxidation catalyst particle of Comparative Example 1 is an example in which the noble metal addition amount is 20% by mass, and the carbon combustion temperature is sufficiently low as 708 ° C., but the value is the same as in Example 3 in which the noble metal addition amount is 15%. It was almost the same. This is presumably because the catalytic properties cannot be improved compared to Example 3 because the amount of precious metal has reached saturation.

  Next, among these exhaust gas purification catalysts of Examples 1 to 11 and Comparative Examples 1 to 4, Examples 1 to 6 in which the average primary particle diameter of silicon carbide particles is 0.035 μm and platinum is used as noble metal particles. FIG. 5 shows the relationship between the ratio (Ms / Mn) of the mass (Mn) of the noble metal particles to the mass (Ms) of the silicon carbide particles and the CO purification temperature and the HC purification temperature in each exhaust gas purification catalyst of Comparative Examples 1 to 3. 4 shows.

  FIG. 4 shows that the presence of the noble metal particles reduces the carbon combustion temperature as compared to the case of the silicon carbide particles alone, and (Ms / Mn) is about 0.005, It was found that the effect of the presence of the noble metal particles appeared when the addition amount of the noble metal particles was about 0.5% by mass relative to the silicon carbide particles. On the other hand, as the addition amount of the noble metal particles increases, the rate of decrease in the carbon combustion temperature decreases, and (Ms / Mn) is 0.15 or more, that is, the addition amount of the noble metal particles is 15 mass% or more with respect to the silicon carbide particles. Even when present, the carbon combustion temperature hardly decreased, and the amount of noble metal reached saturation.

[Example 12]
Each of the oxidation catalyst particles of Example 1 was weighed so that 10.0% by volume, 87.5% by volume of water, and 2.5% by volume of gelatin used as a gelling agent were included. Next, the above oxidation catalyst particles and water were mixed for 12 hours at a rotational speed of 220 rpm in a ball mill using iron-cored resin balls to form a dispersion, and then the gelatin was added to the obtained dispersion. The mixture was added and mixed for 20 minutes to obtain an oxidation catalyst particle dispersion (coating solution) of Example 12.

Next, the silicon carbide honeycomb structure base material was dipped in the oxidation catalyst particle dispersion and then pulled up and dried at 100 ° C. for 12 hours to form a coating film made of oxidation catalyst particles on the honeycomb base material.
Next, the honeycomb substrate on which the coating film was formed was heat-treated under the following conditions.
Fourth stage: temperature = 1000 ° C., holding time = 60 minutes, atmosphere = argon The filter substrate of Example 12 in which the porous film composed of the oxidation catalyst particles of Example 1 was formed on the honeycomb substrate by this heat treatment. Was made.

[Example 13]
A filter substrate of Example 13 was produced in the same manner as Example 12 except that the oxidation catalyst particles of Example 9 were used as the oxidation catalyst particles.
It was.

[Evaluation]
For the filter substrates of Examples 12 and 13, evaluation of the composition of the oxide layer on the surface of the silicon carbide particles forming the porous film, the supported state and average primary particle diameter of the noble metal particles, the pore diameter and the average porosity of the porous film Went.

These evaluation methods are as follows.
(1) Composition of oxide layer The porous film in the filter substrate is cut out for each honeycomb substrate, and carbonized in the oxidation catalyst particles using an X-ray photoelectron analyzer (XPS / ESCA) Sigma Probe (manufactured by VG-Scientific). The composition of the surface of silicon (SiC) particles was analyzed.
(2) Precious metal particle support state and average primary particle diameter A porous membrane in a filter substrate is cut out for each honeycomb substrate and observed with a field emission transmission electron microscope (FE-TEM) JEM-2100F (manufactured by JEOL Ltd.). Thus, the supported state of the noble metal particles was evaluated.
In addition, the average primary particle diameter of the noble metal particles was measured by measuring the primary particle diameter of 100 noble metal particles randomly selected from the field emission transmission electron microscope (FE-TEM) image obtained in the same manner. The average value was defined as the average primary particle diameter of the noble metal particles.

(3) Pore size and average porosity of porous membrane Using a mercury porosimeter device, Pore Master 60GT (manufactured by Quantachrome), the pore size distribution of the porous membrane portion in the filter substrate was measured, and 50% cumulative mercury intrusion volume was accumulated. Was the average pore size of the porous membrane. Moreover, the average porosity of the porous membrane was measured using the same apparatus.
The evaluation results of Examples 12 and 13 are shown in Table 2.

According to Table 2, in the filter substrates of Examples 12 and 13, a porous film made of silicon carbide particles is formed on a honeycomb substrate, and nanometer-sized noble metal particles are formed on the surface of the silicon carbide particles. The oxide layer was supported on the surface of the silicon carbide particles, and the oxide layer covered the noble metal particles.
Moreover, it was confirmed that the porous membranes of Examples 12 and 13 have a pore diameter and a porosity suitable for the porous membrane forming the exhaust gas purification filter.
In Examples 12 and 13, the combustion characteristics of the oxidation catalyst particles were not evaluated by the carbon black oxidation test. However, the oxidation catalyst particles of Examples 1 and 9 used as the raw material for the porous film had good carbon combustion characteristics. And the state of the silicon carbide particles forming the porous film is hardly changed relative to the raw material oxidation catalyst particles.

1 DPF
2 Filter base 3 Partition 4 Gas flow path 4A Inflow cell (inflow side gas flow path)
4B Outflow cell (outflow side gas flow path)
4a Inner wall surface 5 Porous film 11 Oxidation catalyst particle 12 Silicon carbide particle 13 Noble metal particle 14 Amorphous oxide layer 20 Particulate matter (PM)
α, γ End face G Exhaust gas C Purified gas

Claims (5)

An exhaust gas purification filter that purifies the exhaust gas by collecting particulate matter contained in the exhaust gas by passing through a filter base made of a porous body,
The filter base includes a partition wall made of a porous body, an inflow side gas passage formed by the partition and having an inflow side end portion of an exhaust gas containing particulate matter, and the inflow side gas of the filter base. An outflow side gas flow path provided at a position different from the flow path, formed by the partition wall and having an outflow side end of the exhaust gas opened,
A porous film containing oxidation catalyst particles and having a pore diameter smaller than that of the partition wall is formed on at least the surface of the partition wall on the inflow side gas flow path side,
The oxidation catalyst particles have noble metal particles supported on the surface of the silicon carbide particles, and the noble metal particles are supported in a state of being covered with an oxide layer,
A ratio (Mn / Ms) of a mass (Mn) of the noble metal particles to a mass (Ms) of the silicon carbide particles is in a range of 0.005 or more and 0.15 or less. 2. The exhaust gas purification filter according to claim 1, wherein the oxide layer contains amorphous SiO _x C _y (where 0 <x ≦ 3, 0 ≦ y ≦ 3). The oxide layer further contains one or more crystalline materials selected from the group consisting of SiO ₂ , SiO, SiOC ₃ , SiO ₂ C _2, and SiO ₃ C. Item 2. An exhaust gas purification filter according to Item 2.   The exhaust gas purification filter according to any one of claims 1 to 3, wherein an average primary particle diameter of the silicon carbide particles is 0.01 µm or more and 10 µm or less.   The exhaust gas purification filter according to any one of claims 1 to 4, wherein an average primary particle diameter of the noble metal particles is 1 nm or more and 50 nm or less.

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