JP2015009213A - Exhaust gas purification catalyst and exhaust gas purification device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas purification catalyst and an exhaust gas purification device for an internal combustion engine which can achieve both high catalytic activity at a low temperature and high durability at a high temperature by carrying an optimal amount of noble metal particles on silicon carbide particles and is further excellent in production cost by optimizing the amount carried of noble metal particles to silicon carbide particles.SOLUTION: There is provided an exhaust gas purification catalyst 1 which is a catalyst in which a noble metal particle 3 is carried on a surface of a silicon carbide particle 2, wherein the noble metal particle 3 is carried in a state of being covered with an oxide layer 4 and the ratio (Mn/Ms) of the mass (Mn) of the noble metal particle to the mass (Ms) of the silicon carbide particle is in the range of 0.005 or more and 0.15 or less.

Description

  The present invention relates to an exhaust gas purification catalyst and an exhaust gas purification device for an internal combustion engine, and more specifically, carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx) contained in exhaust gas of an internal combustion engine such as an engine. The present invention relates to an exhaust gas purification catalyst that efficiently purifies particulate matter (PM) and the like, and an exhaust gas purification device for an internal combustion engine in which the exhaust gas purification catalyst is disposed in an exhaust gas path of the internal combustion engine.

  Substances such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) contained in exhaust gas discharged from engines such as automobiles cause air pollution, It has caused various environmental problems so far. 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 a conventional exhaust gas purifying apparatus, a ceramic supported by a metal oxide such as alumina, ceria, zirconia, or a metal composite oxide having a perovskite structure represented by a general formula ABO ₃ (where A and B are metal elements) is supported on a catalyst. An exhaust gas purification catalyst is used in which a noble metal element having a catalytic function is supported on the catalyst carrier, and the substance contained in the exhaust gas is brought into contact with the exhaust gas purification catalyst for decomposition treatment (patent) Reference 1).

  In addition, a configuration is also known in which composite particles in which the surface of a noble metal element is supported on a surface of fine particles (supported particles) such as γ-alumina having a high specific surface area and the catalytic activity is increased by increasing the surface area of the noble metal element are used as catalysts. (Patent Document 2).

By the way, when purifying exhaust gas using the above-described conventional exhaust gas purification catalyst, heat is required to develop the catalytic activity of the noble metal element supported on the catalyst carrier. However, for example, at a short time after the engine is started, the exhaust gas purification catalyst is at a low temperature, so that there is a problem that exhaust gas purification is insufficient.
Therefore, as a method for obtaining efficient purification performance at low temperatures, it is conceivable to increase the contact area (contact probability) between the exhaust gas and the noble metal element by increasing the amount of the noble metal element supported on the catalyst carrier. Many of the noble metal elements having a catalytic activity for purifying exhaust gas, such as group 9 elements and group 10 elements, are expensive, which increases the production cost.

In addition, when using composite particles in which noble metal elements are supported on the surface of fine particles such as γ-alumina having a high specific surface area as described above as a catalyst, crystal transition of γ-alumina to α-alumina at a high temperature of 1000 ° C or higher As a result, the specific surface area of the alumina fine particles is remarkably reduced.
This structural change due to crystal transition from γ-alumina to α-alumina is a cause of promoting sintering of the catalyst component (grain growth), and when these catalysts are formed in layers on a substrate, The structural change due to the crystal transition of the catalyst may cause the catalyst layer to peel off or drop off. Therefore, a catalyst using fine particles such as γ-alumina cannot maintain catalytic activity at high temperatures.
In this way, even if the exhaust gas purification catalyst is at a low temperature, the exhaust gas can be stably purified, and furthermore, exhaust gas purification that can achieve both high durability at high temperatures and high catalyst activity at low temperatures. There was a need for a catalyst.

  Therefore, the present inventors conducted intensive research to solve the above problems, and used silicon carbide particles as a catalyst support, and formed noble metal particles on the surface of the silicon carbide particles, and an oxidation formed on the surface. By supporting in a state covered with a physical layer, the catalytic activity from a low temperature is high due to the interaction between the oxide layer and the noble metal particles, and a decrease in the catalytic activity due to sintering of the noble metal particles at a high temperature is suppressed, Therefore, an exhaust gas purifying catalyst capable of reducing the amount of noble metal particles used by being excellent in catalyst characteristics from a low temperature to a high temperature has been proposed (Patent Document 3).

Japanese Patent Laid-Open No. 5-4050 JP 2006-7117 A JP2013-63421A

  However, in the exhaust gas purifying catalyst described in Patent Document 3, no study has been made on a suitable loading amount of the noble metal particles with respect to the silicon carbide 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, high catalytic activity at low temperatures, high durability at high temperatures, Furthermore, an exhaust gas purification catalyst that is excellent in terms of manufacturing cost by optimizing the amount of noble metal particles supported on silicon carbide particles, and an exhaust gas of an internal combustion engine using the exhaust gas purification catalyst An object is to provide a purification device.

  The present inventors efficiently purify carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), particulate matter (PM), etc. contained in exhaust gas of an internal combustion engine such as an engine. As a result of intensive studies on the exhaust gas purification catalyst, noble metal particles are supported on the surface of the silicon carbide particles, and the noble metal particles are supported in a state covered with an oxide layer, and the mass of the silicon carbide particles ( If the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to Ms) is in the range of 0.005 or more and 0.15 or less, high catalytic activity at low temperature and high durability at high temperature are obtained. It was possible to achieve both, and it was found that the amount of noble metal particles supported on the silicon carbide particles was optimized, and the present invention was completed.

  That is, the exhaust gas purification catalyst of the present invention is a catalyst in which noble metal particles are supported on the surface of silicon carbide particles, the noble metal particles are supported in a state of being covered with an oxide layer, and the mass ( The ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to Ms) is in the range of 0.005 or more and 0.15 or less.

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.

  An exhaust gas purification apparatus for an internal combustion engine according to the present invention is an exhaust gas purification apparatus that purifies exhaust gas from the internal combustion engine with one or more types of catalysts disposed in an exhaust gas path of the internal combustion engine. One type is the exhaust gas purification catalyst of the present invention.

According to the exhaust gas purification catalyst of the present invention, the noble metal particles are supported on the surface of the silicon carbide particles, and the noble metal particles are supported in a state of being covered with the oxide layer, and the noble metal with respect to the mass (Ms) of the silicon carbide particles. Since the ratio (Mn / Ms) of the mass (Mn) of the particles is in the range of 0.005 or more and 0.15 or less, high catalytic activity at low temperature due to the interaction at the interface between the noble metal particles and the oxide layer Can be expressed.
In addition, since the silicon carbide particles themselves have high durability at high temperatures and the noble metal particles are supported in a state of being covered with an oxide layer, high durability and similar to those at low temperatures are achieved even in high temperature environments. High catalytic activity can be maintained.
As described above, it is possible to achieve both high catalytic activity at low temperatures and high durability at high temperatures. As a result, high durability is maintained while maintaining high catalytic activity from the low temperature range to the high temperature range. can do.
Furthermore, since the noble metal particles having an optimum loading amount are loaded on the silicon carbide particles, the production cost is excellent while maintaining high catalyst characteristics and high durability.

  According to the exhaust gas purification apparatus for an internal combustion engine of the present invention, since at least one of the catalysts arranged in the exhaust gas path of the internal combustion engine is the exhaust gas purification catalyst of the present invention, a high catalyst from the low temperature region to the high temperature region. High durability can be maintained while maintaining activity. As a result, the reliability of exhaust gas purification of the internal combustion engine can be improved.

It is sectional drawing which shows the noble metal carrying | support silicon carbide particle | grains which are the exhaust gas purification catalyst of one Embodiment of this invention. It is a mimetic diagram showing a porous membrane of one embodiment of the present 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.

An embodiment for carrying out an exhaust gas purification catalyst and an exhaust gas purification apparatus for an internal combustion engine of the present invention will be described.
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 catalyst]
The exhaust gas purifying catalyst of the present embodiment is a catalyst in which noble metal particles are supported on the surface of silicon carbide particles, and the noble metal particles are supported in a state of being covered with an oxide layer, and the mass (Ms) of the silicon carbide particles. The ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to is in the range of 0.005 or more and 0.15 or less.

FIG. 1 is a cross-sectional view showing an exhaust gas purification catalyst 1 of the present embodiment. In this exhaust gas purification catalyst 1, noble metal particles 3 are supported on the surface of silicon carbide particles 2, and the noble metal particles 3 are formed by an oxide layer 4. It is carried in a covered state.
In FIG. 1, the number of the noble metal particles 3 supported on the surface of the silicon carbide particles 2 is one, but the number of the noble metal particles 3 is not limited, and a plurality of noble metal particles 3 are supported. Also good.

As silicon carbide particles 2, the average primary particle diameter is preferably 0.005 μm or more and 5 μm or less, more preferably 0.02 μm or more and 0.1 μm or less, and further preferably 0.035 μm or more and 0.85 μm or less.
Here, when the average primary particle diameter of the silicon carbide particles 2 is less than 0.005 μm, the surface activity of the silicon carbide particles 2 becomes too high, and when the exhaust gas purification catalyst 1 is used at a high temperature, the exhaust gas purification is performed. Since sintering (grain growth) between the catalysts 1 proceeds, the particle diameter becomes coarse, and the catalytic activity decreases, which is not preferable. On the other hand, when the average primary particle diameter exceeds 5 μm, the specific surface area of the silicon carbide particles 2 decreases, and therefore when the optimum amount of noble metal particles is loaded, the distance between the noble metal particles is shortened. When the catalyst is used at a high temperature, sintering (grain growth) between the noble metal particles proceeds and the catalytic activity decreases, which is not preferable.

  As a method for obtaining such silicon carbide particles 2 having an average primary particle diameter of 0.005 μm or more and 5 μm or less, it has high temperature and high activity in a non-oxidizing atmosphere, and it is easy to introduce a fast 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 0.005 μm or more and 5 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.

The noble metal particles 3 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 size of the noble metal particles 3 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 easily aggregates. 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 exhaust gas purification catalyst 1, the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles 3 to the mass (Ms) of the silicon carbide particles 2 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 the exhaust gas purifying catalyst 1 exhibits catalytic characteristics corresponding to the amount of the precious metal particles 3 supported in this range, so that the low temperature region to the high temperature region. This is because it is possible to obtain the exhaust gas purification catalyst 1 that has high catalytic 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 3 and the oxide layer 4 (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 3 is limited because the number of the noble metal particles 3 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 4 reaches saturation. Furthermore, since there are noble metal particles 3 that are not covered with the oxide layer 4, these noble metal particles 3 are likely to be sintered together (grain growth). As a result, the catalyst activity is reduced. Thus, even if the loading amount of the noble metal particles 3 is increased, the catalyst characteristics are not improved, resulting in an increase in cost, which is not preferable.

[Exhaust gas purification device for internal combustion engine]
The exhaust gas purification apparatus for an internal combustion engine according to the present embodiment is an exhaust gas purification apparatus that purifies exhaust gas from the internal combustion engine with one or more types of catalysts disposed in the exhaust gas path of the internal combustion engine. At least one is the exhaust gas purification catalyst of the present embodiment.
This exhaust gas purification device is used for automobiles such as gasoline cars and diesel cars, for example. Further, the exhaust gas purification catalyst only needs to be arranged in the exhaust gas path of the internal combustion engine, and the arrangement method is not particularly limited. For example, the honeycomb base for exhaust gas purification used in automobiles (gasoline cars, diesel cars) can be used. It can carry | support to catalyst support members, such as material and DPF for diesel vehicles, and can arrange | position in an exhaust gas path | route.

This exhaust gas purifying device is preferably formed as a porous layer including a plurality of exhaust gas purifying catalysts of the present embodiment on a substrate provided in an exhaust gas path of an internal combustion engine. Here, the porous layer includes not only a porous film shape which is a continuous film-like structure but also a porous aggregate shape.
The exhaust gas purification catalyst may be in a state where it can exhibit a function as a catalyst, the exhaust gas purification catalyst may be dispersed in the porous layer, or a state in which several exhaust gas purification catalysts are aggregated And may be disposed in the porous layer. Furthermore, several exhaust gas purification catalysts may be aggregated to form a porous continuous film-like structure, or a porous aggregate may be formed. In either case, the porous layer will be referred to. In addition to the exhaust gas purification catalyst of the present embodiment, other catalyst particles, inorganic particles, and metal particles may be included.

  The porosity of the porous layer is preferably 40% or more and 90% or less, more preferably 50% or more and 80% or less. Here, if the porosity is less than 40%, sufficient gas diffusion is not performed in the porous body, and the catalyst may not work effectively. On the other hand, when the porosity exceeds 90%, the mechanical strength of the porous layer is lowered, and therefore, the porous layer itself is easily deformed and peeled off from the base material, and as a result, the catalytic action is lowered. There is a fear.

FIG. 2 is a schematic diagram showing the porous membrane 11 of the present embodiment. The porous membrane 11 is porous as a whole as a result of a plurality of types of exhaust gas purification catalysts 12a and 12b and surface-coated silicon carbide particles 13 being assembled. It became the film form.
Here, in the exhaust gas purification catalyst 12 a, noble metal particles 22 are supported on the surface of the silicon carbide particles 21, and the noble metal particles 22 are supported on the surface of the silicon carbide particles 21 in a state of being covered with the oxide layer 23.
Further, in the exhaust gas purification catalyst 12 b, two noble metal particles 22 are supported on the surface of the silicon carbide particles 21, and these noble metal particles 22 are supported on the surface of the silicon carbide particles 21 while being covered with the oxide layer 23. ing.

The number of the noble metal particles 22 carried by the silicon carbide particles 21 can be carried by three or more in addition to the above one or two.
On the other hand, the surface-coated silicon carbide particles 13 are obtained by covering the surface of the silicon carbide particles 21 with the oxide layer 23, and no noble metal particles 22 are supported on the surface of the silicon carbide particles 21.

As described above, the porous membrane 11 may be composed of a mixture of the exhaust gas purification catalysts 12a and 12b and the surface-coated silicon carbide particles 13 on which the noble metal particles 22 are not supported, and the exhaust gas purification catalysts 12a and 12b. The exhaust gas purifying catalyst on which three or more noble metal particles 22 are supported may be included.
Further, for the exhaust gas purification catalyst and the surface-coated silicon carbide particles, group 3 elements such as silicon (Si), aluminum (Al), boron (B), zirconium (Zr), titanium (Ti), etc., if necessary. It is also possible to contain at least one element selected from group elements, or oxides, carbides, and nitrides thereof alone or in combination. In addition, 80 volume% or more is preferable and, as for the ratio of the silicon carbide in the case of containing another component, 90 volume% or more is more preferable.

When exhaust gas from the internal combustion engine flows into the porous membrane 11 including the exhaust gas purification catalysts 12a and 12b and the surface-coated silicon carbide particles 13, the exhaust gas purification catalysts 12a and 12b constituting the catalyst come into contact with the exhaust gas flowing. The carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), particulate matter (PM), etc. contained in the exhaust gas are converted into CO ₂ by the exhaust gas purification catalysts 12a and 12b. HC is oxidized and decomposed into H ₂ O and CO ₂ , NOx is oxidized into NO ₂ , and carbon constituting PM is oxidized and decomposed into CO ₂ .

Further, since the surface-coated silicon carbide particles 13 also have oxidation / decomposition ability due to the oxygen adsorption / desorption performance of the oxide layer 23 on the surface, the oxidation / decomposition of CO, HC, NOx, PM by the particles themselves. The auxiliary action to the decomposition and the oxidation / decomposition action of the exhaust gas purification catalysts 12a, 12b is caused.
Thereafter, the generated NO ₂ is reduced to N ₂ , and the purified gas from which harmful components and particulate matter are decomposed and removed is discharged into the atmosphere.

[Method of manufacturing exhaust gas purification catalyst and exhaust gas purification device of internal combustion engine]
A method for manufacturing the exhaust gas purifying catalyst and the exhaust gas purifying apparatus for the internal combustion engine of the present embodiment will be described.
In the present embodiment, an exhaust gas purification catalyst may be manufactured first, and then an exhaust gas purification device may be manufactured using the obtained exhaust gas purification catalyst. In the process of manufacturing the exhaust gas purification device, the exhaust gas purification catalyst may be used at the same time. A manufacturing method may be used.
Therefore, in the following description, the manufacturing method is divided into processes, the order of possible processes is shown first, and then details of each process are described.

The method for producing an exhaust gas purification catalyst of the present embodiment includes a base material preparation step ([A] step) for preparing silicon carbide particles as a base material, and a noble metal particle support step for supporting noble metal particles on the surface of the silicon carbide particles. ([B] step) and an oxide layer forming step ([C] step) for forming an oxide layer on the surface of the silicon carbide particles carrying the noble metal particles.
Furthermore, in the method for manufacturing an exhaust gas purification device for an internal combustion engine, the exhaust gas purification catalyst obtained in the oxide layer forming step ([C] step) is applied to the base material that is supported on the base material constituting the exhaust gas purification device. It has a supporting step ([D] step).

These four steps [A] to [D] have the following restrictions in the order. First, the base material preparation step [A] must be first. Next, the oxide layer forming step [C] needs to be performed on the silicon carbide particles on which the noble metal particles are supported in the noble metal particle supporting step ([B] step). ). The reason is that sufficient oxidation catalyst characteristics cannot be obtained even if noble metal particles are supported after forming an oxide layer on silicon carbide particles. Next, the supporting process [D] on the base material is a process specific to the manufacturing method of the exhaust gas purification apparatus of the internal combustion engine, and is not necessary as the manufacturing process of the exhaust gas purification catalyst.
Since the constraints are the above three points, the order can be arbitrarily selected to satisfy these constraints.

In addition, the precious metal particle supporting step [B] and the supporting step [D] on the base material are both similar processes such as dissolution or dispersion in a solvent (dispersion medium), dry removal of the solvent (dispersion medium), and heat treatment. Have. Therefore, in manufacturing the exhaust gas purification apparatus, the noble metal particle supporting step [B] and the supporting step [D] on the base material can be performed in parallel.
From the above, in the manufacturing method of the exhaust gas purification catalyst and the exhaust gas purification device of the internal combustion engine in the present embodiment, the following process order 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.

First, the manufacturing method of the exhaust gas purification catalyst is in the following order of steps.
(1-1): [A] → [B] → [C]
In this method, noble metal particles are supported on silicon carbide particles, and then an oxide layer is formed on the surface of the silicon carbide particles on which the noble metal particles are supported. There is only one type of process that satisfies the above constraints.

Next, the manufacturing method of the exhaust gas purification apparatus is in the following four kinds of steps.
(2-1): [A] → [B] → [C] → [D]
In this method, after producing an exhaust gas purification catalyst (silicon carbide particles carrying precious metal particles and having an oxide layer formed on the surface), the exhaust gas purification catalyst is carried on a substrate.
(2-2): [A] → [B] → [D] → [C]
In this method, silicon carbide particles carrying noble metal particles are formed, the silicon carbide particles carrying the noble metal particles are carried on a substrate, and then an oxide layer is formed on the surface of the silicon carbide particles.
(2-3): [A] → [D] → [B] → [C]
In this method, after silicon carbide particles are supported on a base material, noble metal particles are supported on the surface of the silicon carbide particles, and then an oxide layer is formed on the surface of the silicon carbide particles on which the noble metal particles are supported.
(2-4): [A] → [B] = [D] → [C]
By simultaneously supporting the noble metal particles on the surface of the silicon carbide particles and supporting the silicon carbide particles on the base material, a porous silicon carbide layer supporting the noble metal particles was obtained, and the noble metal particles were supported. In this method, an oxide layer is formed on the surface of the silicon carbide porous layer.

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.
The average primary particle diameter of the silicon carbide particles may be selected in the range of 0.005 μm or more and 5 μm or less in accordance with the required characteristics of the exhaust gas purification catalyst and the exhaust gas purification device. 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 or on the surface of silicon carbide particles that are supported on a substrate constituting an exhaust gas purification device to form a porous layer of silicon carbide particles. This is a step of forming a porous layer made of silicon carbide particles.
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, silicon carbide particles are immersed and dispersed in this solution or dispersion, or a silicon carbide particle porous layer (a substrate carrying silicon carbide particles) is immersed, and then at a temperature of about 60 ° C. to 250 ° C. Dry to remove solvent and dispersion 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 the porous layer 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.
As a result, the noble metal source is reduced and decomposed to become noble metal, and the generated noble metal fine particles adhere to the surface of the silicon carbide particles or the surface of the silicon carbide particles forming the porous layer. On the other hand, noble metal fine particles are not changed in composition even when heat-treated in a reducing atmosphere or in an inert atmosphere, and the surface of the silicon carbide particles or the porous layer is maintained while maintaining the original fine particle state. It adheres to the surface of the silicon carbide particles to be formed.

The temperature and time during the heat treatment may be appropriately selected depending on the type of noble metal source, the noble metal fine particles, the atmospheric conditions, and the like. The range is less than the time. 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, excessive temperature and time are not preferable because there is a possibility that sintering of silicon carbide particles and sintering of generated noble metal particles may be induced.
Through the above steps, it is possible to obtain a porous layer comprising silicon carbide particles having noble metal fine particles supported on the surface or silicon carbide particles having noble metal fine particles supported on the surface.

"Oxide layer formation process"
Oxidizing silicon carbide particles carrying noble metal particles, or silicon carbide particles carried on a substrate and carrying a porous layer of silicon carbide particles carrying noble metal particles, and oxides on the surface of silicon carbide particles carrying noble metal particles Forming a layer to form an exhaust gas purification catalyst or a porous layer made of an exhaust gas purification catalyst.

  As the oxidation treatment, silicon carbide particles on which the above-mentioned noble metal particles are supported, or a silicon carbide particle porous layer on which noble metal particles are supported while being supported on a base material (silicon carbide particles on which noble metal particles are supported are partially separated) The substrate having a layer that has been sintered in the air is at least 600 ° C. and 1000 ° C., preferably 650 ° C. and 900 ° C. in an oxidizing atmosphere such as air and oxygen for 0.5 hour or more. The oxidation treatment is performed for 36 hours or less, preferably 4 hours or more and 12 hours or less to form an oxide layer on the surface of the silicon carbide particles.

Thereby, the noble metal particles are supported on the surface of the silicon carbide particles, and the exhaust gas purification catalyst in which the noble metal particles are covered with the oxide layer, or the noble metal particles are supported on the surface of the silicon carbide particles, and An exhaust gas purifying apparatus comprising a porous layer in which silicon carbide particles in which the noble metal particles are covered with an oxide layer is supported on a substrate can be formed.
In this way, the oxide layer is formed under the above conditions, and when the average primary particle size of the noble metal particles is 1 nm or more and 50 nm or less, the noble metal particles are completely covered without protruding from the oxide layer. An exhaust gas purification catalyst having catalytic activity, or an exhaust gas purification device in which this exhaust gas purification catalyst is supported on a substrate can be formed.

"Supporting process on substrate"
In the supporting process on the substrate, silicon carbide particles as a base material, silicon carbide particles supporting noble metal particles, or silicon carbide particles having an oxide layer formed on the surface supporting noble metal particles are dispersed. A step of preparing a silicon carbide particle dispersion by dispersing in a medium, and applying (coating) the silicon carbide particle dispersion onto a substrate constituting an exhaust gas purification device, followed by drying; And a step of forming a porous layer.
First, any of the silicon carbide particles described above, silicon carbide particles supporting noble metal particles, or silicon carbide particles supporting noble metal particles and having an oxide layer formed on the surface thereof is dispersed in a dispersion medium to obtain silicon carbide particles. A dispersion is prepared.

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 coatability, this dispersion liquid is 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 process of producing this silicon carbide particle dispersion, examples of apparatuses that can disperse silicon carbide particles in a dispersion medium include a kneader, a roll mill, a pin mill, a sand mill, a ball mill, and a planetary ball mill. However, in order to disperse silicon carbide particles in a dispersion medium using a dispersion medium, a sand mill, a ball mill, a planetary ball mill, or the like is preferable.
In addition, as dispersion media such as balls, resin coated bodies made of metals such as steel and lead, sintered inorganic oxides such as alumina, zirconia, silica, and titania, and nitride firing such as silicon nitride are used. Examples of the dispersion medium used in the present embodiment include, but are not limited to, sintered ceramics such as silicon carbide, glass such as soda glass, lead glass, and high specific gravity glass. Therefore, a zirconia having a specific gravity of 6 or more, a resin coated body in which steel is a core material, and the like are preferable.

Next, the above silicon carbide particle dispersion is applied to a substrate constituting an exhaust gas purification apparatus, for example, a honeycomb structure substrate (hereinafter also referred to as a honeycomb substrate), dried, and a coating film ( Coating dry layer).
Here, the “honeycomb structure” in the present embodiment is a structure having a large number of gas flow paths that are partitioned by partition walls and arranged in parallel to each other and open at both ends in the base material. The exhaust gas discharged from the engine flows in from one end side, is purified in contact with the catalyst arranged on the partition wall in the process of flowing through the gas flow path, and is discharged from the other end side.

Here, the above silicon carbide particle dispersion is applied onto a substrate such as a honeycomb substrate by a bar coating method, a slip casting method, a wash coating method, or the like, and dried to form a coating film on the substrate. Alternatively, a substrate such as a honeycomb substrate may be immersed in a silicon carbide particle dispersion, pulled up, and then dried to form a coating film on the substrate.
Further, drying, that is, removal of water and organic solvent may be performed at a temperature of about 100 ° C. to 250 ° C. in an air atmosphere.

Next, a substrate such as a honeycomb substrate on which a coating film containing silicon carbide particles is formed is placed under an inert atmosphere such as nitrogen, argon, neon, or xenon, or under a reducing atmosphere such as hydrogen or carbon monoxide. The heat treatment is performed at 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, the polymer monomer or oligomer in the coating film, the dispersant, the surfactant, the preservative, the stabilizer, the antifoaming agent. The additive such as the leveling agent is dissipated, and the silicon carbide particles are partially sintered to form a neck portion where the particles are bonded to each other, thereby forming a porous film bonded in a porous shape. .

In addition, you may integrate the drying of a dispersion liquid with a heat treatment process as a pre-stage of a heat treatment 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)
The noble metal particle supporting step and the porous film forming step have similar steps. Therefore, in the method for manufacturing an exhaust gas purification apparatus for an internal combustion engine, both steps can be performed in parallel.

First, in the noble metal particle supporting step, a step of preparing a solution in which a noble metal source (a noble metal salt or the like) is dissolved or a dispersion in which noble metal fine particles are dispersed is prepared. In the porous film forming step, a dispersion medium is prepared. Each process is similar. In the noble metal particle supporting step, the step of immersing and dispersing silicon carbide particles in a solution or dispersion is similar to the step of preparing 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 the dispersion medium, it is necessary to select a substance in which the noble metal source can be easily dissolved or dispersed, and water can be particularly preferably used. On the other hand, when an organic solvent or the like is used, a noble metal source that is easily dispersed in the organic solvent or the like may be selected.

  Next, in the porous film forming step, there is a step of applying (coating) the silicon carbide particle dispersion liquid to the partition wall made of a porous body constituting the filter base. In the noble metal particle supporting step, the corresponding step is Absent.

Next, in the noble metal particle supporting step, the step of removing water and the dispersion medium as a solvent and drying is performed, and in the porous film forming step, the partition made of a porous body coated with the silicon carbide particle dispersion is dried. The steps to be performed are similar to each other.
Since these two steps are drying steps and are substantially the same steps, 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, in the noble metal particle supporting step, the step of forming noble metal fine particles by heat treatment in a reducing atmosphere or in an inert atmosphere is composed of a porous body on which a coating dry film is formed in the porous film forming step. The process of forming a porous film by heat-treating the partition walls in a reducing atmosphere or in an inert atmosphere to partially sinter them is similar.
Both of these steps are substantially the same step in that heat treatment is performed in a reducing atmosphere or an inert atmosphere, and can be a single step.
The heat treatment temperature and the heat treatment time may be selected from the range common to both steps in consideration of the precious metal particle production 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 obtained silicon carbide particle dispersion liquid comprising a porous body constituting the filter base. By carrying out the step of applying to the partition walls, the step of drying the obtained coating film, and the step of heat-treating the obtained coating film, the noble metal particle supporting step is incorporated into the porous film forming step, It can be performed as a simultaneous and parallel process in which the porous film forming process is integrated.
As described above, the exhaust gas purification apparatus for the internal combustion engine of the present embodiment can be manufactured.

  As described above, according to the exhaust gas purification catalyst of the present embodiment, the noble metal particles are supported on the surface of the silicon carbide particles, and the noble metal particles are supported in a state of being covered with the oxide layer, and the mass of the silicon carbide particles. Since the ratio (Mn / Ms) of the mass (Mn) of the noble metal particles to (Ms) is in the range of 0.005 or more and 0.15 or less, the interaction at the interface between the noble metal particles and the oxide layer causes a low temperature. Can exhibit high catalytic activity.

In addition, since the silicon carbide particles themselves have high durability at high temperatures and the noble metal particles are supported in a state of being covered with an oxide layer, high durability and similar to those at low temperatures are achieved even in high temperature environments. High catalytic activity can be maintained.
As described above, it is possible to achieve both high catalytic activity at low temperatures and high durability at high temperatures. As a result, high durability is maintained while maintaining high catalytic activity from the low temperature range to the high temperature range. can do.
Furthermore, since noble metal particles having an optimum loading amount with respect to the silicon carbide particles are supported, it is possible to obtain an exhaust gas purification catalyst that is excellent in cost while maintaining high catalyst characteristics and high durability.

In addition, the noble metal particles are covered with an oxide layer on the surface of the silicon carbide particles, and the oxide layer can be bonded to the noble metal particles, and in particular, silicon that is easily bonded to the noble metal particles is included therein. Since the carbide oxide (SiO _x C _y ) is contained, the movement of the noble metal particles on the carrier can be suppressed even at a high temperature, and the sintering (grain growth) of the noble metal particles can be suppressed.
Further, the noble metal particles can suppress the melting of the noble metal particles by chemical bonding with an oxide layer, particularly silicon carbide oxide (SiO _x C _y ).

Silicon carbide particles are stable even at high temperatures, and are represented by ceramics made of metal oxides such as alumina, ceria, zirconia, etc., or a general formula ABO ₃ (where A and B are metal elements). Compared with the metal composite oxide having a perovskite structure, grain growth hardly occurs. Thereby, the interparticle distance of the noble metal particles can be maintained even after being exposed to a high temperature during the production process or use. As a result, contact between noble metal particles can be suppressed, and sintering (grain growth) hardly occurs.

  According to the exhaust gas purification apparatus for an internal combustion engine of the present embodiment, since at least one of the catalysts arranged in the exhaust gas path of the internal combustion engine is the exhaust gas purification catalyst of the present embodiment, the catalytic activity at low temperatures is high and the temperature is high Even if exposed underneath, the catalytic activity does not decrease, and high durability can be maintained. Therefore, carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), particulate matter (PM), etc. contained in the exhaust gas generated from the internal combustion engine can be efficiently oxidized and decomposed and removed. In addition, an exhaust gas purification apparatus for an internal combustion engine having higher reliability can be obtained.

  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 added to a dispersion medium in which ammonium polyacrylate and an antifoaming agent are dissolved as surfactants in 80 g of pure water. In this state, zirconia balls are used as the dispersion medium. The dispersion process used was carried out for 180 minutes.
To the slurry thus obtained, a dinitrodiammine platinum nitric acid solution was added so that platinum was 0.066 g with respect to 1 g of silicon carbide particles, and again a dispersion treatment using a dispersion medium was performed for 30 minutes, The dispersion medium was removed by evaporation and solidification. Then, the exhaust gas purification catalyst of Example 1 was produced by performing heat processing on the following conditions.
First stage: temperature 650 ° C., holding time 240 minutes, atmosphere N ₂
Second stage: temperature 1000 ° C., retention time 360 minutes, atmosphere Ar
Third stage: temperature 700 ° C, holding time 1800 minutes, atmosphere air

[Example 2]
Instead of adding dinitrodiammine platinum nitric acid solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitric acid solution is added so that platinum becomes 0.1 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Example 2 was produced in the same manner as Example 1.

[Example 3]
Instead of adding dinitrodiammine platinum nitrate solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitrate solution is added so that platinum becomes 0.15 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Example 3 was produced in the same manner as Example 1.

[Example 4]
Instead of adding dinitrodiammine platinum nitric acid solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitric acid solution is added so that platinum becomes 0.05 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Example 4 was produced in the same manner as Example 1.

[Example 5]
Instead of adding dinitrodiammine platinum nitrate solution so that platinum is 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitrate solution is added so that platinum is 0.03 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Example 5 was produced in the same manner as Example 1.

[Example 6]
Instead of adding dinitrodiammine platinum nitric acid solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitric acid solution is added so that platinum becomes 0.01 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Example 6 was produced in the same manner as Example 1.

[Example 7]
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 0.8 μm are used, and further dinitrogen so that platinum is 0.066 g with respect to 1 g of silicon carbide particles. Exhaust gas purification catalyst of Example 7 in the same manner as in Example 1 except that a palladium nitrate solution was added so that palladium was 0.066 g per 1 g of silicon carbide particles instead of adding the diammine platinum nitric acid solution. Was made.

[Example 8]
13.5 g of silicon carbide particles with an average primary particle size of 0.8 μm and 0.15 g of silicon carbide particles with an average primary particle size of 0.03 μm, and ammonium polyacrylate and an antifoaming agent as surfactants in 80 g of pure water The dispersion was added to the dissolved dispersion medium, and in this state, a dispersion treatment using zirconia balls as the dispersion medium was performed for 180 minutes.
The slurry thus obtained was added and treated in the same manner as in Example 1 to produce an exhaust gas purification catalyst of Example 8.

[Example 9]
An exhaust gas purification catalyst of Example 9 was produced in the same manner as in Example 1 except that silicon carbide particles having an average primary particle diameter of 15 μm were used instead of silicon carbide particles having an average primary particle diameter of 0.035 μm. .

[Example 10]
An exhaust gas purification catalyst of Example 10 was produced in the same manner as in Example 1 except that the following one-stage heat treatment was performed instead of performing the three-stage heat treatment.
First stage: temperature 1200 ° C, holding time 1800 minutes, atmosphere air

[Comparative Example 1]
Instead of adding dinitrodiammine platinum nitric acid solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitric acid solution is added so that platinum becomes 0.2 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Comparative Example 1 was produced in the same manner as Example 1.

[Comparative Example 2]
Instead of adding dinitrodiammine platinum nitric acid solution so that platinum becomes 0.066 g per 1 g of silicon carbide particles, dinitrodiammine platinum nitric acid solution is added so that platinum becomes 0.001 g per 1 g of silicon carbide particles. Except that, an exhaust gas purification catalyst of Comparative Example 2 was produced in the same manner as Example 1.

[Comparative Example 3]
An exhaust gas purification catalyst of Comparative Example 3 was produced in the same manner as in Example 1 except that the dinitrodiammine platinum nitric acid solution for forming platinum fine particles was not added and platinum fine particles were not supported.

[Evaluation of exhaust gas purification catalyst]
The exhaust gas purification catalysts of Examples 1 to 10 and Comparative Examples 1 to 3 were evaluated.
The evaluation items are as follows.
(1) Composition of oxide layer in exhaust gas purification catalyst Using an X-ray photoelectron analyzer (XPS / ESCA) Sigma Probe (manufactured by VG-Scientific), the oxide layer on the surface of the silicon carbide particles in the exhaust gas purification catalyst A compositional analysis was performed.

(2) Preserved state of noble metal particles and average primary particle size The field state transmission electron microscope (FE-TEM) JEM-2100F (manufactured by JEOL Ltd.) was used to evaluate the supported state of noble metal particles. .
Moreover, the average primary particle diameter of the noble metal particles was determined by measuring the primary particle diameter of 500 noble metal particles randomly selected from field emission type transmission electron microscope images (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) CO purification temperature and HC purification temperature While flowing the simulated exhaust gas shown in Table 1 through the exhaust gas purification catalyst, the simulated exhaust gas was heated at a rate of temperature increase of 10 ° C / min, and carbon monoxide (CO) and The purification rate of each hydrocarbon (HC) was measured. Here, a temperature (T50) at which 50% of carbon monoxide (CO) (or hydrocarbon (HC)) in the simulated exhaust gas is purified is measured at a position downstream of the exhaust gas purification catalyst, and monoxide is obtained. It was used as an index for purification of carbon (CO) (or hydrocarbon (HC)).

Table 2 shows the evaluation results of the exhaust gas purifying catalysts of Examples 1 to 8 and Comparative Examples 1 to 4.

  Next, among these exhaust gas purification catalysts of Examples 1 to 10 and Comparative Examples 1 to 3, the average primary particle diameter of silicon carbide particles was 0.035 μm, and Examples 1 to 6 using platinum 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. 3 shows.

From the observation results of FE-TEM, in the exhaust gas purification catalysts of Examples 1 to 8, 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. It was confirmed that this oxide layer covered the noble metal particles.
Moreover, according to Table 2, the CO purification temperature in the exhaust gas purification apparatuses using the exhaust gas purification catalysts of Examples 1 to 8 is as low as 169 ° C to 205 ° C, and the HC purification temperature is also 179 ° C to 209 ° C. Since it is low, it was confirmed that it has a sufficient purification effect on CO and HC. From these results, in Examples 1 to 8, it was confirmed that the catalyst characteristics were improved by the interface interaction between the noble metal particles and the oxide layer.

Next, in the exhaust gas purification catalysts of Examples 9 and 10, based on the observation results of FE-TEM, noble metal particles of about 60 nanometers 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 was confirmed that this oxide layer covered at least part of the noble metal particles as it was formed.
Further, according to Table 2, the CO purification temperature and the HC purification temperature in the exhaust gas purification apparatuses using the exhaust gas purification catalysts of Examples 9 and 10 were lower than those in Comparative Example 3 in which no precious metal particles were supported. Since it is higher than Examples 1-8, it was confirmed that it has a certain degree of purification effect on CO and HC.

  Thus, the characteristics of the exhaust gas purification catalysts of Examples 9 and 10 are lower than those of Examples 1 to 8 because the particle diameter of the noble metal particles is large, and at least a part of the noble metal particles is an oxide film on the surface of the silicon carbide particles. It is considered that sufficient catalytic activity is not exhibited because it is not covered with the catalyst. The reason is considered to be that in Example 9, the average primary particle diameter of the silicon carbide particles was as large as 12 μm, so that the specific surface area of the silicon carbide particles was low, and sintering (grain growth) between the noble metal particles proceeded. Further, in Example 10, the first stage heat treatment temperature is high, the holding time is long, and the treatment is performed in an air atmosphere, so that it is considered that the sintering (grain growth) of the noble metal particles also progressed.

On the other hand, in the exhaust gas purification catalysts of Comparative Examples 1 and 3, since the amount of noble metal particles is too small or not present at all, the CO purification temperature and the HC purification temperature are high, and a sufficient purification effect on CO and HC is not obtained. confirmed.
Further, the exhaust gas purifying catalyst of Comparative Example 2 is an example in which the precious metal addition amount is 20% by mass, and although the CO purification temperature and the HC purification temperature are sufficiently low, the values thereof are those of Example 3 in which the precious 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, according to FIG. 3, it can be seen that the presence of the noble metal particles causes both the CO purification temperature and the HC purification temperature to decrease as compared with the case of the silicon carbide particles alone, and (Ms / Mn) is 0. It can be seen that the effect of the presence of the noble metal particles appears when about 005, that is, when the addition amount of the noble metal particles is about 0.5 mass% with respect to the silicon carbide particles.
On the other hand, as the addition amount of noble metal particles increases, the rate of decrease in CO purification temperature and HC purification temperature decreases, and (Ms / Mn) is 0.15 or more, that is, the addition amount of noble metal particles is relative to silicon carbide particles. It was found that the CO purification temperature and the HC purification temperature hardly decreased even when 15% by mass or more existed, and the amount of noble metal reached saturation.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 2 Silicon carbide particle 3 Noble metal particle 4 Oxide layer 11 Porous membrane 12a, 12b Exhaust gas purification catalyst 13 Surface-coated silicon carbide particle 21 Silicon carbide particle 22 Noble metal particle 23 Oxide layer

Claims (6)

A catalyst in which noble metal particles are supported on the surface of silicon carbide particles,
The noble metal particles are supported in a state covered with an oxide layer,
The exhaust gas purification catalyst, wherein 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 catalyst 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 3. An exhaust gas purifying catalyst according to Item 2.   The exhaust gas purification catalyst 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 catalyst 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. An exhaust gas purification apparatus for purifying exhaust gas from the internal combustion engine with one or more types of catalysts disposed in an exhaust gas path of the internal combustion engine,
An exhaust gas purifying apparatus for an internal combustion engine, wherein at least one of the catalysts is an exhaust gas purifying catalyst according to any one of claims 1 to 5.

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