JP2013146706A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

Disclosed is an exhaust gas purifying catalyst capable of preventing the occurrence of phosphorus poisoning and maintaining high catalytic activity even after long-term operation.
A first coating layer comprising a carrier substrate, a first coating layer coated on the surface of the carrier substrate, and a second coating layer coated on the surface of the first coating layer. The exhaust gas purifying catalyst, wherein the layer contains a catalyst noble metal and the second coat layer does not contain the catalyst noble metal or contains the catalyst noble metal in an amount smaller than that of the first coat layer.
[Selection figure] None

Description

  The present invention relates to an exhaust gas purification catalyst for an internal combustion engine such as an automobile, particularly a diesel engine.

An exhaust gas purification catalyst for an internal combustion engine such as an automobile oxidizes hydrocarbons (HC) and carbon monoxide (CO) contained in exhaust gas exhausted from the internal combustion engine to form water and carbon dioxide into nitrogen oxides ( NO _x ) is reduced and converted to nitrogen, respectively. As the exhaust gas purifying catalyst, a noble metal supported catalyst in which catalytic noble metal particles such as platinum (Pt), palladium (Pd), rhodium (Rh) are supported on a heat-resistant metal carrier is usually used.

  In order to improve the catalytic activity of the exhaust gas purifying catalyst, it is conceivable to increase the amount of the catalyst noble metal supported. However, such noble metals are expensive, and an increase in the amount of catalyst noble metal supported increases the manufacturing cost of the catalyst.

  In view of such problems, Patent Document 1 discloses nitrogen monoxide, carbon monoxide, and a volatile organic compound as an exhaust gas purification catalyst that improves catalytic activity without increasing the amount of catalyst noble metal to be supported. The exhaust gas treatment catalyst contained is described. According to this document, the catalyst has a coating layer of a carrier made of a porous inorganic compound, the coating layer is composed of a plurality of layers, and the active component composed of one or more kinds of catalyst noble metals is the plurality of the above-mentioned plurality of layers. Rich in the upper layer of the layers, and poor in the lower layer of the plurality of layers. Due to this feature, the catalyst can support the active component concentrated on the upper layer that greatly contributes to the exhaust gas treatment reaction, so that the oxidizing power is improved without increasing the amount of precious metal used in the entire catalyst. I can do it.

By the way, in the exhaust gas purifying catalyst, there is a problem of catalyst poisoning in which components contained in the exhaust gas adhere to the catalyst and reduce the catalytic activity. For example, a sulfur component contained in a trace amount in a fuel of an internal combustion engine such as gasoline is oxidized at the time of combustion of the fuel or oxidized on an exhaust gas purification catalyst to become SO _x . SO _x is because it is an acidic substance, when the NO _x storage-and-reduction type exhaust gas purifying catalyst using _the NO _x storage material, reacts with _the NO _x storage material containing a basic substance to form a sulfate. When the above reaction occurs, the NO _x storage capacity of the NO _x storage material decreases, and as a result, the NO _x purification performance decreases. Such a phenomenon is known as sulfur poisoning.

  Patent Document 2 discloses an exhaust gas that efficiently adsorbs HC and soluble organic components (SOF) in a low temperature range to prevent emission of HC and SOF, suppress generation of sulfate, and prevent sulfur poisoning of an oxidation catalyst. As a purification catalyst, a support substrate, a first support layer formed on the surface of the support substrate and supporting a catalyst metal, and a second support layer formed on the surface of the first support layer and not supporting a catalyst metal An oxidation catalyst for diesel exhaust gas is described. According to this document, the first support layer of this catalyst is characterized in that the adsorbability of components in diesel exhaust gas is lower than that of the second support layer.

Patent Document 3 describes an exhaust gas purifying catalyst comprising a support base, a support layer coated on the surface of the support base, and a Pt and NO _x storage material supported on the support layer. According to this document, the carrier layer has a two-layer structure of an upper layer and a lower layer, and the Pt concentration in the upper layer is higher than the Pt concentration in the lower layer. With this feature, the sulfur component is quickly oxidized to SO _x in the upper layer and adsorbed on the upper NO _x storage material, so that sulfur poisoning by SO _{x on} the lower layer can be suppressed.

  Lubricating oils (engine oils) for internal combustion engines such as gasoline engines or diesel engines contain phosphorus such as zinc dithiophosphate (ZDTP) as an additive to provide functions such as oxidation prevention, corrosion prevention and wear prevention. Acid salts are used. In the case of an oxygen storage oxidation type exhaust gas purifying oxidation catalyst using an oxygen storage material, these phosphates react with an oxygen storage material containing alumina or ceria to form a phosphorus compound. When the above reaction occurs, not only the oxygen storage capacity of the oxygen storage material is lowered, but also gas diffusion into the catalyst is hindered, resulting in a reduction in oxidation performance. Such a phenomenon is known as phosphorus poisoning.

  Patent Document 4 describes an exhaust gas purifying catalyst in which a catalyst layer containing noble metal and γ-alumina particles is formed on the cell surface of a honeycomb-shaped catalyst carrier, and purifies exhaust gas containing phosphorus. According to the document, zirconia particles are dispersed and supported on the outer surface of the γ-alumina particles, and the ratio of the projected area of the zirconia particles to the outer surface of the γ-alumina particles is 80% or less. . Since zirconia is easier to form a phosphorus compound than γ-alumina, phosphorous poisoning of γ-alumina particles can be suppressed.

  Patent Document 5 is an automobile exhaust gas treatment catalyst having excellent resistance to poisoning caused by additives derived from oil and / or fuel, and includes a base material, a base coat, a noble metal-containing first catalyst layer, and a noble metal-containing additional catalyst layer. The catalyst comprising: According to this document, the catalyst has a poison trapping region in which either the first catalyst layer or at least one additional catalyst layer, or all of the noble metal-containing catalyst layer does not contain a noble metal.

JP 2008-188542 A JP 09-155205 A JP 2001-70790 A JP 2004-261681 Special table 2009-501079

  As described above, various techniques have been developed to prevent or suppress the occurrence of catalyst poisoning in the exhaust gas purifying catalyst of an internal combustion engine such as an automobile, but none of these techniques is sufficient. In particular, in the case of phosphorus poisoning, phosphorus adsorbed on the catalyst reacts with a catalyst material such as an oxygen storage material to form a phosphorus compound, so that the catalytic activity of the exhaust gas purification catalyst once poisoned with phosphorus cannot be recovered. Have difficulty. For this reason, in order to suppress a decrease in the catalytic activity (oxidation activity) of the exhaust gas purification oxidation catalyst, it is important to prevent the occurrence of phosphorus poisoning as much as possible.

  Therefore, an object of the present invention is to provide an exhaust gas purifying catalyst that can prevent the occurrence of phosphorus poisoning and maintain high catalytic activity even after long-term operation.

  As a result of various studies on means for solving the above problems, the present inventors have found that there is a certain correlation between the oxidation activity of the exhaust gas purification oxidation catalyst and the penetration depth of phosphorus adsorbed on the catalyst from the catalyst surface. As a result, the present invention was completed.

That is, the gist of the present invention is as follows.
(1) A first coating layer having a carrier substrate, a first coating layer coated on the surface of the carrier substrate, and a second coating layer coated on the surface of the first coating layer Contains a catalyst noble metal, and the second coat layer does not contain the catalyst noble metal or contains the catalyst noble metal in an amount smaller than that of the first coat layer.

  (2) The exhaust gas purifying catalyst according to (1), wherein the thickness of the second coat layer is in the range of 20 to 40 μm.

  (3) The first coat layer and the second coat layer have pores, and the open porosity of the second coat layer is in a range exceeding the open porosity of the first coat layer, (1) or (2) Exhaust gas purification catalyst.

  (4) Any of (1) to (3) above, wherein the concentration of the catalyst noble metal contained in the first coat layer increases as it approaches the bonding surface between the first coat layer and the second coat layer. Exhaust gas purification catalyst.

  (5) The exhaust gas purifying catalyst according to any one of (1) to (4), wherein the catalyst noble metal is selected from the group consisting of Pt, Pd, and Rh.

  According to the present invention, it is possible to provide an exhaust gas purification catalyst capable of preventing the occurrence of phosphorus poisoning and maintaining high catalytic activity even after long-term operation.

1 is a cross-sectional view showing an embodiment (Example 1) of an exhaust gas purifying catalyst of the present invention. FIG. 6 is a cross-sectional view showing another embodiment (Example 2) of the exhaust gas purifying catalyst of the present invention. FIG. 6 is a cross-sectional view showing another embodiment (Example 3) of the exhaust gas purifying catalyst of the present invention. It is sectional drawing which shows the catalyst for exhaust gas purification of a prior art (comparative example). It is a figure which shows the relationship between the phosphorus adsorption amount and phosphorus penetration depth, and oxidation activity in the exhaust gas purification catalyst of a comparative example. A: Relationship between phosphorus adsorption amount and CO-T50; B: Relationship between phosphorus penetration depth from catalyst surface and phosphorus adsorption amount. It is a figure which shows the relationship between the phosphorus penetration depth from the catalyst surface, and CO-T50 in the exhaust gas purification catalyst of Examples 1-3 and a comparative example.

<1. Exhaust gas purification catalyst>
The present invention relates to an exhaust gas purifying catalyst having a support substrate, a first coat layer coated on the surface of the support substrate, and a second coat layer coated on the surface of the first coat layer. .

  In the present specification, the “support substrate” means a material for supporting a coating layer supporting a catalytic noble metal having catalytic activity. In the exhaust gas purifying catalyst of the present invention, the carrier substrate is preferably in the form of a honeycomb, pellets or particles. The carrier substrate preferably contains a heat-resistant inorganic substance such as cordierite or a metal. By using the carrier base material having the above characteristics, the catalytic activity of the exhaust gas purifying catalyst can be maintained even under high temperature conditions.

  In the present specification, the “first coat layer” and the “second coat layer” mean materials for supporting a catalytic noble metal having catalytic activity. In the exhaust gas purifying catalyst of the present invention, it is preferable that the first coat layer and the second coat layer contain a metal oxide having oxygen storage / release ability. Here, the “oxygen storage / release capability” refers to storing oxygen in the exhaust gas when the air-fuel ratio of the exhaust gas is lean to promote the reduction reaction, and storing when the air-fuel ratio of the exhaust gas is rich. It means the ability to promote the oxidation reaction by releasing oxygen. Examples of the metal oxide include, but are not limited to, a metal A oxide selected from the group consisting of cerium (Ce), zirconium (Zr), and aluminum (Al). It is preferable to contain two or more metal A oxides selected from the group consisting of Ce, Zr and Al. The exhaust gas purification performance of the exhaust gas purification catalyst can be enhanced by using the first coat layer and the second coat layer containing the metal A oxide having oxygen storage / release ability.

  In the present specification, the “catalytic noble metal” means a noble metal having catalytic activity, and examples thereof include, but are not limited to, platinum (Pt), palladium (Pd), and rhodium (Rh). Pt, Pd or Rh is preferred. By using the catalyst noble metal, the exhaust gas purification performance of the exhaust gas purification catalyst can be enhanced.

  In the exhaust gas purifying catalyst of the present invention, the catalyst noble metal loading density is preferably in the range of 0.5 to 5% by mass, more preferably in the range of 0.1 to 3% by mass with respect to the total mass of the exhaust gas purifying catalyst. It is more preferable. The mass of the catalyst noble metal supported on the exhaust gas purifying catalyst is determined by inductively coupled plasma (ICP) emission analysis of the metal component in the solution after dissolving the exhaust gas purifying catalyst using acid or the like. Can be determined by

  The present inventors have found that there is a certain correlation between the oxidation activity of the exhaust gas purification oxidation catalyst and the depth of penetration of phosphorus adsorbed on the catalyst from the catalyst surface.

  FIG. 4 is a cross-sectional view of a prior art exhaust gas purification catalyst. As shown in FIG. 4, in the oxidation catalyst 100 for purifying exhaust gas according to the prior art, a catalytic noble metal 4 is supported on a coat layer 2 coated on the surface of a carrier substrate 1. When exhaust gas is circulated above the coat layer 2, phosphorus is adsorbed on the coat layer 2. At this time, when the oxidation activity of the exhaust gas purifying catalyst 100 is measured, the oxidation activity of the catalyst sharply decreases when the phosphorus adsorption amount is in the range of 0 to 0.1% by mass with respect to the total mass of the catalyst. In the range of ˜0.2% by mass, the decrease is gradual, and in the range exceeding 0.2% by mass, the catalyst activity is almost constant (FIG. 5A).

  When the distribution of phosphorus in the exhaust gas purification oxidation catalyst 100 after exhaust gas circulation is examined, phosphorus penetrates from the surface of the coat layer 2 in contact with the exhaust gas, and the thickness is a distance d (depth of penetration) away from the surface. It is adsorbed by the part up to the directional position. At this time, a linear proportional relationship exists between the penetration depth d and the phosphorus adsorption amount c, and the penetration depth d increases as the phosphorus adsorption amount c increases (FIG. 5B).

  In contrast, in the exhaust gas purifying catalyst of the present invention, the first coat layer contains a catalyst noble metal, and the second coat layer does not contain the catalyst noble metal or in an amount smaller than the first coat layer. The catalyst contains a noble metal. Specifically, the supported amount of the catalyst noble metal contained in the first coat layer is preferably in the range of 0.5 to 5% by mass, and 0.1 to 3% by mass with respect to the total mass of the exhaust gas purifying catalyst. More preferably, it is the range. Further, the supported amount of the catalyst noble metal contained in the second coat layer is preferably in the range of 0 to 2% by mass, and in the range of 0 to 1.2% by mass with respect to the total mass of the exhaust gas purifying catalyst. More preferably. It is particularly preferable that the second coat layer does not contain a catalyst noble metal. Due to this feature, even if phosphorus is adsorbed from the catalyst surface to the second coat layer, the catalyst noble metal contained in the first coat layer does not generate phosphorus poisoning, and even after a long-term operation. High catalytic activity can be maintained.

  FIG. 1 is a cross-sectional view showing a first embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst 101 of the present invention includes a carrier substrate 11, a first coat layer 12 coated on the surface of the carrier substrate 11, and a second coat coated on the surface of the first coat layer 12. And a coat layer 13. The first coat layer 12 contains the catalyst noble metal 14, and the second coat layer 13 does not contain the catalyst noble metal or contains the catalyst noble metal 15 in an amount smaller than that of the first coat layer 12. As explained above, the phosphorus adsorption amount c and the phosphorus penetration depth d are proportional to each other, and the phosphorus adsorption amount c of 0 to 0.1% by mass (relative to the total mass of the catalyst) at which the oxidation activity of the catalyst rapidly decreases. Corresponds to a phosphorus penetration depth d of 0 to 20 μm, and the phosphorus adsorption amount c (relative to the total mass of the catalyst) of 0.1 to 0.2 mass% at which the decrease in the oxidation activity of the catalyst is moderate is 20 to 40 μm of phosphorus penetration. Corresponds to depth d. Therefore, the thickness of the second coat layer 13 is preferably in the range of 20 to 40 μm, and more preferably 20 μm. In addition, the thickness of the first coat layer 12 is preferably in the range of 10 to 100 μm, and more preferably in the range of 30 to 60 μm. By having the second coat layer 13 and the first coat layer 12 having the above thicknesses, it is possible to suppress the occurrence of phosphorus poisoning and maintain high catalytic activity over a long period of time.

  FIG. 2 is a cross-sectional view showing a second embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst 102 of the present invention includes a carrier substrate 21, a first coat layer 22 coated on the surface of the carrier substrate 21, and a second coat coated on the surface of the first coat layer 22. And a coat layer 23. The first coat layer 22 contains the catalyst noble metal 24, and the second coat layer 23 does not contain the catalyst noble metal or contains the catalyst noble metal 25 in an amount smaller than that of the first coat layer 22. Further, the first coat layer 22 and the second coat layer 23 have pores 26 and 27, respectively, and the open porosity of the second coat layer 23 is within a range exceeding the open porosity of the first coat layer 22. is there. Here, the thicknesses of the second coat layer 23 and the first coat layer 22 are preferably in the range defined in the first embodiment. In forming the second coat layer, the pores are added by adding 1 to 30% by volume of a pore-forming agent in the range of 1 to 10 μm to the total volume of the slurry of the second coat layer. It can be produced so that the open porosity of the coat layer 23 exceeds the open porosity of the first coat layer 22. Examples of the pore former include organic pore formers such as starch and water-absorbing polymer, and inorganic pore formers such as carbon and silica gel. By having the pores having the above-mentioned open porosity, the exhaust gas efficiently diffuses to the second coat layer 23 through the pores. Therefore, compared with the exhaust gas purification catalyst 101 of the first embodiment, It becomes possible to improve the CO oxidation activity before poisoning.

In the present specification, the “pores” mean both open pores communicating with the outside of the catalyst and closed pores not communicating. The “open porosity” is calculated by the Archimedes method, and the “total porosity” is calculated by calculating the bulk density from the size and weight of the catalyst and subtracting from 1 the value divided by the true density. The relationship between open porosity, total porosity and closed porosity is as follows:
(Open porosity) = (Total porosity)-(Closed porosity)
It is expressed by the following formula.

  FIG. 3 is a cross-sectional view showing a third embodiment of the exhaust gas purifying catalyst of the present invention. The exhaust gas purifying catalyst 103 of the present invention includes a carrier substrate 31, a first coat layer 32 coated on the surface of the carrier substrate 31, and a second coat coated on the surface of the first coat layer 32. And a coat layer 33. The first coat layer 32 contains the catalyst noble metal 34, and the second coat layer 33 does not contain the catalyst noble metal or contains the catalyst noble metal 35 in an amount smaller than that of the first coat layer 32. Further, the concentration of the catalytic noble metal 34 contained in the first coat layer 32 exists so as to increase as it approaches the bonding surface between the first coat layer 32 and the second coat layer 33. Here, the concentration of the catalyst noble metal 34 may increase gradually as it approaches the bonding surface between the first coat layer 32 and the second coat layer 33, or may increase stepwise. Either case is included in the embodiment. Specifically, the amount of catalyst noble metal supported a contained in the portion from the joining surface of the second coat layer 33 and the first coat layer 32 to the position in the thickness direction away from the distance x, and the second Catalyst contained in a portion from the position in the thickness direction, which is a distance x away from the joint surface between the coat layer 33 and the first coat layer 32, to the joint surface between the first coat layer 32 and the carrier substrate 31 The ratio a: b to the noble metal loading b is preferably in the range of 1.5: 1 to 4: 1, and more preferably in the range of 2: 1 to 3: 1. By supporting the catalyst noble metal at the above-mentioned ratio, it is possible to express the catalytic capacity more efficiently, so that the CO before phosphorous poisoning compared with the exhaust gas purification catalyst 101 of the first embodiment. The oxidation activity can be improved.

  As described above in detail, the exhaust gas purifying catalyst of the present invention can suppress a decrease in CO oxidation activity due to phosphorus poisoning. Therefore, by using the exhaust gas oxidation catalyst of the present invention, it is possible to maintain high catalytic activity over a long period of time.

<1. Preparation of exhaust gas purification catalyst>
[Comparative example]
FIG. 4 shows an exhaust gas purifying catalyst of a comparative example. The exhaust gas purifying catalyst 100 of the comparative example has a carrier substrate 1 and a coat layer 2 coated on the surface of the carrier substrate 1. The coat layer 2 contains Pt as the catalyst noble metal 4.

  Commercially available activated alumina powder and dinitrodiammine platinum aqueous solution were mixed with stirring to prepare a dispersed aqueous solution of alumina powder and platinum salt. This aqueous dispersion was dried and calcined at 500 ° C. for 1 hour in the air to obtain Pt-supported alumina powder supporting 3.9 g of platinum per 240 g of alumina powder. Next, a slurry containing Pt-supported alumina powder, ion-exchanged water, and aluminum hydroxide was prepared, and the slurry was applied to the surface of a monolithic carrier made of cylindrical cordierite having a diameter of 104 mm and a length of 153 mm. Wash coated. The amount of Pt supported was 0.65% by mass relative to the total mass of the catalyst.

[Example 1]
FIG. 1 shows the exhaust gas purifying catalyst of Example 1. The exhaust gas purifying catalyst 101 of Example 1 includes a carrier substrate 11, a first coat layer 12 coated on the surface of the carrier substrate 11, and a second coat coated on the surface of the first coat layer 12. Coating layer 13. The first coat layer 12 contains Pt as the catalyst noble metal 14, and the second coat layer 13 contains Pt as the catalyst noble metal 15 in a smaller amount than the first coat layer.

  The exhaust gas purifying catalyst of Example 1 is prepared in the same procedure as in the comparative example. The second coat layer is prepared in the same procedure as the first coat layer except that platinum is not added in the procedure for preparing the coat layer of the comparative example.

[Example 2]
FIG. 2 shows an exhaust gas purification catalyst of Example 2. The exhaust gas purifying catalyst 102 of Example 2 includes a carrier substrate 21, a first coat layer 22 coated on the surface of the carrier substrate 21, and a second coat coated on the surface of the first coat layer 22. Coating layer 23. The first coat layer 22 contains Pt as the catalyst noble metal 24, and the second coat layer 23 contains Pt as the catalyst noble metal 25 in a smaller amount than the first coat layer. Further, the first coat layer 22 and the second coat layer 23 have pores 26 and 27, respectively, and the open porosity of the second coat layer 23 is within a range exceeding the open porosity of the first coat layer 22. is there.

  The exhaust gas purifying catalyst of Example 2 is prepared in the same procedure as in the comparative example. The second coat layer was prepared in the same procedure as the first coat layer except that platinum was not added and the activated alumina powder was changed to another alumina type in the procedure for preparing the comparative coat layer. To do.

[Example 3]
FIG. 3 shows the exhaust gas purifying catalyst of Example 3. The exhaust gas purifying catalyst 103 of Example 3 includes a carrier substrate 31, a first coat layer 32 coated on the surface of the carrier substrate 31, and a second coat coated on the surface of the first coat layer 32. Coating layer 33. The first coat layer 32 contains Pt as the catalyst noble metal 34, and the second coat layer 33 contains Pt as the catalyst noble metal 35 in a smaller amount than the first coat layer. Further, Pt contained in the first coat layer 32 has a higher concentration in the region on the joint surface side with the second coat layer 33 than in the region on the joint surface side with the carrier substrate 31. Exists.

  The exhaust gas purifying catalyst of Example 3 is prepared by the same procedure as in the comparative example. The first coat layer is prepared so as to be two layers of a first upper coat layer and a first lower coat layer. The activated alumina powder is prepared so as to be the same amount in all the coating layers. Platinum is prepared so that the second coat layer: the first upper coat layer: the first lower coat layer = 0: 2: 1.

<2. Performance test>
[Measurement of oxidation activity]
The exhaust gas purifying catalyst of the comparative example was heat-treated at 600 ° C. for 20 hours in an electric furnace in an atmosphere of 1% by mass CO / N ₂ and 10% by mass H ₂ O. The catalyst after the treatment was used as an exhaust gas purification catalyst after the endurance treatment.

0.51 mass% O ₂ ; 14.4 mass% CO ₂ ; 0.41 mass% CO; 0.13 mass% H ₂ ; 0.30 mass% C ₃ H ₆ ; 0.33 mass% The model gas containing NO was circulated at a flow rate of 10 L / min while raising the temperature at 20 ° C./min. The gas temperature upstream of the catalyst when the concentration of CO contained in the gas downstream of the catalyst reaches 50% of the concentration contained in the gas before flowing into the catalyst was defined as CO-T50 (° C). . In this test, a catalyst with a lower CO-T50 value is evaluated as a catalyst with higher CO oxidation performance.

[Measurement of phosphorus adsorption amount]
The catalyst after the gas flow was taken out, the catalyst was divided into 8 equal parts along the gas flow direction from the gas inflow side to the gas discharge side, and each catalyst piece was cut out. Each catalyst piece obtained was ground to obtain powder, and the phosphorus adsorption amount was measured by ICP emission analysis.

[Measurement of phosphorus penetration depth]
The catalyst after the gas flow was taken out, and a catalyst piece of 10 × 10 × thickness direction position (mm) was cut out at a position of 0 mm, 50 mm and 100 mm from the gas inflow side and solidified with an epoxy resin. This catalyst piece was observed from the lateral direction, the depth of phosphorus penetration was measured, and the average value of three points was calculated.

  FIG. 5A shows the relationship between the phosphorus adsorption amount and CO-T50, and FIG. 5B shows the relationship between the phosphorus penetration depth from the catalyst surface and the phosphorus adsorption amount.

  As shown in FIG. 5A, the CO oxidation activity of the catalyst decreased when the phosphorus adsorption amount exceeded 0.1 mass%. Further, as shown in FIG. 5B, phosphorus was poisoned from the surface of the coat layer, and as the amount of phosphorus adsorbed on the catalyst increased, the depth of phosphorus penetration from the coat layer surface increased. Here, in the case of 0.1% by mass, which is the threshold value of the phosphorus adsorption amount at which the CO oxidation activity of the catalyst decreases, the phosphorus penetration depth is about 20 μm.

  Based on the above results, FIG. 6 shows the relationship between the phosphorus penetration depth from the catalyst surface and the CO-T50 in the exhaust gas purifying catalysts of Examples 1 to 3 and Comparative Example.

  As shown in FIG. 6, in the catalyst of the comparative example, when phosphorus was poisoned in the part up to the position in the thickness direction 20 μm away from the catalyst surface, the CO oxidation activity of the catalyst decreased sharply. On the other hand, when phosphorus is poisoned in the part up to the position in the thickness direction 20 to 40 μm away from the catalyst surface, the decrease in the CO oxidation activity of the catalyst is gradual and the thickness direction 40 μm away from the catalyst surface When phosphorus was poisoned up to the position, the CO oxidation activity of the catalyst hardly changed.

  Therefore, when the exhaust gas purifying catalysts of Examples 1 to 3 are tested under the same conditions as described above, the portion leading to the position in the thickness direction 40 μm away from the catalyst surface is the second coat layer. Phosphorus poisoning remains in the second coat layer, and the first coat layer can maintain high CO oxidation activity without being affected by phosphorus poisoning. Here, in the case of the exhaust gas purification catalyst of Example 1, the Pt contained in the second coat layer 13 is less than the Pt contained in the first coat layer 12, so that the exhaust gas purification catalyst of the comparative example Compared with NO, the CO oxidation activity before phosphorus poisoning (initial) is slightly reduced (Fig. 6). On the other hand, in the case of the exhaust gas purification catalyst of Example 2, the exhaust gas reaches the second coat layer 23 through the pores 26 and 27 formed in the first coat layer 22 and the second coat layer 23. In order to diffuse efficiently, the CO oxidation activity before phosphorus poisoning can be improved compared to the exhaust gas purification catalyst of Example 1 (FIG. 6). Similarly, in the case of the exhaust gas purifying catalyst of Example 3, the concentration of Pt contained in the first coat layer 32 approaches the bonding surface between the first coat layer 32 and the second coat layer 33. Since the catalyst capacity can be expressed more efficiently, the CO oxidation activity before phosphorus poisoning can be improved compared to the exhaust gas purification catalyst of Example 1. (Figure 6).

  According to the present invention, it is possible to suppress a decrease in CO oxidation activity due to phosphorus poisoning of an exhaust gas purification catalyst. Therefore, it is useful as an oxidation catalyst for exhaust gas for use in oxidation purification of HC and SOF contained in exhaust gas of gasoline engines and diesel engines.

101 ... Exhaust gas purification catalyst of the present invention
102 ... Exhaust gas purification catalyst of the present invention
103 ... Exhaust gas purification catalyst of the present invention
100 ... Exhaust gas purification catalyst of the prior art
1, 11, 21, 31 ・ ・ ・ Support substrate
2 ... Coat layer
12, 22, 32 ... First coat layer
13, 23, 33 ... Second coat layer
4, 14, 15, 24, 25, 34, 35 ・ ・ ・ Catalyst noble metal
26, 27 ... Porosity

Claims (5)

  A carrier substrate; a first coat layer coated on the surface of the carrier substrate; and a second coat layer coated on the surface of the first coat layer, wherein the first coat layer is a catalyst noble metal. And the second coat layer does not contain the catalyst noble metal or contains the catalyst noble metal in an amount smaller than that of the first coat layer.   2. The exhaust gas purifying catalyst according to claim 1, wherein the thickness of the second coat layer is in the range of 20 to 40 μm.   The exhaust gas according to claim 1 or 2, wherein the first coat layer and the second coat layer have pores, and the open porosity of the second coat layer is in a range exceeding the open porosity of the first coat layer. Purification catalyst.   The exhaust gas purification according to any one of claims 1 to 3, wherein the concentration of the catalyst noble metal contained in the first coat layer increases as it approaches the bonding surface between the first coat layer and the second coat layer. Catalyst. The exhaust gas purifying catalyst according to any one of claims 1 to 4, wherein the catalytic noble metal is selected from the group consisting of Pt, Pd and Rh.

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