JP5966594B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

The present invention relates to an exhaust gas purification catalyst and a method for producing the same .

  Conventionally, in order to purify HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxide), etc. discharged from the engine of an automobile, Pt, Pd, Rh, etc. are used as a support material made of oxide. An exhaust gas purifying catalyst carrying a catalytic metal is used. Since these catalytic metals are rare metals and are expensive, it is required to reduce the amount used. In this regard, even though the catalyst metal is initially dispersedly supported on the support material, when the engine is operated at a high load and exposed to high-temperature exhaust gas, the catalyst metal moves and aggregates on the support material, thereby reducing the surface area. Decreases and decreases active sites. As a result, the contact opportunity between the catalyst metal and the exhaust gas is reduced, and the exhaust gas purification performance of the catalyst is lowered. For this reason, conventionally, a large amount of the catalyst metal is previously supported on the support material in anticipation of performance degradation due to aggregation of the catalyst metal.

  By the way, the exhaust gas purifying catalyst is configured to have, for example, one to three catalyst layers on the cell wall surface of the honeycomb carrier (base material), and the catalyst layer is provided with a catalyst metal, its support material, and a binder. Yes. In this case, pores through which the exhaust gas diffuses and moves are formed between the oxide particles constituting the support material. Thus, in addition to the support material, an oxide powder called a diluent is also added to the catalyst layer. Since this diluent forms pores in the catalyst layer like the support material, a catalyst layer having a good exhaust gas diffusibility can be obtained.

  For example, Patent Document 1 discloses an exhaust gas purifying catalyst having a catalyst layer having a two-layer structure, and the catalyst layer further includes Rh / ZrLa− in which Rh is supported on activated alumina supporting a ZrLa composite oxide. Alumina powder and La-containing activated alumina powder are contained as catalyst materials. In this case, ZrLa-alumina is a support material for Rh, and La-containing activated alumina powder corresponds to the diluent.

JP 2011-62683 A

  As in Patent Document 1, with a catalyst in which a diluent made of La-containing activated alumina is added to a catalyst layer containing Rh, a catalyst layer with a good exhaust gas diffusibility can be obtained. There is another problem. That is, when the catalyst is heated to about 900 ° C. by the exhaust gas, the thermal vibration of Rh increases. As a result, part of Rh moves and adheres from the support material to La-containing activated alumina as a diluent by transpiration or the like. In this case, Rh is dissolved in alumina and deactivated due to further high-temperature heating of the catalyst, and the activity of the catalyst is reduced. In anticipation of this decrease in activity, it is conceivable to increase the amount of Rh supported on the catalyst, but this is disadvantageous in terms of cost.

  Therefore, the present invention prevents the activity of the catalyst from being greatly reduced even when Rh adheres to the diluent by evaporation or the like.

  In order to solve the above problems, the present invention employs a Zr-based composite oxide as a catalyst diluent.

The exhaust gas purifying catalyst presented here includes a catalyst layer containing Rh on a base material, and the catalyst layer containing Rh contains Zr as a main component and at least one of La and Y and contains Ce. A plurality of Zr-based composite oxides not included are contained as a catalyst material. One of the Zr-based composite oxides contained in the Rh-containing catalyst layer contains La as an essential component and is supported on activated alumina. The Rh is supported on a support material in which the Zr-based composite oxide is supported on the activated alumina. That is, the Rh-containing catalyst layer contains the support material on which the Rh is supported and the remaining Zr-based composite oxide of the plurality of types of Zr-based composite oxides.

  Since the Zr-based complex oxide has oxygen ion conductivity, each of the plurality of Zr-based complex oxides releases active oxygen by oxygen ion conduction and contributes to the oxidation purification of HC and CO.

  Here, the Zr-based composite oxide constituting the Rh support material contains La, so that even if the atmosphere (A / F of exhaust gas) changes from lean to rich, it is not reduced so much and is moderately Keep oxidized. This is because, when Rh is supported on a La-containing Zr-based composite oxide, a La—O—Rh bond is easily formed between the Zr-based composite oxide and Rh. This is probably because the oxidation state is hardly affected. As a result, when Rh is reduced, it is disadvantageous for the oxidation of HC and CO. However, according to the present invention, even if it becomes rich, Rh on the Zr-based composite oxide remains in a moderately oxidized state. Therefore, the decrease in HC oxidation ability and CO oxidation ability is reduced. Moreover, since HC and CO are oxidized even in such a rich state, NOx reduction proceeds simultaneously with the oxidation, which is advantageous for NOx reduction purification.

  Further, the La-containing Zr-based composite oxide is supported on activated alumina having a large specific surface area, so that its dispersibility is increased and aggregation can be prevented. As a result, the dispersibility of Rh supported on the Zr-based composite oxide is also increased. This is advantageous for purifying HC, CO, and NOx. In addition, with such a support material, contact between Rh and activated alumina is reduced, so that solid solution of Rh in activated alumina is suppressed, which is advantageous in maintaining catalyst performance.

  Of the plurality of types of Zr-based composite oxides, the remaining Zr-based composite oxides that do not constitute the support material exhibit an effect as a diluent. That is, the remaining Zr-based composite oxide increases the exhaust gas diffusibility of the catalyst layer, thereby increasing the chance of contact between the exhaust gas and Rh. Therefore, it becomes advantageous for purification of exhaust gas.

  Here, the catalyst layer may contain a zirconia-based binder. However, unlike the Zr-based composite oxide as a catalyst material, the binder has a small particle size so as to perform the binder function. This is a factor that hinders gas diffusion. This is a significant difference between a Zr-based composite oxide as a catalyst material and a zirconia-based binder. The number average particle size of the Zr-based composite oxide serving as a diluent is 30 nm to 50 nm, and the number average particle size of the zirconia binder is 8 nm to 15 nm.

  Then, when the thermal vibration of Rh of the support material increases due to the high temperature heat of the catalyst, this Rh may move away from the support material due to transpiration or the like. In that case, at least a part of the moving Rh adheres to the Zr-based composite oxide serving as the diluent. However, although Rh is easily dissolved in alumina, it is difficult to dissolve in Zr-based composite oxides. Therefore, unlike in the case of using alumina as a diluent, deactivation due to high-temperature heating can be avoided. That is, the degree of deterioration of the catalyst due to high temperature heat is small.

  In a preferred embodiment of the present invention, the remaining Zr-based composite oxide of the plurality of types of Zr-based composite oxides has a smaller amount of Rh supported than the Zr-based composite oxide constituting the support material, or Rh loading is zero.

  That is, when Rh is uniformly dispersed and supported on a plurality of types of Zr-based composite oxides, the amount of Rh supported by each Zr-based composite oxide particle is reduced (the concentration of Rh is reduced), and the exhaust gas is reduced. Disadvantageous for purification. Therefore, in this aspect, the Rh loading amount of the Zr-based composite oxide constituting the support material is relatively increased by reducing or reducing the Rh loading amount to the remaining Zr-based composite oxide. .

  In a preferred embodiment of the present invention, the remaining Zr-based composite oxide of the plurality of types of Zr-based composite oxides is supported on activated alumina. As a result, the Zr-based composite oxide can be dispersed, which is advantageous for preventing aggregation of the Zr-based composite oxide when heated at high temperatures. Therefore, it is possible to prevent the exhaust gas diffusibility of the catalyst layer from greatly decreasing and to prevent deactivation of Rh adhering due to transpiration.

  A preferred method for producing an exhaust gas purifying catalyst in which a catalyst layer containing Rh is provided on the base material,
  A step of preparing a support material in which a Zr-based composite oxide containing La is supported on activated alumina;
  A step of preparing a first catalyst material in which Rh is supported on the support material;
  A step of preparing Zr as a main component and a Zr-based composite oxide as a second catalyst material containing at least one of La and Y and not containing Ce;
  A step of forming the catalyst layer containing the Rh on the base material by supporting both the first and second catalyst materials on the base material.

  According to the present invention, the catalyst layer containing Rh on the substrate contains Zr as a main component and a plurality of Zr-based composite oxides containing at least one of La and Y and not containing Ce as a catalyst material. One of the Zr-based composite oxides contains La as an essential component and is supported on activated alumina to constitute a support material, and Rh is supported on the support material. The exhaust gas purification performance of the catalyst is enhanced, and even when Rh moves from the support material to the other by transpiration due to the high temperature heat of the catalyst, at least a part of the Rh adheres to the remaining Zr-based composite oxide. The degree of deterioration of the catalyst due to high temperature heat is reduced.

It is sectional drawing which shows the catalyst layer structure of the catalyst for exhaust gas purification. It is a graph which shows the light-off temperature of an Example and a comparative example.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The following description of the preferred embodiments is merely exemplary in nature and is not intended to limit the invention, its application, or its use.

  The exhaust gas purifying catalyst according to the present embodiment is disposed in an exhaust gas passage of an automobile engine and purifies exhaust gas. FIG. 1 shows a catalyst layer structure of an exhaust gas purifying catalyst 1. A catalyst layer 3 is formed on the exhaust gas passage wall of the substrate (honeycomb carrier) 2. The catalyst layer 3 includes an Rh-containing layer (upper layer) 4 and a Pd-containing layer (lower layer) 5. The Pd-containing layer 5 is formed on the exhaust gas passage wall surface of the substrate 2, and the Rh-containing layer 4 is the Pd-containing layer 5. It is layered on top.

The Rh-containing layer 4 contains, as a catalyst material, Rh-supported ZCN, Rh-supported ZrLa / La alumina (first catalyst material) , and Zr (La, Y) that is a diluent (second catalyst material) . The Pd-containing layer 5 contains Pd-supported La alumina, Pd-supported ZCN, and OSC material as catalyst materials. Each of the Rh-containing layer 4 and the Pd-containing layer 5 contains a zirconia binder (Y-stabilized zirconia containing ₃ mol% of Y ₂ O ₃ ) as a binder.

  The exhaust gas purifying catalyst 1 can be prepared by the following method. That is, the base material 2 is immersed and taken out in a slurry obtained by mixing the catalyst material and the binder constituting the Pd-containing layer 5 with ion-exchanged water. Excess slurry adhering to the substrate 2 is removed by air blowing. Thereafter, in the air, the slurry adhering to the substrate 2 is dried (150 ° C.) and fired (held at a temperature of 500 ° C. for 2 hours). Thereby, the Pd-containing layer 5 is formed on the substrate 2. Next, the base material 2 having the Pd-containing layer 5 is dipped in a slurry obtained by mixing the catalyst material and the binder constituting the Rh-containing layer 4 with ion-exchanged water and taken out. Then, as in the case of the Pd-containing layer 5, the excess slurry adhering to the Pd-containing layer 5 is removed by air blowing, the slurry is dried in the atmosphere (150 ° C.), and baked (held at a temperature of 500 ° C. for 2 hours). To do. Thereby, the Rh-containing layer 4 is formed on the Pd-containing layer 5 of the substrate 2.

<About catalyst material>
The catalyst material will be specifically described.

-Catalyst material of Rh-containing layer 4-
The Rh-supported ZCN as the first Rh component is formed by supporting Rh on a ZrCeNd composite oxide (hereinafter referred to as “ZCN”) by evaporation to dryness using an aqueous rhodium nitrate solution. This ZCN powder can be prepared by a coprecipitation method. That is, an 8 times dilution of 28% by mass ammonia water as a basic solution is mixed with a nitric acid solution obtained by mixing cerium nitrate hexahydrate, zirconyl oxynitrate solution, neodymium nitrate hexahydrate, and ion-exchanged water. To obtain a coprecipitate by neutralization. The solution containing this coprecipitate is centrifuged to remove the supernatant (dehydration), and ion-exchanged water is further added and stirred (water washing). Remove basic solution. The coprecipitate after the final dehydration is dried at 150 ° C. in the air, pulverized, and fired at 500 ° C. for 2 hours in the air. Thereby, ZCN powder can be obtained.

  The Rh-supported ZrLa / La alumina as the second Rh component is formed by supporting Rh on the ZrLa / La alumina as the support material. ZrLa / La alumina is a Zr-based composite oxide containing La in addition to Zr as a main component (hereinafter referred to as “La alumina”) on the surface of an active alumina composite oxide containing La (hereinafter referred to as “La alumina”). ZrLa ”)).

Rh-supported ZrLa / La alumina can be prepared as follows. First, a powder of La alumina containing 4% by mass of La ₂ O ₃ is dispersed in a mixed solution of zirconium nitrate and lanthanum nitrate, and ammonia water is added thereto to form a precipitate. The obtained precipitate is filtered, washed, dried at 200 ° C. for 2 hours, and baked at 500 ° C. for 2 hours to obtain La alumina powder having ZrLa supported on the surface. This is mixed with an aqueous rhodium nitrate solution and evaporated to dryness to obtain Rh-supported ZrLa / La alumina powder.

  ZrLa may further contain Y. A Zr-based composite oxide containing La and Y in addition to Zr as a main component is hereinafter referred to as “ZrLaY”. In the following, a Zr-based composite oxide containing Y in addition to Zr as a main component is referred to as “ZrY”.

  Zr (La, Y) as a diluent is a Zr-based composite oxide (ZrLa, ZrY, or ZrLaY) containing Zr as a main component and containing at least one of La and Y. This Zr (La, Y) can be supported on an activated alumina material such as La alumina.

  The powder of Zr (La, Y) can be prepared as follows. That is, ammonia water is added to a mixed solution of zirconium nitrate and lanthanum nitrate and / or yttrium nitrate to form a precipitate. The obtained precipitate is filtered, washed, dried at 200 ° C. for 2 hours, and baked at 500 ° C. for 2 hours to obtain Zr (La, Y) powder.

-Catalyst material of Pd-containing layer 5-
Pd-supported La alumina is obtained by supporting Pd on La alumina containing 4% by mass of La ₂ O ₃ by evaporation to dryness using an aqueous palladium nitrate solution. Pd-supported ZCN is obtained by supporting Pd on ZCN by evaporation to dryness. The OSC material has oxygen storage capacity that stores oxygen in an oxygen-excess atmosphere and releases oxygen in a reducing atmosphere. In this embodiment, ZCN is adopted as the OSC material.

<Exhaust gas purification performance of exhaust gas purification catalyst>
Hereinafter, the exhaust gas purification characteristics of the exhaust gas purification catalyst according to the embodiment will be described based on examples and comparative examples.

[Examples and Comparative Examples]
As shown in Table 1, the following Examples 1 to 8 and Comparative Examples 1 and 2 differ in the second Rh component or diluent of the Rh-containing layer 4, and the other configurations are the same.

Example 1
Rh-supported ZrLa / La alumina was used as the second Rh component of the Rh-containing layer 4, and ZrLaY / La alumina was used as the diluent.

The composition of ZrLa / La alumina as the second Rh component is ZrO ₂ : La ₂ O ₃ : La alumina = 38: 2: 60 (mass ratio). The composition of ZrLaY / La alumina as a diluent is ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ : La alumina = 32: 3: 5: 60 (mass ratio). Table 2 shows the other components excluding the diluent and the blending amount of each component.

As the substrate 2, a cordierite honeycomb carrier (capacity 1 L) having a cell wall thickness of 3.5 mil (8.89 × 10 ⁻² mm) and 600 cells per square inch (645.16 mm ² ) is used. It was.

-Example 2-
ZrY / La alumina was employed as a diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 1. The composition of ZrY / La alumina is ZrO ₂ : Y ₂ O ₃ : La alumina = 38: 2: 60 (mass ratio).

-Example 3-
ZrLaY was employed as a diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 1. The composition of ZrLaY is ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ = 80: 7.5: 12.5 (mass ratio).

Example 4
ZrLa was adopted as a diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 1. The composition of ZrLa is ZrO ₂ : La ₂ O ₃ = 95: 5 (mass ratio).

-Example 5
Rh-supported ZrLaY / La alumina was adopted as the second Rh component of the Rh-containing layer 4, the same ZrY / La alumina as in Example 2 was adopted as the diluent, and the other configurations were the same as in Example 1. The composition of the second Rh component, ZrLaY / La alumina, is ZrO ₂ : La ₂ O ₃ : Y ₂ O ₃ : La alumina = 32: 3: 5: 60 (mass ratio).

-Example 6
As a diluent for the Rh-containing layer 4, ZrLa / La alumina of ZrO ₂ : La ₂ O ₃ : La alumina = 38: 2: 60 (mass ratio) was adopted, and the other configuration was the same as that of Example 5.

-Example 7-
The same ZrLaY as in Example 3 was used as the diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 5.

-Example 8-
The same ZrLa as in Example 4 was adopted as the diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 5.

-Comparative Example 1-
La alumina was employed as a diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 1.

-Comparative Example 2-
La alumina was employed as a diluent for the Rh-containing layer 4, and the other configuration was the same as in Example 5.

[Exhaust gas purification performance evaluation test]
After bench-aging each of the catalysts of Examples 1 to 8 and Comparative Examples 1 and 2, a core sample having a carrier volume of about 25 mL was cut out from each catalyst. In bench aging, the catalyst was attached to the exhaust pipe of the engine, the engine speed was set so that the exhaust gas temperature at the catalyst inlet was 900 ° C., and the catalyst was exposed to the 900 ° C. exhaust gas for 50 hours. During the engine operation period, engine oil added with P was continuously supplied to the intake manifold by a pump.

  Then, the core sample of the catalyst was attached to the model gas flow reactor, and each light-off temperature T50 (° C.) relating to the purification of HC, CO and NOx was measured. That is, the temperature of the model gas flowing into the catalyst was gradually increased from 100 ° C., and changes in the concentrations of HC and CO in the gas flowing out from the catalyst were detected. T50 (° C.) is the catalyst inlet gas temperature when the purification rate of each component of HC, CO and NOx reaches 50%.

The model gas was A / F = 14.7 ± 0.9. That is, the A / F is forced at an amplitude of ± 0.9 by adding a predetermined amount of fluctuation gas in a pulse form at 1 Hz while constantly flowing the main stream gas of A / F = 14.7. Vibrated. The space velocity SV is 60000 h ⁻¹ , and the heating rate is 30 ° C./min. Table 3 shows the gas composition when A / F = 14.7, A / F = 13.8 and A / F = 15.6.

[Test results]
FIG. 2 shows the results of the exhaust gas purification performance evaluation test. As for any T50 of HC, CO, and NOx, Examples 1 to 8 are lower than Comparative Examples 1 and 2, and it can be seen that the exhaust gas purification performance after high temperature heating is excellent. This is recognized to be due to the fact that Zr-based composite oxide Zr (La, Y) is employed as a diluent. That is, when the thermal vibration of Rh of the first Rh component and the second Rh component increases due to the high temperature heat of the catalyst, the Rh moves to the diluent by transpiration or the like. However, unlike the La alumina of the comparative example, the diluent of the example is a Zr-based composite oxide in which Rh hardly dissolves. Therefore, even if Rh adheres to the Zr-based composite oxide, the Rh Can be avoided. Therefore, the degree of deterioration of the catalyst due to high temperature heat is small.

  According to the comparison between Example 1 and Example 2, the comparison between Example 3 and Example 4, and the comparison between Example 7 and Example 8, ZrLaY is better than ZrY as a diluent. According to a comparison between Example 5 and Example 6, ZrY is better than ZrLa as a diluent. From this, it can be said that the diluent preferably contains Zr as a main component and Y in addition, and more preferably contains La and Y.

  Further, from the comparison between Examples 1 and 3 and the comparison between Example 6 and Example 8, as the diluent, the Zr-based composite oxide is supported on the alumina material rather than the Zr-based composite oxide alone. I understand that is good. Further, when Examples 1 to 8 and Comparative Examples 1 and 2 are compared between catalysts having the same diluent, ZrLa / La alumina is generally better than ZrLaY / La alumina for the support material of the second Rh component.

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 2 Base material 3 Catalyst layer 4 Rh containing layer 5 Pd containing layer

Claims (4)

An exhaust gas purifying catalyst in which a catalyst layer containing Rh is provided on a substrate,
The catalyst layer containing Rh contains Zr as a main component and a plurality of Zr-based composite oxides containing at least one of La and Y and not containing Ce,
One of the Zr-based composite oxides contained in the Rh-containing catalyst layer contains La as an essential component and is supported on activated alumina, and the Zr-based composite is supported on the activated alumina. The above Rh is supported on a support material on which an oxide is supported ,
The Rh-containing catalyst layer contains the support material on which the Rh is supported and the remaining Zr-based composite oxide among the plurality of types of Zr-based composite oxides. Catalyst. In claim 1,
The remaining Zr-based composite oxide among the plurality of types of Zr-based composite oxides has a smaller amount of Rh supported than the one type of Zr-based composite oxide, or the Rh supported amount is zero. Exhaust gas purification catalyst. In claim 1 or claim 2,
The remaining Zr-based composite oxide of the plurality of types of Zr-based composite oxides is supported on activated alumina. A method for producing an exhaust gas purifying catalyst, wherein a catalyst layer containing Rh is provided on a substrate,
  A step of preparing a support material in which a Zr-based composite oxide containing La is supported on activated alumina;
  A step of preparing a first catalyst material in which Rh is supported on the support material;
  A step of preparing Zr as a main component and a Zr-based composite oxide as a second catalyst material containing at least one of La and Y and not containing Ce;
  A process for forming the catalyst layer containing the Rh on the base material by supporting both the first and second catalyst materials on the base material. A method for producing a catalyst.

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