JP2009000648A - Exhaust gas cleaning catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst which does not use Pt and is more excellent in durability than in the case when Pt is used. <P>SOLUTION: The catalyst is a two-layer structural catalyst consisting of a lower catalyst coat layer 2 containing a catalyst powder, wherein Pd is supported on a carrier containing at least a Ce-Zr composite oxide, and a upper catalyst coat layer 3 containing a catalyst powder, wherein Rh is supported by a carrier consisting of at least one of zirconia and the Ce-Zr composite oxide. Pd is supported by the Ce-Zr composite oxide so that PdO with a higher activity is formed from Pd by an oxygen absorb-discharge capacity and a high oxidation activity based on Pt is expressed. Then, the carrier consisting of at least one of zirconia and the Ce-Zr composite oxide is excellent in heat resistance and Rh supported by it is controlled in particle growth. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst such as a three-way catalyst.

Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst that purifies by performing CO and HC oxidation and NO _x reduction simultaneously in exhaust gas at a stoichiometric air-fuel ratio (stoichiometric) has been used. As such a three-way catalyst, for example, a coating layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), palladium (Pd) or the like is formed on the coating layer. Those carrying a noble metal are widely known. In addition, a three-way catalyst that uses ceria having an oxygen storage capacity and expands the purification window is also known.

Of the noble metals, Pt and Pd mainly contribute to the oxidation and purification of CO and HC, Rh mainly contributes to the reduction and purification of NO _x , and Rh has an action of preventing sintering of Pt or Pd. Therefore, it has been found that the combined use of Pt or Pd and Rh suppresses the disadvantage that the activity is lowered due to the reduction of the active site due to sintering, and improves the heat resistance. Therefore, it is known that it is desirable to use Pt or Pd in combination with Rh in the three-way catalyst.

  By the way, in order to respond to the recent tightening of exhaust gas regulations, many two-catalyst systems comprising a startup catalyst and an underfloor catalyst have been adopted. However, in this two-catalyst system, the startup catalyst is installed directly under the engine, so the temperature of the catalyst during use rises considerably compared to the underfloor catalyst, and the effect of suppressing sintering of Pt and Pd by Rh is reduced. . In addition, when Pt and Rh are used in combination, Pt and Rh are alloyed at high temperatures, and it has also been clarified that both Pt oxidation ability and Rh reduction ability are reduced.

  Further, there are unfavorable combinations between the noble metal species and the support species depending on the use conditions. For example, in a catalyst in which Rh is supported on alumina, there is a problem that Rh is dissolved in alumina in a high-temperature oxidizing atmosphere of 900 ° C. or more, and the performance is remarkably deteriorated.

  Three-way catalysts are strongly required to have high temperature durability of 900 ° C or higher. For that purpose, it is an important subject to suppress the deterioration of the catalyst. Furthermore, Rh is extremely scarce in terms of resources, and it is desired to efficiently use Rh and suppress its deterioration to increase heat resistance.

  Therefore, a catalyst layer in which Rh is supported on zirconia is mixed with alumina to form a coat layer. By supporting Rh on zirconia in this manner, solid solution of Rh in alumina described above can be prevented, and deterioration of Rh can be suppressed.

  Furthermore, an exhaust gas purifying catalyst has been proposed in which the coat layer has a two-layer structure and separates and supports plural kinds of noble metals. For example, Japanese Patent Laid-Open No. 05-293376 discloses an exhaust gas purifying catalyst in which Rh is supported on the outermost layer of a coat layer and Pt or Pd is supported on the inner layer. Japanese Laid-Open Patent Publication No. 06-063403 discloses a first coat layer containing Pt or Pd and a second coat layer provided on the first coat layer and containing Rh. In the second coat layer, cerium and An exhaust gas purifying catalyst containing a metal oxide powder mainly containing zirconium has been proposed.

  Japanese Patent Application Laid-Open No. 2004-298813 includes a Pt-supported catalyst in which Pt is supported on alumina on the surface of a support substrate and a ceria-zirconia composite oxide (hereinafter referred to as Ce-Zr composite oxide). A three-way catalyst comprising: a lower catalyst layer; a Ce-Zr composite oxide or an Rh-supported catalyst in which Rh is supported on alumina; and an upper catalyst layer comprising at least one of alumina and Ce-Zr composite oxide. Proposed.

  However, in a use environment where the engine is stopped intermittently, the frequency of the lean atmosphere becomes extremely high. Therefore, even in the case of a three-way catalyst in which two catalyst layers are formed and Pt and Rh are separated and supported, the movement of noble metals is promoted in such an environment, resulting in purification after endurance. There was a problem that the performance deteriorated.

Pd is excellent in oxidation activity in a low temperature range, and efficiently oxidizes and purifies HC in low temperature exhaust gas. Therefore, a three-way catalyst mainly composed of Pd has been developed. However, since Pd is easily alloyed with Rh, it is desirable to carry it separately by the above-described method. Therefore, JP-A-11-151439 discloses a lower coating layer in which Pd is supported on a ceria-rich Ce-Zr composite oxide, and an upper coating layer in which Pt and Rh are supported on a zirconia-rich Ce-Zr composite oxide. A three-way catalyst is proposed.
Japanese Patent Laid-Open No. 05-293376 Japanese Patent Laid-Open No. 06-063403 JP 2004-298813 A Japanese Patent Laid-Open No. 11-151439

  However, in the conventional three-way catalyst, the degree of decrease in purification performance at the time of high temperature durability is still large, and it is required to maintain the purification performance after durability even higher. Thus, as a result of intensive studies, the inventors of the present application have clarified that not only Rh but also Pd, the activity at the time of high-temperature durability greatly decreases when Pt coexists.

  This invention is made | formed in view of the said situation, and makes it the problem which should be solved to make it the catalyst excellent in durability rather than the case where Pt is used, without using Pt.

  The exhaust gas purifying catalyst of the present invention that solves the above problems is characterized in that a carrier substrate, a lower catalyst coat layer formed on the surface of the carrier substrate, and an upper catalyst coat layer formed on the surface of the lower catalyst coat layer The lower catalyst coat layer includes a catalyst powder in which Pd is supported on a support containing at least a Ce-Zr composite oxide, and the upper catalyst coat layer includes a zirconia and Ce-Zr composite. The object is to include a catalyst powder in which Rh is supported on a support made of at least one of oxides.

  The Ce—Zr composite oxide contained in the lower catalyst coat layer preferably has a Ce / Zr atomic ratio greater than 1.

  Further, the Ce—Zr composite oxide contained in the upper catalyst coat layer preferably has a Ce / Zr atomic ratio of less than 1.

  The exhaust gas purifying catalyst of the present invention has a lower catalyst coat layer in which Pd is supported on a carrier containing at least a Ce-Zr composite oxide. By containing at least the Ce-Zr composite oxide in the support, PdO having higher activity is generated from Pd by its oxygen absorption / release ability, and high oxidation activity according to Pt is expressed. Although Pd has lower heat resistance than Rh, it becomes PdO 2 and is supported on the lower layer where the heat of the exhaust gas is difficult to transmit, thereby suppressing Pd grain growth. Accordingly, the lower catalyst coat layer exhibits high oxidation activity even after high temperature durability.

  In one upper catalyst coat layer, Rh is supported on a support made of at least one of zirconia and Ce—Zr composite oxide. A carrier composed of at least one of zirconia and Ce-Zr composite oxide is excellent in heat resistance, and Rh supported thereon also suppresses grain growth. Therefore, even in the upper catalyst coat layer in which the heat of exhaust gas is directly transmitted, the degree of reduction in purification activity during high temperature durability is small, and reduction activity with high Rh is exhibited even after high temperature durability.

  That is, according to the exhaust gas purifying catalyst of the present invention, higher purifying activity is exhibited than when Pt is used after high temperature durability without using Pt.

  If a Ce—Zr composite oxide having a Ce / Zr atomic ratio greater than 1 is used for the lower catalyst coat layer, the oxygen absorption / release capability is further increased, so that Pd is more likely to become PdO 2. Therefore, the grain growth of Pd is further suppressed and the oxidation activity by Pd is further improved.

  Further, if a Ce—Zr composite oxide having a Ce / Zr atomic ratio smaller than 1 is used for the upper catalyst coat layer, the durability of the purification activity is further improved. This is considered to further improve the stability of Rh.

  The exhaust gas purifying catalyst of the present invention comprises a support substrate, a lower catalyst coat layer formed on the surface of the support substrate, and an upper catalyst coat layer formed on the surface of the lower catalyst coat layer.

  As the carrier substrate, a monolith honeycomb substrate formed from a heat-resistant oxide such as cordierite, a metal honeycomb substrate formed from metal, and the like can be used. In addition to the honeycomb shape, the shape may be a foam shape, a pellet shape, or the like.

  The lower catalyst coat layer formed on the surface of the support substrate is formed by supporting Pd on a support containing at least a Ce-Zr composite oxide. This support contains at least the Ce—Zr composite oxide, and the support can be composed of only the Ce—Zr composite oxide. However, Ce-Zr composite oxide has a specific surface area smaller than that of activated alumina or the like, and it may be difficult to support a desired amount of Pd. In such a case, a porous oxide having a large specific surface area such as activated alumina may be used in combination.

  However, as the amount of activated alumina or the like increases, the oxygen absorption / release capability of the entire support decreases, so the amount of Ce—Zr composite oxide is preferably 20% by weight or more of the support. Further, it is preferable that 20% by mass or more of Pd is supported on the Ce—Zr composite oxide.

  The Ce-Zr composite oxide used for the lower catalyst coat layer preferably has a Ce / Zr atomic ratio of greater than 1. That is, in terms of mass%, it is desirable that ceria is contained in the Ce-Zr composite oxide in an amount exceeding 58.3 mass%. By using such a Ce-Zr composite oxide with a large amount of ceria, the oxygen absorption / release capability is further enhanced. Therefore, PdO is more easily generated from Pd, the oxidation activity of Pd is further improved, and the grain growth of Pd during high temperature durability is further suppressed.

  The ceria in the Ce—Zr composite oxide used for the lower catalyst coat layer is desirably 95% by mass or less. When ceria exceeds 95% by mass, the thermal stability of the Ce-Zr composite oxide is lowered, so that the grain growth of the supported Pd is likely to occur and the high temperature durability is lowered. Further, the Ce-Zr composite oxide may contain rare earth element oxides such as Nd, Y, La, and Pr. By including such an oxide, the thermal stability of the Ce—Zr composite oxide is further improved. Even in this case, it is desirable that ceria is contained in an amount of 55% by mass or more.

  The amount of Pd supported on the lower catalyst coat layer is preferably in the range of 0.1 to 20 g per liter of the apparent volume of the catalyst. The “apparent volume” means a true volume in the case of a pellet-shaped carrier substrate, and a volume including the volume of a cell passage in the case of a honeycomb-shaped carrier substrate. If the loading amount of Pd is less than 0.1 g / L, the initial purification activity is not sufficient, and even if the loading exceeds 20 g / L, the purification performance is saturated and wasteful Pd is loaded.

The upper catalyst coat layer formed on the surface of the lower catalyst coat layer is formed by supporting Rh on a carrier made of at least one of zirconia and Ce-Zr composite oxide. A support made of only zirconia or a Ce-Zr composite oxide may be used, or a support made of a mixture of zirconia and Ce-Zr composite oxide may be used. By using a carrier containing at least zirconia, the activity of the steam reforming reaction by Rh is improved, and the reduction activity of NO _x is improved. If other porous oxides such as alumina are used, the activity is greatly reduced by the solid solution of Rh, so other porous oxides should not be used. However, alumina, which is inevitably included as a binder component, is not limited to this.

  The Ce-Zr composite oxide used for the upper catalyst coat layer preferably has a Ce / Zr atomic ratio of less than 1. That is, in terms of mass%, it is desirable that zirconia is contained in the Ce-Zr composite oxide in an amount exceeding 41.7 mass%. Thus, the activity of the steam reforming reaction by Rh is improved by using the Ce-Zr composite oxide containing a large amount of zirconia.

  Further, the Ce—Zr composite oxide of the upper catalyst coat layer may contain an oxide of rare earth elements such as Nd, Y, La, and Pr. By including such an oxide, the thermal stability of the Ce—Zr composite oxide is further improved. Even in this case, it is desirable that zirconia is contained in an amount of 35% by mass or more.

  The amount of Rh supported on the upper catalyst coat layer is preferably in the range of 0.01 to 5 g per 1 L of the apparent volume of the catalyst. If the loading amount of Rh is less than 0.01 g / L, the initial purification activity is not sufficient, and even if the loading exceeds 5 g / L, the purification performance is saturated and wasteful Rh is loaded.

  The amount of formation of the lower catalyst coat layer and the upper catalyst coat layer is not particularly limited. For example, in the case of a honeycomb-shaped carrier substrate, the lower catalyst coat layer is in the range of 5 to 300 g per 1 L apparent volume of the carrier substrate. The total amount of the layer and the upper catalyst coat layer is preferably in the range of 400 g or less per 1 L of the apparent volume of the support substrate. If the coating amount of each of the lower catalyst coat layer and the upper catalyst coat layer is less than the above range, the supported noble metal may grow. If the total coating amount exceeds the above range, the exhaust pressure loss will increase.

  Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples. In the following description, a Ce-Zr composite oxide having a Ce / Zr atomic ratio of less than 1 is referred to as a Zr-Ce composite oxide powder and distinguished from a Ce-Zr composite oxide having a Ce / Zr atomic ratio of greater than 1. To do.

  In Table 1, the structure of the catalyst of each Example and each comparative example is shown collectively.

(Example 1)
A predetermined amount of a commercially available Ce-Zr composite oxide powder (ceria: 60% by weight) is impregnated with a predetermined amount of a palladium nitrate aqueous solution having a predetermined concentration, dried overnight at 120 ° C, and calcined at 500 ° C for 2 hours. Pd / CZ powder carrying Pd was prepared.

  The lower layer slurry was prepared by mixing 70 parts by mass of this Pd / CZ powder, 70 parts by mass of activated alumina powder, alumina sol as a binder and distilled water.

  Meanwhile, a predetermined amount of a commercially available Zr-Ce composite oxide powder (ceria: 20% by mass) is impregnated with a predetermined amount of a rhodium nitrate aqueous solution of a predetermined concentration, dried at 120 ° C overnight, and then fired at 500 ° C for 2 hours. Thus, Rh / ZC powder carrying Rh was prepared. The amount of Rh supported in the Rh / ZC powder is 0.4% by mass.

  The upper layer slurry was prepared by mixing 60 parts by mass of the Rh / ZC powder, 60 parts by mass of the activated alumina powder, alumina sol as a binder and distilled water.

  Next, prepare a cordierite honeycomb-shaped carrier substrate (diameter 103 mm, length 105 mm, cell density 600 cpsi), wash the lower layer slurry first, dry at 120 ° C for 2 hours, and then fire at 500 ° C for 2 hours. Thus, a lower catalyst coat layer was formed. The lower catalyst coat layer was formed in an amount of 150 g per liter of the carrier substrate.

  Next, the upper layer slurry was wash coated, dried at 120 ° C. for 2 hours and then calcined at 500 ° C. for 2 hours to form an upper catalyst coat layer on the surface of the lower catalyst coat layer. The upper catalyst coat layer was formed in an amount of 130 g per liter of the carrier substrate.

  In the three-way catalyst of Example 1 obtained in this way, as shown in FIG. 1, the lower catalyst coat layer 2 supporting Pd was formed on the surface of the carrier substrate 1, and Rh was added to the surface of the lower catalyst coat layer 2. A supported upper catalyst coat layer 3 is formed. The amount of Pd supported is 3 g per liter of the carrier substrate, and the amount of Rh supported is 0.4 g per liter of the carrier substrate.

(Comparative Example 1)
A predetermined amount of a commercially available Zr—Ce composite oxide powder (ceria: 20% by mass) is impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, dried at 120 ° C. overnight, and then calcined at 500 ° C. for 2 hours. Rh / ZC powder carrying Rh was prepared.

  Next, a predetermined amount of palladium nitrate aqueous solution having a predetermined concentration is impregnated with a predetermined amount of Rh / ZC powder, dried at 120 ° C. overnight and then calcined at 500 ° C. for 2 hours to carry Pd—Rh / ZC carrying Pd. A powder was prepared.

  Then, 130 parts by mass of this Pd—Rh / ZC powder, 130 parts by mass of activated alumina powder, alumina sol as a binder and distilled water were mixed to prepare a slurry.

  Next, this slurry was wash-coated on the same carrier substrate as in Example 1, dried at 120 ° C. for 2 hours and then calcined at 500 ° C. for 2 hours to form a catalyst coat layer in which Rh and Pd were co-supported. The catalyst coat layer was formed in an amount of 280 g per liter of the carrier substrate. The amount of Rh and Pd supported is the same as in Example 1.

(Comparative Example 2)
The same amount of the lower catalyst coat layer and the upper catalyst coat layer was formed in the same manner as in Example 1 except that the lower layer slurry and the upper layer slurry were further mixed with a dinitrodiammineplatinum nitrate aqueous solution, respectively. The amount of Pd supported is 3 g per liter of the carrier substrate, the amount of Rh supported is 0.4 g per liter of the carrier substrate, and the amount of Pt supported is 1.5 g per liter of the carrier substrate.

<Test Example 1>
Example 1 and Comparative Example 1-2 were installed in the exhaust system of a gasoline engine burned in a stoichiometric to lean atmosphere (A / F ≈ 15), respectively, and maintained at a catalyst bed temperature of 950 ° C for 50 hours. Went. Next, for each catalyst after the endurance test, the purification rate of HC, CO and NO _x at the time of temperature increase from 200 ° C. to 450 ° C. (temperature increase rate: 10 ° C./min) in a stoichiometric atmosphere was continuously measured. The 50% purification temperature was measured. The results are shown in FIG.

  From FIG. 2, it is clear that the catalyst of Example 1 has a higher purification performance after endurance at high temperatures than the catalyst of Comparative Example 1-2. That is, from the comparison between Example 1 and Comparative Example 1, it is better to carry Rh and Pd separated and supported in two layers, and the high temperature durability is superior. From the comparison between Example 1 and Comparative Example 2, Pt It is clear that the high temperature durability is superior when no is contained.

(Example 2)
An upper layer slurry was prepared in the same manner as in Example 1 except that zirconia powder was used instead of the Zr-Ce composite oxide powder (ceria: 20% by mass). A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 3)
An upper layer slurry was prepared in the same manner as in Example 1 except that the Zr-Ce composite oxide powder (ceria: 10% by mass) was used instead of the Zr-Ce composite oxide powder (ceria: 20% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

Example 4
An upper layer slurry was prepared in the same manner as in Example 1 except that the Zr-Ce composite oxide powder (ceria: 40% by mass) was used instead of the Zr-Ce composite oxide powder (ceria: 20% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 5)
An upper layer slurry was prepared in the same manner as in Example 1 except that the Zr-Ce composite oxide powder (ceria: 50% by mass) was used instead of the Zr-Ce composite oxide powder (ceria: 20% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 6)
A lower layer slurry was prepared in the same manner as in Example 1 except that Ce-Zr composite oxide powder (ceria: 50% by mass) was used instead of Ce-Zr composite oxide powder (ceria: 60% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 7)
A slurry for the lower layer was prepared in the same manner as in Example 1 except that Ce-Zr composite oxide powder (ceria: 70% by mass) was used instead of Ce-Zr composite oxide powder (ceria: 60% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 8)
A lower layer slurry was prepared in the same manner as in Example 1 except that Ce-Zr composite oxide powder (ceria: 80% by mass) was used instead of Ce-Zr composite oxide powder (ceria: 60% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

Example 9
A lower layer slurry was prepared in the same manner as in Example 1 except that Ce-Zr composite oxide powder (ceria: 90% by mass) was used instead of Ce-Zr composite oxide powder (ceria: 60% by mass). did. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used. The amount of Rh and Pd supported is the same as in Example 1.

(Example 10)
A predetermined amount of palladium nitrate aqueous solution having a predetermined concentration was impregnated into a predetermined amount of activated alumina powder, dried at 120 ° C. overnight and then calcined at 500 ° C. for 2 hours to prepare Pd / Al ₂ O ₃ powder supporting Pd. .

Next, 70 parts by mass of Pd / CZ powder prepared in the same manner as in Example 1, 70 parts by mass of Pd / Al ₂ O ₃ powder, alumina sol as a binder and distilled water were mixed to prepare a lower layer slurry. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used instead of the lower layer slurry used in Example 1.

The amount of Rh and Pd supported is the same as in Example 1. Further, Pd in the lower catalyst coat layer is supported on the Ce-Zr composite oxide powder and the Al ₂ O ₃ powder by the same amount.

(Example 11)
20 parts by mass of Pd / CZ powder prepared in the same manner as in Example 1, 80 parts by mass of Pd / Al ₂ O ₃ powder prepared in Example 8, and alumina sol as a binder and distilled water were mixed. A slurry for the lower layer was prepared. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used instead of the lower layer slurry used in Example 1.

The amount of Rh and Pd supported is the same as in Example 1. In the lower catalyst coat layer, 20% of Pd is supported on the Ce—Zr composite oxide powder and 80% is supported on the Al ₂ O ₃ powder.

(Comparative Example 3)
Rh / Al ₂ O ₃ powder supporting Rh was prepared by impregnating a predetermined amount of an active alumina powder with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration, drying at 120 ° C. overnight, and firing at 500 ° C. for 2 hours. .

Next, 60 parts by mass of the same Zr—Ce composite oxide powder (ceria: 20% by mass) as in Example 1, 60 parts by mass of this Rh / Al ₂ O ₃ powder, alumina sol as a binder and distilled water were mixed. A slurry for the upper layer was prepared. A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used instead of the upper layer slurry used in Example 1.

The amount of Rh and Pd supported is the same as in Example 1. The total amount of Rh in the upper catalyst coat layer is supported on Al ₂ O ₃ powder.

(Comparative Example 4)
60 parts by mass of Rh / ZC powder prepared in the same manner as in Example 1, 60 parts by mass of Rh / Al ₂ O ₃ powder prepared in the same manner as in Comparative Example 3, alumina sol as a binder and distilled water were mixed. An upper layer slurry was prepared. A three-way catalyst was prepared in the same manner as in Example 1 except that this upper layer slurry was used instead of the upper layer slurry used in Example 1.

The amount of Rh and Pd supported is the same as in Example 1. Further, the same amount of Rh is supported on the Zr—Ce composite oxide powder and the Al ₂ O ₃ powder in the upper catalyst coat layer.

(Comparative Example 5)
70 parts by mass of Ce-Zr composite oxide powder (ceria: 60% by mass) similar to Example 1, 70 parts by mass of Pd / Al ₂ O ₃ powder prepared in the same manner as Example 10, and alumina sol as a binder And distilled water was mixed and the slurry for lower layers was prepared. A three-way catalyst was prepared in the same manner as in Example 1 except that this lower layer slurry was used instead of the lower layer slurry used in Example 1.

The amount of Rh and Pd supported is the same as in Example 1. Further, the entire amount of Pd in the lower catalyst coat layer is supported on Al ₂ O ₃ powder.

<Test Example 2>
The catalysts of Examples 1 to 11 and Comparative Examples 3 to 5 were subjected to a high temperature durability test in the same manner as in Test Example 1, and the 50% purification temperature was measured in the same manner as in Test Example 1 for each catalyst after the high temperature durability. did. The results are shown in FIGS.

  FIG. 4 shows the results of Examples 1 to 5, and shows the relationship between the amount of ceria in the Zr—Ce composite oxide of the upper catalyst coat layer and the 50% purification temperature. 4 that the amount of ceria in the Zr—Ce composite oxide of the upper catalyst coating layer is preferably less than 40% by mass, that is, the amount of zirconia is preferably 60% by mass or more.

  FIG. 5 shows the results of Example 1 and Examples 6 to 9, and shows the relationship between the amount of ceria in the Ce—Zr composite oxide of the catalyst coat layer and the 50% purification temperature. FIG. 5 shows that the amount of ceria in the Ce—Zr composite oxide of the lower catalyst coat layer is preferably in the range of 60 mass% to 90 mass%.

FIG. 6 shows the results of Example 1, Examples 10 to 11 and Comparative Example 5, and shows the influence of the type of the carrier supporting Pd. FIG. 6 shows that the activity after high-temperature durability decreases as the proportion of Pd supported on the Al ₂ O ₃ powder increases. Pd in the lower catalyst coat layer is supported on the Ce-Zr composite oxide powder. It turns out that is desirable.

It is typical sectional drawing which expands and shows the principal part of the catalyst for exhaust gas purification which concerns on one Example of this invention. It is a graph which shows 50% purification temperature. Is a graph showing the relationship between the amount of CeO ₂ and 50% purification temperatures in the upper catalyst coating layer. Is a graph showing the relationship between the amount of CeO ₂ and 50% purification temperature of lower catalyst coating layer. It is a graph which shows 50% purification temperature. It is a graph which shows 50% purification temperature.

Explanation of symbols

  1: Support base material 2: Lower catalyst coat layer 3: Upper catalyst coat layer

Claims (3)

An exhaust gas purification catalyst comprising: a carrier substrate; a lower catalyst coat layer formed on the surface of the carrier substrate; and an upper catalyst coat layer formed on the surface of the lower catalyst coat layer,
The lower catalyst coat layer includes a catalyst powder in which palladium is supported on a carrier containing at least a ceria-zirconia composite oxide, and the upper catalyst coat layer is rhodium on a carrier made of at least one of zirconia and ceria-zirconia composite oxide. A catalyst for purification of exhaust gas, comprising a catalyst powder on which is supported.   The exhaust gas-purifying catalyst according to claim 1, wherein the ceria-zirconia composite oxide contained in the lower catalyst coat layer has a Ce / Zr atomic ratio larger than 1.   The exhaust gas-purifying catalyst according to claim 1, wherein the ceria-zirconia composite oxide contained in the upper catalyst coat layer has a Ce / Zr atomic ratio smaller than 1.

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