JPH10328566A - Exhaust gas purification catalyst - Google Patents

Abstract

(57) [Problem] To suppress deterioration of catalyst purification performance due to the reaction between Rh and alumina even in a high temperature atmosphere of 900 ° C. or higher,
Exhaust gas purification catalyst with improved heat resistance. The device has a first coat layer carrying Pd20 and a second coat layer formed on the surface of the first coat layer and carrying Rh30. The second coat layer has a specific surface area of 50 to 50%.
The main component is 100 m ² / g θ-alumina. θ-alumina is stable at a temperature of 900 to 1000 ° C., Rh hardly forms a solid solution in alumina crystals, and the sintering of Rh accompanying the phase change and grain growth of alumina is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine of an automobile or the like, and more particularly to an exhaust gas purifying catalyst having improved durability.

[0002]

2. Description of the Related Art As an exhaust gas purifying catalyst conventionally automobiles, three-way catalyst for purifying performing the reduction of the oxidized and NO _x CO and HC in the exhaust gas simultaneously is used in the stoichiometric air-fuel ratio (stoichiometric) . As such a three-way catalyst, for example, a coat layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), palladium (P
Those supporting a catalytic noble metal such as d) are widely known. There is also known a three-way catalyst in which ceria (cerium oxide) having oxygen storage capacity is used in combination, and the window width for air-fuel ratio fluctuation of exhaust gas is widened.

Among the catalytic noble metals, Pt and Pd mainly contribute to the purification of CO and HC by oxidation, and Rh mainly represents NO.
_{In addition} to contributing to the reduction and purification of _x , Rh has an effect of preventing sintering of Pt. Therefore, Pt and Rh
It has been found that the combined use of (1) and (2) suppresses a problem that the activity is reduced due to a decrease in the active site due to sintering, and improves heat resistance. Therefore, it is known that it is desirable to use Pt and Rh together in a three-way catalyst.

[0004] However, with the recent improvement in engine performance and increase in high-speed running, the temperature of exhaust gas has been significantly increased. Therefore, the temperature of the exhaust gas purifying catalyst at the time of use is considerably increased as compared with the conventional case, and it is difficult to suppress sintering of Pt even when Rh coexists. The reason for this is that in a lean atmosphere at a high temperature, Rh forms a solid solution in the γ-alumina of the coating layer, thereby decreasing the activity and also reducing the effect of suppressing Pt sintering. It became clear.

[0005] Therefore, today, there is a strong demand for durability of the three-way catalyst at a high temperature of 900 ° C or more, and for that purpose, it is an important issue to suppress the deterioration of the catalyst due to sintering of the noble metal of the catalyst. Further, Rh is extremely scarce in resources, and it is desired to use Rh efficiently and to suppress its deterioration to increase heat resistance. Therefore, an exhaust gas purifying catalyst in which a coat layer has a two-layer structure and a catalytic noble metal is separately supported has been proposed. For example, JP-A-63-1
Japanese Patent No. 97546 discloses a first alumina coat layer in which a noble metal other than Rh is supported as a lower layer, and an ultrafine zirconia powder supporting Rh and having a particle size of 5000 mm or less is formed in an upper layer.
An exhaust gas purifying catalyst having a second alumina coat layer containing 0 to 50% by weight has been proposed.

[0006] By the addition of the zirconia ultrafine powder, Rh
Is preliminarily supported on zirconia, so that Rh reacts with alumina in a high-temperature atmosphere to suppress the diffusion and deactivation of Rh in the alumina crystals, thereby providing excellent durability.

[0007]

However, when the temperature of the exhaust gas is higher than 900 ° C., the zirconia itself has insufficient heat resistance, so that zirconia sinters.
Rh also sinters accordingly. In addition, a part of Rh reacts with γ-alumina to cause a solid solution of Rh in the alumina crystal, resulting in a problem that performance is remarkably reduced.

The present invention has been made in view of such circumstances, and in an exhaust gas purifying catalyst having a coat layer having a two-layer structure and separating and carrying a catalytic noble metal, Rh and alumina are used even in a high-temperature atmosphere of 900 ° C. or more. It is an object of the present invention to provide an exhaust gas purifying catalyst having improved heat resistance, which can suppress a decrease in catalyst purifying performance due to the above reaction.

[0009]

The feature of the exhaust gas purifying catalyst according to claim 1 which solves the above-mentioned problems is that a carrier base material,
An exhaust gas purifying catalyst comprising: a first coat layer formed on a surface of a carrier substrate and supporting at least one of Pt and Pd; and a second coat layer formed on the surface of the first coat layer and supporting Rh. The second coating layer has a specific surface area of 50 to 10
The main component is 0 m ² / g of θ-alumina.

The exhaust gas purifying catalyst according to claim 2, which further improves the above catalyst, is characterized in that, in the exhaust gas purifying catalyst according to claim 1, at least the second coat layer has an exhaust gas inlet of a carrier substrate. Pt and P are added to 1/6 to 1/3 of the entire length of the carrier substrate from the side end surface to the exhaust gas outlet side.
That is, at least one kind of d is supported at a high concentration.

The exhaust gas purifying catalyst according to claim 3, which further improves the catalyst, is characterized in that, in the exhaust gas purifying catalyst according to claim 1, the second coat layer has a volume of 1 liter of the carrier base material per liter. That is, 0.3 to 1.5 g of Rh is supported.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
The main feature is that at least one of Pt and Pd is supported on the first coat layer of the inner layer, and Rh is supported on the second coat layer of the outer layer. Rh has a specific surface area of 50 to
It is carried on a second coat layer containing 100 m ² / g of θ-alumina as a main component. Since this θ-alumina is more stable at temperatures of 900 to 1000 ° C. than γ-alumina, Rh hardly forms a solid solution in alumina crystals,
Furthermore, sintering of Rh due to phase change of alumina and grain growth is suppressed. Accordingly, it is considered that the activity of Rh is sufficiently maintained even at a high temperature, and the exhaust gas purifying catalyst of the present invention is excellent in durability of the purifying performance.

The carrier substrate may be in the form of a honeycomb or a pellet, and may be made of a heat-resistant inorganic oxide such as cordierite, or a flat sheet of a metal foil and a corrugated sheet, and wound. A metal carrier or the like can be used. The first coat layer coated on the carrier base material surface can be mainly composed of a carrier having a high specific surface area such as γ-alumina. As a result, the P carried on the first coat layer
The dispersibility of t and Pd can be increased. In addition, silica, titania, zirconia, silica-alumina and the like may be contained.

It is preferable that the first coat layer contains ceria. Due to ceria's oxygen storage capacity,
Since the atmosphere fluctuation of rich and lean exhaust gas is reduced, the purification performance is further improved. When ceria is contained in the first coat layer, it is preferably contained as a composite oxide (solid solution) of ceria and zirconia. This further improves the oxygen storage capability of ceria and further enhances the stability of the oxygen storage capability.

The first coat layer carries at least one of Pt and Pd. In the case of Pt, the loading amount is preferably 1.0 g / L or more per liter of the carrier substrate volume. In the case of Pd, it is preferably at least 3.0 g / L per liter of the carrier substrate volume. If the supported amount is less than this range, the purification performance of HC and CO is reduced. The upper limit is not particularly limited, but as the number increases, the effect saturates and the cost increases.

In order to support at least one catalytic noble metal of Pt and Pd on the first coat layer, for example, an alumina powder is loaded with a catalytic noble metal by an impregnation / drying method and then coated on a carrier substrate. Alternatively, a first coat layer may be formed on the surface of the carrier base material from alumina powder or the like, and the first coat layer may be immersed in a catalytic noble metal solution and supported. When the first coat layer contains Pd, the first coat layer preferably further contains Ba. Thereby, sintering of Pd is suppressed and heat resistance is improved.

The second coat layer formed on the surface of the first coat layer mainly has a specific surface area of 50 to 100 m ² / g θ-.
It is made of a carrier containing alumina and Rh. If the specific surface area of θ-alumina is smaller than 50 m ² / g, the dispersibility (active sites) of Rh is reduced, and if it is larger than 100 m ² / g, alumina grain growth (sintering) is promoted.
The θ-alumina can be prepared by heat-treating γ-alumina (including additives such as La and Ba) at 1050 to 1200 ° C.

[0018] As described in claim 3, the second coat layer contains 0.3 to 1.5 g per liter of the volume of the carrier substrate.
Of Rh is preferably carried. As a result, the purification performance is dramatically improved. Rh carried 0.3 g /
If it is less than L, the durability will be low. As the amount of Rh supported on the second coat layer increases, the degree of improvement in purification performance increases. However, when the amount of Rh exceeds 1.5 g / L, the effect is saturated and the cost increases. 0.6
A range of from 1.0 to 1.0 g / L is particularly desirable.

In order to form the second coat layer carrying Rh, there is a method in which Rh is carried by impregnating and drying the θ-alumina powder, the slurry is slurried and coated on the surface of the first coat layer. . In addition, a noble metal can be supported by forming a second coat layer that does not support the noble metal, immersing it in an aqueous solution of a noble metal compound, and baking it. In the latter case, the noble metal is also carried on the first coat layer.

The coating amount of the first coat layer and the second coat layer is 10 coats / liter of the carrier substrate.
It is desirable that the amount is in the range of 0 to 180 g / L and the second coat layer is in the range of 50 to 70 g / L. 250 g /
If it exceeds L, the cells of the honeycomb carrier are undesirably clogged with the alumina coat or the resistance to exhaust gas flow increases. If the respective coating amounts are less than the above ranges, the two-layer structure will not be formed, or the carrying density of the catalytic noble metal will be too high, so that the particles will be easily grown and the purification performance will be reduced.

When the temperature of the exhaust gas is low, such as when the engine is started, it is effective to carry a high concentration of noble metal on the catalyst inlet side, as described in JP-A-6-205983. This technique can be applied to the present invention, and by carrying at least one of Pt and Pd at a high concentration on the catalyst inlet side, the purification performance when the exhaust gas temperature is low is remarkably improved.

However, when at least one of Pt and Pd is supported at a high concentration, there is a problem that sintering becomes easier when used at a high temperature. Therefore, as described in claim 2, at least the second coat layer has Pt and Pd in a part of 1/6 to 1/3 of the entire length of the carrier substrate from the exhaust gas inlet side to the exhaust gas outlet side of the carrier substrate. It is desirable that at least one type is supported at a high concentration.

That is, in the catalyst according to the second aspect of the present invention, at least one of Pt and Pd excellent in the oxidation activity of HC and CO is supported at a high concentration on the catalyst inlet side, so that the catalyst can be used when starting the engine. It shows high purification performance even when the exhaust gas temperature is low. Since the exhaust gas and the catalyst are heated by the reaction heat at this time, the temperature on the downstream side of the catalyst also rises early and participates in the purification reaction, so that HC,
Purification reaction of CO and NO _x are advanced high purification performance can be obtained.

If at least one of Pt and Pd is supported at a high concentration on the catalyst inlet side, sintering may occur in at least one of Pt and Pd when used at a high temperature. However, since the second coat layer contains θ-alumina as a main component, Pt and Pd are less likely to be carried than γ-alumina. Therefore Pt and Pd
When carrying, at least one of Pt and Pd is the second
It is gradually carried not only on the coat layer but also on the first coat layer. As a result, the distribution of Pt and Pd carried in the thickness direction of the coat layer becomes uniform, so that sintering hardly occurs. Thereby, the low-temperature activity is maintained for a long time.

The effect of supporting a high concentration is small when at least one of Pt and Pd is supported at a high concentration is less than 1/6 of the entire length of the carrier base material, and the effect is saturated even when it exceeds 1/3. Cost increases.

[0026]

The present invention will be described more specifically with reference to examples and comparative examples. (Embodiment 1) FIGS. 1 and 2 show the structure of an exhaust gas purifying catalyst of this embodiment. This exhaust gas purifying catalyst comprises a honeycomb-shaped carrier substrate 1 and a first substrate 1 coated on the surface of the carrier substrate 1.
It is composed of an alumina coat layer 2 and a second alumina coat layer 3 coated on the surface of the first alumina coat layer 2.

The first alumina coat layer 2 comprises: a carrier comprising γ-alumina and a ceria-zirconia composite oxide;
And Pd20 carried on this carrier.
The second alumina coat layer 3 includes a carrier made of θ-alumina and Rh30 supported on the carrier. Hereinafter, a method for producing the exhaust gas purifying catalyst will be described, and the detailed description of the configuration will be replaced. The following “parts” means parts by weight.

<Formation of First Alumina Coated Layer> γ-alumina powder containing 3.5% by weight of La (specific surface area: 200 m
² / g), 60 parts of a composite oxide powder of ceria and zirconia (molar ratio Ce / Zr = 1/1), 60 parts of an aluminum nitrate aqueous solution having a concentration of 40% by weight, and amorphous alumina water. A slurry was prepared by mixing and stirring 3 parts of the Japanese product and 100 parts of pure water.

A 1 liter ceramic honeycomb carrier base material 1 is immersed in the slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and fired at 600 ° C. for 1 hour to obtain a honeycomb carrier substrate. A coat layer was formed on the surface of the material 1. The coating layer is made of the honeycomb carrier substrate 1
140 g were formed per liter. Thereafter, the honeycomb substrate was immersed in an aqueous solution of palladium nitrate for 1 hour, pulled up, and dried at 250 ° C. for 1 hour to form a first alumina coat layer 2 supporting 5 g of Pd20 per liter of the honeycomb carrier substrate 1.

<Formation of Second Alumina Coated Layer> γ-alumina powder containing 3.5% by weight of La (specific surface area: 200 m
² / g) was subjected to a heat treatment of heating at 1100 ° C. for 5 hours in an electric furnace, and θ-alumina as a main component and a part of γ-
Θ-alumina powder containing alumina (specific surface area 100 m ²
/ G) was prepared.

The θ-alumina powder was immersed in a rhodium nitrate aqueous solution having a predetermined concentration for 1 hour, and dried at 250 ° C. for 1 hour to carry Rh to prepare Rh-supported θ-alumina powder. The supported amount of Rh is 0.5% by weight. Next, this R
A slurry was prepared by mixing and stirring 40 parts of h-supported θ-alumina powder, 30 parts of an aluminum nitrate aqueous solution having a concentration of 40% by weight, 2 parts of amorphous alumina hydrate, and 20 parts of pure water. .

The honeycomb carrier substrate 1 on which the first alumina coat layer 2 is formed is immersed in this slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and further baked at 600 ° C. for 1 hour. The second alumina coat layer 3 was formed on the surface of the first alumina coat layer 2. The second alumina coat layer 3 is formed in an amount of 60 g per liter of the honeycomb carrier substrate 1, and the amount of supported Rh is 0.3 g per liter of the honeycomb carrier substrate 1.

Example 2 The same procedure as in Example 1 was carried out except that the temperature during the heat treatment in the electric furnace was 1200 ° C., and θ-alumina powder containing θ-alumina as a main component and partially containing α-alumina ( The specific surface area was 50 m ² / g). The same procedure as in Example 1 was repeated except that the first alumina coat layer supporting Pd was formed on the honeycomb carrier base material, and the above-mentioned θ-alumina powder was used. Two alumina coat layers were formed.

Example 3 The same procedure as in Example 1 was carried out except that an aqueous solution of dinitrodiammineplatinic nitric acid was used in place of the aqueous solution of palladium nitrate. One alumina coat layer was formed, and a second alumina coat layer supporting Rh was formed in the same manner as in Example 1.

Embodiment 4 FIG. 3 shows the structure of an exhaust gas purifying catalyst according to this embodiment. In this exhaust gas purifying catalyst, the first
The alumina coat layer 2 carries Pd20, and the second alumina coat layer 3 carries Rh30 and Pd31. 100 parts of γ-alumina powder containing 3.5% by weight of La (specific surface area 200 m ² / g) and 60 parts of a composite oxide powder of ceria and zirconia (molar ratio Ce / Zr = 1/1); 60 parts of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 3 parts of amorphous alumina hydrate, 100 parts of pure water
Was mixed and stirred to prepare a slurry.

A 1 liter ceramic honeycomb carrier base material is immersed in the slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and baked at 600 ° C. for 1 hour. A coat layer was formed on one surface. The coating layer is made of the honeycomb carrier substrate 1
140 g was formed per liter of. Next, on the surface of the honeycomb carrier substrate 1 having this coat layer, a second alumina coat layer 3 supporting Rh30 was formed in the same manner as in Example 1. Then, add 1% aqueous solution of palladium nitrate
After immersion for 2 hours and drying at 250 ° C. for 1 hour, the first alumina coat layer 2 was formed by supporting Pd31 on the second alumina coat layer 3 and Pd20 on the lower coat layer. The supported amount of Pd is 5 g per liter of the honeycomb substrate 1 in total of the first alumina coat layer 2 and the second alumina coat layer 3.

Example 5 γ containing 3.5% by weight of La
-100 parts of alumina powder (specific surface area: 200 m ² / g) and a composite oxide powder of ceria and zirconia (molar ratio Ce)
/ Zr = 1/1), 60 parts of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 60 parts of amorphous alumina hydrate, and 100 parts of pure water are mixed and stirred to prepare a slurry. did.

A 1 liter ceramic honeycomb carrier base material is immersed in this slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and fired at 600 ° C. for 1 hour to obtain a honeycomb carrier base material. A coat layer was formed on the surface. The coat layer was formed in an amount of 140 g per liter of the honeycomb carrier substrate. Next, on the surface of the honeycomb substrate having the coating layer, a second alumina coating layer supporting Rh was formed in the same manner as in Example 1. Thereafter, it is immersed in a dinitrodiammineplatinum nitrate aqueous solution of a predetermined concentration for 1 hour, dried at 250 ° C. for 1 hour, and further carries Pt on the second alumina coat layer, and carries Pt on the lower coat layer. One alumina coat layer was formed. The amount of Pt carried was determined by the first alumina coat layer and the second alumina coat layer.
The total amount of the alumina coat layer is 1.5 g per liter of the honeycomb carrier substrate.

Example 6 In the same manner as in Example 1 except that an aqueous solution of dinitrodiammineplatinic nitrate was used instead of the aqueous solution of palladium nitrate, 1.5 g of Pt was supported per liter of the honeycomb substrate. Further, a second alumina coat layer carrying Rh was formed in the same manner as in Example 1 except that the amount of Rh carried on the θ-alumina powder was 1.0% by weight. The amount of Rh supported is the honeycomb carrier substrate 1.
0.6 g per liter.

Example 7 A honeycomb carrier substrate was loaded with 1.5 g of Pt per liter in the same manner as in Example 1 except that an aqueous solution of dinitrodiammineplatinic nitric acid was used instead of the aqueous solution of palladium nitrate. Further, a second alumina coat layer carrying Rh was formed in the same manner as in Example 1 except that the amount of Rh carried on the θ-alumina powder was 1.7% by weight. The amount of Rh supported is the honeycomb carrier substrate 1.
Is 1.0 g per liter.

Comparative Example 1 A γ-alumina powder containing γ-alumina as a main component and partially containing θ-alumina was prepared in the same manner as in Example 1 except that the temperature during the heat treatment in the electric furnace was 1000 ° C. The specific surface area was 150 m ² / g). Then, using the same honeycomb carrier base material having the first alumina coat layer carrying Pd as in Example 1,
A second alumina coat layer carrying Rh was formed in the same manner as in Example 1 except that the above-mentioned γ-alumina powder was used instead of the alumina powder.

Comparative Example 2 An α-alumina powder (specific surface area 5 m ² / g) was prepared in the same manner as in Example 1 except that the temperature during the heat treatment in the electric furnace was 1300 ° C. Then, using the same honeycomb carrier base material having the first alumina coat layer carrying Pd as in Example 1,
A second alumina coat layer carrying Rh was formed in the same manner as in Example 1 except that the above-mentioned α-alumina powder was used instead of the alumina powder.

(Comparative Example 3) Rh-supported zirconia powder was prepared by immersing zirconia powder in an aqueous solution of rhodium nitrate having a predetermined concentration for 1 hour and drying at 250 ° C for 1 hour to carry Rh. The supported amount of Rh is 2% by weight. Next, this R
Mix 15 parts of h-supported zirconia powder, 40 parts of γ-alumina powder, 30 parts of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 2 parts of amorphous alumina hydrate, and 20 parts of pure water. The slurry was prepared by stirring.

A honeycomb carrier base material having a Pd-supported first alumina coat layer similar to that of Example 1 is immersed in the slurry, pulled up, blown off the excess slurry, and dried at 250 ° C. for 1 hour. By baking at 600 ° C. for 1 hour, a second alumina coat layer was formed on the surface of the first alumina coat layer. The second alumina coat layer is formed at a rate of 60 g per liter of the honeycomb carrier substrate, and the amount of Rh supported is 0.3 g per liter of the honeycomb carrier substrate 1.

Comparative Example 4 Pt was prepared in the same manner as in Example 1 except that an aqueous solution of dinitrodiammineplatinic nitrate was used instead of the aqueous solution of palladium nitrate, and 1.5 g of Pt was supported per liter of the honeycomb substrate. The first to carry
An alumina coat layer was formed. A γ-alumina powder (specific surface area: 150 m ² / g) containing γ-alumina as a main component and partially containing θ-alumina in the same manner as in Example 1 except that the temperature during the heat treatment in the electric furnace was set to 1000 ° C. Prepared.

Example 1 was repeated except that the above-mentioned γ-alumina powder was used in place of the θ-alumina powder using the honeycomb carrier substrate on which the first alumina coat layer was formed.
A second alumina coat layer supporting Rh was formed in the same manner as described above. Comparative Example 5 A Pt-supported Pt was carried out in the same manner as in Example 1 except that an aqueous solution of dinitrodiammineplatinic nitrate was used instead of the aqueous solution of palladium nitrate, and 1.5 g of Pt was supported per liter of the honeycomb support substrate. One alumina coat layer was formed.

The temperature during the heat treatment in the electric furnace was 1300
An α-alumina powder (specific surface area 5 m ² / g) was prepared in the same manner as in Example 1 except that the temperature was changed to ° C. Then, the second alumina supporting Rh was used in the same manner as in Example 1 except that the above-mentioned α-alumina powder was used instead of the θ-alumina powder, using the honeycomb carrier base material on which the first alumina coat layer was formed. A coat layer was formed.

Comparative Example 6 Pt was prepared in the same manner as in Example 1 except that an aqueous solution of dinitrodiammineplatinic nitrate was used instead of the aqueous solution of palladium nitrate, and 1.5 g of Pt was supported per liter of the honeycomb substrate. The first to carry
An alumina coat layer was formed. The zirconia powder is immersed in a predetermined concentration of rhodium nitrate aqueous solution for 1 hour,
After drying for a time, Rh was supported to prepare Rh-supported zirconia powder. The supported amount of Rh is 2% by weight.

Next, this Rh-supported zirconia powder was
, 40 parts by weight of γ-alumina powder, 30 parts of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 2 parts of amorphous alumina hydrate, and 20 parts of pure water are mixed and stirred to prepare a slurry. did. The honeycomb carrier substrate on which the first alumina coat layer is formed is immersed in the slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and further baked at 600 ° C. for 1 hour to obtain the first alumina coat. A second alumina coat layer was formed on the surface of the coat layer. The second alumina coat layer is 60 g per liter of the honeycomb carrier base material.
The amount of Rh thus formed is 0.3 g per liter of the honeycomb carrier substrate.

(Test / Evaluation) Each of the exhaust gas purifying catalysts is housed in a catalytic converter and mounted on an exhaust system of a gasoline engine having a displacement of 2 L, and the inlet gas temperature is 900 ° C. and the air-fuel ratio (A / F) is 14. An actual gas endurance test for 100 hours was performed under the condition of a large fluctuation of 6 ± 1. Thereafter, the gas temperature 400 ° C. enters, under the conditions of the air-fuel ratio 14.6 ± 0.5, was measured purification rate of HC, CO and NO _x. Table 1 shows the results.

[0051]

[Table 1]

When Examples 1 to 7 and Comparative Examples 1 to 6 are compared, in the case of the exhaust gas purifying catalyst of each Example, Pd / R
The purification performance of both the h-based and Pt / Rh-based systems is greatly improved, and the heat resistance in a high-temperature atmosphere such as an exhaust gas temperature of 900 ° C. is remarkably improved. It is apparent that this effect is due to the fact that the upper second coat layer is made of θ-alumina.

That is, Examples 1 to 5 which are Pd / Rh systems
When Examples 2 and 4 are compared with Comparative Examples 1-3, Examples 1-2 and 4
Shows a higher purification rate. Also, when Pt / Rh-based Examples 3 and 5 are compared with Comparative Examples 4 to 6,
5 shows a higher purification rate. Thus, Comparative Example 1
The reason why the purification performances of Comparative Examples 1 to 6 are inferior is that in Comparative Examples 1 and 4, γ-alumina and Rh reacted in a high-temperature atmosphere to cause Rh to form a solid solution in the alumina crystal and further promote Rh sintering. it is conceivable that. In the case of Comparative Examples 2 and 5 using α-alumina, the specific surface area was 5 m ² / g.
, The dispersibility of Rh was low, and the purification performance was low from the beginning before endurance.

Further, Comparative Examples 3 and 6 have higher purification performance after durability than Comparative Examples 1 and 4, but are not sufficient compared with Examples. It is considered that the reason for this is that the sintering of Rh in the second alumina coat layer was promoted due to the low heat resistance of zirconia. On the other hand, in the exhaust gas purifying catalyst of each embodiment, since the thermally stable θ-alumina is used for the carrier of the second alumina coat layer, the reaction between Rh and alumina does not progress, and It is considered that the Rh activity was sufficiently maintained because Rh was hardly dissolved in solid solution and the phase change of alumina and the sintering of Rh accompanying the grain growth were also suppressed.

Further, from the comparison of Examples 3, 6, and 7, the first
It can be seen that the purification performance is improved by further supporting Rh on the alumina coat layer, and the purification performance is improved as the amount of supported Rh increases. This is presumably because, because θ-alumina was used for the carrier of the second alumina coat layer, Rh was hardly dissolved in the alumina crystal, and the effect of suppressing sintering of Rh was effectively exhibited.

Embodiment 8 FIG. 4 shows the structure of an exhaust gas purifying catalyst according to this embodiment. This exhaust gas purifying catalyst comprises a honeycomb-shaped carrier substrate 1 ′, a first alumina coat layer 2 ′ coated on the surface of the carrier substrate 1 ′, and a second alumina coat layer 2 ′ coated on the surface of the first alumina coat layer 2 ′. And an alumina coat layer 3 '.

The first alumina coat layer 2 ′ comprises: a carrier comprising γ-alumina and a ceria-zirconia composite oxide;
And Pt21 supported on this carrier.
The second alumina coat layer 3 'is composed of a carrier made entirely of [theta] -alumina and Rh30 carried on the carrier, and Pd31 is provided on a 1/6 portion of the entire length of the carrier substrate from the end face on the exhaust gas inlet side. Highly supported Pd high loading section 1
0 is formed.

Hereinafter, a method of manufacturing the exhaust gas purifying catalyst will be described, and the detailed description of the structure will be replaced. The following “parts”
Means part by weight. <Formation of First Alumina Coating Layer> 100 parts of the same γ-alumina powder as used in Example 1, 60 parts of a composite oxide powder of ceria and zirconia (molar ratio Ce / Zr = 1/1), and amorphous A slurry was prepared by mixing and stirring 3 parts of alumina hydrate and 100 parts of pure water.

A 1.0 L (honeycomb part: 103 mm in diameter × 120 mm in length) ceramic honeycomb carrier substrate 1 ′ having a volume of 1.0 L was immersed in the slurry, and after being pulled up, excess slurry was blown off and dried at 250 ° C. for 1 hour. Further, the coating was fired at 600 ° C. for 1 hour to form a coat layer on the surface of the honeycomb carrier substrate 1 ′. The coat layer was formed in an amount of 140 g per liter of the honeycomb carrier substrate.

Thereafter, it was immersed in an aqueous solution of dinitrodiammineplatinum nitrate of a predetermined concentration for 1 hour, pulled up, dried at 250 ° C. for 1 hour, and dried at 250 ° C. for 1 L per 1 L of the honeycomb carrier substrate 1 ′.
A first alumina coat layer 2 'supporting 5 g of Pt21 was formed. <Formation of Second Alumina Coating Layer> The θ-alumina powder prepared in the same manner as in Example 1 was immersed in a predetermined concentration of rhodium nitrate aqueous solution for 1 hour, dried at 250 ° C. for 1 hour to carry Rh, and Rh was supported. A supported θ-alumina powder was prepared. The supported amount of Rh is 0.5% by weight.

Next, this Rh-supported θ-alumina powder was
0 parts and an aqueous solution of aluminum nitrate having a concentration of 40% by weight
0 parts, 2 parts of amorphous alumina hydrate, and 20 parts of pure water were mixed and stirred to prepare a slurry. The honeycomb carrier substrate 1 ′ on which the first alumina coat layer 2 ′ is formed is immersed in the slurry, the excess slurry is blown off after pulling up, dried at 250 ° C. for 1 hour, and further baked at 600 ° C. for 1 hour. A second alumina coat layer 3 'was formed on the surface of the first alumina coat layer 2'. 2nd alumina coat layer 3 '
Are formed at a rate of 60 g per liter of the honeycomb carrier base material 1 ′, and Rh is uniformly carried on the whole at a carrying amount of 0.3 g / L.

Next, a portion 20 mm from the end face on the upstream side of the honeycomb carrier substrate 1 ′ (1/1 of the entire length of the honeycomb carrier substrate 1 ′)
6) was immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour, pulled up, dried at 250 ° C. for 1 hour, and Pd at a concentration of 10 g per liter of the honeycomb carrier substrate 1 ′.
The Pd high supporting portion 10 was formed by additionally supporting 31. (Example 9) It is 2 from the upstream end face of the honeycomb carrier substrate 1 '.
The catalyst of this example was prepared in the same manner as in Example 8 except that Pt was additionally supported at a concentration of 10 g / L in place of Pd on the 0 mm portion (1/6 of the entire length of the honeycomb carrier substrate 1 '). Prepared.

(Example 10) Pt was further added at a concentration of 4 g / L instead of Pd to a portion 20 mm from the upstream end face of the honeycomb carrier substrate 1 '(1/6 of the entire length of the honeycomb carrier substrate 1'). A catalyst of this example was prepared in the same manner as in Example 8, except that the catalyst was supported. (Example 11) A portion 40 mm from the upstream end face of the honeycomb carrier substrate 1 '(1/3 of the entire length of the honeycomb carrier substrate 1')
Was prepared in the same manner as in Example 8, except that Pd was additionally supported at a concentration of 10 g / L.

Example 12 Pt was further loaded at a concentration of 4 g / L in place of Pd on a portion 40 mm from the upstream end face of the honeycomb carrier substrate 1 '(1/3 of the entire length of the metal carrier substrate 1'). A catalyst of this example was prepared in the same manner as in Example 8, except that this was done. Example 13 A catalyst of this example was prepared in the same manner as in Example 8, except that Pd was not additionally supported.
This catalyst has a configuration similar to that of the catalyst of Example 3.

Comparative Example 7 γ containing 3.5% by weight of La
-100 parts of alumina powder (specific surface area: 200 m ² / g) and a composite oxide powder of ceria and zirconia (molar ratio Ce)
/ Zr = 1/1), 60 parts of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 60 parts of amorphous alumina hydrate, and 100 parts of pure water are mixed and stirred to prepare a slurry. did.

A 1 liter ceramic honeycomb carrier base material 1 'is immersed in the slurry, pulled up, blown off the excess slurry, dried at 250 ° C. for 1 hour, and further baked at 600 ° C. for 1 hour. One coat layer was formed on the surface of the substrate 1. The coating layer was formed in an amount of 200 g per liter of the honeycomb carrier substrate 1 '. Then, it is immersed in a dinitrodiammine platinum aqueous solution of a predetermined concentration for 1 hour, pulled up, dried at 250 ° C. for 1 hour, further immersed in a predetermined concentration of a rhodium nitrate aqueous solution for 1 hour, pulled up, and dried at 250 ° C. for 1 hour, 1.5 g of Pt and 0.3 g of R per liter of the honeycomb carrier substrate 1 '
Next, 20 mm from the upstream end face of the honeycomb carrier base material 1 ′.
mm portion is immersed in an aqueous solution of palladium nitrate having a predetermined concentration for 1 hour, pulled up, dried at 250 ° C. for 1 hour, and additionally supporting 10 g of Pd per liter of the honeycomb carrier substrate 1 ′ on the portion. A high carrying portion was formed.

Comparative Example 8 An aqueous solution of dinitrodiammine platinum was used in place of the aqueous solution of palladium nitrate, and 10 g of Pt per liter of the honeycomb substrate 1 ′ was applied to a portion 20 mm from the upstream end face of the honeycomb substrate 1 ′. Was prepared in the same manner as in Comparative Example 7 except that a Pt high-carrying portion additionally supporting was formed.

(Test / Evaluation) Each exhaust gas purifying catalyst was housed in a catalytic converter and mounted on an exhaust system of a gasoline engine having a displacement of 2 L, and the inlet gas temperature was 900 ° C. and the air-fuel ratio (A / F) was 14. An actual gas endurance test for 100 hours was performed under the condition of 6 ± 1. Thereafter, the gas temperature 400 ° C. enters, under the conditions of the air-fuel ratio 14.6 ± 0.5, and measuring the average purification rate of HC, CO and NO _x. Further, the temperature of the incoming gas was increased from 250 ° C. to 400 ° C., and the temperature at which the HC purification rate reached 50% was also measured. Table 2 shows the results.

[0069]

[Table 2]

From Table 2, it can be seen that the catalysts of Examples 8 to 12 had higher H at low temperatures even after the durability test than the catalyst of Example 13.
It is clear that the purification activity of C is excellent, and the purification performance is improved by forming the Pd high loading portion 10 or the Pt high loading portion. Also from comparison between Comparative Examples 7 and 8, it can be seen that the purification activity and CO in the low-temperature region, is _the NO _x purification rate is excellent. From this, it can be seen that Rh in the upper layer suppresses solid solution in alumina, and furthermore, sintering of Pd and Pt uniformly supported at a high concentration on the upstream end face is suppressed, and the performance is improved. .

[0071]

That is, according to the exhaust gas purifying catalyst of the first aspect, even in a high temperature atmosphere of 900 ° C. or more, R
The reaction between h and alumina can be suppressed, and the heat resistance is significantly improved. Further, according to the exhaust gas purifying catalyst according to the second aspect, the purification performance at low temperatures such as when starting the engine is improved,
Its durability is also excellent.

Further, according to the exhaust gas purifying catalyst of the third aspect, the effect of suppressing the sintering of Rh is more effectively exhibited, and the purifying performance is further improved.

[Brief description of the drawings]

FIG. 1 is a sectional view of a main part of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is an enlarged view of a main part of FIG. 1, and is a configuration explanatory view of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram of a configuration of an exhaust gas purifying catalyst according to a second embodiment of the present invention.

FIG. 4 is an explanatory view of a configuration of an exhaust gas purifying catalyst according to an eighth embodiment of the present invention.

[Explanation of symbols]

1: Honeycomb carrier substrate 2: First alumina coat layer 3: Second alumina coat layer 20: Pd 3
0: Rh

────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] June 16, 1998

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0031

[Correction method] Change

[Correction contents]

The θ-alumina powder was immersed in a rhodium nitrate aqueous solution having a predetermined concentration for 1 hour, and dried at 250 ° C. for 1 hour to carry Rh to prepare Rh-supported θ-alumina powder. The supported amount of Rh is 0.54% by weight . Next, 40 parts of the Rh-supported θ-alumina powder was added to a concentration of 40 wt%
Of aluminum nitrate aqueous solution, 2 parts of amorphous alumina hydrate, and 20 parts of pure water were mixed and stirred to prepare a slurry.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0060

[Correction method] Change

[Correction contents]

Thereafter, it was immersed in an aqueous solution of dinitrodiammineplatinum nitrate of a predetermined concentration for 1 hour, pulled up, dried at 250 ° C. for 1 hour, and dried at 250 ° C. for 1 L per 1 L of the honeycomb carrier substrate 1 ′.
A first alumina coat layer 2 'supporting 5 g of Pt21 was formed. <Formation of Second Alumina Coating Layer> The θ-alumina powder prepared in the same manner as in Example 1 was immersed in a predetermined concentration of rhodium nitrate aqueous solution for 1 hour, dried at 250 ° C. for 1 hour to carry Rh, and Rh was supported. A supported θ-alumina powder was prepared. The supported amount of Rh is 0.54% by weight .

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masayuki Fukui 41 Toyota Chuo Research Institute, Inc. 41, Changchun Yokomichi 1 Toyota Central Research Laboratory Co., Ltd.

Claims (3)

[Claims] 1. A carrier substrate, a first coat layer formed on the carrier substrate surface and carrying at least one of Pt and Pd, and a second coat layer formed on the surface of the first coat layer and carrying Rh.
An exhaust gas purifying catalyst comprising: a coating layer; and a second coating layer mainly containing θ-alumina having a specific surface area of 50 to 100 m ² / g. 2. The exhaust gas-purifying catalyst, wherein at least the second coat layer has 1/1 of the total length of the carrier substrate from the exhaust gas inlet side end surface of the carrier substrate to the exhaust gas outlet side.
2. The exhaust gas purifying catalyst according to claim 1, wherein at least one of Pt and Pd is supported at a high concentration on 6 to 1/3 of the portion. 3. The exhaust gas purifying catalyst according to claim 1, wherein the second coat layer carries 0.3 to 1.5 g of Rh per liter of the volume of the carrier substrate. .