JP2013027876A - Exhaust gas purifying catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve high efficiency of exhaust gas purification.SOLUTION: An exhaust gas purifying catalyst 1 includes: a base material 2; a lower layer 3, which coats the base material 2, which contains an oxygen storage material and a palladium, and which does not contain a rhodium; and an upper layer 4, which contains a support for coating the lower layer 3 and also rhodium along with a support for supporting the rhodium, and which has a lower oxygen storage capacity compared with that of the lower layer 3.

Description

  The present invention relates to an exhaust gas purifying catalyst containing an oxygen storage material.

Conventionally, a three-way catalyst in which a noble metal is supported on a heat-resistant carrier made of an inorganic oxide such as alumina has been widely used as an exhaust gas purifying catalyst for treating exhaust gas of an automobile or the like. In this three-way catalyst, the noble metal plays a role of promoting the reduction reaction of nitrogen oxide (NO _x ) and the oxidation reaction of carbon monoxide (CO) and hydrocarbon (HC). The heat-resistant carrier plays a role of increasing the specific surface area of the noble metal and suppressing the sintering of the noble metal by dissipating heat generated by the reaction.

  Patent Documents 1 to 3 describe an exhaust gas purifying catalyst using cerium oxide or an oxide containing cerium and another metal element. These oxides are oxygen storage materials having oxygen storage capacity. When the oxygen storage material is used with a three-way catalyst, the previous reduction and oxidation reactions can be optimized. However, as will be described below, even a three-way catalyst using an oxygen storage material achieves good performance both in the state immediately after starting the engine and in the state where the engine is continuously operated. It ’s difficult.

  In the state immediately after starting the engine, the temperature of the catalyst is low. The ability of noble metals to purify exhaust gases under low temperature conditions is lower than the ability of noble metals to purify exhaust gases under high temperature conditions. Therefore, considering the performance immediately after starting the engine, it is advantageous to reduce the heat capacity of the exhaust gas purifying catalyst.

  On the other hand, when the engine is continuously operated, the temperature of the catalyst is sufficiently high. In this case, since the ability of the noble metal to purify the exhaust gas is high, it is advantageous that the exhaust gas purification catalyst contains more oxygen storage material in order to cope with fluctuations in the exhaust gas composition.

  As described above, the performance immediately after starting the engine and the performance in a state where the engine is continuously operated are in a trade-off relationship. Therefore, it is difficult to achieve good performance both in the state immediately after starting the engine and the state in which the engine is continuously operated, and therefore it is difficult to always achieve high exhaust gas purification efficiency.

JP-A-1-281144 Japanese Patent Laid-Open No. 9-155192 JP-A-9-221304

  An object of the present invention is to achieve high exhaust gas purification efficiency.

  According to an aspect of the present invention, a base material, a lower layer that covers the base material and includes an oxygen storage material and palladium, does not include rhodium, and covers the lower layer and rhodium and a carrier that carries the same. In addition, an exhaust gas purifying catalyst comprising an upper layer having a lower oxygen storage capacity than the lower layer is provided.

  According to the present invention, it is possible to achieve high exhaust gas purification efficiency.

1 is a perspective view schematically showing an exhaust gas purifying catalyst according to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing an example of a structure that can be employed in the exhaust gas purification catalyst shown in FIG. 1.

Hereinafter, embodiments of the present invention will be described.
FIG. 1 is a perspective view schematically showing an exhaust gas purifying catalyst according to one embodiment of the present invention. FIG. 2 is a cross-sectional view schematically showing an example of a structure that can be employed in the exhaust gas purifying catalyst shown in FIG.

  The exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 is a monolith catalyst. The exhaust gas-purifying catalyst 1 includes a substrate 2 such as a monolith honeycomb substrate. The substrate 2 is typically made of ceramics such as cordierite.

  A lower layer 3 is formed on the partition wall of the substrate 2. The lower layer 3 includes a first heat-resistant carrier, a first oxygen storage material, and palladium. The lower layer 3 plays a main role of giving the exhaust gas purification catalyst 1 an oxygen storage capability, and also plays a main or auxiliary role of giving the exhaust gas purification catalyst 1 an exhaust gas purification capability of palladium.

  The lower layer 3 does not contain rhodium. In this specification and claims, when it is mentioned that “the layer L1 does not contain the element E”, it is not excluded that an unavoidable amount of the element E is mixed in the layer L1. For example, the layer L1 may include an element E mixed by diffusion from the layer L2 adjacent thereto.

  The first heat-resistant carrier is excellent in thermal stability as compared with the first oxygen storage material. As a material for the first heat-resistant carrier, for example, alumina, zirconia, or titania can be used.

  The first oxygen storage material is, for example, ceria, a composite oxide of ceria and another metal oxide, or a mixture thereof. As the composite oxide, for example, a composite oxide of ceria and zirconia can be used.

  The first oxygen storage material may carry a noble metal such as a platinum group element. Generally, when a trace amount of noble metal is supported on an oxygen storage material, the oxygen storage capacity is improved. For example, a material obtained by supporting platinum on a composite oxide of ceria and zirconia may be used as the first oxygen storage material.

  The coating amount of the lower layer 3 per 1 L of the volume of the base material 2 is, for example, in the range of 20 g / L to 200 g / L. When this coating amount is small, it is difficult to achieve sufficient oxygen storage capacity. Moreover, when this coating amount is large, the heat capacity of the exhaust gas-purifying catalyst 1 increases.

  The mass ratio of the oxide used for the first oxygen storage material to the first heat-resistant carrier is, for example, in the range of 10% by mass to 90% by mass.

  When a composite oxide of ceria and zirconia is used as the first oxygen storage material, the atomic ratio of cerium to zirconium is, for example, in the range of 3 atomic% to 97 atomic%.

  An upper layer 4 is formed on the lower layer 3. The upper layer 4 contains a second heat resistant carrier and rhodium. The upper layer 4 plays a main role in providing the exhaust gas purification catalyst 1 with an exhaust gas purification ability by rhodium.

  The second heat-resistant carrier is excellent in heat resistance compared to the first oxygen storage material. As the material for the second heat-resistant carrier, for example, the materials exemplified for the first heat-resistant carrier can be used.

  The coating amount of the upper layer 4 per 1 L of the volume of the substrate 2 is, for example, in the range of 10 g / L to 200 g / L. When the coating amount is small, the exhaust gas purification ability of the upper layer 4 may be insufficient. Moreover, when this coating amount is large, the heat capacity of the exhaust gas-purifying catalyst 1 increases.

  The upper layer 4 may further contain palladium and / or platinum. In addition, the upper layer 4 can further include a second oxygen storage material. As a 2nd oxygen storage material, the material illustrated about the 1st oxygen storage material can be used, for example.

When the lower layer 3 is omitted from the exhaust gas-purifying catalyst 1, the upper layer 4 has to play both the role of promoting the reduction reaction of NO _x and the oxidation reaction of CO and HC and the role of storing oxygen. . However, in this case, the coating amount of the upper layer 4 is reduced to reduce the heat capacity of the exhaust gas-purifying catalyst 1, and the density of the noble metal is increased to maximize the efficiency of the NO _x reduction reaction and the CO and HC oxidation reaction. In addition, the amount of the oxygen storage material is decreased, and the oxygen storage capacity of the oxygen storage material is decreased.

On the other hand, when the lower layer 3 is interposed between the upper layer 4 and the substrate 2, for example, the upper layer 4 is mainly responsible for promoting the NO _x reduction reaction and the CO and HC oxidation reaction. Alternatively, the lower layer 3 and the upper layer 4 can be shared. And the role which stores oxygen can mainly be given to the lower layer 3. Therefore, a design that maximizes the oxygen storage capacity of the oxygen storage material can be adopted for the lower layer 3. Therefore, even when the coating amount of the lower layer 3 is reduced, a sufficiently large oxygen storage capacity can be achieved. Therefore, for example, even when the coating amount of the upper layer 4 is reduced and the density of palladium and rhodium in the upper layer 4 is increased, the oxygen storage capacity of the exhaust gas-purifying catalyst 1 does not become insufficient. Moreover, since the lower layer 3 is interposed between the upper layer 4 and the substrate 2, the lower layer 3 does not hinder the contact between the upper layer 4 and the exhaust gas.

  Moreover, in order to obtain an exhaust gas purification catalyst having excellent exhaust gas purification performance and low cost, it is advantageous to increase the palladium content as compared with the rhodium content. However, when palladium is contained only in the upper layer 4, it is necessary to increase the coating amount of the layer or increase the density of palladium in the layer. When the coating amount is increased, the heat capacity of the exhaust gas purifying catalyst is increased. When the density of palladium is excessively increased, the oxygen storage capacity of the layer is decreased.

  In the exhaust gas-purifying catalyst 1, in addition to the lower layer 3 containing palladium, the upper layer 3 can also contain palladium. Therefore, if palladium is contained in both the lower layer 3 and the upper layer 3, the palladium content can be increased without increasing the coating amount and excessively increasing the palladium density. If the palladium density is not excessively high, the oxygen storage capacity of the oxygen storage material is improved by the addition of palladium. That is, excellent oxygen storage capacity, exhaust gas purification performance and low cost can be achieved.

  Therefore, by adopting this structure, it is possible to achieve high exhaust gas purification efficiency in both the state immediately after starting the engine and the state in which the engine is continuously operated, and to achieve low cost. Can do.

Various modifications can be made to the exhaust gas-purifying catalyst 1.
For example, the part on the upper layer 4 side in the lower layer 3 may have a higher palladium content than the part on the base material 2 side. In this case, the portion of the lower layer 3 on the substrate 2 side may have a higher oxygen storage capacity (for example, a larger cerium content) than the portion of the lower layer 3 on the upper layer 4 side. At this time, the portion of the lower layer 3 on the base material 2 side may not contain palladium. Moreover, the part by the side of the base material 2 of the lower layer 3 may further contain platinum.

  Of the upper layer 4, the portion on the lower layer 3 side may have a higher rhodium content than the portion on the surface side. In this case, the lower layer 3 side portion may have a smaller palladium content than the surface side portion.

  The lower layer 3 and / or the upper layer 4 may further include an oxide of an alkaline earth metal such as barium, an oxide of a rare earth element such as lanthanum, neodymium, praseodymium, and yttrium, or a mixture thereof. These oxides may form composite oxides and / or solid solutions with other oxides such as ceria.

  The exhaust gas-purifying catalyst 1 may further include a hydrocarbon adsorption layer interposed between the base material 2 and the lower layer 3. The hydrocarbon adsorption layer is a layer containing a hydrocarbon adsorption material such as zeolite. When this structure is adopted, HC emission can be reduced as compared with the case where the structure shown in FIG. 2 is adopted.

  Alternatively, the lower layer 3 may further contain a hydrocarbon adsorbing material such as zeolite. Even when this structure is adopted, HC emission can be reduced as compared with the case where the structure shown in FIG. 2 is adopted. Further, when the lower layer 3 further contains a hydrocarbon adsorbing material, the production of the exhaust gas purification catalyst 1 is simplified as compared with the case where a hydrocarbon adsorbing layer is interposed between the base material 2 and the lower layer 3. can do.

(Example)
Examples of the present invention will be described below.

<Manufacture of catalyst A>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the lower layer 3 has a two-layer structure including a first layer and a second layer.

  First, a cerium zirconium oxide powder having an atomic ratio of cerium to zirconium of 5: 4 was prepared. Hereinafter, this cerium zirconium oxide powder is referred to as oxide powder CZ1.

  Next, 30 g of alumina powder, 30 g of oxide powder CZ1, and an aqueous palladium nitrate solution containing 0.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S1.

  Subsequently, the entire amount of the slurry S1 was coated on the monolith honeycomb substrate 2 made of cordierite. Here, a monolith honeycomb substrate having a length of 100 mm, a volume of 1.0 L, and 900 cells per square inch was used. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the first layer of the lower layer 3 was formed on the monolith honeycomb substrate 2.

  Next, a cerium zirconium oxide powder having an atomic ratio of cerium and zirconium of 2: 7 was prepared. Hereinafter, this cerium zirconium oxide powder is referred to as oxide powder CZ2.

  Next, 30 g of alumina powder, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 1.0 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S2.

  Subsequently, the monolith honeycomb substrate 2 was coated with the entire amount of the slurry S2. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the second layer was formed on the first layer. In this way, a lower layer 3 having a two-layer structure of the first layer and the second layer was obtained.

  Thereafter, 30 g of alumina powder, 30 g of oxide powder CZ2, and an aqueous rhodium nitrate solution containing 0.5 g of rhodium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S3.

  Next, the monolith honeycomb substrate 2 was coated with the whole amount of the slurry S3. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the upper layer 4 was formed on the lower layer 3.

  As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst A.

<Manufacture of catalyst B>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the lower layer 3 has a two-layer structure including a first layer and a second layer.

  First, 30 g of alumina powder and 30 g of oxide powder CZ1 were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S4.

  Subsequently, the same monolith honeycomb substrate 2 as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S4. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the first layer of the lower layer 3 was formed on the monolith honeycomb substrate 2.

  Next, 30 g of alumina powder, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 1.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S5.

  Subsequently, the monolith honeycomb substrate 2 was coated with the entire amount of the slurry S5. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the second layer was formed on the first layer. In this way, a lower layer 3 having a two-layer structure of the first layer and the second layer was obtained.

  Thereafter, the upper layer 4 was formed on the lower layer 3 by the same method as described for the catalyst A. As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst B.

<Manufacture of catalyst C>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the lower layer 3 has a two-layer structure including a first layer and a second layer.

  First, 30 g of alumina powder, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 0.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S6.

  Subsequently, the same monolith honeycomb substrate 2 as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S6. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the first layer of the lower layer 3 was formed on the monolith honeycomb substrate 2.

Thereafter, the second layer of the lower layer 3 and the upper layer 4 were sequentially formed by the same method as described for the catalyst A.
As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst C.

<Manufacture of catalyst D>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method.
First, the lower layer 3 was formed on the monolith honeycomb substrate 2 by the same method as described for the catalyst A.

  Next, 60 g of alumina powder, 60 g of oxide powder CZ2, a palladium nitrate aqueous solution containing 1.0 g of palladium, and a rhodium nitrate aqueous solution containing 0.5 g of rhodium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S7.

  Subsequently, the monolith honeycomb substrate 2 was coated with the whole amount of the slurry S7. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the upper layer 4 was formed on the lower layer 3.

  As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst D.

<Manufacture of catalyst E>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the upper layer 4 has a two-layer structure of a third layer and a fourth layer.

  First, a first layer was formed on the monolith honeycomb substrate 2 by the same method as described for the catalyst A. In this example, the first layer is the lower layer 3.

  Next, a third layer of the upper layer 4 was formed on the lower layer 3 by using the slurry S3 by the same method as described for the upper layer 4 of the catalyst A. Further, a fourth layer was formed on the third layer by using the slurry S2 by the same method as described for the second layer of the catalyst A. As a result, an upper layer 4 having a two-layer structure of the third layer and the fourth layer was obtained.

  As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst E.

<Manufacture of catalyst F>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the lower layer 3 has a two-layer structure of a first layer and a second layer.

  First, 30 g of alumina powder, 30 g of oxide powder CZ1, and a platinum nitrate aqueous solution containing 0.5 g of platinum were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S8.

  In this example, the exhaust gas purification shown in FIGS. 1 and 2 is performed in the same manner as described for the catalyst A except that the slurry S8 is used instead of the slurry S1 for forming the first layer of the lower layer 3. Catalyst 1 for use was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst F.

<Manufacture of catalyst G>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method.
First, 60 g of alumina powder, 30 g of oxide powder CZ1, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 1.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S9.

  Subsequently, the same monolith honeycomb substrate 2 as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S9. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the lower layer 3 was formed on the monolith honeycomb substrate 2.

Thereafter, the upper layer 4 was formed by the same method as described for the catalyst A.
As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst G.

<Manufacture of catalyst H>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method.
First, 30 g of oxide powder and a palladium nitrate aqueous solution containing 0.5 g of palladium were mixed. This mixture was dried at 250 ° C. for 8 hours and subsequently calcined at 500 ° C. for 1 hour. Hereinafter, the powder thus obtained is referred to as powder P1.

  Next, 60 g of alumina powder, 30.5 g of powder P1, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 1.0 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S10.

  Subsequently, the same monolith honeycomb substrate 2 as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S9. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the lower layer 3 was formed on the monolith honeycomb substrate 2.

Thereafter, the upper layer 4 was formed by the same method as described for the catalyst A.
As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst H.

<Manufacture of catalyst I>
In this example, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was produced by the following method. In this example, the lower layer 3 has a two-layer structure of a first layer and a second layer.

  First, 30 g of alumina powder, 30 g of oxide powder CZ1, 6 g of barium sulfate, 3 g of lanthanum carbonate, and an aqueous palladium nitrate solution containing 0.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S11.

  Subsequently, the same monolith honeycomb substrate 2 as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S11. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the first layer of the lower layer 3 was formed on the monolith honeycomb substrate 2.

  Next, 30 g of alumina powder, 30 g of oxide powder CZ2, 6 g of barium sulfate, 3 g of lanthanum carbonate, and an aqueous palladium nitrate solution containing 1.0 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S12.

  Subsequently, the monolith honeycomb substrate 2 was coated with the entire amount of the slurry S12. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the second layer was formed on the first layer. As described above, the lower layer 3 having a two-layer structure of the first layer and the second layer was obtained.

  Thereafter, 30 g of alumina powder, 30 g of oxide powder CZ2, 6 g of barium sulfate, 3 g of lanthanum carbonate, and an aqueous rhodium nitrate solution containing 0.5 g of rhodium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S13.

  Subsequently, the monolith honeycomb substrate 2 was coated with the entire amount of the slurry S13. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the upper layer 4 was formed on the lower layer 3.

  As described above, the exhaust gas-purifying catalyst 1 shown in FIGS. 1 and 2 was completed. Hereinafter, the exhaust gas-purifying catalyst 1 is referred to as catalyst I.

<Manufacture of catalyst J>
In this example, an exhaust gas purifying catalyst was produced by the following method.
First, 30 g of alumina powder, 30 g of oxide powder CZ2, and an aqueous palladium nitrate solution containing 1.5 g of palladium were mixed to prepare a slurry. Hereinafter, this slurry is referred to as slurry S14.

  Subsequently, the same monolith honeycomb substrate as that used in the manufacture of the catalyst A was coated with the whole amount of the slurry S14. The monolith honeycomb substrate was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, a lower layer was formed on the monolith honeycomb substrate.

  Next, the previous monolith honeycomb substrate was coated with the entire amount of the slurry S3. This monolith honeycomb substrate 2 was dried at 250 ° C. for 1 hour and then fired at 500 ° C. for 1 hour. Thereby, the upper layer was formed on the lower layer.

  The exhaust gas purification catalyst was completed as described above. Hereinafter, the exhaust gas-purifying catalyst is referred to as catalyst J.

<Test>
Each of the catalysts A to J was mounted on an automobile having an engine with a displacement of 0.7 L. Next, each automobile was caused to travel a durable travel distance of 60000 km. Thereafter, the emission amount per 1 km of the travel distance was measured for each of non-methane hydrocarbon (NMHC), CO, and NO _{x by} the 10.15 mode method and the 11 mode method. The amount of NMHC discharged is a value obtained by converting a value represented by a volume ratio based on the carbon number equivalent into grams. Moreover, the measured value obtained by the 10 · 15 mode method was multiplied by 88/100, and the measured value obtained by the 11 mode method was multiplied by 12/100 to obtain a total value thereof. The results are summarized in the following table.

As shown in the above table, when the catalysts A to I were used, each of the NMHC, CO, and NO _x emission amounts was smaller than when the catalyst J was used.

  DESCRIPTION OF SYMBOLS 1 ... Exhaust gas purification catalyst, 2 ... Base material, 3 ... Lower layer, 4 ... Upper layer.

As shown in the above table, when the catalysts A to I were used, each of the NMHC, CO, and NO _x emission amounts was smaller than when the catalyst J was used.
The invention described in the original claims is appended below.
[1] A base material, a lower layer that covers the base material and contains an oxygen storage material and palladium, and does not contain rhodium; a lower layer that covers the lower layer and supports rhodium and the carrier; An exhaust gas purifying catalyst characterized by comprising an upper layer having a lower oxygen storage capacity in comparison.
[2] The exhaust gas purifying catalyst according to [1], wherein the upper layer side portion of the lower layer has a higher palladium content than the base material side portion.
[3] The exhaust gas purifying catalyst as described in [2], wherein the substrate side portion has a larger cerium content than the upper layer side portion.
[4] The exhaust gas-purifying catalyst according to [3], wherein the base-side portion does not contain palladium.
[5] The exhaust gas purifying catalyst as set forth in [4], wherein the substrate side portion further contains platinum.
[6] The exhaust gas-purifying catalyst according to any one of items 1 to 5, wherein at least one of the lower layer and the upper layer further contains an alkaline earth metal and / or a rare earth element.
[7] The exhaust gas-purifying catalyst according to item 1, wherein the upper layer further contains palladium.
[8] The exhaust gas purifying catalyst as described in [7], wherein the lower layer portion of the upper layer has a higher rhodium content and a smaller palladium content than the surface portion.

Claims (8)

A substrate;
A lower layer that covers the substrate and includes an oxygen storage material and palladium, and does not include rhodium;
An exhaust gas purifying catalyst comprising: an upper layer which covers the lower layer and includes rhodium and a carrier supporting the rhodium and has a lower oxygen storage capacity than the lower layer.   2. The exhaust gas purifying catalyst according to claim 1, wherein the upper layer side portion of the lower layer has a higher palladium content than the base material side portion.   The exhaust gas purifying catalyst according to claim 2, wherein the substrate side portion has a larger cerium content than the upper layer side portion.   The exhaust gas purifying catalyst according to claim 3, wherein the substrate side portion does not contain palladium.   The exhaust gas purifying catalyst according to claim 4, wherein the portion on the substrate side further contains platinum.   The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein at least one of the lower layer and the upper layer further contains an alkaline earth metal and / or a rare earth element.   The exhaust gas purifying catalyst according to claim 1, wherein the upper layer further contains palladium.   The exhaust gas purifying catalyst according to claim 7, wherein the lower layer portion of the upper layer has a higher rhodium content and a lower palladium content than the surface portion.

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