JP2002045702A - Catalyst for purifying exhaust gas - Google Patents

Catalyst for purifying exhaust gas

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Publication number
JP2002045702A
JP2002045702A JP2000239733A JP2000239733A JP2002045702A JP 2002045702 A JP2002045702 A JP 2002045702A JP 2000239733 A JP2000239733 A JP 2000239733A JP 2000239733 A JP2000239733 A JP 2000239733A JP 2002045702 A JP2002045702 A JP 2002045702A
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Japan
Prior art keywords
catalyst
layer
catalyst layer
exhaust gas
adsorbent
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
JP2000239733A
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Japanese (ja)
Inventor
Koichi Kasahara
Hironori Satou
Kenichi Taki
容規 佐藤
健一 滝
光一 笠原
Original Assignee
Cataler Corp
株式会社キャタラー
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Priority to JP2000239733A priority Critical patent/JP2002045702A/en
Publication of JP2002045702A publication Critical patent/JP2002045702A/en
Pending legal-status Critical Current

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Abstract

PROBLEM TO BE SOLVED: To improve hydrocarbon(HC) purifying performance of a catalyst for purifying exhaust gas having an HC adsorbent layer and a three-way catalyst layer. SOLUTION: This catalyst for purifying exhaust gas comprises the HC adsorbent layer 2 formed on the surface of a base material 1, a lower catalyst layer 3 carrying Pd and formed on the surface of the HC adsorbent layer 2, and an upper catalyst layer 4 carrying Pt and Rh and formed on the surface of the lower catalyst layer 3. The HC adsorbent layer 2 adsorbs hydrocarbons at low temperatures, and hydrocarbons discharged from the HC adsorbent layer 2 are oxidized and purified by Pd in the lower catalyst layer 3. Further, CO and NOx are purified by Pt and Rh contained in the upper catalyst layer 4. The heat deterioration of Pd is suppressed, and a trouble, such as a decline in activity, caused by covering Pt an Rh with Pd is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst using an HC adsorbent and an oxidation catalyst or a three-way catalyst, and more particularly to an exhaust gas purifying catalyst capable of efficiently purifying HC (hydrocarbon) in exhaust gas from a low temperature range. About.

[0002]

2. Description of the Related Art As an exhaust gas purifying catalyst for automobiles, an oxidation catalyst in which a noble metal such as Pt (platinum) is supported on a porous carrier such as alumina has been used. In this oxidation catalyst, HC (hydrocarbon) and CO (carbon monoxide) in exhaust gas
Is oxidized and purified.

A catalyst in which a noble metal such as Pt is supported on a porous carrier such as alumina is used to control the air-fuel ratio to the stoichiometric air-fuel ratio to oxidize CO and HC in exhaust gas and to reduce NO x (nitrogen oxide). Since the reduction is performed at the same time, it is called a three-way catalyst. As such a three-way catalyst, for example, a porous carrier layer composed of γ-alumina is formed on a heat-resistant honeycomb substrate composed of cordierite or the like, and P is formed on the porous carrier layer.
Those supporting noble metals such as t and Rh (rhodium) are widely used.

[0004] However, in the oxidation catalyst and the three-way catalyst,
There is a problem that a catalytic reaction does not occur until the supported noble metal has a temperature equal to or higher than its activation temperature. In order to do such as during-start or during a cold, a noble metal for the temperature of the exhaust gas is low does not reach the activation temperature, the purification of HC and NO x is difficult.

Therefore, as disclosed in, for example, JP-A-5-057148 or JP-A-6-154538,
Exhaust gas purifying devices have been developed in which an HC adsorbent such as zeolite is disposed upstream of an oxidation catalyst or a three-way catalyst in the exhaust gas flow direction. In this exhaust gas purifying device, HC in exhaust gas is first adsorbed by an HC adsorbent at a low temperature, and the adsorbed HC is desorbed at the time of temperature rise and is disposed downstream to be an oxidation catalyst or a three-way catalyst having an activation temperature or higher. The catalyst is oxidized and purified.

Therefore, according to such an exhaust gas purifying apparatus, HC contained in the exhaust gas at the time of a cold state or at the time of starting, etc.
HC is adsorbed by the adsorbent, suppressing emissions. HC released from HC adsorbent at high temperature and HC in exhaust gas are oxidized and purified by oxidation catalyst or three-way catalyst, so HC emission is suppressed from low temperature to high temperature. HC that can be discharged in an unpurified state
The amount can be reduced.

Further, Japanese Patent Application Laid-Open Nos. 11-210451 and 11-210451
Japanese Patent No. 104462 discloses a monolithic catalyst having an integral honeycomb shape, in which a coat layer made of a powder of an HC adsorbent is formed as a lower layer, and a coat layer made of a powder of an oxidation catalyst or a three-way catalyst is formed as an upper layer. Exhaust gas purifying catalysts have been proposed.

In this exhaust gas purifying catalyst, at low temperatures, HC in the exhaust gas passes through the upper layer, which has not reached the activation temperature, and is adsorbed by the lower HC adsorbent, and the adsorbed HC is released as the temperature rises. Then, it is oxidized and purified by the upper oxidation catalyst or the three-way catalyst which is at or above the activation temperature. Therefore, the HC contained in the exhaust gas is adsorbed by the HC adsorbent at the time of cold or start-up, so that the emission is suppressed.
HC released from the adsorbent and HC in the exhaust gas are oxidized and purified by an oxidation catalyst or three-way catalyst.
Can be suppressed.

[0009]

However, in an exhaust gas purifying apparatus in which an HC adsorbent such as zeolite is disposed on the upstream side of an oxidation catalyst or a three-way catalyst, the heat of exhaust gas is reduced by the HC on the upstream side.
Due to the deprivation of the adsorbent, the temperature rise of the downstream oxidation catalyst or the three-way catalyst is hindered, and the time required to raise the activation temperature of the supported noble metal becomes longer, during which the HC purification rate is low. was there.

Regarding the above problems, an exhaust gas purifying catalyst having an HC adsorbent layer formed as a lower layer and an oxidation catalyst layer or a three-way catalyst layer formed thereon is more advantageous. However, in view of the recent tightening of exhaust gas regulations, further improvement in purification performance is desired.

The present invention has been made in view of such circumstances, and it is an object of the present invention to further improve HC purification performance in an exhaust gas purification catalyst including an HC adsorbent layer and an oxidation catalyst layer or a three-way catalyst layer. Aim.

[0012]

The exhaust gas purifying catalyst of the present invention which solves the above-mentioned problems is characterized by a substrate having heat resistance, an HC adsorbent layer formed on the surface of the substrate, and a porous carrier. A lower catalyst layer formed on the surface of the HC adsorbent layer carrying Pd, and an upper catalyst layer formed on the surface of the lower catalyst layer formed by carrying Pt and Rh on a porous carrier. It is in.

In the exhaust gas purifying catalyst, at least one of the lower catalyst layer and the upper catalyst layer has at least Ce.
It is desirable to include an oxide containing

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS When exhaust gas in a low temperature range such as at the time of start-up flows into the exhaust gas purifying catalyst of the present invention, the noble metal carried on the exhaust gas has not reached the activation temperature.
Reaches the bottom HC adsorbent layer without being oxidized and
Adsorbed by the adsorbent. Thereby, the emission of HC is suppressed.

When the temperature of the exhaust gas rises and the noble metal carried becomes higher than the activation temperature, HC in the exhaust gas is oxidized and purified by the noble metal carried on the upper catalyst layer and the lower catalyst layer. The adsorbed HC is released from the HC adsorbent layer, and is oxidized and purified by the noble metal carried when the HC passes through the lower catalyst layer and the upper catalyst layer. Thereby, the emission of HC is suppressed even in a high temperature range.

In the exhaust gas purifying catalyst of the present invention, Pd is contained in the lower catalyst layer. Since Pd has a higher HC oxidizing activity than Pt and Rh, it is possible to efficiently oxidize and purify HC released from the HC adsorbent by including it in the lower catalyst layer formed on the HC adsorbent layer. .

Further, in the exhaust gas purifying catalyst of the present invention, Pt and Rh contained in the upper catalyst layer are excellent in CO oxidizing activity and NO x reducing activity. Therefore, CO in the exhaust gas is oxidized and purified by Pt and Rh contained in the upper catalyst layer,
NO x is reduced and purified.

Further, in the exhaust gas purifying catalyst of the present invention, since the exhaust gas reaches the lower catalyst layer after passing through the upper catalyst layer, the heat of the exhaust gas is not directly transmitted to the lower catalyst layer. Therefore, the probability that Pd contained in the lower catalyst layer is exposed to high heat is low, and the thermal degradation of Pd is suppressed. Further, since Pd is separately carried from Pt and Rh, it is possible to prevent a problem that Pt and Rh are covered with Pd and the activity is reduced. For these reasons, the exhaust gas purifying catalyst of the present invention has excellent durability.

The shape of the substrate can be a pellet shape or a honeycomb shape. As the material, a heat-resistant ceramic such as cordierite, a honeycomb body formed by winding a metal foil, or the like can be used.

As the HC adsorbent, ferrierite, ZSM-
5, zeolites such as mordenite, Y-type zeolite and β-type zeolite can be used. It is also preferable to use a zeolite carrying a noble metal such as Pd or Ag as the HC adsorbent. By supporting the noble metal in this way,
Adsorption of low molecular weight HC is further improved. HC adsorbent layer
It can be formed by adhering the HC adsorbent powder together with a ceramic binder or the like to the surface of the base material and firing it. The thickness of the HC adsorbent layer is not particularly limited, and can be arbitrarily formed within a range that does not increase the ventilation resistance.

The lower catalyst layer comprises Pd supported on a porous carrier, and is formed on the surface of the HC adsorbent layer. As the porous carrier, alumina, silica, silica-alumina, zirconia, titania, or the like can be used.
Among them, γ-alumina having excellent adsorption characteristics and heat resistance is particularly preferable. The amount of Pd carried in the lower catalyst layer is suitably in the range of 0.5 to 10 g per liter of the substrate. If Pd is less than this range, the HC purification rate will decrease, and even if Pd is loaded more than this range, the effect will be saturated and the cost will rise.

The lower catalyst layer preferably contains an oxide containing at least Ce (cerium). Since the oxide containing Ce has an oxygen storage / release capability, the oxygen concentration in the exhaust gas can be stabilized. Therefore, the exhaust gas can be stably brought to a stoichiometric atmosphere, so that the activity of the three-way catalyst layer is significantly improved. Note that ceria can be used as the oxide containing Ce, but it is preferable to use a composite oxide in which ceria is combined with at least one selected from zirconia and yttria. By using such a composite oxide, the thermal stability of the oxygen storage / release capability of ceria is improved, and the durability is improved. Also
Complex oxides with metals such as Nd and Sr can also be used.

The oxide containing Ce can be mixed at a ratio of 20 to 500 parts by weight with respect to 100 parts by weight of the porous carrier. If the oxide containing Ce is less than this range, the effect of mixing cannot be obtained, and if the oxide is mixed beyond this range, the amount of the porous carrier relatively decreases, resulting in a reduction in purification performance.

In order to form the lower catalyst layer, for example, a mixed powder of a porous carrier powder and ceria powder is attached to the surface of the HC adsorbent layer together with a ceramic binder and the like, and after firing, Pd is carried. Good. Alternatively, Pd can be previously supported on a porous carrier powder, mixed with ceria powder or the like, and adhered to the surface of the HC adsorbent layer. The thickness of the lower catalyst layer is not particularly limited, and can be arbitrarily formed within a range that does not increase the ventilation resistance.

The upper catalyst layer comprises Pt and Rh supported on a porous carrier, and is formed on the surface of the lower catalyst layer. As the porous carrier, alumina, silica, silica-alumina, zirconia, titania, and the like can be used as in the case of the lower catalyst layer. Among them, γ-alumina having excellent adsorption characteristics and heat resistance is particularly preferable. The amount of Pt carried in the upper catalyst layer is suitably in the range of 0.5 to 10 g per liter of the base material. The amount of Rh supported per liter of substrate
A range of 0.1 to 10 g is suitable. Supported amount is reduced purification rate of CO and NO x less than this range, effect be larger than this range carrying the cost soars with saturated.

The noble metals supported on the upper catalyst layer are Pt and Rh.
is there. NO only by supporting Pt x Poor purification performance
In addition, simply carrying Rh alone is inferior in CO purification activity
Therefore, by carrying both Pt and Rh,
Purification activity is greatly improved by use.

It is preferable that the upper catalyst layer also contains an oxide containing at least Ce. As a result, the oxygen concentration in the exhaust gas can be stabilized, and the exhaust gas can be stably formed into a stoichiometric atmosphere, so that the activity of the three-way catalyst layer is significantly improved. As in the case of the lower catalyst layer, ceria can be used as the oxide containing Ce, but it is preferable to use a composite oxide in which at least one selected from zirconia and yttria is composited. By using such a composite oxide, the thermal stability of the oxygen storage / release capability of ceria is improved, and the durability is improved. Also N
Complex oxides with metals such as d and Sr can also be used.

The oxide containing Ce can be mixed at a ratio of 20 to 500 parts by weight with respect to 100 parts by weight of the porous carrier as in the case of the lower catalyst layer. If the oxide containing Ce is less than this range, the effect of mixing cannot be obtained, and if the oxide is mixed beyond this range, the amount of the porous carrier relatively decreases, resulting in a reduction in purification performance.

In order to form the upper catalyst layer, for example, a mixed powder of a porous carrier powder and ceria powder is adhered to the surface of the lower catalyst layer together with a ceramic binder and the like, and after firing, Pt and Rh are loaded. I just need. Alternatively, Pt and Rh can be previously supported on a porous carrier powder, mixed with ceria powder or the like, and adhered to the surface of the lower catalyst layer. The thickness of the upper catalyst layer is not particularly limited, and can be arbitrarily formed within a range that does not increase the airflow resistance.

In the exhaust gas purifying catalyst of the present invention, the HC adsorbent is used in an amount of 50 to 300 parts by weight based on 100 parts by weight of the total amount of the porous carrier of the lower catalyst layer and the upper catalyst layer. It is desirable to configure the ratio. If the amount of HC adsorbent is less than this, the HC purification rate in the low-temperature region such as at startup decreases, and if the amount of HC adsorbent exceeds this, the HC released from the HC adsorbent cannot be purified and the HC purification rate decreases. I will be.

The exhaust gas purifying catalyst of the present invention can be used as a three-way catalyst if the air-fuel ratio is controlled near stoichiometry, and can be used as an oxidation catalyst without such control.

[0032]

The present invention will be specifically described below with reference to examples and comparative examples.

(Embodiment 1) FIG. 1 is an enlarged sectional view of a main part of an exhaust gas purifying catalyst according to an embodiment of the present invention. This exhaust gas purifying catalyst comprises a cordierite honeycomb substrate 1,
HC adsorbent layer 2 formed on the wall surface of the honeycomb passage of honeycomb substrate 1, lower catalyst layer 3 formed on the surface of HC adsorbent layer 2, and upper catalyst layer formed on the surface of lower catalyst layer 3 And 4.

The lower catalyst layer 3 uses alumina and a ceria-zirconia composite oxide as carriers, and Pd is supported on alumina. The upper catalyst layer uses alumina and a ceria-zirconia composite oxide as carriers, and Pt and Rh are supported on alumina.

Hereinafter, a method for producing the exhaust gas purifying catalyst will be described, and the detailed description of the structure will be replaced.

A slurry was prepared by mixing 100 g of β zeolite powder, 5 g of silica sol as a binder in solid content, and 150 g of water in a ball mill. Next, a cordierite honeycomb substrate 1 having a capacity of 1000 ml was prepared, immersed in the above slurry, pulled up, and after removing excess slurry, dried at 250 ° C. for 2 hours and calcined at 500 ° C. for 2 hours to obtain HC.
The adsorbent layer 2 was formed. The HC adsorbent layer 2 was formed on the honeycomb substrate 1 in an amount of 105 g.

Next, 100 g of alumina powder, an aqueous solution of palladium nitrate (5 g as Pd) and 100 g of water were mixed,
After evaporation to dryness, the mixture was calcined at 500 ° C. for 1 hour and then pulverized to prepare a Pd—Al 2 O 3 powder in which Pd was supported on alumina powder. This Pd-A
l 2 O 3 powder 105 g and composite oxide CeO 2 -ZrO 2 powder 50
g and 2 g of alumina sol as a binder in solid content
And 100 g of water were mixed with a ball mill to prepare a slurry. Then, the honeycomb substrate 1 having the HC adsorbent layer 2 is immersed in this slurry, pulled up, and after removing excess slurry, dried at 250 ° C. for 2 hours and baked at 500 ° C. for 2 hours to form the lower catalyst layer 3. Formed. 157 g of the lower catalyst layer 3 was formed on the honeycomb substrate 1. The supported amount of Pd is 5 g per honeycomb substrate 1.

Next, 100 g of alumina powder, an aqueous solution of platinum nitrate (5 g as Pt), an aqueous solution of rhodium nitrate (1 g as Rh), and 100 g of water were mixed, and evaporated to dryness.
After calcination for an hour, the mixture was pulverized to prepare a Pt-Rh-Al 2 O 3 powder in which Pt and Rh were supported on alumina powder. And the Pt-Rh-Al 2 O 3 powder 106 g, and CeO 2 -ZrO 2 powder 50g is a composite oxide,
2 g of an alumina sol as a binder in solid content and water 1
Was mixed with a ball mill to prepare a slurry. And a honeycomb substrate 1 having an HC adsorbent layer 2 and a lower catalyst layer 3
Was immersed in this slurry, pulled up, and after removing excess slurry, dried at 250 ° C. for 2 hours and calcined at 500 ° C. for 2 hours to form an upper catalyst layer 4. 158 g of the upper catalyst layer 4 was formed on the honeycomb substrate 1. The supported amount of Pt was 5 g per honeycomb substrate 1, and the supported amount of Rh was honeycomb substrate 1.
To 1 g.

Comparative Example 1 The exhaust gas purifying catalyst of Comparative Example 1 is the same as Example 1 except that Pd, Pt and Rh are supported on the upper catalyst layer 4 without the lower catalyst layer 3. Configuration.

That is, 200 g of alumina powder, an aqueous solution of palladium nitrate (5 g as Pd), an aqueous solution of platinum nitrate (5 g as Pt), and an aqueous solution of rhodium nitrate (1 as Rh)
g) and 200 g of water, evaporate to dryness, bake at 500 ° C for 1 hour and pulverize to prepare Pd-Pt-Rh-Al 2 O 3 powder with Pd, Pt and Rh supported on alumina powder did. This Pd-Pt-Rh-Al 2
211 g of O 3 powder and CeO 2 —ZrO 2 powder as a composite oxide 100
g and alumina sol as a binder in a solid content of 4 g
And 400 g of water were mixed with a ball mill to prepare a slurry.

The HC formed in the same manner as in Example 1
After immersing the honeycomb substrate 1 having the adsorbent layer 2 in this slurry and pulling it up, removing excess slurry,
For 2 hours and calcined at 500 ° C. for 2 hours to form an upper catalyst layer 4. 315 g of the upper catalyst layer 4 was formed on the honeycomb substrate 1. The supported amount of Pd is 5 g per honeycomb substrate 1, the supported amount of Pt is 5 g per honeycomb substrate 1,
The amount of Rh supported is 1 g per honeycomb substrate 1.

Comparative Example 2 An exhaust gas purifying catalyst of Comparative Example 2 was prepared in the same manner as in Example 1 except that the HC adsorbent layer 2 was not formed.

Comparative Example 3 An exhaust gas purifying catalyst of Comparative Example 3 was prepared in the same manner as in Example 1 except that the lower catalyst layer 3 was not formed.

Comparative Example 4 An exhaust gas purifying catalyst of Comparative Example 4 was prepared in the same manner as in Example 1 except that the upper catalyst layer 4 was not formed.

Comparative Example 5 First, on the surface of the honeycomb substrate 1,
The HC adsorbent layer 2 was formed, the upper catalyst layer 4 was formed on the surface of the HC adsorbent layer 2, and the lower catalyst layer 3 was formed on the surface of the upper catalyst layer 4. The forming method and amount of each layer are the same as those in the first embodiment.

(Comparative Example 6) First, the lower catalyst layer 3 was formed on the surface of the honeycomb substrate 1, and the HC adsorbent layer 2 was formed on the surface of the lower catalyst layer 3.
Was formed, and the upper catalyst layer 4 was formed on the surface of the HC adsorbent layer 2. The forming method and amount of each layer are the same as those in the first embodiment.

(Comparative Example 7) First, the lower catalyst layer 3 was formed on the surface of the honeycomb substrate 1, the upper catalyst layer 4 was formed on the surface of the lower catalyst layer 3, and the HC adsorbent layer was formed on the surface of the upper catalyst layer 4. 2 was formed.
The forming method and amount of each layer are the same as those in the first embodiment.

Comparative Example 8 First, the upper catalyst layer 4 was formed on the surface of the honeycomb substrate 1, and the HC adsorbent layer 2 was formed on the surface of the upper catalyst layer 4.
Was formed, and the lower catalyst layer 3 was formed on the surface of the HC adsorbent layer 2. The forming method and amount of each layer are the same as those in the first embodiment.

(Comparative Example 9) First, the upper catalyst layer 4 was formed on the surface of the honeycomb substrate 1, the lower catalyst layer 3 was formed on the surface of the upper catalyst layer 4, and the HC adsorbent layer was formed on the surface of the lower catalyst layer 3. 2 was formed.
The forming method and amount of each layer are the same as those in the first embodiment.

Comparative Example 10 An exhaust gas purifying catalyst of Comparative Example 10 was prepared in the same manner as in Example 1 except that an aqueous rhodium nitrate solution was not added to the slurry for forming the upper catalyst layer 4.

Comparative Example 11 An exhaust gas purifying catalyst of Comparative Example 11 was prepared in the same manner as in Example 1 except that an aqueous solution of platinum nitrate was not added to the slurry for forming the upper catalyst layer 4.

<Test / Evaluation> Each of the catalysts of Examples and Comparative Examples was placed 30 cm directly below the engine of a vehicle equipped with a 2.2 L engine.
At each position, air-fuel ratio (A / F) = 14.6 ± 0.1
When driving in US LA # 4 mode with
C, was measured purification rate of CO and NO x, respectively. Table 1 shows the results.

[0053]

[Table 1]

From Table 1, it can be seen that Comparative Example 1 is inferior in purification performance to Example 1 in spite of carrying the same amount of noble metal as in Example 1. This is probably because Pd covered Pt and Rh, and the activity of Pt and Rh was reduced.

In Comparative Example 2, since there is no HC adsorbent layer 2, the HC purification rate is particularly lower than in Example 1.

In Comparative Example 3, since the lower catalyst layer 3 was not provided,
It is not performed purified by pd, HC, although purification of CO and NO x are both low, and conspicuous particularly low HC purification rate.

In Comparative Example 4, the upper catalyst layer 4 was not provided.
Purification by Pt and Rh is not performed, is lower HC, purification rate of CO and NO x are both.

In Comparative Example 5, since the lower catalyst layer 3 exists on the outermost surface, it is considered that Pd was thermally deteriorated and the HC purification rate was lowered.

In Comparative Example 6, the lower catalyst layer 3 is located below the HC adsorbent layer 2. Therefore, it is considered that the probability that HC released from the HC adsorbent comes into contact with Pd was reduced, and the HC purification rate was reduced.

In Comparative Example 7, the HC adsorbent layer 2 exists on the outermost surface. Therefore, it is considered that HC released from the HC adsorbent is released almost as it is, and thus the HC purification rate has decreased.

In Comparative Example 8, the lower catalyst layer 3 is located above the HC adsorbent layer 2, and the upper catalyst layer 4 is located below the HC adsorbent layer 2. Therefore, HC released from the HC adsorbent is Pd
Comparative Example 6 because the probability of contact with
HC purification rate is higher than However, since the upper catalyst layer 4 is positioned in the lowermost layer, CO and the NO x purification rate is low, HC purification rate also inferior than Example 1.

In Comparative Example 9, as in Comparative Example 7, the outermost surface
The HC adsorbent layer 2 exists. Therefore, HC released from the HC adsorbent is released almost as it is,
It is considered that the purification rate was lowered. Although the HC purification rate is higher than that of Comparative Example 7, it is considered that Pd of the lower catalyst layer 3 located below the HC adsorbent layer 2 contributes to this.

Further, in Comparative Example 10, the upper catalyst layer 4 carries only Pt, so that the NO x purification rate is low. In Comparative Example 11, the upper catalyst layer 4 carries only Rh, so that the CO purification rate is low. .

That is, it is clear that the catalyst of Example 1 has the highest HC, CO and NO x purification rates and extremely excellent purification performance as compared with the comparative examples. It is clear that this is the effect of the configuration described.

[Brief description of the drawings]

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

[Explanation of Signs] 1: Honeycomb base material 2: HC adsorbent layer 3: Lower catalyst layer 4: Upper catalyst layer

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kenichi Taki 7800 Chihama, Daito-cho, Ogasa-gun, Shizuoka F-term in Cataler Co., Ltd. (reference) 3G091 AA02 AA17 AB02 AB03 AB10 BA03 BA14 BA15 BA19 BA39 FA02 FA04 FA12 FA13 FA16 FB02 FB11 FC07 GA01 GA06 GA08 GA20 GB01X GB01Y GB04W GB05W GB05Y GB06W GB07W GB09X GB09Y GB10W GB10X GB17X HA18 4D048 AA06 AA13 AA18 AB01 AB03 AB05 BA03X BA08X BA11X BA19X BA30X BA31BBA BABABA BABA BABA BABX ABAB BABXA BC71B BC72A BC72B BC75A BC75B CA03 CA07 CA09 CA10 DA06 EA18 ZA19B

Claims (2)

[Claims]
1. A heat-resistant base material, an HC adsorbent layer formed on the surface of the base material, and an underlayer formed on a surface of the HC adsorbent layer by supporting Pd on a porous carrier. An exhaust gas purifying catalyst comprising: a catalyst layer; and an upper catalyst layer formed by supporting Pt and Rh on a porous carrier and formed on the surface of the lower catalyst layer.
2. The exhaust gas purifying catalyst according to claim 1, wherein at least one of the lower catalyst layer and the upper catalyst layer contains an oxide containing at least Ce.
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