JPH08164338A - Exhaust gas purifying catalyst for internal combustion engine - Google Patents

Abstract

PURPOSE: To provide a catalyst for preventing discharge of unpurified HC at a cold time of an internal combustion engine and enabling to efficiently decompose HC and NOx in the exhaust gas at the later time. CONSTITUTION: A first layer 2 in which Pd is a catalyst metal, a rare earth oxides layer 3 and a second catalyst layer 4 in which Pt, Rh are catalyst metals are successively formed on surfaces of each HC adsorbent particle 1 consisting of inorganic crystalline molecular sheaves on a carrier 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst for an internal combustion engine.

[0002]

2. Description of the Related Art As a catalyst for purifying HC (hydrocarbons), CO (carbon monoxide) and NOx (nitrogen oxides) in exhaust gas of an internal combustion engine, a first catalyst containing zeolite as a main component on a monolith carrier. It is known that a catalyst layer is provided, and a second catalyst layer containing a noble metal catalyst having a redox ability as a main component is provided on the first catalyst layer (JP-A-2-562).
No. 47). If necessary, P may be added to the first catalyst layer.
Noble metals such as t, Pd, Rh, CeO ₂ (ceria), La
A rare earth oxide such as ₂ O ₃ (lanthanum) is supported, and a noble metal such as Pt, Pd, or Rh is supported on the alumina coat layer of the second catalyst layer, and if necessary, the rare earth oxide or zirconium oxide is supported. Carried.

The exhaust gas purifying catalyst adsorbs HC in the exhaust gas by the zeolite of the first catalyst layer when the internal combustion engine is cold and the air-fuel ratio is in a rich state, and is exhausted by warming up. When the gas temperature and the catalyst temperature rise, the HC desorbed from the first catalyst layer, the HC and CO in the exhaust gas, and the NOx are reduced and oxidized by the second catalyst layer.

[0004]

DISCLOSURE OF THE INVENTION The present inventor
Regarding the function of t, Pd and Rh as the catalytic metal, Pt
Pd and Pd have a high oxidizing ability, but Rh has a lower oxidizing ability. Therefore, Pt and Pd are effective in purifying HC. Moreover, when Pt and Rh are combined, HC is oxidized. At the same time, it has been found that NOx decomposition proceeds relatively efficiently.

However, in the above prior art, the first
When the above Pt, Pd, and Rh are used together as the catalyst metal of the catalyst layer or the catalyst metal of the second catalyst layer, both the HC purification rate and the NOx purification rate are higher than when Pt and Rh are used together. There is a problem of decrease. This is Pt, R
It is considered that h and Pd interfere with each other so as to inhibit their catalytic functions. On the other hand, for example, Pd is the first
It is possible to separate Pd from Pt and Rh by supporting Pt and Rh on the second catalyst layer in the catalyst layer, but Pd is separated from the interface between the first catalyst layer and the second catalyst layer.
And Pt and Rh come into contact with each other, and good results are not always obtained.

Therefore, in the present invention, the above Pd is combined with the above Pt and Rh so that its original catalytic function is exhibited, and the HC purification rate during cold and after warm-up of the internal combustion engine is increased and NOx purification is performed. It tries to increase the rate.

[0007]

Means for Solving the Problem and Its Action As a result of various experiments and studies on the above problems, the present inventor has found that the first catalyst layer containing Pd as a catalyst metal and Pt or Rh
When a layer of a rare earth oxide is interposed between the second catalyst layer containing C as a catalyst metal, each of the catalyst metal and the HC adsorbent effectively exhibits its original function, and It has been found that the object can be achieved, and particularly good results can be obtained when the first catalyst layer, the rare earth oxide layer and the second catalyst layer are formed on each surface of the HC adsorbent particles. Has been completed. Less than,
The invention according to each claim will be specifically described.

<Invention of Claim 1> According to the present invention, an HC adsorbent composed of a powdery inorganic crystalline molecular sieve for adsorbing hydrocarbons in exhaust gas is supported on a carrier, and the above HC A first catalyst layer containing Pd as a catalyst metal is formed on each surface of the adsorbent particles, and a rare earth oxide layer containing a rare earth oxide as a main component is formed on the first catalyst layer. A catalyst for purifying exhaust gas of an internal combustion engine, wherein a second catalyst layer having at least one of Pt and Rh as a catalyst metal is formed on the physical layer.

In the present invention, when the internal combustion engine is cold, HC in the exhaust gas is adsorbed by the HC adsorbent to prevent discharge of unpurified HC. When the exhaust gas temperature and the catalyst temperature rise due to warm-up, desorption of HC adsorbed on each HC adsorbent begins, but on the surface of each HC adsorbent particle, Pd as a catalyst metal is used. Since one catalyst layer is formed, the desorbed HC easily comes into contact with Pd, and therefore Pd efficiently exhibits the catalytic function of oxidatively decomposing the HC. Also in the second catalyst layer, the desorbed HC is decomposed by Pt or Rh. These Pt and Rh are catalytic metals effective for purifying NOx in the exhaust gas, and are desorbed and decomposed. It exerts a catalytic function of reducing and decomposing NOx using HC as a reducing agent. In addition, HC and CO newly discharged from the internal combustion engine are also oxidatively decomposed by the catalytic metals of the first catalyst layer and the second catalyst layer, and the reduction decomposition reaction of NOx proceeds with such an oxidation reaction.

On the other hand, the rare earth oxide present between the first catalyst layer and the second catalyst layer functions as a barrier that prevents the Pt, Rh and Pd from interfering with each other, and these catalyst metals are used. It is possible to prevent deterioration of the catalytic function due to the interaction between the two. On the other hand, if such a barrier also prevents the diffusion of HC in the exhaust gas, the first catalyst layer will not be effectively used for the adsorption / decomposition of HC, but the barrier of the present invention is a rare earth oxide. This rare earth oxide is H
Even if the diffusion of C is slightly hindered, the degree thereof is low, and it does not hinder the adsorption of HC in the exhaust gas by the HC adsorbent in the first catalyst layer. On the contrary, since this rare earth oxide has an O ₂ storage effect, it effectively contributes to the oxidation of HC in the first catalyst layer and the second catalyst layer.

However, in the case of the present invention, since the first catalyst layer, the rare earth oxide layer and the second catalyst layer are formed on the surface of each HC adsorbent particle, Above, oxidative decomposition of HC and reductive decomposition of NOx occur, and the purification rate of each increases.

Here, the carrier may be a monolith carrier or a pellet carrier. Also,
As the inorganic crystalline molecular sieve as the HC adsorbent, aluminosilicate (various zeolites such as Y zeolite, mordenite, ZSM5 and beta zeolite) using Al as a metal forming a crystal skeleton (crystal lattice), Al Alternatively, or together with Al, Ga, C
Other crystalline porous metal-containing silicates using other metals such as e, Mn, and Tb, as well as various materials such as those containing almost only silica and those not containing Si can be adopted.

<Invention of Claim 2> The present invention is the exhaust gas purifying catalyst for an internal combustion engine according to claim 1, wherein the rare earth oxide is CeO _2. Is a catalyst for purifying exhaust gas.

In the present invention, C is used as the rare earth oxide.
The reason why eO ₂ is used is that the O ₂ storage effect of CeO ₂ is high and it is advantageous for the oxidative decomposition of HC.

<Invention of Claim 3> According to the present invention, in the catalyst for purifying exhaust gas of an internal combustion engine according to claim 3, the carrier is a monolith carrier, and the carrier per liter of the carrier is the above. CeO ₂ amount is 15-100g
Is a catalyst for purifying exhaust gas of an internal combustion engine.

In the invention, the reason why the monolith carrier is used is that it is advantageous for contact between the exhaust gas and the catalyst layer, and that the catalyst can be made smaller and lighter and the back pressure is less increased. Also, CeO per liter of carrier is
_The amount of ₂ is 15 or more in order to secure the effect of blocking Pd of the first catalyst layer and Pt or Rh of the second catalyst layer, and the CeO ₂ amount is 100 g or less than that. When the amount increases, HC from the second catalyst layer to the first catalyst layer
This is because it is disadvantageous to the diffusion of.

<Invention of Claim 4> This invention is characterized in that in the exhaust gas purifying catalyst for an internal combustion engine described in claim 1, the HC adsorbent is a crystalline aluminosilicate. It is a catalyst for purifying exhaust gas of an internal combustion engine.

In the present invention, the aluminosilicate is used as the HC adsorbent because it has heat resistance and has a high HC adsorbing ability.

[0019]

According to the invention of claim 1, a first catalyst layer containing Pd as a catalyst metal is formed on each HC adsorbent particle,
Since the second catalyst layer having t or Rh as a catalyst metal and the rare earth oxide layer are formed, and the rare earth oxide layer is interposed between the first catalyst layer and the second catalyst layer, the second catalyst layer From first
While permitting the diffusion of HC into the catalyst layer, it is possible to fully exhibit the catalytic function of Pd while avoiding interference between Pd or Rh and Pd, and preventing the emission of unpurified HC during cold weather. However, the HC and NOx can be efficiently purified, and since the HC and NOx are decomposed and purified on each HC adsorbent particle, a high HC purification rate and a high NOx purification rate can be obtained.

According to the invention of claim 2, high the CeO ₂ from using CeO ₂ as the rare earth oxide O
₂ storage effect can be utilized to oxidative decomposition of the HC, which is advantageous in improvement of the HC purification rate and the NOx purification rate.

According to the invention of claim 3, the CeO
₂ to 15-100 g per liter of monolith carrier
Therefore, while diffusing HC from the second catalyst layer to the first catalyst layer, Pd of the first catalyst layer and Pt or R of the second catalyst layer are formed.
It is advantageous in avoiding contact with h.

According to the fourth aspect of the present invention, since the crystalline aluminosilicate is used as the HC adsorbent, the HC adsorbing ability of the catalyst is increased, which is advantageous for improving the HC purification rate and the NOx purification rate.

[0023]

Embodiments of the present invention will be described below with reference to the drawings.

<Example 1> -Preparation of catalyst-Proton-type Y zeolite powder (Cayban ratio 80) as an HC adsorbent was wash-coated on a honeycomb-shaped monolithic carrier (400 cells / inch ² ) made of cordierite. This wash coat was prepared by combining the Y zeolite and hydrated alumina as a binder at a weight ratio of 100: 20, adding an appropriate amount of pure water, and stirring and mixing to prepare a slurry, and immersing the carrier in the slurry. By repeating a process of pulling up, blowing out excess slurry, and drying, a predetermined amount of Y zeolite is supported on the carrier, and then firing at 500 ° C. for 2 hours is performed.

Next, the wash coat layer was loaded with Pd by an impregnation method to form a first catalyst layer. In this impregnation and loading, the washcoat layer is impregnated with an aqueous solution of palladium nitrate having a predetermined concentration, followed by drying and firing (500 ° C. × 2 hours).

Next, cerium nitrate is dissolved in pure water to prepare a cerium nitrate aqueous solution, the washcoat layer is impregnated with the cerium nitrate aqueous solution, and drying and firing are performed to form an aqueous solution on the first catalyst layer. A CeO ₂ layer was formed as a rare earth oxide layer.

Next, a platinum nitrate-P salt solution (dinitrodiamine platinum (II) nitric acid acidic aqueous solution) and an aqueous rhodium nitrate aqueous solution are mixed, and the mixed aqueous solution is impregnated into the washcoat layer to catalyze Pt and Rh. A second catalyst layer made of metal was formed on the CeO ₂ layer.

Therefore, in the catalyst structure of this example, as shown in FIG. 1, the first catalyst layer 2, the CeO ₂ layer 3 and the second catalyst layer 4 are arranged from the bottom on the surface of each HC adsorbent particle 1 on the carrier. The layers are formed in order.

-Evaluation of catalyst- [NOx purification performance] The carrier 1 was prepared by the above-mentioned catalyst preparation method.
The amount of HC adsorbent supported on the first catalyst layer per liter is 13
Several kinds of catalysts were prepared, in which the amount of Pd carried was 0 g, the amount of Pd carried was 7 g, the amount of CeO ₂ carried was 36 g, and the carried amounts of Pt and Rh in the second catalyst layer were different from each other. Pt and R
The weight ratio with h was 5: 1 in all cases. Then, after subjecting these catalysts to heat treatment, each catalyst was incorporated into a simulated exhaust gas flow device, and the inlet gas temperature of each catalyst was 400 ° C.
The NOx purification rate (NOx C400) at that time was examined. The measurement conditions are as follows.

Heat treatment: 900 ° C. × 50 hours (in air) Simulated exhaust gas: Components N ₂ , CO ₂ , CO, C ₃ H ₆ ,
O ₂ , H ₂ , NO, where A / F = 15.0 and H ₂
10% of O is added. Space velocity; SV = 60000h ^-1 Temperature rise rate of simulated exhaust gas: 30 degrees / min (from room temperature to 500
(Up to ℃)

The reason why the A / F value of the simulated exhaust gas is set to be leaner than the stoichiometric air-fuel ratio is to make purification of NOx by the catalyst difficult and to make the performance difference between the catalysts easy.

The results are shown in FIG. 2 together with those of Comparative Example 1. In Comparative Example 1, after the same carrier as in Example 1 was washed with HC adsorbent, CeO ₂ → Pd → P
The impregnation was carried out in the order of t and Rh, and the order of impregnation was different, but the washcoat method and impregnation method, and the materials were the same as in the examples. Further, in Comparative Example 1, the amount of HC adsorbent carried was 130 g / L, the amount of Pd carried was 7 g / L, C
The amount of eO ₂ carried is 36 g / L, and the amount of Pt and Rh combined is 1.6 g / L.

From the figure, it can be seen that a high NOx purification rate is obtained in the example. This is because the CeO ₂ layer is present between the first catalyst layer (Pd) and the second catalyst layer (Pt, Rh) and prevents interference between Pd and Pt and Rh. Then, looking at the examples, the NOx purification rates are Pt and R
It can be seen that it is governed by the carried amount of h. Also, Pt
And if the total amount of Rh carried is 0.8 g / L or more,
It can be seen that a high NOx purification rate can be obtained. Also, Pt
Even if the total amount of Rh and Rh carried exceeds 1 g, the NOx purification rate is not significantly improved, so the total amount is 1.0
It can be seen that it should be g / L or more.

[HC light-off performance] According to the above-mentioned catalyst preparation method, the amount of HC adsorbent carried on the first catalyst layer per liter of carrier was 130 g, the amount of Pd carried was 7 g, and Pt and Rh of the second catalyst layer were combined. The carried amount is 1.6 g / L,
Several kinds of catalysts having different CeO ₂ loadings were prepared. Then, after subjecting these catalysts to the same heat treatment as in the previous case, each catalyst is installed in a simulated exhaust gas flow device, and the light-off temperature of HC, that is, the exhaust gas temperature when the HC purification rate becomes 50% ( HCT50) was investigated. The measurement conditions are the same as in the case of the evaluation of the NOx purification performance except for the air-fuel ratio of the simulated exhaust gas. The air-fuel ratio of the simulated exhaust gas was varied within a range of ± 0.9 centering on A / F = 14.7. The frequency of this fluctuation is 1 Hz
And

The results are shown in FIG. 3 together with those of Comparative Example 1 above. According to the figure, it is understood that the example has a lower light-off temperature and a higher HC purification performance than the comparative example 1. Then, looking at the examples, the amount of CeO ₂ supported is 15 g /
The light-off temperature of HC at the time of L is the lowest, and the light-off temperature is high when the carried amount is smaller or larger than that. When the amount of CeO ₂ supported is small, the HC light-off temperature is high because Pd of the first catalyst layer and Pt and Rh of the second catalyst layer interfere with each other, and their catalytic functions are sufficiently exhibited. It is considered that the HC light-off temperature becomes high when the CeO ₂ loading amount increases, because the HC to the first catalyst layer is
It is considered that this is because the CeO ₂ hinders the diffusion and migration of Al. Also, from the figure, the amount of CeO ₂ supported is 15 to 100.
It can be seen that a high HC purification rate can be obtained with g / L.

[Regarding Pd Carrying Amount and HC Light-Off Performance, etc.] Proton-type Y zeolite (Cayban ratio 80) powder was made to be 130 g / L by a wash coat method on a cordierite honeycomb monolithic carrier. A plurality of types of catalysts were prepared which were supported and on which the Pd was impregnated in the coating layer in various amounts.
Then, using these catalysts, changes in the HC light-off temperature with respect to changes in the amount of Pd carried were examined. The above washcoat method and impregnation method are the same as the above-mentioned methods for preparing the catalyst, and the measurement conditions and methods for the HC light-off performance are also the same as above.

The results are shown in FIG. According to the figure, the HC light-off temperature decreases as the amount of Pd carried increases, but the decrease of the HC light-off temperature does not proceed so much when the amount exceeds a certain amount. The relationship between the Pd loading amount and the HC light-off performance is the same when the rare earth oxide layer and the second catalyst layer are provided. Therefore, in order to improve the HC light-off performance, the Pd loading amount is 6 g. It can be said that it is appropriate to set it to be / L or more.

<Example 2> -Preparation of catalyst of Example 2- 500 g of ultra-stabilized Y-zeolite (Cayban ratio 30) powder as HC adsorbent, hydrated alumina powder (binder) 1
A slurry was prepared by combining 50 g and 1.5 L of water with stirring and mixing. The same carrier as in Example 1 was wash-coated with this slurry to carry a predetermined amount of the HC adsorbent.

Next, 200 g of a palladium nitrate aqueous solution having a Pd concentration of 4.4 wt% was prepared, and the HC adsorbent coating layer of the above carrier was completely impregnated with it, dried, and then calcined (5
The first catalyst layer was formed on the surface of each HC adsorbent particle by performing the heating at 00 ° C. for 2 hours.

Next, by dissolving cerium nitrate in pure water, an aqueous cerium nitrate solution 6 having a Ce concentration of 6.0 wt% 6 is obtained.
40 g was prepared, the above HC adsorbent layer was completely impregnated with it, dried, and then fired in the same manner,
A CeO ₂ layer was formed on the first catalyst layer of each HC adsorbent particle.

Next, the platinum nitrate-P salt solution and the rhodium nitrate aqueous solution were mixed to give a Pt concentration of 0.8.
7 wt%, Rh concentration 0.17 wt% mixed aqueous solution 200
g, and impregnate all of the above with the HC adsorbent layer,
After drying, each H
A second catalyst layer having Pt and Rh as catalyst metals was formed on the CeO ₂ layer of the C adsorbent particles.

The catalyst obtained has 1.3 L of the above carrier and H.
C adsorbent loading is 150 g / L, Pd loading is 7 g / L
L, CeO ₂ supported amount is 36 g / L, Pt and Rh combined amount is 1.6 g / L (however, Pt: Rh = 5:
It was 1).

-Preparation of catalyst of Comparative Example 2-In the same manner as in Example 2, an HC adsorbent coat layer was formed on a carrier, and Pd was supported thereon by an impregnation method. And
Pt concentration 3.7 wt% and Rh concentration 0.74 wt which are formed by mixing platinum nitrate-P salt solution and rhodium nitrate aqueous solution.
% CeO ₂ powder in 100 g of 100% mixed aqueous solution, water is evaporated by performing mixing and stirring while heating on a hot plate, and baking (500 ° C. × 2 hours).
did. This CeO ₂ powder 10 carrying Pt and Rh
A slurry was obtained by stirring and mixing 0 g, 15 g of hydrated alumina and 300 mL of pure water. Using this slurry, wash coating was performed on the HC adsorbent coating layer of the above carrier.

Accordingly, in the obtained catalyst of Comparative Example 2, the upper and lower two layers were formed on the carrier, the lower layer was the HC adsorbent layer supporting Pd, and the upper layer was C supporting Pt and Rh.
It is an eO ₂ layer. In addition, the carrier is 1.3 L, the HC adsorbent loading amount is 150 g / L, the Pd loading amount is 7 g / L, P
The amount of CeO ₂ loaded with t and Rh is 38 g / L.
(However, Pt: Rh = 5: 1).

—Preparation of Catalyst of Comparative Example 3— As in Example 2, an HC adsorbent coating layer was formed on the carrier, and the same amount of CeO ₂ was prepared by the same impregnation method using an aqueous cerium nitrate solution. Then, the same amount of Pd was carried by the same impregnation method using an aqueous palladium nitrate solution. Therefore, the obtained catalyst of Comparative Example 3 was Pt.
And Rh are different from the catalyst of Example 2.

—Preparation of Catalyst of Comparative Example 4— As in Example 2, an HC adsorbent coating layer was formed on a carrier, and the same amount of CeO ₂ was used by the same impregnation method using an aqueous cerium nitrate solution for the coating layer. Then, the same amount of Pt and Rh was carried by the same impregnation method using a mixed aqueous solution of a platinum nitrate-P salt solution and a rhodium nitrate aqueous solution. Therefore, the obtained catalyst of Comparative Example 4 is different from the catalyst of Example 2 in that Pd is not contained.

-Evaluation of Catalyst- For each of the above catalysts, a V-type 6 cylinder 3000c was used.
It was incorporated into the exhaust system of an automobile equipped with the engine of c, and the exhaust gas purification performance was evaluated in the traveling mode LA-4. The results are shown in Table 1.

[0048]

[Table 1]

According to the table, in Example 2, both the HC purification rate and the NOx purification rate are high. When Example 2 and Comparative Example 2 are compared, the HC purification rate is higher in Example 2 and the NOx purification rate is higher in Comparative Example 2 because in Example 2, Whereas the CeO ₂ layer is formed on one catalyst layer (Pd), in Comparative Example 2, the thick CeO ₂ layer is HC
This is because it is formed so as to cover the entire adsorbent layer.
Comparative Example 3 has a low NOx purification rate because Pt and Rh are absent, and Comparative Example 4 has a low HC purification rate because Pd does not exist.

Example 3 Preparation of Catalysts of Examples and Comparative Examples A catalyst of Example 3 having a similar structure was prepared by the same method as in Example 2. The catalyst of Example 3 is different from the catalyst of Example 2 in that H-type ultra-stabilized Y-type zeolite having a Caban ratio of 80 is used as the HC adsorbent and the amount of CeO ₂ supported is 35 g / L. Moreover, each catalyst of the following comparative examples 5-9 was prepared.

Comparative Example 5 is a catalyst having the same structure as that of Comparative Example 2 above, but the other conditions such as the type of HC adsorbent and the amount of CeO ₂ supported were the same as in Example 3.

The catalyst of Comparative Example 6 is one in which an HC adsorbent layer, a Pd-supporting CeO ₂ layer, and a Pt- and Rh-supporting CeO ₂ layer are formed in this order from the bottom on the carrier. Other conditions such as the type of agent and the amount of CeO ₂ supported were the same as in Example 3.

In the catalyst of Comparative Example 7, an HC adsorbent layer was formed on a carrier, Pt and Rh were supported on the HC adsorbent layer by an impregnation method, and a Pd-supported CeO ₂ layer was formed thereon. Then, Pt and Rh were supported on the CeO ₂ layer by an impregnation method, and other conditions such as the type of HC adsorbent and the amount of CeO ₂ supported were the same as in Example 3.

The catalyst of Comparative Example 8 was prepared by forming an HC adsorbent layer on a carrier and impregnating and supporting CeO ₂ → Pd → Pt and Rh in this order. Other conditions, such as the amount of CeO ₂ supported, were the same as in Example 3.

The catalyst of Comparative Example 9 was obtained by forming an HC adsorbent layer on a carrier and forming a CeO ₂ layer carrying Pd, Pt and Rh on the above HC adsorbent layer by washcoating. Other conditions such as the type of HC adsorbent and the amount of CeO ₂ supported were the same as in Example 3.

-Evaluation of Catalysts-Each of the above catalysts was heat-treated and then incorporated into a simulated exhaust gas flow device, and the inlet gas temperature of each catalyst was 400.
The NOx purification rate (NOx C400) and the light-off temperature of HC (HC T50) at 0 ° C. were examined. The conditions of heat treatment and the conditions and method of measurement in this case were the same as those in Example 1, and the results are shown in Table 2.

[0057]

[Table 2]

According to the table, in Example 3, both the purification performance of HC and the purification performance of NOx are high. Here, Comparative Example 5
6 has a high HC T50 because the thick CeO ₂ layer covers the entire HC adsorbent layer, and Comparative Examples 7 and 8 have a high HC T50 because Pd and the HC adsorbent are separated. In Comparative Examples 8 and 9, it is recognized that NOx C400 is low because Pd is close to Pt and Rh.

[Brief description of drawings]

FIG. 1 is a sectional view showing a catalyst structure of Example 1.

FIG. 2 is a graph showing the relationship between the NOx purification rate and the Pt / Rh loading amount when the catalyst inlet gas temperature is 400 ° C.

FIG. 3 is a graph showing the relationship between the amount of CeO ₂ carried and the light-off temperature in HC purification.

FIG. 4 is a graph showing the relationship between the amount of impregnated Pd and the light-off temperature in HC purification.

[Explanation of symbols]

1 HC Adsorbent Particles 2 First Catalyst Layer 3 CeO ₂ Layer (Rare Earth Oxide Layer) 4 Second Catalyst Layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/86 ZAB 53/94 B01J 29/12 ZAB A B01D 53/36 ZAB 102 B 104 A

Claims (4)

[Claims] 1. An HC adsorbent comprising a powdery inorganic crystalline molecular sieve that adsorbs hydrocarbons in exhaust gas is carried on a carrier, and Pd is adsorbed on each surface of the HC adsorbent particles. A first catalyst layer containing a catalytic metal is formed, a rare earth oxide layer containing a rare earth oxide as a main component is formed on the first catalyst layer, and Pt and Rh of Rh are formed on the rare earth oxide layer. A catalyst for purifying exhaust gas of an internal combustion engine, wherein a second catalyst layer having at least one of them as a catalyst metal is formed. 2. The exhaust gas purifying catalyst for an internal combustion engine according to claim 1, wherein the rare earth oxide is CeO ₂ . 3. The exhaust gas purifying catalyst for an internal combustion engine according to claim 3, wherein the carrier is a monolith carrier, and the amount of CeO ₂ per liter of the carrier is 15 to 100 g. A catalyst for purifying exhaust gas of an internal combustion engine, characterized by: 4. The exhaust gas purifying catalyst for an internal combustion engine according to claim 1, wherein the HC adsorbent is a crystalline aluminosilicate.