JP2003117393A - Exhaust gas purification catalyst - Google Patents

Abstract

(57) [Summary] [Object] To suppress grain growth by suppressing the movement of supported noble metal particles and to suppress a decrease in purification activity after durability. A core body made of a porous oxide has at least
The catalyst particles were composed of catalyst particles 1 supporting Pt and a coating layer 2 formed on the surface of the catalyst particles 1 and made of at least one oxide or composite oxide selected from Al, Zr and Ce. Nuclear body
Since at least a part of the surface of the Pt supported on the Pt is covered with the coating layer 2, the movement of the Pt particles is suppressed even at a high temperature, and the Pt particles aggregate and grow. Is suppressed.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an exhaust gas purifying catalyst for purifying exhaust gas from an automobile engine or the like,
Specifically, it relates to an exhaust gas purifying catalyst having excellent durability.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x has been used as an exhaust gas purifying catalyst for automobiles. As such a three-way catalyst, a coating layer made of γ-alumina is formed on a heat resistant base material made of cordierite or the like, and a noble metal such as platinum (Pt) or rhodium (Rh) is supported on the coating layer. Things are widely known.

By the way, in recent years, the exhaust gas purifying catalyst tends to be installed immediately below the manifold close to the engine, and the exhaust gas temperature becomes high during high-speed traveling. Therefore, the exhaust gas purifying catalyst is often exposed to high temperatures. Has become. However, the conventional exhaust gas-purifying catalyst has a problem that the catalytic performance is deteriorated because the thermal deterioration of γ-alumina proceeds due to the high-temperature exhaust gas, and the catalyst active points are reduced due to the particle growth of the precious metal.

Therefore, for example, Japanese Patent Application Laid-Open No. 4-122441 discloses a method for producing an exhaust gas purifying catalyst in which a noble metal is supported by using alumina which has been heat treated in advance. According to this manufacturing method, since the alumina has already been heat-treated, the resulting exhaust gas-purifying catalyst undergoes almost no thermal deterioration even when exposed to high-temperature exhaust gas, and can prevent grain growth of noble metals. .

In recent years, lean-burn engines, which supply an air-fuel mixture in excess of oxygen, have become the mainstream in order to suppress carbon dioxide emissions. However, in an exhaust gas purifying catalyst produced by the production method disclosed in the above publication, noble metal grain growth is remarkable and catalytic performance is deteriorated when a high temperature of 800 ° C. or higher acts in a lean atmosphere with excess oxygen. There was a problem.

For example, Pt supported on the surface of alumina becomes PtO _{2 in} an atmosphere where oxygen coexists at high temperature, and diffusion and aggregation are promoted by gas phase transfer. Therefore, in a lean atmosphere or stoichiometric atmosphere with excess oxygen, when exposed to high temperatures, grain growth occurs in Pt and the surface area decreases, resulting in a large decrease in catalyst performance.

Therefore, the applicant of the present application has proposed a manufacturing method in which a carrier carrying a noble metal is heat-treated at 800 ° C. or higher in a non-oxidizing atmosphere, as disclosed in JP-A-8-338897. According to this manufacturing method, since the porous carrier is sintered and the pores are shrunk, the supported precious metal is tightly surrounded by the porous carrier. Therefore, since the movement of the noble metal is regulated by the porous carrier even when a high temperature acts in the lean atmosphere, the grain growth of the noble metal can be suppressed.

By the way, since the oxygen concentration in the exhaust gas fluctuates greatly depending on the operating conditions and the like, the purifying activity for oxidation and reduction in the three-way catalyst may become unstable. Therefore, addition of ceria (CeO ₂ ) to the coat layer is performed. Ceria has an oxygen storage capacity (hereinafter referred to as OSC) that stores oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere, and thus stable purification activity can be obtained even if the oxygen concentration in exhaust gas changes.

Further, it is said that a three-way catalyst containing a catalytic metal and ceria tends to lower OSC due to crystal growth of ceria when used at a high temperature of 800 ° C. or higher. Therefore, in order to suppress the crystal growth of ceria and maintain a high OSC, means for adding zirconia (ZrO ₂ ) to ceria has also been developed (JP-A-63-116741 and JP-A-3-1313).
43 publication). Further, JP-A-63-116741 discloses that at least a part of ceria and zirconia is used as a composite oxide or a solid solution. By adding zirconia in this way, heat resistance is improved and OSC after endurance is improved.

Further, JP-A-8-215569 discloses a technique using a Ce-Zr composite oxide prepared from a metal alkoxide. Ce-Zr composite oxide prepared from the metal alkoxide by the sol-gel method, compared to Ce-Zr composite oxide prepared from nitrate, because Ce and Zr are compounded at the atomic or molecular level to form a solid solution, The heat resistance is improved and a high OSC is secured from the initial stage to the end of durability.

Further, Japanese Patent Laid-Open No. 10-202102 discloses Al-
An exhaust gas purification catalyst using a Ce-Zr composite oxide as a carrier is disclosed. According to this exhaust gas purifying catalyst, since Ce and Zr are highly dispersed in Al ₂ O ₃ , the OSC of CeO ₂ can be efficiently extracted. Also, Ce and Zr are Al ₂ O ₃
Since it is a solid solution in Al ₂ O ₃ , the heat resistance of Al ₂ O ₃ is improved, and the heat resistance of the entire composite oxide is improved.

In the exhaust gas purifying catalyst disclosed in JP-A-10-202102, Al, Ce and Zr are uniformly mixed at the atomic or molecular level to form small primary particles, and each particle is almost entirely composed. It is a complex oxide or solid solution. Therefore, this exhaust gas purifying catalyst maintains a stable OSC from the initial use to the endurance at high temperature.

[0013]

However, with the recent improvement in engine performance and the increase in high-speed running, the exhaust gas temperature rises significantly, and the temperature of the exhaust gas-purifying catalyst during use also rises further than in the past. There is. Therefore, JP-A-8-338897
Even with the exhaust gas-purifying catalyst produced by the production method described in the publication, it has been clarified that grain growth occurs in the noble metal when a high temperature exceeding 800 ° C. acts for a long time in a lean atmosphere with excess oxygen. It is considered that this is because the noble metal particles supported outside the pores are not physically and chemically fixed to the carrier and can move freely.

Further, even in a catalyst in which a noble metal is supported on a complex oxide carrier containing CeO ₂ , there is a problem that the OSC is lowered with the grain growth of the noble metals and the purifying activity is lowered.

The present invention has been made in view of the above circumstances, and suppresses the movement of the supported noble metal particles to suppress the grain growth and the deterioration of the purifying activity after the endurance. With the goal.

[0016]

The features of the exhaust gas purifying catalyst of the present invention for solving the above-mentioned problems are that catalyst particles having at least Pt supported on a core made of a porous oxide are formed on the surface of the catalyst particles. And a coating layer made of at least one oxide or complex oxide selected from Al, Zr and Ce.

[0017] karyoplast is preferably made of Al ₂ O ₃ -CeO ₂ -ZrO ₂ composite oxide particles, it is further desirable to further include a PrO _2.

It is also preferable that the core is composed of CeO ₂ -ZrO ₂ composite oxide particles, and the coating layer is a catalyst composed of Rh-supporting ZrO ₂ .

Further, it is desirable that the core body occupies 40 to 80% by weight of the whole, and that the coating layer is formed by a sol-gel method or a coprecipitation method.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION In the exhaust gas purifying catalyst of the present invention,
A coating layer made of at least one oxide or complex oxide selected from Al, Zr, and Ce is formed on the surface of catalyst particles in which at least Pt is supported on a core made of a porous oxide. That is, at least Pt supported on the nucleus
Has at least a part of its surface covered with a coating layer, or a wall of the coating layer is formed around Pt particles. Therefore, the Pt particles are suppressed from moving even at high temperature, and the Pt particles are suppressed from coagulating and growing. Therefore 800 in a lean atmosphere with excess oxygen
Even if a high temperature exceeding 0 ° C. acts for a long time, Pt maintains a highly dispersed state and has many active sites, so that the reduction of the purification activity of the exhaust gas purifying catalyst of the present invention is suppressed.

Al ₂ is used as a nucleus composed of a porous oxide.
_{O 3, ZrO 2, CeO 2} , TiO 2, SiO 2 , etc. can be used various porous oxide, nucleus consisting of CeO ₂ -ZrO ₂ composite oxide particles, or Al ₂ O ₃ -CeO ₂ It is desirable to use a nucleus composed of —ZrO ₂ composite oxide particles. The heat resistance of CeO ₂ is improved by using a core composed of CeO ₂ -ZrO ₂ composite oxide particles, and the OSC is high even after the endurance combined with the effect of suppressing the grain growth of Pt.
Is expressed. Further, in order to obtain a high OSC, it is desirable to support Pt in close proximity to CeO ₂ , but this can be achieved by using such a composite oxide particle as a nucleus.

The CeO ₂ -ZrO ₂ composite oxide is particularly preferably a solid solution. This solid solution has an atomic ratio of 0.3 ≦
The range of Zr / (Ce + Zr) ≦ 0.8 is preferable, and 0.4 ≦ Zr /
The range of (Ce + Zr) ≦ 0.6 is particularly preferable. Zr content is
When it is 30 mol% or less, the action of forming the Zr skeleton in the solid solution crystal is weakened, and it becomes difficult to maintain the cubic crystal of the fluorite structure due to the desorption of oxygen, so oxygen cannot be desorbed and OSC Is reduced.

Further, in the Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide,
Ce and Zr are highly dispersed in Al ₂ O ₃ , so CeO ₂
OSC can be efficiently extracted. And Ce and Zr
There is improved heat resistance of Al ₂ O ₃ since the solid solution in Al ₂ O _3,
The heat resistance of the entire composite oxide is improved. Therefore, a high OSC is exhibited even after the endurance in combination with the grain growth suppressing effect of Pt.

[0024] The Al In ₂ O ₃ -CeO ₂ -ZrO ₂ composite oxide, constitute a small primary particles Al and Ce and Zr are uniformly mixed at the atomic or molecular level, a composite oxide throughout each particle hardly It is a substance or a solid solution. Therefore, the exhaust gas-purifying catalyst of the present invention maintains a stable OSC from the initial use to the endurance at high temperature.

The core made of porous oxide particles is
It is desirable to consist inclusive _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide particles PrO _2. This further improves heat resistance and OSC durability. Although the reason why the heat resistance is improved is unknown, there is a Pr atom, which is the same lanthanoid element as Ce, between Ce atoms in the solid solution, which suppresses the aggregation of Ce atoms at high temperature. It is estimated that it is because of this.

[0026] _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide,
It is desirable to contain at least a CeO ₂ —ZrO ₂ solid solution. By including the CeO ₂ —ZrO ₂ solid solution, phase separation of the composite oxide is less likely to occur, and therefore durability in a high temperature oxidizing atmosphere is further improved. This CeO ₂ -ZrO ₂ solid solution preferably has an atomic ratio in the range of 0.3 ≦ Zr / (Ce + Zr) ≦ 0.8.
The range of 4 ≦ Zr / (Ce + Zr) ≦ 0.6 is particularly preferable. When the Zr content is 30 mol% or less, the action of forming the Zr skeleton in the crystal of the solid solution is weakened, and it becomes difficult to maintain the cubic crystal of the fluorite structure due to the desorption of oxygen. It cannot be detached and OSC decreases.

[0027] _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 metal composition of the composite oxide, an atomic ratio Ce / Zr = 3 / 1~1 / 3, Al / (Ce
+ Zr) = 2 to 10, Ce / Pr = 3/1 to 20/1.

Since the oxygen storage / release capacity depends on the valence change between the trivalent and tetravalent Ce, if Ce / Zr is smaller than 1/3, the absolute amount of Ce will be insufficient and the absolute amount of OSC will be insufficient. , Deterioration easily occurs. When Ce / Zr becomes larger than 3/1,
Since the stabilizing effect of Ce by Zr is insufficient, the effect of suppressing the decrease in OSC after endurance cannot be obtained.

When Al / (Ce + Zr) is less than 2, it becomes difficult to secure a specific surface area after the durability because the absolute amount of Al is insufficient, and the activity of at least Pt supported is greatly reduced by the growth of grains. If Al / (Ce + Zr) exceeds 10, Ce
Since the absolute amount of OSC is insufficient, the absolute amount of OSC is insufficient and the activity decreases.

If Ce / Pr is less than 3, the absolute amount of Ce is insufficient, so that the absolute amount of OSC is insufficient, and if it is more than 20, the absolute amount of Pr is insufficient and durability is deteriorated.

[0031] _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide,
It is desirable that the primary particles be composed of secondary particles that are aggregated. The finer the particle size of the primary particles, the more preferable. However, it is preferable that the primary particle size is 100 Å or less and the BET specific surface area is 30 m ² / g or more. If the primary particle size exceeds 100Å and the BET specific surface area is less than 30 m ² / g, the particle growth of the composite oxide particles at a high temperature becomes remarkable and the absolute amount of OSC decreases. for that reason,
It is not preferable because it is exposed to a lean atmosphere with excess oxygen and the activity is reduced due to grain growth of the catalyst metal.

[0032] _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide,
It is also preferable to perform heat treatment at 500 ° C. or higher in advance. This makes it possible to further suppress the decrease in OSC after endurance. The lower limit of the heat treatment temperature was set to 500 ° C because if it is less than 500 ° C, the effect of suppressing the decrease in OSC due to crystal growth after endurance is small, and if it is about 400 ° C, it takes a long time for heat treatment. is there. On the other hand, the upper limit of heat treatment temperature is 1200 ℃
Is preferred. If the temperature exceeds 1200 ° C, grain growth will be remarkable and the absolute amount of OSC will decrease.

[0033] The _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide may further La, Ba, also be added or conjugated or solid solution of metals such as Fe. These _{metals, Al 2 O 3 -CeO 2 -ZrO} 2 -PrO 2 may be added and mixed to the composite oxide as the sole oxide solution as _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO
_{2 It} can be added as a component of the composite oxide by adding it during the preparation of the composite oxide.

[0034] _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide,
It is desirable to prepare by an alkoxide method or a coprecipitation method. By preparing by the alkoxide method or the coprecipitation method, it is possible to prepare a composite oxide composed of uniform and fine primary particles, and it is possible to easily prepare a composite oxide containing a CeO ₂ -ZrO ₂ solid solution. .

[0035] In the alkoxide method, for example Al, Ce, mixing all of the metal alkoxide of Zr and Pr, preparing _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide by firing after hydrolysis be able to. Further, if all of Al, Ce, Zr, and Pr are not used as the metal alkoxide, if at least one of them is used as the metal alkoxide, the remaining metal can be used as a solution of nitrate or acetylacetate salt.

The metal alkoxide may be any one of methoxide, ethoxide, butoxide, etc., but one having a high solubility in alcohol as a solvent is preferable. Any alcohol can be used as the solvent.

In the coprecipitation method, for example, Al, Ce, Zr and Pr are used.
Of mixing all of the metal water-soluble salt such as nitrate, etc. aqueous ammonia coprecipitation as hydroxide, the _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide by firing it It can be prepared. Further, without using all of Al, Ce, Zr and Pr as the water-soluble salt, if at least one of them is used as the water-soluble salt, the remaining metal can be used as a solid such as a metal powder or an oxide powder. .

The noble metal supported on the nuclei may contain at least Pt, and a noble metal such as Rh, Pd, Ir can also be supported together with Pt. By supporting at least Pt on this nucleus, the OSC speed is increased and the activity is further improved. It is particularly preferable to carry both Pt and Rh. By doing so, the grain growth of Pt can be further suppressed by the partial solid solution of Pt and Rh. Further, the amount of the noble metal supported on the nucleus is not particularly limited, and is the same as in the conventional catalyst.
It is preferably in the range of 0.1 to 20% by weight.

In order to support at least Pt on the nuclide, a conventional supporting method such as an adsorption supporting method or an evaporation-drying method can be used.

The coating layer is composed of at least one oxide or complex oxide selected from Al, Zr and Ce. A
The coating layer may be composed of a single oxide selected from l ₂ O ₃ , ZrO ₂ and CeO _2, but for the same reason as described above, CeO ₂
It is desirable to form from —ZrO ₂ composite oxide or Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide. For the same reason, it is more desirable that the Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide further contains PrO ₂ .

A noble metal can be supported on the coating layer. For example, when the coating layer is made of ZrO _2, it is desirable to support Rh on the coating layer. This further improves the ternary activity. In addition, since Rh does not form a solid solution in ZrO ₂ , the activity does not decrease. In this case, the loading amount of Rh is 0.1-10% by weight.
It is preferably about the same.

In the exhaust gas purifying catalyst of the present invention, the core body occupies 40 to 80% by weight of the whole, and the coating layer 20 to 60% by weight of the whole.
Is desirable. If the content of the nucleolus is less than 40% by weight of the whole, it is considered that the coating layer becomes too thick and the activity of Pt supported on the nucleolus is reduced, and the nucleolus is 80% by weight of the whole.
If Pt exceeds Pt, the coating layer becomes thin and Pt is supported on the nucleus.
It is thought that the particles will move easily and the particles will grow, and the purifying activity after endurance will decrease.

The formation of the coating layer is preferably carried out by a sol-gel method or a precipitation method. As a result, a uniform coating layer can be formed, and grain growth of Pt supported on the core can be well suppressed.

The above exhaust gas purifying catalyst of the present invention can be used for purifying exhaust gas of automobiles as an oxidation catalyst, a three-way catalyst and the like. Its shape can be the same as the conventional one such as a pellet shape or a honeycomb shape, and in the case of a honeycomb-shaped monolith catalyst, a honeycomb substrate formed of heat-resistant ceramics or metal is used as a core supporting at least Pt. The coat layer may be formed from powder, which is an aggregate of particles including a body and a cover layer.

[0045]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples.

Example 1 First, a predetermined amount of zirconium oxynitrate and a predetermined amount of praseodymium nitrate were dissolved in water,
Heat to 80 ° C. with stirring. A predetermined amount of aluminum isopropoxide was added thereto, a predetermined amount of nitric acid was further added and stirring was continued, and a predetermined amount of a solution prepared by dissolving cerium nitrate in ethylene glycol was added at 80 ° C.
Stirring was continued for 48 hours.

The resultant sol was dried in a rotary evaporator, after further 100 hours in a vacuum drying at 110 ° C. in vacuo, and then calcined 2 hours at _{500 ℃ Al 2 O 3 -CeO 2} -ZrO 2 -PrO 2
A complex oxide powder was formed.

The composition ratio of the _{Al 2 O 3 -CeO 2 -ZrO 2} -PrO 2 composite oxide powder of Al, Ce, Zr and Pr is the atomic ratio of Al: Ce:
Zr: Pr = 4: 1: 1: 0.1 and the average particle size is 3μ.
m.

100 g of this composite oxide powder was impregnated with a predetermined amount of a dinitrodiammineplatinum aqueous solution having a predetermined concentration and evaporated to dryness to support Pt. The amount of Pt supported is 1.5 g. In addition, Rh
Was carried. The amount of Rh supported is 0.3 g. Thus, the noble metal-supported composite oxide powder was prepared.

Then, in 1 liter of distilled water under reflux at 85 ° C., zirconium oxynitrate as a zirconia precursor was added.
34 g was put and stirred, and 100 g of the above noble metal-supported composite oxide powder was put therein. Subsequently, 240 g of aluminum triisopropoxide as an alumina precursor was added,
Further, after adding 7 cc of nitric acid, a solution of 55 g of cerium nitrate as a ceria precursor dissolved in 120 cc of ethylene glycol was mixed.

In this state, the mixture was stirred for 5 hours, then vacuum-dried by a rotary evaporator and further baked at 500 ° C. for 2 hours. As a result, a catalyst powder was obtained in which the noble metal-supported composite oxide particles were used as a nucleus, and a total of 100 g of a coating layer made of Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide was formed on the nucleus.

[0052] The catalyst powder is a schematic cross-sectional view as shown in FIG. _{1, Al 2 O 3 -CeO 2} -ZrO 2 -PrO 2 nuclei 10 Pt11 consisting composite oxide particles are carried Catalyst particles 1 and
The surface of the catalyst particles 1 is coated with a coating layer 2 made of Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide.

The catalyst powder thus obtained was molded by a conventional method and then pulverized to 0.5 to 1.5 mm to prepare pellet catalysts.

Example 2 140 g of the noble metal-supported composite oxide powder prepared in Example 1 was used, and the total coating layer was 60 g.
A catalyst powder was prepared in the same manner as in Example 1 except that the pellet catalyst was prepared.

(Example 3) 180 g of the noble metal-supported composite oxide powder prepared in Example 1 was used, and the total coating layer was 20 g.
A catalyst powder was prepared in the same manner as in Example 1 except that the pellet catalyst was prepared.

Example 4 60 g of the noble metal-supported composite oxide powder prepared in Example 1 was used, and the total coating layer was 140 g.
A catalyst powder was prepared in the same manner as in Example 1 except that the pellet catalyst was prepared.

Comparative Example 1 A noble metal-supported composite oxide powder was prepared in the same manner as in Example 1 except that 200 g of the composite oxide powder prepared in Example 1 was loaded with Pt 1.5 g and Rh 0.3 g. Was prepared. Then, this noble metal-supported composite oxide powder was used as a catalyst powder, and using this catalyst powder, a pellet catalyst was prepared in the same manner as in Example 1.

Comparative Example 2 100 g of the noble metal-supported composite oxide powder prepared in Example 1 and 100 g of the composite oxide powder prepared in Example 1 were mixed to prepare a catalyst powder, and this catalyst powder was used. A pellet catalyst was prepared in the same manner as in Example 1.

<Test / Evaluation> Each of the obtained pellet catalysts was filled in an endurance test apparatus, and further, in an N ₂ gas atmosphere containing 10% H ₂ O, an oxidizing atmosphere containing 5% O ₂ and CO were further added. Two
%, A durability test was conducted by heating at 1100 ° C. for 5 hours in an atmosphere in which a reducing atmosphere containing 1% was alternately repeated for 1 minute.

For each pellet catalyst after the durability test, the stoichiometric atmosphere model gas shown in Table 1 was used, and the gas temperature was raised from 100 ° C. to 500 ° C. under the conditions of a catalyst amount of 2 g and a gas flow rate of 10 L / min. Allows HC, CO and NO _x
The purification rate was measured and the 50% purification temperature of each was determined.
The respective results are shown in FIG.

[0061]

[Table 1]

From FIG. 2, the catalysts of Examples 1 to 4 were compared with Comparative Example 1
It was found that the purification activity after endurance was superior to that of ~ 2, and the composition ratios of the carrier components were all the same. The reason for this is that the growth of precious metal particles was suppressed by forming the coating layer. It is clear that there is.

From the comparison of the examples, Examples 1 to 1
It is also clear that No. 2 has a particularly high purifying activity after endurance, and the amount of the core of the noble metal-supported composite oxide is preferably more than 30% by weight and less than 90% by weight.

(Example 5) CeO ₂ produced by the coprecipitation method
75 g of a ZrO ₂ solid solution powder (molar ratio Ce: Zr = 4: 6) was impregnated with a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration, and evaporated to dryness to support Pt. The amount of Pt supported is 1.5 g. Thus, Pt / CeO ₂ —ZrO ₂ powder was prepared.

On the other hand, 75 g of the above Pt / CeO ₂ --ZrO ₂ powder was added to an aqueous zirconium oxynitrate solution and stirred, and the suspension was added to aqueous ammonia. Stir for 5 hours in this state, then vacuum dry on a rotary evaporator,
Further, it was baked at 500 ° C. for 2 hours. As a result, Pt / CeO ₂ −
A powder was obtained in which ZrO ₂ was used as a nucleus, and a total of 75 g of a coating layer made of ZrO ₂ was formed on the nucleus. And 75g of this powder
Impregnated with a predetermined amount of rhodium nitrate aqueous solution of a predetermined concentration,
Evaporated to dryness and loaded with Rh. The amount of Rh supported is 0.3 g.

The catalyst powder obtained was slurried to a volume of 35.
A cc honeycomb substrate was wet coated by a conventional method to form a coat layer, which was used as a monolith catalyst. A coat layer of 6 g was formed.

Comparative Example 3 75 g of ZrO ₂ powder prepared by mixing aqueous ammonia with an aqueous zirconium oxynitrate solution and calcining the obtained precipitate was impregnated with a prescribed amount of an aqueous rhodium nitrate solution having a prescribed concentration, Rh / ZrO ₂ powder was prepared by evaporating to dryness and supporting Rh.

And the Pt / CeO ₂ --Zr prepared in Example 5
O ₂ powder and Rh / ZrO ₂ powder were mixed at a weight ratio of 1: 1 to obtain a catalyst powder, and a monolith catalyst was prepared in the same manner as in Example 5.

<Test / Evaluation> Each of the obtained monolith catalysts was filled in an endurance test apparatus, and under an N ₂ gas atmosphere containing 10% H ₂ O, an oxidizing atmosphere containing 5% O ₂ and CO were further added. Two
A durability test was conducted by heating at 1000 ° C. for 5 hours in an atmosphere in which a reducing atmosphere containing 100% was alternately repeated for 1 minute each.

For each of the monolith catalysts after the durability test, the stoichiometric atmosphere model gas shown in Table 1 was used, and the inlet gas temperature was set to 1 at a gas flow rate of 20 L / min.
The temperature was raised from 00 ° C to 500 ° C and the purification rates of HC, CO and NO _x were measured, and the respective 50% purification temperatures were obtained. The respective results are shown in FIG.

It can be seen from FIG. 3 that the catalyst of Example 5 is superior in purification activity after endurance as compared with Comparative Example 3, and the composition ratios of the carrier components are the same. It is clear that the formation of Pt suppressed the grain growth of Pt.

[0072]

EFFECT OF THE INVENTION According to the exhaust gas purifying catalyst of the present invention, the movement of the noble metal particles carried on the core can be suppressed, and the deterioration of the purifying activity after the endurance can be suppressed.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a catalyst powder prepared in one example of the present invention.

FIG. 2 is a graph showing the 50% purification temperature after durability of the catalysts of Examples and Comparative Examples.

FIG. 3 is a graph showing the 50% purification temperature after durability of the catalysts of Examples and Comparative Examples.

[Explanation of symbols]

1: catalyst particles 2: coating layer 10: core 1
1: Pt

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Claims (6)

[Claims] 1. At least a core made of a porous oxide.
Pt-supported catalyst particles and formed on the surface of the catalyst particles
An exhaust gas-purifying catalyst comprising: a coating layer made of at least one oxide or complex oxide selected from Al, Zr, and Ce. 2. The exhaust gas purifying catalyst according to claim 1, wherein the nuclei are composed of Al ₂ O ₃ —CeO ₂ —ZrO ₂ composite oxide particles. 3. The exhaust gas-purifying catalyst according to claim ₂ , wherein the core further contains PrO ₂ . 4. The exhaust gas-purifying catalyst according to claim 1, wherein the core is made of CeO ₂ —ZrO ₂ composite oxide particles, and the coating layer is made of Rh-supporting ZrO ₂ . 5. The exhaust gas-purifying catalyst according to claim 1, wherein the core accounts for 40 to 80% by weight of the whole. 6. The exhaust gas purifying catalyst according to claim 1, wherein the coating layer is formed by a sol-gel method or a coprecipitation method.