JPH0857315A - Catalyst for purification of exhaust gas - Google Patents

Abstract

PURPOSE: To enhance the NOx removing performance of a catalyst for purification of exhaust gas contg. both of a barium compd. and cerium oxide by carrying at least one of Pd and Pt on each of porous carriers separately contg. the barium compd. and cerium oxide. CONSTITUTION: This catalyst consists of a carrier substrate 1, a 1st catalyst carrying layer 2 formed on the surface of the substrate 1 and contg. at least one of Pd and Pt, cerium oxide and alumina, and a 2nd catalyst carrying layer 3 formed on the surface of the layer 2 and contg. at least one of Pd and Pt, a barium compd. and alumina. The porous carriers of the layers 2, 3 are preferably made of activated alumina and the total amt. of Pd or Pt carried is preferably 0.05-30g per 1l of this catalyst. The contents of the barium compd. and cerium oxide are preferably 0.01-1.0mol and 0.01-3.0mol per 1l of this catalyst, respectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hydrocarbon (HC) contained in exhaust gas discharged from an internal combustion engine of an automobile,
The present invention relates to an exhaust gas purifying catalyst that can purify three components of carbon monoxide (CO) and nitrogen oxide (NOx) at the same time. More specifically, it has the same three-way activity as when rhodium is used without using rhodium (Rh). The present invention relates to an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gas of automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant base material made of cordierite or the like, and a noble metal element such as platinum or rhodium is supported on the porous carrier layer. Things are widely known. Also known is a three-way catalyst in which ceria (cerium oxide) having an oxygen storage effect is used in combination to enhance low temperature activity.

However, platinum and rhodium are expensive, and rhodium is a scarce resource, so considering the worldwide spread of three-way catalysts for automobiles, it is necessary to develop a catalyst that does not use these precious metals. Has been said. Therefore, among the precious metal elements, an exhaust gas purifying catalyst using palladium, which has an exhaust gas purifying property close to that of platinum or rhodium, is conceivable. However, palladium is inferior to the NOx purification performance in the lean atmosphere side rather than the rich atmosphere and the stoichiometric air-fuel ratio (stoichiometric ratio). Therefore, it is considered difficult to purify NOx using only palladium, and it is used as a three-way catalyst in combination with rhodium, which has a high NOx purification performance. However, since rhodium is extremely expensive, it becomes an expensive exhaust gas purifying catalyst.

To improve the NOx purification performance without using rhodium, for example, Japanese Patent Laid-Open No. 2002-2002 is known.
No. 86 discloses that a monolith carrier substrate is coated with a mixture of a catalytically active component composed of palladium, alkaline earth metal, cerium oxide, zirconium oxide and titanium oxide, and a porous carrier composed of activated alumina. A three-way catalyst is disclosed.

Further, JP-A-6-99069 discloses an exhaust gas purifying catalyst containing palladium, alumina, cerium oxide and barium oxide.

[0006]

In the conventional exhaust gas purifying catalyst that does not use rhodium, a basic substance such as an alkaline earth metal is added in order to improve the NOx purifying activity. A barium compound is typically used as the basic substance. With this barium compound, N
The O adsorption amount is increased, and the reaction selectivity of NO among the oxidizing gases (O ₂ , NO) contained in the exhaust gas is improved. In addition, reaction suppression due to HC poisoning under a reducing atmosphere is alleviated. Therefore, the NOx purification performance is improved.

Further, cerium oxide has a property of temporarily storing oxygen and has a function of alleviating fluctuations of the air-fuel ratio in the vicinity of stoichiometry, so that stable ternary activity can be obtained and as a result, purification performance is improved. To do. However, even in the conventional exhaust gas-purifying catalysts containing both cerium oxide and barium compound, the NOx purification performance cannot be said to be sufficiently high, and further improvement is required.

The present invention has been made in view of such circumstances, and it is an object of the present invention to further improve the NOx purification performance of an exhaust gas purification catalyst containing both a cerium oxide and a barium compound.

[0009]

The exhaust gas-purifying catalyst of the present invention for solving the above-mentioned problems is contained in a porous carrier and at least one of palladium and platinum supported on the porous carrier. An exhaust gas purifying catalyst comprising a barium compound and a cerium oxide, characterized in that the barium compound and the cerium oxide are contained separately from each other.

[0010]

In the exhaust gas purifying catalyst of the present invention, the NO adsorption amount is increased by the addition of the barium compound, and the NO reaction selectivity of the oxidizing gas (O ₂ , NO) contained in the exhaust gas is improved. When the exhaust gas becomes a reducing atmosphere, the barium compound alleviates the reaction suppression due to HC poisoning.
This improves the NOx purification performance.

On the other hand, cerium oxide has a function of temporarily storing oxygen, stores oxygen in an oxidizing atmosphere and releases oxygen in a reducing atmosphere. As a result, fluctuations in the atmosphere of the exhaust gas in the vicinity of stoichiometry are alleviated, and as a result catalyst performance is improved. By the way, the present inventors conducted an experiment in which palladium was used as a noble metal catalyst, and barium compounds (acetates) were added to cerium oxide and alumina to measure NO purification performance. As a result, Pd
In the / alumina system, the NO purification performance was improved as the amount of barium compound increased as shown in FIG.
As shown in FIG. 5, it was found that the NO purification performance of the cerium oxide system deteriorates as the amount of barium compound increases.

Further, when the reaction selectivity of the oxidizing gas (O ₂ , NO) in the exhaust gas purification reaction under the stoichiometric condition was investigated, it was found that the barium compound was added to the Pd / alumina catalyst as shown in FIG. The reaction selectivity of NO was improved as the amount of the compound was increased. In other words, as the barium compound amount increases, the NO adsorption amount also increases,
The proportion of NO used in the reaction also increases as compared to O ₂ .

However, as shown in FIG. 7, it became clear that the reaction selectivity of the Pd / cerium oxide-based oxidizing gas decreases as the amount of the barium compound increases.
That is, it was clarified that the addition of the barium compound to the cerium oxide not only impairs the catalytic activity of Pd / cerium oxide, but also that the barium compound does not contribute to the purification of NO.

It is considered that the cause of this is that the oxygen released from the cerium oxide is consumed for the oxidation of HC and CO, and the reduction reaction of NO adsorbed in the vicinity of the barium compound is less likely to occur. Therefore, in the present invention, the barium compound and the cerium oxide are contained separately from each other. Therefore, NO adsorbed near the barium compound
Is less likely to be affected by oxygen released from cerium oxide, and NOx is reduced by reacting smoothly with HC and CO in exhaust gas by the catalytic action of Pd or Pt supported on the porous carrier. Is improved. Further, since cerium oxide sufficiently exhibits the ability to temporarily store oxygen, fluctuations in the atmosphere of exhaust gas are alleviated and the catalytic activity is improved.

[0015]

【Example】

(Specific Examples of the Invention) As the porous carrier, conventionally used refractory inorganic oxides such as alumina, silica, zirconia, and titania can be used. Above all,
Most preferred is activated alumina having a high specific surface area and high heat resistance.

The amount of palladium or platinum supported varies depending on the conditions under which the catalyst is used, but normally the total amount is preferably in the range of 0.05 to 30 g per liter of catalyst. If the amount is less than 0.05 g / L, the purification performance is low, and even if the amount exceeds 30 g / L, the effect is saturated and the cost becomes high. The barium compound content is preferably in the range of 0.01 to 1.0 mol per liter of catalyst. NOx less than 0.01 mol / L
Purification performance is poor, and if the content exceeds 1.0 mol / L, problems such as reduction in strength of the catalyst occur, which is not preferable.

The content of cerium oxide is 1 L of the catalyst.
The range of 0.01 to 3.0 mol is preferable. 0.0
If it is less than 1 mol / L, almost no effect of addition is observed, and if it exceeds 3.0 mol / L, the effect is saturated. The exhaust gas-purifying catalyst of the present invention further includes lanthanum (La), zirconium (Zr), neodymium (N).
It is preferable to include an element such as d). La and Nd improve the NOx purification performance by the same action as the barium compound. In particular, La promotes the reaction between NO and H ₂ , so that the NOx purification performance at low temperatures is improved. Further, since Zr forms a solid solution with cerium oxide, the heat resistance of cerium oxide is improved.

Further, in order to separately contain the barium compound and the cerium oxide, the following forms are possible. (A) As shown in FIG. 1, a carrier substrate 1, a first catalyst supporting layer 2 formed on the surface of the carrier substrate 1 and containing at least one of Pd and Pt, cerium oxide and alumina, and a first catalyst carrier. A structure comprising a second catalyst supporting layer 3 formed on the surface of the layer 2 and containing at least one of Pd and Pt and a barium compound and alumina. (B) As shown in FIG. 2, a carrier substrate 10 and a carrier substrate 1
A first catalyst supporting layer 20 formed on the surface 0 and containing at least one of Pd and Pt, a barium compound, and alumina;
A configuration including a second catalyst supporting layer 30 formed on the surface of the first catalyst supporting layer 20 and containing at least one of Pd and Pt, and cerium oxide and alumina. (C) As shown in FIG. 3, a first catalyst 4 containing at least one of Pd and Pt, a barium compound and alumina,
A structure in which at least one of Pd and Pt and a second catalyst 5 containing cerium oxide and alumina are arranged in series in the exhaust gas flow direction with the first catalyst 4 on the upstream side and the second catalyst 5 on the downstream side. . A configuration in which the two catalysts (D) and (C) are arranged in parallel in the exhaust gas flow direction. (E) A structure in which at least one of Pd and Pt is supported on the granular cerium oxide and the granular barium compound, respectively, and this is mixed with alumina to coat the surface of the carrier substrate.

The form in which the barium compound and the cerium oxide are separately contained is not limited to the above-mentioned form as long as most of the barium compound and the cerium oxide are separated. Further, the barium compound may be separated from the cerium oxide and supported on the porous carrier. Hereinafter, a specific description will be given with reference to examples. (Example 1) 120 g of γ-alumina powder was immersed in an aqueous solution of dinitrodiaminopalladium containing 2 g of Pd, evaporated to dryness, and then dried at 110 ° C. for one day and then in the air.
Baking treatment was performed at 00 ° C. for 3 hours. The obtained Pd-supported powder was immersed in an aqueous solution in which 0.2 mol of a barium acetate compound was dissolved, evaporated to dryness, and then dried at 110 ° C. for a whole day and night,
Furthermore, after performing a baking treatment at 600 ° C. in the atmosphere for 3 hours, Pd
A first powder composed of / Ba / Al ₂ O ₃ was prepared.

On the other hand, cerium oxide (CeO ₂ ) powder 0.
3 mol was immersed in an aqueous solution of dinitrodiaminopalladium containing 1 g of Pd, evaporated to dryness, dried at 110 ° C. for one day and then calcined in air at 600 ° C. for 3 hours to prepare a second powder. The first powder and the second powder were respectively pressed and molded into 0.5 to 1 mm first pellets and second pellets, respectively, and then the whole amount was mixed to obtain an exhaust gas purifying catalyst of this example. Comparative Example 1 Pd-supporting powder prepared in Example 1 and second
The powder was physically thoroughly mixed. This mixed powder was immersed in an aqueous solution in which 0.2 mol of barium acetate was dissolved, evaporated to dryness, dried at 110 ° C. for one day, and further dried in the air at 600 ° C.
A calcination treatment was performed at ℃ for 3 hours. The obtained powder was pressed and molded into pellets of 0.5 to 1 mm to prepare a catalyst of Comparative Example 1.

The content of each element is exactly the same as in Example 1. (Comparative Example 2) The second powder prepared in Example 1 was dipped in an aqueous solution in which 0.2 mol of barium acetate was dissolved, evaporated to dryness, dried at 110 ° C for one day, and further dried in air at 600
A Pd / Ba / CeO ₂ powder was prepared by performing a firing treatment at 3 ° C. for 3 hours. Then, the same Pd-supporting powder as in Example 1 and the above Pd / Ba / CeO ₂ powder were pressed into 0.5 to 1 mm first and second pellets, respectively, and the two types of pellets were mixed. The catalyst of Comparative Example 2 was prepared.

The content of each element is exactly the same as in Example 1. (Pretreatment) For the exhaust model gas shown in Table 1, oxygen and C
Using a model gas in which O / H ₂ (3/1 in molar ratio) was alternately injected at a cycle of 10 minutes, each catalyst of Example 1 and Comparative Examples 1 and 2 was heated at 900 ° C. under a condition of 1 L / min. A pretreatment was performed to stabilize the catalyst by heating for 3 hours.

[0023]

[Table 1] (Evaluation test) With respect to each of the catalysts after the pretreatment, the temperature drop measurement and evaluation using the exhaust model gas shown in Table 2 was performed under the following conditions using the fixed bed flow reactor.

Cooling rate: 12 ° C./min Space velocity (SV): 50,000 / hr

[0025]

[Table 2] The evaluation results are shown by the 50% purification temperature of NO, CO and HC, and are shown in Table 3. Further, the NO purification activity with respect to temperature was measured, and the results are shown in FIG.

[0026]

[Table 3] From Table 3, it is clear that the exhaust gas-purifying catalysts of the Examples have higher catalytic activity than the Comparative Examples. Further, Comparative Example 2 is inferior to Comparative Example 1 in catalytic activity, which is probably because the exhaust gas purifying catalyst of Comparative Example 2 has a larger contact degree between the barium compound and the cerium oxide.

Further, from FIG. 8, the NO purifying activity is also shown in Example 1>
The order of Comparative Example 1> Comparative Example 2 is followed, and the effect of separately supporting the barium compound and the cerium oxide is clear.

[0028]

[Effects of the Invention] According to the exhaust gas purifying catalyst of the present invention, the respective functions of the barium compound and the cerium oxide can be sufficiently exerted without being suppressed from each other, so that it is high without using expensive rhodium. Purification performance can be obtained, and in particular, NOx purification performance can be greatly improved as compared with the conventional case.

[Brief description of drawings]

FIG. 1 is a structural explanatory view of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is a structural explanatory view of an exhaust gas purifying catalyst according to another embodiment of the present invention.

FIG. 3 is a structural explanatory view of an exhaust gas purifying catalyst according to another embodiment of the present invention.

FIG. 4 is a Ba concentration of Pd / alumina-based catalyst and 50 of NO.
It is a graph which shows the relationship of% purification temperature.

FIG. 5: Ba concentration of Pd / ceria catalyst and 50% of NO
It is a graph which shows the relationship of purification temperature.

FIG. 6 is a graph showing the reaction selectivity of oxidizing gas (NO, O ₂ ) when changing the Ba concentration in a Pd / Ba / alumina catalyst.

FIG. 7 is a graph showing the reaction selectivity of oxidizing gas (NO, O ₂ ) when changing the Ba concentration in a Pd / Ba / ceria catalyst.

FIG. 8 is a graph showing the NO purification rate at each temperature of the catalysts of Examples and Comparative Examples of the present invention.

[Explanation of symbols]

1: Support base material 2: First catalyst support layer 3: Second catalyst support layer 4: First catalyst 5: Second catalyst 10: Support base material 20: First catalyst support layer 30: Second
Catalyst support layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Motohisa Saiki, Nagakute-cho, Aichi-gun, Aichi 1st 41st side road, Nagakage, Toyota Central Research Institute Co., Ltd. (72) Minoru Tanabe Nagakute-cho, Aichi-gun, Aichi 1 in 41, Yokomichi, Yokouchi Central Research Institute, Ltd. (72) Inventor Mayuko Nagai 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd.

Claims (1)

[Claims] 1. An exhaust gas purifying catalyst comprising a porous carrier, at least one of palladium and platinum supported on the porous carrier, and a barium compound and a cerium oxide contained in the porous carrier. The barium compound and the cerium oxide are contained separately from each other, and the exhaust gas-purifying catalyst is characterized.