JP2002263493A - Exhaust gas cleaning catalyst and exhaust gas cleaning method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress releasing of hydrogen sulfide having malodor to the atmosphere and to improve heat resistance of an exhaust gas cleaning catalyst by preventing the thermal deterioration of cerium oxide. SOLUTION: In the exhaust gas cleaning catalyst, prepared by forming coating layers 3 and 4 having cerium oxide, barium, activated alumina and a noble metal on the surface of the catalyst support 2, the said barium is fixed on cerium oxide.

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 which is mounted on an exhaust gas purifying apparatus and adsorbs components to be purified in the exhaust gas to purify the exhaust gas.

[0002]

2. Description of the Related Art Conventionally, as an exhaust gas purifying catalyst, for example, as shown in Japanese Patent Publication No. 62-14338,
Activated alumina, cerium, zirconium, at least one of iron and nickel, platinum, palladium and at least one of rhodium mixed with a slurry liquid wash-coated on the catalyst support surface,
A technique for forming a coat layer on the surface of the catalyst carrier by drying and baking the wash coat is already known.

In the catalyst disclosed in this prior art, cerium oxide CeO ₂ is contained in an alumina coat layer (wash coat layer). This cerium oxide CeO ₂ has an oxygen storage capacity effect (when the oxygen O ₂ concentration is high, that is, when the air-fuel ratio is lean side, oxygen O ₂ is absorbed and the oxygen O ₂ concentration is low, that is, the air-fuel ratio Has the effect of releasing oxygen O ₂ on the rich side and contributing to the catalytic reaction).
Based on this oxygen storage effect, eO ₂ widens the range of the air-fuel ratio, widens the region where the catalyst reacts with nitrogen oxides NOx, hydrocarbon HC, etc., and contributes to the improvement of the catalyst performance. Further, the cerium oxide CeO ₂ itself reacts with water to cause a hydrogen gas reaction, which also improves the catalytic performance.

[0004] Gasoline as a currently used vehicle fuel always contains a sulfur component S. This sulfur component becomes sulfur dioxide SO ₂ along with the combustion, and the exhaust gas contains this sulfur dioxide SO ₂ . This sulfur dioxide SO ₂ reacts with hydrogen H ₂ in the catalyst to generate hydrogen sulfide H ₂ S. The hydrogen sulfide H ₂ S is extremely odorous, and it is desired that the generation of the hydrogen sulfide be suppressed by the catalyst itself.

[0005] That is, sulfur dioxide SO ₂ as described above are bonded to the metal M in the catalyst is stored in the form of a metal salt MSO ₄ (trap), the stored SO ₄ is empty natural ratio from a lean atmosphere When the atmosphere changes to a rich atmosphere, it is released as hydrogen sulfide H ₂ S. However, since this release amount is temporarily large, the off-odor becomes remarkable, which is extremely problematic.

The metal M includes, for example, aluminum Al and cerium Ce. Specific examples of the metal salt MSO ₄ include alumina sulfite Al ₂ (SO ₂ )
₄ ) ₃ and cerium sulfite Ce ₂ (SO ₄ ) ₃ .

As a measure for suppressing the generation of hydrogen sulfide H ₂ S, a technique using nickel Ni as a catalyst is known. That is, hydrogen sulfide H ₂ S reacts with nickel Ni in the rich side of the air-fuel ratio, that is, in the reducing atmosphere, generates nickel sulfide and is trapped.

[0008]

However, in such a technology for suppressing the production of hydrogen sulfide H ₂ S using nickel Ni, in Europe and the like, nickel carbonyl having carcinogenicity is produced as a derivative. For this reason, the use of this nickel Ni is prohibited. Therefore, there is a strong demand for the establishment of a technique for suppressing the generation of hydrogen sulfide H ₂ S without using nickel Ni.

In the exhaust gas purifying catalyst described above, cerium oxide CeO ₂ has the property of growing crystals, that is, the crystal structure tends to become large by the heat of the exhaust gas. Here, as the crystal structure increases, the specific surface area of cerium oxide CeO ₂ decreases, and as a result, the above-described oxygen storage effect and hydrogen gas reaction deteriorate. In this way, cerium oxide Ce is grown by crystal growth by heating the catalyst.
O ₂ tends to thermally degrade. For this reason, it is not possible to sufficiently stabilize the low-temperature activity of the catalyst.

The present invention has been made in view of the above circumstances, and a first object of the invention is to provide an exhaust gas purifying catalyst and an exhaust gas capable of suppressing the release of hydrogen sulfide with a strong off-odor into the atmosphere. It is to provide a gas purification method.

A second object of the present invention is to provide an exhaust gas purifying catalyst capable of preventing cerium oxide from thermal degradation and improving heat resistance.

[0012]

According to the first aspect of the present invention,
An exhaust gas purifying catalyst for purifying exhaust gas containing sulfur dioxide discharged from an engine, comprising: a catalyst carrier;
Cerium oxide that adsorbs sulfur dioxide in the exhaust gas to generate sulfate when the engine is in a lean oxidizing atmosphere and shifts to a reducing atmosphere rich in engine air-fuel ratio, and releases the adsorbed sulfur to hydrogen sulfide. And barium, which suppresses sulfur dioxide in the exhaust gas from being stored in the cerium oxide in the form of sulfate, and activated alumina,
And a coat layer having a noble metal and formed on the surface of the catalyst support.

According to a second aspect of the present invention, there is provided an exhaust gas purifying catalyst in which a coating layer having cerium oxide, barium, activated alumina and a noble metal is formed on a surface of a catalyst carrier, wherein the barium is fixed to the cerium oxide. It is characterized by having been made.

According to a third aspect of the present invention, there is provided an exhaust gas purifying method for purifying exhaust gas containing sulfur dioxide discharged from an engine, wherein cerium oxide, barium, activated alumina and a noble metal are coated on a surface of a catalyst carrier. An exhaust gas purifying catalyst having a coat layer is disposed in the middle of an exhaust pipe of an engine, and the engine is operated so that the exhaust gas changes from an oxidizing atmosphere to a reducing atmosphere. The storage of sulfur dioxide in the cerium oxide in an oxidizing atmosphere is suppressed by the barium.

[0015]

According to the first aspect of the present invention, when cerium oxide is in an lean oxidizing atmosphere of the engine air-fuel ratio, it adsorbs sulfur dioxide in the exhaust gas to generate sulfate, and is converted into a reducing atmosphere rich in the engine air-fuel ratio. When the fuel is transferred, the adsorbed sulfur is converted into hydrogen sulfide and released.However, barium suppresses the sulfur dioxide in the exhaust gas from being stored in the cerium oxide in the form of sulfate, so that the engine air-fuel ratio becomes lean. A large amount of hydrogen sulfide is prevented from being discharged when the state changes from rich to rich.

According to the second aspect of the present invention, since barium is fixed to cerium oxide, it is advantageous in preventing a large amount of hydrogen sulfide from being temporarily discharged, and cerium oxide particles are mutually separated. The integration is suppressed, and the crystal growth of cerium oxide is suppressed even when exposed to the heat of the exhaust gas. Therefore, thermal deterioration of cerium oxide can be suppressed, and heat resistance can be improved.

According to the third aspect of the present invention, an exhaust gas purifying catalyst in which a coating layer having cerium oxide, barium, activated alumina and a noble metal is formed on the surface of a catalyst carrier is disposed in the middle of an exhaust pipe of an engine. Since the engine is operated so that the exhaust gas changes from the oxidizing atmosphere to the reducing atmosphere, the barium suppresses the storage of sulfur dioxide in the cerium oxide in the oxidizing atmosphere. However, it is possible to prevent a large amount of hydrogen sulfide from being discharged when the atmosphere is reduced.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of an embodiment of an exhaust gas purifying catalyst according to the present invention will be described below in detail with reference to the accompanying drawings.

(Overall Structure) As shown in FIG. 1, an exhaust gas purifying device 1
2 are interposed. The exhaust gas purifying device 12 includes a housing 14 connected to the exhaust pipe 10 in a front-to-rear communication state, and a housing 14 disposed in the housing 14 and configured to allow exhaust gas to flow therethrough. An exhaust gas purifying catalyst (hereinafter, simply referred to as a catalyst) 1 which is a feature of the present invention.

(First Embodiment) Next, referring to FIG. 2 to FIG. 7, the specific structure of the catalyst 1 of the first embodiment according to the present invention will be described.

As shown in FIG. 2, the catalyst 1 has a large number of through holes 16 extending through the exhaust gas flow direction and penetrating therethrough.
Is provided. As shown in FIGS. 2 and 3, a first coat layer 3 as an alumina coat layer and a second coat layer 4 mainly containing cerium oxide CeO ₂ are sequentially provided on the entire surface of the catalyst carrier 2. Have been.

The catalyst carrier 2 is formed in a so-called honeycomb structure with the above-described through holes 16, and a ceramic such as cordierite is used as a material of the catalyst carrier 2. Note that a heat-resistant metal, a heat-resistant inorganic fiber, or the like may be used instead of the ceramic.

The first coat layer 3 is made of activated alumina γ-
Al ₂ O ₃ as a main component, and its activated alumina γ-Al ₂ O ₃
The precious metal catalyst component is contained therein. The noble metal catalyst component is dispersed in the activated alumina γ-Al
For example, at least one of platinum Pt and rhodium Rh is used as the noble metal catalyst component contained in ₂ O ₃ .

The second coat layer 4 contains cerium oxide CeO ₂ , activated alumina γ-Al ₂ O ₃ , and palladium Pd, and the CeO ₂ particles contain barium Ba and lanthanum La. At least one is fixed.

Such a catalyst is specifically produced as follows. First, activated alumina γ-Al ₂ O ₃ 10
0 g and 100 g of boehmite, 240 cc of water, and nitric acid 1.
Add 0 cc and mix to form a slurry liquid.

Next, the catalyst carrier 2 having a honeycomb structure is immersed in the slurry liquid, and then pulled up to remove excess slurry liquid by air blowing. Then, the catalyst carrier 2 to which the slurry liquid has adhered is dried at a temperature of 130 ° C. for 1.5 hours, and then fired at a temperature of 550 ° C. for 1.5 hours to form a coat layer. Thereafter, the coating layer of the catalyst carrier 2 is impregnated with a solution of gintrodiamine platinum Pt (NO ₂ ) ₂ (NH ₃ ) _{2 and} a solution of rhodium nitrate, and dried at a temperature of 200 ° C. for 1 hour. Thereafter, baking is performed at a temperature of 600 ° C. for 2 hours to complete the formation of the first coat layer 3. in this case,
The amount of alumina (the amount of wash coat) of the first coat layer 3 is 7% by weight with respect to the catalyst carrier 2, and the total amount of noble metals carried is 1.6 g / l (however, Pt: Rh = 5: 1).

Next, a solution of barium nitrate Ba (NO ₃ ) _{2 and} a solution of lanthanum nitrate La (NO ₃ ) ₂ were added to 120 g of cerium oxide CeO ₂ and 50 g of boehmite and mixed. Is dried to a solid mass, which is ground to obtain a powder. As a result, barium Ba and lanthanum L are applied to the surface of the cerium oxide CeO ₂ particles.
At least one kind of a is fixed.

Next, a slurry solution is prepared by adding a palladium chloride solution and 240 cc of water to the powder, and the catalyst carrier 2 on which the first coat layer 3 is formed is immersed in the slurry solution. Excess slurry liquid is removed by air blow. Then, it is dried at a temperature of 130 ° C. for 1 hour, and then baked at a temperature of 550 ° C. for 1.5 hours to form a second coat layer 4 on the first coat layer 3.

In this case, the wash coat amount of the second coat layer 3 was 14% by weight with respect to the catalyst carrier 2, the amount of supported palladium Pd was 1.0 g / l, and the added amounts of lanthanum La and barium Ba were wash. 100% by weight of coat
1 to 10% by weight relative to the addition amount of CeO ₂ is 5 to 30% by weight with respect to 100 wt% weight washcoat.

Therefore, in such a catalyst 1, at least one of lanthanum La and barium Ba is fixed on the surface of each particle of CeO _{2 in} the second coat layer 3,
Since these are intended to suppress the integration of the particles, that is, to suppress the expansion of the particles, the crystal growth of cerium oxide CeO ₂ is suppressed even when the particles are exposed to the high heat of the exhaust gas. In detail, vacancies will be formed in the crystal of cerium oxide CeO ₂ , and these vacancies are densely dispersed due to lattice diffusion of lanthanum La or barium Ba ions, Thereby, cerium oxide Ce
As mentioned above, the movement and distortion of the lattice of O ₂ are regulated to suppress the particle enlargement, and as a result, it is possible to suppress the thermal deterioration in the exhaust gas purification performance of cerium oxide CeO ₂ and improve the heat resistance. And the low-temperature activity can be stabilized.

In the above catalyst, the first coat layer 3
, Platinum Pt is carried on the second coat layer 4, and palladium Pd is carried on the second coat layer 4, and both are carried on different coat layers. For this reason, the platinum Pt and the palladium Pd do not come close to each other, and the platinum Pt and the palladium Pd do not alloy with each other even when exposed to the high heat of the exhaust gas. As a result, the thermal degradation of the catalyst can be suppressed from the viewpoint of preventing alloying.

Further, when rhodium Rh is carried on the first coat layer 3 together with platinum Pt, rhodium Rh having heat resistance is interposed between the platinum Pt. Therefore, the sintering phenomenon of platinum Pt is suppressed, and the thermal deterioration of platinum Pt can be suppressed.
Thus, thermal degradation of the catalyst can be suppressed.

Next, in order to confirm the heat resistance of the catalyst under the following conditions according to the first embodiment, a test was conducted under a certain test condition while comparing with a comparative example which is a conventional exhaust gas purifying catalyst. Was done.

Conditions of the catalyst according to the present embodiment : Addition amount of lanthanum La and barium Ba: 5% by weight with respect to 100% by weight of wash coat Addition amount of cerium oxide CeO ₂ : With respect to 100% by weight of wash coat 14% by weight

Comparative Example 1 First, 240 cc of water, 1 cc of nitric acid, and 60 g of cerium oxide CeO ₂ were added to 100 g of activated alumina γ-Al ₂ O ₃ and 100 g of boehmite, and mixed to form a slurry solution. A lanthanum nitrate solution and a barium nitrate solution are added and stirred so that the coating amount is 21% by weight based on the catalyst carrier (honeycomb structure), and an alumina slurry liquid is obtained.

Next, the catalyst carrier is immersed in this alumina slurry liquid, and then pulled up to remove excess slurry liquid by air blowing.

Next, the catalyst carrier to which the alumina slurry liquid has adhered is dried at a temperature of 130 ° C. for 1 hour.
Firing at a temperature of 0 ° C. for 1.5 hours to form a coat layer on the surface of the catalyst carrier.

Next, the catalyst carrier on which the coating layer was formed was immersed in a solution of platinum Pt, rhodium Rh and palladium Pd at a predetermined concentration, and the same amount of platinum P as in this embodiment was used.
Impregnate with t, rhodium Rh and palladium Pd. Then, the catalyst carrier is dried at 200 ° C. for 1 hour, and then calcined at a temperature of 600 ° C. for 2 hours to obtain a catalyst according to Comparative Example 1. The contents of lanthanum La and barium Ba in Comparative Example 1 were 5% by weight with respect to the washcoat amount of 100% by weight.

Comparative Example 2 100 g of activated alumina γ-Al ₂ O ₃ and boehmite 10
0g, 240cc of water and 1.0cc of nitric acid are added and mixed,
Form a slurry liquid.

Next, a catalyst carrier having a honeycomb structure is immersed in the above slurry liquid, and then pulled up to remove excess slurry liquid by air blowing. Then, the catalyst carrier to which the slurry liquid has adhered is dried at a temperature of 130 ° C. for 1 hour, and then fired at a temperature of 550 ° C. for 1.5 hours to form a coat layer. Thereafter, the coating layer of the catalyst carrier is impregnated with a solution of platinum chloride and a solution of rhodium chloride, dried at a temperature of 200 ° C. for 1 hour, and then fired at a temperature of 600 ° C. for 2 hours to form a first coat. Form a layer. In this case, the amount of alumina (wash coat layer) in the first coat layer was 7% by weight based on the catalyst carrier, and the total amount of noble metals supported was 1.6 g / l.
(However, Pt: Rh = 5: 1).

Next, a solution of palladium chloride was added to 120 g of cerium oxide CeO ₂ and 50 g of boehmite and mixed, and then dried to obtain a solid mass, which was pulverized to obtain a powder.

Next, 240 cc of water is added to the powder to form a slurry liquid, the catalyst carrier having the first coat layer formed thereon is immersed in the slurry liquid, and then pulled up, and excess slurry liquid is removed by air blowing. Remove. Then, it is dried at a temperature of 130 ° C. for 1 hour, and then dried at 550 ° C.
At a temperature of 1.5 hours to form a second coat layer on the first coat layer.

In this case, the amount of alumina (the amount of wash coat) of the second coat layer is 14% by weight with respect to the catalyst carrier, and the amount of supported Pd is 1.0 g / l.

Test Conditions The catalyst capacity was 24 ml for all cases. Before performing the test, aging is performed by heating in the air at 900 ° C. for 50 hours. Under an atmosphere with an air-fuel ratio of A / F of 14.7, the space velocity is set at 60,000 H ⁻¹ , and the exhaust gas temperature at the catalyst inlet is changed to examine the hydrocarbon HC purification rate.

Test Results As a result of such a test, the contents shown in FIG. 4 were obtained. According to this content, it can be understood that the example of the present example shows purification performance from a lower exhaust gas temperature as compared with those of the comparative examples 1 and 2, and the heat deterioration is suppressed and the heat resistance is improved.

Next, the amounts of lanthanum La, barium Ba, and cerium oxide CeO ₂ in the coating layer were examined.

In the case of adding 1% by weight of barium Ba and 14% by weight of cerium oxide CeO ₂ , 5% by weight of barium Ba and cerium oxide CeO ₂ are added .
_In the case of ₂ 14% by weight, in each case of barium Ba 10% by weight and cerium oxide CeO ₂ 14% by weight, the amount of lanthanum La was changed at a temperature of 400 ° C. to change the amount of hydrocarbon H.
The purification performance of C (air ratio A / F: 14.7) was examined.

As a result, the contents shown in FIG. 5 were obtained. According to this content, in each case, when the amount of lanthanum La added was less than 1% by weight or more than 10% by weight, the purification performance was reduced. Therefore, the amount of lanthanum La added is 1 to 10% by weight.
Is preferred.

In the case of adding 1% by weight of lanthanum La and 14% by weight of cerium oxide CeO ₂ , 5% by weight of lanthanum La and cerium oxide CeO are added .
₂ 14% by weight, lanthanum La 10% by weight, and cerium oxide CeO ₂ 14% by weight At 400 ° C., the purifying performance (air-fuel ratio A / F: 14.7) by changing the amount of barium Ba added at 400 ° C. Was examined.

As a result, the contents shown in FIG. 6 were obtained. According to this content, in each case, when the added amount of barium Ba was less than 1% by weight or 10% by weight, the purification performance was reduced. Therefore, the addition amount of barium Ba is also preferably 1 to 10% by weight.

Addition amount of cerium oxide CeO ₂ 400% for each of 1% by weight of lanthanum La and barium Ba, 5% for each of lanthanum La and barium Ba, and 10% for each of lanthanum La and barium Ba. At a temperature of ° C., the purification performance (air-fuel ratio A / F: 14.7) was examined by changing the amount of Ce added.

As a result, the contents shown in FIG. 7 were obtained. According to the contents, in all cases, when the added amount of cerium oxide CeO ₂ was less than 5% by weight or more than 30% by weight, the purification performance was reduced. Therefore, the addition amount of cerium oxide CeO ₂ is preferably 5 to 30% by weight.

It is needless to say that the present invention is not limited to the configuration of the above-described embodiment, but can be variously modified without departing from the gist of the present invention.

For example, in the first embodiment described above, the first and second coating layers 3 and 4 are provided on the surface of the catalyst carrier 1, and the cerium oxide particles of the second coating layer 4 It has been described that by fixing at least one of lanthanum La and barium Ba, cerium oxide CeO ₂ can be prevented from thermal degradation and heat resistance can be improved. Without being limited,
As shown below as a second embodiment, it is configured such that only one coat layer is provided on the surface of the catalyst carrier 1,
Further, the presence of barium Ba can improve not only the heat resistance but also the purifying property of hydrogen sulfide H ₂ S as described above.

(Second Embodiment) Hereinafter, a second embodiment of the exhaust gas purifying catalyst according to the present invention will be described.

First, prior to describing the configuration of the second embodiment, a description will be given of conventional hydrogen sulfide purification.

That is, the sulfur component S is always contained in the gasoline currently used as the vehicle fuel. This sulfur component becomes sulfur dioxide SO ₂ along with the combustion, and the exhaust gas contains this sulfur dioxide SO ₂ . Then, this sulfur dioxide SO ₂ reacts with hydrogen H ₂ in the catalyst, and as shown in the following equation (1), SO ₂ + H ₂ → H ₂ S + O ₂ (1) and hydrogen sulfide H ₂ S Will occur. The hydrogen sulfide H ₂ S is extremely odorous, and it is desired that the generation of the hydrogen sulfide be suppressed by the catalyst itself.

Such generation of hydrogen sulfide H ₂ S is generated by causing a reduction reaction in an atmosphere having a rich air-fuel ratio. In such a reduction reaction, 1 H ₂ S is generated.
The reaction is a mole / 1 mole, and if the concentration is higher than the concentration of sulfur dioxide SO ₂ at the inlet of the catalyst, hydrogen sulfide H ₂ S is not generated, and the problem due to the generation of hydrogen sulfide H ₂ S is relatively small. Things.

However, the above-mentioned sulfur dioxide SO ₂
There was bound to the metal M in the catalyst is stored in the form of a metal salt MSO ₄ (trap), the stored SO ₄ is when the air natural ratio has changed from lean atmosphere to rich atmosphere, the hydrogen sulfide H It is released as ₂ S, but since this release amount is temporarily large, the off-flavor becomes particularly noticeable, which is extremely problematic.

More specifically, when the exhaust gas is in an oxidizing atmosphere, as shown in the following equations (2) and (3), SO ₂ + 1 / 2O ₂ → SO ₃ ... (2) MO + SO ₃ → MSO ₄ .. (3), the metal salt MSO ₄ is generated and accumulated in the catalyst.

It is considered that such a metal salt MSO ₄ is also produced from the process shown in the following equation (4).

MO + SO ₂ + 1 / 2O ₂ → MSO ₄ (4)

Here, examples of the metal M include aluminum Al and cerium Ce. Specific examples of the metal salt MSO ₄ include alumina sulfite Al ₂ (SO _2
4 ) ₃ and cerium sulfite Ce ₂ (SO ₄ ) ₃ .

On the other hand, the metal salt MS thus stored
O ₄ changes as shown in the following equation (5) when the exhaust gas changes from an oxidizing atmosphere to a reducing atmosphere, and hydrogen sulfide H _{2 is changed.}
S is produced and released in large quantities, though temporarily.

MSO ₄ + 4H ₂ → MO + H ₂ S + 3H ₂ O (5)

Specifically, as shown in FIG.
In a state where the vehicle is constantly running in an oxidizing atmosphere of 6.0,
When the sulfur dioxide SO ₂ in the exhaust gas is stored in the catalyst in the form of a metal salt MSO ₄ as shown in the above formula (3) or (4), when the vehicle is stopped from this steady running state and shifts to the idling state, The air-fuel ratio also shifts to a reducing atmosphere of 13.5. When the air-fuel ratio shifts from the oxidizing atmosphere to the reducing atmosphere as described above, the reaction represented by the above formula (5) is performed, and a large amount of hydrogen sulfide H ₂ S with an unusual odor is generated at a time. Such generation of hydrogen sulfide H ₂ S
In the actual running of the vehicle, for example, it occurs when the engine shifts from the running state to the idling state in a garage or at an intersection, and a driver or a pedestrian can easily notice the unpleasant odor due to the hydrogen sulfide H ₂ S, Improvement is desired.

Conventionally, as measures for suppressing the generation of hydrogen sulfide H ₂ S, suppression of a reduction reaction, trapping of hydrogen sulfide H ₂ S, and suppression of formation of a sulfuric acid compound have been studied. In particular, hydrogen sulfide H ₂ S
The mechanism of the trap has been elucidated as follows. That is, sulfur dioxide SO ₂ reacts with cerium oxide CeO ₂ to become cerium sulfite Ce (SO ₄ ) ₂ , and this cerium sulfite Ce (SO ₄ ) ₂ reacts with hydrogen H _{2 in a} reducing atmosphere to form hydrogen sulfide H ₂ S Produces
The hydrogen sulfide H ₂ S is reacted with the metal oxide MOx to replace the metal sulfide MSx, and then trapped in the catalyst. Thus, the hydrogen sulfide H ₂ S is not released to the outside. I have.

As a conventional specific production suppression method using such a hydrogen sulfide H ₂ S trap technique, a technique utilizing nickel Ni as a catalyst is known. That is, in the rich side of the air-fuel ratio, that is, in the reducing atmosphere, hydrogen sulfide H ₂ S reacts with this nickel Ni in the presence of cerium oxide CeO ₂ to generate nickel sulfide and trap hydrogen sulfide H ₂ S. Has been implemented.

However, in such a technique for suppressing the production of hydrogen sulfide H ₂ S using nickel Ni, in Europe and the like, nickel carbonyl having carcinogenicity is derived and produced. Nickel N
Use of i is prohibited. Therefore, this nickel N
There is a strong demand for the establishment of a technique for suppressing the generation of hydrogen sulfide H ₂ S without using i.

In view of the above, the inventor of the present application has developed a technique for trapping hydrogen sulfide H ₂ S using barium Ba instead of nickel Ni. Hereinafter, a second embodiment relating to the technology of trapping hydrogen sulfide H ₂ S with barium Ba will be described in detail.

As shown in FIG. 9, similarly to the first embodiment, the catalyst 5 of the second embodiment has a large number of through-holes 16 extending along the flow direction of the exhaust gas. Is provided. On the entire surface of the catalyst carrier 5, a coating layer 7 containing cerium oxide CeO ₂ as a main component.
Are provided.

This coat layer 7 is made of activated alumina γ-Al
_The main component is ₂ O ₃ , and the active alumina γ-Al ₂ O ₃ contains a noble metal catalyst component and barium oxide BaO. The noble metal catalyst component is contained in the activated alumina γ-Al ₂ O ₃ in a dispersed state, and for the noble metal catalyst component, for example, at least one of platinum Pt and rhodium Rh is used.

Such a catalyst 5 is produced as Experimental Example 1 as follows.

First, 240 g of cerium oxide CeO ₂ is impregnated with a predetermined amount of barium nitrate Ba (NO ₃ ) ₂ , and then calcined to fix barium nitrate Ba (NO ₃ ) ₂ on the cerium oxide CeO ₂ . This barium nitrate Ba (N
O ₃ ) ₂ fixed cerium oxide CeO ₂ powder, activated alumina γ-Al ₂ O ₃ 240 g and boehmite 120 g
1000 cc of water and 10.0 cc of nitric acid are added and mixed,
Stir with a homomixer to form a slurry liquid for washcoat.

Next, the catalyst carrier 6 having a honeycomb structure is immersed in the slurry liquid, and then pulled up to remove excess slurry liquid by air blowing. Then, the catalyst carrier 6 to which the slurry liquid has adhered is dried at a temperature of 600 ° C. for 1 hour, and then fired at a temperature of 550 ° C. in an oxidizing atmosphere to form a coat layer. By this firing, barium nitrate Ba (NO ₃ ) ₂ fixed on cerium oxide CeO ₂ reacts with oxygen O ₂ to form barium oxide BaO.

A predetermined amount of platinum chloride PtCl ₃ , rhodium chloride RhCl ₃ was added to a predetermined amount of 170 cc of water using the alumina-coated catalyst support 6 having barium oxide BaO fixed thereon.
Then, it is dipped in a noble metal solution dissolved in water and then pulled up to remove excess noble metal solution by air blow. Then, after this, the catalyst carrier 6 to which the noble metal solution has adhered is removed by 600
Firing at a temperature of 2 ° C. for 2 hours completes the formation of the coat layer 7.

In this case, the alumina amount (wash coat amount) of the coat layer 7 is 28% by weight with respect to the catalyst carrier 6, and the cerium oxide CeO ₂ is 40% with respect to the wash coat amount.
%, Platinum Pt is 1.0 g / l, rhodium Rh is 0.2 g / l, and barium oxide BaO is 2.5% by weight (3.0 g / l) based on the wash coat amount.

[0078] For the generation amount measurement test contents hydrogen sulfide H ₂ S, as shown in FIG. 10, in the period [1], to start the discharge of the exhaust gas air-fuel ratio 13.5, in the period [2], Air-fuel ratio of 1
After raising to 6.0, the atmosphere was changed to an oxidizing atmosphere, and stored in the catalyst 5. In the period [3], the air atmosphere ratio was returned to the reducing atmosphere of 13.5 to release hydrogen sulfide H ₂ S. . Then, hydrogen sulfide H ₂ S released during this period [3]
Measure the maximum concentration of

Regarding the measurement of the active state of the catalyst 5,
Before performing the test, aging was performed by heating in the air at 900 ° C. for 50 hours. Thereafter, under an atmosphere with an air-fuel ratio of A / F of 14.5, the space velocity was set to 60,000 H ⁻¹ , and the exhaust gas at the catalyst inlet □ was exhausted The purification rate of hydrocarbon HC, specifically, the purification rate at 400 ° C. at the inlet of the catalyst 5 is examined by changing the gas temperature.

[0080] Test Results The results of such testing, with respect to the amount of hydrogen sulfide H ₂ S, is about 50 ppm, this value is the amount of hydrogen sulfide H ₂ S without the addition of barium Ba about 10
It is understood that compared to 0 ppm, a greatly reduced value, that is, the emission of hydrogen sulfide H ₂ S is suppressed to about half. In addition, the active state of the catalyst 5 is about 30%, which can be said to be a slightly improved change compared to about 25%, which is the purification rate when barium Ba is not added. .

According to the results of this test, the experimental example shows that hydrogen sulfide H is higher than that of the catalyst without barium Ba added.
_It can be understood that the effect of suppressing the generation of ₂ S is improved.

Next, the addition amount of barium Ba in the coat layer 7 was examined.

Amount of barium Ba The component of the coat layer 7 was 28% by weight of alumina (wash coat) with respect to the catalyst support 6, and cerium oxide C
eO ₂ is 40% by weight based on the wash coat amount, platinum Pt is 1.0 g / l, and rhodium Rh is 0.2 g / l.
, The amount of barium oxide BaO added was changed as shown in the following table.

[0084]

[Table 1]

That is, in Experimental Examples 2 to 6, the added amount of barium oxide BaO was 3.3: 5.8: 7.5:
As in the case of Experimental Example 1, the amount of generated hydrogen sulfide H ₂ S and the purification performance (air-fuel ratio A / F: 14.5) were examined by changing five types of 11.7: 13.3.

As a result, as shown in FIG. 11, the change in the production amount of hydrogen sulfide H ₂ S and the change in the purification performance were clarified with respect to the change in the addition amount of barium oxide BaO. This FIG.
From the results shown in the above, the optimum range of the added barium oxide BaO, in other words, the range that can be achieved in a state where both the production amount of hydrogen sulfide H ₂ S and the purification performance are compatible, is 3 to 3
It is understood that it is 13% by weight.

Here, the mechanism of suppressing the production of hydrogen sulfide H ₂ S by adding barium oxide BaO is considered as follows.

First, barium oxide BaO is rich on the air-fuel ratio rich side, that is, in a reducing atmosphere of exhaust gas.
As shown in the following formula (6), Ce ₂ (SO ₄ ) ₂ + 4BaO + 17H ₂ → Ce ₂ O ₃ + 4BaS + 17H ₂ O (6), and the inflow of cerium sulfite Ce ₂ (SO ₄ ) ₂ This means that sulfide BaS is generated and hydrogen sulfide H ₂ S is not generated.

The barium sulfide BaS
In the lean side of the air-fuel ratio, that is, in the oxidizing atmosphere of the exhaust gas, BaS + 2O ₂ → BaO + SO ₂ or → BaSO ₄ (7) as shown in the following equation (7), and cerium sulfite Ce ₂ ( Since no SO ₄ ) ₂ is produced, the storage of sulfur dioxide SO ₂ is suppressed.

As described in detail above, in the second embodiment, the coating layer 7 coated on the catalyst carrier 6 of the catalyst 5 is used.
Cerium oxide CeO with barium Ba immobilized
₂ particles, activated alumina γ-Al ₂ O ₃ particles, and platinum P
and a noble metal component of rhodium Rh. Thus, hydrogen sulfide H ₂ with a strong off-flavor
By using harmless barium Ba without using nickel Ni for the generation of sulfur, it is possible to trap the catalyst in the catalyst 5 and effectively suppress its emission to the atmosphere.

The coating layer containing cerium oxide particles having barium immobilized thereon, activated alumina particles, and a noble metal component is also employed in the first embodiment. Also in the embodiment of the present invention, the generation of hydrogen sulfide H ₂ S with a strong off-odor is trapped in the catalyst 1 by using harmless barium without using nickel Ni, and discharge of this into the atmosphere is performed. It can be suppressed effectively.

[0092]

As described above in detail, according to the present invention, it is possible to suppress the release of hydrogen sulfide with a strong off-odor into the atmosphere.

Further, according to the present invention, heat deterioration of cerium oxide can be prevented and heat resistance can be improved.

[Brief description of the drawings]

FIG. 1 is a front sectional view schematically showing a configuration of an exhaust gas purifying catalyst according to the present invention.

FIG. 2 is a vertical cross-sectional view of a catalyst according to a first embodiment of the present invention, taken out and taken on a cross section orthogonal to a flow direction of exhaust gas.

FIG. 3 is a diagram conceptually showing the catalyst of the first embodiment shown in FIG.

FIG. 4 is a graph showing a relationship between an HC purification rate and an exhaust gas temperature at a catalyst inlet in the first embodiment.

FIG. 5 shows the amount of lanthanum added and HC in the first embodiment.
FIG. 3 is a diagram showing a relationship with a purification rate.

FIG. 6 shows the addition amount of barium and HC in the first embodiment.
FIG. 3 is a diagram showing a relationship with a purification rate.

FIG. 7 is a graph showing the relationship between the amount of cerium oxide added and the HC purification rate in the first embodiment.

FIG. 8 is a diagram showing a state in which hydrogen sulfide is discharged.

FIG. 9 is a vertical cross-sectional view showing a second embodiment of an exhaust gas purifying catalyst according to the present invention, which is taken along a cross section orthogonal to the exhaust gas flow direction.

FIG. 10 is a view for explaining the flow atmosphere of exhaust gas when testing the purification state of hydrogen sulfide.

FIG. 11 shows the relationship between the amount of hydrogen sulfide generated and H
FIG. 3 is a diagram showing a relationship with a C purification rate.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 catalyst (first embodiment) 2 catalyst carrier 3 first coat layer 4 second coat layer 5 catalyst (second embodiment) 6 catalyst carrier 7 coat layer 10 exhaust pipe 12 exhaust gas purification device 14 housing 16 through hole

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazunori Ihara 3-1 Shinchi, Fuchu-cho, Aki-gun, Hiroshima Prefecture F-term (reference) 3G091 AB01 AB04 AB06 AB08 AB11 BA07 BA11 BA13 BA20 EA17 FA12 FA18 FB09 FB10 FB12 GA06 GB01W GB03W GB04W GB05W GB06W GB07W GB10W 4D048 AA02 AB01 AB02 AB07 BA09X BA15X BA19X BA30X BA31Y BA32Y BA33X BA34Y BA41X BD02 EA04 4G069 AA03 AA08 BA01A BA01B BA10B BB04A13BC13BCBBC BCBC BCBC FB23 GA02

Claims (3)

[Claims] 1. An exhaust gas purifying catalyst for purifying exhaust gas containing sulfur dioxide discharged from an engine, comprising: a catalyst carrier; and sulfur dioxide in the exhaust gas when the engine is in an oxidizing atmosphere with an engine air-fuel ratio lean. Cerium oxide, which releases sulfate by adsorbing sulfur to form a reducing atmosphere rich in air-fuel ratio of the engine and converts the adsorbed sulfur into hydrogen sulfide, and the cerium oxide in which sulfur dioxide in exhaust gas is converted into sulfate in the form of sulfate. Barium and activated alumina that are stored in
And a coat layer formed on the surface of the catalyst carrier and having a noble metal. 2. An exhaust gas purifying catalyst in which a coat layer having cerium oxide, barium, activated alumina and a noble metal is formed on a surface of a catalyst carrier, wherein the barium is immobilized on the cerium oxide. A catalyst for purifying exhaust gas. 3. An exhaust gas purifying method for purifying an exhaust gas containing sulfur dioxide discharged from an engine, wherein a coat layer containing cerium oxide, barium, activated alumina and a noble metal is formed on a surface of a catalyst carrier. The exhaust gas purifying catalyst is disposed in the middle of the exhaust pipe of the engine, and the engine is operated so that the exhaust gas changes from an oxidizing atmosphere to a reducing atmosphere. Exhaust gas purifying method, wherein the barium suppresses the storage of sulfur dioxide in the exhaust gas.