JPH0871424A - Exhaust gas purification catalyst - Google Patents

Abstract

(57) [Summary] [Purpose] To suppress the emission of ammonia while maintaining the same ternary activity as before. A first porous carrier and a second porous carrier having solid acid points, which are arranged in the vicinity of each other, at least a noble metal supported on the first porous carrier, and at least a second porous carrier supported on the second porous carrier. It is characterized by being composed of nickel.
Ammonia generated on the first porous carrier is selectively adsorbed on the solid acid points of the second porous carrier and decomposed into nitrogen and hydrogen by nickel existing in the vicinity.

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 capable of purifying three components of carbon monoxide (CO) and nitrogen oxide (NOx) at the same time. More specifically, it also includes ammonia (NH ₃ ) which may be generated during reduction purification of NOx. The present invention relates to an exhaust gas purification catalyst that can be purified at the same time.

[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 carrier substrate such as cordierite, and a precious metal catalyst such as Pt, Pd, Rh is supported on the carrier layer. Things are widely known.

For example, Japanese Patent Laid-Open No. 2-107334 discloses a carrier made of a metal oxide of aluminum, cerium, nickel and barium, and P supported on the carrier.
An exhaust gas purification catalyst comprising at least one of t, Pd and Rh is disclosed. According to this exhaust gas purifying catalyst, it is possible to purify CO, HC and NOx in the exhaust gas and prevent the emission of hydrogen sulfide (H ₂ S).

[0004]

By the way, when using the conventional three-way catalyst, a system including an oxygen sensor in the engine and the exhaust gas treatment system is constructed, and the system has an air-fuel ratio (A / F) as a stoichiometric point. It is designed so that CO, HC and NOx in the exhaust gas can be simultaneously purified by controlling the vicinity of (stoichiometric). That is, by detecting the oxygen concentration in the exhaust gas with an oxygen sensor and feedback-controlling the engine based on the detected value, the air-fuel ratio can always be controlled near the stoichiometric point.

However, there are cases where the air-fuel ratio deviates from the stoichiometric point due to fluctuations in engine operating conditions such as frequent repeated acceleration / deceleration during urban driving. And when the air-fuel ratio (A / F) is larger than the stoichiometric point (lean)
Contains a sufficient amount of oxygen, the exhaust gas composition becomes an oxidizing atmosphere, and when the air-fuel ratio (A / F) is smaller than the stoichiometric point (rich), HC and CO in the fuel are large and the exhaust gas composition is reducing. It becomes an atmosphere.

When the exhaust gas having a remarkably reducing atmosphere is introduced into the three-way catalyst, NOx in the exhaust gas is reduced but does not become N ₂ and ammonia (NH ₃ ) may be produced and discharged. was there. The present invention has been made in view of such circumstances, and an object thereof is to suppress the production and discharge of ammonia while maintaining the ternary activity equivalent to that of the conventional one.

[0007]

The exhaust gas-purifying catalyst of the present invention for solving the above-mentioned problems is a first catalyst disposed in the vicinity of each other.
It is characterized by comprising a porous carrier and a second porous carrier having a solid acid point, a noble metal supported on at least the first porous carrier, and at least nickel supported on the second porous carrier.

The exhaust gas purifying catalyst of the second invention is characterized in that at least the first porous carrier further carries cerium oxide. The exhaust gas-purifying catalyst of the third invention is characterized in that the second porous carrier is zeolite. Furthermore, the exhaust gas purifying catalyst of the fourth invention is characterized in that a precious metal and nickel are supported on a porous carrier having solid acid points.

[0009]

In the exhaust gas purifying catalysts of the first to third inventions, CO and HC in the exhaust gas are oxidized and purified in the same manner as in the conventional case by at least the ternary activity of the noble metal supported on the first porous carrier. , NOx is reduced and purified. When the exhaust gas composition is in a remarkably reducing atmosphere, NOx
May not be nitrogen gas (N ₂ ) and ammonia (NH ₃ ) may be generated. However, in the exhaust gas purifying catalyst of the present invention, ammonia is selectively adsorbed on the solid acid points of the second porous carrier, and is decomposed into nitrogen and hydrogen by nickel which is also carried on the second porous carrier and is present in the vicinity. Therefore, the emission of ammonia is suppressed.

Further, when the cerium oxide is supported, the cerium oxide has a property of temporarily storing oxygen, so that it is possible to prevent the atmosphere variation of the exhaust gas composition from being relieved to form a remarkably reducing atmosphere. Therefore, the production of ammonia itself is suppressed. When the second porous carrier is zeolite, a large number of homogeneous solid acid points are present per unit volume, so that the ability to adsorb ammonia is particularly excellent and the emission of ammonia is further suppressed.

In the exhaust gas purifying catalyst according to the fourth aspect of the present invention, the noble metal and nickel are supported on the porous carrier having solid acid points. Therefore, due to the ternary activity of the precious metal, CO and HC in the exhaust gas are oxidatively purified as in the conventional case, and when ammonia is produced, the ammonia is selectively adsorbed to the solid acid point and is near the precious metal. Since the existing nickel is efficiently decomposed into nitrogen and water,
Ammonia emission is suppressed.

[0012]

【Example】

Specific Examples of the Invention In the first to third inventions, examples of the first porous carrier include refractory inorganic oxides having a high specific surface area such as alumina, silica, titania, and zirconia. In particular, activated alumina having high heat resistance and high specific surface area is desirable.

In the first to fourth inventions, platinum (Pt), palladium (Pd), rhodium (R
At least one of h) is used. The amount of the noble metal supported is 0.1% by weight or more, preferably 0.5 to 20% by weight, based on the first porous carrier. When the supported amount of the noble metal is less than 0.1% by weight, the oxidation reaction of CO and HC is reduced, and the reduction reaction of NOx is reduced particularly on the low temperature side, which is not preferable. 20% by weight
If it is carried over, the effect is saturated and the cost becomes high.

The second porous carrier having a solid acid point in the first to third inventions and the porous carrier having a solid acid point in the fourth invention are Bronsted acidic (proton releasing) or Lewis acidic ( An inorganic oxide having an electron pair) is included, and examples thereof include zeolite, silica-alumina, silica-titania, silica-magnesia, and zeolite-type inorganic materials such as SAPO (silicoaluminophosphate). Of these, zeolite is particularly preferable.

Zeolite has the general formula M _x · (Al
₂ O ₃ ) _x · (SiO ₂ ) _Y (M is hydrogen or alkali metal etc.) is a general term for minerals having a unique pore structure. Among them, there are many heat-resistant silicas and the silica / alumina ratio is high. It is particularly desirable that (y / x) is about 10 to 1000. If the silica / alumina ratio is out of this range, the number of solid acid points will decrease and the acid strength will decrease.
Ammonia adsorption capacity is lowered and ammonia is easily discharged. Silica / alumina ratio (y / x) is 10 to 1000
Zeolite β, ZSM-5
Examples include pentasil-type zeolite, mordenite, erionite / offretite, and the like.

The amount of nickel supported on the second porous carrier in the first to third inventions or nickel supported on the porous carrier in the fourth inventions is 0.01 to 1.0 per 1 L of the catalyst. A molar range is desirable. When the amount of nickel supported is less than 0.01 mol / L, it becomes difficult to suppress the emission of ammonia, and even when more than 0.1 mol / L is supported, the effect is saturated and the cost becomes high.

In addition to nickel, the second porous carrier may further carry a noble metal such as platinum, rhodium or palladium. Further, in the exhaust gas purifying catalysts of the first to third inventions, it is desirable that at least the first porous carrier contains cerium oxide, and in the exhaust gas purifying catalyst of the fourth invention, the porous carrier further contains cerium oxide. It is desirable that the oxide is supported. The content of this cerium oxide is not particularly limited, but is 0.0 per 1 L of the catalyst.
A range of 1 to 3.0 mol is suitable. 0.01 mol / L
If it is less, the effect of inclusion cannot be obtained and 3.0 mol / L
Even if it is contained in a larger amount, the effect will be saturated and the amount of the porous carrier will be relatively small, resulting in an adverse effect.

In the first to third inventions, the first porous carrier and the second porous carrier are arranged in the vicinity.
The vicinity means that the ammonia generated in the first porous carrier is the second
A distance that does not prevent adsorption and purification with a porous carrier. As the arrangement of both carriers, for example, a method of arranging the first porous carrier in the front stage of the exhaust gas flow channel and the second porous carrier in the rear stage, or applying the second porous carrier to the carrier substrate, Examples include a method in which the first porous carrier is further applied and arranged on the surface of the second porous carrier layer. There is no particular limitation as long as at least the first porous carrier is on the upstream side of the exhaust gas flow path with respect to the second porous carrier.

As a particularly preferable arrangement form, the first
It is preferable that the porous carrier and the second porous carrier are uniformly mixed and arranged with fine particles. This is because in this case, the ammonia generated in the first porous carrier is adsorbed and purified most efficiently in the second porous carrier. The exhaust gas purifying catalysts of the first to fourth inventions further include NO such as alkali metal, alkaline earth metal or rare earth element.
It is also preferable to contain x absorber. As a result, NOx
The purification performance of is further improved. If zirconia or yttria is included, the heat resistance of cerium oxide can be further improved.

In order to produce the exhaust gas purifying catalysts of the first to third inventions, the second catalyst preliminarily loaded with nickel by an immersion method or the like is used.
A method of coating a porous carrier, a first porous carrier, and a noble metal on a monolith substrate as a slurry and firing the slurry, or a slurry of the second porous carrier and the first porous carrier carrying nickel in advance to form a monolith substrate, etc. There is a method of supporting a noble metal by a dipping method after coating and firing.

In order to produce the exhaust gas purifying catalyst of the fourth aspect of the present invention, a porous carrier supporting nickel by a dipping method or the like, a method of coating a monolith substrate or the like with a slurry of a noble metal as a slurry, and firing it, a porous supporting nickel There is a method in which a porous carrier is made into a slurry, coated on a monolith substrate, etc., baked, and then a precious metal is supported by an immersion method. Hereinafter, a specific description will be given with reference to examples. (Example 1) 120 g of γ-alumina powder was mixed with 0.1 mol of lanthanum carbonate powder and 0.3 mol of cerium oxide powder, and the mixed powder was immersed in an aqueous palladium nitrate solution containing 3 g of Pd and evaporated. After being dried and solidified, it was dried at 110 ° C. for one day, and further subjected to baking treatment at 600 ° C. for 3 hours in the atmosphere. The obtained Pd-supported powder was immersed in an aqueous solution in which 0.2 mol of barium acetate was dissolved, evaporated to dryness, and then 110 ° C.
After being dried for 1 day and 2 days, and further subjected to a calcination treatment in the atmosphere at 600 ° C. for 3 hours, a first powder composed of Pd / barium / ceria / lanthanum / alumina was prepared.

The first powder has γ-alumina as the first porous carrier on which Pd is carried as a noble metal and further contains cerium oxide. Further, it contains lanthanum which is a rare earth element as an NOx absorbent and barium which is an alkaline earth metal. On the other hand, 50 g of a commercially available H-type ZSM-5 zeolite (silica / alumina ratio = 40) was immersed in an aqueous nickel nitrate solution of a predetermined concentration, and then 60
A second powder carrying 10% by weight of nickel was prepared by performing a baking treatment at 0 ° C. for 3 hours. This second powder is the second
This is a structure in which nickel is supported on zeolite as a porous carrier.

Next, the first powder and the second powder were mixed, pressed and molded into pellets of 0.5 to 1 mm to prepare an exhaust gas purifying catalyst of Example 1. The mixing ratio of the first powder and the second powder is such that nickel is 0.1 mol with respect to 120 g of alumina. (Comparative Example 1) Only the first powder prepared in Example 1 was compacted into a pellet and used as an exhaust gas purifying catalyst of Comparative Example 1. Of course, nickel is not included. (Comparative Example 2) A powder obtained by mixing a predetermined amount of commercially available nickel oxide powder with the first powder prepared in Example 1 was compacted and formed into pellets to obtain an exhaust gas purifying catalyst of Comparative Example 2.
The mixing ratio of the nickel oxide powder is a ratio in which nickel is 0.1 mol with respect to 120 g of alumina as in Example 1. (Comparative Example 3) In the same manner as in Example 1 except that silica powder was used instead of zeolite to prepare the second powder,
An exhaust gas purifying catalyst of Comparative Example 3 was prepared. The mixing ratio of the first powder and the second powder composed of silica / nickel is, as in Example 1, 120 g of alumina and 0.1% of nickel.
It is the ratio of molar. (Comparative Example 4) An exhaust gas purifying catalyst of Comparative Example 4 was prepared in the same manner as in Example 1 except that the second powder was prepared by using alumina powder instead of zeolite. The mixing ratio of the first powder and the second powder of alumina / nickel is
As in Example 1, the ratio is such that nickel is 0.1 mol with respect to 120 g of the first powder alumina. (Example 2) γ-alumina powder was immersed in an aqueous dinitrodiaminoplatinum solution having a predetermined concentration, evaporated to dryness, dried at 110 ° C. for one day and then calcined in the atmosphere at 600 ° C. for 3 hours to obtain Pt / A first powder made of alumina was prepared. The supported amount of Pt is 4.0% by weight.

On the other hand, a second powder was prepared in the same manner as in Example 1. The amount of nickel supported on the zeolite was 2 g in terms of metallic nickel with respect to 96 g of zeolite. Then, the first powder and the second powder were mixed in a weight ratio of zeolite / alumina = 4/1, and a pellet-shaped exhaust gas purifying catalyst of Example 2 was prepared in the same manner as in Example 1. (Example 3) An exhaust gas purifying catalyst of Example 3 was prepared in the same manner as in Example 2 except that the weight ratio of nickel to zeolite in the second powder was nickel / zeolite = 5/96. . (Comparative Example 5) A first powder made of Pt / alumina as in Example 2 and a second powder made of only zeolite were mixed and compacted into a pellet, which was used as an exhaust gas purifying catalyst of Comparative Example 5. . The mixing ratio of the first powder and the second powder is zeolite / alumina = 4/1 by weight. (Comparative Example 6) The same Pt / alumina powder as in Example 2 and a second powder made of alumina alone were mixed and compacted into a pellet, which was used as the exhaust gas purifying catalyst of Comparative Example 6. . The mixing ratio of the first powder and the second powder is 4/1 by weight. (Comparative Example 7) A second powder was prepared in the same manner as in Example 1 except that an iron nitrate aqueous solution was used instead of the nickel nitrate aqueous solution, and the second powder was compacted together with the first powder as in Example 2 and then pelletized. As a condition, the exhaust gas-purifying catalyst of Comparative Example 7 was used. The weight ratio of iron supported on the zeolite was iron / zeolite = 2/96, and the weight ratio of the first powder and the second powder was zeolite / alumina = 4/1. (Comparative Example 8) A second powder was prepared in the same manner as in Example 1 except that a copper nitrate aqueous solution was used in place of the nickel nitrate aqueous solution, and the second powder was compacted together with the same first powder as in Example 2 and then pelletized. As a condition, the exhaust gas-purifying catalyst of Comparative Example 8 was used. The weight ratio of copper supported on the zeolite was copper / zeolite = 2/96, and the weight ratio of the first powder and the second powder was zeolite / alumina = 4/1. (Comparative Example 9) Second example similar to Example 1 except that an aqueous solution of cobalt nitrate was used instead of the aqueous solution of nickel nitrate.
A powder was prepared, pressed and molded with the same first powder as in Example 2, and then pelletized to obtain an exhaust gas purifying catalyst of Comparative Example 9. The ratio of cobalt supported on the zeolite was cobalt / zeolite = 2/96 by weight,
The mixing ratio of the first powder and the second powder is zeolite / alumina = 4/1 by weight. (Evaluation Test 1) With respect to each of the catalysts of Example 1 and Comparative Examples 1 to 4, the purification performance of the exhaust model gas was measured under the following conditions using a fixed bed flow reactor.

Exhaust model gas temperature: 300 ° C. space velocity (SV): 50,000 / hr air-fuel ratio: λ = 0.96 (A / F = 14.0)
Equivalent) Note that the exhaust model gas composition of λ = 0.96 is as shown in Table 1.

[0026]

[Table 1] Regarding the purification performance, the purification rates of NO, CO, and HC were measured, the amounts of ammonia discharged were measured, and the ammonia reduction rate was calculated based on the ammonia emissions of the catalyst of Comparative Example 1. The respective results are shown in Table 2.

[0027]

[Table 2] From Table 2, it can be seen that the exhaust gas-purifying catalyst of Example 1 has the same three-way catalytic activity as that of the comparative example, and the ammonia reduction rate is remarkably higher than that of the comparative example. It is clear that this is due to the fact that the zeolite has the second powder in which nickel is supported, and it is clear that this is an effect due to the coexistence of zeolite and nickel. (Evaluation Test 2) With respect to each of the catalysts of Examples 2 to 3 and Comparative Examples 5 to 9, the exhaust model gas purification performance was measured under the following conditions using a fixed bed flow reactor.

Exhaust model gas temperature: 400 ° C. Space velocity (SV): 50,000 / hr Air-fuel ratio: λ = 0.98 (A / F = 14.3)
Equivalent) Note that the exhaust model gas composition of λ = 0.98 is as shown in Table 3.

[0029]

[Table 3] As for the purification performance, the production rates of NO, CO and HC were measured, and the production rates were calculated by measuring the amounts of discharged ammonia and nitrogen gas. The respective results are shown in FIGS. 1 and 2. The constitution of each catalyst is shown in Table 4.

[0030]

[Table 4] From FIG. 1, it is clear that even in a reducing atmosphere of λ = 0.98, each catalyst of Examples has a three-way catalytic activity comparable to that of Comparative Examples. Further, from FIG. 2, even in the reducing atmosphere of λ = 0.98, the catalyst of the example has a higher N ₂ production rate and NH ₃ production than the comparative example.
The production rate is low, and it is clear that this is the effect of supporting Ni on the zeolite. In addition, Example 2 shows better results than Example 1, and it can be seen that the larger the amount of Ni, the better in the range of this experiment.

[0031]

[Effects of the Invention] That is, according to the catalyst for purifying exhaust gas of the present invention, it is possible to prevent generation of ammonia when purifying exhaust gas in a reducing atmosphere while maintaining the same three-way purification activity as conventional ones. Even if ammonia is produced, it is adsorbed / decomposed and purified, so that the emission of ammonia is suppressed.

[Brief description of drawings]

FIG. 1 is a bar graph showing purification rates of exhaust gas-purifying catalysts of Examples and Comparative Examples.

FIG. 2 shows NO purification of exhaust gas purifying catalysts of Examples and Comparative Examples.
Rate and NH₃And N ₂3 is a bar graph showing the generation rate of.

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Claims (4)

[Claims] 1. A first porous carrier and a second porous carrier having solid acid points, which are disposed in the vicinity of each other, a noble metal supported on at least the first porous carrier, and the second porous carrier. An exhaust gas-purifying catalyst comprising at least nickel supported. 2. The exhaust gas-purifying catalyst according to claim 1, wherein at least the first porous carrier further carries cerium oxide. 3. The exhaust gas purifying catalyst according to claim 1, wherein the second porous carrier is zeolite. 4. An exhaust gas purifying catalyst, characterized in that a precious metal and nickel are supported on a porous carrier having solid acid points.