JP3044622B2 - Exhaust gas purification method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an exhaust gas purification method for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas discharged from an internal combustion engine such as an automobile engine. In particular, the present invention relates to an exhaust gas purifying method for purifying combustion exhaust gas with excess oxygen.

(Prior art) Nitrogen oxides, carbon monoxide and hydrocarbons, which are harmful substances in exhaust gas discharged from an internal combustion engine, are, for example, Pt, Rh,
It is removed by a three-way catalyst in which Pd and the like are supported on a carrier. However, with respect to diesel engine exhaust gas, since there is a large amount of oxygen in the exhaust gas, there is no effective catalyst for nitrogen oxide, and exhaust gas purification by the catalyst is not performed.

In gasoline engines, it is necessary to perform lean combustion for the purpose of lowering fuel consumption and reducing carbon dioxide emissions.However, since the exhaust gas of this lean-burn gasoline engine is in an oxygen-excess atmosphere, A conventional three-way catalyst cannot be used, and a method for removing harmful components has not been put to practical use.

As a method for removing nitrogen oxides particularly in such an oxygen-excess exhaust gas, a method of adding a reducing agent such as ammonia, a method of absorbing and removing nitrogen oxides by an alkali, and the like are also known. These methods are not effective methods for use in vehicles that are mobile sources, and their applications are limited.

It is known that zeolite catalysts which ion-exchange or support a base metal, particularly copper, are effective for removing nitrogen oxides in a gas. For example, JP-A-60-125250 proposes a method of directly decomposing nitrogen oxides in exhaust gas without a reducing agent into nitrogen and oxygen by a redox reaction of Cu ions carried on zeolite. However, if a large amount of oxygen is present in the exhaust gas, the redox reaction of Cu ions is hindered, and the decomposition of nitrogen oxides does not proceed. For this reason, the exhaust gas to which the above-mentioned zeolite catalyst in which copper is ion-exchanged or supported is also applicable is limited, and is not suitable for the purpose of purifying the automobile exhaust gas as described above, which is a target of the present invention.

There is also a proposal for purifying exhaust gas of an internal combustion engine, for example, exhaust gas of an automobile engine containing a trace amount of a reducing agent such as unburned carbon monoxide and hydrocarbons (Japanese Patent Application Laid-Open No. Sho 63).
-100919, JP-A-63-283727). This proposes a catalyst containing a base metal in zeolite or the like as a catalyst capable of selectively reducing nitrogen oxides even in an oxygen-excess atmosphere in the presence of the reducing agent.

However, the catalyst according to this conventional proposal is a point that should be further improved in terms of durability at high temperatures, catalytic performance, and the like.
It has not yet been put to practical use.

(Problems to be Solved by the Invention) An object of the present invention is to solve the above-mentioned problems of the prior art, and it is possible to discharge an exhaust gas from an internal combustion engine without using a reducing agent such as ammonia. It is an object of the present invention to provide a method for purifying exhaust gas containing excess oxygen, particularly exhaust gas from automobiles and the like described above, which is suitable for efficiently purifying the exhaust gas and has excellent durability at high temperatures.

(Means for Solving the Problems) As a result of intensive studies on the above problems, the present inventors have completed the present invention.

That is, the present invention provides that the SiO ₂ / Al ₂ O ₃ molar ratio is at least 10
The above zeolite is used as a base material, and by contacting the combustion exhaust gas with a catalyst having copper ions and rare earth elements supported thereon, nitrogen oxides, carbon monoxide and hydrocarbons contained therein are effectively removed. It is intended to provide a method of removing exhaust gas which can be carried out.

 Hereinafter, the present invention will be described in more detail.

The zeolite is generally xM _{2 / n} O.Al ₂ O ₃ .ySiO ₂ zH ₂ O (where n is the valency of a cation, x is a number in the range of 0.8 to 2, y is a number of 2 or more, z is a number of 0 or more), and the zeolite used in the present invention must have a SiO ₂ / Al ₂ O ₃ molar ratio of 10 or more. The SiO ₂ / Al ₂ O ₃ molar ratio is not particularly limited to the upper limit, but if the SiO ₂ / Al ₂ O ₃ molar ratio is less than 10, the heat resistance and durability of the zeolite itself are low, so the catalyst Does not have sufficient heat resistance and durability.
Generally, a SiO ₂ / Al ₂ O ₃ molar ratio of about 10 to 1000 is used.

The zeolite constituting the catalyst used in the present invention may be either a natural product or a synthetic product, and the method for producing these zeolites is not particularly limited. Typically, mordenite, ferrierite, ZSM- 5, ZSM-11, ZS
Zeolites such as M-112 and ZSM-20 can be used. Also Y
Zeolite, such as zeolite and L-type zeolite, may be dealuminated. Further, these zeolites can be used as a catalyst for use in the present invention after ion-exchange to NH ₄ ⁺ type or H type as it is or after being treated with an ammonium salt, a mineral acid or the like.

Representative exhaust gas purifying catalysts used in the present invention include those in which copper ions and rare earth elements are contained by ion exchange, and a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 10 is exemplified. .

Examples of a method for allowing the zeolite to contain a copper ion and a rare earth element include an ion exchange method. The method is not particularly limited, but a method generally used as an ion exchange method can be employed. For example, ion exchange using a mixed aqueous solution containing copper and a rare earth element can be typically exemplified.Rare earth elements may be ion exchanged after copper ions are exchanged, or after rare earth elements are ion exchanged. Copper ions may be ion-exchanged.

The concentration of copper and rare earth element ions in the aqueous solution at the time of ion exchange can be arbitrarily set according to the target ion exchange rate of the catalyst.

As rare earth element ions, for example, La, Ce, Pr, Nd,
Pm, Sm, etc. can be used. Copper and rare earth element ions can be used in the form of a soluble salt. As the soluble salt, for example, nitrate, acetate, oxalate, hydrochloride and the like can be suitably used.

In the above, when exchanging copper ions, ammonia may be added to adjust the pH to increase the content of copper ions. Since copper ions at the ion exchange site are active sites, it is desirable that copper be ion-exchanged at the ion exchange site.

On the other hand, for rare earth elements, when performing ion exchange,
If ammonia is added to an aqueous solution containing a rare earth element to raise the pH, the rare earth element precipitates as a hydroxide and ion exchange becomes difficult, so it is preferable not to add ammonia. It is desirable that the rare earth element is exchanged at the ion exchange site. However, the effect is exhibited even when the hydroxide deposited on the zeolite surface is supported on the zeolite as an oxide by firing.

The ion-exchanged sample is used as a catalyst after solid-liquid separation, washing and drying. Moreover, it can also be used after baking as needed.

The ion exchange amount of copper is expressed as a Cu / Al atomic ratio of 0.01 to
1, preferably in the range of 0.1 to 0.6.
If the atomic ratio of Cu / Al is less than 0.01, the active site copper ions are reduced and sufficient catalytic activity is not obtained. Conversely, if the atomic ratio exceeds 1, excess copper may be present on the zeolite surface in an aggregated state. The above range is preferable because the catalyst activity, heat resistance, and durability may be adversely affected.

The ion exchange amount of the rare earth element is (rare earth element) / A
It is desirable that the atomic ratio is 0.01 or more, preferably 0.01 to 2. If the (rare earth element) / Al atomic ratio is less than 0.01,
The above range is desirable because the effect of the rare earth element coexistence is small and sufficient catalytic performance and durability may not be obtained.

SiO ₂ / Al ₂ O ₃ molar ratio of the exhaust gas purifying catalyst used in the present invention does not change the SiO ₂ / Al ₂ O ₃ molar ratio substantially of zeolite base material used. Further, the crystal structure of the exhaust gas purifying catalyst is not essentially different before and after ion exchange.

The exhaust gas purifying catalyst used in the present invention can also be used by mixing with a binder such as a clay mineral and shaping it.Also, a zeolite is shaped in advance, and copper and rare earth elements are ion-exchanged into the shaped body and contained. It can also be done.
Examples of the binder used for forming the zeolite include clay minerals such as kaolin, attapulgite, montmorillonite, bentonite, allophane, and sepiolite. Alternatively, a binderless zeolite molded body obtained by directly synthesizing a molded body without using a binder may be used. Further, it is also possible to wash coat zeolite on a honeycomb-shaped substrate made of cordierite or metal and use it.

Removal of nitrogen oxides, carbon monoxide and hydrocarbons in the oxygen-excess exhaust gas, the exhaust gas purification catalyst used in the present invention,
It can be carried out by contacting an oxygen-excess exhaust gas containing nitrogen oxides, carbon monoxide and hydrocarbons. The term “excess oxygen-exhaust gas” as used in the present invention refers to an exhaust gas containing oxygen in excess of the amount of oxygen necessary to completely oxidize carbon monoxide, hydrocarbons and hydrogen contained in the exhaust gas. Specific examples of such exhaust gas include exhaust gas emitted from an internal combustion engine of an automobile or the like, particularly exhaust gas in a state where the air-fuel ratio is large (so-called lean region).

The exhaust gas purifying catalyst used in the present invention exhibits catalytic performance equivalent to that of a conventional copper-supported zeolite catalyst even when applied to an exhaust gas containing carbon monoxide, hydrocarbons and hydrogen and not containing excess oxygen. can do.

(Operation) As described above, in the exhaust gas in the absence of a reducing agent, nitrogen oxides are decomposed into oxygen and nitrogen by a redox reaction of copper ions ion-exchanged to zeolite.

On the other hand, in the exhaust gas of an internal combustion engine in which oxygen coexists, nitrogen oxides are formed by the hydrocarbons and carbon monoxide present in trace amounts in the exhaust gas, and the oxygen-containing organic compounds generated by partial oxidation, so that the base metal in the zeolite is reduced. Is reduced above. In this case, as a base metal, a copper ion which is most likely to adsorb nitrogen oxide exhibits the best catalytic performance. Copper ions have low adsorption capacity for nitrogen oxides when they are supported on zeolite by oxides or the like that are not ion-exchange sites or when copper is aggregated, and when they are exposed to high temperatures, they are supported on ion-exchange sites. Since there is a tendency to promote the desorption of the Cu ions, it is desirable that the Cu ions be supported on ion exchange sites in order to exhibit sufficient catalytic performance and durability.

The catalyst used in the present invention is one in which copper ions and rare earth elements coexist in zeolite as described above,
The effects obtained by coexisting rare earth elements are considered as follows.

1) Improve the heat resistance and durability of zeolite itself.

2) The movement and aggregation of copper ions are prevented by interposing rare earth element ions between copper ions in the zeolite.

3) The mild acid point of the rare earth element ions prevents coke formation and converts various hydrocarbons contained in the exhaust gas into active lower olefins and oxygen-containing compounds.

When treated at a high temperature, copper ions ion-exchanged into zeolite are separated from the exchange site, diffuse and aggregate in pores or on the zeolite surface, and it becomes difficult for the nitrogen oxide to react with the reducing component in the exhaust gas. On the other hand, by coexisting rare earth element ions together with copper ions, migration and aggregation of copper ions can be prevented and durability can be improved.

In addition, various hydrocarbons are usually contained in the exhaust gas of an internal combustion engine. Among them, those effective for reducing nitrogen oxides are lower olefins and oxygen-containing organic compounds, such as paraffin and aromatic compounds. Group compounds have low reducing ability to nitrogen oxides. In such a case, the catalyst used in the present invention to which rare earth element ions are added is that the rare earth element forms a mild acid site in the zeolite, and converts a low-reactivity hydrocarbon such as paraffin into a lower-reactivity hydrocarbon. There is an effect that it can be converted to an olefin and an oxygen-containing organic compound. Therefore, by reacting the converted lower olefin and the like with the nitrogen oxide on the copper ion, the hydrocarbon and the nitrogen oxide can be simultaneously removed, and the activity is improved.

Also, mild acid sites formed by the rare earth elements suppress coke formation, thereby improving the durability of the catalyst.

(Effect of the Invention) The exhaust gas purifying catalyst used in the present invention has an effect that nitrogen oxides, carbon monoxide, and hydrocarbons can be simultaneously and efficiently removed from an oxygen-excess exhaust gas,
In addition, there is an effect that the performance is extremely excellent in heat resistance and durability. Therefore, by bringing the catalyst used in the present invention into contact with the exhaust gas, the effect of purifying nitrogen oxides, carbon monoxide and hydrocarbons can be obtained even in an oxygen-excess state.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples.
However, the present invention is not limited to only these examples.

Reference Example 1 An aqueous sodium silicate solution (SiO ₂ ; 250 g /, Na ₂ O; 8) was placed in a stirred overflow type reaction tank (actual volume 1).
2g /, Al ₂ O ₃ ; 2.75g /) and aluminum sulfate aqueous solution (A
l ₂ O ₃ ; 8.76 g /, SO ₄ ; 365 g /) and 1.5 / hr,
It was continuously supplied at a rate of 0.5 / hr. The apparent residence time of the reaction slurry was 30 minutes, the reaction temperature was 30 to 32 ° C, and the pH of the overflowing slurry was 6.3 to 6.6. Solid-liquid separation of discharged slurry,
Washed. As a result of chemical analysis, the obtained wet cake had a composition of 1.6Na ₂ O, Al ₂ O ₃ , 79SiO ₂ , and 460H ₂ O in terms of molar ratio.

2.89 kg of the wet cake thus obtained and 3.5 wt.
A 6.11 kg aqueous sodium hydroxide solution was charged into an autoclave and heated at 165 ° C. for 72 hours to synthesize a zeolite similar to ZSM-5. The zeolite had the following chemical composition, expressed as a molar ratio of oxides on an anhydrous basis.

1.1Na ₂ O, Al ₂ O ₃ , 43SiO ₂ (SiO ₂ / Al ₂ O ₃ = 43) 1 kg of this zeolite is reduced to 1 times the number of Cu atoms and 5 times the number of Ce atoms to the number of Al atoms of the zeolite. 0.
It was added to a mixed aqueous solution of 1 mol / cerium nitrate and 0.1 mol / copper acetate. The mixture was stirred at room temperature for 20 hours to perform an ion exchange treatment. After this operation was repeated three times, the resultant was washed and dried to prepare Catalyst 1.

Catalysts 2 and 3 were prepared in the same manner as above, using La and Nd instead of Ce.

The results of chemical analysis of the resulting catalyst are shown in Table 1 below.

Reference Example 2 A mordenite-type zeolite was synthesized according to the method of Example 5 in JP-A-59-735. This zeolite is
It had the following chemical composition, expressed as a molar ratio of oxides on an anhydrous basis.

1.1Na ₂ O, Al ₂ O ₃ , 17.5SiO ₂ (SiO ₂ / Al ₂ O ₃ = 17.5) 100 g of this zeolite is 1 times the number of Cu atoms and 5 times the number of Ce atoms with respect to the number of Al atoms of the zeolite 0.
It was added to a mixed aqueous solution of 1 mol / cerium nitrate and 0.1 mol / copper acetate. The mixture was stirred at room temperature for 20 hours to perform an ion exchange treatment. After this operation was repeated three times, the resultant was washed and dried to prepare Catalyst 4. The results of chemical analysis of the resulting catalyst are shown in Table 2 below.

Reference Example 3 Ferrilite-type zeolite was synthesized according to the method of Example 1 of JP-A-60-141617. The zeolite had the following chemical composition, expressed as a molar ratio of oxides on an anhydrous basis.

0.3Na ₂ O, 0.7K ₂ O, Al ₂ O ₃ , 17SiO ₂ (SiO ₂ / Al ₂ O ₃ = 17) 100 g of this zeolite is 1 times the number of Cu atoms and 5 times the number of Al atoms of the zeolite So that the number of Ce atoms is 0.
It was added to a mixed aqueous solution of 1 mol / cerium nitrate and 0.1 mol / copper acetate. The mixture was stirred at room temperature for 20 hours to perform an ion exchange treatment. After this operation was repeated three times, the resultant was washed and dried to prepare Catalyst 5. The results of chemical analysis of the obtained catalyst are also shown in Table 2 above.

REFERENCE EXAMPLE 4 200 g of the same zeolite used in Reference Example 1 was added to a 0.1 mol / copper acetate aqueous solution so that the number of Cu atoms was 1 times the number of Al atoms in the zeolite. Then, the pH was adjusted to 11 by adding 2.5% aqueous ammonia. After stirring at room temperature for 20 hours, it was washed and dried to prepare Cu / zeolite.
As a result of chemical analysis, the atomic ratio of Cu / Al was 0.48.

100 g of Cu / zeolite was added to a 0.1 mol / cerium acetate aqueous solution so that the number of Ce atoms was 0.5 times the number of Al atoms of the zeolite, and the mixture was stirred at room temperature for 20 hours to perform ion exchange treatment. Then, solid-liquid separation and drying were performed to prepare Catalyst 6. As a result of chemical analysis, the atomic ratio of Ce / Al was 0.31.

Example 1 Each catalyst was press-molded, pulverized and sized to 42 to 80 mesh. 2 cc of the mixture was filled in a normal-pressure fixed-bed flow reaction tube, and a gas simulating the exhaust gas of a lean burn engine (having a gas composition shown in Table 3) was flowed at a space velocity of 30,000 / hr.

After the pretreatment at 500 ℃ for 30 minutes under the same gas flow,
The temperature was raised from room temperature to 600 ° C. at 5 ° C./min, and the purification activity was measured.

Table 4 shows the purification rates of NO, CO, and C ₃ H ₆ at 400 ° C. The purification rate was calculated as the purification rate for the gases shown in Table 3 by measuring the concentration of each component of the gas after passing through the catalyst.

Each catalyst was treated at 700 ° C. for 5 hours while flowing a gas having the composition shown in Table 3 at a space velocity of 30,000 / hr. Thereafter, the purification rate was measured by the same method as described above, and a durability test was performed.

 Table 4 shows the obtained results.

Comparative Example 1 A performance evaluation test similar to that of Example 1 was performed using the Cu / zeolite obtained in Reference Example 4 (hereinafter referred to as “comparative catalyst”).

 The results obtained are shown in Table 4 above.

From the results in Table 4 above, the catalyst of the comparative example has a low purification rate of new and treated products, whereas the exhaust gas purification catalyst of Example 1 of the present invention has a high purification rate of new and treated products. The excellent effects of the catalyst performance and durability in the exhaust gas purification method of the present invention were confirmed.

Example 2 100 parts by weight of the powder of the catalyst 1 obtained in Reference Example 1, 20 parts by weight of silica sol (10% solid content) and 50 parts by weight of water were mixed.
After stirring, a slurry was prepared.

The clay of the slurry was 250 cps. This slurry was coated on the surface of a cordierite honeycomb substrate having a capacity of 1.7 and about 400 cells, and excess slurry was blown off with an air flow. Next, after drying at 100 ° C. for 3 hours, it was calcined at 500 for 3 hours to prepare a monolith catalyst containing 120 g of a zeolite layer.

This monolithic catalyst is mounted on a metal container and has a displacement of 1.6
Installed in the lean burn engine exhaust system. Nitrogen oxides (NOx), CO and hydrocarbons (HC) simulating urban driving
Was measured.

The average air-fuel ratio of the engine is 22 and the maximum temperature is about 750
° C, test time is 600 hours.

 The results are shown in Table 5.

Comparative Example 2 A monolith catalyst was prepared in the same manner as in Example 2 using the comparative catalyst (Cu / zeolite) obtained in Reference Example 4. Next, the same test as in Example 2 was performed.

 The results are shown in Table 6.

From the results shown in Table 6 above, the catalysts of Comparative Examples before and after the test
While the purification rates after 0, 400, and 600 hours are low, the exhaust gas purification catalysts of the examples of the present invention show the results before and 200, 400, and 600.
Since the purification rate after time was high, it was confirmed that the catalyst had excellent performance and durability in the exhaust gas purification method of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazunari Igawa 1-29-18 Okami, Shinnanyo-shi, Yamaguchi (72) Inventor Shinichi Matsumoto 1-Toyota-cho, Toyota-shi, Aichi Prefecture Inside Toyota Motor Corporation (72 ) Inventor Masayuki Fukui 41 Toyota Chuo Research Institute, Inc., Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory, Inc. In the laboratory (56) References JP-A-2-293050 (JP, A) JP-A-2-251247 (JP, A) JP-A-2-164453 (JP, A)

Claims (1)

(57) [Claims] An exhaust gas purifying catalyst comprising a zeolite having a SiO ₂ / Al ₂ O ₃ molar ratio of at least 10 carrying copper ions and a rare earth element is provided with an excess oxygen containing nitrogen oxides, carbon monoxide and hydrocarbons. A method for removing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust gas, which comprises contacting the combustion exhaust gas.