JP3224054B2 - Exhaust gas purification method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying exhaust gas, and more particularly, to carbon monoxide (CO) and hydrocarbon (HC).
The present invention relates to a method for efficiently purifying nitrogen oxides (NOx) in exhaust gas containing an excess amount of oxygen more than is necessary to oxidize NOx.

[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 an exhaust gas purifying catalyst for automobiles.
As such a catalyst, a catalyst in which a support layer is formed from γ-alumina or the like on a refractory support such as cordierite and a noble metal catalyst such as Pt, Pd, or Rh is supported on the support layer is widely known. I have.

[0003] The purifying performance of such an exhaust gas purifying catalyst greatly varies depending on the air-fuel ratio (A / F) of the engine. That is, on the lean side where the air-fuel ratio is large, that is, on the lean side where the fuel concentration is lean, the amount of oxygen in the exhaust gas increases, and the oxidation reaction for purifying CO and HC is active, but N
The reduction reaction for purifying Ox becomes inactive. Conversely, on the rich side where the air-fuel ratio is low, that is, on the rich side where the fuel concentration is high, the amount of oxygen in the exhaust gas decreases, and the oxidation reaction becomes inactive but the reduction reaction becomes active.

On the other hand, in the case of driving in an urban area, the vehicle frequently starts and stops, and the air-fuel ratio frequently changes within a range from near the theoretical value to an over-lean state. In order to meet the demand for fuel economy in such traveling, it is necessary to operate on the lean side, which supplies an air-fuel mixture as much as possible. Therefore, even on the lean side, N
There is a demand for the development of a catalyst that can sufficiently purify Ox.

[0005]

SUMMARY OF THE INVENTION Accordingly, the applicant of the present application
Previously, a catalyst supporting an alkaline earth metal and Pt has been proposed (Japanese Patent Application No. 4-130904). According to this catalyst, NOx is adsorbed by the alkaline earth metal and is purified by reacting with a reducing gas such as HC. Therefore, the NOx is excellent in NOx purification performance even on the lean side.

However, it has been found that, even with this catalyst, the purification rate of NOx drops to about 50% of the initial value after the durability test. Therefore, it is required to further improve the purification rate after durability before practical use. The present invention has been made in view of such circumstances, and has as its object to purify NOx in exhaust gas containing excess oxygen more efficiently.

[0007]

According to the present invention, there is provided an exhaust gas purifying method for purifying exhaust gas which simultaneously purifies carbon monoxide, hydrocarbons and nitrogen oxides in an exhaust gas in an oxygen-excess atmosphere. A method for purifying exhaust gas, comprising carrying a composite oxide comprising Ba and at least one of Ti, Zr, Ce and Hf and at least one of Pt and Pd on a porous carrier. The catalyst is characterized by contacting the catalyst with an excess oxygen exhaust gas.

Examples of the porous material include alumina, zeolite, zirconia, silica alumina, silica and the like. The carrier may be formed from the porous body itself, or may be used by coating a honeycomb body formed of cordierite, a heat-resistant metal, or the like.

[0009]

In the exhaust gas purifying method of the present invention, Ba is Ti,
A catalyst that is supported as a composite oxide with at least one of Zr, Ce and Hf, and that carries at least one of Pt and Pd is used, and the exhaust gas with excess oxygen contacts and is purified. Here, in the catalyst disclosed in Japanese Patent Application No. 4-130904, barium is supported alone, and it reacts with NOx to form Ba (NO ₃ ) ₂ , whereby N is produced.
It is believed to adsorb Ox. However, the exhaust gas contains SO ₂ generated by the combustion of sulfur (S) contained in the fuel, which is further oxidized to SO ₃ by a catalytic metal in an oxygen-excess atmosphere. And it is also easily converted into sulfuric acid by the water vapor contained in the exhaust gas, and reacts with the barium to form barium sulfate (BaSO4).
It was clarified that ₄ ) suffered poisoning degradation. When barium becomes sulfate in this way, NO
It is considered that x cannot be adsorbed, and as a result, the NOx purification performance of the catalyst after the durability test decreases.

However, in the catalyst used in the present invention, it was clarified that barium sulfate was suppressed from being produced because barium was present as a composite oxide. This is presumed to be because barium atoms are present in the composite oxide lattice in a highly dispersed state, so that even if SO ₃ is adsorbed, it is easily desorbed and crystal growth as barium sulfate is suppressed.

On the other hand, since the above-mentioned composite oxide is basic, the NOx adsorbing ability is not impaired, and shows the same performance as barium alone before the poisoning deterioration by SO ₂ occurs.

[0012]

The present invention will be specifically described below with reference to examples. (Example 1) <Preparation of catalyst> A cerium oxide (CeO ₂ ) powder having a surface area of 100 m ² / g was impregnated with an aqueous barium acetate solution, dried and calcined at 800 ° C to form a Ba-Ce composite oxide powder. .

Next, 100 parts by weight of activated alumina powder,
A slurry is prepared by mixing 30 parts by weight of the above composite oxide powder, 10 parts by weight of alumina sol (alumina content: 10% by weight), and 40 parts by weight of water. The slurry was blown off, dried and calcined at 500 ° C. to form an alumina coat layer supporting the composite oxide. The coating amount is 120 g / liter.

The carrier having the alumina coating layer was immersed in an aqueous solution of dinitrodiammine platinum, dried by blowing off excess water drops, and calcined at 300 ° C. to obtain a catalyst A. The catalyst A had a Ba-Ce composite oxide of 0.3 as barium.
mol / l, 0.3 mol / l as cerium. Further, 2.0 g / liter of platinum is carried. <Evaluation of Purification Performance> The above catalyst was installed in an exhaust passage of a vehicle equipped with a lean burn engine (1.6 liter), and the vehicle was driven in a city running mode (10.15 mode) to measure a NOx purification rate. Next, an endurance test was performed in which the air-fuel ratio (A / F) was 18 and the catalyst-containing gas temperature was 600 ° C. for 100 hours, and then the NOx purification rate was measured under the same conditions as described above. Table 1 shows the results. (Example 2) A slurry was prepared by mixing 100 parts by weight of activated alumina powder, 10 parts by weight of alumina sol (alumina content: 10% by weight), and 40 parts by weight of water, and a cordierite-based honeycomb carrier was added to the slurry. After immersion, excess slurry was blown off, dried and fired at 500 ° C. to form an alumina coat layer. The coating amount is 120 g / liter.

Next, the carrier having an alumina coat layer was impregnated with a mixed aqueous solution of cerium acetate and barium acetate, and was dried by blowing off excess water droplets and then fired at 600 ° C. The catalyst was immersed in an aqueous dinitrodiammine platinum solution, dried by blowing off excess water droplets, and calcined at 300 ° C. to obtain a catalyst B. In the catalyst B, the Ba-Ce composite oxide was 0.2 mol / l as barium and 0.2 mol as cerium.
/ Liter carried. Further, 2.0 g / liter of platinum is carried.

For this catalyst B, the NOx purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1. (Example 3) In order to further enhance the oxygen storage effect, which is another great characteristic of the three-way catalyst, part of Ce in Example 1 was changed to 5
It was substituted by Nb which is a valence element. That is, niobium oxide (Nb ₂ O ₅ ) powder is impregnated with a mixed aqueous solution of barium acetate and cerium acetate, dried, baked at 800 ° C., and
A Ce-Nb composite oxide powder was formed.

Then, a coat layer was formed in the same manner as in Example 1, and platinum was carried thereon to obtain a catalyst C. The catalyst C contained 0.3 mol / l as barium, 0.24 mol / l as cerium, and 0.0 as niobium.
6 mol / liter of Ba-Ce-Nb composite oxide is supported. Further, 2.0 g / liter of platinum is carried.

For this catalyst C, the NOx purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1. (Comparative Example) A carrier having the same alumina coating layer as in Example 2 was immersed in an aqueous barium acetate solution, dried by blowing off excess water droplets, and baked at 600 ° C. Further, the catalyst was immersed in an aqueous solution of dinitrodiammine platinum, blown off with excess water droplets, dried and calcined at 300 ° C. to obtain a catalyst D. The catalyst D contained 0.3 mol / l of barium and 2.0 mol of platinum.
g / liter, and barium is supported in the form of a single oxide.

For this catalyst D, the NOx purification rate was measured in the same manner as in Example 1, and the results are shown in Table 1.

[0020]

[Table 1] <Evaluation> From Table 1, it can be seen that in the catalysts A to C supporting barium as a composite oxide, the degree of reduction in the NOx purification rate after durability was much smaller than that of the catalyst D of the comparative example. On the other hand, when the X-ray diffraction of each catalyst after the durability test was measured, a diffraction line identified as BaSO ₄ was observed in the catalyst D, but was not observed in the catalysts A to C. Therefore, the difference between the catalysts in the NOx purification rate after endurance is B
It is inferred that the presence of aSO ₄ has a great effect.

The catalyst C has NO at both the initial stage and after the endurance.
x Purification rate is superior to Catalyst A and Catalyst B. this is,
It is presumed that a part of Ce ⁴⁺ was changed to Ce ³⁺ by the addition of pentavalent Nb, and the oxygen storage effect was enhanced. In addition,
It should be noted that the catalysts A to D showed no difference in the purification performance of carbon monoxide (CO) and hydrocarbon (HC), and all of them were excellent.

[0022]

According to the exhaust gas purifying method of the present invention, the catalyst used exhibits good NOx purifying performance even after the durability test, and can stably and efficiently purify NOx on the lean side where oxygen is excessive. it can.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) B01D 53/86 B01J 21/00-37/36

Claims (2)

(57) [Claims] An exhaust gas purifying method for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, wherein a porous carrier comprises Ba, Ti, Zr, An exhaust gas purifying method characterized by contacting an exhaust gas with excess oxygen with an exhaust gas purifying catalyst carrying a composite oxide comprising at least one of Ce and Hf and at least one of Pt and Pd. . 2. Exhaust gas in an oxygen-excess atmosphere
Purifies carbon monoxide, hydrocarbons and nitrogen oxides simultaneously
It is a method of purifying exhaust gas, in which a carrier made of a porous material is used.
A composite oxide composed of Ba, Ce and Nb, and Pt and P
exhaust gas purifying catalyst carrying at least one of
The method is characterized by contacting exhaust gas with excess oxygen to the medium
Exhaust gas purification method.