JPH11114424A - Catalyst for exhaust gas purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst for purifying discharge gas which is significantly effective to a exhaust gas containing a large quantity of oxygen and highly durable. SOLUTION: Regarding the catalyst for purifying exhaust gas to be used in oxygen excessive atmosphere, a brownmillerite type compounded oxide having either one of the following formulas A3-a Ba C4-b Db O9 , A2-a Ba C2-b Db O5 , A1-a Ba C2-b Db O4 is used as a catalyst 4 for direct decomposition and absorption of NOx and a reduction catalyst 3 is installed near the catalyst for direction decomposition and adsorption. Each of the consituent elements A, B of the brownmillerite is at least one of elements selected from Ba, Ca, Sr, Mg, and Ce and each of the constituent elements C, D is at least one of elements selected from Y, Dy, Zn, Ti, Mn, Fe, Co, Ni, Cu, Sn, Zr, and Nd. The reduction catalyst is a noble metal group reduction catalyst 3 and contains at least one of elements selected from Ag, Pt, Pd, Ir, and Rh.

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 suitable for exhaust gas from a lean burn internal combustion engine or the like, and more particularly to an exhaust gas purifying catalyst exhibiting a high efficiency of exhaust gas purifying in an oxygen-excess atmosphere.

[0002]

2. Description of the Related Art A conventional three-way catalyst using alumina or zeolite or a three-way catalyst in which a noble metal is supported on an alumina or zeolite carrier has been used as an on-vehicle NOx catalyst.

However, the above three-way catalyst has little effect on a lean burn internal combustion engine having a process in which exhaust gas contains a large amount of oxygen. Perovskite catalysts have been developed for lean-burn internal combustion engines, but have the disadvantage of low reduction rates. Further, as disclosed in Japanese Patent Application Laid-Open No. 5-317652, in a lean state of a lean burn internal combustion engine, nitrogen oxide (NO) is oxidized with a noble metal salt to nitric oxide (NO ₂ ) to form an alkaline earth metal salt ( For example, a catalyst utilizing a mechanism of releasing and reducing adsorbed NOx in the stoichiometric to rich state while adsorbing on Ba, Sr, Ca, Mg) has been put to practical use.

[0004]

SUMMARY OF THE INVENTION In view of the above problems, an object of the present invention is to provide an exhaust gas purifying catalyst which has a remarkable effect on exhaust gas containing a large amount of oxygen and has excellent durability.

[0005]

In order to solve the above-mentioned problems, the present invention provides a catalyst for purifying exhaust gas in an oxygen-excess atmosphere, which is represented by the following formula (1): It is characterized in that the light composite oxide is used as a catalyst for direct decomposition and absorption of NOx, and a reduction catalyst is arranged near the decomposition and absorption catalyst.

A _3-a B _a C _{4-b Db} O ₉ A _2-a B _a C _{2-b Db} O ₅ A _1-a B _a C _{2-b Db} O ₄ ……… (1 )

[0007]

DETAILED DESCRIPTION OF THE INVENTION The present invention is to obtain a highly efficient NOx removal catalyst which can respond to the increasingly stricter exhaust gas regulations. By using a brown-millarite-type complex oxide as a catalyst for decomposing and absorbing NOx and coexisting with a reduction catalyst such as a noble metal salt, an oxide of alkaline earth metal or alkaline earth metal or NOx as an NOx absorbent is used. A higher NOx removal rate than the salt-based catalyst was achieved.

The exhaust gas purifying catalyst according to the present invention allows the direct decomposition reaction and the absorption reaction to proceed simultaneously by selecting an alkaline earth metal as a constituent ion in the brown-millerite type compound. Compared to catalysts that simply use alkaline earth metals, oxides or hydroxides of alkaline earth metals as absorbents, such as those using barium oxide (BaO) as a typical alkaline earth metal-based NO absorbent , Brown Miralite shows high NOx removal rate. The reason is that a catalyst using an alkaline earth metal such as barium oxide (BaO) or an alkali metal or a rare earth metal removes nitrogen oxide (NO) only by absorption, whereas brown miralite does not.
It has the property of removing and decomposing NOx. In other words, Brown Miralite is easy to adsorb NOx due to structural distortions such as oxygen vacancies resulting from structural conversion at high temperature, and the valence of ions in the structure is easy to change. Nitrogen oxide (NO) is decomposed into nitrogen and oxygen without using a reduction catalyst. The adsorbed nitrogen oxide (NO) is reduced by a reduction catalyst such as platinum (Pt), so that the nitrogen oxide (NO) removal rate is further improved.

When an alkaline earth metal is contained in the brown mirror light structure, the brown mirror light has NOx.
In addition to the removal / decomposition properties of NOx, the property of adsorbing alkaline earth metal NOx is added, and the NOx removal rate is further improved. Although the basic structure of brown milalite is established by two components of the constituent elements (sites) A and C in the above general formula, the same structure can be obtained by selecting the components of the constituent elements (sites) B and D. 2 times more NO using heavy barium oxide (BaO)
x Demonstrate the removal rate.

[0010]

【Example】

Invention product 1: Ba _2.85 Ce _0.15 Y _3.8 Dy _0.2 O ₉ General formula Ba _3-a Ce _a Y _4-b Dy _b O ₉ (a = 0.03-1.5, b = 0.04-2)
The raw material powders were weighed and mixed uniformly so that the composition of the above became the above, and calcined at a predetermined temperature to obtain a catalyst powder. An aqueous solution of palladium chloride (PdCl ₂ ) and alumina (Al ₂ O ₃ ) were mixed with the obtained catalyst powder to form a slurry,
The slurry was supported on the surface of a cordierite honeycomb carrier to obtain an exhaust gas purifying catalyst.

Invention 2: Sr _2.85 Ce _0.15 Nd _3.8 Zr _0.2 O ₉ Also, the general formula Sr _3-a Ce _a Nd _4-b Zr _b O ₉ (a = 0.03 to 1.5, b = 0.
The raw material powders were weighed and uniformly mixed so that the composition of 04 to 2) was as above, and calcined at a predetermined temperature to obtain a catalyst powder. A palladium chloride (PdCl ₂ ) solution and alumina (Al ₂ O ₃ ) powder were mixed with the obtained catalyst powder to form a slurry, and the slurry was carried on the surface of a cordierite honeycomb carrier to obtain an exhaust gas purifying catalyst. .

As a comparative example, barium nitrate was prepared by using alumina (Al ₂ O ₃ ) on the surface of a cordierite honeycomb support.
A solution of Ba (NO ₃ ) ₂ and palladium chloride (PdCl ₂ ) was coated to prepare an exhaust gas purifying catalyst supporting each metal of barium (Ba) and palladium (Pd).

FIG. 1 is a side sectional view schematically showing the structure of an exhaust gas purifying catalyst according to the present invention. In the exhaust gas purifying catalyst 2 according to the present invention, particles of a brown miralite structure catalyst 4 for decomposing and absorbing NOx and particles of a noble metal-based reduction catalyst 3 for reducing NOx are provided on the surface of a cordierite honeycomb carrier 5. It is carried evenly dispersed.
Among the exhaust gas purifying catalysts according to the inventions 1 and 2, the catalyst 4
Was confirmed by X-ray diffraction (XRD) to be a composite oxide having a brown-millerite type structure.

Two kinds of simulated gases G1 and G2 as shown in Table 1 assuming operating conditions of a lean-burn internal combustion engine were prepared at a catalyst inlet temperature of 200 to 600 ° C. and an SV (space velocity) of 100,000 / h. A performance test was conducted in which the catalyst was reacted with the exhaust gas purifying catalyst according to the products 1 and 2 of the present invention alternately for 2 minutes. Then the temperature 8
After performing a durability test in which the simulated gases G1 and G2 at 00 ° C. react with the exhaust gas purifying catalysts according to the products 1 and 2 of the present invention for 30 hours, the above-described performance test was repeated.

Table 1 Simulated gas (balance gas is nitrogen) Component NO Oxygen Ethane CO Water vapor Carbon dioxide G1 80ppm 0.5% 2000ppm 0.5% 10% 14.5% G2 800ppm 5% 500ppm 0.1% 10% 13% Catalyst inlet temperature: 200 ~ 600 ° C, SV: 100,000 / h As shown in FIG. 2, from the results of the above-described performance tests, the exhaust gas purifying catalysts according to the products 1 and 2 of the present invention are higher than the comparative products.
NO removal rate, especially initial NO removal rate is 350-55
At 0 ° C. was found to exceed 50%. FIG. 3 shows the NO removal rates before and after the above-mentioned durability test when the catalyst inlet temperature is 400 ° C. The exhaust gas purifying catalyst according to the invention 2 exhibited a high NO removal rate after the durability test as well as before the durability test.

In order to see the NOx removal ability of the exhaust gas purifying catalyst according to the present invention from the actual exhaust gas of the internal combustion engine, a performance test of the exhaust gas purifying catalyst was conducted using a 1.6 liter lean burn internal combustion engine. I got it. The test was carried out by alternately repeating the operation with the stoichiometric air-fuel consumption and the oxygen-excess air-fuel consumption at intervals of 2 minutes on the internal combustion engine bench. FIG. 4 shows the results of the same test. It can be seen that the catalyst exhaust gas purifying catalysts according to the present invention products 1 and 2 exhibit a higher NOx removal rate than the comparative products.

In addition to the products 1 and 2 of the present invention, an exhaust gas purifying catalyst (sample) using the elements of Table 2 for the constituent elements A to D of the general formula (1) and the elements of Table 3 for the reduction catalyst Also performed the same test, and the same good result was obtained.

Table 2 Constituent elements A and B: barium (Ba), calcium (Ca),
Strontium (Sr), magnesium (Mg), cerium (Ce) Constituent elements C, D: yttrium (Y), dysprosium (Dy), zinc (Zn), titanium (Ti), manganese (Mn),
Iron (Fe), cobalt (Co), nickel (Ni), copper (C
u), tin (Sn), zirconium (Zr), neodymium (Nd) Table 3 Reduction catalysts: silver (Ag), platinum (Pt), palladium (P
d), iridium (Ir), rhodium (Rh)

[0019]

As described above, the present invention uses a brown-millerite type composite oxide as a catalyst for decomposing and absorbing NOx as a catalyst for purifying exhaust gas in an oxygen-excess atmosphere.
NOx decomposed and absorbed by the brown-millerite-type composite oxide, in which a reduction catalyst is placed near the absorption catalyst.
Is reduced by a reduction catalyst such as a noble metal,
It exhibits a higher exhaust gas purification rate and longer life than a catalyst using a mere alkaline earth metal, alkali metal, rare earth element metal, or a salt or oxide thereof.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing a result of a performance test of an exhaust gas purifying catalyst according to the present invention.

FIG. 3 is a diagram showing a result of a durability test of an exhaust gas purifying catalyst according to the present invention.

FIG. 4 is a diagram showing a result of a durability test of the exhaust gas purifying catalyst according to the present invention.

[Explanation of symbols]

2: Exhaust gas purification catalyst 3: Precious metal based reduction catalyst 4:
Decomposition / absorption catalyst of brown miralite structure 5: Cordierite honeycomb support

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI B01J 23/656 F01N 3/10 ZABA 23/89 ZAB B01D 53/36 102H F01N 3/10 ZAB B01J 23/64 104A

Claims (4)

[Claims] The present invention relates to a catalyst for purifying exhaust gas in an oxygen-excess atmosphere, wherein a brown-millerite-type composite oxide represented by any of the following general formulas is used as a catalyst for direct decomposition and absorption of NOx. An exhaust gas purifying catalyst comprising a reduction catalyst disposed near an absorption catalyst. A _3-a B _a C _4-b D _b O ₉ A _2-a B _a C _{2-b Db} O ₅ A _1-a B _a C _2-b D _b O ₄ 2. The exhaust gas purifying catalyst according to claim 1, wherein each of the constituent elements A to D of the brown-millerite-type composite oxide is at least one of the following elements. Constituent elements A, B: Ba, Ca, Sr, Mg, Ce Constituent elements C, D: Y, Dy, Zn, Ti, Mn, Fe, Co, Ni, C
u, Sn, Zr, Nd 3. The reduction catalyst is a noble metal-based reduction catalyst,
The exhaust gas purifying catalyst according to claim 1. 4. The noble metal-based reduction catalyst comprises Ag, Pt, Pd, Ir,
The exhaust gas purifying catalyst according to claim 1, wherein the catalyst is at least one of Rh.