JP2002346387A - Exhaust gas cleaning catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust gas cleaning catalyst provided with a catalyst carrier capable of holding a fine catalyst particle shape by suppressing the sintering of catalyst particles and capable of avoiding the deterioration of the catalyst and having excellent durability and high NOx cleaning efficiency. SOLUTION: A nitrogen oxide removing catalyst comprises a composite consisting of perovskite type composite oxide particles represented by the general formula: ABO3 , wherein at least two kinds of elements selected from the element group consisting of Ca, Sr, Ba, Sc, Y, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Ti, Zn, Zr, In, Hf, Ni and Mn are constituent elements A and B, and noble metal catalyst particles formed of at least one kind of a noble metal among Ru, Rh, Pd, Ir, Pt and Au. The exhaust gas cleaning catalyst is constituted by supporting this nitrogen oxide removing catalyst on the intermediate layer formed on the surface of a metal carrier.

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 for purifying nitrogen oxides (NOx) in exhaust gas of an internal combustion engine such as a diesel engine, and more particularly, to a perovskite-type composite oxide. Is a catalyst for decomposing and adsorbing nitrogen oxides, and relates to an exhaust gas purifying catalyst having a nitrogen oxide removing catalyst in which noble metal particles are dispersed on the particle surface.

[0002]

2. Description of the Related Art Conventionally, in order to remove nitrogen oxides contained in exhaust gas discharged from an engine of a vehicle or the like,
Noble metal catalysts such as three-way catalysts have been used.

[0003] The three-way catalyst is composed of a platinum (Pt) -rhodium (Rh) -based or platinum (Pt) -palladium (Pd) -rhodium (Rh) -based mixture. HC) and carbon monoxide (CO) in water (H ₂
O) and carbon dioxide (CO ₂ ), and a reduction reaction to convert nitrogen oxides (NOx) to nitrogen (N ₂ ).

[0004] However, this three-way catalyst has the characteristics of exhibiting high conversion only in an air-fuel ratio window within a narrow range centered on the stoichiometric air-fuel ratio, in addition to problems such as reaction temperature and poisoning. However, there is a problem that the oxygen concentration in the exhaust gas is high, such as when performing lean burn (lean combustion), and the method cannot be applied to lean burn engines and diesel engines.

For this reason, the present inventors disclosed in Japanese Patent Application Laid-Open
JP-A-322452 and JP-A-11-34730 disclose a method in which a composite oxide is used as a catalyst for decomposing and adsorbing nitrogen oxide, and noble metal particles having a function as an oxidation catalyst are dispersed on the particle surface. An exhaust gas purifier equipped with an active and long-life catalyst for removing nitrogen oxides has been proposed.

[0006]

However, when this nitrogen oxide removing catalyst is incorporated in an exhaust gas purifying apparatus,
It is necessary to support the catalyst carrier uniformly and with a high specific surface area representing the total surface area of the catalyst per unit mass. Sometimes, when the particle structure of the catalyst is exposed to a high temperature, sintering occurs, which is a phenomenon that the specific surface area decreases, and as shown in FIG. 4, the specific surface area of the catalyst 20 supported on the carrier 10 decreases. There is.

Further, as shown in the schematic diagram of the surface of FIG. 5, the components of the catalyst 20 are diffused, and the catalyst layer 20 and the carrier 1 are dispersed.
In order to form a diffusion layer (catalyst-alumina diffusion layer) 40 with the alumina layer 10a on the surface of No. 0, a part of the catalyst 20 is deteriorated, so that the catalytic action is weakened and the NOx removal ability is reduced. There is a problem of doing.

The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to form an intermediate layer which has an affinity for both a catalyst and a carrier and is hard to deteriorate the catalyst components. By doing so, exhaust gas with excellent durability and high NOx purification rate is provided with a catalyst carrier that can suppress sintering of catalyst particles and maintain a fine catalyst particle shape, and can avoid deterioration of the catalyst. It is to provide a gas purification catalyst.

[0009]

An exhaust gas purifying catalyst for achieving the above object is a nitrogen oxide removing catalyst comprising a composite of perovskite-type composite oxide particles and noble metal catalyst particles on a metal carrier. And a perovskite-type composite oxide comprising calcium (Ca), strontium (Sr), barium (Ba), scandium (Sc), yttrium (Y), cerium (Ce), Neodymium (Nd), Europium (Eu), Gadolinium (Gd), Terbium (Tb), Dysprosium (Dy), Holmium (H
o), erbium (Er), titanium (Ti), zinc (Z
n), zirconium (Zr), indium (In), hafnium (Hf), nickel (Ni), manganese (M
n) A perovskite-type composite oxide represented by ABO ₃ or the like of a general formula in which at least two kinds of elements in the group of elements are constituent elements A and B, and the noble metal catalyst particles are formed of ruthenium (Ru). , Rhodium (Rh), palladium (P
d), iridium (Ir), platinum (Pt), gold (Au)
And a noble metal of at least one of the elements described above, an intermediate layer is formed on the surface of the metal carrier, and the nitrogen oxide removing catalyst is supported on the intermediate layer.

This intermediate layer is formed of a substance which has an affinity for both the nitrogen oxide removing catalyst and the metal carrier and which hardly deteriorates the nitrogen oxide removing catalyst component.

According to this structure, since the intermediate layer is provided between the nitrogen oxide removing catalyst and the metal carrier, sintering of the nitrogen oxide removing catalyst particles can be prevented. Deterioration due to contact with the metal carrier can be prevented.

[0012] Therefore, both the reduction of the specific surface area and the deterioration of the nitrogen oxide removing catalyst can be prevented, so that an exhaust gas purifying catalyst having a high nitrogen oxide removing rate can be constituted.

In the above exhaust gas purifying catalyst,
The intermediate layer forms a diffusion layer between the metal carrier and the catalyst layer, and is fixed.

Further, in the above exhaust gas purifying catalyst,
The intermediate layer is formed including at least one of zirconium oxide (zirconia), praseodymium oxide, cerium oxide (ceria), barium oxide (barita), titanium dioxide (titania), and rare earth element oxide. And

[0015]

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

FIG. 1 is a schematic diagram showing a cross section of a carrier of an exhaust gas purifying catalyst according to an embodiment of the present invention, and FIG. 2 is a diagram schematically showing a state of a surface of the catalyst in a hierarchical form.

[Structure] The exhaust gas purifying catalyst of the present invention comprises a metal carrier 10 on which perovskite-type composite oxide particles 21
And a nitrogen oxide removing catalyst 20 composed of a composite of precious metal catalyst particles 22. As shown in FIG. 1, an intermediate layer is provided between the metal carrier 10 and the nitrogen oxide removing catalyst 20. 30 is provided.

The metal carrier 10 is made of iron-chromium-aluminum (Fe-Cr-Al) alloy or SYTT (Fe-C
u-Al-Y) alloy or the like. In the example shown in FIG. 2, an alumina layer 10 which is formed of an Fe-20Cr-5Al alloy and is an oxide film of the alloy is formed on the surface of the substrate 10.
a is formed.

The perovskite-type composite oxide particles 21
AB is a constituent element, O is an oxygen element, AB
It is represented by a general formula such as O ₃ , and is composed of alkaline earth metal elements such as calcium (Ca), strontium (Sr) and barium (Ba), and rare earth elements scandium (Sc), yttrium (Y), cerium (Ce), Neodymium (N
d), europium (Eu), gadolinium (Gd),
Terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er) or a metal element such as titanium (Ti), zinc (Zn), zirconium (Zr), indium (In), and hafnium (Hf) ,
It is formed by constituting the above-mentioned A and B with at least two kinds of elements from a group of elements of nickel (Ni) and manganese (Mn).

The noble metal catalyst particles 22 contain at least one element of ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir), platinum (Pt), and gold (Au). Formed.

In the example of FIG. 2, the composite of the perovskite-type composite oxide 21 and the noble metal catalyst particles 22 is BaPr
It is formed of O ₃ .

The intermediate layer 30 is a substance which has an affinity for both the nitrogen oxide removing catalyst 20 and the metal carrier 10 and does not alter the components of the nitrogen oxide removing catalyst 20.
It is formed including at least one of zirconium oxide (ZrO ₂ : zirconia), praseodymium oxide (PrO ₂ ), barium oxide (BaO: barita), titanium dioxide (TiO ₂ : titania), and rare earth element oxide.
In the example shown in FIG. 2, praseodymium oxide (PrO ₂ ), which is one of rare earth element oxides, is employed.

According to this configuration, as shown in FIG. 2, an intermediate layer (praseodymium oxide) 30 is formed on an alumina layer (oxide film on an alloy) 10a on a base material (Fe-20Cr-5Al) 10. A catalyst layer (BaPrO ₃ ) 20 is formed. Further, an alumina-praseodymium oxide layer 15 is formed between the alumina layer 10 a and the intermediate layer 30, and between the intermediate layer 30 and the catalyst layer 20. Is formed with a praseodymium oxide diffusion layer (catalyst surface) 25.

The inclined structure of the components of the alumina-praseodymium oxide layer 15 and the praseodymium oxide diffusion layer 25 makes it possible to firmly fix the alumina layer 10a to the intermediate layer 30 and fix the intermediate layer 30 to the catalyst layer 20. This catalyst structure is firmly maintained,
The durability of the exhaust gas purifying catalyst 1 is enhanced.

[Manufacturing Method] Next, a manufacturing method will be described.

First, praseodymium oxide powder is pulverized and dispersed in acetylacetone, coated on a carrier (substrate) 10 made of an iron-chromium-aluminum (Fe-Cr-Al) alloy by electrophoresis, and The carrier 10 having the intermediate layer 30 formed by the above is obtained.

Next, the perovskite-type composite oxide 21 having a composition of BaPrO ₃ was added to the perovskite-type composite oxide 2.
Noble metal particles 22 of rhodium (Rh) and (Ir) such that the ratio of 1 to noble metal catalyst 22 is 1: 1 to 2 in molar ratio.
Is synthesized, and a powder of the catalyst 20 is dispersed in acetylacetone to prepare a slurry.

A carrier 10 made of an Fe—Cr—Al alloy in which the slurry is coated with an intermediate layer 30 of praseodymium oxide
Is immersed, and the catalyst 20 is supported by electrophoresis. Thereafter, the exhaust gas purifying catalyst 1 in which the catalyst 20 is supported on the intermediate layer 30 is obtained by heat treatment.

Then, the exhaust gas purifying catalyst 1 is placed in a reaction vessel to form an exhaust gas purifying apparatus.

[Measurement Results] The exhaust gas purification catalyst provided with the intermediate layer 20 (Example) and the catalyst 2 without the intermediate layer 20
Exhaust gas purifying devices each having an exhaust gas purifying catalyst carrying only 0 (comparative example) disposed in a reaction vessel,
The simulated gas was reacted with the exhaust gas of a diesel engine to measure the nitrogen oxide removal rate, and the results of comparison are shown in FIG.

According to the measurement results shown in FIG. 4, the nitrogen oxide removal rate in the initial stage of the reaction was 45% in Example and Comparative Example 4
After 1,000 hours, it was 39% in Example and 31% in Comparative Example.

From the results of the measurement, it can be seen that the Example has a higher nitrogen oxide removal rate than the Comparative Example from the beginning of the reaction,
It can be seen that the removal rate hardly decreases even after 1,000 hours.

Then, calcium (Ca), titanium (T
i), perovskite type synthesized using nickel (Ni), manganese (Mn), hafnium (Hf), zinc (Zn), indium (In) and rare earth elements such as scandium (Sc) and yttrium (Y) Similar results were obtained using the composite oxide.

Similar results are obtained in the case of an exhaust gas purification catalyst using zirconium oxide (zirconia), cerium oxide (ceria), barium oxide (barita), titanium dioxide (titania), or a rare earth element oxide as the intermediate layer. showed that.

[Effect] Therefore, according to the exhaust gas purifying catalyst having the above structure, the nitrogen oxide removing catalyst 20 and the metal carrier 1
Since the intermediate layer 30 is provided between 0 and 0, the sintering of the nitrogen oxide removal catalyst particles 20 generated when the nitrogen oxide removal catalyst 20 is supported on the metal carrier 10 can be prevented,
A decrease in the specific surface area of the nitrogen oxide removing catalyst can be prevented.

Further, since the intermediate layer 30 prevents direct contact between the nitrogen oxide removing catalyst 20 and the metal carrier 10, the catalyst 20 can be prevented from being deteriorated.

Therefore, the exhaust gas purifying catalyst is excellent in durability and has a high nitrogen oxide removal rate.

[0038]

As described above, according to the exhaust gas purifying catalyst of the present invention, the following effects can be obtained.

Since the intermediate layer is provided between the nitrogen oxide removing catalyst and the metal carrier, sintering of the nitrogen oxide removing catalyst particles can be prevented when the nitrogen oxide removing catalyst is carried on the metal carrier. Further, it is possible to prevent the nitrogen oxide removing catalyst from being deteriorated by contact with the metal carrier.

[0040] Therefore, both the reduction of the specific surface area of the nitrogen oxide removing catalyst and the deterioration of the nitrogen oxide removing catalyst can be prevented, so that an exhaust gas purifying catalyst having excellent durability and a high nitrogen oxide removing rate is constituted. be able to.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of a catalyst carrier showing a surface of a carrier of an exhaust gas purifying catalyst according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view of the surface of the carrier of FIG.

FIG. 3 is a graph showing the change over time in the NOx removal rate of an exhaust gas purification catalyst having an intermediate layer according to the present invention (Example) and an exhaust gas purification catalyst having no intermediate layer (Comparative Example).

FIG. 4 is a schematic sectional view showing the surface of a carrier of a conventional exhaust gas purifying catalyst.

FIG. 5 is a schematic sectional view of the surface of the carrier shown in FIG. 4;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 10 Substrate (metal carrier) 10a Alumina layer (metal carrier) 15 Alumina-praseodymium oxide layer 20 Catalyst layer (NOx removal catalyst) 25 Praseodymium oxide diffusion layer 30 Intermediate layer

Continued on the front page F-term (reference) 3G091 AB06 BA07 BA14 GA00 GB03W GB04W GB05W GB06W GB07W GB10W 4D048 AA06 AB03 BA02X BA07Y BA08Y BA15X BA16Y BA17Y BA18X BA19Y BA28Y BA30Y BA31Y BA32Y BA33X BA04ABAA BAB BAX BAAY BA38A BB06A BB06B BC09A BC12A BC13A BC13B BC18A BC33A BC35A BC39A BC40A BC43A BC44A BC44B BC50A BC51A BC52A BC62A BC68A BC69A BC70A BC71A BC71B BC72A BC74A BC74B BC75A CA03 CA10 CA13 EC23 EC28

Claims (3)

[Claims] 1. An exhaust gas purifying catalyst comprising a metal carrier and a nitrogen oxide removing catalyst comprising a composite of perovskite-type composite oxide particles and noble metal catalyst particles, wherein the perovskite-type composite oxide With calcium (C
a), strontium (Sr), barium (Ba), scandium (Sc), yttrium (Y), cerium (Ce), neodymium (Nd), europium (Eu),
Gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (E
r), titanium (Ti), zinc (Zn), zirconium (Zr), indium (In), hafnium (Hf),
A perovskite-type composite oxide represented by ABO ₃ or the like of a general formula in which at least two kinds of elements of the group of elements of nickel (Ni) and manganese (Mn) are constituent elements A and B; The catalyst particles were made of ruthenium (Ru), rhodium (Rh), palladium (Pd), iridium (Ir),
It is formed of a noble metal of at least one of platinum (Pt) and gold (Au) elements, an intermediate layer is formed on the surface of the metal carrier, and the nitrogen oxide removing catalyst is supported on the intermediate layer. An exhaust gas purifying catalyst characterized in that: 2. The exhaust gas purifying catalyst according to claim 1, wherein the intermediate layer forms a diffusion layer between the metal carrier and the catalyst layer, and is fixed. 3. The method according to claim 1, wherein the intermediate layer includes at least one of zirconium oxide, praseodymium oxide, cerium oxide, barium oxide, titanium dioxide, and rare earth element oxide. An exhaust gas purifying catalyst according to claim 1.