JP2005177570A - Scr catalyst excellent in characteristic at high temperature - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst which is used in an exhaust gas purifying method for reducing NOx in the exhaust gas from a diesel engine by using ammonia and has excellent denitrification performance particularly in a region of high temperature. <P>SOLUTION: This catalyst is used in the exhaust gas purifying method for reducing nitrogen oxide contained in the exhaust gas from the diesel engine by using ammonia and contains β-type zeolite ion-exchanged by iron and lanthanum. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an exhaust gas-purifying catalyst used for reduction treatment of nitrogen oxides in diesel engine exhaust gas.

  In recent years, exhaust gas regulations relating to nitrogen oxides (hereinafter referred to as “NOx”) have become increasingly severe conditions, and countermeasures have become an urgent issue.

  Conventionally, various methods have been proposed as a method for purifying NOx-containing exhaust gas from a high-temperature combustion system. For exhaust gases such as large boilers which are fixed sources of NOx, a selective catalytic reduction method (hereinafter referred to as “SCR method”) using ammonia as a reducing agent is generally adopted and put into practical use.

In this SCR method, the following reaction formula:
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O,
2NO ₂ + 4NH ₃ + O ₂ → 3N ₂ + 6H ₂ O and NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O
Accordingly, NOx is reduced and converted into harmless nitrogen gas and water vapor.

  As the catalyst for the SCR method (hereinafter referred to as “SCR catalyst”), for example, a metal oxide catalyst such as alumina, zirconia, vanadia / titania, a zeolite catalyst, or the like is employed. In addition, a catalyst containing at least one of titanium, tungsten, vanadium, molybdenum, and iron is used (Patent Document 1), and an iron / ZSM-5 monosilic structure zeolite having silica / alumina having a specific composition is used ( Patent Document 2) has also been proposed.

In addition, for the purpose of reducing nitrous oxide (N ₂ O), which is a global warming substance, with ammonia and removing it from the exhaust gas, pentasil-type zeolite (ZSM-5) supporting iron is used as a catalyst (patent) Similarly, it has been proposed to use β-type zeolite supporting iron (see Patent Document 4).
However, none of these relates to a method for purifying exhaust gas from diesel engines such as trucks and buses that are sources of NOx migration.

  In the SCR method, ammonia used as a reducing agent has an irritating odor and toxicity, and needs to be transported and stored using a pressure vessel. Therefore, there are problems in terms of safety and handling workability. In particular, application to a denitration treatment system for exhaust gas from diesel engines such as trucks and buses is not practical.

  Instead of ammonia, a solid compound at room temperature such as ammonium carbonate, urea, cyanuric acid or melamine which decomposes to generate ammonia can be used. These are preferable because of good handling workability and no safety problems such as leakage, and urea is more practical because it is easily available.

Urea has the following reaction formula:
(NH ₃ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂
Accordingly, ammonia is generated by a hydrolysis reaction, and the ammonia acts as a reducing agent as described above, and a reduction reaction of NOx in the exhaust gas occurs. Since urea is a solid, the supply method to the reaction system is, for example, a method such as spray supply as an aqueous solution.

JP 50-128681 A JP-A-9-103653 Japanese Patent Laid-Open No. 7-60126 JP-A-8-57262

  As described above, in the method of reducing and removing nitrous oxide in exhaust gas with ammonia, it is known per se to use β-type zeolite carrying iron as a catalyst. However, when the catalyst employed in the conventional SCR method is applied to the NOx reduction reaction in the exhaust gas of a diesel engine, NOx is not sufficiently reduced over the entire temperature range of the exhaust gas, particularly in a high temperature range. Since the catalyst performance is insufficient, an SCR catalyst suitable for NOx reduction in the exhaust gas has been demanded.

  Therefore, the subject of this invention is providing the SCR catalyst excellent in the denitration performance in the high temperature area | region especially in the method which carries out reduction processing of NOx by the SCR method for the exhaust gas of a diesel engine.

  Based on the above prior art, the present inventors have intensively studied to obtain an SCR catalyst having high denitration performance particularly in a high temperature region and excellent hydrothermal stability, and as a result, the present invention has been completed.

That is, the present invention
The present invention provides a catalyst for use in an exhaust gas purification method for reducing nitrogen oxides in diesel engine exhaust gas with ammonia, which comprises a β-type zeolite ion-exchanged with iron and lanthanum.

  In the method of purifying exhaust gas by reducing NOx in diesel engine exhaust gas with ammonia, the catalyst of the present invention has excellent denitration performance particularly in a high temperature region.

Hereinafter, the present invention will be discussed in detail.
First, the outline of the diesel engine exhaust gas treatment process by the SCR method using urea as an ammonia source in which the catalyst of the present invention is suitably used will be described with reference to FIG. Note that FIG. 2 is only shown for convenience of explanation, and the SCR method is not limited to this, and various modifications are further required to make it a practical process. It is not intended to limit the application target of the catalyst of the present invention.

  Exhaust gas generated in the diesel engine 1 is discharged to the outside through the first oxidation catalyst 2, the SCR catalyst 3, and the second dioxide catalyst 4 in the exhaust gas pipe. Under the control of the engine control unit 5 based on a preset program, the urea water supply unit 6 pumps the urea water through the urea water supply pipe 7 to the nozzle 8 in the exhaust gas pipe for spraying. The sprayed urea water is mixed with high-temperature exhaust gas and hydrolyzed to produce ammonia, and this ammonia reduces NOx in the exhaust gas by the function of the SCR catalyst 3.

First oxidation catalyst 2, the NO in the exhaust gas is converted to NO _2, thereby adjusting the exhaust gas NO / NO ₂ ratio to be supplied to the SCR catalyst 3, functions decomposed by oxidizing SOF a (SOF) Have The second dioxide catalyst 4 mainly has a function of oxidizing unreacted ammonia.

[Β-type zeolite]
Β-type zeolite is used to prepare the catalyst of the present invention.
Zeolites generally have the following average composition formula:
M _{n / x} Al _n Si _(1-n) O ₂
(Wherein M is a cationic species, x is the valence of M, and n is a number greater than 0 and less than 1)
It is a porous substance which has the composition represented by these. Examples of the M cation species include Na ^{+ and the} like. There are various different types of zeolite such as A-type, X-type, Y-type, ZSM-5, and MOR.

Among them, β-type zeolite has a unit cell composition of the following average composition formula:
M _{m / x} [Al _m Si _(64-m) O ₁₂₈ ] · pH ₂ O
(Wherein M and x are the same as above, m is a number greater than 0 and less than 64, and p is a number greater than or equal to 0)
And is classified as a synthetic zeolite having a tetragonal system. Β-type zeolite generally has a relatively complicated three-dimensional pore structure composed of linear pores having a relatively large diameter and aligned in one direction, and curved pores intersecting the linear pores. It is suitably used as a raw material for the catalyst of the present invention because it has properties such as easy diffusion of cations and diffusion of gas molecules such as ammonia.

The basic structure of the zeolite, the structural unit having an oxygen atom in the 4 vertices of the tetrahedron structure, i.e., [SiO _4/2] units and [AlO _4/2] ^- has a three-dimensional crystal structure consisting of units , the latter [AlO _4/2] ^- units form the cationic species and the ion pair. And since it contains the said ion pair structure, it has ion exchange ability with other cation species.

For the catalyst of the present invention, in particular, the cation species is quaternary ammonium ion (NH ₄ ⁺ ), and the molar ratio of SiO ₂ / Al ₂ O ₃ (hereinafter referred to as “SAR”) is 15 to 300. It is preferable to use β-type zeolite having a composition of preferably 15 to 100, more preferably 15 to 60.

  If the SAR is within the above range, the desorption of Al from the zeolite framework structure due to the moisture contained in the exhaust gas and the exhaust gas temperature occurs, the ion exchange site structure or the framework structure is destroyed, and the durability of the catalyst is stabilized. Thus, a catalyst having high activity can be easily obtained without causing the problem that the catalytic activity is deteriorated and the catalytic activity is likely to be lowered.

The quaternary ammonium ion (NH ₄ ⁺ ) is a β-type used in the present invention because the salt, which is a residual component by-product during ion exchange, does not adversely affect the catalytic activity as a catalyst poison. Preferred as a cationic species for zeolite.

[Ion exchange]
The catalyst of the present invention has the greatest feature in that iron ions (Fe ³⁺ ) are introduced into the β-type zeolite by an ion exchange reaction, and in particular, lanthanum ions (La ³⁺ ) are used. is there.

  In the present invention, the content of iron atoms in the β-type zeolite ion-exchanged with iron and lanthanum (hereinafter referred to as “ion-exchanged zeolite”) is usually 0.15 to 4.1% by weight, preferably 0.2 to 2.0% by weight. %, More preferably 0.50 to 0.75% by weight, and the molar ratio of iron atom / lanthanum atom is usually 1.6 to 6.4, preferably 2.0 to 4.0, more preferably 2.5 to 3.5. Good.

  If the content of iron and lanthanum is within the above range, the catalyst of the present invention can be easily prepared, and a catalyst having sufficiently high catalytic activity, particularly improved catalytic activity in a high temperature region can be obtained. .

  The method for obtaining ion-exchanged zeolite is not particularly limited, and β-type zeolite is used in an ordinary manner using an aqueous solution of an iron-containing compound (eg, ferric nitrate) and an aqueous solution of a lanthanum-containing compound (eg, lanthanum nitrate). And ion exchange treatment.

[Use of support, preparation of support-supported catalyst]
The catalyst of the present invention is preferably a catalyst in which ion-exchanged zeolite is supported on a support, and is practical as a diesel engine exhaust gas purification catalyst.

  As the support, it is preferable to use a flow-through ceramic or metal heat-resistant three-dimensional structure. Examples of the three-dimensional structure include honeycomb monoliths having a cell cross-sectional shape of rectangle, square, triangle, etc., foams, meshes, etc. Among them, ceramic honeycomb monoliths such as cordierite are preferable. The cell density of the honeycomb monolith body is usually about 100 to 900 cpsi, preferably about 300 to 600 cpsi.

  The catalyst of the present invention in a form supported on a support is obtained by immersing the support in a well-known wash coat method, that is, by immersing the support in a slurry containing ion exchange zeolite, and then removing excess slurry by air blowing. Then, after a predetermined weight of ion-exchanged zeolite is supported on the support, it can be prepared by drying and calcining. There are no particular restrictions on the conditions of the washcoat method. The amount of ion-exchanged zeolite supported on the support is a matter designed according to the capacity of the target engine, etc., but is usually 25 to 270 g / L, preferably 35 to 240 g / L per unit volume of the support. L, more preferably about 55 to 190 g / L.

Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples.
[Example]
An aqueous slurry of ion-exchanged zeolite (SAR = 22, iron atom content = 0.50 wt%, lanthanum atom content = 0.38 wt%, iron atom / lanthanum atom (molar ratio) = 3.3) was prepared. A cordierite monolith body (400 cpsi) having square cells is immersed in this slurry, pulled up, excess slurry is removed by air blowing, dried at 180 ° C. for 15 minutes, and then 450 ° C. in an electric furnace. And calcined for 30 minutes to obtain the catalyst of the present invention. The amount of ion-exchanged zeolite supported was 117 g / L.

The NOx reduction performance of diesel engine exhaust gas by NOx reduction of the obtained catalyst was evaluated according to the following evaluation method.
[Evaluation method]
<Preparation of denitration performance measurement sample>
In order to evaluate the denitration performance when the actual equipment was installed, the catalyst was heated at 800 ° C. for 5 hours in an atmosphere of 10 vol% moisture using a gas flow type electric furnace and subjected to durability treatment. The durability-treated catalyst was used as a sample for the following denitration performance measurement.

<Measurement of denitration performance>
Measurement conditions and engine capacity: 8.0L (T / I)
・ Fuel used: Light oil (Sulfur content = less than 50 ppm)
・ Operating conditions: 1500 rpm (Evaluation temperature is set by adjusting the load)
・ Sample evaluation temperature: 5 points of 200 ℃, 300 ℃, 350 ℃, 400 ℃, and 450 ℃ ・ Exhaust gas space velocity: 72,000 h- ¹ (constant)
・ Urea concentration used: 32.5% by weight
・ Urea water spray amount:
(When the evaluation temperature is 200 ° C. and 300 ° C.) Ammonia conversion amount / NOx content in exhaust gas (molar ratio) = 1.0
(When the evaluation temperature is 350 ° C, 400 ° C, 450 ° C) Ammonia equivalent / NOx content in exhaust gas (molar ratio) = 0.8
NOx purification rate measuring device: A sampling device, catalyzed DPF (platinum metal amount: 2 g / L), urea water injection nozzle, endurance treated catalyst, and sampling device are arranged in the engine exhaust gas pipeline in order from the upstream side. We used what was provided.

Measuring method The engine exhaust gas was sampled, and the NOx (IN) concentration (ppm) before the purification treatment was measured. Under the above conditions, urea water was sprayed from the nozzle into the pipe. The exhaust gas after passing through the durability-treated catalyst was sampled, the NOx (OUT) concentration (ppm) after the purification treatment was measured, and the NOx purification rate (%) was calculated by the following equation.

NOx purification rate (%) = [{NOx (IN) −NOx (OUT)} / NOx (IN)] × 100
The measurement results of the NOx purification rate (%) at the five evaluation temperatures are shown in FIG.

[Comparative example]
In place of the ion-exchanged zeolite described in the above example, a catalyst and denitration performance measurement were performed in the same manner as in the example except that a β-type zeolite ion-exchanged with iron having an iron atom content = 0.75% by weight was used. Samples were prepared and the NOx purification rate was measured. The measurement results are shown in FIG.

[Discussion]
From the results of Examples and Comparative Examples, it is clear that the catalyst of the present invention containing lanthanum exhibits excellent denitration performance particularly in a high temperature range where the exhaust gas temperature is 350 to 400 ° C.

It is a graph which shows the NOx purification rate in an Example and a comparative example with the evaluation temperature of a catalyst. It is the schematic which shows an example of the purification process by the SCR method of diesel engine exhaust gas.

Explanation of symbols

1 Diesel Engine 2 First Oxidation Catalyst 3 SCR Catalyst 4 Second Dioxide Catalyst 5 Engine Control Unit 6 Urea Water Supply Unit 7 Urea Water Supply Rod 8 Nozzle

Claims (6)

  A catalyst used in an exhaust gas purification method for reducing nitrogen oxides in diesel engine exhaust gas with ammonia, comprising a β-type zeolite ion-exchanged with iron and lanthanum. The catalyst according to claim 1, wherein the molar ratio of SiO ₂ / Al ₂ O _{3 in} the β-type zeolite is 15 to 300.   The catalyst according to claim 1 or 2, wherein the ion-exchanged β-type zeolite is obtained by ion-exchanging β-type zeolite having a quaternary ammonium ion with iron and lanthanum.   The catalyst according to any one of claims 1 to 3, wherein the content of iron atoms in the ion-exchanged β-type zeolite is 0.15 to 4.1 wt%, and the molar ratio of iron atoms / lanthanum atoms Is a catalyst having a value of 1.6 to 6.4.   The catalyst according to any one of claims 1 to 4, further comprising a support, wherein the ion-exchanged β-type zeolite is supported on the support.   6. The catalyst according to claim 5, wherein the supported amount of the ion-exchanged β-type zeolite is 25 to 270 g / L per unit volume of the support.

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