JP2001198463A - Catalyst for exhaust-gas purification - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a catalyst for exhaust-gas purification superior in the heat resistance, by providing an NOx-occlusion substance for repeating the stable absorption-desorption of NOx even at a high temperature. SOLUTION: This catalyst carries as an NOx-occlusion substance an inorganic compound having a chemical structure reversibly changeable consequently upon NOx absorption on the lean side and NOx desorption on the rich side, and in the catalyst K2WO4, KtaO3, or KVO3 is carried as the NOx-occlusion substance. The NOx-occlusion substance is carried preferably on a carrier comprising alumina, titania, silica, silica-alumina, zirconia, zeolite, ceria, or a mixture thereof, and further a catalytic metal comprising Pt, Pd, Au, Ag, Ph, Ir or a mixture thereof is carried on the 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 exhaust gas emitted from an internal combustion engine of an automobile or the like, and more particularly, to an excellent NOx catalyst at high temperatures.
It relates _the NO _X storage-reduction type exhaust gas purifying catalyst exhibiting _X purification performance.

[0002]

In recent years, from the viewpoint of global protection, to suppress the total amount of carbon dioxide (CO ₂₎ discharged from an internal combustion engine such as an automobile, it is a global issue of suppressing the generation amount of NO _X, the exhaust Gas regulations and fuel economy regulations are being strengthened year by year. As a countermeasure, a lean-burn engine has been developed for the purpose of improving fuel economy, the purpose of purifying the exhaust gas, NO _X storage reduction obtained by adding a function of occluding NO _X in the lean side to the conventional three-way catalyst Three-way catalysts have been developed and have achieved some success with the above issues.

[0003] In this lean burn engine, fuel is normally burned under the condition that the air-fuel ratio (A / F) is lean (excess air), and the fuel is temporarily stoichiometric (stoichiometric air-fuel ratio) to rich (excess fuel). Burn under conditions. HC in exhaust gas
(Hydrocarbons) and CO are efficiently burned and removed by the action of an oxidizing atmosphere and the catalyst in the lean side, whereas, NO _X is in the lean side is absorbed by the occluding material, it temporary stoichiometric-rich condition It is released under the atmosphere and is reduced and purified by the action of the catalyst and its temporary reducing atmosphere.

[0004] By the action of these air-fuel ratio control and _the NO _X storage type three-way catalyst, as a whole, HC simultaneously exhaust gas when improving the fuel economy, CO, can be purified efficiently NO _X. As the NO _X storage material, JP-A-9-24
As described in JP-A No. 8458, JP-A-10-33984, JP-A-10-128114, etc., carbonates and acetates of various alkali metals and alkaline earth metals have been proposed. Potassium acetate has been found to be good.

[0005]

SUMMARY OF THE INVENTION However, these
Previously proposed NO_XThe storage material is NO at high temperature_XOcclusion
Lack of performance, for example, potassium acetate is
NO in the temperature range below about 500 ° C_XOcclusion
Demonstrates performance, but NO above about 500 ° C _XOcclusion
There is a problem that the performance is rapidly reduced.

This is considered to be due to the following reasons. That is, potassium acetate temporarily changes to potassium carbonate at a high temperature. This potassium carbonate is NO _X on the lean side
Reacts with to form nitrates, where the carbonate groups of the anionic groups escape in gaseous form. Accordingly, then when the NO _X is released from the nitrate in the rich side, free potassium cations can not recombine with the original carbonate groups, react directly with TiO ₂ carrier and the cordierite substrate at a high temperature And are firmly connected.

Accordingly, the present invention is provided with _the NO _X storage material can be repeated absorbing and releasing stable NO _X even at high temperatures, and to provide an excellent exhaust gas purifying catalyst in heat resistance.

[0008]

Above object to an aspect of the feature that the chemical structure with the NO _X emissions in NO _X absorption and rich side of the lean side carrying an inorganic compound which reversibly changes as _the NO _X storage material And the exhaust gas purifying catalyst.

In the present invention, the term "reversible change" means that on the lean side, NO _X bonds to the cation group of the inorganic compound to release the anion group of the inorganic compound, and then, on the rich side, the bond between NO _X and the cation group changes. It means that, when cut, these cation groups and anion groups are recombined to return to the chemical structure of the original inorganic compound. In other words, without escaping anionic groups may change as potassium acetate, a change in the binding of the cationic groups of the inorganic compound, and a free and recombination anionic groups, the process for purifying NO _X in the exhaust gas Means to be repeated. Specifically, even after being subjected to "durable heat treatment" as an accelerated test of heat resistance described in the following, while releasing NO _X in the rich side, the inorganic compound of chemical structure when it is first carried the It means that it is identified by powder X-ray diffraction analysis or the like.

[0010] The above-mentioned object is achieved by using K ₂ WO ₄ , KTaO
₃ or KVO ₃ is carried as an NO _x storage material by an exhaust gas purifying catalyst. K ₂ WO ₄ , KTaO ₃ or KVO as storage material
₃ is excellent in _the NO _X storage performance at high temperatures, even after repeated absorption and desorption of NO _X, have been found to maintain the original chemical structure.

[0011]

DETAILED DESCRIPTION OF THE INVENTION The present invention has the chemical structure with the NO _X emissions in NO _X absorption and rich side of the lean side, characterized in that the carrying inorganic compound which reversibly changes as _the NO _X storage material It is an exhaust gas purifying catalyst, and examples of the NO _X storage material include K ₂ WO ₄ , KTaO ₃ , KVO ₃ , and mixtures thereof.

[0012] Such _the NO _X storage material, preferably alumina, titania, silica, silica - is carried alumina, zirconia, zeolite, on a carrier consisting of ceria, or mixtures thereof. As a method of supporting the NO _X storage material on a carrier, for example, carrier powder such as alumina and NO
It can be performed by mixing the _X occluding material powder in a dry or wet manner, and, if necessary, drying and firing.

The loading amount of K ₂ WO ₄ , KTaO ₃ or KVO ₃ is based on 100 g of a carrier such as alumina.
0.01 to 2 mol is preferable in terms of the number of moles of the potassium element,
More preferably, it is 0.05 to 0.5 mol.

Preferably, Pt, Pd,
A catalytic metal consisting of Au, Ag, Ph, Ir, or a mixture thereof is supported. By the action of such a catalytic metal,
This is because the absorption and desorption of combustion and NO _X of HC and CO in the exhaust gas is promoted. The catalyst metal can be supported by a method selected from any methods capable of depositing metal particles on the support, such as a precipitation method, an adsorption method, an ion exchange method, and a reduction precipitation method.

The amount of the supported catalyst metal is preferably 0.1 to 10 g, more preferably 0.2 to 2 g, based on 100 g of a carrier such as alumina.

[0016] Thus with the _the NO _X storage material and the catalytic metal on the support by the catalyst, when used as a general monolith catalysts, as actual catalyst by depositing the catalyst on the monolithic substrate Can be used.

[0017]

EXAMPLE 1 100 g Al ₂ O ₃ , 100 g TiO ₂ , and 4 g
8.9 g of a mixed powder of K ₂ WO ₄ was dispersed in 500 cc of ion-exchanged water and stirred. An aqueous solution of dinitrodiammine Pt nitric acid was added in an amount of ₂ g Pt, stirred for 1 hour, filtered, dried, and then dried. 1 at 300 ° C under air atmosphere
After calcination for a time, a catalyst powder A was obtained.

Example 2 A catalyst powder B was obtained in the same manner as in Example 1 except that 80.4 g of KTaO ₃ was used instead of K ₂ WO ₄ .

Example 3 A catalyst powder C was obtained in the same manner as in Example 1 except that 41.4 g of KVO ₃ was used instead of K ₂ WO ₄ .

Comparative Example 1 A catalyst powder D was obtained in the same manner as in Example 1 except that 52.2 g of K ₂ TiO ₃ was used instead of K ₂ WO ₄ .

Comparative Example 2 A mixed powder of 100 g of Al ₂ O ₃ and 100 g of TiO ₂ was dispersed in 500 cc of deionized water and stirred.
An aqueous solution of dinitrodiammine Pt nitric acid was added in an amount of 2 g Pt, and the mixture was stirred for 1 hour, filtered and dried, and then calcined at 300 ° C. for 1 hour in an air atmosphere to obtain a Pt-supported oxide. This Pt-supported oxide was dispersed in a solution in which 29.4 g of potassium acetate was dissolved in 500 cc of ion-exchanged water.
For 2 hours to obtain a catalyst powder E.

-Durable heat treatment- The obtained catalysts A to E were each compression-molded and then pulverized into pellets having a size of 0.5 to 1.7 mm. These pellet catalysts were subjected to a durable heat treatment in which they were alternately exposed to the following lean and rich model gas atmospheres at 800 ° C. for 1 minute and 4 minutes, respectively, for a total of 5 hours. Rich gas composition: 500ppm NO + 2000ppm HC + 0.6% CO + 1
0% CO ₂ + 0.3% O ₂ +200 ppm SO ₂ + 5% H
₂ O (residual: N ₂ ) Lean gas composition: 500 ppm NO + 2000 ppm HC + 0.1% CO + 1
0% CO ₂ + 6.5% O ₂ +200 ppm SO ₂ + 5% H
₂ O (residual: N ₂ )

[0023] -NO _X purification performance - for pellet catalyst A~E after the above endurance heat treatment, 500 ° C. under a lean atmosphere when switching alternatively 2 minutes each in model gas atmosphere of the lean and rich
Measuring the NO _X purification performance in, N by the following equation
To determine the O _X purification rate. The result is shown in FIG. NO _X purification rate = [(inlet gas concentration - outlet gas concentration) ÷ inflow gas concentration] × 100

-Identification of Potassium Compound- For the pellet catalysts A to E, potassium compounds were identified by powder X-ray diffraction analysis before and after the above-mentioned durability heat treatment. As a result, for the catalysts A to C, only K ₂ WO _{4 and the} like initially supported were detected, and it was confirmed that there was no change in the chemical structure of the compound. On the other hand, from the catalyst D, only K ₂ TiO ₃ added at the time of preparing the catalyst was detected, and from the catalyst E, K ₂ TiO ₃ and K ₂ Ti ₂ O as potassium compounds were detected.
₅ was detected.

From the above results, the following is considered.
In the catalyst A~C according to the invention, even after the endurance heat treatment for 5 hours at 800 ° C., shows the NO _X purification rate of greater than 30%,
And the no change chemical structure of the NO _X storage component that issued absorbing the NO _X is confirmed by a reversible change in chemical structure. The NO _X storage material such as K ₂ WO ₄ of the present invention forms a nitrate compound and releases potassium ions under lean conditions, but WO ₄ , TaO ₃ , and VO ₃ groups have extremely low mobility. It is considered that the compound is chemically stable and can bind to the potassium ion again in the original place under the rich condition, and thus the chemical structure can be reversibly changed.

In the catalyst D using K ₂ TiO ₃ as the NO _X storage material, the bond between potassium and oxygen is strong and cannot be cut off under lean conditions, so NO _X is not chemically bonded and NO _X storage is possible. It is considered that performance cannot be exhibited. In the catalyst D, a slight NO _X purification rate of about 5% was measured, but this was not due to a change in the chemical structure of potassium titanate, but was caused by a simple ionization of NO _X and potassium titanate. action, or believed to be due to Al ₂ O ₃ or the action of TiO ₂ of the NO _X and carrier.

In the case of the catalyst E using potassium acetate as the NO _X storage material, the NO _X purification rate which was considerably lower than that of the catalyst of the present invention, which was less than 20%, was measured. It is considered that potassium titanate was formed by reacting with titania.

[0028]

Can be repeated absorbing and releasing stable in NO _X at a high temperature according to the present invention, it is possible to provide an excellent exhaust gas purifying catalyst in heat resistance.

[Brief description of the drawings]

1 is a graph comparing the NO _X purification performance after endurance heat treatment for the catalyst of Examples and Comparative Examples.

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Claims (2)

[Claims] 1. An exhaust gas purifying catalyst characterized in that an inorganic compound whose chemical structure is reversibly changed as NO _{X is} absorbed on the lean side and NO _{X is} released on the rich side is supported as a NO _X storage material. 2. K ₂ WO ₄ , KTaO ₃ , or KVO ₃
The exhaust gas purifying catalyst, characterized by carrying as _the NO _X storage material.