JP2000262863A - Exhaust gas cleaning catalyst combination device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make up for a part of a chemical reaction required for removing a nitrogen oxide based on a rhodium catalyst by another catalyst and perform a chemical reaction, to begin with, at a low temperature without impairing the intrinsic performance of the rhodium catalyst by combining the catalyst containing a hardly reducible rhodium with a catalyst of a different composition from the former catalyst. SOLUTION: Ammonia water is dripped into an aqueous solution of a mixture of rhodium nitrate, gallium nitrate, zirconium nitrate and aluminum nitrate to produce an aqueous solution containing a coprecipitation product of rhodium, gallium, aluminum and zirconium. After that, the aqueous solution is thermally treated while it is heated/stirred and the obtained solution is filtered by suction to obtain a gel coprecipitation substance. At the same time, the coprecipitation substance is cleaned and dry-ground into fine powder, which is turn, is baked. A catalyst containing a hardly reducible rhodium obtained as described and a catalyst of a different composition from the former catalyst are combined to constitute the device. Further, the catalyst containing the hardly reduible rhodium is arranged in the downstream of the device and the catalyst of a different composition is arranged in the upstream.

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 combination apparatus capable of selectively removing nitrogen oxides even when oxygen is excessively present.

[0002]

2. Description of the Related Art A non-reducible rhodium oxide catalyst proposed by the present applicant (JP-A-11-9998, Japanese Patent Application No. 9-31).
No. 0498) can remove nitrogen oxides with high efficiency even in the presence of an excess of oxygen. However, the operating temperature range is limited, and considering practical conditions, an improvement in activity at particularly low temperatures is desired. The reason why the reaction temperature range is high is that a series of reactions until the two react and are decomposed into nitrogen are manifested only in the high temperature range in the nitrogen oxide reduction action by the hydrocarbon.

[0003]

As described above, although the rhodium catalyst has excellent catalytic performance, it cannot be applied to an exhaust gas purification system which requires activity from a low temperature range. There is.

[0004] The present invention solves the above-mentioned problems, and a part of the reaction required for removing nitrogen oxides based on a rhodium catalyst is supplemented with another catalyst, thereby impairing the original performance of the rhodium catalyst. The purpose is to start the reaction from a low temperature.

[0005]

The present invention has solved the above problems by providing the following constitution.

(1) An exhaust gas purifying catalyst combination device comprising a combination of a catalyst containing non-reducible rhodium and a catalyst having a different composition from the catalyst.

(2) The exhaust gas purifying catalyst combination apparatus according to the above (1), wherein a catalyst containing a non-reducible rhodium is disposed downstream and a catalyst having a different composition from the catalyst is disposed upstream.

(3) The exhaust gas purifying catalyst combination apparatus according to (2), wherein a catalyst mainly composed of a transition metal oxide is disposed on the upstream side.

(4) An exhaust gas purifying catalyst combination apparatus according to the above (2), wherein an alumina catalyst containing a transition metal is disposed on the upstream side.

(5) The exhaust gas purifying catalyst combination apparatus according to the above (2), wherein the amount of the catalyst provided on the upstream side is equal to or less than the amount of the rhodium catalyst provided on the downstream side. .

The exhaust gas purifying catalyst according to the present invention assists a part of the reduction reaction of nitrogen oxides by arranging a catalyst using a transition metal upstream of the above-mentioned structure. Therefore, the present invention can provide a catalyst system having excellent temperature characteristics, and can greatly expand the applicable range.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described below in detail with reference to examples.

Prior to the description of the examples, the preparation of the downstream catalyst and the upstream catalyst will first be described in detail.

(Preparation of downstream catalyst) (Rh-Ga-Zr-
Preparation of Al Oxide Catalyst) A rhodium catalyst as a base has an atomic ratio of rhodium, gallium, zirconium, and aluminum of 1: 10: 5:
An aqueous solution is prepared by mixing rhodium nitrate, gallium nitrate, zirconyl nitrate, and aluminum nitrate so as to obtain a value of 65. While stirring this aqueous solution, aqueous ammonia is added dropwise. The dropping of the aqueous ammonia is performed while measuring the pH of the aqueous solution, and the dropping is stopped when the pH reaches 9.0.
The aqueous solution containing the coprecipitate of rhodium, gallium, aluminum and zirconium formed at this time is left at room temperature for 17 hours. Thereafter, the aqueous solution is heated to 80 ° C., and a heat treatment is performed for 5 hours while stirring. The obtained solution is subjected to suction filtration to obtain a gel-like coprecipitated substance. After washing this gel with 2 liters of pure water,
Dry for 7 hours and crush in a mortar to obtain a fine powder. This powder was calcined at 800 ° C. for 5 hours to synthesize a rhodium-gallium-zirconium-aluminum oxide catalyst (hereinafter referred to as a rhodium catalyst) to be a base catalyst.

(Preparation of Upstream Catalyst) (Preparation of Each Transition Element-Alumina Oxide Catalyst) Each nitrate and alumina were mixed so that the ratio of transition element (iron, cobalt, manganese) to aluminum became 1/10 in atomic ratio. Mix in water. This aqueous solution is heated to about 50 ° C. under reduced pressure with stirring to impregnate the alumina with the transition element.
The obtained powder is dried at 110 ° C. for 17 hours and pulverized in a mortar to obtain a fine powder. This powder was dried at 800 ° C for 5 minutes.
By calcining for a period of time, alumina oxides of the desired transition elements, iron, cobalt and manganese, were obtained.

As the transition element oxide, a commercially available product can be used separately from the above-mentioned preparation.

Next, an embodiment according to the present invention in which the above-described downstream catalyst and upstream catalyst are combined and used will be described.

Example 1 Fe / Al ₁₀ O was used as the upstream catalyst.
x was used, and the rhodium catalyst of the base catalyst was used as the downstream catalyst, and they were combined in a volume ratio of 0.1 cc and 1.4 cc, respectively.

Example 2 Co / Al ₁₀ O was used as the upstream catalyst.
x was used, and the rhodium catalyst of the base catalyst was used as the downstream catalyst, and they were combined in a volume ratio of 0.1 cc and 1.4 cc, respectively.

Example 3 Mn / Al ₁₀ O was used as the upstream catalyst.
x was used, and the rhodium catalyst of the base catalyst was used as the downstream catalyst, and they were combined in a volume ratio of 0.1 cc and 1.4 cc, respectively.

Examples in which the volume ratio between the upstream catalyst and the downstream catalyst in Examples 1 to 3 is changed are shown below.

Example 4 The volume ratio of Co / Al ₁₀ Ox and rhodium catalyst was 0.5 cc and 1.0 cc in the same manner as in Example 2.

Next, Example 5 in which the metal form of the catalyst combined under the same amount of combination as in Example 4 was an oxide
And 6 are shown below. The transition element of the upstream catalyst to be used was a commercial product.

Example 5 Co ₃ O ₄ was used for the upstream catalyst,
The downstream catalyst was combined with a rhodium catalyst in a volume ratio of 0.5 cc and 1.0 cc.

Example 6 MnO ₂ was used as an upstream catalyst and a rhodium catalyst was used as a downstream catalyst in a volume ratio of 0.5 cc and 1.0 cc.

A comparative example was prepared below for comparison with the above example.

Comparative Example 1 Alumina (Al) was used as the upstream catalyst.
Ox), 0.5 cc of a rhodium catalyst as a downstream catalyst, and 1.
The components were combined at a volume ratio of 0 cc.

The performance tests of the catalysts of the respective Examples and Comparative Examples synthesized above were carried out in a normal-pressure fixed-bed flow reactor.
The reaction gas was NO: 1000 ppm, C ₃ H ₆ : 100
_{0ppm, CO: 1200ppm, H 2} : 400pp
_{m, O 2: 6%,} CO 2: 10%, H 2 O: 2.5 liters / mi with 10% N ₂ dilution gases having a composition
At a flow rate of n, the mixture was fed to a catalyst layer having a volume of 1.5 cc and composed of a granular (0.5 mm to 1 mm) catalyst. The space velocity at this time is 100,000 h ^-1 . The performance was evaluated while changing the reaction temperature stepwise under the flow of the reaction gas having this composition. Table 1 shows the maximum NOx purification rate of each catalyst.
Note that the NOx purification rate is a value defined by the following equation.

[0029]

The catalyst combinations of Examples 1 to 3 are as follows:
It was confirmed that the activity on the low temperature side near 450 ° C. to 500 ° C. was significantly improved as compared with the rhodium catalyst having only the downstream catalyst corresponding to the base catalyst (Table 1).

Regarding the effect of the amount of the catalyst combined with the upstream catalyst, the test results of Example 4 show that even if the amount of cobalt is small, the effect of lowering the temperature is lower than that of iron and manganese. Was significantly improved (Table 2).

The activity on the low temperature side is also improved by using an oxide as the elemental form of the catalyst to be combined (Table 3).

Next, as apparent from Comparative Example 1, the catalyst combined with the upstream catalyst has almost no change compared to the activity of the base catalyst when only alumina is used. Is mainly contributing (Table 4). It is to be noted that iron, cobalt, and manganese are optimally added as elements to be added.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

[0038]

As described above, according to the present invention,
As described above, it has been found that the activity on the low-temperature side is greatly improved while maintaining the activity of the rhodium catalyst as a base catalyst, and therefore, an excellent exhaust gas capable of removing nitrogen oxides with higher activity than conventional catalysts A purification catalyst can be provided.

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[Submission date] June 12, 2000 (2006.1.
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[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst combination apparatus capable of selectively removing nitrogen oxides even in the presence of excess oxygen.

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(1) Rhodium nitrate, gallium nitrate and nitric acid
From an aqueous solution of a mixture of zirconyl and aluminum nitrate.
Dropping ammonia water, rhodium, gallium and aluminum
Solution containing a coprecipitate of zirconium
After standing, a warm heat treatment is performed while heating and stirring the aqueous solution.
The resulting solution is filtered by suction to form a gel.
After obtaining the coprecipitated substance and washing the gel, dry powder
Crushing to obtain fine powder and firing this powder
Characterized in that it comprises a catalyst containing rhodium which is hardly reducible and a catalyst having a different composition.
Exhaust gas purification catalyst combining device that.

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Claims (5)

[Claims] 1. An exhaust gas purifying catalyst combination device comprising a catalyst containing a non-reducible rhodium and a catalyst having a different composition from the catalyst. 2. The exhaust gas purifying catalyst combination apparatus according to claim 1, wherein a catalyst containing a non-reducible rhodium is disposed on a downstream side, and a catalyst having a different composition from the catalyst is disposed on an upstream side. 3. The exhaust gas purifying catalyst combination apparatus according to claim 2, wherein a catalyst mainly composed of a transition metal oxide is disposed on the upstream side. 4. The exhaust gas purifying catalyst combination apparatus according to claim 2, wherein an alumina catalyst containing a transition metal is disposed on an upstream side. 5. The exhaust gas purifying catalyst combination apparatus according to claim 2, wherein the amount of the catalyst disposed on the upstream side is equal to or less than that of the rhodium catalyst disposed on the downstream side.