JP3239156B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying catalyst capable of selectively removing nitrogen oxides even in the presence of excess oxygen, and a method for producing the same.

[0002]

2. Description of the Related Art It has been known that a catalyst in which rhodium is dispersed and supported on a metal oxide carrier represented by alumina can remove nitrogen oxides even in the presence of excess oxygen. However, the performance of this type of catalyst for removing nitrogen oxides was limited. There is a limit to the nitrogen oxide removal performance because hydrocarbons in the exhaust gas required to reduce and remove the nitrogen oxides are burned and consumed by excess oxygen, and the nitrogen oxides are reduced. This is because hydrocarbons contributing to the removal are reduced.

In order to solve this problem,
In order to improve the removal performance of nitrogen oxides, instead of holding rhodium on the oxide carrier, stabilize rhodium in a specific oxide and realize it with a normal rhodium catalyst It is necessary to form hard-to-reduce rhodium oxide, which is difficult to form, and to suppress the oxidation reaction of hydrocarbons with oxygen via the oxidation-reduction reaction between rhodium oxide and metal rhodium. By doing so, the hydrocarbon can be effectively used for the reduction and removal of nitrogen oxides, and therefore, a catalyst that can improve the efficiency of removing nitrogen oxides can be obtained.

As a catalyst having such characteristics, the present inventor has previously proposed a catalyst containing rhodium, aluminum and zirconium and a method for producing the same (Japanese Patent Application No. Hei 9 (1994) -209).
164386).

[0005]

Although this catalyst has excellent catalytic performance, it has not been sufficiently considered in terms of production cost and simplification of the production process. The present invention has been made to solve such a problem, and an object of the present invention is to reduce the amount of an alkaline compound used to obtain a catalyst precursor when producing a catalyst. Accordingly, the present invention aims to reduce costs and shorten the manufacturing process.

[0006]

Exhaust gas purifying catalyst SUMMARY OF THE INVENTION The present invention provides is rhodium, aluminum, exhaust gas comprises zirconium and zinc, and the rhodium is present as an oxide of irreducible in the catalyst Purification catalyst
And zirconium / (aluminum + zirconium +
Zinc) in an atomic ratio of 11/16 or less .

[0007] The method for producing an exhaust gas purifying catalyst provided by the present invention is a method of heat treating a precipitate obtained by a coprecipitation method in which an alkaline compound is added to a solution containing rhodium, aluminum, zirconium and zinc.

[0008]

According to the present invention, rhodium is fixed to a specific site of an oxide to produce a non-reducible rhodium oxide which cannot be obtained with a normal supported rhodium catalyst.
For that purpose, the base oxide needs to be in a preferable state, that is, a state having a good crystallinity. One of the methods for this is a method in which a large amount of alkali is used to ripen a catalyst precursor in a strongly alkaline solution.

However, in order to reduce the amount of alkali used as the object of the present invention, it is necessary to improve the crystallinity of the oxide with a substance other than the alkali. The other substance that is effective here is zinc. When zinc is added to the raw material of the catalyst, the crystallinity of the oxide is improved even when a small amount of alkali is used, as in the case where a large amount of alkali is used.

In the present invention, since zinc is used as one of the raw materials, it is possible to reduce the amount of an alkaline compound used to obtain a catalyst precursor when producing a catalyst,
The cost can be reduced, and the manufacturing process can be shortened.

[0011]

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

(Example 1) (Synthesis of Rh-Zn-Zr-Al oxide catalyst) Rhodium nitrate and zinc nitrate such that the atomic ratio of rhodium, zinc, zirconium and aluminum is 1: 10: 70: 80. And an aqueous solution in which zirconyl nitrate and aluminum nitrate were mixed was prepared. While stirring this aqueous solution, aqueous ammonia was added dropwise. PH of aqueous solution is the drop of ammonia water
The dropping was stopped when the pH reached 7.0. The aqueous solution containing the coprecipitate of rhodium, zinc, aluminum and zirconium formed at this time was left at room temperature for 17 hours.

Thereafter, the aqueous solution was heated to 80 ° C. and subjected to a heat treatment for 5 hours while stirring. The obtained solution was subjected to suction filtration to obtain a gel-like coprecipitated substance. After washing this gel with 2 liters of pure water,
C. for 17 hours and crushed in a mortar to obtain a fine powder. This powder was calcined at 800 ° C. for 5 hours to synthesize a rhodium-zinc-zirconium-aluminum oxide catalyst.

Examples 2 to 4 (Rh-Zn-Zr-A)
(1) Synthesis of oxide catalyst; examination of effect of rhodium amount) A rhodium / zinc / zirconium / aluminum oxide catalyst was synthesized in the same manner as in Example 1 while changing the rhodium amount at the following atomic ratio. .

Example 2 Rh / Zn / Zr / Al = 0.5 / 10/70/80 Example 3 Rh / Zn / Zr / Al = 2/10/70/80 Example 4 Rh / Zn / Zr / Al = 4/10/70/80 (Examples 5 to 7) (Synthesis of Rh-Zn-Zr-Al oxide catalyst; examination of influence of zirconium amount) In the same manner as in Example 1, rhodium / zinc / A zirconium-aluminum oxide catalyst was synthesized by changing the amount of zirconium at the following atomic ratio.

Example 5 Rh / Zn / Zr / Al = 1/10/10/140 Example 6 Rh / Zn / Zr / Al = 1/10/40/110 Example 7 Rh / Zn / Zr / Al = 1/10/110/40 (Examples 8 to 10) (Synthesis of Rh-Zn-Zr-Al oxide catalyst; examination of influence of zinc amount) In the same manner as in Example 1, rhodium, zinc, zirconium, An aluminum oxide catalyst was synthesized by changing zinc in the following atomic ratios.

Example 8 Rh / Zn / Zr / Al = 1/20/40/100 Example 9 Rh / Zn / Zr / Al = 1/40/40/80 Example 10 Rh / Zn / Zr / Al = 1/80/40/40 (Comparative Examples 1 to 3) (Synthesis of Rhodium Catalyst Supported on Alumina) As a comparative example, a supported rhodium catalyst using γ · alumina as a conventional general catalyst support was synthesized. Gamma-alumina is immersed in a rhodium nitrate aqueous solution whose concentration is adjusted so that the rhodium content becomes the following atomic ratio, and the powder obtained by evaporation to dryness is calcined at 800 ° C. for 5 hours to carry alumina. Rhodium catalyst was synthesized.

Comparative Example 1 Rh / Al = 1/320 Comparative Example 2 Rh / Al = 1/160 Comparative Example 3 Rh / Al = 1/40 (Comparative Example 4) (Synthesis of Rh-Zr-Al oxide catalyst) A rhodium-zirconium-aluminum composite oxide was produced in exactly the same manner as in Example 1 except that the atomic ratio of rhodium, zirconium, and aluminum was 1:80:80 (that is, zinc was not contained). The catalyst was synthesized.

(Comparative Example 5) (Synthesis of Rh-Zr-Al oxide catalyst; catalyst proposed in Japanese Patent Application No. 9-164386) In Comparative Example 4, ammonia water was dropped at pH = 9.0.
A rhodium-zirconium-aluminum composite oxide catalyst was synthesized in exactly the same manner as in Comparative Example 4 except that the above steps were performed.

The performance test of the synthesized catalyst was carried out in a fixed-bed flow reactor under normal pressure.

The reaction gas is NO: 1000 ppm, C _{3
H 6: 1000ppm, CO: 1200ppm} , H 2:
400 ppm, O ₂ : 6%, CO ₂ : 10%, H ₂ O:
The N ₂ dilution gas with 10% of the composition were used. This gas was granulated at a flow rate of 2.5 l / min (0.5 mm
１1 mm) of the catalyst layer. The space velocity at this time is 100,000h ^-1
It is. After performing a heat treatment at 650 ° C. under the flow of a reaction gas having this composition, the performance was evaluated while changing the reaction temperature stepwise. Table 1 shows the evaluation results and shows the maximum NO _X purification rate of each catalyst. Note that the NO _X purification rate is a value defined by the following equation.

[0022]

(Equation 1)

[0023]

[Table 1]

As is clear from Table 1, Examples 1 to 10
Is, NO _X purification performance seen that overwhelmingly high as compared to the alumina supported catalyst is a typical rhodium catalyst. Furthermore, of the NO _X purification performance Example 1, compared to Comparative Example 4 is a catalyst containing no zinc was obtained in exactly the same process NO _X
It can be seen that the purification performance is overwhelmingly high. It can be seen that the performance is equal to or better than the catalyst of Comparative Example 5 made using a large amount of alkali instead of zinc. The effect can be explained by the X-ray diffraction measurement shown in FIG.

FIG. 1 shows the results of X-ray diffraction measurements of Example 1 and Comparative Examples 4 and 5 using Cu-Kα. In the drawing, the symbol ▼ indicates a diffraction peak due to transition alumina or α-alumina. As can be seen from the figure, the crystallinity of alumina which affects the catalyst activity is very poor in Comparative Example 4 where the activity is low, and good in Comparative Example 5 and Example 1. That is, the effect of Comparative Example 5, which could be prepared by increasing the amount of alkali used, could be achieved by adding zinc as in Example 1.

[0026]

As described above, according to the present invention, as described above, zinc is added as a component to a catalyst which has been prepared by using a large amount of alkali. In this case, the amount of the alkaline compound used in the process can be reduced, and an excellent exhaust gas purifying catalyst can be provided. At the same time, the cost can be reduced and the production process can be shortened.

[Brief description of the drawings]

FIG. 1 is a graph showing the results of X-ray diffraction measurement of Example 1 and Comparative Examples 4 and 5.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94

Claims (2)

(57) [Claims] 1. An exhaust gas purifying catalyst comprising rhodium, aluminum, zirconium and zinc, wherein said rhodium is present in the catalyst as a non-reducible oxide.
Where zirconium / (aluminum + zirconium
+ Zinc) in an atomic ratio of 11/16 or less . 2. A method for producing an exhaust gas purifying catalyst, comprising subjecting a precipitate obtained by a coprecipitation method in which an alkaline compound is added to a solution containing rhodium, aluminum, zirconium and zinc to a heat treatment.