JPH09299792A - Catalytic material for removal of nitrogen oxides(nox), device for treating nox with same and method for removing nox - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a catalytic material capable of removing NOx even in exhaust gas having a high concn. of oxygen from a lean burning engine, a diesel engine, etc., and not requiring a highly poisonous reducing agent such as ammonia, to obtain a device for treating NOx with the catalytic material and to provide a method for removing NOx. SOLUTION: A multiple oxide forming this catalytic material contains Al and Zn as metallic elements, the molar ratio of Zn to Al is >0 to <0.5 and the oxide has a spinel type crystal structure. When NOx-contg. gas is brought into contact with this catalytic material in the presence of hydrocarbon such as ethylene, propylene or propane as a reducing agent, NOx, e.g. in exhaust gas from an automobile is reduced and removed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel catalyst material for removing nitrogen oxides, which can reduce and remove nitrogen oxides in exhaust gas from various combustion engines, for example, and a nitrogen oxide treating apparatus using the same. And a method for removing nitrogen oxides.

[0002]

2. Description of the Related Art In the recent environmental pollution problem, there is an urgent need to develop a method for decomposing and removing harmful substances such as nitrogen oxides in exhaust gas. Exhaust gas purification catalysts include carbon monoxide (CO), hydrocarbons (HC),
A three-way catalyst method for simultaneously removing nitrogen oxides (NOx) and a selective reduction method using ammonia have been put into practical use. Here, as the three-way catalyst, Pd, Pt, and Pd on a heat-resistant carrier such as cordierite coated with γ-alumina.
Those carrying a noble metal such as Rh are generally used. In the ammonia selective reduction method, V, T
Oxides of metals such as i and W are generally used.

[0003]

However, the three-way catalyst has a disadvantage that the noble metal is poisoned by oxygen coexisting in the exhaust gas and the activity is remarkably reduced. The exhaust gas purifying effect can be obtained only in a narrow area. Research on metal ion-exchanged zeolites as a catalyst that functions in an oxygen-excess atmosphere has been advanced, but none of them has been obtained in terms of heat resistance and water resistance.

On the other hand, the ammonia selective reduction method is
It has been put to practical use in stationary combustion devices such as thermal power plants, and it can remove nitrogen oxides even in the presence of oxygen.However, due to the toxicity and dangers of ammonia used,
There is a problem in terms of safety when used in urban areas and mobiles.

The present invention has been made in order to solve the above problems, and is capable of removing nitrogen oxides even in exhaust gas having a high oxygen concentration, such as lean burn engines and diesel engines, and is capable of removing ammonia and the like. An object of the present invention is to provide a catalyst material for removing nitrogen oxides that does not require a highly toxic reducing agent, a nitrogen oxide treatment apparatus using the material, and a method for removing nitrogen oxides.

[0006]

In order to solve the above-mentioned problems, the invention of claim 1 contains Al and Zn as metal elements, and the Zn / Al molar ratio X is 0 <X <0.5. A gist of a nitrogen oxide-removing catalyst material is characterized in that a main component is a composite oxide having a spinel type crystal structure within the range.

A second aspect of the present invention provides a nitrogen oxide treating apparatus, characterized in that the nitrogen oxide removing catalyst material according to the first aspect is used as a part or the whole thereof. To do. The invention according to claim 3 is characterized in that the catalyst for removing nitrogen oxides according to claim 1 is brought into contact with a nitrogen oxide-containing gas and a reducing agent to reduce and remove the nitrogen oxides. The removal method is summarized.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION

(Invention of Claim 1) As a result of intensive research on the above problems, the present inventors have found that aluminum (Al) and zinc (Z
The present invention is based on the finding that a composite oxide containing n) as a main metal element and having a specific crystal structure has a high nitrogen oxide reducing action.

The composite oxide constituting the catalyst material for removing nitrogen oxides of the present invention contains Al and Zn as metal elements, and the Zn / Al molar ratio X is in the range of 0 <X <0.5. And has a spinel type crystal structure. Since this spinel type crystal structure is a solid solution of ZnAl ₂ O ₄ —Al ₂ O ₃ , the material of the present invention has a Zn / Al molar ratio X of 0 <X <
It can be expressed as a solid solution of ZnAl ₂ O ₄ —Al ₂ O ₃ in the range of 0.5.

When the Zn / Al molar ratio X is 0.5 or more, crystals other than the spinel type structure may be generated, and the catalytic activity may be deteriorated. Further, when X = 0, the crystal phase is a γ-alumina simple substance, which is inferior in catalytic activity as compared with the case where Zn is added. The material composed of the complex oxide having the above-mentioned structure is contacted with, for example, the exhaust gas of an internal combustion engine to generate a nitrogen oxide (NO) contained in the exhaust gas.
Similarly, x) can be reduced and removed using, for example, a hydrocarbon contained in the exhaust gas as a reducing agent. Although the mechanism has not been fully clarified yet, Zn
Of charge balance due to addition of oxygen (increased oxygen defects)
However, it is considered that the nitrogen oxide reduction characteristics are improved.

The method for producing the nitrogen oxide-removing catalyst material of the present invention can be obtained, for example, by dissolving nitrates of Al and Zn in pure water, hydrolyzing, and then evaporating to dryness or evaporating to dryness as it is. The method of heat-treating the obtained residue at 500 degreeC or more is mentioned. The most preferable heat treatment temperature here is 600 ° C. to 1000 ° C., whereby a crystal having a spinel structure is sufficiently generated, good powder having a large specific surface area without decomposition is obtained.

In addition to the above manufacturing method, Al and Zn
The oxide can also be synthesized by wet-mixing the oxide with a ball mill, followed by drying and heat treatment. The catalyst material for removing nitrogen oxides of the present invention is not limited to those produced by these production methods.

The catalyst material for removing nitrogen oxides of the present invention is
A solid solution of the general formula (ZnAl ₂ O ₄ -Al ₂ O ₃ ;
As long as the Zn / Al molar ratio X contains a complex oxide represented by 0 <X <0.5) as a main component, its function can be sufficiently exhibited, and unreacted substances of constituent elements and A small amount of a compound that does not conform to the above general formula and further inevitably present impurities may remain. However, in order to improve the removal efficiency of nitrogen oxides, it is preferable to have only the composite oxide represented by the above general formula as a single constituent phase. (Invention of Claim 2) In the invention of Claim 2, the above-mentioned catalyst material for removing nitrogen oxides having a function of removing nitrogen oxides is used as a part or the whole thereof to configure a nitrogen oxide treating apparatus. , For example, by guiding the exhaust gas of an automobile to the nitrogen oxide treatment device and contacting the nitrogen oxide and the reducing agent component such as hydrocarbon in the exhaust gas with the catalytic material for removing nitrogen oxide, It becomes possible to reduce the nitrogen oxides in the gas to nitrogen and effectively purify the exhaust gas.

The structure of the nitrogen oxide treating apparatus may be, for example, one in which the above-mentioned catalyst material is loaded in a heat-resistant container provided in the flow path or the end of the gas to be treated, The catalyst material itself may constitute the entire apparatus. At this time, in order to obtain an effective area that can come into contact with the exhaust gas and enhance the purification efficiency, it is desirable to have various forms such as powder, particles, pellets, and honeycomb shapes. (Invention of Claim 3) In the invention of Claim 3, a hydrocarbon such as ethylene, propylene or propane is used as a reducing agent in the above-mentioned catalyst material for removing nitrogen oxides having a function of removing nitrogen oxides. Then, the nitrogen oxide can be reduced and removed by bringing it into contact with a gas containing the nitrogen oxide.

[0015]

EXAMPLES Examples of the catalyst material for removing nitrogen oxides of the present invention will be described below together with a method for producing the same. First, a method for producing a composite oxide which is a catalyst material for removing nitrogen oxides according to the present embodiment will be described.

[0016] (Example 1) reagent grade aluminum nitrate _{(Al (NO 3) 3 ·} 9H 2 O) and zinc nitrate (Zn (N
O ₃₎ was ₂ · 6H the ₂ O) was dissolved in pure water. At this time Z
The molar ratio X of n / Al was set to 0.05.

Thereafter, 7% aqueous ammonia was gradually added dropwise to this solution to cause hydrolysis, and a precipitate was obtained. Next, the precipitate was evaporated to dryness at 150 ° C. and pulverized in an alumina mortar to obtain a raw material composition.

This was heat-treated at 800 ° C. for 6 hours in an air atmosphere to obtain a synthetic powder. Then, by powder X-ray diffraction, the crystal phase of this synthetic powder was ZnAl ₂ O ₄ -γ-Al ₂
It was confirmed to be an O ₃ solid solution, that is, this synthetic powder was a complex oxide having a spinel type crystal structure.
The crystal phase was confirmed by utilizing the same peak pattern as in JCPDS: 5-669 (shifted to a higher angle side due to solid solution).

(Example 2) The Zn / Al molar ratio X was 0.
Synthetic powder was obtained in the same manner as in Example 1 except that the number was 15. When this synthetic powder was analyzed in the same manner, it was a ZnAl ₂ O ₄ -γ-Al ₂ O ₃ solid solution having a spinel type crystal structure.

Example 3 A Zn / Al molar ratio X is 0.
Synthetic powder was obtained in the same manner as in Example 1 except that No. 28 was used. When this synthetic powder was analyzed in the same manner, it was a ZnAl ₂ O ₄ -γ-Al ₂ O ₃ solid solution having a spinel type crystal structure.

(Example 4) The Zn / Al molar ratio X was 0.
A synthetic powder was obtained in the same manner as in Example 1 except that the amount was 30. When this synthetic powder was analyzed in the same manner, it was a ZnAl ₂ O ₄ -γ-Al ₂ O ₃ solid solution having a spinel type crystal structure.

(Example 5) The molar ratio X of Zn / Al was 0.
A synthetic powder was obtained in the same manner as in Example 1 except that the amount was 40. When this synthetic powder was analyzed in the same manner, it was a ZnAl ₂ O ₄ -γ-Al ₂ O ₃ solid solution having a spinel type crystal structure.

Example 6 The Zn / Al molar ratio X is 0.
A synthetic powder was obtained in the same manner as in Example 1 except that the amount was 47. When this synthetic powder was analyzed in the same manner, it was a ZnAl ₂ O ₄ -γ-Al ₂ O ₃ solid solution having a spinel type crystal structure.

(Comparative Example 1) Example 1 was repeated except that the Zn / Al molar ratio X was 0, that is, the composition did not contain the Zn component.
A synthetic powder was obtained in the same manner as. When this synthetic powder was analyzed in the same manner, γ-Al ₂ O ₃ (JCPDS: 10-425)
Met.

Comparative Example 2 Zn / Al molar ratio X was 0.
5, ie, ZnAl ₂ O ₄ stoichiometric composition, Example 1
A synthetic powder was obtained in the same manner as. This synthetic powder was also analyzed in the same manner and found to be ZnAl ₂ O ₄ (JCPDS: 5-669).

(Experimental Example) Next, an experimental example conducted for confirming the effect of the nitrogen oxide removing catalyst material of the present embodiment will be described. 200 kg / c of the synthetic powders of Examples 1 to 6 and Comparative Examples 1 and 2 obtained by the above-described manufacturing method
After being molded into a constant shape under a pressure of m ² , the molded body was crushed and sized with a standard sieve into a particle size of 26-50 mesh. 0.5 g of the sized powder was packed in a reaction tube made of quartz glass, and NO: 1000 ppm, C ₃
Helium gas containing H ₆ : 1000 ppm and O ₂ : 6% was used at a flow rate of 100 ml / min. (SV: 10,000-
It was distributed at 15000 h ^-1 ).

Then, for the gas coming out of the reaction tube,
Nitrogen oxide reduction characteristics were evaluated using a chemiluminescent nitrogen oxide analyzer and a gas chromatograph. In particular,
The concentration of NOx in the gas discharged from the reaction tube was measured by a chemiluminescence type nitrogen oxide analyzer, and N ₂ , O ₂ , CO, C
The concentrations of O ₂ and C ₃ H ₆ were measured by gas chromatography.
The measured temperature range is 300 to 700 ° C.

Then, based on the obtained measured value, N
The conversion rate from O to N ₂ was determined. Since this conversion rate changes depending on the temperature, the maximum value of the conversion rate is shown in Table 1 below together with the temperature. However, NO conversion rate (%) = {(NO amount supplied−NO amount discharged) / (NO amount supplied)} × 100

[0029]

[Table 1]

As is clear from Table 1, the catalyst materials of Examples 1 to 6, which are within the scope of the present invention, had a high NO conversion rate and exhibited good NO reduction activity. On the other hand, Comparative Examples 1 and 2
Those having a low NO conversion rate are not preferable. Like this
The catalyst material for removing nitrogen oxides of this example has a high function of reducing nitrogen oxides by bringing the nitrogen oxides and a reducing agent component such as hydrocarbon into contact with the catalyst, and is used for internal combustion engines such as automobiles. It is suitable for purification treatment of high-temperature exhaust gas discharged from the exhaust gas.

Therefore, for example, the nitrogen oxide removing catalyst material of this embodiment is provided in the exhaust gas passage as it is as a powder, or is processed into a granule or a pellet, or is formed into a honeycomb shape or the like. By loading the exhaust gas in the exhaust gas purifying apparatus, it is possible to effectively decompose and remove nitrogen oxides in the exhaust gas of an automobile or the like.

It should be noted that the present invention is not limited to the above-described embodiment, and it is needless to say that the present invention can be implemented in various modes without departing from the gist of the present embodiment.

[0033]

As described above in detail, the catalyst material for removing nitrogen oxides comprising the complex oxide of the present invention can reduce and remove nitrogen oxides by using a reducing agent such as hydrocarbon. It is useful as a catalyst for purifying engine exhaust gas. INDUSTRIAL APPLICABILITY The catalyst material for removing nitrogen oxides of the present invention is capable of removing nitrogen oxides even in exhaust gas with high oxygen concentration such as lean burn engine and diesel engine, and requires a highly toxic reducing agent such as ammonia. It has the advantage of not doing.

Therefore, by constructing a nitrogen oxide treating apparatus using this nitrogen oxide removing catalyst material, it is possible to effectively purify exhaust gas from an automobile or the like.

Claims (3)

[Claims] 1. A metal element containing Al and Zn,
A catalyst material for removing nitrogen oxides, characterized in that a Zn / Al molar ratio X is in a range of 0 <X <0.5 and a composite oxide having a spinel type crystal structure is used as a main phase. 2. A nitrogen oxide treating apparatus comprising the catalyst material for removing nitrogen oxide according to claim 1 as a part or the whole thereof. 3. A nitrogen oxide-removing catalyst material according to claim 1, wherein the nitrogen oxide-containing gas and a reducing agent are brought into contact with each other to reduce and remove the nitrogen oxide. Method.