JP3266471B2 - Nitrogen oxide storage material and nitrogen oxide treatment apparatus using the material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel nitrogen oxide storage material capable of storing nitrogen oxide, and a nitrogen oxide treatment apparatus using the material.

[0002]

2. Description of the Related Art Among environmental problems that have recently become a problem,
Decomposes harmful substances such as nitrogen oxides in automobile exhaust gas,
There is an urgent need to develop methods for removal. As an exhaust gas purifying catalyst, a three-way catalyst that simultaneously removes carbon monoxide, hydrocarbons, and nitrogen oxides has been put to practical use. As such a three-way catalyst, a catalyst in which a noble metal such as Pd, Pt, and Rh is supported on a heat-resistant carrier such as cordierite coated with γ-alumina is generally used.

[0003]

However, in a lean burn engine or a diesel engine that burns in an oxygen-excess atmosphere, there is a problem that the three-way catalyst does not function effectively due to oxygen poisoning of a noble metal. Research on zeolites as a catalyst that functions in an oxygen-excess atmosphere has been advanced, but is not sufficient in terms of heat resistance and water resistance. On the other hand, oxide catalysts having excellent heat resistance and the like have been studied, but at present, they have not been put to practical use due to insufficient activity.

[0004] Separately, in fixed combustion devices such as thermal power plants, reduction of nitrogen oxides in exhaust gas having a high oxygen concentration using ammonia as a reducing agent has been put to practical use. Applying this method to a mobile combustion device such as an automobile or a fixed combustion device in an urban area has a problem in terms of safety because ammonia is toxic.

In recent years, as a method for removing nitrogen oxides in an oxygen-excess atmosphere, nitrogen oxides are stored in an occlusion material in a region where fuel is lean, that is, in a region where combustion is performed with excess oxygen, and excess fuel is used. A method has been devised in which nitrogen oxides are released during combustion in the vicinity of (rich) or the stoichiometric air-fuel ratio (stoichiometric), and reduced and removed using a three-way catalyst.

[0006] Substances that occlude this nitrogen oxide include:
Ba-Cu-O-based and Mn-Zr-O-based composite oxides are known, but there is a need for a substance having a larger occlusion amount and exhibiting stable occlusion characteristics even at higher temperatures. The present invention has been made to solve the above-described problem, and temporarily stores nitrogen oxides in high oxygen exhaust gas such as a diesel engine or stores and removes nitrogen oxides released into the atmosphere. It is an object of the present invention to provide a nitrogen oxide storage material which can be used and a nitrogen oxide treatment apparatus using the material.

[0007]

Means for Solving the Problems In order to solve the above problems, the invention of claim 1 provides a nitrogen oxide storage material comprising a hollandite type composite oxide containing at least Al and Sn as main metal elements. Make a summary.

According to a second aspect of the present invention, the composite oxide comprises A
_{Y Al X Sn 8-X O} 16 ( where, A = an alkali metal element or an alkaline earth metal element) and gist of claim 1 nitrogen oxide storage material according represented by.

More specifically, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), calcium (C)
a), strontium (Sr), and barium (Ba). In particular, when the metal element is K, Rb, Cs, Ca,
Sr and Ba are preferable because the hollandite crystal structure is stable.

According to a third aspect of the present invention, there is provided a nitrogen oxide treating apparatus comprising the nitrogen oxide storage material according to the first or second aspect partially or entirely. Specifically, the exhaust gas of various automobiles and the like used when the nitrogen oxide storage material is used in a powder state, a state processed into granules or pellets, or a state formed into a honeycomb shape or the like. A purification device is exemplified.

According to a fourth aspect of the present invention, there is provided a nitrogen oxide treating apparatus according to the third aspect, which is used for treating exhaust gas from an automobile.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION

(Inventions of Claims 1 and 2) The inventors of the present invention have made intensive studies on the above problem, and as a result, a composite oxide containing aluminum (Al) and tin (Sn) and having a hollandite-type crystal structure was obtained. It has a high nitrogen oxide storage effect even under high oxygen concentration,
This has led to the present invention.

The hollandite-type composite oxide has the general formula: A
_{Y BX} C _8-X O ₁₆ (where A = alkali metal element or alkaline earth metal element, B = divalent or trivalent metal element, C =
(Tetravalent metal element). The metal element B substitutes for a part of the metal element C, and forms a one-dimensional tunnel structure with the metal elements B, C and oxygen, and the metal element A exists in the tunnel.

The nitrogen oxide occluding material of the present invention is hollandite containing at least Al and Sn at B and C sites as main metal elements. Of these, the main metal element is Al
And Sn only, the general formula is A _Y Al _X
It is represented by Sn _8-X O ₁₆ . Here, A is an alkali metal element or an alkaline earth metal element. The range of X and Y is not particularly limited, but is preferably about 0.5 to 2.5 from the viewpoint of the stability of the hollandite structure.
That is, if either range is exceeded, the structural stability will be deteriorated due to the difference in the size of the atom.

FIG. 1 shows that A = K as an example of the present invention.
The powder X-ray diffraction pattern when (potassium) and X = Y = 1.8 was shown. The composite oxide having the above-described structure can occlude nitrogen oxide (NO _x ) contained in exhaust gas by being brought into contact with, for example, exhaust gas containing nitrogen oxide. That is, by contacting a hollandite-type composite oxide containing Al and Sn as main constituent metal elements with, for example, exhaust gas containing nitrogen oxide, nitrogen oxide can be occluded.

Although the mechanism of this occlusion has not yet been fully elucidated, on the composite oxide surface of the present invention,
It is considered that NO is oxidized to nitrate or nitrite via NO ₂ and taken into the bulk. As a method for producing the nitrogen oxide storage material of the present invention, for example, an alkali metal element or an alkaline earth metal element, an organometallic compound such as an alkoxide of a main metal element (Al or Sn), and / or a main metal element A method of dissolving an inorganic metal compound such as a nitrate of (Al or Sn) in a solvent such as alcohol and subjecting the residue obtained by evaporating to dryness or evaporating to dryness after hydrolysis, followed by heat treatment at 600 ° C. or higher. . Here, the most preferable heat treatment temperature is 800-12.
The temperature is 00 ° C., whereby hollandite crystals are sufficiently formed, and no hollandite crystals are decomposed and the specific surface area is large.

In addition to the above-mentioned production method, fine tin oxide SnO ₂ is uniformly suspended in alcohol, and the alkoxide of Al and K is dissolved and hydrolyzed in this suspension.
Although there is a method of synthesizing by heat treatment, the present invention is not limited to these manufacturing methods.

However, it is difficult to synthesize the composite oxide of the present invention by a solid phase reaction from an oxide or carbonate of a component element. This is because the ion radii of Al and Sn are significantly different (Al ³⁺ / Sn ⁴⁺ = 0.77). (Invention of claim 3) In the invention of claim 3, the above-mentioned nitrogen oxide storage material having a function of storing nitrogen oxide is used for a part or the whole thereof to constitute a nitrogen oxide treatment apparatus, For example, it is possible to effectively purify the exhaust gas by introducing exhaust gas of an automobile and the like to the nitrogen oxide processing device and storing the nitrogen oxides in the exhaust gas in the nitrogen oxide storage material. Becomes

Specifically, for example, lean fuel
In other words, nitrogen oxides are burned in a region where combustion is performed with excess oxygen,
A device that stores nitrogen oxides in the nitrogen oxide storage material, releases nitrogen oxides during combustion near fuel excess (rich) or near the stoichiometric air-fuel ratio (stoichiometric), and reduces and removes them using a three-way catalyst is known. No. (Invention of claim 4) In particular, the nitrogen oxide storage material used in the present invention has an excellent nitrogen oxide storage capability at high temperatures, and therefore is used as an apparatus for exhausting high-temperature exhaust gas from automobiles and the like. It is suitable.

[0020]

EXAMPLES Examples of the present invention will be described below together with a method of manufacturing the same. The nitrogen oxide storage material of this example uses K as an alkali metal, K and Al and Sn as main metal elements, and K _1.8 Al _1.8 Sn where X = Y = 1.8.
_6.2 O _{16 is} a hollandite-type composite oxide.

<Production Method> First, a method for producing the composite oxide as the nitrogen oxide storage material of this embodiment will be described. Tetra-iso-propoxy tin (Sn (i-OP
r) ₄₎ to isopropanol, and aluminum nitrate _{(Al (NO 3) 3 ·} 9H 2 O) and nitric acid strength potassium (KN
O ₃ ) was dissolved in distilled water such that the metal element ratio, K: Al: Sn, was 1.8: 1.8: 6.2.

Next, an aqueous solution of aluminum nitrate + potassium nitrate was gradually dropped into an isopropanol solution of tetra-iso-propoxy tin to hydrolyze the tetra-iso-propoxy tin. At this stage, Sn (OH) ₄ is generated as a precipitate, but the nitrates of Al and K remain dissolved.

Next, the solution containing the precipitate was heated at 100 ° C.
, And pulverized in an alumina mortar to obtain a raw material composition. This, in the air, and heat treated 6 hours at 800 ° C., to obtain a composite oxide of hollandite type _{K 1.8 Al 1.8 Sn 6. 2 O} 16 of the present embodiment. The specific surface area of the obtained synthetic powder is B
When measured by the ET method, it was a very large value of 32.7 m ² / g.

The thus obtained substance was subjected to powder X-ray diffraction to confirm that the substance was a single hollandite phase. This X-ray diffraction chart is shown in FIG.
Shown in Since it is impossible to confirm that the hollandite phase is a single phase with a microscope or the like, JCPDS
It confirmed using the fact that it was the same as the peak pattern of KTi ₈ O ₁₆ shown in the card (41-1097).

<Experimental Example> Next, an experimental example performed to confirm the effect of the nitrogen oxide storage material of the present embodiment will be described. Powder obtained by the above-described manufacturing method,
After molding at a pressure of 200 kg / cm ² , the molded body was pulverized and sized to 28 to 48 mesh. 2g of sized powder
Into a Pyrex reaction tube, and add 50
At 0 ° C., NO = 1000 ppm, CH ₄ = 1000 pp
The mixed gas of m, O ₂ = 5%, and He = remainder was circulated at 50 ml / min as a model gas of the exhaust gas.

Then, for the gas coming out of the reaction tube,
The nitrogen oxide storage properties were evaluated using a chemiluminescent nitrogen oxide analyzer and a gas chromatograph. In particular,
The NO _x concentration in the gas discharged from the reaction tube was measured by a nitrogen oxide analyzer, and the N ₂ and N ₂ O concentrations were measured by a gas chromatograph every 30 minutes. FIG. 2 shows the measurement results of the NO _X concentration.

As shown in FIG. 2, over the two hours at 500 ° C., no NO _x was detected, and no N ₂ and N ₂ O were detected by gas chromatography. From this, it is clear that the introduced NO gas was completely occluded in the composite oxide of this example despite the high temperature.

[0028] Thus, the nitrogen oxide storage material of the present embodiment, storage capacity is large and the specific surface area is large, and since effectively occlude NO _X even at a high temperature, a high temperature of the internal combustion engine such as an automobile It is suitable for exhaust gas purification processing.
Therefore, for example, the nitrogen oxide storage material of this embodiment
By processing the powder as it is, or processing it into granules or pellets, or forming it into a honeycomb, etc., and filling the exhaust gas purification device provided in the exhaust gas passage, the exhaust gas from automobiles and the like can be effectively exhausted. Nitrogen oxides can be absorbed and removed from the gas.

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 carried out in various modes without departing from the gist of the present embodiment.

[0030]

As described in detail above, the nitrogen oxide storage material comprising the composite oxide of the present invention has a high nitrogen oxide (NO _X ) storage characteristic at high temperatures, for example, in a high oxygen concentration atmosphere. Therefore, it is useful for exhaust gas purification treatment of combustion engines such as diesel engines and lean burn engines, and useful for occlusion and removal of nitrogen oxides released into the atmosphere.

Therefore, the apparatus using the composite oxide can effectively purify exhaust gas and the like of an automobile or the like.

[Brief description of the drawings]

FIG. 1 is a powder X-ray diffraction pattern of a composite oxide (A = K (potassium), X = Y = 1.8) in an example.

FIG. 2 is a graph showing nitrogen oxide storage characteristics of a composite oxide in an experiment of an example.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-9-86907 (JP, A) JP-A-9-75715 (JP, A) JP-A-49-22390 (JP, A) JP-A-7-87 136456 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01J 20/08 B01D 53/56 B01D 53/81 B01D 53/86 B01D 53/94

Claims (4)

(57) [Claims] 1. A nitrogen oxide storage material comprising a composite oxide having a hollandite structure containing at least Al and Sn as main metal elements. 2. The method according to claim 2, wherein the composite oxide is A _Y Al _X Sn _8-X O
_16. The nitrogen oxide storage material according to claim 1, wherein the material is represented by ₁₆ (where A is an alkali metal element or an alkaline earth metal element). 3. A nitrogen oxide treatment apparatus comprising the nitrogen oxide storage material according to claim 1 or 2 partially or entirely. 4. The nitrogen oxide treatment apparatus according to claim 3, which is used for treating exhaust gas of an automobile.