JP2005161225A - Catalyst for purifying exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for purifying exhaust gases that is superior in adsorbing and desorbing performance of HCs and useful for purifying cold HCs. <P>SOLUTION: The catalyst for purifying exhaust gases is a laminated structure where a layer for trapping hydrocarbons and a catalyst layer overlie a honeycomb carrier in this order. The layer for trapping hydrocarbons contains titania nanotubes, while the catalyst layer contains a noble metal and alumina. The titania nanotubes have a pore volume of not less than 0.1 cc/g, a specific surface area of not less than 50 m<SP>2</SP>/g and a pore size of 0.1 to 5 nm. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention is an exhaust gas for purifying hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) in exhaust gas discharged from electrical appliances such as air conditioners and ovens, and internal combustion engines such as automobiles and boilers. The present invention relates to a purification catalyst, and particularly to an exhaust gas purification catalyst that focuses on purification of HC immediately after engine startup (hereinafter referred to as cold HC).

Conventionally, various catalysts for purifying cold HC at the time of starting an engine have been proposed. For example, as seen in a catalyst including an HC trap material, a catalyst that traps HC at a low temperature and releases HC at a high temperature to purify the catalyst. (For example, refer to Patent Document 1)).
The feature of such an exhaust gas purification catalyst is that it contains a zeolite having an HC trap function and a precious metal for purification.

On the other hand, in recent years, a method for producing a titania nanotube that can theoretically be expected to have an HC adsorption function similar to zeolite has been disclosed (see, for example, Patent Documents 2 to 4).
Japanese Patent Laid-Open No. 04-231616 JP 2003-034531 A JP 2002-241129 A Japanese Patent Laid-Open No. 10-152323

However, in such a conventional exhaust gas purification catalyst, when zeolite is used as the HC trap material, a part of the adsorbed HC is desorbed below the activation temperature of the noble metal, and the target HC purification rate in the cold region is increased. There was a problem that it could not be obtained sufficiently.
In order to solve such a problem, an HC trap material that holds HC up to a high temperature is desired. However, in the field of exhaust gas purification, a material with higher performance than zeolite has not been known at present.

On the other hand, although titania nanotubes can be expected to have an HC adsorption function as described above, it has been unclear whether or not the adsorbed HC can be appropriately released and suitably used for exhaust gas purification.
Further, if titania nanotubes are used as an exhaust gas purification catalyst, titanium (Ti) promotes thermal degradation of noble metals and alumina, and there is a problem that exhaust gas purification performance may be reduced.

  The present invention has been made in view of such problems of the prior art, and an object of the present invention is to provide an exhaust gas purifying catalyst that is excellent in HC adsorption and release performance and good in the purification of cold HC. There is to do.

  As a result of intensive studies to achieve the above object, the present inventors have found that the above object can be achieved by appropriately controlling the layer structure, and have completed the present invention.

  That is, the exhaust gas purifying catalyst of the present invention has a laminated structure in which a hydrocarbon trap layer and a catalyst layer are laminated in this order on a honeycomb carrier. The hydrocarbon trap layer contains a titania nanotube, and the catalyst layer is an exhaust gas purifying catalyst containing a noble metal and alumina.

  According to the present invention, it is possible to provide an exhaust gas purifying catalyst that is excellent in HC adsorption and release performance and good in purifying cold HC because the layer structure is appropriately controlled.

Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail. In the present specification, “%” represents mass percentage unless otherwise specified.
As described above, the exhaust gas purifying catalyst of the present invention is formed by coating a hydrocarbon trap layer on a honeycomb carrier and laminating the catalyst layer on the hydrocarbon trap layer. An exhaust gas purifying catalyst having a laminated structure of at least two layers in which catalyst layers are laminated.
The hydrocarbon trap layer contains titania nanotubes, and the catalyst layer contains a noble metal and alumina.

The exhaust gas purifying catalyst of the present invention is a catalyst that adsorbs HC when the exhaust gas level is low, such as after starting the engine, and desorbs and purifies HC as the exhaust gas temperature rises. It has a laminated structure in which titania nanotubes are contained in the inner layer and alumina and a noble metal are contained in the outer layer.
Here, the layer structure is divided into an inner layer and an outer layer, the inner layer contains titania nanotubes, and the outer layer contains alumina and noble metal. The reason why Ti constituting the titania nanotubes is to promote thermal deterioration of alumina and noble metals. Therefore, Ti is separated from the precious metal and alumina to avoid mutual contact and to improve the purification efficiency of the detached HC.

Further, the content of the titania nanotube is not particularly limited, but is preferably 50 to 400 g per liter of the exhaust gas purifying catalyst.
In order to sufficiently adsorb HC when the exhaust gas is cold, it is preferably 50 g or more. If it is less than 50 g, a sufficient amount of HC adsorption may not be realized, and if it exceeds 400 g, a significant increase effect is obtained. It may not be possible.

Furthermore, it is preferable to use the titania nanotube having a pore volume of 0.1 cc / g or more and a specific surface area of 50 m ² / g or more.
In order to adsorb cold HC appropriately, the pore volume and specific surface area are preferably within the above ranges, and if the pore volume is less than 0.1 cc / g, a sufficient amount of HC adsorption cannot be obtained. If the specific surface area is less than 50 m ² / g, a sufficient HC adsorption amount may not be obtained.

In addition, it is preferable that the pore diameter of this titania nanotube is 0.1-5 nm.
In order to trap HC in the exhaust gas, the pore diameter should be within the above range, and if it is less than 0.1 nm, HC cannot enter, and if it exceeds 5 nm, HC may pass through.

Furthermore, it is preferable that the titania nanotube as the catalyst raw material has a desorption peak observed by toluene adsorption / desorption test method even at 400 ° C. or higher.
It is preferable that HC can be adsorbed to a high temperature because of the requirement for an exhaust gas purification catalyst. When HC desorption is observed only at less than 400 ° C., a part of HC is desorbed before the precious metal is activated. As a result, some HC may be unpurified.

In the exhaust gas purifying catalyst of the present invention, the above-mentioned HC trap layer preferably contains alumina, silica or titania and any mixture thereof as a binder. In this case, the weight ratio of the binder to the titania nanotube is 1/100 to 1/5 is preferable.
By using such a binder, it becomes possible to sufficiently support the titania nanotubes on the honeycomb carrier. When the binder / titania nanotube weight ratio is less than 1/100, the titania nanotubes may be peeled off from the carrier. If it exceeds, the amount of titania nanotubes in the layer may be relatively reduced, and the HC adsorption rate may not be sufficiently obtained.

Next, examples of the noble metal to be contained in the catalyst layer include platinum (Pt), palladium (Pd), (rhodium (Rh), and any combination thereof.
The precious metal content is preferably 1 to 10 g per liter of the catalyst. If it is less than 1 g / L, the purification function of desorbed HC may be deteriorated, and if it is contained in excess of 10 g / L, a significant increase effect may not be obtained.

The monolith honeycomb carrier to be used preferably has a GSA (geometric surface area) of 20 to 50 cm ² / cm ³ .
If the GSA is less than 20 cm ² / cm ³ , the contact area with the exhaust gas is small and the NOx trap function may be reduced. If it exceeds 50 cm ² / cm ³ , the thickness of the HC trap layer is insufficient and the HC trap function is reduced. There are things to do.
In addition, the cell cross-sectional shape of the honeycomb carrier is preferably a tetragon to a hexagon.

Further, the alumina to be used is preferably excellent in heat resistance, and lanthanum (La) or cerium (Ce) is preferably supported as is known in general exhaust gas purification catalysts. Further, it is preferable that the alumina carrying Rh carries Zr.
In the exhaust gas purifying catalyst of the present invention, a material generally used in exhaust gas purifying catalysts may be added as a co-catalyst, including ceria, zirconia, sodium (Na), potassium (K) and the like. Alkali metal compounds including alkali metal compounds, magnesium (Mg), calcium (Ca), barium (Ba), and the like can be added.

  In addition, the manufacturing method of a titania nanotube may be the method generally reported, for example, the method of mixing a titania alkoxide solution, surfactant, and water and solidifying, The method of alkali-treating titania.

  EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.

(Example 1)
Titania nanotubes, SiO ₂ sol and water were put into a magnetic ball mill, mixed and ground to obtain a slurry. This was supported on a ceramic honeycomb carrier having GSA = 28.8 cm ² / cm ³ (4 mil, 400 cells).
Next, the activated alumina powder was impregnated with a dinitrodiammine platinum aqueous solution, dried, and fired in air to obtain Pt-supported alumina. Further, Rh-supported alumina powder was obtained by the same operation. Furthermore, these powder and water were put into a magnetic ball mill, mixed and pulverized to obtain a slurry.
Thereafter, the obtained slurry is supported on the titania nanotube supporting carrier, and the titania nanotube is supported on the first layer (inner layer) on the honeycomb carrier side, and the noble metal catalyst layer is supported on the second layer (outer layer). Got.
In this example, the catalyst per 1L, the TiO ₂ 50 g, the Pt and SiO ₂ to 5 g, 2-layer 3g, the Rh and 0.6g and alumina was 200g supported on the first layer.

(Comparative Example 1)
Titania nanotubes, silica sol, Pt-supported alumina powder, Rh-supported alumina powder, and water were put into a magnetic ball mill, mixed and ground to obtain a slurry.
The obtained slurry was supported on a ceramic honeycomb carrier to obtain a catalyst of this example in which titania nanotubes and noble metal were supported in the same layer.

[Performance evaluation]
In order to confirm the improvement effect of desorption HC purification performance by supporting titania on a layer different from the noble metal, a test was conducted under the following conditions. The obtained results are shown in Table 1.
-Durability conditions: inlet 700 ° C x 30 hr, Nissan gasoline 3L, V6 engine-Evaluation conditions: Model gas evaluation-Catalyst capacity: 40cc
Evaluation gas: flow rate = 40 L / min, O ₂ = 5 vol%,
HC (toluene) = 1000 ppmC, H ₂ O = 10 vol%

  From Table 1, it is clear that Example 1 can be purified from a low temperature in the purification activity of HC desorbed from the titania nanotube, and it can be seen that it also meets the object of the present invention.

(Reference Example 1)
The titania nanotube powder used in Example 1 was prepared.

(Reference Example 2)
The titania nanotube powder was fired at 1000 ° C. in advance to prepare a product with reduced specific surface area, pore volume, and pore diameter.

(Reference Example 3)
Zeolite powder (β type) was prepared.

[Test example]
The physical properties of the two types of titania nanotube powders of Reference Examples 1 and 2 were evaluated.
The specific surface area was observed with a BET specific surface area measuring method, the pore volume was measured with an N ₂ adsorption method, and the pore diameter was observed with a transmission electron microscope. The obtained results are shown in Table 2.
The HC adsorption function was evaluated by the following method.
Toluene is volatilized at room temperature, and toluene is allowed to flow and adsorb to the powder in the reaction tube kept at 150 ° C., vacuum degassed, and He gas is introduced. Next, the temperature of the reaction tube is raised, and the amount of desorbed toluene is measured with a mass spectrometer. The obtained results are shown in FIG.

As shown in Table 2 and FIG. 1, when titania nanotubes are used, an effective HC adsorption / desorption reaction may occur when the specific surface area, pore volume, and pore diameter are within the preferred ranges of the present invention. Understand.
Further, since desorbed HC is observed even in a high temperature range as compared to zeolite (see FIG. 1), it can be understood that HC can be maintained at a higher temperature and is suitable for the purpose of the present invention.

Thus, since the titania nanotube of Reference Example 1 suitable for the present invention has an extremely high HC trapping capacity, when it is used as an exhaust gas purification catalyst, for example, the amount supported on the catalyst is small, and the exhaust resistance is reduced. Benefits such as being able to be expected.
Further, since HC can be maintained up to a high temperature, there is a possibility that the amount of noble metal used can be reduced. That is, even when the amount of noble metal supported is reduced and the activation temperature is increased, the HC can be desorbed and purified.

It is a graph which shows the desorption behavior of toluene.

Claims (6)

In the exhaust gas purifying catalyst having a laminated structure in which a hydrocarbon trap layer and a catalyst layer are laminated in this order on a honeycomb carrier,
The hydrocarbon trap layer contains titania nanotubes,
An exhaust gas purifying catalyst, wherein the catalyst layer contains a noble metal and alumina.   2. The exhaust gas purifying catalyst according to claim 1, wherein the titania nanotube content is 50 to 400 g per liter of the catalyst. 3. The exhaust gas purifying catalyst according to claim 1, wherein the titania nanotube has a pore volume of 0.1 cc / g or more and a specific surface area of 50 m ² / g or more.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the titania nanotube has a pore diameter of 0.1 to 5 nm.   The exhaust gas purification catalyst according to any one of claims 1 to 4, wherein the titania nanotube has a desorption peak measured by an adsorption / desorption test method of toluene at 400 ° C or higher.   The hydrocarbon trap layer contains at least one binder selected from the group consisting of alumina, silica and titania, and the weight ratio of the binder to titania nanotube is 1/100 to 1/5. The exhaust gas-purifying catalyst according to any one of claims 1 to 5.

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