JPH10202111A - Catalyst for purifying diesel exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent emission of HC and SOF by efficiently adsorbing HC and SOF at low temperature and further suppress production of sulfates. SOLUTION: This catalyst comprises a coating layer with a three-layer structure comprising a first coating layer which does not bear a catalyst metal, a second coating layer bearing a catalyst metal, and a third coating layer which does not bear a catalyst metal. Since the deposition density of the metal catalyst in the second coating layer is high, the SOF firing performance is improved and SOF purifying performance is improved. Moreover, since the volume ratio of the second coating layer to be brought into contact with SO2 is lowered and the adsorption amount of the SO2 at the time of diffusion, production of sulfates can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention oxidizes and purifies HC (hydrocarbon) and SOF (Soluble Organic Fraction) contained in exhaust gas of a diesel engine (hereinafter referred to as DE) and reduces the amount of diesel particulate emissions. To a diesel exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art With respect to gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of responding to the strict regulations. However, with regard to DE, due to the unique circumstances that harmful components are mainly emitted as particulates, regulations and development of technology are delayed compared to gasoline engines, and the development of an exhaust gas purification catalyst that can reliably purify harmful components is expected. It is rare.

[0003] As a DE exhaust gas purifying apparatus which has been developed to date, a method using a trap (without a catalyst and with a catalyst) and an open type SOF decomposition catalyst are known. Of these, the method using a trap is
It captures diesel particulates and regulates their emissions, and is especially effective for exhaust gas with a high dry suit ratio. However, the method using a trap requires a reprocessing device to incinerate the captured diesel particulates, which causes many practical problems such as cracking of the catalyst structure during regeneration, blockage by ash, or complicated systems. Is leaving.

On the other hand, as an open type SOF decomposition catalyst, a catalyst in which a catalytic metal such as a platinum group metal is supported on a supporting layer of activated alumina or the like as in a gasoline engine is used, as shown in, for example, JP-A-1-171626. And CO
SOF is oxidized and decomposed together with HC and purified. This open-type SOF cracking catalyst has the drawback of low removal rate of dry suits, but the amount of dry suits can be reduced by improving the DE and the fuel itself, and has the great advantage of not requiring a regeneration unit. Therefore, further improvement of technology is expected in the future.

[0005] However, the open type SOF decomposition catalyst can efficiently decompose SOF at a high temperature, but has a drawback that at low temperature conditions, the catalytic action of the catalyst metal is low and the purification performance of the SOF is reduced. Therefore, when the engine is started or idling, the temperature of the exhaust gas is low, and a phenomenon occurs in which undecomposed SOF becomes soot and accumulates in the honeycomb passage. Then, there is a problem that the catalyst is clogged by the deposited soot and the catalyst performance is reduced.

[0006] In the open type SOF decomposition catalyst, SO ₂ in the exhaust gas is also oxidized in a high temperature region to form SO ₃
There is a problem that SO and SO ₄ are generated and become sulfate and consequently the amount of particulates increases. This is SO
₂ is not measured as particulates, but sulfate is measured as particulates.
Especially in DE, there is a lot of oxygen gas in the exhaust gas,
The oxidation reaction of SO ₂ is likely to occur.

Further, it is known that in the open type SOF decomposition catalyst, the catalytic metal is poisoned by sulfur contained in a large amount in DE exhaust gas, and the catalytic activity of the catalytic metal is reduced. That is, SO ₂ generated from sulfur in the fuel reacts with alumina as a catalyst carrier to react with aluminum sulfate (Al
₂ (SO ₄ ) ₃ ) is formed, which covers the catalytic metal and reduces catalytic activity.

On the other hand, in the field of exhaust gas treatment such as boilers, titania (TiO ₂ ), which has excellent resistance to sulfur poisoning, has been developed for a catalyst supporting layer, and a catalyst supporting a catalytic metal such as Pt and V has been developed. Has been provided. However, this type of catalyst has no SOF adsorption property, and HC and S
OF is directly discharged. In view of this, the present applicant has disclosed in Japanese Patent Application Laid-Open No. 4-267282 that a catalyst having a highly adsorptive coat layer such as activated alumina or zeolite and having no catalyst metal is arranged on the upstream side of the exhaust gas flow, and titania, silica, etc. A catalyst device has been proposed in which an exhaust gas purifying catalyst in which a catalyst metal is supported on a carrier having a coating layer with low adsorption property is disposed downstream.

[0009] According to this catalyst device, H in the upstream side catalyst is used.
C and SOF are adsorbed at a low temperature and SO ₂ are adsorbed. However, since the upstream side catalyst has no catalytic metal,
The oxidation of ₂ is prevented. In the downstream catalyst, HC and SOF released from the upstream catalyst at a high temperature are oxidized and purified by the catalyst metal. On the other hand, S
O _{2 is} also released, but the downstream catalyst has low adsorptivity,
Oxidation by adsorption of O ₂ is suppressed, and formation of sulfate is prevented.

[0010] The catalyst device disclosed in the above publication is certainly effective under steady-state conditions. However, when a large amount of SO ₂ adsorbed and accumulated in the upstream catalyst is released at a high temperature and flows into the downstream catalyst at a low temperature, not a small amount of SO ₂ which comes into contact with the catalyst metal of the downstream catalyst is present, and the sulfate is present. There was a problem that was discharged as.

Japanese Patent Application No. 7-320313 (not disclosed at the time of filing of the present application) discloses a catalyst for purifying diesel exhaust gas in which the carrier is divided into two layers and the upper layer does not carry the catalytic metal but only the lower layer carries the catalytic metal. Is disclosed. With such a configuration, HC, SOF and S
O ₂ is adsorbed and accumulated. Since the upper layer has no catalytic metal, HC, SOF and SO ₂ accumulate without being oxidized. HC and SOF become liquid phases, penetrate by capillary action, are adsorbed to the lower layer, and are oxidized and purified. However, SO ₂ is in a gaseous phase even at a low temperature, and is hardly adsorbed compared to HC and SOF, and is difficult to move to the lower layer. Therefore, emission of particulates in a low temperature range is suppressed.

When the temperature of the exhaust gas becomes high, the flow rate of the high-temperature exhaust gas is large, so that HC and SOF adsorbed in the upper layer
And SO ₂ is insufficient time to diffuse to the oxide point of the lower layer is present catalyst metal, HC, SOF and SO ₂ is directly discharged without being excessively oxidized. Therefore, since almost no sulfate is generated, the amount of particulates emitted from the high-temperature exhaust gas itself is small.

[0013]

However, even in a catalyst in which a catalytic metal is supported only in the lower layer as described above, SO ₂
Is inevitable to penetrate to some extent from the upper layer to the lower layer, and the problem that sulfate is generated in the lower layer is inevitable. In addition, since no catalytic metal was exposed, there was a problem that the oxidizing ability of HC and SOF was low.

The present invention has been made in view of the above circumstances, and efficiently adsorbs HC and SOF in a low temperature range to prevent the emission of HC and SOF, further suppresses the production of sulfate, and The purpose is to suppress sulfur poisoning.

[0015]

The features of the catalyst for purifying diesel exhaust gas according to the first aspect of the present invention, which solves the above-mentioned problems, include a honeycomb-shaped carrier substrate and a catalyst metal formed on the cell wall surface of the carrier substrate. A first coat layer that does not carry the catalyst, a second coat layer that is formed on the surface of the first coat layer and carries the catalyst metal, and a third coat layer that is formed on the surface of the second coat layer and does not carry the catalyst metal It is in.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a honeycomb-shaped exhaust gas purifying catalyst, the smaller the opening cross-sectional area of the cell, the larger the surface area and the greater the probability of contact between the exhaust gas and the catalyst metal. However, if the opening cross-sectional area of the cell becomes too small, for example, by increasing the thickness of the coating layer, the airflow resistance of the exhaust gas becomes too high, resulting in a pressure loss. Therefore, the cell density of the carrier substrate is generally set to about 400 cells / inch ^2, and the amount of the coating layer formed is regulated to about 100 to 120 g per liter of the carrier substrate.

The present invention has solved the above-mentioned problems while keeping the same coating layer thickness as before in consideration of such regulations. That is, in the exhaust gas purifying catalyst of the present invention, the catalytic metal is supported only on the intermediate second coat layer. Therefore, the thickness of the second coat layer is smaller than the thickness of the entire coat layer, and if the amount of the catalyst metal in the entire coat layer is to be made equal to the conventional amount, the catalyst metal is carried in the second coat layer at a high density. Will be done.

With this configuration, the carrying density of the catalyst metal in the second coat layer can be, for example, about 10 times that of the one-layer coat and about five times that of the two-layer coat. , The ignition temperature is lowered and the ignition performance is improved, so that the SOF purification performance is improved. Also, the HC purification performance is improved. Then, from the surface layer (third coat layer) to SO
_The penetration of ₂ to some extent is equivalent to the case of the two-layer coating, but since the thickness of the second coating layer supporting the catalyst metal is thin, the volume ratio of the second coating layer in contact with SO ₂ is reduced, In addition, the amount of SO ₂ adsorbed during diffusion is reduced, so that the production of sulfate is suppressed.

As the carrier substrate, a monolithic carrier substrate made of a heat-resistant inorganic substance such as cordierite or a metal carrier substrate made of metal is used. Can be. As the material of the first coat layer and the third coat layer, SO _{2 in} the exhaust gas is used.
It is desirable that the adsorbent has a low adsorptivity of silica (SiO ₂ )
Titania (TiO ₂ ), zirconia (ZrO ₂ ), potassium titanate oxide, a composite oxide thereof, zeolite, or the like can be used. Among them, titania,
A material selected from silica and zeolite is particularly preferred. If such a material is used for the third coat layer, SO ₂
Adsorption and diffusion to the second coat layer can be further reduced, which is further effective in suppressing sulfate formation.

As the material of the second coat layer, the same material as that of the first coat layer can be used.
In particular, alumina or the like having good activity due to the interaction with Pt can be used. Although it is not desirable to use alumina for the first coat layer and the third coat layer, it is possible to use alumina for the second coat layer. Therefore, it is preferable to use alumina for the second coat layer from the viewpoint of catalytic performance.

The thickness of the second coat layer is one of the total coat thickness.
/ 5 to 1/10. When the thickness of the second coat layer is larger than 1/5 of the total coat thickness, sulfate is easily generated and the purification performance of SOF is reduced. When the thickness is smaller than 1/10, the particle diameter of the catalyst metal becomes large, resulting in durability. Decrease. The thickness of the third coat layer is preferably 1/2 to 1/3 of the total coat thickness. When the thickness of the third coat layer is larger than の of the total coat thickness, the purification performance of HC and SOF is reduced. When the thickness is smaller than １ ／, sulfate is easily generated.

The catalyst metal supported on the second coat layer is platinum (Pt), palladium (Pd), rhodium (R
h), noble metals such as silver (Ag), and also base metals such as iron (Fe), nickel (Ni), chromium (Cr), manganese (Mn), cobalt (Co), and copper (Cu).
The choice is made according to the desired oxidizing power. An appropriate amount of the catalyst metal to be carried is about 0.1 to 1.5 g per 1 liter of the catalyst volume in the first to third coat layers as a whole. Also, in the second coat layer, about 2 to 30 g / L is appropriate. If the supported amount is less than this, HC and S
The oxidizing power of the OF becomes insufficient, and even if the OF is carried more, the effect is saturated and a problem in terms of cost may occur.

If a catalyst metal is carried on the third coat layer, there is a positive correlation between the adsorbability of HC and SOF and the adsorbability of SO ₂ , so that SO ₂ is also adsorbed on the third coat layer and rises. When warm, it is not released simply as SO ₂
Oxidation to ₃ increases sulfate production. Therefore, the third coat layer does not carry a catalytic metal,
It simply functions as an adsorption layer.

If the catalytic noble metal is supported on the first coat layer, the structure becomes the same as that of the conventional two-layer coat.
The volume ratio at which the lower layer supporting the catalytic metal is in contact with SO ₂ is large, and the problem that sulfate is generated in the lower layer is inevitable, and the oxidizing ability of HC and SOF is reduced. That is, in the exhaust gas purifying catalyst of the present invention, the third
HC, SOF and SO ₂ are adsorbed and accumulated in the coat layer. Since the third coat layer has no catalyst metal, HC,
SOF and SO ₂ are accumulated without being oxidized. H
C and SOF become a liquid phase, penetrate by capillary action, and are adsorbed to the first coat layer. However, SO ₂ is in a gaseous phase even at a low temperature, and is hardly adsorbed as compared with HC and SOF, and hardly moves to the second coat layer. Therefore, in the exhaust gas purifying catalyst of the present invention, the emission of particulates in a low temperature range is suppressed.

When the temperature of the exhaust gas becomes high, the flow rate of the high-temperature exhaust gas is large, so that the SO 3 adsorbed on the third coat layer
_2, insufficient time to diffuse to the oxide point of the second coating layer catalyst metal is present, SO ₂ is directly discharged without being excessively oxidized. However, since the second coat layer supports the catalyst metal at a high density, the ignition performance of SOF is improved and the purification performance of HC and SOF is high. Therefore, the amount of discharged particulates is reduced.

On the other hand, HC and SOF accumulated from the third coat layer to the first coat layer at a low temperature are efficiently oxidized and purified by the catalyst metal of the second coat layer at a high temperature. Also, the SO accumulated in the third coat layer
₂ is released directly from the third coat layer without reaching the second coat layer and thus without producing sulfate. That is, HC accumulated at low temperatures,
Particulate emissions due to SOF and SO ₂ are also greatly suppressed.

P, which deactivates the catalytic metal oxidizing activity,
Since poisoning substances such as S and Zn are adsorbed to the third coat layer, poisoning of the catalytic metal is prevented.

[0028]

The present invention will be specifically described below with reference to examples and comparative examples. (Embodiment) FIG. 1 is an enlarged sectional view of a main part of an exhaust gas purifying catalyst according to one embodiment of the present invention. This exhaust gas purifying catalyst is a honeycomb-shaped carrier substrate made of cordierite (1).
A first coat layer (2) formed on the surface of the carrier substrate (1), a second coat layer (3) formed on the surface of the first coat layer (2), and a second coat layer And a third coating layer (4) formed on the surface of (3).

The carrier substrate (1) has a diameter of 147 × 95 m
This is an oval honeycomb carrier having a capacity of 1.7 liters, m, length 150 mm, cell number 400 cells / in ² , cell wall thickness 0.15 mm. The lower first coat layer (2) is made of Ti
It is composed of O ₂ and is formed to have a thickness of 50 μm and a thickness of 50 μm per liter of the carrier substrate (1).

The second coat layer (3) is composed of TiO ₂ (30) and Pt (31), and is formed to a thickness of 5 μm and a thickness of 10 μm per liter of the carrier substrate (1). Further, Pt (31) is added in an amount of 1. 1 per liter of the carrier substrate (1).
5 g supported, 30 g / L with only the second coat layer (3)
Carried at a density of Further, the uppermost third coat layer (4) is made of zeolite, and is formed to a thickness of 45 g and a thickness of 50 µm per liter of the carrier substrate (1).

The first coat layer (2) is formed by applying TiO ₂ powder as a slurry to the carrier substrate (1), drying and firing. The second coat layer (3) is made of P
The TiO ₂ powder impregnated and supported with t (31) is attached as a slurry to the surface of the first coat layer (2), and is dried and fired to form. Further, the third coat layer (4) is formed by applying zeolite powder as a slurry to the surface of the second coat layer (3), drying and firing.

(Comparative Example) FIG. 2 shows an exhaust gas purifying catalyst of a comparative example. This exhaust gas purifying catalyst comprises a carrier substrate (1) similar to that of the embodiment and a first substrate formed on the surface of the carrier substrate (1).
It is composed of a coat layer (5) and a second coat layer (6) formed on the surface of the first coat layer (5), and Pt is carried only on the first coat layer (5).

The lower first coat layer (5) is made of TiO.
₂ (50) and Pt (51), each having a thickness of 50 μm and a thickness of 50 μm per liter of the carrier substrate (1). 1.5 g of Pt (51) is supported per liter of the carrier substrate (1). The lower layer, the first
Only the coat layer (5) is supported at a density of 3 g / L, which is 1/10 of the example.

The uppermost second coat layer (6) is made of TiO.
It is composed of _two, support substrate (1) 5 against 1 liter
0 g and a thickness of 50 μm. The first coat layer (5) is made of TiO ₂ previously impregnated with Pt (51).
The powder is adhered to the surface of the carrier substrate (1) as a slurry,
It is formed by drying and firing. The second coat layer (6) is formed by applying TiO ₂ powder as a slurry to the surface of the second coat layer (5), drying and firing.

(Test Example) Each of the exhaust gas purifying catalysts of the embodiment and the comparative example having the above-mentioned configurations was mounted on a 2.6-liter direct-injection DE exhaust system.
pm) while changing the load, the sulfate production amount and the SOF purification rate were measured under the condition of the exhaust gas temperature of 300 ° C. The results are shown in FIGS. 3 and 4, respectively.

3 and 4, it can be seen that the catalyst of the example has a lower sulfate production rate and a higher SOF purification rate than the catalyst of the comparative example.
It is clear that the effect is that Pt is supported only on the coat layer (3).

[0037]

That is, the purification of diesel exhaust gas of the present invention.
According to the catalyst for SO, the SO adsorbed and accumulated at low temperature_TwoIs high
It is difficult to become sulfate when warm. Therefore in the exhaust gas
SO _TwoCan be discharged almost as it is, and HC
And only SOF can be efficiently purified,
Reduce particulate emissions from low to high temperatures
And the temperature window for HC and SOF purification is expanded
It is.

Further, since poisoning substances such as P, S and Zn which deactivate the oxidizing activity of the catalytic metal are adsorbed on the third coat layer, it is possible to prevent the oxidizing power of the catalytic metal from decreasing. Therefore, in the exhaust gas purifying catalyst of the present invention, even if the contact area with the exhaust gas is reduced as compared with the conventional one, the purification performance equal to or higher than that of the conventional one can be obtained. It is advantageous in terms of aspect.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing a main part of an exhaust gas purifying catalyst according to one embodiment of the present invention.

FIG. 2 is a schematic explanatory view showing a main part of an exhaust gas purifying catalyst of a comparative example.

FIG. 3 is a graph showing the amount of sulfate produced by the exhaust gas purifying catalysts of an example and a comparative example.

FIG. 4 is a graph showing SOF purification rates of exhaust gas purifying catalysts of an example and a comparative example.

[Explanation of symbols]

1: carrier base material 2: first coat layer
3: second coat layer 4: third coat layer 30: TiO ₂
31: Pt

Claims (1)

[Claims] 1. A carrier substrate having a honeycomb shape, a first coat layer formed on the cell wall surface of the carrier substrate and not carrying a catalyst metal, and a catalyst metal formed on the surface of the first coat layer and carrying a catalyst metal. A diesel exhaust gas purification catalyst comprising: a second coat layer; and a third coat layer formed on a surface of the second coat layer and not supporting a catalyst metal.