JPH09155205A - Oxidation catalyst for diesel exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent HC and SOF from being discharged by making HC and SOF efficiently adsorbed in a low temp. range and to suppress the generation of sulfate. SOLUTION: This oxidation catalyst is composed of a 1st carrying layer 2 carrying a catalytic metal 20 formed on the surface of a carrier base material 1 and a 2nd carrying layer 3 formed on the 1st carrying layer 2 and carrying no catalytic metal and the 1st carrying layer 2 is constituted so as to be lower than the 2nd carrying layer 3 in the adsorptivity of components in the Diesel exhaust gas. Since SO2 adsorbed at a low temp. on the 2nd carrying layer 3 is discharged without being in contact with the catalytic metal even the temp. becomes high, the generation of sulfate is 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. The oxidation catalyst for diesel exhaust.

[0002]

2. Description of the Related Art Regarding gasoline engines, harmful substances in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with it. 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 exhaust gas purification catalysts that can reliably purify is desired. .

[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 the engine is idling, the temperature of the exhaust gas is low and undecomposed SOF adheres and accumulates in the honeycomb passage. Then, the active layer of the catalyst is covered with the coating layer of SOF that is adhered and deposited, and the catalyst performance is deteriorated.

[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 SO ₃ and SO ₄ are measured as particulates. In particular, in DE, a large amount of oxygen gas is present in the exhaust gas, and 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 in the catalyst supporting layer to produce aluminum sulfate (Al ₂
(SO ₄ ) ₃ ) is formed, which covers the catalytic metal and reduces the 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 in which an oxidation catalyst in which a catalyst metal is supported on a carrier having a coating layer with low adsorption property is disposed downstream is proposed.

[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 catalytic device disclosed in the above publication is certainly effective under steady 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.

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 HC and SOF from being discharged, and well suppresses the production of sulfate, and The purpose is to prevent sulfur poisoning of the oxidation catalyst.

[0012]

The features of the oxidation catalyst for diesel exhaust gas of the present invention for solving the above-mentioned problems are as follows: a carrier substrate; a first carrier layer formed on the surface of the carrier substrate and carrying a catalytic metal; The first supporting layer is formed on the surface of the first supporting layer and does not support the catalytic metal, and the first supporting layer has lower adsorption of the components in the diesel exhaust gas than the second supporting layer.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION As a carrier substrate, a heat-resistant inorganic material such as cordierite or a metal carrier made of metal is used, and the shape thereof may be the same as conventional ones such as a honeycomb type and a pellet type. As the material of the first supporting layer, a material having a low adsorptivity of components in the exhaust gas is used.
O ₂ ), titania (TiO ₂ ), zirconia (Zr
O ₂ ), potassium titanate oxide, or composite oxides thereof can be used. Examples of the catalyst metal supported on the first support layer include noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh), and silver (Ag), as well as iron (Fe), molybdenum (Mo), and tungsten. Base metals such as (W), nickel (Ni), chromium (Cr), manganese (Mn), cobalt (Co), and copper (Cu) are also used, and are selected according to the desired oxidizing power.
The amount of the catalyst metal supported is appropriately 0.1 to 2.5 g per 1 liter of the catalyst volume. Carrying amount is H
It is determined according to the required oxidizing power of C and SOF. However, even if a large amount is supported, the effect may be saturated and a defect in cost may occur.

As the material of the second supporting layer, one having a higher adsorptivity for the components in the exhaust gas than the first supporting layer is used. The materials exemplified for the first supporting layer can be used as long as they have slightly higher adsorptivity than the materials used for the first supporting layer. However, it is desirable to use alumina, silica-alumina, zeolite, or the like, which has a particularly high adsorbability. No catalytic metal is loaded on the second loading layer,
The carrier layer simply functions as an adsorption layer. Therefore, it is desirable that the thickness of the second support layer be in the range of 10 to 50 μm. If the thickness of the second support layer is less than 10 μm, adsorption becomes difficult and the HC and SOF oxidation purification performance deteriorates. Also, if the thickness is more than 50 μm, it becomes difficult for the adsorbed HC and SOF to reach the first supporting layer, so that the HC and SOF are also
The oxidation purification performance of OF deteriorates.

If a catalyst metal is supported on the second support layer, as shown in FIG.
The _second adsorptive a positive correlation, SO ₂ The second
When adsorbed on the carrier layer and heated, it is not simply released as SO ₂ but is oxidized to SO ₃ and the amount of sulfate produced increases. Therefore, the second supporting layer does not support the catalytic metal and merely functions as an adsorption layer.

That is, in the oxidation catalyst of the present invention, HC, SOF and SO ₂ are adsorbed and accumulated in the second supporting layer on the surface layer at low temperature. Since the second supporting layer has no catalytic metal, H
C, SOF and SO ₂ are accumulated without being oxidized. HC and SOF become a liquid phase, permeate by a capillary phenomenon, and are adsorbed even to the first supporting layer. However, SO ₂ is in a gas phase even at a low temperature and is less likely to be adsorbed and move to the first supporting layer as compared with HC and SOF, so most of the SO ₂ is
Adsorbed and accumulated on the carrier layer. Therefore, in the oxidation catalyst of the present invention, the discharge of particulates in the low temperature range is suppressed.

When the temperature of the exhaust gas becomes high, the high-temperature exhaust gas has a high flow velocity, so that the HC adsorbed on the second support layer is
There is not enough time for SOF and SO ₂ to diffuse to the oxidation point of the first supporting layer where the catalyst metal is present, and HC, SOF and SO
₂ is not oxidized so much and is discharged as it is. However, since almost no sulfate is generated, the amount of particulate emissions due to the high temperature exhaust gas itself is small.

On the other hand, HC and SOF accumulated from the second supporting layer to the first supporting layer at low temperature are
It is efficiently oxidized and purified by the catalytic metal of the supporting layer. Further, the SO ₂ accumulated in the second supporting layer is released as it is from the second supporting layer without the second supporting layer becoming a diffusion resistance layer and reaching the first supporting layer, and thus without generating sulfate. That is, the discharge of particulates caused by HC, SOF, and SO ₂ accumulated at low temperature is also greatly suppressed.

Further, in the oxidation catalyst of the present invention, the catalytic metal is supported on the first supporting layer having a low adsorptivity, and the catalyst metal has a high adsorptivity.
Separated from the carrier layer. Therefore, the reaction between SO ₂ and alumina is unlikely to occur, and even if it occurs, it does not affect the catalytic metal. In addition, since poisoning substances such as P, S, and Zn that deactivate the oxidation activity of the catalyst metal are adsorbed on the second support layer, poisoning of the catalyst metal is prevented and the oxidizing power of HC and SOF is improved. The performance continues forever.

[0020]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. (Embodiment) FIG. 1 shows an enlarged cross-sectional view of a main part of an exhaust gas oxidation catalyst according to an embodiment of the present invention. This oxidation catalyst comprises a honeycomb-shaped carrier base material (1) made of cordierite, a first supporting layer (2) formed by coating on the surface of the carrier base material (1),
It is composed of a second supporting layer (3) formed by coating on the surface of the first supporting layer (2).

The carrier substrate (1) is a cylindrical honeycomb carrier having a diameter of 140 mm, a length of 130 mm, a cell number of 400 to 300 cells / in ² , and a cell wall thickness of 0.05 to 0.2 mm. The first support layer of the lower layer (2) is composed of TiO ₂ and Pt, TiO ₂ secondary particles (20) surface Pt (2
1) is highly dispersed and supported. The first supporting layer (2) is formed to have a thickness of 50 g per 1 liter of the carrier substrate (1) and a thickness of 20 μm, and 0.5 g of Pt (21) is supported per 1 liter of the carrier substrate (1). There is. The upper second supporting layer (3) is composed of activated alumina powder (30) only, and is 50 g per 1 liter of the carrier substrate (1) and has a thickness of 20 μm.
Is formed. The first supporting layer (2) was previously made of Pt (2
1) is formed by adhering TiO ₂ powder impregnated and supported on the carrier substrate (1) as a slurry, drying and firing. The second supporting layer (3) is formed by adhering alumina powder as a slurry onto the surface of the first supporting layer (2), drying and firing.

(Comparative Example) FIG. 2 shows a catalyst device of a comparative example. This catalyst device is disclosed in Japanese Patent Laid-Open No. 4-267928, and an upstream catalyst (4) and a downstream catalyst (5) are connected in series to a catalytic converter (6) with a sealing material (6).
0) is stored. The upstream catalyst (4) is made of activated alumina on a carrier substrate similar to that of the embodiment, an upstream coat layer (not shown) is formed, and no catalyst metal is supported. Further, the downstream side catalyst (5) is constituted by forming a downstream side coat layer (not shown) made of TiO ₂ on the same carrier base material as in the example and carrying Pt thereon. The upstream coat layer and the downstream coat layer are formed in an amount of 50 g per 1 liter of the carrier substrate, and Pt is supported in an amount of 0.5 g per liter of the carrier substrate in the downstream catalyst.

(Test Example) The oxidation catalyst and the catalyst device of the above-configured examples and comparative examples were respectively mounted in the exhaust system of the DE, and the exhaust gas temperature was changed while changing the load at a constant engine speed (2000 rpm). The PM (Particulate Matter) purification rate at each exhaust gas temperature was measured under the condition of lowering the temperature from 600 ° C. to 200 ° C. and then again to 600 ° C. The result of the oxidation catalyst of the example is shown in FIG. 3, and the result of the catalyst device of the comparative example is shown in FIG.

From FIGS. 3 and 4, in both the examples and the comparative examples, the PM purification rate is improved as the exhaust gas temperature is lowered. This is due to the SOF adsorption action of the second supporting layer and the upstream catalyst in the low temperature region. However, in the comparative example, the PM purification rate becomes negative when the exhaust gas temperature is changed from 200 ° C. to 600 ° C. again. A negative PM purification rate means that new PM has increased in the catalyst, which means that a large amount of sulfate is produced. However, in the embodiment, the exhaust gas temperature is changed from 200 ° C to 600 ° C again.
However, the PM purification rate is zero, and almost no sulfate is generated.

If you look closely, 600 ° C, 500 ° C,
The PM purification rates at 400 ° C. and 300 ° C. are higher in the catalyst of the example than in the comparative example. It is considered that this is because the catalyst of the example has less sulfur poisoning. From the test results, the catalytic action in Examples and Comparative Examples is explained as follows.

<Oxidation catalyst of Examples> At the initial 600 ° C. and 500 ° C., HC, SOF and SO in exhaust gas
_{Although 2} is adsorbed on the second supporting layer (2), the flow velocity of the exhaust gas is sufficiently high, so that the time for diffusing to the oxidation point of the first supporting layer (20) where Pt (21) exists is insufficient, and
And the probability of SOF being oxidized is low. However, SO ₂ in the exhaust gas is not oxidized but is discharged as it is, and the production of sulfate is small. Therefore, although the PM purification rate is relatively small, it shows a positive purification rate.

Next, in the exhaust gas low temperature range of 400 to 200 ° C., HC and SOF are adsorbed on the second supporting layer (3) as shown in FIG. 5, and since this is a liquid phase, a considerable part is caused by the capillary phenomenon. It moves to the first supporting layer (2) and is adsorbed. On the other hand, SO ₂ in the exhaust gas is also used as the second supporting layer (3).
However, since it is adsorbed in the gas phase, it is difficult to move to the first supporting layer (2), and some of them are discharged as they are. Therefore, due to the accumulation of HC and SOF by adsorption and the absence of sulfate formation,
As shown in, the PM purification rate is sufficiently high.

The reason why the PM purification rate at 600 ° C. is low is that sulphate is produced, and the reason why the PM purification rate is improved as the temperature is lowered is that the sulphate production amount is decreased. And the exhaust gas temperature is 200
When the temperature rises from 600 ° C to 600 ° C, the HC and SOF adsorbed / accumulated on the first supporting layer (2) are gasified and oxidatively purified by Pt (21) of the first supporting layer (2), and CO ₂ and H _{2 2} O is discharged. The HC and SOF adsorbed / accumulated on the second support layer (3) are discharged as they are, but the amount thereof is small. On the other hand, SO ₂ adsorbed on the second supporting layer (3) is absorbed by Pt (2
Without being in contact with 1), that is, without being oxidized, it is released from the second support layer (3) as it is, and almost no sulfate is generated. Further, since the first supporting layer (2) hardly contacts SO ₂ , sulfur poisoning of Pt (21) is also prevented. Therefore, from 200 ℃ again 6
When the temperature is raised to 00 ° C., the purification amount of HC and SOF, the emission amount of HC and SOF, and the production amount of sulfate are offset, and the PM purification rate becomes almost zero, but it does not become negative. <Catalyst device of comparative example> On the other hand, in the catalyst device of the comparative example, H in the exhaust gas was initially at 600 ° C and 500 ° C.
Although C, SOF, and SO ₂ are adsorbed by the upstream catalyst (4), they flow into the downstream catalyst (5) where Pt exists because the flow velocity of the exhaust gas is sufficiently high. However, since the downstream catalyst (5) has low adsorptivity, most of it is not adsorbed and a part of HC and SOF is oxidized by Pt and discharged. The SO ₂ in the exhaust gas is not oxidized, and most of it is discharged as it is, so that the production of sulfate is small. Therefore, although the PM purification rate is relatively small, it shows a positive purification rate.

Next, in the exhaust gas low temperature range of 400 to 200 ° C., HC and SOF are adsorbed by the upstream catalyst (4) as shown in FIG. Most of SO _{2 is} also adsorbed on the upstream catalyst (4) and flows to the downstream as it is. Therefore, the PM purification rate in the low temperature range is not different from that of the example, and shows a high PM purification rate. The PM purification rate at 600 ° C. is low because sulfate generation occurs, and the reason why the PM purification rate improves with temperature decrease is that the sulfate production amount decreases. However, the reason why the PM purification rate at 600 to 300 ° C. is lower than that in the example is due to the decrease in the SOF purification rate due to sulfur poisoning.

However, the temperature of the exhaust gas is 200 ° C. to 600 ° C.
Becomes, the adsorbed HC, SOF, and SO_TwoIs above
Emitted from the flow side catalyst (4) and flows into the downstream side catalyst (5)
I do. Then, the Pt in the downstream catalyst (5) comes into contact with the HC,
And SOF are oxidized to CO _TwoAnd H_TwoDischarge as O
Is done. However, the large amount accumulated in the upstream catalyst (4)
SO_TwoIs also oxidized, producing a large amount of sulfate
I will Therefore, in the comparative example, the temperature rises from 200 ° C to 600 ° C.
When heated, the amount of sulfate produced will be higher than HC and SOF.
The PM purification rate becomes a negative value.
U.

That is, according to the oxidation catalyst of the present embodiment, SO ₂ adsorbed and accumulated at low temperature can be discharged at high temperature without generating sulfate, and PM can be purified in a wide temperature window from low temperature to high temperature. .

[0032]

According to the oxidation catalyst for diesel exhaust gas of the present invention, SO ₂ adsorbed and accumulated at a low temperature hardly produces sulfate at a high temperature. Therefore, SO ₂ in the exhaust gas can be emitted almost as it is, and H 2
Since only C and SOF can be efficiently purified, a high PM purification rate can be obtained from a low temperature to a high temperature, and the temperature window for PM purification is expanded.

Further, since poisoning substances such as P, S and Zn which deactivate the oxidizing activity of the catalytic metal are adsorbed on the second supporting layer, it is possible to prevent the oxidizing power of the catalytic metal from lowering. Therefore, in the oxidation catalyst of the present invention, even if the contact area with the exhaust gas is made smaller than in the conventional case, purification performance equal to or higher than that in the conventional case can be obtained, so that the catalyst volume can be made small, and in terms of weight saving and arrangement space. It is advantageous.

[Brief description of the drawings]

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

FIG. 2 is a front view of a catalyst device of a comparative example.

FIG. 3 is an exhaust gas temperature and P of an oxidation catalyst according to an embodiment of the present invention.
It is a graph which shows the relationship with M purification rate.

FIG. 4 is a graph showing a relationship between an exhaust gas temperature and a PM purification rate of a catalyst device of a comparative example.

FIG. 5 is an explanatory diagram showing a purification mechanism of an oxidation catalyst according to an embodiment of the present invention.

FIG. 6 is an explanatory view showing a purifying mechanism of a catalyst device of a comparative example.

FIG. 7 is a graph showing the correlation between the adsorption amount of HC and SOF and the adsorption amount of SO ₂ .

[Explanation of symbols]

1: carrier substrate 2: first support layer
3: Second support layer 30: Alumina 20: TiO ₂
21: Pt

Claims (1)

[Claims] 1. A carrier substrate, a first carrier layer formed on the surface of the carrier substrate and carrying a catalytic metal, and a second carrier layer formed on the surface of the first carrier layer and carrying no catalytic metal. The oxidation catalyst for diesel exhaust gas, wherein the first support layer has lower adsorption of components in diesel exhaust gas than the second support layer.