JPH06198181A - Catalyst for purifying exhaust gas from diesel engine - Google Patents

Abstract

PURPOSE:To prevent the release of particulates and sulfate and to efficiently remove HC and SOF by combustion by allowing a platinum catalyst and a palladium catalyst to be deposited respectively at the upstream side and the downstream side of the exhaust gas flow in allowing a catalytic metal to be deposited on a catalyst carrier layer. CONSTITUTION:The catalyst for purifying is constituted of a 1st catalyst 1 at the upstream side and a 2nd catalyst at the downstream side, which are placed and arranged in a series parallel to the exhaust gas flow in a case. The 1st catalyst has a 1st catalyst carrier layer 11 composed of titania or the like applied on the surface of a carrier base body 10 of a honeycomb carrier and is made by allowing the palladium catalyst 12 and a rhodium catalyst 13 to be deposited on the layer 11. The platinum catalyst 15 is allowed to be deposited on a 2nd catalyst carrier layer 14 applied on the surface of the 1st catalyst carrier layer 11. The 2nd catalyst 2 similarly has a 2nd catalyst carrier layer 21 composed of gamma-alumina or the like applied on a carrier base body 20 composed of a honeycomb carrier and is made by allowing a radium catalyst 22 and the rhodium catalyst 23 to be deposited on the layer 21.

Description

Detailed Description of the Invention

[0001]

The present invention relates to H contained in exhaust gas from a diesel engine (hereinafter referred to as DE).
The present invention relates to a catalyst that burns and purifies C, CO and SOF (Soluble Organic Fraction), and can reduce the emission amount of diesel particulates and also the emission amount of sulfates.

[0002]

2. Description of the Related Art With respect to gasoline engines, toxic substances in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology capable of coping with the regulations. But DE
With regard to the above, due to the peculiar circumstances in which harmful components are mainly discharged as particulates, the regulations and the development of technology are behind that of the gasoline engine, and the development of an exhaust gas purification device that can surely purify is desired.

As a DE exhaust gas purifying apparatus which has been developed up to now, 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 that uses a trap is
It traps diesel particulates to control emissions, and is particularly effective for exhaust gas with a high proportion of dry suits. However, a regeneration treatment device is required, and many problems remain for practical use, such as cracking of the catalyst carrier during regeneration, blockage due to ash, or complicated system.

On the other hand, the open type SOF decomposition catalyst is, for example, as disclosed in Japanese Patent Application Laid-Open No. 3-38255, a catalyst in which an oxidation catalyst such as a platinum group metal is supported on a catalyst supporting layer such as activated alumina as in a gasoline engine. Used, C
SO in diesel particulate along with O and HC
F is oxidatively decomposed and purified. This open type SOF decomposition catalyst has the drawback that the removal rate of dry suit is low, but the amount of dry suit can be reduced by improving DE and fuel itself, and a great advantage that a regeneration treatment device is unnecessary. Therefore, further improvement of technology is expected in the future.

[0005]

Open type S
Emission of diesel particulates in OF decomposition catalyst
It was difficult to reduce the amount. Ie open
The activated alumina layer formed on the type SOF decomposition catalyst is
SO₂Has the property of adsorbing. Therefore, the elimination of DE
SO contained in gas₂Is adsorbed on the activated alumina layer
When the temperature of the catalyst rises, the SO₂
Are oxidized by the catalytic action of the catalytic metal. Or exhaust
SO in gas₂Are directly oxidized. Therefore SO₂Is
SO₃The amount of particulates increases because it is discharged as
There is a problem of doing. This is SO ₂Is a particulate
SO is not measured as₃Is particulate
This is because it is measured as. Exhaust especially in DE
Sufficient oxygen gas in the gas, SO₂The oxidation reaction of
Easy to get rid of. And SO₃Exists in exhaust gas in large quantities
Reacts readily with water vapor to form sulfate.

The present invention has been made in view of the above circumstances, and an object thereof is to prevent the release of particulates and sulfates and to efficiently burn and remove HC and SOF.

[0007]

[Means for Solving the Problems] First to solve the above problems
An exhaust gas purification catalyst for a diesel engine of the invention is a catalyst for exhaust gas purification of a diesel engine, which comprises a carrier substrate, a catalyst supporting layer formed on the surface of the carrier substrate, and a catalyst metal supported on the catalyst supporting layer. The platinum catalyst is carried on the upstream side of the exhaust gas flow, and the palladium catalyst is carried on the downstream side of the platinum catalyst carrying position.

By the way, the platinum catalyst has strong low-temperature activity and is excellent in purification performance at the time of starting and running at low speed. However, on the other hand, SO ₂ also oxidizes at a low temperature and actively produces sulfate at 300 ° C. or higher. On the other hand, the palladium catalyst has a low purification performance at a low temperature, but exhibits a sufficient purification performance at a high temperature, and the temperature for oxidizing SO ₂ is about 450 ° C. or higher, which is considerably higher than that of the platinum catalyst.

Therefore, in the present invention, the platinum catalyst is loaded on the upstream side and the palladium catalyst is loaded on the downstream side of the platinum catalyst loading position. In this way, when the exhaust gas temperature is as low as about 300 ° C., the platinum catalyst on the upstream side contributes to purification of HC and SOF, and oxidation of SO ₂ does not occur. In addition, since the reaction heat of the catalytic reaction causes the temperature of the downstream side of the carrier substrate to become higher than that of the upstream side, if platinum catalyst is also supported on the downstream side, SO ₂ oxidation occurs there, and sulfate is generated early. Will end up. However, in the present invention, since the palladium catalyst is supported on the downstream side, the formation of sulfate is prevented until the temperature reaches about 450 ° C or higher. Therefore, the release of sulfate at low temperatures is prevented.

It is desirable that the volume ratio of the platinum support portion on the upstream side to the palladium support portion on the downstream side be in the range of 1:10 to 5:10. This is because if the volume ratio on the upstream side is further increased, the amount of sulfate produced increases, and if the volume ratio on the upstream side is smaller than this, the oxidation activity decreases. As the carrier substrate, those conventionally used such as ceramic monolith carriers, ceramic foam carriers, and metal honeycomb carriers can be used.

The catalyst supporting layer may be made of alumina, silica, titania or the like, which has the same structure as the conventional one. In addition,
Silica or titania other than alumina may be used for the downstream catalyst supporting layer on which the palladium catalyst is supported. Since silica and titania do not adsorb SO ₂ , the formation of sulfate is prevented. Silica and titania do not have the disadvantage of covering the palladium catalyst found in alumina and lowering the activity, and can prevent poisoning deterioration of the catalyst metal.

However, it has been clarified that the palladium catalyst has a poor compatibility with silica and that the coexistence with silica tends to lower the oxidation activity. Therefore, claim 3
As described in (1), it is desirable that the palladium catalyst is supported on the mixed catalyst supporting layer of silica and alumina. This can reduce the above problems. In this mixed catalyst supporting layer, the mixing ratio of silica and alumina is preferably in the range of silica: alumina = 9: 1 to 1: 1 by weight. If the proportion of alumina is higher than this, the performance deterioration due to poisoning of the palladium catalyst due to the adsorption of sulfate becomes large, and if the proportion of silica is higher than this, the oxidation activity decreases. Further, when alumina is used for the catalyst supporting layer on the downstream side, it is desirable to use one having a particle size of 0.1 to 10 μm, preferably 0.5 to 5 μm.

However, in the catalyst in which platinum and palladium are separately supported in this way, it is assumed that white smoke is emitted from the exhaust pipe at the time of acceleration from low speed to high speed. The cause of generation of this white smoke is estimated as follows.
That is, HC and SOF are adsorbed on the activated alumina layer at the time of idle when the exhaust gas temperature is low and the gas flow velocity is low.
At the time of acceleration when the exhaust gas temperature is high and the gas flow velocity is high, the adsorbed HC and SOF are desorbed and oxidized by the catalyst metal. Here, the platinum catalyst burns HC and SOF from a low temperature, but when the palladium catalyst has not yet reached the activation temperature, HC and S are fed from the palladium catalyst supporting portion.
OF is discharged in an incomplete combustion state, and this is emitted as white smoke.

Therefore, the exhaust gas purifying catalyst of the second invention is characterized in that a platinum catalyst is further carried on the downstream side of the carrying position of the palladium catalyst carried on the catalyst of the first invention. Due to the platinum catalyst on the most downstream side, H in an incomplete combustion state discharged from the palladium catalyst supporting portion.
Since C and SOF are burned and removed, emission of white smoke is prevented.

The amount of platinum catalyst supported is H, which is white smoke.
It is desirable to minimize the amount of C and SOF within the range where they can be burned and removed. In this case, the carrying amount per unit volume of the carrier substrate is not reduced, but the carrying amount per unit volume is constant, and for example, it is provided only in a portion 5 to 10% of the entire length of the carrier substrate from the end face. It is desirable to reduce the supporting area. This prevents HC, S while preventing the oxidation of SO _2.
It becomes easy to remove OF.

Depending on the purpose, various catalytic metals other than platinum and palladium can be used in combination.

[0017]

In the DE exhaust purification catalyst of the present invention, the upstream side end portion first comes into contact with the exhaust gas during use. SOF is particularly likely to be adsorbed to that part, but HC and SOF are burned and removed by the platinum catalyst that is carried on that part and has excellent low-temperature activity.
Accumulation of C and SOF is prevented. Therefore, at least at the upstream end where the platinum catalyst is supported, HC, SO
Temporary poisoning by F is prevented and the catalytic activity is not lost.

Then, in the portion following the upstream end, SO ₂
Since the oxidation reaction of Pd is carried at a temperature higher than that of the platinum catalyst, SO ₂ is released without being oxidized at a low temperature, and compared with the conventional exhaust purification catalyst in which the platinum catalyst is carried throughout Release of particulates and sulphates is prevented. However, the palladium catalyst has a small activity for HC and SOF at low temperature, and HC and SOF which have not been removed by the catalyst supporting layer are easily adsorbed at the palladium catalyst supporting position. Therefore, when the exhaust gas with a high flow velocity flows at high temperature during acceleration, the absorbed H
C and SOF are released to generate white smoke. Therefore, if a platinum catalyst is also loaded on the downstream side of the palladium catalyst loading position, the desorbed HC and SOF are burned and removed by the platinum catalyst on the downstream side, and are prevented from being released to the outside in an incomplete combustion state. It This prevents white smoke.

When the palladium catalyst is supported on the mixed catalyst supporting layer of silica and alumina, the adsorption of the sulfate and the oxidizing activity of the palladium catalyst and the adsorptivity of the SOF are balanced, and preferable catalytic properties are exhibited.

[0020]

EXAMPLES The present invention will be specifically described below with reference to examples. (1) First Embodiment (Embodiment 1) FIGS. 1 and 2 are schematic vertical sectional views of an exhaust gas purification catalyst for DE according to the present embodiment. The purification catalyst is composed of an upstream first catalyst 1 and a downstream second catalyst 2 which are arranged in parallel in series in the case 100 in parallel with the exhaust gas flow.

The first catalyst 1 arranged on the upstream side of the exhaust gas flow is a carrier substrate 10 made of a cordierite honeycomb carrier, and a first catalyst supporting layer 11 made of titania coated on the surface of the carrier substrate 10. A palladium catalyst 12 and a rhodium catalyst 13 supported on the first catalyst supporting layer 11,
It is composed of a second catalyst supporting layer 14 made of γ-alumina coated on the surface of the catalyst supporting layer 11 and a platinum catalyst 15 supported on the second catalyst supporting layer 14.

The second catalyst 2 arranged on the downstream side includes a carrier substrate 20 made of a cordierite honeycomb carrier,
It is composed of a first catalyst supporting layer 21 made of titania coated on the surface of the carrier substrate 20, a palladium catalyst 22 and a rhodium catalyst 23 supported on the first catalyst supporting layer 21. As shown in Table 1, the second catalyst-supporting layer 14 existing on the outermost surface of the first catalyst 1 is formed at 100 g / liter. The platinum catalyst 1 is formed on the second catalyst supporting layer 14.
5 is carried at 1.5 g / liter.

The first catalyst supporting layer 21 existing on the outermost surface of the second catalyst 2 is formed to 100 g / liter, and the palladium catalyst 22 is 1.0 g / liter and the rhodium catalyst 23 is 0 in the first catalyst supporting layer 21. 0.1 g / liter is supported. The DE exhaust purification catalyst of Example 1 was used as a DE with a naturally aspirated sub-chamber (displacement: 3.6 liters).
It was attached to the exhaust system of and the purification test was performed. In the purification test, the first catalyst 1 is arranged on the upstream side of the exhaust gas flow, the second catalyst 2 is arranged on the downstream side, and after aging for 100 hours at a full load of the rotation speed of 2100 rpm, the rotation speed of 2100 rpm,
Operated under the conditions of a torque of 87 N · m, the gas temperature at that time was
The particulates before and after the catalyst were analyzed at 0 ° C.

The contents of analysis are the amount of particulates before and after the catalyst, the amount of SOF, and the amount of sulfate. A dilution tunnel conforming to EPA was used to collect the particulates. A Soxhlet extractor was used for SOF extraction, and extraction was performed with dichloromethane. Further, the amount of sulfate was analyzed by ion chromatography. The results are shown in Table 2. (Examples 2 to 9 and Comparative Examples 1 to 5) The material of the carrier substrate, the type and coating amount of the catalyst supporting layer, the type of catalytic metal and its supporting amount, and the volume ratio of the first catalyst 1 and the second catalyst 2 were determined. The configuration is the same as that of the first embodiment except that various changes are made as shown in Table 1. Regarding the catalyst of the comparative example, the first catalyst and the second catalyst
The same catalyst is used as the catalyst. Then, these catalysts were tested in the same manner as in Example 1, and the results are shown in Table 2.

[0025]

[Table 1]

[0026]

[Table 2]

From the (Evaluation) table, the catalyst of the comparative example has a large amount of particulates, a low purification rate of SOF, and a large amount of sulfate. Further, in Comparative Examples 2 and 4 in which the palladium catalyst was supported on the first catalyst, the amount of particulates and the amount of sulfate formation were reduced to some extent, but SOF
Is inferior in purification performance.

On the other hand, the catalysts of the respective Examples are excellent in the performance of purifying particulates and SOF, and produce a small amount of sulfate. It is clear that this is due to the fact that the first catalyst carries Pt and the second catalyst carries Pd. Further, it can be seen that there is almost no influence of the carried amount or the volume ratio.
Although two catalysts were used side by side in the first embodiment, the present invention is not limited to this, and Pt is loaded on the upstream side and Pd is loaded on the downstream side in one honeycomb body. The same effect can be expected even with a supported structure. Further, for example, in Example 1 described above, two supporting layers, the first catalyst supporting layer 11 and the second catalyst supporting layer 14, were formed on the first catalyst 1, but only the second catalyst supporting layer 14 supporting the platinum catalyst 15 is formed. Similar results are obtained. (2) Second Example (Example 10) The catalyst of this example is a catalyst comprising a carrier substrate 3 made of a metal carrier and γ-alumina coated on the surface of the carrier substrate 3 as shown in FIG. The support layer 30 includes a platinum catalyst 31 and a palladium catalyst 32 which are supported on the catalyst support layer 30. The carrier substrate 3 is Fe-C.
It is composed of a flat plate made of an r-Al alloy having a plate thickness of 50 μm and a metal carrier formed of a corrugated plate having a diameter of 112 mm and a length of 118 mm.

The surface of the carrier substrate 3 has a total of 150
A catalyst supporting layer 3 coated with g / liter of γ-alumina
0 is formed. The platinum catalyst 31 is supported only within a range of 10 mm from the upstream end surface 33 of the carrier substrate 3 toward the downstream side and within a range of 10 mm from the downstream end surface 34 toward the upstream side, and the supported amount is 1 respectively. It is 0.5 g / liter.

The palladium catalyst 32 is the platinum catalyst 31.
Is carried on the remaining center portion 35 of 98 mm, and the carried amount is 1.8 g / liter. (Embodiment 11) 1.5 g of platinum catalyst is provided only on the upstream end.
The composition is the same as that of Example 10 except that the palladium catalyst was loaded on the remaining 108 mm at a loading amount of 1.8 g / liter. (Comparative Example 6) The structure is the same as that of Example 10 except that the palladium catalyst was not used and the platinum catalyst was uniformly supported on the entire surface at a supported amount of 1.5 g / liter. (Test) The purification catalysts of Examples 10 and 11 and Comparative Example 6 were attached to an exhaust system of a swirl chamber type DE having an exhaust volume of 2400 cc, and SO ₂ SO ₂ when the exhaust gas temperature was 500 ° C. The conversion rate to ₃ was determined by the following formula using a non-dispersive infrared analyzer. The results are shown in Table 3.

Conversion rate to SO ₃ = (SO ₂ concentration of incoming gas−SO ₃ concentration of outgoing gas) × 100 / SO ₂ concentration of incoming gas

[0032]

[Table 3] From Table 3, in Comparative Example 6, since only the platinum catalyst was supported, the SO ₃ conversion rate was high and the sulfate was released. However, in the purification catalysts of Example 10 and Example 11,
Since the palladium catalyst is supported, the conversion rate is low and good results are shown.

Next, each of the purifying catalysts has a displacement of 24
After being attached to the exhaust system of a 00cc swirl chamber type DE and held in an idle state (exhaust gas temperature of 120 ° C) for 6 hours, the engine speed is increased to raise the exhaust gas temperature and the white smoke of gas discharged from the exhaust pipe. The quantities were measured individually. The results are shown in Fig. 4. This test was performed by changing the exhaust gas temperature at the time of measurement. The level of white smoke was measured with a transmission white smoke meter.

From FIG. 4, since the purification catalyst of Comparative Example 6 has the platinum catalyst supported on the entire surface, HC and SOF were completely burned and no white smoke was generated. On the other hand, in the purifying catalyst of Example 11, a particularly large amount of white smoke was emitted when the exhaust gas temperature was around 300 ° C., whereas in the purifying catalyst of Example 10, the amount of white smoke was significantly higher than that of Example 11. Has been reduced to. It is clear that this is an effect of supporting the platinum catalyst also on the downstream end.

In the second embodiment, γ-alumina was used for the catalyst supporting layer of the palladium catalyst, but if a mixed supporting layer of silica and alumina is used, the release of sulfate can be further prevented. Further, even if the catalyst supporting layer of the platinum catalyst is α-alumina and the vanadium catalyst is added to the catalyst metal, the release of sulfate can be further prevented. (3) Third Example (Preparation of slurry) Commercially available silica powder (specific surface area 350
m ² / g), an aqueous solution of aluminum nitrate, an alumina sol, and distilled water were prepared as a slurry A.

Commercially available activated alumina powder (specific surface area 180
m ² / g, particle size 1 to 50 μm), a slurry composed of alumina hydrate, an aqueous solution of aluminum nitrate and distilled water was prepared and designated as a slurry B. Commercially available silica powder (specific surface area 3
50 m ² / g), a slurry comprising silica sol and distilled water was prepared and designated as slurry C. Example 12 Commercially available φ30 × 50,400 cells / in
The honeycomb carrier of No. ² was subjected to water absorption treatment, immersed in the above slurry A, and the excess slurry was blown off, followed by drying and firing to form a silica-alumina mixed catalyst supporting layer. The coating amount at this time was about 100 g per liter of catalyst volume, the weight ratio of silica to alumina was 9: 1, and the particle size of alumina was 5 μm or less.

This carrier was immersed in an aqueous solution of ammine salt of palladium and rhodium, adsorbed and supported for a predetermined time, and then 25
It was dried at 0 ° C. The loading amount of palladium was 1.5 g and rhodium was 0.1 g per liter of the catalyst volume.
It was 3 g. Next, a slurry is prepared by adding a predetermined amount of platinum dinitrodiammine to the above slurry B, and only the inlet side end portion of this catalyst is immersed and the excess slurry is blown off, followed by drying and firing to the inlet side end surface portion. A catalyst supporting layer made of alumina and supporting a platinum catalyst was formed.

In the obtained exhaust gas purification catalyst of this embodiment,
An upstream part made of Pt / Al ₂ O ₃ is formed on the surface of the inlet side (upstream side), and a downstream part made of Pd-Rh / SiO ₂ · Al ₂ O ₃ is formed on the outlet side (downstream side). There is. The volume ratio between the upstream part and the downstream part is 1:10, and the amount of platinum supported on the upstream part is 1.5 g per 1 liter of the catalyst volume. (Example 13) The structure is the same as that of Example 12 except that the volume ratio of the upstream portion and the downstream portion is 3:10. (Example 14) The structure is the same as that of Example 12 except that the volume ratio of the upstream portion and the downstream portion is 5:10. (Example 15) The volume ratio of the upstream part and the downstream part is 0.5: 10.
The configuration is the same as that of the twelfth embodiment except that (Example 16) The structure is the same as that of Example 12 except that the volume ratio of the upstream portion and the downstream portion is 1: 1. (Example 17) A catalyst was prepared in the same manner as in Example 12 except that a slurry in which the ratio of the alumina sol of the slurry A was increased was used. In this catalyst, the weight ratio of silica to alumina in the catalyst supporting layer at the downstream portion is 7: 3, and the volume ratio of the upstream portion and the downstream portion is 3:10. (Example 18) The structure is the same as that of Example 17 except that the weight ratio of silica to alumina in the catalyst supporting layer at the downstream portion is 1: 1. (Example 19) The structure is the same as that of Example 17 except that the weight ratio of silica to alumina in the catalyst supporting layer at the downstream portion is 3: 7. (Example 20) Example 17 except that the weight ratio of silica to alumina in the catalyst carrying layer in the downstream part was 9.5: 0.5.
It has the same configuration as. (Example 21) A catalyst was prepared in the same manner as in Example 12 except that a slurry in which an alumina hydrate was used instead of the alumina sol of the slurry A was used. In this catalyst, the weight ratio of silica to alumina in the catalyst supporting layer in the downstream part is 7: 3.
And the volume ratio between the upstream part and the downstream part is 3:10. Further, the particle size of alumina in the mixed catalyst supporting layer in the downstream portion is 1 to 1
It was 0 μm. Example 22 A catalyst was prepared in the same manner as in Example 12 except that slurry A was mixed with activated alumina powder having a particle size of 10 to 50 μm. The other structure is the same as that of the twenty-first embodiment. (Comparative Example 7) A catalyst supporting layer made of alumina was formed on a honeycomb carrier by using the slurry B, and palladium and rhodium were supported on the entire catalyst supporting layer, and platinum was not supported. It has the same configuration. The amount of palladium carried is 1.5 g / liter, and the amount of rhodium carried is 0.3 g / liter. (Comparative Example 8) 1.5 g instead of palladium-rhodium
The structure is the same as that of Comparative Example 7 except that platinum / liter of platinum is carried. (Comparative Example 9) A catalyst was prepared in the same manner as in Example 17 except that Slurry B was used instead of Slurry A. That is, the structure is the same as that of Example 17 except that the catalyst carrying layer at the downstream portion is made of alumina. (Comparative Example 10) A catalyst was prepared in the same manner as in Example 17 except that Slurry C was used instead of Slurry A.
That is, the structure is the same as that of Example 17 except that the catalyst carrying layer at the downstream portion is made of silica.

Examples 12 to 22 and Comparative Examples 7 to 10
Table 4 summarizes the composition of the catalyst.

[0040]

[Table 4]

(Test 1) Example 13 and Examples 17 to 22
Also, a multi-converter capable of accommodating 12 catalysts of Comparative Examples 9 to 10 was assembled, and a swirl chamber type D having a displacement of 2.4 liters was used.
It was attached to the exhaust system of E. Then, it was operated at 2000 rpm and an inlet gas temperature of 200 ° C. for 10 hours, and the catalyst weight before and after the test was measured to calculate the SOF adsorption amount of each catalyst. In this test, since the incoming gas temperature was 200 ° C., almost no sulfate was generated, and the weight increase amount can be conveniently taken as the SOF adsorption amount.

Next, an endurance test was conducted by operating the above catalyst at 2000 rpm for 100 hours at an inlet gas temperature of 400 ° C., and then C ₂ H ₄ 50 in a C ₂ H ₄ --CO--O ₂ --H ₂ O model gas. The% purification rate temperature was measured. Under this operating condition, the sulfate generated by the catalyst is adsorbed and accumulated in the catalyst supporting layer, which causes a decrease in HC purification performance. The results obtained are shown in Table 5.

[0043]

[Table 5] As is clear from Table 5, as the ratio of alumina in the downstream portion increases, the adsorption of SOF improves and
It can be seen that the SOF adsorption property is rapidly improved only by mixing about 0%.

On the other hand, the HC purification activity is basically better as the proportion of alumina is higher, but the amount of sulfate adsorbed also increases as the proportion of alumina increases, and the HC purification performance deteriorates due to sulfate poisoning. I also understand. That is, in the catalyst of this example, the downstream portion supporting palladium was a mixed catalyst supporting layer of silica and alumina, so that SO
It is clear that the F adsorption performance and the sulfate poisoning resistance are compatible.

From comparison of Examples 17, 21, and 22,
As the alumina particle size increases, SOF adsorption performance and
It can be seen that the HC purification activity has decreased, and it is less than 10 μm.
It is preferable to use the lower alumina. This is aluminum
The smaller the particle size, the higher the dispersion of the catalytic metal, which
It is considered that the number of SOF adsorption sites increases with
It (Test 2) The catalysts of Examples 12 to 16 and Comparative Examples 7 to 8 were used.
Used, N₂-O₂-SO₂-H ₂In O system model gas
The second conversion rate of sulfate at 430 ℃
It was determined in the same manner as in the example. Also, C₂H_Four-CO-O
₂-H₂C in O-based model gas₂H_Four50% purification rate temperature
The degree was also measured. The results are shown in Table 6.

[0046]

[Table 6] From the results of Comparative Examples 7 and 8, the upstream part alone has a high oxidative activity but a high sulfate conversion rate, and the downstream part alone has a low sulphate conversion rate but a low oxidative activity.

However, it is clear that both performances are compatible by forming the downstream portion on the downstream side of the upstream portion. In addition, the catalyst of Example 15 is inferior in oxidation activity to the catalysts of Examples 12 to 14, and the catalyst of Example 16 has a high sulfate conversion rate. Therefore, the volume ratio of the upstream portion and the downstream portion is 1:10 to 5. It is understood that the range of 10 is desirable. By setting the volume of the platinum-supported upstream portion within this range, it becomes possible to improve the oxidation activity for HC as much as the conventional catalyst that supports the platinum catalyst alone while suppressing the amount of sulfate production.

This is because the sulfate formation reaction is more dominant in SV (void velocity) than the HC oxidation reaction, and the sulfate formation reaction is suppressed by increasing the SV by limiting the volume of the platinum-supported upstream portion. It is considered that it is done.

[0049]

[Effects of the Invention] That is, according to the exhaust gas purification catalyst of DE of the present invention, the purification performance of SOF is excellent and the release of particulates and sulfates is prevented.
According to the catalyst of the second aspect of the invention, in addition to the above characteristics, the emission of white smoke due to incomplete combustion HC and SOF is prevented.

Further, since the absolute value of the amount of the platinum catalyst carried is smaller than that of the conventional one, N which is likely to occur in the high temperature state of the catalyst is N.
There is also an additional effect that the generation of O ₂ is reduced and the so-called idle acid odor after running at high speed is prevented. Furthermore the third
According to the catalyst of the present invention, both SOF adsorption performance and sulfate poisoning resistance can be made compatible, and excellent catalyst performance is exhibited.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing the overall structure of an exhaust gas purification catalyst according to a first embodiment of the present invention.

FIG. 2 is an enlarged sectional view of an essential part of an exhaust gas purification catalyst according to a first embodiment of the present invention.

FIG. 3 is an enlarged sectional view of an essential part of an exhaust gas purification catalyst according to a second embodiment of the present invention.

FIG. 4 is a graph showing the relationship between exhaust gas temperature and amount of white smoke in the second embodiment.

[Explanation of symbols]

1: 1st catalyst 2: 2nd catalyst 3,10,
20: Carrier substrate 11, 14, 21, 30: Catalyst support layer 12, 2
2, 32: Palladium catalyst 13, 23: Rhodium catalyst 15, 3
1: Platinum catalyst 33: Upstream end face 34: Downstream end face 3
5: central part

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location B01J 35/04 301 L 8017-4G (72) Inventor Satoshi Kaneko 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Toshiaki Tanaka 1 Toyota-cho, Toyota City, Aichi Prefecture Toyota Automobile Co., Ltd. (72) Inventor Yukimura Yamada 7800 Chihama, Daito-cho, Ogasa-gun, Shizuoka Cataler Industry Co., Ltd. ( 72) Inventor Norihiko Aono 7800, Chihama, Daito-cho, Ogasa-gun, Shizuoka Prefecture Cataler Industry Co., Ltd.

Claims (3)

[Claims] 1. A catalyst for exhaust emission purification of a diesel engine, comprising a carrier substrate, a catalyst supporting layer formed on the surface of the carrier substrate, and a catalyst metal supported on the catalyst supporting layer, the exhaust gas flow comprising: An exhaust gas purification catalyst for a diesel engine, wherein a platinum catalyst is supported on the upstream side of the catalyst, and a palladium catalyst is supported on the downstream side of the position where the platinum catalyst is supported. 2. The catalyst for purifying exhaust gas of a diesel engine according to claim 1, wherein a platinum catalyst is further loaded on the downstream side of the palladium catalyst loading position. 3. The platinum catalyst is supported on a catalyst supporting layer made of alumina, and the palladium catalyst is supported on a mixed catalyst supporting layer made of silica and alumina. 2. A catalyst for purifying exhaust gas of a diesel engine according to 2.