JPH07166854A - Exhaust emission controlling catalytic structure - Google Patents

Abstract

PURPOSE:To provide a high purifying rate of exhaust gas even in the case that a nature of exhaust gas is frequently changed. CONSTITUTION:The first/second catalysts 3, 4 are arranged in series so as to place the former in the downstream and the latter in the upstream, in a flow direction of exhaust gas. An activation temperature range of the first catalyst 3 is wide in a low temperature side from that of the second catalyst 4, and on the other hand, an activation temperature range of the second catalyst 11 is wide in a high temperature side from that of the first catalyst 3, further with mutual activation temperature range partly lapped of both the catalysts 3, 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying catalyst structure.

[0002]

2. Description of the Related Art As a catalyst for purifying engine exhaust gas,
A three-way catalyst that simultaneously oxidizes CO and HC and reduces NOx is known. This is a system in which a precious metal such as Pt is supported on γ-alumina and shows a high purification rate when the engine air-fuel ratio (A / F) is the theoretical air-fuel ratio, but the oxygen concentration in the exhaust gas is When it gets higher, NO
The purification rate of x sharply decreases.

On the other hand, as a NOx purification catalyst,
An oxidation catalyst for oxidizing NO in exhaust gas to NO ₂ is arranged on the upstream side in the exhaust gas flow direction, and a second catalyst made of zeolite for adsorbing / decomposing NO ₂ is arranged on the downstream side,
It is known that NOx is efficiently decomposed and removed (see JP-A-63-49234). In this case, the activation temperature of the first catalyst is higher than that of the second catalyst.

[0004]

However, as described above,
In the case where the NOx oxidation catalyst is arranged on the upstream side in the exhaust gas flow direction and the adsorption / decomposition catalyst is arranged on the downstream side, the exhaust gas passes through the upstream catalyst at the activation temperature of the catalyst and the downstream catalyst. Must be passed at the activation temperature of the catalyst, but it is difficult to bring the exhaust gas to such temperature conditions. Particularly, in the case of an automobile engine, the operating conditions thereof change every moment, so that the exhaust gas rarely reaches the above temperature condition, and the desired exhaust gas purification rate cannot be obtained.

[0005]

Means for Solving the Problem and Its Action As a result of intensive studies on such problems, the present inventor has found that a catalyst in which a noble metal active species is supported on an active species supporting base material such as zeolite has an active temperature The inventors have found that a combination of a high temperature active catalyst and a low temperature active catalyst whose regions partially overlap each other increases the exhaust gas purification rate, and has completed the present invention.

-Invention of Claim 1- That is, the invention of Claim 1 which solves the above problems is
One of the first exhaust gas purifying catalyst and the second exhaust gas purifying catalyst, each of which has a precious metal active species supported on the active species-supporting base material, is on the upstream side in the exhaust gas flow direction, and the other is on the downstream side. The first exhaust gas purifying catalyst has a wider active temperature range on the low temperature side than the second exhaust gas purifying catalyst, and the second exhaust gas purifying catalyst is more than the first exhaust gas purifying catalyst. The exhaust gas purifying catalyst structure is characterized in that the active temperature range is wide toward the high temperature side, and the active temperature ranges of both the exhaust gas purifying catalysts partially overlap each other.

In the present invention, the first exhaust gas purifying catalyst and the second exhaust gas purifying catalyst partially overlap in the activation temperature range, but the former is a low temperature active catalyst and the latter is a high temperature active catalyst. Since they are the catalysts, the total activation temperature range of the both is wider than that of the case where each is used alone. And
Since the activation temperature regions of both catalysts partially overlap, the purification rate of the exhaust gas is further increased in this overlap region.

In this case, when the low temperature active first exhaust gas purifying catalyst and the high temperature active second exhaust gas purifying catalyst are used for purifying exhaust gas of an automobile engine, whichever is located upstream of the exhaust gas flow direction. You may arrange. In the case of an automobile engine, this is because the exhaust gas temperature frequently fluctuates depending on operating conditions (acceleration / deceleration), so that the active temperature range of the first exhaust gas purification catalyst and the second exhaust gas purification catalyst is as described above. This is because there is no great difference in the exhaust gas purification rate regardless of which is on the upstream side. Rather, the exhaust gas purification rate becomes higher as the total activation temperature range of the first exhaust gas purification catalyst and the second exhaust gas purification catalyst combined and the lap region become wider.

Here, as the noble metal active species, P
t, Rh, Ir, and Pd are preferable, and Pt is particularly preferable from the viewpoint of low-temperature activity of the catalyst, and these noble metals can be used in combination. , Pt and Ir are combined,
A combination of Pt, Ir and Rh is preferable, but Pd can be further combined.

As the active species-supporting base material, a porous metal-containing silicate is suitable, but other inorganic porous materials can be used. The above-mentioned porous metal-containing silicate means a crystalline porous body having micropores as represented by aluminosilicate (zeolite) using Al as a metal forming a crystal skeleton. Of course, instead of Al or together with Al, Ga, C
It is also possible to use a metal-containing silicate whose skeleton-forming material is another metal such as e, Mn, or Tb. In addition, regarding aluminosilicate, ZSM-5, ferrierite,
It can be used regardless of the type such as mordenite, A type, X type, Y type, and the Caban ratio is not particularly limited.

With respect to the first exhaust gas purifying catalyst and the second exhaust gas purifying catalyst, however, the former and the second exhaust gas purifying catalyst are made to have the same material composition, and only the latter is subjected to heat treatment, so that the former becomes a low temperature active catalyst. , The latter can be a high temperature active catalyst. For example, Pt as the noble metal active species,
When Ir and Rh are used and a porous metal-containing silicate is used as the active species-supporting base material, the temperature of the second exhaust gas purifying catalyst is 600 to 800 ° C., and the time is 10 to 30.
The heat treatment may be performed for about hr.

In this case, the activation temperature range of the second exhaust gas purifying catalyst becomes higher than that of the first exhaust gas purifying catalyst.
It is considered that the strong oxidizing power of t is suppressed.

Further, the first exhaust gas purifying catalyst and the second exhaust gas purifying catalyst are different from each other in that the material configurations are different from each other as in the inventions according to claim 2 and claim 3. Can be a low temperature active catalyst and the latter a high temperature active catalyst.

-Invention of Claim 2- The invention of Claim 2 which solves the above-mentioned problems is the catalyst structure for purifying exhaust gas according to Claim 1, wherein the first
And the noble metal active species of the second exhaust gas purifying catalyst are P
t, Ir, and Rh, the active species-supporting base material of the first exhaust gas purifying catalyst is a porous metal-containing silicate, and the active species-supporting base material of the second exhaust gas purifying catalyst is porous. It is characterized by being a mixture of a metal-containing silicate and alumina or ceria.

In the present invention, Pt as the noble metal active species has the effect of activating HC in the exhaust gas even at low temperatures, and Ir is the HC activated by Pt by trapping NOx in the exhaust gas. Of the Pt and the suppression of Pt crystal growth contribute to improving the heat resistance of the catalyst. Rh has the function of increasing the heat resistance of the catalyst. Further, the metal-containing silicate advantageously acts on the selective adsorption of NOx. Therefore, the first exhaust gas purification catalyst has high low-temperature activity and excellent heat resistance.

On the other hand, since the second exhaust gas purifying catalyst comprises alumina or ceria in addition to the above metal-containing silicate as an active species supporting base material, its active temperature range is the first.
It is higher than the exhaust gas purification catalyst. That is,
The above-mentioned alumina and ceria are the highest NO of the catalyst.
x Instead of substantially reducing the purification rate, NO
It exhibits a temperature shift action of shifting the activation temperature to the high temperature side while improving the x purification rate.

Although the reason is not always clear, one reason is that Pt as a noble metal active species activates a reducing agent such as HC even at a relatively low temperature due to its strong oxidizing power, so that NOx is changed from a low temperature. In the case of purification, alumina or ceria suppresses the oxidizing power of Pt, and it is considered that the active temperature range shifts to the high temperature side as described above. Further, alumina and ceria improve the heat resistance of the noble metal active species.

When the amount of alumina is small, the effect of shifting the active temperature range is low, and when it is too large, the exhaust gas purification rate of the catalyst is low. From this viewpoint, the amount of alumina in the active species-supporting base material is preferably 5 to 80% by weight, more preferably 15 to 60% by weight. In the case of ceria, from the same viewpoint, it is 5 to 70% by weight, and further 15 to
50% by weight is preferred. Even when both alumina and ceria are added, the total amount may be determined in the same manner as in the case of alumina alone or ceria alone, and the alumina / ceria ratio may be about 0.1 to 10.

In this case, the heat treatment of the second exhaust gas purifying catalyst is a suitable means for shifting the activation temperature range of the second exhaust gas purifying catalyst to a higher temperature side. This is considered to be because the heating causes a change in the noble metal active species in some state, and the interaction with alumina or ceria becomes more prominent. The conditions for the heat treatment in this case may be, for example, a temperature of 550 to 900 ° C. and a time of 10 to 30 hours.

-Invention of Claim 3- The invention according to claim 3 for solving the above-mentioned problems is the catalyst structure for purifying exhaust gas according to claim 1, wherein:
The exhaust gas purifying catalyst contains Pt and Ir as the noble metal active species.
And Rh are used, and a porous metal-containing silicate is used as an active species-supporting base material, and the second exhaust gas purification catalyst is Pt or Ir as a noble metal active species.
And Rh are formed by a mixture of a catalyst powder supported on a porous metal-containing silicate as an active species supporting base material and at least one of alumina and ceria.

The present invention is different from the invention according to claim 2 in that alumina and ceria in the second exhaust gas purifying catalyst are mixed as a part of the catalyst rather than as the active species supporting base material. As in the case of the invention of Item 2, the second
The exhaust gas purifying catalyst has an active temperature range higher than that of the first exhaust gas purifying catalyst due to the addition of alumina or ceria.

The amount of alumina is preferably 5 to 70% by weight, more preferably 10 to 50% by weight from the same viewpoint as in the case of the invention according to claim 2. Further, in the case of ceria, 5 to 60% by weight, more preferably 10 to 50% by weight is suitable. Even when both alumina and ceria are added, the total amount may be determined in the same manner as in the case of alumina alone or ceria alone, and the alumina / ceria ratio may be about 0.1 to 10.

Also in the present invention, the same heat treatment can be applied to the second exhaust gas purifying catalyst as in the second aspect of the invention, whereby the activity of the second exhaust gas purifying catalyst is increased. The temperature range can be shifted to a higher temperature side.

[0024]

Therefore, according to the invention of claim 1,
The first exhaust gas purifying catalyst has a wider active temperature range than the second exhaust gas purifying catalyst on the low temperature side, and the second exhaust gas purifying catalyst has a higher active temperature range than the first exhaust gas purifying catalyst. Since the exhaust gas purifying catalysts have a wide area, and the activation temperature ranges of the two exhaust gas purifying catalysts partially overlap each other, one of the two catalysts is arranged on the upstream side in the exhaust gas flow direction and the other is arranged on the downstream side. Both catalysts work effectively for purification of exhaust gas from a low exhaust gas temperature to a high exhaust gas temperature, and high exhaust gas purification efficiency is obtained even when the characteristics of the exhaust gas change frequently.

According to each invention of claim 2 or 3,
Each of the second exhaust gas purifying catalyst has an active temperature range higher than that of the first exhaust gas purifying catalyst so that the exhaust gas has high exhaust gas characteristics even when the characteristics of the exhaust gas frequently change as in the first embodiment. Purification efficiency can be obtained.

[0026]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an exhaust gas purifying catalyst structure according to an embodiment. That is, in FIG. 1, reference numeral 1 denotes a catalytic converter provided in an exhaust passage 2 of an automobile engine, which includes a first exhaust gas purifying catalyst (hereinafter, simply referred to as a first catalyst) 3 and a second exhaust gas purifying catalyst. (Hereinafter, simply referred to as a second catalyst) 4 are arranged in series so that the former is on the downstream side in the exhaust gas flow direction and the latter is on the upstream side. As shown in FIG. 2, the activation temperature range of the first catalyst 3 is wider than the activation temperature range of the second catalyst 4 on the lower temperature side, while the activation temperature range of the second catalyst 4 is higher than that of the first catalyst 3. The temperature is wider than the temperature range, and the catalysts 1 and 2 partially overlap in the active temperature range.

Next, specific Examples 1 to 8 adopting the above arrangement, Example 9 in which the arrangements of the first catalyst 3 and the second catalyst 4 are replaced, and Comparative Examples 1 and 2 will be described.

<Example 1> For the first catalyst 3 on the downstream side, Pt, Ir and Rh were used as the noble metal active species,
ZSM-5 was adopted as the active species supporting base material.

That is, divalent platinum ammine crystals, iridium trichloride, and rhodium nitrate are used, each having a metal weight ratio of P.
t: Ir: Rh = 30: 6: 1 and the total amount is catalyst 1
The weight was adjusted to 4.5 g per liter, the divalent platinum ammine crystal and rhodium nitrate were dissolved in ion-exchanged water, and iridium trichloride was dispersed in propanol, and then both were mixed. ZS in this mixture
M-5 the _{(SiO 2 / Al 2 O 3} = 70~90) was added,
Stir at room temperature for 2 hours. Then, Pt, Ir, Rh / Z is obtained by instantaneously drying this by the spray drying method.
SM-5 catalyst powder was obtained.

After the catalyst powder was heat-treated (activation treatment) at 200 ° C. for 14 hours in the air, 20% by weight of a binder (hydrated alumina) and an appropriate amount of water were added to make a slurry, which was then slurried. Honeycomb carrier made of cordierite (4
00 cells / inch ² ) with washcoat, then 150
Honeycomb catalyst 3 having a catalyst capacity of 0.8 liter was obtained by performing drying in the air for 3 hours at 500 ° C. and firing in the air for 2 hours at 500 ° C. The amount of the catalyst loaded on the honeycomb carrier was set to 35 to 40% by weight of the weight of the honeycomb.

A rig test was conducted for the activation temperature range of the first catalyst 3 (model exhaust gas corresponding to A / F = 22 was used for SV = 5500).
The NOx purification rate was 25% when the NOx purification rate was measured by flowing it through the catalyst so as to be ⁰ h ^−1.
The active temperature range in which the above was obtained was 170 to 280 ° C. The temperature referred to here is the exhaust gas temperature at the catalyst inlet. Further, hereinafter, the NOx purification rate 2 is all in the active temperature range.
It is a temperature range where 5% or more is obtained.

For the second catalyst 4 on the upstream side, the same noble metal active species as the first catalyst 3 was used, and a mixture of ZSM-5 and alumina was used as the active species supporting base material.

That is, the above ZSM-5 and specific surface area 20
About 0 of γ-alumina (hereinafter, simply referred to as alumina) was mixed so that the ratio of the latter was 20% by weight. On the other hand, divalent platinum ammine crystals, iridium trichloride and rhodium nitrate were weighed under the same conditions as in the case of the first catalyst 3 and dissolved in a solvent to obtain a similar mixed solution. Then, the mixture of ZSM-5 and alumina is added to the mixed solution, and the mixture is stirred at room temperature for 2 hours.
After stirring for a period of time, this was instantaneously dried by a spray drying method to obtain Pt, Ir, Rh / ZSM-5 / alumina catalyst powder.

Then, the catalyst powder was treated in the same manner as in the case of the first catalyst 3 to obtain a honeycomb catalyst 4 having a catalyst capacity of 0.8 liter. When the active temperature range of this catalyst 4 was evaluated under the same conditions as in the case of the first catalyst 3, it was found to be 190 to 3
It was 10 ° C.

<Example 2> In this example, the same catalyst as in Example 1 was used as the first catalyst 3. As the second catalyst 4, Pt, Ir, Rh / ZSM-5.ceria catalyst powder in which ceria (CeO ₂ ) was used instead of alumina in Example 1 was used. That is, as the active species-supporting base material, a mixture of ZSM-5 and alumina such that the ratio of the latter was 40% by weight was used, and otherwise the same treatment as in the second catalyst of Example 1 was performed to form a honeycomb. The second catalyst 4 was obtained. This second catalyst 4
The activation temperature range of was 185 to 305 ° C.

<Example 3> The first catalyst 3 of this example is also the same as Example 1.
Is the same as. The second catalyst 4 was obtained by preparing a honeycomb catalyst under the same conditions and method as those of the first catalyst 3 and subjecting the honeycomb catalyst to a heat treatment A at 700 ° C. for 25 hours in the atmosphere. The activation temperature range of the second catalyst 4 was 190 to 295 ° C.

<Embodiment 4> The first catalyst 3 of this embodiment is also used in Embodiment 1.
Is the same as. Regarding the second catalyst 4, the P described above
The ratio of t, Ir, Rh / ZSM-5 catalyst powder and alumina is 40%.
Used in a catalyst powder mixture prepared by mixing so as to be wt%,
A honeycomb catalyst was obtained under the same conditions and method as in Example 1 except for the above. Then, the honeycomb catalyst is 70
Heat treatment B was performed at 0 ° C. for 12 hours. The activation temperature range of the second catalyst 4 thus obtained was 175 to 310 ° C.

<Embodiment 5> The first catalyst 3 of this embodiment is also the embodiment 1
Is the same as. For the second catalyst 4, ceria was used in place of the alumina in the second catalyst 4 of Example 4 above, except that a honeycomb catalyst was obtained under the same conditions and methods as in Example 4, and the same heat treatment B was performed. gave. The activation temperature range of the second catalyst 4 was 180 to 310 ° C.

<Embodiment 6> The first catalyst 3 of this embodiment is also the embodiment 1
Is the same as. For the second catalyst 4, Pt, Ir, Rh / ZSM-5
The catalyst powder and alumina and ceria were used in a catalyst powder mixture prepared by mixing the latter so that the ratio of the latter was 40% by weight and alumina / ceria = 2/1.
A honeycomb catalyst was obtained under the same conditions and method as in the above case, and the same heat treatment B was performed. The activation temperature range of the second catalyst 4 was 185 to 305 ° C.

<Embodiment 7> The first catalyst 3 of this embodiment is also used in Embodiment 1.
Is the same as. As the second catalyst 4, Pt, Ir / ZSM-5 catalyst powder was used. That is, divalent platinum ammine crystals and iridium trichloride were weighed so that the weight ratio of each metal was Pt: Ir = 2: 1 and the total amount was 4.5 g per liter of catalyst. Iridium trichloride was dissolved in ion-exchanged water and dispersed in propanol, and then both were mixed. ZSM-5 (Si
_{O 2 / Al 2 O 3 =} 70~90) , and the mixture was stirred for 2 hours at room temperature. Then, Pt, Ir / ZSM-5 catalyst powder was obtained by instantaneously drying this by a spray drying method. Then, this catalyst powder was treated in the same manner as the case of the first catalyst 3 to obtain a honeycomb catalyst 4 having a catalyst capacity of 0.8 liter. The activation temperature range of the second catalyst 4 is 200 to 310 ° C.
Met.

<Embodiment 8> The first catalyst 3 of this embodiment is also used in Embodiment 1.
Is the same as. The second catalyst 4 was obtained by preparing a honeycomb catalyst under the same conditions and method as those of the first catalyst 3 and subjecting the honeycomb catalyst to the heat treatment B. The activation temperature range of the second catalyst 4 was 180 to 295 ° C.

<Example 9> In this example, the same first catalyst 3 as in Example 1 was arranged on the upstream side in the exhaust gas flow direction, and the same second catalyst 2 as in Example 3 was arranged on the downstream side.

Comparative Example 1 In this example, two same catalysts as the first catalyst 3 of Example 1 were prepared, and these were arranged in series on the upstream side and the downstream side in the exhaust gas flow direction.

<Comparative Example 2> In this example, two same catalysts as the second catalyst 4 of Example 3 were prepared, and these were arranged in series on the upstream side and the downstream side in the exhaust gas flow direction.

-Purification test-The catalysts of the above-mentioned examples and comparative examples were installed in the exhaust pipe of the engine of an automobile operated in the region of A / F = 16 to 22, and the automobile was run in a predetermined operation mode. HC, C
Each purification rate of O and NOx was measured. The test results are shown in Table 1.

[0047]

[Table 1]

-Test Results-In all of Examples 1 to 9, the CO purification rate and the NOx purification rate were higher than those of Comparative Examples 1 and 2. Further, regarding the HC purification rate, the examples and the comparative examples do not change much. From this, it can be seen that combining two types of catalysts having different activation temperature ranges is effective in improving the purification rate of exhaust gas.

In particular, Examples 4 and 5 are examples in which the total active temperature range of the first catalyst 3 and the second catalyst 4 is wide and the active temperature wrap range of both catalysts is wide, but a high CO purification rate is obtained. And the NOx purification rate. On the other hand, in Example 1, Example 3 or Example 7, the total active temperature range is wide, but the lap region is not so wide, and in Example 8 the lap region is wide but the total active temperature range is total. Since the ratio is not wide, the CO purification rate and the NOx purification rate are higher than those in the comparative examples, but are lower than those in the above-mentioned Examples 4 and 5. From this, it is understood that it is preferable that both the total activation temperature range and the lap region are wide.

Furthermore, in Example 9, the low temperature active first catalyst 3 is provided on the upstream side in the exhaust gas flow direction, and the high temperature active second catalyst 4 is provided on the downstream side. It was comparable to the mode purification rate of Example 3 in which the catalyst arrangement was reversed, and Example 9 was rather superior. From this, it can be seen that the arrangement itself of the first catalyst 3 and the second catalyst 4 does not matter.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an exhaust gas purifying catalyst structure.

FIG. 2 is a graph showing the activation temperature range of the first catalyst and the second catalyst.

[Explanation of symbols]

 1 Catalytic Converter 2 Exhaust Passage 3 First Exhaust Gas Purification Catalyst 4 Second Exhaust Gas Purification Catalyst

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical indication location F01N 3/24 C (72) Inventor Shuji Iwakuni Shinchi Fuchu-cho, Aki-gun, Hiroshima Prefecture Mazda Co., Ltd. ( 72) Inventor Kazuya Komatsu 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda Motor Corporation

Claims (3)

[Claims] 1. One of a first exhaust gas purifying catalyst and a second exhaust gas purifying catalyst, each of which has a precious metal active species supported on an active species-supporting base material, on the upstream side in the exhaust gas flow direction, The other is disposed on the downstream side, the first exhaust gas purifying catalyst has a wider active temperature range on the low temperature side than the second exhaust gas purifying catalyst, and the second exhaust gas purifying catalyst is the first exhaust gas. An exhaust gas purifying catalyst structure, characterized in that the active temperature range is wider toward the high temperature side than that of the purifying catalyst, and the active temperature ranges of both of the exhaust gas purifying catalysts partially overlap each other. 2. The exhaust gas purifying catalyst structure according to claim 1, wherein the noble metal active species of the first and second exhaust gas purifying catalysts are Pt, Ir and Rh, and the first exhaust gas The active species-supporting base material of the purification catalyst is a porous metal-containing silicate, and the active species-supporting base material of the second exhaust gas purification catalyst is a mixture of a porous metal-containing silicate and alumina or ceria. An exhaust gas purifying catalyst structure characterized by the above. 3. The exhaust gas purifying catalyst structure according to claim 1, wherein the first exhaust gas purifying catalyst contains P as a noble metal active species.
t, Ir, and Rh are used, and a porous metal-containing silicate is used as the active species-supporting base material, and the second exhaust gas purifying catalyst contains Pt, Ir, and Rh as the noble metal active species. An exhaust gas purifying catalyst structure comprising a mixture of a catalyst powder supported on a porous metal-containing silicate as an active species supporting base material and at least one of alumina and ceria.