JPH10128068A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exhibit high NOx purifying performance even in a low temp. region by forming an exhaust gas purifying catalyst used for reducing and purifying nitrogen oxide in an excess oxygen containing exhaust gas from a 1st and a 2nd catalyst each obtained by carrying a catalyst noble metal on a porous carrier, and heat treating. SOLUTION: As the exhaust gas purifying catalyst purifying the exhaust gas, the 1st and the 2nd catalyst each obtained by carrying the catalyst noble metal on the porous carrier and heat treating are used. The 1st porous carrier of the 1st catalyst is composed of one kind selected from alumina and silica based porous body and, as the 1st catalyst carried thereon, one kind of platinum, palladium, rhodium is used. On the other hand, as the 2nd porous carrier of the 2nd catalyst, zirconium oxide, titanium oxide or the like is used and, as the 2nd catalyst carried thereon, the same one as the 1st catalyst is used. The 1st catalyst is heat treated in a oxidizing atmosphere and the 2nd catalyst is heat treated in a non-oxidizing atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a diesel engine.
Emissions from internal combustion engines such as
Regarding the exhaust gas purifying catalyst method for purifying the emitted exhaust gas,
More specifically, exhaust gas with excess oxygen,
Carbon monoxide (CO) and hydrogen (H _Two) And carbonized
Required to completely oxidize reducing components such as hydrogen (HC)
Nitrogen oxides in exhaust gas containing excess oxygen
(NO_x) For efficient reduction and purification from low to high temperatures
And a catalyst for purifying exhaust gas.

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NO _x has been used as a catalyst for purifying exhaust gas of automobiles. As such a three-way catalyst, for example, a porous carrier layer made of γ-alumina is formed on a heat-resistant substrate made of cordierite or the like, and platinum (Pt), rhodium (Rh), or the like is formed on the porous carrier layer. What carried the catalyst noble metal is widely known.

On the other hand, in recent years, carbon dioxide (CO ₂ ) in exhaust gas discharged from internal combustion engines such as automobiles has become a problem from the viewpoint of protection of the global environment, and an air-fuel ratio (A / F) of 24 has been solved as a solution. So-called lean burn, in which lean combustion is performed in an oxygen-excess atmosphere expanded as described above, is employed. In this lean burn, the use of fuel is reduced to improve fuel efficiency, and the generation of CO ₂ , which is the combustion exhaust gas, can be suppressed.

However, in the conventional three-way catalyst, when the air-fuel ratio is near the stoichiometric air-fuel ratio (stoichiometric), CO,
HC, simultaneously oxidizing and reducing NO _x, be those for purifying, the three-way catalyst exhibit sufficient purification performance for reduction and removal of the NO _x in an oxygen excess atmosphere in the exhaust gas during the lean-burn Absent. Therefore, development of a catalyst and purification system has been desired can purify NO _x even in an oxygen rich atmosphere.

[0005] By the way, there is an optimum temperature range for the catalytic reaction, and when the temperature is outside the temperature range, the activity decreases. The NO _x reduction reaction by the exhaust gas purifying catalyst is no exception, and the exhaust gas temperature of the automobile greatly varies. Therefore, the temperature range in which the catalytic reaction for reducing NO _x occurs is preferably as wide as possible. Therefore in an oxygen-rich atmosphere, as an exhaust gas purification method for purifying the NO _x over a wide temperature range, for example, as disclosed in JP-A 8-57259 and JP is a method of contacting the exhaust gas to two different types of catalysts are disclosed ing.

In the technique disclosed in the above publication, a high reaction temperature catalyst mainly supporting cobalt on zeolite is arranged on the upstream side of an exhaust gas flow path, and a low reaction temperature catalyst mainly supporting copper on zeolite (Cu / (Zeolite catalyst) is disposed downstream. And by supplying the reducing agent into the exhaust gas, NO _x is reduced and purified primarily reacting the reducing agent and NO _x at the upstream side of the catalyst at high temperatures. On the other hand, in the low temperature range, the reducing agent is partially decomposed by the upstream catalyst to generate active species that are advantageous for NO _x reduction, whereby NO _x is efficiently reduced by the downstream catalyst. Therefore, high NO _x purification activity is obtained in the temperature range of 200 to 600 ° C.

[0007]

[0005] However Cu / zeolite catalyst having a low catalytic reactivity temperature disclosed in the above publication, in the example Cu / ZSM-5 catalyst, as shown in the catalyst C of FIG. 1, NO _x Purification temperature window is 300 ℃
As described above, there is a problem that the NO _x purification rate is low in a low temperature range of about 300 ° C. or less.

[0008] Further, for example, the Cu / ZSM-5 catalyst has a phenomenon that when heat of 600 ° C. or more is applied in an atmosphere containing water vapor, Al in the zeolite is desorbed and Cu ions move. With this change in coordination state, Cu / ZSM-
Since the catalyst No. 5 changes to a state of low chemical reactivity, there is a problem that the NO _x purification rate decreases when used in an atmosphere where a high temperature of 600 ° C. or more acts.

The present invention has been made in view of such circumstances, and has a high NO even in a low temperature range of less than 300 ° C.
_x It is an object of the present invention to provide an exhaust gas purifying catalyst exhibiting purification performance and having excellent heat resistance. Further another object, _the NO _x purification temperature range even when narrow as in the prior art, is to the exhaust gas purification catalyst showing a higher _the NO _x purification performance than conventional in temperature range.

[0010]

According to a first aspect of the present invention, there is provided a catalyst for purifying exhaust gas, which comprises supplying a reducing agent to an exhaust gas containing an excessive amount of oxygen to reduce a nitrogen oxide contained in the exhaust gas. An exhaust gas purifying catalyst used for reducing and purifying an exhaust gas, comprising: a first catalyst heat-treated comprising a first porous carrier and a first catalytic noble metal supported on the first porous carrier; A heat-treated second catalyst comprising a porous carrier and a second catalytic noble metal supported on the second porous carrier,
Is to become.

The exhaust gas purifying catalyst according to claim 1, wherein the first catalyst is heat-treated in an oxidizing atmosphere at a temperature in the range of 600 to 1200 ° C, and the second catalyst is heat-treated in a non-oxidizing atmosphere at 600 to 1200 ° C.
It is desirable that the heat treatment be performed at a temperature of 1200 ° C.
The exhaust gas purifying catalyst according to claim 1, wherein
It is desirable that the catalyst and the second catalyst are arranged in the order of the first catalyst and the second catalyst from upstream to downstream of the exhaust gas channel.

Further, in the exhaust gas purifying catalyst according to the first aspect, the first catalyst and the second catalyst may be arranged in the order of the second catalyst and the first catalyst from upstream to downstream of the exhaust gas passage.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The first porous support of the first catalyst comprises at least one selected from alumina, porous silica, sepiolite and zeolite. These carriers are preferably used because they have excellent adsorptivity for the hydrocarbon compound as a reducing agent. Alumina is α-alumina,
Although γ-alumina can be used, it is preferable to use γ-alumina having high activity.

As the silica-based porous material, porous silica, a layered silica porous material such as FSM and MCM-41, silica gel, and the like can be used. As the zeolite, mordenite, ZSM-5, faujasite, X-type zeolite, Y-type zeolite (X-type and Y-type zeolites are faujasite-type zeolites) and the like can be used. Mordenite, ZSM-5, and ultrastable Y-type zeolite (US-Y) are particularly preferred.

As the first catalytic noble metal supported on the first porous carrier, at least one of platinum (Pt), palladium (Pd) and rhodium (Rh) can be used.
Among them, Pt or R having higher activity in the high temperature region than Pt
It is preferable to use at least one of h. Of course, both Pt and Rh may be used. The amount of the first catalyst noble metal carried is 0.5 to 3 per liter of the first catalyst.
A range of 0 g is desirable. If the amount is less than 0.5 g, NO _x can hardly be purified, and if the amount is more than 30 g, the NO _x purification activity is saturated.

The second porous carrier of the second catalyst may be composed of at least one selected from the group consisting of alumina, silica-based porous material, sepiolite and zeolite, as in the case of the first porous carrier. For example, zirconium oxide (zirconia), titanium oxide (titania), or the like having a low content can be used. As the second catalytic noble metal supported on the second porous carrier, Pt, Pd, R
At least one of h can be used. Among them, Pt having high low-temperature activity is particularly preferable.

The amount of the noble metal carried by the second catalyst is preferably in the range of 0.5 to 30 g per liter of the second catalyst. If less than 0.5 g, NO _x can hardly be purified,
The NO _x purification activity is saturated even if more than 0 g is loaded, so that more loading only causes an increase in cost.
When the first catalyst and the second catalyst are used as they are, their durability is low and their initial activities are high, but their activities gradually decrease as they are used. Therefore, the first catalyst and the second catalyst are both used after being heat-treated. In other words, if the heat treatment is performed at a temperature higher than the temperature actually used, the activity is stabilized because the catalyst noble metal does not grow during use even if the catalyst noble metal grows during the heat treatment.

Since the temperature of the exhaust gas from the diesel engine is approximately 600 ° C. or less,
The range of -1200 degreeC is preferable. If the temperature is lower than 600 ° C., the effect of the heat treatment is hardly obtained, and the noble metal catalyst grows during use, making it difficult to maintain the initial activity. Also 12
If the temperature is higher than 00 ° C., the structure of the porous carrier changes, which is not preferable.

The first catalyst is desirably heat-treated in an oxidizing atmosphere containing oxygen, such as in the air. Due to the heat treatment in an oxidizing atmosphere, the grain growth of the catalytic noble metal becomes non-uniform and the activity shifts to a high temperature side.
The catalyst becomes highly active in a high temperature range of 0 ° C. On the other hand, the heat treatment of the second catalyst is preferably performed in a non-oxidizing atmosphere such as an inert gas, a nitrogen gas, or a vacuum. As a result, the catalytic noble metal grows with a uniform particle size and is stabilized, so that the activity before the heat treatment is relatively maintained, and the second catalyst has a catalytic activity of 15%.
The catalyst becomes highly active in a low temperature range of 0 to 350 ° C.

The behavior of the catalytic noble metal during heat treatment in an oxidizing atmosphere and a non-oxidizing atmosphere is explained as follows. First,
It is known that agglomeration of precious metals on a carrier occurs by collision of precious metals. And for example Pt
It is known that oxidized atmosphere is easier to aggregate than non-oxidized atmosphere. This is because, as expected, the vapor pressure of the oxide of Pt is higher than that of metal Pt,
This is because in an oxidizing atmosphere, the Pt oxide easily moves along the gas phase or the surface of the carrier, and Pt particles collide with each other. However, not all Pt particles on the carrier move at the same speed, and the state of the carrier is non-uniform. For example, the interaction between Pt and the carrier is different due to the difference in position on the carrier, There are Pt that is easy to move and Pt that is difficult to move. Therefore, when heat treatment is performed in an oxidizing atmosphere, collision between Pt particles occurs unevenly.
The particle size distribution of Pt becomes extremely wide, and the activity shifts to a higher temperature side.

Since the grain growth becomes non-uniform due to such a behavior, the heat treatment in an oxidizing atmosphere is more effective when the supported amount of the catalytic noble metal is larger. On the other hand, in a non-oxidizing atmosphere, since Pt exists as metal Pt, the interaction with the carrier is small, and the moving speed of each Pt particle is almost the same. Therefore, the particle size of Pt becomes relatively uniform. However, since metal Pt is harder to move than Pt oxide, the growth rate of the particle size is extremely low. Therefore, Pt relatively maintains the activity when no heat treatment is performed, and the activity does not shift to the high temperature side.

Although the case of Pt has been described above, such a behavior is the same for Pd and Rh. As the reducing agent, a hydrocarbon compound is used. The type is not particularly limited, but if a substance containing a paraffinic hydrocarbon having a large number of carbon atoms is used, NO _x can be effectively purified. Representative examples of such hydrocarbon compounds include light oil and kerosene.

In the case of a diesel engine, the amount of the reducing agent added is preferably in the range of 100 to 10000 ppmC in the exhaust gas. Purifying less than 100ppmC of the NO _x becomes difficult, NOx be added over 10000ppmC
Purification ability is saturated. The reducing agent may be directly added to the exhaust gas, or the air-fuel ratio (A / F) is sometimes made rich so that the exhaust gas contains a large amount of hydrocarbons. You can also.

When the first catalyst and the second catalyst that have been heat-treated as described above are used, the purifying action differs depending on the arrangement order of the first catalyst and the second catalyst in the exhaust gas passage. First, the case where the first catalyst heat-treated in the oxidizing atmosphere is disposed on the upstream side and the second catalyst heat-treated in the non-oxidizing atmosphere is disposed on the downstream side will be described. because of low activity in the range, the reducing agent is adsorbed to the first catalyst at low temperatures, is consumed in the reduction of the NO _x in the second catalyst having high activity at low temperature range desorbed at Atsushi Nobori. Further, the adsorbed reducing agent is cracked when it is desorbed, and is reformed into a low-molecular hydrocarbon that is more effective as a reducing agent, so that the NO _x purification rate is improved.

The one in the catalyst, it is difficult to purify NO _x in a wide range of exhaust gas temperature region, the upstream side of the first catalyst has been heat treated in a highly active oxidizing atmosphere at high temperatures as described above , placing a second catalyst which is heat-treated at high activity in a non-oxidizing atmosphere at a low temperature range on the downstream side, it is possible to purify NO _x over a wide temperature range from low temperature region to high temperature region.

On the other hand, when the second catalyst heat-treated in a non-oxidizing atmosphere having a high activity in a low temperature range is disposed on the upstream side, and the first catalyst heat-treated in an oxidizing atmosphere having a high activity in a high temperature range is disposed on the downstream side. When the exhaust gas temperature is in a low temperature range, the reducing agent burns on the second catalyst on the upstream side and reacts with NO _x to increase the exhaust gas temperature. The exhaust gas is thus the first
Upon reaching the activation temperature range of the catalyst, being purified further NO _x even is in the first catalyst downstream, resulting in high NO _x purification rate is obtained. That even small differences in _the NO _x purification temperature of the first catalyst and the second catalyst, it is possible to obtain a high _the NO _x purification rate at the temperature range. Note that the temperature of the exhaust gas when reaching the first catalyst can be controlled by the supply amount of the reducing agent.

Further, the first heat-treated in an oxidizing atmosphere
When the catalyst is arranged on the upstream side and the second catalyst heat-treated in the non-oxidizing atmosphere is arranged on the downstream side, it is also preferable to arrange the first catalyst heat-treated in the oxidizing atmosphere on the further downstream side. In this way, both characteristics of the above two types of arrangement order appear, and for example, the NO _x purification temperature range at a temperature lower than 300 ° C. is widened, and the NO _x purification activity is further increased in a specific narrow temperature range. Can be enhanced. At this time, it is preferable to supply the reducing agent also to the upstream side of the most downstream first catalyst as necessary.

In some cases, the first catalyst heat-treated in a non-oxidizing atmosphere may be arranged on the upstream side, and the second catalyst treated in an oxidizing atmosphere may be arranged on the downstream side.
As the porous carrier of the catalyst disposed on the upstream side, it is preferable to use one having an adsorbing action, such as alumina, a silica-based porous material, sepiolite and zeolite. As a result, unburned high-boiling hydrocarbons in the exhaust gas are adsorbed by the upstream catalyst, so that N
_Ox purification activity is prevented from being impaired by unburned high boiling hydrocarbons.

The shapes of the first catalyst and the second catalyst are not particularly limited, such as a honeycomb shape and a pellet shape. Also, as the manufacturing method, a conventional manufacturing method can be used. When the first catalyst and the second catalyst are arranged in the exhaust gas passage, the first catalyst and the second catalyst may be integrated, or the separate first and second catalysts may be arranged in series. In the latter case, there may be an interval between the first catalyst and the second catalyst,
You may arrange | position in the state which contact | abutted without an interval.

By adjusting the size of the catalyst, the reducing agent can be used more effectively, and the rate of deterioration of fuel efficiency can be reduced. Further, if hydrocarbons are always present in the catalyst, the oxidation of SO _{2 in} the exhaust gas is also suppressed. Therefore, by using a catalyst having a length such that hydrocarbons are always present in the catalyst, the production of sulfate can be suppressed.

The supply position of the reducing agent may be supplied only to the upstream side of the first catalyst and the second catalyst, or may be supplied also between the first catalyst and the second catalyst. In addition, when supplying also between the first catalyst and the second catalyst, the same reducing agent that is supplied to the upstream side of the first catalyst and the second catalyst may be supplied, or a different type of reducing agent may be supplied. You can also.

[0032]

The present invention will be specifically described below with reference to examples and comparative examples. (1) Preparation of Catalyst A-1 Mordenite (SiO ₂ / Al ₂ O ₃ = 30) Powder 10
0 parts by weight, 35 parts by weight of silica sol (20% by weight of silica), and 135 parts by weight of water were stirred and mixed to prepare a slurry. Next, a cordierite honeycomb carrier substrate (40
(0 cell / in ² , diameter 30 mm, length 25 mm) was prepared, immersed in the slurry, pulled up and blow off excess slurry, dried at 250 ° C., and fired at 650 ° C. to form a mordenite layer. The coating amount of the mordenite is 120 g per liter of the honeycomb carrier substrate.

Next, a honeycomb carrier substrate having a mordenite layer is impregnated with a predetermined amount of a dinitrodiammineplatinum aqueous solution having a predetermined concentration, and after drying the water, it is baked at 300 ° C. for 1 hour to obtain 1 liter of the honeycomb carrier substrate. Pt per 10
g was carried to obtain Catalyst A-1. (2) Preparation of Catalyst A-2 Catalyst A-1 was calcined at 800 ° C. for 3 hours in a nitrogen gas atmosphere to prepare Catalyst A-2. (3) Preparation of Catalyst A-3 The catalyst A-1 was calcined in the air at 800 ° C. for 3 hours,
Catalyst A-3 was prepared. (4) Preparation of Catalyst B-1 100 parts by weight of γ-alumina powder, 20 parts by weight of alumina sol (40% by weight of alumina), and 140 parts by weight of water were mixed by stirring to prepare a slurry. Next, the same honeycomb carrier base material as the catalyst A-1 was prepared, immersed in the slurry, pulled up, and blown off excess slurry.
C. to form a gamma-alumina layer. The coating amount of γ-alumina is 12 per liter of the honeycomb support substrate.
0 g.

Next, the honeycomb carrier substrate having the γ-alumina layer is immersed in an aqueous solution of dinitrodiammine platinum having a predetermined concentration to selectively adsorb Pt, and after drying the water, the substrate is dried.
C. for 1 hour to carry 20 g of Pt per liter of the honeycomb carrier substrate to prepare a catalyst B-1. (5) Preparation of Catalyst B-2 Catalyst B-1 was calcined at 800 ° C. for 3 hours in a nitrogen gas atmosphere to prepare Catalyst B-2. (6) Preparation of catalyst B-3 The catalyst B-1 was calcined at 800 ° C for 3 hours in the air.
Catalyst B-3 was prepared. (7) Preparation of Catalyst C ZSM-5 powder was added to an aqueous copper acetate solution having a predetermined concentration,
After stirring at 0 ° C. for 24 hours, ion exchange with copper ions was performed. Thereafter, the mixture was filtered, washed, evaporated to dryness, and then calcined at 500 ° C. for 1 hour to prepare a Cu / ZSM-5 catalyst powder.

Next, the Cu / ZSM-5 catalyst powder 10
0 parts by weight, 40 parts by weight of silica sol (20% by weight of silica), and 140 parts by weight of water were stirred and mixed to prepare a slurry. Then, a honeycomb carrier base material similar to that of the catalyst A-1 was prepared, dipped in this slurry, pulled up and blow off excess slurry, dried at 250 ° C., fired at 500 ° C., and
A ZSM-5 catalyst layer was formed. The coating amount of the Cu / ZSM-5 catalyst was 120 per liter of the honeycomb support substrate.
g, and the supported amount of Cu at this time is 2 g per liter of the honeycomb carrier base material.

Table 1 summarizes the structure of each of the above catalysts.

[0037]

[Table 1]

(8) Evaluation of NO _x purification activity of each catalyst For each of the above catalysts, a model gas having the composition shown in Table 2 was used.
At a flow rate of 0 L / min (0 ° C., 1 atm), the NO _x purification rate was measured when the incoming gas temperature was raised from 100 to 450 ° C. at a rate of 15 ° C./min. 1 and 2 show the results.
Shown in

[0039]

[Table 2] As it can be seen from FIGS. 1 and 2, catalyst A-1 to 3 and the catalyst B-1 to 3, it is apparent that it is possible to purify NO _x in the low temperature range as compared with the catalyst C. Catalyst A-1
3 shows that the highest NO _x purification rate is higher than that of the catalysts B-1 to B-3, and it is also understood that mordenite is more preferable as the carrier.

Further although the best _the NO _x purification rate is slightly reduced by heat treatment, the catalyst was heat-treated in a nitrogen gas atmosphere (A-2, B-2 ) catalyst without heat treatment _the NO _x purification temperature range (A-1 , B-1) and _the NO _x purification temperature region is equal, the catalyst was heat-treated in the atmosphere (a-3, B-
In 3), the NO _x purification temperature range shifts to a high temperature side of about 30 ° C. That is, the catalysts (A-2, B-2) heat-treated in a nitrogen gas atmosphere exhibit performance in a low temperature range, and the catalysts (A-3, B-3) heat-treated in the atmosphere exhibit performance in a high temperature range. You can see that (9) Preparation of Catalyst B-4 A catalyst B-4 was prepared in the same manner as the catalyst B-1, except that 10 g of Pt was supported per liter of the carrier substrate. (10) Preparation of Catalyst B-5 Catalyst B-4 was calcined at 800 ° C. for 3 hours in a nitrogen gas atmosphere to prepare Catalyst B-5. (11) Preparation of Catalyst B-6 Catalyst B-4 was treated at 800 ° C. in a nitrogen gas atmosphere for 10 minutes.
The mixture was calcined for a time to prepare a catalyst B-6. (12) Preparation of Catalyst B-7 Catalyst B-4 was calcined at 800 ° C. for 3 hours in the air,
Catalyst B-7 was prepared. (13) Preparation of Catalyst B-8 Catalyst B-4 was calcined in the air at 800 ° C. for 10 hours to prepare Catalyst B-8.

Table 1 summarizes the constitution of each of these catalysts. (14) Investigation of the effect of heat treatment First, catalyst C was tested in a model gas shown in Table 2 for 70 days.
Heat treatment was performed by heating at 0 ° C. and 800 ° C. for 5 hours, respectively, and the NO _x purification rates before and after the heat treatment were measured. Fig. 3 shows the results.
Shown in

[0042] As shown in FIG. 3, the catalyst C is _the NO _x purification rate is greatly reduced by heat treatment at 700 ° C. and 800 ° C.,
It turns out that durability is poor. Next, catalysts B-4 and B-
5, B-6, B-7, and B-8 were supplied with a model gas having a composition shown in Table 2 at a flow rate of 30 L / min (0 ° C., 1 atm), and the inlet gas temperature was increased to 15 ° C./mi from 100 to 450 ° C.
The NO _x purification rate when the temperature was raised at the rate of n was measured.
FIG. 4 shows the results.

FIG. 4 shows that the catalyst B-4 has a smaller change in the NO _x purification rate than the catalyst C and has excellent durability even when subjected to heat treatment in a nitrogen gas atmosphere or in the air. Recognize. That is, it is difficult to perform the heat treatment on the catalyst C without deteriorating the NO _x purification activity.
-4 small reduction of the NO _x purification activity by heat treatment, and NO _x by a heat treatment in a nitrogen gas atmosphere and the atmosphere
Since the purification temperature ranges are different, it is possible to optimally arrange the temperature in the exhaust gas flow path. (15) Example 1 The exhaust system of a 2.4-liter diesel engine is shown in FIG.
As shown in the figure, the catalyst A-3 is arranged in series such that the catalyst A-2 is on the downstream side, and the diesel oil is added to the exhaust gas in the amount of 3000 ppmC on each of the upstream sides while the catalyst A-2 is on the downstream side. The NO _x purification rate at each exhaust gas temperature was measured. FIG. 6 shows the results. (16) Example 2 A test was conducted in the same manner as in Example 1, except that the catalyst A-2 was arranged in the upstream side and the catalyst A-3 was arranged in the downstream side, and the results are shown in FIG. (17) Example 3 A test was conducted in the same manner as in Example 1 except that the catalyst B-3 was arranged in series with the upstream side and the catalyst B-2 was arranged on the downstream side, and the results are shown in FIG. (18) Example 4 A test was conducted in the same manner as in Example 1 except that the catalyst B-2 was arranged in series with the upstream side and the catalyst B-3 was arranged with the downstream side, and the results are shown in FIG. (19) Evaluation From FIG. 6 and FIG. 7, the catalysts (A-3, B-3) heat-treated in the air are arranged on the upstream side of the exhaust gas flow path, and the catalysts (A) heat-treated in the nitrogen gas atmosphere are arranged on the downstream side. -2, B-2) (Examples 1 and 3), 300 ° C.
It can be seen that a high NO _x purification rate can be obtained in a wide temperature range in the following low temperature range.

The catalysts (A-2, B-2) heat-treated in a nitrogen gas atmosphere are arranged on the upstream side, and the catalysts (A-3, B-3) heat-treated in the atmosphere are arranged on the downstream side. (Examples 2 and 4), high NO _x at a temperature near 200 ° C.
It turns out that it shows a purification rate. (20) Influence of reducing agent Using catalyst A-3, it is arranged in the exhaust system of a 2.4-liter diesel engine, and propylene (C ₃ H ₆ ), which is an olefinic hydrocarbon, and paraffinic Decane (C ₁₀ H ₂₂ ), which is a hydrocarbon, is used, and 300
While adding to the exhaust gas so that the amount of 0ppmC,
The NO _x purification rate at each exhaust gas temperature was measured. FIG. 8 shows the results.

From FIG. 8, it is clear that the NO _x purification rate is improved by using decane as compared with propylene, and it is preferable to use a paraffinic hydrocarbon having a large number of carbons as the reducing agent. it is obvious.

[0046]

Effects of the Invention] That is, according to the catalyst for purification of exhaust gas according to claim 1, small degree of decrease of the NO _x purifying activity by heat treatment, it has a good catalyst durability by heat treatment. Therefore, by optimally arranging the two types of catalysts in the exhaust gas flow path, the N 2 catalyst can be used in a low temperature range of 300 ° C. or less.
O _x can be purified efficiently, or can be secured even higher _the NO _x purification rate in a specific temperature range.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the temperature of a gas containing a catalyst and the NO _x purification rate.

FIG. 2 is a graph showing the relationship between the temperature of a gas containing a catalyst and the NO _x purification rate.

FIG. 3 shows NO _x before and after heat treatment of a conventional exhaust gas purifying catalyst.
It is a bar graph which shows a purification rate.

FIG. 4 is a graph showing a relationship between a temperature of a gas containing a catalyst and a NO _x purification rate.

FIG. 5 is a schematic explanatory view showing a configuration of an exhaust gas purifying catalyst according to one embodiment of the present invention.

FIG. 6 is a graph showing the relationship between the temperature of a gas containing a catalyst and the NO _x purification rate.

FIG. 7 is a graph showing the relationship between the temperature of the gas containing a catalyst and the NO _x purification rate.

FIG. 8 is a graph showing the relationship between the temperature of the gas containing a catalyst and the NO _x purification rate.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Illumination Kondo 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (72) Inventor Koji Yokota 41, Yokomichi, Toyota Central Research Laboratory Co., Ltd. (72) Inventor Yasuo Takada Ochi-cho, Nagakute-cho, Aichi-gun, Aichi Prefecture 41, Toyoda Central Research Laboratory Co., Ltd. (72) Inventor Masao Kataoka Aichi 41 Toyota Chuo R & D Laboratories Co., Ltd.

Claims (1)

[Claims] An exhaust gas purifying catalyst used for supplying a reducing agent to an exhaust gas containing an excessive amount of oxygen to reduce and purify nitrogen oxides contained in the exhaust gas, comprising: a first porous carrier; The first porous carrier supported on the first porous carrier.
A first catalyst containing a catalytic noble metal and heat-treated; a second porous support; and a second support supported on the second porous support.
An exhaust gas purifying catalyst, comprising: a second catalyst containing a catalytic noble metal and heat-treated.