JP3407901B2 - Exhaust gas purifying catalyst, method for producing the catalyst, and method for purifying exhaust gas - Google Patents

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, a method for producing an exhaust gas purifying catalyst, and an exhaust gas purifying method. More specifically, the present invention relates to exhaust gas in the presence of excess oxygen, that is, exhaust gas. NOx (nitrogen oxide) in the exhaust gas containing oxygen in excess of the amount necessary to completely oxidize reducing substances such as carbon monoxide, hydrogen and hydrocarbons contained in The present invention relates to an exhaust gas purifying catalyst that efficiently purifies exhaust gas, a method for manufacturing an exhaust gas purifying catalyst, and an exhaust gas purifying method.

[0002]

2. Description of the Related Art Conventionally, as an automobile exhaust gas purifying catalyst, oxidation of carbon monoxide (CO) and hydrocarbons (HC) in exhaust gas and reduction of nitrogen oxides (NOx) are simultaneously performed. There are many known three-way catalysts for purifying exhaust gas, which purify exhaust gas. Examples of such a catalyst include, for example, γ-on a heat-resistant carrier such as cordierite.
Alumina slurry is applied and fired to form Pd, Pt, Rh
A catalyst supporting a noble metal such as is typical.

By the way, these exhaust gas purifying catalysts are
The set air-fuel ratio of the engine greatly affects its performance. Especially in a lean mixed atmosphere, that is, on the lean side where the air-fuel ratio is large, the amount of oxygen in the exhaust gas after combustion increases,
Oxidation becomes active and reduction becomes inactive. On the other hand, on the rich side where the air-fuel ratio is small, the amount of oxygen in the exhaust gas after combustion becomes small, and the oxidizing action becomes inactive and the reducing action becomes active.

On the other hand, in recent years, energy saving has been demanded from the viewpoint of resource problems and global warming, and there is a strong demand for low fuel consumption of automobiles. In a gasoline engine, lean burn is attracting attention from the viewpoint of efficient combustion. ing. In this lean burn, the lean side operation in which the air-fuel mixture is burned with an oxygen-rich mixture as much as possible during normal running is performed, and a catalyst capable of sufficiently purifying NOx even on the lean side has been desired.

Therefore, in order to solve these problems, as a catalyst for purifying automobile exhaust gas in such an oxygen-rich atmosphere, exhaust gas purifying for simultaneously oxidizing carbon monoxide and hydrocarbons and reducing nitrogen oxides. Various catalysts for use have been proposed.

For example, a Pt / Al ₂ O ₃ catalyst has been proposed in which platinum is supported on an alumina carrier to simultaneously oxidize carbon monoxide and hydrocarbons and reduce nitrogen oxides.

Further, the applicant of the present application previously mentioned that "exhaust gas which purifies nitrogen oxides in exhaust gas by bringing exhaust gas in excess of oxygen into contact with a catalyst formed by supporting platinum and lanthanum on a porous carrier. Purification method "(Japanese Patent Application No. 3-344781).

[0008]

The former Pt / Al ₂ O ₃ catalyst, however, is
Even if the amount of platinum loaded is increased, the NOx purification rate is at most 2
It was 0 to 30% (air-fuel ratio: A / F = about 22 to 23), and there was a problem that it did not have a practically sufficient purification rate.

In the latter Japanese Patent Application No. 3-344781, the addition of lanthanum enhances the reaction activity of HC and NOx, and can improve the transient characteristics of lean region → stoichi region → lean region, and further, oxidation. Lantern NO
Although it was possible to improve the NOx purification rate during acceleration / deceleration (when A / F fluctuates) by utilizing the x adsorption characteristic, the effect is not yet sufficient with lanthanum, and the additive improves the durability of the catalyst. There was a problem that there is a risk of lowering it.

Therefore, the inventors of the present invention have diligently studied to solve the problems of the prior art as described above, and as a result of various systematic experiments, the present invention has been accomplished.

(Object of the Invention) An object of the present invention is to provide an exhaust gas purifying catalyst which is excellent in the efficiency of purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen excess atmosphere.
An object of the present invention is to provide a method for manufacturing an exhaust gas purification catalyst and an exhaust gas purification method.

Another object of the present invention is to frequently operate within a range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23), such as the running state of a lean-burn internal combustion engine vehicle. An object of the present invention is to provide an exhaust gas purifying catalyst that efficiently purifies NOx, CO, and HC together, a method for manufacturing the exhaust gas purifying catalyst, and an exhaust gas purifying method in a usage state where the gas composition changes.

The present inventors have focused on the following points with respect to the above-mentioned problems of the prior art. That is, first, as a catalyst for efficiently purifying CO and HC, there is an oxidation catalyst in which a catalyst component is supported on alumina. However, this oxidation catalyst is present in the exhaust gas after combustion on the over lean side where the air-fuel ratio is large. The amount of oxygen increases and the reducing action becomes inactive. On the other hand, in the case of a catalyst in which a lanthanum element is added as a catalyst component as in the above-mentioned Japanese Patent Application No. 3-344781, although the effect of improving the purification efficiency of nitrogen oxides is seen, the durability of the catalyst itself is reduced. There is a risk of causing it.

Therefore, the present inventors consider these problems, and improve the catalyst itself by changing the composition and components of the above-mentioned oxidation catalyst itself, which is different from the nitrogen oxide purification rate improving method which has been considered until then. It was considered to solve the above-mentioned problems of the prior art by approaching from the viewpoint. Then, in an oxygen-excess atmosphere or in a usage state where the gas composition changes frequently in the range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23),
As a means to improve the purification efficiency of nitrogen oxides in exhaust gas, increase the adsorption (or storage) sites of nitrogen oxides, increase the adsorption (or storage) sites of hydrocarbons, heat resistance of catalyst components Found that it is necessary to improve.

Then, attention is focused on disposing an adsorbent mainly composed of aluminosilicate, which has solid acidity and molecular sieving property and is excellent in hydrocarbon adsorbing ability, in the vicinity of the oxidation catalyst. It has been found that the purification efficiency of nitrogen oxides in the exhaust gas can be improved in a usage state in which the gas composition frequently changes within the range from near the theoretical air-fuel ratio to over lean (A / F = about 23). The present invention has been accomplished.

[0016]

Means for Solving the Problems (First Invention) An exhaust gas purifying catalyst of the present invention purifies exhaust gas by simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen excess atmosphere. a use catalyst, a catalyst obtained by supporting a catalyst component comprising at least platinum on a carrier made of a porous material, at least one or more selected from alkali metal and alkaline earth metal aluminosilicate having a solid acidic and molecular sieve properties And the hydrocarbon adsorbent supporting only the metal are physically mixed with each other, and are sufficiently dispersed and in close proximity to each other.

[0017]

(Second Invention) The method for producing an exhaust gas purifying catalyst of the present invention is an exhaust gas purifying catalyst for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen excess atmosphere. a method of making at least one member selected at least a catalyst supported catalyst component comprising platinum, the aluminosilicate having a solid acidic and molecular sieves of the alkali metals and alkaline earth metals on a carrier made of a porous material It is characterized in that the above-mentioned hydrocarbon adsorbent carrying only the metal by the ion exchange method is physically mixed and sufficiently dispersed to bring them into close proximity to each other.

(Third invention) An exhaust gas purification method of the present invention is an exhaust gas purification method for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen excess atmosphere, wherein of the exhaust gas, a catalyst obtained by supporting a catalyst component comprising at least platinum on a carrier made of a porous material, the solid acidic and molecular sieves of the having aluminosilicate least one or more selected from alkali metal and alkaline earth metal It is characterized in that the hydrocarbon adsorbent supporting only metal is physically mixed and brought into contact with the exhaust gas purifying catalyst in a state in which they are sufficiently dispersed and in close proximity to each other.

[0020]

The mechanism by which the exhaust gas purifying catalyst of the first invention, the method of manufacturing the exhaust gas purifying catalyst of the second invention, and the exhaust gas purifying method of the third invention exert excellent effects is not always clear yet. However, it is considered as follows.

(Effect of the First Invention) The exhaust gas purifying catalyst of the present invention comprises a catalyst having a catalyst component containing at least platinum supported on a carrier made of a porous material (platinum-supported catalyst), solid acidity and molecular sieving property. And a hydrocarbon adsorbent in which only at least one metal selected from alkali metals and alkaline earth metals is supported on the aluminosilicate having The platinum-supported catalyst is an oxidation catalyst or a three-way catalyst in which a catalyst component containing at least platinum is supported on a porous body, and at least HC, C, etc. of harmful components in exhaust gas in an oxygen excess atmosphere are used.
It has a function of oxidizing and removing O and the like. Further, in the atmosphere, platinum in the catalyst has a function of adsorbing (or occluding) NOx which cannot be decomposed and holding NOx until the exhaust gas becomes close to the stoichiometric air-fuel ratio. Hydrocarbon adsorbent coexist with the supported platinum catalyst is an aluminosilicate der
It has solid acidity and molecular sieving property. When the exhaust gas is in the vicinity of the stoichiometric air-fuel ratio, the adsorbent selectively adsorbs unburned hydrocarbons in the exhaust gas and produces intermediates (partial oxidation products) of reducing active hydrocarbons, Accumulates in the adsorbent. Under an oxygen-excess atmosphere, the activated hydrocarbon reacts with NOx in the exhaust gas to reduce it to N ₂ . The strong acid points in the aluminosilicate that make up the hydrocarbon adsorbent cause coking (carbon deposition) due to the decomposition products of the hydrocarbon, which causes the pores in the aluminosilicate to become clogged, and also aluminosilicate. May lead to structural destruction. In the present invention, since the hydrocarbon adsorbent is loaded with at least one metal selected from alkali metals and alkaline earth metals, it is possible to eliminate extra strong acid points relating to coke formation and to prevent catalyst deterioration. It can be prevented. From this, it is considered that the exhaust gas purifying catalyst of the present invention can purify carbon monoxide, hydrocarbons and nitrogen oxides simultaneously and efficiently even in an oxygen excess atmosphere. Further, the exhaust gas purifying catalyst of the present invention is frequently used in the range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23) like the running state of a vehicle equipped with a lean-burn engine. It is considered that NOx, CO, and HC can be efficiently purified together in a usage state where the gas composition changes.

[0022]

(Operation of Second Invention) The method for producing an exhaust gas purifying catalyst of the present invention has a catalyst in which a catalyst component containing at least platinum is supported on a carrier made of a porous material, and has solid acidity and molecular sieving property. the aluminosilicate is for physically mixing the hydrocarbon adsorbent carried by the alkali metal and ion exchange method at least one kind of metal selected from the alkaline earth metals. At this time, it is considered that by physically mixing the two, it is possible to increase the frequency of the reaction between the active site on Pt where the NOx decomposition reaction occurs and the hydrocarbon activated by the aluminosilicate acid site. . In addition, since aluminosilicate has a large specific surface area, it is considered that the surface area of the entire catalyst can be increased and the sintering of Pt can be suppressed. From this, in the present invention,
Carbon monoxide, hydrocarbons and nitrogen oxides can be purified simultaneously and efficiently even in an oxygen-excess atmosphere. Furthermore, the lean-burn engine can be over lean from near the stoichiometric air-fuel ratio as in the running state of a vehicle equipped with a lean-burn engine. For purification of exhaust gas, which has excellent characteristics that can efficiently purify NOx, CO and HC together in the usage condition where the gas composition changes frequently within the range (A / F = about 23). It is considered that the catalyst can be easily manufactured.

[0024] In the exhaust gas purification method of the present invention (third effect of the invention), an alkaline catalyst obtained by supporting a catalyst component comprising at least platinum on a carrier made of a porous material, the aluminosilicate having a solid acidic and molecular sieve properties Exhaust gas purifying catalyst coexisting with a hydrocarbon adsorbent supporting at least one metal selected from metals and alkaline earth metals is used as in a running state of a vehicle equipped with a lean-burn engine. In addition, if the gas composition is arranged so that the gas composition frequently changes in the range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23), and the contact is made in the vicinity of the stoichiometric air-fuel ratio. with NOx supported catalyst adsorbed and held in an oxygen-rich atmosphere is reduced to N _2, wherein the adsorbent unburned hydrocarbons in the exhaust gas is selectively adsorbed To generate intermediates of hydrocarbons with a reducing activity (partial oxidation products) are held and accumulated in the adsorbent. In an oxygen-rich atmosphere, the activated hydrocarbons retained by this adsorbent react with NOx in the exhaust gas to reduce it to N ₂ , and undecomposed NOx is adsorbed by platinum, and the exhaust gas becomes Hold until the stoichiometric air-fuel ratio is reached. Further, CO and HC are easily oxidized by the present platinum-supported catalyst in the oxygen excess region. As a result, carbon monoxide, hydrocarbons and nitrogen oxides can be purified simultaneously and efficiently even in an oxygen-excess atmosphere. From Over lean (A / F
It is considered that NOx, CO, and HC can be efficiently purified together in a usage state in which the gas composition frequently changes within a range of up to about 23).

[0025]

【The invention's effect】

(Effect of the first invention) The exhaust gas purifying catalyst of the first invention is capable of purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas simultaneously and efficiently in an oxygen excess atmosphere. It can be carried out. Also, in a driving state in which the gas composition changes frequently in the range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23), such as the running state of a vehicle equipped with a lean-burn engine. , NOx, CO and HC can be efficiently purified together.

[0027]

(Effect of the Second Invention) According to the method for producing an exhaust gas purifying catalyst of the second invention, purification of carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas is simultaneously performed in an oxygen excess atmosphere. Moreover, an exhaust gas purifying catalyst that can be efficiently performed can be easily manufactured.

(Effect of the third invention) By the exhaust gas purification method of the third invention, the purification of carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas can be performed simultaneously and efficiently even in an oxygen excess atmosphere. It can be carried out. Also, in a driving state in which the gas composition changes frequently in the range from near the stoichiometric air-fuel ratio to over lean (A / F = about 23), such as the running state of a vehicle equipped with a lean-burn engine. Also, NOx, CO and HC can be efficiently purified together.

[0030]

【Example】

[0031] Hereinafter, the exhaust gas purifying catalyst of the present first invention, a manufacturing method of an exhaust gas purifying catalyst of the second aspect, the invention the exhaust gas purification method of the third aspect of the present invention, and more specifically (example) Will be described.

The carrier used for the platinum-supported catalyst in the present invention is a porous material, and specific examples thereof include alumina, zeolite, zirconia, silica-alumina and silica. The type and physical properties of these porous carriers are not particularly limited, and any porous body conventionally used as a catalyst carrier can be used. Further, these porous bodies may be used by coating them on a monolith made of cordierite, a heat-resistant metal alloy or the like.

The carrier is preferably alumina. When alumina is used, it is possible to maintain a high specific surface area up to a relatively high temperature, and it is possible to suppress grain growth of oxides of platinum and other catalyst components supported as necessary. Furthermore, when Pt is supported on alumina, alumina is Pt.
Since Pt is easily adsorbed, there is an advantage that Pt supported in high dispersion can be stably maintained. Further, as alumina, γ-alumina is suitable. When a hydrate of alumina is heated, it becomes α-alumina through various intermediates. γ-alumina is one of these intermediates and is thermally stable up to about 1100 ° C. Further, γ-alumina has a large specific surface area (several hundred m ² /
g) and is surface active.

The catalyst component supported on the porous body contains at least platinum, and other catalyst components can be supported appropriately depending on the purpose and required performance. The type and physical properties of the catalyst component other than platinum are not particularly limited, and from the arbitrary catalyst components conventionally used as the catalyst component, an appropriate one is selected and used according to the purpose or required performance. be able to. Note that Rh is preferable as the other supported catalyst component. By supporting Rh together with platinum, the activity can be enhanced, and the NOx purification rate and durability can be improved. Furthermore, S
It is possible to obtain a unique effect that the purification rate of Ox can be improved. Also, when oxides of rare earth elements such as lanthanum, cerium, niodymium, lutetium, and yttrium are added as catalyst components, they become NOx adsorption points in a lean atmosphere, which improves transient characteristics and further enhances basicity and low-temperature activity. Has the effect of improving. In addition, rare earth oxides are preferable because they are more stable under hydrothermal conditions than alkaline earth oxides.

The platinum-supported catalyst as a catalyst component of the exhaust gas purifying catalyst of the present invention comprises a carrier composed of the above-mentioned porous body and a catalyst component containing at least platinum. In this way, by using the Pt / porous carrier catalyst, the catalyst functions as an oxidation catalyst for hydrocarbons and carbon monoxide, and
NOx decomposition can be performed while utilizing a partial oxidation product of hydrocarbons in an oxygen excess atmosphere. That is, in this catalyst, the catalyst component is carried on the porous carrier, and at least H of the harmful components in the exhaust gas in the oxygen excess atmosphere is
It has a function of oxidizing and removing C and CO. Further, since the catalyst component contains at least platinum, the platinum is
It has excellent performance as an adsorbent to prevent the emission of Ox, and has the function of holding NOx until the exhaust gas reaches the stoichiometric air-fuel ratio.

Further, by using Pt as an essential catalyst component and using a platinum-supported alumina catalyst, a conventional catalyst in which Cu is ion-exchanged with zeolite (Japanese Patent Laid-Open No. 283727/1988).
As compared with JP-A-3-135437 and JP-A-3-131345), the heat resistance of the catalyst can be remarkably excellent.

Regarding the method for producing this platinum-supported catalyst,
The method is not particularly limited and can be based on a conventional general method. A concrete example of this is as follows. That is, when a porous material such as alumina is immersed in a chloroplatinic acid solution, a dinitrodiammineplatinum aqueous solution or the like, Pt cations are absorbed to take advantage of Pt cations.
It is possible to make a porous body. At this time, when alumina is used as the porous material, P having excellent Pt dispersibility is used.
t-alumina can be made. Further, when it is desired to have good heat resistance and durability, it is preferable to use one having a large specific surface area of alumina. In this case, high Pt dispersibility can be maintained even in a high temperature atmosphere.

The amount of platinum supported is preferably 2 to 10 g / L. When the amount of platinum supported is less than 2 g / l,
This may cause a problem that the function of this catalyst cannot be fully utilized for exhaust gas purification. On the other hand, if the supported amount exceeds 10 g / L, there is a possibility that aggregation may easily occur due to repeated or long-term use, and that the effect of the carrier may not be exerted in the reaction. is there. Particularly, when the supported amount is 2 to 5 g / L, the supported platinum has a high thermal stability and the performance of the carrier can be effectively used in the reaction, which is particularly preferable.

It is preferable that the catalyst component containing at least platinum be supported on the carrier so that the platinum particle size is 100 Å or less. If it is larger than 100Å, it easily aggregates due to heat,
In addition, the metallic property of platinum may become stronger and the catalytic activity may be reduced. Further, when the particle size is 30 to 50 Å, aggregation is not caused, the supported amount can be made sufficient, and the catalytic ability can be further exhibited, which is more preferable.

The hydrocarbon adsorbent used in the present invention is an adsorbent mainly composed of aluminosilicate having solid acidity and molecular sieving property. That is, the acid property and molecular sieve structure of the aluminosilicate can be effectively utilized, and NOx
It has the property of retaining and accumulating a partial oxidation product of a hydrocarbon which effectively acts on the reaction from benzene to N ₂ and supplying the product in the presence of excess oxygen. Specifically, any material may be used as long as it is called zeolite, ZSM-5, mordenite, USY, H-ferrierite, H-offretite, beta zeolite, HL powder, ZSM-8, Zeolite comprising at least one selected from ZSM-11, ZSM-12, Hyper-Y, silicalite, erionite, clinobuchirolite, chabazite, phillipsite, etc. is used.
At this time, since various zeolites have different temperature regions in which adsorption and desorption of hydrocarbons occur, by combining predetermined zeolites by utilizing this property, it is possible to appropriately cope with temperature fluctuations of exhaust gas. Also, this zeolite is
Since it has a large specific surface area (about 500 m ² / g),
The surface area of the entire catalyst can be improved and the sintering of Pt can be suppressed.

Zeolites are composed of a tetrahedral network structure of SiO ₂ and Al ₂ O ₃ , and the individual tetrahedral structures are connected to each other through their corners by oxygen bridges, and the network structure in which passages and cavities penetrate Making the structure. Exchangeable cations (H ⁺ , Na ^+, etc.) are introduced into strong acid points (ion exchange points) having a negative charge in the lattice.
The SiO ₂ / Al ₂ O ₃ molar ratio is preferably 10 to 200. When the molar ratio is less than 10, 600
There is a possibility that the thermal stability will deteriorate at a high temperature of ℃ or more.
On the other hand, if the molar ratio exceeds 200, the amount of Al ₂ O ₃ decreases and the ion exchange amount decreases, so that the ion exchange amount may decrease, that is, the catalytic activity may decrease. Also,
To improve heat resistance at high temperatures, use SiO ₂ / Al
It is preferable to use one having a molar ratio of ₂ O ₃ of 20 or more. As a result, the structural change around aluminum, which is the main cause of thermal deterioration, can be reduced. Of these, Si
ZSM in which the molar ratio of O ₂ / Al ₂ O ₃ is _{30 to 200}
A -5, Y or mordenite structure is particularly preferred. As the zeolite, Cu, alkali metal,
NH ₄ ⁺ and H, which are easy to ion-exchange with alkaline earth metals
It is preferable that ⁺ is attached to the strong acid point. By making the pores large enough to prevent polycyclic aromatic hydrocarbons from entering,
It is difficult to generate coke, and it is possible to prevent structural destruction and catalyst activity reduction due to pore blockage. There are various sizes of hydrocarbons in the exhaust gas, and effective zeolites are available for each type. Further, there are zeolites suitable for each temperature atmosphere due to the difference in adsorption energy to zeolites. Therefore, it is preferable to combine various zeolites according to the atmosphere of the exhaust gas to be purified and to make an adsorbent that forms a partial oxidation product of hydrocarbons in a wide temperature range in consideration of the operating temperature range.

The hydrocarbon adsorbent preferably carries at least one metal selected from alkali metals and alkaline earth metals. That is, the strong acid point, which is the ion exchange point in the aluminosilicate, causes the generation of coke in which a large number of graphite and the like formed by the decomposition of organic substances such as hydrocarbons are bonded, the blockage of pores by the coke, and the structure of the aluminosilicate. It also leads to destruction. Therefore, by supporting at least one metal selected from alkali metals and alkaline earth metals, of the adsorption points of organic compounds such as hydrocarbons, it is possible to eliminate extra strong acid points involved in coke formation. Therefore, it is possible to prevent the generation of coke and prevent the deterioration of the catalyst. At this time, the supported metals are potassium (K), magnesium (Mg), calcium (C
Preferred are a), strontium (Sr), and barium (Ba). The supported amount of the metal is preferably 0.1 to 10% by weight with respect to the hydrocarbon adsorbent mainly composed of aluminosilicate having solid acidity and molecular sieving property. When the supported amount exceeds 10% by weight, the number of acid points required for the reaction is reduced, and the reaction between NOx and HC becomes difficult to proceed.

The exhaust gas purifying catalyst of the present invention comprises a platinum-supported catalyst in which a catalyst component containing at least platinum is supported on a carrier made of a porous material, and the above hydrocarbon adsorbent. The method for producing the exhaust gas purifying catalyst is not particularly limited and can be based on a conventional general method. A brief example of the specific case is as follows.

That is, a Pt / alumina-based platinum-supported catalyst and a hydrocarbon adsorbent are mixed and well physically mixed, and at a predetermined temperature (for example, 550 ° C.) for a predetermined time (for example, 2
Time) Bake. In the present method, by coexisting the platinum-supported catalyst and a hydrocarbon adsorbent mainly composed of aluminosilicate, the active point on Pt where NO _x decomposition reaction occurs and the hydrocarbon activated by the zeolite acid point The frequency of reactions can be increased. Therefore, it is desirable that the both be carefully mixed in a mortar or that they be well mixed in a slurry state. The mixing ratio of the platinum-supported catalyst and the hydrocarbon adsorbent is preferably 50 to 80% by weight of the hydrocarbon adsorbent with respect to the total amount of both. If the mixing amount is less than 50% by weight, the effect of the present invention may not be sufficiently exhibited, and if it exceeds 80% by weight, Pt dispersibility and heat resistance may be deteriorated. In addition, it is preferable that the platinum-supported catalyst and the hydrocarbon adsorbent are well physically mixed and sufficiently dispersed and close to each other.

When it is mounted as an exhaust gas purifying catalyst for automobiles, it is preferably a monolith catalyst. That is, first, one or more alkali metals or alkaline earth metals are loaded on the hydrocarbon adsorbent by the ion exchange method. This is mixed with an alumina slurry containing a rare earth element (for example, a slurry obtained by stirring and mixing active alumina, alumina sol, water, etc.), and a porous monolith carrier such as a cordierite-integrated honeycomb substrate is immersed therein. This is taken out, the excess slurry is blown off with compressed air, dried at 200 ° C., and fired at 600 to 700 ° C. Further, if necessary, after immersing in an aqueous solution of lanthanum nitrate, etc., drying and firing to carry an additive such as La, it is immersed in an aqueous solution of a platinum compound such as dinitrodiammine platinum, chloroplatinic acid, etc. and dried and fired. (200-50
(0 ° C.) to obtain the catalyst of the present invention.

In the exhaust gas purification method according to the present invention,
Exhaust gas in excess of oxygen is made to coexist with a catalyst in which a catalyst component containing at least platinum is supported on a support made of a porous material and a hydrocarbon adsorbent mainly composed of aluminosilicate having solid acidity and molecular sieve property. By contacting with the exhaust gas purifying catalyst porous body, carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas in an oxygen excess atmosphere can be simultaneously purified. At this time, the form of contact between the catalyst and the exhaust gas is not particularly limited. The location of the catalyst in the exhaust gas passage is also not particularly limited, but for example, in the exhaust passage of a lean burn engine,
Even if it is used alone under the floor of the vehicle, it can be arranged under the floor on the downstream side of these in combination with the start catalyst or the three-way catalyst.

Space velocity (S) for introducing exhaust gas into the catalyst layer
V) is preferably in the range of 300,000 to 10,000 hr ^-1 , and the catalyst layer temperature is also preferably 200 to 500 ° C.

As described above, by using the exhaust gas purifying catalyst and the exhaust gas purifying method of the present invention, C in the exhaust gas discharged from the vehicle equipped with the lean burn engine.
O, HC, NOx during steady running (for example, 40 km / h
In both r) and transient states (for example, urban driving simulation state), purification can be performed with high efficiency.

That is, it is considered that the catalyst of the present invention and the catalyst in the method of the present invention act as follows with respect to the composition change of the engine exhaust gas when the vehicle is running.

When the vehicle is stopped, the exhaust gas A / F is theoretical A / F
It is in the vicinity of F and the exhaust gas contains little NOx. At this time, on the catalyst, NOx in the exhaust gas, NOx adsorbed by Pt, and further La ₂ carried as needed.
NOx stored in a mixture of rare earth oxides such as O ₃ and moved to Pt is reduced gas (CO, H ₂ ,
HC) and NO _x is reduced to N ₂ . At this time, the reducing gas is also purified. After the decomposition of the adsorbed NO _x ends, the reducing gas adsorbs on Pt. At the same time, hydrocarbons accumulate in the adsorbent.

During vehicle acceleration, the A / F continuously changes from near the theoretical A / F to lean (A / F = about 23), and a large amount of NO _x exists in the exhaust gas. On the catalyst, NO _x in the exhaust gas is reduced to N ₂ by the reducing gas adsorbed on Pt and the partial oxidation product of hydrocarbon adsorbed on the adsorbent. At this time, the reducing gas is also purified.

During steady-state running, the A / F is in the lean region (A / F = about 23), NO _x in the exhaust gas is less than during acceleration, HC is increased, and part of the NO _x is reduced. It At this time, hydrocarbons are also purified. No reaction NO _x
Are adsorbed and occluded by Pt and other catalyst components (eg lanthanum oxide).

Therefore, the NO _x emission in the vehicle running mode of the urban area or the like, which is a combination of such driving patterns, is reduced by the catalyst of the present invention. Also, HC, CO
With respect to the above, since the catalyst of the present invention has a very excellent oxidizing ability, it exhibits a high purifying ability in the excess of oxygen or near the stoichiometric air-fuel ratio. Further, the more frequently it is exposed to the atmosphere near the theoretical A / F, the more the hydrocarbon is occluded in the adsorbent, and the more effectively it is used for NO _x purification in the oxygen excess atmosphere.

Examples of the present invention will be described below.

First embodiment

First, a commercially available Na-type ZSM-5 is prepared,
This was ion-exchanged with NaCl to form an NH ₄ ⁺ type, and then ion-exchanged with an aqueous solution for ion exchange shown in Table 1 to obtain an adsorbent of this example carrying an alkali metal or an alkaline earth metal. Table 1 shows the amount of the metal-supported material (% by weight based on the hydrocarbon adsorbent).

[0057]

[Table 1]

Γ-alumina (manufactured by WR GRACE: MI-386) containing 4.0% by weight of La and 0.5% by weight of Ce in 300 ml of 3 g / l dinitroammine platinum nitric acid solution 50
g, and further, 12.5 g of ion-exchanged zeolite was added, and the mixture was stirred at room temperature for 5 hours. This was filtered, washed well with distilled water, and dried at 110 ° C. for 5 hours.

Next, the obtained mixture is heated to 500 ° C. in air.
It was calcined for 2 hours, compacted, and processed into pellets of 0.3 to 0.7 mm pieces to prepare the exhaust gas purifying catalyst of this example (Sample Nos. 1 to 5). The catalyst used for evaluation was 0.5 g.

Next, a performance evaluation test of the obtained exhaust gas purifying catalyst was conducted by an activity evaluation method and a durability test method.

First, the activity was evaluated by A / F = 1 shown in Table 2.
Model gas for activity evaluation equivalent to 8 (C ₃ H ₆ , NO, C
_{O, CO 2, H 2 O} , the catalyst in N ₂ balance) in 1
The NO purification rate when the temperature was raised from 00 ° C to 600 ° C was obtained. At the time of evaluation, the purification method in the transition range from stoichiometric to lean of the actual vehicle was estimated by performing pretreatment by exposing the catalyst to an evaluation gas equivalent to A / F = 14.6 at 15 ° C for 15 minutes. did. The obtained results are shown in Table 1.

[0062]

[Table 2]

Next, the durability test was conducted by using a tubular furnace in a model gas for durability test evaluation corresponding to A / F = 22 shown in Table 2 at 800 ° C. for 5 hours. The results obtained are
It shows in Table 1.

For comparison, a comparative catalyst was prepared in the same manner as in the above Example except that no adsorbent was mixed, and the same performance evaluation test as above was conducted. The results are also shown in Table 1.

As is clear from Table 1, the initial activity is remarkably improved by supporting zeolite on Pt / alumina. In particular, it can be seen that the heat resistance is highest when the zeolite adsorbent is mixed with zeolite in which Mg or Ba is ion-exchanged.

Second embodiment

First, a commercially available Na-type ZSM-5 was prepared,
This was ion-exchanged with NaCl to form an NH ₄ ⁺ type, and then calcined in air at 450 ° C. for 5 hours to form a proton type zeolite, which was used as the adsorbent of this example.

Next, the adsorbent mixed amount is shown in Table 3, and the exhaust gas purifying catalyst of this example was prepared in the same manner as in the first example (Sample Nos. 6 to 9).

[0069]

[Table 3]

Next, the performance evaluation test of the obtained exhaust gas purifying catalyst was conducted by the activity evaluation method and the durability test method as in the first embodiment. The results obtained are shown in Table 3.

For comparison, a comparative catalyst was prepared in the same manner as in the above Example except that the adsorbent was not mixed, and the same performance evaluation test as above was conducted. The results are also shown in Table 3.

As is clear from Table 3, the activity is remarkably improved by supporting the zeolite on Pt / alumina. In particular, it can be seen that the activity is highest at the initial stage and after the endurance when the zeolite mixing ratio is 80% by weight.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hideaki Muraki Inventor Hideaki Muraki 41, Nagakute-cho, Aichi Prefecture Automobile Co., Ltd. (72) Inventor, Master Tateishi 7800, Chihama, Daito-cho, Ogasa-gun, Shizuoka Cataler Industry Co., Ltd. (56) Reference JP-A-1-139144 (JP, A) JP-A-2-135126 (JP, A) JP-A-3-131345 (JP, A) JP-A-4-176337 (JP, A) JP-A-5-168939 (JP, A) JP-A-4-267951 (JP, A) (JP-A) 58) Fields surveyed (Int.Cl. ⁷ , DB name) B01J 21/00-38/74 B01D 53/86 B01D 53/94

Claims (3)

(57) [Claims] 1. An exhaust gas purification catalyst for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, the catalyst component containing at least platinum in a carrier made of a porous body. a catalyst supported with a solid acidic and molecular sieve properties obtained by carrying only at least one or more metals selected from alkali metal and alkaline earth metal aluminosilicate having a hydrocarbon adsorbent, but physically mixed The exhaust gas purifying catalyst is characterized in that it is sufficiently dispersed and is in a state of being close to each other. 2. A method for producing an exhaust gas purifying catalyst for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, comprising at least platinum on a support made of a porous body. a catalyst supported catalyst component comprising a hydrocarbon adsorbent at least one or more metal only selected from alkali metal and alkaline earth metal aluminosilicate were carried by the ion exchange method with a solid acidic and molecular sieve properties Are mixed physically and sufficiently dispersed so that they are in close proximity to each other. 3. An exhaust gas purification method for simultaneously purifying carbon monoxide, hydrocarbons and nitrogen oxides in the exhaust gas under an oxygen excess atmosphere, wherein the oxygen excess exhaust gas is applied to a carrier made of a porous body. a catalyst supported catalyst component comprising at least platinum, the solid acidic and molecular sieve properties obtained by carrying only at least one or more metals selected from alkali metal and alkaline earth metal aluminosilicate having a hydrocarbon adsorbent ,
Are mixed physically and brought into contact with exhaust gas purifying catalysts that are sufficiently dispersed and in close proximity to each other.