JP2000262860A - Exhaust gas purification device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve NOx purification performance of an exhaust gas purifying apparatus A provided with a NOx catalyst 1 for purifying NOx even in exhaust gas of an oxygen-excess atmosphere such as a diesel engine without deteriorating fuel efficiency. Plan. SOLUTION: HC and NOx in exhaust gas are adsorbed,
Platinum Pt (third component) is supported by ion exchange on the zeolite (first component) for dehydrogenating the HC. The zeolite and iron Fe for dehydrogenating HC (first component)
And silver Ag (a second component) that oxidizes NO to NO2, to form a catalyst layer 3 on the carrier 2. NO in the exhaust gas is oxidized by silver Ag to NO2, and HC is converted to iron F
e and dehydrogenation with zeolite to carbon fine particles,
They react better with the catalytic activity of platinum Pt.
Cobalt Co may be supported instead of iron Fe.

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 apparatus capable of purifying nitrogen oxides (NOx) even in an exhaust gas having an oxygen-excess atmosphere such as an exhaust gas of a diesel engine.

[0002]

2. Description of the Related Art Conventionally, as this type of exhaust gas purifying apparatus, for example, as disclosed in Japanese Patent Application Laid-Open No. 9-38467, NOx is reduced in exhaust gas of an oxygen-excess atmosphere in an exhaust passage of a diesel engine. Injection device that disposes NOx catalyst and injects fuel (light oil) into the exhaust passage upstream of it, and decomposes higher hydrocarbons contained in the injected fuel into lower hydrocarbons with short carbon chains There is known a catalyst provided with a hydrocarbon cracking catalyst.

That is, in general, the purification rate of the NOx catalyst is considerably lower than, for example, the purification rate of the three-way catalyst near the stoichiometric air-fuel ratio. For example, a large amount of unsaturated hydrocarbons having 2 to 5 carbon atoms is preferably supplied to enhance NOx reduction performance.

[0004]

However, according to the conventional exhaust gas purifying apparatus, the fuel injected into the exhaust passage is discharged without contributing to the combustion, so that the fuel efficiency is deteriorated. In such a case, the amount of fuel injected into the exhaust gas tends to exceed an appropriate amount, and in this case, there is a problem that the amount of unburned hydrocarbons discharged into the atmosphere increases.

[0005] Further, most of the NOx in the exhaust gas is nitric oxide (NO), and this NO is in a metastable state in a normal exhaust temperature state (400 ° C or lower) of a diesel engine and has a reactivity. Low and the decomposition reaction is extremely slow. For this reason, in the case of directly reducing and decomposing NO in exhaust gas as in the above-mentioned conventional exhaust gas purifying apparatus, no matter how much unsaturated hydrocarbon with high reducibility is supplied, N
It has been difficult to sufficiently enhance the Ox purification performance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the NOx purifying performance of a NOx catalyst without deteriorating fuel efficiency.

[0007]

Means for Solving the Problems To achieve the above object, a solution of the present invention is to oxidize NO in exhaust gas once with oxygen (O2) excessively contained to form highly reactive NO2. At the same time, HC in the exhaust gas is dehydrogenated, and this is used to reduce and decompose the NO2.

[0008] More specifically, the invention of claim 1 is directed to an exhaust gas purifying apparatus provided with a catalyst for purifying nitrogen oxides in an exhaust gas in an oxygen-excess atmosphere, wherein the catalyst comprises carbonized gas in the exhaust gas. A first component for dehydrogenating hydrogen, a second component for oxidizing nitrogen monoxide in exhaust gas to nitrogen dioxide, and nitrogen dioxide oxidized by the second component being dehydrogenated by the first component. And a third component that is reduced and purified using hydrocarbons.

According to the above-mentioned structure, first, hydrocarbon (HC) in the exhaust gas is dehydrogenated by the first component of the catalyst to form fine carbon C particles having a high reactivity as a reducing agent. Nitrogen monoxide (NO) in the exhaust gas is oxidized by the second component of the catalyst to become nitrogen dioxide (NO2), which is more reactive than NO, and this NO2 is converted into carbon dioxide by the catalytic activity of the third component. By reacting very effectively with the fine particles, the reductive decomposition of NOx is promoted.

[0010] That is, the NOx purification performance by the catalyst can be improved without wastefully consuming the fuel and deteriorating the fuel efficiency, and the HC emission does not particularly increase.

According to the second aspect of the present invention, the second component of the catalyst comprises:
It is assumed that the hydrocarbon dehydrogenated by the first component is adsorbed. As a result, HC dehydrogenated by the first component of the catalyst, that is, carbon fine particles and HC having a short carbon chain generated along with the generation of the carbon fine particles are adsorbed by the second component,
By easily reacting with NO2 oxidized by the second component, reductive decomposition of NO2 can be further promoted. Also,
The amount of HC emission to the atmosphere can also be reduced.

In the invention of claim 3, at least a part of the first component of the catalyst is supported so as to be close to the third component. Thus, HC dehydrogenated by the first component of the catalyst, that is, carbon fine particles and the like are easily catalyzed by the third component.
Can be further promoted.

According to a fourth aspect of the present invention, the first component of the catalyst according to the third aspect includes a transition metal and a zeolite, and the third component of the catalyst is supported so as to be close to the zeolite. I do. As a result, both the transition metal and the zeolite have a function of adsorbing and oxidizing HC, and the oxidizing function is not so strong. Therefore, the adsorbed HC is converted into carbon dioxide (CO2) and water (H2O). It is not completely oxidized but dehydrogenated to produce carbon fine particles.

In the invention of claim 5, both the second component and the third component of the catalyst are transition metals. In this way, the oxidation of NO in the exhaust gas can be promoted by the catalytic action of the transition metal, and the reaction between NO2 and carbon C can be activated. In addition, it is preferable to use a noble metal among transition metals.

In the invention according to claim 6, the first component of the catalyst contains at least one of iron and cobalt. In this case, a preferable configuration of the first component that dehydrogenates HC in exhaust gas without completely oxidizing HC is realized.

In the invention of claim 7, the second component of the catalyst is silver. In this case, the second oxidation of NO in the exhaust gas is performed.
Preferred configurations of the components are embodied.

According to the invention of claim 8, the third component of the catalyst is platinum. In this case, a preferable configuration of the third component that reduces and decomposes NO2 is embodied.

According to a ninth aspect of the present invention, there is provided an exhaust gas purifying apparatus provided with a catalyst for purifying nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, wherein the catalyst comprises at least one of platinum, silver, iron and cobalt. And zeolite, and the platinum is supported on the zeolite by an ion exchange method.

According to this configuration, first, H in the exhaust gas
C is adsorbed on at least one of iron and cobalt and the zeolite and dehydrogenated, and carbon fine particles are formed and adsorbed on the zeolite or the like. In addition, NO in the exhaust gas is oxidized by silver Ag, and is more reactive than NO.
Becomes 2. And, due to the catalytic activity of platinum Pt, the N
The very effective reaction between O2 and carbon fine particles promotes the reductive decomposition of NOx. Therefore, claim 1
As in the invention of the first aspect, the NOx purification performance can be improved without deteriorating the fuel efficiency, and the amount of HC emission can be reduced.

[0020]

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

FIG. 2 shows a structural example of the NOx catalyst 1 used in the exhaust gas purifying apparatus A according to the embodiment of the present invention.
In the figure, reference numeral 2 denotes a cordierite carrier having a honeycomb structure in which a large number of through holes 2a, 2a, ... extending in parallel with each other in the direction in which the exhaust gas flows are formed. The catalyst layer 3 is formed on the wall surface. Although not shown, the NOx catalyst 1 is arranged, for example, in an exhaust passage of a diesel engine, and is configured to reduce and purify NOx in exhaust gas even in an oxygen-excess atmosphere.

The catalyst layer 3 of the NOx catalyst 1 has platinum P
β-type zeolite carrying ion-exchanged t (third component) (first component), silver Ag (second component) and iron Fe (first component)
Component). The zeolite is not limited to β-type, and various types such as X-type and Y-type can be used. Also, instead of iron Fe, cobalt C
o may be used, and it is also possible to use palladium Pd or rhodium Rh instead of platinum Pt.

When the NOx catalyst 1 is disposed in, for example, an exhaust passage of a diesel engine, HC and NO are first adsorbed by zeolite in exhaust gas in an oxygen-excess atmosphere, and this HC is converted to iron Fe and zeolite. It is dehydrogenated by the oxidizing action to generate carbon C fine particles and HC having a short carbon chain.

At this time, since the oxidizing effect of iron Fe and zeolite is not so strong, HC is completely oxidized and water H
It does not become 2O or carbon dioxide CO2 but adheres to the surface of the iron Fe or zeolite as carbon fine particles. Since such carbon particles are extremely reactive, they become an effective reducing agent and do not desorb until the catalyst 1 reaches a relatively high temperature state as compared with HC. Does not increase.

On the other hand, the NO adsorbed on the zeolite reacts with O2 excessively contained in the exhaust gas by the oxidizing action of silver Ag to form highly reactive NO2. And this NO2
Is reductively decomposed very effectively using the carbon fine particles as a reducing agent by the catalytic activity of platinum Pt, and becomes harmless nitrogen gas N2, carbon dioxide CO2 and water H2O, and is discharged into the atmosphere. Here, the HC dehydrogenated by the zeolite or iron Fe, that is, the HC having a fine carbon particle or a short carbon chain, has a property of being adsorbed by silver Ag.
It is very easy to react with NO2 oxidized by g.

Further, since the platinum Pt is ion-exchanged and supported on the zeolite, the dispersibility of the platinum Pt is increased, and the bias of the electric charge makes it easy to attract NO2. And fine carbon particles are more susceptible to the catalytic action of platinum Pt. That is, the reduction decomposition of NO2 is further promoted by such a loaded state of platinum Pt, silver Ag and the like. Further, since the dehydrogenated HC is adsorbed on silver Ag, the amount of HC discharged into the atmosphere is rather reduced.

Therefore, according to the exhaust gas purifying apparatus A according to this embodiment, iron Fe and zeolite for dehydrogenating HC in exhaust gas and silver A for oxidizing NO to NO2 are used.
g and platinum Pt for reducing and purifying NO2, HC can be dehydrogenated and adsorbed in a highly reactive state even in an oxygen-excess atmosphere, and at the same time, the exhaust gas contains NO is converted into highly reactive NO
Since NO2 can be oxidized to react with the dehydrogenated HC efficiently, the reductive decomposition of NOx can be sufficiently promoted. Moreover, the fuel consumption is not deteriorated, and the HC emission is not increased.

The NOx catalyst 1 of the present invention can be applied not only to a diesel engine but also to, for example, a gasoline lean burn engine or a direct injection stratified combustion engine, and purifies exhaust gas having an oxygen concentration of 4% or more. Can also be adapted. The NOx catalyst 1 may be of a single-layer type, as in this embodiment, or of a stacked type having a plurality of catalyst layers. Further, the present invention is not limited to the honeycomb type monolith catalyst as in this embodiment, and can be applied to a pellet catalyst in which a case is filled with a granular catalyst.

[0029]

Next, a specific embodiment will be described.

(Catalyst of Example) NOx used in this test
In the same manner as in the above embodiment, the catalyst has platinum Pt supported on a β-type zeolite as a base material by an ion exchange method. In addition, silver Ag and iron Fe or cobalt Co are used.
Is carried by, for example, an impregnation method. Specifically, first, a β-type zeolite powder is put into pure water, and a platinum solution (tetraammineplatinum dichloride aqueous solution) is added thereto.
Was added in a predetermined amount, and the mixture was stirred while being heated to a temperature of about 90 ° C., and ion exchange was performed continuously for 6 hours. Thereafter, this was allowed to cool and filtered, and the obtained solid was washed with pure water, and further dried, and the obtained powder was calcined at 350 ° C. for 2 hours to obtain a catalyst powder.

Then, the catalyst powder and the hydrated alumina powder as a binder are put into water at a predetermined ratio, and a predetermined amount of a nitric acid solution such as silver Ag and iron Fe is added thereto, followed by stirring to form a slurry. Obtained. Then, a carrier (having a capacity of 25 mL, 400 cells / square inch, and a wall thickness of 6 mm inch) is immersed in the slurry, and is lifted up to about 10 mm.
The operation of drying at 0 ° C. was repeated, and firing was performed at 500 ° C. for 2 hours in an air atmosphere. As a result, the amount of platinum Pt supported on the carrier was 0.5 g / L, and the amount of silver Ag supported was 10 g / L.
In addition, the loading amount of iron Fe or cobalt Co is 5 g /
L. The concentration of impurities in the catalyst is 1% or less.

(Evaluation of Catalyst) Each of the catalysts adjusted as described above was incorporated into a simulated exhaust gas distribution device, and a maximum NOx purification rate (in this case, NO purification) was passed while simulating exhaust gas having the following composition was circulated. Rate) was measured. In this measurement, the temperature of the gas at the inlet of the catalyst was increased from 150 ° C to 400 ° C.
The temperature was raised to 25 ° C./min at 25 ° C., and the space velocity was set to SV = 56000 / h.

HC (C ₃ H ₆ ): 170 ppmC, NO: 170
ppm, CO: 200ppm, CO2: 2%, H2O: 5%, O
2: 10%, N2: balance.

FIG. 1 shows the results obtained by repeating the above measurement four times using the same test material. In the figure, Pt / Ag-Fe / β is the one carrying iron Fe as the first component, and Pt / Ag-Co / β is iron Fe
Instead of cobalt.

According to the test results shown in FIG. 1, the maximum NOx purification rate by each catalyst increases as the number of measurements increases from one to three times, and is substantially the same for the third and fourth measurements.
It shows a high value of about 22%. As described above, the purifying performance increases as the number of times of measurement increases.
During the measurement up to three times, the HC in the exhaust gas is dehydrogenated,
It is considered that the particles adhered to the catalyst as carbon fine particles. That is, the fine carbon particles adhered to the surface of zeolite or silver Ag as the catalyst exhibit excellent reactivity and strong reducibility, thereby promoting reductive decomposition of NOx.

The final maximum NOx purification rate is about 22
% Is high because NO in the exhaust gas is converted to NO2 by silver Ag.
It is considered that this is due to the fact that platinum Pt is ion-exchanged and supported on zeolite, and is more highly dispersed and supported on an atomic level than impregnation or the like.

Next, in the same catalyst as in the above embodiment,
The change in the NOx purification performance of each catalyst when the supported amount of silver Ag (the amount of added Ag) was changed in the range of 0 to 50 g was examined. As shown in FIG. 3, this result indicates that the amount of silver
0 g / L is the highest, and when it is lower than 0 g / L, it decreases rapidly with the decrease in the supported amount.
It can be seen that the purification performance does not improve any more even at g / L or more, but rather the purification performance gradually decreases as the supported amount increases. This is because when the carrying amount of silver Ag is small, the effect of oxidizing NO in exhaust gas to NO2 becomes insufficient,
It is considered that an excessively large amount causes a reduction in performance due to sintering, and also has some adverse effect on the activity of platinum Pt. Thus, the loading amount of silver Ag is suitably about 8 to 30 g / L, preferably 10 to 20 g / L.
g / L.

FIG. 4 shows the change in NOx purification performance when the amount of iron Fe carried (Fe addition amount) is similarly changed in the range of 0 to 30 g. According to the figure, the NOx purification performance is highest when the carried amount of iron Fe is around 5 g / L, and decreases when it is higher or lower. It is considered that such a tendency is exhibited for the same reason as in the case of silver Ag described above. It should be noted that instead of iron Fe, cobalt C
A similar tendency is observed for the catalyst supporting o.
Accordingly, it is considered that the supported amount of iron Fe is appropriately about 1 to 10 g / L, preferably 2 to 7 g / L.

[0039]

As described above, according to the exhaust gas purifying apparatus according to the first aspect of the present invention, the catalyst of the exhaust gas purifying apparatus is capable of dehydrogenating HC to form carbon fine particles.
A second component that oxidizes NO to NO2 and a third component that reduces and purifies NO2 using the carbon fine particles, so that NO in exhaust gas can be converted into NO2 that is more reactive than NO2. By using the catalytic activity of the third component, NO2 can be extremely effectively reacted with the carbon fine particles having a high reactivity as a reducing agent to promote the reductive decomposition of NOx. Therefore, it is possible to improve the NOx purification performance of the catalyst without causing deterioration in fuel efficiency and increase in HC emission.

According to the second aspect of the present invention, the fine carbon particles and the like dehydrogenated by the first component of the catalyst can be easily reacted with NO2 to promote the reductive decomposition of the NO2, and the HC emission can be reduced. Can also be reduced.

According to the third aspect of the present invention, the reductive decomposition of NO2 by the fine carbon particles and the like dehydrogenated by the first component of the catalyst can be further promoted.

According to the fifth aspect of the present invention, the oxidation of NO in the exhaust gas can be promoted by the transition metal, and the NO2
The reaction between carbon and carbon C can also be activated.

According to each of the sixth, seventh and eighth aspects of the present invention, preferred configurations of the first, second and third components of the catalyst are obtained.

According to the ninth aspect, the same effect as that of the first aspect can be obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing how the NOx purification performance is improved by actually using a NOx catalyst according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a structure of a NOx catalyst according to an embodiment of the present invention.

FIG. 3 is a graph showing a change characteristic of a maximum NOx purification rate when an amount of silver Ag added to a NOx catalyst is changed.

FIG. 4 is a graph showing a change characteristic of a maximum NOx purification rate when an amount of iron Fe added to a NOx catalyst is changed.

[Explanation of symbols]

 A Exhaust gas purifier 1 NOx catalyst 2 Carrier 3 Catalyst layer

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/28 301 F01N 3/28 301Q B01D 53/36 102B 102G (72) Inventor Takahiro Kurokawa Aki-gun, Hiroshima 3-1 Fuchu-cho Shinchi Mazda Co., Ltd. (72) Inventor Yasuhito Watanabe 3-1 Fuchu-cho Shinchi, Aki-gun, Hiroshima Pref. Mazda Co., Ltd. F-term (reference) FB10 GA01 GA06 GA18 GB01W GB01Y GB05W GB06W GB07W GB09W GB09X GB09Y GB17X 4D048 AA06 AB02 BA11X BA30X BA34X BA36X BA37X BB02 CC38 CC46 CC50 EA04 4G069 AA01 AA03 AA08 BA07A BA07B CA13 BCBBC BCBC BC

Claims (9)

[Claims] 1. An exhaust gas purifying apparatus comprising a catalyst for purifying nitrogen oxides in exhaust gas in an oxygen-excess atmosphere, wherein the catalyst is configured to dehydrogenate hydrocarbons in exhaust gas.
A second component for oxidizing nitrogen monoxide in the exhaust gas to nitrogen dioxide, and a nitrogen component oxidized by the second component to the first component.
An exhaust gas purifying apparatus, comprising: a third component that is reduced and purified by using a hydrocarbon dehydrogenated by the component. 2. The exhaust gas purifying apparatus according to claim 1, wherein the second component of the catalyst adsorbs hydrocarbon dehydrogenated by the first component. 3. The exhaust gas purifying apparatus according to claim 1, wherein at least a part of the first component of the catalyst is supported so as to be close to the third component. 4. The catalyst according to claim 3, wherein the first component of the catalyst contains a transition metal and zeolite, and the third component of the catalyst is supported so as to be close to the zeolite. Exhaust gas purification device. 5. The exhaust gas purifying apparatus according to claim 1, wherein both the second component and the third component of the catalyst are transition metals. 6. The exhaust gas purifying apparatus according to claim 1, wherein the first component of the catalyst contains at least one of iron and cobalt. 7. The exhaust gas purifying apparatus according to claim 1, wherein the second component of the catalyst is silver. 8. The exhaust gas purifying apparatus according to claim 1, wherein the third component of the catalyst is platinum. 9. An exhaust gas purifying apparatus provided with a catalyst for purifying nitrogen oxides in an exhaust gas in an oxygen-excess atmosphere, wherein the catalyst includes platinum, silver, at least one of iron or cobalt, and zeolite. The exhaust gas purifying apparatus, wherein the platinum is supported on zeolite by an ion exchange method.