JP2002168117A - Exhaust emission control system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control system capable of efficiently controlling NOx in an excessive oxygen area exhausted from an internal combustion engine and of efficiently controlling CO.HC even if exhaust gas is in a low-temperature range. SOLUTION: In the upstream of an exhaust gas passage in the internal combustion engine, there is disposed an exhaust gas component concentration controlling means of controlling carbon monooxide and hydrogen in exhaust gas and the ratio of hydrogen/carbon monooxide concentration ratio when air-fuel ratio is rich or a stoichiometric air amount. In the downstream of the exhaust gas passage, there is disposed an NOx adsorption reduction catalyst of adsorbing nitrogen oxides when the air-fuel ratio is lean and reducing the adsorbed nitrogen oxides to nitrogen when the air-fuel ratio is rich or the stoichiometric air amount.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hydrocarbons (HC) and carbon monoxide (C) in exhaust gas discharged from internal combustion engines such as automobiles (gasoline and diesel) and boilers.
The present invention relates to an exhaust gas purification system that purifies O) and nitrogen oxides (NOx), and more particularly to a system that efficiently purifies NOx, HC, and CO in an oxygen excess region (lean region) and a low temperature region.

[0002]

2. Description of the Related Art Conventionally, demands for fuel-efficient vehicles have increased due to the problem of depletion of petroleum resources and the problem of global warming, and development of lean-burn vehicles has been attracting attention for gasoline vehicles.

[0003]

However, in lean-burn vehicles, during lean-burn operation, the exhaust gas atmosphere becomes an oxygen-excess atmosphere (lean) compared to the stoichiometric air-fuel state. Is applied, there is a problem that the NOx purification action becomes insufficient due to the influence of excessive oxygen. Therefore, development of a catalyst that can purify NOx even when oxygen becomes excessive has been desired.

[0004] Various catalysts for purifying NOx in the lean region have been proposed. For example, a lean catalyst such as Pt and lanthanum supported on a porous carrier (JP-A-5-168860) has been proposed. Absorb NOx in the area,
A catalyst for releasing and purifying NOx during stoichiometry is disclosed. However, even with such a catalyst, NO
There is a problem that x purification performance may be insufficient.

On the other hand, conventionally, a three-way catalyst for purifying exhaust gas by simultaneously oxidizing CO and HC and reducing NOx has been used as a catalyst for purifying exhaust gasoline engine vehicles. As such a three-way catalyst, for example, a support layer made of γ-alumina is formed on a heat-resistant carrier such as cordierite, and platinum (Pt), palladium (P
Those supporting a catalytic noble metal such as d) and rhodium (Rh) are widely known.

However, as described above, carbon dioxide (CO ₂ ) in exhaust gas discharged from an internal combustion engine such as an automobile has been a problem from the viewpoint of protecting the global environment. As a solution, a lean burn engine and a diesel engine that perform lean burn in a lean region are promising. In these engines, the use of fuel is reduced in order to improve fuel efficiency, and the generation of CO ₂ as combustion exhaust gas can be suppressed. A catalyst for purifying HC, CO and NOx in such a lean region is disclosed in JP-A-9-5.
No. 7098. In this catalyst, H
In order to increase the purification rate of C, CO and NOx, Pt, Rh and Pd are separated and supported. NOx is stored in the NOx storage material and reduced by Pt. To oxidize.

However, the above NOx storage material is not advantageous for NOx adsorption in a very low temperature range (140 ° C. or lower).
Due to its presence, the adsorption amount of NOx may decrease.
Further, it is known that the catalytic activity varies depending on the type of catalytic noble metal, and Pd has a problem that it is more susceptible to poisoning by sulfur oxides present in exhaust gas than Pt. Further, it is known that components that hinder the CO oxidation activity in a very low temperature range are components that coexist with exhaust gas such as NO and HC.

On the other hand, conventionally, in a diesel engine or the like, a catalyst is provided in the middle of an exhaust pipe to purify NOx discharged from an internal combustion engine. As such a NOx purification catalyst, various catalysts such as a zeolite-based catalyst and an alumina-based catalyst have been known, but all have a limited temperature range in which the NOx purification action is exhibited. Therefore, it has been proposed to extend the NOx purification temperature range by combining a plurality of catalysts having different purification temperature ranges. For example, JP-A-6-13425
No. 8 discloses a catalyst device using a plurality of catalysts having different reaction temperatures to obtain the highest activity by changing the amount of cobalt supported on mordenite. The plurality of catalysts are usually arranged such that the high-temperature active catalyst is arranged on the upstream side of the exhaust passage and the low-temperature active catalyst is arranged on the downstream side of the exhaust passage. Japanese Patent Application Laid-Open No. 6-307231 discloses a catalyst device in which a plurality of NOx purification catalysts are arranged in series in the flow direction of exhaust gas so that the oxidizing activity for hydrocarbons increases gradually toward the downstream side. Is disclosed.

[0009] By combining a plurality of catalysts in this way, the purification temperature range is widened, but in a portion where the purification curves of the plurality of catalysts overlap, both NOx purification rates are low.
Even if these are added, there is a problem that a practically sufficient NOx purification rate cannot be obtained, and a temperature range of a valley where the NOx purification rate is low exists. In particular, in a low temperature range where the activity of the catalyst is low, even if a plurality of catalysts are used, each N
Since the amount of purified Ox is small, there is a problem that the NOx purification rate is significantly reduced. Against this background, the present inventors have found that reducing agents (CO and HC), which are very useful for purifying NOx in a high temperature range, prevent purification of NOx in a low temperature range. It was found that the rate did not improve significantly.

The present invention has been made in view of the above-mentioned problems of the prior art, and has as its object to efficiently purify NOx in an oxygen-excess region exhausted from an internal combustion engine and to reduce exhaust gas. Is in the low temperature range.
An object of the present invention is to provide an exhaust gas purification system that efficiently purifies HC. Another object of the present invention is to provide an exhaust gas purification system that exhibits a high or higher NOx purification rate in a wide purification temperature range, particularly in a low temperature range.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, adjusted the concentration of the components of the exhaust gas and adjusted the adsorption and desorption of NOx by changing the temperature of the exhaust gas. As a result, they have found that the above-mentioned problems can be solved, and have completed the present invention.

That is, the exhaust gas purifying system of the present invention comprises:
On the upstream side of the exhaust gas passage of the internal combustion engine, when the air-fuel ratio is rich or stoichiometric, the component concentration of the exhaust gas is 2.0% or less of carbon monoxide, 0.5% or more of hydrogen, and the concentration of hydrogen / carbon monoxide. Exhaust gas component concentration adjusting means having a ratio of 0.5 or more is disposed, and nitrogen oxide is adsorbed downstream of the exhaust gas passage when the air-fuel ratio is lean, and the adsorbed nitrogen oxide is rich or stoichiometric. In this case, a NOx adsorption reduction catalyst for reducing to nitrogen is disposed.

In a preferred embodiment of the exhaust gas purifying system according to the present invention, the exhaust gas component concentration adjusting means adjusts the concentration of carbon monoxide and hydrogen in the rich exhaust gas.
Characterized in that it is a O · H ₂ regulating catalyst.

Further, a preferred embodiment of the exhaust gas purifying system of the present invention is a shift reaction catalyst in which the above-mentioned rich CO.H ₂ adjusting catalyst promotes a reaction represented by the following formula: CO + H ₂ O → CO ₂ + H ₂ . There is,
Alternatively, the catalyst may be a low-H ₂ consumption catalyst having a higher consumption rate of carbon monoxide than the consumption rate of hydrogen in the rich exhaust gas. As still another preferred embodiment, the concentration of carbon monoxide in the rich exhaust gas is reduced. It is a CO reduction catalyst.

Further, in another preferred embodiment of the exhaust gas purifying system of the present invention, the exhaust gas component concentration adjusting means comprises:
It is a NOx adsorption reduction catalyst that adjusts the concentrations of carbon monoxide and hydrogen in rich exhaust gas.

In still another preferred embodiment of the exhaust gas purifying system of the present invention, the upstream NOx adsorption / reduction catalyst has a catalyst state detecting means and a temperature control means, and the upstream NOx adsorption / reduction catalyst has a catalyst state detecting means. The temperature control means controls the temperature of the catalyst and / or the exhaust gas flowing into the catalyst according to the state of the catalyst.

Further, in another preferred embodiment of the exhaust gas purifying system according to the present invention, the temperature control means includes a speed ratio raising means,
At least one kind of heating means selected from the group consisting of an ignition timing heating means, a secondary fuel heating means, a secondary air heating means, an exhaust heating electric heater and an exhaust purification catalyst heating electric heater; Wherein the speed ratio raising means increases the speed ratio of the automatic transmission of the internal combustion engine to raise the temperature of the exhaust gas, and the ignition timing temperature raising means delays the ignition timing from that in normal operation. The secondary fuel temperature raising means secondarily supplies the fuel for temperature raising to the upstream NOx adsorption reduction catalyst inlet side, and the secondary air temperature raising means secondaryly supplies air to the upstream NOx adsorption reduction catalyst. The exhaust gas heating heater heats the exhaust gas flowing into the upstream NOx adsorption reduction catalyst, and the exhaust purification catalyst heating electric heater heats the upstream NOx adsorption reduction catalyst. It is characterized by.

In still another preferred embodiment of the exhaust gas purifying system of the present invention, the temperature control means is a cooling means, and the cooling means supplies a coolant from an upstream side of the upstream NOx adsorption reduction catalyst. It can be sprayed.

Further, the exhaust gas purifying system of the present invention comprises:
An exhaust gas comprising an NOx adsorbing catalyst for adsorbing and desorbing nitrogen oxides by adjusting exhaust gas temperature disposed upstream of an exhaust gas passage of an internal combustion engine, and an oxidation catalyst disposed downstream of the exhaust gas passage. A gas purification system, wherein the NO
x The temperature of the exhaust gas is 1
At 40 ° C., adsorb nitrogen oxides in exhaust gas,
When the temperature is equal to or higher than 00 ° C., the adsorbed nitrogen oxides are desorbed, and the nitrogen oxide / carbon monoxide concentration ratio of the exhaust gas at the NOx adsorption catalyst outlet is 140 ° C. Sometimes it is 0.3 or less.

Further, in another preferred embodiment of the exhaust gas purifying system of the present invention, the exhaust gas component concentration adjusting means provided in the exhaust gas purifying system, the NOx adsorption reduction catalyst and / or the exhaust gas according to claim 10 are provided. The NOx adsorption catalyst and the oxidation catalyst provided in the gas purification system are sequentially arranged from the upstream side of the exhaust gas passage.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an exhaust gas purification system of the present invention will be described in detail. In addition, "%" shows a mass percentage unless otherwise specified. As described above, in the exhaust gas purification system of the present invention, when the air-fuel ratio is rich or stoichiometric, the component concentration of the exhaust gas is 2.0% or less of CO, hydrogen (H ₂₎ ) An exhaust gas component concentration adjusting means for adjusting the exhaust gas component concentration to 0.5% or more and an H ₂ / CO ratio of 0.5 or more is disposed on the NOx adsorption reduction catalyst arranged on the downstream side, and the exhaust gas having the exhaust gas component concentration adjusted is supplied to Purify by inflow.

The exhaust gas purification system of the present invention, the reaction from the adsorption NOx occurring when the rich or stoichiometric to N _2, the present inventors have investigated in detail, most efficiently proceed the reduction gas and the reaction is Although found to be H _2, reaction with H ₂ is effective when the coating to the noble metal of CO takes place is due to the fact that also found that does not express sufficiently. That is, while ensuring the H ₂ concentration at the catalyst inlet, CO 2
If the concentration is reduced, the purification efficiency of NOx will increase. Further, in the exhaust gas purification system of the present invention,
The reason why the concentration of the exhaust gas component flowing into the NOx adsorption reduction catalyst was adjusted to the above-mentioned concentration was that, based on the above findings, further studies were conducted. , The conversion efficiency to N ₂ is sufficient when the H ₂ concentration is 0.5% or more, and the balance between inhibition of CO reaction and promotion of H ₂ reaction is favored by H ₂ / This is due to the finding that the CO concentration ratio is 0.5 or more. Note that such an exhaust gas purification system can be installed, for example, in a configuration as shown in FIG.

Generally, H discharged from an automobile engine
The concentration ratio between ₂ and CO is determined by the thermal equilibrium of the shift reaction represented by the following formula: CO + H ₂ O → CO ₂ + H ₂ .
H ₂ / CO ratio is about 0.3. If the exhaust gas component concentration adjusting means is arranged in the exhaust gas passage, the concentration ratio can be increased, and the NOx purification efficiency of the NOx adsorption reduction catalyst arranged downstream can be improved.

The NOx adsorbing and reducing catalyst adsorbs NOx during lean operation, and absorbs NOx during rich or stoichiometric operation.
Reducing NOx adsorbed on the Ox adsorption and reduction catalyst in _{N 2.}
The catalyst comprises a noble metal based on Pt, Pd or Rh and any combination thereof, a porous carrier such as alumina supporting the noble metal, sodium (Na), potassium (K), rubidium (Rb), cesium (Cs). ), Magnesium (M
g), calcium (Ca), strontium (Sr),
It contains an alkali metal or an alkaline earth metal such as barium (Ba) and an element according to any combination thereof.

It is preferable that the NOx adsorption reduction catalyst has a function as a normal three-way catalyst at the same time.
Ceria, zirconia, lanthanum, etc., which are commonly used in three-way catalysts, may also be included. With the function of the three-way catalyst, the exhaust gas can be efficiently purified even in a state where stoichiometric control is performed during high-load operation such as during acceleration.

Further, the A / F during the lean operation is preferably in a range where the NOx adsorption reaction takes place with high efficiency.
20 to 50 are preferred. Also, A / F during rich operation
It is preferably in the range to be converted with high efficiency the adsorbed NOx into _{N 2,} in particular A / F = 11.0~14.7 are preferred.

The exhaust gas purifying system of the present invention has the above-mentioned N
As the exhaust gas temperature of the Ox adsorption and reduction catalyst inlet is low, high effect of reducing the adsorbed NOx into _{N 2,} in particular from 150 to 25
A high effect is obtained in a temperature range of 0 ° C. In the conventional catalyst, the reaction inhibition by the noble metal coating of CO occurs more strongly as the temperature becomes lower. However, according to the exhaust gas purification system of the present invention, the exhaust gas concentration adjusting means adjusts the CO concentration so that the exhaust gas with a low CO concentration is exhausted. since gas flows into the NOx reduction purification catalyst on the downstream side, it is possible to reduce NOx to N ₂ with high efficiency.

Further, from the above-mentioned findings by the present inventors that when the CO concentration is lowered, the inhibition of the reaction by CO is reduced, and when the H ₂ concentration is increased, the conversion efficiency to N ₂ becomes sufficient.
_The higher the ₂ / CO concentration ratio, the better, and preferably 0.5 or more, and more preferably 1.0 or more.

[0029] As means for adjusting the H ₂ and CO concentration in the exhaust gas, various means are conceivable, for example, a rich CO · adjusting the CO and H ₂ concentration of the rich exhaust gas
H ₂ regulating catalyst can be mentioned.

Specifically, a shift reaction catalyst in which the above-mentioned rich CO.H ₂ adjusting catalyst promotes a reaction represented by the following formula: CO + H ₂ O → CO ₂ + H ₂ . With this shift reaction catalyst, the reaction of oxidizing CO on the left side of the above formula to CO ₂ occurs efficiently, so that high H ₂
An exhaust gas having a / CO concentration ratio is obtained.

The shift reaction catalyst comprises Pt and Rh.
And a cerium (Ce) compound. In general, it is known that a catalyst that promotes the reaction of the above formula is good when a noble metal and ceria are combined.
In the shift reaction catalyst, among the noble metals, Pt and R
h and ceria may be combined, which is based on the findings of the inventors.

A part of the Pt and / or Rh is directly supported on the ceria, or the ceria is supported on activated alumina, and the Pt and / or Rh is further supported thereon.
Is preferably supported. Further, the above-mentioned ceria is preferably as excellent in heat resistance as possible, and may be a composite oxide containing zirconia, lanthanum or the like as generally known. Further, Pd may be further added, and the low-temperature activation function as a three-way catalyst can be improved. The supported amount of ceria may be 5 to 100 g per liter of the catalyst carrier in terms of cerium oxide, but is more preferably 10 to 50 g.

Another example of the rich CO.H ₂ adjusting catalyst is an H ₂ low consuming catalyst having a higher CO consuming rate than the H ₂ consuming rate in the rich exhaust gas. H
₂ , the exhaust gas having a higher H ₂ / CO concentration ratio than the conventional exhaust gas purifying catalyst can flow into the NOx adsorption reduction catalyst.

The above H ₂ low consumption catalyst is, for example, Pt and R
h and a titanium and / or zirconium compound. The addition of titanium and / or zirconium accelerates the CO oxidation reaction, while, on the other hand,
Since the oxidation of H ₂ does not proceed as compared with the oxidation of CO, the resulting exhaust gas has a high H ₂ / CO concentration ratio. The amount of titanium and / or zirconium to be carried may be 5 to 100 g per liter of the catalyst carrier in terms of oxide.
-50 g is more preferred.

Further, as yet another of the rich CO · H ₂ regulating catalyst include CO reduction catalyst for reducing the concentration of CO in the rich exhaust gas. In order to reduce CO in rich exhaust gas, it is necessary to use the above-mentioned CO reduction catalyst efficiently with C
O may be adsorbed, whereby an exhaust gas having a higher H ₂ / CO concentration ratio than a conventional exhaust gas purifying catalyst can be obtained.

The CO reduction catalyst may be Pt, Pd or Rh.
And a noble metal according to any combination thereof, Na,
It is preferable to contain K, Rb, Cs, Mg, Ca, Sr or Ba and a compound containing an alkali metal or an alkaline earth metal according to any combination thereof. Further, these alkali metal and alkaline earth metal compounds are used in an amount of 0.1 to 30 per liter of the catalyst in view of catalyst performance and material cost.
g is preferable. If less than 0.1 g, sufficient C
O adsorption performance cannot be exhibited, CO easily permeates, and if it exceeds 30 g, not only material cost is increased, but also deterioration of precious metals is easily promoted. It is presumed that the alkali metal and alkaline earth metal compounds adsorb CO in the rich exhaust gas and cause a reaction to form a carbonate, resulting in an exhaust gas having a high H ₂ / CO concentration ratio.

In the CO reduction catalyst, the mole number MP of the noble metal and the mole number MA of the alkali metal or alkaline earth metal compound are represented by a ratio MA / MP.
Is preferably in the range of 0.1 to 10.0, more preferably 3.0 to 7.0. In this case, a sufficient CO reduction catalyst effect can be obtained. Further, these may be impregnated and supported before coating on the honeycomb, or may be dipped and impregnated and supported after coating. Note that MA / MP is 10
If it becomes larger, the CO reduction ability may decrease. This reduces the performance of the precious metal and reduces CO
It is assumed that the adsorption performance is reduced. Further, it can be expected that the catalytic activity of the noble metal is reduced and the three-way performance is reduced. On the other hand, if it is less than 0.1, sufficient CO reduction catalyst performance may not be obtained.

Further, it is preferable that the above-mentioned CO reduction catalyst also has a function as a usual three-way catalyst at the same time. Can be. In this case, the exhaust gas can be efficiently purified even in a state where stoichiometric control is performed during a high load operation such as during acceleration. In addition, although details are not clear at present, the coexistence of these in the catalyst not only suppresses the deterioration of the noble metal and the reduction of the specific surface area of the catalyst, but also suppresses the deterioration of the alkali metal and alkaline earth metal compounds. It is assumed that the effect of suppressing occurs.

[0039] The exhaust gas purification system rich CO · H ₂ regulating catalyst used in the present invention, shift catalyst having the function of adjusting the carbon monoxide and hydrogen concentration in the rich exhaust gas, H ₂ low consumption Either one of the catalyst or the CO reduction catalyst may be selected and installed in the exhaust gas passage, or may be installed in combination. Further, each of the above catalyst components is separately applied to one catalyst carrier base material. Things may be installed. In addition, the porous carrier used for each of the above-mentioned catalysts includes, for example, commonly used alumina, silica and titania, and is not particularly limited, and the catalyst carrier substrate is also usually used as a cordierite honeycomb. It may be a carrier substrate or the like.

Further, the exhaust gas component concentration adjusting means includes:
It is not limited to the above three types of catalysts, and other means include a means for regulating the combustion of the internal combustion engine. For example, by lowering the combustion temperature of the internal combustion engine, the reaction represented by the following formula: CO + H ₂ O → CO ₂ + H ₂ ... May be promoted to adjust the H ₂ / CO concentration ratio. In general, H ₂ /
As described above, the CO concentration ratio is determined by the thermal equilibrium of the shift reaction represented by the above equation, and this reaction is more likely to proceed as the exhaust gas temperature is lower. Therefore, if the temperature in the engine cylinder is lowered, the H ₂ concentration can be increased and the CO concentration can be reduced. Further, if the H ₂ O concentration is increased, the shift reaction of the above equation is promoted. Therefore, for example, if the H ₂ O concentration in the engine cylinder is increased, the H ₂ / CO concentration ratio can be increased.

On the other hand, as another means for adjusting the concentrations of H ₂ and CO in the exhaust gas, a NOx adsorption reduction catalyst can be mentioned. In other words, two N on the exhaust gas passage
An exhaust gas purification system in which Ox adsorption reduction catalysts are installed in series is obtained. With such a configuration, the reaction represented by the above equation is promoted in the same manner as when the above-mentioned rich CO · H ₂ adjusting catalyst is used, and the exhaust gas having a high H ₂ / CO concentration ratio is reduced by NOx adsorption reduction on the downstream side. Can be supplied to the catalyst, especially N
Ox purification efficiency can be improved. When a three-way catalyst is installed on the upstream side and heated, NOx on the downstream side
When the air-fuel ratio to rich or stoichiometric to purify the adsorbed NOx adsorption-reduction catalyst, the only with a reducing agent such as CO or H ₂ required for the desorption purification of the adsorbed NOx is merely consumed, rather NOx purification performance may decrease.

When the air-fuel ratio is rich or stoichiometric, the upstream NOx adsorption-reduction catalyst
Preferably, the temperature is controlled at 00 ° C. From this, it adsorbed NOx (nitrogen oxides) is reduced to _{N 2} (nitrogen), and the component concentration of the gas emitted from the outlet of the NOx adsorption-reduction catalyst, CO2.0% or _{less, H} 2 0. It can be 5% or more and the H ₂ / CO ratio is 0.5 or more. This H ₂
The / CO ratio is more preferably 1.0 or more, particularly 10 or more, from the viewpoint of improving the conversion efficiency to N ₂ .
When the catalyst temperature is lower than 200 ° C., H ₂ 0.5% or more can be secured when the air-fuel ratio is rich or stoichiometric.
Since the CO conversion performance is greatly reduced and cannot be reduced to 2.0% or less, the downstream NOx adsorption reduction catalyst
NOx purification performance may not be improved. On the other hand, if the temperature exceeds 500 ° C., N
It becomes large H ₂ consumption at Ox adsorption and reduction catalyst, H ₂ supplied to the NOx adsorption-reduction catalyst disposed on the downstream side may not be ensured.

Further, the upstream NOx adsorption / reduction catalyst includes:
A catalyst state detecting means and a temperature control means can be provided. In this case, the catalyst state detecting means detects the state of the catalyst, and the temperature control means can control the temperature of the catalyst and / or the temperature of the exhaust gas flowing into the catalyst according to the detection result. It is valid. For example, as shown in FIG. 1, an exhaust gas purification system in which a temperature sensor is provided as a catalyst state detection unit and a heater is provided as a temperature control unit can be given. Note that the “catalyst state” detected by the catalyst state detection means refers to, for example, a measurement result of whether or not the temperature of the exhaust gas flowing into the catalyst, the gas amount, and the like are within a predetermined range, and based on this. The temperature control means may be activated.

As the temperature control means, specifically,
When the temperature of the NOx adsorption reduction catalyst on the upstream side is lower than 200 ° C., a means for raising the temperature of the catalyst can be used, and when it exceeds 500 ° C., means for cooling the catalyst can be used. Examples of such a heating means include a speed ratio heating means, an ignition timing heating means, a secondary fuel heating means, a secondary air heating means, an exhaust heating electric heater or an exhaust purification catalyst heating electric heater, and The temperature raising means according to any combination of these can be used. In this case, the speed ratio raising means increases the speed ratio of the automatic transmission of the internal combustion engine to raise the temperature of the exhaust gas.The ignition timing temperature raising means sets the ignition timing higher than in the normal operation. The secondary fuel temperature raising means is configured to supply the fuel for temperature raising to the inlet side of the NOx adsorption reduction catalyst on the upstream side, and the secondary air temperature raising means is configured to supply the fuel for temperature raising upstream to the NOx adsorption reduction catalyst on the upstream side. The exhaust heater electrically heats the exhaust gas flowing into the upstream NOx adsorption reduction catalyst. The exhaust purification catalyst heating electric heater heats the upstream NOx adsorption. Since the reduction catalyst can be heated, it is easy to control the catalyst temperature within a desired range. Further, as the cooling means, for example, a device capable of injecting a coolant from the upstream side of the upstream NOx adsorption reduction catalyst can be used. In this case, it is easy to control the catalyst temperature within a desired range. In addition,
It is provided with the above-mentioned heating means and cooling means, and can be appropriately used depending on the catalyst temperature. The temperature control means can control the temperature of the NOx adsorption reduction catalyst on the downstream side or can also be provided on the catalyst. However, the capacity of the temperature control means must be increased and energy consumption increases. Is not desirable. Further, by using the above-mentioned catalyst state detecting means in combination, unnecessary energy consumption is eliminated and the energy can be used with a minimum necessary amount of energy.

Here, the NOx adsorption reduction catalyst on the upstream side is used as the catalyst.
During lean operation, NOx is adsorbed and
NOx adsorbed on the NOx adsorbing and reducing catalyst during the live operation
To N ₂Catalysts such as Pt, Pd or Rh and
And the noble metal according to any combination of
Porous carrier such as alumina and sodium (Na)
Lium (K), Rubidium (Rb), Cesium (C
s), magnesium (Mg), calcium (Ca),
Alkali such as trontium (Sr) and barium (Ba)
Metal or alkaline earth metal and any combination of these
A catalyst containing such an element can be used. The above alkali metal
And alkaline earth metal compounds,
Since the adsorption capacity of CO is improved, the adsorption NOx-CO reaction
Improve, H₂When the exhaust gas with a high / CO concentration ratio is obtained
Can be inferred.

It is preferable that the NOx adsorption / reduction catalyst also functions as a normal three-way catalyst.
Cerium (Ce), zirconium (Zr), lanthanum (La), and the like which are commonly used in a three-way catalyst can be contained. In particular, it is preferable that the upstream NOx adsorption reduction catalyst contains platinum (Pt) and / or rhodium (Rh) and a cerium compound (such as ceria), and the content of the cerium compound is cerium oxide. 5-100g / L in conversion
Is more preferable. From this, further improved adsorption NOx-CO reactions rich exhaust gas or stoichiometric exhaust gas, can be supplied H 2 _/ CO concentration ratio of high exhaust gas into the downstream catalyst. It is also preferable to use a catalyst containing Pt and / or Rh and a titanium (Ti) and / or zirconium compound. From this, further improved adsorption NOx-CO reactions rich exhaust gas or stoichiometric exhaust gas, can be supplied H 2 _/ CO concentration ratio of high exhaust gas into the downstream catalyst. The content of the titanium and / or zirconium compound is 5 to 100 g / L, particularly 10
More preferably, it is ５０50 g / L. The above N
When the Ox adsorption reduction catalyst has the function of a three-way catalyst, for example, if it is used for an internal combustion engine of a car, even if it is in a state where it is stoichiometrically controlled at a high load operation such as acceleration.
Exhaust gas can be purified efficiently. Further, the cerium compound (such as ceria) has better heat resistance, and may be a composite oxide containing Zr, La, or the like, as is generally known. Further, Pd may be added, and in this case, the low-temperature activation function as a three-way catalyst can be increased.

The above-mentioned upstream NOx adsorption-reduction catalyst can be supported on a porous carrier, and the material thereof is not particularly limited. Examples thereof include alumina, silica, silica-alumina, and titania. be able to. In particular, it is desirable to use alumina having excellent heat resistance and noble metal dispersibility. Further, the porous carrier may be a carrier in which a carrier component such as alumina or silica is coated on a honeycomb carrier substrate or pellet carrier substrate made of cordierite or metal, or a carrier component such as alumina or silica may be used as a honeycomb. It may be formed on a carrier or a pellet carrier. Further, the porous carrier may be the same for the upstream NOx adsorption / reduction catalyst and the downstream NOx adsorption / reduction catalyst, or different carriers may be used. In this exhaust gas purification system, since the temperature of the upstream NOx adsorption reduction catalyst is controlled by heating or the like, it is desirable to install the catalyst in multiple stages from the upstream side to the downstream side. And the temperature may be controlled at the catalyst inlet portion (upstream side). Further, as shown in FIG. 1, three NOx adsorption reduction catalysts can be arranged in series, or a three-way catalyst can be installed at the uppermost stream.

Here, temperature control of an exhaust gas purification system having the above-described catalyst state detecting means and temperature control means will be described with reference to two embodiments. As shown in FIG. 1, the exhaust gas purification system according to the first embodiment includes a temperature sensor as a catalyst state detection unit, a heater and a control device as a temperature control unit, and these are configured to adsorb NOx on the upstream side. It is installed so that the reduction catalyst can be heated. The temperature control of the exhaust gas purification system can be performed according to the flowchart shown in FIG. That is, first, the inlet temperature T0 of the upstream NOx adsorption reduction catalyst is measured by a temperature sensor, and it is determined whether or not the inlet temperature T0 is higher than the specified temperature T [201]. As a result, if T0> T, the heater is turned off [206]. At this time, if T0 ≦ T, the heater is kept ON, and the inlet temperature of the upstream NOx adsorption reduction catalyst when the heater is turned ON is detected [20
4], the detected inlet temperature of the upstream NOx adsorption reduction catalyst is newly set to T0 [205]. After the heater is turned off, when T0 falls and reaches T [207], the temperature is again measured by the temperature sensor [208]. On the other hand, the exhaust gas purification system according to the second embodiment is provided with an injector capable of supplying hydrocarbon fuel and air instead of the heater in the system according to the first embodiment, as shown in FIG. Further, the temperature control of the exhaust gas purification system can be executed according to the flowchart shown in FIG. 4, and can be executed in the same procedure as the flowchart shown in FIG. 2 except that the heating section (heater) is used as an injector.

Next, another exhaust gas purification system of the present invention will be described in detail. Such an exhaust gas purification system is provided with an exhaust gas temperature upstream of an exhaust gas passage.
A NOx adsorption catalyst for adjusting the adsorption and desorption of NOx is disposed, and an oxidation catalyst is disposed downstream of the exhaust gas passage. When the exhaust gas temperature is in a low temperature range, the NOx adsorbing catalyst causes exhaust gas having a low NOx / CO concentration ratio to flow into an oxidation catalyst disposed on the downstream side, and causes HC, C in rich exhaust gas to flow.
O and NOx are efficiently purified.

That is, the NOx adsorption catalyst adsorbs NOx in the exhaust gas when the temperature of the exhaust gas is 140 ° C. from the start of the engine, and desorbs the adsorbed NOx when the temperature of the exhaust gas is 200 ° C. or higher. . At the same time, when the exhaust gas temperature is 140 ° C. from the start of the engine, the exhaust gas component concentration is adjusted so that the NOx / CO concentration ratio of the exhaust gas at the NOx adsorption catalyst outlet becomes 0.3 or less. Note that such an exhaust gas purification system can be installed, for example, in a configuration as shown in FIG.

A porous carrier is used for the NOx adsorption catalyst and the oxidation catalyst, and the material thereof is not particularly limited. Examples thereof include alumina, silica, silica alumina, titania and the like. In particular, it is preferable to use alumina having excellent heat resistance and noble metal dispersibility. Further, the porous carrier may be a carrier obtained by coating a carrier component such as alumina or silica on a honeycomb carrier substrate or a pellet carrier substrate made of cordierite or metal, or a carrier component such as alumina or silica. May be formed on a honeycomb carrier or a pellet carrier. Further, the above NOx
The materials and carrier components of the porous carrier of the adsorption catalyst and the porous carrier of the oxidation catalyst may be the same or different.

The NOx adsorption catalyst contains a component that adsorbs and desorbs NOx by changing the temperature of the exhaust gas, and preferably contains 0.1 to 10 g / L of Pt as one of the components. When the exhaust gas is in the low temperature range, N
When Ox is adsorbed and the exhaust gas temperature rises, the adsorbed N
Ox is desorbed. When the Pt amount is less than 0.1 g / L,
Practically, sufficient activity cannot be obtained, and if it exceeds 10 g / L, the activity does not improve, and there is no effect due to the increase in Pt, which may not be effective.

The Pt can be supported on each of the porous carriers by a conventional method such as an impregnation method, a spraying method, a slurry mixing method, or the like, with the Pt chloride or nitrate.

Further, other N in the NOx adsorbing catalyst
The Ox adsorbent preferably contains an alkali metal, an alkaline earth metal, a rare earth element, and an element of any combination thereof at 1 to 10 g / L. Examples of the alkali metal include lithium (Li) and sodium (N
a), potassium (K), rubidium (Rb), cesium (Cs) and francium (Fr). Also,
The alkaline earth metal refers to a Group 2A element of the periodic table, and includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). As the rare earth elements, scandium (Sc), yttrium (Y), lanthanum (L
a), cerium (Ce), praseodymium (Pr), neodymium (Nd) and the like.

The content of the alkali metal or the like as the NOx adsorbent is 1 to 50 g / L with respect to 1 L of the catalyst-supporting substrate (for example, a honeycomb carrier) used for the NOx adsorption catalyst.
L is preferred. If it is less than 1 g, the amount of NOx adsorbed is small and the NOx adsorbing capacity is also reduced, so that sufficient NOx purification performance cannot be obtained. If it exceeds 50 g, N
The desorption temperature of Ox rises and may not be sufficiently desorbed, and the oxidizing performance is lowered, so that adverse effects such as inhibiting the oxidation of NO to nitrogen dioxide (NO ₂ ) occur. Sometimes.

Further, it is particularly preferable that the NOx adsorption catalyst adsorbs NOx in the exhaust gas when the exhaust gas temperature is 100 to 140 ° C. and lowers the NOx concentration in the exhaust gas. Since the NOx adsorbing catalyst contains the Pt, the alkali metal, or the like as a NOx adsorbent, it adsorbs NOx when the exhaust gas is at 100 to 140 ° C., and causes the oxidation catalyst on the downstream side to emit low NOx / CO concentration. Gas can be sent in, and a decrease in the CO activity of the oxidation catalyst can be prevented. Further, if the temperature of the exhaust gas rises, the adsorbed NOx is desorbed, so that the NOx adsorbing catalyst is regenerated.

The oxidation catalyst disposed downstream of the NOx adsorption catalyst contains a component for improving the low-temperature activity of CO, and serves to oxidize CO, HC and soluble organic compounds (SOF) in lean exhaust gas. On the other hand, it preferably has high activity, and particularly preferably has oxidizing activity immediately after starting the engine.

The oxidation catalyst has a Pt of 0.5 to 20 g /
L is preferably contained, whereby C in the exhaust gas is reduced.
O, HC and SOF can be efficiently oxidized and purified. If the Pt amount is less than 0.5 g / L, sufficient activity cannot be obtained practically. If the Pt amount exceeds 20 g / L, the activity does not improve, and there is no effect of the increased amount. May not be used effectively. As for the method of supporting the Pt on the porous carrier, as described above, a conventional impregnation method, spray method, or the like can be used.

Further, the above-mentioned oxidation catalyst contains 10
Preferably, the content is from 100 to 100 g / L. As the zeolite, mordenite, MFI-type zeolite or β
-Zeolite and the like, and these can be used alone or in any combination. The content of the zeolite is preferably 10 to 100 g / L with respect to the porous carrier, and if it is less than 10 g / L, HC, SO
If the amount of F or particulate (PM) adsorbed is reduced, and if it exceeds 100 g / L, clogging by PM may occur. Further, by containing the above zeolite, HC is adsorbed on the zeolite even when the exhaust gas is at 140 ° C. or lower, so that poisoning by HC is suppressed, Pt oxidation activity is not reduced, and CO
Low temperature activity can be improved. Furthermore, mixing zeolite with the Pt-supported porous carrier component can enhance the effect of suppressing Pt from HC poisoning, rather than coating the Pt-supported porous carrier with the zeolite in a layered manner.

It is preferable that the oxidation catalyst does not contain the alkali metal, alkaline earth metal and rare earth element contained in the NOx adsorption catalyst. Since the oxidation catalyst does not contain the alkali metal or the like as a NOx adsorbent and does not adsorb NOx, when the exhaust gas is in a lean region, it exhibits high oxidation activity, and HC,
It oxidizes and purifies CO. However, the conventional three-way catalyst contains the above alkali metal and the like, and does not exhibit high oxidizing power in a lean region. That is, the conventional three-way catalyst enhances the purifying ability by adding an alkali metal or a rare earth element to the noble metal, but purifies CO, HC and NOx by catalytic reaction at the time of stoichiometry. . Thus, the oxidation catalyst can be expected to have high oxidizing power in a lean region, and is therefore different from the three-way catalyst.

In the exhaust gas purification system of the present invention using the NOx adsorption catalyst and the oxidation catalyst, when the exhaust gas temperature is 140.degree.
NOx is adsorbed on Pt of the adsorption catalyst. In the case where an alkali metal or the like is contained, NOx is also adsorbed on these. Due to these NOx adsorptions, the NOx / CO concentration ratio between the NOx adsorption catalyst and the downstream oxidation catalyst becomes 0.3 or less. The low NOx / CO concentration ratio exhaust gas flows into the oxidation catalyst, CO is oxidized and purified by Pt oxidation activity, and HC and SOF are adsorbed on the zeolite.

Next, when the exhaust gas temperature exceeds 140 ° C.
When the temperature is lower than 00 ° C., in the NOx adsorption catalyst, NOx is adsorbed on Pt as described above, and when an alkali metal or the like is contained, NOx is also adsorbed on Pt. Alternatively, the adsorption equilibrium state is maintained. In the above-mentioned oxidation catalyst, CO is oxidized and purified by the oxidation activity of Pt, and HC and SOF are adsorbed by zeolite or brought into an adsorption equilibrium state. Also,
Depending on the flow rate of the exhaust gas, the oxidation activity of Pt
C and SOF are desorbed and purified.

Further, when the exhaust gas temperature becomes 200 ° C. or more, the adsorbed NOx is desorbed from the NOx adsorbing catalyst, and the NOx adsorbing catalyst is regenerated. Further, in the oxidation catalyst, CO is oxidized and purified by the oxidation activity of Pt, and HC is purified.
And SOF are adsorbed on zeolite and are in an equilibrium state, or are desorbed and purified by the oxidizing activity of Pt.

The arrangement of the NOx adsorbing catalyst and the oxidation catalyst may be multi-stage, or the two catalysts may be integrated,
The NOx adsorption catalyst component and the oxidation catalyst component may be separately applied to one catalyst carrier substrate.

Further, in addition to the two exhaust gas purifying systems described above, an exhaust gas purifying system can be constituted by combining the above-mentioned respective catalysts and the like. Any one of the exhaust gas component concentration adjusting means, the NOx adsorption / reduction catalyst, and the oxidation catalyst may be sequentially arranged from the upstream side of the exhaust gas passage, or may be used instead of the NOx adsorption / reduction catalyst. In addition, the NOx adsorption catalyst may be arranged.
By such a combination, it is possible to simultaneously improve the NOx purification efficiency in the exhaust gas in the lean region and the CO oxidation activity in the low temperature region, and to purify the exhaust gas efficiently. Note that such an exhaust gas purification system can be installed, for example, in a configuration as shown in FIG.

[0066]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

Example 1 Activated alumina powder was impregnated with a dinitrodiammine Pt solution, dried and calcined in air at 400 ° C. for 1 hour to obtain Pt-supported alumina powder (powder A). The Pt concentration of this powder was 3.0%. Rh nitrate
The aqueous solution is impregnated with activated alumina powder, dried and then dried in air.
Baking at 0 ° C for 1 hour, Rh-supported alumina powder (powder B)
I got The Rh concentration of this powder was 2.0%.

576 g of the powder A, 86 g of the powder B,
238 g of activated alumina powder and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry. This slurry is applied to a cordierite-based monolithic carrier (1.7 L, 40 L).
(0 cell), the excess slurry in the cell was removed by an air stream, dried at 130 ° C., and fired at 400 ° C. for 1 hour to obtain a catalyst of 200 g / L of a coat layer. This was impregnated with an aqueous solution of Ba acetate, dried, and calcined at 400 ° C. for 1 hour in the air to obtain a catalyst (catalyst A) having a coat layer of 250 g / L.

The activated alumina powder was impregnated with the cerium nitrate solution, dried and calcined at 600 ° C. for 1 hour in the air to obtain a cerium-supported alumina powder (powder C). The powder C was impregnated with the dinitrodiammine Pt solution, dried and calcined at 400 ° C. for 1 hour in the air to obtain Pt / cerium-supported alumina powder (powder D). The cerium concentration of this powder is 5.0%, P
The t concentration was 3.0%. Powder C is impregnated with an aqueous solution of Rh nitrate, dried and calcined at 400 ° C. for 1 hour in the air.
-A cerium-supported alumina powder (powder E) was obtained. The cerium concentration of this powder was 5.0%, and the Rh concentration was 2.0%.

576 g of the powder D, 86 g of the powder E, 238 g of the activated alumina powder, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry. This slurry was applied to a cordierite monolithic carrier (1.3 L, 400 L).
After drying at 130 ° C and baking at 400 ° C for 1 hour, a catalyst (catalyst B) having a coat layer of 200 g / L was obtained.

The catalyst B was arranged on the upstream side of the exhaust gas passage and the catalyst A was arranged on the downstream side of the exhaust gas passage, and the following test was conducted. The catalyst B corresponds to the shift reaction catalyst.

Comparative Example 1 The same operation as in Example 1 was repeated, except that the cerium nitrate solution of the catalyst B was not used, to obtain a catalyst C of the present example. A catalyst C was arranged on the upstream side of the exhaust gas passage, and a catalyst A was arranged on the downstream side, and the following test was performed.

Example 2 A catalyst D of this example was obtained by repeating the same operation as in Example 1 except that a titania sol was used instead of the cerium nitrate solution of the catalyst B. A catalyst D was arranged upstream of the exhaust gas passage and a catalyst A was arranged downstream of the exhaust gas passage, and the following test was performed. Incidentally, the catalyst D is the _{H 2}
It corresponds to a low consumption catalyst.

Example 3 A catalyst E of this example was obtained by repeating the same operation as in Example 1 except that a zirconium nitrate solution was used instead of the cerium nitrate solution of the catalyst B. The catalyst E was arranged on the upstream side of the exhaust gas passage and the catalyst A was arranged on the downstream side, and the following test was performed. In addition, the said catalyst E
Corresponding to the H ₂ low consumption catalyst.

Example 4 Example 1 was repeated except that a magnesium acetate solution was used instead of the cerium nitrate solution of the catalyst B.
The same operation as in was repeated to obtain a catalyst F of this example. A catalyst F was arranged on the upstream side of the exhaust gas passage, and a catalyst A was arranged on the downstream side, and the following test was performed. The catalyst F corresponds to the CO reduction catalyst.

[Durability test 1] The catalyst of the above combination was mounted on the exhaust system of a 4400 cc engine with a regular gasoline at 750 ° C using a domestic regular gasoline and operated for 50 hours.

[Evaluation Method] The catalyst of the above combination was mounted on the exhaust system of an engine with a displacement of 2,000 cc, and lean (A / F = 20) for 30 sec → rich (A / F = 11.0).
0) 4 sec → stoichiometric (A / F = 14.7) 5 sec
, And the exhaust gas purification rate in this section was determined.
The inlet temperature was set at two levels of 200 ° C and 350 ° C.

Table 1 shows the test results of Examples 1 to 4 and Comparative Example 1.

[0079]

[Table 1]

Example 5 Activated alumina powder was impregnated with an aqueous solution of dinitrodiammine Pt, dried, and dried in air at 400 ° C.
For 1 hour to obtain Pt-supported alumina powder (powder F). The Pt concentration of this powder F was 2.0%.

A magnetic ball mill was charged with 750 parts of powder F, 1250 parts of nitric acid alumina sol (solid content: 20%) and 500 parts of pure water, and mixed and pulverized to obtain a slurry. The grinding time was 1 hour. This slurry was attached to a cordierite-based monolithic carrier (0.3 L, 400 cells / 6 mils), excess slurry in the cells was removed with an air stream, dried at 130 ° C., and calcined at 500 ° C. for 1 hour. The catalyst G of this example having a coat layer of 200 g / L-carrier was obtained.

400 parts of the above powder F and mordenite 350
Part and nitric acid alumina sol (20% as solid content) 125
0 parts and 500 parts of pure water were put into a magnetic ball mill and mixed and pulverized to obtain a slurry. The grinding time was 1 hour. This slurry is applied to a cordierite monolithic carrier (0.5
L, 400 cells / 6 mils), and after removing excess slurry in the cells with an air stream and drying at 130 ° C.,
It was baked at 500 ° C. for 1 hour to obtain a catalyst H of this example having a coat layer of 200 g / L-carrier. The catalyst G was disposed upstream of the exhaust gas passage, and the catalyst H was disposed downstream of the exhaust gas passage.

Example 6 The catalyst G of Example 5 was immersed in an aqueous barium acetate solution, pulled up to blow off excess water, dried at 250 ° C., and calcined at 300 ° C. for 1 hour to obtain Ba. The same operation as in Example 5 was carried out except that the catalyst I was carried, to obtain a catalyst I of the present example. The catalyst I was disposed upstream of the exhaust gas passage, and the catalyst H was disposed downstream of the exhaust gas passage.

(Comparative Example 2) A catalyst was not arranged on the upstream side of the exhaust gas passage, and the catalyst H was arranged only on the downstream side.

Comparative Example 3 An activated alumina powder was impregnated with an aqueous solution of palladium chloride, dried and calcined in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder G). The Pd concentration of this powder G was 2.0%.

750 parts of powder G, 1250 parts of nitric acid alumina sol (20% as solid content) and 500 parts of pure water were charged into a magnetic ball mill, mixed and pulverized to obtain a slurry. The grinding time was 1 hour. This slurry was attached to a cordierite-based monolithic carrier (0.5 L, 400 cells / 6 mils), excess slurry in the cells was removed with an air stream, dried at 130 ° C., and calcined at 500 ° C. for 1 hour. The catalyst J of this example having a coat layer of 200 g / L-carrier was obtained. The catalyst G was arranged on the upstream side of the exhaust gas passage, and the catalyst J was arranged on the downstream side.

Comparative Example 4 A magnetic ball mill was charged with 750 parts of activated alumina, 1250 parts of nitric acid alumina sol (solid content: 20%) and 500 parts of pure water, and mixed and pulverized to obtain a slurry. The grinding time was 1 hour. This slurry is applied to a cordierite monolithic carrier (0.3 L, 400
(Cell / 6 mil), the excess slurry in the cell was removed by an air stream, dried at 130 ° C., and calcined at 500 ° C. for 1 hour to obtain a catalyst of a coat layer of 250 g / L-carrier.

Next, it is immersed in an aqueous solution of dinitrodiammine Pt of a predetermined concentration, pulled up and blow off excess water.
It was dried at 50 ° C. to support Pt. Next, the sample was immersed in an aqueous solution of rhodium nitrate having a predetermined concentration, pulled up to blow off excess water, and dried at 250 ° C. to carry Rh. Pt
And Rh carrying concentration were 2.73% and 0.27%, respectively.
%Met. Furthermore, after immersing this carrier in a barium acetate volume of a predetermined concentration and lifting it to blow off excess water,
It was dried at 250 ° C. and calcined at 300 ° C. for 1 hour to carry Ba, thereby obtaining a catalyst K of this example. The supported amount of Ba was 10 g / L as BaO. The catalyst G was disposed upstream of the exhaust gas passage, and the catalyst K was disposed downstream of the exhaust gas passage.

Table 2 shows the specifications of the catalysts of Examples 5 and 6, and Comparative Examples 2 to 4.

[0090]

[Table 2]

[Evaluation of catalytic activity] The catalytic activities of Examples 5 and 6 and Comparative Examples 2 to 4 were evaluated. Table 3 shows the evaluation conditions of each example and comparative example. For this catalytic activity evaluation,
A model gas simulating an automobile exhaust gas was used. Automatic evaluation was used. As the model gas, a durability model gas and an evaluation model gas were used. Details are shown in Table 3. As the evaluation conditions, the catalyst capacity was 0.02 L for the NOx adsorption catalyst and 0.04 L for the oxidation catalyst.

[0092]

[Table 3]

<Durability Test 2> Examples 5 and 6, Comparative Example 2
Each of the exhaust gas purifying catalysts Nos. 1 to 4 was installed in the order of arrangement shown in each example, with both catalysts in contact with each other.
Then, with an input gas temperature of 500 ° C and a model gas for durability, 1
An endurance test in which the treatment was performed for 0 hour was performed. Gas flow rate is 60L /
min. Met.

<Evaluation A> The exhaust gas purifying catalysts after the durability test 2 were arranged in the order of arrangement shown in each example, with both catalysts in contact with each other. Then, an evaluation model gas is flowed at an incoming gas temperature of 130 ° C.
And CO were analyzed, and the purification rate was calculated by the following equation: purification rate (conversion rate) = {(incoming gas component concentration) − (outgoing gas component concentration)} / incoming gas component concentration × 100.

<Evaluation B> With the combination of the catalysts of Example 5, the exhaust gas purifying catalyst after the endurance test 2 was placed in the model gas flow path in the same manner as in Evaluation A described above, and the catalyst capacity was set as shown in Table 4.
The catalyst activity was evaluated under the following conditions.

[0096]

[Table 4]

In comparison with Comparative Examples 2 to 4, Examples 5 and 6
Has high catalytic activity and high HC and CO even at low temperatures.
The purification performance was shown, and both high HC purification performance and high CO purification performance could be achieved.

Example 7 A catalyst B was impregnated with an aqueous solution of Ba acetate, dried and calcined at 400 ° C. for 1 hour in the air.
The same operation as in Example 1 was repeated to obtain 250 g of the coat layer.
/ L of catalyst (catalyst L) was obtained.

The above-described catalyst L was disposed on the upstream side of the exhaust gas passage, and the catalyst A was disposed on the downstream side thereof, and the following test was performed. The catalyst L corresponds to the NOx adsorption reduction catalyst.

(Comparative Example 5) The following test was conducted with the same catalyst arrangement as in Example 7 except that the temperature of the upstream catalyst inlet was set to 180 ° C during the durability test.

Comparative Example 6 The same operation as in Example 7 was repeated, except that the catalyst B was not impregnated with an aqueous solution of Ba acetate, to obtain a catalyst M of the present example (catalyst M was a three-way catalyst). . The catalyst M is provided upstream of the exhaust gas passage, and the catalyst A is provided downstream thereof.
And the following test was performed. The catalyst M
Corresponds to the NOx adsorption reduction catalyst.

Example 8 The same procedure as in Example 7 was repeated, except that titania sol was used instead of the cerium nitrate solution in the process for producing the catalyst L, and the catalyst N of this example was used.
I got A catalyst N was arranged on the upstream side of the exhaust gas passage, and a catalyst A was arranged on the downstream side thereof, and the following test was performed. Note that the catalyst N corresponds to the NOx adsorption reduction catalyst.

Example 9 A catalyst O of this example was obtained by repeating the same operation as in Example 7 except that a zirconium nitrate solution was used instead of the cerium nitrate solution in the process for producing the catalyst L. A catalyst O was arranged on the upstream side of the exhaust gas passage and a catalyst A was arranged on the downstream side thereof, and the following test was performed. The catalyst O corresponds to the NOx adsorption reduction catalyst.

Example 10 A catalyst P of this example was obtained by repeating the same operation as in Example 7 except that a magnesium acetate solution was used instead of the cerium nitrate solution in the process for producing the catalyst L. The catalyst P was arranged on the upstream side of the exhaust gas passage and the catalyst A was arranged on the downstream side, and the following test was performed.
Note that the catalyst P corresponds to the NOx adsorption reduction catalyst.

Example 11 A catalyst Q of this example was obtained by repeating the same operation as in Example 7 except that a cesium acetate solution was used instead of the barium acetate solution in the process for producing the catalyst L. A catalyst Q was arranged on the upstream side of the exhaust gas passage, and a catalyst A was arranged on the downstream side of the exhaust gas passage, and the following test was performed. The catalyst Q corresponds to the NOx adsorption reduction catalyst.

Example 12 Example 7 was repeated except that the amount of all noble metals was halved in the process for producing the catalyst L.
The same operation as in was repeated to obtain a catalyst R of this example. The following test was conducted by disposing the catalyst L upstream of the exhaust gas passage and two catalysts R downstream thereof. In addition, the said catalyst R is
This corresponds to the NOx adsorption reduction catalyst.

(Example 13) The same operation as in Example 7 was carried out, except that the catalyst M was arranged upstream of the exhaust gas passage, the catalyst L was arranged downstream of the catalyst M, and the catalyst A was further arranged downstream. The following tests were performed.

<Durability Test 3> The exhaust gas purifying catalysts of Examples 7 to 13 and Comparative Examples 5 and 6 were arranged in the combination shown in each example, and mounted on the exhaust system of an engine having a displacement of 4400 cc. The catalyst was operated for 50 hours at a catalyst inlet temperature of 750 ° C. using regular gasoline.

<Evaluation C> The above combination of catalysts was mounted on the exhaust system of an engine with a displacement of 2,000 cc, and lean (A / F = 20) for 30 sec → rich (A / F = 11.1).
0) 4 sec → stoichiometric (A / F = 14.7) 5 sec
, And the exhaust gas purification rate in this section was determined.
At this time, the inlet temperature of the NOx catalyst disposed in the preceding stage is 30
The test was carried out at two levels of 0 ° C. and 400 ° C., and the temperature was set at 180 ° C. at the inlet of the NOx catalyst disposed at the subsequent stage. However, in Comparative Example 5, the inlet temperature of the upstream NOx catalyst was set to 18
0 ° C. In Example 13, the inlet temperature of the catalyst L was 300 ° C. and 400 ° C. Table 5 shows the catalyst specifications and the like of Examples 7 to 13 and Comparative Examples 5 and 6, and Table 6 shows the test results.

[0110]

[Table 5]

[0111]

[Table 6]

Compared with Comparative Examples 5 and 6, the exhaust gas purifying systems of Examples 7 to 13 have higher catalytic activity, exhibit higher NOx purifying performance even in a low temperature range, and have higher HC / CO purifying performance and higher NOx purifying performance. And could be compatible.

(Example 14) Using the catalyst F, a catalyst performance evaluation test described below was carried out.

(Example 15) After the preparation of the catalyst C, the catalyst C was immersed in a magnesium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, dried in an air stream at 150 ° C, and dried at 400 ° C. The mixture was calcined in an air atmosphere to obtain Catalyst 1. The catalyst coating amount at this time was 205 g / L.
MA / MP was 5.3. Using this catalyst 1, a catalyst performance evaluation test described below was performed.

(Example 16) After preparing the above-mentioned catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain Catalyst 2. At this time, the catalyst coating amount was 220 g / L. Also MA
/ MP was 5.6. Using this catalyst 2, a catalyst performance evaluation test described below was performed.

(Example 17) After the preparation of the catalyst C, the catalyst C was immersed in a mixed solution containing barium acetate and magnesium acetate at an arbitrary concentration at a ratio of 3: 1 to remove excess solution in an air stream. , Dried in an air stream at 150 ° C., 400
Calcination was carried out in an air atmosphere at a temperature of 3 ° C. to obtain a catalyst 3. The catalyst coating amount at this time was 215 g / L. MA / M
P was 7.1. Using the catalyst 3, a catalyst performance evaluation test described later was performed.

(Example 18) After preparing the above-mentioned catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and then dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain Catalyst 4. The catalyst coating amount at this time was 205 g / L. Also MA
/ MP was 1.4. Using this catalyst 4, a catalyst performance evaluation test described below was performed.

(Example 19) After the preparation of the above-mentioned catalyst C, the catalyst C was immersed in a sodium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and dried in an air stream at 150 ° C.
It was calcined at 400 ° C. in an air atmosphere to obtain a catalyst 5. The catalyst coating amount at this time was 210 g / L. Also M
A / MP was 6.9. Using this catalyst 5, a catalyst performance evaluation test described below was performed.

(Example 20) After the above-mentioned catalyst C was prepared, it was immersed in a potassium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and then dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain Catalyst 6. The catalyst coating amount at this time was 215 g / L. Also MA
/ MP was 6.8. Using this catalyst 6, a catalyst performance evaluation test described below was performed.

(Example 21) After the above-mentioned catalyst C was prepared, it was immersed in a cesium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain Catalyst 7. The catalyst coating amount at this time was 230 g / L. Also MA
/ MP was 4.6. Using this catalyst 7, a catalyst performance evaluation test described below was performed.

(Example 22) After the above-mentioned catalyst C was prepared, it was immersed in a calcium acetate solution of an arbitrary concentration, excess solution was removed in an air stream, and dried in an air stream at 150 ° C.
It was calcined at 400 ° C. in an air atmosphere to obtain Catalyst 8. The catalyst coating amount at this time was 210 g / L. Also M
A / MP was 7.6. Using this catalyst 8, a catalyst performance evaluation test described below was performed.

(Example 23) After the preparation of the above-mentioned catalyst B, the catalyst B was immersed in a magnesium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, dried in an air stream at 150 ° C, and dried at 400 ° C. Calcination was performed in an air atmosphere to obtain Catalyst 9. The catalyst coating amount at this time was 210 g / L.
MA / MP was 8.5. Using this catalyst 9, a catalyst performance evaluation test described below was performed.

(Example 24) After preparing the above-mentioned catalyst D, the catalyst D was immersed in a magnesium acetate solution of an arbitrary concentration, excess solution was removed in an air stream, dried in an air stream at 150 ° C, and dried at 400 ° C. It was calcined in an air atmosphere to obtain a catalyst 10. The catalyst coating amount at this time was 210 g / L.
MA / MP was 8.5. Using this catalyst 10, a catalyst performance evaluation test described below was performed.

(Example 25) After preparing the above-mentioned catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and then dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain a catalyst 11. The catalyst coating amount at this time was 250 g / L. Also M
A / MP was 13.9. Using this catalyst 11, a catalyst performance evaluation test described below was performed.

(Example 26) After the preparation of the catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain Catalyst 12. The catalyst coating amount at this time was 201 g / L. Also M
A / MP was 0.3. Using this catalyst 12, a catalyst performance evaluation test described below was performed.

(Example 27) After the preparation of the catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and then dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain a catalyst 13. The catalyst coating amount at this time was 211 g / L. Also M
A / MP was 3.1. Using this catalyst 13, a catalyst performance evaluation test described below was performed.

(Example 28) After the preparation of the catalyst C, the catalyst C was immersed in a barium acetate solution having an arbitrary concentration, excess solution was removed in an air stream, and then dried in an air stream at 150 ° C.
It was calcined at 00 ° C. in an air atmosphere to obtain a catalyst 14. The catalyst coating amount at this time was 209 g / L. Also M
A / MP was 9.5. Using this catalyst 14, a catalyst performance evaluation test described below was performed.

<Durability Test 4> Each of the exhaust gas purifying catalysts of Examples 14 to 28 was arranged in an exhaust gas flow path, and was carried out using actual engine exhaust gas. Exhaust gas temperature at catalyst inlet is 700
° C and a 30-hour durability test was performed. In this durability test 4, the deterioration of the catalyst was measured using only the CO reduction catalyst, and the following model gas evaluation was obtained by measuring only the CO reduction performance (CO conversion rate at R / S). is there.

<Evaluation D> An automatic evaluation was used using the following model gas.・ Evaluation model gas Catalyst inlet temperature; 200 ° C Inlet gas composition; lean + rich switching evaluation, SV;
0000Hr ^-1 Table 7 shows the test results of Examples 14 to 28.

[0130]

[Table 7]

The exhaust gas purifying systems of Examples 14 to 28 had high catalytic activity, exhibited high CO purifying performance even in a low temperature range, and were able to achieve both high CO purifying performance and high HC purifying performance.

[0132]

As described above, according to the present invention,
Since the concentration of the components of the exhaust gas is adjusted, and the adsorption and desorption of NOx is adjusted by changing the temperature of the exhaust gas, NOx in the oxygen-excess region exhausted from the internal combustion engine is efficiently purified, and the exhaust gas is cooled in the low-temperature region. Even so, it is possible to provide an exhaust gas purification system that efficiently purifies CO and HC, and an exhaust gas purification system that exhibits a high NOx purification rate that is equal to or higher than a certain value in a wide purification temperature range, particularly in a low temperature range.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing an example of an exhaust gas purification system of the present invention.

FIG. 2 is a flowchart showing temperature control of the exhaust gas purifying catalyst system of FIG.

FIG. 3 is a schematic view showing another example of the exhaust gas purifying catalyst system of the present invention.

FIG. 4 is a flowchart showing temperature control of the exhaust gas purifying catalyst system of FIG. 3;

FIG. 5 is a schematic view showing an example of another exhaust gas purification system of the present invention.

FIG. 6 is a schematic view showing an example of still another exhaust gas purification system of the present invention.

FIG. 7 is a schematic diagram showing an example of another exhaust gas purification system of the present invention.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/22 301 F01N 3/24 L 3/24 3/28 301D 301E 3/28 301 301F 301P B01D 53 / 36 104A ZAB 102H (72) Inventor Maki Uekubo 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Prefecture, Nissan Motor Co., Ltd. (72) Inventor Hiroshi Akama 2 Takaracho, Kanagawa-ku, Yokohama, Kanagawa Prefecture Nissan Motor Co., Ltd. (72) Invention Person Motohisa Kamijo 2 Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Prefecture, Nissan Motor Co., Ltd. (72) Inventor Hironori Wakamatsu 2 Takara-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Prefecture Nissan Motor Co., Ltd. F-term (reference) 3G091 AA02 AA06 AA12 AA17 AB02 AB06 BA14 BA15 BA19 CA04 CA05 CA08 CA15 CA18 CA19 CA23 CB05 DB10 DC01 EA 18 FB01 FB02 FB03 FB11 FB12 GA01 GB02Y GB03Y GB04Y GB05W GB06W GB09W GB17X HA09 HA10 HA12 4D048 AA06 AA13 AA18 AB02 AB07 BA01X BA02X BA07X BA08X BA14X BA15X BA19X BA30X BA33X BA41X CC52 DA02 CC52 DA02

Claims (29)

[Claims] 1. An exhaust gas component concentration of 2.0% or less of carbon monoxide, 0.5% or more of hydrogen, and 0.5% of hydrogen when the air-fuel ratio is rich or stoichiometric on the upstream side of an exhaust gas passage of an internal combustion engine. An exhaust gas component concentration adjusting means for adjusting the carbon monoxide concentration ratio to 0.5 or more, and adsorbing nitrogen oxides when the air-fuel ratio is lean, and adsorbing the nitrogen oxides on the downstream side of the exhaust gas passage. An exhaust gas purification system including a NOx adsorption reduction catalyst that reduces a substance to nitrogen when the substance is rich or stoichiometric. 2. The exhaust gas component concentration adjusting means is a rich CO.H ₂ adjusting catalyst for adjusting the concentrations of carbon monoxide and hydrogen in rich exhaust gas.
An exhaust gas purification system as described. 3. The exhaust gas according to claim 2, wherein the rich CO.H ₂ adjusting catalyst is a shift reaction catalyst that promotes a reaction represented by the following formula: CO + H ₂ O → CO ₂ + H ₂ . Gas purification system. 4. The method according to claim 3, wherein the shift reaction catalyst contains platinum, rhodium and a cerium compound, and the content of the cerium compound is 5 to 100 g / L in terms of cerium oxide. An exhaust gas purification system as described. 5. The low CO ₂ consumption catalyst according to claim 2, wherein the rich CO · H ₂ adjusting catalyst has a higher consumption rate of carbon monoxide than a consumption rate of hydrogen in the rich exhaust gas. An exhaust gas purification system as described. 6. The exhaust gas purifying system according to claim 5, wherein the H ₂ low consumption catalyst contains platinum, rhodium, and a titanium and / or zirconium compound. 7. The exhaust gas purification system according to claim 2, wherein the rich CO.H ₂ adjustment catalyst is a CO reduction catalyst that reduces the concentration of carbon monoxide in rich exhaust gas. 8. The CO reduction catalyst, wherein at least one noble metal selected from the group consisting of platinum, palladium and rhodium, and at least one noble metal selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium and barium. The exhaust gas purification system according to claim 7, further comprising: a compound containing at least one element. 9. A mole number MP of at least one noble metal selected from the group consisting of platinum, palladium and rhodium.
And the molar ratio MA / MP of at least one element selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, and barium, MA / MP, is 0.1 ≦ MA / MP ≦ 1
The exhaust gas purification system according to claim 8, wherein the value is in the range of 0.0. 10. A mole number MP of at least one noble metal selected from the group consisting of platinum, palladium and rhodium.
And the molar ratio MA / MP of at least one element selected from the group consisting of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, and barium, MA / MP, is 3.0 ≦ MA / MP ≦
The exhaust gas purification system according to claim 8, wherein the exhaust gas purification system is in a range of 7.0. 11. A sodium, potassium, rubidium,
A compound containing at least one element selected from the group consisting of cesium, magnesium, calcium, strontium and barium is added in an amount of 0.1 to 30 per liter of the catalyst carrier.
The exhaust gas purification system according to any one of claims 8 to 10, wherein g is included. 12. The exhaust gas purification system according to claim 7, wherein the CO reduction catalyst contains cerium. 13. The method according to claim 8, wherein the CO reduction catalyst contains zirconium.
Exhaust gas purification system according to the two paragraphs. 14. A rich CO.multidot.CO used in the exhaust gas purification system according to claim 3.
A are H ₂ regulating catalyst, shift catalyst having the function of adjusting the carbon monoxide and hydrogen concentration in the rich exhaust gas,
H ₂ rich CO · H ₂ regulating catalyst, characterized in that at least one catalyst selected from the group consisting of low catalyst and CO reduction catalyst. 15. The exhaust gas component concentration adjusting means for adjusting the concentrations of carbon monoxide and hydrogen in the rich exhaust gas.
The exhaust gas purification system according to claim 1, wherein the exhaust gas purification system is an Ox adsorption reduction catalyst. 16. When the air-fuel ratio is rich or stoichiometric, the upstream NOx adsorption reduction catalyst is
The exhaust gas purification system according to claim 15, wherein the system is controlled at a temperature of ° C. 17. The upstream NOx adsorption reduction catalyst,
A catalyst state detection means and a temperature control means, wherein the temperature control means controls a temperature of the catalyst and / or a temperature of exhaust gas flowing into the catalyst according to a state of the catalyst detected by the catalyst state detection means. The exhaust gas purification system according to claim 15, wherein 18. The temperature control means includes a gear ratio heating means, an ignition timing heating means, a secondary fuel heating means, a secondary air heating means, an exhaust heating electric heater and an exhaust purification catalyst heating electric heater. At least one kind of temperature raising means selected from the group consisting of: wherein the speed ratio raising means raises the temperature of exhaust gas by increasing the speed ratio of an automatic transmission of the internal combustion engine. The secondary fuel heating means secondarily supplies the fuel for heating to the upstream NOx adsorption reduction catalyst inlet side, wherein the secondary fuel temperature increasing means delays the ignition timing from that in the normal operation; The exhaust gas purifying catalyst, wherein the air heating means secondarily supplies air to the upstream NOx adsorption reduction catalyst, and the exhaust heating electric heater heats the exhaust gas flowing into the upstream NOx adsorption reduction catalyst. Heating electric heater is upstream NOx adsorption reduction The exhaust gas purification system according to any one of claims 15 to 17, wherein the catalyst is heated. 19. The temperature control means is a cooling means, and this cooling means can inject a coolant from an upstream side of an upstream NOx adsorption reduction catalyst.
The exhaust gas purification system according to any one of Items 17 to 17. 20. The NOx adsorption / reduction catalyst on the upstream side contains platinum and / or rhodium and a cerium compound, and the content of the cerium compound is 5 to 1 in terms of cerium oxide.
The exhaust gas purification system according to any one of claims 10 to 19, wherein the exhaust gas purification system is 00g / L. 21. The upstream NOx adsorption / reduction catalyst,
It contains platinum and / or rhodium and a titanium and / or zirconium compound.
20. The exhaust gas purification system according to any one of the items 19 to 19. 22. An upstream side of an exhaust gas passage of an internal combustion engine,
An exhaust gas purification system comprising: a NOx adsorption catalyst that adsorbs and desorbs nitrogen oxides by adjusting exhaust gas temperature; and an oxidation catalyst disposed downstream of the exhaust gas passage. Is to adsorb nitrogen oxides in the exhaust gas when the temperature of the exhaust gas is 140 ° C. from the start of the engine, and to desorb the adsorbed nitrogen oxides when the temperature of the exhaust gas is 200 ° C. or more; An exhaust gas purifying system, wherein the nitrogen oxide / carbon monoxide concentration ratio of the exhaust gas at the outlet is 0.3 or less when the exhaust gas temperature is 140 ° C. from the start of the engine. 23. The NOx adsorbing catalyst contains 0.1% platinum.
The exhaust gas purification system according to claim 22, wherein the content of the exhaust gas is from 10 to 10 g / L. 24. The NOx adsorbing catalyst contains 1 to 50 g / L of at least one element selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements. 24. An exhaust gas purification system according to claim 23. 25. The NOx adsorption catalyst according to claim 22, wherein the NOx adsorption catalyst adsorbs NOx in the exhaust gas when the exhaust gas temperature is 100 to 140 ° C.
Exhaust gas purification system according to the two paragraphs. 26. The oxidation catalyst according to claim 1, wherein the oxidation catalyst contains 0.5 to 20 platinum.
The exhaust gas purification system according to any one of claims 22 to 25, wherein g / L is contained. 27. The oxidation catalyst converts zeolite to 10
The content is 100 g / L.
6. The exhaust gas purification system according to any one of the above items 6. 28. A NOx adsorption catalyst used in the exhaust gas purification system according to claim 22, wherein nitrogen oxide in the exhaust gas is adsorbed and desorbed depending on the temperature of the exhaust gas. A NOx adsorption catalyst characterized in that: 29. An exhaust gas component concentration adjusting means disposed in the exhaust gas purification system according to claim 1, and / or a NOx adsorption reduction catalyst and / or NOx adsorption disposed in the exhaust gas purification system according to claim 22. An exhaust gas purifying system comprising: a catalyst and an oxidation catalyst arranged sequentially from an upstream side of an exhaust gas passage.