JP3107303B2 - Exhaust gas purification device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for purifying exhaust gas emitted from an internal combustion engine of an automobile or the like, and more particularly to an internal combustion engine operable at a lean air-fuel ratio (lean burn) and an automobile equipped with the internal combustion engine. The present invention relates to a device for purifying exhaust gas discharged from a plant.

[0002]

2. Description of the Related Art Carbon monoxide (CO) and hydrocarbons (HC: H) contained in exhaust gas discharged from an internal combustion engine of an automobile or the like are used.
Hydrocarbons, nitrogen oxides (NOx), and the like adversely affect the human body as air pollutants and cause problems such as hindering plant growth. So far, great efforts have been made to reduce these emissions, and in addition to reducing the amount generated by improving the combustion method of the internal combustion engine, a method of purifying the exhaust gas using a catalyst or the like has been developed. Has been promoted and has achieved steady results. For gasoline engine vehicles, Pt and Rh, which are three-way catalysts, are the main components of activity, and HC and CO
The mainstream is a method using a catalyst that renders harmless by simultaneously oxidizing NO and reducing NOx.

[0003] By the way, due to its characteristics, the three-way catalyst effectively acts only on exhaust gas generated by burning near the theoretical air-fuel ratio called a window. So conventionally,
Although the air-fuel ratio fluctuates according to the driving conditions of the automobile, the fluctuation range is in principle the theoretical air-fuel ratio (A (weight of air) / F (weight of fuel) = 14.7 in the case of gasoline); The air-fuel ratio is represented by A / F = 14.7, but this value varies depending on the fuel type.)
However, if the engine can be operated at an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the fuel efficiency can be improved. Therefore, the development of lean burn combustion technology has been promoted. It is not unusual for cars to burn gas. However, as described above, when purifying lean burn exhaust gas with the current three-way catalyst, HC and CO can be oxidized and purified, but NOx cannot be effectively reduced and purified. Therefore, in order to apply the lean burn method to a large-sized vehicle and extend the burn time of the lean burn method (expansion of the operating range in which the lean burn method is applied), lean burn compatible exhaust gas purification technology is required. Therefore, development of lean burn compatible exhaust gas purification technology, that is, technology for purifying HC, NO, and NOx in exhaust gas containing a large amount of oxygen (O ₂ ), particularly technology for purifying NOx, has been vigorously pursued. .

Japanese Patent Application Laid-Open No. 63-61708 proposes a method in which HC is supplied upstream of lean burn exhaust gas to lower the O ₂ concentration in the exhaust gas to a concentration range in which the catalyst can function effectively, and to extract the ability of the catalyst. ing.

JP-A-62-97630, JP-A-62-106826, and JP-A-62-106826
-117620 Patent Publication No. NOx in the exhaust gas (NO after converted to be amendable to NO ₂ absorption and oxidation) absorbed and removed by contacting with a catalyst having a NOx trapping ability, the passage of the exhaust gas at the time the absorption efficiency is lowered stop H _2, HC etc. methane and gasoline, the stored NOx by using a reducing agent like removed by reduction, method for reproducing NOx absorption capability of the catalyst are shown.

In PCT / JP92 / 01279 and PCT / JP92 / 01330, a NOx absorbent that absorbs NOx when the exhaust gas is lean and releases the absorbed NOx when the oxygen concentration in the exhaust gas is reduced is provided in the exhaust passage. NOx is absorbed when the exhaust gas is lean, and the absorbed NOx is absorbed by the exhaust gas flowing into the NOx absorbent.
_2. Exhaust gas purifying devices have been proposed in which the concentration is reduced and released.

However, in JP-A-63-61708, an exhaust gas composition (O ₂ concentration of about 0.5%) corresponding to an air-fuel ratio (A / F) of about 14.7 at which the catalyst functions is achieved. Requires a large amount of HC. Although the use of the blow-by gas of the present invention is effective, it is not sufficient to treat the exhaust gas during operation of the internal combustion engine. Injecting fuel is not technically impossible, but will result in a reduction in fuel economy, which has been saved by the lean burn method.

Further, Japanese Patent Application Laid-Open Nos. 62-97630 and 62-106826
No., the same 62-117620 discloses, for Upon regeneration of the NOx absorbent by stopping the flow of the exhaust gas a reducing agent such as HC is brought into contact with the NOx absorbent, greatly combustion consumption by O ₂ in the exhaust gas of a reducing agent And the amount of the reducing agent used is drastically reduced. However, an exhaust switching mechanism for providing two NOx absorbents and alternately allowing exhaust gas to flow through them is necessary, and it cannot be denied that the structure of the exhaust treatment device becomes complicated.

Further, in PCT / JP92 / 01279 and PCT / JP92 / 01330, the exhaust gas is always N
Circulated through the Ox absorbent, and when the exhaust gas is lean,
To absorb ox, for regeneration of the absorbent by releasing the NOx absorbed by lowering the O ₂ concentration in the exhaust gas, the switching of the exhaust gas flow is not required, the problems of the method will be eliminated.
However, it is premised that a material capable of absorbing NOx when the exhaust gas is lean and releasing NOx when the O ₂ concentration in the exhaust gas is reduced is used. In the case of this material, NOx
Inevitably, the absorption and release of the carboxylic acid repeats a periodic change in the crystal structure of the absorbent, which requires careful attention to durability. In addition, when the released NOx needs to be treated and is released in a large amount, it is necessary to consider a post-treatment using a three-way catalyst.

[0010]

SUMMARY OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention has a simple structure of an exhaust treatment device,
It is another object of the present invention to provide a device that can effectively remove and detoxify harmful components such as NOx from lean burn exhaust gas of an internal combustion engine, which consumes less reducing agent and has excellent durability.

[0011]

The above object can be attained by the following methods of the present invention.

According to the present invention, in the oxidation-reduction stoichiometry relationship between the components in the exhaust gas, NOx is chemisorbed in a state where the oxidizing agent is large relative to the reducing agent, and when the amount of the reducing agent is equal to or more than the oxidizing agent. A NOx adsorption catalyst that catalytically reduces the NOx adsorbed in the exhaust gas flow path is disposed in the exhaust gas flow path. NOx was chemisorption, then the reducing agent to the oxidizing agent is made the same amount or more states, harmless by reduction to N ₂ by catalytic reaction with a reducing agent adsorbed NOx on adsorption catalyst.

Here, the adsorption catalyst refers to a material that has the ability to adsorb substances such as NOx and also has a catalytic function. In the present invention, it refers to a material having an ability to adsorb and capture NOx, an ability to catalytically reduce NOx, and an ability to catalytically oxidize HC, CO, and the like.

The oxidizing agent is O ₂ , NO, NO ₂ or the like, and is mainly oxygen. The reducing agent was H supplied to the internal combustion engine.
C is a reducing substance such as HC (including oxygen-containing hydrocarbon), CO, H ₂ or the like as a derivative thereof generated by excessive combustion, and HC added to exhaust gas as a reducing component described later.

As described above, HC, CO, H as reducing agents for reducing lean exhaust gas and NOx to nitrogen
_When they come into contact with ₂ etc., they cause a combustion reaction with O ₂ as an oxidizing agent in the exhaust gas. NOx (NO and NO ₂ )
Also reacts with these to be reduced to nitrogen. Usually, the utilization of the reducing agent is low in the presence of oxygen because both reactions proceed in parallel. Especially when the reaction temperature is 500 (depending on the catalyst material)
At a high temperature of ℃ or more, the ratio of the latter becomes considerably large. Therefore, NOx is separated from the exhaust gas in the adsorber (separated from O ₂ at least in the exhaust gas) it is possible to perform reduction to N ₂ of NOx effectively by catalytic reaction with a reducing agent thereafter. In the present invention, NOx in lean exhaust gas is adsorbed and removed by the NOx adsorption catalyst to separate NOx in exhaust gas from O ₂ .

In the present invention, in the oxidation-reduction system composed of the oxidizing agent (O ₂ , NOx, etc.) in the exhaust gas and the reducing agent (HC, CO, H _2, etc.), creating a state of excellence, reducing the adsorbed NOx on adsorption catalyst to N ₂ by catalytic reaction with a reducing agent such as HC.

By the way, NOx in exhaust gas is almost NO and N
Consists of O ₂ . NO ₂ is more reactive than NO. Therefore, adsorption removal and reduction of NO ₂ are easier than NO. Therefore, if NO is oxidized to NO ₂ , N
Adsorption removal and reduction of Ox are facilitated. The present invention is a method of removing oxidized to NO ₂ by O ₂ coexist NOx in lean exhaust gas, NO oxidation means such as adsorption catalyst therefor
It also includes providing an oxidation function or providing an oxidation catalyst before the adsorption catalyst.

In the present invention, the reduction reaction of chemisorbed NOx can be approximately described by the following reaction formula.

M-NO ₃ + HC → MO + N ₂ + CO ₂ +
H ₂ O → MCO ₃ + N ₂ + H ₂ O Here, M is a metal element (the reason why MCO ₃ is used as a reduction product will be described later). The above reaction is an exothermic reaction. Alkali metals and alkaline earth metals are taken as metals M, Na and B respectively.
When the reaction heat is evaluated as a representative, the standard state (1 atm,
(25 ° C.) is as follows.

2NaNO_Three(s) + 5 / 9C_ThreeH₆→ Na_TwoC
O_Three(s) + N_Two+ 2 / 3CO_Two + 5 / 3H_TwoO [-ΔH = 873kjule] Ba (NO_Three)_Two+ 5 / 9C_ThreeH₆→ BaCO_Three(s) + N_Two+
2 / 3CO_Two+ 5 / 3H_TwoO [−ΔH = 751 kjule] Here, s: solid g: gas The thermodynamic quantity of the adsorbed species used was the value of the corresponding solid.

The heat of combustion of C ₃ H ₆ 5/9 mole is 1
070 kjule, and each of the above reactions has a calorific value comparable to the heat of combustion of HC. Naturally, this heat generation is transmitted to the contacting exhaust gas, and a local temperature rise on the surface of the adsorption catalyst is suppressed.

When the NOx trapping agent is a NOx absorbent, the NOx trapped in the bulk of the absorbing agent is also reduced, so that the calorific value increases. Bring. This exotherm shifts the equilibrium of the absorption reaction shown below to the emission side.

[0023]

Even if the concentration of the reducing agent is increased in order to reduce the released NOx quickly and reduce the NOx concentration in the exhaust gas discharged out of the apparatus, the reaction between NO ₂ and HC does not proceed so much in the gas phase. . Therefore, NOx is increased by increasing the reducing agent.
Emissions cannot be reduced sufficiently. It is also conceivable to carry out an operation by a reduction reaction at a stage where the NOx absorption amount is small, but this is not practical because the regeneration frequency of the NOx absorbent increases.

The adsorption catalyst of the present invention traps NOx only in the vicinity of its surface, so that the absolute amount of heat generation is small, and the temperature of the adsorption catalyst is small because it is quickly transmitted to exhaust gas. Therefore, release of the trapped NOx can be prevented.

The NOx adsorption catalyst of the present invention is characterized as a material that captures NOx on its surface by chemisorption and does not generate NOx due to an exothermic reaction upon reduction of NOx. Further, the NOx adsorption catalyst of the present invention is characterized as a material that captures NOx by chemical adsorption on its surface or by chemical bonding near the surface, and does not generate NOx by an exothermic reaction upon reduction of NOx.

The present inventors have determined that at least potassium (K), sodium (Na), magnesium (Mg),
It has been found that the above characteristics can be realized by a NOx adsorption catalyst containing at least one element selected from strontium (Sr) and calcium (Ca) as a part of components.

The exhaust gas purifying apparatus for an internal combustion engine according to the present invention comprises at least one element selected from the group consisting of potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca). The NOx adsorption catalyst, which is included as a part, is disposed in the exhaust gas flow path, and in the oxidation-reduction stoichiometric relationship between the components in the exhaust gas, a state in which the amount of the oxidizing agent is large relative to the reducing agent is formed, and the NOx adsorption catalyst is formed on the adsorption catalyst.
Was chemically adsorbed, then the reducing agent to the oxidizing agent is made the same amount or more states, and characterized in that the detoxification is reduced to N ₂ by catalytic reaction with a reducing agent adsorbed NOx on adsorption catalyst I do.

The exhaust gas purifying apparatus for an internal combustion engine according to the present invention further comprises at least potassium (K), sodium (Na),
A NOx adsorption catalyst containing at least one element selected from magnesium (Mg), strontium (Sr) and calcium (Ca) as a part of components is disposed in the exhaust gas flow path;
In the oxidation-reduction stoichiometry, a state in which the oxidizing agent such as O ₂ is larger than the reducing agent such as HC is used to trap NOx by chemical bonding on the surface of the adsorption catalyst and in the vicinity of the surface. Make the same amount or more, and make NOx trapped by the adsorption catalyst react with the reducing agent to form N ₂
To make it harmless.

Particularly, the following can be suitably applied as the NOx adsorption catalyst in the present invention.

At least one selected from potassium (K), sodium (Na), magnesium (Mg), strontium (Sr) and calcium (Ca), and at least one selected from rare earths such as cerium and the like, platinum, rhodium, A composition comprising a metal and a metal oxide (or composite oxide) containing at least one element selected from precious metals such as palladium, and a composition comprising the composition supported on a porous heat-resistant metal oxide. The composition has excellent SOx resistance in addition to excellent NOx adsorption ability.

In the method of the present invention, the state in which the amount of the reducing agent is equal to or larger than the amount of the oxidizing agent can be prepared by the following method.

The combustion condition in the internal combustion engine is set to the stoichiometric air-fuel ratio or the excess fuel (rich). Further, a reducing agent is added to the lean burn exhaust gas.

The former can be achieved by the following method.

A method of controlling the fuel injection amount according to the output of an oxygen concentration sensor and the output of an intake flow sensor provided in an exhaust duct. In this method, a part of the plurality of cylinders is made excessive and the remaining fuel is made insufficient, and the components in the mixed exhaust gas from all the cylinders have the same amount or larger amount of the reducing agent than the oxidizing agent in the oxidation-reduction stoichiometry Includes methods for creating states.

The latter can be achieved by the following methods.

A method of introducing a reducing agent upstream of the adsorption catalyst in the exhaust gas stream. Gasoline as fuel for internal combustion engine
Light oil, kerosene, natural gas, their reformed products, hydrogen, alcohols, ammonia and the like can be applied. It is also effective to introduce the blow-by gas and the canister purge gas to the upstream of the adsorption catalyst and to introduce a reducing agent such as a hydrocarbon contained therein. In a fuel direct injection type internal combustion engine, it is effective to inject fuel during an exhaust stroke and to input fuel as a reducing agent.

The adsorption catalyst in the present invention can be applied in various shapes. The present invention can be applied as a pellet, plate, granule, or powder, including a honeycomb shape obtained by coating a honeycomb-like structure made of a metal material such as cordierite or stainless steel with an adsorption catalyst component.

In the present invention, the timing for forming a state in which the amount of the reducing agent is equal to or larger than that of the oxidizing agent can be determined by the following methods.

NOx emission during lean operation is determined from an air-fuel ratio setting signal determined by an ECU (Engine Control Unit), an engine speed signal, an intake air amount signal, an intake pipe pressure signal, a speed signal, a throttle opening, an exhaust gas temperature, and the like. When the amount is estimated and the integrated value exceeds a predetermined set value.

When an accumulated oxygen amount is detected by a signal from an oxygen sensor (or A / F sensor) placed upstream or downstream of the adsorption catalyst in the exhaust passage, and the accumulated oxygen amount exceeds a predetermined amount.

As a modified embodiment, when the accumulated oxygen amount during the lean operation exceeds a predetermined amount.

NOx placed upstream of the adsorption catalyst in the exhaust passage
When the accumulated NOx amount is calculated based on the sensor signal, and the accumulated NOx amount during the lean operation exceeds a predetermined amount.

NOx placed downstream of the adsorption catalyst in the exhaust passage
When the NOx concentration during the lean operation is detected based on a signal from the sensor, and the NOx concentration exceeds a predetermined concentration.

In the present invention, as described above, the amount of the reducing agent to be supplied or the time for maintaining the same amount or more of the reducing agent with respect to the oxidizing agent is determined in advance as described above. Although it can be determined in consideration of the characteristics and the like, these can be realized by adjusting the stroke of the fuel injection valve, the injection time and the injection interval.

[0046]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to specific embodiments of the present invention. The present invention is not limited to the following embodiments and examples, and it goes without saying that there are various embodiments within the scope of the idea.

[Adsorption Catalyst] The characteristics of the adsorption catalyst according to the method of the present invention will be described. The characteristics of N-N9 containing Na as an alkali metal and NK9 containing K are as follows.

<< Adsorption catalyst preparation method >> An adsorption catalyst N-N9 was obtained by the following method.

Alumina sol as a binder obtained by mixing alumina powder and boehmite in nitric acid was mixed to obtain a nitric acid acidic alumina slurry. After dipping the honeycomb in the coating liquid, the honeycomb was immediately pulled up, the liquid blocked in the cell was removed by air blow, dried, and subsequently fired at 450 ° C. This operation was repeated until the apparent volume of the honeycomb became 1 L.
Per gram of alumina was coated. A catalytically active component was carried on the alumina-coated honeycomb to obtain a honeycomb-shaped adsorption catalyst. For example, it was impregnated with a cerium nitrate (Ce nitrate) solution, dried, and fired at 600 ° C. for 1 hour. Subsequently, a mixed solution of a sodium nitrate (Na nitrate) solution, a titania sol solution and a magnesium nitrate (Mg nitrate) solution was impregnated, and dried and fired similarly. Furthermore, the mixture was impregnated with a mixed solution of a dinitodiamine Pt nitric acid solution and a rhodium nitrate (Rh nitrate) solution, dried, and baked at 450 ° C. for 1 hour. Finally, Mg nitrate
The solution was impregnated and calcined at 450 ° C. for 1 hour. Ce alumina (Al ₂ O ₃₎ by more than, Mg, Na, Ti, Rh ,
Honeycomb-shaped adsorption catalyst supporting Pt, 2Mg- (0.2R
h, 2.7Pt)-(18Na, 4Ti, 2Mg) -27C
e / Al ₂ O ₃ was obtained. Here, / Al ₂ O ₃ indicates that the active component was supported on Al ₂ O ₃ , and the numerical value before the element symbol is the weight (g) of the indicated metal component supported per 1 L of apparent honeycomb volume. The notation order indicates the order in which the components are carried, and the components separated from the components near Al ₂ O ₃ are carried in order, and the components enclosed in parentheses are carried simultaneously. Incidentally, the loading amount of each active ingredient can be changed by changing the active ingredient concentration in the impregnating solution.

The adsorption catalyst NK9 was prepared by the following method.

A potassium nitrate (K nitrate) solution was used in place of the Na nitrate solution in the preparation of the adsorption catalyst N-N9, and the other method was the same as that for the adsorption catalyst N-N9.
(0.2Rh, 2.7Pt)-(18K, 4Ti, 2M
It was obtained _{g) -27Ce / Al 2 O 3} . In the same manner, the comparative catalyst NR2 2Mg- (0.2Rh, 2.7Pt) -2
7 Ce / Al ₂ O ₃ was obtained.

<< Performance Evaluation Method >> The adsorption catalyst obtained by the above method was heat-treated at 700 ° C. for 5 hours in an oxidizing atmosphere, and then its characteristics were evaluated by the following methods.

A passenger car equipped with a 1.8-liter lean-burn gasoline engine was equipped with a 1.7-liter honeycomb-shaped adsorption catalyst prepared by the method of the present invention and NOx.
The purification properties were evaluated.

<< Characteristics of Adsorption Catalyst >> The adsorbent catalyst N-N9 is mounted, and the A / F = 13.3 rich operation for 30 seconds and the A / F
= 22 was alternately repeated for about 20 minutes to obtain the NOx purification rate aging characteristics of FIG. From the figure, it can be seen that the present adsorption catalyst purifies NOx during the lean operation period. During lean operation, the NOx purification rate gradually decreased to 1
The purification rate of 00% becomes about 40% after 20 minutes. However, this reduced purification rate is 10 times in the rich operation for 30 seconds.
Recovers to 0%. When the lean operation is performed again, the NOx purification ability is restored and the above-described temporal change is repeated. NOx during lean operation even when lean operation and rich operation are repeated multiple times
The rate of reduction of the purification rate over time was unchanged, indicating that the NOx adsorption performance was sufficiently regenerated by the rich operation.

The vehicle speed is kept constant at about 40 km / h (the exhaust gas space velocity (SV) is about 20,000 / h), and the ignition timing is changed to change the NOx concentration in the exhaust gas. Figure 3 was obtained by determining the relationship between the rates.
The NOx purification rate decreases with time, but the lower the NOx concentration, the lower the rate of decrease. The amount of NOx trapped up to the NOx purification rates of 50% and 30% is obtained from FIG.
Becomes

[0056]

[Table 1]

The amount of trapped NOx is substantially constant regardless of the NOx concentration. The fact that the amount of adsorption does not depend on the concentration (pressure) of the adsorbate is a characteristic of chemisorption.

Pt particles are firstly considered as NOx adsorbents in the test adsorption catalyst. When the amount of CO adsorbed, which is frequently used as a means for evaluating the amount of exposed Pt, was evaluated, C
The O adsorption amount (at 100 ° C.) was 4.5 × 10 ⁻⁴ mol.
This value is about 1/100 of the NOx adsorption amount, and it is clear that Pt is not the main player of NOx adsorption.

On the other hand, the BET specific surface area (measured by nitrogen adsorption) measured for each cordierite of the present adsorption catalyst is about 25 m.
It was 28,050 m ² per 1.7 L of honeycomb at ² / g. Further, when the chemical structure of Na of the adsorption catalyst of the present invention was examined, it was mainly determined from the fact that CO ₂ gas was generated and dissolved in the mineral acid and the value of the inflection point in the neutralization titration curve with the mineral acid was used. It was determined that it was present as Na ₂ CO ₃ . If the entire surface is occupied by Na ₂ CO ₃ , 0.275 mol of Na ₂ CO ₃ is exposed on the surface (the specific gravity of Na ₂ CO ₃ is 2.533 g / ml). From this, the volume of one molecule of Na ₂ CO ₃ is determined (Na
Assuming that ₂ CO ₃ is a cube, the area of one surface was determined and this was defined as the area occupied by the surface Na ₂ CO ₃ )). According to the above reaction formula, 0.275 mol of Na ₂ CO ₃ is capable of adsorbing 0.55 mol of NO ₂ . However, the amount of NOx actually removed by the adsorption catalyst of the present invention is on the order of 0.04 mol, which is 1/10 or less. This difference is due to the fact that the BET method evaluates the physical surface area and also evaluates the surface area other than Na ₂ CO ₃ such as Al ₂ O ₃ . The above evaluation shows that the amount of adsorbed NOx is much smaller than the NOx trapping ability of Na ₂ CO ₃ bulk,
This indicates that at least NOx is trapped on the Na ₂ CO ₃ surface or in a limited area near the surface.

In FIG. 3, the rate of reduction of the purification rate decreases from about 20% of the NOx purification rate, which indicates that the reduction reaction by the catalytic function has occurred.

FIG. 4 shows the NOx purification rate immediately after switching from the lean operation to the stoichiometric operation. It can be seen that with the present adsorption catalyst, a NOx purification rate of 90% or more can be obtained immediately after switching to the stoichiometric operation.

FIGS. 5 and 6 show NOx purification characteristics before and after switching from lean to stoichiometric or rich. FIG. 5 shows the NOx concentration at the inlet and outlet of the adsorption catalyst N-N9. FIG. 5A shows the case where the air-fuel ratio is switched from lean at A / F = 22 to rich at A / F = 14.2. It is. At the start of playback immediately after rich switching, A /
Since the exhaust gas NOx concentration at F = 14.2 is high, the inlet NOx concentration in the rich operation greatly increases, and the outlet NOx concentration transiently increases with this. However, the outlet NOx concentration is always significantly lower than the inlet NOx concentration. The regeneration proceeds promptly, and the outlet NOx concentration reaches near zero in a short time. Figure (b) shows A
This is the case where the air-fuel ratio is switched from the lean of / F = 22 to the rich of A / F = 13.2, but the outlet NOx concentration is always much lower than the inlet NOx concentration, as in FIG. The outlet NOx concentration reaches near zero in a short time.

As is clear from the above, the A / F value as a reproduction condition affects the time required for reproduction. The A / F value, time, and amount of reducing agent suitable for regeneration are affected by the composition, shape, temperature, SV value, type of reducing agent, and shape and length of the exhaust passage of the adsorption catalyst. Therefore, reproduction conditions are comprehensively determined in consideration of these.

FIG. 6 shows N and N at the inlet and outlet of the adsorption catalyst NK9.
FIG. 7A shows the Ox concentration. FIG. 7A shows the case where the air-fuel ratio is switched from the lean A / F = 22 to the rich A / F = 14.2, and FIG. 7B shows the lean A / F = 22. A / F = 13.
In this case, the air-fuel ratio is switched to a rich value of 2. However, as in the case of the above-mentioned adsorption catalyst N-N9, the outlet NOx concentration is always much lower than the inlet NOx concentration, and the regeneration of the adsorption catalyst proceeds in a short time. I have. << Basic Characteristics of Adsorption Catalyst >> Using a model gas, basic characteristics, in particular, the influence of the oxygen concentration on the NOx purification rate were evaluated.

A 6 ml small honeycomb of N-N9 is
The sample gas was filled into a φ quartz reaction tube, and the model gas was allowed to flow.
The effect of changing the oxygen concentration in the model gas on the NOx purification rate was studied. The reaction temperature is 3 at the catalyst inlet gas temperature.
00 ° C.

First, O ₂ 5% (volume fraction; the same applies hereinafter),
_{NO600ppm, C 3 H 6 500ppm (} 1500ppm as C
l), a gas having a composition of 1000 ppm of CO, 10% of CO _2, 10% of H ₂ O, and N ₂ Balance was passed, and after 10 minutes when the NOx purification rate was stabilized, only the oxygen concentration was reduced to a predetermined value and held for 20 minutes. Finally, the gas composition was returned to the initial one. The NOx concentration change during this period is reduced to 0%, 0.5%, 0.7%, 1%, 2% and 3%.
FIG. 19 was obtained by changing the species. In FIG. 19, when the oxygen concentration in the lean gas is low, that is, when the oxidizing atmosphere is weakened and the reducing atmosphere is strong, the NOx purification rate tends to increase. This suggests that the present adsorption catalyst purifies NOx by reduction. I have. In FIG. 19, the NOx purification rate is always a positive value, and the present adsorption catalyst does not increase the NOx concentration after passing through the adsorption catalyst regardless of the oxygen concentration.

Next, O ₂ 5%, NO 600 ppm, N ₂ Bal
FIG. 20 is obtained by changing the oxygen concentration in this gas in the same manner while flowing a gas having a composition of ance. The present study is characterized in that the gas does not contain a reducing agent. In FIG. 20, even if the oxidizing atmosphere is weakened by lowering the oxygen concentration, NO
x Purification rate does not improve. This suggests that the present adsorption catalyst purifies NOx by reduction. In FIG. 20, the NOx purification rate does not become negative.
Will not be released.

[Exhaust Gas Purifying Apparatus] FIG. 1 shows the overall structure of an exhaust gas purifying apparatus according to an embodiment of the present invention.

The device according to the present invention comprises an engine 99 capable of lean burn, an air flow sensor 2, a throttle valve 3
, An oxygen concentration sensor (or A / F sensor) 19, an exhaust temperature sensor 21, a NOx adsorption catalyst 18
It is composed of an exhaust system having the above and a control unit (ECU). The ECU is an I / O interface
/ OLSI, arithmetic processing unit MPU, storage device RAM and ROM storing a large number of control programs, timer counter, and the like.

The above exhaust gas purifying apparatus functions as follows. The intake air to the engine is filtered by an air cleaner 1 and then measured by an air flow sensor 2, passes through a throttle valve 3, receives a fuel injection from an injector 5, and is supplied to the engine 99 as an air-fuel mixture. Airflow sensor signal and other sensor signals are E
It is input to the CU (Engine Control Unit).

The ECU evaluates the operating state of the internal combustion engine and the state of the NOx adsorption catalyst by a method described later to determine the operating air-fuel ratio, and controls the injection time of the injector 5 and the like to adjust the fuel concentration of the air-fuel mixture to a predetermined value. Set. The air-fuel mixture sucked into the cylinder is ignited by the ignition plug 6 controlled by a signal from the ECU 25 and burns. The combustion exhaust gas is led to an exhaust purification system. The exhaust purification system is provided with a NOx adsorption catalyst, which purifies NOx, HC, and CO in exhaust gas by the three-way catalytic function during stoichiometric operation, and purifies NOx by NOx adsorption capability during lean operation, and also has a combustion function. This purifies HC and CO. Further, based on the determination and control signal of the ECU, the NOx purification ability of the NOx adsorption catalyst is always determined during the lean operation,
When the Ox purifying ability decreases, the air-fuel ratio of the combustion is shifted to the rich side to recover the NOx adsorbing ability of the adsorption catalyst. Through the above operation, the present apparatus effectively purifies exhaust gas under all engine combustion conditions of lean operation and stoichiometric (including rich) operation.

The fuel concentration (hereinafter referred to as air-fuel ratio) of the air-fuel mixture supplied to the engine is controlled as follows. FIG. 7 is a block diagram showing the air-fuel ratio control method.

The output of the load sensor for outputting a signal corresponding to the depression of the accelerator pedal, the output signal of the intake air amount measured by the air flow sensor, the engine speed signal detected by the crank angle sensor, the exhaust gas temperature signal, and the throttle opening are ECU2 from information such as detected throttle sensor signal, engine coolant temperature signal, starter signal
5 determines the air-fuel ratio (A / F), and this signal is corrected based on the signal fed back from the oxygen sensor to determine the fuel injection amount. At low temperatures, at idle, at high load, etc., the feedback control is stopped by the signals of the sensors and switches. In addition, the air-fuel ratio correction learning function is used to accurately respond to subtle or sudden changes in the air-fuel ratio by the air-fuel ratio correction learning function.

When the determined air-fuel ratio is stoichiometric (A / F = 1)
In the case of 4.7) and rich (A / F <14.7), the injection condition of the injector is determined by the instruction of the ECU, and the stoichiometric and rich operation is performed. On the other hand, lean (A / F> 14.
7) When the operation is determined, the presence or absence of the NOx adsorbing ability of the NOx adsorbing catalyst is determined. When it is determined that the NOx adsorbing catalyst has the adsorbing ability, the fuel injection amount is determined to perform the lean operation as instructed. If it is determined that the NOx adsorption catalyst is not present, the air-fuel ratio is richly shifted for a predetermined period to regenerate the NOx adsorption catalyst.

FIG. 8 shows a flowchart of the air-fuel ratio control. In step 1002, signals for instructing various operating conditions or detecting operating conditions are read. The air-fuel ratio is determined in step 1003 based on these signals,
At 004, the determined air-fuel ratio is detected. Step 10
The magnitude of the air-fuel ratio determined in 05 and the stoichiometric air-fuel ratio are compared. The stoichiometric air-fuel ratio to be compared here is exactly the air-fuel ratio at which the speed of the catalytic reduction reaction of NOx exceeds the trapping speed by adsorption in the adsorption catalyst, and is determined in advance by evaluating the characteristics of the adsorption catalyst. , An air-fuel ratio near the stoichiometric air-fuel ratio is selected. If the set air-fuel ratio is smaller than or equal to the stoichiometric air-fuel ratio, the process proceeds to step 1006 to perform the air-fuel ratio operation as instructed without performing the regeneration operation of the adsorption catalyst. If set air-fuel ratio> stoichiometric air-fuel ratio, the process proceeds to step 1007. Step 1007
Then, an estimation calculation of the NOx adsorption amount is performed. The estimation calculation method will be described later. Subsequently, in step 1008, the estimated NOx
It is determined whether the amount of adsorption is equal to or less than a predetermined limit amount. The limit adsorption amount is set to a value at which NOx in the exhaust gas can be sufficiently purified in consideration of the NOx trapping characteristics of the adsorption catalyst in advance by experiments and the like, and in consideration of the exhaust gas temperature and the temperature of the adsorption catalyst. If NOx adsorbing ability is present, the process proceeds to step 1006, and the air-fuel ratio operation is performed as instructed without performing the regeneration operation of the adsorption catalyst. Step 1 if NOx adsorption capacity is not available
Proceeding to 009, the air-fuel ratio is shifted to the rich side. In step 1010, the rich shift time is counted, and if the elapsed time Tr exceeds a predetermined time (Tr) c, the rich shift ends.

The determination of the NOx adsorption ability can be performed as follows.

FIG. 9 shows N based on various operating conditions during lean operation.
This is a method of integrating and determining the amount of Ox emission.

[0078] estimate the NOx amount E _N adsorbed on the signal and the signal and loading unit time for various engine operating conditions affecting the NOx concentration in the exhaust gas about the operating conditions of the NOx adsorbing catalyst of the exhaust gas temperature such as at step 1007-E01 I do. Integrates the E _N in step 1007-E02, the integrated value ΣEN adsorption amount upper limit value at step 1008-E01 (E
_N ) Compare magnitude with c. If ΣEN ≦ (E _N ) c, integration is continued; if ΣE _N > (E _N ) c, step 100
In 8-E02, the integration is canceled, and the flow advances to step 1009.

FIG. 10 shows a method of making a determination based on the integrated time of the lean operation.

In step 1007-H01, the lean operation time _HL is integrated, and in step 1008-H01, the integrated value ΣH
_A comparison is made between _L and the upper limit value (H _L ) c of the integration time. Σ
Continued integration For _{H L ≦ (H L) c} , ΣH L> (H L) c
In step 1008-H02, the integration is canceled, and the flow advances to step 1009.

FIG. 11 shows a method of making a determination based on an oxygen sensor signal during a lean operation.

In step 1007-O01, the amount of oxygen Q _O in the lean operation is integrated, and in step 1008-O01
Then, the magnitude of the integrated value OQ _O and the upper limit value (Q _O ) c of the integrated oxygen amount are compared. ΣIf Q _O ≤ (Q _O ) c, continue the integration,
If Q _O > (Q _O ) c, the integration is canceled in step 1008 -O 02, and the flow advances to step 1009.

FIG. 12 shows a method of making a determination based on the NOx concentration sensor signal detected at the inlet of the NOx adsorption catalyst during the lean operation.

In step 1007-N01, NOx at the NOx adsorption catalyst inlet is determined based on the NOx concentration sensor signal.
Integrating the amount Q _N. Integrated value [sum] Q _N and the accumulated NOx amount upper limit value at step 1008-N01 compares the magnitude of the (Q _N) c. When ΣQ _N ≤ (Q _N ) c, integration is continued and ΣQ _N >
In the case of (Q _N ) c, the integration is canceled in step 1008-N02, and the flow advances to step 1009.

FIG. 13 shows a method of making a determination based on the NOx concentration sensor signal detected at the outlet of the NOx adsorption catalyst during the lean operation.

At step 1007-C01, the NOx at the inlet of the NOx adsorption catalyst is determined based on the NOx concentration sensor signal.
The concentration CN is detected. Step 1008-C _{N in} C01
And comparing the magnitude of C upper limit of _N (C _N) c. C _N ≦
If (C _N ) c, the detection is continued, and if C _N > (C _N ) c, the process proceeds to step 1009.

FIG. 14 shows another embodiment of the exhaust gas purifying apparatus of the present invention. 1 in that a manifold catalyst 17 is provided in an exhaust duct near the engine.
The tightening of emission control of automobile exhaust gas requires purification of harmful substances such as HC discharged immediately after the engine is started. That is, in the past, the catalyst was discharged untreated until it reached the operating temperature, but this amount needs to be greatly reduced. For this purpose, a method of rapidly raising the temperature of the catalyst to the operating temperature is effective. FIG. 14 shows an apparatus configuration capable of coping with the reduction of HC and CO emissions at the time of engine startup and the purification of exhaust gas in lean and stoichiometric (including rich) operation. In the configuration of FIG. 14, Pt, Rh, CeO
So-called three-way catalysts containing ₂ as a main component and Pd
Or a combustion catalyst containing a combustion active component such as Pd as a central component. In this configuration, at the time of startup, the temperature of the manifold catalyst 17 rises in a short time to purify HC and CO immediately after the startup, and at the time of the stoichiometric operation, both the manifold catalyst and the adsorption catalyst 18 function so that HC, CO,
NOx is purified, and during lean operation, the adsorption catalyst
To adsorb and purify. When the air-fuel ratio is richly shifted during regeneration of the adsorption catalyst, HC and CO as reducing agents reach the adsorption catalyst without undergoing a large chemical change in the manifold catalyst, and are regenerated. It is a great feature of the adsorption catalyst that enables such a configuration.

FIG. 15 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. The difference from the embodiment of FIG. 1 is that the engine 99 is a direct injection type. The device of the present invention can be suitably applied to a direct injection type engine.

FIG. 16 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. 1 and 15 is that a rear catalyst 24 is provided downstream of the adsorption catalyst.
For example, a device in which the HC purification ability is improved by placing a combustion catalyst in the post-catalyst, and a device in which the three-way function at the time of stoichiometry is strengthened by placing a three-way catalyst, are realized.

FIG. 17 shows still another embodiment of the exhaust gas purifying apparatus of the present invention. The difference from FIGS. 1 and 14 to 16 is that fuel is added to the upstream of the adsorption catalyst through the reducing agent injector 23 in response to a rich shift instruction. This method has a great advantage that the operating state of the engine can be set independently of the state of the adsorption catalyst.

Hereinafter, the effects of the present invention will be described with reference to specific examples.

The exhaust gas purification performance of the adsorption catalyst and the device of the present invention was evaluated. A 1.8-liter lean-burn vehicle equipped with an adsorption catalyst and a comparative catalyst was driven on a chassis dynamometer. Both test catalysts were honeycomb-shaped (400 cells / in ² ) having a capacity of 1.7 L and were heat-treated at 700 ° C. for 5 hours in an oxidizing atmosphere, and were placed under the floor. The running was a steady running and a 10-15 mode running based on the domestic exhaust gas regulation measurement method. In the exhaust gas analysis, a method of measuring the concentration of NOx, HC, and CO in the exhaust gas by direct analysis using an automobile exhaust gas measurement device and a method of obtaining CVS (Constant Volume Sampling) by an automobile constant volume dilution sampling device were applied.

In the 10-15 mode running,
10 mode and 15 mode steady running, 10 mode acceleration from 20 km / h to 40 km / h, and 15 mode acceleration from 40 km / h to 60 km / h and 50 km
Lean when accelerating from m / h to 70 km / h (A / F =
22 and 23) Running and the others were stoichiometric running. FIG. 18 shows the last of 10 modes repeated three times when the adsorption catalyst NN9 according to the method of the present invention is mounted, when the adsorption catalyst NK9 is mounted, and when the comparative catalyst NR2 is mounted. The NOx concentrations before and after the adsorption catalyst in the 10 mode and the subsequent 15 mode are shown. The comparative catalyst had the composition shown in Table 2 below.

In FIG. 18, when the adsorption catalysts NN9 and NK9 are mounted, the outlet NOx concentration is lower than the inlet NOx concentration in the entire operation range, and the lean operation and the stoichiometric operation are repeated, so that the adsorption catalyst is effectively operated. It can be seen that it is regenerated and keeps the NOx purification function. On the other hand, in the comparative catalyst NR2, there is a portion where the outlet NOx concentration exceeds the inlet NOx concentration.

CVS obtained with various adsorption catalysts and comparative catalysts
The values are shown in Tables 2 and 3 together with the composition of the adsorption catalyst. The preparation of the adsorption catalyst and the comparative catalyst was performed according to the method described above, but silica sol was used for silicon (Si) as a raw material for preparation. It is presumed that Si exists as silica (SiO ₂ ) or its composite oxide.

[0096]

[Table 2]

[0097]

[Table 3]

[0098]

As is evident from the above, according to the apparatus of the present invention, a NOx adsorption catalyst is provided in the exhaust gas channel, NOx is adsorbed and captured in the oxidizing atmosphere of lean exhaust gas, and a reducing atmosphere is created to regenerate the adsorption catalyst. By doing so, NOx and the like in the lean burn exhaust gas can be purified with high efficiency without significantly affecting the fuel efficiency.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an exhaust gas purifying apparatus according to a method of the present invention, showing a typical embodiment of the present invention.

FIG. 2 is a time-dependent characteristic of a NOx purification rate when a rich operation and a lean operation are alternately repeated by the method of the present invention.

FIG. 3 shows the relationship between the NOx concentration in lean exhaust gas and the NOx purification rate.

FIG. 4 shows the NOx purification rate in stoichiometric exhaust gas.

FIG. 5 shows the relationship between the NOx concentration at the inlet of the adsorption catalyst and the NOx concentration at the outlet when switching from rich (stoichiometric) operation to lean operation.

FIG. 6 shows the relationship between the concentration of NOx at the entrance of the adsorption catalyst and the concentration of NOx at the exit when switching from rich (stoichiometric) operation to lean operation.

FIG. 7 is a block diagram showing a method for controlling an air-fuel ratio.

FIG. 8 is a flowchart showing a method for controlling an air-fuel ratio.

FIG. 9 is a flowchart showing a method of integrating NOx emissions during lean operation.

FIG. 10 is a NOx amount estimation part in the flowchart of FIG. 8;

FIG. 11 is a NOx amount estimation part in the flowchart of FIG. 8;

FIG. 12 is a NOx amount estimation part in the flowchart of FIG. 8;

FIG. 13 is a NOx amount estimation part in the flowchart of FIG. 8;

FIG. 14 is a configuration diagram of an apparatus showing an embodiment provided with a manifold catalyst.

FIG. 15 is a configuration diagram of an apparatus showing an embodiment in a direct injection engine.

FIG. 16 is a configuration diagram of an apparatus showing an embodiment provided with a rear catalyst.

FIG. 17 is an apparatus configuration diagram showing an embodiment in which a reducing agent is added upstream of an adsorption catalyst.

FIG. 18 is a NOx purification characteristic diagram when a mode operation is performed.

FIG. 19 is a graph showing NOx purification characteristics when the oxygen concentration is changed using a model gas.

FIG. 20 is a graph showing NOx purification characteristics when the oxygen concentration is changed using a model gas.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Air cleaner, 2 ... Air flow sensor, 3 ... Throttle valve, 5 ... Injector, 6 ... Spark plug, 7
... Accelerator pedal, 8 ... Load sensor, 9 ... Intake air temperature sensor, 12 ... Fuel pump, 13 ... Fuel tank, 17 ...
Manifold catalyst, 18 adsorption catalyst, 19 oxygen sensor, 20 adsorption catalyst temperature sensor, 21 exhaust gas temperature sensor, 22 NOx concentration sensor, 23 reducing agent injector, 24 post-catalyst, 25 ECU, 26 knock Sensor, 28: Water temperature sensor, 29: Crank angle sensor, 99: Engine.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification code FI F02D 41/04 305 B01D 53/36 103B (72) Inventor Ryota Doi 7-1-1, Omikacho, Hitachi City, Hitachi City, Ibaraki Prefecture Hitachi, Ltd. Hitachi, Ltd. (72) Inventor Toshio Ogawa 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture Hitachi, Ltd.Hitachi Laboratory Co., Ltd. (72) Toshio Yamashita 7-1-1, Omikamachi, Hitachi City, Ibaraki Stock Hitachi, Ltd.Hitachi Research Laboratories (72) Inventor Shigeru Shozuhata 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi, Ltd.Hitachi Research Laboratory Co., Ltd. (72) Kojiro Okude 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 Hitachi, Ltd. Hitachi Research Laboratory (72) Inventor Yuichi Kitahara 2520 Takada, Hitachinaka City, Ibaraki Prefecture Ground: Hitachi, Ltd.Automotive Equipment Division (72) Inventor: Toshifumi Hiratsuka, Hitachinaka City, Ibaraki Prefecture, 2520 Oji Takada Hitachi, Ltd. Automotive Equipment Division (72) Inventor Toshio Manaka 2520 Oji Takaba, Hitachinaka City, Ibaraki Prefecture Hitachi, Ltd. Automotive Equipment Division (56) References JP-A-1-130735 (JP, A) JP-A-3-89942 (JP, A) JP-A-3-202157 (JP, A) JP-A-4-214918 (JP, A) JP-A-8-200049 (JP, A) JP-A-8-121147 (JP, A) JP, A) JP-A-7-310534 (JP, A) Patent 2586739 (JP, B2) Patent 2586738 (JP, B2) European Patent Application Publication 351197 (EP, A2) Preprints 882 of the Society of Automotive Engineers of Japan. , 1988.10.12 [Dynamic beh aviana ana] ysis of three way catalyti c Reaction] traffic safety and environmental pollution Research Center for Inter-industry em ission control pro gram 2 (IIEC-2) Prog ress Report No. 4, 1978 Effect of air-f uel ratio modulati on on conversion e fficiency of three -way catalyst, Mitsubishi automobile (58) investigated the field (Int.Cl. ^7, DB name) B01D 53/94 B01J 23/58 F01N 3/08-3/38 F01N 9/00-11/00 F02D 41/00-41/40

Claims (9)

(57) [Claims] 1. A lean burn air-fuel ratio A / F of 18 or more.
Have a manifold catalyst having the operation is a three-way function or the combustion catalytic function in the exhaust gas line immediately below the engine of the internal combustion engine
, And an exhaust gas purifying apparatus having a NOx trap catalyst on the downstream of the manifold catalyst, the NOx adsorption catalyst, often oxidizing agent to the reducing agent in a redox stoichiometry between the components in the exhaust gas NOx in exhaust gas in a state caught by adsorption as NO ₂ on the catalyst, NO reducing agent to oxidizing agent is trapped on the catalyst in the same amount or more states
_2. An exhaust gas purifying apparatus for an internal combustion engine, wherein ₂ is reduced to N ₂ by a reducing agent in the exhaust gas. 2. A lean burn with an air-fuel ratio A / F of 18 or more.
NOx adsorption catalyst is installed in the exhaust gas passage of the operated internal combustion engine.
And, an exhaust gas purifying apparatus having a combustion catalyst or three-way catalyst on the downstream of the NOx trap catalyst, the NOx adsorption catalyst to the reducing agent in a redox stoichiometry between the components in the exhaust gas NO in the exhaust gas with a large amount of oxidant
The x capture by adsorption as NO ₂ on the catalyst, the reducing agent to the oxidizing agent is reduced to N ₂ by a reducing agent in the exhaust gas to NO _2, which is captured on the catalyst in the same amount or more states An exhaust gas purifying apparatus for an internal combustion engine, comprising: 3. A lean burn with an air / fuel ratio A / F of 18 or more.
Has a catalyst having a function of oxidizing NOx in exhaust gas in the exhaust gas line of an internal combustion engine is operated, NO on the downstream of the catalyst
An exhaust gas purification apparatus having an x-absorption catalyst, wherein the NOx
Adsorber, NOx in exhaust gas while oxidizing agent is more to the reducing agent in a redox stoichiometry between the components in the exhaust gas is trapped by adsorption as NO ₂ on the catalyst, the oxidizing agent On the other hand, NO ₂ trapped on the catalyst in a state where the amount of the reducing agent is equal to or more than the amount of the reducing agent is reduced to N ₂
An exhaust gas purifying apparatus for an internal combustion engine, characterized in that the exhaust gas purifying apparatus reduces the exhaust gas. 4. A lean burn air-fuel ratio A / F of 18 or more.
Exhaust gas having a manifold catalyst having a function of burning hydrocarbons and carbon monoxide exhausted at the time of engine startup in an exhaust gas passage just below the engine of the internal combustion engine to be operated , and having a NOx adsorption catalyst downstream of the manifold catalyst a purification apparatus, the NOx adsorption catalyst, the NOx in the exhaust gas with an oxidizing agent often state as NO ₂ on the catalyst to the reducing agent in a redox stoichiometry between the components in the exhaust gas An internal combustion system characterized in that NO ₂ captured on the catalyst is reduced to N ₂ by a reducing agent in the exhaust gas in a state where the reducing agent is trapped by adsorption and the amount of the reducing agent is equal to or more than that of the oxidizing agent. Engine exhaust gas purification equipment. 5. An exhaust gas purifying apparatus for an internal combustion engine according to claim 4, wherein said manifold catalyst comprises a three-way catalyst. 6. An exhaust gas purifying apparatus for an internal combustion engine according to claim 4, wherein said manifold catalyst comprises a combustion catalyst containing palladium. 7. A lean burn air-fuel ratio A / F of 18 or more.
NOx adsorption catalyst is installed in the exhaust gas passage of the operated internal combustion engine.
And, an exhaust gas purifying apparatus having a combustion catalyst having a hydrocarbon purification performance after flow than the NOx trap catalyst, the NO
x adsorption catalyst, the NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing is higher than the stoichiometric air-fuel ratio is trapped by adsorption as NO ₂ on the catalyst, the air-fuel ratio of the exhaust gas flowing the stoichiometric air-fuel ratio following An exhaust gas purifying apparatus for an internal combustion engine, wherein NO ₂ sometimes trapped on the catalyst is reduced to N ₂ by a reducing agent in the exhaust gas. 8. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said internal combustion engine comprises an internal combustion engine that injects fuel into an intake system of the engine. 9. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said internal combustion engine comprises an internal combustion engine having a direct injection engine.