JP2002089246A - Exhaust emission control method and exhaust emission control device for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and a device for efficiently purifying nitrogen oxides of an internal combustion engine start period. SOLUTION: An NO oxidizing device and a nitrogen oxides trapping catalyst are installed in an exhaust gas passage of an internal combustion engine to purify the nitrogen oxides in exhaust gas of the internal combustion engine start period. To be specific, the NO oxidizing device 3 and the nitrogen oxides trapping catalyst 4 are installed in order in an exhaust pipe 2 from the engine 1. NO exhausted from the engine 1 flows into the NO oxidizing device 3. The NO inside the exhaust gas is oxidized into NO2 by the NO oxidizing device 3. Then the exhaust gas containing the NO2 flows into the nitrogen oxides trapping catalyst 4 and the NO2 is trapped by the nitrogen oxides trapping catalyst.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an exhaust gas purifying method and an exhaust gas purifying apparatus for an internal combustion engine, and more particularly to an exhaust gas purifying catalyst suitable for purifying NOx in exhaust gas discharged from the start of the engine.

[0002]

BACKGROUND OF THE INVENTION JP-A-10-212933 and JP-NOx in exhaust gas chemisorbed as NO ₂ in the lean, the NOx adsorbing catalyst material for reducing the adsorbed NO ₂ to _{N 2} in the stoichiometric or rich exhaust gas purification stream An exhaust gas purification device installed on a road has been proposed.

[0003]

With the stricter regulations on exhaust gas, measures against exhaust gas during operation of the internal combustion engine, particularly at the start-up when the catalyst does not work well, are very important. N at start
Ox is difficult to purify by reaction, and there is a problem that the trapping rate of NOx must be improved in order to sufficiently increase the purification rate.

[0004] It is an object of the present invention to provide a method for operating an internal combustion engine,
An object of the present invention is to provide an exhaust gas purifying method and an exhaust gas purifying apparatus which are excellent in NOx purifying performance in a wide temperature range from room temperature at start-up to 300 ° C. or lower.

[0005]

SUMMARY OF THE INVENTION The present inventors have intensively studied a method and an apparatus for removing NOx from exhaust gas during operation of an internal combustion engine, particularly during a start-up period.

[0006] As a result, the NO contained in exhaust gas discharged from the internal combustion engine during the starting period of the internal combustion engine, after oxidizing the NO ₂ by the oxidation catalyst, NO ₂ in the lean
Was captured by absorption or adsorption on the catalyst, the air-fuel ratio is stoichiometric or rich, the trapped the NOx and are effective method and apparatus for purifying exhaust gas by the NOx trap catalyst for reducing the N ₂ by a reducing agent in the exhaust gas Was found.

[0007] The internal combustion engine start period refers to a certain period or a certain state when the internal combustion engine is started (cold start) without performing warm-up operation. Specifically, the certain period means within 180 seconds immediately after the start of the internal combustion engine. Further, the constant state means that the temperature of the exhaust gas flowing into the NOx trapping catalyst is changed from around room temperature immediately after the start of the internal combustion engine to 30 degrees.
It means a state of 0 ° C. or lower.

[0008] in the exhaust gas, HC, CO, and reducing agent such as _{H 2,} contains an oxidizing agent such as _{O 2.} In the oxidation-reduction reaction, a case where the concentration of the reducing agent and the concentration of the oxidizing agent are not excessive or insufficient is called a stoichiometric air-fuel ratio (stoichiometric). The case where the reducing agent concentration is excessive than the oxidizing agent concentration is called rich. A case where the concentration of the reducing agent is lower than the concentration of the oxidizing agent is called lean.

Specifically, in the internal combustion engine, the weight ratio (air-fuel ratio) of air to fuel is 14.7 for stoichiometry, and 14.7 for stoichiometry.
If less than rich, it is lean, and if it is greater than 14.7, it is lean. In a specific lean burn engine, a lean air-fuel ratio of 20 or more is desirable in consideration of fuel efficiency.

The NOx trapping catalyst has a NOx trapping material for trapping NOx. And the NOx trapping style is
It is roughly divided into adsorption and absorption. Absorption means that NOx is stored inside the catalyst by a chemical reaction. At this time, the form of NOx forms a compound such as a nitrate compound and a nitrite compound. Therefore, when NOx trapping is absorption, it means that NOx is trapped in the NOx absorbent by absorption.

[0011] Adsorption means that NOx is trapped on the surface of the catalyst by chemical adsorption. NO at this time
The form of x includes NO and NO ₂ , but NO ₂ is preferable. Therefore, when NOx trapping is adsorption, NOx
This means that NOx is trapped on the surface of the adsorbent by adsorption. In particular, in the case of capture by NO ₂ adsorption means that NOx is trapped by adsorption in the form of NO ₂ on the adsorbent surface.

Since the above NOx trapping material has solid basicity, it is easy to attract acidic molecules. NO in exhaust gas
When NO and NO ₂ , which are representative components of the x form, are compared, N
O ₂ is more acidic. Therefore, operation of the internal combustion engine,
Especially during the start-up period, NO flowing into the NOx trapping catalyst
When the form of x is previously oxidized from NO to NO _2,
It becomes easy to be trapped by the NOx trapping material.

From the above, it is desirable that the NOx trapping catalyst of the present invention has at least one of an absorbent and an adsorbent. When NO ₂ adsorbent is used, NOx
NO _{2 that} has flowed into the trapping catalyst is trapped on the adsorbent in its original form. Therefore, the NOx auxiliary捉材in an internal combustion engine starting period, NO ₂ adsorbent is particularly preferred.

When the air-fuel ratio in the exhaust gas is stoichiometric or rich, the NOx trapped by the NOx trapping catalyst reduces HC, CO, etc. coexisting in the exhaust gas with the rise in the temperature of the catalyst layer after the start of the internal combustion engine. N by catalytic reaction using an agent
Reduced to ₂ . Accordingly, at the time of starting the internal combustion engine, there the NOx trapping capability in the form of NOx is NO ₂ flowing into the NOx trap catalyst is enhanced.

In the form of NOx discharged from the internal combustion engine, NO and NO ₂ coexist. Therefore, by using a method or an apparatus for converting NO to NO ₂ before flowing into the NOx trapping catalyst, the amount of NOx trapped by the NOx trapping catalyst increases, and the NOx purification performance at the time of starting the internal combustion engine improves. .

[0016] As a method for converting NO to NO _2, there is a method of oxidizing NO to NO ₂ by the coexistence of oxygen in the exhaust gas. Specific examples of this method include a method using a NO oxidation catalyst, a method using an ozone generator, and a method using electron beam irradiation.

In the method using the NO oxidation catalyst, the exhaust gas is N
When flowing through the O oxidation catalyst, an oxidation reaction of NO to NO ₂ occurs. There is also a method of using a NO oxidation catalyst provided with an electric heater. When the exhaust gas temperature is low, the catalyst is electrically heated to obtain a catalyst temperature suitable for NO oxidation.

In the method using an ozone generator, an ozone generator is provided upstream of the NOx trapping catalyst, and N
It converts O into NO ₂ . Specifically, the ozone generator works for a certain time when the internal combustion engine is started, and converts oxygen in the exhaust gas into ozone (formula (1)). And
NO in the exhaust gas reacts with ozone from the ozone generator (formula (2)) and is converted into NO ₂ .

O ₂ → O ₃ (1) NO + O ₃ → NO ₂ + O ₂ (2) As an ozone generator, for example, a discharge tube is installed in an exhaust gas flow path, and the inside of the discharge tube is Some generate ozone when gas flows. Also, by generating ozone outside the exhaust flow path and injecting this ozone into the exhaust flow path, N
O may be oxidized.

In the electron beam irradiation method, an electron beam irradiation device is provided in an exhaust gas flow path, and when the exhaust gas passes through the electron beam irradiation device, NO and oxygen that have been irradiated with the electron beam react with each other to form NO ₂ . It becomes.

Next, specific examples of the catalyst will be described. The NO oxidation catalyst contains Rh and Pt as noble metals on a porous carrier,
Mn. Desirably, a NO oxidation catalyst having Rh, Pt, Pd, and Mn as noble metals on a porous carrier is suitable. Further, it has a porous carrier, Mg, Mn, and Rh and Pt as noble metals, preferably Pd and Ce.
It is particularly preferred to have at least one of

As the porous carrier, in addition to alumina, titanium
Gold such as tania, silica, silica-alumina, magnesia
Oxides and composite oxides of La and Al (La-β-Al
₂O ₃) Can be used. Also,
Alumina is used as a carrier to improve the heat resistance of the NO oxidation catalyst.
L which becomes 95/5 (alumina / La) in a molar ratio of La
a-β-Al₂O₃Is preferred.

When a general three-way catalyst is used as the NO oxidation catalyst, the NO oxidation performance in a low temperature range of 300 ° C. or less during the internal combustion engine start period is not sufficient. Although the detailed reaction mechanism is not clear, the addition of Mn to the noble metal improves the NO oxidation temperature range in a low temperature range of 300 ° C. or lower.

[0024] When the temperature exceeds 900 ° C or higher, the specific surface area of the porous carrier is significantly reduced. It is preferable because the heat resistance of the NO oxidation catalyst containing Mg and Ce is improved. Furthermore, by using the La-β-Al ₂ O ₃ carrier, a decrease in the specific surface area of the carrier is suppressed, and sintering of Mg, Ce and a noble metal, which are active components of the NO oxidation catalyst, is suppressed.

The NOx trapping catalyst uses an alkali metal (Li,
Na, K, Rb, Cs) and alkaline earth metals (Ca, S
r, Ba, Sr), it is possible to use a NOx trapping catalyst containing at least one or more elements as a part of components.

For example, a catalyst in which an alkali metal, titanium, and at least one of Rh, Pt, and Pd are supported on a porous carrier (described in JP-A-10-118458), a noble metal and an alkaline earth metal are supported on a porous carrier a catalyst carrying a composite oxide of titanium (described in JP-a-10-10932), at least K, Na, Mg, Sr, NO 2 adsorptive catalyst comprising one or more elements selected from Ca as part of component (Described in JP-A-10-212933).

Further, a porous carrier, Ce, Zr, W,
Ti, V, Mn, Cr, Mo, Cu, Fe, Ni, Co
And at least one selected from alkali metals (Li, Na, K, Rb, Cs) and alkaline earth metals (Mg, Ca, Sr, Ba). further,
It is also possible to have Ru, Ag, Rh, Pt, Pd, etc.

At least one selected from an alkali metal and an alkaline earth metal, and Zr, Ti, W, V, Mn, C
An NOx adsorbent that is a composite oxide with at least one selected from e, MO, Cu, Fe, Ni, and Co can also be used.

As described above, as the porous carrier, in addition to alumina, metal oxides such as titania, silica, silica-alumina, and magnesia, and composite oxides of La and Al (La-β-Al ₂ O ₃ ) can be used.

As a method for preparing the NO oxidation catalyst and the NOx trapping catalyst, for example, there is a method in which the catalyst is supported on a porous material by a method such as ion exchange, impregnation, and kneading. Further, the shape of the catalyst and the material include those supported on cordierite honeycomb, granules, and those supported on metal honeycomb.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described with reference to specific examples, but the present invention is not limited to these examples.

Embodiment 1 FIG. 1 shows an embodiment of the present invention applied to an automobile. An NO oxidizing device 3 and a NOx trapping catalyst 4 are installed in the exhaust pipe 2 from the engine 1 in this order. NO discharged from the engine 1 is N
It flows into the O oxidation device 3. NO in the exhaust gas is oxidized to NO ₂ in the NO oxidizer 3. Thereafter, exhaust gas containing NO ₂ flows into the NOx trapping catalyst 4, NO ₂ is trapped by the NOx trap catalyst.

[Embodiment 2] FIG. 2 shows another embodiment of the present invention applied to an automobile. An NO oxidation catalyst 5 and a NOx trapping catalyst 4 are provided in this order on an exhaust pipe 2 from the engine 1. NO emitted from engine 1 is N
It flows into the O oxidation catalyst 5. NO in the exhaust gas at the NO oxidation catalyst 5 is converted to NO _2. Thereafter, exhaust gas containing NO ₂ flows into the NOx trapping catalyst 4, NO ₂ is trapped by the NOx trap catalyst.

Third Embodiment FIG. 3 shows another embodiment of the present invention applied to an automobile. An NO oxidation catalyst 6 with an electric heater and NOx are connected to the exhaust pipe 2 from the engine 1.
The trapping catalysts 4 are provided in this order. NO discharged from the engine 1 flows into the NO oxidation catalyst 6 with an electric heater.

The NO oxidation catalyst 6 with an electric heater is electrically heated when the engine is started. The heating temperature is equal to or higher than the temperature at which the NO oxidation catalyst operates. The method of determining the heating temperature is, for example, as follows. The conversion rate for oxidizing NO to NO ₂ with respect to temperature is measured in advance, and the temperature at which the rate of change becomes 80% or more is defined as the heating temperature. When the engine is started, the NO oxidation catalyst is heated by the electric heater so as to reach the heating temperature.

Here, in order to improve the heat conduction of the NO oxidation catalyst, it is preferable to use a metal honeycomb. NO in exhaust gas is generated by the NO oxidation catalyst 6 with an electric heater.
It is converted to NO _2. Thereafter, exhaust gas containing NO ₂ flows into the NOx trapping catalyst 4, NO ₂ is trapped by the NOx trap catalyst.

Fourth Embodiment FIG. 4 shows an example in which a heating control device 9 for controlling the heating of the NOx catalyst 6 with an electric heater is provided. An exhaust gas temperature sensor 8, an NO oxidation catalyst 6 with an electric heater, and N
The Ox trap catalyst 4 is provided in this order.

FIG. 5 shows a control flow of the heating control device 9. When the engine is started (step: S1), the exhaust gas temperature sensor 8 measures the temperature of the exhaust gas flowing into the NO oxidation catalyst 6 with an electric heater (S2). The information on the measured temperature is sent to the engine control unit 9.

The engine control unit 9 has a predetermined temperature T 0 and a temperature T 1 from the exhaust gas temperature sensor 8.
ΔT (= T0−T1) is obtained, and NO
The electric output required to heat the oxidation catalyst 6 is determined (S3).

The engine control unit 9 is shown in FIG.
The map of the electric output with respect to ΔT as shown in FIG. The engine control unit 9 heats the heater-equipped NO oxidation catalyst 6 (S5). If it is determined that ΔT has become equal to or less than zero, the heating of the NO oxidation catalyst with heater 6 is stopped (S6).

Here, the predetermined temperature is a temperature required for the NO oxidation catalyst to sufficiently convert NO to NO ₂ (for example, 80% or more). A method for determining the conversion with respect to temperature is described in Example 4. NO in the exhaust gas is converted into NO ₂ by the NO oxidation catalyst 6 with a heater, flows into the NOx trapping catalyst 4 and is trapped by the NOx trapping catalyst 4.

Fifth Embodiment FIG. 7 shows another example in which a heating control device 9 is further provided in the NOx catalyst 6 with an electric heater. An NO oxidation catalyst 6 with an electric heater and a NOx trapping catalyst 4 are installed in the exhaust pipe 2 from the engine 1 in this order. The NO oxidation catalyst 6 with an electric heater is provided with a catalyst temperature sensor 10.

FIG. 8 shows a control flow of the heating control device 9 in this case. When the engine is started (S1). The catalyst temperature sensor 10 measures the temperature T2 of the NO oxidation catalyst 6 with an electric heater (S2). The information on the measured temperature T2 is sent to the engine control unit 9.

The engine control unit 9 calculates the temperature difference ΔTT (= T) between the predetermined temperature T0 and the catalyst temperature T2.
0−T2) is obtained (S3). The engine control unit 9 is provided with a map (eg, FIG. 9) of ΔTT and the output of the electric heater in advance. Based on the map,
The output of the electric heater is determined from ΔTT. (S4) Δ
If TT is a positive value, NO oxidation catalyst 6 with an electric heater
In order to heat the heater, the output of the electric heater is determined, and the electric heater is controlled to obtain the output (S5). When the engine control unit 9 determines that the value of ΔT has become equal to or less than zero, it stops heating the electric heater (S
6).

NO in the exhaust gas is converted to NO ₂ when flowing through the NO oxidation catalyst 6 with a heater,
By being captured by the capture catalyst 4, NOx in the exhaust gas at the time of starting the engine is purified.

Embodiment 6 FIG. 10 shows another embodiment of the present invention applied to an automobile. An ozone generator 11 and a NOx trapping catalyst 4 are installed in the exhaust pipe 2 from the engine 1 in this order. N discharged from engine 1
O flows into the ozone generator 11. NO in the exhaust gas at an ozone generator 11 is converted into NO _2. Thereafter, the exhaust gas containing NO ₂ flows into the NOx trapping catalyst 4,
NO ₂ is captured by the NOx capture catalyst.

Embodiment 7 FIG. 11 shows still another embodiment of the present invention applied to an automobile. An ozone injection device 12 and a NOx trapping catalyst 4 are installed in the exhaust pipe 2 from the engine 1 in this order. Further, an ozone generator 13 is installed outside the flow path of the exhaust pipe 2,
The ozone generator 13 and the ozone injector 12 are connected.

Ozone is produced by an ozone generator 13 installed outside the flow path of the exhaust pipe 2. Then, ozone is injected into the exhaust gas by the ozone injection device 12, and
O is oxidized to NO ₂ by ozone. Then, NO ₂
Flows into the NOx trapping catalyst 4 and is trapped.

Example 8 An example of the embodiment of the NOx trapping catalyst will be described below. Cordierite honeycomb (4
(00 cells / in2) was coated with 190 g / L of alumina. The alumina-coated honeycomb was impregnated with Ce nitrate dissolved in water, dried at 200 ° C., and then fired at 600 ° C.

Next, a dinitrothodiamine Pt nitric acid solution, a mixed solution of Pd nitrate, Rh nitrate and K acetate were impregnated.
It was dried at 0 ° C and subsequently calcined at 600 ° C. Finally, after impregnating with a mixed solution of Na nitrate, Li nitrate, titania sol and K acetate, it was dried at 200 ° C. and calcined at 600 ° C. for 1 hour.

As described above, with respect to 1 L of honeycomb, Mn: 13.7 g, Pd: 1.5 g, Pt: 2.
8 g, Rh: 0.14 g, K: 15.6 g, Na: 13.
The catalyst of Example 1 containing 8 g, 1.8 g of Li, and 27 g of Ce was obtained.

The first component, the second component and the third component in Table 1 indicate the order of loading, with the smaller number being loaded first. In addition, the supported amount per 1 L of the honeycomb is described before the supported component. For example, 27 Ce indicates that 27 g of Ce is supported on 1 L of honeycomb in terms of metal.

[0053]

[Table 1](Test Example 1) The NOx purification rate of the catalyst
And NO₂Gas type dependence was studied. The test method is as follows
Followed. While flowing the reducing atmosphere model gas,
The temperature was raised from the temperature to 550 ° C. at a rate of 10 ° C./min. reduction
The composition of the atmosphere model gas is CO 0.5%, NO 1
000 ppm, H₂ 0.3%, C₃H ₆ 600p
pm, O₂ 0.5%, CO₂ 5%, H₂O 10
%, N₂ It is left.

Thereafter, the temperature was raised to room temperature while flowing nitrogen gas.
Then, while flowing the reaction gas, 10 ° C./min.
To raise the temperature from room temperature to 350 ° C. The reaction gas is NO
200 ppm, O₂ 5%, N₂ NO gas left
And NO₂ 200 ppm, O ₂ 5%, N₂ Leave
NO₂Two types of gas were used. The catalyst volume is 6c
c, SV was 30,000 / h. The NOx purification rate is
The reduction rate of the NOx concentration before and after flowing through the catalyst layer in the equation (3)
Was.

NOx purification rate = {(NOx concentration before catalyst layer distribution—NOx concentration after catalyst layer distribution) / (NOx concentration before catalyst layer distribution)} × 100 (3) NOx purification rate based on test results Are shown in Table 2. In the example catalyst A, the NOx purification rate is higher when the NO ₂ gas is circulated than when the NO gas is circulated. Therefore, 30
It is clear that converting NO to NO ₂ in a low temperature range of 0 ° C. or lower increases the NOx purification rate.

[0056]

[Table 2] Example 9 Example catalyst B in Table 3 was obtained by the same catalyst preparation method as in Example catalyst A.

[0057]

[Table 3] (Test Example 2) NOx of Example Catalyst A and Example Catalyst B
The state of capture was examined by an in-situ FT-IR (Fourier transform infrared absorption measurement) method. The test method is shown below.

A powder obtained by pulverizing Example Catalyst A or Example Catalyst B is placed on a sample stage. The sample stage was covered with a container to prevent contact with the outside air. This container has a KBr window through which infrared light can pass.

First, the temperature was raised to 500 ° C. in a He gas stream,
The pretreatment gas was circulated for 10 minutes while maintaining the temperature at 500 ° C. The composition of the pretreatment gas was 1 vol% of CO and He residual. After the completion of the pretreatment, the catalyst was cooled in a He gas stream, kept at 300 ° C., and the infrared absorption spectrum of the catalyst before the reaction was measured.

Subsequently, the reaction gas was passed, and the average infrared absorption spectrum was measured for 2 to 3 minutes after the start of the reaction. The infrared absorption spectrum before the reaction gas flow was swept from the infrared absorption spectrum after the reaction gas flow (hereinafter, difference spectrum). The reaction gas was NO 600 volppm, O ₂
5 vol%, the He residue (hereinafter, _{NO-O 2} gas) was.

As a result of the measurement, no nitrate compound was detected in the catalyst A of the example. In addition, since the infrared absorption spectrum coincides with the infrared absorption spectrum obtained when only NO _{2 was} allowed to flow, it was found that NO _{2 was} adsorbed. In addition, the catalyst B of the example is composed of nitric acid B
Since infrared absorption of a and K nitrate was detected, it was found to be absorption. From the above, the catalyst of Example A
Is a catalyst which adsorbs NOx as NO _2, it is evident that the catalyst of Example B is a catalyst that absorbs NOx as nitrate compounds.

Test Example 3 According to Test Example 1 of Example 8, the NOx purification rate by NO ₂ gas was measured. The results are shown in Table 4. Example catalyst B that absorbs NOx is 3
NOx purification performance in low temperature range below 00 ° C is 40-
60%.

Therefore, it is clear that flowing in as NO ₂ enhances NOx purification performance in a low temperature range of 300 ° C. or lower. However, the example catalyst A that adsorbs as NO ₂ is superior to the example catalyst B that absorbs NOx in NOx purification performance in a low temperature range of 300 ° C. or less.

[0064]

[Table 4] Example 10 Example catalyst A and example catalyst B were replaced with 8
The heat resistance was examined by measuring the NOx purification rate in lean and stoichiometric conditions before and after the heat treatment at 30 ° C. for 60 hours.

As test methods, the test gas used was a lean gas simulating lean burn exhaust gas and a stoichiometric gas simulating stoichiometric air-fuel ratio combustion. Each gas composition was as follows.

Lean gas; NOx: 600 ppm;
C ₃ H ₆ : 500 ppm, CO: 0.1%, C
O ₂ : 10%, O ₂ : 5%, H ₂ O: 10%, N
₂ : Residual.

Stoichigas; NOx: 1000 ppm,
C ₃ H ₆ : 600 ppm, CO: 0.5%, CO
_{2: 5%, O 2:} 0.5%, H 2: 0.3%, H
₂ O: 10%, N ₂ : Residual.

The test method followed the following procedure. First, SV: 30,000 / h, 500
The solution was passed through the catalyst layer at 10 ° C. for 10 minutes. Next, after a predetermined temperature of 300 ° C. to 500 ° C., lean gas and stoichiometric gas were alternately passed through the catalyst layer every three minutes, and the NOx purification rate was measured. The measurement time was 30 minutes, the catalyst volume was 6 cc, and the SV was 30,000 / h.

As a result of the test, Table 5 shows the temperature (° C.) and the NOx purification rate (%) one minute after switching from stoichiometric to lean. The results in Table 5 were repeatedly reproduced,
It is clear that NOx trapped in the lean is reduced and purified by the stoichiometry.

NOx 1 minute after lean switching after heat treatment
When the catalysts with the highest purification rates were arranged, the catalysts of Example Catalyst A and Example Catalyst B were arranged in that order. It is clear that the heat resistance of the NOx trapping catalyst is excellent in Example Catalyst A using the adsorbent.

[0071]

[Table 5] Embodiment 11 One embodiment of the NO oxidation catalyst is described below. Beta structure La and A with La / Al molar ratio 5/95
1 of a composite oxide (hereinafter, La-β-Al ₂ O ₃ ) powder was prepared using a cordierite honeycomb (400 cells / inc.).
2) Coated. The coating amount was 190 g per liter of apparent volume of the honeycomb.

This La-β-Al ₂ O ₃ coated honeycomb was impregnated with Ce nitrate dissolved in water and dried at 200 ° C.
Subsequently, firing was performed at 600 ° C. Next, it was impregnated with a mixed solution of dinitrothodiamine Pt nitric acid solution, Pd nitrate, Rh nitrate and Mn nitrate, dried at 200 ° C., and subsequently fired at 600 ° C. Finally, impregnated with Mg nitrate solution and dried at 200 ° C,
A firing treatment was performed at 600 ° C. for 1 hour.

As described above, for 1 L of honeycomb, in terms of metal, Mg: 2.1 g, Pd: 1.5 g, Pt: 2.7.
g, Rh: 0.13 g, Mn: 7.3 g, and Ce: 10 g were obtained.

Also, the catalyst is the same as that of the catalyst C of the example, but Mn
Example Catalyst D which does not carry the compound was obtained. The first in Table 6
The components, the second component, and the third component indicate the order of loading, with the smaller number being loaded first. In addition, the supported amount per 1 L of the honeycomb is described before the supported component. For example, 10 Ce indicates that 10 g of Ce is supported on 1 L of honeycomb in terms of metal.

[0075]

[Table 6] (Test Example 4) NO of Example Catalyst C and Example Catalyst D of heat-durable product fired at 600 ° C and fired at 830 ° C for 60 hours
It was measured conversion to NO ₂ from.

The test method followed the following procedure. From room temperature to 550 ° C while flowing the reducing atmosphere model gas
Up to 10 ° C./min. The composition of the reducing atmosphere model gas is CO 0.5%, NO 1000 ppm,
_{H 2 0.3%, C 3 H} 6 600ppm, O 2 0.5
%, CO ₂ 5%, H ₂ O 10%, N ₂ residue.

Thereafter, the temperature was cooled to room temperature while flowing nitrogen gas, and then the temperature was raised from room temperature to 350 ° C. at a rate of 10 ° C./min while flowing reaction gas. The reaction gas is NO
NO gas containing 200 ppm, 5% of O ₂ , and N ₂ was used. The catalyst volume was 6 cc and the SV was 6500 / h.

The NO conversion rate was defined as the rate of increase of the NO ₂ concentration after flowing through the catalyst layer with respect to the NO concentration at the catalyst inlet (Equation (4)).

NO conversion rate = {(NO ₂ concentration after flowing through the catalyst layer) / (NO concentration before flowing through the catalyst layer)} × 100 (4) Table 7 shows the test results. It is clear that the catalysts of Examples C and D can convert NO to NO ₂ from the conversion (%) in the low temperature range of 300 ° C. or less in both the initial product and the heat-durable product. is there.

In both the initial product and the heat-durable product, the catalyst C of Example carrying Mn was more effective than the catalyst D of Example D carrying no Mn at a NO conversion rate of 150 ° C. or less. Is better. Therefore, it is apparent that Mn enhances the oxidation performance of NO at the initial stage and after the heat endurance.

[0081]

[Table 7] [Example 12] An example of an embodiment in which a NO oxidation catalyst and a NOx trapping catalyst are combined is described below. Example catalyst C was used as the NO oxidation catalyst, and example catalyst A was used as the NOx trapping catalyst.

Test Example 5 Example Catalyst A was arranged downstream of Example Catalyst C. The volume of each catalyst was 6 cc. The gases used in the test were lean gas simulating lean burn exhaust gas and stoichiometric gas simulating stoichiometric air-fuel ratio combustion. Each gas had the following composition.

Lean gas; NOx: 600 ppm,
C ₃ H ₆ : 500 ppm, CO: 0.1%, C
O ₂ : 10%, O ₂ : 5%, H ₂ O: 10%, N
₂ : Residual.

Stoichigas; NOx: 1000 ppm,
C ₃ H ₆ : 600 ppm, CO: 0.5%, CO
_{2: 5%, O 2:} 0.5%, H 2: 0.3%, H
₂ O: 10%, N ₂ : Residual.

The test method is as follows. First, stoichiometric gas was passed through the catalyst layer at an SV of 30,000 / h at 500 ° C. for 10 minutes. Next, after cooling to room temperature, S
V30,000 / h while flowing lean gas
The temperature was raised to ° C. The heating rate was 50 ° C./min. The NOx purification rate at this time was measured. For comparison, a NOx purification rate measurement using only the example catalyst A was also performed.

Table 8 shows the test results. 50-300 ° C
At the upstream of the NOx trapping catalyst (Example Catalyst A)
The NOx purification rate (%) in the case where the O oxidation catalyst (Example catalyst C) was arranged was superior to the case where only the NOx trapping catalyst (Example catalyst A) was used. From the above, it is clear that providing a NO oxidation catalyst upstream of the NOx trapping catalyst enhances NOx purification performance at 300 ° C. or lower.

[0087]

[Table 8] [Embodiment 13] FIG. 12 shows an example of an apparatus in which an NO oxidation catalyst 5 is installed immediately below an engine in an automobile. NO near the joint between engine 1 and exhaust pipe 2
By providing the oxidation catalyst 5, heat loss when the combustion exhaust gas flows through the exhaust pipe is suppressed. Therefore, the NO oxidation catalyst 5 can convert NO to NO ₂ not only during the operation of the engine but also particularly during the engine start period.

[0088]

As described above, according to the exhaust gas purifying method and the exhaust gas purifying apparatus of the present invention, not only during the operation of the internal combustion engine, but also particularly in a wide temperature range from room temperature to 300 ° C. or less during the starting period. NOx purification performance is obtained.

[Brief description of the drawings]

FIG. 1 shows that an NO oxidizer and N
It is a schematic diagram which shows the example of an apparatus provided with the Ox capture catalyst.

FIG. 2 is a schematic diagram showing an example of an apparatus provided with a NO oxidation catalyst and a NOx trapping catalyst.

FIG. 3 is a schematic view showing an example of an apparatus provided with a NO oxidation catalyst with an electric heater.

FIG. 4 is a schematic diagram showing an example of an apparatus provided with an exhaust gas temperature sensor.

FIG. 5 is a diagram illustrating an example of a control flow of an apparatus including an exhaust gas temperature sensor.

FIG. 6 is a diagram showing an example of a map of exhaust gas temperature and electric heater output.

FIG. 7 is a schematic view showing an example of an apparatus provided with a catalyst temperature sensor.

FIG. 8 is a diagram illustrating an example of a control flow of an apparatus having a catalyst temperature.

FIG. 9 is a diagram showing a map example of a catalyst temperature and an electric heater output.

FIG. 10 is a diagram illustrating an example of an apparatus including an ozone generator.

FIG. 11 is a diagram illustrating an example of an apparatus including an ozone generator and an ozone injector.

FIG. 12 is a schematic diagram showing an example of an apparatus in which a NO oxidation catalyst is installed immediately below an engine in an automobile.

[Explanation of symbols]

 Reference Signs List 1 engine 2 exhaust pipe 3 NO oxidation device 4 NOx trapping catalyst 5 NO oxidation catalyst 6 NO oxidation catalyst with electric heater 8 exhaust gas temperature sensor 9 engine control unit 10 catalyst temperature sensor 11 ozone generator 12 ozone injection device

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) F01N 3/10 F01N 3/28 301C 3/28 301 301E B01D 53/36 102G (72) Inventor Masato Kaneda Ibaraki 7-2-1, Omika-cho, Hitachi City, Hitachi, Ltd. Power and Electric Development Laboratory, Hitachi, Ltd. (72) Inventor Toshio Yamashita 7-2-1, Omika-cho, Hitachi City, Ibaraki Prefecture Electric Power / Electricity Development Co., Ltd. Inside the research institute (72) Inventor Yuichi Kitahara 2520 Mao Daiko Takaba, Hitachinaka City, Ibaraki Prefecture Inside Hitachi Automotive Equipment Group 3G091 AA02 AA12 AA17 AB02 AB04 AB06 AB16 BA03 BA14 CA03 CA21 CB02 DA01 DA02 DB10 EA17 EA18 FA02 FA04 FA12 FB02 FB10 FB11 FB12 FC07 GA06 GB01W GB01X GB02W GB04W GB05W GB06W GB07W GB07X GB10X HA10 HA36 HA39 4D048 AA06 AB01 AB02 BA01X BA03X BA07X BA14X

Claims (8)

[Claims] 1. After converting NOx in exhaust gas discharged at least during a starting period of an internal combustion engine into NO ₂ in advance,
Exhaust gas purification method for an internal combustion engine, characterized in that by flowing into the NOx trapping reduction catalyst for reducing the NO ₂ to N _2. 2. An exhaust gas containing NOx discharged from an internal combustion engine is converted into NO ₂ at least during a starting period.
After flowing into NO oxidizing means which can be oxidized to NO and oxidizing NO contained in the exhaust gas to NO ₂ , N
Ox is trapped in the form of NO ₂ and flows into NOx trapping means for trapping and trapping by absorption or adsorption at a lean air-fuel ratio, and NO ₂ trapped at a stoichiometric or rich air-fuel ratio is reduced by a reducing agent in exhaust gas. exhaust gas purification method for an internal combustion engine to reduce the N _2. 3. After NO in the exhaust gas containing NOx discharged to at least the starting period of the internal combustion engine is oxidized to NO _2, the NOx in a lean air-fuel ratio is trapped by adsorption as NO ₂ on the catalyst, the air-fuel ratio the NO ₂ that is trapped on the catalyst at stoichiometric or rich by flowing into the NOx adsorbent catalyst for reducing the N ₂ by a reducing agent in the exhaust gas,
An exhaust gas purifying method for an internal combustion engine for purifying NOx in the exhaust gas. 4. An exhaust gas containing NOx discharged from an internal combustion engine flows into a NO oxidation catalyst that oxidizes NO in exhaust gas discharged from the internal combustion engine into NO ₂ at least during a start-up period. the NOx capture by adsorption as NO ₂ on a catalyst, and the NO ₂ that the air-fuel ratio is trapped on the catalyst at stoichiometric or rich to flow into the NOx trap catalyst to be reduced to N ₂ by a reducing agent in the exhaust gas at, An exhaust gas purifying method for an internal combustion engine for purifying NOx in the exhaust gas. 5. The exhaust gas purifying method for an internal combustion engine according to claim 2, wherein the means for oxidizing NO is a NO oxidation catalyst having Rh, Pt, and Mn as noble metals on a porous carrier. 6. The internal combustion engine according to claim 2, wherein the means for oxidizing NO is an NO oxidation catalyst having Mg on a porous carrier, and Rh, Pt, Pd, and Mn as noble metals. Exhaust gas purification method. 7. NO in exhaust gas discharged from the internal combustion engine at least when the internal combustion engine is in a starting period is reduced to NO ₂
NO oxidation means for converting to NO, and N
A NOx trapping means for trapping and reducing NOx oxidized to O ₂ by absorption or adsorption, wherein the NOx trapped by the NOx trapping means is reduced to N ₂ by a reducing agent in the exhaust gas. Exhaust gas purification device. 8. NO in exhaust gas discharged from the internal combustion engine when the internal combustion engine is at least during a starting period is reduced to NO ₂
And NO oxidation means which can be converted into, the NOx in the exhaust gas which the oxidized via the NO oxidation means to NO _2, captured by absorption or adsorption in the lean air-fuel ratio, NOx air-fuel ratio is trapped in stoichiometric or rich NOx trapping means for reducing NO ₂ to N ₂ by a reducing agent in exhaust gas, and the NO ₂ oxidized by the NO oxidizing means
An exhaust gas purifying apparatus for an internal combustion engine that purifies NOx in the exhaust gas by trapping and reducing the exhaust gas with an x capturing unit.