JP2001003733A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To dispose of exhaust emission at high efficiency over a long period of time by combining a NOx decomposition stage with a catalyst regenerating stage in a suitable time pattern. SOLUTION: An active metal-containing catalyst (catalyst) 7 decomposes NOx when an engine is operated under the lean condition of an air-fuel ratio, so that NOx is contained in the exhaust emission, and the catalyst 7 is gradually oxidized to form an oxide film on the surface of the catalyst 7, so that the NOx decomposition activity of the catalyst 7 is gradually decreased. Such activity decrease is detected by a NOx sensor NS2 installed at an outlet side of the catalyst 7, and when it is known from the NOx concentration that the catalyst activity is unsuitably low or lowered, the air-fuel ratio is put in the rich state. In this state, the oxide film on the surface of the catalyst 7 is removed by reaction and the catalyst 7 is regenerated. The regeneration of the catalyst 7 is determined by monitoring the HC concentration by hydrocarbon sensors CS1, CS2 installed upper-and downstream. When the catalyst is once regenerated the air-fuel ratio is returned to the lean state.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for purifying NOx in exhaust gas of an internal combustion engine using an active metal-containing catalyst.

[0002]

2. Description of the Related Art Various catalysts have been used to treat NOx in exhaust gas of internal combustion engines, for example, diesel engines. However, it is difficult to harmonize the NOx generation temperature range with the effective activation temperature ranges of the individual catalysts, and a single catalyst means efficiently and for a long time over a wide temperature range in the exhaust gas during operation. Satisfactory NOx
Development of exhaust gas treatment technology to achieve purification is required. For purification of exhaust gas, for example, a copper-zeolite catalyst has been studied and studied, but has not been put to practical use due to low catalytic activity and low durability. JP-A-6
No. 50139 discloses a technique in which an air-fuel ratio of inflowing exhaust gas is switched between lean and rich by employing a NOx absorbent. As the NOx absorbent, a material in which at least one selected from alkali metals, alkaline earths and rare earths and platinum are supported on a carrier (for example, alumina) is disclosed. Japanese Patent No. 2600492 discloses that the exhaust gas is always circulated through the NOx absorbent during the operation of the internal combustion engine, and the NOx absorption phase is used when the air-fuel ratio of the exhaust gas is lean, and the NOx release phase is used when the oxygen concentration is low. An exhaust gas purifying device is described. Downstream of this device, the release N
It has been suggested that the Ox-containing exhaust gas is further treated, for example, with a three-way catalyst to improve the purification of residual NOx.

[0003]

DISCLOSURE OF THE INVENTION The present inventors have studied, studied and studied exhaust gas purifying means of practical value that can purify NOx in exhaust gas more efficiently and for a longer period of time. . One result is that when using a metal-containing catalyst that is active against the decomposition of NOx, the conditions under which NOx is generated in the exhaust gas and the hydrocarbons (unburned or externally added) in the exhaust gas Alternatively, by appropriately combining the conditions in which an oxygen-containing compound such as carbon monoxide and a reducing agent such as hydrogen are present, the destruction of NOx and the regeneration of a deactivated or deactivated active metal-containing catalyst can be achieved. The present inventors have found that the above-mentioned processes are performed in a well-balanced manner, and based on this finding, provide here a practical exhaust gas purifying means as described above. The present invention also provides an active metal-containing catalyst and a method (system) useful for such an exhaust gas purifying means. The present invention relates to an engine provided with an exhaust gas recirculation (generally referred to as EGR) system, or an engine provided with a device for supplying a reducing agent upstream of an active metal-containing catalyst, and furthermore a fuel main injection. This is conveniently embodied in an engine in which a richer state of exhaust gas can be developed by injecting a smaller amount of fuel than the main injection before, after, or before or after.

[0004]

Thus, the present invention provides a diesel engine in which exhaust gas is treated with an active metal-containing NOx catalyst. In a diesel engine, when the exhaust gas is in a lean state, NOx contained in the exhaust gas is converted into active metal in the catalyst. And decomposes into nitrogen and oxygen by this reaction, and when it is found that this state has progressed and the catalyst activity has become inappropriately low, or when it has become low, the exhaust gas is made rich and thus the NOx catalyst After the NOx catalyst is regenerated and its NOx decomposing activity is recovered by removing the oxide film or the oxygen and / or sulfur that may be present in the oxide film, the process proceeds to the above-mentioned NOx decomposing step under a lean state. It is characterized by switching, and by combining these NOx decomposition stage and catalyst regeneration stage with an appropriate time pattern, To provide an exhaust gas purifying apparatus which may be treated with efficiency. In implementing the exhaust gas purifying apparatus of the present invention, various means and methods for causing the exhaust gas to be in a rich and lean state as described below can be adopted. That is, the present invention, as one aspect thereof,
In a diesel engine having an exhaust gas recirculation (EGR) system, wherein the exhaust gas is treated with an active metal-containing catalyst before final exhaust, a lean state of the exhaust gas is formed by turning off the EGR system; The exhaust gas purifying device for forming an exhaust gas rich state by turning on an EGR system is also provided. The present invention also provides an exhaust gas purifying apparatus according to another aspect, which includes an active metal-containing NOx catalyst disposed in an exhaust pipe system from a diesel engine, a reducing agent supply device, and an engine. The exhaust gas purifying apparatus for forming a rich state of exhaust gas by supplying a reducing agent on an intake side and / or an exhaust side of the exhaust gas is also provided.

An example of a suitable reducing agent is water.
Oxygen, oxygenated compounds such as carbon monoxide, hydrocarbons, light oil
However, the present invention is not limited to these. Departure
Akira, and even before the main injection of diesel engine fuel
Or a smaller amount of fuel than after the main injection
To form a rich state of exhaust gas by injecting
The exhaust gas purifying apparatus. In the present invention,
Active metal-containing catalysts include copper, cobalt,
At least one selected from rhodium and iridium
Is supported on a metal oxide as a carrier.
is there. Metal oxide carriers include alumina and zirconium.
A, titania, iron oxide and the like. This touch
The medium is a known refractory honeycomb structure, for example,
On the surface of a honeycomb structure in the form of a monolith made of
As a slurry containing a suitable binder, a dip-co
Covered in the exhaust method and placed in the exhaust system from the engine
You. This catalyst is used when the engine is operated under lean air-fuel ratio conditions.
As a result, NOx is contained in the exhaust gas.
In such a state, NOx is typically reduced to NOx ⇒
N_Two+ O_TwoDecomposes as shown by
Is gradually oxidized to form an oxide film on the catalyst surface,
The NOx decomposition activity of the catalyst gradually decreases. Such
Activity reduction is detected from the NOx concentration at the catalyst outlet
Is done. Exhaust at appropriate locations on the inlet and outlet sides of the catalyst
NOx sensor for measuring and detecting NOx concentration in pipe
Are provided, and the detected data signals are supplied to an electronic control unit.
(ECU), the lean and rich states of the present invention
Is output. The activity reduction described above progresses
The data obtained by the NOx sensor as described above,
In particular, the catalyst activity is inappropriate due to the NOx concentration value at the catalyst outlet side.
Low, or when it turns out to be low
Causes the air-fuel ratio rich state to be developed. This state, for example,
For example, the exhaust gas recirculation (EG
R) by turning on the system; or reducing agent
Supplying the reducing agent upstream of the catalyst via a supply device
Or before the main injection of the diesel engine
Or a smaller amount of fuel than after the main injection
By injecting; In this rich state
The exhaust gas contains hydrocarbons (HC) and oxygenated compounds
Material (CO, etc.) and hydrogen are returned as unburned residues or returned
Since it exists as an additive through the base agent supply device,
Oxygen attached or bound to the surface of the
Or oxide film (these are referred to here as "O"_TwoAnd the table
You. ) Is HC + O_Two⇒CO _Two+ H_TwoO; CO + O_Two⇒CO
_TwoH_Two+ O_Two⇒H_TwoO;
As a result, the catalyst activity is restored, and the catalyst is regenerated.
You. Regeneration of the catalyst was provided on its upstream and downstream sides
HC concentration is monitored by hydrocarbon sensor
Judgment by data processing in ECU
Can be. Once the catalyst has been regenerated, the exhaust gas
Returning the air-fuel ratio to the lean state, that is, the EGR system
Or turn off the reductant supply (possibly
Is reduced) or before and / or after the main fuel injection.
Stop additional small amount fuel injection at the time, and NOx
It can be a stage of decomposition. Switching between these
The control can be performed by the ECU. As above
In the present invention, NOx decomposition (purification) using an active metal-containing catalyst is carried out.
) Stage and the regeneration stage of the deactivated catalyst are appropriately patterned in time.
To efficiently treat exhaust gas over a long period of time
Make it possible. An example of an embodiment of the present invention is schematically illustrated in FIG.
I will tell. The diesel engine 1 shown in FIG.
In the middle of the intake pipe 3 extending from the filter 2, an exhaust manifold is provided.
EG for recirculating exhaust gas from Hold
An R pipe 4 is provided. This EGR pipe 4 has
Turn on (recirculate) and turn on the exhaust gas
(Recirculation stop) and recirculation exhaust gas control
An EGR valve 5 provided with
It can be controlled by a signal from the ECU. Also exhaust manifold
The middle of the exhaust pipe 6 from the catalyst is coated with an active metal-containing catalyst.
Fired honeycomb structure 7 (hereinafter referred to for convenience)
It may be referred to as a medium 7. ) Containing a huge part of the diameter
Provided. In the exhaust pipe on the inlet side of the catalyst 7,
To measure the exhaust gas temperature from the engine exhaust manifold
Temperature sensor TS for measuring hydrocarbon concentration (HC) concentration
Hydrocarbon sensor CS₁, NO for NOx concentration measurement
x sensor NS₁Is installed, and the exhaust side of the catalyst 7 is exhausted.
In the trachea, there is a measurement of the hydrocarbon concentration of the treated exhaust gas.
Sensor CS for_TwoFor NOx concentration measurement
NOx sensor NS_TwoIs installed. These sensors
The detected data signal is input to the ECU, and the data
The air-fuel ratio lean state (NOx decomposition stage)
And rich state (regeneration stage of active metal-containing catalyst)
And the signal output therefrom is transmitted to the EGR valve 5.
To operate the EGR valve 5 to turn on, off, and
And the exhaust gas recirculation flow rate is controlled. Fig. 2
It is intended to illustrate and explain the concepts of another aspect of the invention.
You. In this specific example, unlike in FIG.
The system is not used and is reduced to the entrance side of the catalyst 7
Injecting reducing agent to exhaust gas by installing agent supply device
Supply and deactivate or significantly deactivate the catalyst
It is regenerated by the action of the base agent. The reducing agent supply device is
Ink 11, reducing agent supply pump 10, reducing agent supply valve
9, a reducing agent injection nozzle 8. Fig. 2
Also in this case, the exhaust gas is located at a position upstream of the reducing agent injection nozzle 8.
Temperature sensor, and the reducing agent injection nozzle 8 and the catalyst 7
Between the hydrocarbon sensor and NOx, and
Another hydrocarbon sensor, NOx sensor
The sensor is installed and sends each detection data signal to ECU.
And the reducing agent supply pump 10 and the valve 9
Operation and the amount of reductant required for catalyst regeneration is required.
It is made to eject from nozzle 8 for time. However, in FIG.
Indicates sensors and ECUs for simplicity.
Not. Examples of suitable reducing agents are hydrocarbons, especially light
Oil, urea and the like. In the case of FIG. 2, the activity in the catalyst 7 is usually
NOx is decomposed into nitrogen and oxygen by the action of metal,
The catalyst surface is covered with an active metal oxide film,
Sex is lost. This state is a signal from the NOx sensor
And the reducing agent supply pump
A signal is sent to 10 and valve 9 and an appropriate amount of reducing agent
Is injected from the nozzle 8 for an appropriate time to regenerate the deactivated catalyst.
You. The degree of progress of the catalyst regeneration depends on the entrance side of the catalyst 7 and
Compare the data from the hydrocarbon sensor on the outlet side with the ECU.
Is determined. When catalyst regeneration is completed,
From the reducing agent supply pump 10
And the valve 9 and the supply of the reducing agent is stopped. This
The catalyst 7 regenerated as described above is again in the NOx decomposition stage.
Can be used to decompose NOx. the above
NOx decomposition step and catalyst regeneration step
And high efficiency of NOx in exhaust gas for a long time
Can be decomposed and removed. FIG. 3 shows an exhaust gas purification system according to the present invention.
In order to make the exhaust gas rich by introducing a reducing agent
x Add a reducing agent to the engine
Introduced at intake manifold (indicated by Rinj)
FIG. In this case, FIGS.
As mentioned earlier, based on information from various sensors
The ECU controls the supply of the reducing agent, and switches between supply and stop when appropriate.
Replacement is made and supply is adjusted. FIG.
Is the main fuel injection of the engine for purifying the exhaust gas of the present invention.
Before and / or after firing, add a smaller amount than the main injection amount.
Performs additional fuel injection, resulting in rich exhaust gas
This shows the method. In this case as well, various
Based on the information from the sensor, the signal of the additional fuel injection (In
j) is the timing from the ECU to the engine injection system
Well communicated and executed. Active metal used in the present invention
The contained catalyst has the above-mentioned constitution, but its preparation method is known.
is there. For example, it is prepared by the following method. That is,
Copper, cobalt, rhodium or iridium active metals
Metal oxide powder as a carrier in a solution containing a water-soluble salt
(Or fine particles) and stir to mix the carrier surface and its
Apply active metal compound in the vicinity, filter, dry, and
And then pulverize the active metal-supported catalyst powder.
A suitable binder, for example, suspended in water with alumina sol
To make a homogeneous slurry, to which a refractory honeycomb structure
Immerse, pull up, and remove excess adhesive slurry with air jet
Blow off and dry.
It can be reduced in a body atmosphere to obtain an activated catalyst.

[0006]

EXAMPLES Preparation Example of Catalyst A predetermined amount of rhodium nitrate calculated to contain 2.5 g of rhodium as a metal was placed in 300 ml of water, mixed and stirred well, and 150 g of alumina powder was further added to make the mixture homogeneous. Mix well. The excess aqueous phase was drained from the aqueous mixture and the residue was dried in a drying oven at about 100-110 ° C for 4 hours. Then, about 560
580 ° C. hydrogen-containing nitrogen gas flow (1% H ₂ /
Heat treatment was performed in 99% N ₂ ) for 3 hours to obtain a rhodium-supported catalyst. This is lightly crushed and pulverized to a particle size of 150 to 100.
A mesh powder was used. Water, alumina sol binder and this rhodium-supported catalyst powder were mixed in a weight ratio of 70:20:50.
, And sufficiently stirred to form a uniform slurry. The honeycomb carrier made of cordierite was immersed in the slurry, pulled up, and excess deposits were blown off with an air jet.
C was baked for 10 hours and allowed to cool to obtain a mounting catalyst (A). According to the above-mentioned preparation operation, a catalyst (B) containing iridium (Ir) as an active metal component and alumina as a carrier; a catalyst (C) containing iridium (Ir) as an active metal component and titania as a carrier; and copper as an active metal component A catalyst (D) containing (Cu) and zirconia as a carrier was obtained. Example 1 of NOx Treatment The mounting catalyst (A) was loaded into the engine with an EGR system and the NOx catalyst treatment device shown in FIG. 1 and controlled by the ECU to appropriately perform the NOx decomposition step and the catalyst regeneration step. . FIG. 5 is a graph plotting the average NOx emission amount data (relative value display) at the outlet side of the catalyst. The arrow shown along the horizontal axis in FIG. 5 indicates that the catalyst regeneration was performed, and it can be seen that the catalyst activity was recovered with a time lag of several seconds. FIG. 5 shows that reproduction was performed three times at the same interval in 60 seconds.
At the time of actual operation, fluctuation control is performed (usually by data processing of the ECU) so that the regeneration interval becomes shorter or longer according to the load of the engine. Example 2 of NOx treatment As in Example 1, control was performed by an ECU using an engine (without an EGR system) equipped with the catalyst device of the embodiment shown in FIG. 2 instead of the engine and catalyst device of Example 1. When the data of the concentration of exhausted NOx was recorded, the same tendency as that shown in FIG. 5 was observed. Example 3 of NOx treatment Catalysts A (Rh / alumina) and B (Ir) obtained in the above preparation examples
/ Alumina), C (Ir / titania), and D (Cu /
Zirconia) and mounted on the diesel engine device of Example 1 and exhaust gas (under the same operating conditions; 350 ° C.)
The NOx reduction rate was measured for, and relative measurement values as shown in the graph of FIG. 6 were obtained. In this case, the average NOx reduction achieved by each catalyst is 62% for A and 6% for B, based on the NOx emission obtained without a catalyst for comparison.
0%, C was 57%, and D was 48%. As described above, in the present invention, while appropriately switching the air-fuel ratio between rich and lean, the use and regeneration of the function of the catalyst are successfully harmonized,
Enables purification of exhaust gas.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of one embodiment (EGR system) of an apparatus for implementing an exhaust gas purification system of the present invention.

FIG. 2 is a conceptual diagram of another embodiment of an apparatus for implementing the exhaust gas purifying method (introducing a reducing agent) of the present invention.

FIG. 3 is a conceptual diagram of a system of the present invention in which a reducing agent for purifying exhaust gas is introduced at an intake manifold of an engine.

FIG. 4 is a conceptual diagram of a system for producing a rich state by a small amount of fuel injection before and / or after fuel main injection for purifying exhaust gas according to the present invention.

FIG. 5 is a graph showing how the NOx concentration of exhaust gas changes when the purification method of the present invention is implemented.

FIG. 6 is a graph showing relative NOx reduction rate performance according to the present invention.

[Explanation of symbols]

1 diesel engine 2 air filter 3 intake pipe 4 EGR pipe 5 EGR valve 6 exhaust pipe 7 catalyst 8 reducing agent injection nozzle 11 reducing agent tank CS ₁ , CS ₂ hydrocarbon sensor NS ₁ , NS ₂ NOx sensor

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) F01N 3/08 F01N 3/10 ZABA 4G069 3/10 ZAB 3/24 S 3/24 3/28 301C 3 / 28 301 F02D 21/08 301H F02D 21/08 301 41/04 355 41/04 355 41/38 B 41/38 43/00 301G 43/00 301 301N B01D 53/36 104A F term (reference) 3G084 AA01 AA03 AA04 BA05 BA13 BA20 BA24 DA10 DA22 DA27 EA11 FA28 3G091 AA11 AA18 AA28 AB04 AB05 BA11 BA14 BA33 CA13 CA18 CA19 CB02 CB03 DA02 DA03 DB10 EA30 EA33 FB10 FB12 FC02 GA06 GB01W GB01X GB05W GB10X GB15A HAA HAA HAA HAA HAA HAB HAA AB03 AB04 BA01 BA04 BA07 BB01 BB06 DC08 DC15 DE03S DE03Y EA01 EA02 EA05 EA07 EA08 3G301 HA01 HA02 HA04 HA06 HA13 HA15 JA25 JA33 JB09 LA01 LB11 MA01 MA11 MA18 NA08 NE13 NE15 PD01B PD01Z 4D048 AA06 AB02 AB03 AC02 BA03X BA07X BA08X BA33X BA35X BA37Y BA41X BB02 BD03 CC38 DA01 DA02 DA10 4G069 AA10 BA01B BA04B BA05B BC31ABC31ABCBC BC BC

Claims (7)

[Claims] In a diesel engine in which exhaust gas is treated with an active metal-containing NOx catalyst, NOx contained in the exhaust gas when the exhaust gas is in a lean state is reacted with an active metal in the catalyst to produce nitrogen and oxygen. Decomposes, and when this state progresses and the catalyst activity becomes inappropriately low or is found to have become low, by forming a rich state of the exhaust gas, the oxide film of the NOx catalyst or accompanying it, The method is characterized in that the NOx catalyst is regenerated by removing oxygen and / or sulfur which may be present and the NOx decomposition activity is restored, and then the operation is switched to the NOx decomposition stage under the above-mentioned lean condition. The NOx decomposition stage and the catalyst regeneration stage are appropriately time-patterned.
Exhaust gas purifier that can process exhaust gas with high efficiency over a long period of time by combining the components. 2. In a diesel engine having an exhaust gas recirculation (EGR) system in which exhaust gas is treated with an active metal-containing NOx catalyst before final emission, a lean state of the exhaust gas is formed by turning off the EGR system. The exhaust gas purifying apparatus according to claim 1, wherein a rich state of the exhaust gas is formed by turning on the EGR system. 3. An exhaust system comprising an active metal-containing NOx catalyst disposed in an exhaust pipe system from a diesel engine, a reducing agent supply device, and a reducing agent supplied on an intake side and / or an exhaust side of the engine. 2. The exhaust gas purifying apparatus according to claim 1, wherein a rich state of the gas is formed. 4. An oxygen-containing compound such as hydrogen or carbon monoxide,
The exhaust gas purifying apparatus according to claim 1, wherein a rich state of the exhaust gas is formed by adding a reducing agent such as a hydrocarbon or light oil. 5. The exhaust gas purifying apparatus according to claim 1, wherein a rich state of the exhaust gas is formed by injecting a smaller amount of fuel than the main injection before, after or before or after the main fuel injection of the diesel engine. 6. The exhaust gas purifying apparatus according to claim 1, wherein the catalyst has a structure in which at least one active metal selected from copper, cobalt, rhodium and iridium is supported on a metal oxide carrier. . 7. The exhaust gas purifying apparatus according to claim 6, wherein the catalyst is used in a form coated on a refractory honeycomb structure.