JP4359765B2 - Exhaust purification catalytic equipment - Google Patents

Description

  The present invention relates to an exhaust purification catalyst device, and more particularly to a technology of a NOx storage catalyst that purifies NOx with high efficiency.

In recent years, lean burn engines (lean combustion internal combustion engines) capable of combustion in an oxygen-excess atmosphere (lean air-fuel ratio) have been put into practical use in order to improve fuel efficiency, and NOx is generated in the lean burn engine Therefore, a NOx occlusion catalyst that stores NOx in exhaust gas and purifies NOx in exhaust gas during lean combustion is provided in the exhaust passage.
This NOx storage catalyst purifies NOx in exhaust gas by storing NOx on the catalyst in an oxygen-excess atmosphere (lean air-fuel ratio), while storing NOx in an oxygen concentration-reduced atmosphere (stoichio or rich air-fuel ratio). It is known that it has a function of releasing. Specifically, the NOx storage catalyst stores NOx in the exhaust as nitrate in an atmosphere with excess oxygen concentration, while it generates carbonate from the nitrate stored in the NOx storage catalyst and CO in the exhaust in an atmosphere with reduced oxygen concentration. And has a function of releasing NOx.

  By the way, the NOx storage catalyst has a problem that when the NOx storage amount during the lean operation (in the oxygen-excess atmosphere) reaches the saturation amount, NOx cannot be stored thereafter and NOx is discharged to the atmosphere. Therefore, normally, before the NOx storage amount reaches the saturation amount, the engine air-fuel ratio is temporarily switched from the lean air-fuel ratio to the stoichiometric or rich air-fuel ratio, and the exhaust is made into an oxygen concentration-reducing atmosphere. A large amount of reducing agent is generated to release and reduce NOx, thereby recovering the NOx storage capacity of the NOx storage catalyst (referred to as NOx purge).

In particular, in a direct injection lean burn engine capable of directly injecting fuel into the combustion chamber, fuel injection is performed under a lean air-fuel ratio mainly during a compression stroke during low-speed load operation, while intake stroke is mainly performed during high-speed high load operation. In this case, fuel injection is performed under stoichiometric or rich air-fuel ratio, and not only lean air-fuel ratio operation but also stoichiometric operation or rich air-fuel ratio operation is performed. Therefore, the NOx storage catalyst also has a three-way catalyst function. In addition, an O ₂ storage function (also referred to as an oxygen storage function or OSC) is added to increase the purification efficiency by the three-way catalyst function. The O ₂ storage function is a function having a characteristic of storing O ₂ (oxygen) in an oxygen-excess atmosphere and releasing the stored O ₂ into an oxygen concentration-reduced atmosphere. For example, O ₂ such as ceria (Ce) is used. This is realized by adding a storage agent (oxygen storage agent, OSC agent).

That is, by adding an O ₂ storage function together with a three-way catalyst function to the NOx storage catalyst, during stoichiometric operation or rich air-fuel ratio operation, for example, by performing O ₂ feedback control, the exhaust air-fuel ratio is somewhat leaner or richer. Even when the air-fuel ratio is approached, the catalyst atmosphere can be maintained in the vicinity of stoichiometric. Thereby, the three-way catalyst function is exhibited to the maximum, and not only NOx but also HC and CO can be purified well (see Patent Document 1).
JP 2000-27677 A

By the way, in the case where the O ₂ storage function is added to the NOx storage catalyst as described above, if NOx purge is performed with the exhaust gas in an oxygen concentration lowering atmosphere in order to recover the NOx storage capacity, CO and HC supplied to the NOx storage catalyst Such a reducing agent is oxidized by O ₂ released from the O ₂ storage agent by the O ₂ storage function, so that the occluded NOx is not sufficiently released and reduced. That is, most of the reducing agents such as CO and HC are consumed in the oxidation reaction with O ₂ released upstream of the NOx storage catalyst, so that they do not reach the downstream side, and NOx is stored downstream of the NOx storage catalyst. There is a problem that the purge is hardly performed and the NOx occlusion capacity is not sufficiently recovered (refer to part A in FIG. 4). This problem becomes more prominent as the NOx purge time is shorter.

In addition, noble metals such as platinum (Pt), palladium (Pd), and rhodium (Rh) provided in the NOx storage catalyst to promote the catalytic reaction are covered with the O ₂ in a situation where O ₂ exists excessively. The activity is reduced (oxygen poisoning), and this decrease in activity can be recovered by performing NOx purge and removing O ₂ covering the noble metal, but as described above. When the reducing agent is consumed in the oxidation reaction with the occluded O ₂ , there is also a problem that the activity of the noble metal cannot be sufficiently recovered (see part B in FIG. 4).

In this case, as disclosed in Patent Document 1 and the like, it is conceivable to temporarily increase the fuel and increase the reducing agent such as CO and HC so as to reach the downstream side of the NOx storage catalyst. Supply of fuel that does not contribute to engine output is not preferable because it leads to deterioration of fuel consumption.
The present invention has been made to solve such problems, and an object of the present invention is to provide an exhaust capable of increasing the exhaust purification efficiency of a NOx storage catalyst having an O ₂ storage function without deteriorating fuel consumption. It is to provide a purification catalyst device.

In order to achieve the above-described object, the exhaust purification catalyst device according to claim 1 is formed on a single carrier having a plurality of through holes arranged in the exhaust passage and penetrating in the axial direction, and an inner surface defining the through holes. When the exhaust air-fuel ratio is lean, and the exhaust air-fuel ratio is stoichiometric or rich. In the exhaust purification catalyst device carrying the NOx occlusion catalyst for reducing the occluded NOx and reducing it, the carrier layer is substantially the entire region from the upstream side region of the exhaust layer to the downstream side region of the exhaust passage. The oxygen storage agent is supported over the entire area, and the amount of the oxygen storage agent supported is set to be smaller in the upstream region of the exhaust passage than in the downstream region of the exhaust passage.

Further, in the exhaust purification catalyst device according to claim 2, the amount of the oxygen storage agent supported is set so as to gradually increase from the exhaust path upstream side region of the carrier toward the exhaust path downstream side region. It is said.
In the exhaust purification catalyst device according to claim 3, the amount of the noble metal supported is set to be smaller in the upstream region of the exhaust path than the downstream region of the exhaust gas according to the supported amount of the oxygen storage agent. It is characterized by being.

According to the exhaust purification catalyst device of claim 1 of the present invention, the carrier layer carries the oxygen storage agent together with the noble metal and the NOx storage catalyst over substantially the entire region from the exhaust passage upstream region to the exhaust passage downstream region , Since the amount of the oxygen storage agent supported in the upstream region of the exhaust passage of the carrier is smaller than the downstream region of the exhaust passage, the amount of oxygen stored in the oxygen storage agent is in the upstream region of the exhaust passage of the carrier. Therefore, when NOx purge is performed in order to recover the NOx storage capacity of the NOx storage catalyst, reducing agents such as CO and HC are present in the upstream region of the exhaust passage of the carrier. It is preferably prevented from being consumed up by the reaction with oxygen released from the catalyst, and the reducing agent such as CO and HC can be surely reached to the downstream side region of the exhaust path of the carrier.

Therefore, it is not necessary to increase the amount of the reducing agent, that is, the fuel in consideration of the amount consumed by the reaction between the oxygen released from the oxygen storage agent and the reducing agent, and the release and reduction of NOx stored in the NOx storage catalyst are supported by the carrier. This can be implemented sufficiently in the downstream area of the exhaust passage. In addition, oxygen poisoning of the noble metal in the downstream region of the exhaust path of the carrier can be eliminated.
As a result, it is possible to recover the activity of the noble metal together with the NOx occlusion ability of the NOx occlusion catalyst efficiently with the entire catalyst without deterioration of fuel consumption with a simple configuration, and it is possible to improve the exhaust purification efficiency.

Further, according to the exhaust purification catalyst device of claim 2, since the amount of the oxygen storage agent supported is gradually increased from the upstream side region of the exhaust passage toward the downstream side region of the exhaust passage, the efficiency is further increased. The activity of the noble metal can be recovered together with the NOx storage capacity of the NOx storage catalyst.
According to the exhaust purification catalyst device of claim 3, the amount of the noble metal supported is set so that the region on the upstream side of the exhaust passage of the carrier is smaller than the region on the downstream side of the exhaust passage according to the amount of supported oxygen storage agent. Therefore, the amount of noble metal supported can be optimized.

  That is, it has been confirmed that the oxygen storage agent can store oxygen due to the presence of the noble metal. Therefore, the amount of the noble metal supported on the upstream side of the exhaust path of the carrier in the upstream side of the exhaust path depends on the amount of the oxygen storage agent supported By making it smaller than the region, the noble metal can be supported effectively without excess or deficiency.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Referring to FIG. 1, there is shown a schematic configuration diagram of an exhaust purification device including an exhaust purification catalyst device according to the present invention mounted on a vehicle and interposed in an exhaust passage of an engine 1. The configuration of the exhaust emission control device will be described.
As shown in the figure, as an internal combustion engine (hereinafter referred to as an engine) 1, for example, by switching a fuel injection mode, fuel injection in an intake stroke (intake stroke injection) and fuel injection in a compression stroke (compression stroke injection) are performed. In-cylinder injection type spark-ignition gasoline engine that can be used) is adopted. In the in-cylinder injection type engine 1, operation at a lean air-fuel ratio (lean air-fuel ratio operation) as well as operation at a stoichiometric air-fuel ratio (stoichio) or operation at a rich air-fuel ratio (rich air-fuel ratio operation) is facilitated. Is feasible.

As shown in the figure, the cylinder head 2 of the engine 1 is provided with an electromagnetic fuel injection valve 6 together with a spark plug 4 for each cylinder, so that fuel can be directly injected into the combustion chamber. .
An ignition coil 8 that outputs a high voltage is connected to the spark plug 4. Further, a fuel supply device (not shown) having a fuel tank is connected to the fuel injection valve 6 via a fuel pipe 7. More specifically, the fuel supply device is provided with a low pressure fuel pump and a high pressure fuel pump, whereby fuel in the fuel tank is supplied to the fuel injection valve 6 at a low fuel pressure or a high fuel pressure. Can be injected from the fuel injection valve 6 into the combustion chamber at a desired fuel pressure.

An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and one end of an intake manifold 10 is connected so as to communicate with each intake port. Further, an exhaust port is formed in the cylinder head 2 in a substantially horizontal direction for each cylinder, and one end of the exhaust manifold 12 is connected so as to communicate with each exhaust port.
The in-cylinder injection type engine 1 is already known, and therefore, the detailed description of the configuration is omitted.

The intake manifold 10 is provided with an electromagnetic throttle valve 14 for adjusting the amount of intake air and a throttle position sensor (TPS) 16 for detecting the opening θth of the throttle valve 14, and further upstream of the throttle valve 14. Is provided with an air flow sensor 18 for measuring the intake air amount.
On the other hand, an exhaust pipe (exhaust passage) 20 is connected to the exhaust manifold 12, and a NOx storage catalyst unit 30 is interposed in the exhaust pipe 20 as an exhaust purification catalyst device. The exhaust pipe 20 is provided with an O ₂ sensor 22 located upstream of the NOx storage catalyst unit 30.

  The NOx storage catalyst unit 30 is composed of a single carrier 32 having a large number of through holes penetrating in the axial direction, and a support layer made of a refractory inorganic oxide formed on the inner surface defining the through holes, The supporting layer carries a noble metal 36 of any of copper (Cu), cobalt (Co), silver (Ag), platinum (Pt), palladium (Pd), and rhodium (Rh) as an active noble metal, and as a NOx occlusion agent. An alkali metal (for example, potassium (K), sodium (Na)) or an alkaline earth metal (for example, barium (Ba)) is supported.

  Thus, the NOx occlusion catalyst unit 30 basically occludes NOx when the exhaust air-fuel ratio is a lean air-fuel ratio and is in an oxygen concentration excess atmosphere with a high oxygen concentration, and the exhaust air-fuel ratio is a rich air-fuel ratio. The NOx occlusion catalyst function of releasing the occluded NOx when it is in an oxygen concentration lowering atmosphere with a low oxygen concentration and reducing and removing it by the catalytic action of the noble metal 36 is provided.

The NOx occlusion catalyst unit 30 also has a three-way catalyst function by supporting any one of the noble metals 36 on the supporting layer.
Thus, the NOx storage catalyst unit 30 can satisfactorily purify not only NOx but also HC and CO when the exhaust air-fuel ratio is stoichiometric or rich air-fuel ratio.
Further, the NOx storage catalyst unit 30 also has an O ₂ storage function (also referred to as an oxygen storage function, OSC), and an O ₂ storage agent (oxygen storage agent, OSC) such as ceria (Ce) is provided on the support layer. Agent) 34 is carried. That is, the NOx occlusion catalyst unit 30 occludes O ₂ (oxygen) when the exhaust air-fuel ratio is a lean air-fuel ratio and is in an excessive oxygen concentration atmosphere, and the exhaust air-fuel ratio is a rich air-fuel ratio and becomes an oxygen concentration-reduced atmosphere. In some cases, it also has a function of releasing the occluded O ₂ .

Thus, the NOx storage catalyst unit 30 has a target exhaust air / fuel ratio of stoichiometric or rich air / fuel ratio. For example, when the O ₂ feedback control is performed by the O ₂ sensor 22, the exhaust air / fuel ratio becomes lean. Even if it fluctuates near the air-fuel ratio or the rich air-fuel ratio, the fluctuation is adjusted by O ₂ stored in the O ₂ storage agent 34, and the catalyst atmosphere is well maintained near the stoichiometric. Thereby, the three-way catalyst function is exhibited to the maximum.

More specifically, as schematically shown in FIG. 1, in the O ₂ storage agent (indicated by a dot) 34, the exhaust path upstream area 32a of the carrier 32 is smaller than the exhaust path downstream area 32b. The carrying amount is set as follows. That is, the NOx storage catalyst unit 30, towards the O ₂ storage amount is set to be less in the O ₂ exhaust path upstream side region 32a as compared with the storage amount in the exhaust path downstream region 32b.

Further, it has been confirmed that the O ₂ storage agent 34 can occlude O _{2 due} to the presence of the noble metal 36, and here, as schematically shown in FIG. 1, the noble metal 36 (shown by small circles) is supported. The amount is set so that the exhaust path upstream area 32a of the carrier 32 is smaller than the exhaust path downstream area 32b in accordance with the amount of the O ₂ storage agent 34 carried.
The electronic control unit (ECU) 40 includes an input / output device, a storage device (ROM, RAM, etc.), a central processing unit (CPU), etc., and the ECU 40 provides a comprehensive exhaust purification device including the engine 1. Control is performed.

Various sensors such as the TPS 16, the air flow sensor 18, and the O ₂ sensor 22 described above are connected to the input side of the ECU 40, and detection information from these sensors is input.
On the other hand, various output devices such as the fuel injection valve 6, the ignition coil 8, and the throttle valve 14 are connected to the output side of the ECU 40.
Thus, when the target air-fuel ratio of the engine 1 is set, a command signal for the fuel injection amount and fuel injection timing is output to the fuel injection valve 6 in accordance with the target air-fuel ratio, and the command signal for the intake air amount is throttled. The ignition timing command signal is output to the ignition coil 8 in the order of combustion. Accordingly, an appropriate amount of fuel is injected from the fuel injection valve 6 at an appropriate timing, the throttle valve 14 is set to an appropriate opening degree, and spark ignition is performed at an appropriate timing by the spark plug 4.

Actually, the ECU 40 obtains the target average effective pressure Pe corresponding to the engine load based on the throttle opening information θth and the engine rotational speed information Ne from the TPS 16, and at the time of normal operation control, the target average The target air-fuel ratio is set from the effective pressure Pe and the engine speed information Ne.
Further, a fuel injection mode is set from a fuel injection mode setting map (not shown) according to the target average effective pressure Pe and the engine speed Ne.

For example, when both the target average effective pressure Pe and the engine speed Ne are small, the fuel injection mode is set to the compression stroke injection mode (compression lean mode), and the fuel is injected in the compression stroke under the lean air-fuel ratio, and the spark plug 4 When the target average effective pressure Pe increases or the engine speed Ne increases, the fuel injection mode is set to the intake stroke injection mode, and fuel is injected in the intake stroke, and premixed combustion is performed. To be implemented. The intake stroke injection mode includes an intake lean mode that operates at a lean air-fuel ratio (for example, value 18 to value 22), and a stoichiometric feedback mode (SF) that performs the O ₂ feedback control so that the actual air-fuel ratio becomes stoichiometric. / B mode) and an open loop mode (O / L mode) that operates at a rich air-fuel ratio.

  Further, in the exhaust purification device, when the NOx occlusion amount occluded in the NOx occlusion catalyst unit 30 is estimated and it is determined that the NOx occlusion amount has reached a predetermined amount, the air-fuel ratio over a certain period (for example, 2 to 4 sec). As a rich air-fuel ratio, the catalyst atmosphere is the low oxygen concentration atmosphere, and the reducing agent such as CO and HC is discharged to the exhaust passage, and the NOx occluded in the NOx occlusion catalyst unit 30 is released and reduced. That is, NOx purge is performed, and in addition to the above modes, a NOx purge mode for performing the NOx purge is also provided.

Hereinafter, the operation of the exhaust purification device configured as described above and the operation of the exhaust purification catalyst device according to the present invention will be described.
For example, when the operation in the compression lean mode or the intake lean mode is performed and it is determined that the NOx occlusion amount occluded in the NOx occlusion catalyst unit 30 has reached a predetermined amount, the fuel injection mode is switched to the NOx purge mode, The exhaust air / fuel ratio is set to a rich air / fuel ratio, and the catalyst atmosphere is set to the oxygen concentration lowering atmosphere having a low oxygen concentration. Thereby, reducing agents such as CO and HC are discharged to the exhaust passage, and NOx purge is started.

At this time, as described above, the NO 2 storage catalyst unit 30 carries the O ₂ storage agent 34, and the O ₂ storage agent 34 is located in the exhaust passage upstream region 32 a of the carrier 32 in the exhaust passage. The carrying amount is set so as to be smaller than the downstream region 32b.
Therefore, less towards the O ₂ storage amount in the exhaust path upstream side region 32a as compared with the O ₂ storage amount in the exhaust passage downstream regions 32b, even when subjected to NOx purge, the exhaust path upstream side region 32a A reducing agent such as CO and HC is preferably prevented from being consumed due to an oxidation reaction with O ₂ occluded in the O ₂ storage agent 34.

As a result, NOx occluded in the exhaust path upstream area 32a is released and reduced, and a reducing agent such as CO and HC reliably reaches the exhaust path downstream area 32b. Without increasing the amount of fuel to increase the reducing agent, NOx occluded in the exhaust path downstream region 32b is sufficiently released and reduced.
Moreover, the O ₂ covers the noble metal 36 in the situation where O ₂ is present in excess, the activity of the noble metal 36 is reduced, but the phenomenon of (oxygen poisoning) is the occurs, CO during NOx purge, HC, etc. When the reducing agent reliably reaches the exhaust path downstream area 32b, O ₂ covering the noble metal 36 is reliably removed not only in the exhaust path upstream area 32a but also in the exhaust path downstream area 32b by the reducing agent. Is done. Thereby, the activity of the noble metal 36 is sufficiently recovered.

As described above, according to the exhaust gas purification catalyst device of the present invention, the NOx occlusion capacity of the NOx occlusion catalyst unit 30 and the NOx occlusion capacity of the NOx occlusion catalyst unit 30 can be efficiently improved as a whole catalyst with a short NOx purge time and without deterioration in fuel consumption even with a simple NOx purge time. The activity can be recovered, and the exhaust purification efficiency can be improved.
That is, referring to FIG. 2, the experimental result after the NOx purge when the exhaust purification catalyst device according to the present invention is used is shown by the outlet NOx concentration of the NOx storage catalyst unit 30. The conventional case where the O ₂ storage agent 34 is uniformly supported (broken line) by carrying the O ₂ storage agent 34 so that the flow side region 32a is smaller than the exhaust passage downstream side region 32b (solid line). Compared to the above, it is possible to sufficiently recover the NOx storage capacity, and to improve the NOx purification efficiency and thus the exhaust purification efficiency. In FIG. 2, the air-fuel ratio at the time of NOx purge and the NOx concentration at the inlet of the NOx storage catalyst unit 30 are also shown (dashed line).

Here, the amount of the noble metal 36 is set so that the exhaust path upstream region 32a of the carrier 32 is smaller than the exhaust channel downstream region 32b in accordance with the load of the O ₂ storage agent 34. Therefore, the relatively expensive noble metal 36 can be effectively carried on the NOx storage catalyst unit 30 without excess or deficiency.
However, it is not always necessary to change the amount of the noble metal 36 supported, and the noble metal 36 may be uniformly supported on the carrier 32.

By the way, in the above embodiment, the carrier 32 is divided into the exhaust path upstream area 32a and the exhaust path downstream area 32b so that the loading amount of the O ₂ storage agent 34 is different. However, as another example, FIG. as shown in 3, gradually increase (gradient) may NOx storage catalyst unit 30 'as is toward O ₂ storage agent 34 exhaust path downstream region 32b of the support amount from the exhaust path upstream side region 32a of the.

In this way, the NOx occlusion capacity of the NOx occlusion catalyst unit 30 and the activity of the noble metal 36 can be recovered more efficiently in the entire catalyst.
In accordance with this, as shown in the figure, the carrying amount of the noble metal 36 may be gradually increased (gradation) from the exhaust path upstream area 32a to the exhaust path downstream area 32b.

  In the above embodiment, the case where an in-cylinder injection type spark ignition type gasoline engine is used as the engine 1 has been described as an example. However, the present invention is not limited to this, and the engine 1 is a diesel engine or an intake pipe injection type gasoline engine. Also good.

1 is a schematic configuration diagram of an exhaust purification device including an exhaust purification catalyst device according to the present invention. It is a figure which shows the experimental result at the time of using the exhaust purification catalyst apparatus which concerns on this invention. It is a figure which shows the other Example of this invention. It illustrates a conventional problem in the case of adding O ₂ storage function in NOx storage catalyst.

Explanation of symbols

1 engine (internal combustion engine)
4 Spark plug 6 Fuel injection valve 30 30 'NOx storage catalyst unit (exhaust purification catalyst device)
32 Carrier 32a Exhaust path upstream area 32b Exhaust path downstream area 34 O ₂ storage agent (oxygen storage agent)
36 Precious metals 40 Electronic control unit (ECU)

Claims (3)

A single carrier having a large number of through holes arranged in the exhaust passage and penetrating in the axial direction, and a support layer made of a refractory inorganic oxide formed on the inner surface defining the through holes, It carries a noble metal and a NOx storage catalyst that stores NOx in the exhaust when the exhaust air-fuel ratio is lean and releases the stored NOx when the exhaust air-fuel ratio is stoichiometric or rich. In the exhaust purification catalyst device,
The carrier layer carries an oxygen storage agent over substantially the entire region from the upstream side region of the exhaust passage to the downstream side region of the exhaust passage ,
The amount of the oxygen storage agent supported is set in the exhaust gas upstream side region of the carrier smaller than the exhaust gas downstream side region.   The exhaust purification catalyst device according to claim 1, wherein the amount of the oxygen storage agent carried is set so as to gradually increase from the upstream side region of the exhaust path toward the downstream side region of the exhaust path. .   The amount of the noble metal supported is set so that the exhaust path upstream region of the carrier is set to be smaller than the exhaust channel downstream region in accordance with the supported amount of the oxygen storage agent. The exhaust purification catalyst device according to 2.

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