JP2006346525A - Exhaust emission purifying device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission purifying device of an internal combustion engine which is capable of properly preventing the deterioration of exhaust emission control performance of a three way catalyst by an occlusion agent and of controlling the deterioration of NO<SB>x</SB>purification performance. <P>SOLUTION: The exhaust emission purifying device is obtained by adding the occlusion agent and an alkaline stabilizer to the catalyst layer of the NO<SB>x</SB>catalyst, adding palladium (Pd) and an alkaline trap agent to the lower layer (56) in the catalyst layer (54) of the three way catalyst (50) installed at the exhaust emission downstream side of the NO<SB>x</SB>catalyst and adding platinum (Pt) and rhodium (Rh) to the upper layer (58). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification apparatus for an internal combustion engine having a NOx catalyst and a three-way catalyst.

A lean combustion engine such as an in-cylinder injection engine is operated at a lean air-fuel ratio that is leaner than the stoichiometric air-fuel ratio in a predetermined operating range in order to improve fuel consumption characteristics and exhaust characteristics.
In the lean air-fuel ratio operation (hereinafter referred to as “lean operation”) by such a lean combustion type engine, NOx emission increases, and therefore a NOx catalyst is generally provided in the exhaust passage. In the NOx catalyst, during the lean operation, the occlusion agent that is an alkali metal or alkaline earth metal added to the catalyst layer occludes NOx in the exhaust gas, and the occluded NOx is used in the stoichiometric air-fuel ratio operation (hereinafter referred to as stoichiometric operation). Alternatively, release reduction (NOx purge) is performed during rich air-fuel ratio operation (hereinafter referred to as rich operation).

However, the storage agent added to the NOx catalyst is likely to move at a high temperature, and if the catalyst carrier is a cordierite carrier, the catalyst carrier penetrates into the carrier and deteriorates, and the NOx catalyst also scatters. There is a problem that the storage capacity decreases and the storage capacity of NOx decreases.
In addition, when a three-way catalyst is provided on the exhaust downstream side of the NOx catalyst, the storage agent scattered from the NOx catalyst adheres to the three-way catalyst, and the exhaust purification performance of the three-way catalyst also deteriorates. is there. This is because, firstly, the occlusion agent, which is an alkali metal or alkaline earth metal having a high electron donating property, releases electrons, thereby reducing the oxidation ability of the noble metal of the three-way catalyst. It is thought that the occlusion agent that is easy to move covers the surface of the three-way catalyst.

Therefore, in the three-way catalyst provided on the exhaust downstream side of the NOx catalyst, the three-way catalyst is formed by forming an acidic oxide layer to which an acidic oxide having a function of trapping the storage agent is added above the three-way catalyst layer. A technique has been developed in which the occluding agent scattered in the catalyst is captured by the acidic oxide and prevented from entering the three-way catalyst layer (see Patent Document 1).
JP 2003-83052 A

However, in the technique disclosed in Patent Document 1, if the scattered storage material is held in the upper layer of the three-way catalyst, the three-way catalyst layer in the lower layer is covered, so that the exhaust purification performance of the three-way catalyst is lowered. It is not preferable.
Moreover, in the above-mentioned Patent Document 1, there is a configuration in which a catalyst layer to which a noble metal exhibiting ternary performance and an acidic oxide are added is supported on a carrier. Has a problem that its function is lowered by the acidic oxide.

  The present invention has been made to solve such a problem, and the object of the present invention is to satisfactorily prevent the exhaust purification performance of the three-way catalyst from being reduced by the storage agent, and to reduce the NOx purification performance. An object of the present invention is to provide an exhaust emission control device for an internal combustion engine that can be suppressed.

  In order to achieve the above object, in the exhaust gas purification apparatus for an internal combustion engine according to claim 1, at least one of an alkali metal and an alkaline earth metal is added as an occlusion agent to the catalyst layer provided in the exhaust passage of the internal combustion engine. NOx catalyst, provided on the exhaust downstream side of the NOx catalyst, the catalyst layer is composed of an upper layer and a lower layer, at least rhodium (Rh) is added to the upper layer of the catalyst layer, and at least the NOx is added to the lower layer of the catalyst layer And a three-way catalyst to which an acidic oxide having a function of trapping the storage agent scattered from the catalyst is added.

That is, in the exhaust purification device in which the three-way catalyst is provided on the exhaust downstream side of the NOx catalyst, the storage agent scattered from the NOx catalyst is removed by the acidic oxide added to the lower layer of the catalyst layer of the three-way catalyst. Captured in the catalyst underlayer.
The exhaust emission control device for an internal combustion engine according to claim 2 is characterized in that platinum (Pt) is added to the upper layer of the catalyst layer of the three-way catalyst in claim 1.

That is, platinum (Pt) that is adversely affected by the acidic oxide is added to a catalyst layer upper layer different from the catalyst layer lower layer to which the acidic oxide is added.
The exhaust gas purification apparatus for an internal combustion engine according to claim 3 is characterized in that, in claim 1 or 2, an acidic oxide having a function of suppressing scattering of the storage agent is added to the NOx catalyst.

That is, since the acidic oxide that suppresses the scattering of the storage agent is added to the NOx catalyst, the storage of the storage agent from the NOx catalyst is suppressed even at high temperatures.
The exhaust gas purification apparatus for an internal combustion engine according to claim 4 is characterized in that in claim 3, the acidic oxide contained in the lower layer of the three-way catalyst has a higher acidity than the acidic oxide contained in the NOx catalyst. Yes.

That is, an acidic oxide having a high effect of suppressing the movement of the storage agent is added to the three-way catalyst.
The exhaust gas purification apparatus for an internal combustion engine according to claim 5 is characterized in that, in any one of claims 1 to 4, the carrier for the NOx catalyst and the three-way catalyst is a metal carrier.
That is, the support of the NOx catalyst and the three-way catalyst is a metal support that has high thermal conductivity and low heat capacity and is not able to enter the storage agent.

In the exhaust gas purification apparatus for an internal combustion engine according to claim 6, in any one of claims 1 to 5, the storage agent added to the NOx catalyst is potassium (K), and the potassium (K) corresponds to the capacity of the NOx catalyst. In contrast, the NOx catalyst is added in an amount of 20 g / L or more.
That is, 20 g / L, which is a relatively large addition amount of potassium (K) as a storage agent, is added to the NOx catalyst.

  According to the exhaust gas purification apparatus for an internal combustion engine of claim 1 of the present invention using the above means, the occlusion agent scattered from the NOx catalyst is captured by the acidic oxide added to the lower layer of the catalyst layer of the three-way catalyst, thereby occlusion. The functional deterioration of the three-way catalyst due to the agent can be prevented. Further, since rhodium (Rh) is added to an upper layer different from the catalyst layer lower layer to which the acidic oxide is added, the negative effect on the rhodium (Rh) by the acidic oxide can also be prevented. By adding rhodium (Rh) to the upper layer, high HC purification performance when releasing and reducing NOx stored in the NOx catalyst possessed by the rhodium (Rh) can be sufficiently exhibited.

Further, the storage agent trapped in the lower layer of the catalyst layer of the three-way catalyst can also perform NOx purification in the three-way catalyst.
Thereby, the exhaust purification performance fall of the three way catalyst by a storage agent can be prevented favorably, and the fall of the NOx purification performance by endurance can be controlled.
According to the exhaust gas purification apparatus for an internal combustion engine according to claim 2, like rhodium (Rh), platinum (Pt) is also added to a catalyst layer upper layer different from the catalyst layer lower layer to which the acidic oxide is added. Thus, it is possible to prevent the adverse effect of platinum (Pt) on the acidic oxide and to fully demonstrate the high HC purification performance during the lean operation of platinum (Pt). The performance deterioration can be prevented more favorably.

According to the exhaust gas purification apparatus for an internal combustion engine of claim 3, since the acidic oxide that suppresses the scattering of the occluding agent is added to the NOx catalyst, the scattering of the occluding agent from the NOx catalyst is suppressed even at a high temperature. be able to.
Thereby, the fall of NOx purification performance by scattering of the storage agent in a NOx catalyst can be prevented.

According to the exhaust gas purification apparatus for an internal combustion engine according to claim 4, the acidic oxide having a relatively low acidity added to the NOx catalyst can suppress the scattering of the storage agent while sufficiently exhibiting the NOx storage function, The acidic oxide having a relatively high acidity added to the three-way catalyst can surely capture and fix the scattered storage agent.
Thereby, the exhaust purification performance of the NOx catalyst and the three-way catalyst can be maintained well.

According to the exhaust gas purification apparatus for an internal combustion engine of claim 5, the catalyst can be activated early by making the support of the NOx catalyst and the three-way catalyst a metal support having high thermal conductivity and low heat capacity. By preventing the storage agent from entering the carrier, the deterioration of the NOx catalyst and the three-way catalyst can be suppressed.
According to the exhaust gas purification apparatus for an internal combustion engine according to claim 6, the NOx storage performance can be increased by adding a relatively large amount of potassium (K) effective as a storage agent to the NOx catalyst, so that the storage amount of NOx can be increased. Can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Referring to FIGS. 1 to 3, FIG. 1 shows a schematic configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to the present invention, and FIG. 2 shows an NOx catalyst of the exhaust gas purification apparatus for an internal combustion engine according to the present invention. A partially enlarged sectional view is shown, and FIG. 3 shows a partially enlarged sectional view of a three-way catalyst of an exhaust purification device for an internal combustion engine according to the present invention. Hereinafter, a description will be given based on FIG.

As shown in FIG. 1, an engine 1 (internal combustion engine) is a so-called in-cylinder injection engine in which a combustion plug 2 faces a spark plug 4 that performs ignition and an injector 6 that can directly inject fuel.
Further, the combustion chamber 2 communicates with an intake port 8 that extends substantially in the vertical direction of the engine 1 and an exhaust port 10 that extends substantially in the width direction of the engine 1.
The intake port 8 is provided with an intake valve 12 for connecting and blocking the combustion chamber 2 and the intake port 8, and the exhaust port 10 is provided with an exhaust valve 14 for connecting and blocking the combustion chamber 2 and the exhaust port 10. It has been.

The intake port 8 is connected to an intake manifold 16, and the exhaust port 10 is connected to an exhaust manifold 18.
The exhaust manifold 18 is connected to an exhaust pipe 20, and an exhaust purification device 30 is provided in the exhaust pipe 20.
Here, the exhaust emission control device 30 will be described in detail.

Inside the exhaust purification device 30, a NOx catalyst 40 is provided on the exhaust upstream side.
As shown in FIG. 2, the NOx catalyst 40 has a catalyst layer 44 supported on the surface of a metal carrier 42 composed of a large number of cells formed by corrugated plates and flat plates.
The catalyst layer 44 includes a noble metal such as platinum (Pt), palladium (Pd), rhodium (Rh) (for example, Pt and Pd are 0.1 to 10.0 g / L, and Rh is 0.1 -1.0 g / L), a relatively large amount of potassium (K) is added as a storage agent having a function of storing NOx (20 g / L or more, for example, K = 25 g / L in this case). The occlusion agent may be alkali metal or alkaline earth metal such as sodium (Na) and barium (Ba) in addition to potassium (K).

Furthermore, in order to suppress scattering of the storage agent (potassium), acidic catalyst zeolite is added to the catalyst layer 44 as an alkali stabilizer having a function of suppressing the storage of the storage agent (potassium). The alkali stabilizer may be an acidic oxide such as silica (SiO ₂ ), phosphoric acid (H ₃ PO ₄ ), titania (TiO ₂ ), zirconium silicate (ZrSiO ₂ ), in addition to zeolite. I do not care.

Further, the catalyst layer 44 of the NOx catalyst 40 is composed of two layers, a lower layer 46 and an upper layer 48.
Ceria (CeO ₂ ) is added to the catalyst layer lower layer 46. The ceria (CeO ₂ ) has a high O ₂ storage function, and by this O ₂ storage function, the catalyst layer lower layer 46 has a ternary function of platinum (Pt), palladium (Pd), and rhodium (Rh) noble metals. Enhanced.

Further, titania (TiO ₂ ) is added to the catalyst layer upper layer 48. The titania (TiO ₂ ) has a sulfur component (S) adsorption prevention function, and the titania (TiO ₂ ) is added to the catalyst layer upper layer 48 of the NOx catalyst 40, so that the surface of the catalyst layer 44 is added. The sulfur component (S) is not adsorbed and S poisoning is prevented.
The NOx catalyst 40 thus configured mainly stores NOx in an atmosphere where the oxygen concentration is high during the lean air-fuel ratio operation (lean operation) of the engine 1, and during the stoichiometric air-fuel ratio operation (stoichiometric operation) or the rich air-fuel ratio. It has a function of releasing and reducing NOx in an atmosphere in which a large amount of reducing agents such as CO and HC are present during operation (rich operation).

On the other hand, a three-way catalyst 50 is provided in the exhaust purification device 30 on the exhaust downstream side of the NOx catalyst 40.
As shown in FIG. 3, in the three-way catalyst 50, like the NOx catalyst 40, the carrier 52 is a metal carrier, and the catalyst layer 54 is configured and supported on the carrier 52 in two layers of a lower layer 56 and an upper layer 58. ing.

In the catalyst layer lower layer 56, palladium (Pd) and an acidic oxide zeolite are added as an alkali trapping agent having a function of trapping the scattered storage agent. The alkali trapping agent may also be an acidic oxide such as silica (SiO ₂ ), phosphoric acid (H ₃ PO ₄ ), titania (TiO ₂ ), zirconium silicate (ZrSiO ₂ ), in addition to zeolite. However, the alkali trapping agent has a higher acidity than the alkali stabilizer added to the NOx catalyst 40.

Further, platinum (Pt) and rhodium (Rh) are added to the catalyst upper layer 58 (for example, Pt and Pd are 0.1 to 10.0 g / L, and Rh is 0.1 to 1.0 g). / L).
The platinum (Pt) and rhodium (Rh) functions are lowered by the alkali trap agent (zeolite) which is an acidic oxide, so it is added to a layer different from the layer to which the alkali trap agent is added. In addition, during the NOx purge of the NOx catalyst 40 possessed by rhodium (Rh), that is, during the stoichiometric operation or during the rich operation, a high HC purification performance in an atmosphere with a reduced oxygen concentration, and a high HC during the lean operation possessed by platinum (Pt) In order to fully exhibit the purification performance, it is added to the catalyst layer upper layer 58.

Ceria (CeO 2) is added to the catalyst layer 54 of the three-way catalyst 50 in both the lower layer 56 and the upper layer 58, and the three-way function of the three-way catalyst 50 is enhanced.
The three-way catalyst 50 configured as described above has a function of oxidizing CO and HC and reducing NOx.
The operation of the exhaust gas purification apparatus for an internal combustion engine according to the present invention configured as described above will be described below.

Exhaust gas from the engine 1 flows from the exhaust port 10 into the exhaust purification device 30 through the exhaust manifold 18 and the exhaust pipe 20.
At this time, if the engine 1 is in a lean operation, NOx contained in the exhaust gas in a large amount is stored in the form of nitrate by the storage agent (potassium) added to the catalyst layer 44 of the NOx catalyst 40. Further, the sulfur component (S) of the SOx in the exhaust gas tends to be adsorbed by the storage agent (potassium) on the NOx catalyst 40, but the sulfur component (S2) is caused by the action of titania (TiO2) added to the catalyst layer upper layer 48. ) Is suppressed.

When the engine 1 is switched to the stoichiometric operation or the rich operation, the oxygen concentration in the exhaust gas decreases, and the exhaust gas contains a large amount of reducing agent such as CO or HC. When such exhaust flows into the exhaust purification device 30, the nitrate stored in the NOx catalyst 40 is released, and the nitrate reacts with CO or the like by the three-way function or the three-way catalyst 50 that the NOx catalyst 40 has, and nitrogen. NOx is purified by being reduced to (N ₂ ) or the like. Since a relatively large amount of storage agent (potassium) is added to the catalyst layer 44 of the NOx catalyst 40, the NOx purification performance is improved.

Further, CO and HC discharged during the stoichiometric operation and rich operation are oxidized to CO ₂ , H ₂ O, and the like by the three-way catalyst 50.
Since the carriers 42 and 52 of the NOx catalyst 40 and the three-way catalyst 50 are metal carriers having high heat conduction and low heat capacity, the catalyst temperature easily rises and is activated early.
In this way, exhaust purification in the in-cylinder injection type engine 1 is performed by the exhaust purification device 30, but when the temperature of the NOx catalyst 40 rises, the storage agent (potassium) added to the NOx catalyst 40 is also increased. Start to activate.

In the NOx catalyst 40, since the carrier 42 is a metal carrier, it is possible to prevent the storage agent (potassium) from entering the carrier 42, and the catalyst layer 44 of the NOx catalyst 40 has an alkali stabilizer (zeolite) throughout. Therefore, scattering of the storage agent (potassium) can also be suppressed.
Further, even if the storage agent (potassium) is scattered from the NOx catalyst 40, the alkali trap agent (zeolite) added to the catalyst layer lower layer 56 of the three-way catalyst 50 provided on the exhaust downstream side of the NOx catalyst 40. Thus, the scattered storage agent (potassium) can be captured in the catalyst layer lower layer 56 of the three-way catalyst 50. The alkali trap agent (zeolite) added to the catalyst layer lower layer 56 of the three-way catalyst 50 has higher acidity than the alkali stabilizer (zeolite) added to the NOx catalyst 40, and the trapped occlusion agent ( The activity of potassium) is sufficiently suppressed and fixed to the catalyst layer lower layer 56. Further, since the carrier 52 of the three-way catalyst 50 is also a metal carrier, the trapped occlusion agent (potassium) does not enter the carrier 52.

From these things, the functional fall of the three-way catalyst 50 by a storage agent (potassium) can be prevented.
4 and 5, the present example, the conventional example 1 of the NOx catalyst and the three-way catalyst in which the carrier is a cordierite carrier, and the conventional NOx catalyst and the three-way catalyst in which the carrier is a metal carrier. The NOx purification rate and HC purification rate in Example 2 are shown, respectively.

In Conventional Example 1 in which the carrier is a cordierite carrier, the occlusion material enters the carrier and scatters at a certain high temperature, so that the NOx purification rate decreases as shown in FIG.
On the other hand, in Conventional Example 2 in which the carrier is a metal carrier, the NOx purification performance is maintained even at high temperatures as shown in FIG. 4 because the penetration of the occluding agent into the carrier is prevented. Since the scattering of the storage agent is increased and the purification performance of the three-way catalyst is lowered, the purification rate of HC is greatly reduced as shown in FIG.

On the other hand, in this embodiment, in order to suppress the scattering of the storage agent in the NOx catalyst 40 and prevent the purification performance from being lowered by the storage agent in the three-way catalyst 50, as shown in FIGS. A reduction in the HC purification rate can be prevented while maintaining the rate.
Further, since the storage agent (potassium) fixed to the catalyst layer lower layer 56 also stores NOx, when the exhaust purification device 30 becomes durable, the three-way catalyst 50 also exhibits the NOx purification performance.

Thereby, the fall of NOx purification performance by durability can also be prevented. As shown in the graph showing the NOx purification performance with respect to the durability time in the present embodiment of FIG. 6 and the above-described conventional example 1, the NOx purification rate is less decreased due to the increase in the durability time in the present embodiment than in the conventional 1. It is clear from
As described above, in the exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to satisfactorily prevent the exhaust gas purification performance of the three-way catalyst 50 from being reduced by the storage agent (potassium), and to suppress the deterioration of the NOx purification performance due to durability. can do.

Although the description of the embodiment of the exhaust gas purification apparatus for an internal combustion engine according to the present invention is finished above, the embodiment is not limited to the above embodiment.
For example, in the above embodiment, the occlusion agent is a kind of potassium (K), but the present invention is not limited to this, and a plurality of alkali metals or alkaline earth metals may be used.
Moreover, in the said embodiment, although the acidic oxidizing agent was 1 type of zeolite, it is not restricted to this, You may use a some acidic oxide. Further, the acidic oxide includes an acidic composite oxide.

  Moreover, the substance added to the catalyst layer of the NOx catalyst and the three-way catalyst is not limited to the above embodiment, and other substances may be added.

1 is a schematic configuration diagram of an exhaust gas purification apparatus for an internal combustion engine according to the present invention. It is a partial expanded sectional view of the NOx catalyst of the exhaust gas purification apparatus for an internal combustion engine according to the present invention. It is a partial expanded sectional view of the three way catalyst of the exhaust gas purification device of the internal combustion engine which concerns on this invention. It is a graph which shows the NOx purification rate in a present Example, the prior art example 1, and the prior art example 2. FIG. It is a graph which shows the HC purification rate in a present Example and the prior art example 1 and the prior art example 2. FIG. It is a graph which shows the NOx purification performance with respect to the durable time in a present Example and the prior art example 1. FIG.

Explanation of symbols

1 Engine 10 Exhaust port 18 Exhaust manifold 20 Exhaust pipe 30 Exhaust purification device 40 NOx catalyst 42, 52 Carrier 44, 54 Catalyst layer 46, 56 Lower layer 48, 58 Upper layer 50 Three-way catalyst

Claims (6)

A NOx catalyst provided in an exhaust passage of the internal combustion engine, wherein at least one of an alkali metal and an alkaline earth metal is added as a storage agent to the catalyst layer;
Provided on the exhaust downstream side of the NOx catalyst, the catalyst layer is composed of two layers, an upper layer and a lower layer, at least rhodium (Rh) is added to the upper layer of the catalyst layer, and at least the NOx catalyst is scattered from the lower layer of the catalyst layer An exhaust purification device for an internal combustion engine, comprising: a three-way catalyst to which an acidic oxide having a function of trapping a storage agent is added.   The exhaust purification device for an internal combustion engine according to claim 1, wherein platinum (Pt) is added to the upper layer of the catalyst layer of the three-way catalyst.   The exhaust purification device for an internal combustion engine according to claim 1 or 2, wherein an acidic oxide having a function of suppressing scattering of the storage agent is added to the NOx catalyst.   The exhaust gas purification apparatus for an internal combustion engine according to claim 3, wherein the acidic oxide contained in the lower layer of the three-way catalyst has higher acidity than the acidic oxide contained in the NOx catalyst.   The exhaust purification device for an internal combustion engine according to any one of claims 1 to 4, wherein the carrier of the NOx catalyst and the three-way catalyst is a metal carrier.   The storage agent added to the NOx catalyst is potassium (K), and the potassium (K) is added to the NOx catalyst in an amount of 20 g / L or more with respect to the capacity of the NOx catalyst. The exhaust emission control device for an internal combustion engine according to any one of claims 1 to 5.

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