JP2010270695A - Exhaust emission control device and exhaust emission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control device advantageous in fuel consumption and costs for regeneration of a particulate filter and functional recovery of a NOx purification catalyst. <P>SOLUTION: An HC oxidation catalyst 3, the particulate filter 4, and the NOx purification catalyst 5 including a NOx occlusion material are disposed in this order from an upstream side in an exhaust flowing direction in an exhaust passage 2 of a lean-burn engine 1. A catalyst having a large CO production amount when burning particulates is carried in the filter 4, and reduction purification of NOx is carried out in the NOx purification catalyst 5 by using CO produced by combustion of particulates in the filter 4 as a reducing agent. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an engine exhaust purification device and an exhaust purification method.

  The exhaust of diesel engines and lean-burn gasoline engines contains a large amount of NOx (nitrogen oxides) and particulates (black smoke particulates), compared to the exhaust of engines burned in the vicinity of stoichiometry. For this reason, a particulate collection filter and a NOx purification catalyst are often provided in the exhaust passage of such a lean combustion engine.

  In the filter for collecting particulates, for example, a pore passage having a pore diameter of several μm to several tens of μm is formed in a SiC carrier, and the particulate is deposited on the pore passage surface. Then, for example, at every predetermined mileage of the automobile or when it is detected that the filter has reached a predetermined clogged state, the filter is regenerated (to collect and remove the collected particulates), the exhaust temperature The control such as raising is performed.

On the other hand, there are various types of NOx purification catalysts. For example, there is a system that employs a selective reduction type NOx catalyst, supplies urea water or ammonia water to the catalyst from the upstream side of the exhaust flow, and reduces and purifies NOx using ammonia as a reducing agent. Alternatively, when a NOx occlusion catalyst containing NOx occlusion material is employed, the NOx occlusion material is occluded in the NOx occlusion material during lean air-fuel ratio when oxygen in the exhaust gas is excessive, and when the NOx occlusion amount reaches a predetermined value, By controlling the fuel injection of the engine so that the amount of unburned fuel in the exhaust gas increases, the oxygen concentration of the exhaust gas is reduced (by making the air-fuel ratio rich), and NOx is released from the NOx storage material, and the unburned fuel There is a method for reducing and purifying NOx using as a reducing agent. As the NOx reduction catalyst component, TiO ₂ is often used in the former method, and Rh is often used in the latter method.

  In order to burn off particulates, it is generally known to arrange an oxidation catalyst in the exhaust passage upstream of the filter. That is, HC (hydrocarbon) is supplied to the oxidation catalyst, and the temperature of the exhaust gas flowing into the filter is increased by the heat of the oxidation reaction, thereby increasing the temperature of the filter.

  There is also a proposal to regenerate the filter without providing an oxidation catalyst. For example, Patent Document 1 describes that a plasma generator, a particulate collection filter, and a NOx storage catalyst are arranged in the exhaust passage of a diesel engine in order from the exhaust flow upstream side. The plasma generated by the plasma generator converts the oxygen and nitrogen components in the exhaust gas into oxidant components such as ozone and nitrogen dioxide, and this oxidant component partially oxidizes or incompletely deposits particulates on the filter. Combustion produces CO, and this CO is used as a reducing agent to reduce and purify NOx stored in the NOx storage catalyst. Since the oxidant component is generated by plasma, it is said that the particulates can be burned and removed at a relatively low temperature. Further, this document also describes that a CO shift catalyst is disposed between the filter and the NOx storage catalyst, hydrogen is generated by a CO shift reaction, and NOx is reduced and purified using hydrogen as a reducing agent.

Patent Document 2 describes that a burner, a CO generation catalyst, and a particulate filter with a NOx storage catalyst are arranged in this order from the exhaust flow upstream side in the exhaust passage of the diesel engine. The CO generating catalyst is heated to an active temperature by a burner, HC supplied in the exhaust gas is converted to CO by the CO generating catalyst, and NO ₂ released from the NOx storage catalyst is reduced using the CO as a reducing agent. While purifying, the particulates are burned by the released NO ₂ .

JP 2004-84638 A JP 2004-92497 A

  In the case of the methods described in Patent Document 1 and Patent Document 2, a plasma generator or a burner is required, which tends to increase the cost, and requires maintenance to ensure the durability or reliability thereof. On the other hand, in the system in which the temperature of the filter is increased by providing an oxidation catalyst on the upstream side of the filter, a plasma generator and a burner are unnecessary, but NOx is released from the NOx storage catalyst and reduced into the exhaust gas for reduction. Even if HC is supplied, most of the HC is burned by the oxidation catalyst, and the NOx occlusion catalyst disposed on the downstream side of the filter makes the HC insufficient and NOx is not efficiently reduced. Conventionally, the combustion start temperature of particulates has been lowered by providing an oxidation catalyst containing a relatively large amount of Pt in the filter. However, even in that case, the above-mentioned HC is burned by the catalyst on the filter, and NOx reduction by the downstream NOx storage catalyst is not performed efficiently.

  Such a problem can be solved by supplying an excessive amount of HC to the oxidation catalyst and the filter so that the required amount of HC reaches the NOx storage catalyst.

  Therefore, the present invention provides an exhaust purification device and an exhaust purification method that are advantageous in terms of fuel consumption and cost.

  In order to solve the above-mentioned problem, the present invention does not generate CO from particulates by an oxidant component by plasma, and does not generate CO by separately providing a CO generation catalyst. A filter for generating a relatively large amount of CO at the time of particulate combustion is provided in the filter for use.

That is, the means for solving the above problem is an exhaust emission control device for a lean combustion engine,
In the exhaust passage of the engine, an oxidation catalyst that oxidizes HC in the exhaust, a filter that collects particulates in the exhaust, and a NOx purification catalyst that contains a NOx storage material that stores NOx in the exhaust are exhausted. Arranged in order from the upstream side in the flow direction,
The filter carries a catalyst for burning the collected particulates, and the catalyst occupies the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. The proportion of CO is 10% or more in molar ratio,
The NOx purification catalyst contains a catalyst component that reduces and purifies NOx using CO generated by burning particulates in the filter as a reducing agent.

Therefore, when the amount of particulates collected by the filter increases, or when the NOx storage amount of the NOx purification catalyst increases, if the amount of HC in the exhaust gas flowing into the oxidation catalyst is increased, the oxidation The temperature of the exhaust gas flowing into the filter is increased by the heat of oxidation reaction of HC at the catalyst, that is, the temperature of the filter is increased, and particulate combustion by the catalyst starts in the filter. The catalyst of this filter is compared with the combustion of particulates because the proportion of CO in the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. is 10% or more in molar ratio. A large amount of CO is generated.

  On the other hand, in the NOx purification catalyst, NOx is released from the NOx occlusion material by decreasing the oxygen concentration of the exhaust gas by increasing the amount of HC in the exhaust gas. And CO produced | generated by the combustion of the particulates of the said filter becomes a reducing agent, and NOx discharge | released from NOx occlusion material is reduced and purified efficiently.

  Since the catalyst for burning the particulates to generate CO is provided in the filter, the filter particulates can be used without using a plasma generator, and without excessively supplying HC into the exhaust gas for the NOx purification catalyst. The NOx storage capacity of the NOx purification catalyst can be recovered at the same time while recovering the amount of curate that can be collected, which is advantageous in reducing maintenance and reducing costs.

  In a preferred embodiment, the NOx purification catalyst has at least one alkaline earth metal supported on a support as the NOx storage material, and the amount capable of storing NOx at 300 ° C. or higher is 1 g per liter of support. It is the above. By employing an alkaline earth metal as the NOx storage material, the NOx storage capacity of the NOx purification catalyst can be increased. Further, if the amount of NOx that can be stored in the NOx purification catalyst is small, the NOx is released and the frequency of reduction purification is increased, and the fuel consumption is likely to deteriorate. Therefore, it is desirable that the NOx storage capacity is 1 g / L or more. It is. Further, if the amount of HC in the exhaust gas flowing into the oxidation catalyst is increased when the particulate collection amount of the filter is increased, a large amount of CO is generated by combustion of the particulates. It is also advantageous for use without waste in reducing and purifying the water.

  In another preferred embodiment, the catalyst supported on the filter is at least selected from three kinds of Ce-containing oxides, complex oxides containing rare earth metals other than Ce and Zr, and alumina supporting alkali metals. 1 type is contained, and the catalyst noble metal amount per 1 L of filter is 0 g or 0.1 g or less.

That is, Pt and other catalytic noble metals generally have a high oxidizing ability, so that the particulates are easily burned completely (CO ₂ is easily generated), and the amount of CO generated is reduced accordingly. Therefore, the catalyst supported on the filter has a noble metal amount of 0 g or 0.1 g / L or less. Even if the amount of the catalyst noble metal is set in this way, the three types of the Ce-containing oxide, the complex oxide containing the rare earth metal other than Ce and Zr, and the alumina supporting the alkali metal all burn particulates. It works effectively as a catalyst and produces a relatively large amount of CO.

  In this case, the Ce-containing oxide causes an oxygen exchange reaction in which oxygen in the exhaust gas is taken in by valence change of Ce ions and is released as active oxygen, and the active oxygen is used to oxidize particulates to CO. Contribute. The composite oxide containing rare earth metal other than Ce and Zr has high oxygen ion conductivity, transports oxygen ions, and releases them as active oxygen by an oxygen exchange reaction on the oxide particle surface. Contributes to the oxidation of CO to CO. In addition, in the alumina carrying an alkali metal, the alkali metal promotes oxidation of particulates to CO. Examples of rare earth metals other than Ce include Nd, Pr, and La, and examples of alkali metals include K and Na.

  Preferably, when HC is supplied to the oxidation catalyst in order to restore the particulate collection capacity of the filter or the NOx storage capacity of the NOx purification catalyst, the HC is supplied intermittently, so that the exhaust air is exhausted. This is to change the fuel ratio alternately between lean and rich. As a result, oxygen deficiency in the filter can be avoided, which is advantageous for particulate combustion.

In another preferred embodiment, a CO oxidation catalyst for oxidizing CO is provided in the exhaust passage downstream of the NOx purification catalyst. As a result, even if CO is excessively generated by particulate combustion in the filter and a part thereof passes through the NOx purification catalyst, it is oxidized to CO ₂ by the downstream CO oxidation catalyst, and the CO Emission to the atmosphere can be suppressed.

  In this case, the CO oxidation catalyst preferably contains Pd as a catalyst metal. Even when the temperature is relatively low, Pd has a high activity as an oxidation catalyst, which is advantageous for CO oxidation.

  According to still another preferred aspect of the present invention, when the particulates collected by the filter are burned, the fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, and then further expanded. Post-injection in which fuel is injected and supplied in the stroke or exhaust stroke is executed, and unburned fuel is supplied as HC to the oxidation catalyst upstream of the filter by the post-injection. As a result, the temperature of the filter rises due to the oxidation reaction heat of the unburned fuel generated in the oxidation catalyst, and the particulates of the filter burn.

Another means for solving the above problem is that an oxidation catalyst that oxidizes HC in exhaust and a filter that collects particulates in exhaust are sequentially arranged from the upstream side in the exhaust flow direction in the exhaust passage of the lean combustion engine. Set up
In the exhaust gas purification method in which the particulate matter trapped in the filter is burned by oxidizing the HC in the oxidation catalyst and increasing the temperature of the filter by its reaction heat.
A NOx purifying catalyst containing a NOx occlusion material that occludes NOx in the exhaust and a catalyst component for reducing and purifying NOx is disposed in the exhaust passage on the downstream side of the exhaust flow from the filter,
In order to burn the collected particulates in the filter, the proportion of CO in the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. is 10% or more in molar ratio. A catalyst is supported,
The NOx occluded in the NOx purification catalyst is reduced and purified using CO generated by the combustion of particulates in the filter as a reducing agent.

  Therefore, without using a plasma generator and supplying HC excessively into the exhaust gas for NOx purification catalyst, particulates are burned by the catalyst in the filter to generate CO, and this CO is reduced to NOx. It can be used for purification, and the NOx storage capacity of the NOx purification catalyst can be restored at the same time while recovering the particulate collection capacity of the filter, which is advantageous for reduction of maintenance and cost reduction.

  In a preferred embodiment, when the NOx storage amount of the NOx purification catalyst reaches a predetermined value, after the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke, further in the expansion stroke or exhaust stroke. By performing post-injection after injecting and supplying fuel, unburned fuel is supplied to the oxidation catalyst as the HC, and the temperature of the filter is raised by oxidation reaction heat of the unburned fuel.

  Therefore, when the NOx occlusion amount of the NOx purification catalyst reaches a predetermined value, the amount of particulates that can be collected by the filter can be recovered at the same time while the NOx occlusion capability is restored by post-injection of the fuel.

In another preferred embodiment, a CO oxidation catalyst for oxidizing CO is disposed in the exhaust passage downstream of the NOx purification catalyst, and CO passing through the NOx purification catalyst is oxidized to CO ₂ by the CO oxidation catalyst. It is characterized by that. As a result, even if CO is excessively generated by particulate combustion in the filter and a part of the CO passes through the NOx purification catalyst, it is oxidized to CO ₂ by the CO oxidation catalyst, and the CO enters the atmosphere. Can be suppressed.

As described above, according to the present invention, in the exhaust passage of the lean combustion engine, in order from the upstream side in the exhaust flow direction, the oxidation catalyst that oxidizes HC in the exhaust, the filter that collects particulates in the exhaust, A NOx purifying catalyst containing a NOx occlusion material that occludes NOx in the exhaust is disposed, and the filter occupies the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. Since a catalyst having a CO ratio of 10% or more in a molar ratio is carried and NOx is reduced and purified by a NOx purification catalyst using CO generated by burning particulates in a filter, a plasma generator NOx purification at the same time while recovering the filter's particulate collection capacity without using HC and without excessively supplying HC into the exhaust gas for NOx purification catalyst The NOx occlusion capacity of the catalyst can be recovered, which is advantageous for reducing maintenance and reducing costs.

It is a block diagram which shows the exhaust emission purification device of the engine which concerns on embodiment of this invention. It is a flow figure showing an example of control of filter regeneration and NOx discharge concerning an embodiment of the present invention. It is a graph which shows the NOx occlusion ability of the NOx purification catalyst which concerns on embodiment of this invention.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. It should be noted that the following description of the preferred embodiment is merely illustrative in nature, and is not intended to limit the present invention, its application, or its use.

In FIG. 1, reference numeral 1 denotes an automobile diesel engine. In an exhaust passage 2, an HC oxidation catalyst 3 that oxidizes HC in exhaust, a filter 4 that collects particulates in exhaust, and NOx in exhaust. A NOx purification catalyst 5 containing the NOx occlusion material to be occluded and a CO oxidation catalyst 6 for oxidizing CO are sequentially arranged from the upstream side in the exhaust flow direction. The diesel engine 1 is provided with a fuel injection valve 8 that directly injects fuel from the fuel tank 7 into the engine combustion chamber. The filter 4 carries a catalyst for burning the collected particulates. As the catalyst, the proportion of CO in the total amount of CO and CO ₂ generated when carbon is burned at a temperature of 500 ° C. is such that a relatively large amount of CO is generated by burning the particulates. A catalyst having a ratio of 10% or more is employed. The NOx purification catalyst 5 contains a catalyst component that reduces and purifies NOx using CO generated by the combustion of particulates in the filter 4 as a reducing agent.

  For fuel injection control (control of injection timing and injection amount) by the fuel injection valve 8, a controller 9 using a microcomputer is provided. Filter regeneration control for recovering the particulate trappable amount of the filter 4 by burning and removing the particulates of the filter 4, and NOx of the NOx purification catalyst 5 is released and reduced and purified to reduce the NOx purification catalyst 5. The NOx release control for recovering the NOx storage capacity is executed by supplying unburned fuel as HC to the HC oxidation catalyst 3 by fuel injection control.

  For the above-described filter regeneration control and NOx release control, the exhaust passage 2 is provided with an inlet side pressure sensor 11 and an outlet side pressure sensor 12 for detecting the difference in exhaust pressure between the inlet side and the outlet side of the filter 4. On the downstream side of the CO oxidation catalyst 6, a NOx sensor 13 for detecting the NOx concentration of the exhaust gas flowing out of the NOx purification catalyst 5 is provided. That is, the controller 9 determines that the regeneration of the filter 4 is necessary when the particulate collection amount of the filter 4 exceeds a predetermined value based on the differential pressure detected by both the pressure sensors 11 and 12. . Further, the controller 9 needs to restore the NOx storage capacity of the NOx purification catalyst 5 when the NOx storage amount of the NOx purification catalyst 5 exceeds a predetermined value based on the NOx concentration detected by the NOx sensor 13. It is determined that

  Fuel injection control for regeneration of the filter 4 and recovery of the NOx storage capacity of the NOx purification catalyst 5 is performed in the expansion stroke or exhaust stroke after the main injection in which fuel is injected into the combustion chamber of the engine near the top dead center of the compression stroke. Post-injection is performed by supplying fuel. After this, unburned fuel is supplied to the HC oxidation catalyst 3 by injection. The start timing and stop timing of the main injection and post injection are determined and executed based on a signal from a sensor that detects a crank angle (not shown).

  Specifically, in the normal fuel injection control (main injection control) in which the post-injection is not executed, the air-fuel ratio of the engine is controlled to be lean, so that the air-fuel ratio of the exhaust gas is also lean, NOx is stored in the NOx storage material of the NOx purification catalyst 4.

  When regeneration of the filter 4 or recovery of the NOx storage capacity of the NOx purification catalyst 5 is performed, unburned fuel is supplied into the exhaust gas by executing the post-injection, and the unburned fuel is oxidized by the HC oxidation catalyst 3. At that time, the temperature of the exhaust gas flowing into the filter 4 is increased by the oxidation reaction heat generated in the HC oxidation catalyst 3, and the temperature of the filter 4 is increased. Thereby, the particulates collected by the filter 4 can be burned by the catalyst, and the filter 4 can be regenerated. A relatively large amount of CO is generated in the filter 4 by the combustion of the particulates by the catalyst. In the present embodiment, CO produced by the filter 4 is used for NOx purification in the NOx purification catalyst 5.

  That is, when unburned fuel is supplied into the exhaust gas by the post-injection, the oxygen concentration in the exhaust gas is lowered and the air-fuel ratio of the exhaust gas becomes rich, so NOx is released from the NOx purification catalyst 5. The CO generated by the filter 4 serves as a reducing agent that reduces and purifies NOx released from the NOx purification catalyst 5.

  Therefore, even when the unburned fuel supplied into the exhaust gas by the post-injection is oxidized by the HC oxidation catalyst 3 and further oxidized by the filter 4 and the unburned fuel reaching the NOx purification catalyst 5 is reduced, the NOx purification is performed. In the catalyst 5, NOx is reduced and purified by the CO supplied from the filter 4, and it is possible to suppress discharge of unpurified NOx in the NOx release control.

  As is clear from the above description, when the post-injection is executed in order to regenerate the filter 4, the NOx storage capacity of the NOx purification catalyst 5 is restored at the same time, and the NOx storage capacity of the NOx purification catalyst 5 is restored. When the post-injection is executed to recover the amount of particulate matter, the particulate collection capacity of the filter 4 is recovered at the same time.

  Further, when the filter 4 is regenerated, the exhaust gas temperature flowing out from the filter 4 becomes high, so that the NOx purification catalyst 5 arranged on the downstream side of the filter 4 becomes high temperature and the HC component and CO flowing out from the filter 4 Flows into the NOx purification catalyst 5. As a result, the NOx purification catalyst 5 can achieve the desorption of the sulfur component from the NOx occlusion material, that is, recovery from sulfur poisoning simultaneously with the NOx release / reduction purification.

  FIG. 2 shows an example of filter regeneration and NOx release control by the controller 9. In step S1 after the start, the values of the pressure sensors 11, 12 and the NOx sensor 13 are read. In step S2, the particulate collection amount P of the filter 4 is calculated from the values of the pressure sensors 11, 12, and in step S3, the NOx occlusion amount N of the NOx purification catalyst 5 is calculated from the value of the NOx sensor 13.

  In subsequent step S4, it is determined whether or not the particulate collection amount P is equal to or greater than a predetermined value P1 (filter regeneration control start threshold). When P ≧ P1, it is determined that filter regeneration is necessary, and flag F = 1 is set, and post-injection is executed (steps S5 and S6). The flag F = 1 indicates that the filter regeneration by the post injection or the NOx release is being controlled. In a succeeding step S7, it is determined whether or not the particulate collection amount P is equal to or less than a predetermined value P3 (filter regeneration control end threshold). When P ≦ P3, the post-injection is terminated and the flag F = 0 is set (steps S8 and S9).

  When the particulate collection amount P does not reach the predetermined value P1 in step S4, the process proceeds to step S10, and it is determined whether or not the NOx occlusion amount N is equal to or greater than the predetermined value N1. When N ≧ N1, the process proceeds to step S11 to determine whether or not the particulate collection amount P is equal to or greater than a predetermined value P2 (NOx release control allowable threshold). Note that P1 (filter regeneration control start threshold)> P2 (NOx release control allowable threshold)> P3 (filter regeneration control end threshold). When P ≧ P2, the routine proceeds to step S5 and after, and post-injection for NOx release is executed.

  When the NOx occlusion amount N is less than the predetermined value N1 in step S10, or when the particulate collection amount P is less than the predetermined value P2 in step S11, the process proceeds to step S12, and whether or not the flag F = 1 is set. Determine. When F = 1, filter regeneration or NOx release control is being performed by post-injection, so this post-injection control is continued. When F = 0, both filter regeneration control and NOx release control are required. Return as not.

  In the above control example, HC is supplied to the HC oxidation catalyst 3 by post-injection of fuel into the engine combustion chamber. Instead of post-injection, HC is supplied to the exhaust passage 2 upstream of the HC oxidation catalyst 3. An injection device may be arranged, and HC may be supplied to the HC oxidation catalyst 3 by this HC injection device.

<NOx purification performance>
The exhaust purification apparatuses of Examples 1 to 6 and Comparative Examples 1 to 6 were configured, and the CO generating ability and the NOx purification performance of each particulate combustion catalyst were evaluated.

[Catalyst composition]
Example 1
In the model gas flow reactor, the HC oxidation catalyst 3, the filter 4, the NOx purification catalyst 5, and the CO oxidation catalyst 6 were arranged in this order from the upstream side.

The HC oxidation catalyst 3 has a cordierite honeycomb carrier (capacity 12.2) having a cell wall thickness of 4.5 mil (11.43 × 10 ⁻² mm) and 600 cells per square inch (645.16 mm ² ). 5 mL) was used. Then, a honeycomb carrier was coated with a powder obtained by impregnating and supporting Pt and Pd as catalyst metals, dried and fired on a powder obtained by mixing activated alumina, β zeolite, and ZrCeNd composite oxide. The loading amount of each component (the loading amount per liter of the carrier, the same applies hereinafter) is 80 g / L for activated alumina, 60 g / L for β zeolite, 60 g / L for ZrCeNd composite oxide, and 2 g / L for Pt. , Pd is 0.5 g / L. The activated alumina was employed _Al 2 _{O 3} containing _La 2 _{O 3} 5 wt%. A ZrCeNd composite oxide having a composition of ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 77: 19: 4 (molar ratio) was employed.

The filter 4 employs a SiC filter (capacity 25 mL) having a cell wall thickness of 12 mil (30.48 × 10 ⁻² mm) and 300 cells per square inch (645.16 mm ² ). As a curate combustion catalyst, ZrCeNd composite oxide (ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 72: 24: 4 (molar ratio)) was supported. The loading was 40 g / L.

  The NOx purification catalyst 5 has a structure in which two upper and lower catalyst layers are formed on a carrier (capacity 25 mL) having the same cell wall thickness and the same number of cells as the HC oxidation catalyst 3. The lower layer forms a base layer by mixing the activated alumina carrying the catalytic metal Rh and the CePr composite oxide, and the upper layer forms a base layer from the activated alumina carrying the catalytic metal Rh. A solution of Pt as a metal and each solution of Ba, Sr, Mg, and K as a NOx storage material were impregnated, dried and fired, and supported.

The amount of each component supported is 135 g / L for activated alumina and CePr composite oxide for the lower layer, 0.1 g / L for Rh, 50 g / L for activated alumina for the upper layer, 0.2 g / L for Rh, Regarding the components impregnated and supported on both base layers, Pt is 3.5 g / L, Ba is 30 g / L, Sr is 10 g / L, Mg is 10 g / L, and K is 6 g / L. A CePr composite oxide having a composition of CeO ₂ : Pr ₂ O ₃ = 95: 5 (molar ratio) was used.

The CO oxidation catalyst 6 was dried and fired after impregnating and supporting Pt and Pd as catalytic metals on a powder obtained by mixing activated alumina and ZrCeNd composite oxide, and was washed on the same honeycomb carrier as the HC oxidation catalyst 3. The amount of each component supported is 120 g / L for activated alumina, 80 g / L for ZrCeNd composite oxide, 2 g / L for Pt, and 0.5 g / L for Pd. The activated alumina was employed _Al 2 _{O 3} containing _La 2 _{O 3} 5 wt%. A ZrCeNd composite oxide having a composition of ZrO ₂ : CeO ₂ : Nd ₂ O ₃ = 77: 19: 4 (molar ratio) was employed.

-Example 2-
As a particulate combustion catalyst supported on the filter 4, a catalyst (active alumina supported amount: 40 g / L, K supported amount; 10 g / L) in which activated alumina is loaded with K (alkali metal) as a catalytic metal is employed, The other configuration was the same as in Example 1.

-Example 3-
A ZrNd composite oxide (ZrO ₂ : Nd ₂ O ₃ = 88: 12 (molar ratio), supported amount; 40 g / L) was adopted as the particulate combustion catalyst supported on the filter 4, and the others were the same as in Example 1. The configuration.

Example 4
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrCeNd composite oxide as in Example 1 (ZrCeNd composite oxide supported amount; 40 g / L, Pt supported amount; 0.1 g) / L), and the other configuration was the same as in Example 1.

-Example 5
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting K and Pt as catalytic metals on the same active alumina as in Example 2 (active alumina supported amount: 40 g / L, K supported amount; 10 g / L, Pt) The amount was 0.1 g / L), and the other configuration was the same as in Example 1.

-Example 6
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrNd composite oxide as in Example 3 (ZrNd composite oxide supported amount; 40 g / L, Pt supported amount; 0.1 g) / L), and the other configuration was the same as in Example 1.

-Comparative Example 1-
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrCeNd composite oxide as in Example 1 (ZrCeNd composite oxide supported amount; 40 g / L, Pt supported amount; 0.2 g) / L), and the other configuration was the same as in Example 1.

-Comparative Example 2-
As a particulate combustion catalyst supported on the filter 4, a catalyst (active alumina supported amount: 40 g / L, Pt supported amount; 0.2 g / L) obtained by supporting Pt as a catalytic metal on the same active alumina as in Example 2 was used. The other configurations were the same as those in Example 1.

-Comparative Example 3-
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrNd composite oxide as in Example 3 (supported amount of ZrNd composite oxide; 40 g / L, supported amount of Pt; 0.2 g) / L), and the other configuration was the same as in Example 1.

-Comparative Example 4-
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrCeNd composite oxide as in Example 1 (supported amount of ZrCeNd composite oxide; 40 g / L, Pt supported amount; 2 g / L). The other configurations are the same as those in the first embodiment.

-Comparative Example 5-
As the particulate combustion catalyst supported on the filter 4, a catalyst (active alumina supported amount: 40 g / L, Pt supported amount; 2 g / L) obtained by supporting Pt as a catalyst metal on the same active alumina as in Example 2 is used. The other configurations are the same as those in the first embodiment.

-Comparative Example 6
As a particulate combustion catalyst supported on the filter 4, a catalyst obtained by supporting Pt as a catalyst metal on the same ZrNd composite oxide as in Example 3 (ZrNd composite oxide supported amount; 40 g / L, Pt supported amount; 2 g / L) The other configurations are the same as those in the first embodiment.

[NOx storage capacity of NOx purification catalyst]
The NOx storage capacities of the NOx purification catalysts 5 of Examples 1 to 6 and Comparative Examples 1 to 6 were measured. That is, the gas temperature was raised until the catalyst inlet temperature reached a predetermined measurement temperature while flowing the air-fuel ratio lean gas shown in Table 1 through the NOx purification catalyst 5. After the catalyst inlet temperature was stabilized, the air-fuel ratio rich gas shown in Table 1 was switched to flow through the NOx purification catalyst 5 for 5 minutes. Thereafter, the gas was switched to the air-fuel ratio lean gas shown in Table 1, and the lean gas was continuously supplied to the catalyst 5 until the NOx concentration of the gas flowing out from the NOx purification catalyst 5 was saturated. The NOx concentration difference between the inlet and outlet of the NOx purification catalyst 5 after switching to the air-fuel ratio lean gas until the NOx concentration is saturated is integrated to obtain the NOx occlusion amount of the NOx purification catalyst 5 at the measured temperature.

  The NOx purification catalyst 5 was preliminarily aged at a temperature of 750 ° C. for 24 hours in an air atmosphere. The SV value of the gas flowing through the NOx purification catalyst 5 is 80000 / h for both lean and rich. There are three measurement temperatures: 300 ° C, 400 ° C, and 500 ° C. The results are shown in FIG. From the figure, it can be seen that the NOx occlusion amount that can be occluded by the NOx purification catalyst 5 is 1 g or more per liter of carrier in the temperature range of 300 ° C to 500 ° C.

[Measurement of CO production ratio and NOx purification rate]
With respect to Examples 1 to 6 and Comparative Examples 1 to 6, while 5 g / L of carbon black was deposited on the filter 4, while the air-fuel ratio lean gas shown in Table 1 was allowed to flow, the filter inlet temperature reached 500 ° C. The gas temperature was raised. Next, an operation of repeating the cycle of flowing the air-fuel ratio rich gas shown in Table 1 for 60 seconds, then switching to the lean gas shown in Table 1 and flowing it for 60 seconds is performed, and flows out from the filter 4 when the rich gas flows. While measuring the CO amount and CO ₂ amount in the gas, the NOx amount in the gas flowing out from the NOx purification catalyst 5 during the rich gas flow was measured. The gas flow rate is 33 L / min for both, and the SV value is 80000 / h for both the filter 4 and the NOx purification catalyst 5.

Next, in the state where carbon black is not deposited on the filter 4, the same gas flow operation is performed to measure the amount of CO and CO ₂ in the gas flowing out from the filter 4 during the rich gas flow, and NOx during the rich gas flow. The amount of NOx in the gas flowing out from the purification catalyst 5 was measured.

  In any of Examples 1 to 6 and Comparative Examples 1 to 6, the filter 4 is pre-aged to be kept at a temperature of 800 ° C. for 24 hours in an air atmosphere, and the HC oxidation catalyst 3, the NOx purification catalyst 5, and the CO oxidation Catalyst 6 was pre-aged in air at a temperature of 750 ° C. for 24 hours.

Then, by subtracting the measured values of the CO amount and CO ₂ amount in the non-deposited state of carbon black from the measured values of the CO amount and CO ₂ amount in the carbon black deposition state, the CO amount generated by the combustion of the carbon black And CO ₂ amount is calculated, and the CO production ratio (ratio of CO amount to total amount of CO amount and CO ₂ amount CO / (CO + CO ₂ )) at the filter inlet temperature of 500 ° C. is calculated from these values. It calculated about each of 1-6 and Comparative Examples 1-6. Further, the CO production ratio at the filter inlet temperature of 550 ° C. was determined in the same manner.

  Further, based on the measured value of the NOx amount in the carbon black deposition state, the NOx purification rate at the time of rich at the filter inlet temperature of 500 ° C. was calculated. Similarly, based on the measured value of the amount of NOx in the non-deposited state of carbon black, the NOx purification rate when rich at a filter inlet temperature of 500 ° C. was calculated. Further, the NOx purification rate at the time of rich at the filter inlet temperature of 550 ° C. was obtained by the same method.

[Measurement results of CO production ratio and NOx purification rate]
Table 2 shows the measurement results of the CO production ratio and the NOx purification rate.

  In each of Examples 1 to 6, the CO production ratio is 10% or more, and it can be seen that a relatively large amount of CO is produced when particulates are burned by the particulate combustion catalyst of the example. In contrast, in Comparative Examples 1 to 6, the CO production ratio decreases as the amount of Pt supported increases. From Examples 1 to 3, ZrCeNd composite oxide, activated alumina supporting K, and ZrNd composite oxide are all effective as particulate combustion catalysts that generate a relatively large amount of CO even when Pt is not supported. It can be seen from Examples 4 to 6 and Comparative Examples 1 to 6 that the Pt supported amount of the particulate combustion catalyst is preferably suppressed to 0.1 g / L or less.

  In terms of the NOx purification rate when carbon is deposited, all of the examples are higher than the comparative example. The NOx purification rate generally increases as the CO production ratio increases, and it can be seen that the generation of CO by the particulate combustion catalyst in the filter 4 is effective for the reduction and purification of NOx by the NOx purification catalyst 5. In the case of the example, it can be said that the difference between the NOx purification rate without carbon deposition and the NOx purification rate in the carbon deposition state substantially corresponds to the improvement effect of the NOx purification rate due to CO generation in the filter 4.

<About CO oxidation catalyst 6>
The exhaust emission control devices of the following Example 7 and Comparative Example 7 were configured, and the effects of providing the CO oxidation catalyst 6 were confirmed in comparison with the previous Example 2.

-Example 7-
Instead of the NOx purification catalyst 5 and the CO oxidation catalyst 6 of the second embodiment, a one-bed type in which the NOx purification catalyst and the CO oxidation catalyst are arranged in parallel in the exhaust flow direction on one honeycomb carrier is adopted, and the other is the second embodiment. It was the same composition as. As the honeycomb carrier, a cell wall thickness of 3.5 mil (8.89 × 10 ⁻² mm), cordierite made of cordierite with 600 cells per square inch (645.16 mm ² ) (capacity: 25 mL) is adopted, A NOx purification catalyst was constructed in the range of 3/4 of the carrier length from the upstream end of the carrier, and a CO oxidation catalyst was constructed in the remaining 1/4 of the downstream side.

  That is, the lower base layer in which the active alumina supporting the catalytic metal Rh and the CePr composite oxide are mixed within the range of 3/4 of the carrier length from the upstream end of the carrier is formed, and the active metal supporting the catalytic metal Rh is formed thereon. An upper base layer made of alumina was formed. Pt, Rh, Ba, Sr, Mg, and K solutions were impregnated in order to form the NOx purification catalyst only in both the base layers. The remaining downstream quarter of the carrier is dried and fired after impregnating and supporting Pt and Pd as catalyst metals in a powder mixed with activated alumina and ZrCeNd composite oxide to form a CO oxidation catalyst. Washed the dishes.

  The supported amount of each of the active alumina and CePr composite oxide in the lower base layer is 135 g / L, and the active alumina supported amount in the upper base layer is 50 g / L. The supported amount of Pt constituting the NOx purification catalyst is 4.7 g / L, the supported amount of Rh on the lower base layer is 0.1 g / L, the supported amount of Rh on the upper base layer is 0.3 g / L, Ba The supported amount of is 40 g / L, the supported amount of Sr is 13.3 g / L, the supported amount of Mg is 13.3 g / L, and the supported amount of K is 8 g / L. The amount of each component constituting the CO oxidation catalyst is 120 g / L for activated alumina, 80 g / L for ZrCeNd composite oxide, 4 g / L for Pt, and 1 g / L for Pd.

-Comparative Example 7-
In Comparative Example 5, the CO oxidation catalyst 6 was not provided.

[Measurement of CO concentration]
Under the same measurement conditions (filter inlet temperature 500 ° C.) as the measurement of the rich NOx purification rate in the previous carbon black deposition state, for Example 2, each CO concentration at the inlet and outlet of the CO oxidation catalyst 6, and for Example 7 The CO concentration at the outlet of the CO oxidation catalyst and the CO concentration at the outlet of the NOx purification catalyst were measured for Comparative Example 7 where the CO oxidation catalyst 6 was not provided. In any case, the filter 4 was pre-aged at a temperature of 800 ° C. for 24 hours in an air atmosphere, and the other catalysts were pre-aged to be kept at a temperature of 750 ° C. for 24 hours in an air atmosphere. The results are shown in Table 3.

  In Example 2, as can be seen from Table 2, the CO production ratio by the particulate combustion catalyst is large. According to Table 3, the CO concentration at the CO oxidation catalyst inlet of Example 2 is 189 ppm. This is due to the CO that has passed through the NOx purification catalyst 5 without being used for NOx reduction purification. Is. Thus, even when excess CO is generated by the particulate combustion catalyst, the CO concentration at the CO oxidation catalyst outlet of Example 2 is as low as 6 ppm. As can be seen from the CO production ratio of Comparative Example 5 in Table 2, Comparative Example 7 is a case where the amount of CO produced by particulate combustion in the filter is originally small. The concentration is lower than the CO concentration at the outlet of the NOx purification catalyst of Comparative Example 7. Therefore, when a particulate combustion catalyst having a large CO production ratio is adopted for the filter 4, it is preferable to provide a CO oxidation catalyst downstream of the NOx purification catalyst, thereby suppressing CO emission into the atmosphere. It turns out that there is a big effect.

  Example 7 is a one-bed type in which a NOx purification catalyst and a CO oxidation catalyst are provided on one honeycomb carrier, and the CO concentration at the CO oxidation catalyst outlet is 9 ppm. This Example 7 is also a case where the CO production ratio by the particulate combustion catalyst is large as in Example 2. However, even in the case of the one-bed type, by providing the CO oxidation catalyst, it is possible to effectively discharge CO into the atmosphere. It turns out that it can suppress.

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust passage 3 HC oxidation catalyst 4 Filter with particulate combustion catalyst 5 NOx purification catalyst 6 CO oxidation catalyst 8 Fuel injection valve 9 Controller 11 Exhaust pressure sensor 12 Exhaust pressure sensor 13 NOx sensor

Claims (8)

An exhaust emission control device for a lean combustion engine,
In the exhaust passage of the engine, an oxidation catalyst that oxidizes HC in the exhaust, a filter that collects particulates in the exhaust, and a NOx purification catalyst that contains a NOx storage material that stores NOx in the exhaust are exhausted. Arranged in order from the upstream side in the flow direction,
The filter carries a catalyst for burning the collected particulates, and the catalyst occupies the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. The proportion of CO is 10% or more in molar ratio,
The exhaust gas purification apparatus, wherein the NOx purification catalyst contains a catalyst component that reduces and purifies NOx using CO generated by burning particulates in the filter as a reducing agent. In claim 1,
The catalyst supported on the filter contains at least one selected from the group consisting of Ce-containing oxides, complex oxides containing rare earth metals other than Ce and Zr, and alumina supporting alkali metals, The exhaust gas purifying apparatus is characterized in that the amount of the catalyst noble metal per 1 L of the filter is 0 g or 0.1 g or less. In claim 1 or claim 2,
An exhaust gas purification apparatus, wherein a CO oxidation catalyst for oxidizing CO is provided in the exhaust passage downstream of the NOx purification catalyst. In claim 3,
The exhaust gas purification apparatus, wherein the CO oxidation catalyst contains Pd as a catalyst metal. In any one of Claims 1 thru | or 4,
When burning the particulates collected by the filter, after the main injection in which fuel is injected and supplied to the combustion chamber of the engine near the top dead center of the compression stroke, the fuel is injected and supplied in the expansion stroke or exhaust stroke. And the unburned fuel is supplied as the HC to the oxidation catalyst upstream of the filter by the post-injection. In the exhaust passage of the lean combustion engine, an oxidation catalyst that oxidizes HC in the exhaust and a filter that collects particulates in the exhaust are disposed in order from the upstream side in the exhaust flow direction,
In the exhaust gas purification method in which the particulate matter trapped in the filter is burned by oxidizing the HC in the oxidation catalyst and increasing the temperature of the filter by its reaction heat.
A NOx purifying catalyst containing a NOx occlusion material that occludes NOx in the exhaust and a catalyst component for reducing and purifying NOx is disposed in the exhaust passage on the downstream side of the exhaust flow from the filter,
In order to burn the collected particulates in the filter, the proportion of CO in the total amount of CO and CO ₂ produced when carbon is burned at a temperature of 500 ° C. is 10% or more in molar ratio. A catalyst is supported,
An exhaust purification method comprising reducing and purifying NOx occluded in the NOx purification catalyst, using CO generated by combustion of particulates in the filter as a reducing agent. In claim 6,
When the NOx occlusion amount of the NOx purification catalyst reaches a predetermined value, the fuel is injected and supplied in the expansion stroke or the exhaust stroke after the main injection injecting the fuel into the combustion chamber of the engine near the top dead center of the compression stroke. An exhaust purification method comprising: supplying unburned fuel as the HC to the oxidation catalyst by performing post-injection, and increasing the temperature of the filter by oxidation reaction heat of the unburned fuel. In claim 6 or claim 7,
An exhaust gas characterized in that a CO oxidation catalyst for oxidizing CO is disposed in the exhaust passage downstream of the NOx purification catalyst, and CO passing through the NOx purification catalyst is oxidized to CO ₂ by the CO oxidation catalyst. Purification method.

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