JP2008002451A - Exhaust emission control device for diesel engine and exhaust emission control method for diesel engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that an Ag catalyst can not reduce NOx discharged from a diesel engine since reaction intermediate remains and the remaining reaction intermediate is re-oxidized by action of an oxidation catalyst 7 although the Ag catalyst 5 which is a selective contact reduction type catalyst durable against SO<SB>2</SB>in exhaust gas of the diesel engine selectively oxidizes nitrogen oxide (NOx) under existence of hydrocarbon compound (HC). <P>SOLUTION: A reaction intermediate conversion catalyst 6 carrying one or more metals selected out of Cu, V and Fe is arranged in a middle of the Ag catalyst 5 and the oxidation catalyst 7 in a catalytic converter 2 of an exhaust emission control device 1. The reaction intermediate is decomposed by action of the reaction intermediate conversion catalyst 6 and N<SB>2</SB>or the like is formed. Quantity of remaining reaction intermediate is minimized by action of the reaction intermediate conversion catalyst 6 and NOx discharged from the diesel engine can be reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification device and an exhaust gas purification method that effectively reduce harmful nitrogen oxides contained in exhaust gas of a diesel engine and discharge them as nitrogen gas.

Diesel engines emit relatively little carbon dioxide, a greenhouse gas. Therefore, the use of a vehicle equipped with a diesel engine is effective in mitigating global warming. However, diesel engines cause acid rain and photochemical smog and emit relatively large amounts of nitrogen oxides (hereinafter referred to as “NOx”) such as NO and NO ₂ that are harmful to the human body. Therefore, a catalytic system based on selective catalytic reduction (hereinafter referred to as “SCR”) has been developed in order to reduce and remove NOx in the exhaust gas of a diesel engine.

In the SCR catalyst system for reducing and removing NOx described above, NOx contained in the exhaust gas is reduced using a reducing agent. One such SCR catalyst system is an SCR catalyst system that converts urea to ammonia by the action of a catalyst and reduces and removes NOx by this ammonia. Another SCR catalyst system for reducing and removing NOx is based on unburned hydrocarbons (hereinafter referred to as “HC”) in the exhaust gas of a diesel engine or HC in light oil added to the exhaust gas. This is an SCR catalyst system that reduces and removes NOx by the action of a selective reduction catalyst.
JP 09-57063 A JP 09-57064 A

  However, in order to implement the above-described SCR catalyst system for reducing and removing NOx using urea, it is necessary to newly provide a tank for storing urea in the vehicle and to replenish urea. This method is difficult to implement because a supply infrastructure is required.

On the other hand, the SCR catalyst system for reducing and removing NOx by HC has the following problems. An SCR catalyst system that reduces and removes NOx by HC, and promotes the reaction of reducing NOx in the presence of HC. In the case of SCR catalyst, In (indium), Sn (tin), Ag (silver) exhibiting high activity in the presence of moisture. ), Zn (zinc) and other oxides supported on aluminum oxide (hereinafter referred to as “alumina”) represented by the chemical formula Al ₂ O ₃ . Among such SCR catalysts, an SCR catalyst that is generated from sulfur contained in light oil and has durability against SO ₂ contained in exhaust gas is an SCR catalyst that supports Ag on aluminum oxide (hereinafter referred to as “SCR catalyst (hereinafter“ Therefore, from the viewpoint of durability, the Ag catalyst is preferable for the SCR catalyst system that reduces and removes NOx by HC.

However, the Ag catalyst has a problem that the efficiency of selectively reducing NOx is relatively low. This problem will be described with reference to FIG. 11 and FIG. FIG. 11 is a diagram illustrating a reaction process in which NOx contained in exhaust gas of a diesel engine is selectively reduced and removed by the action of an Ag catalyst. FIG. 12 is a diagram for explaining the HC oxidation reaction that occurs on the Ag metal of the Ag catalyst. It is considered that NOx is reduced to nitrogen gas (hereinafter referred to as “N ₂ ”) through a two-stage reaction by the action of the Ag catalyst in the presence of HC. First, NOx is reduced to R-NCO, R-NH ₂ (R is a hydrocarbon group) or the like which is a reaction intermediate (reaction intermediate formation reaction). This reaction intermediate formation reaction occurs on Ag present on the acid sites (O ⁻ ) of alumina in the Ag catalyst. However, when Ag in the Ag catalyst is present as Ag metal other than on the acid sites (O ⁻ ) of alumina supporting Ag, the HC oxidation reaction is promoted on the Ag metal as shown in FIG. The reaction intermediate formation reaction described above hardly proceeds.
Reaction intermediates such as R—NCO and R—NH ₂ produced by the above reaction intermediate formation reaction are present on the acid sites (O ⁻ ) of alumina in the Ag catalyst from NO ₂ as shown in FIG. It reacts with NO ₃ ⁻ (nitrate ion) produced on Ag ions, is oxidized and decomposed, and NO ₃ ⁻ is reduced to produce N ₂ , H ₂ O, CO _{2 and the} like (reaction) Intermediate decomposition reaction).

However, when an Ag catalyst is used, the reaction intermediate formation reaction proceeds rapidly, but the subsequent reaction intermediate decomposition reaction proceeds slowly. Therefore, the above-mentioned reaction intermediate remains in the exhaust gas that has passed through the Ag catalyst. On the other hand, in an exhaust gas purification device for a diesel engine, an oxidation catalyst is provided downstream of the SCR catalyst in the exhaust gas flow path in order to remove unburned HC, carbon monoxide (CO), and the like. Accordingly, the exhaust gas that has passed through the Ag catalyst, including the remaining reaction intermediate, further passes through the oxidation catalyst. At that time, since N ₂ is stable, it does not oxidize even if it passes through the oxidation catalyst, but the remaining reaction intermediate is reoxidized and returned to NOx. This is because the reaction intermediate is not stable. Therefore, in the SCR catalyst system using the Ag catalyst, conventionally, the NOx purification rate cannot be sufficiently increased.
Therefore, the present invention provides a diesel engine exhaust gas purification device that reduces the remaining of reaction intermediates and increases the NOx purification efficiency in a diesel engine exhaust gas purification device that uses an Ag catalyst as an SCR catalyst. For the purpose.

In order to solve the above-mentioned problems, the invention according to claim 1 is characterized in that an Ag catalyst is disposed in the exhaust gas discharge passage, and an oxidation catalyst is disposed downstream of the Ag catalyst in the exhaust gas discharge passage. In the exhaust gas purifying apparatus for a diesel engine, the washcoat layer (hereinafter referred to as “WC layer”) constituting the Ag catalyst carries a porous amount of 0.01% to 20% by weight of Ag. It is a structure, a reaction intermediate purification catalyst is disposed between the Ag catalyst and the oxidation catalyst, and a WC layer constituting the reaction intermediate purification catalyst is made of copper (hereinafter referred to as “Cu”). Alumina supporting at least one element selected from vanadium (hereinafter referred to as “V”) and iron (hereinafter referred to as “Fe”) in an amount of 0.01% or more and 20% or less by weight. Characterized in that it is constituted by a porous structure made of zeolite.
According to the first aspect of the present invention, the reaction intermediate remaining in the exhaust gas after passing through the Ag catalyst can be decomposed by the catalytic action of Cu, V or Fe. Cu carried in the WC layer has a valence varying between +1 and +2, and oxidizes NOx in the exhaust gas. As a result, NO ₃ ⁻ is generated on the Cu surface of the WC layer. When this NO ₃ ⁻ reacts with a reaction intermediate such as R—NCO or R—NH ₂ (hereinafter referred to as “reaction intermediate”), the reaction intermediate is oxidized and decomposed, and NO ₃ ⁻ is reduced. As a result, N _{2 and the} like are generated. Both V and Fe supported in the WC layer have valences varying between +2 and +3, so that NO ₃ ⁻ is similarly generated on the surface. Therefore, the reaction intermediate is oxidized and decomposed as in the case of Cu.

In order to solve the above-mentioned problem, the invention according to claim 2 is the exhaust gas purifying apparatus according to claim 1, wherein the porous structure carrying the Ag is oxidized by the chemical formula Al ₂ O _3. It is made of aluminum (hereinafter referred to as “alumina”). In the present invention, the porous structure is preferably made of γ-Al ₂ O ₃ having a cubic spinel crystal structure.
According to the invention described in claim 2, the porous structure supporting the Ag catalyst is made of alumina which is stable at a high temperature and has a large surface area. Therefore, the ratio of Ag that is dispersed and supported in the alumina porous structure and actually functions as a catalyst is relatively high. Therefore, the catalytic function of the Ag catalyst is enhanced.

In order to solve the above-mentioned problem, the invention according to claim 3 is the exhaust gas purifying apparatus for diesel engine according to claim 2, wherein the porous structure carrying the Ag is mainly of the chemical formula AlOOH · H ₂ O. Γ-Al ₂ O ₃ produced from boehmite represented by the following (hereinafter referred to as “boehmite”).
According to the third aspect of the present invention, the interaction with Ag is stronger than ordinary alumina. As a result, the catalytic function of the Ag catalyst becomes higher.

In order to solve the above-mentioned problems, the invention described in claim 4 is characterized in that the γ-Al ₂ O ₃ is produced by firing boehmite carrying Ag.
According to the fourth aspect of the present invention, since the ratio of Ag in the Ag catalyst that exists as an Ag ion cluster state is increased, NOx can be selectively reduced and purified even with a small amount of HC.

In order to solve the above-mentioned problems, the invention according to claim 5 is the exhaust gas purification apparatus for diesel engines according to any one of claims 1 to 4, wherein the reaction intermediate purification catalyst is configured. The WC layer is made of zeolite carrying Cu in an amount of 0.01% or more and 20% or less by weight. Here, zeolite is a silicate formed by connecting basic units of (SiO ₄ ) ^4- and (AlO ₄ ) ^5- having a tetrahedral structure, and these basic units are apexes of the tetrahedral structure. This is a structure sharing an oxygen atom located at.
According to the invention described in claim 5, NOx can be purified by reacting the reaction intermediate and NO ₃ ⁻ efficiently by the action of Cu supported on the zeolite.

In order to solve the above-mentioned problems, the invention described in claim 6 is characterized in that in the exhaust gas purifying apparatus for a diesel engine described in claim 5, the zeolite supporting Cu is MFI type. Here, the MFI type zeolite is one in which the basic unit of the zeolite forms a ring structure, and the number of oxygen atoms contained in the ring structure is ten.
According to the sixth aspect of the present invention, due to the effect of the pores of the MFI type zeolite, the action of the Cu catalyst for purifying NOx by reacting the reaction intermediate with NO ₃ ⁻ can be enhanced.

In order to solve the above-mentioned problems, the invention according to claim 7 is the exhaust gas purification apparatus for diesel engines according to any one of claims 1 to 4, wherein the reaction intermediate purification catalyst is configured. The WC layer that carries at least one element selected from Cu, V, and Fe in an amount of 0.01% to 20% by weight is supported from γ-Al ₂ O ₃ manufactured from boehmite. It is comprised from the porous structure which becomes.
According to the invention described in claim 7, Cu supported on γ-Al ₂ O ₃ prepared from boehmite, by the action of Fe or V, efficient reaction intermediates and NO ₃ ^- by reacting, NOx can be purified.

In order to solve the above-mentioned problem, an invention described in claim 8 is directed to an exhaust gas purifying apparatus for a diesel engine according to any one of claims 1 to 7, wherein the reaction intermediate purification catalyst is The volume ratio with respect to the Ag catalyst is 20% or more and 100% or less.
According to the eighth aspect of the present invention, a reaction intermediate purification catalyst having a capacity sufficient to decompose the unreacted reaction intermediate is ensured. Further, since the reaction intermediate purification catalyst acts mainly on the unreacted reaction intermediate, it is not necessary to increase the capacity of the reaction intermediate purification catalyst more than necessary.

In order to solve the above-mentioned problems, an invention according to claim 9 is a method for purifying exhaust gas of a diesel engine, and the exhaust gas carries Ag in an amount of 0.01% or more and 20% or less by weight. Purified with an Ag catalyst composed of a WC layer composed of a porous structure that supports at least one element selected from Cu, V, and Fe in an amount of 0.01% to 20% by weight And purifying with a reaction intermediate purification catalyst composed of a WC layer made of a porous structure made of alumina or zeolite, and then purifying with an oxidation catalyst.
According to the ninth aspect of the invention, NOx in the exhaust gas is efficiently reduced to N ₂ by the action of the Ag catalyst and the reaction intermediate purification catalyst, and unburned HC is removed by the action of the oxidation catalyst. Can be oxidized and removed.

According to the exhaust gas purification device for diesel engines (hereinafter referred to as “exhaust gas purification device”) and the exhaust gas purification method for diesel engines (hereinafter referred to as “exhaust gas purification method”) of the present invention, NOx contained in can be efficiently reduced to N ₂ . Therefore, it is possible to reduce the remaining unstable reaction intermediate that is reoxidized by the action of the oxidation catalyst. As a result, the amount of NOx discharged from the diesel engine can be reduced as compared with the conventional case.

  The best mode for carrying out the present invention (hereinafter referred to as “embodiment”) will be described with reference to FIGS. 1A and 1B are diagrams illustrating an exhaust gas purification device according to an embodiment, in which FIG. 1A is a schematic diagram of an exhaust gas purification layer, and FIG. 1B is an enlarged cross-sectional view of a catalyst held by a catalytic converter constituting the exhaust gas purification device. FIG. FIG. 2 is a view showing the structure of the plasma generation unit used in the exhaust gas purification apparatus of the present embodiment.

  First, the structure of the exhaust gas purification apparatus 1 of the embodiment shown in FIG. The exhaust gas purification device 1 includes an exhaust pipe 4, a plasma generation unit 3, and a catalytic converter 2. The plasma generation unit 3 is mainly for oxidizing and purifying particulate matter (hereinafter referred to as “PM”) contained in exhaust gas discharged from a diesel engine by the action of plasma. The catalytic converter 2 purifies NOx, unburned HC, etc. contained in the exhaust gas discharged from the diesel engine. The exhaust pipe 4 is joined to an exhaust manifold (not shown). Exhaust gas discharged from each cylinder of the diesel engine flows into the exhaust pipe 4 via the exhaust manifold.

The exhaust gas discharged from the diesel engine first passes through the plasma generation unit 3 and is sent to the catalytic converter 2 after PM is purified. As shown in FIG. 2, the plasma generating unit 3 includes electrodes 30 made of a metal electrode 31b covered on one side with a dielectric 31a with a predetermined interval therebetween. A gap between the electrodes 30 serves as a plasma generation unit 33. When an alternating high voltage is applied between the electrodes 30 by the high voltage power supply 32, plasma is generated in the plasma generation unit 33 that is a gap between the electrodes 30. PM is oxidized and purified by the plasma excited species generated by the plasma. At the same time, NOx contained in the exhaust gas is oxidized to NO ₂ by the plasma excited species.

  In the present embodiment, the plasma generation unit 3 is used to purify PM, but the embodiment of the present invention is not limited to this. For example, a diesel particulate filter (hereinafter referred to as “DPF”) may be provided downstream of the catalytic converter 2 in the exhaust gas flow path instead of the plasma generation unit 3. The DPF is made of a ceramic porous filter, and separates and collects PM from exhaust gas passing therethrough.

  The exhaust gas from which PM in the exhaust gas is purified by the plasma generation unit 3 is sent to the catalytic converter 2. In the catalytic converter 2, an Ag catalyst 5, a reaction intermediate purification catalyst 6 and an oxidation catalyst 7 are arranged in this order from the upstream side of the exhaust pipe 4 which is an exhaust gas flow path (FIG. 1 (a )reference). Each catalyst of the Ag catalyst 5, the reaction intermediate purification catalyst 6, and the oxidation catalyst 7 has a so-called honeycomb structure, and a large number of cells 25 partitioned by the honeycomb 24 are formed. A corresponding WC layer 20 is formed on the honeycomb 24 (see FIG. 1B). The exhaust gas passes through the cell 25. At that time, the reduction reaction or oxidation reaction for exhaust gas purification is promoted by the catalytic WC layer 20.

  HC, that is, light oil, is added to the catalytic converter 2 as necessary. That is, the HC concentration of the catalytic converter 2 is monitored by a sensor (not shown), and when the unburned HC is sufficiently contained in the exhaust gas, HC is not added. However, when the HC concentration of the catalytic converter 2 is low, HC is added. In the present invention, since NOx is reduced in the presence of HC, the HC concentration of the catalytic converter 2 must always be a predetermined value or more.

In the exhaust gas purifying apparatus 1 of the present embodiment, the Ag catalyst 5 mainly promotes a reaction in which unburned HC reduces NOx and a reaction intermediate is generated (see FIG. 11). Ag contained in the WC layer 20 of the Ag catalyst 5 has the catalytic action. The WC layer 20 of the Ag catalyst 5 is made of a porous structure carrying Ag in an amount of 0.01% or more and 20% or less by weight. Here, although the material which comprises this porous structure is not limited, the thing comprised from an alumina is desirable. In particular, those composed of γ-Al ₂ O _{3 having} a cubic spinel structure are desirable. Since alumina is stable at high temperatures and has a large specific surface area, it is desirable that Ag is supported on the porous alumina structure in that the ratio of Ag exposed to the surface and functioning as a catalyst is relatively high. In particular, since γ-Al ₂ O ₃ formed using boehmite as a starting material has a strong interaction with Ag, the ratio of Ag functioning as a catalyst out of Ag in the Ag catalyst 5 is high. As a result, the reduction reaction of NOx is promoted.

Here, the WC layer 20 constituting the Ag catalyst 5 manufactured using boehmite as a starting material differs in the NOx purification effect as an Ag catalyst depending on the preparation method. The WC layer 20 constituting the Ag catalyst 5 manufactured using boehmite as a starting material has the following two types of preparation methods. One of the preparation methods is to produce from an Ag catalyst powder made of γ-Al ₂ O ₃ produced by firing boehmite supporting Ag. Another preparation method is to produce from Ag catalyst powder in which Ag is supported on γ-Al ₂ O ₃ produced by calcining boehmite. In the present invention, the former preparation method is desirable. The reason for this is that, in the case of the WC layer 20 constituting the Ag catalyst 5 manufactured by this preparation method, among the Ag contained in the WC layer 20, Ag ions are aggregated into a cluster state (Ag ^This is because the ratio of those existing as ( ^{δ +} ) is relatively high.

As shown in FIG. 11, NOx is converted to a reaction intermediate by HC contained in the exhaust gas and purified on the Ag element. This reaction intermediate formation reaction proceeds relatively efficiently when the Ag element supported on the alumina in the Ag catalyst 5 is Ag on the acid point of the alumina. Of the Ag supported on the alumina in the Ag catalyst, all that are present as Ag ions are on the acid sites of the alumina. Therefore, the progress of the reaction intermediate formation reaction is promoted on the Ag ion clusters (Ag ^{δ +} ) of the WC layer 20 described above.
On the other hand, as shown in FIG. 12, when the Ag element supported on the alumina in the Ag catalyst 5 is in a metal state where it is not on the acid point of the alumina, the oxidation reaction due to the combustion of HC is promoted on the Ag metal. , HC is not effectively used in the reaction intermediate formation reaction. As a result, the purification of NOx by the reaction intermediate formation reaction and the subsequent reaction intermediate decomposition reaction shown in FIG. 11 is difficult to proceed. In the case of the Ag catalyst 5 manufactured from the Ag catalyst powder in which Ag is supported on γ-Al ₂ O ₃ manufactured by firing boehmite, the Ag metal ratio in the Ag supported on the alumina in the WC layer 20 is Relatively high. Therefore, the ratio of Ag metal not on the acid point of alumina of the WC layer 20 is also higher than that of the Ag catalyst 5 manufactured from Ag catalyst powder made of γ-Al ₂ O ₃ manufactured by firing boehmite supporting Ag. high. Accordingly, when the Ag catalyst 5 was prepared from the Ag catalyst powder of _γ-Al 2 _{O 3} which is prepared by calcining a boehmite carrying Ag, loading Ag on _γ-Al 2 _{O 3} which is prepared by calcining a boehmite Compared with the Ag catalyst 5 produced from the Ag catalyst powder thus produced, the reaction intermediate formation reaction can be advanced with a smaller amount of HC.

In the exhaust gas purification apparatus 1 according to the embodiment of the present invention, the amount of Ag supported on the Ag catalyst 5 is 0.01% or more and 20% or less with respect to the weight of the WC layer 20 as described above. . For example, when the amount of Ag in this range is supported on alumina, among the Ags supported on the alumina surface, there are relatively many Ag ion clusters (Ag ^{δ +} ) in a high catalytic effect. The catalytic effect of the catalyst 5 as a whole is high. When the amount of Ag is less than 0.01% with respect to the weight of the WC layer 20, the amount of Ag is too small and the Ag catalyst 5 does not sufficiently function as a catalyst. Further, when the Ag amount exceeds 20% with respect to the weight of the WC layer 20, Ag aggregation occurs and Ag ion clusters (Ag ^{δ +} ) exhibiting a catalytic effect among Ag present on the surface of the WC layer 20. Since the number of the state is reduced and the number of the metallic state is increased, the Ag catalyst 5 cannot sufficiently exhibit the catalytic action.

In the exhaust gas purification apparatus 1 of the present embodiment, the reaction intermediate purification catalyst 6 promotes a reaction that decomposes the generated reaction intermediate that is generated by the action of the Ag catalyst 5. NO ₃ ⁻ produced from NOx on the metal supported on the reaction intermediate purification catalyst 6 reacts with the reaction intermediate to promote a reaction in which N ₂ or the like is produced. The WC layer 20 constituting the reaction intermediate purification catalyst 6 is made of alumina or zeolite containing one or more elements selected from Cu, V, and Fe in an amount of 0.01% to 20% by weight. It is constituted by a porous structure. The reason why the weight ratio of these elements is limited is based on the same reason as that of the Ag catalyst 5 described above. Here, the elements Cu, V and Fe have a catalytic action to promote the reaction intermediate decomposition reaction described above.

Here, when the reaction intermediate purification catalyst 6 supports only an element of Cu, the porous structure supporting Cu constituting the WC layer 20 is composed of zeolite or alumina. In the case of zeolite, it is particularly desirable to be MFI type zeolite. This is because the reaction intermediate decomposition reaction described above is further promoted by the effect of fine pores on the surface of the zeolite layer or the MFI type zeolite layer. Further, when the reaction intermediate purification catalyst 6 supports only an element of Fe or V, the porous structure supporting only the element of Fe or V constituting the WC layer 20 is preferably composed of alumina. . This is because the reaction intermediate decomposition reaction described above is further promoted. When the porous structure of the reaction intermediate purification catalyst 6 is composed of the above-described alumina, it is composed of the alumina porous structure of γ-Al ₂ O ₃ formed using boehmite as a starting material. Things are desirable. If manufactured by the same preparation method as the WC layer 20 of the Ag catalyst 5 described above, Cu, V or Fe supported on the WC layer 20 is in a state of exhibiting a high catalytic effect. As a result, the reaction intermediate decomposition reaction is promoted.

  The oxidation catalyst 7 is for accelerating a reaction that oxidizes unreacted HC, CO, and the like. The WC layer 20 of the oxidation catalyst 7 is made of a porous structure made of alumina or the like supporting a noble metal such as Pt.

The operation of the exhaust gas purification device 1 of the present embodiment is as follows. Exhaust gas discharged from each cylinder of the diesel engine is sent to the plasma generation unit 3 through the exhaust pipe 4. Passes through the plasma generating unit 3, PM contained in the exhaust gas while being cleaned by the plasma generating unit 3, NOx contained in the exhaust gas is oxidized into NO _2. The exhaust gas that has passed through the plasma generation unit 3 is sent to the catalytic converter 2. In the catalytic converter 2, NO ₂ in the exhaust gas reacts with unburned HC contained in the exhaust gas or HC contained in the added light oil by the catalytic action of the Ag catalyst 5 and is reduced to change into a reaction intermediate. To do. A part of the generated reaction intermediate is oxidized and decomposed by reacting with NO ₃ ⁻ generated from NO _{2 on} the Ag surface in the WC layer 20 of the Ag catalyst 5 by the catalytic action of the Ag catalyst 5. As a result, N _{2 and the} like are generated. The remaining reaction intermediate reacts with NO ₃ ⁻ generated from NO _{2 on} the Cu, V or Fe surface in the WC layer 20 of the reaction intermediate purification catalyst 6. As a result, N _{2 and the} like are generated. The amount of the remaining reaction intermediate is 0% or suppressed by the action of the reaction intermediate purification catalyst 6. Therefore, it is suppressed that the remaining reaction intermediate is reoxidized and returned to NOx by the catalytic action of the oxidation catalyst 7. Therefore, in the case of a diesel engine provided with the exhaust gas purification device 1 of the present embodiment, NOx emissions are less than conventional.

<Other Embodiments of the Present Invention>
The embodiment of the present invention is not limited to the embodiment shown in FIG. 1, and the constituent elements can be changed without departing from the gist thereof. FIG. 3 is a diagram showing an exhaust gas purification device according to another embodiment of the present invention, in which (a) is a schematic diagram of the exhaust gas purification device according to another embodiment of the present invention, and (b) is a diagram of the present invention. It is a cross-sectional enlarged view of the combined catalyst used in the exhaust gas purification apparatus of another embodiment.
In the catalytic converter 2a of the exhaust gas purification apparatus 1a according to another embodiment of the present invention shown in FIG. 3A, the Ag catalyst 5 and the reaction intermediate purification catalyst 6 of the exhaust gas purification apparatus 1 shown in FIG. 1 are combined. Thus, one composite catalyst 8a is formed. As shown in FIG. 3B, the reaction intermediate purification catalyst WC layer 22 and the Ag catalyst WC layer 21 are laminated with the Ag catalyst WC layer 21 as an upper layer on the honeycomb 24 forming the cells 25 of the composite catalyst 8a. A composite WC layer 20a is formed. In this case, the exhaust gas passing through the cell 25 is first catalyzed by the upper Ag catalyst WC layer 21, and NOx contained in the exhaust gas is reduced to N ₂ by the unburned HC through the above-described reaction intermediate. The Thereafter, the reaction intermediate remaining on the Ag catalyst WC layer 21 is catalyzed by the lower reaction intermediate purification catalyst WC layer 22 and reacts with NO ₃ ⁻ to generate N _{2 and the} like. In the exhaust gas purification device 1a, as in the exhaust gas purification device 1 shown in FIG. 1, the reaction intermediate purification catalyst WC layer 22 is composed of the Ag catalyst WC layer 21 and the oxidation catalyst if the exhaust gas is subjected to a catalytic action. 7 in the middle. Therefore, the exhaust gas purification device 1a has the same operations and effects as the exhaust gas purification device 1 shown in FIG.

  Further, the exhaust gas purification apparatuses 1b and 1c shown in FIGS. 4 and 5 are also included in the embodiment of the present invention. 4 and 5 are diagrams showing an exhaust gas purifying apparatus according to another embodiment of the present invention, in which (a) is a schematic view of the exhaust gas purifying apparatus according to another embodiment of the present invention, and (B) is a cross-sectional enlarged view of the combined catalyst used in the exhaust gas purifying apparatus of another embodiment of the present invention.

In the case of the catalytic converter 2b of the exhaust gas purification apparatus 1b of the embodiment of the present invention shown in FIGS. 4 (a) and 4 (b), the reaction intermediate purification catalyst 6 and the oxidation catalyst 7 of the exhaust gas purification apparatus 1 shown in FIG. The composite catalyst 8b is formed (see FIG. 4A). On the honeycomb 24 forming the cells 25 of the composite catalyst 8b, a composite WC layer 20b in which an oxidation catalyst WC layer 23 and a reaction intermediate purification catalyst WC layer 22 are laminated with the reaction intermediate purification catalyst WC layer 22 as an upper layer, It is formed (see FIG. 4B).
In the case of the catalytic converter 2c of the exhaust gas purification device 1c of the embodiment of the present invention shown in FIGS. 5 (a) and 5 (b), the Ag catalyst 5 and the reaction intermediate purification catalyst of the exhaust gas purification device 1 shown in FIG. 6 and the oxidation catalyst 7 are combined to form a composite catalyst 8c (see FIG. 5A). On the honeycomb 24 forming the cells 25 of the composite catalyst 8c, there is a composite WC layer 20c in which an oxidation catalyst WC layer 23, a reaction intermediate purification catalyst WC layer 22 and an Ag catalyst WC layer 21 are laminated in this order from the honeycomb 24. (See FIG. 5B).

  In the case of the embodiment of the present invention shown in FIGS. 4 and 5, NOx contained in the exhaust gas is first subjected to the catalytic action of the Ag catalyst 5 or the Ag catalyst WC layer 21 and then the reaction intermediate purification catalyst WC layer 22. Catalytic action. Thereafter, the catalytic action of the oxidation catalyst 7 or the oxidation catalyst WC layer 23 is received. Therefore, the exhaust gas purification apparatuses 1b and 1c of these embodiments have the same operations and effects as the exhaust gas purification apparatus 1 of the embodiment shown in FIG. 1, as with the exhaust gas purification apparatus 1a shown in FIG.

  Next, the effect of the exhaust gas purifying apparatus 1 according to the embodiment of the present invention is obtained as a result of the exhaust gas purifying test and the comparative Ag catalyst test using the exhaust gas purifying apparatuses of Examples 1 to 5 and Comparative Examples 1 to 3. This will be explained based on. First, the configuration and manufacturing method of the exhaust gas purifying apparatus and the like of each example and each comparative example will be described. It should be noted that the basic configuration of the exhaust gas purification apparatus of each example and each comparative example is the same as that of the exhaust gas purification apparatus 1 shown in FIG. 1 unless otherwise noted. That is, the exhaust gas purifying apparatus of each example and each comparative example is composed of the plasma generation unit 3 and the catalytic converter 2 as in the exhaust gas purifying apparatus 1 shown in FIG. 1, and the catalytic converter 2 includes the Ag catalyst 5, the reaction The intermediate purification catalyst 6 and the oxidation catalyst 7 are held.

Further, the plasma generation unit 3 used in the exhaust gas purifying apparatus of each example and each comparative example is common and has the following specifications.
In the plasma generation unit 3 of FIG. 2, the size of the electrode 30 is 20 mm × 50 mm, and the metal electrode 31b constituting the electrode 30 is a plate made of SUS316 having a thickness of 1.0 mm. As the dielectric 31a, an alumina plate having a thickness of 0.5 mm was used. The electrodes 30 were arranged so that the plasma generator 33 having a thickness of 0.5 mm could have five layers. In the exhaust gas purification test carried out, in the plasma generation unit 3 of this configuration, a high voltage power supply 32 applied a sine wave having a potential difference between the upper and lower peaks of 7.6 kVp-p and a frequency of 200 Hz between the electrodes 30. Plasma was generated. The maximum electric field strength is 7.6 kV / mm. The output power was 3.1 W and the power density was 1.2 W / cm ³ .
Hereinafter, the configuration and manufacturing method of the exhaust gas purifying apparatus of each example and comparative example will be described.

<Configuration of Exhaust Gas Purifying Device 1 of Example 1>
The Ag catalyst 5 held in the catalytic converter 2 has a honeycomb structure and a size of φ25.4 mm × L60 mm (volume is about 30 mL). The cell density is 62 cells / cm ² . The WC layer 20 of the Ag catalyst 5 has a thickness of 50 to 400 μm and a weight of 300 g / L. The WC layer 20 is composed of a porous structure made of alumina that is supported by boehmite and on which 12 g / L of Ag is supported.
The reaction intermediate purification catalyst 6 held in the catalytic converter 2 has a honeycomb structure and a size of φ25.4 mm × L30 mm (volume is about 15 mL). The cell density is 62 cells / cm ² . The WC layer 20 of the reaction intermediate purification catalyst 6 has a thickness of 25 to 300 μm and a weight of 150 g / L. The WC layer 20 is composed of a porous structure made of MFI-type zeolite ion-exchanged with 7.5 g / L of Cu.
The oxidation catalyst 7 held by the catalytic converter 2 has a honeycomb structure and a size of φ25.4 mm × L30 mm (volume is about 15 mL). The cell density is 62 cells / cm ² . The WC layer 20 of the oxidation catalyst 7 has a thickness of 30 to 200 μm and a weight of 150 g / L. The WC layer 20 is composed of a porous structure made of γ-Al ₂ O ₃ supporting 4.5 g / L of Pt.

<Method for Producing Ag Catalyst 5 of Exhaust Gas Purification Device 1 of Example 1>
Add 125.3 g of boehmite (PURAL SB, manufactured by SASOL) and 6.3 g of silver nitrate (special grade reagent, manufactured by Kojima Chemical Co., Ltd.) to the flask, add 1000 mL of ion-exchanged water, and stir and mix the chemicals in the flask thoroughly. Then, after removing excess water with a rotary evaporator, it is dried at 200 ° C. for 2 hours and further calcined at 600 ° C. for 2 hours to obtain Ag catalyst powder in which Ag is supported on γ-Al ₂ O _3. . 45 g of this Ag catalyst powder, 25 g of an alumina binder (Al ₂ O ₃ concentration: 20%, manufactured by Nissan Chemical Industries, Ltd.) and 150 mL of ion-exchanged water are placed in a polyethylene container, wet-pulverized with alumina balls for 14 hours, and Ag A catalyst slurry was prepared. A cordierite honeycomb with a honeycomb 24 thickness of 0.09 mm was immersed in this Ag catalyst slurry, pulled up, excess was removed by high-pressure air jet, and heated at 200 ° C. for 2 hours. The operation after the immersion in the Ag catalyst slurry was repeated to obtain a predetermined WC amount. After a predetermined amount of Ag was obtained, a cordierite honeycomb was fired at 500 ° C. for 2 hours to produce an Ag catalyst 5 in which γ-Al ₂ O ₃ supporting Ag was supported on the honeycomb 24.

<Method for Producing Ag Catalyst for Comparative Test for Examining Differences in Preparation Method of Ag Catalyst 5>
The WC layer 20 constituting the Ag catalyst 5 of Example 1 is manufactured from an Ag catalyst powder made of γ-Al ₂ O ₃ obtained by firing boehmite supporting Ag as described above. In order to investigate the difference in the preparation method of the Ag catalyst 5, the WC layer 20 constituting the Ag catalyst 5 is manufactured from Ag catalyst powder in which Ag is supported on γ-Al ₂ O ₃ manufactured by first firing boehmite. Catalyst 5 was produced.
This WC layer 20 is manufactured from Ag catalyst powder manufactured by the following process. This Ag catalyst powder was prepared by calcining 125.3 g of boehmite (PURAL SB, manufactured by SASOL) at 600 ° C. for 2 hours and γ-Al ₂ O ₃ and silver nitrate (special reagent grade, manufactured by Kojima Chemical Co., Ltd.) 6 .3 g was put in a flask, 1000 mL of ion exchange water was added, and the chemicals in the flask were thoroughly stirred and mixed. Then, excess water was removed by a rotary evaporator, followed by drying at 200 ° C. for 2 hours, and further at 600 ° C. And baked for 2 hours. From 45 g of this Ag catalyst powder, an Ag catalyst for comparison test was produced through the same steps as the Ag catalyst 5 of Example 1 described above.

<Method for Producing Reaction Intermediate Purifying Catalyst 6 of Exhaust Gas Purifying Device 1 of Example 1>
To a beaker, 420 mL of Na-ZSM-5 type zeolite (Si / Al = 28) was charged with 4000 mL of pure water and stirred at 120 ° C. for 2 hours, and then a 0.1 mol / L copper ammine complex aqueous solution was slowly added dropwise. Thereafter, the mixture was heated and stirred at 90 ° C. for 12 hours to perform ion exchange in which Na ions in the Na—ZSM-5 type zeolite were exchanged for Cu ions. The ion exchanged zeolite was filtered and washed repeatedly with a large amount of ion exchange water. The washed zeolite was dried at 120 ° C. for 24 hours to obtain a Cu catalyst powder composed of MFI-type zeolite ion-exchanged with Cu. 45 g of this Cu catalyst powder, 25 g of alumina binder (Al ₂ O ₃ concentration: 20%) and 150 g of ion-exchanged water were placed in a polyethylene container and wet-pulverized with alumina balls for 14 hours to prepare a Cu catalyst slurry. From this Cu catalyst slurry, the reaction intermediate purification catalyst 6 in which the MFI-type zeolite ion-exchanged with Cu was supported on the honeycomb 24 was manufactured by the same process as the method for manufacturing the Ag catalyst 5 described above.

<Method for Manufacturing Oxidation Catalyst 7 of Exhaust Gas Purifying Device 1 of Example 1>
97 g of γ-Al ₂ O ₃ (AF115, manufactured by Sumitomo Chemical Co., Ltd.) and 4.94 g of dinitrodiamine platinum (manufactured by Kojima Chemical Co., Ltd.) are placed in a flask, and 1000 mL of ion-exchanged water is added. After thoroughly stirring and mixing the chemicals in the flask, the excess water was removed by a rotary evaporator, dried at 200 ° C. for 2 hours, and further fired at 600 ° C. for 2 hours. Pt was γ-Al ₂ O ₃ Pt catalyst powder supported on the catalyst was obtained.
45 g of this Pt catalyst powder, 25 g of silica binder (SiO ₂ concentration: 20%, manufactured by Catalyst Kasei Kogyo Co., Ltd.) and 150 mL of ion-exchanged water are placed in a polyethylene container and wet-ground with an alumina ball for 14 hours to obtain a Pt catalyst slurry. Was made.
From this Pt catalyst slurry, the oxidation catalyst 7 in which γ-Al ₂ O ₃ supporting Pt was supported on the honeycomb 24 was manufactured by the same process as the method for manufacturing the Ag catalyst 5 described above.

<Configuration of Exhaust Gas Purification Device 1 of Example 2>
The Ag catalyst 5 and the oxidation catalyst 7 held in the catalytic converter 2 of the exhaust gas purification device 1 of the second embodiment are the same as the corresponding catalysts of the exhaust gas purification device 1 of the first embodiment. Further, the reaction intermediate purification catalyst 6 of the exhaust gas purification apparatus 1 of the second embodiment has a honeycomb structure and the same size and cell density as the corresponding catalyst of the exhaust gas purification apparatus 1 of the first embodiment. The WC layer 20 of the reaction intermediate purification catalyst 6 has a thickness of 25 to 300 μm and a weight of 150 g / L. The WC layer 20 is composed of a porous structure made of alumina supporting 3 g / L of Fe. The alumina constituting the porous structure of the WC layer 20 of the reaction intermediate purification catalyst 6 is γ-Al ₂ O ₃ produced from boehmite, and the same method as the Ag catalyst 5 of Example 1 described above is used. manufactured.

<Configuration of Exhaust Gas Purification Device 1 of Examples 3 to 5>
The Ag catalyst 5 and the oxidation catalyst 7 held in the catalytic converter 2 of the exhaust gas purification apparatus 1 of Examples 3 to 5 are the same as the corresponding catalysts of the exhaust gas purification apparatus 1 of Examples 1 and 2. Further, the reaction intermediate purification catalyst 6 of the exhaust gas purification apparatus 1 of Examples 3 to 5 has a honeycomb structure and the same size and cell density as the corresponding catalyst of the exhaust gas purification apparatus 1 of Example 1. The thickness and weight of the WC layer 20 of the reaction intermediate purification catalyst 6 of the exhaust gas purification apparatus 1 of Examples 3 to 5 are the same as those of the reaction intermediate purification catalyst 6 of the exhaust gas purification apparatus 1 of Example 2.
The WC layer 20 of the reaction intermediate purification catalyst 6 of Example 3 is composed of γ-Al ₂ O ₃ supporting 3 g / L of V. In the WC layer 20 of the reaction intermediate purification catalyst 6 of Example 4, 3 g / L of Cu is supported on alumina. The WC layer 20 of the reaction intermediate purification catalyst 6 of Example 5 is composed of γ-Al ₂ O ₃ supporting 7.5 g / L of Cu. All of these reaction intermediate purification catalysts 6 are produced by the same method as the Ag catalyst 5 of Example 1.

<Configuration of Exhaust Gas Purification Device of Comparative Example 1>
The Ag catalyst 5 and the oxidation catalyst 7 held in the catalytic converter of the exhaust gas purification apparatus of Comparative Example 1 are the same as the corresponding catalysts of the exhaust gas purification apparatus 1 of Example 1. In the exhaust gas purification device of Comparative Example 1, the reaction intermediate purification catalyst 6 is not held. Instead, the Ag catalyst 5 held in the catalytic converter 2 of the exhaust gas purification device 1 of Embodiment 1 is held in an overlapping manner.

<Configuration of Exhaust Gas Purification Device of Comparative Example 2>
The reaction intermediate purification catalyst 6 held in the catalytic converter of the exhaust gas purification apparatus of Comparative Example 2 is the same as the reaction intermediate purification catalyst 6 held in the catalytic converter 2 of the exhaust gas purification apparatus 1 of Example 2 except for Fe. The load is increased and 37.5 g / L of Fe is supported. Other configurations are the same as those of the exhaust gas purification apparatus 1 of the second embodiment.

<Configuration of Exhaust Gas Purification Device of Comparative Example 3>
The reaction intermediate purification catalyst 6 held in the catalytic converter of the exhaust gas purification apparatus of Comparative Example 3 is the same as that of the reaction intermediate purification catalyst 6 held in the catalytic converter 2 of the exhaust gas purification apparatus 1 of Example 4. The load is increased and 45 g / L of Cu is supported. Other configurations are the same as those of the exhaust gas purification apparatus 1 of the fourth embodiment.

Table 1 summarizes the configurations of the catalysts of the exhaust gas purifying apparatuses of Examples 1 to 5 and Comparative Examples 1 to 3 described above.

  A model exhaust gas having the composition shown in Table 2 was passed through the exhaust gas purification apparatuses of Examples 1 to 5 and Comparative Examples 1 to 3 at a flow rate of 25 L / min, and NOx concentrations before and after the catalyst of each exhaust gas purification apparatus were measured. The NOx purification rate was calculated. Here, the NOx purification rate was calculated using the following equation (1).

  In each exhaust gas purification device, the NOx concentration was measured for each of the gases before and after the Ag catalyst 5 and after the oxidation catalyst 7 to determine the NOx purification rate. Further, for each exhaust gas purification device, the difference in the NOx purification rate between the exhaust gas after the Ag catalyst 5 and the exhaust gas after the oxidation catalyst 7 was determined and used as the reaction intermediate remaining rate. The NOx concentrations of the exhaust gas before and after the Ag catalyst 5 and the exhaust gas after the oxidation catalyst 7 were measured, the NOx purification amount was calculated, and the reaction intermediate remaining rate was determined from the difference in the NOx purification amount. The reaction intermediate residual ratio was determined by the following formula (2).

When the reaction intermediate remaining in the model exhaust gas of the Ag catalyst 5 remains without being decomposed into N ₂ and is sent to the oxidation catalyst 7, the reaction intermediate is oxidized on the oxidation catalyst 7 and returns to NOx.

Hereinafter, measurement results of the NOx purification rates of the exhaust gas purification apparatuses of Examples 1 to 5 and Comparative Examples 1 to 3 will be described.
FIG. 6 is a graph showing the NOx purification rate measurement result when the temperature of the catalytic converter 2 is changed from 50 ° C. to 500 ° C. in the exhaust gas purification apparatus of the first embodiment. FIG. 7 is a graph showing the NOx purification rate measurement result when the temperature of the catalytic converter 2 is changed from 50 ° C. to 500 ° C. in the exhaust gas purification apparatus of Comparative Example 1. Here, in the actual machine, the temperature of the Ag catalyst 5 is usually 200 ° C. or higher, the reaction intermediate purification catalyst 6 is usually 150 ° C. or higher, and the oxidation catalyst 7 is usually 50 ° C. or higher.

As can be seen from FIG. 6, in the case of the exhaust gas purification device 1 of the first embodiment, the NOx purification rate after the Ag catalyst 5 is NOx after the oxidation catalyst 7 in the temperature range of 330 ° C. to 450 ° C. The reaction intermediate remaining rate, which is higher than the purification rate and is the difference at 400 ° C., is maximum, about 20%. Therefore, when the temperature of the catalytic converter 2 is in the temperature range of 330 ° C. to 450 ° C., the reaction intermediate remaining rate is greater than 0%, and some of the reaction intermediate remaining after the Ag catalyst 5 does not react with NO ₃ −. Then, it is sent to the oxidation catalyst 7 as it is.
On the other hand, the NOx purification rate after the oxidation catalyst 7 is higher than the NOx purification rate after the Ag catalyst 5 in the temperature range of the catalytic converter 2 from 50 ° C. to about 330 ° C. and 450 ° C. or more. Therefore, from 50 ° C. to about 330 ° C., the residual ratio of the reaction intermediate is 0%, and all the reaction intermediate remaining after the Ag catalyst 5 is decomposed by reacting with NO ₃ ⁻ and oxidized as it is. It is not sent to the catalyst 7. In this temperature range, part of the NOx that has not been reduced by the action of the Ag catalyst 5 is reduced by the action of the reaction intermediate purification catalyst 6.

As can be seen from FIG. 7, in the case of the exhaust gas purification device of Comparative Example 1, the NOx purification rate after the Ag catalyst 5 is higher than the NOx purification rate after the oxidation catalyst 7 at a high temperature where the catalytic converter temperature exceeds 280 ° C. At 380 ° C., the residual ratio of the reaction intermediate, which is the difference, reaches a maximum of about 70%. Therefore, when the catalytic converter temperature exceeds 280 ° C., the reaction intermediate remaining rate is greater than 0%, and a part of the reaction intermediate remaining after the Ag catalyst 5 does not react with NO ₃ ⁻ and is not decomposed. It is sent to the oxidation catalyst 7 as it is. On the other hand, when the catalytic converter temperature is from 50 ° C. to about 280 ° C., the NOx purification rate after the oxidation catalyst 7 is higher than the NOx purification rate after the Ag catalyst 5. Therefore, from 50 ° C. to 280 ° C., the reaction intermediate remaining ratio is 0%, and all reaction intermediates remaining after the Ag catalyst 5 are decomposed by reacting with NO ₃ ⁻ and remain as the reaction intermediates in the oxidation catalyst. 7 is not sent.

  As can be seen from Table 1, the difference between the exhaust gas purification device 1 of Example 1 and the exhaust gas purification device of Comparative Example 1 is the presence or absence of the reaction intermediate purification catalyst 6. Therefore, in the exhaust gas purification apparatus 1 of Example 1, the critical temperature at which the reaction intermediate residual ratio is higher than 0% is increased from 280 ° C. to 330 ° C. as compared with the exhaust gas purification apparatus of Comparative Example 1. Is due to the action of the reaction intermediate purification catalyst 6 (see FIGS. 6 and 7). Here, as can be seen from the results shown in FIGS. 6 and 7, the NOx purification rate after the Ag catalyst 5 is rapidly increasing around 300 ° C., and the action of the reaction intermediate purification catalyst 6 to raise the critical temperature is as follows. The exhaust gas purification device 1 of Example 1 at about 300 ° C. has a great effect on increasing the NOx purification rate.

  Further, due to the action of the reaction intermediate purification catalyst 6, in the exhaust gas purification apparatus 1 of Example 1, the reaction intermediate remaining rate at a critical temperature of 330 ° C. or higher is suppressed lower than that of the exhaust gas purification apparatus of Comparative Example 1. It has been. Therefore, the NOx purification rate of the exhaust gas purification device 1 of Example 1 is higher than that of the exhaust gas purification device of Comparative Example 1 in a high temperature range of 330 ° C. or higher. The maximum reaction intermediate remaining rate of the exhaust gas purification apparatus 1 of Example 1 is about 20% at 380 ° C., but the reaction intermediate remaining rate of the exhaust gas purification apparatus of Comparative Example 1 at the same temperature is about 20%. 70% (see FIGS. 6 and 7).

  FIG. 8 is a graph comparing the results of measuring the residual ratio of the reaction intermediate when the catalytic converter temperature is 300 ° C. in each of the exhaust gas purifying apparatuses of Examples 1 to 5 and Comparative Examples 1 to 3. As can be seen from FIG. 8, in the exhaust gas purification apparatuses 1 of Examples 1, 4 and 5 having Cu in the WC layer 20 of the reaction intermediate purification catalyst 6, the residual ratio of the reaction intermediate is extremely low and close to 0%. In the case of Examples 2 and 3 having V or Fe in the WC layer 20 of the reaction intermediate purification catalyst 6, the reaction intermediate remaining rate is 15% or less. On the other hand, Comparative Example 1 without the reaction intermediate purification catalyst 6 or Comparative Example 2 having 25 wt% Fe in the WC layer 20 of the reaction intermediate purification catalyst 6 or WC of the reaction intermediate purification catalyst 6 with 30 wt% Cu. The reaction intermediate remaining rate of the exhaust gas purifying apparatus of Comparative Example 3 in the layer 20 is 22% or more.

  From the above, it can be seen that if the exhaust gas purification apparatus 1 of the present embodiment is used, the reaction intermediate remaining in the exhaust gas that has passed through the Ag catalyst 5 can be efficiently decomposed, and the NOx purification rate can be increased. .

<Differences due to Ag catalyst preparation method (Ag catalyst test for comparison test)>
The difference in the NOx purification effect by the preparation method of the Ag catalyst 5 was investigated. For the exhaust gas purification apparatus in which the Ag catalyst 5 in the exhaust gas purification apparatus of Example 1 was replaced with the above-described Ag catalyst 5 for comparison test, model exhaust gas having the composition shown in Table 2 was allowed to flow at a flow rate of 25 L / min. The purification rate of NOx and HC immediately after the Ag catalyst 5 was measured. The Ag catalyst 5 of Example 1 was produced from an Ag catalyst powder made of γ-Al ₂ O ₃ produced by calcining boehmite supporting Ag (hereinafter referred to as “Ag catalyst for Example 1”). It is. The Ag catalyst 5 for comparison test is an Ag catalyst 5 manufactured from Ag catalyst powder in which Ag is supported on γ-Al ₂ O ₃ manufactured by calcining boehmite (hereinafter referred to as “Ag catalyst for comparison test”). . In this test, the measurement was performed in a state where both the Ag catalyst for Example 1 and the Ag catalyst for comparative test were held at 300 ° C.

FIG. 9 is a graph comparing the NOx and HC purification rates after the Ag catalyst of the exhaust gas purification apparatus of the present invention with respect to the Ag catalyst for Example 1 and the Ag catalyst for comparison test.
As can be seen from FIG. 9, the Ag catalyst for Example 1 has a higher NOx purification rate and a lower HC purification rate than the comparative test Ag catalyst. The high NOx purification rate of the Ag catalyst for Example 1 indicates that NOx reacts with HC on the Ag catalyst and is efficiently converted into a reaction intermediate (see FIG. 11).

  On the other hand, the Ag catalyst for Example 1 has a lower HC purification rate than the comparative test Ag catalyst, although the NOx purification rate is high. The ratio of the NOx purification rate to the HC purification rate is about 3.4 for the Ag catalyst for Example 1, whereas it is about 1.9 for the comparative test Ag catalyst, which is almost twice the difference. NOx is selectively reduced by HC and converted into a reaction intermediate (see FIG. 11) to be purified. The NOx purification rate is low and the HC purification rate is high on the comparative test Ag catalyst because the oxidation reaction in which HC burns occurs more on the comparative test Ag catalyst than on the Ag catalyst for Example 1. is there. From the above results, NOx can be selectively reduced efficiently by HC by using the Ag catalyst for Example 1. It can also be seen that by using the Ag catalyst for Example 1, it is possible to purify NOx with a small amount of HC addition compared to the comparative test Ag catalyst.

The reason why NOx can be selectively reduced by HC more efficiently than the comparative test Ag catalyst by using the Ag catalyst for Example 1 is that there is a difference in the state of Ag in the Ag catalyst.
FIG. 10 is a diagram showing the results of measuring the absorption spectra of ultraviolet light and visible light with a spectrophotometer for the Ag catalyst for Example 1 and the Ag catalyst for comparative test, and a diffuse reflection type ultraviolet / visible spectrophotometer device ( JASCO / V-570 manufactured by JASCO Corporation was used. As a light source, a deuterium discharge tube was used as a continuous light source in a wavelength range of 190 to 350 nm, and a tungsten iodine lamp was used as a continuous light source in a wavelength range of 330 to 2500 nm. The absorption spectrum was corrected by measuring the baseline with barium sulfate. In order to remove the adhesion of the organic compound to the object to be measured, the Ag catalyst for Example 1 and the Ag catalyst for comparative test were both preheated in the atmosphere at 500 ° C. for 2 hours. Each of the heat-treated Ag catalyst for Example 1 and the Ag catalyst for comparative test was set in a cell made of quartz glass and measured with the spectrophotometer described above. The absorption spectrum was measured by scanning the wavelength range of 200 to 600 nm at a scanning speed of 100 nm / min.
The absorption peak at a wavelength of 250 nm in Ag supported on Al ₂ O ₃ corresponds to an Ag ion cluster having a high catalytic effect (reference: “Rh-post-doped Ag / Al 2 O 3 as a highly active”). catalyst for the selective reduction of NO with decane ”, Kazuhito Sato et al., Catalysis Communications 4 (2003) 315-319). Absorption at wavelengths of 400 to 600 nm corresponds to Ag metal (reference: “Effect of hydrogen addition on SO2 tolerance of silver-alumina for SCR of NO with propane”, Ken-ichi Shimizu et al. , Journal of Catalysis 239 (2006) 117-124).

  As can be seen from FIG. 10, in the case of the absorption spectrum of the Ag catalyst for Example 1, the peak at a wavelength of 250 nm is relatively high and the absorption at a wavelength of 400 to 600 nm is low. On the other hand, in the case of the absorption spectrum of the comparative Ag catalyst, the peak at a wavelength of 250 nm is relatively low, and the absorption at a wavelength of 400 to 600 nm is relatively high. From this, it can be seen that the Ag catalyst for Example 1 has a higher proportion of Ag contained in the catalyst as Ag ion clusters than the Ag catalyst for comparison test. In the case of the Ag catalyst for Example 1, there are a relatively large number of Ag ion clusters which are active species, and the reaction for selectively reducing NOx by HC on the Ag ion clusters to generate a reaction intermediate is promoted. In the case of the comparative test Ag catalyst, there is a relatively large amount of Ag metal. However, on this Ag metal, a reaction in which HC is burned and oxidized is promoted, so NOx is selectively reduced by HC to generate a reaction intermediate. The reaction does not proceed efficiently.

It is a figure showing the exhaust-gas purification apparatus of embodiment, (a) is the schematic of an exhaust-gas purification layer, (b) is an expanded sectional view of the catalyst hold | maintained at the catalytic converter which comprises an exhaust-gas purification apparatus. It is a figure which shows the structure of the plasma generation unit used for the exhaust-gas purification apparatus of embodiment. It is a figure showing the exhaust-gas purification apparatus of other embodiment of this invention, (a) is the schematic of an exhaust-gas purification apparatus, (b) is an expansion of the catalyst hold | maintained at the catalytic converter which comprises an exhaust-gas purification apparatus. It is sectional drawing. It is a figure showing the exhaust-gas purification apparatus of other embodiment of this invention, (a) is the schematic of an exhaust-gas purification apparatus, (b) is an expansion of the catalyst hold | maintained at the catalytic converter which comprises an exhaust-gas purification apparatus. It is sectional drawing. It is a figure showing the exhaust-gas purification apparatus of other embodiment of this invention, (a) is the schematic of an exhaust-gas purification apparatus, (b) is an expansion of the catalyst hold | maintained at the catalytic converter which comprises an exhaust-gas purification apparatus. It is sectional drawing. In the exhaust gas purification apparatus of Example 1, it is the graph which showed the NOx purification rate measurement result when a catalytic converter temperature is changed from 50 degreeC to 500 degreeC. In the exhaust gas purification apparatus of the comparative example 1, it is the graph which showed the NOx purification rate measurement result when a catalytic converter temperature is changed from 50 degreeC to 500 degreeC. In each exhaust gas purification apparatus of Examples 1-5 and Comparative Examples 1-3, it is the graph which compared the reaction intermediate residual rate measurement result when a catalytic converter temperature is 300 degreeC. It is the figure which compared the purification rate of NOx and HC after Ag catalyst of the exhaust gas purification apparatus of this invention about the Ag catalyst for Example 1, and the Ag catalyst for comparative tests. It is a figure which shows the result of having measured the absorption spectrum of ultraviolet light and visible light about the Ag catalyst for Example 1, and the Ag catalyst for comparative tests with the spectrophotometer. It is a figure explaining the reaction process in which NOx contained in the exhaust gas of a diesel engine is selectively reduced and removed by the action of an Ag catalyst. It is a figure explaining HC oxidation reaction which occurs on Ag metal of Ag catalyst.

Explanation of symbols

1, 1a, 1b, 1c Exhaust gas purification device 2, 2a, 2b, 2c Catalytic converter 3 Plasma generation unit 4 Exhaust pipe 5 Ag catalyst 6 Reaction intermediate purification catalyst 7 Oxidation catalyst 8a, 8b, 8c Composite catalyst 20 WC layer 21 Ag catalyst WC layer 22 Reaction intermediate purification catalyst WC layer 23 Oxidation catalyst WC layer 24 Honeycomb 25 Cell

Claims (9)

In an exhaust gas purifying apparatus for a diesel engine, an Ag catalyst is disposed in an exhaust gas exhaust passage, and an oxidation catalyst is disposed downstream of the Ag catalyst in the exhaust gas exhaust passage.
The washcoat layer constituting the Ag catalyst comprises a porous structure carrying Ag in an amount of 0.01% or more and 20% or less by weight,
A reaction intermediate purification catalyst is disposed between the Ag catalyst and the oxidation catalyst,
The washcoat layer constituting the reaction intermediate purification catalyst is made of alumina or zeolite supporting at least one element selected from Cu, V, and Fe in an amount of 0.01% to 20% by weight. Constituted by a porous structure,
An exhaust gas purification device for diesel engines characterized by In the exhaust-gas purification apparatus for diesel engines described in Claim 1,
The washcoat layer constituting the Ag catalyst is composed of a porous structure made of alumina supporting Ag;
An exhaust gas purification device for diesel engines characterized by In the exhaust-gas purification apparatus for diesel engines described in Claim 2,
The alumina carrying Ag is γ-Al ₂ O ₃ produced from boehmite;
An exhaust gas purification device for diesel engines characterized by In the exhaust-gas purification apparatus for diesel engines described in Claim 3,
An exhaust gas purifying apparatus for a diesel engine, wherein the γ-Al ₂ O ₃ is produced by firing boehmite carrying Ag. In the exhaust-gas purification apparatus for diesel engines as described in any one of Claims 1 thru | or 4,
The washcoat layer constituting the reaction intermediate purification catalyst is composed of a porous structure made of zeolite supporting Cu in an amount of 0.01% to 20% by weight;
An exhaust gas purification device for diesel engines characterized by In the exhaust-gas purification apparatus for diesel engines described in Claim 5,
The zeolite supporting Cu is MFI type;
An exhaust gas purification device for diesel engines characterized by In the exhaust-gas purification apparatus for diesel engines as described in any one of Claims 1 thru | or 4,
The washcoat layer constituting the reaction intermediate purification catalyst is manufactured from boehmite that supports at least one element selected from Cu, V and Fe in an amount of 0.01% to 20% by weight. A porous structure composed of γ-Al ₂ O ₃
An exhaust gas purification device for diesel engines characterized by The exhaust gas purification apparatus for a diesel engine according to any one of claims 1 to 7,
The volume ratio of the reaction intermediate purification catalyst to the Ag catalyst is 20% or more and 100% or less,
An exhaust gas purification device for diesel engines characterized by A method for purifying exhaust gas from a diesel engine,
The exhaust gas,
Purified by an Ag catalyst composed of a washcoat layer composed of a porous structure carrying Ag in an amount of 0.01% to 20% by weight,
It is composed of a washcoat layer made of a porous structure made of alumina or zeolite and supporting at least one element selected from Cu, V, and Fe in an amount of 0.01% or more and 20% or less by weight. After purifying with the reaction intermediate purification catalyst
Purifying with an oxidation catalyst,
A diesel engine exhaust gas purification method characterized by the following.

Images