JP5000221B2 - Exhaust gas purification catalyst and exhaust gas purification device - Google Patents

Description

  The present invention relates to an exhaust gas purification catalyst used for purification of exhaust gas discharged from an internal combustion engine, and an exhaust gas purification device including the exhaust gas purification catalyst.

  Various exhaust gases discharged from internal combustion engines such as automobiles contain nitrogen oxides (hereinafter referred to as NOx) such as nitrogen monoxide and nitrogen dioxide together with water and carbon dioxide. This NOx not only causes air pollution, acid rain, and photochemical smog, but is also concerned about the effects on the human respiratory system and nervous system. For this reason, development of the technique which removes the nitrogen oxide in waste gas efficiently is desired.

  In particular, during lean burn operation in which combustion is performed in a state where oxygen is excessive, generation of NOx becomes a problem. Therefore, as a technique for removing NOx in an oxygen-excess atmosphere, unburned hydrocarbons present in exhaust gas are used as a reducing agent. NOx reduction technology used as a non-patent document 1 and 2 has been proposed. According to this technology, there is no need to newly add components other than exhaust gas components such as ammonia and urea as a reducing agent, and unburned hydrocarbons in exhaust gas are used or new hydrocarbons are added. It is effective because it can be done with

  Various types of NOx reduction catalysts used in the NOx reduction technology have been researched and developed, such as noble metal catalysts, alumina-supported catalysts, and zeolite catalysts. Examples of the NOx reduction catalyst used in the all-lean region include noble metal-based NOx reduction catalysts containing noble metals such as Pt, Pd, Rh and Ir. Among these, Pt-based catalysts are best known, but because of their high hydrocarbon oxidation performance, their selectivity with NOx is poor and their NOx purification performance is low (see Patent Document 1). Furthermore, there is a problem that the NOx purification temperature range is narrow.

  In addition, a catalyst containing a transition metal or a metal composed of a group 12 or 13 element (for example, Ag, Cu, Fe, Co, In, or Ga) has a high NOx purification temperature range and high selectivity with NOx, so that it is high. It is expected to exhibit NOx purification performance (see Patent Document 2).

  However, NOx reduction catalysts composed of the above transition metals and Group 12 and 13 elements are cationic in their catalytic activity, so that the aromatic organic compound contained in the exhaust gas is adsorbed when used at low temperatures, and the performance increases with time. There is a problem that it decreases (see FIG. 10). To solve this problem, it is possible to recover the NOx purification performance by removing the aromatic organic compound adsorbed on the catalyst surface by applying heat to the catalyst and regenerating it. However, on the other hand, new problems such as loss of fuel consumption due to regeneration processing and complicated control system arise.

  In addition, with respect to the problem of deterioration with time due to the adsorbed aromatic organic compound, the adsorbed aromatic organic compound can be reduced by adding a noble metal to the NOx reduction catalyst composed of the transition metal or the group 12 or 13 element. By burning, deterioration with time can be avoided. However, in this case, NOx purification performance deteriorates because the hydrocarbons that reduce NOx are combusted.

Furthermore, it is known that the combustion of hydrocarbons can be promoted by adding Ce to the NOx reduction catalyst composed of the above transition metals and Group 12 and 13 elements. For example, a method of mixing Ag and Ce It has been proposed (see Patent Documents 3 and 4).
Japanese Patent No. 2909553 JP 2003-190788 A JP 2004-322021 A JP 2002-370031 A Iwamoto et al., Applied Catalysis A 222 (2001) 163-181 Environmental Catalyst Handbook, p. 333-358, 384-391

  However, in the methods described in Patent Documents 3 and 4, only purification performance lower than NOx purification performance when only an Ag-based catalyst is used can be obtained. That is, a catalyst that can maintain high NOx purification performance over time has not been known so far. Therefore, it is desired to develop a catalyst that can suppress a decrease in purification performance over time without reducing NOx purification performance.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide an exhaust gas purification catalyst capable of maintaining high NOx purification performance without deterioration over time. Moreover, it is providing the exhaust gas purification apparatus provided with such an exhaust gas purification catalyst.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, a first coat layer comprising a catalyst carrier, a NOx reduction catalyst formed on the catalyst carrier, and a mixture of the NOx reduction catalyst and cerium-containing oxide formed on the first coat layer. According to the exhaust gas purification catalyst having two coat layers, it has been found that high NOx purification performance can be maintained without deterioration over time, and the present invention has been completed. Specifically, the present invention provides the following.

  (1) An exhaust gas purification catalyst used for purification of exhaust gas discharged from an internal combustion engine, comprising a catalyst carrier, a first coat layer comprising a NOx reduction catalyst formed on the catalyst carrier, and the first coat An exhaust gas purification catalyst having a second coat layer made of a mixture of a NOx reduction catalyst and a cerium-containing oxide formed on the layer.

  (2) The exhaust gas purification catalyst according to (1), wherein a weight% of the second coat layer with respect to a total weight of the first coat layer and the second coat layer is 1% or more and 55% or less.

(3) The exhaust gas purification catalyst according to (1) or (2), wherein in the second coat layer, the cerium-containing oxide has a CeO ₂ equivalent weight% with respect to the NOx reduction catalyst of 1% or more and 80% or less.

  (4) In the NOx reduction catalyst, at least one metal element selected from the group consisting of Ag, Cu, Fe, Ga, In, and Co is alumina, zeolite, silica, zirconia, titania, and magnesia. The exhaust gas purification catalyst according to any one of (1) to (3), which is supported on at least one porous body selected from the group consisting of:

  (5) The exhaust gas purifying catalyst according to (4), wherein the weight percent of the metal element in the NOx reduction catalyst is 1% or more and 30% or less.

  (6) The exhaust gas purification catalyst according to any one of (1) to (5), wherein the cerium-containing oxide is cerium oxide.

  (7) The cerium-containing oxide is a mixture and / or composite containing at least one element selected from the group consisting of cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium, lanthanum, yttrium, and hafnium. The exhaust gas purification catalyst according to any one of (1) to (5), which is an oxide.

  (8) An exhaust gas purification device that is provided in an exhaust passage of an internal combustion engine and purifies exhaust gas discharged from the internal combustion engine, the non-thermal plasma reactor being arranged in series sequentially from the upstream side to the downstream side, An exhaust gas purification means and a second exhaust gas purification means are provided. The first exhaust gas purification means includes a catalyst carrier, a first coat layer formed of the NOx reduction catalyst formed on the catalyst carrier, and the first coat layer. An exhaust gas purification apparatus comprising an exhaust gas purification catalyst having a second coat layer made of a mixture of a NOx reduction catalyst and a cerium-containing oxide formed thereon, wherein the second exhaust gas purification catalyst comprises an oxidation catalyst.

  ADVANTAGE OF THE INVENTION According to this invention, the exhaust gas purification catalyst which can maintain high NOx purification performance, without deterioration with time, and the exhaust gas purification apparatus provided with this exhaust gas purification catalyst can be provided.

  Embodiments of the present invention will be specifically described below with reference to the drawings.

<Exhaust gas purification catalyst>
The exhaust gas purification catalyst according to the present embodiment is suitably used for purification of exhaust gas discharged from an internal combustion engine such as an automobile. A schematic diagram showing the configuration of the exhaust gas purifying catalyst according to the present embodiment is shown in FIG. As shown in FIG. 1, the exhaust gas purifying catalyst according to this embodiment includes a catalyst carrier 11, a first coat layer 12 made of a NOx reduction catalyst formed on the catalyst carrier 11, and the first coat layer 12. And a second coat layer 13 made of a mixture of a NOx reduction catalyst and a cerium-containing oxide.

[Catalyst carrier 11]
The catalyst carrier 11 used in the present embodiment is not particularly limited, and a conventionally known catalyst carrier 11 can be used. For example, in addition to honeycomb-like structures made of various materials such as cordierite, alumina, mullite, silicon carbide, and aluminum titanate, pellet-like, plate-like, granular, and powder-like structures can be used. Of these, a cordierite honeycomb structure is preferably used.

[First coat layer 12]
The first coat layer 12 is formed by coating the catalyst carrier 11 with a NOx reduction catalyst. As the NOx reduction catalyst, a conventionally known catalyst can be used. Preferably, at least one metal element selected from the group consisting of Ag, Cu, Fe, Ga, In, and Co is selected from the group consisting of alumina, zeolite, silica, zirconia, titania, and magnesia. Those supported by at least one kind of porous body are used. The reason why these porous bodies are preferably used includes the presence of acid sites having an effect of maintaining a metallic state (cationic) showing NOx purification performance. That is, it is considered that when the active metal is in a cationic state, it exhibits good NOx purification activity. In this respect, the effect of maintaining a metal state (cationic) showing NOx purification performance is exhibited. The above porous body in which the acid sites are present is preferably used. In the case of other carriers, a cationic state which is an active state for NOx purification cannot be taken, and a zero-valent state or an oxide is formed. NOx purification performance cannot be obtained.

  In addition, it is preferable that content of the said metal element in a NOx reduction catalyst is 1 to 30 weight%. When the content of the metal element is less than 1% by weight, the amount of metal that is a catalytically active material is too small to sufficiently reduce NOx. On the other hand, when the content of the metal element exceeds 30% by weight, the metal particles become large and cannot maintain the cationic property, and the NOx purification performance is deteriorated. More preferably, they are 1 weight% or more and 20 weight% or less. The NOx reduction catalyst can be obtained by immersing the porous body in a liquid containing a slurry-like catalytically active substance and then heating and drying.

[Second coat layer 13]
The second coat layer 13 is formed by coating the first coat layer 12 with a mixture of a NOx reduction catalyst and a cerium-containing oxide (hereinafter also referred to as a mixed catalyst). That is, the second coat layer 13 does not support the active metal on the cerium-containing oxide, but is first supported on the porous body to form a NOx reduction catalyst, and then mixed with the cerium-containing oxide in the first coat. It is formed by coating on the layer 12. Specifically, the catalyst carrier 11 on which the first coat layer is formed is immersed in a slurry-like liquid composed of a mixture of a NOx reduction catalyst and a cerium-containing oxide, and then dried by heating.

  As the NOx reduction catalyst, as in the first coat layer 12, at least one metal element selected from the group consisting of Ag, Cu, Fe, Ga, In, and Co is alumina, zeolite, silica, Those supported on at least one porous body selected from the group consisting of zirconia, titania, and magnesia are preferably used. Further, the content of the metal element in the NOx reduction catalyst is the same as that of the first coat layer 12. Note that the NOx reduction catalyst used for the mixed catalyst of the second coat layer 13 and the NOx reduction catalyst of the first coat layer 12 need not have the same metal type, metal amount, and type of porous body, and are different. It may be.

  Examples of the cerium-containing oxide include a mixture and / or a composite oxide containing at least one element selected from the group consisting of cerium, zirconium, praseodymium, neodymium, terbium, samarium, gadolinium, lanthanum, yttrium, and hafnium. Can be mentioned. By including these elements, the durability of the cerium-containing oxide can be improved. A particularly preferable cerium-containing oxide is cerium oxide. Since the exhaust gas purification catalyst 1 of the present embodiment containing these cerium-containing oxides has an appropriate oxidation performance, the adsorbed aromatic organic compound is burned and removed without burning the hydrocarbon of the reducing agent. Can do.

In the second coating layer 13, CeO ₂ in terms of% by weight of the cerium-containing oxide to the NOx reduction catalyst is preferably 80% or less than 1%. When the amount is less than 1% by weight, the amount of the cerium-containing composite oxide is too small to sufficiently suppress the adsorption of the aromatic organic compound. When the amount exceeds 80% by weight, combustion by the cerium-containing composite oxide proceeds excessively, and as a result of burning to hydrocarbons in addition to the adsorbed aromatic organic compound, the reducing agent is added to the NOx reduction catalyst of the first coat layer 12. The NOx purification performance is lowered without reaching the hydrocarbons. More preferably, it is 10% or more and 50% or less.

  Further, the weight% of the second coat layer 13 with respect to the total weight of the first coat layer 12 and the second coat layer 13 is preferably 1% or more and 55% or less. When the amount is less than 1% by weight, the amount of the mixed catalyst is too small to sufficiently suppress the adsorption of the aromatic organic compound. When the amount exceeds 55% by weight, since the amount of the mixed catalyst is too large while the amount of the NOx reduction catalyst is too small, only a purification performance lower than the purification performance of the NOx purification catalyst alone can be obtained. More preferably, it is 3% or more and 40% or less.

[Function and effect]
The exhaust gas purifying catalyst 1 according to the present embodiment has a greater effect when used under all-lean conditions where generation of NOx is particularly concerned, that is, in an oxygen-excess atmosphere. Specifically, the aromatic organic compound attached to the surface is burned (cracked) by the action of the second coat layer 13 formed by a mixed catalyst in which a NOx reduction catalyst and a cerium-containing oxide are mixed. As a result, the catalyst Adsorption of aromatic organic compounds on the surface is suppressed. The aromatic organic compound remaining without being burned in the second coat layer 13 is used as a NOx reducing agent in the first coat layer 12 together with the hydrocarbon. That is, high purification performance can be maintained over time without reducing NOx reduction performance by the burning action of the aromatic organic compound by the cerium-containing oxide in the second coat layer 13.

  The effects of the exhaust gas purification catalyst 1 according to this embodiment will be described in more detail. Like conventional catalysts, catalysts using noble metals such as Pt, Pd, and Rh have poor oxidation performance due to selective reduction with NOx because the oxidation performance is too high, and noble metals are in a metal state with a valence of 0. It is believed that there is. On the other hand, the NOx reduction catalyst 1 according to this embodiment has an appropriate oxidation performance because it has the second coat layer 13 made of a mixed catalyst, and the state of the metal is considered to be ionic. It is done. For this reason, the reaction between the reducing agent and NOx is likely to proceed, and it is considered that the NOx purification performance is good. Although it can be said that the adsorption of the aromatic organic compound is likely to occur because the metal is cationic, the adsorbed aromatic organic compound burns the cerium-containing oxide in the second coat layer 13. Therefore, good NOx reduction performance can be maintained.

  Further, in the exhaust gas purification catalyst 1 according to the present embodiment, since a mixed catalyst in which a cerium-containing oxide and a NOx reduction catalyst are mixed is used, the aromatic organic compound adsorbed on the NOx reduction active point and the cerium-containing catalyst are used. As a result of the increased contact area with the oxide, the aromatic organic compound can be purified efficiently. That is, by adopting a configuration in which the second coat layer 13 made of the mixed catalyst is disposed on the first coat layer 12 made of the NOx reduction catalyst, the exhaust gas purification catalyst that can maintain high NOx purification performance without deterioration over time. 1 is obtained.

  In addition, the aromatic organic compound as used in this specification means the hydrocarbon which has a benzene ring. Hydrocarbons are roughly classified into aromatic organic compounds (with benzene rings) and non-aromatic organic compounds (without benzene rings). For example, aromatic organic compounds in light oil are about 20%. As described above, the exhaust gas purification catalyst 1 according to the present embodiment is characterized by burning the aromatic organic compound adhering to the surface of the catalyst, and therefore contains more aromatic organic compound. It can be said that there is a great effect when fuel is used.

<Exhaust gas purification device>
The schematic block diagram of the exhaust gas purification apparatus 100 which concerns on this embodiment is shown in FIG. As shown in FIG. 2, the exhaust gas purification apparatus 100 according to the present embodiment is provided in the exhaust passage of the internal combustion engine, and is sequentially arranged in series from the upstream side, the nonthermal plasma reactor 101, the first exhaust gas purification means 102, and Second exhaust gas purification means 103 is provided. The first exhaust gas purification means 102 includes the above-described exhaust gas purification catalyst 1, and the second exhaust gas purification means 103 includes an oxidation catalyst.

[Non-thermal plasma reactor 101]
A schematic diagram of a non-thermal plasma reactor 101 constituting the exhaust gas purifying apparatus 100 is shown in FIG. As shown in FIG. 3, the non-thermal plasma reactor 101 is configured to cover one of the opposing metal electrodes with a dielectric and generate plasma in the space between the dielectric and the other metal. As the metal, SUS or the like can be used, and as the dielectric, alumina (Al ₂ O ₃ ) ceramics or the like can be used, but is not limited thereto. Further, the electrode size and the thickness of the plasma space are appropriately selected. Note that electrodes may be stacked so that a plurality of plasma spaces are generated, and metal electrodes may be alternately set as a high electrode and a ground electrode and connected in parallel. Plasma generation conditions such as an alternating sine wave, power, electric field strength, and power density input to the exhaust gas purification apparatus 100 are also set as appropriate.

[First exhaust gas purification means 102]
The exhaust gas purification catalyst provided in the first exhaust gas purification means 102 is the exhaust gas purification catalyst 1 described above. As the exhaust gas purification catalyst 1, a catalyst carrier 11 that uses a cordierite honeycomb structure is preferably used. Moreover, in the exhaust gas purification apparatus 100, although the two exhaust gas purification catalysts 1 are arrange | positioned in series, it is not limited to this. One exhaust gas purification catalyst 1 may be arranged, or two or more different exhaust gas purification catalysts 1 may be arranged in series.

[Second exhaust gas purification means 103]
The type of the oxidation catalyst provided in the second exhaust gas purification means 103 is not particularly limited, and a conventionally known one can be used. The oxidation catalyst has an action of oxidizing and purifying exhaust gas by oxidizing hydrocarbons and carbon monoxide in the exhaust gas. The second exhaust gas purification unit 103 may have the same structure as the first exhaust gas purification unit 102, and may include an oxidation catalyst on a honeycomb-shaped carrier. Specific examples of the oxidation catalyst include those in which an oxidation active component such as Pt, Pd, Rh, Au, or Ag is supported on a carrier such as alumina, silica, silica alumina, titania, ceria, or zeolite. Such an oxidation catalyst can be obtained by immersing the honeycomb carrier in a liquid containing a slurry-like catalytically active substance and then drying by heating.

[Function and effect]
The exhaust gas discharged from the internal combustion engine is introduced into the exhaust gas purification apparatus 100 through the exhaust passage. The exhaust gas introduced from the upstream side contains particulate matter (PM), and this PM is oxidized and removed by the non-thermal plasma reactor 101. The exhaust gas from which PM has been removed by oxidation is introduced into the first exhaust gas purification means 102, and NOx contained in the exhaust gas is reduced and removed by the exhaust gas purification catalyst 1. Next, the exhaust gas from which NOx has been reduced and removed is introduced into the second exhaust gas purification means 103, and hydrocarbons and carbon monoxide contained in the exhaust gas are oxidized and removed. By these series of actions, the exhaust gas discharged from the internal combustion engine is purified.

  Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto. Hereinafter, the upper layer means the second coat layer, and the lower layer means the first coat layer.

<Example 1>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 50%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 16.7%) catalyst]
(A) Boehmite (Pural SB manufactured by SASOL) 125.3g, Silver nitrate (special grade manufactured by Kojima Chemicals Co., Ltd.) 6.3g, and ion-exchanged water 1000g were placed in an eggplant type flask, and excess water was removed with a rotary evaporator. The powder A was obtained by baking at 200 ° C. for 2 hours in a drying furnace and at 600 ° C. for 2 hours in a muffle furnace.

(B) 45 g of the above powder A, 25 g Al binder (Nissan Chemical Co., Ltd.) 25 g (Al ₂ O ₃ concentration 20%), 150 g ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml) and wet pulverized for 14 hours. Thus, slurry B was obtained.

  (C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry B. Next, the honeycomb support was taken out from the slurry B, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supported alumina catalyst C. The amount of washcoat was 250 g / L.

  (D) 22.5 g of the above powder A, 22.5 g of cerium oxide (manufactured by Nikki Co., Ltd.), 25 g of Al binder (same as above), 150 g of ion-exchanged water, and alumina balls in a polyethylene container (250 ml), 14 Slurry D was obtained by wet milling for a period of time.

(E) Ag-supported alumina catalyst B was immersed in the slurry D. Next, the Ag-supported alumina catalyst B was taken out of the slurry and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing at 500 ° C. for 2 hours in a muffle furnace, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 50%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight%: 16.7%) A catalyst was obtained. The amount of the upper layer washcoat was 50 g / L.

[Production of oxidation catalyst]
(A) 97 g of γ-alumina (AF115 manufactured by Sumitomo Chemical Co., Ltd.), 4.94 g of dinitrodiammine platinum (manufactured by Kojima Chemical Co., Ltd.), and 1000 g of ion-exchanged water are placed in an eggplant-shaped flask, and excess water is added using a rotary evaporator. Was removed, and baked in a drying furnace at 200 ° C. for 2 hours and in a muffle furnace at 600 ° C. for 2 hours to obtain powder B.

  (B) 45 g of the above powder B, 25 g of Si binder (Catalytic Chemical Industry Co., Ltd.) (SiO concentration 20%), 150 g of ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml) and wet pulverized for 14 hours to obtain a slurry. It was.

  (C) A cordierite honeycomb support having a honeycomb diameter of 25.4 mm × L 10 mm (5 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry. Next, the honeycomb support was taken out of the slurry and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After the desired loading was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain a Pt-supported alumina catalyst. The amount of washcoat was 150 g / L.

<Example 2>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 33%) catalyst]
As operations (a) and (b), the same operations as (a) and (b) of Example 1 were performed.

  (C) A cordierite honeycomb support having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in the slurry B. Next, the honeycomb support was taken out from the slurry and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, an Ag-supported alumina catalyst E was obtained by calcination in a muffle furnace at 500 ° C. for 2 hours. The amount of washcoat was 150 g / L.

  (D) 40.5 g of the above powder A, 4.5 g of cerium oxide (manufactured by Nikki Co., Ltd.), 25 g of Al binder (same as in Example 1), 150 g of ion-exchanged water, and alumina balls are put in a polyethylene container (250 ml). For 14 hours to obtain slurry F.

(E) Ag-supported alumina catalyst E was immersed in the slurry F. Next, the Ag-supported alumina catalyst E was taken out from the slurry F, and after the excess was removed by air injection, the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 33%) A catalyst was obtained. The amount of the upper layer was 150 g / L.

<Example 3>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 16.7%) catalyst]
As operations (a), (b), and (c), the same operations as (a), (b), and (c) of Example 1 were performed.

  As the operation (d), the same operation as (d) in Example 2 was performed.

(E) Ag-supported alumina catalyst C was immersed in slurry F. Next, the Ag-supported alumina catalyst C was taken out from the slurry F and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 16.7%) A catalyst was obtained. The amount of the upper layer washcoat was 50 g / L.

<Example 4>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 75%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 16.7%) catalyst]
As operations (a), (b), and (c), the same operations as in (a), (b), and (c) of Example 1 were performed.

  (D) 11.25 g of the above powder A, 33.75 g of cerium oxide (manufactured by Nikki Co., Ltd.), 25 g of Al binder (same as in Example 1), 150 g of ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml). For 14 hours to obtain a slurry G.

(E) Ag-supported alumina catalyst C was immersed in the slurry G. Next, the Ag-supported alumina catalyst C was taken out of the slurry G and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After the desired loading was obtained, firing was performed at 500 ° C. for 2 hours in a muffle furnace, and the upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 75%), the lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 16.7%) A catalyst was obtained. The amount of the upper layer washcoat was 50 g / L.

<Example 5>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 3.3%) catalyst]
As operations (a) and (b), the same operations as (a) and (b) in Example 1 were performed.

  (C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry B. Next, the honeycomb support is taken out from the slurry B, and the excess amount is removed by air injection to obtain a desired loading amount, and then calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supporting alumina catalyst H. It was. The amount of washcoat was 290 g / L.

  (D) The same operation as (d) in Example 2 was performed.

(E) Ag-supported alumina catalyst H was immersed in slurry F. Next, the Ag-supported alumina catalyst H was taken out from the slurry F and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 3.3%) A catalyst was obtained. The amount of the upper layer washcoat was 10 g / L.

<Example 6>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 50%) catalyst]
As operations (a) and (b), the same operations as (a) and (b) in Example 1 were performed.

  (C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry B. Next, the honeycomb support was taken out from the slurry B, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supported alumina catalyst I. The amount of washcoat was 150 g / L.

  (D) The same operation as (d) in Example 2 was performed.

(E) Ag-supported alumina catalyst I was immersed in slurry F. Next, the Ag-supported alumina catalyst I was taken out from the slurry F, and the excess was removed by air injection. Then, the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 50%) A catalyst was obtained. The amount of the upper layer was 150 g / L.

<Comparative Example 1>
[Production of 4 wt% Ag / Al ₂ O ₃ catalyst]
(A) 125.3 g of boehmite (Pural SB manufactured by SASOL), 6.3 g of silver nitrate (special grade manufactured by Kojima Chemical Co., Ltd.), and 1000 g of ion-exchanged water are placed in an eggplant type flask, and excess water is removed with a rotary evaporator. The powder A was obtained by baking at 200 ° C. for 2 hours in a drying furnace and at 600 ° C. for 2 hours in a muffle furnace.

(B) 45 g of the above powder A, 25 g Al binder (Nissan Chemical Co., Ltd.) 25 g (Al ₂ O ₃ concentration 20%), 150 g ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml) and wet pulverized for 14 hours. A slurry B was obtained.

  (C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry B. Next, the honeycomb support was taken out from the slurry B, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supported alumina catalyst C. The amount of the upper layer was 300 g / L.

<Comparative example 2>
[Production of mixed 4 wt% Ag / Al ₂ O ₃ + CeO ₂ catalyst (mixing ratio 29: 1)]
(A) The same operation as in Example 1 (a) was performed.

(B) 43.5 g of the above powder A, 1.5 g of cerium oxide (manufactured by Nikki Co., Ltd.), 25 g of Al binder (manufactured by Nissan Chemical Industries, Ltd.) (Al ₂ O ₃ concentration 20%), 150 g of ion-exchanged water, and alumina balls It was put in a polyethylene container (250 ml) and wet pulverized for 14 hours to obtain slurry J.

(C) In this slurry J, a honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed. Next, the honeycomb support was taken out from the slurry, and the excess was removed by air jetting. Then, the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until a predetermined loading amount was obtained. After a desired loading amount was obtained, it was calcined at 500 ° C. for 2 hours in a muffle furnace to obtain a mixed 4 wt% Ag / Al ₂ O ₃ + CeO ₂ catalyst (mixing ratio 29: 1). The amount of washcoat was 300 g / L.

<Comparative Example 3>
[Production of mixed 4 wt% Ag / Al ₂ O ₃ +4 wt% Ag / CeO ₂ catalyst (mixing ratio 29: 1)]
(A) The same operation as (a) in Example 1 was performed.

  (B) 125.3 g of cerium oxide (made by Nikki Co., Ltd.), 6.3 g of silver nitrate (special grade made by Kojima Chemical Co., Ltd.), and 1000 g of ion-exchanged water are placed in an eggplant type flask, and excess water is removed with a rotary evaporator Powder K was obtained by baking at 200 ° C. for 2 hours in a drying furnace and at 500 ° C. for 2 hours in a muffle furnace.

(C) 43.5 g of the above powder A, 1.5 g of the above powder K, 25 g of Al binder (manufactured by Nissan Chemical Industries, Ltd.) (Al ₂ O ₃ concentration 20%), 150 g of ion-exchanged water, and alumina balls in a polyethylene container (250 ml) And wet pulverized for 14 hours to obtain slurry L.

(D) In this slurry L, a honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed. Next, the honeycomb support was taken out from the slurry L and the excess was removed by air jetting. Then, the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After the desired loading was obtained, it was calcined at 500 ° C. for 2 hours in a muffle furnace to obtain a mixed 4 wt% Ag / Al ₂ O ₃ + Ag / CeO ₂ catalyst (mixing ratio 29: 1). The amount of washcoat was 300 g / L.

<Comparative example 4>
[Manufacture of upper layer CeO ₂ (10 g / L), lower layer 4 wt% Ag / Al ₂ O ₃ (290 g / L) catalyst]
As operations (a) and (b), the same operations as (a) and (b) in Example 1 were performed.

  (C) The same operation as (c) of Example 5 was performed.

(D) 45 g of cerium oxide (manufactured by Nikki Co., Ltd.), 25 g of Al binder (manufactured by Nissan Chemical Industries, Ltd.) (Al ₂ O ₃ concentration 20%), 150 g of ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml). For 14 hours to obtain a slurry M.

(E) Ag-supported alumina catalyst H was immersed in the slurry M. Next, the Ag-supported alumina catalyst H was taken out from the slurry M, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After the desired loading was obtained, the catalyst was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an upper layer CeO ₂ (10 g / L) and a lower layer 4 wt% Ag / Al ₂ O ₃ (290 g / L) catalyst. The amount of the upper layer washcoat was 10 g / L.

<Comparative Example 5>
[Production of 4 wt% Ag / CeO ₂ catalyst]
(A) The same operation as in (b) of Comparative Example 3 was performed.

(B) 43.5 g of the powder A, 1.5 g of the powder K, 1.5 g Al binder (Nissan Chemical Co., Ltd.) 25 g (Al ₂ O ₃ concentration 20%), 150 g of ion-exchanged water, and alumina balls are placed in a polyethylene container (250 ml). For 14 hours to obtain a slurry N.

(C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry N and heated for 2 hours. This operation was repeated until the desired loading was obtained. After the desired loading was obtained, it was calcined at 500 ° C. for 2 hours in a muffle furnace to obtain a 4 wt% Ag / CeO ₂ catalyst. The amount of washcoat was 300 g / L.

<Comparative Example 6>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 1%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 16.7%) catalyst]
As operations (a), (b), and (c), the same operations as in (a), (b), and (c) of Example 1 were performed.

  (D) 4.5 g of the above powder A, 40.5 g of cerium oxide (made by Nikki Co., Ltd.), 25 g of Al binder (the same as in Example 1), 150 g of ion-exchanged water, and alumina balls are put in a polyethylene container (250 ml). For 14 hours to obtain a slurry P.

(E) Ag-supported alumina catalyst C was immersed in the slurry P. Next, the Ag-supported alumina catalyst C was taken out of the slurry P, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing at 500 ° C. for 2 hours in a muffle furnace, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 90%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 16.7%) A catalyst was obtained. The amount of washcoat was 50 g / L.

<Comparative Example 7>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 90%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 16.7%) catalyst]
As operations (a), (b), and (c), the same operations as in (a), (b), and (c) of Example 1 were performed.

  (D) 4.5 g of the above powder A, 40.5 g of cerium oxide (made by Nikki Co., Ltd.), 25 g of Al binder (the same as in Example 1), 150 g of ion-exchanged water, and alumina balls are put in a polyethylene container (250 ml). For 14 hours to obtain a slurry P.

(E) Ag-supported alumina catalyst C was immersed in the slurry P. Next, the Ag-supported alumina catalyst C was taken out of the slurry P, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing at 500 ° C. for 2 hours in a muffle furnace, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 90%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 16.7%) A catalyst was obtained. The amount of washcoat was 50 g / L.

<Comparative Example 8>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 0.3%) catalyst]
As the operations (a) and (b), the same operations as (a) and (b) in the above examples were performed.

  (C) A honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in this slurry B. Next, the honeycomb support was taken out from the slurry B, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supported alumina catalyst Q. The amount of washcoat was 299 g / L.

  (D) The same operation as (d) in Example 2 was performed.

(E) Ag-supported alumina catalyst Q was immersed in the slurry F. Next, the Ag-supported alumina catalyst P was taken out of the slurry F, and the excess was removed by air injection. Then, the honeycomb support was heated at 200 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 0.3%) A catalyst was obtained. The amount of washcoat was 1 g / L.

<Comparative Example 9>
[Production of upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (upper layer catalyst weight% = 66%) catalyst]
As operations (a) and (b), the same operations as (a) and (b) in Example 1 were performed.

  (C) A cordierite honeycomb support having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed in the slurry B. Next, the honeycomb support was taken out from the slurry B, and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After a desired loading amount was obtained, it was calcined in a muffle furnace at 500 ° C. for 2 hours to obtain an Ag-supported alumina catalyst R. The amount of washcoat was 100 g / L.

  (D) The same operation as (d) in Example 2 was performed.

(E) Ag-supported alumina catalyst R was immersed in slurry F. Next, the Ag-supported alumina catalyst R was taken out from the slurry F, and after the excess was removed by air injection, the honeycomb support was heated at 500 ° C. for 2 hours. By this operation, a desired loading amount was obtained. After a desired loading amount is obtained, firing in a muffle furnace at 500 ° C. for 2 hours, upper layer 4 wt% Ag / Al ₂ O ₃ + CeO ₂ (CeO ₂ concentration = 10%), lower layer 4 wt% Ag / Al ₂ O ₃ (Upper layer catalyst weight% = 66%) A catalyst was obtained. The amount of washcoat was 200 g / L.

<Comparative Example 10>
[Production of mixed PdO (0.15 g / L) +4 wt% Ag / Al ₂ O ₃ (300 g / L) catalyst]
(A) The same operation as in Example 1 (a) was performed.

(B) 44.85 g of the above powder A, 0.15 g of palladium oxide, 25 g of Al binder (manufactured by Nissan Chemical Industries, Ltd.) (Al ₂ O ₃ concentration 20%), 150 g of ion-exchanged water, and alumina balls in a polyethylene container (250 ml) The slurry S was obtained by wet pulverization for 14 hours.

(C) In this slurry S, a honeycomb support made of cordierite having a honeycomb diameter of 25.4 mm × L 60 mm (30 cc), 400 cells / in 2 and 3.5 mil was immersed. Next, the honeycomb support was taken out from the slurry and the excess was removed by air injection, and then the honeycomb support was heated at 200 ° C. for 2 hours. This operation was repeated until the desired loading was obtained. After the desired loading was obtained, it was calcined at 500 ° C. for 2 hours in a muffle furnace to obtain a mixed PdO (0.15 g / L) +4 wt% Ag / Al ₂ O ₃ (300 g / L) catalyst. The amount of washcoat was 300 g / L.

<Evaluation>
In the performance evaluation of the catalyst obtained in each Example and each Comparative Example, a model gas having a composition as shown in Table 1 was employed. The gas temperature was 300 ° C. and the linear velocity was 5000 h ⁻¹ .

Moreover, the performance evaluation of each catalyst was implemented using the exhaust gas purification apparatus 100 shown in FIG. As the non-thermal plasma reactor 101, the one shown in FIG. 3 was used. SUS316 (thickness: 1.0 mm) was used for the metal, and alumina (Al ₂ O ₃ ) ceramics (thickness: 0.5 mm) was used for the dielectric. The electrode size was 20 × 50 mm, and the thickness of the plasma space was 0.5 mm. In this example, electrodes were stacked so that five plasma spaces were generated, and metal electrodes were alternately set as a high electrode and a ground electrode, and connected in parallel. As a plasma generation condition of the non-thermal plasma reactor 101, an alternating sine wave was set to 200 Hz and input was performed at 7.6 kVp-p. The power at this time is 3.1 W, and the electric field strength is 7.6 kV. mm, and the power density was 1.2 W / cm ³ . However, the nonthermal plasma reactor 101 of the present invention is not particularly limited, and the effects of the present invention are not limited by the configuration of the nonthermal plasma reactor.

  The model gas having a known composition was introduced into the exhaust gas purification apparatus 100, and the gas compositions before and after the introduction were compared to evaluate the NOx purification rate and hydrocarbon (HC) purification rate of each catalyst. A NOx meter was used to measure NOx, and a THC meter and FT-IR were used to measure hydrocarbons. The evaluation results after 5000 seconds of reaction are shown in Table 2.

The investigation of the components adsorbed on the catalyst was performed by the adsorbate desorption measurement. Specifically, using pyrolysis gas chromatograph mass spectrometry, gas components that were desorbed by raising the temperature of the catalyst were separated by gas chromatograph, and then qualitative and quantitative were performed by mass spectrometry. The measurement conditions for pyrolysis gas chromatography mass spectrometry were as follows.
Apparatus: HP6890 / 5972 manufactured by Agilent Technologies.
Sample heating conditions: 50 ° C. × 1 minute → heated to 800 ° C. at 20 ° C./minute
Column: DB-S.
Column heating conditions: 40 ° C. (6 min hold) → Temperature rise to 200 ° C. at 10 ° C./min→Raise temperature to 320 ° C. at 15 ° C./min→Hold at 320 ° C.

<Discussion>
[Effect of CeO ₂ ]
FIG. 4 shows a comparison result of the NOx purification rates of Example 1 and Comparative Example 1 over time. In addition, FIG. 5 shows a comparison result of the HC purification rates of Example 1 and Comparative Example 1 after 5000 seconds of reaction. As shown in FIG. 5, the HC purification rate of Example 1 in which CeO ₂ was introduced was about 20% higher than that of Comparative Example 1. The amount of aromatic organic compound adsorbed on the catalyst surface 5000 seconds after the start of the catalytic reaction (the amount of aromatic organic compound adsorbed per 1 g of catalyst) was 1008 μg / g in Comparative Example 1. In Example 1, it was greatly reduced to 47 μg / g. As described above, in Example 1, by introducing CeO ₂ , the adsorption of the aromatic organic compound to the catalyst surface is suppressed. As a result, as shown in FIG. It was confirmed that deterioration over time due to coking was suppressed.

[Difference in effects of the introduction method of CeO ₂ (comparison in the same amount of CeO _2)]
FIG. 6 shows the comparison results of the NOx purification rates over time of Example 2 and Comparative Examples 1-5. As shown in FIG. 6, Comparative Example 4 in which only the cerium-containing oxide was coated on the upper layer had a lower NOx purification rate over time than Example 2. This is because, in Comparative Example 4, hydrocarbons (HC), which are NOx reducing agents other than aromatic organic compounds, burn, and HC does not reach the lower NOx reduction catalyst, resulting in a reduction in NOx purification performance. It is thought that.

  Further, as shown in FIG. 6, the comparative example 2 consisting only of the mixed catalyst of the NOx reduction catalyst and the cerium-containing oxide had a lower NOx purification rate over time than the example 2. Since this is only the mixed catalyst, the ratio of the NOx reduction catalyst is reduced, so that it is considered that the NOx reduction performance of the NOx reduction catalyst alone has been lowered.

  Furthermore, as shown in FIG. 6, Comparative Examples 3 and 5 in which an active metal was supported on a cerium-containing oxide had a lower NOx purification rate over time than Example 2. This is probably because cerium has a low interaction with the active metal, so that the metal (oxidation) state necessary for the NOx purification activity cannot be maintained, and as a result, the NOx purification performance has deteriorated.

[Effect on NOx purification rate (performance) with respect to CeO ₂ concentration (% by weight) in upper catalyst]
FIG. 7 shows the relationship between the CeO ₂ concentration in the upper catalyst and the NOx purification rate based on the results of the NOx purification rate after 5000 seconds of the reactions of Examples 1, 3, 4, and Comparative Examples 6 and 7. As shown in FIG. 7, when the ceria concentration (% by weight) of the cerium-containing composite oxide in the mixed catalyst is less than 1%, the amount of cerium-containing oxide is too dilute to suppress the adsorption of aromatic organic compounds. It was confirmed that it was not possible. Moreover, when the weight ratio exceeded 80%, combustion by the cerium-containing oxide progressed too much, and it was confirmed that HC as a reducing agent did not reach the lower NOx reduction catalyst and NOx purification performance was lowered. .

[Effect on NOx purification rate (performance) with respect to weight% of upper catalyst]
Based on the results of the NOx purification rate after 5000 seconds of the reactions of Examples 2, 5, 6 and Comparative Examples 8 and 9, the relationship between the weight percent of the upper catalyst and the NOx purification rate is shown in FIG. As shown in FIG. 8, when the weight% of the upper mixed catalyst layer is less than 1% with respect to the total catalyst, the amount of the mixed catalyst layer is too small to sufficiently suppress the adsorption of the aromatic organic compound. Was confirmed. Further, when the weight percentage exceeds 55%, it is confirmed that the amount of the mixed catalyst is too large while the amount of the NOx reduction catalyst is too small, so that the NOx purification performance of the NOx purification catalyst alone is deteriorated. It was.

[Effect of CeO ₂ (reason for not being a noble metal)]
FIG. 9 shows the comparison results of the NOx purification rates over time of Example 2 and Comparative Examples 1 and 10. As shown in FIG. 9, in Comparative Example 10 using Pd, the NOx purification rate with time was much lower than that in Example 2. This is because noble metals (Pt, Pd, Ir, Rh) other than cerium-containing oxides have too strong combustion activity, so that not only aromatic organic compounds but also HC, which is a reducing agent, is burned, resulting in NOx purification. The cause was thought to be a decrease in performance. On the other hand, since the cerium-containing composite oxide has an appropriate combustion performance, it was considered that a good NOx purification rate could be maintained. Also, since cerium has an oxygen scavenging / releasing function, oxygen on cerium is used in the combustion reaction, and it is thought that aromatic organic compounds that are difficult to burn are burned.

It is a schematic diagram of the exhaust gas purification catalyst according to the present embodiment. It is a schematic structure figure of exhaust gas purification device 100 concerning this embodiment. 1 is a schematic diagram of a non-thermal plasma reactor 101. FIG. It is a figure which shows the NOx purification rate in time of Example 1 and Comparative Example 1. It is a figure which shows the HC purification rate of Example 1 and Comparative Example 1. It is a figure which shows the NOx purification rate in time of Example 2 and Comparative Examples 1-5. It is a figure which shows the NOx purification rate of Examples 1, 3, 4 and Comparative Examples 6 and 7. It is a figure which shows the NOx purification rate of Examples 2, 5, 6 and Comparative Examples 8 and 9. It is a figure which shows the NOx purification rate with time of Example 2 and Comparative Examples 1 and 10. It is a figure for demonstrating the mechanism of the conventional NOx reduction catalyst.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Exhaust gas purification catalyst 11 Catalyst carrier 12 1st coat layer 13 2nd coat layer 100 Exhaust gas purification apparatus 101 Nonthermal plasma reactor 102 1st exhaust gas purification means 103 2nd exhaust gas purification means

Claims (4)

An exhaust gas purification catalyst used for purification of exhaust gas discharged from an internal combustion engine,
It includes a catalyst support, a first coating layer formed on the catalyst carrier, and a second coating layer formed on the first coating layer, and
The first coat layer is composed of a NOx reduction catalyst in which Ag is supported on alumina,
The second coat layer is made of a mixture of a NOx reduction catalyst in which Ag is supported on alumina and cerium oxide,
An exhaust gas purification catalyst in which the content of Ag in the NOx reduction catalyst of the first coat layer is the same as the content of Ag in the NOx reduction catalyst of the second coat layer .   2. The exhaust gas purification catalyst according to claim 1, wherein the weight percent of the second coat layer with respect to the total weight of the first coat layer and the second coat layer is 1% or more and 55% or less. 3. The exhaust gas purification catalyst according to claim 1, wherein in the second coat layer, the weight percent of the cerium oxide with respect to the NOx reduction catalyst is 1% or more and 80% or less. The exhaust gas purification catalyst according to any one of claims 1 to 3, wherein the weight percent of Ag in the NOx reduction catalyst is 1% or more and 30% or less.

Images