JP3788141B2 - Exhaust gas purification system - Google Patents

Description

【００４７】
【発明の効果】
以上説明してきたように、本発明によれば、ＨＣとＣＯを選択的に浄化する酸化触媒や三元触媒を、ＮＯｘ浄化触媒の上流側に配置することとしたため、酸素過剰で運転することによる燃費向上効果を十分に享有でき、ＨＣ及びＣＯ成分を効率良く浄化し、特にエンジン始動直後の低温時に排出されるＨＣ及びＣＯを効率良く浄化できる排気ガス浄化システムを提供することができる。
即ち、本発明の排気ガス浄化システムを用いると、酸素過剰雰囲気下の幅広いＡ／Ｆ領域で高いＮＯｘ浄化処理が行なえ、燃費性能を向上できるのみならず、ＨＣ及びＣＯ浄化性能とＮＯｘ浄化性能を高い転化率で両立することができる。
【図面の簡単な説明】
【図１】本発明の排気ガス浄化システムの一実施例を示すシステム構成図である。
【符号の説明】
１ エンジン
２ 酸化触媒／三元触媒
３ ＮＯｘ浄化触媒[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a system for purifying exhaust gas discharged from an internal combustion engine, a combustor, and the like, and particularly, exhaust gas purification capable of purifying nitrogen oxide in lean burn exhaust gas containing excessive oxygen with high efficiency. About the system.
[0002]
[Prior art]
Conventionally, as a catalyst for purifying carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NOx), etc. contained in exhaust gas discharged from an internal combustion engine such as an automobile, a ternary that works at a theoretical air-fuel ratio is used. A catalyst is used. However, the three-way catalyst cannot purify nitrogen oxides when the exhaust gas of the internal combustion engine has excessive oxygen.
As a method of purifying nitrogen oxides when the exhaust gas of the internal combustion engine is excessive in oxygen, Japanese Patent Publication No. 2600609 discloses that NOx is absorbed when the exhaust gas is excessive in oxygen, and the absorbed NOx is converted into NOx. A method is disclosed in which the concentration of oxygen in the exhaust gas flowing into the absorbent is reduced and released, and the purification process is performed.
[0003]
[Problems to be solved by the invention]
However, as described in the above patent publication, NOx is absorbed when the exhaust gas is in excess of oxygen, and the concentration of oxygen in the exhaust gas flowing into the NOx absorbent is reduced, and the absorbed NOx is released. In the method of purifying NOx, HC and CO are used as reducing agents when desorbing and purifying NOx, and in order to sufficiently reduce NOx, HC and CO are used as reducing agents during NOx desorption purification. It is necessary to supply enough CO. For this reason, HC and CO components other than NOx are discharged without being sufficiently purified, and sufficient HC and CO purification performance cannot be obtained.
[0004]
As a solution to this problem, there is a method in which a three-way catalyst is disposed after the NOx storage catalyst to purify it. In such a catalyst system, a catalyst for purifying HC and CO is disposed in the latter stage of the exhaust passage. Therefore, the exhaust temperature at the catalyst inlet is lowered, and there is a problem that sufficient HC and CO purification performance cannot be obtained.
Also, when purifying by releasing the absorbed NOx as described above, if the oxygen concentration is decreased by increasing the HC and CO components in the exhaust gas, the fuel efficiency improvement effect cannot be obtained sufficiently. There are also challenges.
[0005]
The present invention has been made in view of the above-described problems of the prior art, and the object of the present invention is to sufficiently enjoy the fuel efficiency improvement effect by operating with excess oxygen, and to make the HC and CO components efficient. An object of the present invention is to provide an exhaust gas purification system that can purify well and efficiently purify HC and CO discharged at a low temperature immediately after engine startup.
[0006]
[Means for Solving the Problems]
As a result of intensive studies to achieve the above object, the present inventors have arranged an oxidation catalyst or a three-way catalyst for selectively purifying HC and CO on the upstream side of the NOx purification catalyst. Has been found to be achieved, and the present invention has been completed.
[0007]
That is, the exhaust gas purification system of the present invention selectively removes hydrocarbons and carbon monoxide among the reducing components , and also provides an oxidation catalyst and / or a three-way catalyst that hardly purifies hydrogen and ammonia, and a reducing component. An NOx purification catalyst that uses nitrogen oxides for reduction treatment, and the exhaust gas composition has a so-called lean state in which the air-fuel ratio is in an oxygen-excess state, a so-called rich state in which the stoichiometric air-fuel ratio or a fuel-excess state is in a rich state Installed in the exhaust passage of the engine or combustion device ,
The oxidation catalyst and / or the three-way catalyst is disposed upstream of the exhaust gas passage of the NOx purification catalyst,
The selective purification rate of hydrocarbons of the oxidation catalyst and / or the three-way catalyst is 98.5% or more, and the selective purification rate of carbon monoxide is 90% or more .
[0010]
Further, in another preferred embodiment of the exhaust gas purification system of the present invention, the oxidation catalyst or the three-way catalyst contains platinum, palladium and an oxide having solid acidity, and 50% or more of the content of platinum and palladium is more than 50%. The oxide having solid acidity is mixed in the same layer.
[0011]
Furthermore, in another preferred embodiment of the exhaust gas purification system of the present invention, the NOx purification catalyst is a NOx selective reduction catalyst that purifies by reacting nitrogen oxides with a reducing component in the lean state of the air / fuel ratio, NOx occlusion type three-way catalyst that temporarily absorbs nitrogen oxides in the lean state and releases nitrogen oxides in the rich state and purifies nitrogen oxides by reducing components, or the theoretical air-fuel ratio A three-way catalyst that reduces and purifies nitrogen oxides under lean conditions in the vicinity of the catalyst and a catalyst according to any combination thereof.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the exhaust gas purification system of the present invention will be described in detail.
As described above, this exhaust gas purification system includes an oxidation catalyst and / or a three-way catalyst that selectively purifies HC and CO among the reducing components, and a NOx purification catalyst that reduces NOx using the reducing components. The oxidation catalyst and / or the three-way catalyst are arranged on the upstream side of the exhaust passage of the internal combustion engine or the combustion apparatus, and the NOx purification catalyst is arranged on the downstream side thereof.
[0013]
Here, the catalytic reaction in the preceding oxidation catalyst / three-way catalyst will be described.
In the vicinity of the stoichiometric air-fuel ratio and in the excessive fuel region (rich region), the exhaust gas discharged from the engine contains hydrogen (H ₂ ) and ammonia (NH ₃ ) in addition to HC and CO as reducing gas components. . These H ₂ and NH ₃ components are also represented by the following formula H ₂ + O ₂ → H ₂ O in an oxidation catalyst or a three-way catalyst that is usually used.
NH ₃ + O ₂ → N ₂ + H ₂ O
NO + H ₂ → N ₂ + H ₂ O
As a result, the reaction is purified in the same manner as reducing components such as HC and CO.
[0014]
For this reason, as an exhaust purification system, when an oxidation catalyst or a reduction catalyst for purifying HC and CO is arranged at the front stage and a reduction catalyst for purifying NOx is arranged at the rear stage so as to work from a region where the exhaust gas temperature is low, This reducing catalyst is not supplied with a reducing agent for reducing and purifying NOx, and NOx is not sufficiently purified.
Therefore, in the conventional exhaust gas purification system as described in Japanese Patent Publication No. 2600609, HC, which is a reducing component for desorbing and purifying NOx by the NOx storage catalyst arranged at the rear stage in the exhaust passage. , CO, H _2, NH _3, etc. are oxidized in the three-way catalyst on the front stage side in the exhaust passage, and the necessary amount of reducing component is not supplied to the NOx storage catalyst in the rear stage. Even if NOx is desorbed in a state where the oxygen concentration is lowered, sufficient NOx purification performance cannot be obtained.
[0015]
Therefore, in the present invention, as an oxidation catalyst or a three-way catalyst, a catalyst that selectively purifies HC and CO, that is, a specific component that purifies HC and CO among the reducing components but hardly purifies H ₂ and NH ₃ . By using the catalyst, the reducing component necessary for treating the NOx absorbed in the NOx purification catalyst in an excess oxygen atmosphere (lean state) is supplied to the subsequent NOx purification catalyst. .
[0016]
As such an oxidation catalyst or a three-way catalyst, the following formula HC + O ₂ → H ₂ O + CO ₂ (1)
2CO + O ₂ → 2CO ₂ (2)
HC + NO → H ₂ O + N ₂ + CO ₂ (3)
CO + NO → CO ₂ + N ₂ (4)
The selective purification rate of HC and CO at that time is required to be 98.5% and 90% or more , specifically, platinum, palladium and solid acidic A catalyst in which 50% or more of the content of platinum and palladium is mixed in the same catalyst layer as the oxide having solid acidity can be preferably used.
[0017]
The selection purification rate is less than 98.5% in HC, in the less than 90% CO, O2, NO is HC, in addition to reacting with _CO, the following reaction to _{H 2, NH 3 2H 2 +} O 2 → 2H ₂ O… ▲ 5 ▼
4NH ₃ + 3O ₂ → 2N ₂ + 6H ₂ O (6)
4H ₂ + 2NO → 2H ₂ O + N _{2 (} 7)
4NH ₃ + 6NO → 6H ₂ O + 5N ₂ ... (8)
As a result, H ₂ and NH ₃ are consumed by the oxidation reaction.
Further, when the mixed amount of platinum and palladium is less than 50% of these contents, the reactions (5) to (8) are promoted rather than the reactions (1) to (4), and therefore the reducing agent. H ₂ and NH ₃ as components are reduced.
[0018]
Next, the NOx purification catalyst installed in the latter stage will be described.
As this NOx purification catalyst, it is sufficient if NOx can be reduced and purified by a reducing component, but in this system, this reducing component is mainly composed of H ₂ and / or by the action of the above-mentioned specific oxidation catalyst and / or three-way catalyst. or it comes to NH _3.
Examples of the NOx purification catalyst include a NOx selective reduction catalyst, a NOx occlusion type three-way catalyst, a catalyst according to a predetermined three-way catalyst, and any combination thereof, and the NOx selective reduction catalyst has an air-fuel ratio with excess oxygen. NOx is reacted with a reducing component to purify in a clean state (lean state), and the NOx occlusion type three-way catalyst temporarily absorbs NOx in a state where the air-fuel ratio is excessive in oxygen, and the stoichiometric air-fuel ratio and / or rich state The predetermined three-way catalyst has the function of reducing and purifying NOx under lean conditions in the vicinity of the theoretical air-fuel ratio.
[0019]
As the NOx selective reduction catalyst, copper (Cu), cobalt (Co), nickel (Ni), iron (Fe), gallium (Ga), lanthanum (La), cerium (Ce), zinc (Zn), titanium ( Ti), calcium (Ca), barium (Ba), silver (Ag) and mixed elements thereof, and platinum (Pt), iridium (Ir), rhodium (Rh) or palladium (Pd) and mixed noble metals thereof. Mention may be made of a catalyst comprising zeolite or alumina containing at least one.
[0020]
Further, as the NOx occlusion type three-way catalyst, cesium (Cs), barium (Ba), sodium (Na), potassium (K), magnesium (Mg), lanthanum (La) or calcium (Ca) and mixed metals thereof Examples of the catalyst include an element and platinum (Pt), palladium (Pd), rhodium (Rh), and a mixed noble metal element thereof.
[0021]
Furthermore, as the three-way catalyst for reducing and purifying the NOx under lean conditions near the stoichiometric air-fuel ratio, platinum, palladium or rhodium and mixed noble metal elements thereof, cerium, lanthanum, neodymium (Nd) or praseodymium (Pr) A catalyst containing at least one of rare earth elements such as these and mixed elements thereof and zirconia can be used.
[0022]
In the exhaust gas purification system of the present invention, the NOx reduction purification catalyst used in the subsequent stage is selected according to the lean operation conditions in the internal combustion engine or the like, the NOx selective reduction catalyst, the NOx occlusion type three-way catalyst, and the predetermined three-way catalyst. The catalyst can be selected.
[0023]
Further, in the exhaust gas purification system of the present invention, when the above-described oxidation catalyst, three-way catalyst and NOx purification catalyst are used, it is preferably supported by being used on an integral structure type carrier.
As such a monolithic structure type carrier, a monolithic carrier made of a heat-resistant material is preferable. For example, a ceramic material such as cordierite or a metal material such as ferritic stainless steel is used.
[0024]
Since the NOx purification catalyst at the latter stage preferably has a function as a three-way catalyst at the stoichiometric air-fuel ratio, at least a part of noble metals such as Pt, Pd and Rh is supported on the porous substrate. Among them, it is preferable to be supported on alumina.
As the alumina used in this case, one having high heat resistance is preferable, and activated alumina having a specific surface area of about 50 to 300 m ² / g is particularly preferable.
[0025]
Further, for the purpose of improving the heat resistance of alumina, an additive such as a rare earth compound such as cerium and lanthanum or zirconium may be further added as conventionally used in a three-way catalyst.
Furthermore, in order to enhance the function as a three-way catalyst, materials conventionally used for the three-way catalyst may be added. For example, ceria with oxygen storage function and HC adsorption poisoning to precious metals are alleviated. Barium, zirconia that contributes to improving the heat resistance of Rh may be added.
[0026]
In the purification system of the present invention, the amount of the noble metal of the NOx purification catalyst is not particularly limited as long as the NOx absorption function and the three-way function are sufficiently obtained, but as used in a general three-way catalyst, It is preferably in the range of 0.1 to 10 g per liter.
[0027]
In the exhaust gas purification system of the present invention, it is necessary to change the exhaust gas composition in order to achieve good exhaust gas purification. Due to such fluctuation, the air-fuel ratio is changed to an oxygen-excess state (lean state), a stoichiometric air-fuel ratio (stoichiometry). ) And an excess fuel state (rich state).
For example, in order to generate a relatively large amount of H ₂ from the exhaust gas of an engine, it is necessary to perform a concentration fluctuation that increases the fuel concentration. However, such a concentration fluctuation is performed at regular intervals regardless of driving conditions. In addition, in consideration of running conditions, the NOx absorption amount in the NOx purification catalyst (NOx storage type three-way catalyst) is increased, or the NOx purification catalyst can easily reduce and purify NOx ( (Temperature conditions, exhaust gas flow rate conditions, etc.)
[0028]
【Example】
EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, this invention is not limited to these Examples.
FIG. 1 schematically shows the configuration of an exhaust gas purification system constructed in the following embodiment. In this figure, this exhaust gas purification system is configured by installing an oxidation catalyst / three-way catalyst 2 and a NOx purification catalyst 3 in the exhaust system of the engine 1, and the oxidation catalyst / three-way catalyst 2 is the exhaust gas of the engine 1. The NOx purification catalyst 3, which is installed immediately after the outlet and selectively purifies HC and CO in the exhaust gas, and is installed in the subsequent stage, oxidizes and absorbs NOx contained in the exhaust gas in an oxygen-excess atmosphere, Further, when the reducing component is supplied, NOx is purified by a reduction process.
[0029]
Example 1
[Preparation of oxidation catalyst / three-way catalyst]
An aqueous solution of ammonium tungstate and a commercially available titania sol were mixed, and the resulting precipitate was fired to prepare W-Ti oxide powder (powder A) containing equimolar amounts of W and Ti. Further, powder A is impregnated with a dinitrodiammine platinum aqueous solution and a palladium nitrate aqueous solution, dried at 150 ° C. for 12 hours, and then fired at 400 ° C. for 1 hour to obtain Pt, Pd-supported W—Ti oxide powder (powder B). Obtained. The noble metal concentration of the powder B was 4.0% by weight and Pt / Pd = 0.2.
Similarly, the activated alumina powder is impregnated with a dinitrodiammine platinum aqueous solution and a palladium nitrate aqueous solution, dried at 150 ° C. for 12 hours, and then fired at 400 ° C. for 1 hour to obtain a Pt, Pd-supported activated alumina powder (powder C). It was. The noble metal concentration of the powder C was set to 4.0 wt% and Pt / Pd = 0.2 as in the case of the powder B.
[0030]
100 g of powder B, 100 g of powder C and 200 g of nitric acid aqueous solution were put into a magnetic ball mill, mixed and pulverized to obtain a slurry.
This slurry was adhered to a cordierite monolith support (0.1 L, 400 cells / in 2), excess slurry in the cells was removed and dried with an air stream, and calcined at 500 ° C. for 1 hour. A catalyst A having a coat weight of 100 g / L-support was obtained. The amount of noble metal supported was 4.0 g / L-carrier.
[0031]
[Preparation of NOx purification catalyst]
An activated alumina powder was impregnated with a Pd nitrate aqueous solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder B). The Pd concentration of this powder was 5.0% by weight.
An activated alumina powder was impregnated with an aqueous Rh nitrate solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain an Rh-supported alumina powder (powder C). The Rh concentration of this powder was 3.0% by weight.
347 g of powder B, 58 g of powder C, 496 g of activated alumina powder, and 900 g of water were put into a magnetic ball mill, mixed and ground to obtain a slurry liquid. The grinding time was 1 hour. This slurry liquid is attached to a cordierite monolith carrier (1.3 L, 400 cells), excess slurry in the cells is removed with an air flow, dried at 130 ° C., and then baked at 400 ° C. for 1 hour. A layer weight of 200 g / L-carrier was obtained. Further, an impregnation support was carried out on the coated carrier using an aqueous barium acetate solution, dried at 120 ° C. and then calcined at 400 ° C. to prepare a NOx purification catalyst.
[0032]
[Construction of exhaust gas purification system]
Using the oxidation catalyst / three-way catalyst and NOx purification catalyst obtained as described above, as shown in FIG. 1, an oxidation catalyst / three-way catalyst is installed in the front stage of the engine 1, and a NOx purification catalyst is installed in the rear stage. The exhaust gas purification system of this example was constructed.
[0033]
(Example 2)
As the oxidation catalyst / three-way catalyst, one prepared in the same manner as in Example 1 was used. The NOx purification catalyst was prepared by repeating the same operation as in Example 1 except that cesium carbonate was used instead of barium acetate. Both catalysts were installed in the same manner as in Example 1 to construct an exhaust gas purification system of this example.
[0034]
Example 3
The oxidation catalyst / three-way catalyst was prepared as in Example 1. A so-called NOx selective reduction catalyst was used as the NOx purification catalyst. Both catalysts were installed in the same manner as in Example 1 to construct an exhaust gas purification system of this example.
The NOx selective reduction catalyst used was prepared by the following operation.
[0035]
[Preparation of NOx selective reduction catalyst]
Add 1000% of cerium, zirconium, and lanthanum-supported activated alumina powder containing 3 mol% of cerium, 3 mol% of zirconium, and 2 mol% of lanthanum using a palladium nitrate solution to add 2.0 wt% of palladium. After stirring, it was dried in an oven at 150 ° C. for 3 hours and calcined at 400 ° C. for 2 hours in an air atmosphere. 1500 g of this palladium-supported activated alumina, 3 mol% of cerium, 3 mol% of zirconium, 800 g of cerium, zirconium and lanthanum-supported activated alumina powder containing 2 mol% of lanthanum, 460 g of 10 wt% HNO ₃ nitric acid, and 1840 g of water are placed in a ball mill pot. The slurry was charged and pulverized for 8 hours to obtain a slurry. The obtained slurry was applied to a monolith honeycomb carrier substrate (1.3 L, 400 cells), dried, and then fired at 400 ° C. for 2 hours in an air atmosphere. The coating amount at this time was 52 g / piece after firing.
[0036]
Next, γ-alumina as a main component is 3 mol% cerium, 3 mol% zirconium, 2 mol% cerium, zirconium, lanthanum-supporting activated alumina powder 2000 g, 400 wt% nitric acid 400 g, and water 1600 g are ball mill pots. The slurry obtained by pulverizing for 8 hours was applied so that the applied amount after baking was 52 g / piece, dried, and then fired in an air atmosphere at 400 ° C. for 2 hours.
Further, 5.2 kg of 0.2 mol / L copper nitrate or copper acetate solution and 2 kg of zeolite powder were mixed and stirred, followed by filtration. After repeating this three times, drying and firing were performed to prepare a zeolite powder in which Cu was ion-exchanged. 1890 g of this Cu ion-exchanged zeolite powder, 1150 g of silica sol (solid content 20%) and 1100 g of water were put in a magnetic ball mill, and the slurry obtained by pulverization was applied to the above carrier so that the coating amount was 325 g / piece. After drying, the mixture was calcined at 400 ° C. for 2 hours to prepare a catalyst.
[0037]
(Example 4)
The oxidation catalyst / three-way catalyst was prepared as in Example 1. A specific three-way catalyst was used as the NOx purification catalyst. Both catalysts were installed in the same manner as in Example 1 to construct an exhaust gas purification system of this example.
The three-way catalyst used was prepared by the following operation.
[0038]
[Preparation of specific three-way catalyst]
First, activated alumina containing γ-alumina as a main component was impregnated with a cerium nitrate solution and a barium nitrate solution, dried, and then fired at 500 ° C. for 1 hour. At this time, the cerium carrying concentration was 7 wt%, and the barium concentration was 5 wt%. The powder thus obtained was impregnated with an aqueous palladium nitrate solution, dried and then calcined at 400 ° C. for 1 hour to obtain a Pd-supported activated alumina powder. The supported concentration of Pd was 1.00% by weight. A slurry obtained by mixing and pulverizing 700 g of this powder, 300 g of cerium oxide powder, and 1000 g of alumina sol by a ball mill was attached to a monolith support substrate (1.3 L, 400 cells) and fired (400 ° C., 1 hour). The adhesion amount at this time was set to 200 g / L. A three-way catalyst was thus obtained. The amount of Pd in this three-way catalyst was 1.8 g / piece.
[0039]
(Example 5)
The oxidation catalyst / three-way catalyst was prepared in the same manner as in Example 1. As the NOx purification catalyst, two NOx purification catalysts prepared in Example 1 were arranged in tandem in the same catalytic converter. Both catalysts were installed in the same manner as in Example 1 to construct an exhaust gas purification system of this example.
In addition, the normal manifold three-way catalyst used was prepared by the following operation.
[0040]
[Preparation of ordinary Mani three-way catalyst]
An activated alumina powder was impregnated with a Pd nitrate aqueous solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder 1). The Pd concentration of this powder was 17.0% by weight.
An activated alumina powder to which cerium and zirconium were added was impregnated with an aqueous Rh nitrate solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain an Rh-supported alumina powder (powder 2). The Rh concentration of this powder was 3.0% by weight.
[0041]
190.7g of powder 1, 54.0g of powder 2, 49g of cerium oxide powder, 506.3g of activated alumina powder and 1000g of alumina sol were charged into a magnetic ball mill, and mixed and ground for 1 hour to obtain a slurry liquid. This slurry liquid is attached to a cordierite monolith carrier (1.3 L, 400 cells), excess slurry in the cells is removed with an air flow, dried at 130 ° C., and then baked at 400 ° C. for 1 hour. A layer weight of 140 g / L-carrier was obtained.
Further, impregnation support was carried out on the coated carrier using an aqueous barium acetate solution, dried at 120 ° C. and then calcined at 400 ° C. to prepare a catalyst. The amount of noble metal at this time was such that the ratio of palladium / rhodium was 20/1 and the total amount of noble metal was 7 g / L.
[0042]
(Comparative example)
The exhaust gas purification system of this example was constructed by arranging the above-described ordinary manifold three-way catalyst in the front stage and the NOx purification catalyst obtained as follows in the rear stage.
[0043]
An activated alumina powder was impregnated with a Pd nitrate aqueous solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain a Pd-supported alumina powder (powder 1). The Pd concentration of this powder was 17.0% by weight.
An activated alumina powder to which cerium and zirconium were added was impregnated with an aqueous Rh nitrate solution, dried, and then fired in air at 400 ° C. for 1 hour to obtain an Rh-supported alumina powder (powder 2). The Rh concentration of this powder was 3.0% by weight.
[0044]
190.7 g of powder 1 used in Example 1, 54.0 g of powder 2, 49 g of cerium oxide powder, 506.3 g of activated alumina powder and 1000 g of alumina sol were put into a magnetic ball mill, mixed and ground for 1 hour, and then slurryed. A liquid was obtained. This slurry liquid is attached to a cordierite monolith carrier (1.3 L, 400 cells), excess slurry in the cells is removed with an air flow, dried at 130 ° C., and then baked at 400 ° C. for 1 hour. A layer weight of 140 g / L-carrier was obtained.
Further, impregnation support was carried out on the coated carrier using an aqueous barium acetate solution, dried at 120 ° C. and then calcined at 400 ° C. to prepare a catalyst. The amount of noble metal at this time was such that the ratio of palladium / rhodium was 20/1 and the total amount of noble metal was 7 g / L to obtain a NOx purification catalyst.
[0045]
<Performance evaluation>
The exhaust gas purification system in each of the above examples was applied to a passenger car equipped with a direct-injection gasoline engine having a displacement of 1.8 L, and the exhaust gas purification performance was evaluated. The obtained results are shown in Table 1.
[0046]
[Table 1]

[0047]
【The invention's effect】
As described above, according to the present invention, the oxidation catalyst and the three-way catalyst that selectively purify HC and CO are arranged upstream of the NOx purification catalyst, and therefore, by operating with excess oxygen. It is possible to provide an exhaust gas purification system that can sufficiently enjoy the fuel efficiency improvement effect, efficiently purify HC and CO components, and particularly efficiently purify HC and CO discharged at a low temperature immediately after engine startup.
That is, when the exhaust gas purification system of the present invention is used, high NOx purification processing can be performed in a wide A / F region under an oxygen-excess atmosphere, and not only fuel efficiency can be improved, but also HC and CO purification performance and NOx purification performance can be achieved. Both can be achieved at a high conversion rate.
[Brief description of the drawings]
FIG. 1 is a system configuration diagram showing an embodiment of an exhaust gas purification system of the present invention.
[Explanation of symbols]
1 Engine 2 Oxidation catalyst / Three-way catalyst 3 NOx purification catalyst

Claims (7)

NOx purification that selectively removes hydrocarbons and carbon monoxide among the reducing components , and that reduces and eliminates nitrogen oxides using an oxidation catalyst and / or three-way catalyst that hardly removes hydrogen and ammonia. The exhaust gas composition is installed in the exhaust passage of an internal combustion engine or a combustion device that takes a so-called lean state in which the air-fuel ratio is in an oxygen-excess state, a stoichiometric air-fuel ratio or a so-called rich state in which the fuel is excessive ,
The oxidation catalyst and / or the three-way catalyst is disposed upstream of the exhaust gas passage of the NOx purification catalyst,
An exhaust gas purification system characterized in that the selective purification rate of hydrocarbons of the oxidation catalyst and / or the three-way catalyst is 98.5% or more and the selective purification rate of carbon monoxide is 90% or more . The oxidation catalyst or the three-way catalyst contains platinum, palladium, and an oxide having solid acidity, and 50% or more of the content of platinum and palladium is mixed in the same layer as the oxide having solid acidity. The exhaust gas purification system according to claim 1 . 3. The exhaust gas purification system according to claim 2, wherein the oxide having solid acidity is tungsten-titanium oxide. The NOx purification catalyst is a NOx selective reduction catalyst that purifies by reacting nitrogen oxides with a reducing component when the air-fuel ratio is in the lean state, and the air-fuel ratio temporarily absorbs nitrogen oxides when the air-fuel ratio is in the lean state. / Or NOx occlusion type three-way catalyst that releases nitrogen oxides in the rich state and purifies nitrogen oxides by reducing components, or three-way catalyst that reduces and purifies nitrogen oxides under lean conditions near the stoichiometric air-fuel ratio Or it is a catalyst which concerns on these arbitrary combinations, The exhaust gas purification system as described in any one of Claims 1-3 characterized by the above-mentioned. The NOx selective reduction catalyst is at least one element selected from the group consisting of copper, cobalt, nickel, iron, gallium, lanthanum, cerium, zinc, titanium, calcium, barium and silver, and / or platinum, iridium, 5. The exhaust gas purification system according to claim 4 , comprising zeolite or alumina containing at least one noble metal selected from the group consisting of rhodium and palladium. The NOx occlusion type three-way catalyst is at least one metal element selected from the group consisting of cesium, barium, sodium, potassium, magnesium, lanthanum and calcium, and at least selected from the group consisting of platinum, palladium and rhodium The exhaust gas purification system according to claim 4 or 5 , comprising one kind of noble metal element. The three-way catalyst for reducing and purifying nitrogen oxides is at least one kind selected from the group consisting of at least one noble metal element selected from the group consisting of platinum, palladium and rhodium, and cerium, lanthanum, neodymium and praseodymium. The exhaust gas purification system according to any one of claims 4 to 6 , wherein the rare earth element and / or zirconia is included.

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