JP4090547B2 - Exhaust gas purification catalyst - Google Patents

Description

【００３１】
【表２】

【００３２】
【表３】

【００３３】
【発明の効果】
本発明に従い、ＲｈをＰｔの上層に配置し、ＮＯｘ吸蔵材を下層又は下層と上層両方に分散し、かつ窒素酸化物吸蔵材にバリウムとその複合体を使用することにより、酸素過剰な雰囲気においても効率良く窒素酸化物を除去することができる。
【００３４】
以上の説明に関して更に以下の項を開示する。
1. 酸素過剰の雰囲気下で排気ガス中の窒素酸化物（ＮＯｘ）、一酸化炭素（ＣＯ）及び炭化水素（ＨＣ）を浄化する排ガス浄化用触媒において、アルミナと、酸化セリウムまたはセリウムを含む酸素吸蔵放出特性を持つ酸化物とを含む担体と；アルカリ金属、アルカリ土類金属及び希土類元素からなる群から選ばれ、該担体に担持されたＮＯｘ吸蔵材と該担体に担持された白金（Ｐｔ）とを含んでなる第一層（下層）と；アルミナと、酸化セリウム、酸化ジルコニウムまたはセリウム−ジルコニウム複合酸化物とを含む担体に担持されたロジウム（Ｒｈ）を含んでなる第二層（上層）とを有することを特徴とする排ガス浄化用触媒。
2. ＮＯｘ吸蔵材をＲｈを含む上層にも分散させてなる第１項記載の排ガス浄化用触媒。
3. ＮＯｘ吸蔵材がバリウムである第２項記載の排ガス浄化用触媒。
4. ＮＯｘ吸蔵材が、マンガン、鉄、コバルト、ニッケル、銅及び亜鉛からなる群から選ばれた少なくとも一種とバリウム及びバリウムとカリウムの複合体からなる群から選ばれた少なくとも一種類との複合体からなるものである第１項記載の排ガス浄化用触媒。
5. ＮＯｘ吸蔵材が、バリウムと、ジルコニウム及びカリウムからなる群から選ばれた少なくとも一種類との複合体からなるものである第１項記載の排ガス浄化用触媒。
6. 複合酸化物が、酸化セリウムと酸化ジルコニウムとの複合酸化物であり、セリウムとジルコニウムとの比率が原子比でジルコニウム２０〜９９．９％、セリウム０．１〜８０％である第１項記載の排ガス浄化用触媒。
7. 酸化セリウムと酸化ジルコニウムとの複合酸化物が原子比でＬａ０．１〜１０％、Ｎｄ０．１〜１０％、Ｙ又は他の希土類０〜１０％を含むことを特徴とする第６項記載の排ガス浄化用触媒。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst containing nitrogen oxide (NOx), carbon monoxide (CO), and hydrocarbon (HC).
[0002]
[Prior art]
In many cases, exhaust gases from automobiles, power plants, nitric acid manufacturing plants, etc. contain a large amount of NOx, CO, and HC sufficient to cause damage to human health and the environment. In particular, there are many cases where NOx is particularly problematic.
[0003]
Conventionally, a technique called an ammonia selective reduction method has been adopted in order to remove NOx in a fixed exhaust gas generation source such as a nitric acid manufacturing plant or a power plant. This method is generally a method of reducing NOx using a component containing vanadium as a catalyst and ammonia as a reducing agent. The disadvantage of this method is the use of toxic ammonia.
[0004]
Further, a so-called three-way catalyst having Pt, Pd or Rh as a main component is mainly used in an exhaust gas movement source such as an automobile. In automobiles, the ratio of the amount of oxygen (air) required to burn gasoline, which is the fuel for driving the engine, and the amount of gasoline (hereinafter abbreviated as air-fuel ratio) is close to stoichiometrically 1. The reducing agents HC and CO and the oxidizing agents NOx and oxygen contained therein also have almost stoichiometric compositions. The three-way catalyst can remove HC, CO and NOx at the same time using such a composition.
[0005]
However, in recent years, people's interest in environmental issues and energy issues has increased rapidly, and many efforts have been made to counter global warming and ozone depletion as well as impact on human health. In the field of automobiles, so-called lean burn engines have been developed with the goal of low fuel consumption and low pollution. Unlike conventional engines, lean burn engines achieve low fuel consumption by controlling the air-fuel ratio to be extremely high. Naturally, the exhaust gas composition from the lean burn engine is also affected by the oxygen (air) and fuel air-fuel ratio introduced into the engine, resulting in an oxygen-excessive oxidizing atmosphere. Even if HC and CO can be removed, purification of NOx becomes difficult. There is the above-described ammonia method as a technique that can effectively remove NOx even in such an oxidizing atmosphere, but it is obvious that it is difficult to apply to a moving body such as an automobile.
[0006]
In order to solve this problem, recently, application of a NOx storage catalyst has been proposed. This uses a material capable of storing NOx as a catalyst, and temporarily stores NOx when the engine is operated under conditions leaner than stoichiometric. The stored NOx is reduced and removed by forcibly switching the air-fuel ratio instantaneously to fuel-rich for every predetermined period before the NOx storage in the storage material becomes saturated. However, many of the catalysts proposed to date based on such a mechanism are insufficient for NOx storage capacity and heat resistance.
[0007]
[Problems to be solved by the present invention]
Provided is a catalyst excellent in high-temperature heat resistance that can efficiently remove pollutants in exhaust gas discharged from an internal combustion engine and an external combustion engine driven in an air-fuel ratio range from a stoichiometric range to a lean burn range.
[0008]
[Means for Solving the Problems]
As a result of intensive studies, the inventors have dispersed NOx occlusion material mainly composed of barium in the same first layer (lower layer) as Pt, while Rh is separated into a second layer (upper layer) different from the layer containing Pt. Arrangement has led to the invention of a catalyst having a high NOx removal ability not only in a fresh state (after preparation but before high temperature treatment) but also in a high oxygen atmosphere even after high temperature treatment. Further, the NOx occlusion material can be dispersed in a layer containing Rh.
[0009]
The present invention relates to an exhaust gas purifying catalyst for purifying nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbon (HC) in exhaust gas in an oxygen-excess atmosphere. In the exhaust gas purifying catalyst, alumina and cerium oxide or cerium are used. A support containing an oxide having oxygen storage / release characteristics, including: an NOx storage material selected from the group consisting of alkali metals, alkaline earth metals, and rare earth elements, and platinum supported on the support (platinum ( A second layer comprising rhodium (Rh) supported on a carrier comprising alumina and cerium oxide-zirconium oxide or cerium-zirconium composite oxide. And an exhaust gas-purifying catalyst.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, cerium oxide is included in order to maintain the heat resistance of Pt and the ternary activity in the stoichiometric region. Further, alumina and zirconium oxide may be used as the carrier for Rh, and it is desirable to use zirconium oxide stabilized with a rare earth element such as cerium or neodymium in order to provide higher temperature heat resistance.
Barium is effective as the NOx storage material. In order to maintain heat resistance and NOx occlusion ability in a wider temperature range, it is effective to combine an additive such as potassium, zirconium or manganese with barium.
[0011]
In the present invention, more Pt is placed in the vicinity of the NOx storage material, which is important for allowing the NOx storage stage to proceed at a high rate. In an ordinary three-way catalyst, Pt and Rh are dispersed in the same catalyst layer in order to utilize Rh, and a part of the Pt—Rh alloy is made. It is covered with Rh on the particle surface and negatively affects the NO oxidation ability of Pt. Therefore, it is necessary to separate Pt from Rh in order to maintain high NO oxidation activity. In the stage of releasing and reducing NOx, the stored NOx is released all at once, so the NOx reduction rate is important. Needless to say, Rh is effective as a catalyst for this purpose. In order to increase the activity of Rh, a high surface area and high heat resistance carrier is required. Furthermore, in the process of reducing NOx, it is desirable to increase the chance of contacting NOx and Rh as much as possible. Therefore, the layer containing Rh is disposed above the layer containing Pt and NOx storage material. In this case, the NOx occlusion material can be disposed also in the layer containing Rh.
[0012]
In the production of the catalyst, first, cerium oxide can be added to alumina as cerium oxide powder, cerium hydroxide, or cerium carbonate. Further, other cerium compounds may be added by an impregnation method or the like. The cerium compound that can be used is not particularly limited as long as it can be dissolved in water, an organic solvent, or a mixture of water and an organic solvent. In the case of Pt, Pt is dispersed in a mixture of alumina and cerium oxide using a Pt compound by an impregnation method or an ion exchange method. The Pt compound that can be used is not particularly limited as long as it can be dissolved in water, an organic solvent, or a mixture of water and an organic solvent. Barium and its composites are added in the same way as cerium oxide, when insoluble materials such as barium oxide, barium carbonate, and barium hydroxide are used, dissolve in water or an organic solvent or a mixture of water and organic solvent. A thing can be added by the impregnation method or the ion exchange method. The order of addition to alumina is not particularly limited, and any one of cerium, Pt, and NOx storage material may be added first, or two or three may be added simultaneously.
[0013]
Potassium, zirconium, manganese, iron, cobalt, nickel, copper and zinc other than barium can be added by the same method as that for adding barium. The order of addition is not particularly limited. In addition, when adding as a complex of barium and these elements, the compound of barium and these elements is coprecipitation or evaporation to dryness if all the related elements are soluble, otherwise impregnation, ion exchange or kneading. Prepared by the method, and if necessary, dry firing. The resulting composite of barium and other elements is added to the catalyst by the same method of addition as barium.
[0014]
The catalyst of the layer containing Rh is dispersed on the support by an impregnation method or an ion exchange method using an Rh salt soluble in water or an organic solvent. Examples of Rh carriers include alumina, zirconium oxide, or zirconium oxide stabilized by rare earth oxides. As the composition of the stabilized zirconium oxide, the ratio of cerium to zirconium is 20 to 99.9% zirconium and 0.1 to 80% cerium in atomic ratio, and 0 to 10% La and 0 in atomic ratio. May contain -10% Nd, 0-10% Y or other rare earths.
According to the method of the present invention, the exhaust gas purifying catalyst of the present invention having the above-described configuration is brought into contact with an exhaust gas containing nitrogen oxide, carbon monoxide and hydrocarbons, whereby nitrogen oxide in such exhaust gas is obtained. Can be removed effectively, and can be advantageously used for purification of such exhaust gas.
[0015]
【Example】
Example 1
Catalyst No. 1
Preparation of first layer washcoat and its application to honeycomb:
To the commercially available Al ₂ O ₃ (BET specific surface area 200 m ² / g) powder, cerium oxide powder was added and mixed so that the ratio of cerium oxide in the total mixture was 25% by weight. Further, this mixed powder was pulverized with water and acetic acid for 1 hour in a ball mill to form a slurry. A 1 inch diameter, 6.7 cm long core hollowed out from a commercially available 400 cell cordierite honeycomb was dipped into this slurry, then pulled up, and after removing excess slurry with an air gun, it was dried at 105 ° C. for 30 minutes. Furthermore, it baked at 500 degreeC for 1 hour. The amount of the adhered slurry was 150 g per liter of honeycomb after firing.
[0016]
Loading Pt on the first layer honeycomb catalyst:
Based on the water absorption amount of the honeycomb, a solution of [Pt (NH ₃ ) ₄ ] Cl ₂ was prepared so that 3.5 g of Pt could be supported per liter of the honeycomb. After immersing the honeycomb with the slurry in this solution, the excess solution was removed with an air gun, dried at 105 ° C. for 30 minutes, and fired at 500 ° C. for 1 hour to obtain a Pt-supported first layer.
[0017]
Barium loading on first layer honeycomb catalyst:
After supporting Pt, barium acetate was supported by the same method as in the case of Pt so that the supported amount of Ba (in metal conversion) was 25 g / L.
[0018]
Preparation of the second layer washcoat and its stabilized zirconium oxide (BET specific surface area 60 m ² / g, CeO ₂ 15 wt%, ZrO ₂ 75 wt%) powder prepared by the co-precipitation method on the honeycomb Water was added while stirring until wet, and the amount of water absorption per gram was first measured. Based on this water absorption, a solution of Rh (NO ₃ ) ₃ was prepared so that the final Rh content was 1.64% by weight, and the solution was stirred well while adding this solution to zirconium oxide. The resulting mixture was dried at 105 ° C. overnight and calcined in air at 500 ° C. for 2 hours. Subsequently, this powder and Rh-free alumina were mixed at a weight ratio of 1: 1, and pulverized with water and acetic acid for 1 hour in a ball mill to form a slurry. This slurry was applied to the 400-cell cordierite core coated with the first layer of Pt-containing washcoat obtained in the same manner as in the case of the first layer of washcoat, and the barium oxide and zirconium oxide composite was coated. Catalyst No. 2 having a two-layer structure containing 1 was obtained. The amount of the attached second layer washcoat was about 60 g / L, and the amount of Rh contained therein was 0.35 g / L.
[0019]
Example 2
Catalyst No. 2
Catalyst No. 2 having a two-layer structure containing barium in exactly the same manner as in Example 1 except that the amount of barium supported was 75 g per liter of honeycomb catalyst. 2 was obtained.
[0020]
Example 3
Catalyst No. 3
In Example 1, the amount of barium supported in the first layer was 45 g / L, the layer containing Rh was supported on the honeycomb, and then barium was further added in the same manner as in the case where barium was supported in the first layer. The catalyst no. 3 was obtained.
[0021]
Example 4
Catalyst No. 4
Prepared in the same manner as in Example 1 except that manganese was supported from the Mn (NO ₃ ) ₂ solution so as to be 9.2 g / L in terms of Mn before supporting barium on the first layer honeycomb catalyst. Catalyst No. 2 having a two-layer structure containing barium and manganese. 4 was obtained.
[0022]
Example 5
Catalyst No. 5
A double-layer structure containing barium and cobalt was prepared in the same manner as in Example 4 except that cobalt was supported from a Co (NO ₃ ) ₂ solution so that cobalt would be 10.4 g / L instead of manganese. Catalyst No. 5 was obtained.
[0023]
Example 6
Catalyst No. 6
Barium and potassium were prepared in the same manner as in Example 1 except that after loading barium on the first-layer honeycomb catalyst, potassium was loaded from the KNO ₃ solution so that the potassium was 5.3 g / L. Catalyst No. 2 having a two-layer structure containing 6 was obtained.
[0024]
Example 7
Catalyst No. 7
Prepared in the same manner as in Example 3 except that after the manganese and barium were supported on the first layer honeycomb catalyst, potassium was supported from the KNO ₃ solution so as to be 5.3 g / L in terms of K, barium, Catalyst No. 2 having a two-layer structure containing manganese and potassium. 7 was obtained.
[0025]
Comparative Example 1
Catalyst No. R1
To the commercially available Al ₂ O ₃ (BET specific surface area 200 m ² / g) powder, cerium oxide powder was added and mixed so that the proportion of cerium oxide in the total mixture was 30% by weight. This mixed powder was pulverized with water and acetic acid in a ball mill for 1 hour to form a slurry. A 1 inch diameter, 6.7 cm long core hollowed out from a commercially available 400 cell cordierite honeycomb was dipped into this slurry, then pulled up, and after removing excess slurry with an air gun, it was dried at 105 ° C. for 30 minutes. Furthermore, it baked at 500 degreeC for 1 hour. The amount of the adhered slurry was 150 g per liter of honeycomb after firing.
[0026]
Based on the water absorption of the honeycomb, a mixed solution of [Pt (NH ₃ ) ₄ ] Cl ₂ and Rh (NO ₃ ) ₃ was prepared so that 3.5 g of Pt and 0.35 g of Rh could be supported. After immersing the honeycomb with the slurry in this solution, the excess solution was removed with an air gun, dried at 105 ° C. for 30 minutes, and fired at 500 ° C. for 1 hour. Thereafter, based on the amount of water absorbed by the catalyst, barium is supported using barium acetate in the same manner as in the case of Pt and Rh, so that the supported amount of Ba is 25 g / L, and barium is contained. A Pt—Rh catalyst (catalyst No. R1) was obtained.
[0027]
Comparative Example 2
Catalyst No. R2
Except that the supported amount of barium was 75 g per liter of honeycomb catalyst, the catalyst No. R2 was obtained.
[0028]
Evaluation methods:
High temperature treatment:
An electric furnace surrounded by an electric heating body was used around a quartz tube through which a gas can flow, and a catalyst core was placed on the quartz tube, and the temperature was raised to a predetermined temperature in nitrogen. At that temperature, a reducing atmosphere (6% by volume of hydrogen + 94% by volume of nitrogen) and an atmospheric gas of an oxidizing atmosphere (3% by volume of oxygen + 97% by volume of nitrogen) were allowed to flow for 10 minutes to form one cycle, which was repeated for 6 hours. The total flow rate was 3 liters / minute, and the treatment temperature was 800 ° C.
[0029]
Activity test method:
The high temperature treated catalyst core was placed in a normal flow reactor and fixed around the core so that no gas could pass through it. The core was held in the reaction gas composition 1 shown in Table 1 for 15 minutes at 500 ° C., and then the temperature was raised to a predetermined test temperature in the gas. When the temperature became stable, measurement of NOx was started. Switching to gas composition 1 for 3 minutes, then switching to gas composition 2 and holding each for 3 minutes was taken as 1 cycle, and this was repeated 5 cycles. In each cycle, the average value of the NOx purification rate (see the following formula) for 40 seconds after the gas composition 2 was reached was defined as the catalyst purification performance. The test results for each catalyst are shown in Tables 2 and 3.
NOx purification rate = (inlet NOx concentration−outlet NOx concentration) × 100% / inlet NOx concentration
[Table 1]

[0031]
[Table 2]

[0032]
[Table 3]

[0033]
【The invention's effect】
In accordance with the present invention, by placing Rh in the upper layer of Pt, dispersing the NOx storage material in the lower layer or both the lower layer and the upper layer, and using barium and its composite in the nitrogen oxide storage material, Can efficiently remove nitrogen oxides.
[0034]
The following items are further disclosed with respect to the above description.
1. In an exhaust gas purifying catalyst that purifies nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) in exhaust gas under an oxygen-excess atmosphere, alumina and oxygen containing cerium oxide or cerium A carrier comprising an oxide having an occlusion-release characteristic; selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements, and a NOx occlusion material supported on the carrier and platinum (Pt) supported on the carrier A second layer (upper layer) comprising rhodium (Rh) supported on a support containing alumina and cerium oxide, zirconium oxide or a cerium-zirconium composite oxide. An exhaust gas purifying catalyst characterized by comprising:
2. The exhaust gas purifying catalyst according to item 1, wherein the NOx occlusion material is dispersed also in an upper layer containing Rh.
3. The exhaust gas purifying catalyst according to item 2, wherein the NOx storage material is barium.
4. The NOx storage material is a composite of at least one selected from the group consisting of manganese, iron, cobalt, nickel, copper and zinc and at least one selected from the group consisting of barium and a composite of barium and potassium. The exhaust gas-purifying catalyst as set forth in claim 1, comprising:
5. The exhaust gas purifying catalyst according to item 1, wherein the NOx storage material is composed of a composite of barium and at least one selected from the group consisting of zirconium and potassium.
6. The composite oxide is a composite oxide of cerium oxide and zirconium oxide, and the ratio of cerium to zirconium is 20 to 99.9% zirconium and 0.1 to 80% cerium in terms of atomic ratio. The catalyst for exhaust gas purification as described.
7. The composite oxide of cerium oxide and zirconium oxide containing La 0.1 to 10%, Nd 0.1 to 10%, Y or other rare earth 0 to 10% by atomic ratio Exhaust gas purification catalyst.

Claims (6)

An exhaust gas purifying catalyst that purifies nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbons (HC) in exhaust gas in an oxygen-excess atmosphere,
A carrier containing alumina and cerium oxide or an oxide having oxygen storage / release characteristics including cerium; a NOx occlusion material supported on the carrier; and platinum (Pt) supported on the carrier. A first layer (lower layer),
A second layer (upper layer) comprising rhodium (Rh) supported on alumina and a carrier containing cerium oxide, zirconium oxide or cerium-zirconium composite oxide;
The NOx occlusion material is also dispersed in the second layer (upper layer) containing the rhodium (Rh) ,
The NOx storage material are those selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements, and, when the air-fuel ratio is rich, is to reduce and remove occluded NOx, exhaust gas purification Catalyst.   The exhaust gas-purifying catalyst according to claim 1, wherein the NOx storage material is barium.   The NOx storage material is at least one selected from the group consisting of manganese, iron, cobalt, nickel, copper and zinc, and at least one selected from the group consisting of barium, a barium and potassium complex, and The exhaust gas-purifying catalyst according to claim 1 or 2, comprising the composite of the above.   The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the NOx storage material is composed of a composite of barium and at least one selected from the group consisting of zirconium and potassium. .   The composite oxide is a composite oxide of cerium oxide and zirconium oxide, and the ratio of cerium and zirconium is 20 to 99.9% zirconium and 0.1 to 80% cerium in atomic ratio. The exhaust gas-purifying catalyst according to any one of -4.   The composite oxide of cerium oxide and zirconium oxide comprises La 0.1 to 10%, Nd 0.1 to 10%, Y or other rare earth 0 to 10% in atomic ratio. Exhaust gas purification catalyst.