JPH09299748A - Exhaust gas purifying element, production thereof and method for purifying nitrogen oxides - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an exhaust gas purifying element capable of efficiently removing nitrogen oxides from the exhaust gas of combustion, etc., containing excessive oxygen by constituting the element of an electrode containing conductive oxide and a solid electrolyte having oxygen ion conductivity. SOLUTION: This exhaust gas purifying element consists of both the electrode containing the conductive oxide and the solid electrolyte having oxygen ion conductivity. The electrode is constituted of a cathode electrode containing the conductive oxide and an anode electrode consisting of at least either one of Ag, Au, Pt and Pd. The conductive oxide is preferably defined as <=1.0×1.0<-2> Ω.cm at 300k relative resistance, RuO2 , WO2 , Fe3 O4 , MnO2 and ReO2 are exemplified therefor. Stabilized zirconia having high conductivity and especially high strength is preferably used as the solid electrolyte having oxygen ion conductivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying element, a method for manufacturing the same, and a method for purifying nitrogen oxides, which is an atmospheric pollutant nitrogen oxides from a combustion gas containing oxygen. is there.

[0002]

2. Description of the Related Art From the viewpoint of environmental protection, purification of air pollutants is a major social issue. In particular, purification of combustion exhaust gas accompanying the expansion of industrial activities is currently an urgent issue.

Nitrogen oxides contained in combustion exhaust gas discharged from factories, which are fixed sources, and automobiles, which are mobile sources, are said to be the cause of photochemical smog and are gases harmful to the human body. In particular, nitric oxide (NO) is difficult to purify, and has become the most important subject for study. Several methods for purifying nitrogen oxides in combustion exhaust gas have been proposed so far. For example, it is a method of decomposing NOx into N ₂ and H ₂ O on a catalyst using a reducing agent such as ammonia. However, this method is expensive in terms of cost because it uses a reducing agent, and it is necessary to take measures for safety regarding storage of the reducing agent and recovery leakage of unreacted reducing agent. Therefore, this method is effective for large-scale fixed sources, but is not suitable for mobile sources such as automobiles. Up to now, a three-way catalyst device using a noble metal-based component as an active component has been generally used for purifying exhaust gas of a gasoline engine operated near the stoichiometric air-fuel ratio. However, these catalyst devices cannot purify nitrogen oxides in the presence of excess oxygen, and therefore cannot be used for exhaust gas treatment of diesel engines and lean-burn gasoline engines. Recently, a method of removing hydrocarbons using a new catalyst device by using them as a reducing agent has been proposed.
There are many problems such as low x reduction efficiency and insufficient catalyst durability. Therefore, development of a new removal method is desired.

By the way, NO is directly added to N ₂ without using a reducing agent.
The method of decomposing into O ₂ and O ₂ is an ideal treatment method since it requires only passing the exhaust gas through the treatment equipment. However, in the method using a catalyst that has been proposed so far, oxygen generated by decomposition of NOx is not desorbed from the surface of the catalyst, and thus deactivated. In recent years, a method of electrochemically decomposing and removing nitrogen oxides using an element composed of a solid electrolyte having oxygen ion conductivity has been reported in Jounal of Electrochemical Society, 122,
896, (1975). That is, a porous Pt electrode is coated on both sides of scandium oxide-stabilized zirconia, and a gas containing nitrogen oxide containing no oxygen is supplied to one side of the electrode, and both electrodes are formed so that this electrode becomes the cathode electrode. Apply DC voltage. As a result, nitrogen oxides are decomposed into nitrogen and oxygen on the electrode, and the generated oxygen is ionized, and this is moved from the electrode to the counter electrode side through the solid electrolyte from the electrode using the applied voltage as a driving force, and is eliminated from the reaction system ( At this time, oxygen ions flow in the solid electrolyte, so that a current flows in the element and electric energy is consumed). As a result, it is shown that the decomposition of nitrogen oxide can be promoted.

However, in the above method, when excess oxygen is contained as in the combustion exhaust gas, the decomposition reaction of nitrogen oxides does not proceed in the above element, and only the oxygen that is present in excess is ionized, resulting in solid electrolyte. There was a problem of flowing inside.

Recently, it was disclosed in Chemistry Letters P.927 (1994) that nitrogen oxides can be decomposed and removed even in the presence of oxygen by using a device in which a porous Pd electrode is formed on both sides of a solid electrolyte at 700 ° C. It was However, in order to decompose and remove nitrogen oxides, it was necessary to apply a large current having a current density of 500 mA / cm ² or more. That is, in the above element, the nitrogen oxides can be decomposed and removed even in the presence of excess oxygen, but the coexisting oxygen excessively contained in the exhaust gas, which is not related to the decomposition reaction, is ionized and flows in the solid electrolyte at the same time. It was necessary to apply a large amount of electric current to decompose the object. That is, the decomposition and removal efficiency of nitrogen oxides with respect to the amount of current flowing through the element was extremely low.

[0007]

Therefore, it is desired to develop an apparatus capable of efficiently decomposing nitrogen oxides even in the presence of excess oxygen. That is, the object of the present invention is to promote a nitrogen oxide decomposition reaction in the coexistence of oxygen, ionize the decomposed and generated oxygen to permeate the solid electrolyte, and suppress permeation of coexisting oxygen, thereby reducing the amount of current. An object is to provide an exhaust gas purifying element that can be efficiently removed and a method for purifying nitrogen oxides.

[0008]

DISCLOSURE OF THE INVENTION The present invention is to solve the above-mentioned problems, and the present inventors have proposed an exhaust gas purification method comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity. It has been found that nitrogen oxide in the exhaust gas containing oxygen can be efficiently removed with a small amount of current by applying a DC voltage so that the electrode side containing the conductive oxide becomes the cathode electrode using the device.

That is, the present invention relates to an exhaust gas purification element comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity, and also to an electrode containing a conductive oxide and oxygen ion conductivity. In an exhaust gas purifying element comprising a solid electrolyte having, a paste containing a metal element that constitutes a conductive oxide is applied to the solid electrolyte, and an electrode containing a conductive oxide is prepared by firing in an oxygen atmosphere. The present invention relates to a method for producing an exhaust gas purifying element, which further has electrodes on both sides of a solid electrolyte having oxygen ion conductivity, and at least one of the electrodes has conductivity of an exhaust gas purifying element containing a conductive oxide. A gas containing nitrogen oxide is circulated to the electrode side containing oxide, and a voltage is applied so that the electrode becomes a cathode electrode. It relates purification method of nitrogen oxides to symptoms.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to an exhaust gas purifying element comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity. The conductive oxide used in the present invention is an oxide exhibiting metallic or semiconducting conductivity, and its composition is not particularly limited. The method for forming a composite of the conductive oxide and at least one metal selected from silver, gold, platinum, and palladium is not particularly limited.

Further, regarding the constitution of the electrode of the device, the cathode electrode has (a) a conductive oxide, (b) a conductive oxide, and at least one metal selected from silver, gold, platinum, and palladium. Or (a) a conductive oxide, and (b) a conductive oxide, and at least one selected from silver, gold, platinum, and palladium. It may be any one of a composite of metals and (c) at least one metal selected from silver, gold, platinum and palladium.

The present invention is also directed to exhaust gas purification comprising a cathode electrode containing a conductive oxide, an anode electrode containing at least one of silver, gold, platinum and palladium, and a solid electrolyte having oxygen ion conductivity. It relates to a device for use. That is, as the cathode electrode,
(A) conductive oxide, (b) conductive oxide, and silver, gold,
Any one of platinum and a composite of at least one metal selected from palladium may be used, and the anode electrode may be (c) at least one metal selected from silver, gold, platinum, and palladium. good.

The present invention is also directed to exhaust gas purification comprising a cathode electrode made of a conductive oxide, an anode electrode containing at least one of silver, gold, platinum and palladium, and a solid electrolyte having oxygen ion conductivity. It relates to a device for use. That is, the cathode electrode is (a)
It is a conductive oxide, and the anode electrode is (c) silver, gold,
At least one metal selected from platinum and palladium may be used.

The present invention also relates to an exhaust gas purifying element in which a conductive oxide is carried on an electrode containing at least one of silver, gold, platinum and palladium.
In the device, by carrying a conductive oxide having excellent NOx resolution on a noble metal electrode having excellent conductivity, it is possible to improve the electrical conductivity of the entire conductive oxide portion forming the electrode.

The present invention also relates to an exhaust gas purifying element in which a conductive oxide is compounded in an electrode containing at least one of silver, gold, platinum and palladium. In this element, by compounding a conductive oxide having excellent NOx resolution in a noble metal electrode having excellent conductivity, it is possible to improve the electrical conductivity of the entire conductive oxide portion forming the electrode. .

Further, in the present invention, the specific resistance is 1.
It is preferable to use a conductive oxide having a density of 0 × 10 ⁻² Ω · cm or less. Specific resistance of 1.0 × 10 ^-2 Ω at 300K
The composition is not particularly limited as long as it is a conductive oxide having a size of cm or less. For example, ruthenium oxide (Ru
O ₂ ), tungsten oxide (WO ₂ ), iron oxide (Fe ₃
O ₄ ), molybdenum oxide (MoO ₂ ), and rhenium oxide (ReO ₂ ) are preferably used.

As the method for producing the conductive oxide electrode used in the present invention, (1) a paste containing a metal element forming a conductive oxide is applied to a solid electrolyte and baked in an oxygen atmosphere (2) An aqueous solution containing a metal element forming a conductive oxide is applied to a solid electrolyte and baked in an oxygen atmosphere, (3) a metal element forming a conductive oxide is plated on a solid electrolyte or an electrode, and an oxygen atmosphere is formed. A method of firing below is mentioned, but the method is not particularly limited.

The solid electrolyte having oxygen ion conductivity used in the present invention is not particularly limited. Examples of solid electrolytes having oxygen ion conductivity include metal oxides having a fluorite structure, such as stabilized zirconia, stabilized bismuth oxide, and stabilized ceria. Among these, it is preferable to have high conductivity and strength, and particularly preferably to use stabilized zirconia having high strength. Examples of the stabilized zirconia include (a) Y ₂ O ₃ stabilized ZrO ₂ (however, Y ₂ O ₃ concentration:
8-15 mol%) (b) CaO-stabilized ZrO ₂ (however, CaO concentration: 5-
10 mol%) (c) MgO-stabilized ZrO ₂ (however, MgO concentration: 5
10 mol%) (d) CeO ₂ stabilized ZrO ₂ (however, CeO ₂ concentration:
15 to 35 mol%), but is not particularly limited.

The shape of the solid electrolyte is not particularly limited,
Usually, it is used after being molded into a plate shape, a tube shape, or a honeycomb shape, and the thickness thereof is 1 μm to 5 mm, preferably 10 μm to 2 mm from the viewpoint of strength and conductivity. When the thickness is 1 μm or less, a material having sufficient strength cannot be obtained, while when the thickness is 5 mm or more, the moving speed of oxygen becomes slow.

The manufacturing method of these is described below. First, a predetermined concentration of Y ₂ O ₃ , CaO, MgO, or CeO
₍₂₎ Zirconia powder containing a stabilizer is prepared by any of a powder mixing method, a neutralization coprecipitation method, a hydrolysis method, a thermal decomposition method, a hydrothermal decomposition method, or an alkoxide method. This stabilized zirconia powder is press-molded or extrusion-molded to obtain a desired shape. However, in order to facilitate the molding, the organic binder or water may be added and mixed well before molding. After degreasing the binder, the molded body can be fired at, for example, 1400 to 1600 ° C. for 2 to 3 hours to obtain a dense sintered body having no pores.

As the structure of the device of the present invention, a plate-shaped solid electrolyte having metal electrodes on both sides, a tube-shaped solid electrolyte having electrodes on the outer and inner sides, or a solid electrolyte molded in a honeycomb shape can be used. Examples thereof include those including electrodes, but some or all of the respective electrodes are conductive oxides, but are not particularly limited.

In the treatment method of the present invention, a power source is connected so that a DC voltage can be applied between both electrodes of the device, and an exhaust gas containing oxygen and nitrogen oxides is circulated to the electrode side containing conductive oxides. By applying a DC voltage between both electrodes so that the electrode becomes a cathode electrode, nitrogen oxides are decomposed into nitrogen and oxygen, and the generated oxygen is applied as a driving force to the solid electrolyte from above the electrodes. Through to the anode electrode side and is discharged from the anode electrode to the gas phase as oxygen molecules. This is intended to accelerate the decomposition of nitrogen oxides.

The range of applied voltage is 0.5 to 7.0 V / mm with respect to the thickness of zirconia, preferably 1.0 to
It is 5.0 V / mm. When the applied voltage is lower than 0.5 V / mm, the migration of oxygen is too slow and the decomposition reaction of nitrogen oxide does not proceed sufficiently. On the other hand, when the voltage is higher than 7.0 V / mm, diffusion of oxygen from the gas phase cannot catch up, oxygen of the solid electrolyte itself moves instead, and finally the solid electrolyte is destroyed.

The reaction temperature range is 300 to 1200 ° C.
It is preferably 500 to 1000 ° C. At a temperature lower than 300 ° C., the decomposition reaction rate of nitrogen oxides is slow, and the oxygen transfer rate in the solid electrolyte is extremely low, so that sufficient decomposition and removal of nitrogen oxides cannot be performed. On the other hand, when the temperature exceeds 1200 ° C, the reverse reaction of N ₂ + O ₂ → 2NO easily occurs,
N ₂ generated at the electrode reacts with O ₂ again and returns to nitrogen oxide.

[0025]

EXAMPLES Examples will be described below. (Preparation of Stabilized Zirconia Tube) An organic binder and distilled water were added to stabilized zirconia powder with 8 mol% yttria added, and the mixture was thoroughly mixed and then extruded into a tubular shape using a kneading molding machine. This was dried and then baked at 1600 ° C. for 2 hours to obtain a dense stabilized zirconia tube having an outer diameter of 3.8 mmφ and an inner diameter of 2.5 mmφ.

Example 1 An anode electrode was formed inside a stabilized zirconia tube using a palladium paste. This was baked at 1300 ° C. Next, a ruthenium paste was evenly brushed on the outer periphery of the zirconia tube to a length of 25 mm around the outer periphery of the zirconia tube to form a cathode electrode (electrode area of 3 cm ² ) (RuO ₂ at 300 K The specific resistance is 4 × 10 ^-5
Ωcm). This was baked at 800 ° C. in an oxygen atmosphere. Then, fix the platinum wire to the cathode electrode as a lead wire,
Connected to DC power supply. NO 1000pp on the cathode side
m, O ₂ 2%, He balance model gas 50 ml / min.
And He 50 ml / min. Was flown on the anode side. Then, a DC voltage was applied so that the current density in the element was 43.9 mA / cm ² . The reaction temperature is 7 for performance evaluation.
It was carried out at 00 ° C., and the amount of nitrogen present in the gas discharged from the cathode side was analyzed using gas chromatography,
The conversion rate of nitrogen oxides to nitrogen was determined as the removal rate of nitrogen oxides. Table 1 shows the evaluation results.

Example 2 An anode electrode was prepared by using a ruthenium paste instead of the palladium paste on the anode side of Example 1. Using this, evaluation was performed in the same manner as in Example 1.

Example 3 Instead of the ruthenium paste of Example 1, a tungsten paste (the specific resistance of WO ₂ at 300K was 2.9 × 1) was used.
0 ^-3 Ωcm) and baked at 800 ° C in an oxygen atmosphere,
Using this, evaluation was performed in the same manner as in Example 1.

Example 4 Instead of the ruthenium paste of Example 1, the same operation was carried out using a palladium paste to prepare a palladium electrode for both the anode and the cathode. Ruthenium paste was applied on this cathode electrode, and the
Fired at 00 ° C. Using the element having the ruthenium oxide / palladium bilayer electrode as the cathode electrode, evaluation was performed in the same manner as in Example 1.

Example 5 A palladium / ruthenium mixed paste (Pd: Ru = 1: 1 (weight ratio)) was used in place of the ruthenium paste of Example 1, and the same operation was performed.
Baking at 0 ° C. Using the element having the ruthenium oxide / palladium mixed electrode as a cathode electrode, evaluation was performed in the same manner as in Example 1.

Comparative Example 1 Both the anode and cathode electrodes were prepared by using the palladium paste in the stabilized zirconia tube by the method of Example 1. Using this, evaluation was performed in the same manner as in Example 1.

Comparative Example 2 A nickel paste (having a specific resistance of NiO at 300 K of 10 ⁴ Ωcm or more) was used in place of the ruthenium paste of Example 1, and the same operation was performed.
The evaluation was made in the same manner as in

[0033]

[Table 1]

[0034]

By using the element of the present invention, nitrogen oxides can be efficiently removed from exhaust gas such as combustion containing excess oxygen.

Claims (10)

[Claims] 1. An exhaust gas purification element comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity. 2. The exhaust gas purifying element according to claim 1, wherein the electrode comprises a cathode electrode containing a conductive oxide and an anode electrode made of at least one of silver, gold, platinum and palladium. 3. The exhaust gas purifying element according to claim 2, wherein a conductive oxide is supported on the anode electrode. 4. The exhaust gas purifying element according to claim 2, wherein a conductive oxide is compounded in the anode electrode. 5. The specific resistance of the conductive oxide is 300 K and 1.
The exhaust gas-purifying element according to any one of claims 1 to 4, which has a density of 0 × 10 ^-2 Ω · cm or less. 6. Ruthenium oxide (Ru) as a conductive oxide
O ₂ ), tungsten oxide (WO ₂ ), iron oxide (Fe ₃
O ₄ ), molybdenum oxide (MoO ₂ ), and rhenium oxide (ReO ₂ ).
5. The exhaust gas purifying element according to any one of 5 to 10. 7. An exhaust gas purifying element comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity, wherein a paste containing a metal element forming the conductive oxide is applied to the solid electrolyte to form an oxygen atmosphere. A method for producing an exhaust gas purifying element, comprising manufacturing an electrode containing a conductive oxide by firing below. 8. An exhaust gas purifying element comprising an electrode containing a conductive oxide and a solid electrolyte having oxygen ion conductivity, wherein an aqueous solution containing a metal element constituting the conductive oxide is applied to the solid electrolyte to form an oxygen atmosphere. A method for producing an exhaust gas purifying element, comprising manufacturing an electrode containing a conductive oxide by firing below. 9. An exhaust gas purification element comprising an electrode and a solid electrolyte having oxygen ion conductivity, wherein a metal element forming a conductive oxide is plated on the solid electrolyte or the electrode and baked in an oxygen atmosphere. A method for producing an exhaust gas purification element, which comprises producing an electrode containing a conductive oxide. 10. A solid electrolyte having oxygen ion conductivity has electrodes on both sides, and at least one of the electrodes contains nitrogen oxide on the electrode side containing the conductive oxide of the exhaust gas purifying element containing the conductive oxide. A method for purifying nitrogen oxides, characterized in that a gas is circulated and a voltage is applied so that the electrode serves as a cathode electrode.