JPH07185261A - Electrochemical element and manufacturing method thereof - Google Patents

Abstract

PURPOSE:To provide an element to efficiently decompose a nitrogen oxide at low temperature and low current density even in the case the oxygen concentration is high, provide a manufacturing method for the element, and provide a nitrogen oxide decomposing apparatus employing the element, wherein the apparatus decompose nitrogen oxides electrochemically. CONSTITUTION:An electrochemical element is composed of a negative electrode 1 containing a nitrogen oxide adsorptive compound in the presence of oxygen an oxide ion conductive electrolytic substance 2, and a positive electrode 3 put face to face while having the electrolytic substance between the electrode 1 and it. The electrochemical element is manufactured by a printing method. A nitrogen oxide decomposing apparatus is composed of the electrochemical element, a resistor, a DC variable electric power source, a circuit means, a detecting means, a comparing means, and a heating means.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an element for electrochemically decomposing nitrogen oxide into nitrogen and oxygen in a gas atmosphere in which oxygen and nitrogen oxide coexist, and a method for manufacturing the element.

[0002]

2. Description of the Related Art A conventional method of decomposing nitrogen oxides by an electrochemical method is disclosed in, for example, Japanese Patent Laid-Open No. Sho 61-78421.
Both spaces of the positive electrode 12 and the negative electrode 13 through 1 are divided (platinum is used as the electrode material), and the gas to be treated is placed on the negative electrode side.
The positive electrode side is brought into contact with the atmosphere or a reduced pressure gas other than the gas to be treated, and the surface of the platinum electrode on the negative electrode side is coated with the catalyst layer 14 supporting rhodium so that the decomposition reaction proceeds even in the presence of oxygen.

The above-mentioned decomposition apparatus dissociates nitrogen oxides into nitrogen and oxygen on the negative electrode, electrically disperses oxygen in the form of dissociated ions through the solid electrolyte, moves to the positive electrode side, and moves it into the atmosphere or reduced pressure gas. To release. On the other hand, nitrogen remains on the negative electrode side and is released into the gas to be treated. In this way, nitrogen oxides are decomposed.

Further, according to Japanese Patent Laid-Open No. 50-32092, ZrO ₂ -Ca as shown in FIG. 9 is used as a processing device.
A porous noble metal (platinum) membrane electrode 16 is used for both the O-based solid electrolyte 15 and the positive and negative electrodes. Temperature about 500-95
Nitrogen oxide concentration of 50-50 in air at 0 ° C
A decomposition rate of 90% or more is shown in the range of about 00 ppm.

[0005]

However, in the above-mentioned conventional structure, for example, according to the above-mentioned JP-A-61-78421, the decomposition ability of nitrogen oxide in the presence of oxygen is not so high. Operating temperature 700 ℃, oxygen concentration 1
%, NO concentration 1000 ppm, gas flow rate 600 cc / m
In, the decomposition amount is 0.1 μmol / unit area of electrode
cm ² . Moreover, the current density at this time is as high as 200 mA / cm ^{2 although} it is due to the high temperature. On the other hand, Japanese Patent Application Laid-Open No. 50-32092 does not specify the oxygen concentration in detail, and some ambiguity is felt in this respect.

Further, there is a restriction that it is necessary to purposely divide the positive and negative polar spaces in order to increase the decomposition rate.

The present invention solves the above-mentioned problems and, as compared with the prior art using a porous noble metal membrane electrode or a negative electrode provided with a catalyst layer, at a low temperature even in an atmosphere with a high oxygen concentration,
It is an object of the present invention to provide an element or device that decomposes nitrogen oxides efficiently at a low current density.

[0008]

In order to achieve the above object, the present invention has the following constitution.

That is, the electrochemical device is arranged so as to face the negative electrode with a negative electrode containing a compound capable of adsorbing nitrogen oxides in the presence of oxygen, an oxygen ion conductive electrolyte and the electrolyte. The positive electrode does not contain the nitrogen oxide adsorbing compound.

The nitrogen oxide-adsorptive compound contained in the negative electrode is an oxide having a Ba ₂ YCu ₃ O ₇ type structure.

The negative electrode is composed of at least a noble metal and the nitrogen oxide adsorbing compound.

The positive electrode is made of a noble metal. Further, the electrolyte is a stabilized ZrO ₂ composition.

Further, the electrochemical element is arranged so as to face the negative electrode with a negative electrode containing a compound adsorbing a nitrogen oxide in the presence of oxygen, an oxygen ion conductive electrolyte, and the electrolyte therebetween. It was configured to have a positive electrode containing the nitrogen oxide adsorbing compound in the presence of oxygen.

The negative electrode and the nitrogen oxide adsorbing compound contained in the negative electrode are oxides having a Ba ₂ YCu ₃ O ₇ type structure.

The negative electrode and the positive electrode are composed of at least a noble metal and the nitrogen oxide adsorbing compound.

The electrolyte was stabilized ZrO ₂ . Further, on the oxygen ion conductive electrolyte surface, a conductive paste containing a nitrogen oxide adsorbing compound as a negative electrode, or a conductive paste containing no nitrogen oxide adsorbing compound as a positive electrode, or nitrogen as a positive electrode. After printing a conductive paste containing an oxide-adsorptive compound, it is dried and further faces the negative or positive electrode through the electrolyte, and has a polarity opposite to that of the electrode already printed on the surface of the electrolyte. The positive or negative conductive paste was printed, dried, and then fired in an oxidizing atmosphere.

Further, a negative electrode containing a compound capable of adsorbing nitrogen oxides in the presence of oxygen, an oxygen ion conductive electrolyte, and a positive electrode arranged to face the negative electrode via the electrolyte. A configured electrochemical element, or a negative electrode containing a nitrogen oxide-adsorptive compound in the presence of oxygen, an oxygen ion conductive electrolyte, and arranged facing the negative electrode via the electrolyte. Electrochemical element composed of a positive electrode containing a nitrogen oxide adsorbing compound in the presence of oxygen, a resistance for detecting a current as a voltage, and a direct current for applying a voltage to the nitrogen decomposing element It comprises a circuit means in which a variable power source is connected in series, a detecting means for detecting the voltage across the resistor, a comparing means for comparing the output of the detecting means with a set value, and a heating means for heating the nitrogen oxide to an operating temperature. The variable DC power supply is a nitrogen oxide decomposing device for varying the voltage by the comparing means output.

In addition, before the atmosphere containing nitrogen oxides, which contacts the electrochemical device, contacts the device,
The nitrogen oxide decomposing apparatus was provided with a pretreatment means for the atmosphere.

Further, the pretreatment means is a nitrogen oxide decomposing device for removing water, particulate dust or a substance inhibiting nitrogen oxide decomposition.

[0020]

The present invention has the above-described structure, and even in an atmosphere in which oxygen is present in the order of about 10%, the nitrogen oxide-adsorptive compound contained in the negative electrode makes the nitrogen oxide more prone than the negative electrode made of a noble metal. The reactivity of the negative electrode with respect to is increased, and the current density is several mA / cm at low temperature (500 ° C or less).
^{Even at} about ² , the nitrogen oxides will be decomposed into nitrogen and oxygen with higher efficiency than before.

The decomposition reaction of nitrogen oxides starts when the nitrogen oxide adsorbing compound fixes the nitrogen oxides on the negative electrode. On the negative electrode side of the electrochemical device, the nitrogen-oxygen bond of the nitrogen oxide is weakened in the vicinity of the contact interface between the nitrogen oxide adsorbing compound, the noble metal (for example, platinum) and the oxygen ion conductive electrolyte, and oxygen is an oxygen ion. As it passes through the electrolyte, it moves from the negative electrode side to the positive electrode side and is discharged into the gas as oxygen molecules. On the other hand, nitrogen molecules are generated on the negative electrode side and are desorbed into the gas.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will be described below with reference to FIG.

FIG. 1 shows an electrochemical device (hereinafter, h-YBC).
FIG. 3 is a schematic configuration diagram of a system element). 1 is Ba ₂ YCu
Negative electrode composed of ₃ O _7-X and platinum, 2 is an oxygen ion conductive electrolyte composed of ZrO ₂ stabilized by Y ₂ O ₃ ,
3 is a positive electrode made of platinum.

The h-YBC system element was prepared as follows. The positive and negative electrode conductive pastes were alternately printed and dried on the surface of the electrolyte 2 and then baked at 820 ° C. in the atmosphere to form the positive and negative porous electrodes 3 and 1 on the electrolyte 2. Further, a platinum wire to be the lead wire 4 was adhered to the ends of the electrodes (1, 3) to prepare the electrodes. The composition of the negative electrode conductive paste used was platinum vs. Ba ₂ YCu ₃ O.
_The molar ratio of _7-X is approximately 1: 1 but the composition is not necessarily limited to this. Also, the firing temperature is 820 ° C.
It is not limited to.

The electrolyte 2 is Y ₂ O ₃ 8 mol% -ZrO.
₂ , length 25mm, width 12.5mm, thickness 0.5mm
The thickness of the electrodes 1 and 3 was about 20 to 40 μm, and the area was 2.25 cm ² . These conditions are only one example, and are not limited thereto.

For comparison, an element having both positive and negative electrodes made of platinum (hereinafter referred to as platinum element) was also prepared. The electrolyte used for the platinum element is 1 cm x 1 cm, thickness 0.5 m
m, but the other conditions are the same as those of the h-YBC element.

As the Ba ₂ YCu ₃ O _7-X and the platinum conductive paste for the positive electrode, commercially available products were used. The conductive paste for the negative electrode was prepared by mixing a platinum conductive paste and Ba ₂ YCu ₃ O _7-X .

FIG. 2 shows an SEM observation image of the negative electrode 1 after firing. It is clear that it is porous.

3 and 4 show the adsorption characteristics of nitrogen oxides of Ba ₂ YCu ₃ O _7-X (hereinafter NOx, NOx = NO + NO ₂ ) in the presence of oxygen and Ba ₂ at the time of NOx adsorption. The X-ray diffraction pattern of YCu ₃ O _7-X is shown. Adsorption characteristics are measured using a gas flow system and NO at the outlet.
x densitometry. The conditions for each measurement are shown in each figure.

Considering the X-ray diffraction pattern at a NOx concentration of about 5%, Ba ₂ YCu ₃ O _7-X shows an adsorption phenomenon while forming a part of nitrate compound with NOx in the presence of oxygen. Seem. Moreover, it is clear from FIG. 3 that the plot of the adsorption amount and the temperature has the maximum value with respect to the temperature. This suggests an appropriate operating temperature of the h-YBC system element. In addition, Ba ₂ YCu ₃ O _7-X
It was found that the coexistence of Pt and platinum can adsorb NOx at a higher temperature than Ba ₂ YCu ₃ O _7-X alone.
The adsorption characteristic in FIG. 4 was reversible with temperature at a NOx concentration of about 1000 ppm.

Further, even if the Ba site in Ba ₂ YCu ₃ O _7-X is replaced with Sr, which is an alkaline earth element, or the Y site is replaced with another lanthanoid element, NOx adsorption characteristics are exhibited. Was confirmed. Therefore, B
Instead of a ₂ YCu ₃ O _7-X , another Ba ₂ YCu ₃ O ₇
A type oxide may be used. Furthermore, Ba ₂ YCu ₃ O ₇
Even if it is not a type oxide, any compound that can stably absorb or adsorb NOx in oxygen can be applied as the negative electrode material.

As the negative electrode, it is desirable to mix it with platinum because it adsorbs NOx even at high temperatures.
_The composition is a mixture of ₂ YCu ₃ O _7-X and platinum. B
Instead of a ₂ YCu ₃ O _7-X, an oxygen-defective perovskite-type oxide having a high oxygen mobility (for example, LaCo
_0.8 Sr _0.2 O ₃ ) or a composite oxide of iron, manganese, and copper was tried as an electrode material, but good electrical characteristics could not be obtained. In consideration of the actual use environment, the positive electrode 3 is preferably a noble metal such as platinum which has excellent environmental resistance in order to promote the oxidation reaction (positive electrode reaction) of oxygen ions.

Next, the NOx of the h-YBC system element and the platinum element
The decomposition behavior was measured in a gas flow system. The gas flow rate is 200
It was constant at cc / min. The gas composition was mixed and adjusted with NO, oxygen and He as a balance gas, and a chemiluminescence type NOx meter and a gas chromatograph were used for composition analysis. The peak of N ₂ O considered as a by-product was not observed within the detection limit level on the gas chromatograph. The above gases were each supplied from a gas cylinder whose concentration was adjusted. He concentration is 99.999%
That was all.

The concentration analysis capability of the gas chromatograph is ± 10 in consideration of the reproducibility of measurement data and the stability of the equipment.
It is about ppm. Therefore, the concentration of nitrogen and oxygen is 50
At the level of 0 ppm or less, particular attention was paid to quantitative consideration.

FIG. 5 shows a typical relationship between the gas composition and the device current value when NOx is decomposed by the h-YBC system device.

From FIG. 5, it can be seen that when current flows through the h-YBC system element, the nitrogen and oxygen concentrations increase as the NOx concentration decreases. In the presence of oxygen (several percent or more)
In consideration of the detection capability of the gas chromatograph, the change in the oxygen concentration of a trace amount (several tens to several hundreds of ppm) cannot be quantitatively and clearly confirmed. However, an increase in nitrogen (N ₂ ) accompanying NOx decomposition was clearly detected.

NOx by h-YBC system element and platinum element
Specific values of the decomposition behavior are shown in (Table 1) and (Table 2).

[0038]

[Table 1]

[0039]

[Table 2]

In (Table 1) and (Table 2), ΔNOx is the concentration difference between the supply concentration and the outlet concentration, and ΔN ₂ is also the nitrogen concentration difference between the supply concentration and the outlet concentration. A minus sign indicates a decrease in density, and any other sign indicates an increase in density.

From Table 1, it is clear that the h-YBC element decomposes NOx even in the presence of excess oxygen. Because N
This is because the increase (generation) of nitrogen was confirmed with respect to the decrease of Ox.

The amount of NOx decomposed by the h-YBC system element is about four times that of 1% oxygen, although the NOx concentration is different from that of JP-A-61-78421. Further, as compared with the platinum element, it is about 2 times when oxygen is about 0%, about 30 times when oxygen is 1%, and more when oxygen is 10%.

Further, in the h-YBC system element, the ratio between the NOx reduction amount and the generated nitrogen amount is about 2: 1,
It is almost equal to the stoichiometric ratio when considering the decomposition reaction of Equation-1.

2NO → N ₂ + O ₂ (1) N ₂ O, which is considered as a by-product, was not detected by gas chromatography, and the ratio of the NOx reduction amount to the produced nitrogen amount was
Since it is 2: 1, it is considered that there is almost no side reaction other than the formula (1) in the h-YBC system element. This is a great advantage in NOx decomposition.

FIG. 6 shows the overvoltage-current characteristics of the negative electrodes of the h-YBC system element and the platinum element. The overvoltage is measured by a current interruption method (Current) using a reference electrode formed on the electrolyte.
It was measured by the Interruption method). The element used for measurement has an electrolyte of 10 mm x 1
A reference electrode was formed in addition to the positive and negative electrodes with a thickness of 0 mm and a thickness of 0.5 mm. The measured gas atmosphere was about 5000 ppm NO balanced with He, and the measurement temperature was 450 ° C.

FIG. 6 shows that the electrode made of Ba ₂ YCu ₃ O _7-X and platinum has a higher reactivity with respect to NO than that of the electrode having only platinum (large current value). . This is consistent with the results shown in Tables 1 and 2.

Next, an element was prepared with the positive and negative electrodes having the same composition as the negative electrode of the h-YBC system element (hereinafter, referred to as n-
YBC element).

Table 1 shows the NOx decomposition characteristics of the n-YBC element.
According to the same measurement as NO about 540ppm, O ₂ 1
%, The device current was 7.2 ma / cm ² , and the amount of NOx decomposed was about 0.1 μmol / cm ² / min. This value is
It is about 8 times that of a platinum element. Therefore, the n-YBC element can also be used as a nitrogen oxide element.

The manufacturing method of each element described above is the printing method as described in the description of FIG. The formation of electrodes in each element is an important process that affects NOx decomposition. As the method, a sputtering method, a vapor deposition method, a CVD method, a PVD method, and the like can be considered in addition to the printing method, but the printing method is preferable in consideration of cost and simplicity. The firing atmosphere is preferably an oxidizing atmosphere in consideration of the use environment and the structural stability of the obtained electrode.

Next, a schematic structure of an embodiment of the nitrogen oxide decomposing apparatus will be described with reference to FIG. In FIG. 7, 4 is an electrochemical element (hereinafter referred to as a decomposition element), 5 is a resistance for detecting the element current as a voltage, and 6 is h-Y.
DC variable power source for applying voltage to BC element, 7 is circuit means for electrically connecting the h-YBC element 4, resistor 5 and DC variable power source 6 in series, 8 is detection of voltage across resistor 5 Means, 9 is a means for comparing the output value of the detection means 8 with a set value, and 10 is a heating means for heating the h-YBC element to an operating temperature.

In the above nitrogen oxide decomposing apparatus, the decomposing element 4 is exposed to a gas containing NOx, and the heating means 10 holds it at about 450.degree. When a voltage is applied to the decomposition element 4 by the DC variable power source 6, NOx decomposition occurs, and an ionic current flows due to the generated oxygen ions. Oxygen contained in the gas also contributes to the ionic current.

This ionic current is used as a device current for the resistor 5
The detection means 8 detects the voltage across the voltage. If the electrode of the disassembling element 4 is deteriorated or some other influence,
If the element current fluctuates abnormally, the comparison unit 9 compares the preset value with the detection value of the detection unit 8. Therefore, the comparison can detect the abnormality of the element current. The variable DC power supply 6 is activated based on the abnormality detection by the comparison means 9.
-The voltage across the YBC element is controlled so as to eliminate the abnormality.

In this way, the nitrogen oxide decomposing apparatus shown in FIG. 7 exhibits a stable NOx decomposing operation contained in the gas and discharges nitrogen and oxygen into the gas.

The circuit means 7 is a conducting wire, but since the decomposition element is 450 ° C. and the gas containing NOx may be at a high temperature, a noble metal wire such as a platinum wire is preferable at such a high temperature portion. .

The NOx-containing gas may be water, unburned components (hydrocarbons), CO, CO ₂ ,
May contain dust. It is conceivable that these may induce side reactions in the decomposing element as substances that inhibit NOx decomposition, or poison or contaminate the electrode surface. Therefore, it is preferable to remove such NOx decomposition inhibiting substances in advance before the gas reaches the decomposition element. In order to solve this problem, a pretreatment unit such as a filter or a dehumidifier may be provided.

According to the constitutions of the above embodiments, it is possible to obtain an electrochemical device having a NOx decomposition rate superior to that of the conventional one and an inexpensive manufacturing method thereof even in an atmosphere containing oxygen. In addition, stable NOx can be obtained by using the electrochemical device.
A nitrogen oxide decomposing device showing a decomposing operation is obtained.

[0057]

As described above, according to the present invention, an electrochemical element that exhibits an excellent NOx decomposition rate at a lower temperature and a lower current density than in the prior art even in an atmosphere containing oxygen, and an inexpensive production thereof. A method is obtained. Further, if the electrochemical element is used, a nitrogen oxide decomposing apparatus having a stable NOx decomposing operation can be obtained. Therefore, there is an effect of reducing the destruction of the natural environment and the adverse effects on the human body due to NOx.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an electrochemical device according to an embodiment of the present invention.

FIG. 2 is a micrograph of a negative electrode of an electrochemical device according to an example of the present invention.

FIG. 3 shows Ba ₂ YCu ₃ O in one embodiment of the present invention.
_7-X NOx adsorption characteristic diagram

FIG. 4 shows Ba ₂ YCu ₃ O in one embodiment of the present invention.
_The figure which shows the X-ray diffraction pattern at the time of NOx adsorption of _7-X .

FIG. 5 is a diagram showing a schematic change in gas composition and device current during NOx decomposition in an example of the present invention.

FIG. 6 is a diagram showing overvoltage-current characteristics in one example of the present invention.

FIG. 7 is a schematic configuration diagram of a nitrogen oxide decomposing apparatus according to an embodiment of the present invention.

FIG. 8 is a block diagram of a conventional NOx decomposing device.

FIG. 9 is a configuration diagram of a conventional NOx processing device.

[Explanation of symbols]

 4 Electrochemical element (decomposition element) 5 Resistance 6 DC variable power supply 7 Circuit means 8 Detection means 9 Comparison means 10 Heating means

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display area F01N 3/08 C

Claims (13)

[Claims] 1. A negative electrode containing a compound capable of adsorbing nitrogen oxides in the presence of oxygen, an oxygen ion conductive electrolyte, and the nitrogen oxide arranged to face the negative electrode via the electrolyte. An electrochemical device composed of a positive electrode containing no adsorptive compound. 2. The electrochemical device according to claim 1, wherein the nitrogen oxide-adsorptive compound contained in the negative electrode is an oxide having a Ba ₂ YCu ₃ O ₇ type structure. 3. The electrochemical device according to claim 1, wherein the negative electrode comprises at least a noble metal and the nitrogen oxide adsorbing compound. 4. The electrochemical device according to claim 1, wherein the positive electrode is made of a noble metal. 5. The electrochemical device according to claim 1, wherein the electrolyte is stabilized ZrO ₂ . 6. A negative electrode containing a compound capable of adsorbing nitrogen oxides in the presence of oxygen, an oxygen ion-conducting electrolyte, and oxygen present in the presence of oxygen arranged opposite to the negative electrode via the electrolyte. An electrochemical device comprising a positive electrode containing the nitrogen oxide-adsorptive compound. 7. The electrochemical device according to claim 6, wherein the negative electrode and the nitrogen oxide-adsorptive compound contained in the negative electrode are oxides having a Ba ₂ YCu ₃ O ₇ type structure. 8. The electrochemical device according to claim 6, wherein each of the negative electrode and the positive electrode comprises at least a noble metal and the nitrogen oxide adsorbing compound. 9. The electrochemical device according to claim 6, wherein the electrolyte is stabilized ZrO ₂ . 10. A conductive paste containing a nitrogen oxide adsorbing compound as a negative electrode, or a conductive paste not containing a nitrogen oxide adsorbing compound as a positive electrode, or a positive electrode on the surface of an oxygen ion conductive electrolyte. After printing a conductive paste containing a nitrogen oxide-adsorptive compound as an electrode, it is dried, and further facing the negative or positive electrode through the electrolyte, on the surface of the electrolyte, opposite to the electrode already printed. A method for producing an electrochemical element, comprising printing either the positive or negative conductive paste having a polarity, drying the paste, and then firing the paste in an oxidizing atmosphere. 11. A negative electrode containing a compound capable of adsorbing nitrogen oxides in the presence of oxygen, an oxygen ion conductive electrolyte, and a positive electrode arranged facing the negative electrode through the electrolyte. A configured electrochemical element, or a negative electrode containing a nitrogen oxide-adsorptive compound in the presence of oxygen, an oxygen ion conductive electrolyte, and arranged facing the negative electrode via the electrolyte. Electrochemical element composed of a positive electrode containing a nitrogen oxide adsorbing compound in the presence of oxygen, a resistance for detecting a current as a voltage, and a direct current for applying a voltage to the nitrogen decomposing element A circuit means in which a variable power source is connected in series, a detecting means for detecting the voltage across the resistor, a comparing means for comparing the output of the detecting means with a set value, and a heating means for heating the nitrogen oxide to an operating temperature. Nitrogen oxide decomposing device variable DC power source to vary the voltage by the comparing means output. 12. The nitrogen oxide decomposing apparatus according to claim 11, wherein a pretreatment means for the atmosphere is provided before the atmosphere containing the nitrogen oxide that comes into contact with the electrochemical element comes into contact with the element. 13. The nitrogen oxide decomposing apparatus according to claim 11, wherein said pretreatment means removes water, particulate dust or a substance inhibiting nitrogen oxide decomposition.