JPH07306178A - Nitrogen oxide sensor - Google Patents

Abstract

PURPOSE:To obtain a nitrogen oxide sensor excellent in heat resistance and operable under high temperature by composing at least one electrode of a metal oxide, oxygen nitride or a substance containing them. CONSTITUTION:A solid electrolyte is formed of an MgO stabilized ZrO2 pipe 1 having one closed end connected, on the outside thereof, with a first electrode 2 of metal nitride, a current collector 4 of Au mesh, and a lead part 5. A second electrode 3 of Pt is formed on the inside of the ZrO2 pipe 1 and connected with a lead part 6. The ZrO2 pipe 1 is brought into contact, only on the outside thereof, with a gas to be measured and the potential difference is measured between the lead parts 5, 6 thus measuring the concentration of NOx in the gas. Since a metal nitride is employed in the electrode 2, heat resistance and corrosion resistance can be enhanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sensor for detecting the concentration of nitrogen oxides in exhaust gas of, for example, a combustion furnace or an automobile engine.

[0002]

2. Description of the Related Art Methods such as absorptiometry and chemiluminescence have already been put to practical use for the detection of nitrogen oxide concentration. However, these methods have problems that the measuring device is large and expensive, the operation is complicated, and the measurement takes time. Therefore, it is desired to develop a small and simple sensor, and a solid-state sensor has been proposed. The solid-state sensor that has been proposed so far is a semiconductor-type sensor that utilizes the fact that the electrical resistance of an oxide semiconductor changes due to the presence of NO _X gas, and a solid electrolyte is used as a partition wall, and a partition wall is used. A solid electrolyte sensor that measures the equilibrium electromotive force due to the difference in gas partial pressures on both sides of is typical.

However, in the former method, NO_XOther than
Gas selection due to its sensitivity to gases, especially reducing gases
It has the disadvantage of not having sex. On the other hand, the latter method detects
Selection by providing various nitrates as electrodes on the electrodes
The sexuality and sensitivity are improved. For example, Japanese Patent Laid-Open No. 61-1
In 84450, AgI or RbAg was added to the solid electrolyte._FourI_FiveFor
, A solid element type cell with Ag nitrate coated on one of the electrodes.
Sensor is disclosed. This sensor is a density battery type pair
NO between electrodes_X Ag ions in nitrates are
It moves in the body electrolyte and produces an electromotive force that follows the Nernst equation.
Then, measure this electromotive force and_XTo detect the concentration
It The sensor of this structure is NO₂Sensitive to both NO and NO
But AgNO₃Easily dissolves in water and has a melting point of 212 ° C
Therefore, the usage conditions are limited. Against NO
As a sensor with sufficient sensitivity, NASI can be used as a solid electrolyte.
CON (Na₃Zr₂Si₂PO₁₂) With NaNO₂Shading using
A battery-powered sensor is disclosed (Chemistry Letters,
 Vol.1 p587-590 (1992)). Also in this sensor NaNO
₂Has a low melting point of 271 ° C and has deliquescent properties, etc.
It is very difficult to put them into practical use.

As a sensor capable of operating at a relatively high temperature, β / β ″ alumina, which is a Na ion conductor, is used as a solid electrolyte.
A concentration cell type sensor using β / β ″ alumina in which Na ions are replaced by Ba ions and a nitrate containing Ba (NO ₃ ) ₂ or NaNO ₃ and Ba (NO ₃ ) ₂ is disclosed as an electrode. (Denki Kagaku Vol. 59, p465-472 (1991)) In these sensors, the solid electrolyte is Na ion conductor β /
In the sensor using β ″ alumina and Ba (NO ₃ ) ₂ as the electrode, the electromotive force change according to the Nernst equation was not obtained for the NO ₂ concentration. Also, NaNO ₃ and Ba (NO ₃ ) ₂ The concentration cell type sensor using mixed nitrates and β / β ″ alumina in which Na ions are replaced by Ba ions, and the sensor using Ba (NO ₃ ) ₂ as the electrode change NO ₂ concentration at a temperature of 450 ° C. It has been reported that a change in electromotive force according to the Nernst equation can be obtained. Ba (NO ₃ ) ₂ is a salt with a high melting point among nitrates, so it is possible to operate at temperatures around 450 ° C, but since it has deliquescent like NaNO ₂ , it was measured in an atmosphere containing water vapor. There is a problem with long-term stability.

As described above, in a solid-state sensor using a conventional solid electrolyte having sensitivity to nitrogen oxides,
The operating temperature is limited by the melting point of the nitrate used as the electrode material, and under conditions where it is exposed to an atmosphere above the melting point, the nitrate melts or decomposes, destroying the function as a sensor. Was limited. In addition, due to the deliquescent property of nitrate and the property of dissolving in water, there were problems in measurement and long-term stability in an atmosphere containing water. Furthermore, although it has sufficient sensitivity to NO ₂ , it has almost no sensitivity to NO, making it difficult to detect the NO concentration.

[0006]

Therefore, the present invention considers the detection of nitrogen oxide concentration, especially NO concentration, in the exhaust gas of a combustion furnace or an automobile engine, shows sufficient sensitivity to NO, and has a temperature of 500 ° C. It is possible to operate at high temperatures above
Further, the present invention provides a nitrogen oxide sensor which has a function as a sensor and is excellent in heat resistance even when exposed to an atmosphere of 500 ° C. or higher.

[0007]

In order to solve the above-mentioned problems, the nitrogen oxide sensor of the present invention forms at least one pair of electrodes so as to be in contact with an ion conductive solid electrolyte body, and at least one of the electrodes is formed. Nitrogen oxidation in which an electrode is made of a metal nitride or a metal oxynitride, or a substance containing the metal nitride or the metal oxynitride, and which detects the electromotive force between the electrodes that occurs corresponding to the nitrogen oxide concentration. It is an object sensor.

More specifically, the nitrogen oxides of the present invention
NASICON is a Na ion conductor as a solid electrolyte.
(Na₃Zr₂Si₂PO₁₂) Or β / β ″ alumina, or Na-io
Β / β ″ alumina with oxygen substituted by Ba ions, oxygen ions
Zirconia (ZrO)₂ −M₂O₃Or ZrO₂-MO,
M is Yb, Cd, Nd, Ca, Y, Mg, Hf) and bismuth oxide (Bi
₂O₃−M₂O₃Or MO or M₂O_Five, M is Y, Gd, Nb, W, Sr,
Ba), ceria oxide (CeO₂ −M₂O₃Or MO₂, M is Y, Sm)
 , AgX or CuX (X is I, Cl, Br)
Ag ion and Cu ion conductors of genide, and even Ag
Inorganic double salt containing X or CuX, lithium ion conductor
Lithium halide (LiX, X is F, Cl, Br, I)
Which application is possible.

The solid electrolyte body may have a partition wall structure capable of separating the test gas whose nitrogen oxide concentration is to be detected and the atmosphere having a constant atmosphere, or may have a plate-like or rod-like shape without the partition wall structure. Good. When the solid electrolyte body is used as the partition wall, at least one pair of electrodes is formed so as to sandwich the partition wall, and in the structure without the partition wall, at least the pair of electrodes is formed at an arbitrary position on the solid electrolyte body.

The electrode for detecting NOx in the test gas is made of metal nitride or metal oxynitride. Among metal nitrides, Cr, Ti, V, Nb, Mo, Mn, Ta, Ga, Zr,
A metal nitride of at least one of W and Hf or a metal oxynitride in which a part of nitrogen of these metal nitrides is replaced with oxygen is preferable. Each of these metal nitrides is 1000
It has a melting point above ℃ or a decomposition temperature and is excellent in heat resistance. Furthermore, it is almost insoluble in water and has excellent water resistance. Further, substances containing these metal nitrides or metal oxynitrides, for example, a mixture of metal nitrides and metal oxynitrides, a mixture of metal nitrides and metal oxides, a mixture of metal oxynitrides and oxides, or It is formed of a mixture of metal nitride, metal oxynitride, and metal oxide. Further, it may be a substance containing a mixture thereof, for example, a mixture with a substance such as gold or platinum.

The electrode made of metal nitride, metal oxynitride, or a substance containing them is formed by coating the solid electrolyte body by a method such as screen printing and heating after coating. Further, it may be formed by a physical vapor deposition method such as a vacuum vapor deposition method, a sputtering method, a laser ablation method, an ion beam vapor deposition method, an ion plating method, or a chemical vapor deposition method such as a chemical vapor deposition method or a plasma chemical vapor deposition method. Good.
In the production of electrodes by these methods, after forming a metal, heat treatment may be performed in a nitrogen atmosphere or a mixed atmosphere of nitrogen and oxygen to form a metal nitride or a metal oxynitride, or a substance containing these. A metal nitride or a metal oxynitride, or a substance containing them may be directly formed by controlling the atmosphere at that time. Further, the nitride powder may be deposited by plasma spraying or the like. The method is not limited to at least the above method, and any method may be used as long as it is a method of forming an electrode of a metal nitride or a metal oxynitride on the solid electrolyte body, or a material containing these, and further laminating or stacking on the electrode. A conductor such as an embedded metal may be provided.

The other electrode is a noble metal such as Pt, Ag, Au, or Pd, or a conductive ceramic, for example, a perovskite structure oxide represented by ABO ₃ , such as LaCo.
O ₃ , LaNiO ₃ , LaFeO ₄ , LaMnO ₃ or oxides in which some of the A or B sites are replaced by elements such as Sr, or K ₂
It is composed of NiF ₄ type oxide, La ₂ CuO _4, etc.

In addition, the nitrogen oxide sensor of the present invention is not limited to a sensor composed of only a pair of electrodes, but it is further required to detect another gas or to remove the influence of coexisting gas such as oxygen. For example, an electrode made of a carbonate for detecting CO ₂ or CO gas, an electrode made of a sulfate for detecting SO ₂ gas, or the like may be arranged as the electrode.

In the sensor of the present invention, for example, a solid-state battery represented by the following formula is formed. X / solid electrolyte / Y However, X is a metal nitride or a metal oxynitride, or a substance containing these, and Y is a noble metal or conductive ceramics.

When the solid electrolyte body is used as a partition wall, the sensor of the present invention measures the equilibrium electromotive force due to the difference in NO ₂ and NO gas partial pressure on both sides of the partition wall to measure the nitrogen oxide concentration in the test gas. Can be detected. Further, in the structure where the pair of electrodes are not exposed to the partition wall of the solid electrolyte body and are exposed to the test gas, NOx metal nitride or metal oxynitride,
Alternatively, the nitrogen in the test gas is measured by measuring the equilibrium electromotive force caused by the difference in chemical potential between the noble metal electrode, which is the second electrode, or the electrode made of conductive ceramics due to the electrode reaction on the first electrode due to the substance containing them. Oxides can be detected. In particular, in this structure, when oxygen ion conductivity is applied to the solid electrolyte body, by applying an electrode material such that the reaction of oxygen at the second electrode is a four-electron reaction, for example, Pt, oxygen in the test gas is applied. Even if the concentration changes, the electromotive force between the first electrode and the second electrode does not change. This is because there is no difference in the chemical potential of oxygen at the electrode interface between the first electrode and the second electrode. That is, although both electrodes respond to oxygen, the exposed test gas has the same oxygen chemical potential in the same atmosphere. In each of these sensors, a heater for heating the sensor to a predetermined temperature is appropriately arranged as needed.

In an electromotive force type sensor which forms a pair of electrodes on a solid electrolyte body and detects a potential difference caused by a difference in chemical potential between the electrodes, a solid electrolyte body having an ion transport number of about 1 is applied. It Further, the first electrode, which functions as a gas detection electrode in the pair of electrodes provided in the solid electrolyte body, reacts with the gas to be detected existing on the electrode surface, and the chemical potential of the ion conductive carrier of the solid electrolyte body is already increased. It changes with respect to the chemical potential of one electrode to generate a potential difference. There are various possible changes in the chemical potential at the detection electrode with respect to NOx gas. For example, if there is a selective catalytic action for the reaction with the gas to be detected, it is considered effective for the change in chemical potential. At present, the electrode reaction on the first electrode has not been clarified, but metal nitrides or metal oxynitrides have relatively good conductivity, and due to the catalytic action due to the structure containing nitrogen, the chemical potential of It is thought that changes will occur.

As described above, the metal nitride or the metal oxynitride is excellent in heat resistance because the melting point and the decomposition temperature of the material are high, and the electrode of the nitrogen oxide sensor, which is strongly required to be exposed to a high temperature or used at a high temperature. It can be used as a material. It also has excellent corrosion resistance and stability in an atmosphere containing water vapor. The present inventors have, first electrode metal nitride or metal oxynitride provided from these viewpoints into the solid electrolyte body further to prepare a sensor by material containing these, NO ₂ and
It has been found that a nitrogen oxide sensor showing a good response to both NO can be constructed.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings. (Embodiment 1) FIG. 1 is a sectional view of a nitrogen oxide sensor according to a first embodiment of the present invention. The solid electrolyte body of the sensor according to the present embodiment may be either a cation conductor or an anion conductor, but is yttria, calcia, ceria, magnesia or the like in terms of thermal stability and heat resistance. Stabilized or partially stabilized zirconia is preferable, and magnesia-stabilized zirconia is the best from the viewpoint of output stability. A solid electrolyte body is formed by a magnesia-stabilized zirconia tube 1 closed at one end. A first electrode 2 made of metal nitride, a current collector 4 made of Au mesh, and a lead portion 5 are connected to the outside of the zirconia tube. The second electrode 3 made of Pt is formed inside the zirconia tube 1, and the lead portion 6 is formed.
Are connected. Only outer stabilized zirconia tube so as to contact with the gas to be detected, it is possible to measure the concentration of NO _X in the gas to be detected by measuring the potential difference lead portions 5 and 6.

Based on the electromotive force in a 21% oxygen atmosphere of a nitrogen oxide sensor used as a first electrode by applying a paste using various metal nitride powders and firing at 300 to 500 ° C.,
Table 1 shows the changes in electromotive force when 100 ppm NO ₂ and 500 ppm NO were introduced. The nitrogen oxide sensors using any of the metal nitrides as the first electrode showed a response in which the electromotive force increased with respect to NO _{2 and} decreased with respect to NO. As an example, Fig. 2 shows the response characteristics of a nitrogen oxide sensor with TaN as the first electrode at 500 ° C in an air base.
Shown in. The electromotive force increased with respect to NO ₂ and the electromotive force decreased with respect to NO, and good rise response characteristics and return response characteristics were exhibited.

[0020]

[Table 1]

FIG. 3 shows the NOx concentration dependence of the electromotive force change at 500 ° C. in an air base of a nitrogen oxide sensor of Cr ₂ N. The change in the electromotive force of the sensor changes in proportion to the NO ₂ gas concentration and the logarithm of the NO gas concentration, and the electromotive force increases with increasing NO ₂ concentration for NO ₂ and changes with the NO concentration for NO. The electromotive force is decreasing with the increase. The sensor of this example also operates at 550 ° C. and 450 ° C.,
The change in electromotive force was shown in proportion to the logarithm of NOx gas concentration.

(Embodiment 2) In Table 2, Cr, Ti, V, Nb,
Nitrogen nitriding having the same structure as that of the first embodiment in which various metals such as Mo, Mn, Ta, Ga, Zr, W, and Hf are used as targets and the first electrode is formed by the sputtering method using N ₂ + O ₂ gas. 100ppm based on the electromotive force of the object sensor in a 21% oxygen atmosphere
The change of electromotive force at the time of introducing NO ₂ and 500 ppm NO is shown. It was confirmed by an energy dispersive X-ray analyzer that the produced first electrodes all contained nitrogen and oxygen in addition to the metal element. This sensor also showed a response in which the electromotive force increased with respect to NO _{2 and} decreased with respect to NO.

[0023]

[Table 2]

(Embodiment 3) FIG. 4 is an air base of a sensor having a structure similar to that of the first embodiment, in which a paste is applied using NbN and Nb (N, O) powders to form the first electrode 2. Is the response characteristic at 500 ° C. Similar to the example in which TaN is used as the electrode, excellent response characteristics are shown. Incidentally, the sensor structure according to the present invention is not limited to a structure constituted by a zirconia solid electrolyte body which is an oxygen ion conductor, as long as an electromotive force depending on a gas partial pressure is generated,
Na ion conductor, Ba ion conductor, Li ion conductor, Cu
A similar structure may be formed using an ionic conductor such as an ionic conductor.

(Embodiment 4) FIG. 5 is a sectional view of a nitrogen oxide sensor according to another embodiment of the present invention. The solid electrolyte body 7 is made of magnesia-stabilized zirconia. Also in this embodiment, the solid electrolyte body may be either a cation conductor or an anion conductor. A first electrode 8 and a second electrode 9 are provided on one surface of the plate-shaped solid electrolyte body 7. The first electrode 8 is made of Cr ₂ N
It is formed by applying a paste using a mixture with Cr ₂ O ₃ . The second electrode 9 is composed of an electrode that does not respond to NOx, and in this case is composed of Pt. The first electrode 8 and the second electrode 9 are so-called gas electrodes and are formed as porous electrodes. A current collector 10 made of Au is provided on the first electrode 8, and the lead wires 11 and 12 of the first electrode 8 and the second electrode 9 are connected to the measurement circuit. A heater 14 for heating is provided via an insulating layer 13 on the opposite surface of the solid electrolyte on which the electrodes are formed. The solid electrolyte body does not necessarily have to have a plate shape, and may have a cylindrical shape, a thin film formed by sputtering or the like, or a thick film formed by a method such as printing. Further, the pattern shape is not limited to a particular shape.

FIG. 6 shows the NOx concentration dependence of the change in electromotive force of the nitrogen oxide sensor of this example at 500 ° C. in the air base. Also the electromotive force changes in the sensor varies proportionally to the logarithm of the NO ₂ gas concentration and the NO gas concentration, the electromotive force increases with the increase of NO ₂ concentration for NO _2, for the NO NO The electromotive force decreases as the concentration increases. Furthermore, in the sensor of this embodiment, as shown in FIG.
It can be seen that the electromotive force does not change even if the oxygen concentration is changed with the concentration kept constant at 50 ppm or 500 ppm, and is not affected by the oxygen concentration.

(Embodiment 5) With the same structure as in Embodiment 4, Cr (N, O) and Cr ₂ O ₃ or Mo ₂ N and MoN, MoO _{3 are formed on} the first electrode 8.
The NOx response characteristic similar to that of FIG. 6 was also shown in the nitrogen oxide sensor using the mixture of. Fig. 8 shows an air-based nitrogen oxide sensor with a mixture of Mo ₂ N, MoN and MoO _3.
It is the NOx concentration dependence of the electromotive force change at 0 ° C. Also in this embodiment, a change in electromotive force proportional to the logarithm of the NOx concentration is shown, indicating that the NOx sensor operates.

[0028]

As described above, in the nitrogen oxide sensor according to the present invention, the electrode is formed of a metal nitride or a metal oxynitride having excellent heat resistance and corrosion resistance, or a substance containing them, and therefore at least these are used. The operating temperature and heat resistant temperature of the sensor are not restricted by the melting point and decomposition temperature of the substance. Further, since the electrode contains a metal nitride or a metal oxynitride, it is possible to provide a nitrogen oxide sensor which exhibits a good response to any gas of NO ₂ and NO.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a nitrogen oxide sensor according to an embodiment of the present invention.

FIG. 2 is a NO of a nitrogen oxide sensor according to an embodiment of the present invention.
It is a figure which shows the response characteristic with respect to x.

FIG. 3 is a NO of a nitrogen oxide sensor according to an embodiment of the present invention.
It is a figure which shows the characteristic of the electromotive force change with respect to x density | concentration.

FIG. 4 is a NO of a nitrogen oxide sensor according to an embodiment of the present invention.
It is a figure which shows the response characteristic with respect to x.

FIG. 5 is a sectional view of a nitrogen oxide sensor according to another embodiment of the present invention.

FIG. 6 is a diagram showing characteristics of changes in electromotive force with respect to NOx concentration of the nitrogen oxide sensor according to the present example shown in FIG.

7 is a diagram showing the oxygen concentration dependence of the electromotive force in the presence of NO in the nitrogen oxide sensor according to the present example shown in FIG.

FIG. 8 is a graph showing characteristics of changes in electromotive force with respect to NOx concentration in a nitrogen oxide sensor according to another embodiment of the present invention.

[Explanation of symbols]

 1, 7 Solid electrolyte body 3, 9 Second electrode 2, 8 First electrode 5, 6, 11, 12 Lead wire 13 Insulating layer 14 Heater

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location G01N 27/58 Z (72) Inventor Norio Miura 3-17-5 Hirao, Chuo-ku, Fukuoka-shi, Fukuoka Jonan Heights

Claims (8)

[Claims] 1. A pair of electrodes composed of at least a first electrode and a second electrode are formed so as to be in contact with an ion conductive solid electrolyte body, and a pair of electrodes composed of at least a first electrode and a second electrode is generated by an electromotive force generated between the electrodes of the first electrode and the second electrode. In the sensor for detecting the nitrogen oxide concentration in the test gas, at least the first electrode is composed of a metal nitride, a metal oxynitride, or the metal nitride, or a substance containing the metal oxynitride. Characteristic nitrogen oxide sensor. 2. The first electrode is Cr, Ti, V, Nb, Mo, Mn, T
2. The nitrogen oxide sensor according to claim 1, wherein the nitrogen oxide sensor is composed of a nitride of any one of a, Ga, Zr, W, and Hf or an oxynitride. 3. The first electrode is Cr, Ti, V, Nb, Mo, Mn, T
The nitrogen oxide sensor according to claim 1, wherein the nitrogen oxide sensor is composed of a substance containing a nitride of any one of a, Ga, Zr, W, and Hf and an oxynitride. 4. The first electrode is Cr, Ti, V, Nb, Mo, Mn, T
2. The nitrogen oxide sensor according to claim 1, wherein the nitrogen oxide sensor is composed of a nitride of any one of a, Ga, Zr, W, and Hf, or a substance containing any oxynitride and an oxide. 5. The first electrode is Cr, Ti, V, Nb, Mo, Mn, T
2. The nitrogen oxide sensor according to claim 1, wherein the nitrogen oxide sensor is made of a substance containing a nitride of any one of a, Ga, Zr, W, and Hf, an oxynitride, and an oxide. 6. An ion conductive solid electrolyte body is used as a partition wall, at least one pair of electrodes is formed so as to be in contact with the solid electrolyte body with the partition wall interposed therebetween, and one electrode is exposed to a test gas. The nitrogen oxide sensor according to any one of 2, 3, 4, and 5. 7. The ion-conductive solid electrolyte body is provided with at least one pair of electrodes on both sides or one side, and both electrodes are exposed to the same test gas.
The nitrogen oxide sensor according to any one of 1. 8. The ion-conductive solid electrolyte body is an oxygen-ion conductive solid electrolyte.
5. The nitrogen oxide sensor according to any one of 5, 6, and 7.