JPH0875698A - Gas sensor - Google Patents

Abstract

PURPOSE: To enhance the reliability of the gas sensor of a burner particularly to the level of detecting the deterioration of a catalyst layer of sensor characteristics of the maximum subject of a chemical sensor. CONSTITUTION: The gas sensor comprises heating means, further, a solid electrolyte layer 1, a pair of electrodes 2, 3 formed on both sides or the same side of the layer 1, a porous semiconductor metal oxide catalyst coating layer formed on the one side electrode and a third electrode 5 formed on the surface of the catalyst layer. The resistances of the catalyst layer and the electrolyte layer 1 are intermittently measured to detect the deterioration of the layer 4, and the catalyst life can be evaluated. Further, the function of controlling the burner by using an arithmetic circuit, a memory circuit, etc., via an interface by utilizing solid electrolyte output, and transmitting information of carbon monoxide concentration to a place spatially separated by using communication means are provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrochemical sensor for detecting a combustible gas, particularly carbon monoxide, in the general atmosphere or in the exhaust gas of various combustion equipment that uses gas or petroleum as a fuel, and uses a catalyst. The present invention relates to a gas sensor provided with means capable of detecting deterioration of a catalyst, which is the greatest problem in a chemical sensor.

[0002]

2. Description of the Related Art As a conventionally proposed gas sensor, particularly a carbon monoxide detection sensor, an electrode for absorbing carbon monoxide (CO) to oxidize it is provided in an electrolytic solution, and a CO value is calculated from a current value proportional to the CO concentration. These semiconductors are flammable using a method of detecting the concentration (constant potential electrolysis gas sensor), and using a sintered body type n-type semiconductor oxide such as tin oxide (Sn0 ₂ ) sensitized with a trace amount of additional elements such as precious metals. Gas (semiconductor type gas sensor) that uses the characteristic that electric conductivity changes when it comes into contact with a conductive gas (semiconductor type gas sensor), a pair of platinum fine wire of about 20 μm with alumina attached and no precious metal supported Known as a method (catalytic combustion type gas sensor) that detects the difference in heat generation when a combustible gas comes into contact with this element to carry out a catalytic oxidation reaction using a comparative element To have. For example, [Reference 1]
Supervised by Toyoaki Omori: "Practical Encyclopedia of Sensors": Fuji Techno System Chapter 14 Basics of Gas Sensors (Masaki Takeshi Haruta), P1
12-130 (1986) for a detailed description.

In particular, as a gas sensor used to form an electrochemical cell, an oxygen sensor using stabilized zirconia as an electrolyte has been put to practical use for controlling exhaust gas purification of automobiles and is well known. This is a method that utilizes the electromotive force of an oxygen concentration cell. A gas sensor for measuring carbon monoxide and carbon dioxide at the same time as oxygen has been proposed. [Reference 2] JP-A-4-320955, [Reference 3] JP-A-4-320956.

However, in the case of this method, since a concentration cell is formed, it is necessary to introduce a sample gas of a known concentration into one of the electrodes, and although it is highly applicable as an analytical instrument, the sample gas is required for each measurement. Can not be introduced and is not suitable for household appliances. However, this method can correct the deterioration of the electrode each time.

Further, as in the present invention, a method of forming a zirconia electrochemical cell, forming a catalyst layer on one side of an electrode and detecting carbon monoxide is [Reference 4] H.OKAMOTO, H.OBA.
YASIAND T. KUDO, Solid State IonicS, 1, 319 (1
980). The basic principle of the present invention is the same as in Reference 4. However, in Reference 4, platinum / alumina is used as the catalyst.

[0006]

All of these chemical sensors have the following problems. That is, even if a constant potential electrolysis gas sensor, a semiconductor type gas sensor, and a catalytic combustion type gas sensor react indiscriminately to a reducing gas (combustible gas) in principle, even if various measures are taken, basically,
It has the property of detecting hydrogen, alcohol, etc. other than carbon monoxide (CO). That is, it has a drawback that CO selectivity is poor. Further, the sensor and the sensor system are generally expensive, and the signal processing circuit of the sensor is complicated. Further, except for the catalytic combustion type, the controllability is poor because the output of the sensor is non-linear with respect to the CO concentration.

Basically, in the case of a chemical sensor that applies a catalyst, the durability and the selectivity of the catalyst are generally contradictory to each other. In order to increase the selectivity, the operation of the chemical sensor is controlled by the activation control side of the catalytic reaction ( When used on the low temperature side, various poisonous gases are likely to be adsorbed on the surface of the catalyst, and the catalyst deteriorates rapidly and has no durability. On the contrary, if the operation is used on the diffusion control side (high temperature side), the selectivity is deteriorated. For example, in the case of a sensor using an element in which gold particles are supported on αFe ₂ O ₃ doped with Ti ⁴⁺ as a semiconductor type gas sensor, the selectivity of CO for hydrogen and alcohol is excellent but the durability of the catalyst is poor. There is.

This is the most important point, but the fate of the chemical sensor is that when the catalyst deteriorates, there is a fundamental drawback that the sensitivity is basically lowered and the sensor operates unsafely. . From the viewpoint of quality engineering, the situation of carbon monoxide generation is an event in an abnormal environment that rarely occurs, but in this emergency when we do not know when it will occur, the absolute guarantee of sensor reliability is not guaranteed. If this is not the case, the carbon monoxide sensor, which actually affects human life, cannot be applied to a safety system. Although a large number of sensors have been proposed as described in [Reference 1], the fact that they cannot be put into practical use as safety sensors to be mounted on devices has some problems such as the cost of the sensor system, but the essence of chemical sensors This is because no effective solution that could lead to fail-safe has been found for the problem of catalyst deterioration related to.

In particular, the greatest drawback of the method of Document 4 in which an electrochemical cell is formed by using a solid electrolyte having one electrode coated with a catalyst is that catalyst deterioration cannot be detected.

According to the present invention, in addition to the heat resistance that can be installed in the exhaust gas flow path of various domestic combustion equipment, the stability against fluctuating exhaust gas characteristics, downsizing of the sensor itself and high sensitivity are achieved, and the above-mentioned drawbacks are also caused. The purpose is to solve.

[0011]

In order to achieve the above object, the gas sensor of the present invention is provided with a heating means, and is formed on both surfaces or the same surface of the solid electrolyte layer and the solid electrolyte layer. A pair of electrodes, a porous semiconductor metal oxide catalyst coating layer formed on one of the electrodes, and a third electrode formed on the surface of the catalyst layer are used.

Further, by forming an electrochemical cell of P _CO (1), P _O2 (1) / catalyst / electrode (1) / solid electrolyte / electrode (2) / P _CO (1), P _O2 (1) In this solid electrolyte gas sensor, the resistance value between the catalyst-coated layers is intermittently measured by using a porous semiconductor metal oxide as a catalyst and a third electrode formed on the catalyst surface, and the change value of the resistance value is measured. Exceeds the control range Rx, and the resistance (or impedance) between the electrodes of the solid electrolyte is intermittently measured, and when the change value of the resistance value exceeds the control range rX, it is equipped with a means for issuing an alarm. .

Further, the electromotive force of the electrochemical cell is the set value E
When it exceeds x, it is provided with a control means for increasing the rotation speed of the blower fan of the combustion equipment or stopping the combustion.

The electromotive force of the electrochemical cell is the set value Ex
When it exceeds the above, the gas sensor is equipped with a means for transmitting a signal to a second device installed in a place spatially separated from the gas sensor.

The basic structure of the gas sensor will be described below.
As the solid electrolyte layer, Bi ₂ O ₃ -MOO ₃ , which has higher oxygen ion conductivity than stabilized zirconia or stabilized zirconia at low temperature,
An oxygen ion conductor such as CeO ₂ -Sm ₂ O ₃ or a fluoride ion conductor or a proton conductor other than the oxygen ion conductor is formed into a plate shape or formed on a heat resistant insulating substrate such as alumina by sputtering, CVD, or the like. It can be used in a film form by a film method. In an electrochemical cell in which a pair of electrodes are formed on both surfaces or the same surface using a solid electrolyte, if the oxygen concentration between the electrodes is different, an oxygen concentration battery is formed and an electromotive force is generated, and an oxygen sensor element can be configured. .

Further, when one of the electrodes is coated with an oxidation catalyst, a chemical reaction of CO + 1 / 2O ₂ → CO ₂ occurs in the catalyst layer, and strictly speaking, it depends on the gas diffusion rate and the catalytic oxidation reaction rate. The concentration of carbon monoxide reaching the surface is reduced. Oxygen is exclusively adsorbed on the electrode coated with the oxidation catalyst, whereas carbon monoxide and oxygen are adsorbed on the bare electrode without the catalyst coating, and the chemical potential between both electrodes is absorbed here. Difference occurs, a reduction reaction occurs at the electrode on the catalyst layer side and an oxidation reaction occurs at the bare electrode side, and an oxygen concentration battery is formed to obtain an electromotive force. The oxygen concentration is at least a few percent or more even in the atmosphere or under the exhaust gas conditions of combustion equipment, whereas carbon monoxide is at most 1000 PPM.
It is necessary to detect the following, and CO + 1 / 2O ₂ → CO
_Since oxygen is 1/2 mol per mol of carbon monoxide as in ₂ , the difference in oxygen when evaluated as a bulk is small, and the influence of carbon monoxide concentration is dominant in the electromotive force. This increases the carbon monoxide concentration and increases the output. That is, it becomes a carbon monoxide detection sensor. This sensor can detect not only carbon monoxide but also combustible gas.

Generally, the characteristics of the solid electrolyte require heating of the sensor element, which is accomplished by the heating means provided in the sensor. Conventionally known heating means such as a method of paste-printing a resistor on a heat resistant insulating substrate such as alumina can be applied. Further, if necessary, temperature control may be carried out in combination with temperature detection means such as a thermistor for the purpose of accurate temperature control.

Especially when the temperature of the sensor is low and when the catalyst layer is formed on honey, the catalyst layer becomes a resistance against diffusion of oxygen and carbon monoxide, and the reaction rate of the catalyst itself becomes slow. This causes an electromotive force which causes a difference in concentration between oxygen and carbon monoxide. Therefore, a certain temperature or higher is required. Further, when the temperature is further increased, the oxidation reaction rate at the electrode on the side where the catalyst layer is not formed is increased, the effect of the catalyst is apparently lost, and the electromotive force is not generated. Therefore, the temperature of the sensor is preferably in the range of 450 to 550 ° C. when using stabilized zirconia. Platinum, gold, silver, etc. can be used for the electrode, but from the aspect of the adsorption property of oxygen to the electrode related to the response of the sensor, the diffusion property of ions to the electrolyte, and the heat resistance stability, Platinum is preferred.

As the electrode forming method at this time, various methods such as a paste method, a sputtering method, a vacuum vapor deposition method and an electroless plating method can be applied. In addition to basic performance such as sensor responsiveness, electrode characteristics are deeply related to durability, and are important.
Of these methods, electroless plating is particularly preferable because the electrodes are stable and can be atomized and dispersed. Also, in the paste method, excellent characteristics can be obtained by using a paste in which the electrode material is sufficiently atomized.

[0020]

The operation of the sensor in the general atmosphere or in the exhaust gas passage of the combustion device will be described. It is assumed that the sensor element has already been heated to a constant temperature by the built-in heating means. When air containing no combustible gas such as carbon monoxide is sent to the electrode on the solid electrolyte, there is no difference in gas concentration and no electromotive force is generated. Next, when air containing carbon monoxide is sent, carbon monoxide is oxidized in the catalyst layer and does not reach the electrode side where the catalyst layer is formed, so that it occurs between the electrodes on the solid electrolyte. Electricity is generated.

The output of the electromotive force between the electrodes of the solid electrolyte is amplified to extract a signal related to the concentration of carbon monoxide in the air. This increases with increasing carbon monoxide concentration. In the exhaust gas of the combustion equipment, the exhaust conditions change in actual use in relation to combustion control. That is, the exhaust temperature and the exhaust gas composition (oxygen concentration, nitrogen gas concentration, carbon dioxide concentration, water vapor concentration), etc. change greatly over time, but since they do not react in the catalyst layer, they do not affect the sensor output. A stable output related to only combustible gas such as carbon oxide can be obtained. The N-type semiconductor metal oxide is extremely stable as compared with the P-type, but the risk of deterioration due to adhesion of impurities on the surface is not zero depending on the use environment. In the conventional solid electrolyte chemical sensor, the method of confirming the deterioration of the catalyst layer was impossible, but in the present invention, since the catalyst layer uses the conductive N-type semiconductor metal oxide,
The deterioration of the catalyst can be evaluated by measuring the resistance value between the electrodes arranged in the catalyst layer. When this value changes beyond a certain threshold value, or when the resistance (or impedance) between the electrodes of the solid electrolyte exceeds a certain threshold value, calculation of microcomputer, memory, etc. via the input / output interface, In combination with a storage device, the deterioration of the catalyst layer can be estimated and an alarm can be issued. Regarding this threshold, depending on the purpose of use of this gas sensor and the operating environment conditions,
You can set its value.

In addition, depending on the value of the solid electrolyte output, when the output value exceeds a certain set value through the input / output interface in combination with a calculation storage device such as a microcomputer or a memory, it is possible to control the blowing of the combustion equipment. , You can stop the combustion equipment. Conventionally, various methods have been proposed to control combustion equipment by using sensor output, but for gas sensors, fail-safeness related to deterioration of the sensor itself has not been sufficiently secured, so as a final safety sensor. There was a risk and combustion control didn't make much sense. Generally, in combustion equipment, carbon monoxide is easily generated under a condition of insufficient air. Therefore, it is often possible to ensure environmental safety by increasing the air flow rate of the combustion equipment. In addition, when the gas sensor is built into the first device or used in a form close to that of the first device, if the output value exceeds a certain set value by the same method as described above, a position that is spatially separated from the gas sensor by wireless communication means. The second device can be controlled by transmitting a signal to the second device at. Here, the second device refers to a ventilation fan, a combustion device, or the like. However, the second device requires a receiver capable of receiving the signal emitted from the first device, a microcomputer, and an arithmetic and storage device such as a memory.

Further, the communication means can inform a person in a remote place of the abnormality of the carbon monoxide concentration in the room or equipment in which the gas sensor is installed.

[0024]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view of one embodiment of the present invention. In FIG. 1, 1 is a solid electrolyte layer, 2 and 3 are a pair of electrodes, a catalyst layer 4 made of a porous semiconductor oxide is formed on 3 electrodes on one side, and a catalyst layer 4 is formed on the surface of the catalyst layer 4. Three electrodes 5
Are formed. Although the heating means is omitted in FIG. 1, the heater circuit may be installed independently of the present element.
FIG. 2 is also a sectional view of one embodiment of the present invention. In FIG.
Unlike FIG. 1, electrodes 2 and 3 are formed on the same side as the solid electrolyte 1. In addition, the substrate 6 and the heater circuit 7 formed on the substrate are provided on the side opposite to the electrodes formed on the solid electrolyte 1. Other than that, it is the same as FIG.

As a catalyst formed on the electrode 3 on one side,
Various catalysts can be applied only for the purpose of obtaining a gas sensor element, but in the present invention, the material is selected because it has the purpose of detecting the deterioration behavior of the catalyst. A noble metal is superior only in terms of catalytic activity, but a porous semiconductor metal oxide is selected for the purpose of providing a third electrode on the surface of the catalyst layer and detecting the resistance value of the catalyst layer. Semiconductor metal oxides include P type and N type. Comparing the two, the N-type semiconductor oxide is preferable because the P-type is more reactive and the catalytic activity is higher but the stability and durability are conversely inferior. Although the catalytic activity of the N type is inferior, the catalytic layer temperature is set to a high temperature in view of the purpose of the operation of the solid electrolyte and the diffusion resistance of gas.

As N-type, Fe₂O₃, Nb₂0_Five, Nb₂O₃, Ta₂O₃,
CeO₂, ZnO, SnO₂, TiO₂, BaTiO₃, PbTiO ₃, SrTiO₃Oxidation such as
The substances are used alone or as a mixture. These oxides are
It is an insulator at high temperature, but at high temperatures it has the characteristics of a semiconductor.
Show. Also, with these oxides as the main components, Li₂O, Nb
₂O₃, CaO, Gd₂O₃It is also possible to add a small amount of doping material such as
Wear. The porous catalyst layer is formed with a binder such as glass.
Metal oxide and fine powder of organic materials such as polyethylene
It may be prepared by coating and baking the mixed paste.
Inorganic acid salts such as naphthenic acid and octylic acid
Paste with a binder such as methylcellulose
It is also possible to make paste by coating and baking it
Wear. The former method is easier to control the pore size of the porous body
Yes.

As the N-type semiconductor oxide, tin oxide or zinc oxide is applied to a semiconductor type gas sensor and has a track record as a gas sensor. Although copper oxide belongs to the P-type rather than the N-type, it has been found to be particularly effective in the present invention. However, even though they are the same gas sensor, they are completely different in terms of function. That is, in the conventional semiconductor gas sensor, detection is performed by utilizing the increase in surface resistance of the semiconductor metal oxide due to adsorption of gas, whereas in the present invention, it is utilized as an oxidation catalyst. Its electrical conductivity is used only for deterioration detection.

Platinum paste was applied to both sides of a solid electrolyte (10 mm × 10 mm × 0.5 mm) made of yttria-stabilized zirconia by the screen printing method and the mixture was heated at 1000 ° C. for 1 hour.
After firing for a period of time, the lead-wire take-out terminals were placed on the end faces to prepare electrodes of 5 mm square and 5 μm thick. A large number of samples were prepared with the platinum lead wire joined thereto. A paste prepared by using various N-type semiconductor oxides was applied to the sample with a thickness of about 20 μm in an 8 mm square and baked at 700 ° C. for 2 hours to prepare a sensor element.
The paste was a metal oxide powder, which was dispersed with polyethylene fine powder, polyvinyl alcohol, water and the like in a ball mill to form a paste. Regarding the manufactured sensor element,
A test sensor was placed in a flow-through type measuring device, and air containing 1000 PPM of carbon monoxide was sent and tested under the condition that the ambient temperature was 450 ° C. SnO ₂ , Fe ₂ O ₃ , MnO ₂ , SnO
_2, Zn _O, Cu _O, etc. _{Nb 2 O 5, TiO 2,} CeO 2, BaTiO 3, 10
It was confirmed that a sensor output of -20 mV was obtained. Next, when the sensitivity was tightened and the carbon monoxide concentration was set to 200 PPM and the same test was carried out, good output was not obtained in many devices, but in the case of SnO ₂ , ZnO and CuO, it was 2-3 mV.
I checked the output of. In particular, a sensor output of 5 mV was obtained when the composition of SnO ₂ —CuO was 1/1.

FIG. 3 shows the relationship with the output characteristics of the present sensor when the carbon monoxide concentration is changed. An extremely sensitive sensor has been obtained. Furthermore, when a 3 mm square electrode was formed on the catalyst element and the resistance value of the catalyst layer was evaluated, it was in the range of several hundred Ω to several hundred KΩ, and this resistance value was intermittently measured in a general atmospheric environment. Therefore, the deterioration of the catalyst layer can be sufficiently detected.

However, in the presence of a combustible gas such as carbon monoxide, the resistance of the catalyst layer changes and becomes unstable. Therefore, in practice, the output of the solid electrolyte is confirmed and the output of the solid electrolyte is stable. It is necessary to evaluate the resistance of the catalyst layer during the operation, that is, when the combustible gas is not generated. This is a condition for intermittently measuring the resistance value of the catalyst layer.

The deterioration factor of the catalyst layer can be divided into a physical factor and a chemical factor, which can be detected by a change in resistance such as a change in clogging or poisoning due to adhesion of foreign matter. Similarly, by switching the circuit and measuring the resistance between the electrodes arranged in the solid electrolyte, deterioration of the electrodes of the solid electrolyte can be confirmed. This is because the electrode of the solid electrolyte is also a kind of catalyst, and this electrode occupies a large position for deterioration of the solid electrolyte cell. However, compared to the catalyst layer,
Since the electrode of the solid electrolyte is somewhat stable, it is less necessary for practical use, but this further improves safety in an emergency.

[0032]

As described above, according to the gas sensor of the present invention, the following effects can be obtained.

(1) Regarding detection of carbon monoxide, a highly sensitive and small gas sensor can be obtained. It is highly stable and suitable for installation in combustion equipment.

(2) The deterioration behavior of the catalyst layer and the electrode deterioration of the solid electrolyte, which are generally problems in chemical sensors, can be checked, and the life of the gas sensor can be detected. Therefore, the safety is extremely high and the combustion equipment is safe. It can be used as a sensor with high reliability.

(3) Abnormal information of carbon monoxide generation is transmitted to a person who is far from the sensor by the communication means of the sensor, the device containing the sensor or the space in which the sensor is arranged. It is extremely useful for maintaining security.

[Brief description of drawings]

FIG. 1 is a sectional view showing a gas sensor according to an embodiment of the present invention.

FIG. 2 is a sectional view showing a second gas sensor according to an embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the relationship between the carbon monoxide concentration and the output characteristic of the sensor in one example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte 2, 3 A pair of electrodes 4 Catalyst layer 5 3rd electrode on the catalyst layer surface 6 Support substrate 7 Heater circuit as a heating means

Claims (8)

[Claims] 1. A solid electrolyte layer further comprising heating means,
A pair of electrodes formed on both surfaces or the same surface of the solid electrolyte layer, a porous semiconductor metal oxide catalyst coating layer formed on one of the electrodes, and a third electrode formed on the surface of the catalyst layer. Gas sensor consisting of. 2. The gas sensor according to claim 1, wherein an oxygen ion conductor is used as the solid electrolyte. 3. The gas sensor according to claim 1, wherein an N-type semiconductor metal oxide is used as the porous semiconductor metal oxide catalyst. 4. The gas sensor according to claim 1, wherein the porous semiconductor metal oxide catalyst is a catalyst whose main component is one or more oxides selected from the group consisting of tin oxide, zinc oxide and copper oxide. 5. The gas sensor according to claim 1, wherein a protective film made of a glass component is formed on the surface of the third electrode. 6. An electrochemical cell of P _CO (1), P _O2 (1) / catalyst / electrode (1) / solid electrolyte / electrode (2) / P _CO (1), P _O2 (1) is formed. In this solid electrolyte gas sensor, the resistance value between the catalyst-coated layers was intermittently measured by using a porous semiconductor metal oxide as a catalyst and a third electrode formed on the catalyst surface, and the resistance value changed. A gas sensor comprising means for giving an alarm when the value exceeds a control range Rx and when the resistance between the electrodes of the solid electrolyte is intermittently measured, and the change value of the resistance value exceeds the control range rX. 7. The gas sensor according to claim 6, further comprising control means for increasing the rotation speed of the blower fan of the combustion device or stopping the combustion when the electromotive force of the electrochemical cell exceeds the set value Ex. 8. When the electromotive force of the electrochemical cell exceeds a set value Ex, a means for transmitting a signal to a second device installed spatially apart from the gas sensor is provided. Item 6. The gas sensor according to item 6.