JPH102878A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deterioration in a platinum electrode and an oxidation catalyst and improve the durability. SOLUTION: In a gas sensor provided with a heating means 1 and a gas selectively permeable body in the inlet and the outlet of a gas, the gas sensor is produced by forming a pair of platinum electrodes 4 on the surface of an oxygen ion conductive solid electrolytic substance 3, forming a catalytic layer 6 formed on one platinum electrode 4 from an oxidation catalyst 5 and ceramic fibers by paper manufacturing process, and an absorptive layer 8 formed further from an alkali absorbing body 7 and ceramic fibers by paper manufacturing process. Furthermore, an absorptive layer 9 is formed on the other platinum electrode 4 from an alkali absorbing body 7 and ceramic fibers by paper making process.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting flammable gas, especially carbon monoxide, in exhaust gas of various kinds of combustion equipment using general air or gas or oil as fuel. It is stable against a wide range of environmental changes and has excellent characteristics in terms of durability.

[0002]

2. Description of the Related Art Carbon monoxide is a colorless, tasteless and odorless gas which is slightly lighter than air but highly toxic and causes headaches etc. when breathing for 2-3 hours even at a low concentration of about 20 PPM.
At concentrations above M, it takes about 10 minutes, and at concentrations above 6000 PPM, it dies after several minutes of breathing.

[0003] Even in ordinary households, carbon monoxide is generated from instantaneous water heaters, bath kettles, charcoal fires for oil heating appliances and gas heating appliances, and can be used by being built in these appliances or installed indoors. There is a strong demand for an inexpensive, small, and highly reliable carbon monoxide gas detection sensor.

[0004] Conventionally proposed gas sensors, particularly chemical sensors for detecting carbon monoxide, are provided with an electrode that absorbs and oxidizes carbon monoxide in an electrolytic solution, and measures the current value proportional to the carbon monoxide concentration. A method for detecting the concentration of carbon monoxide (a potentiostatic electrolytic gas sensor), using an n-type semiconductor oxide sensitized by adding a trace amount of a metal element such as a noble metal, for example, a sintered body of tin oxide, and these semiconductors are combustible. A method of detecting gas by using the property that the electrical conductivity changes when it comes into contact with a non-reactive gas (semiconductor type gas sensor), with alumina attached to a thin platinum wire of about 20 μm, with and without a noble metal Heating to a certain temperature by using a pair of comparison elements and detecting the difference in heat generated when a combustible gas comes into contact with this element to perform a catalytic oxidation reaction (contact combustion type gas sensor) is known. , Is the [for example Toyoaki Omori supervision:
"Sensor Practical Encyclopedia": Fuji Techno System Chapter 14, Basics of Gas Sensors (Masatoshi Haruta), P112-130
(1986)].

Further, a zirconia electrochemical cell is constituted,
A solid electrolyte type carbon monoxide sensor that detects a carbon monoxide gas by forming a platinum / alumina catalyst layer on one side of the electrode has also been proposed [for example, H.OKAMOTO, H.OBAYASI AND
T. KUDO, Solid State Ionics, 1, 319 (1980)].

The principle of the solid electrolyte type carbon monoxide sensor is based on the fact that a kind of oxygen concentration cell can be formed on the catalyst layer side and the bare side electrode, and oxygen reaches the catalyst layer side electrode as it is. On the bare electrode, both oxygen and carbon monoxide reach, while carbon oxide does not reach.
This carbon monoxide reduces oxygen, an oxygen concentration cell is formed between both electrodes, and an electromotive force output appears.

As for the gas selective permeable membrane, a ceramic gas selective permeable membrane, that is, an inorganic separation membrane has been proposed [for example, Tatsuya Okubo, Seiji Morooka, "Present and Future Development of Inorganic Separation Membrane", Chemical, Engineering Link, 12, 1 (1988, 1
989)], and there has been no proposal for applying an inorganic separation membrane to a gas sensor.

[0008]

The biggest problem of a chemical sensor which has been widely used as a gas sensor in the past is that, despite being a decisive sensor relating to safety, a fail-out detection system is inevitable. It will be. This is due to the fact that, in principle, the signal as a sensor is zero when carbon monoxide is not detected, a signal is output when carbon monoxide is detected, and the output signal is reduced due to deterioration of the sensor. is there.

When a gas sensor is mounted on a combustion device and used for the purpose of detecting incomplete combustion, the risk of incomplete combustion increases more often in a state after the combustion device has been considerably used. At that time, there is a risk that the gas sensor is deteriorating, and there is a problem that incomplete combustion cannot be detected if the output signal is reduced due to the deterioration of the gas sensor.

[0010] The reason why the output of the chemical sensor is reduced, that is, the deterioration is due to the deterioration of the electrode or catalyst which plays a central function of the chemical sensor with time as the reaction proceeds, This deterioration is caused by the reduction of the catalyst by reducing gases such as hydrogen and hydrocarbons present in the exhaust gas of combustion, and the strong adsorption of sulfur-based compounds on the electrode surface, resulting in the detection reaction of carbon monoxide. By being inhibited. In these chemical sensors, noble metals are often used for electrodes or catalysts that play a central role in the sensor function,
These noble metals are susceptible to sulfur-based compounds and silicone-based compounds and are liable to be deteriorated, so that there is a problem that it is very difficult to ensure durability. In addition, hydrocarbons that coexist in the exhaust gas of combustion equipment have a large molecular weight and a large molecular size. Therefore, if they are adsorbed on the surface of a noble metal such as platinum, the adsorption of carbon monoxide is hindered, which has an adverse effect as an interfering gas. There was also a problem.

Further, since the sensor system is essentially not fail-safe, a highly reliable sensor with extremely high durability is required in order to be able to put it into practical use with high reliability. However, there is also a problem that a sensor system that can properly establish durability is not realized.

[0012]

In order to solve the above-mentioned problems, in the gas sensor according to the present invention, gas entering the detection section is caused to flow through a gas selective permeable body.

The function of the gas selective permeator is slightly different depending on the pore size, but if it is less than 10 °, it will not allow molecules of a certain size to permeate due to the molecular sieve effect.
At the 0-1000Å level, it is a Knudsen diffusion region,
It has the property that the permeation rate is inversely proportional to the square root of the molecular weight.
Gases required for detection include carbon monoxide, oxygen, carbon dioxide generated by these reactions, sulfur dioxide as an interfering gas, kerosene vapor, etc. Depending on the size, separation can be performed with high accuracy, or inflow of a polymer can be regulated, so that the poisoning effect on an oxidation catalyst, a platinum electrode, and the like can be reduced. It is desirable that the gas selective permeable body be made of ceramic from the viewpoint of heat resistance.

The heat source for operating the gas sensor is achieved by a heating means provided in the gas sensor, and the temperature is controlled by using a temperature detecting means such as a thermistor and a thermocouple as needed. As the heating means, various means such as a heating wire and a resistance heater film can be applied.As a material used for the resistance heater film, a noble metal-based material such as platinum is desirable in terms of durability, and when a heating wire is used, Iron-chromium type and nickel-chromium type can be used.

The basic idea of the present invention is that if gas other than the gas necessary for detecting carbon monoxide is prevented from touching the gas sensor, semi-permanent durability can be realized, and if semi-permanent durability can be realized, The idea is that fail-out is not a problem in practice.

The ceramic gas selective permeable body will be described below. The ceramic gas permselective is made using what is already commercially available as porous ceramic or porous glass. Porous ceramic or porous glass is used for various applications as a ceramic filter, and it is well known that it is used, for example, for separating yeast from beer. The pore size is 0.05μ
Although it is about m to several μm, since gas permeability cannot be obtained as it is, it is necessary to fill the pores and control the pore diameter.

As a method of controlling the pore diameter, a method of forming a sol-gel film in the pores, or a CVD method of forming a film in the pores by thermal decomposition to control the pores, and the like can be used. Various known film forming methods are applicable. Among these, the pore diameter can be controlled to the pore diameter of the molecular diffusion region of gas by utilizing, for example, a decomposition reaction of metal alkoxide (sol-gel method) or a CVD reaction. It can be controlled to uniform pores of up to about several mm. The size of the pores in this case must be the size of gas molecules, and the movement of gas in the pores is affected by the interaction between the gas and the substance on the surface of the pores. The gas permeation rate basically changes from Knudsen diffusion to molecular sieve diffusion, and is inversely proportional to the square root of the molecular weight of gas or has the property of significantly inhibiting the permeation of large molecules. Thus, large size molecules need to be prevented from passing through the gas selective permeator.

Therefore, the angle is set to 3 to 100 °, practically 5 to 1 °.
It is preferable to use a ceramic tubular gas selective permeable member whose pore diameter is controlled to 00 °, preferably 3 to 10 ° and 100 ° or less, and to basically configure the gas sensor in a state where the sensor element is housed inside. .

That is, the gas contained in the general atmosphere or the exhaust gas of the combustion equipment is initially 3 to 100 ° C.
Practically, the gas diffuses into pores whose pores are controlled to 5 to 100 °, preferably 3 to 10 °, but gas having a molecular size larger than the pore diameter, such as kerosene vapor or silicone oligomer, is selectively permeated by gas. The gas cannot pass through the pores of the body to reach the gas detecting portion inside the sensor element. In addition, reactive gases such as sulfur dioxide (SO ₂ ) and nitrogen dioxide (NO ₂ ) have a large molecular weight, so that they are difficult to diffuse in the pores, and the amount that reaches the gas detecting portion is reduced. Low-molecular gas molecules such as oxygen, carbon monoxide, and nitrogen can freely reach the gas detection unit in a state similar to Knudsen diffusion.

As described above, the size of the pore diameter is extremely important, and if the pore diameter is 100 ° or more, the gas cannot be selectively permeated. It is desirable to set the pore diameter to 10 to 10 °. In this case, gas diffusion becomes a region of molecular sieve and complete permselectivity is obtained. If it is 10 ° or more and 100 ° or more, it becomes a region of Knudsen diffusion and molecules of large size are restricted from flowing. Alternatively, the deterioration of the oxidation catalyst can be prevented, and a longer life can be expected.

The shape of the gas selective permeable body is cylindrical, bag-like,
Various shapes such as a flat plate shape are applicable. Various shapes of oxygen ion conductive solid electrolyte elements can be used in combination with the gas selective permeable body. Various materials can be used as the oxygen ion conductive solid electrolyte, but practically, it is preferable to use yttria-stabilized zirconia.

Further, there is a problem that the platinum electrode used for the solid electrolyte type gas sensor is extremely weak in poisoning with sulfur compounds. If the pores up to the molecular sieve region are realized, the inflow of the sulfur compound is almost completely regulated, but this is practically difficult. In the Knudsen diffusion zone, sulfur compounds,
For example, the inflow of sulfur dioxide is regulated, but the inflow of a part cannot be regulated. To prevent acid gas such as sulfurous acid gas entering the platinum electrode from entering, a ceramic paper layer in which an alkali absorber is mixed with ceramic fibers is located in front of the catalyst layer, or a catalyst in which an alkali absorber is mixed. Preferably, a layer is provided to absorb and remove the acidic gas.

By arranging a metal oxide-based oxidation catalyst such as iron, manganese, copper, nickel, chromium, and cobalt on the noble metal-based oxidation catalyst, the activity of the catalyst layer can be improved with time. In addition, since these metal oxide-based oxidation catalysts also function as an alkali absorber, it is possible to protect the platinum electrode and prevent a decrease in activity due to deterioration over time.

The configuration in which the catalyst is supported in the pores of the gas selective permeator in the form of a plate is also intended to improve the durability. Since the catalyst is disposed in the pores in which gas permeation is regulated, Even in the case where the deterioration hardly progresses and the pore size actually has a distribution, the catalyst in the small-sized portion exhibits extremely stable and continuous functions, leading to a long life.

The detection output of carbon monoxide of the solid electrolyte type gas sensor is maximum around 450 ° C., but this operation is initially set at 500 to 550 ° C., and the temperature is lowered over time. By doing so, it is possible to increase the sensor output over time, which covers the fail-out sensor characteristics.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention is directed to a gas sensor provided with a heating means and a gas selective permeable body at a gas inlet / outlet, wherein a pair of platinum electrodes is formed on the surface of an oxygen ion conductive solid electrolyte. Then, on one of the platinum electrodes, a catalyst layer was formed by making an oxidation catalyst together with ceramic fibers, and then further laminated thereon to form an absorption layer formed by making an alkali absorber together with the ceramic fibers to form an absorption layer. And an absorbent layer formed by paper-making an alkaline absorber together with ceramic fibers.

Since all of the detection gas enters through the gas selective permeator, the entry of an interfering gas having a large molecular weight is prevented or restricted. In particular, even if a part of the acid gas such as sulfur dioxide gas or nitrogen dioxide which adversely affects the poisoning of the platinum electrode is absorbed and removed by the alkali absorber, even if a part of the gas enters, the acid gas reaches the catalyst layer and the platinum electrode. Blocked,
This leads to a longer life. Examples of the alkali absorber include a ceramic mixture containing an alkali metal or alkaline earth metal oxide, hydroxide, or carbonate as a main component, or a ceramic containing a metal oxide that reacts with an acidic gas such as sulfur dioxide or nitrogen dioxide. It is desirable that the mixture or the like is granulated to a particle size of about 10 to 100 mesh before use. The permeability of oxygen gas to the platinum electrode is not affected at all because the catalyst is dispersed in the ceramic fiber without impairing the permeability, and ceramic paper obtained by mixing the catalyst powder with the ceramic fiber is It may be adhered with an inorganic adhesive or fixed and held on a platinum electrode using inorganic fibers.

The electrode used is limited to a platinum electrode system. The electrode may be formed into a thin film by sputtering or electron beam evaporation while masking a required pattern, or may be formed by electroless plating in a required pattern. May be formed. Although the performance is slightly lower, a predetermined pattern may be formed by a thick film printing method by screen printing using a platinum paste.

In the second embodiment, a pair of platinum electrodes is formed on the surface of an oxygen ion conductive solid electrolyte in a gas sensor provided with a heating means and a gas selective permeable body at a gas inlet / outlet. In addition, a film layer was formed by making an oxidation catalyst and an alkali absorber together with ceramic fibers, and an absorption layer was formed by making an alkali absorber together with ceramic fibers on the other platinum electrode.

Although the alkali absorber is randomly mixed with the oxidation catalyst, the effect of absorbing the acidic gas is the same as in the above case, and the poisoning gas does not reach the platinum electrode. Long life expectancy is expected.

The third embodiment is directed to a gas sensor provided with a heating means and a gas selective permeable body at the gas inlet / outlet, wherein a pair of platinum electrodes is formed on the surface of the oxygen ion conductive solid electrolyte, and one of the platinum electrodes is formed. A first catalyst layer formed of a noble metal-based oxidation catalyst together with ceramic fibers is formed thereon, and a second catalyst layer formed by laminating a metal oxide-based oxidation catalyst together with the ceramic fibers is further formed thereon.
On the other platinum electrode, an absorption layer was formed by paper-making an alkali absorber together with ceramic fibers.

As the metal oxide-based oxidation catalyst, a composite oxide or a mixture mainly containing a transition metal oxide such as iron, manganese, copper, nickel, cobalt and chromium is used. It also functions as an alkali absorber that well absorbs acidic gases such as sulfurous acid gas and nitrogen dioxide gas.

The oxidation function of carbon monoxide is higher in the noble metal-based oxidation catalyst, so that when the metal oxide-based oxidation catalyst acting as an alkali absorber and absorbing the acidic gas is deteriorated, the noble metal-based oxidation catalyst is deteriorated. The catalyst exhibits a function as an oxidation catalyst, and can exhibit characteristics of improving the catalytic activity with time. This covers a fail-out property in which the sensor output decreases as the deterioration proceeds.

In the fourth embodiment, a pair of platinum electrodes is formed on the same surface, a coating layer is formed on one of the platinum electrodes by making an oxidation catalyst and an alkali absorber together with ceramic fibers, and the other is formed. A plate-shaped oxygen ion conductive solid electrolyte on which an absorption layer is formed by making an alkaline absorber together with ceramic fibers on a platinum electrode, and a plate-shaped gas selective permeable body on which a heating means such as a heater film is formed on the back surface. Are bonded and sealed with a glass material at the outer periphery of a pair of platinum electrodes, a catalyst layer, and a coating layer to form a flat gas sensor.

The detection gas flows only through the plate-shaped gas selective permeator, and is absorbed by the absorption layer containing the alkali absorber even if the acidic gas in the gas partially passes. An oxygen concentration cell is formed by one platinum electrode facing the coating layer containing the oxidation catalyst and the other platinum electrode facing the absorption layer,
A sensor output corresponding to the concentration of carbon monoxide, that is, an electromotive force is obtained.

In the fifth embodiment, a plate-shaped oxygen ion conductive solid electrolyte having a pair of platinum electrodes formed on the same surface, and an oxidation catalyst in a pore in a region opposed to the one platinum electrode. A plate-shaped gas selective permeable member which is supported and has a heating means such as a heater film formed on the back surface is joined and sealed with a glass material at the outer peripheral portions of a pair of platinum electrodes.

Since the oxidation catalyst is formed in the pores of the gas selective permeable body, the possibility that the oxidation catalyst present in pores having a certain size or less deteriorates is extremely low, and the activity of the catalyst is reduced over a long period of time. Will be maintained.

In the sixth embodiment, alumina or forsterite is used as the gas selective permeable material.

Although it is economical to use inexpensive yttria-stabilized zirconia as the oxygen ion conductive solid electrolyte, a highly reliable gas sensor element using yttria-stabilized zirconia requires zirconia and thermal expansion. It is necessary to use a material having a similar coefficient, and alumina or forsterite is effective because the coefficient of thermal expansion is similar.

In the seventh embodiment, the initial operating temperature is set to 50.
The temperature is set to 0 to 550 ° C., and the temperature is lowered over time based on the operation time of the gas sensor using the set temperature control means. The solid electrolyte gas sensor using yttria-stabilized zirconia has a carbon monoxide
Although the maximum sensor output is shown around ℃, when the sensor temperature is set to a constant value, if the oxidation catalyst and platinum electrode deteriorate over time, the sensor output will gradually decrease. If the sensor output is set to the lower temperature range, the sensor output can be increased by lowering the operating temperature from the initial setting temperature even if the deterioration occurs. can do.

An embodiment of the present invention will be described below with reference to FIGS. (Embodiment 1) FIG. 1 is a conceptual sectional view of a solid electrolyte gas sensor according to Embodiment 1 of the present invention. In FIG. 1, reference numeral 1 denotes a heating means, which may be formed by heating a hot wire of iron chromium or nichrome in a coil shape and heating it by contact, or may be used by forming a resistive film by thick film printing. 1
Accordingly, the gas sensor is heated to a required temperature range of 400 to 500 ° C. Reference numeral 2 denotes a gas selective permeable body, which can be of various shapes such as a tubular shape, a flat plate shape, and a bag shape, and needs to be configured so that all gas flowing into the sensor element portion is performed through the gas selective permeable body 2. There is. In the case of the present embodiment, a configuration is adopted in which a tubular gas selective permeable body 2 is used, and both ends thereof are sealed with a sealing material 10. Alumina sol, colloidal silica,
An adhesive or a glass composition using an inorganic binder such as an alkali metal silicate or a metal phosphate is used.

Next, a method of controlling the pore diameter of the porous body by the sol-gel method, which is an effective method among the methods for producing the gas selective permeable member 2, will be described. In the basic production method, a ceramic porous body having a pore of 1 micron level, which is generally commercially available as a microfiltration membrane, is used as a base material, and the pores are closed.

After hydrolyzing a metal alkoxide such as aluminum isopropoxide or tetraethoxysilane, the solution is subjected to polycondensation under catalytic conditions such as hydrochloric acid to prepare a sol solution. When the sol solution is brought into contact with a hydrophilic porous ceramic having through pores, water is sucked by capillary force, and sol concentration and gelation occur in the pores of the porous ceramic. If necessary, a method of filtering the sol solution using a porous ceramic can also be adopted, and this phenomenon can be used to control the pore diameter.

When the pores of the porous ceramic are brought into contact with an aqueous solution of a sol obtained from a metal alkoxide, specifically, the porous ceramic is dipped in a sol solution for a short time and then dried. Occurs and the pores are closed.
By adjusting the wettability of the pore surface of the porous ceramic, the concentration of the sol, and the immersion time, a uniform pore diameter can be controlled at a level of 2 to several millimeters.

In addition to the sol-gel method, the pore control may be performed by using a CVD method and forming and growing an oxide in the pores of the porous ceramic while thermally decomposing the compound in a flow system. This method is excellent as a method for producing a cylindrical ceramic gas selective permeable body.

The pores thus produced do not allow the passage of high molecular weight gas, and the gas permeability becomes selective due to the interaction with the gel film formed inside the pores. That is, the intermolecular force between the gas molecule and the gel molecule is determined by the orientation force due to the interaction between the permanent dipoles, the induced force between the permanent dipole and the induced dipole, and the dispersion force based on the van der Waals interaction. Selectivity occurs.

Reference numeral 3 denotes an oxygen ion conductive solid electrolyte, which uses yttria-stabilized zirconia or the like. A pair of platinum electrodes 4 are formed on the surface of the oxygen ion conductive solid electrolyte 3. It is characteristically advantageous that the platinum electrode 4 is masked in a predetermined pattern and a thin film is formed by electron beam evaporation, sputtering, or the like, so that a quick response and a large sensor output can be obtained. However, although it is inferior in characteristics, it may be formed by a thick-film printing method or a transfer method in which a screen is printed using a paste and then fired. In FIG. 1, platinum electrodes 4 are formed on the front and back sides of the flat oxygen ion conductive solid electrolyte 3. On one of the platinum electrodes 4, a catalyst layer 6 prepared by mixing the oxidation catalyst 5 together with ceramic fibers, and an absorption layer 8 further laminated thereon and mixed with the alkali absorber 7 ceramic fibers are formed. On the other platinum electrode 4, an absorption layer 9 in which an alkaline absorber 7 is made together with ceramic fibers is formed.

As the oxidation catalyst 5, platinum, palladium,
A noble metal-based catalyst in which a noble metal such as rhodium is supported on a porous carrier, or a mixture or composite oxide catalyst containing a transition metal oxide such as iron, manganese, copper, nickel, cobalt, and chromium can be used. Silica, alumina,
A ceramic fiber such as zirconia and the catalyst powder are dispersed and stirred in a dispersion medium such as water together with an inorganic binder such as alumina sol or colloidal silica, and then paper-made with a filter, pressed and dried to form a sheet. In the case where papermaking is performed using metal oxide fine powder, it is desirable to use granulated particles having a particle size of about 10 to 100 mesh in order to improve the air permeability of the resulting film.

The alkali absorber 7 is used as a porous cured material mainly composed of an oxide, hydroxide, or carbonate of an alkali metal or an alkaline earth metal and made of a binder. It is desirable to use it.

Next, the operation of the solid electrolyte gas sensor formed as described above will be described. The temperature required for the operation of the gas sensor is supplied by the heating means 1 and the gas sensor is installed in the exhaust gas flow path of the combustion gas. Since the combustion gas flows into the detection unit via the gas selective permeable body 2,
The gas flowing into the oxygen ion conductive solid electrolyte 3 is selected, and the sulfurous acid gas, which is a poisoning gas, hardly flows or
Even if it flows in, it will be in an extremely regulated oxygen ion conductive state. Since the gas of oxygen and carbon monoxide required for the detection is not affected by the selection by the gas selective permeable member 2, the gas flows into the oxygen ion conductive solid electrolyte 3, so that the solid electrolyte 3 generates a gas corresponding to the concentration of the carbon monoxide gas. Power is output and can be detected. Even if sulfurous acid gas partially flows in, the absorption layer 8,
The oxidation catalyst 5 and the platinum electrode 4 are absorbed by the alkali absorber 7 of No. 9 and protected.

(Embodiment 2) FIG. 2 is a conceptual diagram of a solid electrolyte gas sensor according to Embodiment 2 of the present invention. FIG.
In the first embodiment described above, the absorption layer 8 in which the alkali absorber 7 is mixed with the ceramic fibers is formed on the catalyst layer 6 in which the oxidation catalyst 5 is mixed with the ceramic fibers. A coating layer 11 composed of a single layer obtained by mixing an alkali absorber 7 with ceramic fibers together with an oxidation catalyst 5 is formed, and the other configuration is the same as that of FIG. In this embodiment, since the coating layer 11 including the oxidation catalyst 5 can be formed thinner than the lamination in the case of the first embodiment, when the oxygen ion conductive solid electrolyte 3 is housed in the gas selective permeable body 2, This has the advantage that the size of the gas sensor can be reduced. Regarding the effect of preventing poisoning by an acidic gas such as sulfurous acid gas, which is a poisoning gas, the effect of protecting the oxidation catalyst 5 is inferior to that of the first embodiment.
Is the same as that of the first embodiment.

(Embodiment 3) FIG. 3 is a schematic cross-sectional view of a solid electrolyte gas sensor according to Embodiment 3 of the present invention. The difference from Embodiments 1 and 2 is that the oxidation catalyst is different. 5 is a configuration of a catalyst layer 6 including a coating layer 5 and a coating layer 11. In Example 3, a second catalyst layer 14 in which a noble metal-based oxidation catalyst 12 was mixed with ceramic fibers was laminated on the first catalyst layer 6 and a metal oxide-based oxidation catalyst 13 was mixed with ceramic fibers. Form and use.

The noble metal-based oxidation catalyst 12 converts one or more noble metal elements such as platinum, palladium and rhodium into γ.
It is used by being supported on a porous carrier such as alumina. It is desirable to use a carrier having a particle size in the range of 10 to 100 mesh from the viewpoint of air permeability and efficient papermaking process.

Examples of the metal oxide-based oxidation catalyst 13 include:
An oxide of a transition metal such as iron, manganese, copper, nickel, chromium, or cobalt, or a composite oxide having a perovskite, spinel structure, or the like is used. These oxidation catalysts
In addition to its catalytic function of oxidizing carbon monoxide, it has an action of absorbing or adsorbing acidic gases such as sulfur dioxide as a poisoning substance. In general, the amount of the metal oxide-based oxidation catalyst used is larger than that of the noble metal-based oxidation catalyst. The catalyst has higher durability. The absorption reaction or adsorption reaction of the acidic gas exhibited by the metal oxide-based oxidation catalyst eventually deteriorates the oxidation catalyst, but conversely covers the noble metal-based oxidation catalyst 12 and the platinum electrode 4 in the lower layer. It has the function of protecting from poison. In general, the activity of the oxidation reaction of carbon monoxide is superior to that of the noble metal-based oxidation catalyst 12. Therefore, in terms of the oxidation reaction of carbon monoxide, it is necessary to configure the oxidation catalyst 12 in a state where there is a margin in terms of oxidation ability. For example, if the upper catalyst layer 14 deteriorates with time, the lower catalyst layer 6 having high oxidation activity functions for the oxidation reaction, which is advantageous in extending the life. The operation as a gas sensor and the effect of the gas selective permeable body 2 are the same as those in the first and second embodiments.

(Embodiment 4) FIG. 4 is a sectional conceptual view of a solid electrolyte gas sensor according to Embodiment 4 of the present invention, and is an example in which the sensor is formed in a flat plate shape. In FIG. 4, a pair of platinum electrodes 4 are formed on the same plane of a flat oxygen ion conductive solid electrolyte 3, and an oxidation catalyst 5 and an alkali absorber 7 are mixed together with ceramic fibers on one of the platinum electrodes 4. A coating layer 11 is formed on the other platinum electrode 4 to form an absorption layer 9 in which an alkali absorber 7 is mixed together with ceramic fibers. In this state, the oxygen ion conductive solid electrolyte 3 and a heater film on the back surface are formed. A gas sensor is formed by joining and sealing the plate-shaped gas selective permeable body 2 on which the heating means 15 is formed by using a glass material 16. As the glass material 16 used here, a material that does not cause a difference in thermal expansion coefficient from the oxygen ion conductive solid electrolyte 3 and the gas selective permeable body 2 is selected and used. The components constituting the gas sensor are manufactured by the same method as described in the first embodiment.

The heating required for driving the sensor is performed by heating means 1
5, the gas selective permeable body 2 is provided with a heating means 1 on one side thereof.
Although 5 is formed, its gas permselectivity is not impaired at all. Oxygen and carbon monoxide required for the detection of carbon monoxide and carbon dioxide formed by the catalytic reaction can enter and exit through the gas selective permeator 2, thereby detecting carbon monoxide and generating an electromotive force output. I do. Regarding the durability as a gas sensor, sulfur dioxide gas or kerosene vapor having a large molecular weight is first protected by the gas selective permeator 2 restricting its inflow, and even if some sulfur dioxide gas flows in, it is absorbed. Since it is absorbed by the alkali absorber 7 of the layer 9 and the coating layer 11, the poisoning of the oxidation catalyst 5 and the platinum electrode 4 is protected, so that a long-life gas sensor can be realized.

(Embodiment 5) FIG. 5 is a schematic cross-sectional view of a solid electrolyte gas sensor according to Embodiment 5 of the present invention. In the configuration of the fifth embodiment, a pair of platinum electrodes 4 is formed on the same plane of the plate-shaped oxygen-ion-conductive solid electrolyte 3, and the oxidation catalyst 17 is placed in pores in a region facing one of the platinum electrodes 4. Heating means 15 which is carried and comprises a heater film on the back surface
Are sealed with a glass material 16 so as to surround the outer periphery of the pair of platinum electrodes 4. The oxidation catalyst 17 supported in the fine pores adsorbs the aqueous solution of the noble metal-based oxidation catalyst by filtering through the gas selective permeable body 2 in a state where only the fine pores are opened and the others are masked and closed. After that, it can be prepared by a wet method using a reducing agent solution such as formalin alkali or sodium borohydride or a dry method of reducing in a hydrogen stream under heating at about 300 to 500 ° C.

Next, the operation of the gas sensor will be described. The heating required for driving the gas sensor is performed by heating means 15.
The gas selective permeable body 2 has a heating means 1 on one side thereof.
Although 55 is formed, its gas permeability is not impaired at all. Of the pair of platinum electrodes 4, oxygen reaches one of the platinum electrodes 4 facing the oxidation catalyst 17 without any problem, but carbon monoxide is completely oxidized by the oxidation catalyst 17 in the pores. Do not reach. Since carbon monoxide and oxygen reach the other platinum electrode 4, carbon monoxide is detected and an electromotive force output is generated.

Since the oxidation catalyst 17 is present in the pores, the catalyst is protected from being poisoned by sulfur dioxide gas or the like as compared with the case where the catalyst is bare, and the effect of preventing the gas selective permeator 2 from being poisoned has already been described. This is the same as in the embodiment described above.

In the case of the constructions of the fourth and fifth embodiments, thermal stability is a problem because the different materials are in contact with each other under heating, and the oxygen ion conductive solid electrolyte 3 has a yttria-stabilized structure. When zirconia is used, it is desirable to use alumina or forsterite having a thermal expansion coefficient close to that of yttria-stabilized zirconia as a material of the gas selective permeable member 2.

The flat gas selective permeable member 2 can be manufactured by the following procedure. First, a plate-shaped porous body made into a porous body by a sintering method and made into a flat plate by a doctor blade method or the like was prepared by the sol-gel method described in Example 1 or C
It is manufactured by closing the pores by a VD method or the like and controlling the diameter. Since the porous body contains fine pores, the air holes have an advantage that they buffer the thermal shock and are strong in the cycle of thermal expansion and contraction.

Next, test results of evaluating the durability of the gas sensor according to the present embodiment described above are shown.

The gas selective permeable body is generally manufactured by a sintering method and used for the purpose of microfiltration as a base of a ceramic filter, and is processed and manufactured. Ceramic filters made by the sintering method
Since the pore diameter is limited to 0.1 micron, in order to be able to use it for the purpose of selective gas permeation, a treatment for closing the pores is required. Therefore, a commercially available alumina microfiltration tube (3.0 / 2.5 mmφ) was used, which was closed with an aluminum polymer to obtain a gas selective permeable material.

The aluminum polymer was prepared by a sol-gel method. That is, it was prepared by hydrolyzing aluminum isopropoxide at 80 ° C. for 24 hours and then conducting a polymerization reaction under boiling for 48 hours under a hydrochloric acid catalyst. In the treatment of the gas selective permeator, the alumina microfiltration tube was immersed in the polymer solution for about 10 seconds, dried for about 8 hours at room temperature, and then baked at 50 K / h to 773 K. The pore size was as large as about 800 °. Next, the mixture was filtered with an alumina polymer through a filter tube for about 3 minutes, dried at room temperature for about 8 hours, and then 773 at 50 K / h.
When the operation of baking to K was repeated several times to control the pore diameter, the boehmite film was changed to γ-alumina.
Evaluation of the molecular weight cutoff and direct confirmation with a scanning electron microscope revealed that the pore diameter was about 50 ° after about four treatments. In this way, a ceramic gas selective permeation tube is produced,
It was processed at a length of 35 cm, cut into a required size with a diamond cutter, and used for a sensor.

A 1.5 × 8 mm (plate thickness: 0.25 mm) zirconia substrate (yttria stabilized zirconia: yttria 8)
(MOL% product) with a substrate temperature of 300 ° C., a platinum electrode is produced by a shutter link so as to face the front and back surfaces with an electrode area of 1 × 6 mm, and a commercially available platinum paste is used.
A 0.1 mm platinum lead wire was joined.

Next, a catalyst layer and an absorption layer were prepared in the following procedure. First, for the catalyst layer, 0.1 wt% of platinum and palladium are supported on γ-alumina having a particle size of 40/60 mesh, and chloroplatinic acid, nitric acid of palladium chloride and hydrochloric acid aqueous solution are adsorbed, and then borohydride A catalyst was prepared by reduction with an aqueous sodium solution,
The mixture was mixed at a basis weight of 00 g / m ² to prepare a 0.5 mm catalyst-containing ceramic sheet.

As an absorbing layer, calcium hydroxide, potassium carbonate, and lime aluminate were mixed in 1/1/1 by weight, and water was added to granulate to obtain a 60/80 mesh powder. In the same manner as described above, an alkali-absorber-containing ceramic sheet having a basis weight of 600 g / m ² and a thickness of 0.3 mm was prepared by mixing with silica-alumina fiber.

After the catalyst-containing sheet and the alkali-absorber-containing sheet were each cut to the same size as the zirconia substrate with a cutter, the alumina-containing zirconia substrate was used. After laminating the containing sheet on top and bonding the sheet containing the alkaline absorber only on the other side, it was stored in a gas selective permeable body (dimension 12 mm) and the platinum lead wire was taken out. Both ends of the tubular gas-permeable member are sealed with a silicate-based adhesive containing a filler to obtain an element having the same configuration as that of the first embodiment. As a comparative example, an element in which only a catalyst-containing sheet was adhered to one surface of a zirconia substrate was referred to as an element B, and an element in which this element was housed in a cylindrical gas selective permeable body in the same configuration as above was manufactured as an element C.

For the devices A, B, and C, the results of evaluating the carbon monoxide concentration in the air at 450 ° C. and the sensor output characteristics using a flow-through gas sensor characteristic evaluation test apparatus are as shown in FIG. FIG. 6 shows that there is no difference in the detection ability of the gas sensor for carbon monoxide.

FIG. 7 shows the temperature characteristics of the element A. It can be seen from FIG. 7 that the output becomes maximum near 400 ° C. Therefore, the element temperature is set to an initial operating temperature of 500 to 550 ° C.
, And by using the set temperature control means, the output can be increased by lowering over time based on the operation time of the sensor. In general, the output of a chemical sensor decreases due to deterioration with time, and the case of a solid electrolyte gas sensor is no exception. Therefore, the above-described means can cover the weak point of fail-out due to the deterioration of the chemical sensor.

Next, in order to evaluate the influence of the durability, the elements A, B, and A were placed in the exhaust gas flow path of the combustion equipment (FF type gas water heater).
C, sulfur dioxide was added to the exhaust gas at 100 PPM, and the change in sensor characteristics was evaluated. The device B deteriorated in about 50 hours, whereas the devices A and C did not show any abnormality even after 2500 hours passed, and the continuous test is still underway, and an effective effect on durability was confirmed.

[0072]

The gas sensor of the present invention is embodied in the form described above, and has the following effects.

(1) Regarding the detection of carbon monoxide, the weak point of the fail-out can be covered, and the reliability of the element configuration is high, which is suitable for installation in a combustion apparatus or the like.

(2) In terms of practical use of a chemical sensor, a gas selective permeable member for restricting the reaching of an interfering gas to a sensor element is used for durability, which has been the biggest problem in the past, and a gas sensor is housed therein. However, the double prevention effect of providing an alkali absorber for absorbing an acid gas having a poisoning effect on the gas sensor is expected to dramatically increase the service life, and a highly reliable gas sensor system can be constructed.

[Brief description of the drawings]

FIG. 1 is a conceptual cross-sectional view of a gas sensor according to a first embodiment of the present invention.

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

FIG. 3 is a conceptual sectional view of a gas sensor according to a third embodiment of the present invention.

FIG. 4 is a conceptual sectional view of a gas sensor according to a fourth embodiment of the present invention.

FIG. 5 is a conceptual sectional view of a gas sensor according to a fifth embodiment of the present invention.

FIG. 6 is a graph showing an evaluation test result of a sensor output characteristic of the gas sensor according to the embodiment of the present invention.

FIG. 7 is a graph showing an evaluation test result of a temperature characteristic of the gas sensor according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 15 Heating means 2 Gas selective permeable body 3 Oxygen ion conductive solid electrolyte 4 Platinum electrode 5, 12, 13, 17 Oxidation catalyst 6, 14 Catalyst layer 7 Alkaline absorber 8, 9 Absorption layer 11 Coating layer

Claims (7)

[Claims] 1. A gas selective permeable member provided at a heating means and a gas inlet / outlet, a pair of platinum electrodes provided on a surface of an oxygen ion conductive solid electrolyte, an oxidation catalyst and ceramic fibers formed on one platinum electrode A gas sensor comprising: a catalyst layer containing: an alkali absorber; and a laminate of an absorption layer containing an alkali absorber and ceramic fibers; and an absorption layer formed on the other platinum electrode and containing an alkali absorber and ceramic fibers. 2. A heating means, a gas selective permeable member provided at a gas inlet / outlet, a pair of platinum electrodes provided on a surface of an oxygen ion conductive solid electrolyte, an oxidation catalyst formed on one platinum electrode, A gas sensor having a coating layer containing an absorber and ceramic fibers, and an absorption layer containing an alkali absorber and ceramic fibers formed on the other electrode. 3. A heating means, a gas selective permeable member provided at a gas inlet / outlet, a pair of platinum electrodes provided on a surface of an oxygen ion conductive solid electrolyte, and a noble metal-based oxidation catalyst formed on one of the platinum electrodes Sensor having a catalyst layer containing metal and ceramic fibers and a catalyst layer containing metal oxide-based oxidation catalyst and ceramic fibers, and an absorption layer formed on the other platinum electrode and containing an alkali absorber and ceramic fibers . 4. A pair of platinum electrodes are provided on the same surface of a plate-shaped oxygen ion conductive solid electrolyte, and a coating layer containing an oxidation catalyst, an alkali absorber and ceramic fibers is formed on one platinum electrode, and the other is formed. A gas sensor in which an absorption layer containing an alkali absorber and ceramic fibers is formed on a platinum electrode, and the oxygen ion conductive solid electrolyte and a gas selective permeable body having heating means formed on the back surface are joined and sealed. 5. A plate-shaped oxygen ion conductive solid electrolyte having a pair of platinum electrodes formed on the same surface, an oxidation catalyst carried in pores in a region facing one of the platinum electrodes, and a heating means on the back surface. A gas sensor in which a gas selective permeable member formed with a seal is joined and sealed. 6. The gas sensor according to claim 4, wherein one of alumina and forsterite is used as the gas selective permeable member. 7. An initial operating temperature is 500 to 550 ° C.,
The gas sensor according to any one of claims 1 to 6, further comprising a set temperature control means for lowering the temperature over time based on the operation time.