JPH11183421A - Contact combustion type gas sensor and method of manufacturing the same - Google Patents

Abstract

(57) [Problem] To provide high detection gas sensitivity even at a lower temperature than before. SOLUTION: An alkali is acted on a mixed aqueous solution of a soluble salt of aluminum and a soluble salt of a catalyst component to prepare a mixed gel of aluminum hydroxide and a hydroxide of a catalyst component, which is heated to be used for a catalyst element. Used as powder raw material. The figure shows the oxidizing activity for methane, a shows the oxidizing activity of the gas detecting element according to the present invention, and b shows that of the gas detecting element manufactured by the conventional method. The temperature at which the oxidation efficiency reaches 100%
It can be seen that the former is lower by about 50-60 ° C. than the latter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas leak alarm which detects an explosive gas leaked into the atmosphere and issues an alarm to prevent an explosion, particularly a contact combustion type gas sensor and its gas detecting element, and The present invention relates to a method for manufacturing a temperature compensation element.

[0002]

2. Description of the Related Art Conventionally, as a combustible gas sensor used for a gas leak alarm, there are a metal oxide semiconductor type using tin oxide, a catalytic combustion type using a noble metal oxide catalyst, and some other types. is there. Among them, the above-mentioned semiconductor type or catalytic combustion type methane gas sensor is used for a city gas alarm which detects leakage of methane gas which is a main component of city gas. Although the catalytic combustion type gas sensor has lower detection sensitivity than the semiconductor type gas sensor, it has features such as less influence by the ambient temperature and humidity than the semiconductor type gas sensor and relatively superior long-term stability. It is used for alarms installed in places where environmental conditions are severe, such as commercial kitchens.

FIG. 10 shows an example of a gas detecting device in which a catalytic combustion type gas sensor is assembled in a bridge circuit. That is, the gas detecting element 1 and the temperature compensating element 2 are connected in series to one side of the bridge, and two fixed resistors 3 and 4 are connected in series to the other side. The gas detecting element 1 is preheated by the power supply from the power supply 5, so that when the combustible gas comes into contact, the oxidation reaction proceeds, and the temperature rise due to the heat increases the electric resistance of the coil. On the other hand, since the oxidation reaction does not proceed in the temperature compensating element 2, the temperature of the element does not rise. As a result, the bridge output voltage detected by the detection unit 6 increases in proportion to the gas concentration, whereby the concentration of the combustible gas can be detected.

FIG. 11 is a cutaway perspective view of a main part of the gas detecting element. That is, a mixture of a porous alumina carrier 8 and a noble metal catalyst 9 is fixed around a coil 7 made of a material having a large resistance temperature coefficient such as platinum. Usually, a catalyst mainly composed of palladium, which is highly active against methane, is added to the gas detection element for detecting methane, but other noble metal catalysts such as platinum and rhodium are added in addition to using palladium alone. In some cases.

Next, a general method for manufacturing a contact combustion type gas sensor will be described. As a method of manufacturing the gas detection element, first, an aqueous inorganic binder and pure water are mixed with activated alumina powder having a specific surface area of 100 to 200 m ² / gr to form a paste, which is applied to the periphery of a platinum coil.
The carrier is fixed to the coil by heating to 0 to 800 ° C. Next, the carrier element is immersed in an aqueous solution of a noble metal salt, dried,
Heat treatment is performed to disperse and support the noble metal catalyst on the carrier. Palladium chloride is usually used as a noble metal salt for the methane sensor. The device subjected to the heat treatment as described above is bonded to two conductive pins embedded on an insulating base, and further assembled by attaching an explosion-proof net around the device.

Next, the temperature compensation element of the contact combustion type gas sensor will be described. The function of the temperature compensation element is literally
An object of the present invention is to provide a device having the same size as a gas detection device and to suppress drift of a bridge output due to a change in ambient temperature. Further, as described below, there is a case where a function of making alcohol vapor in an actual gas atmosphere less sensible is added. This is because ethanol vapor, which is a kind of flammable gas, is contained in cooking steam in a home kitchen, restaurant kitchen, or the like, so that a consumer gas leak alarm does not malfunction due to the steam. Ethanol is easily oxidized on the gas detecting element, but a method is adopted in which the temperature compensating element has an oxidizing activity with respect to ethanol, thereby canceling the output of the gas detecting element and preventing malfunction. For this purpose, an element in which a transition metal oxide having a property of selectively oxidizing ethanol, such as copper oxide or iron oxide, is supported on an alumina carrier is often used as the temperature compensation element.

A general method for manufacturing a temperature compensating element is as follows. Also in the case of the temperature compensation element, the step of fixing the alumina carrier around the platinum coil is the same as that of the gas detection element. Next, the carrier element is impregnated with an aqueous solution of a soluble salt of a transition metal oxide such as copper or iron, dried and heat-treated to disperse and support a transition metal oxide catalyst such as copper oxide or iron oxide on the carrier. Constitute.

[0008]

That is, the conventional combustion method in which a soluble salt aqueous solution of the catalyst component is immersed in the pores of the porous carrier of the activated alumina element is used for the catalytic combustion type gas sensor. The amount has an upper limit depending on the pore volume of the support, and it is difficult to support a sufficient amount of the catalyst component. On the other hand, there is a method of increasing the solution concentration of the soluble salt in order to increase the supported amount, but if the solution concentration is too high, the catalyst salt tends to precipitate in an agglomerated state during drying, and as a result, the catalyst becomes dispersible. It is difficult to carry well.

As described above, in the conventional production method, the amount of the catalyst capable of supporting the catalyst component with good dispersibility on the carrier is limited. For example, in the catalytic combustion type gas sensor for detecting methane, the operating temperature is about 400 ° C. It was necessary to keep it above. However, in the contact combustion type gas sensor, the platinum coil embedded inside the element is used by self-heating, and if used at high temperature for a long period of time, the coil may be gradually thermally damaged and drift may occur over time. There is a problem that there is. Therefore, an object of the present invention is to provide a gas detection system which has a high sensitivity to a detection gas and a low sensitivity to a miscellaneous gas even when operated at a temperature lower than a temperature at which a conventional catalytic combustion type gas sensor is used. An object is to obtain an element and a temperature compensating element.

[0010]

Various studies have been made to obtain a sensor for detecting methane gas with higher sensitivity at a temperature lower than the temperature at which a conventional catalytic combustion type gas sensor is used. I found something good to do. 1) mixing an aqueous solution comprising a mixed salt of a soluble salt of aluminum and a soluble salt of a catalyst component with an alkaline aqueous solution,
A mixed gel of aluminum hydroxide and a hydroxide of the catalyst component is simultaneously precipitated under the condition that the liquid is alkaline. 2) The excess alkali and by-products generated by the neutralization reaction are removed by washing with water. 3) The mixed gel is separated from the mother liquor by a method such as filtration, and then the gel is dried and heated to obtain a homogeneous mixture of alumina (aluminum oxide) and an oxide of the catalyst component. 4) The mixture is ground, and an inorganic binder, water and the like are added to form a paste, which is then attached to a platinum coil and heated to form an element.

As the soluble salt of aluminum, aluminum nitrate is preferable, but acetate, chloride and the like can also be used. As a soluble salt of the noble metal used for the gas detection element, palladium chloride is suitable, but palladium nitrate may be used. Rhodium chloride or nitrate other than palladium can be used. On the other hand, as the transition metal salt used for the temperature compensation element, a nitrate of copper or iron is suitable, but an acetate of these transition metals can also be used. The catalyst component capable of imparting ethanol oxidizing activity to the temperature compensating element includes the above-described copper oxide and iron oxide. In addition, nickel oxide, cobalt oxide, and the like can be used. Similarly, nitrates and acetates can be used.

On the other hand, as the alkali used in the alkaline aqueous solution, sodium carbonate and aqueous ammonia are preferable, but sodium hydroxide can also be used. The precipitate is formed in an alkaline state where the pH of the liquid is 7.0 or more, preferably 8.0 or more. The temperature of the solution at the time of precipitation may be room temperature, but in order to allow the reaction to proceed quickly,
You may heat to about 70 degreeC. When the excess alkali and by-products (such as sodium nitrate and sodium chloride) generated by the neutralization reaction are washed and removed by washing with water, centrifugation or the like, the pH of the mother liquor after washing becomes 7.0 or less. Wash until To separate the mixed gel from the mother liquor,
Conventional filtration methods and freeze-drying methods can be used. The gel separated from the mother liquor is kept at 50 to 100 ° C. to volatilize most of the water, and then heated to 500 to 800 ° C. in an electric furnace to convert aluminum hydroxide to aluminum oxide and to form a catalyst component. Try to convert hydroxide to oxide.
Generally, a gas detection element that detects methane carries palladium as a catalyst component. However, as described above, in order to increase the stability of the methane detection function of the gas detection element, two components of palladium and platinum are used. A system oxidation catalyst may be added.

When an alkaline aqueous solution is applied to an aqueous solution of a soluble salt of platinum, for example, chloroplatinic acid, the hydroxide of platinum does not precipitate. Therefore, in the present invention, a gas detection element comprising a two-component oxidation catalyst is obtained by the following methods 1) to 4). 1) A mixed powder of aluminum oxide and palladium oxide is prepared by the method of the present invention. 2) This mixed powder is immersed in an aqueous solution of chloroplatinic acid and then dried and solidified on a water bath. 3) The dried and solidified solid is placed in an electric furnace at 400-700.
C. to decompose chloroplatinic acid to obtain powder containing aluminum oxide containing a two-component catalyst of palladium oxide and platinum. 4) Using the powder, an element is formed on the coil in the same manner as described above.

That is, in the present invention, a raw material of aluminum oxide (activated alumina) as a carrier and a raw material of a catalyst component are uniformly mixed at an atomic level in the state of an aqueous solution. Things co-precipitate. Therefore, in this mixed gel, the carrier component and the catalyst component are extremely uniformly mixed. The amount of the catalyst component carried is
It can be easily controlled by the amount of the soluble noble metal salt as a starting material. That is, there is no restriction on the amount of the catalyst as in the conventional immersion method, and there is no step of concentrating and precipitating the catalyst salt from the aqueous solution. Therefore, even if the amount of the catalyst is increased, the catalyst component does not condense. Therefore, higher dispersibility of the catalyst can be ensured even when the amount of the supported catalyst is increased as compared with the conventional production method. From these, the catalytic combustion type gas sensor according to the present invention is:
Even if the operating temperature is lowered by about 40 to 50 ° C. as compared with the sensor manufactured by the conventional method, the same methane gas sensitivity and selectivity to ethanol as those of the sensor manufactured by the conventional method can be obtained.

[0015]

EXAMPLE 1 Fabrication of Gas Sensing Element Hereinafter, a method for producing a contact combustion type gas sensor according to the present invention will be described. 1) 1 L of an aqueous solution was prepared by dissolving palladium chloride dihydrate and aluminum nitrate nonahydrate so as to be 0.02 mol / L as palladium and 0.98 mol / L as aluminum. Separately, 1 L of an aqueous solution prepared by dissolving sodium carbonate so as to have a neutralization stoichiometric ratio 1.5 times was prepared. 2) Using a stirrer equipped with a hot plate, heat each solution at 50 ° C
, And a mixed solution of palladium and aluminum was added to the sodium carbonate solution to precipitate a mixed gel of palladium hydroxide and aluminum hydroxide. At this time, the pH of the mother liquor was 8.9. Then, it was aged at 50 ° C. for 1 hour. 3) Subsequently, excess alkali components and sodium chloride and sodium nitrate by-produced by the reaction were removed by washing by centrifugation. Further, centrifugation, addition of pure water, and redispersion were repeated, and the mother liquor was repeatedly washed until the pH became 7.0 or less. 4) Thereafter, the mixed gel was separated from the mother liquor by suction filtration, and then left at room temperature for 72 hours to make the gel into a semi-dry solid. 5) The solid gel was heated in an electric furnace at 100 ° C. for 6 hours and then at 600 ° C. for 6 hours to obtain a mixture of palladium oxide and aluminum oxide. This mixture was ground using a planetary mill to obtain a powder having a center particle size of 15 μm. 6) Colloidal alumina and water were added to this powder to form a paste, which was adhered in beads to a coil made of a platinum wire having a wire diameter of 40 μm. 7) Then, a current was applied to the platinum wire so that the surface temperature of the element became 700 ° C., and this temperature was maintained for 10 minutes to fix the paste to the coil.

Preparation of Temperature Compensating Element 1) Copper nitrate hexahydrate and aluminum nitrate nonahydrate are 0.10 mol / L as copper and 0.9 as aluminum.
1 L of an aqueous solution dissolved to be 0 mol / L was prepared. Separately, 1 L of an aqueous solution prepared by dissolving sodium carbonate so as to be 1.5 times the stoichiometric ratio of neutralization was prepared. 2) Using a stirrer equipped with a hot plate, heat each solution at 50 ° C
, And a mixed solution of copper and aluminum salts was added to the sodium carbonate solution to precipitate a mixed gel of copper hydroxide and aluminum hydroxide. At this time, the pH of the mother liquor was 9.1. Then, it was aged at 50 ° C. for 1 hour. 3) Next, a mixed powder of copper oxide and activated alumina was obtained by washing, filtering, drying and heating in a furnace in the same manner as in the above gas detection element. 4) A paste was prepared under the same conditions as in the method for manufacturing the gas detection element, and an element was formed.

Production of Comparative Element A conventional method for producing a gas detecting element used for comparison will be described below. 1) Colloidal alumina and water are added to activated alumina powder having a specific surface area of 120 m ² / gr and a center particle diameter of 15 μm to form a paste, which is attached to a platinum coil similar to the above in the form of beads, dried, and then dried. Heat at 8 ° C for 8 hours,
Activated alumina was fixed to the coil. 2) Then, the carrier element is immersed in an aqueous solution of palladium chloride, and the element is separated from the liquid.
Heat treatment was performed for 5 hours, and palladium oxide was supported on the carrier. A conventional method for manufacturing a temperature compensation element is as follows. After the activated alumina is fixed to the coil in the same manner as in the conventional method of the gas detection element, the carrier element is immersed in an aqueous solution of copper nitrate, and the element is separated from the liquid.
Heat treatment was performed at 00 ° C. for 5 hours to support copper oxide on the carrier.

Evaluation of catalytic activity In order to evaluate the catalytic activity of the gas detecting element and the temperature compensating element obtained by the present invention, catalytic activity was evaluated by a fixed bed flow method. That is, after the coil was pulled out from the completed device, the device was ground and filled in a quartz reaction tube to form a catalyst layer. The reaction tube was set in an electric furnace, air containing 1% of methane or 0.5% of ethanol was introduced from the inlet, and the concentrations of methane and ethanol in the outlet gas were determined by gas chromatography. By this method, the relationship between the catalyst layer temperature and the oxidation rate of methane or ethanol was determined. The oxidation activity was evaluated at a space velocity of 20,000.
The ^test was performed under the conditions of H- ¹ . Sensor sensitivity evaluation The gas detection element and the temperature compensation element obtained according to the present invention were assembled in a bridge circuit, and the gas sensitivity in 0.4% methane gas and 0.4% ethanol gas was measured. A relationship curve with sensitivity was determined.

FIG. 1 shows the oxidizing activity of the gas detecting element for methane. The curve a in the figure shows the methane oxidation activity of the gas detection element (hereinafter also referred to as the former) manufactured by the method of Example 1,
Further, a curve b is a detection element (hereinafter, referred to as a detection element) according to the above-described conventional manufacturing method.
Methane oxidation activity). FIG.
Therefore, the former has a methane oxidation efficiency of 1 more than the latter.
It can be seen that the temperature to reach 00% is reduced by about 50 to 60 ° C. FIG. 2 shows the oxidation activity of the gas detection element with respect to ethanol. The curve c in the figure shows the former ethanol oxidation activity, and the curve d shows the latter ethanol oxidation activity. From FIG. 2, the temperature at which the efficiency of ethanol oxidation reaches 100% is higher in the former than in the latter, about 40 to 5 times.
It can be seen that the temperature is lowered by 0 ° C.

FIG. 3 shows the applied voltage dependence of methane sensitivity when the gas detection element and the temperature compensation element are assembled in a bridge. The curve e in the figure is the former sensitivity-applied voltage curve, f
Indicates the latter sensitivity-applied voltage curve. From FIG. 3, the sensitivity at the applied voltage of 2.3 V and the sensor temperature of 430 ° C. is almost equal to the sensitivity at the applied voltage of 2.1 V and the sensor temperature of 380 ° C. Therefore, the former is about 50 ° C. lower than the latter. It can be seen that sensitivity equivalent to the latter can be obtained even at the sensor temperature. FIG. 4 shows the applied voltage dependency of the ethanol sensitivity when the gas detection element and the temperature compensation element are assembled in a bridge. The curve g in the figure shows the former sensitivity-applied voltage curve, and the curve h shows the latter sensitivity-applied voltage curve.

In FIG. 4, the ethanol sensitivity shows a tendency to fall to the right with respect to the applied voltage for the following reason. That is, the ethanol oxidation activity on the gas sensing element side is almost saturated with the applied voltage, whereas the ethanol oxidation activity on the temperature compensation element side increases with an increase in the applied voltage (increase in the sensor temperature). This is because when the applied voltage increases, the output of the gas detection element is offset by the output of the temperature compensation element. From FIG. 4, in the latter case, the sensitivity at an applied voltage of 2.3 V and a sensor temperature of 430 ° C. is about 5 m.
In contrast, it can be seen that the output of the former is equivalent to that of the latter at an applied voltage of 2.1 V (sensor temperature of about 380 ° C.). That is, the former shows that the same ethanol sensitivity as the latter can be obtained even when the operating temperature is lowered by about 50 ° C. as compared with the latter.

Example 2 The following step was added to the mixture of palladium oxide and activated alumina which had been subjected to the steps 1) to 5) described in Example 1. 1) The above mixture was added to an aqueous solution in which chloroplatinic acid hexahydrate was dissolved, and the mixture was kept at room temperature with stirring for 2 hours. Here, the amount of chloroplatinic acid added was such that the atomic ratio of palladium to platinum after firing was 1: 0.5. 2) After keeping most of the water in a water bath at 50 ° C. to evaporate most of the water, the mixture was heated in an electric furnace at 600 ° C. for 6 hours to carry platinum on the mixture. 3) After the obtained powder was ground, colloidal alumina and water were added to form a paste, and this paste was adhered to the coil in the same manner as in item 6) of Example 1.

FIG. 5 shows the methane oxidation activity of the gas detection element of the second embodiment. The curve i in the figure shows the methane oxidation activity of the gas detection element of Example 2 and the curve b shows the methane oxidation activity of the gas detection element of the conventional method (the same as the gas detection element of the conventional method of Example 1). FIG. 5 shows that the gas detecting element of Example 2 can obtain 100% methane oxidation efficiency at a low temperature lower by about 40 to 50 ° C. than the gas detecting element of the conventional method. FIG. 6 shows the applied voltage dependency of the methane sensitivity when the gas detection element of the second embodiment and the temperature compensation element of the first embodiment are combined. FIG.
Is a methane sensitivity-applied voltage curve of the combination of the gas detection element of the second embodiment and the temperature compensation element of the first embodiment, and a curve f is the methane sensitivity of the combination of the gas detection element and the temperature compensation element of the conventional method. It is an applied voltage curve. As shown in FIG. 6, the sensor using the gas detection element of the second embodiment is a conventional sensor (applied voltage 2.3 V, sensor temperature: 4).
It can be seen that sensitivity equivalent to that of the conventional sensor can be obtained even at a lower applied voltage (2.17 V) than that of the conventional sensor (ie, 30 ° C.), that is, even at a sensor temperature (400 ° C.) about 30 ° C. lower than the conventional sensor.

Example 3 In the method of manufacturing a gas detection element of Example 1, rhodium trichloride was used as a starting material for the catalyst instead of palladium chloride, and the same procedure as in 1) to 6) of Example 1 was carried out. Then, a gas detection element made of rhodium oxide and aluminum oxide was produced. Further, for comparison, a gas detection element in which rhodium oxide was supported on aluminum oxide was manufactured by a conventional method using rhodium trichloride instead of palladium chloride by the manufacturing method described in the section of manufacturing the comparison element of Example 1. did.

FIG. 7 shows the methane oxidation activity of the gas detecting element of the third embodiment. The curve k in the figure indicates the oxidizing activity of the gas detecting element of Example 3, and the curve 1 indicates the oxidizing activity of the element having rhodium oxide supported on aluminum oxide according to the conventional method. FIG.
Therefore, the gas detection element of Example 3 has a methane oxidation rate of 100
% Is about 40 ° C. lower than that of the conventional gas detection element. A curve m in FIG. 8 indicates a methane sensitivity-applied voltage curve of the sensor of Example 3, and a curve n indicates a gas detection element (catalyst is rhodium oxide) according to a conventional method.
4 shows a methane sensitivity-applied voltage curve of a sensor that combines a temperature compensation element (catalyst is copper oxide) according to a conventional method. From FIG. 8, it is clear that the sensor of Example 3 has a rhodium oxide
It can be seen that methane sensitivity equivalent to that of the sensor according to the conventional method can be obtained even at an applied voltage (2.15 V) corresponding to a sensor temperature (390 ° C.) lower by about 40 ° C. than that of the aluminum oxide element.

Example 4 Three types of temperature compensating elements were prepared by using the following three types of transition metal nitrates instead of copper nitrate hexahydrate in the section of manufacturing temperature compensating elements of Example 1. did. Nitrate mixed solution composition (aluminum nitrate is constant at 0.9 mol / L as aluminum) 1. Iron nitrate hexahydrate 0.2 mol / L as iron 2. Nickel nitrate hexahydrate 0.1 mol / L as nickel Cobalt nitrate hexahydrate 0.3 mol / L as cobalt The production method is 1) to 1) of the production of the temperature compensation element of Example 1.
Same as 4).

FIG. 9 compares the ethanol oxidizing activities of the above three types of temperature compensating elements. Curves o, p, and q show the ethanol compensating elements having iron oxide, nickel oxide, and cobalt oxide supported on aluminum oxide, respectively. The oxidation activity is shown, and curve d shows the ethanol oxidation activity of the conventional temperature compensating element (catalyst is copper oxide). According to FIG.
Although the ethanol oxidation activities of the three types of temperature compensating elements differ slightly depending on the catalyst components, the ethanol oxidizing activities are clearly higher than the temperature compensating element according to the conventional method shown in the curve d of FIG. It turns out that it shows activity.

[0028]

The sensor according to the present invention can be used at an operating temperature lower by about 50 ° C. than the gas sensor manufactured by the conventional method, so that the thermal load on the platinum coil and the catalyst element inside the element is reduced, An advantage is obtained in that stable sensor characteristics can be obtained for a long period of time. Further, according to the production method of the present invention, since the amount of the supported catalyst can be increased without impairing the dispersibility of the catalyst, even if the sensor is exposed to a poisoning component in the actual gas, for example, silicone vapor, the conventional method is used. As compared with the sensor, the output is less reduced due to the poisoning, and the advantage that the reliability is superior can be obtained.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram illustrating methane oxidation activity of a gas detection element according to Example 1.

FIG. 2 is a characteristic diagram illustrating ethanol oxidation activity of the gas detection element according to the first embodiment.

FIG. 3 is a characteristic diagram illustrating methane sensitivity-voltage dependency of the sensor according to the first embodiment.

FIG. 4 is a characteristic diagram for explaining ethanol sensitivity-voltage dependency of the sensor according to the first embodiment.

FIG. 5 is a characteristic diagram illustrating methane oxidation activity of a gas detection element according to a second embodiment.

FIG. 6 is a characteristic diagram illustrating the methane sensitivity-voltage dependency of the sensor according to the second embodiment.

FIG. 7 is a characteristic diagram illustrating methane oxidation activity of a gas detection element according to a third embodiment.

FIG. 8 is a characteristic diagram illustrating methane sensitivity-voltage dependency of the sensor according to the third embodiment.

FIG. 9 is a characteristic diagram illustrating the ethanol oxidation activity of the temperature compensation element according to Example 4.

FIG. 10 is a configuration diagram illustrating an example of a gas detection device.

FIG. 11 is a fragmentary perspective view showing a structural example of a gas detection element.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Gas detection element, 2 ... Temperature compensation element, 3, 4 ... Fixed resistance, 5 ... Power supply, 6 ... Bridge output detection part, 7 ... Platinum coil, 8 ... Alumina carrier, 9 ... Catalyst.

Claims (8)

[Claims] The gas detecting element is palladium, rhodium,
The temperature compensating element consists of a mixture of aluminum oxide and an oxidation catalyst selected from the group consisting of oxides of copper, iron, nickel and cobalt. In at least one of the gas detection element and the temperature compensation element,
A catalytic combustion method characterized by being produced through a process in which an alkali is acted on a mixed aqueous solution of a water-soluble salt of an oxidation catalyst component and a water-soluble salt of aluminum to cause co-precipitation as a hydroxide under conditions of PH 8 or more. Gas sensor. 2. A method for manufacturing a gas detecting element of a catalytic combustion type gas sensor, comprising the following steps (1) to (3). (1) A first step of mixing an aqueous alkaline solution with a mixed aqueous solution of aluminum nitrate and a soluble salt of a noble metal to precipitate a mixed gel of aluminum hydroxide and a hydroxide of a noble metal. (2) A second step of heating the mixed gel after washing to obtain a mixed powder of a noble metal and aluminum oxide. (3) A third step in which the mixed powder is formed into a paste together with a binder, and the paste is applied to the periphery of a platinum coil to form an element. Then, an electric current is applied to the coil to fix the paste to the coil. 3. The method according to claim 2, wherein the noble metal is palladium. 4. The method according to claim 2, wherein the noble metal is rhodium. 5. A method for manufacturing a gas detecting element of a catalytic combustion type gas sensor, comprising the following steps (1) to (4). (1) A first step of mixing an aqueous alkaline solution with a mixed aqueous solution of aluminum nitrate and a soluble salt of a first noble metal to precipitate a mixed gel of aluminum hydroxide and a hydroxide of the first noble metal. (2) A second step of washing and heating and dehydrating the mixed gel to obtain a mixed powder of the first noble metal and aluminum oxide. (3) The mixed powder is immersed in an aqueous solution containing a soluble salt of a second noble metal, dried and heated to decompose the second noble metal salt, and a mixture of aluminum oxide, the first noble metal and the second noble metal A third step of obtaining (4) A fourth step in which the mixture obtained through the third step is pasted together with a binder into a paste, which is applied around a platinum coil to form an element, and then a current is passed through the coil to fix the paste to the coil. . 6. The gas detecting element according to claim 5, wherein the first noble metal is at least one component of palladium or rhodium, and the second noble metal is platinum. Production method. 7. A method for manufacturing a temperature compensation element of a catalytic combustion type gas sensor, comprising the following steps (1) to (3). (1) First, an alkaline aqueous solution is mixed with a mixed aqueous solution of aluminum nitrate and a soluble salt of a transition metal to precipitate a mixed gel of aluminum hydroxide and a hydroxide of a transition metal.
Process. (2) A second step of heating the mixed gel after washing to obtain a mixed powder of a transition metal oxide and aluminum oxide. (3) A third step in which the mixed powder is made into a paste together with a binder, and the paste is applied to the periphery of a platinum coil to form an element, and an electric current is applied to the coil to fix the paste to the coil. 8. The method according to claim 7, wherein the transition metal is at least one of copper, iron, nickel and cobalt.
3. A method for manufacturing a temperature compensation element for a contact combustion type gas sensor according to item 1.