JPH08226909A - Catalytic combustion type carbon monoxide gas sensor - Google Patents

Abstract

(57) [Summary] [Object] To provide a catalytic combustion type carbon monoxide gas sensor having high gas selectivity and having high sensitivity only to carbon monoxide gas. A gas detecting element 3 and a compensating element 4, which are used by being incorporated in a branch side of a bridge circuit, are held by a metal pin 6 which is fixed through a base 5 made of an insulating material and covered with a cap. In the catalytic combustion type carbon monoxide gas sensor, the gas detection element 3 is composed of a resistance temperature detector and an oxidation combustion catalyst layer covering the resistance temperature detector, and the oxidation combustion catalyst layer is an α-ferric oxide carrying a gold catalyst. The compensating element 4 is a sintered body of a mixture of fine powder and alumina powder carrying a platinum catalyst and a palladium catalyst, and the compensating element 4 comprises a resistance temperature detector and a metal oxide layer covering the resistance temperature detector, The metal oxide layer is a sintered body of a mixture of α-ferric oxide fine powder and alumina powder. The element is covered with a cap 8 made of glass fiber filter paper and polyflon filter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic combustion type carbon monoxide gas sensor applicable to detection of incomplete combustion of a gas appliance, and particularly to a carbon monoxide gas sensor having good selectivity for carbon monoxide gas.

[0002]

2. Description of the Related Art Infrared gas sensors, semiconductor gas sensors and catalytic combustion carbon monoxide gas sensors are currently known as sensors for detecting carbon monoxide gas. Infrared gas sensors use absorption of infrared gas, and are highly accurate and highly reliable, but expensive. The semiconductor gas sensor utilizes a change in resistance value due to adsorption of a gas of an oxide semiconductor, and has a considerably high sensitivity, but lacks gas selectivity and is poor in stability. The catalytic combustion type carbon monoxide gas sensor uses the combustion heat of carbon monoxide gas, but it is difficult to detect a gas having a low concentration, and the gas selectivity is not sufficient.

FIG. 6 is a fragmentary sectional view showing a gas detecting element of a conventional catalytic combustion type carbon monoxide gas sensor. A metal oxide carrier 2 such as alumina carrying a catalyst such as platinum and palladium is sintered and fixed around a resistance temperature detector 1 such as a platinum coil. The compensating element has the same structure as the gas detecting element, but copper oxide is carried in place of the platinum and palladium catalysts carried by the gas detecting element.

FIG. 7 is a perspective view of a conventional catalytic combustion type carbon monoxide gas sensor. The gas detecting element 3 and the compensating element 4 are respectively held by two pins 6 which are fixed through the insulating material base 5. The element is covered with a cap 8a having holes 9 for ventilation.
A heat shield plate 7 is erected between the two elements. FIG.
FIG. 3 is a connection diagram of a bridge circuit incorporating a general catalytic combustion type gas sensor. The gas detecting element 3 (resistance value R3) and the compensating element 4 (resistance value R4) are connected in series to the two adjacent branches, and the fixed resistance R is provided on the other two adjacent branches.
1, R2 (which also indicates the resistance value) are connected in series. A power supply E is connected to the bridge circuit, and a load V is connected to the connection point between the elements and the connection point between the fixed resistors.

When the carbon monoxide gas is not present, the bridge circuit is in balance and in the state of R1 × R4 = R2 × R3, and the voltage applied to the load V is 0V. When the atmosphere contains carbon monoxide gas, carbon monoxide burns in the gas detection element 3, the temperature of the platinum coil rises, and its resistance value R4 increases. On the other hand, in the compensating element 4, the carbon monoxide gas does not burn and its resistance value R3 does not change. In this way, the balance of the bridge circuit is broken and the load V
A voltage is applied to.

When the atmosphere contains alcohol gas, the alcohol gas burns in both the gas detecting element 3 and the compensating element 4. Therefore, the resistors R3 and R4 both increase, the balance of the bridge circuit is not lost, and no voltage is generated. In this way, compensation for alcohol gas is performed. The compensating element 4 compensates for temperature. Compensation element 4 or gas detection element 3 depending on changes in room temperature
Even if the temperature of the platinum coil changes, the equilibrium of the bridge circuit is not broken because the temperature coefficients are the same as long as they are at the same temperature. Alumina fixed to the platinum coil has the same shape in the compensating element and the gas detecting element, and retains the combustion heat of these elements and maintains them at the same temperature.

The gas detecting element and compensating element of such a conventional catalytic combustion type carbon monoxide gas sensor are manufactured as follows. Using a platinum wire with a diameter of 60 μm, the outer shape is 0.
A coil having a length of 6 mm, a winding number of 10 turns, and a length of 1.5 mm is manufactured. A paste containing a mixture of alumina powder and alumina sol is attached to the platinum coil and fired at 800 ° C. to fix the alumina carrier to the platinum coil. Impregnate the alumina carrier into an aqueous solution of chloroplatinic acid and palladium chloride,
It is decomposed by heating at 600 ° C. to support a mixed catalyst of platinum and palladium oxide on an alumina carrier.

Similarly, as the compensating element 4, a paste having a mixture of alumina powder and alumina sol is attached to a platinum coil having the same shape as the gas detecting element, and the paste is baked at 800 ° C. and fixed to the platinum coil to form an alumina carrier. An alumina carrier is impregnated in an aqueous solution in which copper sulfate is dissolved and decomposed by heating to support a copper oxide catalyst. The conventional catalytic combustion type carbon monoxide gas sensor as described above has features such as a simple operation principle, relatively long-term stability, and little influence by ambient temperature and humidity. It is widely used for gas detection in the concentration range of 0.1 to 1% for preventing carbon monoxide poisoning with power consumption.

[0009]

However, the conventional catalytic combustion type carbon monoxide gas sensor as described above has a high sensitivity not only to carbon monoxide but also to hydrogen, and it depends on the degree of incomplete combustion. When the ratio of carbon and hydrogen changes, there is a problem that the output also changes. The present invention has been made in view of the above points, and an object thereof is to provide a novel catalytic combustion type carbon monoxide gas sensor having high sensitivity only to carbon monoxide and good gas selectivity.

[0010]

In order to achieve the above-mentioned object, according to the present invention, a gas detecting element and a compensating element, which are used by being incorporated in a side of a bridge circuit, are fixed through a base of an insulator. In a catalytic combustion carbon monoxide gas sensor that is held by a metal pin that is covered with a cap, the gas detection element consists of a resistance temperature detector and an oxidation combustion catalyst layer that covers the resistance temperature detector. The combustion catalyst layer is a sintered body of a mixture of α-ferric oxide fine powder supporting a gold catalyst and alumina powder supporting a platinum catalyst and a palladium catalyst, and the compensating element includes a resistance temperature detector and a temperature measuring resistor. It consists of a metal oxide layer covering the resistor, and the metal oxide layer is α
It shall be a sintered body of a mixture of ferric oxide fine powder and alumina powder. The two elements are formed by mixing each of the mixtures with alumina sol and colloidal silica,
It is good that it is sintered.

The gas detecting element and the compensating element are preferably covered with a cap made of glass fiber filter paper and polyflon filter.

[0012]

According to the present invention, in the gas detecting element, the catalyst in which gold is supported on the ferric oxide carrier having the ability to oxidize and burn carbon monoxide at room temperature is used as the oxidative combustion layer, and therefore, the temperature is 150 ° C. Carbon monoxide can be burned at the following relatively low temperatures. It is also possible to burn hydrogen. On the other hand, since the compensating element of the present invention uses a mixture of α-ferric oxide and alumina which does not carry a catalyst, carbon monoxide cannot be burned, but hydrogen can be burned. . Therefore, when the catalytic combustion type carbon monoxide gas sensor of the present invention is incorporated in a bridge circuit, it has a sensitivity of 1.5 times as much as that of a conventional one at a relatively low temperature for carbon monoxide and a sensitivity for hydrogen. Has a sensitivity less than half that of the conventional one.

Alumina sol and colloidal silica are used as a mixture when a carbon monoxide oxidation combustion catalyst carrying a gold catalyst on α-ferric oxide fine powder and an alumina catalyst carrying platinum and palladium are adhered to the resistance temperature detector coil. By
It becomes possible to sinter at a low temperature, and it is possible to fix the catalyst without deteriorating the performance of the catalyst due to high temperature sintering. Since the gas detection element and the compensating element are covered with the glass fiber filter paper and the cap of the polyflon filter, the filter adsorbs poisoning substances such as NO _x SO _x in the atmosphere, and the poisoning substances are stored in the cap. Since the gas detection element is not touched, it can be expected to protect the element from poisonous substances. Also, since this cap suppresses the temperature fluctuation of the element due to wind,
It is expected that the output stability of the gas sensor will be improved.

[0014]

Embodiments of the gas sensor according to the present invention will be described below with reference to the drawings. FIG. 1 shows a perspective view of a catalytic combustion type carbon monoxide gas sensor according to the present invention. Four pins 6 are fixed to the base 5 made of heat-resistant resin so as to penetrate therethrough. The gas detecting element 3 and the compensating element 4 are held by two pieces each. A heat shield 7 is arranged between the gas sensing element and the compensating element. This is to prevent thermal convection in the sensor and reduce the output fluctuation due to the mounting posture difference.

As the resistance temperature detector, a platinum wire having a diameter of 20 μm was wound at an interval of 100 μm and wound 12 times. Although platinum is used for the metal wire here, it is not limited to this as long as the metal has a large temperature coefficient and a large volume resistivity, and nickel or nickel alloy can be used. The gas sensing element of the present invention comprises a platinum resistance thermometer coil, and a mixture of α-ferric oxide fine powder carrying a gold catalyst and alumina powder carrying platinum and palladium so as to cover the entire coil. The oxidization combustion catalyst layer, which is a unit, is coated, and the compensating element is a resistance temperature sensor coil that is provided with a metal oxide layer that is a sintered body of a mixture of α-ferric oxide fine powder and alumina powder. It is formed by coating.

Such a gas detecting element is manufactured as follows. The method of manufacturing the resistance temperature detector is as follows. A platinum wire 1 having a diameter of 0.6 mm and a length of 1.2 mm and a thickness of 20 μm is wound 12 turns at intervals of 100 μm. This coil has a resistance value of about 6Ω, which is five times or more that of a conventional element. Both ends of the coil are left as 10 mm as leads for the electrodes.

Next, a method of manufacturing the gas detecting element will be described. According to the method disclosed in JP-A-60-238148,
Ferric nitrate pentahydrate 46.04 g and chloroauric acid tetrahydrate 2.4
7 g was added to 600 ml of ultrapure water while stirring, and stirring was continued for 2 hours at a temperature of 70 ° C. even after the addition was completed. continue,
29.04 g of sodium carbonate was dissolved in 400 ml of ultrapure water and stirred at a temperature of 70 ° C. for 1 hour. An aqueous solution in which ferric nitrate and chloroauric acid are dissolved is added to the sodium carbonate aqueous solution with stirring for 10 minutes, and stirring is continued for 1 hour after the addition is completed. The precipitate obtained after stirring is centrifuged several times,
It was thoroughly washed with water and adjusted to a predetermined pH. Then, the product obtained by suction filtration was dried by vacuum freeze drying for 10 hours.
The obtained powder was heat-treated at 400 ° C. for 4 hours in an electric furnace.

Then, γ-alumina powder was added to a liquid obtained by dissolving a predetermined amount of chloroplatinic acid and palladium chloride in pure water, stirred, evaporated to dryness, pulverized, and heat-treated in an electric furnace at 600 ° C. for 1 hour. A carbon monoxide combustion catalyst prepared by mixing the catalyst powder prepared by supporting platinum and palladium on alumina and the catalyst powder prepared by carrying gold on α iron oxide prepared as described above at a ratio of 1: 1 and pulverized well. Was adjusted. This ratio is 1: 0.
A value of 8 to 1.2 is good, and if the α-iron oxide catalyst is large, sintering becomes difficult, and if the alumina-based catalyst is large, the output of the gas sensor is insufficient.

To this carbon monoxide combustion catalyst, a binder mixed with colloidal silica and alumina sol 1: 1 in an amount of 40% by weight was added and mixed to form a paste. This mixture of colloidal silica and alumina sol has the purpose of providing the catalyst powder with an appropriate viscosity and surface tension to facilitate coating and for the purpose of enabling the sintering of a hardly sinterable catalyst at a low temperature. The obtained paste is applied so as to cover the coil portion of the resistance temperature detector, and dried at room temperature for about 2 hours. After drying, heat treatment was performed in an electric furnace at a temperature of 300 to 400 ° C. for 3 to 5 hours. The function of colloidal silica and alumina sol binder makes it possible to obtain a sintered body having sufficient mechanical strength under these conditions. The amount of the binder is preferably 30 to 40% by weight. When the amount is less than the above range, the paste is too hard, and when the amount is more than the above range, the viscosity of the paste is too low.

The compensating element of the present invention is manufactured as follows. Ferric nitrate pentahydrate (48.47 g) was added to 600 ml of ultrapure water with stirring, and the stirring was continued at a temperature of 70 ° C. for 2 hours after the addition was completed. Then, sodium carbonate 29.
04 g was dissolved in 400 ml of ultrapure water and stirred at a temperature of 70 ° C. for 1 hour. An aqueous solution in which ferric nitrate is dissolved is added to the sodium carbonate aqueous solution with stirring over 10 minutes, and stirring is continued for 1 hour after the addition is completed. The precipitate obtained after stirring was centrifuged several times, washed thoroughly with water, and adjusted to a predetermined pH. Then, the product obtained by suction filtration was dried by vacuum freeze drying for 10 hours. The obtained powder is 400 in an electric furnace.
Heat treatment was performed at 4 ° C. for 4 hours. Alumina powder was mixed with this powder in a ratio of 1: 1 and stirred well to be pulverized.

To the mixed powder of α-iron oxide thus prepared, 40 parts of colloidal silica and alumina sol binder are added.
A weight% amount was added and mixed to form a paste. The obtained paste is applied so as to cover the coil portion of the resistance temperature detector 1, and dried at room temperature for about 2 hours. After drying, heat treatment is performed in an electric furnace at a temperature of 300 to 400 ° C. for 3 to 5 hours.

Next, as shown in FIG.
The gas detecting element 3 and the compensating element 4 are connected to and held on the book, and the cap 8 is covered. FIG. 2 is a sectional view of the cap according to the present invention. The cap 8 includes a glass fiber filter paper 82 formed by two wire nets 81 and a polyflon filter 8
It is a structure in which 3 is sandwiched. This cap eliminates the influence of zero-point fluctuation of the output to the sensor due to the wind, and can also prevent the influence of moisture and poisonous substances on the sensor.

FIG. 3 shows the output characteristics of the catalytic combustion type carbon monoxide gas sensor according to the present invention for carbon monoxide. For comparison, the output of the conventional catalytic combustion type carbon monoxide gas sensor is also shown. The solid line a is the output of the gas sensor according to the present invention, and the solid line b is the output of the conventional gas sensor.
A voltage of 1 V is applied to the bridge circuit, a power of 0.1 W is applied to the resistance temperature detector coil, and the temperature of the element is approximately 100 ° C. The product of the present invention in which the room temperature CO oxidation catalyst is mixed can obtain an output 1.5 times that of the conventional product. This is because the gold-supported iron oxide catalyst has 100% CO oxidation combustion power at room temperature.

FIG. 4 shows output characteristics of the catalytic combustion type carbon monoxide gas sensor according to the present invention with respect to hydrogen. For comparison, the output of the conventional catalytic combustion type carbon monoxide gas sensor is also shown. The solid line C is the output of the gas sensor according to the present invention, and the solid line D is the output of the conventional gas sensor. The hydrogen output of the gas sensor of the present invention is less than half that of the conventional one. This is because if the catalyst is not supported on the compensating element, the hydrogen is burned to some extent by the alumina and the bridge output is canceled.

FIG. 5 is a diagram showing the temporal stability of CO output of the catalytic combustion type carbon monoxide gas sensor according to the present invention. For the sake of comparison, the case where a metal cap is covered is also added. Curve E shows the change over time in the output when a filter is attached,
The curve F is the change over time in the output when a metal cap is attached. Regarding the temporal stability, it can be seen that the sensor without a filter deteriorates with time and the output decreases, but the filter with a filter is stable. The filter obtained by stacking the glass fiber filter paper and the polyflon filter can prevent the sensor from being affected by moisture in the atmosphere and by the particles in the atmosphere. Further, it is possible to reduce the influence of poisonous substances on the sensor and to reduce the zero point fluctuation due to wind.

[0026]

According to the present invention, in the gas detecting element, the catalyst in which gold is carried on the ferric oxide carrier having the ability to oxidize and burn carbon monoxide at room temperature is used as the oxidative combustion layer. Since the device uses a mixture of α-ferric oxide and alumina, which does not carry a catalyst, the catalytic combustion type carbon monoxide gas sensor of the present invention has a conventional structure at a relatively low temperature when incorporated into a bridge circuit. It is 1.5 times more sensitive to carbon monoxide and is 1/1
The sensitivity is 2 or less.

Alumina sol and colloidal silica are used as a mixture when a carbon monoxide oxidation combustion catalyst in which a gold catalyst is supported on α-ferric oxide fine powder and an alumina catalyst in which platinum and palladium are supported are fixed to the resistance temperature detector coil. By
It becomes possible to sinter at a low temperature, and it is possible to fix the catalyst without deteriorating the performance of the catalyst due to high temperature sintering. Since the gas detecting element and the compensating element are covered with the glass fiber filter paper and the cap of the polyflon filter, the element can be protected from poisoning substances and reliability is improved. This cap suppresses the temperature variation of the element due to the wind and improves the stability of the output of the gas sensor.

Further, since the catalytic combustion type carbon monoxide gas sensor of the present invention can be operated at room temperature, the electric power required for the operation is small and the load on the power source is small, and the carbon monoxide gas detecting device using this sensor can be used. It can contribute to cost reduction.
Further, even if the compensating element is not specially loaded with a catalyst, it is not affected by alcohol or hydrogen, so that it is possible to contribute to the improvement of the manufacturing yield of the element and the cost reduction.

[Brief description of drawings]

FIG. 1 is a perspective view of a catalytic combustion type carbon monoxide gas sensor according to the present invention.

FIG. 2 is a sectional view of the cap of the catalytic combustion type carbon monoxide gas sensor according to the present invention.

FIG. 3 is a diagram of a bridge output with respect to the carbon monoxide concentration of the catalytic combustion type carbon monoxide gas sensor according to the present invention.

FIG. 4 is a diagram of a bridge output for hydrogen of the catalytic combustion type carbon monoxide gas sensor according to the present invention.

FIG. 5 is a diagram showing changes with time in the output of a catalytic combustion type carbon monoxide gas sensor in which the element according to the present invention is covered with the cap of the present invention.

FIG. 6 is a fragmentary cutaway view showing a gas detection element of a conventional catalytic combustion type carbon monoxide gas sensor.

FIG. 7 is a perspective view of a conventional catalytic combustion type carbon monoxide gas sensor.

FIG. 8 is a connection diagram of a bridge circuit used in a general catalytic combustion gas sensor.

[Explanation of symbols]

 1 resistance thermometer 2 carrier 3 gas detection element 4 compensation element 5 base 7 heat shield plate 6 pin 8 cap 81 wire net 82 glass fiber filter paper 83 polyflon filter E power supply V load R1 resistance R2 resistance

Claims (3)

[Claims] 1. A contact in which a gas detecting element and a compensating element, which are used by being incorporated in a branch side of a bridge circuit, are held by a metal pin which is fixed through a base of an insulator and covered with a cap. In the combustion type carbon monoxide gas sensor, the gas detection element is composed of a resistance temperature detector and an oxidation combustion catalyst layer covering the resistance temperature detector, and the oxidation combustion catalyst layer is an α-ferric oxide fine powder carrying a gold catalyst. , A sintered body of a mixture of an alumina powder carrying a platinum catalyst and a palladium catalyst, wherein the compensating element comprises a resistance temperature detector and a metal oxide layer covering the resistance temperature detector. Is a sintered body of a mixture of α-ferric oxide fine powder and alumina powder, which is a catalytic combustion type carbon monoxide gas sensor. 2. The catalytic combustion type carbon monoxide gas sensor according to claim 1, wherein the gas detecting element and the compensating element are obtained by mixing the mixture of alumina sol and colloidal silica and sintering the mixture. A catalytic combustion type carbon monoxide gas sensor characterized by: 3. The catalytic combustion type carbon monoxide gas sensor according to claim 1, wherein the gas detecting element and the compensating element are covered with a cap made of glass fiber filter paper and a polyflon filter. Type carbon monoxide gas sensor.