JPH09229890A - Catalyst for gas sensor and activation processing method for the catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an activation processing method for increasing activities of a catalyst and generating much higher sensor outputs. SOLUTION: In this activation processing method for the gas sensor catalyst carried by a metal oxide carrier and including at least palladium, the catalyst is processed in air including a flammable gas and water vapor in a first heat treatment, and then in the dry air in a second heat treatment. In the first heat treatment, the catalyst is treated for five or more hours in an ambiance where the temp. of air including 0.5-2% methane gas and water vapor of a dew point of not higher than 30 deg.C is raised to 300-500 deg.C. In the second heat treatment, the catalyst is treated for two or more hours in the dry air of 400-500 deg.C.

Description

Detailed Description of the Invention

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst for a gas sensor containing at least palladium supported on a metal oxide used in a catalytic combustion type gas sensor for detecting a combustible gas, and an activation treatment method thereof.

2. Description of the Related Art FIG. 4 is a bridge circuit diagram of a catalytic combustion type gas detector. The power supply E is connected to the series connection of the two fixed resistors R1 and R2 and the series connection of the gas detection element D and the compensation element C.
(Voltage Vc) Loads W are connected in parallel. When the combustible gas comes into contact with the gas detection element D that has been energized and preheated by applying the voltage Vc to the bridge circuit, combustion occurs, the temperature of the platinum coil rises, and the electric resistance increases in proportion to the gas concentration. Combustion does not occur in the compensating element, and almost no temperature change occurs even if combustible gas comes in. The resulting voltage difference (bridge output) is applied to the load W. Since the bridge output rises in proportion to the gas concentration,
The gas concentration can be detected. FIG. 5 is a fragmentary perspective view showing a gas detecting element of a conventional catalytic combustion gas sensor. A carrier 2 such as alumina carrying a catalyst 3 such as platinum and palladium is fixed around a resistance temperature detector 1 including a platinum coil. The gas detecting element and compensating element of such a conventional catalytic combustion type gas sensor are manufactured as follows. Using platinum wire with a diameter of 60 μm, outline 0.6 m
A coil having m, the number of turns of 10 turns, and a length of 1.5 mm is manufactured. A paste containing a mixture of alumina powder and alumina sol is adhered to the resistance measuring element 1 of the platinum coil and fired at 800 ° C. to fix the alumina carrier 2 to the resistance measuring element 1 of the platinum coil. The alumina carrier 2 is impregnated in an aqueous solution in which chloroplatinic acid and palladium chloride are dissolved and thermally decomposed at 600 ° C., and the mixed catalyst 3 of platinum and palladium oxide is supported on the alumina carrier 2 to manufacture the gas detection element D. . Similarly, in the compensating element, a mixed paste of alumina powder and alumina sol is adhered to a platinum coil and fired at 800 ° C. to fix the alumina carrier to the platinum coil. An alumina carrier is impregnated in an aqueous solution in which copper sulfate is dissolved and thermally decomposed to carry a copper oxide catalyst on the alumina carrier to manufacture a compensating element. The conventional method of manufacturing a catalytic combustion gas sensor as described above is easy to manufacture, has characteristics that pose no practical problems, has relatively long-term stability, is less affected by ambient temperature and humidity, etc. It has the characteristics of and is used with a power consumption of about 0.5 W.

However, in the conventional catalytic combustion type gas sensor as described above, since the step of carrying the catalyst is performed after the production of individual element shapes (supports), the handling of each element is complicated. In addition to the above problem, there is also a problem that activation by thermal decomposition after loading is still insufficient. The present invention has been made in view of the above points, and an object thereof is to provide an activation treatment method for increasing catalyst activity and generating a higher sensor output.

In order to achieve the above object, in a method for activating a gas sensor catalyst which is supported on a carrier made of a metal oxide and contains at least palladium, the catalyst is a flammable gas and steam. The first heat treatment is performed in the air containing H2O, and the second heat treatment is further performed in the dry air thereafter. The first heat treatment is preferably performed for 5 hours or more in an atmosphere in which methane gas is contained in an amount of 0.5 to 2% and water vapor having a dew point of 30 ° C. or lower is heated to 300 to 500 ° C. In addition, the second heat treatment may be performed in dry air at a temperature of 400 to 500 ° C. for 2 hours or more. The metal oxide is preferably γ-alumina or tin oxide. The catalyst is preferably palladium and platinum. Further, in the catalyst for a gas sensor, which is supported on a carrier made of a metal oxide and contains at least palladium, the form of the palladium is palladium oxide in which palladium hydroxide is oxidized.

BEST MODE FOR CARRYING OUT THE INVENTION

Example 1 In this example, a method for activating a catalyst in a catalytic combustion gas sensor for detecting methane gas (CH ₄ ) will be described. Commercially available γ-alumina powder was crushed for 1 hour or more with a Likai machine, and then mixed with a chloroplatinic acid solution dissolved in pure water and an aqueous solution of palladium chloride. At this time, the concentration of the aqueous solution is
The platinum was adjusted to 10 wt% and the palladium was adjusted to 12 wt% with respect to γ-alumina. This mixed solution was stirred for 10 minutes with a stirrer and then dispersed for 5 minutes with an ultrasonic cleaner. After repeating this stirring and dispersion three times, the mixture was evaporated to dryness while stirring in a water bath set at 60 ° C. After evaporating to dryness, this was pulverized with a ball mill to obtain a powder carrying a catalyst raw material for a gas detection element. Transfer the obtained powder to a quartz boat, and by an electric furnace,
After being kept in dry air at 150 ° C. for 1 hour, the temperature was raised to 600 ° C. and heat treatment was performed for 3 hours to thermally decompose the catalyst raw material. FIG.
Is a temperature profile of thermal decomposition of the catalyst raw material. By this thermal decomposition, the catalyst raw material becomes platinum and palladium oxide and becomes a catalyst. This state will be referred to as catalyst powder. At the same time, chlorine is removed from the powder, but 2-3
wt% remains. The above is the same as the conventional catalyst preparation method. The catalyst powder thus produced was activated as follows to improve the catalytic activity. The catalyst powder was inserted into an electric furnace, and first heat treatment was performed for 5 hours in an atmosphere in which dry air was mixed with 1% by volume of methane gas and dew point of 30 ° C. was raised to a predetermined temperature. After that, a second heat treatment was further performed for 2 hours or more at a predetermined temperature (400 ° C.) in dry air. According to X-ray diffraction, palladium of the catalyst powder after the first heat treatment is mainly hydroxide, and its oxidizing ability for CH ₄ is small. A second heat treatment needs to be performed to convert this hydroxide to palladium oxide. Also,
It is extremely important that the palladium oxide that has passed through the hydroxide has a large oxidizing ability with respect to CH ₄ , but the effect of increasing the oxidizing ability is not clear. On the other hand, it was also found that residual chlorine was removed during these heat treatments, resulting in a slight increase in the oxidizing ability for CH ₄ . The oxidizing ability of the thus obtained catalyst powder with respect to CH ₄ was measured using a fixed bed flow method catalyst evaluation apparatus manufactured by Okura Riken Co., Ltd. In this device, air containing a test gas (CH ₄ ) is caused to flow over the element, and the reaction consumption rate of the reacted test gas is measured as a conversion rate (%). 50 mg of catalyst powder was used as a sample, the flow rate of the test gas was 10 sccm, and the concentration of the test gas was 1%. FIG. 1 is a graph showing the temperature of the conversion rate of CH ₄ of the catalyst powder subjected to the activation treatment according to the present invention. Curve a is the case where the first heat treatment temperature is 450 ° C., curve b is the same at 400 ° C., curve c is the same at 350 ° C., and curve d is the case where the activation treatment is not performed. When compared by the temperature at which the conversion rate becomes 90%, the catalyst of the catalytic combustion type gas sensor manufactured by the conventional method is about 370 ° C., but the catalyst for the catalytic combustion type gas sensor manufactured in this example is more than that of the conventional product. Excellent activity from low temperature: 350 ° C treatment at 345 ° C, 400 ° C treatment at 30
The temperature was 325 ° C. when treated at 5 ° C. and 450 ° C. From the above,
It can be estimated that the treatment temperature at which the conversion rate is 90% is lower than that of the conventional catalyst is 400 to 500 ° C. In addition, since the gas sensor is used at a temperature above the conversion rate of 95%,
A heat treatment temperature of 400 ° C. or higher is appropriate. Next, the sensitivity of the gas sensor using the catalyst powder of the activation treatment at 400 ° C. to CH ₄ was examined. The method for producing the catalyst powder for the compensating element used as a pair with the gas detection element is the same as the method for producing the catalyst for the gas detection element, except that the catalyst raw material used is copper nitrate dissolved in pure water. The difference is that the activation process is not performed. The gas detecting element and the compensating element were produced by mixing each catalyst powder with alumina sol, adhering it to a platinum coil, and sintering it at 600 ° C. FIG. 3 is a graph of the bridge output for CH ₄ when the gas sensor using the catalyst powder according to the present invention is incorporated in the bridge circuit. Curve e is the case where the catalyst powder according to the present invention is used, and curve f is the case where the activation treatment is not performed. It can be seen that the sensor using the catalyst manufactured by the method of the present embodiment can obtain a higher output than the sensor manufactured by the conventional method. As described above, when the catalyst activation treatment method of the present invention is used, the output can be increased as compared with the conventional product. Example 2 A method for activating a CH ₄ gas sensor catalyst when tin oxide is used as a carrier will be described below. A commercially available stannic acid powder was heat-treated in the air at 730 ° C. for 3 hours in an electric furnace to prepare a tin oxide powder. This powder was crushed for 1 hour or more with a liquor machine, and then mixed with a chloroplatinic acid solution dissolved in pure water and an aqueous solution of palladium chloride. At this time, the concentration of the catalyst raw material in the aqueous solution was 10 wt% platinum for tin oxide.
So that the palladium content is adjusted to 12 wt%. Stir this mixture with a stirrer for 10 minutes,
Then, disperse in an ultrasonic cleaner for 5 minutes. After repeating this stirring and dispersion three times, the mixture is evaporated to dryness while stirring in a water bath set at 60 ° C. After evaporating to dryness, this was crushed by a ball mill to obtain a catalyst raw material powder for a gas detection element. This was transferred to a quartz boat, heat-treated in an electric furnace in dry air with the same temperature profile as in Example 1, and pyrolyzed to obtain a catalyst powder. The catalyst powder thus produced was activated in the same manner as in Example 1 to improve the catalytic activity. That is, the catalyst powder was inserted into an electric furnace, and 1% by volume of methane gas was mixed with dry air to form a dew point of 3
The first heat treatment was performed for 5 hours in an atmosphere in which 0 ° C. air was heated to a predetermined temperature. After that, a second heat treatment was further performed at a temperature of 400 ° C. for 2 hours in dry air. Then, as in Example 1, the oxidizing power of the catalyst powder with respect to methane gas and the bridge output when incorporated into a bridge circuit as a gas detection element were examined. As a result, almost the same curve as in Example 1 was obtained. That is, even when tin oxide was used as the carrier, the catalyst could be activated more than the conventional product by the activation treatment method of the present invention, and the output of the sensor could be increased as in Example 1.

According to the present invention, in a method for activating a catalyst for a gas sensor, which is supported on a carrier made of a metal oxide and contains at least palladium, the catalyst is carried out in air containing a combustible gas and water vapor. 1 heat treatment,
After that, a second heat treatment is performed in dry air,
The second after the palladium hydroxide is generated by the first heat treatment
It was oxidized by the heat treatment of (1) to generate palladium oxide, and further activated by the added combustible gas. Therefore, the ability to oxidize combustible gas was improved, and the output of the bridge circuit incorporating the gas sensor using the same was increased. In addition, since the activation treatment is performed in the powder state, uniform characteristics can be obtained all at once, and the complexity of activating the individual gas sensors can be avoided.

[Brief description of drawings]

FIG. 1 C of catalyst powder which has been activated according to the present invention
Graph of conversion rate of H ₄ against temperature

FIG. 2 Temperature profile of thermal decomposition of catalyst raw material

FIG. 3 is a graph of bridge output for CH ₄ when a gas sensor using the catalyst powder according to the present invention is incorporated in a bridge circuit.

FIG. 4 is a bridge circuit diagram of the catalytic combustion type gas detection device.

FIG. 5 is a fragmentary perspective view showing a gas detecting element of a conventional catalytic combustion gas sensor.

[Explanation of symbols]

 1 resistance temperature detector 2 carrier 3 catalyst D gas detection element C compensation element R1 resistance R2 resistance E power supply W load

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Koichi Tsuda 1-1-1, Tanabe Shinda, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Fuji Electric Co., Ltd.

Claims (6)

[Claims] 1. A method for activating a catalyst for a gas sensor, which is supported on a carrier made of a metal oxide and contains at least palladium, in which a first heat treatment is carried out in the air containing a combustible gas and water vapor, and thereafter. A method for activating a catalyst for a gas sensor, further comprising performing a second heat treatment performed in dry air. 2. The first heat treatment uses methane gas at 0.5.
4% of air containing ~ 2% and water vapor with dew point below 30 ° C
The method for activating a catalyst for a gas sensor according to claim 1, wherein the method is performed in an atmosphere heated to 00 to 500 ° C. for 5 hours or more. 3. The method for activating a catalyst for a gas sensor according to claim 1, wherein the second heat treatment is performed in dry air at a temperature of 400 to 500 ° C. for 2 hours or more. 4. The activation treatment method for a gas sensor catalyst according to claim 1, wherein the metal oxide is γ-alumina or tin oxide. 5. The method for activating a catalyst for a gas sensor according to claim 1, wherein the catalyst is palladium and platinum. 6. A catalyst for a gas sensor, which is supported on a carrier made of a metal oxide and contains at least palladium, wherein the form of the palladium is palladium oxide in which palladium hydroxide is oxidized.