JPH11316207A - Ammonia gas detection element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ammonia gas detection element hardly poisoned with SOx gas and high in humidity stability. SOLUTION: In an ammonia gas detection element wherein an ammonia gas sensitive layer 2 principally based on a tin oxide semiconductor is provided so as to be covered with a catalyst layer 3, 2-7.5 mol.% lanthanum and 0.002-0.2 atm % gold are added to the tin oxide semiconductor, and the catalyst layer 3 is formed from an alcohol removing catalyst principally based on titanium oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ammonia gas detecting element having an ammonia gas sensitive layer containing a tin oxide semiconductor as a main component and a catalyst layer covering the sensitive layer. The present invention relates to an ammonia gas detecting element used for detecting ammonia leak from a denitration device or monitoring exhaust gas, detecting ammonia leak from an absorption refrigerator using an aqueous ammonia solution as an absorbing solution, monitoring ammonia leak from a sewage treatment facility, and the like.

[0002]

2. Description of the Related Art Ammonia is known to be a harmful gas to the human body, and is liable to have an adverse effect on the environment. Moreover, even a trace amount tends to emit a bad smell and cause the above problem. in case of,
Environmental standards are strictly regulated, and there is a need for high-sensitivity detection at trace and low concentration levels. However, when performing ammonia gas detection, for example, SOx in the combustion exhaust gas, alcohol or water as a solvent causes output fluctuation, and accurate gas detection may be difficult. In view of this, an ammonia gas sensing element provided with an ammonia gas sensitive layer containing a tin oxide semiconductor as a main component and a catalyst layer covering the sensitive layer has been developed. For example, lanthanum oxide or gold is used as the tin oxide semiconductor. Is used as an ammonia gas detecting element having a high sensitivity characteristic to ammonia, and such an ammonia gas detecting element is provided with a coating layer of silica, alumina, zeolite, or the like. It has been studied (for example, see Japanese Patent Application Laid-Open No. 9-105731).

[0003]

However, the above-mentioned ammonia gas detecting element has a characteristic that it is weak against SOx represented by sulfur dioxide (SO ₂ ).
In other words, poisoning in combustion exhaust gas containing a large amount of SOx,
In some cases, the sensitivity may change significantly, making it difficult to accurately detect ammonia gas. Therefore, such an ammonia gas detecting element may not be able to detect ammonia with high reliability depending on the application, and improvement of such characteristics has been desired. Also, measures against malfunctions caused by organic solvents such as alcohol have been strongly desired.

Accordingly, an object of the present invention is to provide an ammonia gas detecting element which suppresses alcohol sensitivity, has high stability against humidity, and is hardly poisoned by SOx gas, in view of the above-mentioned drawbacks.

[0005]

In order to achieve this object, an ammonia gas sensing element according to the present invention is characterized by providing an ammonia gas sensitive layer containing a tin oxide semiconductor as a main component and covering the sensitive layer. A catalyst layer is provided, and the sensitive layer is formed of a tin oxide semiconductor containing lanthanum oxide in an amount of 2 mol% or more.
7.5 mol% and 0.002 atm% to 0.2 a of gold
tm% of tin oxide semiconductor, and the catalyst layer is made of SO containing titanium oxide as a main component.
It is formed from an alcohol removal catalyst which also serves as a protective layer for x.

[Operation and Effect] In other words, since the ammonia sensing layer made of a tin oxide semiconductor is provided, when the ammonia gas sensing element is exposed to an atmosphere in which ammonia gas is present, the ammonia gas passes through the inside of the gas sensing element. Since it diffuses to reach the sensitive layer and reacts by contacting with the tin oxide semiconductor, the apparent resistance value of the sensitive layer changes due to the transfer of electrons accompanying the reaction, and the change in the resistance value is caused by a bridge circuit or the like. Since it is possible to detect the presence of ammonia, it is possible to know the presence of ammonia from the change in the resistance value. At this time, under the condition that the alcohol coexists with ammonia, usually, the alcohol also reacts in the sensitive layer, and there is a possibility that the reaction may be erroneously detected as a reaction by the ammonia. Since a catalyst layer composed of an alcohol removal catalyst containing titanium oxide as a main component is provided, ammonia reaches the sensitive layer, but alcohol is oxidized and removed by the titanium oxide, so that ammonia and alcohol coexist. Even under the conditions, the composition of the gas reaching the sensitive layer is converted to a composition containing ammonia as a main component. Therefore, the sensitive layer can mainly detect only ammonia, and can accurately detect leakage of ammonia.

Further, the present inventors have proposed that a tin oxide semiconductor contains 2 mol% to 7.5 mol% of lanthanum oxide and 0.1 mol of gold.
It has been found that a sensitive layer made of a tin oxide semiconductor added with 002 atm% to 0.2 atm% has a high gas selectivity to ammonia and can increase the humidity stability of its output. According to the above configuration, a gas detection element having excellent gas selectivity and moisture resistance can be provided. Similarly, regarding the catalyst layer, among various catalysts, titanium oxide has a higher SOx than other catalysts such as γ-alumina and high silica zeolite.
The catalyst is found to be highly durable against gas poisoning, and a more stable gas detection element can be provided. Moreover, it was found that even if the titanium oxide was poisoned, the gas response waveform did not show a large change even if it was poisoned, and that a gas detection element excellent in gas response was obtained. That is, as a result, according to the present invention, a gas detection element excellent in gas selectivity, moisture resistance, poisoning resistance, and stability over time could be obtained.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. A mixed aqueous solution in which antimony pentachloride was dissolved in commercially available tin chloride was prepared, an aqueous ammonia solution was added dropwise with stirring, and a precipitate mainly composed of stannic acid was obtained by hydrolysis (coprecipitation method). The resulting precipitate was washed several times with distilled water to remove excess ions such as chloride ions and dried to obtain a stannic acid gel. Next, the stannic acid gel was finely crushed and fired in an electric furnace to obtain a tin oxide semiconductor.
This is further pulverized to form a fine powder, made into a paste using a dispersion medium such as 1.3-butanediol, applied over the noble metal wire 1, dried, and then passed with an electric current through the noble metal wire 1, and dried in air. To obtain a hot-wire type semiconductor gas sensing element composed of only the sensitive layer 2. Next, an aqueous solution of lanthanum nitrate and an aqueous solution of chloroauric acid are sequentially impregnated into the hot-wire type semiconductor gas sensing element, and the noble metal wire is energized and fired to carry lanthanum oxide and gold on the sensitive layer. I let it. Further, a paste of commercially available titanium oxide was applied to the above-mentioned sensitive layer, dried and heated and fired in air to form a catalyst layer made of titanium oxide (see FIG. 1).

This hydrogen gas detecting element was incorporated in a bridge circuit shown in FIG. 2 and used as a gas detecting device. At this time, the sensor output (output) is obtained by the following equation.

V = -E ｛rs / (rs + r0) -r1 /
(R1 + r2)｝ Here, each variable is as follows. V: sensor output E: bridge voltage rs: resistance of the hot-wire semiconductor gas detection element Rs r0: resistance of the fixed resistance R0 r1: resistance of the fixed resistance R1 r2: resistance of the fixed resistance R2

The sensitivity was determined as the difference between the output in the air coexisting with the detected gas and the output in the clean air. When the sensitivity is expressed as the relative sensitivity, the ratio is defined as a ratio where the sensitivity output under a specific condition is set to 1 and the sensitivity under another condition is indicated.

[0012]

EXAMPLE A mixed aqueous solution in which 0.4 atm% of antimony pentachloride was dissolved in commercially available tin chloride was prepared, and an aqueous ammonia solution was added dropwise with stirring, and a precipitate containing stannic acid as a main component was obtained by hydrolysis. (Coprecipitation method). The resulting precipitate was washed several times with distilled water to remove excess ions such as chloride ions and dried to obtain a stannic acid gel. Next, the stannic acid gel was finely crushed and then fired in an electric furnace at 600 ° C. for 4 hours to obtain a tin oxide semiconductor. This is further pulverized to a fine powder, and made into a paste using 1.3-butanediol (manufactured by Wako Pure Chemical Industries, Ltd.) as a dispersion medium.
A spherical coating having a diameter of about 0.45 mm is applied so as to cover a platinum wire coil (wire diameter 20 μm), and after drying, the platinum wire coil 1
Then, a current was passed, and sintering was performed in the air to obtain a hot-wire semiconductor gas detection element Rs including only the sensitive layer 2. Next, an aqueous solution of lanthanum nitrate, and an aqueous solution of chloroauric acid are sequentially impregnated into the hot-wire semiconductor gas sensing element,
The precious metal wire is energized and fired, and lanthanum oxide is applied in 4 mo
1% and 0.02 atm% of gold were loaded on the sensitive layer. Further, a paste of commercially available titanium oxide (titania (manufactured by Wako Pure Chemical Industries, Ltd.)) was applied to the above-mentioned sensitive layer in a thickness of 50 μm, dried, heated and fired in air, and then dried. (A 0.5 mm diameter as a whole) (see FIG. 1). [Example 1] The obtained gas detecting element was incorporated in a bridge circuit, and the sensitivity characteristics of various gases were examined at a sensor voltage of 1.5 V and a load resistance of 5.6 Ω (the temperature of the gas detecting element is equivalent to 330 ° C). The result was as shown in FIG. In other words, it can be seen that this gas detection element can detect ammonia (NH ₃ ) with higher sensitivity than ethanol. In addition, hydrocarbons such as hydrogen (H ₂ ) and ethylene (C ₂ H ₄ ) which is considered to be generated by a dehydration reaction of ethanol (for example, isobutane (i-C ₄
It can be seen that high gas selectivity is also exhibited for H ₁₀ )). In other words, it exhibits high stability against ethylene gas, which is thought to occur when alcohol is decomposed and removed,
It can be seen that alcohol is less likely to be erroneously detected.

Comparative Example 1 Sensitivity characteristics for various gases were examined by changing the amount of lanthanum oxide added to the sensitive layer, and the results are as shown in Table 1. However, in this case, the hot-wire semiconductor gas detection element Rs is obtained by adding the lanthanum and 0.05 atm% of gold to the sensitive layer before forming the catalyst layer.

[0014]

[Table 1]

As a result, it was found that the amount of lanthanum added to the sensitive layer was preferably from 2 mol% to 7.5 mol%.

Comparative Example 2 The sensitivity characteristics were examined in the same manner as in Comparative Example 1 with various amounts of gold added, and the results are shown in Table 2. Here, too, the hot-wire semiconductor gas sensing element Rs
Is the same as the one before forming the catalyst layer and 4 mol%
Lanthanum is added.

[0017]

[Table 2]

As a result, the amount of gold added to the sensitive layer is
It was found that 0.002 atm% to 0.2 atm% is preferable.

Example 2 The gas responsiveness of the above gas detection element was examined, and the result was as shown by the solid line in FIG. still,
For comparison, γ-alumina (manufactured by Kishida Chemical Co., Ltd.) was used instead of the titanium oxide (titania) used in the present invention for the catalyst layer.
And those using high silica zeolite (Ferrierite (manufactured by Catalysis Chemical Industry Co., Ltd.)) were prepared and compared in the same manner as described above. That is, according to the present invention,
It shows that gas response is extremely fast, and ammonia gas can be detected more quickly than when high silica zeolite is used. This is because high silica zeolite has a relatively strong adsorptivity for basic gas, whereas titanium oxide has a relatively weak adsorptivity. In contrast to the above, a detection delay occurs for titanium oxide, whereas titanium oxide can perform detection quickly.

Example 3 Further, the gas responsiveness when the above-described gas detecting element was poisoned by exposure to sulfurous acid gas was examined. Incidentally, the comparative product in Example 2 was similarly examined. As a result, the result was as shown by the broken line in FIG. That is, according to the present invention, it can be seen that the gas responsiveness does not change much even by the poisoning of sulfurous acid gas, and stable detection is possible as compared with γ-alumina, whose responsiveness is reduced by the poisoning. This is considered to be because γ-alumina easily adsorbs SOx, whereas titanium oxide has weak adsorbability. In other words, while γ-alumina easily generates acidic sites on the surface due to this SOx gas adsorption, titanium oxide does not easily generate acidic sites, and the acidic sites easily adsorb ammonia, resulting in responsiveness. Is less likely to occur.

From Examples 2 and 3, it can be seen that a gas detecting element having high gas responsiveness and high stability was obtained.

[Embodiment 4] The dependence of the sensor output on the ammonia gas concentration was examined using the gas detecting element used in Embodiment 1 under various humidity conditions, and the result was as shown in FIG. -30 ° C, 100% RH (absolute humidity 0.38g / m
₃ ) to 40 ° C., 55% RH (28.1 g / m absolute humidity)
It can be seen that a stable output is exhibited even for a wide range of humidity changes up to ₃ ), and that it can be used stably even for a long-term humidity change over one year.

[Embodiment 5] In order to verify the above-mentioned long-term stability, when this gas detection element was actually used outdoors, the output stability was as shown in FIG. 6 (a). Here, the change in the environment is as shown in FIG. In other words, it can be seen that a stable output is actually obtained irrespective of the concentration of ammonia, and highly reliable gas detection can be performed. [Other Embodiments] In the present invention, the noble metal wire is not limited to the platinum wire coil, and other noble metals such as an alloy of platinum and palladium may be used. In addition, the catalyst layer and the sensitive layer can be formed by various other methods. For example, in order to form a catalyst layer by CVD or obtain a metal oxide constituting the sensitive layer, a coprecipitation method may be used instead of impregnating an aqueous solution of a lanthanum salt or a gold salt later.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional perspective view of a hydrogen gas detecting element.

FIG. 2 is a circuit diagram of a gas detection device.

FIG. 3 is a graph showing a sensitivity curve of the gas detection element of the present invention.

FIG. 4 is a graph showing gas responsiveness of a gas detection element.

FIG. 5 is a graph showing the humidity stability of the output of the gas detection element of the present invention.

FIG. 6 is a graph showing long-term stability of the gas detection element of the present invention.

[Explanation of symbols]

 1 Noble metal wire 2 Sensitive layer 3 Silica thin film

Claims (1)

[Claims] 1. An ammonia gas sensing element comprising an ammonia gas sensitive layer containing a tin oxide semiconductor as a main component and a catalyst layer covering the sensitive layer, wherein the sensitive layer is oxidized to a tin oxide semiconductor. 2 mo lantern
1% ~ 7.5mol% and gold 0.002atm% ~
An ammonia gas detection element formed of a tin oxide semiconductor added at 0.2 atm% and wherein the catalyst layer is formed of an alcohol removal catalyst containing titanium oxide as a main component.