JP2571302B2 - Odor gas sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a gas sensor for detecting hydrogen sulfide, methyl mercaptan and trimethylamine, which are malodorous components, with high sensitivity.

[Related Art] An odor gas sensor is required to have a very high sensitivity (for example, 1 ppm or less) as compared with a normal gas sensor (several ppm or more) in order to detect and measure a component causing an odor. As such a gas sensor, a zinc oxide semiconductor gas sensor is used.

This conventional zinc oxide semiconductor gas sensor (5 ') is shown in FIG. In this sensor, platinum electrodes (4a) and (4b) are formed on a substrate (3) made of alumina or the like, and a zinc oxide semiconductor layer (1) is formed thereon. Further, a heater (6) is provided on the back surface of the substrate (3) to keep the sensor temperature constant.
Is formed.

[Problems to be Solved by the Invention] The zinc oxide semiconductor gas sensor as described above is, for example, a fourth type.
The sensitivity to various gases as shown in the figure is shown. The output voltage of this type of semiconductor gas sensor does not become zero even in clean air containing no target gas. Therefore, the sensor sensitivity in the figure indicates the relative value of the net sensor output voltage (the value obtained by subtracting the sensor output voltage in clean air from the sensor output voltage in the target gas) for the target gas. Here, as can be seen from FIG. 4, the conventional zinc oxide semiconductor gas sensor detects a gas other than the gas to be detected (hereinafter referred to as an interfering gas) such as about several tens of ppm of alcohol present at a normal environmental level. Odorous gases such as H ₂
There was a problem that it could not be distinguished from the case where S was detected.

Furthermore, when an organic silicon compound is contained in the atmosphere, the silicon oxide generated by decomposition of the compound adheres to the surface of the zinc oxide semiconductor layer and lowers the activity of the zinc oxide semiconductor layer. There is a problem that the sensitivity to the

The present invention has been made in order to solve the above problems, and can detect only a malodorous gas to be detected even in an interfering gas such as alcohol and contains an organic silicon compound. An object of the present invention is to provide an odor gas sensor whose sensitivity does not decrease even in an atmosphere.

[Means for Solving the Problems] The malodor gas sensor according to the present invention detects a malodor gas selected from the group consisting of hydrogen sulfide, methyl mercaptan and trimethylamine, and comprises a gas-sensitive layer mainly composed of a zinc oxide semiconductor. And a catalyst layer formed on the surface of the gas-sensitive layer and containing at least one metal oxide selected from tungsten, molybdenum, and vanadium.

[Operation] In the offensive odor gas sensor of the present invention, the gas to be detected reaches the gas sensitive layer after the C ₂ H ₅ OH in the gas to be detected is first removed by the catalyst layer containing a metal oxide such as tungsten. Thereby, hydrogen sulfide and the like, which are malodorous gas components, are selectively detected.

FIG. 1 is a perspective view of an odor gas sensor (5) showing one embodiment of the present invention.

Platinum electrodes (4a) and (4b) are formed on an alumina substrate (3), and a paste of zinc oxide powder mixed with, for example, distilled water is applied in layers, and then fired at a high temperature. Thus, a zinc oxide semiconductor layer (1) is formed as a gas sensitive layer. A heater (6) is formed on the back surface of the substrate (3) to keep the sensor temperature constant. This zinc oxide powder is washed with water and dried zinc hydroxide precipitate obtained by neutralizing aqueous zinc nitrate solution with aqueous ammonia, and then drying.
It is obtained by firing in an electric furnace.

Next, a tungsten oxide catalyst layer (2) is formed on the zinc oxide semiconductor layer (1) by the following steps.

(1) SiO ₂ , Al ₂ O ₃ or SiO ₂ —Al ₂ O ₃ was crushed by a ball mill into a powder, and an aqueous solution of ammonium tungstate was used as WO ₃ in a concentration of 0.5 to 5 mol% with respect to the powder. Prepare, mix and soak.

(2) After the above mixture is dried, it is fired at about 700 ° C. in an electric furnace.

(3) The fired substance is mixed well with distilled water to form a paste, which is applied to the entire surface of the zinc oxide semiconductor layer (1).

(4) Next, the whole sensor is heated at about 600 ° C., and a tungsten oxide catalyst layer (2) is formed outside the zinc oxide semiconductor layer (1).
To form

FIG. 2 shows the sensitivity of the malodor gas sensor formed as described above to various gases. The vertical and horizontal axes in FIG. 2 are exactly the same as those in FIG. Comparing FIG. 2 with FIG. 4, the sensitivity to C ₂ H ₅ OH in FIG. 2 is reduced to a level close to that of other gases C ₂ H ₄ . On the other hand, hydrogen oxide (H ₂ S) and methyl mercaptan (CH ₃
A sulfur compound represented by SH), trimethylamine メ チ ル (CH
_{3) 3} N} amines typified by, acetate (CH ₃ COOH acids typified), acetone (CH ₃ COCH ₃ ketones represented by) and acetaldehyde (CH ₃ CHO) in the aldehyde represented by The sensitivity to the type is similar to that of the conventional zinc oxide semiconductor gas sensor shown in FIG. Therefore, it is well understood that the tungsten oxide catalyst layer (2) does not remove the malodorous substance to be detected but selectively decomposes and removes only C ₂ H ₅ OH.

The above-described decomposition reaction of C ₂ H ₅ OH gas by the tungsten oxide catalyst layer is called a so-called intramolecular dehydration reaction of alcohol by a metal oxide, and its chemical reaction formula is as follows.

C ₂ H ₅ OH --- C ₂ H ₄ + H ₂ O (1) The above reaction occurs at a relatively high temperature (about 300 ° C. or higher). At this time, C ₂ H ₄ (olefin-based gas) is generated, but the odor gas sensor of the present invention has a very low sensitivity to this gas as can be seen from FIG.
Therefore, it is found that it is effective to use a metal oxide layer such as a tungsten oxide catalyst layer which decomposes alcohol by the reaction represented by the above formula (1).

Incidentally, the tungsten oxide catalyst layer (2) of the above-mentioned malodorous gas sensor is obtained by applying a 50 μm-thick layer in which tungsten is used as WO _{3 and} 2 mol% is mixed with SiO ₂ .

The tungsten addition amount is determined by the sensitivity ratio of H ₂ S and C ₂ H ₅ OH to the tungsten addition amount shown in Table 1 below = ｛H ₂ S
(0.1 ppm) net sensor output voltage｝ / ｛C ₂ H ₅ OH (10pp
m) was obtained from the relationship of the net sensor output voltage｝. This H
The sensitivity ratio between ₂ S and C ₂ H ₅ OH is hereinafter abbreviated to the sensitivity ratio.

From Table 1, it can be seen that a large sensitivity ratio is obtained in a wide range of the tungsten addition amount of 0.5 to 3 mol%. Further, the relationship between the sensitivity ratio and the amount of addition of molybdenum, vanadium and the like other than tungsten forming the metal oxide catalyst layer was determined. This is shown in Tables 2 and 3 below.

It can be seen from Tables 2 and 3 that, similarly to tungsten, molybdenum and vanadium have good sensitivity ratios as oxides in a wide range of addition amount of 0.5 to 3 mol%. If the amount of the metal forming these metal oxides is 0.5 mol% or less, it is considered that the effect as a catalyst cannot be sufficiently obtained. If it is 3 mol% or more, a sufficient sensitivity ratio cannot be obtained.

Further, when the addition amount of tungsten is 1 mol%, the change in the sensitivity ratio obtained by changing the thickness of the tungsten oxide catalyst layer (2) is shown in Table 4 below.

It can be seen from Table 4 that the thickness of the tungsten oxide catalyst layer is preferably about 10 μm to 100 μm. It is difficult to form a catalyst layer of 10 μm or less to a uniform thickness,
It tends to be insufficient as a catalyst layer. In the case of a catalyst layer having a thickness of 100 μm or more, problems arise in gas diffusion in the layer and mechanical strength of the layer. The change in the sensitivity ratio with respect to the thickness of these catalyst layers was similar for molybdenum and vanadium.

FIG. 3 shows the sensitivity of the odor gas sensor (5) of the present invention to various gases when the sensor temperature was changed from 300 ° C. to 500 ° C. (At this time, H ₂ S is 0.1 ppm, C ₂ H ₅ OH and C
₂ H ₄ is fixed at 1 ppm. ) As can be seen from Figure 3, the maximum sensitivity of the constant relative to H ₂ S are obtained in a wide range of 350 to 400 ° C., shows a kind of plateau region.
Therefore, when controlling this sensor to a constant temperature, for example, about 370 ° C., it is understood that the required temperature control accuracy may be low.

In the above embodiments, tungsten, molybdenum, vanadium and the like are described as specific examples of the metal forming the oxide catalyst layer that causes the intramolecular dehydration reaction of alcohol, but other metals such as thorium, lanthanum, yttrium, and zirconium Even when the oxide catalyst layer was formed by using silicon or the like, an effect similar to that of the above example was obtained. That is, when these oxide catalyst layers were used, the same sensitivity ratio as in the above example could not be obtained, but it was recognized that the sensitivity ratio was considerably improved.

As described above, silicon oxide itself has a function of accelerating the intramolecular dehydration reaction of alcohol, similarly to oxides of metals such as tungsten. Therefore, when the odor gas sensor (5) of the present invention is used in an atmosphere containing an organic silicon compound, silicon oxide generated by decomposition of the organic silicon compound adheres to the surface of the tungsten oxide catalyst layer (2). However, the effect of the tungsten oxide catalyst layer (2) on removing C ₂ H ₅ OH is less likely to be impaired. In addition, since silicon oxide adheres to the surface of the tungsten oxide catalyst layer (2), silicon oxide does not adhere to the inner zinc oxide semiconductor layer (1), and accordingly, the zinc oxide semiconductor layer (1) Does not reduce the activity of Accordingly, the sensitivity of the odor gas sensor (5) of the present invention to odor gas is kept stable.

Table 5 below shows the change in sensitivity before and after operation of the above-described malodorous gas sensor (5) of the present invention in an atmosphere containing an organosilicon compound. For comparison, the sensitivity change of the conventional zinc oxide semiconductor gas sensor (5 ') before and after operation in a similar atmosphere is also shown. This means that the sensitivity of each gas sensor (5) (5 ′) to H ₂ S 1 ppm before and after operating the HMDS [hexamethyldisiloxane {(CH ₃ ) ₃ SiOSi (CH ₃ ) ₃ }] 10 ppm atmosphere for 3 hours is shown. It shows the change. HMDS is the operating temperature of the sensor (about 350 ° C)
To form silicon oxide on the sensor surface.

From Table 5 above, it can be seen that the odor gas sensor (5) of the present invention does not decrease the sensitivity to odor gas compared to the conventional zinc oxide semiconductor gas sensor (5 ') even when silicon oxide adheres to the sensor surface. You.

[Effects of the Invention] As described above, according to the odor gas sensor of the present invention, C ₂ H ₅ OH in the detection gas is removed by the catalyst layer having a metal oxide such as tungsten. Excellent selectivity for C ₂ H ₅ OH of certain hydrogen sulfides, methyl mercaptans and trimethylamine is obtained. Even when used in an atmosphere containing an organic silicon compound, silicon oxide adheres to the surface of the catalyst layer having a metal oxide such as tungsten, so that silicon oxide does not adhere to the internal zinc oxide semiconductor layer. Therefore, the sensitivity of the odor gas sensor of the present invention to odor gas does not decrease.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional perspective view of the odor gas sensor of the present invention, FIG. 2 is a graph showing the sensitivity of the odor gas sensor of the present invention to various gases, and FIG. 3 shows the sensor temperature of the odor gas sensor of the present invention varied. FIG. 4 is a diagram showing sensitivity to various gases at the time, FIG. 4 is a diagram showing sensitivity of a conventionally used zinc oxide semiconductor gas sensor to various gases, and FIG. 5 is a partial cross section of a conventionally used zinc oxide semiconductor gas sensor. It is a perspective view. In the figure, 1 is a zinc oxide semiconductor layer, 2 is a tungsten oxide catalyst layer, and 5 is an odor gas sensor.

Claims (1)

(57) [Claims] An odor gas sensor for detecting an odor gas selected from the group consisting of hydrogen sulfide, methyl mercaptan and trimethylamine, comprising: a gas-sensitive layer mainly composed of a zinc oxide semiconductor; and tungsten formed on the surface of the gas-sensitive layer. A catalyst layer containing at least one metal oxide selected from the group consisting of molybdenum and panadium.