JPH0466857A - bad odor gas sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a gas sensor for detecting and measuring malodorous gas components such as sulfur compounds, amines, carboxylic acids, ketones, aldehydes, etc. with high sensitivity.

[Prior Art] In order to detect and measure components that cause odors, malodorous gas sensors are required to have extremely high sensitivity (for example, I ppm or less) compared to the sensitivity of ordinary gas sensors (several ppm or more). A zinc oxide semiconductor gas sensor is used as such a gas sensor.

This conventionally used zinc oxide semiconductor gas sensor (
5゛) is shown in Figure 5. This sensor has platinum electrodes (4a) (4b) formed on a substrate (3) of alumina or the like, and a zinc oxide semiconductor layer (1) formed thereon.

Furthermore, a heater (6) is formed on the back surface of the substrate (3) in order to keep the sensor temperature constant.

[Problems to be Solved by the Invention] The zinc oxide semiconductor gas sensor described above exhibits sensitivity to various gases as shown in FIG. 4, for example. The output voltage of this type of semiconductor gas sensor does not become zero even in clean air where target gas is not present. Therefore, the sensor sensitivity in the figure shows the relative value of the net sensor output voltage with respect to the target gas (the value obtained by subtracting the sensor output voltage in clean air from the sensor output voltage in the target gas).

As can be seen from Figure 4, the conventional zinc oxide semiconductor gas sensor detects gases other than the target gas (hereinafter referred to as interfering gases), such as several tens of ppm of alcohol, which is present in the normal environment. Foul-smelling gases such as H
There was a problem in that it was difficult to distinguish from the case where 2S was detected.

Furthermore, if the atmosphere contains organosilicon compounds,
The silicon oxide produced by decomposition adheres to the surface of the zinc oxide semiconductor layer and reduces the activity of the zinc oxide semiconductor layer.
As a result, there was a problem in that the sensitivity to malodorous gases was reduced.

This invention was made to solve the above-mentioned problems, and it is possible to detect only the target malodorous gas even in interfering gases such as alcohol, and it is also possible to detect odor gases that contain organic silicon compounds. An object of the present invention is to provide a malodorous gas sensor whose sensitivity does not decrease even in an atmosphere.

[Means for Solving the Problems] A malodorous gas sensor according to the present invention includes a gas sensitive layer mainly made of a zinc oxide semiconductor, and at least one type selected from tungsten, molybdenum, and panadium formed on the surface of the gas sensitive layer. a catalyst layer containing a metal oxide;

[Operation] In the malodorous gas sensor of the present invention, the gas to be detected first reaches the gas sensitive layer after C2H50)1 in the gas to be detected is removed by a catalyst layer containing a metal oxide such as tungsten. Thereby, malodorous gas components such as hydrogen sulfide are selectively detected.

[Example] Figure 1 shows a malodorous gas sensor (5
) is a perspective view of.

Platinum electrodes (4a) (4b) on alumina substrate (3)
) is formed, and a layer of zinc oxide powder mixed with, for example, distilled water to form a base is applied on top of the zinc oxide powder, which is then fired at a high temperature to form a zinc oxide semiconductor layer (1) as a gas-sensitive layer. has been done. Further, a heater (6) is formed on one surface of the substrate (3>) in order to keep the sensor temperature constant. This zinc oxide powder is obtained by neutralizing a zinc nitrate aqueous solution with aqueous ammonia, washing the zinc hydroxide precipitate with water, drying it, and then firing it in an electric furnace.

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

(1) SiO2, Al2O3 or 5i02-A I2
Add ammonium tungstate aqueous solution to the powder obtained by crushing O3 with a ball mill and add 0.5% to the above powder as WO3.
The concentration is adjusted to 5 mol%, mixed and immersed.

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

(3) The fired material is thoroughly mixed with distilled water to form a paste, and this is applied to the entire surface of the zinc oxide semiconductor layer (1).

(4) Next, the entire sensor is heated to about 600°C, and a tungsten oxide catalyst layer (
2) Form.

FIG. 2 shows the sensitivity of the malodorous gas sensor formed as described above to various gases. The vertical and horizontal axes in FIG. 2 are exactly the same as in FIG. 4. Comparing FIG. 2 and FIG. 4, in FIG. 2 the sensitivity to C2H50H has decreased to a level close to that of other gases C2H4. On the other hand, as malodorous substances, hydrogen sulfide (H2S), sulfur compounds represented by methyl mercaptan (CH3SH), amines represented by trimethylamine 1 (C1 + 3) 3Nl, carboxylic acids represented by acetic acid (CH3COO) 1), The sensitivity to ketones represented by acetone (CH3CHOi (3)) and aldehydes represented by acetaldehyde (CH3CHO) is the same as that of the conventional zinc oxide semiconductor gas sensor shown in FIG.

Therefore, it is clearly seen that the tungsten oxide catalyst layer (2) selectively decomposes and removes only C2H50H without removing the malodorous substance to be detected.

Furthermore, C 2 II due to the above tungsten oxide catalyst layer
The decomposition reaction of 50H gas is called an intramolecular dehydration reaction of alcohol by a metal oxide, and its chemical reaction formula is as follows.

C21150H----→C2H4+ tl 20 ・
...(1) Also, the above reactions occur at relatively high temperatures (approximately 300'C or higher). At this time, C2H4 (olefin gas) is generated, but the odor gas sensor of the present invention exhibits very low sensitivity to this gas, as can be seen from FIG. Therefore, it can be seen that it is effective to use a metal oxide layer such as a tungsten oxide catalyst layer that decomposes alcohol by the reaction shown in equation (1) above.

Furthermore, the tungsten oxide catalyst layer (
2) is 2 for 5i02 with tungsten as WO3.
A layer containing a mixture of mole % was applied to a thickness of 50 μm.

This amount of tungsten added is 112S and C2II 50 II for tungsten addition (■) shown in Table 1 below.
It was determined from the relationship: sensitivity ratio - net sensor output voltage of IH2s (0.1 ppm) 1/(net sensor output voltage of C2H50H (10 ppm)). This 112s and C2I
The sensitivity ratio of I501+ will be abbreviated as sensitivity ratio hereinafter.

From Table 1, it can be seen that a large sensitivity ratio is obtained in a wide range of tungsten addition amount from 0.5 to 3 mol %. Furthermore, the relationship between the sensitivity ratio and the amount added was also determined for molybdenum, panadium, etc. other than tungsten, which form the metal oxide catalyst layer. This is shown in Tables 2 and 3 below.

Table 2 Table 3 Table 1 From Tables 2 and 3, molybdenum and panadium are each 05 to 3 mol% as oxides, similar to tungsten.
It can be seen that good sensitivity ratios are obtained over a wide range of addition amounts. It is considered that if the amount of the metal that forms these metal oxides is less than 0.5 mol %, sufficient catalytic effects cannot be obtained. Moreover, if it is 3 mol % or more, a sufficient sensitivity ratio cannot be obtained.

Further, Table 4 below shows the changes in sensitivity ratio obtained by changing the thickness of the tungsten oxide catalyst layer (2) when the tungsten addition amount was 11 mol %.

From this Table 4, the thickness of the tungsten oxide catalyst layer is approximately 10μI! It turns out that 1 to 100 μm is suitable.

It is difficult to form a catalyst layer with a uniform thickness of 10 μm or less, and it tends to be insufficient as a catalyst layer.

A thick catalyst layer of 100 μm or more causes problems with gas diffusion in the layer and mechanical strength of the layer. Changes in the sensitivity ratio with respect to the thickness of these catalyst layers were also similar for molybdenum and panadium.

Furthermore, the malodorous gas sensor of the present invention (5
) is shown in Figure 3. (At this time, H2S is 0.lp
pm, C2H50H and C2H4 are constant at 1 ppm. ) As can be seen from FIG. 3, a constant maximum sensitivity to H2S is obtained over a wide range from 350 to 400 DEG C., indicating a species plateau' region. Therefore, it can be seen that when controlling this sensor to a constant temperature, for example, about 370° C., the required temperature control accuracy may be low.

In the above examples, tungsten is used as the metal forming the oxide catalyst layer that causes the intramolecular dehydration reaction of alcohol.
Although molybdenum, panadium, etc. have been described as specific examples, effects similar to those of the above embodiments can also be obtained by forming the oxide catalyst layer using other metals such as thorium, lanthanum, yttrium, zirconium, etc. or silicon. That is, when these oxide catalyst layers were used, it was found that although it was not possible to obtain a sensitivity ratio equivalent to that of the above example, the sensitivity ratio was considerably improved.

As explained above, silicon oxide itself has the function of promoting the intramolecular dehydration reaction of alcohol, similar to oxides of metals such as tungsten. Therefore, when the malodorous gas sensor (5) of the present invention is used in an atmosphere containing an organosilicon compound, silicon oxide produced by the decomposition of the organosilicon compound adheres to the surface of the tungsten oxide catalyst layer (2). However, C2H50H of the tungsten oxide catalyst layer (2)
Removal action is less likely to be impaired. Furthermore, since silicon oxide adheres to the surface of this tungsten oxide catalyst layer (2), silicon oxide does not adhere to the internal zinc oxide semiconductor layer (1). does not reduce the activity of Therefore, the sensitivity of the malodorous gas sensor (5) of the present invention to malodorous gas is kept stable.

Table 5 below shows the change in sensitivity of the malodorous gas sensor (5) of the present invention described above before and after operation in an atmosphere containing an organosilicon compound. For comparison, the change in sensitivity of a conventional zinc oxide semiconductor gas sensor (5°) before and after operation in a similar atmosphere is also shown. This is to convert each gas sensor (5) (5') into HMDS [hexamethyldisoloxa7 ((CH3)ssiOsi(CHa)3t]
82S before and after operating for 3 hours in a 10ppm atmosphere
1+) shows the change in sensitivity to pllll. Note that HMDS decomposes at the operating temperature of the sensor (approximately 350° C.) and forms silicon oxide on the sensor surface.

Table 5 From Table 5 above, the malodorous gas sensor (5) of the present invention shows no decrease in sensitivity to malodorous gas compared to the conventional zinc oxide semiconductor gas sensor (5°) even if silicon oxide adheres to the sensor surface. I understand.

[Effects of the Invention] As described above, according to the malodorous gas sensor of the present invention, since C2H50)1 in the detected gas is removed by the catalyst layer containing a metal oxide such as tungsten, malodorous gas components (hydrogen sulfide, etc.) are removed. Excellent selectivity for C2H50H can be obtained. Furthermore, even when used in an atmosphere containing an organosilicon compound, silicon oxide adheres to the surface of the catalyst layer containing 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 malodorous gas sensor of the present invention to malodorous gas does not decrease.

[Brief explanation of the drawing]

Fig. 1 is a partial cross-sectional perspective view of the malodorous gas sensor of the present invention, Fig. 2 is a graph showing the sensitivity of the malodorous gas sensor of the present invention to various gases, and Fig. 3 is a graph showing the sensitivity of the malodorous gas sensor of the present invention to various gases. Fig. 4 is a diagram showing the 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. FIG. In the figure, 1 is a zinc oxide semiconductor layer, 2 is a tungsten oxide catalyst layer, and 5 is a malodorous gas sensor. Agent Patent Attorney Takaharu Higashishima

Claims (1)

[Claims] (1) A foul-smelling gas sensor comprising: a gas-sensitive layer mainly made 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 panadium. .