JP2595123B2 - Carbon monoxide gas sensor - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting carbon monoxide gas generated in various burning appliances, factories, and the like.

[Prior Art] A conventional semiconductor-type flammable gas sensor has been used at a high temperature (300 to 450 ° C.) in order to avoid the influence of moisture and oil in the air. However, because the selectivity of carbon monoxide gas cannot be obtained at high temperatures, it must operate at low temperatures (around 100 ° C), heat the sensor appropriately, heat clean it, and remove moisture and oils and fats in the air. Used to avoid the effects. On the other hand, for example, Japanese Patent Application Laid-Open No. Sho 63-45552 introduces a carbon monoxide gas sensor which can be used at a high temperature by adding an alkaline earth metal or the like to tin oxide.

[Problems to be Solved by the Invention] The carbon monoxide gas sensor disclosed in JP-A-63-45552 shows 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 in which no target gas exists. 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. The gas concentration of each target gas is 500 ppm. Here, as seen from Figure 4, H ₂
Is less sensitive to CO (ie, the selectivity to CO is improved), but the extent is still sufficiently low compared to, for example, C ₂ H ₄ (olefin-based gas) in the figure. It cannot be said that it is. Further, it can be seen that the sensitivity to C ₂ H ₅ OH (alcohol) is higher than the sensitivity to CO. Therefore, this sensor has a problem that it cannot be used in an atmosphere in which C ₂ H ₅ OH gas is present.

The present invention has been made in order to solve the problems described above, together with further improve the CO selectivity to H _2, carbon monoxide gas sensor further having sufficient CO selectivity to C ₂ H ₅ OH The purpose is to gain.

[Means for Solving the Problems] A first carbon monoxide gas sensor according to the present invention mainly comprises an oxide of at least one alkaline earth metal selected from beryllium, calcium, strontium and barium. A gas-sensitive layer made of a tin oxide semiconductor; a catalyst layer containing chromium; and a catalyst layer containing at least one metal oxide selected from tungsten, molybdenum, and vanadium.

A second carbon monoxide gas sensor according to the present invention is a gas-sensitive layer mainly composed of a tin oxide semiconductor to which at least one kind of alkaline earth metal oxide selected from beryllium, calcium, strontium and barium is added, And a catalyst layer having at least one metal oxide selected from tungsten, molybdenum, and vanadium.

[Operation] In the first carbon monoxide gas sensor of the present invention, C ₂ H ₅ OH in the detected gas is removed by the catalyst layer having a metal oxide such as tungsten, and the detected gas is removed by the catalyst layer having chromium. It is removed H ₂ in further and reaches the gas-sensitive layer after being removed a portion of the C ₂ H ₅ OH. Thereby, carbon monoxide gas is selectively detected.

In the second carbon monoxide gas sensor of the present invention, C ₂ H ₅ OH in the gas to be detected is removed by the catalyst layer having a metal oxide such as tungsten before reaching the gas-sensitive layer. Thereby, carbon monoxide gas is selectively detected.

Embodiment FIG. 1 (a) is a perspective view of a carbon monoxide gas sensor showing one embodiment of the present invention.

The semiconductor portion (1) is formed in a spherical shape around a coil (4) made of a noble metal as a gas-sensitive layer carrying an alkaline earth metal or the like and containing SnO ₂ as a main component, and then around the semiconductor portion (1). The chromium oxide catalyst layer (2) is formed by the following steps.

(1) An aqueous solution of tin chloride (20% vol) is mixed with, for example, nitrate of chromium in an amount of 0.1 to 2 mol% as chromium based on tin.

(2) Aqueous ammonia is added dropwise to the aqueous solution to precipitate tin and chromium as hydroxides.

(3) The precipitate is washed with water, dried, and fired in an electric furnace.

(4) The fired substance is pulverized to about several μm to obtain a fine powder.

(5) The fine powder is kneaded with water to form a paste, which is applied to the entire surface of the semiconductor portion (1).

(6) The sensor is heated to about 600 ° C. and sintered to form a chromium oxide catalyst layer (2) outside the semiconductor portion (1).

Further, a tungsten oxide catalyst layer (3) is formed around the chromium oxide catalyst layer (2) by the following steps.

(1) At least one powder selected from alumina (silica), silica-alumina, and zeolite powder (several μm) is impregnated with at least one salt of ammonium salts or nitrates such as tungsten, molybdenum, and vanadium by an impregnation method. And 0.1 to 5 mol% with respect to the above powder.

(2) After drying the above mixture, it is fired in an electric furnace.

(3) The fired substance is pulverized to about several μm to obtain a fine powder.

(4) The fine powder is kneaded with water to form a paste, which is applied to the entire surface of the chromium oxide catalyst layer (2).

(5) Heat the sensor to about 600 ° C. and sinter to form a tungsten oxide catalyst layer (3) around the chromium oxide catalyst layer (2).

The carbon monoxide gas sensor (5
FIG. 2 (a) shows the sensitivity to various gases in a).
For comparison, FIG. 2 (b) shows the sensitivity of the carbon monoxide gas sensor provided with only the chromium oxide catalyst layer (2) to various gases.

FIG. 1 (b) shows a carbon monoxide gas sensor (5b) provided with only a tungsten oxide catalyst layer (3) according to a second embodiment of the present invention. FIG. 2 (c) shows the sensitivity of this carbon monoxide gas sensor (5b) to various gases. The vertical and horizontal axes in FIGS. 2 (a), (b) and (c) are exactly the same as those in FIG.

As can be seen by comparing FIG. 4 and FIG. 2 (b).
Sensitivity is reduced to other gases and equal to or less than such C ₂ H ₄ for H _2. This is because the H ₂ is oxidized and removed by chromic oxide catalyst layer. Thus, the chromium oxide catalyst layer it is understood that it is possible to sufficiently improve the CO selectivity to at least H _2. Further, although the decrease in sensitivity to significant C ₂ H ₅ OH Focusing on figure C ₂ H ₅ OH is seen in FIG. 2 (b), when compared with H ₂ and the like, C ₂ H ₅ OH
It can be said that the CO selectivity with respect to is not sufficient. That is, C ₂ H ₅ OH is partially removed by the present chromium catalyst layer.

Next, FIG. 4 is compared with FIG. 2 (c), and it can be seen that C ₂ of the carbon monoxide gas sensor (5b) of the second embodiment of the present invention.
Sensitivity to H ₅ OH at a high temperature region is reduced to equal degree as for C ₂ H _4. This is because C ₂ H ₅ OH has been removed by a later-described reaction with the tungsten oxide catalyst layer. Therefore, according to the carbon monoxide gas sensor (5b) of the second embodiment of the present invention, at least the CO selectivity to C ₂ H ₅ OH can be sufficiently improved. Focusing on Naozu in H ₂ This is exactly the improvement from the conventional example (FIG. 4) is not observed. That is, H ₂ is not removed by the present tungsten oxide catalyst layer.

The decomposition reaction of C ₂ H ₅ OH gas by the tungsten oxide layer is called an intramolecular dehydration reaction of alcohol by an acidic metal oxide, and its chemical reaction formula is as follows.

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, and the carbon monoxide gas sensors (5a) and (5b) for this gas are very low as can be seen from FIGS. 2 (a) and (c). Shows the sensitivity. Therefore, it is understood that the use of an acidic metal oxide layer such as a tungsten oxide layer that decomposes alcohol by a reaction as described in (1) above is effective.

4 and FIGS. 2 (b) and 2 (c) and FIG. 2 (a).
, The carbon monoxide gas sensor (5a) has each catalyst layer, so CO 2 gas for both H ₂ and C ₂ H ₅ OH
The selectivity has been sufficiently improved.

Therefore, the carbon monoxide gas sensor according to the second embodiment of the present invention (5b) is not present H _2, as a carbon monoxide gas sensor by appropriately selecting the operating temperature in an atmosphere C ₂ H ₅ OH is present The CO selectivity is sufficiently satisfied. Further, the carbon monoxide gas sensor (5a) of the first embodiment of the present invention
Even in an atmosphere where H ₂ , C ₂ H ₅ OH and the like exist, the CO selectivity is sufficiently satisfied.

FIG. 3 shows an example of the change in the sensitivity of the carbon monoxide gas sensor with respect to the change in the concentration of each of the gases CO, H ₂ and C ₂ H ₅ OH, that is, one example showing the gas concentration dependence of the sensitivity of the carbon monoxide gas sensor. Here, the carbon monoxide gas sensor (5a) is set to a temperature of about 340
Used under the condition of ° C. That is, the concentration of each gas is changed at point "A" in FIG. 2 (a). As can be seen from FIG. 3, in a generally used gas concentration range, for example, in an atmosphere in which H ₂ or C ₂ H ₅ OH is about 200 ppm,
It can be seen that the target minimum detection amount of about 50 ppm of CO can be sufficiently measured. It is confirmed by the carbon monoxide gas sensor (5b) of the second embodiment of the present invention that the same CO selectivity can be obtained even when the concentration of each gas changes.

The chromium oxide catalyst layer (2) of the above carbon monoxide gas sensor (5a) is a layer in which chromium is mixed at 1 mol% (relative to SnO ₂ ) to a thickness of 100 μm.

The amount of chromium added (1 mol%) in the chromium oxide catalyst layer of the above example was calculated based on the amount of CO added to the amount of chromium shown in Table 1 below.
And the sensitivity ratio of H ₂ (CO / H ₂ sensitivity ratio) = {net sensor output voltage of CO (100 PPM)} / {net sensor output voltage of H ₂ (1000 ppm)}.

It can be seen from Table 1 that the best sensitivity ratio is obtained when the Cr content is about 1 mol%. When the optimum chromium addition amount (1 mol%) obtained as described above is used, the change in the CO / H ₂ sensitivity ratio obtained by changing the thickness of the chromium oxide catalyst layer is shown in Table 2 below. It was shown to.

From Table 2, it can be seen that in order to obtain the best sensitivity ratio, the thickness of the chromium oxide catalyst layer is preferably about 100 μm to 200 μm. When the amount of Cr added was more than 1 mol%, almost the same sensitivity ratio could be obtained by setting the chromium oxide catalyst layer thinner than the above 100 μm. Conversely, if the amount of Cr added is
If less than 10 mol%, the chromium oxide catalyst layer
It may be set thicker than 0 μm.

The tungsten addition amount (2 mol%) of the tungsten oxide catalyst layer in the above example is determined by the sensitivity ratio of CO and C ₂ H ₅ OH to the tungsten addition amount shown in Table 3 below (hereinafter CO / Et-O).
That H sensitivity ratio) = {CO (net of the sensor output voltage of 100 ppm)} {the net sensor output voltage of the H ₂ (1000ppm))} /
Was determined from the relationship.

It can be seen from Table 3 that the best sensitivity ratio is obtained when the added amount of tungsten is about 2 mol%.

When the optimum tungsten addition amount (2 mol%) obtained as described above is used, the change in the CO / Et-OH sensitivity ratio obtained by changing the thickness of the tungsten oxide catalyst layer is described in the following 4th. It is shown in the table.

From Table 4, it can be seen that in order to obtain the best sensitivity ratio, the thickness of the tungsten oxide catalyst layer is preferably about 100 to 300 μm. However, if the thickness of the tungsten oxide catalyst layer or the chromium oxide catalyst layer is about 400 μm or more, the mechanical strength is reduced. Can be The relationship between the added amount of tungsten and the sensitivity ratio of the thickness of the layer was the same as in the case of chromium.

Also, molybdenum and vanadium, which are additives for forming an acidic metal oxide catalyst layer other than tungsten, had a CO / Et-OH sensitivity ratio similar to that of tungsten. Next, the relationship between the added amount of molybdenum and the CO / Et-OH sensitivity ratio is shown in Table 5 below, and the relationship between the added amount of vanadium and the CO / Et-OH sensitivity ratio is shown in Table 6 below.

From Tables 5 and 6, it can be seen that the best sensitivity ratio is obtained when the amount of molybdenum or vanadium added is about 2 mol%.

In the above-described carbon monoxide gas sensor (5a), an example in which the tungsten oxide catalyst layer (3) is provided around the chromium oxide catalyst layer (2) has been described. And tungsten oxide catalyst layer (3)
The same effect as in the above embodiment can be obtained when the positions are replaced.

As can be seen from FIGS. 2 (a) and 2 (c), both of the carbon monoxide gas sensors (5a) and (5b) of the present invention have a relatively high temperature (about 300 to 400 ° C.). The above-mentioned performance is shown when is used. Therefore, a high-temperature purge or the like, which is indispensable for a carbon monoxide gas sensor generally used at about 100 ° C., is not required. Therefore, the configuration of the device is simplified, and a cheap and small device can be obtained.

The carbon monoxide gas sensor described above is a hot-wire type semiconductor gas sensor that measures a change in resistance at both ends of a noble metal coil (4) as an output of the gas sensor. By flowing a constant current through the noble metal coil (4), it serves as a heater for keeping the temperature of the gas sensor constant. FIG. 5 shows an example in which the present invention is applied to a conventional substrate-type semiconductor gas sensor. Each layer is formed in a planar structure on an alumina substrate (7), and a heating heater (6) and a detection output electrode ( It is an example of a carbon monoxide gas sensor (5c) in which 4a) and (4b) are independently provided.

In the above examples, tungsten, molybdenum, vanadium, and the like were described as specific examples as the metal forming the oxide catalyst layer that causes the dehydration reaction of alcohol.However, other metals such as thorium, lanthanum, yttrium, and zirconium were used. Even when an oxide catalyst layer was formed, 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.

[Effect of the Invention] According to the first carbon monoxide gas sensor of the present invention, C ₂ H ₅ OH in the gas to be detected is removed by the catalyst layer having a metal oxide such as tungsten, and the catalyst has chromium. The layer removes H ₂ in the gas to be detected and also removes a part of C ₂ H ₅ OH. Thus obtained a sensor excellent selectivity to H ₂ and C ₂ H ₅ OH of carbon monoxide is operated at high temperatures (about 300 to 400 ° C.).

The second, according to the carbon monoxide gas sensor of the present invention, since the removal of C ₂ H ₅ OH in the detected gas by the catalyst layer having a metal oxide such as tungsten, for C ₂ H ₂ OH carbon monoxide Excellent selectivity for high temperature sensor (300 ~ 400 ℃)
It is obtained by operating with

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) are partial cross-sectional perspective views of first and second carbon monoxide gas sensors of the present invention, respectively, and FIGS. 2 (a) and (c) respectively show the first and second carbon monoxide gas sensors of the present invention. FIG. 2B is a graph showing the sensitivity of the second carbon monoxide gas sensor to various gases for comparison with various gases, and FIG. 2B is a graph showing the sensitivity of the second carbon monoxide gas sensor having only the chromium oxide catalyst layer to various gases. Fig. 3 shows a carbon monoxide gas sensor (5
a) Graph showing gas concentration dependence of sensor sensitivity,
FIG. 5 is a graph showing the sensitivity of a conventional carbon monoxide gas to various gases, and FIG. 5 is a partial cross-sectional perspective view of a substrate-type carbon monoxide gas sensor of the present invention. In the figure, 1 is a semiconductor portion, 2 is a chromium oxide catalyst layer, 3 is a tungsten oxide catalyst layer, and 5a, 5b and 5c are carbon monoxide gas sensors.

Claims (2)

(57) [Claims] 1. A gas-sensitive layer mainly composed of a tin oxide semiconductor to which an oxide of at least one alkaline earth metal selected from beryllium, calcium, strontium and barium is added, a catalyst layer having chromium, and A carbon monoxide gas sensor, comprising: a catalyst layer having at least one metal oxide selected from tungsten, molybdenum, and vanadium. 2. A gas-sensitive layer mainly composed of a tin oxide semiconductor to which an oxide of at least one kind of alkaline earth metal selected from beryllium, calcium, strontium and barium is added, and a gas-sensitive layer composed of tungsten, molybdenum and vanadium. A catalyst layer comprising at least one metal oxide selected from the group consisting of: