JPH0868775A - Sensor for hydrogen sulfide gas - Google Patents

Abstract

PURPOSE: To promptly detect hydrogen sulfide of low concentration by providing the surface of oxygen ion conductive solid electrolyte with a detection pole and a reference pole of precious metal and coating the surface of detection pole with a metal oxide semiconductor layer. CONSTITUTION: Relating to a hydrogen sulfide gas sensor, the surface of oxygen ion conductive solid electrolyte of ZrO2 , CeO2 , etc., with a reference pole and a detection pole, respectively, of precious metal group, and, the surface of detection pole is coated with a metal oxide semiconductor layer. For example, the inside and outside surfaces at the tip of a ZrO2 tube 2 are coated with porous Pt pastes 4 and 4 (detection pole, reference pole), and, Pt wire nets 6 and 6 are partially buried in the pastes 4 and 4, and, Pt wires 8 and 8 are connected, thus an output is taken out. A metal oxide semiconductor layer 10, such as polous WO3 , oxidizes such gas as H2 S, receives oxygen ion required for oxidation from ZrO2 and Pt paste 4, and further, transports the electron generated by oxidation to the Pt paste. This sensor detects H2 S of 1ppm or below in such response time as 2-3 minutes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to H2S, CH3SH,
The present invention relates to detection of hydrogen sulfide-based gas such as (CH3) 2S, and particularly to detection at a low concentration of 1 ppm order.

[0002]

2. Description of the Related Art Hydrogen sulfide gas such as H2S and CH3SH is highly toxic and has a strong malodor, and it is required to be detected at about 1 ppm in order to control toilet ventilation and deodorization or to detect the freshness of food. For the detection of H2S, metal oxide semiconductor gas sensors have been mainly studied, and for example, H2S sensor using SnO2 has been proposed (Yamazoe et al., Sensor and Actuator B, vol. 9, p. 197, 1992, 2nd Chemical Preliminary Collection of Sensor Symposium 287 pages, Hawaii, Honolulu, 1993). However, the metal oxide semiconductor gas sensor has insufficient sensitivity for detecting H2S of 1 ppm or less and its response performance is insufficient.

[0003]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a new hydrogen sulfide-based gas sensor using a solid electrolyte so that a hydrogen sulfide-based gas of 1 ppm or less can be quickly detected.

[0004]

The hydrogen sulfide gas sensor of the present invention is provided with a noble metal reference electrode and a detection electrode on the surface of an oxygen ion conductive solid electrolyte such as ZrO2 or CeO2, and the detection electrode surface is made of metal. It is covered with an oxide semiconductor layer.
The hydrogen sulfide-based gas sensor of the present invention is also provided with a noble metal-based reference electrode and a detection electrode using a mixture of a noble metal and a metal oxide semiconductor on the surface of the oxygen ion conductive solid electrolyte. Here, the oxygen ion conductive solid electrolyte is ZrO2 stabilized with Y2O3 or CaO, or CeO2.
The shape may be a tube shape or a flat plate shape. Since the detection electrode and the reference electrode are noble metal electrodes, and the detection electrode needs electrode activity sufficient to ionize oxygen, Pt, Rh,
Pd, Ir, Ru, Os or an alloy thereof is used. The reference electrode may be a low-activity electrode and does not particularly require oxygen ionization activity, and thus may be an Au or Ag electrode other than the above-mentioned noble metal. The detection electrode is covered with a metal oxide semiconductor layer of WO3, SnO2, In2O3 or the like, and WO3 is particularly preferable.

The role of the metal oxide semiconductor layer is to oxidize hydrogen sulfide-based gas. For example, in the potential method in which the potential between the detection electrode and the reference electrode is used as the detection signal, the sulfide is generated by the mixed potential of the potential due to the ionization reaction of oxygen by the noble metal electrode and the potential due to the oxidation reaction of the hydrogen sulfide-based gas by the metal oxide semiconductor. A potential that depends on the concentration of the hydrogen-based gas is generated. Further, for example, in the current method in which the detection electrode and the reference electrode are short-circuited and the current between them is used as the detection signal, oxygen ions are transported from the reference electrode side and sulfurized by the oxygen ions transported at the interface between the metal oxide semiconductor layer and the solid electrolyte. Oxidize hydrogen gas. Then, the current associated with the movement of oxygen ions is detected and used as a detection signal. In the embodiment, a sensor is shown in which a noble metal such as Pt is coated with a metal oxide semiconductor layer to form a detection electrode, but these may be mixed to form a detection electrode. For example, in the potential method, when a mixture of a metal oxide semiconductor and a noble metal is used to ionize oxygen by the noble metal and oxygen ions are consumed by the metal oxide semiconductor to oxidize hydrogen sulfide or the like, a mixed potential of the two is obtained. In the current method, oxygen ions transported from the reference electrode may be transported to the metal oxide semiconductor through spillover of the surface of the noble metal and consumed for the oxidation reaction of hydrogen sulfide or the like.

Comparing the potential method and the current method, the potential method is preferable, and the inventor has found that the current method is unstable because an overshoot occurs in the detection signal and that the gas concentration dependency of the signal is low. Therefore, the potential method that is stable and has no overshoot is preferable.
In the present invention, hydrogen sulfide-based gas such as H2S of 1 ppm or less can be detected, the response time is 90%, for example, about 2 to 3 minutes, and the low concentration hydrogen sulfide-based gas can be detected quickly.

[0007]

Embodiments FIGS. 1 to 6 show hydrogen sulfide-based gas sensors according to embodiments. Figure 1 shows the structure of the gas sensor used in the experiment. In the figure, 2 is a ZrO2 tube stabilized with Y2O3 (8mol%), and ZrO2 and C stabilized with CaO.
eO2 etc. may be used. Porous Pt pastes 4 and 4 are applied to both inner and outer surfaces of the tip of the ZrO 2 tube 2, and Pt wire nets 6 and 6 are partially embedded in the pastes 4 and 4 to form a Pt wire 8,
8 is connected so that the output can be taken out. A noble metal paste such as Ir, Rh, Pd, or Ru may be used instead of the Pt pastes 4 and 4, and a paste having a low activity such as Au paste may be used particularly on the reference electrode side of the inner surface of the ZrO2 tube 2. Similarly, wire meshes such as Ir, Rh, and Pd may be used instead of the Pt wire meshes 6 and 6. 10 is about 0.
5mm porous WO3 layer, SnO2 or I in place of WO3
Other metal oxide semiconductors such as n2O3 may be used. The role of the metal oxide semiconductor layer 10 is to oxidize a gas such as H2S, and the oxygen ions required for the oxidation are ZrO2 and Pt.
It is necessary to be able to receive the electrons from the paste 4 or the like and transport the electrons resulting from the oxidation to the Pt paste 4 or the like. Therefore, it is preferable that the metal oxide semiconductor layer 10 has a high oxidation activity to hydrogen sulfide and has conductivity. A metal oxide semiconductor such as WO3 may be mixed with the Pt paste 4 on the detection electrode side to form a noble metal / metal oxide semiconductor mixed electrode. 12 is ZrO2 tube 2 at 400 ℃
It is a heater for heating to a moderate operating temperature.

The gas sensor of the example was manufactured as follows. Pt pastes 4 and 4 are applied to both the inner and outer surfaces of the tip of the ZrO 2 tube 2, and Pt wire nets 6 and 6 to which Pt wires 8 are attached in advance are partially embedded in the pastes 4 and 4, and the air 120
Sintered for 30 minutes at 0 ° C. Then, WO obtained by baking ammonium paratungstate in air at 600 ° C. for 5 hours
3 was crushed, and water was added to form a paste, which was applied on the Pt wire net 6 on the detection electrode side and baked in air at 600 ° C. for 4 hours to form a WO 3 layer 10.

The gas sensor of the embodiment can detect a gas such as H 2 S by either the potential between the reference electrode and the detection electrode or the short-circuit current when the reference electrode and the detection electrode are short-circuited. In addition, this sensor is used, for example, with the reference electrode side in contact with a standard atmosphere such as clean air and the detection electrode side in contact with the test atmosphere. In the case of the potential method, the reaction at the detection electrode is to ionize oxygen into oxygen ions and oxidize gas such as H2S with the oxygen ions. Then, the mixed potential associated with these reactions is detected as the potential between the Pr lines 8 and 8. In this case, the reference electrode does not contribute to the reaction but merely generates the reference potential. In the case of the current method, a gas such as H2S is oxidized on the detection electrode side, and oxygen ions necessary for this are transported from the reference electrode side through the ZrO2 tube 2. The role of the reference electrode in this case is to ionize oxygen into oxygen ions.

With respect to the electrode material, in the case of the potential method, all the noble metal materials including Au and Ag can be used as the electrode material on the reference electrode side, and oxygen in the atmosphere is converted to oxygen ions on the detection electrode side. It is possible to use a noble metal material having an activity such that it can be ionized, and all noble metal materials except Au and Ag can be used. In the case of the current method, all the noble metal materials except Au and Ag can be used for both the reference electrode and the detection electrode.

Most of the detection reaction of gas such as H 2 S occurs on the detection electrode side, and the reference electrode side requires only a reaction to ionize oxygen even in the case of the current method. As a result, the structure of the sensor can be simplified as shown in FIG.
In the sensor of FIG. 2, ZrO stabilized with Y2O3 on a flat plate is used.
2 Both surfaces of the substrate 3 are coated with the porous Pt pastes 4 and 4, and the Pt paste 4 is coated with the WO3 layer 10 on the detection electrode side. On the reference electrode side, in addition to the Pt paste 4, the heater 13
Is provided, the voltmeter 14 detects the potential between both electrodes,
The heater power supply 16 drives the heater 13. In this case, the electrode current of the reference electrode is very small, and both the reference electrode and the detection electrode can be brought into contact with the test atmosphere. Oxygen in the atmosphere to be detected is reduced to oxygen ions on the detection electrode side, and the generated oxygen ions oxidize gas such as H2S to generate water vapor or SO2.
And so on. At the reference electrode, dissociation equilibrium occurs due to the ionization of oxygen, and the oxidizing activity to H2S and the like is low, and even if the reference electrode is exposed to the test atmosphere, it does not hinder the detection.

The characteristics of the gas sensor of the embodiment will be described. FIG. 3 shows changes in electromotive force between the detection electrode and the reference electrode when the reference electrode and the detection electrode are heated to about 400 ° C. by the heater 12 and brought into contact with H 2 S up to 12 ppm using the gas sensor of FIG. Indicates. An output of 80 mV or more is obtained for 12 ppm of H2S, and an output of about 30 mV is obtained for 0.6 ppm of H2S. The response time when brought into contact with H2S is as short as about 2 to 3 minutes at 90% response, and the electromotive force returns to the original value in about 20 minutes when returned to the air.

FIG. 4 shows the relationship between the H2S concentration and the steady-state value of electromotive force. The potential of the detection electrode with respect to the reference electrode decreases by about 40 mV for every 10-fold change in H2S concentration, and H2S 0.6 ppm
There is an electromotive force of about 30 mV between air and the air, and
H2S less than 6 ppm can also be detected. 20 pp as interfering gas
I examined SO2 of m and CO2 of 1000ppm, but the same 4
The change in electromotive force for these gases at 00 ° C is SO2
It is -15 mV at 0 ppm and -20 mV at 1000 ppm CO2, which is extremely low sensitivity compared to H2S. Therefore, this sensor has a high selectivity to H2S, and also to H2S derivatives such as CH3SH and (CH3) 2S.
The same sensitivity and response characteristics as 2S were obtained.

The WO3 layer 10 plays a decisive role in the detection of H2S. When the WO3 layer 10 is removed, the characteristics of the sensor are completely changed to the characteristics shown in FIG. In the sensor without the WO3 layer 10, the H2S sensitivity is small, the response time is slow, and the electromotive force does not reach the steady value even 90 minutes after the contact with H2S. Remarkably, in the example, the potential of the detection electrode is lowered by the contact with H2S, whereas in the comparative example from which the WO3 layer 10 is removed, the potential of the detection electrode is increased. This is W
It shows that in the absence of the O3 layer 10, the reaction at the detection electrode is completely different.

The inventor has considered the H2S detection mechanism as follows. The example sensor can be represented as a battery of formula (1). This battery can be expressed as a mixture of the battery of formula (2) and the battery of formula (3). air, Pt ｜ YSZ ｜ Pt, H2S in air (2) air, Pt ｜ YSZ ｜ WO3 (Pt), H2S in air (3) where the potential on the reference electrode side is between oxygen and oxygen ion in equation (4). It is determined by the equilibrium reaction of. O2 + 4e ⁻ = 2O ^{2 −} (4) On the other hand, the Pt paste 4 and the Pt wire mesh 6 are used in the formula (5) for the detection electrode.
Ionization reaction of the oxygen in the mixture proceeds, and the generated oxygen is WO3 layer 1
It is consumed in the oxidation reaction of H2S at 0 (equation (6)). The role of the Pt paste 4 and the Pt wire mesh 6 at the detection electrode is WO3 layer 1
It is to supply oxygen ions to the 0 side and receive electrons generated by oxidation. The potential of the detection electrode is determined by the hybrid potential of the oxygen ionization reaction of equation (5) and the H2S oxidation reaction of equation (6), the reaction currents of both are equal, and the hybrid potential M is determined by the mechanism of FIG. ^{O2 + 4e - → 2O 2- (} 5) H2S + 3O 2- → H2O + SO2 + 6e - (6)

The vertical axis of FIG. 6 represents the potential of the detection electrode, and the horizontal axis represents the reaction currents of equations (5) and (6). Since the reaction currents of the ionization of oxygen of the formula (5) and the oxidation of H2S of the formula (6) are equal, the hybrid potential is determined at the intersection of the polarization curve of H2S and the reduction curve of oxygen. Here, it is assumed that C1 and C2 are H2S and oxygen concentrations, and C2> C1. ppm order H2S
The oxygen concentration changes very little, and the Tafel equation holds for the ionization reaction of oxygen, and the hybrid potential changes linearly with the logarithm of the oxygen reduction current. On the other hand, since the H2S concentration is extremely low, a limiting current is generated, and when the current value is increased, the potential changes rapidly. Therefore, for example, when the oxygen concentration is set to C1 and the H2S concentration is increased from C1 to C2, the mixed potential changes in proportion to the logarithm of the H2S concentration. This is consistent with the result that the potential decreases by 40 mV for every 10-fold increase in H2S concentration. Also, when the oxygen concentration was artificially increased, the potential increased correspondingly, which was consistent with the detection mechanism of FIG.

As already shown, the role of the WO3 layer 10 is to oxidize H2S, and any metal oxide semiconductor having a selective oxidation activity for H2S can be used. Therefore, the material of the metal oxide semiconductor gas sensor that is effective as the H2S detection material can be used in place of the WO3 layer 10, and SnO2 can be used instead of WO3, for example. Further, as shown in FIG. 5, when the WO3 layer 10 is not provided, the potential change with respect to H2S is extremely small. Therefore, both surfaces of the ZrO2 substrate 3 may be brought into contact with the test atmosphere as in the gas sensor of FIG. Further, in FIG. 2, the reference electrode and the detection electrode are provided on both surfaces of the ZrO 2 substrate 3, but they may be integrated on the same surface of the ZrO 2 substrate 3.

In addition to the above, the inventor short-circuits the reference electrode and the detection electrode of the gas sensor of FIG.
Attempted to detect S. However, in this case, H
2S sensitivity is low, for example 40 to 5 for 10 ppm of H2S
At about 0 μA, overshooting often occurred when contacted with H 2 S, and the obtained current value was unstable. Therefore, it is preferable to detect a gas such as H2S from the potential between the detection electrode and the reference electrode rather than the current method.

[0019]

According to the present invention, a low-concentration hydrogen sulfide-based gas of 1 ppm or less can be rapidly detected with a 90% response time of, for example, about 2 to 3 minutes.

[Brief description of drawings]

FIG. 1 is a sectional view of an H2S sensor according to an embodiment.

FIG. 2 is a cross-sectional view of a modified H2S sensor.

FIG. 3: 0.6-12 ppm H 2 at 400 ° C. of the Example
Characteristic diagram showing response characteristics to S

FIG. 4 is a characteristic diagram showing the H2S concentration dependence of the electromotive force at 400 ° C. in the example.

FIG. 5: 40 in the conventional example without WO3 coating
Characteristic diagram showing the relationship between electromotive force at 00 ° C and H2S concentration

FIG. 6 is a characteristic diagram showing an H 2 S detection mechanism in an example.

[Explanation of symbols]

 2,3 Y2O3 stabilized ZrO2 4 Pt paste 6 Pt wire mesh 8 Pt wire 10 WO3 layer 12,13 Heater 14 Voltmeter 16 Heater power supply

Claims (3)

[Claims] 1. A hydrogen sulfide-based gas sensor in which a noble metal-based detection electrode and a reference electrode are provided on the surface of an oxygen ion conductive solid electrolyte, and the detection electrode surfaces are covered with a metal oxide semiconductor layer. 2. On the surface of the oxygen ion conductive solid electrolyte,
A hydrogen sulfide-based gas sensor provided with a reference electrode of a noble metal type and a detection electrode using a mixture of a noble metal and a metal oxide semiconductor. 3. The hydrogen sulfide gas sensor according to claim 1, wherein the metal oxide semiconductor is WO3.