JP2004093144A - Gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain sufficient detection sensitivity by selectively detecting a specific gas. <P>SOLUTION: Sulfuric acid 30 is accommodated as an electrolyte in a solution accommodation tank 20, where one portion of a partition is composed by a gas permeating film 10. The gas permeating film 10 is made of a platinum layer 11, whose outer side has a fiber structure and a solid electrolyte layer 12 whose inside is covered with Nafion (R) or the like, and has a property for permeating a methyl mercaptan gas G to be detected. A counter electrode 40 made of platinum is dipped in sulfuric acid 30. With a specific voltage being applied in between the platinum layer 11 and the counter electrode 40, current flowing between both the electrodes 11, 40 through sulfuric acid 30 is detected by a detection means 50. Since the gas G is efficiently introduced from the fiber-like platinum layer 11 into sulfuric acid 30 via a solid electrolyte layer 12, the quantitative value of the gas G is determined as a current value. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gas sensor, and more particularly, to a potentiostatic gas sensor suitable for detecting living odors such as garbage and bad breath.
[0002]
[Prior art]
In today's environment-conscious environment, various gas sensors are used to collect quantitative data on odors such as odors. In particular, in recent years, demand for low-cost sensors capable of detecting the smell of garbage in refrigerators and garbage disposal facilities and living odors such as bad breath has been expected with the improvement of living standards.
[0003]
2. Description of the Related Art As a sensor for detecting a living odor in daily life, a potentiostatic electrolytic gas sensor capable of directly detecting gas in the air has been proposed. In this gas sensor, a pair of electrodes is immersed in a reaction solution having the property of causing an electrochemical reaction with the gas to be detected, and when a predetermined voltage is applied between the electrodes, the current flowing in the reaction solution is measured. It is based on the principle of doing. If the electrolysis phenomenon is caused while the detection target gas is introduced into the reaction solution, the introduced detection target gas can be quantitatively detected based on the current flowing through the reaction solution. Usually, a reference electrode is further immersed in a reaction solution, and measurement based on a three-electrode method using a potentiostat is often performed.
[0004]
In such a constant potential electrolytic gas sensor, a gas permeable membrane for introducing a gas to be detected is usually formed on a part of a partition wall of a solution storage tank that stores a reaction solution. As the gas permeable membrane, for example, a polymer membrane such as polytetrafluoroethylene (Nafion: registered trademark of DuPont, U.S.A.) and a metal film formed on the surface thereof is used. The metal film portion is used as a working electrode when performing electrolysis.
[0005]
[Problems to be solved by the invention]
However, the conventionally proposed constant potential electrolysis type gas sensor has a problem that the detection sensitivity to ordinary living odor at normal temperature is low. In gas detection applications in factories and research facilities, it is often practically sufficient if detection is possible in a relatively high-temperature atmosphere.However, it is used to detect the smell of garbage and living odors such as bad breath. Requires detection sensitivity at room temperature. Further, the conventionally proposed constant potential electrolysis type gas sensor has a problem that the selectivity of a gas to be detected is low. In particular, methyl mercaptan (a main cause of putrefactive odor of meat), which is a cause of life odor, When used for detection of gases such as trimethylamine (rot odor of fish), acetic acid, acetaldehyde, ethylene, etc., it has been difficult to realize a sensor having selective detection sensitivity to these gases.
[0006]
Therefore, an object of the present invention is to provide a gas sensor capable of selectively detecting a specific gas and obtaining sufficient detection sensitivity.
[0007]
[Means for Solving the Problems]
(1) A first aspect of the present invention relates to a potentiostatic gas sensor,
A gas permeable membrane having a property that the inside is formed of a solid electrolyte layer and the outside is formed of a metal layer having a fiber structure, and has a property of transmitting a gas to be detected.
A solution storage tank in which the gas permeable membrane is part of a partition,
A reaction solution which is contained in the solution containing tank and has a property of causing an electrochemical reaction with the gas to be detected;
A counter electrode immersed in the reaction solution,
A metal layer constituting the inner part of the gas permeable membrane is used as a working electrode, and a predetermined detection voltage is applied between the working electrode and the counter electrode. Detecting means for detecting
Is provided.
[0008]
(2) A second aspect of the present invention provides the gas sensor according to the first aspect,
A reference electrode immersed in the reaction solution is further provided, and the detection means uses a potentiostat to perform current detection using the reference electrode as a reference potential.
[0009]
(3) A third aspect of the present invention provides the gas sensor according to the first or second aspect described above,
A membrane in which a metal layer having a fiber structure is formed on both surfaces of a solid electrolytic layer is used as a gas permeable membrane.
[0010]
(4) A fourth aspect of the present invention provides the gas sensor according to the first to third aspects described above,
As the solid electrolyte layer constituting the gas permeable membrane, an ion conductive polymer membrane having a property of transmitting a gas to be detected is used.
[0011]
(5) According to a fifth aspect of the present invention, in the gas sensor according to the first to fourth aspects,
As the metal layer constituting the gas permeable membrane, a fiber structure layer of metal precipitated in a solution containing a metal salt is used.
[0012]
(6) According to a sixth aspect of the present invention, in the gas sensor according to the first to fifth aspects,
The metal layer is composed of a platinum layer having a fiber structure, the reaction solution is composed of sulfuric acid, and methyl mercaptan can be detected as a detection target gas.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described based on an illustrated embodiment. FIG. 1 is a configuration diagram showing a basic configuration of a gas sensor according to the present invention (a part is shown as a sectional view, and a part is shown as a block diagram). As shown, the basic components of the gas sensor are a gas permeable membrane 10, a solution storage tank 20, a reaction solution 30, a counter electrode 40, and a detecting means 50. The gas permeable membrane 10 is a membrane having a property of transmitting a gas to be detected, and forms a part of a partition wall of the solution storage tank 20. The reaction solution 30 accommodated in the solution accommodation tank 20 is a solution having a property of causing an electrochemical reaction with the gas to be detected, and functions as an electrolytic solution for performing gas detection by a constant potential electrolysis method. Since a part of the partition walls of the solution storage tank 20 is constituted by the gas permeable membrane 10, the detection target gas G existing outside is transmitted through the gas permeable membrane 10 and is transmitted into the solution storage tank 20. Can be. Since the gas permeable membrane 10 does not allow the reaction solution 30 to permeate, the reaction solution 30 does not leak out of the solution storage tank 20.
[0014]
FIG. 1 shows a cross section of each of the gas permeable membrane 10, the solution storage tank 20, and the counter electrode 40. As shown in the figure, the gas permeable membrane 10 has a two-layer structure, in which a metal layer 11 is on the outside and a solid electrolyte layer 12 is on the inside. The metal layer 11 functions as a working electrode for performing the constant potential electrolysis. The counter electrode 40 is immersed in the reaction solution 30, and the detection means 50 applies a predetermined detection voltage between the working electrode made of the metal layer 11 and the counter electrode 40, It has a function of detecting a current that sometimes flows through the reaction solution 30 and flows between the metal layer 11 and the counter electrode 40. The value of the current thus detected indicates the quantitative value of the detection target gas G. This is because the detection target gas G that has passed through the gas permeable membrane 10 causes an electrochemical reaction with the reaction solution 30 and affects an electrolysis phenomenon that occurs in the reaction solution 30. The larger the amount of the detection target gas G introduced into the reaction solution 30, the more the current flowing through the reaction solution 30 and flowing between the metal layer 11 and the counter electrode 40 has a greater effect. By measuring the current value, a quantitative value of the gas G to be detected can be obtained.
[0015]
The gas sensor based on such a principle is a known sensor generally known as a potentiostatic gas sensor, and the gas sensor itself having the basic configuration as shown in FIG. 1 is a known gas sensor already used. is there. Normally, as the solid electrolyte layer 12 constituting the inner layer of the gas permeable membrane 10, for example, a polymer membrane such as polytetrafluoroethylene (NaFion: registered trademark of DuPont of the United States) or the like is used. As the metal layer 11 constituting the outer layer of the gas permeable film 10, a metal thin film such as platinum is used. However, as described above, the conventionally proposed constant potential electrolysis type gas sensor has a problem that the detection sensitivity for general life odor is low and the selectivity of the detection target gas is low.
[0016]
The gas sensor according to the present invention is basically a potentiostatic electrolytic gas sensor conventionally used. However, the inventor of the present application has found that the problems of these conventional gas sensors can be solved by changing the structure of the metal layer 11. That is, the feature of the present invention is that, in the gas sensor having the basic configuration shown in FIG. 1, the metal layer 11 forming the outer portion of the gas permeable membrane 10 is formed of a metal layer having a fiber structure, thereby enabling a specific detection. The point is that the detection sensitivity can be selectively increased with respect to the target gas.
[0017]
In the case of the embodiment shown here, the gas permeable membrane 10 is composed of a solid electrolyte layer 12 made of an ion-conductive polymer membrane “Nafion: Model No. 117” manufactured by DuPont USA, and a metal layer made of platinum having a fiber structure. 11. The reaction solution 30 is made of sulfuric acid, and the counter electrode 40 is made of a mesh plate made of platinum. As will be described later, the gas sensor according to the embodiment shown here has a selectively high detection sensitivity particularly to methyl mercaptan. This is because the solid electrolyte layer 12 made of a polymer film having ionic conductivity and the metal layer 11 made of platinum having a fiber structure can all transmit the molecules of methyl mercaptan, and the sulfuric acid becomes the reaction solution 30. This is because a methyl mercaptan molecule can be introduced therein. In the gas sensor according to the embodiment shown here, the reason why high detection sensitivity can be selectively obtained with respect to methyl mercaptan is that the metal layer 11 has a fiber structure instead of a conventional thin film structure. is there. Specific examples of the metal layer 11 having such a fiber structure and a method for forming the same will be described later in detail.
[0018]
FIG. 1 shows a so-called two-pole detection in which a predetermined detection voltage (constant voltage) is applied between the metal layer 11 and the counter electrode 40 and the current flowing between the two electrodes is measured. In practice, a reference electrode is further added to the reaction solution 30, and the detection means 50 uses a potentiostat to perform current detection by a three-electrode method using the reference electrode as a reference potential. It is preferred that
[0019]
FIG. 2 is a configuration diagram showing a specific embodiment of a type that performs detection by such a triode method (a part is shown as a cross-sectional view, and a part is shown as a block diagram). In the specific embodiment shown in FIG. 2, the gas permeable membrane 10 is arranged as a partition of the case 25, and the upper surface of the case 25 is closed by a lid 26. The room R on the right side partitioned by the case 25 and the gas permeable membrane 10 functions as a solution storage tank 20 shown in FIG. 1, and a reaction solution 30 is stored therein. The reference electrode 45 is immersed in the reaction solution 30 together with the counter electrode 40. As the detecting means 50, a potentiostat 55 is used, and the measurement by the potentiostatic electrolysis method is performed by a triode method using three electrodes of the metal layer 11, the counter electrode 40, and the reference electrode 45. That is, in a state where a predetermined voltage is applied to each electrode with the potential of the reference electrode 45 as a reference potential, the current flowing between the counter electrode 40 and the metal layer 11 is measured. Since the measurement of the potentiostat 55 using the potentiostat 55 by the triode method is a known technique, a detailed description thereof will be omitted.
[0020]
On the other hand, the left room L partitioned by the case 25 and the gas permeable membrane 10 functions as a gas introduction chamber, and is connected to a gas introduction pipe 61 from the pump 60. The pump 60 has a function of sucking outside air to be detected and introducing the sucked outside air into the gas introduction chamber L through the gas introduction pipe 61. Therefore, the detection target gas G contained in the outside air is guided from the pump 60 to the gas introduction chamber L through the gas introduction pipe 61, passes through the gas permeable membrane 10, and is introduced into the reaction solution 30. Will be.
[0021]
In the specific embodiment shown in FIG. 2, the gas permeable membrane 10 includes, as described above, a solid electrolyte layer 12 made of an ion-conductive polymer membrane “Nafion: Model No. 117” manufactured by DuPont, And a metal layer 11 made of platinum having a fiber structure. The polymer film “Nafion” is a cation-conductive solid electrolyte having a chemical structure as shown in FIG. 3, and is a thin film-like sheet in a normal state. However, when this is immersed in water or an electrolytic solution, the portion in [] shown in FIG. ⁺ Are ionized and become movable. Na ⁺ As a result, other cations can move inside and outside the membrane.
[0022]
The metal layer 11 is a fibrous structure made of platinum formed on the polymer film “Nafion”. FIG. 4 is a plan view (a diagram created based on an electron micrograph) of the platinum metal layer 11 having the fiber structure. The actual length of 50 nm is shown below the figure for reference. Since the molecular size of methyl mercaptan serving as the gas G to be detected in this embodiment is 10 nm or less, the fiber structure shown in FIG. 4 has a hollow structure having a dimension that allows the molecules of methyl mercaptan to sufficiently pass therethrough. Will be.
[0023]
On the other hand, the reaction solution 30 (electrolyte solution) in FIG. 2 is composed of sulfuric acid having a concentration of 0.5 M, the counter electrode 40 is composed of a mesh-like plate made of platinum, and the reference electrode 45 is composed of silver. It is composed of a rod-shaped electrode (so-called silver / silver chloride electrode) having a silver chloride film formed on the surface. The reference electrode 45 is for stabilizing the measurement by the potentiostatic electrolysis method, and is preferably made of a material having a property of performing a reversible ion exchange with respect to the reaction solution 30 (in this example, sulfuric acid).
[0024]
Subsequently, the results of detection of five types of gases using the gas sensor shown in FIG. 2 are shown in the graph of FIG. That is, in this graph, five types of gases, which are typical living odors, such as methyl mercaptan, trimethylamine, acetic acid, acetaldehyde, and ethylene, are mixed with nitrogen gas as the detection target gas G, and introduced at a flow rate of 50 ml / min. The detection sensitivity (the value obtained by converting the measured current value into a value per 1 ppm of gas concentration) in the case of the above is converted into a voltage between electrodes (in principle, a detection voltage applied between the metal layer 11 and the counter electrode 40). However, in the case of the three-electrode method shown in FIG. 2, the voltage value between the reference electrode 45 and the metal layer 11) is used as a parameter. As shown in the figure, the detection sensitivity of methyl mercaptan is higher than that of the other four gases over a considerably wide electrode voltage. That is, the gas sensor according to the present embodiment selectively has high detection sensitivity with respect to methyl mercaptan. Such a large detection sensitivity and specific gas selectivity are characteristics never obtained by a conventional general constant voltage electrolytic sensor (a sensor in which the metal layer 11 is a mere metal thin film).
[0025]
6 and 7 are graphs showing quantitative detection characteristics of methyl mercaptan in the gas sensor according to this embodiment. The graph of FIG. 6 shows the flow rate dependency, and the graph of FIG. 7 shows the concentration dependency. First, the graph of FIG. 6 shows the relationship between the flow rate of the gas introduced by the pump 60 and the detected current when the concentration of methyl mercaptan in the nitrogen gas is changed to three types of 2 ppm, 1.5 ppm, and 0.5 ppm. (The voltage between the electrodes is 1.0 V). As shown in the graph, the detection current increases as the gas concentration increases, and the detection current increases as the flow rate increases. On the other hand, the graph of FIG. 7 shows the detected current when the gas flow rate was fixed at 200 ml / min and the concentration of methyl mercaptan in the nitrogen gas was changed from 0.5 ppm to 19 ppm (voltage between electrodes). Is 1.0 V). As shown in the graph, the detected current increases almost linearly with the gas concentration.
[0026]
FIG. 8 is a graph showing the detection response of methyl mercaptan in the gas sensor according to this embodiment. The upper graph shows the results when the gas concentration is 10 ppm, and the lower graph shows the results when the gas concentration is 1.0 ppm (gas flow rate is 200 ml / min). The horizontal axis is the time axis. Specifically, these graphs show changes in the detected current when an experiment in which the introduction and stop of the introduction of the gas by the pump 60 are continuously repeated is performed. The rise time of the graph corresponds to the introduction start time, and the fall time corresponds to the introduction stop time. As shown in the figure, when gas introduction is started, the gas concentration in the sulfuric acid which becomes the reaction solution reaches an equilibrium state in about 1 to 2 minutes, and when the gas introduction is stopped, the reaction also takes about 1 to 2 minutes. It can be seen that the desorption of the gas from the sulfuric acid to be a solution is completed. This indicates that the sensor according to the present invention can perform measurement repeatedly and repeatedly.
[0027]
As described above, by forming the platinum metal layer 11 constituting the inside of the gas permeable membrane 10 into a fiber structure as shown in FIG. 4 instead of the conventional thin film structure, a sufficient detection sensitivity can be obtained conventionally. Sufficient detection sensitivity can be obtained for methyl mercaptan that has not been obtained, and sufficient detection sensitivity can be selectively obtained only for methyl mercaptan as compared with the other four gases. It is considered that the reason is that the platinum metal layer 11 having a fiber structure selectively has good permeability to methyl mercaptan molecules and provides good surface reactivity. That is, in order for the gas G to be detected to electrochemically affect the reaction solution 30, the gas must first pass through the metal layer 11, and The gas must be introduced into the reaction solution 30 via the layer 12 by a chemical reaction. Therefore, the permeability of the detection target gas G and the surface reactivity with the detection target gas G in the metal layer 11 greatly affect the detection sensitivity of the detection target gas G. This time, as a result of changing the structure of the metal layer 11 from the thin film structure to the fiber structure, the permeability and the surface reactivity are greatly changed, and it is considered that the detection sensitivity to methyl mercaptan has been improved.
[0028]
As shown in the graph of FIG. 5, the reason why the detection sensitivity of only methyl mercaptan among the five types of detection target gases G was improved is that the size and structure of the fibrous platinum layer shown in FIG. This is probably because the composition was suitable for improving permeability and surface reactivity. Here, it is difficult at this stage to make a detailed theoretical consideration of what kind of fiber structure should be used to improve the detection sensitivity for what kind of gas, but it is difficult to obtain various forms and sizes. By forming a metal layer 12 having a fiber structure of the following, and by measuring the detection sensitivity for various gases by trial and error, it is possible to specify a fiber structure capable of obtaining a good detection sensitivity for a specific gas. Become.
[0029]
As a method for forming the metal layer 11 having a fiber structure having a desired form and a desired size, it is preferable to adopt a process of depositing a metal in a solution containing a metal salt. This time, in preparing the gas permeable membrane 10 described above, a process of immersing a polymer membrane of “Nafion” in a platinum salt solution and then immersing in a reducing agent solution to precipitate platinum having a fiber structure. Was taken. Specifically, the following process was executed.
[0030]
First, a commercially available polymer film “Nafion: Model No. 117” is cut into a square shape having a side of 2.0 cm to obtain a square film piece. Then, this film piece was immersed in concentrated nitric acid for 30 minutes, rinsed with pure water, and then subjected to a pretreatment of immersing in about 80 ° C. pure water for 2 hours. This pretreatment is a treatment for removing dirt and impurities on the surface of the polymer film and increasing the reactivity.
[0031]
Subsequently, as a solution of a platinum salt, tetraamminedichloroplatinum (Pt (NH ₃ ) ₄ Cl ₂ ) Solution is prepared, and sodium borohydride (NaBH ₄ ) Prepare a solution. Then, the membrane piece subjected to the above pretreatment was immersed in this platinum salt solution for 40 minutes (permeation process). Next, the membrane piece was rinsed with pure water, then placed in a reducing agent solution, and immersed for 120 minutes while stirring at a constant rotation rate using a stirrer (reduction process). Finally, the film piece was immersed in a 0.5 M sulfuric acid solution to remove impurities on the surface and inside (rinsing process).
[0032]
The inventor of the present application conducted an experiment in which some of the above-mentioned process conditions were changed until a fibrous platinum layer (metal layer 11 shown in FIG. 4) that obtained the results shown in the graph of FIG. 5 was obtained. That is, the concentration of the platinum salt solution used in the above process is set to three conditions of 0.001 M, 0.003 M, and 0.005 M, and the concentration of the reducing agent solution is set to 0.01 M, 0.05 M, A sample in which a fibrous platinum layer was formed on a “Nafion” membrane piece was prepared under four conditions of 0.10 M and 0.15 M, and under a total of 12 conditions. As a result, when the concentration of the platinum salt solution was set to 0.005 M and the concentration of the reducing agent solution was set to 0.1 M, the highest detection sensitivity was obtained for methyl mercaptan. The results shown in the graphs of FIGS. 5 to 8 are the results obtained using the samples prepared under the optimum conditions.
[0033]
FIG. 9 is a graph showing the results of measuring the surface roughness of the above 12 kinds of samples, and FIG. 10 is a graph showing the results of measuring the surface areas of the above 12 kinds of samples. The surface roughness (nm) is obtained by bringing a probe supported by a cantilever into contact with the probe vertically above the fibrous platinum layer, scanning the uneven surface of the fibrous platinum layer two-dimensionally, and moving the probe up and down in the vertical direction. Is the value of the square root of the sum of squares. On the other hand, the surface area (unit is an arbitrary scale) indicates the total surface area of the uneven surface of the fibrous platinum layer formed on a region having a predetermined area on the “Nafion” film piece. As shown in this graph, the maximum values of the surface roughness and the surface area were obtained when the concentration of the platinum salt solution was 0.005 M and the concentration of the reducing agent solution was 0.1 M. As far as the above 12 samples are concerned, the result that the highest detection sensitivity is obtained when the sample having the largest surface roughness or surface area is used.
[0034]
As described above, in the process of depositing platinum on a film piece of “Nafion” and forming a platinum layer having a fiber structure, the concentration of the platinum salt solution and the concentration of the reducing agent solution are different from those of the formed fiber structure. It is an important parameter that determines form and size. The inventor of the present application considers the reason as follows. First, when the concentration of the reducing agent solution is fixed at 0.10 M and the concentration of the platinum salt solution is set to three conditions of 0.001 M, 0.003 M, and 0.005 M, which method is used for the platinum deposition process. Let's consider whether such a difference occurs.
[0035]
FIG. 11 is a conceptual diagram of such a deposition process. FIGS. 11 (a), (b) and (c) show the deposition process when the concentration of the platinum salt solution is 0.001M, 0.003M and 0.005M, respectively. The concentration of each of the reducing agent solutions is 0.10M. In the figure, white circles indicate platinum ions, black circles indicate platinum particles (atomic groups), and a substrate supporting these white and black circles indicates a film piece of “Nafion”. First, in a permeation process (a process in which a piece of “Nafion” membrane is immersed in a platinum salt solution), platinum ions (open circles) in the platinum salt are adsorbed on the substrate surface. At this time, the density of the platinum ions adsorbed depends on the concentration of the platinum salt solution, and the higher the concentration, the higher the adsorption density. The platinum ions indicated by white circles in the figure are not fixed on the substrate, but are easily separated in this state. The reduction process (the process of immersing a film piece of “Nafion” on which platinum ions are adsorbed in a reducing agent solution) is a process in which platinum ions are reduced and fixed on the substrate surface as platinum particles (black circles). . In the example shown in FIG. 11, the platinum ions (open circles) adsorbed on the substrate are fixed as platinum particles (black circles) by reduction, as shown in the drawing, by a reduction process using a reducing agent solution having a concentration of 0.10 M. become. The platinum particles (black circles) thus fixed remain on the substrate surface even after the rinsing process.
[0036]
Comparing the platinum layers after the reduction and rinsing processes in FIGS. 11 (a), (b) and (c), the densities of the platinum particles are different from each other. It is based on the difference. In order to obtain a platinum layer having a fiber structure, it is necessary to fix platinum particles having a certain density. As described above, when the concentration of the platinum salt solution is set to 0.005M (FIG. 11 ( c) As shown in FIG. 4, a platinum layer having a fiber structure as shown in FIG. 4 was obtained, and good detection sensitivity was obtained.
[0037]
On the other hand, FIG. 12 (a) shows the state after the permeation process when the concentration of the platinum salt solution is 0.005M (same as the left figure in FIG. 11 (c)). (C), (d), and (e) show the processes of the reduction process and the rinsing process when the concentration of the reducing agent solution is 0.01 M, 0.05 M, 0.10 M, and 0.015 M, respectively. I have. As shown in FIG. 12A, at the stage after the permeation process, platinum ions (open circles) are adsorbed on the surface of the base. Since the concentration of the platinum salt solution was 0.005M, the density of the adsorbed platinum ions was considerably high. However, these platinum ions indicated by white circles are not fixed on the substrate, but are easily separated in this state. In the reduction process, a process is performed in which platinum ions are fixed as platinum particles (black circles) on the substrate surface by reducing platinum ions. The progress of the reduction reaction depends on the concentration of the reducing agent solution. It is considered that the higher the value, the more the reduced platinum ions. 12 (b) to 12 (e) show the state immediately after the reduction process. When the concentration of the reducing agent solution is low, only a small part of the platinum ions (open circles) are reduced and fixed as platinum particles (black circles), whereas when the concentration of the reducing agent solution is high, Most of the platinum ions (open circles) are reduced and fixed as platinum particles (black circles).
[0038]
After the subsequent rinsing process, the fixed platinum particles (black circles) remain, but platinum ions (white circles) are washed out and removed. As a result, when the concentration of the reducing agent solution is low, only a few platinum particles remain, while when the concentration of the reducing agent solution is high, sufficient platinum particles remain. However, the fiber structure required for the present invention is not always obtained when the concentration of the reducing agent solution is the highest. Actually, the ideal fiber structure as shown in FIG. 4 was obtained when the concentration of the reducing agent solution was 0.10 M (FIG. 12 (d)). This is not the case (FIG. 12 (e)). When the concentration was set to 0.15M, which is the highest concentration, not a fiber structure but a so-called dumpling-shaped platinum lump was formed, and good detection sensitivity was not obtained. As described above, the higher the concentration of the reducing agent solution, the better it is not, and there is an optimum concentration value for obtaining a fibrous structure. The reason is that, as shown in FIG. 12 (d), in the case of the optimum concentration value (0.10M in this example), some platinum ions remain unreduced and are removed in the rinsing process. In this way, appropriate voids are formed and a fiber structure is obtained. On the other hand, when the concentration value is too high, all the platinum ions are reduced in the reduction process as shown in FIG. It is considered that these particles are formed into platinum particles, and further, these platinum particles form a lump under the influence of some pressure or load, and finally form a cluster-like platinum lump.
[0039]
After all, when the metal layer 11 is formed by precipitating the metal in the solution containing the metal salt, the concentration of the metal salt solution used in the infiltration process and the concentration of the reducing agent solution used in the reduction process are determined by a combination. The shape of the obtained metal layer will be affected. Therefore, in order to obtain a metal layer 11 having an ideal fiber structure for detecting a specific gas G to be detected, a fiber structure having a predetermined size and shape corresponding to the gas G to be detected is formed. It is sufficient to find an optimal combination of the concentration of the metal salt solution used in the permeation process and the concentration of the reducing agent solution used in the reduction process.
[0040]
At present, when methyl mercaptan is used as the detection target gas G, as described above, the concentration of the platinum salt solution (tetraamminedichloroplatinum) is set to 0.001M, and the reducing agent solution (sodium borohydride) is used. It has been found from experiments that the combination is a concentration of 0.01M. At present, no method has been established to theoretically determine the optimum combination of the type and concentration of the metal salt solution and the type and concentration of the reducing agent solution for the specific gas G to be detected. By forming the metal layer 11 having a fiber structure of various sizes and forms according to the combination of the solution and the concentration value, and measuring the detection sensitivity to various gases, it is optimal to detect a specific detection target gas G. It is possible to create the gas permeable membrane 10.
[0041]
Therefore, in this specification, only an example in which methyl mercaptan is used as the detection target gas G is shown, but the present invention is not limited to a gas sensor for detecting methyl mercaptan, and various other The present invention can be used for a gas sensor that detects a gas of the type described above. The gist of the present invention is that the metal layer 11 serving as the inner layer of the gas permeable membrane 10 has a fiber structure including many irregularities and cavities instead of a conventional flat thin film structure. The point is that a high detection sensitivity can be selectively obtained with respect to a specific gas that is adapted, and the present invention can be implemented in various modes without departing from the spirit of the present invention.
[0042]
Further, in the above-described embodiment, the membrane having a two-layer structure including the solid electrolyte layer on the inner side and the metal layer having the fiber structure on the outer side is used as the gas permeable membrane. A film having a three-layer structure in which a metal layer having a structure is formed may be used as the gas permeable film.
[0043]
【The invention's effect】
As described above, according to the gas sensor of the present invention, a specific gas can be selectively detected, and sufficient detection sensitivity can be obtained.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing a basic configuration of a gas sensor according to the present invention (a part is shown as a cross-sectional view, and a part is shown as a block diagram).
FIG. 2 is a configuration diagram showing a gas sensor according to a specific embodiment of the present invention of a type that performs detection by a triode method (a part is shown as a cross-sectional view and a part is shown as a block diagram).
FIG. 3 is a view showing a chemical structure of a polymer film of “Nafion (registered trademark)” constituting the solid electrolyte layer 12 used in the gas sensor according to one embodiment of the present invention.
FIG. 4 is a plan view (a diagram created based on an electron micrograph) of a platinum layer 11 having a fiber structure used in a gas sensor according to an embodiment of the present invention.
FIG. 5 is a graph showing detection characteristics of five types of gases using the gas sensor according to one embodiment of the present invention.
FIG. 6 is a graph showing a flow rate dependency of a detected value of methyl mercaptan using a gas sensor according to an embodiment of the present invention.
FIG. 7 is a graph showing the concentration dependence of the detected value of methyl mercaptan using the gas sensor according to one embodiment of the present invention.
FIG. 8 is a graph showing the detection response of methyl mercaptan using the gas sensor according to one embodiment of the present invention. The upper graph shows a gas concentration of 10 ppm, and the lower graph shows a gas concentration of 1. The results in the case of 0 ppm (the gas flow rate is 200 ml / min) are shown.
FIG. 9 is a graph showing the results of measuring the surface roughness of twelve samples prepared as the metal layers 11 used in the gas sensor according to the present invention.
FIG. 10 is a graph showing the results of measuring the surface area of twelve samples prepared as the metal layers 11 used in the gas sensor according to the present invention.
FIG. 11 is a conceptual diagram of a deposition process for forming a metal layer 11 used for a gas sensor according to the present invention.
FIG. 12 is another conceptual diagram of a deposition process for forming the metal layer 11 used in the gas sensor according to the present invention.
[Explanation of symbols]
10. Gas permeable membrane
11 ... metal layer
12 ... Solid electrolyte layer
20 ... solution storage tank
25… Case
26 ... Lid
30 ... Reaction solution (electrolyte solution)
40 ... Counter electrode
45 Reference electrode
50 ... Detection means
55 ... Potentiometer
60 ... pump
61 ... Gas inlet pipe
G: Detection target gas
L: Room on the left (gas introduction room)
R: Room on the right (solution storage tank)

Claims (6)

A gas permeable membrane having a property that the inside is formed of a solid electrolyte layer and the outside is formed of a metal layer having a fiber structure, and has a property of transmitting a gas to be detected.
A solution storage tank in which the gas permeable membrane is part of a partition,
A reaction solution that is contained in the solution containing tank and has a property of causing an electrochemical reaction with the gas to be detected,
A counter electrode immersed in the reaction solution,
A predetermined detection voltage is applied between the working electrode made of the metal layer and the counter electrode, and at this time, a current flowing between the metal layer and the counter electrode transmitted through the reaction solution is detected. Detecting means;
A gas sensor comprising: The gas sensor according to claim 1,
A gas sensor further comprising a reference electrode immersed in a reaction solution, wherein the detecting means performs current detection using the reference electrode as a reference potential using a potentiostat. The gas sensor according to claim 1 or 2,
A gas sensor characterized in that a membrane in which a metal layer having a fiber structure is formed on both surfaces of a solid electrolyte layer is used as a gas permeable membrane. The gas sensor according to any one of claims 1 to 3,
A gas sensor, wherein an ion-conductive polymer film having a property of transmitting a gas to be detected is used as a solid electrolyte layer constituting a gas permeable film. The gas sensor according to any one of claims 1 to 4,
A gas sensor, wherein a metal fiber structure layer deposited in a solution containing a metal salt is used as a metal layer constituting a gas permeable membrane. The gas sensor according to any one of claims 1 to 5,
A gas sensor, wherein the metal layer is composed of a platinum layer having a fiber structure, the reaction solution is composed of sulfuric acid, and methyl mercaptan is used as a gas to be detected.

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