Gas detection device

Abstract

The present invention relates to a gas detection device used in a high humidity environment such as a kitchen or a ship's kitchen, and provides a gas detection device having excellent moisture resistance and excellent sensitivity. A gas detection device includes a thin film gas sensor, wherein the gas sensor is formed by having a heater portion, a gas detection portion, and a catalyst portion on a substrate, and the heater portion is energized to heat the gas detection portion and the catalyst portion while detecting a detection target gas, wherein the gas detection device uses a gas sensor, wherein the catalyst portion is composed of a catalyst metal having platinum as a main component supported on a carrier having a transition metal oxide as a main component.

Description

Technical Field

[0001] The present invention relates to a gas detection device, comprising a gas sensor, the gas sensor comprising a heater portion, a gas detection portion whose characteristics change by contact with a detection target gas, and a catalyst portion covering at least a portion of the gas detection portion.

[0002] The gas detection device detects the detection target gas while energizing the heater portion to heat the gas detection portion and the catalyst portion. Background Art

[0003] Such gas detection devices are disclosed in Patent Documents 1 and 2.

[0004] The following description will be given using the gas detection devices described in these documents as examples.

[0005] The gas detection device comprises a gas sensor, a heating control unit for heating and driving the gas sensor, and a gas detection unit for detecting changes in characteristics of a gas detection portion. The heating control unit controls the heating from the heater portion, thereby heating the gas detection portion and a catalyst portion provided on its surface side to an appropriate temperature adapted to the type of the gas to be detected, thereby performing gas detection.

[0006] The gases to be detected include combustible gases such as methane (CH4) and propane (C3H8), and reducing gases such as carbon monoxide (CO) and hydrogen (H2).

[0007] When performing gas detection, the heating control unit pulses electricity to the heater part to heat the gas detection part and the catalyst part. Figure 4 (a), (b) and (c) show the heating method by the heating drive signal. Figure 4 (a) and (b) are diagrams showing heating drive signals when the detection target gas is a combustible gas. Figure 4 (c) is a diagram showing the case where the detection target gas is a combustible gas and a reducing gas.

[0008] Depend on Figure 4 (a) It was also found that the energization of the heater portion is composed of a gas detection step Ts for executing energization and a heating suspension step Tr performed immediately after the gas detection step Ts, and gas detection is repeated at a predetermined gas detection cycle Rt.

[0009] The detection of the detection target gas is generally performed immediately before the energization is stopped as indicated by the black circles in these drawings.

[0010] When detecting combustible gas, Figure 4As shown in (a) and (b), high temperature heating (High) is adopted, and the mixed gas that becomes the interfering gas is burned and removed during detection by the catalyst part.

[0011] Representative interfering gases are hydrogen (H2), ethanol (C2H5OH), and carbon monoxide (CO). Based on this function, the catalyst part is also called an oxidation catalyst layer. Under high-temperature heating, it is possible to detect flame-retardant combustible gases (representatively methane) that pass through the catalyst part and reach the gas detection part.

[0012] Figure 4 The heating drive method shown in (c) is a method in which low-temperature heating (Low) is performed immediately after high-temperature heating (High), and reducing gas (representatively carbon monoxide) is detected just before the power supply of low-temperature heating (Low) is stopped.

[0013] In addition, if Figure 4 As shown in (a), the gas detection step Ts is repeatedly performed at a predetermined gas detection cycle Rt, but in the heating pause step Tr during the gas detection step Ts, the energization of the heater portion is stopped (off).

[0014] In the case of a gas sensor that can be driven by pulse heating and has a small heat capacity of the heated part and high heating responsiveness, the power-on in the gas detection step Ts is performed in a manner where the power-on time is set to about 0.05 seconds to 0.5 seconds. By repeatedly performing this pulse power-on with a gas detection cycle of about 20 seconds to 60 seconds via the heating pause step Tr, power-saving driving can be achieved.

[0015] That is, in this example, the heating in the gas detection step Ts is pulse heating, and the pulse heating is repeatedly performed at a predetermined gas detection cycle Rt with the heating pause step Tr interspersed therewith.

[0016] In this case, the time of the heating pause step Tr is absolutely long, but the gas sensor is heated only for a very short time.

[0017] Such a gas sensor can be driven by so-called pulse heating for gas detection due to its low heat capacity, and thus can be a power-saving gas detection device that can use a battery as a power source.

[0018] Patent Document 1 introduces a technology for preventive maintenance of a gas detection device, and Patent Document 2 introduces a thin film gas sensor that suppresses moisture absorption by a gas sensing layer to maintain high sensitivity.

[0019] If the correspondence with the terms used in the background technology introduced above is expressed as (terms in patent document 1, terms in patent document 2), the gas detection part becomes (sensing layer 57, gas sensing layer 5), and the catalyst part becomes (selective combustion layer 58, gas selective combustion layer 5d).

[0020] The technologies disclosed in these patent documents also reveal that in the gas sensor included in such a gas detection device, a sintered material in which palladium (Pd) or platinum (Pt) as a catalyst metal is supported on an alumina (Al2O3) carrier is used as a catalyst site.

[0021] Furthermore, Patent Document 2 proposes a method of solving the problem of aging-related degradation of a gas sensor caused by humidity by adopting a method as described in the specification. Figure 1 The moisture absorption shown inhibits the drive.

[0022] Prior art literature

[0023] Patent Literature

[0024] Patent Document 1: Japanese Patent Application Publication No. 2013-190232;

[0025] Patent document 2: Japanese Patent Application Publication No. 2007-24509. Summary of the invention

[0026] Problems to be solved by the invention

[0027] Furthermore, the inventors of the present invention discovered a gas sensor having reduced methane sensitivity while studying a gas sensor for a gas detection device used in a high-humidity environment such as a kitchen or a ship's galley.

[0028] The technology disclosed in Patent Document 2 proposes a countermeasure that can be adopted when using a gas detection device in such a high humidity environment, but it requires moisture absorption suppression driving in the heating pause step, which does not meet the purpose of obtaining a power-saving gas detection device.

[0029] Furthermore, when considering the sensor sensitivity to the detection target gas, it is preferable that the sensitivity be as high as possible. However, the inventors' research conducted this time has revealed that there is room for improvement in the conventional type of alumina-supported gas sensors.

[0030] The present invention has been proposed in view of the above-mentioned problems, and its object is to provide a gas detection device which is excellent in moisture resistance and sensitivity for use in a high-humidity environment such as a kitchen or a ship's kitchen.

[0031] Another object is to obtain a gas sensor that can be used in such a gas detection device.

[0032] Means for solving problems

[0033] A first characteristic feature of the present invention is that a gas detection device comprises a gas sensor, wherein the gas sensor comprises a heater portion, a gas detection portion whose characteristics change due to contact with a detection target gas, and a catalyst portion covering at least a portion of the gas detection portion.

[0034] The gas detection device detects the detection target gas by energizing the heater portion to heat the gas detection portion and the catalyst portion.

[0035] The catalyst site is composed of a catalyst metal mainly composed of platinum supported on a carrier mainly composed of a transition metal oxide.

[0036] After in-depth research, the inventors found that the main reason for the sensitivity fluctuation over time in the existing gas detection device is the adsorption / accumulation of water in alumina, which is the main component of the carrier of the catalyst part. The interaction between the alumina carrier and water is strong, and not only does the hydroxyl (OH group) adsorption and chemical adsorption of water increase in a short period of time, but also the water molecules (physical adsorption water) that are not completely dispersed during heating accumulate, and gradually the physically adsorbed water reacts with alumina to form hydrates, causing the alumina to deteriorate.

[0037] As a result, the function of the catalyst site for burning reducing gas and other interfering gases and the gas detection function change. In addition, due to the adsorption / accumulation of moisture, the alumina deteriorates, the dispersion state of the catalyst metal such as palladium carried on the surface deteriorates, the surface area of ​​the catalyst metal decreases, and the function as a catalyst site is also deteriorated and cannot be restored (irreversible change).

[0038] Furthermore, the temperature cannot be raised to a temperature required for detecting the detection target gas. Based on these factors, it is estimated that the sensitivity to the detection target gas described above will change over time.

[0039] Such a finding that "the interaction between the catalyst support and water affects the temporal sensitivity fluctuation of methane" is a completely new finding that is not found in conventional findings.

[0040] Based on these new findings, the inventors studied the material of the support of the catalyst site and selected a transition metal oxide as the main component of the support.

[0041] Generally, zirconium oxide, which is a representative example of transition metal oxides, has a smaller specific surface area than alumina and has not been actively used as a carrier for catalyst sites. The specific surface area of ​​alumina is about 120 m 2 / g, while zirconium oxide is about 30m 2 / g, a difference of about 4 times. Conventionally, since the larger the surface area, the larger the area for interaction with gas, when used as a carrier of a catalyst site, it is believed that alumina has high performance as a catalyst site, while zirconia has low performance.

[0042] However, the inventors refuted this previous understanding and used zirconium oxide and titanium oxide as carriers instead of aluminum oxide to conduct experiments to study the influence of moisture in the air (the "high humidity exposure experiment" described later). It can be confirmed that: compared with aluminum oxide, zirconium oxide and titanium oxide are not prone to sensitivity changes even in high humidity. It is also confirmed that: compared with aluminum oxide, zirconium oxide and titanium oxide can suppress the reduction of sensitivity even in a high humidity environment. It is believed that these are caused by the small interaction between zirconium oxide and titanium oxide and water, and the effect is also the same in transition metal oxides that also have a small interaction with water. Subsequently, the present invention using transition metal oxides as carriers of catalyst sites was completed.

[0043] If we further elaborate on the sensor sensitivity, the inventors' research has shown that even when the same catalyst metal is used, the sensitivity can be improved by simply replacing the carrier of the catalyst part from aluminum oxide to a transition metal oxide, and the sensitivity can be further improved by optimizing the catalyst composition. In addition, when zirconium oxide is used as the carrier, platinum as the catalyst metal shows high sensitivity.

[0044] Therefore, according to the present structure, a gas detection device that performs gas detection using a gas sensor that has a heater portion, a gas detection portion, and a catalyst portion, for example, a gas detection device used in a high humidity environment such as a kitchen or a ship kitchen, can be obtained that has excellent moisture resistance and maintains high sensitivity.

[0045] The gas sensor used in this gas detection device has the following structure.

[0046] The device comprises a heater portion, a gas detection portion whose characteristics change by contact with a detection target gas, and a catalyst portion covering at least a portion of the gas detection portion.

[0047] The catalyst site is a catalyst metal mainly composed of platinum supported on a carrier mainly composed of a transition metal oxide.

[0048] Therefore, as described in the second characteristic configuration of the present invention, the transition metal oxide as the main component of the carrier may be either zirconium oxide or titanium oxide, or both thereof.

[0049] A third characteristic configuration of the present invention is that in the catalyst portion, 0.3 mass % or more and 9 mass % or less of platinum is supported on the transition metal oxide as a carrier as the catalyst metal.

[0050] According to this configuration, moisture resistance can be obtained by using transition metal oxide as a carrier, and as described in the later [Methane Sensitivity Experiment], a gas detection device with high sensitivity can be obtained by using platinum as a catalyst metal compared with the case of using aluminum oxide as a carrier. Here, if the supported concentration of platinum is less than 0.3 mass %, sufficient selective oxidation ability cannot be obtained, and if it is higher than 9 mass %, the oxidation ability becomes too high, and even methane will be burned through the catalyst.

[0051] A fourth characteristic configuration of the present invention is that the catalyst metal further contains one or both of palladium and iridium in addition to the platinum as the main component.

[0052] According to this configuration, by mixing palladium or iridium in combination with platinum, selective oxidizing properties can be obtained as a composite, and a gas detection device with excellent sensitivity can be obtained.

[0053] The fifth characteristic configuration of the present invention is that the following steps are repeatedly performed to detect the above-mentioned detection object gas: a gas detection step, in which power is supplied to the above-mentioned heater portion to heat the above-mentioned gas detection portion and the above-mentioned catalyst portion, and the above-mentioned detection object gas is detected at the same time; and a non-detection step, in which the temperature of the above-mentioned gas detection portion and the above-mentioned catalyst portion is formed to be lower than the temperature of the two portions in the above-mentioned gas detection step.

[0054] In the present configuration, the gas detection device repeatedly performs a gas detection step and a non-detection step, but in the non-detection step, a state is formed in which the temperatures of the gas detection portion and the catalyst portion are lower than the temperatures of the two portions in the gas detection step, so that, for example, even if the temperature at which gas detection can be performed is not reached, the gas detection portion and the catalyst portion can be managed at a temperature that can reduce the influence of water.

[0055] Such a non-detection step can be achieved by combining, for example, heating pause (stopping power supply) and heating (power supply) in any manner. Here, if the temperature of the two parts is managed to be in a state that is not easily affected by water as the purpose of the present invention, the formation of hydrates newly discovered by the inventors can be effectively prevented. Therefore, high sensitivity can be maintained for a long life while suppressing power consumption.

[0056] Although this behavior is similar to the previously described hygroscopic suppression drive, in the present invention, since the carrier constituting the catalyst portion has a transition metal oxide as a main component, the degree of heating can be reduced or the heating frequency can be reduced compared to the hygroscopic suppression drive disclosed in the document. As a result, a practical gas detection device can be realized.

[0057] The sixth characteristic configuration of the present invention is to detect the above-mentioned detection object gas by repeatedly performing the following steps: a gas detection step, energizing the above-mentioned heater portion to heat the above-mentioned gas detection portion and the above-mentioned catalyst portion, and detecting the above-mentioned detection object gas at the same time; and a slight heating step, energizing the above-mentioned gas detection portion and the above-mentioned catalyst portion to set the temperature lower than the temperature of the two portions in the above-mentioned gas detection step.

[0058] By interspersing a slight heating step during the gas detection step, it is not necessary to heat the gas detection part and the catalyst part to a temperature at which gas detection can be performed. A heating state that is warmer than normal temperature (for example, around 50°C) can be formed, thereby reducing the effect of water on the catalyst part, which is the subject of the present invention.

[0059] In addition, by reducing the amount of heating, power consumption can also be suppressed to a low level.

[0060] The seventh characteristic configuration of the present invention is to detect the above-mentioned detection object gas by repeatedly performing the following steps: a gas detection step, energizing the above-mentioned heater portion to heat the above-mentioned gas detection portion and the above-mentioned catalyst portion, and detecting the above-mentioned detection object gas at the same time; and a heating pause step, stopping the energization of the above-mentioned heater portion.

[0061] By adopting this configuration, it is possible to perform gas detection with more power saving without heating the gas sensor at unnecessary timing.

[0062] Here, it is understood that the influence of water on the catalyst site is unlikely to occur in the gas detection step. This is because it is understood that in this step, the gas detection site and the catalyst site are sufficiently heated, and water rarely adheres to the catalyst site.

[0063] In contrast, in the heating pause step, the power supply to the heater portion is stopped, and the temperature of each portion is quickly reduced to room temperature. Therefore, for example, in a gas detection device that performs pulse heating to periodically detect gas, the gas sensor can be placed in an environment (especially a temperature and humidity environment) that is generally greatly affected by water as long as it is heated instantaneously. As a result, it is believed that the problem described above is likely to occur over time. This can also be recognized by the fact that the step of performing adsorption suppression driving in the previously introduced patent document 2 is the heating pause step of the present invention.

[0064] However, in the present invention, by making the main component of the support of the catalyst part into transition metal oxide and the main component of the catalyst metal into platinum, there is no influence of water and high-sensitivity gas detection can be performed.

[0065] An eighth characteristic configuration of the present invention is that the heating time in the gas detection step is shorter than the heating stop time in the heating pause step.

[0066] By adopting this configuration, the heating time is shortened, and highly sensitive gas detection is performed while suppressing power consumption.

[0067] The ninth characteristic configuration of the present invention is that the heating in at least the above-mentioned gas detection step is a pulse heating with a heating time set to 0.05 seconds to 0.5 seconds, and at least the basic heating method of repeatedly performing the pulse heating with a gas detection cycle of 20 seconds to 60 seconds through the above-mentioned heating pause step is executed.

[0068] It should be noted that the basic heating method refers to a normal heating method except for the case where the above-mentioned heating conditions are deviated regularly or irregularly for the purpose of detection delay countermeasures, false alarm suppression, fault diagnosis, and performance improvement.

[0069] According to this configuration, gas detection can be performed with further reduced power consumption.

[0070] In the configuration of the present invention, even when methane detection is performed by battery drive, for example, gas detection can be performed satisfactorily within a predetermined period required of the gas detection device.

[0071] A tenth characteristic configuration of the present invention is that, when detecting the detection target gas, a high-temperature heating step is included for heating the gas detection portion and the catalyst portion to a temperature for methane detection.

[0072] According to this configuration, methane detection can be performed with high moisture resistance and high sensitivity, which is very important for leakage detection of town gas (natural gas), which is one of the gases to be detected. BRIEF DESCRIPTION OF THE DRAWINGS

[0073] [ Figure 1 ] is a diagram showing an overview of a gas detection device;

[0074] [ Figure 2 ] is a graph showing the temporal change of methane sensitivity in a high humidity exposure experiment;

[0075] [ Figure 3 ] is a comparison chart showing the sensitivity of gas detection devices using various catalyst metals;

[0076] [ Figure 4 ] is an explanatory diagram showing a method of heating drive;

[0077] [ Figure 5 ] is an explanatory diagram showing another embodiment of the heating drive;

[0078] [ Figure 6 ] is a diagram showing other embodiments of the gas sensor;

[0079] [ Figure 7 ] is a diagram showing other embodiments of the gas sensor. DETAILED DESCRIPTION

[0080] according to Figure 1 A gas detection device 100 according to this embodiment will be described.

[0081] The gas detection device 100 includes a sensor element 20 (an example of a gas sensor), a heating control unit 12 , and a gas detection unit 13 .

[0082] The gas detection device 100 obtains power from the battery 15 when the battery 15 is mounted thereon, and detects the detection target gas.

[0083] The sensor element 20 is a so-called power-saving gas sensor having a diaphragm structure. Figure 1 It can also be known that the sensor element 20 is composed of a heater layer 6 (an example of a heater part), a gas detection layer 10 (an example of a gas detection part) and a catalyst layer 11 (an example of a catalyst part) on a support layer 5 of a diaphragm structure. Therefore, a structure is formed in which the catalyst layer 11 is exposed to the surrounding environment, and the detection target gas reaches the gas detection layer 10 through the catalyst layer 11. The detection target gas that has arrived contacts with the layer 10, causing its characteristics to change. Here, specifically, the characteristics can be listed as resistance value or voltage value.

[0084] The gas detection device 100 uses the heating control unit 12 to energize the heater layer 6 to heat the gas detection layer 10 to an appropriate temperature suitable for the type of the detection target gas. While maintaining the temperature, the detection target gas is detected according to the changes in the characteristics of the gas detection layer 10.

[0085] When performing methane detection, the catalyst layer 11 is heated to a temperature of 300°C or above by the heater layer 6. Figure 4 (a), (b), (c) show High), so that other interfering gases such as reducing gases such as carbon monoxide or hydrogen are burned, and methane with low activity is allowed to permeate and diffuse to reach the gas detection layer 10. As a result, the detection accuracy of methane is improved.

[0086] Incidentally, when carbon monoxide is detected, the catalyst layer 11 is heated at a low temperature to 50°C to 250°C by the heater layer 6 ( Figure 4 (c) shows Low), so that other mixed gases such as reducing gases such as hydrogen are burned. A part of the carbon monoxide is burned, but most of it is allowed to pass through and diffuse to reach the gas detection layer 10. It should be noted that methane, which has low activity in this low temperature zone, is not detected in the gas detection layer 10.

[0087] In other words, the catalyst layer 11 has the function of making the gas detection device 100 have gas selectivity by heating the interfering gas (non-detection target gas) such as hydrogen gas and alcohol (ethanol) gas other than the detection target gas to an appropriate temperature so that it burns without reaching the gas detection layer 10. In addition, by supplying oxygen to the surface of the gas detection layer 10, it also plays a role in improving the sensitivity.

[0088] (Sensor element)

[0089] The sensor element 20 has a diaphragm structure in which the end of the support layer 5 is supported on the silicon substrate 1. The support layer 5 is formed by stacking a thermal oxide film 2, a silicon nitride (Si3N4) film 3, and a silicon oxide (SiO2) film 4 in sequence. In addition, a heater layer 6 is formed on the support layer 5, and an insulating layer 7 is formed by covering the entire heater layer 6. A pair of bonding layers 8 are formed on the insulating layer 7, and an electrode layer (an example of an electrode) 9 is formed on the bonding layer 8. The heater layer 6 generates heat by energizing, thereby heating the gas detection layer 10 and the catalyst layer 11. The sensor element 20 can have a block structure in which each layer is relatively thick, and the heater layer 6 can also serve as an electrode layer. In addition, a so-called bridge structure can be used as a supporting structure.

[0090] A gas detection layer 10 is formed between a pair of electrode layers 9 on the insulating layer 7. The gas detection layer 10 is a semiconductor layer with metal oxide as a main component. In this embodiment, a mixture with tin oxide (SnO2) as a main component is used as the gas detection layer 10. The gas detection layer 10 changes its resistance value by contacting with the detection target gas. The gas detection layer 10 can be 0.2 to 1.6 thick. μ The film may have a thickness of more than 1.6 μ m thickness (thick film).

[0091] A catalyst layer 11 is formed on the gas detection layer 10 in a form covering the gas detection layer 10. The catalyst layer 11 is formed by supporting a catalyst metal on a carrier having a metal oxide as a main component. The catalyst layer 11 is formed by bonding the metal oxides supporting the catalyst metal to each other via a binder.

[0092] As the catalyst metal, a metal that can oxidize and remove interfering gases (reducing gases such as ethanol or hydrogen) that can cause false detection when detecting the target gas is used. As the catalyst metal, palladium, platinum, and iridium (Ir) can be used, but in this embodiment, at least one of palladium, platinum, and iridium is included.

[0093] Alumina has been mainly used as a carrier for supporting the catalyst metal in the past. In the present embodiment, zirconium oxide is used as a material that is less likely to generate hydroxyl groups on the surface than alumina and that can suppress the adsorption and accumulation of moisture in the air in the catalyst layer 11 .

[0094] As a binder for binding the carrier, metal oxide powders such as zirconium oxide, silica powder, silica sol, magnesium oxide, etc. can be used. If used in a trace amount as a binder, alumina powder or alumina sol can also be used within a range that does not hinder the function of the catalyst layer 11.

[0095] The catalyst layer 11 is a zirconium oxide powder (with a particle size of 1 to 10 μ The catalyst layer 11 is formed by printing a printing paste obtained by mixing a binder and an organic solvent, drying it at room temperature, and then firing it at 500°C for 1 hour. The size of the catalyst layer 11 is set to be sufficient to cover the gas detection layer 10. Screen printing in this way will make the thickness thinner. The specific surface area of ​​the zirconium oxide sintered body formed by this operation is about 30m 2 / g or so.

[0096] The above-mentioned catalyst metal, metal oxide as a carrier, and binder may be used alone or in combination of two or more.

[0097] The amount of the catalyst metal contained in the catalyst layer 11 is preferably 0.3 to 9 mass % relative to the total mass of the catalyst metal and the carrier. When two or more metals are used as the catalyst metal, the total mass of the catalyst metal is preferably 0.3 to 9 mass % relative to the total mass of the catalyst metal and the carrier.

[0098] In addition, when only methane detection is performed, the mass of platinum is preferably set to 0.3 mass % or more and 6 mass % or less.

[0099] (Heating control unit)

[0100] Take the gas detection device 100 for detecting methane as an example. As described above, the heating drive signal in the detection is shown in FIG. Figure 4 (a), (b).

[0101] The heating control unit 12 is configured to perform a conduction behavior of energizing the heater layer 6 (the timing of performing the conduction behavior is the gas detection step Ts in the present invention) and a non-energizing behavior of not energizing the heater layer 6 (the timing of performing the non-energizing behavior is the heating pause step Tr in the present invention). The conduction behavior (gas detection step Ts) is repeated with the gas detection cycle Rt. That is, pulse heating is repeated with the gas detection cycle Rt.

[0102] The heating control unit 12 is configured to change the temperature of the heater layer 6 and to be able to heat the heater layer 6 to a set arbitrary temperature.

[0103] Specifically, the heating control unit 12 receives power from the battery 15 and energizes the heater layer 6 of the sensor element 20 to heat the sensor element 20. The heating temperature, that is, the temperature reached by the gas detection layer 10 and the catalyst layer 11, is controlled by changing the voltage applied to the heater layer 6, for example.

[0104] (Gas detection unit)

[0105] The gas detection unit 13 detects the target gas by measuring the change in the characteristics of the gas detection layer 10 at an appropriate time in the gas detection step Ts. In this embodiment, the gas detection unit 13 measures the resistance value of the gas detection layer 10 by measuring the resistance value (an example of the characteristic) between a pair of electrode layers 9, and detects the concentration of the target gas based on the change in the resistance value.

[0106] (Detection of target gas)

[0107] A case where a combustible gas (detection target gas) such as methane or propane is detected using the gas detection device 100 configured as described above will be described.

[0108] The heater layer 6 is energized by the heating control unit 12, and the gas detection layer 10 and the catalyst layer 11 are heated to 300°C to 500°C for methane detection for 0.05 to 0.5 seconds. Figure 4 The gas detection unit 13 (shown as a black circle just before the energization is stopped in (a) and (b)) measures the resistance value of the gas detection layer 10, and detects the concentration of the combustible gas such as methane and propane based on the resistance value.

[0109] Thereafter, the energization of the heater layer 6 is stopped. Therefore, the gas detection step Ts described above becomes a high-temperature heating step.

[0110] During this period, in the catalyst layer 11 that has reached a high temperature, the reducing gas such as carbon monoxide or hydrogen and other mixed gases are burned by the combustion catalyst of the catalyst metal. In addition, inert combustible gases such as methane and propane penetrate the catalyst layer 11 and diffuse, reaching the gas detection layer 10, and react with the metal oxide (tin oxide) of the gas detection layer 10, causing the resistance value to change.

[0111] As described above, the gas detection device 100 is used to detect combustible gas.

[0112] Furthermore, although the gas detection step Ts is repeatedly performed with a gas detection cycle Rt of 20 seconds to 60 seconds, after the gas detection step Ts, the power supply is stopped (off) as described above (heating suspension step Tr).

[0113] [High humidity exposure test]

[0114] In order to examine the influence of the type of support material on the temporal variation of sensor sensitivity, samples in which only the type of support was changed were prepared, and the temporal variation of sensor sensitivity (methane sensitivity) was measured.

[0115] The following three samples were measured.

[0116] (High temperature exposure test example 1)

[0117] A sample in which 5 mass % of platinum (Pt) and 2 mass % of iridium (Ir) are supported on zirconium oxide (ZrO 2 ) as a carrier.

[0118] (High temperature exposure test example 2)

[0119] A sample in which 5% by mass of platinum (Pt) and 2% by mass of iridium (Ir) are supported on titanium oxide (TiO 2 ) as a carrier.

[0120] (High temperature exposure test example 3)

[0121] A sample in which 5 mass % of platinum (Pt) and 2 mass % of iridium (Ir) are supported on alumina (Al 2 O 3 ) as a carrier.

[0122] Figure 2 The methane sensitivity of the sample subjected to the 50°C, 60%RH exposure test (RCh4 / Rair, obtained by dividing the resistance value RCh4 in 3000ppm methane gas heated at 400°C by the resistance value Rair in air heated at 400°C) changes over time. It should be noted that the methane sensitivity was measured in 20°C, 65%RH clean air.

[0123] Gas detection is performed by repeating the pulse heating described above with a gas detection period Rt (the same is true in the methane sensitivity experiment described later).

[0124] like Figure 2 As shown, in high humidity exposure test examples 1 and 2 where the support was zirconium oxide or titanium oxide, the methane sensitivity did not change over time. In contrast, in high humidity exposure test example 3 (where the support was alumina), the methane sensitivity decreased over time.

[0125] The inventors presume that the main reason is as follows.

[0126] The interaction between the catalyst layer 11 and water includes the following three stages.

[0127] Stage (1): OH group adsorption in the short term, increase in chemically adsorbed water;

[0128] Stage (2): accumulation of water molecules that were not completely dispersed during heating (physical adsorbed water); and

[0129] Stage (3): Adsorbed water reacts with the bulk (carrier) to form hydrates.

[0130] When the heating pause step is not included, the process proceeds from step (1) to (3), and when the heating pause step is included, the process proceeds from step (2) to (3). Therefore, when SiO2 or Al2O3, which has a strong interaction with water, is used as a carrier, (1) is likely to occur in high humidity when the heating pause step is not included, and (2) is likely to occur in high humidity when the heating pause step is included in the method according to the present embodiment, and the process shifts to (3) over time, causing the sensitivity of the gas to be detected to change.

[0131] In this regard, when zirconium oxide or titanium oxide, which has almost no interaction with water, is used as a carrier, (1) is not likely to occur even in high humidity when no heating pause step is included, and (2) rarely occurs even in high humidity when a heating pause step is included. Therefore, hydrates are not formed over time, and there is no change in sensitivity. In addition, gas sensitivity does not depend on humidity.

[0132] [Methane sensitivity experiment]

[0133] In order to compare the methane sensitivity when the type of carrier and the type / amount of catalyst metal were different, 19 samples were prepared in which the type / amount of catalyst metal was mainly changed, and the methane sensitivity under normal environment was measured.

[0134] The methane sensitivity is the same as described in the previous [High Humidity Exposure Test] except for the environmental conditions. That is, the methane sensitivity is RCh4 / Rair, which is obtained by dividing the resistance value RCh4 in 3000ppm methane gas heated at 400℃ by the resistance value Rair in air heated at 400℃.

[0135] The high temperature exposure examples 1 and 2 used in the previous high humidity exposure experiment are samples 16 and 18.

[0136] The samples to be measured are summarized as follows.

[0137] 1. Types of vectors

[0138] Samples 1 to 17: zirconium oxide;

[0139] 18 Titanium oxide;

[0140] 19 Alumina.

[0141] 2. Catalyst Metals

[0142] The catalyst metals to be studied were three types: palladium (Pd), iridium (Ir), and platinum (Pt).

[0143] The supported amounts (mass %) of the catalyst metals in Samples 1 to 19 are shown in Tables 1, 2, 3, and 4 below. Columns without description indicate that no catalyst metal is supported.

[0144] [Table 1]

[0145]

[0146] [Table 2]

[0147]

[0148] [Table 3]

[0149]

[0150] [Table 4]

[0151]

[0152] Further in Figure 3 The methane sensitivity of each sample is shown in FIG.

[0153] The upper part of the figure shows the sample number, and the lower part of the figure shows the type and concentration (mass %) of the metal oxidation catalyst.

[0154] As a result, compared with Sample 19 (alumina support / 7 mass % palladium) corresponding to the prior art, Sample 17 in which only the support was changed to zirconia had improved methane sensitivity.

[0155] Of the two samples, Sample 17 also showed a preferred result of no change in sensitivity in the high humidity exposure test which was also conducted separately.

[0156] Furthermore, among the samples (1 to 17) using zirconium oxide as a support as a comparative object, all the samples showed higher sensitivity to methane than the sample 19.

[0157] In addition, Sample 18, which is a comparative object and uses titanium oxide as a carrier, is more sensitive to methane than Sample 19.

[0158] Compared with the high temperature exposure test example 3 (alumina support / 5 mass % platinum, 2 mass % iridium) described above, samples 1 to 18 showed methane sensitivity higher than that of the high temperature exposure test example 3.

[0159] Among the samples studied, samples (8 and 13) using zirconium oxide as a carrier and containing platinum as the catalyst metal alone showed particularly high methane sensitivity.

[0160] As a result, it was found that a combination of platinum alone as a catalyst metal and zirconium oxide as a carrier is particularly suitable for a gas sensor.

[0161] In addition, if sample 8 and sample 13 are compared, the methane sensitivity of sample 13 with a high platinum concentration is improved, and the higher the platinum concentration, the higher the methane sensitivity. On the other hand, if sample 15 and sample 16 are compared, the higher the platinum concentration is, the lower the methane sensitivity is for the same iridium concentration. From the above results, it can be seen that up to a certain degree of platinum concentration, the higher the platinum concentration, the higher the methane sensitivity is, but if it exceeds a certain concentration, the methane sensitivity decreases instead. In order to obtain high methane sensitivity, there is an appropriate concentration range. It should be noted that the reason why the methane sensitivity is reduced when it exceeds a certain concentration is that the oxidation activity of platinum is increased, and even methane will be burned and oxidized in the catalyst layer. From the above results, it is believed that the concentration of platinum is more than 0.3 mass% and less than 9 mass%, and it is hoped that more than 0.3 mass% and less than 6 mass% is suitable.

[0162] [Other embodiments]

[0163] (1) In the above-mentioned experiments, the significance of the present invention is explained by using the experimental example of methane as an example of combustible gas. As described above, low-carbon hydrocarbon gases such as propane can be detected using the gas detection device involved in the present invention.

[0164] (2) In the above experiment, an example of a catalyst metal-supporting carrier composed of zirconium oxide and titanium oxide was shown, but as described in the previous paragraph

[0034] , since the interaction between transition metal oxides and water is small, transition metal oxides can be used as the catalyst metal-supporting carrier.

[0165] (3) In the case where the carrier is composed of a transition metal oxide, in the above-mentioned experiments, examples of only zirconium oxide or titanium oxide are shown, but the carrier at the catalyst site may be composed of a transition metal oxide as a main component. Here, the main component means 50% by mass or more (when composed of multiple transition metal oxides, the total mass is 50% by mass or more).

[0166] In the present invention, the catalyst metal preferably contains platinum as a main component and the mass % relative to the total mass of the catalyst metal and the carrier is set to 0.3 to 9 mass % in terms of catalyst concentration.

[0167] Here, containing platinum as the main component means that platinum is supported within the above range, and when other metal oxidation catalysts (one or more selected from palladium and iridium) are included, the amount thereof is smaller than the amount of platinum.

[0168] (4) In the previous description, the Figure 4 The heating drive signals shown in (a) and (b) are mainly used to describe the detection of methane. Figure 4 As shown in (c), methane detection may be followed by carbon monoxide detection, or methane and carbon monoxide may be detected alternately. In the detection of carbon monoxide, the gas detection layer 10 and the catalyst layer 11 are heated to 50°C to 250°C. During this period (specifically, Figure 4 The gas detection unit 13 measures the resistance value of the gas detection layer 10 (shown as a black circle in (c) immediately before the energization is stopped), and the concentration of carbon monoxide can be detected based on the resistance value.

[0169] exist Figure 4 In (c), the carbon monoxide detection is performed immediately after the methane detection, but a heating pause step of stopping the power supply to the heater portion may be provided between the two detections.

[0170] (5) In the above-described embodiment, the energization of the heater portion in the gas detection step Ts is pulse energization with a energization time of 0.05 to 0.5 seconds, and the case where this pulse energization is repeated with a gas detection cycle Rt of 20 to 60 seconds via the heating pause step Tr is described.

[0171] This energization method is a method of repeatedly performing so-called pulse heating at a predetermined gas detection cycle Rt, and is an example of a basic energization method in normal times as described above.

[0172] Therefore, when it is possible to detect methane, for example, while executing the basic energization method, the pulse energization (pulse heating) cycle, that is, the gas detection cycle, can be set to an arbitrarily short cycle such as a cycle of 5 to 10 seconds.

[0173] On the other hand, as for the relationship between the heating time in the gas detection step and the heating stop time in the heating pause step, as described above, the former is preferably shorter than the latter in terms of power saving.

[0174] (6) In the above-mentioned embodiment, an example is shown in which the power supply to the heater part in the gas detection step Ts is a pulse power supply with the power supply time set to 0.05 seconds to 0.5 seconds, but as the pulse power supply time, when the gas detection cycle Rt is 20 seconds to 60 seconds, the power supply time can be set to less than 5 seconds.

[0175] (7) Furthermore, in the embodiment shown above, an example is shown in which the heating suspension step Tr of suspending heating is performed after the gas detection step Ts.

[0176] However, according to the gist of the present invention, it is preferable in terms of moisture resistance to energize the heater portion in a time period corresponding to the above-mentioned heating pause step Tr to heat the gas detection portion and the catalyst portion.

[0177] Therefore, during the gas detection step of detecting the detection target gas while energizing the above-mentioned heater portion to heat the gas detection portion and the catalyst portion, it can be first heated to a temperature lower than the temperature reached by the gas detection portion and the catalyst portion in the gas detection step (for example, in the case of only methane detection, it refers to a temperature lower than 100°C and higher than normal temperature, and in the case of accompanied carbon monoxide detection, it refers to a temperature lower than its carbon monoxide detection temperature, and is a temperature lower than 100°C and higher than normal temperature (in the case of carbon monoxide detection at 100°C, it refers to a temperature of about 50°C)), and a slight heating step Trh is performed. Figure 5 An example of performing such a slight heating step is shown. Figure 5 (a) corresponds to Figure 4 (a) shows that the slight heating step Trh is performed immediately after the gas detection step Ts. In the slight heating step Trh, a certain degree of heating is performed by conducting a certain degree of energization. The example shown in the figure is that the slight heating step Trh is combined immediately after the gas detection step Ts, and the gas detection cycle Rt is established. Figure 5(b) and (c) are examples of methane detection, (b) is an example of making the slight heating the constant heating state shown in (a), and (c) is slight heating by pausing heating (stopping power) and heating (powering).

[0178] That is, in a configuration in which a non-detection step without gas detection is performed after a gas detection step Ts for gas detection, the non-detection step can also be set as a heating pause step Tr or a slight heating step Trh, but in the non-detection step, the heating pause (stopping power) and heating (powering) are combined in any manner, and in the non-detection step, a state can be formed in which the temperature of the gas detection part and the catalyst part is lower than the temperature of the two parts in the gas detection step. In this case, if the temperature of the two parts is maintained at a temperature that is not easily affected by water, the formation of hydrates can be hindered. The combination of heating pause (stopping power) and heating (powering) here naturally includes the selection of the timing of the combination and any one or more of the selection of the amount of power. The temperature can change over time.

[0179] Furthermore, the temperature management in the non-detection step may be performed by any means, for example, a means other than energizing the heater portion may be used.

[0180] (8) In the above-mentioned embodiment, the gas sensor (gas detection element 20) constituting a part of the gas detection device 100 has the following structure: Figure 1 The so-called substrate type is shown, but other structures may be used. For example, the insulating layer 7 covering the heater layer 6 may not be provided, and the heater layer 6 may also serve as the electrode layer 9.

[0181] In addition, for example Figure 6 As shown, as a gas sensor 20a, the following structure can also be formed: a gas detection part 23 composed of an oxide semiconductor is formed around a coil 22 of an electrode wire 21 having both an electrode and a heater part, and a catalyst layer (catalyst part) 24, 25 is formed around it. Here, the catalyst layer is set to two layers, but it can also be a single layer. In the case of setting it to two layers, the ratio of the catalyst metal can be changed between the layers. In this case, using platinum as the main component means that the amount of platinum in at least one layer is within the above-mentioned amount range and is greater than the amount of other catalyst metals.

[0182] In addition, if Figure 7 As shown, as a gas sensor 20b, the following structure can also be formed: another electrode 33 is arranged at the center of the coil 32 of the electrode wire 31 which has both an electrode and a heater part, a gas detection part 34 composed of an oxide semiconductor is formed around the coil 32, and a catalyst layer 35 is formed around it.

[0183] It should be noted that the gas detection device 100 can be any gas detection device as follows: it has a gas sensor, which has a heater portion, a gas detection portion whose characteristics change due to contact with the detection target gas, and a catalyst portion covering at least a portion of the gas detection portion, and detects the detection target gas while energizing the heater portion to heat the gas detection portion and the catalyst portion, and is not limited to the embodiments described previously.

[0184] (9) Furthermore, a catalyst portion containing a catalyst metal mainly composed of platinum is supported on a carrier containing such a transition metal oxide, and is provided so as to cover at least a portion of the gas detection portion. This is because it is believed that by providing such a catalyst portion, selective combustion of the interfering gas can be performed.

[0185] Explanation of symbols

[0186] 5: Support layer (substrate);

[0187] 6: Heater layer (heater part);

[0188] 9: electrode layer (electrode);

[0189] 10: Gas detection layer (gas detection part);

[0190] 11: catalyst layer (catalyst part);

[0191] 12: Heating control unit;

[0192] 13: Gas detection unit;

[0193] 15: Battery (power source);

[0194] 20: sensor element (gas sensor);

[0195] 100: Gas detection device.

Claims

1. A gas detection device comprising a gas sensor, the gas sensor comprising a heater portion, a gas detection portion whose characteristics change by contact with a detection target gas, and a catalyst portion provided to cover at least a portion of the gas detection portion, The gas detection device detects the detection target gas by energizing the heater portion to heat the gas detection portion and the catalyst portion. The thickness of the gas detection part is 0.2 to 1.6 μm. The catalyst site is composed of a catalyst metal mainly composed of platinum supported on a carrier mainly composed of a transition metal oxide. The transition metal oxide is zirconium oxide, and the particle size of the zirconium oxide is 1 to 10 μm. As the catalyst metal, in addition to the above-mentioned platinum, iridium is also supported. The gas detection device detects the detection target gas by repeatedly performing the following steps: a gas detection step, energizing the heater portion to heat the gas detection portion and the catalyst portion while detecting the detection target gas; and a heating pause step, stopping energizing the heater portion.

2. The gas detection device according to claim 1, wherein: The catalyst site is formed by supporting 0.3 mass % or more and 9 mass % or less of platinum as the catalyst metal on the carrier.

3. The gas detection device according to claim 2, wherein: The catalyst metal includes palladium in addition to the platinum and iridium.

4. The gas detection device according to claim 1, wherein: The heating time in the gas detection step is shorter than the heating stop time in the heating pause step.

5. The gas detection device according to claim 1, wherein: At least the heating in the above-mentioned gas detection step is pulse heating with a heating time set to 0.05 seconds to 0.5 seconds, and at least the following basic heating method is performed: that is, the pulse heating is repeatedly performed with a gas detection cycle of 20 seconds to 60 seconds through the above-mentioned heating pause step.

6. The gas detection device according to any one of claims 1 to 3, wherein: When the detection target gas is detected, a high temperature heating step of heating the gas detection portion and the catalyst portion to a temperature for methane detection is included.

7. A gas sensor used in the gas detection device according to any one of claims 1 to 3, the gas sensor comprising a heater portion, a gas detection portion whose characteristics change by contact with a detection target gas, and a catalyst portion covering at least a portion of the gas detection portion, The thickness of the gas detection part is 0.2 to 1.6 μm, and the catalyst part is composed of platinum as a main component and iridium as a catalyst metal other than the platinum as a main component supported on a carrier containing zirconium oxide with a particle size of 1 to 10 μm.

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