JP6472168B2 - Contact combustion type gas sensor - Google Patents

Description

  The present invention relates to a catalytic combustion type gas sensor including a detection element that is sensitive to a gas to be detected.

  In the catalytic combustion type gas detection element, a combustion catalyst portion as a gas sensitive portion in which a noble metal catalyst such as platinum is supported on a sintered metal oxide such as alumina is provided on a noble metal wire such as platinum. The detected gas to be detected in the combustion catalyst unit is brought into contact with the noble metal catalyst and burned, and a temperature change that occurs during combustion is detected as a change in the resistance value of the noble metal wire. Since the combustion heat of the gas to be detected is proportional to the concentration of the gas to be detected and the resistance value of the noble metal wire is proportional to the heat of combustion, the gas to be detected is measured by measuring the change in resistance of the noble metal wire due to the combustion of the gas to be detected. Concentration can be measured.

  In addition, since the contact combustion type | formula gas detection element used as the prior art in this invention is a general technique, prior art documents, such as a patent document, are not shown.

  As described above, the conventional catalytic combustion type gas detection element detects a temperature change (change in the resistance value of the noble metal wire) in the gas sensitive part due to the combustion of the gas to be detected, so that the activity of the noble metal catalyst is reduced. This makes it easy to deteriorate the gas sensitive part.

  Accordingly, an object of the present invention is to provide a catalytic combustion type gas detection element in which the durability of the gas sensitive part is improved.

In order to achieve the above object, a catalytic combustion type gas sensor according to the present invention is a catalytic combustion type gas sensor having a detection element that is sensitive to a gas to be detected. The first characteristic configuration of the catalytic combustion type gas sensor is that the detection element is a noble metal wire. A gas-sensitive portion that contacts the gas to be detected, the gas-sensitive portion including an outer layer including a first catalyst carrier that supports a first catalyst of a noble metal having an activity with respect to the gas to be detected; It is in the point comprised by the two-layer structure of the inner layer provided with the 2nd catalyst support | carrier which carry | supports the 2nd catalyst of the noble metal which has higher activity than said 1st catalyst with respect to detection gas.

In this configuration, the gas sensitive part has a two-layer structure of an outer layer and an inner layer. According to this configuration, for a while after starting the catalytic combustion type gas sensor (for example, the time until the first catalyst of the outer layer deteriorates), the gas to be detected diffuses in the outer layer constituting the vicinity of the surface of the gas sensitive portion. . In other words, the first catalyst is heated in the outer layer and undergoes a combustion reaction with the gas to be detected. Since the first catalyst has a lower activity (than the second catalyst), the combustion reaction of the detected gas is not so sensitive. On the other hand, it is considered that an interference gas such as organic silicone gas and sulfur existing in the atmosphere is usually more likely to undergo a combustion reaction in the first catalyst than the detected gas. Therefore, most of the interference gas is removed by burning in the outer layer.

  When a certain amount of time has elapsed after starting the catalytic combustion type gas sensor (for example, a time to the extent that the first catalyst in the outer layer deteriorates), the detected gas passes through the outer layer and reaches the inner layer. The detected gas that reaches the inner layer is the detected gas that has not reacted in the outer layer. In the inner layer, the second catalyst is heated and undergoes a combustion reaction with the gas to be detected. Since the second catalyst has higher activity than the first catalyst, it reacts with the detected gas with high sensitivity. Further, since most of the interference gas burns in the outer layer, it is difficult to reach the inner layer, and the second catalyst is hardly affected by the interference gas.

  Therefore, at the initial stage when the catalytic combustion type gas sensor is activated, the combustion reaction of the gas to be detected can be performed in the outer layer, and the interference gas can be removed. Thereafter, only the combustion reaction of the detected gas can be performed in the inner layer. Therefore, since the deterioration of the second catalyst in the inner layer can be suppressed, the durability of the gas sensitive part can be improved.

  In addition, with the passage of time, the outer layer first catalyst formed on the surface of the gas sensitive portion deteriorates, but even if the first catalyst deteriorates, the particle structure of the outer layer is maintained. Therefore, since the outer layer can serve as a filter layer for the interference gas, the second catalyst in the inner layer is hardly affected by the interference gas even when considering a long span. Since the deterioration of the two catalysts can be suppressed over a long period of time, the durability of the gas sensitive part can be further improved.

  Even if the first catalyst of the outer layer deteriorates, the inner layer includes the second catalyst having high activity, so that the combustion reaction of the detected gas can be performed efficiently.

  Accordingly, the catalytic combustion type gas sensor of the present invention has a gas sensor that has an outer layer and an inner layer as the detection element, and has a stable sensitivity to the gas to be detected in a state where the influence of the interference gas is reduced. It becomes.

  The second characteristic configuration of the catalytic combustion type gas sensor according to the present invention is that the average particle size of the outer layer is 20 to 100 nm and the average particle size of the inner layer is 200 to 1000 nm.

  According to this configuration, the outer layer is a fine layer having a fine particle size, and the inner layer is a porous structure layer formed of a material having a large particle size, so that the gas diffusing speed is different between the outer layer and the inner layer. Can be controlled. Therefore, for example, the interference gas diffuses gently in the outer layer, and most of the interference gas can be reliably burned in the outer layer, so that the interference gas can be reliably removed. In addition, since the detection gas diffuses in the inner layer at a relatively high speed, the inner layer can be efficiently subjected to a combustion reaction with the detection gas.

It is the schematic which shows a gas detection element. It is the schematic of a bridge circuit. It is a principal part schematic of a gas sensitive part. It is a principal part schematic of a gas sensitive part. It is the graph which showed the result of having investigated the gas sensitivity characteristic with respect to methane gas. It is the graph which showed the result of having calculated the degradation rate of the detection element.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIGS. 1 and 2, the catalytic combustion type gas sensor X of the present invention includes a detection element 10 that is sensitive to a gas to be detected (a combustible gas such as hydrogen gas or methane), a change in environment, and the like. A temperature compensation element 20 for correcting a change in the resistance value of the detection element 10 based on a temperature change other than combustion, and fixed resistors R1 and R2 are incorporated in a bridge circuit. The bridge circuit constantly supplies a current of about 90 to 120 mA by the power source E, and maintains the detection element 10 at a temperature at which the gas to be detected is easy to contact and burn.

  The detection element 10 has a gas sensitive part 12 that covers the coiled noble metal wire 11 and is in contact with the gas to be detected. When the detection element 10 is placed in the gas to be detected, heat is generated by energization, whereby the catalyst included therein is heated to react with the gas to be detected, and according to the reaction heat (according to the concentration of the gas to be detected). The output value changes. In the noble metal wire 11, the material, the wire diameter, the coil diameter, the number of coil turns, and the like are the same as those used for the conventional catalytic combustion type gas detection element, and are not particularly limited. Platinum or the like can be used as the material of the noble metal wire 11.

  The gas sensitive part 12 includes an outer layer 12A including a first catalyst carrier that supports a first catalyst of a noble metal having an activity with respect to a detected gas, and a noble metal having a higher activity with respect to the detected gas than the first catalyst. And an inner layer 12B provided with a second catalyst carrier for supporting the second catalyst.

  The first catalyst may be a noble metal catalyst having activity with respect to the gas to be detected, and platinum (Pt), platinum-palladium (PtPd), or the like can be used. The first catalyst carrier is not particularly limited as long as it supports the first catalyst. For example, a metal oxide sintered body such as alumina or silica alumina can be applied.

  The second catalyst may be a noble metal having higher activity than the first catalyst with respect to the gas to be detected. For example, when the first catalyst is platinum or platinum / palladium, the second catalyst is palladium (Pd). Can do. The second catalyst carrier is not particularly limited as long as it supports the second catalyst. For example, a metal oxide sintered body such as alumina or silica alumina can be used.

  Further, as described above, even when the first catalyst and the second catalyst are not different types of precious metals but are the same type of precious metals, the supported amounts are different (the first catalyst supported amount <the second catalyst). The activity can be made different (the activity of the first catalyst <the activity of the second catalyst).

  Moreover, in the gas sensitive part 12, the average particle diameter of the outer layer 12A is 20 to 100 nm, and the average particle diameter of the inner layer 12B is 200 to 1000 nm. That is, in this configuration, the outer layer 12A is a dense layer using a material having a small particle diameter, and the inner layer 12B is a porous structure having a larger gap between particles than the outer layer 12A using a material having a large particle diameter. It has a structure for controlling gas diffusion in each layer in the gas sensitive part 12.

The average particle diameter of each layer can be controlled by changing the temperature at which sintering is performed in a state where a noble metal catalyst is mixed with a metal oxide sintered body.
In order to set the average particle size of the outer layer 12A to 20 to 100 nm, the first catalyst carrier supporting the first catalyst may be fired at about 1100 to 1150 ° C. Moreover, what is necessary is just to calcinate the 2nd catalyst support | carrier which carry | supports a 2nd catalyst at about 1175-1500 degreeC in order to make the average particle diameter of the inner layer 12B 200-1000 nm.

  In this configuration, the gas to be detected (methane gas) constitutes the vicinity of the surface of the gas sensitive portion 12 for a while (for example, the time until the first catalyst of the outer layer 12A deteriorates) after starting the catalytic combustion type gas sensor X. The outer layer 12A is diffused (FIG. 3). That is, in the outer layer 12A, the first catalyst is heated and undergoes a combustion reaction with the gas to be detected. At this time, since the outer layer 12A is a dense layer having a fine particle size, the speed at which the gas to be detected diffuses in the outer layer 12A is relatively slow. Since the first catalyst has a lower activity (than the second catalyst), the combustion reaction of the detected gas is not so sensitive. On the other hand, it is considered that an interference gas such as organic silicone gas and sulfur existing in the atmosphere is usually more likely to undergo a combustion reaction in the first catalyst than the detected gas. Therefore, the interference gas diffuses gently in the outer layer 12A, and most of the interference gas is burned and removed in the outer layer 12A.

  When a certain amount of time (for example, a time when the first catalyst of the outer layer 12A deteriorates) elapses after the catalytic combustion type gas sensor X is activated, the detected gas passes through the outer layer 12A and reaches the inner layer 12B. The detected gas that reaches the inner layer 12B is the detected gas that has not reacted in the outer layer 12A. In the inner layer 12B, the second catalyst is heated and undergoes a combustion reaction with the gas to be detected. Since the second catalyst has higher activity than the first catalyst, it reacts with the detected gas with high sensitivity. Further, since most of the interference gas burns in the outer layer 12A, it becomes difficult to reach the inner layer 12B, and the second catalyst is hardly affected by the interference gas. Since the inner layer 12B is a layer having a porous structure formed of a material having a large particle size, the speed at which the gas to be detected diffuses in the inner layer 12B is relatively fast. Therefore, the inner layer 12B can be efficiently combusted with the detected gas.

  Therefore, at the initial stage when the catalytic combustion type gas sensor X is activated, the combustion reaction of the gas to be detected can be performed in the outer layer 12A and the interference gas can be removed. Thereafter, only the combustion reaction of the gas to be detected can be performed in the inner layer 12B. Therefore, since the deterioration of the second catalyst of the inner layer 12B can be suppressed, the durability of the gas sensitive part can be improved.

  In addition, over time, the first catalyst of the outer layer 12A formed on the surface of the gas sensitive portion 12 deteriorates (FIG. 4). Even if the first catalyst deteriorates, the fine particle structure of the outer layer 12A Is maintained. Therefore, since the outer layer 12A can serve as a filter layer for the interference gas, the second catalyst in the inner layer 12B is hardly affected by the interference gas even when a long-term span is considered. Since the deterioration of the second catalyst can be suppressed over a long period of time, the durability of the gas sensitive part can be further improved.

  Even if the first catalyst of the outer layer 12A is deteriorated, the inner layer 12B is provided with a highly active second catalyst, so that the combustion reaction of the detected gas can be performed efficiently.

  Therefore, the catalytic combustion type gas sensor X of the present invention is stable with respect to the gas to be detected in a state where the influence of the interference gas is reduced because the detection element 10 has the gas sensitive part 12 in which the outer layer 12A and the inner layer 12B are formed. The sensitivity is as follows.

The temperature compensation element 20 is an element for performing temperature compensation of the detection element 10 by being placed in a flammable gas and being energized in the same manner as the detection element 10. It is used to extract only the change of the corresponding output value.
The temperature compensation element 20 is formed, for example, by coating the surface of a noble metal wire equivalent to the detection element 10 with a metal oxide sintered body such as alumina. Since the temperature compensation element 20 does not have a catalyst and combustion of the combustible gas due to the catalytic reaction does not occur, the temperature compensation element 20 is inactive to the gas to be detected. The temperature compensation element 20 generates heat when energized and heats a sintered metal oxide such as alumina covering the periphery thereof, and its resistance value changes due to heat.

  Usually, the contact combustion type gas sensor X includes a detection element 10 that has become high temperature due to heat generated by a combustion reaction that occurs when a combustible gas contacts the catalyst of the detection element 10, and a detection element in which a combustion reaction due to the gas to be detected does not occur. By utilizing the fact that a difference in electrical resistance value occurs between the temperature compensation element 20 and a temperature lower than 10, it is possible to detect the concentration of the combustible gas by offsetting the change in the electrical resistance value due to the ambient temperature.

  The temperature compensation element 20 is used to cancel the influence of changes in the ambient temperature. That is, the temperature change due to the reaction of the sensor element with the gas is as small as several tens of degrees Celsius, and if the ambient temperature changes in the range of 0 ° C. to 40 ° C., for example, the sensor output change due to the reaction with the gas Since the output change cannot be distinguished, the temperature compensation element 20 having the same resistance value as the detection element 10 and having the same resistance change with respect to the ambient temperature is connected in parallel with the detection element 10. The influence of the ambient temperature does not appear on the detection voltage.

[Example 1]
Examples of the present invention will be described.
The detection element 10 in the catalytic combustion type gas sensor X of the present invention was produced by a conventionally known method. That is, platinum or a platinum alloy processed into a coil shape as the noble metal wire 11, 10 wt% of palladium (second catalyst) is supported on alumina (second catalyst carrier) as the inner layer 12 B, and alumina (first catalyst) as the outer layer 12 A. The gas sensitive part 12 carrying 10 wt% of platinum or platinum / palladium (first catalyst) on the carrier) is made to have a substantially spherical shape with an element diameter of about 0.2 mm (the particle diameter of the inner layer 12B is 0.17 mm). . The outer layer 12A was fired at 1150 ° C. to have an average particle size of 20 to 100 nm, and the inner layer 12B was fired at 1300 ° C. to have an average particle size of 200 to 1000 nm.

The detection element 10 produced in this way was incorporated into a bridge circuit, and the gas sensitivity characteristic with respect to methane gas (25% LEL) was examined. In Invention Example 1, the first catalyst of the outer layer 12A was platinum / palladium, and in Invention Example 2, the first catalyst of the outer layer 12A was platinum.
As a comparative example, a gas sensing part of the detection element having a single layer structure was used. The gas sensitive part of Comparative Example 1 is made by supporting 10 wt% platinum / palladium on alumina (fired at 1150 ° C.), and the gas sensitive part of Comparative Example 2 is made by carrying 10 wt% palladium on alumina (1300 Fired at 0 ° C.).
The results are shown in FIG.

  As a result, the inventive examples 1 and 2 had a gas sensitivity of 7 mV or more after 100 days, and the comparative examples 1 and 2 had a gas sensitivity of 6 mV or less after 100 days. Therefore, the inventive examples 1 and 2 in which the gas sensitive part 12 is formed of two layers are recognized as having gas sensitivity superior to the comparative examples 1 and 2 in which the gas sensitive part 12 is formed of one layer. It was.

  Based on the result of the gas sensitivity, the deterioration rate of the detection element 10 was calculated. The deterioration rate was calculated assuming that the deterioration rate of the gas sensitive part with 0 days elapsed was 0%. The results are shown in FIG.

  As a result, Examples 1 and 2 of the present invention had a deterioration rate of about 45 to 55% after 100 days, and Comparative Examples 1 and 2 had a deterioration rate of about 75 to 95% after 100 days. Therefore, it is recognized that the inventive examples 1 and 2 in which the gas sensitive part 12 is formed of two layers have superior durability than the comparative examples 1 and 2 in which the gas sensitive part 12 is formed of one layer. It was.

  INDUSTRIAL APPLICABILITY The present invention can be used for a catalytic combustion type gas sensor including a detection element that is sensitive to a gas to be detected.

X catalytic combustion type gas sensor 10 sensing element 11 noble metal wire 12 gas sensitive part 12A outer layer 12B inner layer

Claims (2)

In a contact combustion type gas sensor equipped with a detection element that is sensitive to the gas to be detected,
The detection element covers a noble metal wire and has a gas sensitive part that comes into contact with the gas to be detected.
The gas sensitive part includes an outer layer including a first catalyst carrier that supports a first catalyst of a noble metal having activity with respect to a gas to be detected;
A catalytic combustion type gas sensor configured with a two-layer structure including a second catalyst carrier supporting a second catalyst of a noble metal having higher activity than the first catalyst with respect to a detected gas.   2. The catalytic combustion gas sensor according to claim 1, wherein the outer layer has an average particle diameter of 20 to 100 nm and the inner layer has an average particle diameter of 200 to 1000 nm.

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