JPH08201340A - Gas sensor element - Google Patents

Abstract

PURPOSE: To exclude the effect of the interference gas and effectively detect the target gas by applying a catalyst layer selectively oxidizing the interference gas not only to the current collector of a detecting electrode but also to a counter electrode. CONSTITUTION: A detecting electrode 4 is covered with a catalyst layer 5 to selectively oxidize the interference gas into the carbon dioxide having little coherence, and the generation of electromotive force via the reaction with the carbon monoxide gas can be prevented. When only the detecting electrode 4 is covered, an electromotive force difference is generated by the reaction with the carbon monoxide gas on a counter electrode 2, and interference cannot be sufficiently excluded. The effect is large when gold or platinum is used as a current collector 4 in particular, the direction of the electromotive force by the carbon monoxide gas becomes opposite to that for the NO gas, and the situation is deteriorated. The counter electrode 2 is also covered with the oxidation catalyst layer 5, the carbon monoxide gas concentrations on the surfaces of both electrodes 4, 5 are made nearly zero, and the electromotive force difference caused by the carbon monoxide gas is not generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure suitable for a gas sensor element for detecting a target gas mainly in the presence of an interference gas in a combustion exhaust gas. It is intended to provide a high-temperature operation type gas sensor which can be used by using.

[0002]

2. Description of the Related Art As a gas sensor for detecting NOx gas, there are known gas sensors of a concentration cell type and a semiconductor type. However, a semiconductor type sensor has a large influence of an interference gas and is not suitable for high-temperature operation. There are drawbacks. On the other hand, in the case of the concentration cell type, by using nitrate for the sub-electrode portion of the detection electrode, the influence of the interference gas is eliminated, and a good selectivity is obtained. However, nitrates having a high melting point are at most 600
℃, and the practical temperature when used for a sensor is at most 4
At present, the temperature is not higher than 50 ° C., and it cannot be said that the heat resistance is sufficient for use by directly inserting into a high-temperature exhaust gas of an automobile or the like. Further, a reference electrode is necessary for a concentration cell type sensor, and since the reference electrode is usually used by exposing the reference electrode to the atmosphere, there is a trouble in configuration and manufacturing.

Therefore, a pair of electrodes having different structures is provided in contact with the solid electrolyte body without providing a reference electrode, and one electrode is used as a detection electrode using an oxide as an auxiliary electrode, and both electrodes are used as a detection target atmosphere. The present inventors have applied for an invention relating to the structure of a NOx gas sensor element based on this configuration by using an electromotive force generated by a chemical potential difference between both electrodes. 352765
issue). The basic configuration is shown in FIG. 1 (hereinafter referred to as a planar type).

[0004] The planar type uses a simple oxide or perovskite oxide as a sub-electrode material of a detection electrode in contact with a solid electrolyte body of an oxygen ion conductor, and further has a noble metal or a conductive oxide as a current collector thereon. Are laminated. The counter electrode has a different configuration from the detection electrode, and is formed of a noble metal as a current collector. In this case, as the current collector, a noble metal such as gold or platinum is easily formed, and it is preferable because it has oxidation resistance.

However, most of the NOx gas in automobile exhaust gas is NO gas, and it is considered that NO gas acts as a reducing gas at high temperatures in the engine exhaust pipe. In the sensor of this configuration, for NO gas,
It is presumed that the chemical potential caused by the reducing property is picked up. Therefore, it has been found that the interference of the carbon monoxide gas, which is also a reducing gas, is large and poses a practical problem. In other words, there are usually several hundreds of
It contains carbon monoxide gas at ppm level or higher, and cannot be ignored.

On the other hand, in the case of a semiconductor gas sensor or the like, it has already been taken to take measures such as providing a catalyst layer which reacts with an interference gas on the surface of a metal semiconductor which is a gas sensitive body to change it into a harmless gas having no interference. It is implemented and known (for example, JP-A-63-63064 and the like).

However, as can be inferred from the prior art, the present inventors cannot effectively detect a target gas with a planar sensor simply by coating the detection electrode with a catalyst layer that renders the interference gas harmless. It was confirmed.

[0008]

SUMMARY OF THE INVENTION The present invention provides a sensor for effectively detecting a target gas by eliminating the influence of coexisting interference gas, which is a practical problem of the above-mentioned planar type sensor. That is the task.

[0009]

In the planar type sensor having the structure shown in FIG. 1, the sub-electrode material 3 provided on the detection electrode current collector 4 on the ion conductor 1 is mainly oxidized and reduced by NOx gas. It is presumed that a chemical potential due to oxygen ions is generated between the detection electrode current collector 4 and the counter electrode 2 to generate an electromotive force. In other words, when the current collectors of both electrodes are made of the same material, if the current collectors of both electrodes are in contact with one oxygen ion conductor, the potential difference caused by the current collectors Does not occur, the cause of the generation of the electromotive force is mainly the chemical potential difference caused by the sub-electrode material. For this reason, it is common practice in the prior art to coat the sensing electrode with a catalyst layer for eliminating interference gas. However, in the planar type sensor, it is not enough to achieve the intended purpose only by covering the detection electrode current collector with the catalyst layer.
The use of platinum or an alloy thereof was recognized as having a serious problem, and it was found that the intended effect could be obtained by the sensor configuration of the present invention.

FIGS. 3 and 5 show examples of a sensor configuration according to the present invention.
Shown in NO containing carbon monoxide gas as an interference gas below
The sensor according to the present invention for detecting NO gas will be described as an example. That is, in the sensor configuration of the present invention, in the planar type configuration, the catalyst layer 5 for selectively oxidizing the carbon monoxide gas, which is an interference gas, is not coated only on the detection electrode current collector 4 but also on the counter electrode 2. In the case of coating or only the sensing electrode current collector, the contact part between the current collector and the solid electrolyte body should not be completely covered with the catalyst layer and part of the current collector should be left uncovered. There are features.

What should be considered in any of the methods is a difference in chemical potential (more strictly, a difference in activity of a substance related to a difference in oxygen ion concentration) between the NO gas and the interference gas in both electrodes. The NO gas to be detected is not oxidized as much as possible, and the detection electrode is coated by using a catalyst layer carrying an oxidation catalyst that selectively oxidizes carbon monoxide gas, which is an interference gas, and converts it into carbon dioxide with low interference. It can be said that trying to prevent the generation of electromotive force due to the reaction with carbon monoxide gas is a natural measure.

However, it has been found that when only the detection electrode is coated as shown in FIG. 7, an electromotive force difference occurs due to the reaction with the carbon monoxide gas at the counter electrode, so that interference cannot be sufficiently eliminated. In particular, when gold, platinum, and alloys thereof are used as the current collector, the effect is large, and the direction of the electromotive force by the carbon monoxide gas is opposite to that of the NO gas, which worsens the situation. (FIG. 9).

This problem can be solved by making the concentration of carbon monoxide gas equal in both electrodes if it is basically caused by a chemical potential difference caused by carbon monoxide gas in both electrodes.

From this point of view, as a first measure, the present inventors have proposed that the counter electrode is also coated with an oxidation catalyst layer as shown in FIG. By doing so, an attempt was made not to generate an electromotive force difference caused by the carbon monoxide gas.

As a second means, as shown in FIG. 5, as the current collector of the detection electrode, the same material as that of the counter electrode is used, or a material having a similar activity to carbon monoxide gas is used.
While contacting a part of it with the oxygen ion conductor,
It was adopted that this portion was not covered with the oxidation catalyst layer but was exposed. In this case, the concentration of carbon monoxide gas at the surfaces of the two electrodes does not become zero, but since the concentration is the same, the electromotive force caused by the carbon monoxide gas is compensated, and there is no difference between the two electrodes. . The present inventors have confirmed that any of the above-described means is actually effective as described in Examples.

In the sensor configuration according to the present invention, the detection target is NO.
Interference gas is not specified as carbon monoxide gas, but other reducing gas (for example, carbon monoxide) is used as the detection target gas. Similarly, reducing interfering gas (NO, hydrocarbon-based gas, etc.) The present invention is also applicable to a case where a problem occurs, and also applicable to a case where an oxidizing gas (for example, ozone) is to be detected, in which interference of the oxidizing gas is made harmless using a reduction catalyst. That is clear from the above discussion. On the other hand, it is also apparent that the configuration of the present invention can be adopted using various ion conductors as long as it is a planar sensor as the ion conductor.

The catalyst layer is basically required to selectively react the interfering gas to render it harmless. The catalyst layer is not limited to a specific catalyst. An alumina-supported platinum catalyst that selectively oxidizes carbon gas was effective. The gas diffusion resistance of the catalyst layer is required to be sufficiently small from the viewpoint of the function of the present sensor.
８０80%) is sufficient. Further, in practical use, it does not prevent adding a means such as providing a heater for self-heating in the sensor and operating the sensor at an optimum temperature.

The sensor of the present invention provides an effective means for eliminating the influence of an interference gas in a planar gas sensor which does not require a reference pole. Its composition is
Not only is it simple and easy to manufacture, but it is versatile in that it can be applied to various detection target gases. In particular, when the detection target gas is NO gas, it is practically effective to eliminate the most problematic interference of carbon monoxide gas.

In the following, the case where CO coexists as an interference gas
The present invention will be described specifically for NO gas detection. (Example 1) As shown in FIG. 10, a Pt electrode 2 was used as a counter electrode current collector and a chromium oxide was used as a detection electrode sub-electrode 3 on an oxygen ion conductor yttria-stabilized zirconia (YSZ) 1. 5 is installed as shown in FIG. 5, and lead wires 6 are connected to both current collectors. The two current collectors were covered with a catalyst layer 5 as shown in the figure. The catalyst layer was formed by mixing a catalyst in which 5 wt% of platinum was supported on pure alumina powder alone with alumina sol, applied to both electrodes, and fired at a temperature of 650 ° C. FIG. 12 shows the result of measuring the sensor output for NO gas and CO gas by raising the temperature of this sensor to 500 ° C. using an indirectly heated heater. This result indicates that there is no CO gas interference.

Example 2 As shown in FIG. 13, the catalyst layer 5 was formed only on the detection electrode side so as to leave a part of the detection electrode current collector 4. The catalyst layer has the same material and the same forming method as in the first embodiment. FIG. 15 shows the results of measuring the sensor output for NO gas and CO gas as in Example 1. It can be seen that even in this configuration, the influence of the CO gas was eliminated.

[0021]

The gas sensor according to the present invention can efficiently detect only the target gas by eliminating the influence of coexisting interfering gas by a simple method, and also has a detection electrode sub-electrode material and a catalyst used for the detection current collector. By appropriately selecting the layer, the influence of the interference gas can be eliminated and the invention can be applied to a gas sensor for various gases.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating a conventional planar sensor.

FIG. 2 is a cross-sectional view of the planar type sensor of FIG.

FIG. 3 is a diagram schematically showing an example of a sensor according to the present invention.

FIG. 4 is a cross-sectional view of the sensor of FIG.

FIG. 5 schematically shows another example of a sensor according to the present invention.

6 is a cross-sectional view of the sensor of FIG. 5 as viewed from the direction of arrows AA.

FIG. 7 is a diagram schematically showing a sensor in which only a detection electrode current collector is entirely covered;

8 is a sectional view of the sensor of FIG.

FIG. 9 is a graph showing a sensor output of the sensor shown in FIG. 7;

FIG. 10 is a schematic diagram of the sensor shown in the first embodiment.

FIG. 11 is a cross-sectional view taken along line AA of FIG. 10;

FIG. 12 is a graph showing a sensor output of the first embodiment.

FIG. 13 is a schematic diagram of the sensor shown in the second embodiment.

FIG. 14 is a sectional view taken along line AA of FIG. 13;

FIG. 15 is a graph showing a sensor output of the second embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ion conductor 2 Counter electrode 3 Detection sub-electrode 4 Detection electrode current collector 5 Catalyst layer 6 Lead wire

Claims (4)

[Claims] 1. A pair of electrodes comprising a counter electrode and a detection electrode having different materials or configurations are provided in contact with a solid ion conductor, and both electrodes are exposed to the same atmosphere to generate an electromotive force generated between the electrodes. In a sensor for detecting gas, a gas sensor element characterized in that both of the electrodes are completely covered with a catalyst layer for selectively oxidizing or reducing an interference gas, or the catalyst layer is partially covered with the catalyst layer. . 2. The gas sensor element according to claim 1, wherein the ionic conductor is an oxygen ionic conductor, and at least one of the electrodes includes an oxide. 3. The method according to claim 1, wherein the electrodes include platinum, gold, a platinum alloy, and a gold alloy.
The gas sensor element according to any one of the preceding claims. 4. The gas sensor element according to claim 1, wherein the catalyst layer contains gold or platinum as a catalyst metal, and the detection target gas is NO.