JP5158968B2 - Contact combustion type gas sensor - Google Patents

Description

  The present invention relates to a catalytic combustion type gas sensor having a sensitive layer on the surface, and more particularly to the structure of the sensitive layer.

In principle, the contact combustion type gas sensor is constructed by forming a sensitive layer made of noble metal particles on the outer periphery of the electrical resistor via an electrical insulating layer as seen in Patent Document 1, and the sensitive layer is combustible. Heat generated by gas contact is detected as a change in the resistance value of the electrical resistor.
When the palladium particles constituting the sensitive layer come into contact with hydrogen sulfide in the environment, there is a problem that the performance as a catalyst is deteriorated, that is, so-called poisoning.

For this reason, a hydrogen sulfide removal filter is disposed at the gas inlet to prevent poisoning. However, depending on the type of gas to be detected, there is a problem that a measurement error occurs due to absorption by the filter.
JP-A-8-226908

  The present invention has been made in view of such problems, and an object of the present invention is to provide a catalytic combustion type gas sensor in which poisoning by hydrogen sulfide is minimized.

  In order to achieve such a problem, in the present invention, a heat sensitive layer made of noble metal particles is formed on the outer periphery of the electric resistor via an electric insulating layer, and heat generated by contact of the combustible gas with the sensitive layer is generated. In the catalytic combustion type gas sensor that detects the change in resistance of the electric resistor, the sensitive layer is composed of particles made of an aggregate of a plurality of palladium crystallites having a crystallite diameter of 25.0 to 26.0 nm.

  According to the present invention, since each particle having a crystallite diameter of 25.0 to 26.0 nm forms a sensitive layer, durability against hydrogen sulfide is improved without causing a decrease in sensitivity, and a decrease in sensitivity due to poisoning is prevented. be able to.

Therefore, details of the present invention will be described below based on the illustrated embodiment.
FIG. 1 shows an embodiment of the gas sensor according to the present invention. The outer periphery of an electric heater 1 in which a resistance wire having a relatively large temperature resistance coefficient such as platinum is shaped like a coil is provided on the outer periphery of the electric sensor 1. A mud body of particles 2 in which palladium (Pd) is attached to porous particles having insulating properties is sintered into a spherical shape, and a sensitive layer 3 is provided.

  The particles 2 constituting the sensitive layer 3 are fired after introducing porous particles such as alumina particles into a solution in which palladium (Pd) is dissolved and reducing the water in the solution as much as possible by a vacuum evaporation method or the like. Then, it is made into a lump or granule, and this lump or granule solid is further pulverized to the same level as the original porous particles, returned to a powder form, and solidified by firing.

  That is, the particles 2 are mud together with an organic viscosity modifier so as to have a predetermined viscosity, and are shaped into a spherical shape on the heater 1 and baked to form the sensitive layer 3 described above.

  The porous particles palladium (Pd) 4 constituting the particles 2 are supported in a state of entering the fine holes 2a or adhering to the surface as shown in FIG. 2 before sintering. The palladium (Pd) of the particle 2 exists as fine particles 4b composed of a plurality of crystallites 4a as shown in FIG.

By the way, the present inventors have found that the resistance to hydrogen sulfide differs depending on the size of the crystallite 4a constituting the fine grain 4b in the course of the production. Based on this knowledge, the size of the crystallite 4a constituting the fine particles 4b of palladium (Pd) adhering to each alumina particle, that is, the crystallite diameter is 28.4 nm, 28.2 nm, 26.0 nm, 24.8 nm, 23.8 A sensor with the sensitive layer 3 changed to nm and 20.1 nm was fabricated, and a certain concentration of combustible gas containing 25 ppm hydrogen sulfide was detected.
The crystallite diameter was measured by measuring the Debye ring with a Laue camera or the like.

  As a result, as shown in FIG. 4, the gas sensor composed of palladium crystallites having a crystallite diameter of 26.0 nm and 24.8 nm as the sensitive layer 3 has a longer sensitivity than gas sensors of other crystallite diameters. It was found to maintain.

  In general, it is known that the crystallite size can be adjusted by controlling the temperature and time during sintering or by the acidity of the solution of the target metal.

As an example, when the sensitive layer 3 is formed by changing the firing temperature from 500 ° C. to 1000 ° C., the particle diameter changes according to the temperature as shown in Table 1. Therefore, by controlling the firing temperature, a highly resistant sensor can be obtained. It can be manufactured efficiently.
table 1

It is a figure which shows typically one Example of the contact combustion type gas sensor of this invention. It is a figure which shows typically the state before baking of the particle | grains which comprise a gas sensor same as the above. It is a figure which shows typically the state after baking of the particle | grains which comprise a gas sensor same as the above. It is a diagram which shows the relationship between the time in the environment where hydrogen sulfide exists, and the change of a sensitivity for every crystallite diameter.

Explanation of symbols

  1 Electric heater 2 Particles 3 Sensitive layer 4 Palladium 4a Palladium crystallite 4b Palladium fine particles

Claims (1)

A contact combustion type gas sensor that forms a sensitive layer made of noble metal particles on the outer periphery of an electric resistor and detects heat generated by contact of the combustible gas with the sensitive layer as a resistance change of the electric resistor. In
The catalytic combustion type gas sensor, wherein the sensitive layer is composed of particles made of an aggregate of a plurality of palladium crystallites having a crystallite diameter of 25.0 to 26.0 nm.

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