JPS6128932B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas detection element, and particularly to a gas detection element suitable for detecting low concentration reducing gas and Freon gas. BACKGROUND ART Gas sensing elements have been known that utilize the fact that the specific resistance of the surface of a metal oxide semiconductor changes when a gas comes into contact with the surface of the metal oxide semiconductor.
For example, when a reducing gas comes into contact with ZnO, SnO ₂ , Fe ₂ O _3, etc., which exhibit N-type semiconductor properties, the resistance value decreases, and when an oxidizing gas comes into contact with it, the resistance value increases. Furthermore, in metal oxide semiconductors exhibiting P-type semiconductor properties, the increase and decrease in resistance value exhibits an inverse relationship. In metal oxide semiconductors such as those mentioned above, the reactivity or selectivity with various gases is determined by the semiconductor surface temperature, surface electron level structure, porosity, pore size, etc., but in general, metal oxide semiconductors alone As a gas-sensitive element, the sensitivity is low and the reducing property is not sufficient. Therefore, attempts have been made to improve gas detection sensitivity by using noble metals such as Pt and Pd as catalysts. That is, methods have been used such as directly adding a noble metal to a metal oxide semiconductor or providing a noble metal supported catalyst layer on a metal oxide semiconductor. When these precious metals are used as catalysts, the gas detection sensitivity for reducing gases such as CO and iso-C ₄ H ₀ is improved compared to the case without catalysts, but the sensitivity is not necessarily sufficient for low concentration gases. I couldn't get it. Furthermore, the detection sensitivity for so-called Freon gas was extremely low. In view of the above points, the present invention provides a gas detection element that has excellent sensitivity to reducing gases such as carbon monoxide, hydrogen, and hydrocarbons, and Freon gas, which is generally used as a refrigerant, and is particularly suitable for detecting low concentration gases. The purpose is to provide. That is, the gas sensing element of the present invention includes a gas sensing element having a pair of electrodes, wherein the gas sensing element is made of a metal oxide semiconductor,
Al ₂ O ₃ , SiO ₂ or SiO ₂ - on the surface of the gas sensitive body
A first catalyst layer made of vanadium or vanadium-molybdenum supported on a carrier made of Al ₂ O ₃ is provided, and further on the first catalyst layer, Al ₂ O ₃ ,
It is characterized in that it is provided with a second catalyst layer made of tungsten supported on a carrier made of SiO ₂ or SiO ₂ -Al ₂ O ₃ . In the case of a vanadium-supported catalyst used as the first catalyst layer, the amount of vanadium supported is
The range is preferably 0.1 to 20% by weight, more preferably 0.5 to 15% by weight, and in the case of a vanadium-molybdenum supported catalyst, the amount of vanadium supported is 0.1 to 20% by weight (more preferably 0.5 to 15% by weight) based on the carrier. % by weight), and the supported amount of molybdenum is preferably 0.05 to 0.5 (more preferably 0.1 to 0.2) expressed as g atomic ratio to vanadium. In this case, the amount of tungsten supported is preferably in the range of 0.1 to 20% by weight, more preferably 0.5 to 15% by weight, based on the carrier. When the composition of each catalyst layer used is within the above range, a remarkable catalytic effect can be obtained, resulting in a highly sensitive gas sensing element. In the present invention, Al ₂ O ₃ , SiO ₂ , or SiO ₂ -Al ₂ O ₃ is used as a carrier component for the catalyst, and diatomaceous earth, silica gel, etc. whose main component is SiO ₂ or Al ₂ O ₃ , Activated alumina and the like are also included. In the present invention, as the gas sensitive material made of a metal oxide semiconductor, any material whose resistance value changes upon contact with a reducing gas can be used. For example, ZnO, SnO ₂ , Fe ₂ O ₃ exhibiting N-type semiconductor properties can be used. etc., and compositions in which small amounts of Sb ₂ O ₃ , Cr ₂ O ₃ , etc. are added thereto. 1 and 2 show one embodiment of the present invention, and FIG. 1 is a cross-sectional view of a cylindrical element,
FIG. 2 is a perspective view of the element mounted on the pin foot. A detailed description will be given below with reference to the drawings. This gas detection element includes a pair of electrodes 2 provided on the outer peripheral surface of a cylindrical insulating substrate 1, a layer 3 made of a metal oxide semiconductor so as to cover the insulator 1 and the electrodes 2, and a layer 3 made of a metal oxide semiconductor. The layer functions as a gas sensitizer. Furthermore, a first layer is formed on the surface of the metal oxide semiconductor layer 3.
A catalyst layer 4 is provided, and a second catalyst layer 5 is further provided thereon.
is provided. These metal oxide semiconductor layer 3 and catalyst layers 4 and 5 are in a porous state. The gas detection element configured as described above is held on the pin foot in a state where it does not come into contact with anything else, for example, as shown perspectively in FIG. In the figure, 6 represents an electrode lead wire, 7 represents an insulating plate, and 8 represents a heater. The heater is designed to heat the gas sensitive body to around 450°C, and was installed to improve the sensitivity of the element. The gas detection element according to the present invention is manufactured, for example, as follows. To prepare the catalyst to be used, first, standard solutions of vanadium, molybdenum, and tungsten are prepared. For example, a standard solution of vanadium is prepared at a predetermined temperature by adding water to ammonium metavanadate and adding oxalic acid under heating. For the molybdenum standard solution, an ammonium molybdate aqueous solution with a predetermined concentration is prepared and used as a standard solution. The tungsten standard solution is prepared by adding ammonium tungstate water and adding oxalic acid as in the case of the vanadium standard solution to obtain a standard solution of a predetermined concentration. When vanadium-molybdenum is supported on a carrier, the vanadium standard solution and molybdenum standard solution prepared above are mixed at a required ratio to prepare a vanadium-molybdenum mixed solution. The carriers used were Al ₂ O ₃ , SiO ₂ , and SiO ₂ -
When a carrier selected from Al ₂ O ₃ is to support vanadium, a predetermined amount of the carrier is immersed in a vanadium standard solution and left overnight. Then, after evaporation to dryness, it is ground into powder and calcined in an electric furnace at 30 to 600°C to obtain the desired vanadium-supported catalyst. When supporting vanadium-molybdenum or tungsten on a carrier, as an immersion solution,
Similar treatment is performed using a vanadium-molybdenum mixed solution or a tungsten solution, respectively. However, in the case of tungsten-supported catalyst, the calcination temperature is
300-800℃ is good. The gas sensitive body is formed as follows. An appropriate amount of a binder such as methyl cellulose is added to the metal oxide semiconductor used for the first catalyst layer and mixed for a certain period of time using a ball mill or the like to form a paste. As shown in FIG. 1, for example, a gold paste is applied and fired to the surface of an insulating substrate 1 on which a pair of electrodes 2 have been previously provided, and after drying, the paste is fired at 600 to 1000°C. After that,
Previously prepared vanadium or vanadium-
The molybdenum-supported catalyst is made into a slurry using an appropriate solvent, coated on the surface of the gas sensitive member 3, dried, and further calcined at 300 to 600°C to form a first catalyst layer. Next, a slurry of a catalyst supporting tungsten is similarly coated and dried on the first catalyst layer, and then
A second catalyst layer is formed by firing at 300 to 800°C. The gas detection element manufactured as described above and having the structure as shown in FIG. 1 is attached and used as shown in FIG. When preparing the catalyst slurry, adding an organic binder such as methyl cellulose, ethyl cellulose, or cellulose acetate in addition to a solvent such as water or alcohol increases the mechanical strength of the formed catalyst layer and improves its impact resistance. Sexuality also improves. Hereinafter, the present invention will be specifically explained by giving examples. Gas detection elements as shown in FIGS. 1 and 2 were manufactured, and the sensitivity (approximately 450° C.) to various gases at a concentration of 500 ppm was measured as RO/Rg. here,
RO is the resistance value shown by the element in air that does not contain the gas to be measured, and Rg is the resistance value shown by the element in air that does not contain the gas to be measured.
This is the resistance value exhibited by the element in air containing air.
The sensing element used is a metal oxide semiconductor.
98ZnO-2Sb ₂ O ₃ composition system (weight ratio) was used, and the first catalyst layer was Al ₂ O ₃ , SiO ₂ or SiO ₂ -
Using Al ₂ O ₃ with vanadium or vanadium-molybdenum supported, and further as a second catalyst layer.
Tungsten is supported on Al ₂ O ₃ , SiO ₂ or SiO ₂ -Al ₂ O ₃ . The amount of vanadium, molybdenum, or tungsten supported on the carrier in each catalyst is shown in Table 1 for each example.
The measurement results (RO/Rg) are also shown in the same manner. _For comparison, _{Al 2 O 3} , SiO ₂ or
Regarding conventional gas detection elements equipped with a catalyst layer in which noble metals are supported on SiO ₂ - Al ₂ O ₃ , various gases can be detected.
The sensitivity to air containing 2000 ppm was measured and the results are shown in Table 2.

【table】

【table】

[Table] As is clear from Tables 1 and 2, the sensitivity of the element according to the present invention to various reducing gas concentrations of 500 ppm is comparable to or higher than the sensitivity of the element using a noble metal catalyst to various reducing gas concentrations of 2000 ppm. It can be seen that the sensitivity is as follows. Furthermore, the element according to the present invention exhibits high sensitivity to a halogenated hydrocarbon gas called Freon. Freon is a partial halide (CHClF ₂ , CHF ₃ ,
CHCl ₂ F, etc.) and complete balogenides (CCl ₂ F ₂ ,
CCl ₃ F, CClF ₃ , etc.) Partial halides have C-H bonds and behave similarly to reducing gases, but complete halides do not have C-H bonds. Because it does not necessarily behave as a reducing gas, it has been considered a type of gas that is difficult to detect with conventional gas detection elements.
However, the device according to the present invention is shown in Table 1.
As CHClF ₂ and CCl ₂ F ₂ are representative and their sensitivity is shown, it is clear that they can be detected with high sensitivity similar to the reducing gases shown in the table. As explained above, the element according to the present invention has excellent sensitivity characteristics at low concentrations with respect to reducing gases and Freon gases, has good high temperature stability and has a long life, and is suitable as a gas detection element for low concentrations of the above gases. I can say that it is.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas detection element according to the present invention,
FIG. 2 is a perspective view showing an example of a device using the gas detection element according to the present invention, 1... cylindrical insulating base, 2... electrode, 3... gas sensitive body, 4... first catalyst layer, 5 ...Second catalyst layer.

Claims (1)

[Claims] 1. A gas sensing element comprising a gas sensitive body having a pair of electrodes, wherein the gas sensitive body is made of a metal oxide semiconductor;
Al ₂ O ₃ , SiO ₂ or SiO ₂ - on the surface of the gas sensitive body
A first catalyst layer made of vanadium or vanadium-molybdenum supported on a carrier made of Al ₂ O ₃ is provided, and further on the first catalyst layer, Al ₂ O ₃ ,
A gas sensing element characterized in that a second catalyst layer is provided in which tungsten is supported on a carrier made of SiO ₂ or SiO ₂ -Al ₂ O ₃ . 2 In the statement of claim 1, the first
A gas sensing element in which the catalyst layer supports vanadium, and the amount of the supported vanadium is 0.1 to 20% by weight based on the carrier. 3 In the description of claim 1, the first
The catalyst layer supports vanadium-molybdenum, and the amount of vanadium supported is 0.1 to 20% by weight.
So, the amount of molybdenum supported is g relative to vanadium.
A gas sensing element with an atomic ratio of 0.05 to 0.5. 4 In the statement of claim 1, the second
A gas sensing element in which the amount of tungsten supported in the catalyst layer is 0.1 to 20% by weight based on the carrier.