JPH0121900B2 - - 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 selectivity 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 ₁₀ is improved compared to the case without catalysts, but the sensitivity is not necessarily sufficient for low concentration gases. was not obtained. 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 present invention provides a gas sensing element comprising a gas sensitive body having a pair of electrodes, wherein the gas sensitive body is made of Al ₂ O ₃ , SiO ₂ , or SiO ₂ −
_Vanadium , _vanadium-
This is a gas sensing element characterized by being made of a ZnO-based metal oxide semiconductor containing a catalyst supported on molybdenum, vanadium-tungsten, or vanadium-molybdenum-tungsten. In the present invention, the content of the catalyst relative to the metal oxide semiconductor is preferably in the range of 0.1 to 20% by weight, more preferably in the range of 0.5 to 15% by weight.
Even if the catalyst content is less than or greater than the above range, the sensitivity tends to be poor. The amount of vanadium supported in the vanadium-supported catalyst is preferably 0.1 to 50% by weight (more preferably 1.0 to 20% by weight) based on the carrier, and vanadium-molybdenum, vanadium-tungsten, or vanadium-molybdenum-tungsten is supported on the carrier. In the catalyst, the amount of vanadium supported is 0.1 to 50% by weight (or even 1.0 to 20% by weight) based on the carrier.
The supported amount of molybdenum and/or tungsten is 0.01 to 0.01 expressed as g atomic ratio to vanadium.
It is preferably 0.5 (more preferably 0.05 to 0.2). Within this range, the element of the present invention exhibits high sensitivity, and outside this range it is difficult to obtain sufficient sensitivity. The catalytic mechanism of the catalyst used in the present invention is thought to proceed through an oxidation-reduction cycle in which vanadium ions are reduced by an oxidizable substance and reoxidized by gaseous oxygen. In addition, the function of molybdenum and tungsten contained together with vanadium is thought to be to generate a solid solution by isomorphic substitution at V ⁵⁺ lattice points, and to increase the amount of V ⁴⁺ and acidity by controlling the valence of the solid solution. . In the present invention, as the gas sensitive material made of a ZnO-based metal oxide semiconductor, any material whose resistance value changes upon contact with a reducing gas can be used. Such ZnO-based metal oxides include ZnO with Sb ₂ ,
Examples include compositions to which trace amounts of O ₃ , Cr ₂ O ₃ and the like are added, and 98ZnO-2Sb ₂ O ₃ is usually used. 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. Hereinafter, a detailed description will be given with reference to the drawings. This gas detection element 4 includes a pair of electrodes 2 provided on the outer peripheral surface of a cylindrical insulating substrate 1, and a layer made of a metal oxide semiconductor containing a catalyst according to the present invention so as to cover the insulating substrate and the electrodes. 3 is formed to constitute a gas sensitive body. Metal oxide semiconductor layer 3
is a porous layer. The gas detection element 4 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 FIG. 2, 5 represents a heater lead wire, 6 represents an electrode lead wire, 7 represents a heating heater, and 8 represents an insulating plate. The heater 7 is designed to heat the gas sensitive body to about 450° C. and is provided to improve sensitivity. Further, the gas detection element according to the present invention is manufactured, for example, as follows. First, a binder and an appropriate amount of H ₂ O as a solvent are added to a metal oxide semiconductor, and the mixture is mixed for a certain period of time using a ball mill or the like to form a paste. Transfer this paste to a chemist and dry it thoroughly at about 100℃. 900 after drying
Bake at ~1000℃. Thoroughly crush the fired sample using a ball mill to form a powder. A predetermined amount of this metal oxide semiconductor powder is taken, a predetermined amount of a separately prepared catalyst is added, and a binder and H ₂ O are added and mixed for a predetermined time in a ball mill to obtain a catalyst-containing metal oxide semiconductor paste. This paste is applied to the outer peripheral surface of a cylindrical insulating substrate 1 on which a pair of electrodes 2 have been previously printed using gold paste or the like, and after drying, it is fired at a temperature of 500 to 600 ℃. Further, the above-mentioned catalyst is prepared, for example, in the following manner. In addition, standard solutions of vanadium, molybdenum, and tungsten are prepared. For example, a standard solution of vanadium is prepared as an ammonium vanadate aqueous solution with a predetermined concentration by adding water to ammonium metavanadate, heating the mixture, and adding oxalic acid. The molybdenum standard solution is prepared as an ammonium molybdate aqueous solution with a predetermined concentration, and the tungsten standard solution is prepared as an ammonium tungstate aqueous solution with a predetermined concentration by adding water to ammonium tungstate and adding oxalic acid in a heated state. When supporting vanadium on a carrier, the carrier is immersed in a vanadium standard solution.
When supporting vanadium-molybdenum, a mixed solution containing a vanadium standard solution and a molybdenum standard solution at a predetermined ratio is prepared, and the carrier is immersed in this mixed solution. When supporting vanadium-tungsten or vanadium-molybdenum-tungsten, similarly prepare a corresponding mixed solution as appropriate,
The carrier may be immersed in this. The mixture is immersed in the solution for a day and night, then evaporated to dryness, crushed in a mortar to form a powder, and calcined in an electric furnace at 300 to 600°C to obtain the desired catalyst. The carrier component of the catalyst used in the present invention is:
_Al2O3 , _{SiO2 or SiO2} - _Al2O3 , _{but Al2O3 ,}
Diatomaceous earth whose main component is SiO ₂ , silica gel,
Activated alumina or the like can be used. 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.
It consists of a 98ZnO-2Sb ₂ O ₃ composition system (weight ratio), and the various catalysts and their contents are shown in Table 1. The obtained results are also shown in Table 1. _For comparison, _{Al 2 O 3} , SiO ₂ ,
Alternatively, similar measurements were performed on a conventional sensing element provided with a catalyst layer in which a precious metal was supported on SiO ₂ -Al ₂ O ₃ . However, in this case, the concentration of each gas to be measured was set to 2000 ppm. Table 2 shows the measurement results.
It was shown to.

【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
Concentration of various reducing gases in devices using noble metal catalysts
It can be seen that the sensitivity is comparable to or higher than that for 2000 ppm. 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 halides (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 traditionally been considered a type of gas that has extremely low sensitivity and is difficult to detect. However, the device according to the present invention has CHClF ₂ and CCl ₂ F ₂ represented in Table 1,
As shown in the sensitivity, it is clear that the same high sensitivity is shown for the reducing gases shown in the table. As described above, the element according to the present invention has excellent sensitivity characteristics and response characteristics at low concentrations of reducing gases and Freon gases, and can be said to be suitable as a gas detection element for low concentrations of the above gases.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing a structural example of a gas detection element according to the present invention, and FIG. 2 is a perspective view showing an example of a device using the element of the present invention. 1... Cylindrical insulator, 2... Electrode, 3 ...Gas sensitive body.

Claims (1)

[Scope of Claims] 1. A gas sensing element comprising a gas sensitive material having a pair of electrodes, wherein the gas sensitive material is made of Al ₂ O ₃ , SiO ₂ , or SiO ₂ −
_Vanadium , _vanadium-
A gas sensing element comprising a ZnO-based metal oxide semiconductor containing a catalyst supported on molybdenum, vanadium-tungsten, or vanadium-molybdenum-tungsten. 2. The gas sensing element according to claim 1, wherein the content of the catalyst relative to the metal oxide semiconductor is 0.1 to 20% by weight. 3. In the statement of claim 1 or 2, the catalyst has vanadium supported on a carrier, and the amount of vanadium supported on the carrier is 0.1 to 50.
Gas sensing element in weight%. 4. In the statement of claim 1 or 2, the catalyst is one in which vanadium-molybdenum, vanadium-tungsten, or vanadium-molybdenum-tungsten is supported on a carrier, and the amount of vanadium supported is 0.1 with respect to the carrier. ~50% by weight, and the supported amount of molybdenum and/or tungsten is 0.01~ expressed in g atomic ratio to vanadium.
Gas detection element with 0.5.