JPS6122897B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas-sensitive element with excellent selectivity. Gas-sensitive elements are known that utilize the fact that the specific resistance of the surface of an oxide semiconductor changes when gas comes into contact with the surface of the oxide semiconductor. For example, when a reducing gas comes into contact with ZnO, SnO ₂ , Fe ₂ O ₃ , etc., which exhibit N-type semiconductivity, the resistance value decreases, and when an oxidizing gas comes into contact with it, the resistance value increases. In addition, in an oxide semiconductor exhibiting P-type semiconductivity, increases and decreases in resistance value exhibit an inverse relationship. The reactivity, or selectivity, of the oxide semiconductors mentioned above with various gases is determined by the semiconductor surface temperature, surface electron level structure, porosity, pore size, etc., but in general, oxide semiconductors alone are sensitive to gases. As an element, the sensitivity is low and the selectivity is not sufficient. Therefore, in oxide semiconductors,
Attempts have been made to increase sensitivity by adding catalysts such as Pt and Pd. In this case, a certain degree of selectivity can be obtained by selecting the element temperature. For example, a gas sensitive material with Pt added has high sensitivity to isobutane gas;
It does not react much to hydrogen gas or carbon monoxide gas. On the other hand, when Pd is added, it exhibits high sensitivity to isobutane gas, hydrogen gas, and carbon monoxide gas. However, both exhibit high sensitivity to alcohol-based gases such as ethyl alcohol gas. Therefore, in the above-mentioned devices, especially in ordinary households, they are sensitive to alcohol-based gases generated by food, hair styling products, cosmetics, etc. in the same way as LPG gas, which has been a big problem in practical use. In view of the above points, an object of the present invention is to provide a highly reliable gas-sensitive element for LPG that is not sensitive to alcohol-based gas and has high sensitivity to isobutane gas. In the present invention, the catalyst carrier such as SiO ₂ or Al ₂ O ₃ has a
~8% by weight of Pd and 0.05~30% of said Pd in molar ratio
This is a gas-sensitive element in which a catalyst layer containing double the amount of P is provided on the surface of a gas-sensitive body made of a tin oxide semiconductor. In addition, in the present invention, the amount of Pd added is set to 0.005 to 8% by weight, because if it is less than 0.005% by weight, the sensitivity to isobutane gas will be small, and if it exceeds 8% by weight, it will become sensitive to alcohol gas. It's for a reason. In addition, the reason why the amount of P added is 0.05 to 30 times the amount of Pd added in terms of molar ratio is because if it is less than 0.05 times, there is no noticeable difference between the sensitivity to isobutane gas and the sensitivity to alcohol gas, so the sensitivity to isobutane gas is This is because the sensitivity to LPG such as isobutane gas decreases when the value exceeds 30 times. The present invention will be explained in detail below using structural examples. First, the gas-sensitive element according to the present invention has a pair of electrodes 2 on the outer peripheral surface of a cylindrical insulating base 1, as shown in cross section in FIG.
A gas sensitive body 3 made of an oxide semiconductor is provided so as to cover the. Furthermore, the gas sensitive body 3
A catalyst layer 4 made of silica-alumina containing predetermined amounts of Pd and P is provided on the surface. Further, the gas-sensitive element configured as described above may be used, for example, as a second
It is assembled on the pin foot as shown perspectively in the figure.
In FIG. 2, 5 represents a lead wire, 6 represents an insulating plate, and 7 represents a heater. The heater 7 is provided to improve the sensitivity of the gas sensitive element, and can be provided as appropriate if necessary. Note that the catalyst layer 4 does not necessarily need to completely cover the surface of the gas sensitive body 3. Further, in the above, silica/alumina is used as a carrier for the catalyst layer, but other materials may be selected as appropriate, and the same effect can be obtained even when no carrier is used. The gas-sensitive element according to the present invention is manufactured, for example, as follows. For example, SnO ₂ − as a tin oxide semiconductor
When using the SiO ₂ system, SnO ₂ and SiO ₂ powder are weighed out at a predetermined composition ratio, mixed, and then water or a binder is added to form a paste, and an insulating substrate 1 on which a pair of electrodes 2 are provided as shown in FIG. After applying and drying
Calcinate at 300-1100℃ to form a gas sensitive body. On the other hand, the catalyst layer is made of e.g. PdCl ₂ or Pd(NO ₃ ) ₂
Add H ₃ PO ₄ NH ₄ H ₂ PO ₄ to make an aqueous solution with an appropriate pH, mix it so that it falls within the composition range of the present invention,
Add silica/alumina powder as a carrier to this to form a slurry. After mixing this slurry well and drying it,
Calcinate at 300°C to 1000°C and crush into fine powder.
Water and a suitable binder are added to this fine powder to form a paste, which is then applied onto the surface of the gas sensitive member, dried and fired at 300 to 1000°C to obtain the gas sensitive element according to the present invention. This catalyst layer has a thickness comparable to that of the gas sensitive layer, and is usually around 100 μm. Note that the catalyst layer has a form in which Pt and P are supported on the surface of the carrier, and most of the volume is made up of carriers such as Al ₂ O ₃ and SiO ₂ . In this embodiment, the SiO ₂ /Al ₂ O _{3 ratio} is about 5 (weight ratio), but a single substance such as SiO ₂ or Al ₂ O ₃ may also be used. Next, various characteristics of the gas-sensitive element according to the present invention as described above were measured and are shown in FIGS. 3 and 4. Figure 3 shows Pd in the gas-sensitive element according to the present invention.
and the sensitivity to 0.2 vol% isobutane gas with respect to the amount of P added. Note that the sensitivity is expressed as Ro/Rg, which is the ratio of the resistance value Ro in air to the resistance value Rg in gas, and the larger the value, the easier the gas detection. Also, Figure 4
Sensitivity Ro/Rg to 0.2vol% alcohol gas
shows. The characteristics shown in Figures 3 and 4 depend on the element temperature.
The value is 350℃. As is clear from FIGS. 3 and 4, when the amounts of Pd and P added in the catalyst layer are within the range of the present invention, the catalyst layer exhibits excellent sensitivity to isobutane gas and is hardly sensitive to alcohol gas. Next, a gas-sensitive element in which no catalyst is used, a catalyst layer containing only P, a catalyst layer containing only Pd, and a catalyst layer containing P and Pd as in the present invention. About 0.2vol%
The sensitivity Ro/Rg to various gases was measured, and the first
Shown in the table.

[Table] As is clear from the results in Table 1, in the case of no catalyst, the overall sensitivity is low and the sensitivity to alcohol gas is higher than to isobutane gas. A similar tendency was observed when a catalyst layer containing only P or only Pd was used, and it was difficult to detect isobutane gas. On the other hand, the gas-sensitive element according to the present invention shows excellent sensitivity only to isobutane gas and hydrogen gas, and can be said to have excellent selectivity with almost no change in sensitivity to alcohol gas. Although the reason for this excellent selectivity in the present invention is not clear, the reactivity is high because Pd and P are mixed, and at least part of the Pd and P added in the catalyst layer manufacturing process is This is thought to be because it is compounded. In addition, in the above embodiments, as the gas sensitive body,
Although a SnO ₂ -SiO ₂ based semiconductor was used, it goes without saying that other tin oxide based semiconductors may be used as needed to achieve similar effects. Further, in the present invention, since the catalyst layer and the gas sensitive body are separated, it is easy to manufacture and has the advantage of having excellent aging characteristics. Furthermore, when Pd and P are mixed in the gas sensitive material, it is thought that the selectivity will be improved compared to the conventional one. As described above, by using the gas-sensitive element according to the present invention, it has high sensitivity to isobutane gas, so it can be said that it is extremely effective in practice as a gas-sensitive element for LPG.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of the configuration of a gas-sensitive element according to the present invention. FIG. 2 is a perspective view showing an example of an apparatus using the gas-sensitive element according to the present invention. FIGS. 3 and 4 are curve diagrams showing characteristic examples of the gas-sensitive element according to the present invention. 3... Gas sensitive body, 4... Catalyst layer.

Claims (1)

[Claims] 1. On the surface of a gas sensitive body made of a tin oxide-based semiconductor having a pair of electrodes, an amount of 0.005 to 8
A gas-sensitive element comprising a catalyst layer containing Pd in a weight percent and P in a molar ratio of 0.05 to 30 times the Pd.