JPH0814557B2 - Method for manufacturing low-alcohol sensitive gas sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a SnO ₂ gas sensor having low alcohol sensitivity.

[Prior Art] Japanese Patent Laid-Open No. 51-130,299 discloses that the surface of a gas sensor is Pt, P
A member of noble metal catalyst such as d, Ir, Rh, etc.
The coating is disclosed. According to the publication, the noble metal catalyst removes interfering gas by catalytic oxidation to enhance gas selectivity, and the removal order by oxidation includes carbon monoxide,
The order is hydrogen, alcohol, imbutane, and methane. Further, JP-A-51-104,397 discloses a gas sensor in which the surface of ZnO is coated with a Pt-Rh composite catalyst. Then, the publication states that the coating slightly increases the isobutane sensitivity.

By the way, actually, the main interfering gas is alcohols such as ethanol, and hydrogen is an important detection target. Therefore, it is necessary to selectively reduce the alcohol sensitivity without impairing the sensitivity to hydrogen. However, the known technology is
No solution is shown.

[Object of the Invention] An object of the present invention is to selectively reduce the alcohol sensitivity without impairing the hydrogen sensitivity.

[Structure of the Invention] The present invention is a method for manufacturing a gas sensor using a gas sensor formed by molding SnO2-based powder, in which a mixed solution of rhodium salt and platinum salt is brought into contact with the molded gas sensor, It is characterized in that a Rh-Pt composite catalyst is added to the surface of the gas sensitive body.

Further, the present invention is a method for manufacturing a gas sensor using a gas sensitive material formed by molding SnO2-based powder, wherein Rh-Pt is added to SnO2.
It is characterized in that a powder to which a composite catalyst is added is prepared, and the gas sensitizer is coated with this powder.

As described above, the addition of the Rh-Pt composite catalyst is performed by impregnating the surface of the gas sensitive body with the Rh-Pt composite catalyst (claim 1), or by adding Rh-Pt to the surface of the gas sensitive body with SnO2.
It is carried out by coating with a powder to which a Pt composite catalyst is added, and the effects are the same. In each case, only the alcohol sensitivity can be selectively reduced while maintaining the hydrogen sensitivity. Rh
The method of adding the -Pt composite catalyst is limited to the above two methods. The added Rh-Pt composite catalyst is supported on SnO2, and the Rh-Pt composite catalyst is supported mainly on the surface of the gas sensitive body.

With addition methods other than this, the alcohol sensitivity cannot be reduced while maintaining the hydrogen sensitivity. For example, Rh-Pt composite catalyst was added to SnO2 powder, and this was used to
The composite catalyst is uniformly added to the entire gas sensor. The characteristics of such a gas sensor are, for example, as shown in FIG. 8, and the hydrogen sensitivity is lower than the sensitivity to alcohol. Moreover, the temperature dependency of the gas sensor increases, and the reliability decreases. On the other hand, when the mixed solution of rhodium salt and platinum salt is brought into contact with the gas sensitive body and the same amount of Rh-Pt composite catalyst is added, the characteristics become as shown in Fig. 2, the alcohol sensitivity is low, and the hydrogen sensitivity is low. Will be higher. In this sensor, the resistance value increases as the sensor temperature increases near 400 ° C, and the increase in output due to the increase in power supply voltage is compensated by the increase in sensor resistance with temperature, and the power supply voltage dependency of the sensor is reduced.

Further, even if the Rh-Pt composite catalyst is added to the surface of the gas sensitive material by vacuum vapor deposition, sputtering or the like, such an effect cannot be obtained. In vacuum deposition and sputtering, a Rh-Pt composite catalyst is layered on the surface of the gas sensor, and SnO2 and Rh
-No interaction with Pt composite catalyst is obtained. And decreasing the alcohol sensitivity while maintaining the hydrogen sensitivity is
This is due to the synergistic action of O2 and the Rh-Pt composite catalyst. This means that the effect of the Rh-Pt composite catalyst depends on whether the carrier is SnO2 or alumina, whether the catalyst is added before or after molding the sensitive body, or added to the surface of the sensitive body. It is clear from the difference depending on whether it is added uniformly over the whole.

In either case of claim 1 and claim 3, Rh
The molar mixing ratio of Pt and Pt is preferably 1/3 ≦ Rh / Pt ≦ 6, more preferably 0.4 ≦ Rh / Pt ≦ 5. The addition amount is
The sum of Rh and Pt per SnO21g of the gas sensor, for example,
It is set to 25 μmol / gSnO2, and the range of 10 times to 1/10 of this is preferable. However, this amount varies depending on the shape of the gas sensitive body, the thickness of the portion to which the Rh-Pt composite catalyst is added, etc., and is not uniformly determined.

[Example] A SnCl ₄ aqueous solution is hydrolyzed with ammonia to precipitate a stannic acid sol. Add water to the precipitate and centrifuge to remove ammonium chloride. Bake in air at 700 ° C for 1 hour,
Grind to obtain SnO ₂ .

SnO ₂ is molded into a gas sensor having the shape shown in FIG.
In the figure, (2) and (4) are noble metal electrodes that also serve as heaters, and (6) is a gas sensitive body, inside of which electrodes (2),
(4) is buried. The thickness of the gas sensor (6) is about 1.5m
m, width about 3mm, length about 2mm, SnO per sensor
_{2 The} amount is about 30 mg. A precious metal catalyst such as Pd, an aggregate such as aluminum, or a convenient binder may be added to SnO ₂ . The Pt-Rh composite catalyst added to the surface and these additives act independently, and there is no composite action between them.
The sensor may have any other shape. When the electrode and the sensitive body can be manufactured separately without embedding the electrode in the gas sensitive body (6), the electrode may be separately mounted after the molding of the sensitive body. However, in the structure of FIG. 1, it is difficult to attach the electrodes after molding, so the electrodes are integrally embedded during molding. The molded gas sensor (6) is heated to 800 ° C. for 5 minutes to be sintered. When the Rh-Pt composite catalyst can be added and the strength of the sensitizer can be maintained and the sensitizer does not deform due to the impregnation of the solution, the sintering may be performed after the impregnation, or the sintering step may be omitted. .

Prepare an aqueous solution of about 0.05 mol / l Rh, Pt, etc. The aqueous solution may be, for example, a mixed solution of rhodium chloride and chloroplatinic acid, and a solution of another salt or another catalyst such as alcohol may be used. 10 μl of the catalyst solution was dropped onto each of the sensitizers (6), dried and heated at 700 ° C. for 10 minutes to thermally decompose the rhodium salt and the like. The state of existence of the catalyst after thermal decomposition is complicated and changes depending on the atmosphere. In all of the following examples, the amount dropped is 10 μl per sensor. The addition amount may be changed about 10 times to 1/10 times, and the catalyst may be added by immersing the sensitizer (6) in the solution without dropping. Also, the dropping may be performed plural times instead of once,
Rhodium and platinum may be added separately. In the above process, the composite catalyst of Rh-Pt is supported on the surface part of the sensitizer (6). The catalyst is distributed on the surface portion (8) of the responsive body (6),
It hardly exists inside. The depth at which the rhodium-platinum catalyst exists is about 0.1 to 0.3 mm. In the impregnation method, the catalyst segregates on the surface of the sensitizer, unless special care is taken.

As another embodiment, the sensitive body (6) before sintering is preliminarily Rh
-Production is carried out by coating with SnO ₂ powder added with Pt composite catalyst and sintering. SnO ₂ used for coating is 10 mg per sensor, and the amount of rhodium is 17 μmol per sensor,
The amount of platinum is 8.5 μmol per sensor. This corresponds to impregnation with a mixed solution of 0.05 mol / l rhodium salt and 0.025 mol / l platinum salt. Since the characteristics of this product were the same as those obtained when the mixed solution was impregnated, the description thereof is omitted.

 As a comparative example, the following gas sensor was prepared.

1 Sn-O2 after burning the stannic acid sol at 700 ℃, Ph-P
t Gas catalyst (6) with uniform addition of composite catalyst.

2 Gas-sensing body (6) after sintering was impregnated with a solution of Rh, Pt, or Pd.

3 A gas sensor (6) coated with alumina carrying a Rh-Pt composite catalyst. In Comparative Example 1, the Rh-Pt composite catalyst was uniformly added to the gas sensor, and the concentration of the Rh-Pt composite catalyst in the cross section of the gas sensor was uniform. In Comparative Example 2, the concentration distribution of the catalyst is the same as that of the Example or the composition is different.
Only, Pt only, or Pd only single component catalysts for comparison. In Comparative Example 3, the carrier of the Rh-Pt composite catalyst was changed from SnO2 to alumina in order to investigate the synergistic action of the Rh-Pt composite catalyst and SnO2.

Five sensors are used, and the characteristics are evaluated in an atmosphere of 20 ° C and a humidity of 65%. First, in Figs. 2-8, 300-500 ℃
The relationship between the heating temperature and the resistance value is shown. Each gas concentration is 20
It is 00 ppm, and is shown based on the resistance value in isobutane at 400 ° C. Table 1 shows the amounts of Rh and Pt added.

Table 1 (Sample in the figure ) Figure Addition amount (μmol / sensor) Figure 2 Rh0.5, Pt0.25 Figure 3 Rh0.5, Pt0.5 Figure 4 Rh0.25, Pt0.5 Figure 5 Rh0. 5 Simple Fig. 6 Pt0.5 Simple Fig. 7 Pt0.5 Pt0.25 Fig. 8 Rh0.5 Pt0.25 (Comparative example with uniform addition) The characteristics in Figs. Different relative sensitivities to other gases, especially hydrogen. Actually, ethanol represents alcohols, and the characteristics of other alcohols such as methanol and isopropanol are similar to those of ethanol.

In the example of FIG. 2, the ethanol sensitivity is selectively reduced, and the hydrogen sensitivity at 400 ° C. is sufficiently higher than the ethanol sensitivity. This tendency is maintained in the embodiment shown in FIGS. 3 and 4.

Selective suppression of ethanol sensitivity is not obtained with other catalysts. In the case of rhodium alone in FIG. 5, the resistance value in ethanol is lower than the resistance value in hydrogen. Next, in the case of platinum alone in Fig. 6, the sensitivity of ethanol is quite high. In the case of a palladium catalyst, although not shown, the suppression of ethanol sensitivity is almost the same as that of rhodium. However, in the complex catalyst of Palladium and Platinum (Fig. 7), the hydrogen sensitivity is lowered.

FIG. 8 shows the characteristics of the raw material SnO ₂ to which the Rh-Pt composite catalyst was uniformly added. Although the amount of rhodium added is the same as that shown in FIG. 2, the relative sensitivity is completely different and the temperature dependence of the sensor is large. Practically, the temperature coefficient of resistance of the sensor is required to be 0 or slightly positive near the detection temperature. When the temperature coefficient of resistance is negative, fluctuations in the power supply voltage synergize with fluctuations in the heating voltage and fluctuations in the detection voltage applied to the sensor, increasing the detection error.

Table 2 shows the relative sensitivity of ethanol to hydrogen and isobutane.

From Table 2, it can be seen that the use of the Rh-Pt composite catalyst can increase the false alarm level due to alcohol by about four times. Regarding the characteristics other than the relative sensitivity, for example, the dependence of the output on the gas concentration and the characteristics over time, the sensor of the example was similar to that of the comparative example. Also, this sensor is
It can be used to detect not only gases such as isobutane and methane but also carbon monoxide.

EFFECTS OF THE INVENTION In the present invention, it is possible to eliminate false alarms due to alcohol in the gas sensor.

[Brief description of drawings]

FIG. 1 is a sectional view of a gas sensor manufactured in the embodiment,
4 to 4 are characteristic diagrams of the gas sensor of the embodiment, and FIG.
FIG. 8 is a characteristic diagram of a conventional gas sensor. (6) …… Gas sensor, (8) …… Surface part.

Claims (4)

[Claims] 1. A method for manufacturing a gas sensor using a gas sensor formed by molding SnO2-based powder, wherein a mixed solution of rhodium salt and platinum salt is brought into contact with the molded gas sensor to obtain a gas sensor. A method for producing a low-alcohol sensitive gas sensor, characterized in that a Rh-Pt composite catalyst is added to the surface portion. 2. The method according to claim 1, wherein the molar ratio of Rh and Pt is 1/3 to 6;
Manufacturing method of low alcohol sensitive gas sensor. 3. A method for manufacturing a gas sensor using a gas sensor formed by molding SnO2-based powder, comprising preparing a powder of Rh-Pt composite catalyst added to SnO2, and coating the gas sensor with this powder. A method of manufacturing a low alcohol sensitive gas sensor, comprising: 4. The method according to claim 3, wherein the molar ratio of Rh and Pt is 1/3 to 6;
Manufacturing method of low alcohol sensitive gas sensor.