JPH04279853A - Gas sensor - Google Patents

Abstract

PURPOSE:To restrict dependency on humidity of a SnO2 gas sensor. CONSTITUTION:A compound catalyst formed of Pt, Pd, Rh and Ir such as in Pt-Ir, Pd-Ir and Pt-Rh-Ir is used, and Ir density in the compound catalyst is set preferably to be 10-50atom%. The Ir restricts dependency on humidity of a gas sensor by decreasing the dependency on humidity of catalytic activity. The Ir compound catalyst is added to SnO2, or to an oxidizing catalyst filter for adjusting relative sensitivity.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a SnO2 gas sensor, and particularly to suppressing its humidity dependence.

0002

2. Description of the Related Art A gas sensor in which a precious metal catalyst is added to SnO2 is well known. Furthermore, in order to adjust the relative sensitivity, a gas sensor in which the surface of a gas sensitive body made of SnO2 is coated with an oxidation catalyst layer is also well known. As is well known, SnO2-based gas sensors have a problem in that their output is dependent on humidity. The inventor studied this problem and succeeded in developing a catalyst material with low humidity dependence, and by using this catalyst, Sn
We succeeded in suppressing the humidity dependence of O2-based gas sensors.

0003

SUMMARY OF THE INVENTION An object of the present invention is to reduce the humidity dependence of a SnO2-based gas sensor.

0004

[Structure of the Invention] This invention uses Pt, Pd,
SnO using at least one member of the group consisting of Rh
In the two-system gas sensor, the catalyst is Pt, Pd,
The present invention is characterized in that it is a composite catalyst of at least one member of the group consisting of Rh and an Ir element. This Ir-containing catalyst (composite catalyst of Pt, Pd, Rh and Ir)
may be added to SnO2, or may be used in the oxidation catalyst layer covering the SnO2 gas receptor. This catalyst may also be added to aggregates when SnO2 is dispersed in aggregates such as alumina. This Ir-based catalyst has a low humidity dependence of activity, reducing the portion of the sensor's humidity dependence that is attributable to catalytic activity. Ir reduces humidity dependence at any content, but the preferred range of Ir content in Pt, Pd, Rh, and Ir is 0.1 to 0.5 in terms of atomic ratio; particularly decreases. A third component such as gold or Re may be added to a composite catalyst using Ir. In that case, the Ir concentration is determined based on the Ir content in Pt, Pd, Rh, and Ir. Ignore the three components. This is because the combined effect of Ir, Pt, etc. is important. Note that the catalytic activity of simple Ir itself is low. The state of existence of this Ir-based catalyst is mainly an alloy metal such as Pt-Ir, but it is not limited to this, and may exist as a composite oxide, for example. The inventor confirmed that the composite catalyst exists as an alloy through X-ray diffraction of the Pt-Ir catalyst.

[0005]

According to the present invention, the humidity dependence of the SnO2 gas sensor can be suppressed.

[0006]

[Embodiment] FIG. 1 shows the structure of a gas sensor according to an embodiment. In the figure, 2 is a substrate made of alumina or the like, which has a built-in heater (not shown). 4 is a SnO2 gas sensitive layer;
A composite catalyst of Ir and at least one member of the group consisting of Pt, Pd, and Rh is added to O2. Note that this composite catalyst will hereinafter be referred to as an Ir composite catalyst. 6 is an oxidation catalyst layer for burning gas such as ethanol and improving relative sensitivity to methane, isobutane, etc. An Ir composite catalyst is also added to the oxidation catalyst layer 6. Reference numerals 8 and 8 denote a pair of electrodes connected to the gas sensitive layer 4.

[0007] The oxidation catalyst layer 6 may not be provided. Such an example is shown in FIG. Further, the Ir composite catalyst may be added only to the oxidation catalyst layer 6. Such an example is shown in FIG. In the figure, 14 is a SnO2 gas sensitive layer to which plain Pd is added.

The manufacturing conditions of the gas sensor shown in FIGS. 1 to 3 are shown. SnCl4 was hydrolyzed with ammonia and the obtained Sn
The acid sol was thermally decomposed at 700°C to form SnO2. This S
A mixed solution of iridium chloride and palladium chloride was added to nO2, and after drying, it was thermally decomposed at 650°C. After thermal decomposition, hydrogen reduction was performed at 200°C to obtain SnO2 supporting a Pd-Ir composite catalyst. γ-alumina (specific surface area 100 m2/gr) was used as a support for the oxidation catalyst layer 6, impregnated with a mixed solution of trihydrate of chloroplatinic acid and rhodium chloride to which iridium chloride was added, and dried at 650°C. It was thermally decomposed and reduced with hydrogen at 150°C. S after adding Ir composite catalyst
A paste of nO2 and γ-alumina was formed, SnO2 was printed on the substrate 2, and the paste was baked at 650°C to obtain a SnO2 gas sensitive layer 4 (thickness: 10 μm). Then, γ-alumina was printed and fired at 650° C. to form an oxidation catalyst layer 6 (film thickness: 20 μm).

The humidity dependence of the obtained gas sensor was evaluated. The results are shown in Tables 1 to 3. The measurement atmosphere is 20℃
dry air and a humidified atmosphere at 20° C. and a water vapor pressure of 20 Torr. Table 1 using the sensor in Figure 1 shows data for methane detection, Table 2 using the sensor in Figure 2 shows data for ethanol detection, and Table 3 using the sensor in Figure 3 shows data for methane detection. The following data is shown.

0010

[Table 1] Humidity dependence (Ir-based composite catalyst is added to both gas sensitive layer and oxidation catalyst layer) Catalyst Composition Pd 100
Pd 72 Pd 65 Pd 55
Pd 0 (added to sensitive body 4) Ir 0
Ir 28 Ir 35 Ir
45 Ir 100 Sensitivity CH4 1000ppm Dry 7.4
7.6 7.8 7.
4 5.6 Wet
10.0 8.7 8
．． 6 8.3 7.2 Rsdry
/Rswet 1.21
1.15 1.10 1.12
1.28CH4 5000ppm Dry
12.2 12.6 13.0
12.6 8.3 〃
Wet 14.5 14.2
14.2 14.0 10
．． 0Rsdry/Rswet 1.
19 1.13 1.09
1.11 1.21ETOH 1000ppm
Dry 9.6 9.5
9.7 10.1 12.5
〃Wet 13.0
11.9 12.0 12.8
18.1Rsdry/Rswet
1.35 1.25 1.24
1.27 1.45* Sensitivity is the ratio of the resistance value in each atmosphere and the resistance value in clean dry air at 20°C.The catalyst is added to the SnO2 gas sensitive body 4, and the amount added is determined by the total amount of catalyst and Sn. The atomic ratio of Pd/Ir is fixed at 0.7Atom%, and the atomic ratio of Pd/Ir is controlled.The catalyst composition shows the Pd/Ir ratio in Atom%. 0.5Atom% Pt-
Added Rh-Ir composite catalyst, catalyst composition is Pt5 in atomic ratio
0:Rh25:Ir25%, sensor temperature 400℃, E
TOH stands for ethanol.

[0011]

[Table 2] Humidity dependence (Ir-based composite catalyst added to gas sensitive layer) Catalyst
Composition Pd 100 Pd 72
Pd 65 Pd 55 Pd
0 (added to sensitive body 4) Ir 0 Ir
28 Ir 35 Ir 45
Ir 100 Sensitivity ETOH 1000ppm Dry 18.2
17.1 17.0 16.
7 21 Wet
34 26 25
26 36 Rsdry/
Rswet 1.85 1
．． 53 1.47 1.56
1.73ETOH 5000ppm Dry
35 34 33
31 34 〃
Wet 55 46
44 44 55 R
sdry/Rswet 1.57
1.36 1.34 1.4
2 1.61* Sensitivity is the ratio of the resistance value in each atmosphere to the resistance value in clean dry air at 20°C.The catalyst is SnO
The amount of addition is fixed at 0.7 Atom% in terms of the atomic ratio between the total amount of catalyst and Sn, the atomic ratio of Pd/Ir is controlled, and the catalyst composition is determined by changing the ratio of Pd/Ir to Atom%.
Displayed in units, sensor temperature is 300℃.

0012

[Table 3] Humidity dependence (Ir-based composite catalyst added to oxidation catalyst layer) Catalyst Composition Pt-Rh 100 Pt-Rh
85 Pt-Rh 75 Pt-Rh 55 P
t-Rh 0 (added to sensitive body 4) Ir
0 Ir 15 Ir 25
Ir 45 Ir 100 Sensitivity
CH4 1000ppm Dry 7.6
7.5 7.4
7.3 5.2 〃
Wet 11.0 9.9
10.0 8.6
7.9Rsdry/Rswet
1.45 1.32 1.2
1 1.18 1.52

CH4 5000ppm Dry 12.
5 12.4 12.2
12.1 7.8 〃
Wet 17.3 15
．． 6 14.5 13.9
11.5Rsdry/Rswet
1.38 1.26 1
．． 19 1.15 1.47ET
OH 1000ppm Dry 10.4
9.8 9.6
9.5 17.5 〃
Wet 17.1 13.5
13.0 12.5 30
Rsdry/Rswet 1
．． 64 1.38 1.35
1.32 1.72* Sensitivity is the ratio of the resistance value in each atmosphere and the resistance value in clean dry air at 20°C, the Ir composite catalyst is added to the oxidation catalyst layer 6, and the amount added is the total amount of catalyst and Al The atomic ratio of Pt-Rh component and Ir component was fixed at 0.5Atom%, and the atomic ratio of Pt-Rh component and Ir component was controlled.
The composition of component h is Pt2:Rh1 in atomic ratio, the sensor temperature is 400°C, and ETOH represents ethanol.

The catalyst composition of the oxidation catalyst layer 6 in Table 3 is Ir10.
The sample with 0% had high ethanol sensitivity. This indicates that the oxidation activity of simple Ir is low and that ethanol removal is insufficient. Therefore, a simple Ir catalyst is not suitable as a material for the oxidation catalyst layer 6. Further, even in the sample in which simple Ir was added to the gas sensitive layer 4 in Table 1, the methane sensitivity was low and the ethanol sensitivity was high. This is also a simple Ir
This indicates that the catalytic activity of the catalyst is low, the promotion of methane adsorption is insufficient, and the removal of ethanol is also insufficient. Therefore, a single Ir catalyst is not suitable as a material for the gas sensitive layer 4. On the other hand, Ir-based composite catalysts such as Pd-Ir and Pt-Rh-Ir have small humidity dependence of the sensor, and relative sensitivity and gas concentration dependence are small compared to plain Pd catalysts and Pt-Rh-Ir-free catalysts. Results comparable to Pd catalysts can be obtained.

Here, the SnO2 gas sensitive body 4 contains Pd-
Although an Ir composite catalyst was used, Pt-Ir composite catalyst or Rh-
An Ir composite catalyst or a Pt-Rh-Ir composite catalyst may be used. Further, although a Pt-Rh-Ir composite catalyst is used for the oxidation catalyst layer 6, a Pd-Ir composite catalyst, a Pt-Ir composite catalyst, or a Rh-Ir composite catalyst may be used.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a gas sensor according to an example.

FIG. 2 is a sectional view of the gas sensor of the example.

FIG. 3 is a cross-sectional view of the gas sensor of the example.

[Explanation of symbols]

2 Board 4 SnO2 layer 6 Oxidation catalyst layer 8 Electrode 14　SnO2 layer

Claims (2)

[Claims] 1. A SnO2-based gas sensor using at least one member of the group consisting of Pt, Pd, and Rh as a catalyst, wherein the catalyst is composed of at least one element of the group consisting of Pt, Pd, and Rh, and Ir. A gas sensor characterized by a composite catalyst with elements. 2. A composite catalyst of at least one member of the group consisting of Pt, Pd, and Rh and an Ir element,
characterized by coating the surface of the SnO2 gas sensitive body,
The gas sensor according to claim 1.