JP2000002680A - Co sensor and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the dependence of the concentration of CO of a CO sensor by adding Ir in a specific unit in terms of a metal to SnO2 as a gas-sensitive material for the CO sensor. SOLUTION: A sintered compact (a chip-shaped sintered compact or a thick- film sintered compact formed on an insulating substrate) which uses SnO2 as a gas-sensitive material is sintered, e.g. at a temp. of 500 to 850 deg.C. After that, the sintered compact is impregnated with the solution of an Ir compound in such a way that the addition amount of Ir to the SnO2 becomes 5 to 500 μg/g SnO2 in terms of a metal. Then, the impregnated Ir compound is decomposed by thermal decomposition, hydrogen reduction or the like, and the Ir is added to the SnO2. At this time, the temperature of the thermal decomposition is set, e.g. at a temp. of about 500 to 850 deg.C. When the Ir at 5 μg/g SnO2 or higher is added, the dependence α on the concentration of Co of a Co sensor is increased. When the addition amount of the Ir is increased, a sensor resistance Rs is increased. The addition amount of the Ir is set at 500 μg/g SnO2 or lower, and a resistance value Rs which is suitable for practical use is obtained. In addition, when an Ir-Pt composite catalyst is added, the dependence on humidity of the CO sensor can be suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a CO sensor using a SnO2 metal oxide semiconductor and a method for manufacturing the same.

[0002]

2. Description of the Related Art It is known that a CO sensor based on SnO2 is alternately changed in temperature between a high temperature range and a low temperature range and CO is detected from an output in a low temperature range. For example, in the case of the applicant's SnO2-based gas sensor TGS203 (TGS203 is a trade name),
A pair of heater / electrodes is embedded in the SnO2 sintered body,
It is operated in a 0 second cycle, and the first 60 seconds are in a high temperature range (maximum temperature of about 300 ° C.), and the second 90 seconds are in a low temperature range (minimum temperature of about 8
0 ° C), and from the sensor signal immediately before the end of the low temperature range, CO
Is detected. SnO2 contains about 2 in terms of metal per gram.
mg of Pd has been added.

This sensor is a typical example of a commercially available CO sensor, where Rs = k · [CO] −α (k is a proportional constant, α is a superscript), where α is the CO concentration dependency and Rs is the sensor resistance. When α is defined by (1), α is about 1. Note that Rs and its reciprocal are used as the sensor output. It is clear that the larger α is, the more reliable the CO sensor is.

[0004]

SUMMARY OF THE INVENTION The object of the present invention is to provide a CO sensor with a C output.
There are improvements in O concentration dependency (claims 1 to 4) and suppression of humidity dependency (claims 2 and 4).

[0005]

The CO sensor according to the present invention is a CO sensor for detecting CO while periodically changing the temperature of SnO2 as a gas-sensitive material.
It is characterized by adding Ir of 0 μg / g SnO2.
Preferably, the Ir is an Ir-Pt composite catalyst, the weight ratio of Ir / Pt in terms of metal is 1/5 to 5, and the amount of addition to SnO2 in terms of metal is 5 to 500 µg / g for both Ir and Pt.
SnO2. The type of the CO sensor is, for example, a temperature higher than room temperature in both the high temperature range and the low temperature range, and power is applied to the heater of the CO sensor in both the low temperature range and the high temperature range. Alternatively, the temperature of the CO sensor is set to, for example, about 300 ° C. in a high temperature range, and the heating time to the high
Second, the holding time to the low temperature range is, for example, 1 to 100 seconds,
In a low temperature range, the heater power may be set to 0, and the heater may be allowed to cool to around room temperature. That is, the type of the CO sensor itself is
Any SnO2-based CO sensor that uses a periodic temperature change may be used.

Further, in the method of manufacturing a CO sensor according to the present invention, a sintered body using SnO 2 as a gas
After sintering at about 850 ° C., a solution of an Ir compound is added to the sintered body, and the amount of Ir added to SnO 2 is 5 to 500 μm in terms of metal.
g / g SnO2, and then the impregnated Ir compound is decomposed by thermal decomposition, hydrogen reduction, or the like, and Ir is added to SnO2. The thermal decomposition temperature is, for example, 500 to 850 ° C.
Degree. The sintered body means a chip-shaped sintered body or a thick-film sintered body formed on an insulating substrate. Preferably, the Ir compound solution is a mixed solution of an Ir compound and a Pt compound, the addition amount of SnO2 in terms of metal per gram of both Sn and Pt is 5 to 500 µg / g SnO2, and the metal content in SnO2 is The weight ratio of Ir / Pt in conversion is 1
/ 5 to 5.

[0007]

According to the present invention, the SnO2-based CO sensor is provided with 5 to 500 .mu.g in terms of metal per 1 g of SnO2.
/ GSnO2 Ir, the amount of addition is preferably 20 to
It is 250 μg / g SnO2. Α is increased by the addition of Ir of 5 μg / g SnO2 or more, and an increase in the amount of added Ir increases the sensor resistance Rs (hereinafter simply Rs).
0 μg / g SnO 2 or less. In this specification, the addition amount of Ir or Pt is shown in terms of metal, and the unit of μg / gSnO2 is omitted and may be expressed as μg / g.

[0008] When Ir is added as an Ir-Pt composite catalyst, a new effect of suppressing humidity dependence as well as increasing α can be obtained. For this purpose, Ir and Pt are set to a weight ratio of Ir / Pt of 1/5 to 5. The suppression of the humidity dependence is an effect peculiar to the Ir-Pt composite catalyst, and may be Ir-Rh, Ir-Ru, Ir-Os,
It cannot be obtained with other combinations such as Ir-Pd, Pt-Ru, Pt-Rh, and Pt-Pd. Ir or Ir-Pt is preferably added by adding an Ir compound solution or a mixed solution of an Ir compound and a Pt compound to the sintered SnO2 sintered body.

According to the present invention, the CO 2 of the SnO 2 -based CO sensor
The concentration dependency α is improved, and the increase in the resistance value Rs is limited to a practically suitable resistance value. Further, when the Ir-Pt composite catalyst is added, the humidity dependency of the CO sensor can be suppressed.

[0010]

FIG. 1 shows the structure of a gas sensor of an embodiment, FIG. 2 shows the structure of a gas sensor of a modification, FIGS. 3 and 4 show the characteristics of the gas sensor of the embodiment, and FIGS. 3 shows the characteristics of the gas sensor shown in FIG. In FIG. 1, reference numeral 2 denotes a gas sensor, 4 denotes a metal oxide semiconductor sintered using SnO2 as a gas-sensitive material, and 6 and 8 denote a pair of coil electrodes serving also as a heater. The metal oxide semiconductor 4 is, for example, a mixture of SnO2 and alumina in a weight ratio of about 1: 1.
After sintering, impregnated with an aqueous solution of Ir nitrate or an aqueous solution of a mixture of Ir nitrate and chloroplatinic acid is thermally decomposed at, for example, 600 ° C in air, and Ir or Ir-Pt is added.

The gas sensor of FIG. 1 has a high temperature range of 60 seconds, a maximum temperature (at the end of the high temperature region) of 300 ° C., a low temperature region of 90 seconds, and a minimum temperature (at the end of the low temperature region) of 80 ° C. It operates in one cycle of 150 seconds, and detects CO from the sensor signal (sensor resistance Rs) immediately before the end of the low temperature range, for example.

As long as SnO2 is used as a gas-sensitive material and the temperature is changed periodically, the structure of the gas sensor itself is arbitrary. For example, as in the gas sensor 12 of the modification of FIG. Glass film 18 is laminated, a thick film heater 20 such as RuO2 is provided, and the film is covered with an insulating film 22 such as a glass film, and the SnO 2 -based metal oxide semiconductor 14 is formed thereon to a thickness of, for example, 20 μ. Form a film. Leads 24, 26 and the like are connected to the heater 20 and the metal oxide semiconductor 14 via electrode pads, respectively. For example, the gas sensor of FIG. 2 applies electric power to the heater 20 for 10 ms to 1 second to heat it to about 300 ° C.
The heater 20 is turned off for a second, and is allowed to cool to around room temperature. During this time, for example, one second after turning off the heater 20 or 10 seconds
CO is detected by using the sensor resistance after seconds or the like.

In the gas sensor 12 shown in FIG.
Sintering the SnO2 based metal oxide semiconductor 14 at a temperature of .degree. C., then impregnating with a solution of an Ir compound or a mixed solution of an Ir compound and a Pt compound, and thermally decomposing at 600.degree.
r or Ir-Pt is added. The effects of Ir and Ir-Pt are shown in FIG.
The gas sensor 2 of FIG. 2 and the gas sensor 12 of FIG.
Ir increases the CO concentration dependency of the sensor resistance, and Ir-Pt increases the CO concentration dependency and suppresses the temperature and humidity dependency. Hereinafter, a production example and characteristics of the gas sensor of FIG. 1 will be described.

An aqueous solution of SnCl4 is neutralized with ammonia,
The precipitate is dried and pyrolyzed in air at 700 ° C. for 1 hour to form S
nO2. To this SnO2, the added amount is 2 mg in metal conversion.
Aqueous solution of Pd was added so as to give Pd / gSnO2, dried and calcined at 600 DEG C. to carry Pd. P instead of Pd
An appropriate noble metal catalyst such as t, Rh, or Au may be added.
Further, sulfur compounds such as V, sulfate ions and thiourea may be added. The catalyst-supported SnO2 is pulverized, mixed with an equal weight of alumina powder, and molded into the shape of the gas sensor of FIG.
For example, sintering is performed at 700 ° C. for 10 minutes in air. Then I
An aqueous solution of a compound such as r, Ir-Pt, Ir-Rh, etc. is dropped into the sensor 2 by a predetermined amount, and heated at 600 ° C. for 10 minutes in the air, for example, to thermally decompose the metal into Ir, Ir-Pt, Ir-Rh. did. Similarly, as a comparative example, an aqueous solution of a salt such as Rh or Pt-Rh, or pure water without a catalyst was dropped in the same manner.
A heat treatment was performed at ℃ to produce a gas sensor of a comparative example. Ir was an aqueous nitrate solution, Pt was an aqueous solution of chloroplatinic acid, and Rh was an aqueous solution of chloride. However, the form at the time of addition is optional,
The thermal decomposition temperature may be, for example, 500 to 850 ° C., and may be changed to metal Ir or metal Ir—Pt by hydrogen reduction or the like instead of thermal decomposition. The amount of catalyst to be added is determined based on the amount of the additive solution and its concentration.
It was converted to the amount added per g of nO2.

After the obtained gas sensor 2 was used under the above-mentioned use conditions for 7 days, the characteristics were measured. The measured characteristics were 100 ppm, 300 ppm, and 1000 pp of CO.
Resistance in m (in an atmosphere of 20 ° C. and 65% relative humidity)
And CO100, 300, and 1000 ppm when the atmosphere is changed from -20 ° C relative humidity 65% to -10 ° C (dew point of about -12 ° C) and 0 ° C (dew point of about -5 ° C).
It is the resistance value inside. The resistance value and the CO concentration dependency α were determined from the measurement at 20 ° C. and 65%, and the temperature and humidity dependency of the gas sensor 2 was measured from the characteristics when the atmosphere was changed to −10 ° C. or 0 ° C. However, the temperature and humidity dependence of the gas sensor is mostly due to the change in the absolute humidity. Since the temperature and humidity dependency is particularly large between the vicinity of room temperature and normal humidity and the low temperature, the temperature and humidity dependency from 20 ° C. to −10 ° C. was measured.

FIGS. 3 to 8 show the results. The number of sensors is five in each case. FIG. 3 shows the characteristics when 60 μg / g of Ir was added, and FIG. 4 shows the characteristics when 60 μg / g of Ir and Pt (added as a mixed aqueous solution of Ir nitrate and chloroplatinic acid) were added. FIG. 5 shows the characteristics of a comparative example in which pure water was dropped in place of Ir or the like and heat-treated at 600 ° C. FIG.
Shows the characteristics when Pt 60 μg / g was added, and FIG.
FIG. 8 shows the characteristics when 32 μg / g was added.
This is the characteristic when a composite catalyst of g / g and Rh of 32 μg / g was added. In each case, the composite catalyst was added as a mixed aqueous solution of a metal salt.

3, 4 and 5 show that Ir and Ir-
It is clear that the addition of Pt increases the CO concentration dependency α, and the addition of Ir-Pt reduces the temperature and humidity dependency.
The increase in α does not occur unless Ir is added, and the improvement in the temperature-humidity dependence occurs only with Ir-Pt.

Table 1 shows the sensor resistance R of various samples (prepared in the same manner as above) at 100 ppm of CO (at 20 ° C. and 65%).
s, CO concentration dependence α in the range of 100 to 1000 ppm of CO at 65% at 20 ° C., 300 ppm of CO at −10 ° C.
Shows the corresponding CO concentration at a sensor resistance of 20 ° C. and 65%. The result is the average of each of the five sensors.

[0019]

Table 1 Sensor characteristics Sample number Sensor resistance α Temperature / humidity dependent catalyst amount (μg / g) Rs (KΩ) 1 None 8 1.0 150 2 Ir 5 10 1.3 180 3 Ir 24 15 1.4 190 4 Ir 60 22 1.5 200 5 Ir 100 50 1.5 210 6 Ir 200 120 1.5 1.5 7 7 Ir 450 270 1.6 180 8 Ir 1000 About 2000 1.5 180 1 809 Ir 60-Pt 60 90 1.5 270 10 Ir 60-Pt 15 1.4 250 11 Ir240-Pt50 170 1.5 250 250 12 Ir24-Pt100 75 1.4 1.4 250 13 Ir100-Pt450 150 1.4 230 14 Ir100-Pt24 130 1.4 1.415 15 Ir60-Rh60 32 1.6 200 Rh32 5 1.0 100 17 Pt60-Rh32 20 1.1 150 18 Pt60 6 1.0 170 19 Pt60-Rh32 20 1.1 150 20 Pt60 6 1.0 170 21 No catalyst * + NonIr 3 0.860 22 No catalyst * + Ir60-Pt60 21 1.3 220

In Table 1, Examples are Sample Nos. 2 to 15 and 22, Sample No. 21 is a comparative example in which neither Pd nor Ir is added to SnO2, and Sample No. 22 is an Ir-containing SnO2 gas sensor 2 in which Pd is not added. This is an example in which a Pt composite catalyst is supported by dropping.

As is clear from Table 1, the addition of Ir is α
Is remarkably increased, and the effect of increasing α is not observed except for Ir. In addition, the increasing effect of α was measured by Ir of sample numbers 9 to 15.
It is also found in -Pt catalysts and Ir-Rh catalysts. The increase in α is Ir
It is already expressed at a concentration of 5 μg / g. However, the sensor resistance increases with the addition of Ir, and when 1000 μg / g is added, the sensor resistance becomes about 2 MΩ, which is an impractical value. Therefore, the Ir addition amount is 5 to 500 μg / g, preferably 20 to 250 μg. / G.

Similarly, from Table 1, it is clear that the addition of the Ir-Pt composite catalyst reduces the temperature and humidity dependence. This effect is not observed except for the Ir-Pt composite catalyst, and the Ir-Pt
Specific to composite catalysts. As is apparent from Sample Nos. 9 to 14, the amount of Pt added to the Ir-Pt catalyst was 5 to 10.
500 μg / g is preferred, and more preferably 10 to 50
0 μg / g. Also, the weight ratio of Ir / Pt (in terms of metal)
Is preferably 5 to 1/5.

The inventor of the present invention has reported that Ir-Pd,
Ir-Ru, Ir-Os, Ir-Re, Pt-Re, Pt-Pd,
Ru, Os and the like were similarly added to the gas sensor 2 (the addition amount was 60 μg / g for each element). However, none of the samples showed improvement in the temperature and humidity dependence, and the increase in α was Ir-P
d, Ir-Re, Ir-Ru and Ir-Os
u, Os, Pt-Re and Pt-Pd could not be obtained.

Sample Nos. 21 and 22 are examples in which the addition of Pd to SnO2 at the powder stage is omitted.
The -Pt composite catalyst has the effect of increasing α, reducing the temperature and humidity dependence, and increasing the sensor resistance Rs. Note that Ir
If an Ir-Pt composite catalyst is added to SnO2 in the powder stage, the effect is reduced.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a gas sensor according to an embodiment.

FIG. 2 is a cross-sectional view of a gas sensor according to a modification.

FIG. 3 shows a gas sensor according to an example in which the addition amount of Ir is 60 μg / g.
Characteristic diagram showing temperature and humidity dependence at 1000 ppm

FIG. 4 shows a gas sensor according to an example in which Ir and Pt were added at 60 μg / g, respectively, from -10 ° C. to 20 ° C. and CO 100 p.
Characteristic diagram showing temperature and humidity dependence at pm to 1000 ppm

FIG. 5 shows a conventional gas sensor in which both Ir and Pt are not added, with CO of 100 ppm to 100 at −10 ° C. to 20 ° C.
Characteristic diagram showing temperature and humidity dependence at 0 ppm

FIG. 6 shows a conventional gas sensor to which Pt was added at 60 μg / g.
Characteristic diagram showing temperature and humidity dependence at 1000 ppm

FIG. 7 shows a conventional gas sensor to which Rh was added at 32 μg / g.
Characteristic diagram showing temperature and humidity dependence at 1000 ppm

FIG. 8 is a characteristic diagram showing the temperature / humidity dependence of a conventional gas sensor containing 60 μg / g of Pt and 32 μg / g of Rh at -10 ° C. to 20 ° C. and 100 ppm to 1000 ppm CO.

[Explanation of symbols]

 2,12 Gas sensor 4,14 Metal oxide semiconductor 6,8 Heater / electrode 16 Insulating substrate 18 Glass film 20 Heater 22 Insulating film 24,26 Lead

Claims (4)

[Claims] 1. A CO sensor for detecting CO while periodically changing the temperature of SnO2 of a gas-sensitive material, wherein said SnO2 has an I of 5 to 500 μg / g SnO2 in terms of metal.
A CO sensor to which r is added. 2. The above-mentioned Ir is an Ir-Pt composite catalyst, the weight ratio of Ir / Pt is 1/5 to 5 in terms of metal, and the amount of addition to SnO2 in terms of metal is 5 to 500 μg for both Ir and Pt. /
2. The CO sensor according to claim 1, wherein the CO sensor is gSnO2. 3. After sintering a sintered body using SnO2 as a gas-sensitive material, impregnating the sintered body with a solution of an Ir compound so that the amount of Ir added to SnO2 is 5 to 500 μg / g SnO2 in terms of metal. And then decompose the impregnated Ir compound,
A method for manufacturing a CO sensor, characterized by adding Ir to said SnO2. 4. The method of claim 1, wherein the Ir compound solution is mixed with an Ir compound and Pt.
A mixed solution of the compounds was added to SnO2 at a metal conversion amount of 5 to 500 .mu.g / g SnO2 for both Ir and Pt, and a weight ratio of Ir / Pt of SnO2 in terms of metal to 1/5 to 5 .mu.g.
4. The method of manufacturing a CO sensor according to claim 3, wherein: