JPH0652245B2 - Exhaust gas sensor - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an improvement in an exhaust gas sensor utilizing a change in resistance value of a metal oxide semiconductor, and is used for detecting an air-fuel ratio of an automobile engine, a stove, a boiler or the like. It is suitable.

[Prior Art] The applicant has proposed an exhaust gas sensor in which a small amount of a noble metal catalyst is added to BaSnO ₃ (Japanese Patent Application No. 59-258,890 and Japanese Patent Application Laid-Open No. 61-137053). Here, BaSnO ₃ has the characteristics that it has excellent durability in a high-temperature reducing atmosphere and detection sensitivity to the air-fuel ratio, and that the detection error due to unreacted components in the exhaust gas is small (Table 1). Then, in this application, the catalyst is added by the impregnation method. The function of the catalyst is to remove unreacted components such as CO and HC in the exhaust gas by catalytic oxidation,
This is to prevent a decrease in detection accuracy due to the presence of unreacted components.

Various methods for adding a catalyst have already been known. For example, JP-A-59-168,353 discloses that Pt is added to TiO ₂ .
The addition of black is disclosed. The significance of the catalyst in the case of TiO ₂ is believed to be to improve the responsiveness to changes in the atmosphere (JP-A-55-136,
941).

The inventor has found that when the Pd addition method is changed from the impregnation method to Pd black addition, the detection sensitivity near the equivalence point increases.

[Problem of the Invention] An object of the present invention is to improve the detection sensitivity in the vicinity of the equivalent point (point of λ = 1) of BaSnO ₃ .

[Structure of the Invention] The exhaust gas sensor of the present invention contains BaSnO ₃ per 1 g thereof,
It is characterized by adding 0.5 to 150 mg of Pd black in terms of metal.

The significance of the addition of Pd black is to improve the sensitivity to the exhaust gas composition near the equivalence point. The effect of addition is BaSnO ₃ 1
It occurs when the amount is 0.5 mg or more per gram, and becomes saturated when the amount is 10 mg or more. This effect occurs only when Pd is added as black and cannot be obtained by the addition by the impregnation method. The effect is small with Pt black and Rh black.

[Examples] Preparation of sample Mixing equimolar amounts of BaCO ₃ and SnO ₂ for 1 hour in air for 4 hours
React at 200 ° C. In this process, BaCO ₃ reacts with SnO ₂ to produce the provskite compound BaSnO ₃ .

The BaSnO ₃ obtained is mixed with the desired amount of Pd black and milled wet. In this way, BaSnO ₃ containing Pd is obtained.
A pair of noble metal electrodes are embedded in the crushed BaSnO ₃ , molded into the shape of the sensor chip shown in FIG. 4, and sintered in air at 1300 ° C. for 4 hours.

Assemble the sensor chip into the sensor of FIG.

In the figure, (2) is the above-mentioned sensor chip, and (4) and (6) are its electrodes. (8) is an insulating substrate made of alumina or the like, and the chip (2) is housed in the recessed portion provided at the end thereof. (10), (1
2) is a base metal external lead, and (14) is an alumina coating for protecting the electrodes (4) and (6).

As comparative examples, those in which Pd black is replaced by Pt or Rh black, and those without a catalyst are manufactured. As a comparative example by the impregnation method, a chip (2) after sintering is impregnated with an aqueous solution of Pd chloride and thermally decomposed at 950 ° C. to be prepared. The Pd content in this example is about 1 mg / g BaSnO _{3 in} terms of metal.
It is. The only difference between the comparative example and the example is the catalyst.

1mg after calcination at 1200 ℃ to show the characteristics of BaSnO ₃
As a comparative example, SnO ₂ or TiO ₂ added with a catalyst of 1 / g and sintered at 1300 ° C.

Characteristics of BaSnO ₃ BaSnO ₃ and Sn to which Pd black of 1 mg / g was added respectively
Regarding O ₂ and TiO ₂ , the sensitivity to oxygen, the sensitivity to combustible gas that has not reacted in the exhaust gas, and the durability to a high-temperature reducing atmosphere are evaluated.

Oxygen sensitivity is representative of the sensitivity to the atmosphere. The measurement was carried out under an N ₂ balance of 700 ° C. with an oxygen partial pressure of 1% to 1%.
It is performed from the change of the resistance value when changing to 0%. Result is,
The resistance value of the sensor is shown by the value of m (hereinafter referred to as the oxygen gradient) when InRS = K + mInPo ₂ .

CO is used as a representative of flammable gas, 700 ℃
With a N ₂ balance system with an oxygen partial pressure of 4.6%, CO concentration of 10
Change from 00ppm to 10000ppm. CO1000
The resistance value ratio between ppm and 10000 ppm is measured, and the sensitivity to CO is evaluated from this value. The ideal value for this value from the oxygen gradient is around 1.02. If this value is larger than 1.02, the CO sensitivity is higher than the oxygen sensitivity,
A detection error occurs due to the presence of unreacted combustible gas.

To evaluate the durability to high temperature reducing atmosphere, 900
The sensor is exposed to the atmosphere having λ of 0.9 and 1.1 at 10 ° C. for 10 minutes alternately for 10 hours. A sensor having low durability irreversibly reduces the resistance by this process. 7 sensors after processing
After returning to 00 ° C. and treating in a measurement atmosphere for 1 hour, the resistance value when λ is 1.15 is measured and the ratio with the value before treatment is determined.

The results are shown in Table 1.

As is clear from the results in Table 1, BaSnO ₃ has a small sensitivity to oxygen, a small detection error due to a combustible gas such as CO,
It is superior to SnO ₂ and TiO _{2 in} terms of both durability to high temperature reducing atmosphere.

Effect of Pd black Figure 1 shows BaSnO ₃ with various types of black added, with lean side (λ = 1.02) and rich side (λ = 0.98).
And shows the resistance value. The measurement temperature is 700 ° C., and the average value is shown for each of the five sensors. The vertical axis represents the resistance value without a unit, and is conveniently shifted for each material.

The ratio of the resistance values of the lean side and the rich side represents the detection sensitivity to changes in the exhaust gas composition centered on the equivalence point. BaSnO ₃ to Pd
When black is added, the detection sensitivity is slightly lowered when the amount added is 0.1 mg / g. When the added amount is 1 mg / g, the resistance value on the rich side is sharply decreased and the sensitivity is increased. And this effect is saturated at the addition of 10 mg / g or more.

A similar effect can be obtained with Pt black, but it is smaller than with Pd and has little effect with Rh black.

Such an effect is obtained only when it is added as black, and the sensitivity is not improved even if Pd is added by the impregnation method.

Increased sensitivity is not only obtained at certain temperatures, but occurs at other temperatures as well. FIG. 2 shows the results at 600 ° C., and FIG. 3 shows the results at 800 ° C. These results are approximately equal to those at 700 ° C.

Other characteristics Table 2 shows changes in sensitivity to CO and propylene by Pd black.

[Effect of the Invention] In the present invention, the addition of Pd black can improve the detection sensitivity near the equivalence point of BaSnO ₃ .

[Brief description of drawings]

1 to 3 are characteristic diagrams of the embodiment, and FIG. 4 is a plan view of an exhaust gas sensor used in the embodiment. In the figure, (2) ... sensor chip, (4), (6) ... electrode.

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP 61-137053 (JP, A) JP 59-168353 (JP, A) JP 55-136941 (JP, A)

Claims (1)

[Claims] 1. An exhaust gas sensor in which a precious metal catalyst is added to BaSnO ₃ , wherein the precious metal catalyst is Pd black, and the addition amount is
An exhaust gas sensor, characterized in that it is 0.5 to 150 mg per 1 g of BaSnO _{3 in} terms of Pd metal.