JPS62187245A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To obtain an oxygen sensor having good durability by forming a lead trap layer of a porous ceramic material so as to have >=0.3cc/g and <=5cc/g pore volume and >=200mu and <=1,000mu thickness. CONSTITUTION:Platinum electrodes 12, 13 are respectively provided on the inside and outside surface of a ceramic pipe 11 essentially consisting of zirconium oxide having closed ends and platinum is deposited by evaporation on the electrode 13 on the outside surface side to form a catalyst layer 14. A metallic oxide such as magnesium spinel is further thermally sprayed thereon to form a protective layer 15 and the lead trap layer 16 having >=0.3cc/g and <=5cc/g pore volume and >=200mu and <=1,000mu thickness is formed on the layer 15 by using the porous ceramic material such as gamma-alumina having a large specific area. Since the thickness of the layer 16 is large, the trapping amt. of the lead component is increased. The lead component contained in an exhaust gas is adsorbed by the gamma-alumina of the layer 16. On the other hand, the exhaust gas is made to arrive quickly at the layer 14 as the pore volume is large. The decrease of the reaction rate is thereby obviated. The durability of the oxygen sensor is thus improved.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an oxygen sensor used for detecting oxygen concentration in exhaust gas in an internal combustion engine, particularly in an air-fuel ratio control device thereof.

(Prior art) As this type of oxygen sensor, for example, there is one having a sensor structure as shown in FIG.
(See Japanese Utility Model Publication No. 04365 and Japanese Utility Model Application Publication No. 59-31054).

To explain the oxygen sensor shown in FIG. 5, a ceramic tube 1 whose main component is zirconium oxide (ZrOz) with a closed end is coated with platinum (platinum) on each part of the inner and outer surfaces.
After applying the Pt) paste, the ceramic tube 1 is fired to form platinum electrodes 2 and 3 for taking out the electromotive force. On the outer surface of the ceramic tube 1, a catalyst layer 4 is further formed by vapor-depositing platinum, and a protective layer 5 is formed by spraying a metal oxide such as magnesium spinel thereon.

Here, while the atmosphere is introduced into the inside of the ceramic tube 1 as a reference gas, the outside of the ceramic tube 1 is made to face the engine exhaust passage and is brought into contact with the exhaust gas, which is the gas to be detected, so that the atmosphere comes into contact with the inner surface. The oxygen concentration in the exhaust gas is detected by generating a voltage between the platinum electrodes 2 and 3 that corresponds to the ratio of the oxygen concentration in the exhaust gas to the oxygen concentration in the exhaust gas that contacts the outer surface.

The catalyst layer 4 promotes the CO+%Oz -COz (7) reaction, and when a rich mixture is combusted, the low concentration of 02 remaining in that area reacts favorably with CO to reduce the 0□ concentration. is set to zero, and the 0zfa degree ratio between the inside and outside of the ceramic tube 1 is increased to generate a large electromotive force. On the other hand, when combustion is performed with a lean mixture, there is a high concentration of 0□ and a low concentration of CO in the exhaust gas, so even if CO and Ot react, there is still 0□ left over, and the 0zfi degree inside and outside of the ceramic tube 1 increases. The ratio is small and almost no voltage is generated.

<Problems to be Solved by the Invention> In an internal combustion engine equipped with an oxygen sensor having such a sensor structure, when leaded gasoline containing a large amount of lead is used, the lead components contained in the exhaust gas may form a protective layer. 5 permeates and covers the surface of the catalyst layer 4, resulting in so-called lead coating. As this poisoning progresses, when the oxygen concentration in the exhaust gas changes to lower than the stoichiometric air-fuel ratio, the reaction between CO and 0□ by the catalyst layer 4 is not sufficiently promoted, as shown by the broken line in FIG. As a result, there is a delay in the rise of the electromotive force, which causes a response delay in the air-fuel ratio switching control from lean to rich based on the signal from the oxygen sensor, and the average value of the air-fuel ratio is biased toward the rich side.
There was a problem in that it adversely affected the fuel efficiency, output, exhaust emissions, etc. of the engine. Incidentally, the solid line in FIG. 7 shows the characteristics in the normal state.

Therefore, as a countermeasure against lead exposure, as shown in FIG. 6, a porous ceramic material such as T-alumina or δ-alumina, which has a larger specific surface area, is used on the protective layer 5, and the pore volume is 0.01. =0.08cc/g, film thickness 20μ~6
A lead trap layer 6 with a thickness of about 0 μm is provided, and lead is trapped with γ-alumina or δ-alumina to suppress lead deposition and improve the life of the oxygen sensor.

By the way, as shown in FIG. 3, the thicker the film thickness of the lead trap N6 described above, the greater the trapping effect on lead.However, in the conventional lead trap layer structure, the pore volume is small, so if the film thickness is increased, There was a problem in that the speed at which the exhaust gas reaches the catalyst layer 4 was slow, resulting in a decrease in responsiveness.

Therefore, the present invention was made in view of the above-mentioned circumstances.
The object of the present invention is to provide an oxygen sensor with even better durability by using a lead trap layer structure that can enhance the lead exposure effect without impairing response performance.

Therefore, in the present invention, the lead trap layer is made of a porous ceramic material with a pore volume of 0.3 cc/g or more and 5 cc/g.
g or less and a thickness of 200 μm or more and 1000 μm or less.

<Function> According to the above configuration, the lead trap layer is thick and can increase the amount of lead component trapped, suppressing lead encrustation in the catalyst layer, and the pore volume is large so that exhaust gas quickly reaches the catalyst layer. Therefore, the responsiveness will not deteriorate.

<Example> Embodiments of the present invention will be described below based on the drawings.

In FIG. 1 showing the first embodiment, platinum electrodes 12 and 13 are respectively provided on the inner and outer surfaces of a ceramic tube 11 mainly composed of zirconium oxide (ZrOz) and having a closed end, as shown in FIG. , a catalyst layer 14 is formed by vapor depositing platinum on the platinum electrode 13 on the outer surface side, a protective layer 15 is formed by spraying a metal oxide such as magnesium spinel, and a lead trap layer is formed on this protective layer 15. 16 is the same as the conventional method.

In the present invention, the lead trap Ji16 is made of a porous ceramic material with a large specific surface area, and has a pore volume of 0.3 cc/g or more and 5 cc/g or less, which is larger than the conventional one, and a thickness of 200 μm or more and 1000 μm or less. It is as follows.

For example, in this example, γ-alumina pellets with a specific surface area of 100rrf/g or more were milled in a ball mill with a particle size of 4 to 10 μm.
The lead trap layer 16 is formed by adding 10 to 40% alumina sol to the powder, applying it to a thickness of 200 to 400 μ on the protective layer 15, and baking it at 500 to 700°C. It is.

According to this configuration, the lead component contained in the exhaust gas is adsorbed by the γ-alumina of the lead trap layer I6, and the adsorption ability thereof becomes even better than that of the conventional case. On the other hand, since the pore volume is large, the exhaust gas quickly reaches the catalyst 7114, so the reaction rate does not decrease. Therefore, the durability of the oxygen sensor can be improved.

Figure 3 shows experimental data on the relationship between the usage time and the deviation of λ (air excess ratio) towards the rich side as the thickness of the lead trap layer 16 changes, and Figure 4 shows the lead trapping layer 16 when the pore volume is changed. This shows experimental data on the relationship between the thickness of the trap JiJ16 and the air-fuel ratio control frequency (reaction speed).
As is clear from the figure, the thickness of the lead trap layer 16 is 20 mm.
It can be seen that the effect is great when the pore volume is 0.0 μ or more and the pore volume is 0.3 cc/g or more. Furthermore, when the pore volume of the lead trap layer 16 exceeds 5 cc/g, the lead adsorption performance begins to be adversely affected, and the thickness of the lead trap layer 16 is 1000 μm.
It has been confirmed that if the lead trapping layer 16 is exceeded, the reaction rate is adversely affected and the lead trap layer 16 is easily peeled off.

FIG. 2 shows a second embodiment, in which the lower layer 16a of the lead I/lamp layer 1G is a catalyst supporting layer in which a catalyst, for example platinum, is supported on γ-alumina, and the upper layer I6b is simply a T-
It is a layer made of only alumina.

In this case, for example, T-alumina pellets with a specific surface area of Loom/g or more are ground with a ball mill to a particle size of 4-10μ, 10-40% alumina sol is added to make a paste, and part of this paste is added with chloroplatinic acid. (0.2 to 2.0 wt % as platinum) is coated on the protective layer 15 to a thickness of 10 to 50 μm and baked at 500 to 700°C.

After this baking treatment, the remaining paste without chloroplatinic acid is applied on top to a thickness of 200-400 μm.
By firing at 00° C., a lead trap layer 16 having a catalyst supporting layer is formed in the lower layer portion 16a.

According to the second embodiment, in addition to the same effects as in the first embodiment, even if the catalyst layer 14 is poisoned and its oxidizing effect is reduced, the catalyst support layer in the lower part 16a of the lead trap layer 16 can catalyze the The catalytic function of the layer 14 can be supplemented, and the durability of the oxygen sensor can be further improved.

Further, in the above embodiment, the lower layer 16a, the lead trap layer 1
6 both showed oxygen sensors using T-alumina,
If δ-alumina is used, an oxygen sensor that is stable even at high temperatures can be obtained.

(Effects of the Invention) As described above, according to the present invention, the lead trap layer is formed with a pore volume of 0.3 cc/g or more and 5 cc/g or less, and a thickness of 200 μm or more and 1000 μm or less. Therefore, it is possible to further enhance the suppressing effect of lead deposition on the catalyst layer without reducing the reaction rate, and not only improve fuel efficiency, output, and exhaust emission characteristics, but also further improve the durability of the oxygen sensor. I can do it.

[Brief explanation of drawings]

Fig. 1 is a sectional view of the main part showing the first embodiment of the present invention, Fig. 2 is a sectional view of the main part showing the second embodiment of the invention, and Fig. 3 is a case where the thickness of the lead trap layer is changed. Fig. 4 shows the relationship between the thickness of the lead trap layer and the reaction rate when the pore volume is varied, and Fig. 5 shows the relationship between the degree of bias toward the rich side and the usage time. A cross-sectional view showing the conventional sensor part of the applied oxygen sensor, Figure 6 is a cross-sectional view of the main part showing a conventional example with a lead trap layer, and Figure 7 shows the output voltage state during normal and lead poisoning. It is a diagram. II...Ceramic tube 12...Platinum electrode on the inner surface side 13...Platinum electrode on the outer surface side 14...Catalyst layer
15... Protection [16... Lead trap layer Patent applicant Japan Electronics Co., Ltd. Nissan Motor Co., Ltd. Agent Patent attorney Fujio Sasashima Figure 5 Figure 6

Claims (2)

[Claims] (1) Platinum electrodes are provided on the inner and outer surfaces of a ceramic tube with a closed end, and a catalyst layer, a protective layer, and a lead trap layer are sequentially laminated on the platinum electrode on the outer surface, and the inner surface is exposed to the atmosphere. In an oxygen sensor that detects the oxygen concentration in exhaust gas by the electromotive force generated between the electrode and the outer surface electrode that is in contact with the exhaust gas of an internal combustion engine, the lead trap layer is made of a porous ceramic material with pores. Volume is 0.3cc/
An oxygen sensor characterized in that it is formed to have a thickness of 200μ to 1000μ and a thickness of 200μ to 1000μ. (2) The oxygen sensor according to claim 1, wherein the lead trap layer has a lower layer that is a catalyst support layer in which a catalyst is supported on a porous ceramic material.