JP2748031B2 - Oxygen sensor for prevention of Si poisoning - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor, and more particularly to an oxygen sensor for controlling an air-fuel ratio used in combination with a three-way catalyst of an exhaust gas purification system for an automobile or the like.

[Prior art and problems] The environment surrounding an oxygen sensor is quite severe. As emissions regulations have been tightened, regulations of 0.4 g / mile No _X have already been implemented in California. Therefore, it is an important necessary condition for the sensor to suppress the variation of the control A / F at the initial stage and to suppress the A / F fluctuation after the endurance.

Furthermore, in recent years, many engine parts use silicon, and the influence of this silicon cannot be ignored.

Accordingly, an object of the present invention is to prevent the control A / F variation at the initial stage, to reduce the A / F fluctuation after the durability, and to prevent the lean shift and the fall responsiveness from being deteriorated due to Si poisoning.

[Means to solve the problem] In order to suppress the control A / F variation at the initial stage and the fluctuation after the endurance, the protective layer on the exhaust gas side of the oxygen sensor should contain a noble metal before the gas arrives at the sensor electrode. Various measures have been taken to completely advance the combustion reaction of unburned components. Among them, we have also found that by providing a protective layer (second protective layer) made of a non-stoichiometric compound, it is possible to maintain a very effective catalyst even after endurance (Japanese Patent Application No. 62-62).
311278). This is a non-stoichiometric compound such as TiO _2-X (x
≤0.4) Particles with electron holes, etc., prevent excessive CO and O ₂ from adsorbing to electrodes, etc. in a rich and lean atmosphere, and have good compatibility with precious metals. Control A / F can be concentrated near λ ≒ 1. The amount of the noble metal in this layer is preferably 2 mol% or less based on the non-stoichiometric compound. If the amount is more than 2 mol%, the emission gradually becomes rich and CO and the like are discharged. Sensor control A /
F has little variation not only in the initial stage but also after the endurance. However, when the Si component was contained in the exhaust gas, although the effect was seen compared with the conventional oxygen sensor element, it shifted considerably to the lean side.

Therefore, in the present invention, the protection layer (including the one carrying a noble metal) contains a component consisting of a Group IIa element in the periodic table (hereinafter, referred to as a "Group IIa component"). The Si layer is allowed to adsorb and react with the protective layer before the Si component in the exhaust gas reaches the active point of the sensor.

This is because the IIa group components in the protective layer, especially Ca, Mg, etc., react with Si contained in the exhaust gas at the temperature under the condition where the sensor is used to generate low melting point crystals. A protective layer (first protective layer) which is located on the inner side and is made of a heat-resistant metal oxide and does not contain a Group IIa component (particularly spinel, Al ₂ O ₃
It is presumed that Si no longer enters the first protective layer and protects the noble metal and the electrode in the first protective layer. In particular, the change products and carbonates containing Ca or Mg form very fine particles, so that the Si component can be prevented from passing through and the activity is high with respect to the Si.

However, when silicon contained in the exhaust gas is mixed at low engine speed, that is, when the temperature is low, the effect of adsorbing Si by IIa group components (especially Ca and Mg compounds) is weakened, and Si enters the protective layer without reacting. I had to do it.
Therefore, when the sensor was exposed to a high temperature, the Si changed to SiO ₂ or the like, and the protective layer was sometimes clogged.

Therefore, in the present invention, the infiltration of Si can be prevented by adding a group IIa component to the protective layer on the side exposed to the exhaust gas and by providing a heater for heating the sensor element. Things. That is, by increasing the temperature of the protective layer by this heater and increasing the adsorption capacity of the IIa group component, Si reacts with the IIa group component and the penetration of only Si can be reduced.

II Regarding the protective layer containing Group a component (second protective layer), II
Ca and Mg are good as group a elements. The composition of the IIa group component includes non-oxides such as chlorides such as CaCl ₂ and MgCO ₃ ,
Carbonates and nitrates are good. However, in the case of an oxygen sensor having a heater, an oxide sensor is effective. In addition, these hydrates such as CaCl ₂ .2H ₂ O, complex compounds such as CaCO ₃ .M
It may be gCO ₃ (dolomite). II Group a component is Al ₂
It is preferable to support on a heat-resistant metal oxide such as O ₃ and titania. In particular, as a non-stoichiometric compound, for example, TiO _2-X (x ≦
0.4), It is preferable to carry out high dispersion loading on La ₂ O _3-X .

As a manufacturing method, for example, a group IIa component is previously supported on titania particles (for example, having an average particle size of 0.1 to 1 μm), applied as a slurry on the first protective layer, and subjected to a heat treatment (for example, 500
To 700 ° C.): a method in which titania particles are applied to the first protective layer, immersed in a Group IIa component solution under reduced pressure or pressure, and then heat-treated. In these cases,
The proportion of the Group IIa component to the refractory metal oxide of the second protective layer is 30 wt% or less, more preferably 20 wt%, in terms of the Group IIa element.
%. If the content exceeds 30 wt%, the response of the sensor gradually deteriorates, that is, clogging starts to occur.

In order to prevent deterioration of the durability of the noble metal as an electrode or a catalyst, it is necessary that at least a part of the refractory metal oxide is present as a non-stoichiometric compound such as TiO _{2 -X} . However, not all need to be non-stoichiometric compounds.
A Group IIa component can be dispersed and supported together with a stoichiometric compound (eg, TiO ₂ , Al ₂ O ₃ , spinel). In this case, the proportion of the non-stoichiometric compound to the stoichiometric compound is preferably 3: 2 or more, more preferably 2: 1 or more.

In addition, noble metal, preferably Pt, is less than 2 mol% based on the non-stoichiometric compound
(1.5 mol% or less), the variation in the control A / F at the initial stage can be further suppressed. In this case, the second protective layer may contain the group IIa component and the noble metal in the same part, but the part carrying the noble metal (first protective part) and the component carrying the IIa group component (A second protection unit). However, in this case, it is particularly necessary to dispose the second protection portion more outside. This second
1, Regarding the heat-resistant metal oxide that constitutes the second protection part,
Both are preferably non-stoichiometric compounds. For the first protection unit, it is necessary that the non-stoichiometric compound is at least 60%. It is preferable that the group IIa component exists independently of the heat-resistant metal oxide constituting the protective layer. It refers to II a group component does not form a inert compound against Si components such as to e.g. MgTiO ₃ react with these metal oxides as "independently".

Further, it is preferable to provide a protective layer (first protective layer) which is located inside the second protective layer so as to directly cover the electrode and does not contain a group IIa component. The first protective layer is also made of a heat-resistant metal oxide, and is particularly preferably spinel (MgO.Al ₂ O ₃ ) or alumina, and is preferably one that is firmly attached by thermal spraying. The inclusion of a noble metal such as Pt and / or Rn, Pd in the first protective layer completely oxidizes and reduces unburned components in the exhaust gas, and provides good control for the sensor.
F will be shown. Note that a third protective layer made of, for example, titania, alumina, spinel, or the like may be further provided outside the second protective layer.

Needless to say, each protective layer needs to have air permeability that does not degrade sensor responsiveness. Therefore, for the first protective layer, for example, the porosity is 10 to 30%, the thickness is 10 to 150 μm,
For the second protective layer, for example, a porosity of 8 to 35% and a thickness of 10 to
It is good to set it to 50 μm.

The sensor element body, for example, the surface of the zirconia solid electrolyte on the measurement electrode side may have a structure having irregularities of 10 μm or more. Excellent protection by preventing peeling of the protective layer.

The main material of the sensor element is ZrO ₂ solid electrolyte,
It may be a TiO ₂ , CoO semiconductor or the like. The non-stoichiometric compound may be lanthanum oxide in addition to titania. The type, material (for example, ceramic), mounting position, and the like of the heater are not limited as long as the above-mentioned effects can be exhibited.

[Example] Example A (Table 1, Sample Nos. 1 to 14) 1. The sensor element body (electrode, first
With protective layer).

Step 1: 5 mol% of 99% pure Y ₂ O ₃ is added to 99% or more pure ZrO ₂ , and the mixture is wet-mixed and calcined at 1300 ° C. for 2 hours.

Process 2: 80% of the particles are 2.5% wet in a ball mill by adding water
Grind to a particle size of μm or less.

Step 3: A water-soluble binder is added, and spherical granulated particles having an average particle size of 70 μm are obtained by spray drying.

Step 4: The powder obtained in step 3 is rubber-pressed and the desired tube (U
After drying and grinding, it is ground to a predetermined shape with a grindstone.

Step 5: A slurry containing a water-soluble sodium binder cellulose glycolate and a solvent is attached to the granulated particles obtained in step 3 on the outer surface.

Step 6: After drying, firing at 1500 ° C. × 2 hrs. Axial length 25mm, outer diameter about 5mmφ, inner diameter about 3m
mφ.

Step 7: Pt measurement electrode layer with a thickness of 0.9 on the outer surface by electroless plating
Deposit to a thickness of μm and then bake at 1000 ° C.

Step 8: forming a first protective layer directly covering the electrode having a thickness of about 100 μm by plasma spraying with MgO · Al ₂ O ₃ (spinel) powder.

Step 9: A Pt reference electrode layer was formed on the inner surface in the same manner as in step 7.

2. Spinel spraying of the element was performed in a 0.05 g / Pt H ₂ PtCl ₆ solution, and the element was evacuated to carry a noble metal (Pt) in the first protective layer. The amount of the carrier is about 0.1 with respect to the metal oxide of the first protective layer.
02 to 0.05 wt%.

3. Add a Group IIa component such as CaCl ₂ · 2H ₂ O (dissolved in pure water) to TiO _2-X powder with an average particle size of about 0.2μm, dry while boiling and then 550 ℃ And heat treated. Sample No.1
1,12 also contained α-Al ₂ O ₃ . The TiO _2-X powder was obtained by treating TiO ₂ particles in a non-oxidizing atmosphere at 600 ° C. or higher in advance. Even if about 0.01 mol% of a noble metal is contained in TiO ₂ , a non-stoichiometric compound can be obtained. When the noble metal was supported on the titania powder, the titania particles were previously placed in a desired noble metal-containing salt solution, dried by boiling, and then heat-treated at 550 ° C. in the atmosphere.

To the device obtained in 4.2, an organic binder and butyl carbitol were added to the powder obtained in 3 and applied and baked with a brush. Baking is 50
Performed at 0 ° C. in a reducing atmosphere.

5. A known sensor was assembled.

Example B (Table 2, Samples Nos. 15 to 27) 1,2. Same as 1 and 2 in A. The device obtained in 3.1 was combined with TiO _2-X powder having an average particle size of about 0.2 μm and an organic binder. Butyl carbitol was added, applied with a brush and dried. Drying was performed at 120 ° C. in the atmosphere.

4. CaCl ₂ · 2H ₂ O was dissolved in water, the coated part of the device obtained in 3 was put in, and vacuum was drawn. At that time, the Ca concentration was changed variously. Thereafter, drying was performed at 100 ° C. in the atmosphere.

Example C (Table 3, Samples 28 to 35) (a) Production of sensor element Same as Example A or B above.

(B) Manufacture of heater etc. 1. A sheet mainly composed of Al ₂ O ₃ was formed to a thickness of 0.8 mm by a doctor blade method.

2. A conductive pattern was printed by a screen printing method using a paste containing W as a main component and an organic binder and a solvent.

3. Further, a 30 μm-thick coating was performed with a paste containing Al ₂ O ₃ as a main component and an organic binder and a solvent.

4. The sheet obtained in step 3 was wound around an insulator ₂ mm in outer diameter containing Al ₂ O ₃ as a main component, 24Hr was removed from the resin at 400 ° C, and fired at 1550 ° C × 2Hrs.

5. A lead wire was soldered to the terminals to obtain a heater.

6. The heater obtained in 1 to 5 was inserted so as not to contact the inside of the bag-shaped element when assembling the element.

Example D (Table 3, Samples 37 to 39) 1. A sheet containing 5 mol% of ZrO ₂ + Y ₂ O ₃ as a main component was 0.8 mm thick.
Was formed by a doctor blade method.

2. The electrodes were printed on both sides with a thickness of 20 μm using a paste containing Pt as a main component, an organic binder and a solvent by a screen printing method.

3. A 30 μm thick coating was made with a paste containing Al ₂ O ₃ as a main component, an organic binder and a solvent, and a small amount of starch or the like to make the electrode more porous so as to cover the electrode.

The same sheet as in 4.1 was coated on both sides with a paste containing Al ₂ O ₃ as a main component and an organic binder and a solvent in a thickness of 30 μm.

A 20 μm heater pattern was printed with the same paste as in 5.2.

6. Step 4 was repeated (however, only the surface on the heater pattern).

The same sheet as in 7.1 was cut into a U-shape, and a spacer sheet was placed between the electrode printing sheet obtained in 1 to 3 and the heater pattern-containing sheet obtained in 4 to 6 and thermocompression-bonded.

8. After baking out the resin at 400 ° C for 24 hours, baking was performed at 1500 ° C for 4 hours.

9. A protective layer carrying titania and IIa group components was formed. Regarding the second protective layer, the sample No. 37 is the sample No. of the example A.
3 and Sample Nos. 38 and 39 correspond to Sample No. 1 of Example B.
Same as in 7,18.

Example E (Table 4, Sample Nos. 40 to 55, 60, 61) 1. Same as 1 (Steps 1 to 9) in Example A.

For samples 45 to 48, the first protective layer was impregnated with a noble metal in the same manner as in Example A-2.

2. TiO ₂ powder (average particle size 0.3μm) with Pt0.05g / ~ 1g /
Dipped in H ₂ PtCl ₆ solution and / or Rh0.05G / of RnCl ₃ · xH ₂ O solution, and left for about 5 minutes at a pressure of 50～100MmHg, Pt or Rh respect TiO ₂ is the 1 mol% equivalent Impregnation. Next, after drying, it was treated in the air at 600 ° C., heat treated, and further made into a paste with an organic binder and a solvent.

3. This paste was applied to the first protective layer and dried at 120 ° C. (first protective part, 20 μm thick).

4. The titania powder was placed in a solution such as CaCl ₂ · 2H ₂ O and dried while boiling. Thereafter, a paste was formed with a water-soluble binder and water. 20 wt% of TiO _{2 in} terms of Group IIa metal
And

5. This paste was applied on the first protection part and baked at 600 ° C. (2nd protection part, 20 μm thickness).

6. The third protection layer is formed on the second protection layer (consisting of the first and second protection parts).
A protective layer was formed. In addition, samples having various multilayer structures as shown in Table 4 were prepared.

7. A known sensor was assembled.

Example F (Table 4, Sample Nos. 56 to 59) 1. Same as 1 to 8 in Example D. However, for Nos. 58 and 59, the noble metal was contained in the protective layer made of Al ₂ O ₃ in the same manner as in Example A-2.

2. The same paste as in Example E-3 was applied and baked in the air at 600 ° C. to form a first protective portion (20 μm).

3. The same paste as in Example E-5 was applied and baked in the air at 600 ° C. to form a second protective portion (20 μm).

4. A pair of supports were mounted on both sides of the element thus obtained by glass seals.

5. A known sensor was assembled.

The present invention is not limited to the above-described embodiment. Various types of air-fuel ratio control oxygen sensors, for example, a full-range air-fuel ratio control sensor provided with a pump or the like (FIG. 7),
Sensors using metal oxide semiconductors such as O ₂ and CoO (No. 8
Figure) can also be applied. In the case of a semiconductor type sensor, for example, a noble metal may be contained in a metal oxide which is a semiconductor, a thermal sprayed layer such as spinel may be provided, and a second protective layer containing a group IIa component may be provided.

1 to 8 show examples of the oxygen sensor according to the present invention. In each of the drawings, 1 is a sensor element main body, 2 is a reference electrode, 3 is a measurement electrode, 4 is a first protective layer, 5 Is a second protective layer, 5a is a refractory metal oxide (especially a non-stoichiometric compound), and 5b is IIa
Group components 6 represent heaters respectively.

[Test] The following tests were performed for each sample.

1. The control A / F at the initial stage of the sensor was measured with a real vehicle. The sensor was mounted on the manifold and controlled by the sensor when it was fixed at a running state of 80 km / Hr × 8 ps. The A / F of the exhaust gas was measured with an air-fuel ratio meter.

2. Attach a sensor to the exhaust pipe (manifold about 1 m downstream), and inject 1Hr (10 cc) of Si oil from the manifold at a rate of 5 cc / 30 min. 3000 rpm (However, No. 28 to 39 for heater type) The engine was operated at 1000 rpm except for [Si test]. The atmosphere was around λ ≒ 1.

3.1 Measures and changes A / F at initial and endurance (ΔA / F)
I asked. The sensor output was monitored by a high-speed response recorder as the response of the sensor (for example, FIG. 10). And
As shown in FIG. 11, an average straight line was connected, and the time (T _LR , T _RL ) between 300 and 600 mV was measured after the Si test.

4. For sample Nos. 28 to 39, the E / G
At rotations of 1000 rpm and 3000 rpm, the heater was energized (except for sample No. 35), and the control state of the sensor was observed.

5. In addition, a thermal cycle test of λ ≒ 1,850 ° C (30 minutes) idle (30 minutes) 1000Hrs was performed with the actual vehicle engine, and control A
/ F was measured.

 The results are shown in Tables 1 to 4 and FIGS.

As is clear from Tables 1 to 4, the comparative samples outside the scope of the present invention showed large A / F fluctuations (Δ
A / F ≧ 0.08), falling response is slow (T _RL ≧ 120m)
S). Also, in the thermal cycle test, the A / F fluctuation is large (ΔA / F = 0.04).

On the other hand, each of the samples of the examples had A / F
The fluctuation is remarkably suppressed (ΔA / F ≦ 0.04), and the response is maintained at a high level (T _RL ≦ 90 mS). Also, in the thermal cycle test, A / F fluctuation is suppressed (ΔA / F ≦ 0.02).

Also, from the results in Table 3, it can be seen that when the rotation speed is low (low temperature) at 1000 rpm and 300 ° C, the heater is not present (Sample No. 3).
5) ΔA / F increases. This is thought to be due to the fact that when Si is mixed at low temperature, the adsorption effect by the IIa group component is weakened. On the other hand, each of the elements Nos. 28 to 34 and Nos. 37 to 39 of this embodiment having the heater can significantly suppress the A / F fluctuation even after the Si test at this low temperature (ΔA / F ≦
0.05).

It was also confirmed that when the IIa group component and the noble metal were present in separate protective portions for the second protective layer, the initial value of A / F tended to be richly controlled (Table 4).

[Effects of the Invention] As described above, according to the present invention, even if the Si component is present in the exhaust gas, A / F can be concentrated while preventing poisoning, and the fall response is excellent. In addition, performance degradation due to Si poisoning can be reliably prevented even at low engine speeds.

[Brief description of the drawings]

1 is a partial cross-sectional view of a sensor showing the operation of the present invention, FIG. 2 is a partial cross-sectional view showing an example of the oxygen sensor of the present invention, and FIGS. 3 and 4 are sensors showing the operation of the present invention. (FIG. 3 is a case without a heater, FIG. 4 is a case with a heater), FIG. 5 is a partial cross-sectional view showing an example (bag shape) of the oxygen sensor of the present invention, FIG. FIG. 7 is a partial cross-sectional view showing an example (plate-like) of the oxygen sensor of the present invention. FIG. 7 is a cross-sectional view showing an example of an oxygen sensor for whole-range air-fuel ratio control (with a pump element) according to the present invention. FIG. 8 is a view showing an example of a semiconductor type oxygen sensor according to the present invention, wherein FIG. 8 (a) is a plan view thereof (a protective layer is omitted), FIG. 8 (b) is a sectional view thereof, and FIG. Figure 9 waveforms showing the output in the control after the test schematic, FIG. 10, FIG. 11 example and measurement time of the waveform of sensor output (T _LR, T _RL) and Diagram for the constant, Figure 12 is a graph showing the initial and change of the control A / F after durability and Figure 13, represents respectively graphs, showing changes in initial and after endurance of (T _LR, T _RL). A: oxygen sensor, B: oxygen sensor element 1: element body, 2: measuring electrode 4: first protective layer, 5: second protective layer 5a: heat-resistant metal oxide (particularly non-chemical Stoichiometric compound) 5b ... II group a component

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroyuki Ishiguro 14-18 Takatsuji-cho, Mizuho-ku, Nagoya, Aichi Prefecture Inside Japan Specialty Ceramics Co., Ltd. No. 18 Japan Special Ceramics Co., Ltd. (56) References JP-A-59-222754 (JP, A) JP-A-60-66154 (JP, A) JP-A-1-97855 (JP, A) JP-A-60 -205343 (JP, A) JP-A-61-137053 (JP, A)

Claims (8)

(57) [Claims] In a sensor for detecting oxygen concentration in exhaust gas, a component composed of a heat-resistant metal oxide and a group IIa element of a periodic table (hereinafter referred to as "IIa") is provided on a side of the sensor element exposed to the exhaust gas. An oxygen sensor comprising a protective layer carrying a group III component), wherein at least a portion of the protective layer is present as a non-stoichiometric compound for the refractory metal oxide. 2. A sensor for detecting the concentration of oxygen in exhaust gas, comprising a heater for heating the sensor element, wherein a heat-resistant metal oxide made of a heat-resistant metal oxide is carried on a side of the sensor element exposed to the exhaust gas. An oxygen sensor comprising a protective layer, wherein at least a part of the protective layer is present as a non-stoichiometric compound for the refractory metal oxide. 3. The protective layer according to claim 1, further comprising a noble metal.
An oxygen sensor according to item 2. 4. A sensor for detecting oxygen concentration in exhaust gas, comprising a protective layer made of a heat-resistant metal oxide and carrying a Group IIa component and a noble metal on a side of the sensor element exposed to the exhaust gas. Characterized in that at least a part of the layer exists as a non-stoichiometric compound with respect to the refractory metal oxide, and the noble metal-supported part is located closer to the electrode than the part supporting the IIa group component. Oxygen sensor. 5. A sensor for detecting the concentration of oxygen in exhaust gas, comprising a heater for heating the sensor element, wherein a heat-resistant metal oxide comprising a heat-resistant metal oxide is provided on the side of the sensor element exposed to the exhaust gas. A protective layer supported thereon, wherein at least a part of the protective layer is present as a non-stoichiometric compound with respect to the refractory metal oxide, and the portion where the noble metal is supported is more electrode than the portion where the group IIa component is supported. An oxygen sensor, which is located in close proximity to a sensor. 6. The method according to claim 1, wherein the non-stoichiometric compound is titania and the titania particles are a protective layer in which a Group IIa component is dispersed and supported. An oxygen sensor according to any of the preceding claims. 7. The oxygen sensor according to claim 1, wherein the main body of the sensor element is made of a ZrO ₂ solid electrolyte. 8. The protective layer containing no Group IIa component contains a noble metal, and the surface of the sensor element body on the side of the measuring electrode is uneven.
The oxygen sensor according to any one of claims 1, 2, 4, and 5, wherein the oxygen sensor has a thickness of 10 µm or more.