JPH02222831A - Oxygen sensor for prevention of poisoning from si - Google Patents

Abstract

PURPOSE:To enable prevention of degrading in a falling response caused by poisoning from Si by arranging a protective layer with a component comprising elements in II (a) group of periodic table supported on the side exposed to an exhaust gas of a sensor element so that at least a part of the protective layer exists as non-stoichiometric compound with respect to heat resistant metal oxide. CONSTITUTION:A protective layer 5 comprising a non-stoichiometric compound 5a is provided to enable the maintaining a highly effective catalyst still after endured. Particles comprising a non-stoichiometric compound 5a have electron holes or the like and can prevent excessive CO and O2 in a rich or lean atmosphere from adsorbing on an electrode or the like and moreover, the particles are easy to conform to precious metal, thereby allowing the concentration of a control A/F in an early period and under an endurance near lambda 1. With a component 5b comprising elements in II (a) group of periodic table contained in the protective layer (including those having precious metal supported) 5, an Si component in an exhaust gas is allowed to react with Si being adsorbed on the protective layer 5 before it reaches an active point of a sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oxygen sensor, and particularly to an oxygen sensor for air-fuel ratio control that is used in combination with a three-way catalyst in an exhaust gas purification system for automobiles and the like.

[Prior art and issues] The environment surrounding oxygen sensors is quite harsh.

Especially as exhaust gas regulations are tightened, 0.4g/s+11
NOx regulations are already in place in California. Therefore, it is an important requirement for the sensor to suppress variations in control A/P at the initial stage and to suppress A/F fluctuations after durability.

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

Therefore, the present invention prevents control A/P fluctuations at the initial stage and reduces A/P fluctuations after durability.

Another purpose is to prevent lean shift and fall responsiveness from deteriorating due to Si poisoning.

[Means for solving the problem] In order to suppress control A/F variations at the initial stage and fluctuations after durability, the protective layer on the exhaust gas side of the oxygen sensor should contain precious metals before the gas reaches the sensor electrode. Various measures have been taken to completely advance the combustion reaction of unburned components. Among these, we also discovered that by providing a protective layer (second protective layer) made of a non-stoichiometric compound, a highly effective catalyst could be retained even after durability (Patent application No. 82
-211278). This is true for non-stoichiometric compounds such as T
Particles consisting of iO2-1 (x≦0.4) have electron holes, etc., and are caused by excess cOl in a rich/lean atmosphere.
In addition to preventing o2 from adsorbing to electrodes, etc., and also being compatible with precious metals, the control A/F during initial and durability periods is set to λ', 1.
It is now possible to concentrate in nearby areas. Preferably, the amount of noble metal in this layer is less than 2 % of the non-stoichiometric compound, and if the amount is more than this, the emission will gradually become rich and CO etc. will be emitted. With such an element, the control A/F of the sensor has less variation not only in the initial stage but also after durability. However, Si
When the component is included in exhaust gas, it is more effective than conventional oxygen sensor elements, but it has shifted considerably to the lean side.

Therefore, in the present invention, by incorporating a component consisting of a group IIa element of the periodic table (hereinafter referred to as +rna group component) into this protective layer (including one carrying a noble metal), This protective layer is designed to adsorb and react with Si until the Si component in the exhaust gas reaches the active site of the sensor.

This is because the Group IIa acid components in the protective layer, such as Ca and Mg, react with St contained in the exhaust gas at the temperature under which the sensor is used, producing crystals with a low melting point. Located on the inside and made of heat-resistant metal oxide IIa
It is presumed that a protective layer (first protective layer) that does not contain a group component (particularly one made of spinel, A1z03) prevents Si from penetrating and protects the noble metal and electrode present in the first protective layer. In particular, chlorides and carbonates containing Ca or Mg form very fine particles, so this S
Highly active against i.

However, if silicon contained in the exhaust gas is mixed in when the engine is running at low speeds, i.e., when the temperature is low, the adsorption effect of Sl by Group IIa acid components (especially Ca and Mg compounds) weakens, and St enters the protective layer without reacting. There was something that happened. Part 8: When the sensor is exposed to high temperatures
In some cases, 1 changed to SiO□, etc., resulting in clogging of the protective layer.

Therefore, in the present invention, the intrusion of this SL can be prevented by further including a group A component in the protective layer on the side exposed to exhaust gas and by providing a heater that heats the sensor element. It is something. In other words, by raising the temperature of the protective layer using this heater and increasing the ability to adsorb the group IIa acid component, SL can react with the group IIa acid component, thereby reducing the intrusion of only St.

Regarding the protective layer (second protective layer) containing a Group IIa acid component, ■
Ca and Mg are suitable as aa elements. Group IIa acid component composition includes non-oxide 2 such as CaCJ2, Mg
Chlorides and carbonates such as CO3.

Nitrates are good. However, in the case of an oxygen sensor equipped with a heater, even oxides are effective. In addition, hydrates of these, such as Ca Ci2 .2H20° complex compounds, such as C
It may also be aCO3.MgCO3 (dolomite).

Group IIa component component A with 204. It is preferable to support it on a heat-resistant metal oxide such as titania. In particular, non-stoichiometric compounds such as TiO2-1 (x≦0.4), La
203-X is preferably supported in a highly dispersed manner.

As for the manufacturing method, 1, for example, titania particles (for example, average particle size 0.1 to 1 μs) are preliminarily supported with a group IIa acid component,
It is applied as a slurry onto the first protective layer and subjected to heat treatment (for example, 5
00 to 700"C); after applying titania particles to the first protective layer, the titania particles are immersed in a group IIa component molten under reduced pressure or under pressure, and then heat treated. In these cases, IIa for the refractory metal oxide of the second protective layer
Family component composition.

30vt% or less in terms of group IIa elements, more preferably 2
It is preferable to set it to 0vt% or less. If it exceeds 30 wt%, the responsiveness of the sensor gradually deteriorates. In other words, clogging begins to occur.

In order to prevent durable deterioration of noble metals used as electrodes or catalysts, it is necessary that at least a portion of the refractory metal oxide be present as a di-nonstoichiometric compound, such as TiO□-8. However, it is not necessary that all the compounds are non-stoichiometric compounds, but stoichiometric compounds (for example, TiO2, A with 204.
It is also possible to carry a compositional dispersion of Group IIa components together with spinel. In this case, the ratio of the non-stoichiometric compound to the stoichiometric compound is 3:2 or more, preferably 2:1.
It is better to make it more than that.

In addition, initial control A can be achieved by containing jl metal, preferably Pt, in an amount of 2 mo1% or less (however, l,5 moffi% or less under rich exhaust gas conditions) relative to the non-stoichiometric compound. The variation in /P can be further suppressed. In this case, the second protective layer and the group IIa component metal may be present in the same part.

However, the part supporting the precious metal (first protective part) and ■a
It may also be configured with a part (second protective part) supporting a group component. However, in that case, it is particularly necessary to arrange the second protection part further outside. This first. It is preferable that all the heat-resistant metal oxides constituting the second protective part be non-stoichiometric compounds. For the first protective part, it is necessary that the non-stoichiometric compound is present at least 60%. Further, it is preferable that the group IIa component exists independently of the heat-resistant metal oxide constituting the protective layer.

"Independently" means that the Group IIa acid component does not react with these metal oxides to form a compound, such as MgTlOs, that is inert to the St acid component.

Further, it is preferable to provide a protective layer (first protective layer) which is located inside the second protective layer and directly covers the electrode, and which does not contain a group a acid component. The first protective layer is also made of a heat-resistant metal oxide, especially when spinel<Mg0-A is 20.
> Preferably, alumina is used, and one that is firmly attached by thermal spraying is preferable.

Inclusion of noble metals such as pt and/or Rh and Pd in this first protective layer completely oxidizes and reduces unburned components in the exhaust gas and provides good control A/F for the sensor. become. Note that the outer side of the second protective layer is made of, for example, titania or alumina.

It may further include a third protective layer made of spinel or the like.

It goes without saying that each protective layer needs to have air permeability to the extent that it does not deteriorate sensor response. Therefore, for the first protective layer, for example, the porosity is 10-30% and the thickness is 10-5%.
For example, for the 0-1 second protective layer, the porosity is 8 to 35%,
The thickness is preferably 10 to 50 μm.

Further, it is preferable that the surface of the sensor element main body, for example, the zirconia solid electrolyte, on the measurement electrode side has a structure in which the unevenness is 10 μm or more. Prevents the protective layer from peeling off and has excellent durability.

In addition to ZrO2 solid electrolyte, the main body material of the sensor element may be TiO2, CoO semiconductor, or the like. In addition to titania, lanthanum oxide may be used as the non-stoichiometric compound. Also, the type and material of the heater (e.g. ceramic)
, mounting position etc.

2 does not matter as long as it can exhibit the above effect.

[Example] Example A (Table 1, Samples 1 to 14) 1. A sensor element body (equipped with an electrode and a first protective layer) is manufactured by the following steps 1 to 9.

Step 1: Add 5% of Y2O3 with a purity of 99% to ZrO2 with a purity of 99% or more.
After adding s+of% and wet mixing, it is calcined at 1300°C for 2 hours.

Step 2: Add water and wet process in a ball mill until 80% of the particles are 2.
Grind to a particle size of 5 μm or less.

Step 3: Add a water-soluble binder and spray dry to obtain spherical granulated particles with an average particle diameter of 70.

Step 4: The powder obtained in Step 3 is rubber pressed to form a desired tubular shape (U-shaped tubular shape), dried, and then ground into a predetermined shape using a grindstone.

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

Step 6: After drying, bake at 1500°C x 211rs. The portion corresponding to the detection portion had an axial length of 25 mm, an outer diameter of approximately 5 mm, and an inner diameter of approximately 3111 mm.

Step 7: A PT measurement electrode layer is formed on the outer surface by electroless plating to a thickness of 0.
9a, and then baked at 1000°C.

Step 8: A first protective layer that directly covers the electrode with a thickness of about 100 μm is formed by plasma spraying MgOφAJ203 (spinel) powder.

Step 9: In the same manner as Step 7, a PT reference electrode layer was formed on the inner surface.

2. The spinel sprayed part of the device was placed in a H2PtCfle solution with Pt of 0.051:/ and evacuated, and the noble metal (pH) was supported in the first protective layer. Approximately 0.02-0.05vt% of the material
It is.

3. Adding Ca to TiO2-1 powder with an average particle size of about 0.2 μm
I such as CJ2 and 2H20 (dissolved in pure water)
Add the group Ia ingredients, boil and dry while stirring, then boil for 55 minutes.
Heat treatment was performed at 0°C. Samples N11l and 12 are α-Ai
'zC)+ was also blended. Furthermore, 'rio2-. The powder.

It was obtained by previously treating T02 particles in a non-oxidizing atmosphere at 800° C. or higher. 0.01 mo for TiO2
Even if it contains about ffi% of noble metal, it can be made into a non-stoichiometric compound. When titania powder was to support a noble metal, titania particles were placed in a desired noble metal-containing salt solution in advance, boiled and dried, and then heat-treated at 550° C. in the atmosphere.

4. To the element obtained in step 2, an organic binder and butyl calpitol were added to the powder obtained in step 3, and the mixture was coated with hJ and baked. Baking was performed in a reducing atmosphere at 500°C.

5. A known sensor assembly was performed.

Example B (Table 2, Samples Na15-27) [,2. T102 with an average grain size of about 0.2M was added to the element obtained in 3.1, which is the same as 1.2 in A above.
-Add organic binder and butyl calpitol to X powder,
It was applied with a brush and dried. Drying was carried out at 120'C in air.

4. CaCJ2.2H20 was dissolved in water, the coated part of the element obtained in 3 was put in, and the mixture was evacuated. At that time, the Ca concentration was variously changed. Thereafter, it was dried in the atmosphere at 100'C.

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

(b) Manufacture of heater, etc. 1, sheet mainly composed of Al1203 with a thickness of 0.81
It was molded using the doctor blade method.

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

3.More A. It was coated with a thickness of 30 μm using a paste containing 03 as the main component and an organic binder and a solvent added thereto.

4. Wrap the sheet obtained in step 3 around an insulator tube with two outer diameters containing A RzOs as the main component and heat it at 400°C for 24 hours.
The resin was removed and baked at 1550°C for 2 hours.

5. Lead wires were soldered with silver to the terminals to obtain a heater.

G. When assembling the element, the heater obtained in steps 1 to 5 was inserted so as not to contact the inside of the bag-shaped element.

Example D (Table 3, Samples 37 to 39) 1. A sheet containing 35 mol % of Z r02 +Y2 as a main component was molded to a thickness of 0.8 am by a doctor blade method.

2. Using pi as the main component by screen printing method.

Electrodes are made with a paste containing an organic binder and a solvent.
Printed on both sides.

3. The main component is Al1203 so as to cover the electrode,
A paste containing an organic binder and a solvent and a small amount of starch to make it porous was coated to a thickness of 30 μI.

On both sides of the same sheet as in 4.1, a paste containing Azoz O3 as a main component and an organic binder and a solvent was coated to a thickness of 30 μm.

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

Steps 6 and 4 were repeated (however, only on the surface above the heater pattern).

A sheet similar to 7.1 was cut into a U-shape, and a spacer sheet was placed between the electrode printed sheet obtained in steps 1 to 3 and the heater pattern embedded sheet obtained in steps 4 to 6, and bonded by thermocompression.

8. 400℃ 24H "1500℃ x 4 after removing resin
1 (r firing was performed.

9. A protective layer supporting titania and a group a component was formed. Regarding this second protective layer, sample Na37 is in sample seed 3 of Example A, and sample Nα38.39 is in Example B.
The same procedure was used for sample Nα17.1g.

Example E (Table 4, samples Nα40-55.80.81)
1. Same as 1 (Steps 1 to 9) of Example A above.

Regarding samples Nα45 to Nα48, the first protective layer was impregnated with a noble metal in the same manner as in Example A-2.

2. TiO2 powder (average particle size 0.3 μm) was converted into P t O
,05g/l~1f/H2PtCJ2. solution and/or
Or Rh 0.05g/i! RhC1'3 #x
) 120 solution and leave it for about 5 minutes under the pressure of 11g of 50-10 servings.

It was impregnated with pt or Rh in an amount equivalent to 1 mof% with respect to TiO2. Next, after drying, it was heat-treated at 600° C. in the atmosphere, and then made into a paste with an organic binder and a solvent.

3. Apply this paste to the first protective layer and heat at 120℃
(first protected part, 208° C. thickness).

4. Titania powder was placed in a solution such as Ca Cj:2 .2H20 and dried while boiling. Thereafter, it was made into a paste using a water-soluble binder and water. For TiO2,
(2) 20vt% in terms of group a metals.

5. This paste was applied on the first protective part and baked at 600°C (second protective part, 20n thick).

B. A third protective layer was appropriately formed on the second protective layer (consisting of the first and second protective parts). In addition, samples with various multilayer structures were prepared as shown in Table 4.

7. A known sensor assembly was performed.

Example F (Table 4, Trial #j No, 5 G~59) 1
, the same as 1 to 8 of the above embodiment. However, regarding the resistance 58°59, a noble metal was contained in the protective layer made of AR203 in the same manner as in Example A-2.

2. A paste similar to 3 of Example E was applied and baked at 600° C. in the atmosphere to form a first protective portion (20 μm).

3. A paste similar to 5 of Example E was applied and baked at 800° C. in the atmosphere to form a second protective portion (20-).

4. A pair of supports were attached to both sides of the element thus obtained using glass seals.

5. A known sensor assembly was performed.

The present invention is not limited to the above-mentioned embodiments, but can be applied to various types of air-fuel ratio control oxygen sensors, such as a wide range air-fuel ratio control sensor equipped with a pump etc. (FIG. 7), or a Ti
Sensors using metal oxide semiconductors such as O2 and Coo (
(Fig. 8) can also be applied. In the case of a semiconductor type sensor 2 For example, a second sensor containing a precious metal in a metal oxide which is a semiconductor, a sprayed layer of spinel or the like, and a second sensor having a group IIa component.
A protective layer may also be provided.

1 to 8 show examples of the oxygen sensor according to the present invention, and in each figure, 1 is the sensor element body, 2 is the reference electrode, 3 is the measurement electrode, 4 is the first protective layer, and 5 is the oxygen sensor according to the present invention. is the second protective layer, 5a is a heat-resistant metal oxide (especially a non-stoichiometric compound)
, 5b represent the IIa group components, respectively.

[test] The following tests were conducted on each sample.

(2) The initial gJA/P of the sensor was measured on an actual vehicle.

The measurement method is to attach a sensor to the manifold and measure 80kj.
/! Control is performed using a sensor when the running condition is fixed at Ir
was measured.

2. Install a sensor upstream and downstream of the exhaust pipe (manifold - 1mm, and then add 5 cc/81 oil from the manifold part.
3000rl)s while injecting 111r (10cc) at a rate of 30 minutes (However, for models with a heater, Na2
The engine was operated at 1000 rpm (except for 8 to 39) [Si test]. The atmosphere was near λζ1.

3.1 was measured, and the change in A/P (Δ
^/P) was calculated. In addition, the sensor output was monitored using a high-speed response recorder to measure the responsiveness of the sensor (for example, Fig. 10). Then, as shown in FIG. 11, the average value is connected with a straight line and the time between 300.80 hV (TLil) is drawn.

TML) was measured after the Si test.

4. Also, regarding sample holes 28 to 39, 1 when injecting Si oil? /C rotation at 11000 rpm and 3000 rpm.

The heater was energized (except for sample N1135), and the control state of the sensor was observed.

5. Also, actual vehicle engine l: Te λw 1, 850°C
(30 minutes) 0 idle (30 minutes) A thermal cycle test of 1o0011rs was conducted and the control A/F was measured.

These results are shown in Tables 1-4 and Figure 12.13.

(Left below) Table 1) When the E/G rotation is 3000 rpm (heater is not energized), the temperature is approximately 550°C.

Also when the EIG rotation is 11000 rpm.

(heater not energized), approximately 400℃ Ru.

The sensor temperature is approximately 400°C (heater energized) and the sensor temperature is approximately 300°C (heater energized). After that, a second protective layer similar to the element cover shown in 0 in the table was formed.

As is clear from Tables 1 to 4, the two comparative samples outside the scope of the present invention had large ^/P fluctuations (Δ^/['≧0.08) and slow fall response in the Si test. (TRL≧120m5). Also, in thermal cycle tests, A/F fluctuations are large (ΔA/I' -0,
04).

On the other hand, in the S1 test, each example sample
A/F fluctuation is significantly suppressed (Δ^/P≦0.04)
, responsiveness is also maintained at a high level (TRL≦90aS).

Also, A/F fluctuations are suppressed in thermal cycle tests (ΔA/F≦0.02).

Also, from the results in Table 3, 11000rp, 30
At low rotation (low temperature) of 0° C., ΔA/P becomes large if no heater is present (sample 35).

This is considered to be because when Si is mixed in at low temperatures, the adsorption effect of group IIa components is weakened.

On the other hand, each element seed 28 of this embodiment equipped with a heater
~34. Na37-39 can significantly suppress A/F fluctuation even after the Si test at this low temperature (ΔA
/P≦0.05).

Regarding the vs2 protective layer, it was also confirmed that the initial value of A/F tends to be richly controlled when the group A acid component noble metal is present in a separate protective part (Table 4).

[Effects of the Invention] As described above, according to the present invention, even if Si components are present in the exhaust gas, the A/P can be concentrated by preventing the poisoning thereof, and the falling response is also excellent. In addition, performance deterioration due to Sil poisoning can be reliably prevented even when the engine rotates at low speeds.

[Brief explanation of the drawing]

FIG. 1 is a partial sectional view of a sensor showing the effect of the present invention. FIG. 2 is a partial cross-sectional view showing an example of the oxygen sensor of the present invention. 3 and 4 are partial sectional views of a sensor showing the effect of the present invention (FIG. 3 is a case without a heater, and FIG. 4 is a case with a heater). FIG. 5 is a partial cross-sectional view showing an example (bag-shaped) oxygen sensor of the present invention. FIG. 6 is a partial cross-sectional view showing an example (plate-shaped) of the oxygen sensor of the present invention. FIG. 7 is a cross-sectional view showing an example of the oxygen sensor for wide range air-fuel ratio control (combined with a pump element) according to the present invention. FIG. 8 is a diagram showing an example of a semiconductor type oxygen sensor according to the present invention, in which FIG. 8(a) is a plan view thereof (however, a protective layer is omitted), and FIG. 8(b) is a sectional view thereof. FIG. 9 is a schematic diagram of waveforms showing the output during control after testing. FIG. 10 and FIG. 11 are diagrams defining an example of the waveform of the sensor output and measurement times (T LR, , T RL ). Figure 12 is a graph showing changes in control #A/F at the initial stage and after durability, and Figure 13 is a graph showing changes in control #A/F at the initial stage and after durability.
Graph showing changes in . respectively. DESCRIPTION OF SYMBOLS 1...Element body 2...Measuring electrode 4...First protective layer 5...Second protective layer 5a...Heat-resistant metal oxide (especially non-stoichiometric compound) 5b... Group IIa components

Claims (8)

[Claims] (1) In a sensor that detects the oxygen concentration in exhaust gas, the side of the sensor element exposed to exhaust gas is made of a heat-resistant metal oxide and consists of a group IIa element of the periodic table (hereinafter referred to as
1. An oxygen sensor comprising a protective layer supporting a group IIa component (referred to as a "group IIa component"), at least a part of which exists as a non-stoichiometric compound with respect to a heat-resistant metal oxide. (2) A sensor that detects the oxygen concentration in exhaust gas is equipped with a heater that heats the sensor element, and the side of the sensor element exposed to the exhaust gas is made of heat-resistant metal oxide IIa.
1. An oxygen sensor comprising a protective layer supporting a group component, at least a part of which exists as a non-stoichiometric compound with respect to a heat-resistant metal oxide. (3) The oxygen sensor according to claim 1 or 2, wherein the protective layer also contains a noble metal. (4) In a sensor for detecting oxygen concentration in exhaust gas, a protective layer made of a heat-resistant metal oxide and supporting a group IIa component and a noble metal is provided on the side of the sensor element exposed to exhaust gas, and at least An oxygen sensor in which a part of the heat-resistant metal oxide exists as a non-stoichiometric compound, and the part on which the noble metal is supported is located closer to the electrode than the part on which the Group IIa component is supported. . (5) A sensor that detects the oxygen concentration in exhaust gas is equipped with a heater that heats the sensor element, and the side of the sensor element exposed to the exhaust gas is made of heat-resistant metal oxide IIa.
A protective layer supporting a group component and a noble metal, at least a part of the protective layer exists as a non-stoichiometric compound with respect to the heat-resistant metal oxide, and the part supporting the noble metal is IIa.
An oxygen sensor characterized by being located closer to an electrode than a portion on which a group component is supported. (6) The oxygen according to any one of claims 1, 2, 4, and 5, wherein the non-stoichiometric compound is titania, and the protective layer is formed by dispersing and carrying a group IIa component on the titania particles. sensor. (7) The oxygen sensor according to any one of claims 1, 2, 4, and 5, wherein the main body of the sensor element is made of ZrO_2 solid electrolyte. (8) The protective layer that does not contain the Group IIa component contains a noble metal, and the surface of the main body of the sensor element on the measurement electrode side has an unevenness of 10 μm.
The oxygen sensor according to any one of claims 1, 2, 4, and 5, which has an oxygen sensor of at least m.