JP2589130B2 - Oxygen sensor element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is used in combination with an oxygen sensor element for detecting the oxygen concentration of various kinds of combustion equipment, in particular, a three-way catalyst for purifying exhaust gas from an internal combustion engine. The present invention relates to an oxygen sensor element for controlling an air-fuel ratio and a method for producing the same.

[Prior art and problems] An oxygen sensor element for air-fuel ratio control comprises a solid electrolyte body having oxygen ion conductivity and a pair of electrodes (reference electrode, measurement electrode) provided on the inner and outer surfaces thereof, and a measurement electrode which comes into contact with exhaust gas. Is generally covered with a porous protective layer to protect the gas from exhaust gas. However, in such a sensor element, a so-called λ point shift, that is, an air excess factor (λ) is shifted due to unburned components contained in exhaust gas, and detection accuracy is reduced. For this reason, it is common practice in exhaust gas purification systems to recirculate and purify exhaust gas, but this requires an additional exhaust gas recirculation device (EGR), which complicates the entire system and properly controls the recirculation state. It is difficult to control, and causes a new problem due to the failure of the device itself. Accordingly, various studies and proposals have been made.

For example, an oxygen sensor element has been proposed in which a layer made of one or both of rhodium and palladium is formed on the surface of the measurement electrode itself (Japanese Patent Application Laid-Open No. Sho 61-30760). However, this type of oxygen sensor element has a problem in durability during use. That is, an alloying reaction occurs between the measurement electrode and the layer made of rhodium or the like, and the action of the layer cannot be stably maintained.

There has also been proposed an oxygen sensor element provided with a measurement electrode made of platinum and rhodium and distributed by changing the ratio of platinum and rhodium. (JP-A-62-198749). However, even in this type of oxygen sensor element, it is similar to the above-mentioned technology that the alloy formation reaction is likely to occur, and there is a concern that the durability may be deteriorated. In addition, responsiveness may be reduced due to the presence of a large amount of rhodium at a high density.

An object of the present invention is to solve such a problem, that is, to develop an oxygen sensor element having excellent durability and capable of maintaining accurate air-fuel ratio control stably for a long period of time.

[Means for Solving the Problems] The present invention has been made as a result of intensive studies in view of such a viewpoint.
The present invention has been completed.

In order to accurately detect the oxygen concentration in the exhaust gas, it is ideally desirable to measure the oxygen concentration in complete combustion, that is, the equilibrium oxygen concentration. However, the exhaust gas system generally has unburned components.
Because it is an incomplete combustion system containing CO, HC, and NOx, a deviation from the equilibrium oxygen concentration occurs due to the presence of the unburned components, and the actual oxygen concentration cannot be detected accurately, resulting in a λ point deviation. . Therefore, the present inventor sets the gas to be measured in an equilibrium oxygen concentration state, detects the oxygen concentration difference from the reference gas, and
In addition, the inventors of the present invention have made various studies to maintain the durability of the invention, and have completed the present invention. The present invention solves the above-mentioned problems by the following means.

(1) An oxygen sensor element in which a reference electrode is provided on one side of the solid electrolyte body and a measurement electrode is provided on the other side, and the measurement electrode is in contact with the gas to be measured. It is covered with a plurality of porous protective layers as main components, and the plurality of protective layers are the first, second, and second protective layers.
The first protective layer is located closer to the measurement electrode than the second and third protective layers, the second protective layer is composed of titania carrying Rh and Pd, and the third protective layer is composed of Pt. An oxygen sensor element comprising at least one of alumina, spinel and magnesia supported.

(2) An oxygen sensor element in which a reference electrode is provided on one side of the solid electrolyte body and a measurement electrode is provided on the other side, and the measurement electrode is in contact with the gas to be measured. It is covered with a plurality of porous protective layers as main components, and the plurality of protective layers are the first, second, and second protective layers.
The first protective layer is located closer to the measurement electrode than the second and third protective layers, the second protective layer is made of titania carrying Rh and Pd, and the third protective layer is made of Pt. An oxygen sensor element comprising at least one of alumina, spinel, and magnesia, and having two or more second and third protective layers, respectively.

[Embodiment and Operation] The element shape or the solid electrolyte shape may be various shapes such as a bag shape, a plate shape, or a tubular shape as long as the front end is closed and the rear end is open, or an electrolyte material fixed on an insulating base material. And the like may be combined to form the same shape as described above. As the solid electrolyte material, for example, a material obtained by adding Y ₂ O ₃ , CaO, or the like as a stabilizer to ZrO ₂ may be used. The reference electrode and the measurement electrode (layered) are both porous, and it is preferable to use a noble metal such as Pt or Pt containing about 2% or less of Rh.

As described above, the measurement electrode must be covered with a plurality of protective layers mainly composed of a metallized material that is thermally stable with respect to the gas to be measured. In addition to protecting the measurement electrode from exhaust gas, preventing exhaust gas reaching the measurement electrode as equilibrium oxygen concentration to prevent a λ point shift, and unburned components (CO, etc.) adsorbing to the measurement electrode and expanding the solid electrolyte This is to prevent peeling from the substrate. The plurality of porous layers include first, second, and third protective layers having different functions.

The first protective layer is located closest to the measuring electrode in the protective layer and is for directly protecting the measuring electrode from exhaust gas. The first protective layer is made of ceramics such as alumina,
It is preferable to use spinel, beryllia, zirconia, or the like or a mixture thereof, and it is particularly preferable to mainly use spinel. Its porosity is 5-20%, preferably 7-20%,
The thickness is preferably 30 to 200 μm, preferably 50 to 130 μm. This is for the purpose of reliably protecting the electrodes without causing a problem in the exhaust gas passage.

The second protective layer must be composed of titania carrying Rh and Pd. NO among unburned components in exhaust gas
_The purpose is to promote the reduction reaction of _X and to compensate for the deviation from the equilibrium oxygen concentration based on the presence of NO _X. An example of the reduction reaction is NO _X + H ₂ → N ₂ + H ₂ O. Titania is TiO
_It is preferable to use a non-stoichiometric titania represented by _X (x = 1.8 or more and less than 2 and preferably 1.95 or more and less than 2). This is because Rh and Pd as a catalyst are supported in a highly dispersed state, the catalytic action can be efficiently exhibited, and the heat resistance is excellent. In particular, it is effective to be able to carry Rh, which is scarce in resources, in a highly dispersed manner. The titania is added to the total amount of the second protective layer.
The content should be 50 wt% or more, preferably 70 wt% or more. Rh, P
In addition to d, other noble metal catalysts may be supported, but it is preferable that the total amount of Rh and Pd is at least 50% or more. NO
_{This is} for sufficiently promoting the reduction reaction of _X. When Rh and Pd are mixed simultaneously, the ratio is 0.05 ≦ Rh / Pd ≦ 1, preferably
It is preferable that 0.2 ≦ Rh / Pd ≦ 1. The amount of the supported catalyst is preferably in the range of 0.1 to 3% by weight based on the constituent material of the second protective layer.
If the amount is less than the lower limit, there is no effect, and if the amount exceeds the upper limit, clogging may occur.

The third protective layer must be composed of at least one of alumina, spinel or magnesia supporting Pt. It promotes the oxidation reaction of CO and HC among the unburned components in the exhaust gas,
This is to correct the deviation from the equilibrium oxygen concentration based on C. As an example of the oxidation reaction, CO + O ₂ → CO ₂ , HC + O ₂ → CO ₂ + H ₂
O. Pt was supported on alumina because Pt was Rh
Consumption is more severe than that of the unburned components.
This is because alumina is supported in a low-dispersion state and its wear and degradation is regulated as much as possible. Alumina is used in an amount of 50% by weight or more, preferably 70% by weight or more based on the total amount of the third protective layer. In order to control the dispersibility, a material containing 50 wt% or less of titania (preferably, non-stoichiometric titania) may be used. If it exceeds 50 wt%, it will be supported in a highly dispersed state, and Pt wear and tear will be severe. Other precious metal catalysts may be supported together with Pt, but the amount of Pt must be at least 70%.
% Is preferable. This is to sufficiently promote the oxidation reaction of CO and HC. The amount of the catalyst carried is preferably in the range of 0.5 to 5% by weight based on the constituent material of the third protective layer. If the amount is less than the lower limit, there is no effect, and if the amount exceeds the upper limit, clogging may occur.

For the second and third protective layers, a transition metal oxide is preferably used as the remaining constituent material. This is for maintaining the carrying state stably. The porosity of the second and third protective layers may be higher than that of the first protective layer. NO _X reduction reaction and CO, while effectively promoting the oxidation reaction of HC, is to prevent the passage resistance and sensor response deterioration of the exhaust gas. The pores of the second and third protective layers may be present as open pores (vents). Also, from a similar point of view, the thickness of the second and third protective layers may be thinner than that of the first protective layer. For example, the thickness may be 10 to 50 μm.

The thickness of each protective layer may be increased or the catalyst content may be increased at the end of the element where a large amount of exhaust gas passes. or,
Protrusions may be present at the interface of each protective layer to enhance the bonding between the layers.

Next, a preferred embodiment and operation of the manufacturing method of the present invention, particularly the processing step on the other surface side (the side on which the measurement electrode is to be formed) of the solid electrolyte substrate will be described.

The solid electrolyte base material is preferably formed by mixing and calcining the raw material powder, pulverizing (2.5 μm or less), then forming secondary particles (20 to 150 μm) by spray drying, and molding into a predetermined shape.

The shape of the electrode is not limited to a normal plating process such as electroplating, chemical plating, etc., but also a normal vapor deposition method such as sputtering,
It may be performed by vapor deposition or screen printing.

As the formation of the first protective layer, various methods such as baking the solution or powder of the material with a brush, dipping, spraying or the like may be mentioned, but plasma spraying is particularly preferable. This is because the adhesion strength between the sprayed powders is high, and the porosity and the pore diameter can be arbitrarily set by appropriately changing the conditions.

The second and third protective layers may be formed by sequentially covering the first protective layer with a paste containing a material for the protective layer and a noble metal component, followed by firing. By simultaneously forming the protective layer and supporting the catalyst,
This is because the catalyst is more firmly supported, scattering at the time of use is prevented, and a long-term stable catalytic action is exhibited. or,
This is because the paste or the like causes the binder and the like to be scattered at the time of firing, so that the desired porosity and pore size can be easily obtained. The paste is obtained by blending a binder, a solvent and the like as usual. The coating method may be any of brush application, dipping, spraying and the like. However, plasma spraying is not suitable. This is because sintering of the protective layer material proceeds during the thermal spraying, and pores cannot be obtained in a desired state (especially as a high porosity). The protective layer material and the supported catalyst may be blended by impregnating the protective layer material powder with a noble metal salt solution. This is to ensure uniform blending. As the material for the protective layer, in addition to the metal oxide, a compound capable of forming a metal oxide by thermal decomposition, such as a hydroxide or a salt, may be used. The particle size of the powder is preferably 2 μm or less. This is because the sinterability is improved and the fixing strength is increased, so that the second and third protective layers are less likely to peel off during use. Preferably it is 0.3 to 1.5 μm. The heat treatment is preferably performed at 700 to 900 ° C. in a non-oxidizing atmosphere. The second and third protective layers may be formed at the same time as the first protective layer, and then may carry a catalyst. For example, the protective layer material may be immersed in a noble metal salt solution, and then dried and fired.

The manufacture of the oxygen sensor element may be performed by a so-called laminating printing method, in addition to the method of forming the constituent elements in a stepwise manner. Laminated printing technology refers to a technology in which the components of the oxygen sensor element are laminated on a predetermined green sheet and printed, and the printed green sheet is applied to a base material and fired and integrated (for example, see Japanese Unexamined Patent Publication No. -222159).

Examples Examples of the present invention will be described below.

FIGS. 1 and 2 show an embodiment. In each of the figures, reference numeral 1 denotes an oxygen sensor element, and this element 1 generates an oxygen concentration difference between a reference gas and a gas to be measured (exhaust gas). The solid electrolyte body 2 to be obtained, a pair of porous electrodes (inner electrode) 3, (outer electrode) 4, formed on the inner and outer surfaces of the solid electrolyte body 2, and a plurality of porous protective layers 5, 6, which cover the outer electrode 4. 7, noble metal catalysts 6a ..., 7a ... uniformly dispersed and supported on the second and third protective layers 6, 7 ...
It is composed of Here, the solid electrolyte body 2 is made of ZrO ₂
Made from those added Y ₂ O _3, the electrodes 3 and 4 are both Pt electrodes, 6a ... is Rh / Pd catalyst, 7a ... is Pt catalyst.

Further, the plurality of porous protective layers are located on the inner side, the first protective layer 5 directly covering the outer electrode 4, the second protective layer 6 located in the middle, and the third protective layer located on the outer side and exposed to exhaust gas. And a protective layer 7. The first protective layer 5 is made of spinel, the second protective layer 6 is made of titania carrying Rh and Pt catalysts 6a, and the third protective layer 7 is made of alumina carrying Pt catalysts 7a.

In FIG. 1, A is an oxygen sensor, 8 is a housing, 9
Is a caulking ring, 10 is a filler, and the tip of the oxygen sensor element is protected by a protective tube.

Next, a production example of the oxygen sensor element of the present invention will be described. The following steps are sequentially performed.

Step 1: ZrO ₂ of more than 99% purity of 99.9% pure Y ₂ O ₃ with 5 mol%
Add, mix and calcine at 1300 ° C for 2 hours.

Step 2: 80% of the particles are wet in a ball mill by adding water
Grind to a particle size of 2.5 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: Rubber pressing, forming into a desired tube (test tube), drying, and grinding with a grindstone into a predetermined shape.

Step 5: After drying, firing at 1500 ° C. × 2 hrs. For the part corresponding to the detector, the axial length is 25 mm, the outer diameter is about 5 mmφ,
The inner diameter was about 3 mmφ.

Process 6: Pt layer thickness 0.9μm on inner and outer surfaces by chemical plating
And then baked at 1000 ° C.

Step 7: forming a first protective layer having a thickness of about 150 μm by plasma spraying with MgO · Al ₂ O ₃ (spinel) powder.

Step 8: After drying, a noble metal-containing titania paste is applied to the surface of the first protective layer, and baked in a reducing atmosphere at 800 ° C. to form a second protective layer having a thickness of about 25 μm having pores of about 2 μm. Form. The above paste is obtained by immersing titania powder in a catalyst solution having a Rh / Pd weight ratio of 1, drying and impregnating it with stirring, and then adding an organic binder and a solvent (butyl carbitol).

Step 9: A γ-alumina paste containing a noble metal is applied to the surface of the second protective layer and baked in an oxidizing atmosphere at 600 ° C.
A third protective layer having a thickness of about 25 μm is formed. The paste is obtained by immersing γ-alumina powder in an H ₂ PtCl ₆ solution, drying and impregnating the powder while stirring, and then adding an organic binder and a solvent (butyl cardol).

Furthermore, using the oxygen sensor element 1 thus manufactured, an oxygen sensor A was obtained by the following steps.

Step 10: After inserting the element 1 into the housing 8, the element 1 is fixed in the housing 8 by inserting a caulking ring 9 and a filler 10 such as talc.

Step 11: A protective tube is arranged so as to cover the front end of the element 1, and the front end of the housing 8 and the rear end of the protective tube are welded.

Step 12: Connect terminals and leads (not shown) to the electrodes,
An oxygen sensor is obtained by covering an outer cylinder (not shown).

[Test Example] The following tests were performed based on the oxygen sensor element of the present invention according to the above-described embodiment, and each evaluation item was examined. In addition, as a comparative example, a device having only a single protective layer made of Pt-supported spinel (without steps 8 and 9) was similarly examined.

The oxygen sensor element was subjected to a durability test using a Bunsen burner. That is, the tip portion (tip portion) of each oxygen sensor element is heated to 700 to 850 ° C. in an incomplete combustion state in which almost no air is introduced, so as to withstand 500 hours.

Evaluation item A: An oxygen sensor equipped with the above heated oxygen sensor element was attached to a combustion tube (inner diameter width 43), and a burner flame was sprayed from a location 1 m away from the sensor to evaluate sensor response.

Evaluation item B: Similarly, attach the oxygen sensor to the specified position on the actual engine of the engine after heating, control the sensor, examine the λ scan value (average control A / F value) located further downstream, and evaluate the λ characteristics. .

Evaluation item C: The state of the element surface was visually evaluated.

The results are shown in the table and FIGS. FIGS. 3 and 4 relate to samples No. 1 and No. 2 and sample No. * 4, respectively.

As is clear from the table and FIGS. 3 and 4, the oxygen sensor element (and oxygen sensor) according to the example shows excellent results for each of the evaluation items A, B, and C as compared with the comparative example. .

[Effects] As described above, according to the present invention, the measurement electrodes can be reliably protected without deteriorating the air permeability by the first to third protective layers, the sensor response and the λ characteristics are excellent, and the air-fuel ratio with high accuracy is achieved. You can maintain control.

Moreover, highly dispersed supported Rh in the second protective layer, as well as promote the reduction reaction of the NO _X of the unburned components by Pd catalyst, of unburned components by low dispersion supported Pt catalyst in the third protective layer Since the oxidation reaction of CO and HC can be promoted, the gas to be measured can be detected as an equilibrium oxygen concentration in a complete combustion state, and ideal detection characteristics can be exhibited.

Therefore, the present invention has succeeded in providing a system which is capable of maintaining high accuracy, that is, sensor control with as small a λ point shift as possible, stably for a long time, and providing a system with extremely excellent purification characteristics by combining with a three-way catalyst. Thus, it is extremely useful in the field of oxygen sensors.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an embodiment of the oxygen sensor element (oxygen sensor) of the present invention, FIG. 2 is an enlarged sectional view of a portion II in FIG. 1, and FIGS. FIG. 3 is a graph showing the relationship between (λ) and output, wherein FIG. 3 shows the relationship before endurance and FIG. 4 shows the relationship after endurance. DESCRIPTION OF SYMBOLS 1 ... Oxygen sensor element, 2 ... Solid electrolyte body 3 ... Reference electrode 4, ... Measurement electrode 5 ... 1st protective layer, 6 ... 2nd protective layer 7 ... 3rd protective layer, 6a, 7a ……catalyst

 ──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Toshiaki Kondo 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Japan Special Ceramics Co., Ltd. (56) References JP-A-63-290956 (JP, A)

Claims (3)

(57) [Claims] An oxygen sensor element comprising a reference electrode on one side of a solid electrolyte body and a measurement electrode on the other side, wherein the measurement electrode is in contact with a gas to be measured. Is covered with a plurality of porous protective layers mainly composed of a material, and the plurality of protective layers are the first, second, and third layers.
The first protective layer is located closer to the measurement electrode than the second and third protective layers, the second protective layer is composed of titania carrying Rh and Pd, and the third protective layer is composed of Pt. An oxygen sensor element comprising at least one of alumina, spinel and magnesia supported. 2. The weight ratio Rh / Pd of Rh and Pd is 0.05 to 1.0.
The oxygen sensor element according to claim 1, wherein 3. The oxygen sensor element according to claim 1, wherein the second protective layer is located closer to the measurement electrode than the third protective layer.