JPH0810209B2 - Oxygen sensor element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention is directed to exhaust gas control of an industrial combustion device that burns various fuels and uses thermal energy, and to improve the use of enzymes at high temperatures such as petrochemical, steel and metallurgy. Regarding the enzyme sensor element for controlling the enzyme concentration in the involved chemical process, metallurgical process, etc., especially by detecting the enzyme concentration in the exhaust gas of automobiles, which have severe operating environment conditions, small tolerance of characteristic changes, and long life. The present invention relates to an enzyme sensor element for controlling air-fuel ratio.

(Prior Art) In order to obtain a good electromotive force characteristic, an enzyme sensor element in which an electrode such as Pt is provided in a solid electrolyte layer is used to protect the electrode from the detection gas, reduce the λ point shift of the oxygen sensor, and obtain a good electromotive force characteristic. Techniques such as are known.

Japanese Patent Publication No. 57-18146 describes that a porous coating film made of a heat-resistant metal oxide and a substance having a catalytic action is provided on the surface of the electrode exposed in the detection gas.

In Japanese Patent Publication No. 57-34900, a porous film that does not carry a catalyst for reacting the gas component of the detection gas is formed on the surface of the electrode exposed in the detection gas, and the gas component of the detection gas is formed on the surface. It is described that a porous coating film carrying a catalyst for reacting with is formed.

In addition, JP-A-56-160653 and JP-A-59-196580.
The publication describes an enzyme concentration cell in which a convex portion of the solid electrolyte is formed on the surface of the solid electrolyte layer and an electrode is provided thereon.

(Problems to be Solved by the Invention) However, the invention described in Japanese Patent Publication No. Sho 57-18146 has a problem that a good electromotive force is generated due to carbon adhesion, desorption of the porous coating, etc. during actual use. The characteristics were not obtained and it could not be used for a long time.

The invention described in JP-B-57-34900 is considered to have been made to improve the drawbacks of the invention described in JP-B-57-18146. However, the life of the invention is not enough to endure practical use. I don't have it.

In addition, JP-A-56-160653 and JP-A-59-196580.
Although the one disclosed in the publication has some effects, it is not sufficient.

It is an object of the present invention to provide an oxygen sensor element that overcomes the above drawbacks of the prior art.

(Means for Solving Problems) In order to achieve the above object, the enzyme sensor element of the present invention has a reference gas contact electrode layer on one surface of the solid electrolyte layer,
A detection gas contact electrode layer is provided on the other surface, which is an enzyme sensor element formed by coating the detection gas contact electrode layer with a porous protective layer, and the solid electrolyte layer surface in contact with the detection gas contact electrode or the like. A plurality of spherical protrusions penetrating the detection gas contact electrode layer and the porous protective layer are formed, and a large number of holes (or also referred to as open voids) of the porous protective layer are reacted with a gas component of the detection gas. The catalyst is dispersed and fixed, and the catalyst dispersed and fixed in the pores consists essentially of platinum, and the weight ratio of the platinum as the catalyst to the volume of the pores is 0.03 g / l to 7 g / l. It is characterized by being l.

Preferably, the upper limit of the weight concentration (g / l) is 5 g / l.

More preferably, the plurality of spherical protrusions are formed in multiple layers on the surface of the solid electrolyte layer.

Still preferably, the platinum catalyst dispersed and fixed in the pores of the porous protective layer is formed by immersing the intermediate product having the porous protective layer formed therein in 0.03 to 7 g / l of H2PtC16 solution.

Still more preferably, in the method for producing an oxygen sensor element according to the present invention, the reference gas contact electrode layer is formed on one surface of the solid electrolyte layer and the detection gas contact electrode layer is formed on the other surface, and the detection gas contact electrode is formed. A method for producing an enzyme sensor element in which a layer is covered with a porous protective layer, wherein particles having a solid electrolyte as a main component are molded, dried, and sintered, and a large number of spherical particles are formed on at least the other surface of the solid electrolyte layer. A solid electrolyte layer forming step of forming a protrusion, and depositing the detection gas contact electrode layer on the other surface of the solid electrolyte layer including the spherical protrusion,
The step of forming a detection gas contact electrode layer for baking, the other surface of the solid electrolyte layer including the spherical protrusion and the detection gas contact electrode layer, the spherical protrusion is the detection gas contact electrode layer and the porous. The porous protective layer forming step of forming the porous protective layer by a plasma spraying method so as to penetrate into the porous protective layer, and at least the intermediate product on which the porous protective layer is formed, H ₂ PtC ₁₆ solution 0.03 It is characterized in that it comprises a step of dispersing and fixing a platinum catalyst which is immersed in ~ 7 g / l and heat-treated to disperse and fix a platinum catalyst for reacting a gas component of a detection gas to the pores of the porous protective layer.

The plasma spraying method is a known technique in which a coating material is melted in high-temperature plasma to form a liquid or gas, and the liquid or gas is sprayed (sprayed) onto the object to be coated.

The spherical protrusions on the surface of the solid electrolyte that are in contact with the detection gas contact electrode layer penetrate into the porous protective layer and prevent the porous protective layer from peeling during use.

Since the catalyst that reacts the gas component of the detection gas dispersed and fixed in the pores of the porous protective layer effectively oxidizes the gas component of the detection gas and maintains the oxidation action stably for a long period of time, the electromotive force characteristic curve shows the theoretical air-fuel ratio. Prevents deviation of the so-called oxygen sensor element from the λ point, and also poisons the detection gas contact electrode layer (Pb, P, etc. when the detection gas contact electrode layer is a Pt layer).
Are preferentially adsorbed to protect the detection gas contact electrode layer from catalyst poisons.

(Preferred embodiment) As a solid electrolyte layer, ZrO ₂ as a stabilizer Y ₂ O ₃ , Ca
The one to which O or the like is added is used. The reference gas contact electrode layer and the detection gas contact electrode layer are preferably Pt or a noble metal electrode layer such as Pt containing Rh of about 2% or less. The solid electrolyte layer may be a tube (test tube) having one end closed and the other end open. The porous layer covering the detection gas contact electrode layer may be a ceramic porous layer such as spinel and has a thickness of about 100 to 180 μm, preferably about 150 μm.

On the surface of the solid electrolyte layer, spherical projections made of solid electrolyte are formed, and the detection gas contact electrode layer is formed thereon. The spherical projections preferably have one layer of granulated particles attached to the surface of the solid electrolyte, but may have two or three layers, and can be integrally formed by sintering after attachment (simultaneous firing or multi-step firing). The size of the granulated particles is preferably 30 to 100 μm, more preferably 50 to 80 μm. The spherical protrusions are distributed so as to leave recesses between them, and together with the recesses form a coupling means for the porous protective layer and contribute to increase the electrode surface area.

The material of the spherical protrusions is preferably the same as that of the solid electrolyte substrate, but the solid electrolyte can achieve the object of the present invention. In some cases, the composition may be a little different from that of the base portion, but it does not matter. For example, if the solid electrolyte substrate is ZrO ₂ −
The Y ₂ O ₃ system and the spherical protrusions may be ZrO ₂ − (CaO, MgO) system, and the solid electrolyte substrate is made of ZrO ₂ − Y ₂ O ₃ system, and the spherical protrusions have the same structure. ZrO ₂ —Y ₂ O ₃ systems having different Y ₂ O ₃ contents may be used.

A catalyst for oxidizing a gas component of the detection gas, such as a platinum group element, is dispersed and fixed in the pores of the ceramic porous layer such as spinel. For that purpose, the ceramic porous layer is impregnated with a solution having a concentration such that the catalyst is sufficiently dispersed and the porous layer does not become clogged after the impregnation, and heat treatment is performed.
If the catalyst is Pt, as a solution Pt is sufficiently dispersed, H ₂
There is PtCl ₆ , and its Pt concentration is preferably 0.03 to 7 g / l (see Examples), and more preferably 0.03 to 5 g / l. When the Pt concentration is less than 0.03 g / l, the detection gas contact electrode layer such as the Pt layer is poisoned by Pb and is at most 7 g / l, and in some cases 5 g / l
If it exceeds, the pores of the ceramic porous layer are clogged, and the responsiveness as an oxygen sensor element deteriorates.

(Example) The Example of the oxygen sensor of this invention is described based on drawing.

FIG. 1 is a cross-sectional view of a tubular (hereinafter, referred to as a test tubular) enzyme sensor element having one end closed and the other end open according to an embodiment of the present invention.

2 and 3 are enlarged sectional views of the oxygen sensor element of the present invention. In the oxygen sensor element shown in FIG. 2, the spherical protrusions (5) are formed as one layer on the solid electrolyte (1) layer, and in FIG. 3, the spherical protrusions (5) are formed on the solid electrolyte (1) layer. It is formed in multiple layers. The solid electrolyte layer (1) is made of ZrO ₂ added with Y ₂ O ₃ and has a test tube shape. The Pt electrode layer (2) in contact with the reference gas is formed on the inner wall of the solid electrolyte layer (1). The Pt electrode layer (3) in contact with the detection gas is formed on the outer wall surface of the solid electrolyte layer (1). The Pt electrode layer (3) in contact with the detection gas is covered by the porous spinel layer (4).

The surface of the solid electrolyte layer has a spherical projection, and this spherical projection (5) is covered by the Pt electrode layer (3) that comes into contact with the detection gas and penetrates into the porous spinel layer (4). Prevent the peeling of the porous spinel layer (4).

Pt particles (6) that oxidize the gas component of the detection gas are dispersed in the porous spinel layer (4) and adhere to the pores (7) of the porous spinel layer (4). Since this Pt particle (6) maintains the oxidizing action of the gas component of the detection gas for a long time, λ
Protects the Pt electrode layer (3) that does not cause point shift and adsorbs catalyst poisons such as Pt and P and contacts the detection gas.

(Manufacturing Example) An example of manufacturing the oxygen sensor element of the present invention will be described.

Process 1. For ZrO ₂ raw material with a purity of 99% or higher, a purity of 99.9% corresponding to 5 mol%
Add Y ₂ O ₃ .

Step 2. Add water and wet-mix in a ball mill for 70 hours and crush.

Step 3. After drying, heat-treat at 1300 ℃ × 2Hr.

Step 4. After heat treatment, pulverize 200Hrs by wet process, add 10Hrs of binder mixture, and spray dry to obtain particle size of 50 ~ 8
0 μm spherical granulated particles are obtained. -(Engineering) 4 Step 5. The powder obtained in (Engineering) 4 is rubber-pressed to form a desired tubular shape (test tubular shape), which is dried and then ground into a predetermined shape with a grindstone.

Step 6. Thereafter, on the outer surface, a slurry in which the water-soluble binder fibrin sodium glycolate and a solvent were added to the granulated particles obtained in (work) 4 was adhered, dried, and calcined at 1500 ° C x 2 Hrs.

Process 7. After firing, Pt layer is 0.9μ thick on the outer surface by chemical plating.
m was deposited and then baked. After that, as a protective coat, plasma spraying was performed with MgO.Al ₂ O ₃ powder to form a porous protective layer.

Process 8. After that, the same chemical plating as in Process 7 was performed on the inner surface, and then the baking treatment was performed.

Step 9. Further, after that, the outer surface of the device was immersed in a H ₂ PtCl ₆ aqueous solution and heat-treated in a 600 ° C. gas furnace to obtain a device.

Process 10. The element was built into a stainless steel fitting and made into a sensor shape.

(Comparative Test) The oxygen sensor element of the present invention according to the above-described manufacturing example and the oxygen sensor element of the comparative example as described below were compared for each evaluation item described below.

Comparative Example 1 Ordinary sensor element manufactured without performing the steps 6 and 9 Comparative Examples 2, 3, 4 and the element manufactured in the same way as the others, Comparative Example 5 Without performing the step 9 Produced element Evaluation item A Output voltage when excess air ratio λ≈1.02 (air is mixed in burner in rich fuel atmosphere) Evaluation item B State of porous spinel layer that covers and protects Pt electrode layer in contact with detection gas. Air-fuel ratio A / F = 12, engine exhaust gas temperature 850 ℃, after 200Hrs durability.

Evaluation item C With 50 mg / gallon of Pb-containing gasoline, the sensor startup response time was measured from the time when the engine exhaust temperature was set to 850 ° C and the fuel-rich atmosphere after 100 Hrs was reached until the electromotive force reached 500 mV.

 The results are shown in Table 1.

Referring to Table 1, Comparative Example 1 is a base material (solid electrolyte layer)
Since there is no protrusion in the layer, the bonding strength between the porous spinel layer and the base material is low, and even if the porous spinel layer does not contain a catalyst, the layer is broken in the evaluation item B.

In Comparative Examples 2 to 4, since the base material does not have a protrusion, the bonding strength between the porous spinel layer and the base material is low, and "the layer is It will "float or peel".

Comparative Example 5 has a structure in which the porous spinel layer itself has a high strength because the base material has a protrusion and has a high bonding strength with the base material and no catalyst. Therefore, although the evaluation item B is "abnormal", the evaluation items A and C are inferior and are not suitable for practical use.

On the other hand, Examples A to F are “no abnormality in the layer” in the evaluation item B and excellent in the evaluation items A and C.

Comparative Example G has a large amount of platinum in its pores because it was immersed in a liquid having a high platinum concentration, and "peeling in the layer" occurred, but no peeling occurred. (It should be noted that the term "breakage" means that a part of the porous spinel layer is cut and has not been peeled.) Examples H to J are platinum blacks dispersed and impregnated in a liquid. In Example H having a low platinum concentration, "there is no abnormality in the layer", and "the layer is slightly cracked" as the platinum concentration increases.

As described above, the oxygen sensor element in which the catalyst is dispersed and fixed in each hole of the porous spinel layer according to the present embodiment (A to F, HL) is the outer surface of the porous spinel layer 4 and the porous spinel layer. 4 does not have a catalyst layer formed at the interface between the base material and the substrate (solid electrolyte layer 1 or Pt electrode layer 3), and since there is no catalyst in the porous matrix (present in the pores), spherical particles Combined with the above effect, the strength of the porous spinel layer 4 itself is high, and the porous spinel layer 4 does not float or peel off from the substrate. Moreover, the output voltage is sufficiently secured,
It also has the effect of being highly resistant to lead. Therefore, according to the present embodiment, a particularly excellent oxygen sensor element having a life that can be practically used is provided.

(Effect of the invention) The enzyme sensor element of the present invention has no λ point shift,
It has a good start-up response, and since the porous protective layer covering the detection gas contact electrode layer does not peel or break, it can be used for a long period of time.

Further, according to the enzyme sensor element of the present invention, there is provided an oxygen sensor exhibiting the above-mentioned effect, which is unlikely to be affected by the catalytic poisoning effect of Pb or the like and the pores of the porous protective layer are less likely to be clogged.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of an embodiment of the oxygen sensor element of the present invention,
2 and 3 are views showing a part of an enlarged cross section of one embodiment of the oxygen sensor element of the present invention. 1 ... Solid electrolyte layer 2 ... Pt electrode layer in contact with reference gas 3 ... Pt electrode layer in contact with detection gas 4 ... Porous spinel layer 5 ... Spherical protrusion 6 ... Pt particle 7 ... Hole

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A-59-196580 (JP, A) JP-A-56-160653 (JP, A) JP-A-59-109854 (JP, A) JP-A-60- 63458 (JP, A) JP 54-89686 (JP, A) JP 50-33892 (JP, A) JP 57-76449 (JP, A)

Claims (1)

[Claims] 1. An enzyme sensor in which a reference gas contact electrode layer is provided on one surface of a solid electrolyte layer, a detection gas contact electrode layer is provided on the other surface, and the detection gas contact electrode layer is covered with a porous protective layer. In the element, on the surface of the solid electrolyte layer in contact with the detection gas contact electrode or the like, a large number of spherical protrusions penetrating the detection gas contact electrode layer and the porous protection layer are formed, and the porous protection layer A catalyst for reacting a gas component of a detection gas is dispersed and fixed to a large number of pores of the catalyst, the catalyst dispersed and fixed to the pores is substantially composed of platinum, and the platinum is the catalyst for the volume of the pores. The enzyme sensor element has a weight ratio of 0.03 g / l to 7 g / l.