JPH07113618B2 - Oxygen gas detector - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an oxygen gas detector for detecting the concentration of an oxygen gas component in the equilibrium state of exhaust gas of an internal combustion engine, and particularly to the gas sensitivity. The present invention relates to an oxygen gas detector using the above metal oxide.

[Prior Art] Conventionally, an oxygen gas detector for detecting the concentration of an oxygen gas component (that is, an air-fuel ratio) in an equilibrium state of exhaust gas of an internal combustion engine has been put into practical use.

Among these, there is one using a gas-sensitive metal oxide having a characteristic that its electric resistance changes when it comes into contact with gas. For example, oxides of transition metal elements such as TiO ₂ , CoO, and NiO can be used as oxygen gas detectors.

The transition metal oxides illustrated here are non-stoichiometric compounds. Then, the amount of charge carriers (holes, electrons) in this non-stoichiometric compound changes depending on the partial pressure of oxygen gas in the surroundings. Therefore, the conductivity changes according to the partial pressure of oxygen gas in the surroundings.

By the way, the oxygen gas detector, for non-equilibrium gas such as exhaust gas of an internal combustion engine, in order to accurately detect the air-fuel ratio, it is necessary to equilibrate the non-equilibrium gas, in order to cope with this In the past, for example, CO in exhaust gas,
An alloy of platinum, which is a catalyst for promoting the oxidation reaction of HC, and rhodium, which is a catalyst for reducing NOx, is made into particles and uniformly dispersed and supported in the gas sensitive layer.

[Problems to be Solved by the Invention] However, it has been found that, in the above catalyst alloy, one of the metals is deposited on the surface of the alloy particles during firing or during use.
For example, when heated in an oxidizing atmosphere, rhodium is deposited on the surfaces of the alloy particles and covers the platinum, reducing the surface area of the platinum and not promoting the oxidation reaction. That is, the output characteristics of the oxygen gas detector largely depend on the rhodium deposition form, and it was not easy to manufacture a stable oxygen gas detector.

[Means for Solving Problems] The present invention employs the following means in order to solve the above problems.

That is, the gist of the present invention is an oxygen gas detector for detecting the concentration of an oxygen gas component in the equilibrium state of exhaust gas of an internal combustion engine, comprising a pair of electrodes and a pair of electrodes covering the pair of electrodes. A porous gas-sensitive layer containing a metal oxide, the electric resistance of which changes according to the concentration of the surrounding oxygen gas component, and an upper layer which covers the gas-sensitive layer, wherein the upper layer promotes a reduction reaction. Oxygen, characterized in that a reduction catalyst layer made of a refractory metal oxide supporting a catalyst and an oxidation catalyst layer made of a refractory metal oxide supporting a catalyst for promoting an oxidation reaction are alternately laminated in one or more layers. On the gas detector.

Here, the electrode is not particularly limited as long as it is a heat-resistant conductor, but usually, tungsten, molybdenum,
A material containing silver, gold or a platinum group as a main component is used.

Examples of the gas-sensitive metal oxide used in the gas-sensitive layer include TiO ₂ , SnO ₂ , CoO, ZnO and Nb ₂ O which are usually used.
Examples thereof include transition metal oxides such as ₅ , Cr ₂ O ₃ , and NiO. In the present invention, any one or a combination of two or more of these may be used.

As the catalyst for promoting the oxidation reaction, the substance may be selected according to the state of use of the oxygen gas detector, but in the present invention, the oxygen gas detector is used to measure the oxygen gas partial pressure in the exhaust gas of an internal combustion engine. Since it is used for, platinum, palladium, etc. can be mentioned.

As for the catalyst that promotes the reduction reaction, like the catalyst that promotes the above-mentioned oxidation reaction, the substance may be selected according to the state of use of the oxygen gas detector, but in the present invention, the oxygen gas detector is Since it is used for measuring the partial pressure of oxygen gas in engine exhaust gas, rhodium, ruthenium, etc. can be mentioned.

The base material of the catalyst layer supporting the catalyst is not particularly limited as long as it is a thermally stable porous material. For example, alumina, magnesia spinel, zirconia, forsterite, cordierite, etc. can be used.

These catalysts are, for example, pyrolyzed after impregnating a solution containing a catalyst component element in a porous catalyst layer base material serving as a carrier.
The catalyst is supported on the catalyst layer by an impregnation method in which the catalyst is dispersed and supported on the catalyst layer base material by reduction or the like. In addition to this impregnation method, a precipitant is added to a salt solution containing a catalyst element to precipitate the catalyst on the surface of the catalyst layer base material that supports the catalyst, or both the catalyst layer base material component and the catalyst component are simultaneously added. Precipitate (coprecipitate)
The catalyst is supported on the catalyst layer by a precipitation method, an ion exchange method that utilizes the ion exchange property of the catalyst layer that is supported, an adsorption method that utilizes adsorption from a solution containing a catalyst component element, or the like.

The oxygen gas detector of the present invention can be produced, for example, by providing a gas sensitive layer or the like on a ceramic substrate by a hybrid technique such as a thick film technique. Alternatively, it may be formed into a disk type, a bead type or the like used in a thermistor or the like without using the thick film technique or the like.

Furthermore, a heating element may be provided in the vicinity of the gas sensitive layer for the purpose of reducing the temperature characteristic fluctuation of the oxygen gas detector during measurement. Then, a part of this heating element and one electrode of the oxygen gas detector may be connected to apply a voltage to the gas sensitive layer to reduce the number of terminals and simplify the measurement circuit.

Further, for the purpose of protecting the gas sensitive layer, a coat layer may be provided so as to overlap the gas sensitive layer. This coat layer is
Adsorbs and captures toxic substances such as lead on gas-sensitive metal oxides and prevents toxic substances from reaching the gas-sensitive layer. The material of the coat layer is not particularly limited as long as it is a thermally stable material, and for example, alumina, magnesia spinel, zirconia or the like can be used.

[Operation] In the present invention, the catalyst that promotes the reduction reaction and the catalyst that accelerates the oxidation reaction are supported by different layers. Therefore, at the time of calcination or use, one catalyst does not deposit on the surface of the other catalyst, and interaction between the catalysts that hinders the other catalyst action does not occur.

Further, since the catalyst that promotes the reduction reaction and the catalyst that promotes the oxidation reaction are in close proximity to each other, it is easy to utilize the products of the reaction of the other party, and a more efficient reaction can be performed. That is,
For example, using oxygen generated by the reduction of NOx, CO,
It oxidizes HC.

Therefore, each of the catalyst functions can be sufficiently exerted,
The gas sensitive reaction becomes stable.

[Effect of the Invention] In the present invention, the interaction of the catalyst does not occur as described above. Therefore, it exhibits excellent performance even in a non-equilibrium gas, and exhibits stable performance over a long period with little deterioration of the catalyst.

Further, since the exhaust gas of the internal combustion engine is in a non-equilibrium state, even if the oxygen concentration in the exhaust gas of the internal combustion engine is directly measured as the oxygen partial pressure, the air-fuel ratio (corresponding to the oxygen concentration partial pressure in the exhaust gas in the equilibrium state) However, in the oxygen gas detector of the present invention, the reducing gas (CO, HC, etc.) in the non-equilibrium exhaust gas is reacted with oxygen in the oxidation catalyst layer that promotes the oxidation reaction, On the other hand, oxidizing gas (NO
Since x) is decomposed into nitrogen and oxygen by the reduction catalyst layer that promotes the reduction reaction, each nonequilibrium gas can be reacted more efficiently than before. That is, in the present invention,
Since the oxygen partial pressure is detected after the gas is changed to a gas closer to the equilibrium state than in the conventional oxygen gas detector, the air-fuel ratio can be accurately detected.

Furthermore, in the present invention, since a refractory metal oxide such as alumina is used as the base material of the catalyst layer, the catalyst layer that is thermally stable even to a gas that reaches a high temperature such as exhaust gas of an internal combustion engine. Can be formed.

[Embodiment] An embodiment of the present invention will be described with reference to the drawings. Note that the scale of each drawing is different for the sake of explanation.

The present embodiment is an oxygen gas detector 10 in which TiO ₂ is used as a gas sensitive layer, and an oxidation catalyst layer in which platinum is supported on alumina and a reduction catalyst layer in which rhodium is supported on alumina are alternately stacked as an upper layer.

As shown in the partially cutaway perspective view of FIG. 1, the oxygen gas detector 10 is formed on a ceramic substrate 12 and the platinum lead wire 14a formed on the ceramic substrate 12 and terminals 13a, 13b, 13e. , 14b, 14e connected to the detection electrodes 16a, 16
b and the electrode pattern 16 such as the thermal resistance electrode 16e, the ceramic substrate 12 and the electrode pattern 16 are laminated on the ceramic substrate 12 and integrated with the ceramic substrate 12, and a ceramic laminated plate 18 having a window portion 18a; Window part 18 of laminated board 18
a, the gas-sensitive layer 20 filled so as to cover the detection electrode patterns 16a and 16b, the ceramic substrate 12 and the gas-sensitive layer 20.
Spherical granulated particles that are dispersed between and prevent the separation of the two
And an upper layer 24 composed of an oxidation catalyst layer 24a and a reduction catalyst layer 24b alternately stacked on the gas-sensitive layer 20, and a coat layer 26 composed of Al ₂ O ₃ laminated on the upper layer 24. There is. In the figure, for the sake of explanation, the oxidation catalyst layer 24 constituting the upper layer 24
The a and the reduction catalyst layer 24b are each one layer, but they may be further layered if necessary.

Next, a manufacturing process of the oxygen gas detector 10 of this embodiment will be described with reference to FIGS.

92 wt% alumina, 3 wt% magnesia, and 5 wt% sintering aid (silica, calcia, etc.) are mixed in a pot mill for 20 hours. Then, the mixture was mixed with polyvinyl butyral 12 wt% and dibutyl phthalate 4 wt% as an organic binder.
Was added, and methyl ethyl ketone, toluene and the like were added as a solvent. Further, the mixture is mixed in a pot mill for 15 hours to obtain a slurry, and the substrate and laminating green sheets 12A and 18A are formed by the doctor plate method.

The shape of the above green sheet is the green sheet 12A for the substrate.
47.8mm × 4.0mm × 0.8mm ^t , with laminated green sheet 18A
It has a size of 47.8 mm × 4.0 mm × 0.26 mm ^t , and a 3.05 mm × 2.0 mm window portion 18a is formed in the lamination green sheet 18A.

Next, platinum black and sponge-like platinum were blended in a ratio of 2: 1, 10 wt% of the material mixture of the green sheet used above was added, and a solvent such as butyl carbidol and Ethocel was added, Use as electrode paste.

Next, an electrode pattern 16 is formed on the substrate green sheet 12A by thick film printing using the electrode paste adjusted in. As the electrode pattern 16, as described above, the detection electrode patterns 16a and 16b, and the thermal resistance electrode pattern 16e that serves as a heater for heating the gas-sensitive portion 20, and the terminal patterns 13a and 13b that serve as the terminals of both patterns 16 described above. , 13e are formed. (Fig. 2 (a), (b)) Then, on the terminal patterns 13a, 13b, 13e, the diameter
The platinum lead wires 14a, 14b, 14e of 0.2 mm are connected to each other (Fig. 3 (a), (b)).

Next, the lamination green sheet 18A is laminated and thermocompression-bonded on the substrate green sheet 12A to form a laminate.
At this time, the window portion 18a of the laminated green sheet 18A,
The tips of the detection electrode patterns 16a and 16b are exposed. Then, 80-150 mesh spherical granulated particles (2 particles made of the same material as the green sheet prepared in the window portion 18a) (2
Secondary particles) 22 are dispersed and adhered, and then the above laminated body is subjected to 1500 ° C.
The ceramic substrate 12 and the ceramic laminated plate 18 which are integrated by firing in the same atmosphere as the atmosphere for 2 hours.
Are formed (Fig. 4 (a), (b)).

When the spherical granulated particles 22 are dispersed and adhered as described above and fired, each particle 22 is dispersed in a single layer as shown in the enlarged view of FIG. 4C to form an uneven surface on the ceramic substrate 12. .

Next, the window portion 18a of the ceramic laminate 18 is filled with a paste of titania particles supporting a small amount of platinum as a gas-sensitive layer, and is oxidized as the oxidation catalyst layer 24a and the reduction catalyst layer 24b which are stacked to form the upper layer 24. The catalyst grain paste and the reduction catalyst grain paste are filled, and each paste is adjusted as follows before filling.

First, a titania grain paste is prepared.

TiO ₂ with an average particle size of 1.2 μm, calcined in the air at 1200 ° C for 1 hour
100 g of powder, 100 g of chloroplatinic acid aqueous solution containing 10 g of platinum
After adding cc and drying in air at 200 ° C. for 24 hours, thermal decomposition was performed in a hydrogen furnace at 700 ° C. for 2 hours to deposit platinum on the surface of the TiO ₂ powder to obtain titania particles.

To 100 g of this titania grain powder, 2 g of ethyl cellulose was added as a binder, and these were added to butycarbitol (trade name of 2- (2-butoxyethoxy) ethanol).
Mix in and adjust to a viscosity of 300 poise to prepare the titania grain paste.

Next, the oxidation catalyst layer paste is prepared.

To 80 g of mixed alumina powder consisting of 96% by weight of alumina, 3% by weight of silica and 1% by weight of CaO and having an average particle size of 0.5 μm,
Add 20g of platinum black or palladium to make a mixed powder. To 100 g of this mixed powder, 2 g of ethyl cellulose was added as a binder, and these were added to butycarbitol (trade name of 2- (2-butoxyethoxy) ethanol).
Mix in and adjust to a viscosity of 300 poise to prepare the oxidation catalyst layer paste.

Next, the reducing catalyst particles are adjusted.

To 80 g of mixed alumina powder consisting of 96% by weight of alumina, 3% by weight of silica and 1% by weight of CaO and having an average particle size of 0.5 μm,
Add 20 g of sponge-like rhodium (rhodium black) or sponge-like ruthenium (ruthenium black) to obtain a mixed powder. To 100 g of this mixed powder, 2 g of ethyl cellulose was added as a binder, and these were mixed in butycarbitol (a trade name of 2- (2-butoxyethoxy) ethanol) to give a viscosity of 300 poise, and a reduction catalyst layer paste. Adjust.

Then, first, the titania grain paste is filled in the window portion 18a by a thick film printing technique, and then a predetermined combination (see Table 1).
Then, the oxidation catalyst layer paste and the reduction catalyst layer paste are stacked and filled in the titania grain paste, and the Al ₂ O ₃ coating layer 2
Stack 4

After that, the laminated body that has undergone the above steps is placed in the atmosphere at 1200 ° C for 1
Let stand for time and bake. (Fig. 5 (a), (b)).

<Experiment> When the reducing catalyst and the reducing catalyst were loaded on different layers (Example), the response speed and durability were examined as follows.

The combination of the reduction catalyst layer and the oxidation catalyst layer formed in the above steps is changed as shown in Table 1 to prepare a sample of the oxygen gas detector of this embodiment. In Table 1, the Pt layer indicates the oxidation catalyst layer formed by using the oxidation catalyst layer paste containing Pt black in the above process, and the Pd layer is also formed by using the oxidation catalyst layer paste containing palladium. Shows the oxidation catalyst layer, the Rh layer also shows a reduction catalyst layer formed by using a reduction catalyst paste containing rhodium, and the Ru layer also shows a reduction catalyst layer formed by using a reduction catalyst paste containing ruthenium. Show.

As a comparative example, as shown in Table 1, a sample having only a gas sensitive layer, a sample in which a reduction catalyst (rhodium 2% by weight) and an oxidation catalyst (platinum 18% by weight) are supported in titania, and an upper layer is a reduction catalyst. A sample carrying an alloy of (rhodium 2% by weight) and an oxidation catalyst (platinum 18% by weight), a sample having an upper layer consisting only of a reduction catalyst (rhodium), and a sample having an upper layer consisting only of an oxidation catalyst (platinum) were prepared. , The same responsiveness as the sample of the example,
The durability was investigated.

After assembling the sample thus prepared in the oxygen gas detector, the response speed and durability of each sample in the non-equilibrium gas are measured by the following experiments. The result of the experiment is also the first
Shown according to the table. In Table 1, the unit of response speed is msec.

-Response speed in non-equilibrium gas The sample S assembled as an oxygen gas detector is exposed to the exhaust gas of a propane gas burner whose exhaust gas is a non-equilibrium gas. This propane gas burner has an exhaust temperature of 350 ° C,
Moreover, the air-fuel ratio is controlled to change between the fuel excess (hereinafter referred to as rich, air fuel ratio λ = 0.9) and the fuel shortage (hereinafter referred to as lean λ = 1.1) every second.
This change of the air-fuel ratio is carried out by adding air or propane gas to the exhaust gas having an exhaust temperature of 500 ° C. after combustion, as shown in FIG.

The voltage applied to the oxygen gas detector is adjusted so that the output of the oxygen gas detector 10 is 1 V when the exhaust gas is in excess of fuel and the output is 0 V when lean.

As the response speed, the time when the output of the oxygen gas detector 10 changes from 300 mV to 600 mV when the atmosphere changes from lean to rich (indicated as 1 → r in the table), and the output when the atmosphere changes from rich to lean Is measured from 600 mV to 300 mV (r → 1 in the table).

-Durability A sample assembled as an oxygen gas detector is attached to an actual vehicle, operated according to a specified durability pattern, and durability is examined from the change in response speed before and after operation. That is, as shown in FIG. 7, the oxygen gas detector S is an exhaust pipe between the engine Eng and the three-way catalyst THC of a commercially available 2000cc three-way catalyst vehicle with EFI.
Attached to Man. Then, the control unit Uni controls the operating state of the engine according to the output of the oxygen gas detector S. The output of the oxygen gas detector S is detected by the circuit as shown in FIG. Here, B is a power supply and Rc is a comparison resistance.

The engine Eng described above is used in the endurance pattern shown in FIG.
Drive for 00 hours. The solid line in the figure shows the temperature of the sample, and the broken line shows the temperature of the exhaust gas.

The response speed described above is measured before and after this operation, and the change is taken as the durability result. That is, it is determined that the sample having less change in response speed before and after the operation has more excellent durability. In Table 1, before operation is described as initial and after operation as after durability test.

From the above experiment, the following was found.

The oxygen gas detectors (sample numbers 6 to 10) of this example are
The durability is superior to the oxygen gas detectors (sample numbers 1 to 5) of the comparative example. Particularly, in the comparative example, the response from lean to rich is extremely reduced after the durability test, whereas in this example, the response is slightly reduced. This is because in the present example, the oxidation catalyst and the reduction catalyst were loaded differently, and there was no interaction between the reduction catalyst and the oxidation catalyst described in [Problems to be Solved by the Invention]. It appears to be.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of an embodiment of the present invention, FIGS. 2 to 5 are illustrations of manufacturing of the embodiment, FIG. 6 is an illustration of a propane gas burner used for a response test, and FIG. FIG. 8 and FIG. 8 are explanatory views of the procedure of a durability test using an oxygen gas detector in an internal combustion engine, and FIG. 9 is a durability pattern diagram thereof. 10 ... Oxygen gas detector, 12 ... Ceramic substrate, 16, 16a, 16b ... Detection electrode, 16e ... Thermal resistance electrode, 18a ... Window, 18 ... Ceramic laminated plate, 20 ... Sensitive gas Layer, 22 ... Spherical granulated particles, 24 ... upper layer, 24a ... Oxidation catalyst layer, 24b ... Reduction catalyst layer

Front Page Continuation (72) Inventor Akio Takami 14-18 Takatsuji-cho, Mizuho-ku, Nagoya, Aichi Nihon Special Ceramics Co., Ltd. (56) Reference JP-A-54-134697 (JP, A)

Claims (3)

[Claims] 1. An oxygen gas detector for detecting the concentration of an oxygen gas component in the equilibrium state of exhaust gas of an internal combustion engine, comprising a pair of electrodes and a gas-sensitive metal oxide covering the pair of electrodes. And a porous gas-sensitive layer whose electric resistance changes according to the concentration of the surrounding oxygen gas component, and an upper layer that covers the gas-sensitive layer, the upper layer carrying a catalyst that promotes a reduction reaction. An oxygen gas detector characterized in that at least one reduction catalyst layer made of a refractory metal oxide and an oxidation catalyst layer made of a refractory metal oxide carrying a catalyst for promoting an oxidation reaction are alternately laminated. . 2. The oxygen gas detector according to claim 1, wherein the catalyst for promoting the oxidation reaction is any one of platinum, palladium, and an alloy of platinum and palladium. 3. The oxygen gas detector according to claim 1 or 2, wherein the catalyst for promoting the reduction reaction is any one of rhodium, ruthenium and an alloy of rhodium and ruthenium.