JPH07107524B2 - 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, and particularly to oxygen using a gas-sensitive metal oxide. Regarding gas detectors.

[Prior Art] Conventionally, an oxygen gas detector for detecting the concentration of an oxygen gas component has been put into practical use.

Among these, there is one that uses a gas-sensitive metal oxide having a characteristic that its electric resistance changes when it comes into contact with oxygen 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, in order to accurately detect the partial pressure of oxygen gas in a non-equilibrium gas such as the exhaust gas of an internal combustion engine, the oxygen gas detector needs to equilibrate the non-equilibrium gas. Therefore, conventionally, a noble metal catalyst such as Pt is made into a powder form and uniformly dispersed and supported in the gas sensitive layer.

[Problems to be Solved by the Invention] However, the catalyst adsorbs a non-equilibrium gas, including oxygen gas, on the surface of the catalyst, reacts on the catalyst surface, and desorbs it as a equilibrium gas to equilibrate the non-equilibrium gas. Turn into. Therefore, even if the oxygen gas partial pressure changes in the gas, the output is delayed for the time from the adsorption of the gas to the catalyst to the desorption.

Therefore, when the amount of the catalyst is increased in order to increase the equilibrium force of the non-equilibrium gas, the equilibration force increases, but the time from adsorption to desorption becomes longer and the responsiveness deteriorates. On the contrary, if the amount of the catalyst is decreased to improve the response, the response is improved, but on the other hand, the equilibrium of the non-equilibrium gas is not sufficiently performed, and correct measurement cannot be performed.

[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 to provide a pair of electrodes, a porous gas-sensitive layer that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electrical resistance that changes according to the concentration of the surrounding oxygen gas component. And an upper layer covering the gas-sensitive layer, wherein the upper layer contains a metal oxide having an oxygen storage property different from that of the gas-sensitive metal oxide and carries a catalyst. It is in.

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.

The oxygen storage metal oxide used in the upper layer has a property of taking in oxygen when the surrounding oxygen gas partial pressure is high, and conversely releasing oxygen when the surrounding oxygen gas partial pressure is low (O ₂ storag).
There is no particular limitation as long as the material has e), but CeO ₂ , La ₂
Oxides, complex oxides, and rare earth element oxides such as O ₃ , Nd ₂ O ₃ , and Eu ₂ O ₃ are suitable and preferred. The above gas-sensitive metal oxide also has an oxygen storage property, but its effect is too strong to reduce the responsiveness, so it cannot be used as the oxygen storage metal oxide of the upper layer. .
This oxygen-storing metal oxide is mixed with the base material of the upper layer described later to form the upper layer.

The upper layer base material 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.

As the catalyst, the substance may be selected according to the state in which the oxygen gas detector is used, etc., but particularly when the oxygen gas detector is used for measuring the partial pressure of oxygen gas in the exhaust gas of an internal combustion engine. One or more selected from the platinum group elements are preferable.

This catalyst is supported on the upper layer by, for example, an impregnation method in which a porous upper layer base material serving as a carrier is impregnated with a solution containing a catalyst component element and then the catalyst is dispersed and supported on the upper layer base material by thermal decomposition, reduction or the like. . In addition to this impregnation method, a precipitating agent is added to a salt solution containing a catalyst element to precipitate the catalyst on the surface of the upper layer base material serving as a carrier, or both the upper layer base material component and the catalyst component are simultaneously precipitated. The catalyst is supported on the upper layer by a precipitation method (coprecipitation), an ion exchange method utilizing the ion exchange property of the upper layer which is a carrier, an adsorption method utilizing 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 further provided on the upper layer. This coat layer adsorbs and captures toxic substances such as lead with respect to gas-sensitive metal oxides and prevents the toxic substances from reaching the gas-sensitive layer. The material of the coat layer is a thermally stable material. There is no particular limitation, and for example, alumina, magnesia spinel, zirconia, etc. can be used.

[Operation] When equilibrating a non-equilibrium gas such as exhaust gas of an internal combustion engine with a catalyst, the catalyst works most efficiently when the atmosphere in the vicinity of the catalyst has a substantially stoichiometric air-fuel ratio.

In the present invention, the oxygen storage metal oxide exists near the catalyst supported on the upper layer. Therefore, when excess unburned components are present in the atmosphere near the catalyst, oxygen is supplied from the oxygen storage metal oxide to the vicinity of the catalyst, while when excess oxygen is present, the oxygen storage metal oxide is oxidized. Oxygen is taken into the material, and the vicinity of the catalyst is maintained at a theoretical mixing ratio with almost no excess unburned components or excess oxygen. Therefore, an atmosphere where the catalyst works most efficiently is always present in the vicinity of the catalyst, and the equilibrium of the non-equilibrium gas can be sufficiently achieved even with a smaller amount of the catalyst than in the conventional case. As a matter of course, it is only the minute portion near the catalyst that is adjusted by the oxygen storage metal oxide, and as a whole, the equilibrium of oxygen partial pressure according to the surrounding atmosphere is adjusted. It is a natural gas. Then, the gas sensitive layer measures the partial pressure of oxygen gas in the equilibrium gas.

The reason why the gas-sensitive metal oxide cannot be used as the oxygen-storing metal oxide is that the gas-sensitive metal oxide absorbs and releases a large amount of oxygen, which deteriorates the responsiveness. is there.

[Effects of the Invention] In the present invention, as described above, the oxygen storage metal oxide in the upper layer makes the atmosphere in the vicinity of the catalyst almost constant. for that reason,
The catalyst always works at maximum capacity. Therefore, even a small amount of the catalyst is sufficient for equilibrating the non-equilibrium gas, and a small amount of the catalyst improves the response delay due to the adsorption and desorption of the catalyst and contributes to resource saving. .

[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.

In this example, titania supporting platinum was used as the gas-sensitive layer, and various oxygen storage metal oxides were mixed with activated alumina as the base material as the upper layer to detect oxygen gas supporting platinum and rhodium as the catalyst. It is a vessel 10.

As shown in the partially broken perspective view of FIG. 1, the oxygen gas detector 10 is formed on a ceramic substrate 12, and the platinum lead wire 14a is formed on the ceramic substrate 12 with terminals 13a, 13b, 13e. Detection electrodes 16a, 16b connected to 14b, 14e
And an electrode pattern 16 such as a thermal resistance electrode 16e, a ceramic laminated plate 18 having a window portion 18a integrated with the ceramic substrate 12 by being laminated on the ceramic substrate 12 and the electrode pattern 16, and the ceramic laminated body. Window 18 of plate 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 laminated on the gas-sensitive layer 20, and a coat layer 26 made of Al ₂ O ₃ laminated on the upper layer 24.

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 to 150 mesh spherical granulated particles (secondary particles) 22 made of the same material as the green sheet adjusted in the window portion 18a are dispersed and adhered, and then the above laminated body is exposed to the atmosphere at 1500 ° C. By firing in the same atmosphere for 2 hours, the integrated ceramic substrate 12 and ceramic laminated plate 18 are formed (Fig. 4 (a) and (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, in the window portion 18a of the ceramic laminated plate 18, a paste of titania particles carrying a small amount of platinum as a gas sensitive layer is filled, and the upper layer paste is filled as the upper layer 24 by stacking. Adjust each paste as follows.

First, a titania particle 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, pyrolysis is 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 upper layer paste is adjusted.

To 1 mol of activated alumina having an average particle size of 0.5 μm, a predetermined amount of a nitrate aqueous solution or chloride aqueous solution containing a metal of an oxygen storage metal oxide (see Table 1) was added, and the mixture was allowed to stand for 10 minutes. Fully dry for 12 hours at ℃,
Alumina powder is obtained by firing in air at 850 ° C. To this alumina powder, a mixed aqueous solution of chloroplatinic acid and chlororhodic acid containing 1 g of platinum and 0.1 g of rhodium was added, and the mixture was placed in the air.
After fully drying at 200 ° C for 24 hours, 2 hours at 700 ° C in a hydrogen furnace
Pyrolysis is performed over a period of time to deposit catalysts platinum and rhodium on the alumina powder. In this way, 2 g of ethyl cellulose was added as a binder to the alumina powder on which the catalyst was deposited, and these were added to butycarbitol (2-
(2-Butoxyethoxy) ethanol (trade name) is mixed to a viscosity of 300 poise to prepare the upper layer paste.

Then, the titania grain paste is first filled in the window portion 18a to a thickness of 200 μm by the thick film printing technique, and then the upper layer paste is stacked on the titania grain paste to a thickness of 100 μm, and further Al ₂ O _{3 is} added. The coat layer 24 is laminated.

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)).
Conventionally, the catalyst amount in the gas sensitive layer is about 5 mol% platinum and about 0.5 mol% rhodium. Therefore, the amount of catalyst contained in the gas-sensitive layer of this example is much smaller than that of the conventional one.

<Experiment> When oxygen storage metal oxide is included in the upper layer (Example)
The response speed and durability in the non-equilibrium gas were investigated as described below.

In the above process, the type and content of the oxygen-storing metal oxide in the upper layer paste are changed as shown in Table 1 to prepare a sample of the oxygen gas detector of this example. In Table 1, the oxygen storage metal oxide content (mol%) is the amount based on activated alumina. As the oxygen-storing metal oxide, CeO ₂ , when using La ₂ O ₃ , the nitrate aqueous solution is used in the above step, and when using Eu ₂ O ₃ and Nb ₂ O ₃ , its chloride is used. Use an aqueous solution.

As a comparative example, a sample having an upper layer containing no oxygen-storing metal oxide was prepared, and the response speed and durability in a non-equilibrium gas were examined in the same manner as the sample of the example. Sample No. A-1 has the same amount of catalyst as in the example, and Sample No. A-1
-2 is 5 g of platinum and 0.5 g of rhodium for 100 g of titania whose catalyst amount is the same as that of the conventional oxygen gas detector, and sample number A-3 has a larger amount of catalyst than the conventional oxygen gas detector, 100 g of titania. On the other hand, 20 g of platinum and 2 g of rhodium are respectively supported.

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 mesc.

-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 detector according to the present embodiment has a
The response speed is similar to that of the conventional oxygen gas detector as shown by -2. This is probably because the oxygen storage metal oxide always keeps the air-fuel ratio in the vicinity of the catalyst in an optimum state, and the function of the catalyst is increased. B-
8, when the content of the oxygen-storing metal oxide is large like B-22, the response speed is relatively slow. This is probably because the oxygen storage effect became too strong.

The oxygen gas detector of this example is superior in durability to the oxygen gas detector of the comparative example. This is because even if the performance of the catalyst itself deteriorates after the durability test, the performance of the catalyst is maximized due to the effect of the oxygen storage metal oxide, and therefore the equilibration ability as the upper layer does not decrease. Seem.

[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

Claims (2)

[Claims] 1. A pair of electrodes, a porous gas-sensitive layer that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electric resistance that changes according to the concentration of the surrounding oxygen gas component, An oxygen gas detector, comprising: an upper layer that covers the gas-sensitive layer, wherein the upper layer contains a metal oxide having an oxygen storage property different from that of the gas-sensitive metal oxide and carries a catalyst. 2. The oxygen-storing metal oxide is CeO ₂ , La
The oxygen according to claim 1, which is either one selected from ₂ O ₃ , Nd ₂ O ₃ and Eu ₂ O ₃ or a composite compound of two or more selected from the metal oxides. Gas detector.