JPS6367556A - Gas detector - Google Patents

Abstract

PURPOSE:To hold an atmosphere nearby the catalyst nearly constant and to make a catalyst work to its maximum capacity all the time by providing oxygen occlusive metal oxide in an upper layer which covers a gas sensing layer. CONSTITUTION:A gas detector 10 consists of a ceramic substrate 12, electrodes 16a and 16b for detection which are formed on the substrate 12, a heat resistance electrode 16e, a ceramic laminate plate 18 which is united with the substrate 12 and has a window part 18a, the gas sensitive layer 20 which are so changed in the window part 18 as to cover the electrode patterns 16a and 16b, and the upper layer 24 which is laminated on the gas layer 20. The oxygen occlusive metal oxide is present nearby the catalyst carried by the upper layer 24. Consequently, the catalyst atmosphere is held at an ideal air fuel ratio.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a gas detector for detecting gas components or their concentrations, and in particular to gas detection using gas-sensitive metal oxides. Concerning vessels.

[Prior Art] Conventionally, oxygen gas detectors, combustible gas detectors, and the like have been put into practical use as gas detectors for detecting the presence of surrounding gas or its concentration.

Some of these use gas-sensitive metal oxides whose electrical resistance changes when they come into contact with gas. For example, transition metal oxides such as TiO2, Coo, and NiO Oxides etc. can be used as oxygen sensors.

The transition gold R oxide exemplified here is a non-chemical compound. The amount of charge carriers (holes, electrons) in this non-stoichiometric compound changes depending on the partial pressure of the surrounding oxygen gas. Therefore, the conductivity changes depending on the partial pressure of the surrounding oxygen gas.

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 above-mentioned oxygen sensor needs to equilibrate the non-equilibrium gas. Conventionally, a noble metal catalyst such as PT is powdered and uniformly dispersed and supported in the gas-sensitive layer.

[Problems to be Solved by the Invention] However, catalysts do not bring non-equilibrium gases into equilibrium by adsorbing non-equilibrium gases, including oxygen gas, on the catalyst surface, causing them to react on the catalyst surface, and desorbing them as equilibrium gases. become Therefore, even if a change in oxygen gas partial pressure occurs in the gas, the output is delayed by the time from when the gas is adsorbed by the catalyst until it is desorbed.

Therefore, if the number of catalysts is increased in order to increase the power to equilibrate a non-equilibrium gas, the power to equilibrate increases, but the time from adsorption to desorption becomes longer and the response deteriorates. On the other hand, if the amount of catalyst is reduced in order to improve responsiveness, while responsiveness will be improved, the non-equilibrium gas will not be sufficiently equilibrated, making accurate measurements impossible.

[Means for solving the problems] The present invention employs the following means to solve the above problems.

That is, the gist of the present invention is to provide a pair of electrodes, a porous material covering the pair of electrodes, containing a gas-sensitive metal oxide, and having an electrical resistance that changes depending on the surrounding gas component and/or its concentration. Gas detection comprising a gas layer and an upper layer covering the gas-sensitive layer, wherein the upper layer contains an oxygen-storing metal oxide different from the gas-sensitive metal oxide and supports a catalyst. It's in the container.

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

The gas-sensitive metal oxide used in the gas-sensitive layer may be selected depending on the gas component to be detected.
Commonly used ones include TiO2, SnO2, Co
Examples include transition metal oxides such as ZnO, ZnO1Nb2O5, Cr2O3, and NiO, and in the present invention, any one or a combination of two or more of these may be used.

The oxygen-storing gold m oxide used in the upper layer has the 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.
There is no particular limitation as long as the material has C
Oxides or double oxides of rare earth elements such as eO2, La2O3, Nd20a, and Eu20s are preferable because their effects are appropriate.The gas-sensitive metal oxides mentioned above also have oxygen storage properties, but the effect is not strong. This oxygen-storing metal oxide cannot be used as the upper layer's oxygen-storing metal oxide because it reduces the response.This oxygen-storing metal oxide is mixed with the upper layer's base material to form the upper layer. Configure.

The base material for the upper layer is not particularly limited as long as it is a thermally stable porous material. For example, alumina, magnesia spinel, zirconia, 7-orsterite, cordierite, etc. can be used.

The above-mentioned catalyst may be selected depending on the state in which the gas detector is used and the gas to be detected, but it is especially important to use the gas detector when measuring the partial pressure of oxygen gas in the exhaust gas of an internal combustion engine. If so, one or more selected from platinum group elements are preferred.

This catalyst is supported on the upper layer by an impregnation method in which, for example, a porous upper layer base material serving as a carrier is impregnated with a solution containing catalyst component elements, and then the catalyst is dispersed and supported on the upper layer base material by thermal decomposition, reduction, etc. . In addition to this impregnating property, a precipitating agent is added to a solution of a salt containing a catalyst element to precipitate the catalyst on the surface of the upper layer base material that serves as a carrier, or to precipitate both the upper layer base material component and the catalyst component simultaneously. The catalyst is supported on the upper layer by a precipitation method (co-precipitation), an ion exchange method that utilizes the ion exchange properties of the upper layer that is a carrier, an adsorption method that utilizes adsorption from a solution containing catalyst component elements, and the like.

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

Furthermore, for the purpose of reducing temperature characteristic fluctuations of the gas detector during measurement, a heating element may be provided near the gas-sensitive layer, and a part of this heating element and one electrode of the gas detector may be connected. A voltage may be applied to the gas-sensitive layer by connecting the two terminals to reduce the number of terminals and simplify the measurement circuit.

Further, for the purpose of protecting the gas-sensitive layer, a coating layer may be provided on top of the upper layer.

This coating layer adsorbs and captures toxic substances such as lead on gas-sensitive metal oxides and prevents the toxic substances from reaching the gas-sensitive layer.The coating layer can be made of a thermally stable material. There is no particular limitation, and for example, alumina, magnesia spinel, zirconia, etc. can be used.

[Operation] When a non-equilibrium gas such as exhaust gas from an internal combustion engine is equilibrated by a catalyst, the catalyst works most efficiently when the atmosphere near the catalyst is approximately at the stoichiometric air-fuel ratio.

In the present invention, an oxygen storage metal oxide is present near the catalyst supported on the upper layer. Therefore, when there is an excess of unburned components in the atmosphere near the catalyst, oxygen is supplied to the vicinity of the catalyst from the oxygen-storing metal oxide; on the other hand, when there is excess oxygen, the oxygen-storing metal oxide Oxygen is incorporated into the material, keeping the vicinity of the catalyst at a stoichiometric mixing ratio with almost no excess unburned components or excess oxygen, therefore,
The vicinity of the catalyst is always an atmosphere in which the catalyst works most efficiently, and non-equilibrium gas can be sufficiently equilibrated even with a smaller amount of catalyst than before. As a matter of course, the upper atmosphere oxygen storage metal oxide adjusts only the minute part near the catalyst, and overall the oxygen partial pressure is adjusted according to the surrounding atmosphere. It is an equilibrium gas. The gas-sensitive layer then measures the partial pressure of oxygen gas in this equilibrium gas.

The reason why gas-sensitive metal oxides cannot be used as oxygen-storing metal oxides is that gas-sensitive metal oxides store and release large amounts of oxygen, which deteriorates responsiveness. be.

[Effects of the Invention] In the present invention, as described above, the oxygen storage metal oxide in the upper layer keeps the atmosphere near the catalyst almost constant. Therefore,
The catalyst always works at its maximum capacity, so even a small amount of catalyst is sufficient to equilibrate non-equilibrium gases, and if the amount of catalyst is small, the response will be delayed due to adsorption and desorption of the catalyst. This contributes to resource conservation as well as improved performance.

[Example] An example of the present invention will be described with reference to the drawings. It should be noted that the scales of each figure are different for explanation purposes.

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

As shown in the partially broken perspective view of FIG. 1, the gas detector 10 includes a ceramic substrate 12, terminals 13a, 13b,
Detection electrodes 16a, 16b and thermal resistance electrodes fiIi! connected to platinum lead wires 14a, 14b, 14e at e! 1
6 e etc. electrode pattern 16 and the above ceramic substrate 1
2 and the electrode pattern 16 to be integrated with the ceramic substrate 12 and having a window 18a; and a window 1 of the ceramic laminate 18.
8a, a gas-sensitive layer 20 filled so as to cover the detection electrode patterns 16a and 16b, and the ceramic substrate 12.
spherical granulated particles 22 dispersed between and the gas-sensitive layer 20 to prevent them from peeling off, an upper layer 24 laminated on the gas-sensitive layer 20, and a coating layer made of Al2O3 coated on the upper layer 24. It consists of 26.

Next, the manufacturing process of the oxygen gas detector 10 according to this embodiment will be explained with reference to FIGS. 2 to 5.

(2) 92 vt% alumina, 3 vt% magnesia, and 5 vt% sintering aids (silica, calcia, etc.) are mixed in a bot mill for 20 hours. Thereafter, 12 νt% of polyvinyl butyral and 4tzt% of dibutyl phthalate were added as organic binders to the mixture, and methyl ethyl getone, toluene, etc. were added as solvents. Further, the mixture is mixed in a pot mill for 15 hours to form a slurry, and green sheets 12A and 18A for substrates and lamination are formed by a doctor blade method.

The shape of the above green sheet is board green sheet 12.
47.8mmX4.0mmX0.8mm' for A, 47.8mmX4.0mmX0 for M layer green sheet 18A
．． 26 mm', and a window 18a of 3.05 mm x 2.0 mm is formed in the 8-layer MII green sheet 18A.

■Next, platinum black and spongy platinum were combined in a ratio of 2:1, and 10% by volume of the green sheet material mixture used in (■) above was added, and a solvent such as butyl calpitol or ethocel was added. In addition, it is used as a paste for electrodes.

■Next, 1! A polar pattern 16 is formed. As described above, the electrode patterns 16 include the detection electrode patterns 16a and 16b, the thermal resistance electrode pattern 16e that serves as a heater for heating the gas sensing section 20, and the terminal patterns 13a and 13b that serve as terminals of both patterns 16. , 13e. (Figure 2 (a), (b)) ■ After that, the above terminal patterns 13a, 13b, 13e
, platinum lead wires 14a, 14b with a diameter of 0.2 mrn,
14e respectively (Figure 3 (a), (b))
.

(2) Next, the f1 layer of the green sheet for lamination 18A is thermocompressed onto the green sheet for substrate 12A to form a laminate. At this time, the tips of the detection electrode patterns 16a and 16b are exposed in the window 18a of the green sheet for lamination 18A. Then, 80 to 150 meshes of spherical granulated particles (secondary particles) 22 made of the same material as the green sheet prepared in step (3) are dispersed and adhered to the window portion 18a, and then the above-mentioned laminate is exposed to the atmosphere at 1500°C. The ceramic substrate 12 and the ceramic laminate 18 are integrally formed by firing in substantially the same atmosphere for 2 hours (FIGS. 4(A) and 4(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 in FIG. 4(C), forming an uneven surface on the ceramic substrate 12. .

■Next, a paste of titania grains carrying a small amount of platinum is filled into the window 18a of the ceramic laminate 18 as a gas-sensitive layer, and the paste for the upper layer is filled as the upper layer 24. Adjust each paste as follows before.

First, titania grain paste is prepared.

Average particle size 1°2μ when calcined for 1 hour at 1200°C in the air
To 100 g of T i O2 powder of m, 100 cc of chloroplatinic acid aqueous solution containing 10 g of platinum was added, and
After drying at °C for 24 hours, it is thermally decomposed at 700 °C for 2 hours in a hydrogen furnace to precipitate platinum on the surface of the TiO2 powder to obtain titania grains.

To 100 g of this titania grain powder, 2 g of ethyl cellulose was added as a binder, and these were mixed in butycarpitol (trade name of 2-(2-butoxyethoxy) ethanol) to make a titania grain paste with a viscosity of 300 boids. Adjust.

Next, a paste for the upper layer is prepared.

Average particle size 0. For 1 mole of activated alumina of 5 μm,
A nitrate aqueous solution or chloride aqueous solution containing an oxygen-storing metal oxide was added to a specified wall (see Table 1), left for 10 minutes, dried in the atmosphere at 120°C for 12 hours, and further exposed to the atmosphere. It is fired at 850°C to form alumina powder. For this alumina powder, 1 g of platinum and 0 rhodium
．． A mixed aqueous solution of chloroplatinic acid and chlororhodic acid containing 1 g was added, dried in the atmosphere at 200°C for 24 hours, and then thermally decomposed in a hydrogen furnace at 700°C for 2 hours.
Platinum and rhodium, which are catalysts, are deposited on the alumina powder. 2 g of ethyl cellulose was added as a binder to the alumina powder with the catalyst precipitated in this way,
These are mixed in butycarpitol (trade name of 2-(2-butoxyethoxy)ethanol) to a viscosity of 300 voids to prepare a paste for the upper layer.

First, the window portion 18a is filled with titania grain paste to a thickness of 200 μm using thick film printing technology, then the upper layer paste is overlaid on the titania grain paste and filled to a thickness of 100 μm, and then a coating layer of Al2O3 is added. WIJ 24
1! I do it.

Thereafter, the laminate that has undergone the above steps is left to stand in the atmosphere at 1200° C. for 1 hour and fired. (Fig. 5 (a), (b)). Conventionally, the amount of catalyst in the gas-sensitive layer was 5 mol% of platinum.
, rhodium 0. It is about 5 mol%. Therefore, the amount of catalyst contained in the gas-sensitive layer of this example is much smaller than that of the conventional one.

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

In the above step (2), samples of the gas detector of this example are prepared by changing the type and content of the M-absorbing metal oxide in the upper layer paste as shown in Table 1.

In Table 1, the content (mol %) of the oxygen storage metal oxide is relative to activated alumina.

In addition, as an oxygen storage metal oxide, CeO2゜La
When 2O5 is used, its nitrate aqueous solution is used in the step (2) above, and when Eu2Qs and Nb2O5 are used, their chloride aqueous solution is used.

Further, as a comparative example, a sample having an upper layer not containing an oxygen storage metal oxide was prepared, and the response speed and durability in a non-equilibrium gas were investigated in the same way as the samples of the example. Sample number A-1 has the same amount of catalyst as the example, and sample number A-1 has the same amount of catalyst as the example.
-2 is Titania 1, which has the same amount of catalyst as a conventional gas detector.
Sample No. A-3 has a larger amount of catalyst than the conventional gas detector, and supports 20 g of platinum and 2 g of rhodium per 100 g of titania.

After the samples thus prepared were assembled into an oxygen sensor, the response speed and durability of each sample in non-equilibrium gas were measured in the following experiment.

The results of the experiment are also shown in Table 1. In Table 1, the unit of response speed is 115ee.

・Response speed in non-equilibrium gas ■ Sample S assembled as an oxygen sensor is exposed to the exhaust gas of a propane gas burner whose exhaust gas is a non-equilibrium gas. The fuel ratio is excessive (hereinafter referred to as rich).
The air-fuel ratio is controlled to vary between λ=0°9) and fuel shortage (hereinafter referred to as lean, λ=1.1).

As shown in FIG. 6, the air-fuel ratio is changed by adding air or propane gas to the exhaust gas after combustion, which has an exhaust temperature of 500°C.

(2) Adjust the voltage applied to the sensor so that when the exhaust gas is in excess fuel, the output of the gas detector 10 is 1V, and when the exhaust gas is lean, the output is Ov.

■ As for the response speed, the output of the gas detector 10 when the atmosphere changes from lean to rich is 300mV to 600mV.
Measure the time for the output to change to V (denoted as l→r in the table) and the time for the output to change from 600+aV to 300mV (denoted as 1--41 in the table) when the atmosphere changes from rich to lean.

・Durability ■ A sample assembled as an oxygen sensor is attached to an actual vehicle, driven in a predetermined durability pattern, and durability is examined from changes in response speed before and after driving. That is, oxygen sensor S
is attached to the exhaust pipe Man between the engine Enz and the three photocatalysts T) and IC of a commercially available 2000 cc three-way catalyst vehicle with EFI, as shown in FIG. The control unit Uni then controls the operating state of the engine according to the output of the oxygen sensor S. The output of the sensor S is detected by a circuit as shown in FIG. Here, B is a power supply and Rc is a comparison resistor.

(2) The above engine Eng is operated for 300 hours according to the durability pattern shown in FIG. Note that the solid line in the figure indicates the temperature of the sample, and the broken line indicates the temperature of the exhaust gas.

The above-mentioned response speed was measured before and after this operation, and the change was taken as the durability result. That is, a sample with little change in response speed before and after operation is judged to have excellent durability. In Table 1, before operation is referred to as initial stage, and after operation is referred to as after durability test.

Table 1 The following was found from the above experiment.

■ The gas detector of this example can handle A-2 even with a small amount of catalyst.
It has a response speed comparable to that of conventional gas detectors as shown in Figure 1. This seems to be because the oxygen storage metal oxide always keeps the air-fuel ratio in the vicinity of the catalyst in an optimal state, which increases the effectiveness of the catalyst. In addition, B-8, B-2
When the content of the oxygen-storing metal oxide is large as in Example 2, the response speed is relatively slow. This is probably because the effect of the vi-storing property becomes too strong.

(2) The gas detector of this example has better durability than the gas detector of the comparative example. This is because even if the performance of the catalyst itself deteriorates after the durability test, the catalyst's performance is maximized due to the effect of the oxygen-storing metal oxide, so the equilibrium ability of the upper layer does not deteriorate. Seem.

[Brief explanation of the drawing]

FIG. 1 is a partially cutaway perspective view of an embodiment of the present invention, FIGS. 2 to 5 are explanatory diagrams of the manufacturing of the embodiment, FIG. 6 is an explanatory diagram of a propane gas burner used for responsiveness testing, and FIG. 8 and 8 are explanatory diagrams of a durability test for using an oxygen sensor in an internal combustion engine, and FIG. 9 is a diagram of its durability pattern. DESCRIPTION OF SYMBOLS 10... Gas detector, 12... Ceramic substrate, 16.16a, 16b---Detection electrode, 16e...
Heat resistance electrode, 18a... Window portion, 18... Ceramic laminate, 20... Gas sensitive layer, 22... Spherical granulated particles, 24... Upper layer, 24a... Oxidation catalyst layer, 24b...Reduction catalyst layer

Claims (1)

[Claims] 1. A pair of electrodes, and a porous gas-sensitive material that covers the pair of electrodes, contains a gas-sensitive metal oxide, and has an electrical resistance that changes depending on the surrounding gas components and/or their concentration. and an upper layer covering the gas-sensitive layer, the upper layer containing an oxygen-storing metal oxide different from the gas-sensitive metal oxide and supporting a catalyst. . 2 The oxygen storage metal oxide is CeO_2, La
The gas detector according to claim 1, which is one selected from _2O_3, Nd_2O_3, Eu_2O_3, or a composite compound of two or more types selected from the metal oxides.