JPS6366448A - Gas detector - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 that change their electrical resistance when they come into contact with gas.For example, TiO2, Coo, NiO
Oxides of transition metal elements such as oxides can be used as oxygen sensors.

The transition metal oxides exemplified here are non-chemical compounds. The amount of charge carriers (holes, electrons) in this non-chemical 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, for example, CO in exhaust gas,
An alloy of platinum, which is a catalyst that promotes the oxidation reaction of HC, and rhodium, which is a catalyst that reduces NOx, is formed into particles and is 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 precipitates on the surface of the alloy particles during firing or use6.
For example, when heated in an oxidizing atmosphere, rhodium precipitates on the surface of the alloy particles and covers the platinum, reducing the surface area of the platinum and preventing the oxidation reaction from accelerating. It is not easy to manufacture a stable sensor because it is highly dependent on the shape.

[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 membrane 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. The upper layer comprises a gas layer and an upper layer covering the gas-sensitive layer, and the upper layer includes at least one reduction catalyst layer supporting a catalyst that promotes a reduction reaction and one or more oxidation catalyst layers supporting a catalyst that promotes an oxidation reaction. There is a gas detector characterized in that 873 are arranged alternately.

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 angry gas layer can be selected depending on the gas component to be detected.
Commonly used materials include TiO2, Sn○2, C
Examples include transition metal oxides such as 0, ZnO, Nb2O5, Cr2O3, and NiO, and in the present invention, any one or a combination of two or more of these may be used.

The catalyst that promotes the oxidation reaction can be selected depending on the conditions in which the gas detector is used and the gas to be detected. When used for pressure measurement, platinum, palladium, etc. can be used.

As with the above-mentioned catalyst that promotes the oxidation reaction, the catalyst that promotes the reduction reaction may be selected depending on the conditions in which the gas detector is used, the gas to be detected, etc., but in particular,
When the gas detector is used to measure the partial pressure of oxygen gas in the exhaust gas of an internal combustion engine, rhodium, ruthenium, etc. can be used.

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, makinesius vinyl, zirconia,
False sprite, cordierite, etc. can be used.

For example, do these catalysts contain catalyst component elements in the porous catalyst layer base material that serves as a carrier? 8 liquid? After the impregnation, the catalyst is supported on the melt Ia by a three-layer method in which the catalyst is dispersed and supported on the catalyst layer base material by thermal decomposition, reduction, or the like. In addition to this impregnating property, a precipitating R agent is added to a solution of a salt containing a catalyst element, and catalyst 3 (
The catalyst Jiq, which is the catalyst, can be precipitated on the surface of the material, or can be precipitated simultaneously with both the catalyst layer base material component and the catalyst component (co-precipitation) using the ion exchange property of the catalyst layer as a carrier. The catalyst is supported on the catalyst layer by an ion exchange method δ, an adsorption method using adsorption from a solution containing catalyst component elements, etc.

The gas detector of the present invention can be fabricated by providing a gas-sensitive layer or the like on a ceramic substrate using a hybrid technique such as a thick layer technique. Alternatively, without using thick film technology etc., a disk type, which is used in thermistors etc.
It may also be formed into a bead shape or the like.

Furthermore, a heating element may be provided near the gas-sensitive layer for the purpose of reducing temperature characteristic fluctuations of the gas detector during measurement. Then, a part of this heating element and one electrode of the gas detector may be connected to apply a voltage to the gas-sensitive layer, thereby reducing the number of terminals and simplifying the measurement circuit.

In addition, for the purpose of protecting the gas-sensitive layer, a coating layer may be provided over the gas-sensitive layer.
The material for the coating layer, which adsorbs and captures toxic substances such as lead on the gas-sensitive gold oxide layer and prevents 1T toxic substances from reaching the gas-sensitive layer, is particularly limited as long as it is a thermally stable material. For example, alumina, magnesia spinel, zirconia, etc. can be used.

[Function] In the present invention, a catalyst that promotes a reduction reaction and a catalyst that promotes an oxidation reaction are supported in different layers. Therefore, during calcination or use, one catalyst does not precipitate on the surface of the other catalyst, and no interaction occurs between the catalysts that would interfere with the catalytic action of the other catalyst.

Furthermore, since the catalyst that promotes the reduction reaction and the catalyst that promotes the oxidation reaction are close to each other, it is easy for each to utilize the products of the other's reaction, and a more efficient reaction can be performed. That is,
For example, Co and HC are oxidized using oxygen generated by the N Ox (7) Q element.

Therefore, each catalytic action can be fully developed, and
Anger gas reaction becomes stable.

[Effects of the Invention] In the present invention, no catalyst interaction occurs as described above. Therefore, it exhibits excellent performance even in non-equilibrium gases, and exhibits stable performance over a long period of time with little deterioration of the catalyst.

[Example] An example of the present invention will be described with reference to the drawings. r4, the scale of each figure is different for explanation purposes.

This example is an oxygen gas detector 10 in which TiO2 is used as the 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 formed as the upper layer.

The gas detector 10 includes a ceramic substrate 12, terminals 13a, 13b, 1
3e: detection electrodes 16a, 16b and thermal resistance electrode 16e connected to platinum lead wires 14a, 14b, 14e;
The ceramic substrate 12 is laminated on the ceramic substrate 12 and on the electrode pattern 16.
a ceramic laminate @ 18 that is integrated with the ceramic laminate @ 18 and has a window 18a; and a window 18a of the ceramic laminate 18;
was filled so as to cover the detection electrode patterns 16a and 16b! 8 gas Jρ20 and the ceramic substrate 12
and the spherical granulated particles 22 that are dispersed between the spherical granules 22 and the gas-sensitive layer 20 to prevent the oxidation of the two, and the acid (acrylic acid aj'' 724a and reduction catalyst rf
The upper layer 24 consisting of 424 b and the honorable A! 24 and a coat layer 26 made of A I 203 laminated thereon. In addition, in the figure, for the sake of explanation, the oxidation catalyst layer 242i and the reduction diverter medium layer 24b that constitute the upper layer 24 each have 11 layers, but they may be further stacked if necessary.

Next is the present example! ! ! The manufacturing process of the raw gas detector 10 will be explained with reference to FIGS. 2 to 512.

(2) 92 wt% alumina, 3 wt% magnesia, and 5 wt% sintering aid (silica, calcia, etc.) are mixed in a bot mill for 20 hours. Thereafter, 12 νt% of polyvinyl butyral and 4% by weight of dibutyl phthalate were added as organic binders to the mixture, and 11% of methyl ethyl ketone, toluene, etc. were added as bulking agents. Further, it was mixed for 15 hours in a bot mill to form a slurry, and then prepared using the doctor blade method for substrates: J and for lamination, Green Sea 1-12A, 1.
The shape of the green sheet 9 above that forms 8A is 47.3mm x 4.0m+ for substrate green sheet 1-12A.
r+'<0.8mm', 1lTJH green sheet 1
8A, 47.8mmX4.0mmX0.26mm', and.

The lamination green sheet 18A has a thickness of 3.05 nm.
X2. A window portion 18a of 0 mm is formed.

■Next, platinum black and spongy platinum were mixed at a ratio of 2:1, and 10 wt% of the green sheet material mixture used in (■) above was added, and a solvent such as butyl calpitol and ethocel was added. and make it a paste for xi.

■ Next, use the electrode paste prepared in step ■ to print the electrode pattern 1 on the substrate green sheet 12A by thick film printing.
form 6. The electrode patterns 16 include the detection electrode patterns 16a and 16b as described above, the thermal resistance electrode pattern 16e which serves as a heater for heating the gaseous gas section 20, and the terminal patterns 13a and 13b which serve as terminals of both of the patterns 16. , 13e. (Figure 2 (a), (b)) ■ After that, the above terminal patterns 13a, 13b, 13e
, platinum lead wires 14a, 14b, 1 with a diameter of 0.2 mm
4e (Fig. 3 (a) and (b)).

(2) Next, the lamination green sheet 18A is laminated and thermocompressed onto the substrate green sheet 12A to form a laminate. At this time, the window portion 1 of the green sheet for lamination 18A
8a, the tips of the detection electrode patterns 16a and 16b are exposed. Then, 80 to 150 meshes of spherical granulated particles (secondary particles) 22 made of the same material as the green sheet prepared in step (I) are dispersed and adhered to the window portion 18a, and then the laminate is heated to 1500°C in the air. The ceramic substrate 12 and the ceramic laminate 18 are integrally formed by firing for 2 hours in the same atmosphere as shown in FIG.
b), (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. .

(2) Next, a paste of titania grains supporting a small amount of platinum is filled into the window portion 18a of the ceramic laminate 18 as an angry gas layer, and the oxidation catalyst layer 24 constituting the upper layer 24 is stacked.
a. The reduction catalyst layer 24b is filled with oxidation catalyst particle paste and reduction catalyst particle paste, but before filling, each paste is adjusted as follows.

First, titania grain paste is prepared.

Average particle size 1°2μ when calcined for 1 hour at 1200°C in the air
100 cc of chloroplatinic acid aqueous solution containing 10 g of platinum was added to 100 g of TiO2 powder of 100 g, and after drying in the air at 200°C for 24 hours, thermal decomposition was performed at 700°C for 2 hours in a hydrogen furnace, and the surface of the TiO2 powder was Platinum is precipitated 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, prepare the catalyst layer paste in [7f].

Mixed alumina powder 80 with an average particle size of 0.5 μm, consisting of 96% by weight of alumina, 3% by weight of silica, and 01% by weight of Ca.
20 g of platinum black or palladium is added 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 mixed in butycarpitol (trade name of 2-(2-butoxyethoxy) ethanol) to a viscosity of 300 voids to form an oxidation catalyst layer paste. Adjust.

Next, reduced catalyst particles are prepared.

Average particle size: 0.96% by weight of alumina, 3% by weight of silica, and 01% by weight of Ca. 5 μm mixed alumina powder 8
To 0 g, 20 g of spongy rhodium (rhodium black) or spongy ruthenium (ruthenium black) is added to form a mixed powder. This mixed powder 100
2 g of ethyl cellulose as a binder and mixed in butycarpitol (trade name of 2-(2-butoxyethoxy)ethanol),
Adjust the reduced catalyst layer paste to the viscosity of the boyz.

First, the titania grain paste is filled into the window portion 18a using thick film printing technology, and then the oxidation catalyst layer paste and the reduction catalyst layer paste are stacked and filled on the titania grain paste in a predetermined combination (see Table 1). Then, a coat NJ24 of Al2O3 is applied II.

Thereafter, the laminate that has undergone the above steps is left to stand in the atmosphere at 1200° C. for 1 hour and fired. (Figure 5 (a), (b))
.

Experiment> When the reduction catalyst and the reduction catalyst are supported on different layers (
Regarding Example), the response speed and durability were investigated as follows.

Samples of the gas detector of this example were prepared by changing the combination of the reducing catalyst layer and the oxidizing catalyst i layer formed in the above step (2) as shown in Table 1. In Table 1, the Pt layer is
This shows an oxidation catalyst layer formed using an oxidation catalyst layer paste containing Pt black in the process of , the Pd layer also shows an oxidation catalyst layer formed using an oxidation catalyst layer paste containing palladium, and the Rh layer also shows an oxidation catalyst layer formed using an oxidation catalyst layer paste containing palladium. A reduced catalyst layer formed using a reduced catalyst paste containing rhodium is shown, and the Ru layer also shows a reduced catalyst layer formed using a reduced catalyst paste containing ruthenium.

As a comparative example, as shown in Table 1, a sample containing only sensitive gas 71, a reduction catalyst (2% by weight of rhodium) in titania,
and an oxidation catalyst (platinum 18% by weight);
(wt%) sample, the upper layer is a reduction catalyst (rhodium) layer only, the upper layer is an oxidation catalyst (platinum)
A sample consisting only of the above was prepared and examined for responsiveness and durability in the same manner as the sample in the example.

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 response speed is 11 seconds.

・Response speed in non-equilibrium gas■ Sample S assembled as a viz sensor is exposed to the exhaust gas of a propane gas burner whose exhaust gas is a non-equilibrium gas.The exhaust temperature of this propane gas burner is 350°C.
And the air-fuel ratio is controlled to change every second between excess fuel (hereinafter referred to as rich, air-fuel ratio λ = 0°9) and fuel deficiency (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.

■Adjust the voltage applied to the sensor so that the output of the gas detector 10 is 1■ when the exhaust gas is in excess of fuel, and the output is O■ when the exhaust gas is lean.

■ As for the response speed, the output of the gas detector 10 when the atmosphere changes from lean to rich is 300mV to 600s.
The time to change to +V (denoted as l - hr in the table) and the output when the atmosphere changes from rich to lean are 600
Time to change from mV to 30001V (in the table, r-+
1) is measured.

・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 Eng and the three-way catalyst IC of a commercially available 2000cc 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 Re is a comparison resistance.

(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.

(2) The gas detectors of this example (sample numbers 6 to 10) are superior in durability to the gas detectors of comparative examples (sample numbers 1 to 5). In particular, in the comparative example, the responsiveness from lean to rich was extremely decreased after the durability test, whereas in this example, the responsiveness decreased only slightly. This is because in this example, the oxidation catalyst and the reduction catalyst are supported differently,
This seems to be because there is no interaction between the reduction catalyst and the oxidation catalyst as described in [Problems to be solved by the invention].

[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. 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 comprising at least one reduction catalyst layer supporting a catalyst for promoting a reduction reaction and one or more oxidation catalyst layers supporting a catalyst for promoting an oxidation reaction. A gas detector characterized by being laminated with. 2. The gas detector according to claim 1, wherein the catalyst that promotes the oxidation reaction is platinum, palladium, or an alloy of platinum and palladium. 3. The gas detector according to claim 1 or 2, wherein the catalyst that promotes the reduction reaction is rhodium, ruthenium, or an alloy of rhodium and ruthenium.