KR870001034B1 - Gas detecting apparatus - Google Patents

Abstract

The two sensors are pref. secured and suspended by lead wires in a cut area in an inuslating support plate of glass epoxy resin or A1203 having Cu foil conductor patterns on its front and rear surfaces. The sensors may be on opposite faces of the support, or in parallel on the same surface. The drive circuit for the device pref. comprises (i) a separate resistance-voltage converting circuit connected to each sensor; (ii) an alarm level setting circuit connected to the reference resistance converter; (iii) a voltage comparator comparing signals from the CO sensor and the alarm level setting circuit; and (iv) an alarm circuit.

Description

Gas detector

1 is a cross-sectional view of an example of a gas detection element of the present invention.

2 is an example of a device assembled with the device.

3 is a graph showing the sensitivity characteristics of various gases of the device using the Pd (1.0wt%)-Al ₂ O ₃ catalyst.

4 is a graph showing the sensitivity characteristics of the SnO ₂ thin film in the present invention.

* Explanation of symbols for the main parts of the drawings

1: insulation support 2: electrode

3: Sn ₂ O ₃ thin film 4: catalyst layer

5: insulation plate 6: pin

7: lead wire 8: heater

The present invention relates to a gas detection element, which is highly sensitive to a low concentration of reducing gas, and has carbon monoxide (CO) at a low temperature of room temperature to about 120 ° C, methane (CH ₄ ) and propane (C) at a high temperature of 350-450 ° C. _It relates to a gas detection element for detecting ₃ H ₈ ) selectively.

Conventionally, a gas detection element using a sintered body of a metal oxide semiconductor such as SnO ₂ , ZnO, Fe ₂ O ₃ , which exhibits N-type semiconductor characteristics, is known to detect a reducing gas in the atmosphere. When connected, the electric conductivity increases, that is, the resistance decreases.

On the other hand, in recent years, research on thin-film devices instead of the sintered body-type gas detection devices has been promoted in response to the demand for miniaturization and multifunctionality of devices in response to the trend of systemization on the premise of effective use of energy. The device of this type has a structure in which the metal oxide semiconductor as described above is coated with a thin film forming method such as spot method, vapor deposition method, or CVD method to form a thin film.

In the gas detection device of either the sintered body type or the thin film type, in general, precious metals such as platinum (Pt) and palladium (Pd) are generally used because only a metal oxide semiconductor has a low sensitivity and sufficient selectivity as a gas detection device. Attempts to increase the sensitivity of the device using the catalyst. That is, a method of directly adding Pt and Pd to a metal oxide semiconductor or forming a catalyst layer supporting Pt and Pd on a metal oxide semiconductor is taken.

According to this method, sensitivity is improved as compared with the case where there is no catalyst, but it does not show sufficient sensitivity with respect to a low concentration of reducing gas. Furthermore, when various reducing gases are mixed, it is extremely difficult to detect only one kind of reducing gas with high selectivity because malfunction of the device is caused by the influence of other reducing gases. In particular, even at low concentrations such as CO, it is extremely difficult to detect a gas that adversely affects the human body while excluding a malfunction caused by another reducing gas.

Furthermore, when assuming that the gas detection device is used in a general home, it is important to eliminate the malfunction caused by alcohol vapor.

An object of the present invention is to provide a thin film type gas detection element which is highly sensitive to a low concentration of reducing gas and can selectively detect various reducing gases depending on the use temperature.

The gas detection device of the present invention thermally decomposes an organic compound containing a metal atom on the surface of an insulating support on which a pair of electrodes is mounted to coat the electrode to form a tin oxide thin film or a tin oxide thin film containing niobium or antimony. And a layer of a catalyst in which at least one of palladium, platinum and rhodium is supported on aluminum oxide is coated on the thin film.

1 and 2 show an example of the present invention. FIG. 1 is a cross-sectional view of a cylindrical element, and FIG. 2 is a perspective view of the element, and the present invention will be described in detail below with reference to the drawings.

First, (1) of FIG. 1 is a cylindrical insulating support made of alumina or mullite, and a pair of electrodes 2 are provided on the outer circumferential surface of the support 1. The support 1 and the electrode 2 are coated with a thin film of tin oxide 2 (SnO ₂ ) or a thin film 3 of SnO ₂ containing an oxide of niobium (Nb) or antimony (Sb) as an impurity and thereon. The gas detection device of the present invention is manufactured by covering and stacking the whole with a thick catalyst layer 4.

At this time, the film thickness of the thin film 3 is preferably 1000 kPa-1 mu m. When the thickness of the film is more than 1 mu m, the sensitivity to the reducing gas is lowered, and if it is less than 1000 kPa, the sensitivity is lowered and the imbalance increases. In addition, the thickness of the thick catalyst layer 4 is preferably in the range of 10-50 占 퐉, but beyond this range, the catalytic effect such as sensitivity and selectivity is lowered.

The device of the present invention configured as described above is provided so as not to be connected to each other on the pin 6 erected on the insulating plate 5 as shown in FIG. Reference numeral 7 denotes a lead wire for an electrode, and 8 denotes a heater, and a heater 8 is provided to adjust the surface temperature (operating temperature) of the element.

The thin film 3 according to the present invention is a SnO ₂ thin film or SnO containing Nb or Sb formed by thermal decomposition of an organic compound containing Sn or a mixture of an organic compound containing Nb or Sb and an organic compound containing Sn. ₂ thin film. The thin film 3 is manufactured as follows.

First, a tin metal soap (e.g., 2 ethyl hexane tin) or a resin salt containing Sn, an alkoxide of tin (ROSn (where R is an alkyl group)) or an organometallic compound of tin (RSn: where R is an alkyl group or an allyl group) Sn-containing organic compounds or mixtures containing a predetermined amount of Nb- or Sb-containing organic compounds are dissolved using a suitable solvent such as toluene, benzel or n-butyl alcohol to prepare a sample solution having a predetermined concentration of Sn. Manufacture. It is preferable to make Sn concentration into the range of 1.0-20 weight%.

Thereafter, the outer surface of the insulating support 1 having the pair of electrodes 2 is covered with the sample solution and left in air for about 30 minutes to 1 hour, and then heated to a temperature of about 120 ° C. to vaporize the solvent. Then, when the whole is heated to a temperature of 400-700 ° C. in air for 30 minutes to 1 hour, the organic compound containing Sn is thermally decomposed and Sn is oxidized to form a Sn0 ₂ thin film. Due to the Sn concentration of the sample solution to be used can not be determined by any one, but the coating and firing process is repeated about 1-4 times to form a SnO ₂ thin film.

In the case where the thin film 3 to which Nb and Sb is added as an impurity is produced, both Nb and Sb are possible as donors.

Nb, Sb is preferably set to an amount that is an atomic ratio (Nb / Sn or Sb / Sn) in the range of 0.005-0.05 with respect to Sn.

A thick membrane catalyst layer 4 is formed on the Sn0 ₂ thin film 3 prepared as described above using the following coating method.

In the catalyst layer 4 of the present invention, any one of palladium (Pd), platinum (Pt), and rhodium (Rh) or palladium-platinum (Pd-Pt) and palladium-rhodium (Al ₂ O ₃ ) Pd-Rh) and platinum-rhodium (Pt-Rh) are prepared from a catalyst supporting one.

The catalyst is prepared as follows.

First, a chloride such as H ₂ PtCl ₆ · 6H ₂ O, PdCl ₂ , RhCl ₃ · 3H ₂ O, or an ammonium salt such as (NH ₄ ) ₂ PtCl ₆ , (NH ₄ ) ₂ PdCl ₆ , (NH ₄ ) ₃ RhCl ₆ To prepare a Pd, Pt, Rh aqueous solution of the desired concentration.

Al ₂ O ₃ on Pd, Pt, the time to the Rh each alone supported by immersing the Al ₂ O ₃ in a predetermined amount to each of the aqueous solutions, and further Pd-Pt, Pd-Rh, a Pt-Rh Al ₂ O ₃ In the case of supporting it on the substrate, an aqueous solution of Pd, Pt, and Rh is mixed at a predetermined ratio to prepare a mixed solution, and then a predetermined amount of Al ₂ O ₃ is immersed.

After the mixture was sufficiently stirred and mixed, the mixture was dried under reduced pressure for 1-2 hours, and then dried by heating to about 100 ° C. It is crushed with a powder grinder and put into a quartz crucible and fired at a temperature of 400-800 ° C. In this way, a catalyst having a predetermined amount of Pd, Pt, Rh, Pd-Pt, Pd-Rh, and Pt-Rh supported on Al ₂ O ₃ is prepared.

At this time, a Pd, Pt, when the loading amount of Rh are each alone, together in the range of 0.05-20.0% by weight, based on the weight of Al ₂ O ₃ in a range other than the above are the improved sensitivity of the element to the Al ₂ O ₃ Does not contribute. In the case of supporting Pd-Pt, Pd-Rh, Pt-Rh on Al ₂ O _3, in the case of Pd-Pt, Pd-Rh, Pd is 0.05-20.0% by weight based on the weight of Al ₂ O ₃ , Pt, Rh The atomic ratio (Pt / Pd or Rh / Pd) to Pd is 0.05-1.0, in the case of Pt-Rh, 0.05-20.0% by weight relative to the weight of Al ₂ O ₃ of Pt, and the atomic ratio of Pt to Rt ( Rh / Pt) is preferably in the range of 0.05-1.0.

It does not contribute to the improvement of the sensitivity of the manufactured element in the range other than the above.

In this way, the prepared catalyst is prepared into a paste using an aqueous solution of aluminum hydroxy chloride or the like as a binder, coated on a SnO ₂ thin film to a predetermined thickness, dried, and calcined at a temperature of 300-400 ° C., according to the present invention. To form a catalyst layer.

Example 1

1) Formation of SnO ₂ thin film.

2-ethyl hexane tin was dissolved in n-butanol to prepare a sample solution having a Sn content of 10% by weight.

As shown in FIG. 1, the outer surface of the insulating support 1 provided with the pair of electrodes 2 was coated and left in air for 1 hour, and then heated to 120 ° C. to evaporate n-butanol. The whole was then calcined at 400 ° C. in air for 1 hour. The coating-firing process was repeated three times to form a SnO ₂ thin film having a thickness of about 0.3 μm.

2) formation of catalyst layer

(NH ₄ ) ₂ PdCl ₆ was dissolved in water to prepare an aqueous solution having a Pd content of 1.0% by weight. The aqueous solution was immersed in an Al ₂ O ₃ fraction having a surface area of about 100 m ² / g and sufficiently stirred. The Al ₂ O ₃ fractions were separated by a sieve and dried under reduced pressure for 1.5 hours to remove moisture and then evaporated to dryness. Thereafter, the resultant was pulverized with a powder grinder, and the obtained powder was placed in a quartz crucible and fired at 400 ° C.

The catalyst powder was added to an aqueous aluminum hydroxy chloride solution (Al ₂ O ₃ 1%) to prepare a paste. The paste was coated on a SnO ₂ thin film, then dried, and the whole was baked at 400 ° C. A catalyst layer of Pd-supported Al ₂ O _{3 having} a thickness of 20 μm was formed.

3) Measurement of sensitivity characteristics

From the gas detection device of the present invention manufactured as described above, a device as shown in FIG. 2 was assembled, and was used to fabricate 200 ppm of CO, H ₂ , CH ₄ , C ₃ H ₈ and 1000 ppm of C _2. For H ₅ OH gas, the sensitivity with respect to the operating temperature of the device was measured by Rair / Rgas. Here, Rair is a resistance value seen by the device in air that does not contain the measurement gas, and Rg as is a resistance value seen by the device in air containing the gas at the indicated concentrations, respectively.

Larger Rair / Rgas means higher sensitivity.

The above results are collectively shown in FIG.

In addition, FIG. 4 shows the sensitivity of the gas detection device having only a SnO ₂ thin film without a Pd-Al ₂ O ₃ catalyst layer provided for comparison.

은 CH₄, □은 C₃H₈, ●은 C₂H₅OH를 각기 표시한다.In the figure, ○ is CO, △ is H ₂ ,

Is CH ₄ , □ is C ₃ H ₈ , and ● is C ₂ H ₅ OH.

As is apparent from FIG. 4, the SnO ₂ thin film-only device is less sensitive overall and has a higher sensitivity of C ₂ H ₅ OH at all operating temperatures. However, the device of the present invention having a Pd-Al ₂ O ₃ catalyst layer formed thereon, as is apparent from FIG. 3, has a high sensitivity as a whole and a high sensitivity, and an operating temperature at which the sensitivity starts to appear generally moves to a low temperature. The sensitivity characteristics for each gas change to indicate gas selectivity by operating temperature.

Considering C ₂ H ₅ OH in FIG. 3, for a gas concentration of 1000 ppm, its sensitivity is maximum at about 200 ° C. but below about 120 ° C. it is lower than its sensitivity to 200 ppm CO. Further, the sensitivity is gradually lowered beyond 200 ° C, and lower than the sensitivity for 200 ppm of H ₂ , CH ₄ , C ₃ H ₈ at about 420 ° C or higher.

Therefore, since the alarm concentrations of CH ₄ and C ₃ H ₈ are more than 2000 ppm, if 1000 ppm of C ₂ H ₅ OH is present, and if the operating temperature is about 350 ° C. or higher, there will be no malfunction due to C ₂ H ₅ OH and CH ₄ You can set the alarm level for C ₃ H ₈ . In addition, when the operating temperature is lower than 120 ° C., the CO alarm level of 200 ppm can be set without causing malfunction by 1000 ppm of C ₂ H ₅ OH.

Example 2

In the same manner as in Example 1, various SnO ₂ thin films were formed, and various thick catalyst layers were formed thereon, to fabricate the device of the present invention. Each operation on them in the same manner as in Example 1 measured the sensitivity of the temperature (Rair / Rgas). The sensitivity of each device at the operating temperatures of 100 ° C, 350 ° C and 400 ° C is collectively shown in the table.

In the SnO ₂ thin film, Nb and Sb are 0.01 equivalents in atomic ratio with respect to Sn, respectively. In the catalyst, each of Pd, Pt, and Rh supported on A ₂ lO ₃ was 1.0 wt% based on the weight of Al ₂ O ₃ , and in the case of Pd-Pt, Pd-Rh, The supported amount of Al ₂ O ₃ was 1.0% by weight, and Pt and Rh were 0.5 in an atomic ratio to Pd.

For Pt-Rh, Pt was present at 1.0 weight percent relative to Al ₂ O ₃ , and Rh was present at an atomic ratio of 0.5 to Pt.

As is clear from the table, the device of the present invention exhibits high sensitivity especially to CO and has a value of 100 times or more at any concentration of 200 ppm. It is also 10-70 times more sensitive to CH ₄ and C ₃ H ₈ . Moreover, the sensitivity of C ₂ H ₅ OH is gradually lowered at 200 ° C. or higher, and at 420 ° C. or higher, the sensitivity of 1000 ppm C ₂ H ₅ OH is lower than that of 200 ppm CH ₄ , C ₃ H ₈ .

As described above, the gas detection element of the present invention exhibits high sensitivity to a low concentration of reducing gas, and has excellent sensitivity to CO in a low temperature region of about 120 ° C, and a high temperature of about 350-450 ° C. In the zone, CH ₄ , C ₃ H ₈ has high sensitivity and good selectivity. Therefore, by setting the operating temperature to the above arbitrary temperature, it is useful to eliminate various malfunctions based on the presence of C ₂ H ₅ OH and to detect various reducing gases.

Claims (1)

In the gas detection element in which a metal oxide semiconductor and a catalyst layer are provided on the surface of the insulating support 1 on which the pair of electrodes 2 are mounted, a metal atom is provided on the surface of the insulating support 1 on which the pair of electrodes 2 is mounted. Pyrolysing the organic compound containing the electrode to form the second tin oxide thin film or the tin oxide thin film (3) containing niobium or antimony, and then carrying at least one of palladium, platinum and rhodium on aluminum oxide. A gas detection device characterized in that it is produced by coating the thin film with the resulting thick catalyst layer (4).

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