GB2166549A - Gas detecting elements and process for producing the same - Google Patents

Abstract

A gas detecting element capable of detecting with high sensitivity and selectivity a reducing gas having a low concentration is obtained by providing a thick catalyst layer 4 having one or more metals of the platinum group, on an In-Sn-O thin film 3 disposed on an insulating substrate 1. The thin film is formed by drying and firing a layer of a solution of organic compounds containing In and Sn. The catalyst layer is formed by drying and firing a slurry made of a carrier power mixed with a solution of at least one compound of a platinum-group metal. <IMAGE>

Description

SPECIFICATION Gas detecting elements and process for producing the same BACKGROUND OF THE INVENTION This invention relates to gas detecting elements and to processes for producing the same and more particularly to a gas detecting element capable of detecting a reducing gas in air with high sensitivity and selectivity and a process for producing the same.

Heretofore, known elements for detecting a reducing gas in air are those gas detecting elements wherein a sintered body of a metal oxide semiconductor exhibiting N-type semiconductor characteristics such as SnO2, ZnO, or Fe2O3 is used. In such elements, when these metal oxide semiconductors come into contact with a reducing gas, their electric conductivity is increased. A gas is detected by measuring the change of electric resistance value.

In recent years, compact and multifunctional elements have been sought. Studies have been carried out on the use of thin film-type elements in place of the sintered body-type gas detecting elements described above. Such a thin film-type element has a thin film structure obtained by depositing a metal oxide semiconductor having gas sensitivity as described above by a thin filmforming process such as sputtering, vapor deposition, or CVD.

In both sintered body-type gas detecting elements and thin film-type gas detecting elements, the use of a metal oxide semiconductor alone generally provides a gas detecting element having low sensitivity and insufficient selectivity. For this reason, ordinarily noble metals such as platinum (Pt) and palladium (Pd) are being used as catalysts with the aim of increasing the sensitivity of the element. That is, Pt or Pd is directly added to the metal oxide semiconductor.

Alternatively, a catalyst layer supporting Pt or Pd is formed on the metal oxide semiconductor.

When such measures are carried out, the sensitivity is improved as compared with that in the case wherein no catalyst is used. However, the gas detecting element still does not exhibit sufficient sensitivity to detect a reducing gas having a low concentration. Further, when various reducing gases are present, highly sensitive and selective detection of a particular reducing gas is extremely difficult because erroneous functioning of the element is induced by the influence of other reducing gases. Particularly in the case of gases which adversely affect a human body even at a low concentration, such as CO, it has been extremely difficult to eliminate erroneous functioning due to the other reducing gases in order to detect them.Furthermore, when the gas detecting element is to be used in a general home, the elimination of erroneous functioning of the gas detecting element due to miscelianeous gases, particularly alcohol vapor is an important problem.

While an element having a stannic oxide thin film (Japanese Patent Laid-Open Publn. No.

38641/1984) and an element having an indium oxide thin film (Japanese Patent Laid-Open Publn.

No. 38642/1984) are known as gas detecting elements heretofore proposed, even greater improvement of sensitivity as well as selectivity is desired.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a thin film-type gas detecting element by which a reducing gas having a low concentration can be detected with high sensitivity, particularly a detecting element by which carbon monoxide (CO) can be selectively detected with high sensitivity at a low temperature range (from room temperature to about 1200C), and methane (CH4) and propane (C3Hs) can be selectively detected with high sensitivity at a high temperature range (from 350 to 450"C).

It is another object of the present invention to provide a process for producing a thin film-type gas detecting element.

According to one aspect of the present invention, there is provided a gas detecting element comprising an insulating substrate provided with a pair of electrodes, an In-Sn-O thin film so provided on the surface of said insulating substrate as to cover said electrodes, and a catalyst layer laminated on said thin film and comprising a carrier and at least one of platinum group metals supported on said carrier.

According to another aspect of the present invention, there is provided a process for producing gas detecting element which comprises: (1) a first step of forming an In-Sn-O thin film on the surface of an insulating substrate provided with a pair of electrodes by applying as a coating a thin film-forming starting-material solution obtained by dissolving an organic compound containing In and an organic compound containing Sn in a solvent, onto the surface of said insulating substrate so as to cover said electrodes, drying said applied solution, and thereafter firing resulting structure to pyrolyze said organic compounds; and (2) a second step of laminating a catalyst layer on said thin film by applying onto said thin film a slurry containing a catalyst powder obtained from an aqueous solution containing one or more compounds of platinum group metal wherein a carrier powder is immersed therein, drying said slurry, and thereafter firing the resulting structure.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the present invention, it is believed that the invention will be better understood from the following detailed description taken in connection with the accompanying drawings, in which: Figure 1 is a cross-sectional view of a gas detecting element according to the present invention; Figure 2 is a perspective view of a gas detecting device in whch the element shown in Fig. 1 is used; and Figure 3 is a graph indicating the variation of sensitivity with compositional ratio of In and Sn.

DETAILED DESCRIPTION OF THE INVENTION Figures 1 and 2 illustrate an example of an element according to the present invention. Fig. 1 is a cross-sectional view of a cylindrical element, and Fig. 2 is a perspective view showing one example of the state of use of the element.

Referring to Fig. 1, the outer cylindrical surface of a cylindrical substrate 1 made of an insulating material such as alumina or mullite is provided with a pair of electrodes 2. The substrate 1 and the electrodes 2 are covered with an In-Sn-O thin layer 3. The thin layer 3 is covered with a catalyst layer 4. Thus an element of the present invention is formed.

The film thickness of the In-Sn-O thin film is preferably in the range of from 1,000 A to 1 micrometer. If the film thickness is more than 1 micrometer, its sensitivity to reducing gases is reduced. If the film thickness is less than 1,000 A, sufficient sensitivity cannot be obtained. The thickness of the catalyst layer 4 is preferably in the range of from 10 to 50 micrometers. If the thickness of the catalyst layer is outside this range, catalytic effects such as those on sensitivity and selectivity are reduced.

As shown in Fig. 2, the thus constituted element of the present invention is mounted and retained on pins 6 vertically provided on an insulating plate 5 in such a state that the element does not come into contact with other parts. Lead wires 7 are provided for electrodes. A heater 8 is provided centrally through the element in order to adjust the surface temperature (working temperature) of the element.

A process for producing an In-Sn-O thin film will be described.

The In-Sn-O thin film according to the present invention is formed by pyrolyzing an organic compound containing In and an organic compound containing Sn. Specific amounts of Incontaining organic compounds such as In metal soat (e.g., indium oxtylate), In-containing resin salts, alkoxides and Rln wherein R is an alkyl or aryl group, and specific amounts of Sncontaining compounds such as Sn metal soap (e.g., tin octylate), Sn-containing resin salts, alkoxides and RSn wherein R is an alkyl or aryl group are dissolved in suitable solvents such as toluene, benzene and n-butyl alcohol to prepare thin film-forming starting-material solutions having desired concentrations.

A solution of this character is then applied to the outer cylindrical surface of the insulating substrate 1 having a pair of electrodes 2. It is allowed to stand for a specific period of time (ordinarily from 30 minutes to one hour) in air. Thereafter, the coated substrate is heated to a suitable temperature (ordinarily about 120"C) to evaporate the solvent used and then fired for from 30 minutes to one hour in air at a temperature of from 400"C to 700"C. Thus, the organic compound containing In and the organic compound containing Sn are pyrolyzed to form an In-Sn-O thin film.

The number of repetitions of this application-firing step varies depending upon the concentration of the starting material solution used and thus cannot be generally specified. When the application-firing step (a first step) is repeated from once to four times, a thin film having a specific film thickness is formed. It is believed that when the content of Sn in the thin film is less than about 10% of In in atomic ratio percent ((Sn/ln)X100), Sn acts as an impurity of indium oxide, whereas when the content of Sn is 10% or more, polycrystals of indium oxide and tin oxide are formed.

In a second step, a catalyst layer 4 is then laminated on the thus formed thin film 3 by the following process.

The catalyst layer 4 according to the present invention comprises a catalyst wherein at least one of platinum group metals, e.g. any one of palladium (Pd), platinum (Pt) and rhodium (Rh) or any one of palladium-platinum (Pd-Pt), palladium-rhodium (Pd-Rh) and platinum-rhodium (Pt-Rh) is supported on a carrier, e.g., aluminum oxide (AI,O,).

This catalyst is prepared as follows: First, chlorides such as H2PtCI6.6H2O, PdCI2 and RhC13.3H2O or ammonium salts such as (NH4)2PtCl6, (NH4)2PdCI6, and (NH4)3RhCI6 are used to prepare an aqueous solution containing Pd, Pt or Rh at a specific temperature. When Pd, Pt or Rh alone is supported on Al203, a specified amount of Al2O3 is immersed in the respective solutions. When Pd-Pt, Pd-Rh or Pt-Rh is supported on Awl203, aqueous solutions containing Pd, Pt or Rh are mixed at a specific ratio to prepare a mixture, and a specific amount of Awl203 is immersed in the mixture.

Both components are thorougly stirred and mixed. The mixture is then dried for, for example, 1 to 2 hours under reduced pressure and heated and dried at a temperature of about 100"C.

The dried material is ground into a powder, for example, in a mortar. The powder is placed in a quartz crucible and fired at a temperature of from 400" to 800"C. As a result, a catalyst wherein a predetermined amount of Pd, Pt, Rh, Pd-Pt, Pd-Rh or Pt-Rh is supported on AI2O3 is obtained.

The amount of Pd, Pt or Rh supported on AT203 is as follows: when each component is used, it is preferable that Pd, Pt or Rh be in the range of from 0.05% to 20.0% by weight based on the weight of Awl203. If the amount is outside this range, the amount of Pd, Pt or Rh will not contribute to the improvement of the sensitivity of the element.The amount of Pd-Pt, Pd-Rh, or Pt-Rh supported on Al2O3 is as follows: in the cases of Pd-Pt and Pd-Rh, it is preferable that Pd be present in an amount of from 0.05% to 20.0% by weight based on the weight of Al2O3, and Pt or Rh be present in an atomic ratio of Pt or Rh to Pd (Pt/Pd or Rh/Pd) of from 0.05 to 1.0; and in the case of Pt-Rh, it is preferable that Pt be present in an amount of from 0.05% to 20.0% by weight based on the weight of Al2O3 and Rh be present in an atomic ration of Rh to Pt (Rh/Pt) of from 0.05 to 1.0.

The thus prepared catalyst is then slurried by using, for example, an aqueous solution containing aluminum hydroxychloride or the like as a binder. This slurry is applied onto the thin film and dried to obtain a prescribed thickness. Thereafter, the whole assembly is fired at a temperature of from 300 to 400 C to form a catalyst layer according to the present invention.

The following non-limiting examples are set forth to more fully illustrate the present invention.

EXAMPLE 1 Indium octylate and tin octylate were used as starting materials for an In-Sn-O thin film.

These materials were so dissolved in toluene that the content of metal atoms became 50% in atomic ratio percent ((Sn/ln)X 100), whereby a starting-material solution was obtained.

This solution was then applied onto the outer cylindrical surface of a cylinder of an insulating substrate 1 previously provided with a pair of electrodes 2 as shown in Fig. 1. The substrate thus coated was allowed to stand for one hour in air and thereafter heated to a temperature of 1200C to evaporate off the toluene.

The resulting structure was then fired for one hour at a temperature of 500 C in air. This application-firing step was repeated three times to form a thin film having a thickness of about 3,000 A.

Thereafter, a catalyst layer was formed on this thin film. First, PdCI2 was dissolved in water to form an aqueous solution containing 1.0% by weight of Pd. AI2O3 fine powder having a surface area of about 100 m2/g was immersed in the aqueous solution, and the mixture was thorougly stirred. The mixture was dried for 1.5 hours under reduced pressure to remove moisture and then evaporated to dryness. The dried material was then ground in a mortar and the resulting powder was placed in a quartz crucible and fired at a temperature of 400"C.

This catalyst powder was placed in an aluminum hydroxychloride aqueous solution (1% Awl203) with water to form a slurry. This slurry was applied onto the In-Sn-O thin flm and dried. The structure thus obtained was fired at a temperature of 400"C. Thus, a Pd-AI2O2 catalyst layer having a thickness of 20 micrometers and containing 1.0% by weight of Pd in supported state was formed.

In a similar manner, elements were prepared wherein Pt-AI2O2 or Rh-AI2O3 catalysts as well ad Pd-Pt-AI203, Pd-Rh-AI2O3 and Pt-Rh-AI2O3 catalysts were used.

In catalysts wherein any one of Pd, Pt and Rh was supported on AI2O3, the amount of Pd, Pt or Rh was 1.0% by weight based on the weight of AI2O3. In the cases of Pd-Pt and Pd-Rh, the amount of Pd supported was 1.0% by weight based on the weight of AI2O2, and the atomic ratio of Pt or Rh to Pd was 0.5. In the case of Pt-Rh, the amount of Pt was 1.0% by weight based on the weight of AI2O3, and the atomic ratio of Rh to Pt was 0.5.

A device as shown in Fig. 2 was assembled by using each of thus prepared gas detecting elements wherein the type of each catalyst layer was different. The sensitivity to CO, H2, CH4, and C3H,, having concentrations of 200 ppm and C2H5OH having a concentration of 1,000 ppm was measured as Ral,/Rgas using each device. The measurement was carried out at working temperatures of 100 C and 400"C. As used herein, R, is the resistance value exhibited by elements in air containing no measuring gas, and Rgas is the resistance value exhibited by elements in air containing the above gases having respective concentrations. Accordingly, the larger the ratio Ra,,/Rgas is, the higher is the sensitivity.

The results obtained are shown in the following Table.

Table

Sensitivity (Rair/Rgas) Working Composition of Tempera- Catalysts CO H2 C3H8 CH4 C2H5OH ture ( C) 200 ppm 200 ppm 200 ppm 200 ppm 1,000 ppm Pd-Al2O3 2,200 1.0 1.0 1.0 900 Pt-Al2O3 2,400 1.7 1.0 1.0 1,100 Rh-Al2O3 2.300 1.5 1.0 1.0 1,000 100 Pd-Pt-Al2O3 2,800 2.5 1.0 1.0 2,100 Pd-Rh-Al2O3 2,500 2.1 1.0 1.0 2,000 Pt-Rh-Al2O3 2,900 2.7 1.0 1.0 2,600 Pd-Al2O3 1.3 12.0 18.0 24.0 6.0 Pt-Al2O3 1.2 22.0 28.0 17.0 7.5 Rh-Al2O3 1.2 18.0 26.0 16.0 7.0 400 Pd-Pt-Al2O3 1.5 20.0 29.0 26.0 8.0 Pd-Rh-Al2O3 1.4 19.0 28.0 24.0 8.5 Pt-Rh-Al2O3 1.6 24.0 29.0 23.0 9.0 As can be seen from the Table, in all catalysts, the sensitivity of CO (200 ppm) is higher than that of C2H5OH (1,000 ppm) at a working temperature of 100 C. It is apparent that the gas detecting elements exhibit extremely high sensitivity to CO gas. On the other hand, it is apparent that the sensitivity of C3H8 and CH4 (200 ppm) is higher than that of C2H5OH (1,000 ppm) at a working temperature of 400"C.

EXAMPLE 2 Elements were produced as in Example 1 except that the contents of In and Sn in the In-Sn-O thin film were varied. The sensitivity to the percent content of Sn to In was measured under the same conditions as described in Example 1.

The results obtained by using an element having a Pd-A12O2 catalyst as a catalyst layer to measure the sensitivity to CH4 (200 ppm) gas at a working temperature of 400"C are shown in Fig. 3 as one example. As can be seen from Fig. 3, when the atomic ratio percent content ((Sn/ln)X100) is zero, i.e., the thin film is free of Sn and contains only In203 (In-O system), the sensitivity is low. The sensitivity is increased as the percent content of Sn is gradually increased. However, the sensitivity is gradually decreased fom about 50%. When the percent content is 100, i.e., the thin film is free of In and contains only SnO2 (Sn-O system), the sensitivity is approximately the same as that of the thin film having a percent content of zero.

As can be seen from the foregoing, the In-SN-O thin film provides higher sensitivity as compared with the thin film formed from in203 (In-O system) only or SnO2 (Sn-O system) only.

Particularly, when the atomic ratio percent of Sn to In is about 50%, the gas detecting element exhibits excellent sensitivity. Such a tendency was similarly observed even when the type of the catalyst layer and gases to be measured were varied.

As can be also seen from the results of Examples set forth above, the gas detecting element of the present invention exhibits high sensitivity to reducing gases of low concentration and shows excellent sensitivity to CO gas at a low temperature range of from room temperature to about 1200C and high sensitivity to CH4 and C2H8 at a high temperature range of from 350" to 450"C. Thus, the gas detecting element of the present invention exhibits excellent selectivity.

Accordingly, by varying the working temperature, erroneous functioning due to miscellaneous gases such as C2H5OH can be eliminated to achieve highly sensitive detection of various reducing gases.

Claims (17)

1. A gas detecting element comprising an insulating substrate provided with a pair of electrodes, an In-Sn-O thin film so provided on the surface of said insulating substrate as to cover said electrodes, and a catalyst layer laminated on said thin film and comprising a carrier and at least one of platinum group metals supported on said carrier.

2. The gas detecting element according to claim 1 wherein said thin film is prepared by pyrolyzing an organic compound containing In and an organic compound containing Sn.

3. The gas detecting element according to claim 2 wherein said organic compound containing In is an indium metal soap, and said organic compound containing Sn is a tin metal soap.

4. The gas detecting element according to claim 1 wherein said thin film has a film thickness in the range of from 1 ,000 A to 1 micrometer.

5. The gas detecting element according to claim 1 wherein said platinum group metal is selected from the group consisting of Pt, Pd, Rh and mixtures thereof.

6. The gas detecting element according to claim 1 wherein said carrier is AI2O2.

7. The gas detecting element according to claim 1 wherein said catalyst layer has a thickness of from 10 to 50 microns.

8. A process for producing a gas detecting element which comprises: (1) a first step of forming an In-Sn-O thin film on the surface of an insulating substrate provided with a pair of electrodes by applying as a coating a thin film-forming starting-material solution obtained by dissolving an organic compound containing In and an organic compound containing Sn in a solvent, onto the surface of said insulating substrate so as to cover said electrodes, drying said applied solution, and thereafter firing the resulting structure to pyrolyze said organic compounds; and (2) a second step of laminating a catalyst layer on said thin film by applying onto said thin film a slurry containing a catalyst powder obtained from an aqueous solution containing one or more compounds of platinum group metal wherein a carrier powder is immersed therein, drying said slurry, and thereafter firing the resulting structure.

9. The process according to claim 8 wherein said organic compound containing In is an indium metal soap, and said organic compound containing Sn is a tin metal soap.

10. The process according to claim 8 wherein the pyrolysis of said organic compound containing In and said organic compound containing Sn is carried out at a temperature of from 400" to 700"C.

11. The process according to claim 8 wherein said In-Sn-O thin film has a film thickness of from 1,000 A to 1 micrometer.

12. The process according to claim 8 wherein said first step is repeated several times until the thin film having a desired thickness is obtained.

13. The process according to claim 8 wherein said compound of platinum group metal is a chloride or ammonium salt of Pt, Pd or Rh.

14. The process according to claim 8 wherein said carrier powder is Al2O3 powder.

15. The process according to claim 8 wherein said catalyst layer has a thickness of from 10 to 50 microns.

16. A gas detecting element substantially as hereinbefore described with reference to the accompanying drawing.

17. A process for producing a gas detecting element, substantially as hereinbefore described with reference to the accompanying drawing.