JP5134599B2 - Catalyst for CO gas sensor, paste, and CO gas sensor - Google Patents

Description

  The present invention relates to a catalyst, and more particularly to a combustion catalyst for a combustible gas sensor that detects a combustible gas such as carbon monoxide (CO) gas. The invention also relates to a method for producing such a catalyst, a paste comprising such a catalyst, and a carbon monoxide (CO) gas sensor comprising such a catalyst.

  Generally, a catalytic combustion type gas sensor is known as a combustible gas sensor for detecting a combustible gas such as hydrogen or carbon monoxide (CO).

  The catalytic combustion type combustible gas sensor has a gas detection unit provided with a platinum coil covered with a catalyst called a combustion catalyst, which is a composite of a noble metal such as gold and an oxide. At the time of use of the sensor, in this detection unit, for example, a combustible gas such as carbon monoxide (CO) that has flowed into the sensor reacts with oxygen in the air by a catalytic action to generate heat of oxidation. This also increases the temperature of the platinum coil. Therefore, the concentration of combustible gas can be measured by measuring the resistance change caused by the temperature rise of the platinum coil as a detection signal.

  Further, recently, a thermoelectric type combustible gas sensor using a thermoelectric conversion element instead of a platinum coil as a detection unit is known. In this type of gas sensor, a thermoelectric conversion element is used to detect heat generated by combustion of combustible gas generated by catalytic action, and this is converted into a voltage signal. Therefore, the concentration of combustible gas can be measured by the output of the voltage signal.

  In order to increase the reaction activity of such a combustion catalyst for a combustible gas sensor, various reports have been made so far. For example, it has been reported that a combustion catalyst with high reaction activity can be obtained by supporting fine gold particles of 3 wt% on titanium oxide (Non-patent Document 1).

Nishibori M. Tajima K .; Shin W., et al. , Izu N .; , Itoh T. , Matsubara I .; , Tsubota S. , J .; Ceram. Soc. Jpn. 115 (1), 37-41, 2007)

  By the way, in order to increase the detection sensitivity of the combustible gas sensor, so-called “selectivity” in which only the measurement target gas is selectively detected is important. For example, in the case of a carbon monoxide (CO) gas sensor, it is necessary to selectively detect only carbon monoxide (CO) without being affected by interfering substances (for example, hydrogen gas) present in the measurement environment. .

  However, the combustion catalyst described in Non-Patent Document 1 described above exhibits good reaction activity with respect to carbon monoxide (CO) gas, but in the presence of hydrogen gas, the combustion catalyst with respect to carbon monoxide (CO) gas. It has been pointed out that there is a problem that selectivity is lowered. Accordingly, there is a demand for the development of a combustion catalyst that exhibits high reaction activity with respect to carbon monoxide (CO) gas and that has good selectivity with respect to carbon monoxide (CO) gas.

  The present invention has been made in view of such a background, and in the present invention, high selectivity for carbon monoxide (CO) gas is maintained while maintaining high reaction activity for carbon monoxide (CO) gas. It is an object to provide a catalyst for the CO gas sensor shown. Moreover, it aims at providing the manufacturing method of such a catalyst, the paste containing such a catalyst, the CO gas sensor containing such a catalyst, and the manufacturing method of a CO gas sensor.

In the present invention,
Including gold particles and metal oxide particles,
The gold particle is contained in an amount of 10 wt% or more and less than 40 wt% with respect to the total weight, and a catalyst for a CO gas sensor is provided.

  In the catalyst, the gold particles may have an average particle size ranging from 10 nm to 20 nm.

  In the catalyst, the metal oxide particles may have an average particle size in a range of 20 nm to 100 nm.

  In the catalyst, the metal oxide particles may include at least one material selected from the group consisting of cobalt oxide, titanium oxide, nickel oxide, iron oxide, aluminum oxide, and silicon oxide.

Further, in the present invention, a method for producing a catalyst for a CO gas sensor, comprising gold particles and metal oxide particles,
Providing a colloidal gold solution and metal oxide particles;
In the catalyst finally obtained, the gold colloid solution, the metal oxide particles, Preparing an aqueous solution comprising:
Heating the aqueous solution while controlling the pH of the aqueous solution to remove moisture, thereby obtaining a coagulum;
Firing the coagulated product in a temperature range of 300 ° C to 400 ° C;
A method for producing a catalyst for a CO gas sensor is provided.

  Here, in the method, the pH may be in the range of 6.5 to 7.5.

  In the method, the gold particles contained in the gold colloid solution may have an average particle diameter in the range of 1 nm to 5 nm.

  In the method, the metal oxide particles may have an average particle size in the range of 5 nm to 30 nm.

Furthermore, in the method for producing a catalyst for the above-described CO gas sensor, after the step of calcining the solidified product in a temperature range of 300 ° C. to 400 ° C.,
You may have the step which heat-processes the obtained baked material in the range of 150 to 250 degreeC in the atmosphere containing hydrogen and oxygen.

  The heat treatment step may be performed in a range of 10 minutes to 24 hours.

  The heat treatment step may be performed in an atmosphere containing 1 vol% or less of hydrogen.

In the present invention, a paste containing a matrix medium and a catalyst,
A paste is provided in which the catalyst is a catalyst according to the present invention having the aforementioned characteristics.

  In the paste, the matrix medium is selected from the group consisting of at least one solvent selected from the group consisting of terpineol, butyl carbitol acetate, and isopropyl alcohol, and ethyl cellulose, acrylic resin, and polyvinyl alcohol. And at least one organic binder.

Moreover, in the present invention, a CO gas sensor including a detection unit provided with a combustion catalyst,
A CO gas sensor is provided in which the combustion catalyst is a catalyst having the aforementioned characteristics.

  Here, at least the combustion catalyst may be previously heat-treated in an atmosphere containing hydrogen and oxygen in the range of 150 ° C. to 250 ° C.

Further, in the present invention, a method for manufacturing a CO gas sensor including a detection unit in which a combustion catalyst is installed,
The combustion catalyst is
Providing a colloidal gold solution and metal oxide particles;
In the finally obtained combustion catalyst, the gold colloid solution and the metal oxide particles so that the weight ratio of gold to the sum of the weight of gold and the weight of metal oxide particles is 10 wt% or more and less than 40 wt%. Preparing an aqueous solution comprising:
Heating the aqueous solution while controlling the pH of the aqueous solution to remove moisture, thereby obtaining a coagulum;
Calcining the solidified product in a temperature range of 300 ° C. to 400 ° C. to obtain a combustion catalyst;
A method of manufacturing a CO gas sensor is provided.

  Here, the CO gas sensor manufacturing method may further include a step of heat-treating the obtained combustion catalyst in a range of 150 ° C. to 250 ° C. in an atmosphere containing hydrogen and oxygen.

  The present invention provides a catalyst for a CO gas sensor that exhibits high selectivity for carbon monoxide (CO) gas while maintaining high reaction activity for carbon monoxide (CO) gas. It is also possible to provide a method for producing such a catalyst, a paste containing such a catalyst, a CO gas sensor containing such a catalyst, and a method for producing a CO gas sensor.

It is a TEM photograph which shows one form of the catalyst by this invention. It is the graph which showed an example of the output voltage in 1 vol% CO + air (a) or 1 vol% hydrogen + air (b) of the sensor sample containing the catalyst by this invention. It is a flowchart at the time of manufacturing the catalyst by this invention. It is a schematic block diagram of an example of the CO gas sensor by this invention. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% carbon monoxide (CO) + dry air gas of the sample which concerns on Example 1 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% hydrogen + dry air gas of the sample which concerns on Example 1 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% carbon monoxide (CO) + 1vol% hydrogen + dry air gas of the sample which concerns on Example 1 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% carbon monoxide (CO) + dry air gas of the sample which concerns on Example 2 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% hydrogen + dry air gas of the sample which concerns on Example 2 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% carbon monoxide (CO) + 1vol% hydrogen + dry air gas of the sample which concerns on Example 2 in each temperature. It is the graph which showed the time-dependent change of the voltage signal with respect to 1 vol% carbon monoxide (CO) + dry air gas in each temperature of sample A preliminarily heat-treated at 150 ° C.

  The present invention will be described in detail below.

  In a catalyst containing gold particles and oxide particles (hereinafter referred to as “gold-oxide catalyst”), in order for the catalyst to exhibit high reaction activity with respect to carbon monoxide (CO) gas, It is necessary to be in a highly dispersed state. Therefore, conventionally, extremely fine particles have been actively used as the gold particles contained in the catalyst so that a highly dispersed state can be obtained relatively easily. For example, the typical particle size of gold particles is about several nm at the maximum. Along with this, it has been considered that the maximum amount of gold contained in the combustion catalyst is several wt% at the maximum. This is because when more gold is present in the combustion catalyst, the gold particles tend to aggregate and the possibility of grain growth increases. When such gold grain growth occurs, the effect of dispersing fine gold particles in the catalyst is lost.

  However, as a result of earnest research and development, the inventors of the present application have developed carbon monoxide (CO) in an environment in which an interfering substance (for example, hydrogen gas) exists in the case of a conventional combustion catalyst with a low gold content. ) It has been found that sufficient selectivity for gas cannot be obtained (for example, Non-Patent Document 1 described above). Further, the inventors of the present application have come to consider that this is because a combustion catalyst having a low gold content as in the prior art does not sufficiently secure an active site for carbon monoxide (CO) gas. And in order to improve the selectivity with respect to the carbon monoxide (CO) gas of a combustion catalyst, contrary to the conventional idea, a gold particle is more actively contained in the catalyst, and the carbon monoxide (CO) gas. The inventors found that it is effective to increase the number of reaction active sites with respect to the present invention, and reached the present invention.

  That is, the present invention provides a catalyst for a CO gas sensor comprising gold particles and metal oxide particles, wherein the gold particles are contained in an amount of 10 wt% or more and less than 40 wt% based on the total weight. Is done. The maximum amount of gold particles is less than 40 wt% with respect to the entire catalyst. As a result of experiments, if too much gold particles are contained in the catalyst, the gold particles easily aggregate together. It is because it became clear that the selectivity of a catalyst fell. (On the other hand, the minimum amount of gold particles is set to 10 wt% or more with respect to the entire catalyst. Because it becomes worse.)

  In particular, the gold particles are preferably contained in an amount of 10 wt% to 35 wt% with respect to the total weight, and more preferably in the range of 10 wt% to 20 wt%.

FIG. 1 shows a TEM photograph of the catalyst according to the present invention. Small particles that have a particle size of about 10 to 20 nm and appear black are gold particles, and larger gray particles that have a particle size of about 100 nm are metal oxide particles. From this photograph, it can be seen that the gold particles are relatively uniformly dispersed around the metal oxide particles. In the example of this photograph, the metal oxide is cobalt oxide (Co ₃ O ₄ ), and the gold content is 10 wt% with respect to the entire catalyst.

  Since such a catalyst can provide a large number of reaction active sites for carbon monoxide (CO) gas, the selectivity for carbon monoxide (CO) gas can be significantly improved while maintaining high reaction activity. Can do.

  FIG. 2 shows an example of selectivity of the sensor sample including the catalyst shown in FIG. 1 with respect to carbon monoxide (CO) gas. A catalyst composed of 10 wt% gold particles and cobalt oxide was used. FIG. 2A shows the measurement result of the output voltage in dry air containing 1 vol% CO, and FIG. 2B shows the measurement result of the output voltage in dry air containing 1 vol% hydrogen. is there. The measurement temperature is 220 ° C. for all.

  From this result, it can be seen that the catalyst according to the present invention has good sensitivity to CO gas. Further, in the measurement results, the sensitivity to hydrogen is sufficiently low compared to CO gas, and it can be seen that the catalyst according to the present invention has significant selectivity to CO gas.

  In addition, the catalyst according to the present invention has an additional feature that when applied to a CO gas sensor, deterioration is relatively difficult to occur. A conventional catalyst in which fine gold particles having a particle size of about several nanometers are supported at a low content, the gold particles are deteriorated in a relatively short time because the absolute amount of the gold particles is small. There is a problem of inferiority. However, since the catalyst according to the present invention contains a relatively large amount of gold particles, the rate of deterioration is relatively relaxed and the catalyst can be used stably for a longer time.

  Furthermore, the catalyst according to the invention has the additional feature of being excellent in initial stability. That is, in the conventional catalyst in which fine gold particles having a particle size of about several nanometers are supported at a low content, the gold particles are more active, so that the gold particles are likely to move to other positions. Tend. Therefore, in a sensor including a conventional catalyst as a combustion catalyst, the measured value may become unstable immediately after the start of use (until the arrangement of the gold particles is stabilized). On the other hand, since the catalyst according to the present invention contains a relatively high content of gold particles, the movement of the gold particles is relatively difficult to occur, and in a sensor including such a catalyst, the sensor is stable from the beginning of use. Can be used for

  In the catalyst according to the present invention, the average particle size of gold is preferably in the range of 10 nm to 20 nm. Here, in the present application, the “average particle diameter” of gold was measured by the following method. First, an electron microscope (TEM) photograph of fine particles is taken at a magnification of about 50000 to 200000 times. In the photograph, 10 gold particles having the most universal size and shape are selected and their diameters are measured. The obtained 10 diameter data were averaged to obtain the average particle diameter of gold.

  In the catalyst according to the present invention, the average particle diameter of the metal oxide serving as a support for gold particles is not particularly limited as long as it is larger than that of the gold particles. This is because if the average particle diameter of the metal oxide exceeds 100 nm, it may be difficult to uniformly disperse small gold particles. However, when there is a technique for uniformly dispersing gold particles around the metal oxide, the average particle diameter of the metal oxide may exceed 100 nm. In the present application, the “average particle diameter” of the metal oxide was measured in the same manner as in the case of gold described above.

  In the catalyst according to the present invention, the material of the metal oxide is not particularly limited. The metal oxide may be, for example, cobalt oxide, titanium oxide, nickel oxide, iron oxide, aluminum oxide, silicon oxide, or a mixture thereof.

(Method for producing catalyst according to the present invention)
Next, an example of a method for producing a catalyst according to the present invention having the above-described features will be described. It will be apparent to those skilled in the art that the method described below is only an example, and that the catalyst according to the present invention may be produced by another method.

  FIG. 3 shows an example of a production flow of the catalyst according to the present invention. The method for producing a catalyst according to the present invention includes a step of preparing a colloidal gold solution and metal oxide particles (step S110), and a gold catalyst with respect to the sum of the weight of gold and the weight of metal oxide particles in the finally obtained catalyst. A step of preparing an aqueous solution containing the gold colloid solution and the metal oxide particles so that the weight ratio is 10 wt% or more and less than 40 wt% (step S120), and the pH of the aqueous solution is controlled A step of obtaining a coagulated product by heating the aqueous solution to remove moisture (step S130) and a step of firing the coagulated product in a temperature range of 300 ° C. to 400 ° C. (step S140) are included.

  In the conventional combustion catalyst as described above, it is necessary to control the particle size of gold to a size of several nanometers at the time of production, and complicated and precise control is required. Further, it is extremely difficult to uniformly disperse such fine gold particles with respect to the oxide carrier. For example, gold particles having a particle size of several nanometers have a very high activity, and thus may cause a problem that the gold particles easily aggregate when dispersed in a carrier.

  On the other hand, the process for producing a catalyst according to the present invention does not include a conventional process requiring complicated and advanced control. Thus, the method according to the invention makes it possible to produce a gold-oxide catalyst relatively easily.

  Hereafter, each step of the manufacturing method of the catalyst by this invention is demonstrated in detail.

(Step S110)
First, in step S110, a gold colloid solution and metal oxide particles are prepared as raw materials.

  Any gold colloid solution prepared by a known method may be used. The colloidal gold solution may be of the mercaptosuccinic acid type, for example. The gold particles contained in the colloidal solution have an average particle diameter in the range of 1 nm to 10 nm, for example, and the average particle diameter is, for example, 5 nm. The concentration of the gold particles contained in the colloidal solution is not particularly limited, but the concentration of the gold particles is, for example, 2 wt%.

  The material for the metal oxide particles is not particularly limited, and the materials described above may be used. In addition, the average particle diameter of the gold oxide particles is not particularly limited, but if a particle having a very large average particle diameter is used, the particle diameter of the finally obtained support oxide may be extremely large. Therefore, the average particle size is preferably 50 nm or less. The average particle diameter of the metal oxide particles is, for example, in the range of 10 nm to 30 nm.

(Step S120)
Next, in step S120, an aqueous solution containing the aforementioned gold particles and metal oxide particles is prepared. This is performed, for example, by adding the aforementioned gold particles and metal oxide particles to a container (eg, a beaker) containing distilled water. In this case, in the finally obtained catalyst, the gold colloid solution is used so that the weight ratio of gold to the sum of the weight of gold and the weight of metal oxide particles is 10 wt% or more and less than 40 wt%. The metal oxide particles are mixed.

(Step S130)
Next, in step S130, the aqueous solution containing gold particles and metal oxide particles is heated to evaporate moisture. The heating temperature is preferably in the range of 50 ° C to 90 ° C, for example 70 ° C.

  In this step, the pH of the aqueous solution is monitored during the evaporation of moisture, and the pH is adjusted to be maintained within a predetermined range. For example, the pH of the aqueous solution is maintained at around 7 (for example, pH 6.5 to 7.5). Various acids and alkalis are used to adjust the pH. As the alkali, for example, sodium hydroxide or ammonia solution is used.

  The pH of the aqueous solution is controlled in order to suppress the aggregation of gold during the water evaporation process. For example, when a mercapto succinic acid type colloidal gold solution is used, if the pH of the aqueous solution is significantly out of the range of 6.5 to 7.5, the aggregation of gold is promoted and the final catalyst powder in the catalyst powder is obtained. There is a high possibility that the dispersibility of gold is deteriorated.

  After most of the water is evaporated by heating, the solidified material in the container is completely dried. This may be done, for example, by holding the container in a drying oven set at 100 ° C.

  Thereafter, the dried coagulum is recovered from the container.

(Step S140)
Next, the recovered solidified product is fired in an atmospheric furnace. The firing temperature is preferably in the range of 300 ° C to 400 ° C. Although the firing time depends on the firing temperature, it is about 30 minutes to 5 hours.

  Through the above steps, a powdered catalyst containing gold particles and metal oxide particles and containing gold in an amount of 10 wt% or more and less than 40 wt% based on the total weight is obtained.

  As described above, in the method according to the present invention, it is not necessary to perform the complicated and highly accurate control as in the prior art when the catalyst is manufactured. Thus, the process according to the invention makes it possible to produce the catalyst according to the invention relatively easily.

(Second aspect of the present invention)
In the present invention, it is also possible to provide a paste containing a catalyst having the aforementioned characteristics. Such a paste can be used, for example, when installing a combustion catalyst in a detection part of a carbon monoxide (CO) gas sensor. Hereinafter, an example of a method for preparing a paste according to the present invention will be described.

  The paste according to the present invention can be obtained by adding the aforementioned catalyst to a matrix medium constituting the paste. The form of the catalyst to be added is not particularly limited, and the form of the catalyst may be powder, particle, or mass.

  The matrix medium is not particularly limited as long as the catalyst can be dispersed. The matrix medium is at least one solvent selected from the group consisting of terpineol, butyl carbitol acetate, and isopropyl alcohol, and at least one organic selected from the group consisting of ethyl cellulose, acrylic resin, and polyvinyl alcohol. It may contain a binder. In this case, the mixing ratio of the solvent and the organic binder is not particularly limited, and various mixing ratios can be adopted depending on the combination of both materials. The matrix medium may be, for example, a mixture of terpineol and ethyl cellulose. In this case, the mixing ratio of terpineol and ethyl cellulose is not particularly limited, and the weight ratio of ethyl cellulose to the entire matrix medium may be, for example, in the range of 0 to 10 wt% (for example, 5 wt%).

  Further, the mixing ratio of the matrix medium and the catalyst is not particularly limited. The weight ratio between the matrix medium and the catalyst may be, for example, 1: 1 to 4: 1 (for example, 2: 1).

  In addition, it is preferable that a matrix medium has the property to burn out by baking at the temperature of 400 degrees C or less. When a matrix medium having a burning temperature higher than 400 ° C. is used, when the sensor detection unit is manufactured using such a paste, that is, when the paste is applied to the sensor detection unit and fired, This is because there is a risk of affecting the components. However, it will be apparent that if such a component has a higher heat-resistant temperature, a matrix medium with a burning temperature higher than 400 ° C. may be used.

(Third aspect of the present invention)
In another aspect of the present invention, a carbon monoxide (CO) gas sensor provided with a catalyst having the above-described characteristics in a detection unit is provided.

  FIG. 4 schematically shows an example of such a CO gas sensor. 4A is a schematic top view of the CO gas sensor, and FIG. 4B is a schematic cross-sectional view taken along the line AA of FIG.

  The CO gas sensor 400 includes a substrate 410, a support film 413, a heater 415, a thermoelectric film 420, an insulating layer 430, and a combustion catalyst 440.

  The substrate 410 is made of, for example, silicon. The thickness of the substrate 410 is not particularly limited, but is 350 nm, for example. Note that a portion of the substrate 410 where the support film 413 is installed is cut out, and the support film 413 is exposed when viewed from the substrate 410 side. The support film 413 is made of, for example, silicon nitride. Actually, after a substrate such as silicon is nitrided and silicon nitride is formed on the surface, the substrate is removed to obtain the structure shown in the figure.

  The support film 413 has a role of supporting the heater 415 and the thermoelectric film 420 disposed on the support film 413. The thickness of the support film 413 is not particularly limited, but is about 280 nm, for example. The heater 415 has a role of heating the vicinity of the combustion catalyst 440 and promoting the combustion reaction of CO gas in the combustion catalyst 440. The arrangement of the heater 415 shown in the drawing is an example, and the heater 415 may be arranged on the support film 413 in any arrangement pattern.

  The thermoelectric film 420 has a role of converting heat generated when carbon monoxide (CO) gas is oxidized by the combustion catalyst 440 into an electric signal. The thermoelectric film 420 is made of, for example, silicon germanium. In the illustrated example, the thermoelectric film 420 has a thickness of about 450 nm. In the example shown in the figure, the thermoelectric film 420 also serves as an electrode for taking out the output voltage (or output current), and therefore a separate electrode is not shown. However, it will be apparent that an electrode connected to the thermoelectric film may be arranged separately from the thermoelectric film 420.

  The combustion catalyst 440 is composed of the catalyst according to the present invention having the above-described characteristics. The combustion catalyst 440 is configured, for example, by applying and solidifying the above-described paste according to the present invention. The thickness of the combustion catalyst 440 is, for example, about 10 μm.

  The insulating layer 430 has a role of preventing direct contact between the combustion catalyst 440 and the thermoelectric film 420, and is made of, for example, silicon dioxide. The thickness of the insulating layer 430 is, for example, about 300 nm.

  In the present application, a portion of the CO gas sensor 400 excluding the combustion catalyst 440 may be particularly referred to as a “thermoelectric device”.

  The above configuration is an example, and it will be apparent to those skilled in the art that various other configurations of the CO gas sensor can be used.

  Such a CO gas sensor according to the present invention has a high reaction activity with respect to carbon monoxide (CO) gas and a high selectivity with respect to carbon monoxide (CO) gas due to the significant characteristics of the catalyst according to the present invention as described above. can get. In addition, since deterioration is less likely to occur compared to conventional combustion catalysts, it can be used stably over a long period of time.

  When measuring the CO gas concentration in an actual environment using a CO gas sensor, it is desirable that such a CO gas sensor has a high response speed to the CO gas and can display the measurement result as quickly as possible. . Therefore, when the catalyst according to the present invention is applied as a combustion catalyst for a CO gas sensor, it is preferable to take measures to increase the cleanliness of the catalyst in advance. Thereby, the activity of the catalyst is further improved, and the response speed to the CO gas can be increased.

  For example, the catalyst is subjected to the following heat treatment (hereinafter, also referred to as “preliminary heat treatment”) to remove impurities such as oxides and carbides remaining on the catalyst surface, thereby improving the cleanliness of the catalyst. Can be increased.

  “Preliminary heat treatment” is performed in an atmosphere containing hydrogen and oxygen. By performing the treatment in an atmosphere containing both hydrogen and oxygen, it is possible to burn and remove carbides and the like present as impurities on the catalyst surface.

  The concentration of oxygen in the atmosphere is not particularly limited, but is, for example, in the range of 1 vol% to 99 vol%. For example, when air is used as the atmospheric gas, the oxygen concentration is about 21 vol%. On the other hand, the concentration of hydrogen in the atmosphere needs to be less than 4 vol% (for example, 1 vol%) when air is used as the atmospheric gas. Since the lower explosion limit of hydrogen is 4 vol% in air, there is a safety problem if the hydrogen concentration is higher than 4 vol%. Note that when an atmosphere containing a certain concentration of oxygen, which is different from the air atmosphere, is selected, the concentration of hydrogen needs to be less than 1/5 of the oxygen volume contained in the atmosphere.

  Usually, the preliminary heat treatment is performed by holding the catalyst according to the present invention for a predetermined time at a temperature of about 150 ° C to 250 ° C. The time for the preliminary heat treatment is, for example, in the range of 10 minutes to 24 hours, for example, 1 hour.

  Such a preliminary heat treatment may be carried out in the state of a catalyst, but it will be understood by those skilled in the art that the catalyst may be carried out after applying the catalyst to a CO gas sensor (for example, immediately before using the CO gas sensor). Is obvious.

  Examples of the present invention will be described below.

Example 1
A CO gas sensor sample was prepared using the gold-oxide catalyst according to the present invention, and its detection characteristic (selectivity) for carbon monoxide (CO) gas was evaluated.

(Preparation of gold-oxide catalyst)
A catalyst composed of 10 wt% gold-cobalt oxide was prepared by the following method.

First, 2.75 g of a colloidal gold solution containing gold particles having an average particle diameter of 3 nm (mercaptosuccinic acid type: gold content 2 wt%) is placed in a container, and the solution is stirred. 0.5 g (average particle size 15 nm) was added. Furthermore, 10 ml of distilled water was added to this solution. This mixed solution was kept at 70 ° C. and stirred at a rotational speed of 180 rpm to 3000 rpm to evaporate water. During this period, water droplets adhering to the container wall were wiped off as appropriate. During the evaporation, the pH of the mixed solution was appropriately monitored using a pH test paper, and the pH of the mixed solution was maintained at around 7. (For this reason, 1N sodium hydroxide solution or 1N ammonia solution was added to the mixed solution as necessary.)
After the water of the mixed solution was almost evaporated, the container was placed in a drier set at 100 ° C. and held for 30 minutes or more to completely dry the coagulum in the container. Thereafter, the solidified product was recovered from the container, put in a muffle furnace, kept at 300 ° C. for 2 hours in the atmosphere, and fired. By this treatment, a catalyst powder composed of 10 wt% gold-cobalt oxide was obtained.

XRD analysis of the obtained catalyst powder was performed using an X-ray diffractometer (RINT2100, manufactured by Rigaku Corporation). From the obtained diffraction peak results, the particle size of gold was calculated using the following Scherrer equation.

d = kα / βcos θ Formula (1)

Here, d is the crystal grain size, λ is the X-ray wavelength, β is the half width, and θ is the diffraction angle. K is a constant and k = 0.9.

  From equation (1), the particle size of gold was estimated to be about 15 nm.

(Preparation of paste)
A dispersion medium prepared by adding 10 wt% of ethyl cellulose to terpineol was mixed with the catalyst powder prepared by the method described above to prepare a paste. The mixing weight ratio of the dispersion medium and the catalyst was 2: 1.

(Production of CO gas sensor sample)
First, the paste was applied onto a thermoelectric device (substrate size: 4 mm long × 4 mm wide) using a syringe (amount applied: 0.003 μL). Next, this thermoelectric device was placed in a muffle furnace and held at 400 ° C. for 2 hours. As a result, a CO gas sensor sample (hereinafter referred to as “sample according to Example 1”) was obtained.

  The configuration of the thermoelectric device is as shown in Table 1.

(Sample characterization)
Using the sample according to Example 1 manufactured by the above-described method, its detection characteristics with respect to carbon monoxide (CO) gas were evaluated. Evaluation was performed as follows.

  Gas tank A filled with air, gas tank B filled with dry air gas containing 1 vol% carbon monoxide (CO) (hereinafter referred to as “mixed gas 1”), and dry air containing 1 vol% hydrogen Gas tank C filled with gas (hereinafter referred to as “mixed gas 2”) and dry air gas containing 1 vol% carbon monoxide (CO) and 1 vol% hydrogen (hereinafter referred to as “mixed gas 3”) Is prepared.

  First, the sample according to Example 1 is heated to a predetermined temperature (200 ° C. to 240 ° C.) and maintained at that temperature. In this state, air is supplied from the gas tank A to the sample according to the first embodiment (flow rate 200 ccm), and measurement of the voltage value output from the sample is started. After confirming that the output voltage is almost 0 (approximately 40 seconds after the supply of the mixed gas), the air supply from the gas tank A is stopped, and the sample according to the first embodiment is supplied from the gas tank B. A mixed gas 1 is supplied. Further, after about 200 seconds, the supply of the mixed gas 1 from the gas tank B to the sample is stopped, and air is supplied from the gas tank A to the sample again. The change with time of the output voltage obtained from the sample during this period is measured.

  The same measurement was performed for the mixed gases 2 and 3, and the detection characteristics for the respective supply gases in the sample according to Example 1 were evaluated.

  5 to 7 show the measurement results. In each figure, the horizontal axis is the elapsed time, and the vertical axis is the output voltage from the sample. FIG. 5 shows the results obtained with the mixed gas 1 (1 vol% CO + dry air), FIG. 6 shows the results obtained with the mixed gas 2 (1 vol% hydrogen + dry air), and FIG. It is the result obtained in the mixed gas 3 (1 vol% CO + 1 vol% hydrogen + dry air).

  From FIG. 5, it can be seen that at each temperature of 200 ° C. to 240 ° C., a substantially constant output voltage is obtained during the period (about 30 seconds to about 200 seconds) in which the mixed gas 1 is supplied. In particular, when the temperature of the sample was 240 ° C., the output voltage reached about 2390 mV. On the other hand, in the case of the mixed gas 2, as shown in FIG. 6, at any temperature, the output voltage from the sample was extremely small and about 0.5 mV at the maximum (when the temperature was 240 ° C.). In the case of the mixed gas 3 containing both carbon monoxide and hydrogen, the voltage output from the sample is the output voltage obtained in the mixed gas 1 (that is, the output voltage shown in FIG. 5), as shown in FIG. Almost equal.

  Table 2 shows the output voltage V1 (the value of the plateau part in FIG. 5) obtained with the mixed gas 1 and the output voltage V2 (the value of the plateau part in FIG. 6) obtained with the mixed gas 2 at each temperature. The ratio V1 / V2 of both is shown.

  From these results, at any temperature in the range of 200 ° C. to 240 ° C., the ratio V1 / V2 is in the range of 4.2 to 12.6, and in the sample according to Example 1, the CO gas On the other hand, it turns out that it has high selectivity.

(Example 2)
A CO gas sensor sample was prepared using the gold-oxide catalyst according to the present invention, and its detection characteristic for carbon monoxide (CO) gas was evaluated.

  A catalyst composed of 20 wt% gold-cobalt oxide was prepared by the following method.

  First, 6.25 g of a gold colloid solution (mercaptosuccinic acid type: gold content 2 wt%) containing gold particles having an average particle diameter of 3 nm is placed in a container, and the solution is stirred. 0.5 g (average particle size 15 nm) was added. Furthermore, 10 ml of distilled water was added to this solution. This mixed solution was kept at 70 ° C. and stirred at a rotational speed of 180 rpm to 3000 rpm to evaporate water. During this period, water droplets adhering to the container wall were wiped off as appropriate. During the evaporation, the pH of the mixed solution was appropriately monitored using a pH test paper, and the pH of the mixed solution was maintained at around 7. (For this reason, 1N sodium hydroxide solution or 1N ammonia solution was added to the mixed solution as necessary.)

  After the water of the mixed solution was almost evaporated, the container was placed in a drier set at 100 ° C. and held for 30 minutes or more to completely dry the coagulum in the container. Thereafter, the solidified product was recovered from the container, put in a muffle furnace, kept at 300 ° C. for 2 hours in the atmosphere, and fired. By this treatment, a catalyst powder composed of 20 wt% gold-cobalt oxide was obtained.

  The obtained catalyst powder was subjected to XRD analysis using an X-ray diffractometer (RINT2100, manufactured by Rigaku Corporation). From the obtained diffraction peak result, the particle size of gold was calculated using the Scherrer equation described above, and the particle size of gold was estimated to be about 18 nm.

(Preparation of paste)
A dispersion medium prepared by adding 10 wt% of ethyl cellulose to terpineol and a 20 wt% gold-cobalt oxide catalyst powder prepared by the above-described method were mixed to prepare a paste. The mixing weight ratio of the dispersion medium and the catalyst was 2: 1.

(Production of CO gas sensor sample)
First, the above-described paste was applied onto a thermoelectric device having the same configuration as that of Example 1 using a syringe (amount applied: 0.003 μL). Next, this thermoelectric device was placed in a muffle furnace and held at 300 ° C. for 2 hours. As a result, a CO gas sensor sample (hereinafter referred to as “sample according to Example 2”) was obtained.

(Sample characterization)
Using the produced sample according to Example 2, the same evaluation as in Example 1 was performed, and the detection characteristics (selectivity) of the sample with respect to carbon monoxide (CO) gas were evaluated.

  8 to 10 show the measurement results. In each figure, the horizontal axis is the elapsed time, and the vertical axis is the voltage value output from the sample. FIG. 8 shows the results obtained with the mixed gas 1 (1 vol% CO + dry air), FIG. 9 shows the results obtained with the mixed gas 2 (1 vol% hydrogen + dry air), and FIG. It is the result obtained in the mixed gas 3 (1 vol% CO + 1 vol% hydrogen + dry air).

  From FIG. 8, it can be seen that, at each temperature of 200 ° C. to 240 ° C., a substantially constant output voltage is obtained during the period of supplying the mixed gas 1 (between about 30 seconds and about 200 seconds). In particular, when the temperature of the sample was 240 ° C., the output voltage reached about 3310 mV. On the other hand, in the case of the mixed gas 2, as shown in FIG. 9, the output voltage from the sample was extremely small at any temperature and was less than 0.5 mV at the maximum (when the temperature was 240 ° C.). In the case of the mixed gas 3 containing both carbon monoxide and hydrogen, as shown in FIG. 10, the voltage output from the sample is the output voltage obtained in the mixed gas 1 (ie, the output voltage shown in FIG. 5). Almost equal.

  Table 3 shows the output voltage V1 (the value of the plateau part in FIG. 8) obtained with the mixed gas 1 and the output voltage V2 (the value of the plateau part in FIG. 9) obtained with the mixed gas 2 at each temperature. The ratio V1 / V2 of both is shown.

  From these results, the ratio V1 / V2 is in the range of 8.5 to 9.7 at any temperature in the range of 200 ° C. to 240 ° C., and the sample according to Example 2 is carbon monoxide. It can be seen that it has high selectivity for (CO) gas.

(Comparative Example 1)
A CO gas sensor sample was prepared using a conventional gold-oxide catalyst, and its detection characteristics for carbon monoxide (CO) gas were evaluated.

  A catalyst composed of 3 wt% gold-titanium oxide was prepared by the following method.

  First, 400 ml of distilled water was put in a container, and 0.125 g of gold chloride (III) acid tetrahydrate was added thereto, and dissolved by heating and stirring at a temperature of 70 ° C. and a rotation speed of 100 rpm using a hot stirrer. . Further, while maintaining the mixed solution at 70 ° C. and stirring at 100 rpm, the pH of the mixed solution was monitored using a pH test paper, and the pH of the mixed solution was adjusted to around 7. A 1N sodium hydroxide solution was used to adjust the pH. After the pH of the mixed solution reached 7, after holding the mixed solution for about 1 hour, 2 g of titanium oxide was added and stirred at 250 rpm. Further, the mixed solution was heated and stirred at 70 ° C. and 250 rpm for 1 hour, and then the heating and stirring was stopped and the mixture was allowed to cool to room temperature.

  The mixed solution at room temperature was treated with a centrifuge at 6000 rpm for 10 minutes, and then the supernatant solution was separated from the mixed solution. Distilled water was added to the remaining solid, and the pH of this solution was monitored again, and the pH of the solution was adjusted to around 7. The solution was further treated with a centrifuge at 6000 rpm for 10 minutes, and then the supernatant solution was separated from the solution. This operation was repeated until the pH of the solution when distilled water was added to the solid content was about 7.0.

  Next, the coagulated product was collected, put into a drier set at 100 ° C., and held for 12 hours or more to completely dry the coagulated product. Thereafter, the solidified product was taken out from the container, put in a muffle furnace, and kept at 400 ° C. for 4 hours. By this treatment, a catalyst powder composed of 3 wt% gold-titanium oxide was obtained.

(Paste adjustment)
A dispersion medium prepared by adding 10 wt% of ethyl cellulose to terpineol and 3 wt% gold-titanium oxide catalyst powder prepared by the above-described method were mixed to prepare a paste. The weight mixing ratio of the dispersion medium and the catalyst powder was 4: 1.

(Production of CO gas sensor sample)
First, the above-described paste was applied onto a thermoelectric device having the same configuration as that of Example 1 using a syringe (amount applied: 0.003 μL). Next, this thermoelectric device was placed in a muffle furnace and held at 300 ° C. for 2 hours. As a result, a CO gas sensor sample (hereinafter referred to as “sample according to Comparative Example 1”) was obtained.

(Sample characterization)
Using the produced sample according to Comparative Example 1, the same evaluation as in Example 1 was performed, and the detection characteristic (selectivity) of the sample with respect to carbon monoxide (CO) gas was evaluated.

  Table 4 shows the output voltage V1 (plateau value) obtained with the mixed gas 1 and the output voltage V2 (plateau value) obtained with the mixed gas 2 at 150 ° C. and 200 ° C. The ratio V1 / V2 is shown.

  From these results, at any temperature, the ratio V1 / V2 is in the range of 1.7 to 1.8, and the sample according to Comparative Example 1 is good against carbon monoxide (CO) gas. It was found that the selectivity was not shown.

(Comparative Example 2)
A CO gas sensor sample (hereinafter referred to as “sample according to Comparative Example 2”) was prepared by the same method as in Example 2, and its detection characteristics for carbon monoxide (CO) gas were evaluated. However, in Comparative Example 2, a catalyst composed of 40 wt% gold-cobalt oxide prepared by the following method was used as the catalyst used for the CO gas sensor sample.

  First, 20 g of a gold colloid solution (mercapto succinic acid type: gold content 2 wt%) containing gold particles having an average particle diameter of 3 nm is put in a container, and the solution is stirred. 0.6 g of a particle size of 15 nm) was added. Furthermore, 10 ml of distilled water was added to this solution. This mixed solution was kept at 70 ° C. and stirred at a rotational speed of 180 rpm to 3000 rpm to evaporate water. During this period, water droplets adhering to the container wall were wiped off as appropriate. During the evaporation, the pH of the mixed solution was appropriately monitored using a pH test paper, and the pH of the mixed solution was maintained at around 7. (For this reason, 1N sodium hydroxide solution or 1N ammonia solution was added to the mixed solution as necessary.)

  After the water of the mixed solution was almost evaporated, the container was placed in a drier set at 100 ° C. and held for 30 minutes or more to completely dry the coagulum in the container. Thereafter, the solidified product was recovered from the container, put in a muffle furnace, kept at 300 ° C. for 2 hours in the atmosphere, and fired. By this treatment, a catalyst powder composed of 40 wt% gold-cobalt oxide was obtained.

  XRD analysis of the obtained catalyst powder was performed using an X-ray diffractometer (RINT2100, manufactured by Rigaku Corporation). From the obtained diffraction peak result, the particle size of gold was calculated using the Scherrer equation described above, and the particle size of gold was estimated to be about 35 nm.

  In the sample according to Comparative Example 2, other production conditions (paste preparation conditions, paste installation conditions on the thermoelectric device, etc.) are the same as in Example 2.

(Sample characterization)
Using the produced sample according to Comparative Example 2, the same evaluation as in Example 1 was performed, and the detection characteristics (selectivity) of the sample with respect to carbon monoxide (CO) gas were evaluated.

  Table 5 shows the output voltage V1 (plateau value) obtained with the mixed gas 1 and the output voltage V2 (plateau value) obtained with the mixed gas 2 at 200 ° C. and 240 ° C. The ratio V1 / V2 is shown.

  From these results, the ratio V1 / V2 is in the range of 0.6 to 4.0, and the sample according to Comparative Example 2 does not show good selectivity for carbon monoxide (CO) gas. I understood it.

(Example 3)
A catalyst composed of 30 wt% gold-cobalt oxide was prepared by the following method.

First, 10.71 g of a gold colloid solution (mercapto succinic acid type: gold content 2 wt%) containing gold particles having an average particle diameter of 3 nm is put in a container, and the solution is stirred. 0.5 g (average particle size 15 nm) was added. Furthermore, 10 ml of distilled water was added to this solution. This mixed solution was kept at 70 ° C. and stirred at a rotational speed of 180 rpm to 3000 rpm to evaporate water. During this period, water droplets adhering to the container wall were wiped off as appropriate. During the evaporation, the pH of the mixed solution was appropriately monitored using a pH test paper, and the pH of the mixed solution was maintained at around 7. (For this reason, 1N sodium hydroxide solution or 1N ammonia solution was added to the mixed solution as necessary.)
After the water of the mixed solution was almost evaporated, the container was placed in a drier set at 100 ° C. and held for 30 minutes or more to completely dry the coagulum in the container. Thereafter, the solidified product was recovered from the container, put in a muffle furnace, kept at 300 ° C. for 2 hours in the atmosphere, and fired. By this treatment, a catalyst powder composed of 30 wt% gold-cobalt oxide was obtained.

  Next, using this catalyst, a paste was prepared by the same method as in Example 2 described above, and a CO gas sensor sample was produced (hereinafter referred to as “sample according to Example 3”).

  Using the produced sample according to Example 3, the same evaluation as in Example 1 was performed, and the detection characteristics (selectivity) of the sample with respect to carbon monoxide (CO) gas were evaluated.

  Table 6 shows the output voltage V1 (the value of the plateau part) obtained with the mixed gas 1 and the output voltage V2 (the value of the plateau part) obtained with the mixed gas 2 at each temperature, and the ratio V1 / V2.

  From these results, the ratio V1 / V2 is in the range of 5.9 to 6.5 at any temperature in the range of 200 ° C. to 240 ° C., and the sample according to Example 3 is carbon monoxide. It can be seen that it has high selectivity for (CO) gas.

(Comparative Example 3)
A CO gas sensor sample (hereinafter referred to as “sample according to Comparative Example 3”) was prepared by the same method as in Example 2, and its detection characteristics for carbon monoxide (CO) gas were evaluated. However, in Comparative Example 3, a catalyst composed of 3 wt% gold-cobalt oxide prepared by the following method was used as the catalyst used for the CO gas sensor sample.

  First, 1.55 g of a gold colloid solution containing gold particles having an average particle diameter of 3 nm (mercaptosuccinic acid type: gold content 2 wt%) is placed in a container, and the solution is stirred. 1.0 g (average particle size 15 nm) was added. Furthermore, 10 ml of distilled water was added to this solution. This mixed solution was kept at 70 ° C. and stirred at a rotational speed of 180 rpm to 3000 rpm to evaporate water. During this period, water droplets adhering to the container wall were wiped off as appropriate. During the evaporation, the pH of the mixed solution was appropriately monitored using a pH test paper, and the pH of the mixed solution was maintained at around 7. (For this reason, 1N sodium hydroxide solution or 1N ammonia solution was added to the mixed solution as necessary.)

  After the water of the mixed solution was almost evaporated, the container was placed in a drier set at 100 ° C. and held for 30 minutes or more to completely dry the coagulum in the container. Thereafter, the solidified product was recovered from the container, put in a muffle furnace, kept at 300 ° C. for 2 hours in the atmosphere, and fired. By this treatment, a catalyst powder composed of 3 wt% gold-cobalt oxide was obtained.

  XRD analysis of the obtained catalyst powder was performed using an X-ray diffractometer (RINT2100, manufactured by Rigaku Corporation). From the obtained diffraction peak results, the gold particle size was calculated using the Scherrer equation described above, and the gold particle size was estimated to be about 10 nm or less.

  Next, using this catalyst, a paste was prepared by the same method as in Example 2 described above, and a CO gas sensor sample was produced (hereinafter referred to as “sample according to Comparative Example 3”).

  Using the sample according to Comparative Example 3 that was produced, the same evaluation as in Example 1 was performed, and the detection characteristics (selectivity) of the sample with respect to carbon monoxide (CO) gas were evaluated. Here, however, the test temperatures were 220 ° C. and 240 ° C.

  Table 7 shows the output voltage V1 (the value of the plateau part) obtained with the mixed gas 1 and the output voltage V2 (the value of the plateau part) obtained with the mixed gas 2 at each temperature, and the ratio V1 / V2.

  From these results, the ratio V1 / V2 is in the range of 2.7 to 3.9 at any temperature in the range of 220 ° C. and 240 ° C., and the sample according to Comparative Example 3 is carbon monoxide. It can be seen that good selectivity to (CO) gas is not exhibited.

Example 4
Next, the response speed of the CO gas sensor sample to CO was examined.

  First, heat treatment was performed on the CO gas sensor sample manufactured by the method of Example 2 described above. The heat treatment atmosphere was an air atmosphere containing 1 vol% hydrogen. The heat treatment temperature was 150 ° C., 200 ° C., or 250 ° C., and the heat treatment time (the holding time of each temperature described above) was 1 hour. The CO gas sensor samples obtained by such heat treatment are referred to as Sample A (150 ° C.), Sample B (20 ° C.), and Sample C (250 ° C.), respectively.

  Using the CO gas sensor samples A to C obtained after the heat treatment, the same measurement as in Example 1 described above is performed, and the response speed of each sample A to C with respect to CO is determined as a sample not subjected to the heat treatment (that is, the implementation). The case of the sample according to Example 2 was compared and evaluated.

  FIG. 11 shows, as an example, measurement results obtained in the mixed gas 1 (1 vol% CO + dry air) in the sample A (heat treatment temperature 150 ° C.). Measurement temperature is 200 degreeC, 220 degreeC, and 240 degreeC.

  Table 8 shows the response speed at each measurement temperature (200 ° C., 220 ° C. and 240 ° C.) in the mixed gas 1 (1 vol% CO + dry air) for the samples A to C and the sample according to Example 2. Are shown together. The response speed was represented by two indices “T90” and “T65”. Here, T90 means the time when the response voltage reaches 90% of the maximum output voltage value (plateau value), and T65 means 65% response of the maximum output voltage value (plateau value). It means the time when voltage is reached.

  From the results of Table 8, in the sample according to Example 2 where heat treatment is not performed, T65 is about 10 to 11 seconds, whereas in any of Samples A to C, T65 is 2 to 4 It can be seen that it is in the second range. That is, by performing heat treatment on the CO gas sensor (precisely, the catalyst included in the CO gas sensor) in the range of 150 ° C. to 250 ° C. in advance, the response speed to CO gas is higher than when no heat treatment is performed. It turns out that it improves.

  The present invention can be applied to a CO gas sensor or the like that measures carbon monoxide (CO) concentration.

400 CO gas sensor 413 Support film 415 Heater 420 Thermoelectric film 430 Insulating layer 440 Combustion catalyst.

Claims (6)

The gold particles and the metal oxide particles to a catalyst for including CO gas sensor,
The gold particles are contained in an amount of 10 wt% or more and less than 40 wt% with respect to the total weight ,
The metal oxide particles include at least one material selected from the group consisting of cobalt oxide, titanium oxide, nickel oxide, iron oxide, aluminum oxide, and silicon oxide,
The catalyst for a CO gas sensor , wherein the catalyst is preheated in the range of 150 ° C. to 250 ° C. in an atmosphere containing hydrogen and oxygen .   The catalyst for a CO gas sensor according to claim 1, wherein the gold particles have an average particle diameter in the range of 10 nm to 20 nm.   The catalyst for a CO gas sensor according to claim 1 or 2, wherein the metal oxide particles have an average particle diameter in a range of 20 nm to 100 nm. A paste comprising a matrix medium and a catalyst,
The said catalyst is a catalyst as described in any one of Claims 1 thru | or 3. The paste characterized by the above-mentioned. The matrix medium is at least one solvent selected from the group consisting of terpineol, butyl carbitol acetate, and isopropyl alcohol, and at least one selected from the group consisting of ethyl cellulose, acrylic resin, and polyvinyl alcohol. The paste according to claim 4 , further comprising an organic binder. A CO gas sensor including a detection unit provided with a combustion catalyst,
The combustion catalyst is
Including gold particles and metal oxide particles,
The metal oxide particles include at least one material selected from the group consisting of cobalt oxide, titanium oxide, nickel oxide, iron oxide, aluminum oxide, and silicon oxide,
The gold particles are contained in an amount of 10 wt% or more and less than 40 wt% with respect to the total weight,
The CO gas sensor , wherein the combustion catalyst is preheated in a range of 150 ° C. to 250 ° C. in an atmosphere containing hydrogen and oxygen .