JPH06130016A - Nitric oxide gas sensor - Google Patents

Abstract

PURPOSE:To provide a nitric oxide gas sensor which is simple in structure, inexpensive, sensitive to nitric oxide gas, and has improved selectivity. CONSTITUTION:A metal oxide semiconductor part 3, where at least one type of oxide selected from Ca, Sr, and Ba is added to tin oxide semiconductor as an admixture by 1-20mol%, is provided and a tin oxide catalyst layer 4 where oxide Cr of copper and chrome are carried at the outer periphery of the metal oxide semiconductor part 3 by 1-10mol%. and 0.1-10mol%, respectively, are provided, thus constituting a semiconductor-type nitric oxide gas sensor 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitric oxide gas sensor.

[0002]

2. Description of the Related Art Conventionally, as a nitric oxide gas sensor for measuring the concentration of nitric oxide (NOx) gas, a constant potential electrolysis type,
Those using methods such as a chemiluminescence method, an infrared absorption method, and an ultraviolet absorption method are known.

[0003]

However, in the above-mentioned constant potential electrolysis type, since a liquid such as an electrolytic solution is used, the apparatus is complicated, and maintenance such as replenishment of the solution must be frequently performed. There are drawbacks. Furthermore, even in the case of chemiluminescence type, infrared absorption type, ultraviolet absorption type,
Since a relatively complicated device configuration such as a light source and its detection system is required, the device is large and expensive. On the other hand, a so-called semiconductor type nitric oxide gas sensor has not been put to practical use in the past, and if a conventional semiconductor type sensor is used, other interfering gases (hydrocarbon, hydrogen, carbon monoxide,
The gas selectivity to ethanol, etc.) was poor and it was impossible to put it to practical use.

Therefore, the present application is to provide a nitric oxide gas sensor which has a simple structure and is inexpensive, and which is highly sensitive to nitric oxide gas and excellent in selectivity.

[0005]

To achieve this object, the nitric oxide gas sensor according to the present invention is characterized in that tin oxide semiconductor contains calcium (Ca) and strontium (S).
r), at least 1 selected from barium (Ba)
There is a metal oxide semiconductor portion to which a kind of oxide is added as an additive, and a tin oxide catalyst layer supporting oxides of copper (Cu) and chromium (Cr) on the outer periphery of the metal oxide semiconductor portion. , Its action and effect are as follows.

[0006]

[Function] That is, the nitric oxide gas sensor of the present application is
In addition to the metal oxide semiconductor part that mainly contributes to element detection,
Formed around the circumference and has good selectivity for interfering gases
And a tin oxide catalyst layer. Metallic acid
The oxide semiconductor portion is described below.
For semiconductors, calcium (Ca), strontium (S
r), at least one selected from barium (Ba)
Nitric oxide (NO
x) sensitivity is increased and sufficient sensitivity for detection is obtained.
And methane (CH _Four), Hydrocarbon gas
The selectivity for is improved. About tin oxide catalyst layer
Is an oxide of chromium (Cr) in the tin oxide layer that is the medium.
Add mainly hydrogen (H₂) Reduced gas sensitivity
Then, an oxide of copper (Cu) is added to the carbon monoxide (C
O) and ethanol (C₂H_FiveLow sensitivity to (OH) gas
Good selectivity can be obtained (see the experimental results shown below).
See).

[0007]

Therefore, the nitric oxide gas sensor of the present invention is highly sensitive to nitric oxide gas and is excellent in selectivity with respect to other gases. Accurate gas detection without detection can be performed. Further, structurally, the structure is similar to that of the conventional semiconductor gas sensor, so that the structure is simple and maintenance and inspection is very easy.

[0008]

【Example】

[Structure of Sensor] FIG. 1 shows the semiconductor type nitric oxide (NO) of the present application.
x) Shows the gas sensor 1. As shown in the figure, this sensor 1 has tin oxide (SnO ₂ ) on a platinum (Pt) coil 2.
And a tin oxide catalyst layer 4 formed on the outer layer side thereof. More specifically, the metal oxide semiconductor portion 3 is formed of a tin oxide (SnO ₂ ) sintered body, and the sintered body 3 contains calcium (Ca), strontium (Sr), or barium (Ba). At least one oxide selected from the above is added as an additive. On the other hand, the tin oxide catalyst layer 4
Is a carrier layer mainly composed of tin oxide (SnO ₂ ), which carries oxides of chromium (Cr) and copper (Cu).

[Method for Manufacturing Sensor] First, the sensor
The manufacturing method will be described. 1) Using tin tetrachloride, prepare an aqueous solution with a certain concentration,
Add a predetermined amount of antimony chloride to the aqueous solution.
After drying the precipitate of tin hydroxide obtained by dropping monia water,
The valence was controlled by firing in an electric furnace at 700 ° C for 2 hours.
Tin oxide (SnO ₂) Get. Grind this into fine powder
Knead with water to make a paste and attach to a platinum (Pt) coil
After depositing and drying, heat at 600 ° C for 1 hour to obtain a sintered body.
It 2) Alkaline earth metal (calcium) as an additive
(Ca), strontium (Sr), barium (Ba)
One or more) selected from
Is adjusted to be 1 to 20 mol% with respect to tin oxide
Then, the sintered body is impregnated. After drying it at 600 ℃
The metal oxide semiconductor portion 3 is formed by heating for 1 hour. 3) Adjust the tin oxide catalyst layer 4 in the same manner as above.
Chromium nitrate against tin oxide in diluted tin tetrachloride aqueous solution
0.1-10 mol% of copper nitrate to tin oxide
Then, they are mixed so as to be 1 to 10 mol%. Aqueous ammonia is added dropwise to this aqueous solution to form hydroxide as hydroxide.
Of copper, chromium and copper, and after drying at 600 ° C in an electric furnace
Bake for 1 hour. This is crushed into a fine powder and kneaded with water.
To form a paste, which is used for the metal oxide semiconductor portion 3
Apply to the entire circumference of the surface, dry and heat at 600 ° C for 1 hour.
A medium layer is formed.

[Sensor Circuit] The sensor 1 is used by being incorporated in the Wheatstone circuit 5 shown in FIG.
In the figure, a series resistor 5a is a resistor connected in series as a load resistor for the sensor 1, and resistors 5b and 5c.
Are reference resistors connected in series with each other to determine the reference potential of this circuit. The sensor 1 and the series resistor 5a are parallel to the other reference resistors 5b and 5c with respect to the power source 5d,
The output of this sensor can be obtained in the form of a voltage (mV) due to the potential difference between the midpoints A and B of the resistors, respectively.

[Sensor Characteristics] Regarding the semiconductor type nitric oxide gas sensor of the present application, the characteristics of additives and sensor output added to the metal oxide semiconductor portion 3, the tin oxide catalyst layer 4 on the metal oxide semiconductor portion 3 will be described below. The sensor sensitivity characteristics of the sensor having the configuration will be described in order.

[Relationship between Addition of Additives and Sensor Sensitivity] Calcium (Ca) as an additive is added to the metal oxide semiconductor portion 3.
The relationship between the sensor temperature and the sensitivity characteristic for nitric oxide gas (NO) (representing nitric oxide in this gas) when barium (Ba) and strontium (Sr) are added is shown in FIGS. 5 (vertical axis: sensor output, horizontal axis: sensor temperature). Here, as the sensor temperature to be studied, 200 to 400 ° C. was targeted. The reason for this is that at low temperatures, the oil and moisture in the gas have a bad effect, and in general, the semiconductor gas sensor has a temperature of 350 to 500 ° C.
It is often used in the
This is because it cannot be used unless the temperature is 0 ° C or higher.

As a result, as shown in FIGS. 3 to 5, in the sensor temperature range of 200 to 400.degree.
Although the characteristics of the sensor output change depending on the added amount have their respective characteristics, it can be seen that the addition of these additives makes them sensitive to nitric oxide. When these additives are not added, the sensor temperature shows a (-) value below 300 ° C.
At a temperature of 00 ° C or higher, the value is almost zero and there is no sensor sensitivity.

On the other hand, in FIG. 6, the sensor temperature is kept constant (33
The relationship between the addition ratio of each additive and the sensor output in the state of (0 ° C.) is shown (vertical axis: sensor output, horizontal axis: additive addition amount). When selecting the sensor temperature,
Taking into consideration the sensitivity responsiveness to nitric oxide gas and the selectivity with other gases, the temperature was set at 330 ° C., and the NO sensitivity change depending on the addition amount of each additive was examined. As a result, when the addition amount of each additive was 1 to 20 mol%, the additive was sufficiently sensitive to nitric oxide, and particularly large sensitivity was obtained mainly at 2 to 10 mol%.

[Gas selection of sensor equipped with tin oxide catalyst layer
Optional] Hydrogen (H₂),Carbon monoxide
(CO), ethanol (C₂H _FiveLow sensitivity to OH)
Oxidation with tin oxide catalyst layer 4 provided to reduce
The gas selectivity of the nitrogen gas sensor will be described below.
It First, tin oxide containing chromium for hydrogen gas
The effect of the catalyst layer 4 will be described, and next, carbon monoxide and
Of the tin oxide catalyst layer 4 containing copper for tanol gas
The effect will be described.

A Chromium Oxide-Containing Catalyst Layer A sensor containing chromium in the catalyst layer contains nitric oxide (N
O) sensitivity to 50 ppm and hydrogen (H ₂ ) 1000
ppm, carbon monoxide (CO) 1000 ppm, ethanol (C ₂ H ₅ OH) 1000 ppm sensitivity ratio (N
O / H ₂ , NO / CO, NO / C ₂ H ₅ OH)
The change of is shown in Table 1 (barium oxide (BaO) 5mo
The metal oxide semiconductor portion 3 which is a sensitive portion added with 1% is coated with the tin oxide catalyst layer 4 to form a sensor. ).

[0017]

[Table 1]

As can be seen from Table 1, chromium plays a role of reducing the sensitivity to hydrogen gas. Addition amount 0.
The sensitivity ratio is 1 or more over the concentration range of 1 to 10 mol%. Even at 10 mol% or more, the sensitivity ratio with hydrogen is 1 or more, but the NO sensitivity itself is reduced, which makes it practically difficult to use.

B Copper Oxide-Containing Catalyst Layer Next, the effect of the catalyst layer containing copper for the purpose of reducing the sensitivity to carbon monoxide and ethanol will be described.
Table 2 shows the change in the ratio of the sensitivity to nitric oxide gas 50ppm and the sensitivity to hydrogen 1000ppm, carbon monoxide 1000ppm, and ethanol 1000ppm when copper was added to the tin oxide catalyst layer containing chromium (2 mol%). The same as described in a).

[0020]

[Table 2]

As shown in Table 2, copper plays a role in reducing the sensitivity to carbon monoxide and ethanol.
The sensitivity ratio is 1 or more over the concentration range of 1 to 20 mol% which is the object of study. Especially 5-10 mol
It can be seen that the selectivity is excellent in%. 20mo
If it is 1% or more, the NO sensitivity itself is greatly reduced, which is a practical problem.

In summary, the NO / H ₂ sensitivity ratio is 1
The above range of chromium is Cr: 0.
It is 1 mol% (sensitivity ratio = 2.1) to 10 mol% (sensitivity ratio = 2.2). As can be seen from Table 2 due to the addition of copper, the NO / H ₂ sensitivity ratio gradually deteriorates.
Chromium 0.1m when ol% to 10mol% is added
The sensitivity ratio was 1 or more in the range of ol% to 10 mol%. Therefore, chromium works effectively in the range of 0.1 mol% to 10 mol%. In addition, NO / CO, NO / C ₂ H ₅ O
The range of copper in which the sensitivity ratio of H is 1 or more is 2 mol of chromium.
In the case of addition by%, it is 1 to 20 mol%. Although the sensitivity ratio of NO / CO and NO / C ₂ H ₅ OH gradually deteriorates due to the decrease or increase of the added amount of chromium (in the range of 0.1 to 10 mol%), the above-mentioned copper 1 mol% to 10 mol% %
In, each sensitivity ratio was 1 or more. Therefore, copper shows good results in the range of 1 mol% to 10 mol%.

[Sensor Temperature and Gas Selectivity] Hereinafter, the tin oxide catalyst layer 4 is formed on the metal oxide semiconductor portion 3 to which each additive is added.
The characteristics of the sensor output of the configuration provided with will be described. The characteristics of barium added as an additive are shown in FIGS. FIG. 7 shows the relationship between sensor temperature and sensor output, and FIG. 8 shows the relationship between gas concentration and sensor output. Sensor specifications Additives: barium (Ba): 5 mol%, catalyst layer inclusions: chromium (Cr): 2 mol%, copper (Cu): 7 mol% Sensor temperature shown in FIG.

As a result, as shown in FIG. 7, the sensitivity of this sensor to nitric oxide (NO) and nitrogen dioxide (NO ₂ ) is the highest near 300 ° C. or lower, and gradually decreases in the temperature range higher than that. ing. On the other hand, in other gases, the sensitivity decreases as the temperature rises. Therefore, the selectivity is sufficiently maintained at 300 to 400 ° C. Further, as shown in FIG. 8, sensitivity to nitric oxide gas is sufficiently high from 5 ppm, and the sensitivity is high, and hydrogen (H ₂ ), carbon monoxide (CO), ethanol (C ₂ H _{2 It} can be seen that separation from each gas of ₅ OH) and methane (CH ₄ ) is also sufficiently good. By the way, in the above-described examples, the above-mentioned four kinds of gases are examined as interfering gases, but the examination of these gases can cover the examination of other gas components, which is substantially sufficient. A practical nitric oxide gas sensor can be obtained.

In the above examples, barium was used as an additive, but examples using calcium and strontium as additives are shown in FIGS. 9 and 10. Each of them has the following mixing ratio. Sensor specifications (shown in FIG. 9) Additives; Calcium (Ca): 3 mol%, catalyst layer inclusions; Chromium (Cr): 5 mol%, Copper (Cu): 2 mol% Sensor specifications (shown in FIG. 10) ) Additives: Strontium (Sr): 7 mol%, catalyst layer-containing substances: Chromium (Cr): 1 mol%, Copper (Cu): 10 mol% The change in sensor output with respect to each sensor temperature is shown. Although there is a slight difference in the temperature dependence of the sensitivity of nitric oxide (NOx) depending on the added calcium, barium, and strontium, in the range of 300 to 400 ° C., the interfering gases hydrogen, carbon monoxide, ethanol, and methane are Sensitivity is low and nitric oxide selectivity is obtained.

[Other Embodiments] Other embodiments of the present application are listed below. (A) As a method for adding barium, calcium, and strontium to the metal oxide semiconductor part 3, in addition to the impregnation method described in the examples, each solution is mixed with a tin chloride solution in the process of obtaining the tin oxide. It is also possible to carry out the addition by a method of coprecipitating together with aqueous ammonia, and any method may be used. Further, although ammonium salts and nitrates were used in the above-mentioned examples in such a metal addition step, any salts such as nitrates, that is, any water-soluble salts may be used. (B) In the above embodiment, a single catalyst layer 4 was formed and chromium and copper were mixed in this layer, but it may be formed as a separate catalyst layer. In this case,
The sensor has a three-layer structure. (C) Further, in the above embodiment, the hot-wire type sensor configuration is shown as shown in FIG. 1, but the sensor configuration may be a substrate type as shown in FIGS. 11 and 12. In FIG. 11, a noble metal electrode 10 formed in a rectangular wave shape is adopted instead of the noble metal coil in the above-mentioned hot wire type structure, and the noble metal electrode 10 has a substrate portion 11 on the lower surface side and a metal portion on the upper surface side. The oxide semiconductor portion 12 is formed, and the tin oxide catalyst layer 13 is further formed on the upper surface thereof. In the case of this example, the sensor 1 is heated by the heat generation of the noble metal electrode 10. On the other hand, the one shown in FIG. 12 is provided with a special heater 14 for heating the sensor in the configuration of FIG. It should be noted that reference numerals are added to the claims for convenience of comparison with the drawings, but the present invention is not limited to the configurations of the accompanying drawings by the entry.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a semiconductor-type nitric oxide gas sensor of the present application.

FIG. 2 is a diagram showing a configuration of a detection circuit.

FIG. 3 is a diagram showing temperature characteristics of a metal oxide semiconductor part to which calcium is added.

FIG. 4 is a diagram showing temperature characteristics of a metal oxide semiconductor portion to which barium is added.

FIG. 5 is a diagram showing temperature characteristics of a metal oxide semiconductor part to which strontium is added.

FIG. 6 is a diagram showing the relationship between the ratio of additives added and sensitivity.

FIG. 7 is a diagram showing sensor characteristics in which a catalyst layer is provided on a barium-doped metal oxide semiconductor portion.

8 is a diagram showing concentration characteristics with respect to a detection gas in the sensor of FIG.

FIG. 9 is a diagram showing sensor characteristics in which a catalyst layer is provided on a metal oxide semiconductor portion to which calcium is added.

FIG. 10 is a diagram showing sensor characteristics in which a catalyst layer is provided on a metal oxide semiconductor part to which strontium is added.

FIG. 11 is a view showing another embodiment of the semiconductor type nitric oxide gas sensor of the present application.

FIG. 12 is a view showing still another embodiment of the semiconductor type nitric oxide gas sensor of the present application.

[Explanation of symbols]

 1 Nitric oxide gas sensor 3 Metal oxide semiconductor part 4 Tin oxide catalyst layer

Claims (2)

[Claims] 1. A tin oxide semiconductor containing calcium (Ca),
A metal oxide semiconductor part (3) to which at least one oxide selected from strontium (Sr) and barium (Ba) is added as an additive, and copper on the outer periphery of the metal oxide semiconductor part (3). A semiconductor type nitric oxide gas sensor provided with a tin oxide catalyst layer (4) carrying oxides of (Cu) and chromium (Cr). 2. The addition ratio of the additive to the tin oxide semiconductor in the metal oxide semiconductor part (3) is 1 to 2.
It is 0 mol%, and the addition ratio of the oxide to tin oxide in the tin oxide catalyst layer (4) is copper (Cu).
The nitrogen oxide gas sensor according to claim 1, which is 1 to 10 mol% with respect to, and 0.1 to 10 mol% with respect to chromium (Cr).