JP2918390B2 - Nitrogen oxide detection sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the detection of nitrogen oxides used in fields such as nitrogen oxide reduction technology and nitrogen oxide decomposition technology.

[0002]

2. Description of the Related Art Methods for measuring the concentration of nitrogen oxides in exhaust gas and the like include a chemiluminescence system, an infrared absorption system,
An ultraviolet absorption system, a constant potential electrolysis system, a constant potential coulomb system and the like are known. On the other hand, oxide semiconductor sensors (particularly SnO
₂ ) is known, which has a proven track record in durability.

[0003]

Here, the chemiluminescence type has the highest accuracy and reliability and is considered to be the most effective method at present. However, an ozone generator, a photomultiplier tube,
It requires a high voltage source, etc., and has limitations in miniaturization, price reduction, and maintenance conditions. On the other hand, the absorption method has a high detection threshold and requires correction. When strict accuracy is not required, the constant potential electrolysis method is simple, but there are difficulties in changing the electrode performance with time and maintaining the electrolyte solution. The devices used in these methods have the problem of costing hundreds of thousands of yen to several million yen, and have high measurement accuracy, but require correction due to drift, It has the disadvantage of lacking properties. On the other hand, in the above-described oxide semiconductor sensor, although it has durability, in detecting nitrogen oxides in the atmosphere or combustion exhaust gas,
It has the disadvantage of poor selectivity for nitrogen oxides.

Accordingly, it is an object of the present invention to provide a nitrogen oxide detection sensor which has a relatively simple structure, is inexpensive, and has good selectivity to nitrogen oxide.

[0005]

In order to achieve the above object, a characteristic feature of a nitrogen oxide detection sensor is that the nitrogen oxide detection sensor has electrical conductivity or semi-conductivity.

[0006]

Embedded image

(A is one or more elements selected from Group IIa elements, B is one or more elements selected from divalent transition metal elements, δ is −0.3 to +0.6) The gas detecting section mainly composed of the complex oxide represented by the formula (1) and an electrode electrically connected to the gas detecting section. The operation and effect are as follows.

[0008]

The composite oxide employed in the gas detecting portion of this sensor reversibly generates a B (NO ₃ ) ₂ phase by a gas-solid reaction with a nitrogen oxide gas. This reaction is dependent on the concentration of nitrogen oxides in the gas phase,
It is a release reaction. Here, since the generated B (NO ₃ ) ₂ is a non-conductive substance, the electric resistance of the gas detector itself changes according to the concentration of nitrogen oxide in the gas phase. This change in the electric resistance value is detected using an electrode provided in the gas detection unit, and the sensor functions as a nitrogen oxide sensor. Since this compound is formed for the first time in the presence of nitrogen oxides, it is less sensitive to other gases such as hydrogen and alcohol.

[0009]

That is, in the sensor of the present invention, the change in the electric resistance value of the gas detecting portion is measured through the electrode, and the concentration of only the nitrogen oxide gas contained in the atmosphere or the combustion exhaust gas is measured. Can be detected with good selectivity. Furthermore, it is configured as a simple and inexpensive device. To explain the selectivity in more detail, in the conventionally proposed semiconductor type sensor, flammable gases such as hydrogen, carbon monoxide, hydrocarbons, alcohols and the like become interference components, so that these components Under coexistence, it was difficult to detect only nitrogen oxide gas with good selectivity, but from the above-described detection principle, the sensor of the present invention was able to greatly improve this conventional problem. .

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings in the order of sensor structure, sensor manufacturing method, and sensor characteristics.

1. Sensor structure FIG. 1 shows the configuration of the nitrogen oxide detection sensor 1 of the present application. The sensor 1 is provided with a thin-film gas detector 3 on the upper side of the MgO substrate 2, and a pair of Pt current flowing electrodes 4 and these current A Pt voltage detection electrode 5 for the electrode 4 is provided. Further, a ceramic heater plate 6 as a heating means is provided below the substrate 2. By the way, the main component of the above-mentioned gas detection unit 3 is

[0012]

Embedded image It is BaCuO _2.5 .

That is, with this sensor configuration, a change in the electric resistance value of the gas detecting section 3 is detected. Here, when the gas detection unit 3 is formed in a thin film shape as in this embodiment, when the nitrogen oxide contained in the gas to be inspected in the entire gas detection unit reacts with this part, the gas detection is performed. The proportion of the portion where the resistance is increased by the solid-phase reaction is increased, and the influence of diffusion of nitrogen oxides in the gas detection portion can be reduced. As a result, it is possible to improve the speed of absorption and release of nitrogen oxides (the response speed of the sensor). Accordingly, the sensitivity to low-concentration nitrogen oxides and the response of sensitivity to changes in nitrogen oxide concentration are improved. Furthermore, regarding the electrode configuration, by adopting the above configuration, the current also flows to the high resistance portion in the gas detection unit, and the electric resistance change of the gas detection unit due to the nitrogen oxide gas can be effectively detected. it can.

2. Manufacturing method of sensor The manufacturing method of the composite oxide thin film constituting the gas detection unit 3 will be described below. 1. Ba and Cu nitrates are used as starting materials, and these materials are blended in a predetermined ratio. 2. Then calcined at 500 ° C, pulverized and 750 ° C in air
For 5 hours to obtain a sintered body. 3. Using the obtained sintered body as a target, a thin film-shaped gas detector 3 is formed by a laser ablation method.
Table 1 shows the film forming conditions for laser ablation.

[0015]

[Table 1]

The gas detecting section 3 is formed on the substrate 2 in this way, and the detecting section 3 is provided with an electrode 4.5.

3. Sensor Characteristics 3-1 Nitrogen oxide sensitivity characteristics Sensitivity characteristics were measured as follows. A constant voltage is applied to the ceramic heater plate 6, the gas detection unit 3 is maintained at 450 ° C., and a current of 10 mA is applied to the current supply electrode 4 to bring a gas containing various components into the air into a predetermined concentration. The voltage was measured according to 5, and the electric resistance value was obtained. FIG.
Each gas _{(NO, NO 2, C 2} H 5 OH, CH 3 OH,
H ₂ ). In the drawing, the resistance value in air is represented by R ₀ (Ω), and the resistance value in each gas is represented by R (Ω).
In the figure, the horizontal axis represents the concentration (in ppm) of each gas, and the vertical axis represents the response rate (log (R / R ₀ )).

As a result, this sensor shows sufficient sensitivity characteristics to NO and NO ₂ and does not show sensitivity to other gases to a predetermined concentration (about 800 ppm). Therefore, it is possible to select and detect nitrogen oxides. Comparing this result with the case of SnO ₂ as a representative of the semiconductor sensor shown in FIG. 7, it can be understood that the selectivity of the sensor of the present invention is high.

3-2 Nitrogen Oxide Sensitivity of Pt Carrying the sensor 1 described above, the interference component (C ₂ H ₅ O
H, CH ₃ OH, H ₂ ) by oxidation reaction
An oxidation catalyst layer 7 having a platinum group element such as t or Pd is formed (Pt in active alumina is 1%
The mixed and dispersed material is printed by a screen printing method to a thickness of 500 μm and formed by baking at 600 ° C. for 24 hours) is also effective for detecting nitrogen oxides. The configuration of this sensor 10 is shown in FIG. Further in FIG.
The measurement results corresponding to FIG. 2 of the sensor 10 are shown. As a result, the selectivity to the interference component is improved (the lower limit of the concentration for detecting the interference component is increased to 1500 ppm), and the sensitivity to the NO and NO ₂ components is improved.
That is, it can be seen that by providing the catalyst layer 7 containing a platinum group element such as Pt and Pd on the composite oxide, it is possible to suppress the influence of a high concentration of interference components.

4. EXPERIMENTAL EXAMPLES As a study of a material suitable as a composite oxide constituting the gas detection unit 3, experimental results on various materials performed by the inventor are shown below. The sensor configuration was the same as that of FIG. 1, and the results of the sensitivity characteristics were arranged as in FIG. FIG. 5 shows the results for NO ₂ and FIG. 6 shows the results for NO. In these figures, R ₀ indicates a resistance value when NO ₂ or NO does not exist, and R indicates a resistance value when NO ₂ or NO exists at a certain concentration. Table 2 shows the composition of the composite oxide sample studied. 5 and 6, each number corresponds to the number of each sample.

[0021]

Embedded image

[0022]

[Table 2]

As a result, it was found that any of the samples could detect nitrogen oxides. Therefore, from the above detection principle and crystal structure, as a composite oxide that can be employed in the gas detection unit 3 of the present application,

[0024]

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It can be seen that the composite oxide represented by In the formula, as the element A, Ca, Sr, Ba
And at least one element selected from the group IIa elements (may be a mixture or a solid solution of two or more elements). As the element B, Cr, Mn, Fe, C
One or more elements selected from divalent transition metal elements such as o, Ni, and Cu (a mixture or a solid solution of two or more elements) may be used. Here, since the valence of B can change to some extent, the amount of oxygen can change in the range of 1.7 to 2.6 (δ is −0.3 to 0.6). Further, regarding the combination of A, B, and δ, this material should be electrically conductive or semiconductive depending on the sensor configuration and the detection principle configuration.

[Another embodiment] Another embodiment will be described below. (1) Configuration of Oxidation Catalyst Layer In the above embodiment, an example in which the oxidation catalyst layer 7 in which Pt is supported on activated alumina is provided, but this may be configured as follows. a. A platinum group element such as Pt, Pd, etc.
% And mixed. b. A platinum group element such as Pt and Pd is supported on the inner surface of the composite oxide.

(2) Electrode Configuration In the above embodiment, an example has been described in which a pair of current flowing electrodes 4 and a pair of voltage detection electrodes 5 are provided to detect a change in resistance. In the meantime, a constant voltage may be applied to detect the current between the pair of current detection electrodes. Further, as a method of forming the electrodes, it is possible to use a normal two-terminal electrode (in which the current electrode and the voltage electrode are common). In this case, in the case of the gas detection unit according to the present invention, the nitrogen oxide is used. Since the high-resistance phase is partially generated by the absorption of the gas, the current preferentially flows through only the low-resistance phase, and the change in the resistance value of the gas detection unit that can be detected from the electrode may be small. Therefore, the configuration disclosed in the embodiment is preferable. However, as the function of the electrode, any configuration may be used as long as it can detect a change in resistance value occurring in the gas detection unit.

(3) Heating means In order to improve the responsiveness and recovery of the electrical resistance value of the nitrogen oxide detection sensor to changes in the concentration of nitrogen oxide gas, the sensor is heated to 450 ° C. or higher as in the embodiment. For this purpose, it is preferable to provide an arbitrary heating means in addition to the ceramic heater plate 6 of the embodiment. By providing a heating means, nitrogen oxide absorption and
The release rate becomes sufficiently large, and the responsiveness and recovery of the electrical resistance value of the nitrogen oxide detection sensor to changes in the concentration of the nitrogen oxide gas can be improved. However, if a change in the resistance value can be ensured, detection can be performed even without this heating means. A self-heating type is also conceivable.

(4) Film Forming Method As a thin film forming method for forming the gas detecting portion, in addition to the above-mentioned methods, a physical vapor deposition method such as a vacuum vapor deposition method and a sputtering method, and a chemical vapor deposition method such as MO-CVD and chloride CVD. There is also a vapor deposition method.

(5) Material of Substrate In addition to MgO, SrTiO ₃ can be used as the material of the substrate.

(6) Sensor Configuration The gas detector 3 can exhibit the function of detecting nitrogen oxides even when an arbitrary structure such as a lump or the like is adopted in addition to a thin film.

In the claims, reference numerals are provided for convenience of comparison with the drawings, but the present invention is not limited to the configuration shown in the attached drawings.

[Brief description of the drawings]

FIG. 1 is a diagram showing a sensor configuration.

FIG. 2 is a diagram showing sensitivity characteristics to gas.

FIG. 3 is a diagram showing a configuration of a sensor including a catalyst layer carrying Pt.

FIG. 4 is a view showing gas sensing characteristics of a sensor having a catalyst layer.

FIG. 5 is a diagram showing the gas sensitivity characteristics of each composite oxide to NO _{2 in} an experimental example.

FIG. 6 is a graph showing gas sensitivity characteristics of each composite oxide of the experimental example to NO.

FIG. 7 is a view showing gas sensitivity characteristics of SnO ₂ .

[Explanation of symbols]

 2 Substrate 3 Gas detector 4 Electrode 5 Electrode 7 Oxidation catalyst layer

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Masamichi Ipponmatsu Osaka Gas Co., Ltd. 4-1-2, Hirano-cho, Chuo-ku, Osaka-shi, Osaka (56) Reference JP-A-3-200057 JP-A-3-200058 (JP, A) JP-A-49-49698 (JP, A) JP-A-50-144391 (JP, A) JP-A-51-15492 (JP, A) Sensors and Actors B, 9 , (1992), p17-23 (58) Fields investigated (Int. Cl. ⁶ , DB name) G01N 27/12 JICST file (JOIS)

Claims (4)

(57) [Claims] (1) having electrical conductivity or semi-conductivity; (A is one or more elements selected from Group IIa elements, B is one or more elements selected from divalent transition metal elements, and δ is −0.3 to +0.6). A gas detector (3) containing a composite oxide as a main component, and electrodes (4, 5) electrically connected to the gas detector (3). Sensor. 2. The nitrogen oxide detection sensor according to claim 1, wherein A is a group IIa element containing Ba, and B is a transition metal element containing Cu. 3. The nitrogen oxide detection sensor according to claim 1, wherein an oxidation catalyst layer supporting a platinum group element is provided on at least a surface side of the gas detection section. 4. The nitrogen oxide detection sensor according to claim 1, wherein the gas detection section is a thin film provided on a flat substrate.