JPH07225213A - Solid-electrolyte type nox sensor - Google Patents

Abstract

PURPOSE:To use an NOx sensor at normal temperature. CONSTITUTION:A solid electrolyte body 2 is molded in a disk shape. A net body made of platinum is fixed as a working electrode 4 at the bottom surface. A gold wire is fixed to the working electrode 4 as a lead wire 6. Silver paste is provided as a reference electrode 10 on the upper surface of the solid electrolyte body 2. A gold wire is connected by compression and fixed as a lead wire 8 through the silver paste.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a NOx sensor for measuring nitrogen oxide NOx. Nitrogen oxide measurement is used for monitoring of combustion exhaust gas such as flue gas and automobile exhaust gas, combustion control, and environmental monitoring of atmospheric air, tunnel air, indoor air, high-rise air, and the like.

[0002]

2. Description of the Related Art As a method for measuring nitrogen oxides, several methods are adopted according to the purpose of measurement. As a method for measuring with a nitrogen oxide generation source, a chemiluminescence method, a non-dispersive infrared absorption method, an infrared absorption method and a potentiostatic electrolysis method have been adopted as JIS as an NOx automatic measuring instrument and are widely used. For emissions sources, it is necessary to measure all nitrogen oxides NOx, including NO and NO ₂ . The composition ratio of nitrogen oxides in the gas in the emissions source NO ₂ / NO = 10~30 / 90~
70.

As a NOx measuring method used for environmental monitoring, the Salzman absorption spectrophotometric method is adopted as an official method,
The chemiluminescence method is also adopted in JIS as an automatic measuring instrument. The environmental standard is defined by the NO ₂ concentration, and the daily average value of 1-hour value is specified to be 0.04 to 0.06 ppm or less.

For the measurement of NOx by the ultraviolet absorptiometry, a method of air-oxidizing NO at room temperature and high pressure has been put into practical use. NO + (1/2) O ₂ → NO _{2 In the} NOx measurement by the infrared absorptiometry or the chemiluminescence method, NO ₂ is converted into NO by a reducing agent, and the total NO measurement method is put into practical use. For the reduction, for example, carbon is reduced at 150 to 400 ° C. as in the following formula. 2NO ₂ + C → 2NO + CO ₂

Although no practical NOx sensor is available yet, research reports have been increasing rapidly in recent years. No already reported
There are roughly two types of x sensors. One is an oxide semiconductor type, and a sensor mainly made of WO ₃ is the mainstream, but there are still unsolved problems in stability, reproducibility, and moisture interference, and it is far from practical use. Another sodium ion conductor (NASICON) a sensor for the solid electrolyte, which is noteworthy, sensitivity ratio NO ₂ / NO = 2~
At 10/1, there is little interference due to CO ₂ , O _2, etc., the linearity is good, and almost Nernst slope is shown ("
CHEMICAL SENSOES "1993 Vol.9 Supplement A, pp.81-8
See 4). The working temperature of the sensor using NASICON is 150 ° C.

There is an attempt of a galvanic sensor for measuring NOx by measuring a current flowing between a working electrode and a comparison electrode using a solid electrolyte (BUNSEKI KAGAKU, Vol. 32, 541-5).
46 (1983)). In the case of a galvanic sensor, the current flowing between both electrodes through the solid electrolyte is measured, so that the smaller the resistance of the solid electrolyte, the higher the sensitivity. Therefore,
In the case of an inorganic solid electrolyte, it is heated and a minute current is measured,
Determine the NOx concentration. In this case, the current and the NOx concentration are proportional.

[0007]

The automatic measuring instrument which has been put into practical use as an emission source has high precision, is expensive, and is large in size.
However, there is a demand for a sensor type that is inexpensive, has high durability, and has a high response speed even if the accuracy is somewhat low for control and automobiles. The automatic measuring instruments for stationary observation used for environmental monitoring have high accuracy, but they are also expensive and large. However, there is no measuring instrument suitable for simple measurement such as observation carried by the observer and arbitrarily measuring, high-level air observation by airplane or balloon, unmanned sea observation, observation in tunnel or indoor, and here also sensor A type is desired.

A solid electrolyte type NOx sensor whose main material is NASICON has a sensitivity ratio of NO ₂ / NO = 1/1 and detects NO ₂ and NO with the same sensitivity, and the total nitrogen oxide NO
Efforts have been made to detect x, but at present they will be difficult. Therefore, in order to use the solid electrolyte type NOx sensor whose main material is NASICON as the NOx sensor for the emission source, it is necessary to convert NO into NO ₂ and also as an environmental NOx sensor (measuring only NO ₂ ). To use it, there is NO interference and it cannot be used.

Both the oxide semiconductor and the solid electrolyte need to be operated at a high temperature, and the operating temperature is, for example, about 3 in the former case.
The temperature is 00 ° C, and the latter is about 150 ° C. Therefore, it requires a heating unit and is not implemented as a portable sensor.
An object of the present invention is to provide a NOx sensor that can be used at room temperature.

[0010]

The NOx sensor of the present invention is a potentiometric type, in which a working electrode is provided on one of a pair of surfaces of a solid electrolyte body and a reference electrode is provided on the other surface. The working electrode side is exposed so as to come into contact with the sample gas, and the reference electrode side is brought into contact with the reference gas containing no NOx.

A preferred example of the solid electrolyte is silver halide, and a preferred silver halide is silver chloride, silver bromide or silver iodide. The operating temperature can be from room temperature to a temperature below the melting point of the solid electrolyte. Practically, the operation at room temperature which does not require a heating part is preferable. Even if silver halide is used for the solid electrolyte, it is not limited to a simple silver halide, but may be a mixture, a mixed crystal or a melt of silver halide and a nitrate, a nitrite or another halogen compound.

In the case of a galvanic sensor, the ionic current flowing between both electrodes through the solid electrolyte is measured,
In order to supply the ions, the material of the electrode is limited depending on the solid electrolyte. However, the NOx sensor of the present invention is a potentiometric type, and since an ionic current does not flow through the solid electrolyte body, the material of the electrode is not restricted. When silver halide is used for the solid electrolyte, the working electrode is platinum or gold, and the reference electrode is platinum, gold or silver.

[0013]

The NOx sensor of the present invention using the solid electrolyte has a sensitivity ratio of NO ₂ / NO = 1000/1 or more at room temperature, and has an overwhelmingly high sensitivity to NO ₂ . Also C
The influence of interference on O ₂ , SO ₂ , and O ₂ is also small. Therefore, NO ₂ can be selectively measured without being affected by NO in environmental measurement.

Potentiometric method (potential difference measuring method)
Is a measurement method in which a potential difference E between the working electrode potential e ₁ and the comparison electrode potential e ₂ is measured, and almost no current flows in the solid electrolyte body. Here, e ₂ has a constant potential when it is in contact with a constant composition gas such as air. However, since e ₁ becomes the potential e ₁ ′ corresponding to the NOx concentration, theoretically ΔE (= e ₁ −
By measuring e ₁ '), the concentration can be obtained from the following Nernst equation. ΔE = (2.303RT / nF) log (C ₁ / C ₂ ) where R is a gas constant, T is an absolute temperature, n is the number of charges, and F is
Is the Faraday constant, and C ₁ and C ₂ are the potentials e ₁ and e ₁ ', respectively.
Is the NOx concentration.

[0015]

EXAMPLE FIG. 1 shows an example. The sensor 1 is shown in FIG.
It is shown in (A). The solid electrolyte body 2 is formed in a disk shape having a diameter of 10 mm and a thickness of 5 mm, and a platinum mesh body of about 100 mesh is fixed to the bottom surface as a working electrode 4, and a lead wire 6 is attached to the working electrode 4. A gold wire with a diameter of 0.01 mm is fixed. Solid electrolyte body 2
A silver paste is provided as a reference electrode 10 on the upper surface of
The diameter of the lead wire 8 is 0.0 through the silver paste.
A 1 mm gold wire is crimped and fixed.

In this sensor 1, as shown in FIG. 1B, an inorganic adhesive 14 is applied to the tip of the glass tube 12 so that the reference electrode 10 side is inside the glass tube 12 and the working electrode 4 side is exposed. It is fixed airtightly. The glass tube 12 is a Pyrex glass tube having an outer diameter of 10 mm.

The silver halide solid electrolyte body 2 of the sensor 1 of FIG. 1 can be manufactured by any of the pressing method, the melting method and the plating method. An example of molding an AgCl solid electrolyte body by a pressing method is as follows. A platinum net is laid on the bottom of a mold having a solid electrolyte body-shaped recess to be molded, and AgCl powder is put on the platinum mesh body for about 1 t / t. Press at cm ² . Simultaneously with the pressing, the net body is fixed to the solid electrolyte body 2 to form the working electrode 4. The molded solid electrolyte body 2 is taken out of the mold, and a gold wire is bonded as a lead wire 8 to the other surface of the solid electrolyte body 2 by pressure bonding and silver paste to form a reference electrode 10, and the working electrode 4 is also formed. A gold wire is fixed as the lead wire 6.

A method for evaluating the sensor shown in FIG. 1B will be described. The evaluation device used was that shown in FIG. In FIG. 2, a glass tube 12 having a solid electrolyte body NOx sensor 1 fixed at its tip is set on an upper portion of a tube 16 of an evaluation device. In the tube 16 of the evaluation apparatus, a measurement section furnace heater 18 is provided in a portion where the sensor 1 is provided, and a residual heater furnace heater 20 is provided in a lower portion of the tube 16. The heaters 18 and 20 are temperature-controlled at the same temperature by a pair of temperature sensor and temperature controller 22, 24, respectively. From the bottom of the tube 16, NO ₂ or N entered the cylinder 26
An O measurement gas or an interference gas such as SO ₂ or CO ₂ and the air contained in the cylinder 28 are supplied through a gas mixer 30. The gas mixer 30 has a gas flow rate of about 0.5 liter / min, and the measurement gas mixing ratio is 0, 20, 40, 8
Adjust to 0 or 100%. The lead wire 6 from the working electrode of the sensor 1 and the lead wire 8 from the reference electrode are guided to the amplifier 32, and the electromotive force due to the sample gas is detected.
It is output to the recorder 34.

The evaluation results at room temperature (25 ° C.) are shown in FIG. When AgCl is used as the solid electrolyte body and AgI
Was evaluated for the case of using. Both are functional to NO ₂ at room temperature, and are nonfunctional to NO. Also CO ₂ ,
It was also unfunctional to SO ₂ .

FIG. 4 shows an example of the exhaust gas measuring device. As the NOx sensor 1, the one shown in FIG. 1 (A) can be used. The NOx sensor 1 is mounted with the dust removal filter 40 in contact with the working electrode side thereof,
Dust is prevented from adhering to the surface of the sensor 1.
The dust removing filter 40 is made of, for example, glass fiber,
As the exhaust gas passes through the filter 40, dust is removed. The reference electrode side of the sensor 1 is covered so as not to come into contact with the sample gas.

The sensor 1 and the filter 40 are housed in a tubular body 42, and a tip end portion of the tubular body 42 on the filter 40 side extends as a thin tube, and the tip end is opened to serve as a sampling probe 44 and an exhaust gas flue 46. Has been inserted into. The tubular body 42 is used by being fixed to the exhaust gas flue 46 with a flange 48.

A suction pump 52 is connected to the base end of the cylindrical body 42 via a flow control valve 50. An ozone supply port is provided on the upstream side of the filter 40 in the middle of the cylindrical body 42, and ozone-containing air is introduced by an ozone generator 54 at a flow rate of 1% or less of the sample flow rate. . The ozone generator 54 is a silent discharge ozone generator, and instrument air or atmospheric air is supplied to the ozone generator 54 via a flow rate control valve 56, and the oxygen in the supplied air is about 10 due to silent discharge. % Is converted to ozone. 5
Reference numeral 8 is a high voltage transformer of the ozone generator 54.

In the measuring apparatus shown in FIG. 4, the air containing about 10% ozone from the ozone generator 54 is 1% of the sample gas flow rate.
The exhaust gas is sucked from the sampling probe 44 from the suction pump 52 at a flow rate of, for example, about 10 l / min while being supplied at a flow rate of about the following. NO _{2 in} sample gas
Nitrogen oxides other than the above are oxidized by ozone to NO ₂ and the sample gas from which dust is removed through the filter 40 contacts the working electrode side of the sensor 1, and the electromotive force thereof is detected by the amplifier 32 and amplified. Is output.

FIG. 5 shows an example of a NOx measuring device which is convenient for atmospheric measurement. Also in this case, the NOx sensor 1 shown in FIG. 1A can be used. The ozone generator 60 is an ultraviolet ozone generator and is provided with a low pressure mercury lamp 61. Low-pressure mercury lamp 6
1 needs to emit light of 185 nm. A suction pump 68 is provided through a dust filter 66 in a flow path connected to the probe 64 for introducing the atmosphere, and the suction pump 6
The air sucked in 8 is guided to the ozone generator 60, and the sample gas passing through the ozone generator 60 contacts the working electrode side of the NOx sensor 1. The ultraviolet rays generated by the low-pressure mercury lamp generate ozone according to the following equation, and NO is oxidized into NO ₂ . O ₂ + UV (185 nm) → 2O O + O ₂ → O ₃ NO + O ₃ → NO ₂ + O ₂ It is convenient to use if the part surrounded by the chain line is assembled as a measuring unit.

The ultraviolet ozone generator is suitable for environmental measurement because it produces less ozone than the silent discharge type. When the ozone generator is turned on, total nitrogen oxide NOx can be measured, and when the ozone generator is turned off, only NO ₂ can be measured. Thus, the NOx of the present invention
The sensor has a selectively sensitive to NO _2, NOx measurement and NO ₂ by switching the on-off the ozone generator
It is possible to switch the measurement.

[0026]

According to the present invention, a working electrode is provided on one surface of a pair of surfaces of a solid electrolyte body, a reference electrode is provided on the other surface, and the working electrode side is brought into contact with a sample gas by a potentiometric method. Since the potential difference between both electrodes is measured, it is possible to obtain a NOx sensor which is functional to NO ₂ and nonfunctional to NO at room temperature or at low heating conditions. Since it does not need to be heated to a high temperature as in the past, it can be realized as a battery-powered NOx monitor.
It can also be used as an unmanned monitor used at sea or in the sky.

Since the potentiometric method is used for the measurement, it has the following advantages over the galvanic method. (1) Since the potential difference depends only on the interaction of the three-phase interface of the gas, the electrode, and the solid electrolyte, it can be measured at room temperature by the potentiometric method. On the other hand, in the galvanic method, the resistance is reduced by heating and the sensitivity is improved, and the resistance is large at room temperature, which makes measurement difficult. (2) With the potentiometric method, the temperature can be corrected according to the Nernst equation, so the measurement technique is easy.
However, in the galvanic method, it is difficult to correct the sensor temperature change because the resistance value changes depending on the material.

[Brief description of drawings]

FIG. 1 is a diagram showing a NOx sensor according to an embodiment,
(A) is a front sectional view, and (B) is a front sectional view showing the NOx sensor attached to a glass tube.

FIG. 2 is a schematic configuration diagram showing a NOx sensor evaluation apparatus of the same embodiment.

FIG. 3 is a diagram showing output characteristics of the NOx sensor of the same example.

FIG. 4 is a schematic configuration diagram showing an example of a NOx measuring device using the NOx sensor of the present invention.

FIG. 5 is a schematic configuration diagram showing another example of a NOx measuring device using the NOx sensor of the present invention.

[Explanation of symbols]

 1 NOx sensor 2 AgCl solid electrolyte body 4 Working electrode 6,8 Lead wire 10 Reference electrode

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takanori Uebayashi 1 Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto City, Kyoto Prefecture Shimazu Corporation Sanjo Factory

Claims (1)

[Claims] 1. A working electrode is provided on one surface of a pair of surfaces of a solid electrolyte body, and a reference electrode is provided on the other surface, and the working electrode side is exposed so as to come into contact with a sample gas,
A potentiometric NOx sensor, wherein the reference electrode side is brought into contact with a reference gas containing no NOx.