JP4009017B2 - Nitrogen oxide sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nitrogen oxide sensor for detecting a nitrogen oxide concentration.
[0002]
[Prior art]
NOx discharged from combustion equipment such as automobiles and industrial furnaces is regarded as one of the polluting gases that destroy the global environment, and its reduction is desired. For this reason, there is an increasing need for detection of NOx concentration for each generation source, and combustion control based on the NOx concentration. Further, NOx concentration, NO in the exhaust gas, Without being influenced by the concentration ratio of these nitrogen oxide gas which varies by the combustion state for the sum of the nitrogen oxides consisting of NO ₂ or the like, detects the NOx concentration It is required to do.
[0003]
As the electromotive force type sensor for detecting the NOx concentration, the present inventors have proposed JP-A-6-194605, JP-A-6-216698, JP-A-6-216699, and the like. However, in these configurations, it is difficult to detect the NOx concentration with high accuracy because of the mutual interference between NO and NO ₂ gas mixed in the test gas.
[0004]
In view of this, the present inventors have proposed a nitrogen oxide sensor having a NOx conversion pump electrode for efficiently converting NO and NO ₂ mixed in the test gas into NO gas or NO ₂ gas (Japanese Patent Application). Hei 9-329637). However, the NOx conversion pump electrode of the nitrogen oxide sensor of this configuration discharges or pumps oxygen into the can chamber when converting NO and NO ₂ mixed in the test gas into NO gas or NO ₂ gas. Therefore, the oxygen concentration fluctuation in the can chamber is large, and it is difficult to accurately detect the NOx concentration.
[0005]
[Problems to be solved by the invention]
Thus, this nitrogen oxide sensor have been proposed, it is difficult to accurately detect the NOx concentration consisting NO and NO _2.
An object of this invention is to provide the nitrogen oxide sensor which can detect a NOx density | concentration accurately by reducing the oxygen concentration fluctuation | variation in the can chamber connected to detection object atmosphere gas.
[0006]
[Means for Solving the Problems]
As shown in the figure, the nitrogen oxide sensor according to the present invention has a solid electrolyte substrate 10 and 11 having oxygen ion conductivity, a nitrogen oxide conversion electrode 1 having activity against nitrogen oxide and oxygen, and oxygen only. A sensor in which a counter electrode 2 having activity, a nitrogen oxide detection electrode 3 having activity against nitrogen oxides and oxygen, and a counter electrode 4 having activity only on oxygen are formed, and electrodes 1 to 3 are gas diffusion resistors. The body 12 has a structure formed in the same can chamber 13 made of a solid electrolyte substrate having oxygen ion conductivity communicating with the detection target gas atmosphere.
[0007]
The nitrogen oxide conversion electrode 1 and the counter electrode 2 of the present invention are formed in a can chamber 13 that communicates with the detection target atmospheric gas, and NO and NO ₂ flowing in through the gas diffusion resistor 12 are either NO single gas or NO _{2. It} has a pumping capacity sufficient to be converted into a single gas, and has means for applying a predetermined voltage or passing a current between the electrodes.
[0008]
The nitrogen oxide detection electrode 3 is formed of an electrode material that is active against nitrogen oxides and oxygen, and is disposed in a can chamber 13 that communicates with the detection target atmospheric gas. The counter electrode 4 is formed of an electrode material having activity only for oxygen. The counter electrode 4 is formed so as to be in contact with the atmosphere and is disposed at a position facing the nitrogen oxide detection electrode 3 with the solid electrolyte substrate 11 interposed therebetween. The nitrogen oxide detection electrode 3 and the counter electrode 4 are provided with means for measuring a potential difference based on the concentration change of NO or NO ₂ gas.
The gas diffusion hole is set to a size large enough for nitrogen oxides composed of NO and NO ₂ in the detected atmosphere gas to be converted into NO or NO ₂ single gas by the nitrogen oxide conversion electrode 1 and the counter electrode 2. Is done.
[0009]
The oxygen concentration control electrodes 5 and 6 for controlling the oxygen concentration in the can chamber are formed at a position where one electrode is in contact with the measurement atmosphere, and the other electrode 5 is disposed so as to be in contact with the atmosphere. The oxygen control pump electrodes 5 and 6 are active only with respect to oxygen, and are formed of an electrode material having a pumping capacity sufficient to make the oxygen concentration in the test gas constant. Are provided with means for applying a predetermined voltage or passing a current.
[0010]
The oxygen concentration detection sensor electrode 7 is formed in the can chamber 13 and is formed at a position facing the counter electrode 4 provided at a position in contact with the atmosphere with the solid electrolyte substrate 11 interposed therebetween. The oxygen concentration in the can chamber 13 is measured by a potential difference between the nitrogen oxide detection electrode 3 and the counter electrode 4 generated based on a change in the oxygen concentration. The nitrogen oxide detection electrode 3 includes means for measuring a potential difference based on a change in oxygen concentration.
[0011]
In the configuration of the present invention, the nitrogen oxide conversion electrode 1 and the counter electrode 2 are formed in the same can chamber 13 communicating with the detection target gas atmosphere. The inside of the can chamber 13 is controlled by the oxygen concentration control pump electrodes 5 and 6 so that the oxygen concentration becomes constant. The potential difference generated between the nitrogen oxide detection electrode 3 and the counter electrode 4 of the nitrogen oxide sensor of the present invention is generated by a mixed potential of a potential difference generated based on a change in oxygen concentration and a potential difference generated based on a change in NO and NO ₂ concentrations. Therefore, the potential difference generated based on the NO and NO ₂ concentrations changes as the oxygen concentration in the can chamber fluctuates.
[0012]
In the nitrogen oxide sensor of this configuration, since the nitrogen oxide conversion electrode 1 and the counter electrode 2 are formed in the same can chamber 13, even when a voltage is applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2. The variation of the oxygen concentration in the can chamber 13 is small, and the NO and NO ₂ concentrations can be detected with high accuracy.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the nitrogen oxide sensor of the present invention will be described with reference to the drawings. FIG. 1 shows a basic configuration of a nitrogen oxide sensor of the present invention.
The nitrogen oxide sensor of the present invention includes a solid electrolyte substrate 10 and 11 having oxygen ion conductivity, a nitrogen oxide conversion electrode 1 having activity against nitrogen oxide and oxygen, a counter electrode 2 having activity only against oxygen, An oxygen nitrogen oxide, a nitrogen oxide detection electrode 3 having activity with respect to oxygen, and a counter electrode 4 having activity only with oxygen are formed. Further, a gas diffusion resistor 12 for restricting the flow of the test gas is formed at the test gas introduction port 14. The electrodes 1 to 3 are formed in the same can chamber 13 composed of the solid electrolyte substrates 10 and 11 having oxygen ion conductivity communicating with the detection target gas atmosphere by the gas diffusion resistor 12, and the counter electrode 4 is in contact with the atmosphere. And is disposed at a position facing the nitrogen oxide detection electrode 3 with the solid electrolyte substrate 11 therebetween.
[0014]
The ion conductors are oxygen ion conductors 10 and 11, and a solid electrolyte substrate or bismuth oxide or the like obtained by adding a stabilizer to an oxide such as zirconium oxide, hafnium oxide, thorium oxide, or cerium oxide can be applied. From the viewpoint of thermal stability and chemical stability, it is preferable to use stabilized zirconia using a stabilizer such as yttrium oxide, magnesium oxide, calcium oxide or the like.
[0015]
The gas diffusion resistor 12 is large enough to convert nitrogen oxide composed of NO and NO ₂ in the detected atmospheric gas into a single gas of NO or NO ₂ by the nitrogen oxide conversion electrode 1 and the counter electrode 2. Needs to be set to The gas diffusion / diffusion hole 14 may be formed with one fine hole or may be a porous body composed of a plurality of holes.
[0016]
The nitrogen oxide conversion electrode 1 and the nitrogen oxide detection electrode 3 are Pt, Rh, Ir, Pd, Ru, Au, Ag, Cr, Ni, Mn, Fe, Cu, active against oxygen, NO, and NO ₂ . Of W, Zn, Sn, Re, Mo, or the like, an alloy or a simple composition containing at least one element, a noble metal-oxide cermet, a transition metal oxide, or the like is used.
[0017]
The counter electrode 2 and the counter electrode 4 are formed of an electrode material such as a noble metal such as Pt, Rh, Ir, or Pd having an activity on oxygen, an alloy thereof, or a cermet of a noble metal and a metal oxide. The electrode material is preferably a Pt-based material from the viewpoint of thermal stability and chemical stability.
[0018]
The nitrogen oxide sensor of the present invention preferably has a structure including oxygen concentration control pumps 5 and 6 and an oxygen concentration detection sensor electrode 7 as shown in FIG. This is because by providing these electrodes, it is possible to detect the oxygen concentration in the can chamber 13 and control the oxygen concentration.
The oxygen concentration control pumps 5 and 6 and the oxygen concentration detection sensor electrode 7 are made of electrode materials such as Pt, Rh, Ir, Pd and other precious metals having an activity on oxygen, and alloys thereof, and cermets of precious metals and metal oxides. Formed with. The electrode material is preferably a Pt-based material from the viewpoint of thermal stability and chemical stability.
The structures shown in FIGS. 2 to 4 are also within the scope of the idea of the nitrogen oxide sensor of the present invention.
[0019]
In the example shown in FIG. 3, the oxygen concentration control electrode 6 is positioned in the can chamber 13, the nitrogen oxide conversion electrode 1 and the counter electrode 2 are opposed to each other via the nitrogen oxide detection electrode 3 and the can chamber 13 ′, and the counter electrode 4. Is disposed outside the can chamber 13 '. Since the oxygen ion conductor that separates the can chamber 13 and the can chamber 13 'is porous (diffusion resistor) at the portion of the nitrogen oxide conversion electrode 1, the test gas passes through the electrode 1 and NO. The conversion efficiency into NO ₂ gas can be improved.
In the example shown in FIG. 4, an alumina insulating layer 8, a counter electrode 2, a zirconia layer 9 and a nitrogen oxide conversion electrode 1 are laminated on a nitrogen oxide detection electrode 3 as shown in the figure.
[0020]
Since the nitrogen oxide sensor of the present invention uses a solid electrolyte substrate that exhibits oxygen ion conductivity at a high temperature, it is necessary to heat the sensor to a predetermined temperature. As a heating means, a self-heating device may be added and heated to a predetermined temperature, or may be heated in high-temperature exhaust gas.
[0021]
In the nitrogen oxide sensor of the present invention, the detection target atmospheric gas flows into the can chamber 13 through the front chamber diffusion resistor 12. The nitrogen oxide gas composed of NO and NO ₂ flowing into the can chamber 13 is oxidized or reduced by applying a voltage between the nitrogen oxide conversion electrode 1 and the counter electrode 2 to be converted into NO single gas or NO ₂ single gas. The The oxygen concentration in the can chamber 13 is controlled by the oxygen concentration control pump electrodes 5 and 6 and the oxygen concentration detection sensor 7. In the nitrogen oxide sensor of the present invention, when a voltage is applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2, oxygen pumping is performed only in a can chamber controlled to a predetermined oxygen concentration. Therefore, oxygen does not flow from the atmospheric air or the like, and even if a voltage is applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2, the oxygen concentration in the can chamber hardly fluctuates, and the atmosphere gas to be detected NO and NO ₂ concentrations can be accurately detected.
[0022]
【Example】
Example 1
An embodiment of the nitrogen oxide sensor of the present invention will be described below with reference to FIG.
A 6 mol% yttria-stabilized zirconia green sheet (thickness: 200 μm) produced by the doctor blade method was cut into a width of 10 mm and a length of 80 mm. At the time of cutting, the zirconia green sheets 15 and 16 were provided with open ports 17 and 18 for introducing air. Further, the green sheet 12 was formed with a diffusion hole 14 and a can chamber 13 for restricting the introduction of the detection target gas.
Further, a Pt / Rh electrode (nitrogen oxide conversion electrode 1) and a Pt electrode (counter electrode 2) were formed on the surface of the can chamber 13 of the sheet 10 by a screen printing method. Further, a Pt / Rh electrode (nitrogen oxide detection electrode 3) and a Pt electrode (oxygen concentration detection sensor electrode 7) are formed on the surface of the can chamber of the sheet 11 by a screen printing method, and a Pt electrode is formed on the open surface of the atmosphere. (Counter electrode 4) was formed.
[0023]
Next, after attaching a Pt ribbon wire having a width of 1 mm, a thickness of 0.05 mm and a length of 10 mm to each electrode, zirconia green sheets were superposed in the order of 19 to 20. When the zirconia green sheets were overlapped, a 200 μm-thick theobromine sheet 21 was inserted into the openings 17 and 18 and the diffusion holes 14 and the can chamber 13 to maintain the sheet shape.
The laminated molded body was degreased at 600 ° C. for 5 hours (temperature raising / lowering rate 2 ° C./min). The sample after degreasing was calcined in an atmosphere of N ₂ /5% O ₂ at 1400 ° C. for 5 hours (temperature raising / lowering rate 2 ° C./min) to prepare a sample. Tubular furnace so that the sample gas into the test gas inlet and the atmosphere inlet can be filled with NO50 ppm / O ₂ 5%, NO ₂ 50 ppm / O ₂ 5%, 25 ppm NO + 25 ppm NO ₂ /5% O ₂ And heated to 600 ° C.
[0024]
At the time of measurement, a direct current voltage of 0.5 V was applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2 so that the nitrogen oxide conversion electrode 1 was an anode and the counter electrode 2 was a cathode. The electromotive force generated between the detection electrode 3 and the counter electrode 4 was measured. The oxygen concentration in the can chamber was calculated from the Nernst equation by measuring the electromotive force generated between the oxygen concentration detection sensor electrode 7 and the counter electrode 4.
Further, as a comparative example, a sample in which the counter electrode 2 was printed on the air release surface of the zirconia green sheet 10 was produced, and the same measurement as in the example was performed.
[0025]
FIG. 6 shows a response curve when gas of 50 ppm NO, 50 ppm NO ₂ , 25 ppm NO + 25 ppm NO ₂ is introduced into the can chamber 13. When any gas was introduced, the electromotive force responded in the positive direction, the electromotive force values were almost the same, and it was confirmed that NO gas was sufficiently converted to NO ₂ gas. Moreover, compared with the comparative example, the electromotive force between the nitrogen oxide detection electrode 3 and the counter electrode 4 was significantly increased.
[0026]
In FIG. 7, a DC voltage of 0.1 to 0.5 V was applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2 so that the nitrogen oxide conversion electrode 1 was an anode and the counter electrode 2 was a cathode. The relationship between the applied voltage and the oxygen concentration obtained from the electromotive force generated between the oxygen concentration detection sensor electrode 7 and the counter electrode 4 and the electromotive force value between the nitrogen oxide detection electrode 3 and the counter electrode 4 is shown. At this time, the test gas was NO ₂ 50 ppm, the oxygen concentration was 5%, and the sensor element temperature was 600 ° C. In the nitrogen oxide sensor of the present invention, the NO ₂ electromotive force increased as the applied voltage between the nitrogen oxide conversion electrode 1 and the counter electrode 2 was increased. Further, even when the applied voltage was increased, the oxygen concentration in the can chamber hardly changed.
On the other hand, the NO ₂ electromotive force of the nitrogen oxide sensor of the comparative example was smaller than that of the example, and the applied voltage was 0.3 V or more and the NO ₂ electromotive force was reduced. Further, the oxygen concentration in the can chamber increased as the applied voltage was increased, and was 65% at an applied voltage of 0.5V.
[0027]
(Example 2)
FIG. 8 is a structural diagram of the nitrogen oxide sensor of Example 2. The zirconia green sheet was cut in the same manner as in Example 1. Next, similarly to Example 1, oxygen concentration control pump electrodes 5 and 6, a nitrogen oxide detection electrode 3, a counter electrode 4, and an oxygen concentration detection sensor electrode 7 were formed. Further, an alumina insulating layer 8, a counter electrode 2, a 6 mol% yttria stabilized zirconia layer 9, and a nitrogen oxide conversion electrode 1 were sequentially laminated on the nitrogen oxide detection electrode 3 by a screen printing method. The printed green sheet was laminated, degreased and fired in the same manner as in Example 1 to produce a nitrogen oxide sensor. The produced nitrogen oxide sensor was evaluated in the same manner as in Example 1.
[0028]
FIG. 9 shows a response curve when 50 ppm NO, 50 ppm NO ₂ , 25 ppm NO + 25 ppm NO ₂ gas is introduced into the can chamber. When any gas was introduced, the electromotive force responded in the positive direction, the electromotive force values were almost the same, and it was confirmed that NO gas was sufficiently converted to NO ₂ gas. Moreover, compared with the comparative example, the electromotive force between the nitrogen oxide detection electrode 3 and the counter electrode 4 was significantly increased, and the same result as in Example 1 was obtained.
[0029]
In FIG. 10, a DC voltage of 0.1 to 0.5 V was applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2 so that the nitrogen oxide conversion electrode 1 was an anode and the counter electrode 2 was a cathode. The relationship between the applied voltage and the oxygen concentration obtained from the electromotive force generated between the oxygen concentration detection sensor electrode 7 and the counter electrode 4 and the electromotive force value between the nitrogen oxide detection electrode 3 and the counter electrode 4 is shown. In the nitrogen oxide sensor of this example, the oxygen concentration change caused by applying a voltage between the nitrogen oxide conversion electrode 1 and the counter electrode 2 was larger than that in Example 1, but the change was more than that in the comparative example. Was much smaller. Further, it was confirmed that the NO ₂ electromotive force of the nitrogen oxide sensor of this example was larger than that of the comparative example, and the same tendency as in Example 1 was observed.
[0030]
In the nitrogen oxide sensor of the present invention, when a voltage is applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2, oxygen pumping is performed only in a can chamber controlled to a predetermined oxygen concentration. Therefore, oxygen does not flow from the atmospheric air or the like, and even if a voltage is applied between the nitrogen oxide conversion electrode 1 and the counter electrode 2, the oxygen concentration in the can chamber hardly fluctuates, and the atmosphere gas to be detected NO and NO ₂ concentrations can be accurately detected.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a nitrogen oxide sensor of the present invention.
FIG. 2 is a view showing a cross section of the nitrogen oxide sensor of the present invention.
FIG. 3 is a cross-sectional view of the nitrogen oxide sensor of the present invention.
FIG. 4 is a view showing a cross section of the nitrogen oxide sensor of the present invention.
FIG. 5 is a view showing a cross section of the nitrogen oxide sensor of the example.
FIG. 6 is a diagram showing a NOx response curve at 600 ° C. when 50 ppm NO, 50 ppm NO ₂ , 25 ppm NO + 25 ppm NO ₂ gas is introduced into the can chamber of the nitrogen oxide sensor of the present invention.
FIG. 7 is a graph showing the relationship between the voltage applied to the nitrogen oxide conversion electrode, the NO 2 sensitivity, and the oxygen concentration in the can of the nitrogen oxide sensor of the present invention.
FIG. 8 is a view showing a cross section of the nitrogen oxide sensor of the example.
FIG. 9 is a graph showing a NOx response curve at 600 ° C. when 50 ppm NO, 50 ppm NO ₂ , 25 ppm NO + 25 ppm NO ₂ gas is introduced into the can chamber of the nitrogen oxide sensor of the present invention.
FIG. 10 is a graph showing the relationship between the voltage applied to the nitrogen oxide conversion electrode, the NO 2 sensitivity, and the oxygen concentration in the can of the nitrogen oxide sensor of the present invention.
[Explanation of symbols]
1, 3, 5, 6, 7 Electrode 2, 4 Counter electrode 10, 11 Solid electrolyte substrate 12 Diffusion resistor 13 Can chamber

Claims (1)

A solid oxide substrate having oxygen ion conductivity, a nitrogen oxide conversion electrode having activity against nitrogen oxide and oxygen ( 1 ) , a counter electrode of the nitrogen oxide conversion electrode having activity only against oxygen ( 2 ) , nitrogen oxidation nitrogen oxide sensing electrode active against objects and oxygen (3), and only one having an active oxygen, nitrogen oxide sensing electrode that will be located in a separate room leading to the atmosphere which is shielded from the test gas counter ( 4 ) is a nitrogen oxide sensor formed, which is connected to a gas atmosphere to be detected through a gas diffusion resistor and is converted into nitrogen oxide in the same can chamber made of a solid electrolyte substrate having oxygen ion conductivity. electrodes (1), a counter electrode of the nitrogen oxides conversion electrode (2), and nitrogen oxide sensors which nitrogen oxide sensing electrode (3) is characterized that you have made form.

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