WO2010010978A1

WO2010010978A1 - Nox sensor

Info

Publication number: WO2010010978A1
Application number: PCT/KR2008/004296
Authority: WO
Inventors: Jinsu Park; Jae Hoon Hwang
Original assignee: Cios Inc.
Priority date: 2008-07-23
Filing date: 2008-07-23
Publication date: 2010-01-28

Abstract

Disclosed is a NOx sensor which gives an output of the same direction with respect to NO and NO2 with the same sensitivity, thereby precisely measuring total NOx concentration, and also which can be reacted with only NO2 according to a forming method of an oxide sensing electrode. The NOx sensor comprises an oxygen ion conductive solid electrolyte 10; one or more oxide sensing electrodes 20 formed at the oxygen ion conductive solid electrolyte 10; a first noble metal electrode 30 formed at the oxide sensing electrode 20; a second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10; and an electromotive force lead line 50 for the electrical connection of the oxide sensing electrode 20 and the second noble metal electrode 40. The oxide sensing electrode 20 and the second noble metal electrode 40 are connected in parallel to each other through the first metal electrode 30.

Description

[DESCRIPTION]

[invention Title] NOX SENSOR

[Technical Field]

The present invention relates to a NO_x sensor, and more particularly, to a NO_x sensor which adapts output directions of sensor signals with respect for NO and NO₂ to be identical with each other thereby precisely measuring total NO_x concentration, and also which can react with only NO₂ according to a forming method of an oxide sensing electrode.

[Background Art]

When nitrogen contained in combustion air and fuel is combined with oxygen by influence of temperature and the like, nitrogen oxide is generated, and the nitrogen oxide including NO, NO₂ and N₂O₃ is expressed as NO_x.

Particularly, the NO₂ is a maroon colored poisonous gas having a pungent smell. The NO and NO₂ accounting for a major percent of the whole NO_x are mainly generated from a transportation means and becomes a cause of air pollution.

Therefore, there is an increased necessity of measuring NO_x concentration.

There has been proposed various types of NO_x sensors for measuring the NO_x concentration. First of all, an equilibrium potential type NO_x sensor will be described.

In the equilibrium potential type NO_x sensor, an electrochemical cell is composed of a sensing electrode and a noble metal electrode on a solid electrolyte. A sensing electrode is formed from solid nitrate in which nitrogen oxide is contained and a noble metal electrode is formed from a material which allows activity of conductive ions in the solid electrolyte to be constant. NO_x concentration of equilibrium potential type NO_x sensor is measured by using electromotive force generated from the electrochemical cell.

However, since the sensing electrode of the equilibrium potential type NO_x sensor has a low melting point, there is problem that it is limited to operate in a high gas temperature. Substantially, Ba (NO₃ )₂ used for forming the sensing electrode of the equilibrium potential type NO_x sensor has a melting point of 592 ^°C .

Secondly, as shown in Fig. 1, there has been proposed amperometric type NO_x sensor. In the amperometric type NO_x sensor, all of nitrogen oxides (NO_x) are converted into NO by means of an oxygen pumping cell 150, and the amperometric type NO_x sensor measures the currents generated by oxygen ions O^2" obtained from the decomposition of the NO into N₂ and O₂ and thus estimate the NO_x concentration.

The atnperometric type NO_x sensor is comprised of an oxygen pumping cell formed at an upper stream, at which exhaust gas is introduced, so as to convert the NO₂ into the NO, and a measuring cell for resolving the NO into nitrogen and oxygen and measuring the currents induced from the resolved oxygen .

In the amperometric type NO_x sensor, if the whole amount of the NO₂ can be converted into the NO by the oxygen pumping cell, it is possible to obtain the currents proportional to the whole amount of the NO_x. However, in measuring the currents by the oxygen ions, variation due to the temperature is high, and also since the measured currents become so small in the condition that the concentration is a few hundred ppm or less, it is difficult to measure the total NO_x concentration.

Thirdly, as an another conventional NO_x sensor for measuring the NO_x concentration, there has been proposed a mixed potential type NO_x sensor as shown in Fig. 2.

Referring to Fig. 2, the mixed potential type NO_x sensor includes an oxygen ion conductor 110 using stabilized zirconia,- an oxide sensing electrode 120 formed at one side of the oxygen ion conductor 110; a first noble metal electrode 130 formed at the oxide sensing electrode 120; and a second noble metal reference electrode 140. And the mixed potential type NO_x sensor is characterized by measuring electromotive force between both ends of the first noble metal electrode 130 and the second noble metal reference electrode 140.

In the mixed potential type NO_x sensor, the oxide sensing electrode 120 has reactivity with both NO_x and oxygen, while the noble metal reference electrode 140 has reactivity only with oxygen. Thus the voltages difference between the noble metal reference electrode 140 and the oxide sensing electrode

120 is proportional to the NO_x concentration contained in the gas. Therefore, if a difference of electromotive force is measured, a NO_x amount can be measured.

That is, in the mixed potential type NO_x sensor, if the

NO₂ is existed, reactions take place at the oxide electrode 120 according to the following formulas (1) and (2) , and if the NO is existed, reactions take place at the oxide electrode 120 according to following formulas (3) and (4) :

In case of NO₂ : NO₂ + 2e^~ → NO + 0^2" (1)

O^2" → 1/2O₂ + 2e^" (2)

In case of NO : NO + O^{2 "} → NO₂ + 2e^~ ( 3 ) 1/2O₂ + 2e^~ → 0^2" ( 4 )

As represented by the formulas (1) to (4) , a sign of the electromotive force generated between the noble metal electrode and the sensing electrode in the presence of the NO presence is contrary to that of the electromotive force generated between the noble metal electrode and the sensing electrode in the presence of the NO₂. Therefore, in case that the NO and NO₂ are mixed together like in the exhaust gas, the NO_x sensor using a mixed potential principle has a disadvantage that it is difficult to measure the total NO_x concentration due to the characteristic that the electromotive forces tend to move in opposite direction for NO and NO₂ exposure.

Fig. 3 is a graph showing the electromotive force in case that NiO as the sensing electrode is formed in the mixed potential type NO_x sensor, and Fig. 4 is a graph showing the electromotive force in case that CuO as the sensing electrode is formed in the mixed potential type NO_x sensor. With reference to Figs . 3 and 4 , it can be understood that accuracy of the NO_x sensor is deteriorated due to the reduction of the electromotive force caused by the increase in NO concentration, although the NO₂ concentration is constantly maintained or increased. It means that the simple mixed potential type NO_x sensor can not be used in the mixed gas of the NO and NO₂.

In actuality, in case that the NiO is used as the sensing electrode, as shown in Fig. 3, if the NO concentration is changed from lOppm to lOOppm, a change in the electromotive force amounts to -6.5mV, and if the NO₂ concentration is changed from lOppm to lOOppm, the amount of change in the electromotive force is 83.7mV. As shown in Fig. 4, in case that the CuO is used as the sensing electrode, if the NO concentration is changed from lOppm to lOOppm, a change in the electromotive force amounts to -3.4mV, and if the NO₂ concentration is changed from lOppm to lOOppm, the amount of change in the electromotive force is 66.OmV.

As described above, there is a problem that, although the NO has a high concentration, the conventional mixed potential type NO_x sensor can not sense it.

To solve the above problem, there has been proposed a multi-layer structure, in which a conversion cell 160 is provided at an inlet port through which measuring gas is introduced so that the NO_x is converted to a single gas and thus unified into NO or NO₂ so as to measure the total NO_x concentration, as shown in Fig. 5. In the above-mentioned method, the unifying process for converting the NO₂ to the NO or converting the NO to the NO₂ should be performed. However, since the conversion cell 160 has a limit to convert the whole mixture gas to the NO or NO₂, it is difficult to precisely measure the total NO_x concentration.

Meanwhile, in an apparatus 200 of reducing nitrogen oxide gas by injecting a catalytic material like urea, it is essential to precisely measure the NO_x concentration. Here the selectively catalytic reduction material like urea directly reacts with NOx in the exhaust to reduce the nitrogen oxide into nitrogen and oxygen.

Further, in the apparatus for reducing nitrogen oxide gas, since the injection amount of the catalytic material is changed according to the amount of NO and NO₂ concentration in the exhaust, it is necessary to exactly measure the individual concentrations of NO and NO₂. However, in the conventional NO_x sensor, it is not possible to separately calculate the NO and NO₂ concentration.

[Disclosure] [Technical Problem]

The object of the present invention is to provide a NO_x sensor which gives an output of the same direction with respect to NO and NO₂ with the same sensitivity, thereby precisely measuring total NO_x concentration in a gaseous environment of high temperature and low concentration, and also which can be reacted with only NO₂ thereby precisely and stably measuring the NO₂ concentration. It is another object of the present invention to separately calculate NO and NO₂ concentration using two or more NO_x sensor, so that an exact amount of catalytic material can be injected. [Technical Solution]

To achieve the objects, the present invention provides a NO_x sensor, comprising an oxygen ion conductive solid electrolyte 10; one or more oxide sensing electrodes 20 formed at the oxygen ion conductive solid electrolyte 10; a first noble metal electrode 30 formed at the oxide sensing electrode 20; a second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10; and an electromotive force lead line 50 for electrically connecting the oxide sensing electrode 20 and the second noble metal electrode 40, wherein the oxide sensing electrode 20 and the second noble metal electrode 40 are connected in parallel to each other.

Preferably, the oxide sensing electrode 20 and the second noble metal electrode 40 connected in parallel to each other are electrically connected with other oxide sensing electrode 20 and second noble metal electrode 40 connected in parallel to each other, and the total NO_x or NO₂ concentration is obtained by measuring the voltages between two electrical nodes when constant currents is applied. Preferably, the NO₂ concentration is measured, if the oxide sensing electrodes 20 are formed from the same materials, and total NO_x concentration is measured, if the oxide sensing electrodes 20 are formed of different materials.

Preferably, the oxide sensing electrodes 20 are formed at both upper and lower surfaces of the oxygen ion conductive solid electrolyte 10, and two or more oxide sensing electrodes 20 can be formed at one surface of the oxygen ion conductive solid electrolyte 10. Preferably, the oxygen ion conductive solid electrolyte 10 is formed from one of the selected from ; stabilized zirconia, CeO₂ or ThO₂, and the oxide sensing electrode 20 is formed from one or more oxides selected from NiO, CuO, NiO-YSZ, LaCoO₃ or 2CuO -Cr₂O₃, and the first and second noble metal electrodes 30 and 40 are formed from platinum or gold.

[Advantageous Effects]

Therefore, according to the NO_x sensor of this invention, the directions of sensor signals with respect for NO and NO₂ is adapted to be identical with the same sensitivity, thereby precisely measuring the total NO_x concentration even in an environment of mixture gas of NO and NO₂ and also the NO_x sensor can be made to react with only NO₂ thereby precisely and stably measuring the NO₂ concentration. Further, according to the present invention, it can separately calculate NO and NO₂ concentration using two or more NO_x sensor, so that an exact amount of catalytic material can be injected to selectively reduce the NO_x in the exhaust. [Description of Drawings]

The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic view of a conventional amperometric type NO_x sensor.

Fig. 2 is a schematic view of a conventional mixed potential type NO_x sensor. Fig. 3 is a graph showing electromotive force in case that NiO as an oxide sensing electrode is formed for the mixed potential type NO_x sensor of Fig. 2.

Fig. 4 is a graph showing the electromotive force in case that CuO as the oxide sensing electrode is formed for mixed potential type the NO_x sensor of Fig. 2.

Fig. 5 is a schematic view of another conventional mixed potential type NO_x sensor.

Fig. 6 is a schematic view of a typical NO_x sensor according to the present invention. Fig. 7 is a graph of voltages in case that a positive oxide sensing electrode is formed from NiO and a negative oxide sensing electrode is formed from CuO in the NO_x sensor of Fig. 6.

Fig. 8 is a schematic view of another NO_x sensor according to the present invention where two oxide electrodes are made of the same material .

Fig. 9 is a graph showing electromotive force in case that NiO as an oxide sensing electrode is formed for the NO_x sensor of Fig. 8.

Fig. 10 is a schematic view of a NO_x sensor in which an oxide sensing electrode is not connected with a noble metal electrode in parallel.

Fig. 11 is a graph of voltages in case that NiO as an oxide sensing electrode is formed for the NO_x sensor of Fig. 10.

[Detailed Description of Main Elements] 100: NO_x sensor

10: oxygen ion conductive solid electrolyte 20, 20-1, 20-2: oxide sensing electrode 30: first noble metal electrode 40: second noble metal electrode 50: electromotive force lead line

[Best Mode]

Hereinafter, the embodiments of the present invention will be described in detail with reference to accompanying drawings .

A NO_x sensor 100 of the present invention includes an oxygen ion conductive solid electrolyte 10; one or more oxide sensing electrodes 20 formed at the oxygen ion conductive solid electrolyte 10; a first noble metal electrode 30 formed at the oxide sensing electrode 20; a second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10; and an electromotive force lead line 50 for the electrical connection of the oxide sensing electrode 20 and the second noble metal electrode 40. The oxide sensing electrode 20 and the second noble metal electrode 40 are connected in parallel to each other.

In the NO_x sensor 100 of the present invention, the oxide sensing electrode 20 and the second noble metal electrode 40 connected in parallel to each other are electrically connected with other oxide sensing electrode 20 and second noble metal electrode 40 connected in parallel to each other.

And, in the NO_x sensor 100, the total NOx and NO₂ concentration is measured from the voltages measured when constant given currents is applied.

In the total NO_x sensor 100 of the present invention, since the oxide sensing electrode 20 and the second noble metal electrode 40 are connected in parallel to each other, and thus direction and magnitude of voltages change upon exposure to both NO and NO₂ is identical, it is possible to solve the problem of measurement error caused by the mixed potential type NOx sensor where two kinds of gases in a mixed gas of the NO and NO₂ have a different directional response from each other.

The first noble metal electrode 30 formed at the oxide sensing electrode 20 and the second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10 can be formed from the same materials, and the first noble metal electrode 30 formed at the oxide sensing electrode 20 can be electrically connected with the second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10.

The oxide sensing electrode 20 and the second noble metal electrode 40 can be a catalytic electrode for the oxygen ion, and thus they are indicated by different numerical references.

The oxygen ion conductive solid electrolyte 10 is formed from one of the component selected from ; stabilized zirconia, CeO₂ or ThO₂, and the oxide sensing electrode 20 is formed from one or more oxides selected from NiO, CuO, NiO-YSZ, LaCoO₃ or 2CuO -Cr₂O₃.

The NO_x sensor 100 of the present invention can be formed into various types in terms of structures and materials. The total NOx or NO₂ concentration can be measured according to a forming method of an oxide sensing electrode, as will be described below.

Fig. 6 is one of the schematic pictures of a total NO_x sensor 100 according to the present invention. In the NO_x sensor 100 of Fig. 6, the oxide sensing electrodes 20 are formed at both upper and lower surfaces of the oxygen ion conductive solid electrolyte 10, and the oxide sensing electrodes 20 are formed from different materials 20-1 and 20- 2, respectively.

As shown in Fig. 6, in case that the oxide sensing electrodes 20-1 and 20-2 are respectively formed from different materials, the NO_x sensor 100 of the present invention can measures the total NO_x concentration.

Fig. 7 is a graph of voltage responses from the NO_x sensor of Fig. 6, wherein the positive oxide sensing electrode 20-1 is formed from NiO, and the negative oxide sensing electrode 20-2 is formed from CuO, and the oxygen ion conductive solid electrolyte 10 is formed from stabilized zirconia, and the first and second noble metal electrodes 30 and 40 are formed from platinum, and constant currents of 2μA is applied between two nodes of electrical wire 50 at a temperature of 700 ^°C and an oxygen partial pressure of 10%. As shown in Fig. 7, it can be seen that output values from the NOx sensor of the present invention are proportional to the total NO_x concentration.

Fig. 8 is a schematic view of another example of NO_x sensor according to the present invention, wherein the oxide sensing electrodes 20 are formed from the same materials and also formed at both upper and lower surfaces of the oxygen ion conductive solid electrolyte 10.

Fig. 9 is a graph showing the electromotive force from the NO_x sensor 100 of Fig. 8, wherein the oxide sensing electrodes 20 are formed from NiO, and the oxygen ion conductive solid electrolyte 10 is formed from stabilized zirconia, and the first and second noble metal electrodes 30 and 40 are formed from platinum, and constant currents of 2μA is applied at a temperature of 700 "C and an oxygen partial pressure of 10%.

As shown in Fig. 9, in the NO_x sensor 100 of the present invention, if the oxide sensing electrodes 20 are formed from the same materials, the voltage responses are caused only by the NO₂ concentration regardless of its NO concentration.

In other words, in the NO_x sensor 100 of the present invention, if the oxide sensing electrodes 20 are formed from the same materials, the NO₂ concentration in the gas mixture can be is measured, and if the oxide sensing electrodes 20-1 and 20-2 are formed from the different materials, the total NO_x concentration can be measured if we select appropriate amount of currents between two electrical nodes.

And as shown in Figs. 6 and 8, the oxide sensing electrodes 20 may be formed at both upper and lower surfaces of the oxygen ion conductive solid electrolyte 10, and also two or more oxide sensing electrodes 20 may be formed at one surface of the oxygen ion conductive solid electrolyte 10.

Fig. 10 is a schematic view of a NO_x sensor which does not have the parallel connection of the second noble metal electrode 40 for the comparison with the NO_x sensor 100 of the present invention. Unlike the NO_x sensor of the present invention, in the NO_x sensor of Fig. 10, the oxide sensing electrode 20 is not connected with the second noble metal electrode 40 in parallel, and only the oxide sensing electrode 20 is connected with a lead line 240.

Fig. 11 is a graph of voltage response from the NO_x sensor of Fig. 10 wherein the oxide sensing electrode 220 is formed from NiO, and the oxygen ion conductive solid electrolyte 210 is formed from stabilized zirconia, and the noble metal electrode 230 is formed from platinum, and constant currents of lOμA is applied at a temperature of 700 ^°C and an oxygen partial pressure of 10%. It can be seen that, when the NO_x does not exist, background voltages does not maintain constant to decrease continuously, which gives rise to a lack of accuracy.

However, in the NO_x sensor of the present invention (referring to Fig. 6) , when the oxide sensing electrode 20 is connected with the second noble metal electrode 40 in parallel, the background voltages maintain constant even though NOx in the gas is not present, and thus the accuracy can be promoted.

As described above, the NO_x sensor of the present invention can be applied to various fields. In more detail, the NO_x sensor of the present invention can be applied to an apparatus 200 for reducing nitrogen oxide so as to detect the NO_x and inject a catalytic material, thereby reducing the nitrogen oxide in exhaust gas.

In the apparatus for reducing nitrogen oxide, since each amount of catalytic material for removing the NO and the NO₂ is different from each other, there is a problem that it is difficult to inject the exact amount of catalytic material, even though the total NO_x concentration is measured. Therefore, the apparatus for reducing nitrogen oxide of the present invention is provided with two kinds of NO_x sensors 100, so that one NO_x sensor measures the NO₂ and the other NO_x sensor measures the total NO_x. Then, the measured NO₂ concentration is subtracted from the total NO_x concentration in order to obtain the NO concentration. Thus, it is possible to precisely control the amount of catalytic material to be injected.

Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.

[industrial Applicability]

According to the NO_x sensor of the present invention, as described above, the directions of sensor signals with respect to NO and NO₂ is adapted to be identical with each other, thereby precisely measuring total NO_x concentration in the gas even in an adverse environment of high temperature and low concentration, and also the NO_x sensor can be made to react with only NO₂, thereby precisely and stably measuring the NO₂ concentration only. Further, an apparatus 200 for reducing nitrogen oxide according to the present invention can separately calculate NO and NO₂ concentration using two or more NO_x sensor, so that an exact amount of catalytic material can be injected.

Claims

[CLAIMS]

[Claim l]

A NO_x sensor, comprising of: an oxygen ion conductive solid electrolyte 10; one or more oxide sensing electrodes 20 formed at the oxygen ion conductive solid electrolyte 10; a first noble metal electrode 30 formed at the oxide sensing electrode 20; a second noble metal electrode 40 formed at the oxygen ion conductive solid electrolyte 10; and an electromotive force lead line 50 for the electrical connection with the oxide sensing electrode 20 and the second noble metal electrode 40, wherein the oxide sensing electrode 20 and the second noble metal electrode 40 are connected in parallel to each other.

[Claim 2]

The NO_x sensor according to claim 1, wherein the oxide sensing electrode 20 and the second noble metal electrode 40 connected in parallel to each other are electrically connected with other oxide sensing electrode 20 and second noble metal electrode 40 connected in parallel to each other, and NO_x or NO₂ concentration is measured by voltages measured when constant currents is applied.

[Claim 3 ]

The NO_x sensor according to claim 2, wherein the oxide sensing electrodes 20 are formed from the same material to measure the NO₂ concentration only.

[Claim 4]

The NO_x sensor according to claim 2, wherein the oxide sensing electrodes 20 are formed from the different material to measure the total NOx concentration in the mixture gas .

[Claim 5] The NO_x sensor according to claim 2, wherein the oxide sensing electrodes 20 are formed at both upper and lower surfaces of the oxygen ion conductive solid electrolyte 10.

[Claim 6]

The NO_x sensor according to claim 2, wherein two or more oxide sensing electrodes 20 are formed at one surface of the oxygen ion conductive solid electrolyte 10.

[Claim 7]

The NO_x sensor according to claim 1, wherein the oxygen ion conductive solid electrolyte 10 contains at least one of the components selected from; stabilized zirconia , CeO₂ or ThO₂

[Claim 8]

The NO_x sensor according to claim 1, wherein the oxide sensing electrode 20 contains at least one of the components selected from NiO, CuO, NiO-YSZ, LaCoO₃ or 2CuO -Cr₂O₃.

[Claim 9]

The NO_x sensor according to claim 1, wherein the first and second noble metal electrodes 30 and 40 are formed of platinum or gold.