JP2005121666A - Nox sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a NOx concentration measurement technique for continuously and accurately measuring the concentration of NOx in a gas under measurement without being affected by the concentration of oxygen or its change. <P>SOLUTION: This sensor is provided with a first internal void 6 into which the gas under measurement is introduced in order to know the concentration of NOx in the gas, a second internal void 8 into which the atmosphere in the internal void 6 is further introduced, a first oxygen pump means for controlling oxygen partial pressure in the internal void 6, a second oxygen pump means for pumping out oxygen in the internal void 8, and a current detection means 32 for detecting a pump electric current flowing through the second oxygen pump means operating. The concentration of NOx in the gas is found from the value of the pump current Ip detected by the detection means 32. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a measurement method and a measurement apparatus for a predetermined gas component in a gas component to be measured, and particularly to a measurement method and various gas sensors in which the gas component to be measured has bound oxygen, and in particular, the gas to be measured is a combustion gas. The present invention relates to a sensor that measures NOx in such a gas as a gas component to be measured, and a method that can advantageously measure such NOx.

  Conventionally, various measuring methods and apparatuses have been proposed in order to know the concentration of a desired gas component in a gas to be measured. For example, as a method for measuring NOx in a gas to be measured such as combustion gas, Using the NOx reducing property of Rh, the electromotive force between these electrodes was measured using a sensor in which a Pt electrode and an Rh electrode were provided on an oxygen ion conductive solid electrolyte such as zirconia. Techniques are known. However, such a sensor not only greatly changes the electromotive force due to the change in the oxygen concentration contained in the combustion gas as the gas to be measured, but also the change in the electromotive force is small with respect to the change in the NOx concentration. In order to extract NOx reducibility, CO or other reducing gas is indispensable, and in general, CO is generated under a fuel-poor combustion condition where a large amount of NOx is generated. Since the amount becomes lower than the generation amount of NOx, there is a drawback that measurement cannot be performed with the combustion gas formed under such combustion conditions.

  Also, a set of electrochemical pump cell and sensor cell composed of a Pt electrode and an oxygen ion conductive solid electrolyte, and another set of electrochemical pump cell and sensor cell composed of an Rh electrode and an oxygen ion conductive solid electrolyte were combined, Methods for measuring NOx based on the difference between the pump current values are disclosed in Japanese Patent Application Laid-Open Nos. Sho 63-38154 and 64-39545. Further, in JP-A-1-2777751 and JP-A-2-15443, two pairs of electrochemical pump cells and sensor cells are prepared, and a sensor composed of one pair of pump cells and sensor cells is used for NOx. The limit current is measured under the partial pressure of oxygen that is not reduced, and the pump current is measured under the partial pressure of oxygen at which NOx is reduced by the sensor consisting of the other pair of pump cells and sensor cells. Or using a sensor consisting of a pair of pump cells and sensor cells, and switching the oxygen partial pressure in the gas to be measured between the oxygen partial pressure at which NOx is reduced and the oxygen partial pressure at which it cannot be reduced. There have been proposed methods for measuring the above.

FIG. 25 is a principle diagram of these conventional methods. Two independent first and second sensor elements 61 and 62 are connected to an external measurement gas existence space via diffusion resistance parts 63 and 64, respectively. It consists of internal cavities 65 and 66 and electrochemical pump cells 67 and 68 using a solid electrolyte. The first sensor element 61 obtains the oxygen concentration by pumping only oxygen under diffusion-controlled conditions, and multiplying the pump current Ip ₁ at that time by the sensitivity coefficient K ₁ . The second sensor element 62 pumps both oxygen and NOx under diffusion-limited conditions in the presence of an electrode or catalyst capable of reducing NOx, and multiplies the pump current Ip ₂ at that time by a sensitivity coefficient K ₂ , The total amount of oxygen concentration and NOx is determined. Therefore, the NOx concentration: Cn is obtained by the following equation.
Cn = K ₂ · Ip ₂ −K ₁ · Ip ₁

  However, in these NOx measurement methods, the limit current value is mostly a current due to oxygen contained in a large amount, and the current based on the target NOx is usually very small, so the difference between the two large current values. As a result, a small current value corresponding to NOx is obtained, and there is a problem that continuous measurement cannot be performed, responsiveness is inferior, accuracy is inferior, etc., when switching with a pair of sensors and measuring. In the case of a system using two sets of sensors, if the oxygen concentration in the gas to be measured changes greatly, an error is likely to occur in the measured value. When the concentration changed greatly, it could not be used. This is because the oxygen concentration dependency of the pump current of one sensor is different from the oxygen concentration dependency of the pump current of the other sensor. For example, in the case of an automobile, the oxygen concentration of the exhaust gas is approximately several percent under the operating condition where the air-fuel ratio is 20, whereas the NOx concentration is several hundred ppm, and NOx is 1/100 of oxygen. Therefore, the difference in the limit current value with respect to the oxygen concentration change is larger than the limit current change due to the NOx to be measured. It is. Also, if the diffusion rate limiting means of the pump cell is clogged by the oil combustion product in the exhaust gas, the pump current will be changed and the accuracy will be lost, or if the exhaust gas temperature changes greatly, the measured value will be abnormal. It happened. Furthermore, if there is a difference in the temporal change in the characteristics of the two sets of sensors, it becomes an error as it is, and there is a disadvantage that it cannot withstand long-time use.

  As described above, oxygen present in the gas to be measured causes various problems when measuring NOx, and also when measuring other gas components to be measured other than NOx, the measurement accuracy is improved. A similar problem such as lowering is caused, and there is a strong desire to solve the problem.

JP 63-38154 A JP-A-64-39545 Japanese Patent Laid-Open No. 1-277751 JP-A-2-1543

  Here, the present invention has been made in order to eliminate the disadvantages in the conventional measurement method of gas components such as NOx, and the problem to be solved is the purpose in the gas to be measured. The desired gas component in the gas to be measured, in particular the bonded oxygen-containing gas component, can be measured continuously and with good responsiveness without being affected by the oxygen concentration or its change. It is in providing a measuring method and a measuring apparatus. Further, a further object of the present invention is to provide a NOx sensor capable of measuring in a wide range that can target a combustion gas from a fuel rich region to an excessive air region as a gas to be measured, and so on. Another object of the present invention is to provide a NOx measurement method having a wide measurement area.

  In the present invention, in order to solve the problems as described above, a measured gas containing a measured gas component to be measured is placed in the first internal space from the external measured gas existence space. Leading under a predetermined diffusion resistance and controlling the amount of oxygen in the measured gas in the first internal space, the measured gas is substantially used for measuring the target measured gas component amount. The controlled atmosphere in the first internal space is less than the first internal space under a predetermined diffusion resistance. To the second internal space, and in the second internal space, the amount of the measured gas component existing in the atmosphere in the second internal space is measured. The gist of the method for measuring the predetermined gas component is as follows.

  Note that, according to such a preferred embodiment of the measurement method according to the present invention, the amount of oxygen in the gas to be measured guided to the first internal space is controlled so that the substantial amount of oxygen in the first internal space is substantially reduced. The atmosphere has a negligible oxygen concentration.

  According to another preferable aspect of the measuring method according to the present invention, the control of the oxygen amount in the gas to be measured in the first internal space is realized by the pumping action of oxygen by the electrochemical cell. It becomes.

  According to one of the advantageous aspects of the measuring method of the present invention, the measured gas component containing the measured gas component having the bound oxygen to be measured in the first internal space from the external measured gas existence space. The measurement gas is guided under a predetermined diffusion resistance, the amount of oxygen in the measurement gas is controlled in the first internal space, and the measurement gas is measured for the amount of the measurement gas component having bound oxygen. Is controlled to a predetermined atmosphere in which the gas component to be measured cannot be substantially reduced or decomposed, and the atmosphere in the controlled first internal space is It leads from the internal space into the second internal space under a predetermined diffusion resistance, and reduces or measures the gas component to be measured existing in the atmosphere of the second internal space in the second internal space. It decomposes and oxygen generated at that time is electrochemical cell The pump current flowing through the electrochemical cell is detected by pumping with oxygen pumping, and the measured gas component amount in the measured gas is obtained from the detected value. .

  According to another preferred embodiment of the measurement method according to the present invention as described above, the amount of oxygen in the gas to be measured guided to the first internal space is controlled, and the first internal The atmosphere has an oxygen concentration that can be substantially ignored in the void.

  According to another preferred embodiment of the measuring method according to the present invention, the oxygen amount in the gas to be measured in the first internal space is different from that of the electrochemical cell. It will be realized by the pumping action of oxygen by the electrochemical cell.

  By the way, in the measuring method according to the present invention, a measured gas containing a measured gas component having bound oxygen to be measured is introduced into a predetermined diffusion resistance in the first internal space from the external measured gas existence space. The oxygen partial pressure in the atmosphere in the first internal cavity is reduced by the pumping action of oxygen by the first electrochemical cell to the first internal cavity. The measured gas component is controlled to a predetermined low value at which the gas component to be measured cannot be substantially reduced or decomposed in the heating environment, while the atmosphere in the controlled first internal space is changed to the second internal space. And oxygen is pumped out by a pumping action of oxygen by the second electrochemical cell to the second internal space, and the partial pressure of oxygen in the atmosphere in the second internal space is reduced. Heating the second internal cavity Controlled to a predetermined value at which the gas component to be measured can be reduced or decomposed under the boundary, the gas component to be measured present in the atmosphere in the second internal space is reduced or decomposed and generated The pumping current is pumped out at the same time, so that the pump current flowing by pumping oxygen by the second electrochemical cell is detected, and the measured gas component amount in the measured gas is obtained from the detected value. The gist of the measuring method is characterized by this.

  Furthermore, the measurement method according to the present invention allows a measurement gas containing a measurement gas component having bound oxygen to be measured to be measured under a predetermined diffusion resistance from the external measurement gas existence space to the first internal space. And the oxygen partial pressure in the atmosphere in the first internal cavity is increased by the pumping action of oxygen by the first electrochemical cell to the first internal cavity. The gas component to be measured is controlled to a predetermined low value that cannot be substantially reduced or decomposed in the environment, and the atmosphere in the controlled first internal space is communicated with the first internal space. And is led to a second internal cavity filled with a porous body having a predetermined diffusion resistance, and further by the pumping action of oxygen by the second electrochemical cell to the second internal cavity Pumps oxygen out of the second internal cavity The oxygen partial pressure in the atmosphere is controlled to a predetermined value at which the gas component to be measured can be reduced or decomposed in the heating environment of the second internal space, and the atmospheric pressure in the second internal space is adjusted. By reducing the existing gas component to be measured and pumping out the oxygen generated at that time, the pump current flowing by pumping oxygen by the second electrochemical cell is detected and detected. The gist of the measurement method is that the amount of the gas component to be measured in the gas to be measured is obtained from the value.

  According to a preferred aspect of the measurement method according to the present invention, the partial pressure of oxygen in the atmosphere in the first internal space is detected, and the power supply voltage is changed based on the detected value, so that the first By controlling the pumping action of oxygen by the electrochemical cell, the oxygen partial pressure in the atmosphere in the first internal space is controlled to a predetermined constant value.

  According to another preferred embodiment of the measuring method according to the present invention, the pumping of oxygen by the second electrochemical cell applies a voltage having a magnitude that gives a diffusion limit current of the gas component to be measured. This is performed by supplying power from a constant voltage power source, and the oxygen partial pressure in the atmosphere in the second internal space is controlled at the limit current value.

  Furthermore, according to one of the advantageous embodiments according to the present invention, the heating temperature of the second internal space is equal to or higher than the heating temperature of the first internal space.

  Furthermore, according to another advantageous embodiment according to the invention, the oxygen partial pressure in the atmosphere in the second internal space is equal to the oxygen partial pressure in the atmosphere in the first internal space. Or less than that.

In such a measurement method according to the present invention, the gas component to be measured having bound oxygen, which is a measurement target, is advantageously any one of NOx, CO _2, and H ₂ O.

  The gist of the present invention is also a measuring device for a predetermined gas component in a gas to be measured as described below, and the above-described measuring method can be advantageously implemented in such a measuring device. Is.

  That is, the apparatus for measuring a predetermined gas component in a gas to be measured according to the present invention reduces or decomposes the gas component to be measured having bound oxygen in the gas to be measured, and the amount of oxygen generated at that time. A measurement device configured to determine the amount of the gas component to be measured in the gas to be measured by measuring the first internal space communicated with the external space to be measured gas; and the gas to be measured; A first diffusion rate controlling means for guiding a measurement gas containing the measurement gas component from the existence space to the first internal space under a predetermined diffusion resistance; and a first oxygen ion conductive solid electrolyte; Using an electrochemical cell composed of a pair of electrodes provided in contact therewith, the first internal space is pumped by oxygen by energization between the pair of electrodes, and the first internal space. The oxygen partial pressure in the atmosphere First oxygen pump means for controlling the gas component to be measured to a predetermined low value at which the gas component to be measured cannot be substantially reduced or decomposed; and a second internal space communicated with the first internal space; A second diffusion rate controlling means for guiding a controlled atmosphere in the first inner space into the second inner space under a predetermined diffusion resistance; a second oxygen ion conductive solid electrolyte and An electrochemical cell composed of a pair of electrodes provided in contact with each other, pumping out oxygen in the atmosphere in the second internal space by energization between the pair of electrodes, and oxygen content in the atmosphere The pressure is controlled to a predetermined value at which the NOx component can be reduced or decomposed, and the NOx component present in the atmosphere in the second internal space is reduced or decomposed, and oxygen generated at that time is reduced. The second oxygen pump hand that pumped out at the same time An apparatus for measuring a predetermined gas component in a gas to be measured, characterized by comprising: current detection means for detecting a pump current flowing by the pump operation of the second oxygen pump means. is there.

  According to such a preferred aspect of the measuring apparatus according to the present invention, a catalyst that generates oxygen by reducing or decomposing the gas component to be measured is disposed in the second internal space.

  According to another preferable aspect of the measuring apparatus according to the present invention, the catalyst is in the second internal space of the pair of electrodes constituting the electrochemical cell in the second oxygen pump means. Also serves as an electrode.

  Furthermore, according to one of the other preferable aspects of the measuring apparatus according to the present invention, the sensor device includes a sensor element including the first and second oxygen ion conductive solid electrolytes, and the sensor element is integrated. The first and second internal cavities, the first and second diffusion rate limiting means, and the first and second oxygen pump means are integrally provided. Will be.

  According to one advantageous aspect of the measuring apparatus of the present invention, the sensor element has a slit flat space having a predetermined diffusion resistance that opens to an external space where the gas to be measured exists. In the flat space, the first and second diffusion rate-limiting means are configured, and the measurement gas existence space opening side portion of the flat space is defined as the first internal space, The first oxygen pump means is provided there, and further, the portion of the flat space on the back side of the first internal space is defined as the second internal space, and the second oxygen space is provided in the second space. An oxygen pump means is provided.

  According to another aspect of the measuring apparatus according to the present invention, the first and second oxygen ion conductive solid electrolytes are composed of the same oxygen ion conductive solid electrolyte layer or different. Each of the oxygen ion conductive solid electrolyte layers is constituted.

  Furthermore, in the measuring apparatus according to the present invention, a porous body having a predetermined diffusion resistance is filled in the opening of the flat space in the sensor element, or the first internal space and / or the A configuration in which a porous body having a predetermined diffusion resistance is filled in the second internal space is appropriately employed.

  In addition, according to a preferred aspect of the above-described measuring apparatus of the present invention, there is further provided a heating means capable of heating each of the first internal space and the second internal space to a predetermined temperature. Therefore, even when the temperature of the gas to be measured is low, the measuring device can be operated effectively, and the predetermined gas component in the gas to be measured can be advantageously decomposed. It is.

  Further, in the present invention, in order to solve the above-described problem, a sensor element including an oxygen ion conductive solid electrolyte is used as an integral structure, and a catalyst provided in the sensor element is used in a gas to be measured. A measuring apparatus for reducing or decomposing a gas component to be measured having bound oxygen and determining the amount of gas component to be measured in the gas to be measured by measuring the amount of oxygen generated at that time. a) a first internal space provided in an integrated structure of the sensor element and communicating with an external space to be measured gas; and (b) below a predetermined diffusion resistance from the space to be measured. A first diffusion rate controlling means for guiding a gas to be measured containing the gas component to be measured to the first internal space; and (c) communicating with the first internal space in an integrated structure of the sensor element. And provided separately from the first internal space, A second internal space in which a medium is disposed; and (d) second diffusion rate controlling means for guiding the atmosphere in the first internal space to the second internal space under a predetermined diffusion resistance. (E) heating means capable of heating each of the first internal space and the second internal space to a predetermined temperature; and (f) an oxygen ion conductive solid electrolyte of the sensor element in contact therewith. Using an electrochemical cell composed of a pair of electrodes provided, oxygen is pumped or pumped into and from the first internal space by energization between the pair of electrodes. The first oxygen that controls the oxygen partial pressure in the atmosphere in the void to a predetermined low value at which the gas under measurement cannot be substantially reduced or decomposed in the heating environment of the first void. Pump means; and (g) oxygen ion conductive solid electrolysis of the sensor element. And an electrochemical cell comprising a pair of electrodes provided in contact therewith, by energizing between the pair of electrodes, pumping out oxygen in the atmosphere in the second internal space, The oxygen partial pressure is controlled to a predetermined value at which the gas component to be measured can be reduced or decomposed by the catalyst under the heating environment of the second internal space, and the oxygen partial pressure in the second internal space is controlled. A second oxygen pump means for reducing or decomposing a gas component to be measured present in the atmosphere and pumping out oxygen generated at the same time; and (h) a pump of the second oxygen pump means. The gist of the present invention is also a measuring device having current detecting means for detecting a pump current flowing by operation.

  Similarly, in the present invention, a sensor element including an oxygen ion conductive solid electrolyte is used as an integral structure, and a gas component to be measured in the gas to be measured is reduced or decomposed by a catalyst provided in the sensor element. The measuring device is configured to determine the amount of gas component to be measured in the gas to be measured by measuring the amount of oxygen generated at that time, and (a) provided in an integral structure of the sensor element. A first internal space communicating with an external measurement gas existence space; and (b) a measurement gas component having bound oxygen to be measured from the measurement gas existence space under a predetermined diffusion resistance. (D) a first diffusion rate controlling means for guiding the gas to be measured to the first internal space; and (c ′) an integrated structure of the sensor element, the first internal space communicating with the first internal space, and the catalyst inside A second internal space, and (d ′) the second internal space A second diffusion rate-controlling means composed of a porous body having a predetermined diffusion resistance and filled in the space; and (e) the first internal space and the second internal space are each defined in the predetermined space. Heating means capable of heating to a temperature, and (f) an electrochemical cell comprising an oxygen ion conductive solid electrolyte of the sensor element and a pair of electrodes provided in contact therewith, Oxygen is pumped or pumped into the first internal space by energization, and the oxygen partial pressure in the atmosphere in the first internal space is reduced under the heating environment of the first internal space. A first oxygen pump means for controlling the gas component to be measured to a predetermined low value at which the gas component cannot be substantially reduced or decomposed; and (g) an oxygen ion conductive solid electrolyte of the sensor element and the first oxygen pump means Electricity consisting of a pair of electrodes By using an electrical cell between the pair of electrodes, oxygen in the atmosphere in the second internal space is pumped out, and the oxygen partial pressure in the atmosphere is heated to heat the second internal space. Under the environment, the gas component to be measured is controlled to a predetermined value that can be reduced or decomposed by the catalyst, and the gas component to be measured existing in the atmosphere in the second internal space is reduced or decomposed. A second oxygen pump means for simultaneously pumping oxygen generated at that time, and (h) a current detection means for detecting a pump current flowing by the pump operation of the second oxygen pump means, A gist of the measuring device is also provided.

  According to a preferred aspect of the measuring apparatus according to the present invention, the measuring apparatus further includes oxygen partial pressure detecting means for detecting an oxygen partial pressure in the atmosphere in the first internal space, Based on the oxygen partial pressure value detected by the pressure detecting means, by controlling the amount of current flowing between the pair of electrodes of the electrochemical cell in the first oxygen pump means, the inside of the first internal space Therefore, the oxygen partial pressure in the first internal space can be controlled more accurately and easily.

  According to another preferred aspect of the measuring device according to the present invention, a reference gas existence space is provided in the integral structure of the sensor element independently of the first and second internal spaces, An oxygen ion conductive solid electrolyte extending between the reference gas existence space and the first internal space; and a reference electrode provided in contact with the solid electrolyte located in the reference gas existence space; The oxygen partial pressure detecting means is constituted by an electrochemical cell comprising a measurement electrode provided in contact with the solid electrolyte located in the first internal space, and the presence of the reference gas The void is preferably opened at the atmospheric exposure portion of the sensor element, and the atmospheric air is introduced as a reference gas into the void where the reference gas exists through the opening.

  Further, according to one of the preferred embodiments of the present invention, the electrochemical cell in the second oxygen pump means includes oxygen extending between the second internal space and the reference gas existing space. An ion conductive solid electrolyte, a first pump electrode provided in contact with the solid electrolyte located in the second internal space, and a solid electrolyte located in the reference gas existence space. A second pump electrode and advantageously in the electrochemical cell of the second oxygen pump means the oxygen ion conducting solid electrolyte and in the electrochemical cell of the oxygen partial pressure detection means The oxygen ion conductive solid electrolyte is composed of an integral oxygen ion conductive solid electrolyte, and the second pump electrode and the reference electrode provided on the solid electrolyte are used as a common electrode.

  According to one advantageous aspect of the measuring device according to the present invention, the first pump electrode disposed in the second internal space of the electrochemical cell constituting the second oxygen pump means. Is the same as the catalyst, and simplifies the device manufacturing process.

  In the present invention, such a first pump electrode is advantageously composed of a porous cermet made of a metal and a ceramic capable of reducing or decomposing the gas component to be measured having bound oxygen. Therefore, by adopting such an electrode structure that also serves as a catalyst, the gas component to be measured can be effectively reduced or decomposed.

  According to one aspect of the present invention, the catalyst is in the second internal space in the vicinity of the first pump electrode of the electrochemical cell constituting the second oxygen pump means. It is arranged or provided on the first pump electrode of the electrochemical cell constituting the second oxygen pump means.

  Further, according to a different preferred embodiment of the measuring device according to the present invention, the heating means is disposed in the sensor element so as to be biased toward the second internal space, and the first means The second internal space is configured to be heated to a higher temperature than the internal space. By providing such a heating means, the second internal space can be maintained in an environment (particularly temperature) where the gas component to be measured can be easily reduced or decomposed.

  Furthermore, according to another different preferred embodiment of the measuring apparatus according to the present invention, the diffusion resistance in the second diffusion rate-limiting means is larger than the diffusion resistance in the first diffusion rate-limiting means, Thus, it is possible to effectively avoid the influence on the measured value of the gas component to be measured based on clogging by the oil combustion product.

Furthermore, according to another advantageous aspect of the measuring device according to the present invention, the gas component to be measured having bound oxygen to be measured is any one of NOx, CO ₂ and H ₂ O. .

  By the way, the present invention also includes a first oxygen pump means for controlling the oxygen partial pressure in the measurement gas led from the measurement gas existence space to a predetermined value, and the first oxygen pump means. Then, oxygen in the gas to be measured is pumped out from the gas to be measured having a partial pressure of oxygen, and the oxygen partial pressure in the atmosphere is controlled to a predetermined value at which the NOx component can be reduced or decomposed. A second oxygen pump means for reducing or decomposing NOx components present in the atmosphere in the second internal space and for simultaneously pumping out the oxygen generated at that time. The gist of the NOx sensor is characterized in that the NOx concentration present in the gas to be measured is obtained by detecting the pump current flowing by the pump operation of the pump means.

  Furthermore, the present invention comprises a solid electrolyte and a pair of electrodes provided in contact therewith, and one of the pair of electrodes is disposed in a first internal space into which a gas to be measured is introduced. It consists of a first electrochemical pump cell, a stationary electrolyte, and a pair of electrodes provided in contact therewith, and one of the pair of electrodes is arranged in a second internal space for NOx measurement. An electrode having a second electrochemical pump cell, wherein the NOx reducing power of the electrode disposed in the second internal space of the second electrochemical pump cell is disposed in the first internal space And the oxygen concentration in the first internal space is adjusted by the first electrochemical pump cell, while the second electrochemical pump cell is used for adjusting the oxygen concentration in the first electrochemical pump cell. To detect the NOx concentration in the second internal space. Also NOx sensor according to symptoms, and its gist.

  According to one of the desirable embodiments of the NOx sensor according to the present invention, the first internal cavity and the second internal cavity are communicated with each other, and the first electrochemical pump cell An atmosphere having an oxygen concentration adjusted in this manner is guided from the first internal space to the second internal space.

  Therefore, according to the method and apparatus for measuring a predetermined gas component in the gas to be measured according to the present invention, the target gas is measured without being affected by the oxygen concentration in the gas to be measured or its change. It was possible to measure the concentration of the measured gas component accurately, and it was possible to measure continuously continuously with good response and accurate measurement for a long time. There is no influence by the object, and it is possible to easily and accurately measure the amount of NOx contained in a wide range of combustion gases from the excessive fuel area to the excessive air area as the measurement gas. It is possible to find very significant industrial significance.

  Hereinafter, in order to clarify the present invention more specifically, the configuration of the present invention will be described in detail with reference to a specific example of NOx measurement shown in the drawings targeting NOx as a gas component to be measured. And

  First, FIGS. 1 and 2 clarify a typical example of a NOx sensor according to a measuring apparatus according to the present invention, in which FIG. 1 is a plan view of such a sensor, and FIG. These are the principal part expansion explanatory drawings in the AA cross section in FIG.

  In these drawings, reference numeral 2 denotes a sensor element having an elongated plate-like body shape. As is apparent from FIG. 2, the sensor element 2 includes a plurality of dense and airtight oxygen ion conductive solids. A plate-shaped body having an integral structure is formed by laminating the electrolyte layers 4a, 4b, 4c, 4d, and 4e. Each of the solid electrolyte layers 4a to 4e is formed using a known oxygen ion conductive solid electrolyte material such as zirconia porcelain. In addition, the sensor element 2 having an integral structure is manufactured by integrally firing a laminate of unfired solid electrolyte layers in the same manner as in the past.

  In the integrated sensor element 2, a first internal space 6 and a second internal space 8 each having a rectangular planar shape are provided on the tip side of the element, and the second internal space 8. And a reference air introduction passage 10 serving as a reference gas existence space in a form independent of the first and second internal cavities 6, 8. The reference air introduction passage 10 is opened at the end of the sensor element 2 on the base side and communicated with the atmosphere. Here, in the first and second internal cavities 6 and 8 and the reference air introduction passage 10, the corresponding vacant spaces formed in the solid electrolyte layer 4b are covered with the upper and lower solid electrolyte layers 4a and 4c. By this, it is formed in a state that is located on substantially the same plane. Further, a first diffusion rate controlling passage 12 as a first diffusion rate controlling means for communicating the first inner space 6 with the external space to be measured gas is provided through the solid electrolyte layer 4a. An external measurement gas is guided into the first internal space 6 through the first diffusion-controlling passage 12 under a predetermined diffusion resistance. Furthermore, a second diffusion rate controlling means for communicating the two internal cavities 6 and 8 through the solid electrolyte layer 4b located between the first internal cavities 6 and the second internal cavities 8. A second diffusion-controlled passage 14 is provided, and the atmosphere in the first internal space 6 is passed through the second diffusion-controlled passage 14 under the predetermined diffusion resistance. 8 can be introduced.

In addition, an inner electrode (pump electrode) 16 made of a rectangular porous Pt electrode is provided in contact with the exposed portion of the solid electrolyte layer 4a in the first inner space 6, and further the inner electrode. 16 is provided with an outer electrode (pump electrode) 18 made of a porous Pt electrode having the same rectangular shape so as to be in contact with the outer surface portion of the solid electrolyte layer 4a corresponding to the electrode 16. The electrolyte layer 4a constitutes an electrochemical cell in the first oxygen pump means, that is, a first electrochemical pump cell. Accordingly, a desired voltage is applied between the two electrodes 16 and 18 of the first electrochemical pump cell by the external variable power source 20, and a current flows in a predetermined direction, whereby the first internal space is provided. The oxygen in the atmosphere in 6 is pumped into the external space to be measured gas, or, conversely, oxygen is pumped into the first internal space 6 from the external space to be measured gas. It has become. Here, the porous Pt electrode is composed of a cermet electrode made of Pt as an electrode metal and ZrO ₂ as a ceramic.

  Further, a portion of the solid electrolyte layer 4c exposed in the first internal space 6 is provided with a measurement electrode 22 made of a rectangular porous Pt electrode in contact therewith, while the solid electrolyte layer 4c A reference electrode 24 made of a similar porous Pt electrode is provided in contact with the portion exposed in the reference air introduction passage 10, and includes the measurement electrode 22, the reference electrode 24, and the solid electrolyte layer 4 c. An electrochemical cell as an oxygen partial pressure detecting means, that is, an electrochemical sensor cell is constructed, and as is well known, the atmosphere in the first internal space 6 and the reference air in the reference air introduction passage 10 are known. The electromotive force generated between the measurement electrode 22 and the reference electrode 24 is measured by a potentiometer 26 based on the oxygen concentration difference between the first internal space 6 and the first internal space 6. The oxygen partial pressure of It is adapted to be. Based on the value of the oxygen partial pressure in the atmosphere in the first internal space 6 detected by the potentiometer 26, the voltage of the variable power source 20 is controlled, so that the first internal space 6 The oxygen partial pressure in the inner atmosphere can be maintained at a constant value.

  Furthermore, a rectangular internal pump electrode (first pump electrode) 28 is provided on and in contact with the solid electrolyte layer 4 c so as to be located in the second internal space 8. The internal pump electrode 28 is composed of a porous cermet made of Rh, which is a metal capable of reducing NOx, and zirconia as a ceramic, thereby reducing NOx present in the atmosphere in the second internal space 8. While functioning as a possible NOx reduction catalyst, a constant voltage is applied from the constant voltage power source 30 to the reference electrode (second pump electrode) 24 disposed in the reference air introduction passage 10, whereby the second The oxygen in the atmosphere in the internal space 8 is pumped into the reference air introduction passage 10. Therefore, the reference electrode 24 also functions as one of the pump electrodes. The reference electrode 24, the internal pump electrode 28, and the solid electrolyte layer 4c serve as an electrochemical cell that provides a second oxygen pump means, that is, the second electrode. This is an electrochemical pump cell. The pump current flowing by the pump operation of this electrochemical pump cell, in other words, the second oxygen pump means, is detected by the ammeter 32. The constant voltage power source 30 described above has a limit current against the pumping of oxygen generated at the time of NOx decomposition in the second electrochemical pump cell under the limited NOx inflow by the second diffusion rate limiting passage 14. A voltage of a given magnitude can be applied.

  In the sensor element 2, an alumina insulating layer 34 is integrally laminated in a form sandwiched between the solid electrolyte layers 4c and 4e and surrounded on three sides by the solid electrolyte layer 4d. A heater 36 is embedded in the insulating layer 34 to generate heat by external power feeding. As shown in FIG. 2, the heater 36 is disposed so as to be biased toward the second internal space 8 located on the front end side of the sensor element 2. The second internal space 8 is heated to a higher temperature, in other words, the internal pump electrode 28 is heated to a higher temperature than the inner electrode 16 and the measurement electrode 22. For example, when the gas temperature of the gas to be measured changes between 300 ° C. and 850 ° C., the inner electrode 16 and the measurement electrode 22 in the first internal space 6 are changed to 400 ° C. to 600 ° C. The heater 36 is arranged so that the internal pump electrode 28 in the internal space 8 is heated to 700 ° C. to 900 ° C.

In the sensor element 2 having such a configuration, the tip end side is disposed in the measured gas existence space, whereby the measured gas is supplied to the first diffusion rate limiting device provided in the sensor element 2. Through the passage 12, it is led into the first internal cavity 6 under a predetermined diffusion resistance. Then, the gas to be measured introduced into the first internal space 6 is caused by applying a predetermined voltage between the two electrodes 16 and 18 constituting the first electrochemical pump cell. Due to the pumping action of oxygen, the oxygen partial pressure is controlled to a predetermined value, for example, 10 ⁻⁶ atm.

By the way, in order to keep the oxygen partial pressure in the atmosphere in the first internal space 6 at a predetermined constant value, as described above, in the electrochemical sensor cell, based on the well-known Nernst equation. The electromotive force between the measurement electrode 22 and the reference electrode 24 is measured by a potentiometer 26, and is set to, for example, 203 mV (500 ° C.) between the two electrodes 16 and 18 of the first electrochemical pump cell. A method of controlling the voltage (variable power supply 20) applied to the capacitor is adopted, and this makes it possible to easily control the target oxygen partial pressure of 10 ⁻⁶ atm. That is, the voltage of the first electrochemical pump cell may be controlled so that an electromotive force corresponding to the difference between the desired oxygen concentration in the first internal space 6 and the oxygen concentration of the reference air is obtained. The first diffusion control passage 12 narrows down the amount of oxygen in the gas to be measured that diffuses and flows into the measurement space (first internal space 6) when a voltage is applied to the first electrochemical pump cell. The first electrochemical pump cell functions to suppress the current flowing through the first electrochemical pump cell.

Further, in the first internal space 6, the oxygen content in which NOx in the atmosphere is not reduced by the inner electrode 16 and the measurement electrode 22 even in the heating environment by the gas to be measured outside, and also in the heating environment by the heater 36. A state under pressure, for example, a state under oxygen partial pressure in which the reaction of NO → 1 / 2N ₂ + 1 / 2O ₂ does not occur is formed. However, if NOx in the gas to be measured (atmosphere) is reduced in the first internal space 6, it is impossible to accurately measure NOx in the second internal space 8. In the first internal space 6, it is necessary to form a situation in which NOx cannot be reduced by the components involved in the reduction of NOx (at least the components of the inner electrode 16 of the first electrochemical pump cell). It is.

In this way, the gas to be measured whose oxygen partial pressure is controlled in the first internal space 6 passes through the second diffusion-controlling passage 14 and passes through the second internal space under a predetermined diffusion resistance. It will be led into the place 8. Then, the gas to be measured introduced into the second internal space 8 has oxygen between the internal pump electrode 28 and the reference electrode 24 constituting the second electrochemical pump cell. A predetermined voltage, for example, 449 mV (700 ° C.) is applied in a direction to be pumped from the site 8 to the reference air introduction passage 10 side, thereby receiving an oxygen pumping action. In particular, at the three-phase interface of the internal pump electrode 28, the oxygen concentration further decreases, the oxygen concentration becomes 10 ⁻¹⁰ atm, and NOx is reduced around the internal pump electrode 28 that also functions as a NOx reduction catalyst. It is controlled in a state where, for example, a reaction of NO → 1 / 2N ₂ + 1 / 2O ₂ is induced. At this time, the current flowing through the second electrochemical pump cell is determined by the oxygen concentration in the atmosphere guided to the second internal space 8, that is, the oxygen concentration in the atmosphere in the first internal space 6, and the internal pump. The value is proportional to the sum of the oxygen concentration generated by reducing NOx at the electrode 28, but the oxygen concentration in the atmosphere in the first internal space 6 is controlled to be constant. The current flowing through the second electrochemical pump cell is proportional to the NOx concentration. The NOx concentration corresponds to the amount of NOx diffusion limited in the second diffusion rate limiting passage 14, and thus the NOx concentration can be measured.

  For example, if the oxygen concentration in the atmosphere in the first internal space 6 is 0.02 ppm and the NO concentration of the gas to be measured is 100 ppm, the oxygen concentration generated by reduction of NO is 50 ppm and the first internal space. 6 and the oxygen concentration in the atmosphere in the atmosphere of 0.02 ppm: A current corresponding to 50.02 ppm flows, and therefore, the pump current value in the second electrochemical pump cell was almost reduced in NO. It represents the quantity and therefore does not depend on the oxygen concentration in the gas to be measured.

  Here, the principle of the present invention will be described in more detail with reference to FIG. In FIG. 24, the gas to be measured is introduced into the first internal space 6 via the first diffusion-controlled passage 12, and the oxygen partial pressure in the first internal space 6 is the oxygen concentration control means. The first electrochemical pump cell 56 is controlled to a predetermined, desirably low value, where NOx is not reduced. The atmosphere in the first internal space 6 in which the oxygen partial pressure is controlled is communicated with the first internal space 6 via the second diffusion-controlled passage 14. NOx is reduced in the second internal space 8 and oxygen produced in the second internal space 8 is circulated in the second internal space 8 using a second electrochemical pump cell 58 under gas diffusion-controlled conditions. Further, the amount of NOx in the gas to be measured is measured based on the value of the current flowing through the second electrochemical pump cell 58.

In this method, the NOx concentration: Cn is obtained by Cn = K · Ip ₃ −A. However, K is a sensitivity coefficient, Ip ₃ is a current value flowing through the second electrochemical pump cell 58, and A is a constant caused by a small amount of oxygen remaining in the first internal space 6. Here, Ip ₃ is mostly due to oxygen generated by decomposition of the NOx component in the gas to be measured, and in a state in which the influence of oxygen in the gas to be measured is excluded compared to the conventional method, It can accurately measure even a very small amount of NOx. The outer electrode provided opposite to the electrode exposed to the internal space may be in a state where oxygen can be released, and may be, for example, in the air.

Incidentally, FIG. 3 shows a first specific example relating to the control of the electrode heating temperature and the oxygen partial pressure in each internal space, where it was co-fired with a ZrO ₂ substrate at a temperature of 1400 ° C. The cermet-shaped Pt electrode having a volume ratio of ZrO ₂ and Pt of 40:60 and the cermet-shaped Rh electrode having a volume ratio of ZrO ₂ and Rh of 40:60 Reducibility is shown. As is clear from FIG. 3, the Pt electrode does not exhibit reducing properties unless it is under high temperature and low oxygen partial pressure, whereas the Rh electrode exhibits reducing action under low temperature and high oxygen partial pressure. It is understood.

The present invention utilizes the above-described properties, and the oxygen concentration and temperature in the first internal space 6, specifically, the Pt electrode disposed in the first internal space 6. The temperature of the (inner electrode 16 and measurement electrode 22), that is, the temperature of the first electrode is controlled and set in a region where NO is not reduced by the first electrode. For example, as shown in the figure, when the oxygen partial pressure in the first internal space ⁶ is set to 10 ⁻⁶ atm, the gas temperature of the gas to be measured is set to 900 ° C. (substantially the highest exhaust gas temperature of the automobile). Even so, the arrangement and electric power of the heater 36 are set so that the temperature in the first internal space 6 is 650 ° C. or lower. Then, the gas to be measured whose oxygen partial pressure is controlled to 10 ⁻⁶ atm passes through the second diffusion-controlling passage 14 and enters the second internal space 8. In the place 8, a voltage of 450 mV is applied between the internal pump electrode 28 and the reference electrode 24, and oxygen is pumped from the second internal space 8 into the reference air introduction passage 10, and the internal pump electrode 28. The oxygen partial pressure at the three-phase interface is set to approximately 10 ⁻¹⁰ atm (the electromotive force between the internal pump electrode 28 and the reference electrode 24 corresponds to 450 mV). Thus, as shown in FIG. 3, the Rh electrode as the internal pump electrode 28, that is, the second electrode, can reduce NO at 410 ° C. or higher under an oxygen partial pressure of 10 ⁻¹⁰ atm. The temperature of the second internal space 8, specifically, the internal pump electrode 28 as the second electrode, is such that the gas temperature of the gas to be measured is 300 ° C. (approximately the lowest temperature of the exhaust gas temperature of the automobile). Sometimes, the arrangement and electric power of the heater 36 are set so as to be 410 ° C. or higher. Thus, NO is reduced at the three-phase interface of the internal pump electrode 28 as the second electrode, and current flows due to the generation of oxygen due to the reduction, and the current value is proportional to the concentration of NO. .

That is, in the first specific example described above, the oxygen partial pressure in the first internal space 6 is 10 ⁻⁶ atm, and the second internal space 8, particularly the internal pump electrode 28 (second electrode). ) Is controlled to 10 ⁻¹⁰ atm, and the inner electrode 16, the measurement electrode 22, and the second inner space provided in the first inner space 6 are controlled. When the temperature of the internal pump electrode 28 provided in the station 8 is within the range of the total change in the exhaust gas temperature of the automobile, for example, in the range of 300 ° C. to 900 ° C., the inner electrode 16 and the measuring electrode 22 The arrangement and power of the heater 36 are set so that the electrode 28 has a temperature of 410 ° C. or higher.

  The temperature of the sensor element 2 can be set to a target temperature by controlling the electric power of the heater 36 in accordance with the temperature of the gas to be measured, for example, the exhaust gas temperature. When the heater 36 is arranged close to the element tip side from the lower side of the element exposed to the exhaust gas toward the other end side, the tip side tends to become higher temperature, and therefore the internal pump electrode 28 is arranged. The heater power can be controlled by arranging the second internal space 8 on the tip side of the element and the first internal space 6 on which the inner electrode 16 and the measurement electrode 22 are disposed away from the tip of the element. The above temperature setting can be easily performed only by applying a predetermined voltage.

Further, FIG. 4 clarifies a specific example for explaining the setting of the heater arrangement in the sensor element. That is, in FIG. 4, in the ZrO ₂ solid electrolyte substrate having a width of 4.2 mm, a thickness of 1.3 mm, and a length of 62 mm, the size of the heat generating part is a width of 3.6 mm and a length of 5 mm. Yes, a normal temperature resistance: 8Ω of a platinum heater embedded in a position 1 to 6 mm from the tip of the element is attached to the exhaust pipe of an automobile, a voltage of 12 V is applied to the platinum heater, and 300 ° C. and 900 ° C. The position and temperature from the tip of the sensor element when exposed to an exhaust gas at 0 ° C. are shown.

  As is clear from FIG. 4, the maximum temperature of exhaust gas: 900 ° C., and the position where the sensor element is 650 ° C. or less is a position of 5.2 mm or more from the tip of the element. (Inner electrode 16 and measurement electrode 22) are arranged, and the position where the minimum gas temperature is 300 ° C. and 410 ° C. or higher is the position of 0 to 6.2 mm. If the pump electrode 28) is arranged, a NOx sensor according to the present invention capable of setting the temperature as shown in FIG. 3 can be realized. The temperature distribution to the sensor element can be arbitrarily set by appropriately selecting the heater power (resistance value), size (length), and arrangement position.

FIG. 5 shows the relationship between the NO concentration and the pump current value (diffusion limit current value: Ip) in the NOx sensor described above, where the pump current value (Ip) with respect to the change in NO concentration. Is changing linearly. Therefore, the NO concentration can be easily known by measuring the Ip value. In this case, the Ip value when NO = 0 ppm is 0.03 μA, which means that the oxygen concentration in the atmosphere in the first internal space 6 is 10 ⁻⁶ atm and the atmosphere in the second internal space 8. That is, this is the pump current necessary for pumping out oxygen corresponding to the difference between the oxygen concentration at the three-phase interface of the internal pump electrode 28 and 10 ⁻¹⁰ atm. In consideration of sensitivity: 30 μA / 2000 ppm = 0.015 μA / ppm, this pump current is not an obstacle to concentration measurement of several tens of ppm, but it is a problem as an error factor in measuring NO concentration of several ppm. There is a risk of becoming. For this reason, such a pump current value is preferably close to zero as much as possible, and such a pump current value reduces the oxygen partial pressure in the atmosphere in the first internal space 6 substantially by NOx. In a situation where it cannot be performed, the oxygen partial pressure in the atmosphere in the first internal space 6 and the oxygen partial pressure in the atmosphere in the second internal space 8, that is, the internal pump, are reduced as much as possible. This is realized by making the oxygen partial pressure of the three-phase interface in the electrode 28 equal.

FIG. 6 shows a second specific example relating to the control of the oxygen partial pressure in the two internal spaces and the control of the temperature of each electrode heated in these spaces. There, both the oxygen partial pressure in the first internal space 6 and the oxygen partial pressure in the second internal space 8, that is, the oxygen partial pressure at the three-phase interface of the internal pump electrode 28 as the second electrode, 10 ⁻⁸ atm. The heater 36 is set so that the first electrode (inner electrode 16 and measurement electrode 22) is 490 ° C. or lower and the second electrode (internal pump electrode 28) is 430 ° C. or higher.

  The relationship between the NO concentration and the pump current (Ip) in the NOx sensor controlled in this way is shown in FIG. Here, the oxygen partial pressure in the first internal space 6 and the oxygen partial pressure at the three-phase interface of the second electrode (internal pump electrode 28) in the second internal space 8 are set to the same value. Therefore, the Ip value when NO = 0 ppm is 0 μA.

FIG. 8 shows a third specific example relating to the control of the oxygen partial pressure and the electrode heating temperature, in which the first electrode (inner electrode 16, measurement electrode 22) and the second electrode are shown. (Internal pump electrode 28) are both Pt electrodes, and the oxygen partial pressure in the first internal space 6 and the second electrode (internal pump electrode in the second internal space 8). 28) The oxygen partial pressure at the three-phase interface is set to 10 ⁻⁶ atm. The heater 36 is set so that the first electrode has a temperature lower than 650 ° C. and the second electrode has a temperature higher than 650 ° C.

Further, FIG. 9 shows a fourth specific example relating to the control of the oxygen partial pressure and the electrode heating temperature, in which the first electrode (inner electrode 16, measurement electrode 22) and the second electrode are shown. (Internal pump electrode 28) are both Pt electrodes, and the oxygen partial pressure in the first internal space 6 is 10 ⁻⁶ atm, and the oxygen in the second internal space 8 is The partial pressure, specifically, the oxygen partial pressure at the three-phase interface of the second electrode (internal pump electrode 28) is set to 10 ⁻¹⁰ atm. The heater 36 is set so that the first electrode is 650 ° C. or lower and the second electrode is 430 ° C. or higher.

  In the case of the fourth specific example, when NO = 0 ppm, the pump current flows as described above, and the value thereof is the oxygen partial pressure in the first internal space 6 and the second partial pressure. If the internal space 8, that is, the oxygen partial pressure at the three-phase interface of the second electrode is made constant, it is easy to calibrate from a constant value. FIG. 10 shows the relationship between the NO concentration and the pump current in the NOx sensor according to the fourth specific example.

  Thus, in the above-described specific example of the present invention, the gas to be measured is guided into the first internal space, and the oxygen partial pressure and temperature at which NOx is not reduced are set in the first internal space. However, the oxygen pumping operation of the first electrochemical pump cell controls the oxygen concentration in the gas to be measured to a constant value, while the gas to be measured having a constant oxygen concentration in the first internal space is controlled to a second value. The temperature and oxygen concentration at which NOx is reduced at the three-phase interface of the NOx reduction catalyst (internal pump electrode = first pump electrode) disposed in the second internal space, Since the second electrochemical pump cell is operated with oxygen pumping and the pump current proportional to the NOx concentration in the second electrochemical pump cell is measured, it is not affected by the oxygen concentration in the gas to be measured. Effectively measures NOx Than it has become to be able to.

  In the above-described NOx sensor as the measuring apparatus according to the present invention, the first internal space is located behind the first diffusion rate-limiting passage (means), and further, the second diffusion rate-limiting passage (means) is located behind the first internal space. Therefore, even if clogging due to oil combustion products occurs in the first diffusion-controlled passage, clogging is unlikely to occur in the second diffusion-controlled passage. Then, the relationship between the diffusion resistance D1 of the first diffusion-controlled passage and the diffusion resistance D2 of the second diffusion-controlled passage is set to a relationship of D1 + α << D2 in consideration of the diffusion resistance change α due to clogging. If this is done, the NOx measurement value due to clogging has no effect. That is, if set in this way, even if the first diffusion rate-limiting passage is clogged, the pump current for keeping the oxygen concentration in the first internal space constant is reduced, and the NOx is reduced. Since the substantial diffusion control part is the second diffusion control path, there is no influence on the measured value.

  In the specific example described above, the first electrodes (16, 22) arranged in the first internal space 6 are constituted by Pt cermet electrodes, and are arranged in the second internal space 8. The second electrode (28) to be formed is composed of a cermet electrode of Rh, or both the first and second electrodes are composed of a cermet electrode of Pt. For example, if a cermet electrode made of Au or an alloy of Au and Pt is used for the first electrode and a Rh cermet electrode is used for the second electrode, the reducibility of the Au / Pt cermet electrode This is preferable because the oxygen partial pressure in the first internal space and the degree of freedom in setting the temperature are increased. In addition, the electrode metal of such an electrode can be appropriately selected from known ones. For example, both the first and second electrodes are Au electrodes, and the second Au electrode is formed on the second Au electrode. Further, a catalyst body in which a NOx reducing metal is supported is laminated on a porous body of a ceramic such as an Rh or Pt electrode or alumina, or both the first and second electrodes are made Pt electrodes, It is also possible to arrange an Rh catalyst electrode on the second Pt electrode, or to make a difference only in the temperature of the first and second electrodes.

  In any case, the first and second electrodes are preferably cermet electrodes composed of an electrode metal and appropriate ceramics. In particular, as shown in the case of using the second electrode that also serves as a NOx reduction catalyst. In this case, a porous cermet electrode made of a metal capable of reducing known NOx such as Rh or Pt and ceramics is desirable. In addition, the NOx reduction catalyst is close to the internal pump electrode 28 disposed in the second internal space 8 in the second electrochemical pump cell for pumping out oxygen in the second internal space 8. As shown in FIG. 11, porous alumina or the like carrying a NOx reduction catalyst made of Rh or the like is laminated on the first pump electrode 28 by printing or the like, so that the NOx reduction catalyst 42 is layered. It may be configured. Further, in the sensor element as shown in FIG. 11, the heater 36 is disposed on the second internal space 8 side, and the second internal space 8 has a higher temperature than the first internal space 6. From the point of being heated, the NOx reduction catalyst 42 works more effectively.

  Further, in the present invention, when exhaust gas with excess fuel is targeted as the gas to be measured, the amount of emission of unburned components such as CO and HC becomes large, and the reaction with NOx causes a measurement error. For example, as shown in FIG. 12, the first internal space 6 is filled with an oxidation catalyst body 38 made of porous alumina or the like, and the first internal space 6 is filled with the gas to be measured. It is preferable to oxidize unburned components such as CO and HC. In this case, the polarity of the first electrochemical pump cell is opposite to that in the case of excess air, and the oxygen is taken into the first internal space 6 from the measured gas atmosphere. Thus, the arrangement of the oxidation catalyst body 38 in the first internal space 6 is effective in eliminating the influence of reducing gas such as CO and HC even if the gas to be measured is in an atmosphere with excess air. It is valid.

  The arrangement of the oxidation catalyst body 38 in the first internal space 6 is not necessarily filled as illustrated, for example, the solid electrolyte between the first diffusion-controlled passage 12 and the measurement electrode 22 is used. Printing may be applied on the layer 4 c or on the inner electrode 16. That is, it is only necessary to oxidize unburned components such as CO and HC in the measurement gas before the measurement gas reaches the second internal space 8.

In order to realize such an oxidation reaction, it is not always necessary to dispose the oxidation catalyst body 38, and it goes without saying that the oxidation reaction is promoted if the inner electrode 16 has an oxidation catalytic property. Even if, for example, an Au electrode or an Au / Pt alloy electrode is used, it can be achieved by setting the oxygen partial pressure and temperature in the first internal space 6 and the oxidizable conditions of these unburned components. To get. The conditions also include an advantageous condition for NOx measurement in which the NOx is not reduced and the oxygen partial pressure is as low as possible and close to the oxygen partial pressure of the second internal space 8. For example, if it is 500 ° C., it can be oxidized at 10 ⁻¹⁰ atm or more, and if it is 600 ° C., it can be sufficiently oxidized if it is 10 ⁻¹⁵ atm or more.

  In such a NOx measuring method, the atmosphere to be measured is adjusted in the first internal space 6 prior to the measurement of the NOx amount by the reduction of NOx, and the atmosphere effective for measuring the NOx amount. Specifically, oxygen is pumped or pumped from the atmosphere in the first internal space 6 by the pumping action of oxygen by the first electrochemical cell. The oxidation of unburned components such as carbon monoxide (CO) and hydrocarbons (HC) contained in the gas to be measured is performed at a constant value as close as possible to the oxygen partial pressure of the second internal space 8. The value is controlled. In this way, unburned components such as carbon monoxide (CO) and hydrocarbons (HC) are oxidized, thereby avoiding a reaction between such unburned components and NOx. Therefore, the NOx concentration can be measured more accurately. When measuring the NOx concentration in the gas burned under excessive fuel conditions, the amount of unburned components such as carbon monoxide and hydrocarbons becomes extremely large. This is particularly effective for the gas to be measured.

  The oxidation of unburned components in the gas to be measured as described above is not only in the case of exhaust gas with excess fuel, but also in the case of exhaust gas with insufficient fuel remaining slightly. Needless to say, this is effective in eliminating the influence on the measured value.

  Further, as shown in FIG. 13, the oxidation catalyst body 38 is arranged in a layered manner on the solid electrolyte layer 4 a so as to be located at a portion of the first diffusion-controlled passage 12 on the external measurement gas existence space side. In this case, it is mainly effective in removing errors due to CO and HC when the air is excessive. However, when the fuel is excessive, the total amount of CO and HC cannot be oxidized because the oxygen in the gas to be measured is too small. Further, in the arrangement form of the oxidation catalyst body as shown in FIG. 14, a gas introduction path 40 is provided by further laminating solid electrolyte layers 4f and 4g outside the solid electrolyte layer 4a, and an oxidation catalyst is provided therein. The medium 38 can also be disposed. In this case, the oxidation catalyst body 38 is disposed at a higher temperature than in the example of FIG. is there.

  Further, in order to control the oxygen concentration at the three-phase interface of the second electrode (28) that also serves as the NOx reduction catalyst in the second internal space 8, from the second electrode (28) to the reference electrode 24 side. In addition to the exemplary method of pumping oxygen, an electrochemical pump cell is formed between the outer electrode 18 and the second electrode (28) of the first electrochemical pump cell, and the outer electrode 18 side The second internal space 8 is pumped with oxygen, and the NOx reduction catalyst electrode (28) in the second internal space 8 is used for pumping oxygen separately. Based on the electromotive force between the NOx reduction catalyst electrode and the reference electrode 24, an electrode for pumping is provided from the pumping electrode to the reference electrode 24 side, the outer electrode 18 side, or a separate pumping electrode. Pump to the Method and controls the oxygen concentration of the three phase boundary of the Ox reduction catalyst electrode can also be employed.

  Furthermore, as a method for controlling the oxygen partial pressure in the first internal space 6, a method for changing the applied voltage from start to finish based on the detection value of the electromotive force by the electrochemical sensor cell as in the above example, Even a method in which a constant voltage is applied between a pair of electrodes 16 and 18 of one electrochemical pump cell, or a method in which one electrochemical cell is time-shared and used as a pump cell and a sensor cell. There is no problem.

  Further, the reference electrode 24 does not necessarily need to be communicated with the atmosphere through the reference air introduction passage 10, but forms a space for storing oxygen pumped out from the second electrode (28), and the reference space is used as the reference space. It is also possible to arrange the electrode 24.

  Furthermore, in the present invention, the second internal space and the second diffusion rate limiting means may be configured in a space filled with a porous body. More specifically, instead of the configuration of the second internal space 8 and the second diffusion-controlled passage 14 in the sensor element 2 shown in FIG. 2, a configuration as shown in FIG. 15 can be adopted. That is, the second diffusion rate-limiting means made of the porous body 44 such as porous alumina is filled in the space following the first internal space so as to be in contact with the first internal space 6. As a result, a second internal space is formed at the same time, thereby simplifying the shape of the internal space. Here, the diffusion resistance of the second diffusion rate limiting means (44) is configured to be larger than the diffusion resistance of the first diffusion rate limiting path 12, and the atmosphere in the first internal space 6 is However, it is made not to be influenced by the atmosphere in the second internal space.

  Further, the filling of the porous body 44 may be a form in which printing is applied on the inner pump electrode 28 as shown in FIG. By adopting such a configuration, the porous body 44 becomes the second diffusion rate-limiting means, and the porous body itself becomes the second internal space and is filled in the internal space.

  In the sensor element 2 having such a configuration, the first electric power is set so that the electromotive force between the measurement electrode 22 and the reference electrode 24 in the first internal space 6 is 203 mV (500 ° C.). The voltage applied between the two electrodes 16 and 18 of the chemical pump cell is controlled, and furthermore, the electromotive force between the internal pump electrode 28 and the reference electrode 24 constituting the second electrochemical pump cell is 449 mV. In a state where the predetermined voltage is controlled to be (700 ° C.), the NO concentration and the pump current (Ip) change linearly as shown in FIG. Therefore, the NO concentration can be known by measuring the Ip value.

  In addition to the configuration of the sensor element shown in FIG. 15, a NOx reduction catalyst is further supported in the porous structure of the second diffusion rate controlling means 44, or CO or HC is contained in the first internal space 6. Even if an oxidation catalyst that oxidizes unburned components such as these is arranged, the same NO detection characteristic can be obtained, and thus various configurations can be added as necessary.

  Furthermore, the electrodes (16, 22) arranged in the first internal space 6 and the electrodes (28) in the second internal space (second diffusion rate-limiting means 44) have their NOx reducing power. A material whose relationship is “weak (electrode in the first internal space 6) ≦ (electrode in the second internal space) is strong” is appropriately selected and used. In addition, a NOx reduction catalyst layer may be disposed on the electrode (28) in the second internal space.

  The reference electrode, the position of the reference gas introduction hole, and the type of the reference gas are such that the electromotive force between the measurement electrode and the reference electrode in the first internal space 6 is in the range according to the Nernst equation, It is selected as appropriate.

  Further, as the heating means, the relationship between the electrode temperature in the first internal space 6 and the electrode temperature in the second internal space (second diffusion control means 44) is “low (first internal space The electrode is appropriately selected so that “the electrode in the void 6) <(the electrode in the second void) is high”.

  Furthermore, in the NOx sensor according to the present invention, the first in a slit flat space having a predetermined diffusion resistance provided in the sensor element so as to open to the external space to be measured gas. It is also possible to configure the diffusion rate limiting means and the second diffusion rate limiting means, an example of which is shown in FIG.

  That is, in FIG. 17, the sensor element 2 is laminated by including six oxygen ion conductive solid electrolyte layers 4a, 4b, 4c, 4h, 4d, and 4e as in the sensor element of FIG. The tip of the second solid electrolyte layer 4b from the top in the figure is cut out into a rectangular shape over a predetermined length among the plurality of solid electrolyte layers. As a result, a narrow flat space 50 having a predetermined diffusion resistance that opens at the tip of the sensor element 2 is extended over a predetermined length in the longitudinal direction of the sensor element 2 with a predetermined width. ing. In short, the flat space 50 is in the form of a long rectangular plane, and is open to an external space to be measured gas in one short side.

  Then, the gas to be measured outside is introduced into the flat space 50 from the opening and reaches the inner part of the flat space 50 under a predetermined diffusion resistance. Such a flat space 50 itself constitutes the first diffusion rate limiting means and the second diffusion rate limiting means. Further, the flat space 50 is provided with the inner electrode (pump electrode) 16 constituting the first electrochemical pump cell at the opening side portion thereof, so that the portion becomes the first inner space 6. On the other hand, the flat space 50 part on the back side of the first internal space 6 is defined as the second internal space 8, and the internal pump electrode 28 constituting the second electrochemical pump cell is provided there. Is disposed.

  In addition, here, the reference air introduction passage 10 is formed by a notch portion of the solid electrolyte layer 4h, opens at the end portion on the base side of the sensor element 2, and communicates with the atmosphere. Further, a reference electrode 24 is disposed in the reference air introduction passage 10 to constitute an electrochemical sensor cell between the measurement electrode 22 provided in the flat space 50 and as one of pump electrodes. Functions together with the internal pump electrode 28 to form a second electrochemical pump cell.

  The other parts of the NOx sensor shown in FIG. 17 are the same as the sensor element structure of the NOx sensor shown in FIG. 2 described above. And Further, specific structures of the flat space 50 and the reference air introduction passage 10 provided in the sensor element 2 are shown in detail in FIGS. 10 and 11 of Japanese Patent Publication No. 5-18059. Referring to those figures, the sensor element structure of FIG. 17 should be understood.

  Also, FIG. 18 shows one of the modifications of the sensor element 2 shown in FIG. 17 described above, in which a porous body 52 having a predetermined diffusion resistance is provided in the opening of the flat space 50. Is filled. Through such a porous body 52, the gas to be measured is guided from the external space to be measured gas into the flat space 50 under a predetermined diffusion resistance, and is configured at the opening side portion of the flat space 50. If oxygen pumping is performed by the first oxygen pump means in the first internal space 6, a more reliable diffusion rate control for the first internal space 6 can be realized, and CO and An advantage that an unburned component such as HC can be effectively oxidized in the porous body 52 occurs.

Furthermore, in the specific examples of the operation of the NOx sensor described above, any temperature reduction is provided between the first internal space 6 and the second internal space 8 to effectively decompose and reduce NOx. Although the operation can be realized, it is not always necessary to provide a difference between the temperature of the atmosphere in the first internal space 6 and the temperature of the atmosphere in the second internal space 8. For example, referring to the example shown in FIG. 3, if the temperature of the first internal space 6 and the second internal space 8 are both set to 600 ° C., the inside of the first internal space 6 While the Pt electrode (16) provided in FIG. ⁶ cannot reduce NOx under an oxygen partial pressure of approximately 10 ⁻⁶ atm or more, the Rh electrode (28) provided in the second internal space 8 NOx is reduced under an oxygen partial pressure of 10 ⁻⁵ atm or less. Accordingly, the oxygen partial pressure in the atmosphere in the first internal space 6 is set to 10 ⁻⁶ atm or more, and the oxygen partial pressure in the atmosphere in the second internal space 8 is set to 10 ⁻⁵ atm or less. If set, it is possible to measure NOx even if the temperature in the first and second internal spaces 6 and 8 is the same 600 ° C.

Further, as described above, if an electrode having no NOx reducing property such as an Au electrode, an Au / Pt alloy electrode or an electrode having a low reducing property is employed, the temperature of the first internal space> the second internal space This makes it possible to measure NOx even under the above temperature conditions. For example, in the case of an alloy electrode containing 1% Au in Pt, NOx is not reduced at 10 ⁻¹⁵ atm even at a temperature of 800 ° C., so the temperature of the first internal space is 800 ° C. and the oxygen partial pressure is 10 ⁻ to ¹⁰ atm, the second temperature internal space 600 ° C., the oxygen partial pressure is set to 10 ^-10 atm, it's becomes possible to measure the NOx.

FIG. 19 shows the relationship between the electromotive force and the oxygen partial pressure at 600 ° C., and the first internal space 6 is set so that the electromotive force of the measurement electrode 22 constituting the electrochemical sensor cell is 150 mV. If the oxygen partial pressure in the atmosphere inside is controlled, the oxygen partial pressure in the first internal space 6 is approximately 10 ⁻⁴ atm. For this reason, the first internal space 6 Inside, NOx is not reduced. On the other hand, when 450 mV is applied to the internal pump electrode 28 disposed in the second internal space 8 constituting the second electrochemical pump cell, the oxygen partial pressure at the three-phase interface is about 10 ⁻¹¹ atm. Therefore, NOx is reduced by the internal pump electrode 28, and oxygen generated during the reduction becomes a pump current of the second electrochemical pump cell and can be detected. The relationship between the NO concentration in the NOx sensor controlled in this way and the pump current (Ip) is shown in FIG.

In each of the above specific examples, NOx is the target gas component to be measured. Of course, the present invention is not limited to other bonds other than NOx that are affected by oxygen present in the gas to be measured. It goes without saying that the present invention can also be effectively applied to the measurement of oxygen-containing gas components such as H ₂ O and CO ₂ .

Incidentally, in the case of measuring H ₂ O (moisture) in the gas to be measured using the sensor element 2 as shown in FIGS. 1 and 2, first, the atmosphere in the first internal space 6 is In the pumping of oxygen by the first electrochemical pump cell, the oxygen partial pressure is controlled to a value as low as possible and H ₂ O does not decompose, for example, 10 ⁻¹⁰ atm. Next, the gas to be measured (atmosphere in the first internal space 6) whose O ₂ concentration is controlled to a certain low value passes through the second diffusion-controlled passage 14 and enters the second internal space 8. Led. A constant voltage power source 30 for applying a predetermined voltage between both electrodes 24 and 28 of the second electrochemical pump cell is set to 1.5 V, and oxygen is pumped from the internal pump electrode 28 toward the reference electrode 24. . At this time, H ₂ O in the atmosphere in the second internal space 8 is decomposed into H ₂ and O _2, and the decomposed O ₂ is also pumped out simultaneously. In such a pump operation by the second electrochemical pump cell, the pump current detected by the ammeter 32 is proportional to the H ₂ O concentration in the atmosphere in the second internal space 8. become. Since the oxygen in the gas to be measured has been reduced to 10 ⁻¹ atm (0.0001 ppm) in the first internal space 6, most of the values are based on the decomposition of H ₂ O. FIG. 22 shows the relationship between the H ₂ O concentration and the pump current (Ip) when both the first internal space 6 and the second internal space 8 are heated to 700 ° C.

Incidentally, in the measurement of such H ₂ O, to decompose of H ₂ O in the atmosphere, it may be a pump voltage of the second electrochemical pumping cell above 1V, the it from voltage becomes lower, H ₂ O cannot be decomposed sufficiently. In addition, it is desirable that the pump voltage be 3 V or less. If the pump is operated at a higher voltage for a long time, the solid electrolyte material (for example, ZrO ₂ ) is decomposed to cause a problem such as a decrease in strength. To provoke. As this pump voltage, 1.2V to 2.5V is preferably adopted.

Even when the gas component to be measured is CO ₂ , the measurement can be performed in the same manner as in the case of H ₂ O using the sensor element 2 having the configuration shown in FIGS. Is possible. That is, when measuring CO ₂ , first, the oxygen partial pressure in the atmosphere in the first internal space 6 is controlled to 10 ⁻¹⁰ atm so that CO ₂ is not decomposed and the oxygen partial pressure value is as low as possible. Is done. Then, the atmosphere in which the O ₂ concentration is controlled to a constant low value is led into the second internal space 8 through the second diffusion rate-determining passage 14, and is a constant voltage power source set to 1.5V. Oxygen is pumped from the internal pump electrode 28 toward the reference electrode 24 by the pumping operation of the second electrochemical pump cell by the power supply from 30. At this time, CO ₂ in the second internal space 8 is decomposed into CO and O ₂ , and this decomposed O ₂ is also pumped out at the same time, and the pump current at this time is proportional to the CO ₂ concentration. It is. Since the oxygen in the gas to be measured is reduced to 10 ⁻¹⁰ atm (0.0001 ppm) in the first internal space 6, most of the oxygen pumped out from the second internal space 8. Is a value based on the decomposition of CO2. FIG. 23 shows the relationship between the CO ₂ concentration and the pump current (Ip) when both the first internal space 6 and the second internal space 8 are heated to 700 ° C.

Even in the case of such CO ₂ measurement, in order to decompose CO ₂ in the atmosphere, the pump voltage of the second electrochemical pump cell may be set to 1 V or more, and the voltage becomes lower than that. , CO ₂ cannot be sufficiently decomposed. The pump voltage is desirably 3 V or less. If the pump is operated for a long time at a voltage higher than that, problems such as decomposition of the solid electrolyte material are caused. A more desirable range of the pump voltage is 1.2V to 2.5V.

As described above, even in the case of gas components having bonded oxygen such as H ₂ O and CO ₂ , they are decomposed in the second internal space 8 in the same manner as NOx, and O ₂ generated thereby is reduced. By detecting the pump current value (Ip) when pumping outside, the concentration of these gas components can be obtained. In order to actively decompose the gas components to be measured such as H ₂ O and CO ₂ , an appropriate decomposition catalyst may be disposed in the second internal space 8 in the same manner as in the case of NOx. Is possible.

In addition, the present invention is particularly advantageously employed when the gas component having bound oxygen such as NOx, H ₂ O, CO _{2 and the} like is used as a gas component to be measured, but is not limited thereto. The present invention can be applied to any gas component that is not affected by oxygen in the gas to be measured. For example, H ₂ and NH ₃ are combusted by oxygen in the gas to be measured at the time of measurement, and there is a risk that accurate measurement thereof may be difficult. If removed in the internal space, the concentration of H ₂ and NH ₃ can be accurately measured in the second internal space.

Note that the concentration measurement of H _2, NH ₃ various measurement gas components, such as present in the atmosphere in the second internal space, known suitable detecting means is selected, specifically, H ₂ In this measurement, a proton pump having the same configuration as that of the oxygen pump means is used. That is, the proton pump is composed of a proton ion conductive solid electrolyte layer and a pair of electrodes provided in contact therewith, and H ₂ is supplied from the second internal space to the outside by energization between the pair of electrodes. By pumping and detecting the pump current at that time, the H ₂ concentration is obtained. In the case of measuring NH ₃ , it decomposes in the second internal space to generate H ₂ and N _2, and the generated H ₂ is pumped out by the proton pump. By detecting the pump current, the NH ₃ concentration can be known.

  As described above, it goes without saying that the present invention can be implemented in a mode in which various changes, modifications, improvements, and the like are added based on the knowledge of those skilled in the art. It should be understood that all fall within the scope of the present invention without departing from the spirit of the invention.

It is plane explanatory drawing which shows an example of the sensor element which comprises the NOx sensor which concerns on the measuring apparatus according to this invention. It is a principal part expansion explanatory drawing in the AA cross section of the sensor element shown by FIG. It is a graph which shows one specific example of the control system of the electrode heating temperature and oxygen partial pressure in each internal space. It is a graph which shows the relationship between the heating temperature of the sensor element in the different exhaust gas temperature, and the distance from an element front-end | tip. It is a graph which shows the relationship between NO density | concentration and pump current (Ip) in the example of control shown by FIG. It is a graph which shows the 2nd specific example which concerns on control of the electrode heating temperature and oxygen partial pressure in each internal space. 7 is a graph showing the relationship between the NO concentration and the pump current (Ip) in the specific example shown in FIG. 6. It is a graph which shows the 3rd specific example which concerns on control of the electrode heating temperature and oxygen partial pressure in each internal space. It is a graph which shows the 4th specific example which concerns on control of the electrode heating temperature and oxygen partial pressure in each internal space. 10 is a graph showing the relationship between NO concentration and pump current (Ip) in the specific example shown in FIG. 9. FIG. 3 is a partial view corresponding to FIG. 2, showing an example in which a NOx reduction catalyst is stacked on an internal pump electrode. FIG. 3 is a partial view corresponding to FIG. 2, showing an example in which an oxidation catalyst body is disposed in a first internal space. FIG. 3 is a partial view corresponding to FIG. 2 showing another example of arrangement of the oxidation catalyst body. It is the fragmentary view corresponding to FIG. 2 which shows the other example of another arrangement | positioning of an oxidation catalyst body. FIG. 3 is a partial view corresponding to FIG. 2, showing specific examples of different configurations of the second internal space and the second diffusion rate-limiting means. 16 is a graph showing the relationship between NO concentration and pump current (Ip) in the specific example shown in FIG. 15. FIG. 3 is a cross-sectional explanatory view corresponding to FIG. 2, showing a different example of sensor elements constituting the NOx sensor according to the measuring apparatus according to the present invention. FIG. 18 is a cross-sectional explanatory view corresponding to FIG. 17 showing a modification of the sensor element shown in FIG. 17. It is a graph which shows the relationship between the electromotive force in 600 degreeC, and oxygen partial pressure. NO concentration in the embodiment in which the temperature of the first internal space and the second internal space is 600 ° C., and the applied voltages to the first electrochemical pump cell and the second electrochemical pump cell are different. It is a graph which shows the relationship of pump current. FIG. 16 is a partial view corresponding to FIG. 2, showing a specific example of a configuration different from that shown in FIG. 15 with respect to the second internal space and the second diffusion rate limiting means. A first internal space and a second temperature of internal space in the 700 ° C., is a graph showing the relationship between the H ₂ O concentration and the pumping current (Ip) in the embodiment was measured in H ₂ O . A first internal space and a second temperature of internal space in the 700 ° C., which is a graph showing the relationship between the CO ₂ concentration and the pumping current (Ip) in the embodiment was measured in CO _2. It is sensor element cross-sectional explanatory drawing for demonstrating the principle of this invention. It is sensor element cross-sectional explanatory drawing for demonstrating the principle of the conventional method.

Explanation of symbols

2 Sensor elements 4a, 4b, 4c, 4d, 4e, 4f, 4g, 4h Solid electrolyte layer 6 First internal space 8 Second internal space 10 Reference air introduction passage 12 First diffusion rate-limiting passage 14 Second Diffusion-controlled passage 16 inner electrode 18 outer electrode 20 variable power supply 22 measurement electrode 24 reference electrode 26 potentiometer 28 internal pump electrode 30 constant voltage power supply 32 ammeter 34 alumina insulating layer 36 heater 38 oxidation catalyst body 40 gas introduction path 42 NOx reduction Catalyst 44, 52 Porous body 50 Flat space 56 First electrochemical pump cell 58 Second electrochemical pump cell

Claims (2)

  First oxygen pump means for controlling the oxygen partial pressure in the measurement gas led from the measurement gas existence space to a predetermined value, and measurement of the oxygen partial pressure controlled by the first oxygen pump means Oxygen in the gas to be measured is pumped out from the gas, and the oxygen partial pressure in the atmosphere is controlled to a predetermined value at which the NOx component can be reduced or decomposed, so that the atmosphere in the second internal space A second oxygen pump means for reducing or decomposing NOx components present in the gas and simultaneously pumping out the oxygen generated at that time, and a pump that flows by the pump operation of the second oxygen pump means A NOx sensor characterized in that a NOx concentration existing in a gas to be measured is obtained by detecting an electric current. A first electrochemical cell comprising a solid electrolyte and a pair of electrodes provided in contact therewith, wherein one of the pair of electrodes is disposed in a first internal space into which a gas to be measured is introduced. A second electrochemical cell comprising a pump cell, a stationary electrolyte and a pair of electrodes provided in contact therewith, one of the pair of electrodes being disposed in a second internal space for NOx measurement A NOx reducing power of an electrode disposed in the second internal space of the second electrochemical pumping cell than a NOx reducing power of an electrode disposed in the first internal space. The first electrochemical pump cell adjusts the oxygen concentration in the first internal cavity while the second electrochemical pump cell adjusts the oxygen concentration in the second internal cavity. NOx concentration characterized by detecting the NOx concentration of Support.

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