JP2010261855A - Nitrogen oxide sensor and nitrogen oxide detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an NOx sensor for directly decomposing NOx and detecting NOx with high selectivity even under high oxygen concentration. <P>SOLUTION: A nitrogen oxide sensor for detecting nitrogen oxides in a gas to be detected includes an electrochemical cell having the following elements; (a) a solid electrolyte; (b) a first electrode including a nitrogen oxide decomposition catalytic phase having a perovskite type oxide expressed as LaSrMO<SB>3</SB>as a principle body to be exposed to the gas to be detected; and (c) a second electrode opposed to the first electrode via the solid electrolyte and arranged in such a way as to be cut off from the gas to be detected. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a nitrogen oxide sensor and a method for detecting nitrogen oxides.

As a sensor for detecting and measuring nitrogen oxides such as nitrogen monoxide and nitrogen dioxide (hereinafter also simply referred to as NOx) contained in vehicle exhaust gas, etc., a ceramic solid electrolyte is provided with a cathode and an anode. Sensors using arranged electrochemical cells have been developed. As such a NOx sensor, a limiting current type sensor that detects a current generated by decomposition of NOx and a mixed potential type sensor are known. The limiting current type sensor has a multi-chamber structure in order to control the oxygen concentration to a low concentration, and detects the current flowing through the electrochemical cell based on O ²⁻ ions generated by NOx decomposition (Patent Literature). 1). The mixed potential sensor detects the NOx concentration in the test gas by measuring the potential generated when the NO ₂ reduction reaction and the oxygen oxidation reaction occur simultaneously at the electrode / solid electrolyte interface. Yes (Patent Document 2).

  On the other hand, as a NOx sensor, a direct detection type NOx sensor for directly detecting NOx by decomposing (reducing) NOx with high selectivity has been studied (Patent Document 3).

Japanese Patent Laid-Open No. 9-288084 JP 2002-162381 A JP 11-148916 A

However, the limiting current type ensures NOx measurement accuracy and detection sensitivity by a multi-chamber structure for controlling the O ₂ concentration and feedback control of the oxygen concentration. In addition, the hybrid potential type sensor has a simple structure, but since oxygen is always involved in the reaction, it is important to have a structure that allows oxygen concentration control to produce a sensor with stable sensitivity. ing.

  Considering the simplicity of the structure and the cell fabrication accuracy, the NOx direct detection type sensor is considered preferable, but it has not been possible to detect NOx with high selectivity even under a high oxygen concentration.

  Accordingly, an object of the present invention is to provide a NOx sensor that can directly decompose NOx and detect NOx with high selectivity even under a high oxygen concentration.

In order to solve the above-mentioned problems, the inventors of the present invention studied selective NOx decomposition catalyst materials for various materials. As a result, the perovskite oxide represented by ABO ₃ type has a high NOx level even under a high oxygen concentration. As a result, the present invention has been completed. That is, according to the present invention, the following means are provided.

According to the present invention, a nitrogen oxide sensor that detects nitrogen oxides in a gas to be detected, the following elements:
(A) solid electrolyte, (b) perovskite oxide represented by ABO ₃ (A represents two or more elements selected from rare earth elements, alkaline earth metal elements and alkali metal elements, respectively, A first electrode including a nitrogen oxide decomposition catalyst phase mainly comprising a transition metal element and exposed to the gas to be detected; and (c) the solid electrolyte. There is provided a sensor comprising: an electrochemical cell having a second electrode facing the first electrode via the second electrode and arranged to be cut off from the gas to be detected.

  In the sensor of the present invention, it is preferable that the first electrode includes the nitrogen oxide decomposition catalyst phase. Further, the first electrode may be configured to be exposed to the detected gas whose oxygen concentration is not controlled. Furthermore, the sensor of the present invention may further comprise a temperature control means for controlling the temperature of the electrochemical cell to a predetermined temperature of 600 ° C. or less that is capable of ensuring ion conductivity in the solid electrolyte.

  According to the present invention, there is provided a method for detecting nitrogen oxides using the sensor according to the present invention, wherein the first electrode of the electrochemical cell is exposed to a gas to be detected. There is provided a method including a detecting step of detecting nitrogen oxide in the detection gas based on a current value generated by applying a predetermined voltage to the first electrode and the second electrode.

  In the detection method of the present invention, the detection step may be performed at a predetermined temperature that is equal to or higher than a temperature at which the ionic conductivity of the solid electrolyte can be secured and is 600 ° C. or lower. The detection step may include exposing the first electrode to the detected gas having an oxygen concentration of 21 vol% or less.

It is a figure which shows an example of the electrochemical cell of the NOx sensor of this invention. It is a figure which shows the outline of an example of the direct detection type sensor which is a NOx sensor of this invention. It is a figure which shows an example of the measurement state of the to-be-detected gas by a direct detection type sensor. Shows the combined polarization curve of the base gas in Example 1 (21vol% _{O 2)} and NOx-containing gas _{(500ppmNO 2 + 21vol% O 2} ). It is a figure which shows together the polarization curve of various base gas and NOx containing gas. The current value corresponding to NOx reduction when the oxygen concentration is 21% ([current value at the time of NOx + O ₂ reduction] − [current value at the time of O ₂ reduction] −100% when the oxygen concentration is 5% and 10%) It is a figure which compares the electric current value equivalent to NOx reduction | restoration at the time of%. It is a figure which shows the relationship between temperature and NOx selectivity.

  The present invention relates to a NOx sensor and a NOx detection method using the NOx sensor. According to the NOx sensor of the present invention, since NOx can be highly selectively decomposed and detected even under a high oxygen concentration, a practical direct detection type NOx sensor can be provided. Further, according to the NOx detection method of the present invention, NOx can be measured even under a high oxygen concentration, so that NOx can be detected and measured without necessarily requiring control of the oxygen concentration of the gas to be detected. Further, according to the NOx detection method of the present invention, NOx can be detected and measured under a high oxygen concentration even at a low temperature of 600 ° C. or lower. For this reason, NOx can be detected at a low temperature and with good detection sensitivity and accuracy not only in the form in which NOx in the gas to be detected is directly detected, but also in the aspect for detecting the limit current and the aspect for detecting the mixed potential.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings as appropriate. FIG. 1 shows an outline of an example of the NOx sensor of the present invention, FIG. 2 shows an outline of an example of a direct detection type sensor, and FIG. 3 shows an example of a measurement state of a gas to be detected.

(NOx sensor)
The NOx sensor 2 of the present invention includes an electrochemical cell 10 shown in FIG. The electrochemical cell 10 includes a solid electrolyte 12 and at least one pair of a first electrode 16 and a second electrode 18 facing each other with the solid electrolyte 12 interposed therebetween.

NOx that the NOx sensor of the present invention is a detection target is nitric oxide (NO), nitrogen dioxide (NO _2), nitrous oxide (dinitrogen monoxide) (N ₂ O), dinitrogen trioxide (N ₂ O ₃ ), nitrogen oxides such as dinitrogen tetroxide (N ₂ O ₄ ), dinitrogen pentoxide (N ₂ O ₅ ) and the like. Among these, the NOx sensor of the present invention preferably detects at least one of nitrogen monoxide and nitrogen dioxide, preferably both.

  The gas to be detected applied to the NOx sensor of the present invention is sufficient if it contains NOx, and various combustion gases and the like can be targeted. Among these, it is preferable to use the exhaust gas of an automobile, which is considered to be one of the main causes of air pollutants, as a detection gas.

(Electrochemical cell)
(Solid electrolyte)
The solid electrolyte 12 can be used without particular limitation as long as it has oxygen ion conductivity. As the solid electrolyte 12, for example, a zirconia-based solid electrolyte (typically a ZrO ₂ —M ₂ O ₃ solid solution or a ZrO ₂ —MO solid solution: where M is preferably Y, Yb, Gd, Ca, or Mg). ), Ceria based solid electrolyte (typically CeO ₂ —M ₂ O ₃ solid solution or CeO ₂ —M solid solution: where M is preferably Y or Sm), bismuth oxide based solid electrolyte (typically Bi ₂ O ₃ —WO ₃ solid solution), or LaGaO ₃ compounds having a perovskite structure. A zirconia-based solid electrolyte is preferable from the viewpoints of stability and oxygen ion conductivity when exhaust gas from an internal combustion engine (engine) such as an automobile is used as a detection gas. Among these, stabilized zirconia in which yttria, magnesia or calcia in an amount of 3 to 10 mol% of the whole is dissolved is particularly preferable.

(First electrode)
The first electrode 14 is provided in contact with (in close contact with) the solid electrolyte 12. The first electrode 14 functions as a detection electrode for detecting NOx. The first electrode 14 has both oxygen ion conductivity and electron conductivity, and has a nitrogen oxide decomposition catalyst phase 16 made of one or more materials having NOx decomposition catalyst activity. (Hereinafter simply referred to as catalyst phase 16). The first electrode is preferably composed of the catalyst phase 16 and has no coating layer on the surface layer exposed to the gas to be detected. Such a catalyst phase 16 may be constituted by using, for example, two or more kinds of materials, but is preferably constituted by using a material satisfying these with a single material. By using such a material, NOx can be detected with high selectivity and high responsiveness. That is, the catalyst phase 16 may be formed of, for example, an oxygen ion conductive material, an electron conductive material, and a NOx decomposition catalytically active material, but preferably has a perovskite oxide, more preferably a perovskite oxide. It is mainly composed of a type oxide, and more preferably substantially consists of a perovskite type oxide. Such a perovskite oxide has both oxygen ion conductivity and electron conductivity, and has NOx decomposition catalytic activity. Since the first electrode 14 has the catalyst phase 16 containing such a perovskite oxide, the first electrode 14 is exposed to the detection gas containing NOx, and the electrochemical cell 2 is exposed to the first electrode 14. When a voltage is applied to and electrons are introduced, the following events occur.

(1) When electrons flow into the first electrode 14 from the external circuit, the electrons are diffused into the catalyst phase 16.
(2) The catalyst phase 16 exposed to the gas to be detected causes the adsorbed NOx to preferentially react with electrons to be reduced. O ²⁻ generated by this reduction diffuses in the catalyst phase 16, reaches the oxygen ion conductive solid electrolyte 12, diffuses, and reaches the second electrode 18.

  The above event is considered to be common to perovskite oxides, and the perovskite oxides can be applied as the material of the catalyst phase 16 in the sensor of the present invention. It is considered that oxygen vacancies present in the perovskite structure contribute to the decomposition of NO by the catalytic activity of the perovskite oxide. Therefore, it is considered that the oxygen vacancies contribute to the reduction reaction of NOx also in the electrochemical decomposition of NOx. In addition, since the perovskite oxide has both oxygen ion conductivity and electron conductivity and also has NOx decomposition catalytic activity, the above event is high with respect to NOx even under high oxygen concentration. It is thought to occur with selectivity. As a result, it is considered that sufficient detection sensitivity and response speed can be secured even under a high oxygen concentration.

The perovskite oxide is represented by ABO ₃ , where A is two or more elements selected from rare earth elements, alkaline earth metal elements, and alkali metal elements, and B is selected from transition metal elements. 1 type or 2 types or more are represented. Considering the formation of oxygen vacancies that contribute to the sensor function, it is preferable that A represents one or more selected from Sr, Mg, Ca and Ba in addition to La. In consideration of the stability of oxygen vacancies, A is preferably La and Sr. In consideration of the amount of oxygen vacancies, it is preferable that A includes two types of elements having different valences, and the mol% of elements having a large valence is preferably larger than the mol% of elements having a small valence. For example, when A is a perovskite oxide A1pA2qBO ₃ represented by an element A1 having a higher valence and an element A2 having a lower valence, p is preferably 0.6 or more and 0.8 or less, Is preferably 0.2 or more and 0.4 or less. A1 is preferably a valence 3 metal element, typically La, and A2 is preferably a valence 2 metal, typically Sr.

  The B preferably represents one or more selected from the group consisting of Ni, Fe, Co, Mn, Cr, Cu, Rh and V. In a perovskite oxide having two or more elements as B, the combination of these elements is determined in consideration of oxygen ion conductivity, electron conductivity, and catalytic activity. From the viewpoint of catalytic activity, Co and It is preferable to use Mn. In addition, when two or more elements are contained as B, the mol% (y) of the most abundant elements is preferably 0.6 or more and less than 1, and the total mol% (1− y) is preferably more than 0 and 0.4 or less.

Such perovskite type oxides are typically La-Sr-Ni-O perovskite oxide, La-Sr-Fe-O perovskite oxide, La-Sr-Co-O perovskite oxide, La-Sr. -mn-O perovskite oxide, a perovskite type oxide containing La-Sr-Co-Mn- O perovskite oxide _{La x Sr 1-x Co y} Mn 1-y O 3 (0 <x <1,0 ≦ y ≦ 1). In this perovskite oxide _{La x Sr 1-x Co y} Mn 1-y O 3, a 0.6 ≦ x ≦ 0.8, more preferably from 0.6 ≦ y ≦ 1, more preferably 0.6 ≦ y <1. According to the perovskite oxide _{La x Sr 1-x Co y} Mn 1-y O 3, the oxygen concentration is in the order of 5% to 20%, it is possible to detect the 50ppm level of NOx. Furthermore, NOx can be detected even at 10 ppm.

  Such a perovskite oxide can be synthesized according to a conventional method. For example, there may be mentioned a method in which an aqueous solution of each salt of La and Sr (acetate or nitrate, etc.) and an aqueous solution of an element salt of B site are mixed and dried, and then fired.

  The first electrode 14 may be composed of only the catalyst phase 16 or may have the catalyst phase 16 as a matrix (parent phase) or a dispersed phase as long as at least the catalyst phase 16 is included as a continuous phase. It may be. The first electrode 14 may contain noble metals such as palladium (Pd), platinum (Pt), and rhodium (Rh) as long as the selective NOx decomposition and detection ability by the catalyst phase 16 is not impaired. Good.

(Second electrode)
The second electrode 18 is provided in contact with (in close contact with) the solid electrolyte 12 so as to face the first electrode 14 through the solid electrolyte 12. The second electrode 18 is a counter electrode of the first electrode 14 as a detection electrode, and functions as a reference electrode or a reference electrode. The second electrode 18 need only be electronically conductive, and its constituent material is not particularly limited. Examples of the electron conductive material include noble metals belonging to the platinum group element (typically Pt, Pd, Rh), other noble metals (typically Au, Ag), and highly conductive base metals (eg, Ni). It is done. Further, alloys based on any of these metals (for example, Pt—Rh, Pt—Ir, etc.) can be mentioned. In addition, metal oxides such as nickel oxide, cobalt oxide, copper oxide, lanthanum manganese knight, lanthanum cobaltite, lanthanum chromite and the like can also be mentioned. The second electrode 18 can include one or more of these electronically conductive materials. The second electrode 18 may include an oxygen ion conductive material. As the oxygen ion conductive material, the same material as that used for the solid electrolyte 12 can be used. Examples thereof include zirconia stabilized with yttria or scandium oxide, ceria stabilized with gadolinium oxide or samarium oxide, and lanthanum gallate. Considering the balance between the electronic conductivity of the second electrode 18 and the adhesion to the solid electrolyte 12, the oxygen ion conductive material of the second electrode 18 is more than 0% by mass of the total mass of the electrode 18. It is preferable to set it as the range of 10 mass% or less, More preferably, they are 1 mass% or more and 5 mass% or less.

(Production of electrochemical cell)
The electrochemical cell 10 is produced from the solid electrolyte 12, the first electrode 14, and the second electrode 18 according to a conventional method. For example, the electrochemical cell 10 can be an electrolyte support type or an electrode support type. In the case of the electrolyte support type, typically, the composition for the first electrode 14 is applied to one surface of the solid electrolyte 12 after firing or before firing by screen printing or the like, and then fired. Thus, a stacked body of the solid electrolyte 12 and the first electrode 14 can be formed. Similarly, the second electrode 18 and the like can be formed on the solid electrolyte 12. The electrochemical cell 10 is used in various modes depending on the mode of the NOx sensor. In either mode, the first electrode 14 is exposed to a gas to be detected that is intended to detect NOx. Configured.

  The first electrode 14 and the second electrode 18 are each provided with a current collector made of a conductive material such as a metal as necessary, and configured to be able to apply a voltage from the outside. The NOx sensor of the present invention is preferably used in a state where a voltage is applied between the first electrode 14 and the second electrode 18. By detecting the value of the current flowing between the electrodes in the voltage application state, NOx can be detected highly selectively. Accordingly, a preferred usage mode of the NOx sensor of the present invention is a current detection type NOx sensor such as a direct detection type NOx sensor or a limit current type NOx sensor. The configuration to which electrolysis can be applied is not particularly limited. For example, as shown in FIG. 2, a current collector made of Pt or the like is applied to a part of the surface of the first electrode 14, and an external power source is connected by a lead wire. Connected to. Similarly, a lead wire is connected to the second electrode 18. In addition, when the 2nd electrode 18 is formed only with a metal etc., it is not necessary to form a collector.

(Mode of NOx sensor)
The NOx sensor 2 of the present invention can take various usage modes. That is, a known limit current type NOx sensor, a mixed potential type NOx sensor, a direct detection type NOx sensor, or the like can be adopted. Among these, the NOx sensor 2 of the present invention is preferably used for a current detection type sensor that detects a current generated when a voltage is applied to the first electrode 14 and the second electrode 18.

  Since the NOx sensor 2 of the present invention can detect NOx even under a high oxygen concentration, it is preferably used as a direct detection type NOx sensor. Since the direct detection type NOx sensor detects NOx even under a high oxygen concentration, the direct detection type NOx sensor is usually provided with a configuration in which it is exposed to a gas to be detected whose oxygen concentration is not previously controlled within a predetermined range. The oxygen concentration of the gas to be detected to which the first electrode 14 is exposed in the direct detection type NOx sensor is not particularly limited. However, according to the NOx sensor 2 of the present invention, NOx can be detected well even under a high oxygen concentration. 0.5% or more is preferable, and 1% or more is more preferable. Furthermore, it can be 5% or more. More preferably, it is 10% or more, More preferably, it is 15% or more. Moreover, although an upper limit is not specifically limited, It is preferable that it is 21% or less which is the oxygen concentration in air | atmosphere.

  The direct detection type NOx sensor 2 is typically configured such that the first electrode 14 is directly exposed to a gas to be detected to detect NOx, and the second electrode 18 includes a first electrode 18. The electrode 14 is configured to be in a state where the electrode 14 is exposed to the detected gas or in a state where the electrode 14 can be blocked. Appropriate contact form between the first electrode 14 and the like of the electrochemical cell 10 and the gas to be detected is to form a necessary partition so as to form a flow path of the gas to be detected for ensuring such a contact form. Can be obtained. The partition wall is preferably made of a material having sufficient insulation and heat resistance in the operating temperature range of the NOx sensor 2. Examples thereof include ceramic materials such as alumina, magnesia, mullite, and cordierite.

  An example of the direct detection type NOx sensor 2 is shown in FIG. In the embodiment shown in FIG. 2, a partition wall for partitioning a space that requires the second electrode 18 and a space that requires the first electrode 14 is formed on the second electrode 18 side of the solid electrolyte 12. In addition, the interruption | blocking state of the 1st electrode 14 and the 2nd electrode 18 is not limited to this, The partition may be formed in the 1st electrode 14 side.

  The NOx sensor 2 of the present invention may be a limiting current type NOx sensor. Since the NOx sensor 2 of the present invention has high selectivity with respect to NOx, even if it is a detection target gas whose oxygen concentration is controlled to be low in advance, it can detect NOx with good sensitivity. The limiting current type NOx sensor is configured such that the first electrode 14 is exposed to a gas to be detected whose oxygen concentration has been adjusted to a predetermined range in advance. Typically, the limiting current type NOx sensor is provided with a first cavity that has an oxygen pump function to preliminarily adjust the oxygen concentration of the gas to be detected, and a gas to be detected in which the oxygen concentration is adjusted. A second cavity for detecting NOx in the gas to be detected is provided. The first electrode 14 is exposed to the gas to be detected in the second cavity. Note that the second electrode 18 is configured to be shielded from the detection gas to which the first electrode 14 is exposed.

The NOx sensor 2 of the present invention can include temperature control means for controlling the temperature of the electrochemical cell 10. By providing the temperature control means, the ion conductivity of the solid electrolyte 12 and the selectivity of NOx (for example, NO ₂ ) to O _{2 in} the first electrode 14 or the electrochemical cell 10 can be adjusted appropriately. The temperature control means includes, for example, heating (or cooling) means for heating or cooling the electrochemical cell 10 or the vicinity thereof, or both of them, and a temperature sensor for detecting the temperature of the electrochemical cell 10 or the vicinity thereof. And a control circuit for outputting a control signal for controlling the temperature of the electrochemical cell 10 or the like based on a signal from the temperature sensor. For example, as shown in FIG. 3, the heating means (heater) can be configured to heat the electrochemical cell 10 and a region including the gas to be detected that reaches the electrochemical cell 10.

  The temperature control means preferably controls the temperature of the electrochemical cell 10 or the vicinity thereof to 700 ° C. or less in consideration of the NOx selectivity. More preferably, it is 600 degrees C or less. When it is 600 ° C. or lower, the NOx selectivity can be made 70% or higher. Further, considering NOx selectivity, it is more preferably 550 ° C. or less, further preferably 500 ° C. or less, and further preferably 450 ° C. or less. On the other hand, considering the ionic conductivity of the solid electrolyte 12, the temperature is preferably 450 ° C. or higher, more preferably 500 ° C. or higher. Moreover, a preferable temperature range is 450 degreeC or more and 700 degrees C or less, More preferably, it is 450 degreeC or more and 600 degrees C or less.

  The NOx sensor 2 of the present invention can include voltage application means 20 that applies a voltage between the first electrode 14 and the second electrode 18. Specifically, an external circuit in which electrons flow into the first electrode 14 can be provided. The voltage applying means 20 is provided in the electrochemical cell 10 through the lead wires attached to the electrodes 14 and 18 as already described. The voltage applying means 20 can also be provided with a potentiostat for keeping the voltage at a constant value. The NOx sensor 2 of the present invention can also include current detection means such as an ammeter for measuring the current flowing between the first electrode 14 and the second electrode 18.

(NOx detection method)
The NOx detection method of the present invention can include the following NOx detection step using the NOx sensor of the present invention. That is, based on the current value generated when a predetermined voltage is applied to the first electrode 14 and the second electrode 18 with the first electrode 14 of the electrochemical cell 10 exposed to the gas to be detected. A detection step of detecting NOx in the detection gas.

  In the NOx detection step, the first electrode 14 is exposed to the gas to be detected. As already described, the oxygen concentration in the gas to be detected may differ depending on the mode of the NOx sensor 2. When the NOx sensor 2 of the present invention is used in the form of a direct detection type NOx sensor, it may be 5% or more, or 15% or more. Moreover, when using with the aspect of a limiting current type NOx sensor, 0.01% or less may be sufficient.

  Note that the gas to be detected to which the first electrode 14 is exposed may be one in which the oxygen concentration is controlled within a predetermined concentration range. The NOx concentration of the gas to be detected to which the first electrode 14 is exposed, that is, the total concentration of one or more types of nitrogen oxide gases may be 1000 ppm or less, and at least 10 ppm or more. Is preferred.

  In the detection step, the electrochemical cell 10 is performed at a temperature or higher that can ensure the ionic conductivity of the solid electrolyte 12. From the viewpoint of NOx selectivity, it is preferable to carry out at a temperature as already described. That is, it is preferably 700 ° C. or lower. This is because when it is 700 ° C. or lower, the NOx selectivity can be secured at 50% or more. Considering the NOx selectivity, it is more preferably 600 ° C. or lower. More preferably, it is 550 degrees C or less, More preferably, it is 500 degrees C or less. Most preferably, it is 450 degrees C or less.

  In the detection step, a voltage is applied to the first electrode 14 and the second electrode 18. Specifically, a voltage is applied from an external circuit so as to supply electrons to the first electrode 14. The magnitude (absolute value) of the applied voltage is not particularly limited, but can be a voltage of about 1 V or less, for example. More preferably, it can be set to −100 mV or more and 0 mV or less.

  By performing the detection step, NOx in the gas to be detected can be detected and measured as a current value, and the NOx concentration can be measured. According to the configuration of the electrochemical cell 10, NOx can be detected with high selectivity against a large excess of oxygen, so that the influence of oxygen concentration is suppressed or avoided, and NOx is accurately and accurately reproduced. Measure well. Moreover, according to the structure of the electrochemical cell 10, since NOx can be detected with a high NOx selectivity at 600 ° C. or lower, it is possible to detect NOx better at a lower temperature than in the prior art. Furthermore, when the detection process is performed in the form of a direct detection type sensor, a simple configuration can be adopted, and complicated oxygen concentration control can be avoided.

  EXAMPLES Hereinafter, although an Example is given and an example is demonstrated concretely, this invention is not limited to a following example.

  In the following examples, an electrochemical cell was produced and a direct detection type NOx sensor was produced. The NOx concentration was measured under various conditions using the prepared NOx sensor.

(1) Preparation of YSZ pellets YSZ powder (zirconia in which 8 mol% of Y ₂ O ₃ was dissolved, manufactured by Tosoh Corporation) was uniaxially pressed using a tablet molding machine having a diameter of 10 mm, and then cold isostatic pressure Pressurized (CIP) and fired at 1530 ° C. for 2 hours in an air atmosphere to produce a pellet having a diameter of about 9 mm and a thickness of 1 mm.

(2) Production of sensing electrode and counter electrode Next, a paste in which a perovskite oxide (La _0.6 Sr _0.4 Co _0.98 Mn _0.02 O ₃ ) was mixed with α-terpineol on one side of the produced pellet. Printing was performed using a 7 mm diameter screen. Further, platinum paste (TR7095, manufactured by Tanaka Kikinzoku Co., Ltd.) was printed on the opposite surface of the pellet using a screen having a diameter of 3 mm. Then, this was baked at 1000 degreeC in air | atmosphere for 2 hours, the detection electrode (1st electrode) and the counter electrode (2nd electrode) were formed, and it was set as the electrochemical cell.

(3) Current collector attachment A Pt wire was joined to the detection electrode surface of the obtained electrochemical cell with a Pt paste (TR7905, manufactured by Tanaka Kikinzoku Co., Ltd.) in the form shown in FIG. Time baking, current collector and lead wire were attached.

(4) Attaching the magnesia tube A magnesia tube (MgO, outer diameter × inner diameter × length = 6 mm × 4 mm × 350 mm) was used as a partition wall for partitioning the detection electrode and the counter electrode with respect to the gas to be detected. The magnesia tube was bonded to the electrochemical cell using an inorganic adhesive so that the detection electrode and the counter electrode were exposed to different gas atmospheres in the form illustrated in FIG. 2 to obtain a direct detection type NOx sensor.

(5) Preparation of gas to be detected Gases 1 to 6 having various compositions shown in Table 1 were prepared as gases to be detected. That is, as the gases 1 to 6, gas cylinders of pure oxygen (O ₂ ), pure nitrogen (N ₂ ), and 1000 ppm nitrogen dioxide (NO ₂ , a mixed gas of pure nitrogen and 1000 ppm (0.1%) NO ₂ ) are used. Using a mass flow meter, the concentration was adjusted by adjusting the flow rate of each cylinder.

(6) Measurement method The prepared NOx sensor is put in a glass tube with a flange in the manner illustrated in FIG. 3, and the gases adjusted in Table 1 are circulated, and a detection electrode and a counter electrode using a potentiostat. A potential difference was applied between and the current value was read. In addition, the temperature was raised to 600 ° C. using a heater, the flow rate was 100 cm ³ / min using a mass flow meter, and the applied potential difference was 0 to −300 mV.

(1) for oxygen to assess the selective detectability rating NOx selective detectability of NOx in an atmosphere in large excess, based gas (gas 1) and 500PpmNO ₂ (gas 4) in each atmosphere A polarization curve was prepared. That is, in a base gas (21 vol% O ₂ ) atmosphere, after confirming that the potential between the detection electrode and the counter electrode is stable, a potential difference is given between the detection electrode and the counter electrode, and the relationship between the potential and the current value is plotted on the horizontal axis. The voltage and the vertical axis were recorded and plotted as current values. Similarly, polarization curve measurement in a 500 ppm NO ₂ atmosphere was measured. These results are shown in FIG. In FIG. 4, the reduction current is shown as a negative current value.

In FIG. 4, the current value due to the gas 1 is a current value related only to O ₂ reduction, and the current value due to the gas 4 represents the current value related to O ₂ reduction and NO ₂ reduction. Therefore, a current value obtained by subtracting the current value of gas 1 from the current value of gas 4 is a current value related to NO ₂ reduction in gas 4. As shown in FIG. 4, when the applied voltage was increased, the current values of gas 1 and gas 4 were increased, but it was found that 500 ppm of NOx could be measured even under a high partial pressure of 21% (210000 ppm). Moreover, the result of calculating the NO ₂ selectivity from the current value at the time of applying the −50 mV potential shown in FIG. 4 is about 70% as shown in Table 2,

(2) Influence of oxygen concentration In order to confirm the influence of oxygen concentration on NOx detection ability, the polarization curves of gases 1 to 3 with oxygen concentrations of 5%, 10% and 21% and oxygen concentrations of 5%, 10 % And 21% and 500 ppm NO ₂ gas 4-6 together with the polarization curves were drawn together. The results are shown in FIG. Further, when the applied voltage is −50 mV and the oxygen concentration is 21%, the current value corresponding to NOx reduction ([NOx + current value during O ₂ reduction] − [current value during O ₂ reduction]) The current values corresponding to NOx reduction when the oxygen concentration was 5% and 10% when compared with 100% were compared. The results are shown in FIG.

As shown in FIG. 5, NOx could be detected under a high oxygen concentration at any voltage application. In particular, it was found that the influence of the oxygen concentration was small when a voltage smaller than 50 mV was applied. As shown in FIG. 6, assuming that the NOx reduction current value at O ₂ 21% concentration is 100, the NOx reduction current value at O ₂ 5% concentration is 126%, and the NOx reduction current value at O ₂ 10% concentration is It was 113%, and it was found that the influence of the oxygen concentration was suppressed even when the excess was large.

In the following examples, regarding the gas 1 and gas 4 shown in Table 1, the heater set temperature is 450 ° C. to 700 ° C., the flow rate is 100 cm ³ / min using a mass flow meter, and the applied potential difference is −50 mV. Were similarly operated according to Example 1 to obtain each polarization curve. The NOx selectivity from the obtained polarization curve was calculated. The results are shown in FIG.

  As shown in FIG. 7, it was found that even when the temperature was 700 ° C. or lower, a good NOx selectivity was exhibited, and a much higher selectivity was obtained by lowering the temperature. NOx selectivities of 75% at 600 ° C, 85% at 550 ° C, 95% at 500 ° C and nearly 100% at 450 ° C were obtained.

  From the above examples, it was found that the NOx sensor of the present invention can detect NOx while suppressing the influence well even in the presence of a large excess of oxygen. It was also found that NOx can be detected with a high selectivity even at a low temperature of 700 ° C. or lower.

2 NOx sensor, 10 electrochemical cell, 12 solid electrolyte, 14 first electrode, 16 catalyst phase, 18 second electrode, 20 temperature control means

Claims (11)

A nitrogen oxide sensor for detecting nitrogen oxide in a gas to be detected,
The following elements:
(A) a solid electrolyte,
(B) Perovskite oxide represented by ABO ₃ (A is two or more elements selected from rare earth elements, alkaline earth metal elements and alkali metal elements, respectively, and B is selected from transition metal elements) A first electrode exposed to the gas to be detected, and (c) the first electrode through the solid electrolyte. A second electrode facing the one electrode and arranged to be cut off from the gas to be detected;
An electrochemical cell having,
A sensor.   The first electrode includes a nitrogen oxide phase mainly composed of the perovskite oxide, wherein A represents at least La and represents one or more selected from Sr, Mg, Ca, and Ba. The sensor according to claim 1.   In the first electrode, the B is mainly composed of the perovskite oxide that represents one or more selected from the group consisting of Ni, Fe, Co, Mn, Cr, Cu, Rh, and V. The sensor according to claim 1, comprising a nitrogen oxide phase.   The first electrode includes a nitrogen oxide phase mainly composed of the perovskite oxide in which the A represents La and Sr, and the B represents Co and Mn. Sensor. The first _{electrode, La x Sr 1-x Co} y Mn 1-y O 3 (0 <x <1,0 ≦ y ≦ 1) is the perovskite type oxide of nitrogen oxide decomposing catalyst mainly comprising a The sensor according to claim 1, comprising a phase.   The sensor according to claim 1, wherein the first electrode includes a nitrogen oxide decomposition catalyst phase.   The sensor according to claim 1, wherein the first electrode is configured to be exposed to the detected gas whose oxygen concentration is not controlled.   Furthermore, the temperature control means which controls the temperature of the said electrochemical cell to the predetermined temperature which is more than the temperature which can ensure the ionic conductivity in the said solid electrolyte, and is 600 degrees C or less is provided in any one of Claims 1-7 The sensor described. A method for detecting nitrogen oxides using the nitrogen oxide sensor according to claim 1,
In a state where the first electrode of the electrochemical cell is exposed to a gas to be detected, a current value is generated by applying a predetermined voltage to the first electrode and the second electrode of the nitrogen oxide sensor. And a detection step of detecting nitrogen oxide in the gas to be detected based on the method.   The method according to claim 9, wherein the detecting step is performed at a predetermined temperature that is equal to or higher than a temperature at which the ionic conductivity of the solid electrolyte can be ensured and is equal to or lower than 600 ° C.   The method according to claim 9 or 10, wherein the detecting step includes exposing the first electrode to the gas to be detected having an oxygen concentration of 21% or less.