JP2019020299A - Ammonia sensor element and ammonia sensor - Google Patents

Abstract

To provide an ammonia sensor element and an ammonia sensor capable of accurately detecting ammonia concentration under oxygen coexistence.SOLUTION: There are provided an ammonia sensor element 1 including a proton conductive solid electrolyte 2, a detection electrode 3, a reference electrode 4 and a measurement gas chamber 5, and an ammonia sensor including the ammonia sensor element. The solid electrolyte 2 includes a measurement gas surface 21 contacting a measurement gas Gm, and a reference gas surface 22 contacting a reference gas Gb. The detection electrode 3 is formed on the measurement gas surface 21 of the solid electrolyte 2. The reference electrode 4 is formed on the reference gas surface 22 of the solid electrolyte 2. The measurement gas chamber 5 faces the measurement gas surface 21 of the solid electrolyte 2 and has a gas introduction part 51. At least a part of a boundary surface 31 of the detection electrode 3 with the measurement gas chamber 5 contains at least one sort of electrode material selected from a group consisting of SnO, AlO, WO, ZnO with Au, Pt-Au, TiO, SnO, Sb solid-solved.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an ammonia sensor element having a proton conductor and an ammonia sensor.

  An ammonia sensor is used to detect ammonia in a mixed gas such as combustion gas or exhaust gas. For example, a method for purifying NOx contained in exhaust gas from an internal combustion engine such as an engine using ammonia is known. In order to perform this purification efficiently, measurement of ammonia concentration by an ammonia sensor is required. An ammonia sensor is required to have a performance capable of accurately detecting ammonia in a gas such as exhaust gas.

  Patent Document 1 discloses an internal space into which a gas to be measured is introduced under a predetermined diffusion resistance, a proton conductive solid electrolyte layer, an active electrode having high ammonia decomposition activity, and a reference electrode having low ammonia decomposition activity. An ammonia detection element having the following is disclosed. Each electrode is formed in a solid electrolyte layer. As an active electrode, it is disclosed to use a porous cermet electrode mainly composed of Ni, Pt or the like. The active electrode is also called a sensing electrode.

  In the ammonia detection element having such a configuration, the amount of gas flowing into the internal space is controlled by the diffusion resistance layer. Therefore, if the ammonia decomposition reaction at the detection electrode proceeds smoothly, the ammonia concentration in the internal space decreases. As a result, even if the potential between the reference electrode and the detection electrode is swept, the current flowing in the solid electrolyte body is rate-limited, and a limit current having a constant current value is generated. By detecting this limiting current, the ammonia concentration can be measured.

JP 2010-48596 A

  However, the detection electrode mainly composed of Ni, Pt or the like has high oxidation activity as well as decomposition activity against ammonia. Therefore, in the presence of oxygen, the ammonia to be detected is oxidized and consumed by the catalytic action of the detection electrode. As a result, the limit current does not appear and the ammonia concentration in the gas cannot be accurately detected.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an ammonia sensor element and an ammonia sensor that can accurately detect the ammonia concentration in the presence of oxygen.

One aspect of the present invention is a proton conductive solid electrolyte body (2) having a measurement gas surface (21) in contact with a measurement gas (Gm) and a reference gas surface (22) in contact with a reference gas (Gb). )When,
A sensing electrode (3) formed on the measurement gas surface of the solid electrolyte body;
A reference electrode (4) formed on the reference gas surface of the solid electrolyte body;
A measurement gas chamber (5) that faces the measurement gas surface of the solid electrolyte body and into which the measurement gas is introduced from a gas introduction part (51),
At least a part of the interface (31) of the detection electrode with the measurement gas chamber is made of SnO ₂ , Al ₂ O ₃ , WO ₃ , ZnO in which Au, Pt—Au, TiO ₂ , SnO ₂ , and Sb are dissolved. The ammonia sensor element (1) contains at least one electrode material selected from the group consisting of:

  Another aspect of the present invention is an ammonia sensor (7) having the ammonia sensor element.

  In the ammonia sensor element, at least a part of the interface with the measurement gas chamber in the detection electrode contains the specific material. Such a detection electrode exhibits low ammonia oxidation activity while exhibiting ammonia decomposition activity. That is, the detection electrode can achieve both a catalytic oxidation inhibiting effect on ammonia and an electrochemical decomposition effect on ammonia.

  Therefore, even when the measurement gas contains oxygen, consumption of ammonia in the measurement gas by the detection electrode is suppressed, and the detection electrode can extract protons from the ammonia. As a result, the ammonia sensor element can accurately detect a current resulting from proton conduction based on decomposition of ammonia. As a result, the ammonia sensor element and the ammonia sensor including the same can detect the ammonia concentration with high accuracy.

As described above, according to the above aspect, it is possible to provide an ammonia sensor element and an ammonia sensor that can accurately detect the ammonia concentration in the presence of oxygen.
In addition, the code | symbol in the parenthesis described in the means to solve a claim and a subject shows the correspondence with the specific means as described in embodiment mentioned later, and limits the technical scope of this invention. It is not a thing.

FIG. 3 is a cross-sectional view in the longitudinal direction of the ammonia sensor element according to the first embodiment. II-II arrow directional cross-sectional view in FIG. FIG. 2 is an enlarged cross-sectional view of a detection electrode having a two-layer structure of an electrode base layer and an electrode surface layer in the first embodiment. The expanded sectional view of the detection electrode which has an electrode base layer and the electrode surface layer which covers this in Embodiment 1. FIG. FIG. 4 is an enlarged cross-sectional view of a detection electrode having a single layer structure in Embodiment 2. The schematic diagram of the analyzer in Experimental example 1. FIG. The figure which shows the mass spectrometry result of the gas which passed Pt powder | flour in Experimental example 1. FIG. The figure which shows the mass spectrometry result of the gas which passed Au powder in Experimental example 1. FIG. The figure which shows the mass spectrometry result of the gas which passed the Pt-Au powder in Experimental example 1. It shows the results of mass spectrometry of gas passing through the Al ₂ O ₃ powder in Experimental Example 1. Sectional drawing of the simple element in Experimental example 2. FIG. The schematic diagram of the ammonia detection apparatus in Experimental example 2. FIG. The figure which shows the relationship between the voltage and electric current value of a simple element in the experiment example 2 under the ammonia distribution | circulation and the non-ammonia distribution | circulation. Sectional drawing of the ammonia sensor in Embodiment 3. FIG. The schematic diagram of the exhaust gas purification system using the ammonia sensor in Embodiment 3. FIG.

(Embodiment 1)
An embodiment relating to an ammonia sensor element will be described with reference to FIGS. As illustrated in FIGS. 1 and 2, the ammonia sensor element 1 includes a solid electrolyte body 2, a detection electrode 3, a reference electrode 4, and a measurement gas chamber 5.

  The shape of the ammonia sensor element 1 is not particularly limited, but is, for example, a long plate shape as illustrated in FIG. In the present specification, of both ends in the longitudinal direction X of the ammonia sensor element 1, an end portion on the side exposed to the measurement gas Gm is referred to as a tip end 111, and an opposite end portion is referred to as a base end 112. The longitudinal direction may be referred to as the axial direction. In FIG. 1, the left side is the distal end side in the axial direction X, and the right side is the proximal end side in the axial direction X. The direction perpendicular to the axial direction and in which the solid electrolyte body 2, the detection electrode 3, the reference electrode 4, and the like are laminated is referred to as a thickness direction Y. The thickness direction Y may be referred to as a stacking direction. A direction orthogonal to both the axial direction X and the thickness direction Y is referred to as a width direction Z. Hereinafter, this embodiment will be described in detail.

  The solid electrolyte body 2 contains a proton conductive solid electrolyte. The proton conductive solid electrolyte is preferably made of a perovskite oxide. In this case, since the solid electrolyte body 2 exhibits excellent proton conductivity, the sensitivity of the ammonia sensor element 1 is improved.

  The perovskite oxide is not particularly limited, but is doped with rare earth elements such as Y and Yb. Strontium zirconate, calcium zirconate, barium zirconate, strontium cerate, calcium cerate, barium cerate, etc. Is exemplified. The solid electrolyte body 2 can contain at least one of these perovskite oxides.

  From the viewpoint that the detection temperature range of the ammonia sensor element 1 is further expanded, among these, barium zirconate and strontium zirconate are preferable, and those doped with rare earth elements are more preferable. Specifically, there are barium zirconate doped with Y, strontium zirconate doped with Yb, and the like.

  As illustrated in FIGS. 1 and 2, the solid electrolyte body 2 has a measurement gas surface 21 and a reference gas surface 22. The measurement gas surface 21 is a surface in contact with the measurement gas Gm to be measured. The reference gas surface 22 is a surface in contact with the reference gas Gb.

  The measurement gas surface 21 faces the measurement gas chamber 5, and the detection electrode 3 is formed on the measurement gas surface 21. The detection electrode 3 is provided in the measurement gas chamber 5 and is in contact with the solid electrolyte body 2.

  As illustrated in FIGS. 1 and 2, the ammonia sensor element 1 can have a reference gas chamber 6. The reference gas surface 22 of the solid electrolyte body 2 faces the reference gas chamber 6, and the reference electrode 4 is formed on the reference gas surface 22. The reference electrode 4 is provided in the reference gas chamber 6 and is in contact with the solid electrolyte body 2. That is, in the ammonia sensor element 1, the reference electrode 4, the solid electrolyte body 2, and the detection electrode 3 are laminated in this order.

In the detection electrode 3, at least a part of the interface 31 with the measurement gas chamber 5 is made of SnO ₂ , Al ₂ O ₃ , WO ₃ , ZnO in which Au, Pt—Au, TiO ₂ , SnO ₂ , and Sb are dissolved. Contains at least one selected from the group. Since these materials have low oxidation activity against ammonia, they are hereinafter referred to as “low-active electrode materials” as appropriate. That is, the low active electrode material is at least one selected from the group consisting of SnO ₂ , Al ₂ O ₃ , WO ₃ and ZnO in which Au, Pt—Au, TiO ₂ , SnO ₂ , and Sb are dissolved. . Pt—Au is an alloy of Pt and Au.

Low active electrode material, Au, Pt-Au, preferably contains at least one TiO _2, SnO _2, Sb is selected from the group consisting of SnO _2, Al ₂ O ₃ were dissolved. In this case, the detection electrode 3 can achieve both the electrochemical decomposition effect on ammonia and the catalytic oxidation suppression effect at a higher level. From the viewpoint of compatible at a higher level, at least a low active electrode material, Au, Pt-Au, a mixture of TiO ₂ and SnO _2, Sb is selected from the group consisting of SnO _2, Al ₂ O ₃ was dissolved More preferably, it contains at least one selected from the group consisting of Au, Pt—Au, a mixture of TiO ₂ and SnO _2, and Al ₂ O ₃ . Note that the mixture of TiO ₂ and SnO ₂ may contain a composite oxide of Ti and Sn.

  The detection electrode 3 only needs to contain a low activity electrode material in at least a part of the interface 31 between the detection electrode 3 and the measurement gas chamber 5, and the entire detection electrode 3 may contain a low activity material. Good. The detection electrode 3 can further contain a proton conductive solid electrolyte such as a noble metal such as Pt, Rh, and Pd and a perovskite oxide in addition to the low-active electrode material.

  When the detection electrode 3 contains a noble metal, the electrode performance of the detection electrode 3 is improved. That is, the progress of ammonia electrolysis at the detection electrode 3 can be promoted. Precious metals such as Pt, Rh, and Pd can be said to be highly active electrode materials with high ammonia decomposition activity. The highly active electrode material does not include Au or Pt—Au.

  Further, when the detection electrode 3 contains a proton conductive solid electrolyte, the three-phase interface between the electrode material, the solid electrolyte, and the gas phase (specifically, ammonia) is increased, and the ammonia decomposition activity is improved. . As a result, the reaction resistance of the detection electrode 3 is further reduced, and the ammonia decomposable temperature range in the ammonia sensor element 1 can be expanded. In this case, the adhesion between the detection electrode 3 and the solid electrolyte body 2 can be improved.

  Examples of the proton conductive solid electrolyte used for the detection electrode 3 are the same as those of the solid electrolyte body 2 described above. The detection electrode 3 and the proton conductive solid electrolyte of the solid electrolyte body 2 may be the same or different.

  As illustrated in FIGS. 3 and 4, the detection electrode 3 preferably has an electrode base layer 32 and an electrode surface layer 33. The electrode base layer 32 preferably contains a highly active electrode material such as a noble metal, and the electrode surface layer 33 preferably contains the above-described low activity electrode material. In this case, the electrode base layer 32 exhibits an excellent electrochemical decomposition effect on ammonia, and the electrode surface layer 33 exhibits a catalytic oxidation inhibiting effect on ammonia. However, the electrode surface layer 33 can also exhibit an electrochemical decomposition effect, although to a lesser extent than the electrode base layer. The detection electrode 3 having the electrode base layer 32 and the electrode surface layer 33 can sufficiently electrolyze ammonia while suppressing catalytic oxidation of ammonia.

The electrode base layer 32 contains at least one selected from the group consisting of Pt, Rh, and Pd among the noble metals from the viewpoint that both the electrochemical decomposition effect and the catalytic oxidation suppression effect can be achieved at a higher level. It is more preferable that Pt is further contained. The electrode surface layer 33 is selected from a group of metal oxides, that is, a group consisting of SnO ₂ , Al ₂ O ₃ , WO ₃ , and ZnO in which TiO ₂ , SnO ₂ , and Sb are dissolved, among the low active electrode materials. it is more preferable to contain at least one, a mixture of TiO ₂ and SnO _2, more preferably in Sb contains at least one selected from the group consisting of SnO _2, Al ₂ O ₃ were dissolved, TiO ₂ It is even more preferable to contain at least one of a mixture of SnO ₂ and Al ₂ O ₃ .

  Both the electrode base layer 32 and the electrode surface layer 33 can contain a proton conductive solid electrolyte. It is preferable that at least the electrode base layer 32 contains a proton conductive solid electrolyte. Of the electrode base layer 32 and the electrode surface layer 33, the electrode base layer 33 has a higher electrochemical decomposition activity with respect to ammonia, and when the electrode base layer 32 contains a proton conductive solid electrolyte, the decomposition activity can be further increased. Because it can.

  The formation pattern of the electrode surface layer 33 is illustrated in FIGS. 3 and 4. 3 and 4 show the detection electrode 3 on the solid electrolyte body 2 formed in the measurement gas chamber 5.

  As illustrated in FIG. 3, the electrode surface layer 33 can be laminated on the surface of the electrode base layer 32. The electrode surface layer 33 is formed on the surface of the electrode base layer 32 in the stacking direction Y. The electrode surface layer 33 is not formed on the surface in the axial direction X of the electrode base layer 32, and the surface in the axial direction X is exposed to the measurement gas chamber 5. Further, the electrode surface layer 33 is not formed on the surface of the electrode base layer 32 in the width direction Z and is exposed to the measurement gas chamber 5. That is, of the interface between the electrode base layer 32 and the measurement gas chamber 5, the interface in the stacking direction Y is stacked on the electrode surface layer 33, and the other interfaces are exposed in the measurement gas chamber 5. In this case, the detection electrode 3 can sufficiently exhibit the electrochemical decomposition effect and the catalytic oxidation suppression effect, and the electrode surface layer 33 can be easily formed.

  Further, as illustrated in FIG. 4, the electrode base layer 32 can be covered with the electrode surface layer 33. That is, all the surfaces of the electrode base layer 32 in the stacking direction Y, the axial direction X, and the width direction Z can be covered with the electrode surface layer 33. The electrode base layer 32 is completely covered by the electrode surface layer 33 and is not exposed to the measurement gas chamber 5. In this case, it is possible to further improve the catalytic oxidation suppression effect while the detection electrode sufficiently achieves the electrochemical decomposition effect and the catalytic oxidation suppression effect. That is, the catalytic oxidation of ammonia can be further suppressed while sufficiently showing the electrolysis effect on ammonia.

  The reference electrode 4 can have the same configuration as the detection electrode 3 described above. That is, the reference electrode 4 can contain an electrode material and a proton conductive solid electrolyte. However, the type of electrode material in the reference electrode 4, the type of proton conductive solid electrolyte, and the like may be the same as or different from those of the detection electrode 3. It is also possible to use noble metals such as Pt, Rh, and Pd as the electrode material of the reference electrode.

  As illustrated in FIG. 1, the measurement gas chamber 5 is an internal space into which the measurement gas Gm is introduced. The measurement gas Gm is a mixed gas that may contain ammonia such as exhaust gas. When the measurement gas Gm is exhaust gas, the measurement gas Gm can include oxygen, nitrogen, carbon dioxide, nitrogen oxides, ammonia, hydrogen, hydrocarbons, water, and the like. The measurement gas chamber 5 includes a space surrounded by, for example, the solid electrolyte body 2, the first spacer 12, the insulator 13, and the like.

  The first spacer 12 and the insulator 13 are made of an electrically insulating ceramic such as alumina. The first spacer 12 and the insulator 13 are formed of a dense body that does not substantially transmit gas. It is preferable that the first spacer 12 and the insulator 13 are integrally sintered.

  The measurement gas chamber 5 has a gas introduction part 51. The gas introduction part 51 serves as an inlet for the measurement gas Gm into the measurement gas chamber 5. As illustrated in FIG. 1, the gas introduction part 51 can be provided at the tip 111 of the ammonia sensor element 1, for example. The formation position of the gas introduction part 51 may not be the tip 111. The gas introduction part can be provided in a contact area with the measurement gas in the ammonia sensor element 1. Preferably, the gas introduction part 51 is provided closer to the tip 111 than the center in the longitudinal direction X of the ammonia sensor element 1. In this case, the measurement gas Gm can be easily introduced from the gas introduction part 51 without controlling the flow of the measurement gas.

  The ammonia sensor element 1 preferably has a gas diffusion layer 34. In this case, the limiting current type ammonia sensor element 1 can be obtained. The gas diffusion layer 34 is a part that controls the amount of the measurement gas Gm that reaches the detection electrode 3. The gas diffusion layer 34 is made of, for example, porous ceramics. The measurement gas Gm diffuses through a large number of small holes in the gas diffusion layer 34. The amount of the measurement gas Gm that reaches the detection electrode 3 can be controlled by the resistance at the time of diffusion, and a limit current can be developed.

  Examples of the material of the porous ceramic in the gas diffusion layer 34 include titanium oxide, alumina, tungsten oxide, tin oxide, and zinc oxide. Among these, titanium oxide is preferable from the viewpoint that the oxidation of ammonia as a detection gas can be suppressed in the presence of oxygen.

  As illustrated in FIG. 1, the gas diffusion layer 34 can be formed, for example, in the gas introduction part 51 of the measurement gas chamber 5. In this case, the amount of the measurement gas Gm reaching the detection electrode 3 can be controlled by controlling the amount of the measurement gas Gm flowing into the measurement gas chamber 5.

  Although illustration of a structure is abbreviate | omitted, it is also possible to laminate | stack a gas diffusion layer on a detection electrode. That is, the detection electrode may be covered with the gas diffusion layer. In this case, the amount of measurement gas reaching the detection electrode can be controlled without providing a gas diffusion layer in the gas introduction part of the measurement gas chamber.

  The reference gas chamber 6 is an internal space into which the reference gas Gb is introduced. The reference gas chamber 6 includes a space surrounded by the solid electrolyte body 2, the second spacer 14, and the ceramic heater 15, for example. The second spacer 14 is formed of a dense body that does not substantially transmit gas. The ceramic heater 15 has a heat generating portion 151 inside, and the periphery of the heat generating portion 151 is formed by a dense body that does not substantially allow gas to pass therethrough. The reference gas surface 22 faces the reference gas chamber 6, and the reference electrode 4 can be formed in the reference gas chamber 6.

  By providing the reference gas chamber 6, the reference electrode 4 can be partitioned from the external gas atmosphere. Thereby, it can suppress that the solid projectile contained in the external gas atmosphere collides with the reference electrode 4, and the reference electrode 4 is damaged. Note that the ammonia sensor element 1 does not necessarily have the reference gas chamber 6.

  The reference gas chamber 6 can have a gas introduction part 61. The gas introduction part 61 serves as an inlet for the reference gas Gb into the reference gas chamber 6. As illustrated in FIG. 1, the gas introduction part 61 can be provided at the base end 112 of the ammonia sensor element 1, for example. The position of the gas introduction part 61 in the reference gas chamber 6 can be appropriately changed in the same manner as the gas introduction part 51 in the measurement gas chamber 5 described above. The reference gas Gb is, for example, the atmosphere, but may be the same type of gas as the measurement gas Gm, such as exhaust gas.

  The second spacer 14 and the ceramic heater 15 forming the reference gas chamber 6 are formed of an electrically insulating ceramic such as alumina. The ceramic heater 15 has a heat generating portion 151 inside, and the heat generating portion 151 is sandwiched between the first insulating layer 152 and the second insulating layer 153. The first insulating layer 152 and the second insulating layer 153 are made of dense alumina, for example. The solid electrolyte body 2 is disposed between the measurement gas chamber 5 and the reference gas chamber 6.

  The ammonia sensor element 1 is formed, for example, by laminating and firing various ceramic sheets for forming the solid electrolyte body 2, the first spacer 12, the gas diffusion layer 34, the insulator 13, the second spacer 14, and the heater 15. can get. Note that various electrode pastes for forming the detection electrode 3 and the reference electrode 4 are applied to the ceramic sheet for forming the solid electrolyte body 2 before lamination. In addition, the ceramic sheet for forming the first insulating layer 152 or the second insulating layer 153 of the heater 15 is an electrode paste for forming the heating portion 151 and various lead portions for applying a voltage to the heating portion. Apply.

  The detection electrode paste is obtained, for example, by mixing an electrode material such as a low activity electrode material or a high activity electrode material, a proton conductive solid electrolyte added as necessary, and an organic binder, and kneading the mixture sufficiently. It is done. The reference electrode paste can be prepared in the same manner as the detection electrode paste.

In the ammonia sensor element 1 illustrated in FIGS. 1 and 2, the measurement gas Gm is introduced from the outside of the ammonia sensor element 1 into the measurement gas chamber 5 through the gas diffusion layer 34. In the measurement gas chamber 5, ammonia in the measurement gas Gm is decomposed by the following reaction formula (I) at the detection electrode 3 to generate protons.
_{^{2NH 3 → 6H + + 6e -}} ··· (I)

Protons generated in the detection electrode 3 are conducted in the solid electrolyte body 2 to reach the reference electrode 4, and water is generated in the reference electrode 4 by the following reaction formula (II).
6H ⁺ + 3/2 O ₂ + 6e ⁻ → 3H ₂ O (II)

  When the ammonia sensor element 1 has the gas diffusion layer 34, when the reaction of the above formulas (I) and (II) proceeds smoothly in the detection electrode 3 and the reference electrode 4, the ammonia detection electrode 3 is moved to. The diffusion of is the rate-limiting reaction. This is because the supply of ammonia is limited by the gas diffusion layer 34. Therefore, a limit current depending on the ammonia concentration is observed between the detection electrode 3 and the reference electrode 4. Based on this limit current, the ammonia sensor element 1 can detect the ammonia concentration.

  As illustrated in FIGS. 1 and 2, in the ammonia sensor element 1, at least a part of the interface 31 with the measurement gas chamber 5 in the detection electrode 3 contains a specific low activity electrode material. Such a detection electrode 3 exhibits low ammonia oxidation activity while exhibiting ammonia decomposition activity. That is, the detection electrode 3 can achieve both a catalytic oxidation suppression effect on ammonia and an electrochemical decomposition effect on ammonia.

  Therefore, even when the measurement gas Gm contains oxygen together with ammonia, consumption of ammonia in the measurement gas Gm by the detection electrode 3 can be prevented, and the detection electrode 3 can extract protons from ammonia. . Thereby, the ammonia sensor element 1 can detect accurately the electric current resulting from the conduction of protons based on the decomposition of ammonia. As a result, the ammonia sensor element 1 can accurately detect the ammonia concentration.

(Embodiment 2)
In this embodiment, an ammonia sensor element in which the detection electrode is formed of a single layer will be described. Of the reference numerals used in the second and subsequent embodiments, the same reference numerals as those used in the above-described embodiments represent the same components as those in the above-described embodiments unless otherwise indicated.

  As illustrated in FIG. 5, the detection electrode 3 can be formed of a single layer. In this case, not only the interface 31 of the detection electrode 3 with the reference gas chamber 5, but also the entire detection electrode 3 can contain the above-described low activity electrode material.

  When the entire detection electrode 3 contains the above-described low-active electrode material as in this embodiment, it is preferable to contain a noble metal such as Pt, Rh, or Pd together with the low-active electrode material. Thereby, the fall of electrode performance can be suppressed.

  Moreover, it is preferable that the detection electrode 3 contains a proton conductive solid electrolyte. In this case, the ammonia decomposition activity is improved because the three-phase interface is increased.

(Experimental example 1)
In this example, the oxidative degradation inhibiting effect of various low-active electrode materials on ammonia is examined by mass spectrometry. The same analysis is performed for Pt for comparison.

  FIG. 6 shows an outline of the analyzer 8. As illustrated in FIG. 6, the analyzer 8 includes a quartz sample tube 81, gas tube fixtures 82 and 83 that close the openings at both ends of the sample tube 81, and gas tube fixtures 82 and 83. Gas pipes 84 and 85 that penetrate therethrough and a heater 86 that heats the inside of the sample pipe 81 are provided. The gas pipe fixtures 82 and 83 are made of stainless steel.

A measurement method will be described. First, as illustrated in FIG. 6, the sample powder 87 to be measured is packed in the sample tube 81. As the sample powder 87, Au powder, Pt—Au powder, Al ₂ O ₃ powder, and Pt powder were used, respectively.

  Next, He gas was flowed into the sample tube 81 in order to remove moisture and gas adhering to the sample powder 87. He gas is introduced into the sample tube 81 from the upstream gas tube 84, passes through the sample powder 87, and is discharged from the downstream gas tube 85. At this time, the inside of the sample tube 81 was heated to 800 ° C. at a temperature rising rate of 10 ° C./min by the heater 86 and then allowed to cool naturally.

Next, in the state where He gas containing 5000 ppm NH ₃ and He gas containing 5 % by volume of O ₂ are circulated in the sample tube 81, the inside of the sample tube 81 is heated at a rate of 10 ° C./min. The temperature was raised to 800 ° C. At this time, the gas component discharged from the downstream gas pipe 85 was detected by a mass spectrometer. The results are shown in FIGS.

FIG. 7 shows the results of Pt powder for comparison. FIG. 8 shows the result of the Au powder of the low activity electrode material. FIG. 9 shows the result of the Pt—Au powder of the low activity electrode material. FIG. 10 shows the result of the Al ₂ O ₃ powder of the low activity electrode material.

As can be seen from FIG. 7, with Pt, ammonia rapidly decreases around the temperature of 150 ° C. and reaches the detection limit. On the other hand, the amount of NO, N ₂ , and H ₂ O increases with the decrease in ammonia. This indicates that ammonia is decomposed and consumed by Pt. That is, Pt indicates that the activity of oxidizing ammonia is high in the presence of oxygen.

On the other hand, as is known from FIGS. 8 to 10, the oxidative decomposition of ammonia is suppressed in the low active electrode materials such as Au, Pt—Au, and Al ₂ O ₃ .

  Thus, according to this example, it can be seen that the catalytic oxidation of ammonia in the presence of oxygen can be suppressed by using the low active electrode material for the detection electrode.

(Experimental example 2)
In this example, ammonia is detected using a simple element of an ammonia sensor element having a detection electrode containing a low active electrode material. In this example, the simple element 10 illustrated in FIG. 11 is used. The simple element 10 includes a solid electrolyte body 2, a detection electrode 3, a reference electrode 4, and a measurement gas chamber 5.

The solid electrolyte body 2 is made of a sintered body of SrZr _0.9 Yb _0.1 O _3-δ that is proton conductive. Hereinafter, SrZr _0.9 Yb _0.1 O _3-δ is referred to as SZY as appropriate. The solid electrolyte body 2 has a disk shape.

The detection electrode 3 has an electrode base layer 32 and an electrode surface layer 33 formed on the surface thereof.
The electrode base layer 32 contains Pt as an electrode material, and further contains SZY as a proton conductive solid electrolyte. The electrode surface layer 33 contains SnO ₂ and TiO ₂ as a low active electrode material, and further contains SZY as a proton conductive solid electrolyte. Low active electrode material consists SnO ₂ and TiO ₂ sintered powder containing a composite oxide of SnO _2, TiO _2, and Sn and Ti.

  The reference electrode 4 has the same configuration as the electrode base layer 32, contains Pt as an electrode material, and further contains SZY as a proton conductive solid electrolyte.

  Current collectors 301 and 401 to which lead wires 302 and 402 are welded are attached to the detection electrode 3 and the reference electrode 4, respectively. The lead wires 302 and 402 are made of Pt wires, and the current collectors 301 and 401 are made of Pt nets.

  A diffusion cap 50 is attached to the solid electrolyte body 2 on the detection electrode 3 side, and a measurement gas chamber 5 is formed. The diffusion cap 50 is a lid-like member having a cylindrical portion and an axial end surface thereof closed. The diffusion cap 50 is made of alumina, and is fixed to the solid electrolyte body 2 with an inorganic adhesive 509.

  A pinhole 501 having a diameter of 0.5 mm is formed in the center of the closed end face of the diffusion cap 50. The pinhole 501 is a gas introduction part 51 to the measurement gas chamber 5 and serves as a gas diffusion layer that limits the amount of gas flowing into the measurement gas chamber.

  In producing the simple element 10, first, the detection electrode 3 and the reference electrode 4 were formed by printing and baking each electrode material paste on the solid electrolyte body 2. Current collectors 301 and 401 to which lead wires 302 and 402 were welded were attached to the detection electrode 3 and the reference electrode 4, respectively.

  Next, a diffusion cap was attached to the detection electrode 3 side of the solid electrolyte body 2 and fixed with an inorganic adhesive 509. In this way, the simple element 10 illustrated in FIG. 11 was produced.

  Next, the simple element 1 with the extension leads attached to the lead wires 302 and 401 was inserted into the quartz tube 810 as illustrated in FIG. Then, both ends of the quartz tube 810 were sealed with silicone rubber plugs 820 and 830. Glass tubes 840 and 850 penetrating the rubber plugs are inserted into the silicone rubber plugs 820 and 830. In FIG. 12, a part of the configuration of the simple element 10 is omitted for convenience of drawing.

After the quartz tube 810 was installed in the electric tubular furnace 860, N ₂ gas containing O ₂ having a concentration of 5% by volume was circulated in the quartz tube 810 at a flow rate of 300 ml / min. The gas flows into the quartz tube 810 from the upstream glass tube 840 and is discharged from the downstream glass tube 850 to the outside. With the gas flowing in the quartz tube 810, the temperature inside the quartz tube 810 was raised to a temperature of 500 ° C. at a heating rate of 5 ° C./min by an electric tubular furnace 860.

  Next, the extension lead of the simple element 1 was connected to a potentiogalvanostat. The potential was swept from the open circuit voltage to +1 V at a sweep rate of 2 mV / sec, and the current at this time was measured. The voltage is a potential with respect to the reference electrode.

Then, the N ₂ gas containing a concentration of 5% by volume of O _2, and N ₂ gas containing NH ₃ in 100 ppm, was circulated in the quartz tube 810 at a flow rate of 300 ml / min. Then, the potential was swept from the open circuit voltage to +1 V at a sweep speed of 2 mV / sec, and the current at this time was measured. FIG. 13 shows the relationship between voltage and current when the ammonia concentration is 0 and when the ammonia concentration is 100 ppm.

  As can be seen from FIG. 13, when the ammonia concentration is 0, the current value hardly rises even when the potential is swept positively. On the other hand, in the presence of ammonia at a concentration of 100 ppm, an increase in current value is observed by sweeping the potential positive. This is because electrolysis occurs in the detection electrode 3 of the element 10. When the potential is further swept, a region where the current value hardly changes is reached.

  This is because the ammonia concentration in the measurement gas chamber 5 decreases due to electrolysis, the current flowing in the solid electrolyte body 2 is rate-limited, and a limiting current is generated. That is, when the detection electrode 3 contains a low-active electrode material at the interface, a limit current appears even in the presence of oxygen, and the ammonia concentration can be detected with high accuracy.

(Embodiment 3)
In this embodiment, an embodiment of an ammonia sensor will be described with reference to FIGS. As illustrated in FIG. 14, the ammonia sensor 7 includes an ammonia sensor element 1. The configuration of the ammonia sensor element 1 can be the same as that of the above-described embodiment. FIG. 14 shows a vertical cross section of the ammonia sensor, the right side in the axial direction X is shown as a cross section, and a part of the left side is not a cross sectional view but an external view.

  The ammonia sensor element 1 is assembled in a substantially cylindrical housing H1 via an insulator H2 or the like. The outer periphery of the housing H1 is fixed to a measurement gas pipe P such as an exhaust gas pipe. Thereby, the tip in the axial direction X of the ammonia sensor 7 is disposed in the measurement gas pipe P and exposed to the measurement gas flowing in the pipe.

  A bottomed cylindrical element cover C1 is provided on the tip side in the axial direction of the ammonia sensor 7 (that is, the lower side in FIG. 14). In the ammonia sensor 7, the tip end side in the axial direction X of the ammonia sensor element 1 protrudes from the housing H1 and is covered with an element cover C1. The element cover C1 can be formed by a double structure of an inner cover C1 and an outer cover C2, for example. Thereby, it can prevent that the water etc. adhere to the ammonia sensor element 1, and are damaged. The element cover C1 is fixed by crimping to the housing H1.

  The base end side in the axial direction X of the ammonia sensor 7 (that is, the upper side in FIG. 14) protrudes from the housing H1 and is accommodated in the atmosphere-side cover C2.

  The detection electrode inside the ammonia sensor element 1 is electrically connected to the detection electrode terminal S11. The detection electrode terminal S11 is connected to the signal line S13 via the connection terminal S12 and the like, and is connected to an external current difference detection circuit (not shown).

  The reference electrode inside the ammonia sensor element 1 is electrically connected to the reference electrode terminal S21. The reference terminal electrode S21 is connected to the signal line S23 via the connection terminal S22 and the like, and is connected to an external current difference detection circuit (not shown).

  The signal lines S13 and S23 are insulated and held by a rubber bush at the opening on the base end side of the atmosphere-side cover C2.

  The heater lead inside the ammonia sensor element 1 is connected to a heater electrode terminal (not shown), and the heater electrode terminal is connected to an energization line (not shown) via a connection terminal (not shown). The energization line is connected to a heater control circuit in an engine control unit (that is, ECU) (not shown).

  The ammonia sensor 7 is used in an exhaust gas purification system illustrated in FIG. FIG. 15 shows a usage example of the ammonia sensor 7, and the usage method is not limited to this example.

  FIG. 15 shows an outline of an exhaust gas purification system of a diesel engine. As illustrated in FIG. 15, the exhaust gas discharged from the exhaust manifold E2 of the diesel engine E1 passes through the oxidation catalyst E3. The unburned hydrocarbon HC, carbon monoxide CO, and nitric oxide NO are oxidized by the oxidation catalyst E3.

Next, the exhaust gas passes through the diesel particulate filter E4 (that is, DPF). This DPF removes particulate matter PM. Next, the exhaust gas passes through the selective reduction catalyst E5 (that is, SCR). In this SCR, NOx is reduced to harmless N ₂ and H ₂ O and discharged.

  A urea injection nozzle E53 is provided on the inlet side of the SCR. The urea injection nozzle E53 injects urea water pumped up from the urea tank D52 by the urea pump E51 into the exhaust gas. This makes it possible to reduce NOx in the SCR.

  As illustrated in FIG. 15, the ammonia sensor 7 can be disposed, for example, on the outlet side of the SCR. Thereby, the concentration of ammonia contained in the exhaust gas after passing through the SCR can be monitored. And based on the detection result of the ammonia sensor 7, the drive of the urea injection nozzle E53 can be controlled in order to optimize the SCR.

  The present invention is not limited to the above embodiments, and can be applied to various embodiments without departing from the scope of the invention. For example, not only the detection electrode 3 but also other electrodes can be formed on the measurement gas surface 21 of the solid electrolyte body 2 of the ammonia sensor element 1. Examples of the other electrode include a monitor electrode for monitoring the concentration of oxygen or NOx. In Embodiment 1, the gas introduction part 61 of the reference gas chamber 6 is formed on the proximal end side in the axial direction, but may be formed on the distal end side. This is because the measurement gas and the reference gas may be different but may be the same.

DESCRIPTION OF SYMBOLS 1 Ammonia sensor element 2 Solid electrolyte body 3 Detection electrode 31 Interface 32 Electrode base layer 33 Electrode surface layer 4 Reference electrode 5 Measurement gas chamber

Claims (10)

A proton conductive solid electrolyte body (2) having a measurement gas surface (21) in contact with the measurement gas (Gm) and a reference gas surface (22) in contact with the reference gas (Gb);
A sensing electrode (3) formed on the measurement gas surface of the solid electrolyte body;
A reference electrode (4) formed on the reference gas surface of the solid electrolyte body;
A measurement gas chamber (5) that faces the measurement gas surface of the solid electrolyte body and into which the measurement gas is introduced from a gas introduction part (51),
At least a part of the interface (31) of the detection electrode with the measurement gas chamber is made of SnO ₂ , Al ₂ O ₃ , WO ₃ , ZnO in which Au, Pt—Au, TiO ₂ , SnO ₂ , and Sb are dissolved. An ammonia sensor element (1) containing at least one electrode material selected from the group consisting of: At least a part of the interface of the detection electrode with the measurement gas chamber is at least one selected from the group consisting of SnO ₂ and Al ₂ O ₃ in which Au, Pt—Au, TiO ₂ , SnO ₂ , and Sb are dissolved. An ammonia sensor element containing any electrode material.   The detection electrode has an electrode base layer (32) formed on the solid electrolyte body, and an electrode surface layer (33) laminated on the electrode base layer, and the electrode surface layer is made of the electrode material. Item 3. The ammonia sensor element according to Item 1 or 2.   The ammonia sensor element according to claim 3, wherein the electrode base layer contains a noble metal.   The ammonia sensor element according to claim 4, wherein the electrode base layer contains at least one selected from the group consisting of Pt, Rh, and Pd as the noble metal.   The ammonia sensor element according to any one of claims 3 to 5, wherein the electrode base layer contains a proton conductive solid electrolyte.   The ammonia sensor element according to claim 1 or 2, wherein the entire detection electrode contains the electrode material.   The ammonia sensor element according to claim 7, wherein the detection electrode contains a proton conductive solid electrolyte.   Furthermore, the ammonia sensor element of any one of Claims 1-8 which has a gas diffusion layer (34) which controls the quantity of the measurement gas which reaches | attains the said detection electrode.   An ammonia sensor (7) comprising the ammonia sensor element according to any one of claims 1 to 9.

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