JP6867921B2 - Ammonia concentration measuring device, ammonia concentration measuring system, exhaust gas treatment system, and ammonia concentration measuring method - Google Patents

Description

The present invention relates to an ammonia concentration measuring device, an ammonia concentration measuring system, an exhaust gas treatment system, and an ammonia concentration measuring method.

Conventionally, an ammonia sensor for detecting the ammonia concentration in a gas to be measured such as an exhaust gas of an automobile has been known. For example, Patent Document 1 describes a multi-gas sensor including an ammonia sensor unit in which a pair of electrodes are arranged on a solid electrolyte body. Further, as a characteristic of the electromotive force EMF of a mixed potential cell having a solid electrolyte and a pair of electrodes, the following formula (2) based on the mixed potential formula is known (for example, Non-Patent Document 1).

Patent No. 5204160

D. Schonauer et al., Sensors and Actuators B vol.140 (2009), p.585-590

However, when the inventors have investigated, in the actual sensor element, the electromotive force EMF, ammonia concentration p _NH3, relationship of the oxygen concentration p _O2, H ₂ O concentration p _{H2 O} was in some cases not be a formula (2) as .. _{Therefore, when the ammonia concentration p NH3} is derived using the equation (2) in the hybrid potential type ammonia sensor, the ammonia concentration in the gas to be measured may not be derived accurately.

The present invention has been made to solve such a problem, and an object of the present invention is to more accurately derive the ammonia concentration in the gas to be measured.

The present invention has taken the following measures to achieve the above-mentioned main object.

The ammonia concentration measuring device of the present invention
The concentration of ammonia in the gas to be measured using a sensor element having a mixed potential cell having a solid electrolyte body, a detection electrode disposed on the solid electrolyte body, and a reference electrode disposed on the solid electrolyte body. It is an ammonia concentration measuring device that measures
An electromotive force acquisition unit that acquires information on the electromotive force of the hybrid potential cell when the detection electrode is exposed to the gas to be measured, and an electromotive force acquisition unit.
An oxygen concentration acquisition unit that acquires information on the oxygen concentration of the gas to be measured, and an oxygen concentration acquisition unit.
An ammonia concentration derivation unit that derives the ammonia concentration in the gas to be measured based on the information on the acquired electromotive force, the information on the acquired oxygen concentration, and the relationship of the following formula (1).
EMF = αlog _a (p _NH3 ) -βlog _b (p _O2 ) + B (1)
(However,
EMF: Electromotive force of the hybrid potential cell α, β, B: Constant a, b: Arbitrary bottom (where a ≠ 1, a> 0, b ≠ 1, b> 0)
p _NH3 : Ammonia concentration in the gas to be measured p _O2 : Oxygen concentration in the gas to be measured)
It is equipped with.

This ammonia concentration measuring device is based on the information on the electromotive force of the mixed potential cell of the sensor element, the information on the oxygen concentration in the gas to be measured, and the relationship of the above equation (1), and the ammonia concentration in the gas to be measured. Is derived. In this way, by using the above formula (1), for example, the ammonia concentration in the gas to be measured can be derived more accurately than when the above formula (2) is used. Here, the derivation of the ammonia concentration based on the relation of the formula (1) may be performed by using the relation of the formula (1), and is not limited to the derivation of the ammonia concentration using the relation of the formula (1) itself. .. For example, the ammonia concentration may be derived based on a modified formula (1). Further, the relationship between the values of each variable (EMF, p _NH3 , p _O2 ) in the equation (1) may be stored as a map, and the ammonia concentration may be derived based on the map. Further, the constants α, β, and B are values that are determined according to the sensor element, and can be obtained in advance by, for example, an experiment.

The ammonia concentration measuring system of the present invention includes the sensor element and the above-mentioned ammonia concentration measuring device. Therefore, this ammonia concentration measuring system can obtain the same effect as the above-described ammonia concentration measuring device of the present invention, for example, the effect of more accurately deriving the ammonia concentration in the gas to be measured.

In the ammonia concentration measuring system of the present invention, the detection electrode may contain Au-Pt alloy as a main component. The Au-Pt alloy is suitable as the main component of the detection electrode because a mixed potential is likely to occur at the three-phase interface between the solid electrolyte and the gas to be measured. In this case, the detection electrode has a density (= Au abundance [atom%] / Pt) measured using at least one of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The abundance amount [atom%]) may be 0.3 or more. When the degree of enrichment is 0.3 or more, the hybrid potential can be generated more reliably. However, the degree of concentration may be a value of 0.1 or more.

In the ammonia concentration measuring system of the present invention, the sensor element may include a heater that raises the temperature of the hybrid potential cell to a driving temperature of 450 ° C. or higher and 650 ° C. or lower. In this ammonia concentration measurement system, the solid electrolyte can be appropriately activated by setting the driving temperature to 450 ° C. or higher. Further, in this ammonia concentration measuring system, by setting the driving temperature to 650 ° C. or lower, it is possible to suppress a decrease in measurement accuracy due to combustion of ammonia. The driving temperature may be 600 ° C. or lower.

The exhaust gas treatment system of the present invention includes an exhaust gas path which is a path of exhaust gas of an internal combustion engine as the gas to be measured and an ammonia concentration measuring system of any of the above-described embodiments, and the sensor element is the exhaust gas path. It is arranged in. Therefore, this exhaust gas treatment system can obtain the same effect as the above-mentioned ammonia concentration measuring system of the present invention, for example, the effect of more accurately deriving the ammonia concentration in the gas to be measured.

The exhaust gas treatment system of the present invention includes one or more oxidation catalysts arranged in the exhaust gas path, and the sensor element has the exhaust gas more than the one or more arranged in the most upstream of the one or more oxidation catalysts. It may be arranged on the downstream side of the route. By doing so, the sensor element has a component (for example, at least one of hydrocarbon and carbon monoxide) that is present in the gas to be measured and affects the measurement accuracy of the ammonia concentration after being oxidized by the oxidation catalyst. The measurement gas will reach. Therefore, this exhaust gas treatment system can more accurately derive the ammonia concentration in the gas to be measured.

The method for measuring ammonia concentration of the present invention is
Ammonia concentration in a gas to be measured using a sensor element having a mixed potential cell having a solid electrolyte body, a detection electrode disposed on the solid electrolyte body, and a reference electrode arranged on the solid electrolyte body. It is a measurement method of
An electromotive force acquisition step for acquiring information on the electromotive force of the hybrid potential cell when the detection electrode is exposed to the gas to be measured, and
An oxygen concentration acquisition step for acquiring information on the oxygen concentration of the gas to be measured, and
A concentration derivation step for deriving the ammonia concentration in the gas to be measured based on the information on the acquired electromotive force, the information on the acquired oxygen concentration, and the relationship of the following formula (1).
EMF = αlog _a (p _NH3 ) -βlog _b (p _O2 ) + B (1)
(However,
EMF: Electromotive force of the hybrid potential cell α, β, B: Constant a, b: Arbitrary bottom (where a ≠ 1, a> 0, b ≠ 1, b> 0)
p _NH3 : Ammonia concentration in the gas to be measured p _O2 : Oxygen concentration in the gas to be measured)
Is included.

In this method of measuring the ammonia concentration, the ammonia concentration in the gas to be measured can be derived more accurately based on the relationship of the formula (1), as in the case of the above-mentioned ammonia concentration measuring device. In this ammonia concentration measuring method, various aspects of the above-mentioned ammonia concentration measuring device, ammonia concentration measuring system and exhaust gas treatment system may be adopted, or steps for realizing each of these functions may be added. Good.

Explanatory drawing of exhaust gas treatment system 2 of engine 1. Explanatory drawing of ammonia concentration measurement system 20. The flowchart which shows an example of the ammonia concentration derivation processing routine. The flowchart which shows an example of a constant derivation process. _{The graph which shows the relationship between the ammonia concentration p NH3} [ppm] of the sensor elements 1 and 2 and the electromotive force EMF [mV]. _{The graph which shows the relationship between the oxygen concentration p O2} [%] of the sensor elements 1 and 2 and the electromotive force EMF [mV]. The graph which shows the electromotive force EMF actually measured in the sensor element 1 and the electromotive force EMF derived from the equation (4). The graph which shows the electromotive force EMF actually measured in the sensor element 2 and the electromotive force EMF derived from the equation (5). _{The graph which shows the relationship between the H 2} O concentration p _H2O [%] of the sensor elements 1 and 2 and the electromotive force EMF [mV].

Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an explanatory diagram of an exhaust gas treatment system 2 of an engine 1 according to an embodiment of the present invention. FIG. 2 is an explanatory diagram of the ammonia concentration measuring system 20.

The exhaust gas treatment system 2 is a system that treats the exhaust gas of the engine 1 as the gas to be measured. The engine 1 is a diesel engine in this embodiment. As shown in FIG. 1, the exhaust gas treatment system 2 includes an exhaust gas path 3 connected to the engine 1 and an ammonia concentration measuring system 20 including a gas sensor 30 arranged in the exhaust gas path 3. The exhaust gas treatment system 2 includes a DOC (Diesel Oxidation Catalyst) 4, a DPF (Diesel particulate filter) 5, an injector 6, and an SCR (Selective Catalytic Reduction) from the upstream to the downstream of the exhaust gas. , Selective reduction catalyst) 7, gas sensor 30, and ASC (Ammonia Slip Catalyst) 8 are arranged in this order. DOC4 is one of the oxidation catalysts included in the exhaust gas treatment system 2, and converts HC and CO in the exhaust gas into water and carbon dioxide to detoxify them. DPF5 captures PM in the exhaust gas. The injector 6 is a device that injects at least one of ammonia and a substance capable of producing ammonia (for example, urea) into an exhaust pipe and sends it to SCR7. In the present embodiment, the injector 6 injects urea, and the injected urea is hydrolyzed to produce ammonia. The SCR 7 uses ammonia supplied into the exhaust pipe by the injector 6 to reduce nitrogen oxides (NOx) in the exhaust gas and decompose them into _{harmless N 2} and H _{2 O.} The exhaust gas after passing through the SCR 7 flows in the pipe 10. The gas sensor 30 is attached to the pipe 10. The ASC 8 is arranged downstream of the pipe 10. ASC8 is one of the oxidation catalysts included in the exhaust gas treatment system 2, and is also called a post-stage DOC as opposed to a DOC4 (pre-stage DOC). That is, the exhaust gas treatment system 2 of the present embodiment has two oxidation catalysts, DOC4 and ASC8. Further, the gas sensor 30 is arranged on the downstream side of the DOC4 arranged in the uppermost stream among the one or more (two in this case) oxidation catalyst included in the exhaust gas treatment system 2. ASC8 oxidizes excess ammonia in the exhaust gas that has passed through SCR7 and decomposes it into _{harmless N 2} and H _{2 O.} The exhaust gas after passing through ASC8 is released into the atmosphere, for example.

The ammonia concentration measuring system 20 includes the gas sensor 30 described above and an ammonia concentration measuring device 70 electrically connected to the gas sensor 30. The gas sensor 30 is configured as an ammonia sensor that generates an electric signal according to the excess ammonia concentration contained in the gas to be measured in the pipe 10 after passing through the SCR 7. Further, the gas sensor 30 also has a function as an oxygen sensor that generates an electric signal according to the oxygen concentration in the gas to be measured, and is configured as a multi-sensor. The ammonia concentration measuring device 70 derives the ammonia concentration in the gas to be measured based on the electric signal generated by the gas sensor 30 and transmits it to the engine ECU 9. The engine ECU 9 controls the amount of urea injected from the injector 6 into the exhaust pipe so that the detected excess ammonia concentration approaches zero. Hereinafter, the ammonia concentration measuring system 20 will be described in detail.

As shown in FIG. 1, the gas sensor 30 is fixed in the pipe 10 with the central axis of the gas sensor 30 perpendicular to the flow of the gas to be measured in the pipe 10. In the pipe 10, the central axis of the gas sensor 30 is perpendicular to the flow of the gas to be measured in the pipe 10 and is tilted by a predetermined angle (for example, 45 °) with respect to the vertical direction (vertical direction in FIG. 1). It may be fixed to. As shown in the enlarged cross-sectional view of FIG. 1, the gas sensor 30 includes a sensor element 31 and a protective cover 32 that covers and protects the front end side (lower end side of FIG. 1) which is one end side in the longitudinal direction of the sensor element 31. It includes an element fixing portion 33 for sealing and fixing the sensor element 31, and a nut 37 attached to the element fixing portion 33. Further, one end side of the sensor element 31 is covered with a porous protective layer 48.

The protective cover 32 is a bottomed tubular cover that covers one end of the sensor element 31, and although it is a single cover in FIG. 1, for example, it may be a double or more cover having an inner protective cover and an outer protective cover. .. The protective cover 32 is formed with a plurality of holes for allowing the gas to be measured to flow through the protective cover 32. One end of the sensor element 31 and the porous protective layer 48 are arranged in a space surrounded by the protective cover 32.

The element fixing portion 33 includes a cylindrical main metal fitting 34, a ceramic supporter 35 sealed in the through hole inside the main metal fitting 34, and ceramics such as talc sealed in the through hole inside the main metal fitting 34. It includes a green compact 36 obtained by molding a powder. The sensor element 31 is located on the central axis of the element fixing portion 33, and penetrates the element fixing portion 33 in the front-rear direction. The green compact 36 is compressed between the main metal fitting 34 and the sensor element 31. As a result, the green compact 36 seals the through hole in the main metal fitting 34 and fixes the sensor element 31.

The nut 37 is fixed coaxially with the main metal fitting 34, and a male screw portion is formed on the outer peripheral surface. The male threaded portion of the nut 37 is inserted into the mounting member 12 which is welded to the pipe 10 and has a female threaded portion on the inner peripheral surface. As a result, the gas sensor 30 can be fixed to the pipe 10 with one end side of the sensor element 31 and the protective cover 32 protruding into the pipe 10.

The sensor element 31 will be described with reference to FIG. The cross-sectional view of the sensor element 31 of FIG. 2 shows a cross section of the sensor element 31 along the central axis in the longitudinal direction (cross section along the vertical direction of FIG. 1). The sensor element 31 includes a base 40 made of an oxygen ion conductive solid electrolyte, and a detection electrode 51 provided on one end (lower end in FIG. 1, left end in FIG. 2) side of the sensor element 31 and on the upper surface of the base 40. It includes an auxiliary electrode 52, a reference electrode 53 provided inside the base 40, and a heater unit 60 for adjusting the temperature of the base 40.

The base 40 has four layers, a first substrate layer 41, a second substrate layer 42, a spacer layer 43, and a solid electrolyte layer 44 _{, each of which is composed of an oxygen ion conductive solid electrolyte layer such as zirconia (ZrO 2).} However, it has a plate-like structure laminated in this order from the lower side in FIG. The solid electrolyte forming these four layers is dense and airtight. The periphery of the portion of the base 40 existing in the protective cover 32 is exposed to the gas to be measured introduced in the protective cover 32. Further, in the base portion 40, a reference gas introduction space 46 is provided between the upper surface of the second substrate layer 42 and the lower surface of the solid electrolyte layer 44 at a position where the side portion is partitioned by the side surface of the spacer layer 43. Has been done. The reference gas introduction space 46 is provided with an opening on the other end side (right end side in FIG. 2), which is a position far from one end side of the sensor element 31. For example, the atmosphere is introduced into the reference gas introduction space 46 as a reference gas for measuring the ammonia concentration and the oxygen concentration. Each layer of the base 40 may be a substrate made of a zirconia solid electrolyte (yttria-stabilized zirconia (YSZ) substrate) to which _{yttria (Y 2} O _{3) is added in an amount of 3 to 15 mol% as a stabilizer.}

The detection electrode 51 is a porous electrode arranged on the upper surface of the solid electrolyte layer 44 in FIG. 2 of the base 40. The detection electrode 51, the solid electrolyte layer 44, and the reference electrode 53 constitute a hybrid potential cell 55. In the hybrid potential cell 55, a hybrid potential (electromotive force EMF) is generated in the detection electrode 51 according to the concentration of a predetermined gas component in the gas to be measured. Then, the value of the electromotive force EMF between the detection electrode 51 and the reference electrode 53 is used to derive the ammonia concentration in the gas to be measured. The detection electrode 51 is composed mainly of a material that generates a hybrid potential according to the ammonia concentration and has a detection sensitivity to the ammonia concentration. The detection electrode 51 may contain a noble metal such as gold (Au) as a main component. The detection electrode 51 preferably contains an Au-Pt alloy as a main component. Here, the principal component means the component having the largest abundance (atm%, atomic weight ratio) among all the contained components. The detection electrode 51 has a density (= Au abundance [atom%] / Pt abundance [= Au abundance [atom%] / Pt abundance] measured using at least one of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Atom%]) is preferably a value of 0.3 or more. When the degree of enrichment is 0.3 or more, the hybrid potential can be generated more reliably. The density of the detection electrode 51 is the surface density of the surface of the noble metal particles of the detection electrode 51. The Au abundance [atom%] is determined as the Au abundance on the surface of the noble metal particles of the detection electrode 51. Similarly, the abundance of Pt [atom%] is determined as the abundance of Pt on the surface of the noble metal particles of the detection electrode 51. The surface of the noble metal particles may be the surface of the detection electrode 51 (for example, the upper surface of FIG. 2) or the fracture surface of the detection electrode 51. For example, when the surface of the detection electrode 51 (upper surface of FIG. 2) is exposed, the density can be measured on that surface, so the measurement may be performed by XPS. However, the degree of concentration may be measured by AES. On the other hand, when the detection electrode 51 is covered with the porous protective layer 48 as in the present embodiment, the fracture surface of the detection electrode 51 (the fracture surface along the vertical direction in FIG. 2) is measured by XPS or AES. And measure the degree of enrichment. As the concentration value increases, the abundance ratio of Pt on the surface of the detection electrode 51 decreases, so that ammonia in the gas to be measured can be suppressed from being decomposed by Pt around the detection electrode 51. Therefore, the larger the concentration value, the better the accuracy of deriving the ammonia concentration in the ammonia concentration measuring system 20. Specifically, the degree of concentration is preferably a value of 0.1 or more, and more preferably a value of 0.3 or more. There is no particular upper limit to the concentration value, and for example, the detection electrode 51 may not contain Pt. Further, the entire detection electrode 51 may be composed of Au.

The auxiliary electrode 52 is a porous electrode arranged on the upper surface of the solid electrolyte layer 44 like the detection electrode 51. The auxiliary electrode 52, the solid electrolyte layer 44, and the reference electrode 53 constitute an electrochemical concentration cell 56. In the concentration cell 56, an electromotive force difference V, which is a potential difference according to the oxygen concentration difference between the auxiliary electrode 52 and the reference electrode 53, is generated. Then, the value of this electromotive force difference V is used to derive the oxygen concentration (oxygen partial pressure) in the gas to be measured. The auxiliary electrode 52 may be a noble metal having catalytic activity. For example, as the auxiliary electrode 52, Pt, Ir, Rh, Rd, or an alloy containing at least one of them can be used. In this embodiment, the auxiliary electrode 52 is Pt.

The reference electrode 53 is a porous electrode arranged on the lower surface of the solid electrolyte layer 44, that is, on the side of the solid electrolyte layer 44 opposite to the detection electrode 51 and the auxiliary electrode 52. The reference electrode 53 is exposed in the reference gas introduction space 46, and the reference gas (here, the atmosphere) in the reference gas introduction space 46 is introduced. The potential of the reference electrode 53 serves as a reference for the above-mentioned electromotive force EMF and electromotive force difference V. The reference electrode 53 may be a noble metal having catalytic activity. For example, as the reference electrode 53, Pt, Ir, Rh, Rd, or an alloy containing at least one of them can be used. In this embodiment, the reference electrode 53 is Pt.

The porous protective layer 48 covers the surface of the sensor element 31 including the detection electrode 51 and the auxiliary electrode 52. The porous protective layer 48 plays a role of suppressing, for example, moisture in the gas to be measured from adhering to the sensor element 31 to cause cracks. The porous protective layer 48 contains, for example, any one of alumina, zirconia, spinel, cordierite, titania, and magnesia as a main component. In the present embodiment, the porous protective layer 48 is made of alumina. Although not particularly limited, the film thickness of the porous protective layer 48 is, for example, 20 to 1000 μm, and the porosity of the porous protective layer 48 is, for example, 5% by volume to 60% by volume. The sensor element 31 does not have to include the porous protective layer 48.

The heater unit 60 plays a role of temperature control for heating and keeping the base 40 (particularly the solid electrolyte layer 44) warm in order to activate the solid electrolyte of the base 40 and enhance the oxygen ion conductivity. The heater unit 60 includes a heater electrode 61, a heater 62, a through hole 63, a heater insulating layer 64, and a lead wire 66. The heater electrode 61 is an electrode formed so as to be in contact with the lower surface of the first substrate layer 41. The heater electrode 61 is connected to the heater power supply 77 of the ammonia concentration measuring device 70.

The heater 62 is an electric resistor formed in a manner of being sandwiched between the first substrate layer 41 and the second substrate layer 42 from above and below. The heater 62 is connected to the heater electrode 61 via a lead wire 66 and a through hole 63, and generates heat when power is supplied from the heater power supply 77 through the heater electrode 61 to heat the base 40 forming the sensor element 31. Keep warm. The heater 62 is configured to be able to control the output of the hybrid potential cell 55 and the concentration cell 56 (particularly the solid electrolyte layer 44) so as to have a predetermined driving temperature by using a temperature sensor (here, the temperature acquisition unit 78). .. Since the solid electrolyte layer 44 of the hybrid potential cell 55 can be appropriately activated, the driving temperature is preferably 450 ° C. or higher. Further, the driving temperature is preferably 650 ° C. or lower because it is possible to suppress a decrease in measurement accuracy due to combustion of ammonia. The driving temperature may be 600 ° C. or lower. The heater insulating layer 64 is an insulating layer made of porous alumina formed on the upper and lower surfaces of the heater 62 by an insulator such as alumina.

The ammonia concentration measuring device 70 is a device that measures the ammonia concentration in the gas to be measured by using the sensor element 31. Further, the ammonia concentration measuring device 70 also serves as a control device for the sensor element 31. The ammonia concentration measuring device 70 includes a control unit 72, an electromotive force acquisition unit 75, an oxygen concentration acquisition unit 76, a heater power supply 77, and a temperature acquisition unit 78.

The control unit 72 controls the entire device, and is configured as a microprocessor including, for example, a CPU and a RAM. The control unit 72 includes a storage unit 73 that stores a processing program and various data. The electromotive force acquisition unit 75 is a module that acquires information on the electromotive force EMF of the hybrid potential cell 55. In the present embodiment, the electromotive force acquisition unit 75 is configured as a voltage detection circuit connected to the detection electrode 51 and the reference electrode 53 of the mixed potential cell 55 to measure the electromotive force EMF. The oxygen concentration acquisition unit 76 is a module for acquiring information on the oxygen concentration in the gas to be measured. In the present embodiment, the oxygen concentration acquisition unit 76 is connected to the auxiliary electrode 52 and the reference electrode 53 of the concentration cell 56, and is configured as a voltage detection circuit for measuring the electromotive force difference V as information on the oxygen concentration. There is. The electromotive force acquisition unit 75 and the oxygen concentration acquisition unit 76 output the electromotive force EMF and the electromotive force difference V measured by each to the control unit 72. The control unit 72 derives the ammonia concentration in the gas to be measured based on the electromotive force EMF and the electromotive force difference V. The heater power supply 77 is a power supply that supplies electric power to the heater 62, and the output is controlled by the control unit 72. The temperature acquisition unit 78 is a module that acquires a value (here, a resistance value) related to the temperature of the heater 62. The temperature acquisition unit 78 is connected to the heater electrode 61, for example, and acquires the resistance value of the heater 62 by passing a minute current and measuring the voltage at that time.

Although not shown in FIG. 2, each electrode of the detection electrode 51, the auxiliary electrode 52, and the reference electrode 53 has a plurality of lead wires formed toward the other end (right side in FIG. 2) of the sensor element 31. It is one-to-one conducting. The electromotive force acquisition unit 75 and the oxygen concentration acquisition unit 76 measure the electromotive force EMF and the electromotive force difference V, respectively, via the lead wires.

Subsequently, the measurement of the ammonia concentration by the ammonia concentration measuring system 20 thus configured will be described. FIG. 3 is a flowchart showing an example of the ammonia concentration derivation processing routine executed by the control unit 72. The control unit 72 stores this routine in, for example, a storage unit 73, and when an ammonia concentration derivation command is input from the engine ECU 9, the control unit 72 repeatedly executes this routine in a predetermined cycle (several msec to several tens of msec, etc.), for example. .. The control unit 72 controls the output of the heater power supply 77 in advance to generate heat of the heater 62, and sets the temperature of the hybrid potential cell 55 and the concentration cell 56 to a predetermined driving temperature (for example, 450 ° C. or higher and 650 ° C. or lower). It is controlled so that it becomes (the temperature). The control unit 72 controls the output of the heater power supply 77 so that the temperature (here, the resistance value) of the heater 62 acquired by the temperature acquisition unit 78 becomes a predetermined value, so that the mixed potential cell 55 and the concentration cell The temperature of 56 is controlled to be a predetermined driving temperature.

When the ammonia concentration derivation processing routine is started, the control unit 72 first performs an electromotive force acquisition step of acquiring information on the electromotive force EMF of the hybrid potential cell 55 via the electromotive force acquisition unit 75 (step S100). In the present embodiment, the control unit 72 acquires the value of the electromotive force EMF measured by the electromotive force acquisition unit 75 as it is. The control unit 72 executes the ammonia concentration derivation processing routine basically in a state where the exhaust gas from the engine 1 circulates in the pipe 10 and the exhaust gas also circulates in the protective cover 32. Therefore, the control unit 72 acquires the electromotive force EMF of the hybrid potential cell 55 in a state where the detection electrode 51 is exposed to the gas to be measured. Here, in the mixed potential cell 55, an electrochemical reaction such as oxidation of ammonia in the gas to be measured and ionization of oxygen occurs at the three-phase interface between the detection electrode 51, the solid electrolyte layer 44, and the gas to be measured, and the detection electrode 51 Generates a mixed potential. Therefore, the electromotive force EMF is a value based on the ammonia concentration and the oxygen concentration in the gas to be measured.

Further, the control unit 72 performs an oxygen concentration acquisition step of acquiring information on the oxygen concentration of the gas to be measured via the oxygen concentration acquisition unit 76 (step S110). In the present embodiment, the control unit 72 acquires the electromotive force difference V of the concentration cell 56 from the oxygen concentration acquisition unit 76. Here, in the concentration cell 56, an electromotive force difference V is generated between the auxiliary electrode 52 and the reference electrode 53 according to the difference between the oxygen concentration in the gas to be measured and the oxygen concentration in the atmosphere in the reference gas introduction space 46. Occurs. _{Hydrocarbons, NH 3} , CO, NO, and NO ₂ in the gas to be measured are redox-reduced by the catalytic action of Pt, which is the auxiliary electrode 52. However, the concentration of these gas components in the gas to be measured is usually very small compared to the oxygen concentration in the gas to be measured, so even if these redoxes occur, the oxygen concentration in the gas to be measured will be high. Has little effect. Therefore, the electromotive force difference V becomes a value based on the oxygen concentration in the gas to be measured. The control unit 72 may perform either step S100 or step S110 first, or may perform in parallel.

Subsequently, the control unit 72 determines the ammonia concentration in the gas to be measured based on the information on the electromotive force EMF acquired in step S100, the information on the oxygen concentration acquired in step S110, and the relationship of the following formula (1). The concentration derivation step for deriving the above is performed (step S120), and this routine is terminated. The relationship of the formula (1) is stored in advance in, for example, the storage unit 73.

EMF = αlog _a (p _NH3 ) -βlog _b (p _O2 ) + B (1)
(However,
EMF: Electromotive force of hybrid potential cell 55 α, β, B: Constant a, b: Arbitrary bottom (where a ≠ 1, a> 0, b ≠ 1, b> 0)
p _NH3 : Ammonia concentration in the gas to be measured p _O2 : Oxygen concentration in the gas to be measured)

In step S120, the control unit 72 substitutes the value of the electromotive force EMF acquired in step S100 into “EMF” in the equation (1). The control unit 72 includes a electromotive force difference V obtained in step S110, a correspondence relationship between the electromotive force difference V and the oxygen concentration p _O2 stored in advance in the storage unit 73, the oxygen concentration p _O2 based on It is derived and the derived value is substituted into _{"p O2" in Eq. (1).} Then, the control unit 72 derives _{the ammonia concentration p NH3 in the equation (1).} The unit of the electromotive force EMF may be, for example, [mV]. Further, the ammonia concentration p _NH3 is the volume fraction of ammonia in the gas to be measured, and the oxygen concentration p _O2 is the volume fraction of oxygen in the gas to be measured. The units of p _NH3 and p _O2 may be, for example, a value expressed in parts per million [ppm], a value expressed in percentage [%], or a dimensionless value (for example, 10). If it is%, the value may be 0.1). The _{units of p NH3} and p _{O 2} may be different. The bases a and b may be, for example, a value of 10 or a Napier number e. The constants α, β, and B are values determined according to the sensor element 31, and may have different values depending on the sensor element 31. The constants α, β, and B can be obtained in advance by, for example, an experiment described later. The constants α and β may satisfy α: β ≠ (2/3): (1/2). The constants α and β may be positive values. _{It should be noted that the derivation of the ammonia concentration p NH3} performed by the control unit 72 based on the relationship of the equation (1) may be performed using the relationship of the equation (1), and the ammonia concentration using the equation (1) itself. It is not limited to the derivation of. For example, the storage unit 73 may store the equation (1) itself, or the following equation (1)'that is a modification of _{the equation (1) so that the left side is only "p NH3".} You may. Further, the storage unit 73 stores _{the relationship between the values of each variable (EMF, p NH3} , p _O2 ) in the equation (1) as a map, and the control unit 72 derives the _{ammonia concentration p NH3 based on the map.} You may.

As described above, in the present embodiment, the control unit 72 derives _{the ammonia concentration p NH3} in the gas to be measured based on the relationship of the above formula (1). Thereby, for example, the ammonia concentration in the gas to be measured can be derived more accurately than in the case of using the above formula (2). This will be described below.

As described above, the above equation (2) is known as a characteristic of the electromotive force EMF of the mixed potential type ammonia sensor. However, where the inventor has examined the actual sensor element (sensor element 31), the electromotive force EMF, ammonia concentration p _NH3, in the expression (2) as relationship of the oxygen concentration p _O2, H ₂ O concentration p _{H2 O} It didn't happen. For example, from the coefficients _{of the p NH3} term and the p _O2 term on the right side of the equation (2), the _{degree of influence of the ammonia concentration p NH3 on} the electromotive force EMF (NH ₃ sensitivity) and the oxygen concentration p _{O2 on} the electromotive force EMF. There should be a relationship of NH ₃ sensitivity: O ₂ coherence = (2/3): (1/2) with the degree of influence (O ₂ coherence), but there are cases where such a relationship does not occur. It was. Further, Equation (2) According in H ₂ O concentration p influence of the electromotive force EMF of _{H2 O} (H ₂ O coherent) but should exist, H ₂ of fact the measurement gas O 2 concentration p _{H2 O} The electromotive force EMF hardly changed even if.

At the three-phase interface of the mixed potential cell 55, the above-mentioned oxidation of ammonia and ionization of oxygen in the gas to be measured are represented by the anode reaction represented by the following reaction formula (a) and the following reaction formula (b). Cathodic reaction is occurring. The reaction formulas (a) and (b) can also be expressed as reaction formulas (a)'and (b)', respectively. _{In addition, "O o} " in the reaction formula (a) represents ^{the oxygen ion (O 2-} ) existing in the oxygen site in the solid electrolyte layer 44. Further, the fourth term on the right side of the reaction formula (a) indicates that oxygen ions (O ^2- ) are absent (deficient) at the oxygen sites in the solid electrolyte layer 44.

When the anode reaction and the cathode reaction occur simultaneously at the three-phase interface point of one detection electrode (for example, the detection electrode 51), a local battery is formed and an electromotive force EMF appears in the mixed potential cell (for example, mixed). Potential cell 55). Then, the electromotive force EMF at that time should theoretically follow the equation (2). _{For example, the coefficient "2/3" of the ammonia concentration p NH3 in} the above formula (2) is a value based on the coefficient "2/3" _{of NH 3 on} the left side of the reaction formula (a). Similarly, the equation (2) Oxygen concentration coefficient "1/2" on p _O2 and _{H 2} O concentration p _{H2 O} coefficients "1", the coefficient of the left side of the O ₂ of each reaction formula (b) "1 / 2 ”and a value based on the coefficient“ 1 ” _{of H 2} O on the right side of the reaction formula (a).

However, in an actual sensor element, it was confirmed by experiments that the relationship between the variables does not follow the equation (2) as described above, but the equation (1). The inventor thought that the reason why it _{should be substituted as p NH3} , p _O2 , and p _H2O in the formula (2) is not the concentration in the gas to be measured but the partial pressure felt by the three-phase interface. .. _{Assuming that the NH 3} partial pressure, O ₂ partial pressure, and H ₂ O partial pressure felt by the three-phase interface of the detection electrode are p _NH3 ^* , p _O2 ^* , and p _H2O ^* , respectively, the following equation (A1) holds. This can also be derived from Eq. (2). The actual electromotive force EMF is considered to follow this equation (A1) instead of the equation (2). On the other hand, since it is not possible to directly know the partial pressures that the three-phase interface feels, p _NH3 ^* , p _O2 ^* , and p _H2O ^* _{, p NH3} , p _O2 , and p _H2O in the gas to be measured can be obtained from Eq. It is necessary to derive the formula used. The inventor thought that the above equation (1), which is an equation using _{p NH3} , p _O2 , and p _H2O based on the equation (A1), could be explained as follows.

First, consider the hybrid potential equation from a microscopic point of view. _{As described above, the partial pressures lnp NH3} ^* , lp _O2 ^* , and lp _H2O ^* at the three-phase interface of the detection electrode do not match the _{partial pressures lnp NH3} , lp _O2 , and lp _H2O of the atmosphere (measured gas). Rather than reaching the three-phase interface directly from the gas phase, the electrochemical reaction is adsorbed from the gas phase to the surface of the detection electrode, diffused on the surface of the detection electrode, reaches the three-phase interface, and undergoes an electrochemical reaction. This is because the product undergoes a dynamic change in that it leaves the state of being adsorbed on the surface of the detection electrode. Now consider the _{product H 2} O of the anodic reaction. It is considered that the generated H ₂ O is adsorbed on the detection electrode and then desorbed in the gas phase. _{Since a large amount of H 2} O is present in the gas to be measured, it is considered _{that the H 2} O generated by the anodic reaction cannot be immediately desorbed from the surface of the detection electrode. Therefore, the H ₂ O partial pressure p _{H2 O} ^* to three-phase interface feel when H ₂ O is adsorbed greater than H ₂ O partial pressure p _{H2 O} in the gas to be measured, always following equation (A2) is satisfied It is thought that there is. _{Further, the H 2} O concentration in the gas to be measured (exhaust gas in this case) is usually about 5 to 15%, and the total pressure is constant at 1 atm. Therefore, for safety, the H ₂ O concentration is said to be 1 to 20%. Considering that it changes in a wide range, the following equation (A3) holds.

p _H2O ^* ＞ p _H2O (A2)
0.01 atm <p _H2O <0.2 atm (A3)

Next, the detection electrode surface H ₂ O consider whether p _{H2 O} ^* is happens to when p _{H2 O} is changed in a state that adsorbed. First, of the H ₂ O to the three-phase interface feel, the H ₂ O adsorbed to the detection electrode is denoted by H ₂ O (ad), denoted of H ₂ O vapor and H ₂ O (gas). Further, the partial pressure of _{H 2} O adsorbed on the detection electrode is referred to _{as p H 2 O (ad),} and the partial pressure of the gas phase H ₂ O is referred to as p _{H 2 O (gas).} Therefore, it can be expressed _{as p H2O} ^* = p _{H2O (ad)} + p _{H2O (gas).} The p _{H2 O (ad),} min adsorbed to the detection electrode of of H ₂ O in the measurement gas, the anode reaction (the reaction formula (a), (a) ' ) of H ₂ O produced by The amount adsorbed on the detection electrode is included. p _{H2O (gas)} includes the portion of H ₂ O in the gas to be measured that exists at the three-phase interface in the gas phase state and the portion of _{H 2 O generated by the anodic reaction that is in the gas phase state.} , Is included. The following _{equations (A4) and (A5) are established between H 2} O (ad) and H ₂ O (gas) with the equilibrium constant K = (constant), and p _H2O ^* is this equation (A4), ( It seems that it changes according to A5), but it is not actually the case. This is because p _H2O changes within the range of the above equation (A3), whereas p _{H2O (ad)} cannot change once the adsorption of H ₂ O to the detection electrode is stable and once reaches a steady state (= 1 atm). It is thought that this is the reason. Here, the _{reason why pH2O (ad)} is considered to be 1 atm in the steady state will be described. Adsorbed on the detecting electrode H ₂ O (ad) is not a gas phase, the amount of H ₂ O (ad) is represented by exactly is not a partial pressure activity a _{H2O (ad).} Then, H ₂ O to (ad) is regarded as the solid active amount a _{H2 O (ad)} is equal to 1 (that is, regardless of the amount of adsorption of the sensing electrode activity of 1), activity 1 to the partial pressure 1atm It can be regarded as equivalent.

From the above, p _{H2O (ad)} can be regarded as 1 atm. In addition, p _{H2 O (gas)} is likely to be about 0.01 to 0.2 atm as in Eq. (A3), _{but since there is H 2} O (ad) on the surface of the detection electrode that can be regarded as 1 atm, it is a concern. It is considered that the H ₂ O of the phase is less likely to contribute to the reaction, and the H ₂ O partial pressure p _{H 2 O (gas)} of the gas phase felt by the three-phase interface is considerably smaller than 0.01 to 0.2 atm. Be done. Therefore, it is considered that p _{H2O (ad)} >> p _{H2O (gas)} is established, and p _{H2O (gas)} is a negligibly small value. Thus, even if p _{H2 O} is changed in a state where the detection electrode surface H ₂ O is adsorbed, p _{H2 O} ^* is considered to be constant as the following equation (A6). As a result, the above formula (A1) can be regarded as the following formula (A7). That is, the partial pressure p _H2O ^* _{of H 2} O felt by the three-phase interface can be regarded as having no influence on the electromotive force EMF (H _{2 O coherence).}

Next, consider the hybrid potential equation from a macroscopic point of view. If the total pressure of the gas to be measured is 1 atm, the concentration and the partial pressure are equal. _Therefore , p _{NH3, p O2} , and p _{H2 O} will be described below as the partial pressures. From the above formula (A3), the following formula (A8) can be derived. Further, the following formula (A9) can be derived from the above formula (A6). From the following equations (A8) and (A9), the following equation (A10) is established. Further, the ratio of lnp _H2O ^* and lnp _H2O is defined as the pressure adjustment coefficient δ, and δ is defined by the equation (A11). From equation (A10), δ satisfies -1 <δ <1. Similarly, _{the ratio of lnp NH3} ^* to lnp _NH3 is defined as the pressure adjustment coefficient δ', and δ'is defined by the equation (A12). The pressure adjustment coefficients δ and δ'are values unique to the sensor element, for example, depending on the composition and structure of the detection electrode.

-4.6 <lnp _H2O <-1.6 (A8)
lnp _H2O ^* ≒ 0 (A9)
｜ lnp _H2O ^* ｜ ＜ ｜ lnp _H2O ｜ (A10)
δ = lnp _H2O ^* / lnp _H2O (A11)
δ'= lnp _NH3 ^* / lnp _NH3 (A12)

By transforming the equation (A1) using the pressure adjustment coefficients δ and δ', the following equation (A13) can be derived. In the following formula (A13), "lnp _H2O ^* = δ × lnp _{H2O " based on the above formula (A11) is substituted in the formula (A1), and “lnp NH3} ^* = δ'× lnp _NH3 based on the above formula (A12) is substituted. Substituting, and further substituting "lnp _O2 ^* = lnp _O2 ". In existing O ₂ sensors and SOFCs, it is well known that the relationship between oxygen concentration and electromotive force follows the Nernst equation, so it is clear that _{"lnp O2} ^* = lnp _{O2" holds.}

_{Since "lnp H2O} ^* = δ × lnp _H2O = 0" holds from the equations (A6) and (A11), the equation (A13) can be expressed as the following equation (A14), and the equation (A14) is further expressed by the equation (A15). )It can be expressed as. The constants A and B are values unique to the sensor element, for example, depending on the composition and structure of the detection electrode. Then, in the equation (A15), the base of the logarithm is set to arbitrary values a and b, and the coefficients of each term on the right side are expressed as constants α and β, so that the above equation (1) is derived.

Unlike the equation (2), this equation (1) _{does not always have the above-mentioned relationship of p NH3} sensitivity: p _O2 sensitivity = (2/3): (1/2), and H ₂ O interference. It can express that there was almost no sex. Therefore, by using the formula (1), the ammonia concentration p _NH3 can be derived more accurately than in the formula (2). The formula (1) is applicable not only when the total pressure of the gas to be measured is 1 atm but also when the total pressure is about 1 atm (for example, 0.9 atm to 1.10 atm). Further, the formula (1) can be applied even when the total pressure of the gas to be measured is other than about 1 atm.

The constants α, β, and B in the equation (1) can be obtained in advance by experiments as follows, for example. FIG. 4 is a flowchart showing an example of the constant derivation process. In the constant derivation process, first, for the sensor element 31 to which the constant is derived, the first electromotive force measurement process for acquiring the first electromotive force data _{showing the correspondence between the ammonia concentration p NH3 and the electromotive force EMF is performed a plurality of times (} Step S200). Specifically, first, the _{sensor element 31 is exposed to the gas to be measured in which the oxygen concentration p O2} and the ammonia concentration p _NH3 are adjusted to predetermined values, the electromotive force EMF at that time is measured, and the ammonia concentration p _NH3 is generated. The correspondence with the electric power EMF is acquired as the first electromotive force data. Next, while keeping the oxygen concentration p _O2 in the measured gas at the same value (constant), the ammonia concentration p _NH3 in the measured gas was changed to measure the electromotive force EMF multiple times, and similarly, a plurality of electromotive force EMFs were measured. Acquires the first electromotive force data of. When a plurality of first electromotive force data are acquired in this way, a constant α is derived based on the acquired data (step S210). Specifically, the relationship between the logarithmic log _a (p _{NH3 ) of the ammonia concentration p NH3} and the electromotive force EMF in a plurality of first electromotive force data acquired by a plurality of first electromotive force measurement processes is linear (linear). The slope when approximated by the function) is derived as a constant α. Approximation is performed based on, for example, the least squares method. As described above, by executing the first electromotive force measurement process a plurality of times with the oxygen concentration constant in step S200, it becomes easy to derive the constant α in step S210.

Next, for the sensor element 31 whose constant is to be derived, the second electromotive force measurement process for acquiring the second electromotive force data representing the correspondence between _{the oxygen concentration p O2 and the electromotive force EMF is performed a plurality of times (step S220).} This plurality of second electromotive force measurement processes is performed in the same manner as in step S200, except that _{the ammonia concentration p NH3} in the gas to be measured is kept constant and the oxygen concentration p _{O2 in the gas to be measured is changed.} be able to. Then, the constant β is derived based on the plurality of second electromotive force data acquired by the plurality of second electromotive force measurement processes (step S230). In this process, as in the process of step S210, the constant β is derived as the slope when the relationship between the logarithmic log _b (p _{O2 ) of the oxygen concentration p O2} and the electromotive force EMF is approximated by a straight line (linear function). As described above, by executing the second electromotive force measurement process a plurality of times with the ammonia concentration constant in step S220, it becomes easy to derive the constant β in step S230.

Then, the constant B is derived based on the constants α and β derived in steps S210 and S230, and at least one of one or more first electromotive force data and one or more second electromotive force data (step). S240), this process is terminated. For example, the derived constants α and β, the logarithm log _a (p _{NH3 ) of the ammonia concentration p NH3} in the first electromotive force data, the logarithm log _b (p _O2 ) of the fixed oxygen concentration p _O2 , and the electromotive force EMF. And may be substituted into the equation (1) to derive the constant B. At this time, the average value of the constants B derived for each of the plurality of first electromotive force data may be the constant B of the equation (1). Similarly, the constant B may be derived based on one or more second electromotive force data. Further, the average value of the constant B derived based on the first electromotive force data and the constant B derived based on the second electromotive force data may be derived as the constant B of the equation (1).

The first electromotive force data and the second electromotive force data are both measured in a state where the mixed potential cell 55 is raised to the same predetermined drive temperature by the heater 62. Further, as can be seen by comparing the equation (1) and the equation (A15), the constants α and β also change depending on the temperature T of the hybrid potential cell 55, that is, the driving temperature when the sensor element 31 is used. Therefore, when one sensor element 31 is used at a plurality of drive temperatures, the constants α and β of the equation (1) are derived for each of the plurality of drive temperatures and stored in the storage unit 73, for example. Keep it. Then, when the control unit 72 performs the ammonia concentration derivation process, the constants α and β corresponding to the driving temperature of the sensor element 31 are used. Since the value of the constant B may change depending on the drive temperature when the sensor element 31 is used, the constant B may be derived for each of the plurality of drive temperatures and stored in the storage unit 73, for example. Good.

Here, the correspondence between the components of the present embodiment and the components of the present invention will be clarified. The solid electrolyte layer 44 of the present embodiment corresponds to the solid electrolyte of the present invention, the detection electrode 51 corresponds to the detection electrode, the reference electrode 53 corresponds to the reference electrode, and the mixed potential cell 55 corresponds to the mixed potential cell. The electromotive force acquisition unit 75 corresponds to the electromotive force acquisition unit, the oxygen concentration acquisition unit 76 corresponds to the oxygen concentration acquisition unit, and the control unit 72 corresponds to the ammonia concentration derivation unit. In the present embodiment, an example of the ammonia concentration measuring method of the present invention is also clarified by explaining the operation of the ammonia concentration measuring device 70.

According to the exhaust gas treatment system 2 of the present embodiment described in detail above, the ammonia concentration measuring device 70 is in the gas to be measured as compared with the case where the above formula (2) is used, for example, based on the relationship of the formula (1). Ammonia concentration can be derived more accurately.

Further, since the detection electrode 51 contains Au-Pt alloy as a main component, a mixed potential at the three-phase interface between the solid electrolyte layer 44 and the gas to be measured is likely to occur. Further, the detection electrode 51 can more reliably generate a mixed potential when the density measured using at least one of XPS and AES is 0.3 or more.

Further, by setting the driving temperature of the hybrid potential cell 55 to 450 ° C. or higher, the solid electrolyte layer 44 can be appropriately activated. Further, by setting the driving temperature of the hybrid potential cell 55 to 650 ° C. or lower, it is possible to suppress a decrease in measurement accuracy due to combustion of ammonia.

Furthermore, the exhaust gas treatment system 2 includes one or more oxidation catalysts (DOC4 and ASC8) arranged in the exhaust gas path 3, and the sensor element 31 is arranged in the most upstream of the one or more oxidation catalysts. It is arranged on the downstream side of the exhaust gas path 3 with respect to the DOC 4. As a result, the sensor element 31 is measured after a component (for example, at least one of hydrocarbon and carbon monoxide) existing in the gas to be measured and affecting the measurement accuracy of the ammonia concentration is oxidized by the oxidation catalyst. The gas will arrive. Therefore, the exhaust gas treatment system 2 can more accurately derive the ammonia concentration in the gas to be measured.

It goes without saying that the present invention is not limited to the above-described embodiment, and can be implemented in various aspects as long as it belongs to the technical scope of the present invention.

For example, in the above-described embodiment, the detection electrode 51 and the reference electrode 53 are arranged on the solid electrolyte layer 44, but the detection electrode 51 and the reference electrode 53 may be arranged not only on the solid electrolyte layer 44 but also on the solid electrolyte body. For example, the detection electrode 51 and the reference electrode 53 may be arranged on the upper surface and the lower surface of the solid electrolyte body in which a plurality of solid electrolyte layers are laminated. Further, in the above-described embodiment, the reference electrode 53 also serves as the reference electrode of the mixed potential cell 55 and the reference electrode of the concentration cell 56, but the reference electrode 53 is not limited to this and is referred to by the mixed potential cell 55 and the concentration cell 56. The electrode may be present separately.

In the above-described embodiment, the sensor element 31 is provided with the concentration cell 56 so that the oxygen concentration can be measured, but the present invention is not limited to this. The sensor element 31 does not have to include the concentration cell 56 (specifically, the auxiliary electrode 52). In this case, the ammonia concentration measuring device 70 may acquire information on the oxygen concentration from other than the sensor element 31. For example, the ammonia concentration measuring device 70 obtains information on the oxygen concentration from another sensor (for example, an oxygen sensor, an A / F sensor, or a NOx sensor) that can detect the information on the oxygen concentration arranged in the exhaust gas path 3. You may get it. Alternatively, the ammonia concentration measuring device 70 may acquire information on the oxygen concentration not only from the sensor but also from another device (for example, the engine ECU 9). When the ammonia concentration measuring device 70 acquires information on the oxygen concentration from another sensor arranged at a position different from the sensor element 31 in the exhaust gas path 3, the ammonia concentration measuring device 70 is different from the sensor element 31. It is preferable to derive the ammonia concentration in consideration of the deviation of the measurement time (time delay C) due to the difference in the mounting position with the sensor. Specifically, in the exhaust gas path 3, the time required for the gas to be measured to flow from the one located upstream to the one located downstream of the sensor element 31 and another sensor is set as a time delay C, and the ammonia concentration is measured. It is preferable that the apparatus 70 derives the ammonia concentration in consideration of this time delay C. For example, when another sensor exists upstream of the sensor element 31, the control unit 72 stores a plurality of oxygen concentration values acquired from the other sensor at predetermined intervals in the storage unit 73 by a time delay C. .. Then, each time the control unit 72 acquires the electromotive force EMF from the sensor element 31, the control unit 72 reads out the oldest oxygen concentration value at that time (= the value acquired in the past by the time delay C) from the storage unit 73. , The ammonia concentration is derived based on the acquired electromotive force EMF, the read oxygen concentration value, and the equation (1). The ammonia concentration measuring device 70 can derive the ammonia concentration more accurately by considering the time delay C in this way.

In the above-described embodiment, the engine 1 is a diesel engine, but a gasoline engine may be used.

In the above-described embodiment, the ammonia concentration measuring device 70 is a device different from the engine ECU 9, but the ammonia concentration measuring device 70 may be a part of the engine ECU 9.

Hereinafter, an example in which the ammonia concentration measuring method is specifically executed will be described as an example. The present invention is not limited to the following examples.

[Manufacturing of sensor elements 1 and 2]
A sensor element used for measuring the ammonia concentration by an ammonia concentration measuring device was manufactured. First, as each layer of the base 40, four unfired ceramic green sheets containing a zirconia solid electrolyte containing 3 mol% of yttria as a stabilizer as a ceramic component were prepared. A plurality of sheet holes and necessary through holes used for positioning during printing and laminating are formed in advance on this green sheet. Further, the green sheet serving as the spacer layer 43 is provided with a space serving as a reference gas introduction space 46 in advance by punching or the like. Then, a pattern printing / drying process for forming various patterns on each ceramic green sheet corresponding to the first substrate layer 41, the second substrate layer 42, the spacer layer 43, and the solid electrolyte layer 44 is performed. went. Specifically, a pattern such as the detection electrode 51 made of Au-Pt alloy, the auxiliary electrode 52 made of Pt, the reference electrode 53, each lead wire, and the heater portion 60 was formed. The pattern printing was performed by applying a pattern forming paste prepared according to the characteristics required for each formation target onto a green sheet using a known screen printing technique. After the pattern printing and drying were completed, the adhesive paste for laminating and adhering the green sheets corresponding to each layer was printed and dried. Then, the green sheets on which the adhesive paste was formed were laminated in a predetermined order while being positioned by the sheet holes, and crimped by applying predetermined temperature and pressure conditions to form a single laminated body. A laminate having the size of the sensor element 31 was cut out from the laminate thus obtained. Then, the cut-out laminate was fired in a tubular furnace at 1100 ° C. for 2 hours in an air atmosphere. As a result, a sensor element 31 in which the detection electrode 51, the auxiliary electrode 52, and the reference electrode 53 were formed in the solid electrolyte layer 44 was obtained. Next, the porous protective layer 48 was formed on the surface of the sensor element 31 by dipping and firing using a slurry containing alumina. In this way, a plurality of sensor elements 31 were manufactured to be used as sensor elements 1 and 2. The density of the noble metal surface on the fracture surface of the detection electrodes 51 of the sensor elements 1 and 2 was measured by AES and found to be a value of 1.05. The porosity of the porous protective layer 48 was 40%. In the subsequent tests, the drive temperature when the sensor element 1 was used was 480 ° C, and the drive temperature when the sensor element 2 was used was 600 ° C.

[Experiment 1: Derivation of constant α]
Steps S200 and S210 of the above-mentioned constant derivation process were performed for each of the sensor elements 1 and 2. Specifically, the oxygen concentration p _O2 in the measurement gas 10%, and fixed of H ₂ O concentration p _{H2 O} at 5%, the ammonia concentration p _NH3 varied as shown in Table 1, each of the electromotive force EMF Was measured. The component (base gas) other than the above of the gas to be measured was nitrogen, and the temperature was 120 ° C. The gas to be measured was circulated in a pipe having a diameter of 70 mm, and the flow rate was set to 200 L / min. _{FIG. 5 is a graph showing the relationship between the ammonia concentration p NH3} [ppm] of the sensor elements 1 and 2 and the electromotive force EMF [mV]. The horizontal axis of FIG. 5 is represented by a logarithmic scale. From FIG. 5, it was confirmed that the relationship between the logarithm of the ammonia concentration p _{NH3 and the electromotive force EMF when the oxygen concentration p O2} is fixed in each of the sensor elements 1 and 2 can be approximated by a straight line. .. Further, from the slope of the approximate straight line shown in FIG. 5, the value 45.9 was derived as the constant α of the sensor element 1, and the value 27.9 was derived as the constant α of the sensor element 2.

[Experiment 2: Derivation of constant β]
For each of the sensor elements 1 and 2, steps S220 and S230 of the above-mentioned constant derivation process were performed. Specifically, the ammonia concentration p _NH3 in the gas to be measured is 10 ppm, the fixed of H ₂ O concentration p _{H2 O} at 5%, the oxygen concentration p _O2 is varied as shown in Table 2, each of the electromotive force EMF It was measured. Other conditions were the same as in Experiment 1. _{FIG. 6 is a graph showing the relationship between the oxygen concentration p O2} [%] of the sensor elements 1 and 2 and the electromotive force EMF [mV]. The horizontal axis of FIG. 6 is represented by a logarithmic scale. From FIG. 6, it was confirmed that in each of the sensor elements 1 and 2, the relationship between the logarithm of the oxygen concentration p _{O2 and the electromotive force EMF when the ammonia concentration p NH3} is fixed can be approximated by a straight line. .. Further, from the slope of the approximate straight line shown in FIG. 6, a value of 37.0 was derived as the constant β of the sensor element 1, and a value of 15.9 was derived as the constant β of the sensor element 2.

[Experiment 3: Derivation of constant B]
Based on the data obtained in Experiments 1 and 2, the constant B of each of the sensor elements 1 and 2 was derived by performing step S240 of the constant derivation process described above. The average of the constant B derived for each of the plurality of first electromotive force data obtained in Experiment 1 and the constant B derived for each of the plurality of second electromotive force data obtained in Experiment 2. As a value, a constant B was derived. As a result, the value −68.9 was derived as the constant B of the sensor element 1, and the value −7.1 was derived as the constant B of the sensor element 2.

The 1-3 above experiment, as the relationship of each variable (EMF, p _NH3, p _O2) in the sensor element 1, the following equation (3) is derived, each variable (EMF in the sensor element 2, p _NH3, p _The following equation (4) was derived as the relationship of O2). In the equations (3) and (4), the bases a and b of the equation (1) are the Napier numbers e. Further, in the formulas (3) and (4), the unit of the electromotive force EMF is [mV], the unit of the ammonia concentration p _NH3 is [ppm], and the unit of the oxygen concentration p _O2 is dimensionless (for example, if it is 10%, the value is a value). 0.1).

EMF = 45.9 ln (p _NH3 ) -37.0 ln (p _O2 ) -68.9 (3)
EMF = 27.9 ln (p _NH3 ) -15.9 ln (p _O2 ) -7.1 (4)

[Evaluation test]
For the sensor element 1, the measured values of the electromotive force EMF were measured under the same conditions as in Experiment 1 except that _{the ammonia concentration p NH3} and the oxygen concentration p _{O2 in the gas to be measured were variously changed.} Then, the actually measured value of the electromotive force EMF was compared with the electromotive force EMF derived from the equation (3). The results are shown in Table 3 and FIG. A similar test was performed on the sensor element 2. The results are shown in Table 4 and FIG. In FIGS. 7 and 8, the line connecting each point of the actually measured electromotive force EMF is represented by a solid line, and the line connecting each point of the electromotive force EMF derived from the equations (3) and (4) is broken line. It is represented by. From Tables 3 and 4 and FIGS. 7 and 8, it was confirmed that the actually measured values of the electromotive force EMF and the electromotive force EMF derived based on the equations (3) and (4) accurately match. _{Further, the ratio of the NH 3} sensitivity to the measured value of the electromotive force EMF and the O ₂ coherence does not always have a relationship of (2/3): (1/2), and the relationship of Eq. (2) always holds. I was also able to confirm that this was not the case. _{From these results, it was confirmed that the ammonia concentration p NH3} can be derived more accurately by using the formula (1) as compared with the case of using the formula (2).

[Experiment 4: _{Confirmation of H 2} O coherence]
For each of the sensor elements 1, 2, 10 ppm ammonia concentration p _NH3 in the gas to be measured, the oxygen concentration p _O2 and fixed in 10%, and of H ₂ O concentration p _{H2 O} was varied as shown in Table 5, respectively The electromotive force EMF was measured. Other conditions were the same as in Experiment 1. _{FIG. 9 is a graph showing the relationship between the H2O} concentration p H2O [%] of the sensor elements 1 and ₂ and the electromotive force EMF [mV]. From Figure 9, both of the sensor elements 1 and 2, hardly changes electromotive force EMF be H ₂ O concentration p _{H2 O} in the measurement gas is changed (H little ₂ O coherence) is confirmed It was. _{That is, it was confirmed that the term of the H 2} O concentration p _{H 2O in} the formula (2) is not consistent with the relationship between the actual electromotive force EMF and the H ₂ O concentration p _{H 2O.}

1 engine, 2 exhaust gas treatment system, 3 exhaust gas path, 4 DOC, 5 DPF, 6 injector, 7 SCR, 8 ASC, 9 engine ECU, 10 piping, 12 mounting members, 20 ammonia concentration measurement system, 30 gas sensor, 31 sensor Element, 32 protective cover, 33 element fixing part, 34 main metal fitting, 35 supporter, 36 green compact, 37 nut, 40 base, 41 1st substrate layer, 42 2nd substrate layer, 43 spacer layer, 44 solid electrolyte layer, 46 Reference gas introduction space, 48 Porous protective layer, 51 Detection electrode, 52 Auxiliary electrode, 53 Reference electrode, 55 Mixed potential cell, 56 Concentration cell, 60 Heater part, 61 Heater electrode, 62 Heater, 63 Through hole, 64 Heater insulation layer, 66 lead wire, 70 ammonia concentration measuring device, 72 control unit, 73 storage unit, 75 electromotive force acquisition unit, 76 oxygen concentration acquisition unit, 77 heater power supply, 78 temperature acquisition unit.

Claims (8)

The concentration of ammonia in the gas to be measured using a sensor element having a mixed potential cell having a solid electrolyte body, a detection electrode disposed on the solid electrolyte body, and a reference electrode disposed on the solid electrolyte body. It is an ammonia concentration measuring device that measures
An electromotive force acquisition unit that acquires information on the electromotive force of the hybrid potential cell when the detection electrode is exposed to the gas to be measured, and an electromotive force acquisition unit.
An oxygen concentration acquisition unit that acquires information on the oxygen concentration of the gas to be measured, and an oxygen concentration acquisition unit.
An ammonia concentration derivation unit that derives the ammonia concentration in the gas to be measured based on the information on the acquired electromotive force, the information on the acquired oxygen concentration, and the relationship of the following formula (1).
EMF = αlog _a (p _NH3 ) -βlog _b (p _O2 ) + B (1)
(However,
EMF: Electromotive force of the hybrid potential cell α, β, B: Constant a, b: Arbitrary bottom (where a ≠ 1, a> 0, b ≠ 1, b> 0)
p _NH3 : Ammonia concentration in the gas to be measured p _O2 : Oxygen concentration in the gas to be measured)
Ammonia concentration measuring device equipped with. With the sensor element
The ammonia concentration measuring device according to claim 1 and
Ammonia concentration measurement system equipped with. The detection electrode contains Au-Pt alloy as a main component.
The ammonia concentration measuring system according to claim 2. The detection electrode has a density (= Au abundance [atom%] / Pt abundance [= Au abundance [atom%] / Pt abundance] measured using at least one of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). atom%]) is a value of 0.3 or more,
The ammonia concentration measuring system according to claim 3. The sensor element includes a heater that raises the hybrid potential cell to a driving temperature of 450 ° C. or higher and 650 ° C. or lower.
The ammonia concentration measuring system according to any one of claims 2 to 4. The exhaust gas path, which is the path of the exhaust gas of the internal combustion engine as the gas to be measured, and
The ammonia concentration measuring system according to any one of claims 2 to 5.
With
The sensor element is arranged in the exhaust gas path.
Exhaust gas treatment system. The exhaust gas treatment system according to claim 6.
One or more oxidation catalysts disposed in the exhaust gas path,
With
The sensor element is arranged on the downstream side of the exhaust gas path with respect to the one or more oxidation catalysts arranged in the uppermost stream.
Exhaust gas treatment system. Ammonia concentration in a gas to be measured using a sensor element having a mixed potential cell having a solid electrolyte body, a detection electrode disposed on the solid electrolyte body, and a reference electrode arranged on the solid electrolyte body. It is a measurement method of
An electromotive force acquisition step for acquiring information on the electromotive force of the hybrid potential cell when the detection electrode is exposed to the gas to be measured, and
An oxygen concentration acquisition step for acquiring information on the oxygen concentration of the gas to be measured, and
A concentration derivation step for deriving the ammonia concentration in the gas to be measured based on the information on the acquired electromotive force, the information on the acquired oxygen concentration, and the relationship of the following formula (1).
EMF = αlog _a (p _NH3 ) -βlog _b (p _O2 ) + B (1)
(However,
EMF: Electromotive force of the hybrid potential cell α, β, B: Constant a, b: Arbitrary bottom (where a ≠ 1, a> 0, b ≠ 1, b> 0)
p _NH3 : Ammonia concentration in the gas to be measured p _O2 : Oxygen concentration in the gas to be measured)
Ammonia concentration measuring method including.

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