NOx Sensor

Abstract

To provide an NOx sensor capable of precisely detecting proper NOx concentration by reducing the estimated variation in NO concentration resulting from the reaction of NH3 mixed in the measured gas, and increasing the correction accuracy of sensor output.SOLUTION: A NOx sensor S includes: a sensor element 1 that detects the NOx concentration in a measured gas G including Nox and NH3; and a sensor control unit 10 that controls the detection operation. The sensor element 1 includes: a gas introduction part 3; and a pump cell 1p and a sensor cell 1s which are formed in a gas measuring chamber 2. The gas introduction part 3 has a diffusion resistance layer 31 provided in contact with the gas measuring chamber 2 and a porous protective layer formed over the outside 4; and an NH3 oxidation suppression layer 41 exhibiting weaker Lewis acidity than other layers constituting the porous protective layer 4 on the outermost side of the porous protective layer 4.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a NOx sensor for detecting the concentration of NOx in a measurement gas. [Background technology]

[0002] For example, a NOx sensor for a vehicle includes a sensor element using a solid electrolyte, and detects the NOx concentration by introducing exhaust gas to be measured into the sensor element and measuring the current flowing through the solid electrolyte due to the decomposition of nitrogen oxides (NOx). The sensor element generally has a plurality of cells with electrodes formed on the surface of the solid electrolyte, and detects the oxygen (O 2 ) is pumped to adjust the oxygen concentration to a predetermined low level, and then the NOx contained in the measurement gas is detected by a sensor cell installed downstream.

[0003] On the other hand, in detecting NOx concentration, ammonia (NH 3 ) can be a cause of error. As a countermeasure, Patent Document 1 describes a method in which the measurement gas is a mixture of NOx and NH 3 NH that is added to the NOx output when the exhaust gas contains 3 A system has been proposed to measure the original NOx concentration, taking into account the output of the exhaust gas. 2 Concentration and H 2 O concentration and the actual NH 3 The concentration of NH outside the sensor 3 From the concentration, NH 3 By calculating the derived NO concentration and subtracting it from the total NOx concentration, the original NOx concentration can be calculated. Such a system is used, for example, in a urea SCR system for feedback control based on the NOx concentration in exhaust gas. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-215334 A Summary of the Invention [Problem to be solved by the invention]

[0005] The system described in Patent Document 1 is a NH 3 External NH measured by a sensor, etc. 3 Concentration and NH 3 The relationship between NO concentration and O in the exhaust gas 2 Concentration or H 2 Focusing on the change in the concentration of NH 3 This estimates the NO concentration derived from the oxygen and corrects the sensor output. 2 If the concentration is high or H 2 When the O concentration is low, NH 3 Part of the nitrogen (N 2 ) is more likely to undergo a reaction to change to NH 3 The NO concentration is the concentration of NH outside the sensor. 3 is lower than the concentration.

[0006] For this reason, in the system of Patent Document 1, the sensor outside NH 3 NH vs. concentration 3 The ratio of NO concentration to NH 3 Sensitivity and O 2 Concentration or H 2 The relationship with O concentration is examined and stored in advance. 2 Concentration or H 2 NH calculated from O concentration 3 Using the sensitivity, 3 The NO concentration was calculated. 2 Concentration or H 2 The O concentration is calculated using the air-fuel ratio detected in the pump cell. 3 Not only does it require more steps to calculate sensitivity, but it also requires 2 Concentration or H 2 NH due to environmental factors other than O concentration or sensor factors 3It became clear that the variation in sensitivity was not negligible and became a factor in deviation of the sensor output.

[0007] The present invention has been made in view of the above-mentioned problems, and is directed to a method for detecting NH 3 The present invention aims to provide a NOx sensor capable of reducing the estimated variation in NOx concentration caused by the above and capable of detecting the actual NOx concentration with high accuracy. [Means for solving the problem]

[0008] One aspect of the present invention is NOx and NH 3 A NOx sensor (S) comprising: a sensor element (1) for detecting a NOx concentration in a measurement gas (G) containing: The sensor element is a measurement gas chamber (2) into which the measurement gas is introduced via a gas inlet (3); a solid electrolyte body (11) constituting a chamber wall of the measurement gas chamber; a pump cell (1p) having a pump electrode (21) on a surface of the solid electrolyte body facing the measurement gas chamber, for adjusting the oxygen concentration in the measurement gas by oxygen pumping; a sensor cell (1s) having a sensor electrode (22) on a surface of the solid electrolyte body facing the measurement gas chamber and outputting a detection signal based on a NOx concentration in the measurement gas having an adjusted oxygen concentration, The gas inlet portion has a diffusion resistance layer (31) provided in contact with the measurement gas chamber, and a porous protective layer (4) provided on the outer side of the diffusion resistance layer. The outermost layer of the porous protective layer is made of NH 3 The NOx sensor has an oxidation suppression layer (41). Effect of the Invention

[0009] In the NOx sensor having the above configuration, NOx and NH 3When the measurement gas containing NH reaches the surface of the gas inlet of the sensor element, the outermost layer 3 The gas flows from the oxidation prevention layer into the porous protective layer, passes through the diffusion resistance layer, and is taken into the measurement gas chamber. In the measurement gas chamber, oxygen contained in the measurement gas is ionized in the pump cell and pumped out, and NH 3 The NO generated is decomposed together with the original NOx in the sensor cell, and the original NOx concentration is returned to NH 3 The output is increased by the NOx concentration resulting from the

[0010] Here, NH 3 The material that constitutes the oxidation prevention layer exhibits weaker Lewis acidity than the other layers that constitute the porous protective layer, and is highly resistant to NH 3 The NH 3 This has the effect of suppressing the oxidation of NH, which has a smaller molecular weight than NOx. 3 passes through the gas inlet more quickly and reaches the pump cell, where NH 3 This contributes to improving sensitivity. In addition, the NH 3 The oxidation of NH is less susceptible to environmental factors, and fluctuations in the gas ratio passing through the gas inlet are suppressed. 3 The sensitivity variation is reduced. As a result, 3 Sensitivity is stable and NH 3 By performing correction based on the NOx concentration derived from the above, the actual NOx concentration can be calculated with high accuracy.

[0011] As described above, according to the above embodiment, NH 3 It is possible to provide a NOx sensor capable of reducing the estimated variation in NOx concentration caused by the above and detecting the original NOx concentration with high accuracy. In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention. [Brief description of the drawings]

[0012] [Figure 1] 3 is a longitudinal cross-sectional view showing the configuration of a gas inlet portion of a sensor element which is a main part of the NOx sensor in the first embodiment. FIG. [Diagram 2] 3 is a longitudinal sectional view showing a schematic configuration of a gas detection section of the sensor element in the first embodiment. FIG. [Diagram 3] 1 is a longitudinal sectional view showing an overall configuration of a NOx sensor in a first embodiment. [Figure 4] 3 is a longitudinal cross-sectional view for explaining the gas detection principle introduced in the gas detection unit in the first embodiment. FIG. [Diagram 5] FIG. 4 is a diagram showing the relationship between the ceramic material constituting the gas inlet part and Lewis acidity in the first embodiment. [Figure 6] FIG. 4 is a graph showing the relationship between the Lewis acidity of the constituent material of the gas inlet section and the NH 3 sensitivity in the first embodiment. [Figure 7] FIG. 4 shows the results of measuring the amount of NH3 adsorption by the constituent materials of the gas inlet part according to the first embodiment using a thermal desorption method. [Figure 8] FIG. 4 is a longitudinal cross-sectional view for explaining the difference in effect depending on the constituent material of the NH3 oxidation prevention layer in the gas inlet part in the first embodiment. [Figure 9] FIG. 2 is a diagram showing the relationship between the Lewis acid site of the constituent material (θ-alumina) of the gas inlet part and the reaction of NH3, and the change in concentration of various gases depending on temperature in the first embodiment. [Figure 10] FIG. 2 is a diagram showing the relationship between the constituent material (α-alumina) of the gas inlet part and the reaction of NH3, and the change in concentration of various gases with temperature in the first embodiment. [Figure 11] FIG. 4 is a graph showing the relationship between the concentration of NH3 introduced into the gas detection unit and the sensor output in the first embodiment. [Figure 12] FIG. 11 is a graph showing the difference in NH 3 sensitivity variation depending on the constituent material of the gas inlet section, comparing θ-alumina with α-alumina in the first embodiment. [Figure 13] FIG. 2 is a graph showing the contribution of the temperature or film thickness of the porous protective layer to NH 3 sensitivity in a comparison between θ-alumina and α-alumina in the first embodiment. [Figure 14] FIG. 2 is a graph showing the contribution of the density or crystallinity of the porous protective layer to NH3 sensitivity in a comparison between θ-alumina and α-alumina in the first embodiment. [Figure 15] FIG. 2 is a graph showing the contribution of gas flow rate to NH3 sensitivity in a comparison between θ-alumina and α-alumina in the first embodiment. [Figure 16] FIG. 2 is a graph showing the contribution of NH3 concentration to NH3 sensitivity in a comparison between θ-alumina and α-alumina in the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] (Embodiment 1) A first embodiment of a NOx sensor will be described with reference to the drawings. 2 and 3, the NOx sensor S of this embodiment includes a sensor element 1 and a sensor control unit 10. A sensor main body 1A incorporating the sensor element 1 is installed, for example, in an exhaust gas passage of an internal combustion engine such as a vehicle engine, and detects the concentration of NOx (i.e., nitrogen oxides) contained in exhaust gas G using the sensor element 1. The sensor control unit 10 is installed outside the exhaust gas passage and controls the detection operation by the sensor element 1. As shown in FIG. 4, exhaust gas G contains NOx and NH 3 (i.e., ammonia) is included in the measurement gas.

[0014] The NOx sensor S is, for example, a selective catalytic reduction (SCR) sensor for purifying NOx. This system is used in the NOx reduction system to detect the NOx concentration downstream of the catalyst and to control the supply of reducing agent upstream of the catalyst. 3 is used as a reducing agent, unreacted NH 3 When the residual NOx and NH 3 In some cases, these may be introduced into the sensor element 1 in a mixed state.

[0015] 1 and 2, the sensor element 1 has a measurement gas chamber 2 formed inside one end side (for example, the left end side in FIG. 2) which serves as a gas detection section, and exhaust gas G is introduced into the measurement gas chamber 2 from the outside of the sensor element 1 through a gas inlet part 3 having a multi-layer structure. The sensor element 1 is configured as, for example, a limiting current type sensor, and includes a solid electrolyte body 11 which constitutes the chamber wall of the measurement gas chamber 2, a pump cell 1p, and a sensor cell 1s. NH 3 When NOx and NH are mixed, the output from sensor element 1 is based on the original NOx. 3 The reference gas chamber 5 is disposed on the opposite side of the solid electrolyte body 11 from the measurement gas chamber 2.

[0016] The pump cell 1p has a pump electrode 21 on the surface facing the measurement gas chamber 2, and O 2 Pumping (hereinafter, appropriately referred to as oxygen pumping) is performed to discharge or introduce oxygen (i.e., oxygen) as a reference gas into or from the atmosphere A side, thereby adjusting the oxygen concentration in the exhaust gas G. The sensor cell 1s has a sensor electrode 22 on a surface facing the measurement gas chamber 2, and is configured to output a detection signal based on the NOx concentration in the exhaust gas G whose oxygen concentration has been adjusted. The sensor element 1 may also include a monitor cell 1m disposed in parallel with the sensor cell 1s.

[0017] At this time, in the pump cell 1p, the O contained in the exhaust gas G is converted into 2 is oxide ions (i.e., O 2- ) at the pump electrode 21. As shown in an enlarged view in FIG. 3 is oxidized and converted to NO. 3 Due to the oxidation process, NH 3 The NOx derived from these is detected by the sensor cell 1s. The NOx is decomposed by the catalytic action of the sensor electrode 22, and the generated oxide ions move through the solid electrolyte body 11, causing a current corresponding to the NOx concentration to flow.

[0018] 1, the gas inlet 3 is composed of multiple layers having gas permeability, specifically, a diffusion resistance layer 31 provided in contact with the measurement gas chamber 2 and a porous protective layer 4 provided on the outside thereof. The porous protective layer 4 is the outermost layer, and has a NH 3 The oxidation prevention layer 41 is included. 3 The oxidation prevention layer 41 prevents NH contained in the exhaust gas G around the sensor element 1 from oxidizing when the exhaust gas G is introduced into the measurement gas chamber 2 from the gas introduction portion 3. 3 It has the effect of suppressing the oxidation of NH 3 By increasing the sensitivity and suppressing the variation, the NH 3 This makes it possible to stabilize the output from the device.

[0019] Specifically, the Lewis acidity, which will be described in detail later, can be used as an index of the strength of Lewis acidity. 3 The constituent materials of the oxidation prevention layer 41 are selected so that the Lewis acidity of the constituent materials is smaller than the Lewis acidity of the constituent materials of the other layers of the porous protective layer 4. 3 The ratio of the Lewis acidity of the material constituting the oxidation prevention layer 41 to the largest Lewis acidity of the materials constituting the other layers of the porous protective layer 4 is preferably adjusted to 1 / 2 or less. 3 NH by oxidation prevention layer 41 3 The oxidation of the above-mentioned can be effectively suppressed.

[0020] The porous protective layer 4 is made of NH 3 The oxidation suppression layer 41 is configured to protect the sensor element 1 from the exhaust gas G introduced therein, in combination with the functions of other layers disposed on the inner side of the oxidation suppression layer 41, and to enable accurate detection. The configuration of the other layers is not necessarily limited, and can be appropriately set so as to obtain the desired functions. Specifically, the other layers may be NH 3A trap layer 42 for removing poisonous components may be provided between the oxidation prevention layer 41 and the diffusion resistance layer 31. In this case, NH 3 The thickness of the oxidation prevention layer 41, the trap layer 42, etc. can be appropriately set according to manufacturing or functional needs.

[0021] The constituent materials of the other layers constituting the porous protective layer 4 are not necessarily limited, but may include, for example, transition alumina. Transition alumina is aluminum oxide (Al 2 O 3 ), metastable phases of θ-alumina, δ-alumina, γ-alumina, etc. are preferred. These porous particles made of transition alumina and having a relatively large specific surface area are suitable as materials for the other layers constituting the porous protective layer 4. Transition alumina is obtained by thermal decomposition of an aluminum compound or alumina hydrate, and has a plurality of phases with different crystal structures and physical properties. For example, when cubic γ-alumina is heat-treated, it undergoes a phase transformation via δ-alumina to monoclinic θ-alumina, improving the crystallinity while reducing the specific surface area. More preferably, θ-alumina is used as a material for the other layers.

[0022] Here, "Lewis acidity" refers to the ability of the Lewis acid to receive an unshared electron pair from a base. The material constituting the gas introduction section 3 is an insulating ceramic material such as an oxide or compound containing a metal or nonmetal element, and preferably a material mainly composed of a metal oxide is used. A ceramic material with a relatively weak Lewis acidity has a relatively low ability to adsorb a base having an unshared electron pair, and a ceramic material with a relatively strong Lewis acidity has a relatively high ability to adsorb a base having an unshared electron pair. Specifically, when the base NH 3 The known temperature programmed desorption method (hereinafter referred to as NH 3 -TPD method) to determine the amount of Lewis acid sites. 3 By measuring the amount of adsorption, the strength of Lewis acidity can be expressed.

[0023] As shown in Figs. 5 and 6, NH 3 The Lewis acidity of various ceramic materials can be calculated by the NH 3 It can be calculated based on the TPD curve and is defined as follows: Lewis Acidity: NH2 from room temperature to 800℃ by thermal desorption 3 Accumulation value of signal intensity per unit specific surface area in adsorption amount measurement (arbitrary unit)

[0024] NH in Figure 7 3 The -TPD curve is 3 -TPD method was used to measure the amount of NH released when the temperature was increased from room temperature to 800°C. 3 The Lewis acidity is the NH 3 It is calculated by calculating the total area of ​​the NH-TPD curve, i.e., the cumulative signal intensity in the measurement temperature range. 3 The intensity of the detection signal at each temperature in the -TPD curve is the ratio of NH 3 In other words, the amount of NH 3 The upper limit of the measurement temperature range is set to a temperature higher than the upper limit of the normal operating temperature range of the sensor element 1.

[0025] Figure 7 shows the NH 3 The Lewis acidity of other ceramic materials can be calculated in the same manner. 3 The amount of adsorption is the amount of desorbed NH 3 Therefore, for various ceramic materials, 3 By calculating the Lewis acidity from the -TPD curve, it is possible to effectively determine the NH 3 It is possible to know the adsorption ability of the material.

[0026] By using Lewis acidity, it is possible to compare the strength of Lewis acidity of various ceramic materials. In general, metal ions exposed on the surface of metal oxides act as Lewis acid sites, so that basic NH 3 When a material contains NH, some of it is adsorbed to the Lewis acid site and easily oxidized. In this case, the material with fewer Lewis acid sites, that is, the material with lower Lewis acidity, has weaker Lewis acidity and is more susceptible to oxidation. 3 Therefore, the outermost layer of the porous protective layer 4 is made of a material having a lower Lewis acidity than the other layers. 3 By including the oxidation prevention layer 41, the NH 3 Suppresses the oxidation of NH 3 It is possible to improve the sensitivity.

[0027] At this time, NH 3 The weaker the Lewis acidity of the oxidation-prevention layer 41 compared to other layers, the greater the amount of NH 3 As described above, the oxidation suppression effect of NH 3 It is preferable that the ratio of the Lewis acidity of the material constituting the oxidation prevention layer 41 to the highest Lewis acidity of the material constituting the other layers of the porous protective layer 4 is adjusted to 1 / 2 or less. 3 NH in the oxidation inhibition layer 41 3 The oxidation of NH is further suppressed, and the variation in the composition of the exhaust gas G passing through the gas inlet portion 3 is suppressed. 3 It is possible to improve the estimation accuracy of the NOx concentration derived from NH 3 The configuration of the gas introduction section 3 is preferably adjusted so that the ratio of the Lewis acidity of the constituent material of the oxidation prevention layer 41 to the highest Lewis acidity of the constituent materials of the other layers is 1 / 4 or less.

[0028] In that case, NH 3The oxidation prevention layer 41 is selected based on the relationship between various oxides and Lewis acidity, as shown in Fig. 5. For example, when the constituent material of the other layers contains transition alumina, a ceramic material having a lower Lewis acidity than transition alumina, such as θ-alumina (Lewis acidity: 12.8) or γ-alumina (Lewis acidity: 8.1), specifically a material having a Lewis acidity of less than 8, is used to form the NH 3 The oxidation prevention layer 41 can be formed from such ceramic materials. 2 ), titanium oxide (titania; TiO 2 ), boron oxide (B 2 O 3 ), α-alumina (α-Al 2 O 3 Metal oxide materials such as boron oxide (BO) and zinc oxide (ZnO) can be used. All of these ceramic materials have a Lewis acidity of 6 or less, and the Lewis acidity ratio to θ-alumina is 1 / 2 or less. Boron oxide, α-alumina, and zinc oxide all have a Lewis acidity of 3 or less, and the Lewis acidity ratio to θ-alumina is 1 / 4 or less, and the Lewis acidity ratio to γ-alumina is 1 / 2 or less. ZrO 2 ;Lewis Acidity: 6.0 TiO 2 ;Lewis Acidity: 4.8 B 2 O 3 ;Lewis Acidity: 3.0 α-Al 2 O 3 ;Lewis Acidity: 2.0 ZnO;Lewis acidity: 1.2

[0029] α-alumina is a stable phase of Al with a rhombohedral crystal structure. 2 O 3 The transition alumina is transformed into a stable phase of α-alumina by heat treatment at high temperature. The material of the diffusion resistance layer 31 arranged on the innermost side of the gas inlet part 3 is not necessarily limited, but is preferably NH 3The oxidation prevention layer 41 may be made of a material that exhibits Lewis acidity equal to or less than that of the oxidation prevention layer 41. In order to suppress the variation in the composition of the exhaust gas G passing through the gas inlet 3, the diffusion resistance layer 31 disposed on the inner side of the porous protective layer 4 may also be made of a material that exhibits Lewis acidity equal to or less than that of the oxidation prevention layer 41. 3 It is desirable to suppress oxidation as much as possible. For example, α-alumina is suitable as a material for the diffusion resistance layer 31 from the viewpoint of Lewis acidity and structural stability. 3 Both the oxidation prevention layer 41 and the diffusion resistance layer 31 may be made of α-alumina.

[0030] Figure 6 shows the NH 3 -Lewis acidity based on the TPD curve (i.e., see Figure 7) and NH 3 As shown in the figure, the Lewis acidity of α-alumina is significantly reduced compared to θ-alumina and γ-alumina, which are transition aluminas. 3 The sensitivity is improved and the sensor variation is reduced. 3 The sensitivity is the NH 3 It shows the sensitivity. NH 3 Oxidation prevention layer 41 (α-alumina): 500 μm Trap layer 42 (θ-alumina): 300 μm Diffusion resistance layer 31 (α-alumina): 1 mm (1000 μm)

[0031] Here, NH 3 The sensitivity is determined according to the configuration of the gas inlet 3. In FIG. 6, the NH 3 The median sensitivity (solid line) is shown. The sensor variation (dotted line) indicates a range of ±4.7σ (>99.99%) where σ is the standard deviation. As shown in Figure 7, γ-alumina is a NH 3 The oxidation tendency is between α-alumina and θ-alumina, with a lower peak at higher temperatures than θ-alumina. 3 NH in the temperature range where oxidation does not proceed3 The results show that there is no significant difference between α-alumina and α-alumina. 3 The relationship with sensitivity will be described later.

[0032] In this way, the gas inlet 3 is NH 3 By providing the oxidation suppression layer 41, NH 3 This makes it possible to detect NOx accurately while suppressing the effects of oxidation. An example of the configuration of the NOx sensor S and details of the control by the sensor control unit 10 will be described below.

[0033] (Overall configuration of NOx sensor S) 3, the NOx sensor S has a sensor element 1 inserted and held inside a cylindrical housing H, a sensor main body 1A having an element cover C1 and an atmosphere cover C2, and a sensor control unit 10 arranged outside the sensor main body 1A. The axial direction of the housing H is the longitudinal direction Y of the sensor element 1, and both ends protruding outward from the housing H are housed inside the element cover C1 and atmosphere cover C2 fixed to both ends of the housing H, respectively. The element cover C1 is arranged so as to cover the outer periphery of the tip end side of the sensor element 1 (i.e., the lower end side in FIG. 3), and the atmosphere cover C2 is arranged so as to cover the outer periphery of the base end side of the sensor element 1 (i.e., the upper end side in FIG. 3).

[0034] The housing H is fixed to the passage wall of the exhaust gas passage (not shown), and the tip side of the sensor element 1 housed in the element cover C1 is positioned to protrude into the exhaust gas passage. The element cover C1 has, for example, a double-cylinder structure having a bottom surface, and the outer cover C11 and the inner cover C12 have a plurality of gas circulation holes C131, C132 on their side surfaces, respectively, and the inner cover C12 has a gas circulation hole C133 on its bottom surface. In addition, the cylindrical atmosphere cover C2 has a plurality of gas circulation holes C21 on its side surface. As a result, the exhaust gas G flowing through the exhaust gas passage is taken into the inside of the element cover C1 and reaches the tip side of the sensor element 1, and the atmosphere A taken into the inside of the atmosphere cover C2 reaches the base end side of the sensor element 1.

[0035] The sensor element 1 has a gas detection section at its tip end, housed in the element cover C1, exposed to exhaust gas G, which is the gas to be measured, and detects NOx in the exhaust gas using the atmosphere A taken in from its base end, housed in the atmosphere cover C2, as a reference gas. A control signal is input from the external sensor control unit 10 to the gas detection section, or a detection signal is output from the gas detection section, via a lead wire L1 electrically connected to a terminal section at the base end of the sensor element 1. The base end of the sensor element 1 is clamped by a spring terminal section L2 provided at the end of the lead wire L1.

[0036] (Configuration of sensor element 1) 2, in the gas detection section of the sensor element 1, a pump cell 1p is provided upstream in the gas flow direction X, and a sensor cell 1s and a monitor cell 1m are provided downstream of the pump cell 1p. The pump cell 1p has a pair of electrodes 21, 51 exposed to the exhaust gas G introduced into the measurement gas chamber 2 and the atmosphere A introduced into the reference gas chamber 5, respectively, and similarly, the sensor cell 1s and the monitor cell 1m have a pair of electrodes 22, 51 and a pair of electrodes 23, 51. In addition, the sensor element 1 has a heater 6 built in at a position corresponding to the gas detection section, and the operation of the gas detection section and the heater 6 is controlled by the sensor control section 10.

[0037] In the sensor element 1, a shielding layer 13, a spacer layer 12, a solid electrolyte body 11, a spacer layer 14, and a heater insulating layer 61 are arranged in this order, with the stacking direction being orthogonal to the gas flow direction X (i.e., the up-down direction in FIG. 2). The spacer layer 12, which forms a space to become the measurement gas chamber 2, and the shielding layer 13 are arranged on one surface side of the solid electrolyte body 11, and the spacer layer 14, which forms a space to become the reference gas chamber 5, and the heater insulating layer 61 constituting the heater 6 are arranged on the other surface side of the solid electrolyte body 11.

[0038] The measurement gas chamber 2 communicates with the space inside the element cover C1 (see, for example, FIG. 3 described above) in which the exhaust gas G exists, via a diffusion resistance layer 31 embedded in the tip surface (i.e., the left end surface in FIG. 2) of the spacer layer 12. The diffusion resistance layer 31, together with the porous protective layer 4 disposed on its outside, constitutes the gas inlet 3 to the measurement gas chamber 2, and is adjusted so that the exhaust gas G is introduced under a predetermined diffusion resistance. The porous protective layer 4 is formed so as to cover the entire surface on the tip side which becomes the gas detection portion of the sensor element 1, and a part of the tip surface located outside the diffusion resistance layer 31 constitutes the gas inlet 3.

[0039] The solid electrolyte body 11 is made of a zirconia-based solid electrolyte material having oxide ion conductivity. As the zirconia-based solid electrolyte material, for example, partially stabilized zirconia or stabilized zirconia containing a stabilizer such as yttria can be used. On the surface of the solid electrolyte body 11 facing the measurement gas chamber 2, a pump electrode 21 of the pump cell 1p is formed on the upstream side adjacent to the diffusion resistance layer 31, and a sensor electrode 22 of the sensor cell 1s and a monitor electrode 23 of the monitor cell 1m are arranged in parallel on the downstream side of the pump cell 1p. On the surface of the solid electrolyte body 11 facing the reference gas chamber 5, a reference electrode 51 common to these cells is formed.

[0040] In this embodiment, the reference electrode 51 is provided as a common electrode, but it may be provided for each cell. Also, a plurality of solid electrolyte bodies 11 may be provided facing the reference gas chamber 5. In this case, the reference electrode 51 for each cell may be provided on the surface of a different solid electrolyte body 11 facing the reference gas chamber 5.

[0041] The electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m may be porous cermet electrodes containing a precious metal or a precious metal alloy material and a zirconia-based solid electrolyte. The sensor electrode 22 of the sensor cell 1s is made of an electrode material having decomposition activity for the NOx to be detected. For example, an electrode containing platinum and rhodium (hereinafter, appropriately referred to as a Pt-Rh electrode) may be used as such an electrode.

[0042] The pump electrode 21 of the pump cell 1p has oxygen decomposition activity and NH 3 The reference electrode 51 is configured using an electrode material having an oxidation activity of 0.1 to 100% and no NOx decomposition activity. For example, an electrode containing platinum and gold (hereinafter, appropriately referred to as a Pt-Au electrode) can be used as such an electrode. The monitor electrode 23 of the monitor cell 1m is also configured using the same electrode material as the pump electrode 21. The reference electrode 51 can be configured as an electrode containing platinum (hereinafter, appropriately referred to as a Pt electrode).

[0043] The sensor electrode 22 is not limited to a Pt-Rh electrode, and may be made of an electrode material having a similar effect. For example, an alloy material in which platinum is mixed with metals such as palladium (Pd), iron (Fe), cobalt (Co), or nickel (Ni) is used. The alloy ratio in the Pt-Rh electrode and the Pt-Au electrode can be appropriately selected.

[0044] The reference gas chamber 5 in which the common reference electrode 51 is disposed extends to the base end side (e.g., the upper end side in FIG. 3) of the sensor element 1, and communicates with the space inside the atmosphere cover C2 (e.g., see FIG. 3 described above) in which the atmosphere A exists through a reference gas inlet opening on the end face of the reference gas chamber 5. A heating element 62 that generates heat when electricity is applied is embedded inside a heater insulating layer 61 that forms the bottom wall of the reference gas chamber 5, forming the heater 6. The heating element 62 is disposed corresponding to the portions where the electrodes of the pump cell 1p, the sensor cell 1s, and the monitor cell 1m are formed, and is capable of heating the entire tip side of the element that serves as the gas detection portion. The heater insulating layer 61, the shielding layer 13, and the spacer layers 12 and 14 can be formed using insulating ceramic materials such as alumina.

[0045] (Configuration of sensor control unit 10 and detection principle) The sensor control unit 10 receives a command from a vehicle engine control unit (hereinafter, ECU) not shown in the figure and controls the detection operation of the sensor element 1. The sensor control unit 10 has a detection control unit 100, a NOx detection unit 101, and a heater control unit 102. The detection control unit 100 controls the operation of the pump cell 1p, the monitor cell 1m, and the sensor cell 1s. Specifically, the detection control unit 100 applies a predetermined voltage for oxygen pumping to the pump cell 1p, and applies a predetermined voltage for NOx detection to the sensor cell 1s. In addition, a predetermined voltage is applied to the monitor cell 1m to enable monitoring of the oxygen concentration remaining in the exhaust gas G after oxygen pumping.

[0046] Specifically, the NOx detection unit 101 includes a detection signal acquisition unit 101A, a NOx concentration calculation unit 101B, and a correction amount calculation unit 101C. The detection signal acquisition unit 101A acquires, for example, a difference value between the current signals flowing between the electrodes of the sensor cell 1s and the monitor cell 1m as a detection signal. Here, the exhaust gas G that reaches the sensor cell 1s and the monitor cell 1m is adjusted to a predetermined low oxygen concentration by oxygen pumping in the pump cell 1p, and in the monitor cell 1m, the oxygen remaining in the adjusted exhaust gas G is decomposed and ionized, so that a current based on the remaining oxygen flows. In the sensor cell 1s, a current based on the oxygen remaining in the exhaust gas G and the decomposition of NOx contained in the exhaust gas G flows. Therefore, by using a difference value between the currents detected in the sensor cell 1s and the monitor cell 1m, the influence of the remaining oxygen can be eliminated.

[0047] The NOx concentration calculation unit 101B corrects the NOx concentration based on the detection signal acquired by the detection signal acquisition unit 101A using the correction amount calculated by the correction amount calculation unit 101C to calculate the original NOx concentration. 3 (i.e., real NH 3 Therefore, in the correction amount calculation unit 101C, the output derived from NH 3 The NOx concentration derived from the above is calculated as a correction amount, and by subtracting it from the pre-correction NOx concentration based on the detection signal as shown in the following formula (1), the original NOx concentration can be obtained. [Original NOx concentration] = [Uncorrected NOx concentration] - [NH 3 NOx concentration due to The original NOx concentration calculated by the NOx concentration calculation unit 101B is output to an external ECU and used for feedback control in the SCR system.

[0048] In the correction amount calculation unit 101C, the actual NH 3 The concentration is the NH 3 The NOx concentration is, for example, a concentration of NH3 The sensor can detect the amount of oxygen in the atmosphere. At this time, as shown in the following formula (2), NH 3 The NOx concentration derived from the NH 3 Sensitivity and actual NH detected outside the sensor 3 It is calculated using the concentration. [NH 3 NOx concentration derived from 3 Sensitivity] × [Actual NH 3 Concentration]...(2)

[0049] The heater control unit 102 controls the heater temperature for heating the sensor element 1, and activates each cell of the sensor element 1 and causes the NH 3 In the oxidation prevention layer 41, NH 3 The power supply to the heater 6 is controlled so that the temperature is such that the thermal decomposition of NH is suppressed. Specifically, it is desirable to control the temperature of the regions in which the pump cell 1p, the monitor cell 1m, and the sensor cell 1s are formed to a predetermined temperature, for example, 500° C. or higher, so that the temperatures are suitable for the pumping and detection operations, respectively. 3 The oxidation inhibition layer 41 is formed by absorbing NH contained in the exhaust gas G. 3 It is desirable to maintain the temperature at, for example, 650° C. or less, preferably 600° C. or less, so that the oxidation suppression effect of the present invention can be obtained and thermal decomposition can be suppressed.

[0050] The outer surface layer of the gas inlet portion 3 is NH 3 The temperature of the oxidation prevention layer 41 rises due to thermal conduction from the components of the chamber wall of the measurement gas chamber 2, and is usually lower than the temperature of each cell disposed inside the sensor element 1. Therefore, it is desirable to appropriately set the heater temperature and the layer structure of the gas inlet part 3 so that each part is in an appropriate temperature range, taking into consideration the thermal conductivity from the heater 6, etc.

[0051] (Configuration of gas introduction section 3) In FIG. 1, the gas inlet 3 is provided with NH 3The porous protective layer 4 includes an oxidation suppression layer 41, and NH contained in the exhaust gas G is 3 The porous protective layer 4 suppresses oxidation of the sensor element 1 and protects the sensor element 1 from poisons, moisture, etc. 3 The porous protective layer 4 has a multi-layer structure consisting of an oxidation prevention layer 41 and at least one other layer, and here has a two-layer structure with different main functions and materials. In this case, the outer layer of the porous protective layer 4 that contacts the exhaust gas G is NH 3 The oxidation suppression layer 41 is mainly composed of NH 3 It has an oxidation inhibitory function. 3 An inner layer formed between the oxidation prevention layer 41 and the diffusion resistance layer 31 constitutes a trap layer 42, and has a main function of trapping poisoning substances.

[0052] The trap layer 42 is configured to be capable of trapping poisoning components in the exhaust gas G, for example, compounds containing components such as P, Si, Ca, and Zn contained in the oil. For this purpose, the trap layer 42 is desirably configured with a porous material having a large specific surface area, and can be configured with, for example, transition alumina such as θ-alumina or γ-alumina. Transition alumina usually has a BET specific surface area of ​​100 m 2 / g or more, and the particles themselves are porous, making them suitable for forming a trap layer 42 with a high porosity. From the viewpoint of achieving both the structural characteristics and thermal stability required for trap layer 42, it is preferable to use θ-alumina.

[0053] The diffusion resistance layer 31 may be configured to have a desired diffusion resistance so that the exhaust gas G is introduced into the measurement gas chamber 2 through the gas inlet 3. In this case, the sensor element 1 exhibits limiting current characteristics, and the NOx concentration can be detected based on the limiting current flowing through the sensor cell 1s while the exhaust gas G is maintained at a predetermined low oxygen concentration by oxygen pumping of the pump cell 1p. Specifically, the thickness of the diffusion resistance layer 31 is appropriately set to a desired diffusion resistance, for example, within a range of 5 mm or less and a porosity within a range of 10% to 90%. Preferably, the thickness of the diffusion resistance layer 31 is within a range of 0.5 mm to 2.5 mm, and the diffusion resistance layer 31 can be made of, for example, α-alumina.

[0054] The thickness and porosity of the porous protective layer 4 are set to a desired value within a range that does not impede the flow of the exhaust gas G through the diffusion resistance layer 31. 3 The thickness of the trap layer 42 can be appropriately set so as to obtain the oxidation suppression function and the trapping function. Preferably, the thickness of the trap layer 42 can be set in the range of 5 μm to 1000 μm, and the porosity can be set in the range of 50% to 90%. Although the trapping function of the trap layer 42 improves when the layer thickness is increased, depending on the constituent material, the trapping function may be improved by increasing the thickness of the trap layer 42. 3 This may affect the oxidation suppression function, and may also slow the diffusion of exhaust gas G, resulting in reduced responsiveness. On the other hand, increasing the porosity may facilitate the diffusion of exhaust gas G, but may reduce the trapping function. In general, it is preferable that the trap layer 42 is thinner than the diffusion resistance layer 31 and has a higher porosity. More preferably, the thickness of the trap layer 42 can be set appropriately within the range of 50 μm to 500 μm.

[0055] NH 3 The oxidation prevention layer 41 constitutes the outer layer of the porous protective layer 4 and prevents NH contained in the exhaust gas G present around it. 3 is suppressed from being oxidized and converted to NOx when it is taken into the gas inlet 3. 3The thickness of the oxidation prevention layer 41 can be set, for example, in the range of 50 μm to 1200 μm, and the porosity can be set in the range of 10% to 50%. By making the porosity smaller than that of the trap layer 42, the water resistance can be improved, and the water contained in the exhaust gas G in contact with the outer surface layer portion of the porous protective layer 4 can be prevented from penetrating into the inside through the gas inlet portion 3. More preferably, the NH 3 By setting the thickness of the oxidation prevention layer 41 in the range of 100 μm to 1000 μm, the desired NH 3 Sensitivity and NH 3 The oxidation inhibition function can be stably achieved.

[0056] (NH 3 Sensitivity settings) Here, the NH 3 -Lewis acidity based on the TPD method, selection of the constituent material of the gas inlet 3, and NH 3 The relationship between the sensitivity and the NH concentration will be examined with reference to Figs. 8 to 11. As shown in Fig. 4, when exhaust gas G is introduced into the gas inlet 3, it diffuses through the porous protective layer 4 and the diffusion resistance layer 31 in sequence, and reaches the pump electrode 21 of the pump cell 1p disposed in the measurement gas chamber 2. NH 3 When is oxidized, the reactions of the following formulas (11) to (13) are thought to proceed. 4NH 3 +5O 2 →4NO+6H 2 O (11) 4NH 3 +6NO→5N 2 +6H 2 O (12) 4NH 3 +4NO+O 2 →4N 2 +6H 2 O (13)

[0057] At this time, as shown in the right diagram of Fig. 8, when the entire porous protective layer 4 is made of θ-alumina, the reactions of the above formulas (11) to (13) proceed in the porous protective layer 4. That is, according to the above formula (11), NH3 Part of O 2 When it reacts with 2 O is produced, and the NO produced is converted to NH by the reaction of the above formula (12) or (13). 3 When reacted with, N 2 and H 2 The reaction of formula (11) above is O 2 The higher the concentration, the higher the H 2 The lower the O concentration, the easier it is to proceed. When NO is generated, the reaction of the above formula (12) or (13) is more likely to proceed, and the generated NO is converted to N 2 As the proportion of 3 The resulting NOx concentration will be lower.

[0058] This is because the Lewis acidity of θ-alumina is high, and basic NH 3 This is because θ-alumina has a structure that easily adsorbs NH 3 The reactivity is dominated by oxidation reactions, and the composition of the exhaust gas after passing through the gas inlet 3 changes depending on the temperature. The gas inlet 3 is heated to, for example, 550°C to 650°C by the heat from the heater 6 built into the sensor element 1, and the θ-alumina has a structure in which the hydroxyl groups on the surface are eliminated, exposing aluminum ions that become Lewis acid sites. H adsorbed at these Lewis acid sites 2 O to NH 3 is bonded to NH 4 + When adsorbed as O 2 It reacts with and is eliminated as NO.

[0059] On the other hand, when the porous protective layer 4 of the gas inlet 3 is made of α-alumina, which has low Lewis acidity, as shown in the left diagram of Fig. 8, the oxidation reaction does not proceed as in θ-alumina, since there are no Lewis acid sites on the surface. 3 The reactivity is dominated by the decomposition reaction, which is a reversible reaction. In the temperature range of 550°C to 650°C, NH 3is relatively stable. When the temperature rises above 600°C, NH 3 decreases gradually, and above 650℃, N 2 and H 2 O increases rapidly. 2 O is NH 3 H generated by decomposition of 2 O 2 When the temperature is lowered from 650°C, NH 3 Therefore, preferably, the porous protective layer 4 is kept at a temperature of 600° C. or less to prevent the NH 3 The decomposition reaction of the above can be almost completely suppressed.

[0060] In addition, the NH 3 The -TPD curves are shown for various alumina samples, respectively. 3 The adsorption test was carried out using a -TPD device under the following conditions: As a pretreatment, the temperature was raised to 800°C in a He atmosphere, the adsorbed gas of the alumina sample was exhausted, and then the temperature was lowered to 50°C in a He atmosphere, and NH 3 The gas discharged from the device at each temperature was introduced into a detector, and the desorbed NH 3 As shown in the figure, θ-alumina reacts with NH 3 The signal intensity increases sharply and shows a peak in the range up to 800 °C, whereas γ-alumina shows a peak in the range from 300 °C to 800 °C, and the maximum peak intensity is almost half that of θ-alumina. 3 The signal intensity peaks around 400°C, whereas α-alumina shows a peak of NH 3 The signal strength is almost flat, and the NH 3 It can be seen that almost no

[0061] These relationships are shown in Figures 9 and 10 based on the NH 3The FT-IR method is a detection method using a Fourier transform infrared spectrophotometer, and the measurement gas (NH 3 Gas analysis can be performed using the infrared absorption spectrum of 100 ppm (concentration: 5000 ppm).

[0062] As shown in FIG. 8, when the reactions of the above formulas (11) to (13) proceed in the gas inlet portion 3, NH 3 Some of the 2 In that case, NH 3 and NO, N 2 Due to the difference in molecular weight between NH and NH2, the diffusion rate differs, and the ratio of NH2 in the gas when it is taken into the measurement gas chamber 2 changes. 3 It was found that this was the main cause of the variation in sensitivity. 3 As shown below, the diffusion coefficient of NO, which has a larger molecular weight, is larger, resulting in a difference in the diffusion ratio between NO and N. 2 is considered to be roughly equivalent to NO, but because its molecular weight (28) is slightly smaller than that of NO, its diffusion coefficient is slightly smaller and its diffusion ratio is slightly larger. NO: molecular weight (30), diffusion coefficient (5.48), diffusion ratio (1) NH 3 : molecular weight (17), diffusion coefficient (4.12), diffusion ratio (1.33)

[0063] That is, when the diffusion ratio of NO is 1, the diffusion ratio of NH 3 The diffusion ratio of NH is 1.33 at the gas inlet 3. 3 If oxidation of NO were not to occur, 1.33 times more NH would be produced than NO. 3 However, the O2 diffuses through the gas inlet 3 into the measurement gas chamber 2. 2 Concentration or H 2 There is no effect of the O concentration. 3 is converted to NO in the pump cell 1p by a reaction similar to that of the above formula (11), and is further converted to N2 and O 2 and O 2 When the electrons become ionized, an electric current flows. NO+2e - →1 / 2N 2 +O 2 (14)

[0064] Therefore, as shown in Figure 11, NH in the exhaust gas G 3 The sensor output derived from changes depending on the diffusion ratio, and NH 3 The more the oxidation of NH is suppressed, the less NH detected by the analyzer 3 At this time, the sensor output (vertical axis) and analyzer NH 3 The concentration (horizontal axis: ppm) is expressed by [NH 3 NOx concentration derived from 3 The ratio of these corresponds to NH 3 In other words, when the above formula (2) is transformed into the following formula (21), NH 3 The larger the diffusion ratio, the greater the sensitivity of NH 3 Sensitivity is improved. [NH 3 Sensitivity]=[NH 3 NOx concentration derived from] / [actual NH 3 Concentration]...(21)

[0065] Therefore, as described above, NH 3 The oxidation suppression layer 41 is made of α-alumina or the like having low Lewis acidity, and the NH 3 By suppressing the oxidation of NH 3 The effect of environmental factors on the sensitivity can be reduced, and the sensitivity variation can be suppressed. In this case, the desired NH 3 By providing the trap layer 42 made of θ-alumina or the like in the porous protective layer 4 within a range where oxidation suppression effect can be obtained, the poisoning resistance can be improved.

[0066] FIG. 12 shows the results of comparing the case where the entire porous protective layer 4 of the gas inlet 3 is made of θ-alumina and the case where the entire porous protective layer 4 is made of α-alumina. 3 In the case where the oxidation suppression layer 41 is included as an outer layer, NH 3 The median sensitivity and the sensitivity variation due to environmental factors and sensor factors are shown for comparison. The diffusion resistance layer 31 of the gas inlet 3 is made of α-alumina. The thickness of each layer is the same as that of the NH 3 The gas inlet 3 is the same as that used in the calculation of the sensitivity, and the NH 3 The evaluation environment for the median sensitivity was as follows: (Evaluation environment) Flow velocity: 10m / s Gas temperature: Room temperature Gas atmosphere: Air NH 3 Concentration: 100ppm

[0067] As shown on the left side of FIG. 12, the NH 3 The median sensitivity was 0.91, less than 1, and ranged from 0.6 to 1.2 for NH 3 The sensitivity variation has increased. This is because 3 This is because the sensor-related variation included in the sensitivity variation, i.e., the variation (±31%) due to the structure of the porous protective layer 4, becomes large. In addition, although the repetition variation (±1.58%), which is a frequency factor, is relatively small, the variation due to environmental factors (gas flow rate dependency: ±12.5%, NH 3 Concentration dependency (±6%) becomes larger, and the accuracy of NOx detection becomes lower overall.

[0068] In contrast, as shown on the right side of Figure 12, NH composed of α-alumina 3 The gas inlet portion 3 including the oxidation prevention layer 41 as the outer layer of the porous protective layer 4 is 3 The median sensitivity exceeded 1.1, and the sensitivity variation was greatly reduced to a range of 1.0 to 1.2. This is because, as shown in Figures 13 and 14, 3 As shown by comparing the sensitivity contributions, the NH3 In the porous protective layer 4 including the oxidation prevention layer 41, NH 3 This is due to the smaller effect on sensitivity.

[0069] As shown in the upper diagram of FIG. 13, in response to the temperature change of the porous protective layer 4, the θ-alumina exhibits a tendency to increase in NH 3 Although the sensitivity decreases, α-alumina has a high NH 3 13, the θ-alumina has a larger NH 3 Although the sensitivity is reduced, α-alumina has a high NH 3 There is almost no effect on sensitivity.

[0070] Furthermore, as shown in the upper diagram of FIG. 14, in response to the change in density of the porous protective layer 4, the θ-alumina exhibits a tendency to increase in NH 3 Although the sensitivity is reduced, α-alumina has a high NH 3 14, the change in the crystallinity of the porous protective layer 4, that is, the decrease in the baking temperature of the porous protective layer 4, causes the NH 3 It was found that the sensitivity tends to decrease. 3 It is desirable to set the baking temperature of the porous protective layer 4 so as to obtain sensitivity and to maintain the crystallinity of the porous protective layer 4 .

[0071] As shown in Fig. 15, in response to the change in the environmental factors, θ-alumina exhibited a decrease in NH 3 The sensitivity increases, but α-alumina is NH 3 There is almost no effect on sensitivity. Also, as shown in Figure 16, 3 With respect to the change in concentration, θ-alumina exhibits a 3 As the concentration increases from 100 ppm to 300 m / s, the NH 3 Although the sensitivity is reduced, α-alumina is NH 3 There is almost no effect on sensitivity.

[0072] From these results, the gas inlet 3 is set to a fixed value of NH 3 It is preferable to combine the layers so that the sensitivity falls within a predetermined range. 3 NH without the oxidation suppression layer 41 3 Based on the sensitivity α, the NH 3 It is desirable to configure the sensitivity to be 1α or more, and preferably to be in the range of 1.1α to 1.3α.

[0073] NH 3 The sensitivity is the amount of NO passing through the porous protective layer 4 as described above. 2 and N.H. 3 For example, when the entire porous protective layer 4 is made of θ-alumina, 3 When the theoretical value of the sensitivity α is used as a reference (α=1), the NH 3 The theoretical sensitivity is 1.33. Therefore, the NH 3 In consideration of oxidation, the gas inlet portion 3 is configured as follows: NH 3 It is desirable to configure the sensitivity to be in the range of about 1.1α to 1.3α.

[0074] This allows the NH 3 More of these diffuse into the measurement gas chamber 2, increasing the sensor output and 3 By suppressing oxidation, the sensitivity variation is also reduced, and a stable sensor output is obtained. On the other hand, when the entire porous protective layer 4 is made of θ-alumina, the sensitivity variation due to the configuration of the gas inlet portion 3 and changes in the environment becomes large, and the NH 3 The median sensitivity is believed to be below the theoretical value.

[0075] In this way, the porous protective layer 4 is formed of α-alumina. 3 By providing the oxidation suppression layer 41, NH 3 The sensitivity fluctuation is suppressed. Therefore, NH3 Set the sensitivity as a fixed value, NH 3 It is possible to calculate a correction amount derived from the NOx concentration, and further, it is possible to accurately detect the original NOx concentration using the calculated correction amount.

[0076] The present invention is not limited to the above-mentioned embodiments, and can be applied to various embodiments without departing from the gist of the present invention. For example, in the above-mentioned embodiment, the porous protective layer 4 in the gas inlet portion 3 of the sensor element 1 is made of NH 3 Although a two-layer structure including the oxidation prevention layer 41 and the trap layer 42 has been described, a multi-layer structure having three or more layers with different configurations may also be used.

[0077] In the above embodiment, the sensor element 1 has a three-cell structure including a pump cell, a sensor cell, and a monitor cell, but may have a two-cell structure including a pump cell and a sensor cell. Furthermore, the measurement gas chamber 2 can be partitioned into an upstream chamber in which the pump cell is accommodated and a downstream chamber in which the sensor cell and the monitor cell are accommodated. In this way, the structure of the sensor element 1 is not necessarily limited to that shown in the above embodiment, and can be modified as appropriate. [Explanation of symbols]

[0078] S NOx sensor 1 Sensor element 1p Pump cell 1s sensor cell 10 Sensor control unit 2. Measurement gas chamber 3 Gas inlet 31 Diffusion resistance layer 4 Porous protective layer NH 41 3 Oxidation suppression layer

Claims

1. NOx and NH 3 A NOx sensor (S) comprising: a sensor element (1) for detecting a NOx concentration in a measurement gas (G) containing: The sensor element is a measurement gas chamber (2) into which the measurement gas is introduced via a gas inlet (3); a solid electrolyte body (11) constituting a chamber wall of the measurement gas chamber; a pump cell (1p) having a pump electrode (21) on a surface of the solid electrolyte body facing the measurement gas chamber, for adjusting the oxygen concentration in the measurement gas by oxygen pumping; a sensor cell (1s) having a sensor electrode (22) on a surface of the solid electrolyte body facing the measurement gas chamber and outputting a detection signal based on a NOx concentration in the measurement gas having an adjusted oxygen concentration, The gas inlet portion has a diffusion resistance layer (31) provided in contact with the measurement gas chamber, and a porous protective layer (4) provided on the outer side of the diffusion resistance layer. The outermost layer of the porous protective layer is made of NH 3 A NOx sensor having an oxidation suppression layer (41).

2. The Lewis acidity, which indicates the strength of the Lewis acidity, was measured by temperature-programmed desorption of NH 3 When expressed as a cumulative value of signal intensity per unit specific surface area in the adsorption amount measurement, the above NH 3 2. The NOx sensor according to claim 1, wherein the Lewis acidity of the constituent material of the oxidation inhibition layer is smaller than the Lewis acidity of the constituent materials of the other layers.

3. 3. The NOx sensor according to claim 2, wherein a ratio of the Lewis acidity of the material of the NH 3 oxidation inhibition layer to the largest Lewis acidity of the materials of the other layers is 1 / 2 or less.

4. The sensor control unit acquires a detection signal output from the sensor cell, and calculates NH 3 The NOx detection unit (101) calculates an original NOx concentration by subtracting the NOx concentration derived from NH 3 The NOx concentration derived from the NH 3 Sensitivity and NH detected outside the sensor 3 The NOx sensor according to any one of claims 1 to 3, wherein the NOx concentration is calculated using

5. The gas inlet is 3 The sensitivity is such that the porous protective layer is 3 NH without oxidation suppression layer 3 5. The NOx sensor according to claim 4, wherein the sensitivity α is in the range of 1.1α or more and 1.3α or less.

6. The sensor control unit includes a heater control unit (102) that controls a heater temperature for heating the sensor element, The heater control unit is configured to control the temperature of the pump cell and the sensor cell of the sensor element at or above a temperature at which the pump cell and the sensor cell are activated, and the NH 3 NH in the oxidation prevention layer 3 The NOx sensor according to any one of claims 1 to 5, wherein the temperature is controlled to be equal to or lower than a temperature at which thermal decomposition of

7. 7. The NOx sensor according to claim 1, wherein the diffusion resistance layer is made of α-alumina.

8. The NOx sensor according to any one of claims 1 to 7, wherein the material of the other layer includes transition alumina.

9. The porous protective layer is formed by adding the above-mentioned NH 3 9. The NOx sensor according to claim 1, further comprising a trap layer (42) provided between the oxidation prevention layer and the diffusion resistance layer, the trap layer being made of θ-alumina.

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