JP2001083117A - Nitrogen oxide gas sensor - Google Patents

Abstract

(57) Abstract: The always total NOx concentration ratio of NO / NO ₂ in NOx is varied in the exhaust gas, and can be detected by simple means. SOLUTION: The oxygen concentration in the gas detection chamber (5) is set to 0.1.
The NOx concentration is maintained at vol% or higher, and the NOx concentration is detected by converting NOx gas into NO gas by a catalyst disposed in the gas detection chamber (5).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitrogen oxide gas sensor using a solid electrolyte for detecting the concentration of a test gas in a measurement atmosphere. In particular, it is a nitrogen oxide gas sensor that can measure the total concentration of nitrogen oxides in automobile exhaust gas in real time.

[0002]

2. Description of the Related Art In recent years, there has been a great demand for a sensor capable of measuring NOx in vehicle exhaust gas as a total NOx concentration without being affected by the presence of other gases. The present inventor has already proposed a high-temperature operation type mixed potential type NOx sensor using a zirconia solid electrolyte which is an oxygen ion conductor. One basic configuration of the sensor is disclosed in Japanese Unexamined Patent Publication No. 9-2740.
No. 11 discloses this. This takes a form in which a gas detection chamber consisting of a space is formed inside a sensor element using a green sheet of a zirconia solid electrolyte. On the zirconia solid electrolyte in the gas detection chamber, a detection electrode that is active at least for NOx and oxygen and a reference electrode that is active for oxygen and not active for NOx are provided. Further, an electrochemical oxygen pump is provided on the zirconia solid electrolyte forming the gas detection chamber, and while oxygen is supplied from the atmosphere, NOx is electrochemically converted to NO ₂ by the oxygen pump. . The electrochemically converted NO ₂ can be measured as the total NOx concentration at the hybrid potential of the sensing electrode. Furthermore, this N
An example of separately installing an oxygen pump called a conversion electrode having a large activity for NOx in order to improve the Ox conversion efficiency is shown in SAE Paper 1999-01-1280.

[0003] JP-A-9-274011 and SAE Pape
The total NOx sensor disclosed in r 1999-01-1280 is a method of converting NOx to NO ₂ to electrochemically. Conversely, a method of electrochemically converting NOx to NO has also been proposed. The sensor structure is disclosed in Japanese Patent Application Laid-Open No. 9-329578. An oxygen pump is provided in the sensor front chamber near the gas inlet so as to constantly exhaust oxygen in the sensor internal space. A detection electrode and / or a reference electrode are installed in the rear chamber of the sensor, and an oxygen electrode for detecting the oxygen concentration in the rear chamber is also provided. In this configuration, NOx is electrochemically converted into NO at the oxygen pump cathode in the sensor front chamber.

However, the above-mentioned electrochemical NOx
Although the conversion efficiency is excellent, the conversion efficiency is excellent, but there is a disadvantage that the sensor structure is complicated. In addition, the former NO ₂
In the method of detecting the total NOx concentration by converting to NO, conversion becomes difficult in principle as the temperature becomes higher. Further, in the latter method, during the operation of the engine when the oxygen in the exhaust gas becomes close to zero (rich operation), the oxygen concentration in the gas detection chamber becomes almost completely zero because the oxygen pump drive is in the oxygen discharge direction. . Therefore, there is no problem in converting NOx to NO in this method, but it is difficult to detect the converted NO with a mixed potential. That is, in the hybrid potential detection, respective electrode reactions of oxygen and NO are required, but the electrode reaction of oxygen is lost, which causes a decrease in output and a decrease in response speed. Furthermore, HC in exhaust gas
Such as reducing gas cannot be excluded from oxidation, resulting in electrode deterioration and output abnormality.

On the other hand, also in a manner using a mixed potential-type sensor element formed on a solid electrolyte, and the NO / NO ₂ ratio of NOx in the measurement gas constant, has been proposed a method of detecting the total NOx I have. As disclosed in Japanese Patent Application Laid-Open No. 10-2881, a means for supporting a catalyst on a honeycomb structure has been proposed as a pre-stage pretreatment device in order to keep the NO / NO ₂ ratio constant. In this method, the first-stage honeycomb catalyst body is heated to 900 ° C. in an electric furnace, and the second-stage hybrid potential sensor element is heated to 500 ° C. in another electric furnace for detection. However, this method cannot reduce the size of the sensor system and cannot be used as an actual vehicle-mounted sensor. Further, in the exhaust gas of a vehicle, the oxygen concentration in the exhaust gas fluctuates greatly, and this measurement system cannot maintain the oxygen concentration necessary for generating a hybrid potential.
In particular, when the engine is operating in a rich or stoichiometric region, there is almost no oxygen in the exhaust gas, and no method for supplying oxygen from the outside is disclosed. Furthermore, even if the NO / NO ₂ ratio is constant when NOx passes through the honeycomb catalyst, there is a very high possibility that the NO / NO ₂ ratio changes while reaching the sensor element in the subsequent stage.

[0006]

As described above [0006] As a conventional total NOx sensor vehicle, to detect at all times the total NOx concentration in NO / NO ₂ ratio is varied in the NOx in the exhaust gas, the sensor It has a complicated structure and cannot cope with fluctuations in oxygen concentration and removal of other gases such as HC. In particular, in the measurement system of the pretreatment method using a catalyst, it is practically impossible to reduce the size of the sensor system, and in the exhaust gas, the oxygen concentration in the exhaust gas fluctuates greatly, which is necessary for generating a mixed potential of the subsequent detection element. The oxygen concentration could not be maintained. Further, when NOx passes through the honeycomb catalyst, NO / NO ₂
Even when the ratio is constant, NO /
There is a high possibility that the NO ₂ ratio changes, and there is a problem that accurate measurement is not performed. The present invention has been made in view of such a problem.

[0007]

In view of the above problems, the present inventor proposes means for solving the problems by the following means. That is, a zirconia solid electrolyte having oxygen ion conductivity has a sensing electrode active at least for NOx and oxygen, and a sensing electrode active for oxygen paired with the sensing electrode.
An inactive reference electrode is provided at Ox, a gas detection chamber having a gas inlet formed so as to surround at least the detection electrode is provided, and the oxygen concentration in the gas detection chamber is set to 0.1.
The NOx gas in the measurement gas is converted to NO gas by a catalyst placed in the gas detection chamber while maintaining the vol% or more,
An object of the present invention is to provide a nitrogen oxide gas sensor characterized by detecting a NOx concentration in a measurement gas by measuring a potential difference between the detection electrode and a reference electrode. As a result, the sensor structure becomes simpler than in the past, and even if there is no oxygen concentration in the measurement gas, the total NOx concentration can be detected without being affected by other gases.

In the sensor according to the present invention, a catalyst formed in the gas detection chamber has a catalyst layer (see FIG. 1) arranged at least opposite to the detection electrode, and at least a detection electrode among the electrodes installed in the gas detection chamber. NOx can be efficiently used by using a catalyst layer (see FIG. 2) laminated thereon or a porous catalyst body (see FIG. 3) filled in a gas detection chamber.
Is converted to NO, and the total NOx can be detected. In the example of the sensor structure shown in FIGS. 1, 2, and 3, the reference electrode paired with the detection electrode is installed in the gas detection chamber. However, as shown in FIG. It may be arranged in the duct.

Further, as a configuration of the present invention, the gas detection chamber is formed by arranging at least a first zirconia solid electrolyte provided with a detection electrode and a second zirconia solid electrolyte so as to face the first zirconia solid electrolyte. The zirconia solid electrolyte has an oxygen pump electrode that is active on oxygen and inactive on NOx on the front and back of the zirconia solid electrolyte. A nitrogen oxide gas sensor is provided for detecting oxygen as described above while supplying oxygen in the atmosphere into a gas detection chamber. 5 and 6 show structural examples showing this sensor configuration. This sensor configuration makes it possible to easily supply oxygen from the atmosphere to the gas detection chamber. Also,
As shown in FIG. 7, one or more reference electrodes are disposed in an air reference duct that is isolated from the gas detection chamber and the measurement gas atmosphere on the first zirconia solid electrolyte, and communicates with the atmosphere. It is also possible to perform output correction using the potential difference between the detection electrodes and the potential difference between the reference electrode and the oxygen detection electrode. By adopting this method, even if the oxygen concentration in the gas detection chamber fluctuates to some extent, the total N
Ox concentration can be detected.

[0010] Alternatively, as shown in FIG. 8, the first zirconia solid electrolyte is separated from the gas detection chamber and the atmosphere of the measurement gas by an air reference duct in the air reference duct.
At least one reference electrode is arranged, and the oxygen pump voltage is feedback-controlled so that the oxygen concentration in the gas detection chamber is kept almost constant using the potential difference between the oxygen detection electrode installed in the gas detection chamber and the reference electrode. By doing so, fluctuations in the oxygen concentration in the gas detection chamber can be made extremely small. Since NOx conversion, which is the gist of the present invention, is a gas equilibrium reaction utilizing catalytic properties, the pump driving direction can be made independent of the conversion efficiency by using an oxygen pump inert to NOx.

Further, as shown in FIG. 9, the gas detection chamber is made into two continuous chambers, and an oxygen pump which is active only for oxygen is installed in a front chamber having a gas inlet which is in contact with a measurement gas atmosphere.
A structure in which at least a catalyst and a detection electrode are arranged in a rear chamber that communicates with the front chamber by a gas communication port. Other gas removal becomes easier, and the detection accuracy of the sensor is improved.

In a sensor configuration in which a reference electrode is formed in an atmospheric reference duct, one of the electrodes of the oxygen pump electrode is simultaneously installed in the atmospheric reference duct, and oxygen is driven by the oxygen pump to release oxygen. A diffusion resistance of the atmospheric reference duct is set so as not to control the diffusion. This configuration is shown in FIG. 10, which simplifies the sensor structure by using only one duct structure.

On the other hand, the electrodes formed at least in the gas detection chamber of the above-described oxygen pump are composed of Pt, Au, Pd, and Pt.
-Au alloy or Pt-Pd alloy is used to convert NOx in the gas detection chamber electrochemically even if the oxygen pump is driven in the oxygen supply direction, that is, catalytic conversion. Other gases such as HC can be oxidized without lowering the efficiency of the process. Further, the detection electrode Pt-Rh alloy formed on the first zirconia solid electrolyte, Pt-Ru _{alloy, Cr 2 O 3, NiCr 2} O 4, Mg
Cr ₂ O _4, FeCr by a _second electrode member comprising at least one or more of O _4, a larger NO output.

In the sensor configuration of the present invention, the total NOx detection can be performed more efficiently by integrating the self-heating heater substrate with the sensor element and maintaining the catalyst temperature in the gas detection chamber at 600 ° C. or higher. Further, as in the structure shown in FIG. 11, the catalyst heater is operated at a temperature higher than the detection electrode side heater temperature by the two heater substrates integrally formed with the gas detection chamber of the sensor element interposed therebetween. In this case, the optimum temperature can be set for each of the detection electrodes, and the efficiency of the total NOx detection can be increased.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described with reference to the sensor structure diagram of FIG. This sensor configuration has a zirconia solid electrolyte 1 having oxygen ion conductivity as described above.
Above is a hybrid potential type sensor comprising a detection electrode 2 having activity on both NOx and oxygen, and a reference electrode 3 having activity on oxygen and not having activity on NOx. Zirconia solid electrolyte 1
When the detection electrode 2 and the reference electrode 3 are formed and exposed to a measurement gas atmosphere containing NOx, the outputs are opposite to each other between the case of only NO ₂ gas and the case of only NO gas as shown in FIG. Becomes This is because two kinds of reactions (Equation 1,
This is a principle that arises because the exchange of electrons in equations 2 and 3 and equation 4) is reversed. In this state, NOx becomes NO
It is clear that the total NOx concentration in the exhaust gas containing NO and NO ₂ , whose abundance ratio fluctuates, cannot be measured.

(In the case of NO detection) O ₂ + 4e ⁻ → 2O ² −... ( ² ) + 2O ²⁻ → 2NO ₂ + 4e ⁻ (2) (In the case of NO ₂ detection) 2O ²⁻ → O ₂ + 4e ⁻ … (Equation 3) 2NO ₂ + 4e ⁻ → 2NO + 2O ²⁻ (Equation 4)

Therefore, a minute space is formed inside the sensor element as shown in FIG. 1, and the catalyst 4 is disposed therein. Thereby, NOx (NO and N) in the minute space 5 (gas detection chamber) is
O ₂ ) is unique (thermal equilibrium) depending on its temperature and oxygen concentration
The NO / NO ₂ ratio is determined. Equations 5 and 6 show the equilibrium reaction equations at this time. K in Equation 6 is an equilibrium constant in the reaction of Equation 5. If there is no catalytic action, it takes time to reach the equilibrium state, and the fluctuating NOx concentration in the exhaust gas cannot be measured in real time. Figure 2 shows the catalyst formation position.
It may be laminated on the detection electrode 2 as shown in FIG. Further, as shown in FIG. 3, the catalyst body 4 may be filled and formed in the gas detection chamber. However, in this case, it is necessary to optimize the porosity of the catalyst body 4.

NO + 1 / 2O ₂ = NO ₂ (Equation 5) [NO] = [NO ₂ ] / (K [O ₂ ] ^1/2 ) (Equation 5) Equation 6)

With the above-described sensor configuration, for example, when the oxygen concentration in the exhaust gas is several percent or more, it is possible to detect the total NOx concentration by optimizing the operating temperature. That is, as shown in FIG. 13, if the oxygen concentration and the reaction temperature are appropriately controlled, it is basically possible to almost completely turn into NO by the catalytic action. The problem is that it is easier to have NO in the absence of oxygen, but oxygen is needed to produce a hybrid potential. In addition, oxygen must be present in the gas detection chamber to oxidize and remove other gases such as HC in the exhaust gas.

Therefore, as shown in FIG. 5, oxygen is supplied into the gas detection chamber 5 from the atmosphere by the oxygen pump 6 and the oxygen concentration is maintained at an appropriate temperature so that NOx can be turned into NO. In the case of FIG. 5, an oxygen concentration detection electrode is not formed in the gas detection chamber 5, and optimization design of the gas diffusion resistance of the gas inlet and the supply speed of the oxygen pump is required. Although the oxygen pump 6 is formed in the second solid electrolyte 1 'in the structure of FIG. 5, it may be formed in the first solid electrolyte 1 at the same time as the detection electrode 2 as shown in FIG. In particular, when the reference electrode 3 is installed in the air reference duct, the air introduction duct and the air reference duct can be shared. However, in this case, it is necessary that the supply of oxygen in the atmospheric duct is not in a diffusion-controlled state by driving the oxygen pump.

In the sensor structure of FIG. 8, the oxygen pump driving voltage is fed back while detecting the oxygen concentration in the gas detection chamber 5 to always maintain the oxygen concentration in the gas detection chamber 5 at an optimum value. In this case, although the structure is slightly complicated, the oxygen concentration in the gas detection chamber can be easily maintained at the optimum concentration in consideration of NOx conversion and NOx detection. In addition, oxygen electrode 7
As a development of FIG. 5 in which no gas is installed, it is possible to easily remove the reducing gas by making the gas detection chamber 5 two or more chambers. As shown in FIG. 9, the gas detection chamber 5 is separated into a front chamber and a rear chamber, and a catalyst layer 4 is installed in each of them. Further, by setting the gas diffusion resistance of the gas introduction port smaller than that of the gas communication port, HC can be excluded irrespective of the oxygen concentration fluctuation in the exhaust gas.

The higher the operating temperature of the sensor, the more
Although it is clear that NO is easily formed, if the mixed potential output is too high, the output is reduced and the temperature characteristics are contradictory. Furthermore, if the position of the catalytic converter and the detecting electrode are far apart, the gas equilibrium will shift and the NO ratio will decrease. FIG.
As described above, two heater substrates are formed integrally with the gas detection chamber interposed, and the heater load on the catalyst layer side is increased and the heater load on the detection electrode side is reduced to optimize the temperature of the NOx converter and the NOx detector. can do. The catalyst temperature is 60
The temperature is 0 ° C or higher, preferably 650 ° C to 800 ° C.

As described above, the optimal setting of the sensor operating temperature and the oxygen concentration in the gas detection chamber is determined by the balance between the NOx conversion efficiency, the NO sensitivity of the detection electrode, and the ability to remove and oxidize the reducing gas in the measurement gas. It is clear from FIG. 13 that the efficiency of NO formation in the gas equilibrium reaction is higher as the oxygen concentration is lower and the reaction temperature is higher. On the other hand, the mixed potential element NO
Sensitivity to the NO ₂ is the is larger for NO ₂ as seen in the example of FIG. 12 is common. Therefore, the conversion efficiency of NOx needs to be at least 95% or more. In order to satisfy this conversion efficiency of 95%,
The catalyst temperature and the oxygen concentration upper limit are determined as shown in Table 1. The NO sensitivity of a hybrid potential type single element is usually 500 to 8
0.1% oxygen or more at 00 ° C. is required. If the element temperature is too low, the gas response speed will be significantly deteriorated. If the element temperature is too high, the sensitivity is significantly reduced. According to the relationship between the NO sensitivity of the element and the oxygen concentration, the oxygen concentration needs to be 0.1% or more. If the oxygen concentration is too low, the gas response speed will be significantly reduced. There is no restriction on the high oxygen concentration side. Further, when a reducing gas such as HC is present in the measurement gas, it is necessary to have an oxygen concentration sufficient to eliminate the oxidation. In ordinary engine exhaust gas, it is necessary that the oxygen concentration in the gas detection chamber be at least 1% or more based on the HC concentration in the exhaust gas. These conditions are summarized in Table 1. From this, in the present invention, the oxygen concentration needs to be 0.1% to 20%, and is desirably 1 to 20% in exhaust gas where a reducing gas is present. The operating temperature is set at 7
Considering at a temperature of 00 ° C. or lower, the content is more preferably set to 1 to 10%.

[0024]

[Table 1]

The configuration of the sensor of the present invention has been described above. Such a sensor element is manufactured as follows. As the zirconia solid electrolyte, generally used yttria-added zirconia is generally used, but is not particularly limited in the present invention. The zirconia (ZrO ₂ ) green sheet is used for manufacturing the sensor of the present invention. In a general method for producing a green sheet, zirconia powder to which a predetermined amount of yttria has been added is subjected to ball mill mixing with an organic binder such as PVA, a plasticizer and an organic solvent. At this time, in order to improve the dispersion of the powder particles,
A dispersant may be added. The zirconia slurry thus obtained is formed on a PET film to a thickness of several hundred microns by a sheet forming machine using a doctor blade. When this is dried and the solvent is evaporated, it is very flexible, and when the sheets are pressed together while heating, they can be strongly adhered to each other. In order to form an internal space in this green sheet laminate,
It can be formed by filling the green sheet with theobromine or the like which sublimates at a temperature lower than the degreasing temperature at which the organic binder is oxidized and removed.

Generally, electrodes, lead conductors, heaters, temperature sensors, and the like are formed on the green sheet by screen printing. As the detection electrode, a mixed potential type detection electrode material having sensitivity to at least NO is used.
In the noble metal system, a Pt-Rh alloy and a Pt-Ru alloy are particularly suitable. In metal oxides, Cr ₂ O ₃ , NiCr ₂ O ₄ ,
It is preferable to use among MgCr ₂ O ₄ and FeCr ₂ O ₄ .
If the oxygen pump is installed in the gas detection room, NOx
It is important not to have activity. For that purpose, the oxygen pump electrode formed in the gas detection chamber is
It is preferable to use Pt, Au, Pd, a Pt-Au alloy, or a Pt-Pd alloy. Further, as a catalyst formed in the gas detection chamber, Pt, Au, Pd, Rh and alloys thereof, manganese oxide, cobalt oxide, iridium oxide, chromium oxide, and the like are applied. The catalyst body formed in the gas detection chamber is preferably arranged near the NOx detection pole. Although the arrangement method is as described above, it is particularly important to control the porosity of the catalyst body or the catalyst layer in order to increase the conversion efficiency. The porosity is desirably 10 to 50%, preferably 20 to 40%. further,
In order to adjust the porosity, alumina particles and zirconia particles are dispersed and added. In particular, by adding zirconia particles, the porosity of the catalyst body is adjusted and at the same time, the bonding strength with the zirconia solid electrolyte substrate is enhanced, which contributes to improvement of reliability. The amount of alumina particles and zirconia particles added is 10 to 95 vol.
%, Preferably 30 to 90 vol%. These catalyst bodies or catalyst layers are formed by kneading the catalyst powder together with an organic binder to form a paste and form it by screen printing or the like, but, of course, is not limited to this manufacturing method.

Hereinafter, the present invention will be described in detail with reference to specific examples, but the present invention is not limited to the scope of these examples.

[0028]

[Embodiment 1] The sensor structure shown in FIG. 1 was manufactured as follows. First, a green sheet of zirconia to which 6 mol% of yttria was added was prepared by the above-described method. The thickness of the zirconia green sheet was about 200 μm. This was cut to the size before the firing of the sensor substrate, and was performed at the same time as opening the window in the portion where the gas detection chamber and the air duct were formed. Electrodes and lead conductors formed respectively were applied to these cut green sheets by screen printing. As the detection electrode, a paste obtained by mixing and kneading a zirconia solid electrolyte powder, an organic binder, and an organic solvent was used mainly for NiCr ₂ O ₄ powder whose particle size was adjusted by ball milling. Pt paste was used for the reference electrode, oxygen pump electrode, and lead conductor. A Pt-Pd (5 wt%) paste was used for printing the catalyst layer. This paste was prepared by kneading a Pt-Ru alloy fine powder and a zirconia powder with an organic binder or the like. The green sheets on which these prints were formed were sequentially overlapped while inserting a Pt wire into the lead take-out portion so as to be in the laminated state shown in FIG. 1, and lamination was performed by isostatic pressing in warm water.
This laminate was degreased at about 600 ° C.
The firing was performed in a high temperature atmosphere of 00 ° C.

The sensor sample thus prepared was heated and maintained at 650 ° C. in a soaking section in a quartz tube set in an electric furnace. The potential difference between the detection electrode and the reference electrode via the Pt lead wire of the sensor was captured by a digital multimeter and recorded on a computer. NO + O ₂ (1%) + N ₂ (remainder) as a measurement gas in a quartz tube heated and maintained at 650 ° C
Mixed gas I, NO ₂ + O ₂ (1%) + N ₂ (remainder) mixed gas II
Were introduced. However, the measurement was performed while changing the gas concentrations of NO and NO _{2 to 20} , 50, 100, 200, and 500 ppm. FIG. 14 shows the NOx concentration dependency of the sensor output at that time. As can be seen here, NO gas, N
Almost the same output is obtained in the minus direction regardless of the O ₂ gas. That is, NOx (NO + NO ₂ ) in the measurement gas
It can be seen that NO is converted into NO by the sensor structure of the present embodiment and detected as the total NOx concentration.

[Embodiment 2] As shown in FIG.
A sample having the sensor structure shown in FIG. 8 was manufactured. That is, the difference from the sensor structure manufactured in Example 1 is that the oxygen pump anode is formed in the gas detection chamber, and the oxygen pump cathode is formed in a duct communicating with the atmosphere. Further, in the structure of FIG. 8, the pump concentration is feedback-controlled so that the oxygen concentration is detected and the oxygen concentration in the gas detection chamber is kept constant. In addition, an integrated heater substrate was installed on the sensor element so that the sample could be self-heated. In order to compare performance with this sample, a self-heating type sample in which a heater substrate was integrated with the sensor structure of FIG. 1 manufactured in Example 1 was also manufactured. The oxygen pump electrode of this sample
An alloy electrode of Pt-Pd (5%) was used. In addition, in order to improve oxygen pumping performance, zirconia solid electrolyte powder was added at the same time, and the addition amount was adjusted so as to form a porous electrode. In this example, the sample having the structure of FIG. 6 was A, the sample having the structure of FIG. 8 was B, and the comparative sample having the structure of FIG. Note that a constant voltage was always applied to the oxygen pump voltage of Sample A.

Samples A, B and C were set in a gas sensitivity measuring apparatus, and heating was controlled by a self-heater so that the temperature of the sensor element became 650 ° C. At this time, NO (200
ppm) + O ₂ + N gas mixture of ₂ (balance) and _{NO 2 (200ppm) + O 2} + N
_The sensor output was measured using the ₂ (remainder) mixed gas. At this time, the oxygen concentration in the mixed gas was set to 0, 0.5, 1, 5,
The effect of the sensor output on the oxygen concentration in the measurement gas was investigated by changing to 10 vol%. FIG. 15 shows the measurement results. From this result, in the sample of the present embodiment, the influence of the oxygen concentration in the measurement gas can be reduced by the addition of the oxygen pump and the feedback control of the oxygen pump. That is, it can be seen that, in fact, since the oxygen concentration always fluctuates in the exhaust gas of the vehicle, the measurement accuracy of the inventive sensor of this embodiment is greatly improved.

Example 3 A sample having the sensor structure shown in FIG. 1 was manufactured in the same manner as in Example 1. In this example, the detection electrodes of these samples were changed to the materials shown in Table 2 and 300 ppm of NO gas (O ₂ : 2%, N ₂ : balance)
And sensor outputs for NO ₂ gas (O ₂ : 2%, N ₂ : balance) were measured. Incidentally, zirconia solid electrolyte powder is dispersedly added to each electrode material in order to adjust the porosity of the electrode. Table 2 shows the results measured in the same manner as in Example 1.
Are shown together. These detection electrodes can be used as a total NOx sensor for NO detection.

[0033]

[Table 2]

Example 4 A sample having the sensor structure shown in FIG. 1 was produced in the same manner as in Example 1. In this embodiment, the oxygen pump electrodes of these samples were changed to the materials shown in Table 3 and 300 ppm of NO gas (O ₂ : 2%, N ₂ :
The sensor output was measured for the remaining portion) and NO ₂ gas (O ₂ : 2%, N ₂ : remaining portion). Incidentally, zirconia solid electrolyte powder is dispersedly added to each electrode material in order to adjust the porosity of the electrode. Table 3 summarizes the results measured in the same manner as in Example 3. For comparison, a sample in which a detection electrode material having NOx activity was laminated on an oxygen pump electrode or an oxygen pump electrode was simultaneously measured. From these, it can be seen that the total NOx sensor for NO detection can be efficiently provided by each oxygen pump electrode of this embodiment having no activity on NOx.

[0035]

[Table 3]

Example 5 In the same manner as in Example 2, self-heating type sensor samples A, B and C were prepared again. The influence of these samples on the sensor output of HC (reducing gas) was examined as follows. Each sample was set in a measuring device, and the sensor output was measured while flowing a measurement gas containing no oxygen. NO (200 ppm) and NO (2
(00 ppm) + C ₃ H ₆ (1000 ppm) was used. The sensor operating temperature was 650 ° C. Table 4 shows the results. Thus, the sensor structure capable of introducing oxygen into the gas detection chamber is based on HC in exhaust gas.
It can be seen that it is very effective for not being affected by (reducing gas).

[0037]

[Table 4]

Example 6 A sample having the sensor structure shown in FIG. 1 was manufactured in the same procedure as in Example 1. However, in all of the samples of this example, the self-heating heater substrate was formed integrally with the sensor element. Further, the catalyst materials shown in Table 5 and additives for adjusting the porosity were used as the materials for the catalyst layer. Further, in order to confirm the effect of the catalyst layer, a sample having no catalyst layer was prepared at the same time. These samples were heated and held using a self-heater so that the sensor temperature became 650 ° C. 300 ppm NO gas (O ₂ : 1%,
N ₂ : balance) and NO ₂ gas (O ₂ : 1%, N ₂ : balance)
Was prepared, and the sensor output of each sample was measured. Table 5 shows the results. It can be seen that the total NOx can be detected with these catalyst materials.

[0039]

[Table 5]

[0040]

According to the present invention, a detection electrode active on NOx and oxygen and a reference electrode active on oxygen and not active on NOx are provided on the zirconia solid electrolyte, and a gas detection chamber is provided so as to surround the detection electrode. While maintaining the oxygen concentration of 0.1% or more, preferably 1% or more, NOx gas in the measurement gas is converted to NO gas by a catalyst disposed in the gas detection chamber, and the gas is supplied between the detection electrode and the reference electrode. By measuring the potential difference between NOx and NOx in high-temperature exhaust gas, NOx
And even if the oxygen concentration in the exhaust gas is zero, the total NOx concentration can be measured without being affected by the reducing gas in the exhaust gas.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration example of a sensor of the present invention.

FIG. 2 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 3 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 4 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 5 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 6 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 7 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 8 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 9 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 10 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 11 is a diagram showing another configuration example of the sensor of the present invention.

FIG. 12 is a graph showing an output characteristic example of a conventional hybrid potential sensor element.

FIG. 13 is a graph showing a curve indicating a calculated value of a thermal equilibrium concentration of NOx gas.

FIG. 14 is a graph showing NOx output characteristics of the sensor of the present invention in Example 1.

FIG. 15 is a graph showing output characteristics indicating the influence of the oxygen concentration in the measurement gas in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte 2 Detection electrode 3 Reference electrode 4 Catalyst layer 5 Detection chamber 6 Oxygen pump 7 Oxygen electrode

Claims (11)

[Claims] A zirconia solid electrolyte having oxygen ion conductivity is provided with a detection electrode active at least for NOx and oxygen, and a reference electrode paired with the detection electrode, which is active on oxygen and not active on NOx. A gas detection chamber having a gas inlet formed so as to surround the detection electrode is provided, and while the oxygen concentration in the gas detection chamber is maintained at 0.1% or more, a catalyst disposed in the gas detection chamber causes a gas contained in the gas to be measured. A nitrogen oxide gas sensor comprising: converting NOx gas into NO gas; and measuring a potential difference between the detection electrode and a reference electrode to detect a NOx concentration in the measurement gas. 2. A catalyst layer in which a catalyst formed in a gas detection chamber is disposed so as to face at least a detection electrode, a catalyst layer laminated on at least the detection electrode among the electrodes, or a porous material filled in the gas detection chamber. The nitrogen oxide gas sensor according to claim 1, wherein the nitrogen oxide gas sensor is made of any one of a catalytic catalyst. 3. A gas detection chamber is formed by arranging a second zirconia solid electrolyte so as to face at least the first zirconia solid electrolyte provided with a detection electrode, and forming the gas detection chamber on both sides of the second zirconia solid electrolyte. NOx active in oxygen
An oxygen pump electrode in the gas detection chamber is provided as an anode, while the oxygen pump electrode disposed in an oxygen introduction duct leading to the atmosphere is used as a cathode while the oxygen in the atmosphere is supplied to the gas detection chamber. The nitrogen oxide gas sensor according to claim 1, wherein the measurement is performed. 4. A method according to claim 1, wherein one or more reference electrodes are disposed on the first zirconia solid electrolyte in an air reference duct isolated from the gas detection chamber and the measurement gas atmosphere and communicating with the atmosphere.
4. The nitrogen oxide gas sensor according to claim 1, wherein output correction is performed using the potential difference between the reference electrode and the detection electrode and the potential difference between the reference electrode and the oxygen detection electrode. 5. One or more reference electrodes are disposed on the first zirconia solid electrolyte in an air reference duct isolated from a gas detection chamber and a measurement gas atmosphere and communicating with the atmosphere.
The oxygen pump voltage is feedback-controlled so as to keep the oxygen concentration in the gas detection chamber substantially constant using a potential difference between the oxygen detection electrode installed in the gas detection chamber and the reference electrode. Item 6. The nitrogen oxide gas sensor according to item 1 or 3. 6. The gas detection chamber is composed of two continuous chambers, and an oxygen pump that is active only for oxygen is installed in a front chamber having a gas introduction port in contact with a measurement gas atmosphere, and is connected to the front chamber through a gas communication port. A structure in which at least the catalyst and the detection electrode are arranged in a chamber, wherein a gas diffusion resistance of a gas inlet is larger than a gas communication port.
6. The nitrogen oxide gas sensor according to any one of 5. 7. In the sensor configuration in which the reference electrode is formed in an atmospheric reference duct, one of the electrodes of the oxygen pump electrode is simultaneously installed in the atmospheric reference duct, and oxygen is driven by driving the oxygen pump. 7. The nitrogen oxide gas sensor according to claim 3, wherein a diffusion resistance of the atmospheric reference duct is set so that diffusion control is not performed by the atmospheric reference duct. 8. An electrode formed at least in a gas detection chamber of the oxygen pump is composed of Pt, Au, Pd, and Pt-Au.
8. The nitrogen oxide gas sensor according to claim 3, wherein the nitrogen oxide gas sensor is made of any one of an alloy and a Pt-Pd alloy. 9. The detection electrode formed on the first zirconia solid electrolyte is a Pt-Rh alloy, a Pt-Ru alloy, Cr ₂ O ₃ , N
_{iCr 2 O 4, MgCr 2 O} 4, FeCr 2 nitrogen oxides gas sensor according to any one of claims 1 to 8, characterized in that it consists of at least one or more of O _4. 10. The sensor configuration according to claim 1, wherein a self-heating heater substrate is integrated and a catalyst temperature in a gas detection chamber is maintained at 600 ° C. or higher. Nitrogen oxide gas sensor. 11. A heater configured to operate at a catalyst-side heater temperature higher than a detection-electrode-side heater temperature by two heater substrates integrally formed with a gas detection chamber of the sensor element interposed therebetween. The nitrogen oxide gas sensor according to claim 10.