JPH1172476A - Nitrogen oxide gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a potential difference type NOx sensor low in sensor impedance, having a noble metal electrode good in electrode conductivity and excellent in NOx sensitivity characteristics by using an alloy electrode consisting of platinum and rhodium or a thermet electrode consisting of platinum, rhodium and zirconia as a gas detection electrode of a sensor. SOLUTION: A sensor is formed by using a green sheet 1 of zirconia doped with 8 mol. of Y2 O3 as an oxygen ion conductor. The green sheet is formed by a doctor blade method and has a thickness of 0.3 mm. This green sheet is cut into a sample size of 4×6 mm to be used. As a material of a detection electrode 2, paste prepared by adding a predetermined amt. of an org. binder and a predetermined amt. of an org. solvent to an alloy powder of Pt and Rh to knead them therewith is prepared. The addition amt. of Rh is set to 5% by wt. of the total amt. of Pt and Rh. Zirconia is further added to this paste in order to adjust electrode porosity. Pt is applied to a reference electrode 3 by paste printing so as to form a pair on the surface of the zirconia sheet 1 along with the detection electrode 2 and zirconia for adjusting an electrode structure is dispersed and added in the same way as the detection electrode 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid-state sensor for detecting a nitrogen oxide gas, and more particularly to a discharge N of a general combustor.
O _X and the measurement of indoor environmental, and more particularly to nitrogen oxide sensor suitable the NO _X sensing in automobile exhaust gas are exposed to high temperatures.

[0002]

BACKGROUND ART zirconia solid electrolyte substrate is provided sensing electrode and its counter electrode, a gas sensor system for measuring the potential difference between the electrodes of the NO _X concentration has already been reported. For example, Japanese Patent Application Laid-Open Nos. 7-198167 and 8-4334
No. 6 discloses CdMn ₂ O ₄ and NiCr ₂ on a zirconia solid electrolyte substrate as an oxygen ion conductor.
This is a configuration in which a detection electrode made of a composite oxide of O ₄ and a Pt counter electrode are provided, and it can be said that it has heat resistance that can be used even in a high-temperature atmosphere.

On the other hand, a noble metal-based electrode can be expected as a detection electrode having a structure having sufficient heat resistance even in a high-temperature atmosphere such as in automobile exhaust gas. In that respect, Pt electrodes have already been used as electrodes in automobile λ sensors and wide area air-fuel efficiency sensors, and their high reliability in actual use has been demonstrated. Noble metal-based electrodes have many advantages, such as their chemical stability, ease of fabrication, and reduction in electrode impedance. The cited in JP-A-8-271476 as a sensor example relating to the NO _X gas using a noble metal sensing electrode to the zirconia solid electrolyte substrate are those shown below. One is U.S. Pat.
No. 25, it is Rh to provide NO _X sensitivity to a gray-scale potential difference type oxygen sensor (λ sensor) of an automobile.
FIG. 2 illustrates a sensor provided with an alumina overcoat layer impregnated with. However, in this structure R
The role of the overcoat layer impregnated with h is NO _X decomposition catalyst layer, it is clear that the oxygen itself generated by decomposing NO _X are sensed by Pt sensing electrode. The second one is disclosed in JP-A-59-91358, in which an electrode made of a noble metal such as Pt, Rh, Pd, and Au is provided on a zirconia solid electrolyte substrate, and an N ₂ O decomposition catalyst such as Co ₃ O ₄ is used. A detection electrode laminated or carried on the electrodes is formed, and the potential difference between the electrodes is measured. However, considering the case of measuring NO _{X in} the exhaust gas of an automobile, NO and NO ₂ are target gases and are not measured by N ₂ O. Further, the potential difference with respect to the low-concentration gas is very small, and is substantially equal to the potential difference in the actual concentration region in exhaust gas (several 1000 ppm or less).

[0004] its role to say that using noble metal sensing electrode in this way conventional gray potentiometric NO _X sensor,
Simply NO _X is as a cracking catalyst, or not acting only as a collector for collecting the charges decomposition reaction in the catalyst layer.
As also described in further Hei 8-271476 discloses, in NO _X sensor using a conventional noble metal sensing electrode potential difference is small, a large dependence on the oxygen concentration in the detection gas atmosphere, further NO _X At present, it can only be applied in the direction of disassembly.

[0005]

[0007] As described above, although the potentiometric NO _X sensor using an oxide electrode has a large sensitivity obtained, the sensor electrode resistance is high, therefore, the current collector in the sensing electrode Must be formed to reduce the electrode area. On the other hand, the missing was measured as the NO _X potential at the electrodes of noble metal which is electrically good conductor as an electrode material. It has only a slight sensitivity with N ₂ O (laughing gas). In addition, in the noble metal-based electrode, since the potential difference itself has oxygen partial pressure dependency, accurate O ₂ concentration control is required. In view of these problems, the sensor impedance is low, a good noble metal electrodes of the electrode conductivity and it is an object to provide an excellent potentiometric NO _X sensor of the NO _X sensitivity. Furthermore even when applied to an exhaust gas of an automobile, it is within the present problem of measuring the NO _X concentration without being affected by the ambient oxygen partial pressure.

[0006]

In view of the above problems, we have solved the problems by the following means. That is, the present invention is (1) oxygen ion conductor and the detection electrode provided on the zirconia solid electrolyte substrate is, the sensing electrode and the insensitive platinum counter electrode, platinum in the NO _X of the solid electrolyte substrate paired A nitrogen oxide gas sensor of a type that measures a potential difference between a reference electrode and an alloy electrode or platinum made of platinum and rhodium as a gas detection electrode of the sensor.
Nitrogen oxide gas sensor characterized by using a cermet electrode composed of rhodium and zirconia, and (2) an alloy electrode composed of platinum and rhodium and platinum, rhodium, cermet electrode composed of zirconia,
The object is solved by providing a further nitrogen oxide gas sensor characterized by using a detection electrode in which the amount of rhodium contains at least 0.5 wt% with respect to the total amount of platinum and rhodium.

Also, an alloy comprising the above-mentioned platinum and rhodium
An electrode or a support made of platinum, rhodium and zirconia
-Combining the electrode with the sensor structure
NO _XAs a detection sensor, (3) the oxygen ion conductor
Installed inside a sensor substrate consisting of a luconia solid electrolyte substrate
To perform gas detection by introducing a test gas into a closed can chamber
In the formula, the gas flowing into the can chamber leads to the test gas atmosphere.
A first canister with a mouth, or communicating with the first canister
A sensor comprising a second can chamber,
Oxygen discharge or oxygen pumping provided in the canister and the second canister
Electrode and oxygen concentration in the first or second chamber
And NO or NO in the first can chamber.
_TwoNO converted to_XDetection electrode for detecting
Platinum counter electrode or detection electrode formed in the same chamber as
Reference oxygen concentration across the zirconia solid electrolyte substrate
Platinum counter electrode formed through a duct that maintains the temperature
And the detection electrode is the platinum described above.
And rhodium alloy electrodes or platinum, rhodium,
Nitrogen formed by a zirconia cermet electrode
Oxide gas sensor, and (4) the detection electrode is formed
Oxygen concentration in a certain chamber is oxygen and NO at the sensing electrode._X
From mixed potential for NO_XConcentration to produce a potential difference
Providing a nitrogen oxide gas sensor featuring a controlled system
NO by offering_XThe electrode itself, which is the noise of detection
Oxygen partial pressure dependency can be substantially eliminated.

[0008] In more detail, Pt and rhodium
NO_XIt is used as a catalyst, but its alloy is
Detection electrode (oxygen and NO_XActive)
It is not. The present invention claims that
The invention electrode is used on a principle different from the conventional gray potential difference.
Where it is. That is, NO_XThe reaction of the sensing electrode is N
O_X(NO, NO_TwoNO) as oxidation / reduction reaction_XAnd oxygen
Is determined simultaneously with the mixed potential (NO of the detection electrode_XWhen
O_TwoElectrode potential (potential difference with respect to the counter electrode)
Output) is used as the output. Sensor configuration
As shown in FIGS. 1 and 2, the detection electrode 2 and the counter electrode 3 (N
O_XZirconia solid electrolyte substrate
If it is above, it is not bound by the arrangement method. Just the detection pole
Oxygen exists in the atmosphere and only needs to form a mixed potential
It is. Counter electrode 3 is NO under operating conditions _XEven if you are insensitive
And therefore usually only made of platinum or an electrode set
Formed with zirconia added to platinum for weaving control
Is done. Of course, in the configuration shown in FIG.
May be fixed to the atmosphere, for example. The configuration shown in FIG.
NO on the counter electrode 3 side_XIf no exists, NO_X
For example, the Pt-Rh alloy electrode of the present invention can be used.
Obviously, this is within the scope of the present invention. Figure
1, FIG. 2 and FIG.
5 indicates that the counter electrode 3 is isolated from the test gas
Indicate the isolation wall to be made.

Under such conditions, in the case of a conventionally reported oxide electrode of NiCr ₂ O ₄ or the like, the conductivity of the electrode film itself is low and a current collector for capturing a reaction charge is formed under the electrode. Needed. Since the electrode impedance of the oxide electrode itself is large, for example, when the oxide electrode is used in a vehicle, noise easily occurs and it is difficult to ensure accuracy. However, even if an attempt is made to increase the electrode size, there is a problem that the potential difference cannot be measured efficiently without a current collector because the conductivity of the electrode itself is low.

On the other hand, noble metal electrode although conductive electrode is good, it can detect the NO _X as mixed potential has not been found. Conventionally, a potentiometric precious metal that has been used in the NO _X sensor is its catalytic, or which was just current collector is as described above. The present invention is an alloy film of Pt and Rh in the NO _X sensing electrode,
The catalytic activity of Rh has been added to the oxygen adsorption capacity of Pt is held on the same electrode is based on the idea of measuring the NO _X difference by mixed potential described above. Therefore, the alloy dispersibility (addition concentration) of Rh should have a correlation with the sensitivity, and such a result has been obtained in fact.

However, the noble metal-based electrode is active on oxygen itself. For example, if the detection of the concentration potential difference method is performed with the structure as shown in FIG. 3, the fluctuation of the oxygen concentration on the detection electrode 2 side is directly detected. Therefore, accurate oxygen partial pressure control is required in the detection electrode atmosphere. In the region where the oxygen concentration is near zero,
Actually, there is no choice but to measure with an oxygen concentration sensor. In this oxygen concentration region, the output dependence on the oxygen concentration is very large, and accurate concentration control is practically impossible. On the other hand, the oxygen concentration dependence in the detection of mixed potential-type is the application type of the present invention is very small, substantially oxygen concentration control hardly affect the NO _X output be very rough. Therefore, even under the environment considered to be used in vehicles conventionally, the Pt-R
h-alloy electrodes are practically applicable.

FIGS. 7 and 8 show NO and NO ₂ in vehicle exhaust gas.
The shows a sensor structure that can be detected as the total NO _X. NO and NO ₂ in the exhaust gas are gasified into either NO or NO ₂ by the oxygen pumping electrode installed in the first chamber in the first chamber, and the potential difference is detected by the electrode of the present invention in the second chamber. . That is, when detecting the NO _X as NO ₂ can carry out the oxidation of NO do oxygen汲included in the first bottom chamber by pumping electrodes. Conversely, when NO is detected by reduction of NO ₂ , the pump operation voltage is reversed to discharge oxygen. In any case, the oxygen concentration in the first chamber is feedback-controlled by an oxygen sensor installed in the second chamber connected to the first chamber. By incorporating the mixed potential detection method of the aforementioned sensor structure shown in FIGS. 7 and 8, a large oxygen partial pressure dependence is greatly alleviated conventional noble metal electrode itself has, capable of total NO _X sensing as a vehicle sensor Applied to

Furthermore, no a detection electrode provided on the zirconia solid electrolyte is an oxygen ion conductor, the activity in the NO _X on the solid electrolyte constituting the detection electrode and the counter P
A nitrogen oxide gas sensor for measuring a potential difference between the electrode and a reference electrode, wherein a gas detection electrode of the sensor is an electrode in which a rhodium layer is laminated on a platinum layer or zirconia is dispersed and added in the laminated electrode. And the oxygen concentration of the measurement atmosphere is 0.05 to 21 vol%.
A nitrogen oxide gas sensor characterized by being controlled to a predetermined value having an arbitrary width.

[0014] The effects of the present invention are as follows, taking into account the situation in which it is used in a general room to the situation in which it is used in automobile exhaust gas. In method for measuring the NO _X concentration at a potential difference, the present invention is a Pt-Rh alloy electrode, by using the cermet electrode with those of the laminated electrode or that of zirconia, very non-conventional noble metal-based electrode A large detection output was obtained. Therefore, NO _X concentration measurement accuracy is greatly improved. Pt-Rh
By using an alloy electrode, a laminated electrode thereof, or a cermet electrode thereof with zirconia, the conductivity of the electrode itself was improved, and the necessity of forming a current collector on the sensing electrode was eliminated. The method of integrally sintering with the zirconia green sheet eliminates the problems of the evaporation of the electrode material and the poor adhesion, which are conventionally found in the oxide electrode material. By disposing the electrode of the present invention in a chamber having a somewhat controlled oxygen concentration, the dependence of the electrode itself on the oxygen partial pressure is largely eliminated, and the measurement accuracy in actual sensor driving is greatly improved.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS A known zirconia solid electrolyte substrate is used. In general, a screen printing method is used for forming an electrode for the present invention. In the screen printing method, a green sheet can be used for a substrate to be printed. Of course, a sintered substrate can also be used,
It is a very advantageous point to use a green sheet. This is because a complicated laminated structure can be easily formed in order to obtain an arbitrary shape, and the adhesion to the oxide electrode can be increased as compared with the case of using a sintered substrate. However, the present invention is not particularly limited to green sheets. Further, the forming method by screen printing is not limited, and a method such as a sputtering thin film electrode or a colloid solution coating may be used.

In the screen printing method, the alloy electrode material composed of platinum and rhodium or the cermet electrode material composed of platinum, rhodium and zirconia is used in the present invention to convert the powder into an organic binder such as PVA and a solvent or dispersant. And a kneaded paste is used. Platinum and rhodium may use respective powders or an alloy powder thereof. If the mixed paste of platinum powder and rhodium powder is fired at a high temperature of 1200 ° C. or more, the alloy is completely alloyed. This is because sintering of the zirconia green sheet requires firing at 1300 ° C. or higher. A method of further adding zirconia to platinum and rhodium is obtained by co-precipitating a material powder in a system in which a zirconic acid solution is directly added to a Pt acid solution (similarly for a rhodium solution) at the time of generating the material powder. Zirconia is simultaneously produced by adding Y ₂ O ₃ and imparting ionic conductivity to itself. Addition of zirconia is also effective for controlling the sintered structure of the electrode. The addition amount of zirconia is adjusted according to the firing shrinkage of the zirconia green sheet and the desired electrode structure. Generally, 1 to 20 wt% is added to the electrode metal component, preferably 5 to 15 wt%. The amount is preferred on the electrode structure. Platinum in the present invention is very effective in improving the is facilitated also NO _X sensitivity characteristic that the rhodium electrodes are formed on the zirconia green sheet laminated sintered sensor substrate. For example, it is effective to use a highly ion-conductive zirconia sheet to which 8 mol of Y ₂ O ₃ is added as the sensor substrate. In an actual sensor, the addition amount of Y ₂ O ₃ in the zirconia green sheet is determined from both the substrate strength characteristics and the long-term stability. That is, a Y ₂ O ₃ composition which does not cause crystal transformation and has high strength so that there is no problem in long-term stability is desirable.

In the present invention, an electrode comprising a laminate of a platinum layer and a rhodium layer, or platinum, or a cermet electrode material in which zirconia is dispersed and added to this electrode is used in a screen printing method to convert the material powder into an organic binder such as PVA. A paste kneaded with the solvent, dispersant and the like is used. First, a Pt film is formed by printing, sputtering, or the like in order to manufacture an electrode of a laminate. Thereafter, a Rh film is further formed by printing or vapor deposition. In particular, in the case of a Rh vapor-deposited film, an extremely thin layer is formed, so that the film thickness needs to be controlled.
In the case of forming the Rh layer can further improve the NO _X sensitivity characteristic by oxidizing the Rh layer. As a method of forming an oxide layer of Rh, a method of baking in the air after printing the Rh layer, or a method of forming Rh by an evaporation method.
When a layer is formed, a slight amount of oxygen may be added to the deposition atmosphere. The characteristics can be further improved by further adding a third noble metal to the Pt-Rh electrode. For example, Ru, Ir, Pd, Au, A
By adding g, the gas response performance can be significantly improved. This electrode structure is considered to be because became fine without reducing the NO _X activity of Pt-Rh electrode. This indicates that the electrode impedance is substantially further reduced. On the other hand, it was found to also have a large NO _X activity in alloy or mixed phase electrode of iridium or iridium and rhodium. In particular, in the case of an iridium electrode, a very fine electrode structure is obtained, and characteristics with a high gas response speed are obtained. These are produced in the same manner as the above-mentioned Pt-Rh-based electrode. That is, a paste using iridium powder may be printed on a clean sheet and fired. Also, an electrode is similarly prepared by forming an alloy powder or a mixed powder of iridium and rhodium into a paste. However, in this case, a mixed phase in which the Rh phase is partially separated and precipitated may be formed. Separately, the sensing electrode is made of a mixed phase of platinum and rhodium oxide, or Ru, Ir, Pd,
A mixture of at least one oxide of Ag, Ni, and Cr may be used. The oxygen concentration in the measurement atmosphere is 0.5
It may be within the range of ~ 21 vol%. Therefore, the oxygen concentration is set to 0.
It will be controlled to 5 to 21 vol%.

A description will be given below with reference to a detailed embodiment. (Example 1) Basic manufacturing method and characteristics of the present invention A sensor sample having the structure shown in FIG. 1 was manufactured using a zirconia green sheet 1 to which 8 mol of Y ₂ O ₃ was added as an oxygen ion conductor. This green sheet has a thickness of 0.3 mm produced by a doctor blade method. This zirconia green sheet was cut into a sample size of 4 mm × 6 mm for use. A paste was prepared by adding and kneading a predetermined amount of an organic binder and an organic solvent to an alloy powder of Pt and Rh as a material of the detection electrode 2. The amount of Rh added is P
The content was 5 wt% based on the total amount of t and Rh. Zirconia is further added to this paste to adjust the electrode porosity. On the reference electrode 3, Pt was paste-printed on the surface of the zirconia sheet 1 so as to form a pair with the detection electrode 2.
As with the detection electrode 2, zirconia is dispersed and added for adjusting the electrode structure. The thus prepared green sample was fired at 1400 ° C., and after attaching the Pt lead wires 4a and 4b to the electrodes 2 and 3, the NO and NO ₂ gas sensitivity was evaluated. In the gas sensitivity evaluation, a quartz tube was placed in an electric furnace, a sample was inserted into the quartz tube, and a potential difference between the detection electrode 2 and the reference electrode 3 was measured while flowing a measurement gas. As a measurement gas, 4% of O ₂ and 50 ppm of NO or NO ₂ were added to an N ₂ base, and the measurement was performed at a total flow rate of 5 L / min. The measurement was performed at an ambient temperature of 600 ° C. by controlling the electric furnace with a thermocouple provided near the sensor sample. FIG. 4 shows the result of the NO _x concentration dependence of the sensor output with respect to NO ₂ and NO. As can be seen from the results, the sensitivity to NO ₂ shows an output equal to or higher than that of the conventionally reported NiCr ₂ O ₄ detection electrode.
Further, it can be seen that it has sensitivity to NO.

Example 2 A sensor sample manufactured in the same manner as in Example 1 was prepared by changing the Rh composition ratio. However, for adjusting the composition ratio of Pt and Rh, a mixed powder of Pt powder and Rh powder was used. The composition ratio of Rh is 0.1%, 0.5%, 1.0%, 3.0%, 5.0%, 7.0 with respect to the total amount of Pt and Rh.
%, 50%, and 100%. The sensitivity was measured using the same apparatus as in Example 1, the ambient temperature was 600 ° C., and the total gas flow rate was 5
The sensitivity of NO: 50 ppm or NO ₂ : 50 ppm was evaluated every L minutes. The results are shown in Table 1 and FIG. From this result, it can be seen that when the Rh composition ratio is 0.5% or more, large sensitivity to NO ₂ is obtained. On the other hand, for NO, 0.5%
It turns out that it has sensitivity in more than 50%.

[0020]

[Table 1]

Example 3 A sample was prepared in substantially the same manner as in Example 1, but in this example, electrodes were printed on a zirconia green sheet and then assembled into a sensor structure shown in FIGS. 7 and 8. Between the substrate 6 for oxygen pump zirconia solid electrolyte facing the NO _X sensor and the substrate 7 of the oxygen sensor zirconia solid electrolyte, the gas for measurement first
Inlet 12 and a second inlet 13 facing away therefrom
The first can chamber 14 and the second can chamber 15 are formed with a spacer 19 having The substrate 6 has oxygen pump electrodes 9a, 9
The b a first bottom chamber 14 side has on its front and back, the substrate 7 is NO _X sensor detection electrode 10a and the counter electrode NO _X counter 10
b, and an oxygen sensor detection electrode 11a and an oxygen sensor counter electrode 11b as a counter electrode thereof are provided on the front and back sides. On the other hand, the oxygen electrode 9b
Was exposed to the first bottom chamber 14, NO _X sensor detection electrode 1
0a and the oxygen sensor detection electrode 11a are exposed in the second can chamber 15. In the example of FIG. 8, the oxygen sensor detection electrode 11a
Both the oxygen sensor counter electrode 11b and the oxygen sensor counter electrode 11b are exposed in the second can chamber 15, but the example of FIG. 8 has the same configuration except for this point. NO _X opposing the substrate 6 via the spacer 20
A reference atmosphere duct partition wall 8b for sensors and oxygen sensors is provided, an oxygen pump oxygen introduction duct 17 is formed, and an oxygen pump oxygen introduction duct partition body 8a opposed to the substrate 7 via a spacer 21 is provided. , making NO _X sensor and the oxygen sensor reference atmosphere duct 16. Both ducts 16,
Reference numeral 17 denotes a reference atmosphere (atmosphere) 18. Potential V ₁ of the between the NO _X sensor detection electrode 10a and the NO _X counter, the potential difference V ₂ between the oxygen pump electrodes 9a, 9b and the oxygen sensor detection electrode 11a and its counter electrode 11b is measured.

[0022] In the sensor structure of the present embodiment shown in FIGS. 7 and 8, by adjusting the oxygen concentration in the exhaust gas introduced into the can chamber 14 pumping electrode 9a, by 9b, performs single gasification of NO _X. To NO or NO ₂ single gasified NO _X in the can chamber 15 communicating with Pt-Rh (5%) electrode 11
The potential difference is detected by a and 11b. At this time, the oxygen concentration in the can chamber 15 is set to a predetermined concentration range by the pump electrodes 11a and 11b. The can chamber present invention electrode 10a formed on 15, is detected as a single output V ₁ of the NO or NO ₂ by 10b. Here, the N concentration when the oxygen concentration in the can chamber 15 is set to a concentration range of 4% to 50%.
And O ₂ detection method, in the case of NO detection method (NO: 25pp
m) and (NO ₂ : 25 ppm) were evaluated for total NO _X output characteristics. NO _X in the exhaust gas by combination of the present invention the electrode on the sensor structure as shown in this embodiment as can be seen from the results shown in FIG. 6 (NO and NO ₂₎ is the total NO
_It can be seen that not only the _X concentration is detected but also the large oxygen concentration dependency of the Pt-Rh detection electrode itself is removed, and stable NO _X detection can be performed. That is, in the NO ₂ detection method, the oxygen 4 having the largest oxygen concentration dependency in the measurement range.
% In the vicinity, even coarse control of ± 1% at an oxygen concentration NO _X: low concentration range sensitivity ± 2.5 ppm accuracy 50ppm can be ensured. On the other hand, in the NO detection method, the output is almost saturated in a region where the oxygen concentration is high, and it can be seen that there is no problem if the oxygen is substantially 10% or more.

(Example 4) Y ₂ as an oxygen ion conductor
A sensor sample having the structure shown in FIG. 9 was manufactured using a zirconia green sheet to which 8 mol of O ₃ was added. This green sheet has a thickness of 0.3 mm produced by a doctor blade method. The zirconia green sheet 25 was cut into a sample size of 4 mm × 6 mm for use. A Pt paste was prepared by adding a predetermined amount of an organic binder (for example, PVA) to Pt as a detection electrode material and kneading the mixture. Further, a predetermined amount of an organic binder (for example, PVA) is added to Rh as the detection electrode material, and the mixture is kneaded to obtain Rh.
Paste created. Pt on this green sheet
The paste 26 was formed by screen printing, and after drying in an oven, the Rh paste 27 was formed by printing. On the reference electrode 28, a Pt paste was printed on the back surface of the zirconia sheet 25 so as to form a pair with the detection electrodes 26 and 27. The green sample prepared in this way was 14
After baking at 00 ° C. and attaching a Pt lead wire to the electrode, the NO and NO ₂ gas sensitivity was evaluated. The Rh thickness of the obtained sample was approximately 3 μm and porous. For the gas sensitivity evaluation, a quartz tube was placed in an electric furnace, a sample was inserted into the quartz tube, and the potential difference between the detection electrodes 26 and 27 and the reference electrode 28 was measured while flowing a measurement gas. Measurement gas is O ₂ 4 on N ₂ base
% And 300 ppm of NO or NO ₂ and a total flow rate of 5 L
Measurements were taken every minute. The measurement temperature is controlled by an electric furnace using a thermocouple provided in the vicinity of the sensor sample.
C. was performed. Table 2 shows the results of sensor outputs for NO ₂ and NO, along with comparisons with conventional oxide electrodes. As can be seen from the results, the sensitivity to NO ₂ shows an output equal to or higher than that of the conventionally reported NiCr ₂ O ₄ detection electrode. Further, it can be seen that it has sensitivity to NO.
Incidentally, those obtained by dispersing and adding zirconia (10 wt%) to each paste and producing the detection electrodes in the same manner as described above also showed almost the same results as in Table 2.

[0024]

[Table 2]

(Example 5) A sensor sample manufactured in the same manner as in Example 4 was prepared. However, the same Pt electrode and the reference electrode are formed in the NO _X sensing electrode. Rh vapor deposition was performed on the Pt detection electrode by a sputtering method. Rh film thickness extrapolated from the relationship between deposition time and Rh film thickness and 50 ppm
FIG. 10 shows the relationship between the NO sensitivity and the 50 ppm NO ₂ sensitivity. As can be seen, the sensitivity increases from the extrapolated film thickness of about 10 °, and the sensitivity tends to saturate at about 40 °. As described above, it can be seen that a very high sensitivity can be obtained also by providing an extremely thin Rh layer by vapor deposition.

(Example 6) A sample was prepared in substantially the same manner as in Example 4, except that Pt-Rh-X was used as a detection electrode material, and that noble metals of X = Ru, Ir, Pd, Au, and Ag were each 1 to 10 wt%. % Was added. It was measured NO _X sensitivity of these samples in the same manner as in Example 4. Table 3 shows the results. Also NO _X sensitivity sufficiently large in any case, it can be seen that are and reduced electrode impedance.

[0027]

[Table 3]

(Example 7) A sensor sample was prepared in the same manner as in Example 4, and then subjected to an oxidation treatment at 850 ° C. However, in this embodiment, Pt-Rh and P
In t-Rh-X, X = Ru, Pd, Ir, A
g, Ni, and Cr were used. Table 4 shows the results. In the detection electrode of the obtained sample, the Rh oxide and the oxide of the third additional element were confirmed by X-ray diffraction or XPS surface analysis. In any case, it can be seen that the sensitivity is greatly improved by performing the oxidation treatment as compared to before the oxidation treatment.

[0029]

[Table 4]

Example 8 A sensor sample was manufactured in the same manner as in Example 4. An Ir electrode and an alloy electrode of Ir and Rh were used as the detection electrode in this example. Table 5 shows the results of these sensitivity measurements. As shown here, a sufficiently large NO
_It can be seen that _X sensitivity is obtained and characteristics with good gas response performance are obtained.

[0031]

[Table 5]

FIG. 11 shows Pt-Rh according to the fourth embodiment.
(3 wt%) showing the oxygen concentration dependency of the NO _X sensor sensitivity with detection electrodes. From FIG. 11, the oxygen concentration is 0.05-2.
It can be seen that it should be controlled to a predetermined value having an arbitrary width of 1 vol%. If it is less than 0.05%, the gas response speed becomes extremely slow, which is not preferable. In addition, H in exhaust gas is
The concentration required to oxidize and remove C and CO is preferably
0.5 to 21 vol%, considering the sensor output,
More preferably, it is 0.5 to 5 vol%.

[Brief description of the drawings]

FIG. 1 is a front view showing a basic element configuration example (same plane arrangement) of an electrode of the present invention.

FIG. 2 is a front view showing a basic element configuration example (front and back arrangement) of the electrode of the present invention.

FIG. 3 is a front view showing an application example using the electrode of the present invention.

Figure 4 is a graph showing the element output characteristic of the Pt-Rh (5%) electrode of the present invention (NO _X concentration dependent)
EMF stands for Electromotive Force.

FIG. 5 is a graph showing Rh and NO ₂ sensitivity of the electrode of the present invention.
It is a graph which shows composition dependence.

FIG. 6 is a graph showing the control oxygen concentration dependency in the total NO _X sensor structure.

FIG. 7 is a sectional view showing an example of a total NO _X sensor structure to which the electrode of the present invention is applied.

FIG. 8 is a sectional view showing a second example of the total NO _X sensor structure to which the electrode of the present invention is applied.

FIG. 9 is a cross-sectional view of a sensor in which Pt and Rh are stacked.

FIG. 10 is a diagram showing a sensor output with respect to a Rh film thickness (Å).

FIG. 11 is a diagram showing a sensor output with respect to an oxygen concentration.

[Explanation of symbols]

1 zirconia solid electrolyte substrate 2 sensing electrode (Pt-Rh alloy according to the present invention, the cermet electrode) 3 counter (reference electrode) 4a, 4b leads 5 partition wall 6 oxygen pump zirconia solid electrolyte 7 NO _X sensor and zirconia oxygen sensor oxygen introducing solid electrolyte 8a oxygen pump duct partition wall 8b NO _X sensor and the oxygen sensor reference atmosphere duct partition wall 9a, 9b oxygen pumping electrode 10a NO _X sensor detection electrode 10b NO _X counter 11a oxygen sensor detection electrode 11b oxygen sensor counter electrode 12 measurement gas inlet (first) 13 gas inlet (second) 14 first cans chamber 15 second cans chamber 16 NO _X sensor and the reference atmosphere duct 17 oxygen pump oxygen inlet duct 18 standard atmosphere oxygen sensor (air)

Claims (11)

[Claims] A detection electrode provided on the zirconia solid electrolyte substrate is 1. A oxygen ion conductor, and the sensing electrode and the solid electrolyte insensitive platinum counter electrode, platinum reference electrode in the NO _X on a substrate paired A nitrogen oxide gas sensor of a type for measuring a potential difference between the two, wherein an alloy electrode composed of platinum and rhodium or a cermet electrode composed of platinum, rhodium alloy and zirconia is used as a gas detection electrode of the sensor. Oxide gas sensor. 2. An alloy electrode comprising platinum and rhodium or a cermet electrode comprising platinum and rhodium alloy and zirconia, wherein a detection electrode having a rhodium content of at least 0.5 wt% with respect to the total amount of platinum and rhodium is used. The nitrogen oxide gas sensor according to claim 1, wherein: 3. A can chamber, which is provided in a pair of sensor substrates made of a zirconia solid electrolyte which is an oxygen ion conductor, and into which a test gas is introduced, receives gas from a gas inlet communicating with a test gas atmosphere. A first can chamber, or a sensor having a structure including the first can chamber and a second can chamber communicating with the first can chamber, wherein an oxygen discharging or oxygen pumping electrode provided in the first can chamber and the second can chamber is provided. Means for controlling the oxygen concentration in the first or second chamber, and N in the first chamber.
A detection electrode for detecting an O or NO ₂ on the converted NO _X, a duct that maintains a reference oxygen concentration across the zirconia solid electrolyte substrate in which the sensing electrode and the platinum counter electrode or the detection electrode formed on the same cans chamber in contact A nitrogen oxide gas sensor constituted by a platinum counter electrode formed so as to pass through the above, and the detection electrode is constituted by an alloy electrode composed of platinum and rhodium or a cermet electrode composed of platinum, rhodium alloy and zirconia. 4. A claim 4, wherein the concentration controlled manner so that the oxygen concentration in the can chamber that is formed in the sensing electrode produces NO _X difference from the mixed potential of oxygen and NO _X in the detection electrode The nitrogen oxide gas sensor as described in the above. The potential difference between the sensing electrode provided on the zirconia solid electrolyte is wherein oxygen ion conductor, and the sensing electrode and the solid Pt reference electrode having no active the NO _X on the electrolyte paired The gas detection electrode of the sensor is an electrode in which a rhodium layer is laminated on a platinum layer or a cermet electrode in which zirconia is dispersed and added in the laminated electrode. Yes,
A nitrogen oxide gas sensor wherein the oxygen concentration in the measurement atmosphere is controlled to a predetermined value having an arbitrary width of 0.05 to 21 vol%. 6. The nitrogen oxide gas sensor according to claim 5, wherein the rhodium layer of the detection electrode is a deposition layer. The potential difference between the sensing electrode provided on the zirconia solid electrolyte 7. is an oxygen ion conductor, and the sensing electrode and the solid Pt reference electrode having no active the NO _X on the electrolyte paired The gas detection electrode of the sensor is an alloy made of platinum and rhodium or an alloy obtained by adding a third metal element to the gas detection electrode, and the third metal element is made of Ru. , Ir, P
A nitrogen oxide gas sensor comprising at least one of d, Au, and Ag, and wherein the oxygen concentration in the measurement atmosphere is controlled to a predetermined value having an arbitrary width of 0.05 to 21 vol%. The potential difference between the sensing electrode provided on the zirconia solid electrolyte 8. is an oxygen ion conductor, and the sensing electrode and the solid Pt reference electrode having no active the NO _X on the electrolyte paired A nitrogen oxide gas sensor of the type wherein the gas detection electrode of the sensor is a mixed phase of platinum and rhodium oxide or a third phase of platinum and rhodium oxide.
Wherein at least one of the oxides of the metal elements is mixed, and the third metal element is Ru, Ir, Pd, A
A nitrogen oxide gas sensor comprising any one of g, Ni, and Cr, and wherein the oxygen concentration in the measurement atmosphere is controlled to a predetermined value having an arbitrary width of 0.05 to 21 vol%. 9. The nitrogen oxide gas sensor according to claim 7, wherein zirconia is dispersed and added to the detection electrode. 10. The method according to claim 7, wherein the ratio of (Ph + third element) / (Pt + Rh + third element) is 0.01 to 0.5. Item 10. A nitrogen oxide gas sensor according to item 8. The potential difference between the 11. The sensing electrodes provided on the zirconia solid electrolyte on an oxygen-ion conductor, and the sensing electrode and the solid Pt reference electrode having no active the NO _X on the electrolyte paired The gas detection electrode of the sensor is an electrode made of iridium, an electrode made of an alloy or a mixed phase of iridium and rhodium, or a sensor obtained by dispersing and adding zirconia in the electrode. A nitrogen oxide gas sensor, which is a met electrode and wherein the oxygen concentration in the measurement atmosphere is controlled to a predetermined value having an arbitrary width of 0.05 to 21 vol%.