JP4036676B2 - NOx gas detection method and apparatus - Google Patents

Description

[0001]
[Field of the Invention]
The present invention relates to NOx detection using an alkali metal ion conductor, and more particularly to improvement of long-term stability of a NOx sensor used.
[0002]
[Prior art]
The inventors have developed a NOx sensor using an alkali metal ion conductor such as NASICON. Such a NOx sensor detects ppb level NOx, and is necessary for building a network for monitoring air pollution or reducing traffic pollution. As is well known, since NO can be converted to NO2 by a catalytic converter, the sensor only needs to be able to detect at least NO2 and is not required to be interfered by CO2 or H2O.
[0003]
The inventors have provided a NOx sensor in which a detection electrode, a reference electrode, and a counter electrode are provided on an alkali metal ion conductor, alkali metal nitrite is added only to the counter electrode, and the potential of the detection electrode is made negative with respect to the reference electrode. Proposed (Japanese Patent Laid-Open No. 11-271266). This sensor is hardly affected by H 2 O or CO 2 and can detect ppb order NO 2. The electrode reaction is as follows, for example.
Na ⁺ + NO 2 + e ⁻ → Na NO 2 (Detection electrode) (1)
NaNO2 → Na ⁺ + NO2 + e ⁻ (Counter electrode) (2)
This sensor is heated to about 150 ° C., and when NO2 touches the detection electrode, NaION is replenished from NASICON to produce NaNO2, and accordingly, NaNO2 is decomposed at the counter electrode and the alkali metal nitrite of the counter electrode is consumed. . As a result, the sensor output decreased in a short period and did not reach at least one month of the target life.
[0004]
In order to compensate consumption of the alkali metal nitrite which is the active material of the counter electrode, the inventors applied a reverse bias voltage with the detection electrode as a positive electrode and a negative electrode to decompose the metal nitrite accumulated in the detection electrode, It has been proposed to transport Na ions from the detection electrode to the counter electrode (Japanese Patent Laid-Open No. 2001-194337). The idea here is to apply a reverse bias voltage to the sensor periodically to initialize the sensor so that the states of the detection electrode and the counter electrode do not change from the initial state. The inventor then considered extending the sensor life while allowing consumption of metal nitrite and the like at the counter electrode, and arrived at the present invention.
[0005]
[Problems of the Invention]
A basic object of the present invention is to provide a new detection method and apparatus capable of extending the life of a NOx sensor (claims 1 to 6 ).
[0006]
[Structure of the invention]
In the NOx gas detection method of the present invention, a detection electrode, a counter electrode, and a reference electrode are provided on an alkali metal ion conductor, a sensor in which alkali metal nitrite or nitrate is contained in the counter electrode, and the detection electrode is used as a reference electrode. In the method of detecting a NOx gas from the current between the counter electrode and the detection electrode by applying a negative potential to the detection electrode, the negative potential is applied to the detection electrode with a duty ratio of 1/3 or less and a pulse width of 3 seconds to 10 seconds. Second, with a pulse period of 30 seconds to 1 minute , applied in a pulse manner. After the initial reduction in sensitivity has been completed and the sensor sensitivity has reached a stable range, from the current between the counter electrode and the detection electrode at the time of pulse application. The NOx gas is detected, and the detection electrode is made equipotential with respect to the reference electrode or the electrical connection with the reference electrode is interrupted except when the pulse is applied (Claim 1).
[0007]
Preferably, the electrical connection between the detection electrode and the reference electrode is interrupted except when the pulse is applied. Particularly preferably, the detection electrode is electrically disconnected from both the reference electrode and the counter electrode except when a pulse is applied. Here, “electrically connected” or “electrically disconnected” means electrically connected or disconnected by an auxiliary circuit of the sensor, and is not related to being connected by an ionic conductor in the sensor. .
[0008]
Preferably, NOx gas is detected from the current in the last half 1/3 of the pulse and immediately before the end of the pulse .
[0009]
Further, the NOx gas detection device of the present invention is provided with a detection electrode, a counter electrode, and a reference electrode on an alkali metal ion conductor, and a sensor containing alkali metal nitrite or nitrate is used as the counter electrode, and the detection electrode is referred to. In an apparatus for detecting NOx gas from a current between the counter electrode and the sensing electrode by applying a negative potential to the electrode, the negative potential is applied to the detection electrode with a duty ratio of 1/3 or less and a pulse width of 3 seconds. 10 seconds, pulse period 30 seconds to 1 minute , and applied in a pulsed manner, and the detection electrode is equipotential with respect to the reference electrode, or is electrically connected to the reference electrode, except during the pulse application. The pulse driving means for cutting off and the initial sensitivity decrease are completed, and after the sensor sensitivity reaches a stable range, NOx gas is detected from the current between the counter electrode and the detection electrode when the pulse is applied. With current detection means It characterized the door (claim 4).
[0010]
Preferably, the detection electrode is electrically disconnected from the reference electrode except when the pulse is applied, and particularly preferably, the detection electrode is electrically disconnected from both the reference electrode and the counter electrode except when the pulse is applied.
Preferably, the current detection means detects NOx gas from the current in the last half of the pulse 1/3 to immediately before the end of the pulse (claim 5).
Particularly preferably, the duty ratio is 1 / 4-1 / 20 (claim 6).
[0011]
[Operation and effect of the invention]
In the NOx gas detection method and apparatus of the present invention, the detection electrode and the reference electrode are always kept at the same potential or electrically cut off, and the pulse is negative with respect to the reference electrode as a reference at a duty ratio of 1/3 or less. The NOx gas is detected from the current between the counter electrode and the detection electrode when a pulse is applied. In anticipation, since a pulsed current flows between the counter electrode and the detection electrode, the consumption of the active material of the counter electrode should decrease in proportion to the duty ratio, and the lifetime of the sensor should be increased accordingly. However, in reality, when a potential is applied in a pulsed manner between the detection electrode and the reference electrode (pulse drive), the current value between the counter electrode and the detection electrode is continuously applied to the detection electrode (continuous) Unlike the driving), the NOx sensitivity higher than the continuous driving can be obtained by the pulse driving, and therefore the lower detection limit concentration can be lowered when detecting a small amount of NOx gas in the atmosphere. Further, even when a potential was applied in a pulsed manner to the detection electrode, the sensitivity decreased, for example, for about 10 days after the start of use of the sensor, as in the case where the potential was continuously applied to the detection electrode. However, when a potential was applied in a pulsed manner to the detection electrode, a stable range of sensitivity appeared after the initial sensitivity drop. Since the sensitivity stabilization period lasts, for example, 30 days or more, the life of the NOx sensor can be extended by using, for example, a stable region (claims 1, 4 ).
[0012]
Here, when the detection electrode is cut off from the reference electrode except when the pulse is applied, no current flows between the detection electrode and the counter electrode except when the pulse is applied.
[0013]
The response waveform when a pulse is applied to the detection pole is divided into two parts, and there is a peak of sensor current that has a weak correlation with the NOx concentration in the first half. When this peak elapses, the sensor current has a linear relationship with the NOx concentration. Therefore, when NOx gas is detected from the current in the latter half of the pulse, the NOx concentration can be detected with high accuracy (Claims 3 and 5 ). The absolute value of the time derivative of the sensor current on the falling side (second half of the peak) of this peak is smaller as the NOx concentration is higher. Therefore, it is possible to detect the NOx concentration from this, and it is not impossible to detect other than the sensor current in the second half of the pulse.
[0014]
Even if the sensor is used while applying a negative potential in a pulsed manner to the detection electrode, the initial decrease in sensor sensitivity is not much different from the case where a potential (bias voltage) is continuously applied to the detection electrode. However, if the detection electrode is connected to the counter electrode in a pulsed manner, a stable range of sensor sensitivity occurs thereafter. Therefore, the NOx concentration can be accurately detected by aging the sensor during a period in which the initial sensitivity deterioration occurs and then starting detection of NOx .
[0015]
The pulse period is, for example, 30 seconds to 1 minute from the point that the NOx concentration is to be detected every 30 seconds to 1 minute . Next, the peak of the sensor current is terminated during the pulse, so that the NOx concentration can be measured accurately, the pulse width is set to 3 seconds to 10 seconds , and the duty ratio is set to 1/4 to extend the life of the sensor. ~ 1/20 is preferred. This condition is to make the sensor life as long as possible at a practical detection interval and to take out the sensor current after the end of the initial peak at the time of pulse-on .
[0016]
【Example】
With reference to FIGS. 1-8, the NOx gas detection method and NOx gas detection apparatus of an Example are demonstrated. Hereinafter, for the sake of simplicity, the detection of NO2 will be described assuming that NOx gas is NOx, and NO is detected by being converted to NO2 by an appropriate converter.
[0017]
In the NOx gas detection device 2 of FIG. 1, 4 is a NOx sensor, 6 is an insulating substrate such as alumina, 8 is a heater using a Pt film, a ruthenium oxide film or the like, and 10 is a heater power source. The NOx sensor 4 is heated to, for example, 150 ° C. (generally 100 to 200 ° C.) by the heater 8.
[0018]
Reference numeral 12 denotes a sodium ion conductor such as NASICON, which may be a lithium ion conductor or an alkali metal ion conductor. Reference numeral 14 denotes a detection electrode, which is made of a noble metal film such as gold. Reference numeral 16 denotes a counter electrode, which is a film obtained by adding an alkali metal nitrite such as NaNO2 or NaNO3 or an alkali metal nitrate to a noble metal such as gold. The type of alkali metal in the nitrate or nitrite is preferably the same as the type of alkali metal ion used in the ionic conductor 12 because the sensor 4 is driven by current. The alkali metal nitrate or alkali metal nitrite is, for example, The saturated aqueous solution is added dropwise to the counter electrode 16. Reference numeral 18 denotes a reference electrode, which is similarly made of a noble metal film such as gold. Reference numeral 20 denotes a sealing portion made of glass or the like, which is used for shielding the reference electrode 18 from the atmosphere to be detected (atmosphere), but the sealing portion 20 may not be provided. The structure of the NOx sensor 4 is known by Japanese Patent Laid-Open Nos. 2001-194337 and 11-271266 by the inventors. A third component such as alkaline earth metal nitrite may be added to the counter electrode from the viewpoint of improving moisture resistance.
[0019]
A reference power source 22 applies a bias voltage of about −100 to −200 mV to the detection electrode 14 at a potential based on the reference electrode 18 with a predetermined duty ratio in a pulse manner. Reference numeral 24 denotes a relay, which is an example of a switch, and may be a semiconductor switch or the like as long as the voltage loss is negligible with a low resistance. When the relay 24 is opened, the detection electrode 14 is electrically disconnected from the counter electrode 16 and the reference electrode 18, and when the relay 24 is closed, the detection electrode and the counter electrode are electrically connected, and the detection electrode is connected to the counter electrode. It is placed at a predetermined potential. An operational amplifier 26 keeps the potential of the detection electrode 14 negative with respect to the reference electrode 18 by about −100 to −200 mV when the relay 24 is closed. An ammeter 28 is connected to the counter electrode 16 and outputs a current value at a predetermined timing in the latter half of the pulse when the relay 24 is closed and a bias voltage is applied to the detection electrode 14 in a pulsed manner. Instead of providing the relay 24 between the operational amplifier 26 and the reference power source 22, a relay 25 may be provided between the ammeter 28 and the counter electrode 16, or both of the relays 24 and 25 may be provided. Also, a function generator or the like may be provided instead of the reference power supply 22 without providing the relay 24. In this way, the output pulse of the function generator keeps the detection pole at a negative potential with respect to the reference pole, and the detection pole is kept at the same potential as the reference pole during other periods.
[0020]
A pulse generator 30 generates a pulse at a predetermined cycle of about 30 seconds to 1 minute, and a waveform shaping unit 32 shapes the pulse into a square wave pulse with a predetermined width of about 3 seconds to 10 seconds. To drive the relay 24 to turn it on / off. The relay drive 34 sends a sampling signal to the ammeter 28 at a predetermined timing in the latter half of the pulse (immediately before the end of the pulse or in the latter half 1/3 of the pulse width). The current value at this point is output. The concentration converter 36 converts this current value into a NOx concentration. Since the current value in the embodiment is determined by the sum of the offset current at the NOx concentration of 0 and the current proportional to the NOx concentration, the concentration converter 36 calculates the offset current value for each sensor 4 and the measured current value. A coefficient for multiplying the value excluding the offset current and converting it to the NOx concentration is stored.
[0021]
In the embodiment, by intermittently turning on the relay 24 at a predetermined duty ratio, a potential is applied between the detection electrode 14 and the reference electrode 18 only during that period, and the detection electrode 14 and the counter electrode 16 are connected. I have to. However, a bias voltage may be constantly applied between the detection electrode 14 and the reference electrode 18, the electric circuit between the detection electrode 14 and the counter electrode 16 may be intermittently connected, and the others may be cut off.
[0022]
FIG. 2 shows the bias potential applied to the detection electrode and the sensor output. The potential of the detection electrode is indicated with reference to the reference electrode, for example, about −100 mV to −200 mV, the sensor is driven with a period T, and when the ON time for turning on the relay 24 is T1, and the OFF time is T2, this duty The ratio (T1 / T) is preferably 1/3 or less, more preferably 1/4 to 1/20. The on-time T1 is preferably 3 seconds or longer and 10 seconds or shorter, more preferably 5 seconds or longer and 10 seconds or shorter. The period T determines the NOx detection period and is preferably 30 seconds or more and 1 minute or less, for example, 30 seconds or 1 minute.
[0023]
In the embodiment, as shown in FIG. 2 (1), the potential E is applied to the detection pole by a square wave pulse, but the waveform of the potential applied to the detection pole is not limited to the square wave. For example, in FIG. 2 (2), the initial peak of the pulse is terminated during the first wide pulse, and the second narrow pulse is added to this at intervals of about 1 to 5 seconds, for example. The sensor current is sampled in synchronization with the second pulse.
[0024]
Fig. 2 (3) shows the sensor output. In the example, the current flowing from the detection electrode to the counter electrode is the sensor output, and the current value in the first half of the pulse is very weakly correlated with the NOx concentration. Since linearity with the NOx concentration is obtained, the current value in the latter half of the pulse is sampled. Further, in the embodiment, the purpose is not to forcibly return the Na ions that have moved from the counter electrode to the detection electrode but to send the consumption of Na ions at the counter electrode, so that the detection electrode can be used while the pulse is not applied. And the electrical connection between the counter electrode and the current is forcibly fixed to zero. Note that the term “second half of the pulse” means that if the pulse waveform is not a simple square wave as shown in FIG. 2 (2), the first pulse and the second pulse are regarded as a part of one pulse as a whole. It means the timing of applying a negative potential to the detection pole and the latter half of the pulse as a whole.
[0025]
3 and 4 show sensor outputs when a bias voltage of -150 mV is applied to the detection pole in a pulsed manner. Hereinafter, the results when NaNO2 is added to the counter electrode will be described. The waveforms in FIGS. 3 and 4 are obtained when the period T is 38 seconds, the pulse voltage application time T1 is 8 seconds, the sensor temperature is 150 ° C., and the potential of the detection electrode with respect to the reference electrode is −150 mV. Is. Further, the NO2 concentration is about 40 ppb, and the data is about 10 days after the start of use of the sensor under these conditions. When a bias voltage of −150 mV is applied to the detection pole in a pulsed manner, after the peak of the sensor output occurs, the peak is rapidly attenuated, and a region approaching a steady value can be seen. The range where the peak is almost finished and approaches the steady value of the sensor current is a region where the NOx concentration can be measured. When the sensor temperature is changed from 150 ° C. in FIG. 3 to 200 ° C. in FIG. 4, the peak width is rapidly reduced, and NOx can be detected with a shorter pulse. However, when the sensor temperature is increased, the consumption amount of the active material of the counter electrode increases in the case of the same pulse width.
[0026]
FIG. 5 shows the response waveform on the seventh day from the start of use of the sensor when the sensor temperature is 150 ° C. and the detection electrode is driven by applying a bias voltage of −150 mV to the reference electrode for 8 seconds in a cycle of 38 seconds. Show. This response waveform is measured by extending a normal pulse width of 8 seconds to 38 seconds for one period. The difference between the current values of NO2 238ppb and NO2 2ppb is the net sensor output, and these differences are small at the peak immediately after the pulse application, and the difference in current value increases as the peak ends and approaches the steady value. To do. Therefore, the sensor output to be sampled is preferably in the second half of the pulse, and particularly preferably after the peak of the sensor current is almost finished.
[0027]
FIG. 6 shows the relationship between the NO2 concentration and the sensor output under the condition that the sensor temperature is 150 ° C. and the detection electrode is set to −150 mV with respect to the reference electrode for 8 seconds in a cycle of 38 seconds. This is data on the fifth day of use of the sensor, and there is an offset current of about 10 nA, and the sensor output has a linear relationship with the NO2 concentration. Therefore, if the offset current value and the NO2 concentration per sensor current are stored for each sensor, the NOx concentration can be obtained even if the sensor is replaced.
[0028]
FIG. 7 shows the relationship of the sensor output with respect to the potential of the detection electrode when the sensor temperature is 150 ° C. and the bias voltage is always applied between the detection electrode and the reference electrode. As the absolute value of the bias voltage increases, the sensor output increases, and accordingly, the consumption of the active material of the counter electrode is accelerated, and the sensor output decreases when the bias voltage is 0 side from −100 mV. -100 to -200 mV is preferred with respect to the pole.
[0029]
FIG. 8 shows the time-lapse data of the sensor for 41 days. The vertical axis shows the relative NO2 sensitivity relative to the start of use of the sensor that has not been aged, and the first day as a reference. Here, NO2 200 ppb is the detection target. The sensor temperature is 150 ° C., and in the comparative example, the potential of the detection electrode is always kept at −150 mV, and in the embodiment, a potential of −150 mV is applied to the detection electrode in a pulse of 38 seconds for 8 seconds with respect to the reference electrode. The counter electrode was connected to the detection electrode during the pulse, and the current value at 5 seconds from the pulse application was used as the sensor output.
[0030]
The sensitivity on the first day is 0.4 nA / ppb in the example and 0.1 nA / ppb in the comparative example. It can be seen that when a bias potential is applied to the detection electrode in a pulsed manner, the value of the sensor current differs between pulse application and continuous application, and the mechanism for determining the output current of the sensor differs between pulse application and continuous application.
[0031]
For about 10 days from the start of use, the sensitivity similarly decreases in both the example and the comparative example. However, in the embodiment, after about 10 days from the start of use, the sensor sensitivity reaches a stable range, and thereafter, the sensor sensitivity is stable for at least 30 days or more, and highly reliable NOx can be detected. On the other hand, in the comparative example, there is no stable range of the sensor output, and the sensor sensitivity decreases monotonously. Therefore, when the sensor is aged for about 10 days under pulse driving conditions and then the measurement of the NOx concentration is started, the NOx concentration can be stably measured.
[Brief description of the drawings]
FIG. 1 is a block diagram of a NOx gas detection device of an embodiment. FIG. 2 is a waveform diagram of a detection electrode potential and a sensor output in the embodiment. (1) is a waveform of a detection electrode potential E in the embodiment. 2) shows the waveform of the detected pole potential in the modified example, and (3) shows the waveform of the sensor output in the example.
FIG. 3 is a diagram showing the waveform of the sensor output when the sensor temperature is 150 ° C., the detection potential is −150 mV, the drive cycle is 38 seconds, and the pulse application time is 8 seconds. FIG. FIG. 5 is a diagram showing the waveform of the sensor output when the drive potential is 38 seconds and the pulse application time is 8 seconds at a polar potential of −150 mV. FIG. 5 is a sensor temperature of 150 ° C., a detected polar potential of −150 mV, and a drive cycle of 38 Fig. 6 shows sensor output waveforms in NO2 2ppb and NO2 238ppb when the second is set. Fig. 6 Sensor temperature 150 ° C, detection pole potential -150mV, drive cycle 38 seconds, pulse application time 8 seconds Fig. 7 shows the relationship between the sensor output and NO2 concentration. Fig. 7 shows the relationship between the detected electrode potential and the sensor output when the sensor temperature is 150 ° C and the detected electrode potential is continuously applied. 8] Sensor temperature 150 ° C, detection potential -15 Diagram showing sensor output progress in NO2 200ppb at 0mV, drive cycle 38 seconds, pulse application time 8 seconds.
2 NOx gas detection device 4 NOx sensor 6 Insulating substrate 8 Heater 10 Heater power supply 12 Na ion conductor 14 Detection electrode 16 Counter electrode 18 Reference electrode 20 Sealing unit 22 Reference power supply 24, 25 Relay 26 Operational amplifier 28 Ammeter 30 Pulse generation unit 32 Waveform shaping unit 34 Relay drive 36 Concentration conversion unit

Claims (6)

Using a sensor in which a detection electrode, a counter electrode, and a reference electrode are provided on an alkali metal ion conductor and an alkali metal nitrite or nitrate is contained in the counter electrode,
In the method of detecting NOx gas from the current between the counter electrode and the sensing electrode by applying a negative potential to the sensing electrode with respect to the reference electrode,
Applying the negative potential to the detection electrode in a pulse manner with a duty ratio of 1/3 or less, a pulse width of 3 seconds to 10 seconds, and a pulse period of 30 seconds to 1 minute ,
After the initial decrease in sensitivity is reached and the sensor sensitivity reaches a stable range, NOx gas is detected from the current between the counter electrode and the detection electrode when the pulse is applied,
The NOx gas detection method, wherein the detection electrode is equipotential with respect to the reference electrode or the electrical connection with the reference electrode is interrupted except when the pulse is applied.   2. The NOx gas detection method according to claim 1, wherein electrical connection between the detection electrode and the reference electrode is interrupted except when the pulse is applied.   3. The NOx gas detection method according to claim 1, wherein NOx gas is detected from a current in the first half of the pulse and immediately before the end of the pulse. A sensing electrode, a counter electrode, and a reference electrode are provided on an alkali metal ion conductor, and a sensor containing alkali metal nitrite or nitrate is used for the counter electrode,
In a device for detecting NOx gas from a current between the counter electrode and the detection electrode by applying a negative potential to the detection electrode with respect to the reference electrode,
The negative potential is applied to the detection electrode in a pulse manner with a duty ratio of 1/3 or less, a pulse width of 3 seconds to 10 seconds, and a pulse period of 30 seconds to 1 minute . A pulse driving means for making the detection electrode equipotential with respect to the reference electrode or interrupting the electrical connection with the reference electrode;
And a current detecting means for detecting NOx gas from the current between the counter electrode and the sensing electrode at the time of applying the pulse after the initial decrease in sensitivity is completed and the sensor sensitivity reaches a stable range. NOx gas detection device.   5. The NOx gas detection apparatus according to claim 4, wherein the current detection means detects NOx gas from a current in the last 1/3 of the pulse and immediately before the end of the pulse. Characterized in that said duty ratio is set to 1 / 4~1 / 20, NOx gas detecting device according to claim 4 or 5.

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