EP0698207A1 - Electrochemical sensor to determine the oxygen concentration in gaz mixtures - Google Patents

Electrochemical sensor to determine the oxygen concentration in gaz mixtures

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Publication number
EP0698207A1
EP0698207A1 EP95910419A EP95910419A EP0698207A1 EP 0698207 A1 EP0698207 A1 EP 0698207A1 EP 95910419 A EP95910419 A EP 95910419A EP 95910419 A EP95910419 A EP 95910419A EP 0698207 A1 EP0698207 A1 EP 0698207A1
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EP
European Patent Office
Prior art keywords
measuring electrode
bismuth
sensor according
oxygen
electrode
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
EP95910419A
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German (de)
French (fr)
Inventor
Karl-Hermann Friese
Hans-Martin Wiedenmann
Frank Stanglmeier
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Robert Bosch GmbH
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Robert Bosch GmbH
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Publication of EP0698207A1 publication Critical patent/EP0698207A1/en
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
    • G01N27/4071—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/407—Cells and probes with solid electrolytes for investigating or analysing gases
    • G01N27/4075—Composition or fabrication of the electrodes and coatings thereon, e.g. catalysts

Definitions

  • Electrochemical sensor for determining the oxygen concentration in gas mixtures
  • the invention is based on an electrochemical sensor for determining the oxygen concentration in gas mixtures according to the preamble of the main claim.
  • the EMF is measured at the electrodes, a large voltage being generated when the lambda value of the gas mixture is less than the stoichiometric ratio. This is a for the electrode exposed to the sample gas
  • the functionality of catalysts in exhaust gas detoxification systems of internal combustion engines can be monitored by measuring the oxygen contained in the exhaust gas downstream of the catalytic converter in the direction of flow of the exhaust gas. If the catalytic converter is not functional, the oxygen in the exhaust gas remains downstream of the catalytic converter incompletely implemented and there are unreacted oxidizable and reducible fuel components in the exhaust gas.
  • the incomplete implementation means that the residual oxygen in the exhaust gas is significantly higher than the thermodynamic equilibrium.
  • the sensor according to the invention should respond to such increased residual oxygen, for example. It must therefore be ensured that the changes in the oxygen partial pressure can be detected at the three-phase boundary of the measuring electrode.
  • the oxygen is converted by the catalytic converter, so that the thermodynamic equilibrium pressure is at least approximately downstream of the catalytic converter, which reduces the potential difference at the measuring electrode and leads to a lambda jump similar to a conventional lambda probe.
  • the lambda jump shifts to higher lambda values.
  • a disadvantage of this sensor is that there is no potential difference when the lambda values ⁇ 1 (rich exhaust gas) are adjusted. In this lambda range, the sensor is therefore not suitable for monitoring a catalytic converter.
  • the sensor according to the invention with the characterizing features of the main claim has the advantage that the oxygen in non-equilibrium gas mixtures can be measured with the cross-sensitivity to oxidizable gas components largely being switched off.
  • the sensor is particularly suitable for monitoring the degree of conversion of catalysts in exhaust gas detoxification systems of internal combustion engines with exhaust gases regulated to lambda values ⁇ 1. Conclusions about the aging of the catalyst can be drawn from the degree of conversion of the catalyst.
  • Platinum-bismuth measuring electrode uses the properties of bismuth, which has a strong tendency to adsorb oxygen, whereby the oxygen can also be dissociatively adsorbed.
  • bismuth hinders adsorption of other gas components, such as CO, in favor of good oxygen adsorption.
  • the catalysis of the oxidation of CO and HC which occur in exhaust gases from internal combustion engines, is hindered or completely suppressed, so that a mixed potential is formed at the measuring electrode.
  • the measuring electrode designed in this way essentially reacts to oxygen and in this respect represents a non-equilibrium electrode or mixed potential electrode
  • Catalysis of the platinum-inhibiting effect of the bismuth was found within a range with a bismuth content of 1 to 50 mol% based on the total amount of platinum and bismuth.
  • Particularly favorable properties with regard to the formation of mixed potential and reaction times could be determined with a bismuth content of ' 5 to 20 mol% based on the total amount of bismuth and platinum.
  • FIG. 1 shows a cross section through a sensor according to a first exemplary embodiment
  • FIG. 2 shows a cross section through a sensor according to a second exemplary embodiment, the sensor having two electrodes exposed to the gas mixture
  • FIG. 3 shows a characteristic curve of the sensor according to FIG. 2.
  • the sensor according to Figure 1 consists of a
  • Solid ion electrolyte 10 conducting oxygen ions for example made of Zr0 2 stabilized with Y2O3, on which a first measuring electrode 11 exposed to the gas mixture with a porous protective layer 12 lying thereon is applied.
  • the protective layer 12 consists, for example, of Al2O3.
  • a reference channel 14 is provided in the solid electrolyte 10, in which a reference electrode 15 is positioned facing the measuring electrode 11.
  • a heating device 17 embedded in an electrical insulation 16 is also integrated in the solid electrolyte 10. The heating device 17 is designed as a resistance heater.
  • the measuring electrode 11 is a porous platinum cermet electrode which is printed on the solid electrolyte 10, for example by means of screen printing.
  • the reference electrode 15 is also a platinum cermet electrode, which, for example, likewise, is laminated onto a corresponding solid electrolyte film before the solid electrolyte 10 is laminated together was applied by screen printing.
  • the sensor according to FIG. 1 with the platinum cermet electrodes 11, 15 and the heating device 17 is sintered at approximately 1400 ° C. after printing on the corresponding solid electrolyte films and laminating them together.
  • a bismuth-containing paste is applied to the measuring electrode 11, for example by brushing on.
  • the sensor is then heated with the Wismu layer applied to the measuring electrode 11 at a temperature of, for example, 800 to 900 ° C. in an oxygen atmosphere.
  • the heating temperature of the bismuth layer make sure that the bismuth does not evaporate before it combines with the platinum or a stable oxide on the
  • the protective layer 12 is applied, for example, by means of plasma spraying.
  • the sensor described in FIG. 1 is expanded by a second measuring electrode 21 exposed to the gas mixture, the second measuring electrode 21 also being covered with a porous protective layer 22.
  • the second measuring electrode 22 is an electrode which catalyzes the equilibrium setting of the gas mixture.
  • An electrode material that achieves such an effect is, for example, platinum or a platinum alloy with rodium or palladium.
  • the oxygen partial pressure is set to the thermodynamic equilibrium pressure by oxidation.
  • FIG. 3 show a measurement signal U1 of the catalytically inactive measurement electrode 11 and a measurement signal U2 of the catalytically active measurement electrode 21 measured in the flow direction of the exhaust gas downstream of a catalyst (not shown).
  • the EMF of the catalytically inactive measuring electrode 11 was also recorded, which is shown as measuring signal U1 '.
  • the measurement signal Ul 'recorded upstream of the catalytic converter characterizes the characteristic curve of a catalytic converter with an efficiency of 0%, ie it indicates a total catalytic converter failure. Because of the oxygen dissociating at the catalytically inactive measuring electrode 11, there is a small potential difference between the reference electrode 15 and the measuring electrode 11.
  • the oxygen partial pressure is always vanishingly small, regardless of the state of the catalyst used for the afterburning.
  • the oxygen partial pressure corresponds to the thermodynamic equilibrium pressure, because the electrode material of the measuring electrode 21 ensures an at least approximately complete conversion of the oxidizable components with the oxygen.
  • the oxygen partial pressure in the exhaust gas increases. None changes at the three-phase boundary of the catalytically active measuring electrode 21 because the electrode material continues to ensure that the oxidizable constituents are at least approximately completely converted before the gas reaches the three-phase boundary.
  • the oxygen partial pressure depends on the catalytic activity of the catalyst. If the catalytic converter is fully effective, the exhaust gas is at least approximately in the equilibrium state downstream of the catalytic converter. Accordingly, no or very little oxygen can be dissociated from the material of the measuring electrode 11, as a result of which a large potential difference arises at the measuring electrode 11. In this case, the measurement signal U 1 of the catalytically inactive measurement electrode 11 takes approximately the course of the
  • Measurement signal U2 of the catalytically active measuring electrode 21 As the degree of conversion of the catalyst decreases, the oxygen partial pressure in the exhaust gas rises, so that the potential difference decreases at the three-phase limit of the catalytically inactive electrode 11.
  • the dashed curve shown shows the course of the measurement signal U 1 with an efficiency of the catalyst of approximately 90%.
  • the sensor according to the invention is therefore suitable for monitoring the functionality of catalysts which are used in exhaust systems of internal combustion engines which are regulated to a rich exhaust gas (lambda ⁇ 1).

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  • Chemical & Material Sciences (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Health & Medical Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Molecular Biology (AREA)
  • Analytical Chemistry (AREA)
  • Biochemistry (AREA)
  • General Health & Medical Sciences (AREA)
  • General Physics & Mathematics (AREA)
  • Immunology (AREA)
  • Pathology (AREA)
  • Measuring Oxygen Concentration In Cells (AREA)

Abstract

The proposal is for an electrochemical sensor for determining the oxygen concentration in gas mixtures. The sensor has a solid oxygen-ion-conducting electrolyte (10) having a measuring electrode (11) which does not catalyse the equilibrium setting of the gas mixture and is exposed to the gas mixture. The measuring electrode contains platinum and bismuth. These materials ensure that the free oxygen contained in the gas mixture is preferentially adsorbed without reacting with the other gas components like CO and HC.

Description

Elektrochemischer Sensor zur Bestimmung der Sauerstoffkonzentration in GasgemischenElectrochemical sensor for determining the oxygen concentration in gas mixtures
Stand der TechnikState of the art
Die Erfindung geht aus von einem elektrochemischen Sensor zur Bestimmung der Sauerstoffkonzentration in Gasgemischen nach der Gattung des Hauptanspruchs .The invention is based on an electrochemical sensor for determining the oxygen concentration in gas mixtures according to the preamble of the main claim.
Bei den bekannten elektrochemischen Sensoren zur Bestimmung der Sauerstoffkonzentration wird die EMK an den Elektroden gemessen, wobei eine große Spannung erzeugt wird, wenn der Lambda- ert des Gasgemisches kleiner als das stöchiometrische Verhältnis ist. Dabei wird für die dem Meßgas ausgesetzte Elektrode ein dieIn the known electrochemical sensors for determining the oxygen concentration, the EMF is measured at the electrodes, a large voltage being generated when the lambda value of the gas mixture is less than the stoichiometric ratio. This is a for the electrode exposed to the sample gas
Gleichgewichtseinstellung des Gasgemisches katalysierendes Material, beispielsweise Platin verwendet. Die katalysierenden Eigenschaft des Platins bewirken, daß der im Gasgemisch vorkommende freie Sauerstoff sich mit den oxidierbaren Bestandteilen des Gasgemisches verbindet, wodurch der Sensor auch empfindlich gegenüber den oxidierbaren Gaskomponenten wird. Die Querempfindlichkeit von elektrochemischen Sensoren ist jedoch bei bestimmten Anwendungsfällen störend.Equilibrium of the gas mixture catalyzing material, such as platinum used. The catalyzing property of the platinum causes the free oxygen occurring in the gas mixture to combine with the oxidizable components of the gas mixture, which also makes the sensor sensitive to the oxidizable gas components. The cross sensitivity of electrochemical sensors is, however, disruptive in certain applications.
Beispielsweise kann die Funktionsfähigkeit von Katalysatoren in Abgasentgiftungsanlagen von Brennkraftmaschinen dadurch überwacht werden, daß der in Strömungsriehtung des Abgases stromab vom Katalysator im Abgas enthaltene Sauerstoff gemessen wird. Ist der Katalysator nicht funktionsfähig, so bleibt der Sauerstoff im Abgas stromab vom Katalysator unvollständig umgesetzt und es befinden sich noch nicht umgesetzte oxidierbare und reduzierbare Treibstoffanteile im Abgas. Die unvollständige Umsetzung bedeutet, daß der Restsauerstoff im Abgas wesentlich höher ist als es dem thermodynamisehen Gleichgewicht entspricht. Auf einen solchen erhöhten Restsauerstoff soll beispielsweise der erfindungsgemäße Sensor ansprechen. Es muß also dafür gesorgt werden, daß an der Dreiphasengrenze der Meßelektrode die Änderungen des Sauerstoffpartialdrucks erfaßt werden können. Dazu ist aus der DE-OS 23 04 464 bekannt, eine die Gleichgewichtseinstellung des Gasgemisches nicht katalysierende Elektrode aus Gold oder Silber herzustellen. Diese Elektrodenmaterialien bewirken, daß an der Elektrode zwischen dem Sauerstoff und den oxidierbaren bzw. reduzierbaren Gaskomponenten eine Konkurrenzreaktion stattfindet. Dadurch wird selbst bei eingeregelten hohen Lambda- erten der im Abgas mitgeführte freie Sauerstoff durch die Konkurrenzreaktion mit beispielsweise CO weitgehend umgesetzt, so daß nur wenig Sauerstoff die Dreiphasengrenze der Meßelektrode erreicht, wodurch ein annähernd gleichbleibend hohes positives Potential von niedrigen bis zu hohen Lambda-Werten erhalten bleibt. Bei einem intaktem Katalysator wird der Sauerstoff vom Katalysator umgesetzt, so daß stromab vom Katalysator zumindest annähern der thermodynamische Gleichgewichtsdruck vorliegt, wodurch sich die Potentaldifferenz an der Meßelektrode verringert und es zu einem Lamda-Sprung ähnlich einer konventionellen Lambda-Sonde kommt. Mit abnehmendem Konvertierungsgrad des Katalysators verschiebt sich der Lambda-Sprung zu höheren Lambda-Werten. Nachteilig bei diesem Sensor ist, daß bei eingeregelten Lambda-Werten < 1 (fettes Abgas) keine Potentialdifferenz auftritt. In diesem Lambda-Bereich ist der Sensor somit zur Überwachung eines Katalysators nicht geeignet. Vorteile der ErfindungFor example, the functionality of catalysts in exhaust gas detoxification systems of internal combustion engines can be monitored by measuring the oxygen contained in the exhaust gas downstream of the catalytic converter in the direction of flow of the exhaust gas. If the catalytic converter is not functional, the oxygen in the exhaust gas remains downstream of the catalytic converter incompletely implemented and there are unreacted oxidizable and reducible fuel components in the exhaust gas. The incomplete implementation means that the residual oxygen in the exhaust gas is significantly higher than the thermodynamic equilibrium. The sensor according to the invention should respond to such increased residual oxygen, for example. It must therefore be ensured that the changes in the oxygen partial pressure can be detected at the three-phase boundary of the measuring electrode. For this purpose, it is known from DE-OS 23 04 464 to produce an electrode made of gold or silver that does not catalyze the equilibrium of the gas mixture. These electrode materials cause a competitive reaction to take place at the electrode between the oxygen and the oxidizable or reducible gas components. As a result, even with regulated high lambda values, the free oxygen carried in the exhaust gas is largely converted by the competition reaction with, for example, CO, so that only a little oxygen reaches the three-phase limit of the measuring electrode, as a result of which an approximately constant high positive potential from low to high lambda values preserved. In the case of an intact catalytic converter, the oxygen is converted by the catalytic converter, so that the thermodynamic equilibrium pressure is at least approximately downstream of the catalytic converter, which reduces the potential difference at the measuring electrode and leads to a lambda jump similar to a conventional lambda probe. As the degree of conversion of the catalytic converter decreases, the lambda jump shifts to higher lambda values. A disadvantage of this sensor is that there is no potential difference when the lambda values <1 (rich exhaust gas) are adjusted. In this lambda range, the sensor is therefore not suitable for monitoring a catalytic converter. Advantages of the invention
Der erfindungsgemäße Sensor mit den kennzeichnenden Merkmalen des Hauptanspruchs hat den Vorteil, daß der Sauerstoff in Nichtgleichgewichtsgasgemiεchen unter weitestgehender Ausschaltung der Querempfindlichkeit gegenüber oxidierbaren Gaskomponenten meßbar ist. Besonders geeignet ist der Sensor zur Überwachung des Konvertierungsgrades von Katalysatoren in Abgasentgiftungsanlagen von Brennkraftmaschinen bei auf Lambda-Werte < 1 eingeregelten Abgasen. Über den Konvertierungsgrad des Katalysators lassen sich Rückschlüsse auf die Katalysatoralterung ziehen.The sensor according to the invention with the characterizing features of the main claim has the advantage that the oxygen in non-equilibrium gas mixtures can be measured with the cross-sensitivity to oxidizable gas components largely being switched off. The sensor is particularly suitable for monitoring the degree of conversion of catalysts in exhaust gas detoxification systems of internal combustion engines with exhaust gases regulated to lambda values <1. Conclusions about the aging of the catalyst can be drawn from the degree of conversion of the catalyst.
Mit den in den Unteransprüchen aufgeführten Maßnahmen sind vorteilhafte Weiterbildungen und Verbesserungen des erfindungsgemäßen Sensors möglich. DieWith the measures listed in the subclaims, advantageous developments and improvements of the sensor according to the invention are possible. The
Platin-Wismut-Meßelektrode nutzt die Eigenschaften des Wismut, das eine starke Neigung zur Sauerstoffadsorption hat, wobei der Sauerstoff auch dissoziativ adsorbiert werden kann. In Kombination mit Platin behindert Wismut eine Adsorption anderer Gaskomponenten, wie beispielsweise CO zugunsten einer guten Sauerstoffadsorption. Dadurch wird die Katalyse der Oxidation von CO und HC, die in Abgasen von Verbrennungsmotoren vorkommen, behindert oder völlig unterdrückt, so daß sich an der Meßelektrode ein Mischpotential ausbildet. Die derart ausgebildete Meßelektrode reagiert im wesentlichen auf Sauerstoff und stellt insofern eine Nichtgleichgewichtselektrode beziehungsweise Mischpotentialelektrode dar. Die diePlatinum-bismuth measuring electrode uses the properties of bismuth, which has a strong tendency to adsorb oxygen, whereby the oxygen can also be dissociatively adsorbed. In combination with platinum, bismuth hinders adsorption of other gas components, such as CO, in favor of good oxygen adsorption. As a result, the catalysis of the oxidation of CO and HC, which occur in exhaust gases from internal combustion engines, is hindered or completely suppressed, so that a mixed potential is formed at the measuring electrode. The measuring electrode designed in this way essentially reacts to oxygen and in this respect represents a non-equilibrium electrode or mixed potential electrode
Katalyse des Platin inhibierende Wirkung des Wismut konnte innerhalb eines Bereiches mit einem Wismut-Anteil von 1 bis 50 mol% bezogen auf die Gesamtmenge von Platin und Wismut festgestellt werden. Besonders günstige Eigenschaften hinsichtlich der Mischpotentialbildung und Reaktionszeiten konnte bei einem Wismut-Anteil von '5 bis 20 mol% bezogen auf die Gesamtmenge von Wismut und Platin ermittelt werden.Catalysis of the platinum-inhibiting effect of the bismuth was found within a range with a bismuth content of 1 to 50 mol% based on the total amount of platinum and bismuth. Particularly favorable properties with regard to the formation of mixed potential and reaction times could be determined with a bismuth content of ' 5 to 20 mol% based on the total amount of bismuth and platinum.
Zeichnungdrawing
Zwei Ausführungεbeispiele der Erfindung sind in der Zeichnung dargestellt und in der nachfolgenden Beschreibung näher erläutert. Es zeigen Figur 1 einen Querschnitt durch einen Sensor nach einem ersten Ausfuhrungsbeispiel, Figur 2 einen Querschnitt durch einen Sensor nach einem zweiten Ausfuhrungsbeispiel, wobei der Sensor über zwei dem Gasgemisch ausgesetzte Elektroden verfügt und Figur 3 eine Kennlinie des Sensors gemäß Figur 2.Two exemplary embodiments of the invention are shown in the drawing and explained in more detail in the description below. 1 shows a cross section through a sensor according to a first exemplary embodiment, FIG. 2 shows a cross section through a sensor according to a second exemplary embodiment, the sensor having two electrodes exposed to the gas mixture, and FIG. 3 shows a characteristic curve of the sensor according to FIG. 2.
Ausführungsbeispieleembodiments
Der Sensor gemäß Figur 1 besteht aus einemThe sensor according to Figure 1 consists of a
Sauerstoffionenleitenden Festelektrolyt 10, beispielsweise aus mit Y2O3 stabilisiertem Zr02, auf dem eine dem Gasgemisch ausgesetzte erste Meßelektrode 11 mit einer darüber liegende porösen Schutzschicht 12 aufgebracht ist. Die Schutzschicht 12 besteht beispielsweise aus AI2O3. Im Festelektrolyt 10 ist ein Referenzkanal 14 vorgesehen, in dem der Meßelektrode 11 zugewandt eine Referenzelektrode 15 positioniert ist. Im Festelektrolyt 10 ist ferner eine in eine elektrische Isolation 16 eingebettete Heizeinrichtung 17 integriert. Die Heizeinrichtung 17 ist als Widerstandsheizung ausgeführt.Solid ion electrolyte 10 conducting oxygen ions, for example made of Zr0 2 stabilized with Y2O3, on which a first measuring electrode 11 exposed to the gas mixture with a porous protective layer 12 lying thereon is applied. The protective layer 12 consists, for example, of Al2O3. A reference channel 14 is provided in the solid electrolyte 10, in which a reference electrode 15 is positioned facing the measuring electrode 11. A heating device 17 embedded in an electrical insulation 16 is also integrated in the solid electrolyte 10. The heating device 17 is designed as a resistance heater.
Die Meßelektrode 11 ist eine poröse Platin-Cermet Elektrode, die beispielsweise mittels Siebdruck auf den Festelektrolyt 10 aufgedruckt ist. Die Referenzelektrode 15 ist ebenfalls eine Platin-Cermet-Elektrode, welche vor dem Zusammenlaminieren des Festelektrolyten 10 auf eine entsprechende Festelektrolytfolie, beispielsweise ebenfalls im Siebdruckverfahren aufgebracht wurde. Der gemäß Figur 1 vorliegende Sensor mit den Platin-Cermet-Elektroden 11, 15 und der Heizeinrichtung 17 wird nach dem Bedrucken der entsprechenden Festelektrolytfolien und dem Zusammenlaminieren bei ca. 1400°C gesintert.The measuring electrode 11 is a porous platinum cermet electrode which is printed on the solid electrolyte 10, for example by means of screen printing. The reference electrode 15 is also a platinum cermet electrode, which, for example, likewise, is laminated onto a corresponding solid electrolyte film before the solid electrolyte 10 is laminated together was applied by screen printing. The sensor according to FIG. 1 with the platinum cermet electrodes 11, 15 and the heating device 17 is sintered at approximately 1400 ° C. after printing on the corresponding solid electrolyte films and laminating them together.
Nach dem Sintern wird auf die Meßelektrode 11 eine wismuthaltige Paste, beispielsweise mittels Aufpinseln aufgetragen. Danach wird der Sensor mit der auf der Meßelektrode 11 aufgetragenen Wismu -Schicht bei einer Temperatur von beispielsweise 800 bis 900°C in einer Sauerstof atmosphäre erhitzt. Bei der Wahl der Aufheiztemperatur der Wismut-Schicht ist darauf zu achten, daß das Wismut nicht verdampft, bevor es sich mit den Platin verbindet beziehungsweise ein stabiles Oxid auf derAfter sintering, a bismuth-containing paste is applied to the measuring electrode 11, for example by brushing on. The sensor is then heated with the Wismu layer applied to the measuring electrode 11 at a temperature of, for example, 800 to 900 ° C. in an oxygen atmosphere. When choosing the heating temperature of the bismuth layer, make sure that the bismuth does not evaporate before it combines with the platinum or a stable oxide on the
Oberfläche bildet. Nach dem Aufbringen der Wismut-Schicht bzw. nach dem Herstellen einer Wismut-Platin-Verbindung in der Meßelektrode 11 wird die Schutzschicht 12 beispielsweise mittels Plasmaspritzen aufgebracht.Surface forms. After the bismuth layer has been applied or a bismuth-platinum compound has been produced in the measuring electrode 11, the protective layer 12 is applied, for example, by means of plasma spraying.
Bei dem in Figur 2 dargestellten Sensor ist der in Figur 1 beschriebene Sensor durch eine zweite dem Gasgemisch ausgesetzte Meßelektrode 21 erweitert, wobei die zweite Meßelektrode 21 ebenfalls mit einer porösen Schutzschicht 22 bedeckt ist. Bei der zweiten Meßelektrode 22 handelt es sich im Gegensatz zur ersten Meßelektrode 11 um eine die Gleichgewichtseinstellung des Gasgemisches katalysierende Elektrode. Ein eine derartige Wirkung erzielendes Elektrodenmaterial ist beispielsweise Platin oder eine Platinlegierung mit Rodium oder Palladium. An der zweiten Meßelektrode 22 wird der Sauerstoffpartialdruck durch Oxidation auf den thermodynamisehen Gleichgewichtsdruck eingestellt. Die in Figur 3 dargestellten Kennlinien des Sensors gemäß Figur 2 zeigen ein Meßsignal Ul der katalytisch inaktiven Meßelektrode 11 und ein Meßsignal U2 der katalytisch aktiven Meßelektrode 21 gemessen in Strömungsrichtung des Abgases stromab von einem nicht dargestellten- Katalysator. Mit einem in Strömungsrichtung des Abgases stromauf vom Katalysator angeordneten Sensor wurde ebenfalls die EMK der katalytisch inaktiven Meßelektrode 11 aufgenommen, welche als Meßsignal Ul ' dargestellt ist. Das stromauf vom Katalysator aufgenommene Meßsignal Ul ' charakterisiert die Kennlinie eines Katalysators mit einem Wirkungsgrad von 0%, d.h., es zeigt einen totalen Katalysatorausfall an. Aufgrund des an der katalytisch inaktiven Meßelektrode 11 dissoziierenden Sauerstoffs liegt zwischen Referenzelektrode 15 und Meßelektrode 11 ein geringer Potentialunterschied vor.In the sensor shown in FIG. 2, the sensor described in FIG. 1 is expanded by a second measuring electrode 21 exposed to the gas mixture, the second measuring electrode 21 also being covered with a porous protective layer 22. In contrast to the first measuring electrode 11, the second measuring electrode 22 is an electrode which catalyzes the equilibrium setting of the gas mixture. An electrode material that achieves such an effect is, for example, platinum or a platinum alloy with rodium or palladium. At the second measuring electrode 22, the oxygen partial pressure is set to the thermodynamic equilibrium pressure by oxidation. The characteristic curves of the sensor according to FIG. 2 shown in FIG. 3 show a measurement signal U1 of the catalytically inactive measurement electrode 11 and a measurement signal U2 of the catalytically active measurement electrode 21 measured in the flow direction of the exhaust gas downstream of a catalyst (not shown). With a sensor arranged upstream of the catalytic converter in the flow direction of the exhaust gas, the EMF of the catalytically inactive measuring electrode 11 was also recorded, which is shown as measuring signal U1 '. The measurement signal Ul 'recorded upstream of the catalytic converter characterizes the characteristic curve of a catalytic converter with an efficiency of 0%, ie it indicates a total catalytic converter failure. Because of the oxygen dissociating at the catalytically inactive measuring electrode 11, there is a small potential difference between the reference electrode 15 and the measuring electrode 11.
Das an der katalytisch aktiven Meßelektrode 21 aufgenommene Meßsignal U2 hat einen Lambda-Sprung in Gleichgewichtszustand bei Lambda = 1. An der katalytisch aktiven Meßelektrode 21 ist der Sauerstoffpartialdruck unabhängig vom Zustand des für die Nachverbrennung verwendeten Katalysators immer verschwindend klein. Der Sauerstoffpartialdruck entspricht hierbei dem thermodynamisehen Gleichgewichtsdruck, weil das Elektrodenmaterial der Meßelektrode 21 eine zumindest annähernd vollständige Umsetzung der oxidierbaren Bestandteile mit dem Sauerstoff gewährleistet.The measuring signal U2 recorded at the catalytically active measuring electrode 21 has a lambda jump in the equilibrium state at lambda = 1. At the catalytically active measuring electrode 21, the oxygen partial pressure is always vanishingly small, regardless of the state of the catalyst used for the afterburning. The oxygen partial pressure corresponds to the thermodynamic equilibrium pressure, because the electrode material of the measuring electrode 21 ensures an at least approximately complete conversion of the oxidizable components with the oxygen.
Ist der Katalysator nicht mehr wirksam, so steigt der Sauerstoffpartialdruck im Abgas an. An der Dreiphasengrenze der katalytisch aktiven Meßelektrode 21 ändert sich dabei nichts, weil das Elektrodenmaterial weiterhin für eine zumindest annähernd vollständige Umsetzung der oxidierbaren Bestandteile sorgt, bevor das Gas an die Dreiphasengrenze gelangt. An der katalytisch inaktiven Meßelektrode 11 hingegen hängt der Sauerstoffpartialdruck von der katalytischen Aktivität des Katalysators ab. Ist der Katalysator voll wirksam, so befindet sich das Abgas stromab vom Katalysator zumindest annähernd im Gleichgewichtszustand. Vom Material der Meßelektrode 11 kann demzufolge auch kein oder nur sehr wenig Sauerstoff dissoziiert werden, wodurch sich eine große Potentialdifferenz an der Meßelektrode 11 einstellt. In diesem Fall nimmt das Meßsignals Ul der katalytisch inaktiven Meßelektrode 11 annähernd den Verlauf desIf the catalytic converter is no longer effective, the oxygen partial pressure in the exhaust gas increases. Nothing changes at the three-phase boundary of the catalytically active measuring electrode 21 because the electrode material continues to ensure that the oxidizable constituents are at least approximately completely converted before the gas reaches the three-phase boundary. On the catalytically inactive measuring electrode 11 on the other hand, the oxygen partial pressure depends on the catalytic activity of the catalyst. If the catalytic converter is fully effective, the exhaust gas is at least approximately in the equilibrium state downstream of the catalytic converter. Accordingly, no or very little oxygen can be dissociated from the material of the measuring electrode 11, as a result of which a large potential difference arises at the measuring electrode 11. In this case, the measurement signal U 1 of the catalytically inactive measurement electrode 11 takes approximately the course of the
Meßsignals U2 der katalytisch aktiven Meßelektrode 21 an. Bei nachlassendem Konvertierungsgrad des Katalysators steigt der Sauerstoffpartialdruck im Abgas an, so daß an der Dreiphasengrenze der katalytisch inaktiven Elektrode 11 die Potentialdifferenz abnimmt. Den Verlauf des Meßsignals Ul bei einem Wirkungsgrad des Katalysators von etwa 90% zeigt die dargestellte gestrichelte Kurve.Measurement signal U2 of the catalytically active measuring electrode 21. As the degree of conversion of the catalyst decreases, the oxygen partial pressure in the exhaust gas rises, so that the potential difference decreases at the three-phase limit of the catalytically inactive electrode 11. The dashed curve shown shows the course of the measurement signal U 1 with an efficiency of the catalyst of approximately 90%.
Der erfindungsgemäße Sensor eignet sich somit zur Überwachung der Funktionsfähigkeit von Katalysatoren, welche in Abgasanlagen von Brennkraftmaschinen eingesetzt sind, die auf ein fettes Abgas (Lambda < 1) eingeregelt sind. The sensor according to the invention is therefore suitable for monitoring the functionality of catalysts which are used in exhaust systems of internal combustion engines which are regulated to a rich exhaust gas (lambda <1).

Claims

Ansprüche Expectations
1. Sensor zur Bestimmung der Sauerstoffkonzentration in Gasgemischen mit einem sauerstoffionenleitenden1. Sensor for determining the oxygen concentration in gas mixtures with an oxygen ion-conducting
Festelektrolyt, welcher eine die Gleichgewichtseinstellung des Gasgemisches nicht katalysierende Meßelektrode aufweist, die dem Gasgemisch ausgesetzt ist, dadurch gekennzeichnet, daß einer platinhaltigen Meßelektrode (11) ein Stoff zugesetzt ist, welcher die katalytische Wirkung derSolid electrolyte which has a measuring electrode which does not catalyze the equilibrium of the gas mixture and which is exposed to the gas mixture, characterized in that a substance is added to a platinum-containing measuring electrode (11) which has the catalytic effect of the
Meßelektrode (11) zu inhibieren vermag, derart, daß ein zumindest annähernd dem freien Sauerstoff entsprechendes Potential an der Meßelektrode (11) ausbildbar ist.Measuring electrode (11) is able to inhibit in such a way that an at least approximately free oxygen potential can be formed on the measuring electrode (11).
2. Sensor nach Anspruch 1, dadurch gekennzeichnet, daß der der platinhaltigen Meßelektrode (11) zugesetzte Stoff Wismut ist.2. Sensor according to claim 1, characterized in that the platinum-containing measuring electrode (11) added substance is bismuth.
3. Sensor nach Anspruch 2, dadurch gekennzeichnet, daß der Wismut-Anteil 1 bis 50 mol% bezogen auf die Gesamtmenge von3. Sensor according to claim 2, characterized in that the bismuth portion 1 to 50 mol% based on the total amount of
Platin und Wismut beträgt.Is platinum and bismuth.
4. Sensor nach Anspruch 3, dadurch gekennzeichnet, daß der Wismut-Anteil 5 bis 20 mol% bezogen auf die Gesamtmenge von Platin und Wismut beträgt.4. Sensor according to claim 3, characterized in that the bismuth fraction is 5 to 20 mol% based on the total amount of platinum and bismuth.
5. Sensor nach einem der Ansprüche 2 bis 4, dadurch gekennzeichnet, daß das Wismut ganz oder teilweise durch Antimon ersetzt ist.5. Sensor according to one of claims 2 to 4, characterized in that the bismuth is replaced in whole or in part by antimony.
6. Sensor nach einem der Ansprüche 2 bis 5, dadurch gekennzeichnet, daß das Wismut ganz oder teilweise durch Blei ersetzt ist. 6. Sensor according to one of claims 2 to 5, characterized in that the bismuth is completely or partially replaced by lead.
7. Sensor nach Anspruch 1, dadurch gekennzeichnet, daß eine weitere dem Gasgemisch ausgesetzte Meßelektrode (21) vorgesehen ist, welche eine die Gleichgewichtseinstellung des Gasgemisches katalysierende Wirkung besitzt.7. Sensor according to claim 1, characterized in that a further exposed to the gas mixture measuring electrode (21) is provided which has a catalyzing effect of the equilibrium of the gas mixture.
8. Sensor nach Anspruch l, dadurch gekennzeichnet, daß eine einem Referenzgas ausgesetzte Referenzelektrode (15) vorgesehen ist. 8. Sensor according to claim l, characterized in that a reference electrode (15) exposed to a reference gas is provided.
EP95910419A 1994-03-14 1995-02-28 Electrochemical sensor to determine the oxygen concentration in gaz mixtures Withdrawn EP0698207A1 (en)

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DE4408361A DE4408361C2 (en) 1994-03-14 1994-03-14 Electrochemical sensor for determining the oxygen concentration in gas mixtures
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PCT/DE1995/000253 WO1995025276A1 (en) 1994-03-14 1995-02-28 Electrochemical sensor to determine the oxygen concentration in gaz mixtures

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