EP1277047A1 - Sensor element operated with a preliminary catalysis - Google Patents

Sensor element operated with a preliminary catalysis

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Publication number
EP1277047A1
EP1277047A1 EP01921199A EP01921199A EP1277047A1 EP 1277047 A1 EP1277047 A1 EP 1277047A1 EP 01921199 A EP01921199 A EP 01921199A EP 01921199 A EP01921199 A EP 01921199A EP 1277047 A1 EP1277047 A1 EP 1277047A1
Authority
EP
European Patent Office
Prior art keywords
gas
sensor element
diffusion barrier
coarse
element according
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.)
Withdrawn
Application number
EP01921199A
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German (de)
French (fr)
Inventor
Roland Stahl
Gerhard Hoetzel
Harald Neumann
Johann Riegel
Lothar Diehl
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Robert Bosch GmbH
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Robert Bosch GmbH
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Filing date
Publication date
Application filed by Robert Bosch GmbH filed Critical Robert Bosch GmbH
Publication of EP1277047A1 publication Critical patent/EP1277047A1/en
Withdrawn legal-status Critical Current

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Classifications

    • 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
    • G01N27/4071—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure
    • G01N27/4072—Cells and probes with solid electrolytes for investigating or analysing gases using sensor elements of laminated structure characterized by the diffusion barrier
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N33/00—Investigating or analysing materials by specific methods not covered by groups G01N1/00 - G01N31/00
    • G01N33/0004—Gaseous mixtures, e.g. polluted air
    • G01N33/0009—General constructional details of gas analysers, e.g. portable test equipment
    • G01N33/0011—Sample conditioning
    • G01N33/0013—Sample conditioning by a chemical reaction

Definitions

  • the invention relates to a sensor element of a gas sensor with a means for pre-catalysis for determining gas components in gas mixtures according to the preamble of claim 1.
  • Amperometric gas sensors for determining the concentration of gas components in the exhaust gases of internal combustion engines are usually operated according to the so-called limit current principle.
  • a limit current situation is only reached if the electrochemical pump cells in the gas sensor are able to pump out the entire content of the gas to be determined (for example oxygen) present in the measuring gas from the measuring gas space of the gas sensor.
  • the gas to be determined for example oxygen
  • oxygen-evacuating gas sensor this must also be ensured with an atmospheric oxygen content of approximately 20% by volume.
  • a diffusion barrier is integrated between the gas outlet opening of the sensor element and the measuring gas space which contains the electrochemical pump cell.
  • a gas sensor is described in the patent specification DE 37 28 289 Cl, which contains a diffusion barrier with a platinum content of up to 90% by weight.
  • the main disadvantage of this is the large amount of platinum required for this, which has a negative effect on the production costs of the gas sensor.
  • the gas sensor according to the invention with the characterizing features of claim 1 has the advantage; that gas constituents of a gas mixture can be determined very precisely even with combustion mixtures set to be rich, despite the associated lack of oxygen.
  • a coarse-pored, catalytically active area upstream of the diffusion barrier is created in that a protective layer formed over the electrodes arranged on the large area of the sensor element additionally also covers the gas outlet opening.
  • FIG. 1 shows a cross section through the large area of the sensor element according to the invention in accordance with a first exemplary embodiment
  • FIG. 2 shows a cross section through the large area of the sensor element in accordance with a second exemplary embodiment
  • FIG. 3 shows a cross section through the large area of the sensor element in accordance with a further exemplary embodiment.
  • FIG. 1 shows the basic structure of a first embodiment of the present invention.
  • 10 designates a planar sensor element of an electrochemical gas sensor, which has, for example, a plurality of oxygen ion-conducting solid electrolyte layers 11a, 11b, 11c, lld, lle and llf.
  • the solid electrolyte layers 11a-11f are designed as ceramic foils and form a planar ceramic body.
  • the integrated shape of the planar ceramic body of the sensor element 10 is produced by laminating together the ceramic films printed with functional layers and then sintering the laminated structure in a manner known per se.
  • Each of the solid electrolyte layers 11a-11f is made of solid ion material which conducts oxygen ions, such as, for example, Y 2 O 3 partially or fully stabilized ZrO 2 .
  • the sensor element 10 contains a measurement gas space 13 and, for example, in a further layer plane 11 an air reference channel 15, which at one end leads out of the planar body of the sensor element 10 and is connected to the air atmosphere.
  • an outer pump electrode 20 is arranged on the solid electrolyte layer 11a, which can be covered with a porous protective layer (not shown) and which is arranged in a ring shape around a gas inlet opening 17.
  • the associated inner pump electrode 22 On the side of the solid electrolyte layer 11a facing the measuring gas space 13 there is the associated inner pump electrode 22, which is also designed in a circular shape adapted to the circular geometry of the measuring gas space 13. Both pump electrodes 20, 22 together form a pump cell.
  • a measuring electrode 21 is located in the measuring gas space 13 opposite the inner pump electrode 22. This is also designed, for example, in the form of a ring.
  • An associated reference electrode 23 is arranged in the reference gas channel 15. Measuring and reference electrodes 21, 23 together form a Nernst or concentration cell.
  • all electrodes used contain a catalytically active material, such as platinum, the electrode material being used as a cermet for all electrodes in a manner known per se in order to interact with the ceramic Sintering foils.
  • a resistance heater 39 is also embedded in the ceramic base body of the sensor element 10 between two electrical insulation layers. The resistance heater is used to heat the sensor element 10 to the necessary operating temperature.
  • a porous diffusion barrier 12 is arranged upstream of the measuring gas chamber 13 in the diffusion direction of the measuring gas of the inner pump electrode 22 and the measuring electrode 21.
  • the porous diffusion barrier 12 forms a diffusion resistance with respect to the gas diffusing to the electrodes 21, 22.
  • a basic prerequisite for the functional humidity of an amperometric gas sensor is that the electrochemical pump cell of the sensor element is always able to remove the entire oxygen content from the measuring gas space 13 even at high oxygen concentrations.
  • the maximally occurring oxygen content is the atmospheric with approx. 20 vol. %.
  • a diffusion barrier 12 is connected upstream of the measuring gas space 13 and thus also the inner pump electrode 22, which leads to a reduction in the oxygen content in the measuring gas space 13 by gas phase diffusion.
  • the other gas components occurring in the exhaust gas are also subject to diffusion and the composition of the gas atmosphere present in the measuring gas space 13 is dependent on the diffusion rate of the individual gas components. Especially with a rich exhaust gas, this leads to a strong accumulation of hydrogen in the measurement gas space 13 and thus to a falsified measurement value of the gas sensor.
  • the hydrogen content in the exhaust gas can be reduced if it is on a catalytically active one
  • the diffusion barrier 12 has a coarsely porous, catalytically active area 14. This is in front of the diffusion barrier 12 in the direction of flow of the gas mixture.
  • the porosity is selected so that only an insignificant diffusion resistance is opposed to the penetrating gas mixture; however, the layer thickness should not be less than a certain minimum in order to allow the gas mixture to come into intensive contact with the catalytically active surface of the coarse-pored area.
  • the coarse porous catalytically active region 14 contains metals such as Pt, Ru, Rh, Pd, Ir or a mixture thereof as catalytically active components.
  • the catalytically active components can either be added as a powder to a printing paste, from which the coarse porous catalytically active area 14 is produced by means of a printing process, or the catalytic activation takes place by impregnating the already sintered coarse porous catalytically active area with a metal salt solution and a subsequent heat treatment in a manner known per se.
  • FIG. 2 shows a second embodiment of the sensor element according to the invention, FIG. 2 showing a section of the sensor element shown in FIG. 1.
  • the coarse-porous, catalytically active region 14a at least partially encompasses the space upstream of the diffusion barrier 12, but, as shown in FIG. 2, it can also occupy the entire region between the diffusion barrier 12 and the gas inlet opening 17.
  • the lengthened path of the penetrating gases within the coarse porous catalytically active region 14a ensures a catalytic equilibrium between the gas components. This is particularly important because, for example, the equilibrium of the water gas equilibrium is slow under the conditions prevailing in the exhaust gas.
  • FIG. 3 shows a further embodiment of the sensor element according to the invention, FIG. 3 likewise showing a section of the sensor element shown in FIG. 1.
  • the outer pump electrode 20 arranged on the large surface of the sensor element has a coarse-pored protective layer
  • the protective layer 16 covered, which the electrode before the entry of solid contaminants conditions, such as soot particles, protects. If the protective layer 16 is provided with catalytically active components and additionally applied over the gas inlet opening 17, the area of the protective layer 16 covering the gas inlet opening 17 serves as a coarse porous area of the diffusion barrier 12. This arrangement is characterized by simple manufacture, since no additional process step is necessary is.
  • one or more substances are added to the coarse porous catalytically active region 14, 14a, 16, which remove sulfur oxides from the penetrating exhaust gas.
  • This can be barium nitrate, for example.
  • a catalytically active and coarse porous area of a diffusion barrier for pre-catalysis in exhaust gas sensors is not limited to the exemplary embodiments listed, but also in multi-chamber sensors, in sensors with several pump and concentration cells or sensors with gas inlet openings arranged on the end face can be used.
  • a coarse-porous catalytically active layer 14, 14a, 16 can also be subordinated to the fine-porous region of the diffusion barrier 12.

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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 invention relates to a sensor element for determining the concentration of gas components in gaseous mixtures, especially in the exhaust gases of internal combustion engines. Said sensor element comprises at least one chamber for the gas to be measured (13) and at least one gas inlet (17) via which the gas mixture is fed to the chamber for the gas to be measured (13). The sensor element is further provided with a diffusion barrier (12) interposed between the gas inlet (17) and the chamber for the gas to be measured (13). The diffusion barrier (12) has at least one area (14, 14a, 16) that contains a catalytically active material for establishing the equilibrium in the gaseous mixture and is divided up into a coarse pored and a fine pored section.

Description

Sensorelement mit VorkatalyseSensor element with pre-catalysis
Die Erfindung betrifft ein Sensorelement eines Gassensors mit einem Mittel zur Vorkatalyse zur Bestimmung von Gaskomponenten in Gasgemischen nach dem Oberbegriff des Anspruchs 1.The invention relates to a sensor element of a gas sensor with a means for pre-catalysis for determining gas components in gas mixtures according to the preamble of claim 1.
Stand der TechnikState of the art
Amperometrische Gassensoren zur Bestimmung der Konzentration von Gasbestandteilen m den Abgasen von Verbrennungsmotoren werden üblicherweise nach dem sogenannten Grenzstromprinzip betrieben. Eine Grenzstromsituation wird jedoch nur dann erreicht, wenn die im Gassensor befindlichen elektrochemischen Pumpzellen in der Lage sind, den gesamten im Meßgas vorhandenen Gehalt des zu be- stimmenden Gases (beispielsweise Sauerstoff) aus dem Meßgasraum des Gassensors abzupumpen. Dies muß im Falle eines sauerstoffabpumpenden Gassensors auch bei einem atmosphärischen Sauerstoffgehalt von ungefähr 20 Vol.% gewährleistet sein. Da die üblichen in Gassensoren zum Einsatz kommenden elektrochemischen Pumpzel- len dafür keine ausreichende Pumpleistung aufweisen, wird zwischen der Gasemtrittsoffnung des Sensorelements und dem Meßgasraum, der die elektrochemische Pumpzelle beinhaltet, eine Diffusionsbarriere integriert. An dieser bildet sich bedingt durch die daran stattfindende Gasphasendiffusion ein Konzentrations- gradient zwischen externem Gasgemisch und der Gasatmosphare des Meßgasraums aus. Dies hat zur Folge, daß auch andere Gasbestandteile des Gasgemischs der Diffusion unterliegen und sich aufgrund deren unterschiedlicher Diffusionsgeschwindigkeiten eine in ihrer Zusammensetzung veränderte Meßgasatmosphare im Meßgas- räum des Sensorelements einstellt.Amperometric gas sensors for determining the concentration of gas components in the exhaust gases of internal combustion engines are usually operated according to the so-called limit current principle. However, a limit current situation is only reached if the electrochemical pump cells in the gas sensor are able to pump out the entire content of the gas to be determined (for example oxygen) present in the measuring gas from the measuring gas space of the gas sensor. In the case of an oxygen-evacuating gas sensor, this must also be ensured with an atmospheric oxygen content of approximately 20% by volume. Since the usual electrochemical pump cells used in gas sensors do not have sufficient pump power for this purpose, a diffusion barrier is integrated between the gas outlet opening of the sensor element and the measuring gas space which contains the electrochemical pump cell. Due to the gas phase diffusion taking place there, a concentration gradient forms between the external gas mixture and the gas atmosphere of the Sample gas space. The result of this is that other gas components of the gas mixture are also subject to diffusion and, due to their different diffusion speeds, a measuring gas atmosphere with a different composition is established in the measuring gas space of the sensor element.
Dies wirkt sich vor allem nachteilig auf die Meßgenauigkeit von Lambdasonden aus, da diese bei einem KraftstoffÜberschuß im Abgas (fettes Abgas) deutlich abweichende Lambdawerte ermitteln. Die Ursache hierfür ist, daß der in einem fetten Abgas vorhandene Wasserstoff wegen seines kleinen Molekuldurchmessers eine sehr hohe Diffusionsgeschwindigkeit aufweist und sich im Meßgasraum des Sensorelements anreichert. Wird das Abgas noch vor Eintritt in den Gassensor einer katalytisch aktiven Oberflache aus- gesetzt, so reagieren oxidierende Bestandteile im Abgas mit dem Wasserstoff und die Meßgenauigkeit der Abgassensoren verbessert sich merklich.This has a disadvantageous effect on the measuring accuracy of lambda probes, since they determine significantly different lambda values if there is an excess fuel in the exhaust gas (rich exhaust gas). The reason for this is that the hydrogen present in a rich exhaust gas has a very high diffusion speed due to its small molecular diameter and accumulates in the measuring gas space of the sensor element. If the exhaust gas is exposed to a catalytically active surface before it enters the gas sensor, oxidizing components in the exhaust gas react with the hydrogen and the measuring accuracy of the exhaust gas sensors improves noticeably.
In der Patentschrift DE 37 28 289 Cl wird ein Gassensor be- schrieben, der eine Diffusionsbarriere mit einem Platingehalt von bis zu 90 Gew.% beinhaltet. Nachteilig daran ist vor allem die große dafür erforderliche Platinmenge, die sich negativ auf die Herstellungskosten des Gassensors auswirkt.A gas sensor is described in the patent specification DE 37 28 289 Cl, which contains a diffusion barrier with a platinum content of up to 90% by weight. The main disadvantage of this is the large amount of platinum required for this, which has a negative effect on the production costs of the gas sensor.
Aufgabe der vorliegenden Erfindung ist es, mit geringen Mengen an Platin und ohne Veränderung des Diffusionsverhaltens herkömmlicher Diffusionsbarrieren eine Gleichgewichtseinstellung der Gaskomponenten zu ermöglichen, noch bevor diese die elektrochemische Pumpzelle des Sensorelements erreichen. Vorteile der ErfindungIt is an object of the present invention to enable equilibrium adjustment of the gas components with small amounts of platinum and without changing the diffusion behavior of conventional diffusion barriers, even before they reach the electrochemical pump cell of the sensor element. Advantages of the invention
Der erfindungsgemäße Gassensor mit den kennzeichnenden Merkmalen des Anspruchs 1 hat den Vorteil; daß Gasbestandteile eines Gas- gemischs auch bei fett eingestellten Verbrennungsgemischen trotz des damit verbundenen Sauerstoffmangels sehr genau bestimmt werden können. Dies wird dadurch erreicht, daß die Diffusionsbarriere einen vorgelagerten grobporösen Bereich aufweist, der ein katalytisch aktives Material beinhaltet und einen feinporösen Bereich, der den eigentlichen Diffusionswiderstand bildet. Diese Anordnung ermöglicht eine katalytische Reaktion der Gasbestandteile untereinander, noch bevor diese die elektrochemische Pumpzelle des Sensorelements erreichen.The gas sensor according to the invention with the characterizing features of claim 1 has the advantage; that gas constituents of a gas mixture can be determined very precisely even with combustion mixtures set to be rich, despite the associated lack of oxygen. This is achieved in that the diffusion barrier has an upstream coarse porous area that contains a catalytically active material and a fine porous area that forms the actual diffusion resistance. This arrangement enables a catalytic reaction of the gas components with one another even before they reach the electrochemical pump cell of the sensor element.
Durch die in den Unteranspruchen aufgeführten Maßnahmen sind weitere vorteilhafte Weiterbildungen und Verbesserungen des im Hauptanspruch angegebenen Sensorelements möglich. Wird beispielsweise der Diffusionsbarriere nicht nur eine grobporose Schicht vorgelagert, sondern der gesamte Bereich zwischen Gas- eintrittsoffnung und Diffusionsbarriere mit einem grobporosen und katalytisch aktiven Material ausgefüllt, so wird die katalytische Wirkung der Schicht weiter verstärkt, ohne daß der Diffusionswiderstand in nennenswertem Umfang steigt.The measures listed in the subclaims further advantageous developments and improvements of the sensor element specified in the main claim are possible. If, for example, not only a coarse-porous layer is placed in front of the diffusion barrier, but the entire area between the gas inlet opening and the diffusion barrier is filled with a coarse-porous and catalytically active material, the catalytic effect of the layer is further increased without the diffusion resistance increasing to any appreciable extent.
In einer weiteren vorteilhaften Ausfuhrungsform wird ein grobpo- roser, der Diffusionsbarriere vorgelagerter und katalytisch aktiver Bereich dadurch erzeugt, daß eine über den auf der Großflache des Sensorelements angeordneten Elektroden ausgebildete Schutzschicht zusatzlich auch die Gasemtrittsoffnung bedeckt. Dies ist eine besonders für den Herstellungsprozeß vorteilhafte Losung. ZeichnungIn a further advantageous embodiment, a coarse-pored, catalytically active area upstream of the diffusion barrier is created in that a protective layer formed over the electrodes arranged on the large area of the sensor element additionally also covers the gas outlet opening. This is a particularly advantageous solution for the manufacturing process. drawing
Drei Ausfuhrungsbeispiele der Erfindung sind in der Zeichnung dargestellt und in der nachfolgenden Beschreibung naher er- läutert. Es zeigen Figur 1 einen Querschnitt durch die Großflache des erfindungsgemaßen Sensorelements gemäß einem ersten Ausfuhrungsbeispiel, Figur 2 einen Querschnitt durch die Großflache des erfindungsgemaßen Sensorelements gemäß einem zweiten Ausfuhrungsbeispiel und Figur 3 einen Querschnitt durch die Großflache des erfindungsgemaßen Sensorelements gemäß einem weiteren Ausfuhrungsbeispiel .Three exemplary embodiments of the invention are shown in the drawing and are explained in more detail in the following description. 1 shows a cross section through the large area of the sensor element according to the invention in accordance with a first exemplary embodiment, FIG. 2 shows a cross section through the large area of the sensor element in accordance with a second exemplary embodiment and FIG. 3 shows a cross section through the large area of the sensor element in accordance with a further exemplary embodiment.
AusfuhrungsbeispieleExemplary embodiments
Die Figur 1 zeigt den prinzipiellen Aufbau einer ersten Ausfuhrungsform der vorliegenden Erfindung. Mit 10 ist ein planares Sensorelement eines elektrochemischen Gassensors bezeichnet, das beispielsweise eine Mehrzahl von Sauerstoffionenleitenden Festelektrolytschichten 11a, 11b, 11c, lld, lle und llf aufweist. Die Festelektrolytschichten lla-llf werden dabei als keramische Folien ausgeführt und bilden einen planaren keramischen Korper. Die integrierte Form des planaren keramischen Korpers des Sensorelements 10 wird durch Zusammenlaminieren der mit Funktionsschichten bedruckten keramischen Folien und anschließendem Sin- tern der laminierten Struktur in an sich bekannter Weise hergestellt. Jede der Festelektrolytschichten lla-llf ist aus Sauerstoffionenleitendem Festelektrolytmaterial, wie beispielsweise mit Y203 teil- oder vollstabilisiertem Zr02 ausgeführt.FIG. 1 shows the basic structure of a first embodiment of the present invention. 10 designates a planar sensor element of an electrochemical gas sensor, which has, for example, a plurality of oxygen ion-conducting solid electrolyte layers 11a, 11b, 11c, lld, lle and llf. The solid electrolyte layers 11a-11f are designed as ceramic foils and form a planar ceramic body. The integrated shape of the planar ceramic body of the sensor element 10 is produced by laminating together the ceramic films printed with functional layers and then sintering the laminated structure in a manner known per se. Each of the solid electrolyte layers 11a-11f is made of solid ion material which conducts oxygen ions, such as, for example, Y 2 O 3 partially or fully stabilized ZrO 2 .
Das Sensorelement 10 beinhaltet einen Meßgasraum 13 und beispielsweise in einer weiteren Schichtebene lld einen Luftreferenzkanal 15, der an einem Ende aus dem planaren Korper des Sensorelements 10 herausfuhrt und mit der Luftatmosphare in Verbindung steht. Auf der dem Meßgas unmittelbar zugewandten Großflache des Sensorelements 10 ist auf der Festelektrolytschicht 11a eine äußere Pumpelektrode 20 angeordnet, die mit einer nicht dargestellten porösen Schutzschicht bedeckt sein kann und die kreisringformig um eine Gaseintrittsoffnung 17 herum angeordnet ist. Auf der dem Meßgasraum 13 zugewandten Seite der Festelektrolytschicht 11a befindet sich die dazugehörige innere Pumpelektrode 22, die angepaßt an die kreisringformige Geometrie des Meßgasraums 13 ebenfalls kreisringformig ausgeführt ist. Beide Pumpelektroden 20, 22 bilden zusammen eine Pumpzelle.The sensor element 10 contains a measurement gas space 13 and, for example, in a further layer plane 11 an air reference channel 15, which at one end leads out of the planar body of the sensor element 10 and is connected to the air atmosphere. On the large surface of the sensor element 10 directly facing the measuring gas, an outer pump electrode 20 is arranged on the solid electrolyte layer 11a, which can be covered with a porous protective layer (not shown) and which is arranged in a ring shape around a gas inlet opening 17. On the side of the solid electrolyte layer 11a facing the measuring gas space 13 there is the associated inner pump electrode 22, which is also designed in a circular shape adapted to the circular geometry of the measuring gas space 13. Both pump electrodes 20, 22 together form a pump cell.
Gegenüber der inneren Pumpelektrode 22 befindet sich im Meßgasraum 13 eine Meßelektrode 21. Auch diese ist beispielsweise kreisringformig ausgeführt. Eine dazugehörige Referenzelektrode 23 ist im Referenzgaskanal 15 angeordnet. Meß- und Referenzelektrode 21, 23 bilden zusammen eine Nernst- bzw. Konzentrationszelle .A measuring electrode 21 is located in the measuring gas space 13 opposite the inner pump electrode 22. This is also designed, for example, in the form of a ring. An associated reference electrode 23 is arranged in the reference gas channel 15. Measuring and reference electrodes 21, 23 together form a Nernst or concentration cell.
Um zu gewahrleisten, daß an den Elektroden eine Einstellung des thermodynamischen Gleichgewichts der Meßgaskomponenten erfolgt, enthalten alle verwendeten Elektroden ein katalytisch aktives Material, wie beispielsweise Platin, wobei das Elektrodenmaterial für alle Elektroden in an sich bekannter Weise als Cermet eingesetzt wird, um mit den keramischen Folien zu versintern.In order to ensure that an adjustment of the thermodynamic equilibrium of the measuring gas components takes place on the electrodes, all electrodes used contain a catalytically active material, such as platinum, the electrode material being used as a cermet for all electrodes in a manner known per se in order to interact with the ceramic Sintering foils.
In den keramischen Grundkorper des Sensorelements 10 ist ferner zwischen zwei elektrischen Isolationsschichten ein Widerstandsheizer 39 eingebettet. Der Widerstandsheizer dient dem Aufheizen des Sensorelements 10 auf die notwendige Betriebstemperatur.A resistance heater 39 is also embedded in the ceramic base body of the sensor element 10 between two electrical insulation layers. The resistance heater is used to heat the sensor element 10 to the necessary operating temperature.
Innerhalb des Meßgasraums 13 ist in Diffusionsrichtung des Meßgases der inneren Pumpelektrode 22 und der Meßelektrode 21 eine poröse Diffusionsbarriere 12 vorgelagert. Die poröse Diffusionsbarriere 12 bildet einen Diffusionswiderstand bezüglich des zu den Elektroden 21, 22 diffundierenden Gases aus. Wie schon eingangs erwähnt, ist eine Grundvoraussetzung für die Funktionstuchtigkeit eines amperometrischen Gassensors, daß auch bei hohen Sauerstoffkonzentrationen die elektrochemische Pump- zelle des Sensorelements stets in der Lage ist, den gesamten Ge- halt an Sauerstoff aus dem Meßgasraum 13 zu entfernen. Der dabei maximal auftretende Sauerstoffgehalt ist der atmosphärische mit ungefähr 20 Vol . % . Da dieser jedoch zu einer Überlastung der elektrochemischen Pumpzelle fuhrt, wird dem Meßgasraum 13 und damit auch der inneren Pumpelektrode 22 eine Diffusionsbarriere 12 vorgeschaltet, die zu einer Reduzierung des Sauerstoffgehal- tes im Meßgasraum 13 durch Gasphasendiffusion fuhrt.A porous diffusion barrier 12 is arranged upstream of the measuring gas chamber 13 in the diffusion direction of the measuring gas of the inner pump electrode 22 and the measuring electrode 21. The porous diffusion barrier 12 forms a diffusion resistance with respect to the gas diffusing to the electrodes 21, 22. As already mentioned at the beginning, a basic prerequisite for the functional humidity of an amperometric gas sensor is that the electrochemical pump cell of the sensor element is always able to remove the entire oxygen content from the measuring gas space 13 even at high oxygen concentrations. The maximally occurring oxygen content is the atmospheric with approx. 20 vol. %. However, since this leads to an overload of the electrochemical pump cell, a diffusion barrier 12 is connected upstream of the measuring gas space 13 and thus also the inner pump electrode 22, which leads to a reduction in the oxygen content in the measuring gas space 13 by gas phase diffusion.
Allerdings unterliegen auch die anderen im Abgas vorkommenden Gasbestandteile der Diffusion und die Zusammensetzung der im Meßgasraum 13 vorliegenden Gasatmosphare ist abhangig von der Diffusionsgeschwindigkeit der einzelnen Gaskomponenten. Dies fuhrt vor allem bei einem fetten Abgas zu einer starken Anreicherung von Wasserstoff im Meßgasraum 13 und damit zu einem verfälschten Meßwert des Gassensors. Der Wasserstoffgehalt im Abgas laßt sich jedoch verringern, wenn an einer katalytisch aktivenHowever, the other gas components occurring in the exhaust gas are also subject to diffusion and the composition of the gas atmosphere present in the measuring gas space 13 is dependent on the diffusion rate of the individual gas components. Especially with a rich exhaust gas, this leads to a strong accumulation of hydrogen in the measurement gas space 13 and thus to a falsified measurement value of the gas sensor. However, the hydrogen content in the exhaust gas can be reduced if it is on a catalytically active one
Oberflache der Wasserstoff mit oxidierenden Gasen wie Sauerstoff und Kohlendioxid umgesetzt wird und somit eine thermodynamische Gleichgewichtseinstellung der Gasbestandteile untereinander gewahrleistet ist.Surface of the hydrogen is reacted with oxidizing gases such as oxygen and carbon dioxide and thus a thermodynamic equilibrium between the gas components is ensured.
Um eine derartige Vorkatalyse zu bewerkstelligen, weist die Diffusionsbarriere 12 einen grobporosen, katalytisch aktiven Bereich 14 auf. Dieser ist der Diffusionsbarriere 12 in Stromungsrichtung des Gasgemischs vorgelagert. Die Porosität ist so ge- wählt, daß dem eindringenden Gasgemisch nur ein unwesentlicher Diffusionswiderstand entgegen gesetzt wird; die Schichtdicke sollte jedoch ein gewisses Minimum nicht unterschreiten, um einen intensiven Kontakt des Gasgemischs mit der katalytisch aktiven Oberflache des grobporosen Bereiches zu ermöglichen. Der grobporose katalytisch aktive Bereich 14 enthalt als katalytisch aktive Komponenten Metalle wie Pt, Ru, Rh, Pd, Ir oder eine Mischung derselben.In order to accomplish such a pre-catalysis, the diffusion barrier 12 has a coarsely porous, catalytically active area 14. This is in front of the diffusion barrier 12 in the direction of flow of the gas mixture. The porosity is selected so that only an insignificant diffusion resistance is opposed to the penetrating gas mixture; however, the layer thickness should not be less than a certain minimum in order to allow the gas mixture to come into intensive contact with the catalytically active surface of the coarse-pored area. The coarse porous catalytically active region 14 contains metals such as Pt, Ru, Rh, Pd, Ir or a mixture thereof as catalytically active components.
Wahrend des Herstellungsprozesses können die katalytisch aktiven Komponenten entweder einer Druckpaste, aus der der grobporose katalytisch aktive Bereich 14 mittels eines Druckvorganges erzeugt wird, als Pulver zugesetzt werden oder die katalytische Aktivierung erfolgt durch das Imprägnieren des bereits gesinter- ten grobporosen katalytisch aktiven Bereichs mit einer Metallsalzlosung und einer anschließenden Wärmebehandlung auf an sich bekannte Weise.During the manufacturing process, the catalytically active components can either be added as a powder to a printing paste, from which the coarse porous catalytically active area 14 is produced by means of a printing process, or the catalytic activation takes place by impregnating the already sintered coarse porous catalytically active area with a metal salt solution and a subsequent heat treatment in a manner known per se.
In Figur 2 ist eine zweite Ausfuhrungs form des erfindungsgemaßen Sensorelements dargestellt, wobei die Figur 2 einen Ausschnitt des in Figur 1 dargestellten Sensorelements darstellt. Hier umfaßt der grobporose katalytisch aktive Bereich 14a zumindest teilweise den der Diffusionsbarriere 12 vorgelagerten Raum, er kann aber auch, wie in Figur 2 dargestellt, den gesamten Bereich zwischen Diffusionsbarriere 12 und Gaseintrittsoffnung 17 einnehmen. Durch die so verlängerte Weglange der eindringenden Gase innerhalb des grobporosen katalytisch aktiven Bereichs 14a ist eine katalytische Gleichgewichtseinstellung der Gaskomponenten untereinander gewahrleistet. Dies ist vor allem deshalb wichtig, weil beispielsweise die Gleichgewichtseinstellung des Wassergasgleichgewichts unter den im Abgas vorherrschenden Bedingungen nur langsam erfolgt.FIG. 2 shows a second embodiment of the sensor element according to the invention, FIG. 2 showing a section of the sensor element shown in FIG. 1. Here, the coarse-porous, catalytically active region 14a at least partially encompasses the space upstream of the diffusion barrier 12, but, as shown in FIG. 2, it can also occupy the entire region between the diffusion barrier 12 and the gas inlet opening 17. The lengthened path of the penetrating gases within the coarse porous catalytically active region 14a ensures a catalytic equilibrium between the gas components. This is particularly important because, for example, the equilibrium of the water gas equilibrium is slow under the conditions prevailing in the exhaust gas.
In Figur 3 ist eine weitere Ausfuhrungsform des erfindungsgema- ßen Sensorelements dargestellt, wobei die Figur 3 ebenfalls einen Ausschnitt des in Figur 1 dargestellten Sensorelements darstellt.FIG. 3 shows a further embodiment of the sensor element according to the invention, FIG. 3 likewise showing a section of the sensor element shown in FIG. 1.
Die auf der Großflache des Sensorelements angeordnete äußere Pumpelektrode 20 ist dabei mit einer grobporosen SchutzschichtThe outer pump electrode 20 arranged on the large surface of the sensor element has a coarse-pored protective layer
16 bedeckt, die die Elektrode vor dem Zutritt fester Verunreini- gungen, wie beispielsweise Rußpartikel, schützt. Wird die Schutzschicht 16 mit katalytisch aktiven Komponenten versehen und zusatzlich über der Gaseintrittsoffnung 17 aufgebracht, so dient der die Gaseintrittsoffnung 17 bedeckende Bereich der Schutzschicht 16 als grobporoser Bereich der Diffusionbarriere 12. Diese Anordnung zeichnet sich durch eine einfache Herstellung aus, da kein zusatzlicher Verfahrensschritt notig ist.16 covered, which the electrode before the entry of solid contaminants conditions, such as soot particles, protects. If the protective layer 16 is provided with catalytically active components and additionally applied over the gas inlet opening 17, the area of the protective layer 16 covering the gas inlet opening 17 serves as a coarse porous area of the diffusion barrier 12. This arrangement is characterized by simple manufacture, since no additional process step is necessary is.
Da die Einstellung des Gleichgewichtes der Gaskomponenten durch Schwefeloxide im Abgas gehemmt wird, wird dem grobporosen katalytisch aktiven Bereich 14, 14a, 16 darüber hinaus eine oder mehrere Substanzen zugemischt, die Schwefeloxide aus dem eindringenden Abgas entfernen. Dies kann beispielsweise Bariumnitrat sein.Since the adjustment of the equilibrium of the gas components is inhibited by sulfur oxides in the exhaust gas, one or more substances are added to the coarse porous catalytically active region 14, 14a, 16, which remove sulfur oxides from the penetrating exhaust gas. This can be barium nitrate, for example.
Es ist ausdrucklich anzumerken, daß sich die Anwendung eines katalytisch aktiven und grobporosen Bereiches einer Diffusionsbarriere zur Vorkatalyse bei Abgassensoren nicht auf die aufgeführten Ausfuhrungsbeispiele beschrankt ist, sondern auch bei Mehr- kammersensoren, bei Sensoren mit mehreren Pump- und Konzentrationszellen oder Sensoren mit stirnseitig angeordneten Gaseintrittsoffnungen zum Einsatz kommen kann. Darüber hinaus kann eine derartige grobporose katalytisch aktive Schicht 14, 14a, 16 auch dem feinporosen Bereich der Diffusionsbarriere 12 nachge- ordnet sein. It is expressly to be noted that the use of a catalytically active and coarse porous area of a diffusion barrier for pre-catalysis in exhaust gas sensors is not limited to the exemplary embodiments listed, but also in multi-chamber sensors, in sensors with several pump and concentration cells or sensors with gas inlet openings arranged on the end face can be used. In addition, such a coarse-porous catalytically active layer 14, 14a, 16 can also be subordinated to the fine-porous region of the diffusion barrier 12.

Claims

Ansprüche Expectations
1. Sensorelement zur Bestimmung der Konzentration von Gaskomponenten in Gasgemischen, insbesondere in Abgasen von Verbrennungsmotoren, mit mindestens einem Meßgasraum und mindestens einer Gaseintrittsoffnung, über die das Gasgemisch dem Meßgasraum zufuhrbar ist, und mindestens einer zwischen Gaseintrittsoffnung und Meßgasraum angeordneten Diffusionsbarriere, wobei die Diffusionsbarriere mindestens einen Bereich aufweist, der ein katalytisch aktives Material zur Einstellung des Gleichgewichtes im Gasgemisch beinhaltet, dadurch gekenzeichnet, daß die Diffusionsbarriere (12) einen grobporosen und einen feinporosen Ab- schnitt aufweist.1.Sensor element for determining the concentration of gas components in gas mixtures, in particular in exhaust gases from internal combustion engines, with at least one sample gas space and at least one gas inlet opening via which the gas mixture can be fed to the sample gas space, and at least one diffusion barrier arranged between gas inlet opening and sample gas space, the diffusion barrier at least has a region which contains a catalytically active material for adjusting the equilibrium in the gas mixture, characterized in that the diffusion barrier (12) has a coarse porous and a fine porous section.
2. Sensorelement nach Anspruch 1, dadurch gekennzeichnet, daß sich der grobporöse Abschnitt (14, 14a, 16) der Diffusionsbarriere (12) auf einer der Gaseintrittsöffnung (17) zugewandten Seite der Diffusionsbarriere (12) und der feinporöse Bereich auf einer dem Meßgasraum (13) zugewandten Seite der Diffusionbarriere (12) befindet. 2. Sensor element according to claim 1, characterized in that the coarse-porous section (14, 14a, 16) of the diffusion barrier (12) on one of the gas inlet opening (17) facing side of the diffusion barrier (12) and the fine-porous area on one of the measuring gas space ( 13) facing side of the diffusion barrier (12).
3. Sensorelement nach Anspruch 1 oder 2, dadurch gekennzeichnet, daß der grobporose Abschnitt (14, 14a, 16) der Diffusionsbarriere (12) das katalytisch aktive Material enthalt.3. Sensor element according to claim 1 or 2, characterized in that the coarse-pored section (14, 14a, 16) of the diffusion barrier (12) contains the catalytically active material.
4. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der grobporose Abschnitt (14a) der Diffusionsbarriere (12) die Gaseintrittsoffnung (17) im wesentlichen ausfüllt.4. Sensor element according to one of the preceding claims, characterized in that the coarse-pored section (14a) of the diffusion barrier (12) essentially fills the gas inlet opening (17).
5. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der grobporose Abschnitt (16) der Diffusionsbarriere (12) auf der dem Gasgemisch ausgesetzten Außenflache des Sensorelements aufgebracht ist und daß der grobporose Abschnitt (16) der Diffusionsbarriere (12) eine auf der Au- ßenflache des Sensorelements angeordnete Außenelektrode (20) und die Gaseintrittsoffnung (17) bedeckt.5. Sensor element according to one of the preceding claims, characterized in that the coarse porous section (16) of the diffusion barrier (12) is applied to the exposed surface of the gas mixture of the sensor element and that the coarse porous section (16) of the diffusion barrier (12) one on the Outer electrode (20) arranged on the outer surface of the sensor element and covers the gas inlet opening (17).
6. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß der grobporose Abschnitt (14, 14a, 16) der Diffusionsbarriere (12) bis zu 10 Gew.%, vorzugsweise 2 Gew.% des katalytisch aktiven Materials in feinstverteilter Form enthält.6. Sensor element according to one of the preceding claims, characterized in that the coarse-pored section (14, 14a, 16) of the diffusion barrier (12) contains up to 10% by weight, preferably 2% by weight, of the catalytically active material in finely divided form.
7. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß das katalytisch aktive Material ein Metall aus der Gruppe Pt, Ru, Rh, Pd, Ir oder eine Mischung davon enthalt.7. Sensor element according to one of the preceding claims, characterized in that the catalytically active material contains a metal from the group Pt, Ru, Rh, Pd, Ir or a mixture thereof.
8. Sensorelement nach einem der vorhergehenden Ansprüche, dadurch gekennzeichnet, daß die Diffusionsbarriere (12) ein Material enthalt, das Schwefeloxide aus dem Gasgemisch entfernt.8. Sensor element according to one of the preceding claims, characterized in that the diffusion barrier (12) contains a material that removes sulfur oxides from the gas mixture.
9. Sensorelement nach Anspruch 8, dadurch gekennzeichnet, daß das Material, das Schwefeloxide aus dem Gasgemisch entfernt, Ba- riumnitrat ist. 9. Sensor element according to claim 8, characterized in that the material which removes sulfur oxides from the gas mixture is barium nitrate.
EP01921199A 2000-03-21 2001-03-15 Sensor element operated with a preliminary catalysis Withdrawn EP1277047A1 (en)

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