EP0852820A2

EP0852820A2 - Electrode material for hydrocarbon sensors

Info

Publication number: EP0852820A2
Application number: EP96933384A
Authority: EP
Inventors: Ulrich Guth
Original assignee: Heraeus Electro Nite International NV
Current assignee: Heraeus Electro Nite International NV
Priority date: 1995-09-25
Filing date: 1996-09-25
Publication date: 1998-07-15
Also published as: US6090249A; WO1997012413A2; WO1997012413A3; JP2001513188A

Abstract

The invention pertains to an electrode material for potentiometric or amperometric electrochemical sensors. The material in question has the chemical composition Ln1-zA1-xBxO3 in which Ln is at least one lanthanide cation or a mixture of rare-earth cations; A is at least one trivalent transition metal; and B is at least one tri- or bivalent redox-stable cation.

Description

Electrode material for hydrocarbon sensors

The invention relates to a novel electrode material for hydrocarbon sensors and a novel sensor and a method for its production.

As is known, the concentration of unburned fuels in oxygen-containing gases can be determined in the combustion gas stream in m-situ by sensors, such as on a solid electrolyte, e.g. Yttrium stabilized Zιrκonαιoxιd, two Elektroαen, which react in different ways to the sample gas. The potential of one electrode is largely determined by the equilibrium oxygen partial pressure αes Gaeses, that of the other predominantly determined by the partial pressure of the fuel gas, so that a voltage can be measured between the electrons in the same gas, which depends on the hydrocarbon concentration depends. Gold and alloys of gold and platinum are preferably used as CH-sensitive electrodes (e.g. A. Vogel, G. Baier, V. Schule, Sensors and Actuators 15-16 (1993) 147-150).

ORIGINAL DOCUMENTS - 4 -

0.5, a range from 0.15 to 0.25 and in particular from 0.19 to 0.21 having proven advantageous.

The parameter z is either equal to 0 or is in the range from 0.01 to 0.29, from 0.3 to 0.6 or from 0.19 to 0.4.

The hydrocarbon sensitivity of the electrode can be increased by selecting the parameter z over compounds with z = 0 if this is desired. Through the targeted generation of a sub-stoichiometry of the Ln ion, oxide ion vacancies are formed in the oxygen partial lattice of the rounding, which means that other electrode mechanisms than those for connections with z = 0 are responsible for the sensitivity.

The sensitivity of the electrode is further increased in the range z = 0.3 to z = 0.6. Element A can be present with increasing z m of an oxidic phase next to the mixed oxide, so that the electrode is present as a whole m mixed phase.

The element oαer the element B mixture can also be heterogeneous as oxide neoen αem Mischoxiα present at a level of 0, 1 to 70. ⁵

A sensor according to the invention for combustible gases, in particular for hydrocarbons, has a solid electrolyte and at least 2 electrodes, of which at least one electrode contains an electro material with the advantageous features described so far.

According to one embodiment of the invention, the second electrode has the same chemical composition as the first electrode. A good sensitivity to fuel gas is then achieved by providing means for generating a temperature difference between the first and the second electrode. In operation, the temperature difference between transition metal and B is at least a three- or two-valent redox-stable cation, an electrode with a perovskite structure for an electrochemical sensor can be created which, after sintering on a solid electrolyte or a ceramic carrier material, is stable for a long time even in an aggressive high-temperature environment.

A particularly good sensitivity to fuel gases is achieved (fuel gas generally means gaseous and oxiαieroare components under the operating conditions of the sensor) if the element or element mixture A and / or the element or element mixture has a germicidal catalytic activity .

Here msoesonαere is advantageous if A em element or a mixture of elements from the group manganese, chromium, cobalt, iron, titanium. The element or the element mixture B is preferably selected from the group consisting of gallium, aluminum, magnesium, calcium, gadolinium and other redox-stable rare earth elements.

It is particularly advantageous if A is manganese or chromium or a mixture or both and if an element or mixture of gallium, aluminum and magnesium. An electrode material has been found to be particularly advantageous, where α is A and B is gallium.

Furthermore, it is advantageous if Ln is a lanthanide or a mixture of lanthanides, especially lanthanum itself has particularly advantageous properties in the context of the present invention.

The parameter x is in the range from 0.001 to 3.99, in particular 0.1 to 0.9, preferably 0.1 to 0.8 or 0.2 to - 6 -

The carrier material can be a solid electrolyte, but it can also directly on an oxide ceramic carrier material, such as. B. AI2O3 can be printed and a solid electrolyte can be arranged above or next to it. H _: 0 and / or organic solvents are advantageous as solvents, hydrophilic or organic solvents can be used. A safe, complete oxidation of the electrode material is ensured if the reaction takes place in air or oxygen. If the reaction product forms a sinter cake, an additional annealing step can be carried out after comminuting the reaction product, in which the complete homogeneous reaction of the components is ensured. The component Ln: 0 _:. can also be a mineral such as cerite for industrial production. A composition which essentially corresponds to monazite is particularly advantageous.

A sensor according to the invention or a sensor which is produced according to the advantageous method can be used in particular as a hydrocarbon sensor for use in the exhaust gas of a combustion point, the combustion point being an internal combustion engine with internal or external combustion (in particular a gasoline engine or a diesel engine) or a heating system, such as an oil or natural gas heater.

In this case, a good hydrocarbon sensitivity can be achieved if the solid electrolyte is from about 8 mole .- * _Y: is _made: 0 ₃ ZrO vollstabi¬ lisiertem. By doping with Y _; 0 _; , empty spaces are generated in the partial oxygen lattice of the solid electrolyte.

However, it is also possible to place the electrodes on a solid electrolyte with a lower degree of doping at Y _: 0 _:. or the corresponding proportion of another cation with lower value the first and second electrodes are advantageously between 100 ° C and 200 ° C.

In the case of sensors whose electrodes do not have an identical chemical composition, the second electrode is advantageously of the chemical composition Ln -, C _y D0-, where Ln has already been explained above, C em is alkaline earth metal and D is at least trivalent transition metal. C is in particular strontium. D is advantageously manganese and / or chromium. Ln can be a different element or a different element mixture in the second electrode than in the first Ξlektroαe.

The parameter y is advantageously in the range 0.01 to 0.9, in particular 0.02 to 0.7, 0.05 to 0.5, 0.1 to 0.3 or 0.2

A method for producing a sensor for combustible gas using an electrode material according to the invention comprises the following steps:

• Contacting the starting products containing Ln, A and B, preferably as Ln 0, A 0 and B 0, optionally with solvents;

• reacting the mixture at about 1,350 ° C to 1,650 ° C to form a reaction product;

• crushing the reaction product;

• making a paste; and

• Printing and baking the paste on a carrier material. Instead of the oxides, the respective citrate or nitrate conversion of the starting materials can, for example, also be selected as the starting material. Example 3:

To produce an oxygen-sensitive perovskite electrode, the compound La ₀ gosSrc.oosCrOs from the oxides L3 ₂ ÜJ H _: 0, Cτz0 ₂ and SrC0 _{3 is} weighed in a stochiometric ratio and mixed 20 mm in a ball mill. The mixture is then converted into air in a aluminum oxide crucible at 1400 ° C. for 20 h. The sinter cake obtained is milled and annealed at 1650 ° C for 30 mm.

The electrode materials according to Example 1 and Example 2 can in a simple, electronics-compatible layer technology such as. B. screen printing on a solid electrolyte and then used in addition to a so-called equilibrium electrode, for example made of platinum.

The electrode material according to example 3 forms a mixed oxide of the perovskite type with negligible fuel gas sensitivity and can be used in addition to the electrodes according to example 1 and example 2 instead of the plate electrode. The sensors, which are produced with αen electrodes according to Example 1 and Example 2 as a fuel gas-sensitive electrode and according to Example 3 as equilibrium electrodes, do not show the voltage jump at λ = 1, which can be measured with platelectrodes, for example, characterizes the conventional λ probes. A fuel gas sensitive sensor thus arises which, when used in the exhaust gas of an automobile engine, can be used constantly in the range around λ = 1 in the case of a regulated catalytic converter. The output signal of the sensor according to the invention is essentially dependent on the concentration of fuel gases in this exhaust gas, that is to say of hydrocarbons which have never been completely burnt or afterburned by the catalytic converter. The constant transition from λ> 1 to λ <1 and vice versa due to the control processes of the engine control action, e.g. B. Mg or Ca, ie with a lower vacancy concentration to apply.

It is also possible to reduce the vacancy concentration of the starting solid electrolyte by chemical modification with cations of equal or higher value. It is advantageous to add TιO_ or Nb; 0-. It is also possible to use a completely different type of solid electrolyte. B. NASCION.

The invention is described in more detail below with reference to the preparation of three exemplary electrode materials according to the invention.

Example 1:

The oxides La _: 0 were used to produce the compound LaCr - ₀ Ga .0 _. , H_0, Cr_0. and Ga_0 weighed in a stochiometric ratio and mixed 20 mm in a ball mill. The mixture is then reacted in a sintered corundum crucible at 1400 ° C. for 20 hours in air. The sinter cake obtained is milled and annealed at 1650 ° C for 30 mm. The complete formation of the desired product can be demonstrated with the aid of X-ray diffractometer images.

Example 2:

To produce the compound LaCr-Al 0, the oxides La ₂ O-HO, Cr 0 and Al.O are weighed in a stochiometric ratio and mixed 20 mm in a ball mill and reacted in air at 1400 ° C. for 20 hours in a corundum crucible. The sinter cake obtained is ground and subjected to an annealing process at 1650 ° C. for 30 mm in order to obtain the desired compound as pure as possible. - 10 - numbers 21 to 28; 39, 41, 42, 44, 45; 57 to 71, 74, 76, 77, 79; and 92 and finally to be understood as redox-stable cations, for example Ga, Al, Sc, Mg and Ca.

does not or does not significantly impair the output signal of the sensors according to the invention.

It can also be combustible gas sensors αurch two completely glei che burning gas-sensitive electrode as LaCr ₆ Gao _^ produce 0 _3, by driving one of the two electrodes at a temperature be¬ where the Brenngassensitivitat disappear 'and the electrode sufficiently katalytiscn is active). This electrode then becomes an oxygen electrode. The second electrode is driven at a temperature at which the electrode material is not yet catalytically active. It is therefore in this Elektroαe αie fuel gas sensitivity maintained. For this purpose, a sensor can be manufactured in which both electrodes are arranged on the same substrate, but a temperature gradient is set via the sensor, so that a temperature difference between the electrodes of 100 K to 150 K results. This temperature gradient can be generated in particular by a heating conductor printed on the carrier material of the sensor. This embodiment has the advantage that both electrodes can be applied to the substrate or to the solid electrolytes in one production step.

It is also possible to apply a voltage to the electrochemical cells with the fuel gas sensitive electrodes according to example 1 or example 2 on the one hand and the oxygen electrodes according to example 3 or a platinum electrode and thus to force a current flow which is in a clear connection with the concentration of the Fuel gas stent. This mode of operation of the sensors according to the invention is known per se and is referred to as the amperometric mode of operation.

In the context of the above description, the elements with the atomic numbers 57 to 71 are to be considered as lanthanides, the elements with the atomic numbers as trivalent transition metals. - 12 -

7. Electrode material according to one of the preceding claims, characterized in that B em element or a mixture of elements from the group Ga, Al, Mg.

8. Electrode material according to one of the preceding claims, characterized in that A is Cr.

9. electrode material according to one of the preceding claims, characterized in that B is Ga.

10. Electrode material according to one of the preceding claims, characterized in that Ln em element or a mixture of elements from the group La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb or Lu.

11.Electrode material according to one of the preceding Ansprü¬ che, characterized in that Ln em element or a mixture of elements from the group of rare earth elements and alkaline earth metals.

12.Electrode material according to one of the preceding claims, characterized in that Ln is La.

13.Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.001 to 0.99.

14. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.01 to 0.9.

15. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.1 to 0.8.

16. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.2 to 0.5.

Claims

claims

1. Electrode material for potentiometric or amperometric electrochemical sensors, with the chemical composition Ln _:. _z A - vBvO- where Ln is at least one lanthanide cation or a mixture of rare earth cations, A is at least one trivalent transition metal and B is at least one trivalent or divalent redox-stable cation.

2.Electrode material according to claim 1, characterized in that A em element or a mixture of elements with ge ringer catalytic activity.

3.Electrode material according to one of the preceding claims, characterized in that B em element or eme Mi¬ mixture of elements with low catalytic activity

4.Elektroαenmaterial naen one of the preceding claims, characterized in that A em element or eme mixture of elements from the group Mn, Cr, Co, Fe, Ti, Ni

5.Electrode material according to one of the preceding claims, characterized in that B em element or eme mixture of elements from the group consisting of Ga, Al, Sc, Mg or Ca.

6.Electrode material according to one of the preceding claims, characterized in that A is Mn or Cr. - 14 - temperature difference between the first electrode and the second electrode are provided.

26. Sensor according to one of the preceding claims 23 to 25, characterized in that in operation the temperature difference between the first electrode and the second electrode is ^η 00K to 200K.

27. Sensor according to claim 23, characterized in that the second electrode has the chemical composition has, wherein C is at least one alkaline earth metal and D is at least one trivalent transition metal.

28. Sensor according to claim 27, characterized in that C is Sr.

29. Sensor according to one of the preceding claims 27 or 28, characterized in that D is Mn and / or Cr.

30. Sensor according to one of the preceding claims 27 to 29, characterized in that y is 0.001 to 0.9.

31. Sensor according to one of the preceding claims 27 to 30, characterized in that y is 0.02 to 0.7.

32. Sensor according to one of the preceding claims 27 to 31, characterized in that y is 0.05 to 0.5.

33.Sensor according to one of the preceding claims 27 to 32, characterized in that y is 0.1 to 0.3.

34.Sensor according to one of the preceding claims 27 to 33, characterized in that y is 0.2 to 0.4.

35. Method for producing a sensor for combustible gases, in particular for hydrocarbons, characterized by the following steps: contacting the starting products containing Ln, A and B. O 97/12413 PC17EP96 / 04184

- 13 -

17. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.15 to 0.25.

18. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.19 to 0.21.

19. Electrode material according to one of the preceding claims, characterized in that B is also present as an oxide heterogeneously in addition to the mixed oxide in a proportion of 0.01 ~ to 70 ^c .

20. Electrode material according to one of the preceding claims, characterized in that z is in the range from 0.01 to 0.29.

21. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.3 to 0.6.

22. Electrode material according to one of the preceding claims, characterized in that x is in the range from 0.19 to 0.4.

23. Sensor for combustible gases, in particular for hydrocarbons, with a solid electrolyte and with at least two electrodes, characterized in that at least one electrode contains an electrode material according to claims 1 to 22.

24. Sensor according to claim 23, characterized in that the second electrode has the same chemical composition as the first electrode.

25. Sensor according to one of the preceding claims 23 or 24, characterized in that means for generating a temperature - 16 -

43.Method according to one of the preceding claims 35 to 42, characterized in that Ln ₂ 0 ₃ is essentially Mona¬ zit.

44.Use of a sensor according to one of claims 23 to 34 or manufactured according to one of claims 32 to 40 as a hydrocarbon sensor for the exhaust gas of a burning point.

45.Use of a sensor according to claim 44 as a hydrocarbon sensor for the exhaust gas of an internal combustion engine with internal or external combustion.

46.Use of a sensor according to claim 45 as a hydrocarbon sensor for the exhaust gas of a heating system.

, preferably as Ln ₂ 0 _{3 /} A2O3, and B ₂ 0 ₃ , optionally with solvents;

Reacting the mixture at about 1350 ° C. to 1650 ° C. to form a reaction product; Crushing the reaction product; Making a paste; and printing and baking the paste on a substrate

36. The method according to claim 35, characterized in that the carrier material is a solid electrolyte.

37.Method according to one of the preceding claims 35 or

36, characterized in that the solvent is H _: 0 and / or organic solvent.

38.Method according to one of the preceding claims 35 to

37, characterized in that the organic solvent is hydrophobic or hydrophilic.

39.Method according to one of the preceding claims 35 to

38, characterized in that the reaction takes place in air or oxygen.

40.Method according to one of the preceding claims 35 to

39, characterized in that the reaction product forms a sinter cake.

41.Method according to one of the preceding claims 35 to

40, characterized in that after the comminution of the reaction product, the annealing at about 1400 ° C to 1650 ° C follows.

42.Method according to one of the preceding claims 35 to

41, characterized in that Ln _: 0 ₃ is essentially a cerium earth.