JPH06331593A - Oxygen sensor - Google Patents

Abstract

PURPOSE:To control air/fuel ratio accurately when temperature is low by forming an electrode with a precious metal constituent where platinum particles are coated with platinum precious metal other than for improving catalyst activity. CONSTITUTION:Electrodes 2 and 3 formed on the inside and outside surfaces of a zirconia tube (substrate) 1 consisting of oxygen ion conductive solid electrolyte which mainly consists of zirconium oxide are constituted by precious metal paste (electrode material paste) which is formed by including approximately 60-80% platinum and 20-40% ceramic constituent. Then, the platinum particle which is the main constituent of the electrode material paste coated with at least one type of platinum precious metals other than platinum such as rhodium, palladium, and ruthenium are used as the electrodes 2 and 3, thus effectively improving its catalyst activity and the low-temperature activity of the title sensor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxygen sensor, and more particularly to a technique for improving the low temperature activity of an oxygen sensor that generates an electromotive force according to an oxygen partial pressure ratio.

[0002]

2. Description of the Related Art Conventionally, as an oxygen sensor using an oxygen ion conductive solid electrolyte, there is one having a sensor part structure as shown below (Japanese Patent Laid-Open No. 58-20436).
No. 5, Japanese Utility Model Publication No. 59-31054, etc.). That is, platinum P is formed on the inner surface and the outer surface of a zirconia tube having a closed tip formed of an oxygen ion conductive solid electrolyte containing zirconium oxide ZrO ₂ as a main component.
A t-electrode is formed, and a porous ceramics protective layer is formed further outside the platinum Pt electrode on the outside.

In such a structure, while a reference gas (for example, the atmosphere) is introduced into the inner cavity of the zirconia tube,
The outside of the zirconia tube is brought into contact with the gas to be detected (for example, exhaust gas from an internal combustion engine), and the oxygen partial pressure of the reference gas that contacts the inner surface and the oxygen partial pressure of the gas that contacts the outer surface are determined according to the ratio. By generating an electromotive force between the electrodes, the oxygen partial pressure (oxygen concentration) of the gas to be detected is detected.

[0004]

By the way, in the zirconia type oxygen sensor as described above, since the internal resistance of the sensor element is high at low temperature, as shown in FIG. 3, the rich output is lowered and the lean output is floated. Occurs.
For this reason, the air-fuel ratio feedback control for measuring the exhaust air-fuel ratio of the internal combustion engine using the oxygen sensor and performing feedback control of the fuel supply amount to the engine so as to bring the measurement result close to the target air-fuel ratio is executed from a low temperature. Then, there is a problem that the control point shifts to the lean side due to the sensor output characteristic at the low temperature, and the exhaust property and the drivability are deteriorated.Therefore, in order to ensure the accuracy of the air-fuel ratio control at the time of cooling, the oxygen It has been desired to improve the low temperature characteristics of the sensor.

The present invention has been made in view of the above circumstances, and by improving the catalytic activity of the electrode, it is possible to improve the output characteristics at low temperature and enable highly accurate air-fuel ratio feedback control from low temperature. An object is to provide an oxygen sensor.

[0006]

Therefore, in the oxygen sensor according to the present invention, electrodes are formed on the inner and outer surfaces of a substrate made of an oxygen ion conductive solid electrolyte, respectively, and an electrode on one surface brought into contact with a reference gas and An oxygen sensor that generates an electromotive force according to the oxygen partial pressure ratio between the other surface electrode that is in contact with the detection gas and a precious metal component formed by coating platinum powder with a platinum-based precious metal other than platinum. The electrodes are included to form the electrodes.

[0007]

According to the above oxygen sensor, the platinum is coated on the platinum powder by forming an electrode containing a precious metal component obtained by coating the platinum powder with a platinum-based precious metal other than platinum such as rhodium, palladium, and ruthenium. Other platinum-based noble metals improve the catalytic activity of the platinum electrode.

[0008]

EXAMPLES Examples of the present invention will be described below. Referring to FIG. 1 showing an oxygen sensor structure of an example, a zirconia tube 1 having a closed tip formed of an oxygen ion conductive solid electrolyte containing zirconium oxide ZrO ₂ as a main component.
Electrodes 2 and 3 for extracting electromotive force are formed on the inner surface and the outer surface of the (base), respectively.

On the outside of the electrode 3 on the outer surface side,
A porous ceramic protective layer 4 for protecting the electrode 3 is formed. In such a configuration, while the reference gas (for example, the atmosphere) is introduced into the inner cavity of the zirconia tube 1, the outside of the zirconia tube 1 is brought into contact with the gas to be detected (for example, the exhaust gas of the internal combustion engine), and the reference gas contacting the inner surface is The oxygen partial pressure (oxygen concentration) of the gas to be detected is generated by generating an electromotive force according to the ratio of the oxygen partial pressure and the oxygen partial pressure of the gas to be detected in contact with the outer surface between the electrodes 2 and 3. Can be detected.

Here, the oxygen sensor having the above structure is manufactured as follows. First, the zirconia tube 1 is produced and is temporarily calcined. On the other hand, a precious metal paste (electrode material paste) to which ceramic powder is added is prepared. Here, the precious metal paste has a precious metal content of 60-80.
%, And a ceramic content of about 20 to 40%. The noble metal content contains platinum Pt as a main component, and rhodium Rh and palladium P are provided around the platinum Pt particles.
At least one platinum-based noble metal other than platinum such as d and ruthenium Ru is coated (see FIG. 2).

The particle size of the platinum Pt particles is preferably 20 μm or less. For example, platinum powder Pt coated with rhodium Rh decomposes rhodium chloride by mixing platinum Pt particles with an aqueous solution of rhodium chloride and stirring the mixed solution while heating at a temperature at which the rhodium chloride is decomposed. Then, it is obtained by concentrating.

Rhodium Rh for platinum Pt particles,
The addition ratio of platinum-based noble metals other than platinum such as palladium Pd and ruthenium Ru is preferably about 1 wt% to 20 wt%. Next, the precious metal paste is applied to the zirconia tube 1 by printing, and then the precious metal paste and the zirconia tube 1 are simultaneously sintered at a high temperature of about 1100 to 1300 ° C. to form an electrode on the surface of the zirconia tube 1. .

As a method for applying the noble metal paste to the zirconia tube 1, there are dipping, brush coating, spray injection, etc. in addition to the above printing, but printing is particularly preferable. As described above, when the platinum electrode is formed by including the noble metal component in which the platinum-based noble metal other than platinum such as rhodium Rh, palladium Pd, and ruthenium Ru is coated around the platinum Pt powder, the platinum electrode catalyst is formed. The activity is improved by rhodium Rh, palladium Pd, ruthenium Ru, etc. coated around platinum Pt particles, and the low temperature characteristics of the oxygen sensor are improved.

Here, even if platinum precious metal paste is prepared by mixing platinum Pt powder with particles of platinum-based precious metal other than platinum Pt such as rhodium Rh, palladium Pd, ruthenium Ru, only platinum Pt is a precious metal component. Although the catalytic activity can be improved as compared with the case where the electrode is formed as described above, by coating the platinum Pt particles around the platinum Pt particles as described above, noble metal components such as rhodium Rh, palladium Pd, and ruthenium Ru can be added to the platinum Pt. The particles can be surely collected around the particles, so that the addition of the noble metal components such as rhodium Rh, palladium Pd, and ruthenium Ru has a great effect of improving the characteristics (see FIG. 4).

As described above, if the catalyst activity is improved and the low temperature operation is promoted, if the oxygen concentration is provided to detect the oxygen concentration in the exhaust gas of the internal combustion engine, the amount of oxygen in the exhaust gas immediately after the engine is started. Oxygen concentration can be measured accurately,
The air-fuel ratio feedback control based on the information on the oxygen concentration can be performed at an early stage to improve the exhaust quality at the engine start (when the engine is cold).

[0016]

As described above, according to the oxygen sensor of the present invention, the platinum Pt powder contains a noble metal component formed by coating a platinum-based noble metal such as rhodium Rh, palladium Pd, and ruthenium Ru other than platinum. Since the electrode is formed with, the catalytic activity of platinum can be improved by the coated platinum-based noble metal, and still,
The effect that the platinum-based noble metal capable of improving the catalytic activity of platinum can be reliably collected around the platinum particles, and thus the catalytic activity of the electrode can be effectively improved and the low temperature characteristics of the oxygen sensor can be improved. There is.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of an oxygen sensor structure.

FIG. 2 is a diagram showing a structure of platinum particles forming an electrode.

FIG. 3 is a diagram showing an output characteristic of an oxygen sensor.

FIG. 4 is a diagram for explaining the effect of coating.

[Explanation of symbols]

 1 Zirconia tube 2,3 Electrode 4 Protective layer

Claims (1)

[Claims] 1. An electrode is formed on each of the inner and outer surfaces of a substrate made of an oxygen ion conductive solid electrolyte, and the electrode on one surface is in contact with a reference gas and the electrode on the other surface is in contact with a gas to be detected. An oxygen sensor that generates an electromotive force according to the oxygen partial pressure ratio, characterized in that the electrode is formed by including a noble metal component obtained by coating a platinum-based noble metal other than platinum around platinum powder. Sensor.