Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-02-28
2003-08-19
Tung, T. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S293000, C204S426000, C205S781000
Reexamination Certificate
active
06607643
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a NO
x
gas detecting apparatus. More particularly, the present invention relates to a NO
x
gas detecting apparatus capable of detecting a nitrogen oxide gas (a NO
x
gas) contained in exhaust gas discharged from boilers, automobiles and the like, or in living environments with high sensitivity.
2. Description of Related Art
When voltage is applied between electrodes that are sintered onto each side of an oxygen ion conductive solid electrolyte (hereinafter referred to simply as a “solid electrolyte”), such as a stabilized zirconia as a typical example, then an oxygen pumping action is produced in that oxygen is discharged from one electrode (cathode) to the other electrode (anode).
Also, when electrodes that are sintered onto each side of a solid electrolyte (hereinafter referred to as a “cell”) are exposed in an atmosphere including an oxygen bound gas, such as NO
x
, SO
x
, H
2
O, CO
2
and the like, and when voltage is applied between the electrodes, the oxygen bound gas is decomposed around the cathode and the oxygen generated thereby is discharged to the anode by an oxygen pumping action of the solid electrolytes. Simultaneously, current follows from the anode to the cathode in proportion to the concentration of the oxygen bound gas decomposed.
The voltage at which this oxygen pumping action is initiated differs depending on each oxygen bound gas. In addition, even in the case of the same oxygen bound gas, the voltage at which an oxygen pumping action is initiated in association with gas decomposition differs depending on a material constructing the electrodes (especially the cathode) disposed onto the electrolyte. Further, when heated in an atmosphere of a low partial pressure of oxygen, an oxygen bound gas is decomposed generally into oxygen and other components which have been bounded to the oxygen. The partial pressure of oxygen of which decomposition rate reaches a certain value differs depending on each oxygen bound gas.
Taking advantage of an oxygen pumping action of a cell and difference in decomposition of an oxygen bound gas owing to a gas diffusion rate determining body, voltage applying conditions, electrode materials, or partial pressure conditions of oxygen, a gas detecting apparatus capable of detecting concentration of a specific oxygen bound gas contained in a measurement gas to be measured may be produced. Especially, a NO
x
gas detecting apparatus for detecting concentration of a NO
x
gas contained in a measurement gas has been put into actual use as a sensor for monitoring an amount of NO
x
gas contained in exhaust gas discharged from combustion equipment and combustion facilities such as automobiles and boilers.
It is required that a NO
x
detecting apparatus detect a NO
x
gas contained in exhaust gas in a minute amount independent of influence of oxygen which is a main component of the exhaust gas. To meet this end, a NO
x
detecting apparatus comprises a gas diffusion rate determining body, an oxygen pumping cell, and a NO
x
gas detecting cell. The gas diffusion rate determining body is constructed for restricting gas diffusion flowing into the oxygen pumping cell and the NO
x
detecting cell by providing a small pinhole or a porous body at an upstream stage of the oxygen pumping cell. The oxygen pumping cell is a cell for selectively removing nothing but oxygen from the measurement gas through the use of an oxygen pumping action of the solid electrolyte. Accordingly, for an cathode of the oxygen pumping cell, an electrode having a high activity relative to an oxygen gas but is inactive or low active relative to a NO
x
gas is used. As such an electrode, for example, a cermet electrode composed of a Pt-Au alloy and a ceramic component (hereinafter referred to as a “Pt-Au electrode”) is known.
The NO
x
detecting cell is a cell for decomposing NO
x
contained in the measurement gas from which oxygen has been removed whereby a value of the current flowing between the electrodes at that time is measured. Accordingly, for a cathode of the NO
x
detecting cell, an electrode having a high activity relative to NO
x
gas is used. As such an electrode, for example, a cermet electrode composed of Pt and a ceramic component (hereinafter referred to as a “Pt electrode”), a cermet electrode composed of a Pt-Rh alloy and a ceramic component (hereinafter referred to as a “Pt-Rh electrode, ” and the like are known). (See, for example, Japanese Unexamined Patent Publication No. HEI 11-183434 for reference.)
A NO
x
gas is apt to decompose when heated in a low oxygen atmosphere containing a reduction component such as an uncombusted fuel. Consequently, in order to find out about an accurate NO
x
concentration in exhaust gas with the NO
x
gas detecting apparatus, the NO
x
concentration needs to be measured immediately after oxygen has been removed from the measurement gas and before the measurement gas is influenced by other components. To meet this end, in the NO
x
detecting apparatus, the NO
x
detecting cell is usually disposed at a downstream stage of the oxygen pumping cell in adjacent thereto.
The NO
x
gas detecting apparatus constructed as described above is produced generally by printing paste containing an electrode material on a surface of a green sheet containing a solid electrolyte, laminating it onto another integrally, and then sintering it in an atmosphere at a high temperature (in the case of a zirconia-base solid electrolyte for example at 1,400° C. or higher).
In the technical back ground described above, Pt is active for both an oxygen gas and a NO
x
gas, but becomes active only for oxygen by adding Au to Pt. Accordingly, a Pt-Au alloy is especially suitable as an electrode material of an oxygen pumping cell. On the other hand, Au which is contained in a Pt-Au alloy has a low temperature resistance (the melting point of 1,064° C.), and therefore heating a Pt-Au alloy at a high temperature causes Au contained in the alloy to scatter in all directions.
Due to the above reasons, there is a problem in the case of using a Pt-Au electrode as the cathode of the oxygen pumping cell, and a Pt electrode as the cathode of the NO
x
detecting cell. When a green sheet constructing the oxygen pumping cell and a green sheet constructing the NO
x
detecting cell are laminated and sintered at a high temperature, Au contained in the cathode of the oxygen pumping cell scatters and adheres to the Pt electrode of the NO
x
detecting cell. This causes the NO
x
-reducing ability of the Pt electrode to be reduced.
Further, fuel used for automobiles or combustion facilities such as a boiler contains various impurities. Also, in exhaust gas discharged from combustion facilities, there exist components derived from additives contained in engine oil. These components also cause a problem of reducing electrode activity drastically if they adhere to the Pt electrode used in the NO
x
detecting cell no matter how minute the amount is.
Alternatively, a Pt-Rh electrode that contains a large amount of Rh relative to Pt (about 40 wt %) may be used as the electrode of the NO
x
detecting cell, because Rh has a high NO
x
reducing ability.
However, since Rh is strongly bounded to oxygen, if the electrode is left in an oxidation atmosphere, oxygen is adsorbed to Rh thereby forming an oxide film on the surface thereof. This causes a problem of lowering the NO
x
reducing ability. Here, in order to improve the NO
x
reducing ability, the oxide film formed on the surface of Rh needs to be removed so that Rh is used as a metal. Yet, there is a problem in that the reduction process takes a long time.
In addition, even if the reduction process is carried out, Rh again adsorbs oxygen with a lapse of time. Consequently, when voltage is applied to the NO
x
detecting cell, oxygen gas that has been adsorbed within the Pt-Rh electrode is discharged gradually immediately after the activation. Consequently, the NO
x
detecting cell appears to output as if there existed a NO
x
gas although there is n
Inaba Tadashi
Masuoka Yumi
Nakamura Tadashi
Saito Toshitaka
Saji Keiichi
Kabushiki Kaisha Toyota Chuo Kenkyusho
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tung T.
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