NOx-decomposing electrode and NOx concentration-measuring apparatus

Disclosed is a NOx concentration-measuring apparatus for measuring a NOx concentration by using a main pumping cell including an electrode (inner pumping electrode and outer pumping electrode) having no or low decomposing/reducing ability with respect to NOx to control an oxygen concentration in a measurement gas to have a predetermined value which substantially does not affect measurement of NOx component, and using a detecting electrode having certain or high decomposing/reducing ability with respect to NOx to decompose NOx so that an amount of oxygen produced during this process is measured, wherein the detecting electrode is a cermet electrode composed of an alloy of Pt--Rh and a ceramic component, and Pt and Rh are contained in a weight ratio of Pt:Rh=20:80 to 1:99.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a NOx-decomposing electrode having certain
 or high decomposing/reducing ability with respect to NOx so that NOx is
 decomposed to produce oxygen during this process. The present invention
 also relates to a NOx concentration-measuring apparatus for measuring NOx
 contained, for example, in atmospheric air and exhaust gas discharged from
 vehicles or automobiles.
 2. Description of the Related Art
 A technique has been hitherto known as the method for measuring NOx in a
 measurement gas such as combustion gas, in which the NOx-reducing ability
 of Rh is utilized while using a sensor comprising a Pt electrode and an Rh
 electrode formed on an oxygen ion-conductive solid electrolyte such as
 zirconia to measure an electromotive force generated between the both
 electrodes.
 However, such a sensor suffers the following problem. That is, the
 electromotive force is greatly changed depending on the change in
 concentration of oxygen contained in the combustion gas as a measurement
 gas. Moreover, the change in electromotive force is small with respect to
 the change in concentration of NOx. For this reason, the conventional
 sensor tends to suffer influence of noise.
 Further, in order to bring out the NOx-reducing ability, it is
 indispensable to use a reducing gas such as CO. For this reason, the
 amount of produced CO is generally smaller than the amount of produced NOx
 under a lean fuel combustion condition in which a large amount of NOx is
 produced. Therefore, the conventional sensor has a drawback in that it is
 impossible to perform the measurement for a combustion gas produced under
 such a combustion condition.
 In order to solve the problems as described above, for example, Japanese
 Laid-Open Patent Publication No. 8-271476 discloses a NOx sensor
 comprising pumping electrodes having different NOx-decomposing abilities
 arranged in a first internal space which communicates with a measurement
 gas-existing space and in a second internal space which communicates with
 the first internal space, and a method for measuring the NOx concentration
 in which the O.sub.2 concentration is adjusted by using a first pumping
 cell arranged in the first internal space, and NO is decomposed by using a
 decomposing pumping cell arranged in the second internal space so that the
 NOx concentration is measured on the basis of a pumping current flowing
 through the decomposing pump.
 Further, Japanese Laid-Open Patent Publication No. 9-113484 discloses a
 sensor element comprising an auxiliary pumping electrode arranged in a
 second internal space so that the oxygen concentration in the second
 internal space is controlled to be constant even when the oxygen
 concentration is suddenly changed.
 A cermet electrode composed of Rh/ZrO.sub.2 is used for the NOx-decomposing
 electrode of the NOx sensor as described above. When the cermet electrode
 composed of Rh/ZrO.sub.2 is used for the NOx-decomposing electrode, a
 phenomenon has been observed, in which the sensitivity is lowered in
 accordance with the increase in operation time.
 Rh tends to be oxidized in a range of 500 to 1000.degree. C., and it
 repeats the oxidation reaction and the reduction reaction depending on the
 oxygen concentration in the atmosphere. The NOx-decomposing electrode is
 peeled off from the solid electrolyte substrate due to the change in
 volume of Rh caused by the repetition of the oxidation reaction and the
 reduction reaction. As a result, the impedance of the pumping cell is
 increased during the use of the gas sensor, and the increase in impedance
 consequently causes the decrease in sensitivity to NOx.
 SUMMARY OF THE INVENTION
 The present invention has been made taking the foregoing problems into
 consideration, an object of which is to provide a NOx-decomposing
 electrode which makes it possible to effectively suppress the oxidation
 reaction and the reduction reaction of Rh, and which makes it possible to
 suppress the adsorption of NOx at a low temperature and the formation of
 alloy together with any other metal element (for example, Au).
 Another object of the present invention is to provide a NOx
 concentration-measuring apparatus which makes it possible to effectively
 suppress the oxidation reaction and the reduction reaction of Rh contained
 in a NOx-decomposing electrode, which makes it possible to suppress the
 adsorption of NOx at a low temperature and the formation of alloy together
 with any other metal element (for example, Au), and which makes it
 possible to stabilize the impedance and stabilize the measurement
 sensitivity.
 According to the present invention, there is provided a NOx-decomposing
 electrode having certain or high decomposing/reducing ability with respect
 to NOx so that NOx is decomposed to produce oxygen during this process,
 wherein the electrode is a cermet electrode composed of an alloy of Pt--Rh
 and a ceramic component, and Pt and Rh are contained in a weight ratio of
 Pt:Rh=20:80 to 1:99.
 The cermet electrode, which is composed of the alloy of Pt--Rh and the
 ceramic component, is used as the NOx-decomposing electrode. By doing so,
 it is possible to effectively suppress the oxidation reaction and the
 reduction reaction of Rh contained in the NOx-decomposing electrode.
 Especially, in the present invention, the ratio between Pt and Rh is
 Pt:Rh=20:80 to 1:99 in the weight ratio. Therefore, it is possible to
 suppress the adsorption of NOx at a low temperature and the formation of
 alloy together with any other metal element (for example, Au).
 It is preferable that the ratio between Pt and Rh contained in the
 NOx-decomposing electrode is Pt:Rh=10:90 to 1:99 in weight ratio.
 According to another aspect of the present invention, there is provided a
 NOx concentration-measuring apparatus comprising an oxygen pump including
 an electrode having no or low decomposing/reducing ability with respect to
 NOx, the oxygen pump being used to control an oxygen concentration in a
 measurement gas to have a predetermined value at which NO is not
 substantially decomposable, and a NOx-decomposing electrode having certain
 or high decomposing/reducing ability with respect to NOx, the
 NOx-decomposing electrode being used to measure a NOx concentration by
 decomposing NOx and measuring an amount of oxygen produced during this
 process, wherein the NOx-decomposing electrode is a cermet electrode
 composed of an alloy of Pt--Rh and a ceramic component, and Pt and Rh are
 contained in a weight ratio of Pt:Rh=20:80 to 1:99.
 According to the present invention, it is possible to effectively suppress
 the oxidation reaction and the reduction reaction of Rh contained in the
 NOx-decomposing electrode. Further, it is possible to suppress the
 adsorption of NOx at a low temperature and the formation of alloy together
 with any other metal element (for example, Au). In the present invention,
 it is especially preferable that the ratio between Pt and Rh is
 Pt:Rh=10:90 to 1:99 in weight ratio.
 The above and other objects, features, and advantages of the present
 invention will become more apparent from the following description when
 taken in conjunction with the accompanying drawings in which a preferred
 embodiment of the present invention is shown by way of illustrative
 example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An illustrative embodiment of the NOx concentration-measuring apparatus
 including the NOx-decomposing electrode according to the present invention
 (hereinafter simply referred to as "NOx concentration-measuring apparatus
 according to the embodiment") will be explained below with reference to
 FIGS. 1 to 7.
 As shown in FIGS. 1 and 2, the NOx concentration-measuring apparatus 10
 according to this embodiment has a substrate 14 comprising, for example,
 five stacked solid electrolyte layers 12a to 12e composed of ceramics
 based on the use of oxygen ion-conductive solid electrolytes such as
 ZrO.sub.2.
 The substrate 14 is constructed as follows. That is, first and second
 layers from the bottom are designated as first and second substrate layers
 12a, 12b respectively. Third and fifth layers from the bottom are
 designated as first and second solid electrolyte layers 12c, 12e
 respectively. A fourth layer from the bottom is designated as a spacer
 layers 12d.
 A space (reference gas-introducing space) 16, into which a reference gas
 such as atmospheric air to be used as a reference for measuring oxides is
 introduced, is formed between the second substrate layer 12b and the first
 solid electrolyte layer 12c, the space 16 being comparted by a recess
 formed on a lower surface of the first solid electrolyte layer 12c and a
 recess formed on an upper surface of the second substrate layer 12b.
 A first chamber 18 for adjusting the partial pressure of oxygen in a
 measurement gas, and a second chamber 20 for finely adjusting the partial
 pressure of oxygen in the measurement gas and measuring oxides, for
 example, nitrogen oxides (NOx) in the measurement gas are formed and
 comparted between a lower surface of the second solid electrolyte layer
 12e and an upper surface of the first solid electrolyte layer 12c.
 The NOx concentration-measuring apparatus 10 according to this embodiment
 has a space section 22 which is formed at a forward end of the spacer
 layer 12d. A forward end opening of the space section 22 constitutes a
 gas-introducing port 24. The space section 22 communicates with the first
 chamber 18 via a first diffusion rate-determining section 26. The first
 chamber 18 communicates with the second chamber 20 via a second diffusion
 rate-determining section 28.
 In this embodiment, each of the first and second diffusion rate-determining
 sections 26, 28 gives a predetermined diffusion resistance to the
 measurement gas to be introduced into the first chamber 18 and the second
 chamber 20 respectively. In the illustrative arrangement shown in FIG. 1,
 each of the first and second diffusion rate-determining sections 26, 28 is
 formed as a vertically extending slit having a predetermined
 cross-sectional area capable of introducing the measurement gas. Both of
 the vertically extending slits are formed at substantially central
 portions of the spacer layer 12d in the widthwise direction.
 It is also preferable that a porous member composed of ZrO.sub.2 or the
 like is arranged and packed in the slit of the second diffusion
 rate-determining section 28 so that the diffusion resistance of the second
 diffusion rate-determining section 28 is larger than the diffusion
 resistance of the first diffusion rate-determining section 26. It is
 preferable that the diffusion resistance of the second diffusion
 rate-determining section 28 is larger than that of the first diffusion
 rate-determining section 26. However, no problem occurs even when the
 former is smaller than the latter.
 The atmosphere in the first chamber 18 is introduced into the second
 chamber 20 under the predetermined diffusion resistance via the second
 diffusion rate-determining section 28.
 The space section 22 functions as a clogging-preventive section for
 avoiding the clogging of particles (for example, soot and oil combustion
 waste) produced in the measurement gas in the external space, which would
 be otherwise caused in the vicinity of the inlet of the first chamber 18.
 Accordingly, it is possible to measure the NOx component more accurately.
 The NOx concentration-measuring apparatus 10 according to this embodiment
 has an inner pumping electrode 30 which is composed of a porous cermet
 electrode (for example, a cermet electrode of Pt.ZrO.sub.2 containing 1
 wt. % Au) formed on an inner wall surface of the first chamber 18. An
 outer pumping electrode 32 is formed on an upper surface portion
 corresponding to the inner pumping electrode 30, of the upper surface of
 the second solid electrolyte layer 12e. An electrochemical pumping cell,
 i.e., a main pumping cell 34 is constructed by the inner pumping electrode
 30, the outer pumping electrode 32, the second solid electrolyte layer 12e
 interposed between the both electrodes 30, 32, the first solid electrolyte
 layer 12c, and the spacer layer 12d.
 The inner pumping electrode 30 is formed mutually continuously to surround
 the inner wall surface of the first chamber 18. The inner pumping
 electrode 30 is constructed by continuously forming an electrode section
 30a which is arranged on the lower surface of the second solid electrolyte
 layer 12e as the upper surface of the wall surface of the first chamber
 18, an electrode section 30b which is arranged on the upper surface of the
 first solid electrolyte layer 12c as the lower surface of the inner wall
 surface, and electrode sections 30c, 30d (electrode section 30d is not
 shown) which are arranged on the side surfaces of the spacer layer 12d as
 the side surfaces of the inner wall surface.
 The inner pumping electrode 30 may be constructed by using only any one of
 the electrode section 30a on the upper surface and the electrode section
 30b on the lower surface. Alternatively, the inner pumping electrode 30
 may be constructed by using three in total of the electrode sections 30a,
 30b on the both upper and lower surfaces and any one of the electrode
 sections 30c, 30d on the side surfaces.
 A desired control voltage (pumping voltage) Vp1 is applied between the
 inner pumping electrode 30 and the outer pumping electrode 32 of the main
 pumping cell 34 by the aid of an external variable power source 40 to
 allow a pumping current Ip1 to flow in a positive or negative direction
 between the outer pumping electrode 32 and the inner pumping electrode 30.
 Thus, the oxygen in the atmosphere in the first chamber 18 can be pumped
 out to the external space, or the oxygen in the external space can be
 pumped into the first chamber 18.
 A measuring electrode 42, which is composed of a porous cermet electrode
 having a substantially rectangular planar configuration, is formed on a
 portion of the upper surface of the first solid electrolyte layer 12c for
 forming the first chamber 18, the upper surface portion being disposed in
 the vicinity of the second diffusion rate-determining section 28 (the
 lower surface electrode section 30b of the inner pumping electrode 30 is
 not formed on the portion). A reference electrode 44 is formed on a lower
 surface portion exposed to the reference gas-introducing space 16, of the
 lower surface of the first solid electrolyte layer 12c. An electrochemical
 sensor cell, i.e., a controlling oxygen partial pressure-detecting cell 46
 is constructed by the measuring electrode 42, the reference electrode 44,
 and the first solid electrolyte layer 12c.
 The controlling oxygen partial pressure-detecting cell 46 is operated as
 follows. That is, an electromotive force is generated between the
 measuring electrode 42 and the reference electrode 44 on the basis of a
 difference in oxygen concentration between the atmosphere in the first
 chamber 18 and the reference gas (atmospheric air) in the reference
 gas-introducing space 16. The partial pressure of oxygen in the atmosphere
 in the first chamber 18 can be detected by measuring the electromotive
 force by using a voltmeter 48.
 That is, the voltage V1 generated between the reference electrode 44 and
 the measuring electrode 42 is an electromotive force of the oxygen
 concentration cell generated on the basis of the difference between the
 partial pressure of oxygen of the reference gas introduced into the
 reference gas-introducing space 16 and the partial pressure of oxygen of
 the measurement gas in the first chamber 18. The voltage V1 has the
 following relationship known as the Nernst's equation.
EQU V1=RT/4F.ln(P1(O.sub.2)/PO(O.sub.2))
 R: gas constant;
 T: absolute temperature;
 F: Faraday constant;
 P1(O.sub.2): partial pressure of oxygen in the first chamber 18;
 P0(O.sub.2): partial pressure of oxygen of the reference gas.
 Therefore, the partial pressure of oxygen in the first chamber 18 can be
 detected by measuring the voltage V1 generated on the basis of the
 Nernst's equation by using the voltmeter 48.
 The detected value of the partial pressure of oxygen is used to control the
 pumping voltage of the variable power source 40 by the aid of a feedback
 control system 50. Specifically, the pumping operation effected by the
 main pumping cell 34 is controlled so that the partial pressure of oxygen
 in the atmosphere in the first chamber 18 has a predetermined value which
 is sufficiently low to control the partial pressure of oxygen in the
 second chamber 20 in the next step.
 Each of the inner pumping electrode 30 and the outer pumping electrode 32
 is composed of an inert material having a low catalytic activity on NOx
 such as NO contained in the measurement gas introduced into the first
 chamber 18. Specifically, the inner pumping electrode 30 and the outer
 pumping electrode 32 may be composed of a porous cermet electrode. In this
 embodiment, the electrode is composed of a metal such as Pt and a ceramic
 material such as ZrO.sub.2. Especially, it is necessary to use a material
 which has a weak reducing ability or no reducing ability with respect to
 the NO component in the measurement gas, for the inner pumping electrode
 30 and the measuring electrode 42 disposed in the first chamber 18 to make
 contact with the measurement gas. It is preferable that the inner pumping
 electrode 30 and the measuring electrode 42 are composed of, for example,
 a compound having the perovskite structure such as La.sub.3 CuO.sub.4, a
 cermet comprising a ceramic material and a metal such as Au having a low
 catalytic activity, or a cermet comprising a ceramic material, a metal of
 the Pt group, and a metal such as Au having a low catalytic activity. When
 an alloy composed of Au and a metal of the Pt group is used as an
 electrode material, it is preferable to add Au in an amount of 0.03 to 35%
 by volume of the entire metal component.
 The NOx concentration-measuring apparatus 10 according to this embodiment
 further comprises a detecting electrode 60 having a substantially
 rectangular planar configuration and composed of a porous cermet
 electrode, the detecting electrode 60 being formed at an upper surface
 portion for forming the second chamber 20, separated from the second
 diffusion rate-determining section 28, of the upper surface of the first
 solid electrolyte layer 12c. An alumina film, which constitutes a third
 diffusion rate-determining section 62, is formed to cover the detecting
 electrode 60. An electrochemical pumping cell, i.e., a measuring pumping
 cell 64 is constructed by the detecting electrode 60, the reference
 electrode 44, and the first solid electrolyte layer 12c.
 The detecting electrode 60 is composed of a porous cermet comprising
 zirconia as a ceramic material and a metal capable of reducing NOx as the
 measurement gas component. Accordingly, the detecting electrode 60
 functions as a NOx-reducing catalyst for reducing NOx existing in the
 atmosphere in the second chamber 20. Further, the oxygen in the atmosphere
 in the second chamber 20 can be pumped out to the reference
 gas-introducing space 16 by applying a constant voltage Vp2 between the
 detecting electrode 60 and the reference electrode 44 by the aid of a DC
 power source 66. The pumping current Ip2, which is allowed to flow in
 accordance with the pumping operation performed by the measuring pumping
 cell 64, is detected by an ammeter 68. Details of the detecting electrode
 60 will be described later on.
 The constant voltage (DC) power source 66 can apply a voltage of a
 magnitude to give a limiting current to the pumping for oxygen produced
 during decomposition in the measuring pumping cell 64 under the inflow of
 NOx restricted by the third diffusion rate-determining section 62.
 On the other hand, an auxiliary pumping electrode 70, which is composed of
 a porous cermet electrode (for example, a cermet electrode of Pt.ZrO.sub.2
 containing 1 wt. % Au), is formed on an inner wall surface portion for
 forming the second chamber 20, of the lower surface of the second solid
 electrolyte layer 12e. An auxiliary electrochemical pumping cell, i.e., an
 auxiliary pumping cell 72 is constructed by the auxiliary pumping
 electrode 70, the second solid electrolyte layer 12e, the spacer layer
 12d, the first solid electrolyte layer 12c, and the reference electrode
 44.
 The auxiliary pumping electrode 70 is based on the use of a material having
 a weak reducing ability or no reducing ability with respect to the NO
 component contained in the measurement gas, in the same manner as in the
 inner pumping electrode 30 of the main pumping cell 34. In this
 embodiment, for example, it is preferable that the auxiliary pumping
 electrode 70 is composed of a compound having the perovskite structure
 such as La.sub.3 CuO.sub.4, a cermet comprising a ceramic material and a
 metal having a low catalytic activity such as Au, or a cermet comprising a
 ceramic material, a metal of the Pt group, and a metal having a low
 catalytic activity such as Au. Further, when an alloy comprising Au and a
 metal of the Pt group is used as an electrode material, it is preferable
 to add Au in an amount of 0.03 to 35% by volume of the entire metal
 components.
 The auxiliary pumping electrode 70 is formed mutually continuously to
 surround the inner wall surface of the second chamber 20, in the same
 manner as the inner pumping electrode 30 described above. The auxiliary
 pumping electrode 70 is constructed by continuously forming an electrode
 section 70a which is arranged on the lower surface of the second solid
 electrolyte layer 12e as the upper surface of the wall surface of the
 second chamber 20, an electrode section 70b which is arranged on the upper
 surface of the first solid electrolyte layer 12c as the lower surface of
 the wall surface, and electrode sections 70c, 70d (electrode section 70d
 is not shown) which are arranged on the side surfaces of the spacer layer
 12d as the side surfaces of the wall surface.
 The auxiliary pumping electrode 70 may be constructed by using only any one
 of the electrode section 70a on the upper surface and the electrode
 section 70b on the lower surface. Alternatively, the auxiliary pumping
 electrode 70 may be constructed by using three in total of the electrode
 sections 70a, 70b on the both upper and lower surfaces and any one of the
 electrode sections 70c, 70d on the side surfaces.
 A desired constant voltage Vp3 is applied between the reference electrode
 44 and the auxiliary pumping electrode 70 of the auxiliary pumping cell 72
 by the aid of an external DC power source 74. Thus, the oxygen in the
 atmosphere in the second chamber 20 can be pumped out to the reference
 gas-introducing space 16.
 Accordingly, the partial pressure of oxygen in the atmosphere in the second
 chamber 20 is allowed to have a low value of partial pressure of oxygen at
 which the measurement of the amount of the objective component is not
 substantially affected, under the condition in which the measurement gas
 component (NOx) is not substantially reduced or decomposed. In this
 embodiment, owing to the operation of the main pumping cell 34 for the
 first chamber 18, the change in amount of oxygen introduced into the
 second chamber 20 is greatly reduced as compared with the change in the
 measurement gas. Accordingly, the partial pressure of oxygen in the second
 chamber 20 is accurately controlled to be constant.
 Therefore, in the case of the NOx concentration-measuring apparatus 10
 constructed as described above, the measurement gas, which has been
 controlled for the partial pressure of oxygen in the second chamber 20, is
 introduced into the detecting electrode 60.
 As shown in FIG. 2, the NOx concentration-measuring apparatus 10 according
 to this embodiment further comprises a heater 80 for generating heat in
 accordance with electric power supply from the outside. The heater 80 is
 embedded in a lower portion of the second substrate layer 12b. The heater
 80 is provided in order to increase the conductivity of oxygen ion. A
 insulative layer 82 composed of alumina or the like is formed to cover
 upper and lower surfaces of the heater 80 so that the heater 80 is
 electrically insulated from the first and second substrate layers 12a,
 12b.
 The heater 80 is arranged over the entire portion ranging from the first
 chamber 18 to the second chamber 20. Accordingly, each of the first
 chamber 18 and the second chamber 20 is heated to a predetermined
 temperature. Simultaneously, each of the main pumping cell 34, the
 controlling oxygen partial pressure-detecting cell 46, and the measuring
 pumping cell 64 is also heated to a predetermined temperature and
 maintained at that temperature.
 Next, the operation of the NOx concentration-measuring apparatus 10
 according to the embodiment of the present invention will be explained. At
 first, the forward end of the NOx concentration-measuring apparatus 10 is
 disposed in the external space. Accordingly, the measurement gas is
 introduced into the first chamber 18 under the predetermined diffusion
 resistance via the first diffusion rate-determining section 26. The
 measurement gas, which has been introduced into the first chamber 18, is
 subjected to the pumping action for oxygen, caused by applying the
 predetermined pumping voltage Vp1 between the outer pumping electrode 32
 and the inner pumping electrode 30 which construct the main pumping cell
 34. The partial pressure of oxygen is controlled to have a predetermined
 value, for example, 10.sup.-7 atm. The control is performed by the aid of
 the feedback control system 50.
 The first diffusion rate-determining section 26 serves to limit the amount
 of diffusion and inflow of oxygen in the measurement gas into the
 measuring space (first chamber 18) when the pumping voltage Vp1 is applied
 to the main pumping cell 34 so that the current flowing through the main
 pumping cell 34 is suppressed.
 In the first chamber 18, a state of partial pressure of oxygen is
 established, in which NOx in the atmosphere is not reduced by the inner
 pumping electrode 30 even in an environment of being heated by the
 external measurement gas and being heated by the heater 80. For example, a
 condition of partial pressure of oxygen is formed, in which the reaction
 of NO.fwdarw.1/2N.sub.2 +1/2O.sub.2 substantially does not occur, because
 of the following reason. That is, if NOx in the measurement gas
 (atmosphere) is reduced in the first chamber 18, it is impossible to
 accurately measure NOx in the second chamber 20 disposed at the downstream
 stage. In this context, it is necessary to establish a condition in the
 first chamber 18 in which NOx is not reduced by the component which
 participates in reduction of NOx (in this case, the metal component of the
 inner pumping electrode 30). Specifically, as described above, such a
 condition is achieved by using, for the inner pumping electrode 30, the
 material having a low ability to reduce NOx, for example, an alloy of Au
 and Pt.
 The gas in the first chamber 18 is introduced into the second chamber 20
 under the predetermined diffusion resistance via the second diffusion
 rate-determining section 28. The gas, which has been introduced into the
 second chamber 20, is subjected to the pumping action for oxygen, caused
 by applying the constant voltage Vp3 between the reference electrode 44
 and the auxiliary pumping electrode 70 which constitute the auxiliary
 pumping cell 72 to make fine adjustment so that the partial pressure of
 oxygen has a constant and low value of partial pressure of oxygen.
 The second diffusion rate-determining section 28 serves to limit the amount
 of diffusion and inflow of oxygen in the measurement gas into the
 measuring space (second chamber 20) when the constant voltage Vp3 is
 applied to the auxiliary pumping cell 72 so that the pumping current Ip3
 flowing through the auxiliary pumping cell 72 is suppressed, in the same
 manner as performed by the first diffusion rate-determining section 26.
 The measurement gas, which has been controlled for the partial pressure of
 oxygen in the second chamber 20 as described above, is introduced into the
 detecting electrode 60 under the predetermined diffusion resistance via
 the third diffusion rate-determining section 62.
 When it is intended to control the partial pressure of oxygen in the
 atmosphere in the first chamber 18 to have a low value of the partial
 pressure of oxygen which does not substantially affect the measurement of
 NOx, by operating the main pumping cell 34, in other words, when the
 pumping voltage Vp1 of the variable power source 40 is adjusted by the aid
 of the feedback control system 50 so that the voltage V1 detected by the
 controlling oxygen partial pressure-detecting cell 46 is constant, if the
 oxygen concentration in the measurement gas greatly changes, for example,
 in a range of 0 to 20%, then the respective partial pressures of oxygen in
 the atmosphere in the second chamber 20 and in the atmosphere in the
 vicinity of the detecting electrode 60 slightly change in ordinary cases.
 This phenomenon is caused probably because of the following reason. That
 is, when the oxygen concentration in the measurement gas increases, the
 distribution of the oxygen concentration occurs in the widthwise direction
 and in the thickness direction in the first chamber 18. The distribution
 of the oxygen concentration changes depending on the oxygen concentration
 in the measurement gas.
 However, in the case of the NOx concentration-measuring apparatus 10
 according to this embodiment, the auxiliary pumping cell 72 is provided
 for the second chamber 20 so that the partial pressure of oxygen in its
 internal atmosphere always has a constant low value of the partial
 pressure of oxygen. Accordingly, even when the partial pressure of oxygen
 in the atmosphere introduced from the first chamber 18 into the second
 chamber 20 changes depending on the oxygen concentration in the
 measurement gas, the partial pressure of oxygen in the atmosphere in the
 second chamber 20 can be always made to have a constant low value, owing
 to the pumping action performed by the auxiliary pumping cell 72. As a
 result, the partial pressure of oxygen can be controlled to have a low
 value at which the measurement of NOx is not substantially affected.
 NOx in the measurement gas introduced into the detecting electrode 60 is
 reduced or decomposed around the detecting electrode 60. Thus, for
 example, a reaction of NO.fwdarw.1/2N.sub.2 +1/2O.sub.2 is allowed to
 occur. In this process, a predetermined voltage Vp2, for example, 430 mV
 (700.degree. C.) is applied between the detecting electrode 60 and the
 reference electrode 44 which construct the measuring pumping cell 64, in a
 direction to pump out the oxygen from the second chamber 20 to the
 reference gas-introducing space 16.
 Therefore, the pumping current Ip2 flowing through the measuring pumping
 cell 64 has a value which is proportional to a sum of the oxygen
 concentration in the atmosphere introduced into the second chamber 20,
 i.e., the oxygen concentration in the second chamber 20 and the oxygen
 concentration produced by reduction or decomposition of NOx by the aid of
 the detecting electrode 60.
 In this embodiment, the oxygen concentration in the atmosphere in the
 second chamber 20 is controlled to be constant by means of the auxiliary
 pumping cell 72. Accordingly, the pumping current Ip2 flowing through the
 measuring pumping cell 64 is proportional to the NOx concentration. The
 NOx concentration corresponds to the amount of diffusion of NOx limited by
 the third diffusion rate-determining section 62. Therefore, even when the
 oxygen concentration in the measurement gas greatly changes, it is
 possible to accurately measure the NOx concentration, based on the use of
 the measuring pumping cell 64 by the aid of the ammeter 68.
 According to the fact described above, almost all of the pumping current
 value Ip2 obtained by operating the measuring pumping cell 64 represents
 the amount brought about by the reduction or decomposition of NOx.
 Accordingly, the obtained result does not depend on the oxygen
 concentration in the measurement gas.
 The detecting electrode 60 formed in the second chamber will now be
 explained in detail below. For example, when a cermet electrode of
 Rh/ZrO.sub.2 was used as the detecting electrode 60, a phenomenon was
 observed, in which the sensitivity was decreased in accordance with the
 increase in operation time.
 This phenomenon was caused by the increase in impedance of the measuring
 pumping cell 64. When the NOx concentration-measuring apparatus with the
 increased impedance was observed, it was recognized that the contact area
 was decreased between the detecting electrode 60 and the first solid
 electrolyte layer 12c. That is, it is assumed that the increase in
 impedance is caused by the decrease in contact area between the detecting
 electrode 60 and the first solid electrolyte layer 12c.
 Accordingly, an experiment (conveniently referred to as "first illustrative
 experiment") was carried out. In the first illustrative experiment, the
 way of change of the weight of the alloy (sample) composed of Pt and Rh
 depending on the increase in heat was measured by using a thermo-balance
 while changing the weight ratio between Pt and Rh. Obtained results are
 shown in FIG. 3.
 In FIG. 3, a curve "a" represents a characteristic in the case of Rh=100
 wt. %. A curve "b" represents a characteristic in the case of Pt/Rh=1 wt.
 %/99 wt. %. A curve "c" represents a characteristic in the case of Pt/Rh=5
 wt. %/95 wt. %. A curve "d" represents a characteristic in the case of
 Pt/Rh=10 wt. %/90 wt. %. A curve "e" represents a characteristic in the
 case of Pt/Rh=20 wt. %/80 wt. %. A curve "f" represents a characteristic
 in the case of Pt/Rh=25 wt. %/75 wt. %.
 According to the experimental result shown in FIG. 3, it is understood that
 in the case of Rh=100 wt. % (curve "a"), the weight increase caused by the
 oxidation of Rh (Rh.sub.2 O.sub.3) is observed in a range from about
 500.degree. C. to about 800.degree. C., the weight is decreased as the
 metallization is started again from about 1100.degree. C., and the weight
 is returned to the original weight at about 1200.degree. C.
 Similarly, it is understood that in the case of Pt/Rh=1 wt. %/99 wt. %
 (curve "b"), the weight increase caused by the oxidation of Rh (Rh.sub.2
 O.sub.3) is observed in a range from about 600.degree. C. to about
 900.degree. C., the weight is decreased as the metallization is started
 again from about 1000.degree. C., and the weight is returned to the
 original weight at about 1150.degree. C.
 FIG. 4 summarizes the range of weight increase caused by the oxidation, and
 the range of the weight decrease caused by the reconversion into metal as
 described above.
 According to FIGS. 3 and 4, it is understood that in the case of the
 detecting electrode of Rh=100 wt. % (curve "a"), the weight increase,
 i.e., the volume increase remarkably occurs in the operation temperature
 range of 500.degree. C. to 700.degree. C.
 The practical use or operation of the NOx concentration-measuring apparatus
 10 is usually carried out by setting the element temperature at about
 700.degree. C. Therefore, for example, if the detecting electrode 60 is
 constructed by using the cermet electrode of Rh=100 wt. % (curve "a"), the
 volume decrease occurs due to the reconversion into metal of Rh as caused
 by the oxygen pumping effected by the detecting electrode 60 during the
 operation of the sensor. The oxidation of Rh (Rh.sub.2 O.sub.3) occurs
 immediately after the stop of the operation of the sensor, because the
 element temperature is still not less than 600.degree. C. although the
 oxygen pumping is stopped, resulting in the occurrence of the volume
 increase of Rh.
 When the series of volume increase and volume decrease are repeated, then
 the detecting electrode 60 is partially peeled off from the first solid
 electrolyte layer 12c, and the contact area is decreased between the
 detecting electrode 60 and the first solid electrolyte layer 12c. In this
 situation, it is postulated that the impedance of the measuring pumping
 cell 64 is increased, and the sensitivity to NOx is decreased.
 However, as understood from FIG. 3, when the detecting electrode 60 is
 constructed by using the cermet electrode which is composed of the alloy
 of Pt--Rh and the ceramic component, the volume increase occurs in the
 range deviated from the operation temperature range T. Therefore, the
 peeling off phenomenon as described above is not caused by the detecting
 electrode 60. That is, it is possible to suppress the oxidation reaction
 and the reduction reaction of Rh in the practical use.
 Another illustrative experiment (conveniently referred to as "second
 illustrative experiment") was carried out, in which NOx
 concentration-measuring apparatuses 10 were manufactured while changing
 the ratio of Pt/Rh contained in the detecting electrode 60 respectively.
 Limiting current characteristics, which were obtained when the NO
 concentration was 1000 ppm, were plotted for the five NOx
 concentration-measuring apparatuses 10. FIG. 5 shows the limiting current
 characteristics which were obtained when the heat was applied to an
 element (substrate 14) to be a temperature of 700.degree. C.
 In FIG. 5, a curve "a" represents a characteristic in the case of Rh=100
 wt. %. A curve "b" represents a characteristic in the case of Pt/Rh=1 wt.
 %/99 wt. %. A curve "c" represents a characteristic in the case of
 Pt/Rh=10 wt. %/90 wt. %. A curve "d" represents a characteristic in the
 case of Pt/Rh=20 wt. %/80 wt. %. A curve "e" represents a characteristic
 in the case of Pt/Rh=25 wt. %/75 wt. %.
 The horizontal axis represents the electromotive force of the oxygen
 concentration cell, generated on the basis of the difference between the
 partial pressure of oxygen of the reference gas to be introduced into the
 reference gas-introducing space 16 and the partial pressure of oxygen in
 the atmosphere to make contact with the detecting electrode 60 disposed in
 the second chamber 20. The horizontal axis equivalently corresponds to the
 oxygen concentration of the measurement gas.
 According to FIG. 5, in the second illustrative experiment, an
 approximately constant pumping current (pumping current corresponding to a
 NO concentration =1000 ppm) flows in each of the apparatuses corresponding
 to Rh=100% (curve "a"), Pt/Rh=1 wt. %/99 wt. % (curve "b"), Pt/Rh=10 wt.
 %/90 wt. % (curve "c"), and Pt/Rh=20 wt. %/80 wt. % (curve "d"), in a
 substantial detection range D (400 mV to 600 mV). Therefore, it is
 understood that NOx can be accurately measured in the case of these
 apparatuses.
 Although the apparatus, which corresponds to Pt/Rh=25 wt. %/75 wt. % (curve
 "e"), involves no problem in view of the practical use, it is feared that
 the pumping current somewhat disperses in this apparatus, and it is
 difficult for this apparatus to obtain a certain accuracy for the
 measurement of NOx.
 Such a fear may be caused because the cermet electrode of this apparatus,
 which is composed of the alloy of Pt--Rh and the ceramic component, is
 inferior in NOx-decomposing/reducing ability as compared with the Rh
 cermet electrode.
 Thus, in this embodiment, it is approved that the detecting electrode 60 is
 constructed by using the cermet electrode composed of the alloy of Pt--Rh
 and the ceramic component, and the optimum value of the amount of addition
 of Rh in the Pt--Rh alloy is such that the ratio of Pt and Rh is
 Pt:Rh=20:80 to 1:99, preferably Pt:Rh=10:90 to 1:99 in weight ratio.
 An illustrative experiment (conveniently referred to as "third illustrative
 experiment") will now be described. In the third illustrative experiment,
 NOx concentration-measuring apparatuses 10 were manufactured while
 changing the ratio of Pt/Rh contained in the detecting electrode 60
 respectively. Observation was made for the change in sensitivity to NOx
 during the practical use of the five NOx concentration-measuring
 apparatuses 10. Experimental results are shown in FIG. 6.
 In FIG. 6, a curve "a" represents a characteristic in the case of Rh=100
 wt. %. A curve "b" represents a characteristic in the case of Pt/Rh=25 wt.
 %/75 wt. %. A curve "c" represents a characteristic in the case of
 Pt/Rh=20 wt. %/80 wt. %. A curve "d" represents a characteristic in the
 case of Pt/Rh=10 wt. %/90 wt. %. A curve "e" represents a characteristic
 in the case of Pt/Rh=1 wt. %/99 wt. %.
 The following judgement may be acknowledged in the third illustrative
 experiment. That is, those in which the ratio of sensitivity change is not
 deteriorated up to -6% at a point of time at which the durable time
 exceeds 1000 hours are durable. Those in which the ratio of sensitivity
 change exceeds -6% at a point of time at which the durable time exceeds
 1000 hours are not durable.
 Based on this judgement standard, it is understood that the apparatus
 corresponding to Rh=100 wt. % (curve "a") is fairly inferior in
 durability, because the ratio of sensitivity change is deteriorated up to
 -6% after the passage of about 250 hours from the start of the experiment.
 It is understood that the apparatus corresponding to Pt/Rh=25 wt. %/75 wt.
 % (curve "b") is slightly inferior in durability, because the ratio of
 sensitivity change is deteriorated up to -6% after the passage of about
 820 hours from the start of the experiment.
 On the other hand, it is understood that the apparatuses corresponding to
 Pt/Rh=20 wt. %/80 wt. % (curve "c"), Pt/Rh=10 wt. %/90 wt. % (curve "d"),
 and Pt/Rh=1 wt. %/99 wt. % (curve "e") are excellent in durability,
 because the ratio of sensitivity change is not deteriorated up to -6% even
 after the passage of 1000 hours from the start of the experiment.
 As described above, the NOx concentration-measuring apparatus 10 according
 to the embodiment of the present invention uses the cermet electrode
 composed of the alloy of Pt--Rh and the ceramic component, as the
 detecting electrode 60 for constructing the measuring pumping cell 64.
 Therefore, the oxidation and the reconversion into metal of Rh contained
 in the detecting electrode 60 are suppressed. Even when the operation time
 of the NOx concentration-measuring apparatus 10 is increased, the increase
 in impedance is not brought about, which would be otherwise caused by the
 decrease in contact area between the detecting electrode 60 and the first
 solid electrolyte layer 12c.
 In this embodiment, the ratio between Pt and Rh is Pt:Rh=20:80 to 1:99,
 preferably Pt:Rh=10:90 to 1:99 in weight ratio. Therefore, it is possible
 to suppress the adsorption of NOx at a low temperature and the formation
 of alloy together with any other metal element (for example, Au), and it
 is possible to remarkably improve the durability.
 In other words, the NOx concentration-measuring apparatus 10 according to
 this embodiment makes it possible to stabilize the impedance and stabilize
 the measurement sensitivity.
 Next, a modified embodiment of the NOx concentration-measuring apparatus 10
 according to the embodiment of the present invention will be explained
 with reference to FIG. 7. Components or parts corresponding to those shown
 in FIG. 1 are designated by the same reference numerals.
 As shown in FIG. 7, the NOx concentration-measuring apparatus 10a according
 to this modified embodiment is constructed in approximately the same
 manner as in the NOx concentration-measuring apparatus 10 according to the
 foregoing embodiment (see FIG. 2). However, the former is different from
 the latter in that a measuring oxygen partial pressure-detecting cell 90
 is provided in place of the measuring pumping cell 64.
 The measuring oxygen partial pressure-detecting cell 90 comprises a
 detecting electrode 92 formed on an upper surface portion for forming the
 second chamber 20, of the upper surface of the first solid electrolyte
 layer 12c, the reference electrode 44 formed on the lower surface of the
 first solid electrolyte layer 12c, and the first solid electrolyte layer
 12c interposed between the both electrodes 92, 44.
 In this embodiment, an electromotive force (electromotive force of an
 oxygen concentration cell) V2 corresponding to the difference in oxygen
 concentration between the atmosphere around the detecting electrode 92 and
 the atmosphere around the reference electrode 44 is generated between the
 reference electrode 44 and the detecting electrode 92 of the measuring
 oxygen partial pressure-detecting cell 90.
 Therefore, the partial pressure of oxygen in the atmosphere around the
 detecting electrode 92, in other words, the partial pressure of oxygen
 defined by oxygen produced by reduction or decomposition of the
 measurement gas component (NOx) is detected as a voltage value by
 measuring the electromotive force (voltage V2) generated between the
 detecting electrode 92 and the reference electrode 44 by using a voltmeter
 94.
 The NOx concentration-measuring apparatus 10a according to this modified
 embodiment also uses the cermet electrode composed of the alloy of Pt--Rh
 and the ceramic component, as the detecting electrode 92 for constructing
 the measuring oxygen partial pressure-detecting cell 90. As a result, the
 oxidation and the reconversion into metal of Rh contained in the detecting
 electrode 92 are suppressed. Even when the operation time of the NOx
 concentration-measuring apparatus 10a is increased, the increase in
 impedance is not brought about, which would be otherwise caused by the
 decrease in contact area between the detecting electrode 92 and the first
 solid electrolyte layer 12c.
 Also in the NOx concentration-measuring apparatus 10a according to this
 modified embodiment, the ratio between Pt and Rh is Pt:Rh=20:80 to 1:99,
 preferably Pt:Rh=10:90 to 1:99 in weight ratio. Therefore, it is possible
 to suppress the adsorption of NOx at a low temperature and the formation
 of alloy together with any other metal element (for example, Au), and it
 is possible to remarkably improve the durability.
 It is a matter of course that the NOx-decomposing electrode and the NOx
 concentration-measuring apparatus according to the present invention are
 not limited to the embodiments described above, which may be embodied in
 other various forms without deviating from the gist or essential
 characteristics of the present invention.