Abstract:
A NO x  gas detecting apparatus including an oxygen pumping cell for removing oxygen from a measurement gas, and a NO x  detecting cell positioned downstream from the oxygen pumping cell to detect concentration of NO x  in the measurement gas, the NO x  detecting cell being configured to measure current which flows when oxygen generated from reducing NO x  is pumped, wherein the NO x  detecting cell has a NO x  detecting cathode made of an electrode material including at least one alloy selected from the group consisting of a Pt-Pd alloy, a Pt-Au-Pd alloy, and a Pt-Pd-Rh alloy.

Description:
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 no NO x  gas, which results in a problem in that data with repeatability are not obtained. Accordingly, it is difficult to detect a NO x  gas at a low concentration. 
     Further, the amount of oxygen gas adsorption to Rh greatly changes depending on oxygen concentration in the atmosphere and the lapse of time, and the rate of adsorption/desorption is rather slow. Due to these reasons, even if the measurement gas supplied to the NO x  gas detecting cell is controlled to be constant in its oxygen concentration by using the oxygen pumping cell, it is difficult to keep the Pt-Rh electrode always at a constant state under the conditions where oxygen concentration in the atmosphere fluctuates depending on driving state as in the case of automobiles. Hence, it is difficult to faithfully monitor the state in which abrupt changes occur in the combustion state and thus in the amount NO x  gas generated. 
     Also, this NO x  detecting apparatus is provided with a gas diffusion rate determining body at an upstream stage of the oxygen pumping cell. Therefore, when a measurement gas is supplied to the NO x  detecting cell and voltage applied between the electrodes is increased, a limiting current characteristic is exhibited, that is, the output current remains generally constant irrespective of applied voltage. The minimum voltage at which the output voltage becomes constant (hereinafter referred to as a “limiting current generating voltage”) differs depending on the types of oxygen bound gas. For example, exhaust gas contains H 2 O and CO 2  other than NO x  gas, and H 2 O and CO 2  are less decomposable than NO x  so that its limiting current generating voltage is higher than that of NO x . 
     Also, the limiting current generating voltage relative to oxygen bound gas differs depending on the composition of the electrodes. In the case of detecting concentration of NO x  in the exhaust gas using the NO x  detecting cell, the cathode of the NO x  detecting cell having a high limiting current generating voltage relative to NO x  makes it difficult to detect NO x  at a low concentration with high accuracy. This is because H 2 O and CO 2  are decomposed along with NO x  thereby to produce oxygen. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a NO x  gas detecting apparatus capable of detecting a NO x  gas accurately and stably over a long period of time independent of adherents to the cathode of the NO x  detecting cell, such as Au, impurities contained in exhaust gas, and the like. 
     Another object of the present invention is to provide a NO x  gas detecting apparatus which is excellent in startability, and which is capable of faithfully detecting abrupt fluctuations in the NO x  concentration. 
     Yet, another object of the present invention is to provide a NO x  gas detecting apparatus capable of accurately detecting NO x  at a low concentration even if the measurement gas contains other oxygen bound gases such as H 2 O, CO 2  and the like besides NO x . 
     To achieve the objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, in the present invention, a NO x  gas detecting apparatus comprises an oxygen pumping cell for removing oxygen from a measurement gas, a NO x  detecting cell for measuring NO x  contained in the measurement gas from which oxygen has been removed. The NO x  detecting cell comprises, as the cathode, an electrode composed of at least one selected from the group consisting of: a Pt-Pd alloy; a Pt-Au-Pd alloy; and a Pt-Pd-Rh alloy. 
     The NO x  gas detecting apparatus of the present invention configured as constructed above uses an alloy including Pd as the cathode of the NO x  detecting cell so that the activity relative to NO x  is enhanced drastically. In addition, the NO x  gas detecting apparatus allows the activity relative to NO x  not to be lowered even if Au or other impurities contained in exhaust gas adhere to the electrode. Further, the Pt-Pd alloy or the Pt-Au-Pd alloy is excellent in oxidation resistance and fast in the adsorption/desorption rate as compared with the Pt-Rh alloy. Therefore, the NO x  gas detecting apparatus is capable of detecting NO x  accurately and stably over a long period of time. Also, the NO x  gas detecting apparatus is excellent in startability and capable of faithfully detecting abrupt fluctuations in the NO x  concentration, and also excellent in responsivity and repeatability. Further, through optimization of the electrode composition, the NO x  gas reducing ability may be further improved as compared with the Pt electrode or the Pt-Rh electrode, thereby allowing a NO x  gas to be detected in a minute amount accurately. 
     In the case that the NO x  gas detecting apparatus comprises, as the cathode, an electrode composed of a Pt-Pd-Rh alloy, since Rh is further added to the Pt-Pd alloy, the limiting current generating voltage relative to a NO x  gas is lowered, and thus a NO x  gas may be discharged at a low voltage. As the result, even if the measurement gas contains oxygen bound gases (for example H 2 O) other than NO x , little influence is exerted on output current generated when NO x  is discharged, and therefore NO x  is detected at a low concentration with higher accuracy. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present invention and, together with the description, serve to explain the objects, advantages and principles of the invention. In the drawings, 
     FIG. 1 is a cross sectional view showing a NO x  detecting apparatus consistent with the present invention; 
     FIG. 2 is a view showing current-voltage characteristics with respect to an oxygen gas exhibited by oxygen pumping cells each having a Pt-Au electrode as a cathode; 
     FIG. 3 is a view showing current-voltage characteristics with respect to an oxygen gas+a NO gas exhibited by an oxygen pumping cell having a Pt-Au electrode as a cathode; 
     FIG. 4 is a view showing current-voltage characteristics with respect to a NO gas exhibited by NO x  detecting a cell having a Pt electrode or a Pt-Rh electrode as a cathode; 
     FIG. 5 is an oblique view showing an electrode examining cell; 
     FIG. 6 is a view showing current-voltage characteristics with respect to a NO gas exhibited by electrode examining cells each having, as a cathode, an electrode composed of Pt with addition of Au and various noble metals other than Au; 
     FIG. 7 is a view showing current-voltage characteristics with respect to a NO gas exhibited by electrode examining cells each having, as a cathode, a Pt-Au-Pd electrode with a different addition amount of Pd; 
     FIG. 8 is a view showing current-voltage characteristics with respect to a NO gas exhibited by electrode examining cells each having, as a cathode, a Pt-Au-Pd electrode with a different addition amount of Pd; 
     FIG. 9 is a current-voltage characteristics with respect to a NO gas exhibited by electrode examining cells each having, as a cathode, a Pt-Au-Pd electrode with a different addition amount of Au; 
     FIG. 10 is a view showing the relation between an addition amount of Pd and an addition amount of Au in order to obtain a Pt-Au-Pd alloy having reducing ability equal to or better than that of Pt; 
     FIG. 11 is a view showing the relation between a NO gas concentration and an output current of a NO x  gas detecting apparatus having a Pt-Pd electrode as a cathode of a NO x  detecting cell; 
     FIG. 12 is a view showing responsivity of a NO x  gas detecting apparatus having a Pt-Pd electrode or a Pt-Rh electrode as a cathode of a NO x  detecting cell; 
     FIG. 13 is a cross sectional view showing a NO x  gas detecting apparatus used in the experiments; 
     FIG. 14 is a view showing current-voltage characteristics exhibited by a detecting cell having, as a cathode, an electrode composed of Pt-10 wt % ZrO 2  (1 g)+Pd (0.3 g)+Rh (0.3 g) (Sample No. 45); 
     FIG. 15 is a view showing the relation between a compound amount of Rh and resistance of a detecting cell; 
     FIG. 16 is a view showing the relation between a compound amount of Rh and a peak current; 
     FIG. 17 is a view showing current-voltage characteristics exhibited before conducting the heat treatment by the detecting cell having, as a cathode, an electrode composed of Pt-10 wt % ZrO 2  (1 g)+Pd (0.3 g)+Rh (0.3 g) (Sample No. 45); 
     FIG. 18 is a view showing current-voltage characteristics exhibited after conducting the heat treatment by the detecting cell having, as a cathode, an electrode composed of Pt-10 wt % ZrO 2  (1 g)+Pd (0.3 g)+Rh (0.3 g) (Sample No. 45); 
     FIG. 19 is a view showing the relation between a composition amount of Rh in a Pt-Pd-Rh electrode and a falling rate of output current due to the heat treatment; 
     FIG. 20 is a view showing relation between a composition amount of Rh in the Pt-Pd-Rh electrode and a limiting current generating voltage after the heat treatment; 
     FIG. 21 is a view showing relation between an output current of a NO x  detecting apparatus having a Pt-Pd-Rh electrode as a cathode and a NO gas concentration; and 
     FIG. 22 is a view showing responsivity of a NO x  detecting apparatus having a Pt-Pd electrode or a Pt-Pd-Rh electrode as a cathode of the NO x  detecting cell. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detailed description of preferred embodiments of the present invention will now be given referring to the accompanying drawings. In FIG. 1, there is shown a cross section of a NO x  gas detecting apparatus (hereinafter referred to as a “detecting apparatus”) consistent with a first preferred embodiment of the present invention. In FIG. 1, the detecting apparatus  10  is constructed of solid electrolytes  12   a - 12   h , insulating layers  14   a ,  14   b ,  14   c , each of which are disposed between the solid electrolytes  12   d  and  12   e,  between the solid electrolytes  12   d  and  12   f,  and between the solid electrolytes  12   c  and  12   g,  respectively, and an insulating layer  40   a  disposed at the undermost layer. 
     Disposed inside the detecting apparatus  10  is a gas introducing chamber  16  and a gas discharging chamber  18 . The gas introducing chamber  16  is a closed space provided with a gas introducing section  16   a  from which a measurement gas is introduced thereinto by gas diffusion. Inner walls of the gas introducing chamber  16  is constructed of the solid electrolytes  12   b,    12   c,    12   f  and  12   d.  Also, the gas discharging chamber  18  is a closed space provided with a gas discharging section  18   a  for discharging an oxygen gas which has been discharged thereinto through the solid electrolytes  12   d  and  12   f.  Inner walls of the gas discharging chamber  18  is constructed of the solid electrolytes  12   d,    12   e,    12   f,    12   g  and  12   h.    
     In addition, the detecting apparatus  10  comprises an oxygen gas supplying cell  20 , an oxygen pumping cell  24 , an oxygen reference electrode generating cell  28 , an oxygen monitoring cell  32 , a NO x  detecting cell  36 , and a heater section  40 . 
     The oxygen gas supplying cell  20  comprises the solid electrolyte  12   a  as well as an anode  22   a  and a cathode  22   b  disposed on each side thereof. The anode  22   a  and the cathode  22   b  are connected to a power source (not illustrated) for applying a predetermined voltage. The oxygen gas supplying cell  20  is a cell for supplying oxygen to the measurement gas introduced into the gas introducing chamber  16 , and is disposed in the vicinity of the gas introducing section  16   a.  The anode  22   a  and the cathode  22   b  are disposed on the inner wall of the gas introducing chamber  16  and the outer wall of the detecting apparatus  10 , respectively. 
     The oxygen pumping cell  24  comprises the solid electrolyte  12   d  as well as an anode  26   a  and a cathode  26   b  disposed on each side thereof. The anode  26   a  and the cathode  26   b  are connected to a power source (not illustrated) for applying a predetermined voltage. The oxygen pumping cell  24  is a cell for selectively discharging, to the gas discharging chamber  18 , nothing but oxygen that exists in the measurement gas which has been introduced into the gas introducing chamber  16 . The oxygen pumping cell  24  is disposed at a downstream stage of the oxygen supplying cell  20 . The anode  26   a  and the cathode  26   b  are disposed on the inner wall of the gas discharging chamber  18  and the inner wall of the gas introducing chamber  16 , respectively. 
     The oxygen reference electrode generating cell  28  comprises the solid electrolyte  12   b  as well as an anode  30   a  and a cathode  30   b  disposed on each side thereof. The anode  30   a  and the cathode  30   b  are connected to a power source (not illustrated) for applying a predetermined voltage. The oxygen reference electrode generating cell  28  is a cell for forming a reference electrode that will be necessary upon measuring an amount of residual oxygen remaining in the measurement gas after the oxygen pumping cell  24  has removed oxygen therefrom. The oxygen reference electrode generating cell  28  is disposed at a downstream stage of the oxygen pumping cell  24 . The anode  30   a  is disposed at the interface between the solid electrolytes  12   a  and  12   b , and the cathode  30   b  is disposed on the inner wall of the gas introducing chamber  16 . Further, the solid electrolyte  12   b  is provided with an oxygen discharging exit  28   a  adjacent to the oxygen reference electrode generating cell  28  so that the oxygen collected around the anode  30   a  flows back into the gas introducing chamber  16 . 
     The oxygen monitoring cell  32  comprises the solid electrolyte  12   b,  the electrode  34   a  disposed on the inner wall of the gas introducing chamber  16 , and the anode  30   a.  In other words, the oxygen monitoring cell  32  is constructed to share the anode  30   a  with the oxygen reference electrode generating cell  28 . The electrode  34   a  and the anode  30   a  are connected to a voltmeter (not illustrated) for measuring potential difference which occurs between the two electrodes. The oxygen monitoring cell  32  is a cell for measuring an amount of residual oxygen remaining in the measurement gas with the anode  30   a  of the oxygen reference electrode generating cell  28  as the reference electrode, and is disposed in adjacent to the oxygen reference electrode generating cell  28 . 
     The NO x  detecting cell  36  comprises the solid electrolyte  12   f  as well as an anode  38   a  and a cathode  38   b  disposed on each side thereof. The anode  38   a  and the cathode  38   b  are connected to a power source (not illustrated) for applying a predetermined voltage, and to an ammeter (not illustrated) for measuring current flows between the two electrodes. The NO x  detecting cell  36  is a cell for decomposing a NO x  gas contained in the measurement gas to measure an amount of oxygen generated at that time, and is disposed at a downstream stage of the oxygen pumping cell  24  adjacent thereto. The anode  38   a  is disposed on the inner wall of the gas discharging chamber  18 , while the cathode  38   b  is disposed on the inner wall of the gas introducing chamber  16 . 
     The heater section  40  is constructed of a heater  40   b  made from a metal and a ceramic component and disposed within an insulation layer  40   a.  The heater section  40  is used to heat the solid electrolytes  12   a ,  12   b,    12   d  and  12   f  until they exhibit an oxygen pumping action, or to keep the detecting apparatus  10  at a predetermined temperature. In the example shown in FIG. 1, the heater section  40  is disposed at a lower edge of the detecting apparatus  10 . 
     Here, a material for forming the solid electrolytes  12   a - 12   h  does not have to be limited to any specific one as long as it exhibits oxygen ion conductivity. Zirconia-base solid electrolytes (ZrO 2 -M 2 O 3  solid solution or ZrO 2 -MO solid solution, provided M=Y, Yb, Gd, Mg or the like), ceria-base solid electrolytes (CeO 2 -M 2 O 3  solid solution or CeO 2 -MO solid solution, provided M=Y, Sm or the like), and bismuth oxide-base solid electrolytes (Bi 2 O 3 -WO 3  solid solution or the like) are some suitable examples. 
     Especially, zirconia-base solid electrolytes are suitable as a material for the solid electrolytes  12   a - 12   h  in view of the stability in exhaust gas. Also, ZrO 2  with addition of Y 2 O 3  in an amount of 5 to 8 mol % has excellent thermal impact resistance and high oxygen ion conductivity so that it is especially suitable as a material for the solid electrolytes  12   a - 12   h.    
     The insulating layers  14   a ,  14   b  and  14   c  are used to electrically separate the oxygen pumping cell  24  and the NO x  detecting cell  36 . Accordingly, a suitable material for the insulating layers  14   a ,  14   b  and  14   c  has high insulation resistance at the temperature at which the solid electrolytes  12   a - 12   h  exhibit the oxygen pumping action. Alumina, spinel, mullite, cordierite, and the like are some suitable examples. 
     Further, regarding the electrodes used in the NO x  detecting cell  36 , at least the cathode  38   b  needs to be a cermet electrode composed of a Pt-Pd alloy or a Pt-Au-Pd alloy with a ceramic component (hereinafter, the former is referred to as a “Pt-Pd electrode” and the latter is referred to as a “Pt-Au-Pd electrode”). 
     When a Pt-Pd alloy or a Pt-Au-Pd alloy is used to construct part of the cathode  38   b,  it is preferred that an amount of Pd added to Pt (=100×Pd/(Pt+Pd)) is 1 wt % or more. If Pd is added to Pt in an amount less than 1 wt %, the cathode  38   b  becomes less active relative to NO x , which is not desirable. In addition, in order to obtain the cathode  38   b  which has activity equally high, or higher than a conventionally used Pt electrode or Pt-Rh electrode, it is preferred that an amount of Pd added to Pt be 90 wt % or less. More preferably, Pd is added in an amount of 5 to 40 wt %. 
     Also, when a Pt-Au-Pd alloy is used to construct the cathode  38   b,  by making a weight ratio of Pd to Au (hereinafter referred to as a “Pd/Au ratio”) 1.67 or more, the thus obtained electrode becomes more active relative to NO x  as compared with the Pt-Au electrode which does not contain Pd. By making the Pd/Au ratio 6.67 or more, an electrode of which activity relative to NO x  is generally equal to that of the Pt electrode or the Pt-Rh electrode is obtained. Further, if the addition amount of Pd and the addition amount of Au in the Pt-Au-Pd alloy are set within the diagonally shaded area shown in FIG. 10, an electrode of which activity relative to NO x  is equally high, or higher than the Pt electrode or Pt-Rh electrode is obtained. 
     The ceramic component constructing other part of the cathode  38   b  is added in order to enhance adherence between the cathode  38   b  and the solid electrolyte 12 f.  Accordingly, the composition and the addition amount may be decided arbitrarily as long as good adherence is obtained between them. Here, if the addition amount of ceramic is too large, the conductivity of the cathode  38   b  is undesirably lowered. Normally, a ceramic component having the same composition with the solid electrolyte 12 f  is added to the cathode  38   b  in a degree of 10 to 20 wt %. 
     Here, a material forming the other electrodes does not have to be limited to any specific one as long as it exhibits high activity relative to an oxygen gas and an oxygen bound gas. Yet, in order to suppress the decomposition of NO x  gas, the cathode  26   b  of the oxygen pumping cell  24  needs to be composed of a material having high activity relative to an oxygen gas but no or low activity relative to a NO x  gas. A Pt-Au electrode is one suitable example. Due to the same reasons, it is required that electrodes exhibiting high activity only relative to an oxygen gas be used as the anode  30   a  and the cathode  30   b  of the oxygen reference electrode generating cell  28 , and also as the electrode  34   a  of the oxygen monitoring cell  32 . 
     The insulating layer  40   a  of the heater section  40  is constructed of a material having high insulation resistance at a temperature at which the solid electrolytes  12   a - 12   h  exhibit an oxygen pumping action. Alumina, spinel, mullite, cordierite, and the like are some suitable examples. The heater 40 b  is normally composed of metal with excellent oxidation resistance (for example, Pt and the like) and a ceramic component. 
     Next, description is given to a general scheme to detect a NO x  gas contained in exhaust gas using the detecting apparatus  10  shown in FIG.  1 . First, the whole body of the detecting apparatus  10  is placed in exhaust gas. Then, the detecting apparatus  10  is heated using the heater section  40  such that it is maintained at a temperature at which the solid electrolytes  12   a - 12   h  exhibit an oxygen pumping action (for example 700° C.). In this case, the measurement gas diffuses and flows into the gas introducing chamber  16  through the gas introducing section  16   a.    
     Under this condition, a predetermined voltage (for example, from 0.3 to 0.6 V) is applied between the anode  22   a  and the cathode  22   b  of the oxygen gas supplying cell  20 . Here, in the case of exhaust gas in a lean atmosphere (an excess oxygen atmosphere), oxygen contained in the exhaust gas outside the detecting apparatus  10  is supplied to the gas introducing chamber  16  through an oxygen pumping action of the solid electrolyte  12   a . On the contrary, in the case of exhaust gas in a rich atmosphere (an excess fuel atmosphere), the exhaust gas contains little oxygen but usually contains H 2 O gas of 5 to 10%. Consequently, this H 2 O gas is decomposed around the cathode  22   b  and the oxygen generated through the decomposition is supplied to the gas introducing chamber  16  by oxygen pumping action. 
     A NO x  gas has a characteristic that it is apt to decompose into a nitrogen gas and an oxygen gas when heated at a high temperature under a low oxygen atmosphere. Accordingly, in order to accurately detect a NO x  gas contained in the measurement gas at a low concentration, decomposition of the NO x  gas needs to be suppressed until the measurement gas reaches the NO x  detecting cell  36 . In the detecting apparatus  10  consistent with this preferred embodiment, the oxygen gas supplying cell  20  supplies oxygen into the gas introducing chamber  16 . Therefore, even if the atmosphere of the exhaust gas changes to lean or to rich, inside the gas introducing chamber  16  is controlled to be a lean atmosphere at all times, thereby suppressing decomposition of NO x  gas. 
     Thereafter, a mixture gas of the measurement gas flowing through the gas introducing section  16   a  and the oxygen gas supplied from the oxygen gas supplying cell  20  diffuses and reaches the oxygen pumping cell  24 . The cathode  26   b  of the oxygen pumping cell  24  is formed of a material that exhibits high oxygen reducing ability but no NO x  reducing ability at a predetermined voltage (for example, 0.3 V) or less. Consequently, when an adequate voltage is applied between the anode  26   a  and the cathode  26   b,  nothing but an oxygen gas contained in the measurement gas is selectivity discharged to the gas discharging chamber  18  by an oxygen pumping action. 
     In addition, the discharge state of oxygen at this time is monitored by the oxygen reference electrode generating cell  28  and the oxygen gas monitoring cell  32 . That is, when a predetermined voltage (for example, 0.5 V) is applied between the anode  30   a  and the cathode  30   b  of the oxygen reference electrode generating cell  28 , oxygen gas contained in the measurement gas which exists in the gas introducing chamber  16  is collected to the side of anode  30   a  by the oxygen pumping action. Since the anode  30   a  is disposed at the interface between the solid electrolytes  12   a  and  12   b,  the side of anode  30   a  is assumed to be in an oxygen atmosphere of almost 100%. Consequently, by measuring a potential difference generated between the anode  30   a  and the electrode  34   a  of the oxygen monitoring cell  32  with the anode  30   a  as the reference electrode, it is possible to measure the partial pressure of oxygen in the measurement gas that has passed through the oxygen pumping cell  24 . 
     In the actual measurement of a NO x  gas concentration, it is preferred that an applied voltage to the oxygen pumping cell  24  be controlled such that an electromotive force of the oxygen monitoring cell  32  is made constant (for example, 0.3 V). This makes it possible to supply the measurement gas to be supplied to the NO x  detecting cell  36  such that the measuring gas contains no oxygen gas or oxygen gas at a certain concentration. 
     Then, the oxygen gas which has been controlled its oxygen concentration diffuses and reaches the NO x  detecting cell  36 . Here, when a predetermined voltage (for example, 0.5 V) is applied to the anode  38   a  and the cathode  38   b  of the NO x  detecting cell  36 , the NO x  gas first adheres to the cathode  38   b , and then decomposes at the interface between the cathode  38   b  and the solid electrolyte  12   f.  Oxygen generated by the NO x  gas decomposition is discharged to the gas discharging chamber  18  by an oxygen pumping action of the solid electrolyte  12   f.  At this time, current flows between the anode  38   a  and the cathode  38   b  in proportion to the concentration of the NO x  gas. Hence, by measuring the value of the current with a not illustrated ammeter, the NO x  gas concentration is measured as well. 
     Here, in the case where an oxygen gas contained in the measurement gas has been completely removed by the oxygen pumping cell  24 , the NO x  gas concentration may be known directly from the value of the current flowing through the NO x  detecting cell  36 . On the other hand, in the case where the measurement gas supplied to the NO x  detecting cell  36  has been controlled to have a certain oxygen concentration value, the current value flowing through the NO x  detecting cell  36  is corrected using a value of residual oxygen concentration detected by the oxygen monitoring cell  32 , whereby the NO x  gas concentration contained in the measurement gas is detected with high accuracy. 
     Next, description is given to operations of the detecting apparatus  10  consistent with this preferred embodiment of the present invention. As the cathode  26   b  of the oxygen pumping cell  24 , generally, a Pt-Au electrode which is active relative to oxygen and inactive relative to NO x  is used. FIG. 2 shows current-voltage characteristics with respect to an oxygen gas exhibited by the oxygen pumping cell  24  having a Pt-Au electrode as the cathode  26   b.  FIG. 2 shows the results obtained when the oxygen pumping cell  24  was heated to 800° C. and measurement gases with different oxygen concentrations were supplied. From FIG. 2, it is apparent that the Pt-Au electrode exhibited the limiting current characteristic with respect to oxygen: That is, the output current increased in proportion to the voltage and then became saturated. It is also apparent that a high output current was obtained at a relatively low voltage. These findings show that the Pt-Au electrode has high activity relative to oxygen. 
     Further, FIG. 3 shows current-voltage characteristics with respect to a NO gas exhibited by the oxygen pumping cell  24  having a Pt-Au electrode as the cathode  26   b.  FIG. 3 also shows the results obtained when the oxygen pumping cell  24  was heated to 800° C. and the measurement gases containing oxygen of 0.1% and NO gases of different concentrations were supplied. From FIG. 3, it is apparent that the Pt-Au electrode exhibited the limiting current characteristic with respect to the NO x  concentration: That is, when the voltage exceeded 0.3 V, the output current became saturated. It is also apparent that the output current gradually increased as the voltage increased. These findings show that the Pt-Au electrode has low activity relative to a NO x  gas. 
     Unlike a Pt-Au electrode, on the other hand, a Pt electrode or a Pt-Rh electrode is known to have high activity relative to a NO x  gas. FIG. 4 shows current-voltage characteristics with respect to a NO gas exhibited by the NO x  detecting cell  36  having a Pt electrode or a Pt-Rh electrode as the cathode  38   b.  FIG. 4 shows the results obtained when the detecting apparatus  10  comprising the NO x  detecting cell  36  was heated to 800° C. and measurement gases containing oxygen of 1% and NO gasses of various concentrations were supplied to the detecting apparatus  10  while applying a voltage of 0.3 V to the oxygen pumping cell  24 . From FIG. 4, it is apparent that the Pt electrode or the Pt-Rh electrode exhibited the limiting current characteristic with respect to a NO x  gas when the voltage exceeded about 0.3 V. That is, the output current increased in proportion to the voltage and became saturated. It is also apparent that a high output current was obtained at a relatively low voltage. 
     Accordingly, the Pt electrode or the Pt-Rh electrode naturally has high NO x  reducing ability as shown in FIG.  4 . However, if Au adheres thereto, the NO x  reducing ability lowers drastically and characteristics as shown in FIG. 3 is exhibited. As the result, it becomes difficult to detect a NO x  at a low concentration with high accuracy. 
     The same phenomena occurs in the case when impurities in exhaust gas adhere to the Pt electrode or the Pt-Rh electrode. Such decrease in the reducing ability is considered to occur due to the following reason. That is, Au or impurities in exhaust gas adhere to the surface of Pt, whereby NO x  adsorption/desorption ability of the Pt surface lowers, and thus the NO x  reducing ability is lowered. 
     On the contrary, if a Pt-Pd electrode or a Pt-Au-Pd electrode is used as the cathode  38   b  of the NO x  detecting cell  36 , it is possible to drastically restrict lowering of the NO x  gas reducing ability due to adherents to the Pt surface, such as Au, impurities in exhaust gas, and the like. Also, depending on the alloy component, it is possible to obtain an electrode with higher NO x  reducing ability than that of Pt electrode. This is considered that because Pd having a higher NO x  adsorption ability is added to Pt, the electrode adsorbs NO x  gas more easily. 
     Further, the Pt-Pd alloy or the Pt-Au-Pd alloy is less oxdizable and fast in the oxygen gas adsorption/desorption as compared with the Pt-Rh alloy. Therefore, it does not occur that an oxygen gas is discharged from inside the electrode gradually right after activation. Accordingly, the detecting apparatus  10  comprising a Pt-Pd electrode or a Pt-Au-Pd electrode as the cathode  38   b  of the NO x  detecting cell  36  is excellent in the startability and capable of detecting a NO x  gas at a low concentration with high accuracy. In addition, this detecting apparatus  10  is capable of faithfully monitoring even in the condition where abrupt change occurs in the combustion state or the NO x  concentration. 
     Hereinafter, description is given to a detecting apparatus consistent with a second preferred embodiment of the present invention. The detecting apparatus consistent with this preferred embodiment comprises, as a cathode of a NO x  detecting cell, a cermet electrode composed of a Pt-Pd-Rh alloy and a ceramic component (hereinafter referred to as a “Pt-Pd-Rh electrode”). 
     Here, when a Pt-Pd-Rh alloy is used to construct part of the cathode, it is preferred that an amount of Pd added to Pt (=100×Pd/(Pt+Pd)) be 1 wt % or more. If the addition amount of Pd is less than 1 wt %, the cathode becomes less active relative to NO x , which is undesirable. In addition, in order to obtain the cathode having activity equally high, or higher than a conventionally used Pt electrode or Pt-Rh electrode, it is preferred that an amount of Pd added to Pt be 90 wt % or less. More preferably, Pd is added in an amount of 5 to 40 wt %. 
     Also, when the Pt-Pd-Rh alloy is used, it is preferred that an addition amount of Rh (=100×Rh/(Pt+Pd+Rh)) be 30 wt % or less. If an addition amount of Rh exceeds 30 wt %, the electrode resistivity and the cell resistivity increase, and thus the limiting current generating voltage becomes high, which is nor desirable. In addition, an amount of oxygen gas adheres to the electrode increases, thereby undesirably causing the startability of the detecting cell to be decreased. 
     Also, the Pt-Pd-Rh electrode may be used as it is sintered to the solid electrolytes, Yet, it is preferred that heat treatment be conducted as follows after sintering the electrodes. That is, the electrodes are heated at a predetermined temperature for a predetermined period of time in an atmosphere while applying a predetermined voltage between the electrodes. The heat treatment as above achieves an effect of lowering the limiting current generating voltage of the Pt-Pd-Rh electrode relative to NO x  gas. The most suitable heat treatment condition differs depending on the electrode component, yet, for example, the following condition achieves a sufficient effect: the heat treatment temperature of 900° C., the applied voltage of 1 V, and the heating time of about 10 minutes. 
     The ceramic component constructing other part of the cathode is added in order to enhance adherence between the cathode and the solid electrolyte. Accordingly, the composition and the addition amount may be decided arbitrary as long as good adherence is obtained between them. Normally, a ceramic component having the same composition with the solid electrolyte onto which the cathode is sintered is added to in a degree of 10 to 20 wt %. In these two points, the detecting apparatus of this embodiment is the same as the detecting apparatus  10  of the first preferred embodiment. In addition, the detecting apparatus has the same configuration as the detecting apparatus  10  of the first preferred embodiment other than the cathode of the detecting cell, so description thereof is omitted. 
     Next, description is given to operation of the detecting apparatus of this preferred embodiment. The adsorption/desorption of an oxygen bound gas, the dissociation of oxygen, and the ability to ionize oxygen differ depending on the electrode material. That is, Pt is a metal, which is weak in the NO x  adsorption, but strong in the dissociation of oxygen from the NO x  gas and the ability to ionize oxygen. Pd is a metal, which is strong in the NO x  adsorption ability, but weak in the dissociation of the NO x  gas, and also low in the ability to ionize oxygen. On the other hand, Rh is a metal which is the strongest in the NO x  adsorption among noble metals, and the dissociation of the NO x  gas and the ability to ionize oxygen is low. 
     Accordingly, an electrode composed solely of Pt is weak in the NO x  adsorption so that adherents have great influence. Yet, if Pd which has strong NO x  adsorption is added to Pt, influence of adherents is reduced. In addition, the NO x  which adheres to the electrode is dissociated into nitrogen and oxygen, and the oxygen dissociated therefrom is ionized. As the result, a Pt-Pd electrode has high NO x  reducing ability than that of a Pt electrode. 
     However, the limiting current generating voltage relative to an oxygen bound gas differs depending on the electrode component and the type of the oxygen bound gas. For example, in the case of the Pt-Pd electrode described above, the limiting current generating voltage relative to a NO x  gas is about 0.3 V, which is relatively high. The exhaust gas contains oxygen bound gases, other than NO x , such as H 2 O, CO 2  and the like in a large amount. Generally, the limiting current generating voltage with respect to H 2 O or CO 2  is higher than that of NO x . Accordingly, in the case where the limiting current generating voltage of the electrode with respect to NO x  is high, not only NO x  but also H 2 O, CO 2  and the like are discomposed simultaneously thereby to produce oxygen. When detecting a NO x  gas at a low concentration contained in exhaust gas, the thus produced oxygen will be an measurement error factor, which is not negligible. 
     On the contrary, Rh adsorbs a NO x  gas and more strongly. Hence, adding Rh to the Pd electrode even in a small amount, the NO x  reducing ability is sufficiently enhanced. As the result, the limiting current generating voltage of the Pt-Pd-Rh electrode relative to a NO x  gas becomes lower than that of the Pt-Pd electrode. Therefore, influence exerted by decomposition of H 2 O and CO 2  on the output current is reduced so that NO x  is measured accurately at a low concentration. 
     Thereafter, heat treatment may be conducted as follows. That is, after the Pt-Pd-Rh electrode has been sintered to the surface of the solid electrolyte, it is heated in an atmosphere while applying voltage. By conducting the heat treatment, the limiting current generating voltage relative to a NO x  gas further shifts toward a lower voltage. It is considered that by heating the electrode while applying a voltage, a thin oxide film is formed on the surface of the Pt-Pd-Rh electrode, and this oxide film controls the diffusion rate of oxygen. 
     WORKING EXAMPLE 1 
     A sample of electrode paste was made by adding Au and other noble metal powder to Pt in order to seek for a electrode material of which NO x  gas reducing ability does not decrease even when Au is adhered to Pt. 
     First, ZrO 2  green sheets were made in the following procedures. That is, METOLOSE (manufactured by Shin-Etsu Chemical Co., Ltd.) was added to 8Y-ZrO 2  powder (manufactured by TOSOH Corporation) as binder in an amount of 4 wt %, and then stirred and mixed. Next, glycerine of 1.5 wt % and water of 2.1 wt % were added and further mixed. The thus obtained ceramic paste was subjected to a kneading process repeatedly for 5 to 6 times. Then, the ceramic paste was molded by extrusion molding using a molding machine with dies attached thereto, whereby ZrO 2  green sheets each having a 1.0 mm thickness were obtained. 
     Next, electrode paste having predetermined composition was printed on the front surface (a measurement electrode) and the rear surface (a reference electrode) of the ZrO 2  green sheets by screen printing. Then, they were sintered in an atmosphere under the condition of 1,430° C. for one hour to make cells for examining electrodes. The electrode examining cells had a shape as shown in FIG. 5, and the area of the electrode was 3×4 mm. Further, the component of the electrode paste used for constructing the measurement electrode and the reference electrode is shown in Table 1. 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                  Sample 
                   
                 Reference 
               
               
                 No. 
                 Measurement Electrode 
                 Electrode 
               
               
                   
               
             
             
               
                  1 
                  Pt-10 wt % ZrO 2   
                 Pt-10 wt % ZrO 2   
               
               
                 2 
                 Pt-10 wt % ZrO 2 -3 wt % Au 
                 Pt-10 wt % ZrO 2   
               
               
                 3 
                 Pt-10 wt % ZrO 2 -3 wt % Au-10 wt % Ru 
                 Pt-10 wt % ZrO 2   
               
               
                 4 
                 Pt-10 wt % ZrO 2 -3 wt % Au-10 wt % Ir 
                 Pt-10 wt % ZrO 2   
               
               
                 5 
                 Pt-10 wt % ZrO 2 -3 wt % Au-10 wt % Pd 
                 Pt-10 wt % ZrO 2   
               
               
                 6 
                 Pt-10 wt % ZrO 2 -3 wt % Au-10 wt % Rh 
                 Pt-10 wt % ZrO 2   
               
               
                   
               
             
          
         
       
     
     To the thus obtained electrode examining cells, the measurement gas of which gas composition: 0.15%NO/N 2 , gas temperature: 700° C., and gas flow rate: 2 L per minute was supplied in order to examine the current-voltage characteristics. The results are shown in FIG.  6 . Here, in FIG. 6, the first quadrant shows characteristics of the measurement electrode relative to a NO x  gas, and the third quadrant shows characteristics of the reference electrode relative to a NO x  gas. 
     From the current-voltage characteristics shown in the first quadrant of FIG. 6, it is apparent that in the case of the sample No.1 (the electrode paste of the Pt-10 wt %ZrO 2  composition), the current flowed in proportion to the voltage and the pumping current was large. On the other hand, in the case of the sample No.2 (the electrode paste of the Pt-3 wt % Au-10 wt %ZrO 2  composition), the current did not increase abruptly until the applied voltage was in the vicinity of 0.6 V. This result shows that Pt had high NO reducing ability by itself, but by adding Au to Pt, the NO reducing ability was decreased drastically. 
     The sample No. 3, which was made by further adding Ru to the electrode paste of the sample No. 2, the sample No. 4, which was made by further adding Ir to the electrode paste of the sample No. 2, and the sample No. 6, which was made by further adding Rh to the electrode paste of the sample No. 2, all exhibited the generally identical current-voltage characteristics to the sample No. 2. The reason that the sample No.3 and the sample No. 4 exhibited the characteristics as shown in FIG. 6 is assumed as follows. That is, Ir and Rh added respectively thereto might have been scattered away upon sintering and there was no residuals left in the electrodes. Further, the sample No. 6, which was made by adding Rh exhibited hysteresis, a line that did not pass through the origin. This indicates that, similarly to the Pt-Au electrode (sample No. 2), Au which adhered to the electrode hindered the pumping current from flowing, and Rh which was added thereto caused nitrogen gas accumulated at the electrode to be emitted. 
     On the contrary, the sample No. 5, which was made by further adding Pd to the electrode paste of the sample No. 2 exhibited the most similar current-voltage characteristics to the sample No. 1. This result shows that the NO reducing ability was lowered due to the addition of Au but recovered by further adding Pd to the Pt-Au electrode. 
     WORKING EXAMPLE 2 
     In order to examine an appropriate amount of Pd to be added to a Pt-Au electrode to enhance NO x  gas reducing ability, electrode examining cells were made by further adding Pd in an amount of 0 to 0.5 g to electrode paste of 1 g having a composition of Pt-3 wt % Au-10 wt %ZrO 2 . The procedures for making the electrode examining cells were the same as the working example 1. Further, the component of the electrode paste used herein is shown in Table 2. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                  Sample 
                  Measurement Electrode 
                 Pd Addition 
                 Reference 
               
               
                 No. 
                 Component (1 g) 
                 Amount (g) 
                 Electrode 
               
               
                   
               
             
             
               
                  11 
                  Pt-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 12 
                 Pt-3 wt % Au-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 13 
                 Pt-3 wt % Au-10 wt % ZrO 2   
                 0.05 
                 Pt-10 wt % ZrO 2   
               
               
                 14 
                 Pt-3 wt % Au-10 wt % ZrO 2   
                 0.10 
                 Pt-10 wt % ZrO 2   
               
               
                 15 
                 Pt-3 wt % Au-10 wt % ZrO 2   
                 0.20 
                 Pt-10 wt % ZrO 2   
               
               
                 16 
                 Pt-3 wt % Au-10 wt % ZrO 2   
                 0.50 
                 Pt-10 wt % ZrO 2   
               
               
                 17 
                 Pd-3 wt % Au-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                   
               
             
          
         
       
     
     The thus obtained electrode examining cells were examined for the current-voltage characteristics under the same condition as the working example 1. The results are shown in FIG.  7 . Here, in FIG. 7, the first quadrant shows characteristics of the measurement electrode relative to a NO x  gas, and the third quadrant shows characteristics of the reference electrode relative to a NO x  gas. 
     From the current-voltage characteristics shown in the first quadrant of FIG. 7, the following is apparent. That is, in the case of the sample No. 13 made by adding Pd in an amount of 0.05 g to the electrode paste of 1 g having composition of Pt-3 wt % Au-10 wt % ZrO 2  (Pd/Au ratio=1.67), the output current was higher than that of the sample No. 12 (composition: Pt-3 wt % Au-10 wt % ZrO 2 ) having no Pd when the same voltage was applied. As the addition amount of Pd increased, the current changed more linearly with the voltage (the characteristics determined by resistance of the zirconia electrolyte), and the voltage required for NO gas reducing decreased, which meant enhancement of NO reducing ability. 
     Specifically, the sample No. 15, which was made by adding Pd in an amount of 0.2 g (Pd/Au ratio=6.67) exhibited the generally same characteristics as the Pt electrode (the sample No. 11). The sample No. 16, which was made by adding Pd in an amount of 0.5 g (Pd/Au ratio=16.67) exhibited better NO reducing ability than that of the Pt electrode. On the other hand, the sample No. 17 having composition of Pd-3 wt % Au-10 wt % ZrO 2  (Pd/Au ratio=29.41) merely exhibited the generally same current-voltage characteristics as the Pt electrode. 
     WORKING EXAMPLE 3 
     Assuming that Au which scatters and adheres to the Pt electrode of the NO x  detecting cell  36  lowers NO x  reducing ability, the amount of Au adheres is considered to be 0.1 wt % or less. Here, electrode examining cells were made by further adding Pd in an amount of 0 to 0.2 g to the electrode paste of 1 g having composition of Pt-0.1 wt % Au-10 wt %ZrO 2 . The procedures for making the electrode examining cell were the same as the working example 1. Further, the component of the electrode paste used herein is shown in Table 3. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                  Sample 
                  Measurement Electrode 
                 Pd Addition 
                 Reference 
               
               
                 No. 
                 Component (1 g) 
                 Amount (g) 
                 Electrode 
               
               
                   
               
             
             
               
                  21 
                  Pt-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 22 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 23 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                  0.005 
                 Pt-10 wt % ZrO 2   
               
               
                 24 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                  0.01 
                 Pt-10 wt % ZrO 2   
               
               
                 25 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                  0.05 
                 Pt-10 wt % ZrO 2   
               
               
                 26 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                 0.1 
                 Pt-10 wt % ZrO 2   
               
               
                 27 
                 Pt-0.1 wt % Au-10 wt % ZrO 2   
                 0.2 
                 Pt-10 wt % ZrO 2   
               
               
                   
               
             
          
         
       
     
     The thus obtained electrode examining cells were examined for the current-voltage characteristics under the same condition as the working example 1. The result are shown in FIG.  8 . Here, in FIG. 8, the first quadrant shows characteristics of the measurement electrode relative to a NO x  gas, and the third quadrant shows characteristics of the reference electrode relative to a NO x  gas. 
     From the current-voltage characteristics shown in the first quadrant of FIG. 8, the following is apparent. That is, in the case of the sample No. 23 made by adding Pd in an amount of 0.005 g to the electrode paste of 1 g having composition of Pt-0.1 wt % Au-10 wt % ZrO 2  (Pd/Au ratio=5.0), the output current was higher than that of the same No. 22 having no Pd (composition: Pt-0.1 wt % Au-10 wt % ZrO 2 ). As the addition amount of Pd increased, the current changed more linearly with the voltage (the characteristics determined by resistance of the zirconia electrolyte), and the voltage required for NO gas reducing decreased, which meant enhancement of the NO x  reducing ability. 
     Specifically, it was found that the sample Nos. 24-27, which were made by adding Pd in an amount of 0.1 g or more (Pd/Au ratio≧10.0) exhibited NO x  reducing ability generally the same as, or better than the Pt electrode (the sample No. 21) despite Au added thereto. 
     WORKING EXAMPLE 4 
     In order to examine an appropriate amount of Au to be added to the Pt-Pd electrode of the NO x  detecting cell  36 , which causes NO x  reducing ability to be lowered, electrode examining cells were made by further adding Au in an amount of 0 to 0.5 g to electrode paste of 1 g having composition of Pt-50 wt % Pd-10 wt %ZrO 2 . The procedures for making the electrode examining cells were the same as the working example 1. Further, the component of the electrode paste used herein is shown in Table 4. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                  Sample 
                  Measurement Electrode 
                 Au Addition 
                 Reference 
               
               
                 No. 
                 Component (1 g) 
                 Amount (g) 
                 Electrode 
               
               
                   
               
             
             
               
                  31 
                  Pt-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 32 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 — 
                 Pt-10 wt % ZrO 2   
               
               
                 33 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 0.05 
                 Pt-10 wt % ZrO 2   
               
               
                 34 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 0.10 
                 Pt-10 wt % ZrO 2   
               
               
                 35 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 0.20 
                 Pt-10 wt % ZrO 2   
               
               
                 36 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 0.30 
                 Pt-10 wt % ZrO 2   
               
               
                 37 
                 Pt-50 wt % Au-10 wt % ZrO 2   
                 0.50 
                 Pt-10 wt % ZrO 2   
               
               
                   
               
             
          
         
       
     
     The thus obtained electrode examining cells were examined for the current-voltage characteristics under the same condition as the working example 1. The results are shown in FIG.  9 . Here, in FIG. 9, the first quadrant shows characteristics of the measurement electrode relative to a NO x  gas, and the third quadrant shows characteristics of the reference electrode relative to a NO x  gas. 
     From the current-voltage characteristics shown in the first quadrant of FIG. 9, the following is apparent. That is, in the case of the sample No. 37 made by adding Au in an amount of 0.5 g to the electrode paste of 1 g having composition of Pt-50 wt % Pd-10 wt % ZrO 2  (Pd/Au ratio=1.0), the voltage required for NO reducing increased drastically. On the other hand, in the case of the sample No. 36 made by adding Au in an amount of 0.3 g (Pd/Au ratio=1.67), the voltage required for NO reducing decreased as compared to the sample No. 37. 
     As the addition amount of Au decreased, the current changed more linearly with the voltage (the characteristics determined by resistance of the zirconia electrolyte), and the voltage required for NO x  gas reducing decreased, which meant enhancement of NO x  reducing ability. In other words, it was found that the sample No. 34, which was made by adding Au in an amount of 0.1 g (Pd/Au ratio=5.0) exhibited drastic improvement in the NO x  reducing ability, and that the sample Nos. 33 and 32, which were made by adding Au in an amount of 0.05 or less (Pd/Au ratio≧10.0) exhibited better NO x  reducing ability than the Pt electrode (the sample No. 31). 
     Based on the findings obtained in the working examples 1-4, consideration was made to find out an appropriate amount of Pd to be added to Au in order to achieve equal or better NO x  reducing ability as compared with the Pt electrode. The results are shown in FIG.  10 . As is apparent from the working examples 1-4, equal or better NO x  reducing ability as compared with the Pt electrode was obtained when the addition amount of Pd and the addition amount of Au in the Pt-Au-Pd alloy were kept within the diagonally shaded area shown in FIG.  10 . 
     WORKING EXAMPLE 5 
     The detecting apparatus  10  shown in FIG. 1 was made in the following procedures. That is, first ZrO 2  green sheets each having a 0.5 mm thickness was made in the same procedures as the working example 1. Then, Pt paste having composition of Pt-10 wt % ZrO 2  was printed on the front surface and the rear surface of a ZrO 2  green sheet by screen printing. In this way, the oxygen gas supplying cell  20  was made. Similarly, Pt paste having composition of Pt-1 wt % Au-10 wt % ZrO 2  was printed on the front surface and the rear surface of the ZrO 2  green sheets by screen printing. In this way, the oxygen reference electrode generating cell  28  and the oxygen monitoring cell  32  were made. 
     Next, Pt paste having composition of Pt-1 wt % Au-10 wt % ZrO 2  was printed on the front surface of a ZrO 2  green sheet by screen printing, thereby constructing the cathode  26   b,  while Pt paste having composition of Pt-10 wt % ZrO 2  was printed on the rear surface thereof thereby constructing the anode  26   a.  In this way, the oxygen pumping cell  24  was made. 
     Similarly, Pt paste having composition of Pt-20 wt % Pd-10 wt % ZrO 2  was printed on the front surface of a ZrO 2  green sheet by screen printing thereby forming the cathode  38   b , while Pt paste having composition of Pt-10 wt % ZrO 2  was printed on the rear surface thereof thereby forming the anode  38   a.  In this way, the NO x  detecting cell  36  was made. 
     Next, in the same procedures except that Al 2 O 3  powder was used as source powder, an alummina green sheet having a 0.2 mm thickness was made. On the front surface of this alummina green sheet, Pt paste having composition of Pt-10 wt %ZrO 2  was printed to construct the heater  40   b.  In this way, the heater section  40  was made. 
     Next, each cell and the heater section  40  made above were laminated and bounded together in the following order so as to have a cross section as shown in FIG.  1 . That is, the oxygen gas supplying cell  20  was laid over the oxygen reference electrode generating cell  28  and the oxygen monitoring cell  32 , which were in turn laid over to the oxygen pumping cell  24 , which was in turn laid over the NO x  detecting cell  36 , which was in turn laid over the heater section  40 . Here, between the oxygen pumping cell  24  and the NO x  detecting cell  36 , there were inserted alummina sheets  14   a ,  14   b  and  14   c  each having a thickness of 200 μm in order to insulate the two cells. 
     The thus obtained layered product was sintered in an electric furnace, whereby the detecting apparatus  10  was obtained. Here, the sintering was conducted in an atmosphere. In addition, the sintering pattern was as follows: {circle around (1)} the temperature was raised up to 450° C. at the temperature raising rate of 50° C. per hour and then the layered product was left at the temperature of 450° C. for an hour in order to degrease the binder added thereto, {circle around (2)} the temperature was raised up to 1,430° C. at the temperature raising rate of 100° C. per hour and the layered product was sintered for one hour, {circle around (3)} the electric furnace was turned off for slow cooling. 
     Next, performance tests were conducted on the thus obtained detecting apparatus  10 . The performance tests were conducted with an oxygen gas testing apparatus using exhaust gas. This testing apparatus is capable of varying the excess air rate in a gas atmosphere from λ=1.5 to 0.8 by mixing isobutane and air. The lean (λ=1.2) gas atmosphere and the rich (λ=0.9) atmosphere obtained by the testing apparatus were injected a NO x  gas of 0 to 2,000 ppm thereby to produce measurement gases. The measurement gases were supplied to the detecting apparatus  10  thereby to measure output current of the NO x  gas detecting cell  36 . Here, a NO gas was injected to the measurement gases using an injector at an interval of 50 ppm or 100 ppm such that the NO x  gas concentration was between a range of 0 and 200 ppm. 
     The operation conditions of the detecting apparatus 10 was as follows. 
     Heating temperature of heater: 700° C., 
     Current of oxygen gas supplying cell: 1 mA 
     Applied voltage to oxygen gas reference electrode generating cell: 0.5 V 
     Applied voltage to oxygen pumping cell: 0.3 V 
     Applied voltage to NO x  detecting cell: 0.6 V 
     Excess air rate of exhaust gas: λ=1.2, 0.9 
     Temperature of exhaust gas: 450° C. 
     Shown in FIG. 11 is the output current of the NO x  detecting cell  36  when the NO gas concentration changed in the gas atmosphere with the excess air rate of λ=0.9, 1.2. As is apparent from FIG. 11, the detecting apparatus  10  consistent with this preferred embodiment generated output current in proportion to the NO gas concentration irrespective of the change in the atmosphere between lean and rich. This is because the atmosphere in the gas introducing chamber  16  was maintained under a lean state by the oxygen gas supplying cell  20 . In addition, it is also because a Pt-Pd electrode was used as the cathode  38   b  of the NO x  detecting cell  36 , whereby decrease in activity relative to NO x  due to Au adhesion was suppressed. 
     WORKING EXAMPLE 6 
     The detecting apparatus  10  made in the working example 5 was examined for its responsivity to a NO x  gas. As a comparative example, a detecting apparatus was made using a conventionally-used electrode having composition of Pt-40 wt % Rh-10 wt % ZrO 2  as a cathode of a NO x  detecting cell to examine its responsivity to a NO x  gas. Here, to examine the responsivity, first, a NO gas was injected into a gas atmosphere of an excess air rate of λ=1.2 to produce a measurement gas, the measurement gas was supplied to the NO x  detecting cell, and then the NO x  gas concentration was abruptly changed from 500 ppm to 1,000 ppm. Here, change in the output current of the NO x  detecting cell generated at this time was examined to evaluate the responsivity. The results are shown in FIG.  12 . 
     In the case of the detecting apparatus comprising the NO x  detecting cell using the Pt-Rh electrode as the cathode, upon abruptly changing the NO x  gas concentration rate to 1,000 ppm, the output current exceeded 10 μA and dropped to about 8 μA a few minutes after. Thereafter, the output current gradually decreased as time went by. These findings show that in the case of the Pt-Rh electrode, influence of the adsorbed oxygen was large, and therefore, it was difficult to detect abrupt change in the NO concentration faithfully. On the contrary, in the case of detecting apparatus comprising the NO detecting cell using the Pt-Pd electrode as the cathode, the output current responded immediately to the change in the NO gas concentration. This indicates the capability of detecting the change in the NO x  gas concentration faithfully. 
     WORKING EXAMPLE 7 
     In order to determine an optimum addition amount of Rh of a Pt-Pd-Rh electrode, detecting apparatuses were made using Pt-Pd-Rh electrodes with different Rh addition amounts as the cathodes, and the detecting apparatuses were examined for their current-voltage characteristics. Shown in FIG. 13 is a cross section of the detecting apparatus  50  used in the experiment. 
     First, ZrO 2  green sheets each having a thickness of 0.5 mm were made in the same procedures as the working example 1. Next, as shown in FIG. 13, an anode  56   a  and a cathode  56   b  constructing a pumping cell  54  were printed on a green sheet  52   b  by screen printing, and so were an anode  60   a  and a cathode  60   b  constituting a detecting cell  58 . Next, green sheets each having a pre-determined shape were laminated over each side of the green sheet  52   b  thereby forming a predetermined projections and depressions on the surface of the green sheet  52   b.  Further, green sheets  52   a  and  52   c  were laminated over each side of the green sheet  52   b.  Thereafter, by sintering the layered product in an atmosphere, the detecting apparatus  50  was obtained. 
     Here, the sintering condition was as follows. First, the temperature was raised at the temperature raising rate of 50° C. per hour to keep the layered product was at the temperature of 450° C. for one hour, then the temperature was raised at the temperature raising rate of 100° C. per hour to keep the layered product at 1,430° C. for one hour, and finally the layered product was slowly cooled in an atmosphere. The composition of the paste used to form electrodes of each cell is shown in Table 5. Here, it should be noted that in this preferred embodiment and the working examples 8-10, Au is adhered to the cathode  60   b  of the detecting cell through the sintering process described above. 
     
       
         
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                  Sample 
                  Pumping Cell 
                 Detecting Cell 
               
             
          
           
               
                  No. 
                  Cathode 
                 Anode 
                 Cathode 
                 Anode 
               
               
                   
               
             
          
           
               
                  41 
                  Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                   
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                   
                 ZrO 2   
               
               
                 42 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.05 g) 
                 ZrO 2   
               
               
                 43 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.1 g) 
                 ZrO 2   
               
               
                 44 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.2 g) 
                 ZrO 2   
               
               
                 45 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.3 g) 
                 ZrO 2   
               
               
                 46 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.4 g) 
                 ZrO 2   
               
               
                 47 
                 Pt-3% Au- 
                 Pt-10% 
                 Pt-10% ZrO 2   
                 Pd 
                 Rh 
                 Pt-10% 
               
               
                   
                 10% ZrO 2   
                 ZrO 2   
                 (1 g) 
                 (0.3 g) 
                 (0.6 g) 
                 ZrO 2   
               
               
                   
               
             
          
         
       
     
     Next, current-voltage characteristics of each of the thus obtained detecting apparatuses were measured. The measurement was performed under the condition where the gas temperature: 700° C., the gas atmosphere: 0.1%NO/N 2 , and the gas flow rate: 2 L per minute. In the measurement, the current-voltage characteristics were measured three times consecutively while changing the voltage applied to the detecting cell from 0 V to 1.2 V at the voltage application rate of 10 mV per second. 
     In FIG. 14, the measurement results of the sample No. 45 (the Rh addition amount of 20 wt %) are shown. As is apparent from FIG. 14, in the case where Rh was further added to the Pt-Pd electrode, a peak was observed in the current value in the first measurement. This peak was caused because the two current values were added together: one derived from oxygen generated upon the NO decomposition and the other derived from oxygen that had been adhered to the electrode and ejected therefrom. However, no peak was observed in either second or third measurement and rather there was exhibited limiting current characteristic corresponding to the oxygen concentration derived by the NO gas decomposition. These findings show that the Pt-Pd-Rh electrode was a little inferior in terms of the startability, yet capable of measuring the NO gas concentration accurately provided that it was used consecutively. Although not illustrated, the other sample Nos. 42-44, 46 and 47 to which Rh was added exhibited the similar results. That is, no peak caused by ejection of adsorbed oxygen was observed in the second measurement and after. 
     Next, based on the measurement results showing the current-voltage characteristics of the sample Nos. 41-47, the resistance was calculated from the current at the voltage of 0 V to 0.1 V to examine the relation between the composition amount of Rh and the resistance of the detecting cell. The results are shown in FIG.  15 . Up to the Rh composition amount of 0.4 g relative to Pt-10 wt %ZrO 2  (1 g)-Pd(0.3 g) (the sample Nos. 41-46, the Rh addition amount of 0 to 25 wt %), the resistance remained generally constant. However, when the Rh composition amount was 0.6 g, the resistance increased rapidly. The angle of the rising edge in the current-voltage characteristics, as shown in FIG. 14, largely depends on the resistance of the detecting cell. The larger the resistance is, the smaller the angle of the rising edge will be. That is, increase in the resistance of the detecting cell results in increase in the limiting current generating voltage. Accordingly, it is preferred that an upper limit be provided on the Rh composition amount such that an intended limiting current voltage is obtained. 
     Shown in FIG. 16 is the relation between the peak current which was detected in the first measurement of the current-voltage characteristics and the Rh composition amount. As is apparent from FIG. 16, the peak voltage increased in proportion to the increase in the Rh composition amount. This is because a larger amount of oxygen adheres to the electrode as the Rh composition amount increases. Increase in the peak voltage results in decrease in the startability. Accordingly, it is preferred that an upper limit be provided on the Rh composition amount such that an intended startability is obtained. 
     WORKING EXAMPLE 8 
     In accordance with the same procedures as the working example 7, seven different types of detecting apparatuses (the sample Nos. 41-47) were made with different addition amounts of Rh to examine influence on current-voltage characteristics exerted by heat treatment. The heat treatment was conducted by heating each of the manufactured detecting apparatuses to 900° C. in an atmosphere and by applying voltage of 1 V for ten minutes. 
     Next, each sample before the heat treatment and after the heat treatment was subjected to the following operation and the operation was repeated three times. That is, first voltage was applied to the sample while varying the voltage from 0 V to 1.2 V at the voltage application rate of 10 mV per second under the condition where the gas temperature: 700° C., the gas atmosphere: 0.2%NO/N 2 , the gas flow rate: 2 L per minute, whereby the oxygen adhered to the electrode was caused to be ejected. Thereafter, the current-voltage characteristics were measured under the same condition. FIGS. 17 and 18 respectively show the current-voltage characteristics of the detecting apparatus of the sample No. 45 measured before and after the heat treatment. 
     Before the heat treatment, the sample No. 45 exhibited the limiting current generating voltage of about 0.3 V and the current value in the limiting current region was about 0.05 mA. On the other hand, after the heat treatment, the limiting current voltage was about 0.2 V and the current value in the limiting current region was about 0.025 mA, which meant that the limiting current region was expanded and the current was lowered. In addition, even if the same heat treatment was conducted again on the sample which had been heat treated already, the change in the output current was small. 
     FIG. 19 shows the relation between the Rh composition amount and the falling rate of the output current. Here, the “falling rate of the output current” refers to the value obtained by the expression (output current after heat treatment/output current before heat treatment)×100. From FIG. 19, it is apparent that the output current fell as the Rh composition amount increased. In addition, when the Rh composition amount is 0.2 g or more (the Rh addition amount of 14.3 wt %), the output current fell more than 50%, whereby the falling rate of the output current became saturated so that further increase in the Rh composition amount did not cause any further drop in the output current. 
     FIG. 20 shows the relations of the Rh composition amount with the limiting current generating voltage before and after the heat treatment respectively. In the case of the samples before the heat treatment, when the Rh composition amount was less than 0.4 g (the Rh addition amount of 25 wt %), the limiting current generating voltage slightly fell as the Rh composition amount increased. However, when the Rh composition amount was 0.6 g, the limiting current generating voltage exceeded 0.3 V. On the other hand, after conducting the heat treatment on the sample containing Rh added thereto, the limiting current generating voltage fell as compared with that of before the heat treatment. Moreover, the falling rate the limiting current generating voltage was apt to increase as the Rh composition amount increased. Especially, when the Rh composition amount was 0.2 g to 0.4 g (the Rh addition amount of 14.3 to 25 wt %), the limiting current generating voltage dropped to about 0.2 V and the limiting current region expanded. 
     The above findings indicate that by conducting heating treatment in which the Pt-Pd-Rh electrode was heated in an atmosphere while applying voltage, the limiting current generating voltage shifts toward a lower voltage. This is because by conducting heating treatment on the Pt-Pd-Rh electrode, there is formed a thin oxide film having Rh 2 O 3  as a main component on the surface of the electrode. This oxide film is considered to control the diffusion rate of oxygen in the detecting cell. 
     WORKING EXAMPLE 9 
     Detecting apparatus  10  having the configuration as shown in FIG. 1 was made in the same procedures as the working example 5 except that the cathode  38   b  of the NO x  detecting cell  36  was constructed of Pt paste having composition of Pt-10%ZrO 2  (1 g)+Pd (0.3 g) and Rh (0.3 g) combined thereto. Then, performance tests of the thus obtained detecting apparatus  10  were carried out. Here, the operation conditions of the detecting apparatus  10  were the same as that of the working example 5 except that the voltage applied to the NO x  detecting cell was 0.4 V. 
     Shown in FIG. 21 is the output current of the NO x  detecting cell  36  when the NO gas concentration was changed in the gas atmosphere with the excess air rate of λ=0.9, 1.2. As is apparent from FIG. 21, the detecting apparatus  10  consistent with this preferred embodiment generated output current in proportion to the NO gas concentration irrespective of change in the atmosphere between lean and rich. This is because the atmosphere in the gas introducing chamber  16  was maintained under a lean state by the oxygen gas supplying cell  20 . In addition, this is also because a Pt-Pd-Rh electrode was used as the cathode  38   b  of the NO x  detecting cell  36 , whereby the limiting current generating voltage shifted toward a lower voltage. As the result, although the voltage applied to the NO x  detecting cell  36  was lowered, output current was obtained in correspondence to the NO x  concentration. 
     WORKING EXAMPLE 10 
     The detecting apparatuses  10  having the configuration as shown in FIG. 1 were made in the same procedures as the working example 5 except that the cathode  38   b  of the NO x  detecting cell  36  was constructed of Pt paste having composition of Pt-10%ZrO 2  (1 g)+Pd (0.3 g) and Rh combined in different amounts thereto. Here, in this preferred embodiment, there were four different Rh composition amounts: 1 g, 0.1 g, 0.3 g and 0.6 g (0 wt %, 7.7 wt %, 20 wt %, and 33.3 wt % respectively in terms of the Rh addition amount). Then, the thus obtained detecting apparatuses  10  were examined for the responsivity. The results are shown in FIG.  22 . 
     In the case where a Pt-Pd electrode was used as the cathode  38   b  of the NO x  detecting cell  36 , the output current responded to change in the NO x  gas concentration immediately, and thus detection of NO x  was carried out accurately. On the other hand, in the case where a Pt-Pd-Rh electrode was used as the cathode  38   b  of the NO x  detecting cell  36 , the output current exceeded 10 μA in response to abrupt change in the NO x  gas concentration to 1,000 ppm, and thereafter gradually decreased as the time went by. In addition, the larger the Rh composition amount was, the larger the change in the output current was in response to the abrupt change in the NO x  gas concentration and the longer it took before the output current stabilized. This is because the difference in the Rh composition amount resulted in the change in the ejection time of the oxygen gas adhered to the electrode. 
     The foregoing findings show that the larger the Rh composition amount in the Pt-Pd-Rh electrode was, the less the responsivity to the NO x  would be. Accordingly, in order to obtain an intended responsivity, the Rh composition amount needs to be set to an optimum value. 
     The foregoing description have been given to the preferred embodiments of the present invention in detail. However, the present invention is not limited to the above preferred embodiments and various modifications are possible without departing from the spirit and the scope of the present invention. 
     For example, in the above preferred embodiments, the detecting apparatus  10  comprises an oxygen gas supplying cell  20 . Yet, the oxygen supplying cell  20  may be omitted in the case where the measurement gas is always maintained in a lean state. Also, in the above preferred embodiment, the residual oxygen concentration is measured with the use of the oxygen reference electrode generating cell  28 . Instead, a gas of which oxygen concentration is known (for example atmosphere) may be introduced into one of the cells in the detecting apparatus  10  so that it functions as a reference electrode upon measuring the residual oxygen concentration. 
     Further, in the above preferred embodiment, description has been given mainly to a so-called “current-type” NO x  detecting apparatus, which causes NO x  contained in a measurement gas to be decomposed and measures the current generated upon the decomposition to determine the NO x  gas concentration. Yet, electrode materials having Pt-Pd composition consistent with the present invention are equally applicable also to a so-called “electromotive force type” NO x  gas detecting apparatus to achieve the equal effect. This type of apparatus measures an electromotive force generated between a measurement electrode to which a measurement gas is introduced and a reference electrode so as to determine the NO x  gas concentration. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in the light of the above teachings or may be acquired from practice of the invention. The embodiments chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.