NITROGEN OXIDE SENSOR

Abstract: A nitrogen oxide sensor having at least one pair of electrodes, i.e., a first electrode and a second electrode, set in such a state that the electrodes come into contact with an oxide ion conducting solid electrolyte, wherein at least the first electrode is made of an oxide of a Group 7a or 8 element, particularly selected from among Mn, Fe, Co and Ni, or a substance containing the oxide, or a composite oxide (including oxygen-deficient one) containing a Group 7a or 8 element, particularly represented by the formula: ABO 3 , AB 2 O 4 , A 2 BO 4 or ACBO 4 , or a substance containing the composite oxide. The oxide or composite oxide used as the electrode material is active for oxygen and NO x but little exhibits catalytic activity for CO 2 . Accordingly, the above sensor can act effectively without being affected by change in CO 2 concentration even at a temperature of 600 °C or above, and therefore can detect the concentration of NO x with a high accuracy, when set in the flow of high-temperature exhaust gas.

Description: Field of the Invention The present invention relates to a sensor to detect NOx concentration in an exhaust gas generated from combustion furnace, automobile engine, etc. Prior Art Continuous monitoring, feedback control of the combustion condition based on the monitoring result or the optimalization of the control of the desulfurization equipment have been examined to reduce NOx exhausted from combustion furnace, automobile engine, etc. Therefore, a small sized solid and simple solid type sensor with high sensitivity is needed to be used for the control. Especially, for the feedback of the combustion condition, a sensor which works stablely in a high temperature exhausted gas of several hundreds °C. A semiconductor type sensor utilizing a phenomena that the electric resistance of a gas sensor composed of oxide semiconductor changes in proportion to the concentration of NOx or a solid electrolyte sensor measuring the electromotive force caused by the difference in the partial pressure of a gas at each electrodes which are formed on an ion conductive solid electrode are the representative sensors being proposed up to now. However, there is a drawback in a semiconductor type sensor that enough sensitivity can not be obtained at a temperature above 500°C where the physical absorption of the test gas does not occur, since it utilizes the phenomena of the change of electric resistance of the gas sensor caused by the physical absorption. On the other hand, a solid electrolyte sensor, using AgI or RbAg 4 I 5 as a solid electrolyte and an electrode coated by silber nitrate is disclosed (The Japanese Laid-open Patent Publication Sho61-184450). This sensor is a concentration cell type, wherein Ag ion in the nitrate migrates in the solid electrolyte by the difference of NOx concentration between the electrodes and causes the electromotive force which complies with Nernst formula and detects NOx concentration by the measurement of said electromotive force. A concentration cell type sensor based on the same principle using NASICON (Na 3 Zr 2 Si 2 PO 12 ) as a solid electrolyte and NaNO 3 as electrodes is disclosed. (Chemistry Letters, vol.1, p.587 - 590 (1992)) Furthermore, a concentration cell using Na ion conductive β /β " alumina or β /β " alumina, in which Na ion is replaced by Ba ion and using Ba(NO 3 ) 2 cr mixture of NaNO 3 and Ba(NO 3 ) 2 as an electrode is disclosed in Denki Kagaku, vol.59, (1991) p.465 ∼ 472. In these sensors which detect NOx concentration by the measurement of the electromotive force which complies with Nernst formula, nitrates or nitrites are used as an electrode material and then the durable temperature of the sensor is limited by the melting point of nitrates or nitrites. Even when Ba(NO 3 ) 2 , having the highest melting point is used as an electrode, the durable temperature is below 592°C . Furthermore, due to the diliquescence property of nitrates and nitrites, performance and stability of the sensor were not enough when used in the test gas containing water vapor. For resolving above issues, a NOx sensor using an oxide of 6a Group element and oxides of perovskite or pseudo perovskite as an electrode material is proposed. These sensors have a good thermal durability due to the high melting point and decomposition temperature of the oxides. However, the response of the NOx sensor of this type is different from that of the sensor using nitrate as an electrode which complies with Nernst formula and depends on the change of electromotive force caused by the catalytic activity of the electrode for the test gas and oxidation reduction reactions on the electrode and hence the performance of the sensor depends largely on the oxides. For example, electrodes using oxides of perovskite or pseudo perovskite structure containing Sn or Cu respond not only to NOx but also CO 2 and there are issues that the exact detection of NOx concentration is not possible, since it is affected very much by the CO 2 concentration. And a sensor using oxides of 6a Group element and perovskite structure containing Ti as an electrode works well around 500 °C , but its sensitivity to NOx rapidly decreases above 600°C and there are issues in the reduction of resolution and accuracy of NOx concentration. U. LAWRENCE ET AL.: "Investigations on the determination of NO with galvanic ZrO2 solid electrolyte cells", FRE-SENIUS JOURNAL OF ANALYTICAL CHEMISTRY, vol. 349, pages 679 to 683 - discloses to use a detecting electrode which is made of Perovskite type oxide with the composition La 1-x Sr x MeO 3 (x = 0.1...0.5; Me = Cr, Mn, Fe, Co). DE 42 25 775 A (HAEFELE UMWELTVERFAHRENSTECHNIK) 10 February 1994 - discloses to use an electrode which is made of the composition Ln 1-x AMO 3 (Ln = Sc, Y, La; M = Cr, Mn, Fe, Co; A = zweite Hauptgruppe). US-A-4 927 517 (Mizutani et al.) 22 May 1990 - discloses a NOx sensor consisting essentially of first and second electrodes in contact with an oxygen conductive solid electrolyte, wherein the first electrode is made of oxides of Mn, Fe, Co or Ni. Disclosure of the Present Invention The present invention provides a NOx sensor which does not respond to CO 2 and has a good sensitivity to NOx at a temperature above 600°C. The present invention proposes a NOx sensor consisting essentially of first and second electrodes in contact with an oxygen conductive solid electrolyte, wherein said sensor is capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes, characterized in that the first electrode is made of a material selected from the group consisting of MgMn 2 O 4 , TiMn 2 O 4 , CrMn 2 O 4 , CoMn 2 O 4 , CdMn 2 O 4 , (CdAl)Mn 2 O 4 , (CdAl) MnCr 2 O 4 , CdFe 2 O 4 , ZnFe 2 O 4 , CrFe 2 O 4 , Ca 2 MnO 4 , Mg 2 MnO 4 , Fe 2 MnO 4 , Ni 2 MnO 4 , Zn 2 MnO 4 , Al 2 MnO 4 , Sb 2 MnO 4 , La 2 MnO 4 , Cr 2 CoO 4 , CdGaMnO 4 or YFeMnO 4 . In a more concrete form of the NOx sensor of the present invention, zirconia (ZrO 2 -M 2 O 3 or ZrO 2 -MO, where M is Yb, Cd, Nd, Ca, Y, Mg, Hf) bismuth oxide (Bi 2 O 3 -M 2 O 3 or MO or M 2 O 3 , where M is Y, Gd, Nb, W, Sr, Ba) and ceria oxide (CeO 2 -M 2 O 3 or MO 2 , where M is Y, Sm) are used as a solid electrolyte. A solid electrolyte can be either a separation wall structure which separates the test gas to be detected of NOx concentration from the constant atmospheric gas such as air, a plate or rod shape. In case that a solid electrolyte is a separation wall structure, the first and second electrode are placed on each surface of the wall and in the case that a solid electrolyte is not a separation wall structure, the first and second electrodes are placed at any place on the solid electrolyte. The first electrode to be formed on an electrolyte is made of a material selected from the group consisting of MgMn 2 O 4 , TiMn 2 O 4 , CrMn 2 O 4 , CoMn 2 O 4 , CdMn 2 O 4 , (CdAl)Mn 2 O 4 , (CdAl) MnCr 2 O 4 , CdFe 2 O 4 , ZnFe 2 O 4 , CrFe 2 O 4 , Ca 2 MnO 4 , Mg 2 MnO 4 , Fe 2 MnO 4 , Ni 2 MnO 4 , Zn 2 MnO 4 , Al 2 MnO 4 , Sb 2 MnO 4 , La 2 MnO 4 , Cr 2 CoO 4 , CdGaMnO 4 or YFeMnO 4 . The oxygen content expressed by the chemical formula can be a stoichiometric value including a non-integral number caused by the oxygen deficiency. If required, an electric collector composed of rare metals such as Pt, Au, Pd, Ir, Rh, Ru or its alloy as an electro-conductive material can be formed to keep electrical connection. The first electrode of a hybrid oxide is formed on a solid electrolyte by coating using screen printing, etc., and sintering and also it can be formed by physical deposition method such as vacuum deposition, lasor ablation, ion beam deposition and ionplating method and chemical deposition method such as chemical vapour phase deposition and plasma chemical vapour phase deposition. The second electrode is composed of a substance comprising rare metals, such as Pt, Ag, Au, Pd, Ir, Rh and Ru, its alloy, electro-conductive ceramics, for example, oxides of perovskite structure expressed by ABO 3 , such as LaCoO 3 , LaNiO 3 , LaFeO 3 , wherein a part of A or B site can be replaced by an element such as Sr, an oxide having a structure of K 2 NiF 4 and a substance which can be an oxygen electrode such as La 2 CuO 4 or a substance which can give a definite chemical potential to oxygen. However, in a sensor wherein both the first and second electrode are exposed in the test gas, it is required that the change of the electrical potential caused at the first electrode should not be cancelled by the change of the electrical potential caused by the second electrode and it is preferred that it is composed of a material which does not cause a change of electrical potential for NO and NO 2 or a material which cause a change of electrical potential in an opposite direction to that caused at the first electrode. In an electromotive force type sensor, wherein a pair of electrodes is formed on a solid electrolyte and the difference of the electrical potential caused by the difference in the chemical potential of the electrodes, the first electrode which works as a detecting electrode, reacts with the test gas existing on the surface of the electrode and as a result, the difference in the electrical potential is caused by the change of the chemical potential of the ion conductive carrier of a solid electrolyte to the chemical potential of the other electrode, which complys with the Nernst formula. However, the change of the electromotive force in the present invention does not comply with the Nernst formula, because the number of the electrons obtained from the slope of the dependency of the electromotive force on the concentration and temperature are not integral numbers. In a NOx sensor according to the present invention, the change of the electromotive force is different for NO 2 and NO. The electromotive force increases depending on the increase of the concentration of NO 2 and decreases depending on the increase of NO concentration. Judging from the change of the electromotive force corresponding to the change of the concentration of the test gas, it is thought that the reduction reaction of the test gas occurs on the electrode when the electromotive force increases and oxydation reaction occurs on the electrode when the electromotive force decreases. Furthermore, the dependency of the electromotive force on the concentration of NOx is different in the presence and absence of oxygen, wherein the slope of the dependency of the electromotive force on NOx concentration in the presence of oxygen is larger than that in the absence of oxygen; that is oxygen is involved in the electrode reaction. Namely the response of the sensor according to the present invention is obtained by the following reactions. On the first electrode, an electrochemical reaction between oxygen and NOx is concomitantly occurs and the change of the electromotive force is caused by the hybrid electropotential of the local battery formed on the first electrode. Namely, a hybrid electric potential is caused for NO 2 by the following reactions on the first electrode NO 2 + 2e → NO + O 2- O 2- → ½ O 2 + 2e and a hybrid electric potential is caused for NO by the following reactions on the first electrode ½ O 2 + 2e → O 2- NO + O 2- → NO 2 + 2e To obtain the electromotive force by the hybrid electric potential, it is necessary that the two reactions occur concomitantly on the electrode either for NO 2 and NO and the hybrid electric potential can not be obtained; if it is active for a reaction and not active for another reaction. The first electrode of the NOx sensor according to the present invention can act as an oxygen electrode and has enough activity for the reactions of (2) and (4). Furthermore, it also has a high catalytic activity for NOx and is active for the reactions of (1) and (2). Namely, since it is active both for oxygen and NOx, the change of the electromotive force is obtained by the hybrid electric potential caused by the electrochemical reaction between oxygen and NOx which occurs concomitantly on the first electrode and occurs also at 600°C , and hence a good performance is obtained. On the other hand, it is thought that a hybrid electric potential is caused for CO 2 gas by the same reaction for NO 2 . However, the catalytic activity of the electrode according to the present invention is low for CO 2 and electro-chemical reaction between oxygen and CO 2 does not occur concomitantly on the first electrode and hence it does not respond to CO 2 Brief Explanation of the Figures Fig. 1 is a cross section of an example of a NOx sensor according to the present invention. Fig. 2 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to comparative example 1. Fig. 3 is a figure showing the dependency of the change of the electromotive force on the NOx concentration in an example of a NOx sensor according to example 1. Fig. 4 is a figure showing the dependency of the change of the electromotive force on the CO 2 concentration, according to example 1. Fig. 5 is a figure showing the dependency of the change of the electromotive force on the NOx sensor according to comparative example 2. Fig. 6 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention. Fig. 7 is a figure showing the dependency of the change of the electromotive force on NOx concentration in an example of a NOx sensor according to the present invention. Preferred Embodiment of the Present Invention Example 1 (Comparative Example) Fig. 1 shows a cross section of a NOx sensor in an example according to the present invention. A solid electrolyte 1 can be any oxgen ion conductive material, however, in view of thermal stability and resistance, zirconia, wholly or partially stabilized by yttria, calcia or magnesia, etc. is preferred. In this example, zirconia stabilized by 8 mol% of yttria was used. On one surface of a plate solid electrolyte, the first electrode 2 and second electrode 3 are placed. The first electrode 2 is composed of oxides of 7a or 8a Group element. The first electrode was prepared by sputtering using oxide as a target. After preparation of a film by sputtering, heat treatment was conducted for 1 h at 900°C in the air. The second electrode 3 is composed of an electrode which does not respond to NOx and Pt is used in this case. The first electrode 2 and the second electrode 3 is a so called gas electrode and formed as a porous electrode. An electric collector 4 of Pt is placed on the first electrode 2 and the cable 5 and 6 of the second electrode 3 is connected with the measurement circuit. The change of the electromotive force at the introduction of 100ppm of NO 2 and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4% oxygen at 600°C as a standard is shown in Table 1. The NOx sensor, whichever oxides are used, responds to NO 2 resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force. Among oxides, the sensor using Co as the first electrode showed the heighest sensitivity. Figs. 2 and 3 show the dependency of the electromotive force of a NOx sensor using oxides of Co as the first electrode on NOx concentration. Fig. 4 shows the dependency of the electromotive force of a NOx sensor using oxides of Co as the first electrode on CO 2 concentration at 600°C . The sensor does not respond to CO 2 and the electromotive force does not change even when CO 2 concentration changes. The dependency of the electromotive force on NOx concentration and the response characteristic to CO 2 were the same for the sensor using other oxides shown in this example. The Change of the Electromotive Force corresponding to 100ppm NO 2 and 500ppm No Electrode Material The Change of the Electromotive Force (mV) 100ppm NO 2 500ppm NO Mn 2 O 3 16.9 -10.5 Fe 2 O 3 28.7 -11.5 CO 3 O 4 64.3 -38.1 NiO 43.8 -10.1 Example 2 (comparative example) A NOx sensor was prepared using hybrid oxides expressed by the same method as explained in Example 1. Hybrid oxides expressed by ABO 3 used as the first electrode were prepared by sintering a mixture of a single oxide of each element composing of hybrid oxides. It was confirmed by the X-ray diffraction that these hybrid oxides are composed of a single phase or a mixed phase of oxides of either A or B. The change of electromotive force at the introducing of 100ppm of NO 2 and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4% oxygen at 600 °C as a standard is shown in Table 2. A NOx sensor using hybrid oxides expressed by ABO 3 as the first electrode responds to NO 2 resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO 2 and NO was proportional to the logarithm of the concentration. Furthermore, any of the sensors did not respond to CO 2 . Fig. 5 shows the dependency of the electromotive force of a NOx sensor at 600 °C on NOx concentration. The Change of the Electromotive Force corresponding to 100ppm NO 2 and 500ppm NO Electrode Material The Change of the Electromotive Force (mV) 100ppm NO 2 500ppm NO MgMnO 3 37.4 -18.2 SrMnO 3 66.1 -11.4 CaMnO 3 30.5 -14.5 BaMnO 3 43.6 -12.8 YMnO 3 25.3 -11.8 CrMnO 3 32.2 -10.0 CoMnO 3 36.8 -17.4 NiMnO 3 52.4 -25.7 AgMnO 3 32.1 -13.5 ZnMnO 3 57.5 -35.6 LaMnO 3 44.8 -25.7 NdMnO 3 35.4 -18.7 SmMnO 3 39.0 -10.7 (YSr)MnO 3 22.6 -10.5 GaFeO 3 21.8 - 9.2 LaFeO 3 49.3 -19.8 LaCoO 3 45.2 -20.1 LaNiO 3 22.4 -11.2 SrNiO 3 28.6 -10.9 Eample 3 A NOx sensor was prepared using the hybrid oxides expressed by AB 2 O 4 and indicated in Table 3 as the first electrode by the same method as shown in Example 2. The change of the electromotive force at the introduction of 100ppm of NO 2 and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4% oxygen at 600 °C as a standard is shown in Table 3. In this example, it was also confirmed that the sensor responds to NO 2 resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO 2 and NO was proportional to the logarithm of the concentration. Any sensor did not responds to CO 2 at the first electrode. In Fig. 6, the dependency of the electromotive force of a NOx sensor using CdMn 2 O 4 as the first electrode on NOx concentration at 600°C is shown. The Change of the Electromotive Force corresponding to 100ppm NO 2 and 500ppm NO Electrode Material The Change of the Electromotive Force (mV) 100ppm NO 2 500ppm NO MgMn 2 O 4 35.5 -16.1 TiMn 2 O 4 25.4 -11.8 CrMn 2 O 4 28.2 -17.1 CoMn 2 O 4 29.7 -15.9 CdMn 2 O 4 47.9 -30.6 (CdAl)Mn 2 O 4 46.8 -25.6 (CdAl)(MnCr) 2 O 4 10.8 -19.4 CdFe 2 O 4 18.0 -10.2 ZnFe 2 O 4 34.5 -18.6 CrFe 2 O 4 26.7 -12.3 Example 4 A NOx sensor was prepared using the hybrid oxides expressed by A 2 BO 4 and indicated in Table 4 as the first electrode by the same method as explained in Example 2 and the change of the electromotive force at the introduction of 100ppm of NO 2 and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4% oxygen at 600 °C as a standard is shown in Table 4. In this example, it was also confirmed that the sensor responds to NO 2 resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO 2 and NO was proportional to the logarithm of the concentration. In Fig. 7, the dependency of the change of electromotive force at 600°C of a NOx sensor using La 2 MnO 2 as the first electrode is shown. The Change of the Electromotive Force corresponding to 100ppm NO 2 and 500ppm NO Electrode Material The Change of the Electromotive Force (mV) 100ppm NO 2 500ppm NO Ca 2 MnO 4 33.1 -14.1 Mg 2 MnO 4 29.5 -12.2 Fe 2 MnO 4 33.8 -14.0 Ni 2 MnO 4 56.7 -10.2 Zn 2 MnO 4 50.0 -17.6 Al 2 MnO 4 28.6 -11.9 Sb 2 MnO 4 58.2 -10.4 La 2 MnO 4 44.8 -25.7 Cr 2 CoO 4 55.1 -18.8 Example 5 A NOx sensor was prepared using the hybrid oxides expressed by ABCO 4 and indicated in Table 5 as the first electrode by the same method as explained in Example 1 and the change of the electromotive force at the introduction of 100ppm of NO 2 and 500ppm of NO taking the value of the electromotive force under the atmospheric circumstance of 4% oxygen at 600 °C as a standard is shown in Table 5. In this example it was also confirmed that the sensor responds to NO 2 resulting in the increase of the electromotive force and to NO in the decrease of the electromotive force and the change of the electromotive force to the concentration of NO 2 and NO was proportional to the logarithm of the concentration. Either of the electrodes did not respond to CO 2 . The Change of the Electromotive Force to 100ppm NO 2 and 500ppm NO Electrode Material The Change of the Electromotive Force (mV) 100ppm NO 2 500ppm NO CdGaMnO 4 44.7 -26.3 YFeMnO 4 40.3 -16.6 Possible Utilization of the Invention in the Industry Since hybrid oxides which have no catalytic activity for CO 2 are used as the electrode material, it is possible to detect NOx concentration correctly without being affected by the fluctuation of CO 2 concentration. Furthermore, those hybrid oxides are active both for oxygen and NOx and hence the sensor looks at a temperature above 600 °C and detects NOx concentration with high accuracy.

Claims: A NOx sensor consisting essentially of first and second electrodes in contact with an oxygen conductive solid electrolyte, wherein said sensor is capable of detecting a concentration of NOx in a gas by converting an amount of NOx concentration into an electromotive force level between the first and second electrodes, characterized in that the first electrode is made of a material selected from the group consisting of MgMn 2 O 4 , TiMn 2 O 4 , CrMn 2 O 4 , CoMn 2 O 4 , CdMn 2 O 4 , (CdAl)Mn 2 O 4 , (CdAl) MnCr 2 O 4 , CdFe 2 O 4 , ZnFe 2 O 4 , CrFe 2 O 4 , Ca 2 MnO 4 , Mg 2 MnO 4 , Fe 2 MnO 4 , Ni 2 MnO 4 , Zn 2 MnO 4 , Al 2 MnO 4 , Sb 2 MnO 4 , La 2 MnO 4 , Cr 2 CoO 4 , CdGaMnO 4 or YFeMnO 4 . The NOx sensor of claim 1, characterized in that said first and second electrodes are formed on one side of said oxygen conductive solid electrolyte.

Inventors: HASEI MASAHARU, KUROSAWA HIDEYUKI, NAKANOUCHI YUKIO, HASEI, MASAHARU, KUROSAWA, HIDEYUKI, NAKANOUCHI, YUKIO, HASEI, Masaharu, KK Riken, Kumagaya Office, KUROSAWA, Hideyuki, KK Riken, Kumagaya Office, NAKANOUCHI, Yukio, KK Riken, KumagayaOffice
Applicants: RIKEN KK, KABUSHIKI KAISHA RIKEN
Application year: 1996
Main classification: G01N27
IPC codes: G01N27