Method for measuring nitrogen oxide

Abstract: A method for measuring nitrogen oxide (NOx) in a gas by decomposing NOx in the gas and measuring an amount of oxygen generated by the decomposition includes: introducing the gas to a first inner space (2) via a first diffusion resistance (1) connected with an external space, controlling an oxygen partial pressure in an atmosphere in the first inner space to be a predetermined low value at which NO is not decomposed substantially by a main pump means (60), introducing a controlled atmosphere of the first inner space into a second inner space (9) connected with the first inner space via a second diffusion resistance (8), and converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by an electric signal conversion means. A first oxygen concentration detecting means (12) is disposed for detecting an oxygen partial pressure in the second inner space, and a control voltage to be applied to the main pump means is adjusted on the basis of an output of the first oxygen concentration detecting means.

Description: Background of the Invention and Related Art Statement The present invention relates to a method for measuring nitrogen oxide. Particularly, the present invention provides a method for measuring a very small amount of nitrogen oxide in a gas containing a great amount of interfering gas with a high preciseness. With regard to a method for obtaining NOx amount in a gas to be measured by measuring an amount of oxygen generated by decomposing NOx in the gas to be measured, there has been, for example, a method disclosed in European Patent Laid Open No. 0678740A1 as shown in Fig. 13. In Fig. 13, 80 is a NOx sensor having a main pump means 60 and an electric signal conversion means 70. The main pump means 60 includes an electrochemical pump cell 6 comprising a substrate 3 comprising an oxygen ion conductive solid electrolyte and an inner pump electrode 5 and an outer pump electrode 4 formed on the inner surface and outer surface of the substrate, respectively. The main pump means 60 treats oxygen contained in the measuring gas introduced from an external space by pumping processing on the basis of a control voltage applied between the inner pump electrode 5 and the outer pump electrode 4 by a power source 7. The electric signal conversion means 70 includes a pair of detection electrodes 11 and 14 formed on the side where the measuring gas that was treated by pumping processing by the main pump means is introduced. The electric signal conversion means 70 generates an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the measuring gas after treated by the pumping processing by the main pump means 60. There has been used the NOx sensor 80 for measuring an amount of nitrogen oxide in a gas by introducing the gas to a first inner space 2 connected with the external space by means of a first diffusion resistance 1, controlling an oxygen partial pressure in an atmosphere in the first inner space 2 to be a predetermined low value at which NO is not decomposed substantially by the main pump means 60, introducing a controlled atmosphere of the first inner space 2 into a second inner space 9 connected with the first inner space 2 by means of a second diffusion resistance 8, and converting the atmosphere into an electric signal corresponding to the quantity of oxygen generated by decomposition or reduction of NOx contained in the atmosphere by the electric signal conversion means 70. Incidentally, in Fig. 13, a potential difference detection means 40 measures an electromotive force V1 corresponding to a difference in oxygen partial pressure between an electrode 10 exposed in the first inner space 2 and an electrode 11 exposed in a space 15 where a standard gas is present. A voltage applied to the main pump means 60 is adjusted so that the electromotive force V1 is controlled to have a predetermined value. However, it is difficult by this method to completely remove an influence of an interfering gas, for example, oxygen gas, carbonic acid gas (CO 2 ), steam, or the like, in an exhaust gas of an internal engine. Because of a difference between a gas concentration in the side facing the first inner space 2 of an inner pump electrode 5 of the electrochemical pump cell 6 and that in the side of the outer pump electrode 4 , a disorder of an atmosphere in the first inner space 2 by pulsation of a gas to be measured, an error caused by decomposition of water, a change of electrode activity or diffusivity of each gas component with a change of a temperature of a sensor element, a change of standing of electrodes and electrolyte, a discordance between an average value of an electrode potential of the electrochemical pump cell 6 and an average of a gas concentration, etc., it was difficult to maintain an oxygen partial pressure of a gas introduced into the second inner space 9 to be a certain value. When NOx in a gas having a high oxygen partial pressure is measured, an oxygen partial pressure of a gas introduced into the second inner space 9 is changed to be higher. According to the change, a base line of an electric signal (a pump current value Ip in Fig. 13) by an electric signal conversion means 70 corresponding to NOx amount is also changed. Particularly, it causes a large error when a very small amount of NOx is measured. On the contrary, when a steam amount of a gas to be measured increases, a hydrogen amount in the first inner space 2 in an equilibrium condition, a balance between hydrogen and oxygen in the second inner space 9 is lost by the difference in diffusion coefficients, a part of oxygen generated by decomposition of NOx is consumed by combining with hydrogen, thereby causing an error in measuring NOx. Summary of the Invention The present invention solves the problems of a conventional measuring method and aims to provide a highly precise method for measuring an nitrogen oxide in a gas, the method has little influence of a concentration of oxygen or steam in the gas and little fluctuation by a temperature change. According to the present invention, there is provided a method and a sensor for measuring nitrogen oxide (NOx) in a gas as in claim 1 and claim 5 respectively. In a sensor according to the invention, it is not necessary to keep an atmosphere of the first inner space constant. The atmosphere of the first inner space is controlled only to keep constant an atmosphere of a peripheral portion of an entrance of the second inner space. Accordingly, a change of a composition of a gas to be measured is roughly dealt with by controlling a voltage by the main pump means or a feedback control (a proportional control and/or a differential control) by an oxygen concentration detecting means arranged in the first inner space. To a small declination for a relatively long period of time, the voltage applied to the main pump means is controlled with using an output by a means for detecting an atmosphere in the peripheral portion of an entrance of the second inner space. An oxygen partial pressure of the second inner space relates to a substantially integrated value of an atmosphere of the first space for a predetermined period of time by a diffusion resistance of the entrance of the second inner space and a capacity of the second inner space. When an integrated value taken for the longer period of time is required, it is desirable to integrate an output of an oxygen concentration detecting means by an electric circuit. All influences of factors of various changes can be compensated by controlling an atmosphere in the peripheral portion of the entrance of the second inner space, which makes a very stable measurement with a high precision possible. Typically, an electric signal conversion means is an electrochemical pump cell or an electrochemical sensor cell in the present invention. Brief Description of the Drawings Fig. 2 is an explanatory view showing an embodiment of a basic constitution of a sensor used for a method of the present invention. Fig. 6 is an explanatory view showing another embodiment of a basic constitution of a sensor used for a method of the present invention. Fig. 7 is an explanatory view showing yet another embodiment of a basic constitution of a sensor used for a method of the present invention. Fig. 8 is a cross-sectional view and a circuit showing an embodiment of a sensor to be used in the present invention. Fig. 9 is a graph showing a comparison of a change of pump current values of an electrochemical pump cell corresponding to an oxygen partial pressure in an exhaust gas by a method of the present invention and a conventional method. Fig. 10 is a graph showing a comparison of a change of pump current values of an electrochemical pump cell corresponding to an oxygen partial pressure in an exhaust gas by a method of the present invention and a conventional method. Fig. 11 is a graph showing a comparison of a change of pump current values of an electrochemical pump cell corresponding to a steam concentration in an exhaust gas by a method of the present invention and a conventional method. Fig. 12 is a graph showing a comparison of a change of electromotive force of an electrochemical sensor cell corresponding to an oxygen partial pressure in an exhaust gas by a method of the present invention and a conventional method. Fig. 13 is an explanatory view showing an embodiment of a constitution of a conventional sensor. Detailed Description of the Preferred Embodiment In Fig. 2, a gas containing a nitrogen oxide and oxygen which interferes the nitrogen oxide is introduced into a first inner space 2 through a first diffusion resistance 1. A main pump means 60 includes an electrochemical cell 6 having a first oxygen ion conductive solid electrolyte 3 such as zirconia and a pair of inner and outer pump electrodes 5 and 4, respectively. The inner electrode 5 of the electrochemical cell 6 is exposed in the first inner space 2. A control voltage is applied to the electrochemical cell 6 from a power source 7, thereby controlling an atmosphere in the first inner space 2 so as to contain little oxide. At this time, the first inner space 2 may be provided with a pair of electrochemical cells (not shown) so as to control the atmosphere since a voltage applied to the electrochemical pump cell 6 is controlled by an output of the electrochemical cells. Almost all oxygen (interfering component) is removed from a gas to be measured in such a manner. The gas is introduced into a second inner space 9 through a second diffusion resistance 8. An electrochemical cell 12 is an oxygen concentration detecting means having a second oxygen ion conductive solid electrolyte 19 such as zirconia and a pair of electrodes 10 and 11. The electrode 10 is exposed in the second inner space 9. The output of the electrochemical cell 12 is fed back to a power source 7 which applies the output of the electrochemical cell 12 to the electrochemical pump cell 6 so that the output of the electrochemical cell 12 coincides with a predetermined value. Accordingly, an oxygen partial pressure in the second inner space 9 is less influenced by a composition of a gas to be measured in comparison with the case in which the output is controlled only by signals in the first inner space 2. Thus obtained gas to be measured in the second inner space 9, for example, nitrogen monoxide, is decomposed as shown by the following formula by a catalytic action or a reducing action by electrification on a detection electrode 14 of the electrochemical pump cell 13 which is an electric signal conversion means 70, the detection electrode 14 being exposed in the second inner space 9. As a result, a generated oxygen is sent to a space 15 where a standard gas is present through the detection electrode 14 and a reference electrode 11. An amount of NOx is obtained by a value of a current generated by the transference of the oxygen. [Formula 1] 2NO → N 2 + O 2 According to the invention, the oxygen concentration detecting means formed in the second inner space 9 is configured as an oxygen pump cell 12. In Fig. 2, an oxygen partial pressure in the second inner space 9 is detected as a limited diffusion current by a predetermined voltage applied to the portion between the electrode 10 and the electrode 11. The output is fed back to the power source 7 which applies the output to the first electrochemical cell 6 so that a value of the limited diffusion current coincides with a predetermined value. Fig. 6 shows an embodiment in which the electrochemical pump cell 13 of a constitution shown in Fig. 2 is substituted by an electrochemical sensor cell 55 as an electric signal conversion means 70. The others are the same as in Fig. 2. Accordingly, when a sensor having a structure shown in Fig. 6 is used, a gas to be measured in a second inner space 9 is generated by a catalytic action on a detection electrode 14 of the electrochemical sensor cell 55 which is an electric signal conversion means 70, the detection electrode 14 being exposed in the second inner space 9. As a result, an amount of NOx is obtained from an electromotive force generated between a detection electrode 14 and a reference electrode 11. Fig. 7 shows an embodiment in which a porous protective layer 57 was disposed on a detection electrode 14 in a constitution shown in Fig. 2. In this embodiment, a second inner space 9 and a second diffusion resistance 8 are formed with using a porous body. The other constitutions are the same as in Fig. 2. In such a constitution of a sensor element, the second inner space 9 and the second diffusion resistance 8 may be unitarily formed with using a porous ceramic and disposed in a third inner space 17. Further, the second inner space 9 and the second diffusion resistance 8 may be substituted by adjustment of porosity (rate of pores, diameter of pores, etc.) of an electrode 14 of signal conversion means 70 which is exposed in a gas to be measured. In Fig. 7, it is possible to unitarily form the second inner space 9 and the second diffusion resistance 8 with using a porous ceramic and dispose them in a first inner space 2. Alternatively, the second inner space 9 and the second diffusion resistance 8 may be substituted by adjustment of porosity (rate of pores, diameter of pores, etc.) of an electrode 14 of signal conversion means 70 which is exposed in a gas to be measured. Fig. 8 shows an embodiment of a measuring circuit of the present invention. A gas to be measured is introduced into a first inner space 2 through a first diffusion resistance 1. There, almost all oxygen in the gas is removed by an electrochemical pump cell 6 (main pump means 60). As a control voltage applied between an outer pump electrode 4 and an inner pump electrode 5 of the electrochemical pump cell 6, an output compared with a predetermined voltage of a power source 23 by a comparator 22 is used with an electromotive force between the inner pump electrode 5 and a reference electrode 11 being subjected to an impedance conversion by an amplifier 21. The gas to be measured obtains an oxygen partial pressure almost same as a predetermined one by this. The gas enters a second inner space 9 through a second diffusion resistance 8. A predetermined voltage is applied to a electrochemical cell 12 which is an oxygen concentration detection means disposed in the second inner space 9 from a power source 24. A pump current flowing at that time is detected by a resistance 25 and an amplifier 26, made to pass an integrated circuit of a resistance 27 and a condenser 28, and compared with a predetermined volume of a power source 29 by a comparator 30. A voltage to be applied to the electrochemical pump cell 6 is adjusted by the output. An atmosphere in the second inner space 9 is stabilized by this. NOx is decomposed by an electrode 14 having a catalytic activity, and an amount of generated oxygen is detected as a pump current of an electrochemical pump cell 13. Incidentally, an oscillation of the aforementioned integrated circuit can be avoided by setting a time constant for an adequate value. The inner pump electrode 5 in the first inner space 2 and the electrode 10 in the second inner space 9 are preferably made of a material having a low catalytic activity to NOx, for example, Au or Au-Pt alloy. The electrode 14 which detects NOx is preferably made of a material having a high catalytic activity to NOx such as Rh. [ Example ] In each of the case of the sensor and circuit shown in Fig. 8, in which a zirconia porcelain was used as a solid electrolyte, and the case of the conventional sensor and circuit shown in Fig. 13, an exhaust gas of a gasoline-combustion engine was measured for NOx amount therein. A change of pump current values of the electric signal conversion means 70 (electrochemical pump cell 13) according to a concentration of an interfering gas component in the exhaust gas was compared to each other. A voltage V1 between the inner pump electrode 5 and the reference electrode 11, a terminal voltage V2 at the electrochemical cell 12, and a terminal voltage V3 at the electrochemical pump cell 13 were adjusted as in Table 1. The results are shown in Figs. 9 - 11. Incidentally, Fig. 13 shows a sensor in which an oxygen partial pressure in the first inner space 2 is controlled only by a potential difference detection means 40. One of the electrodes of the potential difference detection means 40 is exposed in the first inner space 2. Example 1 (Fig. 9) Example 2 (Fig. 10) Example 3 (Fig. 11) V1 300 mV 300 mV 300 mV V2 400 mV 400 mV 400 mV V3 450 mV 430 mV 430 mV As Fig. 9 shows, when an oxygen partial pressure in a gas to be measured ascended, a pump current flowing in the electric signal conversion means (electrochemical pump cell) 70 was increased as a curved line B in a measuring method having a conventional constitution shown in Fig. 13, thereby increasing an error. On the other hand, in a straight line A based on a method of the present invention shown in Fig. 8, an oxygen partial pressure dispersed in the second inner space 9 is detected and negatively fed back to the electrochemical pump cell 6. Accordingly, a pump current flowing in the electric signal conversion means 70 (electrochemical pump cell 13) is not changed even if an oxygen partial pressure in a gas to be measured changes. Thus, a measuring preciseness (a change of an offset current) is sharply improved. However, since there is a large difference between an electromotive force and an applied voltage V3 of the electric signal conversion means in this embodiment, an offset value of the pump current is not zero. In Fig. 10, when an oxygen partial pressure in a gas to be measured in the conventional constitution shown in Fig. 13 ascended, a pump current flowing in the electric signal conversion means (electrochemical pump cell) 70 was increased as shown by a curved line D, thereby increasing an error. On the other hand, in the method of the present invention shown in Fig. 8, a pump current flowing in the electric signal conversion means 70 (electrochemical pump cell 13) is not changed as shown by a straight line C even if an oxygen partial pressure in a gas to be measured changes. Further, an electromotive force of the electric signal conversion means was equalized with an applied voltage V3 when an oxygen partial pressure in the gas to be measured was zero. As a result, an offset value of a pump current of the electric signal conversion means was zero at that time. As Fig. 11 shows, when a steam concentration in a gas to be measured ascended in a conventional constitution shown in Fig. 13, a pump current flowing in the electric signal conversion means (electrochemical pump cell) 70 was decreased as a curved line F, thereby increasing an error. The reason seems that because a diffusion coefficient of hydrogen generated by dissociation of water in the first inner space 2 is larger than that of oxygen, hydrogen enters the second inner space 9 more than the oxygen. Accordingly, an equilibrium oxygen partial pressure in the second inner space is lowered, and a pump current required for maintaining a predetermined oxygen partial pressure in the second inner space 9 is decreased. On the other hand, in a method of the present invention shown in Fig. 8, since the electrochemical cell 12 plays a part of adjustment including a difference in diffusion coefficients based on a difference in kinds of gases or a temperature change in the second diffusion resistance 8, a pump current flowing in the electric signal conversion means 70 (electrochemical pump cell 13) is not changed as a straight line E even if an oxygen partial pressure in a gas to be measured changes. Then, in each of the case of the sensor and circuit shown in Fig. 6 (present invention), in which a zirconia porcelain was used as a solid electrolyte, and the case of the conventional sensor and circuit shown in Fig. 13, an exhaust gas of a gasoline-combustion engine was measured for NOx amount therein in the same manner as described above. A change of electromotive force of the electric signal conversion means 70 (electrochemical sensor cell 55) according to a oxygen partial pressure in the exhaust gas was compared to each other. Incidentally, in a sensor shown in Fig. 13, the electrochemical pump cell was substituted by an electrochemical sensor cell as the electric signal conversion means 70. Incidentally, in Fig. 6 (present invention), V1 and V2 were set to be 300 mV and 400 mV, respectively. In Fig. 13 (conventional method) V1 was set to be 300 mV. The results are shown in Fig. 12. As Fig. 12 shows, when an oxygen partial pressure in a gas to be measured in the conventional constitution shown in Fig. 13 ascended, an electromotive force in the electric signal conversion means (electrochemical sensor cell) 70 was decreased as shown by a curved line H, thereby increasing an error. On the other hand, in the method of the present invention shown in Fig. 6, an electromotive force in the electric signal conversion means 70 (electrochemical sensor cell 55) is not changed as shown by a straight line G even if an oxygen partial pressure in a gas to be measured changes. Accordingly, NOx can be measured very precisely. As obvious from the above description, the present invention gives a very precise method for measuring NOx, having little influence of oxygen or steam concentration in a gas to be measured and little fluctuation by a temperature change, and is very useful as a means for improving an air environment, or the like.

Claims: A method of measuring concentration of NOx as a component of a measurement gas by means of a NOx sensor having first and second inner spaces (2,9), comprising the steps of: introducing the measurement gas containing the NOx component from an external measurement-gas space into said first inner space (2), via a first diffusion resistance (1); controlling the partial pressure of oxygen in the measurement gas within said first inner space (2) to a level at which NO is not decomposed, by means of a main electrochemical pump cell (6) of the sensor; introducing the measurement gas from said first internal space (2) into said second inner space (9) via a second diffusion resistance (8); determining the concentration of the NOx component present in the atmosphere in said second inner space (9) by decomposition or reduction so as to derive oxygen from the NOx component and measuring the oxygen in said second inner space by means of electric signal conversion means (13) of the sensor which has an electrode (14) in said second inner space (9) and gives an electrical output, while controlling the oxygen partial pressure of the atmosphere in said first and second inner spaces such that the concentration of the NOx component determined in said second inner space (9) by means of said electrical signal conversion means (13) corresponds to a concentration of the NOx component in said measurement gas; characterized by detecting oxygen concentration in gas in said second inner space (9) which has passed from said first inner space (2) via said second diffusion resistance, by means of a second electrochemical pump cell (12) of the sensor which has an electrode (10) in said second inner space (9) and adjusts oxygen concentration of gas in said second inner space, and adjusting said control voltage applied in said main pump cell (6) in dependence on the measurement output of said second pump cell (12) so as to maintain the measurement output of said second pump cell (12) constant. A method according claim 1, wherein said electric signal conversion means is a third electrochemical pump cell, and the determination of NOx is carried out on the basis of a current for pumping of said third electrochemical pump cell. A method according to claim 1 or 2, wherein said electrodes (10,14) of said electric signal conversion means (13) and said second electrochemical pump cell (12) respectively are both exposed to the same gas atmosphere in said second inner space. A method according to claim 1 or 2, wherein said electrode (14) of said electric signal conversion means is covered by a porous protective layer (57) providing diffusion resistance and said electrode (10) of said second electrochemical pump cell (12) is adjacent said porous protective layer (57). A NOx sensor for measuring nitrogen oxide (NOx) in gas, having a first inner space (2) and a first diffusion resistance (1) for introducing gas to be measured from outside the sensor to said first inner space (2), a main electrochemical pump cell (6) for controlling oxygen partial pressure in said first inner space (2), means for applying a control voltage to said main electrochemical pump cell in order to effect oxygen pumping thereby, a second inner space (9) connected to said first inner space via a second diffusion resistance (8), electrical signal conversion means (13) having an electrode (14) in said second inner space (9), for effecting decomposition or reduction of nitrogen oxide in said second inner space (9) and measuring the amount of oxygen produced by the decomposition or reduction by producing an electrical output corresponding to the amount of oxygen produced, characterized by a second electrochemical pump cell (12) having an electrode (10) in said second inner space (9) arranged for adjusting oxygen concentration in gas in said second inner space (9) which has passed from said first inner space via said second diffusion resistance (8), means for applying electrical power to effect pumping of oxygen by said second electrochemical pump cell (12) and for obtaining thereby an electrical measurement output dependent on the oxygen concentration at said electrode (10) thereof, and means for adjusting said control voltage applied in said main electrochemical pump cell (6) in dependence on said measurement output of said second electrochemical pump cell (12) so as to maintain said measurement output of said second electrochemical pump cell (12) at a constant level. A NOx sensor according to claim 5, wherein said electrodes (10,14) of said electric signal conversion means (13) and said second electrochemical pump cell (12) respectively are both exposed to the same gas atmosphere in said second inner space. A NOx sensor according to claim 5, wherein said electrode (14) of said electric signal conversion means is covered by a porous protective layer (57) providing diffusion resistance and said electrode (10) of said second electrochemical pump cell (12) is adjacent said porous protective layer (57).

Inventors: INA NORIYUKI, KATO NOBUHIDE, INA, NORIYUKI, KATO, NOBUHIDE
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1997
Main classification: G01N27
IPC codes: G01N27