- Email: [email protected]
A limiting current type sensor for hydrocarbons
Sensors and Actuators B 66 Ž2000. 9–11 www.elsevier.nlrlocatersensorb
A limiting current type sensor for hydrocarbons Y. Eguchi, S. Watanabe, N. Kubota, T. Takeuchi ) , T. Ishihara, Y. Takita Department of Applied Chemistry, Faculty of Engineering, Oita UniÕersity, 700 Dannoharu, Oita 870-1192, Japan Received 30 July 1998; received in revised form 25 January 1999; accepted 2 March 1999
Abstract A hydrocarbon ŽHC. sensor, composed of yttria-stabilized zirconia ŽYSZ. as a solid electrolyte and catalytically active and passive material for HC as electrodes, is proposed. The active and passive electrodes were attached to the same side of a YSZ disk as cathodes. A Pt electrode was applied to the other side of YSZ as anode. The diffusion of gas to the cathodes was limited by a pinhole of a cover attached to YSZ and the sensor showed a limiting current behavior by applying voltage. The difference between the limiting current, D I, of two cathodes increased linearly with the concentration of HC such as methane and isobutane, which suggests that HC concentration can be obtained by measuring D I. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; Hydrocarbon; Limiting current; YSZ
1. Introduction
2. Experimental
Limiting current type oxygen sensors have been used for monitoring exhaust gas from automobiles w1x. The sensor element consists of yttria-stabilized zirconia ŽYSZ. and a gas diffusion-limiting device attached to the outside of the cathode. By applying voltage, the sensor shows a limiting current, which depends on oxygen concentration. Recently, limiting current type sensors began to be applied to gases other than oxygen. Reinhardt et al. w2x proposed an amperometric sensor for NO that has two electrodes. On the other hand, Narducci et al. w3x and Lee et al. w4x reported limiting current type CO 2 and CO sensors, respectively. Recently, hydrocarbon ŽHC. in exhaust gas from cars was strictly regulated in California, USA. Similar regulations may be applied in other countries in the future. One of the expected uses of HC sensors is to monitor the deterioration of three-way automotive catalysts. Hibono et al. reported HC sensors consisting of a high temperaturetype proton conductor and YSZ as a solid electrolyte and Pd and Au as electrodes w5,6x. In this report, a limiting current type HC sensor, which has two types of cathodes and a cover for limiting gas diffusion, is proposed and examined.
2.1. Fabrication of sensor elements YSZ powder ŽHSY-8. was pressed into a pellet and sintered at 15008C for 1 h. The powder was kindly supplied by Daiichi, Japan. The YSZ disk Ž16 mm in diameter, 0.74 mm in thickness. was employed as a solid electrolyte. Metal pastes ŽTanaka, Japan. such as gold and palladium were used as two kinds of cathodes with 0.2 cm2 area and a platinum paste was utilized as an anode with 0.5 cm2 area. Platinum paste ŽTR-706. was mixed with Pt black ŽTanaka. and platinum ŽII. chloride ŽTanaka. and Au paste ŽT-60. was mixed with Au powder ŽAY-5022, Tanaka.. Platinum and gold wires were used as output terminals for electrodes. Electrodes were baked at 9008C for 30 min and the cover for a gas diffusion limiting, which was made of machinable ceramics ŽMitui mining material, MACERiTE-SP, Japan., was attached to the outside of the cathodes with ceramic adhesive ŽULTRA-TEMP 516, Aremco, USA.. The size of a pinhole of the cover was 0.3 mm in diameter. Fig. 1 shows the schematic diagram of the sensor structure. 2.2. Measurement of sensor characteristics
)
Corresponding author.
The experiment was carried out in the flow of a mixture of nitrogen, oxygen and 0–2000 ppm HC, such as methane
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 4 6 5 - 7
Y. Eguchi et al.r Sensors and Actuators B 66 (2000) 9–11
10
Fig. 1. Schematic diagram of the sensor structure.
or isobutane. The sensor characteristics were measured by potentiostat ŽHA-301, Hokuto Denko, Japan. and digital multimeter ŽR6451A, Advantest, Japan.. The electrical measurement of two cathodes was made alternately. In other words, when one cathode was measured, electrical circuit of the other cathode was opened.
3. Results and discussion Fig. 2 shows the I–V characteristics of sensor elements with Pd and Au electrodes in the flow of a mixture of nitrogen, 2.5% oxygen and 0 or 2000 ppm HC at 7008C, where i Au and i Pd are the currents of Au and Pd electrodes, respectively. When the sample gas contains no HC, the limiting current corresponds only to O 2 concentration. In this case, the limiting current, I Pd , of the Pd electrode coincides with the IAu of the Au electrode, which means that the limiting currents for both electrodes are restricted by the same pinhole. When HC was introduced, I Pd and IAu decreased because of the oxygen consumption by HC. Although Au is passive to HC, IAu decreased about the same as I Pd . It seems that IAu was affected by Pd electrode because both cathodes were enclosed within the
Fig. 2. I – V characteristics of sensor element with Pd and Au electrodes at 7008C in the flow of a mixture of nitrogen, 2.5%-fixed oxygen and HC. iAu ŽI: HC 0 ppm, ^: methane 2000 ppm, `: isobutane 2000 ppm.; i Pd ŽB: HC 0 ppm, ': methane 2000 ppm, v: isobutane 2000 ppm..
Fig. 3. Relationship between limiting current and isobutane concentration at 7008C at 1.2 V of applied potential. `: Au electrode, v: Pd electrode.
same space. It is seen in Fig. 2 that in the case of isobutane, the dependence of the limiting current on applied potential shows single and double plateaus for Pd and Au electrodes, respectively. It is considered that the second plateau of Au is due to combustion products such as H 2 O and CO 2 decomposing to H 2 and CO. However, on the Pd electrode, it is considered that the decomposition scarcely occurs. The result in Fig. 3 shows limiting current measured at applied potential of 1.2 V with varying isobutane concentration. The results show that I Pd depends largely on isobutane concentration, while IAu shows limited dependence. The difference between I Pd and IAu seems to be the result of the difference in decomposition characteristics of H 2 O and CO 2 on Pd and Au. This difference can be applied to measure HC concentration. The difference between the limiting currents, D I, of the two cathodes is defined as D I s IAu y I Pd . Fig. 4 represents the relationship between D I and HC concentration at applied potential of 1.2 V at 7008C. As
Fig. 4. Relationship between D I and HC concentration at 7008C at 1.2 V of applied potential. ^: Isobutane, I: methane.
Y. Eguchi et al.r Sensors and Actuators B 66 (2000) 9–11
11
over 8% oxygen may be due to deterioration of palladium electrode. 4. Conclusions
Fig. 5. Relationship between D I and O 2 concentration at 7008C at 1.5 V of applied potential in the flow of a mixture of nitrogen, oxygen and 1500 ppm HC. ^: Isobutane, I: methane.
shown in the figure, D I increases linearly with HC concentration. This means that HC concentration can be obtained by measuring D I. The slope for isobutane in Fig. 4 is larger than that for methane because reactivity of isobutane is higher than that of methane and the amount of oxygen required for isobutane is larger than that for methane. The present sensor utilizes the difference between the limiting currents, D I, of two cathodes. Therefore, the influence of oxygen concentration will be canceled out. Indeed, the experiment on D I at 1.5 V in the flow of a mixture of nitrogen, oxygen and 1500 ppm HC shows almost independent relation on oxygen concentration in the range of 2.5–7.5% as in Fig. 5. The sharp increase in D I
A limiting current type sensor, which has two cathodes and a cover with a pinhole, is proposed. The limiting current of the sensor decreased with increasing HC concentration in the sample gas containing HC because of consumption of oxygen by HC. The limiting current, IAu , changed through two steps, while I Pd remained constant. The difference between the limiting current, D I, of two cathodes at 1.2 V of applied potential was linearly proportional to HC concentration and did not depend on oxygen concentration in the range of 2.5–7.5% because D I cancels out the influence of oxygen concentration. This sensor may be suitable for automobiles with lean burn engine system. References w1x T. Takeuchi, I. Igarashi, in: Chemical Sensor Technology Vol. 1 Kodansha, Tokyo, 1988, pp. 79–95. w2x G. Reinhardt, S.I. Somov, U. Schonauer, U. Guth, Gopel, Proc. 8th ¨ ¨ Int. Conf. Solid-State Sensors and Actuators and Eurosensors IX Ž1995. 799. w3x D. Narducci, L. Facheris, C.M. Mari, Sens. Actuators B, 24–25 Ž1995. 636. w4x J.-H. Lee, H. Kim, B.-K. Kim, Solid State Ionics 86–88 Ž1996. 1087. w5x T. Hibono, A. Masegi, H. Iwahara, Res. Chem. Intermed. 21 Ž2. Ž1995. 181. w6x T. Hibono, Y. Kuwahara, Y. Kuroki, T. Oshima, R. Inoue, S. Kitanoya, T. Fuma, Solid State Ionics 104 Ž1997. 163.
A limiting current type sensor for hydrocarbons Y. Eguchi, S. Watanabe, N. Kubota, T. Takeuchi ) , T. Ishihara, Y. Takita Department of Applied Chemistry, Faculty of Engineering, Oita UniÕersity, 700 Dannoharu, Oita 870-1192, Japan Received 30 July 1998; received in revised form 25 January 1999; accepted 2 March 1999
Abstract A hydrocarbon ŽHC. sensor, composed of yttria-stabilized zirconia ŽYSZ. as a solid electrolyte and catalytically active and passive material for HC as electrodes, is proposed. The active and passive electrodes were attached to the same side of a YSZ disk as cathodes. A Pt electrode was applied to the other side of YSZ as anode. The diffusion of gas to the cathodes was limited by a pinhole of a cover attached to YSZ and the sensor showed a limiting current behavior by applying voltage. The difference between the limiting current, D I, of two cathodes increased linearly with the concentration of HC such as methane and isobutane, which suggests that HC concentration can be obtained by measuring D I. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Gas sensor; Hydrocarbon; Limiting current; YSZ
1. Introduction
2. Experimental
Limiting current type oxygen sensors have been used for monitoring exhaust gas from automobiles w1x. The sensor element consists of yttria-stabilized zirconia ŽYSZ. and a gas diffusion-limiting device attached to the outside of the cathode. By applying voltage, the sensor shows a limiting current, which depends on oxygen concentration. Recently, limiting current type sensors began to be applied to gases other than oxygen. Reinhardt et al. w2x proposed an amperometric sensor for NO that has two electrodes. On the other hand, Narducci et al. w3x and Lee et al. w4x reported limiting current type CO 2 and CO sensors, respectively. Recently, hydrocarbon ŽHC. in exhaust gas from cars was strictly regulated in California, USA. Similar regulations may be applied in other countries in the future. One of the expected uses of HC sensors is to monitor the deterioration of three-way automotive catalysts. Hibono et al. reported HC sensors consisting of a high temperaturetype proton conductor and YSZ as a solid electrolyte and Pd and Au as electrodes w5,6x. In this report, a limiting current type HC sensor, which has two types of cathodes and a cover for limiting gas diffusion, is proposed and examined.
2.1. Fabrication of sensor elements YSZ powder ŽHSY-8. was pressed into a pellet and sintered at 15008C for 1 h. The powder was kindly supplied by Daiichi, Japan. The YSZ disk Ž16 mm in diameter, 0.74 mm in thickness. was employed as a solid electrolyte. Metal pastes ŽTanaka, Japan. such as gold and palladium were used as two kinds of cathodes with 0.2 cm2 area and a platinum paste was utilized as an anode with 0.5 cm2 area. Platinum paste ŽTR-706. was mixed with Pt black ŽTanaka. and platinum ŽII. chloride ŽTanaka. and Au paste ŽT-60. was mixed with Au powder ŽAY-5022, Tanaka.. Platinum and gold wires were used as output terminals for electrodes. Electrodes were baked at 9008C for 30 min and the cover for a gas diffusion limiting, which was made of machinable ceramics ŽMitui mining material, MACERiTE-SP, Japan., was attached to the outside of the cathodes with ceramic adhesive ŽULTRA-TEMP 516, Aremco, USA.. The size of a pinhole of the cover was 0.3 mm in diameter. Fig. 1 shows the schematic diagram of the sensor structure. 2.2. Measurement of sensor characteristics
)
Corresponding author.
The experiment was carried out in the flow of a mixture of nitrogen, oxygen and 0–2000 ppm HC, such as methane
0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 9 9 . 0 0 4 6 5 - 7
Y. Eguchi et al.r Sensors and Actuators B 66 (2000) 9–11
10
Fig. 1. Schematic diagram of the sensor structure.
or isobutane. The sensor characteristics were measured by potentiostat ŽHA-301, Hokuto Denko, Japan. and digital multimeter ŽR6451A, Advantest, Japan.. The electrical measurement of two cathodes was made alternately. In other words, when one cathode was measured, electrical circuit of the other cathode was opened.
3. Results and discussion Fig. 2 shows the I–V characteristics of sensor elements with Pd and Au electrodes in the flow of a mixture of nitrogen, 2.5% oxygen and 0 or 2000 ppm HC at 7008C, where i Au and i Pd are the currents of Au and Pd electrodes, respectively. When the sample gas contains no HC, the limiting current corresponds only to O 2 concentration. In this case, the limiting current, I Pd , of the Pd electrode coincides with the IAu of the Au electrode, which means that the limiting currents for both electrodes are restricted by the same pinhole. When HC was introduced, I Pd and IAu decreased because of the oxygen consumption by HC. Although Au is passive to HC, IAu decreased about the same as I Pd . It seems that IAu was affected by Pd electrode because both cathodes were enclosed within the
Fig. 2. I – V characteristics of sensor element with Pd and Au electrodes at 7008C in the flow of a mixture of nitrogen, 2.5%-fixed oxygen and HC. iAu ŽI: HC 0 ppm, ^: methane 2000 ppm, `: isobutane 2000 ppm.; i Pd ŽB: HC 0 ppm, ': methane 2000 ppm, v: isobutane 2000 ppm..
Fig. 3. Relationship between limiting current and isobutane concentration at 7008C at 1.2 V of applied potential. `: Au electrode, v: Pd electrode.
same space. It is seen in Fig. 2 that in the case of isobutane, the dependence of the limiting current on applied potential shows single and double plateaus for Pd and Au electrodes, respectively. It is considered that the second plateau of Au is due to combustion products such as H 2 O and CO 2 decomposing to H 2 and CO. However, on the Pd electrode, it is considered that the decomposition scarcely occurs. The result in Fig. 3 shows limiting current measured at applied potential of 1.2 V with varying isobutane concentration. The results show that I Pd depends largely on isobutane concentration, while IAu shows limited dependence. The difference between I Pd and IAu seems to be the result of the difference in decomposition characteristics of H 2 O and CO 2 on Pd and Au. This difference can be applied to measure HC concentration. The difference between the limiting currents, D I, of the two cathodes is defined as D I s IAu y I Pd . Fig. 4 represents the relationship between D I and HC concentration at applied potential of 1.2 V at 7008C. As
Fig. 4. Relationship between D I and HC concentration at 7008C at 1.2 V of applied potential. ^: Isobutane, I: methane.
Y. Eguchi et al.r Sensors and Actuators B 66 (2000) 9–11
11
over 8% oxygen may be due to deterioration of palladium electrode. 4. Conclusions
Fig. 5. Relationship between D I and O 2 concentration at 7008C at 1.5 V of applied potential in the flow of a mixture of nitrogen, oxygen and 1500 ppm HC. ^: Isobutane, I: methane.
shown in the figure, D I increases linearly with HC concentration. This means that HC concentration can be obtained by measuring D I. The slope for isobutane in Fig. 4 is larger than that for methane because reactivity of isobutane is higher than that of methane and the amount of oxygen required for isobutane is larger than that for methane. The present sensor utilizes the difference between the limiting currents, D I, of two cathodes. Therefore, the influence of oxygen concentration will be canceled out. Indeed, the experiment on D I at 1.5 V in the flow of a mixture of nitrogen, oxygen and 1500 ppm HC shows almost independent relation on oxygen concentration in the range of 2.5–7.5% as in Fig. 5. The sharp increase in D I
A limiting current type sensor, which has two cathodes and a cover with a pinhole, is proposed. The limiting current of the sensor decreased with increasing HC concentration in the sample gas containing HC because of consumption of oxygen by HC. The limiting current, IAu , changed through two steps, while I Pd remained constant. The difference between the limiting current, D I, of two cathodes at 1.2 V of applied potential was linearly proportional to HC concentration and did not depend on oxygen concentration in the range of 2.5–7.5% because D I cancels out the influence of oxygen concentration. This sensor may be suitable for automobiles with lean burn engine system. References w1x T. Takeuchi, I. Igarashi, in: Chemical Sensor Technology Vol. 1 Kodansha, Tokyo, 1988, pp. 79–95. w2x G. Reinhardt, S.I. Somov, U. Schonauer, U. Guth, Gopel, Proc. 8th ¨ ¨ Int. Conf. Solid-State Sensors and Actuators and Eurosensors IX Ž1995. 799. w3x D. Narducci, L. Facheris, C.M. Mari, Sens. Actuators B, 24–25 Ž1995. 636. w4x J.-H. Lee, H. Kim, B.-K. Kim, Solid State Ionics 86–88 Ž1996. 1087. w5x T. Hibono, A. Masegi, H. Iwahara, Res. Chem. Intermed. 21 Ž2. Ž1995. 181. w6x T. Hibono, Y. Kuwahara, Y. Kuroki, T. Oshima, R. Inoue, S. Kitanoya, T. Fuma, Solid State Ionics 104 Ž1997. 163.