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Improvement of sensing performances of zirconia-based total NOx sensor by attachment of oxidation-catalyst electrode
Solid State Ionics 175 (2004) 503 – 506 www.elsevier.com/locate/ssi
Improvement of sensing performances of zirconia-based total NOx sensor by attachment of oxidation-catalyst electrode Takashi Onoa,b,*, Masaharu Haseia, Akira Kunimotoa, Norio Miurac a R & D Department, Riken Corporation, Kumagaya-shi, Saitama 360-8522, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan c Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan b
Accepted 11 April 2004
Abstract The sensing performance of the mixed-potential-type total NOx sensor based on laminated yttria-stabilized zirconia (YSZ) sheets was improved by the attachment of oxidation-catalyst electrode. It was found that pure Pt was the best material for the catalyst electrode among the four Pt-based metals examined. The addition of 20 vol.% resin to Pt-electrode paste was quite effective to improve the catalytic activity of the electrode. The obtained sensor could measure total NOx concentration down to 10 ppm at 600 8C. The use of the oxidation-catalyst electrode was found to decrease largely the interference of reducing gases (C3H8 and CO) to the NOx sensitivity. D 2004 Elsevier B.V. All rights reserved. PACS: 82.47.Rs; 82.45.Xy; 82.45.Jn Keywords: NOx sensor; Oxidation-catalyst; YSZ; Mixed potential; Automobile exhaust
1. Introduction Lately, compact low-cost and reliable on-board NOx sensors have been strongly required to improve a catalyst diagnosis system for automobiles [1–3]. So far, we have been developing the mixed-potential-type zirconia-based sensors using oxide-sensing electrode [4–7]. These sensors are confirmed to be able to detect NOx content in the test gas in the low concentration range even below ca. 100 ppm at rather high temperatures. This sensor, however, has the problem that its accuracy in NOx detection is not high when reducing gases, such as hydrocarbons and CO, coexist in the gas phase. Thus, in this study, we tried to search for the oxidationcatalyst electrode material which can convert effectively the reducing gases existing in car exhausts into noninterference * Corresponding author. R & D Department, Riken Corporation, Kumagaya-shi, Saitama 360-8522, Japan. Tel.: +81 48 527 2001; fax: +81 48 527 2010. E-mail address: [email protected] (T. Ono). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.04.044
gases. We also examined the performances of the NOx sensor attached to the selected oxidation-catalyst electrode.
2. Design of laminated YSZ-based NOx sensor As a practical total NOx sensor for automobiles, we designed here a laminated yttria-stabilized-zirconia (YSZ)based sensor attached with an oxidation-catalyst electrode. Fig. 1 shows a cross-sectional view of the sensor designed. The green sheets of yttria-stabilized zirconia (YSZ, 6 mol% Y2O3 doped) were fabricated by means of doctor-blade method. Each thick-film electrode of precious metals (or metal oxides) was screen-printed on the green sheets obtained. Then, these green sheets were laminated and fired to make the sensing device given in Fig. 1. The size of sensing device was 70 mm in length, 5 mm in width and 3 mm in thickness. The size of the oxidation-catalyst electrode was 4 mm in length, 3 mm in width and 3 Am in thickness. The oxidation-catalyst electrode was formed in the gas inlet leading to the inner cavity. Test gases were let to flow into
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T. Ono et al. / Solid State Ionics 175 (2004) 503–506
Fig. 1. Cross-sectional view of the laminated-type NOx sensor using an oxidation-catalyst electrode.
the inner cavity by passing through the porous oxidationcatalyst electrode layer. Only the reducing gases such as C3H8 and CO can be adsorbed on the surface of the oxidation-catalyst electrode. Oxygen gas can be supplied electrochemically to the oxidation-catalyst electrode side from the air duct side by applying the fixed voltage of 500 mV between the oxidation-catalyst electrode and the counterelectrode exposed to the air duct. Then, the adsorbed reducing gases are chemically and/or electrochemically oxidized and converted into noninterference gases (CO2 and H2O). Both NO and NO2 are also let to flow into the inner cavity, and all NO gas can be converted to NO2 by the NOx conversion electrode settled in the upper side of the cavity. Thus, the total NOx content in the test gas can be measured with the NOx -sensing electrode installed at the bottom of the inner cavity.
3. Experimental To examine the activity of the various oxidation-catalyst electrodes, a pumping-cell-type device was separately fabricated. The cell was fabricated by firing YSZ green sheet (18 mm in diameter) on which the oxidation-catalyst electrode (anode) and the Pt counterelectrode (cathode) were screen-printed. The size of each electrode was 6 mm in length, 6 mm in width and 3 Am in thickness. Pure Pt and Pt (+5 wt.% Rh), Pt (+5 wt.% Pd), Pt (+5 wt.% Ru) alloys were used as candidate materials for oxidation-catalyst electrodes. The porosity of the electrode layer was adjusted by adding resin powder into each electrode-paste. The obtained pumping cell was fixed on the end of YSZ tube by means of glass sealant after a Pt lead wire was connected to each electrode to construct the whole cell. The evaluation cell thus fabricated was heated up to 600 8C in an electric furnace, and then a fixed voltage of 500 mV was applied between the two electrodes of the cell by means of a potentiostat. The difference in electric current (oxidation current: IC3H8) measured under the two conditions, i.e., in 5000 ppm C3H8 (diluted with 5 vol.% O2 and 94.5 vol.% N2) flow and in the flow of 5 vol.% O2 and 95 vol.% N2,
was evaluated as an oxidation-catalyst activity for each electrode. By using the best material selected above for the catalyst electrode, the laminated-type NOx sensor was fabricated. The sensing device was made up to the assembly structure by mounting an alumina tube, a metal fitting and a protective cap. The whole sensor was connected electrically with a control unit consisting of electric circuits. The potential difference between the NOx -sensing electrode using Cr2O3 and the Pt reference-electrode was measured as a sensing signal. This sensing signal was calculated by a control unit based on the output data of the oxygen-sensing electrode. The latter was installed near the NOx -sensing electrode to cancel out the dependence of the sensor output on the concentration of oxygen. Then, the sensing performances of the sensor with or without the oxidation-catalyst electrode were evaluated in the flow of the various test gases.
4. Results and discussion First, the oxidation current (IC3H8) of the pumping-type cell using each of precious-metal-catalyst electrode was evaluated. It was confirmed that the Pt-catalyst electrode gave the highest current value (ca. 420 AA), while the alloy electrodes examined gave significantly lower values (ca. 300 AA). Thus, hereafter, pure Pt was used as the catalyst electrode material for the NOx sensor. By the way, the oxidation current value seems to indicate the oxidationcatalyst activity of the electrode toward the reducing gases. This point can be explained as follows. When the reducing gas is consumed at the surface of the catalyst electrode, the concentration of oxygen in the vicinity of the catalyst electrode can be decreased. Such a decrease in oxygen concentration can increase the pumping rate of oxygen from the counterelectrode side to the catalyst electrode side when the reducing gas is coexisting in the test gas. This seems to be caused by the generation of reverse electromotive force based on the formation of oxygen concentration cell across the present pumping cell under the condition that the constant voltage is applied between the air counterelectrode and the catalyst electrode. Fig. 2 shows the influence of resin content (vol.%) in the Pt paste on the oxidation current in the flow of 5000 ppm C3H8 diluted with 5 vol.% O2 and 94.5 vol.% N2 at 600 8C. It is seen that the maximum value of IC3H8 appears at the content of 20 vol.%. Fig. 3 shows the complex impedance spectra measured in 5000 ppm C3H8 in the frequency range 20 mHz–100 kHz at 600 8C for each of the cells attached with oxidation-catalyst electrodes formed by the paste having different resin content. A large semicircular arc was observed in each spectrum. The resistance value (ZV) at the intersection of the semiarc with the real axis at lower frequencies varied with the resin content, and then the minimum resistance (3.2 kV) was obtained at 20 vol.%.
T. Ono et al. / Solid State Ionics 175 (2004) 503–506
Fig. 2. Influence of resin content on the oxidation current.
505
Fig. 4. Relationship between sensor output and NOx concentration at 600 8C for the laminated sensor attached with the Pt-catalyst electrode.
This ZV value of kV level seems to be ascribable to the electrode reaction resistance at the interface between YSZ and Pt. These results indicate that the density of reaction site at three-phase boundary (Pt/YSZ/gas) is highest when the catalyst electrode layer was fabricated by using the Pt paste containing 20 vol.% resin. Thus, hereafter, we examined the sensing performances of the laminated sensor attached with the Pt catalyst electrode fabricated by using the Pt paste added with 20 vol.% resin. Fig. 4 shows the relationship between the sensor output and the NOx concentration at 600 8C. The sensor output to both NO and NO2 gave the positive direction, and almost same output values were seen at any concentration examined. It is noteworthy that the output value is still about 4 mV even at NO (NO2 ) concentration of 10 ppm. Furthermore, it was confirmed that the 63% response time
Fig. 3. Complex impedance spectra of the pumping type cell using each of the Pt-catalyst electrodes fabricated by using the various pastes containing different resin contents.
Fig. 5. Influence of concentration of reducing gases. (A) C3H8 and (B) CO on the sensor output.
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T. Ono et al. / Solid State Ionics 175 (2004) 503–506
to 100 ppm NOx for the sensor attached with the Pt-catalyst electrode was less than 2 s. The response rate was hardly affected by the attachment of the oxidation-catalyst electrode. The dependence of the sensor output on concentration of C3H8 is given in Fig. 5(A). In this case, the C3H8 concentration was changed between 0–5000 ppm, while NO concentration was fixed at 100 ppm in the absence of oxygen. Inasmuch as the oxygen concentration in exhaust gas is usually very low when the reducing gases are coexisting in the test gas, the concentration of oxygen was set to 0 vol.% for the test gas condition mentioned above. Fig. 5(A) shows that, in the absence of the oxidationcatalyst electrode, a strong interference on the output signal is observed at C3H8 concentrations above ca. 1000 ppm. On the other hand, in the case of the sensor attached with the Pt-catalyst electrode, its output was hardly affected by the presence of C3H8 up to 3000 ppm. Fig. 5(B) gives the sensor output under the CO-existing condition. In this case, the CO concentration was changed between 0–5 vol.%, while NO concentration was fixed at 100 ppm in the absence of oxygen. The output of the sensor without the oxidation-catalyst electrode was interfered largely by the presence of CO in the concentration range above ca. 1000
ppm, while the output of the sensor attached with the catalyst-electrode was hardly affected by the presence of CO up to 1 vol.%. In conclusion, the use of Pt-oxidation-catalyst electrode is very effective in decreasing the interference of reducing gases on the NOx sensor output. Thus, the present sensor can be practically applied to an on-board NOx sensor if its long-term stability is ensured.
References [1] F. Menil, V. Coillard, C. Lucat, Sensors and Actuators B 67 (2000) 1. [2] N. Docquier, S. Candel, Progress in Energy and Combustion Science 28 (2002) 107. [3] N. Miura, M. Nakatou, S. Zhuiykov, Electrochemistry Communications 4 (2002) 284. [4] N. Miura, H. Kurosawa, M. Hasei, G. Lu, N. Yamazoe, Solid State Ionics 86–88 (1996) 1069. [5] A. Kunimoto, M. Hasei, Y. Yan, Y. Gao, T. Ono, Y. Nakanouchi, SAE Paper No. 1999-01-1280 (1999). [6] T. Ono, M. Hasei, A. Kunimoto, T. Yamamoto, A. Noda, JSAE Review 22 (2001) 49. [7] N. Miura, S. Zhuiykov, T. Ono, M. Hasei, N. Yamazoe, Sensors and Actuators B 83 (2002) 222.
Improvement of sensing performances of zirconia-based total NOx sensor by attachment of oxidation-catalyst electrode Takashi Onoa,b,*, Masaharu Haseia, Akira Kunimotoa, Norio Miurac a R & D Department, Riken Corporation, Kumagaya-shi, Saitama 360-8522, Japan Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan c Advanced Science and Technology Center for Cooperative Research, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan b
Accepted 11 April 2004
Abstract The sensing performance of the mixed-potential-type total NOx sensor based on laminated yttria-stabilized zirconia (YSZ) sheets was improved by the attachment of oxidation-catalyst electrode. It was found that pure Pt was the best material for the catalyst electrode among the four Pt-based metals examined. The addition of 20 vol.% resin to Pt-electrode paste was quite effective to improve the catalytic activity of the electrode. The obtained sensor could measure total NOx concentration down to 10 ppm at 600 8C. The use of the oxidation-catalyst electrode was found to decrease largely the interference of reducing gases (C3H8 and CO) to the NOx sensitivity. D 2004 Elsevier B.V. All rights reserved. PACS: 82.47.Rs; 82.45.Xy; 82.45.Jn Keywords: NOx sensor; Oxidation-catalyst; YSZ; Mixed potential; Automobile exhaust
1. Introduction Lately, compact low-cost and reliable on-board NOx sensors have been strongly required to improve a catalyst diagnosis system for automobiles [1–3]. So far, we have been developing the mixed-potential-type zirconia-based sensors using oxide-sensing electrode [4–7]. These sensors are confirmed to be able to detect NOx content in the test gas in the low concentration range even below ca. 100 ppm at rather high temperatures. This sensor, however, has the problem that its accuracy in NOx detection is not high when reducing gases, such as hydrocarbons and CO, coexist in the gas phase. Thus, in this study, we tried to search for the oxidationcatalyst electrode material which can convert effectively the reducing gases existing in car exhausts into noninterference * Corresponding author. R & D Department, Riken Corporation, Kumagaya-shi, Saitama 360-8522, Japan. Tel.: +81 48 527 2001; fax: +81 48 527 2010. E-mail address: [email protected] (T. Ono). 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.04.044
gases. We also examined the performances of the NOx sensor attached to the selected oxidation-catalyst electrode.
2. Design of laminated YSZ-based NOx sensor As a practical total NOx sensor for automobiles, we designed here a laminated yttria-stabilized-zirconia (YSZ)based sensor attached with an oxidation-catalyst electrode. Fig. 1 shows a cross-sectional view of the sensor designed. The green sheets of yttria-stabilized zirconia (YSZ, 6 mol% Y2O3 doped) were fabricated by means of doctor-blade method. Each thick-film electrode of precious metals (or metal oxides) was screen-printed on the green sheets obtained. Then, these green sheets were laminated and fired to make the sensing device given in Fig. 1. The size of sensing device was 70 mm in length, 5 mm in width and 3 mm in thickness. The size of the oxidation-catalyst electrode was 4 mm in length, 3 mm in width and 3 Am in thickness. The oxidation-catalyst electrode was formed in the gas inlet leading to the inner cavity. Test gases were let to flow into
504
T. Ono et al. / Solid State Ionics 175 (2004) 503–506
Fig. 1. Cross-sectional view of the laminated-type NOx sensor using an oxidation-catalyst electrode.
the inner cavity by passing through the porous oxidationcatalyst electrode layer. Only the reducing gases such as C3H8 and CO can be adsorbed on the surface of the oxidation-catalyst electrode. Oxygen gas can be supplied electrochemically to the oxidation-catalyst electrode side from the air duct side by applying the fixed voltage of 500 mV between the oxidation-catalyst electrode and the counterelectrode exposed to the air duct. Then, the adsorbed reducing gases are chemically and/or electrochemically oxidized and converted into noninterference gases (CO2 and H2O). Both NO and NO2 are also let to flow into the inner cavity, and all NO gas can be converted to NO2 by the NOx conversion electrode settled in the upper side of the cavity. Thus, the total NOx content in the test gas can be measured with the NOx -sensing electrode installed at the bottom of the inner cavity.
3. Experimental To examine the activity of the various oxidation-catalyst electrodes, a pumping-cell-type device was separately fabricated. The cell was fabricated by firing YSZ green sheet (18 mm in diameter) on which the oxidation-catalyst electrode (anode) and the Pt counterelectrode (cathode) were screen-printed. The size of each electrode was 6 mm in length, 6 mm in width and 3 Am in thickness. Pure Pt and Pt (+5 wt.% Rh), Pt (+5 wt.% Pd), Pt (+5 wt.% Ru) alloys were used as candidate materials for oxidation-catalyst electrodes. The porosity of the electrode layer was adjusted by adding resin powder into each electrode-paste. The obtained pumping cell was fixed on the end of YSZ tube by means of glass sealant after a Pt lead wire was connected to each electrode to construct the whole cell. The evaluation cell thus fabricated was heated up to 600 8C in an electric furnace, and then a fixed voltage of 500 mV was applied between the two electrodes of the cell by means of a potentiostat. The difference in electric current (oxidation current: IC3H8) measured under the two conditions, i.e., in 5000 ppm C3H8 (diluted with 5 vol.% O2 and 94.5 vol.% N2) flow and in the flow of 5 vol.% O2 and 95 vol.% N2,
was evaluated as an oxidation-catalyst activity for each electrode. By using the best material selected above for the catalyst electrode, the laminated-type NOx sensor was fabricated. The sensing device was made up to the assembly structure by mounting an alumina tube, a metal fitting and a protective cap. The whole sensor was connected electrically with a control unit consisting of electric circuits. The potential difference between the NOx -sensing electrode using Cr2O3 and the Pt reference-electrode was measured as a sensing signal. This sensing signal was calculated by a control unit based on the output data of the oxygen-sensing electrode. The latter was installed near the NOx -sensing electrode to cancel out the dependence of the sensor output on the concentration of oxygen. Then, the sensing performances of the sensor with or without the oxidation-catalyst electrode were evaluated in the flow of the various test gases.
4. Results and discussion First, the oxidation current (IC3H8) of the pumping-type cell using each of precious-metal-catalyst electrode was evaluated. It was confirmed that the Pt-catalyst electrode gave the highest current value (ca. 420 AA), while the alloy electrodes examined gave significantly lower values (ca. 300 AA). Thus, hereafter, pure Pt was used as the catalyst electrode material for the NOx sensor. By the way, the oxidation current value seems to indicate the oxidationcatalyst activity of the electrode toward the reducing gases. This point can be explained as follows. When the reducing gas is consumed at the surface of the catalyst electrode, the concentration of oxygen in the vicinity of the catalyst electrode can be decreased. Such a decrease in oxygen concentration can increase the pumping rate of oxygen from the counterelectrode side to the catalyst electrode side when the reducing gas is coexisting in the test gas. This seems to be caused by the generation of reverse electromotive force based on the formation of oxygen concentration cell across the present pumping cell under the condition that the constant voltage is applied between the air counterelectrode and the catalyst electrode. Fig. 2 shows the influence of resin content (vol.%) in the Pt paste on the oxidation current in the flow of 5000 ppm C3H8 diluted with 5 vol.% O2 and 94.5 vol.% N2 at 600 8C. It is seen that the maximum value of IC3H8 appears at the content of 20 vol.%. Fig. 3 shows the complex impedance spectra measured in 5000 ppm C3H8 in the frequency range 20 mHz–100 kHz at 600 8C for each of the cells attached with oxidation-catalyst electrodes formed by the paste having different resin content. A large semicircular arc was observed in each spectrum. The resistance value (ZV) at the intersection of the semiarc with the real axis at lower frequencies varied with the resin content, and then the minimum resistance (3.2 kV) was obtained at 20 vol.%.
T. Ono et al. / Solid State Ionics 175 (2004) 503–506
Fig. 2. Influence of resin content on the oxidation current.
505
Fig. 4. Relationship between sensor output and NOx concentration at 600 8C for the laminated sensor attached with the Pt-catalyst electrode.
This ZV value of kV level seems to be ascribable to the electrode reaction resistance at the interface between YSZ and Pt. These results indicate that the density of reaction site at three-phase boundary (Pt/YSZ/gas) is highest when the catalyst electrode layer was fabricated by using the Pt paste containing 20 vol.% resin. Thus, hereafter, we examined the sensing performances of the laminated sensor attached with the Pt catalyst electrode fabricated by using the Pt paste added with 20 vol.% resin. Fig. 4 shows the relationship between the sensor output and the NOx concentration at 600 8C. The sensor output to both NO and NO2 gave the positive direction, and almost same output values were seen at any concentration examined. It is noteworthy that the output value is still about 4 mV even at NO (NO2 ) concentration of 10 ppm. Furthermore, it was confirmed that the 63% response time
Fig. 3. Complex impedance spectra of the pumping type cell using each of the Pt-catalyst electrodes fabricated by using the various pastes containing different resin contents.
Fig. 5. Influence of concentration of reducing gases. (A) C3H8 and (B) CO on the sensor output.
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T. Ono et al. / Solid State Ionics 175 (2004) 503–506
to 100 ppm NOx for the sensor attached with the Pt-catalyst electrode was less than 2 s. The response rate was hardly affected by the attachment of the oxidation-catalyst electrode. The dependence of the sensor output on concentration of C3H8 is given in Fig. 5(A). In this case, the C3H8 concentration was changed between 0–5000 ppm, while NO concentration was fixed at 100 ppm in the absence of oxygen. Inasmuch as the oxygen concentration in exhaust gas is usually very low when the reducing gases are coexisting in the test gas, the concentration of oxygen was set to 0 vol.% for the test gas condition mentioned above. Fig. 5(A) shows that, in the absence of the oxidationcatalyst electrode, a strong interference on the output signal is observed at C3H8 concentrations above ca. 1000 ppm. On the other hand, in the case of the sensor attached with the Pt-catalyst electrode, its output was hardly affected by the presence of C3H8 up to 3000 ppm. Fig. 5(B) gives the sensor output under the CO-existing condition. In this case, the CO concentration was changed between 0–5 vol.%, while NO concentration was fixed at 100 ppm in the absence of oxygen. The output of the sensor without the oxidation-catalyst electrode was interfered largely by the presence of CO in the concentration range above ca. 1000
ppm, while the output of the sensor attached with the catalyst-electrode was hardly affected by the presence of CO up to 1 vol.%. In conclusion, the use of Pt-oxidation-catalyst electrode is very effective in decreasing the interference of reducing gases on the NOx sensor output. Thus, the present sensor can be practically applied to an on-board NOx sensor if its long-term stability is ensured.
References [1] F. Menil, V. Coillard, C. Lucat, Sensors and Actuators B 67 (2000) 1. [2] N. Docquier, S. Candel, Progress in Energy and Combustion Science 28 (2002) 107. [3] N. Miura, M. Nakatou, S. Zhuiykov, Electrochemistry Communications 4 (2002) 284. [4] N. Miura, H. Kurosawa, M. Hasei, G. Lu, N. Yamazoe, Solid State Ionics 86–88 (1996) 1069. [5] A. Kunimoto, M. Hasei, Y. Yan, Y. Gao, T. Ono, Y. Nakanouchi, SAE Paper No. 1999-01-1280 (1999). [6] T. Ono, M. Hasei, A. Kunimoto, T. Yamamoto, A. Noda, JSAE Review 22 (2001) 49. [7] N. Miura, S. Zhuiykov, T. Ono, M. Hasei, N. Yamazoe, Sensors and Actuators B 83 (2002) 222.