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Reduction of nitrogen oxide by a steam electrolysis cell using a proton conducting electrolyte
SOLID STATE IONlcS
ELSEVIER
Solid State Ionics 86-88
(1996)
603-607
Reduction of nitrogen oxide by a steam electrolysis proton conducting electrolyte Tetsuro Kobayashi”‘“,
Shinya Morishita”,
Katsushi Abe”, Hiroyasu Iwaharab
“Toyota Central R&D Labs., Nagakute, hCenter for Integruted
Research
in Science
and Engineering,
cell using a
Nagoya
Aichi 480-I I, Japan University,
Furo-cho,
Chikusa-ku,
Nagoya
464, Japan
Received 28 August 1995; accepted 9 October 1995
Abstract Removal of nitrogen oxide (NO) by electrochemical reduction was studied using a steam electrolysis cell with platinum electrodes and a proton conductor SrZr,,, Yb,,,O,_, as the electrolyte. Without NO, the cell produced H, with nearly 100% current efficiency in the temperature range 380-590 “C. With NO fed to the cathode, NO was effectively reduced. The main products were N,O and N, at low current densities, and N, and NH, at higher current densities. When 0, was mixed into the cathode gas, NO was not reduced on the pure Pt cathode. However, ‘Pt sponge + Sr/Al,O,’ catalyst on the Pt cathode showed a good catalytic ability for the reduction of NO supplied with 0,. Keywords:
Proton; Ionic conductivity; Nitrogen oxide; Reduction; Zirconium compound; Strontium compound
1. Introduction NO is one of the polluting components from combustion engines and its removal is a very important issue. To date, chemical catalysts have been mainly used for this purpose, converting NO into N, using hydrocarbons and CO in the exhaust as the reducing agents. However, this method is difficult when engines are operated under the lean-burn (02rich) condition which is more fuel-efficient but leaves very little of the reductive components. New methods are required for the effective reduction of NO even under lean-burn conditions [ 11. In this study, we investigated an electrochemical way of removing NO. Electrochemical methods have been studied using *Corresponding author. 0167-2738/96/$15.00 PI/
Copyright 01996
SOl67-2738(96)00216-O
ZrO, [2,3] and aqueous HClO, solution [4] as electrolytes. The cell using ZrO,, which is an oxygen-ion conductor, removed NO by electrochemical reduction [2,3]. The rate of this reaction was not sufficient because the reaction is purely electrochemical. A stronger reducing capability will be required, especially when 0, coexists. The cell with HClO, removed NO efficiently [4]. This system, however, cannot be used in high-temperature conditions such as in the exhaust of automobiles. In this study, a high-temperature solid proton conductor was used as the electrolyte. Fig. 1 shows a schematic diagram of the cell. Water vapor is electrochemically oxidized on the anode and the produced protons are transported through the solid electrolyte to the cathode. On the cathode, protons are first reduced to atomic hydrogen, which is known to be a strong reducing agent. Therefore, in this cell, NO is
Elsevier Science B.V. All rights reserved
604
T. Kobqushi
et al. / Solid State lonics 86-88
anode
Proton
conductor
cathode
Fig. I. Schematic
diagram of a steam electrolysis cell for the
reduction of nitrogen oxide.
expected to be reduced not only electrochemically but also chemically. As a water source for the anode, water vapor in the exhaust gases can be utilized. Therefore, this cell can be a simple but effective NO-removal device for the exhaust.
2. Experimental Fig. 2 shows a cross-section of the experimental apparatus. A closed one-end tube of sintered ceramics SrZr,,,Yb,,,Oj_, (supplied by TYK) was used as the proton conductor [S], where cy is the mole fraction of oxygen ion vacancies. This proton conductor is chemically stabler than other proton conductors based on SrCeO, and BaCeO, in an atmosphere including CO, [5,6], which is one of the components in the exhaust gas. Platinum was plated
(1996)
603-607
on both sides of the electrolyte by an electroless plating method and served as the electrodes. The areas of the anode and the cathode were 6 cm* and 5 cm’, respectively. The thickness of both electrodes was 2 pm. The anode chamber and cathode chamber were formed as shown in the figure. 100% H,O gas was introduced into the anode chamber at a rate of 200 ml/min. Inert gases with or without NO and O2 were fed to the cathode at a rate of 30 ml/min. Direct current was applied to the cell galvanostatically. Experiments were carried out in the temperature range 380-590 “C under atmospheric pressure. Gas chromatography with porapak Q and molecular sieve 5A columns was empIoyed to analyze the concentrations of HZ, N,, 0, and N,O. The concentrations of NO, and NH, were measured using the Saltzman method and an absorbing-type gas detector (GASTEC Co. No. 3L, 3M), respectively. Electrocatalyst ‘Pt sponge + Sr/Al,O,’ was employed on the Pt plated cathode for the removal of NO when 0, was mixed with the cathode gas. The Pt sponge was prepared by thermal decomposition of (NH,),PtCl,. Sr/Al,O, was prepared by the following process: y-Al,O, powder was added into 0.25 mol/l Sr(NO,), solution and stirred. After evaporating water while stirring, the resulting solid was dried at 110 “C. It was then calcined at 800 “C to remove NO.,. Sr in this catalyst accounted for 2.5 mmol/gA120,. Pt sponge and Sr/Al,O, were mixed in a proportion of 50 to 50 by volume with a terpine oil binder. The mixture was then spread on the Pt plated cathode and calcined at 800 “C.
,electrolyte
(SrZr, ,Yb,, ,OJ~li 1 -anode
chamber
-cathode
‘anode
(Pt)
‘cathode ‘glass 7
Fig, 2. Cross-section
of the experimenta
chamber
(Pt) packing
alumina tube
apparatus.
3. Results and discussion 3.1. Currenr efficiency
ofHz production
by sream
electrolysis
Pure Ar gas was introduced to the cathode as a carrier gas. The H, production rate was measured at 450 “C. The result is shown in Fig. 3. The broken line represents 100% efficiency calculated by Faraday’s law. Efficiencies greater than 90% were obtained in the current densities from 0 to 4 mA/cm’. The same results as in Fig. 3 were obtained at all temperatures. This indicates that when water vapor is supplied to the anode the present system can effi-
T. Kobapashi et al. I Solid State Ionics 86-88
theoreti~l line, 1’.
1
I-’
(1996) 603-607
the increase of the current density, and reached 100% at current densities higher than 1.2 mA/cm2. The main products were N,O and N, at low current densities, and N, and NH, at high current densities. NH, was almost exclusively produced at high current densities, as shown in Fig. 5. The total amount of nitrogen in the products was equal to that of the removed NO. 3.3. Reduction
Current
density
( mA cm-* )
Fig. 3. H, production rate at various current densities. Temp.: 450 “C. The broken line represents 100% efficiency calculated by Faraday’s law.
ciently produce hydrogen reducing agent for NO. 3.2. Reduction
which can be utilized as a
of NO under O,-free atmosphere
Fig. 4 shows the NO-removal efficiency when 1000 ppm NO in He gas was introduced to the cathode at 450 “C. (The NO removal efficiency is defined as the ratio of the amount of removed NO to that of supplied NO.) The efficiency increased with
605
of NO with small amount of 0,
Fig. 6 shows NO- and O,-removal efficiencies when a small amount of 0, (1000 ppm) was mixed to the cathode gas ( 1000 ppm NO in He) at 450 “C. At lower current densities, 0, was reduced predominantly. After most of the 0, was reduced, NO reduction started. The product distribution of the NO reduction was similar to that of Fig. 4. The total amount of nitrogen in the products was also equal to that of the removed NO. The preferable reduction of 0, over NO was observed throughout the experimental temperature range. Related electrochemical reactions are shown in the following formula: 2NO(g) + 4H’ + 4e
+ N,(g) + 2H,O(g)
= 1.51
(1)
2NO(g) + 2H+ + 2e- + N,O(g)
+ H,O(g)
= 1.23
Current density ( n~4 cme2 ) Fig. 4. NO-removal efficiency* and concentrations of the products at various current densities (low current densities). Feed gas: 1000 ppm NO in He. Temp.: 450°C. *:NO - removal efficiency = Removed NO/Supplied NO.
E”,V
E”,V (2)
Current density ( mA em-* ) Fig. 5. NO-removal efficiency* and concentrations of the products at various current densities (high current densities). Feed gas: 1000 ppm NO in He. Temp.: 450°C. *:NO - removal efficiency = Removed NO/Supplied NO.
606
T. Kobayushi
et al.
I Solid
efficiency*
and concentrations
86-88
E”,V= 1.09 + H,O(g)
= 0.55
(3)
E”,V
t
-&-
“Pi sponge + Sr/Al~O~” on Pt plated cathode
-o--
pure PI plated cathode
(4)
where E” is the standard potential versus reversible hydrogen potential (RHE) at 450 “C. Reduction reactions having higher standard potentials usually proceed first. In this cell, however, reactions ( 1) and (2), of which the standard potentials are higher than that of reaction (3), did not proceed until reaction (3) was finished. Therefore, the main NO reduction paths are possibly not the electrochemical reactions (1) and (2) since 0, reduction is considered to be electrochemical. Therefore, chemical reactions with atomic hydrogen are rather probable or even dominant as the NO reduction mechanism. 3.4. Reduction
603-607
O,(NO).
50
NO(g) + 5Hf + 5eP + NH,(g)
(1996)
of the products at various current densities. Feed gas: 1000 ppm NO and 1000
ppm O2 in He. Temp.: 450°C. *:O,(NO) - removal efficiency = RemovedO,(NO)/Supplied
O?(g) + 4H’ + 4eP + 2H,O(g)
lonics
density ( mA cme2 )
Current Fig. 6. 03- and NO-removal
State
of NO with excess
of O2
Fig. 7 shows the temperature dependence of the NO-removal efficiency when a larger amount of 0, (8%) was mixed to the cathode gas (1000 ppm NO). The current density was 2.4 mA/cm2, which is only 1.9% of the current density necessary to reduce all the 0, in the supplied gas. NO was not removed at all on the pure Pt plated cathode as expected. In the cell with the ‘Pt
Temperature ( “c ) Fig. 7. Temperature dependence of NO-removal efficiency* for the cell with the pure Pt cathode and the cell with catalyst on the Pt cathode. Feed gas: 1000 ppm NO and 8% O2 in He. Current density: 2.4 mA/cm’. *:NO - removal efficiency = Removed NO/Supplied NO.
sponge + Sr/A1207’ catalyst on the Pt plated cathode, however, NO was removed at temperatures lower than 500 “C. The NO-removal efficiency reached 20% at 350 “C. In the outlet gas, no product except for N?, was detected. The NO-removal rate and the N2-production rate were plotted in Fig. 8. This figure indicates that 50% of the removed NO was reduced to N,. The rest of the removed NO is considered to be absorbed into the catalyst because alkaline-earth elements such as Ba and Sr are known to absorb NO
T.
Kobayashi
et al.
I Solid
State
Ionics
86-88
(1996)
603-607
607
I”“““‘I”“““‘I”“““’
“Pt sponget Sr/AIzO>” 1PC1Sr(Zr,Yb)03 1pt cell
NO removal
1
300
400
500
I
600
Temperature ( “c ) Fig. 8. NO-removal
rate and N,-production
rate for the cell with ‘Pt sponge + Sr/AIZ03’
catalyst. Feed gas: 1000 ppm NO and 8% Oz in He.
Current density: 2.4 mA/cm’.
forming nitrate, i.e. Ba(NO,), and Sr(NO,), when O2 coexists [7-91. Under an open circuit condition, however, no NO was removed. It is interesting that even NO absorption did not proceed until direct current was applied. Absorbed NO into Sr might be reduced to N, and this reduction would enhance further absorption of NO. In any case, Sr surely worked as a co-catalyst for NO reduction because no NO was reduced in its absence.
4. Conclusion A steam electrolysis call was constructed with the proton conductor SrZrO.uYb,,,O,_, as the electrolyte. The present system efficiently produced H, by steam electrolysis. When NO was supplied to the cathode, NO was removed effectively. The reduction products were N,O, N, and NH, depending on the current density. Combining the ‘Pt sponge+ Sr/ Al,O,’ catalyst with this electrolysis cell made it possible to reduce NO even under oxidizing atmospheres. Further studies in this area are expected to improve the performance of this cell as a NOremoval device.
Acknowledgments The authors are grateful to TYK Co. for supplying electrolytes and to Dr. Y. Morimoto for useful discussions.
References ]I] SAE SP-938, ‘Automotive Emissions and Catalyst Technology’. ISBN I-56091 -302-9. [Z] T.M. Giir and R.A. Huggins. J. Electrochem. Sot. 126 (1979) 1067. [3] T. Hibino, J. Applied Electrochemistry 25 ( 1995) 203. [4] S.H. Langer, Platinum Metals Rev. 36 (1992) 202. [5] T. Yajima, H. Suzuki, T. Yogo and H. Iwahara, Solid State Ionics 51 (1992) 101. [6] M.J. Scholten et al., The Electrochem. Sot. l83rd Meeting (Honolulu, HI) 93( I ) (1993) 1625. [7] M. Machida, K. Yasuoka, K. Eguchi and H. Arai, J. Chem. Sot. Chem. Commun. (1990) 1165. 181 N. Miyoshi et al., SAE paper 950809 ( 1995). [9] N. Takahashi et al., Catalysis Today 27 (1996) 63.
ELSEVIER
Solid State Ionics 86-88
(1996)
603-607
Reduction of nitrogen oxide by a steam electrolysis proton conducting electrolyte Tetsuro Kobayashi”‘“,
Shinya Morishita”,
Katsushi Abe”, Hiroyasu Iwaharab
“Toyota Central R&D Labs., Nagakute, hCenter for Integruted
Research
in Science
and Engineering,
cell using a
Nagoya
Aichi 480-I I, Japan University,
Furo-cho,
Chikusa-ku,
Nagoya
464, Japan
Received 28 August 1995; accepted 9 October 1995
Abstract Removal of nitrogen oxide (NO) by electrochemical reduction was studied using a steam electrolysis cell with platinum electrodes and a proton conductor SrZr,,, Yb,,,O,_, as the electrolyte. Without NO, the cell produced H, with nearly 100% current efficiency in the temperature range 380-590 “C. With NO fed to the cathode, NO was effectively reduced. The main products were N,O and N, at low current densities, and N, and NH, at higher current densities. When 0, was mixed into the cathode gas, NO was not reduced on the pure Pt cathode. However, ‘Pt sponge + Sr/Al,O,’ catalyst on the Pt cathode showed a good catalytic ability for the reduction of NO supplied with 0,. Keywords:
Proton; Ionic conductivity; Nitrogen oxide; Reduction; Zirconium compound; Strontium compound
1. Introduction NO is one of the polluting components from combustion engines and its removal is a very important issue. To date, chemical catalysts have been mainly used for this purpose, converting NO into N, using hydrocarbons and CO in the exhaust as the reducing agents. However, this method is difficult when engines are operated under the lean-burn (02rich) condition which is more fuel-efficient but leaves very little of the reductive components. New methods are required for the effective reduction of NO even under lean-burn conditions [ 11. In this study, we investigated an electrochemical way of removing NO. Electrochemical methods have been studied using *Corresponding author. 0167-2738/96/$15.00 PI/
Copyright 01996
SOl67-2738(96)00216-O
ZrO, [2,3] and aqueous HClO, solution [4] as electrolytes. The cell using ZrO,, which is an oxygen-ion conductor, removed NO by electrochemical reduction [2,3]. The rate of this reaction was not sufficient because the reaction is purely electrochemical. A stronger reducing capability will be required, especially when 0, coexists. The cell with HClO, removed NO efficiently [4]. This system, however, cannot be used in high-temperature conditions such as in the exhaust of automobiles. In this study, a high-temperature solid proton conductor was used as the electrolyte. Fig. 1 shows a schematic diagram of the cell. Water vapor is electrochemically oxidized on the anode and the produced protons are transported through the solid electrolyte to the cathode. On the cathode, protons are first reduced to atomic hydrogen, which is known to be a strong reducing agent. Therefore, in this cell, NO is
Elsevier Science B.V. All rights reserved
604
T. Kobqushi
et al. / Solid State lonics 86-88
anode
Proton
conductor
cathode
Fig. I. Schematic
diagram of a steam electrolysis cell for the
reduction of nitrogen oxide.
expected to be reduced not only electrochemically but also chemically. As a water source for the anode, water vapor in the exhaust gases can be utilized. Therefore, this cell can be a simple but effective NO-removal device for the exhaust.
2. Experimental Fig. 2 shows a cross-section of the experimental apparatus. A closed one-end tube of sintered ceramics SrZr,,,Yb,,,Oj_, (supplied by TYK) was used as the proton conductor [S], where cy is the mole fraction of oxygen ion vacancies. This proton conductor is chemically stabler than other proton conductors based on SrCeO, and BaCeO, in an atmosphere including CO, [5,6], which is one of the components in the exhaust gas. Platinum was plated
(1996)
603-607
on both sides of the electrolyte by an electroless plating method and served as the electrodes. The areas of the anode and the cathode were 6 cm* and 5 cm’, respectively. The thickness of both electrodes was 2 pm. The anode chamber and cathode chamber were formed as shown in the figure. 100% H,O gas was introduced into the anode chamber at a rate of 200 ml/min. Inert gases with or without NO and O2 were fed to the cathode at a rate of 30 ml/min. Direct current was applied to the cell galvanostatically. Experiments were carried out in the temperature range 380-590 “C under atmospheric pressure. Gas chromatography with porapak Q and molecular sieve 5A columns was empIoyed to analyze the concentrations of HZ, N,, 0, and N,O. The concentrations of NO, and NH, were measured using the Saltzman method and an absorbing-type gas detector (GASTEC Co. No. 3L, 3M), respectively. Electrocatalyst ‘Pt sponge + Sr/Al,O,’ was employed on the Pt plated cathode for the removal of NO when 0, was mixed with the cathode gas. The Pt sponge was prepared by thermal decomposition of (NH,),PtCl,. Sr/Al,O, was prepared by the following process: y-Al,O, powder was added into 0.25 mol/l Sr(NO,), solution and stirred. After evaporating water while stirring, the resulting solid was dried at 110 “C. It was then calcined at 800 “C to remove NO.,. Sr in this catalyst accounted for 2.5 mmol/gA120,. Pt sponge and Sr/Al,O, were mixed in a proportion of 50 to 50 by volume with a terpine oil binder. The mixture was then spread on the Pt plated cathode and calcined at 800 “C.
,electrolyte
(SrZr, ,Yb,, ,OJ~li 1 -anode
chamber
-cathode
‘anode
(Pt)
‘cathode ‘glass 7
Fig, 2. Cross-section
of the experimenta
chamber
(Pt) packing
alumina tube
apparatus.
3. Results and discussion 3.1. Currenr efficiency
ofHz production
by sream
electrolysis
Pure Ar gas was introduced to the cathode as a carrier gas. The H, production rate was measured at 450 “C. The result is shown in Fig. 3. The broken line represents 100% efficiency calculated by Faraday’s law. Efficiencies greater than 90% were obtained in the current densities from 0 to 4 mA/cm’. The same results as in Fig. 3 were obtained at all temperatures. This indicates that when water vapor is supplied to the anode the present system can effi-
T. Kobapashi et al. I Solid State Ionics 86-88
theoreti~l line, 1’.
1
I-’
(1996) 603-607
the increase of the current density, and reached 100% at current densities higher than 1.2 mA/cm2. The main products were N,O and N, at low current densities, and N, and NH, at high current densities. NH, was almost exclusively produced at high current densities, as shown in Fig. 5. The total amount of nitrogen in the products was equal to that of the removed NO. 3.3. Reduction
Current
density
( mA cm-* )
Fig. 3. H, production rate at various current densities. Temp.: 450 “C. The broken line represents 100% efficiency calculated by Faraday’s law.
ciently produce hydrogen reducing agent for NO. 3.2. Reduction
which can be utilized as a
of NO under O,-free atmosphere
Fig. 4 shows the NO-removal efficiency when 1000 ppm NO in He gas was introduced to the cathode at 450 “C. (The NO removal efficiency is defined as the ratio of the amount of removed NO to that of supplied NO.) The efficiency increased with
605
of NO with small amount of 0,
Fig. 6 shows NO- and O,-removal efficiencies when a small amount of 0, (1000 ppm) was mixed to the cathode gas ( 1000 ppm NO in He) at 450 “C. At lower current densities, 0, was reduced predominantly. After most of the 0, was reduced, NO reduction started. The product distribution of the NO reduction was similar to that of Fig. 4. The total amount of nitrogen in the products was also equal to that of the removed NO. The preferable reduction of 0, over NO was observed throughout the experimental temperature range. Related electrochemical reactions are shown in the following formula: 2NO(g) + 4H’ + 4e
+ N,(g) + 2H,O(g)
= 1.51
(1)
2NO(g) + 2H+ + 2e- + N,O(g)
+ H,O(g)
= 1.23
Current density ( n~4 cme2 ) Fig. 4. NO-removal efficiency* and concentrations of the products at various current densities (low current densities). Feed gas: 1000 ppm NO in He. Temp.: 450°C. *:NO - removal efficiency = Removed NO/Supplied NO.
E”,V
E”,V (2)
Current density ( mA em-* ) Fig. 5. NO-removal efficiency* and concentrations of the products at various current densities (high current densities). Feed gas: 1000 ppm NO in He. Temp.: 450°C. *:NO - removal efficiency = Removed NO/Supplied NO.
606
T. Kobayushi
et al.
I Solid
efficiency*
and concentrations
86-88
E”,V= 1.09 + H,O(g)
= 0.55
(3)
E”,V
t
-&-
“Pi sponge + Sr/Al~O~” on Pt plated cathode
-o--
pure PI plated cathode
(4)
where E” is the standard potential versus reversible hydrogen potential (RHE) at 450 “C. Reduction reactions having higher standard potentials usually proceed first. In this cell, however, reactions ( 1) and (2), of which the standard potentials are higher than that of reaction (3), did not proceed until reaction (3) was finished. Therefore, the main NO reduction paths are possibly not the electrochemical reactions (1) and (2) since 0, reduction is considered to be electrochemical. Therefore, chemical reactions with atomic hydrogen are rather probable or even dominant as the NO reduction mechanism. 3.4. Reduction
603-607
O,(NO).
50
NO(g) + 5Hf + 5eP + NH,(g)
(1996)
of the products at various current densities. Feed gas: 1000 ppm NO and 1000
ppm O2 in He. Temp.: 450°C. *:O,(NO) - removal efficiency = RemovedO,(NO)/Supplied
O?(g) + 4H’ + 4eP + 2H,O(g)
lonics
density ( mA cme2 )
Current Fig. 6. 03- and NO-removal
State
of NO with excess
of O2
Fig. 7 shows the temperature dependence of the NO-removal efficiency when a larger amount of 0, (8%) was mixed to the cathode gas (1000 ppm NO). The current density was 2.4 mA/cm2, which is only 1.9% of the current density necessary to reduce all the 0, in the supplied gas. NO was not removed at all on the pure Pt plated cathode as expected. In the cell with the ‘Pt
Temperature ( “c ) Fig. 7. Temperature dependence of NO-removal efficiency* for the cell with the pure Pt cathode and the cell with catalyst on the Pt cathode. Feed gas: 1000 ppm NO and 8% O2 in He. Current density: 2.4 mA/cm’. *:NO - removal efficiency = Removed NO/Supplied NO.
sponge + Sr/A1207’ catalyst on the Pt plated cathode, however, NO was removed at temperatures lower than 500 “C. The NO-removal efficiency reached 20% at 350 “C. In the outlet gas, no product except for N?, was detected. The NO-removal rate and the N2-production rate were plotted in Fig. 8. This figure indicates that 50% of the removed NO was reduced to N,. The rest of the removed NO is considered to be absorbed into the catalyst because alkaline-earth elements such as Ba and Sr are known to absorb NO
T.
Kobayashi
et al.
I Solid
State
Ionics
86-88
(1996)
603-607
607
I”“““‘I”“““‘I”“““’
“Pt sponget Sr/AIzO>” 1PC1Sr(Zr,Yb)03 1pt cell
NO removal
1
300
400
500
I
600
Temperature ( “c ) Fig. 8. NO-removal
rate and N,-production
rate for the cell with ‘Pt sponge + Sr/AIZ03’
catalyst. Feed gas: 1000 ppm NO and 8% Oz in He.
Current density: 2.4 mA/cm’.
forming nitrate, i.e. Ba(NO,), and Sr(NO,), when O2 coexists [7-91. Under an open circuit condition, however, no NO was removed. It is interesting that even NO absorption did not proceed until direct current was applied. Absorbed NO into Sr might be reduced to N, and this reduction would enhance further absorption of NO. In any case, Sr surely worked as a co-catalyst for NO reduction because no NO was reduced in its absence.
4. Conclusion A steam electrolysis call was constructed with the proton conductor SrZrO.uYb,,,O,_, as the electrolyte. The present system efficiently produced H, by steam electrolysis. When NO was supplied to the cathode, NO was removed effectively. The reduction products were N,O, N, and NH, depending on the current density. Combining the ‘Pt sponge+ Sr/ Al,O,’ catalyst with this electrolysis cell made it possible to reduce NO even under oxidizing atmospheres. Further studies in this area are expected to improve the performance of this cell as a NOremoval device.
Acknowledgments The authors are grateful to TYK Co. for supplying electrolytes and to Dr. Y. Morimoto for useful discussions.
References ]I] SAE SP-938, ‘Automotive Emissions and Catalyst Technology’. ISBN I-56091 -302-9. [Z] T.M. Giir and R.A. Huggins. J. Electrochem. Sot. 126 (1979) 1067. [3] T. Hibino, J. Applied Electrochemistry 25 ( 1995) 203. [4] S.H. Langer, Platinum Metals Rev. 36 (1992) 202. [5] T. Yajima, H. Suzuki, T. Yogo and H. Iwahara, Solid State Ionics 51 (1992) 101. [6] M.J. Scholten et al., The Electrochem. Sot. l83rd Meeting (Honolulu, HI) 93( I ) (1993) 1625. [7] M. Machida, K. Yasuoka, K. Eguchi and H. Arai, J. Chem. Sot. Chem. Commun. (1990) 1165. 181 N. Miyoshi et al., SAE paper 950809 ( 1995). [9] N. Takahashi et al., Catalysis Today 27 (1996) 63.