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The low temperature limit of the application of solid electrolyte potentiometry in heterogeneous catalysis
Solid State Ionics 112 (1998) 75–78
The low temperature limit of the application of solid electrolyte potentiometry in heterogeneous catalysis ¨ a , H.-G. Lintz a ,b , *, G. Valentin b J. Bruck a
b
¨ Chemische Verfahrenstechnik, Universitat ¨ Karlsruhe, Kaiserstraße 12, D-76128 Karlsruhe, Germany Institut f ur ´ Laboratoire des Sciences du Genie Chimique, CNRS - Groupe ENSIC 1, rue Grandville, F-54001 Nancy-Cedex, France Received 16 February 1998; accepted 18 June 1998
Abstract Solid Electrolyte Potentiometry is an electrochemical method to characterise the state of solid catalysts under working conditions. Its application is temperature limited due to polarization effects at the catalyst electrode. A new operational definition of this temperature limit is proposed which is based on the measurements of exchange current densities. 1998 Elsevier Science B.V. All rights reserved. Keywords: SEP; Electrochemical cell; Catalytic electrode; Oxidic catalyst
1. Introduction Following a proposition by C. Wagner [1], solid electrolyte electrochemical cells may be operated to investigate the behaviour of catalysts in situ. Since the pioneering work of Vayenas and Saltsburg [2] such arrangements have been used two-fold. Either the application of an outer potential influences the catalysed reaction, leading to the surprising effect of Non-Faradaic Electrochemical Modification of the Catalytic Activity (NEMCA) [3], now well understood due to the work of the group in Patras, or the catalyst is characterised by its oxygen activity under working conditions measured by the technique known as Solid Electrolyte Potentiometry (SEP) [4]. Both techniques have been widely applied on metal
*Corresponding author. [email protected]
Fax:
133-3-8332-2975;
e-mail:
catalysts. The NEMCA effect was observed in 45 catalytic reactions using various solid electrolytes [5]. SEP studies on metals are summarized for instance in a recent article by I. Metcalfe [6]. In both cases the electrochemical cell has the form: reactive gas phase ucatalyst electrodeu solid electrolyte ureference electrodeu O 2 / N 2 , the reference electrode consisting mainly of a porous platinum film. SEP has also been used to investigate the behaviour of oxidic catalysts in a limited number of studies [7–11]. The reaction rate and oxygen activity were monitored simultaneously in order to correlate catalyst selectivity with oxygen activity and / or the stable phase composition of the catalyst under working conditions. Its use and limits in the case of oxidic catalysts have been discussed in detail elsewhere [12]. The application of SEP is useful only if the combined kinetic and potentiometric measurements
0167-2738 / 98 / $ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00201-X
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck
76
can be made simultaneously at the same temperature of catalytic interest. Now, SEP measurements have been proven to be possible only down to a lower temperature limit, below which polarization effects inhibit the formation of a stable potential. In the following we shall discuss an operational definition of this limit temperature below which potentiometric results become questionable.
2. Measurements under equilibrium conditions Stabilised zirconia, an oxygen ion conducting solid electrolyte, is mainly used in SEP measurements and all results reported in the following refer to YSZ. Thus the potential-determining reaction may be written as: O 2 1 4e 2 ⇔2O 22
Fig. 1. Temperature dependence of the equilibrium potential DE*; (catalyst electrode: Pt, reference electrode: Pt).
(1)
It takes place at the three phase border line gas– electrode–electrolyte. If the oxygen at the interfaces is in equilibrium with the gas phase oxygen on both sides of the electrochemical cell, i.e. there is no chemical reaction, its partial pressure at the catalyst side, P M O 2 , is related to its value at the reference side, P RO 2 , via a Nernst equation:
S D
PM R?T O2 ]] ]] DE* 5 ln 4F P RO 2
(2)
Eq. (2) is respected within a large range of partial pressures and temperature. Typical results describing the temperature dependence are represented in Figs. 1 and 2. The agreement between the values calculated by use of the Nernst equation and the experimentally determined ones is quite satisfactory. However, at temperatures below 520 K in the case of platinum (Fig. 1) or 640 K in the case of vanadia– titania (Fig. 2) no stable values of the potential are obtained even after 24 h, due to polarization effects. In the case of platinum, we characterised the quality of the porous electrodes by exchange current measurements and were able to correlate limit temperature, exchange current and BET surface of the electrode (characteristic of the extension of the three phase border line). We came to the conclusion [13], that the lower temperature limit of the application of
Fig. 2. Temperature dependence of the equilibrium potential DE*; (catalyst electrode: V2 O 5 / TiO 2 , reference electrode: Pt).
SEP under equilibrium conditions is due to activation polarization and that it may be lowered only by thorough preparation of the electrodes. However, the low temperature limits obtained so far in different cases (Table 1) often remain rather high for the study of heterogeneously catalysed reactions.
3. Measurements in reactive systems In the case where the catalyst electrode is exposed to a reacting gas mixture, the Nernst equation may be written in the form:
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck Table 1 Experimentally determined values of the low temperature limit, qL , for the use of solid electrolyte electrochemical cells under equilibrium conditions Catalyst electrode
qL / 8C
Reference
Pt Copper oxide Copper oxide11 mol% Na Vanadia–titania Copper molybdate Mo–V–Cu mixed oxide
250 420 400 370 415 420
[14] [15] [15] [16] [17] [17]
S D
a 2O RT DE 5 ] ln ] 4F p RO 2
(3)
Eq. (3) constitutes the operational definition of the oxygen activity a O of atomic oxygen on the catalyst under operating conditions. We first restricted the combined kinetic and potentiometric measurements to the temperature domain above the limit determined under equilibrium conditions. However, for reasons of catalytic interest, we additionally performed measurements at temperatures somewhat lower than this limit. It turned out that stable values of the potential DE could equally be obtained and there appeared no dramatic change in the measured quantities or the nature of the interrelations shown. The reasons were found through the determination of exchange currents under reacting conditions. The values obtained by current–potential measurements or cyclic voltammetry (5 mV/ s, [16]) showed a substantial increase up to one order of magnitude relative to the values determined under equilibrium conditions. Typical results are summarized in Table 2. At 3908C, the temperature limit under equilibrium conditions, the exchange current density is equal to Table 2 Exchange current densities at different temperatures and gas phase compositions (catalyst electrode: vanadia–titania, reference electrode: Pt)
q / 8C
Gas phase
i O /(nA / cm 2 )
390 390 350 300
O2 / N2 O 2 / NO / NH 3 / N 2 O 2 / NO / NH 3 / N 2 O 2 / NO / NH 3 / N 2
10 119 77 42
77
10 nA / cm 2 , using a mixture of 20 mbar O 2 in N 2 at the catalyst electrode and air at the reference side. The density is related to the geometrical area of the solid electrolyte. Maintaining the same partial pressure of oxygen but adding an equimolar mixture of NH 3 and NO (c i 50.01 mol / m 3 ) this value is increased to 119 nA / cm 2 . Even after a temperature decrease of 908 below the temperature limit, a stable potential difference is obtained and the exchange current density remains 4 times higher than under the equilibrium conditions at the low temperature limit. The observed increase in exchange current densities under reactive conditions is not restricted to the NH 3 –NO–O 2 system. Similar results are obtained in the partial oxidation of organic compounds on oxidic catalysts [15,17].
4. Discussion and conclusions Balian et al. [18] have published potentiometric measurements on copper oxides under reactive conditions – the oxidation of hydrogen – at temperatures as low as 2508C, far beneath the limit determined by Hildenbrand [15] under equilibrium conditions (cf. Table 1). In addition, we have found in the continuation of the latter work that the kinetic indication of a phase transition in the copper–oxygen system could be confirmed by SEP down to 2808C. The extension of potentiometric measurements beyond the equilibrium-determined limit thus seems to be justified. It is interesting to note that observations related in the literature [19,20] indicate that the response time of electrochemical cells to variations of the gas phase composition is considerably shortened if the catalyst electrode is in contact with a reactive mixture. This gives a strong hint that the rate of a potential determining reaction and thus its time constants of the formation of the potential may be altered by the presence of reactants in the gas phase. In addition, in the case of vanadia–titania catalyst, the results of which were reported in [2], independent conductivity measurements under reaction conditions have shown that the potentiometrically determined oxygen activity is a real measure of the oxidation state of the catalyst within the whole temperature range investigated.
78
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck
Without any speculation about the physical reasons for the lowering of the limit temperature below its value determined under equilibrium conditions, we feel that a cautious extension of the potentiometric measurements to values below this limit are possible. Therefore we propose the following operational definition of the limitations of SEP in a reactive system: Potentiometric measurements are possible down to temperatures where the exchange current densities under reactive conditions are of the same order of magnitude as at the lowest temperature where under equilibrium conditions, a stable electrode potential is still obtainable.
References [1] C. Wagner, Adv. Catal. 21 (1970) 323. [2] C.G. Vayenas, H. Saltsburg, J. Catal. 57 (1979) 296. [3] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303.
[4] H.-G. Lintz, C.G. Vayenas, Angew. Chem. Int. Ed. Engl. 28 (1989) 708. [5] C.G. Vayenas, S.I. Bebelis, Solid State Ionics 94 (1997) 267. [6] I.S. Metcalfe, Catal. Today 20 (1994) 283. [7] S. Pizzini, C.M. Mari, L. Zanderighi, Gazz. Chim. Ital. 110 (1980) 389. [8] E.M. Breckner, S. Sundaresan, J.B. Benziger, Appl. Catal. 30 (1987) 277. [9] H.-H. Hildenbrand, H.-G. Lintz, Appl. Catal. 65 (1990) 241. [10] P.D. Petrolekas, I.S. Metcalfe, J. Catal. 152 (1995) 147. ¨ [11] J. Bruck, M. Brust, H.-G. Lintz, Ber. Bunsenges. Phys. Chem. 99 (1995) 1509. [12] H.-H. Hildenbrand, H.-G. Lintz, Ber. Bunsenges. Phys. Chem. 95 (1991) 1191. ¨ [13] E. Hafele, H.-G. Lintz, Solid State Ionics 23 (1987) 235. [14] M. Oerter, Ph. D. Thesis, Karlsruhe 1994. [15] H.-H. Hildenbrand, Ph. D. Thesis, Karlsruhe 1991. ¨ [16] J. Bruck, Ph. D. Thesis, Karlsruhe 1995. [17] M. Estenfelder, Ph. D. Thesis, Karlsruhe 1998. [18] A. Balian, G. Hatzigiannis, D. Eng, M. Stoukides, J. Catal. 145 (1994) 526. [19] A. Caneiro, M. Bonnat, J. Fouletier, J. Appl. Electrochem. 11 (1981) 83. [20] J.E. Anderson, Y.B. Graves, J. Appl. Electrochem. 12 (1982) 335.
The low temperature limit of the application of solid electrolyte potentiometry in heterogeneous catalysis ¨ a , H.-G. Lintz a ,b , *, G. Valentin b J. Bruck a
b
¨ Chemische Verfahrenstechnik, Universitat ¨ Karlsruhe, Kaiserstraße 12, D-76128 Karlsruhe, Germany Institut f ur ´ Laboratoire des Sciences du Genie Chimique, CNRS - Groupe ENSIC 1, rue Grandville, F-54001 Nancy-Cedex, France Received 16 February 1998; accepted 18 June 1998
Abstract Solid Electrolyte Potentiometry is an electrochemical method to characterise the state of solid catalysts under working conditions. Its application is temperature limited due to polarization effects at the catalyst electrode. A new operational definition of this temperature limit is proposed which is based on the measurements of exchange current densities. 1998 Elsevier Science B.V. All rights reserved. Keywords: SEP; Electrochemical cell; Catalytic electrode; Oxidic catalyst
1. Introduction Following a proposition by C. Wagner [1], solid electrolyte electrochemical cells may be operated to investigate the behaviour of catalysts in situ. Since the pioneering work of Vayenas and Saltsburg [2] such arrangements have been used two-fold. Either the application of an outer potential influences the catalysed reaction, leading to the surprising effect of Non-Faradaic Electrochemical Modification of the Catalytic Activity (NEMCA) [3], now well understood due to the work of the group in Patras, or the catalyst is characterised by its oxygen activity under working conditions measured by the technique known as Solid Electrolyte Potentiometry (SEP) [4]. Both techniques have been widely applied on metal
*Corresponding author. [email protected]
Fax:
133-3-8332-2975;
e-mail:
catalysts. The NEMCA effect was observed in 45 catalytic reactions using various solid electrolytes [5]. SEP studies on metals are summarized for instance in a recent article by I. Metcalfe [6]. In both cases the electrochemical cell has the form: reactive gas phase ucatalyst electrodeu solid electrolyte ureference electrodeu O 2 / N 2 , the reference electrode consisting mainly of a porous platinum film. SEP has also been used to investigate the behaviour of oxidic catalysts in a limited number of studies [7–11]. The reaction rate and oxygen activity were monitored simultaneously in order to correlate catalyst selectivity with oxygen activity and / or the stable phase composition of the catalyst under working conditions. Its use and limits in the case of oxidic catalysts have been discussed in detail elsewhere [12]. The application of SEP is useful only if the combined kinetic and potentiometric measurements
0167-2738 / 98 / $ – see front matter 1998 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00201-X
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck
76
can be made simultaneously at the same temperature of catalytic interest. Now, SEP measurements have been proven to be possible only down to a lower temperature limit, below which polarization effects inhibit the formation of a stable potential. In the following we shall discuss an operational definition of this limit temperature below which potentiometric results become questionable.
2. Measurements under equilibrium conditions Stabilised zirconia, an oxygen ion conducting solid electrolyte, is mainly used in SEP measurements and all results reported in the following refer to YSZ. Thus the potential-determining reaction may be written as: O 2 1 4e 2 ⇔2O 22
Fig. 1. Temperature dependence of the equilibrium potential DE*; (catalyst electrode: Pt, reference electrode: Pt).
(1)
It takes place at the three phase border line gas– electrode–electrolyte. If the oxygen at the interfaces is in equilibrium with the gas phase oxygen on both sides of the electrochemical cell, i.e. there is no chemical reaction, its partial pressure at the catalyst side, P M O 2 , is related to its value at the reference side, P RO 2 , via a Nernst equation:
S D
PM R?T O2 ]] ]] DE* 5 ln 4F P RO 2
(2)
Eq. (2) is respected within a large range of partial pressures and temperature. Typical results describing the temperature dependence are represented in Figs. 1 and 2. The agreement between the values calculated by use of the Nernst equation and the experimentally determined ones is quite satisfactory. However, at temperatures below 520 K in the case of platinum (Fig. 1) or 640 K in the case of vanadia– titania (Fig. 2) no stable values of the potential are obtained even after 24 h, due to polarization effects. In the case of platinum, we characterised the quality of the porous electrodes by exchange current measurements and were able to correlate limit temperature, exchange current and BET surface of the electrode (characteristic of the extension of the three phase border line). We came to the conclusion [13], that the lower temperature limit of the application of
Fig. 2. Temperature dependence of the equilibrium potential DE*; (catalyst electrode: V2 O 5 / TiO 2 , reference electrode: Pt).
SEP under equilibrium conditions is due to activation polarization and that it may be lowered only by thorough preparation of the electrodes. However, the low temperature limits obtained so far in different cases (Table 1) often remain rather high for the study of heterogeneously catalysed reactions.
3. Measurements in reactive systems In the case where the catalyst electrode is exposed to a reacting gas mixture, the Nernst equation may be written in the form:
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck Table 1 Experimentally determined values of the low temperature limit, qL , for the use of solid electrolyte electrochemical cells under equilibrium conditions Catalyst electrode
qL / 8C
Reference
Pt Copper oxide Copper oxide11 mol% Na Vanadia–titania Copper molybdate Mo–V–Cu mixed oxide
250 420 400 370 415 420
[14] [15] [15] [16] [17] [17]
S D
a 2O RT DE 5 ] ln ] 4F p RO 2
(3)
Eq. (3) constitutes the operational definition of the oxygen activity a O of atomic oxygen on the catalyst under operating conditions. We first restricted the combined kinetic and potentiometric measurements to the temperature domain above the limit determined under equilibrium conditions. However, for reasons of catalytic interest, we additionally performed measurements at temperatures somewhat lower than this limit. It turned out that stable values of the potential DE could equally be obtained and there appeared no dramatic change in the measured quantities or the nature of the interrelations shown. The reasons were found through the determination of exchange currents under reacting conditions. The values obtained by current–potential measurements or cyclic voltammetry (5 mV/ s, [16]) showed a substantial increase up to one order of magnitude relative to the values determined under equilibrium conditions. Typical results are summarized in Table 2. At 3908C, the temperature limit under equilibrium conditions, the exchange current density is equal to Table 2 Exchange current densities at different temperatures and gas phase compositions (catalyst electrode: vanadia–titania, reference electrode: Pt)
q / 8C
Gas phase
i O /(nA / cm 2 )
390 390 350 300
O2 / N2 O 2 / NO / NH 3 / N 2 O 2 / NO / NH 3 / N 2 O 2 / NO / NH 3 / N 2
10 119 77 42
77
10 nA / cm 2 , using a mixture of 20 mbar O 2 in N 2 at the catalyst electrode and air at the reference side. The density is related to the geometrical area of the solid electrolyte. Maintaining the same partial pressure of oxygen but adding an equimolar mixture of NH 3 and NO (c i 50.01 mol / m 3 ) this value is increased to 119 nA / cm 2 . Even after a temperature decrease of 908 below the temperature limit, a stable potential difference is obtained and the exchange current density remains 4 times higher than under the equilibrium conditions at the low temperature limit. The observed increase in exchange current densities under reactive conditions is not restricted to the NH 3 –NO–O 2 system. Similar results are obtained in the partial oxidation of organic compounds on oxidic catalysts [15,17].
4. Discussion and conclusions Balian et al. [18] have published potentiometric measurements on copper oxides under reactive conditions – the oxidation of hydrogen – at temperatures as low as 2508C, far beneath the limit determined by Hildenbrand [15] under equilibrium conditions (cf. Table 1). In addition, we have found in the continuation of the latter work that the kinetic indication of a phase transition in the copper–oxygen system could be confirmed by SEP down to 2808C. The extension of potentiometric measurements beyond the equilibrium-determined limit thus seems to be justified. It is interesting to note that observations related in the literature [19,20] indicate that the response time of electrochemical cells to variations of the gas phase composition is considerably shortened if the catalyst electrode is in contact with a reactive mixture. This gives a strong hint that the rate of a potential determining reaction and thus its time constants of the formation of the potential may be altered by the presence of reactants in the gas phase. In addition, in the case of vanadia–titania catalyst, the results of which were reported in [2], independent conductivity measurements under reaction conditions have shown that the potentiometrically determined oxygen activity is a real measure of the oxidation state of the catalyst within the whole temperature range investigated.
78
¨ et al. / Solid State Ionics 112 (1998) 75 – 78 J. Bruck
Without any speculation about the physical reasons for the lowering of the limit temperature below its value determined under equilibrium conditions, we feel that a cautious extension of the potentiometric measurements to values below this limit are possible. Therefore we propose the following operational definition of the limitations of SEP in a reactive system: Potentiometric measurements are possible down to temperatures where the exchange current densities under reactive conditions are of the same order of magnitude as at the lowest temperature where under equilibrium conditions, a stable electrode potential is still obtainable.
References [1] C. Wagner, Adv. Catal. 21 (1970) 323. [2] C.G. Vayenas, H. Saltsburg, J. Catal. 57 (1979) 296. [3] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303.
[4] H.-G. Lintz, C.G. Vayenas, Angew. Chem. Int. Ed. Engl. 28 (1989) 708. [5] C.G. Vayenas, S.I. Bebelis, Solid State Ionics 94 (1997) 267. [6] I.S. Metcalfe, Catal. Today 20 (1994) 283. [7] S. Pizzini, C.M. Mari, L. Zanderighi, Gazz. Chim. Ital. 110 (1980) 389. [8] E.M. Breckner, S. Sundaresan, J.B. Benziger, Appl. Catal. 30 (1987) 277. [9] H.-H. Hildenbrand, H.-G. Lintz, Appl. Catal. 65 (1990) 241. [10] P.D. Petrolekas, I.S. Metcalfe, J. Catal. 152 (1995) 147. ¨ [11] J. Bruck, M. Brust, H.-G. Lintz, Ber. Bunsenges. Phys. Chem. 99 (1995) 1509. [12] H.-H. Hildenbrand, H.-G. Lintz, Ber. Bunsenges. Phys. Chem. 95 (1991) 1191. ¨ [13] E. Hafele, H.-G. Lintz, Solid State Ionics 23 (1987) 235. [14] M. Oerter, Ph. D. Thesis, Karlsruhe 1994. [15] H.-H. Hildenbrand, Ph. D. Thesis, Karlsruhe 1991. ¨ [16] J. Bruck, Ph. D. Thesis, Karlsruhe 1995. [17] M. Estenfelder, Ph. D. Thesis, Karlsruhe 1998. [18] A. Balian, G. Hatzigiannis, D. Eng, M. Stoukides, J. Catal. 145 (1994) 526. [19] A. Caneiro, M. Bonnat, J. Fouletier, J. Appl. Electrochem. 11 (1981) 83. [20] J.E. Anderson, Y.B. Graves, J. Appl. Electrochem. 12 (1982) 335.