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Electrochemical promotion of ethylene and propylene oxidation on Pt deposited on yttria–titania–zirconia
Solid State Ionics 136–137 (2000) 833–837 www.elsevier.com / locate / ssi
Electrochemical promotion of ethylene and propylene oxidation on Pt deposited on yttria–titania–zirconia a b a b, P. Beatrice , C. Pliangos , W.L. Worrell , C.G. Vayenas * a
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 -6272, USA b Department of Chemical Engineering, University of Patras, Patras GR-26500, Greece
Abstract The effect of electrochemical promotion (NEMCA) was investigated for the oxidation of ethylene and propylene on Pt deposited on 4.5 mol% Y 2 O 3 –10 mol% TiO 2 –85.5 mol% ZrO 2 (YZTi10) which is believed to be an ionic conductor with some n-type electronic conductivity in reducing environments. It was found that YZTi10 can induce the effect of electrochemical promotion for both C 2 H 4 and C 3 H 6 oxidation with apparent Faradaic efficiency, L, values up to 10 3 . The rate enhancement ratio, r 5 r /r o , however was found to be much smaller than in the cases where 8 mol% Y 2 O 3 -stabilized– ZrO 2 (YSZ) or even TiO 2 is used as the solid electrolyte and O 22 donor. The origin of this difference is not clear. 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical promotion; Oxidation; Ethylene; Propylene; Faradaic efficiency
1. Introduction The catalytic performance (activity, selectivity) of metals interfaced with solid electrolytes can be affected significantly upon external current or potential application. The electrochemically induced change in catalytic rate can be several orders of magnitude higher than the rate of ion transport through the solid electrolyte. This phenomenon is known as ‘non-Faradaic electrochemical modification of catalytic activity’ (NEMCA) [1,2] or electrochemical promotion [3]. It has been studied for more than 55 catalytic systems [1,2] using various metal or
*Corresponding author. Tel.: 130-61-997-756; fax: 130-61997-269. E-mail address: [email protected] (C.G. Vayenas).
metal-oxide catalyst-electrode films (Pt, Pd, Rh, Ag, Ni, IrO 2 , RuO 2 , etc.) interfaced with O 22 , Na 1 , H 1 , F 2 ionic conductors, mixed electronic–ionic conductors (TiO 2 , CeO 2 ) [4,5] or aqueous alkaline solutions (LiOH, KOH) [6] for several groups of catalytic reactions (i.e. oxidations, reductions, hydrogenations, decompositions, etc.). Work in this area has been reviewed recently [2,7]. The importance of NEMCA in catalysis has been addressed by several authors [3,8,9] and a considerable number of research groups has made an important contribution in this area [10–14]. The observed promotional phenomena have been shown to be due to the electrochemically controlled reverse spillover of ionic species migrating between the solid electrolyte support and the catalyst surface [1,2,15,16]. Consequently, an ‘effective’ electrochemical double layer [17,18] is, thus, established on
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00518-X
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P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
the catalyst surface, which changes the catalystelectrode work function, eF, by D(eF ) 5 e DVWR
(1)
where VWR , is the catalyst (working)-electrode potential with respect to the reference electrode. Eq. (1) has been confirmed by Kelvin probe [15] and UPS [19] measurements and shows that any change of the catalyst potential is accompanied by a change in the catalyst work function. The magnitude of Electrochemical Promotion is given by the Faradaic efficiency, L, and the rate enhancement ratio, r defined from [1,2,7]:
L 5 Dr /(I / 2F ); r 5 r /r o
(2)
where, Dr5r2r o , is the electrochemically induced change in catalytic rate, I, is the applied current and F is the Faraday constant. L and r values as high as 3310 5 [16] and 150 [2] have been observed, respectively. Yttria-stabilized zirconia doped with 5–20 mol% titania is a mixed conductor, exhibiting oxygen ion and n-type conductivity under reducing atmospheres and elevated temperature [22–28]. The NEMCA behavior of both ethylene and propylene oxidation on Pt deposited on ionically conductive yttria-stabilized zirconia (YSZ) has been extensively investigated [2,29,30]. Also the NEMCA behavior of ethylene oxidation on Pt deposited on titania, a mixed conductor, has shown a rate increase up to 20 times higher than the unpromoted catalytic rate [4]. The goal of this work was to investigate whether YZTi10, a mixed conductor, can also induce NEMCA behavior for these reactions.
2. Experimental The experimental setup used in this study consists of the flow system, the reactor and the gas analysis unit. A schematic of this is shown in Fig. 1. The reactor corresponds to the ‘single pellet’ design [1,17,20,21] and has been described in detail elsewhere [1,2,7,17,20], together with the metal film deposition technique. The Pt-metal film deposited onto the one side of YSZ pellets serve as the working electrode, while two Au films, deposited on the other side of the disk,
Fig. 1. Schematic of single-pellet reactor and of the solid electrolyte pellet.
serve as the reference and counter electrode. All the electrodes are exposed to the same gas mixture. Gold behaves as a good pseudoreference electrode, since previous NEMCA studies have shown a small variation of its potential (,0.1 V) over the range of gaseous compositions used in the present study [20]. In all cases, a series of blank experiments was carried out to confirm that the catalytic activity of the Au electrodes was negligible in comparison to that of the Pt catalyst. The 4.5 mol% yttria –10 mol% titania–zirconia powder was produced using a co-precipitation process from ZrOCl 2 ?2H 2 O (Alfa AESAR), YCl?6H 2 O (Aldrich Chemical Company) and TiCl 4 (Aldrich Chemical Company). The precipitated powder was crushed to 2200 mesh and calcined at 8008C for 12 h in air. The powder was isopressed into discs approximately 1-mm thick, and sintered at 16008C for 12 h in air. The resulting discs were impervious although a small volume of isolated enclosed pores were seen by SEM. XRD showed the material to be only one phase, a cubic zirconia structure, with no evidence of the formation of a zirconium titanate
P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
phase that forms above the solubility limit. The 4.5 mol% yttria–10 mol% titania–zirconia material is abbreviated as YZTi10 in this work. The counter and reference electrodes were prepared by applying a thin coating of Engelhard A1118 Au paste and calcining at 8508C for 1 h in air. The same procedure was used to make the working electrode from Engelhard A-1121 Pt paste. The sample was mounted in the continuous flow ‘single pellet’ reactor as shown in Fig. 1. The reactant gases, pure (99.99%) He and mixtures of O 2 / He, C 2 H 4 / He, and C 3 H 6 / He were manufactured and certified by Air Liquide. Gas flow-rates between 150–200 cm 3 / min were used. A Perkin–Elmer Sigma 300 Gas Chromatograph with a Poropak N column was used to analyse the reactant and product gases. In addition, the CO 2 concentration was monitored with an Anarad IR gas analyser.
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The extracted exchange current densities, I0 , are of the order of 1 mA and increase with an apparent activation energy of 21–29 kcal / mol. Fig. 3a, b show the effect of applied current I and equivalent rate, I / 2F, of O 22 supply to the catalystelectrode on the increase in the rate of C 3 H 6 oxidation, expressed in mol O / s. Fig. 3a refers to positive currents (O 22 supply to the catalyst) and Fig. 3b refers to negative currents (O 22 removal from the catalyst). In both cases the rate increases, i.e. the reaction exhibits electrophobic behaviour (L .0) for positive currents and electrophilic behaviour (L ,0) for negative ones. As shown in Fig. 3a and 3b the effect is clearly non-Faradaic with L values up to 1000 (I .0) and down to 21000 for I ,0.
3. Results and discussion
3.1. Propylene oxidation Fig. 2 shows current-overpotential plots of the Pt catalyst-electrode in the presence of propylene (PC 3 H 6 52 kPa) and oxygen (PO 2 510 kPa). The anodic and cathodic transfer coefficients extracted from the linear log I2UWR (Tafel) dependence for uUWR u between 0.2 and 1 V are of the order of 0.25.
Fig. 2. Current-overpotential plots during C 3 H 6 oxidation on Pt / YZTi10.
Fig. 3. Effect of applied current on the increase in the rate of C 3 H 6 oxidation on Pt / YZTi10 for positive (a) and negative (b) currents.
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Fig. 6. Effect of constant applied potential (22 V) on the rate oscillation of C 2 H 4 oxidation on Pt / YZTi10. Fig. 4. Current–overpotential plots during C 2 H 4 oxidation on Pt / YZTi10.
3.2. Ethylene oxidation Fig. 4 shows current overpotential plots of the Pt catalyst in presence of ethylene (PC 2 H 4 52 kPa) and oxygen (PO 2 59 kPa). The behaviour is quite similar to that obtained in presence of propylene–oxygen mixtures (Fig. 2). Fig. 5 shows that, as well known from the catalytic literature [31], C 2 H 4 oxidation on Pt is an oscillatory reaction over a wide range of gaseous composition and temperature. The oscillating pattern can be relatively simple (Fig. 5) or more complex (Fig. 6). As shown in Fig. 6 the rate oscillations can be influenced by the applied potential, as also well known from the case of C 2 H 4 oxidation on Pt / YSZ [32].
Fig. 7. Transient effect of applied negative potential on the rate of C 2 H 4 oxidation on Pt / YZTi10.
Fig. 7 shows the effect of applied potential under conditions where the reaction does not exhibit oscillatory behaviour. Negative potential application causes a two-fold enhancement in the rate ( r 52) with a Faradaic efficiency L 5 2250. The reaction thus exhibits electrophilic behaviour which is typical of C 2 H 4 oxidation on Pt under relatively low O 2 / C 2 H 4 ratios [33].
4. Conclusions
Fig. 5. Open-circuit oscillations in the rate of C 2 H 4 oxidation (expressed in %CO 2 in the reactor exit) and in the open-circuit potential of the Pt / YZTi10 catalyst.
TiO 2 -doped-YSZ (YSZTi10) can be used to induce the effect of electrochemical promotion on Pt-catalyst electrodes, both for the oxidation of C 3 H 6 and C 2 H 4 . The electrochemical promotion behaviour is qualitatively similar with that observed when using YSZ or TiO 2 as the O 22 donor, but the effect is less
P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
pronounced. At this point it is not clear if this is due to the morphology of the catalyst electrodes used in the present study or due to the ionic conduction properties of YSZ Ti10 itself.
Acknowledgements Funding for this project was provided by NSF Grant INT-9600288. The authors would like to thank Dr. Yoshihara Uchimoto of Kyoto University for his instructions with the material preparation technique.
References [1] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. [2] C.G. Vayenas, M.M. Jaksic, S. Bebelis, S.M. Neophytides, in: J.O’M. Bockris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, Vol. 29, Plenum Press, New York, 1995, pp. 57–202. [3] J. Pritchard, Nature (London) 343 (1990) 592. [4] C. Pliangos, I.V. Yentekakis, S. Ladas, C.G. Vayenas, J. Catal. 159 (1996) 189. [5] P.D. Petrolekas, S. Balomenou, C.G. Vayenas, J. Electrochem. Soc. 145 (4) (1998) 1202. [6] S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, C.G. Vayenas, Nature (London) 370 (1994) 45. [7] C.G. Vayenas, I.V. Yentekakis, in: G. Ertl, H. Knotzinger, J. Weitcamp (Eds.), Handbook of Catalysis, VCH Publishers, Weinheim, 1997, pp. 1310–1338. [8] J.O’M. Bockris, Z.S. Minevski, Electrochim. Acta 39 (1994) 1471. [9] B. Grzybowska-Swierkosz, J. Haber, in: Annual Reports on the Progress of Chemistry, Vol. 91, The Royal Society of Chemistry, Cambridge, 1994, p. 395. [10] T.I. Politova, V.A. Sobyanin, V.D. Belyaev, React. Kinet. Catal. Lett. 41 (1990) 321. [11] L. Basini, C.A. Cavalca, G.L. Haller, J. Phys. Chem. 88 (1994) 10853.
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[12] I. Harkness, R.M. Lambert, J. Catal. 152 (1995) 211. [13] P.C. Chiang, D. Eng, M. Stoukides, Interface 139 (1993) 683. [14] E. Varkaraki, J. Nicole, E. Plattner, Ch. Comninellis, J. Appl. Electrochem. 25 (1995) 978. [15] C.G. Vayenas, S. Bebelis, S. Ladas, Nature (London) 343 (1990) 625. [16] S. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125. [17] C.A. Cavalca, G. Larsen, C.G. Vayenas, G.L. Haller, J. Phys. Chem. 97 (1993) 6115. [18] C.G. Vayenas, S. Ladas, S. Bebelis, I.V. Yentekakis, S.G. Neophytides, Y. Jiang et al., Electrochim. Acta 39 (1994) 1849. [19] W. Zipprich, H.-D. Wiemhoefer, U. Voehrer, W. Goepel, Ber. Bunsenges. Phys. Chem. 99 (1995) 1406. [20] I.V. Yentekakis, S. Bebelis, J. Catal. 137 (1992) 278. [21] J. Nicole, D. Tsiplakides, S. Wodiuning, Ch. Comninellis, J. Electrochem. Soc. 144 (12) (1997) 312. [22] K. Swider, The electrical properties of bulk and thin film yttria-stabilized zirconia–titania, Ph.D. Thesis, University of Pennsylvania, 1992. [23] W.L. Worrell, P. Han, Y. Uchimoto, P.K. Davies, A new single-component solid oxide fuel cell, in: M. Doklya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), The Electrochemical Society, Pennington, NJ, 1995, p. 50. [24] L.S.M. Traqueia, T. Pagnier, F.M.B. Marques, J. Eur. Ceram. Soc. 17 (1997) 1019. [25] M.T. Colomer, P. Duran, A. Caballero, J.R. Jurado, Mat. Sci. Eng. A229 (1997) 114. [26] K. Kobayashi, Y. Kai, S. Yamaguchi, N. Fukatsu, T. Kawashima, Y. Iguchi, Solid State Ionics 93 (1997) 193. [27] M.T. Colomer, L.S.M. Traqueia, J.R. Jurado, F.M.B. Marques, Mat. Res. Bull. 30 (No.4) (1995) 515. [28] H. Naito, H. Arashi, Solid State Ionics 53 (1992) 436. [29] S.I. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125. [30] A. Kaloyiannis, C. Vayenas, J. Catal. 182 (1999) 37. [31] C.G. Vayenas, B. Lee, J. Michaels, J. Catal. 66 (1980) 36. [32] I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170. [33] S. Bebelis, M. Makri, A. Buekenhoudt, J. Luyten, S. Brosda, P. Petrolekas, C. Pliangos, C.G. Vayenas, Solid State Ionics (2000) in press.
Electrochemical promotion of ethylene and propylene oxidation on Pt deposited on yttria–titania–zirconia a b a b, P. Beatrice , C. Pliangos , W.L. Worrell , C.G. Vayenas * a
Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104 -6272, USA b Department of Chemical Engineering, University of Patras, Patras GR-26500, Greece
Abstract The effect of electrochemical promotion (NEMCA) was investigated for the oxidation of ethylene and propylene on Pt deposited on 4.5 mol% Y 2 O 3 –10 mol% TiO 2 –85.5 mol% ZrO 2 (YZTi10) which is believed to be an ionic conductor with some n-type electronic conductivity in reducing environments. It was found that YZTi10 can induce the effect of electrochemical promotion for both C 2 H 4 and C 3 H 6 oxidation with apparent Faradaic efficiency, L, values up to 10 3 . The rate enhancement ratio, r 5 r /r o , however was found to be much smaller than in the cases where 8 mol% Y 2 O 3 -stabilized– ZrO 2 (YSZ) or even TiO 2 is used as the solid electrolyte and O 22 donor. The origin of this difference is not clear. 2000 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical promotion; Oxidation; Ethylene; Propylene; Faradaic efficiency
1. Introduction The catalytic performance (activity, selectivity) of metals interfaced with solid electrolytes can be affected significantly upon external current or potential application. The electrochemically induced change in catalytic rate can be several orders of magnitude higher than the rate of ion transport through the solid electrolyte. This phenomenon is known as ‘non-Faradaic electrochemical modification of catalytic activity’ (NEMCA) [1,2] or electrochemical promotion [3]. It has been studied for more than 55 catalytic systems [1,2] using various metal or
*Corresponding author. Tel.: 130-61-997-756; fax: 130-61997-269. E-mail address: [email protected] (C.G. Vayenas).
metal-oxide catalyst-electrode films (Pt, Pd, Rh, Ag, Ni, IrO 2 , RuO 2 , etc.) interfaced with O 22 , Na 1 , H 1 , F 2 ionic conductors, mixed electronic–ionic conductors (TiO 2 , CeO 2 ) [4,5] or aqueous alkaline solutions (LiOH, KOH) [6] for several groups of catalytic reactions (i.e. oxidations, reductions, hydrogenations, decompositions, etc.). Work in this area has been reviewed recently [2,7]. The importance of NEMCA in catalysis has been addressed by several authors [3,8,9] and a considerable number of research groups has made an important contribution in this area [10–14]. The observed promotional phenomena have been shown to be due to the electrochemically controlled reverse spillover of ionic species migrating between the solid electrolyte support and the catalyst surface [1,2,15,16]. Consequently, an ‘effective’ electrochemical double layer [17,18] is, thus, established on
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 00 )00518-X
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P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
the catalyst surface, which changes the catalystelectrode work function, eF, by D(eF ) 5 e DVWR
(1)
where VWR , is the catalyst (working)-electrode potential with respect to the reference electrode. Eq. (1) has been confirmed by Kelvin probe [15] and UPS [19] measurements and shows that any change of the catalyst potential is accompanied by a change in the catalyst work function. The magnitude of Electrochemical Promotion is given by the Faradaic efficiency, L, and the rate enhancement ratio, r defined from [1,2,7]:
L 5 Dr /(I / 2F ); r 5 r /r o
(2)
where, Dr5r2r o , is the electrochemically induced change in catalytic rate, I, is the applied current and F is the Faraday constant. L and r values as high as 3310 5 [16] and 150 [2] have been observed, respectively. Yttria-stabilized zirconia doped with 5–20 mol% titania is a mixed conductor, exhibiting oxygen ion and n-type conductivity under reducing atmospheres and elevated temperature [22–28]. The NEMCA behavior of both ethylene and propylene oxidation on Pt deposited on ionically conductive yttria-stabilized zirconia (YSZ) has been extensively investigated [2,29,30]. Also the NEMCA behavior of ethylene oxidation on Pt deposited on titania, a mixed conductor, has shown a rate increase up to 20 times higher than the unpromoted catalytic rate [4]. The goal of this work was to investigate whether YZTi10, a mixed conductor, can also induce NEMCA behavior for these reactions.
2. Experimental The experimental setup used in this study consists of the flow system, the reactor and the gas analysis unit. A schematic of this is shown in Fig. 1. The reactor corresponds to the ‘single pellet’ design [1,17,20,21] and has been described in detail elsewhere [1,2,7,17,20], together with the metal film deposition technique. The Pt-metal film deposited onto the one side of YSZ pellets serve as the working electrode, while two Au films, deposited on the other side of the disk,
Fig. 1. Schematic of single-pellet reactor and of the solid electrolyte pellet.
serve as the reference and counter electrode. All the electrodes are exposed to the same gas mixture. Gold behaves as a good pseudoreference electrode, since previous NEMCA studies have shown a small variation of its potential (,0.1 V) over the range of gaseous compositions used in the present study [20]. In all cases, a series of blank experiments was carried out to confirm that the catalytic activity of the Au electrodes was negligible in comparison to that of the Pt catalyst. The 4.5 mol% yttria –10 mol% titania–zirconia powder was produced using a co-precipitation process from ZrOCl 2 ?2H 2 O (Alfa AESAR), YCl?6H 2 O (Aldrich Chemical Company) and TiCl 4 (Aldrich Chemical Company). The precipitated powder was crushed to 2200 mesh and calcined at 8008C for 12 h in air. The powder was isopressed into discs approximately 1-mm thick, and sintered at 16008C for 12 h in air. The resulting discs were impervious although a small volume of isolated enclosed pores were seen by SEM. XRD showed the material to be only one phase, a cubic zirconia structure, with no evidence of the formation of a zirconium titanate
P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
phase that forms above the solubility limit. The 4.5 mol% yttria–10 mol% titania–zirconia material is abbreviated as YZTi10 in this work. The counter and reference electrodes were prepared by applying a thin coating of Engelhard A1118 Au paste and calcining at 8508C for 1 h in air. The same procedure was used to make the working electrode from Engelhard A-1121 Pt paste. The sample was mounted in the continuous flow ‘single pellet’ reactor as shown in Fig. 1. The reactant gases, pure (99.99%) He and mixtures of O 2 / He, C 2 H 4 / He, and C 3 H 6 / He were manufactured and certified by Air Liquide. Gas flow-rates between 150–200 cm 3 / min were used. A Perkin–Elmer Sigma 300 Gas Chromatograph with a Poropak N column was used to analyse the reactant and product gases. In addition, the CO 2 concentration was monitored with an Anarad IR gas analyser.
835
The extracted exchange current densities, I0 , are of the order of 1 mA and increase with an apparent activation energy of 21–29 kcal / mol. Fig. 3a, b show the effect of applied current I and equivalent rate, I / 2F, of O 22 supply to the catalystelectrode on the increase in the rate of C 3 H 6 oxidation, expressed in mol O / s. Fig. 3a refers to positive currents (O 22 supply to the catalyst) and Fig. 3b refers to negative currents (O 22 removal from the catalyst). In both cases the rate increases, i.e. the reaction exhibits electrophobic behaviour (L .0) for positive currents and electrophilic behaviour (L ,0) for negative ones. As shown in Fig. 3a and 3b the effect is clearly non-Faradaic with L values up to 1000 (I .0) and down to 21000 for I ,0.
3. Results and discussion
3.1. Propylene oxidation Fig. 2 shows current-overpotential plots of the Pt catalyst-electrode in the presence of propylene (PC 3 H 6 52 kPa) and oxygen (PO 2 510 kPa). The anodic and cathodic transfer coefficients extracted from the linear log I2UWR (Tafel) dependence for uUWR u between 0.2 and 1 V are of the order of 0.25.
Fig. 2. Current-overpotential plots during C 3 H 6 oxidation on Pt / YZTi10.
Fig. 3. Effect of applied current on the increase in the rate of C 3 H 6 oxidation on Pt / YZTi10 for positive (a) and negative (b) currents.
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P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
Fig. 6. Effect of constant applied potential (22 V) on the rate oscillation of C 2 H 4 oxidation on Pt / YZTi10. Fig. 4. Current–overpotential plots during C 2 H 4 oxidation on Pt / YZTi10.
3.2. Ethylene oxidation Fig. 4 shows current overpotential plots of the Pt catalyst in presence of ethylene (PC 2 H 4 52 kPa) and oxygen (PO 2 59 kPa). The behaviour is quite similar to that obtained in presence of propylene–oxygen mixtures (Fig. 2). Fig. 5 shows that, as well known from the catalytic literature [31], C 2 H 4 oxidation on Pt is an oscillatory reaction over a wide range of gaseous composition and temperature. The oscillating pattern can be relatively simple (Fig. 5) or more complex (Fig. 6). As shown in Fig. 6 the rate oscillations can be influenced by the applied potential, as also well known from the case of C 2 H 4 oxidation on Pt / YSZ [32].
Fig. 7. Transient effect of applied negative potential on the rate of C 2 H 4 oxidation on Pt / YZTi10.
Fig. 7 shows the effect of applied potential under conditions where the reaction does not exhibit oscillatory behaviour. Negative potential application causes a two-fold enhancement in the rate ( r 52) with a Faradaic efficiency L 5 2250. The reaction thus exhibits electrophilic behaviour which is typical of C 2 H 4 oxidation on Pt under relatively low O 2 / C 2 H 4 ratios [33].
4. Conclusions
Fig. 5. Open-circuit oscillations in the rate of C 2 H 4 oxidation (expressed in %CO 2 in the reactor exit) and in the open-circuit potential of the Pt / YZTi10 catalyst.
TiO 2 -doped-YSZ (YSZTi10) can be used to induce the effect of electrochemical promotion on Pt-catalyst electrodes, both for the oxidation of C 3 H 6 and C 2 H 4 . The electrochemical promotion behaviour is qualitatively similar with that observed when using YSZ or TiO 2 as the O 22 donor, but the effect is less
P. Beatrice et al. / Solid State Ionics 136 – 137 (2000) 833 – 837
pronounced. At this point it is not clear if this is due to the morphology of the catalyst electrodes used in the present study or due to the ionic conduction properties of YSZ Ti10 itself.
Acknowledgements Funding for this project was provided by NSF Grant INT-9600288. The authors would like to thank Dr. Yoshihara Uchimoto of Kyoto University for his instructions with the material preparation technique.
References [1] C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. [2] C.G. Vayenas, M.M. Jaksic, S. Bebelis, S.M. Neophytides, in: J.O’M. Bockris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, Vol. 29, Plenum Press, New York, 1995, pp. 57–202. [3] J. Pritchard, Nature (London) 343 (1990) 592. [4] C. Pliangos, I.V. Yentekakis, S. Ladas, C.G. Vayenas, J. Catal. 159 (1996) 189. [5] P.D. Petrolekas, S. Balomenou, C.G. Vayenas, J. Electrochem. Soc. 145 (4) (1998) 1202. [6] S. Neophytides, D. Tsiplakides, P. Stonehart, M.M. Jaksic, C.G. Vayenas, Nature (London) 370 (1994) 45. [7] C.G. Vayenas, I.V. Yentekakis, in: G. Ertl, H. Knotzinger, J. Weitcamp (Eds.), Handbook of Catalysis, VCH Publishers, Weinheim, 1997, pp. 1310–1338. [8] J.O’M. Bockris, Z.S. Minevski, Electrochim. Acta 39 (1994) 1471. [9] B. Grzybowska-Swierkosz, J. Haber, in: Annual Reports on the Progress of Chemistry, Vol. 91, The Royal Society of Chemistry, Cambridge, 1994, p. 395. [10] T.I. Politova, V.A. Sobyanin, V.D. Belyaev, React. Kinet. Catal. Lett. 41 (1990) 321. [11] L. Basini, C.A. Cavalca, G.L. Haller, J. Phys. Chem. 88 (1994) 10853.
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[12] I. Harkness, R.M. Lambert, J. Catal. 152 (1995) 211. [13] P.C. Chiang, D. Eng, M. Stoukides, Interface 139 (1993) 683. [14] E. Varkaraki, J. Nicole, E. Plattner, Ch. Comninellis, J. Appl. Electrochem. 25 (1995) 978. [15] C.G. Vayenas, S. Bebelis, S. Ladas, Nature (London) 343 (1990) 625. [16] S. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125. [17] C.A. Cavalca, G. Larsen, C.G. Vayenas, G.L. Haller, J. Phys. Chem. 97 (1993) 6115. [18] C.G. Vayenas, S. Ladas, S. Bebelis, I.V. Yentekakis, S.G. Neophytides, Y. Jiang et al., Electrochim. Acta 39 (1994) 1849. [19] W. Zipprich, H.-D. Wiemhoefer, U. Voehrer, W. Goepel, Ber. Bunsenges. Phys. Chem. 99 (1995) 1406. [20] I.V. Yentekakis, S. Bebelis, J. Catal. 137 (1992) 278. [21] J. Nicole, D. Tsiplakides, S. Wodiuning, Ch. Comninellis, J. Electrochem. Soc. 144 (12) (1997) 312. [22] K. Swider, The electrical properties of bulk and thin film yttria-stabilized zirconia–titania, Ph.D. Thesis, University of Pennsylvania, 1992. [23] W.L. Worrell, P. Han, Y. Uchimoto, P.K. Davies, A new single-component solid oxide fuel cell, in: M. Doklya, O. Yamamoto, H. Tagawa, S.C. Singhal (Eds.), Proceedings of the Fourth International Symposium on Solid Oxide Fuel Cells (SOFC-IV), The Electrochemical Society, Pennington, NJ, 1995, p. 50. [24] L.S.M. Traqueia, T. Pagnier, F.M.B. Marques, J. Eur. Ceram. Soc. 17 (1997) 1019. [25] M.T. Colomer, P. Duran, A. Caballero, J.R. Jurado, Mat. Sci. Eng. A229 (1997) 114. [26] K. Kobayashi, Y. Kai, S. Yamaguchi, N. Fukatsu, T. Kawashima, Y. Iguchi, Solid State Ionics 93 (1997) 193. [27] M.T. Colomer, L.S.M. Traqueia, J.R. Jurado, F.M.B. Marques, Mat. Res. Bull. 30 (No.4) (1995) 515. [28] H. Naito, H. Arashi, Solid State Ionics 53 (1992) 436. [29] S.I. Bebelis, C.G. Vayenas, J. Catal. 118 (1989) 125. [30] A. Kaloyiannis, C. Vayenas, J. Catal. 182 (1999) 37. [31] C.G. Vayenas, B. Lee, J. Michaels, J. Catal. 66 (1980) 36. [32] I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170. [33] S. Bebelis, M. Makri, A. Buekenhoudt, J. Luyten, S. Brosda, P. Petrolekas, C. Pliangos, C.G. Vayenas, Solid State Ionics (2000) in press.