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Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation
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Catalysis Communications 9 (2008) 17–20 www.elsevier.com/locate/catcom
Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation Antonio de Lucas-Consuegra a, Fernando Dorado a, Jose´ Luis Valverde a, Reda Karoum b, Philippe Vernoux b,* a
b
Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain Universite´ de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS, Universite´ Lyon 1, 69626 Villeurbanne, France Received 26 March 2007; received in revised form 26 April 2007; accepted 27 April 2007 Available online 10 May 2007
Abstract This study exposes, for the first time, the effect of Electrochemical Promotion (EPOC) by potassium cations on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization drastically increased the catalytic rate for more than 11 times by decreasing the light-off temperature by almost 40 C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds on Pt surface that enhanced the adsorption of O2 at the expense of CO. The achieved results demonstrated that Electrochemical Promotion of Catalysis could be applied to improve the performance of catalytic converters at lower temperatures. 2007 Elsevier B.V. All rights reserved. Keywords: CO oxidation; Electrochemical Promotion; NEMCA effect; Platinum catalyst; Potassium promoter
1. Introduction Despite the encouraging achievements in catalytic abatement of harmful emissions from automotives, the exhaust released from a car engine during the first minutes after a ‘‘cold-start’’ is a critical issue for the performance of catalytic converters, especially at high CO concentrations [1]. Also, new and fuel-efficient engines generate colder exhaust gases than current engines, resulting in slower heating of the catalyst. This places new demands on the low temperature activity for the catalytic converters used in future emission abatement systems [2]. Several technologies including, for example, burner or electrically heated catalysts [3], exhaust gas ignition [4] and hydrocarbons traps [5] have been developed to improve the efficiency of the catalyst, decreasing the light-off temperature (temperature at which the conversion *
Corresponding author. Tel.: +33 4 72 43 15 87; fax: +33 4 72 43 16 95. E-mail address: [email protected] (P. Vernoux).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.04.038
of the pollutants exceeds 50%). Nevertheless, these systems are expensive, with difficult diagnosis and highly complex [6]. The use of electrochemistry to activate and control a reaction process is an expanding domain, because it enables to improve in a very pronounced manner the catalytic performance of a metal catalyst. This process, called Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion of Catalysis (EPOC) was discovered and developed by Vayenas et al. [7,8]. It is based on the control, by an applied potential, of the catalyst work function, due to electrochemical pumping of ions (promoters) from a solid electrolyte [9]. Previous studies have explored the utility of electrochemical promotion to improve the catalytic activity of several metals for CO oxidation using YSZ [10–15], Na–bAl2O3 [16,17], and CaF2 [18] as solid electrolytes. Nevertheless, all these studies have been carried out between 300 and 700 C, which is a high temperature range for the practical treatment of exhaust gases.
18
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
The aim of this work was to demonstrate that EPOC could be used to decrease the operating temperature window of a metal catalyst. Thus, it is reported for the first time, the effect of Electrochemical Promotion by potassium on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization was investigated between 190 and 340 C, which is a temperatures range compatible with the treatment of exhaust emissions. 2. Experimental 2.1. Electrochemical catalyst preparation The electrochemical catalyst consisted of a porous and continuous thin Pt film (geometric area of 1.12 cm2) deposited on a side of a 17 mm diameter and 1 mm thickness K–b00 Al2O3 disk (Ionotec). The Pt film (1.24 mg Pt/cm2) was deposited, as described in detail elsewhere [19], by successive steps of deposition and thermal decomposition (650 C for 1 h) of a H2PtCl6 precursor solution. Gold counter and reference electrodes were deposited on the opposite side by application and calcination of thin coatings of gold paste. The dispersion of the Pt film was determined by measuring its reactive oxygen uptake at 350 C via the isothermal titration technique [8]. The reactive oxygen uptake was No = 3.5 · 107 mol O, which led to a metal dispersion of 5%. 2.2. Catalytic activity measurements for CO oxidation The catalytic activity measurements were carried out within a specific quartz reactor, as described in a previous study [20]. Mixtures of CO (Air Liquide, 1% in He, purity: 99.95%), O2 (Air Liquide, purity: 99.95%) and He (Air Liquid, purity: 99.95%) were used as feed to the reactor. The gas composition was controlled, by mass flow controllers (Brooks). The feed composition was as follows: 5000 ppm CO, 5000 ppm O2, He balance, overall flow rate 6.5 L/h. Reactants and products were analyzed using an on-line micro gas-chromatograph (Varian CP-2003), while CO2 concentration in the effluent from the reactor was also continuously monitored by an infrared (IR) analyzer (Horiba VA-3000). Before catalytic activity measurements, the Pt film was pretreated under H2 at 450 C for 1 h in order to reduce platinum. Cyclic voltammetry measurements were performed with the potentiostat–galvanotat at 250 C under reaction conditions, and were recorded at a sweep rate of 2 mV/s. 3. Results and discussion CO oxidation activities of the electrochemical catalyst Pt/K–b00 Al2O3 was investigated through temperature-programmed reaction experiments (light-off measurements) under application of five different catalyst potentials (Fig. 1). CO oxidation was strongly temperature dependent, increasing conversion from 1% to 90% between 190 and
Fig. 1. Light-off curves of the electrochemical catalyst Pt/K–bAl2O3 under different catalyst potentials. Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
340 C. Full conversion of CO was never reached because a part of the flow bypassed the Pt film. It is well known that at low temperature the oxidation of CO is inhibited by CO adsorption [21]. As the reaction temperature increases, the adsorbed molecules of CO start to desorb and oxygen is activated by dissociative adsorption whereby CO is oxidised. The operating temperature window at which this activation takes place depends on several factors, e.g., the type of metal catalyst, the CO/O2 ratio, or the total reactant flow. Nevertheless, it can be observed in Fig. 1 that for a given system, the electrochemistry can be used to decrease the reaction temperature at which this activation initiates. Thus, as the catalyst potential decreased to lower values the activation begun at lower temperatures. A decrease in the catalyst potential involves a decrease in the catalyst work function, as already discussed in previous studies [22–25]. This is the result of the migration of positively charged potassium species from the electrolyte through the catalyst surface, which modifies the chemisorption properties of the metal. Then, a decrease in the catalyst potential of the Pt catalyst electrode enhanced CO oxidation rate by increasing the coverage of electron-acceptor species (O2), at the expense of electron-donor (CO). Therefore, negative overpotentials led to the observed activation of the catalyst. Assuming that at VWR = 2 V there was no promoter on the catalyst (hK = 0), the promotional phenomenon could be characterized by the rate enhancement ratio (q) which was defined by the following equation: q ¼ r=r0
ð1Þ
where r0 is the catalytic reaction rate under unpromoted conditions (2000 mV), and r is the catalytic reaction rate under promoted conditions (VWR < 2000 mV). Fig. 2 shows the variation of the maximum value of the rate enhancement ratio (obtained for each light-off curve, qmax) and the light-off temperature (temperature corresponding to 50% of conversion, T50) vs. the applied catalyst potential (VWR). It could be observed that as the catalyst potential
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
Fig. 2. Effect of catalyst overpotential (VWR) on both the maximum rate enhancement ratio (qmax) and the light-off temperature (T50). Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
decreased, qmax increased and T50 decreased, showing a purely electrophilic NEMCA behaviour. The electrochemical addition of potassium promoter species on the catalyst can drastically improve the catalytic activity since a negative overpotential of 2 V increases the CO oxidation rate by a factor 11. At the same time, the value of the light-off temperature (T50) drops by almost 40 C. An other important point is that the temperature corresponding to the beginning of the catalytic activity (5% of CO conversion) decreases from about 240 C without promoters (+2 V) to 210 C in the presence K+ cations (2 V). Furthermore the maximum electrical power used for activating the catalyst was very small, i.e., 1 lW/cm2 which would also preserve the operation life of the electrochemical cell. These promotional effects are among the highest that have ever been obtained for CO oxidation by Electrochemical Promotion. In addition, all these results have been obtained at a lower temperature range than previously reported [10–18] with annealed pastes, which clearly demonstrates that the electrochemical cell based on Pt impregnated catalyst supported on K–b00 Al2O3 solid electrolyte presents a significant potential for the abatement of automotive exhaust gas. Our results are in agreement with a recent study of chemical promotion of Pt/Al2O3 by alkalis for CO oxidation, which demonstrated that potassium is one of the best promoters for this reaction [26]. The promotional phenomenon was also investigated in a dynamic way by Cyclic Voltammetry carried out at 250 C under reaction conditions. Fig. 3 shows the variation of the current and the Turnover Frequency (TOF) vs. a linear variation (2 mV/s) on the applied catalyst potential between 2 and 2 V. It can be observed that starting from a potassium-free Pt surface, i.e., from positive overpotential of +0.5 V, and then decreasing catalyst potential, there was a continuous decrease of the current with the appearance of a cathodic peak centred on 0.5 V. At this point,
19
Fig. 3. Current and Turnover frequency (TOF) vs. catalyst potential (VWR) during cyclic voltammetry at 250 C. Scan rate: 2 mV/s. Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
the maximum rate of ions supply to the catalyst was achieved which led to the highest slope in the TOF increase. Nevertheless, the maximum TOF value was attained during the anodic polarization at 0.5 V, because of at this point the highest coverage of promoter was achieved (transition from negative to positive current). Above 0.5 V during the anodic polarization, the K+ ions returned to the solid electrolyte (positive currents) leading to a TOF decrease. The anodic peak centred on 0.75 V represents the maximum ions migration rate from the catalyst to the solid electrolyte. As already reported in previous studies with Pt catalyst supported on sodium conductors under C3H6 + O2 [27] and C3H6 + O2 + NO [19] atmospheres, the cathodic and anodic peaks could be associated to the maximum rate of formation and decomposition of Pt surface compounds, between the promoter ions and the different species presented in the feed. Considering that the reaction mixture in contact with the electrochemical catalyst Pt/K–bAl2O3 is composed of O2, CO and CO2, one can suggest that the promoter compound could be a potassium carbonate as already observed by XPS on Pt/K–bAl2O3 under CO + H2 atmosphere [23]. Nevertheless, we can not rule out the formation of potassium oxides since the presence of strongly bonded oxygen on K covered Pt (1 1 1) has been reported before to form either a K–O layer [28] or a planar K2O [29]. Curves obtained in different cycles clearly showed that the promotional phenomena were reproducible and reversible. However, additional experiments should be done in order to identify the real nature of these promoter species. 4. Conclusion This study clearly demonstrated that Electrochemical Promotion by potassium ions is a suitable technique to improve the efficiency of Pt catalysts in the low temperature CO oxidation process. The effect of catalyst polariza-
20
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
tion strongly increased the catalytic activity, decreasing the light-off temperature below 250 C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds that enhanced the adsorption of O2 at the expense of CO. References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12]
M. Skoglundh, E. Fridell, Top. Catal. 28 (2004) 79. R.W. Macabe, J.M. Kisenyi, Chem. Ind. 15 (1995) 605. K. Kollman, J. Abthoff, W. Zahn, Automot. Eng. 102 (10) (1994) 17. T. Ma, N. Collings, T. Hands, SAE Paper 920400. J.K. Hochmuth, P. Burk, C. Tolentino, M.J. Mignano, SAE Paper 930739. R.G. Silver, J.C. Summers, W.B. Williamson, Stud. Surf. Sci. Catal. 71 (1991) 167. C.G. Vayenas, S. Bebelis, S. Ladas, Nature (London) 343 (1990) 625. C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, in: Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion Metal-Support Interactions, Kluwer Academic/Plenum, New York, 2001. C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170. J. Poppe, S. Volkening, A. Schaak, E. Schutz, J. Janek, R. Imbihl, Phys. Chem. 1 (1999) 5241. C.G. Vayenas, S. Neophytides, in: Catalysis, vol. 12, Royal Society of Chemistry Cambridge, 1996, p. 199.
[13] C. Karavasilis, S. Bebelis, C.G. Vayenas, Mater. Sci. Forum 76 (1991) 175. [14] T.I. Politova, G.G. Gal‘vita, V.D. Belyaev, V.A. Sobyanin, Catal. Lett. 44 (1997) 75. [15] O.A. Marina, V.A. Sobyanin, Catal. Lett. 13 (1992) 67. [16] I.V. Yentekakis, G. Moggridge, C.G. Vayenas, R.M. Lambert, J. Catal. 146 (1994) 292. [17] C.G. Vayenas, M.M. Jaksic, S. Bebelis, S.G. Neophitides, in: J.O.M. Bokris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, vol. 29, Plenum, New York, 1996, p. 57. [18] I.V. Yentekakis, C.G. Vayenas, J. Catal. 149 (1994) 238. [19] F. Dorado, A. de Lucas-Consuegra, P. Vernoux, J.L. Valverde, Appl. Catal. B 73 (2007) 42. [20] P. Vernoux, F. Gaillard, L. Bultel, E. Siebert, M. Primet, J. Catal. 208 (2002) 412. [21] A. To¨rncrona, M. Skoglundh, P. Thorma¨hlen, E. Fridell, E. Jobson, Appl. Catal. B 14 (1997) 131. [22] A.J. Urquhart, F.J. Williams, R.M. Lambert, Catal. Lett. 103 (2005) 137. [23] A.J. Urquhart, J.M. Keel, F.J. Williams, R.M. Lambert, J. Phys. Chem. B. 107 (2003) 10591. [24] R.M. Lambert, A. Palermo, F.J. Williams, M.S. Tikhov, Solid State Ionics 136 (2000) 677. [25] G. Pitselis, P. Petrolekas, C.G. Vayenas, Ionics 3 (1997) 110. [26] Y. Minemura, M. Kuriyama, S. Ito, K. Tomishige, K. Kunimori, Catal. Commun. 7 (2006) 623. [27] P. Vernoux, F. Gaillard, C. Lopez, E. Siebert, Solid State Ionics 175 (2004) 609. [28] G. Pirug, H.P. Bonzel, G. Brode´n, Surf. Sci. 122 (1982) 1. [29] E.L. Garfunkel, G.A. Somorjai, Surf. Sci. 115 (1982) 441.
Catalysis Communications 9 (2008) 17–20 www.elsevier.com/locate/catcom
Electrochemical activation of Pt catalyst by potassium for low temperature CO deep oxidation Antonio de Lucas-Consuegra a, Fernando Dorado a, Jose´ Luis Valverde a, Reda Karoum b, Philippe Vernoux b,* a
b
Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Campus Universitario s/n, 13071 Ciudad Real, Spain Universite´ de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS, Universite´ Lyon 1, 69626 Villeurbanne, France Received 26 March 2007; received in revised form 26 April 2007; accepted 27 April 2007 Available online 10 May 2007
Abstract This study exposes, for the first time, the effect of Electrochemical Promotion (EPOC) by potassium cations on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization drastically increased the catalytic rate for more than 11 times by decreasing the light-off temperature by almost 40 C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds on Pt surface that enhanced the adsorption of O2 at the expense of CO. The achieved results demonstrated that Electrochemical Promotion of Catalysis could be applied to improve the performance of catalytic converters at lower temperatures. 2007 Elsevier B.V. All rights reserved. Keywords: CO oxidation; Electrochemical Promotion; NEMCA effect; Platinum catalyst; Potassium promoter
1. Introduction Despite the encouraging achievements in catalytic abatement of harmful emissions from automotives, the exhaust released from a car engine during the first minutes after a ‘‘cold-start’’ is a critical issue for the performance of catalytic converters, especially at high CO concentrations [1]. Also, new and fuel-efficient engines generate colder exhaust gases than current engines, resulting in slower heating of the catalyst. This places new demands on the low temperature activity for the catalytic converters used in future emission abatement systems [2]. Several technologies including, for example, burner or electrically heated catalysts [3], exhaust gas ignition [4] and hydrocarbons traps [5] have been developed to improve the efficiency of the catalyst, decreasing the light-off temperature (temperature at which the conversion *
Corresponding author. Tel.: +33 4 72 43 15 87; fax: +33 4 72 43 16 95. E-mail address: [email protected] (P. Vernoux).
1566-7367/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.04.038
of the pollutants exceeds 50%). Nevertheless, these systems are expensive, with difficult diagnosis and highly complex [6]. The use of electrochemistry to activate and control a reaction process is an expanding domain, because it enables to improve in a very pronounced manner the catalytic performance of a metal catalyst. This process, called Non-Faradaic Electrochemical Modification of Catalytic Activity (NEMCA effect) or Electrochemical Promotion of Catalysis (EPOC) was discovered and developed by Vayenas et al. [7,8]. It is based on the control, by an applied potential, of the catalyst work function, due to electrochemical pumping of ions (promoters) from a solid electrolyte [9]. Previous studies have explored the utility of electrochemical promotion to improve the catalytic activity of several metals for CO oxidation using YSZ [10–15], Na–bAl2O3 [16,17], and CaF2 [18] as solid electrolytes. Nevertheless, all these studies have been carried out between 300 and 700 C, which is a high temperature range for the practical treatment of exhaust gases.
18
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
The aim of this work was to demonstrate that EPOC could be used to decrease the operating temperature window of a metal catalyst. Thus, it is reported for the first time, the effect of Electrochemical Promotion by potassium on Pt catalyst for low temperature CO oxidation process. The effect of catalyst polarization was investigated between 190 and 340 C, which is a temperatures range compatible with the treatment of exhaust emissions. 2. Experimental 2.1. Electrochemical catalyst preparation The electrochemical catalyst consisted of a porous and continuous thin Pt film (geometric area of 1.12 cm2) deposited on a side of a 17 mm diameter and 1 mm thickness K–b00 Al2O3 disk (Ionotec). The Pt film (1.24 mg Pt/cm2) was deposited, as described in detail elsewhere [19], by successive steps of deposition and thermal decomposition (650 C for 1 h) of a H2PtCl6 precursor solution. Gold counter and reference electrodes were deposited on the opposite side by application and calcination of thin coatings of gold paste. The dispersion of the Pt film was determined by measuring its reactive oxygen uptake at 350 C via the isothermal titration technique [8]. The reactive oxygen uptake was No = 3.5 · 107 mol O, which led to a metal dispersion of 5%. 2.2. Catalytic activity measurements for CO oxidation The catalytic activity measurements were carried out within a specific quartz reactor, as described in a previous study [20]. Mixtures of CO (Air Liquide, 1% in He, purity: 99.95%), O2 (Air Liquide, purity: 99.95%) and He (Air Liquid, purity: 99.95%) were used as feed to the reactor. The gas composition was controlled, by mass flow controllers (Brooks). The feed composition was as follows: 5000 ppm CO, 5000 ppm O2, He balance, overall flow rate 6.5 L/h. Reactants and products were analyzed using an on-line micro gas-chromatograph (Varian CP-2003), while CO2 concentration in the effluent from the reactor was also continuously monitored by an infrared (IR) analyzer (Horiba VA-3000). Before catalytic activity measurements, the Pt film was pretreated under H2 at 450 C for 1 h in order to reduce platinum. Cyclic voltammetry measurements were performed with the potentiostat–galvanotat at 250 C under reaction conditions, and were recorded at a sweep rate of 2 mV/s. 3. Results and discussion CO oxidation activities of the electrochemical catalyst Pt/K–b00 Al2O3 was investigated through temperature-programmed reaction experiments (light-off measurements) under application of five different catalyst potentials (Fig. 1). CO oxidation was strongly temperature dependent, increasing conversion from 1% to 90% between 190 and
Fig. 1. Light-off curves of the electrochemical catalyst Pt/K–bAl2O3 under different catalyst potentials. Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
340 C. Full conversion of CO was never reached because a part of the flow bypassed the Pt film. It is well known that at low temperature the oxidation of CO is inhibited by CO adsorption [21]. As the reaction temperature increases, the adsorbed molecules of CO start to desorb and oxygen is activated by dissociative adsorption whereby CO is oxidised. The operating temperature window at which this activation takes place depends on several factors, e.g., the type of metal catalyst, the CO/O2 ratio, or the total reactant flow. Nevertheless, it can be observed in Fig. 1 that for a given system, the electrochemistry can be used to decrease the reaction temperature at which this activation initiates. Thus, as the catalyst potential decreased to lower values the activation begun at lower temperatures. A decrease in the catalyst potential involves a decrease in the catalyst work function, as already discussed in previous studies [22–25]. This is the result of the migration of positively charged potassium species from the electrolyte through the catalyst surface, which modifies the chemisorption properties of the metal. Then, a decrease in the catalyst potential of the Pt catalyst electrode enhanced CO oxidation rate by increasing the coverage of electron-acceptor species (O2), at the expense of electron-donor (CO). Therefore, negative overpotentials led to the observed activation of the catalyst. Assuming that at VWR = 2 V there was no promoter on the catalyst (hK = 0), the promotional phenomenon could be characterized by the rate enhancement ratio (q) which was defined by the following equation: q ¼ r=r0
ð1Þ
where r0 is the catalytic reaction rate under unpromoted conditions (2000 mV), and r is the catalytic reaction rate under promoted conditions (VWR < 2000 mV). Fig. 2 shows the variation of the maximum value of the rate enhancement ratio (obtained for each light-off curve, qmax) and the light-off temperature (temperature corresponding to 50% of conversion, T50) vs. the applied catalyst potential (VWR). It could be observed that as the catalyst potential
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
Fig. 2. Effect of catalyst overpotential (VWR) on both the maximum rate enhancement ratio (qmax) and the light-off temperature (T50). Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
decreased, qmax increased and T50 decreased, showing a purely electrophilic NEMCA behaviour. The electrochemical addition of potassium promoter species on the catalyst can drastically improve the catalytic activity since a negative overpotential of 2 V increases the CO oxidation rate by a factor 11. At the same time, the value of the light-off temperature (T50) drops by almost 40 C. An other important point is that the temperature corresponding to the beginning of the catalytic activity (5% of CO conversion) decreases from about 240 C without promoters (+2 V) to 210 C in the presence K+ cations (2 V). Furthermore the maximum electrical power used for activating the catalyst was very small, i.e., 1 lW/cm2 which would also preserve the operation life of the electrochemical cell. These promotional effects are among the highest that have ever been obtained for CO oxidation by Electrochemical Promotion. In addition, all these results have been obtained at a lower temperature range than previously reported [10–18] with annealed pastes, which clearly demonstrates that the electrochemical cell based on Pt impregnated catalyst supported on K–b00 Al2O3 solid electrolyte presents a significant potential for the abatement of automotive exhaust gas. Our results are in agreement with a recent study of chemical promotion of Pt/Al2O3 by alkalis for CO oxidation, which demonstrated that potassium is one of the best promoters for this reaction [26]. The promotional phenomenon was also investigated in a dynamic way by Cyclic Voltammetry carried out at 250 C under reaction conditions. Fig. 3 shows the variation of the current and the Turnover Frequency (TOF) vs. a linear variation (2 mV/s) on the applied catalyst potential between 2 and 2 V. It can be observed that starting from a potassium-free Pt surface, i.e., from positive overpotential of +0.5 V, and then decreasing catalyst potential, there was a continuous decrease of the current with the appearance of a cathodic peak centred on 0.5 V. At this point,
19
Fig. 3. Current and Turnover frequency (TOF) vs. catalyst potential (VWR) during cyclic voltammetry at 250 C. Scan rate: 2 mV/s. Reaction conditions: CO = 5000 ppm, O2 = 5000 ppm, He balance, total flow rate = 6.5 L/h.
the maximum rate of ions supply to the catalyst was achieved which led to the highest slope in the TOF increase. Nevertheless, the maximum TOF value was attained during the anodic polarization at 0.5 V, because of at this point the highest coverage of promoter was achieved (transition from negative to positive current). Above 0.5 V during the anodic polarization, the K+ ions returned to the solid electrolyte (positive currents) leading to a TOF decrease. The anodic peak centred on 0.75 V represents the maximum ions migration rate from the catalyst to the solid electrolyte. As already reported in previous studies with Pt catalyst supported on sodium conductors under C3H6 + O2 [27] and C3H6 + O2 + NO [19] atmospheres, the cathodic and anodic peaks could be associated to the maximum rate of formation and decomposition of Pt surface compounds, between the promoter ions and the different species presented in the feed. Considering that the reaction mixture in contact with the electrochemical catalyst Pt/K–bAl2O3 is composed of O2, CO and CO2, one can suggest that the promoter compound could be a potassium carbonate as already observed by XPS on Pt/K–bAl2O3 under CO + H2 atmosphere [23]. Nevertheless, we can not rule out the formation of potassium oxides since the presence of strongly bonded oxygen on K covered Pt (1 1 1) has been reported before to form either a K–O layer [28] or a planar K2O [29]. Curves obtained in different cycles clearly showed that the promotional phenomena were reproducible and reversible. However, additional experiments should be done in order to identify the real nature of these promoter species. 4. Conclusion This study clearly demonstrated that Electrochemical Promotion by potassium ions is a suitable technique to improve the efficiency of Pt catalysts in the low temperature CO oxidation process. The effect of catalyst polariza-
20
A. de Lucas-Consuegra et al. / Catalysis Communications 9 (2008) 17–20
tion strongly increased the catalytic activity, decreasing the light-off temperature below 250 C. The promotional phenomenon was found to be reversible due to the formation and decomposition of potassium compounds that enhanced the adsorption of O2 at the expense of CO. References [1] [2] [3] [4] [5] [6] [7] [8]
[9] [10] [11] [12]
M. Skoglundh, E. Fridell, Top. Catal. 28 (2004) 79. R.W. Macabe, J.M. Kisenyi, Chem. Ind. 15 (1995) 605. K. Kollman, J. Abthoff, W. Zahn, Automot. Eng. 102 (10) (1994) 17. T. Ma, N. Collings, T. Hands, SAE Paper 920400. J.K. Hochmuth, P. Burk, C. Tolentino, M.J. Mignano, SAE Paper 930739. R.G. Silver, J.C. Summers, W.B. Williamson, Stud. Surf. Sci. Catal. 71 (1991) 167. C.G. Vayenas, S. Bebelis, S. Ladas, Nature (London) 343 (1990) 625. C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides, in: Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion Metal-Support Interactions, Kluwer Academic/Plenum, New York, 2001. C.G. Vayenas, S. Bebelis, I.V. Yentekakis, H.-G. Lintz, Catal. Today 11 (1992) 303. I.V. Yentekakis, C.G. Vayenas, J. Catal. 111 (1988) 170. J. Poppe, S. Volkening, A. Schaak, E. Schutz, J. Janek, R. Imbihl, Phys. Chem. 1 (1999) 5241. C.G. Vayenas, S. Neophytides, in: Catalysis, vol. 12, Royal Society of Chemistry Cambridge, 1996, p. 199.
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