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Comparison of methanol oxidations on Pt, Pt|Ru and Pt|Sn electrodes
Journal of Electroanalytical Chemistry 444 (1998) 95 – 100
Comparison of methanol oxidations on Pt, Pt Ru and Pt Sn electrodes Yu Morimoto a,*, Ernest B. Yeager b b
a Electrochemistry Lab., Toyota Central R&D Labs., Inc., Nagakute, Aichi 480 -11, Japan The Ernest. B. Yeager Center for Electrochemical Sciences and Chemistry Department, Case Western Reser6e Uni6ersity, 10900 Euclid A6e., Cle6eland, OH 44106, USA
Received 4 August 1997; received in revised form 21 October 1997
Abstract Electrochemical oxidations of methanol were compared on smooth or high area platinum, platinum tin and platinum ruthenium electrodes. On smooth platinum, while ruthenium promoted the catalytic activities steadily according to the coverage, tin showed an enhancing effect only for a short period of time because of the dissolution of tin from the surface. On high area platinum, tin seemed much stabler and exhibited long-lasting enhancing effects as well as ruthenium. The reasons for the inconsistent effects of tin among past studies are discussed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Methanol oxidation; Platinum; Tin; Ruthenium
1. Introduction Studies on the electrochemical oxidation of methanol are of great importance and excellent review articles are available [1–3]. It is widely agreed that, as a single component catalyst, platinum is the only element to show a significant electrocatalytic activity on methanol oxidation and, therefore, has been studied most extensively. The catalytic activity of platinum, however, is still too low to consider direct methanol fuel cells as a practical power source. Therefore, varieties of studies have been conducted to promote the electrocatalytic activity of platinum. Adding secondary elements has been one of the most common and successful methods [1]. Ruthenium and tin have been the elements most widely used to add to platinum. The effect of ruthenium seems very consistent throughout the various preparation methods, such as alloying [4 – 7], electrodeposition [8–10], and carbon supported [11,12] or unsupported [13] dispersed particles. On the contrary, using various preparation techniques, the results using * Corresponding author. Fax: + 81 561 63 6136; e-mail: [email protected] 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 5 6 3 - 9
tin as the secondary element vary in the effects on methanol oxidation from significantly enhancing (codeposition and bulk alloy [14], upd [15], electrodeposition [16], co-deposition [17]) to moderate (immersion and co-precipitation [18,19], immersion [20]), no (bulk alloy [21]) or even negative (upd and co-deposition [22]). Recently Ross’s group [21,23–25] observed a significant enhancing effect of tin for COad oxidation but no effect for methanol oxidation using well-defined alloy surfaces. They also presented a model in which COad resulting from the dehydrogenation of methanol is in a different state from that directly adsorbed from gaseous CO. In the previous paper [26], the authors studied COad oxidation on Pt Ru and Pt Sn electrodes. Both ruthenium and tin promoted COad oxidation but the mechanisms of the promotion seemed different. While ruthenium is effective at higher potentials and oxidizes the COad that would not be oxidized easily on pure platinum, tin mainly further promotes the oxidation of the COad that could be oxidized at more negative potentials on pure platinum. The latter kind of COad was found more on high area platinum than on smooth electrodes, and the authors suggested that, therefore,
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the morphology of platinum surfaces plays a important role on the effect of tin. In the present paper, the effects of ruthenium and tin on methanol oxidation were studied by electrodepositing these metals on smooth or platinized platinum electrodes. The roles of the two elements in methanol oxidation are compared, and the reasons for inconsistent effects of tin among past studies are discussed.
2. Experimental Preparation for the electrodes is described in our previous paper [26]. In some cases, after deposition, ruthenium or tin was partially removed electrochemically to obtain various stages of the coverage. Then the electrodes were washed with hydrogen-saturated water and quickly transferred to an electrochemical measurement cell. Before dipping the electrodes into the electrolyte, all cables between the electrodes and the potentiostat were connected and the potential was preset as 50 mV vs. RHE. This procedure enables us to avoid both further oxidative removal of ruthenium or tin by exposing the electrodes to the oxidative environment. Electrochemical and in situ infrared spectroscopic measurements were conducted in the same way as described previously [26].
3. Results and discussion
3.1. Smooth Pt Ru, Pt Sn electrodes 3.1.1. Voltammograms in 3 M sulfuric acid After depositing ruthenium on smooth platinum, ruthenium was partially stripped by electrochemical oxidation at the potential holding at 1100 mV. The cyclic voltammograms in 3 M sulfuric acid are shown
Fig. 1. Cyclic voltammograms of a smooth Pt Ru electrode in 3 M H2SO4 after Ru stripping by positive potential holding.
Fig. 2. Cyclic voltammograms of a smooth Pt Sn electrode in 3 M H2SO4 at 10 mV s − 1. (a) Whole potential range for 1st and 2nd sweeps, (b) hydrogen region for 1st to 12th sweeps.
in Fig. 1. The initial voltammogram was identical with that of pure ruthenium. The voltammogram then became gradually more like that of pure platinum according to the progress of ruthenium removal. The potential of the Pt Sn electrode was cycled between 50 and 1100 mV in 3 M sulfuric acid. The cyclic voltammograms are shown in Fig. 2. Tin was oxidatively removed by the positive sweep, which is confirmed by the recovery of the hydrogen adsorption–desorption features of the voltammogram in the following cycle. After 12 cycles, the hydrogen desorption voltammogram became almost identical with that of pure platinum, which indicates that tin was almost completely removed from the surface as shown in Fig. 2(b).
3.1.2. Voltammograms in 3 M sulfuric acid with methanol Pure platinum was electrochemically cleaned in a blank electrolyte and then transferred to the methanolcontaining electrolyte by the procedure described in the previous section. The voltammogram during the first positive sweep after 1 min of potential holding at 50 mV is shown in Fig. 3. Up to 120 mV, the voltam-
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Fig. 3. Voltammograms of Pt in 3 M sulfuric acid at 10 mV s − 1 and 25°C; (——) with 0.1 M methanol, (- - -) without methanol.
Fig. 5. Voltammograms of a smooth Pt Sn electrode in 3 M H2SO4 with 1 M CH3OH at 10 mV s − 1.
mogram was identical with that in the blank electrolyte, and then oxidation of methanol was observed. It can be concluded safely, therefore, that this procedure could avoid the accumulation of partially oxidized methanol adsorbate on the electrode surface before the potential sweep. This was confirmed in in situ infrared spectroscopic studies [27]. Voltammograms of Pt Ru electrodes in 3 M sulfuric acid with 1 M methanol are shown in Fig. 4. As the degree of ruthenium coverage decreases, a methanol oxidation current peak emerges at 750 mV and grows up to 30 s of stripping, followed by a decrease and then the emergence and growth of another peak at 930 mV, where the methanol oxidation peak on pure platinum would be also seen. This suggests that the peaks at 750 and 930 mV represent the methanol oxidations by the interaction of platinum and ruthenium, and the singular function of platinum, respectively. The voltammograms for methanol oxidation on Pt Sn are affected by the progress of tin removal as shown in Fig. 5. Up to eight cycles, the methanol oxidation current increased in the whole range of the
potential. After eight cycles, the current decreased in the lower potential range (500–850 mV) but increased in the higher range (850–1100 mV), which makes the peak sharper and more like that of pure platinum. This effect of tin towards methanol oxidation is consistent with the observation that tin promotes the oxidation of COad only in the lower potential range.
Fig. 4. Voltammograms for a smooth Pt Ru electrode in 3 M H2SO4 with 1 M CH3OH at 100 mV s − 1.
3.1.3. Chronoamperometry in 3 M sulfuric acid with methanol Fig. 6 gives methanol oxidation currents during the potential holding at 500 mV in 3 M sulfuric acid with 1 M methanol for pure Pt, Pt Ru (after stripping for 30 s) and Pt Sn (after seven cycles). The electrodes were conditioned in another cell and were transferred as described earlier. On Pt Ru, although the methanol oxidation current immediately after the start was lower than on Pt, higher and more stable methanol oxidation is subsequently observed. Obviously, ruthenium showed a stable enhancing effect on methanol oxidation. On the other hand, Pt Sn showed similar behavior with
Fig. 6. Chronoamperometry of smooth Pt, Pt Ru (Ru stripped at 1.1 V for 45 s) and Pt Sn (Sn stripped by seven potential cycles) in 3 M H2SO4 with 1 M CH3 OH at 500 mV.
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Fig. 8. In situ infrared spectra of a smooth Pt Ru electrode in 3 M H2SO4 with 0.1 M CH3OH. Reference state, 100 mV. Fig. 7. In situ infrared spectra of polycrystalline platinum electrode in 3 M H2SO4 with 0.1 M CH3OH. Reference state, 100 mV.
Pt Ru for a short period in the early stages, but the current decreased to the same level with that of pure platinum. The enhancing effect of tin did not seem stable. Tin seemed to be removed during the potential holding, because the voltammogram after 2 min of potential holding at 500 mV was found to be almost identical with that of pure platinum. It is likely that this instability of tin on platinum is one of the reasons that the effects of tin toward methanol oxidation have been inconsistent in the literature.
significantly by the oxidative potential cycles, and ruthenium seemed extremely stable on the high area platinum surface. In the case of tin, the voltammograms are shown in Fig. 11. Tin was removed gradually by the potential cycles, but the removal rate was found to be much slower than from the smooth surface when Fig. 2(b) and Fig. 10(b) are compared. Therefore, tin also seemed much stabler on the high area platinum surface than on the smooth one. This result agrees with that obtained by Campbell and Parsons [20].
3.1.4. In situ infrared spectroscopic studies In situ infrared absorption spectra for Pt, Pt Ru (after stripping for 30 s) and Pt Sn (after seven cycles) are shown in Figs. 7 – 9, respectively. While on pure platinum linearly bonded COad formation is clearly indicated by the peaks around 2070 cm − 1, peaks around this wave number on Pt Ru and Pt Sn are not as well distinguished. Iwasita et al. [5] suggested lower COad coverage on Pt Ru. The present results suggest that COad coverage is lower on Pt Sn as well. CO2, which is the final product of methanol oxidation, emerges at 400 mV for Pt and Pt Ru and at 340 mV for Pt Sn. These results indicate the enhancing effect of tin for methanol oxidation at lower potentials. This effect can be attributed to the enhanced catalytic activity of tin toward COad [26]. 3.2. High area Pt Ru, Pt Sn electrodes 3.2.1. Voltammograms in 3 M sulfuric acid The voltammograms of high area Pt Ru are shown in Fig. 10. The voltammogram of Pt Ru was not affected
Fig. 9. In situ infrared spectra of a smooth Pt Sn electrode in 3 M H2SO4 with 0.1 M CH3OH at various potentials. Reference potential, 100 mV.
Y. Morimoto, E.B. Yeager / Journal of Electroanalytical Chemistry 444 (1998) 95–100
Fig. 10. Cyclic voltammograms of a high area Pt Ru electrode in 3 M H2SO4 at 10 mV s − 1.
3.2.2. Chronoamperometry in 3 M sulfuric acid with methanol The electrode potential of high area Pt, Pt Ru and Pt Sn was held at 500 mV in 3 M sulfuric acid with 1 M
99
Fig. 12. Chronoamperometry of high area Pt, Pt Ru and Pt Sn electrodes in 3 M H2SO4 with 1 M CH3OH at 500 mV.
methanol. Unlike the smooth electrodes as shown above, Pt Sn, as well as Pt Ru, showed a much steadier methanol oxidation current than pure platinum, as shown in Fig. 12. It is likely that this steady enhancing effect of tin is brought about by the fact that tin is stabler on the high area platinum than on smooth platinum.
4. Conclusion Ruthenium and tin were electrodeposited on smooth and high area platinum electrodes to examine the effects of these metals toward methanol oxidation. On smooth platinum, while ruthenium promoted the catalytic activities steadily according to the coverage, tin showed an enhancing effect only for a short period of time, and it diminished quickly probably because of dissolution of tin from the surface. On high area platinum, tin seemed much stabler and exhibited long-lasting enhancing effects as well as ruthenium. Not only ruthenium but also tin has enhancing effects for both COad and methanol oxidations. The effects of tin, however, are susceptible to the potential range and the morphology of the platinum surface. On high area platinum, which has more sites for the COad than tin can promote to oxidize [26], tin is also more stable. This susceptibility of tin could be one of the reasons for the inconsistency of the effects of tin towards methanol oxidation in previous studies.
References
Fig. 11. Cyclic voltammograms of a high area Pt Sn electrodes in 3 M H2SO4 at 10 mV s − 1. (a) Whole potential range for 1st cycle, (b) hydrogen region for positive sweeps.
[1] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [2] J.M. Leger, C. Lamy, Ber. Bunsenges. Phys. Chem. 94 (1990) 1021. [3] T. Iwasita-Vielstich, in: Advances in Electrochemical Science and Engineering, vol. 1, VCH, New York, 1990, pp. 27 – 171.
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[4] M. Watanabe, T. Suzuki, S. Motoo, Denki Kagaku 38 (1970) 927. [5] T. Iwasita, F.C. Nart, W. Vielstich, Ber. Bunsenges. Phys. Chem. 94 (1990) 1030. [6] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., E.J. Cairns, J. Phys. Chem. 97 (1993) 12020. [7] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., E.J. Cairns, J. Electrochem. Soc. 141 (1994) 1795. [8] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 267. [9] J. Sobkowski, K. Franaszczuk, J. Electroanal. Chem. 327 (1992) 231. [10] M. Shibata, S. Motoo, J. Electroanal. Chem. 209 (1986) 151. [11] M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem. 199 (1986) 311. [12] S. Wasmus, W. Vielstich, J. Appl. Electrochem. 23 (1993) 120. [13] J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan, S.A. Weeks, J. Electroanal. Chem. 240 (1988) 133. [14] M.M.P. Janssen, J. Moolhuysen, Electrochim. Acta 21 (1976) 861. [15] M. Watanabe, Y. Furuuchi, S. Motoo, J. Electroanal. Chem. 191 (1985) 367.
.
.
[16] B.D. McNicol, R.T. Short, A.G. Chapman, J. Chem. Soc. Faraday I 72 (1976) 2735. [17] K.J. Cathro, J. Electrochem. Soc. 116 (1969) 1608. [18] T. Frelink, W. Visscher, J.A.R. van Veen, Electrochim. Acta 39 (1994) 1871. [19] B. Bittins-Cattaneo, T. Iwasita, J. Electroanal. Chem. 238 (1987) 151. [20] S.A. Campbell, R. Parsons, J. Chem. Soc. Faraday Trans. 88 (1992) 833. [21] A.N. Haner, P.N. Ross, J. Phys. Chem. 95 (1991) 3740. [22] B. Beden, F. Kadirgan, C. Lamy, J.-M. Leger, J. Electroanal. Chem. 125 (1981) 71. [23] K. Wang, H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., Electrochim. Acta 41 (1995) 2587. [24] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., J. Phys. Chem. 99 (1995) 8945. [25] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., Catalysis Lett. 36 (1996) 1. [26] Y. Morimoto, E.B. Yeager, J. Electroanal. Chem. 441 (1998) 77. [27] Y. Morimoto (in preparation).
Comparison of methanol oxidations on Pt, Pt Ru and Pt Sn electrodes Yu Morimoto a,*, Ernest B. Yeager b b
a Electrochemistry Lab., Toyota Central R&D Labs., Inc., Nagakute, Aichi 480 -11, Japan The Ernest. B. Yeager Center for Electrochemical Sciences and Chemistry Department, Case Western Reser6e Uni6ersity, 10900 Euclid A6e., Cle6eland, OH 44106, USA
Received 4 August 1997; received in revised form 21 October 1997
Abstract Electrochemical oxidations of methanol were compared on smooth or high area platinum, platinum tin and platinum ruthenium electrodes. On smooth platinum, while ruthenium promoted the catalytic activities steadily according to the coverage, tin showed an enhancing effect only for a short period of time because of the dissolution of tin from the surface. On high area platinum, tin seemed much stabler and exhibited long-lasting enhancing effects as well as ruthenium. The reasons for the inconsistent effects of tin among past studies are discussed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Methanol oxidation; Platinum; Tin; Ruthenium
1. Introduction Studies on the electrochemical oxidation of methanol are of great importance and excellent review articles are available [1–3]. It is widely agreed that, as a single component catalyst, platinum is the only element to show a significant electrocatalytic activity on methanol oxidation and, therefore, has been studied most extensively. The catalytic activity of platinum, however, is still too low to consider direct methanol fuel cells as a practical power source. Therefore, varieties of studies have been conducted to promote the electrocatalytic activity of platinum. Adding secondary elements has been one of the most common and successful methods [1]. Ruthenium and tin have been the elements most widely used to add to platinum. The effect of ruthenium seems very consistent throughout the various preparation methods, such as alloying [4 – 7], electrodeposition [8–10], and carbon supported [11,12] or unsupported [13] dispersed particles. On the contrary, using various preparation techniques, the results using * Corresponding author. Fax: + 81 561 63 6136; e-mail: [email protected] 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 5 6 3 - 9
tin as the secondary element vary in the effects on methanol oxidation from significantly enhancing (codeposition and bulk alloy [14], upd [15], electrodeposition [16], co-deposition [17]) to moderate (immersion and co-precipitation [18,19], immersion [20]), no (bulk alloy [21]) or even negative (upd and co-deposition [22]). Recently Ross’s group [21,23–25] observed a significant enhancing effect of tin for COad oxidation but no effect for methanol oxidation using well-defined alloy surfaces. They also presented a model in which COad resulting from the dehydrogenation of methanol is in a different state from that directly adsorbed from gaseous CO. In the previous paper [26], the authors studied COad oxidation on Pt Ru and Pt Sn electrodes. Both ruthenium and tin promoted COad oxidation but the mechanisms of the promotion seemed different. While ruthenium is effective at higher potentials and oxidizes the COad that would not be oxidized easily on pure platinum, tin mainly further promotes the oxidation of the COad that could be oxidized at more negative potentials on pure platinum. The latter kind of COad was found more on high area platinum than on smooth electrodes, and the authors suggested that, therefore,
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the morphology of platinum surfaces plays a important role on the effect of tin. In the present paper, the effects of ruthenium and tin on methanol oxidation were studied by electrodepositing these metals on smooth or platinized platinum electrodes. The roles of the two elements in methanol oxidation are compared, and the reasons for inconsistent effects of tin among past studies are discussed.
2. Experimental Preparation for the electrodes is described in our previous paper [26]. In some cases, after deposition, ruthenium or tin was partially removed electrochemically to obtain various stages of the coverage. Then the electrodes were washed with hydrogen-saturated water and quickly transferred to an electrochemical measurement cell. Before dipping the electrodes into the electrolyte, all cables between the electrodes and the potentiostat were connected and the potential was preset as 50 mV vs. RHE. This procedure enables us to avoid both further oxidative removal of ruthenium or tin by exposing the electrodes to the oxidative environment. Electrochemical and in situ infrared spectroscopic measurements were conducted in the same way as described previously [26].
3. Results and discussion
3.1. Smooth Pt Ru, Pt Sn electrodes 3.1.1. Voltammograms in 3 M sulfuric acid After depositing ruthenium on smooth platinum, ruthenium was partially stripped by electrochemical oxidation at the potential holding at 1100 mV. The cyclic voltammograms in 3 M sulfuric acid are shown
Fig. 1. Cyclic voltammograms of a smooth Pt Ru electrode in 3 M H2SO4 after Ru stripping by positive potential holding.
Fig. 2. Cyclic voltammograms of a smooth Pt Sn electrode in 3 M H2SO4 at 10 mV s − 1. (a) Whole potential range for 1st and 2nd sweeps, (b) hydrogen region for 1st to 12th sweeps.
in Fig. 1. The initial voltammogram was identical with that of pure ruthenium. The voltammogram then became gradually more like that of pure platinum according to the progress of ruthenium removal. The potential of the Pt Sn electrode was cycled between 50 and 1100 mV in 3 M sulfuric acid. The cyclic voltammograms are shown in Fig. 2. Tin was oxidatively removed by the positive sweep, which is confirmed by the recovery of the hydrogen adsorption–desorption features of the voltammogram in the following cycle. After 12 cycles, the hydrogen desorption voltammogram became almost identical with that of pure platinum, which indicates that tin was almost completely removed from the surface as shown in Fig. 2(b).
3.1.2. Voltammograms in 3 M sulfuric acid with methanol Pure platinum was electrochemically cleaned in a blank electrolyte and then transferred to the methanolcontaining electrolyte by the procedure described in the previous section. The voltammogram during the first positive sweep after 1 min of potential holding at 50 mV is shown in Fig. 3. Up to 120 mV, the voltam-
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Fig. 3. Voltammograms of Pt in 3 M sulfuric acid at 10 mV s − 1 and 25°C; (——) with 0.1 M methanol, (- - -) without methanol.
Fig. 5. Voltammograms of a smooth Pt Sn electrode in 3 M H2SO4 with 1 M CH3OH at 10 mV s − 1.
mogram was identical with that in the blank electrolyte, and then oxidation of methanol was observed. It can be concluded safely, therefore, that this procedure could avoid the accumulation of partially oxidized methanol adsorbate on the electrode surface before the potential sweep. This was confirmed in in situ infrared spectroscopic studies [27]. Voltammograms of Pt Ru electrodes in 3 M sulfuric acid with 1 M methanol are shown in Fig. 4. As the degree of ruthenium coverage decreases, a methanol oxidation current peak emerges at 750 mV and grows up to 30 s of stripping, followed by a decrease and then the emergence and growth of another peak at 930 mV, where the methanol oxidation peak on pure platinum would be also seen. This suggests that the peaks at 750 and 930 mV represent the methanol oxidations by the interaction of platinum and ruthenium, and the singular function of platinum, respectively. The voltammograms for methanol oxidation on Pt Sn are affected by the progress of tin removal as shown in Fig. 5. Up to eight cycles, the methanol oxidation current increased in the whole range of the
potential. After eight cycles, the current decreased in the lower potential range (500–850 mV) but increased in the higher range (850–1100 mV), which makes the peak sharper and more like that of pure platinum. This effect of tin towards methanol oxidation is consistent with the observation that tin promotes the oxidation of COad only in the lower potential range.
Fig. 4. Voltammograms for a smooth Pt Ru electrode in 3 M H2SO4 with 1 M CH3OH at 100 mV s − 1.
3.1.3. Chronoamperometry in 3 M sulfuric acid with methanol Fig. 6 gives methanol oxidation currents during the potential holding at 500 mV in 3 M sulfuric acid with 1 M methanol for pure Pt, Pt Ru (after stripping for 30 s) and Pt Sn (after seven cycles). The electrodes were conditioned in another cell and were transferred as described earlier. On Pt Ru, although the methanol oxidation current immediately after the start was lower than on Pt, higher and more stable methanol oxidation is subsequently observed. Obviously, ruthenium showed a stable enhancing effect on methanol oxidation. On the other hand, Pt Sn showed similar behavior with
Fig. 6. Chronoamperometry of smooth Pt, Pt Ru (Ru stripped at 1.1 V for 45 s) and Pt Sn (Sn stripped by seven potential cycles) in 3 M H2SO4 with 1 M CH3 OH at 500 mV.
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Fig. 8. In situ infrared spectra of a smooth Pt Ru electrode in 3 M H2SO4 with 0.1 M CH3OH. Reference state, 100 mV. Fig. 7. In situ infrared spectra of polycrystalline platinum electrode in 3 M H2SO4 with 0.1 M CH3OH. Reference state, 100 mV.
Pt Ru for a short period in the early stages, but the current decreased to the same level with that of pure platinum. The enhancing effect of tin did not seem stable. Tin seemed to be removed during the potential holding, because the voltammogram after 2 min of potential holding at 500 mV was found to be almost identical with that of pure platinum. It is likely that this instability of tin on platinum is one of the reasons that the effects of tin toward methanol oxidation have been inconsistent in the literature.
significantly by the oxidative potential cycles, and ruthenium seemed extremely stable on the high area platinum surface. In the case of tin, the voltammograms are shown in Fig. 11. Tin was removed gradually by the potential cycles, but the removal rate was found to be much slower than from the smooth surface when Fig. 2(b) and Fig. 10(b) are compared. Therefore, tin also seemed much stabler on the high area platinum surface than on the smooth one. This result agrees with that obtained by Campbell and Parsons [20].
3.1.4. In situ infrared spectroscopic studies In situ infrared absorption spectra for Pt, Pt Ru (after stripping for 30 s) and Pt Sn (after seven cycles) are shown in Figs. 7 – 9, respectively. While on pure platinum linearly bonded COad formation is clearly indicated by the peaks around 2070 cm − 1, peaks around this wave number on Pt Ru and Pt Sn are not as well distinguished. Iwasita et al. [5] suggested lower COad coverage on Pt Ru. The present results suggest that COad coverage is lower on Pt Sn as well. CO2, which is the final product of methanol oxidation, emerges at 400 mV for Pt and Pt Ru and at 340 mV for Pt Sn. These results indicate the enhancing effect of tin for methanol oxidation at lower potentials. This effect can be attributed to the enhanced catalytic activity of tin toward COad [26]. 3.2. High area Pt Ru, Pt Sn electrodes 3.2.1. Voltammograms in 3 M sulfuric acid The voltammograms of high area Pt Ru are shown in Fig. 10. The voltammogram of Pt Ru was not affected
Fig. 9. In situ infrared spectra of a smooth Pt Sn electrode in 3 M H2SO4 with 0.1 M CH3OH at various potentials. Reference potential, 100 mV.
Y. Morimoto, E.B. Yeager / Journal of Electroanalytical Chemistry 444 (1998) 95–100
Fig. 10. Cyclic voltammograms of a high area Pt Ru electrode in 3 M H2SO4 at 10 mV s − 1.
3.2.2. Chronoamperometry in 3 M sulfuric acid with methanol The electrode potential of high area Pt, Pt Ru and Pt Sn was held at 500 mV in 3 M sulfuric acid with 1 M
99
Fig. 12. Chronoamperometry of high area Pt, Pt Ru and Pt Sn electrodes in 3 M H2SO4 with 1 M CH3OH at 500 mV.
methanol. Unlike the smooth electrodes as shown above, Pt Sn, as well as Pt Ru, showed a much steadier methanol oxidation current than pure platinum, as shown in Fig. 12. It is likely that this steady enhancing effect of tin is brought about by the fact that tin is stabler on the high area platinum than on smooth platinum.
4. Conclusion Ruthenium and tin were electrodeposited on smooth and high area platinum electrodes to examine the effects of these metals toward methanol oxidation. On smooth platinum, while ruthenium promoted the catalytic activities steadily according to the coverage, tin showed an enhancing effect only for a short period of time, and it diminished quickly probably because of dissolution of tin from the surface. On high area platinum, tin seemed much stabler and exhibited long-lasting enhancing effects as well as ruthenium. Not only ruthenium but also tin has enhancing effects for both COad and methanol oxidations. The effects of tin, however, are susceptible to the potential range and the morphology of the platinum surface. On high area platinum, which has more sites for the COad than tin can promote to oxidize [26], tin is also more stable. This susceptibility of tin could be one of the reasons for the inconsistency of the effects of tin towards methanol oxidation in previous studies.
References
Fig. 11. Cyclic voltammograms of a high area Pt Sn electrodes in 3 M H2SO4 at 10 mV s − 1. (a) Whole potential range for 1st cycle, (b) hydrogen region for positive sweeps.
[1] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257 (1988) 9. [2] J.M. Leger, C. Lamy, Ber. Bunsenges. Phys. Chem. 94 (1990) 1021. [3] T. Iwasita-Vielstich, in: Advances in Electrochemical Science and Engineering, vol. 1, VCH, New York, 1990, pp. 27 – 171.
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[4] M. Watanabe, T. Suzuki, S. Motoo, Denki Kagaku 38 (1970) 927. [5] T. Iwasita, F.C. Nart, W. Vielstich, Ber. Bunsenges. Phys. Chem. 94 (1990) 1030. [6] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., E.J. Cairns, J. Phys. Chem. 97 (1993) 12020. [7] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., E.J. Cairns, J. Electrochem. Soc. 141 (1994) 1795. [8] M. Watanabe, S. Motoo, J. Electroanal. Chem. 60 (1975) 267. [9] J. Sobkowski, K. Franaszczuk, J. Electroanal. Chem. 327 (1992) 231. [10] M. Shibata, S. Motoo, J. Electroanal. Chem. 209 (1986) 151. [11] M. Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem. 199 (1986) 311. [12] S. Wasmus, W. Vielstich, J. Appl. Electrochem. 23 (1993) 120. [13] J.B. Goodenough, A. Hamnett, B.J. Kennedy, R. Manoharan, S.A. Weeks, J. Electroanal. Chem. 240 (1988) 133. [14] M.M.P. Janssen, J. Moolhuysen, Electrochim. Acta 21 (1976) 861. [15] M. Watanabe, Y. Furuuchi, S. Motoo, J. Electroanal. Chem. 191 (1985) 367.
.
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[16] B.D. McNicol, R.T. Short, A.G. Chapman, J. Chem. Soc. Faraday I 72 (1976) 2735. [17] K.J. Cathro, J. Electrochem. Soc. 116 (1969) 1608. [18] T. Frelink, W. Visscher, J.A.R. van Veen, Electrochim. Acta 39 (1994) 1871. [19] B. Bittins-Cattaneo, T. Iwasita, J. Electroanal. Chem. 238 (1987) 151. [20] S.A. Campbell, R. Parsons, J. Chem. Soc. Faraday Trans. 88 (1992) 833. [21] A.N. Haner, P.N. Ross, J. Phys. Chem. 95 (1991) 3740. [22] B. Beden, F. Kadirgan, C. Lamy, J.-M. Leger, J. Electroanal. Chem. 125 (1981) 71. [23] K. Wang, H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., Electrochim. Acta 41 (1995) 2587. [24] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., J. Phys. Chem. 99 (1995) 8945. [25] H.A. Gasteiger, N. Markovic´, P.N. Ross Jr., Catalysis Lett. 36 (1996) 1. [26] Y. Morimoto, E.B. Yeager, J. Electroanal. Chem. 441 (1998) 77. [27] Y. Morimoto (in preparation).