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J. Phys. Chem. C 2008, 112, 7186-7192
Potential Oscillation Generated by Formic Acid Oxidation in the Presence of Dissolved Oxygen Mitsunobu Kikuchi, Sho Miyahara, Yoshiharu Mukouyama, and Hiroshi Okamoto* DiVision of Science, School of Science and Engineering, Tokyo Denki UniVersity, Hatoyama, Saitama 350-0394, Japan ReceiVed: July 27, 2007; In Final Form: January 25, 2008
The effect of dissolved oxygen (0-1 mM) on the potential oscillation generated by the oxidation of formic acid is investigated with polycrystalline platinum at 315 K in a sulfuric acid solution containing 1 or 0.1 M formic acid. With an increase in the oxygen concentration, an induction period appears with 1 M formic acid at a current density of 4.8 mA/cm2 or greater, while it becomes short with 0.1 M formic acid at most current densities producing oscillation. A fluctuation appears and increases in amplitude and period during the oscillation with 1 M formic acid, and in potential during the induction period with 1 and 0.1 M formic acid. The duration of an entire oscillation decreases, but the maximum current for the appearance of oscillation hardly changes. With 1 M formic acid, the duration of the peak potential increases where small oscillations are observed. The addition of oxygen has two opposite effects on the current: the anodic current is increased because of oxidation of the adsorbed CO, and the cathodic current is increased because of electrochemical reduction of oxygen. The reasons for the oscillation behavior found upon the addition of dissolved oxygen are discussed on the basis of this fact and the voltammogram results.
1. Introduction It is widely accepted that the main framework of the reaction mechanism for the electrochemical oxidation of methanol, formaldehyde, and formic acid on platinum is the dual path mechanism.1,2 The mechanism consists of a direct path involving a reactive intermediate, probably adsorbed formate,3-5 and an indirect path involving a strongly adsorbed intermediate, adsorbed CO.6-8 Under galvanostatic conditions, the oxidation of the three substances shows potential oscillation,9-30 where the formation and oxidation of the adsorbed CO plays a crucial role.11-16,19,22-25,30-32 We therefore expected that the oscillation behavior would change when an oxidizing agent such as oxygen was added during oscillation. With this idea in mind, we recently30 investigated the effect of oxygen on the potential oscillation generated by the oxidation of formaldehyde. We found that the chaotic oscillation persists, that both periodic and chaotic oscillations show fluctuations in amplitude, and that the period length almost doubles in the presence of dissolved oxygen. We then investigated the effect of oxygen on the oscillation generated during the oxidation of formic acid, where no formation of oxidized substances other than carbon dioxide by oxygen is possible, unlike the oxidation of formaldehyde. Thus, the reaction system is simple, at least in the first approximation. The oxidation of formic acid, however, may not give as much information as that of formaldehyde because the former seems to produce only periodic oscillationssor at least to show chaotic oscillations only with difficulty.11-21 We have nevertheless been able to observe some characteristic changes due to oxygen addition and explain some of them with reference to the voltammograms. This paper describes the effect of oxygen on the potential oscillation generated by the oxidation of formic * Corresponding author: fax +81-49-296-2960; e-mail okamoto@ u.dendai.ac.jp.
acid and discusses reasons for this effect on the basis of voltammogram measurements obtained under various conditions. 2. Experimental Section Experiments were conducted in a three-electrode cell isolated from the surrounding air. The working electrode was a platinum mesh or a platinum wire (Tanaka Kikinzoku Group), both with a purity of 99.99%. If a charge of 0.210 mC/cm2 was assumed for the monolayer of adsorbed hydrogen,33 the true surface areas for both were estimated to be 2.5 cm2, but roughness factors were 1.25 and 1.16, respectively. The reference electrode was a normal hydrogen electrode (NHE). The counterelectrode was a platinum wire in a glass tube with bubbling nitrogen gas separated by a glass frit from the reaction solution. The solution temperature was 315 K (42 °C). The working electrode was pretreated by heating it in a hydrogen flame for about 10 s to remove organic contaminants. Before each run, it was further cleaned by repeatedly applying a triangular potential sweep between 0.05 and 1.4 V. The supporting electrolyte solution was 0.5 mol/dm3 (M) sulfuric acid (Kanto Chemical Co., Inc. Ultrapur or Wako Pure Chemical Industries, Ltd. Super Special Grade) diluted with Millipore Milli Q water. The formic acid solution was prepared by adding formic acid (Merck, Suprapur) to the supporting electrolyte solution. Safety precautions should be taken for the handling of formic acid. Formic acid is corrosive and combustible. Liquid and mist cause severe burns to all body tissues. Inhalation may cause lung damage. Vapor is irritating to eyes and respiratory tract. Potential health effects in more detail and first aid are found in Material Safety Data Sheet F5956. To control the concentration of dissolved oxygen, nitrogen gas (Nippon Sanso Corp., over 99.9999%) mixed with oxygen gas (Japan Fine Products Corp., grade G1, over 99.99995%) or with air (Japan Fine Products Corp., grade G1) by use of a
10.1021/jp075940l CCC: $40.75 © 2008 American Chemical Society Published on Web 04/17/2008
HCOOH Potential Oscillation with Oxygen
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Figure 2. Potential oscillation patterns generated by the oxidation of 1 M formic acid in the presence of 1 mM dissolved oxygen at 12 mA (a) and its enlargements in the initial (b) and final (c) stages.
Figure 1. Potential oscillation patterns generated by the oxidation of 1 mol dm-3 (M) formic acid in the absence of dissolved oxygen at a current of 12 mA and a temperature of 315 K. The oscillation pattern changes with time (a-f). (Inset) Enlargement of a sharp peak.
thermal mass flow control system (Nippon Tylan Corp., to an accuracy of within (0.25%), was bubbled through the solution before the measurement and passed over the solution during the measurement. Dissolved oxygen at concentrations of 0.20 mM or less was confirmed with a dissolved oxygen sensor (Horiba Ltd., OM-14). Incidentally, a dissolved oxygen concentration of 1 mM is the saturated point under 105 Pa (1 atm) of pure oxygen at 315 K. We used a function generator (Hokuto-Denko Co., Ltd., HB105) and a potentiostat/galvanostat (Hokuto-Denko Co., Ltd, HA-501G). The time sequence of the potential or current values was acquired through an AD converter (National Instruments Corp., PCI-6034E) and saved in a personal computer after averaging of 100 values acquired at a sampling rate of 100 kHz or slower. At the same time, it was recorded with an X-T or X-Y recorder. 3. Results 3.1. Potential Oscillations in the Presence of Dissolved Oxygen (1 M Formic Acid). For reference, the oscillation pattern generated by the oxidation of 1 M formic acid in the absence of dissolved oxygen is shown in Figure 1, where the applied current is 12 mA (current density 4.8 mA/cm2) and the temperature is 315 K. Although the appearance of the oscillation pattern differs slightly from run to run, on the whole the oscillation pattern changes with time as follows. First, it is very sharp and relaxational with large amplitudes and periods (Figure 1a). A detailed examination of the sharp-looking potential peak proves it to be divided into a few small oscillations, as shown in the inset. Then the large oscillation, remaining relaxational, slightly changes its shape as the amplitudes and periods decrease together with decreases in the number of small oscillations observed in the peak potential region to one or zero (Figure 1b). Next, by contrast, the amplitudes and periods increase, and
the small oscillations observed at and near the peak potential increase in number but decrease in amplitude, becoming almost flat (Figure 1c). Then a sharp projection appears just before the broad peak potential drops (Figure 1d) and its number increases, producing rather irregular patterns (Figure 1e) before and after a regular pattern (Figure 1f). Finally, the potential jumps up to over 1.2 V to terminate the oscillation. At a current higher than 12 mA and lower than 18 mA, which is the maximum current at which at least one period of oscillation appears, the oscillation pattern change was similar to that at 12 mA, although the duration of the entire oscillation decreased. At a current lower than 12 mA, an induction period appeared before the emergence of an oscillation, which was similar to that shown in Figure 1b, though with long periods frequently lasting several tens of seconds and which continued longer than 1 h. Similar but not identical oscillation pattern changes with increasing applied current were demonstrated by Schell and co-workers,12,13 who carried out experiments under conditions where the formic acid concentration was 2 M, in either 0.5 or 1 M sulfuric acid at 30 or 50 °C, and a rotating electrode (4000 rpm) was used. When 1 mM dissolved oxygen was present in a solution containing 1 M formic acid, at the moment of 12 mA current application, a sharp spike appeared that was not followed by an oscillation for ca. 24 min; in other words, an induction period was observed prior to the occurrence of a continuing oscillation, as shown in Figure 2a. The oscillation pattern change resembles that of the latter half (seen in Figure 1d-f) of the entire oscillation in the absence of dissolved oxygen; namely, that observed after periods and amplitudes began to increase, although the periods are longer than they were without oxygen. Thus, the first half of the entire oscillation observed without dissolved oxygen appears to become an induction period in its presence. The oscillation at first has small amplitudes and long periods, both of which fluctuate, with several small oscillations at and near the peak potential region, and later changes to a large-amplitude oscillation with many small oscillations. There are five items characterizing the oscillation observed with an increased dissolved oxygen concentration and with 1 M formic acid. First, the induction period tends to increase from zero to ca. 22 min at 0.2 mM and hardly changes with further increases of concentration, as shown in Figure 3, where the applied current is 12 mA. Second, the potential fluctuates both
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Figure 3. Dependence of the induction period, tind, on the concentration of dissolved oxygen, [O2], with 1 M at 12 mA.
Figure 5. Potential oscillation patterns generated by the oxidation of 0.1 M formic acid in the presence of dissolved oxygen. Shown are the patterns in the presence of 0.6 mM dissolved oxygen at 4 mA (a) and at 3 mA (b), and those in the presence of 1 mM dissolved oxygen at 3 mA (c) and 2 mA (d).
Figure 4. Potential oscillation patterns generated by the oxidation of 0.1 M formic acid in the absence of dissolved oxygen. Shown are the patterns just after the induction period (a) and just before oscillation cessation (b) at 5 mA, and those at 4 mA (c, d).
during the induction period, especially near the beginning of oscillation, and during the oscillation, especially at the start, where the oscillation periods also fluctuate. Fluctuation in amplitude was also observed in the potential oscillation during the oxidation of formaldehyde in the presence of dissolved oxygen.30 Third, the duration of the oscillation including the induction period tends to decrease, for example, from ca. 62 to ca. 30 min at 12 mA, when the dissolved oxygen concentration is increased from 0 to 1 mM (Figures 1 and 2). The duration of oscillation excluding the induction period naturally also decreases. Fourth, at least in the latter half of the entire oscillation, the duration of the peak potential region increases and small oscillations are observed. Sometimes the small oscillations show a period doubling and a very short irregular-looking pattern, as in Figure 2c. And fifth, the maximum current, 18 mA, for the appearance of oscillation tends to show hardly any change. 3.2. Potential Oscillations in the Presence of Dissolved Oxygen (0.1 M Formic Acid). When the concentration of formic acid was 0.1 M, an induction period was observed even in the absence of dissolved oxygen at a current of up to 5 mA, which was very near 6 mA, approximately the maximum current for the appearance of oscillation. At 5 mA, at the moment of current application, a sharp potential spike appears and is followed by an induction period of about 4 min, and then, as Figure 4a shows, the potential exhibits sharp relaxational oscillations with large amplitudes and long periods. The oscillation pattern does not change with time, unlike that of 1
M formic acid, but the periods decrease steadily (Figure 4b). A detailed examination of the sharp oscillation peak reveals that it divides into 3-5 peaks as time passes. At 4 mA, after an induction period of about 22 min, the potential first shows relaxational oscillations (Figure 4c); then the periods decrease and small oscillations appear just before the potential drops (Figure 4d); and finally the potential jumps, reaching over 1.2 V. At a current lower than or equal to 3 mA, the induction period was longer than 1 h. Chen et al.14 also found that the oscillation pattern changed little during the oxidation of 0.1 M formic acid at 25 °C, although their oscillation pattern was somewhat different from ours. In the presence of 0.6 mM dissolved oxygen with 0.1 M formic acid at 4 mA, after an induction period of ca. 6.5 min, only a single oscillation appears and then the potential jumps to over 1.2 V, as shown in Figure 5a. The induction period and the oscillation duration are very short compared to those seen in the absence of dissolved oxygen, which were ca. 22 and 13 min, respectively. The potential fluctuates at least during the induction period, especially near the beginning of the oscillation. At 3 mA with 0.6 mM dissolved oxygen, as shown in Figure 5b, the induction period becomes longer, ca. 40 min, and the potential oscillates twice and rises sharply. The potential fluctuation is also observed, at least during the induction period. In the presence of 1 mM dissolved oxygen at 3 mA (Figure 5c), the induction period, ca. 20 min, is shorter than that in the presence of 0.6 mM dissolved oxygen at the same current (Figure 5b), and the number of oscillations is reduced to one. At 2 mA with 1 mM dissolved oxygen, a very long induction period of ca. 56 min is observed with one oscillation (Figure 5d). The potential fluctuates at least during the induction period, especially near the beginning of oscillation. The results obtained with 0.1 M formic acid with an increase in the dissolved oxygen concentration are, in summary, as follows. First, the induction period decreased steadily at a fixed current. Second, the potential fluctuated, at least during the induction period, especially near the beginning of oscillation. Third, the duration of oscillation or the number of oscillations decreased. Fourth, the small oscillations observed at and near the peak potential with 1 M formic acid were not observed. Finally, the maximum current, 6 mA, for the occurrence of at least one period of oscillation hardly changed at all.
HCOOH Potential Oscillation with Oxygen
Figure 6. Voltammograms for the oxidation of 1 M (a) and 0.1 M (b) formic acid in the absence and presence of dissolved oxygen (1 mM). The sweep rate is 0.1 V s-1. Current peaks I and II are indicated. In the presence of dissolved oxygen, peak I appears with 1 M formic acid and becomes more distinct with 0.1 M formic acid.
3.3. Voltammograms Measured in the Presence of Dissolved Oxygen. Figure 6 shows repeating voltammograms obtained after 10 triangular potential sweeps for the oxidation of 1 and 0.1 M formic acid with and without 1 mM dissolved oxygen, at a sweep rate of 0.1 V s-1 and in the potential range between 0.05 and 1.4 V. Although large current peaks are observed in the negative sweep direction, we are not concerned about them because the oscillation potential does not exceed 0.9 V, meaning that the large current peaks observed after the reduction of platinum oxides and the complete disappearance of the adsorbed CO due to oxidation have nothing to do with the oscillation phenomena. As shown in Figure 6a, the voltammogram for the oxidation of 1 M formic acid hardly exhibits a current peak (peak I) at ca. 0.55 V in the positive sweep direction in the absence of dissolved oxygen, whereas it shows a clear peak in the presence of 1 mM dissolved oxygen. Because the adsorbed CO is formed during the sweep at a potential lower than ca. 0.5 V and is a surface poison at a potential lower than ca. 0.7 V, this can be explained in terms of the amount of the adsorbed CO. Thus, in the absence of dissolved oxygen, this amount is so large that peak I is hardly visible, while in its presence, adsorbed CO decreases and peak I is clear. On the other hand, as Figure 6b shows, the voltammogram of 0.1 M formic acid shows a clear peak I even in the absence of dissolved oxygen, indicating that little adsorbed CO is formed during the sweep. In the presence of 1 mM dissolved oxygen, the voltammogram shows a larger peak I than in its absence, indicating that an even smaller amount of adsorbed CO is present. Incidentally, the current peak (peak II) at ca. 0.9 V observed in the positive sweep direction does not change its height whether dissolved oxygen is present or absent for both 1 and 0.1 M formic acid. Figure 6b also shows a reduction current that is below 0.4 V in the positive sweep direction and below 0.3 V in the negative sweep direction and is due to the electrochemical reduction of oxygen. This current was also observed in the presence of 1 M formic acid, though found to be slight upon magnification of the current scale. Because the adsorbed CO is present in the potential range just stated above, the electrochemical reduction of oxygen probably proceeds in the presence of the adsorbed CO, although it may depend on the amount of adsorbed CO, which is examined later. It is known34-36 that oxygen reduction occurs by two overall pathways, a direct four-electron reduction and a “peroxide” pathway that involves hydrogen peroxide as an intermediate. Markovic´ et al.37 showed by using a rotating ring-disc electrode
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Figure 7. Voltammograms for the oxidation of 1 M formic acid in the absence and presence of dissolved oxygen (1 mM) at sweep rates of 3 V s-1 (a) and 0.03 V s-1 (b). At 3 V s-1, the voltammograms are identical in the absence and presence of dissolved oxygen, whereas at 0.03 V s-1, they are different, especially around the peak I potential.
that the electrochemical reduction of oxygen proceeds almost entirely through the direct four-electron pathway at potentials more positive than ca. 0.2 V, where the oscillation is observed. Shao et al.38 showed by surface-enhanced infrared absorption spectroscopy (SEIRAS) that the surface reaction intermediate is superoxide anion, O2-(ads), during the reduction in a basic solution. Anderson et al.39 showed by the analysis of activation energies that adsorbed peroxyl radical, OOH(ads), is the surface reaction intermediate in an acidic solution. In spite of these findings, we assume, for simplicity, that the surface reaction intermediate is adsorbed atomic oxygen, O*, because we are not concerned about a specific species in this paper. Because the behavior of the adsorbed CO is closely related to the oscillation phenomenon, we then investigated the mechanism of the decreased amount of adsorbed CO due to oxygen addition. There are two possible mechanisms30 in the first approximation. One is that adsorbed oxygen reacts with formic acid in the solution or with temporarily adsorbed formic acid to hinder formation of adsorbed CO:
O* + HCOOH* f CO2 + H2O + 2*
(1)
The other is that adsorbed oxygen reacts with adsorbed CO to remove the latter:
O* + CO* f CO2 + 2*
(2)
Here, asterisks are adsorption sites and O* and CO* are adsorbed species. We will show below that reaction 2 occurs to reduce the amount of adsorbed CO. At a fast sweep rate of 3 V s-1 in the solution containing 1 M formic acid, the voltammograms are almost identical, particularly peaks I and II, in the absence and presence of dissolved oxygen (1 mM), as shown in Figure 7a. Therefore, adsorbed CO is formed in the same amount during the fast sweep, irrespective of the presence of dissolved oxygen. Thus, the amount does not decrease because of additional dissolved oxygen. On the other hand, at a slow sweep rate of 0.03 V s-1, the voltammograms in the absence and presence of dissolved oxygen (1 mM) differ completely in the positive sweep direction, as shown in Figure 7b. Without dissolved oxygen, the current is almost zero until 0.8 V is reached in the positive sweep direction, while in the presence of dissolved oxygen, the current begins to increase at a potential of 0.35 V. These results illustrate that the adsorbed CO is formed in almost the same amount at potentials less than 0.35 V with dissolved oxygen present as without it, and it is slowly oxidized at higher potentials. It can therefore be said that the decrease in the amount of the adsorbed CO on addition of dissolved oxygen is due to slow reaction 2.
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Figure 8. Changes in the currents observed while the potential Eh is held at 0.2 V after it was held at 1.4 V for 5 s. The first and second numbers in parentheses are, respectively, the concentrations of dissolved oxygen (millimolar) and formic acid (molar). The currents become stationary with fluctuations within 100 s. That of 1 M formic acid is smaller in absolute value than that of 0.1 M.
3.4. Oxygen Reduction Currents at a Constant Potential. We have found that the formation of adsorbed CO is hardly influenced by the presence of dissolved oxygen at potentials lower than 0.35 V, as shown in Figures 6 and 7. We have also investigated the effect of formic acid on the electrochemical reduction of dissolved oxygen. Figure 8 shows current changes with time for 1000 s at a holding potential, Eh, of 0.2 V in the presence of 1 mM dissolved oxygen after the potential was held at 1.4 V for 5 s. When the formic acid concentration is zero, the observed current fluctuates considerably and becomes stationary at approximately -1.2 mA within 100 s (dotted line), which is almost the same as that for 0.1 M formic acid (thin solid line). When the formic acid concentration is 1 M, the current fluctuates slightly and becomes stationary at approximately -0.6 mA, in about 100 s (thick solid line). Incidentally, in the absence of both oxygen and formic acid we have confirmed that the current is zero (dotted-dashed line). The figure shows that a current with a larger absolute value, corresponding to a greater consumption of dissolved oxygen, fluctuates more. We consider that the current fluctuation therefore may appear as a result of fluctuation of the dissolved oxygen concentration, which also causes the current fluctuation observed in a solution containing formaldehyde and oxygen.30 The stationary current in the presence of 1 M formic acid has a smaller absolute value than in the absence of formic acid. This indicates that the reduction current of oxygen is hindered by the adsorbed CO, which probably covers the surface almost fully, as deduced from the voltammogram in Figure 7b. Such a current decrease has been observed as a result of methanol addition.40-42 On the other hand, the current is not decreased even when the surface is partially covered with adsorbed CO, as in the case of 0.1 M formic acid. It can therefore be said that the electrochemical reduction of oxygen is not hindered until the surface is almost fully covered with adsorbed CO. It is known37,43 that, at potentials between ca. 0.2 and ca. 0.5 V, the current due to electrochemical reduction of the dissolved oxygen is diffusion-limited, at least in the absence of formic acid. The diffusion-limiting current is expected to decrease in proportion to the reciprocal square root of time, according to the Cottrell formula, whereas the observed current becomes almost stationary within 100 s. This is probably because the fluctuation of the dissolved oxygen concentration incessantly disrupts the diffusion layer. The dependence of the stationary current on the concentration of dissolved oxygen at 0.2 V is shown in Figure 9a. For both
Kikuchi et al.
Figure 9. Dependence of the apparent stationary current at 0.2 V on the concentrations of formic acid and dissolved oxygen (a), and that on Eh and the concentration of formic acid in the presence of 1 mM dissolved oxygen (b). The current increases in absolute value with the increasing dissolved oxygen concentration. It depends hardly at all on Eh at a potential equal to or lower than 0.3 V.
Figure 10. Voltammograms measured without formic acid in the absence (a) and presence (b) of 1 mM dissolved oxygen. In the presence of oxygen, the reduction current flows until 0.9 V in the positive sweep direction.
1 and 0.1 M formic acid, the current increases in absolute value linearly with dissolved oxygen concentration, and in the absence of formic acid, the current increases in the same way as with 0.1 M formic acid. The dependence of the stationary current on Eh is shown in Figure 9b. When formic acid is absent, the current is approximately -1.2 mA and hardly depends on Eh at all when Eh is less than 0.7 V in the presence of 1 mM dissolved oxygen. When formic acid is present, the currents are approximately -1.2 mA with 0.1 M formic acid and approximately -0.6 mA with 1 M formic acid, and they hardly depend on Eh when Eh is less than 0.3 V. At Eh ) 0.4 V, where the adsorbed CO is not oxidized if dissolved oxygen is absent,44 the currents for both 1 and 0.1 M formic acid decreased with time even after 1000 s, so that the plotted values are transient ones at 1000 s. The reason for the current decrease with time is probably mainly the continued, but slow, formation of adsorbed CO at 0.4 V. The observed currents should be a mixture of oxygen reduction and formic acid oxidation currents at potentials between 0.4 and 0.9 V, because the oxygen reduction current is observed at potentials lower than 0.9 V without formic acid, as shown in Figure 10. 4. Discussion The reasons for the change in oscillation behavior observed when dissolved oxygen was added, based on the voltammogram results and the deduced mechanism for the decrease in adsorbed CO due to oxygen addition, are dealt with below. We will consider the fluctuations, the maximum currents for the oscillation appearance, the induction periods, the duration of an entire oscillation, and the duration of small oscillations, in this order. When the dissolved oxygen concentration was increased, the oscillation tended to fluctuate in amplitude and period with 1
HCOOH Potential Oscillation with Oxygen M formic acid (Figure 2), as did the potential in the induction period with 1 and 0.1 M formic acid (Figures 2 and 5). This phenomenon seems to be related to the current fluctuation observed at a fixed potential (Figure 8). We suppose that the reason for these fluctuations is the same as that for the current fluctuationsthat is, the fluctuations in the oscillation and the induction period probably result from the fluctuation of the dissolved oxygen concentration. The maximum currents at which at least one period of oscillation occurs, which were 18 mA with 1 M formic acid and 6 mA with 0.1 M formic acid, depended hardly at all on the dissolved oxygen concentration, whether the formic acid was at 1 or 0.1 M. This seems related to the fact that the peak II currents observed at ca. 0.9 V in the voltammograms showed hardly any change with increases in the dissolved oxygen concentration with either 1 or 0.1 M formic acid (Figure 6). We think it natural that the peak current mainly determines, or at least restricts, the maximal currents because an applied current above the peak current causes the potential to rise above its peak potential. When no dissolved oxygen was present, the induction period was zero with 1 M formic acid at currents of at least 12 mA; an induction period was present with 0.1 M formic acid at most currents producing oscillation. With increasing dissolved oxygen concentration, the induction period with 1 M formic acid increased at a concentration of ca. 0.2 mM and tended to remain constant at higher concentrations, while with 0.1 M formic acid it decreased steadily at most currents producing oscillation. In discussing the reasons for this behavior, we should keep in mind that oxygen addition has two opposite effects on the current and, therefore, on the oscillation behavior. One effect is the increase in anodic current due to increased formic acid oxidation (the direct path of the dual path mechanism) because the adsorbed CO is oxidized by oxygen (reaction 2). If this effect is dominant during oscillation, the potential should decrease to maintain the applied current, resulting in a long induction period. The other effect is a decrease in the anodic current due to the electrochemical reduction of oxygen. If this effect prevails during oscillation, the induction period decreases because the potential should increase to satisfy the applied anodic current. It is thought that, with 1 M formic acid, after the first sharp oscillation, the potential increases to produce the next oscillation under constant current conditions. In the absence of dissolved oxygen, the potential increases rapidly due to surface deactivation by the increased amount of adsorbed CO. On the other hand, when dissolved oxygen is present, it increases very gradually because of the slow increase in the amount of adsorbed CO, whose consumption (reaction 2) proceeds simultaneously. As a result, the induction period appears in the presence of dissolved oxygen with 1 M formic acid. With an increasing dissolved oxygen concentration, the reduction of oxygen becomes noticeable because more adsorbed CO is oxidized, disappearing from the surface to produce more reaction sites for oxygen reduction. Thus, the two effects are in balance, and the induction period is almost constant at dissolved oxygen concentrations above 0.2 mM. With 0.1 M formic acid, an induction period was observed even in the absence of dissolved oxygen. This is probably because the formation velocity of the adsorbed CO is low compared to that for 1 M formic acid due to the low concentration of formic acid, which is supported by the clear appearance of peak I even in the absence of dissolved oxygen (Figure 6b). The reason why the induction period decreased steadily with increasing oxygen concentrations is probably that
J. Phys. Chem. C, Vol. 112, No. 18, 2008 7191 electrochemical reduction contributes to the current more than formic acid oxidation due to the low concentration of formic acid (compared to 1 M formic acid conditions). The fluctuation of the dissolved oxygen concentration may also contribute to the decreased induction period. Specifically, when the reduction current increases, the potential also increases to satisfy the applied current, perhaps far enough to enter the potential region where the potential is accelerated to increase, resulting in the appearance of oscillation. The duration of the entire oscillation decreased with increasing dissolved oxygen concentration for both 1 and 0.1 M formic acid. This is probably because the dissolved oxygen oxidizes the adsorbed CO, a substance essential for the appearance of oscillation,11-13 and decreases its amount. The fluctuation of the dissolved oxygen concentration may also contribute to a decrease in the duration because the reduction current may happen to increase, and, in order to maintain the applied current, the potential may increase suddenly above 0.9 V and never decrease. As to increases in the duration of small oscillations observed in the potential part of peak potential with 1 M formic acid, we have as yet found no relevant facts, but we offer the following speculative description. The small oscillations appear at potentials as high as 0.8 V, where poisonous hydroxides or oxides, or both, are formed on platinum. When the adsorbed CO is oxidized and removed from the surface at a comparatively high potential, producing active reaction sites, the potential decreases to maintain the applied current. At the same time, hydroxides or oxides may be formed, and the potential is thus forced up to maintain the applied current. When the dissolved oxygen is also present, the oxygen reduction current also has the effect of forcing up the potential. In this way, in the presence of dissolved oxygen, the potential is generally kept high and the duration of the peak potential is prolonged. Conclusions We investigated the effect of dissolved oxygen on the potential oscillation generated by the oxidation of formic acid on platinum and discussed the reasons for this behavior mainly on the basis of the voltammograms. With increasing concentration of dissolved oxygen from 0 to 1 mM, we found that (1) an induction period appeared with 1 M formic acid at a current density of 4.8 mA/cm2 or above, but became shorter with 0.1 M formic acid at most currents producing oscillation; (2) a fluctuation appeared and increased in amplitude and period during oscillation with 1 M formic acid and in potential during the induction period with 1 and 0.1 M formic acid; (3) the duration of an entire oscillation decreased; (4) with 1 M formic acid there was an increase in the duration of the peak potential where small oscillations were observed; and (5) the maximum current for the appearance of oscillation hardly changed. The voltammogram measurement showed that, with an increase in the dissolved oxygen concentration, the amount of adsorbed CO decreased due to its oxidation with (adsorbed) oxygen and the electrochemical reduction of oxygen proceeded in the presence of adsorbed CO and simultaneously with the oxidation of formic acid. The peak II current hardly changed at all. The current at a fixed potential was found to fluctuate. On the basis of these facts, the reasons for the oscillation behavior observed upon the addition of oxygen were discussed. Acknowledgment. This work was partially supported by the Research Institute for Science and Technology of Tokyo Denki University under Grants Q06M-07 and Q06M-08.
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