Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at

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J. Phys. Chem. B 2006, 110, 16559-16566

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Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy Gabor Samjeske´ ,†,‡ Atsushi Miki,§ Shen Ye,‡ and Masatoshi Osawa*,‡ CREST, Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan, Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan, and Graduate School of EnVironmental Earth Science, Hokkaido UniVersity, Sapporo 060-0801, Japan ReceiVed: March 27, 2006; In Final Form: June 13, 2006

Surface-enhanced infrared absorption spectroscopy (SEIRAS) combined with cyclic voltammetry or chronoamperometry has been utilized to examine kinetic and mechanistic aspects of the electrocatalytic oxidation of formic acid on a polycrystalline Pt surface at the molecular scale. Formate is adsorbed on the electrode in a bridge configuration in parallel to the adsorption of linear and bridge CO produced by dehydration of formic acid. A solution-exchange experiment using isotope-labeled formic acids (H12COOH and H13COOH) reveals that formic acid is oxidized to CO2 via adsorbed formate and the decomposition (oxidation) of formate to CO2 is the rate-determining step of the reaction. The adsorption/oxidation of CO and the oxidation/reduction of the electrode surface strongly affect the formic acid oxidation by blocking active sites for formate adsorption and also by retarding the decomposition of adsorbed formate. The interplay of the involved processes also affects the kinetics and complicates the cyclic voltammograms of formic acid oxidation. The complex voltammetric behavior is comprehensively explained at the molecular scale by taking all these effects into account.

1. Introduction

structures.) Adsorption of formate from formic acid solution

The electrocatalytic oxidation of methanol, formaldehyde, and formic acid (HCOOH) on Pt and Pt-based alloys has received strong interest due to their prospect being used as fuels for low temperature fuel cells. The oxidation of formic acid on Pt, the simplest reaction system, has extensively been studied as a prototype of the reactions as summarized in many review articles.1 However, detailed mechanistic understanding at the molecular level has not fully emerged yet. It is generally accepted that formic acid is oxidized to CO2 via the so-called dual-pathway mechanism: a direct path to CO2 via a reactive intermediate and an indirect path via a poisoning species which is oxidized to CO2 at high potentials.1 Infrared reflection-absorption spectroscopy (IRAS) clearly demonstrated that the poisoning species in the indirect path is adsorbed CO produced by dehydration of formic acid.2,3 On the other hand, the reactive intermediate in the direct path has not been defined yet. It has long been assumed that a carboxylic acid species adsorbed via the carbon atom (-COOH) is the intermediate and its adsorption is rate-determining.1 Sun et al.4 observed an IR absorption band at 1750 cm-1 on Pt(111) in HClO4 solution containing formic acid and assigned it to the reactive intermediate (possibly the CdO stretching mode of COOH). Nevertheless, this band is observed only on Pt(111). Furthermore, CO adsorbed at hollow sites on Pt(111) exhibits a band at almost the same position (1775 cm-1).5 Another possible candidate for the reactive intermediate is a formate species (HCOO).6,7 (Note that these two species have the same composition but different * Corresponding author. E-mail: [email protected]. † CREST, Japan Science and Technology Agency. ‡ Catalysis Research Center, Hokkaido University. § Graduate School of Environmental Earth Science, Hokkaido University.

HCOOH f HCOO(a) + H+ + e-

(1)

has been observed on Pd8 and Ir7 electrodes by IRAS. Recently, we observed the adsorption of formate on Pt electrodes for the first time by using surface-enhanced infrared absorption spectroscopy (SEIRAS) in an attenuated total reflection (ATR) mode (an ATR measurements with an IR-transparent prism/thin metal electrode/solution geometry).9 By using isotope-labeled formic acids (H12COOH and H13COOH) we further demonstrated that formic acid is oxidized to CO2 via adsorbed formate and that the decomposition (oxidation) of formate yielding CO2

HCOO(a) f CO2 + H+ + e-

(2)

is the rate-determining step.10 Our proposed reaction mechanism was successfully used to explain potential and current oscillations observed during galvanostatic and potentiostatic formic acid oxidation, respectively.10,11 In the present study, we focus our attention on voltammetric behavior of formic acid oxidation on Pt. Despite the rather simple reaction scheme, cyclic voltammograms exhibit several (one to five) oxidation peaks depending on experimental conditions, including the potential range to be examined, sweep rate, sweep direction, supporting anion, and formic acid concentration.1,12-15 The complex voltammetric behavior is analyzed at the molecular scale by employing ATR-SEIRAS. ATR-SEIRAS is more favorable than IRAS for elucidating kinetic and dynamic aspects of formic acid oxidation due to the high surface-sensitivity and free mass-transport advantages.16 The high surface-sensitivity advantage enables real-time monitoring of the reaction with time resolutions ranging from

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milliseconds to seconds with negligible interference from the bulk solution. The free mass-transport advantage is more essential in the present study.17 In IRAS measurements using a thin solution layer confined between the working electrode and an IR-transparent cell window, mass transport of reactants and products between the thin layer and the reservoir is seriously suppressed, and thus, the concentrations of the reactants in the thin layer are considerably reduced during spectral measurements, which makes the detection of the reactive intermediates difficult. ATR-SEIRAS is free from this problem and has a higher possibility to detect the real reactive intermediate. Since the adsorption of formate is commonly observed in the electrooxidation of methanol18 and formaldehyde,17 the detailed mechanistic studies of formic acid oxidation would give valuable information for understanding the oxidation of methanol and formaldehyde. 2. Experimental Section The experimental details of the ATR-SEIRAS have been described elsewhere.16,19,20 The ATR prism, which serves as the cell window and substrate on which a thin Pt film electrode is deposited, was a Si hemicylinder (1 cm in radius and 2.5 cm long, Pier Optics, Japan). A thin (∼50 nm) Pt film electrode was deposited on the total-reflecting plane of the Si prism by an electroless plating process. 9,17 The spectroelectrochemical cell used was a three-electrode design similar to that reported before.20 A Pt gauze served as the counter electrode, and the reference electrode was the reversible hydrogen electrode (RHE) in the supporting electrolyte solution (0.5 M H2SO4). Electrode potential was controlled with a potentiostat (EG&G PARC model 263A). The electrode surface was cleaned prior to use by repeating potential excursions between 0.05 and 1.4 V in the supporting electrolyte. The geometrical area of the electrode surface in contact with the solution was 1.77 cm2, and the real surface area was 10.8 cm2, estimated from hydrogen adsorption/ desorption peaks in the cyclic voltammogram (CV) by assuming 210 µC cm-2 for monolayer hydrogen adsorption. The large roughness factor of ∼6 arises from a surface roughness in nanometer scale,17 but the film was as shiny as well-polished Pt surfaces. A Bio-Rad FTS-60A/896 or DigiLab FTS-7000 Fourier transform infrared spectrometer equipped with an MCT detector and a homemade single-reflection accessory (incident angle of 70°) was used to record spectra. The spectrometer was operated in rapid scanning mode with a spectral resolution of 4 cm-1. Typically, five interferograms were coadded to each spectrum, which required 0.98 s. Spectra are shown in the absorbance units defined as A ) -log(I/I0), where I and I0 represent the intensities of the IR signals reflected from the Pt surface in the electrolyte solutions with and without formic acid, respectively. The supporting electrolyte solution was prepared from Milli-Q water (>18 MΩ) and analytical grade H2SO4 (Suprapure, Merck) and was deaerated with Ar prior to use. HCOOH (analytical grade, Wako Pure Chemicals), H13COOH (13C 99%, Cambridge Isotope Laboratories), and DCOOD (D 99.5%, Merck) were used as received without further purification. All measurements were carried out at room temperature (23 °C). 3. Results 3.1. Cyclic Voltammetry. Figure 1 shows cyclic voltammograms (CVs) for a Pt electrode recorded in 0.5 M H2SO4 + 0.1 M HCOOH at sweep rates of 50, 5, and 0.3 mV s-1 (from top to bottom). Ohmic drops (solution resistance R ≈ 5 Ω) were compensated in recording the CVs by using the iR-compensation

Figure 1. Cyclic voltammograms for a Pt electrode recorded in 0.5 M H2SO4 + 0.1 M HCOOH at sweep rates of 50, 5, and 0.3 mV s-1 (from top to bottom).

Figure 2. Series of time-resolved SEIRA spectra of the Pt electrode surface acquired simultaneously with the cyclic voltammogram in Figure 1 (top, at 50 mV s-1). Time-resolution was 0.98 s.

mode of the potentiostat. At 50 mV s-1, two peaks are observed at 0.6 and 0.9 V on the positive-going sweep from 0.1 V, and a peak and a shoulder are observed at 0.75 and 0.5 V, respectively, on the reverse negative-going sweep. Following earlier studies,1a,12,13,15 these peaks and shoulder are denoted as peaks I, II, IV, and V in the order of appearance in the CV. The peak potential of peak III is located at 1.5 V 1a,13,15 and is not depicted in this figure. By slowing the sweep rate to 5 mV s-1, peak I is suppressed, and peak II is negatively shifted to 0.8 V (or two peaks are merged). On the negative-going sweep, peak IV is largely suppressed compared to peaks II and V and slightly shifted in the positive direction. Peak V also is slightly shifted in the positive direction. These trends become more significant at the further slow sweep rate of 0.3 mV s-1, where peak IV is observed only weakly as a shoulder at 0.8 V. The voltammetric behavior is consistent with earlier studies on polycrystalline Pt surfaces.1,12-15 3.2. SEIRAS Measurements Coupled to Cyclic Voltammetry. A series of SEIRA spectra of the electrode surface recorded simultaneously with the CV at 50 mV s-1 (Figure 1, top) is shown in Figure 2. A spectrum recorded at 0.1 V in the pure electrolyte before injecting formic acid was used as the reference. Seven bands are observed at 3650, ∼3500 (negative peak), 2055-2075, 1800-1850, 1620 (negative peak), 1323, and 1180-1200 cm-1 in the spectral range of 1000-4000 cm-1. The noisy feature around 1100 cm-1 is due to the low transmittance of the prism. The spectral range below 1000 cm-1

IR study of Electrooxidation of HCOOH on Pt could not be observed due to the very strong absorption by the prism. The spectral feature observed here is identical to that of a CO-covered Pt surface in the same supporting electrolyte solution9 except for the presence of additional bands at 1323 and 1180-1200 cm-1 (the latter band was observed after CO oxidation). The bands at 2055-2075 and 1800-1850 cm-1 are assigned to linear- and bridge-bonded CO (denoted as COL and COB, respectively). The negative bands at ∼3500 and 1620 cm-1 are assigned to the OH stretching, ν(OH), and HOH bending, δ(HOH), modes, respectively, of interfacial water.20 Since water molecules are repelled from the interface by the adsorption of CO, these bands are observed as the negative peaks. The band at 3650 cm-1 also is assigned to ν(OH) of water. The very high frequency and sharp feature (fwhm of 50 cm-1) indicate that the water molecules showing this band are free from hydrogen bonding. Since the sharp ν(OH) mode is observed only when CO is adsorbing, we attributed this band to isolated water molecules near or in the CO monolayer.9 The corresponding δ(HOH) mode of the isolated water is hardly observed due to its weak intensity. The very weak band at 1180-1200 cm-1, which was observed on a clean Pt electrode surface in the pure electrolyte with larger intensity, is undoubtedly ascribed to adsorbed (bi)sulfate.21 The most interesting and important finding in Figure 2 is the band at 1323 cm-1. This band was shifted to 1295 and 1305 cm-1 when DCOOD and H13COOH were used, respectively, instead of H12COOH, implying that the adsorbed species contains both hydrogen and carbon atoms. A similar band is observed at almost the same position (typically, 1340 cm-1) in catalytic formic acid decomposition on metal and metal-oxide surfaces in ultrahigh vacuum (UHV) and gas phase, and has been assigned to the symmetric O-C-O stretching mode of formate bonded to two Pt atoms with two oxygen atoms (that is, in a bridging conformation).22-25 Following the studies in UHV and gas phase, we assign the 1323 cm-1 band to formate adsorbed on the Pt electrode with the bridging conformation. The frequency is remarkably lower than the corresponding mode of formate aion in aqueous solution (1380 cm-1), suggesting that formic acid is chemisorbed on Pt by releasing a proton and an electron (eq 1). Although formate has the antisymmetric O-C-O stretching mode around 1580 cm-1, this mode is not observed in Figure 2. On the basis of the surface selection rule in SEIRAS (same as in IRAS),16,26 the absence of this mode is consistent with the adsorption of formate in the bridging conformation. No other adsorbate bands were detected in the present study. To find the correlation between the CV and SEIRA spectra acquired simultaneously at a potential sweep rate of 50 mV s-1, the intensities of COL, COB, and formate bands taken from Figure 2 are plotted in Figure 3 (b, c, and d, respectively) as a function of the applied potential. The intensity data for adsorbed (bi)sulfate is not shown due to its very weak intensity for quantitative analysis. At the very beginning of the positive-going potential sweep, the COL band increases and COB band decreases in intensity at 0.05-0.25 V due to the partial site change of CO from bridge sites to atop sites.27 After reaching a maximum around 0.3 V, the COL band decreases in intensity gradually at 0.3-0.7 V and then steeply at more positive potentials. The COB band shows a similar potential dependence, and both COL and COB bands completely disappear around 0.9 V by the oxidation of CO. Concomitantly with the CO oxidation, the formate band appears at about 0.3 V and increases in intensity at more positive potentials. The formate band reaches

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Figure 3. Cyclic voltammogram recorded in 0.5 M H2SO4 + 0.1 M HCOOH at 50 mV s-1 (a), and the integrated band intensities of COL (b), COB (c), and formate (d) taken from the SEIRA spectra acquired simultaneously with the CV (Figure 2). Panel e represents the baseline levels of the surface spectra at 2500 cm-1 with (b) and without (O) formic acid in the solution.

a maximum at 0.9 V and decreases to zero at 1.3 V. On the reverse negative potential sweep, the formate band reappears around 0.85 V associated with the reduction of surface oxides (vide infra), reaches a maximum at ∼0.7 V, and then decreases gradually. The CO bands reappear at about 0.5 V and increase in intensity with decrease in potential. It is worth noting that oxidation current is observed only when formate is adsorbing on the surface. The formate band intensity versus potential curve is very similar to the CV although the oxidation current is not proportional to the band intensity. These results suggest that formic acid is oxidized via adsorbed formate and that CO adsorption/oxidation and surface oxidation/reduction greatly affect the formic acid oxidation by blocking the adsorption of formate. The (bi)sulfate band is observed at 0.4-0.75 V only on the negative potential sweep where CO is absent and the formate coverage is not so high, implying that (bi)sulfate is adsorbed less strongly than CO and formate. Chen et al.28 showed clearly that (bi)sulfate is desorbed from the surface by the adsorption of formate. The baseline of the spectrum can give additional information on some reactions involved. The closed circles in Figure 3e represent the potential dependence of the baseline level at 2500 cm-1 taken from Figure 2, while the open circles in the same figure represent the corresponding data measured in the pure

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Figure 4. Cyclic voltammograms recorded in 0.5 M H2SO4 + 0.1 M HCOOH and the integrated band intensities of COL, COB, and formate taken from the sets of time-resolved spectra of the Pt electrode surface acquired simultaneously with the voltammograms. Potential sweep rates: (a) 5 and (b) 0.3 mV s-1. The dashed traces in panel a are the data for a sweep rate of 50 mV s-1 (same as in Figure 3).

electrolyte. In the absence of formic acid in the solution, the baseline decreases (that is, the reflectivity of the electrode increases) at potentials less positive than 0.4 V due to the adsorption of hydrogen17,29 and rises at potentials more positive than 0.8 V due to the oxidation of the electrode.17 The decrease in the baseline around 0.8 V on the negative-going sweep is due to the reduction of oxides on the electrode surface. By the addition of formic acid into the solution (0.1 M), the baseline shift by hydrogen adsorption is totally suppressed due to the adsorption of CO (panels b and c). The adsorbed CO scarcely changes the baseline in the double-layer potential range. An interesting finding in this figure is that surface oxidation is suppressed by the addition of formic acid. A similar result has been obtained also by quartz-crystal microbalance (QCM) measurements coupled to cyclic voltammetry.30 The adsorption of formate is believed to suppress surface oxidation by blocking the adsorption of water or OH, oxygen sources for surface oxidation. On the other hand, the reduction of surface oxides is scarcely affected by the addition of formic acid. The slightly higher baseline level at 0.4-0.7 V on the negative potential sweep may be due to the adsorption of formate. It is also noted that peak IV occurs during the reduction of surface oxides and not after the complete reduction. Therefore, peak IV cannot be explained simply by the appearance of a clean surface; rather, the result suggests that some very active sites are created by surface reduction. The spectral features observed at the slower potential sweep rates of 5 and 0.3 mV s-1 were essentially identical to those at 50 mV s-1. The band intensity data for the slower potential sweep rates are shown in Figure 4 (a for 5 mV s-1 and b for 0.3 mV s-1) together with CVs recorded simultaneously. For comparison, the spectral data for the potential sweep at 50 mV

s-1 were replotted in Figure 4a by dashed curves by omitting data points for clarity. The intensities of the CO bands become larger as the sweep rate is decreased, indicating that full CO coverage is not reached at low potentials and the kinetics of CO formation is slow. The increase in CO coverage suppresses formate adsorption and can result in the suppression of peak I. It is also found that the potential at which CO is completely oxidized is shifted negatively as the sweep rate decreases (∼1, ∼0.8, and ∼0.7 V at 50, 5, and 0.3 mV s-1, respectively). The complete CO oxidation at less positive potentials enables the adsorption of formate at less positive potential and results in the negative shift of peak II. On the other hand, the potential of CO formation on the negative-going sweep is shifted positively as the potential sweep rate is decreased. As a result, the hysteresis of the CO band intensity versus potential curves becomes smaller. The sweep-rate dependence of CO oxidation/ formation reflects the slow kinetics of these processes. The emerging potential of the formate band on the negative-going potential sweep also depends on sweep rate (being shifted from ∼0.8 V at 5 and 50 mV s-1 to 0.95 V at 0.3 mV s-1). Since formate is not adsorbed on the fully oxidized surface as is found in the potential range of 0.8-1.4 V on the negative-going sweep, the shift of the emerging potential on the negative-going potential sweep can be ascribed to the slow kinetics of the surface reduction. 3.3. Isotopic Substitution of Adsorbed Formate. As shown in Figure 3, the formate band intensity reaches a maximum at 0.9 V at 50 mV s-1 where peak II appears. The coincidence of the peak potentials of the band intensity and oxidation current appears to be a good evidence for formic acid oxidation via formate. However, the coincidence is accidental. The formate band intensity plot for the positive-going sweep splits into two

IR study of Electrooxidation of HCOOH on Pt

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Figure 5. (a) Time-resolved SEIRA spectra representing isotopic substitution of adsorbed formate for a quick solution exchange from 0.5 M H2SO4 + 0.1 M H13COOH to 0.5 M H2SO4 + 0.1 M H12COOH at 0.7 V. The solution was changed at about 101 s after stepping up the electrode potential from 0.05 to 0.7 V (shown by arrow). Time resolution used was 80 ms. (b) Semilogarithmic plot of the band intensity of 13C-formate at 1305 cm-1 versus time after the solution exchange at 0.7 (b) and 1.0 V (O). Time was set at 0 when the 12C-formate band appeared, and the band intensities were normalized to that before the solution exchange for each measurement.

components at slower sweep rates (Figure 4). The low-potential component is shifted negatively as the sweep rate is decreased, while the high-potential component is not shifted. The vibrational frequency of the formate band was constant within the experimental accuracy (4 cm-1) over the potential range examined, and thus the adsorbed states of formate are identical for the two components. However, the two components can be clearly differentiated by the residence time of adsorbed formate on the surface. Figure 5 shows a set of 80-ms time-resolved SEIRA spectra in the 1250-1400 cm-1 region showing the spectral change caused by a quick solution exchange from H13COOH to H12COOH at a fixed potential of 0.7 V (the supporting electrolyte was 0.5 M H2SO4, and the formic acid concentration was 0.1 M for both solutions). The symmetric O-C-O stretching mode of 13C-formate at 1305 cm-1 is replaced by that of 12C-formate at 1323 cm-1 soon after the solution exchange at the time indicated by the arrow. The isotopic substitution of the adsorbed formate is approximately represented by the first-order kinetics as can be seen from the semilogarithmic plot of the band intensity against time as shown in Figure 5b (closed circles). From the slope of the plot, a rate constant of ∼3 s-1 is calculated. Since the rate of the isotopic substitution of adsorbed formate was very sensitive to the period of time required for full exchange of the solution, the rate constant might not be very accurate. Nevertheless, it is large enough to explain the current of 4 mA measured during the solution-exchange experiment by assuming the decomposition (oxidation) of formate being the rate-determining step in formic acid oxidation.10 In marked contrast, the isotopic substitution is 1 order of magnitude slower at 1.0 V (open circles) at which the surface is partially oxidized (Figure 3e). The slow isotopic substitution at the higher potential explains the decrease in oxidation current at the positive potential side of peak II. 3.4. SEIRAS Measurements Coupled to Chronoamperometry. Figure 6 shows chronoamperometric curves and the corresponding spectral data for potential steps from 0.1 V to the values indicated in the figure. Oxidation current keeps constant at 0.5 V in the time region examined, whereas it decreases at less positive potentials and increases at more positive potentials. The band intensity plots show that the oxidation and formation of CO are equilibrated at 0.5 V and the oxidation overwhelms the formation at higher potentials. The change in oxidation current is well correlated with those in the intensities of CO and formate bands. That is, as in the case of CV experiments, the decrement (increment) of CO

Figure 6. Chronoamperometric curves for potential steps from 0.1 V to the values indicated (top) and integrated band intensities of COL, COB, and formate plotted against the time after the potential steps. The spectral data were taken from sets of time-resolved SEIRA spectra acquired simultaneously with the corresponding chronoamperometric curves. Solution: 0.5 M H2SO4 + 0.1 M HCOOH; time resolution for spectral acquisition: 0.98 s.

coverage increases (decreases) formate coverage and results in the increment (decrement) of oxidation current. An exception is the result obtained at 0.8 V, where current decays gradually after reaching a maximum despite the constant formate band intensity. The decay may be due to the partial oxidation of the

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Figure 7. (a) Cyclic voltammogram for a 12CO-covered Pt electrode recorded in 0.5 M H2SO4 + 0.1 M HCOOH at a sweep rate of 50 mV s-1. The CO monolayer was established in the pure supporting electrolyte by bubbling CO gas at 0.05 V for 10 min, and the voltammogram was recorded after removing dissolved CO and adding H13COOH to be 0.1 M. (b) Selected SEIRA spectra of the electrode surface acquired during the voltammetric measurement. (c) Plot of the integrated band intensities of 12COL and 13C-formate against potential (solid traces, for positive-going sweep). The dashed trace represents the oxidative removal of a 12CO monolayer in the pure electrolyte.

Pt surface because surface oxidation delays the decomposition of formate as has already been shown in Figure 5b (open circles). The intensity plots of IR bands also show the presence of some fast processes. The sudden decrease in the COB band intensity at the very beginning implies that the site change of CO from bridge sites to atop sites occurs very quickly. The sharp increase in the formate band intensity soon after the potential step to 0.4 and 0.5 V indicates that the adsorption of formate is also very fast. The slower increment at higher potentials is apparently due to the gradual increase in vacant sites created by CO oxidation. 3.5. Contribution of the CO Path. Dual-path mechanism assumes that formic acid is oxidized to CO2 via both the reactive intermediate (formate) and CO. In a previous publication,10 we showed that the isotopic substitution of adsorbed CO for a solution exchange from H13COOH to H12COOH is very slow at 0.6 V where the formation and oxidation of CO are nearly equilibrated (Figure 6) and thus concluded that the contribution of the indirect path is negligible. A further support was obtained by examining the oxidation of H13COOH on a 12CO-covered Pt electrode, the results of which are shown in Figure 7. After having established a 12CO monolayer on a Pt electrode at 0.05 V by bubbling 12CO gas through 0.5 M H2SO4 and removed the remaining 12CO in the solution with Ar, H13COOH was injected into the solution to be 0.1 M at the same potential. Figure 7a shows the CV of the system recorded at a sweep rate of 50 mV s-1 without ohmic drop compensation. The CV is essentially identical to that shown in Figure 1 (topmost) recorded at the same sweep rate except for three features: total suppression of peak I, positive shift of peak II on the positive-going sweep, and a sharp spike at 0.82 V on the negative potential sweep. The former two features are due to the higher CO coverage (full coverage), and the last is due to the ohmic drop. In Figure 7b, several selected time-resolved spectra recorded simultaneously with the CV are shown. On the positive potential sweep, the 12CO bands at ∼2060 and ∼1860 cm-1 decrease in intensity by the oxidation of CO, and the 13C-formate band appears at 1305 cm-1. If the CO path were active, isotopic substitution of adsorbed CO should occur as in the case of formate (Figure 5). However, the 13CO bands appear at 2002 and ∼1780 cm-1 only on the negative-going sweep after the preadsorbed 12CO is totally oxidized. The result implies that

the CO-path is not active also in the peak II range. Accordingly, the term “dual-path mechanism” is not very suitable for formic acid oxidation although CO produced by the side reaction greatly affects formic acid oxidation indeed. In Figure 7c, the band intensities of the linear 12COL and formate are plotted as a function of the applied potential (for the positive scan only). The dashed trace in the figure represents the oxidative stripping of a 12CO monolayer established by CO bubbling and measured at the same sweep rate in the pure electrolyte (without both CO and formic acid). Adsorbed CO is completely oxidized at about 0.8 V in the pure electrolyte, whereas it can survive up to 1 V in the presence of formic acid in the solution. The delay of CO oxidation is not due to the supply of CO from formic acid in the solution as evidenced by the spectra shown in Figure 7b. The most plausible explanation of the positive shift of CO oxidation is that the adsorption of formate blocks sites for adsorption of oxygen species (H2O or OH) required for CO oxidation. 4. Discussion We did not detect any signals corresponding to adsorbed -COOH that has long been speculated to be the reactive intermediate in formic acid oxidation; instead we observed the adsorption of formate (Figure 2). Figures 3, 4, and 7 clearly show that an oxidation current flows only when formate is adsorbing on the electrode surface. The isotopic substitution experiment (Figure 5) demonstrates that adsorbed formate is continuously replaced by formate newly supplied from formic acid with a rate fast enough to explain the oxidation current. All the data strongly support that formic acid is oxidized to CO2 via adsorbed formate. Figure 6 (bottom) shows that formate coverage saturates very quickly, indicating that the adsorption is fast and equilibrated with the decomposition (oxidation) of formate. Thus, the decomposition of adsorbed formate is believed to be the rate-determining step in the direct path. Although the proposed reaction scheme via adsorbed formate is different from the general consensus (via COOH), it is worth noting that this reaction scheme is identical to that of formic acid oxidation on Pt in UHV and ambient gas phase (adsorbed CO is also formed as a byproduct).23 As found from the experimental data given in the former section, CO formation/oxidation and surface oxidation/reduction

IR study of Electrooxidation of HCOOH on Pt

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greatly affect formic acid oxidation by preventing formate adsorption and retarding the decomposition of formate. The voltammetric behavior of formic acid oxidation can basically be explained as follows. The slight oxidation of adsorbed CO on the positive-going sweep yields peak I by creating vacant sites for the direct path via formate, and the following faster CO oxidation at higher potentials further increases formate coverage and results in peak II. The oxidation of the electrode surface retards the formate decomposition, and thus, current decreases at the positive side of peak II. On the negative-going sweep, the reduction of the electrode surface provides active sites for the formate path to yield peaks IV and V. The CO adsorption decreases the current on the negative side of peak V by blocking active sites. The observed sweep rate dependence of CV (Figure 1) can be ascribed to the slow kinetics of CO formation/oxidation and surface oxidation/reduction. Masstransfer limitation is known to scarcely affect the voltammogram.14,15 The interplays of the involved processes have some additional effects on the kinetics. Figure 7c shows that CO oxidation is suppressed by the adsorption of formate, which in turn suppresses formic acid oxidation. Formate adsorption also suppresses the surface oxidation as can be seen from the baseline analysis shown in Figure 3e, which enables formic acid oxidation in the wave II region where the surface is easily oxidized in the pure electrolyte. Formate-formate and formate-CO interactions also can affect the kinetics of formic acid oxidation at the positive potential side of peak I. The decrease in current at the positive side of peak I (the so-called negative differential resistance, NDR) has mostly been explained by assuming the adsorption of some second poisoning species such as water, OH, and supporting anions.12-14,31,32 However these assumptions were made without knowing the fact that formate is adsorbed. Since formate is adsorbed on Pt more strongly than water and (bi)sulfate (and possibly OH), the latter species would not play dominant roles for the NDR. Very recently, we suggested that the NDR can be explained without assuming any poisoning species other than CO by the analogy with the decompositions of formate and acetate on metal surfaces in UHV. Several UHV studies have shown that formic acid and acetic acid are adsorbed on metal surfaces to yield formate and acetate, respectively, and the rates of the formate and acetate decompositions are remarkably suppressed at high coverage or by coadsorption of other species.33 The explanation for this phenomenon has been that the frustrated rotation of the adsorbed carboxylate species through which they are decomposed (Figure 8) is sterically hindered by the occupation of adjacent sites by coadsorbates including themselves.33d,e The kinetics of the decomposition is given by a nonlinear rate equation.33b,f By adopting the secondorder kinetics which assumes random occurrence of the reaction, the kinetics of the electrooxidation of formic acid is given by

i∝-

dθformate ) kθformate(1 - θCO - 2θformate) dt

(3)

where i is current, k is the rate constant, and θformate and θco are coverages of formate and CO (both COL and COB), respectively. The factor 2 of θformate reflects the occupation of two adjacent sites by formate. The parenthesis on the right side of the equation is the coverage of vacant sites and represents the necessity of adjacent vacant sites for formate decomposition. (Note that the isotopic exchange shown in Figure 5 is represented by firstorder kinetics since the coverage of vacant sites is constant during the measurement.) Since θformate is increased almost

Figure 8. Decomposition of formate, yielding CO2 through frustrated rotation. The scissoring of the C-H bond occurs when formate is largely tilted from the surface normal, and thus an adjacent vacant site is necessary for the decomposition of formate.

linearly with potential without changing θco so significantly in the peak I region (Figure 3), eq 3 predicts that the current increases first and then decreases as the potential is increased. That is, formate is the reactive intermediate but suppresses the decomposition of neighboring formate at high coverage. The kinetics equation also suggests that adsorbed CO suppresses formic acid oxidation not only by blocking active sites but also by retarding the decomposition of formate. Mokouyama et al.34 mathematically demonstrated that this kinetics model well simulates peak I. The last problem we should discuss is the origin of peak IV. Formic acid oxidation is accelerated by the reduction of surface oxides and gives peak IV. From QCM measurements coupled to cyclic voltammetry, Wilde and Zhang30 observed that the reduction of surface oxides is accelerated in the presence of formic acid and proposed that the reaction of formic acid with adsorbed OH or Pt-OH accelerates surface reduction and gives peak IV. However, it should be noted that their CV and QCM measurements were seriously affected by ohmic drops. The very sharp current spike at about 0.7 V (vs SCE) on the negativegoing sweep in their CV clearly shows the effect of ohmic drop (similar CVs were observed also in our measurements when ohmic drops were not compensated as shown in Figure 7a, for example). Since the actual potential across the interface (φ ) E - iR) jumps negatively at the current spike, the surface reduction should appear to be accelerated. In fact, our baseline analysis without ohmic drop (Figure 3e) shows that formic acid does not affect the reduction of surface oxides. Regarding this issue, it is worth noting that peak IV becomes larger as the positive limiting potential is increased.15,35 Since the placeexchange between surface Pt atoms and adsorbed OH (Pt-OH f OH-Pt f O-Pt) occurs at surface oxidation potentials36 and gives adatoms and holes on the reduction,37 it seems reasonable to assume that the adatoms and holes produced by surface reduction are active sites for formic acid oxidation in the peak IV region. The oxidation current at peak IV remarkably deceases as the potential sweep rate is reduced, suggesting that the active sites created by the reduction of surface oxide are not very stable and deactivated during the potential sweep. The reaction mechanism proposed in the present study can well explain several electrochemical measurements reported in the literature. In the context of the discussion above, a simple CV should be obtained if the effect of the surface activation is removed by employing very slow potential sweep or by limiting the potential sweep to the potentials where the place-exchange between surface Pt atoms and adsorbed OH does not occur. Okamoto et al.15 showed that a single peak appears around 0.64 V on both positive- and negative-going potential sweeps at very slow sweep rates (peaks corresponding to I and IV). The CV is asymmetric with a sharp decrement of current at the lower potential side of the peak, which is apparently due to the adsorption of CO. Lu et al.14 obtained a nearly symmetric single peak centered at 0.57 V by minimizing the effects of both CO

16566 J. Phys. Chem. B, Vol. 110, No. 33, 2006 adsorption and surface activation by employing a potential step technique. The symmetric peak can be well explained by eq 3 (by setting θco ) 0). The latter authors reported two additional results: (1) The Tafel slope at the negative potential side of the peak is approximately 120 mV/decade and (2) the oxidation current decreases when the electrolyte is changed from HClO4 to H2SO4. The Tafel slope of 120 mV/decade indicates that a one-electron transfer process is rate determining and is consistent with our proposed reaction mechanism (the decomposition of formate, eq 2, being rate determining). The anion effect apparently can be ascribed to the adsorption of (bi)sulfate that is stronger than that of perchlorate. Since (bi)sulfate is adsorbed less strongly than formate, the adsorption of (bi)sulfate does not interfere the adsorption of formate but can suppresses formate decomposition by blocking the adjacent vacant sites. Very recently, Chen et al.28 argued that the contribution of the formate path to the total oxidation current is less than 25% by assuming the oxidation current being proportional to formate coVerage (that is, the first-order kinetics) and proposed a third reaction path via an unknown intermediate that runs parallel to the CO and formate paths. The triple-pathway mechanism was proposed from the nonlinear relationship between the formate band intensity and oxidation current. However, the oxidation current is not necessarily proportional to the formate band intensity (coverage) because the coverage of the adsorbed reactive intermediate should be inversely proportional to the residence time on the surface (equivalently, the reaction rate). Furthermore, the nonlinear relationship can be explained by the second-order kinetics eq 3. Although further study on this issue is necessary, we did not obtain in the present study any data that support the triple-pathway mechanism. 5. Conclusion In situ ATR-SEIRAS coupled to cyclic voltammetry and chronoamperometry revealed that formic acid is oxidized to CO2 via adsorbed formate. The voltammetric behavior of formic acid oxidation is well explained by assuming that the rate of the decomposition (oxidation) of formate to CO2, the rate-determining step, depends on the coverage of formate. The coverage dependence arises from the necessity of adjacent vacant sites for the decomposition of formate. Essentially, formic acid oxidation exhibits a single current peak around 0.6 V but is greatly affected by the adsorption/oxidation of CO and surface oxidation/reduction through blocking the adsorption of formate and retarding the decomposition of adsorbed formate. The interplays of the involved processes also affect the CV. The sweep rate dependence of the CV is ascribed to the slow kinetics of CO adsorption/oxidation and surface oxidation/reduction. Acknowledgment. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Basic Research No. 18350038 and for Scientific Research on Priority Areas 417), Japan Science and Technology Agency, Research Institute of the Innovative Technology for the Earth, and the Asahi Glass Foundation. References and Notes (1) (a) Capon, A.; Parsons, R. J. Electroanal. Chem. 1973, 44, 239. (b) Beden, B.; Le´ger, J. M.; Lamy, C. In Modern Aspects of Electrochemistry; Bockris, J. O.’M., Conway, B. E., White, R. E., Eds.; Plenum Press: New York, 1992; Vol. 22; pp 97. (c) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 75. (d) Sun, S.-G. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; pp 243. (e) Markovic´, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117.

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