Dynamics of CO Oxidation on a Polycrystalline Platinum Electrode: A

shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. ... Forgot ACS...
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Dynamics of CO Oxidation on a Polycrystalline Platinum Electrode: A Time-Resolved Infrared Study Gabor Samjeske´, Kei-ichi Komatsu, and Masatoshi Osawa* Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan ReceiVed: January 20, 2009; ReVised Manuscript ReceiVed: March 18, 2009

Electro-oxidation of CO adsorbed on a polycrystalline Pt electrode in the potential region of hydrogen absorption is examined by fast time-resolved surface-enhanced infrared absorption spectroscopy coupled to voltammetry or chronoamperometry. Oxidation dynamics at a weak preoxidation peak around 0.5 V (vs RHE) and the main oxidation peak around 0.7 V observed in stripping voltammetry are focused. IR spectra show that the shift of bridge-bonded CO to atop sites triggers the partial oxidation of CO adsorbed at atop sites to yield the preoxidation peak. It is also shown that CO on terraces becomes very mobile in the main oxidation region after some amount of CO being oxidized via a nucleation-and-growth mechanism and that terrace CO is oxidized faster than CO adsorbed at step edges. The result is interpreted in terms of a Langmuir-Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species (most likely OH) adsorbed at steps. The oxidation mechanism is essentially identical to that proposed in some earlier studies, but more convincing spectroscopic evidence of the mechanism is presented. 1. Introduction Oxidation of CO on Pt has been a subject of extensive study due to its fundamental and practical importance. In direct methanol fuel cells (DMFCs), for example, CO is formed on Pt catalysts by dehydrogenation of methanol and inhibits methanol oxidation.1 Therefore, oxidative removal of adsorbed CO is essential to improve the performance of DMFCs. It is well established in heterogeneous catalysis that adsorbed CO is oxidized to CO2 via a Langmuir-Hinshelwood type mechanism involving adsorbed CO and an oxygen-containing species.2,3 In CO oxidation on the Pt(335) stepped surface, Yates and co-workers3 showed, by using IR reflection absorption spectroscopy (IRAS), that CO adsorbed on terraces is more readily oxidized than CO adsorbed at step edges. Since oxygen atoms, the oxygen source for CO oxidation, are preferentially adsorbed at steps, the result convincingly demonstrates the occurrence of the oxidation at steps by surface diffusion of terrace CO. This mechanism has been adapted also to electrocatalytic CO oxidation by replacing oxygen atom with OH that can be supplied from water (H2O + * f OHads + H+ + e-, where * stands for a free site on the surface).4-9 The increase in the apparent rate constant with the increase of defect sites, such as steps and kinks, suggests the preferential oxidation at defects.10 Fast surface diffusion of adsorbed CO, needed for the oxidation of CO at defects, has been suggested by potential step chronoamperometry,5-8,11 NMR,12 and surface-enhanced IR absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS).13 In situ, real-time monitoring of CO oxidation on stepped single-crystal8b and polycrystalline13 Pt surfaces by IRAS and ATR-SEIRAS, respectively, were interpreted in terms of this mechanism. However, the details of the reaction mechanisms proposed from the two independent IR studies are different. Lebedeva et al.8b concluded that terrace CO is preferentially oxidized at steps leaving CO at step edges behind, * To whom correspondence should be addressed. E-mail: osawam@ cat.hokudai.ac.jp. Fax: +81-11-706-9124.

while Kunimatsu et al.13 concluded that terrace CO diffuses to step edges and the step-edged CO is oxidized. The different conclusions indicate the need for further investigation. There still remains another question regarding the small preoxidation peak observed in stripping voltammetry at a potential about 0.2 V lower than the main CO oxidation peak. The preoxidation peak is observed only for high-coverage CO adlayers established at potentials in the hydrogen adsorption region (E < 0.2 V vs RHE).14-17 The origin of the preoxidation peak has been discussed extensively, and the following several explanations were given: preferential oxidation of bridge-bonded CO,13,14 oxidation of a weakly adsorbed state4,15 or kinetically unstable state,17 preferential oxidation at or near defect sites,7c,16,18 lowering of the activation energy at high CO coverage,19 and oxidation driven by a potential-induced rearrangement of CO.20 Regarding the last explanation, scanning tunneling microscopy (STM) on Pt(111) electrode surface showed that, in the presence of CO in the solution, the CO adlayer established at 0.05 V has a structure of p(2 × 2)-3CO (θCO ) 0.75) and is converted to (19 × 19)R23.4°-13CO (θCO ) 0.684) at E > 0.2 V.21 CO molecules are adsorbed at atop and threefold hollow sites in the former and at atop and bridge sites in the latter. The site conversation of the multibonded CO was confirmed by IRAS16a,21-24 and sum frequency generation (SFG).25 Although slightly different results were obtained by surface X-ray scattering (SXS) at high potentials,4a the potential-dependent CO coverage deduced by STM is in good agreement with that determined electrochemically.16a,24 Yoshimi et al.20 proposed that the change in the structure of the CO adlayer generates vacant sites for the adsorption of water and facilitates CO oxidation at the prepeak. However, an alternative interpretation that the partial oxidation results in the rearrangement of adsorbed CO also would be possible. As summarized, it is still not clear why and how CO is partially oxidized at such a low potential. In the present report, we discuss the dynamics of CO oxidation on a polycrystalline Pt electrode in 0.5 M H2SO4 at both the preoxidation and main oxidation peaks on the basis of fast time-resolved ATR-SEIRAS coupled to cyclic voltammetry

10.1021/jp900582c CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009

Dynamics of CO Electro-oxidation on Pt

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or potential step chronoamperometry. ATR-SEIRAS is more suitable than IRAS for dynamic studies owing to several advantages: higher sensitivity, free mass transport, and fast cell response.26 After describing the global features of CO oxidation under potential sweep conditions, the origin of the preoxidation peak is discussed first and then CO oxidation dynamics at higher potentials is examined. Finally, we propose a mechanistic model for CO oxidation in which terrace CO is oxidized first at defect sites and step-edge CO is oxidized later. The model is essentially identical to that proposed by Lebedeva et al.,8b,c but more convincing spectroscopic evidence of the mechanism is presented. 2. Experimental Section Experimental details of ATR-SEIRAS were described elsewhere.26-28 Briefly, a thin (∼50 nm) Pt film chemically deposited on the total reflecting plane of a 60° triangular Si ATR prism29 was used as the working electrode. Infrared radiation from a FT-IR spectrometer (Bio-Rad FTS-60A/896) was focused at the electrode/solution interface through the prism at an incident angle of 60°, and the totally reflected beam was detected with a liquid-N2-cooled MCT detector (Bio-Rad). The spectrometer was operated at a spectral resolution of 4 cm-1 and time resolution of 80 ms (single scan) except otherwise noted. Spectra are shown in absorbance units defined as -log(I/I0), where I and I0 represent spectra of the CO-covered and CO-free surfaces, respectively. The electrochemical cell was a glass one with a Pt mesh counter electrode and a trapped hydrogen electrode (reversible hydrogen electrode, RHE).28 The Pt-coated Si prism was attached to the cell by sandwiching an O-ring (i.d. of 1.4 cm). All potentials are given with respect to RHE. The electrolyte solution (0.5 M H2SO4) was prepared from Milli-Q water (>18 MΩ) and H2SO4 (Suprapure, Merck). After deoxygenating of the solution with Ar, the electrode surface was cleaned by cycling the electrode potential between 0.05 and 1.5 V until a stable voltammogram was obtained. After a reference spectrum of the clean surface at 0.05 V was taken, the solution was bubbled with CO gas at 0.05 V for 10 min to establish a CO adlayer on the Pt surface, and finally CO dissolved in the solution was purged by Ar bubbling. Electrochemical measurements were carried out with a potentiostat/ galvanostat (EG&G PAR model 263A). Current densities are given with respect to the real surface area calculated from the charge for underpotential deposition of hydrogen (Hupd) by assuming 210 µC cm-2 for monolayer adsorption. 3. Results and Discussion 3.1. General Features of CO Oxidation under Potential Sweep Conditions. Figure 1 (solid trace) shows a cyclic voltammogram (CV) for a CO-covered Pt thin film electrode recorded at 50 mV s-1, which exhibits a weak peak at 0.5 V (preoxidation peak) and a strong peak at 0.72 V (main peak) with a shoulder at 0.8 V. The coverage of the preadsorbed CO is calculated to be 0.9 by integrating the CO oxidation peaks after subtracting the CV for the successive second potential sweep (i.e., the CO-free surface, dotted trace in the figure) as the background. The CV for the CO-free Pt film electrode is very similar to those for bulk polycrystalline Pt electrodes.30 A set of time-resolved SEIRA spectra of the CO-covered Pt electrode surface acquired simultaneously with the CV is shown in Figure 2. Although the time resolution used was 80 ms (i.e., 4 mV intervals), spectra at every 20 mV interval were plotted in the figure for clarity. The very strong band at 2050-2085 cm-1 and a broad, weaker band at 1845-1880 cm-1 are assigned

Figure 1. Cyclic voltammogram for a CO-covered Pt electrode in COfree 0.5 M H2SO4 at 50 mV s-1. Solid trace, first sweep; dotted trace, successive second sweep. The CO monolayer was established at 0.05 V, and then dissolved CO in the solution was purged with Ar prior to the measurement.

to the C-O stretching modes of linearly bonded CO at atop sites (COL) and bridge-bonded CO (COB), respectively. These bands disappear at about 0.7 V due to the oxidative stripping of adsorbed CO. Concomitant with the CO stripping, the band at 3640 cm-1, assigned to the O-H stretching mode of water on top of the CO adlayer,29a,31 disappears and the S-O stretching modes of (bi)sulfate adsorbed on the Pt surface32 emerge at ∼1200 and 1150 cm-1. The negative-going peaks at 3500 and 1610 cm-1 are the O-H stretching and H-O-H bending modes, respectively, of water that was removed from the surface by the adsorption of CO.29a,31 To facilitate the comparison of the IR spectra with the CV, the integrated band intensities of COL, COB, and (bi)sulfate are plotted in Figure 3a as a function of applied potential. The intensity of adsorbed (bi)sulfate shown here is the sum of the two bands at ∼1200 and 1150 cm-1, which is nearly proportional to the coverage of (bi)sulfate.31 Starting from 0.05 V, the band intensity of COL is constant up to 0.5 V and slightly increases at the preoxidation peak despite the partial (∼10%) oxidation, which is followed by a quick decrease at E > 0.6 V. On the other hand, the band intensity of COB decreases at 0.3-0.5 V and also at E > 0.75 V. Comparison with the CV (black, solid trace in the figure) suggests that the main CO oxidation peak at 0.72 V and the shoulder at the positive side chiefly arise from the oxidation of COL and COB, respectively. The band intensity of (bi)sulfate almost saturates before COB oxidation, implying that the coverage of COB is small. The origin of the prepeak is discussed in the next section. Figure 3b is a plot of the vibrational frequency of COL against the applied potential. The vibrational frequency of COB also is potential dependent but not shown in the figure because the band center is unclear due to the broad, asymmetric shape of this band. As is well-known, the vibrational frequency of COL is blue-shifted as the potential is made more positive at a rate of 30 cm-1 V-1. The potential-dependent shift of the vibrational frequency is explained in terms of the electric field effect at the interface (the vibrational Stark tuning effect)33,34 and/or the change in the back-donation of electrons from the metal to the 2π* orbital of adsorbed CO.35 The frequency starts to deviate from the linear dependence at 0.45 V and becomes constant in the preoxidation peak region. At higher potentials from 0.6 to 0.66 V, the frequency is blue shifted again at a slightly smaller rate of 18 cm-1 V-1 and then rapidly red shifted associated with the rapid CO oxidation at higher potentials. The red shift associated with the oxidation is ascribed to the decrease in the dipole-dipole interaction among COL molecules.36,37 Interac-

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Figure 2. Eighty-millisecond time-resolved IR spectra of the Pt surface acquired simultaneously with the CV in Figure 1 (first sweep, solid trace). Only the spectra at every 0.4 s (i.e., every 20 mV) interval are shown for clarity. A spectrum of the CO-free surface collected at 0.05 V before bubbling the solution with CO was used as the reference.

Figure 4. Plot of the integrated band intensities of COL and COB as a function of the total CO coverage, θCO.

Figure 3. Potential dependence of the integrated band intensities of COL (red), COB (blue), and (bi)sulfate (green) (a), and of the vibrational frequency of COL (b). All data, including the CVs shown in (a) with black solid and dotted traces, were taken from the experimental data in Figures 1 and 2.

tions of CO molecules through the surface (electronic interactions with the surface) also can shift the vibrational frequency, but this effect is smaller than the dipole coupling.36-38 In Figure 4, the intensities of the COL and COB bands in Figure 3a are replotted as a function of the total CO coverage (θCO). The intensity of the COL band is proportional to θCO at θCO < 0.8 and decreases at higher θCO. On the other hand, the COB intensity is nearly constant up to θCO ∼ 0.8 and increases at higher θCO. The result is very similar to that reported by Heinen et al.39 The nonlinear relationship between the band intensity and θCO at high θCO has been observed also in UHV and ascribed to the screening effect by the electronic polarizability of CO.40 3.2. Origin of the Preoxidation Peak. The preoxidation peak completely disappears after adsorbed CO was partially oxidized by scanning the potential up to 0.6 V and back to 0.05 V as shown in Figure 5a (red trace). The same behavior has been observed also on Pt single-crystal surfaces.4 The partial oxidation reduces the frequency of the COL band by 6 cm-1 and expands

the linear potential-dependent region of this band up to 0.66 V (Figure 5c). The results suggest that the CO adlayer established at 0.05 V is a highly compressed metastable phase and relaxed to a more stable, less compressed one by the partial oxidation at the prepeak. Consistent with the data shown in Figures 3 and 4, the intensity of the COL band increases and that of the COB band decreases associated with the partial oxidation (Figure 5b). Kunimatsu and co-workers13,14 ascribed the prepeak to the preferential oxidation of COB from the decrease of the COB band intensity. However, it is well-known that COB is shifted to atop sites as the potential is made more positive due to the weakening of the Pt-CO bonding.41 Heinen et al.39 explained the increase of the COL intensity and the decrease of the COB intensity, observed for a potential step from 0.06 to 0.6 V, in terms of the site conversion. However, the red shift of the COL vibration by the partial oxidation (Figure 5c) indicates the decrease of the COL-COL dipole coupling, i.e., the decrease of the partial coverage of COL. Note that the COL band intensity increases as the coverage is reduced at high θCO as shown in Figure 4. Apparently, both explanations are insufficient. We show below that the spectral changes at the prepeak arise from the two different processes. Figure 6 shows the transients of the integrated intensities of the COL and COB bands and the peak frequency of the COL band after stepping the potential from 0.05 to 0.6 V. The intensity of COB is suddenly dropped by the potential step (within 80 ms, the time-resolution used) and then further reduced slightly, while the COL band intensity gradually increases with

Dynamics of CO Electro-oxidation on Pt

Figure 5. (a) CV for a CO-covered Pt electrode in 0.5 M H2SO4 at 50 mV s-1. (b, c) Changes in the integrated intensities of the COL and COB bands, and in the vibrational frequency of COL, respectively, measured during the potential cycling in (a) with a time resolution of 0.4 s. Blue curves and dots corresponds to the first cycle reversed at 0.6 V, and red curves and dots corresponds to the second cycle.

Figure 6. Changes in the integrated intensities of the COL (red) and COB (blue) bands and in the frequency of the COL vibration, ν(COL), for a potential step from 0.05 to 0.6 V. Time resolution used for IR spectroscopy was 80 ms.

some delay accompanied by a gradual red shift from 2085 cm-1. It is worth noting that the frequency of the COL vibration soon after the potential step (2085 cm-1) is identical to that measured by extrapolating its linear potential dependence to 0.6 V (Figure 3b); that is, COL is scarcely affected soon after the potential step. The results show that the dynamics at 0.6 V is composed of two processes: an initial very fast process that decreases the intensity of the COB band and a following slow process that increases the intensity of the COL band. The latter slow process is apparently the oxidation of COL as is found from the gradual red shift of the COL vibration. In fact, Heine et al.39 showed that COL is preferentially oxidized for a prolonged oxidation at 0.6 V. The decrease of the COB band intensity clearly shows the decrease in the partial coverage of COB. Nevertheless, the band intensity starts to decrease at 0.3-0.4 V where CO oxidation is negligible (Figure 3). Heinen et al.39 also observed the decrease of the COB intensity at 0.3 V in their potential step

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Figure 7. Selected time-resolved IR spectra of the Pt electrode collected during CO stripping at 50 mV s-1. Potential range: (a) 0.63-0.71 V, (b) 0.72-0.75 V, and (c) 0.76-0.79 V. Red traces connecting peak maxima are only for eye guidance. Time resolution: 80 ms.

experiment. It should also be noted that the decrease of this band is as fast as the bridge-to-atop site conversion,41a and too fast to be ascribed to the oxidation of COB (it takes a few seconds for both COL and COB oxidation in the main oxidation region as shown in Figures 3 and 5, where 0.1 V scan requires 2 s). The site conversion model proposed by Heinen et al.39 is plausible in this sense for the initial very fast process. However, no concomitant very fast spectral changes expected from the site conversation are observed for COL. The puzzle can be solved by assuming that COB is kicked away from the surface (i.e., desorbed) by the repulsive interaction with preadsorbed COL before reaching to atop sites. Desorption of COB generates vacant sites, which enables the adsorption of OH or water and can result in the oxidation of COL. The partial oxidation of COL generates further vacant sites and enables the surface diffusion of adsorbed CO so to reduce the intermolecular repulsion, by which the relaxation of the initial compressed CO adlayer to a less compressed, more stable phase becomes possible. Since the stabilization of the CO adlayer retards CO oxidation, the partial CO oxidation can give a peak in the CV. Accordingly, we ascribe the preoxidation peak to the partial oxidation of COL triggered by the potential-induced site conversion of COB. 3.3. CO Oxidation at the Main Peak. After passing through the preoxidation peak, oxidation current starts to increase again at 0.6 V and the band intensity of COL starts to decrease, while the frequency of the COL vibration continues to be blue shifted up to 0.66 V (Figure 3). The blue shift indicates that the CO adlayer is oxidized without changing dipole-dipole interaction, i.e., via a (hole) nucleation-and-growth mechanism (a LangmuirHinshelwood mechanism without or with negligible surface diffusion of CO).14,37 At higher potentials, the band intensity of COL quickly decreases accompanied by a red shift, indicating that the kinetics of CO oxidation at higher potentials is different from that at lower potentials. To see the oxidation process at the higher potentials in more detail, IR spectra in the potential range of 0.63-0.79 V, selected from Figure 2, are displayed in Figure 7. Initially, the COL band is asymmetric with a broad tail extending toward the lowfrequency side. The low-frequency tail disappears first (shown by a blue thick arrow). Kunimatsu et al.13 assigned the main

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peak and the tail to COL adsorbed on terraces and at step edges, respectively. However, this assignment is questionable because CO adsorbed at step edges should be invisible at high θCO due to the intensity transfer to terrace CO.3,38,42-45 In fact, step-edge CO was identified at low CO coverage, as will be shown later. It should also be noted that an absorption band is symmetric only when adsorbed molecules are perfectly ordered or homogeneously dispersed over the surface by repulsive intermolecular interactions (i.e., lattice-gas).36 Perfect order of CO is not conceivable on polycrystalline surfaces with many defects, and hence we assign the low-frequency tail to terrace COL in the near vicinity of defects where dipole coupling is reduced and interpret the disappearance of the tail that CO oxidation is initiated at terrace sites near defects. With further increase of the potential, the COL vibration shifts to lower frequencies accompanied by noticeable broadening and weakening. It is worth noting that this band becomes symmetric in the potential range a (0.65-0.71 V). The symmetric broad shape is indicative of homogeneous dispersion of COL over the surface,36 and the red shift indicates the increase of intermolecular spacing, which signals fast surface diffusion of CO. In the higher potential range b (0.72-0.75 V), a shoulder appears at the lower frequency side of the band, which increases in intensity and shifts to higher frequencies with the increase of potential (i.e., with decrease of θCO). The red-shifted highfrequency component and the blue-shifted low-frequency component are merged at around 0.75 V to yield a single broad symmetric band, which is weakened and shifts to an asymptotic value of 2060 cm-1 (the vibrational frequency of singleton) at more positive potentials. COB exhibits a similar behavior as COL. The broad envelope of the COB band is composed of two components at 1880 and 1820 cm-1 at 0.63 V. The higher frequency component is red shifted, and the lower frequency component is blue shifted with increasing potential, and eventually they are merged into a single band at around 0.73 V. The spectral changes of the COB vibration are not very clear in this figure but were observed more clearly with a time resolution of 0.4 s (averaging of five spectra) owing to the better signal-to-noise ratio (Supporting Information). The spectral features observed in Figure 7 are very similar to the coverage-dependent spectra of CO adsorbed on stepped Pt surfaces in UHV3,42,44 and also under electrochemical environment8b,38,45 except for the difference in vibrational frequency arising from differences in environment, surface structure, and/or applied potential. Hence the assignments of the observed bands are straightforward. The higher and the lower frequency components for both COL and COB are ascribed, respectively, to CO adsorbed on terraces and at step edges.38,42-46 The spectra in Figure 7 clearly demonstrate that terrace CO is oxidized more readily than step-edge CO. Although step-edge COL is invisible at high coverage, this does not mean the absence of step-edge COL. Taking into account that CO is adsorbed preferentially at defects,3,8b,38,42,45 the missing of the band corresponding to step-edge COL should be interpreted that the intensity is merely diminished by the intensity transfer at high θCO as supported by theory.43,44 On the other hand, the coverage of COB is much smaller than that of COL, and hence the stepedge COB can be observed even at full coverage owing to less intensity transfer to terrace COB. 3.4. Potential Step Chronoamperometry. The preferential oxidation of terrace CO and the broadening and red shift of the COL band in the main oxidation region are consistent with a Langmuir-Hinshelwood type mechanism for CO oxidation. In earlier studies, this mechanism has been examined mostly by

Samjeske´ et al.

Figure 8. (a) Current transient for a potential step from 0.6 to 0.7 V recorded after holding the potential at 0.6 V for 10 s. Red dotted trace is the result of fitting to the mean-field approximation (eq 1). (b) Change in the integrated band intensities of the COL (red), COB (blue), and (bi)sulfate (green) taken from the time-resolved IR spectra acquired simultaneously with (a) (Figure 9).

potential step chronoamperometry on stepped surfaces.5-8 To confirm that the mechanism is operative also on polycrystalline surfaces, we also employed potential step chronoamperometry. Figure 8a (solid trace) shows the current transient for a potential step from 0.6 to 0.7 V recorded after holding the potential at 0.6 V for 10 s for avoiding the complexity arising from the partial oxidation at the prepeak and also for using the linear relationship between the intensity of the COL band and θCO (Figure 4). As shown by the dotted trace in the figure, the observed current transient is reasonably fitted to the mean-field approximation for the Langmuir-Hinshelwood mechanism6-8

i(t) )

Qk exp(-k(t - tmax)) [1 + exp(-k(t - tmax))]2

(1)

where i(t) is the current transient, Q is the total charge for CO oxidation, k is the reaction rate constant, and tmax is the time at which the maximum current is observed in the transient. Note that the mean-field approximation model assumes fast surface diffusion and homogeneous spatial distribution of adsorbed CO (i.e., lattice gas). No reasonable fits were obtained by spontaneous and progressive nucleation-and-growth kinetics. IR spectra selected from a set of 80-ms time-resolved spectra acquired simultaneously with the current transient are shown in Figure 9. The changes in spectral features are essentially identical to those observed under potential sweep conditions (Figure 7), and the broadening and red shift of the COL band are found to occur very quickly. Both the electrochemical and SEIRAS measurements consistently demonstrate a LangmuirHinshelwood type mechanism. We should address a comment on the slight deviation of the mean-field approximation form the measured current transient in the longer time region in Figure 8a. Such a tailing is often observed and was explained in terms of the long surface diffusion of terrace CO to steps.8c This explanation may be reasonable, but it is noted that COL is oxidized faster than COB

Dynamics of CO Electro-oxidation on Pt

Figure 9. Time-resolved IR spectra of the Pt electrode for a potential step from 0.6 to 0.7 V recorded with a time resolution of 80 ms. Spectra at every 0.4 s interval are shown for clarity. The CO monolayer was established at 0.05 V and held at 0.6 V for 10 s before the potential step.

J. Phys. Chem. C, Vol. 113, No. 23, 2009 10227 to occur at inner corners of steps. The concentration gradient of CO on terraces generated by the oxidation at steps and also repulsive intermolecular interactions facilitate the diffusion of terrace CO to steps, at which the oxidation occurs continuously with adsorbed OH repeatedly supplied from water. After terrace CO being oxidized almost completely, step-edge CO is finally oxidized at the steps or by diffusing over terraces to other steps.8b,c The two possibilities for step-edge CO oxidation cannot be discussed by IR because the singleton frequencies of CO at terrace and step-edge sites are very close.38,42,43 CO oxidation may occur also at kinks, but it could not be clarified in the present study because no bands for CO adsorbed at kink sites were observed in the expected spectral range lower than 1800 cm-1.47 The reaction mechanism above is essentially identical to that of Lebedeva et al.8b,c proposed from electrochemical and IRAS experiments on well-defined stepped surfaces. However, it should be noted that any clear spectroscopic evidence of the fast CO diffusion (the homogeneous dispersion of terrace CO) was not obtained in their IRAS study8b because their oxidation experiments were conducted at low θCO (0.33), under which the broadening and systematic red shift of the terrace CO vibrations induced by surface diffusion cannot be observed. We guess that the experimental condition was intentionally designed for observing the step-edge COL band clearly. Very recently, Kunimatsu and co-workers13 examined CO oxidation on a polycrystalline Pt with ATR-SEIRAS and concluded that terrace CO is oxidized after diffusing to step edges. The conclusion different from ours comes from the different assignment of the COL band that is broadened and red shifted associated with the oxidation. That is, they assigned this band to step-edge CO and ascribed the decrease of this band to the oxidation of step-edge CO, while we assigned it to terrace CO and ascribed the decrease of this band to the preferential oxidation of terrace CO. As has been discussed in section 3.3, our assignment is consistent with earlier IR studies using welldefined stepped surfaces,3,38,42-46 and the vibration of COL at step edges locates at a lower frequency. If terrace CO were oxidized after diffusing to step edges as they proposed, CO oxidation should not obey the Langmuir-Hinshelwood type kinetics assuming homogeneous CO distribution. 4. Conclusion

Figure 10. Proposed mechanism for CO electro-oxidation on Pt. A polycrystalline surface was modeled by a (111) terrace and a (110) step. The hatched circle represents a Pt atom in the step through. See text for details.

as is evident from the intensity transients of the COL and COB bands shown in Figure 8b taken from Figure 9. Accordingly, the different oxidation rates of COL and COB also can be a reason for the tailing. 3.5. Mechanistic Model for CO Oxidation. As a summary of the previous section, a mechanistic model of CO oxidation deduced from the electrochemical SEIRAS study is illustrated in Figure 10, where the polycrystalline surface is modeled by a stepped surface with (111) terraces and (110) steps, the most active step toward CO oxidation.8b,9 The oxygen source for CO oxidation (OH is generally assumed) is very likely to be adsorbed preferentially at steps (troughs of (110) step or inner corners of other steps),8,10 although no spectroscopic evidence exists for this. Terraces and step edges are fully occupied by CO initially, and the oxidation of terrace CO is initiated at steps as evidenced by the quick disappearance of the low-frequency tail of the COL band ascribed to terrace CO near defects (Figure 7). Since step-edge CO is less active, CO oxidation is believed

Electro-oxidation of CO on a polycrystalline Pt electrode at the preoxidation and main oxidation peaks was examined by ATR-SEIRAS. The high time resolution (80 ms) allowed us to analyze the CO oxidation dynamics. The dynamics at the prepeak is composed of an initial very fast process and a successive slow process. The potential-induced site conversion of COB results in the COB desorption in the initial process, while COL is oxidized in the successive slow process. The oxidation of COL facilitates the surface diffusion of adsorbed CO, by which the initially formed highly compressed metastable CO adlayer is relaxed to a less compressed, more stable adlayer to reduce repulsive intermolecular repulsion. At the main oxidation peak, CO is oxidized initially via a nucleation-and-growth mechanism, and then the kinetics shifts to a LangmuirHinshelwood type one owing to the faster diffusion of CO at lower coverage. The fast CO diffusion was manifested itself by broadening and red shift of the terrace COL band. In the Langmuir-Hinshelwood process, CO on terraces is oxidized faster than CO at step edges. Acknowledgment. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan

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(Grant-in-Aid for Basic Research No. 18350038 and Grant-inAid for Scientific Research on Priority Area “Strong PhotonMolecule Coupling Fields (No. 470)”), and also by The Iwatani Naoji Foundation. G.S. acknowledges Japan Society for the Promotion of Science (JSPS) for a Postdoctoral Fellowship for Foreign Researchers (No. 18 · 06343). Supporting Information Available: Time-resloved IR pectra for CO oxidation measured with a time resolution of 0.4 s. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998. (2) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994. (3) (a) Yates, J. T. J. Vac. Sci. Technol., A 1995, 13, 1359. (b) Xu, J. Z.; Henriksen, P.; Yates, J. T. J. Chem. Phys. 1992, 97, 5250. (c) Xu, J. Z.; Yates, J. T. J. Chem. Phys. 1993, 99, 725. (4) (a) Markovic´, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. B 1999, 103, 487. (b) Markovic´, N. M.; Lucas, C. A.; Grgur, B. N.; Ross, P. N. J. Phys. Chem. B 1999, 103, 9616. (5) (a) Petukhov, A. V.; Akemann, W.; Friedrich, K. A.; Stimming, U. Surf. Sci. 1998, 404, 182. (b) Maillard, F.; Eikerling, M.; Cherstiouk, O. V.; Schreier, S.; Savinova, E.; Stimming, U. Faraday Discuss. 2004, 125, 357. (6) Bergelin, M.; Herrero, E.; Feliu, J. M.; Wasberg, M. J. Electroanal. Chem. 1999, 467, 74. (7) (a) Koper, M. T. M.; Jansen, A. P. J.; van Santen, R. A.; Lukkien, J. J.; Hilbers, P. A. J. J. Chem. Phys. 1998, 109, 6051. (b) Koper, M. T. M.; Lebedeva, N. P.; Hermse, C. G. M. Faraday Discuss. 2002, 121, 301. (c) Housmans, T. H. M.; Hermse, C. G. M.; Koper, M. T. M. J. Electroanal. Chem. 2007, 607, 69. (8) (a) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2002, 524, 242. (b) Lebedeva, N. P.; Rodes, A.; Feliu, J. M.; Koper, M. T. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 9863. (c) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938. (9) Samjeske´, G.; Xiao, X. Y.; Baltruschat, H. Langmuir 2002, 18, 4659. (10) (a) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. Electrochem. Commun. 2000, 2, 487. (b) Lebedeva, N. P.; Koper, M. T. M.; Herrero, E.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2000, 487, 37. (11) Gutierrez, A. C.; Pinheiro, A. L. N.; Leiva, E.; Gonza´lez, E. R.; Iwasita, T. Electrochem. Commun. 2003, 5, 539. (12) (a) Kobayashi, T.; Babu, P. K.; Gancs, L.; Chung, J. H.; Oldfield, E.; Wieckowski, A. J. Am. Chem. Soc. 2005, 127, 14164. (b) Kobayashi, T.; Babu, P. K.; Chung, J. H.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. C 2007, 111, 7078. (13) Kunimatsu, K.; Sato, T.; Uchida, H.; Watanabe, M. Electrochim. Acta 2008, 53, 6104. (14) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R. Langmuir 1986, 2, 464. (15) Wieckowski, A.; Rubel, M.; Gutie´rrez, C. J. Electroanal. Chem. 1995, 382, 97. (16) (a) Lopez-Cudero, A.; Cuesta, A.; Gutie´rrez, C. J. Electroanal. Chem. 2005, 579, 1. (b) Lopez-Cudero, A.; Cuesta, A.; Gutie´rrez, C. J. Electroanal. Chem. 2006, 586, 204. (c) Cuesta, A.; Couto, A.; Rinc´on, A.; Pe´rez, M. C.; Lopez-Cudero, A.; Gutie´rrez, C. J. Electroanal. Chem. 2006, 586, 184.

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