In Situ, Time-Resolved Reflectance Spectroscopy in the Microsecond

In Situ, Time-Resolved Reflectance Spectroscopy in the Microsecond. Domain: Oxidation of Adsorbed Carbon Monoxide on Polycrystalline. Pt Microelectrod...
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Langmuir 2006, 22, 10389-10398

10389

In Situ, Time-Resolved Reflectance Spectroscopy in the Microsecond Domain: Oxidation of Adsorbed Carbon Monoxide on Polycrystalline Pt Microelectrodes in Aqueous Solutions† Ping Shi, Iosif Fromondi, and Daniel A. Scherson* Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078 ReceiVed May 26, 2006. In Final Form: August 4, 2006 The dynamics of the electrooxidation of adsorbed CO, COads, on polycrystalline Pt microelectrodes has been examined in CO-saturated 0.5 M H2SO4 and 0.5 M HClO4 aqueous solutions, using in situ, time-resolved, normalized differential reflectance spectroscopy (λ ) 633 nm). Attention was focused on the unique dependence of COads oxidation on the potential at which the adsorbed full CO monolayer is assembled (i.e., hydrogen adsorption/desorption vs the double-layer region) using both fast linear scan voltammetry and potential step techniques. As evidenced from the data collected, COads oxidation at a fixed potential proceeds at slower rates when the monolayer is formed in the doublelayer region compared to when it is formed in the hydrogen adsorption/desorption region. Possible explanations for this effect are discussed.

Introduction Applications of in situ, time-resolved spectroelectrochemical and structural techniques to the study of electrode/electrolyte interfaces down to the millisecond domain have reached a relatively high level of maturity over the past few years. Particularly noteworthy are recent advances in the areas of Fourier transform infrared (FTIR)1-3 and UV-vis reflectance spectroscopy,4-7 potential step surface diffraction,8 EXAFS,9 and highspeed video scanning tunneling microscopy (STM)10-12 as temporal probes of interfacial events. The development and implementation of methods for the study of much faster heterogeneous electron-transfer processes require careful analysis of the factors that govern the rates at which interfacial potential control can be achieved. Some of these issues have been discussed by McCreery et al.,13 Amatore et al.,14 and Xu15 in connection with reactions involving solution phase and adsorbed species. In particular, an ideal polarizable electrodeelectrolyte interface may be represented by a capacitor, C, due †

Part of the Electrochemistry special issue.

(1) Samjeske, G.; Miki, A.; Ye, S.; Yamakata, A.; Mukouyama, Y.; Okamoto, H.; Osawa, M. J. Phys. Chem. B 2005, 109, 23509-23516. (2) Zhou, Z. Y.; Tian, N.; Chen, Y. J.; Chen, S. P.; Sun, S. G. J. Electroanal. Chem. 2004, 573, 111-119. (3) Miki, A.; Ye, S.; Senzaki, T.; Osawa, M. J. Electroanal. Chemistry 2004, 563, 23-31. (4) Sagara, T.; Fukuda, M.; Nakashima, N. J. Phys. Chem. B 1998, 102, 521527. (5) Brevnov, D. A.; Finklea, H. O. J. Electrochem. Soc. 2000, 147, 34613466. (6) Yamada, T.; Nango, M.; Ohtsuka, T. J. Electroanal. Chem. 2002, 528, 93-102. (7) Flatgen, G.; Krischer, K.; Ertl, G. J. Electroanal. Chem. 1996, 409, 183194. (8) Tamura, K.; Wang, J. X.; Adzic, R. R.; Ocko, B. M. J. Phys. Chem. B 2004, 108, 1992-1998. (9) Rose, A.; South, O.; Harvey, I.; Diaz-Moreno, S.; Owen, J. R.; Russell, A. E. Phys. Chem. Chem. Phys. 2005, 7, 366-372. (10) Magnussen, O. M.; Polewska, W.; Zitzler, L.; Behm, R. J. Faraday Discuss. 2002, 121, 43-52. (11) Magnussen, O. M.; Zitzler, L.; Gleich, B.; Vogt, M. R.; Behm, R. J. Electrochim. Acta 2001, 46, 3725-3733. (12) Labayen, M.; Haak, C.; Magnussen, O. M. Phys. ReV. B 2005, 71. (13) Robinson, R. S.; McCreery, R. L. J. Electroanal. Chem. 1985, 182, 6172. (14) Amatore, C.; Maisonhaute, E.; Simonneau, G. J. Electroanal. Chem. 2000, 486, 141-155. (15) Xu, C. Ph.D. Dissertation, Department of Chemistry, University of Illinois at Urbana-Champaign, 1992.

to the double layer, in series with the cell resistance, R, due to the electrolyte. For a disk electrode of radius ro embedded in an insulating surface, theory predicts that the RC constant of the cell is proportional to ro (e.g., for ro ) 12.5 µm), such as for the disk used in the experiments to be described in this work, and assuming reasonable values for the electrolyte conductivity, RC is on the order of a fraction of a microsecond, which is thus shorter than the rise time of commercial potentiostats. Efforts in our laboratory seek to combine in situ spectroscopic techniques with microelectrodes as a first step toward monitoring interfacial dynamics in the submicrosecond domain. Attention has been centered on the use of second harmonic generation (SHG),16,17 and, more recently, reflectance spectroscopy18 for studies involving the oxidation of adsorbed CO, COads, on single-crystal Pt(111) microfacets and on Pt(poly), respectively. Microelectrodes offer, in addition, rates of mass transport that can exceed those of conventional convective electrodes, such as rotating disks (RDE) and channels (or tubes). In particular, theoretical calculations show that the mass-transport coefficient for a static disk microelectrode of ro ) 5 µm is equivalent to that of an RDE rotating at rates higher than ca. 15 000 rpm.19 This aspect is of key importance to the study of redox-active solution-phase species. From an optical viewpoint, the exciting radiation, either at low power for reflectance spectroscopy or medium to high power for SHG and Raman, can and in many cases must be focused on the electrode surface to yield optimum signals. The diffraction limit in the UV-visible region is about 1 to 2 µm and thus is of the same order of magnitude as microelectrodes themselves. In summary, a combination of focused optical beams, microelectrodes, and fast means to achieve potential control across the interface, such as charge injection,13 afford ideal conditions for the study of interfacial dynamics in the submicrosecond regime. (16) Pozniak, B.; Mo, Y.; Scherson, D. A. Faraday Discuss. 2002, 121, 313322. (17) Pozniak, B.; Mo, Y. B.; Stefan, I. C.; Mantey, K.; Hartmann, M.; Scherson, D. A. J. Phys. Chem. B 2001, 105, 7874-7877. (18) Fromondi, L.; Shi, P.; Mineshige, A.; Scherson, D. A. J. Phys. Chem. B 2005, 109, 36-39. (19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001.

10.1021/la061497e CCC: $33.50 © 2006 American Chemical Society Published on Web 10/10/2006

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This article examines the oxidation of COads on a polycrystalline Pt, Pt(poly), microelectrode in CO-saturated 0.5 M H2SO4 and 0.5 M HClO4 aqueous solutions using in situ normal incidence reflectance spectroscopy, ∆R/R ) [R(Esam) - R(Eref)]/R(Eref), where R(Ei) is the reflectance measured at a potential Ei (i ) ref (reference) and sam (sampling) potentials), coupled with fast scan linear voltammetry and potential step techniques. This latter technique was employed by Love and Lipkowski,20 Lebedeva et al.,21,22 and others23,24 to examine COads on Pt single-crystal and polycrystalline electrodes of much larger dimensions. Preliminary experiments in this laboratory involving Pt microelectrodes showed that for small overpotentials the reaction rates of COads oxidation are much slower than the time required for a conventional potentiostat, such as the one used in this study, to achieve interfacial potential control. Because the changes in ∆R/R with CO coverage, θCO, are very small, the rates of COads oxidation were determined by optically monitoring the formation of Pt oxide or, more precisely, an oxygen-containing species such as PtOH, for which the corresponding changes in ∆R/R are much larger. For simplicity, however, the generalized term “oxide” will be used throughout this article without implying a specific stoichiometry. As has been reported by others25 and confirmed in our work,26 the coverage of Pt oxide, θox, correlates linearly with changes in ∆R/R over a wide range of θox. Because the rates of Pt oxide formation are exceedingly fast, the observed optical changes can therefore be attributed to the much slower rates of COads oxidation. Advantage will be taken of this approach to examine the electrochemical oxidation of COads on Pt and, in particular, the dependence of the rate on the potential at which the saturated layer of COads is assembled. Experimental Section Reflectance measurements were performed using the same equipment described in our previous communication.18 As specified therein, ∆R/R data were collected using a low-power HeNe laser (λ ) 633 nm) aimed normal to the electrode surface. An optical splitter oriented 45° with respect to the laser beam and a microscope objective were interposed between the source and the electrode. After passing through the objective, the beam reflected from the electrode was directed via high-reflection mirrors to a condensing lens that focused the light onto either a battery-biased Si PIN detector (Newport, model 818-BB-20) connected to a battery-operated current preamplifier (Hamamatsu C6438-01) or a high-speed amplified Si detector (ThorLabs PDA155). Also outlined in our previous paper18 are the methods employed for the preparation and cleaning of Pt(poly) microelectrodes (25 µm diameter); therefore, they will not be repeated here. All experiments were performed at room temperature in either Ar-purged (Praxair 5.0) or CO (Matheson Tri-Gas)-saturated ultrapure aqueous (18.3 MΩ water, Barnstead water purifier) 0.5 M H2SO4 (Ultrex) or 0.5 M HClO4 (Ultrex) in a three-electrode system using a SCE reference electrode placed in a separate compartment connected to the main body of the cell (a 1 cm path length quartz cuvette) via Teflon tubing to avoid contamination (chloride ion). A cleaning step was included in the potential step protocols to remove accumulated surface impurities induced by the enhanced mass-transport rates associated with microelectrodes. (20) Love, B.; Lipkowski, J. ACS Symp. Ser. 1988, 378, 484-496. (21) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938-12947. (22) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2002, 524, 242-251. (23) Petukhov, A. V.; Akemann, W.; Friedrich, K. A.; Stimming, U. Surf. Sci. 1998, 404, 182-186. (24) Jiang, J. H.; Kucernak, A. J. Electroanal. Chem. 2002, 533, 153-165. (25) Conway, B. E.; Angerstein-Kozlowska, H.; Laliberte, L. H. J. Electrochem. Soc. 1974, 121, 1596-1603. (26) Shi, P.; Fromondi, I.; Chen, Y.; Scherson, D. A. Anal. Chem., to be submitted for publication.

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Figure 1. Cyclic voltammogram (thin curve, left ordinate) and ∆R/R vs E curve (Eref ) 0.2 V, thick curve, right ordinate) recorded simultaneously for a 25-µm-diameter Pt microdisk electrode in deaerated 0.5 M H2SO4 in the potential range -0.25 < E < 1.2 V at a scan rate ν ) 100 V/s. The curves represent the average of 3579 identical replicate experiments or acquisitions (acq).

Results and Discussion Shown in Figure 1 are cyclic voltammetry (scan rate, ν ) 100 V/s, thin curve, left ordinate) and ∆R/R (Eref ) 0.2 V, thick curve, right ordinate) versus E curves recorded simultaneously for a Pt(poly) microdisk electrode in deaerated 0.5 M H2SO4 in the potential range of -0.25 < E < 1.2 V vs SCE. These data are in agreement with those reported earlier for much larger electrodes25 and more recently for similar Pt microelectrodes under virtually identical conditions in this laboratory.18 In agreement with results reported elsewhere,25,26 proper quantitative analysis of the coulometric and optical data yields a direct proportionality between ∆R/R and Qn, defined as the relative oxide charge in units of e/Pt surface atom (or, equivalently, the extent of surface oxidation) for Qn e 1. Because the rates of oxide formation are exceedingly high and indeed well beyond the reach of present techniques, it becomes possible to measure the rate at which the interface achieves potential control by monitoring the optical signal while applying a potential step from a value in the double-layer region (i.e., a nominally bare surface) to a value at which oxide is formed. A very similar approach was introduced by McCreery et al. for Au electrodes within the double-layer region.13 Shown in Figure 2 are ca. 10 000 averaged acquisitions (Acq), current (upper subpanels, solid lines), and ∆R/R versus E curves (lower subpanels, jagged curves) recorded simultaneously following the application of a potential step between Eini ) 0.2 (τini ) 24 ms) and Efin ) 0.8 V (τfin ) 10 ms) in Ar-purged 0.5 M H2SO4 (left panel), 0.5 M HClO4 (middle panel), and 0.1 M HClO4 (right panel) followed by a cleaning step at Ecl ) 1.1 V versus SCE (τcl ) 40 µs), where τi represents the time spent at the specified potential (i.e., initial, ini; final, fin; and cleaning, cl; see Scheme 1). Also displayed in the lower subpanels in this Figure are curves obtained upon subtracting from the current transient (solid lines) exponential fits to the decay background (dash lines) in the corresponding upper subpanels. Further evidence that the oscillatory behavior observed is not caused by artifactual electronic coupling was obtained by blocking the optical detector, in which case the response following the application of the potential step displayed no oscillations. In fact, similar oscillations were found earlier in this laboratory for potential step experiments involving microfacetted single-crystal

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Figure 2. Chronoamperometric (solid curves, upper subpanels) and ∆R/R vs E curves (jagged curves, lower subpanels) recorded simultaneously for a potential step between Eini ) 0.2 V (τini ) 24 ms) and Efin ) 0.8 V (τfin ) 10 ms) followed by a cleaning step at Ecl ) 1.1 V vs SCE (τcln ) 40 µs) (acq ) ca. 10 000) in Ar-purged 0.5 M H2SO4 (left panel), 0.5 M HClO4 (middle panel), and 0.1 M HClO4 (right panel). The dashed lines in each of the upper subpanels represent exponential fits to the current decay transients. The smooth curves shown in each of the lower subpanels represent the oscillatory component of the current (i.e., following removal of the exponential decay in the upper subpanels; see the text for details). Scheme 1. Potential Protocol for Potential Step Measurementsa

a See the text for details. E represents the applied potential, and τ represents the polarization time at the specified E.

Pt electrodes using second harmonic generation (SHG) as the optical probe.17 On the basis of their rather large magnitude, the oscillations in ∆R/R are due to modulations in the extent of surface oxidation induced by potential instabilities across the interface. A cursory inspection of the curves further indicates that the oscillations in the current are in phase with the corresponding optical response and occur within the first 50 µs for concentrated solutions (i.e., 0.5 M) and over much longer times (>180 µs) and at higher amplitude for the more dilute, and thus more resistive, solution (i.e., 0.1 M). Such times are still longer than the rise time of the potentiostat as specified by the vendor. In particular, the magnitude of the largest oscillation in the lower left panel (i.e., ∆R/R ca. 2.0) corresponds to an initial overshoot of ca. 50 mV toward more positive potentials, followed by yet another overshoot of the same magnitude toward more negative potentials with respect to the desired end value. This behavior is consistent with the view that the oxide formed under these conditions can be (at least in part) reversibly reduced within the specified potential region. Fortunately, as the experiments to be presented in this work will show, the oxidation of COads occurs over a much longer time domain, and therefore potential problems associated with these oscillations can be safely neglected. In summary, these results eloquently demonstrate that optics affords a means of monitoring “true” interfacial potentials with very high time resolution.

Figure 3. Cyclic voltammogram (thin curve, left ordinate) and ∆R/R (Eref ) 0.2 V, scattered data, right ordinate) vs E for a 25µm-diameter Pt microdisk electrode in CO-saturated 0.5 M H2SO4 in the region of -0.25 < E < 1.2 V at a scan rate ν ) 2 V/s (146 acqs). The ∆R/R data at this relatively slow scan rate was collected using a low-pass RC filter; acq ) 146. The solid line through the scattered data represents the 5-point AA smoothed ∆R/R data.

Shown in Figure 3 are the cyclic voltammogram (thin curve, left ordinate) and ∆R/R (Eref ) 0.2V, scattered points, right ordinate) versus E recorded simultaneously for the same Pt(poly) microdisk electrode in CO-saturated 0.5 M H2SO4 in the range of -0.25 < E < 1.2 V at a scan rate of ν ) 2 V/s. As clearly seen, the onset of COads oxidation occurs at ca. 0.7 V, a potential at which the current increases markedly and ∆R/R exhibits a sudden drop. At more positive potentials, the decrease in ∆R/R becomes virtually identical to that found in solutions devoid of CO. Moreover, as indicated therein, once the oxide undergoes full reduction, the response during the scan in the negative direction reaches values that appear to be higher than those found for the subsequent scan in the positive direction, followed by a decay toward lower ∆R/R values for E < 0.3 V. This behavior suggests a gradual adsorption of CO on the surface or, more strictly, a replacement of adsorbed H by adsorbed CO. Furthermore, the fact that features attributed to hydrogen adsorption/ desorption can still be seen in the voltammogram indicates that continuous cycling at this relatively high scan rate does not allow

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Figure 4. ∆R/R vs t (10-point AA smoothed data, jagged thick curve, left ordinate, Eref ) -0.25 V) upon application of a potential pulse between -0.25 and 0.9 V vs SCE (smooth curve). The open circles (right ordinate) represent the optical data on an expanded scale about ∆R/R ) 0; acq ) 6300. The thin line along the scattered data represents the best exponential fit.

for the surface to become saturated with adsorbed CO at potentials negative to the double-layer region. Because the changes in the optical signal are indeed small, it becomes necessary to average many replicates of the same measurement in order to improve the signal-to-noise ratio and, more specifically, to identify the minimum time for CO to achieve full surface saturation at the beginning of each oxidation run to decrease the overall measurement time. Assuming that CO adsorption on an atomically smooth surface proceeds under strict diffusion control, theory predicts that in the

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absence of kinetic hindrances (i.e., the unit value for the sticking coefficient of CO) the time required for CO to achieve surface saturation would be about 0.16 s (Appendix). Two types of in situ optical experiments were performed in order to determine minimum adsorption times to reach full CO coverage involving in each case potential step techniques. The first strategy was inspired by the small but nevertheless consistent ∆R/R decay associated with the CO readsorption process during the scan in the negative direction in the region of E < 0.3 V shown in Figure 3. More specifically, a potential step was applied to the electrode from Eini ) -0.25 V to Efin ) 0.9 V (square wave in Figure 4) and held at that value for 0.1 s. Subsequently, E was stepped back to -0.25 V and held there for 0.8 s while recording ∆R/R as a function of time. As shown in Figure 4 on an expanded scale (right ordinate), the optical decay lasted ca. 0.6 s (best-fit line through open circles), which is of the same order of magnitude as that predicted by the simple theoretical model. The second tactic relies on the fact that the COads layer hinders Pt oxidation, an effect that gives rise to changes in ∆R/R of up to 1% and thus are about an order of magnitude larger that those associated with CO adsorption (i.e., as small as 0.05%, see above). Shown in the left panel in Figure 5 are chronoamperometric (top subpanel) and ∆R/R versus time curves (lower subpanel) recorded simultaneously in neat (i.e., Ar-purged, curves a) and CO-saturated 0.5 M H2SO4 (curves b) using the potential protocol in Scheme 1 for Eads (or Eini) ) 0.2 V (τads ) 1 s), Eox ) 0.8 V (τox ) 10 ms), and Ecl ) 1.1 V versus SCE (τcl ) 50 µs) (curve a, acq ) 8795; curve b, acq ) 3501). The ∆R/R versus time curve found in CO-free solutions (curve a) showed a fast drop followed by a slow decay over the first 2 ms, an effect that was not observed when identical experiments were performed in CO-

Figure 5. (Left panels) Chronoamperometric (upper subpanel) and ∆R/R vs t curves (lower subpanel) recorded simultaneously in neat, Ar-purged (curves a, acq ) 8795), and CO-saturated 0.5 M H2SO4 (curves b, acq ) 3501) for a potential step between Eads ) 0.2 V (for 1 s) and Eox ) 0.8 V vs SCE (for 10 ms). At the end of the oxidation step, the potential was stepped to Ecl ) 1.1 V (for 50 µs) to clean the surface. (Right panels) Chronoamperometric (upper subpanel) and ∆R/R vs t curves (10-point AA smoothed data, lower subpanel) recorded simultaneously in a CO-saturated 0.5 M H2SO4 solution for a potential step between Eads ) 0.2 V and Eox ) 0.8 V vs SCE (for 10 ms) for times of adsorption of 0.2 s (curves a, 3310 acqs), 0.5 s (curves b, 3866 acqs), 1.1 s (curves c, 6610 acqs) and 1.76 s (curves d, 3342 acqs). At the end of the oxidation step, the potential was stepped to Ecl ) 1.1 V (for 50 µs) to clean the surface.

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Figure 7. Expanded section of the cyclic voltammogram (i vs E, thick line, left ordinate) shown in the upper panel of Figure 6 over the potential region of 0.82 < E < 1.02 V and -1/R(∂R/∂E) vs E curve (thin line, right ordinate) over the same potential region obtained by taking the derivative with respect to E of the fitted optical data in the upper panel of Figure 6. Scheme 2. Potential Waveform Employed for Fast Linear Scan Voltammetric Studies of COads Oxidation on a Pt(poly) Microelectrode Figure 6. Simultaneous cyclic voltammogram (solid smooth curves, left ordinate) and ∆R/R vs E measurement (Eref ) 0.2V, solid jagged curves, right ordinate) for a 25-µm-diameter Pt microelectrode in CO-saturated 0.5 M H2SO4 at 100 V/s (upper panel, acq ) 2165) and 1000 V/s (lower panel, acq ) 1977). For both sets of measurements, 2.3 s was allowed for CO readsorption at E ) -0.26V between every cycle. Similar measurements recorded in the same solution free of CO (purged with Ar) are also shown as dashed curves (>10 000 acqs for both 100 and 1000 V/s) for comparison.

free 0.5 M HClO4 (not shown in this Figure). Although somewhat speculative, it may be suggested that the slower decay stage is related in some yet unexplained way to bisulfate adsorption/ desorption. In contrast, the corresponding decrease in ∆R/R in CO-saturated solutions (curve b) was found to occur over a much longer time domain regardless of the electrolyte. In brief, the presence of COads delays oxide formation but does not affect the final state of the surface. For the second approach, the electrode was polarized at Eads (Eini) (e.g., 0.2 V) and then stepped to a value high enough for COads to be oxidized, Eox (e.g., 0.8 V) (τox ) 10-50 ms) followed by a very short pulse τcl ) 40-50 µs at Ecl ) 1.1 V (Scheme 1). Subsequently, the potential was stepped to Eads, and the entire protocol was repeated thousands of times. The length of time the electrode was polarized at Eads, τads, was varied, and the corresponding chronoamperometric and ∆R/R versus time curves were recorded following application of the potential step, yielding, after averaging, curves such as those shown in the right panel of Figure 5. As the adsorption time, τads, increased, the reflectance decay rate decreased (as a result of surface oxide formation), and the peak current, which has contributions due both to COads oxidation and surface oxide formation, was delayed. For τads long enough for the full CO monolayer to form, both the current and ∆R/R transients remained unchanged. On the basis of the data in these Figures, τads ) 1.1 s is sufficient to achieve CO surface saturation. Measurements were then performed by selecting different values of Eads yielding similar minimum τads values (i.e., ca. 1s) for the same CO surface saturation conditions

to be reached. Therefore, for all subsequent measurements a value of τads g 1 s was employed. Dynamics of COads Oxidation. Two types of approaches were used to examine dynamic aspects of COads oxidation on Pt(poly) microelectrodes: voltammetry at very high scan rates and potential step techniques. Voltammetry at High Scan Rates. Shown in Figure 6 are simultaneous voltammetric (smooth solid curves, left ordinate) and ∆R/R versus E measurements (jagged solid curves, right ordinate) for CO oxidation on the Pt(poly) microelectrode in CO-saturated 0.5 M H2SO4 at 100 V/s (upper panel, 2165 acqs) and 1000 V/s (lower panel, 1977 acqs). For both of these measurements, τads was set at 2.3 s to allow CO to reach surface saturation at Eads ) -0.26 V between every cycle (Scheme 2). Also shown in the same panels in this Figure (dashed curves) are the results obtained in Ar-purged solutions devoid of CO under otherwise identical conditions. It becomes evident from these data that the onset potential for the oxidation of COads coincides with the onset potential for oxide formation as signaled by the drop in ∆R/R, even at scan rates as high as 1000 V/s. The decrease in ∆R/R in this case originates solely from surface oxide formation, which, as shown elsewhere, is linearly correlated with the charge associated with that process.25,26 Therefore, the derivative of ∆R/R in this region represents, within a constant, the current component associated purely with oxide formation. Shown in Figure 7 are plots of the derivative of the best fit to the ∆R/R data, -1/R(∂R/∂E) (thin line, right ordinate) and the corresponding voltammetric curve (thick line, left ordinate) collected at 100 V/s over the potential region where the sharp current peak due to COads oxidation is found (upper panel, Figure 6). As evident from these results, the leading edge and the peak

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Figure 8. (Upper panel) Simultaneous cyclic voltammogram (left ordinate) and ∆R/R vs E (Eref ) 0.2 V, right ordinate) for a 25µm-diameter Pt microelectrode in CO-saturated 0.5 M H2SO4 at 100 V/s for CO adsorption potentials of Eads ) 0.2 V (thick lines, 1712 acqs) and -0.26 V (thin lines, 2165 acqs). A time of 2.3 s at Eads was allowed for full CO readsorption between every cycle. (See the text.) (Lower panel) Expanded view of the data in the upper panel in the potential range of 0.8 < E < 1.0 V.

in the -1/R(∂R/∂E) versus E curve closely follow the voltammetric curve in that potential region. Excellent agreement between the optical and electrochemical measurements was also found for experiments performed in 0.5 M HClO4 (Supporting Information). An analysis of the data collected at 1000 V/s (not shown here) yielded curves that were displaced by a potential that corresponds to times of 10-20 µs, which are shorter than the time required for the potentiostat to achieve interfacial potential control, a few tens of microseconds. It may thus be concluded that COads oxidation appears to coincide with oxide formation at least within times as short as tens of microseconds. In other words, the rates of oxide formation are determined only by the rates of COads oxidation, and hence the latter can be determined directly from the ∆R/R data. We are particularly interested in examining the dependence of the rate of COads oxidation on the potential at which the COads monolayer is formed using the methodology described above. Shown in the upper panel of Figure 8 are simultaneous cyclic voltammograms recorded at a scan rate of 100 V/s (left ordinate) and ∆R/R versus E measurements (right ordinate) for CO oxidation on a Pt(poly) microelectrode in CO-saturated 0.5 M H2SO4 for Eads ) 0.2 V (thick curves) and Eads ) -0.26 V (thin curves) using τads ) 2.3 s (see above). As evidenced from an

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expanded view of these data in the region of COads oxidation (lower panel in this Figure), the onset and peak potentials for CO adsorbed in the double-layer region (0.2V) appear to be shifted by ca. 10 mV (or, equivalently, by a time of 0.1 ms at the scan rate employed) toward more positive values compared to the corresponding parameters for CO adsorbed in the hydrogen adsorption/desorption region (-0.26 V). A similar shift in potential, although larger in value, has been observed for linear scan voltammograms recorded at much slower scan rates for significantly larger electrodes.27,28 Although ohmic drop effects could distort the curves somewhat, the qualitative differences observed cannot be explained on that basis. Most importantly, however, because COads oxidation is kinetically hindered, the application of linear scan techniques is difficult to interpret quantitatively because both time and potential effects must be considered. In contrast, techniques based on the potential step offer a unique advantage in that the reaction progress can be monitored at a fixed (over)potential. Potential Step. Shown in Figure 9 are chronoamperometric (upper subpanels) and ∆R/R versus time curves (lower subpanels) recorded simultaneously following a potential step from Eads ) 0.2 V to various Eox values in the ranges of 0.72 e Eox e 0.80 V (left panel) and 0.80 e Eox e 0.83 V (right panel). For these experiments, τads was set at 1.1 s before application of the step. As evident from these data, an increase in Eox leads to a corresponding increase in the rate at which ∆R/R decays. Furthermore, the curves recorded for small overpotentials display a clear delay before the onset of oxidation occurs, which is shortened as the overpotential is increased. As also indicated, no oxidation was found for Eox ) 0.72 V over the period of time examined. It is important to note that the step in ∆R/R found immediately after the potential step was applied is due to the Stark shift29 (i.e., the change in ∆R/R induced by the applied potential at constant COads coverage, which occurs over a time period much faster than that associated with the oxidation of COads). This effect is responsible for the sloping character of ∆R/R versus E found during the scan in the positive direction for E < 0.7 V in Figure 3 and also in a previous paper.18 Curves i in the right panel of Figure 9 represents a current transient (upper), and the ∆R/R versus time (lower) profile collected under conditions in which the solution was not saturated with CO and, hence, τads ) 1.1 s was insufficient for the coverage of CO on the surface to reach unity. The sudden drop in ∆R/R following application of the step supports the view that the surface has not reached full CO coverage, but most importantly, the lack of full coverage appears to increase the rate of oxidation of COads. A useful way of displaying the optical data in Figure 9 is by taking the derivative of the fitted curves, as shown for five Eox values in the range of 0.76 e Eox e 0.80 V versus SCE in the upper panel (Figure 10), which represent the rate at which COads is oxidized. As shown in detail elsewhere,26 these data can be used to determine the contribution to the measured chronocoulometric current (see curves a, c, and e in the middle panel of Figure 10) due to Pt surface oxidation (curves a′, c′, and e′ in the middle panel of Figure 10) and hence, by difference, the contribution due predominantly to the oxidation of COads (lower panel of Figure 10). It must be stressed first that the spike in the current (curves a, c, and e in the middle panel of Figure 10) observed at very small times is due to double-layer charging and (27) Couto, A.; Rincon, A.; Perez, M. C.; Gutierrez, C. Electrochim. Acta 2001, 46, 1285-1296. (28) Jambunathan, K.; Hillier, A. C. J. Electroanal. Chem. 2002, 524, 144156. (29) Lambert, D. K. Electrochim. Acta 1996, 41, 623-630.

Oxidation of Adsorbed CO on Pt Microelectrodes

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Figure 9. Chronoamperometric (upper subpanels) and smoothed (10-point AA smoothed data) ∆R/R vs t curves (lower subpanels) recorded simultaneously following a potential step from a fixed CO adsorption potential of Eads ) 0.2 V to various oxidation potentials in the range of 0.72 e Eox e 0.80 V (left panel: curve a, 0.80; b, 0.79; c, 0.78; d, 0.77; e, 0.76; and f, 0.72 V vs SCE) and 0.80 e Eox e 0.83 V (vs SCE right panel: curve g, 0.80; h, 0.81; i, 0.82; and j, 0.83 V vs SCE) in CO-saturated 0.5 M H2SO4 except for curves i, which were collected in solutions for which the concentration of CO was below saturation. (See the text for details.) A time of 1.1 s was allowed for full CO readsorption before application of the (subsequent) step. Numbers of acquisitions: 3501 (a and g), 3817 (b), 9170 (c), 3657 (d), 3290 (e), 1779 (f), 3538 (h), 6837 (i), and 8067 (j).

second that the smaller currents observed at the longer times (beyond the hump) are due to the oxidation of bulk CO (see below). These data allows for the instantaneous coverage of COads to be determined solely on the basis of optical information. Specifically, if one assumes for simplicity that a single COads molecule occupies a single site on the surface, then the sum of the coverages of COads, θCO, oxidized Pt, θox, and bare areas, θbare, of the electrode should always add up to unity (i.e., θox + θCO + θbare ) 1). Hence for an initial coverage θCO ) 1 as reported in the literature,30 θox over regions of the electrode not covered by COads following application of a potential step to Ef is prescribed at all times by that potential

θox(t) 1 - θCO(t)

) θox(Ef)

(1)

where θox(Ef) (i.e., the (final) coverage of surface oxide, which is numerically equal to the relative oxide charge as defined by Conway25 in units of e/Pt atom, Qn) can be determined from data collected in the absence of CO. Embodied in this model is the fact that rates of oxide formation on bare patches on the surface are so fast that equilibrium in areas not covered by CO is achieved instantaneously. On the basis of the values of -∆R/R versus E collected in the absence of CO in solution, θox ) -(∆R/R × 103)/4.7,26 and hence from eq 1

θCO (t) ) 1 +

(

)

∆R/R × 103 4.7θox(Ef)

(2)

This analysis makes it possible to convert transient optical data such as those in the lower left panel of Figure 9 into a θCO(t) (30) Brummer, S. B.; Ford, J. I. J. Phys. Chem. 1965, 69, 1355-1362.

versus t plot, as illustrated in Figure 11, where the initial step caused by the Stark shift following application of the potential step was subtracted. Shown in Figure 12 are chronoamperometric (upper panel) and ∆R/R versus time curves (lower panel) recorded simultaneously following a potential step from Eads ) -0.26 V to various Eox values in the range of 0.70 e Eox e 0.80 V versus SCE, allowing once again a 1.1 s time interval for CO readsorption to surface saturation before application of the step. By analogy to the results obtained for Eads ) 0.20 V, the decay rate of ∆R/R increases as the overpotential is increased; however, the curves in this case do not show any discernible delay. In fact, the charging currents overlap with the faradaic process(es) in this very short time domain. This constitutes an important difference, which will be discussed later in this article. Also noteworthy is the fact that the oxidation of COads appears to occur at lower overpotentials than those found for Eads ) 0.20 V. It is important to stress that the current observed at 50 ms (beyond the scale used in the Figure, i.e., 30 µA) is twice the steady-state value, which is consistent with that predicted by eq A2 in the Appendix for this time (in dimensionless variables) based on the dimensions of the electrode and the solubility and diffusion coefficient of CO (Appendix). Corresponding data, such as that shown in Figure 10, are given for Eads ) -0.26 V (i.e., the hydrogen adsorption/desorption region) in Figure 13. A better illustration of the contrasting behavior found between the two adsorption potentials is given in Figure 14, where the results obtained for both measurements (i.e., θCO versus t (upper panel) and converted current due to oxide formation versus t data (lower panel)) overlap. Particularly interesting is the fact that for Eads ) 0.2 V a value of Eox ) 0.72 V is not sufficient to oxidize COads in this time domain. At Eads ) -0.26 V, however, the process begins even for values of Eox as low as 0.70 V over the same time period. The transition from

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Figure 11. Plot of θCO(t) vs t derived from ∆R/R vs t data shown in the lower left panel of Figure 9 after subtraction of the step due to the Stark shift following application of the potential step for a series of final potentials. The solid lines represent the best fits to the data. The notation is defined as in the caption of Figure 9.

Figure 10. (Upper panel) Derivatives of the fitted optical curves, -1/R(∂R/∂t) vs t, corresponding to the optical data for five Eox values in the range of 0.76 e Eox e 0.80 V vs SCE shown in Figure 9 for Eads ) 0.20 V. Curve a, 0.80 V; b, 0.79 V; c, 0.78 V; d, 0.77 V; and e, 0.76 V. (Middle panel) Total current as a function of time for the data in Figure 9 for Eox ) 0.80 V (a), 0.78 V (c), and 0.76 V (e) and the corresponding components that are purely due to surface oxide formation derived from the data in the upper panel, a′, c′, and e′. (Lower panel) Contributions to the total current (predominantly) due to the oxidation of COads determined from the difference between the curves labeled a, c, and e and the corresponding curves a′, c′, and e′ in the middle panel.

one type of behavior to the other appears to be smooth as judged from the results of experiments in which Eox was fixed at 0.77 V while Eads was varied in the range of -0.26 e Eads e 0.20 V (raw data and derivative curves in Figures 15 and 16, respectively). These data strongly support the view that the rates of COads oxidation decrease rather markedly as Eads increases from hydrogen adsorption into the double-layer region. Insight into the nature of the adsorption sites of CO on Pt as a function of the applied potential was gained by Kunimatsu et al., who examined CO adsorption on Pt(poly) using polarization modulation FTIR reflection-absorption spectroscopy.31-33 On the basis of their studies, these authors concluded that CO adsorbs predominantly in a linear fashion in the double-layer region with the amount of bridge-bonded CO being negligible. However, for potentials in the hydrogen adsorption/desorption region CO is adsorbed not only linearly but also in bridge-type sites. Other (31) Kunimatsu, K.; Golden, W. G.; Seki, H.; Philpott, M. R. Langmuir 1985, 1, 245-250. (32) Kunimatsu, K.; Seki, H.; Golden, W. G.; Gordon, J. G.; Philpott, M. R. Langmuir 1986, 2, 464-468. (33) Kunimatsu, K.; Shimazu, K.; Kita, H. J. Electroanal. Chem. 1988, 256, 371-385.

Figure 12. Current vs t (upper panel) and ∆R/R vs t curves (lower panel) recorded simultaneously following a potential step from a fixed CO adsorption potential of Eads ) -0.26 V to various oxidation potentials in the range of 0.70 e Eox e 0.80 V vs SCE (0.80 (a), 0.78 (b), 0.76 (c), 0.72 (d), and 0.70 V (e)) allowing 1.1 s for CO readsorption to full coverage before application of the step. The solid lines in the lower panel represent the best fits to the data. Numbers of acquisitions: 3584 (a), 3288 (b), 4106 (c), 8687 (d), and 3971 (e).

studies34 have also provided evidence for two different pathways for adsorbed CO oxidationsan island mechanism predominant for CO adsorbed in the double-layer potential region and a random (34) Leiva, E. P. M.; Santos, E.; Iwasita, T. J. Electroanal. Chem. 1986, 215, 357-367.

Oxidation of Adsorbed CO on Pt Microelectrodes

Figure 13. (Upper panel) Derivatives of the fitted optical curves, -1/R(∂R/∂t) vs t, corresponding to the optical data for five Eox values in the range 0.70 e Eox e 0.80 V vs SCE shown in Figure 12, for Eads ) -0.26 V. Curve a: 0.80 V, b: 0.78 V, c: 0.76 V, d: 0.72 V and e: 0.70 V. (Middle panel) Total current as a function of time for the data in Figure 12 for Eox ) 0.80 V (a), 0.78 V (b), 0.76 V (c), and the corresponding components due purely to surface oxide formation derived from the data in the upper panel in this figure, a′, b′, c′. (Lower panel) Contributions to the total current due (predominantly) to the oxidation of COads determined from the difference between the curves labeled as a, b and c and the corresponding curves a′, b′ and c′ in the middle panel in this figure.

mechanism for CO adsorbed in the hydrogen region. In the latter case, the removal of bridged-bonded CO by oxidation, which requires much lower overpotentials, creates evenly distributed vacancies over the entire electrode surface, thereby facilitating the oxidation of linearly bonded CO. Evidence in support of the presence of CO islands on the surface was provided by Weaver et al., who studied the electrooxidative stripping behavior of CO on Pt(111), Pt(110), and Pt(100) using in situ reflection absorption infrared spectroscopy (IRAS).35-37

Conclusions The influence of the adsorption potential (i.e., double layer versus hydrogen adsorption/desorption region) of a full monolayer of CO on Pt(poly) microelectrodes in aqueous acidic electrolytes has been examined using in situ UV-visible reflectance spectroscopy. This method allows for the instantaneous coverage of CO to be determined directly from changes in the optical signal induced by Pt surface oxidation, from which the rates of COads oxidation can be deduced. Furthermore, it makes it possible (35) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095-5102. (36) Chang, S. C.; Weaver, M. J. Surf. Sci. 1990, 230, 222-236. (37) Chang, S. C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582-4594.

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Figure 14. (Upper panel) Plots of the best fit θCO(t) vs t curves in Figure 11 for Eads ) 0.20 V (thick lines) (Eox ) 0.80 V (a2), 0.78 V (b2). 0.76 V (c2), and 0.72 V (d2)) and for Eads ) -0.26 V (thin lines) (Eox ) 0.80 V (a1), 0.78 V (b1). 0.76 V (c1), 0.72 V (d1), and 0.70 V (e1)) derived from the optical data in the lower panel of Figure 12. (Lower panel) Plots of converted current due to oxide formation shown in the middle panels of Figures 10 and 13 for Eads ) -0.26 V (thin curves) and Eads ) 0.2 V (thick curves). The notation is same as that for the upper panel in this figure.

to extract from the total chronocoulometric response the current due to Pt surface oxidation based solely on the reflectance data to yield by difference that contribution to the current attributed predominantly to the oxidation of COads. The results obtained revealed differences in the rates of oxidation of COads induced by the potential at which the CO saturated layer is formed. Specifically, for adsorption potentials Eads in the double-layer region, the reaction proceeds at slower rates than for Eads in the hydrogen adsorption/desorption region. This behavior has been tentatively ascribed to differences in the spatial disposition of CO molecules on the surface during oxidation. Acknowledgment. This work was supported by a grant from the NSF.

Appendix The steady-state current density values, iss, that can be achieved for a microelectrode of radius ro and for a rotating disk electrode (RDE) are given by iss ) 4nFDoC*o/πro, and iss ) 0.62nFDo2/3ω1/2ν - 1/6Co*, respectively, where n is the number of electrons

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most aqueous solutions), and Co* and Do are the bulk concentration and diffusion coefficient of the reactant, respectively. For CO in aqueous solutions, Do ) 1.5 × 10-5 cm2/s;38 hence, equating the expressions above yields

ω1/2ro )

4Do1/3ν1/6 4 × 0.025 × 0.464 ) ) 2.48 × 10-2 cm π0.62 3.14 × 0.62 (A1)

Therefore, for a disk with a radius of 1.25 × 10-3 cm, such as that used in our work, the corresponding equivalent rotation rate ω that will produce the same flux at the limiting current (or steady state) may be shown to be about 3750 rpm. Another important aspect that must properly addressed is the time required for steady state to be achieved for a microelectrode under the conditions of our experiments. An empirical formula that provides rather accurate values for the current normalized by the limiting current, iss, for a microelectrode following a potential step that instantaneously decreases the concentration of the reactive species to zero at the boundary is given by -1/2 i ) f(τ) ) 0.7854 + 0.8862τ-1/2 + 0.2146e-0.7823 τ iss (A2)

Figure 15. Current vs t (upper panel) and smoothed (10-point AA smoothed data) ∆R/R vs t curves (lower panel) recorded simultaneously following a potential step from various CO adsorption potentials Eads ) 0.2 V (a), 0 V (b), -0.1 V (c), and -0.26 V (d) to a fixed oxidation potential Eox ) 0.77 V vs SCE, allowing 1.1 s for CO readsorption to surface saturation before application of the step. Numbers of acquisitions: 4318 (a), 3048 (b), 4964 (c), and 4680 (d).

where τ ) 4Dot/ro2 is a dimensionless time.19 For Do ) 1.5 × 10-5 cm2/s and ro ) 1.25 × 10-3 cm, τ ) 4 × 1.5 × 10 - 5t/(1.25 × 10 - 3)2 ) 38.4t. For example, for t ) 1 s, τ ) 38.4; hence, on the basis of the above equation, the actual current is about 10% higher than that expected at steady state. To estimate the time required for a monolayer to form, one can assume for simplicity that the system has achieved steady state. This assumption overestimates the time required because the flux will be much higher prior to steady state being achieved. For solution-phase CO oxidation in CO-saturated aqueous solutions (Co* ) 0.9 mM38), on the basis of a steady-state current for the microelectrode, iss ) 4nFDoCo*ro ) 15 nA, the flux would be given by

flux )

4DoCo* mol 4(1.5 × 10-5)(0.9 × 10-6) iss ) ) ) nFA πro cm2s π(1.25 × 10-3) 4.32 mol nmol × 10-8 2 ) 13.8 2 (A3) π cm s cm s

A monolayer corresponds to ca. 220 µC/cm2 or, equivalently, (220 × 10-6/F) mol/cm2. This translates to 2.2 nmol/cm2; hence, it would take about 0.16 s for the monolayer to form, provided there are no kinetic hindrances, and both the sticking coefficient and the surface roughness are 1. Notice that limiting current conditions prevail during the oxidation of bulk CO and also during the adsorption of CO on bare Pt. Figure 16. Derivative of the fitted optical curves, -1/R(∂R/∂t) vs t corresponding to the optical data for the four Eads values shown in Figure 15 for Eads ) 0.2 V (a), 0.0 V (b), -0.1 V (c), and -0.26 V (d) for a common value of Eox ) 0.77 V.

Supporting Information Available: Simultaneous CV and ∆R/R versus E curves for a 25-µm-diameter Pt microelectrode in HClO4. This material is available free of charge via the Internet at http:// pubs.acs.org. LA061497E

involves in the reaction, F is the Faraday constant, ro is the radius of the microdisk, ν is the kinematic viscosity (ca. 0.01 cm2/s in

(38) Caram, J. A.; Gutierrez, C. J. Electroanal. Chem. 1991, 305, 259-274.