Study of CO Oxidation on Polycrystalline Pt Electrodes in Acidic

Jul 17, 2011 - pairs are considered to be stable or less mobile, forming OHad ... be better assigned to the “probe” of the local COad coverage rat...
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Study of CO Oxidation on Polycrystalline Pt Electrodes in Acidic Solution by ATR-SEIRAS Yan-Gang Yan,*,†,‡ Yao-Yue Yang,† Bin Peng,† Souradip Malkhandi,‡ Andreas Bund,‡ Ulrich Stimming,‡ and Wen-Bin Cai*,† †

Shanghai Key Laboratory for Molecular Catalysis and Innovative Materials and Department of Chemistry, Fudan University, Shanghai 200433, China ‡ Physik-Department E19, Technische Universit€at M€unchen, D-85748 Garching, Germany

bS Supporting Information ABSTRACT: Adsorption and electro-oxidation of CO on a polycrystalline Pt electrode in acidic solutions were systematically revisited by in situ attenuated-total-reflection surfaceenhanced infrared absorption spectroscopy (ATR-SEIRAS) in conjunction with related Gram Schmidt response analysis. CO was either adsorbed in the double-layer region, i.e., 0.45 V (RHE) (denoted as CO@DL) or in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE) (denoted as CO@UPD). The results indicate that the CO@UPD and H2Ofree coexisted structure (or simply costructure) forms only at a sufficiently high global CO coverage (H2Ofree denotes hydrogen-bonding-broken water); In contrast, the CO@DL and H2Ofree costructure forms in an earlier adsorption phase, less dependent on the global CO coverage. The partial oxidation of CO from solution and weakly adsorbed COL at the active sites is suggested to yield a prepeak that occurs with the relaxation of the COad-H2Ofree costructure and the disorganization of the outer water net layers. In the main oxidation process, the oxidation of CO@UPD tends to proceed via the “mean-field approximation” kinetics due to the high COad mobility resulting from the oxidation prepeak. The oxidation process of CO@DL is, however, likely via the “nucleation and growth” kinetics due to the good stability of the local CO@DL and H2Ofree costructured islands. The H2Ofree can be better assigned to the “probe” of the local COad coverage rather than the main oxygenated species for COad oxidation according to the spectral results for both CO@UPD and CO@DL.

1. INTRODUCTION Carbon monoxide is the simplest C1 molecule related to low temperature fuel cells and is an important model system for fundamental studies on the electrocatalysis of C1 molecules.1 CO oxidation is one of the most extensively studied reactions at both solid/gas2,3 and solid/liquid interfaces.1,4 7 Crystalline defects such as steps and kinks are known to be more reactive than regular sites at flat terraces at both catalysis reaction interfaces.2 7 In the solid/gas heterogeneous catalysis, the surface CO is oxidized to CO2 via a Langmuir Hinshelwood type mechanism involving adsorbed CO and oxygen-containing species.8 The same mechanism has been extended to the electrocatalytic CO oxidation, except that otherwise the adsorbed OH species (OHad) replaces the adsorbed O species (Oad) in the former.1,4,7 14 For the CO oxidation, surface diffusion of adsorbed CO (COad) and/or OHad play a key role,6,7,9,10,13,15 17 as a matter of fact, the kinetics of the CO oxidation is controlled by the interplay of surface mobility and reactivity.9 There are two typical but inconsistent kinetic models for the CO oxidation at Pt electrodes: (1) in the “mean field approximation” model,7,10,13 the diffusion rates of the reaction-pairs (COad and OHad) are much faster than the reaction rates, then the reaction-pairs will be distributed homogeneously on the surface during the oxidation process; r 2011 American Chemical Society

(2) in the “nucleation and growth” model,18 20 the reactionpairs are considered to be stable or less mobile, forming OHad and COad domains/phases on the surface, and propagation of the reaction fronts occurs at the boundary between OHad and COad domains. In previous reports,1,7,10,13,18 25 researchers dealt partly and separately with this complex issue on different types of Pt electrodes under different conditions of CO adsorption, leading largely to the diversity in explanations. Another open question is about the well-known prepeak (the so-called preignition or preoxidation wave) prior to the main CO oxidation in CO stripping voltammograms when the CO was predosed at a potential of the hydrogen adsorption/desorption region.1,12,22,25 31 The origin of the prepeak has been argued extensively, with the following explanations proposed: the shift of bridge-bonded CO (COB) to atop sites (COL) that triggers the partial oxidation of COL,31 preferential oxidation of COB,22,25 oxidation of a weakly adsorbed state1,26 or a kinetically unstable state,30 preferential oxidation at or near defect sites,12,27 29 lowering of the activation energy at high CO coverage,32 and oxidation driven by a potential-induced rearrangement of CO.33 Received: May 7, 2010 Revised: July 2, 2011 Published: July 17, 2011 16378

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The Journal of Physical Chemistry C Specifically, in recent ATR-SEIRAS investigations, Hanawa et al.34 proposed that the prepeak can be attributed to the oxidation of COB based on the decrease of the COB band during the prepeak process; Samjeske et al.,31 however, assigned this observation34 to the shift of COB to COL and attributed the prepeak to the oxidation of partial COL triggered presumably by the potential-induced site conversion of COB. Therefore, a reasonable clarification for the origin of the prepeak is needed. In order to get a better understanding of the COad electrooxidation on Pt electrode, including the origin of the prepeak and the inconsistent kinetic models for the main-oxidation process, we decided to revisit this classical but controversial topic through a systematic infrared spectroscopic study and a reexamination of previous proposals. Attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) is quite suitable for dynamic studies of adsorption and oxidation of CO, because of its high signal sensitivity, simple surface selection rule, unrestricted mass transport and fast cell response.25,31,34 39 In the present work, the oxidation of CO predosed at two potentials, one in the double-layer region, i.e., 0.45 V (RHE) and the other in the hydrogen underpotential deposition region, i.e., 0.1 V (RHE), was systematically investigated by using this technique and its related Gram Schmidt response analysis. The Gram Schmidt algorithm is a vector orthogonalization technique commonly used in linear algebra,40 as applied to LC/FTIR or GC/FTIR spectrometry.41,42 In ATR-SEIRAS measurements, the evanescent waves of the IR radiation can penetrate into the solution for some hundred nm and the SEIRAS selectively probes the vicinity of the electrode surface, ca. 5 nm.36,43 Therefore, the interferogram signal includes the information of COad and H2O close to and/or directly adsorbed on the surface. The Gram Schmidt intensity is reconstructed from interferogram signal collected during a series of time-resolved SEIRA spectra before the Fourier Transform is done. This technique is capable of removing redundant information from a set of vectors, and can quickly and sensitively provide the information of variation of adsorbed layer and/or interfacial structure in our real time electrochemical ATR-SEIRAS study. A detailed picture of the CO oxidation process at Pt film electrodes was proposed based on our results and their comparison with those from other groups obtained by IR, STM, SXS, or DEMS.1,25,27,31,34,38,39 It should be pointed out that the “mean field approximation” and “nucleation and growth” models are two ideal kinetics models, which may be best suited for describing the oxidation behavior on a Pt single crystal (like Pt(111)) surface than on polycrystalline Pt surfaces. However, we have no better way to obtain the real-time detailed surface vibrational signals on Pt(111) including the H2Ofree band under otherwise the same electrochemical conditions with the conventional external IRAS measurement. Since a polycrystalline Pt electrode is often assumed to consist largely of (111)-oriented facets of different domains, the “mean field approximation” and “nucleation and growth” models were frequently used to qualitatively explain the CO oxidation processes on polycrystalline Pt electrodes in the literature.9,18,19,44 For these reasons, polycrystalline film Pt electrodes are used in our experiments owing to their strong SEIRA effect, more or less (111)-orientation preferred nature.

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2. EXPERIMENTAL DETAILS A SEIRA-active Au nanoparticle film (ca. 60 nm-thick) was chemically deposited on the total-reflecting plane of a hemicylindrical Si prism via an electroless plating method.37 A Pt overlayer was electrodeposited galvanostatically onto the Au base film (working area 1.33 cm2) at 0.4 mA cm 2 for 250 s in 4 mM H2PtCl6 3 6H2O + 0.7 M Na2HPO4 solution with a saturated calomel electrode (SCE) and a Pt mesh serving as the reference and counter electrodes, respectively. With this procedure a virtually pinhole-free Pt overlayer can be obtained.37 The average mass equivalent thickness is calculated to be ca. 8.5 nm based on ICP-AES measurements (details can be found in our previous paper37). Assuming 210 μC cm 2 for the hydrogen adsorption charge at an ideally smooth electrode, the roughness factor of the as-prepared electrode was around 5. The subsequent (spectro)-electrochemical measurements were performed in 0.1 M HClO4 purged with high-purity Ar before the CO was predosed. The CO was predosed at 0.1 V (denoted as CO@UPD) and 0.45 V (denoted as CO@DL), respectively. The adsorption rate of CO depends chiefly on the concentration of dissolved CO near the surface in the solution. The CO bubbling was controlled into the spectral electrochemical cell in a very gentle way on purpose, and a very slow increase in dissolved CO concentration could be expected. A CHI 660B electrochemistry workstation (CH Instrument, Shanghai) was employed for the potential/current control with a reversible hydrogen electrode (RHE) and a Pt mesh serving as the reference and counter electrodes, respectively. All electrode potentials in this work are cited versus the reversible hydrogen electrode (RHE), unless stated otherwise. A Nicolet Magna-IR 760 FTIR or Varian 3100 FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT detector was used for real-time SEIRAS measurements and was operated at a resolution of 4 or 8 cm 1 with different acquisition times (depending on the measurement) for each spectrum. In situ spectroscopy on Pt nanofilm electrodes was run in the so-called Kretschmann attenuated-total reflection (ATR) configuration (prism/thin metal film/solution), details of which can be found elsewhere.36 Unpolarized infrared radiation from a ceramic source was focused at the interface by being passed through the prism at an incident angle of ca. 65°, and the totally reflected radiation was detected. Real-time spectra were collected during the potential program. All the spectra in this paper are shown in absorbance units defined as -log (I/I0), where I and I0 represent the intensities of the reflected radiation at the sample and reference potentials, respectively. For the Gram Schmidt response, the basis vectors were taken at the start of the FTIR experiment in all experiments. The basis sets described the initial interfacial states for CO adsorbed at 0.1 V (section 3.2) and 0.45 V (section 3.3), respectively. The Gram Schmidt response was automatically calculated with its basis vector by computer during the FTIR measurements. All experiments were performed at room temperature. The theory of Gram Schmidt data treatment is shown in section 3 of the Supporting Information. 3. RESULTS AND DISCUSSIONS 3.1. Adsorption of CO. Adsorption of CO@UPD.. Figure 1 shows in situ spectra of the process of CO adsorption at 0.1 V (CO@UPD). Three bands in the regions from 3620 to 3650 cm 1, from 2006 to 2071 cm 1 and from 1768 to 1872 cm 1 are 16379

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Figure 1. Series of time-resolved SEIRA spectra of CO on Pt nanofilm electrode in 0.1 M HClO4 collected sequentially during the process of CO adsorption at 0.1 V. The time resolution used and shown is 4.7 s. The reference spectrum was taken at a Pt electrode before introducing CO in 0.1 M HClO4 at 0.1 V.

Figure 2. Selected spectra at (a) 324.4, (b) 333.9, (c) 386.4, (d) 424.6, and (e) 634.4 s from Figure 1. The intensities for spectra (a) and (b) have been multiplied by a factor of 10.

attributed to the νOH of non-hydrogen-bonded H2Ofree (probably on top of CO adlayer with certain chemical bonding between them37,43,45), νCO of linearly/atop (COL),1,27,37,46 48 and νCO of bridge (COB) and multiply (COM) bonded CO on Pt, respectively. For a clear assignment of the broad band from 1768 to 1872 cm 1, five spectra selected from Figure 1 are redisplayed in Figure 2. The asymmetric shape of the broad band consists of a COB band from 1800 to 1872 cm 11,27,37,46 48 and a COM band at ca. 1768 cm 1.1,27,46,47 The asymmetric broad band observed for this polycrystalline Pt film electrode is hardly separable because the dipole dipole coupling between adjacent COad makes an intensity transfer from COM to COB or from COM and COB to COL.49 For this reason, COM and COB bands

Figure 3. Selected integrated intensities, center frequencies and full width at half height (FWHHS) of IR bands of COad and H2Ofree from Figure 1. The intensity for δHOH has been multiplied by a factor 20.

(denoted as COB+M) will be counted and integrated as a whole in discussing related spectral results. Figure 3 shows that the band intensity of COB+M is initially higher than that of COL; however, at the time ca. 385 s the COB+M band goes lower and at the same instance the spectral feature of H2Ofree starts to appear. Then the frequencies of H2Ofree and COL bands increase rapidly, until they reach the highest values at ca. 425 s. Meanwhile, the fwhh of the COB+M band increases until ca. 460 s and then decreases with its frequency significantly shifted toward 1870 cm 1. The COad-H2Ofree costructure may form and stabilize until ca. 550 s. Then COad-H2Ofree costructure at saturated CO coverage is relatively stable, with four peaks at 16380

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Figure 4. Series of time-resolved SEIRA spectra of CO on a Pt nanofilm electrode in 0.1 M HClO4 collected sequentially during the process of CO adsorption at 0.45 V. The time resolution used and shown is 2.8 s. The reference spectrum was taken at a Pt electrode before introducing CO in 0.1 M HClO4 at 0.45 V.

3650 (νOH of H2Ofree), 2071 (νCOL), 1872 (νCOB), and 1768 cm 1(νCOM). Three origins could lead to the variations of COL and COB+M bands during the adsorption process: first, preferential adsorption of COB+M at low potentials and that of COL at high COad coverages;46,49 52 second, site shifting effects of coadsorbed water molecules which can make the partial COB+M shift to COL;43,52 54 third, the increasing dipole dipole coupling between adjacent COad resulting in an intensity transfer from the lower frequency component to the higher one.49 52 As can be seen from the selected spectra at 324.4 (a) and 333.9 s (b) of Figures 1 and 2, the frequencies of νCOad, i.e., 2006 (νCOL), 1805 (νCOB), and 1768 cm 1 (νCOM), are very low and no H2Ofree could be identified. Figure 3 shows that the fwhh of the νCOL band in the initial adsorption phase is much wider than that upon saturated CO adsorption. These indicate that the local CO coverage may be very low in the initial adsorption phase. The H2Ofree occurs only when the global COad coverage increases to a certain threshold, although this value is hard to calculate because of the nonlinearity of coverage dependent IR band absorption at low COad coverages. Adsorption of CO@DL.. Figures 4 and 5 show the time-revolved SEIRAS results for the process of CO adsorption at 0.45 V (CO@DL). In this case, one can see that the H2Ofree and COad bands occur almost at the same time in the initial adsorption phase where the intensity of the COL band is higher than that of the COB+M band. With increasing global coverage, the band intensities and frequencies for adsorbed species increase and saturate at about 180 s. There are no pronounced variations of COad bands’ shape during the entire CO@DL adsorption process, obviously different from that in the case of CO@UPD. It is also noted that the spectral features recorded at 0.45 V for the COad adlayers initially formed at 0.1 V (CO@UPD) are somewhat different from those for the COad adlayers directly formed at 0.45 V (CO@DL) (section 1 of the Supporting Information). In the initial (10 20 s) adsorption phase, the fwhh of the COL band is ca. 20 cm 1 for CO@DL which is significantly lower than that for CO@UPD, i.e., ca. 30 cm 1. Meanwhile, the

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Figure 5. Selected integrated intensities, center frequencies and full width at half height (FWHHS) of IR bands of COad and H2Ofree on Pt nanofilm electrode in 0.1 M HClO4 collected sequentially during the process of CO adsorption at 0.45 V from Figure 4.

corresponding frequencies of the νOH (H2Ofree) and νCOL bands are 3644 and 2040 cm 1, respectively, much higher than those in the case of CO@UPD (Figure 3), even considering the potential-induced frequency shift. Furthermore, plots of νCOL frequencies vs normalized COad band intensities (the latter more or less reflects the global COad coverage) adapted from Figures 1 and 4 are compared in Figure S2 in the Supporting Information, showing again different CO adlayer structural variations during the controlled CO adsorption processes at the above two different potentials. Therefore, the local COad coverage for CO@DL should be much higher than that for CO@UPD in the initial adsorption phase, leading to the emergence of the corresponding νOH (H2Ofree) band at a much lower global CO@DL coverage. Briefly, a much higher global COad coverage threshold is required for initializing the H2Ofree-CO@UPD costructure, and the costructure is sensitive to the global COad coverage. In contrast, the relatively stable CO@DL-H2Ofree costructure can form in the very initial adsorption phase, far less sensitive to the global COad coverage. For the first-time, we have identified the different interfacial spectral features for CO@DL and CO@UPD adsorptions by using in situ IR spectroscopy. 3.2. Oxidation of CO@UPD. Origin of the Prepeak. Figures 6 and 7 show the time-resolved SEIRAS results for CO@UPD at Pt electrode during potential cycling from 0.1 to 1.1 V and back to 0.03 V at 50 mV s 1. The voltammogram exhibits a peak at about 0.66 V (prepeak) and a main oxidation peak at 0.75 V, with features similar to those reported in literature.26,27,30,32 The insert of Figure 6 shows that the shape of the νCOB+M band is sensitive to the structure and coverage of COad during the oxidation process, like in the adsorption process (Figures 1, 2, and 4). This section focuses on the nature of the prepeak covering the potential region (a) from ca. 0.4 V to ca. 0.64 V and the potential region (b) from ca. 0.64 V to ca. 0.7 V in Figure 7. In the potential region (a), the current starts to increase at about 0.4 V while the COB+M band intensity decreases slightly and the COL band intensity is largely unchanged. In the potential region (b), COL band intensity increases slightly and then decreases sharply while the COB+M band intensity keeps decreasing. At the same time, the center frequencies of νOH, νCOL, and νCOB+M decrease 16381

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Figure 6. Series of time-resolved SEIRA spectra of CO on a Pt nanofilm electrode in 0.1 M HClO4 (saturated with CO) collected sequentially during a potential sweep cycling 0.1 V f 1.1 V f 0.03 V at 50 mV s 1. The time resolution used and shown is 0.2 s. The Pt electrode was predosed with CO at 0.1 V in CO-saturated 0.1 M HClO4 solution for 30 min. The reference spectrum was taken at 1.1 V. Inset: enlarged representation of the spectra in the region from 1950 to 1500 cm 1 which is highlighted by the blue frame.

and the fwhh of νCOL increases (see Figure 7). The νCOL frequency increases linearly with potential with a typical Stark tuning rate of 28.4 cm 1 V 1 from 0.1 to 0.64 V22 and then decreases as the potential continues going up. Interestingly, the Gram Schmidt intensity also decreases at about 0.64 V and gets its bottom at ca. 0.7 V. The decrease of COB+M in potential region (a) may arise from one or more of the following factors: (1) the decrease of the infrared cross-section of νCOB+M induced by a decreasing back-donation from the Pt d-orbital to the CO 2π* orbital at more positive potentials,34,55 which can reversibly happen in the potential region below 0.23 V;34 (2) the site shift from COB to COL and/or from COM to COB and COL at more positive potential due to preferential adsorption of COB+M at low potentials and COL at high potentials;33,39,43,46,49 52,56,57 (3) the oxidation of COB and/or COM at low potentials;25,34 (4) the exchange of COad with dissolved CO. Since the decrease of the COB+M band intensity is more pronounced in CO-saturated solution than in CO-sparged solution (Figures 8 and 9) and the band intensity change is indeed reversible over the potential range from 0.05 to 0.23 V,34 factors (1) and (4) should be mainly responsible. In fact, on Pt(111) electrode, the (2  2)3CO structure formed at 0.1 V can stay up to ca. 0.5 V at a slow potential sweep rate in the CO-saturated solution according to the study of IRAS and SXS.1,33,27 The COad adstructure may not vary in the potential region (a) due to the fast potential sweep in our case. As for the potential region (b), the weakening of the COB+M band can be better explained with factors (2) and/or (3) as mentioned above, i.e., the site shift of COB+M and/or the oxidation of COB+M;25,33,34,39,43,46,52 57 the increase and subsequent decrease of the COL band can be mainly

Figure 7. Upper panel: Cyclic voltammogram recorded at 50 mV s 1 during in situ SEIRAS measurement along with selected potential dependent integrated intensities for IR bands of COad and H2Ofree. Lower panel: plots of potential-dependent Gram Schmidt intensity and center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensity for δHOH has been multiplied by a factor of 5. All other conditions are the same as in Figure 6.

ascribed to the coeffect of the site shift from COB+M to COL and the oxidation of partial COL. 16382

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Figure 8. Series of time-resolved SEIRA spectra of CO on a Pt nanofilm electrode in 0.1 M HClO4 collected sequentially during a potential sweep from 0.1 to 1.1 V at 50 mV s 1. The time resolution used and shown is 0.2 s. The CO was adsorbed at 0.1 V in 0.1 M HClO4 solution for 30 min and then the dissolved CO was purged with Ar for 3 h. The reference spectrum was taken at 1.1 V.

On the basis of the above argument, the processes involved in the prepeak with increasing potential could be envisioned as follows. First, the COad weakly adsorbed on the crystalline defects such as steps and kinks1 (denoted as active sites) could be oxidized at ca. 0.4 V (mainly COL). The preferential oxidation of COL for CO@UPD will be further proved in the ATR-SEIRAS measurement in conjunction with potential step from 0.1 to 0.55 V (vide infra). Then, the CO in solution and the COad near the active sites competitively diffuse to and oxidize at active sites. Since the higher CO concentration in the solution leads to a higher oxidation current,34 the role of the COad diffusion and oxidation is significant only when the dissolved CO close to the active sites is greatly consumed. As considerable amounts of COad species diffuse to and oxidize at active sites at ca. 0.64 V in Figure 7 (corresponding to the turning point at the this potential in the Gram Schmidt intensity plot or the νCOL frequency vs E plot), the local coverage of COad decreases, resulting in a relaxation of the COad and H2Ofree co-structure. When that occurs, the center frequencies of νOH, νCOL, and νCOB+M decrease and the fwhh of νCOL increases (Figure 7). The relaxation of the costructure provides some free Pt sites at terraces (which are inactive sites in this potential region1,34) and then new COL species adsorb on these terrace sites directly from the solution. This is why the νCOL band intensity has a more pronounced increase when the concentration of CO in solution is higher34 or the relaxation time is longer.31 As can be reflected by Gram Schmidt intensity response, it increases slightly due to the minor change of the interfacial structure due to, e.g., partial oxidation of COad and oxidation or desorption of H2Ofree in the potential region (a). The decrease of Gram Schmidt intensity from ca. 0.64 to 0.70 V could be ascribed to the adsorption of dissolved CO onto the nonactive sites that were released due to the relaxation and change of the costructure. The resultant rearrangement of interfacial water layers43 may also contribute to the decrease of the Gram Schmidt intensity.

Figure 9. (upper) Cyclic voltammogram, first cycle (solid line) and second cycle (dash line), of a CO-predosed Pt electrode recorded at 50 mV s 1 during in situ SEIRAS measurement along with selected potential dependent integrated intensities for IR bands of COad and H2Ofree; (lower) plots of potential-dependent Gram Schmidt intensity and center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensity for δHOH has been multiplied by a factor 3. All the other conditions are the same as in Figure 8.

It is also interesting to study comparatively the prepeak in a CO-sparged electrolyte. Figure 8 shows four bands for νOH of H2Ofree, νCOL, νCOB+M, and δHOH of H2Ofree, the same as those observed in the CO-saturated solution except for lower 16383

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Figure 10. Series of time-resolved SEIRA spectra (0.0 s f 9.0 s) of CO on a Pt electrode in 0.1 M HClO4 collected sequentially during a potential holding at 0.1 V for 2 s, followed by step from 0.1 to 0.65 V. The time resolution used and shown is 0.1 s. The CO was adsorbed at 0.1 V in 0.1 M HClO4 solution for 30 min and then the dissolved CO was purged with Ar for 3 h. The reference spectrum was taken at 20 s.

frequencies of νOH and νCOad bands and weaker νCOM band due to the lower COad coverage.27 Again, the shape of the νCOB+M band is sensitive to the coverage of COad and the costructure during the oxidation process in Figure 8. Meanwhile, the quantitative analysis for the detailed spectral information is shown in the Figure 9. Similar to Figure 7, four potential regions herein can be identified, i.e., (a) 0.3 to 0.44 V, (b) 0.44 to 0.61 V, (c) 0.61 to 0.73 V, and (d) above 0.73 V. Potential regions (a) and (b) correspond to the weaker prepeak, and potential regions (c) and (d) to the main oxidation peak, respectively. In Figure 9, during the anodic potential scan the current increases very gently at ca. 0.3 V in the potential region (a) and is relatively pronounced in the potential region (b). At the beginning of the potential region (b), the νCOL frequency as well as the Gram Schmidt intensity starts to decrease gently. Before this change, the Stark tuning rate of νCOL is ca. 32 cm 1 V 1 from 0.1 to 0.44 V. The COL band intensity slightly decreases while the COB+M band intensity keeps virtually unchanged in the potential region (b) of Figure 9. This indicates that the COL is easier to be oxidized than COB+M, in agreement with previous reports.31,39 In the CO-sparged solution, the COL at active sites is oxidized first at 0.3 V, and the COad near active sites starts to move subsequently. Then the relaxation of the costructure occurs at about 0.44 V (corresponding to a turning point at this potential in the Gram Schmidt intensity plot or the νCOL frequency vs E plot in Figure 9). In the case of CO-saturated solution in Figure 7, this variation of the costructure occurs at 0.64 V, mainly because the diffusion and oxidation of COL was compensated to some extents by the readsorption of CO from solution. The longer variation time (potential region (b) of Figure 9) is due to lower electrocatalytic activity of the electrode at lower potentials than that in potential region (b) of Figure 7. Similar explanation may also apply to the less pronounced decrease of the Gram Schmidt intensity in Figure 9 than that in Figure 7. In the above two cases of CO@UPD, higher global COad coverages should be responsible for partial COad oxidation at such low potentials, i.e., the potential region (a). The reasons are: first, the COad adsorption heat is lower at a higher coverage, e.g., on Pt (h k l) in aqueous electrolyte, that may vary from ca. 150 ( 15 kJ/mol at low coverage to ca. 65 ( 15 kJ/mol at saturated coverage;1 second, the structure of high-coveraged CO@UPD, is

Figure 11. (upper) Current transient of a CO-predosed Pt electrode for a potential step from 0.1 to 0.65 V along with IR bands of COad and H2Ofree. The insert shows the potential programs, potential step from E1 (0.1 V, 2s) to E2 (0.65 V, t2); (lower) plots of time-dependent Gram Schmidt intensity, center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensities for νOH and νCOB+M have been by a factor 2. All other conditions are the same as in Figure 10.

not stable, in which COB could shift to COL and/or COM to COB at potentials higher than 0.3 V.1,33,52 Nevertheless, the formation of oxygenated species (adsorbed OH or activated H2O, simply denoted as OHad) on the active sites is essential for the oxidation of partial CO@UPD in the potential region (a) prior to the relaxation of the COad-H2Ofree costructure.43 The relaxation of the costructure may deter the OHad reformation at such low potentials, which may account for the observations in Figure 5 of 16384

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Figure 12. (upper) A current transient of a CO-predosed Pt electrode for a potential step from 0.1 to 0.55 V (the time scale for the data was from 0 to 95 s) along with IR bands of COad and H2Ofree. (lower) plots of time-dependent Gram Schmidt intensity, center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensity for νOH has been multiplied by a factor of 2.

ref 31 by Samajeke et al., i.e., the absence of the prepeak in the second cycle (red trace) between 0.05 and 0.60 V, and the unchanged Stark tuning rate of COL until the start of the main oxidation. In summary, based on combined time-resolved SEIRAS and the Gram Schmidt response analyses on the CO@UPD prepeak in both CO-saturated and CO-free electrolytes, a more detailed and reasonable explanation is proposed for the prepeak. The oxidation of partial CO at the active sites, including the weakly adsorbed COL and/or the CO near the active sites diffused from the solution yields the prepeak, occurring with the relaxation of the costructure and related interfacial water layers. The oxygenated species involved in the prepeak at low potentials is likely to form only on the active sites before the relaxation. Although we may agree with Osawa’s group31 on that the prepeak may be ascribed to the partial oxidation of COL, it seems to us that the triggering mechanism (i.e., the potential-induced site conversion of COB by assuming that COB is kicked away from the surface by the repulsive interaction with preadsorbed COL before reaching atop sites.) they proposed lacks sufficient evidence. For the first time we provide solid in situ spectral evidence in conjunction with Gram Schmidt response analysis to clarify the details involved in the prepeak. Main Oxidation Process of CO@UPD. After the prepeak region, the main oxidation current increases apparently from 0.7 V with a peak at 0.75 V for CO-saturated solution in Figure 7, or from 0.61 V with a peak at 0.73 V for CO-sparged solution in Figure 9, in which the CO could also be activated and oxidized at terrace sites,1 not just at the active sites. The surface CO coverage, θCO,

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defined as QCO/2(QH), was estimated to be ca. 0.81 for CO@UPD in Figure 9, where QCO is the CO oxidation peak charge calibrated with the subsequent cyclic voltammetric curve as the baseline and QH is the hydrogen desorption charge in the absence of adsorbed CO.27,48 In the potential region (c) of Figures 7 and 9, the νOH, νCOL, and νCOB+M bands get weakened quickly, accompanied with significant red shift and band broadening. In both cases, the νCOL band decreases prior to the νCOB+M band which indicates that COL is easier to oxidize than COB+M. Meanwhile, a steep increase of the Gram Schmidt intensity was observed during the main oxidation of COad due to the drastic change of the interfacial structure. The significant red shift and broadening of νOH, νCOL and νCOB+M bands suggest that the local coverage of H2Ofree and COad quickly and continually decrease during the main CO@UPD oxidation process due to the decrease of dipole dipole interaction.51,52 In other words, the surface diffusion of COad is fast and the spatial distribution of COad is rather homogeneous. The mobility of COad is significantly enhanced probably due to the relaxation and change of the costructure and the disorganization of outer water layers in the prepeak region. The Gram Schmidt intensity reached its maximum at 0.75 V in COsaturated solution in Figure 7 (or at 0.73 V in CO-sparged solution in Figure 9), while the COad bands and the currents still can be seen at this potential (In fact, the main oxidation currents touch the bottom at 0.80 V in Figure 7, and 0.77 V in Figure 9, respectively). In other words, the interfacial structure variation completes before the COad adlayer is completely oxidized. It indicates that small amount of COad residue species does not affect the interfacial structure due to its quite low local coverage, fast surface diffusion and relatively homogeneous spatial distribution. The main oxidation process of CO@UPD at constant potentials was further probed by time-resolved ATR-SEIRAS in conjunction with two potential-step experiments, and the results are shown in Figures 10 and 11 (where the potential is stepped from 0.1 to 0.65 V within the main oxidation region) and Figure 12 (where the potential is stepped from 0.10 to 0.55 V within the prepeak region). For the former, the activation and relaxation process only takes 0.5 s in Figure 11, in which the νCOL band intensity immediately decreases accompanied with the increase of its fwhh while the νCOB+M band intensity slightly decreases due to the site shift and oxidation. In contrast, the Gram Schmidt intensity first increases after the potential step, then decreases due to the relaxation of the costructure and then increases quickly again due to the COad oxidation. For the latter, the activation and relaxation process takes ca. 6 s, as indicated by the Gram Schmidt intensity plot (green curve). In the both oxidation processes in Figures 11 and 12, the νCOL band quickly decreases, broadens and red-shifts while COB+M gets slower oxidation (it is not oxidized completely even at 100 s in Figure 12). The lower oxidation rates of COB+M tail the current transient in Figures 11 and 12.31 The spectral results indicate that the initial current spike in Figure 11 is not just a purely pseudocapacitive charge, but also involves the oxidation of partial COad.39 The results of main oxidation of potential step experiments also show that local coverage of H2Ofree and COad quickly and continually decrease due to the decrease of dipole dipole interaction.51,52 In other words, the surface diffusion of COad is fast and the spatial distribution of COad is rather homogeneous. Another obvious conclusion is that the COL is much easier to oxidize than the 16385

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Figure 13. Series of time-resolved SEIRA spectra of CO on Pt nanofilm electrode in 0.1 M HClO4 collected sequentially during a potential sweep cycling from 0.45 to 1.1 V at 50 mV s 1. The time resolution used and shown is 0.2 s. The CO was adsorbed at 0.45 V in 0.1 M HClO4 solution for 30 min and then the dissolved CO was purged with Ar bubbling for 3 h. The reference spectrum was taken at 1.1 V.

Figure 14. (upper) Cyclic voltammogram, first cycle (solid line) and second cycle (dash line), of a CO-predosed Pt electrode recorded at 50 mV s 1 during in situ SEIRAS measurement along with selected potential dependent integrated intensities for IR bands of COad and H2Ofree; (lower) plots of potential-dependent Gram Schmidt intensity, center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensities for νCOB+M and δHOH have been multiplied by a factor 2 and 5 respectively. All other conditions are the same as in Figure 13.

COB+M not only in the prepeak but also in the main oxidation peak for CO@UPD. This fact is very important in discussing the origin of the prepeak in the above section (supra infra).

Figure 15. (upper) Cyclic voltammogram of a CO-predosed Pt electrode recorded at 50 mV s 1 during in situ SEIRAS measurement along with selected potential dependent integrated intensities for IR bands of COad and H2Ofree; (lower) plots of potential-dependent Gram Schmidt intensity and center frequencies and FWHHS for IR bands of COad and H2Ofree. The intensities for νCOB+M and δHOH have been multiplied by a factor 2 and 10, respectively. Series of time-resolved SEIRA spectra (not shown) of CO on Pt electrode in CO-saturated 0.1 M HClO4 collected sequentially during a potential sweep cycling from 0.45 to 1.1 V at 50 mV s 1.

Regarding the role of the H2Ofree, Kunimatsu et.al.25,34 suggest that the H2Ofree is activated and directly involved in 16386

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The Journal of Physical Chemistry C oxidizing the COad because of nearly concurrent decrease of these two species. However, according to our spectral results that the band of H2Ofree disappears much earlier than that of COad (the H2Ofree disappears 6.5 s in advance in Figure 11 or 1 min in advance Figure 12), a better explanation should be that the H2Ofree is just the indicator of the local COad coverage and is not directly involved in the COad oxidation. In other words, the H2Ofree will disappear when the local coverage of COad is lowered down sufficiently in agreement with that observed in CO adsorption process. Briefly, the spectroelectrochemical results demonstrate that the main oxidation of CO@UPD proceeds largely via the “meanfield approximation” kinetics rather than via the “nucleationand-growth” kinetics in the framework of the Langmuir Hinshelwood mechanism.7,10,13 In the main oxidation process, the COad could have a high mobility and homogeneous distribution which could result from the relaxation of the costructure in the prepeak, and the oxidation of COL is faster than that of COB+M. The H2Ofree can be better assigned to a probe of the local COad coverage. 3.3. Oxidation of CO@DL. The time-resolved SEIRAS results of CO@DL oxidation in CO-free 0.1 M HClO4 during a potential scan from 0.45 to 1.1 V are shown in Figures 13 and 14. The shape of the COB+M band does not change significantly in the whole oxidation process in Figure 13, in contrast to that in Figures 6 and 8, suggesting that the local COad and H2O costructure is rather stable in the oxidation process. The current starts to increase at about 0.71 V, and then peaks at 0.79 V with a shoulder at 0.76 V in Figure 14. The surface coverage of CO@DL is ca. 0.76 as evaluated from Figure 14, slightly smaller than that of CO@UPD, in reasonable agreement with the previous reports.27,48 In the oxidation region (a) from 0.71 to 0.78 V, the band intensities for H2Ofree and COad decrease synchronously with nearly constant νCOL, νCOB+M and νOH frequencies. It is interesting to note that both the Stark tuning rate of νCOL and the Gram Schmidt intensity keep nearly unchanged in the initial oxidation phase (potential region (a)), which is totally different from that found in the case of CO@UPD. In the potential region (b) of Figure 14, the IR bands intensities of COad and H2Ofree decrease quickly accompanied with the slight red shift and broadening of the bands, and the Gram Schmidt intensity varies synchronously with the stripping of the remaining COad. In Figure 14, although nearly half of the COad is estimated to be oxidized in the potential region (a) by the integration of the CO stripping peak, we do not see the red-shift and broadening of the COad bands and the increase of Gram Schmidt intensity. It means that the interfacial structure including the local COad coverage hardly changes in this initial oxidation phase, totally different from that in the case of CO@UPD. In the potential region (b), the minor red shift and broadening of the COad bands in response to the oxidation of the remaining COad further indicate that the local COad coverage changes very little in the main oxidation phase. At the near end of oxidation process, the νCOL frequency is about 2076 cm 1 at 0.84 V which is much higher than that in the case of CO@UPD, i.e., 2041 cm 1 at 0.73 V in Figure 9, even with a calibration of the Stark effect. This indicates that the local COad coverage is high even when most of COad is oxidized. Moreover, the Gram Schmidt intensity in Figure 14 varies synchronously with the stripping of the remaining COad in stark contrast to the case of CO@UPD oxidation in which the Gram Schmidt

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intensity reaches the higher plateau prior to the complete consumption of COad. In other words, in the late oxidation stage, despite a very small global CO coverage, CO molecules are locally dispatched on Pt surfaces in the form of smaller CO islands rather than homogeneously distributed as in the meanfiled approximation model. When the solution is saturated with CO (Figure 15), the oxidation behavior is similar to the case in CO-free solution except for additionally small contribution from solution CO in the potential region (a) of Figure 15. In the potential region (b) of Figure 15, the intensities for IR bands of COad and H2Ofree start to decrease in response to the COad oxidation. Furthermore, the Gram Schmidt intensity increases quickly in main oxidation in the potential region (c) of Figure 15, but the frequency and the fwhh of the νCOL band are almost unchanged. These spectral results also suggest that either the local coverage of COad is high and less mobile or the CO@DL and H2Ofree costructured islands/domains with shrinking sizes are mobile but possess a relative stable local coverage during the main oxidation; the COL and COB+M are oxidized nearly simultaneously at the boundary of COad and H2Ofree islands/ domains; the variation of the interfacial structure completes with the total stripping of COad. Briefly, the results of the CO@DL oxidation in both CO-free and CO-saturated electrolytes indicate that the OHad could form at the active sites and the reactions just occur around CO@DL and H2Ofree costructured islands. Subsequent propagation of the reaction fronts occurs at the boundary between OHad and COad domains. Only at the end of CO oxidation process does the variation of interfacial structure complete and the H2Ofree disappear, owing to the relatively stable local costructure within the islands. This process approximately follows the “nucleation and growth model” kinetics.18 20 Again, the H2Ofree species serves likely as an indicator of the local COad coverage.

4. CONCLUSIONS Adsorption and electro-oxidation of CO@UPD and CO@DL on a polycrystalline Pt electrode were systematically revisited by using real-time (resolution down to 100 ms) ATR-SEIRAS in conjunction with Gram Schmidt response analysis and reexamination of diverse proposals in literature. In the case of CO@UPD, the partial oxidation of the weakly adsorbed COL and the CO transferred from solution is suggested to yield the prepeak that occurs with the relaxation of the COad and H2Ofree costructure and related interfacial water layers. At low potentials of the prepeak region, oxygenated species only produces on the active sites before the relaxation. The main oxidation process of CO@UPD may be qualitatively described via “meanfield approximation” kinetics model due to the faster diffusion and rather homogeneous distribution of COad resulted from the relaxation and the disorganization involved in the prepeak. In contrast, the rather stable costructure of local CO@DL and H2Ofree islands formed in the very initial adsorption phase and least dependent on the global coverage enables the CO@DL oxidation to proceed approximately via the “nucleation and growth” kinetics. The interfacial structure varies synchronously with the stripping of CO@DL. In both cases, the H2Ofree may be better assigned as a probe for the local COad coverage rather than the oxygenated species to directly oxidize COad. 16387

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’ ASSOCIATED CONTENT

bS

Supporting Information. Comparison of spectra at 0.45 V for the CO@UPD and the CO@DL in the presence of solution CO; comparison of the plots of νCOL frequencies vs normalized COad band intensities during adsorption process of CO@UPD and CO@DL; Gram Schmidt orthogonalization and Gram Schmidt reconstruction in in situ SEIRAS measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (W.-B.C.); [email protected] (Y.-G.Y.).

’ ACKNOWLEDGMENT This work is supported by NSFC (Nos. 2083305, 20873031, and 21073045) and STCSM (Nos. 08JC1402000 and 08DZ2270500). Y.-G.Y. thanks the Alexander von Humboldt Foundation for a Postdoctoral Fellowship and Prof. Nenad M. Markovic, Prof. Jacek Lipkowski, and Dr. Hans-Hermann Belz (Thermo Fisher Scientific GmbH) for their scientific discussions. ’ REFERENCES (1) Markovic, N. M.; Ross, P. N., Jr. Surf. Sci. Rep. 2002, 45, 117–229, and references cited therein. (2) Ertl, G; Neumann, N.; Streit, K. M. Surf. Sci. 1977, 64, 393–410. (3) Engel, T.; Ertl, G. Adv. Catal. 1979, 28, 1–78. (4) Akemann, W.; Friedrich, K. A.; Linke, U.; Stimming, U. Surf. Sci. 1998, 402 404, 571–575. (5) Lebedeva, N. P.; Koper, M. T. M.; Herrerro, E.; Feliu, J. M.; Van Santen, R. A. J. Electroanal. Chem. 2000, 487, 37–44. (6) Lebedeva, N. P.; Rodes, A.; Feliu, J. M.; Koper, M. T. M.; Van Santen, R. A. J. Phys. Chem. B 2002, 106, 9863–9872. (7) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; Van Santen, R. A. J. Phys. Chem. B 2002, 106, 12938–12947. (8) Gilman, S. J. Phys. Chem. 1964, 68, 70–80, and references cited therein. (9) Maillard, F.; Eikerling, M.; Cherstiouk, O. V.; Schreier, S.; Savinova, E.; Stimming, U. Faraday Discuss. 2004, 125, 357–377. (10) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R. A. J. Electroanal. Chem. 2002, 524 525, 242–251. (11) Markovic, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. J. Phys. Chem. B 1999, 103, 487–495. (12) Housmans, T. H. M.; Hermse, C. G. M.; Koper, M. T. M. J. Electroanal. Chem. 2007, 607, 69–82. (13) Bergelin, M.; Herrero, E.; Feliu, J. M.; Wasberg, M. J. Electroanal. Chem. 1999, 467, 74–84. (14) Samjeske, G.; Xiao, X. Y.; Baltruschat, H. Langmuir 2002, 18, 4659–4666. (15) Cherstiouk, O. V.; Simonov, P. A.; Zaikovskii, V. I.; Savinova, E. R. J. Electroanal. Chem. 2003, 554 555, 241–251. (16) Kobayashi, T.; Babu, P. K.; Gancs, Ho Chung, L. J.; Oldfield, E.; Wieckowski, A. J. Am. Chem. Soc. 2005, 127, 14164–14165. (17) Kobayashi, T.; Babu, P. K.; Panakkattu, K.; Ho Chung, J.; Oldfield, E.; Wieckowski, A. J. Phys. Chem. C 2007, 111, 7078–7083. (18) Mc'Callum, C.; Pletche, D. J. Electroanal. Chem. 1976, 70, 277–290. (19) Love, B.; Lipkowski, J. In Electrochemical Surface Science; Soriaga, M., Ed.; ACS Symposium Series; ACS: Washington, DC, 1988, Vol. 378, 474. (20) Korzeniewski, C.; Kardash, D. J. Phys. Chem. B 2001, 105, 8663–8671.

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