Potential-Dependent Chemisorption of Carbon Monoxide at a Gold

Dec 9, 2009 - The potential (E)-dependent spectral behavior of both C−O and ... by the DFT slab model are in good agreement with the observation by ...
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J. Phys. Chem. C 2010, 114, 403–411

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Potential-Dependent Chemisorption of Carbon Monoxide at a Gold Core-Platinum Shell Nanoparticle Electrode: A Combined Study by Electrochemical in Situ Surface-Enhanced Raman Spectroscopy and Density Functional Theory Pu Zhang,† Jun Cai,† Yan-Xia Chen,*,† Zhi-Qiang Tang,† Dong Chen,† JinLong Yang,† De-Yin Wu,‡ Bin Ren,‡ and Zhong-Qun Tian‡ Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, 230026, China, and State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen UniVersity, Xiamen, 361005, China ReceiVed: September 2, 2009; ReVised Manuscript ReceiVed: October 29, 2009

The potential (E)-dependent spectral behavior of both C-O and Pt-CO stretching vibrations from a saturated COad layer at a Au core-Pt shell nanoparticle film electrode has been examined in a wide potential window by surface-enhanced Raman spectroscopy (SERS). The Stark slopes of C-O stretching for linear-(COL) and bridge-(COB) binding CO adsorbates are positive and increase toward more negative potentials, which is explained by a synergetic effect from the potential-induced site conversion, the consequent changes in dipolesdipole coupling interactions among the nearby CO oscillators in addition to the potential-dependent chemical bonding changes. The Stark slopes of the Pt-CO stretching of COL and COB are negative and roughly constant (ca. -5 and -20 cm-1/V) throughout the potential regime examined, which is mainly attributed to the potential-dependent changes in the Pt-CO chemical bonding. From the measured potential-dependent frequencies of the Pt-CO stretching, the potential-induced changes in Pt-CO bond length are estimated to be 0.005 (0.01) Å/V for COL (COB) and the changes in CO binding energies (∆Eb/∆E) are ca. 0.20 eV/V (for COL) and 0.37 eV/V (for COB), respectively. The higher ∆Eb/∆E for COB than that for COL reveals that the chemical bonding of Pt-COB is more sensitive to the changes in the interfacial electric field, as is consistent with theoretical predictions. From the DFT calculations using two different (2 × 2)-3CO slab models with Pt(111), we found that the overall trends of the potential-dependent frequencies of Pt-CO and C-O stretching predicted by the DFT slab model are in good agreement with the observation by SERS. However, great discrepancies in the Stark slopes between the calculations and experimental results exist; possible origins for such differences have been discussed. 1. Introduction Carbon monoxide molecules are the common reactants or reaction intermediates in various industrial relevant processes, such as the anode reactions in fuel cells, removal of car exhaust gases, methanation, and Fischer-Tropsch processes.1,2 It is found that CO molecules adsorb strongly on the surfaces of most Pt-based catalysts commonly used in such processes and play a very important role in the reaction kinetics.3 To have a better understanding of the nature of CO chemisorptions as well as to improve the catalytic efficiency, extensive studies of the chemisorption of CO at Pt-based metal surfaces have been carried out both theoretically4-15 and experimentally,16-25 especially by vibrational spectroscopic techniques. In electrochemical environments, the effect of electrode potential on the adsorption/oxidation of CO at the Pt electrode is the central focus of such a topic. On the basis of the wellconfirmed experimental evidence on the C-O stretching vibration from CO adsorbed at the Pt electrode as well as the corresponding theoretical calculations, some consensus on the Pt-CO surface bonding is established: (i) CO can adsorb at atop (COL), 2-fold bridge (COB), and 3-fold hollow (COH) sites * To whom correspondence should be addressed. E-mail: yachen@ ustc.edu.cn. † University of Science and Technology of China. ‡ Xiamen University.

at Pt surfaces, and at more negative potentials higher coordination sites for CO are favored;26 (ii) there is a linear dependence of C-O stretching frequency with electrode potential (the socalled “electrochemical” Stark effect);27 and (iii) toward lower COad surface coverages (θCO), C-O stretching frequency (νC-O) decreases and the corresponding Stark slope (dνC-O/dE) increases.27-29 In most previous experiments carried out in electrochemical environments, both the electrode potential and the COad surface coverage (as well as the fractional surface coverage of COL, COB, and COH) change simultaneously. Because of the interference of the spectral behavior of C-O stretching vibration by the strong dipolesdipole coupling effects among the adsorbed CO molecules, direct derivation of surface bonding information from the spectral behavior of C-O stretching vibration is very difficult.30 On the other hand, the dipolesdipole coupling effects of the metal-adsorbate vibration are ca. 2 orders smaller than those of the corresponding C-O vibtation;31 thus, it is more straightforward to derive the metal-chemisorbate bonding parameters, such as bonding strength and bonding length from Pt-CO vibration. It is believed that an accurate determination of the metalsCO stretching within the anharmonic approximation could be used to indirectly estimate the adsorption energies of CO on metal surfaces.14

10.1021/jp908478m  2010 American Chemical Society Published on Web 12/09/2009

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However, density functional theory (DFT) calculations using a cluster model by Wasileski et al. showed that, with the decrease in the electric field from 0.5 to -0.3 V/Å, the binding energy of COL to Pt decreases slightly; further increasing the electric fields negatively from -0.3 to -1.0 V/Å leads to an increase in the binding energy of COL.9,10 On the other hand, the calculated binding energy of COH decreases monotonically with the negative shift in the electric field from 0.5 to -1.0 V/Å. Furthermore, their calculations showed that both dνPt-COL/ dE and dνPt-COH/dE are negative at positive and mild negative fields, while at a large negative field (F < -0.3 V/Å), dνPt-COL/ dE is positive and dνPt-COH/dE is nearly zero. The opposite potential dependence for νPt-COL and Eb with the maximum of νPt-COL appearing at the potential where Eb is smallest is surprising.9,10 In contrast to the above-mentioned prediction by the DFT studies with a cluster model, recent DFT studies using slab models13-15 show no maximum of νPt-COL appearing at mild negative potentials. In contrast, periodic DFT calculations give much smaller Stark slopes for dνPt-CO/dE and dνC-O/dE than those from the experimental observation and DFT calculations using the cluster model. In addition, periodic DFT calculations show that the absolute value of dνPt-CO/dE (denoted as |dνPt-CO/dE|) is larger for atop sites than for high-coordinated sites,14 which is at variance with the prediction by DFT using the cluster model. The latter shows that dνPt-CO/dE for hollow sites is ca. twice than that for atop sites.10 Up to now, it is not possible to judge which prediction is more realistic since no direct experimental evidence is available so far. The field-dependent metal-adsorbate vibration, normally inaccessible to infrared adsorption spectroscopy (IRAS), can be obtained in the electrochemical interface by SERS.21 Recently, we have developed a thin layer flow cell for electrochemical in situ SERS study.32 Exploiting the “intensity borrowing strategy”, we have successfully observed the Raman spectra of both Pt-CO and C-O stretching vibrations at a Au core-Pt shell nanoparticle film electrode.32 To have a better understanding of the potential-dependent Pt-CO surface bonding and to correctly evaluate the above-mentioned discrepancies between the experimental and theoretical studies as well as the discrepancies among different theoretical approaches, we have carried out systematic studies on the potential-induced changes of the vibrational properties of Pt-CO and C-O stretching modes of a saturated CO adlayer at a Pt electrode in a wide potential range (from -1.2 to 0.55 V) using electrochemical in situ SERS combined with a thin layer flow cell and the periodic DFT calculations. The broad potential window in the SERS study is realized by carrying out the measurements with a COadsaturated electrode surface under a continuous flow of COsaturated electrolyte and by the change in the pH of the electrolyte. The implications of the potential-induced changes in the peak frequency, binding energy, and equilibrium Pt-CO bond length will be discussed based on the complementary information of both Pt-CO and C-O stretching vibrations and DFT calculations. 2. Experimental Section 2.1. Experimental Setup. The design of the electrochemical flow cell for SERS study was largely based on the knowledge of the spectroelectrochemical flow cell for EC-IRAS studies with attenuated total reflection configuration developed by Chen et al.33 and has been described in detail in ref 32. The electrolyte flowing through the cell can be switched between different electrolytes reservoirs, and the flow rate is controlled by

Zhang et al. hydraulic pressure in the reservoirs. In this experiment, the flow rate was 50 µL/s. All measurements were performed at ambient temperature (25 ( 3 °C). A thin Au foil (thickness ) 50 µm) and a saturated calomel electrode (SCE) were used as the counter electrode (CE) and the reference electrode (RE). All potentials are reported with respect to a normal hydrogen electrode (NHE) in this paper. Electrochemical measurements were conducted using a CHI631B electrochemical workstation (CH Instruments, Shanghai, China). SERS measurements were carried out with a confocal microprobe Raman system (LabRam I from Dilor, France) using an air-cooled CCD and a He-Ne laser operating at 632.8 nm. The laser power delivered at the sample (with a beam diameter of ca. 2 µm) was approximately 5 mW. The microscope attachment was based on an Olympus BX40 system using a long working length (8 mm) 50× objective. An 1800 g/mm grating was used, and the spectral resolution is 1 cm-1. 2.2. Chemicals and Preparation of 55 nm [email protected] nm Pt Nanoparticle Film Electrode. HAuCl4 (A. R.), H2PtCl6 (A. R.), sodium citrate (A. R.), ascorbic acid (A. R.), Na2SO4 (A. R.), H2SO4 (G. R.), and NaOH (G. R.) were purchased from Shanghai Reagent Corporation, China. Millipore Milli-Q water (18.2 MΩ/cm) was used throughout the study. Before SERS measurements, all the electrolyte solutions were deaerated by continuous N2 (4N, from Linde Gas China) purging. COsaturated solution was achieved by prebubbling the supporting electrolyte with pure CO (99.9%, from Linde Gas China) for 15 min in the electrolyte reservoir and with continuous purging during the experiments. Pt nanoparticles (55 nm [email protected] nm) were prepared by coating a thin layer (0.7 nm) of Pt over 55 nm Au nanoparticles following the report by Tian’s group.34 In brief, after synthesizing the Au nanoparticles with a diameter of ca. 55 nm by reducing AuCl4- using sodium citrate, 30 mL of sol containing 55 nm Au seeds were mixed with 0.76 mL of 1 mM H2PtCl6 and heated to 80 °C for several minutes. Ascorbic acid (0.4 mL, 10 mM) was then slowly dropped into the above mixtures through a syringe controlled by a step motor under vigorous stirring. The mixtures were further stirred for about 20 min to ensure the complete reduction of H2PtCl6. The nanoparticles display an ellipsoidal shape with a uniform size distribution, as demonstrated in the scanning electron microscope (SEM) image (inset in Figure 1). After that, the 55 nm Au core-0.7 nm Pt shell (denoted as 55 nm [email protected] nm Pt or Au@Pt hereafter) sol was centrifuged three times to remove excess reactants. The remaining sol (5 µL) was then cast on a smooth Pt electrode (diameter ca. ) 1.5 mm) and dried in a desiccator, and this procedure was repeated three times to ensure that the Pt electrode was completely covered by Au@Pt nanoparticles. The cyclic voltammograms (CVs) of 55 nm [email protected] nm Pt nanoparticles coated Pt electrode (denoted as 55 nm [email protected] nm Pt/Pt hereafter) in 0.5 M H2SO4, 0.5 M Na2SO4, and 0.5 M NaOH are plotted in Figure 1a, which are in good agreement with literature reports,35,36 confirm that the nanoparticle electrodes exhibit the electrochemical behavior of polycrystalline Pt and that our Raman flow cell works well for electrochemical measurements. 2.3. Experimental Protocol. The potential-dependent SERS experiments of the saturated COad layer at 55 nm [email protected] nm Pt/Pt were conducted as following. The electrode surface was first saturated with the COad layer by flowing CO-saturated 0.5 M H2SO4 solution through the cell when holding the electrode potential at 0.05 V for 15 min. The CVs from -0.2 to 0.55 V at a scan rate of 1 mV/s and the corresponding Raman spectra

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Figure 2. Two different Pt(111) (2 × 2)-3CO models used in the periodic DFT calculations. (a) COL + 2COH: one atop + one fcc hollow + one hcp hollow CO molecules. (b) COL + 2COB: one atop + two bridge CO molecules.

Figure 1. Cyclic voltammograms of 55 nm [email protected] nm Pt/Pt in 0.5 M H2SO4 (solid line), Na2SO4 (dashed line), and NaOH (dashed-dotted line) solutions (scan rate ) 10 mV/s) (a) and CO-saturated 0.5 M H2SO4 (solid line), Na2SO4 (dashed line), and NaOH (dashed-dotted line) solutions (scan rate ) 1 mV/s) (b). Inset: SEM image of 55 nm [email protected] nm Pt core-shell nanoparticles.

in the range from 1600 to 2250 cm-1 were then recorded at a time resolution of 50 s per spectrum (50 mV per spectrum). Then after, the grating was switched to cover the region from 250 to 1050 cm-1 and the same potential program was repeated to record the spectral signal of the Pt-CO stretching vibration. All the measurements were done under continuous flowing with CO-saturated H2SO4. After scanning back to -0.1 V in 0.5 M H2SO4, the electrolyte solution is switched to CO-saturated 0.5 M Na2SO4 when holding at -0.1 V for ca. 5 min. In between the cell and the connecting tubes is carefully flushed with 0.5 M Na2SO4 to make sure that the solution exchange is complete. The electrode potential is then scanned negatively from -0.1 to -1.2 V and then back. After that, the electrode potential was held at -0.55 V, and the electrolyte was switched to COsaturated 0.5 M NaOH; the CV from -0.45 to -1.2 V was also recorded (Figure 1b). During the potential scan in neutral and basic solutions, the Raman spectra of C-O and Pt-CO stretching vibrations were also recorded as similar to the case in acidic solution. It should be emphasized that the lower potential limit chosen in such measurements is to avoid the bubbles from the fast hydrogen evolution reaction (HER), and the upper potential limit is chosen to avoid any COad oxidation that may cause the change in the coverage of the CO adlayer, as is confirmed by the CVs recorded. The potential chosen for the solution switch is judged from the CVs measured above (Figure 1) to make sure that the CO adlayer is stable in both solutions during the electrolyte switch. Note that the small cathodic current at lower potentials in each solution is from the hydrogen evolution reactions that take place at the free sites in between the saturated adsorbed CO molecules. 3. Computational Details The first-principles calculations were performed with the plane-wave based Vienna ab initio simulaton package (VASP).37,38 The exchange-correlation energy was calculated within the generalized gradient appoximation (GGA) proposed by Perdew, Burke, and Ernzerhof.39 The interaction between atomic cores and electrons was described by means of the projector augmented wave (PAW) approach,40 and the cutoff energy was set to 400 eV. Ionic iterations were performed until the forces on the ions were less than 0.003 eV/Å. Brillouin zone integrations

were performed on a (5 × 5 × 1) grid of Monkhorst-Pack points.41 Fractional occupancies were calculated using a firstorder Methfessel-Paxton smearing function with a width of 0.2 eV.42 The Pt(111) surface was modeled using a 2 × 2 super cell composed of four metal layers with 4 Pt atoms per layer for a total of 16 Pt atoms in the unit cell. Each slab is separated from its periodic image in the z direction by a vacuum space (∼10 Å) to avoid interactions between slabs. The relative positions of the Pt atoms have been fixed initially as those in the bulk, with an optimized lattice parameter of 3.99 Å, which agrees well with the experimental value of 3.92 Å. The calculations were made with two different (2 × 2)-3CO structures, both with three adsorbed CO molecules in the unit cell (corresponding to the coverage of 0.75 ML, Figure 2). A structure with one atop and two bridge CO molecules (COL + 2COB) has been optimized and compared with another structure containing one atop, one fcc, and one hcp hollow CO molecules (COL + 2COH). For the (COL + 2COB) structure, the degrees of freedom of the C and O atoms in the plane parallel to the surface of the slab have been constrained to the ideal atop and bridge sites because a full relaxation of the C and O atoms along all coordinates shows that this structure (COL + 2COB)) is not energetically stable. An electric field perpendicular to the slab was imposed by introducing a planar dipole layer in the vacuum using the method implemented in VASP.43 Vibrational frequencies were calculated by diagonalization of the dynamical matrix. 4. Results and Discussion 4.1. Potential-Dependent SER Spectral Behavior of the Saturated CO Adlayer at Au@ Pt. Figure 3 shows representative sets of potential-dependent Raman spectra of Pt-CO and C-O stretching from the COad layer at 55 nm [email protected] nm Pt/ Pt in CO-saturated 0.5 M H2SO4 (Figure 3a,b), 0.5 M Na2SO4 (Figure 3c,d), and 0.5 M NaOH (Figure 3e,f). The higherfrequency bands in the region from 1970-2090 and 1770-1860 cm-1 are assigned to the intramolecular vibrations of linearly (C-OL) and bridge-bonded (C-OB) CO, respectively.28,44 The lower-frequency band located at ca. 480-490 cm-1 and the small shoulder at ca. 410 cm-1 are attributed to metal-adsorbate stretching vibrations of linearly (Pt-COL) and bridge-bonded (Pt-COB) CO based on previous studies.21,44-46 The band at ca. 982 cm-1 in Na2SO4 is from the symmetric vibration of SO42- in Na2SO4 located in the solution phase rather than in the chemically adsorbed state,47,48 as inferred by the fact that the band frequency of sulfate ion does not change with electrode potentials. It should be mentioned that the lack of the characteristic C-O stretching vibration from CO adsorbed at the Au surface (above 2130 cm-1)34 confirms that the 0.7 nm thick Pt overlayer on the Au core prepared in this study is pinhole-free. In Figure 3c,e, only one broad band in the low-frequency region is observed at E < -0.7 V. Furthermore, it is seen that,

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Figure 4. Curve-fitting analysis of the Pt-CO band at -0.7 V in COsaturated 0.5 M Na2SO4 (a) and NaOH (b) (solid line, raw spectrum; dashed line, fitted spectrum).

Figure 3. Representative SER spectra of Pt-CO (a, c, e) and C-O (b, d, f) from adsorbed CO on 55 nm [email protected] nm Pt/Pt as a function of electrode potential in CO-saturated 0.5 M H2SO4 (a, b), 0.5 M NaOH (c, d), and 0.5 M Na2SO4 (e, f). The spectra were taken with 50 s per spectrum.

in 0.5 M Na2SO4, the peak frequency of the Pt-CO band increases with electrode potentials from -1.2 to -0.7 V. However, when the potential changes from -0.7 to -0.2 V, the higher-frequency side of this broad band (corresponding to Pt-COL vibration) becomes more and more prominent than that of the lower-frequency side (for Pt-COB vibration) and both display a red shift in peak frequencies as the electrode potential increases. One may think that -0.7 V is the potential for the maximum νPt-COL, which decreases with potential scans to either direction, as predicted by DFT calculations using the cluster model.10 However, after careful examination of the potentialdependent SERS behavior of the CO adlayer in basic solution (Figure 3e), we did not observe a similar phenomenon. On the other hand, the well-identified bands for both C-OL and C-OB in the higher-frequency region recorded under the same condition in 0.5 M Na2SO4 (Figure 3d) suggest that the broad feature centered at ca. 460 cm-1 should come from the superimposition of the Pt-COL and Pt-COB bands. To correctly understand the potential-dependent spectral behavior, a deconvolution procedure is exploited to pick out the individual Pt-COL and Pt-COB bands. For those Raman bands with a well-recognized peak maximum for both Pt-COL and Pt-COB peaks, the deconvolution is done by choosing the

maximum as the peak center for each vibration from different adsorption configurations, assuming a Gaussian-Lorenzian type band shape, while for those Raman bands recorded below -0.7 V where only one broad feature with a single peak maximum is recognizable, the deconvolution is done by assuming that the Pt-CO band intensities scale roughly with that of the corresponding intramolecular C-O stretching vibrations. Examples of the Pt-COL,B bands recorded at -0.7 V before (solid line) and after (dotted line) spectral deconvolution with spectra recorded in 0.5 M Na2SO4 and 0.5 M NaOH are shown in Figure 4. By such data manipulation, information on the peak frequencies and integral band intensities as well as full widths at halfmaximum (fwhm) are extracted from the SERS spectra of C-OL,B and Pt-COL,B stretching vibrations and is plotted as a function of electrode potential in Figure 5. From Figure 5a, it is seen that there is a monotonic increase in the peak frequencies of the C-OL,B and a decrease in the peak frequencies of Pt-COL,B with electrode potential in all three solutions. We found that the slope of dνC-OL/dE is different in various potential regions. In 0.5 M H2SO4, dνC-OL/dE is 32 cm-1/V; in 0.5 M Na2SO4 and 0.5 M NaOH, dνC-OL/dE is ca. 92 cm-1/V in the potential region from -1.2 to -0.15 V, which is about 3 times higher than that from -0.2 to 0.55 V in 0.5 M H2SO4. In the neutral and alkaline solutions, dνC-OB/dE is 40 cm-1/V from -0.9 to -0.15 V and 137 cm-1/V from -1.2 to -0.9 V; however, a comparison with that in acidic solution is not possible due to the barely discernible C-OB band in acidic solution, as observed in the present study. The lack of the C-OB band in acidic solutions is probably due to the smaller Raman cross section as well as the low fractional surface coverage of COB. On the other hand, in all the three solutions, dνPt-COL/dE and dνPt-COB/dE are roughly constant throughout the potential regime examined with values of -5 and -20 cm-1/V, respectively. It can be easily found out that this is just because of the big difference in the slopes of dνPt-COL/dE and dνPt-COB/dE, which leads to the mergence of Pt-COL and Pt-COB bands at large negative potentials. Furthermore, it is seen that, with the negative shift of electrode potential, the band intensities of C-OB and Pt-COB increase, while those of C-OL and Pt-COL decrease (Figure 5b) in all three solutions, accompanying with the increase of bandwidths for all the bands (Figure 5c). Such spectral changes clearly reveal that, toward negative potentials, site conversion from COL to COB occurs and the COad layer structure becomes more disordered. Except for the case that, in acidic solution, the C-OB band is not observed, while the Pt-COB band appears, the alternation of Pt-COL,B band intensities in both alkaline and neutral solutions throughout the potential regime examined follows approximately with the corresponding changes of C-OL,B bands. The total band intensities (IC-OL + IC-OB or IPt-COL + IPt-COB) decrease with electrode potentials, and this

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Figure 5. Peak frequencies (a), integrated band intensities (b), and fwhm’s (c) of Pt-COL, Pt-COB, C-OL, and C-OB from adsorbed CO on 55 nm [email protected] nm Pt/Pt as a function of electrode potential in 0.5 M H2SO4 (O), Na2SO4 (0), and NaOH (3).

could be explained by the potential-dependent decrease (increase) fractional coverage of COL (COB) in addition to the smaller Raman cross section of COB comparing to that of COL.49 In 0.5 M NaOH, though the potential-dependent peak frequencies and bandwidths are quite similar to those in 0.5 M Na2SO4, the intensity ratios of IPt-COL/IPt-COB and IC-OL/IC-OB are higher in NaOH than that in Na2SO4 (Figures 3-5) at E < -0.7 V. To make sure that such spectral changes are not due to artifacts, we have repeated the experiments in 0.5 M Na2SO4 and 0.5 M NaOH carefully and found that such spectral behavior is quite reproducible. Because the only differences in these two electrolytes are anions, the smaller COB population in NaOH comparing to the case in 0.5 M Na2SO4 must be due to effects from the anions. Spendelow et al. have suggested that, even at negative potentials, it is possible for OH- anions to specifically adsorb at defects sites.50 One consequence from OH- adsorptions is that the potential-induced COL to COB conversion is inhibited. Thus, the trend in the band intensity changes is not as clear as that in 0.5 M Na2SO4. Besides, so far, we are not sure whether nonadsorbing anions in the electrochemical double layer affect the COad layer structure. 4.2. Potential Dependency of the C-O Stretching Frequency. As pointed out in the Introduction, the central goal of this work is to provide vibrational spectroscopic data of COad on Pt surfaces in a broad potential window and to understand the potential-dependent frequency changes of both the C-O and the Pt-CO stretching vibrations. Eariler studies suggest that two main origins, that is, the potential-dependent metaladsorbate chemical bonding and the Stark effect induced by the variance of eletrostatic field in the double layer, can contribute to the so-called “electrochemical” Stark effects on the frequencies of adsorbate on electrode surfaces.6 Ab initio calculations revealed that, for oriented CO close to a surface, the occupancy of the 2π* orbital can be changed by the applied electric field.51 This implies that a change in chemical bonding is inseparable from the vibrational Stark effect. In the DFT calculations by Lozovoi et al.,52 it was found that surface charge always tends to climb to the top of most protruding surface atoms, as this decreases the total electrostatic energy. Thus, it

is suggested that the chemical and physical nature are of the same origin at the molecular level. A Stark slope of 32 cm-1/V for dνC-OL/dE for a saturated CO adlayer in 0.5 M H2SO4 observed in this study agrees well with previous observations.21,27 DFT calculations reveal that the Fermi level (EF) of a neutral Pt surface lies between the 5σ and 2π* levels of the CO molecule.13 Applying a negative potential upshifts the Fermi levels of the Pt electrode relative to those of COad molecules; consequently, the gap between the EF and 2π* levels decreases and that between the EF and 5σ levels increases. Thus, at more negative electrode potentials, the back-donation is enhanced, while the donation is suppressed; this leads to the net transfer of some electronic charges from Pt to COad when reducing the electrode potential. At positive potentials, one would expect the opposite effect. Hence, the decrease in νC-OL with electrode potentials with a Stark slope of 32 cm-1/V for a saturated CO adlayer in acidic solution is believed to be dominated by the increase in the d(Pt)f2π*(CO) back-donation toward negative potentials. In alkaline or neutral solutions and in the lower potential ranges from -0.2 to -1.2 V, the Stark slope of dνC-OL/dE observed is ca. 92 cm-1/V, which is remarkably larger than that at higher potentials (-0.2 to 0.55 V) in acidic solutions (∼32 cm-1/V). This trend is in good agreement with a previous report by Zou et al.21 From inspections of the spectral data shown in Figure 3, it is noticed that the higher dνC-OL/dE appears in the potential region where the fractional coverage of COB increases notably, while that of COL decreases, as inferred from the corresponding band intensity changes (Figure 5b). This clearly reveals that some adsorbed CO molecules at atop sites move to bridge or hollow sites. In previous studies on the dipolesdipole coupling effects of CO adsorbed at Pt electrode surfaces,30,32,53 it was found that (i) strong dipolesdipole coupling effects exist among the nearby COad oscillators, (ii) dipolesdipole effects cause a marked blue shift in the C-OL peak frequencies, (iii) the closer the stretching frequencies, the stronger the dipoles dipole coupling effects, and (iv) the decrease in the total COad surface coverage will lead to a reduction in the dipole-dipole coupling interaction, which consequently reduces the corre-

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Figure 6. Potential-dependent Pt-CO bond length (a) and binding energy (b) changes (vs Eb at -0.15 V) of COL (0) and COB (O) calculated from eq 4.

sponding screening effect of dipole-dipole coupling on the electric field. Thus, the Stark tuning rate of the C-O stretching increases as the coverage decreases. Under the present conditions, the decrease in the total surface coverage of COad with electrode potential is less likely. The major reasons for this are that (i) throughout the potentialdependent Raman measurements, the electrolyte solution flushing through the cell is always saturated with CO; (ii) CO adsorption energy increases toward negative potentials (see Figure 6; this will be further discussed in section 4.3); (iii) under the present experimental conditions, OH- is the only species whose adsorption at the Pt surface may be stronger than that of CO, while the effect from the competitive adsorption of OHwith CO is negligible because, at the same electrode potential, the Raman spectral behavior is similar in CO-saturated 0.5 M NaOH and 0.5 M Na2SO4 solutions; and (iv) the experiments were done with a CO-saturated adlayer at nanoparticle surfaces. Small nanoparticle surfaces limit the formation of large domain structures as those for CO adsorbed at single-crystalline Pt surfaces, and potential-induced phase transition (adlayer domain structure change) is less likely. The contribution of the competitive adsorption of OH- to the larger Stark tuning rate toward negative potentials can be explicitly excluded based on the fact that OH- adsorption strength decreases toward negative potentials due to the electrostatic repulsion, while the Stark slopes increases. On the basis of the above analysis, we propose that the much larger dνC-OL/dE at more negative potentials may probably come from the reduction of the fractional surface coverage of COL species due to the conversion of COL to COB coordination, which reduces the dipolesdipole coupling interactions among neighboring COL molecules and corresponding COL peak frequencies. On the other hand, the dνC-OB/dE value (∼137 cm-1/V) in the potential region from -1.2 to -0.9 V is much larger than that (∼40 cm-1/V) from -0.9 to -0.2 V. This is probably due to the conversion from bridge to hollow site adsorption configuration at E < -0.9 V. In alkaline and neutral solutions, it was found that, in the potential region from -1.2 to -0.9 V, dνC-OB/dE (∼137 cm-1/V) is larger than dνC-OL/dE (∼92 cm-1/ V). This trend is in good agreement with the literature reports measured at higher potentials in acidic solutions, for example, for CO adsorption on Pt(111) at 0-0.6 V in 0.1 M HClO4, where

Zhang et al. the vibrational Stark tuning rates are found to be ∼30 to 35, 40-45, and 50-60 cm-1/V for COL, COB, and COH, respectively.27 However, in the potential region from -0.9 to -0.2 V, dνC-OB/dE (∼40 cm-1/V) is much smaller than dνC-OL/dE (∼92 cm-1/V); this is in obvious contrast with previous results. Such disprepancies can be easily rationalized by the potentialinduced site conversion and consequent variation in the fractional surface coverages of COL, COB, and COH, which consequently changes the extent of dipolesdipole coupling effects among the nearby C-O oscillators and the Stark slopes. 4.3. Potential Dependency of the Pt-CO Stretching Frequency and Its Implication of Equilibrium Bonding Length and Pt-CO Binding Energy. In contrast to the large and variable Stark slopes for the C-O stretching vibrations, throughout the potential regime examined (-1.2 to 0.55 V), the dνPt-COL/dE and dνPt-COB/dE are roughly constant in all the three solutions and with values of -5 and -20 cm-1/V, respectively (Figure 5a). The much smaller |dνPt-COL/dE| and |dνPt-COB/dE| comparing to the corresponding dνC-OL/dE (32-90 cm-1/V) and dνC-OB/dE (40-137 cm-1/V) can be rationalized by the fact that both the 5σ(CO)fsp(Pt) donation and the d(Pt)f2π*(CO) back-donation contribute to the Pt-CO bonding,14 while the alternation in electrode potentials leads to opposite changes in these two components, which offsets each other. The constant values for both dνPt-COL/dE and dνPt-COB/ dE reveal that the effects of the changes in fractional surface coverages of COL and COB species on the Pt-CO stretching frequencies are negligible. This further confirms that the Pt-CO vibration frequencies are mainly determined by the Pt-CO chemical bonding and the effects of dipolesdipole coupling interaction are negligible.31 In surface science and electrocatalysis, information on metalsCO bond length and binding energies is of great significance to understand the behavior of CO adsorption, diffusion, and oxidation at metal surfaces. Unfortunately, up to now, it is very difficult to directly measure these two parameters using available experimental techniques in an electrochemical environment.54 Thus, great efforts have been made to figure out whether it is possible to derive such information from the metal-chemisorbates vibrational frequencies. As described in the Introduction, previous DFT calculations with the cluster model found that, from positive to mild negative field, the binding energyof COL at Pt decreases, while from mild negative to more negative field, it increases.10 Furthermore, they found that curves of νPt-COL and Eb as a function of electrode potential just change in the opposite direction with the maximum νPt-COL appears at the potential where Eb is lowest. By decomposition of the Pt-CO vibrational frequencies into individual orbital and steric repulsion components, the positive slope of dνPt-COL/dE at F < -0.3 V/Å (ca. 0.2 V vs NHE, assuming that the double layer thickness is ca. 3 Å and the pzc of the Pt-CO system is ca. 1.1 V vs NHE)10,55 is found to originate from the offsetting field-dependent contributions from d(Pt)f2π*(CO) back-donation and 5σ(CO)fsp(Pt) donation together with the electrostatic repulsion between the negative charged surface and the negative charged COad due to surface-bond polarization (df2π* backdonation prevails over 5σfsp donation). Hence, the authors claimed that there is no simple relationship between the field (F)-dependent binding energies and the Pt-CO frequencies and the binding energy is a property of the metal-chemisorbate system as a whole, which should include adsorption-induced changes in both Pt-CO and C-O bonds, while the Pt-CO is a local property of surface bond itself.10,56

Chemisorption of CO at a Au Core-Pt Shell NP Electrode However, the maximum of νPt-COL was not observed in the present study even at the potential as negative as -1.2 V, which is much more negative than the potential (ca. 0.2 V) predicted by DFT calculations using the cluster model. Hence, we believe that there should be a relatively simple relationship between the metal-CO stretching and the adsorption energies of CO on metal surfaces. Although determining the absolute binding energies in an electrochemical environment is difficult with reasons as listed above,54 here, we will introduce a method to roughly evaluate potential-induced changes of Pt-CO binding energies (∆Eb) and equilibrium bond length (∆req) from the measured vibrational frequencies. In many cases, such parameters are as powerful as those of the absolute values of binding energies and equilibrium bond length for predicting the trend in the change in kinetics adsorption, desorption, or oxidation processes. A number of DFT calculations demonstrated that the Bagder’s rule, that is, the vibration frequency (ν) changes in a reciprocal manner to the corresponding bonding length (r) at equilibrium, applies to the case for adsorbed CO at the Pt surface.13-15,49,56 On the basis of this relationship, the field-induced variation of the equilibrium bond length ∆req is given by

∆req ) -k∆ν

(1)

where k is a constant and ∆ν is the corresponding potentialinduced change in vibrational frequency. Unfortunately, in an electrochemical environment, the value of k could not be directly determined by experiment. From several DFT calculations,13-15,49 it was estimated that the values of k are ca. (1 ( 0.1) × 10-3 and (5 ( 0.4) × 10-4 Å/cm-1 for Pt-COL and Pt-COB stretching vibrations, respectively. Thus, using eq 1, the potential-induced changes in equilibrium bond length can be easily deduced from the measured Pt-CO peak frequencies. On the other hand, the frequencies of the Pt-CO stretching vibration can be written as a function of the changes in Pt-CO binding energy (∆Eb) and the displacement of the Pt-CO bond length (∆req) from the equilibrium using the harmonic oscillator approximation56,57



hν ) pω ) p

2∆Eb µ∆(r2eq)

(2)

where µ is the reduced mass of the Pt-CO system (28 g/mol). 2 ) can be expressed as From eq 1, ∆(req

∆(r2eq) ) 2req∆req ) -2reqk∆ν

(3)

Hence, the changes in the binding energies can be expressed by the corresponding changes in Pt-CO vibration frequencies as follows

∆Eb ) 2π2µν2∆(r2eq) ) -4π2µkν2req∆ν

(4)

Because the change of equilibrium bond length induced by the electric field is very small (e.g., for ∆E ) 1 V, ∆req/req ) 0.2%), taking the equilibrium bond length in vacuum, that is, 1.85 and 1.45 Å for Pt-COL and Pt-COB, respectively, as req in eq 4, will give a very good approximation for evaluating the binding energies.58 From the measured νPt-CO ∼ E dependencies and eqs 1 and 4, the changes of equilibrium bonding length and binding energy and from -0.2 to -1.2 V (vs Eb at -0.15 V) have been calculated (Figure 6). The potential-dependent changes in the binding energy (Pt-CO bonding length) for COL and COB are estimated to be ca. 0.20 and 0.37 eV/V (0.005 and 0.01 Å/V), respectively. Obviously, the potential-induced changes for the

J. Phys. Chem. C, Vol. 114, No. 1, 2010 409 Pt-CO bonding length are rather small. The decrease in the binding energy with the increase in the potentials probably originates from a stronger decrease in the d(Pt)f2π*(CO) backdonation, which overweighs the increase in the 5σ(CO)fsp(Pt) donation in addition to the reduced Pauli repulsion toward positive potentials. The higher slope of ∆Eb/∆E for COB than that for COL reveals that potential-dependent changes in binding energy are more pronounced for COB than for COL and adsorption at high-coordinated sites is more favored at negative potentials, which agrees well with present experimental results that the intensity of COL (COB) bands (or fractional surface coverage of COL (COB)) decreases (increases) toward negative potentials as well as with the previous theoretical predictions.14,15 4.4. Comparison of Present Experimental Results with Periodic DFT Calculations. To have a better understanding of the discrepancies of the potential-dependent C-O and Pt-CO stretching frequencies between the experimental results and the predictions from various DFT calculations, we have carried out DFT calculations for the Pt(111)-CO system. Our recent studies reveal that, except for small changes in the peak frequencies and great differences in the SERS intensities, the potentialdependent SERS spectral behavior of CO at 55 nm [email protected] nm Pt core-shell nanoparticle electrodes is quite similar to what is observed on roughened polycrystalline Pt electrodes; for example, we found that, when one increases the thickness of Pt overlayers of 1, 2, 4, 8, and 20 MLs, the changes in the C-OL stretching frequency are within 4 cm-1.34 The small frequency shift may probably originate from the strain and electronic effect of the Au core on the Pt shell, as confirmed by systematic DFT calculations by Kitchin et al. who found that the electronic properties of Pt overlayers supported on a Au substrate exhibit largely that of pure Pt, except that the d-band shape and center are slightly altered.59,60 On the basis of these facts, we think that using CO adsorption at Pt(111) as a model substrate can qualitatively predict the trends of potential-dependent vibrational properties of CO at Au core-Pt shell nanoparticle electrodes. As described in the Computational Details, the (COL + 2COH) structure is more stable than the (COL + 2COB) configuration. The full optimization will lead to the conversion from the latter to the former. In the present study, the species at the Au@Pt nanoparticle film electrode are mainly linearly and multiply (either adsorbed at bridge or hollow site) bonded CO; hence, the two (2 × 2)-3CO slab models were used in our calculations for comparison. The calculated electric-field-dependent Pt-CO and C-O stretching frequencies and the corresponding Stark slopes for CO adsorbed in both (COL + 2COH) and (COL + 2COB) at Pt(111) are shown in Figure 7. To facilitate the comparison, the peak frequencies at one selected potential, that is, -0.1 V, as well as the Stark slopes from the experiments and calculations are listed in Table 1. From inspections of Figure 7 and Table 1, we found that the calculated peak frequency of C-OL (C-OB) stretching at -0.4 V/Å is ca. 2074 (1889) cm-1, which is slightly higher than the present SERS observation (ca. 2065 (1859) cm-1) at -0.1 V. Furthermore, the values of dνC-OL,B/dE calculated from DFT using the slab models (e.g., 7.5 and 8.7 cm-1/V for dνC-OL/dE and dνC-OB/dE) are much smaller than what were observed by SERS in the present study. On the other hand, the peak frequencies and Stark tuning rates of C-O stretching from our present calculations using the (COL + 2COH) structure agree well with the results from recent periodic DFT calculations.14 In contrast, earlier DFT cluster calculations gave a dνC-OL/dE of 40 cm-1/V.10 This is much closer to the Stark tuning rate typically observed in our

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Zhang et al. is infinitesimal, whereas a profound shift in the spectral frequencies due to the coverage-induced change in the dipolesdipole coupling interaction cannot be negligible, at least for the C-O stretching mode. On the other hand, though the surface coverage effect was considered in our present and previous DFT calculations using the periodical slab model, significant differences in the Stark slopes between the theoretical predictions and our present observation still exist. This obviously cannot be attributed to the effect from the Au core. All these discrepancies suggest that the effects of water and ions in the double layer that is in direct contact with the saturated CO adlayer may not be negligible. Further DFT calculations with an improved model, according to the above guidelines for such systems, are necessary in order to well predict/understand the potential-dependent vibrational frequencies and the Stark slopes of CO adsorbed at Pt electrodes.

Figure 7. Potential-dependent Pt-CO and C-O frequencies calculated by periodic DFT using the (a) COL + 2COH structure and (b) COL + 2COB structure.

experiments. It should be mentioned that the latter calculations are carried out at the zero-coverage limit. This also partly explains why the absolute values of νC-O from the cluster model are much smaller than that from our present experimental observation and periodic DFT calculations.10 As for the intermolecular Pt-CO stretching, it was found that the calculated Stark slope of dνPt-COL,B/dE (ca. 2.2-2.8 cm-1/V) by the slab model is much smaller than what is observed in the present study (ca. 5-20 cm-1/V). This is similar to the trend for the C-O stretching modes, as already discussed above. Furthermore, our DFT calculations using the slab model predicted that the absolute value of the Stark slope of metal-adsorbate stretching vibrations decreases in the order of |dνPt-COL/dE| (ca. 2.8 cm-1/V) > |dνPt-COB/dE| (ca. 2.2 cm-1/V). This also agrees well with recent periodic DFT calculations by other groups.14 However, such a trend contradicts our experimental observation by SERS where |dνPt-COL/dE| (ca. 6 cm-1/ V) < |dνPt-COB/dE| (ca. 20 cm-1/V). In contrast, the DFT calculation using the cluster model predicts an order of |dνPt-COL/ dE| (ca. 4.5 cm-1/V) < |dνPt-COH/dE| (ca. 7.7 cm-1/V), which agrees roughly with the present observation.10 The differences in the peak frequencies and the Stark slopes between the DFT calculation and present experimental observation suggest that either the model or the method used for the DFT calculations needs to be improved. Previous calculations done with the cluster model assumed that the surface coverage

5. Conclusions SERS studies on the chemisorption of CO at Au@Pt nanoparticle electrodes in a wide potential window were carried out in CO-saturated solution with different pHs using a flow cell. We found that, throughout the potential regime examined, dνC-OL,B/dE is positive, while that of dνPt-COL,B/dE is negative. The Stark slopes of dνPt-COL,B/dE are roughly constant throughout the potential regime examined, while the Stark slopes of dνC-OL,B/dE increase toward negative potentials. The absolute values of dνPt-COL,B/dE are typically 5 to 20 times smaller than that for dνC-OL,B/dE, and dνC-OB/dE can be smaller or larger than that of dνC-OL/dE in different potential regions. Qualitatively, all the potential-dependent spectral behaviors can be rationalized by the delicate changes in the counterweighing effects of the chemical bonding (d(Pt)f2π*(CO) back-donation and 5σ(CO)fsp(Pt) donation) and the dipoles dipole coupling; the latter originates from potential-induced changes in the fractional surface coverage of COL and COB species. Based on the fact that the influence of the dipolesdipole coupling interaction on the Pt-CO vibration is negligible, the measured dνPt-COL,B/dE can directly serve as an indicator for the potential-induced changes in bonding strength and bond length of Pt-CO, from which we estimate the potential-induced changes in the binding energies and bonding lengths of COL (COB) of ca. 0.20 (0.37) eV/V and 0.005 (0.01) Å/V, respectively. The larger ∆Eb/∆E of COB than that of COL reveals that the chemical bonding of COB is much more sensitive to the changes in the interfacial electric field than that of COL.

TABLE 1: Computational and Experimental Vibrational Frequencies and Stark Slopes of Pt-CO and C-O Stretching Modes systema

sites

COL + 2COH present work

atop hollow atop bridge atop hollow atop hollow atop bridge

COL + 2COB present work cluster model10 COL + 2COH14 experiment this work

νPt-CO (cm-1)b E ) -0.1 V

dνPt-CO/ dE (cm-1/V)c

νC-O (cm-1)b E ) -0.1 V

dνC-O/ dE (cm-1/V)c

474 381 449 423 411 292

-3.2 -2.5 -2.8 -2.2 -4.5 -7.7

2081 1810 2074 1889 1952 1663 2076 1809 2065 1859

7.2 7.9 7.5 8.7 40 42.5 7.0 8.6 32 40

488 415

-5 -20

a COL + 2COB refers to the structure with one atop and two bridge CO molecules (Figure 2a); COL + 2COH refers to the structure containing one atop, one fcc, and one hcp hollow CO molecules (Figure 2b). Experiment refers to the experimental results for saturated COad on the Au@Pt nanoparticle electrode measured by SERS. b Vibrational frequencies at -0.1 V vs NHE corresponds to ca. -0.4 V/Å in the DFT calculations, assuming that the double layer thickness is ca. 3 Å and the potential of zero charge of the Pt-CO system is ca. 1.1 V vs NHE. c Computational Stark slopes converted to potential-based values, assuming that the double layer thickness is ca. 3 Å.

Chemisorption of CO at a Au Core-Pt Shell NP Electrode Furthermore, we have also carried out peoridic DFT calculations on the field-depdendent vibrational frequencies and Stark slopes for the saturated CO adlayer, and careful comparison of the experimental data with the present and previous DFT calculations has been made. We found that Stark slopes of dνPt-COL,B/dE and dνC-OL,B/dE predicted by DFT calculations with the slab model are much smaller than the experimental observation. On the other hand, the Stark slopes calculated using the cluster model are in approximate agreement with our present experimental observation in acid solution, but it gives much smaller values for that in neutral (basic) solution and the absolute peak frequencies. The predicted order of the Stark slope of metal-adsorbate stretching vibrations, that is, |dνPt-COL/dE| > |dνPt-COB/dE|, as predicted by periodic DFT calculations, contradicts our experimental observation as well as that from DFT calculations using the cluster model. The differences between the DFT calculation and present experimental observation suggest that either the model or the method used for the DFT calculations needs to be improved. Because CO is one of the most important molecules in the heterogeneous catalytic reactions, we believe that systematic study of the vibrational spectroscopic behavior of COad will greatly improve both the fundamental understanding of such CO adsorption/desorption processes and the kinetics of heterogeneous catalytic reactions involving COad as intermediates, spectators, or poisoning species. We hope the present study will boost wide interests and further investigations on such topics. Acknowledgment. This work was supported by grants of the National Natural Science Foundation of China (NSFC) (Project No. 20773116), the 100 Talents Program of The Chinese Academy of Sciences, the 973 program from the Ministry of Science and Technology of China (Project No. 2010CB923302), and the State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University (Project No. 200706) and by the Program for Changjiang Scholars and Innovative Research Team (Team No. IRT0756) in the University “PCSIRT” from Education Ministry of China. Supporting Information Available: Discussion of cyclic voltammograms in Figure 1a,b and electrochemical mass spectroscopy results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Beden, B.; Leger, J. M.; Lamy, C. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O’M., Conway, B. E., Eds.; Plenum Press: New York, 1992. (2) Chorkendorff, J. W.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics; Wiley-VCH: Weinheim, Germany, 2006. (3) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 117. (4) Korzeniewski, C.; Stanley, P.; Schmidt, P. P.; Severson, M. W. J. Chem. Phys. 1986, 85, 4153. (5) Mehandru, S. P.; Anderson, A. B. J. Phys. Chem. 1989, 93, 2044. (6) Lambert, D. K. Electrochim. Acta 1996, 41, 623. (7) Curulla, D.; Clotet, A.; Ricart, J. M.; Illas, F. Electrochim. Acta 1999, 45, 639. (8) Koper, M. T. M.; van Santen, R. A. J. Electroanal. Chem. 1999, 476, 64. (9) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 3518. (10) Wasileski, S. A.; Weaver, M. J.; Koper, M. T. M. J. Electroanal. Chem. 2001, 500, 344.

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