8920
J. Phys. Chem. B 1999, 103, 8920-8925
Sum Frequency Generation of CO on (111) and Polycrystalline Platinum Electrode Surfaces: Evidence for SFG Invisible Surface CO Steve Baldelli,†,§ Nenad Markovic,§ Phil Ross,§ Yuen-Ron Shen,‡,§ and Gabor Somorjai*,†,§ Departments of Chemistry and Physics, UniVersity of California at Berkeley, Berkeley, California 94720, and Materials Science DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 ReceiVed: May 19, 1999; In Final Form: August 23, 1999
The vibrational spectroscopy sum frequency generation (SFG) is used to investigate the adsorption of carbon monoxide on the single crystal (111) and polycrystalline platinum surfaces. By varying the frequency and polarization of the light beams, different surface species of CO species are probed. SFG signal intensities for different polarization indicate that adsorbed CO polarizability is significantly perturbed from the gas-phase molecule. The SFG signal of CO disappears well below the main oxidation potential of CO to CO2. The disappearance of the CO signal is interpreted as a transformation in the CO layer to a state which is invisible to SFG. The invisible state is suggested to be CO with the bond axis nearly parallel to the platinum surface.
Introduction Carbon monoxide adsorption on platinum electrodes is an extensively studied system.1-7 Infrared spectroscopy is particularly useful for investigating this system since the CO signal is intense and the vibrational frequency provides information on the interaction with the surface. CO adsorbed on top-sites, linearly bonded to one platinum atom, has vibrational frequencies from 2040 to 2080 cm-1. CO coordinated to two or three platinum atoms has frequencies in the 1840-1800 cm-1 and 1750-1700 cm-1, ranges, respectively.1 CO on platinum, (111) oriented, single-crystal electrodes occurs in all three sites depending on surface coverage and applied potential.3 On the polycrystalline platinum electrode surface, CO appears to occupy only top sites.4,5 Further, the vibrational frequency for CO at a given site is coverage and potential dependent because of adsorbate-substrate and adsorbate-adsorbate interactions and electric field (Stark) effects.2,4,5,8 There are, however, discrepancies in comparing voltammetry and previous infrared spectroscopy experiments, especially in correlating small changes in CO coverage in the preoxidation potential region with the infrared intensities.1,4 The use of sum frequency generation (SFG) in these experiments has the advantage that SFG is only sensitive to the electrode surface and therefore there is no need to use potential modulation or subtractive normalization to obtain surface sensitivity. The results presented here using SFG indicate that CO adsorbed on the platinum surface occupies both 3-fold and topsite positions on Pt(111)1 and only top site on polycrystalline platinum electrodes. On Pt(111) the peak at 1785 cm-1 (3-fold) is observed only with ssp polarization (s-SF, s-vis., p-ir), and top site at 2070 cm-1 is of similar intensity in both the ssp and ppp spectra. The SFG signal of CO disappears at a potential between 0.45 and 0.50 V (vs Pd/H reference electrode), which is before the main oxidation peak (0.75 V) in the anodic scan. The integrated anodic charge indicates the surface still contains * Corresponding author: e-mail
[email protected]. † Department of Chemistry, UC Berkeley. ‡ Department of Physics, UC Berkeley. § Lawrence Berkeley National Laboratory.
∼90% of a saturated monolayer at the potential where the signal disappears. This result suggests that upon oxidation of a small amount of the adsorbed CO adsorbed at 50 mV there is a transformation of the remaining CO into a state which is invisible to SFG. Background. SFG is a nonlinear, vibrational spectroscopic technique, sensitive only to the interface region between two centrosymmetric phases.9-11 The technique involves combining a fixed visible beam at frequency, ωvis, and a tunable infrared beam, ωir, on the surface of the electrode generating a third beam at ωSF, where ωvis + ωir ) ωSF. As the infrared frequency is scanned and comes into a vibrational resonance an increase in sum frequency signal, ωSF, is detected. More specifically, the sum frequency signal intensity is given by
ISFR
|∑
|
2 χ(2) IJKE(ωvis)JE(ωir)K
JK
(1)
where χ(2) is the surface macroscopic susceptibility. ISF is the SFG signal intensity and E(ωvis) and E(ωir) are the electric fields of the visible and infrared beams at the surface, respectively. The surface light field intensities are determined using Fresnel’s equations. The macroscopic susceptibility is related to the molecular hyperpolarizability, β, through
χ(2)IJK ) N〈β(2)IJK〉
(2)
Here, 〈β(2)IJK〉 is the molecular hyperpolarizability orientation averaged in surface coordinates, IJK; β(2) is the product of the Raman transition matrix elements, Rij, and the infrared dipole transition matrix elements, µk; and N is the number of molecules.
β(2)ijk ) 〈g|Rij|v〉〈v|µk|g〉
(3)
The indices ijk are the molecular coordinate system which can be transformed to the surface coordinates (IJK) through Euler relations. In eq 3, v is the first excited vibrational state and g is the ground vibrational state. Thus, from eq 3, SFG spectra are interpreted in terms of linear Raman and infrared spectra. Further, the intensity of a peak in an SFG spectrum is proportional to N2 and is affected by the orientation of the
10.1021/jp991649x CCC: $18.00 © 1999 American Chemical Society Published on Web 10/01/1999
Sum Frequency Generation of CO
Figure 1. Electrochemical cell for SFG experiments.
adsorbed molecules. SFG detects only polar oriented molecules at the interface11-14 and is typically sensitive to coverages as low as ∼5% of a monolayer.11,15-17 Experimental Section The SFG system is briefly described here. The output of a Nd:YAG laser (1064 nm), with a 20 ps pulse width and 20 Hz repetition rate, is frequency doubled to 532 nm in a lithium borate crystal. Part of the 532 nm beam is used to pump an optical parametric/difference frequency generation system and generate tunable infrared light from ∼1400-3800 cm-1 with a bandwidth of 7-10 cm-1. The other portion of the 532 nm beam is used as visible input for the surface SFG experiment.18 The infrared and visible beams are spatially and temporally coincident on the platinum surface at an incident angle of 45° and 33°, respectively. The energy density is ∼10 mJ/cm2 and 3.5 mJ/cm2 for the infrared and 532 nm beam at normal incidence outside the cell. The reflected infrared light is monitored with a joule meter (Gentec) and used to normalize the SFG signal. The sum frequency beam is separated from the 532 nm and spatially and spectrally filtered with a notch filter (Kaiser), a color filter (Omega), and a 0.1 m monochromator (ISA). The signal is detected with a Hamamatsu PMT (R647P), and the data is collected with a Stanford Research boxcar/averager (SRS250). The entire system, i.e., wavelength scanning, data collection, is computer controlled with a homemade Labview program. The presented data are an average of three scans each 50 shot/point with error bars at one standard deviation. The data are corrected for fluctuations in the 532 nm and infrared by dividing the sum frequency signal by infrared and 532 nm beam energy at each point. Spectral intensities are normalized to the intensity of the 2070 cm-1 peak of the ppp spectrum. SFG spectra do not require a background subtraction, as do infrared spectra,3,19 since SFG does not detect molecules in the bulk electrolyte. The electrochemical experiments are conducted in a conventional thin-layer cell shown in Figure 1.4 The cell is made of
J. Phys. Chem. B, Vol. 103, No. 42, 1999 8921 quartz with Teflon valves and a central Kel-F shaft. The shaft holds the platinum electrode and allows the electrode to be pushed against and withdrawn from the cell window, see Figure 1. The cell and its components are cleaned with 50/50 v/v concentrated HNO3/H2SO4 solution, rinsed several times with triple-distilled water, steam cleaned, and rinsed with electrolyte before use. The electrode is prepared by flame-annealing a platinum disk with a platinum wire spot-welded to the back face. Flame annealing involves heating the platinum disk in a hydrogen/air flame for 4-6 min to a white-hot color. While still white hot, the sample is transferred into a glass chamber with flowing ultrahigh purity (UHP) nitrogen or argon gas for approximately 10 min. From this chamber, the platinum is immediately covered with a drop of triple-distilled water and transferred to the electrochemical cell. The electrolyte solution is 0.5 M H2SO4 (J. T. Baker) purged for a minimum of 20 min with UHP argon. The system is considered to be clean when the voltammetry of the platinum electrode displays the characteristic peaks of a clean surface (Figure 2A).20,21 Oxidation and roughening of the surface is avoided by keeping the potential below 1.1 V. A saturated CO solution is prepared by bubbling CP grade CO through the electrolyte for 10-15 min. The surface is saturated with CO by adsorbing CO for ∼15 min at 0.05 V vs Pd/H reference. The adsorption procedure is performed with the electrode retracted from the window. While still under potential control, the CO saturated solution is replaced with clean, O2 free electrolyte and the electrode is then pressed against the window for the SFG experiment. The trapped electrolyte layer is estimated to be 1-10 µm thick. The counter electrode is a platinum wire and the reference electrode is Pd/H which is approximately +0.08 V versus a normal hydrogen electrode (nhe) in acid solution. CO oxidative stripping experiments are conducted in a hanging meniscus electrode configuration for accurate current measurements. This experiment involves inverting the platinum electrode, with the drop of water on it, so that this drop forms a meniscus with the electrolyte solution. This configuration allows the electrolyte to contact only the polished oriented (111) face of the crystal. In the stripping experiment, CO is adsorbed at 0.05 V for 15 min to create a saturation coverage. The solution is then purged of CO while holding the potential at 0.05 V. Next, the potential is stepped to the holding potential for 15 min. After this holding period, the potential scan for CO stripping is continued to 1.0 V, and the anodic charge for CO stripping is obtained as a function of holding potential. Results SFG spectra for CO/Pt(111) in the CO stretch region are taken with various input/output polarization combinations. Results of the polarization dependence spectra of CO on Pt(111) are presented in Figure 3A,B. Because of the selection rules, the spp spectra, spp (s-polarized sum frequency, p-polarized visible, and p-polarized infrared) and psp spectra are featureless with only a weak background signal present. Figure 2C,D are the ppp and ssp spectra. The ppp spectrum taken with the specified beam geometry indicates one peak at 2070 cm-1, which is assigned to CO linearly, bound to one platinum atom, top-site geometry.1 The ssp spectrum shows two peaks: one at 1785 cm-1 and one at 2070 cm-1. The peak at 1785 cm-1 is characteristic of CO coordinated to three platinum atoms in a 3-fold hollow site.1 The 2070 cm-1 peak is also assigned to top-site CO. The peak intensity ratio of top-site to 3-fold is 2.5: 1. Further, the top-site peak intensity is nearly the same for ppp and ssp polarization. The spectra in Figure 4 are CO on
8922 J. Phys. Chem. B, Vol. 103, No. 42, 1999
Baldelli et al.
Figure 2. (a) Cyclic voltammetry of clean Pt(111) in 0.5 M H2SO4. (b)Stripping voltammetry of CO on Pt(111) 50 mV/s. Dashed line is clean Pt(111) on retrace after stripping.
Figure 3. SFG spectra of CO on Pt(111) electrode at 0.05 V vs Pd/H for different polarization combinations. A ) spp, B ) psp, C ) ppp, and D ) ssp.
polycrystalline platinum, where only the 2070 cm-1 peak is present, characteristic of top site CO. The intensity in the ssp spectrum is 0.8 of that in the ppp spectrum. The data in Figure 5 presents the SFG peak intensity as a function of electrode potential. The SFG intensity remains relatively constant until about 0.55 V. The voltammetry shown in Figure 4A is representative of a clean Pt(111) electrode in 0.5 M H2SO4 solution. Below 0.25 V, the broad voltammetric feature is due to underpotential deposition of hydrogen or OH. The peak at ∼0.35 V is due to anion adsorption. On this surface, the main peak for oxidation of CO to CO2 occurs at ∼0.75-0.8 V (vs Pd/H)22 as seen in the voltammetry curve in Figure 4B. There is measurable current starting at ∼0.4-0.5 V, referred to as the preoxidation wave. This preoxidation region accounts for approximately 10% of the total oxidation current generated in CO oxidation.5,23,24 The results of CO stripping experiments where the potential is held at different values for 15 min are summarized in Figure 5. There
are two features to notice here. First, the potential of the main oxidation peak is only slightly affected ( δRxx/δQ ) δRyy/δQ and δRij/δQ. It have been seen that for CO adsorbed on a metal surface polarizability increases by a factor of ∼2-6 when compared to the gas phase.8 Therefore, the elements (δRxx/δQ, δRyy/δQ) in the Raman tensor are larger when the CO is adsorbed on the surface rather than in the gas phase. The SFG results are consistent with this, where the polarizability increase is about 2.4 times for adsorbed CO at top sites and 6 times for CO at 3-fold coordinated sites. Further, since s-polarized (Ey) light has significant intensity at the metal surface, SFG is able to access these components of the Raman tensor. Increased polarizability in the x-y direction is most likely due to the platinum d-band electrons which are donated back into the 2π* orbital of the adsorbed CO molecules. The electron density of the 2π* orbital is outside of the bond axis, consistent with the polarizability becoming more spherical.30 Adsorption of CO on the metal modifies the polarizability from onedimensional (free molecule) to more three-dimensional (surface adsorbed), that is the polarizability becomes less directional. The electrochemistry of CO on platinum surfaces is an extensively studied system.6,31 While some experimenters have observed a disappearance of CO before the main oxidation peak, the phenomenon is not generally noted.1,5,23 Recent in situ X-ray diffraction experiments of CO on Pt(111) indicate the ordered p(2 × 2)-3CO structure disappears near 0.6 V.24 The voltammetry results in Figure 5 indicate the surface of Pt contains nearly 90% of the CO coverage adsorbed at saturation while the SFG signal has decreased to below detection, i.e., the CO signal disappears when about 10% of the CO is oxidized in the preoxidation region. This is an indication that most of the CO on the surface is invisible to SFG after the potential is stepped from 0.05 V to above 0.5 V. A similar result has been reported previously with infrared spectroscopy.5,23 Two possible mech-
Baldelli et al. anisms of CO disappearance are considered if CO is invisible to SFG and infrared. First, CO is tilted such that its dipole is tilted nearly parallel with the metal surface. Thus, the infrared metal surface selection rule is operative.25,32,33 Second, the infrared cross-section has dramatically decreased for most of this bound CO layer. Calculations based on the interaction of the CO dipole with the high surface electric field of the electrode surface indicate infrared cross-section changes are only on the order of 10%,8 thus the tilted CO model is favored. A simple calculation based on the dipole selection rules for a metal surface25,32,33 indicates the CO must be tilted >75° for a signal decrease by a factor of 50, which would be near the detection limit for these experiments. These results and the reexamination of previous experiments1,5,23,34 present a case that infrared dipole spectroscopies do not necessarily detect all of the CO present on the electrode surface. That is, the CO peak in the infrared or SFG spectrum disappears 100-200 mV before the main oxidation wave occurs in the voltammetry,1,5,23 Figure 5. By using these two independent techniques, SFG and cyclic voltammetry, there is a clear discrepancy as to the coverage of surface CO. SFG and previous infrared experiments clearly demonstrate the absence of a CO peak on Pt(111) electrode while the current measured during the voltammetry is only about 10% of the total current measure for a saturation coverage of CO. This naturally leads to the conclusion that SFG and infrared spectroscopy do not sense the majority of the CO on the surface. CO that is not detected by SFG and infrared spectroscopy but is still on the surface is considered to be lying flat with the C-O bond axis nearly parallel to the platinum surface. More information is obtained about the nature of CO on the Pt(111) electrode when considering SFG intensity and previous X-ray diffraction experiments.24 At potentials below the preoxidation region CO is observe to form a 2 × 2-3 CO structure with a surface coverage of ∼0.75 CO/Pt.35 CO in this 2 × 2-3 CO structure must be oriented with the CO bond axis perpendicular to the platinum surface to produce the X-ray scattering observed. However, after the preoxidation wave at ∼0.55 V, the two-dimensional ordering is lost and the surface assumes an indeterminate structure. As indicated above, these changes are consistent with the SFG data. Under the current experimental conditions, SFG sensitivity is ∼0.02 of a monolayer and the signal-to-noise ratio is ∼100:1. Therefore, the SFG signal detected at < 0.5 V represents the saturation coverage of CO (0.75 CO/Pt). If only 10% of the surface CO were detected at potentials
