Infrared Spectral Comparison of Electrochemical Carbon Monoxide

Langmuir 2002, 18, 3233-3240

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Infrared Spectral Comparison of Electrochemical Carbon Monoxide Adlayers Formed by Direct Chemisorption and Methanol Dissociation on Carbon-Supported Platinum Nanoparticles Sungho Park,† YuYe Tong,‡,§ Andrzej Wieckowski,‡ and Michael J. Weaver*,† Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, and Department of Chemistry, University of Illinois, Urbana, Illinois 61801 Received September 4, 2001. In Final Form: January 31, 2002 A comparison is made between electrochemical infrared reflection-absorption spectra for carbon monoxide adlayers formed on carbon-supported platinum nanoparticle films by direct chemisorption from solution CO and via methanol dissociation. In addition to the importance of the C/Pt materials as electrocatalysts in methanol-based fuel cells, clarifying the nature of the extent of CO formation from the latter solute is motivated by the use of methanol as a source of chemisorbed CO at these and related interfaces. As in previous studies, commercial C/Pt materials were employed, having metal loadings from 10 to 60%, corresponding to nanoparticle diameters from ca. 2 to 9 nm. Ultrathin C/Pt films with excellent infrared as well as voltammetric characteristics were prepared by physical deposition onto gold. Absolute C-O stretching (νCO) bands as a function of electrode potential, CO coverage, and nanoparticle size were obtained upon solution CO dosing, as usual, by subtracting a reference spectrum measured following CO electrooxidation. Such “absolute” absorbance spectra, however, could not readily be obtained in the presence of methanol solute, obliging the utilization instead of “bipolar” potential-difference infrared (PDIR) spectra where the reference spectrum is also acquired within the potential region where the adlayer is stable. The interpretation of such bipolar PDIR data is complicated for the present systems by the broad asymmetric shape of the component νCO bands, exacerbated for thicker films by their anomalous optical properties. Nevertheless, a detailed understanding of the methanol PDIR spectra, including interpretation of unexpected potential-dependent band intensities and complex bipolar band shapes, was achieved by comparison with corresponding absolute as well as bipolar spectra from solution CO dosing. This analysis, along with cyclic voltammetry, also indicates that the methanol dissociation yields only intermediate-coverage CO adlayers. These spectral findings invite a reassessment of some conclusions from an earlier infrared study (Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J.-M.; Lamy, C. J. Phys. Chem. B 2000, 104, 5803), including a comparison of 13C NMR data for the C/Pt-methanol systems in relation to the infrared properties.

Introduction Carbon-supported nanoparticles of Pt-group metals, especially platinum itself, constitute intriguing electrocatalytic materials from both practical and fundamental standpoints. A key example of their technological significance concerns methanol fuel cells, where such materials offer high surface area/volume ratios and efficient electrooxidation characteristics.1 A fundamentally interesting feature of these commercial C/Pt materials is the ability to alter the average Pt nanoparticle diameter (d) over the range ca. 2-10 nm, which can yield marked variations in their electrocatalytic, adsorptive,2,3 and other physicochemical properties. In particular, the Illinois group has exploited their high metal surface area to undertake extensive solid-state NMR * Corresponding author. E-mail: [email protected]. † Purdue University. ‡ University of Illinois. § Present address: Dept. of Chemistry, Georgetown University, Washington DC 20057. (1) For example: Carrette, L.; Friedrich, K. A.; Stimming, U. ChemPhysChem 2000, 1, 162. (2) (a) Mukerjee, S.; McBreen, J. J. Electroanal. Chem. 1998, 448, 163. (b) McBreen, J.; Mukerjee, S. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; Chapter 49. (3) For example: (a) Frelink, T.; Visscher, W.; van Veen, J. A. R. J. Electroanal. Chem. 1995, 382, 65. (b) He, C.; Kunz, H. R.; Fenton, J. M. J. Electrochem. Soc. 1997, 144, 970.

measurements in electrochemical environments.4-6 These studies have included 195Pt NMR investigations of cleansurface materials5 as well as 13C NMR examinations for carbon monoxide layers formed by methanol dissociative chemisorption.6 The former results show a sensitivity of the Pt electronic properties to particle size, while the 13C chemical shift and spin-lattice relaxation measurements indicate the occurrence of increasing local density of states associated with Pt-CO dπ-2π* back-bonding for smaller nanoparticles, especially for d < 4 nm.6 An initial comparison was also undertaken between the nanoparticle size-dependent 13C NMR properties and corresponding electrochemical infrared spectra for the C-O stretching (νCO) vibration, formed within modified C/Pt films on a carbon substrate from dissociative methanol chemisorption.7 The latter measurements also indicated an increasing extent of 2π* back-bonding for the smaller nanoparticles from the observation of greater νCO redshifts.7 The Purdue group has recently undertaken electrochemical infrared reflection-absorption spectroscopic (4) Tong, Y. Y.; Oldfield, E.; Wieckowski, A. Anal. Chem. 1998, 70, 518A. (5) Tong, Y. Y.; Rice, C.; Godbout, N.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 1999, 121, 2996. (b) Tong, Y. Y.; Babu, P. K.; Wieckowski, A.; Oldfield, E., in preparation. (6) Tong, Y. Y.; Rice, C.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2000, 122, 1123. (7) Rice, C.; Tong, Y. Y.; Oldfield, E.; Wieckowski, A.; Hahn, F.; Gloaguen, F.; Leger, J.-M.; Lamy, C. J. Phys. Chem. B 2000, 104, 5803.

10.1021/la0113825 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/12/2002

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(EC-IRAS) measurements for carbon-supported Pt nanoparticle (C/Pt) films on gold.8-10 Electrodes exhibiting welldefined IRAS properties were found to be produced by physical deposition of ultrathin (monolayer-level) C/Pt films (vide infra).8,10 Aside from examining the dissociative chemisorption of small organic molecules,9 detailed potential (E)-dependent IRAS measurements have recently been made for CO adlayers formed on these films from solution-phase CO as a function of coverage and nanoparticle size.8 Significantly, absolute νCO absorbance spectra could readily be obtained for monolayer-level films, using a potential-difference infrared (PDIR) tactic where the bulk-phase interferences are removed by subtracting a spectrum obtained subsequently at a potential where complete CO electrooxidation occurs.11 This enables nanoparticle size-dependent νCO band shapes as well as frequencies to be obtained, along with accurate coveragedependent Stark-tuning (νCO-E) slopes. Comparison of these parameters with those for corresponding potentialand coverage-dependent νCO spectra for low-index and stepped monocrystalline electrodes, along with related data for chargeable high-nuclearity Pt carbonyl clusters, indicates that the particle size-dependent νCO redshifts are primarily due to decreases in the average Pt surface coordination number for the smaller nanoparticles, especially for d < 4 nm.8 Armed with such detailed EC-IRAS findings for CO chemisorption on C/Pt, the comparison of this behavior with corresponding data obtained under the same conditions for methanol is of interest, especially given the central relevance of the latter to fuel-cell technology, along with its common use as a convenient source of adsorbed CO.6,7 Such a comparison is undertaken in the present paper. A complication is that unlike solution CO dosing, absolute νCO spectra are difficult to obtain for methanol dissociation on the C/Pt films due to the large amount of CO2 produced upon methanol electrooxidation, undermining the film integrity in the thin layer (vide infra). As in the earlier infrared study of methanol dissociation on C/Pt films undertaken by the Illinois group,7 νCO spectral data can still be obtained by using the common PDIR approach where the reference as well as sample spectrum is obtained in the potential region where the adsorbed CO is stable. However, the bipolar νCO spectra obtained from the partly overlapping Stark-shifted bands at the two potentials chosen can obscure band shape and other details and yield systematically incorrect peak frequencies. As demonstrated below, these complications are especially severe for the present C/Pt systems, which can result in incorrect spectral data interpretation. We show here how these spectral complications can be assessed and understood for methanol dissociation by examining corresponding PDIR data obtained from solution CO dosing, exploiting our ability to acquire coveragedependent absolute as well as potential-difference bipolar spectra for the latter. Overall, the results indicate substantial nanoparticle size-dependent differences in the νCO-E spectra for solution CO and methanol adsorption, which are identified as due chiefly to the lower CO coverages formed in the latter case. In addition to providing (8) Park, S.; Wasileski, S. A.; Weaver, M. J. J. Phys. Chem. B 2001, 105, 9719. (9) Park, S.; Xie, Y.; Weaver, M. J. Langmuir, submitted. (10) Park, S.; Tong, Y. Y.; Wieckowski, A.; Weaver, M. J. Electrochem. Commun. 2001, 3, 509. (11) This PDIR method, originally introduced by the Purdue group and dubbed “single potential alteration infrared spectroscopy” (SPAIRS) (ref 12), is commonly utilized to obtain absolute (i.e., unipolar) spectra for CO and other adsorbates on monocrystalline Pt-group and other planar electrodes (ref 13).

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a more complete spectral interpretation, they also prompt a reassessment of some conclusions from the earlier infrared study.7 Experimental Section All experiments were performed at Purdue. Most experimental details of the EC-IRAS measurements are available elsewhere.13 The FTIR spectrometer was a Mattson RS-2 instrument, with a custom-built external reflection compartment containing the narrow-band MCT detector. Typically, infrared spectra were obtained by acquiring 100 interferometer scans (at 4 cm-1 resolution) at each electrode potential. The metal gold disk substrate (ca. 1.0 cm diameter), mounted on a glass plunger by wrapping with Teflon tape, was pressed against the CaF2 window forming the base of the spectroelectrochemical cell so to create the optical thin layer. The C/Pt films were prepared by depositing a dilute suspension (3 mg/mL) of the material (purchased from E-Tek, Inc., Natrick, MA) via a microsyringe onto the polished gold substrate diluting with additional water droplets so as to spread the C/Pt evenly over the surface, followed by drying by argon flow for ca. 3 min and rinsing with a jet of ultrapure water to remove loosely held particles.10 The resulting surface contains a thin, relatively uniform, and reproducible C/Pt film which provides excellent voltammetric and IRAS characteristics, the latter being aided by its highly reflective nature.10 (This procedure differs from that used earlier,7 partly in that the use of Nafion/tetraethyl phosphate “filler” in the deposited film is now avoided.) The C/Pt metal loadings utilized here, 60, 30, 20, and 10%, are quoted by the supplier (E-Tek) as having average nanoparticle diameters of 8.8, 3.2, 2.5, and 2.0 nm, respectively. Histograms of the asreceived samples analyzed by transmission electron microscopy (at Illinois) yielded similar diameters, specifically 8.6 ( 0.7, 2.7 ( 0.4, and 2.3 ( 0.4 nm for the 60, 20, and 10% metal loadings, respectively. Carbon monoxide (99.99% min, Matheson Gases) was dispensed from an aluminum cylinder (to avoid iron carbonyl impurities), methanol (Spec grade) was from Fisher, and supporting electrolytes were prepared from double-distilled H2SO4 or HClO4 (G.F. Smith) using ultrapure water from a Millipore MilliQ system. Electrode potentials were measured and are reported here versus a Ag/AgCl (sat. KCl) reference electrode (Bioanalytical Systems).

Results and Discussion Cyclic Voltammetry. As a prelude to describing the IRAS data, it is desirable to examine the cyclic voltammetric (CV) responses within the hydrogen adsorption and double-layer regions for the various C/Pt film loadings. More specifically, the degree to which hydrogen adsorption is attenuated in the presence of methanol compared with solution carbon monoxide itself provides information regarding the extent of methanol dissociative chemisorption. Figures 1 and 2 show representative CV data obtained (at 50 mV s-1) in 0.05 M H2SO4 over the potential range -0.25 to 0.4 V. Parts A-D refer to C/Pt films with metal loadings of 60, 30, 20, and 10%, respectively. The charge densities contained under the reversible hydrogen adsorption-desorption region (-0.25 to 0 V), ca. 250-500 µC cm-2, are not greatly different from those, ca. 200250 µC cm-2, measured for planar Pt electrodes, thereby corresponding roughly to 1-2 “equivalent monolayers” of surface Pt sites. The CV responses shown are highly reproducible following a few initial cycles. Similar CV data were obtained for thinner (i.e., submonolayer) as well as thicker films, even though the nature of the IRAS (12) Corrigan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (13) For example: (a) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (b) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (c) Tang, C.; Zou, S.; Severson, M. W.; Weaver, M. J. J. Phys. Chem. B 1998, 102, 8796.

Electrochemical CO Adlayers on Pt Nanoparticles

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Figure 1. Comparison between cyclic voltammograms (50 mV s-1) for C/Pt films on gold in 0.05 M H2SO4 (solid traces) and with saturated CO adlayers following solution CO dosing (dashed traces), for the following metal loadings: (A) 60%, (B) 30%, (C) 20%, (D) 10%. Electrode potentials are vs Ag/AgCl.

Figure 3. (A) Absolute absorbance spectra for saturated CO layers (from solution CO) on C/Pt films in 0.05 M H2SO4 at -0.2 V vs Ag/AgCl with metal loadings as indicated. (B) Plots of νCO peak frequencies (extracted from data as in A) versus electrode potential for saturated CO layers on C/Pt films with metal loadings as indicated.

Figure 2. As in Figure 1, but before and after exposure to 0.1 M methanol for 20 min at 0 V (solid and dashed traces, respectively).

responses can differ significantly (vide infra).10 Worthy of mention is the progressive attenuation of the “strongly adsorbed H” peak (i.e., the features appearing at less negative potentials) with decreasing particle size. The hydrogen CV characteristics for the largest particles, however, approach those observed for polycrystalline Pt electrodes. The dashed traces in Figure 1A-D were obtained in the presence of near-saturated CO solutions formed by sparging briefly with gaseous CO. As expected, the voltammetric hydrogen adsorption features are entirely removed by the chemisorbed CO, yielding “flat” currentpotential traces that reflect residual capacitance contributions from the CO-covered Pt particles, the carbon support, and possibly also the gold substrate. The corresponding dashed traces in Figure 2A-D were obtained after adding 0.1 M methanol and holding for 15 min at 0 V. While the hydrogen adsorption features are clearly attenuated, especially for the higher particle loadings, comparison with Figure 1A-D indicates that the extent of blockage is incomplete. As the effect arises predominantly from methanol dissociation to form chemisorbed CO, one can surmise from Figure 2A-D that the CO coverage relative to the “saturated level” achieved with solution CO, XCO, is about 0.4-0.6 (cf. ref 6). The extent of voltammetric H blockage slowly increases with time, although remaining decidedly incomplete even after a few hours of exposure to methanol. Infrared Spectra of Nanoparticle Films: Solution CO Dosing. We now describe the salient features of Pt

nanoparticle size-dependent infrared spectra obtained upon dosing with solution CO. While some of these findings are detailed elsewhere,8 a brief description forms a necessary precursor to the interpretation of related IRAS data for methanol dissociation, considered in the next section. Figure 3A shows representative absorbance spectra in the C-O stretching (νCO) region for thin (ca. 1 monolayer) C/Pt films for each of the metal loadings considered here, obtained at -0.2 V versus Ag/AgCl in 0.05 M H2SO4. (Essentially identical results were obtained in 0.1 M HClO4.) The spectra, obtained following CO solution sparging, each refer to a saturated CO adlayer as also confirmed from the voltammetric data (vide supra). The reference spectrum was obtained subsequently at 0.65 V following complete CO electrooxidation in each case, so that the infrared bands in Figure 3A reflect purely the νCO absorbance properties at -0.2 V. Plotted alongside (Figure 3B) are the νCO peak frequencies, νPCO, versus the electrode potential, E, in the range (ca. -0.25 to 0.1 V) where no significant reduction or oxidation processes occur. Several features of Figure 3A,B are worthy of mention here. The νPCO values at a given potential, consistent with atop Pt-CO coordination,14 are seen to redshift progressively with decreasing metal loading and hence nanoparticle size, especially for particle diameters d < 4 nm. This effect, discussed further below, has been ascribed primarily to a systematic diminution in the average Pt surface coordination number with decreasing particle size.8 However, the νPCO-E (i.e., Stark tuning) slopes, 27 ( 3 cm-1 V-1, are essentially independent of the particle size (Figure 3B). The νCO band shapes, which are virtually invariant with potential, feature a long “tail” toward lower wavenumbers. This feature, not seen on conventional planar Pt electrodes, has been attributed in part to nanoparticle dispersion and fragmentation.8 In addition to the main atop CO band, a weak broad feature centered (14) (a) Note that this vibrational assignment, along with the ca. 2050 cm-1 band to atop CO coordination, has been unassailably identified on Pt(111) electrodes by means of atomic-resolution scanning tunneling microscopy combined with EC-IRAS measurements (ref 14b). (b) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648.

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Figure 4. Selected absolute absorbance spectra for various relative CO coverages (XCO) as indicated, from dilute CO solutions on (A) 60% loading and (B) 20% loading films at -0.2 V in 0.05 M H2SO4.

at ca. 1850 cm-1 is discernible for the larger particles, indicative of bridging CO.14 Also evident for the larger particles is a mild absorbance “dip” at frequencies slightly higher than νPCO. As discussed elsewhere,10 this is a vestige of the anomalous “antiabsorbance” optical effects, commonly seen for metal particle arrays,15 which are manifested as decidedly “bipolar” νCO band shapes for thicker Pt nanoparticle films (vide infra). Last, the markedly greater νCO band absorbances observed for the highest metal loadings (Figure 3A) arise partly from the occurrence of significant “infrared intensity enhancement” as well as from larger Pt surface areas.8 Given that methanol dissociation generally produces subsaturated CO coverages, it is also appropriate to examine the nature of the absolute νCO absorbance spectra obtained from solution CO dosing as a function of the CO coverage relative to saturation, XCO. Parts A and B of Figure 4 show typical νCO spectra for XCO e 1.0, as indicated, for C/Pt films with 60 and 20% loading, respectively, again at -0.2 V in 0.05 M H2SO4. The lower coverages were obtained by dosing with diluted (ca. 10-5 M) CO solutions for controlled times, usually e2 min.8 (The XCO values are readily obtained from the absorbance of the 2345 cm-1 band due to thin-layer CO2, evident upon CO electrooxidation at 0.65 V, relative to the value for a saturated adlayer.8) Substantial (up to ca. 60 cm-1) νPCO redshifts are seen for the 60% loading film with decreasing XCO (Figure 4A). These redshifts are markedly larger than those (ca. 30 cm-1) expected on the basis of diminishing dipole-dipole coupling, as observed on structurally uniform surfaces such as Pt(111).13a Most likely, they reflect in part the preferential binding of CO to edge and other low-coordination Pt sites at lower XCO, which are known to yield redshifted νPCO values relative to terrace binding.8,16 The containment of the low-coverage νCO bands within the redshifted tail “envelope” of the saturated adlayer (XCO ) 1) spectrum further suggests that such a distribution of coordination sites is primarily responsible (15) For example, see: Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71, 1967. (16) Brandt, R. K.; Hughes, M. R.; Bowget, L. P.; Truszkowski, K.; Greenler, R. G. Surf. Sci. 1993, 286, 15.

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for the band shape of the latter. Similar, although less marked (ca. 40 cm-1), νPCO redshifts are observed with decreasing XCO for the 20% loading film (Figure 4B). Overall, the observed sensitivity of νPCO to XCO clearly indicates the need to consider this factor when interpreting spectra for partial-coverage adlayers from methanol dissociation (vide infra). Another significant feature, detailed in ref 8, concerns the corresponding νPCO-E dependencies obtained at lower XCO values on the Pt nanoparticles. Briefly, the behavior is more complex than that for saturated adlayers (Figure 3). At potentials ( 0 V even for moderate dosed CO coverages (XCO ∼ 0.5) on the present Pt nanoparticles,8 the extraction of coverage estimates from such νPCO-E behavior appears unjustified. This point of view is affirmed upon recalling the systematic errors and uncertainties in extracting true νPCO values from the fixed-interval PDIR spectra, already noted. Despite these complications, the systematic dependence of νPCO at a given potential on the nanoparticle size, already noted for saturated CO adlayers (Figures 3B and 9B), is roughly mimicked in the methanol dissociation

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case (Figure 9A). In this respect, then, the behavior of the latter intermediate-XCO adlayers formed by methanol dissociation is qualitatively compatible with that for the better-defined case of solution CO dosing. Implications for the Correlation of 13C NMR and IRAS Behavior. In some respects, the present observation of markedly different νCO spectral characteristics for C/Pt films dosed with solution CO to saturation and exposed to methanol, together with the recognition of the PDIR spectral ambiguities incumbent for the latter, complicates the interpretation of such methanol-based data in terms of Pt nanoparticle structure and bonding. Moreover, the recognition that the major dissimilarities in the νCO spectra for direct chemisorption and methanol dissociation result from the presence of intermediate CO coverages in the latter case introduces a further element of uncertainty. Given that similar methanol-based PDIR data have recently been utilized by the Illinois group to interpret the nature of nanoparticle size-dependent CO bonding in comparison with related 13C NMR results,7 it is therefore appropriate to reexamine briefly these earlier conclusions in the light of the present findings. Two significant aspects of the earlier infrared data in this respect, also evident in Figure 9A, are that progressively increasing νPCO redshifts along with enlarged Stark tuning slopes are obtained for smaller Pt nanoparticles. Both of these observations were interpreted in ref 7 as being due to enhanced Pt-CO dπ-2π* back-donation for the smaller nanoparticles, associated with enhancement in the Fermi-level local density of states (LDOS) as gleaned from the 13C NMR data.6,7 As is evident in Figures 3B and 9B, the absorbance spectra for the CO adlayers produced by direct solution CO dosing exhibit a roughly similar, albeit distinct, νPCO-d dependence to those observed upon methanol dosing, even though the νPCO values themselves are significantly different. In ref 8, the latter results were analyzed by drawing comparisons with related νPCO-E data for CO on low-index and stepped monocrystalline Pt electrodes and for high-nuclearity Pt carbonyl cluster solutes of known structure. This analysis enabled the νPCO-d dependence at a fixed electrode potential, along with corresponding band shape behavior, to be identified at least partly with a transition from predominately terrace to edge-site CO coordination for d < 4 nm. Interestingly, it is just this nanoparticle diameter range, from d ≈ 4-2 nm where a change from chiefly “flat” Pt terraces to lower coordination number Pt atoms forming edge (or “step”) sites is anticipated from metal atompacking considerations.8 The ca. 20 cm-1 νPCO redshifts seen for saturated CO adlayers in diminishing the particle size from d ) 4 nm to d ) 2 nm are nicely consistent with the predominant presence of edge-site coordination for the smaller particles, being in harmony with wellestablished νPCO-Pt coordination number correlations (vide supra).16 Further insight into such surface structural effects is evident from detailed density functional calculations for CO bound to various Pt surface sites reported by Nørskov and co-workers.20 In accordance with experimental observations, they found larger binding energies and redshifted νCO frequencies for CO bound atop to step versus terrace sites. The stronger CO binding on the step (and kink) sites was identified clearly as being due to increased Fermi-level LDOS compared to terrace coordination, (20) Hammer, B.; Nielson, O. H.; Nørskov, J. K. Catal. Lett. 1997, 46, 31.

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thereby enhancing the degree of dπ-2π* back-donation. As already mentioned, the 13C NMR data also yield larger LDOS values for the smallest Pt nanoparticles in a manner which correlates with the νPCO-d behavior. Thus, for d ) 2.0, 2.5, and 8.8 nm, for which (for saturated CO adlayers) νPCO ) 2042, 2048, and 2057 cm-1 at -0.2 V (Figure 3), the 2π LDOS values are 7.4, 6.7, and 6.5 Ry-1 mol-1,6,7 i.e., exhibiting a semiquantitative correlation. In ref 7, however, a roughly similar correlation between the νPCO-d and LDOS behavior deduced from methanol dosing was interpreted instead in terms of a carbonsupport effect. One piece of evidence favoring this explanation comes from clean-surface LDOS data for 2.5 and 8.8 nm particles derived from 195Pt NMR, which suggest that while the smaller particle possesses higher Fermilevel LDOS, a fraction (about 1/3) of the surface Pt atoms are influenced by metal-support interactions.5b No such interactions, however, were found for the larger Pt particle, suggesting that the origins of the LDOS-d and νPCO-d dependencies may lie partly in varying metal-carbon interactions as well as from more intrinsic surfacestructure effects. An unambiguous resolution of this issue, however, will have to await the receipt of IRAS and NMR data obtained on free-standing, and preferably structurally well-characterized, nanoparticles. The second size-dependent infrared property noted in ref 7, the apparently correlated increase in the Starktuning slope for the smallest nanoparticles, appears from the present findings to be an artifact arising probably from systematically varying CO coverages associated with

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the nuances of methanol dissociation. Thus, the saturated CO adlayers yield essentially invariant νPCO-E slopes with decreasing particle size (Figure 3B). While markedly larger Stark-tuning slopes are observed at E > 0 V for subsaturated CO-dosed adlayers, no simple correlation with nanoparticle size is discernible.8 Consequently, the deduction of ref 7 that the Stark tuning-particle size dependency observed upon methanol dosing also arises from enhancement in dπ-2π* back-donation now appears to be questionable. Regardless of these uncertainties, however, undertaking detailed comparisons between IRAS and NMR data, especially for structurally well-characterized nanoparticle systems, clearly has much to offer for the development of our fundamental understanding of electronic structure and bonding. Even for the present nanoparticle systems, given the new-found availability of detailed νCO absorbance spectra from direct CO dosing,8 it would seem worthwhile to undertake more comprehensive comparisons with NMR data obtained under compatible conditions. Acknowledgment. This work is supported at Purdue by the National Science Foundation (Analytical and Surface Chemistry Program) and by a grant to the Purdue Industrial Associates Program from Monsanto/Pharmacia Corp. Y.Y.T. acknowledges support from the Department of Energy, administered via the Fredrick Seitz Materials Research Laboratory (U. Illinois). LA0113825