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J. Phys. Chem. B 2001, 105, 3518-3530
Field-Dependent Chemisorption of Carbon Monoxide on Platinum-Group (111) Surfaces: Relationships between Binding Energetics, Geometries, and Vibrational Properties as Assessed by Density Functional Theory Sally A. Wasileski,† Marc T. M. Koper,*,‡ and Michael J. Weaver*,† Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907, and Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, EindhoVen UniVersity Of Technology, 5600 MB EindhoVen, The Netherlands ReceiVed: September 13, 2000; In Final Form: February 8, 2001
The field-dependent frequency behavior of the metal-adsorbate (νM-CO) as well as the intramolecular (νCO) vibration of carbon monoxide chemisorbed in atop and threefold-hollow sites on three platinum-group (111) metal surfacessPt, Ir, and Pdsis explored in relation to the metal-chemisorbate (M-CO) binding energetics and geometries by means of Density Functional Theory (DFT) calculations for finite clusters. This overall objectiveshaving particular importance in electrochemical systemssof linking field-dependent vibrational, energetic, and geometric properties of the M-CO bond, prompted by the availability of potential-dependent νM-CO data at Pt-group electrodes from Raman spectroscopy, provides an opportunity to assess in quantumchemical terms these surface-adsorbate binding parameters in relation to the extensively studied intramolecular CO vibration. The binding energies (-Eb) tend to increase toward negative fields (F), especially for hollowsite binding. An energy decomposition into specific orbital and steric interactions shows that this effect is driven primarily by enhanced π-back-donation, although offset by progressively weaker σ-donation along with greater surface-chemisorbate steric repulsion. Although these individual orbital and steric interactions exert similar effects on the νM-CO frequencies, the overall νM-CO-F dependencies are notably different, typically displaying a broad maximum at moderate/large negative fields (ca. -0.3 to -0.5 V Å-1). Unlike the bindingenergy behavior, these nonmonotonic νM-CO-F dependencies correlate roughly with the corresponding F-dependent M-CO equilibrium bond lengths, rM-CO. A decomposition of the field-dependent νM-CO and rM-CO behavior into individual interactions exhibits close parallels, with π-bonding acting to markedly blueshift νM-CO and decrease rM-CO, being offset increasingly toward more negative fields by the effects of σ-bonding and steric repulsion. In contrast, the monotonically red-shifted νCO frequencies and the correspondingly elongated C-O bond lengths, rCO, found toward negative fields arise chiefly from the wellknown effects of dπ-2π* back-donation. A common correlation is observed between the field-dependent νCO and rCO values for each of the metal-CO systems and even uncoordinated CO. The likely role of electrostatic factors in the νM-CO-F dependencies is also considered: the increasing M f CO charge polarization seen toward negative fields can account qualitatively for the νM-CO-F maxima. A semiquantitative agreement is evident with electrode potential-dependent νM-CO and νCO vibrational data, although νM-CO-F maxima have yet to be observed experimentally.
Introduction Understanding the nature of carbon monoxide binding to transition metals is an issue that transcends surface chemistry,1 inorganic and organometallic chemistry,2 and even biochemistry.3 An intriguing and widely studied feature concerns the sensitivity of the CO vibrational properties, especially the C-O stretching frequency (νCO), to the electrostatic as well as chemical and geometric environments. An important class of system where all three factors necessarily play important roles in metal-CO binding is electrochemical interfaces. The large ranges of electrode potential (often 2-3 V) commonly accessed at metal-electrolyte interfaces correspond to very high and variable electrostatic fields (ca. 108 V cm-1). These fields can yield substantial alterations in adsorbate binding energies and * Corresponding author. † Purdue University. ‡ Eindhoven University Of Technology.
other properties, such as vibrational spectra. Indeed, examining in detail the potential-dependent intramolecular frequencies for chemisorbed CO (and also for nitric oxide) obtained by in-situ infrared spectroscopy at ordered metal-solution interfaces with corresponding measurements on uncharged metal surfaces in ultrahigh vacuum (UHV) has helped to establish a close behavioral link between these two metal-based interfacial environments, emphasizing the central role played by the surface potential.4 Elucidating the manner in which the binding of simple molecular chemisorbates (of which carbon monoxide and nitric oxide are archetypical examples) depends on the applied potential, and hence interfacial field, therefore is an issue commanding substantial interest in electrochemistry, as well as other branches of surface science. The quantum-chemical understanding of chemisorbate bonding on transition-metal surfaces is currently being advanced considerably by the emergence of Density Functional Theory (DFT), utilizing either finite metal clusters or periodic slabs as
10.1021/jp003263o CCC: $20.00 © 2001 American Chemical Society Published on Web 04/05/2001
Carbon Monoxide on Platinum-Group (111) Surfaces models.5,6 These approaches have been shown to yield bindingsite energies, intramolecular vibrational frequencies, and even spatial adlayer structures for chemisorbed CO and NO on lowindex Pt-group surfaces that are in reasonable concordance with UHV-based experimental data.7-18 Aside from this impressive capability of DFT for describing the observed chemisorbate structure and binding properties, the use of cluster models and molecular orbital theory enables one to discern specific orbital and other contributions to the metal-adsorbate and intramolecular potential-energy surfaces.13,14,17,18 Consequently, such calculations offer important opportunities for attaining a more complete understanding of chemisorbate bonding, especially when compared to spectroscopic and other experimental data. Several finite cluster studies involve applying variable external electric fields, so to mimic the effect of altering the electrode potential in electrochemical systems.14-18 Our interest in this topic stems centrally from a desire to understand how the experimental vibrational behavior is related to the quantum-chemical nature of surface bonding. Our initial studies along these lines have focused on CO and NO chemisorption, prompted by their archetypical importance along with the longstanding interest of the Purdue group in their electrochemical vibrational properties. Although the examination of CO chemisorption by DFT-based methods has already been the subject of a number of detailed studies,7-17 their application to furthering the quantum-chemical understanding of spectroscopic and other experimental data is still in its infancy. We have recently surveyed by cluster DFT calculations the fielddependent bonding of CO and NO at all five hexagonal Ptgroup metal surfacessPt(111), Ir(111), Pd(111), Rh(111), and Ru(0001)sin order to elucidate the quantum-chemical factors that control the observed metal- (and field-) dependent variations in the energetically preferred binding-site geometries and the νCO frequencies, as deduced from vibrational and other data.17,18 Among other things, the DFT results provide a quantumchemical rationale for the increasing preference for atop versus multifold CO coordination observed experimentally in the series Pd < Pt ∼ Rh < Ir ∼ Ru and the greater tendency for multifold binding seen toward more negative interfacial fields.17,18a We have now undertaken field-dependent cluster DFT calculations of the metal-chemisorbate (νM-CO) along with the intramolecular CO stretching (νCO) modes on Pt-group (111) surfaces. This was prompted by the availability of potentialdependent νM-CO as well as νCO data on Pt-group electrodes from the Purdue group, accessed by means of surface-enhanced Raman spectroscopy (SERS) for thin films on gold substrates.19-21 The direct information on potential-dependent surface bonding provided by the νM-CO vibration, essentially inaccessible to infrared spectroscopy in view of its low frequency (e500 cm-1), provides insight well beyond what is evident from the intramolecular vibration, which has been the focus of previous DFT studies. Interestingly, the experimental νM-CO and νCO frequencies exhibit opposite dependencies on the electrode potential, E, yielding “Stark-tuning slopes”, dνM-CO/dE and dνCO/dE, that are negative and positive, respectively.19,20 Reported herein is a detailed DFT analysis of such fielddependent CO vibrational properties on three Pt-group metal surfacessPt(111), Ir(111), and Pd(111)sin both atop and threefold-hollow coordination geometries. These surfaces were chosen in view of the observed strong propensity for atop and hollow coordination on Ir(111) and Pd(111), respectively, with Pt(111) yielding intermediate behavior, a trend mimicked well by the DFT calculations.18 Attention is focused on the role of specific orbital and electrostatic interactions in determining the
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3519 field-dependent CO binding energetics, coordination, and bondlength geometries in relation to the vibrational properties, including strategies aimed at deconvolving the coupled influence of M-CO and C-O bonding. The procedures used to calculate the metal surface-CO (M-CO) as well as C-O vibrational properties involve computing iteratively the equilibrium bond distances at each field. As such, the results furnish a more complete picture of the field-dependent CO binding properties than that attained in our earlier broad survey.18a An unexpected finding is the prediction of nonmonotonic νM-CO-field dependencies. The more general significance of such metal-adsorbate Stark-tuning behavior to our understanding of field-dependent surface bonding is also considered. Computational Methods The DFT calculational procedures largely followed those described in detail in ref 18. Briefly, most calculations utilized a 13-atom cluster model of the (111) surfaces, arranged in two hexagonal layers containing seven and six atoms, with the interatomic distance fixed at the experimental bulk-phase value. A single CO molecule is oriented normal to the hexagonal surface plane. The former and latter planes provide the atop and (hcp) hollow binding geometries so that the metal-CO configurations achieve C3V symmetry, desirable for computational efficiency. Some calculations for atop CO utilized also a larger 25-atom cluster, arranged as three layers with (13,6,6) metal atoms. As is conventional in finite-cluster model calculations,13-18 variable homogeneous external fields were applied along the C3V axis (i.e., along the M-C-O bond direction). The calculations utilized the Amsterdam Density Functional package (ADF 2.3.0, Department of Theoretical Chemistry, Vrije Universiteit, Amsterdam, 1997).22 Slater-type functions are used to represent the atomic orbitals. To enhance computational efficiency, we kept frozen the innermost atomic shells of all atoms (up to and including the following orbitals: C 1s, O 1s, Na 1s, Cl 2p, Pt 5p, Ir 5p, and Pd 4p), as these core electrons do not contribute significantly to the chemical bonding. The Pt, Ir, and Pd basis sets are of double-ζ quality; in addition, the C, O, Na, and Cl basis sets were augmented by polarization functions. The Kohn-Sham one-electron equations were solved in the so-called DFT-GGA approximation. The Vosko-WilkNusair form of the local density approximation23 was used in combination with the BP86 functional for the generalized gradient approximation (GGA).24 Relativistic effects within the cores were accounted for self-consistently by first-order perturbation theory. All cluster calculations were carried out in the spin-restricted mode. The νM-CO and νCO stretching frequencies were calculated using the AnharmND program,25 from a potential-energy surface consisting of nine points for displacements of ca. ( 0.2 Å from the equilibrium bond lengths. This relatively wide range ensures that we obtain a reasonable description of the first vibrational excited state and any anharmonic contributions. The potentialenergy surface for the metal-chemisorbate bond at each external field was obtained by fixing the intramolecular bond length at its coordinated equilibrium value, obtained iteratively by altering also the former bond distance. For the intramolecular potentialenergy surface, the chemisorbate center of mass was fixed at the coordinated equilibrium position. These procedures to effectively decouple the intramolecular stretching from the metal-chemisorbate vibration are approximately valid since the latter has a considerably (5-10-fold) lower frequency.
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TABLE 1: Metal-Dependent CO Binding Parameters at Zero External Field Pt atop relaxeda rM-CO /Å rCO /Å Eb /eVc E(st) /eVd E(orb) /eVe E(A1) /eVf E(E) /eVf
const.b
1.900 1.900 1.159 1.140 -1.26 -1.25 7.19 9.79 -8.45 -11.03 -5.46 * -7.26* -2.96 * -3.74*
Pt hollow relaxeda
const.b
1.488 1.488 1.190 1.140 -1.28 -1.17 5.92 12.38 -7.21 -13.55 -2.31* -6.82* -5.06* -6.84*
Ir atop relaxeda
Ir hollow
const.b
relaxeda
1.900 1.900 1.165 1.140 -1.20 -1.18 8.44 11.80 -9.64 -12.98 -6.65 -8.95 -3.06 -4.07
const.b
1.644 1.644 1.189 1.140 -1.00 -0.89 6.90 13.14 -7.90 -14.03 -2.38* -6.70* -5.08* -6.83*
Pd atop
Pd hollow
relaxeda const.b
relaxeda const.b
1.970 1.156 -0.87 0.99 -1.86 -0.03 -1.78
1.554 1.177 -1.09 -0.40 -0.89 1.88 -2.73
1.970 1.140 -0.87 3.19 -4.06 -1.56 -2.45
1.554 1.140 -1.03 4.41 -5.45 -1.49 -4.03
a Binding-energy parameters obtained for the “bond-relaxed” condition, where the C-O bond length, rCO, has a true equilibrium value in the chemisorbed state, as listed (see text). b Binding-energy parameters obtained for the “bond-constrained” condition, where r CO is fixed at the uncoordinated equilibrium value, 1.140 Å (see text). c Overall binding energy, for either the “bond relaxed” or the “bond constrained condition, as noted. d Steric repulsion component of binding energy. e Total orbital interaction component of binding energy. f “Bond-prepared” orbital components, obtained by fixing the bare cluster electronic configuration so to yield a difference, a16e4, with respect to that of the ligated cluster appropriate for uncoordinated CO (see text). Values marked with an asterisk (for Ir hollow and Pt atop and hollow) denote cases where this electronic configuration differs from the ground state.
Results and Discussion Significance of Field-Dependent Parameters. Before presenting the field-dependent DFT results, it is helpful to clarify briefly the meaning and significance of the “external interfacial field”, F, in UHV and electrochemical systems.4 By definition, the condition F ) 0 necessarily applies to a vanishingly small chemisorbate coverage on a “clean” (uncharged) metal in UHV. Nonzero fields can be induced at such metal-UHV interfaces either by coadsorption of dipolar and/or ionizable adsorbates26,27 or (to a limited extent) by applying a variable external field using a nearby “counter-electrode”.28 At electrochemical interfaces, much larger field variations (typically ca. 108 V cm-1) are induced by altering the electrode potential, E. While the relationship between the changes in field, ∆F, and applied potential, ∆E, is relatively straightforward (vide infra), the electrode potential corresponding to F ) 0 is not so readily evaluated.4 This is partly because the field is influenced by dipolar solvent as well as free (electronic/ionic) charge σm, and only the latter component vanishes at the potential of zero charge, Epzc , where σm ) 0 so that E(F)0) * Epzc.4 Nevertheless, the zero-field electrode potential, E(F)0), can be defined as the point where the surface potential is equivalent to that for the corresponding clean metal-UHV interface, as evaluated by the work function Φ(F)0), according to4,18a
E(F)0) ) Φ(F)0)/e - Eref(abs)
(1)
where e is the electronic charge and Eref(abs) is the “absolute” potential of the reference electrode on the vacuum scale. A consistently close concordance between the intramolecular frequencies for CO (and NO) chemisorbed at corresponding electrochemical and UHV-based metal interfaces is obtained, at least for high adlayer coverages, when the former are evaluated at such “equivalent surface potentials”.28 Admittedly, the situation is not as straightforward at low chemisorbate coverages, where “local electrostatic” and other influences from coadsorbed solvent can also influence the vibrational properties.4b Nevertheless, the experimental potential- (and hence field-) dependent vibrational frequencies at electrochemical interfaces, and by implication other chemisorbate bonding properties, are determined to a large extent by the external electrostatic field, which can be evaluated and linked in relatively simple fashion to corresponding parameters for metal-UHV systems. This situation therefore justifies consideration of field-dependent bonding and vibrational properties as deduced from DFT with relevant experimental parameters at metal surfaces in electrochemical and UHV environments on a common basis.
Metal-Dependent Bonding Energetics. We will first consider the metal-dependent binding-site energetics at zero external field, F ) 0. Table 1 summarizes binding-energy parameters obtained for CO in atop and hollow coordination sites on the (111) surfaces of platinum, iridium, and palladium, as mimicked by the M13 cluster configurations.18a While the overall binding energies (-Eb) are roughly comparable to those obtained in our earlier survey,18a the present calculations involve a complete energy optimization of the bond geometries. The BP86 GGA functional used in the present study usually yields smaller -Eb values than those from the PW91 functional employed for most slab calculations so far.29,30 Trial calculations for atop CO on the present Pt13 cluster using the PW91 functional indeed yielded slightly (ca. 0.15 eV) higher binding energies than with BP86, although the field-dependent Eb behavior is essentially unaffected. (Moreover, the bond lengths and vibrational frequencies, of primary interest here, calculated with these two functionals are essentially identical, within 1%.) The -Eb values obtained here (Table 1) tend to be significantly (ca. 0.2-0.5 eV) smaller than those of the experiment, even though the latter low-coverage values are often uncertain. For atop CO on Pt(111), for example, measurements of the adsorption heat, ∆Had, yielded -∆Had ≈ 1.8 eV at low coverages,31 larger than the present -Eb value, 1.25 eV. More significantly, the present BP86-GGA functional yields relatiVe binding energies for atop versus hollow CO coordination that are at least in semiquantitative concordance with experiment.18a Thus, the observed strong energetic preference for atop binding on Ir(111) and hollow-site binding on Pd(111) in UHV and electrochemical environments, established by vibrational spectroscopy for both surfaces32,33 and also by photoelectron diffraction in the latter case,34 is mimicked by the estimated Eb site differences, both about 0.2 eV. For Pt(111), the DFT calculations yield closely similar Eb values for atop and hollowsite binding at zero field (Table 1). This is consistent with the small (ca. 0.1 eV) binding-site preference known for atop versus bridging CO coordination at low coverage,35 even though the energy difference between atop and hollow sites is unknown (but see below). Of particular interest here, as in ref 18a, is the subdivision of the overall binding energies into individual orbital and other interaction terms, as demonstrated earlier.36 Such an energy decomposition is readily made with the present ADF-cluster approach, which for C3V symmetry can be expressed as17,18a
Eb ) E(st) + E(orb) ) E(st) + E(A1) + E(E) + rest
(2)
Here E(st) arises from Pauli repulsion and other steric interac-
Carbon Monoxide on Platinum-Group (111) Surfaces tions,37 and E(orb) denotes orbital interactions. The latter term is decomposed further in eq 2 into contributions having A1 and E symmetry, along with the A2 and other residual components, labeled “rest”. In the present case,17 the major E(A1) and E(E) terms correspond predominantly to 5σ CO-M donation and 2π* M-CO back-donation, respectively, in the “frontier orbital” picture.12-15 Table 1 includes these individual components of Eb; note that the sum of the E(A1) and E(E) terms is generally close to E(orb), the small ( 0 denotes dipoles with the positive sign outward from the metal surface.) As might be expected and as noted previously,14,15,17 µD becomes less positive/more negative toward negative fields, reflecting more pronounced π-back-donation along with weaker σ-donation.56 Interestingly, the µD values for atop CO on Pt(111) and Ir(111) switch sign at moderate negative fields and on Pd(111) at farther negative fields (Figure 13), mirroring roughly the location of the corresponding νM-CO maxima (Figures 5-7). Included for comparison in Figure 13 are µD-F plots computed for a strongly electropositive atom, sodium, and an electronegative atom, chlorine, both bound in Pt(111) hollow sites (open, filled diamonds, respectively). As expected, these two species yield strongly positive and negative µD values, respectively. The calculated Stark tuning behavior for adsorbed sodium and chlorine also reflect their “cation-like”
J. Phys. Chem. B, Vol. 105, No. 17, 2001 3527 and “anion-like” properties, yielding large negative (ca. -90 cm-1 V-1 Å) and positive (ca. 70 cm-1 V-1 Å) average dνM-A/dF values, respectively. Following the above simple electrostatic argument, then, one might associate negative and positive Stark-tuning slopes with “anion-like” and “cation-like” adsorbate polarization. On this basis, then, one is tempted to interpret the dνM-CO/dF sign reversal for atop CO toward large negative fields as reflecting the emergence of an “electron-rich” (or “anion-like”) chemisorbate, resulting from substantial π-backdonation, with the opposite (“electron-poor”) situation pertaining at more positive fields. In other words, the positive dνM-CO/dF values obtained for atop CO at large negative fields can be thought as arising partly from a positive electrostatic contribution, with the opposite being the case at more positive fields. The dνM-CO/dF maximum may therefore signal in part the fielddependent “amphoteric” properties of chemisorbed CO. Further, the larger positive dνM-CO/dF slopes seen at larger negative fields for atop CO on Ir(111) versus Pt(111) (Figures 3 and 1) are consistent with the larger negative µD values seen on the former metal (Figure 13), also associated with greater fieldinduced π-back-donation (vide supra). The lack of a νM-CO-F maximum for hollow-site CO on Pt(111) (Figure 8) might be considered surprising on this basis, given that markedly lower (i.e., less positive/more negative) µD values are observed than those for atop CO, consistent with the greater π-back-donation in the former geometry. However, a likely offsetting factor is the sharply more negative Eb values seen toward negative fields for hollow versus atop Pt(111)/CO, favoring progressively larger νM-CO values under these conditions, thereby accounting for the observed lack of νM-CO-F sign reversal for hollow-site binding. These issues will be examined in greater detail elsewhere.43 Comparison with Experimental Vibrational Spectra. Given the detailed links between field-dependent bond energetics, geometries, and vibrational frequencies furnished by DFT, it is of obvious interest to compare the last quantities with experimental potential-dependent vibrational spectra for electrochemical and related interfaces. We have recently made such comparisons for potential-dependent intramolecular frequencies of chemisorbed CO and NO on hexagonal-packed Pt-group surfaces.18 The zero-field νCO and νNO frequencies were found to be in good agreement with the DFT predictions (within 2-3%), although only a semiquantitative concordance was evident for the intramolecular frequency-field (“Stark-tuning”) slopes. We consider here a more restricted comparison along these lines for chemisorbed CO on Pt, Ir, and Pd, encompassing νM-CO as well as νCO vibrations. Table 2 summarizes field-dependent vibrational frequencies from the present DFT calculations in comparison with experimental values in electrochemical (EC) and UHV environments, obtained from infrared (IR) and SERS data. These include atop νCO values at zero external field, obtained for low CO coverages (θ e 0.1) on Pt(111) and Ir(111) in UHV, and also corresponding data at the aqueous electrochemical interface, determined at an equivalent surface potential, E(F)0), to that for the (uncharged) metal-UHV interface as prescribed by eq 1 (vide supra).4 Corresponding hollow-site νCO values are also included on Pd(111). While hollow-site CO binding on Pt(111) is not observed in UHV, the electrochemical system exhibits a mixture of hollow and atop coordination at high coverages and lower potentials, most definitively in the (2 × 2)θ ) 0.75 adlayer structure characterized by IR/STM,48,49 which provides the νCO(F)0) estimate included in Table 2. Although the DFT
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TABLE 2: Comparison with Experimental Field-Dependent Vibrational Frequencies systema
binding site
νCO(F)0) b (cm-1)
dνCO/dF c (cm-1 V-1 Å)
νM-CO(F)0) b (cm-1)
dνM-CO/dF c (cm-1 V-1 Å)
Pt(111)-DFT Pt(111)-UHV Pt(111)-EC Pt(poly)-EC Pt(111)-DFT Pt(111)-EC Ir(111)-DFT Ir(111)-UHV Ir(111)-EC Ir(poly)-EC Pd(111)-DFT Pd(111)-DFT Pd(111)-UHV Pd(111)-EC
atop atop atop atop hollow hollow atop atop atop atop atop hollow hollow hollow
2000 2084d 2082e ∼2080f 1715 ∼1820g 1955 2030h 2040e i 2007 1815 1825j ∼1870k
120 75l 120m ∼120m,n 130 ∼200o 115
408 465p
-13.5
460 f 288
-24r -23
427
-18
505q 315 234
-25r -13 -20
150e i 129 135 ∼150k
DFT refers to the present calculations; others are experimental systems as noted for specific data entries (UHV ) ultrahigh vacuum, EC ) aqueous electrochemical environment; “Pt(poly)” or “Ir(poly)” refer to Pt or Ir film on gold utilized to obtain SER spectral data). b Vibrational frequency at zero external field, at low CO coverage except where noted. Values for electrochemical systems evaluated at the “equivalent surface potential”, E(F)0), to the work function of the corresponding metal-UHV interface [see eq 1 and ref 4], as noted in footnotes for specific systems. c Vibrational frequency-field slope, evaluated either at F ) 0 (for DFT and UHV entries) or for EC systems over range of electrode potentials, usually between ca. 0 to 0.5 V vs SHE [i.e., ca. 1.5 to 0.5 V negative of E(F)0)]. Experimental Stark-tuning slopes converted to field-based values quoted by using eq 2 with di ) 3 Å. d From ref 35, as noted in ref 4b. e From ref 4b, evaluated at E(F)0) taken as 1.1 V vs SHE. f Evaluated from SER spectra for high CO coverage (ref 19), at ca. 1.0 V vs SHE. g Extracted from high-coverage infrared data [containing hollow-site νCO band, for (2 × 2) adlayer] in ref 49, evaluated at E(F)0) ≈ 1.1 V vs SHE and extrapolated to low coverage by assuming similar coverage dependencies in hollow and atop sites (the latter is known, ref 49). h From ref 50. i Values omitted due to significant disparity between SERS and infrared data (see ref 20). j From ref 33a (data at 300 K). k From ref 33b, as quoted in ref 4b, evaluated at E(F)0) ≈ 1.0 V vs SHE. l Direct experimental value, from ref 51. m From ref 4b. n From high-coverage SER spectra (ref 19). o From the dνCO/dE value for a (2 × 2) adlayer, (ref 49). p High-coverage value, from ref 52. q From high-coverage SER spectra (ref 20). r Slope evaluated (from SER spectra for high CO coverage) over electrode potential range ca. -0.2 to 0.7 V vs SHE (corresponding to approximate F range of -0.3 to 0 V cm-1 Å). a
νCO(F)0) values are ca. 4% lower than the experimental UHV and EC frequencies, similarly small disparities are observed for other Pt-group interfaces, also reflected in the slightly lower νCO value calculated here for uncoordinated CO (2115 cm-1) in comparison with experiment (2145 cm-1).18a The experimental dνCO/dF data listed in Table 2 include a value, 75 cm-1V-1Å, obtained for low atop CO coverages on Pt(111) by Lambert and co-workers by applying a small external field across the metal-UHV interface.51 The other dνCO/dF values refer to electrochemical interfaces and are obtained from the measured dνCO/dE slopes. [These typically refer to E-E(F)0) values, eq 1, from ca. -0.5 to -1.0 V, i.e., for moderate negative fields, i.e., with F ≈ -0.15 to -0.35 V Å-1, and are converted to dνCO/dF values by using eq 2 with di ) 3 Å.] The atop DFT and experimental dνCO/dF values are seen to be roughly similar, although the value observed in UHV is about 2-fold smaller than that obtained from the electrochemical data. (Possible reasons for this discrepancy have been discussed.39,51 These include differences in the interfacial field profile in the UHV and electrochemical systems brought about by double-layer solvation.) The experimental dνCO/dF value for hollow-site CO on Pt(111), noticeably (ca. 1.5-fold) larger than that for atop CO, is not mimicked quantitatively by DFT, the computed value being only slightly larger for hollow versus atop binding (Table 2).54 On the other hand, the DFT and experimental dνCO/dF values for hollow-site CO on Pd(111) are in reasonable agreement (Table 2). [In addition, curvature in experimental atop νCO-E data, essentially in accordance with the DFT νCO-F nonlinearities (Figure 3), is evident over a sufficiently wide electrode potential range.53] The experimental zero-field νM-CO data given in Table 2 include an entry, 465 cm-1, extracted from IRAS for high atop coverages on Pt(111) in UHV and a similar electrochemical value obtained on polycrystalline Pt from SERS. (No experimental νM-CO data are available for hollow-site CO on Pt or for atop or hollow CO on Pd.) Both νM-CO values for atop CO
on Pt are significantly higher than the DFT estimate, 408 cm-1. A similar discrepancy is seen between the measured νM-CO(F)0) value for atop CO on polycrystalline Ir, again from SERS, and the DFT value on Ir(111), although DFT correctly predicts the higher frequency observed with respect to atop Pt (Table 2). One likely source of the systematically smaller DFT estimates of νM-CO relative to experiment lies in coupling between the surface vibration and metal phonon modes,55 which are not considered in the present calculations. The experimental dνM-CO/dF slope, -24 cm-1V-1Å, extracted from νM-CO-E SERS data for atop CO on polycrystalline Pt,19 is ca. 2-fold larger than the DFT estimate for F ≈ 0, although the corresponding experimental and DFT estimates on iridium20 are in reasonable agreement (Table 2). However, the range of E - E(F)0) values to which the experimental SERS data refer, from ca. -1.5 V to -0.3 V, corresponding to F values between -0.5 and -0.1 V Å-1, encompasses fields approaching the DFT νM-CO-F maxima (Figure 3A). Admittedly, the SERS-active polycrystalline Pt and Ir are not strictly hexagonal surfaces, the data refer to high rather than low CO coverages, and there are significant uncertainties in the E(F)0) values (presumed here to be ca. 1.0 V vs SHE).4 Nevertheless, there is no experimental evidence at present for νM-CO-F maxima, uniformly negatiVe dνM-CO/dE (and hence dνM-CO/dF) slopes being obtained on these and other metal surfaces.19,20 This disparity suggests that the influence of π backdonation in increasing the νM-CO frequency, as well as the M-CO bond strength, toward more negative fields may prevail over the influence of σ-donation and steric interactions to a greater extent than that described by the present DFT calculations. In any case, given the obvious limitations of the present finitecluster model in describing potential-dependent electrochemical systems, quantitative agreement in not to be expected. It would clearly be of interest to undertake detailed periodic slab
Carbon Monoxide on Platinum-Group (111) Surfaces calculations, incorporating coadsorbed solvent and so on, to mimic more precisely the electrochemical interface. Concluding Remarks Together with our earlier related work,17,18 the present study is believed to add significantly to our understanding of the quantum-chemical factors that influence the potential-dependent bonding properties of carbon monoxide on Pt-group surfaces. Several key points are worth reiterating here. Although the changes in “internal” C-O bonding incurred upon chemisorption complicate somewhat interpretation of the binding-energy parameters in terms of the M-CO bond itself, a useful approach entails calculating also “bond-constrained” parameters where the C-O bond is fixed in its uncoordinated geometry, thereby focusing attention on the nature of the M-CO interactions. This tactic aids significantly the elucidation of metal- as well as fielddependent bonding trends. While the field-dependent DFT νM-CO and νCO frequencies correlate closely with the equilibrium bond lengths, no such clear-cut relationship is seen with the corresponding M-CO bond energies. The νM-CO-F dependencies display in most cases a maximum at moderate negative F values, even though atop CO displays a -Eb-F minimum, and hollow-site CO shows monotonically increasing -Eb values under these conditions. The origins of these unexpected behavioral differences can be explored by decomposing the field-dependent frequencies into individual steric and orbital components, in an analogous fashion to the binding energies. The peaked νM-CO-F behavior is seen to be determined by a interplay between π-back-donation and σ-donation along with steric repulsion, which act to blueshift and red-shift νM-CO, respectively, toward negative fields. Indeed, these indiVidual orbital and steric contributions are seen to influence the field-dependent M-CO well shape (νM-CO) in a manner roughly similar to that of the well depth (-Eb); only the differences in their combination are responsible for the dissimilarities in the νM-CO-F and -Eb-F dependencies. Further insight into the factors responsible for the overall νM-CO-F behavior is obtained from the field-dependent M-CO charge polarization, as gleaned from bond dipole moments. The progressively greater M f CO charge-transfer deduced toward negative fields on this basis suggests the importance of electrostatic metal-adsorbate interactions: such charge polarization tends to favor positive rather than negative dνM-CO/dF slopes, i.e., corresponding to “anion-like” versus “cation-like” Stark-tuning behavior. The overall picture that emerges from the present analysis of field-dependent surface bonding for this archetypical “πacceptor” chemisorbate is that several factors contribute importantly to the overall binding energetics and the metaladsorbate vibrations which are absent from, or at least only indirectly reflected in, the properties of the well-studied intramolecular vibration. The limited νM-CO-E data that are available so far exhibit only negative Stark-tuning slopes, suggesting a more prevailing influence from field-dependent π-back-donation. Nevertheless, the scrutiny of surfaceadsorbate bonding by means of such low-frequency vibrational measurements combined with DFT calculations constitutes an intriguing new avenue for exploring electrosorption. Acknowledgment. This research program is supported by the Petroleum Research Fund and the U.S. National Science Foundation (to M.J.W.) and by a fellowship of the Royal Netherlands Academy of Arts and Sciences (to M.T.M.K).
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