Influence of Dipole–Dipole Interactions on Coverage-Dependent

Article pubs.acs.org/Langmuir

Influence of Dipole−Dipole Interactions on Coverage-Dependent Adsorption: CO and NO on Pt(111) Prashant Deshlahra,† Jonathan Conway,† Eduardo E. Wolf,† and William F. Schneider*,†,‡ †

Department of Chemical and Biomolecular Engineering and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: Density functional theory (DFT) calculations of energetic, geometric, vibrational, and electrostatic properties of different arrangements of CO and NO at quarter and half monolayer coverage on Pt(111) are presented. Differences in the extents of electron back-donation from the Pt surface to these molecules cause the low-coverage adsorbate dipoles to have opposite signs at atop and more highly coordinated bridge or fcc sites. These dipoles of opposite sign occupy adjacent positions in the experimentally observed atop−bridge or atop−fcc high -coverage arrangements, leading to attractive electrostatic interactions and concomitant changes in dipole moments, bond lengths, and vibrational frequencies. The interaction energies are estimated by charge partitioning to extract individual dipoles from the mixed arrangement and by calculations of field−dipole interactions. These estimated dipole interactions contribute significantly (20−60%) to the DFT-calculated relative stability of mixed arrangements over atop-, bridge-, or fcc-only arrangements and thus play an important role in coverage-dependent adsorption. We further extend these analyses to a range of molecules with varying dipole moments and show that the general nature of these interactions is not limited to CO and NO.

1. INTRODUCTION Interactions between adsorbates at a heterogeneous surface introduce coverage dependence into adsorption energies,1−4 influence adsorbate site preferences and the distribution of adsorbates on a surface,5,6 and can contribute to poisoning and the promotion of chemical reactions.7 Electronic interactions that arise from competition between adsorbates for surface states tend to be short-ranged and can be described in terms of the d-band center models.8−10 Strain interactions arise from adsorbate-induced expansion or compression of the surface and can be longer-ranged.11 When adsorption creates significant surface dipoles, longer-ranged electrostatic interactions can appear. In this work, we consider the interactions of such dipoles and their influence on adsorbate binding, vibrational spectroscopy, and site preferences. Gas molecules with or without a net intrinsic dipole moment exchange charge density with the surface on which they adsorb, leading to a modified adsorbate dipole.12,13 The interactions of these adsorbate dipoles with external electric fields can have structural and energetic consequences relevant to catalysis. © 2012 American Chemical Society

Field−dipole interactions arise in electrochemical or electrocatalytic systems,14−16 electric fields induced by charge transfer at metal support interfaces of supported catalysts,17−19 and model catalytic reaction studies in field-ion microscopes20 as well as on single-crystal surfaces using electroreflectance vibrational spectroscopy.21 These interactions can significantly affect the adsorbate stability12,13,15,22,23 and reaction barriers13,22 on the catalyst surface. Electrochemical field−dipole effects have been studied in several experiments and simulations. Shifts in the vibrational frequency of adsorbates with the electric field, known as the vibrational Stark effect, are well known.16 However, the effect of coverage on the energetics of these interactions and the effect of the local environment leading to different Stark tuning rates on electrochemical and UHV systems is not well understood. Received: March 6, 2012 Revised: April 22, 2012 Published: April 30, 2012 8408

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

2. COMPUTATIONAL DETAILS Periodic supercell DFT calculations were performed using the Vienna ab initio simulation package (VASP).32 The PW91 implementation33,34 of the generalized gradient approximation was used with the projector augmented wave35,36 method to describe atom cores and plane waves included to a cutoff energy of 400 eV to expand the valence electronic states. The Pt(111) surface was modeled using a 2 Pt × 2 Pt supercell with 4 Pt layers and an approximately 4-Pt-layer-thick vacuum region. A 6 × 6 × 1 Monkhorst-Pack37 k-point mesh was used to sample the first Brillouin zone. One or two adsorbates were placed on one side of the slab in different arrangements. Gasphase molecules were simulated in a 20 Å × 20 Å × 20 Å box by including spin polarization and only the Γ point. We applied the linear extrapolation correction proposed by Mason et al.31 to correct for the GGA underprediction of the CO HOMO− LUMO gap. For this purpose, CO adsorption energies were calculated using three different PAW atom cores. The details of these corrections are described in the Supporting Information and elsewhere.12 Average adsorption energies are calculated from

An adsorbate surface dipole will interact with the electric field induced by neighboring dipoles. These dipolar interactions between adsorbates play a role in coverage-dependent shifts in vibrational frequencies and intensities, as has been observed experimentally,24 described using semiclassical models,24,25 and shown in some DFT calculations.26,27 Repulsive dipole−dipole interactions have been used in lattice-gas models to describe coverage-dependent adsorption energies.28 On the molecular level, these effects have been estimated using a simplified approach of a uniform electric field interacting with a rigid adsorbate or transition-state dipole mainly to investigate the role of alkali promoters in catalytic reactions.22,23 Detailed studies of these interactions, including their coverage dependence and sensitivity to adsorbate polarizability, have not been reported. CO and NO on Pt are good models for studying adsorbate− adsorbate interactions and are relevant to heterogeneous catalytic reactions of industrial and environmental importance. Their adsorption behaviors have been studied using a range of experimental surface science techniques as well as theoretical studies. CO preferentially adsorbs at atop sites on Pt(111) at coverages sufficiently low that adsorbate−adsorbate interactions are weak. At 1/4 monolayer (ML) coverage, adsorbed CO forms a p(2 × 2)-CO ordered structure.29 At higher coverage, CO starts to occupy mixed adsorption sites. For example, at 1/2 ML coverage a stable c(4 × 2)structure consisting of an equal number of atop and bridge sites has been identified using lowenergy electron diffraction (LEED), infrared reflection absorption spectroscopy (IRAS), and electron energy loss spectroscopy (EELS).30 NO preferentially occupies fcc hollow sites at low coverage and an equal number of atop and fcc hollow sites at 1/2 ML coverage, as indicated by STM studies and accurately predicted by DFT calculations.26 IRAS experiments at high coverage, however, show only an absorption band corresponding to atop NO because of a significantly weaker infrared absorption coefficient of the fcc NO.26 In this work, we report plane wave, supercell DFT-GGA calculations to evaluate the adsorption geometry, energy, and vibrational frequency of different 1/4 and 1/2 ML coverage arrangements of CO and NO on Pt(111). The results, after applying an extrapolation correction to CO,31 correctly predict the experimentally observed trends in site preference. Changes in adsorption properties with coverage and adsorbate arrangements show interesting similarities between CO and NO, indicating attractive dipole−dipole interactions at high coverage. We investigate these interactions by partitioning the charge densities to calculate individual dipole moments of atop and higher-coordinated adsorbates in mixed arrangements and estimate their interaction energies by calculating the electric field induced by each dipole at neighboring sites and the resulting field−dipole interaction. The results show that in contrast to the commonly observed increasing dipolar repulsion with coverage, attractive dipolar interactions between these molecules at high coverage contribute significantly to the stability of mixed-site arrangements. We then compare field− dipole interactions and dipole−dipole interactions for several small adsorbate molecules on Pt(111) with dipole moments varying in sign and magnitude. This study elucidates how surface dipoles and resulting electrostatic interactions evolve with coverage and their role in adsorbate binding, geometry, and vibrational frequencies.

ΔE0 =

E Pt − ads − E Pt − nEads n

(1)

where EPt−ads, EPt, and Eads are the total energies of the Pt slab with adsorbed molecules, the bare Pt surface, and the gas-phase adsorbate respectively, and n is the number of molecules adsorbed. We calculated vibrational frequencies by diagonalizing the dynamic matrix computed using two-sided finite differences of the energy gradients. To calculate the gradients, each adsorbate atom was perturbed one by one in all three Cartesian directions by ±0.01 Å. The much heavier Pt atoms were not perturbed. The relative infrared absorption intensities Ik of the adsorbed layer are proportional to the square of the dipole derivative:38 ⎛ ∂μ ⎞2 Ik ∝ ⎜ z ⎟ ⎝ ∂rvib ⎠

(2)

We evaluated the derivatives by calculating the adsorbate dipole moments at the equilibrium position, which are perturbed along the normal vibrational mode.39 Here, μz is the adsorbate dipole in the direction normal to the Pt surface and rvib is the magnitude of perturbation for the vibrational mode. We computed the z-component dipole moments as the first moments of the spatial charge distribution obtained from the DFT calculation μz =

∫V

zρel dV +

∑ atoms

znunnu

(3)

where z is the z-coordinate position inside the supercell, ρel is the negative charge density of electrons, V is the supercell volume, znu is position of atoms, and nnu is the positive charge of the atom cores. For CO and NO, which adsorb through C and N on Pt(111), positive dipole moments correspond to Cδ−−Oδ+ and Nδ−−Oδ+ charge distributions, respectively. Similarly, negative dipole moments correspond to Cδ+−Oδ− and Nδ+−Oδ−distributions. For configurations with more than one adsorbate in a single supercell unit, the charge distribution was partitioned as described in the Supporting Information to calculate individual dipole moments of each adsorbate. 8409

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

with the experimental IRAS frequency of 2100 cm−1 for 1/4 ML atop CO on Pt(111).46 With increased adsorbate coordination, the vibrational frequency and intensity decrease, whereas both the CO and NO bond lengths increase. This inverse relation between the bond length and vibrational frequency is commonly referred to as Badger’s rule.47 3.1.2. Half Monolayer (1/2 ML) Coverage. Table 2 shows different adsorbate arrangements and the corresponding

3. RESULTS AND DISCUSSION 3.1. CO and NO Adsorption on the Pt(111) Surface. NO differs electronically from CO by the addition of an electron to the diatomic 2π* antibonding orbital. The two molecules, however, have interesting similarities in the trends in their adsorption properties on metal surfaces and in changes with coverage. 3.1.1. Quarter Monolayer (1/4 ML) Coverage. Table 1 compares GGA-computed atop, bridge and hollow site CO and

Table 2. DFT-Calculated Average Adsorption Energy (ΔE0) per Molecule, Average Dipole Moment (μ0) per Molecule, Bond Length (l), Vibrational Frequency (ν), and Squared Dipole Derivatives Representing the Vibrational Intensity (Ik) per Unit Supercell for Different 1/2 ML Arrangements of CO and NO on Pt(111)

Table 1. GGA-Calculated 1/4 ML Adsorption Energy (ΔE0) per Molecule, Dipole Moment (μ0) per Molecule, Bond Length (l), Vibrational Frequency (ν), and Squared Dipole Derivatives Representing the Infrared Absorption Intensity (Ik) per Unit Supercell of CO and NO at Different Adsorption Sites on Pt(111)

a

GGA results corrected for underpredicted CO excitation energy. Details in Supporting Information.

NO adsorption at 1/4 ML coverage and their adsorption energies, dipole moments, C−O or N−O bond lengths, vibrational frequencies and infrared absorption intensities. As previously described, the extrapolated GGA results correctly recover the CO preference for atop over bridge and hollow adsorption at 1/4 ML. NO preferentially adsorbs at fcc hollow site at this coverage, consistent with other calculations26,40 and experiments.41 NO adsorbs bent on atop sites and has a lower adsorption energy compared to that of the linearly adsorbed fcc NO. Previous work shows that fcc and hcp hollow sites differ little in their CO and NO adsorption characteristics;29,40,42 we limit ourselves to the fcc sites here. The computed atop CO and NO dipole moments are positive, corresponding to Cδ−−Oδ+ and Nδ−−Oδ+ charge distributions, and have magnitudes that are slightly different from the gas-phase dipole moments of +0.028 and +0.035 eÅ, respectively. Dipole moments of both molecules reverse their signs at the more highly coordinated bridge and hollow sites, independent of the extrapolation correction. This change in the sign of the dipole moment reflects increased electron density at the oxygen end of the adsorbate and is consistent with the increased back-donation to the adsorbate 2π* energy levels at higher coordination sites. The trends in CO and NO vibrational frequencies at different adsorption sites are in agreement with previous calculations43−45 and experiments, but the actual values for CO are slightly lower than in the experiments.46 The extrapolated atop CO vibrational frequency of 2107 cm−1 is higher than the uncorrected GGA value of 2073 cm−1 and in better agreement

a

GGA results corrected for underpredicted CO excitation energy. Details in Supporting Information.

properties per CO or NO molecule at 1/2 ML coverage on Pt(111). Both are experimentally observed to populate mixed adsorption sites at 1/2 ML coverage. CO forms a c(4 × 2)ordered arrangement in which half of the CO molecules occupy atop and the other half occupy bridge sites.30 NO, however, prefers a p(2 × 2)-2NO arrangement with equal numbers of occupied atop and fcc hollow sites.26 Consistent with the experimental observations, we find that mixed-site arrangements are significantly more stable than atop−atop or hollow− hollow arrangements at this coverage. For CO, however, the uncorrected GGA energy of the experimentally known atop− bridge arrangement is almost the same as the atop−fcc one. After the extrapolation correction is applied, the energy difference between the two mixed-adsorbate arrangements increases and the experimentally observed c(4 × 2) atop− bridge becomes the more stable arrangement. The computed average dipole moments per molecule in the mixed arrangements are smaller than the 1/4 ML values because the positive atop and dipole moments and negative bridge or fcc dipole moments are in opposition. These neighboring dipoles of opposite sign compensate for each other, leading to small values in the average calculations, even though the individual molecules have larger dipole moments. In the 1/2 ML mixed arrangements, the atop CO and NO bond lengths decrease and bridge and fcc bond lengths increase relative to 8410

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

their 1/4 ML values. In the atop−atop, bridge−bridge, and fcc− fcc arrangements, the opposite pattern is observed, with atop bond lengths increasing and bridge−fcc bond lengths decreasing. Thus, a positive atop dipole increases and a negative fcc dipole decreases the bond length of its neighbor in all arrangements. These trends are consistent with a positive dipole creating a negative electric field (and vice versa) as described below and suggest that such dipole interactions can be important at these coverages. Consistent with the trends in bond length and Badger’s rule,47 the 1/2 ML mixed-arrangement vibrational frequencies increase relative to those of the 1/4 ML at atop site and decrease at bridge and fcc sites. For example, in the atop− hollow mixed NO arrangement the atop frequency increases by +23 cm−1 and the fcc frequency decreases by −66 cm−1. The coverage-dependent frequency shifts are commonly ascribed to dynamic dipole coupling with the dipole moments of neighboring molecules vibrating in or out of phase.25 This effect can be separated out by carrying out the mixedarrangement vibrational frequency calculations one molecule at a time while keeping the neighboring dipole fixed. In agreement with the results reported by Aizawa et al.,26 we find that the dynamic dipole coupling accounts for only a small fraction (+9 and −11 cm−1 for atop and fcc NO, respectively) of the total shift (+23 and −66 cm−1, respectively). The large total shifts and corresponding trends in bond length indicate a more prominent static dipole−dipole interaction, which we further analyze below. The infrared absorption intensity of highly coordinated sites decreases significantly in the mixed arrangements, as shown in Table 2. In the atop−bridge CO configuration, the ratio of the atop vibrational intensity to the bridge mode vibrational intensity is about 2.4, in good agreement with IRAS experiments by Schwiezer et al.,30 who report a peak area ratio of 2.7 for atop and bridge CO spectra in the c(4 × 2)ordered arrangement at 95 K. In the case of NO, the coverage has an even more significant effect on the vibrational intensity. At 1/2 ML,the intensity of the fcc hollow NO vibration at 1466 cm−1 is an order of magnitude smaller than the atop NO intensity at 1745 cm−1, even though equal numbers of the two types of adsorbates are present. This result explains why only atop NO bands are seen in the infrared spectra of NO adsorbed on Pt at 1/2 ML or higher coverage.26 3.2. Dipole Interactions in Mixed CO and NO Arrangements. The 1/2 ML mixed-site arrangements of CO and NO consist of neighboring dipoles of opposite sign that interact attractively, opposite to the uniformly repulsive dipole−dipole interactions expected between same-sign adsorbate dipoles at high coverage. The influence of these attractive interactions on the CO and NO dipoles can be observed in the computed coadsorbate-induced charge density differences, defined as

Figure 1. Charge density difference isosurfaces, showing induced dipoles in atop−fcc mixed 1/2 ML arrangements of (a) CO and (b) NO on Pt(111).

positive charge accumulation (i.e., loss of electron density) near the O atom and negative charge accumulation near the C or N atom of the atop adsorbates. Thus, the positive atop CO and NO dipoles (Cδ−−Oδ+ and Nδ−−Oδ+) are actually enhanced (i.e., larger in magnitude) at 1/2 ML over 1/4 ML because of the influence of neighboring fcc adsorbates. The ρdiff isosurfaces similarly show an enhancement in the negative dipole moments of fcc CO and NO because of the presence of atop adsorbates of opposite polarization. To calculate the magnitude of the enhanced dipoles in these mixed-adsorbate arrangements, we partition the charge density distribution obtained from periodic supercell calculations into charge-neutral regions using partitioning planes. The local dipole moments are calculated as the first moment of the partitioned charge density distribution in each prismatic region, as described by eq 3. Figure 2 shows the partitioning of the mixed CO and NO charge densities into atop and fcc regions. In the case of linear adsorbates, the partitioning planes are drawn to bisect the line connecting an atop adsorbate and its nearest neighbors, and the shape of prismatic regions thus depends on the symmetry of the system. Atop−fcc and atop− bridge CO arrangements with three and four nearest neighbors, respectively, lead to triangular and diamond-shaped regions. The atop−fcc NO arrangement, however, is partitioned into rectangular regions to accommodate the bent atop NO as shown in Figure 2b. After initial partitioning by bisecting planes, slight adjustments are made by symmetrically expanding one of the two partitioned regions and contracting the other to obtain unique charge-neutral partitions. Details of the partitioning methods are described in the Supporting Information. The dipole moment for different arrangements, including the mixed ones calculated by partitioning, trend linearly with the net charge transfer from the Pt surface to adsorbed CO or NO as calculated by Bader’s method48 (Supporting Information), confirming that the partitioning scheme yields consistent dipole moments. Figure 3 summarizes the evolution of CO and NO dipole moments with the surface arrangement. The red upward arrows in the leftmost column and the blue downward arrows in the rightmost column represent positive atop dipoles and negative bridge or fcc dipoles at 1/4 ML coverage, respectively. The pair of arrows in the middle column shows how these dipoles

ρdiff ( r ) = ρPt − mixedads ( r ) − ρPt − atopads ( r ) − ρPt − fccads ( r ) →

→

+ ρPt ( r ) →

→

→

(4)

where ρ is the charge density of each system calculated by single-point calculations on atom positions fixed to the optimized geometry of the mixed arrangement. ρdiff is thus the change in the charge density of the 1/4 ML atop and fcc adsorbates in response to the population of neighboring mixed adsorption sites. The isosurfaces of ρdiff in Figure 1 show 8411

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

Figure 2. Schematic showing the partitioning of charge density between atop and fcc adsorbates in half monolayer arrangements of (left) CO and (right) NO. Details in Supporting Information.

these attractive interactions could be a determining factor in the experimentally observed stability of the mixed arrangements at 1 /2 ML. To estimate these interactions in a periodic system quantitatively, the dipole−dipole interaction can be treated as the interaction of a dipole with the electric field created by a dipole at a neighboring adsorption site; we consider two such models in the following section. 3.3. Estimation of Dipolar Interaction Energies. Figure 4 shows a schematic of an ideal classical dipole consisting of

Figure 3. Schematic evolution of CO (top) and NO (bottom) dipole moments on Pt(111) at various 1/4 ML (Table 1) and 1/2 ML (Table 2) arrangements. Red upward arrows represent positive dipole moments, blue downward arrows represent negative dipole moments, and lengths correspond to dipole magnitudes. The mixed-arrangement dipole moments are obtained by charge partitioning as described in Figure 2. Figure 4. Schematic showing an ideal dipole and an electric field Find induced by it at a distance r in the plane of the dipole.

change as they come together to form the different 1/2 ML arrangements listed in Table 2. When the adsorbates are restricted to one type of site (atop, fcc, or bridge for CO; atop or fcc for NO), the dipole moments at 1/2 ML are uniformly diminished relative to those at 1/4 ML, diminishing the unfavorable repulsion between parallel dipoles. In the mixed configurations, however, the adsorbate dipole moments are of opposite sign and are actually enhanced, introducing an attractive electrostatic interaction between neighboring dipoles. The positive atop adsorbate dipole stabilizes the adjacent negative dipole at the fcc or bridge position by creating a negative electric field and vice versa, which is consistent with the trends observed when a uniform external field is applied to induce changes in the adsorbate dipole moments, adsorption energy, and vibrational frequency.12,13 The enhanced dipole moments of mixed arrangements are consistent with the bond lengths and vibrational frequency changes shown in Tables 1 and 2 because CO and NO molecules with a larger (more positive) dipole have shorter bonds and higher frequencies whereas an increasingly negative dipole moment leads to longer bonds and lower vibrational frequencies. The opposite dipoles in the mixed arrangements interact attractively and contribute to stabilizing these structures. In fact,

charges +q and −q separated by a distance d (resulting in a net positive dipole moment in the z direction) and the electric field created by it at a distance r in the plane bisecting the dipole. The electric field components within the bisecting plane cancel each other, but they add up in the normal direction. Thus, a positive dipole creates a net negative electric field Find at a neighboring adsorption site and vice versa. The field strength induced by a single dipole at a distance r ≫ d is given by49 μ1 F ind = − 4πε0r 3 (5) where μ1 is the dipole moment, ε0 is the permittivity of free space, and r is the distance from the center in the plane of the dipole. The first-nearest-neighbor (1NN) electrostatic interaction energy ΔEINN int due to the dipole-induced electric field can be estimated (assuming r ≫ d) as μμ 1NN ΔE int = −μ2 F2ind = n1NN 1 2 3 4πε0r (6) 8412

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

Table 3. CO and NO Adsorption Arrangements at 1/2 ML Corresponding to the Number of First Neighbors (n1NN) and the total Dipole Moments of the Two Neighbors (μ1and μ2) and Their Corresponding First Neighbor (ΔEINN int ) and Total (ΔEint ) Dipole Interaction Energies per Supercell Calculated Using Equations 6 and 10, Respectively adsorption configuration

n1NN (eÅ)

μ1 (eÅ)

atop−atop bridge−bridge fcc−fcc atop−bridge atop−fcc

2 2 2 4 3

0.014 −0.083 −0.087 0.179 0.256

atop−atop fcc−fcc atop−fcc

2 2 3

0.026 −0.030 0.180

μ2 (eÅ)

ΔEINN int (eV)

ΔEtotal int (eV)

0.014 −0.083 −0.087 −0.248 −0.342

2.5 × 10−4 0.009 0.010 −0.049 −0.110

5.6 × 10−4 0.020 0.022 −0.128 −0.252

0.026 −0.030 −0.175

8.7 × 10−4 0.001 −0.039

0.002 0.003 −0.091

CO

NO

where n1NN is the number of first nearest neighbors. The induced electric field dies off as 1/r3 and is relatively small at the adsorbate−adsorbate separations characteristic of 1/4 ML or lower CO or NO coverage on Pt. In the 1/4 ML arrangement, the adsorbates have six first nearest neighbors but at a relatively large distance (distributed over four supercell units). ΔEint INN per supercell unit for the 1/4 ML dipole moments listed in Table 1 ranges from +8.2 × 10−5 eV for atop CO (smallest dipole moment) to +0.002 eV for fcc CO (largest dipole moment). These interactions are thus relatively negligible. The 1/2 ML arrangements have fewer nearest neighbors but at shorter distances. n1NN = 2 in single-site 1/2 ML arrangements, 3 in p(2 × 2) atop−fcc mixed arrangements, and 4 in the c(4 × 2) atop−bridge arrangement. ΔE1NN int per supercell unit calculated for these arrangements is shown in Table 3. The 1NN dipolar interactions are much greater in these configurations. These estimates do not include contributions from long-range dipole interactions and thus are lower limits of interaction energies. In periodic systems, the electric field induced by all dipoles surrounding an adsorption site adds up and can lead to field strengths large enough to affect adsorption on that site. Although the contribution from each neighbor decreases with distance, the number of such neighbors increases and can contribute significantly to the total interaction. Dipoles of surface adsorbates will be either on the same plane, as in the ideal case shown in Figure 4, or at slightly different heights if different types of sites such as atop and hollow are occupied simultaneously. The collective electric field induced at an atop site by a periodic array of fcc adsorbate dipoles can be calculated more accurately by analyzing the electrostatic potential distribution obtained from the DFT calculations. We placed a range of adsorbates (CO, NO, Cl, OH and Na; fully relaxed adsorbate and top layer Pt atoms) on an fcc hollow site in a p(2 × 2) supercell and calculated the change in electrostatic potential ϕdiff induced by these adsorbates as the difference between the potential in the presence (ϕads) and the absence (ϕ0) of the fcc adsorbate at the (x, y) position of the empty atop site: ϕdiff,atop(z) = ϕads(x = xatop , z) − ϕ0(x = xatop ,

Figure 5. (a) Change in atop site electrostatic potential due to adsorption at the fcc hollow site on Pt(111) in a p(2 × 2) arrangement. The induced electric field is given by the negative gradient of this change in potential. (b) Electric field induced by the fcc adsorbate (calculated as described in part a) at the atop site vs the dipole moment of the fcc adsorbate on the Pt(111) surface.

The magnitude of the induced field is the negative gradient of the induced potential change:

y = yatop ,

y = yatop ,

z)

F ind = −∇ϕdiff (7)

(8)

As expected, the negative dipole moment of fcc-bound CO induces a positive electric field at a neighboring atop site, and the positive OH dipoles induce a negative field. The electric field induced by other fcc adsorbates was also calculated in this manner. Figure 5b shows the atop-induced electric field in V Å−1 versus the fcc dipole moment in e Å for all adsorbates; the

Figure 5a shows the changes in electrostatic potential at the atop site induced by fcc-adsorbed CO and OH. The four vertical dotted lines show the Pt positions in the four-layer slab. Just beyond the top Pt layer, the potential changes linearly with the z coordinate because of the adsorbate-induced electric field. 8413

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

Table 4. Stabilities of 1/2 ML Mixed Arrangements Relative to Atop-, Bridge-, or fcc-Only Arrangements (ΔE0,diff) and the Dipole Interaction Contributions to These Stabilities (ΔEtotal int,diff), as Calculated Using Equation 11

two are observed to be approximately linearly related according to ind Fatop = −2.88μfcc

(9)

Deviations from eq 9 are small, and the correlation coefficient for the fitting in Figure 5b is close to 1. The induced field is thus nearly independent of factors such as the size or height of the adsorbate, and the substrate relaxation and depends primarily on the dipole moment. The collective interaction energy ΔEtotal int per supercell unit of these periodic adsorbate dipoles in this arrangement is then given by total ind ΔE int = −μatop Fatop = 2.88μatop μfcc

ΔE0,diff (eV)

ΔEtotal int,diff (eV)

atop−bridge CO atop−fcc CO atop−fcc NO

−0.43 −0.40 0.42

−0.14 −0.27 −0.09

stability of the mixed configurations. Dipolar interactions are thus not the only factor favoring the mixed arrangements. Other potential contributions include substrate relaxation26 and through-space steric repulsion.10 To estimate the former, we calculated the differences in NO adsorption energies on an unrelaxed Pt slab and found its contribution to ΔE0,diff to be only −0.02 eV; similarly small effects are expected for CO. We estimate steric interactions by calculating the gas-phase CO and NO energies of different arrangements at atom positions identical to those of the relaxed, adsorbed molecules. These gas-phase molecules do not have appreciable dipoles, so these calculations primarily capture through-space steric repulsion. The estimated stability contributions for atop−bridge and atop−fcc CO arrangements are −0.27 and −0.18 eV, respectively. The sum of this steric component and the dipole interaction ΔEtotal int,diff in Table 4 is nearly equal to the total ΔE0,diff, so that these two evidently determine the relative stability of different CO arrangements at 1/2 ML coverage. The calculated NO gas-phase interaction energy evaluated this way is +0.17 eV, implying additional stability instead of repulsion between more closely packed molecules. This result likely reflects some electron pairing between odd-electron NO in the gas phase and is thus not actually representative of the steric interaction. 3.4. Dipolar Interactions with Molecules Other Than CO and NO. Other adsorbates can exhibit significantly larger surface dipoles than CO and NO, increasing their sensitivity to field−dipole and dipole−dipole interactions. We previously showed that the electric field effect on adsorption energies is dominated by dipolar and polarization contributions that appear as first- and second-order terms in the electric field:12 1 ΔE(F ) = ΔE0 − μ0 F − αF 2 (12) 2 Here, ΔE0 is the zero-field adsorption energy, μ0 is the zerofield dipole moment, and α is the molecule polarizability. A positive electric field stabilizes a positive dipole and destabilizes a negative dipole. The second-order term is always stabilizing, but it becomes important only at large electric fields and for molecules with small μ0. Table 5 summarizes the gas-phase dipole moment μ0g, zerofield adsorption energy ΔE0, adsorbed dipole moment μ0, and polarizability α of eight adsorbates in addition to CO and NO at different adsorption sites on Pt(111). The μ0g and ΔE0 values were obtained using eqs 1 and 3, respectively. The μ0 and α values, however, were obtained by applying external electric fields between −1 and +1 V/Å in DFT calculations and fitting the field-dependent adsorption energies to eq 12 as described elsewhere.12 Atomic and symmetric molecular adsorbates have no gas-phase dipole, but adsorption always induces some surface dipole moment. O, H, and Cl have small μ0 values because they adsorb in close proximity to the surface as a result of their small sizes. Na, however, has a large positive μ0 and a

(10)

ΔEtotal int

We also calculated alternatively by estimating the field induced at the fcc site as a function of an atop dipole moment leading to similar interaction energies. Estimated this way, ΔEtotal int neglects smaller repulsive contributions from parallel dipoles at larger distances (for example, atop−atop repulsion) that are amplified over their 1/4 ML values by the increase in the magnitude of the dipoles. Furthermore, atop NO adsorbs bent and thus has a horizontal component to its dipole that is not included in the model. These factors are expected to be small relative to those captured by eq 10, and neglecting them does not alter the general conclusions. Similar dipolar interaction correlations can be computed for adsorbate configurations other than fcc−atop. We considered 1 /2 ML arrangements with a single type of site occupied and the c(4 × 2) atop−bridge CO configuration. In all cases, the arrangements with a larger number of nearest neighbors also have a longer nearest-neighbor distance, thus decreasing the field induced by individual dipoles. These two factors tend to cancel each other out for different arrangements at the same coverage and to yield interaction energies that approximately follow eq 10. Further details are given in the Supporting Information. As a simplification, we neglect these small configurational differences and use eq 10 to estimate the dipole interactions in all 1/2 ML arrangements. Table 3 summarizes the local dipoles and 1NN and total dipolar interaction energies estimated from eqs 6 and 10, respectively, for various 1/2 ML CO and NO arrangements. The first-neighbor dipole interaction energies in all cases are directionally the same but 2 to 3 times smaller than the total interactions incorporated in ΔEtotal int . Repulsions between dipoles of the same sign at equivalent sites lead to positive interaction energies; attractions between dipoles of opposite sign in the mixed arrangements introduce negative interaction energies. Furthermore, the dipoles are diminished in same-sign case to reduce the repulsive interactions, but the dipoles are amplified in the opposite-sign case to enhance the attractive interactions. These interactions play a significant role in promoting particular CO and NO ordered structures at 1/2 ML coverage. Table 4 reports the stability of the mixed arrangements per supercell unit relative to the corresponding atop and bridge-/ fcc-only arrangements calculated as ΔE0,diff or the difference between adsorption energies listed in Table 2. For example, atop − fcc ΔE0,diff = 2ΔE0atop − fcc − ΔE0atop − atop − ΔE0fcc − fcc

arrangement

(11)

The corresponding contribution of dipolar interactions to these total relative stabilities, calculated as ΔEtotal int,diff from the ΔEint values listed in Table 3, is also reported. The electrostatic dipole− dipole interactions account for about 20−60% of the relative 8414

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

Table 5. DFT-Calculated Gas-Phase Dipole Moment (μ0g), Zero Field Adsorption Energy (ΔE0), Zero-Field Adsorbed Dipole Moment (μ0), and Polarizabiliy (α) of Molecules Adsorbed on Pt(111) Subjected to an External Electric Field adsorbate CO

μ0g (eÅ)

adsorption site

ΔE0 (eV)

μ0 (eÅ)

α (eÅ2/V)

0.028

atop bridge fcc atop fcc atop fcc fcc fcc fcc fcc bridge disigma ethylidene atop

−1.63 −1.74 −1.77 −1.52 −1.93 −2.45 −2.13 −1.26 −0.59 −2.25 −1.27 −0.69 −1.06 −1.31 −0.69

0.026 ↑ −0.110 ↓ −0.137 ↓ 0.065 ↑ −0.056 ↓ −0.026 ↑ 0.278 ↑ −0.014 ↓ 0.010 ↑ 0.619 ↑ 0.057 ↑ −0.083 ↓ 0.220 ↑ 0.272 ↑ 0.467 ↑

0.15 0.12 0.11 0.18 0.12 0.12 0.10 0.00 0.00 0.27 0.12 0.01 0.17 0.18 0.19

NO

0.035

OH

0.312a

O H Na Cl O2 C2H4

0b 0b 0c 0b 0 0 0 0.309

NH3

Table 6. Electric Field Induced by Different fcc Adsorbates on Pt(111) in a p(2 × 2) Supercell at an Empty Atop Site (Find atop) Calculated Using Equation 10 and the Vibrational Frequency (ν), Bond Length (l), and Adsorption Energy (ΔECO ads ) of CO Adsorbed on This Atop Site CO adsorbed on atop site fcc adsorbate

−1 Find atop (V Å )

l (Å)

ν (cm−1)

ΔECO ads (eV)

Na OH H empty O NO CO

−1.643 −0.608 −0.029 0 0.040 0.218 0.259

1.187 1.163 1.157 1.157 1.155 1.154 1.152

1893 2022 2072 2073 2073 2073 2089

−1.51 −1.46 −1.57 −1.63 −1.49 −1.44 −1.48

a

The gas-phase molecule corresponds to isolated OH. bThe gas-phase molecule corresponds to diatomic molecules. cThe gas-phase molecule corresponds to an isolated Na atom.

large α, consistent with the large atomic radius of the alkalis and their propensity to donate their loosely bound outer electron. The resulting electrostatic effect on reactants is well known in the alkali promotion of catalytic ethylene oxidation23 and ammonia synthesis.22 The ethylene gas-phase dipole moment is zero by symmetry, but it develops a large positive dipole in both di-σ-bonded and ethylidene configurations. Similarly, ammonia adsorbed on Pt(111) has a large positive dipole indicating high sensitivity to the external electric field according to eq 12. For example, a −1 V Å−1 electric field decreases the NH3 adsorption energy from a zero-field value of 0.69 to 0.31 eV, whereas a 1 V Å−1 field increases the adsorption energy to 1.26 eV at 1/4 ML coverage. We considered OH to be both a bent atop adsorbate and an upright fcc adsorbate. Whereas CO and NO are good π acids because of their low-lying, vacant π* states, OH has filled π states and is a π base. The OH dipole moment hence responds in an opposite way to adsorption at fcc and atop sites compared to that of CO and NO. It has a relatively large positive dipole of 0.31 eV in the gas phase that almost completely disappears upon atop adsorption but is largely retained upon adsorption on an fcc hollow site. These interactions would promote mixedsite adsorption at high OH coverages. To explore dipolar effects on coadsorption, we performed calculations for atop-adsorbed CO in a p(2 × 2) supercell in the presence of a subset of fcc adsorbates from Table 5. Table 6 lists these fcc adsorbates, the electric field induced by them at an empty atop site according to eq 10, and the atop CO vibrational frequency, bond length, and adsorption energy in the presence of this fcc coadsorbate in the field of the fcc adsorbates: CO ΔEads = E Pt − ads − CO − E Pt − ads − ECO

Figure 6. Change in the vibrational frequency and bond length of atop CO as a function of the dipole moment of the fcc coadsorbate on Pt(111). The dashed tangent line shows the expected trends determined from the electric field induced by the fcc dipole (eq 9) and the Stark tuning rate of atop CO.11

the fcc adsorbate. The dipole moment decreases along the x axis (positive to negative) corresponding to a gradually increasing electric field induced on the atop-adsorbed CO. The CO vibrational frequency increases and the bond length decreases with increasing electric field (or decreasing dipole moment), consistent with the effect of an externally applied uniform electric field12 (Supporting Information). Unlike the external field case, however, the variations are not linear. The red shift induced by the field of a positive dipole is larger than that expected from a uniform external field of the same strength, whereas the blue shift due to the negative dipole is slightly smaller. These deviations suggest that additional through-metal and through-space effects as described above modify the dipolar interactions. The changes in adsorption energy do not show any specific trend with the dipole moment of the fcc adsorbate, which again suggests that there are other electronic and geometric contributions to surface adsorption energies, which are different from the electrostatic effects described here. The dipole contribution to the coadsorbate-induced change in CO adsorption energies can be calculated from eq 10. Assuming the atop CO dipole to be unperturbed from its 1/4 ML value with no coadsorbate, we would expect the contributions to be repulsive for positive fcc dipoles of Na, OH, and H and attractive for negative fcc O, NO, and CO. However, as seen from the mixed arrangements of CO in Figure 3, its dipole itself changes significantly in the presence of a coadsorbate. For example, we expect fcc Na to polarize the atop CO toward

(13)

Here, EPt−ads−CO is the energy of the combined system, EPt−ads is the energy of a Pt surface with just the fcc adsorbate, and ECO is the energy of a free CO molecule. Figure 6 shows the change in the CO vibrational frequency and bond length as a function of the surface dipole moment of 8415

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

(3) Getman, R. B.; Xu, Y.; Schneider, W. F. Thermodynamics of Environment-Dependent Oxygen Chemisorption on Pt (111). J. Phys. Chem. C 2008, 112, 9559−9572. (4) Getman, R. B.; Schneider, W. F.; Smeltz, A. D.; Delgass, W. N.; Ribeiro, F. H. Oxygen-Coverage Effects on Molecular Dissociations at a Pt Metal Surface. Phys. Rev. Lett. 2009, 102, 076101. (5) Yeo, Y. Y.; Vattuone, L.; King, D. A. Energetics and Kinetics of CO and NO Adsorption on Pt {100}: Restructuring and Lateral Interactions. J. Chem. Phys. 1996, 104, 3810−3821. (6) Wu, C.; Schmidt, D. J.; Wolverton, C.; Schneider, W. F. Accurate Coverage-Dependence Incorporated into First-Principles Kinetic Models: Catalytic NO Oxidation on Pt(111). J. Catal. 2012, 286, 88−94. (7) Marshall, S. T.; Medlin, J. W. Surface-Level Mechanistic Studies of Adsorbate-Adsorbate Interactions in Heterogeneous Catalysis by Metals. Surf. Sci. Rep. 2011, 66, 173−184. (8) Mortensen, J. J.; Hammer, B.; Nørskov, J. K. A Theoretical Study of Adsorbate−Adsorbate Interactions on Ru (0001). Surf. Sci. 1998, 414, 315−329. (9) Iṅ oğlu, N.; Kitchin, J. Simple Model Explaining and Predicting Coverage-Dependent Atomic Adsorption Energies on Transition Metal Surfaces. Phys. Rev. B 2010, 82, 045414. (10) Mason, S. E.; Grinberg, I.; Rappe, A. M. Adsorbate-Adsorbate Interactions and Chemisorption at Different Coverages Studied by Accurate Ab Initio Calculations: CO on Transition Metal Surfaces. J. Phys. Chem. B 2006, 110, 3816−2322. (11) Schmidt, D.; Chen, W.; Wolverton, C.; Schneider, W. F. Performance of Cluster Expansions of Coverage-Dependent Adsorption of Atomic Oxygen on Pt(111). J. Chem. Theory Comput. 2011, 8, 264−273. (12) Deshlahra, P.; Wolf, E. E.; Schneider, W. F. A Periodic Density Functional Theory Analysis of CO Chemisorption on Pt (111) in the Presence of Uniform Electric Fields. J. Phys. Chem. A 2009, 113, 4125−4133. (13) Hyman, M. P.; Medlin, J. W. Theoretical Study of the Adsorption and Dissociation of Oxygen on Pt(111) in the Presence of Homogeneous Electric Fields. J. Phys. Chem. B 2005, 109, 6304−6310. (14) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. FieldDependent Electrode-Chemisorbate Bonding: Sensitivity of Vibrational Stark Effect and Binding Energetics to Nature of Surface Coordination. J. Am. Chem. Soc. 2002, 124, 2796−2805. (15) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. Metal Electrode−Chemisorbate Bonding: General Influence of Surface Bond Polarization on Field-Dependent Binding Energetics and Vibrational Frequencies. J. Chem. Phys. 2001, 115, 8193−8203. (16) Lambert, D. K. Vibrational Stark Effect of Adsorbates at Electrochemical Interfaces. Electrochim. Acta 1996, 41, 623−630. (17) Jochum, W.; Eder, D.; Kaltenhauser, G.; Kramer, R. Impedance Measurements in Catalysis: Charge Transfer in Titania Supported Noble Metal Catalysts. Top. Catal. 2007, 46, 49−55. (18) Hayek, K.; Kramer, R.; Paal, Z. Metal-Support Boundary Sites in Catalysis. Appl. Catal., A 1997, 162, 1−15. (19) Deshlahra, P.; Schneider, W. F.; Bernstein, G. H.; Wolf, E. E. Direct Control of Electron Transfer to Surface-CO Bond in a Pt/TiO2 Catalytic Diode. J. Am. Chem. Soc. 2011, 133, 16459−16467. (20) McEwen, J. S.; Gaspard, P.; De Decker, Y.; Barroo, C.; de Bocarme, T. V. Catalytic Reduction of NO(2) with Hydrogen on Pt Field Emitter Tips: Kinetic Instabilities on the Nanoscale. Langmuir 2010, 26, 16381−16391. (21) Luo, J. S.; Tobin, R. G.; Lambert, D. K. Electric Field Screening in an Adsorbed Layer: CO on Pt (111). Chem. Phys. Lett. 1993, 204, 445−450. (22) Mortensen, J. J.; Hammer, B.; Nørskov, J. K. Alkali Promotion of N2 Dissociation over Ru (0001). Phys. Rev. Lett. 1998, 80, 4333− 4336. (23) Linic, S.; Barteau, M. A. On the Mechanism of Cs Promotion in Ethylene Epoxidation on Ag. J. Am. Chem. Soc. 2004, 126, 8086−8087. (24) Severson, M. W.; Stuhlmann, C.; Villegas, I.; Weaver, M. J. Dipole-Dipole Coupling Effects upon Infrared Spectroscopy of

negative dipole moments and fcc CO to polarize it toward more positive dipole moments and modify the interaction energy accordingly. Detailed analyses of dipole moments by charge partitioning and other effects such as steric repulsion and binding competition need to be performed to determine the contribution of each of these effects, as shown here for the case of CO and NO.

4. CONCLUSIONS We report DFT supercell calculations of the coveragedependent evolution of surface dipoles of CO and NO on Pt(111). With increasing proximity, the same-sign dipoles repel and diminish in magnitude whereas opposite-signed dipoles are enhanced by the favorable attractive interactions. These changes are also reflected in the bond lengths and vibrational frequencies at different 1/2 ML adsorbate arrangements. The magnitude of the dipole moments in the mixed arrangements calculated by charge partitioning and the resulting interaction energy contribute significantly but not exclusively to the stability of these arrangements, with the other most significant effect being steric repulsion. Such dipole interactions are shown to exist for a range of molecules and can even be significantly larger for several molecules that have larger low-coverage dipoles than CO and NO. These results show that electrostatic interactions between adsorbates can be appreciable at surfaces, can act to bias toward certain site preferences, but can also act in concert with through-space (steric), surface relaxation, and through-metal interactions to determine the overall adsorption behavior. DFT is a useful tool for disentangling these interactions, as demonstrated here.

■

ASSOCIATED CONTENT

* Supporting Information S

Correction of CO adsorption properties by extrapolation. Scheme for partitioning the charge density of mixed-adsorbate arrangements. Comparison of CO and NO dipole moments to Bader charge analysis. Calculation of dipole interaction in different 1/2 ML arrangements. Coverage-dependent vibrational Stark effect. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Financial support from the National Science Foundation, Chemical, Bioengineering, Environmental, and Transport Systems (CBET) under grant 0854324 and computing resources provided by the University of Notre Dame Center for Research Computing are gratefully acknowledged.

■

REFERENCES

(1) Thomas, J. M.; Thomas, W. J. In Principles and Practice of Heterogeneous Catalysis; VCH Publishers: New York, 1996. (2) van Santen, R. A.; Neurock, M. In Molecular Heterogeneous Catalysis: A Conceptual and Computational Approach; Wiley-VCH: Weinheim, Germany, 2006. 8416

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417

Langmuir

Article

Compressed Electrochemical Adlayers: Application to the Pt (111)/ CO System. J. Chem. Phys. 1995, 103, 9832−9843. (25) Scheffler, M. The Influence of Lateral Interactions on the Vibrational Spectrum of Adsorbed CO. Surf. Sci. 1979, 81, 562−570. (26) Aizawa, H.; Morikawa, Y.; Tsuneyuki, S.; Fukutani, K.; Ohno, T. A Density-Functional Study of the Atomic Structures and Vibrational Spectra of NO/Pt (111). Surf. Sci. 2002, 514, 394−403. (27) Lin, X.; Haas, K. C.; Schneider, W. F.; Trout, B. L. Chemistry of Sulfur Oxides on Transition Metals I: Configurations, Energetics, Orbital Analyses, and Surface Coverage Effects of SO2 on Pt (111). J. Phys. Chem. B 2002, 106, 12575−12583. (28) McEwen, J. S.; Payne, S. H.; Kreuzer, H. J.; Kinne, M.; Denecke, R.; Steinrück, H. P. Adsorption and Desorption of CO on Pt (111): A Comprehensive Analysis. Surf. Sci. 2003, 545, 47−69. (29) Feibelman, P. J.; Hammer, B.; Nørskov, J. K.; Wagner, F.; Scheffler, M.; Stumpf, R.; Watwe, R.; Dumesic, J. A. The CO/Pt (111) Puzzle. J. Phys. Chem. B 2001, 105, 4018−4025. (30) Schweizer, E.; Persson, B. N. J.; Tüshaus, M.; Hoge, D.; Bradshaw, A. M. The Potential Energy Surface, Vibrational Phase Relaxation and the Order-Disorder Transition in the Adsorption System Pt {111}-CO. Surf. Sci. 1989, 213, 49−89. (31) Mason, S. E.; Grinberg, I.; Rappe, A. M. First-Principles Extrapolation Method for Accurate CO Adsorption Energies on Metal Surfaces. Phys. Rev. B 2004, 69, 161401. (32) Kresse, G.; Furthmuller, J. Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15−50. (33) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, Molecules, Solids and Surfaces - Applications of the Generalized Gradient Approximation for Exchange and Correlation. Phys. Rev. B 1992, 46, 6671− 6687. (34) Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation-Energy. Phys. Rev. B 1992, 45, 13244−13249. (35) Blochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953−17979. (36) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758− 1775. (37) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (38) Martorell, B.; Clotet, A.; Fraxedas, J. A First Principle Study of the Structural, Vibrational and Electronic Properties of Tetrathiafulvalene Adsorbed on Ag(110) and Au(110) Surfaces. J. Comput. Chem. 2010, 31, 1842−1852. (39) Wilson, E. B. In Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra; Dover Publications: New York, 1980. (40) Getman, R. B.; Schneider, W. F. DFT-Based Characterization of the Multiple Adsorption Modes of Nitrogen Oxides on Pt (111). J. Phys. Chem. C 2007, 111, 389−397. (41) Matsumoto, M.; Fukutani, K.; Okano, T.; Miyake, K.; Shigekawa, H. Study of the Adsorption Structure of NO on Pt(111) by Scanning Tunneling Microscopy and High-Resolution Electron Energy-Loss Spectroscopy. Surf. Sci. 2000, 454−456, 101−105. (42) Curulla Ferré, D.; Niemantsverdriet, J. W. Vibrational Stark Tuning Rates from Periodic DFT Calculations: CO/Pt (111). Electrochim. Acta 2008, 53, 2897−2906. (43) Lozovoi, A. Y.; Alavi, A. Vibrational Frequencies of CO on Pt(111) in Electric Field: A Periodic DFT Study. J. Electroanal. Chem. 2007, 607, 140−146. (44) Gil, A.; Clotet, A.; Ricart, J. M.; Kresse, G.; García-Hernández, M.; Rösch, N.; Sautet, P. Site Preference of CO chemisorbed on Pt (111) from Density Functional Calculations. Surf. Sci. 2003, 530, 71− 87. (45) Xu, Y.; Getman, R. B.; Shelton, W. A.; Schneider, W. F. A FirstPrinciples Investigation of the Effect of Pt Cluster Size on CO and NO Oxidation Intermediates and Energetics. Phys. Chem. Chem. Phys. 2008, 10, 6009−6018.

(46) Gajdoš, M.; Eichler, A.; Hafner, J. CO Adsorption on ClosePacked Transition and Noble Metal Surfaces: Trends from Ab Initio Calculations. J. Phys.: Condens. Matter 2004, 16, 1141−1164. (47) Cioslowski, J.; Liu, G.; Mosquera Castro, R. A. Badger’s Rule Revisited. Chem. Phys. Lett. 2000, 331, 497−501. (48) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360. (49) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1991.

8417

dx.doi.org/10.1021/la300975s | Langmuir 2012, 28, 8408−8417