A Density Functional Theory Study on Carbon Monoxide Adsorption

Article pubs.acs.org/JPCC

A Density Functional Theory Study on Carbon Monoxide Adsorption on Platinum−Osmium and Platinum−Ruthenium−Osmium Alloys Nicholas Dimakis,*,† Thomas Mion,† and Eugene S. Smotkin‡ †

Department of Physics and Geology, University of Texas-Pan American, Edinburg, Texas 78539, United States Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States

‡

S Supporting Information *

ABSTRACT: Periodic density functional theory calculations on carbon monoxide (CO) adsorbed atop on platinum−osmium binary alloys (PtOs2 and PtOs4) and the platinum−ruthenium−osmium tertiary alloy (PtRu2Os2) are used to elucidate the changes in the C−O and C−Pt bonds upon alloying Pt with Ru/ Os atoms. As Pt is alloyed with Ru/Os atoms, the adsorbate internal bond (C−O bond) and the adsorbate−metal bond (C−Pt bond) strengthen following the substrate trends of PtOs4 > Pt > PtOs2 > PtRu2Os2 and Pt > PtOs4 > PtOs2 > PtRu2Os2, respectively. These trends are manifested by the corresponding C−O and C−Pt stretching frequencies and the CO adsorption energy variations. Here, we establish a theoretical framework based on the π-attraction σ-repulsion mechanism to explain the above results. This model correlates the charges, polarizations, and electron densities of the adsorbate CO orbitals, and the sp/d populations of the adsorbing Pt atom. For the systems studied here, the traditional theoretical model of 5σ-donation/2π*-back-donation with the metal substrate bands is not always sufficient to explain the relative C−O and C−Pt bonds strengths. alloys25 and by scanning tunneling microscopy of COads on the PtCo alloy.33 Liu et al.31 using potential- and coveragedependent PM-IRAS of COads atop on single-phase polycrystalline arc-melted Pt-based binary PtOs and tertiary PtRuOs alloy electrodes observed νCO reduction following the substrate trend of PtRuOs(65/25/10) > PtRuOs(8/1/1) > PtOs(8/2) > Pt at 0.1% CO coverage. Ishikawa et al.15 using density functional theory (DFT) on small Pt and PtOs clusters observed νCO, C− Pt stretching frequency (νCPt), and COads enthalpy of adsorption (Eads) reductions upon Pt alloying with Os. These reductions were not sensitive to the Os location in the PtOs alloy cluster. As the Pt mole fraction in the PtOs alloy was reduced, Ishikawa et al. observed a minor νCO increase (by about 3 cm−1) and νCPt reduction accompanied by Eads increase. The weakening of the COads internal bond has been correlated with changes in the charges and polarizations of the CO molecular orbitals (MOs)34−39 as well as the metal bands of the substrate.40 The first theoretical interpretation is by the so-called “Blyholder model” by Bagus and Pacchioni,35 which only considers the effect of the 5σ and 2π* CO frontier MOs and their interactions with the substrate metal d-bands as 5σ → dz2 and dxz,yz → 2π* (also known as 5σ-donation/2π*back-donation). However, this description differs from the one presented in the original Blyholder paper, where the entire adsorbate π-system was considered and the 5σ MO was

1. INTRODUCTION Platinum fcc alloys are the core of prevalent direct methanol fuel cell (DMFC) anode catalyst.1−14 During methanol oxidation, CO is strongly adsorbed (COads) on the catalyst surface,15,16 reducing the number of Pt sites available for the oxidative adsorption of methanol.17 Experiments indicate that at low CO coverage (ultrahigh vacuum), COads is in the atop site.18,19 Binary alloys of Pt with oxophilic metals such as Ru20,21 and Sn22,23 have been investigated in the past to serve as DMFC anode catalysts of improved electrocatalytic activity with respect to pure Pt. However, Pt−Os24−26 electrodes exhibit higher catalytic activity for DMFC operation with respect to Pt-based Ru and Sn alloys.27 Moreover, Reddington et al.13 using combinatorial chemistry proposed a borohydride prepared quaternary PtRuOsIr alloy catalyst with significantly higher activity than borohydride prepared PtRu alloy catalyst.28,29 Os could replace Ru in Pt-based alloys, if it was not for the toxicity of Os.30 The effect of CO adsorption on binary PtOs (as a function of the Pt mole fraction in the alloy) and tertiary PtRuOs alloys is still poorly understood. The electronic structure of the Pt and Pt-based alloys can be probed by examining the effect of the CO adsorption on the catalyst surface. Modulated infrared absorption spectroscopy (PM-IRAS) of COads on Pt surfaces shows C−O stretching frequency (ν CO) reduction upon CO adsorption, thus indicating weakening of the internal C−O bond of the adsorbate (COads internal bond).31,32 At low CO coverage, CO is exclusively adsorbed on the Pt site of the Pt-based alloy, as shown by PM-IRAS of COads on PtRu, PtOs, and PtRuOs © 2012 American Chemical Society

Received: July 26, 2012 Revised: September 10, 2012 Published: September 17, 2012 21447

dx.doi.org/10.1021/jp3074199 | J. Phys. Chem. C 2012, 116, 21447−21458

The Journal of Physical Chemistry C

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Figure 1. The unit cell of the CO/PtRu2Os2 periodic slab. Ru atoms are depicted in black and Os atoms in light gray.

The sole repulsive nature of the σ̃ -system has been questioned. In the original π−σ model, the σ-repulsion is based on (1) the absence of σ-donation to the metal and (2) the assumption of a completely filled d̃σ orbital.36,38 Föhlisch et al. observed a concomitant charge reduction in the CO region of the σ̃-system and of the dz2-band, which implies the absence of a σ → dz2 donation mechanism. However, in our past report,41we showed that if adsorption is considered in a stepwise fashion, σ-donation concomitant with a reduction of the dz2 population is possible. Moreover, Kresse et al.,44 using the ab initio DFT VASP program45 for COads on Pt(111), discuss the importance of the 5σ → dz2 donation mechanism for atop adsorption. The partial occupation of the d̃σ orbital was verified by Rangelov et al.46 using inverse photoemission spectra on CO adsorbed on Ni, Pd, and Pt surfaces. Additionally, Aizawa and Tsuneyuki47 using periodic plane wave DFT calculations on COads on pure Pt(111) verified the above experimental observations. This work is an extension of our prior theoretical work on CO/Pt and CO/PtRu alloys.41 Here, we elucidate the effect of COads on binary PtOs and tertiary PtRuOs alloys using periodic DFT. Changes on the νCO, νCPt, and Eads of COads on PtOs2, PtOs4, and PtRu2Os2 alloys are correlated with charge transfers between the CO molecule and the metal substrate and polarizations within the CO unit, where we consider all hybrid orbitals from energies as low as the 4σ̃ to as high as the 2π̃*. Additionally, the effect of the Pt sp- and d-band center energy shifts (denoted as Pt-esp and Pt-ed, respectively) on the COads and C−Pt bonds as Pt is alloyed with Ru and Os atoms is also discussed.

assumed constant between the free and the adsorbed CO.34 In the Blyholder model, the weakening of the COads internal bond is attributed to the 2π*-back-donation domination over the 5σdonation. More specifically, the depletion of the 5σ MO strengthens the COads internal bond, whereas populating the 2π* MO weakens the COads internal bond. Both donation and back-donation processes contribute to the formation of a stable C−metal bond. The Blyholder model failed to explain changes on C−Pt bonding as Pt was alloyed with Ru atoms.41 The second theory is by Nilsson et al.,36 Bennich et al.,37 and Föhlisch et al.,38,39,42 where CO adsorption on metal surfaces is attributed to π-bonding and σ-repulsion (π−σ model). Their theory is based on experimentally measuring the electronic structure of N2 and CO molecules adsorbed on Ni(100) and Cu(100) surfaces by using X-ray emission spectroscopy (XES). The XES data were complemented by quantum mechanical calculations. Föhlisch et al. found that both π-bonding and σrepulsion effects increase with the number of coordinated metal atoms.38 In the π−σ model, the C−metal π-bonding is ascribed to the metal−CO hybrid tilde-type orbitals: the 1π̃, d̃π, and 2π̃*. In the original π−σ model description, the d̃π orbital is a hybrid of the unperturbed 1π and 2π* CO MOs mixed with the metal dxz,yz band. First-order perturbation theory applied to the unperturbed orbitals of the π-system (the CO 1π, 2π*, and the substrate dxz,yz band) accounts for charge exchange between the CO molecule and the metal substrate. Second-order perturbation theory elucidates charge polarization within the CO unit. In the π−σ model, the COads internal bond is weakened due to higher 1π̃ polarization toward the carbon atom with respect to free CO.36,43 The π−σ model does not assume direct 2π* back-donation from the substrate metal bands to the unperturbed 2π* as the Blyholder model. The overall π̃ interaction is bonding to the metal surface. The σ-repulsion in the original π−σ model is ascribed to 4σ̃, 5σ̃, and d̃σ orbitals, whereas the last orbital is a hybrid of the unperturbed 5σ CO MO and the dz2 substrate band. Here, the σ-repulsion is primarily an effect ascribed to charge redistribution in the CO region of the σ̃ orbitals rather than an extent of the σ-donation to the metal substrate.39 The abovementioned effect diminishes the weakening of the COads internal bond due to the changes in the π̃-system. Moreover, the σ̃-system minimizes C−Pt bonding, which is caused by the π̃ attractive interaction.

2. MODELS AND COMPUTATIONAL METHODS Similar to our latest report for CO/Pt and CO/PtRu alloys,41 a three-layer periodic slab is used to model the Pt(100). The CO is placed atop as a c(4 × 4) overlayer (Figure 1). Low CO coverage (θCO = 1/8) is assumed, to avoid possible CO−CO interactions, which would affect C−O and C−Pt distances and corresponding stretching frequencies. Pt, Ru, and Os atoms are fixed in the Pt crystallographic positions and are not allowed to move during geometry optimization. Relaxation effects will be examined in a future work. Ru and Os atoms are only located at the top layer. Periodic DFT calculations on PtOs2, PtOs4, and PtRu2Os2 slabs with and without atop adsorbed CO molecule are performed using the CRYSTAL0648 program, which 21448



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employs Gaussian type function basis sets centered at the atoms. Normal mode spectrum is obtained by CRYSTAL06 at the Gamma point (k = 0).49 The “modified” version of the hybrid B3LYP functional was employed, which consists of the same exact and exchange functionals as the original B3LYP functional,50 while replacing the VWN correlation functional with the VWN551 functional. The innermost orbitals of the Pt, Ru, and Os atoms are described by effective core pseudopotentials.52 The effective valence basis sets for these atoms are as follows: Pt atoms are described by the [4s4p2d] basis set, whereas Ru and Os atoms are described by the [4s3p2d] basis set. The latter basis set is developed from the [6s5p3d2fg]53 atomic basis set, by dropping functions with exponents less than 0.1 and concurrently removing f and g functions from the corresponding original atomic basis set. The Ru basis set of this work was used previously.41 For carbon and oxygen atoms, the original 6-311++G** basis set54 described as [5s4p1d] is contracted to [4s3p1d] for each element. The metal−carbon interaction is more accurately described by adding an additional d orbital to the carbon basis set (i.e., [4s3p2d] basis set for the carbon atom). All basis sets are optimized for crystalline calculations.55 Brillouin zone integrations are performed on a 12 × 12 Monkhorst−Pack grid.56 The Fermi energy and the density matrix are evaluated on a denser grid of 24 × 24 points (Gilat grid).57,58 SCF convergence is achieved by employing Anderson quadratic mixing,59 coupled with additional mixing of the occupied with the virtual orbitals. Pt, Ru, and Os orbital populations are calculated using Mulliken population analysis.60 The Fermi level is directly calculated by CRYSTAL06. In this work, due to fine differences in the C−O and C−Pt distances among the systems studied, the corresponding νCO and νC−Pt calculations are performed in the C−O fragment optimized geometry of the three-layer slab without omitting the third layer, as done previously.41 Charges in the CO region of the adsorbate orbitals are obtained by integrating the corresponding partial density-of-states (DOS) spectrum within appropriate energy regions. Charge density difference maps are obtained using the XcrySDen graphical package.61

Figure 2. Pt DOS spectrum for clean (a) Pt, (b) PtOs binary alloys, and (c) the tertiary PtRu2Os2 alloy. Arrows denote major electron transfers among Pt, Os, and Ru sp/d-bands, as Pt is alloyed with Ru and Os atoms. Middle graph: Values in square brackets refer to the x PtOs4 alloy. The following electron transfer is not shown: sp→d within Os atoms, where x is 1.00 e, 2.00 e, and 1.03 e for PtOs2, PtOs4, and PtRu2Os2 alloys, respectively. Thick horizontal bars represent the Ptband center-of-masses. The horizontal dashed line is the Fermi level.

occupied part of the 2π̃* orbitals, and the Ptc-s, -p, and -d orbitals for Pt, PtOs2, PtOs4, and PtRu2Os2 slab models with and without an adsorbed CO. An sp → d electron transfer within the Pt atoms of the third layer is also observed (Figure 2a,b). For the PtOs binary alloys, electrons transferred via Ossp → Pt-sp increase along with the increase of the Os mole fraction in the PtOs alloy. However, as the Os mole fraction increases, the Os-sp → Pt-d electron transfer reduces (Figure 2b). As Pt is alloyed with both Ru and Os atoms forming the PtRu2Os2 tertiary alloy, the Ru electronic configuration becomes s0.40p0.13d6.82 (the electronic configuration for isolated Ru atoms is d7s1), indicating Ru sp- and d-band electron reductions by 0.47 and 0.18 e per Ru atom, respectively. It is not possible to identify the individual effects, which cause these electron reductions. The Ru electron reduction of the combined Ru sp/d-bands is mainly due to Ru-sp/d → Pt-sp transfer to the upper two layers of the substrate (Figure 2c). Additionally, the Os sp-band increase, associated with the Os dband reduction, is attributed to Os-sp → Pt-sp (upper two layers) and Os-sp → Os-d electron transfers. For the PtRu2Os2 alloy, an overall reduction of the Pt d-band population is observed; however, for the third layer, a Pt-sp → Pt−d electron transfer is observed (Figure 2a,c). Table 1 shows that for clean substrates the Ptc-sp population increases following the substrate trend PtRu2Os2 > PtOs4 > PtOs2 > Pt, which contributes to the repulsion of the adsorbate via σ-repulsion. Center-of-Bands. Figure 3 shows the stretching frequencies νCO and νCPt, the Eads, the Pt-ed, and the Pt-esp band centers for CO/Pt, CO/PtOs2, CO/PtOs4, and CO/PtRu2Os2. Moreover, the Pt-ed and Pt-esp for the corresponding clean substrates are also shown. The values of these properties (with the exception of the Pt-esp) are summarized in Table 2. Hammer et al.40 ascribe the Eads reduction with the downshift in energy of the Pt-ed. Moreover, in accordance with the Blyholder model, the downshift of the Pt-ed is indicative of stronger COads internal

3. RESULTS AND DISCUSSION 3.1. Binary PtOs Alloys and the Tertiary PtRu2Os2 Alloy. Electron Transfers. Upon alloying Pt with Ru/Os atoms on the surface layer of the fcc lattice (Figure 1), the Os electronic configuration becomes s0.82p0.04d6.50, s0.79p0.04d6.50, and s0.80p0.04d6.51 for the PtOs2 and PtOs4 binary alloys, and the Pt2Ru2Os2 tertiary alloy, respectively (the electronic configuration for isolated Os atoms is d6s2). These observations translate to substantial depletion of the Os sp-band by 1.14, 1.17, and 1.16 e per Os atom of the unit cell (Figure 1) for PtOs2, PtOs4, and PtRu2Os2 alloys, respectively. Concomitantly, the Os d-band is increased by 0.50, 0.50, and 0.51 e per Os atom for the above-mentioned alloys, respectively. Figure 2 shows the Pt sp- and d-band DOS spectra for Pt, PtOs2, PtOs4, and Pt2Ru2Os2 alloys, along with major electron transfers among Pt, Os, and Ru sp- and d-bands. For the PtOs2 binary alloy, electrons are transferred concurrently from the Os spband to the Pt sp- and d-bands of the upper two layers and to the Os d-band (Figure 2b). The Os-sp → Os-d electron transfer is responsible for increasing the sp orbital populations of the central Pt atom (Ptc), and the Os-sp → Os-d electron transfer for increasing the Os d-band population. Table 1 shows the CO contributions to the adsorbate 4σ̃, 5σ̃, 1π̃, d̃π, d̃σ, the 21449



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Table 1. CO Contributions to the Adsorbate 4σ̃, 5σ̃, 1π̃, d̃π, d̃σ, the Occupied Part of the 2π̃* Orbitals, and the Ptc-s, -p, and -d Orbitals for CO/Pt, CO/PtOs2, CO/PtOs4, and CO/PtRu2Os2 Slab Modelsa molecule/atom CO

Ptc

orbital 4σ̃ 5σ̃ d̃σ 1π̃ dπ̃ 2π̃* 6s 6p 5dz2 5dxzb 5dxyc 5d

CO/Pt

CO/PtOs2

CO/PtOs4

CO/PtRu2Os2

1.791 1.591 0.367 3.650 0.240 0.664 0.78 (0.83) 0.45 (0.34) 1.43 (1.88) 1.80 (1.77) 1.83 (1.72) 8.74 (8.84)

1.789 1.594 0.361 3.651 0.250 0.643 0.80 (0.86) 0.57 (0.40) 1.47 (1.89) 1.78 (1.78) 1.82 (1.71) 8.75 (8.85)

1.756 1.618 0.334 3.632 0.310 0.629 0.83 (0.94) 0.66 (0.48) 1.50 (1.91) 1.77 (1.73) 1.83 (1.72) 8.76 (8.80)

1.774 1.611 0.351 3.645 0.351 0.656 0.83 (0.95) 0.63 (0.51) 1.51 (1.91) 1.76 (1.73) 1.84 (1.73) 8.78 (8.81)

a

The CO contributions are directly calculated via the CRYSTAL06 program by DOS spectrum integration at appropriate energy ranges. The Pt orbital populations are calculated by Mulliken population analysis. The values in parentheses refer to corresponding populations at clean substrates. b Average value (5dxz + 5dyz)/2 assumed for Pt atoms. cAverage value (5dxy + 5dx2−y2)/2 assumed for Pt atoms.

3a,c). Upshifts/downshifts of the Pt-ed do not always correspond to reductions/increases of the corresponding overall d-electron density of the substrate. For example, for the PtOs2 binary alloy, the Pt-ed of the clean PtOs2 substrate is slightly upshifted by about 0.006 eV with respect to Pt (Table 2). However, this small upshift does not reflect the substantial electron transfers to the Pt d-band discussed in the last section, which change the Pt-ed DOS shape as shown in Figure 2a,b. The Pt-esp band centers of the clean substrate are upshifted following the substrate trend Pt > PtOs2 > PtOs4 > PtRu2Os2 (Figure 3e). Similar to the Pt-ed discussion above, the Pt-esp shifts are not related to an actual translation (in energy) of the corresponding band, but rather to charge transfers among different energy regions in the DOS spectrum. 3.2. C−O, C−Pt Optimal Geometries, Vibrational Frequencies, and Eads. The C−Pt bond lengths do not vary inversely with respect to their corresponding stretching frequencies νCPt as otherwise expected (Table 2). This mismatch may be due to DFT inability (at the functional and basis set employed here) to resolve C−Pt interatomic distance variations at the order of about 10−3 Å. Therefore, C−Pt bond strengths variations are more accurately determined by examining the corresponding stretching frequency values, which depend on the second derivative of the C−Pt bond length. DFT calculated frequencies are systematically overestimated by DFT; appropriate scaling factors can be applied for quantitative comparison with experimental observations.62 However, such parameters are still not known for the functional/basis set pair employed here. The νCPt changes

Figure 3. (a) Stretching frequencies νCO and νCPt, (b) Eads, (c) Pt-ed, and (d) Pt-esp band centers for CO/Pt, CO/PtOs2, CO/PtOs4, and CO/PtRu2Os2 alloys. Empty circles denote band center values for the corresponding clean substrates.

bond through reduced overlap of the unperturbed 2π* CO MO with the substrate d-band. In this work, the d-band center argument cannot be used to correctly predict the relative strength of the COads internal bond among the systems studied. For example, for CO/PtRu2Os2 relative to CO/Pt, the d-band center argument predicts a stronger COads internal bond due to the Pt-ed downshift of the corresponding clean substrate, which cannot be reconciled with the observed reduced νCO (Figure

Table 2. Calculated C−O and C−Pt Distances and Corresponding Stretching Frequencies, νCO, νCPt, Fermi Energies, Eads, and Pt-ed for the DFT Geometrically Optimized CO/Pt, CO/PtOs2, CO/PtOs4, and CO/PtRu2Os2 Periodic Slab Modelsa

a

property

CO/Pt

CO/PtOs2

CO/PtOs4

CO/PtRu2Os2

dC−O (Å) νCO (cm−1) dC−Pt (Å) νCPt (cm−1) EFermi (eV) Eads (eV) Pt-ed (eV)

1.137 2142 1.879 446 −5.52 (−5.55) −1.81 −3.20 (−3.34)

1.137 2140 1.890 434 −5.43 (−5.42) −1.46 −3.24 (−3.33)

1.137 2142 1.889 443 −5.32 (−5.50) −1.72 −3.55 (−3.42)

1.138 2129 1.887 382 −5.34 (−5.39) −1.37 −3.30 (−3.37)

The values in parentheses refer to corresponding clean substrates properties. 21450



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Figure 4. σ CO DOS for CO adsorbed on pure Pt and alloy surfaces as calculated by CRYSTAL06. CO DOS (thick black solid line) is factor decomposed into contributions from C (thin blue solid line) and O (red dashed line) atomic orbitals.

Figure 5. π CO DOS for CO adsorbed on pure Pt and alloy surfaces as calculated by CRYSTAL06. CO DOS (thick black solid line) is factor decomposed into contributions from C (thin blue solid line) and O (red dashed line) atomic orbitals.

along with the Eads (Figure 3a,b). The adsorbate C−O distance is invariant of alloying Pt with Os atoms (i.e., no change is observed within 10−3 Å). This result is in agreement with past

DFT cluster calculations for COads on (Pt3)(Os2Pt5) and (Pt3)(Os4Pt3) two-layer clusters.15 However, for CO/PtOs2 relative to CO/Pt, νCO is slightly downshifted by 2 cm−1, 21451



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Table 3. Charge Differences for COads on PtOs2, PtOs4, and PtRu2Os2 Alloys with Respect to CO/Pt for the CO Contribution to the Adsorbate 4σ̃, 5σ̃, 1π̃ , d̃π, d̃σ, and the Occupied Part of the 2π̃* Orbitals per Carbon and Oxygen Atoma substrate PtOs2

PtRu2Os2

C

O

C

O

C

O

4σ̃ 5σ̃ d̃σ

−0.003 0.004 −0.010 0.006 0.002 −0.007

0.001 −0.001 0.001 −0.004 0.013 −0.015

0.012 −0.019 −0.031 0.018 0.004 −0.013

−0.044 0.046 0.001 −0.035 0.067 −0.022

0.016 −0.014 −0.019 0.020 0.029 0.001

−0.033 0.034

1π̃ d̃π 2π̃* a

PtOs4

CO contribution

−0.024 0.082 −0.009

These values are directly calculated by CRYSTAL06 program via DOS integration in the appropriate energy regions.

the 4σ̃ orbital and the d̃σ-band being C−O antibonding. The C−O bonding type of the 5σ̃ orbital is of a special case. The 5σ̃ orbital appears as C−O bonding, antibonding, or nonbonding depending on the orbital’s polarization in its CO region (vide infra). The d̃σ-band is a hybrid of the unperturbed 5σ CO MO with the metal dz2-band. Upon CO adsorption on pure Pt, the 4σ and 5σ contributions to the adsorbate 4σ̃ and 5σ̃ orbitals, respectively, are diminished with respect to the corresponding MO populations of the free CO. These effects are indicative of electron donation from the CO to the metal sp- and dz2-bands and strengthen the COads internal bond.41 In the free CO, the 4σ and 5σ MOs polarize toward oxygen and carbon atoms, respectively. However, upon CO adsorption, the 4σ̃ polarizes toward carbon, while the reverse effect is observed for the 5σ̃ orbital.36 Figure 5 shows that the CO contribution to the d̃σband polarizes toward carbon.39 The increased 4σ̃ polarization toward carbon increases the orbital’s C−O antibonding character.64 However, the 5σ̃ orbital changes from C−O antibonding in the free CO to nonbonding in the CO/Pt.41 This observation is attributed to the 5σ̃ polarization toward oxygen, which reduces carbon 2s population (5σ is antibonding between carbon 2s and oxygen 2pz).39 Overall, changes in the σ̃-system strengthen the COads internal bond. The 4σ̃ and 5σ̃ orbitals are bonding to the metal substrate. This bonding effect is minimized by the d̃σ-band, which is C−Pt antibonding. The C−Pt antibonding character of the d̃σ-band is not enough to counterbalance C−Pt bonding caused by the 4σ̃ and 5σ̃ orbitals. This is due to the partial occupancy of the d̃σ-band, which reduces the orbital’s C−Pt repulsion. Therefore, changes in the σ̃-system result in the formation of a C−Pt bond. This effect has been explained in our previous report41 and by others.44 π̃-System. The π̃-system consists of the 1π̃ orbital, the d̃πband, and the part of the 2π̃*-band below the Fermi level. The d̃π-and 2π̃*-bands are formed by mixing of the unperturbed 1π and 2π* CO MOs with the dxz,yz-bands of the metal. The d̃π in this work and in our previous report41 differs from the d̃π in the original π−σ model, which does not contains contributions from the unperturbed 2π* CO MO. The 1π̃ orbital and the d̃πband are C−O bonding, whereas the 2π̃*-band is C−O antibonding. Moreover, the 1π̃ and the part of the 2π̃* below the Fermi level are C−Pt bonding, while the d̃π is C−Pt antibonding. Upon CO adsorption on pure Pt, charge is transferred from the CO region of the 1π̃ orbital toward the metal dxz,yz-band. Concomitantly, the 1π̃ polarization toward oxygen is reduced. The resultant charge in the CO region of the 1π̃ and d̃π orbitals is less than the 1π population of the free CO (i.e., 3.89 e and 4.0 e for CO/Pt and free CO, respectively,

indicating weakening of the COads internal bond (Table 1), in agreement with past PM-IRAS23 and DFT calculations.15 Concomitantly, the C−Pt bond weakens as verified by νCPt downshift (and the lowering of the Eads), in agreement with past DFT reports.15 As the Os mole percent in the alloy composition is increased, the νCO is now up shifted by 2 cm−1, thus matching the corresponding value of the CO/Pt case. However, for CO/ PtOs4 relative to CO/Pt, the COads internal bond is considered slightly stronger due to the shortening of the corresponding C−O bond by about 3 × 10−4 Å. This result is in partial agreement with the past cluster DFT calculations, where increasing the Os mole percent in the PtOs alloy resulted in a slightly stronger COads internal bond (by about 3 cm−1) but still weaker relative to CO/Pt.15 For CO/PtOs4 relative to CO/ PtOs2, the C−Pt bond strengthens as indicated by the upshift of the νCPt (and the Eads). The strengthening of the C−Pt bond upon increasing the Os mole percent in the PtOs alloy has been observed in the past.15 However, Ishikawa et al.15 draw no trend between νCPt and the Eads. This was probably due to the small clusters employed in their calculations, which makes the Eads calculation less accurate. Here, the periodic slab calculations show no trend between the Os mole percent in the PtOs alloy and the COads and C−Pt bond strengths. CO absorption on the PtRu2Os2 tertiary alloy causes maximum COads and C−Pt bond weakening among all systems examined here. For the CO/PtRu2Os2 case, the νCO, νCPt, and Eads appear at their lowest values of 2129 cm−1, 382 cm−1, and 1.37 eV, respectively (Table 2). The downshift of the νCO for CO/ PtRu2Os2 relative to CO/Pt (by 13 cm−1) is in agreement with Liu et al.,23 who observed a similar downshift by 15 cm−1 for CO/PtRuOs (80:10:10) relative to CO/Pt. The increase of the Fermi level for the adsorbate−substrate system relative to the clean substrate indicates that the adsorbate molecule is an electron donor to the substrate lattice. This occurs for the pure Pt, the PtOs4, and the PtRu2Os2, whereas for the PtOs2 the CO is an electron acceptor to the substrate (Table 2). The calculated Eads for the CO/Pt is agreement with past reports and within the range of the latest experimental value of 1.89 ± 0.20 eV reported by Yeo et al.63 for pure Pt at (111) face and low CO coverage. 3.3. CO MOs Hybridization with Pt sp/d-Bands of the Substrate. Figures 4 and 5 show the σ and π CO DOS spectra, respectively, for COads on Pt and Pt-based alloys discussed here. Additionally, the CO DOS is factor decomposed into contributions from carbon and oxygen atomic orbitals. 3.3.1. CO Adsorbed on Pure Pt (CO/Pt). σ̃-System. The σ̃system consists of the 4σ̃ and 5σ̃ orbitals, and the d̃σ-band, with 21452



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Figure 6. Contour plots of charge density differences for CO/PtOs2 relative to CO/Pt (top row), CO/PtOs4 relative to CO/PtOs2 (middle row), and CO/PtOs4 relative to CO/Pt (bottom row) at appropriate energy regions. Charge flows from the blue areas to the red areas. Seven contours are drawn in linear scale from −0.001 to 0.001 e/a03.

PtOs2 relative to CO/Pt, CO/PtOs4 relative to CO/PtOs2, and CO/PtOs4 relative to CO/Pt, in various energy ranges. Charge density difference maps obtained within particular energy intervals help us to isolate the effect of particular tilde-type orbital(s) from contributions of other orbitals between the systems examined. For example, charge density differences obtained within the energy interval of [−0.6, −0.4] hartrees (i.e., [−16.33, −10.88] eV) relative to the Fermi levels provide information only about the 4σ̃ orbital (Figure 4). In the contour map, the red area indicates charge surplus and the blue area charge deficiency (i.e., charge flows from blue to red areas). For CO/PtOs2-4σ̃, an approximate 0.003 e reduction is observed in the orbital’s carbon region relative to CO/Pt-4σ̃ (Table 3). This effect reduces the overall charge in the CO region of the 4σ̃ orbital and the orbital polarization toward carbon. The former observation is indicative of increased 4σ donation to the substrate, which strengthens both the COads internal bond and the C−Pt bond for CO/PtOs2 relative to CO/Pt. However, the latter observation is indicative of reduced carbon 2s population, and thus stronger COads internal bond and weaker C−Pt bond for the above-discussed systems. For

Table 1). These effects are indicative of a weaker COads internal bond. The CO regions in the adsorbate d̃π- and 2π̃*-bands, respectively, are populated by back-donation from the metal dxz,yz-bands. The partial occupancy of the 2π̃*-band (C−O antibonding) further weakens the COads internal bond. Similar to the σ̃-system, the overall π̃-system is bonding to the surface. The π̃- and σ̃-systems compete with each other, leading to a weaker COads internal bond.36,41 3.3.2. CO Adsorbed on the PtOs2 Alloy (CO/PtOs2). σ̃System. Table 3 shows the charge differences in the carbon and oxygen regions of the σ̃- and π̃-orbitals for the CO/PtOs2, CO/ PtOs4, and CO/PtRu2Os2 relative to CO/Pt. These quantities are directly related to the σ̃- and π̃-orbitals polarization changes for CO adsorbed on PtOs and PtRu2Os2 alloys relative to CO/ Pt. Tables 1 and 3 show that alloying Pt with Os and Ru/Os atoms minimally changes the CO contributions to the σ̃-and π̃systems, as well as the corresponding orbital polarizations. Therefore, in this work, to avoid premature judgments on the relative strength of the COads internal bond and the C−Pt bond upon alloying is also important to study the charge density differences among the systems examined here. Figure 6 shows the contour plots of the charge density differences for CO/ 21453



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reductions are observed at the orbital’s carbon and oxygen regions, respectively (Table 3). Charge reduction of the C−O antibonding 2π̃* orbital strengthens the COads internal bond and weakens the C−Pt bond. However, the substantial increase of the 2π̃* polarization toward carbon attenuates the abovementioned changes for both bonds (Figure 6, top row, 2π̃*). For CO/PtOs2 relative to CO/Pt, an overall charge reduction of 6 × 10−4 e is observed in the carbon region of the 1π̃ and 2π̃* orbitals, which strengthens the COads internal bond and weakens the C−Pt bond. For CO/PtOs2 relative to CO/Pt, a competition between the σ̃- and the π̃-system results in a slightly weaker COads internal bond, accompanied by a substantially weaker C−Pt bond. The overall increased charge in the carbon region of the 4σ̃ and 5σ̃ orbitals more than offsets the overall reduced charge in the carbon region of the 1π̃ and 2π̃* orbitals, thus weakening the COads internal bond (higher charge in the carbon region of these orbitals reduces C−O bonding).39 The C−Pt bond weakening is evident by the observed charge deficiency (large blue area at Figure 6, top row, overall) of C−Pt bonding character between the carbon and the metal. We now examine the use of the Blyholder model for CO/ PtOs2 relative to CO/Pt. We recall that the Blyholder model only considers charge changes in the unperturbed 5σ and 2π* CO MOs, where both MOs are C−O antibonding orbitals. For CO/PtOs2 relative to CO/Pt, the increased charge of the 5σ MO is indicative of reduced charge donation to the substrate bands (i.e., reduced C−Pt bonding). However, the reduced 2π* MO charge (by about 0.02 e, Table 1), which is indicative of decreased back-donation from the substrate bands to the 2π*, more than offsets the increased charge of the 5σ MO. Therefore, for CO/PtOs2 relative to CO/Pt, the Blyholder model predicts a stronger COads internal bond, which cannot be reconciled with the observed νCO downshift (Table 2). However, it correctly predicts a weaker C−Pt bond between the two structures examined here. 3.3.3. CO Adsorbed on the PtOs4 Alloy (CO/PtOs4). σ̃System. As the Os mole percent in the PtOs alloy is increased, the charge in the CO region of the 4σ̃ orbital is reduced. However, there is no trend drawn between the 4σ̃ polarization variations and the Os mole percent in the PtOs alloy: For CO/ PtOs4 relative to CO/PtOs2, the 4σ̃ polarization toward carbon is increased, while for CO/PtOs2 relative to CO/Pt, the same polarization is reduced (Table 3). The increased 4σ̃ polarization toward carbon attenuates the COads internal bond strengthening and augments the C−Pt bond strengthening, which are caused by the charge reduction in the CO region of the 4σ̃ orbital, in agreement with Figure 6 (middle row, 4σ̃). However, this is exactly opposite to what is observed for CO/PtOs2-4σ̃ relative to CO/Pt-4σ̃ (i.e., weaker COads and C−Pt bonds). The charge in the CO region of the 5σ̃ orbital is increased along with the Os mole percent in the PtOs alloy (Table 1) and is indicative of C−Pt bond weakening for CO/PtOs4 relative to CO/PtOs2. Similar to the 4σ̃ orbital, there is also no trend drawn between the 5σ̃ polarization variations and the Os mole percent in the PtOs alloy. For CO/PtOs4 relative to CO/PtOs2, the 5σ̃ polarization toward oxygen is increased (Table 3). This polarization change alone strengthens the COads internal bond and reduces C−Pt bonding. Figure 6 (middle row, 5σ + 1π) shows decreased charge in the region between carbon and the metal (blue area), indicating that the combined effect of 5σ̃ and 1π̃ orbitals due to increased Os alloying is a weaker C−Pt bond (vide infra). Here, the reduced charge in the carbon region of

CO/PtOs2 relative to CO/Pt, a competition between these two effects leads to stronger COads internal bond and weaker C−Pt bond due to changes in the 4σ̃ orbital. Figure 6 (top row, 4σ̃) shows a blue area in the region between carbon and the metal (i.e., charge deficiency), which is indicative of reduced charge in a C−Pt bonding orbital. Therefore, when changes in the orbital’s charge and polarization are small, it is important to also study the spatial distribution of the orbital’s charge density. Contrary to the 4σ̃ case, the charge in the carbon region of the CO/PtOs2-5σ̃ is increased by about 0.003 e relative to CO/ Pt-5σ̃, while minimal charge change is observed in the orbital’s oxygen region between the two systems (∼0.001 e, Table 3). The 0.003 e increase corresponds to an increase in the carbon 2s population, which augments the C−O antibonding character of the 5σ̃ orbital. This effect weakens the COads internal bond and more than offsets the strengthening of the COads internal bond, which is caused by the changes in the 4σ̃ orbital. Moreover, it contributes to the weakening of the C−Pt bond. Figure 6 shows the charge density difference in the energy regions occupied by 5σ̃ and 1π̃ orbitals. These orbitals are located adjacent in the energy spectrum, and their contributions partially overlap in energy (Figures 4 and 5); thus is not possible to accurately obtain separate charge density difference maps for each of these orbitals. For CO/PtOs2 relative to CO/ Pt, Figure 6 (top row, 50 + 1π) shows that the combined effect of 5σ̃ and 1π̃ orbitals leads to the weakening of both the COads internal bond and the C−Pt bond (vide infra). Charge reduction by about 0.01 e is observed in the carbon region of the CO/PtOs2-d̃σ relative to CO/Pt-d̃σ, accompanied by minimal charge change in the orbital’s oxygen region (by about 0.001 e, Table 3). The d̃σ orbital has mostly metallic character.39 Therefore, the above-discussed charge reduction strengthens the C−Pt bond, while the COads remains mostly unaffected. Here, the strengthening of the C−Pt bond, caused by changes in the d̃σ, more than offsets the weakening of the C−Pt bond caused by changes in the 4σ̃ and 5σ̃ orbitals. However, for CO/PtOs2 relative to CO/Pt, the presence of increased Ptc-sp and -dz2 orbital populations (by about 0.18 e, Table 1), which correspond to increased σ−dz2,sp antibonding states, finally weakens the C−Pt bond. In general, for the σ̃system, the strength of the C−Pt bond for the systems examined here depends majorly on the changes in the d̃σ orbital and the orbital populations of the Ptc and less on the changes in the 4σ̃ and 5σ̃ orbitals. Therefore, the CO/PtOs2-σ̃ system weakens both the COads internal bond and the C−Pt bond relative to CO/Pt-σ̃. The weakening of the COads internal bond is in agreement with the slightly increased net charge in the carbon region of the 4σ̃ and 5σ̃ orbitals (∼8 × 10−4 e). π̃-System. For CO/PtOs2-1π̃ relative to CO/Pt-1π̃, the charge in the carbon region of the 1π̃ orbital is increased (Table 3). This effect translates to a small charge increase in the CO region of the orbital (∼0.002 e, Table 2) and reduces the orbital polarization toward oxygen (Table 3). The former effect strengthens the COads internal bond and weakens the C−Pt bond, while the latter effect attenuates these bond changes. For CO/PtOs2-d̃π relative to CO/Pt-d̃π, a charge increase is observed in the oxygen region of the orbital (Table 3). Figure 6 (top row, d̃π) shows that changes in the d̃π orbital, for CO/ PtOs2 relative to CO/Pt, minimally affect the COads and the C−Pt bonds (d̃π is mostly an oxygen lone pair state). This statement applies to all systems examined in this work; therefore, changes in the d̃π are not discussed further. For CO/ PtOs2-2π̃* relative to CO/Pt-2π̃*, 0.007 e and 0.015 e charge 21454



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Figure 7. Contour plots of charge density differences for CO/PtRus2Os2 versus CO/Pt (top row) and CO/PtRus2Os2 versus CO/PtOs2 (bottom row) at appropriate energy regions. Charge flows from the blue areas to the red areas. Seven contours are drawn in linear scale from −0.001 to 0.001 e/a03.

the substrate trends PtOs4 > Pt > PtOs2 and Pt > PtOs4 > PtOs2, respectively. π̃-System. The charge in the CO region of the 1π̃ orbital is reduced as the Os mole percent in the PtOs is increased (∼0.02 e, Table 1). This is the exact opposite effect when compared to the charge in the CO region of CO/PtOs2-1π̃ relative to CO/ Pt-1π̃. The 1π̃ charge depletion (in the orbital’s CO region) is indicative of COads internal bond weakening and C−Pt bond strengthening. Figure 6 (middle row, 5σ + 1π) shows that this charge depletion is associated with charge reduction in the oxygen atom region of the orbital, accompanied by charge increase in the carbon atom region of the orbital. This observation is in agreement with the reduced 1π̃ polarization toward oxygen (Table 3), which attenuates the COads internal bond weakening and augments the C−Pt bond strengthening. The 1π̃ polarization toward oxygen reduces as the Os mode percent in the PtOs alloy is increased (Table 3). For CO/PtOs4 relative to CO/PtOs2, the C−Pt weakening due to changes in the 5σ̃ more than offsets the changes in the 1π̃ (Figure 6, middle row, 5σ + 1π). The charge in the CO region of the occupied part of the 2π̃* is reduced as the Os mole percent in the PtOs alloy is increased (∼0.014 e, Table 1). This charge reduction could be prematurely interpreted as a stronger COads internal bond and a weaker C−Pt bond. However, the appropriate contour map (Figure 6, middle row, 2π̃*) shows no charge change between carbon and oxygen, accompanied by charge deficiency (blue area) between carbon and the metal. The former result is indicative of a constant COads internal bond, whereas the latter result is indicative of a weaker C−Pt bond. Therefore, the orbital polarization, and in some cases the charge density spatial

the 5σ̃ is not counterbalanced by the increased charge in carbon region of the 4σ̃ orbital. Therefore, for CO/PtOs4 relative to CO/PtOs2, the combined effect of the 4σ̃ and 5σ̃ orbitals strengthens both the COads and the C−Pt bonds. The charge in the carbon region of the d̃σ orbital is reduced as the Os mole percent in the PtOs alloy is increased (by about 0.02 e, Table 3). Similar to the discussion in the last subsection, this effect alone strengthens the C−Pt bond. For CO/PtOs4 relative to CO/PtOs2, the increased σ-repulsion caused by the increase of the overall Ptc-sp and -dz2 orbital populations (by about 0.14 e, Table 1) does not counterbalance the strengthening of the C−Pt bond, which is caused by the d̃σ orbital changes. This result is verified by examining the contour plot of the charge difference map for all occupied orbitals for CO/PtOs4 relative to CO/PtOs2 (Figure 6, middle row, overall). This map shows that the blue area just below carbon (i.e., charge deficiency) has the shape of σ-type C−Pt antibonding orbital. Thus, for CO/PtOs4 relative to CO/ PtOs2, the σ̃-system strengthens both the COads and the C−Pt bond. We also briefly examine the changes in the σ̃-system for CO/ PtOs4 relative to CO/Pt. In this case, the overall charge in the carbon region of the 4σ̃ and 5σ̃ orbitals is reduced, which is indicative of stronger COads internal bond. Figure 6 (bottom row, overall) shows the contour plot of the charge difference for CO/PtOs4 relative to CO/Pt for all occupied orbitals. This contour map is identical to the corresponding contour map for CO/PtOs2 relative to CO/Pt (Figure 6, top row, overall) and was interpreted as reduced C−Pt bonding. Therefore, changes in σ̃-system strengthen the COads and C−Pt bonds following 21455



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former case, the area between carbon and oxygen has less electron density than in the latter case, as indicated by the large red area between carbon and oxygen (Figure 7, bottom, 5σ + 1π). This effect is attributed to the increased polarization of the combined 5σ + 1π orbitals toward carbon for the CO/ PtRu2Os2 relative to CO/PtOs2 case. However, the same polarization reverses direction for CO/PtRu2Os2 relative to CO/Pt, which is verified by the red area away from oxygen region (Figure 7, top, 5σ + 1π). For CO/PtRu2Os2-d̃σ relative to CO/Pt-d̃σ and CO/PtOs2d̃σ, the charge in the carbon region of the d̃σ orbital is reduced (Table 3). This effect alone strengthens the C−Pt bond. However, the contour map difference between CO/PtRu2Os2 and CO/PtOs2 (Figure 7, lower row, overall) shows a red area between carbon and the metal, which is indicative of increased σ-type repulsion between the metal and the adsorbate. Therefore, for CO/PtRu2Os2 and CO/PtOs2, the C−Pt repulsion caused by overall increased charge in the Ptc-sp and -dz2 orbitals more than offsets the C−Pt attraction caused by changes in the d̃σ orbital, leading to an overall weaker C−Pt bond. Therefore, for the σ̃-system alone, the COads internal bond and the C−Pt bond are the weakest for CO/PtRu2Os2 relative to all systems examined here. π̃-System. For CO/PtRu2Os2-1π̃ relative to CO/Pt-1π̃ and CO/PtOs2-1π̃, charge decreases of 0.004 e and 0.006 e, respectively, are observed in the orbital’s CO region, accompanied by a reduction of the orbital’s polarization toward oxygen. This situation is similar to CO/PtOs4-1π̃ relative to CO/PtOs2-1π̃ and was interpreted as a weaker COads internal bond and stronger C−Pt bond between the two systems. The charge in the CO region of the 2π̃* orbital is reduced by 0.008 e for CO/PtRu2Os2 relative to CO/Pt and increased by 0.013 e for CO/PtRu2Os2 relative to CO/PtOs2. An increased/ reduced charge of the 2π̃* C−O antibonding orbital could be interpreted as weakening/strengthening of the COads internal bond. However, for the “fine” charge differences here, the orbital polarization, as well as the contour plot of the charge differences between the systems compared here, must be examined. For CO/PtRu2Os2 relative to CO/Pt, the corresponding contour plot (Figure 7, top row, 2π̃*) shows a charge polarization around oxygen, whereas for CO/PtRu2Os2 relative to CO/PtOs2 a red area between carbon and oxygen is observed. Therefore, changes in the 2π̃* orbital for CO/ PtRu2Os2 relative to CO/Pt minimally affect the COads internal bond. However, for CO/PtRu2Os2 relative to CO/PtOs2, changes in the 2π̃* weaken the COads internal bond in accordance with the increased charge discussed above. The overall charge change in the carbon region of the 1π̃ and 2π̃* is positive for CO/PtRu2Os2 relative to CO/PtOs2 and to CO/Pt, which is indicative of weaker COads internal bond. The C−Pt bond for CO/PtRu2Os2 is the weakest among all systems studied, as confirmed by the appropriate contour maps (Figure 7, overall).

distribution, must be examined in addition to any charge changes in the orbital’s CO region. For CO/PtOs4 relative to CO/PtOs2, changes in the 1π̃ orbitals dominate over charges in 2π̃* orbitals, thus leading to a weaker COads internal bond and a stronger C−Pt bond. The same result appears for the π̃-system of CO/PtOs4 relative to CO/Pt. In this case, the overall charge change in the carbon region of the 1π̃ and 2π̃* between the two systems is positive, which is indicative of an overall COads internal bond weakening and C−Pt bond strengthening. Therefore, changes in the π̃-system strengthen the COads internal bond and weaken the C−Pt bond following the substrate trend PtOs2 > Pt > PtOs4. A competition between the σ̃- and the π̃-systems for CO/ PtOs4 relative to CO/PtOs2 leads to a slightly stronger COads internal bond and a much stronger C−Pt bond (Table 2). Here, the increased overall charge in the carbon region of the 1π̃ and 2π̃* orbitals does not counterbalance the reduced overall charge in the carbon region of the 4σ̃ and 5σ̃ orbitals. This same result appears for CO/PtOs4 relative to CO/Pt and is interpreted as increased C−O bonding between the two systems. Changes in the C−Pt bond have been discussed in the last σ̃-systems subsection. Therefore, the COads internal bond and the C−Pt bond strengthen following the substrate trends of PtOs4 > Pt > PtOs2 and Pt > PtOs4 > PtOs2, respectively. The Blyholder model fails to predict the correct trend of the COads and C−Pt bonds among CO/PtOs4, CO/PtOs2, and CO/Pt. For example, for CO/PtOs4 relative to CO/PtOs2, the increased charge in the 5σ MO more than offsets the reduced charge in the 2π* MO, leading to weaker COads and C−Pt bonds. This is in disagreement with the observed increased νCO and νCPt (Table 2). 3.3.4. CO Adsorbed on PtRu2Os2 Alloy (CO/PtRu2Os2). Figure 7 shows the contour plots of the charge density differences for CO/PtRu2Os2 relative to CO/Pt and CO/ PtRu2Os2 relative to CO/PtOs2, in various energy ranges. σ̃-System. The charge in the CO region of the 4σ̃ orbital is reduced for CO/PtRu2Os2 relative to CO/Pt and CO/PtOs2 (Table 1), accompanied by increased orbital polarization toward carbon. This situation is similar to CO/PtOs4-4σ̃ relative to CO/POs2-4σ̃, which was interpreted as stronger COads and C−Pt bonds between the two systems. Figure 7 (top and bottom row, 4σ̃) shows charge flow from the CO region of the 4σ̃ orbital toward the C−Pt region of the 4σ̃ orbital, in agreement with the above-discussed 4σ̃ charge changes (in the orbital’s CO region) and polarization. For CO/PtRu2Os2 relative to CO/Pt and CO/PtOs2 the charge in the CO region of the 5σ̃ orbital is increased concomitantly with increased 5σ̃ polarization toward oxygen (Tables 1 and 3). Similar to the discussions in the previous subsections, the overall charge change in the carbon region the 4σ̃ and 5σ̃ orbitals is a measure of the COads internal bond relative strength due to changes in the σ̃-system. Here, this charge change is positive (for CO/ PtRu2Os2 relative to CO/Pt and CO/PtOs2), which is indicative of reduced C−O bonding. Figure 7 shows the charge density difference in the energy regions of the 5σ̃ and 1π̃ orbitals for CO/PtRu2Os2 relative to CO/Pt (Figure 7, top, 5σ + 1π) and CO/PtOs2 (Figure 7, bottom, 5σ + 1π). In both cases, the combined effect of the 5σ + 1π orbitals weakens the C−Pt bond as indicated by the blue area between the carbon atom and the substrate. However, the charge density difference maps of CO/PtRu2Os2 relative to CO/Pt and CO/PtRu2Os2 relative to CO/PtOs2 show a different density profile in the region between oxygen and carbon. More specifically, in the

4. CONCLUSION The effect of CO adsorption on binary PtOs2 and PtOs4 alloys and the tertiary PtRu2Os2 alloy was studied using the nonlocal DFT method on periodic slabs (Figure 1). As Pt is alloyed with Ru/Os atoms, the COads internal bond and the C−Pt bond strengthen following the substrate trends of PtOs4 > Pt > PtOs2 > PtRu2Os2 and Pt > PtOs4 > PtOs2 > PtRu2Os2, respectively, as indicated by changes in the DFT calculated properties νCO, νCPt, and Eads. Here, CO contributions, polarizations, and 21456



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(10) Ley, K. L.; Liu, R.; Pu, C.; Fan, Q.; Leyarovska, N.; Segre, C.; Smotkin, E. S. J. Electrochem. Soc. 1997, 144, 1543−1548. (11) Chrzanowski, W.; Wieckowski, A. Langmuir 1997, 13, 5974− 5978. (12) Chrzanowski, W.; Kim, H.; Wieckowski, A. Catal. Lett. 1998, 50, 69−75. Chrzanowski, W.; Kim, H.; Tremiliosi-Filho, G.; Wieckowski, A.; Grzybowska, B.; Kulesza, P. J. New Mater. Electrochem. Syst. 1998, 1, 31−38. (13) Reddington, E.; Sapienza, A.; Gurau, B.; Viswanathan, R.; Sarangapani, S.; Smotkin, E. S.; Mallouk, T. E. Science 1998, 280, 1735−1737. (14) Anderson, A. B.; Grantscharova, E.; Seong, S. J. Electrochem. Soc. 1996, 143, 2075−2082. (15) Ishikawa, Y.; Liao, M. S.; Cabrera, C. R. Surf. Sci. 2002, 513, 98− 110. (16) Tong, Y. Y.; Kim, H. S.; Babu, P. K.; Waszczuk, P.; Wieckowski, A.; Oldfield, E. J. Am. Chem. Soc. 2001, 124, 468−473. (17) Beden, B.; Lamy, C.; Bewick, A.; Kunimatsu, K. J. Electroanal. Chem. 1981, 121, 343−347. (18) Baro, A.; Ibach, H. J. Chem. Phys. 1979, 71, 4812−4816. (19) Steininger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264− 282. (20) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267− 273. (21) Kabbabi, A.; Faure, R.; Durand, R.; Beden, B.; Hahn, F.; Leger, J.-M.; Lamy, C. J. Electroanal. Chem. 1998, 444, 41−53. (22) Gonzalez, M. J.; Hable, C. T.; Wrighton, M. S. J. Phys. Chem. B 1998, 102, 9881−9890. (23) Frelink, T.; Visscher, W.; Cox, A. P.; Van Veen, J. A. R. Electrochim. Acta 1995, 40, 1537−1543. (24) Petrii, A. O.; Kalinin, V. D. Russ. J. Electrochem. 1999, 35, 699− 707. (25) Crown, A.; Moraes, I. R.; Wieckowski, A. J. Electroanal. Chem. 2001, 500, 333−343. (26) Orozco, G.; Gutiérrez, C. J. Electroanal. Chem. 2000, 484, 64− 72. (27) Zhu, Y.; Cabrera, C. R. Electrochem. Solid-State Lett. 2001, 4, A45−A48. (28) Rauhe, B. R.; McLarnon, F. R., Jr.; Cairns, E. J. J. Electrochem. Soc. 1995, 142, 1073−1084. (29) Bockris, J. O’M.; Wroblowa, H. J. Electroanal. Chem. 1964, 7, 428−451. (30) Santiago, E. I.; Giz, M. J.; Ticianelli, E. A. J. Solid State Electrochem. 2003, 7, 607−613. (31) Liu, R.; Iddir, H.; Fan, Q.; Hou, G.; Bo, A.; Ley, K. L.; Smotkin, E. S.; Sung, Y. E.; Kim, H.; Thomas, S. J. Phys. Chem. B 2000, 104, 3518−3531. (32) Dimakis, N.; Iddir, H.; Diaz-Morales, R. R.; Liu, R.; Bunker, G.; Chung, E. H.; Smotkin, E. S. J. Phys. Chem. B 2005, 109, 1839−1848. (33) Gauthier, Y.; Schmid, M.; Padovani, S.; Lundgren, E.; Bus, V.; Kresse, G.; Redinger, J.; Varga, P. Phys. Rev. Lett. 2001, 87, 036103− 036106. (34) Blyholder, G. J. Phys. Chem. 1964, 68, 2772−2778. (35) Bagus, P. S.; Pacchioni, G. Surf. Sci. 1992, 278, 427−436. (36) Nilsson, A.; Weinelt, M.; Wiell, T.; Bennich, P.; Karis, O.; Wassdahl, N. Phys. Rev. Lett. 1997, 78, 2847−2850. (37) Bennich, P.; Wiell, T.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Stöhr, J.; Samant, M. Phys. Rev. B 1998, 57, 9274−9284. (38) Föhlisch, A.; Nyberg, M.; Hasselström, J.; Karis, O.; Pettersson, L. G. M.; Nilsson, A. Phys. Rev. Lett. 2000, 85, 3309−3312. (39) Fö hlisch, A.; Nyberg, M.; Bennich, P.; Triguero, L.; Hasselström, J.; Karis, O.; Petterssonm, L. G. M.; Nilsson, A. J. Chem. Phys. 2000, 112, 1946−1958. (40) Hammer, B.; Morikawa, Y.; Norskov, J. K. Phys. Rev. Lett. 1996, 76, 2141−2144. (41) Dimakis, N.; Cowan, M.; Hanson, G.; Smotkin, E. J. Phys. Chem. C 2009, 113, 18730−18739.

electron densities of the adsorbate CO orbitals from energies as low as the 4σ̃ to as high as the occupied part of the 2π̃* and the sp/d populations of the adsorbing Pt atom were used to elucidate changes in the COads and C−Pt bonds among the systems studied. Because of a minimal change in the CO contribution to the adsorbate σ̃- and π̃-system orbitals among the systems examined here, it is important to also study the polarizations and the spatial distribution of the adsorbate electron densities. Moreover, changes in the d̃ π orbital minimally affect the COads and C−Pt bonds. The relative strength of the COads internal bond is governed by the overall charge change in the carbon region of the 4σ̃, 5σ̃, 1π̃, and 2π̃* orbitals, where a positive factor stands for more C−O antibonding. However, the contour maps are examined for determining the relative strength of the C−Pt bond. There is no universal trend drawn between the Os mole percent in the PtOs alloy and the orbital polarizations of the σ̃- and π̃-system orbitals. The Blyholder model is insufficient to predict the correct bond strength of the COads and C−Pt bonds for CO adsorbed on PtOs binary alloys. Concomitantly, the d-band center argument might also predict erroneous COads internal bond strengths, because it only considers variations of the Pt-ed with the 5σ and 2π* frontier orbitals.

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ASSOCIATED CONTENT

S Supporting Information *

Optimized Gaussian basis sets for osmium, carbon, and oxygen atoms. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to the comments from the reviewers that resulted in a step improvement in the manuscript. Funding for ESS was provided by ARO contract W911NF-08-C-0037. Calculations were performed using the High Performance Computing Cluster at the University of Texas-Pan American.

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REFERENCES

(1) Watanabe, M.; Vehida, M.; Motoo, S. J. Electroanal. Chem. 1987, 229, 395−406. (2) Hamnet, A.; Kennedy, B. J. Electrochim. Acta 1988, 33, 1613− 1618. (3) Troughton, G. L.; Hamnett, A. Bull. Electrochem. 1991, 7, 488− 492. (4) Gasteiger, H. A.; Markovic, N.; Ross, P. N.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020−12029. (5) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Electroanal. Chem. 1993, 361, 269−273. (6) Krausa, M.; Vielstich, W. J. Electroanal. Chem. 1994, 379, 307− 314. (7) Gasteiger, H. A.; Markovic, N.; Ross, P. N. J. Phys. Chem. 1995, 99, 16757−16767. (8) Hogarth, M. P.; Hards, G. A. Platinum Met. Rev. 1996, 40, 150− 159. (9) Frelink, T.; Visscher, W.; van Veen, J. A. R. Langmuir 1996, 12, 3702−3708. 21457



Article

(42) Föhlisch, A.; Hasselström, J.; Bennich, P.; Wassdahl, N.; Karis, O.; Nilsson, A.; Triguero, L.; Nyberg, M.; Pettersson, L. G. M. Phys. Rev. B 2000, 61, 16229−16240. (43) Nilsson, A.; Pettersson, L. G. M. Surf. Sci. Rep. 2004, 55, 49− 167. (44) Kresse, G.; Gil, A.; Sautet, P. Phys. Rev. B 2003, 68, 073401− 073404. (45) Kresse, G.; Furthmüller, J. Comput. Mater. Sci. 1996, 6, 15; Phys. Rev. B 1996, 54, 11169−11186. (46) Rangelov, G.; Memmel, N.; Bertel, E.; Dose, V. Surf. Sci. 1991, 251−252, 965−970. (47) Aizawa, H.; Tsuneyuki, S. Surf. Sci. 1998, 399, L364−L370. (48) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; et al. CRYSTAL06; http://www.crystal.unito.it/. (49) Pascale, F.; Zicovich-Wilson, C. M.; Lopez, F.; Civalleri, B.; Orlando, R.; Dovesi, R. J. Comput. Chem. 2004, 25, 888−897. (50) Becke, A. D. J. Phys. Chem. 1993, 98, 5648−5652. (51) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200− 1211. (52) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123−141. (53) Dirk, A.; Diplomarbeit (1989) and http://www.theochem.unistuttgart.de. (54) Hehre, W. J.; Radom, L.; Schleyer, P. V. R.; Pople, J. A. Ab initio Molecular Orbital Theory; Wiley: New York, 1987. (55) Doll, K. Surf. Sci. 2004, 573, 464−473. (56) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188−5192. (57) Gilat, G.; Raubenheimer, L. J. Phys. Rev. 1966, 144, 390−395. (58) Gilat, G. J. Comput. Phys. 1972, 10, 432−465. (59) Anderson, D. G. J. Assoc. Comput. Mach. 1965, 12, 547−560. (60) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833−1840. (61) Kokalj, A. Comput. Mater. Sci. 2003, 28, 155−168 (code available from http://www.xcrysden.org/). (62) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502−16513. (63) Yeo, Y. Y.; Vattuone, L.; King, D. A. J. Chem. Phys. 1997, 106, 392−401. (64) Nilsson, A.; Pettersson, L. G. M. Surf. Sci. Rep. 2004, 55, 49− 167.

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A Density Functional Theory Study on Carbon Monoxide Adsorption

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