Adsorption of Carbon Monoxide on PlatinumâRuthenium, Platinum

Article pubs.acs.org/JPCC

Adsorption of Carbon Monoxide on Platinum−Ruthenium, Platinum−Osmium, Platinum−Ruthenium−Osmium, and Platinum− Ruthenium−Osmium−Iridium Alloys Nicholas Dimakis,*,† Fernando A. Flor,† Nestor E. Navarro,‡ Andres Salgado,† and Eugene S. Smotkin§ †

Department of Physics, University of Texas Rio Grande Valley, Edinburg, Texas 78539, United States Department of Chemistry, University of Texas Rio Grande Valley, Edinburg, Texas 78539, United States § Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States ‡

S Supporting Information *

ABSTRACT: Ternary and quaternary PtRuOs and PtRuOsIr alloys are promising alternatives to the binary PtRu alloys that serve as efficient anode catalysts for methanol and hydrogen air fuel cells. The efficiency of these catalysts is correlated to the adsorption of CO molecules on their surfaces. In this work, we study CO adsorption on a series of PtRu, PtOs, PtRuOs, and PtRuOsIr alloys and on pure Pt, Os, Ir, and Ru using periodic density functional theory. Systematically, we vary the location of the alloy atoms in the substrate and the alloy Pt mole percent. As CO is adsorbed on PtRu, PtRuOs, and PtRuOsIr alloys, the CO internal adsorbate bond and the C−Pt surface bond weaken on average (for alloy configurations of the same Pt mole percent) along with the decrease of the Pt mole fraction in the alloy. However, the frozen substrate calculations show that these bonds are about invariant of alloying Pt with Os atoms, with the exception of PtOs configurations with Os atoms in the middle layer, whereas relaxing the substrate surface may lead to stronger C−O and C−Pt bonds due to alloying Pt with Os. The C−O and C−Pt overlap populations are correlated with the carbon stype vacancies and the overall s, p, and d vacancies of the adsorbing metal, for the C−O and C−Pt bonds, respectively: Hybridization defects are attributed to the cases of concomitant increase of the overlap populations and downshifts of the corresponding stretching frequencies. Changes in the CO internal adsorbate bond are explained using a phenomenological model based on the modified π-attraction σ-repulsion scheme and is compared with the traditional 5σ donation−2π* back-donation mechanism. This model successfully ascribes the C−O internal adsorbate bond strength to the carbon and oxygen atom contributions of the σ and π hybrid CO-substrate orbitals for the majority of the systems examined here.

1. INTRODUCTION

Pt‐(CO)ads + M‐(H 2O)act

An atomistic understanding of adsorbate bonding is of importance to surface science, electrochemistry, and catalysis with respect to adsorbates as reaction intermediates and as probes of substrate coadsorbate structure.1,2 For example, adsorption of carbon monoxide (COads) on Pt-based mixed metal catalysts is of interest to the fuel cell community3−13 as a probe of surface structure in hydrogen air fuel cells and as a reaction intermediate in direct methanol fuel cells (DMFCs).14−17 COads poisons active Pt sites for methanol oxidation. COads oxidation is not important in hydrogen air fuel cells because the anode potentials are never sufficiently positive. In hydrogen air fuel cells, it is more an issue of COads competing with hydrogen for active sites, rather than CO oxidation. In a DMFC, COads is oxidized by a bifunctional mechanism that requires the use of Pt−M mixed-metal anode catalysts as follows:18,19 © 2016 American Chemical Society

→ Pt + M + CO2 + 2H+ + 2e−

(1) 18,20,21

22,23

where M is an oxophilic metal such as Ru, Sn, and Os.24−27 At low overpotentials, activation of water by metal M is rate-limiting.15 In addition to facilitating the bifunctional mechanism, M also decreases the COads enthalpy of adsorption on Pt, by altering Pt metal electronic states (ligand effect).19 Ley et al. suggested mixed metal Pt-based catalysts with metal M having M−O bond energies similar to the Pt−C bond energy (≈ 590 kJ mol−1).9 The Ley diagram (Figure 1) suggests Mo, Ru, Os, and Sn as excellent candidates for alloying with Pt. Although Os is ideal for M, its low solubility limit (∼20%) in Pt and the need for higher mole fractions of oxophilic promoters at the surface (∼50%) demand introduction of an oxophilic diluent. The Ley diagram suggests Ru as a diluent. The Received: February 29, 2016 Revised: April 18, 2016 Published: April 22, 2016 10427

DOI: 10.1021/acs.jpcc.6b02086 J. Phys. Chem. C 2016, 120, 10427−10441

Article

The Journal of Physical Chemistry C

occupied part of the 2π̃*-band (C−O antibonding) are bonding to the surface, whereas the d π̃ -band (C−O bonding), which is typically an oxygen lone pair, is antibonding. The π̃system weakens the COads internal bond due to decreased 1π̃ polarization toward oxygen and charge transfer to the CO region of the C−O antibonding 2π̃*-band. The extended and the original π−σ model use the same tilde-type orbitals to describe π-attraction; however, some differences are noteworthy. For example, the d π̃ -band of the original π−σ model contains 2π* components,49 whereas the d π̃ -band of our extended π−σ model is mainly composed of 1π and dxz,yx-band character. The σ̃-system of the extended π−σ model is ascribed to the 4σ̃ and 5σ̃ orbitals and the d σ̃ -band. The d σ̃ -band is a hybrid of the 5σ CO molecular orbital with the substrate dz2-band, and mostly of metallic character. The 4σ̃ and 5σ̃ orbitals are bonding to the surface, whereas the d σ̃ -band is antibonding. The 4σ̃ orbital and the d σ̃ -band are C−O antibonding, whereas the 5σ̃ orbital is nonbonding for the CO/Pt case.40 The overall effect of the σ̃-system as described here is attractive to the surface, in contrast to its sole repulsive nature in the original π−σ model, which assumes that all σ-type orbitals below the Fermi level are filled.49 Rangelov et al.52 using inverse photoemission spectra measurements for CO/Ni, CO/Pd, and CO/Pt verified the partial filling of the d σ̃ -band below the Fermi level. Moreover, the original π−σ model interpreted the observed charge decrease in the CO regions of the 4σ̃ and 5σ̃ orbitals and the decrease in the substrate dz2-band population, as absence of σ-type charge donation to the surface. However, we reported that decreases in σ-donation and substrate dz2-band population is not only possible, but in agreement with the increased charge to the CO region of the π̃-system via the substrate sp-band.40 The σ̃-system through charge donation (transfer) from the CO to the substrate and possible polarization changes for the 4σ̃ and 5σ̃ orbitals relative to the corresponding molecular orbitals of the free CO strengthen the COads internal bond. However, this bond strengthening is more than offset by changes in π̃-system, leading to a weaker COads internal bond relative to free CO. In the past, the extended π−σ model was used to correlate the COads internal bond strength with the net change of the CO contributions to the carbon 2s and pxy atomic orbitals of the σ̃- and the π̃-systems, respectively, as,42,53

Figure 1. Metal−oxygen bond dissociation energy (D°298K (M−O) in diatomic molecules versus group. Periods are determined by symbols. Reproduced from ref 9 with permission. Copyright 1997 ECSThe Electrochemical Society.

enormous compositional phase space of possible ternary and quaternary mixed metal catalysts prompted the development of combinatorial screening of electrocatalysts.28 The PtRuOsIr quaternary mixed metal catalyst evolved from these combinatorial studies. Ir enhances C−H activation, which is ratelimiting at higher DMFC anode potentials (or lower cell voltages).15 Upon adsorption on the Pt, the C−O internal bond weakens and a stable C−M dative bond is formed. Polarization modulated infrared reflection absorption spectroscopy measurements on COads on Pt29−31 and Pt-based alloys (PtRu, PtOs, and PtRuOs)27,29 showed alloy induced downshift of the COads stretching frequency (νCO) relative to free CO. Computational simulations for COads on Pt-group metals32−37 and on Pt-based alloys such as PtRu,38−40 PtOs, and PtRuOs alloys41,42 verified the νCO downshift. Several phenomenological models correlate renormalized CO orbitals (i.e., CO adsorbate orbitals) with the strength of the COads internal bond and the C−Metal surface bond. Bagus and Pacchioni43 ascribed the frontier CO orbitals 5σ and 2π* with the weakening of the COads internal bond as a 5σ donation concurrent with 2π* back-donation from the substrate d-bands. Their frontier orbital model improves upon the “Blyholder model”44 which only considered changes in the π-adsorbate system. In the frontier orbital model, the charges in the 5σ and 2π* COads molecular orbitals (i.e., Q5σ and Q2π*, respectively) are correlated with the νCO as, νCO ∝ −(Q 5σ + Q 2π*)

σ̃ π̃ νCO ∝ −(Q C2s + Q Cp ) xy

(2)

(3)

Here, eq 3 is modified as,

The frontier orbital model explains COads on Ni, Fe, Cr, Ti and Pt,45 but fails for COads on PtOs and PtRuOs alloys having Ru and Os surface atoms.41 Hammer et al.46 used the frontier orbital model to correlate the shift of the substrate d-band center-of-mass with the CO adsorption energy (Eads). An upshift of the d-band center increases the overlap of the substrate d-band with the renormalized 2π* CO molecular orbital, thus weakening the COads internal bond. The frontier orbital model was challenged by Nilsson and coworkers47−51 with a π-attraction and σ-repulsion model (π−σ model) that uses hybrid adsorbate−substrate tilde-type orbitals to elucidate adsorption. We extended the π−σ model by inclusion of the attractive parts of the σ-type orbitals (extended π−σ model, vide infra).40 In our extended π−σ model, the πattraction is ascribed to the 1π̃ orbital and the d π̃ - and 2π̃*-bands (π̃-system). The 1π̃ orbital (C−O bonding) and the

π̃ 4σ ̃ 5σ ̃ d σ̃ νCO ∝ −(Q O2s + Q C2s + Q C2s + Q Cp ) xy

(4)

which improves the description of the 4σ̃ C−O antibonding orbital (i.e., bonding between carbon 2s and oxygen 2pz and antibonding between oxygen 2s and carbon 2s). In this work, we examine CO adsorption on PtRu, PtOs, PtRuOs, and PtRuOsIr alloys, as well as on Pt, Os, Ir, and Ru surfaces with a focus on the ligand effect. For best catalytic performance, the composition of the Pt-based alloy substrate is such that CO is adsorbed weakly on its surface, thus reducing surface poisoning. However, for DMFCs, this alloy must contain oxophilic metals on its surface for water activation, in accordance with the bifunctional mechanism. CO/PtRuOsIr has been experimentally studied by Reddington et al.28 using combinatorial chemistry and found Pt44Ru41Os10Ir5 alloy to 10428


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be the preferred site (Figure S1). This result, which is in agreement with experiments, is in contrast to past computational calculations for CO/Pt(111), where DFT was found to erroneously predict the hollow site to be the preferred COads site at low coverages (“CO adsorption puzzle”58). Table S1 shows the DFT calculated C−O and C−Pt optimized distances, the corresponding stretching frequencies νCO, νCPt, the Eads for CO/Pt(111), and corresponding experimental data. DFT59−61 under the CRYSTAL0962 program is used to calculate electronic, structural, and vibrational properties of COads on Pt, Os, Ir, Ru, and Pt-based alloys. The CRYSTAL09 program employs Gaussian type function basis sets centered at the atoms. The νCO and νCMetal are calculated at the Gamma point (k = 0).63 The hybrid B3LYP semiempirical functional64 and the PBE0 nonempirical/parameter-free functional are used in this work. The innermost orbitals of the substrate atoms are described by Stuttgart-Dresden ECP,65,66 which accounts for mass-velocity and Darwin relativistic corrections. For Ru atoms, 28 core electrons have been substituted with ECP, whereas for Pt, Ru, and Ir atoms, the number of ECP core electrons is Z-60. The use of ECP provides similar accuracy as the all-electron basis sets at a fraction of CPU time. Effective valence basis sets for substrate atoms employed here are as follows: The Pt atoms are described by the optimized-for-crystalline calculations basis sets of [4s4p2d],67 whereas Ru, Os, and Ir atoms by the [4s3p2d] basis set. The Ir atom basis set developed for these calculations is shown in the Supporting Information (Table S2), whereas the Ru and Os basis sets are from past works.40−42 For carbon and oxygen atoms, the [4s3p2d] basis sets are used. The basis sets were kept at this size to avoid linear dependencies, which arise due to small exponents being present in Gaussian-type basis sets. Brillouin zone integrations are performed on a 12 × 12 Monkhorst−Pack grid,68 whereas the Fermi energy and the density matrix use the 24 × 24 grid (Gilat grid).69,70 The atomic orbital populations are calculated using Mulliken population analysis.71 Charge density plots were obtained using the XCrySDen graphical package.72 SCF energy convergence was obtained using Anderson quadratic mixing,73 with additional mixing of the occupied with the virtual orbitals and smearing of the Fermi surface with a Gaussian smearing of 0.005 hartree. The SCF threshold energy between geometry optimization steps was set to 10−10 to 10−9 hartrees: This value is much smaller than the default value of 10−7 hartrees used by CRYSTAL09. An rms default value of 0.0012 Å is set for geometry optimizations. A pruned grid, with 75 radial and 974 angular points, was used as the integration grid (XLGRID keyword). Tighter truncation criteria were used for the bielectronic integrals: The overlap thresholds for exchange and Coulomb integrals were set to 10−8 (TOLINTEG keyword set to 8 8 8 8 16 relative to the default value of 6 6 6 6 12). The stability of the final geometry conformation was assured by performing postgeometry optimizations for the final structure (FINALRUN keyword set to 4). The COOP values are directly calculated by CRYSTAL09. The Eads was calculated with treatment of the basis set superposition error (BSSE),74 which arises due to finite size of the crystalline basis sets used here. The BSSE error is minimized using the counterpoise correction75 by including “ghost” atoms (i.e., massless atoms) in the fragment SCF energy calculations of the adsorbate−substrate structure. Calculated Eads values using the above treatment for the BSSE error appear to be less negative in energy than corresponding uncorrected values.53 Here, for CO/Pt, this shift was found to be about 0.1 eV for DFT calculations using the B3LYP and PBE0 functionals.

have significantly higher activity than the binary P50tRu50 alloy, which typically serves an anode catalyst on DMFCs. Here, changes on the COads internal bond, the C−Pt bond, and the CO interaction with the substrate surface as Pt is alloyed with Ru, Os, and Ir atoms are correlated with the alloy mole percent and the distribution of the Ru, Os, and Ir atoms in the Pt-based alloy. Gurau et al.54 have found that PtRuIr and PtOsIr are poor fuel cell anode catalysts and, thus, will not be examined here.

2. MODELS AND COMPUTATIONAL METHODS All substrates are modeled as three-layer periodic slabs. More specifically, the Pt and Ir metals and the Pt-based alloys PtRu, PtOs, PtRuOs, and PtRuOsIr are modeled as (100) fcc lattices, whereas the Ru and Os metals as (0001) hexagonal lattices. The lattice parameters of the Pt, Os, Ir, and Ru substrates are experimentally reported values from crystallography. The Pt-based alloys are constructed by atomic substitution of Pt with Ru/Os/Ir atoms: Substrate atoms remain locked in the Pt, experimentally measured, crystallographic positions. However, we calculated the lattice parameter for bulk Pt and found it to be slightly larger than the experimentally reported value of 3.924 Å (i.e., 4.044 and 3.966 Å, for B3LYP and PBE0 calculations, respectively). Substrate relaxation effects are examined for a limited number of configurations here. In these cases, the upper two layers are free to move during the optimization process. High efficiency Pt-based alloy catalysts are single-faced fcc lattices.54 To avoid an overwhelming number of alloying configurations, the amounts of Ru, Os, and Ir used in the PtRu, PtOs, PtRuOs, and PtRuOsIr alloys of this work are limited to the solubility limits of these elements in fcc Pt (Figure 2).54

Figure 2. Periodic three-layered slabs with CO overlayers (shown by orange-line rectangles) as follows: (a) CO/Pt[Ir] as c(4 × 4) overlayer and (b) CO/Ru[Os] as (3 × 3) overlayer. Gray and blue atoms are located at the adsorbing and middle layers, respectively. (c) Ternary chart shows the PtRu, PtOs, PtRuOs, and PtRuOsIr alloy configurations used here. Alloys that contain Ir atoms are shown as triangles, while all others as circles. The trapezoidal area within the dotted lines approximately defines the region of Pt-based alloys at single fcc face. Moreover, we examine COads on PtRu and PtOs alloys, by varying the amount of alloying elements and their position within the fcc lattice. The COads is placed atop as (4 × 4) and (3 × 3) overlayers to the fcc and hexagonal lattices, respectively (i.e., 1/8 ML and 1/9 ML coverages, respectively, Figure 2) and is free to move during geometry optimizations. The atop site was found to be the preferred site for all calculations here. We will examine other adsorption sites in the future. The low CO coverage of this work eliminates possible CO−CO interactions that would affect C−O and C−Metal distances, as well as the stretching frequencies νCO and νCMetal and corresponding crystal orbital overlap populations (COOP).55 However, we also examine the effect of the CO coverage for a single PtRu alloy. Consistent with our past reports,39−42,53 we choose the (100) as the substrate face for Pt and Pt-based alloys. Moreover, we also performed DFT calculations for COads on a two-layer Pt(111) substrate at 1/9 ML CO coverage using the PBE0 functional,56,57 where we found the atop adsorption to

3. RESULTS AND DISCUSSION 3.1. CO Adsorption on Pt, Os, Ir, and Ru Metals. Table 1 shows the DFT calculated C−O and C−Metal distances, νCO, νCMetal, and Eads for COads on Pt, Os, Ir, and Ru surfaces and the corresponding past experimental measurements, where available. Our calculated values are in close agreement with the experimental results, including calculated properties from CO/ Pt(111) (Table S1). Experimental measurements are not 10429


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Table 1. DFT Calculated C−O and C−Metal Optimized Bond Lengths, Corresponding Stretching Frequencies νCO and νCMetal, and Eads for COads Atop on Pt, Ru, and Ir Surfaces of Figure 2 Using the B3LYP Functional, and Compared with Priory Reported Experimental Measurementsa Metal Pt (100) property C−O (Å) νCO (cm−1) C−Metal (Å) νCMetal (cm−1) Eads (eV)

Os (0001)

Ir (100)

calc.

exp.

calc.

calc.

1.138 (1.136) 2132 (2171) 1.877 (1.849) 441 (475) −1.40 (−2.03)

1.15 ± 0.10b,78

1.144 (1.141) 2080 (2125) 1.939 (1.917) 441 (471) −1.46 (−1.95)

1.147 (1.144) 2063 (2107) 1.900 (1.880) 474 (501) −1.98 (−2.50)

2080c,80 1.85 ± 0.10b,78 480d,83 −2.33e,84

Ru (0001) exp.

-i 2027j,81 -i 479−483j,81 −1.63k,85

calc.

exp.

1.155 (1.153) 2003 (2042) 1.887 (1.862) 439 (468) −1.56 (−2.02)

1.10 ± 0.05f,79 1980−2080g,82 1.93 ± 0.04f,79 445g,82 −1.66h,86

a

The values in parentheses refer to calculations under the PBE0 functional. No experimental measurements are available for CO/Os. bLow energy electron diffraction (LEED) for CO/Pt (111) at 0.35−0.5 ML surface coverages. cInfrared reflection−absorption spectroscopy (IRAS) at ultra high vacuum at nonsaturation coverages. dElectron energy loss spectra (EELS) measurements for CO/Pt(111) at 0.17 ML coverage. eSingle crystal adsorption calorimetry at CO coverages of 0.05−0.25 ML. fLEED measurement at 0.33 ML coverage. gEELS measurements at 0.07 ML coverage. h Thermal desorption spectroscopy (TDS) for up to 0.2 ML coverages. iNo data available. jEELS measurements at low coverage (0.5 L). kTDS measurements for CO/Ir(111) at 0.25 ML coverage.

Figure 3. (a) νCO (left) and C−O (right); (b) C−O COOP, (c) νCMetal (left) and C−Metal (right); COOP of (d) C−Metal (left) and O−Metal (right); (e) Eads (left) and ed (right); and (f) frontier orbital model, using eq 2 (left) and extended π−σ model, using eq 4 (right) for COads on Pt, Os, Ir, and Ru surfaces of Figure 2. Solid lines and dashed lines show calculations under the B3LYP and PBE0 functionals, respectively. Experimental values are shown by circles.

available for CO/Os, for the C−O and C−Metal distances of the CO/Ir, and in some cases, for COads on Pt and Ir at the 100 face (e.g., experimental CO/Pt(100)-νCPt data are not available). In the last case, our values are compared with the corresponding reported values for the 111 face. The accuracy of the B3LYP functional for periodic DFT calculations of solids has been questioned by Paier et al.76 They reported B3LYP calculated lattice parameters and atomization energies overestimated by 1% and underestimated by about 17%, respectively (versus reported experimental values). Alternatively, the PBE0 functional, which describes the homogeneous

gas exactly, was preferred for calculations of these properties. For CO/Pt(111) and COads atop, Stroppa et al.33 reported improved C−O distances using the PBE0 functional relative to other functionals (0.8% error). Soini et al.77 studied CO adsorption on large Pt clusters and reported C−O and C−Pt distances to be among the smallest versus calculations with other functionals. In this work, for CO/Pt(100), the B3LYP calculated νCO is closer to the experimental value versus the corresponding PBE0 calculated value. The opposite is observed for νCPt and Eads (Table 1). We also performed calculations for CO/Pt using the B3LYP functional by allowing the upper-two 10430


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Table 2. CO Contributions to the Adsorbate 4σ̃ ,5σ̃ , and 1π̃ Orbitals and d π̃ , d σ̃ , and 2π̃*-Bands, the s and p Carbon and Oxygen Orbital Valence Populations, and the Adsorbing Metal s, p, and d Orbital Populations for COads on Pt, Os, Ir, and Ru Using the B3LYP Functionala substrate molecule/atom CO

orbital/band

Pt

Os

Ir

Ru

4σ̃

dσ̃

1.761 ↓ 1.581 ↑ (0.065) 0.367

1.541 ↓ 1.767 ↑ (0.179) 0.322

1.417 ↓ 1.889 ↑ (0.258) 0.280

1.858 ↑b 1.484 ↓b(0.592) 0.345

1π̃

3.639

3.774

3.797

3.792

0.183

0.120

0.579 (0.227)

0.902 (0.413)

1.79 4.38 1.47 2.32 0.96 0.07 7.02

1.77 4.43 1.61 2.54 0.66 0.40 6.80

5σ̃

↓ ↑

dπ̃

0.189

2π̃*

0.670 (0.170)

0.647 (0.228)

s p s p s p d

1.80 4.30 1.54 2.32 0.78 0.51 8.67

1.79 4.36 1.51 2.33 0.99 0.08 7.08

0.109 ↑

↑ Oxygen Carbon Absorbing Metal

a Up and down arrows, show orbital polarization directions towards oxygen and carbon atoms, respectively. The free CO 4σ and 5σ orbitals polarize towards oxygen and carbon atoms, respectively. Numbers in parentheses are contributions of carbon s to the 5σ̃ orbitals and carbon pxy to the 2π̃*-band. bFor CO/Ru, 4σ̃ and 5σ̃ polarizations are in the same directions as 4σ and 5σ free CO.

defects (vide infra).90 Therefore, changes on νCO prevail over changes on corresponding C−O bond lengths for describing the strength of the COads internal bond. For covalent bonds, the following relationship between bond energy and COOP is given91

substrate layers to relax during geometry optimizations. In this case, the C−Pt distance shortened by 0.03 Å relative to the corresponding frozen substrate calculations, and the νCPt and | Eads| were increased by 34 cm−1 and 0.56 eV, respectively. Changes in the C−O distance due to substrate relaxation are at the order of 10−4 Å, whereas νCO increased by 10 cm−1. Similar shifts due to substrate surface relaxation are observed for the COads on the alloy substrates of this work (vide infra). Figure 3 shows the variations of the adsorbate C−O and C− Metal bond lengths, the corresponding νCO and νCMetal, Eads, C−O, C−Metal, and O−Metal COOP, d-band centers (ed), the frontier model, and the extended π−σ model for COads on Pt, Os, Ir, and Ru periodic slabs of Figure 2. These calculated values follow the same trends for both B3LYP and PBE0 functionals and this statement is extended to the alloy calculations of this work (vide infra). A minor exception to this rule is when comparing the Eads of CO/Ru and CO/Pt between B3LYP and PBE0 calculations: The PBE0 calculations show Eads to be the same for both cases, whereas B3LYP calculations show a slightly stronger adsorption for CO/Ru relative to CO/Pt. The strength of the C−X bond is associated with changes in the C−X bond length, the corresponding stretching frequency, and the C−X COOP for X = O, Metal. The CO interaction with the surface is also measured by changes in the Eads. Bond lengths and corresponding stretching frequencies are commonly related through an inverse relationship known as the Badger’s rule.87−89 Although, the Badger’s rule holds for changes between C−O bond lengths and corresponding νCO variations for COads on Pt, Ru, Os, and Ir metals (Figure 3a), it may fail for CO adsorption on selected Pt-based alloys. Kaupp et al. reported that shorter bonds are not necessarily stronger bonds due to possible sp hybridization

Ek =

ΔH COOPk ∑i COOPi

(5)

where Ek is k-th bond energy, ΔH is the heat of atomization, COOPi is the COOP overlap for the i-th bond, and the summation runs over all bonds for the structure. It is important to state that COOP cannot be used to predict bond strengths for mostly ionic and long-range interactions. For the structures of this work, due to low CO coverages, changes on the C−O COOP are expected to be along with changes in the νCO. However, for CO/Os relative to CO/Ir, changes in the νCO and the C−O bond length, both indicative of stronger C−O bond for the CO/Os, are in contrast with the decreased CO/Os C− O COOP, which could be interpreted as C−O bond weakening (Table 2 and Figure 3). This mismatch is due to sp hybridization defects. For CO/Ir, the increased carbon s-type vacancies do not contribute efficiently to hybridization with the carbon p-type orbitals; thus they weaken the COads internal bond relative to CO/Os. This effect has been observed for HnF3−nSn−HnFn−3, n = 1−3.90 In general, changes in the νCO opposite to changes in the C−O COOP are indicative of sp hybridization defects, and this statement is extended to the C− Metal surface bond by including the adsorbate d-orbitals (i.e., spd hybridization). Loffreda et al.92 observed no correlation between the adsorbate−substrate bond length and the corresponding COOP for NO/Rh and NO/Pd. 10431


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The Journal of Physical Chemistry C Table 2 shows, among others, the CO contributions to the σ̃ and π̃ orbitals/bands, the s and p orbital populations for oxygen and carbon atoms, and the s, p, and d orbital populations for the adsorbing metal using the B3LYP functional. Similar results are obtained using the PBE0 functional. Figure 4 shows the

for CO/Pt relative to adsorption on the other metals (Figure 3c,d). Therefore, the C−Pt bond for CO/Pt is primarily ionic, whereas strong covalent character is evident for the C−Metal bond for adsorption on the Os, Ir, and Ru substrates. This is consistent the substantially less overall s, p, and d-orbital vacancies of the adsorbing Pt relative to the corresponding vacancies of the adsorbing Os, Ir, and Ru (Table 2). 3.1.1. Frontier Orbital Model and Extended π−σ Model. Table 2 shows that CO adsorption on Ru is dominated by charge back-donation from the substrate toward the CO region of the 2π̃*-band, in agreement with the increased COads carbon p-type orbital population. For CO/Ru, the νCO is downshifted the most relative to COads on the other three metals of this work (i.e., COads internal bond appears the weakest at CO/Ru). However, for CO adsorption on Pt, Os, and Ir, charge transfer from the CO regions of the 4σ̃ and 5σ̃ orbitals to the substrate surface, coupled with changes in the orbitals’ internal polarizations, play an important role in describing the COads internal bond strength. Figure 3f shows the behavior of the frontier orbital model (eq 2) and the extended π−σ model (eq 4) for CO adsorption on Pt, Os, Ir, and Ru surfaces. For CO/Os relative to CO/Ir, the extended π−σ model predicts slightly weaker COads internal bond, in contrast to upshift in the νCO. However, in this case, changes in the νCO are opposite to C−O COOP changes, which is indicative of hybridization defects, as stated before. The frontier orbital model fails to correctly predict changes in the COads internal bond strength between COads on Ru and Ir. We examine the changes in the σ̃-system between these two cases by recalling that the frontier orbital model only considers charge changes for the CO contribution of the 5σ̃ orbital. For CO/Ru relative to CO/Ir, the substantial charge transfer from the CO region of the 5σ̃ -CO/Ru toward the substrate is interpreted by the frontier orbital model as COads internal bond strengthening. However, the 5σ̃ orbital is a C−O nonbonding orbital for CO/ Pt,40 whereas it changes to slightly antibonding for COads on Os, Ir, and Ru, due to increased carbon s-type contribution to this orbital, which enhances the orbital’s antibonding character (Table 2). Moreover, for CO/Ru, the 4σ̃ and 5σ̃ orbitals polarize in the same direction as the corresponding orbitals of the free CO, which is indicative of COads internal bond weakening49 in contrast to bond strengthening predicted by the frontieorbital model. In general, the failure of the frontier orbital model is attributed to the fact that it only considers charge changes for the CO contributions to the 5σ̃ orbital and the 2π̃*-band without incorporating information from the remaining orbitals/bands of the σ̃-and π̃-systems and their polarizations within the CO unit. Therefore, for CO/Os relative to CO/Ir, the agreement of the frontier model with changes in the νCO is fortuitous. 3.2. CO Adsorption on Bimetallic PtRu and PtOs Alloys, Tertiary PtRuOs Alloys, and Quaternary PtRuOsIr Alloys. Figures 5 and 6 show the variations of the COads internal bond properties (i.e., C−O bond length, νCO, and C− O COOP) and C−Pt bond properties (i.e., C−Pt bond length, νCPt, C−Pt COOP, and Eads), respectively, for PtRu, PtOs, PtRuOs, and PtRuOsIr alloys for the configurations of Figure 2. The COads internal bond and C−Pt bond properties have been calculated with both the B3LYP and PBE0 functionals, where a systematic shift of these properties has been observed between these two calculations. Figure 7 shows the variations of the CO contributions to the π̃- and σ̃- system orbitals/bands, as well as

Figure 4. Contour plot of the charge density difference for CO/Pt calculated using the PBE0 functional relative to the B3LYP functional of the occupied orbitals per configuration. Charge flows from the blue areas to the red areas. Ten contours are drawn in a linear scale from −0.005 to 0.005 e/a03.

contour plot of the charge density difference for CO/Pt calculated using the PBE0 versus the B3LYP functional. The red areas (i.e., charge excesses) between oxygen and carbon and carbon and Pt show stronger C−O and C−Pt bonds, respectively, for the PBE0 calculations versus the B3LYP calculations, in agreement with trends from C−O and C−Pt calculated properties from Table 1. For CO/Os versus CO/Ir, the C−O COOP decrease is along with the decreased carbon atom s-type vacancies (Table 2), and this statement applies to COads on Os, Ir, and Ru. However, this argument fails for CO/ Pt versus CO/Ir. The C−O COOP values between these two systems show increased C−O orbital overlap for CO/Pt relative to CO/Ir, which is attributed to the significantly shorter C−O bond of the CO/Pt in agreement with the increased CO/ Pt O−Metal destabilizing interaction (Figure 3d). Figure 2 shows that C−O bond weakens following the substrate trend Ru > Ir > Os > Pt, as verified by νCO decrease and corresponding C−O bond length increase. We now focus on C−Metal bond trends for COads on Pt, Ru, Os, and Ir. Here, the Badger’s rule cannot associate changes between the C−Metal bond length and the νCMetal (Figure 3c). For example, using the B3LYP functional, for COads on Pt, Os, and Ru (Figure 3c), the νCMetal appears at about 440 ± 1 cm−1, whereas the corresponding C−Metal bond length varies from 1.88 Å in CO/Pt to 1.94 Å in CO/Os. Moreover, changes in the νCMetal are not always correlated with changes in the |Eads| in an analogous relationship (Figure 3c,e, B3LYP calculations). The lack of correlation between νCMetal and Eads has been reported.40,93 This is because νCMetal and Eads are derived from a local and global property of the potential energy surface, respectively, thus may not be always correlated. Therefore, the Eads is interpreted as an interaction of the adsorbate CO with the entire substrate surface, whereas νCMetal emphasizes the localized interaction of the adsorbate with the adsorbing metal atom. Here, the CO adsorption on the substrate surface, as described by Eads and νCMetal changes, shows a substantially stronger adsorption on Ir and weaker on Pt, Os, and Ru (Figure 3). The C−Metal distances and C−Metal COOP are minimal 10432


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the frontier orbital model (eq 2) and the extended π−σ model (eq 4) for the systems of this work using the B3LYP functional. Figures 5−7 also show the variation of the above properties for the average quantities of various configurations under the same Pt mole percent. 3.2.1. CO Adsorption on PtRu. Figure 5 shows that for CO/ PtRu, the COads internal bond weakens along with the increased Ru concentration in the PtRu alloy, as verified by C−O bond length elongations, C−O COOP decreases, and νCO downshifts for the average values of the above properties per Pt mole percent. The observed weakening of the COads internal bond due to alloying Pt with Ru atoms is in agreement with past corresponding infrared spectroscopy measurements and computational calculations. More specifically, Liu et al.27 using infrared spectroscopy on linearly bound COads on Pt and PtRu surfaces (0.1% CO coverage and 0.05 V electrode potential) reported νCO downshift of about 8 and 23 cm−1 for COads on Pt relative to PtRu(80:50) and PtRu(50:50), respectively. Periodic DFT calculations on CO/PtML/Ru(0001) alloys by Koper et al.93 show COads internal bond weakening relative to CO/Pt(111), as verified by the increased C−O bond length by 0.05 Å and the νCO downshift by 24 cm−1 for CO adsorbed atop at 1/3 ML surface coverage. Ishikawa et al.38 observed similar trends in DFT calculations for COads two-layer (Pt3) (Pt7) and (Pt3) (Ru3Pt4) clusters, where νCO decreased by 23 cm−1 due to alloying, while the C−O bond lengths only increased by 0.01 Å. Figure 6 shows that when the Pt mole fraction of CO/PtRu decreases, the C−Pt bond weakens. This is consistent with C−Pt elongations, C−Pt COOP, |Eads| decreases, and νCPt downshifts for average values of these properties per Pt mole percent (Figure 6). This statement also applies for CO adsorbed on the PtOs, PtRuOs, and PtRuOsIr alloys of this work. The concomitant decreases of the O−Pt COOP and Pt mole percent in the Pt-based alloys are in agreement with the above-discussed weakening of the C−Pt bond (Figure S2, top). Finally, the upshift of the Pt ed along with the decreased |Eads| are in agreement with the d-band center argument for the average values of these properties per Pt mole percent for COads on the Pt-based alloys of this work (Figures 6d and S2 bottom). It is important to examine how the Ru atoms placement in the PtRu alloy substrate affects the adsorbate properties C−O and C−Pt. Figure 8 shows the Pt-based alloy substrate configurations used here. At 83 Pt mole percent (i.e., Pt20Ru4 configurations), 4 Ru atoms are placed in the adsorbing, middle, and bottom substrate layers (Figure 8c,d). In the CO/ Pt20Ru4 case, the COads internal bond appears substantially weaker, when the Ru atoms are placed in the adsorbing substrate layer relative to their placement in the middle and bottom layers, as evidenced by changes in the C−O bond length, νCO, and C−O COOP (Figure 5). Table 3 shows the effects of the Ru placements in the Pt20Ru4 alloy for the CO contributions to the π̃- and σ̃-systems, the oxygen and carbon s and p orbital populations, the C−O COOP, and the νCO. When Ru atoms are placed in adsorbing layer, the weakening of the COads internal bond is mostly due to the increased backdonation to the CO region of the 2π̃*-band, which decreases C−O overlap and increases the C−O bond length (Table 3 and Figure 5). Figure 9 shows trends drawn between the carbon stype vacancies and C−O COOP and the overall s, p, and d vacancies of the adsorbing metal and the C−Pt COOP. The decreased C−O overlap in this case is in agreement with the decreased carbon s-type vacancies relative to CO/Pt.

Figure 5. (a) C−O, (b) νCO, and (c) C−O COOP per Pt mole fraction for the COads on Pt and Pt-alloy slabs of Figure 2. Colors and shapes indicate alloy substrate layers that contain Ru and/or Os atoms using the B3LYP functional calculations. Red dashed-dotted lines and blue dashed lines are trend lines drawn using the average values (where applicable) of the above properties for COads on alloys of the same Pt mole fraction and the CO/Pt, for B3LYP and PBE0 functionals, respectively. For PtRuOsIr alloys, dotted circles around the square points refer to Ir 8.33 mol %, whereas this value is half for points without a circle. 10433


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Figure 6. (a) C−Pt, (b) νCPt, (c) C−Pt COOP, and (d) Eads per Pt mole fraction for the COads on Pt and Pt-alloy slabs of Figure 2. Colors and shapes indicate alloy substrate layers that contain Ru and/or Os atoms using the B3LYP functional. Red dashed-dotted lines and blue dashed lines are trend lines drawn using the average values (where applicable) of the above properties for COads on alloys of the same Pt mole fraction and the CO/Pt, for B3LYP and PBE0 functionals, respectively. For PtRuOsIr alloys, dotted circles around the square points refer to Ir 8.33 mol %, whereas this value is half for points without a circle.

bottom layers, as indicated by changes in the C−Pt bond length, the νCPt, and Eads (Figure 6a,b,d). The above statement also applies to CO adsorption on Pt16Ru8 and Pt12Ru12 and is in agreement with past periodic DFT calculations by Liu and Nørskov.94 As the Pt mole percent in the PtRu alloy is reduced to 67 and 50% (i.e., Pt16Ru8 and Pt 12 Ru 12 configurations, respectively) the weakest CO adsorptions are observed on substrates with Ru atoms equally distributed between the middle and bottom layers relative to other PtRu substrate configurations under the same Pt mole percent (Figure 6b,d). For CO/Pt16Ru8, we also examine the case of adsorption on a substrate with all Ru atoms in the middle layer, which is shown to be the second best choice as a catalyst in terms of weak CO adsorption on surface at 67 Pt mole percent. For CO/PtRu, the weakest CO adsorption is observed on the Pt12Ru12 alloy with Ru atoms distributed as (0) (6)(6).

When Ru atoms are placed in the middle and bottom layers of the Pt20Ru4 alloy, the observed νCO downshifts relative to CO/Pt (i.e., COads internal bond weakening) are contrasted with the slightly higher C−O COOP (Ru atoms in the middle layer) and invariant C−O COOP (Ru atoms in the bottom layer) relative to CO/Pt (Table 3). In these cases, the COads internal bond length appears almost unchanged relative to CO/ Pt (Figure 5a), whereas changes in the C−O COOP among these different CO/Pt20Ru4 alloy configurations are in agreement with changes in their carbon s-type vacancies (Figure 9a). The significantly increased carbon s-type vacancies of these configurations relative to the one with Ru placed in the adsorbing layer indicate that this COads internal bond weakening is due to hybridization defects. The location of the Ru atoms in the Pt20Ru4 alloy also affects the C−Pt bond. The CO molecule is adsorbed substantially weakly on the Pt20Ru4, when Ru atoms are located in the middle layer relative to corresponding adsorption with Ru atoms in the adsorbing and 10434


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Figure 7. CO contributions to the (a) 5σ̃ orbital, (b) overall σ̃ system, (c) 2π̃*-band, (d) π̃-bonding, and the (e) frontier orbital, and the extended (f) π−σ model per Pt mole fraction for the COads on Pt and Pt-alloy slabs of Figure 2 using the B3LYP functional. Colors and shapes indicate alloy substrate layers that contain Ru and/or Os atoms. Red dashed lines are trend lines drawn using the average values (where applicable) of the above properties for CO adsorption on alloys of the same Pt mole fraction and the CO/Pt. For PtRuOsIr alloys, dotted circles around the square points refer to Ir 8.33 mol %, whereas this value is half for points without a circle. 10435


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Table 3. DFT Calculations Using the B3LYP Functional for the CO Contributions to the Adsorbate 4σ̃ , 5σ̃ , and 1π̃ Orbitals and d π̃ , d σ̃ , 2π̃*-Bands, the Carbon and Oxygen s and p Orbital Populations, and the C−O COOP and νCO for Various CO/Pt20Ru4 Alloy Configurationsa Ru location molecule/atom CO

Oxygen Carbon C−O COOP νCO (cm−1)

orbital/band

Adsorbing

Middle

Bottom

4σ̃ 5σ̃ dσ̃ 1π̃ dπ̃ 2π̃* s p s p

1.782 1.593 0.338 3.674 0.109 0.682 1.78 4.30 1.61 2.37 0.668 (0.677) 2117 (2159)

1.780 1.525 0.330 3.679 0.089 0.644 1.79 4.30 1.51 2.30 0.699 (0.702) 2123 (2164)

1.772 1.569 0.329 3.663 0.118 0.630

1.54 2.31 0.695 (0.696) 2130 (2170)

a

For CO/Pt, the carbon and oxygen charges, the C−O COOP, and νCO are 5.99 e, 8.13 e, 0.695, and 2132 cm−1, respectively. The values in parentheses refer to PBE0 calculations.

was minimal. A similar statement applies to corresponding calculations for COads on PtOs, PtRuOs, and PtRuOsIr alloys. Moreover, the relaxation of the substrates increased νCO and | Eads| by about 20 cm−1 and 0.35 eV, respectively, but did not change the trends drawn from the frozen substrate calculations. We must state that surface relaxation calculations on the CO/ Pt12Ru12 with no Ru atoms on the surface layer did not reveal Ru atom migration to the adsorbing layer. Figure 10 shows the charge density differences for COads on the best catalysts for DMFC operation per PtRu and per PtOs, PtRuOs, and PtRuOsIr compared with COads on Pt and on the best DMFC catalyst per PtRu, respectively. A similar figure with the best catalysts for hydrogen air fuel cells is shown in the Supporting Information (Figure S3). For CO adsorbed on the best DMFC PtRu catalyst relative to CO/Pt, the blue region between the carbon atom and the adsorbing metal is indicative of decreased adsorption on this PtRu substrate relative to Pt (Figure 10a). For CO/PtRu, the CO contributions to the 2π̃*-band and 5σ̃ orbital slightly increases and substantially decreases, respectively, as the Pt mole percent in the alloy decreases for the average values of these properties per Pt mole percent (Figure 7a,c). The frontier orbital model, as expressed by eq 2, shows COads internal bond strengthening as the Pt mole percent in the alloy decreases in direct disagreement with the observed νCO downshift discussed above. Figure 7b,d shows that the COads π-bonding and σ-antibonding monotonically decrease along with the decrease of the Pt mole percent in the PtRu alloy (average values per Pt mole percent). These competing effects and changes in the polarizations within π̃- and σ̃-system components lead to a weaker COads internal bond. We must state that the summation of the CO contributions to the 4σ̃ orbital and d σ̃ -band shows similar trends as the CO σ antibonding of Figure 7b (Figure S4). We also tested the extended π−σ model as described by eq 4 for COads on the Pt16Ru8 alloy with all Ru atoms located in the middle layer with varying CO coverage. As the coverage

Figure 8. The fcc Pt-based alloy configurations per layer. (a and b) 2, (c and d) 4, and (e and f) 6 same-type alloy atoms located at the appropriate substrate layer; (g) 2 mixed-type alloys atoms. Colors are as follows: Pt atoms are blue, Ru/Os are brown and black, and CO is red. CO overlayers are shown by green-line rectangles.

However, this Pt12Ru12 alloy cannot serve as DMFC anode catalyst due to the absence of oxophilic metals in the adsorbing layer as required by the bifunctional mechanism (eq 1, COads oxidation). Alternatively, this same Pt12Ru12 alloy could be used as anode catalyst for hydrogen air fuel cells, for which the bifunctional mechanism is far less important than the ligand effect.94 Therefore, both fuel cells may use the Pt12Ru12 alloy as catalyst of improved efficiency relative to Pt, but under different configurations. For use in DMFCs, the Pt12Ru12 with Ru atoms equally distributed in the three layers could be used as anode catalyst, whereas for hydrogen air fuel cells, the Ru configuration of (0)(6)(6) is preferred (Figure 6b,d). Similar to the CO/Pt case, we calculated the C−O, C−Pt, νCO, νCPt, and Eads of COads on the best Pt12Ru12 substrates for DMFC and hydrogen air fuel cells by allowing the upper-two substrate layers to relax during the geometry optimization process. We found that surface relaxation upshifted νCO by about 14−6 cm−1, whereas the effect on the C−O bond length 10436


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Figure 10. Contour plots of charge density differences of the occupied orbitals per configuration using the B3LYP functional for the best DMFC catalysts (i.e., weak COads and oxophilic atoms at the adsorbing layer). Frozen substrates are used. The alloy atoms arraignments for the Pt12Ru12, Pt12Ru6Os5, Pt19Os5, and Pt12Ru8Os2Ir2 substrates are Ru (4)(4)(4), Os (4)(0)(1), Ru (0)(4)(2) and Os (4)(0)(2), and Ru (0) (4)(4) and Os and Ir (2)(0)(0), respectively. Charge flows from the blue areas to the red areas. Ten contours are drawn in a linear scale from −0.005 to 0.005 e/a03. For CO/Pt, Eads are −1.40 and −1.96 eV for frozen and relaxed substrate calculations, respectively (B3LYP calculations). Numbers in square brackets show Eads values under surface relaxation calculations.

Figure 9. DFT calculations showing (a) the carbon 2s population relative to C−O COOP and (b) the adsorbing atom overall s, p, and d vacancies for COads on Pt (black point) and Pt-based alloys of this work (colored points) relative to C−Pt COOP. Colors and shapes indicate alloy substrate layers that contain Ru and/or Os atoms using the B3LYP functional. Red dashed-dotted lines and blue dashed lines are trend lines drawn using the B3LYP and PBE0 functionals, respectively. For PtRuOsIr alloys, dotted circles around the square points refer to Ir 8.33 mol %, whereas this value is half for points without a circle.

this case, the weaker COads internal bond for the second case is due to small increases of the 4σ̃ polarization toward carbon, which are not accounted for by eq 4. 3.2.2. CO Adsorption on PtOs. CO/PtOs has been studied before using cluster42 and periodic41 DFT for PtOs alloys with Os atoms located only in the adsorbing substrate layer. In this work, we expand the scope of CO/PtOs, by considering substrate configurations with Os atoms located at any of the three layers, at 8 and 21 Os mole percent (CO/Pt22Os2 and CO/Pt19Os5, respectively). For the CO/Pt19Os5, configurations (Figure 8c,d), an Os atom is always located at the bottom layer at an image location relative to the adsorbing atom. For CO/ PtOs, the calculated νCO values are approximately invariant of alloying Pt with Os atoms (i.e., constant C−O internal adsorbate bond), with the exception of the substrate configurations with Os atoms in the middle layer (frozen substrate, Figure 5b). The weakest CO adsorptions on PtOs are observed for configurations that contain Os atoms in the middle layer, irrespective of the Pt mole percent in the PtOs alloy (Figure 6b,d). For DMFCs, Os atoms must be also located at the absorbing layer of the PtOs alloy. In the case of PtOs atoms with Os at the adsorbing layer, the CO/Pt19Os5, shows weaker

increased to 1/4−1/2 ML, νCO was upshifted and C−O COOP was decreased. As stated before, these two opposite effects are indicative of hybridization defects as verified by the decrease in the carbon 2s population as the CO coverage increases, leading to weaker C−O bond. The extended π−σ model (eq 4) predicts C−O bond weakening in contrast to changes in the νCO but in agreement with changes in COOP (Table S3). A similar effect has been observed for CO coverage studies for adsorption on pure Pt.40 The extended π−σ model provides a good description of the COads internal bond strength for CO/PtRu for the average values per Pt mole percent (Figure 7f). However, there are also cases that eq 4 is not sufficient to describe changes in the COads internal bond strength, drawn by νCO variations. For example, for CO/Pt16Ru8, eq 4 predicts constant COads internal bond strengths between adsorption on the substrate configurations of Ru atoms equally distributed between the middle and bottom layers (νCO ∼ 2115 cm−1, B3LYP calculations) and the case where all Ru atoms are located in the middle layer (νCO ∼ 2106 cm−1, B3LYP calculations), in contrast to changes in the νCO. In 10437


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PtRuOsIr is as weak as adsorption on PtRu, and thus, they are good candidates to serve as anode catalysts for DMFC and hydrogen air fuel cells. We now examine COads on PtRuOsIr. Due to the low levels of Ir in the quaternary alloy,28 we limit the Ir to a single atom in the bottom layer (at an imaging location to the adsorbing metal) or two Ir atoms at the adsorbing layer (Figure 8g). The addition of Ir to the PtRuOs alloy has minimal effect on the C− Pt bond. Figures 10 and S3 show that the best PtRuOsIr catalyst is as good as the best PtRuOs catalyst. We recall that for DMFCs, Ir enhances C−H activation and Os promotes water adsorption, and thus, both must be located in the adsorbing layer, which is in agreement with our proposed Pt12Ru8Os2Ir2 catalyst shown in Figures 10 and S3. In the last subsection, we found that the CO adsorption on the most efficient PtRu catalyst is governed by increased back-donation to the CO region of the 2π̃*-band accompanied by increased change in the CO region of the 4σ̃ orbital: Both of these effects weaken the COads internal bond relative to adsorption on Pt (Figure 5). However, this is not the case for adsorption on the most efficient DMFC PtOs, PtRuOs, and PtRuOsIr catalysts, where the decreased charge on the CO region of the 2π̃*-band relative to CO/Pt is indicative of COads internal bond strengthening. In these cases, the observed COads internal bond weakening relative to adsorption on Pt is due to the increases in the oxygen 2s of the 4σ̃ orbitals (i.e., increased C− O antibonding), which offsets increased π̃-system bonding. Figure 7e shows that the frontier orbital model fails to predict changes in the COads internal bond due to alloying Pt with Ru and Os and Pt with Ru, Os, and Ir atoms. However, the extended π−σ model correctly predicts changes for this bond, with the exception of two cases: the CO/Pt14Ru6Os4 and CO/ Pt13Ru6Os4Ir with Ru and Os placed as (0)(4)(2) and (4)(0) (0), respectively, and the Ir atom at the bottom layer. For these cases, the B3LYP calculated C−O COOP values (and the carbon s-type vacancies, Figure 9a) are at the maximum value of 0.704 (for CO/Pt, C−O COOP is 0.695), which in turn maximize the C−O sp hybridization defect among all Pt-alloy systems of this work. Similar to the CO/PtRu case, the 4σ̃ polarization toward carbon, which is fully accounted for by the present form of the extended π−σ model, contributes to the weakening of the COads internal bond.

CO adsorption relative to CO/Pt22Os2. The contour plot of the charge density difference for COads on the best DMFC PtOs and PtRu catalysts shows red areas (i.e., charge excesses) between carbon and oxygen and carbon and the adsorbing Pt metal, which are indicative of stronger C−O and C−Pt bonds, respectively for COads on PtOs relative to PtRu (Figure 10a,c). Therefore, PtOs catalysts are less efficient than PtRu catalysts for DMFC operation, and thus, not preferred. This statement also applies to hydrogen air fuel cells (Figure S3). Moreover, surface relaxation calculations for CO/Pt19Os5 (best DMFC PtOs alloy) show that νCO and |Eads| are upshifted by 39 cm−1 and 0.9 eV, respectively, relative to corresponding frozen substrate calculations, and thus, PtOs alloys are less efficient anode catalysts relative to both PtRu and Pt. We now examine changes in the COads internal bond strength for CO/Pt19Os5 with Os atoms located in the middle layer relative to CO/Pt (frozen substrate calculations). In this case, the νCO downshift is indicative of COads internal bond weakening, accompanied by minor increase in the C−O COOP. Here, the decreased CO contribution to the 2π̃*-band and the increased overall COads π-bonding are indicative of stronger COads internal bond relative to CO/Pt (Figure 7c,d). However, changes in the σ̃-system more than offset changes in the π̃-system. More specifically, increased charge in the CO region of the 5σ̃ orbital weakens the COads internal bond in agreement with the frontier orbital model and the extended π−σ model. We must state that the Badger’s rule correlating C−O and νCO fails for CO/Pt22Os2 and CO/Pt19Os5 with Os atoms located in the middle layer (Figure 5a,b). 3.2.3. CO Adsorption on PtRuOs and PtRuOsIr. The study of COads on PtRu alloys revealed that high efficiency PtRuOs and PtRuOsIr DMFC and hydrogen air fuel cell anode catalysts must have high content of Ru atoms in the alloy (the alloy Pt mole percent should not be less than about 50 for fcc phase). Moreover, for hydrogen air fuel cells, high efficiency anode catalysts do not contain Ru atoms in the adsorbing layer. Concurrent alloying of Pt with Ru and Os atoms (and Ir atoms for PtRuOsIr alloys) weakens the C−Pt bond as verified by νCPt downshifts and |Eads| decreases along with decreased Pt mole percent in the Pt-based alloy (Figure 6b,d), with the exception of CO/Pt16Ru6Os2 configuration with Ru and Os placed as (4) (0)(2) and (0)(0)(2), respectively. In this case, the νCPt is upshifted relative to CO/Pt, whereas the Eads is unaffected of the alloying. The presence of Ru atoms in the adsorbing layer significantly strengthens the C−Pt bond relative to the other two CO/Pt16Ru6Os2 configurations presented here that do not contain Ru atoms in this layer. The weakest CO adsorptions are observed for CO/Pt12Ru8Os6, with Ru and Os atoms at (0)(4) (2) and (4)(0)(2), respectively, and for CO/Pt12Ru8Os4 with Ru and Os atoms at (0)(4)(4) and (4)(0)(0), respectively (50 Pt mole percent). Both of these configurations contain Os atoms in the absorbing layer, which can serve as water promoters for use in DMFCs. The contour map charge differences for COads on the best DMFC PtRuOs and PtRu catalysts show small changes in the C−O and C−Pt bond strengths. More specifically, the blue area between carbon and Pt is indicative of slightly weaker C−Pt bond. The small red area on the oxygen side is due to excess charge from the σ̃system, which will weaken the C−O bond. However, this effect is more than offset by decreased carbon pxy which strengthens the C−O bond. The same statement applies for COads on best PtRuOsIr catalyst (Figure 10d). The surface relaxation calculations reveal that CO adsorption on PtRuOs and

4. CONCLUSION The CO adsorption on Pt, Os, Ir, and Ru metals and on the Ptbased alloys PtRu, PtOs, PtRuOs, and PtRuOsIr has been studied using periodic DFT. The B3LYP and PBE0 functionals were used for analysis of the COads internal bond and C−Metal bond. The COads internal bond strength has been correlated with changes in the C−O distance, νCO, and C−O COOP, whereas the C−Metal distance, νCmetal, C−Metal COOP, and Eads were used as a measurement of the CO interaction with the substrate surface. We observed that both functionals provided qualitative agreement with corresponding experimental measurements. The C−O, C−Metal, corresponding stretching frequencies, and Eads are systematically shifted between the two functional calculations. The C−O and C−Metal COOP, which measure orbitals overlaps between two atoms, were correlated with the carbon s-type and the overall s, p, and dvacancies of the adsorbing metal, respectively. Opposite trends between νCO and C−O COOP and νCMetal and C−Metal COOP are indicative of hybridization defects, which weaken the C−O and the C−Metal bonds, respectively. Moreover, 10438



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DFT PBE0 calculations for CO/Pt(111) predict the atop site to be the correct adsorption site in agreement with experiments. We systematically vary the Pt mole percent in Pt-based alloys, as well as the location of the alloy atoms in the mixedmetal substrates. The frontier orbital model, which only considers changes in the populations of 5σ and 2π* CO molecular orbitals, fails to predict changes in the COads internal bond for COads on PtRu, PtRuOs, and PtRuOsIr, whereas it is acceptable for COads on PtOs. However, the extended π−σ model (eq 4) correctly predicts changes in the COads internal bond strength for adsorption on the majority of the systems examined here. This model considers changes in the CO contributions and polarizations of the entire π̃- and σ̃-systems, in contrast to the limited information contained in the frontier orbital model. CO coverage studies on a single Pt16Ru8 alloy revealed C−O hybridization defects as coverage increases, which is indicative of weaker C−O bond and correctly described by the extended π−σ model. Surface relaxation calculations for COads on Pt and on limited number of Pt-based alloys reveal a systematic upshift of the νCO, νCPt, and Eads. Weak CO adsorption on the substrate surface is used as an indication of the catalyst’s efficiency. In this sense, the highest efficiency PtRu, PtOs, PtRuOs, and PtRuOsIr catalysts are identified and their properties are compared to each other. For the highest efficiency Pt-based catalysts, adsorption of CO is substantially stronger on the PtOs surface than the corresponding adsorption on the surfaces of the other three Pt-based alloys. We identified the structure of the highest efficiency DMFC and hydrogen air fuel cell PtRu, PtRuOs, and PtRuOsIr catalysts. All of these substrates are at 50 Pt mole percent with Ru atoms and for PtRuOs, and PtRuOsIr Ru atoms are not present at the adsorbing layer. Here, the PtRuOs and PtRuOsIr with Os and Ir atoms at the adsorbing layer offer about the same CO adsorption relative to the PtRu catalyst, when surface relaxations effects are included. Moreover, the presence of the oxophilic Os atoms in the adsorbing layer will promote water activation, which is necessary for DMFC applications, in agreement with the bifunctional model. Although from our analysis here the adsorption on the best PtRu, PtRuOs, and PtRuOsIr shows no differences in terms of CO adsorption strength on the substrate surface, the presence of the Ir atoms in the adsorbing layer accelerates methanol oxidation, and thus increases the PtRuOsIr catalyst efficiency for use in DMFC applications. As was found for mixed metal alloys for nickel metal hydride batteries, this works suggests that there may be a fundamentally rational reason why ternary and quaternary mixed metal anode should be developed for DMFC and hydrogen air fuel cell anodes.

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AUTHOR INFORMATION

Corresponding Author

*Address: 1201 W. University Drive, Edinburg, TX 78539. Phone: (956) 665-8761. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding was provided by the Army Research Office Grant W911NF-12-1-0346.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b02086. Information about CO/Pt(111), the Ir basis set, figures on O−Pt COOP, ed, and 4σ̃ + d σ̃ , the CO coverage effect on Pt16Ru8, and contour maps for the best hydrogen air fuel cell anode catalysts (PDF) 10439


Article

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