density functional theory study of pt m alloy surface ... · important surface property that...

Density Functional Theory Study of Pt3M Alloy Surface Segregationwith Adsorbed O/OH and Pt3Os as Catalysts for Oxygen ReductionReactionHo-Cheng Tsai,† Ted H. Yu,†,‡ Yao Sha,† Boris V. Merinov,*,† Pu-Wei Wu,§ San-Yuan Chen,§

and William A. Goddard, III*,†

†Materials and Process Simulation Center, California Institute of Technology, 1200 East California Blvd., m/c 139-74, Pasadena,California 91125, United States‡Department of Chemical Engineering, California State University, Long Beach, California 90840, United States§Department of Materials Science and Engineering, National Chiao Tung University, Hsin-Chu 300, Taiwan ROC

*S Supporting Information

ABSTRACT: Using quantum mechanics calculations, we have studied the segregation energy withadsorbed O and OH for 28 Pt3M alloys, where M is a transition metal. The calculations foundsurface segregation to become energetically unfavorable for Pt3Co and Pt3Ni, as well as for themost other Pt binary alloys, in the presence of adsorbed O and OH. However, Pt3Os and Pt3Irremain surface segregated and show the best energy preference among the alloys studied for bothadsorbed species on the surface. Binding energies of various oxygen reduction reaction (ORR)intermediates on the Pt(111) and Pt3Os(111) surfaces were calculated and analyzed. Energybarriers for different ORR steps were computed for Pt and Pt3Os catalysts, and the rate-determining steps (RDS) were identified. It turns out that the RDS barrier for the Pt3Os alloycatalyst is lower than the corresponding barrier for pure Pt. This result allows us to predict a betterORR performance of Pt3Os compared to that of pure Pt.

1. INTRODUCTION

The efficiency of the oxygen reduction reaction (ORR) at thecathode of a proton exchange membrane fuel cell (PEMFC) isa critical issue for its commercial application.1−3 To date,although many catalysts have been tested for improving theORR efficiency, including Pt-based alloys4−13 and nonpreciousmetal catalysts,14−18 the best ORR catalysts remain Pt and Pt-based binary alloys, such as Pt3Ni and Pt3Co.

9,13 Although theexact mechanism responsible for the superior ORR perform-ance of the Pt3Ni alloy has not been clearly identified yet, it wasrealized that in alloyed Pt3M electrocatalysts, where M is atransition metal, the surface segregation of Pt atoms, where thesurface layer is 100% Pt (so-called Pt-skin) and the second layerhas ∼50% of the alloyed solutes9,19 (e.g., Co, Ni), is animportant surface property that contributes significantly to theimproved ORR catalytic activity.6,8,9,13,19,20 Previous studies ofbare Pt3M(111) alloy surfaces21,22 predicted the Pt surfacesegregation for a number of Pt3M alloys (see Figure 1). Surfacesegregation energy was studied using quantum mechanics(QM) calculations by comparing the energy of the “surfaceuniform” slab, where the slab layers are all 75% Pt, and thedesirable “surface segregated” slab, where the top layer is purePt, and the second layer is enriched in solute metals.22 Pt3Coand Pt3Ni were predicted to be surface segregating (see Figure1), while Pt3Fe was only slightly surface segregating, inagreement with experiments performed.13,23 However, during

the ORR process, various adsorbates are formed on the catalyticsurfaces. Such adsorbates, as Oad and OHad, have been knownto accumulate on the Pt surface as ORR intermediates.24,25

According to ref 26, when one of those adsorbates, Oad, is onthe Pt surface, Pt3Co and Pt3Ni are no longer energeticallyfavorable to have surface segregation. These adsorbates can lead

Received: July 16, 2014Revised: October 15, 2014Published: October 22, 2014

Figure 1. Surface segregation of Pt3M binary alloys.22

Article

pubs.acs.org/JPCC

© 2014 American Chemical Society 26703 dx.doi.org/10.1021/jp507103c | J. Phys. Chem. C 2014, 118, 26703−26712

pubs.acs.org/JPCC

to undesirable leaching of the alloy solute onto the catalystsurface,26−29 which can alter the surface segregation and causethe alloyed solute leaching from the surface into the electrolyte.For instance, Co atoms in the Pt3Co catalyst were found todissolve into the electrolyte during an extended fuel celloperation,30−32 and the electrolyte is filled with undesirable Coproducts. Consequently, the surface layers become essentiallyCo-free, and only Pt atoms are presented in the surface regime(so-called Pt-skeleton structure13) reflecting a Pt-like electro-catalytic behavior. A similar behavior of the Ni leaching hasbeen observed in Pt3Ni.

33 This metal dissolution and surfaceoxide formation lead to the descending activity and stability.4

Therefore, it is important to find Pt alloy catalysts where thesolute metal is energetically favored to remain subsurface, whenthe surface is exposed to adsorbed ORR species.In this paper, we have examined the segregation energy for

28 Pt3M alloys with adsorbed Oad and OHad on the surfaceusing QM calculations. Looking ahead, we say that calculationsfound surface segregation to become energetically unfavorablefor Pt3Co and Pt3Ni, as well as for the most other Pt binaryalloys, in the presence of adsorbed Oad and OHad. However,Pt3Os and Pt3Ir remain surface segregated and show the bestenergy preference among the alloys studied for both adsorbedspecies on the surface. PtIr materials have been studied earlier(see, for instance, refs 34 and 35); therefore, we selected thePt3Os system for our further theoretical investigation. Bindingenergies of various ORR intermediates and reaction energybarriers for various pathways of different ORR mechanisms onthe Pt3Os(111) surface were calculated, analyzed, anddiscussed.

2. THEORETICAL METHODSIn our theoretical study, we used QM calculations to investigatesurface metal segregation, the unique binding-site preferencesdue to the placement of sublayer alloying atoms for allintermediates involved in the ORR on the Pt and Pt3Os(111)surfaces, and the consequent changes to the reaction barriersand ORR mechanisms.All QM calculations were carried out using the SeqQuest

software36 with optimized double-ζ plus polarization qualityGaussian-type orbitals and the Perdew−Becke−Ernzehof(PBE)37 functional of density functional theory (DFT) in thegeneralized gradient approximation (GGA).38,39 Small corenorm-conserving angular momentum projected pseudopoten-tials40−44 were employed in our calculations. All calculationswere performed with spin optimization, and the average spin

was ∼2. The reciprocal space grid was 5 × 5 × 0 for the slabcalculations and 12 × 12 × 12 for the bulk lattice constantcalculations. We used four layer 2 × 2 cell slabs for thesegregation study, while six layer 2 × 2 cell slabs were appliedfor the binding energy and most reaction barrier calculations.Therefore, each layer contains four atoms, and the surfacecoverage is ∼1/4. We consider the 1/4 coverage to be largeenough to avoid irrelevant cross cell interaction, while it stillkeeps the unique properties of the segregated Pt3Os surface.For the O hydration step, 3 × 3 and 2 × 4 cell slabs were usedfor Pt and Pt3Os, respectively, to avoid artificial hydrogenbonding. The periodic cell parameter of the slab corresponds tothat of the optimized Pt3Os bulk structure with the latticeconstant of 3.94 Å, slightly smaller than 3.98 Å for Pt. Allcharges were obtained from the Mulliken population analysis.To represent the effects of solvent polarization, an implicit

model45 based on the Poisson−Boltzmann approximation wasapplied.46−48 We showed that our implicit solvation modelcorrectly reproduces the solvation energies of Oad, Had, OHad,and H2Oad calculated using explicit water layers.45 TheMulliken charges were used as inputs to our solvation model.The binding energies (BE) are calculated as the energy gain

for species to adsorb to the surface, i.e.,

= − −+E E EBEgas surf A,gas surf,gas A,gas

where A is an adsorbate. For the solution phase, the solventstabilization is added directly to the binding energy in order toisolate the influence of water. This leads to

= − −+E E EBEsolv surf A,solv surf,solv A,gas

The solvent phase binding energy does not include the solventeffect of the adsorbate itself, because most of the species areradicals that do not have well-defined solvation energies forcomparison. The lack of these energy data for radical speciesdoes not affect the barrier calculations, since the reactants,products, and transition states are all surface species and thedifferences in individual solvation energies eventually cancelout.All barriers are calculated as the energy difference between a

transition state and surface reactant sites, using the nudgedelastic band (NEB) method.49,50 We disregard the energy thatreactants need to migrate from the globally preferred sites tothe reacting sites, as these barriers would correspond only tovery low surface coverage. This condition is not typical for theconventional fuel cell operation. However, to avoid misunder-standing and show certain details of the ORR mechanism, the

Figure 2. Surface segregation energy with adsorbed O (left) and OH (right).

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp507103c | J. Phys. Chem. C 2014, 118, 26703−2671226704

spontaneous O migration step has been added to our figuresrepresenting possible ORR mechanisms. While both Lang-muir−Hinshelwood and Eley−Rideal mechanisms may occur inORR at electrode potentials near 1.23 eV (the reductionpotential of the ORR established by the Nernst equation),51

here we consider only the Langmuir−Hinshelwood typereactions. The Eley−Rideal mechanism of ORR is consideredelsewhere (see, for instance, refs 52 and 53).

3. RESULTS AND DISCUSSION3.1. Surface Segregation Effect. We have performed QM

calculations to examine 28 Pt binary alloys to identify goodsegregating alloys that maintain the favorable segregatingproperty when the oxidative species, Oad and OHad, areadsorbed on the surface. For calculating the segregation energy,we used the lowest energy Oad and OHad surface binding sitesfrom the previous Pt3Ni study.

20

Figure 2 displays the segregation energy with adsorbed Oadand OHad on the surface. For the OH adsorbed surfacesegregation, six solute metals, showing favorable segregationenergies for the corresponding Pt3M alloys, are Os, Re, Ir, Ru,Tc, and Rh. For the Oad adsorbed surface segregation, only twosolute metals, Ir and Os, show favorable segregation energies.All of these solute metals are considered difficult to oxidize andhave more positive reduction potential versus hydrogen. All butone (Tc) are considered as noble metals. As a general rule,segregation becomes unfavorable when Oad and OHad areadsorbed on a metal that is easily oxidized.The segregation energy can vary significantly, when Oad or

OHad is adsorbed on the surface. For example, withoutadsorbed species, the best five segregation energies were Ptalloyed with Re, W, Os, Tc, and Mo.22 However, thesegregation energy of Pt3W and Pt3Mo changes from stronglyfavorable to strongly unfavorable with adsorbed Oad and OHad,as both W and Mo are known to easily react to form oxides.Our segregation result is consistent with results of othertheoretical investigations, in which a similar approach was usedto study some Pt-based alloy and core−shell catalysts,34,35 aswell as with the experimental results obtained by Abrams et al.for Pt deposit on the Au foil.27 They found that the Au atomsmigrated to the surface because of a lower Au surface energy.

However, exposing the sample to air makes the deactivatedsample partially reactive again. This reactivation was ascribed tothe adsorbed O from the air, which favors the Pt to segregateback to the surface (adsorbate-induced surface segregation).Our calculation on the Pt3Au alloy also indicates that Pt showsbetter segregation property with adsorbed O on the surface(see Figure 2). We found only a few Pt binary alloys withfavorable surface segregation energy in the presence ofadsorbed Oad or OHad. Out of them, only Pt3Os and Pt3Irshow surface segregation in the presence of both adsorbedspecies. Therefore, these alloys are predicted to be moreresistant to solute metal leaching, than Pt3Co or Pt3Ni.However, according to the Pt−Os phase diagram,54 only 20%Os can be mixed into the Pt structure, whereas Ir cannot bemixed into the Pt structure at all.55 Despite this disappointinginformation, synthesis of PtOs nanoparticles even with themolar ratio of 1:156 and PtIr nanoparticles with the molar ratioof 3:157,58 were reported. In this paper, we carried outcomputations of binding energies and energy barriers of theORR intermediates on the Pt3Os(111) alloy surface.

3.2. Binding Energy of ORR Intermediates. 3.2.1. Bind-ing Site Notation. Generally, a closest packed (111) surface offcc structured metals has four types of sites:On-top, bonded to one Pt (μ1), denoted as t,Bridging, between two Pt (μ2), denoted as b,Bridging, between three Pt (μ3-fcc) but in the fcc position

(not above atoms of the top or second layer), denoted as f,Bridging, between three Pt (μ3-hcp) but in the hcp position

(above atoms of the second layer), denoted as h.However, due to strong segregation, the Pt3Os(111) surface

has 100% Pt in the first layer, 50% Os and 50% Pt in the secondlayer, and 25% Os and 75% Pt in the four remaining layers. Wefind that the binding energies of the ORR intermediates to thepure Pt layer strongly depend on the nature of the second layeratoms. Figure 3 shows notations of these sites and details of thedifferences between various sites.For the first and second layers, there are two types of top

sites: t1 with one Os neighbor in the second layer and t2 withtwo Os neighbors. Considering also the third layer, we candistinguish t1a with no Os in the third layer directly beneath the

Figure 3. Binding sites on the Pt3Os(111) surface (a). For top sites t1 and t2, the triangle indicates the sublayer atoms (b). t1 has one Os atombeneath it, while t2 has two. For bridge sites, the bridge itself is shown as the thick black line, while the two terminals of the black line connect thetwo surface atoms forming the bridge site. The trapezoid beneath are sublayer atoms. b0-b3 have from 0 to 3 Os atoms in the sublayer. An fcc site isin the center of a surface triangle (shown as solid triangle). f1 and f2 differ in the sublayer triangle beneath the surface triangle: f1 has one Os atombeneath, while f2 has two. An hcp site is also in the center of a surface triangle; it has one sublayer atom beneath: for h1 it is Os, while for h2 it is Pt.



surface and t1b with one Os. All t2 sites are the same (see Figure3).For the top two layers, there are four μ2 bridge sites,

depending on the number of Os atoms underneath: b0, b1, b2,and b3 with 0, 1, 2, and 3 Os atoms in the second layer. Withadding the third layer, there appear two subtypes for b1, b2, andb3, depending on the distance to the Os atom in the third layer.We denote the subtypes closer to the third layer Os as b1a, b2a,b3a and the others as b1b, b2b, b3b, respectively (Figure 3).Considering only two top layers, two fcc sites can be

distinguished: f1 and f2 with one and two Os atoms in thesublayer triangle, but adding the third layer splits f1 into f1a andf1b with f1a on top of the third layer Os and f1b on top of thethird layer Pt.Similarly, there are two hcp sites related to the two top

layers: h1 and h2. Here h1 is on top of the Os sublayer, while h2is on top of the Pt sublayer. Adding the third layer splits the h1site into h1a and h1b with one Os atom and without Os atoms inthe projected triangle of the third layer atoms, as shown inFigure 3.3.2.2. Binding Energies of ORR Intermediates. First, we

studied the preference of Had, Oad, OHad, H2Oad, O2ad, OOHad,and HOOHad ORR intermediates on various binding sitesshown in Figure 3. Tables 1 and 2 list binding energies of theabove-mentioned intermediates on the Pt3Os(111) alloysurface in gas phase and solution, respectively. It should benoted that considering energetics (binding energies andreaction barriers) related to ORR on the Pt3Os(111) surfaceand comparing it with ORR energetics on pure Pt, more

attention should be paid to the solvation phase, because thisphase better corresponds to the PEMFC operating conditionsthan the gas phase.

H Binding. For the pure Pt surface, the binding energy of Had

is 2.70−2.80 eV in gas phase and 2.81−2.87 eV in solvatedphase. The strongest binding energy corresponds to the topsite.The preferred binding site for Had on the Pt3Os(111) surface

is also the top site, t1b in gas phase, with a binding energy of2.65 eV, followed by t2 and t1a with a binding energy of 2.57and 2.55 eV, respectively. Under solvation, the preferred sitebecomes t2 with a binding energy of 2.84 eV, followed by t1b,b2b, b3b, t1a, f1a, b3a, f2, and b0 with binding energies of 2.63−2.79eV. Thus, for Pt3Os, Had can migrate relatively easily in alldirections to react with other ORR species.

O Binding. On pure Pt, Oad binds strongly to the fcc sitewith a net energy of 3.66 eV in the gas phase and 4.36 eV in thesolvated phase. The huge solvation stabilization arises fromelectrostatics due to appearance of the strong dipole at surfaceOad atoms.For Pt3Os in the gas phase, the strongest binding of Oad is at

the f1a site, 3.55 eV, followed by the f2 site with a bindingenergy of 3.48 eV. With solvent, the f2 and f1b sites becomedominant with stronger binding energies, 5.18 and 4.97 eV,respectively. All other binding sites are significantly less stablethan f2. This means Oad, formed from O2ad dissociation,strongly prefers to occupy the f2 site and most probably nofurther migration occurs to other sites.

Table 1. Binding Energies (eV) of Various ORR Species at Different Sites on Pt3Os and Pt20 in the Gas Phase

site H O OH O2a OOHb H2O

c H2O2a

Pt3Ost1a −2.55 −2.33 −1.86 −0.95 (−0.94) −0.16t1b −2.65 −2.50 −1.95 −1.06 (−0.94) −0.19t2 −2.57 −2.57 −2.03 −1.02 (−0.84) −0.17b0 −2.53 −3.05 −1.84 −0.49 −0.25b1a −2.49 −3.03 −1.87 −0.36 −0.33b1b −2.51 −3.02 −1.91 −0.39 −0.26b3a −2.46 −3.02 −2.16 −0.53 −0.37b3b −2.38 −2.87 −2.06 −0.47 −0.38b2a −2.48 −3.11 −2.20 −0.59 −0.37b2b −2.44 −3.02 −1.99 −0.44 −0.32f2 −2.52 −3.48 −1.86 −0.29f1a −2.49 −3.55 −2.17 −0.50f1b −2.42 −3.34 −2.04 −0.30h1a −2.45 −3.07 −2.12 −0.36h1b −2.40 −3.05 −2.02 −0.31h2 −2.48 −3.21 −1.90 −0.34Best −2.65 −3.55 −2.20 −0.59 −1.06 −0.19 −0.38

PtT −2.80 −2.50 −2.23 −1.06 −0.22b −2.70 −3.10 −2.25 −0.40 −0.27f −2.72 −3.66 −2.22 −0.46h −2.70 −3.28 −2.28 −0.35Best −2.80 −3.66 −2.28 −0.46 −1.06 −0.22 −0.27

aThe center of the O−O bond is used to denote the binding sites of O2 and H2O2. Thus, b means that two O atoms are located approximately onthe top of the surface atoms with the O−O bond center at the b site. The f and h binding sites are defined as one O atom is located at the top siteand the other at the b site with the O−O bond center at the f or h site, respectively. bThe position of the first O atom is used to denote the bindingsites of OOH with the second O atom close to the b site. The numbers in parentheses correspond to the OOH binding at the t1a, t1b, t2 sites with thesecond O atom close to the f1b, f1a, f2 sites.

cThe position of the O atom is used to denote the binding sites of H2O with the O atom at the top site ofPt and two O−H bonds parallel to the surface.



OH Binding. On pure Pt, OHad has almost the same bindingenergy at all sites, 2.22−2.28 eV in the gas phase and 2.57−2.77eV in solution.For Pt3Os, the most preferred site in gas phase is b2a followed

by f1a, b3a, h1a and b3b with binding energies of 2.06−2.20 eV. Insolution, t2 is the most preferred site with a binding energy of2.63 eV, followed by slightly weaker binding at the b2a site, 2.60eV; f1b, 2.59 eV; f1a and h1a, both 2.56 eV; b3a, 2.50 eV. Thebinding energy of OH at all possible sites ranges within ∼0.2and 0.4 eV for Pt and Pt3Os, respectively. This probablyindicates the ability for easier OHad migration on pure Pt andharder OHad migration on the Pt3Os surface.In the gas phase, both O and OH binding on Pt3Os are

weaker than on Pt, 3.55 vs 3.66 eV and 2.20 vs 2.28 eV,respectively. In solution, OH still binds weaker on the Pt3Os(111) surface (2.63 vs 2.77 eV), whereas the O bindingbecomes stronger (5.18 vs 4.36 eV). A possible reason for thischange in solution might be a strong dependence of the solventeffect on a charge dipole. The dipole related to the O bindingon Pt3Os is obviously stronger than that related to the OHbinding (Figure S1, Supporting Information). Furthermore, thecharge dipole between Os and Pt makes the solvent effect forPt3Os more significant than for an almost uniformly distributedslab of pure Pt.O2 Binding. For pure Pt, we find that the binding energy of

O2ad is 0.46 eV in the gas phase and 0.87 eV in solution, withthe difference for various sites ranging within 0.11 and 0.17 eVfor the gas phase and solution, respectively.

For Pt3Os, O2ad prefers to bind to the surface at the b2a sitewith a binding energy of 0.59 eV in the gas phase followed byb3a, f1a, b0 and b3b with similar binding energies of 0.53, 0.50,0.49, and 0.47 eV, respectively. Overall, the correspondingbinding energies vary from 0.29 to 0.59 eV. In the solvatedPt3Os phase, the strongest binding energy, 1.06 eV, is for the f1asite of O2. The following energetically favored sites are b2a (0.94eV), b3a and b1b (0.85 and 0.84 eV, respectively), f1b (0.79 eV).Such a strong binding of O2ad at the f1a site should impede O2ad

migration to other sites.OOH Binding. For pure Pt, OOHad binds to the top sites

with the terminal O bonded to the Pt and the OOHad planeparallel to the surface. OOHad prefers to have the O−O bondheading to an adjacent Pt atom. This leads to a binding energyof 1.06 eV in the gas phase and 1.52 eV in solution.On the Pt3Os(111) surface, OOHad binds to the surface at

the t1b site with a similar strength, 1.06 eV, in the gas phase, asin pure Pt. The corresponding binding energy of OOHad atother stable binding sites, t1a and t2, are 0.95 and 1.02 eV,respectively. In solution, OOHad binds to the Pt3Os(111)surface significantly stronger at all stable binding sites, by 0.5−0.6 eV, than in the gas phase. The preferred site is t2 with abinding energy of 1.67 eV, which is by ∼0.15 eV stronger thanthe corresponding binding energy for pure Pt. It should also benoted that the OOHad binding at the t2 site with the O−Obond toward f2 is more stable than heading to another Pt atom,but the first O atom still binds to the t2 top site. It is unstablefor the first O atom to bind to other sites. Thus, once formed,

Table 2. Binding Energies (eV) of Various ORR Species at Different Sites on Pt3Os and Pt20 in Solution

site H O OH O2a OOHb H2O

c H2O2a

Pt3Ost1a −2.67 −3.08 −2.40 −1.46 (−1.67) −0.45t1b −2.79 −3.18 −2.46 −1.61 (−1.44) −0.49t2 −2.84 −3.34 −2.63 −1.61 (−1.32) −0.52b0 −2.63 −3.75 −2.32 −0.77 −0.53b1a −2.57 −4.36 −2.36 −0.68 −0.58b1b −2.59 −4.78 −2.37 −0.84 −0.52b3a −2.64 −3.87 −2.50 −0.85 −0.62b3b −2.68 −4.24 −2.34 −0.74 −0.60b2a −2.62 −3.86 −2.60 −0.94 −0.62b2b −2.69 −4.46 −2.34 −0.77 −0.54f2 −2.63 −5.18 −2.41 −0.77f1a −2.66 −4.25 −2.56 −1.06f1b −2.54 −4.97 −2.59 −0.79h1a −2.58 −4.21 −2.56 −0.75h1b −2.42 −4.63 −2.39 −0.59h2 −2.63 −3.88 −2.32 −0.74Best -2.84 −5.18 −2.63 −1.06 −1.67 −0.52 −0.62

Ptt -2.87 −3.09 −2.77 −1.52 −0.58b −2.82 −3.73 −2.63 −0.73 −0.61f −2.85 −4.36 −2.57 −0.87h −2.81 −3.92 −2.64 −0.70Best −2.87 −4.36 −2.77 −0.87 −1.52 −0.58 −0.61

aThe center of the O−O bond is used to denote the binding sites of O2 and H2O2. Thus, b means that two O atoms are located approximately onthe top of the surface atoms with the O−O bond center at the b site. The f and h binding sites are defined as one O atom is located at the top siteand the other at the b site with the O−O bond center at the f or h site, respectively. bThe position of the first O atom is used to denote the bindingsites of OOH with the second O atom close to the b site. The numbers in parentheses correspond to the OOH binding at the t1a, t1b, t2 sites with thesecond O atom close to the f1b, f1a, f2 sites.

cThe position of the O atom is used to denote the binding sites of H2O with the O atom at the top site ofPt and two O−H bonds parallel to the surface.



OOHad will probably not migrate on the surface, but the O−Obond could be differently orientated.H2O Binding. Similar to the case of pure Pt, H2Oad binds

only to the top sites (t1a, t1b, and t2) on the Pt3Os(111) surfacewith very similar binding energies, 0.16−0.19 eV in the gasphase, and 0.45−0.52 eV in solution. These values are veryclose to the corresponding values for pure Pt, 0.22 eV in the gasphase and 0.58 eV in solution. The difference between thebinding energies in the gas phase and solution is close to thevalue of the solvent stabilization of bulk H2Oad, 0.40 eV. Sincethe surface H2Oad does not bind to bridge, fcc, or hcp sites,migration of H2Oad from one top site to the others is throughadsorption and dissociation. The migration barrier can beestimated as 0.10−0.20 eV (the 0.50−0.60 eV binding energyminus the 0.40 eV solvation of H2Oad, H2O is consideredalways solvated).HOOH Binding. Only the bridge sites are available for the

HOOHad binding on the Pt(111) and Pt3Os(111) surfaces.Two oxygen atoms bind to two neighboring top sites with theO−O bond parallel to the Pt−Pt bond. For Pt3Os, the bindingenergies vary from 0.25 to 0.38 eV and 0.52 to 0.62 eV in thegas phase and solution, respectively. Both are close to theHOOHad binding energy of pure Pt, 0.27 eV in the gas phaseand 0.61 eV in solution.Summarizing this section, we can say that the Pt3Os binding

energies show strong site dependence. However, we do not findany obvious trend for the binding energies of the O-containingspecies, in particular for the O and OH binding energy insolution. Thus, in the case of the Pt3Os(111) alloy surface, it ishard to decide whether Pt3Os is a good ORR catalyst basedonly on the binding energies of the ORR intermediates.Reaction barrier calculations might help to make a morethorough conclusion.3.3. Reaction Barriers and Possible ORR Mechanisms.

Generally, eight fundamental steps (Figure 4) can beconsidered in the ORR mechanisms:

(1) H2 dissociation: →H 2H2ad ad(2) O2 dissociation: →O 2O2ad ad(3) OH formation: + →O H OHad ad ad(4) hydration: + →O H O 2OHad 2 ad ad(5) OOH formation: + →O H OOH2ad ad ad(6) OOH dissociation: → +OOH OH Oad ad ad(7) H - OOH dissociation: + →OOH H 2OHad ad(8) H2O formation: + →OH H H Oad ad 2 ad

By including these fundamental steps into an overall ORRmechanism, we distinguish three chemical processes:59

O−O bond activation, which can occur via two mechanisms:O2 dissociation (2) and OOH formation (5) followed by OOHdissociation (6);OH formation proceeds via two mechanisms as well: OH

formation (3), O hydration (4), and H−OOH dissociation (7);OH consumption: There is only one mechanism, H2O

formation (8) for this process.A good catalyst must provide low barriers for all of these

three processes and for pathways connecting them.Starting from the preferred sites, we calculated the barriers

for all eight steps on the Pt3Os(111) surface in the gas phaseand solution. These barriers and the corresponding barriers forpure Pt20 are shown in Tables 3 and 4. The pathway with the

lowest reaction barriers is used for the final determination ofthe ORR mechanism. The potential energy surface, includingbarriers and geometry insets for the OOH-form-hydrmechanism and O2-diss-hydr mechanism

60 for Pt3Os and Pt,is shown in Figure 5.

3.4. Reaction Barriers and ORR Mechanisms for Pt3Osin the Gas Phase. O−O Bond Activation. For pure Pt,OOHad formation with a barrier of 0.28 eV is followed byOOHad dissociation with a lower barrier of 0.14 eV, whereas thebarrier for the direct dissociation is 0.58 eV.In the Pt3Os case, the OOHad formation barrier is 0.34 eV,

and the barrier for the OOHad dissociation is only 0.09 eV,whereas the barrier for the direct O2ad dissociation is 1.36 eV,much higher than the barrier for the OOHad formation. Thus, itis preferable for the ORR pathway to proceed via the OOHformation step.

OH Formation. The barrier for the direct OHad formation is0.72 and 0.57 eV for Pt and Pt3Os, respectively. This issignificantly higher compared to the OHad formation via the Ohydration step, 0.29 and 0.23 eV for Pt and Pt3Os, respectively.

OH Consumption. The H2Oad formation step proceeds witha small barrier of 0.11 eV for Pt and 0.09 eV for Pt3Os.

Figure 4. Eight ORR fundamental steps.

Table 3. Reaction Barriers (eV) for Pt20 and Pt3Os in theGas Phase

reaction barriers Pt Pt3Os

H2 dissociation 0.00 0.03O2 dissociation 0.58 1.36OH formation 0.72 0.57O hydration 0.29 0.23OOH formation 0.28 0.34OOH dissociation 0.14 0.09H-OOH dissociation 0.18 0.24H2O formation 0.11 0.09

Table 4. Reaction Barriers (eV) for Pt20 and Pt3Os in theSolvated Phase

reaction barriers Pt Pt3Os

H2 dissociation 0.00 0.05O2 dissociation 0.00 0.16OH formation 1.09 0.90O hydration 0.50 0.48OOH formation 0.19 0.00OOH dissociation 0.00 0.00H-OOH dissociation 0.04 0.41H2O formation 0.17 0.35



Summarizing these three processes, we propose the followingORR mechanism for Pt and Pt3Os in the gas phase:

+ → =EO H OOH ( 0.28eV for Pt and 0.34 for Pt Os)2 a 3

→ + =EOOH O OH ( 0.14 eV for Pt and 0.09

eV for Pt Os)a

3

+ → =EO H O 2OH ( 0.29eV for Pt and 0.23

eV for Pt Os)2 a

3

+ → =EOH H H O ( 0.11 eV for Pt and 0.09

eV for Pt Os)2 a

3

Figure 5a shows the potential energy surface of thismechanism for Pt and Pt3Os. It starts from O2 gas labeled asO2(g) and represents the sequential steps of the ORR thatfollows the four-electron mechanism: O2 + 4H+ + 4e− →2H2O. For the purpose of conservation of atoms, we include4H+ in the first step, although the proton successively comes tothe cathode from the electrolyte. The reactants and productsinvolved in each reaction step are marked by bold font. Therate-determining step (RDS) is O-hydration with a barrier of0.29 eV for Pt and OOH formation with a barrier of 0.34 eV forPt3Os. Although the RDS barrier for Pt3Os is higher than forPt, the energy barrier for the O hydration reaction on thePt3Os(111) surface is lower (0.23 eV) than for Pt.

3.5. Reaction Barriers and ORR Mechanism for Pt3Osin Solvated Phase. O−O Bond Activation. Since solventstrongly favors O2 dissociation, this mechanism becomespreferable for the materials considered here. O2 can easilydissociate to form Oad at the fcc site with no barrier for Pt andwith a barrier of 0.16 eV for Pt3Os, where Oad is at the top site(Figure S2, Supporting Information). The solvent effect on theO2 dissociation is stronger on Pt3Os than on Pt (see Tables 3and 4), which is related to the stronger solvent effect on the Oadbinding on Pt3Os than on Pt. On Pt3Os, O2 first migrates fromthe f1a site to the b2a site and then dissociates to two adsorbedoxygen at the t1b and t2 sites. The O atom adsorbed at the topsite generates a stronger dipole than the O atom adsorbed atthe fcc site, which results in the stronger solvent effect on Pt3Oscompared to pure Pt.For comparison, OOHad formation and OOHad dissociation

on the Pt3Os surface are barrierless, which favors the ORRpathway to go via these steps.

OH Formation. Proceeding from the above conclusion, thesecond step should be O hydration because the barrier for thedirect OHad formation is 1.09 and 0.90 eV for Pt and Pt3Os,respectively. The O hydration step with a barrier of 0.50 eV forPt and 0.48 eV for Pt3Os is significantly favorable.

OH Consumption. The H2O formation reaction on thePt3Os (111) surface occurs with a barrier of 0.35 eV, while purePt has a lower barrier of 0.17 eV.Summarizing the above discussion, we come to conclusion

that the O2-diss-hydr mechanism60 is favorable for Pt, but for

Pt3Os, both the OOH-form-hydr mechanism and O2-diss-hydrmechanism in solvated phase are feasible, although the OOH-form-hydr mechanism with the barrierless OOHad formationand OOHad dissociation steps looks more preferable than theO2-diss-hydr mechanism:

− →

=

EO O bond activation: O 2O (

0.00 eV for Pt, and 0.16 eV for Pt Os)2 a

3

+ → =EO H OOH ( 0.19 eV for Pt and 0.00

eV for Pt Os)2 a

3

Figure 5. Potential energy surfaces including reaction barriers for theOOH-form-hydr mechanism in gas phase (a), OOH-form-hydrmechanism in solution (b), and O2-diss-hydr mechanism in solution(c) for Pt and Pt3Os.



→ + =EOOH O OH ( 0.00 eV for Pt and 0.00

eV for Pt Os)a

3

+

→ =E

OH formation: O H O

2OH ( 0.50 eV for P and 0.48eV for Pt Os)2

a 3

+ →

=E

OH consumption: OH H H O

( 0.17 eV for Pt and 0.35 eV for Pt Os)2

a 3

Figure 5b,c shows the potential energy surfaces of the OOH-form-hydr and O2-diss-hydr mechanisms for Pt and Pt3Os insolution. The RDS for both mechanisms is the O hydrationstep with a barrier of 0.50 and 0.48 eV for Pt and Pt3Os,respectively. Spontaneous Oad migration to the most stable fccsite after the OOH dissociation step or the O2 dissociation stepis included in the figure as well.On the basis of this result, the ORR catalytic activity of the

Pt3Os alloy can be estimated and compared to that of pure Pt.We find that Pt3Os should show about 2 times better catalyticactivity than pure Pt (e−0.48ev/kT/e−0.50ev/kT = 2.18) at 80 °C.The electrochemical analysis of dealloyed Pt2Os nanoparticles(19% Os), performed in our previous work,61 revealed the 2-fold better mass activity and 3.5-fold better specific activitycompared to pure Pt, along with improved stability. Nilekar etal.7 calculated the OH−OH repulsive energy and correlated itwith the measured kinetic current density for Pt0.8M0.2/Pd(111), where M = Au, Pd, Rh, Re, and Os. Under 0.8 V,the current density for Pt0.8Os0.2/Pd (111) is about 3 timesbetter than for Pt/Pd(111) (47 vs 17 mA/cm2). Pt alloys withOs and Re were found to be more reactive, than the others dueto their high OH−OH and OH−O repulsive energy. Anotherpossible way to explain the better performance of Pt3Os iseither by the smaller size of the Os atom, which results in acompressed strain (strain effect) for the surface Pt atoms, or theelectronic interaction (ligand effect) of the subsurface Os atomswith the surface Pt atoms. In general, the strain and ligandeffects occur simultaneously and it is hard to separate them.Both effects are manifested in the interatomic matrix elementdescribing bonding interaction between an atom and its nearestneighbors.62 For the Pt3Os catalyst, the electronic structure ofthe surface Pt is modified by the underlying Os atoms and thePt−Pt bond distance is compressed to 2.76 Å compared to 2.81Å for the Pt bulk structure. Nørskov et al. developed the d-bandmodel,62−64 which was applied to connect the surface chemicalproperties of bimetallic alloy catalysts with their electronicstructures. For compressive strain, the interatomic distancesbecome smaller and the overlapping of metal d-states increases.The increased d-state overlapping results in the increased d-band width which is highly correlated with the position of thed-band center due to the fact the d-band filling changesnegligibly upon the formation of the bimetallic surfaces. Tomaintain the filling, the broader d-band downshifts the d-bandcenter which results in weaker adsorbate bonding. On thePt3Os(111) surface, the downshift of the surface d-band centerrelative to pure Pt(111) is ∼0.35 eV (Figure S3, SupportingInformation), whereas according to Stamenkovic et al.,8 thisdownshift should be ∼0.2 eV and the O/OH bindings shouldbe by 0.1−0.2 eV weaker6,8,29 to reach the maximum oxygenreduction activity. Both above-mentioned effects reduce the Oand OH binding energies (the O binding energy on thePt3Os(111) surface is by 0.11 eV weaker than for Pt(111), 3.55

vs 3.66 eV, and the OH binding energy for Pt3Os(111) is by0.08 eV weaker than for Pt(111), 2.20 vs 2.28 eV, see Table 1)in gas phase. However, in solution, which is more relevant tothe PEMFC operating conditions, O binds more strongly onPt3Os than on pure Pt (see Table 2), but the computed RDSbarrier is lower for Pt3Os, 0.48 vs 0.50 eV for Pt. Therefore,knowing the binding energies of the intermediates is not alwayssufficient to reliably predict the ORR activity. Reaction barriershave to be considered as well to better understand the ORRkinetics. The result obtained in our study indicates that PtOsmaterials might be considered as potential ORR catalysts.

4. CONCLUSION

Surface segregation of a number of Pt3M alloys wasinvestigated. Only a few Pt binary alloys demonstrate favorablesurface segregation energy in the presence of adsorbed O orOH. Out of them, only Pt3Os and Pt3Ir show surfacesegregation in the presence of both adsorbed species. Thisassumes that unlike other Pt-based binary alloys, Pt3Os andPt3Ir might be stable under fuel cell operating conditions. Westudied systematically the binding site preference of all reactionintermediates involved in ORR on Pt3Os in the gas phase andsolution. The binding energies of adsorbates on the alloysurface show the strong sublayer dependence. Reaction barriersfor the eight ORR fundamental steps were also calculated forPt3Os both in the gas phase and solution, and compared tothose for pure Pt. According to our result, Pt3Os has a slightlylower energy barrier for the ORR RDS than pure Pt, whichshould result in better catalytic activity compared to pure Pt.This conclusion is in agreement with our earlier performedexperimental study of PtOs catalysts,61 in which we found thatthe ORR mass and specific activities of dealloyed Pt2Osmaterials are 2 and 3.5 times better than the correspondingactivities of pure Pt. In addition, these materials show goodelectrochemical stability. Thus, PtOs alloys might be consideredas promising ORR catalysts.

■ ASSOCIATED CONTENT

*S Supporting InformationFigures showing charge dipoles related to the O and OHbinding on Pt3Os(111) surface, O2 dissociation on Pt(111) andPt3Os(111) surfaces, and densities of states and d-band centersfor Pt(111) and Pt3Os(111) surface layers. This material isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: [email protected] (B.V.M.).*E-mail: [email protected] (W.A.G.).

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by the National Science Foundation(Grant CBET-1067848, Caltech) and the Caltech and TaiwanEnergy Exchange (CTEE) collaborative program funded by theNational Science Council of Taiwan (Grant NSC 103-3113-P-008-001). The facilities of the Materials and Process SimulationCenter used in this study were established with grants fromDURIP-ONR, DURIP-ARO and NSF-CSEM.



http://pubs.acs.org

mailto:[email protected]

mailto:[email protected]

■ REFERENCES(1) Appleby, A. J.; Foulkes, F. R. Fuel Cell Handbook; Van NostrandReinhold: New York, 1989.(2) O’Hayre, R. P.; Cha, S. W.; Colella, W.; Prinz, F. B. Fuel CellFundamentals; 2nd ed.; John Wiley & Sons: New York, 2006.(3) Kordesch, K.; Simader, G. Fuel Cell and Their Applications; VCH:New York, 1996.(4) Hwang, S. J.; Kim, S. K.; Lee, J. G.; Lee, S. C.; Jang, J. H.; Kim, P.;Lim, T. H.; Sung, Y. E.; Yoo, S. J. Role of Electronic Perturbation inStability and Activity of Pt-Based Alloy Nanocatalysts for OxygenReduction. J. Am. Chem. Soc. 2012, 134, 19508−19511.(5) Debe, M. K. Electrocatalyst Approaches and Challenges forAutomotive Fuel Cells. Nature 2012, 486, 43−51.(6) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T.P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.;Nørskov, J. K. Alloys of Platinum and Early Transition Metals asOxygen Reduction Electrocatalysts. Nat. Chem. 2009, 1, 552−556.(7) Nilekar, A. U.; Xu, Y.; Zhang, J.; Vukmirovic, M. B.; Sasaki, K.;Adzic, R. R.; Mavrikakis, M. Bimetallic and Ternary Alloys forImproved Oxygen Reduction Catalysis. Top. Catal. 2007, 46, 276−284.(8) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J. J.; Ross, P. N.;Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Nørskov, J. K. Changingthe Activity of Electrocatalysts for Oxygen Reduction by Tuning theSurface Electronic Structure. Angew. Chem. 2006, 118, 2963−2967.(9) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P.N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen ReductionActivity on Pt3Ni(111) Via Increased Surface Site Availability. Science2007, 315, 493−497.(10) Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.; Yu, C.F.; Liu, Z. C.; Kaya, S.; Nordlund, D.; Ogasawara, H.; Toney, M. F.;Nilsson, A. Lattice-Strain Control of the Activity in Dealloyed Core-Shell Fuel Cell Catalysts. Nat. Chem. 2010, 2, 454−460.(11) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.;Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F.Platinum Monolayer Fuel Cell Electrocatalysts. Top. Catal. 2007, 46,249−262.(12) Yang, H. Platinum-Based Electrocatalysts with Core-ShellNanostructures. Angew. Chem., Int. Ed. 2011, 50, 2674−2676.(13) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J.;Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends inElectrocatalysis on Extended and Nanoscale Pt-Bimetallic AlloySurfaces. Nat. Mater. 2007, 6, 241−247.(14) Othman, R.; Dicks, A. L.; Zhu, Z. Non Precious Metal Catalystsfor the PEM Fuel Cell Cathode. Int. J. Hydrogen Energy 2012, 37, 357−372.(15) Bashyam, R.; Zelenay, P. A Class of Non-Precious MetalComposite Catalysts for Fuel Cells. Nature 2006, 443, 63−66.(16) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. High-Performance Electrocatalysts for Oxygen Reduction Derived fromPolyaniline, Iron, and Cobalt. Science 2011, 332, 443−447.(17) Gong, K. P.; Du, F.; Xia, Z. H.; Durstock, M.; Dai, L. M.Nitrogen-Doped Carbon Nanotube Arrays with High ElectrocatalyticActivity for Oxygen Reduction. Science 2009, 323, 760−764.(18) Chen, Z. W.; Higgins, D.; Yu, A. P.; Zhang, L.; Zhang, J. J. AReview on Non-Precious Metal Electrocatalysts for PEM Fuel Cells.Energy Environ. Sci. 2011, 4, 3167−3192.(19) Stamenkovic, V.; Schmidt, T. J.; Ross, P. N.; Markovic, N. M.Surface Composition Effects in Electrocatalysis: Kinetics of OxygenReduction on Well-Defined Pt3Ni and Pt3Co Alloy Surfaces. J. Phys.Chem. B 2002, 106, 11970−11979.(20) Sha, Y.; Yu, T. H.; Merinov, B. V.; Shirvanian, P.; Goddard, W.A., III Mechanism for Oxygen Reduction Reaction on Pt3Ni Alloy FuelCell Cathode. J. Phys. Chem. C 2012, 116, 21334−21342.(21) Ma, Y. G.; Balbuena, P. B. Pt Surface Segregation in BimetallicPt3M Alloys: A Density Functional Theory Study. Surf. Sci. 2008, 602,107−113.(22) Yu, T. H.; Sha, Y.; Merinov, B. V.; Goddard, W. A., III ImprovedNon-Pt Alloys for the Oxygen Reduction Reaction at Fuel Cell

Cathodes Predicted from Quantum Mechanics. J. Phys. Chem. C 2010,114, 11527−11533.(23) Gauthier, Y. Pt-Metal Alloy Surfaces: Systematic Trends. Surf.Rev. Lett. 1996, 3, 1663−1689.(24) Sepa, D. B.; Vojnovic, M. V.; Vracar, L. M.; Damjanovic, A.Different Views Regarding the Kinetics and Mechanisms of OxygenReduction at Pt and Pd Electrodes. Electrochim. Acta 1987, 32, 129−134.(25) Sepa, D. B.; Vojnovic, M. V.; Damjanovic, A. ReactionIntermediates as a Controlling Factor in the Kinetics and Mechanismof Oxygen Reduction at Platinum-Electrodes. Electrochim. Acta 1981,26, 781−793.(26) Ma, Y.; Balbuena, P. B. Surface Segregation in Bimetallic Pt3M(M = Fe, Co, Ni) Alloys with Adsorbed Oxygen. Surf. Sci. 2009, 603,349−353.(27) Abrams, B. L.; Vesborg, P. C. K.; Bonde, J. L.; Jaramillo, T. F.;Chorkendorff, I. Dynamics of Surface Exchange Reactions between Auand Pt for HER and HOR. J. Electrochem. Soc. 2009, 156, B273−B282.(28) Mayrhofer, K. J. J.; Hartl, K.; Juhart, V.; Arenz, M. Degradationof Carbon-Supported Pt Bimetallic Nanoparticles by SurfaceSegregation. J. Am. Chem. Soc. 2009, 131, 16348−16349.(29) Stephens, I. E. L.; Bondarenko, A. S.; Perez-Alonso, F. J.; Calle-Vallejo, F.; Bech, L.; Johansson, T. P.; Jepsen, A. K.; Frydendal, R.;Knudsen, B. P.; Rossmeisl, J.; Chorkendorff, I. Tuning the Activity ofPt(111) for Oxygen Electroreduction by Subsurface Alloying. J. Am.Chem. Soc. 2011, 133, 5485−5491.(30) Maillard, F.; Dubau, L.; Durst, J.; Chatenet, M.; Andre, J.;Rossinot, E. Durability of Pt3Co/C Nanoparticles in a Proton-Exchange Membrane Fuel Cell: Direct Evidence of Bulk CoSegregation to the Surface. Electrochem. Commun. 2010, 12, 1161−1164.(31) Dubau, L.; Maillard, F.; Chatenet, M.; Guetaz, L.; Andre, J.;Rossinot, E. Durability of Pt3Co/C Cathodes in a 16 Cell PEMFCStack: Macro/Microstructural Changes and Degradation Mechanisms.J. Electrochem. Soc. 2010, 157, B1887−B1895.(32) Colon-Mercado, H. R.; Popov, B. N. Stability of Platinum BasedAlloy Cathode Catalysts in PEM Fuel Cells. J. Power Sources 2006, 155,253−263.(33) Colon-Mercado, H. R.; Kim, H.; Popov, B. N. Durability Studyof Pt3Ni1 Catalysts as Cathode in PEM Fuel Cells. Electrochem.Commun. 2004, 6, 795−799.(34) Ma, Y.; Balbuena, P. B. Role of Iridium in Pt-Based AlloyCatalysts for the ORR: Surface Adsorption and Stabilization Studies. J.Electrochem. Soc. 2010, 157, B959−B963.(35) Ramírez-Caballero, G. E.; Ma, Y.; Callejas-Tovar, R.; Balbuena,P. B. Surface Segregation and Stability of Core−Shell Alloy Catalystsfor Oxygen Reduction in Acid Medium. Phys. Chem. Chem. Phys. 2010,12, 2209−2218.(36) Schultz, P. SeqQuest, Electronic Structure Code; Sandia NationalLaboratory, Albuquerque, NM, http://dft.sandia.gov/Quest/.(37) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized GradientApproximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.(38) Perdew, J. P.; Zunger, A. Self-Interaction Correction to Density-Functional Approximations for Many-Electron Systems. Phys. Rev. B1981, 23, 5048−5079.(39) Ceperley, D. M.; Alder, B. J. Ground-State of the Electron-Gasby a Stochastic Method. Phys. Rev. Lett. 1980, 45, 566−569.(40) Goddard, W. A., III New Foundation for the Use ofPseudopotentials in Metals. Phys. Rev. 1968, 174, 659−662.(41) Hamann, D. R. Generalized Norm-Conserving Pseudopoten-tials. Phys. Rev. B 1989, 40, 2980−2987.(42) Melius, C. F.; Goddard, W. A., III Ab Initio Effective Potentialsfor Use in Molecular Quantum Mechanics. Phys. Rev. A 1974, 10,1528−1540.(43) Melius, C. F.; Olafson, B. D.; Goddard, W. A., III Fe and Ni Ab-Initio Effective Potentials for Use in Molecular Calculations. Chem.Phys. Lett. 1974, 28, 457−462.(44) Redondo, A.; Goddard, W. A., III; McGill, T. C. Ab InitioEffective Potentials for Silicon. Phys. Rev. B 1977, 15, 5038−5048.



http://dft.sandia.gov/Quest/

(45) Sha, Y.; Yu, T. H.; Liu, Y.; Merinov, B. V.; Goddard, W. A., IIITheoretical Study of Solvent Effects on the Platinum-CatalyzedOxygen Reduction Reaction. J. Phys. Chem. Lett. 2010, 1, 856−861.(46) Baker, N. A.; Sept, D.; Joseph, S.; Holst, M. J.; McCammon, J.A. Electrostatics of Nanosystems: Application to Microtubules and theRibosome. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 10037−10041.(47) Holst, M.; Saied, F. Multigrid Solution of the Poisson-Boltzmann Equation. J. Comput. Chem. 1993, 14, 105−113.(48) Holst, M. J.; Saied, F. Numerical Solution of the NonlinearPoisson-Boltzmann Equation: Developing More Robust and EfficientMethods. J. Comput. Chem. 1995, 16, 337−364.(49) Mills, G.; Jonsson, H.; Schenter, G. K. Reversible WorkTransition State Theory: Application to Dissociative Adsorption ofHydrogen. Surf. Sci. 1995, 324, 305−337.(50) Mills, G.; Jonsson, H. Quantum and Thermal Effects in H2Dissociative Adsorption: Evaluation of Free Energy Barriers inMultidimensional Quantum-Systems. Phys. Rev. Lett. 1994, 72,1124−1127.(51) Keith, J. A.; Anton, J.; Kaghazchi, P.; Jacob, T. ModelingCatalytic Reactions on Surfaces with Density Functional Theory. InDeutschmann, O. Modeling and Simulation of Heterogeneous CatalyticReactions: From the Molecular Process to the Technical System; JohnWiley & Sons: New York, 2013.(52) Bonnet, N.; Otani, M.; Sugino, O. Self-Poisoning DynamicalEffects in the Oxygen Reduction Reaction on Pt (111) from a Top-Down Kinetic Analysis. J. Phys. Chem. C 2014, 118, 13638−13643.(53) Jinnouchi, R.; Kodama, K.; Hatanaka, T.; Morimoto, Y. FirstPrinciples Based Mean Field Model for Oxygen Reduction Reaction.Phys. Chem. Chem. Phys. 2011, 13, 21070−21083.(54) Voronova, L. I.; Polyakova, V. P.; Savitskii, E. M. Alloys of theSystem Pt–Os. Russ. Metall. 1984, 5, 201−203.(55) Raub, E.; Plate, W. Tempering and Decomposition of Platinum-Iridium Alloys. Z. Metallkunde 1956, 47, 688−693.(56) Moore, J. T.; Chu, D.; Jiang, R. Z.; Deluga, G. A.; Lukehart, C.M. Synthesis and Characterization of Os and Pt-Os/CarbonNanocomposites and Their Relative Performance as MethanolElectrooxidation Catalysts. Chem. Mater. 2003, 15, 1119−1124.(57) Bing, Y.; Liu, H.; Zhang, L.; Ghosh, D.; Zhang, J.Nanostructured Pt-Alloy Electrocatalysts for PEM Fuel Cell OxygenReduction Reaction. Chem. Soc. Rev. 2010, 39, 2184−2202.(58) Hu, S.; Xiong, L. P.; Ren, X. B.; Wang, C. B.; Luo, Y. M. Pt-IrBinary Hydrophobic Catalysts: Effects of Ir Content and Particle Sizeon Catalytic Performance for Liquid Phase Catalytic Exchange. Int. J.Hydrogen Energy 2009, 34, 8723−8732.(59) Yu, T. H.; Hofmann, T.; Sha, Y.; Merinov, B. V.; Myers, D. J.;Heske, C.; Goddard, W. A., III Finding Correlations of the OxygenReduction Reaction Activity of Transition Metal Catalysts withParameters Obtained from Quantum Mechanics. J. Phys. Chem. C2013, 117, 26598−26607.(60) Sha, Y.; Yu, T. H.; Merinov, B. V.; Shirvanian, P.; Goddard, W.A., III Oxygen Hydration Mechanism for the Oxygen ReductionReaction at Pt and Pd Fuel Cell Catalysts. J. Phys. Chem. Lett. 2011, 2,572−576.(61) Lee, Y. J.; Hsieh, Y. C.; Tsai, H. C.; Lu, I. T.; Wu, Y. H.; Yu, T.H.; Merinov, B. V.; Goddard, W. A., III; Wu, P. W. Dealloyed Pt2OsNanoparticles for Enhanced Oxygen Reduction Reaction in AcidicElectrolytes. Appl. Catal., B 2014, 150−151, 636−646.(62) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G. Role ofStrain and Ligand Effects in the Modification of the Electronic andChemical Properties of Bimetallic Surfaces. Phys. Rev. Lett. 2004, 93,156801.(63) Hammer, B.; Nørskov, J. K. Theoretical Surface Science andCatalysisCalculations and Concepts. Adv. Catal. 2000, 45, 71−129.(64) Kitchin, J. R.; Nørskov, J. K.; Barteau, M. A.; Chen, J. G.Modification of the Surface Electronic and Chemical Properties ofPt(111) by Subsurface 3d Transition Metals. J. Chem. Phys. 2004, 120,10240−10246.



density functional theory study of pt m alloy surface ... · important surface property that...

Documents