Atomic Oxygen Absorption into Pt-Based Alloy Subsurfaces

J. Phys. Chem. C 2008, 112, 5057-5065

5057

Atomic Oxygen Absorption into Pt-Based Alloy Subsurfaces Zhihui Gu and Perla B. Balbuena* Department of Chemical Engineering, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed: December 18, 2007; In Final Form: January 13, 2008

Density functional theory is used to study atomic oxygen absorption into subsurfaces of various platinumbased alloys to investigate the effects of alloyed metals on the thermodynamics and kinetics of this process. The results show that alloying Pt with Ir and Rh increases the absorption energy barrier compared with pure Pt, making the O migration to the subsurface more difficult, and that the addition of Co and Ru to Pt can decrease the reverse energy barrier of absorption making O unstable in the subsurface. By comparison of the atomic oxygen absorption in Pt3X alloys with that in PtX alloys, it is found that both the nature of the alloyed metal species and the alloy composition have effects on the atomic oxygen absorption kinetics. Analyses of the atomic oxygen absorption into the subsurface of ternary alloys such as Pt2IrRu show that compared with pure platinum the presence of the two added metals can increase the barriers for surface diffusion and that for O absorption while simultaneously decreasing the reverse energy barrier, therefore inhibiting the atomic oxygen absorption process more effectively. Pt “skin” monolayers on Pt alloys show increases in the atomic oxygen absorption energy barrier and in their corresponding reverse energy barrier.

1. Introduction The durability of Pt or Pt-based alloy cathode catalysts in an acidic environment during proton-exchange membrane fuel cells (PEMFC) operation is one of the main challenges for their longterm applications, as the electrode active surface loss of cathode results in a substantial decrease of fuel cell performance.1 Losses of electrocatalyst active surface due to increases in the average catalyst cluster size were detected during PEMFC operation by comparing the fresh and aged cathode catalysts through highresolution transmission electron microscopy images.2 The presence of Pt and Cr (one of the alloy metals) in the cathode outlet water3 and Pt cations inside the proton exchange membrane observed by UV detection2 indicate that dissolution of the cathode catalyst is another main reason for cathode active surface loss. The dissolved platinum species can redeposit on the Pt particles to form larger particles following an Ostwald ripening process3,4 and also can diffuse out of the membrane electrode assembly becoming deposited on the polymer membrane,1 concomitant with a decrease in platinum cathode active surface. The precipitation of Pt in the electrolyte membrane close to the cathode and at the catalyst/membrane interface due to reduction of dissolved platinum species such as Pt2+ by permeating hydrogen1,5 and the presence of platinum cations in the PEMFC cathode outlet water indicate that Pt dissolution is one of the current challenging problems in catalyst design.1 Experimental reports on Pt and Pt-alloy cathode catalysts showed that Pd and Rh are more soluble than Pt at the same experimental conditions,6 in agreement with our previous density functional theory (DFT) studies,7 and the metal dissolution rate of a Pt alloy with a PtM ratio of 3:1 is lower than that of a Pt alloy with PtM ratio of 1:1 (M ) Co, Ni).5 Improved durability of PtCo/C in PEMFC operation was also determined by comparing the performance with that of pure platinum.8 It was reported that the equilibrium concentration of dissolved platinum in perchloric acid electrolyte is a function of the cell * To whom correspondence should be addressed. E-mail: balbuena@ tamu.edu.

potential measured versus the standard hydrogen electrode.9 In the range of potentials from 0.85 to 1.15 V, a half monolayer (ML) of oxygen atoms is adsorbed on the cathode catalyst,10,11 and from 1.15 to 1.2 V, a second half oxygen atom monolayer is formed on the catalyst surface and it is suggested that a place exchange of adsorbed oxygen atoms with the surface platinum atoms takes place with further formation of an absorbed oxygen layer.10,12 Thus, the place exchange would make the oxygen atoms diffuse into the subsurface, leaving the platinum atoms exposed to the electrolyte for further oxidation and dissolution. It has been reported that atomic oxygen can be found in the subsurface of transition metals such as Pt, Pd, and Ru during surface reactions in various experimental environments,13-18 and subsurface oxygen may be further considered as the initial stage in the corrosion and growth of metal oxides.19 Under electrochemical conditions, subsurface oxygen can be found in the oxidation of polycrystalline platinum, and the concentration of subsurface oxygen has been found to depend on the cycling rate by cyclic voltammetry experiments.20,21 Possible structures of the initial stages of the oxidized surface have been also proposed in which the oxygen atom migrates into subsurface by place exchange between Pt atom and atomic oxygen10,12 to form platinum oxide. However, the microscopic understanding of this complex process is at its infancy. It is important to remark that the initial stages of formation of surface oxide correlate with Pt dissolution, and both oxygen coverage and Pt dissolution are functions of potential.1 Moreover, many other studies have also linked the initial step of surface oxide formation with the initial corrosion stages.19 On the other hand, single-crystal studies of the gas-phase adsorption of oxygenated compounds have proven to be fundamental22-24 not only to understand oxygen reactivity but also to guide the design of Pt-alloy electrocatalysts to improve activity. By analogy, it is expected that fundamental studies of the incorporation of oxygen into the subsurface of relevant crystallographic facets will be also important to elucidate the initial stages of oxide growth and their relationship with the metal corrosion process. Previously,

10.1021/jp711875e CCC: $40.75 © 2008 American Chemical Society Published on Web 03/11/2008

5058 J. Phys. Chem. C, Vol. 112, No. 13, 2008

Gu and Balbuena

TABLE 1: (part a) Calculated Lattice Constants (Å), Binding Energies (eV) of 0.25 ML Atomic Oxygen on fcc and hcp Hollow Sites, and Absorption Energies (eV) of 0.25 ML Inside Tetrahedral Sites beneath hcp Hollow Sites of Pt-X (111) Alloy Surfaces (X ) Co, Ni, Ru, Rh, Pd, Ir). (part b) Calculated Diffusion and Absorption Barriers (in eV) for the Respective Forward and Reverse Processes of PtX(111) Alloy Surfaces Shown in Parts a and b of Figure 2. The Barriers Corresponding to Pure Pt(111) Surfaces are Also Shown for Comparison Part a

Pt-Co Pt-Ni Pt-Ru Pt-Rh Pt-Pd Pt-Ir Pt

lattice constant

fcc-1 2Pt-1X

fcc-2 1Pt-2X

hcp-1 2Pt-1X

hcp-2 1Pt-2X

tetra-1 3Pt-1X

tetra-2 1Pt-3X

3.801 3.800 3.895 3.911 3.960 3.926 3.977

-4.97 -4.95 -5.00 -4.80 -4.72 -4.58 -4.61

-5.17 -4.96 -5.56 -4.99 -4.40 -4.94

-4.72 -4.55 -4.86 -4.49 -4.37 -4.35 -4.21

-5.13 -4.95 -5.51 -4.84 -4.34 -4.77

-0.60 -0.68 -2.31 -2.16 -2.63 -2.15 -2.66

-3.74 -3.74 -3.39 -2.90 -2.61 -2.55

Pt-Co Pt-Ni Pt-Ru Pt-Rh Pt-Pd Pt-Ir Pt

diffusion barrier (forward)

Part b diffusion barrier (reverse)

absorption barrier (forward)

absorption barrier (reverse)

0.25 0.39 0.30 0.52 0.45 0.43 0.60

0.21 0.38 0.20 0.28 0.08 0.19 0.20

1.40 1.36 2.12 2.58 2.07 2.88 2.22

0.01 0.16 0.00 0.64 0.34 0.66 0.70

we reported the subsurface oxygen formation process in Pt subsurfaces and the effects of the chemical environment on the subsurface oxygen formation.25,26 Here, we present a kinetic and thermodynamic investigation of the atomic oxygen absorption into the (111) subsurfaces of Pt-based alloys with various compositions and analyze the effects of the alloyed metal on the absorption process. Transition metals Co, Ni, Ru, Rh, Pd, and Ir are the alloyed species tested to find the best metal that alloyed with Pt is able to inhibit atomic oxygen absorption.

of oxygen adsorption on different hollow sites (hcp and fcc; hcp ) hexagonal close packed) were investigated according to their corresponding binding energies, and the stabilities of oxygen absorption in the subsurface sites and inside tetrahedral or octahedral sites were evaluated according to their respective absorption energies. The binding energies (Eb) of oxygen adsorption on Pt and Pt-alloy surfaces and absorption energies (Eab) in subsurfaces are defined by the equation:

E ) EPtnO - EPtn - EO 2. Computational Details All calculations were performed using spin polarized DFT in the generalized gradient approximation and the PerdewBurke-Ernzerhof functional27 for electron exchange and correlation. The Vienna Ab Initio Simulation Package (VASP)28,29 was used to solve the Kohn-Sham equations in a simulation cell with periodic boundary conditions using plane wave basis sets in which the Blo¨ch’s all electron projector augmented method30 with frozen core approximation is implemented.31 Energy cutoff of 400 eV is used for all calculations, and the convergence criterion for structure relaxation is that the force acting on each atom becomes less than 0.01 eV/Å. The Monkhorst-Pack scheme32 was employed for the k-point sampling and the calculated bulk equilibrium lattice constant of face-centered cubic (fcc) Pt and fcc Pt-based alloys. The calculated lattice constant of fcc Pt is 3.977 Å with the converged k-mesh of 14 × 14 × 14, which agrees within 1.45% of the experimental value of 3.92 Å,33 and the calculated lattice constants of various Pt-based alloys with same k-mesh of 14 × 14 × 14 are listed in the Results and Discussion section. The (111) surfaces of Pt and Pt alloys are modeled in the supercell approach using five layer Pt or Pt-based alloy slabs in which the three top layers are allowed to relax and the two bottom layers are fixed at the calculated bulk positions. The vacuum space is approximately equal to 12 Å above the top layers. For atomic oxygen adsorption on surfaces and absorption into the subsurfaces, we investigated 0.25 ML coverages at various surfaces and inside subsurfaces. Monkhorst-Pack grid of 5 × 5 × 1 was used for corresponding (2 × 2) cells. The stabilities

EPtnO is the energy of the complex where oxygen is adsorbed on the Pt or Pt-alloy surface or the energy of the complex or where oxygen is absorbed in Pt or Pt-alloy subsurfaces. EPtn is the energy of clean Pt or Pt-alloy surfaces, and EO is the energy of an isolated oxygen atom at its ground state. EPtnO and EPtn were calculated with the above-mentioned calculation methods and corresponding k-meshes, and EO was calculated with a single oxygen atom in a cubic unit cell with 8 Å side length. The climbing image nudged elastic band method34-36 was employed to investigate the saddle points and minimum energy paths for oxygen diffusion into subsurfaces. In this report, the (2 × 2) cells corresponding to 0.25 ML oxygen coverage were used to study the minimum energy paths, transition states, and corresponding energy barriers of oxygen absorption into the subsurfaces. 3. Results and Discussion 3.1. Atomic Oxygen Absorption into (111) Subsurfaces of Pt-Based Bimetallic Alloys (PtX, X ) Co, Ni, Ru, Rh, Pd, Ir). The calculated lattice constants of fcc Pt-X alloys are listed in Table 1a. PtX alloys have smaller lattice constants than that of pure Pt in all the studied alloys involving transition metals with smaller lattice constants. For the PtX alloy surfaces there are various different fcc and hcp three-fold sites. Here, we refer to fcc-1 and hcp-1 as fcc and hcp sites, respectively, which comprise two Pt atoms and one X atom, and fcc-2 and hcp-2 as fcc and hcp sites, respectively, which comprise one Pt atom and two X atoms. For the subsurface tetrahedral site underneath

Oxygen Absorption into Alloy Subsurfaces

J. Phys. Chem. C, Vol. 112, No. 13, 2008 5059 TABLE 3: Atomic Oxygen Charge at hcp Site, in Transition-State Site, and inside Subsurface Tetrahedral Site and Charge Changes (∆C) of Atomic Oxygen during Absorption from hcp-2 to tetra-2. Negative or Positive ∆C Means Oxygen Obtains or Loses Electron Density hcp-2 TS tetra-2 ∆Chcp-TS ∆Ctetra-TS ∆Ctotal

Figure 1. Top view of fcc-1, fcc-2, hcp-1, and hcp-2 sites on PtX alloy surfaces. Pt atoms are in gray, and alloyed atoms are in blue. Only the top two layers are shown in this figure for better visualization.

TABLE 2: Pt-O and X-O Bond Lengths (RPtO/Å and RXO/Å, Respectively) of Atomic Oxygen Atom Adsorbed on fcc-2 and hcp-2 Sites of Pt-X Alloys. See Text and Figure 1 for Definition of Sites fcc-2 RXO hcp-2 RXO

RPtO 1.862 RPtO 1.876

Pt-Co

Pt-Ni

Pt-Ru

Pt-Rh

Pt-Pda

Pt-Ir

2.082 1.853 2.072 1.859

2.046 1.987 2.033 1.984

2.070 2.004 2.105 1.997

2.054 2.077 2.048 2.144

2.022 2.029 2.008 2.009

2.084 2.102

a For Pt-Pd, the Pt-O and X-O bonds correspond to adsorption on fcc-1 and hcp-1.

the hcp site, which is the most energetically stable subsurface site for atomic oxygen,25 there are two different types of the sites. We define tetra-1 and tetra-2 as the subsurface tetrahedral sites which are comprised of three Pt atoms and one X atom beneath the hcp-1 site and one Pt atom and three X atoms beneath the hcp-2 site, respectively (Figure 1). The binding energy (Eb) of atomic oxygen adsorbed on the fcc and hcp hollow sites and the absorption energy (Eab) of atomic oxygen absorbed in the subsurface tetrahedral sites were calculated as explained in the previous section. Table 1a shows the binding energies (Eb) on various fcc and hcp sites and absorption energies (Eab) inside two different subsurface sites. The fcc-2 sites are the most energetically stable sites for the atomic oxygen adsorption on PtX alloy surfaces except in the PtPd case; for example, the binding energy of 4.94 eV on fcc-2 is larger than those on other sites in PtIr, whereas the binding energy of 4.72 eV on fcc-1 site is larger than those in other sites in the PtPd case, in agreement with previous reports.37 Comparing the two hcp sites, we found that hcp-2 sites are energetically more favorable than those in hcp-1 sites in most of the studied alloys except in the PtPd alloy in which hcp-1 and hcp-2 have very similar binding energies. Both the fcc-2 and hcp-2 sites are composed of one Pt atom and two alloyed metal atoms. Table 2 shows the metal-O bond lengths of adsorbed atomic oxygen on fcc-2 and hcp-2 sites. From the data we find that the bond lengths of Pt-O are longer than those of the alloyed metal-O bond (RXO) except in the Pt-Pd case, which could explain why in the PtPd alloy the most favorable site is fcc-1 instead of fcc-2: more Pt atoms in fcc-1 than in fcc-2 and the RPtO distance is shorter than RPdO. Compared with the binding energies on fcc-2 sites of alloys (fcc-1 of PtPd) with that of pure platinum, the binding energies are more negative on alloy surfaces than those on a pure platinum surface, indicating that certain alloy species such as Co, Ni, Ru, Rh, and Ir can thermodynamically favor the atomic oxygen adsorption on Pt-based alloy metallic surfaces. Table 1a also shows that the absorption energy of tetra-2 is more negative than that of tetra-1 except for PtPd in which tetra-1

PtCo

PtNi

PtRu

PtRh

PtPda

PtIr

Pt

-0.92 -1.00 -1.07 -0.08 0.06 -0.02

-0.89 -1.03 -1.06 -0.14 0.03 -0.11

-0.92 -1.06 -1.06 -0.14 -0.00 -0.14

-0.80 -1.06 -1.01 -0.26 -0.05 -0.31

-0.78 -0.91 -0.92 -0.13 0.01 -0.12

-0.85 -1.16 -1.05 -0.31 -0.11 -0.44

-0.74 -1.07 -0.95 -0.33 -0.12 -0.45

a For Pt-Pd, hcp-1 and tetra-1 are noted instead of hcp-2 and tetra-2.

and tetra-2 have very similar absorption energies, indicating that tetra-2 is the most energetically stable subsurface site in most cases. Therefore, the atomic oxygen inside tetra-2 sites (tetra-1 of PtPd) was considered the final state in the subsequent absorption process investigations. Figure 2a shows the minimum energy paths for atomic oxygen migration from the most energetically stable surface adsorption sites to the most energetically stable subsurface sites. Atomic oxygen diffuses from fcc sites (A in Figure 2a) to hcp sites (B in Figure 2a) across bridge sites, and then oxygen migrates into subsurface sites (D in Figure 2a). The surface diffusion energy barriers on PtX alloys (Table 1b) are lower than that of 0.60 eV on pure Pt, and the corresponding reverse energy barriers (Table 1b) are similar or higher than that of 0.20 eV except on PtPd, indicating that the atomic oxygen diffusion on (111) surfaces of Pt-based alloys with Co, Ni, Ru, Rh, Pd, and Ir is kinetically easier than that on pure platinum surfaces. We note that although that the accuracy of our estimated barriers permits us only to identify trends, the results are still useful in a comparative analysis. Higher absorption energy barriers and reverse absorption energy barriers lower than those of pure Pt can kinetically prevent atomic oxygen absorption into the subsurface. The results that follow indicate that the introduction of a second transition-metal species could be an effective way to prevent atomic oxygen absorption into the subsurface. Since we have shown for pure Pt that the energy barrier of absorption into the subsurface is much higher than that of the atomic oxygen surface diffusion on the surface,25 here we investigate oxygen absorption starting from the hcp-2 to the tetra-2 subsurface site (most energetically stable sites) in PtX alloys and compare the effects of the alloyed specie X on the absorption process. Figure 2b shows the minimum energy path of atomic oxygen absorption into the subsurface from hcp-2 to tetra-2 sites in various Pt-X alloys (hcp-1 to tetra-1 in PtPd). O in the transition state is closer to the hcp surface site than to the subsurface site (see inset in Figure 2a). As shown in Table 1b, the absorption energy barriers in PtIr (2.88 eV) and PtRh (2.58 eV) are higher than that in pure Pt (2.22 eV) (Figure 2b, from hcp-2 to tetra-2), indicating that the introduction of Ir and Rh can increase the absorption energy barrier, thus kinetically helping to prevent O absorption into the subsurface. The reverse absorption energy barriers of PtIr and PtRh are 0.66 and 0.64 eV, respectively, which are close to the 0.70 eV value of pure Pt, indicating that the introduction of 50% Ir or Rh do not significantly decrease this barrier. The absorption energy barriers of PtCo, PtRu, PtNi, and PtPd (Table 1b) are lower than the 2.22 eV value of pure Pt. However, the reverse absorption energy barriers of Pt-Co, PtRu, Pt-Ni, and Pt-Pd (Table 1b) are lower than the 0.70 eV value of pure Pt, indicating that the introduction of 50% Co, Ru, Ni, and Pd decreases the energetic stabilities of subsurface


Gu and Balbuena

Figure 2. Minimum energy path of atomic oxygen diffusion from the fcc-2 surface site to the subsurface tetrahedral/tetra-2 site in PtX(111) surface cell at the atomic oxygen coverage of 0.25 ML. (a) Pathway shows migration from fcc-2 to hcp-2 into tetra-2 (fcc-1 to hcp-1 into tetra-1 in PtPd). (b) Migration from hcp-2 into tetra-2 (hcp-1 into tetra-1 in PtPd). Key: A, O adsorbed on fcc-2; B, O adsorbed on hcp-2; C, O in hcp-2 (transition-state site); D, O absorbed inside tetra-2. For the insets in part a, Pt atoms are in gray, alloyed atoms are in blue, and O atom is in red.

Figure 3. Top view of fcc-I, fcc-II, hcp-I, hcp-II, and hcp-III sites on Pt3X alloy surface. Pt atoms are in gray and alloyed atoms are in blue. Only the top two layers are shown in this figure for better visualization.

oxygen in those alloys. Especially in the Pt-Co and Pt-Ru cases, the reverse energy barriers are close to zero, indicating that from the kinetic point of view O almost cannot exist inside Pt-Co and Pt-Ru subsurfaces (at least at 0.25 ML surface coverage), and therefore, these alloys also may prevent atomic oxygen absorption into the subsurface. In our previous reports,24,25 we discovered that the absorption energy barrier increases with increasing coverage due to the

higher oxygen repulsion and higher geometric lattice deformation at high oxygen coverages. The geometric deformations of PtX alloys during the absorption process are listed in Table 1a of the Supporting Information. However, in contrast to those of pure Pt surfaces, the deformations of alloy lattices do not follow a clear trend in relation to their corresponding absorption energy barriers. As another indicator of the O migration process, Table 3 shows the changes in the oxygen charge (∆C, defined as difference of O charges when O is located at each of two sites) during the absorption process in PtX alloys, which were calculated using the Bader charge density analysis method.38 Atomic oxygen becomes more charged when going from the hcp-2 to the transition state (TS) in PtIr and PtRh alloys (Table 3), which have higher absorption energy barriers than other alloys (Figure 2 and Table 1b), indicating that oxygen withdrawing more electron density from the metal atoms contributes to higher absorption energy barriers. Note also that the charges on the adsorbed oxygen atom indicated in the first row of Table 3 reflect the oxidative tendency of the alloyed metal; thus, it is higher for Co, Ni, and Ru alloys than for Rh, Ir, and Pd alloys, and the lowest negative charge corresponds to the pure Pt surface (the less favorable to oxidation). At the transition-state site most metals show similar degree of charge on the oxygen atom, and again, a different behavior is found between the group of alloys


J. Phys. Chem. C, Vol. 112, No. 13, 2008 5061

Figure 4. Minimum energy path of O diffusion from the fcc-I surface site to the subsurface tetrahedral/tetra-I site in Pt3X(111) surface cell at O coverage of 0.25 ML. Key: A, O adsorbed on fcc-I; B, O adsorbed on bridge site; C, O adsorbed on hcp-I; D, O in hcp-I (transition-state) site; E, O absorbed inside tetra-I. Insets color key: Pt in gray, alloyed atoms in blue, and O in red.

TABLE 4: (part a). Calculated Binding Energies (eV) of 0.25 ML Atomic Oxygen on fcc and hcp Hollow Sites, and Absorption Energies (eV) of 0.25 ML Inside Tetrahedral Sites beneath hcp Hollow Sites of Pt3X Alloys (X ) Co, Ru, Rh, Ir). See Text and Figure 3 for Definition of Sites. (part b) Calculated Diffusion and Absorption Barriers (in eV) for the Respective Forward and Reverse Processes of Pt3X(111) Alloy Surfaces Shown in Figure 4. The Barriers Corresponding to Pure Pt(111) Surfaces Are Also Shown for Comparison

Pt3Co Pt3Ru Pt3Rh Pt3Ir Pt

fcc-I 2Pt-1X

fcc-II 3Pt

hcp-I 2Pt-1X

-4.97 -5.15 -4.87 -4.84

-4.47 -4.23 -4.49 -4.23

-4.64 -4.78 -4.50 -4.44

-4.61

Pt3Co Pt3Ru Pt3Rh Pt

Part a hcp-II 3Pt -4.17 -3.96 -4.14 -3.91 -4.21

hcp-III 2Pt-1X

tetra-I 2Pt-2X

tetra-II 4Pt

tetra-III 3Pt-1X

-4.65 -4.92 -4.51 -4.58

-3.48 -3.11 -3.12 -2.82

-1.73 -2.05 -2.44 -2.22 -2.66

-2.89 -2.75 -2.76 -2.55





0.55 0.44 0.61 0.60

0.22 0.08 0.24 0.20

1.39 1.79 1.81 2.22

0.23 0.11 0.43 0.70

containing Co, Ni, and Ru and that of Rh, Ir, and pure Pt, with respect to the variation of charge from the transition state to the subsurface site (the negative charge on O increases further for the first group whereas it decreases for the most noble metals). As a result, there is a correlation between the magnitude of the energy barrier for adsorption and the total charge change involved in the process (last row in Table 3). However, other factors such as the high extent of lattice geometric deformation also contribute to the increase in the absorption energy barriers; therefore, the energy barrier does not increase monotonically with an increase in charge transfer. For the reverse process from tetra-2 to TS, the charge changes of atomic oxygen are smaller than the corresponding absorption from hcp-2 to the TS. Clear negative charges increasing from tetra-2 to TS in PtIr and pure Pt contribute to higher reverse energy barriers (Table 3, Table 1b, and Figure 2). 3.2. Atomic Oxygen Absorption into (111) Subsurfaces of Pt3X (X ) Co, Ru, Rh, Ir). According to the above investigation, the introduction of Ir and Rh can increase the energy barrier of O absorption into the subsurface and the introduction of Co and Ru can decrease the reverse energy barrier of O absorption. Both effects: increased absorption energy barrier and decreased reverse absorption energy barrier could kinetically inhibit O absorption into the subsurface. To understand the effect of alloy composition, we also investigated Pt-based bimetallic alloys with

3:1 ratios. The calculated lattice constants of Pt3Co, Pt3Ru, Pt3Rh, and Pt3Ir are 3.927, 3.950, 3.971, and 3.969 Å, respectively, in fair agreement with the corresponding available experimental lattice constants of Pt3Co and Pt3Ir, which are 3.8539 and 3.90 Å.40 On Pt3X surfaces there are various types of three-fold hollow sites (fcc and hcp) due to the different compositions (Figure 3). Here, we designate fcc-I to fcc sites comprising two Pt atoms and one X atom; fcc-II are fcc sites composed of three Pt atoms; hcp-I are hcp sites comprising two Pt atoms and one X atom and located above the X atom; hcp-II are hcp sites consisting of three Pt atoms and located above a Pt atom; hcpIII are hcp sites composed of two Pt atoms and one X atom and located above a Pt atom. Also, there are three types of tetrahedral subsurface sites which are defined by their compositions: tetra-I (two Pt atoms and two X atoms) located beneath hcp-I; tetra-II (four Pt atoms) beneath hcp-II; and tetra-III (three Pt atoms and one X atom) beneath hcp-III. Table 4a lists the binding energies (Eb) of atomic oxygen at 0.25 ML coverage on fcc and hcp sites and absorption energies (Eab) of atomic oxygen absorption inside subsurface tetrahedral sites. The fcc-I (2Pt + X) sites are the most energetically stable sites for O adsorption on the catalyst surface in all four Pt alloys according to their more negative binding energies (Table 4a). Adsorptions on hcp-I and hcp-III (2Pt + X) sites have similar binding energies, more negative than those on the hcp-II (3Pt) site.


Gu and Balbuena for pure Pt, 0.60 eV. Then O migrates from the hcp-I site into the tetra-I site (E, Figure 4), and in the transition state O is located close to the hcp-I transition-state site (D, Figure 4). The energy barriers from hcp-I to tetra-I in Pt3Co, Pt3Ru, Pt3Rh, and Pt3Ir (Table 4b) are lower than the 2.22 eV value of pure Pt.29 Compared with the corresponding X component of the PtX alloys in the previous section, each of the absorption energy barriers of Pt3X (Table 4b) are lower than the corresponding ones of PtX (X ) Co, Ru, Rh, Ir), Table 1b. One reason could be the weaker binding energies on Pt3X surfaces than those on PtX surfaces (Table 1a and Table 4a) which would make easier for O to migrate to the subsurface. However, as discussed in our previous work24,25 other factors such as lattice reorganization and charge transfer during the migration process may contribute to the absorption barriers, but the specific effects are not easy to quantify in alloys. The reverse absorption energy barriers of Pt3Co, Pt3Ru, Pt3Rh, and Pt3Ir (Table 4b) are smaller than 0.the 70 eV value of pure Pt, indicating that the introduction of 25% alloyed Co, Ru, Rh, and Ir can decrease the energetic stability of subsurface O. Compared with the PtX alloys (X ) Co, Ru, Rh, Ir), Pt3Ir and Pt3Rh have lower absorption energy barriers than PtIr and PtRh, whereas Pt3Co and Pt3Ru have higher reverse energy barriers than PtCo and PtRu, indicating that the amount of alloyed transition metal also has particular effects on the absorption kinetics. 3.3. Atomic Oxygen Absorption into (111) Subsurfaces of Pt2IrX (X ) Co, Ru). The results in Section 3.1 regarding O absorption into alloy PtX subsurfaces showed that the introduction of 50% Ir can increase the absorption energy barrier and the introduction of 50% Co or Ru can decrease the reverse energy barrier of atomic oxygen absorption. In this section, we present the O absorption behavior in ternary Pt alloys, Pt2IrCo and Pt2IrRu, to investigate the combined effects of two alloyed metals. We selected Co and Ru because of their extremely low reverse absorption barriers (thus making O unstable in the subsurface), but other elements such as Ni are also promising. Similar to the cases in the above two sections, there are various adsorption and absorption sites due to different atomic compositions. Here, we define that fcc-i and hcp-i are those fcc and hcp sites consisting of one Pt, one Ir and one alloyed atom (X ) Co, Ru), fcc-ii and hcp-ii are fcc and hcp sites comprising two Pt atoms and one Ir atom, and fcc-iii and hcp-iii are fcc and hcp sites composed of two Pt atoms and one alloyed atom (X ) Co, Ru, Figure 5). The absorption subsurface sites tetra-i (two Pt atoms, one Ir atom, and one alloyed X atom), tetra-ii (two Pt atoms and two Ir atoms), and tetra-iii (two Pt atoms and two alloyed X atoms) are located underneath hcp-i, hcp-ii, and hcp-iii sites, respectively (Figure 5). Table 5a shows the

Figure 5. Top view of fcc-i, fcc-ii, fcc-iii, hcp-i, hcp-ii, and hcp-iii sites on Pt2IrX alloy (111) surface. Pt atoms are in gray, Ir atoms are in brown, and alloyed atoms (X ) Co, Ru) are in blue. Only the top two layers are shown in this figure for better visualization.

The introduction of a second species (Co, Ru, Rh, and Ir) in Pt3X also increases the binding energies compared with that of pure Pt (Eb in fcc-I more negative than that in fcc sites of pure Pt, Eb in hcp-I more negative than in Pt hcp, Table 4a). According to the absorption energies displayed in Table 4a, the most energetically stable subsurface site is the tetra-I comprising two Pt atoms and two alloyed metal atoms located beneath the hcp-I site. It is interesting that the absorption energies for the tetra-I and tetra-III sites are consistently stronger than those in pure Pt due to the presence of the X elements having higher affinity for oxygen, whereas the ones in tetra-II sites (composed of all Pt atoms) are always weaker than those in pure Pt subsurfaces. The weaker absorption energies in the tetra-II sites follow a behavior similar to the decrease of the surface adsorption energies for O adsorbed on Pt-skin surfaces where the subsurface contains other transition metals with higher affinity for oxygen.41 Thus, the interaction of Pt atoms with X transition-metal atoms (both in the tetra-II subsurface sites as in the Pt-skin surface) reduces the strength of their interaction with atomic oxygen, yielding weaker absorption or adsorption energies. Figure 4 shows the minimum energy path of the O absorption into the subsurface site from the fcc-I site (most energetically stable on-surface adsorption site) to the most energetically stable tetra-I subsurface site. O diffuses from the fcc-I site (A, Figure 4) to hcp-I (C, Figure 4) through the bridge site (B, Figure 4) which is formed by one Pt atom and one alloyed X atom. The energy barriers for the on-surface diffusion are lower than that

TABLE 5: (part a) Calculated Binding Energies (eV) of 0.25 ML Atomic Oxygen on fcc and hcp Hollow Sites and Absorption Energies (eV) of 0.25 ML inside Tetrahedral Sites beneath hcp Hollow Sites of Pt2IrX Alloys (X ) Co, Ru). See Text and Figure 5 for Definition of Sites. (part b) Calculated Diffusion and Absorption Barriers (in eV) for the Respective Forward and Reverse Processes of Pt2IrX(111) Alloy Surfaces Shown in Figure 6. The Barriers Corresponding to Pure Pt(111) Surfaces Are Also Shown for Comparison Part a Pt2IrCo Pt2IrRu

Pt2IrCo Pt2IrRu Pt

fcc-i

fcc-ii

fcc-iii

hcp-i

hcp-ii

hcp-iii

tetra-i

tetra-ii

tetra-iii

-5.28 -5.42

-4.80 -4.58

-4.43 -4.76

-5.07 -5.21

-4.56 -4.30

-4.44 -4.65

-2.78 -2.46

-1.89 -2.05

-2.99 -2.65





0.89 0.83 0.60

0.07 0.05 0.20

1.79 2.49 2.22

0.34 0.35 0.70


J. Phys. Chem. C, Vol. 112, No. 13, 2008 5063

Figure 6. Minimum energy path of O migration from the fcc-i surface site to the subsurface tetrahedral/tetra-iii site in Pt2IrX(111) surface cell at O coverage of 0.25 ML. Key: A, O adsorbed on fcc-i; B, O adsorbed on bridge site; C, O adsorbed on hcp-iii; D, O in hcp-iii site (transition state); E, O absorbed inside tetra-iii. Insets color key: Pt in gray, Ir in brown, alloyed atom in light blue, and O in red.

Figure 7. Three-fold adsorption sites on Pt skins of Pt3Ru and Pt2IrRu. (a) fcc-a, fcc-b, hcp-a, and hcp-b on Pt skin of Pt3Ru. (b) fcc-A, fcc-B, fcc-C, hcp-A, hcp-B, hcp-C on Pt skin of Pt2IrRu. Pt atoms are in gray, Ir atoms are in brown, and Ru atoms are in blue. Only the top three layers are shown in this figure for better visualization.

binding and absorption energies of atomic oxygen. The tetraiii (PtPtXX, X ) Co, Ru) is the most energetically stable subsurface site for O absorption in Pt2IrCo and Pt2IrRu with absorption energies of -2.99 and -2.65 eV, respectively. fcc-i sites are the most energetically stable sites for O surface adsorption according to their corresponding binding energies. Figure 6 illustrates the minimum energy path for O migration from fcc-i to tetra-iii. Similarly to the previous cases, O diffuses to hcp-iii (C in Figure 6) first, and then it migrates into the tetra-iii subsurface site (E in Figure 6). The surface diffusion energy barriers of Pt2IrCo and Pt2IrRu (Table 5b) are higher than the energy barrier of 0.60 eV on a pure Pt surface. The reverse energy barriers for O surface diffusion of Pt2IrCo and Pt2IrRu (Table 5b) are much smaller than the 0.2 eV value of pure platinum, indicating that O adsorption on hcp-iii is kinetically very unstable. The absorption energy barriers from hcp-iii to tetra-iii subsurface sites and the corresponding reverse energy barriers for Pt2IrCo, Pt2IrRu, and pure Pt are shown in Table 5b. Thus, the introduction of alloyed Ir and Ru to pure Pt can increase the energy barrier for absorption and decrease the reverse energy barrier, indicating that the combination of Ir and Ru can kinetically inhibit O absorption. Though the absorption barrier from hcp-iii to tetra-iii of Pt2IrCo is lower than that of Pt, its reverse absorption energy barrier (for migration from the subsurface site to the surface) is lower than that of pure platinum, suggesting that subsurface O may be more

Figure 8. Minimum energy path of atomic oxygen diffusion from the most energetically stable surface site to the most stable subsurface tetrahedral site in various Pt-based alloys at O coverage of 0.25 ML. Key: A, O adsorbed on fcc-i; B, O adsorbed on bridge site; C, O adsorbed on hcp-iii; D, O in hcp-iii site; E, O absorbed inside tetra-iii.

unstable in the ternary alloy than inside a pure Pt subsurface site. Such ternary alloy has already been successfully tested for fuel cell electrode applications.42 3.4. Atomic Oxygen Absorption into (111) Subsurfaces of Pt-Skin/Pt2IrRu. The formation of a Pt “skin” has been observed for Pt-based alloy cathode catalysts, which probably results from the alloyed atoms tending to leach out of the alloys during fuel cell operation in an acidic electrolyte, while the Pt atoms may segregate and rearrange on the cathode surface.43-45 Pt skin can also be formed by annealing the alloys at high temperature46 due a strong tendency of the Pt atom to segregate to the surface when Pt is alloyed with most other transition metals.47,48 The Pt skin thickness is estimated to be less than a few layers.44,46 Here, Pt skins are modeled as a Pt monolayer on top of Pt3Ru (111) and Pt2IrRu(111) substrates having the same lattice constants as those in the corresponding Pt alloys. The three-fold surface adsorption sites for alloys with different compositions are shown in Figure 7. Figure 7a shows an fcc-a site vertically above an alloyed atom in the third layer and an fcc-b site vertically above a Pt atom in the third layer, while there is an hcp-a site above an alloyed atom in the second layer and an hcp-b site above a Pt atom in the second layer. The tetrahedral subsurface sites of tetra-a (three Pt atoms and one


Gu and Balbuena

TABLE 6: (part a) Calculated Binding Energies (eV) of 0.25 ML Atomic Oxygen on fcc and hcp Hollow Sites of Pt Skins and Absorption Energies (eV) of 0.25 ML inside Tetrahedral Sites beneath hcp Hollow Sites of Pt3Ru and Pt2IrRu with Pt Skins, Respectively. See Text and Figure 7 for Definition of Sites. (part b) Calculated Diffusion and Absorption Barriers (in eV) for the Respective Forward and Reverse Processes of Pt skin/Pt2IrRu and Pt skin/Pt3Ru alloy (111) Surfaces Shown in Figure 8. The Barriers Corresponding to Pure Pt (111) Surfaces Are Also Shown for Comparison Pt skin/Pt2IrRu Pt skin/Pt3Ru

fcc-A -4.28 fcc-a -4.36

Pt skin/Pt2IrRu Pt skin/Pt3Ru Pt

fcc-B -4.20 fcc-b -4.16

fcc-C -4.07 hcp-a -3.97

Part a hcp-A hcp-B -3.90 -3.81 hcp-b tetra-a -3.86 -2.71

hcp-C -3.86 tetra-b -2.15

tetra-A -2.28

tetra-B -2.08

tetra-C -1.74





0.59 0.65 0.60

0.21 0.15 0.20

2.96 2.30 2.22

1.34 1.14 0.70

alloyed atom) and tetra-b (four Pt atoms) are located beneath hcp-a and hcp-b sites, respectively, in surfaces of Pt3Ru alloys with Pt skins. For Pt2IrRu with Pt skin, Figure 7b shows three types of fcc sites: a fcc-A site vertically above an alloyed Ru atom in the third layer, a fcc-B site vertically above an Ir atom in the third layer, and a fcc-C site vertically above a Pt atom in the third layer. Figure 7b also shows three types of hcp sites: an hcp-A site above an alloyed Ru atom in the second layer, an hcp-B above an Ir atom in the second layer, and an hcp-C site above a Pt atom in the second layer. There are three types of subsurface sites located beneath corresponding hcp sites (hcpA, hcp-B, and hcp-C): tetra-A, tetra-B, and tetra-C, respectively. The binding energies on Pt skins and absorption energies into subsurface sites of atomic oxygen in a Pt3Ru alloy with Pt skin are listed in Table 6a. The most energetically stable adsorption sites of Pt skin on Pt3Ru alloys are fcc-a sites according to their most negative binding energies, illustrating the specific effects that the alloyed atoms in the second layer or third layer have on atomic oxygen adsorption (Table 6a). The most energetically stable subsurface sites are tetra-a sites based on their most negative absorption energies, indicating the effect that the specific composition of the tetrahedral subsurface sites has on absorption (Figure 7a and Table 6a). For Pt2IrRu alloys with Pt skin, the most energetically stable on-surface adsorption sites and subsurface absorption sites are fcc-A and tetra-A (three Pt atoms and one alloyed Ru atom) sites based on their most negative binding energies and absorption energies (Table 6a). Minimum energy paths of O diffusion from the most stable Pt skin sites (fcc-a, fcc-A) to the most stable subsurface sites (tetra-a, tetra-A) are shown in Figure 8, in which minimum energy paths of other types of Pt-based alloys are also included for comparison. Similarly to the above Pt alloy cases, O diffuses from fcc-a (fcc-A in Pt2IrRu with Pt skin) to hcp-a (hcp-B in Pt2IrRu with Pt skin) first on the Pt skin surface of Pt3Ru, and then it migrates into the subsurface site. The surface diffusion on Pt skins above Pt3Ru and Pt2IrRu are very similar to the surface diffusion on pure Pt surfaces as indicated by their forward and reverse energy barriers (Table 6b). For the O absorption process from Pt skin above Pt3Ru into the subsurface, the energy barrier (2.30 eV, Table 6b) is slightly higher than the 2.22 eV value of pure Pt with a reverse energy barrier of 1.14 eV larger than the 0.70 eV value of pure Pt, indicating that O is kinetically more stable in the subsurface of Pt3Ru with Pt skin. Compared with Pt3Ru without Pt skin, both the absorption energy barrier and its corresponding reverse energy barrier of the Ptskin/Pt3Ru surface are higher due to the presence of the Pt skin layer. Pt skin/Pt2IrRu has a higher absorption energy barrier that of pure Pt (Table 6b), indicating that the Pt2IrRu surface can inhibit the O absorption process according to this criterion, and its reverse energy barrier is also higher

than that of pure platinum. Pt2IrRu alloy without Pt skin has a much smaller reverse absorption energy barrier (Table 5b) compared with that of Pt2IrRu with Pt skin, indicating that the existence of the Pt skin layer increases the reverse energy barrier. 4. Conclusions Adding transition metals (Co, Ni, Ru, Rh, Pd, and Ir) to pure Pt can influence the magnitudes of the O absorption energy barrier and reverse the energy barrier, as inferred from the minimum energy path in which O diffuses from the most energetically stable fcc site to the hcp site above the most energetically stable subsurface site, and then it migrates into the subsurface site. In the Pt alloys with 50% alloyed transition metals, the introduction of Ir and Rh increases the absorption energy barrier compared with pure Pt surfaces which can kinetically inhibit O absorption, whereas the introduction of Co and Ru can decrease the reverse energy barrier of absorption, making subsurface oxygen more unstable than that inside a pure Pt subsurface. The geometric deformation analysis of the alloys during the atomic oxygen absorption could not show a clear relationship between the magnitude of deformation and the absorption energy barrier, though the energetic cost of lattice deformation is one reason explaining the increase of the absorption energy barrier. Charge analysis of atomic oxygen during its migration to the subsurface indicates that alloys with higher absorption energy barriers or higher reverse energy barriers show more electron density transfer to atomic oxygen at some point in the absorption or during the corresponding reverse process. Comparison of the O absorption process in Pt3X alloys with that in PtX alloys suggests that both the nature of the alloyed metal species and the alloy composition have effects on the O absorption kinetics. Atomic oxygen absorption into the Pt2IrRu subsurface shows that the introduction of two types of alloyed metals such as Ir and Ru can simultaneously increase the O absorption energy barrier and decrease the reverse energy barrier compared with pure Pt, which can inhibit O absorption more effectively. Pt skins on Pt alloys increase both O absorption energy barriers and their corresponding reverse energy. Although the reported absorption and diffusion barriers have not been calculated under an applied potential, we estimate that the results are extremely valid to establish trends regarding the behavior of alloys in comparison with that of pure Pt surfaces. Acknowledgment. This work was supported by the Department of Energy, Grant No. DE-FG02-05ER15729. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC03-

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Atomic Oxygen Absorption into Pt-Based Alloy Subsurfaces

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