First-Principle Study of the Adsorption and Dissociation of O2 on Pt

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J. Phys. Chem. C 2009, 113, 20657–20665

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First-Principle Study of the Adsorption and Dissociation of O2 on Pt(111) in Acidic Media Lihui Ou, Fan Yang, Yuwen Liu, and Shengli Chen* Hubei Electrochemical Power Sources Key Laboratory, Department of Chemistry, Wuhan UniVersity, Wuhan 430072, China ReceiVed: June 25, 2009; ReVised Manuscript ReceiVed: September 7, 2009

The adsorption and dissociation of O2 on the Pt(111) surface in both the absence and the presence of the hydrated proton were investigated using ab initio DFT calculations to evaluate the role of the proton in the initial steps of the Pt-catalyzed oxygen reduction reaction (ORR) in acid solutions. The results from geometric optimization and electronic structure and minimum energy path calculations indicated that, although in both cases, a t-b-t configured chemisorption state serves as the most stable molecular precursor for the dissociation of O2, the formation of this precursor state and its dissociation are substantially altered in the presence of the hydrated proton. The interactions of O2 with the hydrated proton inhibit the formation of the t-b-t precursor state but facilitate its dissociation. In the presence of the hydrated proton, the t-b-t molecular chemisorption of O2 is preceded by a metastable end-on chemisorption state that is protonated while the t-b-t state itself is not protonated. That is, the chemisorption of O2 on Pt in acid solution may undergo a sequential protonation and deprotonation process. It is also shown that the transformation from the end-on state to the t-b-t state is nearly a nonactivated process with the reaction energy larger in amount than the activation energy required for the subsequent dissociation. The formation of the end-on state via a proton-coupled electron-transfer process is, therefore, identified as the ratedetermining step in the adsorption and dissociation processes of O2 on the Pt(111) surface in acidic media. The present calculation results may provide a link between the long disputed Damjanovic’s view and the Yeager’s view on the mechanism of the initial steps in ORR. 1. Introduction The oxygen reduction reaction (ORR) on Pt plays a crucial role in electrochemical energy conversion.1,2 Despite intensive research over the past a few decades,2-9 it remains a great challenge so far for electrochemists to formulate the initial steps for Pt-catalyzed ORR due to a number of open questions that remain, for example, whether the oxygen dissociates before or after it is hydrogenated, which one of the adsorption step and the dissociation step is the rate-determining step (rds), and so on. The main reason for these mechanistic uncertainties lies in the lack of reliable information on the sites and configuration for O2 adsorption and the interaction between the proton and different adsorption states at the Pt/ electrolyte interface.2 In the past two decades, many molecular details have been revealed for gas-phase adsorption and dissociation of O2 on the Pt surface by applying various surface science techniques and ab initio theoretical calculations. Physical and chemical adsorption of molecular O2 as well as dissociative atomic adsorption were implied by results of NEXAFS, EELS, photoemission spectroscopy, and other surface science techniques (detailed review can be found in ref 2a and references therein). The physisorption of O2 occurs at temperatures below 50 K, whereas the molecular chemisorption of O2 takes place at temperatures between 50 and 150 K, which is believed to be accompanied by a charge transfer from the Pt substrate to the O2 molecule. Two chemisorbed molecular species, the nonmagnetic peroxo (O22-) state and the paramagnetic superoxo (O2-) state, were inferred by EELS and NEXAFS, respectively. Recent ab initio * To whom correspondence should be addressed. E-mail: slchen@ whu.edu.cn. Phone: +8627-68754693. Fax: +8627-68754693.

theoretical calculations on the Pt(111)/O2 system confirmed the existence of two distinct molecular chemisorbed precursors, namely, a top-bridge-top (t-b-t) configured paramagnetic superoxo precursor and a nonmagnetic peroxo species adsorbed with slightly tilted hollow-bridge configurations (t-fcc-b/t-hcpb).10 The calculated data, such as the distance between adsorbates and the Pt surface, the O-O bond lengths, and the bond stretching frequencies, show good agreement with those estimated from the experimental measurements, indicating that ab initio theoretical calculations can provide reliable information on the adsorption of O2 on the Pt surface. Due to the essential inaccessibility of the electrode/electrolyte interface to conditions of low temperatures and ultravacumm, it is practically difficult to experimentally obtain direct molecular information on the adsorption and dissociation of O2 during the electrochemical reduction. The ab initio calculations thus become a valuable alternative for unveiling the molecular mechanism of ORR. Indeed, the mechanism of the Pt-catalyzed ORR has been the subject of a few recent ab initio studies. Using density functional theory (DFT) calculations, the adsorption and dissociation of O2 were investigated, respectively, by Anderson and his colleagues7 with a dual-Pt-atom cluster representing the electrode and a hydrated hydronium cluster of H7O3+ to model the acidic solution and by Hyman and Medlin,8 who used a Pt(111) periodical slab to model the electrode and a H5O2+ cluster to model the acid solution. These two studies both showed that t-b-t bridge adsorption between two Pt atoms is the most favored adsorption state for O2 in the presence of a hydrated hydronium ion, seemingly to support Yeager’s view5 on the ORR mechanism that bridge molecular adsorption is necessary for the direct 4-electron reduction of O2. These DFT calculations also showed that

10.1021/jp9059505 CCC: $40.75  2009 American Chemical Society Published on Web 11/04/2009

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the presence of a hydronium ion kinetically prevents the adsorbed O2 precursor from direct dissociation. Instead, a protonated precursor of bridge-adsorbed OOH* is formed prior to the dissociation step. As far as the rate-determining step (rds) is concerned, however, disagreements can be found in these two DFT studies. The calculations by Anderson et al. suggested that the formation of OOH* is the rds, which supports the proposition by Damjanovic et al.3 that the first proton-coupled electron-transfer step is the rds in ORR, whereas the results from Hyman and Medlin’s calculations showed that the dissociation of OOH* requires higher active energy than its formation, thus tending to support Yeager’s view. Recently, Wang and Balbuena9 have performed an ab initio molecular dynamics (MD) simulations study on ORR at a Pt(111) periodical slab at 350 K in the presence of a H7O3+ hydronium cluster. By incorporating ab initio calculations on the electron-electron and electron-nuclei interactions into the classic molecular dynamics method, ab initio MD simulations could provide direct images of the reaction trajectory (bond breaking and forming processes) at finite temperature. It was shown by these authors that the adsorption of the O2 molecule is accompanied by a proton transfer, forming an end-on OOH* intermediate, which can dissociate with negligible activation barrier. The formation of end-on OOH* species via a proton-coupled electron-transfer process is thus considered the rds in ORR, well supporting Damjanovic’s view. Recent ab initio calculations studies of ORR by Nørskov et al.11 and by Janik et al.12 are also worthwhile to mention. In these studies, the authors have attempted to incorporate the influence of the electrode potential in DFT calculations on the reaction energies of possible steps in ORR. The results can very much advance our understanding on the potential dependence of ORR rates and can also provide clues for designing better ORR electrocatalysts. However, the mechanistic details, for example, elementary reaction steps, are not appropriately dealt with in these studies. The calculations are based on presumed reaction steps and reactive intermediates. For example, the authors in ref 12 calculated the potential dependent reaction energies and activation barriers for the single-step reduction of adsorbed O2* to OOH*. To do so, the authors have simply assumed that the adsorption of O2 and the proton transfer are separated steps. It seems clear now from previous ab initio calculations that the proton transfer is involved in the chemisorption of O2 on the Pt surface in acid media and OOH-like precursors are likely formed prior to the dissociation step. However, it remains not clear whether an end-on molecular state of OOH* bonded to one Pt atom or bridge OOH* bonded to multiple Pt sites is the stable precursor for the following dissociation step. In addition, whether the formation of OOH* or its dissociation is the rds remains a question. The details on how a nearby hydronium ion may affect the adsorption and dissociation of O2 also require further clarification. Answers to these questions would greatly help the desire of efficient ORR electrocatalysts, which is presently one of the major limitations in the progress of the state-of-the-art fuel cell technologies. In this paper, we conducted detailed DFT calculations on the geometric and electronic structures and the minimum energy paths for the adsorption and dissociation of O2 on the Pt(111) surface in the presence and the absence of a hydrated hydronium ion so that the possible reaction intermediates and reaction pathways may be revealed. The results indicated that both end-on and bridge-configured molecular

Ou et al. chemisorption states are likely formed in the initial steps of ORR, with the protonated end-on state serving as the precursor state for the formation of the unprotonated bridge state and the latter as the precursor state for the dissociation. The transformation from the end-on state to the bridge one is almost nonactivated, and the proton-coupled electrontransfer step in which end-on OOH* forms is thus identified as the rds in the adsorption and dissociation of O2 on Pt(111) in acidic media. These findings seem to indicate that both the Damjanovic and the Yeager views, which have been long in dispute and almost equally quoted in various ORR studies, are operative but only partly for Pt-catalyzed ORR. 2. Models and Methods Calculations were performed in the framework of DFT on periodic supercells, using the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional13 for exchange-correlation and ultrasoft pseudopotentials14 for nuclei and core electrons. The Kohn-Sham orbitals were expanded in a plane-wave basis set with a kinetic energy cutoff of 30 Ry and the charge-density cutoff of 300 Ry. The Fermi-surface effects have been treated by the smearing technique of Methfessel and Paxton, using a smearing parameter15 of 0.02 Ry. Calculations were carried out with spin polarization, which is essential to properly represent the electronic structure of molecular O2. The PWSCF codes contained in the Quantum ESPRESSO distribution16 were used to implement all calculations, while figures of the chemical structures were produced with the XCRYSDEN17-19 graphical package. In most of the calculations, we used (2 × 2) three-layer fcc(111) slabs with an experimental equilibrium lattice constant of Pt (3.923 Å) to model the Pt(111) surface. Some results are obtained on (2 × 4) and (3 × 3) slabs for comparison. Brillouin zone (BZ) integrations were performed with the special-point technique, using (4 × 4), (4 × 2), and (3 × 3) uniformly shifted k-meshes for (2 × 2), (2 × 4), and (3 × 3) slabs, respectively. Vacuum layers 12 Å in thickness were added above the top layer of slabs in all cases. The Pt atoms in the bottom two layers are fixed at the experimental bulk positions, whereas the top layer is allowed to relax, and all the other structural parameters have been optimized so as to minimize the total energy of the system. Structural optimization was performed until the Cartesian force components acting on each atom were brought below 10-3 Ry/bohr and the total energies were converged to within 10-5 Ry. The climbing image nudged elastic band method (CI-NEB)20,21 was employed to locate the saddle points and minimum energy paths (MEPs). The transition-state images from the NEB calculations were optimized using the quasi-Newton method, which minimizes the forces to find the saddle point. Geometry optimization was performed for each intermediate point in MEPs, in which the bottom two layers of metal atoms were fixed while the top layer of metal atoms and all other nonmetal atoms were allowed to relax. Zero-point energy (ZPE) corrections were applied to the calculations of the activation energy barriers. To consider the ZPE contributions to the activation energy barriers, the vibrational properties were studied using density functional perturbation theory within the linear response.22 The PHONONS code16 was used to calculate ZPEs. The calculation procedure can be briefly described as follows. The dynamical matrices are first obtained for the initial state, the transition state, and the final state in a reaction path. The dynamical matrices are then Fouriertransformed to real space with which the force-constant matrices

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TABLE 1: Geometric Parameters and the Adsorption Energies for Various Adsorption States of the Pt/O2 System physisorption t-f-b chemisorption t-h-b chemisorption t-b-t chemisorption

RPt-O2 (Å)

RO-O (Å)

Ead (eV)

3.76 1.91 1.93 1.92

1.24 1.39 1.37 1.36

-0.072 -0.42 -0.34 -0.45

can be constructed, which are used to obtain the phonon density of states. Finally, ZPEs for various states are evaluated by phonon density of states. 3. Results and Discussions 3.1. Adsorption and Dissociation of O2 on Pt(111) in the Absence of Hydrated Hydronium Ion. 3.1.1. Adsorption States Determined by Geometric Optimization. A comparison between the adsorption and dissociation processes of O2 in the presence and the absence of a hydronium ion would be useful in evaluating the role of a proton in ORR. Therefore, we first conducted DFT calculations on the gas-phase adsorption and dissociation of O2 on Pt(111) by performing a series of geometry optimizations on the “Pt(111) + O2” system with a variety of initial Pt-O2 distances (z, measured as the perpendicular distance between the center of O-O bond and the Pt slab surface) and initial configurations of O2 over the Pt surface (e.g., fcc-bridge-hcp, top-bridge-top, top-fcc-bridge, and top-hcpbridge). Three bridge-configured molecular chemisorption states (top-fcc-hollow-bridge, top-hcp-hollow-bridge, and top-bridgetop) as well as a physisorption state and a dissociative adsorption state were identified in these geometric optimizations. The physical or dissociative adsorption occurs, respectively, as z is initially larger than 2.4 Å or smaller than 1.5 Å, while molecular chemisorptions occur at initial Pt-O2 distances between 1.6-2.3 Å. The equilibrium structures of these adsorption states are shown in Figure S1 in the Supporting Information. The binding sites of the atomic O in the dissociative adsorption state depend on the number of the Pt atoms on the surface. For 2 × 2 Pt(111) slab, the two atomic oxygens are located at a fcc hollow site and a top site, whereas the two O atoms occupy two hollow sites when a 2 × 4 Pt slab is used. This indicates that the coverage has a critical impact on the adsorption configuration of atomic O on the Pt(111) surface. However, the optimized states for adsorptions of molecular O2 are almost the same for 2 × 2 and 2 × 4 slabs. Table 1 gives the geometric parameters and the adsorption energies associated with these adsorption states. The adsorption energies are calculated with the energies of the bare Pt(111) slab (EPt) and the isolated oxygen molecule (EO2) as the references according to

Ead ) EPt-O2 - EPt - EO2

(1)

where EPt-O2 refers to the total energies of the optimized Pt/O2 system. The physisorption state is characterized by its almost identical O-O bond length (RO-O) to the isolated O2 molecule (1.237 Å) and the very small adsorption energy, whereas the molecular chemisorpion states are manifested by the relatively shorter Pt-O2 equilibrium distances, the significantly elongated O-O bond lengths, and the much increased adsorption energies. The geometric configurations and the corresponding adsorption energies obtained from present geometric optimizations are very similar to those obtained by Eichler and Hafner,10 who have

conducted a systematic calculation on the potential energy of the “Pt(111)/O2” system as a function of the distance z and the bond length of the O2 molecule along various reaction channels. According to the values of the adsorption energy, the t-b-t chemisorption state should represent the most stable molecular adsorption state. The physical and chemical states are also characterized by the adsorption-induced changes in the Lo¨wdin charges and the local densities of states (LDOS). The Lo¨wdin charge (the number of valence electrons) of each atom in the supercell system can be obtained from the Lo¨wdin population analysis based on the projected electron densities of states. The total Lo¨wdin charges for a certain component (e.g., Pt slab, O2 molecule, etc.) in the systems were obtained by making a sum of the Lo¨wdin charges of all atoms in it. Table 2 gives the charge gains (∆q) for various components in different supercell systems, which were calculated by subtracting the Lo¨wdin charge of the corresponding component in its isolated form from that in the optimized adsorption structures. A positive value of ∆q will imply a gain of electrons by the component. It can be seen that a loss of Lo¨wdin charge of ca. 0.48 e by the Pt(111) slab and a charge gain of about the same amount (0.45 e) by the O2 molecule occur upon the formation of the t-b-t molecular chemisorption state in the Pt/O2 system, whereas negligible charge redistribution is seen in the physisorption state. Further analysis indicates that the charge loss of the Pt slab occurs mainly in its d orbitals, and the gained charges in O2 mainly go to p orbitals (Table S1 in the Supporting Information). This means that a d-π charge transfer between the Pt substrate and O2 molecule occurs in the chemisorption process. It should be pointed out that the values of ∆q given in Table 2 may not represent the exact charge gain/loss for each component in the supercell systems since some parts of the “delocalized” or “bonding” charges may be missed in such calculations. These charges may be located in regions that are not included in the atomic orbitals within which the LDOS are integrated to obtain Lo¨wdin charges. In DFT calculations, the atomic orbitals used for charge calculations are arbitrary to some extent.23 In addition, some charges may also spill out into the vacuum region in some cases.24 Due to the same reasons, the Lo¨wdin charges do not satisfy a sum rule. For example, the Lo¨wdin charge gain of O2 does not exactly match the value of charge loss by the Pt slab in magnitude. However, this would not deny the fact that charge transfer does occur. From the LDOS curves of the isolated Pt slab and the O2 molecule (Figure 1a), it can be easily seen that the molecular orbitals for p electrons in O2 (pσ, pπ, and pπ*) are distributed over an energy interval where the d band of Pt roughly spans and that the Fermi energy level of the Pt slab falls into the gap between the spin-splited anti-π molecular orbitals (pπ*) in the O2 molecule. Therefore, electron transfer between the d orbitals of the Pt slab and the pπ* orbitals of the O2 molecule is expected when they are brought close to each other. As shown in Figure 1b, the LDOS curves for the oxygen molecule and the Pt slab in the physisorption state are almost identical to their isolated states, implying weak interaction and little charge transfer between the Pt substrate and O2 molecule. The LDOS curves for the t-b-t chemisorption state (Figure 1e), however, differ significantly from those in Figure 1a,b. The chemisorption results in significant reduction on the spin-splitting of molecular orbitals (sσ, sσ*, pπ, and pπ*) and reduction on the energy splitting between the bonding and antibonding molecular orbitals in O2, implying a weakened bond strength and elongated bond length of the O2 molecule upon chemisorption. It is also seen

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TABLE 2: Charge Gains (∆q) of the Pt Slab, O2 Molecule, and H3O+(H2O)2 Cluster in Various Adsorption States of Different Supercell Systems Pt(111)/O2/H7O3+

Pt(111)/O2 Pt slab O2 H7O3+

phys.

t-b-t chem.

diss.

end-on chem.

t-b-t chem.

diss.

∼0 ∼0

-0.4820 +0.4501

-1.0660

-0.6650 +0.4182 +0.2851

-0.8690 +0.6938 +0.1913

-1.6400

Figure 1. Local density of states for the Pt slab and O2 molecule in (a) their isolated states, (b) the physisorption state of the Pt/O2 system, (c) the physisorption state of the Pt/O2/H7O3+ system, (d) the end-on chemisorption state of the Pt/O2/H7O3+ system, (e) the chemisorption state of the Pt/O2 system, and (f) the t-b-t chemisorption state of the Pt/O2/H7O3+ system.

from Figure 1e that the pπ and pπ* orbitals of O2 are largely broadened and overlap considerably with each other, indicating that strong d-π interactions are involved in the chemisorption of O2 on the Pt surface. 3.1.2. Pathways for the Adsorption and Dissociation of O2. Figure 2 gives the minimum energy paths (MEPs) for the transformations from the physisorbed O2 to the chemisorbed molecular O2 of the t-b-t configuration and the subsequent dissociation of the t-b-t adsorbed O2 determined using the NEB method. The MEPs for the formation of the t-b-t chemisorption state and its dissociation are given here because the similar t-b-t molecular state is found also to be the most stable chemisorption state in the presence of a hydronium ion (see the following sections). It can be seen that rather a small activation energy (ca. 0.052 eV) is involved in the transformation from the physisorption state to the chemisorption molecular state. That says that the electron transfer accompanied chemisorption of O2 on the Pt surface is a rather facile process. The dissociation of the t-b-t chemisorption state requires a relatively high activation barrier (ca. 0.63 eV). Due to the strong exothermicity of the chemisorption process, however, the net barrier lying

Figure 2. Minimum energy paths (MEPs) for the transformations from the physisorption state to the t-b-t chemisorption state and the subsequent dissociation of the t-b-t adsorbed O2 in the gas phase.

between the physisorbed precursor and the dissociated atomic state is only 0.20 eV, which might be easily overcome by the thermoactive process at ambient temperatures.

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Figure 3. Various geometric configurations of the Pt(111)/O2/H7O3+ system: (a) the initial configuration for the geometric optimizations, (b) the optimized physisorption state, (c) the optimized end-on chemisorption state, (d) the optimized t-b-t bridge chemisorption state, and (e) the dissociative atomic chemisorption state.

3.2. Adsorption of O2 on Pt(111) in the Presence of Hydrated Proton. 3.2.1. Model for the SolWated Proton. To simulate the reaction at the electrochemical interface in acid solution and evaluate the role of a proton in the reaction, we used a hydrated hydronium ion cluster of H3O+ · · · (H2O)2 to model the solvated proton. Although the solvated proton is usually denoted as H3O+, it is now generally believed that H3O+ binds at least to one additional water molecule, forming the so-called Zundel cluster (H5O2+).25 There should also be existing additional solvation of H5O2+ through hydrogen bonding, which will more or less affect the reactivity of the proton in solutions. In the present H3O+ · · · (H2O)2 cluster model, the transferable proton is bounded to a water molecule to form the core of the hydronium ion that is hydrated with two other water molecules (Figure 3a). One reason for us to use such a proton model is because it has been also used in previous ab initio studies of ORR.7,9 Using the same model enables us to compare the results of present DFT calculations with previous studies, especially with the recent ab initio MD simulation study by Wang and Balbuena.9 In addition, the rationality of the use of the H3O+ · · · (H2O)2 cluster to represent the solvation of a proton in solution was also verified by the equilibrious structures of supercell systems of Pt(111)/H3O+ · · · (H2O)n and Pt(111)/O2/ H3O+ · · · (H2O)n with n varying from 0 to 3. To perform DFT calculations on these systems, a positive charge is introduced to the supercells to form a hydrated proton. In the mean time, a homogeneous negative background charge of the same amount will be imposed automatically into the supercells so that the entire periodic supercell remains neutral to avoid the divergence of Coulomb energy. It is known that the background charge will distort the distribution of electrostatic potential in the system and makes contribution to the DFT total energy. However, the geometric structures of the adsorbates at/ near the slab surface would not be substantially affected because the background charge mainly affects the potential distribution in the vacuum region.26 A question may be asked whether the positive charge is added to the hydrated proton or it is distributed uniformly in the supercell. We have performed the Lo¨wdin charge analysis on an isolated H7O3+ and the Pt(111)/H7O3+ system. It was found that the Lo¨wdin charge of H7O3+ in the Pt(111)/H7O3+ system is ca. 23.7733 e, which is very close to the calculated Lo¨wdin charge for the isolated H7O3+ (23.7667 e). This makes us believe that the charge will be added to the hydrated proton, which will be energetically more favored than uniformly distributing the charge in the entire supercell system. The geometric optimization of the “Pt/H3O+” system produced an adsorbed hydrogen on the Pt surface and a water

molecule (Figure S2a, Supporting Information). Such an instability of the bare H3O+ at the Pt(111)/H3O+ interface was also shown by Hyman and Medlin.8 This seems to imply that the proton can be reduced or specifically adsorbed on an uncharged Pt surface, which is obviously in conflict with the general belief in electrochemistry that a proton is not a specific adsorption ion and can only be reduced to an adsorbed hydrogen atom on a negatively charged Pt electrode in acid solution. As shown in Figure S2b (Supporting Information), the spontaneous proton transfer from the hydronium ion to the Pt(111) surface can be prevented by replacing H3O+ with the Zundel cluster H3O+ · · · H2O, implying that the Zundel cluster can stabilize the proton at the Pt/solution interface. As shown in Figure S2c (Supporting Information), the core hydronium ion changes its orientation at the interface upon replacing the Zundel cluster with H3O+ · · · (H2O)2, indicating that further hydration of the Zundel cluster is necessary to model the hydrated proton at the Pt/solution interface. Further addition of one hydration water to the H3O+ · · · (H2O)2 cluster, however, brings about negligible change in the configuration of the core hydronium ion at the interface (Figure S2d, Supporting Information). This seems to tell that H3O+ · · · (H2O)2 would be able to model the hydration of a proton in the present situation. In geometric optimizations of Pt(111)/O2/H3O+ · · · (H2O)n systems with initial configurations that lead to the most stable t-b-t chemisorption state (see the next subsection for details), a substantial change on the final equilibrious structure is also found when replacing the Zundel cluster with H3O+ · · · (H2O)2. For the Zundel cluster, the optimization leads to a proton transfer from the core hydronium ion to the chemisorbed oxygen molecule, forming a stable chemisorption state of bridge OOH* (Figure S3a, Supporting Information). For the H3O+ · · · (H2O)2 cluster, however, a t-b-t configured chemisorption state of O-O* is obtained without an obvious proton transfer occurring in the final optimized structure (Figure S3b, Supporting Information). This means that the Zundel cluster alone cannot stabilize the hydronium ion at the O2-adsorbed Pt/solution interface. When adding one more hydration water molecule to the H3O+ · · · (H2O)2 cluster, no substantial change in the configuration of the core hydronium ion and the O2 molecule can be seen in the optimized structures (Figure S3c, Supporting Information). These results provide further support for the rationality to use H3O+ · · · (H2O)2 (will be denoted as H7O3+ for simplicity) to represent the hydration interaction of a proton in acid solution for studying the O2 adsorption on Pt(111).

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TABLE 3: Geometric and Energetic Parameters for Various Adsorption States of the Pt(111)/O2/H7O3+ System RPt-O2 (Å) RO-O (Å) RO-H (Å) Ead′ (eV) physisorption end-on chemisorption t-b-t chemisorption

3.48 2.04 1.94

1.24 1.34 1.44

2.10 1.31 1.79

0 -0.31 -0.57

3.2.2. Adsorption States of O2 on Pt(111) in the Presence of Hydrated Hydronium Ion. Similar to the Pt/O2 system in the gas phase, physisorption and molecular chemisorption as well as dissociative atomic chemisorption states were also identified for the Pt(111)/O2/H7O3+ system through geometry optimizations with different initial Pt-O2 distances and O2 orientations (Figure 3a). The physisorption state (Figure 3b) can be trapped when placing O2 above the Pt surface with distances of z > 3.00 Å. As the initial Pt-O2 distance is shorter than 1.50 Å, the dissociation of O2 occurs, which is accompanied by a proton transfer from the H7O3+ cluster to one of the adsorbed oxygen atoms, forming an adsorbed OH at a top site and an O atom at the fcc hollow site of the Pt surface (Figure 3e). Unlike that in the gas phase, the dissociation always produces a topsite adsorbed OH regardless of the size of the Pt slab, indicating that the OH group prefers to adsorb on the top site of Pt. For the Pt(111)/O2/H7O3+ system, two molecular chemisorption states were identified through geometric optimizations when the initial Pt-O2 distances were between 1.50 Å and 3.00 Å, with one having a tilted end-on configuration (Figure 3c) and the other one having a t-b-t bridge configuration (Figure 3d). The end-on adsorption was obtained when O2 is initially in the topfcc-bridge or top-hcp-bridge configuration, whereas the bridge adsorption occurs by using the initial configurations of fccbridge-hcp and top-bridge-top. The t-b-t molecular chemisorption state obtained in the presence of a hydrated proton is very similar to the corresponding one obtained in the gas-phase Pt(111)/O2 system. In the presence of a hydronium ion, the corresponding molecular chemisorbed states of t-f-b and t-h-b configurations seen in the gas-phase Pt(111)/O2 system were not obtained. We have also performed DFT geometric optimization of a Pt(111)/O2/H3O+ · · · (H2O)12 supercell system in which a 3 × 3 Pt slab is used and the twelve H2O molecules were initially arranged as a two-water bilayer. The t-b-t configured state without proton transfer was also identified as the most stable chemisorption. Table 3 gives the geometric and energetic parameters for various adsorption states of the Pt(111)/O2/H7O3+ system. Because the relaxation of water molecules contributes a significant portion to the energy variation in the geometric optimization, it is not straightforward to calculate the absolute adsorption energy in the presence of a hydrated hydronium ion. Therefore, the total energy of the physisorption state was used as a reference to obtain a relative adsorption energy Ead′. For instance, the value of Ead′ for a certain adsorption state is the difference between the total energies of this state and the physisorption state. Similar to that in the gas-phase Pt(111)/O2 system, the physisorption results in negligible change in the O-O bond length as compared with the isolated O2 molecule. However, the presence of a proton affects the chemisorption states significantly. For instance, the O-O bond in the t-b-t chemisorption state is 1.44 Å in the presence of a hydrated proton, whereas it is 1.36 Å in the gas-phase t-b-t state. The energy difference between the t-b-t chemisorption state and the physisorption state is 0.57 eV in the Pt(111)/O2/H7O3+ system, whereas it is 0.38 eV in the gas-phase Pt(111)/O2 system. These results imply that the presence of a proton results in more

pronounced Pt-O2 interaction in the t-b-t chemisorption state. As indicated by the values of adsorption energy, the t-b-t adsorption state is much more stable than the end-on one. The relatively weak Pt-O2 interaction in the end-on state is also evidenced by the shorter O-O bond length (Table 3), less charge transfer between the Pt substrate and the adsorbed O2 molecule (Table 2), and less reduction of the spin-splitting of the molecular orbital than that in the end-on state (Figure 1d). Thus, it can be concluded that the t-b-t configured adsorption is the most favorite molecular chemisorption state of O2 on Pt(111) in both the gas phase and the solution environments. The LDOS maps for O2 and Pt in the physisorption state in the presence of a hydrated proton (Figure 1c) are very similar to those in the gas-phase physisorption state (Figure 1b) and their isolated states (Figure 1a). However, the LDOS maps of the chemisorption states in the presence of a hydrated proton show obvious differences from that in the gas phase. For instance, the t-b-t chemisorption states in the presence of a hydronium ion show more reduced spin-splitting of bonding sσ and antibonding sσ* orbitals (nearly disappearing) and further reduction of the energy splitting between these orbitals (Figure 1f) as compared with the gas-phase t-b-t chemisorption state. This infers a more pronounced stretching of the O2 molecule. In addition, molecular pπ and pπ* orbitals of O2 become more pronouncedly broadened in the presence of a hydrated proton, indicating enhanced d-π interaction between the Pt substrate and the adsorbed O2 molecule. A particularly interesting characteristic in the LDOS in Figure 1f is that the molecular orbitals derived from the p electrons of O2 extend significantly to a high energy level, and a sharp localized antibonding π* orbital peak occurs at an energy level near 5 eV. This should be a result of strong coupling between the d band of the substrate and the antibonding π* orbital of the O2 molecule, which results in a split-off state at the upper level of the d band above the Fermi level.27 Actually, the peak at an energy level of ∼5 eV is also seen in the LDOS curve of O2 in the gas-phase t-b-t chemisorption state (Figure 1e), but with a rather small height. This means that the charge donation from d orbitals of Pt to the antibonding π* orbitals of O2 is more pronounced in the presence of a hydrated proton, in agreement with the Lo¨wdin charge data given in Table 2. As shown in Figure 1d, the endon chemisorption results in less reduction of spin-splitting of bonding sσ and antibonding sσ* and less broadening of p orbitals than the t-b-t adsorption. In addition, the DOS peak at an energy level of ∼5 eV is almost unseen in the end-on state, indicating that the Pt-O2 interaction in the end-on state is much weaker than that in the t-b-t states. The population analysis of the dissociative state of the Pt(111)/O2/H7O3+ system revealed that the Pt substrate loses ∼1.64 electrons as compared with the isolated Pt slab, which is much larger than that in the molecular chemisorption state. This implies that the dissociation involves further electron transfer. The energy difference between the dissociated state and the physisorption state is about 0.98 eV in the presence of a hydrated proton, which is much larger than that in the gas phase (0.43 eV), seemingly to tell that the presence of a hydrated proton could facilitate the dissociation process. This is further confirmed by the results of MEPs given later on. 3.2.3. Protonation of the Chemisorbed O2 Molecules. As shown in Table 3, the nearest O-H distance between the adsorbed O2 and the hydrated proton is 1.79 Å in the t-b-t chemisorption state, which is much larger than the typical O-H bond length but is in the range of the hydrogen-bonding distances in aqueous solution. This implies that the t-b-t adsorbed

Adsorption and Dissociation of O2 on Pt(111)

Figure 4. Difference in electron density for the (a) end-on and (b) t-b-t chemisorption states of the Pt(111)/O2/H7O3+ system.

Figure 5. Minimum energy paths (MEPs) for the transformations from physisorbed O2 to the chemisorbed molecular O2 of the t-b-t configuration and the subsequent dissociation of the t-b-t adsorbed O2 in the presence of a hydrated proton.

O2 is not protonated. However, the hydrogen-bonding interaction between the adsorbed O2 molecule and the hydrated proton renders the t-b-t state energetically more stable than that in the gas phase, as manifested by the increased adsorption energy. In the end-on chemisorption state, the nearest O-H distance between the adsorbed O2 and the hydrated proton is 1.31 Å, which is much shorter than a usual hydrogen-bonding distance in aqueous solution, indicating that a (at least a partial) proton transfer is involved in the end-on chemisorption. The different O2-proton interactions in these two chemisorption states are also indicated by the difference in electron densities given in Figure 4. In the end-on state, there is a significant electron density increase in the O-H region between O2 and H7O3+, whereas such an electron accumulation is not seen in the t-b-t state. As seen in Table 2, the charge variation of the hydrated proton in the end-on state is larger than that in the t-b-t state. These electronic properties clearly indicate that the O2-H7O3+ interaction is stronger in the end-on state than the t-b-t state. 3.2.4. Reaction Pathways for the Adsorption and Dissociation of O2 on Pt(111) in the Presence of Hydrated Proton. Figure 5 shows the minimum energy paths (MEPs) for the formation of the most stable t-b-t molecular chemisorption state and its dissociation in the Pt(111)/O2/H7O3+ system. It can be seen that the physisorption state is separated from the t-b-t chemisorption state by an activation barrier of ca. 0.13 eV, which is larger than that in the gas phase (0.052 eV), whereas the subsequent dissociation of the t-b-t adsorbed O2 requires an

J. Phys. Chem. C, Vol. 113, No. 48, 2009 20663 activation energy of ca. 0.23 eV, which is much smaller than the 0.63 eV observed in the gas phase. The decrease of the dissociative activation energy of the t-b-t chemisorption state in the presence of a hydrated proton may be easy to understand when considering the elongated O-O bond (Table 3) and the increased d-π* electron transfer, as stated in previous sections. A question is why the activation barrier for the chemisorption is higher in the Pt(111)/O2/H7O3+ system in which the energy difference between the t-b-t chemisorption state and the physisorbed precursor is much larger (Table 3). The answer can be found from the fine structures of the MEP curve for the chemisorption in the Pt(111)/O2/H7O3+ system, which differs significantly from the corresponding one in the gas phase (Figure 2), especially in the branch following the formation of the activation transition state. It can be seen that the transformation from the activation transition state to the stable t-b-t chemisorbed state of O2 in the Pt(111)/O2/H7O3+ system is not accompanied by a continuous energy falling. Instead, a relatively level region is formed in the middle of the energy path, which is both preceded and followed by a continuous energy-falling section. The occurrence of such an energy plateau in MEP infers that a relatively stable intermediate state is involved in the course of the formation of the t-b-t chemisorption state. As indicated by the given geometric configurations for the intermediate points along the MEP in Figure 5, the states associated with the energy plateau are very similar to the end-on chemisorption state obtained in geometric optimization of the Pt(111)/O2/H7O3+ system (Figure 3c). We found that the end-on chemisorption state for Pt(111)/ O2/H7O3+ system was indeed obtainable in a geometric optimization with the activation transition state prior to the energy plateau as the initial configuration. Thus, we may say that the first activation barrier in Figure 5 is actually associated with the transformation from the physisorption state to the end-on chemisorption state. The reaction energy for this transformation is about 0.2 eV, which is smaller than the chemisorption energy of 0.38 eV in the gas-phase Pt/O2 system; therefore, higher activation energy is required. The higher activation energy for the formation of the end-on adsorbed OOH* seems to tell that the proton-coupled electron transfer to the molecular O2 is somewhat a slower process compared with the simple electrontransfer process. The chemisorption necessarily involves translation, rotation, and stretching of the O2 molecule above the Pt surface. Such molecular reconfigurations would be relatively easier for the O2 molecule alone but could become difficult in the presence of a hydrated proton because the accompanying proton transfer renders the O2 molecule and the hydrated proton as a whole to undergo the reconfigurations. As shown in Figure 5, the transformation from the states associated with the energy plateau to the stable t-b-t chemisorption state is almost a nonactivated process involving no obvious activation barrier. This is further confirmed by the results from a separated MEP search in which the end-on adsorption state (Figure 3c) serves as the precursor and the t-b-t adsorption state as the product. The corresponding transformation activation energy was identified to be as low as 0.025 eV (Figure 6), which is indeed negligible. To this end, we may say that the end-on adsorption state is only a metastable state. Actually, the metastability of the end-on adsorption is also implied by the energy evolutions in geometric optimizations of the Pt(111)/O2/H7O3+ system that leads to the stable t-b-t chemisorption state. It was found that these geometric optimizations mostly involve an energy plateau, during which the system has configurations very similar to those of the end-on chemi-

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Figure 6. Minimum energy path (MEP) for the transformation from the end-on state to the t-b-t state in the presence of a hydrated proton.

Figure 7. Energy evolution during the geometric optimization leading to the stable t-b-t chemisorption state.

sorption state. An example of such energy evolutions is given in Figure 7, which shows a well-defined energy plateau prior to the formation of the final t-b-t chemisorption state. Another example of such energy evolutions is given in the Supporting Information (Figure S4), which was obtained during the geometric optimization of the Pt(111)/O2/H7O3+ system with a 2 × 4 Pt slab. In addition, we have calculated the zero-point energies (ZPEs) of adsorbates in different adsorption states in the above MEPs. The calculated ZPEs were then used to correct the calculated activation energies shown above. As shown in Table 4, the effects of ZPEs are not significant. These corrections do not alter the general trends of the reaction paths shown above. 3.3. Further Discussions on the Mechanism and Kinetics of ORR. The above DFT calculations have shown that the chemisorption of O2 on the Pt(111) surface in the presence of a hydrated proton involves two steps. The first step is a proton-

Ou et al. coupled electron-transfer process in which an end-on OOH* is formed. Such a protonated intermediate state is only metastable and would transform to a more stable unprotonated state with a t-b-t bridge configuration through a nearly nonactivated process. The formation of the end-on OOH* state agrees with the observation in recent ab initio MD simulation study by Wang and Balbuena.9 If simply comparing the heights of the activation barriers in the MEPs given in Figure 5, the dissociation step may be considered as the rds in the earlier stages of ORR because it requires an activation energy much larger than the adsorption process. This is contradictory to the finding in Wang and Balbuena’s ab initio MD simulations,9 which showed that the formation of the end-on adsorbed OOH* state requires higher activation energy than its dissociation. However, one would find that the formation of OOH* is indeed the slower step by carefully inspecting the whole energy path in Figure 5. Although the t-b-t precursor state and the dissociative state are separated by a considerable activation barrier, the height of this activation barrier is significantly lower than the energy difference between the metastable end-on state OOH* and the stable t-b-t state. Because the transition from the end-on state to the t-b-t state requires little activation energy, the energy released in this transformation is enough for the system to overcome the energy barrier for the subsequent dissociation. That is, the dissociation of the end-on adsorption state is apparently a nonactivated process. We thus can say that the whole adsorption-dissociation reaction of O2 on Pt(111) in the presence of a hydrated proton involves only one activation barrier that requires extra energy for the O2 molecule to overcome, namely, the chemisorption barrier. As a comparison, two activation barriers have to be overcome by the O2 molecule to become dissociated in the gas phase (Figure 2), and the dissociation is the slower step. Therefore, the results of present DFT calculations agree well with Wang and Balbuena’s MD simulations and also support the Damjanovic’s proposition on the ORR mechanism that has been quoted in most of the ORR kinetic studies. In addition, our calculations also support Yeager’s view on ORR that the bridge-adsorbed O2 molecule and its dissociation are necessarily involved in the direct four-electron reduction of ORR. To this end, we may conclude that the proton transfer plays a key role in the early stage of Pt-catalyzed ORR and the first proton-coupled electron-transfer step, in which an end-on chemisorption state OOH is formed, is the rds in the initial steps of ORR. Therefore, we may be able to formulate the rds in the early stage of Pt-catalyzed ORR as

Pt + O2 + H+ + e- f PtOOH*

(1a)

Such a reaction equation for the rds step has been proposed in most of the kinetics-related ORR studies to explain various electrochemical kinetic data obtained in Pt-catalyzed ORR in solutions of various pHs,2-4 for example, the 120 mV/dec Tafel slope and the nearly first reaction orders in O2 pressure and

TABLE 4: Activation Energy Barriers for the Reaction Path in the Pt(111)/O2 and Pt(111)/O2/H7O3+ Systems with and without ZPE Correction activation energy (eV) systems Pt(111)/O2 Pt(111)/O2/H7O3+

reaction path

without ZPE correction

with ZPE correction

phys.-t-b-t chem. t-b-t chem.-diss. phys.-end-on chem. t-b-t chem.-diss. end-on chem.-t-b-t chem.

0.052 0.63 0.13 0.23 0.025

0.055 0.61 0.15 0.28 0.021

Adsorption and Dissociation of O2 on Pt(111) proton concentration at high current densities. The present DFT calculations thus provided a solid theoretical base for these electrochemical kinetic results. 4. Conclusion The DFT geometric optimization and electronic structure and minimum energy path calculations have revealed that the proton transfer plays a key role in the early stage of Pt-catalyzed ORR. Two types of molecular chemisorbed intermediates, a protonated end-on species OOH* and an unprotonated bridge species OO*, are formed prior to the dissociation of O2 in the presence of the hydrated proton. The OOH* state serves as the precursor for the formation of the OO* state. That is, the O2 molecule undergoes, first, a protonation then deprotonation process during the course of the chemisorption. The presence of the hydrated proton inhibits the molecular chemisorption of O2 on Pt(111) but leads to enhanced interaction between the O2 and the Pt substrate in the final t-b-t adsorption state. The deprotonation step from the end-on state to the final t-b-t state is almost nonactivated, which implies that the end-on OOH* is only metastable. Moreover, the energy gained in such a nonactivated transformation is much larger than the activation energy required for the subsequent dissociation so that no apparent barrier exists between the end-on OOH* state and the dissociative state. Therefore, the proton-coupled electron-transfer process for the formation of OOH* is identified as the rds in the adsorption and dissociation processes of O2 on the Pt(111) surface in acidic media. The present calculation results may provide a link between the long disputed Damjanovic’s view and the Yeager’s view on the mechanism of the initial steps in ORR. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (NSFC Nos. 50632050 and 20573082), the State Education Ministry of China under the program for New Century Excellent Talents in Universities of China (NCET-06-0612), and the Natural Science Foundation of Hubei Province for Outstanding Young Researchers. Supporting Information Available: The optimized structures of Pt(111)/O2, Pt(111)/H3O+ · · · (H2O)n, and Pt(111)/O2/ H3O+ · · · (H2O)n supercell systems; the energy evolution during the course of a typical geometric optimization of the Pt(111)/ O2/H7O3+ system with a 2 × 4 Pt slab; and the Lo¨wdin charge variations in s, p, and d orbitals for the t-b-t chemisorption state in Pt/O2 and Pt/O2/H7O3+ systems. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493. (b) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6,

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