How Potentials of Zero Charge and Potentials for Water Oxidation to

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How Potentials of Zero Charge and Potentials for Water Oxidation to OH(ads) on Pt(111) Electrodes Vary With Coverage Feng Tian, Ryosuke Jinnouchi, and Alfred B. Anderson* Department of Chemistry, Case Western ReserVe UniVersity, 10900 Euclid AVenue, CleVeland, Ohio 44106-7078 ReceiVed: June 8, 2009; ReVised Manuscript ReceiVed: July 23, 2009

Self-consistent potential-dependent quantum chemical calculations on the formation of OH(ads) from H2O(ads) oxidation on the Pt(111) surface in acidic solution have been performed using a unified theory for the electrochemical interface. It is found that the potentials of zero charge (PZC) are relatively insensitive to H2O(ads) coverage and, when adsorbed OH is present, are very sensitive to the OH(ads)/H2O(ads) ratio. The PZC generally increases as the degree of oxidation of the surface is increased by higher OH(ads) coverage. Standard reversible potentials for the formation of 1/6 monolayer (ML) OH(ads) from oxidation of H2O(ads) for initial H2O coverage values of 1/6, 1/3, 1/2, and 2/3 ML were calculated and found to be respectively 0.71, 0.59, 0.65, and 0.63 V on the SHE scale. These calculated reversible potentials agree well with experimental measurements of onset potentials for H2O oxidation, but it is argued using experimental measurements of the groups of Orts and Watanabe that the 0.59 V value corresponds to the room temperature interface. The reversible potentials for oxidizing the last 1/6 ML of H2O(ads) at 1/3, 1/2, and 2/3 ML initial H2O coverage are calculated to be respectively 1.25, 1.29, and 1.21 V. The high reversible potentials are attributed to the loss of stabilizations from hydrogen bonding when H2O(ads) is oxidized to OH(ads) and suggest that high coverage for OH(ads) beyond 1/3 ML can exist only at high electrode potentials. However, the experimental results of Watanabe and our preliminary theoretical studies show that the OH(ads) is oxidized to O(ads) at about 0.8 V, well before such high potentials are reached. H2O2(aq) + 2H+(aq) + 2e-(Uo3) h 2H2O(l)(Uo3 ) 1.763 V)

Introduction The oxygen reduction reaction (ORR) on platinum cathodes in proton exchange membrane fuel cells (PEMFC) has been and continues to be a topic of active research1 The mechanistic details of the two-electron reduction to hydrogen peroxide and the four-electron reduction to water are not yet fully resolved. For fuel cell applications, the four-electron process is sought, and two paths are possible as reviewed by Yeager: a direct 4-electron pathway and a pathway comprised of two 2-electron reactions.2 The direct 4-electron pathway leads to water

O2(g) + 4H+(aq) + 4e-(Uo1) h H2O(l)(Uo1 ) 1.229 V) (1)

The standard reversible potential (in parentheses) is for the standard hydrogen electrode (SHE) scale. In the other path, O2 is first reduced to hydrogen peroxide

O2(g) + 2H+(aq) + 2e-(Uo2) h H2O2(aq)(Uo2 ) 0.695 V) (2)

Then the hydrogen peroxide is reduced to water in a second two-electron step * Corresponding author. E-mail: [email protected]. Phone: 216-3685044. Fax: 216-368-3006.

(3) The direct 4-electron pathway with low overpotential is preferred for fuel cell applications because of the higher limit for the operating cathode potential and because no hydrogen peroxide is formed at potentials greater than 0.7 V. Hydrogen peroxide corrodes both the catalyst carbon support and the polymer proton exchange membrane. It has been difficult to obtain experimental evidence for the identities and potential dependencies of intermediates that form during the course of oxygen reduction. Steps for hydrogen oxidation reactions and for ORR on platinum electrodes have been modeled in this group using a local reaction center (LRC) approach to calculate potential-dependent activation energies for the electron transfers.3-9 An assumption used throughout these studies was that species underwent the electron transfer reactions when in the adsorbed state. There are four steps in the reduction of oxygen to water, and each might be influenced in different ways by bonding to different electrocatalyst surfaces. On different electrocatalysts, different electron transfer steps might in principle determine the onset potential for O2 reduction. A proposed series of equilibrium steps for the direct 4-electron reduction on platinum electrodes with one available adsorption site, modeled as coordinatively unsaturated Pt, was5

Pt-O2 + H+(aq) + e-(Uo4) h Pt-OOH

10.1021/jp905377d CCC: $40.75  2009 American Chemical Society Published on Web 09/14/2009

(4)

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Pt-OOH + H+(aq) + e-(Uo5) h Pt-OHOH

(5)

Pt-OHOH + H+(aq) + e-(Uo6) h Pt-OH + H2O

(6) Pt-OH + H+(aq) + e-(Uo7) h Pt-OH2

(7)

When more coordinatively unsaturated sites are present, the following mechanism was suggested:7 O2(bridging) + H+ + e-(Uo8) h OOH(bridging)

(8)

OOH(bridging) h O(bridging) + OH(one-fold)

(9)

O(bridging) + H+ + e-(Uo10) h OH(one-fold)

(10) 2OH(one-fold) + 2H+ + 2e-(Uo11) h 2H2O(one-fold) (11) In these two studies activation energy and reversible potential predictions were made using calculated internal energies for neutral surfaces without surface charging or solvation beyond coordinating three water molecules to the hydronium ion by hydrogen bonding.5,7 The reverse of reactions given in eqs 10 and 11 are steps for H2O oxidation, which is the theme of this paper. The oxidation of water to form adsorbed OH and O is a topic of contemporary research. Depending on the reversible potentials for these reactions, the surface adsorbed O and OH may cause overpotentials for oxygen cathodes in fuel cells. The hydrogen electrode operates very close to 0.0 V, so in a hydrogen fuel cell the onset potential for rapidly increasing current will be predominately determined by the oxygen reduction step with the lowest reversible potential. Predicted internal energies of activation, Ea, were small at the reversible potentials for the cases examined, ∼0.2 eV and less and they increased as the potentials were increased positive from the reversible potentials.3-10 In a fuel cell, additional voltage reduction is caused by a combination of electron transfer activation energies and diffusion resistance for oxygen and proton transport resistance in the membrane electrolyte. Maximum power is obtained at around 0.5 V, which is much smaller than the 1.229 V standard reversible potential. From experimental measurements, the origin of the slow kinetics at the cathode has been unclear, in part because of uncertainty concerning the reaction mechanism.11 However, H2O(ads) oxidation is known to take place at low potentials relative to 1.229 V. What is learned about the reversible potential for water oxidation to OH(ads) bears directly on the overpotential for oxygen cathodes since its reduction is almost certainly a step in O2 reduction. On platinum electrodes, as the potential is swept positive, oxidation of water in acid electrolyte commences at ∼0.55 to 0.7 V11-15

H2O(ads) h OH(ads) + H+(aq) + e-(Uo12)

(12)

This reaction is the same as the reaction in eqs 7 and 11. Thermodynamic analysis of OH adsorption voltammograms for

Pt(111) electrodes in 0.1 M perchloric acid by Orts and coworkers suggests that the maximum OH(ads) coverage that can be achieved is 0.45 ML.14 This coverage was based on integrating the current densities during sweeps from the double layer to just past the current density peak at 0.8 V and assigning all current to OH(ads) formation by eq 12. Recent work from the Watanabe group yields, by deconvolution of X-ray photoelectron signals, the coverage OH and O as functions of potential on a Pt(111) electrode in 0.1 M HF.15 These authors assumed a structure model where for the double layer region 2/3 ML H2O is adsorbed in a (3 × 3) pattern; as the potential increased, up to 1/3 ML OH(ads) could form at 0.80 V. At 0.85 V O(ads) started to form and by about 0.95 V the OH(ads) coverage dropped to 0.1 ML while the O(ads) coverage reached 0.25 ML. Both OH(ads) coverage and O(ads) coverage increased at higher potentials, up to 1.1 V for a total coverage of about 2/3 ML. It is possible that the assumption of the Orts group, that all current goes into OH formation, resulted in overestimating the OH(ads) coverage at potentials above 0.75 V. Due to the role OH(ads) likely plays in the ORR mechanism, as an intermediate in the four-electron reduction of water, which one would like to reduce to water at low overpotential, the reversible potential and potential-dependent activation energies for OH(ads) formation by eq 12 have already motivated theoretical work. The reversible potential for this reaction on Pt(111) in acid was found to be about 0.68 V at 1/4 monolayer OH coverage.14 The local reaction center (LRC) model has produced reversible potentials close to this.9 The linear Gibbs energy relationship (LGER), which predicts reversible potentials for reactions of molecules specifically adsorbed on electrode surfaces by perturbing standard bulk solution values with adsorption bond strengths, has also given accurate values for this coverage.16,17 However, this method does not include surface charging or solvation. The LGER approach, based density functional slab band calculations, that showed OH adsorbed relatively more weakly than H2O on Pt skins on Pt alloys, and has thereby provided at least partial explanation for the higher water oxidation potential on the skin and the reduced overpotential for O2 reduction.16 Other theory groups are also working actively on understanding the mechanism of water oxidation and O(ads) and OH(ads) reduction on platinum. Pitsch and co-workers have continued with the LRC approach to calculate internal energies by increasing the size of the surface model and have obtained reasonable reversible potentials for H2O(ads) oxidation to OH(ads) and OH(ads) oxidation to O(ads) on Pt(111).18 The potential-dependent activation energies were used to generate rate constants and, with assumptions about adsorbate interactions, predictions about coverage of H2O, OH, and O at different potentials via dynamic Monte Carlo simulations and a mean field model. Norskov and co-workers have calculated Gibbs energies for extended surfaces with intermediates adsorbed on them, but with still no surface charging or solvation.19 Subsequently, the Norskov group extended the model by adding an electric field.20 Then the model was enhanced by charging the surface while employing water multilayers with a constant diffuse neutralizing background charge, to yield calculated free energies as functions of electrode potential.20 There have been numerous related studies.21-23 Missing from the abovementioned theoretical works have been systematic evaluations of the effects of changes in surface coverage, interactions with coadsorbed molecules, solvation, and double-layer structure on the potentials of OH(ads) reduction.

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Recently a unified fully self-consistent theoretical approach to the electrochemical interface was developed in this lab and encoded in a computational program, Interface 1.024,25 It is a two-dimensional density functional band theory employing atomic orbitals and, unlike three-dimensional band theories, it allows the potential to be determined easily and unambiguously from the Fermi level energy. The surface potential is changed by adding charge to the translational unit cell and the counter charges are handled self-consistently by a modified PoissonBoltzmann distribution of ions in the double layer and the whole interface is immersed in a dielectric continuum. The theory has been applied to reactions specified in eqs 8, 10, and 11 above as well as the formation of under-potential-deposited H on Pt(111), all at 1/4 ML coverage. It was demonstrated in ref 24 that, for accurate reversible potential predictions, hydrogen bonding stabilizations of the reaction intermediates by coadsorbed water molecules must be included. Gibbs energies for adsorbed O2, OOH, O, OH, and H2O were calculated as functions of electrode potential. The calculated work function for the standard hydrogen electrode was 4.43 eV and the potential was determined as 4.43 V minus the Fermi energy divided by e, the unit of electron charge, a positive number. The potential was changed by adding or subtracting fractional electrons in the translational unit cells used in the calculations, which was followed by fully self-consistent variational calculations. A reversible potential for each surface reaction determined as the potential at which the Gibbs energies of the reactant and product were equal. In ref 24 a value of 0.47 V was predicted for the reaction

H2O(l) h OH(ads) + H+(aq) + e-(Uo12)

(13)

In that study the state of the left-hand side of eq 13 was a water molecule and the Pt(111) surface, both immersed in the dielectric medium with ionic concentrations corresponding to pH ) 0.0 monoprotic acid. The energy of this system changes with changes in surface charge and electrode potential. The H2O(l) molecule in eq 13 was modeled as a ring of four molecules immersed in the dielectric medium and its energy is independent of the surface charging. One fourth of the ring’s Gibbs energy was used in calculating the equilibrium potential. The righthand side of the equation was 1/4 ML OH(ads) immersed in the dielectric medium and the energy of the interface changes with changes in surface charge and electrode potential. The H+(aq) was modeled as H3O+(H2O)3 surrounded by the dielectric medium and its energy is independent of surface charging. The reaction in eq 12, where H2O is adsorbed on the surface, allows changes in the water molecule structure and adsorption bond strength as the surface charge and electrode potential are changed and, therefore, we believe better represents the state of the water molecule during oxidation. This is why the predicted reversible potential is expected to be closer to the onset potentials for water oxidation seen in potential scans. As shown below, water oxidation potentials calculated by eq 12 are higher than those calculated by eq 13 and are closer to experiment. The primary purpose of the present study is to use Interface 1.0 to learn what happens to the H2O(ads) oxidation potential as (i) the coverage of OH increases and (ii) the amount of water molecules is increased to the point of forming two water layers, representing a saturated surface using the same model as Watanabe and co-workers assumed in ref 15. Based on the calculated results, an analysis is made of the experimental findings of the Watanabe and Orts groups. The secondary purpose of the work is to calculate the potential of zero charge

Figure 1. (A) Three-layer slab model used for the calculations consisting of 18 Pt atoms for modeling the Pt(111) surface, where a × b ) 3 × 3. (B) Top view of rectangular 3 × 3 transitional unit cell.

(PZC) for each of the interface structures to learn, for various amounts of adsorbed H2O and OH, the behavior of this parameter, which quantifies a concept that has been called “one of the most fundamental ideas in electrochemistry”.26 All calculations will include solvation and self-consistent doublelayer treatment. The results are for pH ) 0.0 and 298.15 K. Computational Details The overall procedure is to minimize, using variational theory, the Gibbs energies of models of charged electrochemical interfaces as functions of their structures, and then determine the electrode potentials. The procedure is repeated using different surface charges until enough data are obtained. Interface 1.0 is a fully self-consistent computational program that employs periodic density functional theory (DFT) with linear combination of pseudoatomic orbitals (LCPAO)27 and norm-conserving pseudopotentials (NCPP).28 The molecular properties from the density functional theory are combined with a dielectric continuum model, to take electrolyte into account, and a modified Poisson-Boltzmann (MPB) equation.29,30 Differently from Ohtani and Sugino,30 we use an adaptive cavity function that defines electrostatic and nonelectrostatic interactions. This new feature allows the accurate prediction of solvation free energies of molecules and ions when in solution,24 and when adsorbed at the electrochemical interface, to yield accurate predictions of reversible potentials.24,25 The revised Perdew-Burke-Ernzerhof (RPBE) exchange functional developed in the Nørskov group is used with the generalized gradient approximation (GGA).31 The choice of RPBE is based on M-O bond strength calculations in ref 31, where M-O bond strengths were found to be more accurate than the overestimates that came from the Perdew-Burke-Ernzerhof (PBE) functional.32 For the two-dimensional periodic calculations in this work, a three-layer slab translational cell consisting of 18 Pt atoms (Figure 1) was used in simulating the Pt(111) surface. The slab translational unit cell was rectangular 3 × 3. The bottom layer of platinum atoms was frozen in the previously calculated bulk lattice positions with lattice constant 4.03 Å.24 A 3 × 6 × 1 k-point mesh for Monkhorst-Pack sampling was used in Brillouin-zone integrations.33 An energy criterion of 5 × 10-7 a.u. and a force criterion of 0.05 eV/Å were used to define the respective electronic energy and structure convergences. This combination of slab model, k-point mesh, energy criteria, and force criteria was demonstrated to be satisfactory for making predictions of reversible potentials in the previous work.24,25 Regarding solvation and the double layer, a solution/slab/ vacuum model was employed. Ionic radii used in the MPB dielectric continuum theory were set to 3.0 Å, as in refs 24 and 25. In Interface 1.0 the cations and anions are equal in radius and are charged +1 and -1. The ion concentration used for



this study was 1.0 M, corresponding to pH ) 0.0 monoprotic acid, also as in the earlier work. To predict reversible potentials at different pH values for 1.0 M electrolytes of this type, the Nernst equation is used. The electrode potentials, on the standard hydrogen electrode scale, were evaluated as functions of the Fermi levels by the following equation:

U)-

εF + φSHE e

(14)

where φSHE is the thermodynamic work function of the standard hydrogen electrode. The value 4.43 eV, calculated previously using Interface 1.0, was employed.24 There have been numerous experimental and theoretical estimates of φSHE, covering the range from 4.28 to 4.80 eV.34-36 We now give an overview of the theory; full details are in ref 25. The free energy, G, is divided into the following components:

G ) K + Exc + Ees + Ωss,nonel + Ωis,nonel - TSe - TSi (15) where K, Exc, and Ees are the kinetic, exchange-correlation, and electrostatic energies, respectively; Ωss,nonel and Ωis,nonel are the respective free energies from noneletrostatic solute-solvent and ion-solute interactions; T is temperature; and Se and Si are entropies of the electrons and ions, respectively. A frequency analysis is performed using the optimized structures of the neutral systems to get the thermal corrections to the free energies that are used for all of the potentials studied. When the thermal correction is included, the total free energy of neutral system is written as

G ) K + Exc + Ees + Ωss,nonel + Ωis,nonel - TSe TSi + Hs,corr-TSs

(16)

where Hs,corr - TSs are, in the present study, determined using vibration eigen-energies in the standard statistical formulas. For charged systems, the mass conversation correction term is included and the total free energy of charged system becomes

G ) K + Exc + Ees + Ωss,nonel + Ωis,nonel - TSe TSi + Hs,corr-TSs + Ωmc (17) where

Ωmc ) -εFNe + µ+N+ + µ-N-

(18)

and εF is the Fermi level energy, Ne is the number of electrons, N+ and N- are the number cations and anions in solution, and µ+ and µ- are their respective chemical potentials. Results and Discussion H2O on Pt(111): Coverage, Structure, and Potential of Zero Charge. On Pt(111), adsorptions of 2/3 ML H2O in a 2-dimensional hexagonal arrangement, i.e. an ice-like structure, have been seen by low energy electron diffraction (LEED), helium atom diffraction (HAD), and scanning tunneling microscopy (STM).36-39 The bilayer (Figure 2d) adsorption

Figure 2. Top and side views of H2O adsorptions at various coverages on the Pt(111) surfaces: (a) 1/6 ML, (b) 1/3 ML, (c) 1/2 ML, (d) 2/3 ML MD structure, (e) 2/3 ML noncontact structure, and (f) 2/3 ML H-down structure.

structure of 2/3 ML H2O on Pt(111) was proposed by Doering and Madey40 and hereafter this is called the MD structure. However, recent X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) studies suggested that an H-down structure (Figure 2f) rather than MD structure is preferred in vacuum.41 Previous Interface 1.0 calculations for the vacuum condition showed that at 2/3 ML H2O coverage the H-down structure is 0.02 eV more stable than the MD structure,25 and this result qualitatively agrees with the XPS/XAS studies. However, when the double layer was included in the calculations by using the MPB theory and dielectric continuum model, the MD structure became 0.02 eV more stable than H-down structure. This stabilization may be associated with the pointing-out into solution of the hydrogen atoms in the MD structure, which might lead to stronger electrostatic bonding to the double layer. In any event, the difference is small and a calculated energy criterion may be insufficient for choosing the more probable structure. However, as will be shown below, the PZC is sensitive to the water structure and it may provide a useful criterion for choosing the correct structure from two or more structures that have insignificant differences in energy. Potentials of zero charge for 1/6, 1/3, 1/2, and 2/3 ML H2O coverage (all structures are in Figure 2) are in Table 1, along with the shifts relative to having no adsorbed H2O. It is evident that the PZC is sensitive to coverage and structure. In the cases

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TABLE 1: Calculated Potentials of Zero Charge, PZC (V vs SHE), and Their Shifts Relative to Clean Pt(111), ∆PZC for Different Degrees of H2O(ads) Coveragea structure

a

b

c

d

e

f

1/6 1/3 1/2 2/3 2/3 2/3 H2O coverage 0 PZCb 1.13b 0.54 0.42 0.30 -0.35 0.74 0.34 ∆PZC 0 -0.59 -0.71 -0.83 -1.48 -0.39 -0.79 a

Structures are in Figure 2. b Reference 25.

of 1/6, 1/3, and 1/2 ML H2O coverage, there is 1/6 ML of oxygen lone-pair donation bonds to the surface; for 1/3 ML and 1/2 ML the additional H2O molecules are more distant from the surface. The ∆PZCs are therefore quite close to one another, ranging from -0.59 to -0.83 V and the respective PZCs are 0.54, 0.42, and 0.30 V, which are close to the experimental estimates. However, in the MD structure, at 2/3 ML H2O coverage, there is 1/3 ML of lone-pair donation bonds to the surface and so ∆PZC is about double, -1.48 V. These findings support the conclusion in the review by Thiel and Madey that H2O adsorption with the O-down structure decreases the surface work function.42 For 2/3 ML H2O coverage, the earlier study found a third local minimum (Figure 2e), in addition to the MD and H-down structures, and it has almost the same energy as the other two.25 This structure has all oxygen atoms ∼3.6 Å above the Pt(111) surface and has no OH bonds pointing down toward the surface. It will be called the noncontact structure. The MD, H-down, and noncontact structures give PZC values of -0.35, 0.34, and 0.74 V, respectively. The PZC at 2/3 ML H2O coverage is very sensitive to the surface structure. In the MD case, the lone-pair donation bonding of half of the water molecules causes the PZC to be less than the 1.13 V value when water is not adsorbed, as determined in ref 25. The H-down structure has a combination of lone-pair donation and hydrogen bonding interactions with the Pt surface, resulting in a higher PZC. Finally, the noncontact case has the highest PZC, though, due to the proximity of the water layer, it still less than for the bare surface in the dielectric medium. It is expected that, as the surface is charged positively, the MD structure should become more stable. Calculations show this happens when between 0.1 e and 0.2 e charge is added to the transitional unit cell. From the electrostatic viewpoint, work functions are rationalized using surface dipoles.26 Large changes can occur in the work function and Fermi energy when the direction of the water dipole is changed. However, the water orientation does not have a big influence on the internal energies. In the extreme case, rotating a water molecule in the vacuum changes the dipole moment in a given direction between zero and the molecular dipole value but does not change the internal energy at all. We note that the water dipole orientation is insufficient to explain all of the results in Table 1. For example, water molecules at 1/6 ML coverage have their dipoles nearly parallel to the surface and the lone-pair donation, which creates a dipole nearly perpendicular to the surface, better explains the 0.54 eV decrease in work function form the value when no water is adsorbed. The measured potential of zero charge in 1.0 M H+(aq) is around 0.4 V.25 The 2/3 ML coverage models might well represent the saturated interface, though possibly only for low temperatures, and, with this in mind, the H-down structure, with its PZC of 0.34 V, would be favored over the MD and noncontact interface structures. However, at room temperature the weakly adsorbed water molecules are likely to form interfacial structures with surface vacancies. The 1/6, 1/3, and 1/2 ML structures have calculated PZC values of 0.54, 0.42, and 0.30 V, respectively.

All are in the range of experimental estimates and, as discussed below, there is reason to select the 1/3 ML result. H2O and OH on Pt(111): Coverage, Structure, and Potential of Zero Charge. Mixed OH and H2O adsorption on metal surfaces has been intensely studied theoretically and experimentally.43 However, their adsorption at an electrochemical interface with a double-layer present is not well understood. Here the coverage of OH(ads) and H2O(ads) on Pt(111) are varied using the MPB with dielectric continuum theory. The surface systems are in Figure 3, showing the range of OH(ads) coverage is 1/6 ML to 2/3 ML in steps of 1/6 ML, and at each coverage there are additional entries where up to 1/2 ML H2O(ads) is added to the surface, so that all possible coverage combinations from 1/6 ML to 2/3 ML of O-containing molecules are included. For 1/6 ML OH(ads), the calculated Pt-O bond length of 2.02 Å, which is close to the value of 2.11 ( 0.04 Å determined in a LEED study by Seitsonen et al.44 The calculated Pt-O-H angle of 105.0 deg with dielectric continuum included is close to 107 deg previously calculated by Michaelides and Hu using the PW91 functional for 1/6 ML on-top adsorption of OH in vacuum.45 For 1/6, 1/3, 1/2, and 2/3 ML OH(ads) coverage, without water molecules present, OH positions were assumed to be the same as the H2O positions in Figure 2. The adsorbed OH were oriented to maximize the H-bond interactions on Pt(111), a structure feature noted in earlier DFT studies reported by Karlberg and co-workers.46,47 Based on pairwise interactions for a surface with 1/4 ML OH(ads) and 1/4 ML H2O(ads), our own VASP calculations showed that, on Pt(111), OH2 · · · OH hydrogen bonds have 0.57 eV strength and H2O · · · H2O hydrogen bonds have 0.26 eV strength.17 PZCs of Pt(111) with adsorbed OH-containing structures, shown in Figure 3, are presented in Table 2, along with values of ∆PZC. It is evident that, as the coverage of OH(ads) is increased from 1/6 ML to 2/3 ML, the PZC increases over the range 1.29-1.90 V when no adsorbed water molecules are included. This is due to the increase in work function as the surface is oxidized, forming Ptδ+ and OHδ-. The work function is a measure of the Fermi level and responds to changes in the surface platinum atom charge. The average delocalized partial charge on the Pt atoms increases as the OH(ads) coverage increases, and therefore the work function increases. The hydrogen bonding between the adjacent OH(ads) groups owes its strength to the partial negative charge on the O atoms, and it leads to stabilization of the adsorbed OHδ- at the highest coverage. When OH(ads) coverage is 1/6 ML, adding 1/6 ML of H2O causes a 0.57 V decrease in the PZC because of the added 1/6 ML of lone-pair donation bonds added to the surface. Adding a second 1/6 ML of H2O results in a relatively small change due to the second 1/6 ML of H2O(ads) not being in direct contact with Pt atoms in the surface. However, adding a third 1/6 ML H2O(ads) brings a 0.79 V further reduction in the PZC, all the way down to -0.05 V because in this case the coverage of lone-pair-donating water molecules increases from 1/6 ML to 1/3 ML. When OH(ads) coverage is 1/3 ML, adding 1/6 ML H2O decreases the PZC by 0.50 V and adding another 1/6 ML H2O decreases the PZC by an additional 0.58 V. In both cases the water molecules are bonded to the surface by lone-pair donation. Finally, the PZC decreases 0.48 V when 1/6 ML H2O is added to the surface with 1/2 ML OH(ads). Again the water molecules form lone-pair donation bonds with the surface.


J. Phys. Chem. C, Vol. 113, No. 40, 2009 17489 atom, causes a ∼0.5 to ∼0.8 V decrease in the PZC. When no OH(ads) is present, the first 1/6 ML of H2O adsorbed by lonepair donation and when coverage is 1/3 ML and 1/2 ML H2O the number of lone-pair donation bonds remains 1/6 ML and the PZCs decrease by 0.59, 0.71, and 0.83 V, respectively, all about the same. The PZCs vary considerably at the maximum 2/3 ML coverage of OHn, n ) 1 and 2. In those cases when there is 1/6 ML OH(ads), 1/3 ML OH(ads), 1/2 ML OH(ads), or 2/3 ML OH(ads) present, the respective PZCs are -0.05, 0.36, 1.19, and 1.90 V. It is concluded that the PZC follows a trend of increasing values as the OH(ads) coverage and, therefore the level of surface oxidation, increases. This is the natural consequence of the Fermi level decreasing as the surface metal atoms become increasingly positively polarized. Reversible Potentials for Water Oxidation on Pt(111) (eq 12) as a Function of OH(ads) and H2O(ads) Coverage. Reversible potentials were found by two series of calculations. First, the Gibbs energy of the reactant, H2O(ads), was calculated as a function of the Pt(111) electrode potential and the results were graphed. Second, the Gibbs energy of the product of the oxidation reaction, OH(ads) + H+(aq) + e- was graphed as a function of potential. The potential where these curves cross marks U°, the potential at which the reactions are in equilibrium. All possible combinations of surface H2O(ads) + OH(ads) coverage were considered, from 1/6 ML up to 2/3 ML. Figure 4 shows these curves and their crossing points and marks indicating the previously calculated PZCs. The corresponding structures are already presented in Figures 2 and 3. Potentials were changed by adding -0.10 e, 0.00 e, +0.10 e, +0.20 e, +0.30 e, +0.40 e, +0.50 e, where the electron charge unit e is positive, to the translational unit cell in most of the cases. Two figures, the first and last, required the use of additional charges, -0.20 e and -0.30 e, in order to obtain accurate crossing points. In Figure 4 each “before oxidation” curve goes through a broad maximum within 0.2 V of the PZC. Adding surface charge of either sign stabilizes the adsorbed systems. The same is true for the surface species after oxidation: adding the energy of the electron would change the shapes of the “after oxidation” curves in the figure. Table 3 summarizes all of the reversible potentials for OH(ads) formation from H2O oxidation in Figure 4. It may be seen that for each initial 1/6, 1/3, 1/2, and 2/3 ML H2O(ads) coverage, when no OH is coadsorbed, the reversible potentials lie in a narrow range of 0.59 to 0.71 V. All are close to the observed onset potential for water oxidation on platinum electrodes in acid electrolyte. The reversible potential for forming the second 1/6 ML of OH(ads) depends on the H2O coverage. When the initial H2O coverage is 1/6 ML, the potential for its oxidation is 1.25 V. The high potential is attributed to the loss of 1/6 ML of stable hydrogen bonds to OH(ads). As mentioned above, these bond strengths are about 0.57 eV, and the reaction

OH2(ads) · · · OH(ads) h OH(ads) · · · OH(ads) + H+(aq) + e-(Uo14) Figure 3. Top and side views of various OH-containing adsorptions on the Pt(111) surfaces. a. 1/6 ML OH; b. 1/3 ML OH; c. 1/2 ML OH; d. 2/3 ML OH; e. 1/6 ML OH + 1/6 ML H2O; f. 1/6 ML OH + 1/3 ML H2O; g. 1/6 ML OH + 1/2 ML H2O; h. 1/3 ML OH + 1/6 ML H2O; i. 1/3 ML OH + 1/3 ML H2O; j. 1/2 ML OH + 1/6 ML H2O.

Overall, it is concluded for the surfaces with OH(ads) that each 1/6 ML increase in H2O adsorption, where the adsorption bond is by lone-pair donation from the O atom to a surface Pt

(19)

has two hydrogen bonds on the left and one on the right, which means the electron must be stabilized to make up for the loss in hydrogen bonding stability on the right-hand side of eq 19, and this causes the high potential. In the cases 1/3 and 1/2 ML H2O(ads) the potentials are lower, 0.74 and 0.56 V, respectively because there is no overall change in the number of hydrogen bonds.

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TABLE 2: Calculated Potentials of Zero Charge, PZC (V vs SHE), and Their Shifts Relative to Clean Pt(111), ∆PZC for Different Combinations of OH(ads) and H2O(ads) Coveragea structure OH coverage H2O coverage PZC ∆PZC a

0 0 1.13b 0

a

b

c

d

e

f

g

h

i

j

1/6 0 1.29 0.16

1/3 0 1.44 0.31

1/2 0 1.67 0.54

2/3 0 1.90 0.77

1/6 1/6 0.72 -0.41

1/6 1/3 0.74 -0.39

1/6 1/2 -0.05 -1.18

1/3 1/6 0.94 -0.19

1/3 1/3 0.36 -0.77

1/2 1/6 1.19 0.06

Structures are in Figure 3. b Reference 25.

When the initial OH(ads) coverage is 1/3 ML, and the H2O(ads) coverage is 1/6 ML and 1/3 ML, the oxidation potential is again high at 1.29 and 1.30 V, due to the loss of 1/6 ML of hydrogen bonds to OH(ads). Finally, when the initial OH(ads) coverage is 1/2 ML and H2O coverage is 1/6 ML, the potential, 1.21 V is high because of the loss of 1/6 ML hydrogen bonds to adsorbed OH groups. Looking down the fifth column in Table 3, it is concluded that, for the adsorption patterns chosen, the first 1/3 ML of OH(ads) forms at a potential of about 0.6 V. Additional OH(ads) forms at a much higher potential, around 1.2 or 1.3 V. At this potential it is expected that at least some of the OH(ads) will have been oxidized to O(ads). However, the model calculations do not seem to express the slow increase in reversible potential for OH(ads) formation as the OH(ads) coverage increases. To account for this we propose that at room temperature for the electrochemical interface a full 2/3 ML coverage of OHn, n ) 1-2, is unlikely. All experimental determinations of 2/3 ML H2O(ads) coverage were in vacuum at low temperatures. Since the adsorption free energy of water on the surface is negative only for less than 2/3 ML coverage,25 there will be vacant adsorption sites. This idea can be extended to provide an interpretation for the increase in potential for OH(ads) formation with its increasing coverage. The lowest potential, adjusted for pH ) 1.0, for forming the first 1/6 ML OH(ads) is 0.53 V at 1/3 ML H2O(ads) coverage. This is in good agreement with the 0.55 V onset potential for OH(ads)14 formation in pH ) 1.0 acid electrolyte, and even if the average water coverage is 1/2 or 2/3 ML, the predicted potential increases by no more than 0.06 V. Comparing panels f and g in Figure 3, both with 1/6 ML OH(ads), it can be seen that each OH(ads) enjoys three hydrogen bonds to water molecules in both panels but in the second one, with 1/2 ML H2O, the last 1/6 ML of H2O is only hydrogen-bonded to adsorbed water molecules, and such hydrogen bonds are about half the strength.17 Consequently, the next 1/6 ML of OH(ads) will form at a higher potential of about 0.68 V, adjusted for pH ) 1.0. Additional computations including randomness in surface structure and coverage should yield a smooth coverage versus potential relationship approximating that seen in slow-scan voltammograms. Modeling more points on the curve would, however, require calculations with large translational unit cells. Concluding Comments In the present model, there are two potential regimes on Pt(111) electrodes in pH ) 0.0 aqueous electrolyte: as the potential is increased through the double layer region, which extends from ∼0.3 V to ∼0.6 V, OH(ads) begins to form at around 0.6 V until 1/3 ML coverage is reached. Upon further oxidation, hydrogen bonds between adsorbed OH and neighboring adsorbed OH and H2O molecules are lost, and the molecular products of oxidation are less stable. This means that the electron must become more stable at equilibrium, which means the electrode potential is higher. Cyclic voltammograms show a new

reduction peak at about 1.04 V.15 Based on the work of the Orts group,14 this peak might be correlated with the completion of 2/3 ML OH(ads) coverage following the 0.45 ML coverage at the end of the 0.8 V peak. Its potential would be about 0.15 V less, adjusted for pH ) 1.0, than our predicted values for cases where 1/6 ML of hydrogen bonds is lost, seemingly in good agreement. However, present results support an interpretation that takes into account the new results of Watanabe and co-workers showing that from 0.70 to 0.85 V the amount of OH(ads) increases from 0.1 ML to about 1/3 ML and then starts to drop as O(ads) begins to form at about 0.85 V.15 Preliminary results using Interface 1.0 are in agreement, indicating that OH(ads) is oxidized to O(ads) in the 3-fold site at about 0.76 V at pH ) 1.0. Experimentally, the total observed coverage of H2O, OH, and O reaches nearly 2/3 ML at 0.80 V and remains at that value up to 0.90 V, then it drops quickly to around 0.45 ML at 0.95 V, remains at this value until about 1.00 V, and then rises to about 2/3 ML at 1.05 V.15 The decrease in overall coverage of H2O(ads), OH(ads), and O(ads) from 0.9 to 1.0 V is due to a decrease in water adsorption, its coverage approaching zero, and OH adsorption, its coverage droping from 0.35 to 0.10 ML. It is possible that O(ads) in 3-fold sites passivates three surface Pt atoms toward water adsorption and that the increase in the amount of OH(ads) and O(ads) that begins at 1.0 V corresponds to some of the adsorbed O atoms exchanging with surface Pt atoms, exposing them to water adsorption and oxidation to OH(ads) and O(ads). Place exchange between O and surface Pt atoms was suggested by Conway.15,48 Further work is underway to confirm or not these hypotheses by using interface 1.0 to determine reversible potentials for these steps. Another of the findings of the Watanabe group work is the low H2O coverage on Pt(111) at potentials less than 0.6 V, a potential range where only a small amount of OH(ads) is expected to be on the surface. This is probably because the weakly adsorbed water is lost from the surface in the ex situ XPS measurements. As shown in ref 25, at the free energies of adsorption on Pt(111) in vacuum calculations are within 0.1 to 0.3 eV positive for coverage from 1/4 to 2/3 ML, and so H2O(ads) will evaporate. Evidently in the Watanabe experiments most of the H2O(ads) evaporates when electrodes are prepared at potentials less than the potential where OH(ads) begins to form. At this point the Watanabe results show that the amount of H2O(ads) increases at the same rate as the amount of OH(ads) increases until 2/3 ML coverage is reached. It is likely that this water is able to stay on the surface even in vacuum during the XPS measurements because of stability gained through hydrogen bonding to the OH(ads), about 0.57 eV per bond. This study has also shown that the PZC is sensitive to changes in adsorbed species. Three energetically nearly degenerate structures were calculated for the 2/3 ML H2O(ads) surface and only one of them, the H-down structure, had a calculated PZC that closely matched current experimental estimates. Thus, the PZC may provide a criterion for selecting among calculated



Figure 4. Free energies, G(eV) for reactants and products, reversible potentials, Urev(V) highest point of reactant free energy, and potential of zero charge, PZC, for reactions as shown. Reactant and product structures are shown in Figures 2 and 3and the results are for pH ) 0.0 electrolyte. The 2/3 ML H2O figure is for the MD structure.

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TABLE 3: Reversible Potentials (V) as Functions of Coverage (ML) of H2O(ads) and OH(ads) as Determined from Crossing Points in Figure 4 OH + H2O coverage

initial OH coverage

0 1/6 1/3 1/2

1/6

1/3

1/2

2/3

0.71

0.59 1.25

0.65 0.74 1.29

0.63 0.56 1.30 1.21

structures when their calculated energies are not significantly different. However, it is possible that a rigid 2/3 ML coverage pattern can exist only at very low temperature where it is frozen in; it is expected that under common conditions of electrochemical measurements the pattern is disrupted and adsorption site vacancies form. The predicted PZC is nearly unchanged when water coverage is reduced to 1/2 ML and 1/3 ML. This suggests that the PZC might not change much at temperatures where adsorption site vacancies form, but proof would require dynamical calculations. It was found that the PZC increased systematically and significantly as the OH(ads) coverage of the surface increased, a simple consequence of increasing the average oxidation state of the surface metal atoms. Finally, the reversible potentials for the reactions studied here are strategically dependent on hydrogen bonding. An accurate description of the electrochemical interface will not generally be possible without complete representation of hydrogen bonding interactions. We close with note on the computations. The RPBE functional was developed by the Norskov group to overcome the extra strong metal-oxygen bond strengths obtained when density functional calculations are performed with the frequently employed PBE and PW91functionals.31 The results in this paper demonstrates the importance of using an accurate functional and including enough water molecules so that no strong hydrogen bonds between adsorbed molecules are left out. Acknowledgment. This work was supported by the National Science Foundation, Grant No. CHE-0809209 and by Toyota Central R&D laboratories, Inc. References and Notes (1) Kinoshita, K. Electrochemical Oxygen Technology; Wiley: New York, 1992. (2) Yeager, E. B. Electrochim. Acta 1984, 29, 1527–1537. (3) Anderson, A. B.; Kang, D. B. J. Phys. Chem. A 1998, 102, 5993– 5996. (4) Anderson, A. B.; Albu, T. V. J. Am. Chem. Soc. 1999, 121, 11855– 11863. (5) Anderson, A. B.; Albu, T. V. J. Electrochem. Soc. 2000, 147, 4229– 4238. (6) Kostadinov, L. N.; Anderson, A. B. Electrochem. Solid State Lett. 2003, 6, E30–E33. (7) Anderson, A. B.; Cai, Y.; Sidik, R. A.; Kang, D. B. J. Electroanal. Chem. 2005, 580, 17–22. (8) Zhang, T.; Anderson, A. B. Electrochim. Acta 2007, 53, 982–989. (9) Zhang, T.; Anderson, A. B. J. Phys. Chem. C 2009, 113, 3197– 3202.

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JP905377D

How Potentials of Zero Charge and Potentials for Water Oxidation to

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