DSpace at VNU: Hydronium Adsorption on OOH Precovered Pt(111) Surface: Effects of Electrode Potential

11, 2983–2989, 2011 Hydronium Adsorption on OOH Precovered Pt111 Surface: Effects of Electrode Potential Do Ngoc Son1 2 ∗, Bach Thanh Cong2, and Hideaki Kasai1 1Graduate School of Engine

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Nanoscience and Nanotechnology Vol 11, 2983–2989, 2011

Hydronium Adsorption on OOH Precovered

Pt(111) Surface: Effects of Electrode Potential

Do Ngoc Son1 2 ∗, Bach Thanh Cong2, and Hideaki Kasai1

1Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan

2Department of Physics, Hanoi University of Science, 334 Nguyen Trai, Hanoi, Vietnam

Using the Density Functional Theory-based total energy calculations, the hydronium adsorption on

the OOH precovered Pt(111) surface is studied The electrode potential is modeled by varying the

electron afﬁnity of the reduction center [OOH+H3O(H2O)]+ Two possible structures of this reduction

center on the Pt surface are HOOH+2H2O and 2(OH)+2H2O Evidently, the dissociation of HOOH

into 2(OH) can be accomplished by changing the electrode potential to the higher value by 0.16 V

The activation energy for the dissociation is approximately 0.1 eV The optimized structures are also

obtained

Keywords: Hydrated Hydronium Ion, Platinum Surface, Electrode Potential, Electron Afﬁnity,

Activation Energy

1 INTRODUCTION

The Pt(111) surface has long been the best

electrocat-alyst for the oxygen reduction (ORR) due to its role

as the rate controlling reaction in electrochemical energy

conversion.1–3 Fully understanding the mechanism of the

oxygen reduction on the Pt surface is highly desirable in

both fundamental and applicable aspects Over the Pt

sur-face in aqueous perchloric and sulfuric acid media, the

oxygen reduction to water can be observed in four-electron

steps in the presence of hydronium ion H3O+, which

comes from the membrane4 5 or appears by the charge

defects in water created by excess protons6 on the

cath-ode of proton exchange membrane fuel cells The overall

ORR is

O2+ 4H3O++ 4e− 6H2O (1)

The four-electron steps of intermediate reactions, which

were introduced in our previous work,7 are proposed as

follows

O2 O2 ad Molecular adsorption (2)

O2 2Oad Dissociative adsorption (DA) (3)

Oad+ H3O++ e− OHad+ H2O

Reductive transition (RT) (4)

O2 ad+ 2H3O++ 2e− 2OHad+ 2H2O

Reductive adsorption (RA) (5)

∗ Author to whom correspondence should be addressed.

OHad+ H3O++ e− 2H2O Reductive desorption (RD)

(6) Reactions (2)–(6) represent two adsorption pathways: One is through the dissociative adsorption (3) that forms adsorbed oxygen atom (Oad on the surface, followed by

a reductive transition (4) from Oad to adsorbed hydroxyl (OH)ad In this pathway, the ﬁrst electron transfers after

DA of O2; the other pathway is through the molecular adsorption (2), followed by a reductive adsorption (5) in which first electron transfers after the O2 adsorption In both cases, the reaction is completed by the reductive desorption of (OH)ad (6) Experimental and density func-tional theory (DFT) investigations found that O2favorably adsorbs on clean Pt(111) surface, as expressed in (2), in two possible configurations The first is a surface-parallel configuration centered over a bridge site as a -bound

paramagnetic O2− superoxo (bridge) state The second is

a tilted conﬁguration over a three fold fcc hollow site in a

-bound nonmagnetic O2 −peroxo (hollow) state.8–16 The superoxo state is more stable than the peroxo state Both adsorption states are observed at high coverages, but only the peroxo state is observed at low coverages.17 The DA (3) was studied by Yotsuhashi et al.,18wherein O2 dissoci-ates into two Oad atoms as O2approaches to the Pt surface

at a positive value of potential energy corresponding to the O–O distance about 2 Å and the center of mass of O2to Pt surface about 1.5 Å The reductive transition (4) has been done in the work of Son et al.7

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It is important to notice that there are two electrons in

the reductive adsorption reaction (5), which can be further

divided into the following two steps In the ﬁrst electron

reduction step,

O2 ad+ H3O++ e− OadOH+ H2O (7)

O2 ad gets molecularly adsorbed before the proton is

transferred from the hydronium ion to O2 ad, and

simulta-neously reduces one electron to form an adsorbed end-on

intermediate OadOH on the Pt surface with Oadon top site

The reaction (7) was also investigated in Ref [7]

The second electron reduction can proceed through the

following serial reduction and/or direct reduction in which

the reductive adsorption is continued by succeeding to (7)

in the presence of another hydronium ion

Serial reduction: The relatively stable intermediate

OadOH can be decomposed into adsorbed oxygen and

hydroxyl6with a small barrier yielding coadsorbed oxygen

and hydroxyl (Oad+ (OH)ad,

OadOH Oad+ OHad (8) before interacting with another hydronium and reduces

second electron to form 2(OH)ad as the following reactions

Oad+ OHad+ H3O++ e− Oad+ H2Oad+ H2O (9)

Oad+ H2Oad 2OHad (10) The decomposition is primarily driven by the

chemisorption of hydroxyl.19

Direct reduction: The OadOH intermediate in (7) can

directly interact with another hydronium ion and reduce

the second electron with the following reaction,6

OadOH+ H3O++ e− HOadOH+ H2O (11)

to form an end-on adsorbed hydrogen peroxide HOadOH

This peroxide readily dissociates homolytically to form

2(OH)ad as shown in reaction (12),

HOadOH 2OHad (12)

In this paper, the research shall be focused on the

intermediate reactions (11) and (12) taking into account

the electrode potential of the Pt surface using the

Den-sity Functional Theory Understanding the dependence of

mechanism, structures, and activation energies on the

elec-trode potential is one of main research topics in the

sur-face chemical reaction sciences The formation of (OH)ad

by the electrooxidation of H2O and its potential

depen-dence on Pt electrodes has been studied with a view toward

understanding its mechanism using the non-charge

self-consistent Atom Superposition and Electron

Delocaliza-tion Molecular Orbital models.20 21 This model is high

accuracy for predicting the changes in bond

polariza-tions and hybridizapolariza-tions as a function of electrode

poten-tial However, it was not applicable to predict accurate

potentials, reaction energies, and activation energies for redox reactions under the electrode potential Therefore,

we need a charge self-consistent theory for these

prob-lems An ab initio charge self-consistent theory has been

developed by Anderson’s research group.22 The electrode potential was modeled by varying the electron afﬁnities

of the reactant complex The transition state structures and activation energies were reported for the four one-electron steps of oxygen reduction In order to investigate the effects of electrode potential, an alternative approach would be either to charge the Pt layer, or put the system under the inﬂuence of an external ﬁeld By charging the

Pt layer, the work function of metal is varied A strong electric ﬁeld across the double layer results in the sur-face charge First-principles computational approach to a charged surface and interface, and the solution–electrode interface, under a bias voltage (an electrode potential)

in the framework of slab models were also introduced

in the work of Otani et al.23 24 The method is called

the Effective Screening Medium-Based First Principles Molecular Dynamics The dependence of activation ener-gies on the electrode potential can also be revealed by using phenomenological models for relating electrode cur-rents to activation Gibbs energies These models is based

on the view point of expanding the activation Gibbs ener-gies about the reversible potential One of these mod-els is the linear approximation, which gives rise to the Butler-Volmer equations and provides insight into the lin-ear regions of Tafel plots of the log of the current density

as a function of the electrode overpotential.25 The other is the harmonic model, which has linear and quadratic terms and a parameter attributed to the solvent reorganization energy accompanying the electron transfer Marcus and others have developed this model based on the transfer equations for characterizing outer-sphere electron-transfer reactions where electron tunneling is associated with a sudden charge in the redox state.26–28

In this work, we study reactions (11) and (12) on the same footing utilizing the Density Functional Theory based on exchange correlation functional, which has been recognized as good method in describing chemical sys-tems When the electron is loaded into the slab, the charge mainly goes to the Pt surface causing the change in the Fermi level The charge will then transfer from the Pt sur-face to the reduction center and enhances its electron affin-ity by varying the applied electrode potential The electron affinity then modifies the optimized geometry of the reduc-tion center

This paper is organized as follows: In Section 2, we give details of the computational method used in this study In Section 3, we sequentially present the simulation results for the reductive transition and the reductive adsorption, and lastly in Section 3, we draw the conclusions

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2 COMPUTATIONAL METHOD

We perform the DFT-based total energy calculations

utiliz-ing the Vienna ab initio simulation package VASP.29–31For

the exchange correlation energy, we use the

Perdew-Burke-Ernzerhof (PBE) derivatives of the generalized gradient

approximation,32which is good for the non-uniform charge

density systems The electron-ion interaction is described

using the projector-augmented-wave (PAW) method33 34

with plane waves up to the cutoff energy of 400 eV

Calcu-lations for the reactions (11) and (12) on the same footing

are performed with a three-layer slab of Pt(111) (4× 4),

and a vacuum space of 14.65 Å in the supercell Each Pt

layer consists of 16 atoms with nearest neighbor distance

of 2.78 Å All Pt atom positions in slab are ﬁxed in the

simulation This causes an error of 0.01 eV in the total

energy calculations as compared with the ﬁrst-Pt layer

relaxed case We have chosen sufﬁciently large supercell

to avoid interactions between hydronium ions and

oxy-gen atoms of different unit cells The surface Brillouin

zone integration is done using the special point sample

technique of Monkhorst and Pack with 5× 5 × 1

sam-pling meshes.35 The cutoff energy and the vacuum space

and the sampling meshes are tested to ensure the

con-vergence of calculations In addition, water molecules are

included in the periodic supercell since the water molecule

has a large dipole moment, which gives rise to the

non-negligible effect on the energetic of hydronium ion on the

Pt surface The dipole correction is taken into account in

the simulation.36 37

3 RESULTS AND DISCUSSION

In our previous paper,7 the optimized structure for the

OOH+ H2O formation was found, in which OOH is an

end-on adsorbed on the Pt(111) surface If the next

hydro-nium ion H3O+approaches the OOH+ H2O complex, the

Fig 1 (a) Model for the calculation of potential energy surface The distances from the hydrogen H2 to the Pt surface and to the hydronium ion O3 are denoted by Pt–H and O–H, respectively (b) The potential energy surface as a function of Pt–H and O–H At the minimum energies Min1= −017 eV

at (Pt–H= 3.7 Å, O–H = 1.7 Å), and Min2 = −157 eV at (Pt–H = 2.9 Å, O–H = 2 Å), the HOOH and the separated 2(OH) are formed, respectively.

The dissociation of HOOH into 2(OH) follows the least energy pathway, the arrowed curve.

hydronium ion shall be hydrated by the water molecule before reacting with OOH In this paper, we focus the research on the reaction (11), in which the hydronium ion H3O+ is hydrated by one water molecule, OOH+

H3O+(H2O)+ e−, on the Pt(111) surface We also

investi-gate the effects of electrode potential on the dissociation of HOOH into 2(OH) following the reaction (12) on the same footing The optimized structures, the minimum energies, the activation energies for the HOOH formation and for the dissociation of HOOH into 2(OH), can be obtained by studying the potential energy for the hydrated hydronium ion toward the Pt surface A slab model in Figure 1(a) is used for the calculation of the potential energy surface To avoid confusion, just few important indexes of Pt atoms (labeled 1, 2, and 3) are shown in this figure The opti-mized structure of the OOH intermediate adsorbed with an end on the Pt surface, which was obtained in Ref [7], has the bond distances of Pt1–O1= 2.75 Å, Pt2–O2 = 2.02 Å, O1–O2= 1.44 Å, and O1–H1 = 1.01 Å The distance Pt– H2 of hydrogen H2 to the Pt surface is denoted by Pt–H, and O–H denotes for the distance from H2 to the hydro-nium oxygen O3 The potential energy surface, in general, can be obtained when the H3O+(H2O) complex is moved toward the Pt surface along the slab normal to the oxygen O2 of the OOH intermediate However, in details, for each fixed value of Pt–H, O–H is varied by moving the hydro-nium oxygen O3 along the extended line of O2–H2 In this calculation, Pt atoms and H2 and O3 are fixed while all the others are allowed to relax Fixing Pt atoms causes an error in the total energy calculation of 0.01 eV The total energy of the Pt(111)-OOH+H3O+(H2O)+e−complex as

a function of O–H and Pt–H is obtained and presented in Figure 1(b) This ﬁgure is partitioned into three different areas for three different states of the complex denoted by Phase 1, 2, and 3 by the black curves

In Phase 1, the OOH+H3O+(H2O)+e−complex, which

has a large Pt–H above 3.3 Å and a small O–H less than

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1 Å, is in the initial state corresponding to the isolated

H3O+(H2O) over the Pt–OOH

For the large distances of Pt–H (greater than 3.1 Å) and

O–H (greater than 1 Å), the OOH+H3O+(H2O)+e−

com-plex passes through its structure in Phase 2 The hydrated

hydronium ion H3O+(H2O) is closed enough to the OOH

intermediate, and there is the transfer of proton H2 from

H3O+(H2O) to OOH to form HOOH weekly adsorbed on

the Pt surface The minimum energy, Min1= −017 eV at

(Pt–H= 3.7 Å, O–H = 1.7 Å), corresponds to the HOOH

formation This value of minimum energy is originated

from a transition state on the interface (the black line) of

Phase 2 and Phase 3 at (Pt–H= 3.3 Å, O–H = 2.6 Å) The

optimized structure of HOOH+2H2O and the

correspond-ing total charge density are shown in Figures 2(a) and (b),

respectively At Min1, O1 is side-adsorbed on Pt1 while

O2 is on the top of Pt2 O1H1 is almost parallel to the

Pt surface, while O2H2 is in the direction of the surface

normal, and angles (H1O1O2)= 101 and (H2O2O1)=

100 The bond distances of the HOOH formation are

listed in Table I This table shows that O1–H1 is 0.07 Å

shorter, while O1–O2 is 0.02 Å longer, than those of

Fig 2 (a) The optimized structure of the HOOH + 2H 2 O formation on the Pt surface (b) The contour plot of the total charge density of the HOOH +2H 2 O complex (c) The optimized structure of the 2(OH) +2H 2 O formation on the Pt surface (d) The contour plot of the total charge density

of the 2(OH) + 2H 2 O complex.

OOH in the initial state, respectively The calculated dis-tance of O1–O2 is in good agreement with the results obtained in Refs [38–39] The total charge density of the HOOH formation is shown in Figure 2(b), where HOOH is interpreted as a set of the identical center roundish curves closest to the Pt surface The outer most contour curve describes the bonding of HOOH with 2H2O and also the adsorbed state of the HOOH+ 2H2O complex on the Pt surface

As shown in Figure 1(b), there is a gradual transition from Phase 1 to Phase 2 on varying O–H for large enough Pt–H Independently, the OOH+ H3O+(H2O)+ e−

com-plex can also transit directly from Phase 1 to Phase 3 for its initial state with a short enough distance of Pt–H (less than 3.3 Å) Phase 3 is interpreted as the deep well interfacing with Phase 1 for small O–H and with Phase 2 for large O–H As stated before that the calculation of the potential energy surface is obtained by varying each ﬁxed value of Pt–H and O–H is altered to ﬁnd the minimum energies Thus, for small distances of Pt–H lower than that at Min1 (Pt–H= 3.7 Å, O–H = 1.7 Å) of the HOOH formation, H2 is interpreted as being forced to move toward the Pt

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Table I List of parameters of the HOOH + 2H 2 O and the 2(OH) + 2H 2 O.

Intermediates Pt–O1 (Å) Pt–O2 (Å) O1–H1 (Å) O2–H2 (Å) O1–O2 (Å) Minimum energy (eV) Fermi energy of Pt Slab (eV)

∼ 150 b

surface This enforcement is possible by varying the

elec-trode potential.39 When H2 is forced to lower positions,

the reduction center [OOH+ H3O(H2O)]+ passes through

its structure in Phase 3 The minimum energy, Min2=

− 157 eV at (Pt–H = 2.9 Å, O–H = 2 Å), corresponds

to the two separated OH formation on the Pt surface,

2(OH)+ 2H2O The optimized structure of 2(OH)+ 2H2O

and the corresponding total charge density are presented

in Figures 2(c) and (d), respectively The energy difference

of Min1 and Min2 is 1.4 eV Figure 2(c) shows that O1

and O2 adsorb on the top of Pt1 and Pt2, respectively, and

the hydrogen H1 intervenes between O1 and O2

separat-ing these two oxygen atoms The bond distances in the

2(OH)+ 2H2O formation are also pointed in Table I One

found that Pt–O1 and Pt–O2 are decreased and shorten

by 0.98 Å and 0.7 Å, correspondingly, compared to those

of HOOH+ 2H2O It means that O1 and O2 are much

stronger bound on the Pt surface The bond distance of

O2-H2 is the same for both cases while O1–H1 and O1–O2

increases by 0.03 Å and 1.18 Å, respectively, compared to

those of HOOH+ 2H2O Additionally, the obtained value

of O1–O2 is comparable to that in the work of Ref [39]

The signiﬁcant change in the O1–O2 distance occurs by

the intervention of the hydrogen H2 between O1 and O2

This intervention stems from H2 was forced toward the

Pt surface, which can be done by changing the electrode

potential making the variation in the electron afﬁnity of

the reduction center [OOH+ H3O(H2O)]+, which shall be

discussed later in this manuscript Additionally, it can be

seen that O1–H1 is 0.04 Å shorter than that of OOH

in the initial state This change in O1–H1 stabilizes the

partially occupied ∗ orbitals of oxygen O1 The total

charge density of the 2(OH)+ 2H2O formation is shown

in Figure 2(d), where 2(OH) is presented by two sets of

the identical center roundish curves, while in the case of

HOOH+ 2H2O is only one set, closest to the Pt slab

sur-face The presentation of the total charge densities could

help us to distinguish the difference between the HOOH

formation and the 2(OH) formation The outer most

con-tour curve describes the bonding of 2(OH) with 2H2O,

also the adsorbed state of the 2(OH)+ 2H2O complex on

the Pt surface

Figure 1(b) also shows that there is a possibility for the

reduction center transit from Phase 2 to Phase 3 along the

least reaction pathway as described by the arrowed curve

The transition must overcome a barrier at the interface of these two phases This transition is understood as the dis-sociation of HOOH into 2(OH) The activation energy for this dissociation is directly related to the classical barrier height, i.e., the energy difference between the maximum energy of the saddle point at (Pt–H= 3.0 Å, O–H = 1.2 Å) and the energy Min1 Thus, the transition from Phase 2

to Phase 3 corresponding to the dissociation of HOOH into 2(OH) must overcome an activation energy of about 0.1 eV

As aforementioned, the potential energy surface was obtained by varying Pt–H and O–H For each ﬁxed Pt–H, O–H is gradually changed by ﬁxing O3 at different posi-tions to obtain the optimized structures of the reaction center This method is equivalent to applying the elec-trode potential onto the Pt slab If the elecelec-trode potential

is altered, the Fermi level of the slab is varied, leading to modify the amount of electron transfer from the Pt slab

to the reduction center [OOH+ H3O(H2O)]+ adsorbed on the surface, and hence, modifying the electron affinity of the reduction center Due to the thermal fluctuations, if the electron affinity matches the electron chemical potential of the Pt slab, the reduction center will pass through its struc-tures that are of HOOH+ 2H2O or 2(OH)+ 2H2O The electron chemical potential of the Pt electrode is the neg-ative of the work function.39 Thus, the electron affinity,

EA, is

EA = − = −Evacuum− EFermi (13) Here, the vacuum level and the Fermi level of Pt slab are denoted byEvacuumandEFermi, respectively In order to obtain the vacuum level, the potential energy as a function

of z along the surface normal is calculated The obtained

curve presented in Figure 3 is the local potential of the Pt slab, in which the average vacuum level is approximately

Evacuum= 469 eV.

Different structures of the reduction center were obtained, which are of HOOH+2H2O and 2(OH)+2H2O The corresponding Fermi levels for these structures, as shown in Table I, are −0.55 eV and −0.71 eV, respec-tively Using Eq (13), the electron afﬁnities of these two structures can be obtained, correspondingly,EA1= −524

eV and EA2 = −54 eV The work functions are also

deduced that are 1= 524 eV and 2= 54 eV,

respec-tively The difference of the two electron afﬁnities is 0.16 eV

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Fig 3 Local potential of the Pt(111) slab The curve is ﬂat for z ranging

from 1 to over 4 The average of the ﬂat part of the local potential

corresponds to the average vacuum level of the Pt slab The average

vacuum level is 4.69 eV.

On the scale of the standard hydrogen electrochemical

potential, the electrode potentialU is given as

As determined by (14), 2(OH) is formed at the electrode

potential of 0.8 V for2= 54 eV This value of the

elec-trode potential is within the 1.23 V reversible potential

on the standard hydrogen scale, and closed to the

work-ing voltage of the cathode of the proton exchange

mem-brane fuel cells around 0.8 V on Platinum electrodes.40

The electrode potential of 0.8 V is 0.2 V greater than of

the OH formation from water decomposition on platinum

electrodes

The difference of electrode potentials of the two

struc-tures in Phase 3 and Phase 2,U , is

U = U2− U1=2− 1

e = 016 V (15)

From the above analysis, it is found that the dissociation

of HOOH into 2(OH) can be done by applying a more

positive potential of 0.16 V onto the Pt electrode

In order to understand how much charge variation of the

reduction center corresponding to the 0.16 eV difference

in the electron afﬁnity, the Bader charge shall be analyzed

in the following section

In Table II, the Bader charge of atoms in the reduction

center was calculated The Bader charge of H2 is zero in

both cases, and H2 remains its role as a proton of the

reduction center Oxygen atoms gain charge while other

Table II Bader charge of the HOOH + 2H 2 O (Phase 2) and the 2(OH) + 2H 2 O (Phase3); (−) charge loss, (+) charge gain.

Structure O1 (e) O2 (e) O3 (e) O4 (e) H1 (e) H2 (e) H3 (e) H4 (e) H5 (e) H6 (e) Total gain (e) Phase 2 +09893 +09902 +19848 +19605 −09997 0 −09988 −09994 −09989 −09994 +09286

Phase 3 +13347 +14124 +19975 +19701 −09999 0 −09988 −09995 −09986 −09984 +17195

hydrogen atoms loose its charge The charge gain of oxy-gen atoms attains from hydrooxy-gen atoms and from the Pt slab The total charge gain of the reduction center from the

Pt slab is approximately+093e and +1.72e for Phase 2

and Phase 3, respectively These amounts of charge mainly

go to oxygen atoms O1 and O2, as shown in Table II When the charge of O1 and O2 increases, the repulsive force between these oxygen atoms also increases, pushing them far from with each other, and hence, giving the space between O1–O2 for H2 to occupy The distance O1–O2

is increased, as shown in Table I, to form two separated

OH adsorbed on the Pt surface When the Pt electrode is applied with the more positive potential of 0.16 V com-pared to the case of the HOOH formation in Phase 2, the Fermi level of the electrode is decreased from−0.55 eV to

−0.71 eV, increasing the electron transfer from the elec-trode into the reduction center This change in the electron transfer varies the electron afﬁnity of the reduction center, arranging the optimal structures in Phase 2 and Phase 3

as the electron afﬁnity of the reduction center matches the electron chemical potential of the Pt slab As have seen

in the above analysis, the O1–O2 is stretched, while O1– H1 is shrunken, compared to OOH in the initial state The stretching of the O1–O2 bond stabilizes the partially occu-pied∗ orbitals of O1 and O2, while the shrinking of the

O1–H1 bond stabilizes the partially occupied ∗ orbitals

of O1 and O2, increasing the electron afﬁnity of the reduc-tion center as obtained in the upper part

The effects of the electric double layer and the trode potential are incorporated by introducing excess elec-trons to a metal-solution interface slab model,22 where

a compensating uniform background charge was artiﬁ-cially inserted to the vacuum layer to measure the applied electrode potential In this work, a comparison of two adsorbed states in Phase 2 and Phase 3 are mainly con-cerned with The hydrated hydronium ion is formed as adsorbed on the OOH precovered Pt(111) surface so that

an electron of hydrogen atom H2 is transferred to the Pt-OOH This model is neutral, and hence, the uniform back-ground charge is not required.41

4 CONCLUSION

In this work, we have studied the formation of HOOH and 2(OH) on the Pt surface and the transition of the former into the latter on the same footing and clariﬁed the effects

of the electrode potential using the density functional the-ory On varying the position of proton, the potential energy surface was obtained by changing the bond distance of the

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proton and the hydronium oxygen to ﬁnd the optimized

structures of the reduction center [OOH+H3O(H2O)]+ By

this way, the electron afﬁnity of the reduction center was

varied, and hence, the electrode potential takes its effects

The change in the electron afﬁnities of HOOH+2H2O and

2(OH)+ 2H2O is 0.16 eV corresponding to the 0.16 V

variation of the electrode potential This research evidently

shows that the dissociation of HOOH into 2(OH) can be

done by changing the electrode potential The activation

energy for the decomposition of HOOH into 2(OH) is

about 0.1 eV

Acknowledgments: This work is partly supported by

the Ministry of Education, Culture, Sports, Science and

Technology of Japan (MEXT) through the Grants-in-Aid

for Scientiﬁc Research (A19206007) and (A20246011)

and through the Special Coordination Funds for the Global

Center of Excellence (COE) program (H08) “Center of

Excellence for Atomically Controlled Fabrication

Tech-nology.” One of the authors (Do Ngoc Son)

acknowl-edges the funding from the Marubun Research Promotion

Foundation

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Received: 31 January 2010 Accepted: 15 March 2010

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