SERS study of hydrogen peroxide electroreduction on a Pb-modified Au electrode

JOURNAL OF RAMAN SPECTROSCOPYJ. Raman Spectrosc. 2005; 36: 715–724Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jrs.1339

SERS study of hydrogen peroxide electroreductionon a Pb-modified Au electrode

Xiao Li and Andrew A. Gewirth∗

Department of Chemistry and Fredrick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA

Received 17 September 2004; Accepted 26 November 2004

The mechanism of the electroreduction of H2O2 on a Pb-modified Au surface was examined by surface-enhanced Raman spectroscopy. The measurements show the presence of lead peroxide, PbOOH+, with themaximum intensity at potential just positive of underpotentially deposited Pb, and bimetallic dihydroxide,AuPb(OH)2, at potential just positive of that where peroxide is reduced and Pb islands maximally formedon the surface. All the modes for these two species decay rapidly and even disappear as the reductioncurrent flows. Both species are absent in solutions not containing either Pb2+ or H2O2. The edge of Pbislands on the Au surface is thus shown to be the active site for this catalysis process. These results furtherunderscore the important role of M–OH species in the peroxide electroreduction reaction. Copyright 2005 John Wiley & Sons, Ltd.

KEYWORDS: surface-enhanced Raman scattering; hydrogen peroxide; electroreduction reaction; lead-modified goldelectrode; electrocatalysis

INTRODUCTION

The electrocatalysis of dioxygen reduction has an importantrole in fuel cells, metal–air batteries and corrosion.1 Fordecades, the kinetics of this electroreduction process havebeen studied extensively using various techniques, especiallyelectrochemical methods.2 However, a clear mechanism forthis process has still not been achieved, which hindersthe search for better and more active catalysts. In acidsolution, oxygen reduction proceeds via either a four-electron pathway [Eqn (1)] on materials such as Pt or Ag:

O2 C 4HC C 4e� ��! 2H2O E° D 1.229 V vs NHE�1�

or a peroxide intermediate-based two-electron pathway onmaterials such as Au as shown in Eqns (2) and (3):

O2 C 2HC C 2e� ��! H2O2 E° D 0.67 V �2�

H2O2 C 2HC C 2e� ��! 2H2O E° D 1.77 V �3�

Surfaces modified with monolayers or submonolayersof underpotentially deposited (upd) foreign metal adatoms

ŁCorrespondence to: Andrew A. Gewirth, Department ofChemistry and Fredrick Seitz Materials Research Laboratory,University of Illinois at Urbana-Champaign, Urbana, Illinois 61801,USA. E-mail: [email protected]/grant sponsor: National Science Foundation;Contract/grant number: CHE-0237683.

have long been interesting for their unique properties, whichare typically different from those of either the bulk depositor the original metal surface.3 – 5 Relevant to oxygen andperoxide reduction, Pb,6 Tl7 or Bi8 upd on an Au surfacecan greatly enhance the reduction of both H2O2 and O2. Theenhancement occurs only within a narrow potential range,corresponding to specific structures present on the electrodesurface. In the case of Pb upd on Au(111), electrochemicalcharacterization of this system in the presence of peroxidehas shown that the catalytic activity occurs in a narrowpotential region associated with the most positive Pb updpeaks.9

The Pb upd process on an Au surface has been extensivelystudied using electrochemical methods.10 – 17 The Pb updbehavior is solvent dependent.18,19 and specific adsorption ofanions can complicate the process.20 Other aspects of Pb updon gold have been structurally characterized by STM.21 – 23

On an Au(111) surface, Pb islands form before the onsetof the full monolayer structure.24,25 On an Au(100) surface,the Pb deposition starts in steps, and a commensuratec(2 ð 2)-Pb adlattice is formed on flat terraces. At lowerpotentials, a Pb-c(3

p2 ð p

2)R45° superlattice structureswas observed on Au(100) before the formation of a fullmonolayer.26 Theoretical studies suggest that the adsorptionof Pb decreases in the order �110� > �100� > �111�.27

Pb upd acts as a catalyst for the electroreduction ofboth H2O2 and O2 on Au surfaces.3,28,29 However, Pb updon Ag partially inhibits the four-electron reduction of O2.6

Copyright 2005 John Wiley & Sons, Ltd.

716 X. Li and A. A. Gewirth

The catalytic activity of Pb upd towards O2 and H2O2

electroreduction is also sensitive to the crystallographicorientation of the Au surface.3,15 Pb does not act as a catalystfor this process on Au(110),29 but is active on both the(111) and (100) faces of Au, as indicated by both the positiveshift of the half-wave potentials and the increased limitingcurrent.6 We have shown that the potential of maximumdioxygen and H2O2 electroreduction is associated with anAu surface decorated with Pb islands.24,25,30 We furthersuggested that the active sites are at the edge of the islandswhere a heterobimetallic bridge between the Au substrateand the Pb adatom may be formed by peroxide.25

Although the structure of the catalytically active Pb-modified Au surface is now understood, there is littleunderstanding as yet concerning the mechanism of O—Obond cleavage and H2O2 electroreduction on a Pb-modifiedAu surface. Recently, we used surface-enhanced Ramanscattering (SERS) along with detailed calculations to examinethe peroxide reduction process on Bi-modified Au surfaces.31

In that case, peroxide reduction was found to proceed viathe formation of a Bi—OH moiety formed via spontaneousdecomposition of peroxide on the surface. In this work, weutilized the SERS technique to examine species present on aPb-modified electrode surface during hydrogen peroxideelectroreduction. Correlation of species identified in thespectroscopy with the electrochemical response providesinsight into intermediates in the electroreduction reaction,which helps to clarify the mechanism.

EXPERIMENTAL

All solutions were prepared from ultrapure water (Milli-Q UVplus, Millipore; 18.2 M� cm). Reagent-grade PbO(Aldrich Chemical), H2O2 (30%, Fisher) and HClO4 (70%,Baker, UltrexII) were used for preparing solutions. Cyclicvoltammograms were obtained in a glass electrochemicalcell using an AFRDE5 (Pine Instrument) potentiostat. Thepolycrystalline gold was first annealed for 3 min in ahydrogen flame and quenched with ultrapure water. A goldwire (Alfa, 99.9985%, 0.5 mm diameter) was used as thecounter electrode and an Ag/AgCl electrode was used as thereference electrode. All potentials are reported with respectto NHE. The solutions were deoxygenated with N2 for 1 hand an N2 atmosphere was maintained in the cell duringall electrochemical experiments. Rotating disk electrode datawere obtained using a Pine Model MSRX rotator equippedwith a collet to hold the gold crystal.

Au crystal preparation and SERS instrumentation havebeen described previously.31 The solution was deaeratedwith N2 during all experiments. Spectra were obtainedbetween 150 to 1200 cm�1 at 0.1 V intervals. The systemwas allowed to equilibrate for 2 min at each potential beforeacquiring spectra. The spectral resolution was estimated to be5 cm�1. The baseline for each SERS spectrum was correctedprior to presentation.

Calculations of vibrational wavenumbers for different Pbcompounds were performed using the Gaussian 94 programpackage at the DFT (B3LYP) level of theory using theLANL2DZ basis set.32

RESULTS

Electrochemical behaviorPb UPDFigure 1 shows the cyclic voltammogram of Pb upd on(a) Au(111)25 and (b) Au(poly) in solution containing 1.0 mM

Pb2C and 0.1 M HClO4. Above 0.5 V, no features associatedwith Pb deposition is observed on the Au(111) surface. Asthe potential is swept negatively, only double-layer currentis observed until the sharp peak C appears at �0.03 V.According to previous studies using STM and AFM,24,33 Pbislands begin to form on the surface at a potential positiveof peak C, and they coalesce to the full Pb monolayer atpotentials negative of peak C. On the reverse scan, twocorresponding stripping peaks, A1 and A2, are observed.Small features at more positive potentials are associatedwith the Pb stripping from Au step edges.

On the SERS-active Au(poly) surface, the electrochemicalbehavior, shown in Fig. 1(b), is substantially different fromthat on Au(111). First, peak C, the Pb deposition peak onAu(111), is split into two peaks, at �0.08 V and 0.16 V.Second, the corresponding stripping peaks A1 and A2 shiftfurther away from each other, and settle at �0.04 V and0.24 V, respectively. These differences are associated with Pbupd on faces in addition to (111) on the polycrystalline Au

(b)

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0204060 A1

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-0.2 0.0 0.2 0.4 0.6 0.8-25-20-15-10-505

101520

Potential/V vs. NHE

Cur

rent

/µA

Figure 1. Cyclic voltammogram of 1.0 mM Pb2C C 0.1 M HClO4

on (a) Au(111) and (b) SERS-active Au(poly) surfaces. Scanrate, 25 mV s�1. The cyclic voltammogram on Au(111) isadapted from Ref. 25.

Copyright 2005 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2005; 36: 715–724

SERS study of H2O2 electroreduction 717

surface. For example, Pb upd exhibits peaks around �0.1and 0.2 V on Au(110) and Au(311).11,15 Finally, the featuresin the voltammogram obtained from polycrystalline Au arebroader than those observed on a single-crystalline surface,a behavior observed previously.34

H2O2 electroreductionFigure 2 shows hydrogen peroxide reduction on (a) Pb-modified Au(111) and (b) Au(poly) electrodes in solutioncontaining 10 mM H2O2 C 1.0 mM Pb2C C 0.1 M HClO4. Posi-tive of 0.4 V, there is essentially no current due to H2O2

reduction on the Au(poly) surface. The H2O2 reduction cur-rent begins to flow at 0.3 V, and reaches the maximum at�0.15 V. Hydrogen evolution occurs at more negative poten-tials. As the full Pb monolayer formed at potentials negativeof peak C, we suggest that a Pb full monolayer also hascertain activities towards peroxide reduction. By way ofcomparison, the Au(111) surface exhibits both a sharper andmore positive onset for peroxide reduction; this change isassociated with changes in the upd voltammetry betweenthe two surfaces shown in Fig. 1

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Potential/V vs. NHE

j D

-1 (

mA

-1cm

2 )

ω-1/2 (s-1/2)

Figure 2. Linear sweep voltammogram obtained from (a) theAu(111) surface (adapted from Ref. 25) and (b) the SERS-activeAu surface both rotating at 400 rpm in a solution containing10 mM H2O2 C 0.1 mM Pb2C C 0.1 M HClO4. The dashed line isthe current without Pb2C. The inset in (b) is the Levich plot forH2O2 reduction. Peak current was measured at the maximumof the catalytic current at each rotation speed.

Compared with the near zero H2O2 reduction currenton bare Au surface, shown as the dashed line in Fig. 2(b),peroxide electroreduction is greatly catalyzed by the Pb-modified Au surface at �0.15 V. The negative shift of ca70 mV of the potential of the maximum catalytic activityrelative to the upd features in the voltammetry is causedby the iR drop due to the high catalytic current, which hasbeen observed previously.35 The inset in Fig. 2(b) showsthe Ketoucky–Levich plot for H2O2 reduction on a Pb-modified Au(poly) surface. Below 1000 rpm, the maximumreduction current increases linearly with the square root ofthe angular velocity of the disk, which is characteristic ofa totally mass transfer-limited process. As the rotation rateincreases beyond 1000 rpm, the current becomes saturated,which indicates that the reaction is kinetically limited. Asimilar behavior was also observed on the Au(111) surface.25

This plot can be fitted into the Ketoucky–Levich Eqn (4).For electrode processes with slow kinetics, the currentdensity can be expressed as

1/jD D 1/jK C 1/0.26nω�1/2 �4�

where jD is the diffusion-limited current density (mA cm�2),n is the number of electrons exchanged in the H2O2

reduction reaction, jK is the kinetic current and ω is therotation rate. The constant 0.26 comes from evaluating0.62 F D0

2/3��1/6C0, where F is the Faraday constant, D0

is the diffusion coefficient of H2O2 �1.71 ð 10�5 cm�1 s�1�,27,� is the kinematic viscosity �9.97 ð 10�5 cm2 s�1�, and C0

is the bulk concentration of H2O2�10.0 ð 10�6 mol cm�3�.’Fitting the data in the inset of Fig. 2(b) to Eqn (4) yieldsjK D 3.8 mA cm�2 and n D 1.6 �R2 D 0.97�.

SER spectraSERS was used to investigate the interface during theelectrocatalytic process. Shown in Fig. 3 are the SER spectraof electrochemically roughened Au immersed in solutionscontaining (a) 0.1 M HClO4, (b) 1.0 mM Pb2C C 0.1 M HClO4

and (c) 10 mM H2O2 C 0.1 M HClO4. The anodic scan isshown on the left of each potential scale from �0.7 to 0.7 V(vs NHE), and the cathodic scan is on the right. Only thatpart of the spectrum with features is shown.

SERS with perchlorate aloneIn a solution containing 0.1 M HClO4, four peaks, labeled4, 5, 7 and 8, are observed, as indicated by arrows inFig. 3(a). Peak 5 at 927 cm�1 is well understood to be thesymmetric stretch mode of the perchlorate ion �s�ClO4

��.37

The intensity of this peak increases as the potential movespositively and decreases as the potential scans back. Thisbehavior has been observed previously and is attributed tothe enhanced adsorption of perchloric ion on the Au surfaceat high potentials.31,38

Peak 4 at 850 cm�1 is assigned to the twisting mode ofwater in the clathrate structure formed by the perchlorate



Figure 3. SER spectra from Au(poly) in a solution containing (a) 0.1 M HClO4, (b) 1.0 mM Pb2C C 0.1 M HClO4 and (c)10 mM H2O2 C 0.1 M HClO4. The anodic scan from �0.7 to 0.7 V is shown on the left of each panel and the cathodic scan is shownon the right.

above the Au surface, υ�H2O�.39,40 At room temperature, anHClO4Ð5.5H2O clathrate (I) structure forms at the interface,which decays rapidly perpendicular to the surface.41

Peak 7 at 998 cm�1 is assigned to the asymmetric O–Hstretch mode of the hydronium ion, �2�H3OC�.37,42 Theintensity of this peak decreases as the potential increasesand eventually disappears at positive potentials, which iscoincident with the absence of this mode at potential positive0.0 V in previous studies.31 Interestingly, the adsorptionof hydronium at low potential was also observed ona Pt(111) surface using infrared spectroscopy.43 At themost negative potentials in the cathodic scan, peak 7 isobserved again, which may due to the increasing attractionbetween hydronium ion and the negative charged Ausurface.

Peak 8 at 1035 cm�1 belongs to the symmetric stretchmode of perchlorate, �s�ClO3�, which is adsorbed with a C3�

coordination geometry on the Au surface.44,45 The intensityof this peak decreases as the potential increases, which maycaused by the possible change of adsorption geometry ofperchlorate on the surface.

SERS with added Pb2C

With the addition of Pb2C, the spectra shown in Fig. 3(b)exhibit three peaks, labeled 5, 7 and 8. Peak 5 at 930 cm�1

is assigned to the �s�ClO4�� mode because of its energy

and potential dependence are identical with those for thesame band seen in blank solution discussed above. Peak7 at 998 cm�1, the �2�H3OC� mode, exhibits low intensitythroughout the potential region studied, which is caused by



the interference of the Pb deposition process occurring on theAu surface. Peak 8, the �s�ClO3� mode at 1035 cm�1, exhibitslower intensity in the cathodic than in the anodic scan.

Peak 4 at 850 cm�1, associated with the H2O twistingmode in the clathrate structure, is not observed. Thedisappearance is due to the collapse of the perchlorateclathrate structure near the Au surface with the additionof lead ions, which has been observed previously with theaddition of Bi3C ion.31

SERS with added H2O2Figure 3(c) shows the SER spectra of roughened Au in solu-tion containing 10 mM H2O2 C 0.1 M HClO4 under potentialcontrol. Three peaks are observed, labeled 3, 5 and 7. Peak7 at 998 cm�1, the �2�H3OC� mode, exhibits the same poten-tial dependence as in the solution not containing H2O2 withlow intensity at positive potentials. Peak 5 at 930 cm�1, the�s�ClO4

�� mode, shows near constant intensity throughoutthe potential region. Peak 8, �s�ClO3�, is absent with thepresence of H2O2 in solution. Peak 4 at 850 cm�1, associatedwith the twisting mode of H2O, is also not observed, whichis probably due to the collapse of the clathrate structure.

The new peak observed at 840 cm�1, peak 3, is thecharacteristic O–O stretch mode of H2O2. The intensityand position of this peak changes little with potential,as previously observed.31 Note that the behavior of peak3 is substantively different from that of peak 4, υ�H2O�,arising from the perchlorate clathrate structure near theelectrode surface, particularly with regard to the potentialdependence of peak position. The wavenumber of peak3 is essentially independent of potential, whereas peak 4exhibits considerable potential dependence. This shows thatthese features probably arise from different species at theinterface.

SERS with both Pb2C and H2O2Figure 4 shows the SER spectra obtained from Au(poly)in a solution containing 10 mM H2O2 C 1.0 mM Pb2C C 0.1 M

HClO4. In the presence of Pb2C and H2O2, six peaks areobserved at 680 cm�1 (peak 1), 722 cm�1 (peak 2), 932 cm�1

(peak 5), 973 cm�1 (peak 6), 998 cm�1 (peak 7) and 1053 cm�1

(peak 9).Peak 5 at 932 cm�1 is assigned to the �s�ClO4

�� stretchmode. Its intensity increases as the potential increases, abehavior observed in the solution not containing eitherPb2C or H2O2, as discussed above. Peak 7 at 998 cm�1,the �2�H3OC� mode, is observed only between �0.3 and0.4 V. The disappearance of this mode at high potential wasobserved in the blank solution with and without H2O2.At potentials lower than �0.2 V, peak 7 disappears asH2O2 reduction occurs on the surface covered with Pbislands.

Four new peaks, labeled 1, 2, 6 and 9, are observed inthe presence of both Pb2C and H2O2. Peak 9 at 1053 cm�1 isassigned as a mode associated with PbO2

2�.46 The intensity

Figure 4. SER spectra from Au(poly) in a solution containing10 mM H2O2 C 1.0 mM Pb2C C 0.1 M HClO4. The anodic scanfrom �0.7 to 0.7 V is shown on the left of each panel and thecathodic scan is shown on the right.

of this peak maximizes at high potentials and decreases atlow potentials in the cathodic scan, which will be discussedlater in detail. Because of the small energy difference betweenpeaks 8 [1035 cm�1, �s�ClO3�] and 9, we cannot be absolutelysure if peak 8 is present or not. However, the potentialdependence of peak 9 found with both Pb2C and H2O2 isdifferent from that observed in the absence of Pb2C, leadingus to assign this feature as a new peak distinct from peak 8.

No peaks are observed in the region between 800 and900 cm�1. The absence of peak 4 at 850 cm�1 corresponds tothe collapse of the clathrate structure in the presence of bothH2O2 and Pb2C. We did not observe the O–O stretch mode ofH2O2, which is observed with the presence of only peroxide.The disappearance of this mode suggests that H2O2 is notpresent as a stable species near the surface in the presence ofPb ion in the solution within the potential region examinedhere.

There are three more new peaks (1, 2 and 6) at 680, 722and 972 cm�1. However, assignment of these peaks is not sostraightforward, as there are no easy literature precedents onwhich we can rely. We now begin to address the assignmentof these peaks. First, as these peaks are not observed ineither pure perchlorate solution or in the solutions withonly one additive, it is unlikely that they are associatedwith perchlorate compounds. Pb�ClO4�2 exhibits featuresat 640 and 920 cm�1 due to the perchlorate group, and nomodes were reported near what we observed.47 Second, sinceAu oxides and Au hydroxides (with only Au present) areobserved only between 400 to 600 cm�1, peaks 1, 2 and 6 donot originate from species on the bare Au surface or from Auoxide. Hence it is reasonable to conclude that these peaks arefrom species associated with Au, Pb and H2O2 present on ornear the surface.



Figure 5. Ball-and-stick geometry of compounds of Pb, Au, O and H. The calculated vibrational wavenumbers of these species arelisted in Table 1. Au, Pb, O and H atoms are striped, solid, open and no balls, respectively.

The vibrational modes associated with Pb oxides, suchas Pb2O2 (cyclic), PbO, Pb�O2�, Pb2�O2� and Pb3O4, havebeen reported previously and are always found below600 cm�1.48 – 50 Hydroxide has long been thought to play animportant role in the reduction of H2O2 on varies surfaces.1

Unfortunately, no mode of Pb hydroxide has been reportedat the energy of peaks 1, 2 and 6.32,51 – 53 We previouslysuggested that Pb island edges are most active for H2O2

reduction, and an Au–Pb heterobimetallic site is probablythe catalytic site of this catalysis.25

To clarify the origins of peaks 1, 2 and 6, we performedcalculations of vibrational modes for possible Au, Pb, Oand H compounds. Shown in Fig. 5 are the 15 possiblehydroxide and peroxide compounds considered here. Forsimplicity, chemisorption of species on the Au surface ismodeled through interaction of species with one Au atom,since realistic modeling of the Pb island overlayer on an Ausurface would require prohibitive computational resources.Bimetallic (Au–Pb and Pb–Pb) dihydroxide, lead hydroxide,lead dihydroxide, cyclic lead dihydroxide, lead peroxide,lead diperoxide and lead hydrogen peroxide near or on thesurface are shown in Fig. 5(a)–(o), respectively.

The calculated vibrational modes from these models arelisted in Table 1. Interestingly, the O–H bending vibrationmodes of the bimetallic dihydroxide [Fig. 5(b)] at 645and 723 cm�1 are close to the experimental peak 1 and2 wavenumbers at 680 and 722 cm�1, respectively. AsFig. 6(a) shows that the intensities of both peaks 1 and 2exhibit the same potential dependence, it is highly likely thatthey are modes originating from the same moiety, whichsupports the calculation result that both peaks belong tothe bimetallic hydroxide. Furthermore, the O–O stretch

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Figure 6. Potential dependence of SER spectral intensity fromFig. 4: (a) peak 1, υ(PbOH) (ž), and peak 2, υ(AuOH) (°), ofAuPb(OH)2 on Au surface, and (b) peak 6, �(O–O) of PbOOHC

(�), and peak 9, ��PbO22�� (�), overlaid with the cathodic scan

at 25 mV s�1 obtained during the experiment (solid line).

mode of lead peroxide cation [Fig. 5(k)] at 1003 cm�1 isclose to the energy of peak 6 observed at 968 cm�1. There



Table 1. Results of vibrational wavenumber (cm�1) calculations for Pb, Au species

Speciesa �(Au–O) �(Pb–O) �as(Pb–O) υ(Pb–O–H)b υ(Au–O–H) �(O–O) �(O–H)

(a) AuPb(OH)2 438 493 624 794 36273788

(b) Au3Pb(OH)2 353 569 645 723 37993821

(c) Pb2�OH�2 583 435 780 37773805

(d) Au2Pb2�OH�2 355 339 557 3822612 3826

(e) AuPbOH 476 491 3718(f) Pb(OH)2 609 543 458 3819

3822(g) AuPb(OH)2 482 423 608 3753

614 3753(h) Pb(OH)2 cycle 609 543 398 459 3820

3822(i) AuPb(OH)2 cycle 517 717 561 3717

781 3739(j) PbOOH 448 1251 747 3612(k) PbOOHC 429 1369 1003 3549(l) AuPb(OOH) 300 869 3808(m) Pb(OOH)2 424 1258 908 3618

471 1303 861 3670(n) AuPb(OOH)2 321 330 1243 870 3213

1248 874 3214(o) Pb�H2O2� 543 458 609 3820

3822

a (a), (b), etc. Correspond to the geometry shown in Fig. 5(a), (b), etc.b In compounds with —OOH moieties, this column shows υ(Pb–O–O).

is no other calculated mode within the 100 cm�1 region ofpeak 6 and 50 cm�1 for both peaks 1 and 2. Therefore, wetentatively assign peaks 1, 2 and 6 to υ(PbOH) and υ(AuOH)in AuPb(OH)2, and �(O–O) in PbOOHC, respectively.

Potential-dependent behaviors of peaks 1, 2, 6 and 9. Figure 6shows the SERS intensity of (a) peak 1 (filled circle) and peak2 (open circle), which are assigned to υ(PbOH) and υ(AuOH),respectively, in AuPb(OH)2 on the Au surface, and (b) peak6, �(O–O) in PbOOHC (open triangle), and peak 9, ��PbO2

2��(open square), in the cathodic scan as a function of potentialimposed on the voltammogram of the H2O2 electroreductioncurrent obtained using a linear scan rate of 25 mV s�1 in thespectroelectrochemical cell. This voltammogram is differentfrom that given in Fig. 2 owing to the absence of a rotatingworking electrode.

In Fig. 6(a), peaks 1 and 2 both exhibit the same potentialdependence. The intensities of these peaks achieve a localmaximum at ca 0.3 V and then plateau until ca �0.15 V.These potentials are those correlated with the formation ofPb islands on the Au(111) surface, and this result suggeststhat peroxide impinging on these islands results in the

formation of what we assigned as a bimetallic HOAu–PbOHspecies. Below �0.2 V, the intensities of υ(PbOH) andυ(AuOH) decrease rapidly and eventually disappear, whichis correlated with the formation of a Pb full monolayer onthe surface.

In Fig. 6(b), peak 9, the ��PbO22�� mode, exhibits

maximum intensity at 0.5 V. As the potential is sweptnegatively, the intensity decreases sharply as the potential isswept negative of �0.1 V and eventually disappears below�0.4 V. Peak 6, the �(O–O) mode of PbOOHC, roughlyfollows the behavior of peak 9.

DISCUSSION

The electrochemical behavior of the SERS-active polycrys-talline Au surface is similar to that found for single-crystalAu(111). Both surfaces exhibit features due to the Pb updprocess, although the polycrystalline material exhibits awell-understood complexity due to the presence of manydifferent faces. Both surfaces also exhibit catalytic activitytoward H2O2 reduction with a maximum current foundaround �0.2 V. On the Au(111) surface, the maximum in



catalytic current is associated with the formation of close-packed Pb islands atop the Au surface.24 We further showedthat the locus of catalytic activity was the edges of the Pbislands.25 The correspondence between the RDE behavior forthe Au(poly) and Au(111) surfaces suggests that Pb islandsare also involved with the catalytic activity on the polycrys-talline material. The questions addressed here relate to thenature of intermediates in peroxide electroreduction.

SERS measurementsThe SERS measurements provide unique insight into thenature of species present on the surface during the courseof the peroxide electroreduction event. In particular, themeasurements reported here show that Pb and Au oxygenspecies are present on the surface during electroreduction ofperoxide.

SERS measurements obtained from Au(poly) in thepresence of peroxide but absence of Pb ions show thepresence of a feature at 860 cm�1, �(O–O), which is attributedto the peroxide in solution. The intensity of this mode changeslittle throughout the potential region examined here, whichis consistent with the inactivity of bare Au towards peroxidereduction, as discussed previously.31 However, when bothPb and H2O2 are present on the surface, the free peroxide�(O–O) stretch is no longer observed. Although SERSmeasurements always suffer from indeterminacy relatingto enhancement, the absence of this band does suggest thatfree peroxide is no longer a major species at or near theelectrode surface. We found similar behavior in our study ofperoxide reduction on Bi-modified Au surfaces.31

In the presence of peroxide and Pb, the results presentedabove showed that there were a number of additionalspecies present at the electrode surface. Specifically, SERSrevealed two lower energy modes (peaks 1 and 2) whichwere assigned to OH species on the surface. Additionally,the spectroscopy showed what are probably a peroxo species(peak 6) and a superoxo species (peak 9). The intensities ofthese modes exhibit a similar potential dependence withrelatively large intensities at positive potentials which thendisappear, commensurate with the onset of electroreductionactivity. This behavior strongly suggests that these speciesare intermediates in the electroreduction process.

The presence of metal hydroxide species during theelectroreduction of peroxide has been observed previouslyon both bare Pt and Bi upd modified Au surfaces. In thelatter case, we suggested that the hydroxo species formed asa consequence of spontaneous decomposition of the peroxideto form two Bi hydroxides.31 These hydroxides were thenreduced as a consequence of electron transfer across theinterface. A possible Bi2O2 tetracyclic intermediate was alsosuggested on the basis of the spectroscopy.

In the present case, we also observed bands associatedwith Pb–OH species. This again suggests that spontaneousdecomposition of H2O2 on the Pb island step edge occurs.One of the possible assignments for bands 1 and 2 involves

a mixed PbOH–AuOH species and it is possible thatthe decomposition of the peroxide takes place in thisheterobimetallic system. Unfortunately, detailed calculationsor model compound work that would confirm the identityof this species are not yet available.

The spectroscopy presented above also shows bandsthat correspond to a Pb–OOHC species. The assignment ofthis band also relies on a somewhat tenuous calculation.However, the wavenumber of the band (972 cm�1) isreasonably close to that found for free peroxide (860 cm�1)and suggests that an oxygen-containing species at thisoxidation state is present on the surface. If this assignmentis correct, then the initial interaction of peroxide with thePb-modified Au surface might occur via end-on attack of theperoxide by the Pb island edges.

The SERS revealed the presence of one more species asso-ciated with PbO2

2� on the electrode surface. Since the initialO-containing species was peroxide, the presence of a super-oxide species suggests that some level of disproportionationhas occured. Peroxide is known to disproportionate in acidicmedia to form water and O2. The superoxo species mightform as a consequence of O2 reduction. We note that a super-oxo species was observed during the electroreduction of O2

on the Bi upd modified Au surface. Unfortunately, studies ofthe interaction of O2 with the Pb surface, which would helpto confirm this proposed mechanism, are not yet in hand.

Mechanistic considerationsHydroxide has long been suggested to play an importantrole in dioxygen electroreduction on metal surfaces.54,55 Onthe basis of the experimental results discussed above, thecatalytic activity of Pb-modified Au surface towards H2O2

can be explained by the participation of Pb in several waysduring the electroreduction process. First, Pb ions interactstrongly with H2O2, which is indicated by the observationof the PbOOHC mode. Second, the formation of Pb(OH)2 athigh potentials indicates a strong interaction between Pb andOH. Third, as Pb islands form on the Au surface at certainpotentials, H2O2 can be dissociatively adsorbed on the edgeof the islands by forming hydroxide through interaction withboth Pb and Au atoms on the surface. As no electroreductioncurrent is measured on bare Au surface, a strong interactionbetween Pb and the O of H2O2 or OH is crucial for theactivity.

The mechanism of H2O2 electroreduction on a Pb-modified Au surface is proposed in Fig. 7. In the initialstate, there is only a bare Au surface with Pb ions and H2O2

in the solution. As a result of interaction between peroxideand Pb ions, PbOOHC forms in the solution. With potentialcontrol, Pb upd begins, Pb islands or adlattice forms on theAu surface, and H2O2 is chemisorbed on the edge of Pbislands by forming hydroxide with both Pb and Au atoms,as a result of the cleavage of the O—O bond. The Pb and Auhydroxide will be further reduced to water by the proton inthe solution.



Ene

rgy

(eV

)

Reaction coordinate σ

H2O2 + 2H+ + 2e → 2H2O

Figure 7. Proposed hydrogen peroxide reduction mechanism on Pb-modified Au surface. The pale gray, dark gray, black and whiteballs represent Au, Pb, O and H atoms, respectively.

Interestingly, our previous study of H2O2 electroreduc-tion on a Bi-modified Au surface suggests the formation ofBi(OH)2 on the surface as an intermediate in the process,31

which is coincident with the observation of bimetal hydrox-ide and dihydroxide during the electroreduction process onPb-modified Au surface. These further imply the impor-tance of hydroxide formation during the dioxygen reductionprocess. Additionally, the studies of H2O2 reduction on aBi-modified Au surface suggest a crucial value of 6 A forthe Bi—Bi distance for the catalytic activity. Based on a sim-ple model calculations of one Pb adsorption on an fcc siteof a (2 ð 2) Au(111) slab, it is estimated that the distancesbetween Pb and neighboring Au atoms is in a range 4–6 Adepending on the direction. Clearly, further simulation ofH2O2 on a Pb island on a larger Au(111) [such as (3 ð 3)]surface can provide more insight into the adsorption andreduction process. Unfortunately, the calculation of a largeAu slab is very time consuming and this is still in progress.

CONCLUSION

The SERS measurements strongly suggest the formation oftwo intermediates in the electroreduction of H2O2 to wateron a Pb upd modified Au surface. The first intermediate,PbOOHC, is observed with its intensity maximizing justbefore the deposition of Pb on the Au surface. The secondis the formation of bimetallic dihydroxide, AuPb(OH)2, onthe Pb islanded Au surface. The bending vibrational mode,υ(OH), of this intermediate has local maximum intensity at apotential just positive of that of H2O2 reduction, and it decaysrapidly until it disappears as the reduction current flows. The

reduction of this hydroxide complex leads to OH�, whichcombines with HC in the acidic solution to form water.

Bound hydroxide has been implied in dioxygen electrore-duction on other materials, such as Ag-, Pt- and Bi-modifiedAu surfaces in acid. These results strongly point to an impor-tant role for a metal hydroxide as an intermediate in thecatalysis of peroxide electroreduction, because this hydrox-ide forms as a consequence of O—O bond cleavage.

AcknowledgmentsX.L. thanks the Department of Chemistry for financial support in theform of a Carl Shipp Marvel fellowship and a Chester W. HannumScholarship. The authors thank R. Strange and J. O. White of the LaserLaboratory in the Frederick Seitz Materials Research Laboratoryat the University of Illinois for their assistance with Raman dataacquisition. The Laser Laboratory is funded by Department ofEnergy grant DE-FG02-91ER45439 through the Materials ResearchLaboratory at the University of Illinois. This work was funded bythe National Science Foundation (CHE-0237683).

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SERS study of hydrogen peroxide electroreduction on a Pb-modified Au electrode