Surface Science 605 (2011) 1459–1462

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Structural dependence of intermediate species for the hydrogen evolution reactionon single crystal electrodes of Pt

Masashi Nakamura ⁎, Toshiki Kobayashi, Nagahiro HoshiDepartment of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan

⁎ Corresponding author.E-mail address: mnakamura@faculty.chiba-u.jp (M.

0039-6028/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.susc.2011.05.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 April 2011Accepted 12 May 2011Available online 19 May 2011

Keywords:PlatinumHydrogen atomInfrared absorption spectroscopyElectrochemical methods

Adsorbedhydrogen andwaterweremeasuredduring thehydrogenevolution reaction (HER) on the lowandhighindex planes of Pt in 0.5 M H2SO4 using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at theatop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridgeH) is found on Pt(100). The band intensity of the adsorbedhydrogen depends on temperature, indicating that thebands are due to the intermediate species for the HER. The band of the atop H appears on stepped surfaces with(110) step, whereas the asymmetric bridge H is observed on Pt(211)=3(111)–(100) and Pt(311)=2(111)–(100) that have (100) step. The absence of the atop H on Pt(100), Pt(211), and Pt(311) can be attributed to therelative stability of the bridge site.

Nakamura).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Efficient hydrogen production from water is necessary for thedevelopment of new energy sources such as fuel cells. Waterelectrolysis is one of the important methods for the hydrogenevolution. Pt electrodes have extremely low overpotentials for thehydrogen oxidation reaction (HOR) and the hydrogen evolutionreaction (HER); Pt is useful for the electrocatalysts of fuel cells andwater electrolysis. The HOR/HER processes have been studied fordecades [1–4]. The activity for the HER depends on the pH, electrodematerials and the surface structure of the electrode. Adsorbedhydrogen (Had) is the key intermediate species for the HER on Ptelectrode in acidic solution [5]. Two types of Had are suggested: theunderpotential deposited hydrogen (Hupd) as a spectator species andthe overpotential deposited hydrogen (Hopd) as an intermediatespecies [6].

Vibrational spectroscopy can identify the adsorption site of Had.Infrared reflection absorption spectroscopy (IRAS) foundHad at the atopsite of Pt in the Hopd and Hupd regions [7–9]. The band appears at2090 cm−1 on polycrystalline Pt and Pt(111) [7]. According to thereport of another group, the bandwas not found on defect-free Pt(111),and another band appeared at 2020 cm−1 on Pt(100) and Pt(11 1 1) inthe Hupd region [8]. However, adsorbed CO is often produced by thereduction of dissolved carbonate species or CO2, and the oxidation ofcarbon contamination, giving IR band around 2020 cm−1 [9]. Recently,surface enhanced infrared absorption spectroscopy (SEIRAS) suggeststhat the atop Hopd at 2080 cm−1 is a reactive intermediate during HER

on polycrystalline Pt electrode [5], whereas the structural dependenceand band assignment of Hopd are controversial on Pt single crystals.

IR spectra indicate that water is adsorbed on the Pt surface duringthe HER [10]. Water preferentially binds at the atop sites of the Ptsurface through the oxygen lone pair [11]. The adsorption energy ofadsorbedwater on Pt is comparable to that of adsorbed hydrogen [12].The IR band of hydronium cation (H3O+) appears on Pt and the bandintensity depends on the electrode potential [5,10]. Interfacial waterplays an essential role for proton transfer and proton hydration duringthe HER.

The relationship between the HER activity and the adsorption siteof the intermediate Hopd is unclear. It is important to examine thestructural dependence of the Hopd using well-defined surfaces. In thispaper, we have studied the structural and the temperature depen-dence of the intermediate Hopd and adsorbed water on Pt electrodesusing IRAS and the density functional theory (DFT) calculations. Weidentified the IR band of the intermediate Hopd on the low and highindex planes of Pt. The adsorption site of the Hopd strongly depends onthe surface structure of Pt during the HER.

2. Experimental

Platinum single crystals were prepared by the method of Clavilieret al. [13]. The samples were hydrogen-flame annealed, cooled inAr+H2 or Ar, and transferred to the IR cell after protecting the surfacewith a droplet of ultrapurewater (Milli-Q Advantage). It is known thatthe reconstruction is induced on several Pt single-crystal surfaces[14–17]. For Pt(110) and Pt(211), the unreconstructed surfaces wereprepared according to the procedure described elsewhere [16,17]. Thesolution was prepared with H2SO4 (Merck Suprapur) and ultrapurewater. The reference electrode was the reversible hydrogen electrode

1460 M. Nakamura et al. / Surface Science 605 (2011) 1459–1462

(RHE). The infrared beam was incident at an angle of 60°. The IR cellwas attached to a Fourier transform IR spectrometer (JASCO FT/IR-6100) with a liquid nitrogen-cooled mercury cadmium telluride(MCT) detector. The spectra were collected with p-polarized light at aresolution of 4 cm−1. All IR spectra were obtained using subtractivelynormalized interfacial Fourier transform spectroscopy (SNIFTIRS).Reference potential is set at 0.8 V (RHE) to oxidize CO that is producedfrom contamination. Total 1024 scans were co-added in 8 cycles of128 scans at both reference and sample potentials.

DFT calculations of the IR frequencies were carried out with theGaussian 03 program using the LANL 2MB ECP basis for Pt and the6-31G** basis for H at the BLYP level. The metal surfaces are modeledby three layered clusters consisting of 20 and 28 metal atoms. ThePt–H distance is optimized while the metal clusters are always keptfixed at the bulk structure. DFT calculations of the adsorption energieswere carried out with the Vienna ab initio simulation program (VASP)[18]. The structure models were comprised of a 6 (low index planes)and 10 (Pt(211)) layer Pt slab with a 1×1 surface unit cell. The slabswere separated by approximately 1 nm of vacuum. The first Brillouinzone was sampled with a 10×10×1 k-point mesh within theMonkhorst–Pack scheme [19]. Ionic cores were described by ultrasoftpseudopotentials and the Kohn–Sham one-electron valence stateswere expanded in a basis of planewavewith a cutoff energy of 400 eV.Electron exchange and correlation were described within the PW91generalized gradient approximation (GGA) [20]. The calculated latticeconstant of bulk Pt was 0.398 nm, which was within the error of 1.5%of the experimental value. The adsorption energy is defined as theenergy difference per H atom between the adsorbed system and thesum of the Pt slab and H2 molecule.

3. Results and discussion

We measured voltammograms of Pt electrodes in 0.5 M H2SO4

used in this study. The voltammograms were identical with thosereported previously [13,21–23]. Fig. 1 shows the potential depen-dence of the IRAS spectra of the low index planes of Pt in 0.5 M H2SO4

at 298 K. A positive going band is observed between 1600 and1630 cm−1 on all the surfaces. These bands are assigned to δHOH ofadsorbed water. These bands show the red shift at positive potential.We reported that the frequency shift of δHOH on Pt groupmetals is dueto the charge transfer from the water lone pair to the electrode [10].Charge transfer from the oxygen lone pair to the metal causes a smallexpansion of the HOH angle, whichmeans the decrease of the bending

Fig. 1. Potential dependence of the IRAS spectra of Pt(111), Pt(100), and Pt(110) in0.5 M H2SO4. Reference potentials are 0.5 V (Pt(111)), 0.8 V vs. RHE (Pt(100), Pt(110)).

frequency. The frequency of δHOH on Pt(110) is lower than those on Pt(111) andPt(100),which indicates thatwater is strongly adsorbed onPt(110). The adsorption energy of water at steps is higher than those atterraces [24]. The band intensities of δHOH on Pt(100) and Pt(110)increase at negative potentials. The increase of the band intensity onPt(100) below 0.05 V is due to the coupling with the band of Had asdiscussed below. On Pt(110), the adsorption of (bi)sulfate anion above0.2 V inhibits the adsorption of water [25,26]. Since (bi)sulfate anion isadsorbed on Pt(111) and Pt(100) above 0.4 V and 0.3 V, respectively,the coverage of water is unchanged between 0 and 0.2 V [26–28].

A broad band is found at 1750 cm−1, which is assignable to theasymmetric bending mode of the hydronium cation (H3O+) [29]. Theband intensity of H3O+ decreases below 0.05 V because of thedecrease of H+ concentration neighboring to the electrode surface bythe hydrogen evolution. The band intensity of H3O+ also depends onthe surface structure. This structural dependence may be due to thedifference of the orientation of H3O+ and the HER mechanism. Thenegative going band at 1235 cm−1 is assigned to the symmetric SO3

stretching mode of adsorbed (bi)sulfate anion at the referencepotential.

Small positive going band appears at 2081 cm−1 on Pt(110) at 0 Vwhere the HER occurs. This band is assigned to νPt–H of theintermediate Hopd at the atop site (atop H) of Pt. The frequency ofνPt–H is identical to that on polycrystalline Pt reported previously[5,7]. On the other hand, a tail appears in the higher-frequency regionof δHOH band below 0.05 V on Pt(100). The tail includes the νPt–H bandof the intermediate Had at asymmetric bridge site (bridge H) onPt(100), as discussed below. However, no bands of Had is observed onPt(111). DFT calculation predicts that the adsorbed hydrogen prefersthe three-fold hollow site (hollow H) on the Pt(111). Previous high-resolution energy-loss spectroscopy (HREELS) studies show dipoleactive modes of the hollow H at 904 and 1224 cm−1[30]. Under theconditions of our experiments, these bands could not be observedbecause of the overlap with the (bi)sulfate band (1235 cm−1) and thetransmission limit of the IR window. We tried to measure the band ofhollow H in 0.1 M HF that has no IR active species around 1200 cm−1.However, no band was found around 1200 cm−1 even in 0.1 M HF.The band intensity of hollow H may be too weak to be detected usingIR spectroscopy, because the dipole component of the hollow H alongthe surface normal is small or dynamic dipole is effectively screenedby surface atoms [31]. Since the band intensity of δHOH does notdecrease during HER, the intermediate Hopdis coadsorbed with wateron the Pt electrode.

Fig. 2 shows the temperature dependent IRAS spectra on the lowindex planes of Pt at 0 V. The intensity of 2080 cm−1 band on Pt(110)

Fig. 2. Temperature dependent IR spectra of Pt(111), Pt(100), and Pt(110) in 0.5 MH2SO4 at 0 V (RHE). Reference potentials are 0.8 V (RHE).

Fig. 3. IR spectra of high index planes of Pt in 0.5 M H2SO4 at 0 V (RHE). Referencepotentials are 0.8 V (RHE).

Fig. 4. Cluster models for DFT calculations and optimized structures for H adsorbed onPt(110) and Pt(100).

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and that of 1630 cm−1 band on Pt(100) increase with the increase ofthe temperature. The enhancement of the band intensity on Pt(110)shows the increase of the coverage of atop H. The exchange currentdensity of the HER/HOR gets higher at higher temperatures [2]. Thecoverage of the intermediate Had may relate to the activity of the HER.SEIRAS study also shows the quantitative relation between thecoverage of the atop H and the kinetics of HER [5]. The increase of1630 cm−1 band on Pt(100) is attributed to the vibrational couplingof νPt–H (bridge H) with δHOH, supporting that the tailed at 1630 cm−1

on Pt(100) includes the band of Had during the HER. The δHOH band at1620 cm−1 on Pt(111) and Pt(110), which does not include the νPt–Hband, is symmetric and independent of the temperature. Thetemperature dependence of νPt–H supports that the observed bandis the intermediate Had of the HER.

Pt(110)=2(111)–(111), which has high step density, is the mostactive surface for the HER [2,32,33]. The exchange current density ofthe HER/HOR on Pt(110) is higher than those on Pt(111) and Pt(100)because of a different reactionmechanism [2]. The fact that the atop Hdoes not appear on Pt(111) and Pt(100) suggests that the HER/HORpathway with a high reaction-rate goes through the atop H. Stepstructure may be important for the activation of the HER. We examinethe step structural dependence of the intermediate Hopd using thefollowing three series of the high index planes:

n−1ð Þ 111ð Þ− 110ð Þseries : Pt 331ð Þn = 3; Pt 553ð Þn = 5

Table 1Calculated frequencies of νPt–H at each site.

Atop H onPt(110)

Asymmetric bridge H onPt(100)

Symmetric bridge H onPt(100)

Calc. 2173 cm−1 1672 cm−1 1329 cm−1

IR 2081 cm−1 1630 cm−1 –

n 100ð Þ− 110ð Þseries : Pt 210ð Þn = 2; Pt 310ð Þn = 3

n 111ð Þ− 100ð Þseries : Pt 311ð Þn = 2; Pt 211ð Þn = 3

where n denotes the number of terrace atomic rows. Fig. 3 shows theIR spectra of the high index planes in 0.5 M H2SO4 at 0 V at 298 K. TheδHOH of the adsorbed water is observed at 1620 cm−1 on all thesurfaces. The atop H appears on (n-1)(111)–(110) and n(100)–(110)series at 2080 cm−1, whereas the band of the asymmetric bridge Happears at high frequency side of 1620 cm−1 on n(111)–(100) series.The weak band of the asymmetric bridge H is also observed on Pt(310) with (100) terrace. These facts support that the atop H isadsorbed on (110) structure. On the series of (n-1)(111)–(110), thenarrowest surface of the terrace width is the (110) plane, i.e., the stepstructure is identical to that of (110). The step structure of n(100)–(110) has a local (110) orientation. The appearance of the atop H on Pt(331), Pt(553), Pt(210), and Pt(310) is a quite reasonable result.

DFT calculations were performed to confirm the band assignment ofthe intermediate Had. Calculated frequencies of νPt–H at each site arelisted in Table 1. Fig. 4 shows cluster models using frequencycalculations and optimized structures for H adsorbed on Pt(110) andPt(100). The band frequencies observed on Pt(110) and Pt(100) agreewith the calculated values at the atop and asymmetric bridge sites.

We calculate the structural dependence of the adsorption energyof hydrogen on the low index planes at 1 ML. We consider the threedifferent adsorption sites of adsorbed hydrogen: the atop, symmetricbridge, and hollow sites. Table 2 shows the adsorption energies ofHopd on Pt surfaces. Had on Pt(111) prefers the hollow site to the atopand bridged sites, which is consistent with the previous theoreticaland experimental reports [12,30]. The most stable site of hydrogen isthe symmetric bridge site on Pt(100) and Pt(110). Hupd on Pt(100)and Pt(110) will be located at the symmetric bridge site. We could notfind the stable structure for the hollow H on Pt(110). The stableadsorption sites estimated from DFT calculations are different from

Table 2Comparison of the adsorption energy Ead (meV) for Had on Pt(hkl) by DFT calculations.

Atop H Bridge H Hollow H

Pt(111) 394 364 440Pt(100) 416 672 311Pt(110) 533 615 –

Pt(211) 399 635 322

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those observed using IR on Pt(100) and Pt(110). The intermediatespecies do not have to go through the most stable adsorption site.However, we can speculate the structural dependence of intermediateHad on Pt(110) and Pt(100) as follows. In the case of Pt(110), thedifference of the adsorption energy between the atop and bridge sitesis small (82 meV). The intermediate Hopd will be located at the atopsite during HER. On the other hand, the bridge site on Pt(100) is morestable by N250 meV than the atop and hollow sites. HER on Pt(100)proceeds via the intermediate state at asymmetric bridge site close tothemost stable bridge site. The adsorption energy is also calculated onunreconstructed Pt(211). The bridge site on Pt(211) stronglystabilizes Hopd as is the case of Pt(100). Therefore, the appearance ofasymmetric bridge H on n(111)–(100) is due to the strong stability ofthe bridge site.

4. Conclusion

We revealed the structural dependence of intermediate Hopd

during the HER. The atop H was observed on Pt(110) and the severalstepped surface with (110) step, whereas the asymmetric bridge Happears on Pt(100) and the surfaces with (100) step. According to DFTcalculations, the symmetric bridge site on Pt(100) and Pt(211)strongly stabilizes Had.

Acknowledgment

This work was supported by a grant-in-aid (KAKENHI) for YoungScientists (B) no. 22710099 and New Energy and Industrial Technol-ogy Development Organization (NEDO).

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