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Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt
Surface Science 605 (2011) 1459–1462
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Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt Masashi Nakamura ⁎, Toshiki Kobayashi, Nagahiro Hoshi Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
a r t i c l e
i n f o
Article history: Received 21 April 2011 Accepted 12 May 2011 Available online 19 May 2011 Keywords: Platinum Hydrogen atom Infrared absorption spectroscopy Electrochemical methods
a b s t r a c t Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H2SO4 using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands 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 the relative stability of the bridge site. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Efficient hydrogen production from water is necessary for the development of new energy sources such as fuel cells. Water electrolysis is one of the important methods for the hydrogen evolution. Pt electrodes have extremely low overpotentials for the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER); Pt is useful for the electrocatalysts of fuel cells and water electrolysis. The HOR/HER processes have been studied for decades [1–4]. The activity for the HER depends on the pH, electrode materials and the surface structure of the electrode. Adsorbed hydrogen (Had) is the key intermediate species for the HER on Pt electrode in acidic solution [5]. Two types of Had are suggested: the underpotential deposited hydrogen (Hupd) as a spectator species and the overpotential deposited hydrogen (Hopd) as an intermediate species [6]. Vibrational spectroscopy can identify the adsorption site of Had. Infrared reflection absorption spectroscopy (IRAS) found Had at the atop site of Pt in the Hopd and Hupd regions [7–9]. The band appears at 2090 cm− 1 on polycrystalline Pt and Pt(111) [7]. According to the report of another group, the band was not found on defect-free Pt(111), and another band appeared at 2020 cm− 1 on Pt(100) and Pt(11 1 1) in the Hupd region [8]. However, adsorbed CO is often produced by the reduction of dissolved carbonate species or CO2, and the oxidation of carbon contamination, giving IR band around 2020 cm− 1 [9]. Recently, surface enhanced infrared absorption spectroscopy (SEIRAS) suggests that the atop Hopd at 2080 cm− 1 is a reactive intermediate during HER
⁎ Corresponding author. E-mail address: [email protected] (M. Nakamura). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.05.014
on polycrystalline Pt electrode [5], whereas the structural dependence and band assignment of Hopd are controversial on Pt single crystals. IR spectra indicate that water is adsorbed on the Pt surface during the HER [10]. Water preferentially binds at the atop sites of the Pt surface through the oxygen lone pair [11]. The adsorption energy of adsorbed water on Pt is comparable to that of adsorbed hydrogen [12]. The IR band of hydronium cation (H3O +) appears on Pt and the band intensity depends on the electrode potential [5,10]. Interfacial water plays an essential role for proton transfer and proton hydration during the HER. The relationship between the HER activity and the adsorption site of the intermediate Hopd is unclear. It is important to examine the structural dependence of the Hopd using well-defined surfaces. In this paper, we have studied the structural and the temperature dependence of the intermediate Hopd and adsorbed water on Pt electrodes using IRAS and the density functional theory (DFT) calculations. We identified the IR band of the intermediate Hopd on the low and high index planes of Pt. The adsorption site of the Hopd strongly depends on the surface structure of Pt during the HER. 2. Experimental Platinum single crystals were prepared by the method of Clavilier et al. [13]. The samples were hydrogen-flame annealed, cooled in Ar + H2 or Ar, and transferred to the IR cell after protecting the surface with a droplet of ultrapure water (Milli-Q Advantage). It is known that the reconstruction is induced on several Pt single-crystal surfaces [14–17]. For Pt(110) and Pt(211), the unreconstructed surfaces were prepared according to the procedure described elsewhere [16,17]. The solution was prepared with H2SO4 (Merck Suprapur) and ultrapure water. The reference electrode was the reversible hydrogen electrode
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(RHE). The infrared beam was incident at an angle of 60°. The IR cell was attached to a Fourier transform IR spectrometer (JASCO FT/IR6100) with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. The spectra were collected with p-polarized light at a resolution of 4 cm − 1. All IR spectra were obtained using subtractively normalized interfacial Fourier transform spectroscopy (SNIFTIRS). Reference potential is set at 0.8 V (RHE) to oxidize CO that is produced from contamination. Total 1024 scans were co-added in 8 cycles of 128 scans at both reference and sample potentials. DFT calculations of the IR frequencies were carried out with the Gaussian 03 program using the LANL 2MB ECP basis for Pt and the 6-31G** basis for H at the BLYP level. The metal surfaces are modeled by three layered clusters consisting of 20 and 28 metal atoms. The Pt–H distance is optimized while the metal clusters are always kept fixed at the bulk structure. DFT calculations of the adsorption energies were 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 slabs were separated by approximately 1 nm of vacuum. The first Brillouin zone was sampled with a 10 × 10 × 1 k-point mesh within the Monkhorst–Pack scheme [19]. Ionic cores were described by ultrasoft pseudopotentials and the Kohn–Sham one-electron valence states were expanded in a basis of plane wave with a cutoff energy of 400 eV. Electron exchange and correlation were described within the PW91 generalized gradient approximation (GGA) [20]. The calculated lattice constant 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 the energy difference per H atom between the adsorbed system and the sum 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 those reported previously [13,21–23]. Fig. 1 shows the potential dependence 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 and 1630 cm − 1 on all the surfaces. These bands are assigned to δHOH of adsorbed water. These bands show the red shift at positive potential. We reported that the frequency shift of δHOH on Pt group metals is due to the charge transfer from the water lone pair to the electrode [10]. Charge transfer from the oxygen lone pair to the metal causes a small expansion of the HOH angle, which means the decrease of the bending
Fig. 1. Potential dependence of the IRAS spectra of Pt(111), Pt(100), and Pt(110) in 0.5 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) and Pt(100), which indicates that water is strongly adsorbed on Pt (110). The adsorption energy of water at steps is higher than those at terraces [24]. The band intensities of δHOH on Pt(100) and Pt(110) increase at negative potentials. The increase of the band intensity on Pt(100) below 0.05 V is due to the coupling with the band of Had as discussed below. On Pt(110), the adsorption of (bi)sulfate anion above 0.2 V inhibits the adsorption of water [25,26]. Since (bi)sulfate anion is adsorbed 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 the asymmetric bending mode of the hydronium cation (H3O +) [29]. The band intensity of H3O + decreases below 0.05 V because of the decrease of H + concentration neighboring to the electrode surface by the hydrogen evolution. The band intensity of H3O + also depends on the surface structure. This structural dependence may be due to the difference of the orientation of H3O + and the HER mechanism. The negative going band at 1235 cm − 1 is assigned to the symmetric SO3 stretching mode of adsorbed (bi)sulfate anion at the reference potential. Small positive going band appears at 2081 cm − 1 on Pt(110) at 0 V where the HER occurs. This band is assigned to νPt–H of the intermediate Hopd at the atop site (atop H) of Pt. The frequency of νPt–H is identical to that on polycrystalline Pt reported previously [5,7]. On the other hand, a tail appears in the higher-frequency region of δHOH band below 0.05 V on Pt(100). The tail includes the νPt–H band of the intermediate Had at asymmetric bridge site (bridge H) on Pt(100), as discussed below. However, no bands of Had is observed on Pt(111). DFT calculation predicts that the adsorbed hydrogen prefers the three-fold hollow site (hollow H) on the Pt(111). Previous highresolution energy-loss spectroscopy (HREELS) studies show dipole active modes of the hollow H at 904 and 1224 cm − 1[30]. Under the conditions of our experiments, these bands could not be observed because of the overlap with the (bi)sulfate band (1235 cm − 1) and the transmission limit of the IR window. We tried to measure the band of hollow 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 using IR spectroscopy, because the dipole component of the hollow H along the surface normal is small or dynamic dipole is effectively screened by surface atoms [31]. Since the band intensity of δHOH does not decrease during HER, the intermediate Hopdis coadsorbed with water on the Pt electrode. Fig. 2 shows the temperature dependent IRAS spectra on the low index 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 M H2SO4 at 0 V (RHE). Reference potentials are 0.8 V (RHE).
M. Nakamura et al. / Surface Science 605 (2011) 1459–1462
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Fig. 4. Cluster models for DFT calculations and optimized structures for H adsorbed on Pt(110) and Pt(100).
nð100Þ−ð110Þseries : Ptð210Þn = 2; Ptð310Þn = 3 nð111Þ−ð100Þseries : Ptð311Þn = 2; Ptð211Þn = 3
Fig. 3. IR spectra of high index planes of Pt in 0.5 M H2SO4 at 0 V (RHE). Reference potentials are 0.8 V (RHE).
and that of 1630 cm − 1 band on Pt(100) increase with the increase of the temperature. The enhancement of the band intensity on Pt(110) shows the increase of the coverage of atop H. The exchange current density of the HER/HOR gets higher at higher temperatures [2]. The coverage of the intermediate Had may relate to the activity of the HER. SEIRAS study also shows the quantitative relation between the coverage of the atop H and the kinetics of HER [5]. The increase of 1630 cm − 1 band on Pt(100) is attributed to the vibrational coupling of 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 at 1620 cm − 1 on Pt(111) and Pt(110), which does not include the νPt–H band, is symmetric and independent of the temperature. The temperature dependence of νPt–H supports that the observed band is the intermediate Had of the HER. Pt(110) = 2(111)–(111), which has high step density, is the most active surface for the HER [2,32,33]. The exchange current density of the HER/HOR on Pt(110) is higher than those on Pt(111) and Pt(100) because of a different reaction mechanism [2]. The fact that the atop H does not appear on Pt(111) and Pt(100) suggests that the HER/HOR pathway with a high reaction-rate goes through the atop H. Step structure may be important for the activation of the HER. We examine the step structural dependence of the intermediate Hopd using the following three series of the high index planes: ðn−1Þð111Þ−ð110Þseries : Ptð331Þn = 3; Ptð553Þn = 5
Table 2 Comparison of the adsorption energy Ead (meV) for Had on Pt(hkl) by DFT calculations.
Table 1 Calculated frequencies of νPt–H at each site. Atop H on Pt(110) Calc. IR
−1
2173 cm 2081 cm− 1
Asymmetric bridge H on Pt(100) −1
1672 cm 1630 cm− 1
where n denotes the number of terrace atomic rows. Fig. 3 shows the IR spectra of the high index planes in 0.5 M H2SO4 at 0 V at 298 K. The δHOH of the adsorbed water is observed at 1620 cm − 1 on all the surfaces. 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 H appears 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 is adsorbed on (110) structure. On the series of (n-1)(111)–(110), the narrowest surface of the terrace width is the (110) plane, i.e., the step structure 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 of the intermediate Had. Calculated frequencies of νPt–H at each site are listed in Table 1. Fig. 4 shows cluster models using frequency calculations and optimized structures for H adsorbed on Pt(110) and Pt(100). The band frequencies observed on Pt(110) and Pt(100) agree with the calculated values at the atop and asymmetric bridge sites. We calculate the structural dependence of the adsorption energy of hydrogen on the low index planes at 1 ML. We consider the three different adsorption sites of adsorbed hydrogen: the atop, symmetric bridge, and hollow sites. Table 2 shows the adsorption energies of Hopd on Pt surfaces. Had on Pt(111) prefers the hollow site to the atop and bridged sites, which is consistent with the previous theoretical and experimental reports [12,30]. The most stable site of hydrogen is the 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 not find the stable structure for the hollow H on Pt(110). The stable adsorption sites estimated from DFT calculations are different from
Symmetric bridge H on Pt(100) 1329 cm –
−1
Pt(111) Pt(100) Pt(110) Pt(211)
Atop H
Bridge H
Hollow H
394 416 533 399
364 672 615 635
440 311 – 322
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those observed using IR on Pt(100) and Pt(110). The intermediate species do not have to go through the most stable adsorption site. However, we can speculate the structural dependence of intermediate Had on Pt(110) and Pt(100) as follows. In the case of Pt(110), the difference of the adsorption energy between the atop and bridge sites is small (82 meV). The intermediate Hopd will be located at the atop site during HER. On the other hand, the bridge site on Pt(100) is more stable by N250 meV than the atop and hollow sites. HER on Pt(100) proceeds via the intermediate state at asymmetric bridge site close to the most stable bridge site. The adsorption energy is also calculated on unreconstructed Pt(211). The bridge site on Pt(211) strongly stabilizes Hopd as is the case of Pt(100). Therefore, the appearance of asymmetric bridge H on n(111)–(100) is due to the strong stability of the 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 several stepped surface with (110) step, whereas the asymmetric bridge H appears on Pt(100) and the surfaces with (100) step. According to DFT calculations, 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 Young Scientists (B) no. 22710099 and New Energy and Industrial Technology Development Organization (NEDO). References [1] J.O'.M. Bockris, A.K.N. Reddy, Modern Electrochemistry, Vol. 2, Plenum Press, New York, 1970. [2] N.M. Markovic, B.N. Grgur, P.N. Ross, J. Phys. Chem. B 101 (1997) 5405. [3] B.E. Conway, J. Barber, S. Morin, Electrochim. Acta 44 (1998) 1109. [4] G. Jerkiewicz, Prog. Surf. Sci. 57 (1998) 137.
[5] K. Kunimatsu, H. Uchida, M. Osawa, M. Watanabe, J. Electroanal. Chem. 587 (2006) 299. [6] D. Strmcnik, D. Tripkovic, D. van der Vliet, V. Stamenkovic, N.M. Markovic, Electrochem. Commun. 10 (2008) 1602. [7] R.J. Nichols, A. Bewick, J. Electroanal. Chem. 243 (1988) 445. [8] H. Ogasawara, M. Ito, Chem. Phys. Lett. 221 (1994) 213. [9] N. Nanbu, F. Kitamura, T. Ohsaka, K. Tokuda, J. Electroanal. Chem. 485 (2000) 128. [10] M. Nakamura, H. Kato, N. Hoshi, J. Phys. Chem. C 112 (2008) 9458. [11] A. Michaelides, V.A. Ranea, P.L. De Andres, D.A. King, Phys. Rev. Lett. 90 (2003) 216102. [12] G.W. Watson, R.P.K. Wells, D.J. Willock, G.J. Hutchings, J. Phys. Chem. B 105 (2001) 4889. [13] J. Clavilier, R. Faure, G. Guinet, R.J. Durand, J. Electroanal. Chem. 107 (1979) 205. [14] C.A. Lucas, N.M. Markovic, P.N. Ross, Phys. Rev. Lett. 77 (1996) 4922. [15] A. Nakahara, M. Nakamura, K. Sumitani, O. Sakata, N. Hoshi, Langmuir 23 (2007) 10879. [16] M. Nakamura, N. Sato, N. Hoshi, J.M. Soon, O. Sakata, J. Phys. Chem. C 113 (2009) 4538. [17] N.M. Markovic, B.N. Grgur, C.A. Lucas, P.N. Ross, Surf. Sci. 384 (1997) L805. [18] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558; G. Kresse, J. Furthmuller, Comput. Mat. Sci. 6 (1996) 15; G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169; G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1785. [19] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [20] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [21] J. Clavilier, K. El Achi, A. Rodes, J. Electroanal. Chem. 272 (1989) 253. [22] N.M. Marković, N.S. Marinković, R.R. Ažić, J. Electroanal. Chem. 241 (1988) 309. [23] N. Furuya, M. Shibata, J. Electroanal. Chem. 467 (1999) 85. [24] M.L. Grecea, E.H.G. Backus, B. Riedmuller, A. Eichler, A.W. Kleyn, M. Bonn, J. Phys. Chem. B 108 (2004) 12575. [25] T. Iwashita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Electrochim. Acta 40 (1995) 53. [26] N. Hoshi, A. Sakurada, S. Nakamura, S. Teruya, O. Koga, Y. Hori, J. Phys. Chem. B 106 (2002) 1985. [27] F.C. Nart, T. Iwashita, M. Weber, Electrochim. Acta 39 (1994) 961. [28] F.C. Nart, T. Iwashita, M. Weber, Electrochim. Acta 39 (1994) 2093. [29] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed. John Wiley & Sons, New York, 1986. [30] S.C. Badescu, K. Jacobi, Y. Wang, K. Bedurftig, G. Ertl, P. Salo, T. Ala-Nissila, S.C. Ying, Phys. Rev. B 68 (2003) 205401. [31] J.E. Reutt, Y.J. Chabal, S.B. Christman, J. Electron, Spectrosc. Relat. Phenom. 44 (1987) 325. [32] N. Hoshi, Y. Asaumi, M. Nakamura, K. Mikita, R. Kajiwara, J. Phys. Chem. C 113 (2009) 16843. [33] R. Kajiwara, Y. Asaumi, M. Nakamura, N. Hoshi, J. Electroanal. Chem. (2011) 16843, doi:10.1016/j.jelechem.2011.03.011.
Contents lists available at ScienceDirect
Surface Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u s c
Structural dependence of intermediate species for the hydrogen evolution reaction on single crystal electrodes of Pt Masashi Nakamura ⁎, Toshiki Kobayashi, Nagahiro Hoshi Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi-cho 1-33, Inage-ku, Chiba 263-8522, Japan
a r t i c l e
i n f o
Article history: Received 21 April 2011 Accepted 12 May 2011 Available online 19 May 2011 Keywords: Platinum Hydrogen atom Infrared absorption spectroscopy Electrochemical methods
a b s t r a c t Adsorbed hydrogen and water were measured during the hydrogen evolution reaction (HER) on the low and high index planes of Pt in 0.5 M H2SO4 using infrared reflection absorption spectroscopy. Hydrogen is adsorbed at the atop site (atop H) on Pt(110) during the HER, whereas adsorbed hydrogen at the asymmetric bridge site (bridge H) is found on Pt(100). The band intensity of the adsorbed hydrogen depends on temperature, indicating that the bands 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 the relative stability of the bridge site. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Efficient hydrogen production from water is necessary for the development of new energy sources such as fuel cells. Water electrolysis is one of the important methods for the hydrogen evolution. Pt electrodes have extremely low overpotentials for the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER); Pt is useful for the electrocatalysts of fuel cells and water electrolysis. The HOR/HER processes have been studied for decades [1–4]. The activity for the HER depends on the pH, electrode materials and the surface structure of the electrode. Adsorbed hydrogen (Had) is the key intermediate species for the HER on Pt electrode in acidic solution [5]. Two types of Had are suggested: the underpotential deposited hydrogen (Hupd) as a spectator species and the overpotential deposited hydrogen (Hopd) as an intermediate species [6]. Vibrational spectroscopy can identify the adsorption site of Had. Infrared reflection absorption spectroscopy (IRAS) found Had at the atop site of Pt in the Hopd and Hupd regions [7–9]. The band appears at 2090 cm− 1 on polycrystalline Pt and Pt(111) [7]. According to the report of another group, the band was not found on defect-free Pt(111), and another band appeared at 2020 cm− 1 on Pt(100) and Pt(11 1 1) in the Hupd region [8]. However, adsorbed CO is often produced by the reduction of dissolved carbonate species or CO2, and the oxidation of carbon contamination, giving IR band around 2020 cm− 1 [9]. Recently, surface enhanced infrared absorption spectroscopy (SEIRAS) suggests that the atop Hopd at 2080 cm− 1 is a reactive intermediate during HER
⁎ Corresponding author. E-mail address: [email protected] (M. Nakamura). 0039-6028/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2011.05.014
on polycrystalline Pt electrode [5], whereas the structural dependence and band assignment of Hopd are controversial on Pt single crystals. IR spectra indicate that water is adsorbed on the Pt surface during the HER [10]. Water preferentially binds at the atop sites of the Pt surface through the oxygen lone pair [11]. The adsorption energy of adsorbed water on Pt is comparable to that of adsorbed hydrogen [12]. The IR band of hydronium cation (H3O +) appears on Pt and the band intensity depends on the electrode potential [5,10]. Interfacial water plays an essential role for proton transfer and proton hydration during the HER. The relationship between the HER activity and the adsorption site of the intermediate Hopd is unclear. It is important to examine the structural dependence of the Hopd using well-defined surfaces. In this paper, we have studied the structural and the temperature dependence of the intermediate Hopd and adsorbed water on Pt electrodes using IRAS and the density functional theory (DFT) calculations. We identified the IR band of the intermediate Hopd on the low and high index planes of Pt. The adsorption site of the Hopd strongly depends on the surface structure of Pt during the HER. 2. Experimental Platinum single crystals were prepared by the method of Clavilier et al. [13]. The samples were hydrogen-flame annealed, cooled in Ar + H2 or Ar, and transferred to the IR cell after protecting the surface with a droplet of ultrapure water (Milli-Q Advantage). It is known that the reconstruction is induced on several Pt single-crystal surfaces [14–17]. For Pt(110) and Pt(211), the unreconstructed surfaces were prepared according to the procedure described elsewhere [16,17]. The solution was prepared with H2SO4 (Merck Suprapur) and ultrapure water. The reference electrode was the reversible hydrogen electrode
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(RHE). The infrared beam was incident at an angle of 60°. The IR cell was attached to a Fourier transform IR spectrometer (JASCO FT/IR6100) with a liquid nitrogen-cooled mercury cadmium telluride (MCT) detector. The spectra were collected with p-polarized light at a resolution of 4 cm − 1. All IR spectra were obtained using subtractively normalized interfacial Fourier transform spectroscopy (SNIFTIRS). Reference potential is set at 0.8 V (RHE) to oxidize CO that is produced from contamination. Total 1024 scans were co-added in 8 cycles of 128 scans at both reference and sample potentials. DFT calculations of the IR frequencies were carried out with the Gaussian 03 program using the LANL 2MB ECP basis for Pt and the 6-31G** basis for H at the BLYP level. The metal surfaces are modeled by three layered clusters consisting of 20 and 28 metal atoms. The Pt–H distance is optimized while the metal clusters are always kept fixed at the bulk structure. DFT calculations of the adsorption energies were 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 slabs were separated by approximately 1 nm of vacuum. The first Brillouin zone was sampled with a 10 × 10 × 1 k-point mesh within the Monkhorst–Pack scheme [19]. Ionic cores were described by ultrasoft pseudopotentials and the Kohn–Sham one-electron valence states were expanded in a basis of plane wave with a cutoff energy of 400 eV. Electron exchange and correlation were described within the PW91 generalized gradient approximation (GGA) [20]. The calculated lattice constant 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 the energy difference per H atom between the adsorbed system and the sum 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 those reported previously [13,21–23]. Fig. 1 shows the potential dependence 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 and 1630 cm − 1 on all the surfaces. These bands are assigned to δHOH of adsorbed water. These bands show the red shift at positive potential. We reported that the frequency shift of δHOH on Pt group metals is due to the charge transfer from the water lone pair to the electrode [10]. Charge transfer from the oxygen lone pair to the metal causes a small expansion of the HOH angle, which means the decrease of the bending
Fig. 1. Potential dependence of the IRAS spectra of Pt(111), Pt(100), and Pt(110) in 0.5 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) and Pt(100), which indicates that water is strongly adsorbed on Pt (110). The adsorption energy of water at steps is higher than those at terraces [24]. The band intensities of δHOH on Pt(100) and Pt(110) increase at negative potentials. The increase of the band intensity on Pt(100) below 0.05 V is due to the coupling with the band of Had as discussed below. On Pt(110), the adsorption of (bi)sulfate anion above 0.2 V inhibits the adsorption of water [25,26]. Since (bi)sulfate anion is adsorbed 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 the asymmetric bending mode of the hydronium cation (H3O +) [29]. The band intensity of H3O + decreases below 0.05 V because of the decrease of H + concentration neighboring to the electrode surface by the hydrogen evolution. The band intensity of H3O + also depends on the surface structure. This structural dependence may be due to the difference of the orientation of H3O + and the HER mechanism. The negative going band at 1235 cm − 1 is assigned to the symmetric SO3 stretching mode of adsorbed (bi)sulfate anion at the reference potential. Small positive going band appears at 2081 cm − 1 on Pt(110) at 0 V where the HER occurs. This band is assigned to νPt–H of the intermediate Hopd at the atop site (atop H) of Pt. The frequency of νPt–H is identical to that on polycrystalline Pt reported previously [5,7]. On the other hand, a tail appears in the higher-frequency region of δHOH band below 0.05 V on Pt(100). The tail includes the νPt–H band of the intermediate Had at asymmetric bridge site (bridge H) on Pt(100), as discussed below. However, no bands of Had is observed on Pt(111). DFT calculation predicts that the adsorbed hydrogen prefers the three-fold hollow site (hollow H) on the Pt(111). Previous highresolution energy-loss spectroscopy (HREELS) studies show dipole active modes of the hollow H at 904 and 1224 cm − 1[30]. Under the conditions of our experiments, these bands could not be observed because of the overlap with the (bi)sulfate band (1235 cm − 1) and the transmission limit of the IR window. We tried to measure the band of hollow 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 using IR spectroscopy, because the dipole component of the hollow H along the surface normal is small or dynamic dipole is effectively screened by surface atoms [31]. Since the band intensity of δHOH does not decrease during HER, the intermediate Hopdis coadsorbed with water on the Pt electrode. Fig. 2 shows the temperature dependent IRAS spectra on the low index 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 M H2SO4 at 0 V (RHE). Reference potentials are 0.8 V (RHE).
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Fig. 4. Cluster models for DFT calculations and optimized structures for H adsorbed on Pt(110) and Pt(100).
nð100Þ−ð110Þseries : Ptð210Þn = 2; Ptð310Þn = 3 nð111Þ−ð100Þseries : Ptð311Þn = 2; Ptð211Þn = 3
Fig. 3. IR spectra of high index planes of Pt in 0.5 M H2SO4 at 0 V (RHE). Reference potentials are 0.8 V (RHE).
and that of 1630 cm − 1 band on Pt(100) increase with the increase of the temperature. The enhancement of the band intensity on Pt(110) shows the increase of the coverage of atop H. The exchange current density of the HER/HOR gets higher at higher temperatures [2]. The coverage of the intermediate Had may relate to the activity of the HER. SEIRAS study also shows the quantitative relation between the coverage of the atop H and the kinetics of HER [5]. The increase of 1630 cm − 1 band on Pt(100) is attributed to the vibrational coupling of 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 at 1620 cm − 1 on Pt(111) and Pt(110), which does not include the νPt–H band, is symmetric and independent of the temperature. The temperature dependence of νPt–H supports that the observed band is the intermediate Had of the HER. Pt(110) = 2(111)–(111), which has high step density, is the most active surface for the HER [2,32,33]. The exchange current density of the HER/HOR on Pt(110) is higher than those on Pt(111) and Pt(100) because of a different reaction mechanism [2]. The fact that the atop H does not appear on Pt(111) and Pt(100) suggests that the HER/HOR pathway with a high reaction-rate goes through the atop H. Step structure may be important for the activation of the HER. We examine the step structural dependence of the intermediate Hopd using the following three series of the high index planes: ðn−1Þð111Þ−ð110Þseries : Ptð331Þn = 3; Ptð553Þn = 5
Table 2 Comparison of the adsorption energy Ead (meV) for Had on Pt(hkl) by DFT calculations.
Table 1 Calculated frequencies of νPt–H at each site. Atop H on Pt(110) Calc. IR
−1
2173 cm 2081 cm− 1
Asymmetric bridge H on Pt(100) −1
1672 cm 1630 cm− 1
where n denotes the number of terrace atomic rows. Fig. 3 shows the IR spectra of the high index planes in 0.5 M H2SO4 at 0 V at 298 K. The δHOH of the adsorbed water is observed at 1620 cm − 1 on all the surfaces. 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 H appears 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 is adsorbed on (110) structure. On the series of (n-1)(111)–(110), the narrowest surface of the terrace width is the (110) plane, i.e., the step structure 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 of the intermediate Had. Calculated frequencies of νPt–H at each site are listed in Table 1. Fig. 4 shows cluster models using frequency calculations and optimized structures for H adsorbed on Pt(110) and Pt(100). The band frequencies observed on Pt(110) and Pt(100) agree with the calculated values at the atop and asymmetric bridge sites. We calculate the structural dependence of the adsorption energy of hydrogen on the low index planes at 1 ML. We consider the three different adsorption sites of adsorbed hydrogen: the atop, symmetric bridge, and hollow sites. Table 2 shows the adsorption energies of Hopd on Pt surfaces. Had on Pt(111) prefers the hollow site to the atop and bridged sites, which is consistent with the previous theoretical and experimental reports [12,30]. The most stable site of hydrogen is the 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 not find the stable structure for the hollow H on Pt(110). The stable adsorption sites estimated from DFT calculations are different from
Symmetric bridge H on Pt(100) 1329 cm –
−1
Pt(111) Pt(100) Pt(110) Pt(211)
Atop H
Bridge H
Hollow H
394 416 533 399
364 672 615 635
440 311 – 322
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those observed using IR on Pt(100) and Pt(110). The intermediate species do not have to go through the most stable adsorption site. However, we can speculate the structural dependence of intermediate Had on Pt(110) and Pt(100) as follows. In the case of Pt(110), the difference of the adsorption energy between the atop and bridge sites is small (82 meV). The intermediate Hopd will be located at the atop site during HER. On the other hand, the bridge site on Pt(100) is more stable by N250 meV than the atop and hollow sites. HER on Pt(100) proceeds via the intermediate state at asymmetric bridge site close to the most stable bridge site. The adsorption energy is also calculated on unreconstructed Pt(211). The bridge site on Pt(211) strongly stabilizes Hopd as is the case of Pt(100). Therefore, the appearance of asymmetric bridge H on n(111)–(100) is due to the strong stability of the 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 several stepped surface with (110) step, whereas the asymmetric bridge H appears on Pt(100) and the surfaces with (100) step. According to DFT calculations, 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 Young Scientists (B) no. 22710099 and New Energy and Industrial Technology Development Organization (NEDO). References [1] J.O'.M. Bockris, A.K.N. Reddy, Modern Electrochemistry, Vol. 2, Plenum Press, New York, 1970. [2] N.M. Markovic, B.N. Grgur, P.N. Ross, J. Phys. Chem. B 101 (1997) 5405. [3] B.E. Conway, J. Barber, S. Morin, Electrochim. Acta 44 (1998) 1109. [4] G. Jerkiewicz, Prog. Surf. Sci. 57 (1998) 137.
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