Adsorption of bisulfate anion on a Pt(111) electrode: A comparison of in-situ and ex-situ IRAS

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J Electroanal. Chem., 358 (1993) 337-342 Elsevier Sequoia S .A., Lausanne JEC 02971PN Preliminary note

Adsorption of bisulfate anion on a Pt(111) electrode : a comparison of in-situ and ex-situ IRAS Hirohito Ogasawara, Yuichiro Sawatari, Junji Inukai and Masatoki Ito Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku Yokohama 223 (Japan) (Received 9 June 1993; in final form 22 June 1993)

INTRODUCTION Recent studies of bisulfate ion adsorption on platinum polycrystalline and Pt(111) single crystal electrodes using in-situ infrared reflection absorption spectroscopy (IRAs) have attracted particular attention [1-5] . Since inconsistencies exist in the band assignments for the adsorbates, the orientation is still unclear for the adsorption structures of bisulfate ions on platinum electrodes in sulfuric acid solutions [2,3] . Therefore, detailed studies are needed in order to determine the local orientation using in-situ IRAs, possibly combined with theoretical calculations . Infrared spectroscopy is suitable for determining local structures of adsorbates or admolecules on an electrode . However, it does not provide information on the long-range ordering of anions on the surface . To understand this long-range surface structure of adsorbed bisulfate ions, ex-situ studies using low energy electron diffraction (LEED) were recently carried out [6] . These ex-situ LEED experiments may cover the in-situ IRAs data, but it is still unclear whether the emersed (ex-situ) electrode surface retains its immersed (in-situ) conditions . Information to fulfill this environmental gap is evidently needed . In this paper, we present in-situ and ex-situ IRAS data of bisulfate ion adsorption on a Pt(111) electrode at various electrode potentials . The spectra were fully examined for determining the orientation of bisulfate adsorption on Pt(111). Ab-initio molecular orbital calculations were carried out to confirm the assignment of the band observed on an electrode surface. Further ex-situ LEED observation was carried out together with ex-situ IRAs measurements, to bridge in-situ and ex-situ conditions . The anomalously sharp spike currents and negative onset of the current associated with the potential sweep on a Pt(111) electrode surface are discussed in relation to the adsorption of the anion on the electrode surface . 0022-0728/93/$06 .00 © 1993 - Elsevier Sequoia S .A . All rights reserved

338 EXPERIMENTAL

In-situ measurements In-situ IRAS measurements were carried out with a thin layer electrochemical cell . A Pt(111) single crystal electrode was treated with a hydrogen-flame annealing followed by quenching into Ultrapure water . Sample preparation and characterization in the in-situ experiments are described elsewhere [7]. The in-situ IR cell was attached to a Bio-rad FTS-45RD Fourier transform infrared spectrometer with a liquid nitrogen-cooled MCT detector . The spectra were normally obtained by the subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) technique with an 8 cm' resolution . The background potential of the electrode was held at 50 mV (vs . SHE) where no anion adsorption occurs . The sample potential was changed stepwise from 200 to 1100 mV .

Ex-situ measurements A UHV instrument was constructed with the instrumental concepts similar to those reported by Hubbard [8], Ross [9] and Kolb [10]. The detailed designing will be described elsewhere [11] . Briefly, it consists of a main chamber (base pressure 5 x 10" Torr) and an auxiliary chamber (base pressure I x 10 -9 Torr) with an electrochemical cell . A Pt(111) single crystal surface was prepared by At' bombardment (5 x 10 -6 Ton for 30 min) and thermal annealing at 600°C under 5 x 10' Torr of oxygen for 1 h followed by vacuum annealing . The structure and the cleanliness of the surface was characterized by LEED and AES . The freshly prepared Pt(111) surface was transferred into an auxiliary chamber . After filling the auxilliary chamber with Ultrapure argon gas (99 .9995%) up to atmospheric pressure the electrochemical cell was introduced for immersion of the sample . Following electrochemical characterization and polarization at a certain potential, -3 the sample was emersed and the auxiliary chamber was evacuated to less than 10 Torr by use of a sorption pump and to 10 - s Torr by a cryopump . LEED and ex-situ IRAS measurements were subsequently carried out . The ex-situ IRAS experiments were carried out in the auxiliary chamber with a Perkin-Elmer 1720X Fourier transform infrared spectrometer with a liquid nitrogen-cooled MCT detector. In order to gain a high sensitivity in IRAS measurements, inflared radiation was used at near-grazing incidence to the metal surface (85-88° to the surface normal) . The spectra were normally obtained with an 8 cm - ' resolution . All solutions were prepared with Ultrapure water (Milli-Q SP-TOC system, Millipore) and super special grade sulfuric acid (Junsei Chemical) . In this paper the potentials are quoted against the standard hydrogen electrode (SHE) . RESULTS AND DISCUSSION

Voltammetry and LEED studies Figure 1 shows voltammetric curves for a Pt(111) electrode in a 0 .5 M H2SO4 solution prepared with the anneal-quenching method (Fig . 1(a)), and with Ar`-



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M CO Z W O Z W Q 0 V

200 (a)

400 600 E/mV vs. SHE

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i 200

50 pA/cm2

400 600 Eht V vs . SHE

(b)

Fig . 1 . Voltammetric curves of a Pt(111) in a 0 .5 M H 2 SO4 solution . (a) Flame annealed ; (b) UHV prepared . Scan rate: 50 mVs - ' . bombardment and annealing under UHV (Fig . 1(b)), respectively . Both voltammetric curves accord well with the earlier results [12] . Figure 2(a) shows a LEED pattern of a clean Pt(111) surface before a voltammetric measurement . A clear (1 x 1) pattern from the Pt(111) surface is seen . After the voltammetric measurement, the Pt(111) electrode was emersed at a certain potential, transferred back to the main UHV chamber and characterized by LEED . At 200 and 300 mV, very weak and diffused (2 x 2) patterns were observed . However, at potentials above 400 mV, x R30° LEED patterns were clearly observed. Figure 2(b) is the x 11) - R30° pattern at 500 mV where the image was the most distinct . A similar (Vi X C)-R30° was observed for a Pt(111) electrode surface emersed at more positive potentials .

(v

(v v ) -



340

Fig. 2 . LEED patterns. (a) Clean Pt(111)-(lxl), E=138 eV; (b) Pt(111} (yXy5')-R300 -HSO4 , E=96 eV.

In-situ and ex-situ IRAS studies

Figure 3(a) shows in-situ IRAS of bisulfate ions adsorbed on a Pt(111) electrode in a 0 .5 M sulfuric acid solution as a function of electrode potential . No absorption band was observed until 300 mV, where a broad band started to appear at 1200 cm -1 and increased in intensity until 900 mV with a blue shift . The large half widths of the band from 300 to 400 mV suggest inhomogeneous adsorption of bisulfate ions possibly at step sites. Thus, at hydrogen region, 50 - 300 mV, the surface covered with hydrogen holds such a negative charge that bisulfate ions cannot be adsorbed on the

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1400

1300 1200 Wavenumbers/cm'

1100

400

1300 1200 Wavenumbers/cm'

00

lal Fig. 3 . (a) Potential dependence of in-situ IRAS for a Pt(l11) electrode in a 0 .5 M H zSO4 solution . (b) Emersion potential dependence of ex-situ IRAS for the Pt(11l) from a 0.5 M H 2 SO4 solution .

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electrode surface . At 300 mV, bisulfate ions started to be adsorbed . As the potential was swept in a more positive direction, the coverage of the bisulfate ion on the surface increased until 900 mV at saturation, where the bisulfate ions formed fairly ordered structures on the surface since the IRAS exhibited a sharp band width. The radiochemical data obtained in a solution more dilute than 10 -3 M showed that bisulfate ions start to be adsorbed on a Pt(111) electrode at 400 mV, reach maximum coverage at 800 mV, and are desorbed at 1100 mV [6] . Our present IR results coincide with those with small difference in potentials . We have examined IRAS measurements in different concentrations of sulfuric acid down to 0 .1 mM . Even in the case of a 0 .1 mM solution, IRAS results are roughly similar to the present results in a 0 .5 M solution except that the adsorption potential of bisulfate ions are slightly higher (400 mV) and the maximum intensity is reduced to one-fourth due to the smaller coverage . Therefore, our result at lower coverages accords well with the previous radiochemical result reported by Gamboa-Aldeco et al . [6] . We assign the band in Fig . 3(a) to the SO 3 symmetric stretch of bisulfate ions adsorbed on a Pt(111) electrode surface because of the following reasons . From ab-initio molecular orbital calculation of a bisulfate ion, the HOMO is mainly localized on oxygen lone-pairs, and the lone-pair orbital exhibits antibonding character for O-S bond [13] . Therefore, the lone-pair electrons are favorable for bond formation with metal atoms, and the donation of the lone-pair electrons to Pt orbitals should cause strengthening of the O-S bond . This means bisulfate ions should exhibit a high frequency shift upon adsorption on the electrode surface . In order to confirm a frequency shift upon adsorption of an anion on a positively charged electrode surface, ab-initio MO calculations of a model molecule (Li` plus HSO4 ) were also carried out in relation to the HSOa ion . The calculated frequencies of a bisulfate ion and a LiHSO 4 model molecule coincide well with those experimental values . It is notable that the symmetric stretch (1040 cm 1 ) showed high frequency shift (to 1133 cm - ') upon Li' addition to the anion. This supports the conclusion that the large frequency shift of SO 3 symmetric stretching band is attributed to a strong interaction of anions and a positively charged electrode surface . Only the symmetric stretch was observed at = 1200 - 1277 cm - ', while the asymmetric stretch was invisible . Under the surface selection rule, the asymmetric doubly-degenerated stretch should be polarized in the surface plane . Consequently, two orientations of the bisulfate ion on the surface are possible : either through one oxygen group (OH) or three oxygen atoms . Figure 3(b) shows the ex-situ IRAS of bisulfate ions adsorbed on the Pt(111) surface . After the single crystal electrode was emersed out from a 0 .5 M H2SO4 solution at the potentials of 200 and 300 mV, a band appeared at 1264 cm - ' . However, once the electrode was emersed at a more positive potential than 400 mV, the band appeared at 1274 cm - ' . Therefore, the IRAS spectra of bisulfate on the surface pointed out the distinct difference of the adsorption states below and above 400 mV . This potential, 400 mV, corresponds to where the x R30°

(vi vi)-

34 2

structure is formed . Therefore, we ascribe the (' x F) - R30° LEED pattern to the adsorption structure of bisulfate ions. Now we compare our in-situ IRAS results with those of ex-situ IRAS results . At lower potentials than 300 mV, no bisulfate ions were adsorbed on a Pt(111) electrode . In-situ IRAS result revealed a remarkable increase in intensity between 400 and 500 mV . Not only of the adsorption intensity, but also the discontinuous features of the band-center position and the half width of the absorption band appeared at 400 mV by in-situ WAS measurements . It is interesting to note that anomalous spike currents occur at around 450 mV. Some drastic changes are supposed to be taking place at this potential . However, we also briefly note that the potential dependence in band center frequency is only observed by in-situ IRAS in an electrochemical environment . (IT X R30° structure should be related to the local symmetry between oxygen terminals and the hexagonal surface (111) geometry . The assignment of the band at = 1200 - 1277 cm - ' to symmetric stretch suppolts the model that bisulfate ions take a pseudo C3v structure on the threefold sites on a Pt(111) electrode .

0) -

CONCLUSIONS

Bisulfate ions, (1) start to be absorbed at 300 mV on a Pt(111) electrode in 0 .5 M H 2S04, ( 2) form (Vi x i)-R30° structure at more positive potential than 400 mV and (3) take pseudo C 3V structure on the Pt(111) surface and bond through single or three oxygen atoms . ACKNOWLEDGEMENTS

The authors wish to thank the Japan Private School Promotion Foundation, for supporting the present research . REFERENCES 1 K. Kunimatsu, M .G . Samant and H Seki, J. Electroanal. Chem., 58 (1989) 185 . 2 K. Kunimatsu, M .G. Samant and H Seki, J. Electroanal. Chem., 280 (1990) 391 . 3 P .W . Faguy, N . Markovic, R .R . Adzic, C.A . Fierro and E .B . Yeager, J. Electroanal. Chem., 295 (1990)24S . 4 T . Iwashita and F.C . Nart, J. Electroanal. Chem., 322 (1992) 289 . 5 F.C . Nart and T. Iwashita, J Electroanal. Chem., 322 (1992) 289 . 6 ME . Gamboa-Aldeco, H Henero, P .S . Zelenay and A. Wieckowski, J. ElectroanaL Chem, 348 (1993) 451 ; A. Wieckowski, private communication . 7 S . Watanabe, Y. Kinomoto, M . Takahashi and M . Ito, J. Electron Spectrosc., 54/55 (1990) 1205 . 8 A.T . Hubbard, Acc. Chem. Res, 13 (1980) 117 . 9 P .N. Ross Jr ., Surf. Sci., 102 (1981) 463 . 10 D .M . Kolb, Z . Phys . Chem ., New Folge, 154 (1987) 179 . 11 H . Ogasawara, J. Inukai and M. Ito, to be published . 12 J. Clavilier, R. Fame, G . Guinet and R. Durand, J Electroanal. Chem ., 107 (1980) 205. 13 Y. Sawatari, J . Inukai and M . Ito, J. Electron Spectrosc., submitted .