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Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption
Journal of Electroanalytical Chemistry 463 (1999) 109 – 115
Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption F. Huerta a, E. Morallo´n a, C. Quijada a, J.L. Va´zquez a,*, L.E.A. Berlouis b b
a Departamento de Quı´mica Fı´sica, Uni6ersidad de Alicante, Apartado 99, 03080 Alicante, Spain Department of Pure and Applied Chemistry, Uni6ersity of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
Received 28 July 1998; received in revised form 15 November 1998
Abstract The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles. © 1999 Elsevier Science S.A. All rights reserved. Keywords: UV-visible reflectance spectroscopy; Low index single crystal surfaces; Adsorbed cyanide; Platinum
1. Introduction Many optical methods have been used to investigate the adsorption-desorption processes that take place on metal surfaces. Among them, the technique of electroreflectance (ER) has been applied widely to the study of several metal electrolyte interfaces [1 – 5]. However, few papers discuss the application of the technique to platinum electrolyte interface electrodes and in particular, to that of the single crystal. One of the first studies in this area was carried out by Bewick and Tuxford [6,7] who studied the potential dependence of the surface reflectivity in the UV-visible spectral range for a polycrystalline platinum electrode in sulphuric acid medium. They argued that the strongly bound hydro* Corresponding [email protected].
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gen, which produces an increase in reflectivity, could correspond to the hydrogen embedded in the electronic surface of the metal. On the other hand, they reported that the weakly adsorbed hydrogen has the more conventional optical behaviour characteristic of a chemisorbed species on the surface of the metal. These conclusions were proposed taking into account that the two peaks observed in the cyclic voltammogram of a platinum electrode correspond to the underpotential deposition (upd) of hydrogen. Molina and Parsons [5] studied by electroreflectance the behaviour of the low index planes of platinum in sulphuric and perchloric acid solutions. These authors found that the characteristics of the reflectance change in the hydrogen region differ from those in the oxygen region and suggest that the so-called ‘‘anomalous’’ region for the Pt(111) surface could be attributed to hydrogen adsorption. Kazarinov et al. [8] employed electroreflectance and
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phase electro-modulation of the reflected light to study the joint adsorption of hydrogen and of anions on smooth polycrystalline platinum. On calculation of the residual charge on adatoms such as Clad and Brad as a function of potential, they were able to deduce that the halide species with different values of residual charge were present simultaneously on the surface. The optical electroreflectance method has been also used in the study of the chemisorption and oxidation of small organic molecules on noble metal electrodes [9– 11]. Thus, Nakabayashi et al. [11] studied the reaction mechanism of the electrooxidation of formaldehyde on a platinum electrode by the combination of electrochemical and optical techniques. Caram and Gutie´rrez [9,10] studied the chemisorption of CO, methanol and ethanol on platinum electrodes by potential-modulated reflectance spectroscopy and cyclic voltammetry. On the other hand, it has been noted that the adsorption and oxidation of simple amino acids on platinum electrodes gives adsorbed cyanide as the main product [12,13]. The study of the interaction of the CN − with the surface of the platinum electrode by the electroreflectance method could well provide additional information on the nature and transformation that occurs during this process. It is obvious therefore that the adsorption of hydrogen on platinum single crystal electrodes has been the subject of extensive research. The aim has been to understand the electrochemistry that occurs at these surfaces and in particular, the origin and nature of the reversible peaks appearing at potentials more positive of the hydrogen adsorption-desorption region on a
Pt(111) electrode in acidic and alkaline media. The present work is thus devoted to the study of a Pt(111) electrode in different acid and alkaline media and to the examination of the adsorption of cyanide at the Pt(111) surface using a combination of in-situ UV-visible electroreflectance and cyclic voltammetry.
2. Experimental The Pt(111) electrode was prepared following the method developed by Clavilier et al. [14]. The electrode had an area about 4–5 mm in diameter. A clean and well-ordered surface was obtained after flame annealing and cooling of the sample in a H2 + N2 stream [15]. The electrode surface was protected with a droplet of ultrapure water in equilibrium with both gases and transferred to the electrochemical cell. The acid solutions (0.1 M HClO4 and 0.5 M H2SO4) were prepared from concentrated acid (Merck suprapur) while the alkaline solutions (0.1 M Na2CO3 and 0.1 M NaOH) were prepared from Merck p.a chemicals. KCN solutions were prepared from a Merck pro labo chemical. In all cases the water used was from a Millipore Alpha-Q system (resistivity\18 MV cm − 1). The spectroelectrochemical cell was designed so that the electrode could be kept face down during the electroreflectance measurements [5]. After verifying the voltammogram of the single crystal using the dipping technique, the electrode was then immersed in the electrolyte solution for the optical experiments. The experimental set-up for the optical experiments is
Fig. 1. Schematic diagram of the electroreflectance set-up used in this work.
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shown in Fig. 1. The light was conducted to the electrode surface using a lens and mirrors. The incidence angle was 45° and the response was evaluated for both s and p polarised incidence light in the UV-visible range (365 –550 nm). Light from a 150 W Xenon arc lamp was fed through an f/3.4 Applied Photophysics monochromator to give the desired wavelength for the experiments. A photomultiplier tube (PMT) was placed at the specular angle to collect the reflected light. The output of the PMT was amplified and fed into an EG&G Model 5209 lock-in amplifier and signal processor which recovered the quotient DR/R and fed it into the controlling microcomputer. A Farnell LFM4 sine square wave oscillator produced the modulating square wave of 50 mV p-p at a frequency between 43 and 777 Hz as well as the reference signal for the lock-in amplifier. The potential was scanned during the electroreflectance experiments at a sweep rate of 4 mV s − 1. Cyanide adlayers were obtained by the immersion of a clean Pt(111) electrode into a 0.1 M KCN solution under open circuit conditions for 2 min according to the method used in Huerta et al. [16]. Under these conditions, the coverage obtained for adsorbed cyanide was close to saturation. The reference electrode employed in all the measurements was a reversible hydrogen electrode (RHE) prepared by electrochemically loading a Pd foil with hydrogen immediately prior to the experiments.
3. Results and discussion
3.1. Sulphuric acid Fig. 2(a) shows the well-known voltammogram for a Pt(111) electrode in 0.5 M H2SO4 recorded in the cell used for the electroreflectance measurements. The voltammogram of this platinum surface shows two characteristic adsorption regions at potentials more positive of the hydrogen evolution reaction (her). The first zone (from 0.06 to 0.33 V) corresponds to the hydrogen adsorption-desorption processes. The second one, called the unusual or anomalous state, takes place between 0.33 V and 0.5 V and is shifted to more positive potentials when the concentration of sulphuric acid decreases [17], thus revealing the role of the specific adsorption of anions in this region. Fig. 2(b and c) shows the electroreflectance spectra of this Pt(111) electrode in 0.5 M H2SO4 for the s- and p-polarised light. In both these instances, the ER responses are very similar but a larger signal, as expected [4], was obtained in the case of the p-polarised radiation. Fig. 2(b and c) shows that during the positive sweep from 0.06 V, the initial region shows a decrease in the ER signal. This is an artefact due to the surface being covered with hydrogen bubbles as the modulation
111
Fig. 2. (a) Voltammogram for a Pt(111) electrode in 0.5 M H2SO4, n =50 mV s − 1. Electroreflectance spectra of a Pt(111) electrode in 0.5 M H2SO4 obtained with (b) s-polarised light and (c) p-polarised light at 440 nm. Modulation frequency =43 Hz. n =4 mV s − 1.
amplitude of 50 mV p-p here pushes the potential into the her. This greatly affected the quality of the reflected light. Taking this into account, and especially as the signal in this region remained flat on the negative-going sweep, one can infer that the spectrum should not change significantly until the potential reaches a value close to 0.3 V. This potential region (between 0.06 and 0.3 V) corresponds clearly to the zone observed in the voltammogram of Fig. 2(a) associated with the hydrogen adsorption-desorption processes. From 0.3 V, the ER signal increases with potential up to a maximum at ca. 0.5 V and then decreases monotonically with further increase in potential, reaching a value close to zero at ca. 0.7 V for both polarisations. The maximum of the ER signal at ca. 0.5 V corresponds to the so-called ‘anomalous process’ observed in the cyclic voltammogram (Fig. 2(a)) and it is worth noting that both the forward and reverse sweeps in the electroreflectance data gave a very similar behaviour in this potential region. This indicates the highly reversible nature of the process taking place here. As the frequency of modulation was increased from 43 to 777 Hz, a decrease in the electroreflectance signal in the potential region of 0.3– 0.8 V was observed, signifying a lowering in the sensitivity of the modulating signal to probing charge transfer processes occurring at the interface. It is worth
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re-emphasising here the strong parallelism observed in the voltammogram and the ER spectrum in the potential region between 0.3 and 0.5 V. From IR spectroscopy, it has been observed that bisulphate is the main anionic species both in solution and in the interfacial region in sulphuric acid medium [18 – 20]. This zone in the voltammogram has been attributed to the adsorption and desorption of bisulphate anions [18–21]. This interaction leads to an increase in the electroreflectance signal which must therefore be as a result of the partial transference of electrons from the adsorbed bisulphate anions to the platinum surface. The electroreflectance spectra show that this process is clearly different from that obtained in the hydrogen region.
3.2. Perchloric acid Fig. 3 shows the voltammogram in the cell used for the optical measurements and the ER spectra (s- and p-polarised lights) recorded in 0.1 M HClO4 for the Pt(111) electrode. The voltammogram (Fig. 3(a)) has a well-defined zone between 0.05 and 0.4 V which corresponds to the usual zone of adsorption-desorption of hydrogen. The second region between 0.5 and 0.9 V shows a broad structure leading to a sharp narrow peak
Fig. 3. (a) Voltammogram for a Pt(111) electrode in 0.1 M HClO4, n = 50 mV s − 1. Electroreflectance spectra of a Pt(111) electrode in 0.1 M HClO4 obtained with (b) s-polarised light and (c) p-polarised light at 550 nm. Modulation frequency = 43 Hz. n= 4 mV s − 1.
Fig. 4. Electroreflectance spectra of a Pt(111) electrode in 0.1 M HClO4 at different frequencies.
at ca. 0.8 V and these features correspond clearly to a highly reversible process. The ER spectra for a Pt(111) in 0.1 M HClO4 is presented in Fig. 3(b and c) (s- and p-polarised light respectively). The ER signal during the positive sweep is very similar to that obtained during the reverse negative sweep, again indicating that the process(es) giving rise to the ER signal are very reversible. As in the case of the sulphuric acid solution, a strong correspondence exists with the cyclic voltammogram. Thus, in the hydrogen adsorption-desorption region (between 0.05 and 0.35 V) there is almost no change in the ER signal. This optical characteristic for the hydrogen adsorption-desorption region was also observed in the case of sulphuric medium in the same region. From 0.3 V, a potential that corresponds to the beginning of the double layer region, a gradual increase in ER signal with potential occurred leading to a peak located at ca. 0.8 V. This signature was observed for both the s- and p-polarisations during the positive going sweep but was less evident on the reverse sweep. The presence of an ER signal in this region reflects that the process occurring here is different from that occurring in the potential region between 0.05 and 0.35 V, i.e., the hydrogen adsorption-desorption region. From purely voltammetric data this region was attributed in this electrolyte to the adsorption of hydroxyl ions [22]. However, no spectroscopic evidence of this assignation has been obtained. The ER signal appearing between 0.05 and 0.8V has the same sign as that obtained in sulphuric acid solution corresponding to the adsorption-desorption of bisulphate anions. Then, as in the case of sulphuric acid medium, the change in ER signal could tentatively be attributed to the adsorption of an anionic species. However, since the perchlorate anions do not adsorb on the platinum surface, the only anion able to do so would be an oxygenated species originating from the decomposition of water, i.e. OH − [22–24]. The peak at 0.8 V however merits further discussion. Fig. 4 shows the dependence of the ER spectra on the modulation frequency carried out in perchloric acid medium. Although the general trend observed was
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similar to that in sulphuric acid, a notable difference here was that there was a larger relative decrease in the peak at 0.8 V with increasing modulation frequency with respect to the broader feature between 0.3 and 0.7 V. This shows clearly that the origin of the process giving rise to the electroreflectance signal at 0.8 V differs from that observed at less positive potentials. The difference in frequency dependence between these two regions suggests a kinetically related process which could correspond to different rates of adsorption/desorption at different surface sites in these two regions.
3.3. Cyanide adsorption in acidic media Fig. 5 shows the electroreflectance and cyclic voltammogram obtained in a 0.1M HClO4 solution for a cyanide-covered Pt(111) electrode. Following the cyanide adsorption procedure, the electrode was rinsed with ultrapure water to avoid the introduction of cyanide in the spectroelectrochemical cell [16]. The adsorption of cyanide partially blocks the hydrogen adsorption-desorption processes. However, the voltammetric wave corresponding to the usual adsorption of hydrogen in the test solution of 0.1 M HClO4 appears over a wide potential range, extending into the double layer zone (Fig. 2(a)). The wave between 0.3 and 0.6 V
Fig. 5. (a) Voltammogram, n = 50 mV s − 1; (b) electroreflectance spectrum obtained with s-polarised light and (c) electroreflectance spectrum obtained with p-polarised light for a CN − covered Pt(111) electrode in 0.1 M HClO4. Modulation frequency = 43 Hz (50 mV p-p). n= 4 mV s − 1. l=440 nm.
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is associated with the presence of cyanide on the Pt(111) surface. The adsorbed cyanide is thus stable and it is not oxidised until a potential of ca. 1.5 V is reached [16]. Fig. 5(b and c) show the electroreflectance spectra for the s- and p-polarised light obtained under the same conditions as that of the voltammogram of Fig. 5(a). A good correspondence between the voltammogram and the ER spectrum was again observed here. Two welldefined zones can be distinguished in the ER spectra. The first zone occurs at potentials lower than 0.6 V, where the ER signal is large and negative, with a minimum value observed at ca. 0.45 V. This ER signal is associated clearly with the wave that appears in the voltammogram over the same potential range. The decrease in the potential range between 0.2 and 0.6 V can thus be associated with the presence of adsorbed cyanide on the platinum surface (see Fig. 3 to compare the ER signal for the Pt(111) surface free of cyanide). This decrease in the ER signal may indicate that the adsorbed cyanide decreases the electronic density of the platinum surface as a consequence of back-donation from the platinum to the cyanide [25]. The same behaviour has been observed in sulphuric acid medium for a cyanide-covered Pt(111) electrode. The peak in the ER spectra for adsorbed cyanide on Pt(111) occurs very close to the transition potential (ca. 0.5 V vs. RHE) where on the positive going sweep, N-bound cyanide changes to C-bound cyanide [26–28]. There is then a very abrupt change in the intensity of the ER signal. From density functional theory (DFT) calculations, it has been shown that the potential energy of the PtCN − species is 0.86 eV below that of the PtNC − with corresponding experimental vibrational frequencies of the cyanide being 2150 and 2070 cm − 1 respectively, in good agreement with sum frequency generation (SFG) measurements [26,27]. It is clear therefore that the rapid change in the ER intensity with the orientation of the adsorbed CN − occurs is entirely in line with changes occurring in the electronic configuration of that surface. The bonding of the cyanide to transition metal electrode surfaces is essentially ionic [29], with the CN axis parallel to the electric field direction [30]. Thus, for N-bound cyanide, the dipole moment will increase as the electrode potential is made more positive. On the other hand, for C-bound cyanide, the dipole is now oriented against the electric field and a decrease in the bond polarisation is found [16]. The distance of closest approach and hence the interfacial potential experienced by the adsorbed C-bound cyanide is thus considerably less than that of the N-bound cyanide and so, its effect on the metal surface electron density considerably reduced [27]. An alternative explanation for the change in ER signal could be a change in the microstructure of the surface. The cyanide is reported to adsorb in a hexago-
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nal (2 3 ×2 3)R30° arrangement with an additional CN in the centre of the ring [31,32]. The transition in the region of 0.5 V vs. RHE is explained by Friedrich et al. [31] in terms of a structural transformation into a metastable (2×2) structure with corresponding changes in the bonding or cyanide coverage of the surface. On the other hand, the explanation advanced by Kim et al. [32] is that a structural transition to a ( 7 × 7)R19° occurs in a narrow potential range centred near 0.5 V. It was further postulated that the centre of the hexagon was not a CN species but a hydronium cation and strong evidence for this was found in a parallel study of CN adsorption from KClO4 solutions [32]. It has furthermore been observed that cyanide adsorbs at non-equivalent sites on the platinum surface, viz. symmetric atop sites and near-top sites [33] which could lead to electronic variations in the electrode surface. Nevertheless, of these two alternatives, the change in the orientation of the adsorbed cyanide fits in more readily with the large intensity changes found in our electroreflectance data in the 0.5 V region. The structural transitions could well lead to changes in the ER intensity but it is doubtful whether the magnitudes would be comparable. The second zone observed in Fig. 5(b and c) occurs at potentials higher than 0.8 V and a maximum appears in the ER spectra. This maximum corresponds to the voltammetric wave observed in Fig. 5(a) and is associated with adsorption of oxygen-containing species [16]. The results obtained by electroreflectance in cyanide are in agreement with the interpretation given above in the case of perchloric acid.
3.4. Alkaline solutions Fig. 6 shows the cyclic voltammogram for a Pt(111) electrode in 0.1 M NaOH and the electroreflectance spectrum using p-polarised light. The cyclic voltammogram of Fig. 6(a) shows that for this platinum surface, three zones can again be distinguished clearly [34]. The region between 0.06 and 0.4 V corresponds, as in the case of acid media, to hydrogen adsorption-desorption processes. However here, the adsorbed hydrogen comes from water rather than from protons in solution and associated with this will be an extensive network of hydrogen-bonded water dipoles to the adsorbed hydrogen [22]. The second zone is the double layer region between 0.4 and 0.65 V. In the third zone, the broad peak between 0.65 and 0.9V would again appear to correspond to the adsorption/desorption of oxygenated species which is very reversible. The experimental 1/R((R/(E) data in this medium (Fig. 6(b)) was considerably more noisy than the previous ER data, making the interpretation that much more difficult. On the other hand, on integration, the DR/R signal shows the three distinct regions (dotted line in Fig. 6(b))
Fig. 6. (a) Voltammogram for a Pt(111) electrode in 0.1 M NaOH, n =50 mV s − 1. (b) Electroreflectance spectrum of a Pt(111) electrode in 0.1 M NaOH obtained with p-polarised light at 550 nm, dotted line: integrated data obtained in the positive sweep. Modulation frequency =43 Hz (100 mV p-p). n= 4 mV s − 1.
observed in the cyclic voltammogram of Fig. 6(a), particularly on the positive sweep. A slowly increasing background is found as the potential is made more positive, with small steps as the potential is swept into the different regions. These small changes in the modulated reflectance data would suggest that the nature of the species in the proximity of the electrode surface and hence, their electron-donating or withdrawing capability, varies very little with the applied potential and is thus concerned only with the reorientation of hydrogen-bonded surface species and water dipoles. This is in sharp contrast to the ER data in perchloric and sulphuric acid media where an increase in the ER data was observed beyond ca 0.35 V, the start of the double layer region. Thus in alkaline solutions, either water or oxygenated species such as OH − anions may always be present on the surface of the electrode in the different potential regions.
4. Conclusions The electroreflectance technique has been used to analyse the adsorption characteristics of a Pt(111) electrode in acidic and alkaline solutions. In sulphuric and
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perchloric acids, in the zone corresponding to the usual adsorption-desorption of hydrogen, the electroreflectance signal is practically zero and does not change. However, very different behaviour is observed at more positive potentials. In the case of sulphuric acid solutions, an increase in the ER signal is observed and this is associated with the adsorption-desorption of bisulphate anions. This result indicates that the adsorption of bisulphate increases the electronic density of the platinum surface. In the case of perchloric acid, the increase in the ER signal in this region is attributed to the adsorption of oxygenated species. The electroreflectance spectrum obtained for a cyanide-covered Pt(111) surface shows two clearly defined zones. At potentials lower than 0.6 V, a strong decrease in the ER signal was observed. This may indicate that the electronic density of the platinum surface is decreased as a consequence of back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated electroreflectance spectra showed that small changes obtained corresponded well to the features observed in the cyclic voltammogram.
Acknowledgements C. Quijada and E. Morallo´n are indebted to the ‘Ministerio de Educacio´n y Cultura’ for the award of their F.P.I. postdoctoral fellowships. F. Huerta thanks the ‘Generalitat Valenciana’ for a thesis grant.
References [1] R.H. Muller (Ed.) Advances in Electrochemistry and Electrochemical Engineering, Vol. 9, Wiley, 1973, and references therein. [2] D.M. Kolb, in: A.F. Silva (Ed.), Trends in Interfacial Electrochemistry, 1986, pp. 301–330. [3] M.A. Barret, R. Parsons, Symp. Faraday Soc. 4 (1970) 72. [4] D.M. Kolb, in: R.J. Gale (Ed.), Spectroelectrochemistry: Theory and Practice, Ch. 4, Plenum Press, New York, 1988, p. 87, and references therein. [5] F.V. Molina, R. Parsons, J. Chimie Phys. 88 (1991) 1339. [6] A. Bewick, A.M. Tuxford, J. Electroanal. Chem. 47 (1973) 255.
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[7] A. Bewick, A.M. Tuxford, Symp. Faraday Soc. 4 (1970) 114. [8] V.E. Kazarinov, A. M. Funtikov, G.A. Tsirlina, Soviet Electrochem. 27 (1991) 278. [9] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 323 (1992) 213. [10] J.A. Caram, C. Gutie´rrez, J. Electroanal. Chem. 291 (1990) 289. [11] S. Nakabayashi, I. Yogi, N. Sugiyama, K. Tamura, K. Uosaki, Surf. Sci. 386 (1997) 82. [12] F. Huerta, E. Morallo´n, F. Cases, A. Rodes, J.L. Va´zquez, A. Aldaz, J. Electroanal. Chem. 421 (1997) 179. [13] F. Huerta, E. Morallo´n, F. Cases, A. Rodes, J.L. Va´zquez, A. Aldaz, J. Electroanal. Chem. 431 (1997) 269. [14] J. Clavilier, D. Armand, S.G. Sun, M. Petit, J. Electroanal. Chem. 205 (1986) 267. [15] A. Rodes, M.A. Zamakhchari, K. ElAchi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [16] F. Huerta, E. Morallo´n, J.L Va´zquez, A. Aldaz, Surf. Sci. 396 (1998) 400. [17] J. Clavilier, K. El Achi, A. Rodes, Chem. Phys. 141 (1990) 1. [18] P.W. Faguy, N. Markovic, R.R. Adzic, C.A. Fierro, E.B. Yeager, J. Electroanal. Chem. 289 (1990) 245. [19] H. Ogasawara, Y. Sawatari, J. Inukai, M. Ito, J. Electroanal. Chem. 358 (1993) 337. [20] F.C. Nart, T. Iwasita, J. Electroanal. Chem. 322 (1992) 289. [21] J. Clavilier, J.M. Orts, R. Go´mez, J.M. Feliu, A. Aldaz, Prog. Electrochem. Soc. 94 (1994) 167. [22] K. Al Jaaf-Golze, D.M. Kolb, D.A. Scherson, J. Electroanal. Chem. 200 (1986) 353. [23] U.W. Hamm, D. Kramer, R.S. Zhai, D.M. Kolb, J. Electroanal. Chem. 414 (1996) 85. [24] F.T. Wagner, P.N. Ross Jr., J. Electroanal. Chem. 250 (1988) 301. [25] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1986. [26] A. Tadjeddine, A. Peremans, A. Lerille, M. Zheng, W.Q. Tadjeddine, J. Chem. Soc. Faraday Trans. 92 (1996) 3823. [27] A. Tadjeddine, P. Guyot-Sionnest, Electrochim. Acta 36 (1991) 1849. [28] F.J. Huerta, E. Morallon, C. Quijada, J.L. Vazquez, A. Aldaz, Electrochim. Acta. 44 (1998) 943 – 948. [29] M.R. Philpot, P.S. Bagus, C.J. Nelin, H. Seki, J. Electron. Spectroc. Relat. Phenom. 45 (1987) 169. [30] K.A.B. Lee, K. Kunimatsu, J.G. Gordon, W.G. Golden, H. Seki, J. Electrochem. Soc. 134 (1987) 1676. [31] K.A. Friedrich, W. Daum, C. Klu¨nker, D. Knabben, U. Stimmimg, H. Ibach, Surf. Sci. 335 (1995) 315. [32] Y.G. Kim, S.L. Yau, K. Itaya, J. Am. Chem. Soc. 118 (1996) 393. [33] C. Stuhlmann, I. Villegas, M.J. Weaver, Chem. Phys. Lett. 219 (1994) 319. [34] E. Morallo´n, J.L. Va´zquez, A. Aldaz, J. Electroanal. Chem. 334 (1992) 323.
Potential modulated reflectance spectroscopy of Pt(111) in acidic and alkaline media: cyanide adsorption F. Huerta a, E. Morallo´n a, C. Quijada a, J.L. Va´zquez a,*, L.E.A. Berlouis b b
a Departamento de Quı´mica Fı´sica, Uni6ersidad de Alicante, Apartado 99, 03080 Alicante, Spain Department of Pure and Applied Chemistry, Uni6ersity of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
Received 28 July 1998; received in revised form 15 November 1998
Abstract The electroreflectance (ER) technique has been applied to the study of a Pt(111) electrode in different acidic and alkaline electrolytes. The variation in the normalised potential derivative of the reflectance signal, 1/R(dR/dE) is observed in sulphuric and perchloric acid media as the potential is scanned 1.0 V positive from the hydrogen evolution region. In both these media, the usual hydrogen adsorption-desorption region corresponds to a zone in which the ER signal is practically zero and does not change with the potential. However, in the anomalous region, an increase in the ER signal was observed indicating that here, the adsorbed species increased the electron density of the platinum surface. The ER spectrum recorded for the cyanide-covered Pt(111) surface showed a large decrease in the electron density of the platinum surface and this is attributed to electron back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated ER spectrum showed relatively small changes which corresponded to the features in the cyclic voltammogram and were attributed to the reorientation of the hydrogen-bonded surface species and water dipoles. © 1999 Elsevier Science S.A. All rights reserved. Keywords: UV-visible reflectance spectroscopy; Low index single crystal surfaces; Adsorbed cyanide; Platinum
1. Introduction Many optical methods have been used to investigate the adsorption-desorption processes that take place on metal surfaces. Among them, the technique of electroreflectance (ER) has been applied widely to the study of several metal electrolyte interfaces [1 – 5]. However, few papers discuss the application of the technique to platinum electrolyte interface electrodes and in particular, to that of the single crystal. One of the first studies in this area was carried out by Bewick and Tuxford [6,7] who studied the potential dependence of the surface reflectivity in the UV-visible spectral range for a polycrystalline platinum electrode in sulphuric acid medium. They argued that the strongly bound hydro* Corresponding [email protected].
author.
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gen, which produces an increase in reflectivity, could correspond to the hydrogen embedded in the electronic surface of the metal. On the other hand, they reported that the weakly adsorbed hydrogen has the more conventional optical behaviour characteristic of a chemisorbed species on the surface of the metal. These conclusions were proposed taking into account that the two peaks observed in the cyclic voltammogram of a platinum electrode correspond to the underpotential deposition (upd) of hydrogen. Molina and Parsons [5] studied by electroreflectance the behaviour of the low index planes of platinum in sulphuric and perchloric acid solutions. These authors found that the characteristics of the reflectance change in the hydrogen region differ from those in the oxygen region and suggest that the so-called ‘‘anomalous’’ region for the Pt(111) surface could be attributed to hydrogen adsorption. Kazarinov et al. [8] employed electroreflectance and
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phase electro-modulation of the reflected light to study the joint adsorption of hydrogen and of anions on smooth polycrystalline platinum. On calculation of the residual charge on adatoms such as Clad and Brad as a function of potential, they were able to deduce that the halide species with different values of residual charge were present simultaneously on the surface. The optical electroreflectance method has been also used in the study of the chemisorption and oxidation of small organic molecules on noble metal electrodes [9– 11]. Thus, Nakabayashi et al. [11] studied the reaction mechanism of the electrooxidation of formaldehyde on a platinum electrode by the combination of electrochemical and optical techniques. Caram and Gutie´rrez [9,10] studied the chemisorption of CO, methanol and ethanol on platinum electrodes by potential-modulated reflectance spectroscopy and cyclic voltammetry. On the other hand, it has been noted that the adsorption and oxidation of simple amino acids on platinum electrodes gives adsorbed cyanide as the main product [12,13]. The study of the interaction of the CN − with the surface of the platinum electrode by the electroreflectance method could well provide additional information on the nature and transformation that occurs during this process. It is obvious therefore that the adsorption of hydrogen on platinum single crystal electrodes has been the subject of extensive research. The aim has been to understand the electrochemistry that occurs at these surfaces and in particular, the origin and nature of the reversible peaks appearing at potentials more positive of the hydrogen adsorption-desorption region on a
Pt(111) electrode in acidic and alkaline media. The present work is thus devoted to the study of a Pt(111) electrode in different acid and alkaline media and to the examination of the adsorption of cyanide at the Pt(111) surface using a combination of in-situ UV-visible electroreflectance and cyclic voltammetry.
2. Experimental The Pt(111) electrode was prepared following the method developed by Clavilier et al. [14]. The electrode had an area about 4–5 mm in diameter. A clean and well-ordered surface was obtained after flame annealing and cooling of the sample in a H2 + N2 stream [15]. The electrode surface was protected with a droplet of ultrapure water in equilibrium with both gases and transferred to the electrochemical cell. The acid solutions (0.1 M HClO4 and 0.5 M H2SO4) were prepared from concentrated acid (Merck suprapur) while the alkaline solutions (0.1 M Na2CO3 and 0.1 M NaOH) were prepared from Merck p.a chemicals. KCN solutions were prepared from a Merck pro labo chemical. In all cases the water used was from a Millipore Alpha-Q system (resistivity\18 MV cm − 1). The spectroelectrochemical cell was designed so that the electrode could be kept face down during the electroreflectance measurements [5]. After verifying the voltammogram of the single crystal using the dipping technique, the electrode was then immersed in the electrolyte solution for the optical experiments. The experimental set-up for the optical experiments is
Fig. 1. Schematic diagram of the electroreflectance set-up used in this work.
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shown in Fig. 1. The light was conducted to the electrode surface using a lens and mirrors. The incidence angle was 45° and the response was evaluated for both s and p polarised incidence light in the UV-visible range (365 –550 nm). Light from a 150 W Xenon arc lamp was fed through an f/3.4 Applied Photophysics monochromator to give the desired wavelength for the experiments. A photomultiplier tube (PMT) was placed at the specular angle to collect the reflected light. The output of the PMT was amplified and fed into an EG&G Model 5209 lock-in amplifier and signal processor which recovered the quotient DR/R and fed it into the controlling microcomputer. A Farnell LFM4 sine square wave oscillator produced the modulating square wave of 50 mV p-p at a frequency between 43 and 777 Hz as well as the reference signal for the lock-in amplifier. The potential was scanned during the electroreflectance experiments at a sweep rate of 4 mV s − 1. Cyanide adlayers were obtained by the immersion of a clean Pt(111) electrode into a 0.1 M KCN solution under open circuit conditions for 2 min according to the method used in Huerta et al. [16]. Under these conditions, the coverage obtained for adsorbed cyanide was close to saturation. The reference electrode employed in all the measurements was a reversible hydrogen electrode (RHE) prepared by electrochemically loading a Pd foil with hydrogen immediately prior to the experiments.
3. Results and discussion
3.1. Sulphuric acid Fig. 2(a) shows the well-known voltammogram for a Pt(111) electrode in 0.5 M H2SO4 recorded in the cell used for the electroreflectance measurements. The voltammogram of this platinum surface shows two characteristic adsorption regions at potentials more positive of the hydrogen evolution reaction (her). The first zone (from 0.06 to 0.33 V) corresponds to the hydrogen adsorption-desorption processes. The second one, called the unusual or anomalous state, takes place between 0.33 V and 0.5 V and is shifted to more positive potentials when the concentration of sulphuric acid decreases [17], thus revealing the role of the specific adsorption of anions in this region. Fig. 2(b and c) shows the electroreflectance spectra of this Pt(111) electrode in 0.5 M H2SO4 for the s- and p-polarised light. In both these instances, the ER responses are very similar but a larger signal, as expected [4], was obtained in the case of the p-polarised radiation. Fig. 2(b and c) shows that during the positive sweep from 0.06 V, the initial region shows a decrease in the ER signal. This is an artefact due to the surface being covered with hydrogen bubbles as the modulation
111
Fig. 2. (a) Voltammogram for a Pt(111) electrode in 0.5 M H2SO4, n =50 mV s − 1. Electroreflectance spectra of a Pt(111) electrode in 0.5 M H2SO4 obtained with (b) s-polarised light and (c) p-polarised light at 440 nm. Modulation frequency =43 Hz. n =4 mV s − 1.
amplitude of 50 mV p-p here pushes the potential into the her. This greatly affected the quality of the reflected light. Taking this into account, and especially as the signal in this region remained flat on the negative-going sweep, one can infer that the spectrum should not change significantly until the potential reaches a value close to 0.3 V. This potential region (between 0.06 and 0.3 V) corresponds clearly to the zone observed in the voltammogram of Fig. 2(a) associated with the hydrogen adsorption-desorption processes. From 0.3 V, the ER signal increases with potential up to a maximum at ca. 0.5 V and then decreases monotonically with further increase in potential, reaching a value close to zero at ca. 0.7 V for both polarisations. The maximum of the ER signal at ca. 0.5 V corresponds to the so-called ‘anomalous process’ observed in the cyclic voltammogram (Fig. 2(a)) and it is worth noting that both the forward and reverse sweeps in the electroreflectance data gave a very similar behaviour in this potential region. This indicates the highly reversible nature of the process taking place here. As the frequency of modulation was increased from 43 to 777 Hz, a decrease in the electroreflectance signal in the potential region of 0.3– 0.8 V was observed, signifying a lowering in the sensitivity of the modulating signal to probing charge transfer processes occurring at the interface. It is worth
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re-emphasising here the strong parallelism observed in the voltammogram and the ER spectrum in the potential region between 0.3 and 0.5 V. From IR spectroscopy, it has been observed that bisulphate is the main anionic species both in solution and in the interfacial region in sulphuric acid medium [18 – 20]. This zone in the voltammogram has been attributed to the adsorption and desorption of bisulphate anions [18–21]. This interaction leads to an increase in the electroreflectance signal which must therefore be as a result of the partial transference of electrons from the adsorbed bisulphate anions to the platinum surface. The electroreflectance spectra show that this process is clearly different from that obtained in the hydrogen region.
3.2. Perchloric acid Fig. 3 shows the voltammogram in the cell used for the optical measurements and the ER spectra (s- and p-polarised lights) recorded in 0.1 M HClO4 for the Pt(111) electrode. The voltammogram (Fig. 3(a)) has a well-defined zone between 0.05 and 0.4 V which corresponds to the usual zone of adsorption-desorption of hydrogen. The second region between 0.5 and 0.9 V shows a broad structure leading to a sharp narrow peak
Fig. 3. (a) Voltammogram for a Pt(111) electrode in 0.1 M HClO4, n = 50 mV s − 1. Electroreflectance spectra of a Pt(111) electrode in 0.1 M HClO4 obtained with (b) s-polarised light and (c) p-polarised light at 550 nm. Modulation frequency = 43 Hz. n= 4 mV s − 1.
Fig. 4. Electroreflectance spectra of a Pt(111) electrode in 0.1 M HClO4 at different frequencies.
at ca. 0.8 V and these features correspond clearly to a highly reversible process. The ER spectra for a Pt(111) in 0.1 M HClO4 is presented in Fig. 3(b and c) (s- and p-polarised light respectively). The ER signal during the positive sweep is very similar to that obtained during the reverse negative sweep, again indicating that the process(es) giving rise to the ER signal are very reversible. As in the case of the sulphuric acid solution, a strong correspondence exists with the cyclic voltammogram. Thus, in the hydrogen adsorption-desorption region (between 0.05 and 0.35 V) there is almost no change in the ER signal. This optical characteristic for the hydrogen adsorption-desorption region was also observed in the case of sulphuric medium in the same region. From 0.3 V, a potential that corresponds to the beginning of the double layer region, a gradual increase in ER signal with potential occurred leading to a peak located at ca. 0.8 V. This signature was observed for both the s- and p-polarisations during the positive going sweep but was less evident on the reverse sweep. The presence of an ER signal in this region reflects that the process occurring here is different from that occurring in the potential region between 0.05 and 0.35 V, i.e., the hydrogen adsorption-desorption region. From purely voltammetric data this region was attributed in this electrolyte to the adsorption of hydroxyl ions [22]. However, no spectroscopic evidence of this assignation has been obtained. The ER signal appearing between 0.05 and 0.8V has the same sign as that obtained in sulphuric acid solution corresponding to the adsorption-desorption of bisulphate anions. Then, as in the case of sulphuric acid medium, the change in ER signal could tentatively be attributed to the adsorption of an anionic species. However, since the perchlorate anions do not adsorb on the platinum surface, the only anion able to do so would be an oxygenated species originating from the decomposition of water, i.e. OH − [22–24]. The peak at 0.8 V however merits further discussion. Fig. 4 shows the dependence of the ER spectra on the modulation frequency carried out in perchloric acid medium. Although the general trend observed was
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similar to that in sulphuric acid, a notable difference here was that there was a larger relative decrease in the peak at 0.8 V with increasing modulation frequency with respect to the broader feature between 0.3 and 0.7 V. This shows clearly that the origin of the process giving rise to the electroreflectance signal at 0.8 V differs from that observed at less positive potentials. The difference in frequency dependence between these two regions suggests a kinetically related process which could correspond to different rates of adsorption/desorption at different surface sites in these two regions.
3.3. Cyanide adsorption in acidic media Fig. 5 shows the electroreflectance and cyclic voltammogram obtained in a 0.1M HClO4 solution for a cyanide-covered Pt(111) electrode. Following the cyanide adsorption procedure, the electrode was rinsed with ultrapure water to avoid the introduction of cyanide in the spectroelectrochemical cell [16]. The adsorption of cyanide partially blocks the hydrogen adsorption-desorption processes. However, the voltammetric wave corresponding to the usual adsorption of hydrogen in the test solution of 0.1 M HClO4 appears over a wide potential range, extending into the double layer zone (Fig. 2(a)). The wave between 0.3 and 0.6 V
Fig. 5. (a) Voltammogram, n = 50 mV s − 1; (b) electroreflectance spectrum obtained with s-polarised light and (c) electroreflectance spectrum obtained with p-polarised light for a CN − covered Pt(111) electrode in 0.1 M HClO4. Modulation frequency = 43 Hz (50 mV p-p). n= 4 mV s − 1. l=440 nm.
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is associated with the presence of cyanide on the Pt(111) surface. The adsorbed cyanide is thus stable and it is not oxidised until a potential of ca. 1.5 V is reached [16]. Fig. 5(b and c) show the electroreflectance spectra for the s- and p-polarised light obtained under the same conditions as that of the voltammogram of Fig. 5(a). A good correspondence between the voltammogram and the ER spectrum was again observed here. Two welldefined zones can be distinguished in the ER spectra. The first zone occurs at potentials lower than 0.6 V, where the ER signal is large and negative, with a minimum value observed at ca. 0.45 V. This ER signal is associated clearly with the wave that appears in the voltammogram over the same potential range. The decrease in the potential range between 0.2 and 0.6 V can thus be associated with the presence of adsorbed cyanide on the platinum surface (see Fig. 3 to compare the ER signal for the Pt(111) surface free of cyanide). This decrease in the ER signal may indicate that the adsorbed cyanide decreases the electronic density of the platinum surface as a consequence of back-donation from the platinum to the cyanide [25]. The same behaviour has been observed in sulphuric acid medium for a cyanide-covered Pt(111) electrode. The peak in the ER spectra for adsorbed cyanide on Pt(111) occurs very close to the transition potential (ca. 0.5 V vs. RHE) where on the positive going sweep, N-bound cyanide changes to C-bound cyanide [26–28]. There is then a very abrupt change in the intensity of the ER signal. From density functional theory (DFT) calculations, it has been shown that the potential energy of the PtCN − species is 0.86 eV below that of the PtNC − with corresponding experimental vibrational frequencies of the cyanide being 2150 and 2070 cm − 1 respectively, in good agreement with sum frequency generation (SFG) measurements [26,27]. It is clear therefore that the rapid change in the ER intensity with the orientation of the adsorbed CN − occurs is entirely in line with changes occurring in the electronic configuration of that surface. The bonding of the cyanide to transition metal electrode surfaces is essentially ionic [29], with the CN axis parallel to the electric field direction [30]. Thus, for N-bound cyanide, the dipole moment will increase as the electrode potential is made more positive. On the other hand, for C-bound cyanide, the dipole is now oriented against the electric field and a decrease in the bond polarisation is found [16]. The distance of closest approach and hence the interfacial potential experienced by the adsorbed C-bound cyanide is thus considerably less than that of the N-bound cyanide and so, its effect on the metal surface electron density considerably reduced [27]. An alternative explanation for the change in ER signal could be a change in the microstructure of the surface. The cyanide is reported to adsorb in a hexago-
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nal (2 3 ×2 3)R30° arrangement with an additional CN in the centre of the ring [31,32]. The transition in the region of 0.5 V vs. RHE is explained by Friedrich et al. [31] in terms of a structural transformation into a metastable (2×2) structure with corresponding changes in the bonding or cyanide coverage of the surface. On the other hand, the explanation advanced by Kim et al. [32] is that a structural transition to a ( 7 × 7)R19° occurs in a narrow potential range centred near 0.5 V. It was further postulated that the centre of the hexagon was not a CN species but a hydronium cation and strong evidence for this was found in a parallel study of CN adsorption from KClO4 solutions [32]. It has furthermore been observed that cyanide adsorbs at non-equivalent sites on the platinum surface, viz. symmetric atop sites and near-top sites [33] which could lead to electronic variations in the electrode surface. Nevertheless, of these two alternatives, the change in the orientation of the adsorbed cyanide fits in more readily with the large intensity changes found in our electroreflectance data in the 0.5 V region. The structural transitions could well lead to changes in the ER intensity but it is doubtful whether the magnitudes would be comparable. The second zone observed in Fig. 5(b and c) occurs at potentials higher than 0.8 V and a maximum appears in the ER spectra. This maximum corresponds to the voltammetric wave observed in Fig. 5(a) and is associated with adsorption of oxygen-containing species [16]. The results obtained by electroreflectance in cyanide are in agreement with the interpretation given above in the case of perchloric acid.
3.4. Alkaline solutions Fig. 6 shows the cyclic voltammogram for a Pt(111) electrode in 0.1 M NaOH and the electroreflectance spectrum using p-polarised light. The cyclic voltammogram of Fig. 6(a) shows that for this platinum surface, three zones can again be distinguished clearly [34]. The region between 0.06 and 0.4 V corresponds, as in the case of acid media, to hydrogen adsorption-desorption processes. However here, the adsorbed hydrogen comes from water rather than from protons in solution and associated with this will be an extensive network of hydrogen-bonded water dipoles to the adsorbed hydrogen [22]. The second zone is the double layer region between 0.4 and 0.65 V. In the third zone, the broad peak between 0.65 and 0.9V would again appear to correspond to the adsorption/desorption of oxygenated species which is very reversible. The experimental 1/R((R/(E) data in this medium (Fig. 6(b)) was considerably more noisy than the previous ER data, making the interpretation that much more difficult. On the other hand, on integration, the DR/R signal shows the three distinct regions (dotted line in Fig. 6(b))
Fig. 6. (a) Voltammogram for a Pt(111) electrode in 0.1 M NaOH, n =50 mV s − 1. (b) Electroreflectance spectrum of a Pt(111) electrode in 0.1 M NaOH obtained with p-polarised light at 550 nm, dotted line: integrated data obtained in the positive sweep. Modulation frequency =43 Hz (100 mV p-p). n= 4 mV s − 1.
observed in the cyclic voltammogram of Fig. 6(a), particularly on the positive sweep. A slowly increasing background is found as the potential is made more positive, with small steps as the potential is swept into the different regions. These small changes in the modulated reflectance data would suggest that the nature of the species in the proximity of the electrode surface and hence, their electron-donating or withdrawing capability, varies very little with the applied potential and is thus concerned only with the reorientation of hydrogen-bonded surface species and water dipoles. This is in sharp contrast to the ER data in perchloric and sulphuric acid media where an increase in the ER data was observed beyond ca 0.35 V, the start of the double layer region. Thus in alkaline solutions, either water or oxygenated species such as OH − anions may always be present on the surface of the electrode in the different potential regions.
4. Conclusions The electroreflectance technique has been used to analyse the adsorption characteristics of a Pt(111) electrode in acidic and alkaline solutions. In sulphuric and
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perchloric acids, in the zone corresponding to the usual adsorption-desorption of hydrogen, the electroreflectance signal is practically zero and does not change. However, very different behaviour is observed at more positive potentials. In the case of sulphuric acid solutions, an increase in the ER signal is observed and this is associated with the adsorption-desorption of bisulphate anions. This result indicates that the adsorption of bisulphate increases the electronic density of the platinum surface. In the case of perchloric acid, the increase in the ER signal in this region is attributed to the adsorption of oxygenated species. The electroreflectance spectrum obtained for a cyanide-covered Pt(111) surface shows two clearly defined zones. At potentials lower than 0.6 V, a strong decrease in the ER signal was observed. This may indicate that the electronic density of the platinum surface is decreased as a consequence of back-donation from the platinum to the adsorbed cyanide. In alkaline solutions, the integrated electroreflectance spectra showed that small changes obtained corresponded well to the features observed in the cyclic voltammogram.
Acknowledgements C. Quijada and E. Morallo´n are indebted to the ‘Ministerio de Educacio´n y Cultura’ for the award of their F.P.I. postdoctoral fellowships. F. Huerta thanks the ‘Generalitat Valenciana’ for a thesis grant.
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