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In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on a platinum electrode
185
J. Electroanal. Gem., 272 (1989) 185-194
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on a platinum electrode Part 2. Mildly acid and alkaline sodium sulfate solutions K. Kunimatsu Research Institute for Catalysis, Hokkaido
University, Sapporo 060 (Japan)
M.G. Samaut and H. Seki * IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099
(U.S.A.)
(Received 27 February 1989; in revised form 22 May 1989)
ABSTRACT Adsorption of sulfate/bisulfate ions and water molecules on a platinum electrode has been studied in a mildly acid solution (pH = 3.4) of 0.5 M Na,SO, and H,S04 and an alkaline solution (pH = 11.5) of 0.5 M Na,SO, and NaOH by potential difference FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy). In the acidified sodium sulfate solution, sulfate ions and water molecules are adsorbed on top of the adsorbed hydrogen layer at 0.05 V/RHE. As the potential becomes more positive they are desorbed and replaced by bisulfate ions. The desorption of the sulfate ions is completed in the oxygen region. The number of sulfate ions and water molecules replaced by one bisulfate ion has been found to be constant throughout the entire potential range. In contrast to pure sulfuric acid solutions, the asymmetric S-O stretching frequency of the adsorbed bisulfate ions shows much smaller potential dependence. In the alkaline sodium sulfate solution, adsorption of sulfate ions increases continuously from 0.05 V/RHE to more positive potentials and saturates in the oxygen region, and no desorption of sulfate ions has been observed. The sulfate ion adsorption is accompanied by adsorption of water molecules. Only a very weak bisulfate band has been detected in the oxygen repion.
INTRODUCTION
In our previous papers [1,2] the potential dependent coadsorption of bisulfate and sulfate ions on a platinum electrode in sulfuric acid was reported by detecting the infrared active asymmetric S-O stretching vibration of the adsorbed HSO; and SOi- around 1200 cm-’ and 1100 cm-‘, respectively. The coadsorption was * To whom correspondence should be addressed. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
186
observed throughout the potential range studied between 0.05 V and 1.35 V/RHE. The sulfate ion adsorption was greatly reduced above 1.0 V/RHE, while the bisulfate ion adsorption continues to increase in the same potential range. Furthermore, the relative band intensities of the bisulfate and sulfate ions adsorbed on the Pt electrode appear not to be proportional to the relative concentration of the bisulfate and sulfate ions in 0.5 M sulfuric acid. In the present report, we have studied the adsorption of sulfate and bisulfate ions on a Pt electrode in 0.5 M sodium sulfate containing a small amount of sulfuric acid or sodium hydroxide. In these Na,SO, solutions of higher pH, the concentration of the sulfate ion is far greater than that of the bisulfate ion and we can study the coadsorption of bisulfate/sulfate ions over different potential regions by changing the pH level. Related electrochemical papers have been given in the previous reports [1,2] and are not repeated here. EXPERIMENTAL
Experimental details have been given in the previous reports [1,2]. Briefly, the sodium sulfate solutions were prepared from Na,SO, (Aldrich, gold label) and Nanopure water (Barnstead Nanopure system). The sodium sulfate solution was made acidic or alkaline by the addition of an appropriate amount of sulfuric acid or sodium hydroxide. The solutions thus prepared were (1) 0.5 M Na,SO, + 0.01 M H,SO, (pH = 3.4) and (2) 0.5 M Na,SO, + 0.0025 M NaOH (pH = 11.5). In solution (l), the ratio of the concentrations of bisulfate and sulfate ions, [SO:-]/[HSO;], is approximately 49, while in solution (2) the amount of bisulfate ion is negligible. All potentials are referenced to the RHE. RESULTS AND DISCUSSION
Mildly acid Na,SO,
Figure 1 shows the cyclic voltammogram of Pt in 0.5 M Na,SO,,+ 0.01 M H,SO, observed at the scan rate of 50 mV/s. The essential features of the voltammogram are very similar to those in sulfuric acid except that the hydrogen region is a little broader and extends almost up to 0.35 V. Figure 2 shows the potential dependence of the spectra for the spectral region between 1050 and 1900 cm- ‘. All spectra are referenced to the spectrum at 0.05 V. The spectral region for the asymmetric S-O stretching vibrations of HSO; and SO:-, has a bipolar characteristic with an upward bisulfate band around 1200 cm-’ and a downward sulfate band around 1100 cm-‘. This is significantly different from the case of the sulfuric acid where both bands increased with increasing potential [1,2]. We have confirmed that the water band and the bisulfate/sulfate bands are due to species adsorbed on the Pt electrode surface both by comparing the spectra taken by s- and p-polarized light and by examining the effect of CO preadsorption as was done in the case of the sulfuric acid solutions [1,2]. Additional measurements of the spectra by p-polarized light to longer wavelength using ZnSe windows showed
187 Pt/0.5MNa&O~+O,OIM
t!&O,
Fig. 1. Linear potential sweep voltammogram of Pt in 0.5 M Na2S04 +O.Ol M H2S04. Fig. 2. Potential dependence of the infrared spectra of ,HSO; , SOiadsorbed on Pt in 0.5 M Na,SO., +O.Ol M H,SO,.
and H,O(H-O-H
bending mode)
the bands around 1050 cm-’ and 950 cm-’ of the bisulfate ion. They have the same polarity as the 1200 cm-’ band and similarly, show a slight shift in frequency to higher wavenumbers with increasing potential. We note that there is a strong correlation in the potential dependence of the intensities of the three bands corresponding to the bisulfate/sulfate asymmetric S-O stretching modes and the H-O-H bending modes. This is seen in Fig. 3, where the potential dependence of the integrated band intensities of the bisulfate/sulfate and water bands are plotted, and in Fig. 4, where the intensities of the water band and the sulfate band are shown to be linearly dependent on the intensity of the bisulfate band. The fact that the water and the sulfate bands are of the opposite polarity with respect to the bisulfate band indicates that these two species are present on the Pt electrode at 0.05 V in the hydrogen region and desorb as the potential is made more positive. Assuming their intensity falls to zero at positive potentials, we conclude that the actual potential dependence of the three bands must be as shown in Fig. 5. A possible interpretation of this result is that the sulfate ion, adsorbed in the hydrogen region, is hydrated. Since the ratio of the sulfate ions and water molecules being replaced by the bisulfate ion is constant into the oxygen region as shown by Fig. 4, the sulfate ion and its hydration molecules as a unit must be replaced by a bisulfate ion as the electrode potential is made more positive. Figures 3 and 5 suggest that this process must take place in two stages. In the transition from the
188
0 HSO; l
0
so:-
so,‘- o” ?
0
II3-
0
J
F
E
2
5
r/lr.
0 /
H20
I
0.5
1.0
1.5 Intensity
E /V(RHE)
(a. u.). HSO;
Fig. 3. Potential dependence of the integrated intensities of the H-O-H bending mode of water molecules, the asymmetric S-O stretching bands of HSOi and SOiadsorbed on Pt in 0.5 M Na,SO, +O.Ol M H,SO,. Fig. 4. A plot of the integrated intensity of the bisulfate ions.
band intensities
of sulfate ions and of water molecules
against
the
hydrogen region to the double layer region, roughly 40% of the sulfate ions are replaced by or converted to bisulfate ions. The structure of the adsorbed hydrated sulfate ion is very likely to be different from that in solution. As one of the simplest model we can consider, a water molecule and sulfate ion hydrogen bonded to the hydrogen on platinum is shown in Fig. 6. The Pt surface is positively charged in the double layer region (see Fig. 8) so it is not unreasonable that after the hydrogen on the Pt surface is desorbed, the
E /V(RHE) Fig. 5. Potential dependence of the integrated band intensities of adsorbed sulfate molecules referred to the oxygen region, and of bisulfate ions referred to 0.05 V.
ions and water
189
Fig. 6. The proposed bonding mechanism of the coadsorption 0.05 V in 0.5 M Na,SO, +O.Ol M H,SO,.
of hydrogen,
Hz0
and SOi-
ion on Pt at
water molecule which is hydrogen bonded to the sulfate ion, attaches itself directly to the Pt via the oxygen. Another possibility is that the adsorbed sufate ion is associated with a fixed number of adjacently adsorbed water molecules which are desorbed as the sulfate ion is converted into or replaced by a bisulfate ion as the potential is made more positive. Again the simplest case would be one water molecule per sulfate ion. The potential dependence of the asymmetric S-O stretching frequencies of bisulfate and sulfate ions, respectively, determined from the peak positions of the bipolar bands in Fig. 2 are shown in Fig. 7. The asymmetric S-O stretching frequency of the bisulfate ions is almost constant at 1204 cm-’ in the hydrogen region and increases by only 5 cm- ’ in the double layer region to a relatively constant value of 1209 cm-’ in the oxygen region. The potential dependence of the bisulfate band position in the solution is significantly different from that in sulfuric acid [2] where the S-O stretching frequency increased by as much as 100 cm-‘/V in the double layer region and reached a maximum at 0.85 V.
P?/o.flM N02So4 +o.ol
M H2S04
HSa; (ads.1
0
I
I
0.5
1.0
I
I
1.5
E/V(RHEl
Fig. 7. Potential dependence of the asymmetric adsorbed on Pt in 0.5 M Na,SO, +O.Ol M H,SO,.
S-O
stretching
frequencies
of HSO;
and SO:-
Fig. 8. Potential dependence of the surface charge density on a platinized Pt electrode in 0.5 M Na,SO, acidified by sulfuric acid (pH = 2.2), and made alkaline by NaOH (pH = 12) [3].
190
We now compare the dependence of the S-O stretching frequency to the surface charge density determined by Frumkin and Petry [3] which is reproduced in Fig. 8 for a solution with a pH = 2.2, quite close to the pH of the solution used in the present study (pH = 3.4). It is interesting that the potential dependence of the S-O stretching frequency of the adsorbed sulfate and bisulfate ions shows no similarity to the potential dependence of the surface charge density. This is in strong contrast to the results of the highly acidic case reported earlier [2] where there is a close similarity between the behavior of the frequency and the surface charge density. The difference between potential dependence of the frequency of the sulfate and bisulfate band is much less for this solution.
Alkaline Na,SO, The equilibrium concentration of the bisulfate ion in the alkaline solution is practically zero and this is reflected in Fig. 9 which shows the potential dependence of the spectra for the bisulfate/sulfate region. We see only the broad, asymmetric S-O stretching band of sulfate ions which develops its intensity as the potential is made more positive and whose peak frequency shifts by a very small amount to a higher value. A very weak band, barely detectable, around 1230 cm-’ appears only at higher potentials in the oxygen region and is assigned to the adsorbed bisulfate ions. Measurements of the spectra to lower wavenumbers using ZnSe windows do not show the bisulfate bands around 1050 cm-’ or 900 cm-‘.
P1/0.5M
Na&O,
Pt/Q.SMNQps04 (pH=11.5)
,
1400
I
I
1300 1200 Wownumbers
I
1100 /cm-’
I
loo0
I
I
I
1300
1200
1100
Wavenumbers /cm-’
Fig. 9. Potential dependence of the asymmetric in 0.5 M Na,SO, + NaOH @H = 11.5). Fig. 10. The p- and s-polarized infrared Na,SO, +NaOH @H = 11.5) solution.
I
1400
S-O stretching band of SO:-
spectra of SOi-
at the interface
adsorbed on Pt electrode
between
Pt and 0.5 M
191
The intensity of the sulfate band is reduced greatly when s-polarized light is used instead of p-polarized light and the band position is also shifted to a lower wavenumber by ca. 40 cm-’ as shown in Fig. 10. The band position of the s-polarized spectrum is close to the band position of free sulfate ions [1,4] which is located at 1105 cm- ‘. The s-polarized spectrum suggests that there is a change in concentration of the sulfate ions between 0.05 V and 0.5 V in the thin solution layer between the Pt electrode and the CaF, window. The change in concentration in the solution is most likely due to double layer charging. The upward sense of the s-polarized spectrum means that the bulk sulfate concentration increases between 0.05 V and 0.5 V which is somewhat puzzling. According to the data of Frumkin and Petry [3], shown in Fig. 8, the surface charge density increases by ca. 12 PC/cm* between 0.05 V and 0.5 V which implies an increased adsorption of the sulfate ion on the electrode surface. For the thin layer cell, into which diffusion of additional solute is very slow, we would expect the bulk concentration to decrease. Furthermore, it is a little surprising to have an increased sulfate adsorption all the way into the oxygen region where the Pt surface is always negatively charged in this potential region. Figure 11 shows the potential dependence of the peak intensity of the sulfate band observed by p-polarized light. Also shown in Fig. 11 is the cyclic voltammogram observed at a scan rate of 50 mV/s toward the positive potential. The band intensity increases all the way from the hydrogen to the oxygen region and becomes constant. The increased sulfate ion adsorption as the potential becomes more positive cannot be explained in the usual way by the potential dependence of the surface charge density shown in Fig. 8. There should be a chemical effect that makes sulfate ion adsorption possible on Pt which is negatively charged.
N
I
Pt/0.5M Na2S04 lpH.ll.5)
Fig. 11. (A) The linear potential sweep voltammogram taken at 50 mV/s and (B) potential dependence the peak intensity of the asymmetric S-O stretching band of SO:adsorbed on Pt in 0.5 Na,SO, +NaOH (pH =11.5) solution.
of M
192 0 Pt-O-H---~-S
t
0 0
Fig. 12. The adsorption scheme of sulfate ions on Pt in the oxygen region in the alkaline 0.5 M sodium sulfate solution.
In contrast to the earlier results, the sulfate ions do not desorb even in the oxygen region in this alkaline sodium sulfate solution. As previously shown [2] adsorbed sulfate ions are replaced by bisulfate ions in the oxygen region in sulfuric acid as well as in the acidified 0.5 M sodium sulfate in the present study; see Fig. 5. The weak band detected around 1230 cm-’ in Fig. 9 does not show that bisulfate ions appear on the Pt surface at highly positive potentials above 1.1 V, but this has practically no effect on the adsorption of sulfate ions in the oxygen region. This suggests that the nature of the Pt surface in the oxygen region is different between the acidic and in the alkaline solutions. In the acidic solutions, the presence of an adsorbed oxygen layer explains the preference of bisulfate ion adsorption over sulfate ion adsorption, while the presence of the adsorbed sulfate ions at high positive potentials in the alkaline solution implies that the surface “oxygen species” is not O(a) but is more likely to be OH(a), on which sulfate ions can adsorb via hydrogen bonding as shown in Fig. 12. The reason why we have the increased sulfate ion adsorption throughout the hydrogen, double layer and the oxygen region despite the presence of negative surface charge on Pt is likely to be due to the formation of surface OH species in the hydrogen and double layer region. Indeed, as seen in Fig. 11, the double layer region is not clearly defined in the linear potential sweep voltammogram of this solution. Therefore, we are led to conclude that the oxygen region overlaps with the hydrogen region and the Pt surface gets covered by OH as the potential becomes more positive. Furthermore, we can hypothesize that the appearance of the bisulfate ion band, in spite of almost a complete absence of bisulfate ions in the solution, may be the result of a reaction of the following type: Pt-OH-SO;-
= Pt-0-HSO,-
In this reaction the sulfate ion reacts with the surface hydroxide group and forms a bisulfate ion which is attached by hydrogen bonding to the surface Pt-0. The band position shifts to higher wavenumber by ca. 17 cm-’ and becomes constant at 1130 cm-’ by 0.5 V, as shown in Fig. 13, in which the data from this solution are compared with those observed in sulfuric acid and the acidified 0.5 M sodium sulfate. The potential dependence of the asymmetric S-O stretching frequency of the adsorbed sulfate ions is very similar in all the solutions with different pH values studied so far. Finally, Fig. 14 shows the spectral change in the H-O-H bending region with the development of the sulfate band as the potential becomes more positive. It is interesting to note that, contrary to the acidic solutions studied so far, the water
Pt/0.5M 1
NagSO4
Pt/lHSa;/SO:-)
1200 -
i .I
f I ZQCQ 1500 Wavenumbers /cm-’
I 1000
Fig. 13. Potential dependence of the asymmetric S-O stretching frequenies of HSOL and SOi- adsorbed on Pt in (0) 0.5 M H2S04 @H = 0.29), (X)0.005 M H,SO, (pH =1.24), (A) 0.5 M Na2S04 +0.05 M H,SO, @H = 3.4) and (0) 0.5 M Na,SO, +NaOH (pH = 11.5). Fig. 14. The asymmetric S-O stretching bands of SOi- and H-O-H adsorbed on Pt in 0.5 M Na,SO, + NaOH (pH = 11.5).
bending mode of water molecules
band grows upward and the band position shifts to a higher wave number as compared to the acidic solutions by ca. 15 cm-‘. The positive intensity of the water band suggests clearly that we have more water molecules at more positive potentials compared to 0.05 V and the state of the water molecules is different, more perturbed, on platinum in the alkaline solution. The increase in the number of surface water molecules in close relation to the increase of the surface density of sulfate ions suggests that the sulfate ions may be hydrated by the water molecules.
SUMMARY
The present in-situ infrared spectroscopic study of the Pt/OS M Na,SO, interface has shown a very interesting adsorption/desorption behavior of hydrogen, water molecules, sulfate and bisulfate ions, and adsorbed 0 and OH. It is particularly interesting that the infrared spectroscopic data has strongly suggested that coadsorption of H, H,O and SO,‘- take place in the hydrogen region in the acidic 0.5 M Na,SO,. The surface bonding scheme proposed to explain such coadsorption is new for specific adsorption of anions, since the sulfate ion is not directly in contact with the Pt surface but is indirectly in contact via adsorbed hydrogen and water molecules.
194 ACKNOWLEDGEMENTS
We wish to express our thanks to J.G. Gordon, 0. Melroy and and M.R. Philpott for their interest, support and comments. The technical assistance of G.L. Borges and B.A. Hoenig was indispensible for execution of the experiments. This work was supported in part by the Office of Naval Research. REFERENCES 1 2 3 4
K. Kunimatsu, M. Samant, H. Seki and M.R. Philpott, J. Electroanal. Chem., 243 (1988) 203. K. Kunimatsu, M. Samant and H. Seki, J. Electroanal. Chem., 258 (1989) 163. A.N. Frumkin and O.A. Petry, Electrochim. Acta, 15 (1970) 391. K. Nakamoto, Infrared and Raman Spectrum of Inorganic and Coordination Compounds, 4th ed., Wiley-Interscience, New York, 1986.
J. Electroanal. Gem., 272 (1989) 185-194
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
In-situ FT-IR spectroscopic study of bisulfate and sulfate adsorption on a platinum electrode Part 2. Mildly acid and alkaline sodium sulfate solutions K. Kunimatsu Research Institute for Catalysis, Hokkaido
University, Sapporo 060 (Japan)
M.G. Samaut and H. Seki * IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099
(U.S.A.)
(Received 27 February 1989; in revised form 22 May 1989)
ABSTRACT Adsorption of sulfate/bisulfate ions and water molecules on a platinum electrode has been studied in a mildly acid solution (pH = 3.4) of 0.5 M Na,SO, and H,S04 and an alkaline solution (pH = 11.5) of 0.5 M Na,SO, and NaOH by potential difference FT-IRRAS (Fourier transform infrared reflection absorption spectroscopy). In the acidified sodium sulfate solution, sulfate ions and water molecules are adsorbed on top of the adsorbed hydrogen layer at 0.05 V/RHE. As the potential becomes more positive they are desorbed and replaced by bisulfate ions. The desorption of the sulfate ions is completed in the oxygen region. The number of sulfate ions and water molecules replaced by one bisulfate ion has been found to be constant throughout the entire potential range. In contrast to pure sulfuric acid solutions, the asymmetric S-O stretching frequency of the adsorbed bisulfate ions shows much smaller potential dependence. In the alkaline sodium sulfate solution, adsorption of sulfate ions increases continuously from 0.05 V/RHE to more positive potentials and saturates in the oxygen region, and no desorption of sulfate ions has been observed. The sulfate ion adsorption is accompanied by adsorption of water molecules. Only a very weak bisulfate band has been detected in the oxygen repion.
INTRODUCTION
In our previous papers [1,2] the potential dependent coadsorption of bisulfate and sulfate ions on a platinum electrode in sulfuric acid was reported by detecting the infrared active asymmetric S-O stretching vibration of the adsorbed HSO; and SOi- around 1200 cm-’ and 1100 cm-‘, respectively. The coadsorption was * To whom correspondence should be addressed. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
186
observed throughout the potential range studied between 0.05 V and 1.35 V/RHE. The sulfate ion adsorption was greatly reduced above 1.0 V/RHE, while the bisulfate ion adsorption continues to increase in the same potential range. Furthermore, the relative band intensities of the bisulfate and sulfate ions adsorbed on the Pt electrode appear not to be proportional to the relative concentration of the bisulfate and sulfate ions in 0.5 M sulfuric acid. In the present report, we have studied the adsorption of sulfate and bisulfate ions on a Pt electrode in 0.5 M sodium sulfate containing a small amount of sulfuric acid or sodium hydroxide. In these Na,SO, solutions of higher pH, the concentration of the sulfate ion is far greater than that of the bisulfate ion and we can study the coadsorption of bisulfate/sulfate ions over different potential regions by changing the pH level. Related electrochemical papers have been given in the previous reports [1,2] and are not repeated here. EXPERIMENTAL
Experimental details have been given in the previous reports [1,2]. Briefly, the sodium sulfate solutions were prepared from Na,SO, (Aldrich, gold label) and Nanopure water (Barnstead Nanopure system). The sodium sulfate solution was made acidic or alkaline by the addition of an appropriate amount of sulfuric acid or sodium hydroxide. The solutions thus prepared were (1) 0.5 M Na,SO, + 0.01 M H,SO, (pH = 3.4) and (2) 0.5 M Na,SO, + 0.0025 M NaOH (pH = 11.5). In solution (l), the ratio of the concentrations of bisulfate and sulfate ions, [SO:-]/[HSO;], is approximately 49, while in solution (2) the amount of bisulfate ion is negligible. All potentials are referenced to the RHE. RESULTS AND DISCUSSION
Mildly acid Na,SO,
Figure 1 shows the cyclic voltammogram of Pt in 0.5 M Na,SO,,+ 0.01 M H,SO, observed at the scan rate of 50 mV/s. The essential features of the voltammogram are very similar to those in sulfuric acid except that the hydrogen region is a little broader and extends almost up to 0.35 V. Figure 2 shows the potential dependence of the spectra for the spectral region between 1050 and 1900 cm- ‘. All spectra are referenced to the spectrum at 0.05 V. The spectral region for the asymmetric S-O stretching vibrations of HSO; and SO:-, has a bipolar characteristic with an upward bisulfate band around 1200 cm-’ and a downward sulfate band around 1100 cm-‘. This is significantly different from the case of the sulfuric acid where both bands increased with increasing potential [1,2]. We have confirmed that the water band and the bisulfate/sulfate bands are due to species adsorbed on the Pt electrode surface both by comparing the spectra taken by s- and p-polarized light and by examining the effect of CO preadsorption as was done in the case of the sulfuric acid solutions [1,2]. Additional measurements of the spectra by p-polarized light to longer wavelength using ZnSe windows showed
187 Pt/0.5MNa&O~+O,OIM
t!&O,
Fig. 1. Linear potential sweep voltammogram of Pt in 0.5 M Na2S04 +O.Ol M H2S04. Fig. 2. Potential dependence of the infrared spectra of ,HSO; , SOiadsorbed on Pt in 0.5 M Na,SO., +O.Ol M H,SO,.
and H,O(H-O-H
bending mode)
the bands around 1050 cm-’ and 950 cm-’ of the bisulfate ion. They have the same polarity as the 1200 cm-’ band and similarly, show a slight shift in frequency to higher wavenumbers with increasing potential. We note that there is a strong correlation in the potential dependence of the intensities of the three bands corresponding to the bisulfate/sulfate asymmetric S-O stretching modes and the H-O-H bending modes. This is seen in Fig. 3, where the potential dependence of the integrated band intensities of the bisulfate/sulfate and water bands are plotted, and in Fig. 4, where the intensities of the water band and the sulfate band are shown to be linearly dependent on the intensity of the bisulfate band. The fact that the water and the sulfate bands are of the opposite polarity with respect to the bisulfate band indicates that these two species are present on the Pt electrode at 0.05 V in the hydrogen region and desorb as the potential is made more positive. Assuming their intensity falls to zero at positive potentials, we conclude that the actual potential dependence of the three bands must be as shown in Fig. 5. A possible interpretation of this result is that the sulfate ion, adsorbed in the hydrogen region, is hydrated. Since the ratio of the sulfate ions and water molecules being replaced by the bisulfate ion is constant into the oxygen region as shown by Fig. 4, the sulfate ion and its hydration molecules as a unit must be replaced by a bisulfate ion as the electrode potential is made more positive. Figures 3 and 5 suggest that this process must take place in two stages. In the transition from the
188
0 HSO; l
0
so:-
so,‘- o” ?
0
II3-
0
J
F
E
2
5
r/lr.
0 /
H20
I
0.5
1.0
1.5 Intensity
E /V(RHE)
(a. u.). HSO;
Fig. 3. Potential dependence of the integrated intensities of the H-O-H bending mode of water molecules, the asymmetric S-O stretching bands of HSOi and SOiadsorbed on Pt in 0.5 M Na,SO, +O.Ol M H,SO,. Fig. 4. A plot of the integrated intensity of the bisulfate ions.
band intensities
of sulfate ions and of water molecules
against
the
hydrogen region to the double layer region, roughly 40% of the sulfate ions are replaced by or converted to bisulfate ions. The structure of the adsorbed hydrated sulfate ion is very likely to be different from that in solution. As one of the simplest model we can consider, a water molecule and sulfate ion hydrogen bonded to the hydrogen on platinum is shown in Fig. 6. The Pt surface is positively charged in the double layer region (see Fig. 8) so it is not unreasonable that after the hydrogen on the Pt surface is desorbed, the
E /V(RHE) Fig. 5. Potential dependence of the integrated band intensities of adsorbed sulfate molecules referred to the oxygen region, and of bisulfate ions referred to 0.05 V.
ions and water
189
Fig. 6. The proposed bonding mechanism of the coadsorption 0.05 V in 0.5 M Na,SO, +O.Ol M H,SO,.
of hydrogen,
Hz0
and SOi-
ion on Pt at
water molecule which is hydrogen bonded to the sulfate ion, attaches itself directly to the Pt via the oxygen. Another possibility is that the adsorbed sufate ion is associated with a fixed number of adjacently adsorbed water molecules which are desorbed as the sulfate ion is converted into or replaced by a bisulfate ion as the potential is made more positive. Again the simplest case would be one water molecule per sulfate ion. The potential dependence of the asymmetric S-O stretching frequencies of bisulfate and sulfate ions, respectively, determined from the peak positions of the bipolar bands in Fig. 2 are shown in Fig. 7. The asymmetric S-O stretching frequency of the bisulfate ions is almost constant at 1204 cm-’ in the hydrogen region and increases by only 5 cm- ’ in the double layer region to a relatively constant value of 1209 cm-’ in the oxygen region. The potential dependence of the bisulfate band position in the solution is significantly different from that in sulfuric acid [2] where the S-O stretching frequency increased by as much as 100 cm-‘/V in the double layer region and reached a maximum at 0.85 V.
P?/o.flM N02So4 +o.ol
M H2S04
HSa; (ads.1
0
I
I
0.5
1.0
I
I
1.5
E/V(RHEl
Fig. 7. Potential dependence of the asymmetric adsorbed on Pt in 0.5 M Na,SO, +O.Ol M H,SO,.
S-O
stretching
frequencies
of HSO;
and SO:-
Fig. 8. Potential dependence of the surface charge density on a platinized Pt electrode in 0.5 M Na,SO, acidified by sulfuric acid (pH = 2.2), and made alkaline by NaOH (pH = 12) [3].
190
We now compare the dependence of the S-O stretching frequency to the surface charge density determined by Frumkin and Petry [3] which is reproduced in Fig. 8 for a solution with a pH = 2.2, quite close to the pH of the solution used in the present study (pH = 3.4). It is interesting that the potential dependence of the S-O stretching frequency of the adsorbed sulfate and bisulfate ions shows no similarity to the potential dependence of the surface charge density. This is in strong contrast to the results of the highly acidic case reported earlier [2] where there is a close similarity between the behavior of the frequency and the surface charge density. The difference between potential dependence of the frequency of the sulfate and bisulfate band is much less for this solution.
Alkaline Na,SO, The equilibrium concentration of the bisulfate ion in the alkaline solution is practically zero and this is reflected in Fig. 9 which shows the potential dependence of the spectra for the bisulfate/sulfate region. We see only the broad, asymmetric S-O stretching band of sulfate ions which develops its intensity as the potential is made more positive and whose peak frequency shifts by a very small amount to a higher value. A very weak band, barely detectable, around 1230 cm-’ appears only at higher potentials in the oxygen region and is assigned to the adsorbed bisulfate ions. Measurements of the spectra to lower wavenumbers using ZnSe windows do not show the bisulfate bands around 1050 cm-’ or 900 cm-‘.
P1/0.5M
Na&O,
Pt/Q.SMNQps04 (pH=11.5)
,
1400
I
I
1300 1200 Wownumbers
I
1100 /cm-’
I
loo0
I
I
I
1300
1200
1100
Wavenumbers /cm-’
Fig. 9. Potential dependence of the asymmetric in 0.5 M Na,SO, + NaOH @H = 11.5). Fig. 10. The p- and s-polarized infrared Na,SO, +NaOH @H = 11.5) solution.
I
1400
S-O stretching band of SO:-
spectra of SOi-
at the interface
adsorbed on Pt electrode
between
Pt and 0.5 M
191
The intensity of the sulfate band is reduced greatly when s-polarized light is used instead of p-polarized light and the band position is also shifted to a lower wavenumber by ca. 40 cm-’ as shown in Fig. 10. The band position of the s-polarized spectrum is close to the band position of free sulfate ions [1,4] which is located at 1105 cm- ‘. The s-polarized spectrum suggests that there is a change in concentration of the sulfate ions between 0.05 V and 0.5 V in the thin solution layer between the Pt electrode and the CaF, window. The change in concentration in the solution is most likely due to double layer charging. The upward sense of the s-polarized spectrum means that the bulk sulfate concentration increases between 0.05 V and 0.5 V which is somewhat puzzling. According to the data of Frumkin and Petry [3], shown in Fig. 8, the surface charge density increases by ca. 12 PC/cm* between 0.05 V and 0.5 V which implies an increased adsorption of the sulfate ion on the electrode surface. For the thin layer cell, into which diffusion of additional solute is very slow, we would expect the bulk concentration to decrease. Furthermore, it is a little surprising to have an increased sulfate adsorption all the way into the oxygen region where the Pt surface is always negatively charged in this potential region. Figure 11 shows the potential dependence of the peak intensity of the sulfate band observed by p-polarized light. Also shown in Fig. 11 is the cyclic voltammogram observed at a scan rate of 50 mV/s toward the positive potential. The band intensity increases all the way from the hydrogen to the oxygen region and becomes constant. The increased sulfate ion adsorption as the potential becomes more positive cannot be explained in the usual way by the potential dependence of the surface charge density shown in Fig. 8. There should be a chemical effect that makes sulfate ion adsorption possible on Pt which is negatively charged.
N
I
Pt/0.5M Na2S04 lpH.ll.5)
Fig. 11. (A) The linear potential sweep voltammogram taken at 50 mV/s and (B) potential dependence the peak intensity of the asymmetric S-O stretching band of SO:adsorbed on Pt in 0.5 Na,SO, +NaOH (pH =11.5) solution.
of M
192 0 Pt-O-H---~-S
t
0 0
Fig. 12. The adsorption scheme of sulfate ions on Pt in the oxygen region in the alkaline 0.5 M sodium sulfate solution.
In contrast to the earlier results, the sulfate ions do not desorb even in the oxygen region in this alkaline sodium sulfate solution. As previously shown [2] adsorbed sulfate ions are replaced by bisulfate ions in the oxygen region in sulfuric acid as well as in the acidified 0.5 M sodium sulfate in the present study; see Fig. 5. The weak band detected around 1230 cm-’ in Fig. 9 does not show that bisulfate ions appear on the Pt surface at highly positive potentials above 1.1 V, but this has practically no effect on the adsorption of sulfate ions in the oxygen region. This suggests that the nature of the Pt surface in the oxygen region is different between the acidic and in the alkaline solutions. In the acidic solutions, the presence of an adsorbed oxygen layer explains the preference of bisulfate ion adsorption over sulfate ion adsorption, while the presence of the adsorbed sulfate ions at high positive potentials in the alkaline solution implies that the surface “oxygen species” is not O(a) but is more likely to be OH(a), on which sulfate ions can adsorb via hydrogen bonding as shown in Fig. 12. The reason why we have the increased sulfate ion adsorption throughout the hydrogen, double layer and the oxygen region despite the presence of negative surface charge on Pt is likely to be due to the formation of surface OH species in the hydrogen and double layer region. Indeed, as seen in Fig. 11, the double layer region is not clearly defined in the linear potential sweep voltammogram of this solution. Therefore, we are led to conclude that the oxygen region overlaps with the hydrogen region and the Pt surface gets covered by OH as the potential becomes more positive. Furthermore, we can hypothesize that the appearance of the bisulfate ion band, in spite of almost a complete absence of bisulfate ions in the solution, may be the result of a reaction of the following type: Pt-OH-SO;-
= Pt-0-HSO,-
In this reaction the sulfate ion reacts with the surface hydroxide group and forms a bisulfate ion which is attached by hydrogen bonding to the surface Pt-0. The band position shifts to higher wavenumber by ca. 17 cm-’ and becomes constant at 1130 cm-’ by 0.5 V, as shown in Fig. 13, in which the data from this solution are compared with those observed in sulfuric acid and the acidified 0.5 M sodium sulfate. The potential dependence of the asymmetric S-O stretching frequency of the adsorbed sulfate ions is very similar in all the solutions with different pH values studied so far. Finally, Fig. 14 shows the spectral change in the H-O-H bending region with the development of the sulfate band as the potential becomes more positive. It is interesting to note that, contrary to the acidic solutions studied so far, the water
Pt/0.5M 1
NagSO4
Pt/lHSa;/SO:-)
1200 -
i .I
f I ZQCQ 1500 Wavenumbers /cm-’
I 1000
Fig. 13. Potential dependence of the asymmetric S-O stretching frequenies of HSOL and SOi- adsorbed on Pt in (0) 0.5 M H2S04 @H = 0.29), (X)0.005 M H,SO, (pH =1.24), (A) 0.5 M Na2S04 +0.05 M H,SO, @H = 3.4) and (0) 0.5 M Na,SO, +NaOH (pH = 11.5). Fig. 14. The asymmetric S-O stretching bands of SOi- and H-O-H adsorbed on Pt in 0.5 M Na,SO, + NaOH (pH = 11.5).
bending mode of water molecules
band grows upward and the band position shifts to a higher wave number as compared to the acidic solutions by ca. 15 cm-‘. The positive intensity of the water band suggests clearly that we have more water molecules at more positive potentials compared to 0.05 V and the state of the water molecules is different, more perturbed, on platinum in the alkaline solution. The increase in the number of surface water molecules in close relation to the increase of the surface density of sulfate ions suggests that the sulfate ions may be hydrated by the water molecules.
SUMMARY
The present in-situ infrared spectroscopic study of the Pt/OS M Na,SO, interface has shown a very interesting adsorption/desorption behavior of hydrogen, water molecules, sulfate and bisulfate ions, and adsorbed 0 and OH. It is particularly interesting that the infrared spectroscopic data has strongly suggested that coadsorption of H, H,O and SO,‘- take place in the hydrogen region in the acidic 0.5 M Na,SO,. The surface bonding scheme proposed to explain such coadsorption is new for specific adsorption of anions, since the sulfate ion is not directly in contact with the Pt surface but is indirectly in contact via adsorbed hydrogen and water molecules.
194 ACKNOWLEDGEMENTS
We wish to express our thanks to J.G. Gordon, 0. Melroy and and M.R. Philpott for their interest, support and comments. The technical assistance of G.L. Borges and B.A. Hoenig was indispensible for execution of the experiments. This work was supported in part by the Office of Naval Research. REFERENCES 1 2 3 4
K. Kunimatsu, M. Samant, H. Seki and M.R. Philpott, J. Electroanal. Chem., 243 (1988) 203. K. Kunimatsu, M. Samant and H. Seki, J. Electroanal. Chem., 258 (1989) 163. A.N. Frumkin and O.A. Petry, Electrochim. Acta, 15 (1970) 391. K. Nakamoto, Infrared and Raman Spectrum of Inorganic and Coordination Compounds, 4th ed., Wiley-Interscience, New York, 1986.