- Email: [email protected]
In situ FTIR study of pyridine adsorbed on a polycrystalline gold electrode
PII:
Electrochimica Acta, Vol. 43, Nos 21±22, pp. 3297±3301, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00049-8 0013±4686/98 $19.00 + 0.00
In situ FTIR study of pyridine adsorbed on a polycrystalline gold electrode Yasunari Ikezawa*, Tatsuro Sawatari, Takahiro Kitazume Hiroshi Goto and Koji Toriba Department of Chemistry, Faculty of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-Ku, Tokyo 171, Japan (Received 2 September 1997; in revised form 19 January 1998) AbstractÐIn situ IR spectra of pyridine adsorbed on a polycrystalline gold electrode in 1 M NaF have been measured. An in-plane ring symmetrical vibration due to vertically adsorbed pyridine was observed at around 1605 cmÿ1 accompanying a decrease of the same mode band at 1596 cmÿ1 due to dissolved pyridine. The position and shift of the bands due to vertically adsorbed pyridine were independent of the concentration of pyridine, leading us to deduce that it is caused by a weak Stark eect and/or an eect of the electrode potential on the nonbonding orbital of the nitrogen atom. The vertically adsorbed pyridine was observed at 0.7 V (vs Ag/AgCl), at which potential detection of adsorbed pyridine by an electrochemical method has not been reported. A decrease in an asymmetric band at 1446 cmÿ1 due to dissolved pyridine was also observed at more positive potentials. # 1998 Elsevier Science Ltd. All rights reserved Key words: pyridine, adsorption, IRAS, FTIR, gold electrode.
INTRODUCTION Pyridine adsorption on polycrystalline and single crystal gold electrodes has drawn considerable attention as a possible model of adsorbate coordination to a surface by electrochemical methods [1± 5]. The most distinct coordination would involve either direct binding of the pyridine ring to the surface (¯at orientation) or a surface bond through the nonbonding orbital of the nitrogen atom in the pyridine ring (vertical orientation). At negative potentials, the ¯at orientation is favorable; the vertical orientation increases as the potential is changed in the positive direction. Recently, IRAS techniques have been used to study various processes at the electrode±solution interface. These techniques oer detailed information about the molecular orientations of adsorbed species and their lateral interactions and amounts. In the present study, we present in situ FTIR spectra on pyridine adsorption at a polycrystalline *Author to whom correspondence should be addressed. E-mail: [email protected]
gold/solution interface. The nature of the adsorbed species is discussed on the basis of spectral data obtained at dierent potentials and pyridine concentrations and the results are compared with those obtained by electrochemical methods. EXPERIMENTAL The electrochemical cell used was the same as that described previously [6]. The IR window was CaF2 beveled at 658. The spectroscopic measurements were made with a JIR 5500 spectrometer (JEOL) equipped with an MCT detector (JUDSON). A modi®ed ATR attachment and a polarizer were used. Potential control and current measurement were accomplished with an H-151 (Hokuto Denko) potentiostat connected to a personal computer. The working electrode was a polycrystalline Au electrode (10 mm in diameter) mounted on the bottom face of a PTFE rod. The electrode was mirror®nished with 0.05 mm alumina powder and cleaned by steam washing. The counter-electrode used was a Pt wire and an Ag/AgCl (saturated KCl) electrode was used as a reference electrode. The electrolytic
3297
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solution was prepared with NaF (Wako) and Millipore water, 1 M NaF was used because of the conductivity of the thin layer cell. Subtractively normalized interfacial FT-JR (SNIFTIR) spectra were obtained in the presence of pyridine by recording the sample and reference spectra alternatively at two dierent potentials, 500 interferograms (2 scans sÿ1, 4 cmÿ1 resolution) were recorded at each potential with the voltage being switched every 20 scans. The interferograms were added, transformed and used to calculated a relative change of the electrode re¯ectivity, which is de®ned as: DA ÿlogR E1 =R E2 , where DA is relative adsorbance, R(E1) and R(E2) are the electrode re¯ectivity at potentials E1 and E2 (reference), respectively. Prior to the measurements, the Au electrode was cleaned by repeated oxidation reduction cycles from ÿ0.7 to 1.4 V (vs Ag/AgCl) until a reproducible voltammogram in 1 M NaF under nitrogen gas was obtained. After cleaning, pyridine was added immediately at ÿ0.7 V. The Au electrode was pushed against the CaF2 window and SNIFTIRS measurements were performed. RESULTS AND DISCUSSION Identi®cation of the adsorbate and solution features The p-polarized light spectra with a reference potential at ÿ0.7 V (vs Ag/AgCl) in 1 M NaF containing 10 mM pyridine are shown in Fig. 1. A bipolar band around 1596 and 1605 cmÿ1, a negative goingband at 1446 cmÿ1 and a bipolar band around 1481 and 1487 cmÿ1 appeared as the potential was changed in the positive direction. A broad negative going-band at 1400 cmÿ1 was also observed in pyridine-free solution and may be due to carbonate anion in the solution. In order to assign these bands, the same measurements were performed in 10 mM pyridine-d5 solution, as shown in Fig. 2. The bands observed at 1562, 1554 and 1309 cmÿ1 are shifted in a lower wave number direction in comparison with pyridine-h5, indicating that these bands are clearly due to pyridine-d5. The band positions are shown in Table 1 along with liquid pyridine data [7]. The isotopic shifts are similar to those of liquid samples, the bipolar and positive goingbands were assigned to an in-plane symmetric ring stretch [nsym(C±C)] and the negative going-band is assigned to an inplane asymmetric ring stretch [nasy(C±C)] according to previous assignments [7]. The band corresponding to 1487 cmÿ1 of pyridineh5 was not observed, which would be expected at around 1340 cmÿ1, since the intensity is expected to be weak (Table 1). According to the surface selection rule, only vibrations having a dipole component normal to the surface can interact with the p-polarized light. In
Fig. 1. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine with a reference potential of ÿ0.7 V. The addition of pyridine was performed at ÿ0.7 V.
Fig. 2. IRA spectra of adsorbed pyridine-d5 on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine-d5 with a reference potential of ÿ0.7 V. The addition of pyridine was performed at ÿ0.7 V.
FTIR study of pyridine
3299
Table 1. IR data for pyridine and pyridine-d5 Neat pyridine/cmÿ1 Class
np
A1 A1 B1
n4 n9 n18
h5 vs s vs
1583 1482 1439
vs w s
Adsorbed pyridine/cmÿ1
d5
D
h5
d5
D
1530 1340 1301
53 142 138
1599±1605 1481±1497 1446
1558±1562 ÿ 1311
42
vertically orientated pyridine (Pyv) with the nitrogen atom facing the electrode, the in-plane symmetric modes can be observed, but the in-plane asymmetric modes and the out of plane modes cannot be observed. In order to clarify the origin of the bands, spolarized light spectra were measured (Fig. 3). Both negative going-bands at 1596 and 1446 cmÿ1 were observed and the positive going-bands at 1605 and 1487 cmÿ1 were not observed. From the surface selection rule, these negative going-bands were assigned to the loss of solution pyridine in the thin layer solution resulting from increasing adsorption as the potential is altered from the base to more positive sample values. The loss of solution pyridine increases when changing the potential in the positive direction. In Fig. 3, the positive going-bands observed by p-polarized light were not detected. On the surface selection rule, it is reasonable that these bands assigned to the in-plane symmetrical mode are due to the vertically adsorbed pyridine. Therefore, the bipolar bands observed in Fig. 1
135
were composed with the positive going part due to Pyv and the negative going part due to solution pyridine. Similar bipolar bands have been reported for a sulfate/gold electrode system [8]. For the bipolar band around 1596 and 1605 cmÿ1, the bands due to adsorption of pyridine are located at wave numbers higher than those of the solution species. Blue shifts of the positive going-bands at 1605 cmÿ1 were observed with increasing potential and the band intensities increased as the potential was changed in the positive direction. For the bipolar band around 1481 and 1487 cmÿ1, the bands due to adsorption of pyridine are located at wave numbers lower than those of the solution species. Blue shifts of the positive going-bands at 1487 cmÿ1 were also observed with increasing potential and, as a result, the bands are overlapped completely with the dissolved pyridine at positive potentials. Consequently, the band intensity is not increased with increasing potential. It should be pointed out that the ¯at oriented adsorbed pyridine (PyF) cannot be observed on the base of the surface selection rule, however this type of adsorption also causes the loss of solution pyridine. BANDS DUE TO PYRIDINE IN SOLUTION The intensity ratios of the negative going-bands at 1596 and 1446 cmÿ1 in p-polarized light (Fig. 1) were dierent from those in s-polarized light (Fig. 3). It was presumed that a part of the intensity of the 1595 cmÿ1 band is due to Pyv at the reference potential. An IRA spectra measurement with a reference potential of ÿ0.8 V was performed (Fig. 4). The band intensity of 1594 cmÿ1 is smaller than that of Fig. 1, indicating that the 1596 cmÿ1 band in Fig. 1 is due to loss of solution pyridine and Pyv at the reference potential. COMPARISON WITH ELECTROCHEMICAL METHODS
Fig. 3. IRA spectra by s-polarized light. Other conditions were the same as in Fig. 1.
In a previous study for pyridine adsorption at a polycrystalline electrode, chronocoulometry, radiochemistry and Raman spectroscopy were used. The chronocoulometry technique is more precious than radiochemical method and Raman spectroscopy for ideally polarized electrode [5]. In the chronocoulometric study, pyridine on a polycrystalline Au electrode was not adsorbed at about ÿ0.66 V in
3300
Y. Ikezawa et al. IRAS study, however, the intermediate orientation could not be detected. The amount of loss of solution pyridine is proportional to the amount of all adsorbed pyridine. Hence, the band intensities at 1605 cmÿ1 due to Pyv with p-polarized light and at 1446 cmÿ1 due to the loss of solution pyridine with s-polarized light were plotted as a function of the electrode potential, as shown in Fig. 5. Assuming that the orientation of adsorbed pyridine is only the vertical type at around 0.3 V, the dierence in the two curves corresponds to adsorbed pyridine other than Pyv; this may be the ¯at type. The ¯at type is stable at negative potentials with the same results as the electrochemical method. PYRIDINE CONCENTRATION DEPENDENCY ON IRAS
Fig. 4. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine with a reference potential of ÿ0.8 V. The addition of pyridine was performed at ÿ0.7 V.
pyridine solutions lower than 5 mM, PyF increased up to ÿ0.21 V with increasing positive potential and Pyv increased gradually up to 0.46 V in the potential region more positive than ÿ0.21 V [2]. On the other hand, Pyv appeared at ÿ0.7 V in this IRAS study, clearly dierent from the chronocoulometry results. In another chronocoulometric study on a single crystalline (100)Au electrode, other orientations, presumably intermediate between the ¯at and the vertical orientation, have been observed at intermediate surface concentrations [3]. In the present
Fig. 5. Relative band intensities as a function of potential. w: 1601 cmÿ1 band with p-polarized light, q: 1446 cmÿ1 band with s-polarized light.
In order to clarify the pyridine concentration dependency on the amounts of Pyv, IRA spectra with a reference potential of ÿ0.7 V were measured in 1 and 3 mM pyridine solutions, as shown in Figs 6 and 7, respectively. The 1446 cmÿ1 band corresponding to the total amount of adsorbed pyridine and the 1605 cmÿ1 band due to Pyv decreased with decreasing pyridine concentration. In the chronocoulometry study, the amount of ¯at oriented pyridine as a function of the electrode potential was dependent of the bulk pyridine concen-
Fig. 6. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 1 mM pyridine. Other conditions were the same as in Fig. 1.
FTIR study of pyridine
3301
®eld in the inner double layer. Another possibility is due to a change in the charge density of the nonbonding orbital in the nitrogen atom with potential changes. In a chronocoulometric study on an Au (110) electrode, pyridine molecules were adsorbed vertically, as explained by the Frumkin isotherm [4]. The lateral interaction parameter in the equation depends on the electrode potential; it may be related to the band shift observed by IRAS. CONCLUSION The IRA spectra of pyridine adsorbed on a polycrystalline Au electrode in 1 M NaF containing pyridine were measured. A symmetric vibration observed around 1603 cmÿ1 was due to vertically adsorbed pyridine and a band shift with changing electrode potential was observed. The potential induced band shift of an in-plane symmetric ring stretch was observed in all concentrations, it seems that the shift is caused by a weak Stark eect or the change of the charge density of the nonbonding orbital in the nitrogen atom with potential changes. Fig. 7. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 3 mM pyridine. Other conditions are the same as in Fig. 1.
tration, however, the amount of vertically adsorbed pyridine was independent [5]. The shift of the band due to Pyv was independent of the pyridine concentration, however a slight potential dependency of the band position (approximately 6 cmÿ1/V) was observed in all pyridine concentrations, indicating that the shift is caused only by potentials rather than by lateral interaction between adsorbed pyridine or dipole coupling. A weak Stark eect [9] is expected because only a part of the pyridine ring of Pyv is aected by the electric
REFERENCES 1. B. E. Conway, R. G. Barradas, P. G. Hamilton and J. M. Parry, J. Electroanal. Chem. 10, 485 (1965). 2. L. Stolberg, J. Richer and J. Lipkowski, J. Electroanal. Chem. 207, 213 (1986). 3. L. Stolberg and J. Lipkowski, J. Electroanal. Chem. 238, 333 (1987). 4. L. Stolberg, J. Lipkowski and D. E. Irish, J. Electroanal. Chem. 296, 171 (1990). 5. J. Lipkowski, L. Stolberg, S. Morin and D. E. Irish, J. Electroanal. Chem. 355, 147 (1993). 6. Y. Ikezawa, H. Saito, H. Fujisawa and G. Toda, J. Electroanal. Chem. 240, 281 (1988). 7. D. A. Long, F. S. Mur®n and E. L. Thomas, Trans. Faraday Soc. 59, 12 (1963). 8. M. Weber and F. C. Nart, Langmuir 12, 1895 (1996). 9. D. K. Lambert, Solid State Commun. 51, 297 (1984).
Electrochimica Acta, Vol. 43, Nos 21±22, pp. 3297±3301, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S0013-4686(98)00049-8 0013±4686/98 $19.00 + 0.00
In situ FTIR study of pyridine adsorbed on a polycrystalline gold electrode Yasunari Ikezawa*, Tatsuro Sawatari, Takahiro Kitazume Hiroshi Goto and Koji Toriba Department of Chemistry, Faculty of Science, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-Ku, Tokyo 171, Japan (Received 2 September 1997; in revised form 19 January 1998) AbstractÐIn situ IR spectra of pyridine adsorbed on a polycrystalline gold electrode in 1 M NaF have been measured. An in-plane ring symmetrical vibration due to vertically adsorbed pyridine was observed at around 1605 cmÿ1 accompanying a decrease of the same mode band at 1596 cmÿ1 due to dissolved pyridine. The position and shift of the bands due to vertically adsorbed pyridine were independent of the concentration of pyridine, leading us to deduce that it is caused by a weak Stark eect and/or an eect of the electrode potential on the nonbonding orbital of the nitrogen atom. The vertically adsorbed pyridine was observed at 0.7 V (vs Ag/AgCl), at which potential detection of adsorbed pyridine by an electrochemical method has not been reported. A decrease in an asymmetric band at 1446 cmÿ1 due to dissolved pyridine was also observed at more positive potentials. # 1998 Elsevier Science Ltd. All rights reserved Key words: pyridine, adsorption, IRAS, FTIR, gold electrode.
INTRODUCTION Pyridine adsorption on polycrystalline and single crystal gold electrodes has drawn considerable attention as a possible model of adsorbate coordination to a surface by electrochemical methods [1± 5]. The most distinct coordination would involve either direct binding of the pyridine ring to the surface (¯at orientation) or a surface bond through the nonbonding orbital of the nitrogen atom in the pyridine ring (vertical orientation). At negative potentials, the ¯at orientation is favorable; the vertical orientation increases as the potential is changed in the positive direction. Recently, IRAS techniques have been used to study various processes at the electrode±solution interface. These techniques oer detailed information about the molecular orientations of adsorbed species and their lateral interactions and amounts. In the present study, we present in situ FTIR spectra on pyridine adsorption at a polycrystalline *Author to whom correspondence should be addressed. E-mail: [email protected]
gold/solution interface. The nature of the adsorbed species is discussed on the basis of spectral data obtained at dierent potentials and pyridine concentrations and the results are compared with those obtained by electrochemical methods. EXPERIMENTAL The electrochemical cell used was the same as that described previously [6]. The IR window was CaF2 beveled at 658. The spectroscopic measurements were made with a JIR 5500 spectrometer (JEOL) equipped with an MCT detector (JUDSON). A modi®ed ATR attachment and a polarizer were used. Potential control and current measurement were accomplished with an H-151 (Hokuto Denko) potentiostat connected to a personal computer. The working electrode was a polycrystalline Au electrode (10 mm in diameter) mounted on the bottom face of a PTFE rod. The electrode was mirror®nished with 0.05 mm alumina powder and cleaned by steam washing. The counter-electrode used was a Pt wire and an Ag/AgCl (saturated KCl) electrode was used as a reference electrode. The electrolytic
3297
3298
Y. Ikezawa et al.
solution was prepared with NaF (Wako) and Millipore water, 1 M NaF was used because of the conductivity of the thin layer cell. Subtractively normalized interfacial FT-JR (SNIFTIR) spectra were obtained in the presence of pyridine by recording the sample and reference spectra alternatively at two dierent potentials, 500 interferograms (2 scans sÿ1, 4 cmÿ1 resolution) were recorded at each potential with the voltage being switched every 20 scans. The interferograms were added, transformed and used to calculated a relative change of the electrode re¯ectivity, which is de®ned as: DA ÿlogR E1 =R E2 , where DA is relative adsorbance, R(E1) and R(E2) are the electrode re¯ectivity at potentials E1 and E2 (reference), respectively. Prior to the measurements, the Au electrode was cleaned by repeated oxidation reduction cycles from ÿ0.7 to 1.4 V (vs Ag/AgCl) until a reproducible voltammogram in 1 M NaF under nitrogen gas was obtained. After cleaning, pyridine was added immediately at ÿ0.7 V. The Au electrode was pushed against the CaF2 window and SNIFTIRS measurements were performed. RESULTS AND DISCUSSION Identi®cation of the adsorbate and solution features The p-polarized light spectra with a reference potential at ÿ0.7 V (vs Ag/AgCl) in 1 M NaF containing 10 mM pyridine are shown in Fig. 1. A bipolar band around 1596 and 1605 cmÿ1, a negative goingband at 1446 cmÿ1 and a bipolar band around 1481 and 1487 cmÿ1 appeared as the potential was changed in the positive direction. A broad negative going-band at 1400 cmÿ1 was also observed in pyridine-free solution and may be due to carbonate anion in the solution. In order to assign these bands, the same measurements were performed in 10 mM pyridine-d5 solution, as shown in Fig. 2. The bands observed at 1562, 1554 and 1309 cmÿ1 are shifted in a lower wave number direction in comparison with pyridine-h5, indicating that these bands are clearly due to pyridine-d5. The band positions are shown in Table 1 along with liquid pyridine data [7]. The isotopic shifts are similar to those of liquid samples, the bipolar and positive goingbands were assigned to an in-plane symmetric ring stretch [nsym(C±C)] and the negative going-band is assigned to an inplane asymmetric ring stretch [nasy(C±C)] according to previous assignments [7]. The band corresponding to 1487 cmÿ1 of pyridineh5 was not observed, which would be expected at around 1340 cmÿ1, since the intensity is expected to be weak (Table 1). According to the surface selection rule, only vibrations having a dipole component normal to the surface can interact with the p-polarized light. In
Fig. 1. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine with a reference potential of ÿ0.7 V. The addition of pyridine was performed at ÿ0.7 V.
Fig. 2. IRA spectra of adsorbed pyridine-d5 on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine-d5 with a reference potential of ÿ0.7 V. The addition of pyridine was performed at ÿ0.7 V.
FTIR study of pyridine
3299
Table 1. IR data for pyridine and pyridine-d5 Neat pyridine/cmÿ1 Class
np
A1 A1 B1
n4 n9 n18
h5 vs s vs
1583 1482 1439
vs w s
Adsorbed pyridine/cmÿ1
d5
D
h5
d5
D
1530 1340 1301
53 142 138
1599±1605 1481±1497 1446
1558±1562 ÿ 1311
42
vertically orientated pyridine (Pyv) with the nitrogen atom facing the electrode, the in-plane symmetric modes can be observed, but the in-plane asymmetric modes and the out of plane modes cannot be observed. In order to clarify the origin of the bands, spolarized light spectra were measured (Fig. 3). Both negative going-bands at 1596 and 1446 cmÿ1 were observed and the positive going-bands at 1605 and 1487 cmÿ1 were not observed. From the surface selection rule, these negative going-bands were assigned to the loss of solution pyridine in the thin layer solution resulting from increasing adsorption as the potential is altered from the base to more positive sample values. The loss of solution pyridine increases when changing the potential in the positive direction. In Fig. 3, the positive going-bands observed by p-polarized light were not detected. On the surface selection rule, it is reasonable that these bands assigned to the in-plane symmetrical mode are due to the vertically adsorbed pyridine. Therefore, the bipolar bands observed in Fig. 1
135
were composed with the positive going part due to Pyv and the negative going part due to solution pyridine. Similar bipolar bands have been reported for a sulfate/gold electrode system [8]. For the bipolar band around 1596 and 1605 cmÿ1, the bands due to adsorption of pyridine are located at wave numbers higher than those of the solution species. Blue shifts of the positive going-bands at 1605 cmÿ1 were observed with increasing potential and the band intensities increased as the potential was changed in the positive direction. For the bipolar band around 1481 and 1487 cmÿ1, the bands due to adsorption of pyridine are located at wave numbers lower than those of the solution species. Blue shifts of the positive going-bands at 1487 cmÿ1 were also observed with increasing potential and, as a result, the bands are overlapped completely with the dissolved pyridine at positive potentials. Consequently, the band intensity is not increased with increasing potential. It should be pointed out that the ¯at oriented adsorbed pyridine (PyF) cannot be observed on the base of the surface selection rule, however this type of adsorption also causes the loss of solution pyridine. BANDS DUE TO PYRIDINE IN SOLUTION The intensity ratios of the negative going-bands at 1596 and 1446 cmÿ1 in p-polarized light (Fig. 1) were dierent from those in s-polarized light (Fig. 3). It was presumed that a part of the intensity of the 1595 cmÿ1 band is due to Pyv at the reference potential. An IRA spectra measurement with a reference potential of ÿ0.8 V was performed (Fig. 4). The band intensity of 1594 cmÿ1 is smaller than that of Fig. 1, indicating that the 1596 cmÿ1 band in Fig. 1 is due to loss of solution pyridine and Pyv at the reference potential. COMPARISON WITH ELECTROCHEMICAL METHODS
Fig. 3. IRA spectra by s-polarized light. Other conditions were the same as in Fig. 1.
In a previous study for pyridine adsorption at a polycrystalline electrode, chronocoulometry, radiochemistry and Raman spectroscopy were used. The chronocoulometry technique is more precious than radiochemical method and Raman spectroscopy for ideally polarized electrode [5]. In the chronocoulometric study, pyridine on a polycrystalline Au electrode was not adsorbed at about ÿ0.66 V in
3300
Y. Ikezawa et al. IRAS study, however, the intermediate orientation could not be detected. The amount of loss of solution pyridine is proportional to the amount of all adsorbed pyridine. Hence, the band intensities at 1605 cmÿ1 due to Pyv with p-polarized light and at 1446 cmÿ1 due to the loss of solution pyridine with s-polarized light were plotted as a function of the electrode potential, as shown in Fig. 5. Assuming that the orientation of adsorbed pyridine is only the vertical type at around 0.3 V, the dierence in the two curves corresponds to adsorbed pyridine other than Pyv; this may be the ¯at type. The ¯at type is stable at negative potentials with the same results as the electrochemical method. PYRIDINE CONCENTRATION DEPENDENCY ON IRAS
Fig. 4. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 10 mM pyridine with a reference potential of ÿ0.8 V. The addition of pyridine was performed at ÿ0.7 V.
pyridine solutions lower than 5 mM, PyF increased up to ÿ0.21 V with increasing positive potential and Pyv increased gradually up to 0.46 V in the potential region more positive than ÿ0.21 V [2]. On the other hand, Pyv appeared at ÿ0.7 V in this IRAS study, clearly dierent from the chronocoulometry results. In another chronocoulometric study on a single crystalline (100)Au electrode, other orientations, presumably intermediate between the ¯at and the vertical orientation, have been observed at intermediate surface concentrations [3]. In the present
Fig. 5. Relative band intensities as a function of potential. w: 1601 cmÿ1 band with p-polarized light, q: 1446 cmÿ1 band with s-polarized light.
In order to clarify the pyridine concentration dependency on the amounts of Pyv, IRA spectra with a reference potential of ÿ0.7 V were measured in 1 and 3 mM pyridine solutions, as shown in Figs 6 and 7, respectively. The 1446 cmÿ1 band corresponding to the total amount of adsorbed pyridine and the 1605 cmÿ1 band due to Pyv decreased with decreasing pyridine concentration. In the chronocoulometry study, the amount of ¯at oriented pyridine as a function of the electrode potential was dependent of the bulk pyridine concen-
Fig. 6. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 1 mM pyridine. Other conditions were the same as in Fig. 1.
FTIR study of pyridine
3301
®eld in the inner double layer. Another possibility is due to a change in the charge density of the nonbonding orbital in the nitrogen atom with potential changes. In a chronocoulometric study on an Au (110) electrode, pyridine molecules were adsorbed vertically, as explained by the Frumkin isotherm [4]. The lateral interaction parameter in the equation depends on the electrode potential; it may be related to the band shift observed by IRAS. CONCLUSION The IRA spectra of pyridine adsorbed on a polycrystalline Au electrode in 1 M NaF containing pyridine were measured. A symmetric vibration observed around 1603 cmÿ1 was due to vertically adsorbed pyridine and a band shift with changing electrode potential was observed. The potential induced band shift of an in-plane symmetric ring stretch was observed in all concentrations, it seems that the shift is caused by a weak Stark eect or the change of the charge density of the nonbonding orbital in the nitrogen atom with potential changes. Fig. 7. IRA spectra of adsorbed pyridine on a polycrystalline Au electrode in 1 M NaF containing 3 mM pyridine. Other conditions are the same as in Fig. 1.
tration, however, the amount of vertically adsorbed pyridine was independent [5]. The shift of the band due to Pyv was independent of the pyridine concentration, however a slight potential dependency of the band position (approximately 6 cmÿ1/V) was observed in all pyridine concentrations, indicating that the shift is caused only by potentials rather than by lateral interaction between adsorbed pyridine or dipole coupling. A weak Stark eect [9] is expected because only a part of the pyridine ring of Pyv is aected by the electric
REFERENCES 1. B. E. Conway, R. G. Barradas, P. G. Hamilton and J. M. Parry, J. Electroanal. Chem. 10, 485 (1965). 2. L. Stolberg, J. Richer and J. Lipkowski, J. Electroanal. Chem. 207, 213 (1986). 3. L. Stolberg and J. Lipkowski, J. Electroanal. Chem. 238, 333 (1987). 4. L. Stolberg, J. Lipkowski and D. E. Irish, J. Electroanal. Chem. 296, 171 (1990). 5. J. Lipkowski, L. Stolberg, S. Morin and D. E. Irish, J. Electroanal. Chem. 355, 147 (1993). 6. Y. Ikezawa, H. Saito, H. Fujisawa and G. Toda, J. Electroanal. Chem. 240, 281 (1988). 7. D. A. Long, F. S. Mur®n and E. L. Thomas, Trans. Faraday Soc. 59, 12 (1963). 8. M. Weber and F. C. Nart, Langmuir 12, 1895 (1996). 9. D. K. Lambert, Solid State Commun. 51, 297 (1984).