Electrochemical and Fourier Transform Infrared Spectroscopy Studies

and Department of Chemistry, Acadia University, Wolfville, Nova Scotia B0P 1X0, Canada. Received February 17, 1997. In Final Form: June 16, 1997X...
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Langmuir 1997, 13, 4737-4747

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Electrochemical and Fourier Transform Infrared Spectroscopy Studies of Benzonitrile Adsorption at the Au(111) Electrode Aicheng Chen,† Jocelyn Richer,‡ Sharon G. Roscoe,§ and Jacek Lipkowski*,† Guelph-Waterloo Center for Graduate Study in Chemistry, Guelph Campus, University of Guelph, Guelph, Ontario N1G 2W1, Canada, Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, E3B 6C2, Canada, and Department of Chemistry, Acadia University, Wolfville, Nova Scotia B0P 1X0, Canada Received February 17, 1997. In Final Form: June 16, 1997X The subtractively normalized interfacial Fourier transform infrared technique has been employed to study the adsorption of benzonitrile (BN) at the Au(111) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed BN molecules at positive potentials. Our studies show that BN molecules are totally desorbed from the Au(111) surface at potentials more negative than -0.6 V (SCE) and they adsorb at the gold surface at more positive potentials. At potentials more negative than 0.05 V (SCE), the adsorption has an associative character. The BN molecules are initially oriented flat (π-bonded) on the electrode surface and progressively reorient from the flat to a vertical (N-bonded) state when the electrode potential approaches the potential of zero charge. This change of the surface coordination is gradual and apparently involves a progressive change of the tilt angle. When the potential is greater than 0.05 V (SCE), the character of BN adsorption becomes dissociative and the adsorbed molecules partially hydrolyze to form benzamide (BA). The adsorbed layer becomes a mixture of BN and BA molecules. The ratio of BN to BA molecules decreases as the electrode potential increases. The optical properties of CaF2 prisms and flat windows have also been investigated. Our results show that flat windows should not be used to study physical features such as orientation and coordination of adsorbates. However, they are useful to extract quantitative information about the surface concentration of adsorbed species and to determine the composition of the interfacial region.

1. Introduction Fourier transform infrared (FTIR) spectroscopy is a powerful in situ technique to study electrified interfaces.1-3 To date, this technique has found three major applications. The first application is to identify intermediates and products of an electrode reaction. FTIR spectroscopy has been widely used to study electrocatalytic oxidation mechanisms of small organic molecules at noble metal electrodes.4-6 The second application is to study the orientation and coordination of molecules adsorbed at metal surfaces, for example, CO to platinum group metals,7-9 difluorobenzene on polycrystalline platinum,10 isoquinoline11 and flavine adenosine12 on mercury, and acetonitrile13 on gold. The third involves investigations of ionic adsorption and the surface properties of water.9,14-16 In * Corresponding author. † University of Guelph. ‡ University of New Brunswick. § Acadia University. X Abstract published in Advance ACS Abstracts, August 1, 1997. (1) Pons, S. J. Electroanal.Chem. 1983, 150, 495. (2) (a) Bewick, A.; Pons, S. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden and Son: New York, 1985, Vol. 12, Chapter 1. (b) Pender, C. A study of adsorption on electrodes using infrared spectroscopy. Ph.D. Thesis, University of Southampton, 1987. (3) Chen, A.; Sun, S. G.; Yang, D.; Pettinger, B.; Lipkowski, J. Can. J. Chem. 1996, 74, 2321. (4) Chen, A.; Sun, S. G. Chem. J. Chin. Univ. 1994, 15, 401 and 548. (5) Sun, S. G.; Chen, A. J. Electroanal.Chem. 1992, 323, 319. (6) Edens, G. J.; Hamelin, A.; Weaver, M. J. J. Phys. Chem. 1996, 100, 2322. (7) Chang, S-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (8) Jiang, X.; Weaver, M. J. Surf. Sci. 1992, 275, 237. (9) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (10) Pons, S.; Bewick, A. Langmuir 1985, 1, 141. (11) Blackwood, D.; Korzeniewski, C.; McKenna,W.; Li, J.; Pons, S. ACS Symp. Ser. 1988, No. 378, 338. (12) Birss, V. I.; Hinman, A. S.; McGarvey, C. E.; Segal, J. Electrochim. Acta, 1994, 39, 2449.

S0743-7463(97)00158-3 CCC: $14.00

most of these applications, in situ FTIR studies were concerned with either the identification of species generated at the electrode surface or the determination of the characteristic of their surface coordination. Less work has been done to develop FTIR spectroscopy as a tool for quantitative analysis of adsorbed species which could provide information about the composition of the interfacial region. There are only a few papers in which such quantitative studies were reported.3,14,17 In this work, we have employed in situ FTIR spectroscopy to investigate the adsorption of benzonitrile (BN) at the Au(111) electrode surface. We have recently reported thermodynamic studies of benzonitrile (BN) adsorption at the Au(111) surface.18 The results of these studies indicate that BN adopts a flat π-bonded surface coordination at the negatively charged interface with low bulk concentrations and a vertical (N-bonded) orientation at a positively charged surface with higher bulk concentrations.18 However, we had difficulties to determine whether BN molecules assume the vertical orientation or whether they are oxidized at these positive potentials. We now want to resolve this issue with the help of in situ FTIR spectroscopy. The macroscopic description of the surface properties of adsorbed BN will be augmented and refined by the molecular interpretation of the FTIR herein. (13) Faguy, P. W.; Fawcett, W. R.; Liu, G.; Motheo, A. J. J. Electroanal. Chem. 1992, 339, 339. (14) Eden, G. J.; Gao, X.; Weaver, M. J. J. Electroanal. Chem. 1994, 375, 357. (15) Faguy, P. W.; Marinkovic´, N. S.; Adzˇic´, R. R. J. Electroanal. Chem. 1996, 407, 209. (16) Ataka, K.-I.; YoEsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10669. (17) Corrigan, D. S.; Krauskopf, E. K.; Rice, L. M.; Wieckowski,A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 1596. (18) (a) Richer, J.; Iannelli, A.; Lipkowski, J. J. Electroanal. Chem. 1992, 324, 339. (b) Richer, J. Ph.D. Thesis, University of Guelph, Canada, 1990.

© 1997 American Chemical Society

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The adsorption of BN at metal surfaces has been investigated by a number of techniques.2,17-27 Spectrochemical studies of surface-enhanced Raman spectroscopy (SERS)19 and electromodulated infrared spectroscopy (EMIRS)2a showed that BN was vertically oriented (Nbonded) with respect to the gold electrode surface at potentials E greater than 0.4 V (SCE) in 1 M H2SO4 aqueous solutions containing 0.02 M benzonitrile. SERS experiments indicated also that BN is N-bonded to the surface of a Ag electrode.20 However, comprehensive EMIRS studies of adsorption of fluorosubstituted benzonitriles by Pender2b demonstrated that these molecules may assume both a flat π-bonded surface orientation and a vertical N-bonded orientation. Ultrahigh vacuum (UHV) studies carried out by Solomun et al.21-23 showed that BN molecules assume a flat π-bonded surface orientation when adsorbed at the Au(100) surface from the gas phase. However, other UHV experiments demonstrated that the surface coordination of a BN molecule to a metal surface depends on the nature of the metal and/or presence of other co-adsorbed species such as oxygen.24-27 Apparently, the character of BN adsorption at a surface of a metal electrode depends on the specific experimental conditions. The objectives of our present work are therefore 3-fold: (i) to employ the surface selection rules of IR spectroscopy to investigate the potential-induced reorientation of the adsorbed molecules; (ii) to use IR spectroscopy to determine the potential range within which the molecules are stable at the electrode surface and to determine the potential of the onset of BN oxidation; (iii) to correlate the surface concentrations of BN molecules determined from electrochemical studies in ref 18 with the intensities of selected IR bands for adsorbed molecules. 2. Experimental Section Reagents. All solutions were prepared from Milli-Q water with a resistivity higher than 17 MΩ cm. The supporting electrolyte was 0.1 M KClO4 (ACS Certified from Fisher Scientific) purified according to the procedure described in our previous papers.18,28 The concentration of BN (HPLC grade 99.9% pure from Aldrich Chemical Co.) and benzamide (BA) (ACROS Organics) used in this study was 0.03 M. The electrolyte solution was deaerated by purging with argon for about 20 min before starting the measurements, and argon was allowed to flow over the solution at all times. Cell and Electrodes. A syringe type IR cell with a CaF2 prism or a flat window was used in the in situ FTIR studies. A Au(111) single-crystal electrode which was grown, cut, and polished in our laboratory was used as the working electrode. The working electrode was flame annealed before each experiment. For the in situ FTIR experiments, the working electrode was pushed against the CaF2 window to form a thin-layer configuration. A platinum foil was used as the counter electrode, and the reference electrode was an external saturated calomel electrode (SCE) connected to the cell through a salt bridge. All experiments were performed at 20 ( 2 °C. (19) (a) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5040. (b) Gao, P.; Gosztola, D. J.; Leung, W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211. (20) Holze, R. Electroanalysis 1993, 5, 997. (21) Solomun, T.; Christmann, K.; Baumga¨rtel, H. J. Phys. Chem. 1989, 93, 7199. (22) Solomun, T.; Christmann, K.; Newmann, A.; Baumga¨rtel, H. J. Electroanal. Chem. 1991, 309, 355. (23) Solomun, T.; Newmann, A.; Christmann, K. Ber. Bunsenges Phys. Chem. 1991, 95, 95. (24) Kordesch, M. E.; Feng, W.; Stenzel, W.; Weaver, M. J.; Conrad, H. J. Electron Spectrosc. Relat. Phenom. 1987, 44, 149. (25) Feng, W. M.; Stenzel, W.; Conrad, H.; Kordesch, M. E. Surf. Sci. 1989, 211, 1044. (26) Kishi, K.; Okino,Y.; Fujimoto, Y. Surf. Sci. 1986, 176, 23. (27) Nakayama, T.; Inamura, K.; Inoue, Y.; Ikeda, S.; Kishi, K. Surf. Sci. 1987, 179, 47. (28) Richer, J.; Lipkowski, J. J. Electrochem. Soc. 1986, 133, 121.

Chen et al. Experimental Procedures. The in situ FTIR experiments were carried out on a Nicolet 20SX/C FTIR apparatus equipped with a MCT-B detector cooled by liquid nitrogen. The sample compartment of the FTIR apparatus was purged throughout the experiment using CO2- and H2O-free air provided by the Puregas Heatless Dryer. The electrode potential was controlled by a PAR 173 potentiostat. Equipment and data processing procedures used to determine the differential capacity and charge density have been described in our previous publications.18,28 The subtractively normalized interfacial Fourier transform infrared (SNIFTIR) technique was employed to record the IR spectra.1,3 The spectra were determined using a multiple potential step (MPS) procedure in which the electrode potential was stepped periodically between the reference potential E1 and the sample potential E2. During each step, n interferograms were acquired at the potential E1 and E2. The acquisition was delayed for 30 s after each potential change to allow the interface to reach thermodynamic equilibrium at that potential. The change of the electrode potential was synchronized with the acquisition of the interferograms by connecting the external trigger port of the PAR 173 potentiostat to the communication port of the DX 486 computer of the FTIR instrument. This procedure was repeated m times until a total number of N ) n × m interferograms were acquired at each of the two potentials. Typical values of n and m employed in this study were n ) 100 and m ) 20. The interferograms were added, Fourier transformed, and used to calculate a relative change of the electrode reflectivity, which is defined as:

∆R/R ) [R(E2) - R(E1)]/R(E1)

(1)

where R(E1) and R(E2) are the electrode reflectivity at potentials E1 and E2, respectively. The spectra were recorded with a resolution of 4 cm-1. The IR incident beam was normal to the prism, but at an angle about 60° to the flat window. A transmission spectrum was determined by squeezing the investigated solution between two flat CaF2 windows to form a thin layer.

3. Results and Discussion 3.1. Electrochemical Results. In order to determine suitable reference and sample potentials for the FTIR studies and to facilitate further discussion of the spectroscopic data, we will review briefly what we have learned about the character of BN adsorption at the Au(111) surface from differential capacity and chronocoulometric measurements described previously.18 The measurements were conducted within the double layer region of the Au(111) electrode, which in a neutral solution corresponds to ∼-0.8 V < E < ∼0.6 V (SCE). For 0.1 M KClO4 + 0.03 M BN and 0.1 M KClO4 solutions Figure 1 shows (A) the differential capacity curves, (B) absolute charge densities, and (C) surface concentrations of BN calculated from charge density data in ref 18. Solid lines and open points show the curves recorded in the presence of BN; dotted lines show the curves recorded for the BN-free supporting electrolyte. At potentials more negative than -0.65 V (SCE), the differential capacity and charge-potential curves recorded in the presence of BN molecules merge with the corresponding curves recorded for the organicfree electrolyte, and the surface concentration of BN drops to zero. These features indicate that BN molecules are totally desorbed from the electrode surface at these negative potentials. A large broad peak on the differential capacity curve and a step on the charge-potential plot indicate the onset of BN adsorption. Their positions correlate well with the potential at which the first inflection point is observed on the surface concentration plot. The differential capacity curve for BN adsorption displays a minimum within the potential range -0.1 < E < 0.2 V (SCE) in which a plateau is seen on the Γ versus E plot. The potential of zero charge for the BN-covered electrode is equal to 0.15 V, and hence

Benzonitrile Adsorption on Au

Figure 1. (A) Differential capacity curves recorded using a 5 mV (root mean square) sine wave modulated at 25 Hz and a 5 mV s-1 sweep rate. (B) Absolute charge density vs electrode potential plots. (C) Gibbs excess versus potential plot, determined for a Au(111) electrode in 0.1 M KClO4 (dotted lines) and 0.1 M KClO4 + 0.03 M BN solutions (open points and solid line).

it is located at the plateau of the surface concentration plot (or within the minimum on the differential capacity curve). The potential of zero charge (pzc) for the BNcovered electrode is about 0.12 V more negative than the pzc for the electrode that is free of adsorbed organic molecules. These results are consistent with the parallel (π-bonded) adsorption of BN molecules at the negatively charged surface (at E < 0.15 V (SCE)). The surface concentration of BN corresponding to the plateau region is very close to the maximum packing density for π-bonded molecules estimated to be 3.3 × 10-10 mol cm-2. In addition, the small negative shift of the pzc is consistent with the orientation of the adsorbed BN molecule in which the component of its permanent dipole moment in the direction normal to the surface is small.18 At E > 0.2 V (SCE), that is at a positively charged surface, the differential capacity increases, a second step appears on the charge potential curve, and the surface concentration of the adsorbed molecules rises. The increase of the capacity and the surface concentration of adsorbed molecules suggests that adsorbed BN molecules undergo a potential-induced reorientation from the π-bonded to the vertical N-bonded surface coordination. However, this behavior is apparently pH dependent and the “reorientation” could be shifted toward more positive potentials in solutions of low pH.18 We will return to this point later. Apparently, the behavior of BN molecules at positive potentials is complex and a full explanation of its character requires spectroscopic investigations. All these features will be examined by the following FTIR investigations. 3.2. FTIR Studies. General Properties of IR Spectra. To facilitate the interpretation of our electroreflection spectra, we also measured the spectrum of pure BN using a thin layer transmission cell. We also made an

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Figure 2. Comparison of (a) the transmission spectrum of pure BN, (b) transmission and (c, d) SNIFTIR spectra for 0.03 M BN + 0.1 M KClO4 solution at the Au(111) electrode acquired using E1 ) -0.75 V (SCE) and (c) E2 ) -0.05 V and (d) E2 ) +0.35 V. Table 1. Vibrational Frequencies (cm-1) and Assignments mode

transmission

vibrational assignment

a1 a1 b2 a1 b2 b2

2229 vs 1599 s 1582 w 1491 s 1448 s 1288 w

CtN stretch ring stretch ring stretch ring stretch ring stretch β(CH)

attempt to measure the transmission spectrum for the aqueous solution of BN. Due to a low solubility of BN and strong absorption of IR light by water, only the CN stretch band could be seen in this spectrum. This band is shown in Figure 2. The spectrum for pure BN is compared with two selected SNIFTIR spectra in Figure 2. One of these spectra was measured at E2 ) -0.05 V, which corresponds to a negatively charged surface and the plateau on the surface concentration plot; the other spectrum was measured at E2 ) 0.35 V, which corresponds to the positively charged surface and to the rising section of the surface concentration plot. A compilation of the transmission frequencies, along with vibrational assignments,21 is given in Table 1. Within the investigated wavelength range, all IR bands have either a1 or b2 symmetry. The BN molecule exhibits a planar structure and belongs to a C2v point group. Consequently, a1 and b2 bands correspond to vibrations in the plane of the molecule; a1 bands correspond to changes of the dipole moment in the direction parallel to the C2 axis and b2 bands correspond to changes of the dipole moment in the direction normal to the C2 axis. The SNIFTIR spectra in Figure 2, and all the other electroreflectance spectra reported in this work, were acquired using the reference potential E1 ) -0.75 V (SCE) where BN molecules are totally desorbed from the electrode surface. Under these conditions, the measured changes of the reflectivity (∆R/R) represent the difference between the absorption spectrum of the Γ molecules that are in solution at potential E1 and the spectrum of the molecules that are adsorbed at the electrode surface at

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potential E2. When a linearly polarized light is used in the experiment, the measured (∆R/R) can be expressed as

(∆R/R) ) 2.3Γ[cos2 θ(E1) (E1) - cos2 θ(E2) (E2)] (2) where R is the electrode reflectivity, θ is the angle between directions of the electric field of the photon and the direction in which the dipole moment of the molecule changes,  is the molar absorption coefficient, and Γ is the surface concentration of the adsorbed molecules. At potential E1, the BN molecules are desorbed from the electrode surface and are randomly oriented. The function cos θ has to be averaged over all possible orientations and the result is 〈cos2 θ(E1)〉random ) 1/3. At potential E2 the magnitude of cos θ(E2) is determined by the orientation of the adsorbed molecule. For a molecule adsorbed at a gold electrode, surface selection rules state that the vibration will be allowed if a change in the dipole derivative has a nonzero component in the direction normal to the surface.29 The BN molecules are likely to assume either a flat (π-bonded) or a vertical (N-bonded) surface coordination. In the first case, all vibrations corresponding to a1 and b2 bands will be parallel to the surface and their component in the direction normal to the surface will be equal to zero. The second term in eq 2 will then be equal to zero and the measured change of the electrode reflectivity will give the absorption difference spectrum of Γ molecules desorbed into the solution (solution species). The sign of the spectrum will be positive. In the second case, the change of the dipole moment corresponding to a1 bands will have a significant contribution in the direction normal to the surface while the change of the dipole moment corresponding to b2 bands will be predominantly in the direction parallel to the surface. The vibrations corresponding to a1 bands will be surface active while vibrations corresponding to b2 bands should remain surface inactive. Equation 2 shows that the absorption bands corresponding to the adsorbed species should have a negative sign. The bands corresponding to the adsorbed species should also be frequency shifted with respect to the bands of the solution species either due to the Stark effect or due to a different chemical environment of adsorbed molecules. Consequently, for this orientation, a1 bands should appear as bipolar features on ∆R/R spectrum, with the positive section corresponding to the absorption by the solution species and the negative section to the absorption by the adsorbed molecules. We will now use these general principles to discuss the results of our spectroscopic experiments. In Figure 2, the SNIFTIR spectrum for E2 ) -0.05 V displays essentially only one strong band at 2235 cm-1 corresponding to CN stretch. Its position corresponds well to the band recorded for the aqueous solution of BN in transmission. It is blue shifted by 6 cm-1 with respect to the band measured in the transmission mode for pure BN. It is predominantly positive although it displays some negative features indicating that it has a weak bipolar character. The evolution of the bipolar character of this band with the electrode potential will be discussed in the next section. The broad peak seen at about 1620 cm-1 may be assigned to the vibration of H2O.3,13,16 The potential-induced change of the water bands may be a result of either a substitution of water molecules at the surface by the adsorbed molecules of BN or a change in the concentration of ions in the diffuse part of the double (29) Moskovits, M. J. Chem. Phys. 1982, 77 (9), 4408.

Figure 3. SNIFTIR spectra in the CtN stretch region for BN adsorbed at the Au(111) electrode from 0.1 M KClO4 + 0.03 M BN solutions using (A) p-polarized and (B) s-polarized infrared beams. For each spectrum, the reference potential E1 was equal to -0.75 V/SCE, and the value of E2 is indicated in the figure.

layer at each potential E2 and E1. The other bands are apparently lost in the background. The spectrum recorded for E2 ) 0.35 V shows, in addition to the CN stretch band, strong bands in the 1300-1700 cm-1 region (Figure 2). The CN stretch band becomes apparently bipolar and dominated by negative features. This behavior indicates that the adsorbed BN molecules undergo a potential-induced reorientation at the Au(111) electrode surface. Four strong positive bands centered at 1679, 1666, 1621, and 1573 cm-1 appear in the SNIFTIR spectrum, which are absent in the transmission spectrum. In addition, a bipolar peak located between 1400 and 1450 cm-1 has no corresponding band in the transmission spectrum. The appearance of these new bands indicates that new species are formed at potential E2. This behavior suggests that the potential-induced reorientation of the adsorbed BN molecules is coupled with a change in their chemical state. In light of this, we will discuss our SNIFTIR spectra in two sections. First we will consider the CtN stretch region to examine the orientation of the adsorbed BN molecules and then we will discuss the ring stretch region to describe the change of the chemical state of adsorbed molecules. Orientation of Adsorbed Molecules. Figure 3 shows a series of SNIFTIR spectra measured in the CtN stretch region with E2 varied from -0.35 to +0.45 V using p-polarized (A) and s-polarized infrared beams (B). The electric field of an s-polarized photon is equal to zero at the electrode surface, and hence adsorbed BN is optically inactive for this polarization.29-32 In contrast, the electric field of the photon has a nonzero value in the bulk of the thin layer. If the adsorbed molecules are desorbed from the electrode surface, they remain in the thin layer and become randomly oriented. The desorbed molecules (30) Samant, M. G.; Kunimatsu, K.; Seki, H.; Philpott, M. R. J. Electroanal. Chem. 1990, 280, 391. (31) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095. (32) Watanable, S.; Inukai, J.; Ito, M. Surf. Sci. 1993, 293, 1.

Benzonitrile Adsorption on Au

become therefore IR active. Only positive bands centered at 2235 cm-1 due to the CtN stretch can be observed in the spectra at potentials more negative than -0.1 V using either p-polarized or s-polarized infrared beams. Therefore, the positive band corresponds to the IR absorption from solution species, and adsorbed BN molecules are IR inactive. All these features show that BN is oriented flat (π-bonded) to the surface at these negative potentials. We start to see a bipolar peak at the potential -0.05 V using a p-polarized infrared beam. The frequency of the negative band depends on the electrode potential E2, with the positive shift of E2 causing a blue shift of the band. The bipolar features become more intense with increasing electrode potential. All these features show that BN reorients from a flat to a vertical orientation at high potentials. This change of orientation is gradual and suggests that the tilt angle for the adsorbed molecule progressively increases when potential becomes more positive. The evolution of the SNIFTIR spectra with potential indicates that BN molecules are π-bonded at the negatively charged surface and assume a tilted orientation around the pzc and a vertical N-bonded orientation at the positively charged electrode. There is another feature of the SNIFTIR spectra that is important to note. For the s-polarized beam, the intensity of the CtN stretch decreases at potentials more positive than 0.05 V, and hence the spectra recorded using p-polarized light become predominantly negative. These features indicate that the reorientation of adsorbed BN molecules is coupled with a surface reaction, in which the adsorbed BN molecules are converted into another species. Identification of the Species Formed at Positive Potentials. It is well-known that nitriles may easily be hydrolyzed to amides according to the following general scheme:33

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Figure 4. Comparison of the spectra of benzamide (BA) and benzonitrile (BN): (a) transmission spectrum of BA; (b-d) SNIFTIR spectra obtained at E1 ) -0.75 and E2 ) +0.45 V from 0.1 M KClO4 + 0.03 M BA solutions using (b) s-polarized and (c) p-polarized infrared beams, and (d) from 0.1 M KClO4 + 0.03 M BN solutions using p-polarization.

and that this reaction is particularly convenient in a variant wherein hydrolysis is induced under mildly basic conditions by hydrogen peroxide. An OH radical, which is the surface equivalent of H2O2, is likely to be formed at a gold electrode at potentials more positive than +0.4 V (SCE). Hence, a surface reaction between the adsorbed BN molecules and the OH species may be the reaction that converts BN into benzamide (BA) at the more positive potentials. Further support for a surface hydrolysis reaction may be given by the results reported for the hydrolysis of acetonitrile.34 A slow hydrolysis was measured under mild conditions at room temperature catalyzed by Hg2+ with the conversion of acetonitrile to acetamide in a high yield and purity as determined by Raman spectroscopic measurements. The reaction, carried out in a bulk medium of acetonitrile and water, proceeds with acetonitrile in the primary solvation sphere of Hg2+ reacting with water. The initial complex between acetonitrile and Hg2+ was determined to be an ion-dipole complex, which is very stable. A covalent bond was excluded, since no Raman frequency attributable to the metal-nitrogen stretch was observed. Acetamide, the major product, was found to

bind to Hg2+ via the N atom as shown by an intense 300 cm-1 peak attributed to the Hg2+-N stretch which reflects the great affinity of mercury for nitrogen ligands. Mercury acetamide complexes appear to be much more stable than mercury acetonitrile toward hydrolysis reactions as negligible amounts of acetic acid were found. Therefore, it is likely that a similar type of mechanism occurs with the positively charged Au(111) surface catalyzing the hydrolysis of benzonitrile (BN) to benzamide (BA). To test this hypothesis, we measured the transmission spectrum of pure BA and the SNIFTIR spectra of 0.03 M BA + 0.1 M KClO4 solution. Figure 4 shows the comparison of the transmission spectrum for BA and the SNIFTIR spectra at E2 ) +0.45 V, recorded using s-and p-polarized light. To facilitate interpretation of the results, the SNIFTIR spectrum of BN (p-polarized light) is also included in Figure 4. Only the 1300-2300 cm-1 range is reported in this figure since this region contains all the major absorption bands of the BA molecule. Five bands can be observed in the transmission spectrum. The band centered at 1402 cm-1 can be assigned to the C-N stretching mode. The band at 1660 cm-1 is due to the CdO stretch, whereas the band at 1625 cm-1 corresponds to the deformation of NH2, and the other two bands at 1578 and 1449 cm-1 are due to the ring stretch.35 These five bands can also be seen in the SNIFTIR spectrum (b) using an s-polarized infrared beam. However, the frequency of these bands is slightly shifted due to solvent effects and hydrogen bond formation. In spectrum c a bipolar peak appears between 1400 and 1450 cm-1 instead of a monopolar peak centered at 1411 cm-1 using a p-polarized infrared beam. This indicates that the adsorbed BA is vertical (N-bonded) to the surface. Spectrum d which was measured in a solution initially containing only BN, becomes similar to spectrum c at this positive potential. Since

(33) Roberts, J. D. Modern Organic Chemistry W. A., Benjamin, Inc.: New York, 1967; p 471. (34) Sze, Y.-K.; Irish, D. E. Can. J. Chem. 1975, 53, 427.

(35) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. In Introduction to Infared and Raman Spectroscopy; Academic Press: New York, 1990; pp 319-322.

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Figure 5. SNIFTIR spectra in the range between 1330 and 1530 cm-1 for BN adsorption at the Au(111) electrode using a p-polarized infrared beam. For each spectrum, the reference potential E1 was equal to -0.75 V, and the value of E2 is indicated in the figure.

spectrum c was measured in a BA solution, the results indicate that BN was hydrolyzed to BA at this potential. To determine the potential for the onset of BN hydrolysis, one has to find the potential of the initial appearance of the bipolar band centered at 1430 cm-1 in the spectrum recorded for the solution of BN molecules. Figure 5 shows the region from 1330 to 1530 cm-1 of

Chen et al.

SNIFTIR spectra for BN, measured using a p-polarized IR beam with E2 varied from -0.35 to +0.45 V. At potentials more negative than 0.05 V the spectral features are lost in the noisy background. The bipolar peak at 1430 cm-1 appears at the potential +0.05 V, which indicates that hydrolysis of BN begins at this potential. The intensity of the bipolar peak increases significantly with each increment in the potential, which shows that the conversion of BN into BA progresses with potential. The bipolar character of this band indicates that BA formed by hydrolysis of BN remains on the surface as an adsorbed molecule. This point is further supported by the fact that the frequency of the negative branch of this band is blue shifted when E2 increases. Figure 6A shows that the frequency of the C-N stretch of the newly formed BA increases linearly with potential. For comparison, Figure 6B shows the potential dependence of the frequency of CtN stretch of the BN molecule. The frequency shift is 18 cm-1 per volt for the C-N stretch and 10 cm-1 per volt for the CtN stretch. These values are comparable to the shifts of CtN stretch frequency observed by Pender for fluoro-substituted benzonitrile.2b Correlation between the Integrated Band Intensity and Surface Concentration of Adsorbed Molecules. It is interesting to compare the integrated IR intensity of the bands at 2235 and 1409 cm-1 with the surface concentration of adsorbed molecules determined by chronocoulometry.18 Such a comparison is particularly straightforward for the spectra acquired using s-polarized light. In that case, the electric field of the photon at the surface is equal to zero, and hence ads(E2) is equal to zero as well. Equation 2 simplifies then to

∆R/R ) 2.3Γdes(E1)

(3)

∫ (∆R/R) dν ) 2.3Γ ∫des(E1) dν

(4)

and

The absorption bands are positive and the band intensity

Figure 6. Dependence of frequency of the negative peak on the electrode potential of (A) the C-N stretch band of adsorbed BA and (B) the CtN stretch band of adsorbed BN.

Benzonitrile Adsorption on Au

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Figure 7. Comparison of the surface concentration of BN obtained in 0.1 M KClO4 + 0.03 M BN solutions (a) by the thermodynamic method18 and (b, c) the integrated infrared intensity of the bands at (b) 2235 and (c) 1409 cm-1. The inset presents the absolute charge density as a function of the electrode potential acquired in (d) the neutral solution and (e) the acidic solution at pH ) 2.4.

is proportional to the product of the surface concentration of molecules adsorbed at potential E2 and the integrated molar absorption coefficient of molecules in the bulk of the investigated solution. In the present case, the intensity of the band at 2235 cm-1, corresponding to the CtN stretch in BN, is proportional to the surface concentration of adsorbed BN molecules. Likewise, the intensity of the band at 1409 cm-1, corresponding to C-N stretch in BA, is proportional to the surface concentration of BA molecules formed by the surface hydrolysis of BN. Consequently, the correlation of the integrated intensities of the CtN stretch and the C-N stretch bands with the surface concentrations of adsorbed molecules determined from chronocoulometric experiments provides a means to estimate the fraction of the total surface concentration that corresponds to adsorbed BN and the fraction that corresponds to adsorbed BA species. Figure 7 shows a plot of the surface concentration versus potential (curve a), the dependence of the integrated IR intensity on the electrode potential for the CtN stretching mode (curve b), and the C-N stretch of the new band at 1409 cm-1 (curve c). When the potential is more positive than -0.6 V, the CtN stretching intensity rises quickly and follows the change in the surface concentration, while the intensity of the C-N stretch band initially remains at zero. The CtN stretching intensity attains a limiting value in the potential range where the plateau is observed on the surface concentration plot. At potential ∼0.05 V, just before the surface concentration starts to rise further, the intensity of the CtN band begins to decrease. Coincidentally, the new C-N stretch band at 1409 cm-1 appears in the SNIFTIR spectrum. The decrease of the CtN stretch intensity correlates very well with the increase of the C-N band intensity. Increasing the potential further results in a continually decreasing CtN stretch intensity, while the intensity of the new band

Figure 8. SNIFTIR spectra in the CtN stretch region for BN adsorbed at the Au(111) electrode from 0.1 M KClO4 + 0.03 M BN solutions using (A) a p-polarized infrared beam with a prism and (B) a flat window. For each spectrum, the reference potential E1 was equal to -0.75 V/SCE, and the value of E2 is indicated in the figure.

increases significantly. It is clear that, at more positive potentials, the decrease in the CtN stretch intensity is due to the hydrolysis of CtN group, resulting in the formation of BA on the electrode surface. At potentials

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Figure 9. (A) SNIFTIR spectra in the CtN stretch region at the Au(111) electrode from 0.1 M KClO4 + 0.03 M BN solutions acquired with a flat window without any polarizer. (B) Comparison of the surface concentration of BN and the integrated intensity determined using a flat window. The inset presents the integrated absorptivity of the CtN stretch band.

more positive than 0.05 V, the organic film at the electrode surface becomes a mixture of BN and BA molecules with the ΓBN:ΓBA ratio progressively decreasing with increasing electrode potential. It is useful to note that the CtN stretch band displays bipolar features at a potential as low as E ) -0.05 V. The oxidation of BN molecules is therefore preceded by a reorientation from the π-bonded to the N-bonded state. We propose that the following sequence of events takes place as the potential increases from the negative limit. At negative potentials when the electrode is negatively charged, BN molecules are π-bonded to the gold surface. As the electrode potential approaches pzc, the adsorbed BN molecules undergo a progressive reorientation (a progressive change of the tilt angle) from the π-bonded to the vertical N-bonded coordination. When the electrode potential is about to cross the pzc, hydrolysis of BN molecules begins and BA is formed at the surface. The hydrolysis apparently involves the N-bonded state of the adsorbed BN molecule, and the adsorbed BA formed by this reaction is also N-bonded. The conversion of adsorbed BN into adsorbed BA progresses as the potential becomes more positive. We have already mentioned above that the conversion of BN into BA is pH dependent. Chronocoulometry experiments were made to investigate the dependence of pH of the solution on the formation of BA from BN. The inset to Figure 7 shows plots of the absolute charge densities σM versus E for solutions containing the KClO4 electrolyte and 0.03 M BN in both pH ) 7 and pH ) 2.4. The shape of these two curves is very similar, and the two curves merge at potentials more negative than -0.1 V. A steeply sloped section starts at E ) +0.2 V in the charge density curve measured with the neutral solution. The

position of this quick rise shifts by about 0.2 V in the positive direction when the pH is decreased to 2.4, which shows that the formation of BA is pH dependent. This strong pH effect may explain why in EMIRS studies of a gold polycrystalline electrode in 1 M H2SO4 + 0.02 M BN solution, Bewick and co-workers2a observed that the intensity of the CtN stretching mode on a surface decreased only at a very high potential around 1.4 V. 3.3. Comparison of the Optical Properties of CaF2 Prisms and Flat Windows. Faguy and Fawcett36 gave a thorough discussion of the advantages of using CaF2 prisms versus flat disks. The prism is constructed so that normal incidence angles at each of the two side faces result in an incidence angle of about 80° at the metal solution interface, which ensures a near maximum enhancement of the mean square electric field strength of the photon at the electrode surface. In addition, the reflection losses for the prism are small, and the square transmissivity is close to unity. In contrast, for the 80° angle of incidence at the flat window, the angle at the electrode surface is only 45° and hence the enhancement of the mean square electric field strength at the electrode surface is less than half of the maximum value. In addition, there are significant reflection losses of the beam so that the square transmissivity is about 0.5. Figure 8 shows how the geometry of the IR window affects the shape of the SNIFTIR spectra of the CtN stretch region for a 0.3 M BN in 0.1 M KClO4 solution acquired using a prism (A) and a flat window (B). Although all these spectra were measured in the same solution using a p-polarized infrared beam, there are significant differences between the spectra recorded using the prism and (36) Faguy, P. W.; Fawcett, W. R. Appl. Spectrosc. 1990, 44, 1309.

Benzonitrile Adsorption on Au

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Figure 10. (A, B) SNIFTIR spectra in the region from 1300 to 1000 cm-1 acquired with a flat window: (A) 0.1 M KClO4 + 0.03 M BN and (B) 0.1 M KClO4. (C) The integrated intensity vs the absolute charge density for the supporting electrolyte with 0.03 M BN (O) and without BN (0).

the flat window. Only positive bands are observed with a flat window while a bipolar feature is seen on the spectra recorded with a prism. These results indicate that when a flat window is used, the mean square electric field strength of the photon at the electrode surface is so weak that the adsorbed molecules are effectively optically inactive and the spectrum is dominated by the absorption bands of molecules desorbed from the electrode surface into the bulk of the solution at potential E1. In contrast, when the prism is used to acquire the spectra, the field enhancement at the electrode surface is strong enough for the intensities of absorption bands of molecules adsorbed at the electrode surface at potential E2 to be of a comparable magnitude with the intensities of the bands of molecules desorbed into the bulk of the solution at potential E1 and therefore the spectra are bipolar. These features indicate that CaF2 flat windows should not be used to study physical properties such as the characteristics of surface coordination of adsorbates. However, flat windows may be used successfully to extract analytical information about the surface concentration of adsorbates. The spectra in Figure 8B, acquired using a flat window and p-polarized light, are similar to the spectra recorded with the prism and s-polarized radiation, shown earlier in Figure 3B. When the flat window is used, the field strength at the electrode surface is so weak that the second term in eq 2 is essentially equal to zero for both p- and s-polarized radiation and the measured change of the electrode reflectivity is described by eqs 3 and 4 for all polarizations. Therefore, with a flat window, the polarizer may be removed and the SNIFTIR spectra may be recorded using nonpolarized light. In this case the beam intensity is significantly enhanced and the signal to the noise ratio (S/N) is improved. Figure 9A displays a family of SNIFTIR spectra in the CtN stretch region acquired using a flat window and a

nonpolarized light. The S/N for these spectra is much better than that obtained from the spectra recorded using either s-polarized or p-polarized light (shown previously in Figures 3B and 8B, respectively). To make our point stronger, we calculated the integrated intensities of the bands at 2235 and 1409 cm-1 in spectra acquired with a flat window. These bands correspond to the CtN stretch of the nitrile group and C-N stretch of the amide group, respectively. Figure 9B shows the intensities and the surface concentrations of adsorbed molecules determined from chronocoulometric experiments. These results are presented in the same way as the results of measurements with a prism and an s-polarized light, shown earlier in Figure 7. Qualitatively, the two figures display the same dependence of the integrated band intensity on the electrode potential. At potentials sufficiently negative with respect to pzc, the intensity of the CtN stretch correlates very well with the surface concentration of BN. This correlation is apparently better than that previously observed in Figure 7. At potentials more positive than 0.05V, BA is formed and the intensity of the CtN stretch decreases while the intensity of C-N stretch increases. There are some minor quantitative differences between the change of the integrated intensities displayed in Figures 7 and 9. Due to the better S/N in the measurements performed using the flat window, we consider the results presented in Figure 9 to be a better representation of the changes of the surface concentration of BN and BA molecules. For potentials more negative than 0.05 V, the integrated intensities and the Gibbs excess data may be used to calculate the integrated absorptivity A for adsorbed BN molecules:

A)

1 2.3Γ

δν ∫band ∆R R

(5)

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The integrated intensities are plotted against the electrode potential in the inset in Figure 9B. Their values are essentially independent of potential and are approximately equal to 1.1 × 107 cm mol-1. It is interesting to compare the integrated absorptivities determined from the potential modulation experiments with the values obtained by means of transmission infrared measurements as defined by

A)

1 bc

I

∫band log I0 dν

(6)

where b is the thickness of the cell, c is the bulk concentration of BN, and Io and I are the incident and transmitted beam intensities. The integrated absorptivities, determined by eq 6, are equal to 1.3 × 106 cm mol-1. The values determined from transmittance are smaller than the values determined from the change of the electrode reflectivity. Such differences between absorptivities determined in the transmission and reflection experiments are expected when the thickness of the thinlayer cavity of the spectroelectrochemical cell is comparable to the wavelength of the infrared radiation, as reported previously in the literature.3,37,38 At this point, we can reasonably assume that the integrated absorptivity of BN at potentials E > 0.05 V is equal to the average absorptivity given in the inset to Figure 9 B and use this value to calculate the surface concentrations of BN at these positive potentials from the measured integrated CtN stretch band intensity. The surface concentrations of adsorbed BA may then be calculated by subtracting the surface concentration of BN from the total surface concentration of adsorbed molecules determined from chronocoulometric experiment. The surface concentrations of BN and BA calculated in this way are plotted in Figure 11 and will be discussed later. Overall, our SNIFTIR results substantiate the surface concentration data determined using the thermodynamic method18 and demonstrate that these two methods are complementary. The absolute surface concentrations of adsorbed ions or molecules cannot be obtained by IR spectroscopy, because the absorption coefficients measured by transmission FTIR and by SNIFTIR may differ significantly. However, IR spectroscopy can be used to identify the nature of adsorbed species and provides a good measure of the relative change in electrode coverage by this species. If an adsorbate reacts at the electrode surface to form a new species, the IR spectroscopy provides valuable information about the change in surface coverage for the adsorbate and new species. In contrast, the thermodynamic method provides information about the total surface concentration of adsorbed molecules which is the sum of the concentration of the adsorbate and the new species. The thermodynamic measurements must be combined with the IR measurements in order to give a complete description of the composition of the adsorbed layer. In such a case we can obtain not only the total concentration of molecules adsorbed at the surface but also information regarding their chemical nature and, for more than one species adsorbed at the interface, the fractional composition of each surface species. Another advantage of the flat window is that it transmits a wider frequency range than a prism window. Although the low-frequency cutoff for a CaF2 prism is 1250 cm-1, the range is extended to 950 cm-1 for a CaF2 flat window. Since potassium perchlorate has been used as a supporting (37) Corrigan, D. S.; Weaver, M. J. J. Phys. Chem. 1986, 90, 5300. (38) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 239, 55.

Figure 11. (a) Plot of the total Gibbs excess of adsorbed molecules from a 0.03 M BN + 0.1 M KClO4 solution at Au(111), (b) the surface concentration of adsorbed BN molecules, and (c) the surface concentration of adsorbed BA molecules. The segments of the CtN stretch and C-N stretch bands show the potential limits corresponding to the flat and vertical adsorption of benzonitrile and the range corresponding to bezamide formation. The inset shows the dependence of the electrosorption valency for BN adsorption at the Au(111) surface on the electrode potential, taken from ref 18.

electrolyte, it is useful to look at the spectral behavior of ClO4- ions. A number of research groups have studied the feature of ClO4- ion adsorption,9,38,39 but with different conclusions. Perchlorates absorb IR around 1107 cm-1 and hence the IR spectra of ClO4- can be acquired using a flat window. Figure 10 shows a series of SNIFTIR spectra recorded for 0.1 M KClO4 + 0.03 M BN (A) and 0.1 M KClO4 solutions without BN (B) in the region between 1300 and 1000 cm-1. A broad negative band centered at 1107 cm-1, which corresponds to the IR absorption by perchlorate ions, can be seen in the spectra. We have already emphasized earlier that for the flat window the photon field strength at the surface is weak and hence this band corresponds to the solution rather than to the adsorbed perchlorate ion. Its negative sign indicates that more perchlorate ions are present in the thin layer at potential E2 than at the more negative potential E1. The IR intensity for ClO4- ions significantly increases with increasing potential. Figure 10C shows the plots of the integrated IR intensity versus the absolute charge density measured by chronocoulometry for the corresponding solutions in the presence and absence of BN. The linear character of this dependence is consistent with the recent results by Marinkovic et al.39 This relationship shows that the concentration of the perchlorate ion in the optical cavity of the thin layer cell changes as the potential is stepped from E1 to E2. This potential step, ∆E, causes a charge ∆σM to pass from the gold electrode in the thin layer cavity to the auxiliary electrode (39) Marinkovic´, N. S.; Calvente, J. J.; Kova´cˇova`, Z.; Fawcett, W. R. J. Electrochem. Soc. 1996, 143 (8), L171.

Benzonitrile Adsorption on Au

outside this cavity. To charge the electrode surface, a migration current flows between the two electrodes and ions are exchanged between the thin layer cavity and the bulk of the electrolyte.39 For the potassium perchlorate electrolyte, the fraction of the charge transferred by perchlorate ions is equal to the product of ∆σM and the transport number for ClO4-. The linear dependence between the integrated band intensity and the charge is therefore expected. 4. Summary The subtractively normalized interfacial Fourier transform infrared spectroscopy and electrochemical method were employed to study the adsorption of benzonitrile at the Au(111) electrode surface. Major features of BN adsorption on the Au(111) electrode are summarized in Figure 11 which shows the potential dependence of (a) the total concentration of adsorbed molecules, (b) the surface concentration of adsorbed BN, and (c) the surface concentration of BA formed by hydrolysis of BN at the electrode surface. The surface concentration plots are divided into three regions corresponding to the flat (πbonded) and the vertical-tilted (N-bonded) orientation of adsorbed BN molecules and the region in which BN molecules react to form BA. For each region, the spectroscopic signature for each orientation of the molecule and for the appearance of adsorbed BA molecules is shown in the figure. The results show that BN molecules assume a flat π-bonded orientation at the negatively charged electrode surface. When the electrode potential approaches pzc the molecules undergo a progressive reorientation to assume a vertical N-bonded surface coordination. The change of the orientation is gradual and involves a progressive increase of the tilt angle. At potentials close to pzc, hydrolysis of adsorbed BN molecules begins and BA is formed. The newly formed BA molecules remain in the adsorbed state apparently N-bonded to the gold surface. The conversion of BN into BA is progressive,

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and when the electrode potential increases, the surface concentration of BN molecules decreases while the surface concentration of BA rises. The inset to Figure 11 shows that the change of the character of BN adsorption by moving from the negatively to the positively charged electrode surface is reflected also in the electrosorption valency versus electrode potential plot. The plot of electrosorption valency may be seen as consisting of two sections. The first, corresponding to the region in which BN is stable at the electrode surface is characterized by a small slope. The second, corresponding to potentials where BN reacts to form BA, has a much larger slope. Overall, our work shows excellent agreement between the results of thermodynamic studies of BN adsorption based on chronocoulometric measurements of the electrode charge density and spectroscopic investigations of this system. The thermodynamic method provides information concerning the surface concentration of adsorbed molecules and the energetics of their adsorption. The SNIFTIR technique provides information on the adsorbed molecules and whether adsorption has an associative or dissociative character. In the latter case, the IR technique can identify species formed at the electrode surface. When more than one species are adsorbed at the electrode surface, the complete description of the composition of the adsorbed layer may be achieved only by combining the thermodynamic and the spectroscopic studies. The thermodynamic method gives the total concentration of adsorbed molecules. The spectroscopic data give the surface concentrations of individual species present at the electrode surface. Acknowledgment. This work was supported by a grant from Natural Sciences and Engineering Research Council of Canada. We express our gratitude to Dr. Celia Pender for sending us a copy of her Ph.D. thesis. LA9701586