J . Chem. SOC., Faraday Trans. I , 1988, 84(2), 433-439 Raman Spectra of Aniline adsorbed on an Ag Electrode in Acidic Solutions Hitoshi Shindo and Chizuko Nishihara National Chemical Laboratory for Industry, Tsukuba Research Center, Yatabe, Ibaraki 305, Japan The adsorption of aniline on an Ag electrode in acidic aqueous solutions has been studied by surface-enhanced Raman scattering. Adsorption, both as neutral and protonated forms, was observed to depend upon the electrode potential. The choice of halide ions used as the supporting electrolyte as well as the pH of the solution affected the stability of the protonated form on the surface. The geometry of adsorption was also studied by the change in intensities of the Raman bands upon adsorption. It was concluded that the anilinium ion is adsorbed Coulombically in at least two different geometries on halide ions which remain on the electrode surface.Surface-enhanced Raman scattering (SERS) has been widely applied to the studies of molecular structures of solid/solution interfaces as well as solid/gas interfaces.' This technique will greatly help electrochemists to draw realistic pictures of molecules and atoms on electrode surfaces. We have employed SERS in studying the structures of adsorbates on an Ag electrode during the reduction of nitrobenzene in neutral and alkaline aqueous solutions. 2-5 The Raman spectrum of weakly adsorbed aniline was observed in the more cathodic potential range, while at more anodic potentials the electrode was covered with other adsorbates such as trans-azobenzene.2 The geometry of adsorption of aniline was studied by the change in intensities of Raman bands of each symmetry species upon adsorption.Using SER electromagnetic enhancement factors derived by Creighton6, and assuming approximately C,, symmetry for the adsorbate, it was concluded that aniline is adsorbed face-on on the ele~trode.~ In the present work adsorption of aniline was studied in acidic conditions, where the molecule takes a protonated form in the solution phase. Experimental Sample solutions were prepared by dissolving aniline (0.005 or 0.01 mol dm-3) in de- ionised and distilled water and HX(X = C1, Br or I) was added. The solution was purged of dissolved oxygen by bubbling with pure nitrogen. Aniline (Nakarai Chemicals, SP grade) was purified by distillation.The following chemicals were used as purchased : HC1 (Kokusan Chemical Works, GR, 35 YO); HBr (Nakarai Chemicals, GR, 47 YO); HI (Nakarai Chemicals, GR, 55 YO) ; D,O (Wako Pure Chemical Industries, 99.75 YO D) ; C,D,NH2 (Aldrich, Gold Label, 99 % D) ; DCl (Aldrich, Gold Label, 99 % D); DI (Aldrich, Gold Label, 99 YO D). Anilinium halides (powder) were synthesized from chemicals described above by a conventional method. The design of the electrochemical cell used for Raman measurements was based on that introduced by Watanabe.g As a modification the counter-electrode, a platinum wire, was held in a compartment separated from the bulk solution by a glass frit. Mixing of by-products at the counter-electrode with the sample solution was thus avoided.For example, oxidation of halide ions and polymerization of aniline occurs at the counter- electrode. Experiments with D20 solutions were performed in a smaller cell with a design 43 3434 Aniline adsorbed on an Ag Electrode used by Fleischmann et a1." The counter-electrode was separated in this case, too. A saturated calomel electrode (SCE) was used as the reference electrode. The electrode potential is quoted us. SCE throughout this paper. Origin and pretreatments of the Ag working electrode (99.99 % purity) were described previ~usly.~ The oxidation and reduction cycle for the activation of SERS was made by potential steps. Formation of silver halides was performed at + 0.2,O.O and - 0.2 V (vs. SCE) for solutions with C1-, Br- and I-, respectively.An argon-ion laser (Coherent, CR-8) was used for the excitation of Raman scattering. In most cases the 514.5 nm line was used at 50 mW. The scattered light was observed in the 90" direction and was analysed with a double-dispersion Raman spectrometer (JASCO, R-800). Resolution of 5 cm-' was usually used to obtain the spectra shown in this paper. However, all the frequencies given in the figures were obtained by separate measurements with a resolution of 2cm-'. Other details are given in our previous Results and Discussion Adsorption of Aniline In the first experiment a 0.01 mol dm-3 aqueous solution of aniline with 0.1 mol dm-3 HCl was used. The spectrum shown in fig. 1(a) was observed when the electrode potential was set to -0.3 V immediately after the oxidation of Ag at 0.2 V for 3 s.Three sharp and strong bands were observed at 1027, 1006 and 793 cm-l. They are easily assigned to v18a, vI2 and v, modes of anilinium ion adsorbed on the electrode. Throughout this paper Wilson notation'' is used for the numbering of vibrational modes of the phenyl ring. For the anilinium ion in the solution phase these modes are observed at 1030, 1008 and 797cm-', respectively. The frequency shifts upon adsorption are rather small. Contribution by the Raman bands of the ion in the solution is almost negligible in fig. 1 (a), since they are weaker than those of the adsorbed ion by more than one order of magnitude. The broad band at around 840 cm-' is assigned to the vlOa mode. This mode is weakly observed at 837cm-' in the Raman spectrum of C,H, NHlCl- in the solid phase, although the band is too weak to be detected in the solution phase.The enhancement and broadening of the vlOa band was observed previously in the case of adsorbed aniline.3 When the electrode potential was stepped to a more cathodic range, deprotonation of the ion occurred. The two bands in fig. 1 (b) at 1023 and 996 cm-' are assigned to v~~~ and v,, bands of adsorbed aniline in the neutral form. The broad vlOa band is observed at 822 cm-l. The deprotonation occurs at ca. -0.5 V. At this potential, coadsorption of aniline in neutral and protonated forms is observed. The Raman band of adsorbed C1- is observed at ca. 240 cm-'. Fig. 2 shows the change in intensity of the Raman band with the electrode potential in a blank experiment without aniline.The curves (a)-(e) were obtained in a series while the voltage was changed stepwise from -0.2 V towards the more cathodic range. A marked decrease in intensity occurred at ca. -0.5 V. At -0.6 V the peak nearly disappeared. The result is in accordance with the results of Wetzel et aZ.12 in neutral solution. In our experiment addition of aniline to the HCl solution did not change the behaviour of the Raman band of c1-. The fact that the decrease in intensity of Raman bands of the anilinium ion and C1- occurs in the same potential range suggests a relation between the two ionic species. As reported by Wetzel et aZ.,12 I- is adsorbed on Ag more strongly than C1- and Br- in the more cathodic potential range. When we used 0.1 mol dmW3 HI in place of HCl the protooated form of aniline remained the main adsorbate down to -0.9 V.The Raman band of adsorbed I- was also observed at this voltage. It is very likely that the anilinium ion is absorbed on the Ag electrode by Coulombic interaction with halide ions which remain on the surface, depending upon the potential.H. Shindo and C. Nishihara 435 n I 1 I 1 1 1000 900 800 Raman shiftlcm-' Fig. 1. Raman spectra of aniline adsorbed in acidic conditions. Aniline 0.005 mol dm-3; HCl 0.1 mol dm-3, (a) at -0.3 V us. SCE adsorption of C,H,NHi was observed; (b) at -0.6 V adsorption of C,H,NH, was observed. 300 200 Raman shiftlcm-' Fig. 2. Raman spectra of C1- adsorbed on Ag at various potentials. The spectra were obtained in the order ( a x e ) . (a) -0.2 V, (b) -0.3 V, (c) -0.4 V, ( d ) -0.5 V, (e) -0.6 V us.SCE.436 Aniline adsorbed on an Ag Electrode " - I 1 1 1600 1400 1200 1000 800 600 400 Raman shift/cm-' Fig. 3. Raman spectra of anilinium ion in solid and adsorbed states. (a) C,H,NH,Br (solid); (b) C,H,NHi adsorbed at -0.4 V from a 0.005 mol dm-, aqueous solution of aniline with 0.1 mol dm-, HBr; (c) C,H,NHi adsorbed at -0.7 V in a 0.005 mol dm-, solution of aniline with 0.1 mol dm-, HI. The pH of the solution has a large effect on the stability of the anilinium ion on the surface. In the case of 0.1 mol dm-3 HC1 solution, the coadsorption of aniline in the two forms was observed at -0.5 V. When 0.01 mol dm-3 HC1 was used, while keeping [Cl-] = 0.1 mol dm-3 by addition of KC1, the coadsorption was observed at -0.4 V.On the other hand, only the ionised form was observed even at -0.8 V for a solution with 1.0 mol dm-3 HCl. Raman Spectra of the adsorbed Anilinium Ion In fig. 3 (b) and (c) are shown the Raman spectra of anilinium ions adsorbed on Ag from 0.005 mol dm-3 solutions of aniline in 0.1 mol dm-3 HBr and HI, respectively. In order to study the structure of the adsorbed ion in detail, the vibrational spectra of anilinium halides in solid and solution phases were also studied. As the Raman bands of out-of- plane modes are very weak in the solution spectra, the Raman spectrum of anilinium bromide in powder form is shown in fig. 3(a). The assignments in fig. 3(a) were made by studying Raman and infrared spectra of C,H5NH3X (X = C1, Br, I), C6H5ND3X and C6D,NH3X in solid and solution phases.H.Shindo and C. Nishihara 437 Raman shift/cm-' Fig. 4. Raman spectra of C,H,NDi adsorbed on an Ag electrode observed at -0.9 V in a 0.005 mol dm-3 solution of aniline in D,O with 0.1 mol dm-3 DI. Virtually all the H atoms of the amino group are replaced with D atoms. (a) 1750-400 cm-l; (b) 2600-2050 cm-'. - - tn 0- a w > 7. vv N Otn N o , . . 1 I 1 1 1 1600 1200 800 400 Raman shift/cm-' Fig. 5. Raman spectrum of C,D,NHi adsorbed on an Ag electrode observed at -0.9 V in a 0.01 mol dm-3 aqueous solution of C,D,NH, with 0.1 rnol dm-3 HI. Ab initio calculations of vibrational frequencies were also performed. The details of the assignment will be reported in a separate paper and only the results are shown here. Most Raman bands in fig. 3(b) and ( c ) are easily assigned by comparing the spectra with fig.3(a). In order to confirm the assignment, experiments with deuterated compounds were performed also. The Raman spectra of adsorbed C,H,NDi are shown in fig. 4. The spectra were observed at - 0.9 V for a 0.005 mol dm-, solution of aniline in D,O with 0.1 mol dm-, DI. Virtually all H atoms of the amino group were quickly replaced with D atoms. The Raman band of the N-D stretching vibration is observed at ca. 2150 cm-l. A corresponding band was observed for adsorbed C,H,NHi in the experiment in H,O. However, we cannot conclude that this is the N-H stretching band of the anilinium ion, since the observation of C-H stretching bands of contaminants has been reported', in the same frequency range.On the other hand, the broad band in fig. 4 is safely assigned to the N-D stretching band, since it is improbable that the C-H bonds of contaminants are easily replaced with C-D. The N-D stretching band is observed in the 2350-2100 cm-' range for C,H,ND,I powder. The NH, bending band (6,) is observed at ca. 1540 cm-l in fig. 3(b), while the ND,438 Aniline ahorbed on an Ag Electrode bending mode (S,?) is observed at 1125 cm-l in fig. 4(a). The C-N stretching mode is clearly observed in both spectra. Compared to the band at 1629 cm-l in fig. 3(a) the vst, mode of the adsorbed ion at 1653 and 1660 cm-l in fig. 3 (b) and (c), respectively, seems to be too high in energy. Chen et aL1* reported that the bending mode of adsorbed water is sometimes observed at a frequency as high as 1640 cm-l when halide ions are used in large concentrations.However, the bands in fig. 3(b) and (c) do not belong to water, since a similar band is observed at 1650 cm-l, as shown in fig. 4(a) in a D,O solution. The bands do not come from contaminants either, since they are shifted to a lower frequency, as indicated in fig. 5 when C6D,NHi was used with 0.1 mol dm-3 HI. The band certainly belongs to the phenyl ring. The reason for the large shift to higher frequencies upon adsorption is still under investigation. All other assignments in fig. 3-5 are in good agreement with each other. Geometry of Adsorption In the previous r e p ~ r t , ~ geometry of adsorption of aniline was discussed by assuming that the molecule has nearly C,, symmetry. The assumption is more readily applicable to the anilinium ion since the -NHi group rotates freely around the C-N axis just as in the case of the methyl group of toluene.Here, again, the (T, plane was chosen to be perpendicular to the plane of the phenyl ring. Thus, phenyl in-plane vibrations belong to a, or b, symmetry species, while out-of-plane modes belong to b, or a, species. Creighton' calculated the surface Raman enhancement factors for molecules adsorbed at the surface of a metal sphere. He also suggested that the result applies qualitatively to adsorption on roughened electrode and aggregated colloid surfaces. According to his calculations for a C,, molecule, the b, modes are least enhanced when the molecule lies flatly on the surface, while the a, modes are least enhanced when the molecule stands up on the metal with the molecular axis perpendicular to the surface. The former is the case for aniline adsorbed on Ag in the neutral and alkaline condition^.^ As shown in fig.6(a), the molecule most probably adsorbs with n-electrons interacting with the metal atoms on the surface. The results in acidic conditions showed a marked difference. In fig. 3 (c), b, bands such as vet,, v,, and v l g t , are clearly visible, while a, modes such as vlea and vtOa are relatively weak. In neutral conditions the results were opposite. It is suggested that the anilinium ion stands up rather than lies flatly on the electrode surface. The result shown in fig. 3 (b) is an intermediate case, where a, modes as well as b, modes have considerable intensities.It is noted that b, Raman bands such as v16, and v,,, are observed neither in fig. 3(b) nor in (c). They were observed for neutral aniline lying flatly on the s~rface.~ According to the calculation by Creighton,' the b, modes are least enhanced if the molecule lies side-on on the electrode. The side-on adsorption does not seem very likely, but it is not at all impossible considering an interaction between an H atom in the ortho position of the phenyl ring and a halide ion on the surface. However, the b, Raman bands of the anilinium ion are weak in the first place, as shown in fig. 3(a), and it is difficult to evaluate the surface Raman enhancement factors upon adsorption. As is seen in fig. 3(a), the a, Raman bands generally have smaller intensities than the b, modes.The fact that b, Raman bands are stronger than a, modes in fig. 3(c) does not necessarily mean that 'the enhancement factor' for b, modes is larger than that for a, modes. However, it is safe to say that the anilinium ion takes at least two different forms of adsorption and that the case of fig. 3(b) is closer to the face-on adsorption. Let us now consider the reason why the adsorption geometry differs with the choice of halide ions. As discussed above, I- is adsorbed more strongly than Br- and CI- on the surface of Ag.12 If compared at the same potential, the surface concentration of halide ions is largest when I- is used. When a halide ion has a large concentration on theH . Shindo and C . Nishihara 439 Fig. 6. Proposed geometries of adsorption of aniline: (a) face-on adsorption in the neutral form; ( c ) end-on adsorption in the protonated form making bonds with three halide ions on the surface ; (b) an intermediate case in which the anilinium ion makes bonds with two (or one) halide ions.surface, it is probable that the anilinium ion makes bonds with up to three halide ions as demonstrated in fig. 6(b) and (c). Neutron diffraction analyses of the structures of crystalline anilinium bromidel5- l6 support the structure shown in fig. 6 (c). In the case of fig. 6(b) the angle of the molecular plane to the electrode surface is not fixed because of the rotation around the C-N axis. On the other hand, the angle is fixed at 90" in the case of fig. 6(c). In this case the u2 Raman bands are least enhanced.We propose that the adsorption geometry of the anilinium ion is close to fig. 6 ( c ) when the surface concentration of the halide ion is very large. When the potential is swept to a more cathodic range, the halide ion desorbs and the geometry of adsorption of aniline becomes closer to face-on adsorption. In the most cathodic potential range no halide ion remains on the surface, and aniline does not absorb stably in the protonated form. Face- on adsorption in the neutral form prevails instead. The presence of adsorbates on the surface very often has a strong effect on the selectivity of electrode reacti~ns,~' although the mechanisms are not clear in most cases. We have studied the adsorption of aniline in relation to the reduction of nitrobenzene. It is generally said that nitrobenzene is reduced to phenylhydroxylamine (four-electron reduction) in alkaline solutions, while six-electron reduction to aniline prevails in acidic solutions." It is very likely that the adsorbed anilinium ion is working as an effective proton donor on the surface, facilitating the six-electron reduction.References 1 See, e.g. Spectroscopic Studies of Adsorbates on Solid Surfaces, Surf. Sci., 1985, 158. 2 H. Shindo and C. Nishihara, Surf. Sci., 1985, 158, 393. 3 H. Shindo, J. Chem. SOC., Faraday Trans. I , 1982, 82, 45. 4 C. Nishihara and H. Shindo, J. Electroanal. Chem., 1986, 202, 231. 5 H. Shindo, in Recent Advances in Electro-organic Synthesis, Proc. 1st Int. Symp. Electro-organic 6 J. A. Creighton, Surf. Sci., 1983, 124, 209. 7 J. A. Creighton, Surf. Sci., 1985, 158, 211. 8 T. Sakai and H. Terauchi, Acta Crystallogr., Sect. B, 1981, 37, 2101. 9 T. Watanabe, J. Metal Finish. SOC. Jpn, 1982, 33, 96. Synthesis, Kurashiki, 1986, ed. S. Torii (Kodansha-Elsevier, Tokyo-Amsterdam, 1987), p. 405. 10 M. Fleischmann, P. J. Hendra and A. J. McQuillan, Chem. Phys. Lett., 1974, 26, 163. 11 E. B. Wilson, Phys. Rev., 1934, 45, 706. 12 H. Wetzel, H. Gerischer and B. Pettinger, Chem. Phys. Lett., 1981, 78, 392. 13 B. Pettinger, M. R. Philpott, J. G. Gordon 11, J. Chem. Phys., 1981, 74, 934. 14 T. T. Chen, J. F. Owen, R. K. Chang and B. L. Laube, Chem. Phys. Lett., 1982, 89, 356. 15 G. Fecher, A. Weiss, W. Joswig and H. Fuess, Z. Naturforsch., Teil A , 1981, 36, 956. 16 G. Fecher, A. Weiss and G. Heger, Z. Naturforsch., Teil A, 1981, 36, 967. 17 R. Jansson, Chem. Eng. News, 1984, 62 (47), 43. 18 W. Kemura and T. M. Krygowski, in Encyclopedia of Electrochemistry of Elements, ed. A. J. Bard (Dekker, New York, 1979): vol. XIII, chap. 2. Paper 7/369; Received 26th February, 1987