J. Chem. SOC., Faraday Trans. I , 1986, 82, 45-51 Raman Spectroscopic Observation of Adsorbates on Ag during Electrochemical Reduction of Nitrobenzene Hitoshi Shindo National Chemical Laboratory for Industry, Tsukuba Research Center, Yatabe, Ibaraki 305, Japan Raman spectroscopic observation of the surface of a silver electrode during potentiostatic reduction of nitrobenzene in neutral and alkaline aqueous solutions has revealed adsorption of at least four chemical species at different potentials. At - 0.5 V (us. SCE), where the reduction starts, trans-azobenzene was detected with two other species. Raman bands of adsorbed aniline were observed at more cathodic potentials. When aniline was adsorbed from its aqueous solution, three of the above four species except azobenzene were detected on the electrode.Adsorbed aniline lies flat on the surface. Since its discovery by Fleischmann et aI.,l surface enhanced Raman scattering (SERS)2~ has been applied to the studies of the structure of molecules in their adsorption states. Acquisition of such data is most welcome in the fields of heterogeneous catalysis and electrochemical synthesis where surface adsorbates play important roles. In situ analysis of interfacial structure with Raman spectroscopy seems to have its greatest advantage there. The reduction of nitrobenzene is one of the most studied reactions in electrochemistry. Various products, including dimeric compounds, are selectively by altering reaction conditions. However, the mechanism of the change in the selectivity with change of voltage, pH, solvent, electrode materials, etc.has not yet been clarified in detail owing to the lack of information on the chemical structure of the interfacial region. The author has applied SERS to the study of adsorbates on a silver electrode during potentiostatic reduction of nitrobenzene in neutral and alkaline aqueous solutions. The main products are phenylhydroxylamine89 and aniline in these solutions. Experimental Experimental details of the measurement of Raman spectra have been given in previous reports1O* l1 and are only briefly described here. A silver plate (Furu-Uchi Chemicals, 99.99 % ) was polished to a mirror finish with alumina suspension (Baikalox, 0.05 pm) and was used as the working electrode after ultrasonic cleaning. A saturated calomel electrode (SCE) was used as the reference electrode.A platinum wire was used as the counter electrode and was placed in a separate compartment. As pretreatments of the silver electrode, hydrogen generation at -2.0 V (us. SCE, 20 s) and several oxidation- reduction cycles (ORC) between -0.2 and +0.3 V ( 3 s) were performed by potential steps. Silver is first oxidized to form AgCl and, then, reduced to Ag in the ORC. The measurements of Raman spectra using the 514.5 nm line of an argon ion laser [Coherent Radiation CR-8, 50 mW) and a double dispersion Raman spectrometer (JASCO R-800) were performed during potentiostatic reduction of nitrobenzene at given potentials. The sample solutions were prepared by dissolving nitrobenzene [(2.0-5.0) x 10-3 mol dm-3] and KCl (0.1 mol drn-,), a supporting electrolyte, in distilled water.The solutions were purged of dissolved oxygen by nitrogen bubbling. Buffer reagents (H,BO, 4546 H. Shindo and NaOH) were used when required. When strongly alkaline solutions were prepared, NaOH or KOH (0.1 mol dm-3) were used. Nitrobenzene (Wako Pure Chemical, GR grade) was purified by washing first with dilute H,SO,, then with dilute NaOH, followed by vacuum distillation. Aniline (Nakarai Chemicals, GR grade) was purified by vacuum distillation. Phenylhydroxylamine was synthesized by a literature method1, and was purified by recrystallizing from water. The main impurity was azoxybenzene. Phenylhydroxylamine was fairly stable when kept under vacuum in a refrigerator. Results and Discussion Reduction of Nitrobenzene Prior to Raman measurement, the electrochemistry of nitrobenzene was checked using the rotating ring-disc electrodes method.13 An unbuffered neutral solution of nitro- benzene with 0.1 mol dmP3 KCl was used first.Both ring and disc electrodes (Ag) were pretreated by H, generation. Nitrobenzene (2.0 x mol dmP3) was reduced mainly to PhNHOH (4e-reduction) in the potential range between - 0.5 and - 0.9 V. The product was detected by its electrochemical oxidation to PhNO at the ring electrode. When the potential of the disc electrode was scanned in a more cathodic range, formation of PhNH, (6e-reduction) occurred and became dominant at - 1.6 V. Use of KOH (0.1 mol dm-3) instead of KCl did not alter the shape of the current-potential curve extensively.The product molecules were identified from their U.V. absorption spectra in alkaline solution. Results of detailed electrochemical analysis for the alkaline solution will be reported e1~ewhere.l~ In the first series of Raman experiments nitrobenzene (5.0 x lop3 mol dmP3) was reduced in a neutral solution without buffer reagents. Changing the concentration of the reactant from (5.0 to 2.0) x lod3 mol dm-3 did not affect the results described below. As stated in the previous paragraph the reduction occurs at ca. - 0.5 V and at more cathodic potentials. Only weak Raman bands of the parent molecule and water were observed at -0.4 V and at more anodic potentials. When the voltage was set to -0.5 V and to more cathodic values, the reduction occurred and Raman spectra shown in fig.1 were observed. Each curve was obtained independently after cleaning by H, generation and an ORC treatment. The measurement was completed within the first six minutes of the potentiostatic reduction. At least four different chemical species, A-D, are observed in fig. 1. The species A which is strongly observed at -0.5 V has already been assigned to trans-azobenzene in the author’s previous reports.lo9 l 1 3 l5 To be accurate, spectra of two types of trans-azobenzene with different degrees of interaction with the surface are simultaneously observed15 as species A in fig. l ( a ) . The one with weaker surface interaction gave a spectrum very similar to that of the molecule in methanol solution. Large resonance enhancement is not expected in the tail of the n-n* transition of trans-azobenzenelG at 514.5 nm.The spectrum of the other species indicates a marked difference in the relative intensities of the Raman bands of vN=N and v ~ - ~ vibrations, which suggests a kind of complexation of azobenzene with the metal. The latter species becomes dominant with time. When the potential was lowered from - 0.5 to - 0.7 V, azobenzene was reduced to hydrazobenzene and the Raman bands of A disappeared. On the other hand, Raman bands of species D grew toward the more cathodic potentials as shown in fig. 1 (b)-(d). The bands assigned to species B and C were strongly observed in the potential range between -0.5 and -0.8 V. The possibility of observing adsorption of product molecules was checked first. Phenylhydroxylamine and aniline are expected as the final products of the reaction system.In addition, hydrazobenzene is also a possible reaction product, since its pre- cursor, azobenzene, has been detected on the electrode. The Raman spectra of the threeElectrochemical Reduction of Nitrobenzene 47 I D D 996 1600 1400 1200 1000 Raman shiftlcm-’ Fig. 1. Raman spectra of adsorbates on Ag observed during potentiostatic reduction of nitrobenzene (5 x lop3 mol dm-3) in a neutral aqueous solution [KCl 0.1 mol dmp3): (a) -0.5 V (cs. SCE); (b) -0.8 V; (c) - 1.0 V; ( d ) - 1.2 V. The spectral resolution is 5 cm-l. See ref. (1 1) and (15) for the frequencies and assignment of the bands denoted ‘A’. candidates are shown in fig. 2. Vibrational assignment for aniline has been given in the literatureI7-l9 and is reproduced in table 1.Apart from vibrations of the amino group the assignment seems mostly applicable to the other two molecules because they all have a common structure. The two bands of species D in fig. 1 (d) at 996 and 1025 cm-l resemble the bands in the same region in fig. 2(a) and (b), which are assigned to ring deformation (vI2) and C-H in-plane bending (v18J vibrations. If there are any Raman bands ofD in fig. 1 (d) in the frequency range 1100-1 500 cm-l they are obscured by the bands of other species. No C-H or N-H stretching vibration was clearly observed in the higher frequency range. Further comparison with the reference spectra was made in the lower frequency range. As will be seen in fig. 3 ( 4 , the lower frequency bands of D also resemble those in fig.2(a) and (b). It seems appropriate to assign D to adsorbed aniline or hydrazobenzene. However, it is difficult to determine which of the two is the correct answer just by discussing slight differences between the reference spectra. Adsorption of molecules generally causes changes in peak positions and intensities of Raman bands. It is noticeable in fig. 1 (a) and (b) that a large and broad background is observed from 1100-1700 cm-’. This might be attributable to Raman bands of carbon as observed by Mahoney et al.,l However, it is unlikely that the background came from impurities in the solution such as CO, and CO:-. Blank experiments without nitrobenzene did not show any such background even with addition of CO, or K,CO, (1 .O x lo-, mol dmp3) to the solution.The background appears only when Raman bands of A, B and C are strongly observed. Photodecomposition of adsorbates might be the cause of the background.48 H. Shindo 4 ----I- I 1 6 . L 1400 1000 600 2 00 Raman shift/cm-' Fig. 2. Raman spectra of several product molecules in nitrobenzene reduction: (a) aniline (neat liquid) ; (b) hydrazobenzene (solid) ; (c) phenylhydroxylamine (solid). The asterisks in (c) indicate Raman bands of trans-azoxybenzene as an impurity. The spectral resolution is 5 ern-'. See table I and ref. (1 7)-( 19) for vibrational assignment of aniline. Adsorption of Aniline In order to identify the adsorbates observed in fig. 1, adsorption of aniline was studied. The molecule was adsorbed from its 1.0 x lo-, mol dm-3 neutral aqueous solution.Potassium chloride (0.1 mol dm-3) was added. Following H, generation at - 2.0 V (20 s) and oxidation of Ag at +0.3 V (3 s), the potential was set to - 1.0 V. The Raman spectrum in fig. 3(b) was obtained. The neat spectrum of aniline is reproduced in fig. 3 (a). Peak positions of the bands in the two spectra [(a) and (b)] are in good agreement, although discrepancies are noticed in the relative intensities. The peak positions and approximate intensities of the two spectra are summarized in table 1 together with their assignment. If we neglect coupling of the -NH, vibrations with vibrations of the phenyl ring, we can assume C,, symmetry for the Ph-N group and discuss the geometry of adsorption from the change in intensity of the Raman spectra.Creighton,, proposed expressions of electromagnetic enhancement factors, depending upon symmetry species, of Raman intensities of adsorbed molecules and discussed the orientation of adsorbed pyridine on an Ag electrode. By comparing intensities of the Raman bands of aniline in its 'free' and adsorbed states, as shown in table 1, it is noticed that a, and b, bands are most enhanced by the adsorption, while the b, bands are least enhanced. It is thus concluded that at - 1 .O V and under the assumption of C,, symmetry, aniline is adsorbed on Ag with the phenyl ring lying flat on the surface. The nitrogen lone pair and phenyl 71 electrons are both likely to interact with the surface. However, the interaction seems to be weak since the change in peak positions on adsorption is rather small.When the electrode potential was stepped from - 1.0 to -0.6 V, the spectrum in fig. 3 (c) was observed. Marked differences are noticed, especially in the higher frequencyElectrochemical Reduction of Nitrobenzene Table 1. Assignment of aniline Raman bands 49 frequency and intensityC typea assignmentb neat/cm-' adsorbed/cm-l phenyl in-plane a1 b2 out-o f-plane a2 others 8a C-C stretch 19a C-C stretch C-N stretch 9a C-H bend 18a C-H bend 12 ring def. 1 ring breathing 6a ring def. 19b C-C stretch 14 C-C stretch 9b C--H bend 6b ringdef. C-N bend 10a C-H bend 16a ring def. 17b C-H bend 4 C-Hbend 10b C-H bend 11 C-H bend C-N bend NH, bend 2 x NH, wag? 2 x 10b ring def. 1602 s 1500 w 1278 m 1175 m 1027 s 996 s 814 s 530 m 1467 w 1341 vw 1154 m 619m 386 m 826 m 412 vw 883 vw 755 w 506 w 231 m 1618 w 1387 vw - - 1602 s - - 1175 w 1023 s 996 s ? - - - - 617 w - 822 s 409 m 884 m 749 m 698 w 508 m - - 1290 w - a By neglecting H atoms of -NH, group C,, symmetry was assumed.From ref. (1 8) [partly from ref. (19)]. Wilson numbering [ref. (20)] was used. Present work (resolution 2 cm-l). Intensity: s, strong; m, medium; w, weak; vw, very weak. range. It may be possible to explain the change of the spectrum from (b) to (c) in the frequency range below 900 cm-l as that caused by the change in geometry of adsorption. However, it is difficult to give such an explanation for the change in the 1100-1500 cm-l range. We therefore consider below the possibility of different adsorbates. Cyclic voltammograms indicate that aniline does not react in the potential range employed.However, it is possible that a limited amount of aniline is oxidized on the surface to form other adsorbates. Let us now compare the spectra of adsorbates derived from aniline with those observed during reduction of nitrobenzene. The spectrum in fig. 3(d) was observed during the reduction at -0.9 V in a neutral solution (5 x mol dm-3). The Raman bands of species D are in good agreement with the bands in fig. 3(b). The four other bands of species B and C in fig. 3 ( d ) or in fig. 1 in the 1100-1450 cm-l region, on the other hand, agree well with those observed in fig. 3 ( c ) . It is likely that the same species B, C and D are observed in fig. 3(b)-(d). The species D is assigned to adsorbed aniline lying flat on the electrode. The assignment of the bands at 1531 and 1558 cm-l in fig.3(c) is not known yet. Since species B and C appeared both in the reduction of nitrobenzene and adsorption of aniline, we should think about the molecules which would appear on the way when50 H . Shindo 996 1602 I 822 1027 I 1600 1400 1200 1000 800 600 400 Raman shiftlcm-' Fig. 3. Raman spectra of adsorbates derived from aniline and nitrobenzene: (a) aniline (neat); (b) adsorbed aniline (1 x loA2 mol dm-3 as.) at - 1 .O V (us. SCE); (c) following (b) the potential was changed to -0.6 V; ( d ) adsorbates (B, C and D in fig. 1) observed during reduction of nitrobenzene ( 5 x mol dm-3) at -0.9 V in a neutral solution. The spectral resolution is 5 cm-l [2 cm-l for curve (a)].we go from nitrobenzene to aniline in the reduction path. However, Raman spectra of B and C do not resemble those of nitrosobenzene, nor phenylhydroxylamine. The possi- bility of other species, for instance, dissociatively adsorbed species, should also be checked. Kishi et aZ.23 reported the observation, by X.P.S. measurement of the N 1s band, of PhNH(ads), PhN(ads) and PhNO(ads) on Ni and Fe which were derived from nitrobenzene and aniline. Assignment of B and C to such species seems to explain well the behaviour of adsorbates observed on the Ag electrode. For instance, formation of azobenzene on the surface can be explained by dimerization of the adsorbates. However, no direct evidence for the assignment is available at present. Dependence on pH In the first series of experiments, nitrobenzene was reduced in a neutral solution without buffer reagents.In this case consumption of protons in the reaction increases the local pH in the vicinity the electrode. The Raman measurements with buffer solutions of pH 8.15 and 9.45 resulted in observation of similar adsorbates A-D on the electrode. Essentially the same process occurs on the surface as in the case without buffer reagents. Some differences are noticed, however. During the reduction at -0.5 V, Raman bandsElectrochemical Reduction of Nitrobenzene 51 of azobenzene were more weakly observed and those of aniline were more strongly observed in the buffered solutions. When the behaviour of adsorbates is quantitively studied, controlling the pH will be more important.Cyclic voltammograms of nitrobenzene in alkaline (0.1 mol dm-3 KOH or NaOH) solutions clearly indicated a larger contribution of binuclear molecules, azobenzene and hydrazobenzene, in the reaction system. Stronger Raman bands of azobenzene were observed at -0.5 V in this case than in neutral solutions. In the previous section the author concluded that the species D is adsorbed aniline. There still remains a possibility that Raman bands of hydrazobenzene are overlapped with those of aniline. Hydrazobenzene seems to be stereochemically less favourable as an adsorbate on a flat surface. Because of its low solubility in water, however, it is probable that the molecule produced via azobenzene remains in the vicinity of the electrode in the form of organic multilayers.The author thanks the referees, whose valuable advice greatly helped improve the discussions in this paper. References 1 M. Fleischmann, P. J. Hendra and A. J. McQuillan, Chem. Phys. Lett., 1974, 26, 163. 2 Surface Enhanced Raman Scattering, ed. R. K. Chang and T: E. Furtak (Plenum Press, New York, 1982). 3 J. A.'Creighton, in Vibrational Spectroscopy of Adsorbates, ed. R. F. Willis (Springer-Verlag. Berlin, 4 C. L. Wilson and H. V. Udupa, Trans. Electrochem. SOC., 1952, 99, 289. 5 R. H. McKee and C. J. Brockman, Trans. Electrochem. Soc., 1932, 62, 203. 6 R. C. Snowdon, J . Phys. Chem., 1911, 15, 797. 7 K. Sugino and T. Sekine, J. Electrochem. Soc., 1957, 104, 497. 8 W. Kemula and T. M. Krygowski, in Encyclopedia of Electrochemistry of Elements, ed. A. J. Bard 9 G. Kokkinidis and K. Juttner, Electrochim. 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Catal., 1979, 68, 228. 1980). (Dekker, New York, 1979), vol. XIII, chap. 2. Paper 412070; Received 7th December, 1984