Theoretical Study on Electroreduction of p-Nitrothiophenol on Silver

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Theoretical Study on Electroreduction of pNitrothiophenol on Silver and Gold Electrode Surfaces Liu-Bin Zhao, Jia-Li Chen, Meng Zhang, De-Yin Wu, and Zhong-Qun Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp512957c • Publication Date (Web): 10 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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The Journal of Physical Chemistry

Theoretical Study on Electroreduction of p-Nitrothiophenol on Silver and Gold Electrode Surfaces Liu-Bin Zhao*, †,Jia-Li Chen‡, Meng Zhang‡, De-Yin Wu*, ‡, and Zhong-Qun Tian‡ † Department of Chemistry, School of Chemistry and Chemical Engineering, Southwest University, Chongqing400715, China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China Corresponding Authors [email protected]; [email protected] Abstract: The electro-reduction of p-nitrothiophenol (PNTP) on gold and silver electrodes has been investigated by means of density functional theory. A combination of thermodynamic calculations and surface Raman/IR spectral simulations has allowed us to reveal the reaction mechanism and reaction products of electro-reduction of PNTP on metal electrodes. Firstly, thermodynamic calculations are carried out to calculate the standard electrode potentials of PNTP and its possible intermediates. The potential energy curves of PNTP reduction as a function of the applied potential are obtained on the basis of the calculated standard electrode potentials of the elementary electrochemical reactions. Secondly, surface vibrational spectral simulation is performed to provide theoretical assignments of reaction products for the in situ Raman/IR experimental studies of electro-reduction of PNTP. The most interesting finding in the reaction product identified by IR spectroscopy is PATP, however, Raman spectroscopy show that the main product is p,p′-dimercaptoazobenzene (DMAB). The difference between IR and Raman measurements arises from that the incident laser used in Raman measurement can induce the formation of DMAB by photo-reduction of PNTP or photo-oxidation of PATP. Finally, the reaction mechanism of electro-reduction of PNTP is compared with its photo-reduction mechanism.

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Introduction Nitrobenzene reduction is very important reaction in industrial production of aniline1-3 and water pollution treatment.4-6 Nitrobenzene reduction can be driven by either electrochemical cathodic polarization or light irradiation or catalytic hydrogenation. The electro-reduction of nitrobenzene is one of the most classical model systems and has drawn large amount of attention in electrochemical reaction mechanism investigation for multiple electron transfers and proton transfers, various reaction intermediates and products, and different reaction pathways are involved in nitrobenzene reductions.7-10 The general accepted reaction network for the reduction of aromatic nitro compounds is based on the electrochemical model presented by Haber.11-12 Two different routes are proposed for this reaction. In the more direct route, the aromatic nitro compound is reduced to the nitroso compound and then to the corresponding hydroxylamine. Finally, the hydroxylamine is further reduced to the aniline derivative (horizontal pathway in Scheme 1). In the second route, the nucleophilic hydroxylamine react with electrophilic nitroso to give the azoxy compound, which is consecutive steps to the azo, hydrazo, and aniline compounds (vertical pathway in Scheme 1).3 The latter route is favorable under alkaline conditions. As seen in Scheme 1, the electro-reduction of nitrobenzene is extremely complicate for various intermediates. It is quite difficult to characterize every single reaction species by using traditional electrochemical methods.


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Scheme 1.Schematic illustration of reaction networks of electro-reduction of an aromatic nitro compound to the corresponding aniline. An important task in electrochemical reaction mechanism investigation is to identify the intermediates and products during the reaction. Surface-enhanced vibrational spectroscopy, including surface-enhanced Raman scattering (SERS)13-14 spectroscopy and surface-enhanced infrared adsorption (SEIRA) spectroscopy,15 can provide in situ probes of electrochemical interfaces. These techniques open a window for us to obtain electrode-electrolyte interfacial information and study the electrochemical reaction processes at the molecular level. Two different mechanisms, the electromagnetic (EM) and chemical enhancement (CE), are proposed to contribute to the total enhancement in SERS and SEIRS. The EM mechanism originates from the



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collective excitation of free electrons on metal surfaces and gives rise to great enhancement of local electromagnetic field due to surface plasmon resonance.16 The CE mechanism is contributed from the chemisorption interaction, the photon-driven charge transfer (CT) between adsorbate and substrate, and the coupling effect between the electron-hole pair and adsorbed molecules.17-18 The ability to amplify spectral signal from molecules adsorbed on metal surfaces allows SERS and SEIRA to detect not only the adsorption/desorption behavior, but also the redox process of the surface species by providing direct identification of the adsorbed intermediates and/or products formed in multi steps. When combined with conventional electrochemical measurements, surface vibrational spectroscopy can be applied to identify the intermediates and evaluate the reaction pathways of the electrochemical reaction.14 The p-nitrothiophenol (PNTP) molecule, which has a thiol group substituted at the para position of nitrobenzene, can form a self-assembled monolayer (SAM) on metal surfaces and metal nanoparticles and its electrochemical19-24 and photochemical25-33 reduction have been studied by various spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS),34-36 SERS,19-23, 26 and SEIRA.19, 24 In situ FTIR spectroscopy has been used to study the electro-reduction of PNTP adsorbed on gold and silver electrodes.19, 24 A nitro-to-amino reduction was evaluated from the spectral changes. Combined cyclic voltammetry and in situ SERS studies of the electro-reduction of nitrobenzene on gold and silver electrodes has been reported.19, 22 Although the reaction products were identically assigned to PATP in these studies, the spectral behaviors of the reaction products on gold22 and silver19 surfaces are significantly different. It is noted that the two surface-enhanced vibrational spectroscopies, SERS and SEIRA, give distinct spectral properties of PNTP and PATP adsorbed on metal surfaces. The surface IR spectra of PNTP and PATP are quite similar to their normal IR spectra.37-38 However, the surface Raman spectra of PNTP and PATP often exhibit abnormal signals which are different from their normal Raman spectra.39-40 Our recent theoretical and experimental studies demonstrate that both PNTP and its reduction product PATP can transform to DMAB with visible light irradiation.41-47 The laser used in SERS measurements can easily trigger the probe molecules to occur photoreactions.42, 44 As seen from above, the identification of reduction products of PNTP on gold and silver electrodes is still controversial. The lack of thermodynamic and kinetic data about elementary electrochemical reactions was expected to be solved separately. In this case, theoretical calculations can provide detailed reaction properties and standard spectra of possible existing surface species which help us for a better understanding of the complicated electrode interface processes. In the present paper, a combination of thermodynamic and spectroscopic investigation is carried out to study the electro-reduction of PNTP by means of density functional theory (DFT) calculations. The potential energy curves of PNTP reduction as a function of the applied electrode potential are obtained on the basis of the calculated standard electrode potentials of the elementary electrochemical reactions. The most interesting problem is why the final reaction product is identified to be PATP by IR spectroscopy and DMAB by Raman spectroscopy. Accordingly, we simulate the surface Raman and IR spectra of reaction species during PNTP reduction, and the theoretical spectra are compared with reported experimental results to give assignments of reaction intermediates and products.

Computational details


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The metallic cluster model was employed to investigate the electro-reduction of PNTP on gold and silver surfaces. Molecules adsorbed on the metal electrode surfaces were modeled as the metal-molecule complexes. Density functional theory (DFT) calculations were carried out with the hybrid exchange-correlation functional B3LYP48-49 and generalized gradient approximation functional PW91PW91.50 The basis sets for C, H,N, O, and S atoms of investigated molecules were 6-311+G**, including the polarization function to all the atoms and the diffuse function to C, N, O, and S atoms.51-52 For all metal atoms, the valence electrons and the inner shells were described by the basis set, LANL2DZ, and the corresponding relativistic effective core potentials, respectively.53-54 The solvent effect was considered by integral equation formalism polarization continuum model (PCM).55 Water with dielectric constant (ε = 78.3) was chosen as the solvent. Electronic excitation was examined through the time-dependent DFT (TD-DFT) approach TD-B3LYP, which is efficient and offers an orbital picture for a physical understanding of the excitation process.56 The TD-DFT results were used to predict the charge transfer (CT) direction of reactants on metal electrodes. The photoinduced electron transfer processes can be visualized from analysis of the molecular orbitals involved in the CT transitions, i.e., the photoinduced reduction is associated with a metal-to-molecule CT while the photoinduced oxidation is associated with a molecule-to-metal CT.45 Surface Raman and IR spectra are simulated on the basis of the optimized structures. Note that only chemical enhancement is considered in the present theoretical calculations. For vibrations with the same irreducible representation, the CE effect should be considered as the main factor to influence their relative Raman intensities, whereas the EM enhancement only magnifies the CE effect on their SERS signals.14 To make sure that all the structures reported here were the minima on potential energy surfaces, we verified that there is no imaginary frequency. A scaling factor of 0.981 was used in B3LYP/6-311+G** level calculation.41, 57 IR intensities and Raman activities of the optimized structures were obtained from the integral adsorption coefficient and the differential Raman scattering cross sections within the double harmonic approximation.58 The scaled quantum mechanics force field (SQMF) procedure was used to assign all the fundamental bands.59 To allow a direct comparison with experimental spectra, the simulated IR and Raman spectra were presented in terms of a Lorentzian expansion with a line width of 10 cm-1and an excitation wavelength of 632.8 nm was used in the calculation of Raman spectra. The frequency calculations were performed to obtain the thermodynamic data. The standard electrode potentials were calculated from the Gibbs free energy changes at temperature of 298.15 K and pressure of 1 atm. The electro-reduction of PNTP contains multiple electron transfers and proton transfers. For a typical one-electron-one-proton transfer reaction,

AO + H + + e − → AR H

(1)

The Gibbs free energy change of eq. 1 depends on the electrode potential U and solution pH through the chemical potentials of e− and H+.

H + + e- → 1 2 H 2 ( g )

(2)

It is difficult to calculate the absolute energies of an electron in the electrode and a proton in the electrolyte.60 By setting the reference potential to be that of the standard hydrogen electrode (SHE), we can relate the chemical potential for the reaction (H+ + e−) to that of 1/2 H2. This means



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that, at pH = 0 in the electrolyte and 1 bar of H2 in the gas phase at 298 K, the reaction free energy of eq. 1 is equal to that of the hydrogenation reaction (eq. 3) at an electrode potential of U = 0 relative to the standard hydrogen electrode.60-61

AO + 1 2 H 2 ( g ) → AR H

(3)

The standard electrode potentials of eq. 1 can be then calculated from the change in Gibbs free energy of eq. 3,

∆G 0 = −nFE 0

(4)

We introduce the effect of a bias on all states involving an electron in the electrode, by shifting the energy of this state by –eU, where U is the electrode potential. The pH effect is considered from the Nernst equation. This allows us to calculate the free energy of eq. 1 at arbitrary solution pH and electrode potential U,

∆G (U , pH) = ∆G (U = 0, pH) − neU = − nFE − neU RT = − nF ( E 0 − ln[H + ] − neU F 0 = − nFE + 0.0592nF ⋅ pH − neU

(5)

All calculations including structure optimization and frequency computation were carried out by using Gaussian 09 package.62 The optimized structures of reaction species on gold and silver surfaces can be found in Figures S1 and S2 in the Supporting Information.

Results and discussion Potential energy curves of electro-reduction of PNTP Figure 1 shows the elementary reaction steps of electro-reduction of PNTP to PATP. In this scheme, PNTP is reduced to PATP by a six-electron-six-proton reduction process through p-nitrosothiophenol (PNSTP) and p-hydroxylaminothiophenol (PHATP). The calculated thermodynamic data for the reactions in Figure 1 are given in Table 1. As shown in Figure 1, PNTP receives two electrons and two protons to give PNTP(2H), which undergoes an irreversible dehydration reaction to produce PNSTP.10 On gold electrodes, the calculated standard electrode potentials of PNTP/PNTP(1H) and PNTP(1H)/PNTP(2H) are −0.36 and 0.23 V vs. SHE. The one-electron reduction potential of nitrobenzene was predicted to be – 1.29 V vs. SHE.63 The reduction potential increases a lot as the proton transfer is coupled with electron transfer. Tsutsumiet al.34 and Nielson et al.35 studied the electrochemical behaviors of PNTP on gold electrodes by cyclic voltammetry. A cathodic wave was observed at –0.4 V vs. statured calomel electrode (SCE) in a pH 2 buffer solution and –0.65 V vs. SCE in a pH 7 buffer solution. By referring to the standard hydrogen electrode, the measured reduction potential is about 0 V in pH 0 solution. This value is close to the calculated standard electrode potential of PNTP/PNTP(2H). PNTP(2H) is not a stable intermediate, which transforms to PNSTP by losing one water molecule. The dehydration reaction is an exothermic (∆H= –23.2 kcal/mol) and entropy increasing reaction (∆S= 36.0 cal/mol·K), with a Gibbs free energy change of –34.0 kcal/mol. PNSTP undergoes two-electron-two-proton transfers to give PHATP through the intermediate PNSTP(H). The calculated standard electrode potentials of PNSTP/PNSTP(H) and PNSTP(H)/PHATP are predicted at 0.46 and 0.34 V vs. SHE. Experimentally, the cathodic wave


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of PNSTP/PNTP pair cannot be observed in the first scan in voltammograms, because the reduction potential of PNSTP is more positive than that of PNTP.64 In the first reverse scan, an anodic peak can be found corresponding to the re-oxidation of PHATP to PNSTP, which is partnered with a cathodic peak in the second negative-going scan. The reversible redox voltammetric peak of PNSTP/PHATP pair was observed at 0.2 V vs. SCE in a pH 2 buffer solution34 and –0.175 V vs. AgCl/Ag in a pH 8 buffer solution.64 By referring to the standard hydrogen electrode, the measured reduction potentials are about 0.444 V in pH 2 solution and 0.022 V in pH 8 solution. The calculated redox potentials of PNSTP/PHATP pair in pH 2 and 8 solutions are 0.282 V and –0.074 V.

Figure 1. Elementary reaction steps for electro-reduction of PNTP to PATP. PNTP, p-nitrothiophenol; PNSTP, p-nitrosothiophenol; PHATP, p-hydroxylaminothiophenol; PATP, p-aminothiophenol. The reduction of PHATP to PATP is an irreversible process, with a free energy change of –57 kcal/mol. Note that the calculated standard electrode potentials decrease as PHATP > PNSTP > PNTP, which means that PNTP is directly reduced to PATP when the electrode potential reaches the reduction potential of PNTP. This can rationalize that only a broad cathodic wave was observed in the voltammograms of PNTP.19, 22, 34-35, 64 Figure 2 shows the Gibbs free energies of PNTP to PATP as a function of the applied potential in a pH 7 solution according to eq. 5 and the calculated standard electrode potentials listed in Table 1. The potential energy curves of reduction of PNTP to PATP on a silver electrode were given in Figure S3 in Supporting Information. As seen from Figure 2, the Gibbs free energies of reaction species decrease with the negative-going movement of the electrode potential. By comparing the reduction potentials for each one-electron-one-proton elementary reactions, we find the reduction of PNTP to PNTP(1H) is the



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rate determining step during the overall six-electron-six-proton transfer process, which has the most negative reduction potential.7-8 Also note that the reductions of PNTP to PNSTP and PHATP to PATP are irreversible reactions while the reduction of PNSTP to PHATP is reversible.

Figure 2. Calculated Gibbs free energies of electro-reduction of PNTP on a gold electrode in a neutral solution (pH = 7) as a function of the applied potential. The electrode potentials are referred to the standard hydrogen electrode. Table 1. Calculated Enthalpies ∆H0, Entropies ∆S0, Gibbs Free Energies∆G0, and Standard Electrode Potentials E0 for Electro-reduction of PNTP to PATP on Gold Electrode. Reaction PNTP → PNSTP PNTP→ PNTP(2H) PNSTP → PHATP PHATP → PATP PNTP →PNTP(1H) PNTP(1H) → PNTP(2H) PNTP(2H)→ PNSTP PNSTP → PNSTP(H) PNSTP(H) → PHATP PHATP → PATP(NH) PATP(NH) → PATP

∆H0 (kcal/mol)

∆S0 (cal/mol·K)

∆G0 (kcal/mol)

E0cal. (V)

−26.54 −3.30 −24.86 −56.96 6.83

8.21 −27.76 −28.19 7.83 −8.24

−31.06 2.90 −18.53 −61.37 8.25

0.67 −0.06 0.40 1.33 −0.36

−10.13

−19.52

−5.35

0.23

−23.24 −13.52 −11.35 −26.73 −33.40

35.97 −13.05 −15.14 22.59 −15.08

−33.96 −10.66 −7.87 −31.43 −29.94

0.46 0.34 1.36 1.30

a

Rescaled to the standard hydrogen electrode in pH 0 solution –0.4 V vs. SCE in a pH 2 buffer solution from ref. 34 c 0.2 V vs. SCE in a pH 2 buffer solution from ref. 34 b


E0exp. (V)a −0.04b 0.56c

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In alkaline medium the nitroso compound may act as an electrophilic center and the hydroxylamine compound can react with it with the formation of azoxy compound, which can be further reduced to azo, hydrazo, and finally aniline species.7-10 Figure 3 shows the elementary reaction steps of electro-reduction of DMAOB to PATP. In this scheme, DMAOB is formed by the condensation reaction of PNSTP and PHATP. DMAOB is then reduced to DMAB, DMHAB, and PATP through sequential electron and proton transfers. The calculated thermodynamic data for these elementary reactions in Figure 3 are given in Table 2.

Figure 3. Elementary reaction steps for electro-reduction of DMAOB to PATP. DMAOB, p,p′-dimercaptoazoxybenzene; DMAB, p,p′-dimercaptoazobenzene; DMHAB, p,p′-dimercaptohydrazobenzene. As shown in Figure 3, DMAOB obtains two electrons and two protons to give DMAOB(2H), which undergoes an irreversible dehydration reaction to produce DMAB. The calculated standard electrode potentials of DMAOB/DMAOB(1H) and DMAOB(1H)/DMAOB(2H) are −0.24 and 0.37 V vs. SHE. The standard electrode potential of DMAOB/DMAOB(2H) was predicted to be 0.07 V more positive than for the PNTP/PNTP(2H) redox pair at −0.06 V. This is in agreement with the voltammograms of electro-reduction of nitrobenzene on a gold electrode.7 DMAOB(2H) is not a stable intermediate, which transforms to DMAB by losing one water molecule. The irreversible dehydration reaction is an exothermic (∆H= –33.9 kcal/mol) and entropy increasing (∆S= 39.9 cal/mol·K) reaction, with a Gibbs free energy change of –45.8 kcal/mol.



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Figure 4. Calculated Gibbs free energies of electro-reduction of DMAOB on gold electrode in a neutral solution (pH = 7) as a function of the applied electrode potential. The electrode potentials are referred to the standard hydrogen electrode. The reduction of DMAB to DMHAB is a reversible process with a standard electrode potential of 0.22 V. The reversible redox peaks corresponding to DMAB/DMHAB pair in the second scan of voltammograms of nitrobenzene in basic solution.7 The two-electron-two-proton reduction of DMAB to PATP is an irreversible process with a Gibbs free energy change of −47.3 kcal/mol. Note that the calculated standard electrode potentials decrease as DMHAB > DMAB > DMAOB, which means that DMAOB is directly reduced to PATP when the electrode potential reaches the reduction potential of DMAOB. Table 2.Enthalpies ∆H0, Entropies ∆S0, Gibbs Free Energies∆G0, and Standard Electrode Potentials E0 for Electro-reduction of DMAOB to PATP on Gold Electrode. Reaction

∆H0 (kcal/mol)

∆S0 (cal/mol·K)

∆G0 (kcal/mol)

E0 (V)

DMAOB→ DMAB DMAOB→ DMAOB(2H) DMAB→ DMHAB DMHAB→2PATP DMAOB→DMAOB(1H) DMAOB(1H) → DMAOB(2H) DMAOB(2H) →DMAB DMAB→DMAB(H) DMAB(H)→ DMHAB DMHAB → PATP + PATP(H) PATP(H)→PATP

−44.45 −10.58 −16.96 −39.99 2.53 −13.11 −33.87 −1.57 −15.39 −6.59 −33.40

8.09 −31.77 −30.09 17.70 −13.17 −18.60 39.86 −12.59 −17.49 32.78 −15.08

−48.94 −3.18 −10.07 −47.34 5.42 −8.60 −45.75 1.15 −11.21 −17.40 −29.94

1.06 0.07 0.22 1.03 −0.24 0.37 −0.05 0.49 0.76 1.30


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Figure 4 shows the Gibbs free energies of DMAOB to PATP as a function of the applied electrode potential in a pH 7 solution according to eq. 5 and the calculated standard electrode potentials listed in Table 2. The potential energy curves of reduction of DMAOB to PATP on a silver electrode were given in Figure S4 in Supporting Information. The potential energy curves in Figure 4 are quite similar to that in Figure 2 except that the reduction potential of DMAOB is positive to that of PNTP and the reduction potential of DMAB is negative to that of PNSTP. We also note that the reduction of DMAOB to DMAOB(1H) is the rate determining step during the reduction of DMAOB to PATP, which has the most negative reduction potential.7 Surface Raman and IR Spectra of PNTP and PATP Surface enhanced Raman and IR spectroscopy had been used to characterize the reaction products and study the reaction mechanism of electro-reduction of PNTP on gold and silver electrodes. Figure 5 shows the simulated IR spectra of PNTP and PATP on gold and silver surfaces. The surface IR spectra of reaction intermediates are given in Figure S5 and S6 in the Supporting Information. The IR spectra of PNTP adsorbed on gold and silver surfaces are dominated with a strong band at 1339 (1332) cm-1, which is assigned to the –NO2 symmetric stretching, νs(NO2). The moderate band at 1543 (1532) cm-1 is assigned to the –NO2 asymmetric stretching, νas(NO2). The weak bands at 1586 (1584) and 1605 (1598) cm-1 are assigned to the benzene ring C−C stretching mode. A nitro-to-amino reduction can be inferred from the appearance of the C−N stretching, νC−N at 1281 (1272) cm-1 and amino scissoring, δ(NH2) at 1633 (1632) cm-1. These two peaks can be hardly detected by Raman spectroscopy.65 Meanwhile the C−C stretching mode at 1584 cm-1 blue-shifts to 1606 cm-1. A new peak appears at 1494 cm-1 is assigned to the C−H in-plane bending mode.

Figure 5. Simulated IR spectra of PNTP and PATP adsorbed on gold and silver surfaces.



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The simulated surface IR spectra match well with the in situ IR spectra obtained during electrochemical reduction of PNTP.19, 24 Matsuda et al. studied the electro-reduction of PNTP on a silver electrode by in situ surface-enhanced IR spectroscopy.19 As the negative movement of electrode potential, they observed that the intensity of IR bands that attributed to PNTP (1346, 1520, and 1572 cm-1) decrease and the intensity of IR bands that assigned to PATP (1271, 1491, 1599, and 1628 cm-1) increase. Similarly, Futamata et al. studied the electro-reduction of PNTP on gold electrodes by attenuated total reflection (ATR)-IR spectroscopy.24 They found that IR band intensities from PNTP at 1572, 1505, and 1348 cm-1decreased with negative moving electrode potentials, while the new peaks at 1594 and 1487 cm-1increasesfor PATP. The appearance of vibrational modes in surface IR spectra is subject to the surface dipole selection rule which can be used to infer the adsorption configurations of adsorbates. This states that only those vibrational modes which give rise to an oscillating dipole perpendicular to the surface are IR active and give rise to an observable absorption band. For PNTP, the νas(NO2)mode should not appear in the IR spectra when PNTP is adsorbed on metal surfaces with a perpendicular configuration. In contrast, the νs(NO2) mode should not appear in the IR spectra when PNTP is adsorbed with a flat configuration. Since both νs(NO2) and νas(NO2) modes can be observed in the surface IR spectra,37 PNTP should follow in an inclined orientation on gold and silver surfaces. According to the surface selection rule, the electro-reduction product PATP also adopts an inclined configuration because the relative IR intensities of νC−N and δ(NH2) modes in the SEIRA of PATP

are weaker than its normal IR spectrum.38, 65 Figure 6 presents the simulated Raman spectra of PNTP and PATP on gold and silver surfaces. The surface Raman spectra of reaction intermediates are given in Figure S7 and S8 in the Supporting Information. The Raman spectra of PNTP on gold and silver are dominated by three strong bands at 1068 (1070), 1339 (1332), and 1586 (1584) cm-1. They are assigned to the C−S stretching mode νC−S, the −NO2 symmetric stretching νs(NO2), and the parallel C−C stretching νC−C, respectively. The weak band at 1098 (1102) cm-1 is attributed to the C−N stretching νC−N. The simulated surface Raman

spectra of PATP have two strong bands at 1074 (1077) and 1606 (1608) cm-1. They are assigned to the C−S stretching mode νC−S and the parallel C−C stretching mode, respectively. The weak bands at 1275 and 1632 cm-1 are assigned to the C−N stretching mode νC−N and the amine scissoring mode δ(NH2) which are very strong in the IR spectra of PATP as shown in Figure 5, and the bands at 1180 and 1491 cm-1 can be attributed to C−H in-plane bending modes.


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Figure 6. Simulated Raman spectra of PNTP and PATP adsorbed on gold and silver surfaces. An excitation wavelength of 632.8 nm was used in the simulated Raman spectra. The simulated surface Raman spectra of PNTP and PATP on gold surfaces well reproduce the in situ SERS spectra of PNTP reduction on a gold electrode.22 When the applied electrode potential goes negatively, the Raman peaks from the reactant PNTP decrease, and the Raman intensities of the product increases. Firstly, the most intense Raman peak from νs(NO2) gradually disappears. Secondly, the relative intensities of 1570 and 1590 cm-1 change with the decrease of electrode potential.22 According to our calculations, these two peaks are the C−C stretchings of PNTP

and PATP as shown in Figure 6. Thirdly, the doublet peaks around 1080 cm-1 turns to a singlet peak when PNTP is reduced to PATP. The reason is that the C−N stretching of PNTP at 1098 cm-1 blue-shifts to 1281 cm-1 as the C−N bond length decreases from 1.476 Å in PNTP to 1.393 Å in PATP. The above spectral changes were also observed in the in situ SERS study of Au-Pt-Au catalyzed hydride reduction of PNTP by NaBH4.66 The perfect matching between our theoretical simulation and experimental measurement make us believe that PNTP is reduced to PATP, however, another experimental study on PNTP reduction on gold electrodes shows entirely different results.23 Futamata applied ATR surface-plasmon-polariton Raman spectroscopy to study the reduction of PNTP on a gold electrode. The SERS spectra was dominated with strong Raman peaks at 1141, 1390, and 1432 cm-1. These characteristic SERS signals were also detected in the electro-reduction of PNTP on silver electrodes19-20 and attributed to a nitro-to-amino reduction product PATP based on the original assignment by Osawa et al.65 As seen in Figure 6, the simulated Raman spectra of PNTP and PATP on silver (bottom) are highly similar to that of PNTP and PATP on gold (top). Nevertheless, the experimental SERS spectra of PNTP39 and PATP65 on silver are quite different from our simulations and show



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abnormal peaks at 1142, 1391, and 1440 cm-1, which do not appear in the normal Raman spectra of PNTP and PATP. In our previous study, we proved that these peaks arise from neither PATP nor PNTP, and in fact they are attributed to a new surface species DMAB, which is formed by the photo-oxidation of PATP or the photo-reduction of PNTP during SERS experiments.41, 43 We also noted that the appearance of these abnormal peaks strongly depends on the laser power, the laser wavelength, the electrode potential, the solution pH, the surrounding atmosphere, as well as the metal substrate.42, 45-47 During the SERS experiments, the incident laser can induce the photo-reduction of PNTP or the photo-oxidation of PATP. In the SERS study of PNTP reduction, two kinds of Raman spectra were observed. The reduction product with SERS signatures at ~1080 and ~1600 cm-1 should be assigned to the nitro-to-amino product PATP.19, 22, 66 While the reduction product characterized with SERS signals at 1140, 1390, and 1440 cm-1 should be assigned to the dimer product DMAB.20-21, 23 Comparison of electro-reduction and photo-reduction of PNTP Table 3 presents the vertical transition energies and oscillator strengths of low-lying charge transfer (CT) excited states of PNTP, PATP and their reaction intermediates by visible-light irradiation (excitation energy less than 3.0 eV).The CT transitions diagram is shown in Figure 7. As seen in Table 3 and Figure 7, a metal-to-molecule CT is predicted for PNTP, PNSTP, DMAOB, and DMAB, and a molecule-to-metal CT is predicted for PATP, PHATP, and DMHAB by TD-DFT calculations. In our previous study, the photo-induced metal-to-molecule CT transitions are directly related to the photo-reduction reactions, and the photo-induced molecule-to-metal CT transitions are related to the photo-oxidation reactions. Table 3. Calculated Vertical Transition Energy (∆E, eV in Unit) and Oscillator Strength (f) of the Low-lying Photon-driven CT States of Adsorbates on Silver and Gold Surfaces Molecule PNTP PNSTP PHATP PATP DMAOB DMAB DMHAB

∆E / eV

f

Au

Ag

Au

Ag

2.35 2.19 2.29 2.20 2.29 2.27 2.27

1.76 1.63 2.59 2.48 1.85 1.85 2.51

0.0609 0.0681 0.0462 0.0595 0.1799 0.3466 0.1057

0.0508 0.0446 0.0218 0.0317 0.1050 0.1487 0.0292

Assignment M→m M→m m→ M m→ M M→m M→m m→ M

M and m stand for metal and molecule, respectively. Scheme 2 compares with the reaction mechanisms for the electro-reduction and photo-reduction of PNTP based on the calculated standard electrode potentials and CT excitation energies. PNTP can be directly reduced to PATP through PNSTP and PHATP or undergoes an indirect pathway by the formation of ring coupling products DMAOB, DMAB, and DMHAB. The electro-reductions of PNTP to PNSTP and PHATP to PATP are irreversible processes, while the electro-reduction of PNSTP to PHATP is a reversible reaction. The calculated Gibbs free energy changes for PNTP to PNSTP, PNSTP to PHATP, and PHATP to PATP are −31.06 (−31.14), −18.53 (−14.87), and −61.37 (−60.10) kcal/mol on gold (silver). The calculated reduction potentials of


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PNTP, PNSTP, and PHATP decrease as PNTP < PNSTP < PHATP. When the applied potential reaches the reduction potential of PNTP, PNTP is directly reduced to PATP through a six-electron transfer process.19, 34

Figure 7. Diagram of photo-induced CT transitions of PNTP on gold and silver within visible light irradiation. The blue and red lines present the electronic state on gold and silver, respectively. The solid and dashed lines are the HOMOs and LUMOs of the surface complexes. The dotted lines are the metal Fermi levels. PHATP can re-oxidized to PNSTP when the positive movement of the electrode potential. A reversible redox wave can be observed in the voltammograms of PNTP.64 In basic solution, PNSTP condense with PHATP to produce DMAOB, which can be reduced to DMAB, DMHAB, and PATP.7 The reduction of DMAOB to DMAB is an irreversible reaction with the Gibbs free energy change of −49.07 (−48.94) kcal/mol on gold (silver). The reduction of DMAB to DMHAB is a reversible process with the Gibbs free energy change of −5.98 (−10.07) kcal/mol on gold (silver). DMHAB can be further reduced to two molecules of PATP by cleavage N−N bond. The calculated Gibbs free energy changes for DMHAB to PATP are −46.03 (−47.34) kcal/mol on gold (silver). The calculated reduction potentials of DMAOB, DMAB, and DMHAB decreases as DMAOB < DMAB < DMHAB. It is noted that DMAB is not a stable reaction product during the electro-reduction of PNTP as the reduction potential of DMAB is more positive than that of its precursor DMAOB.



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Scheme 2. Reaction pathways of reduction of PNTP and oxidation of PATP on silver surfaces. eand hν denotes an electrochemical and photochemical reaction respectively. With light irradiation, the photo-induced metal-to-molecule CT leads to reduction of PNTP to PNSTP then to PHATP. The calculated CT transition energies of PNTP and PNSTP are 2.35 (1.76) and 2.19 (1.63) eV on gold (silver). However, PHATP is not able to be further reduced to PATP under visible light. As seen in Figure 7, the LUMO level of PHATP is much higher than those of PNTP and PNSTP. As a result, the energy of visible light does not match the required metal-to-molecule CT energy. The calculated transition energy required for exciting an electron from silver to PHATP is 3.32 eV. When increasing the incident light energy, nitrobenzene can be directly reduced to aniline with ultraviolet light irradiation.67 PHATP cannot be reduced to PATP by visible light, however, it can be re-oxidized to PNSTP through the photo-induced molecule-to-metal CT transition. The calculated molecule-to-metal transition energy for PHATP is 2.29 (2.59) eV on gold (silver). Thus PNSTP and PHATP accumulate on the metal surfaces and react to DMAOB, which can be further reduced to DMAB and DMHAB through the photo-induced metal-to-molecule CT transition. The calculated molecule-to-metal transition energies for DMAOB and DMAB are 2.29 (1.85) and 2.27 (1.85) eV on gold (silver). DMHAB cannot be reduced to PATP because of its relative high LUMO level. Thus DMAB is a relative stable product under the photoreaction condition. It should be also noted that the CT transition energies of surface complexes on gold fall to the energy region of 2.2 to 2.4 eV, which is very close the interband transition energy of gold.68-69 In this case, the damping of plasmon oscillation reduces the possibility of CT transitions. As a result, the photo-reduction of PNTP on gold substrates is more difficult to take place than on silver.27 This is the reason that Zhu et al observed the Raman signals from PATP other than DMAB in their SERS study on


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electro-reduction of PNTP on gold electrode.22 Conclusions The electro-reductions of p-nitrothiophenol on gold and silver electrodes have been investigated by means of density functional theory. Integration of thermodynamic calculations and surface Raman and IR spectral simulations has allowed us to reveal the electro-reduction mechanism and reaction products. The electro-reduction of PNTP involves two possible reaction pathways. In the first reaction route, PNTP is directly reduced to PATP through PNSTP and PHATP. The calculated standard electrode potentials decrease in an order of E0(PHATP/PATP) > E0(PNSTP/PHATP) > E0(PNTP/PNSTP). The one-electron-one-proton reduction of PNTP to PNTP(1H) is the rate determining step during the reduction of PNTP to PATP. In the second reaction route, the condensation of PNSTP and PHATP gives DMAOB, which can be further reduced to DMAB, DMHAB, and PATP. The calculated standard electrode potentials decrease in an order of E0(DMHAB/PATP) > E0(DMAB/DMHAB) > E0(DMAOB/DMAB). The one-electron-one-proton reduction of DMAOB to DMAOB(1H) is the rate determining step during the reduction of DMAOB to PATP. Theoretical investigation of thermodynamic properties can be used to assign the redox peaks observed in cyclic voltammetry and to infer electrochemical reaction mechanism. The surface Raman and IR spectra of PNTP, PATP, and the intermediates on gold and silver surfaces are simulated and assigned, and they are compared with the in situ SERS and SEIRA experiments. When PNTP is reduced to PATP, the simulated IR spectra well reproduce the experimental results. The disappearance of nitro symmetric stretching at ~1335 cm-1 of PNTP and appearance of amino scissoring at ~1275 cm-1 are followed with the negative movement of applied potentials. However, the simulated Raman spectra of the product PATP are significantly different from the experimental observation. According to our calculation, the electro-reduction product of PNTP characterized by SERS is DMAB. The difference between IR and Raman measurements arises from the fact that the incident laser used in Raman measurement can induce the formation of DMAB by the photo-reduction of PNTP or the photo-oxidation of PATP. Thus, the reaction mechanism of electro-reduction and photo-reduction of PNTP are compared. Both PNTP and PATP can transform to DMAB under visible light irradiation. The photoreactions occurred in SERS experiments may lead to incorrect judgments of electrode interface processes. This should attract great attentions in the future SERS researches. Acknowledgement This work is supported by the National Natural Science Foundation of China (Nos.21373712 and21321062), the Fundamental Research Funds for the Central Universities (SWU114076), and the Open Funds of State Key Laboratory of Physical Chemistry of Solid Surfaces (Xiamen University No. 201416) (1) Blaser, H.U. A Golden Boost to an Old Reaction. Science 2006, 313, 312-313. (2) Corma, A.; Serna, P. Chemoselective Hydrogenation of Nitro Compounds with Supported Gold Catalysts. Science 2006, 313, 332-334. (3) Corma, A.; Concepción, P.; Serna, P. A Different Reaction Pathway for the Reduction of Aromatic



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Theoretical Study on Electroreduction of p-Nitrothiophenol on Silver

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