Investigation on Electrochemical Redox of Hydroquinone-Fourier Transform Infrared Spectroelectrochemistry Techniques

Abstract

Abstract The electrochemical behavior of hydroquinone (QH2) was studied and the reasonable electrochemical oxidation mechanisms were proposed in acetonitrile, neutral unbuffer and basic solutions based on the techniques of cyclic voltammetry (CV), in situ FT-IR spectroelectrochemistry (FT-IR), cyclic voltabsorptometry (CVA), derivative cyclic voltabsorptometry (DCVA) and reconstructed i-E curves. In acetonitrile solution, the electrochemical behavior of hydroquinone shows a couple of irreversible redox peaks in cyclic voltammogram. One negative-going band at 1510 cm−1 and three positive-going bands at 1656, 1595 and 1318 cm−1, which respectively corresponded to QH2, Q, were observed in FT-IR 3D map. The results suggested that the oxidation of QH2 was 2e-2H+ process and the final oxidation product was benzoquinone (Q). In neutral unbuffer solution, it displayed an anodic peak (A1) and two cathode peaks (C1, C2) for one cycle. For continuous three cycles, a new oxidation peak (A2), corresponding to the negative potential reduction peak (C2), appeared. In IR 3D map, six IR absorption peaks, 1510 cm−1 for QH2; 1656 and 1318 cm−1 for Q; 1341 cm−1 for Q−.; 1495 and 1272 cm−1 for Q2− were observed, respectively. The results indicated that the final oxidation product was also Q. In reduction process, one part of Q was reduced to QH2 at more positive potential (C1) and the other was to Q2− at more negative potential (C2). When 2 equiv of OH− were added to the solution, only a couple of redox peaks (Q2−/Q) appeared. In addition, a change of hydrogen bond by the negative-going bands at 2140 cm−1 was observed, which was due to the absorption of Q2−…D2O or Q−.…D2O. So the results indicated that this process is H–bonding coupled-electron transfer. Based on reconstructed current-potential (i-E) curve, we can confirm that the electron transfer of Q2− shows two-step one-electron transfer process.