Theoretical Study on Electroreduction of <i>p</i>-Nitrothiophenol on Silver and Gold Electrode Surfaces

The electroreduction 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 electroreduction of PNTP on metal electrodes. First, thermodynamic calculations were 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. Second, surface vibrational spectral simulation was performed to provide theoretical assignments of reaction products for the in situ Raman/IR experimental studies of electroreduction of PNTP. The most interesting finding in the reaction product identified by IR spectroscopy is PATP; however, Raman spectroscopy shows that the main product is p,p′-dimercaptoazobenzene (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 photoreduction of PNTP or photo-oxidation of PATP. Finally, the reaction mechanism of electroreduction of PNTP was compared with its photoreduction mechanism.