A DFT and SERS study of synergistic roles of thermodynamics and kinetics during the electrocatalytic reduction of benzyl chloride at silver cathodes

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The reductive cleavage of carbon-halogen bonds is one of the classical model systems of concerted versus non-concerted bond breaking electron transfer in molecular electrochemistry, but most studies did not consider the influence of adsorption of reaction intermediates on this phenomenon. We performed density functional theory (DFT) calculations to understand the electrochemical reduction of benzyl chloride at silver electrodes through predicting the surface adsorption effect of the reactant and of follow-up intermediates and analyzing the energetics of the overall dissociative electron transfer. The ensuing DFT calculations of surface-enhanced Raman spectroscopic (SERS) signals provided some key information for characterizing different surface species at different potentials at the molecular level. The most intense and broad spectral peak observed near 800 cm−1 in the electrochemical SERS was thus assigned to the CH2 wagging vibration closely associated with the co-adsorption of benzyl radical and chlorine. After the dissociation of the C-Cl benzyl chloride bond, the chloride anion adsorbs at the three-fold hollow site on silver electrodes, as indicated by a low vibrational frequency and a weak Raman signal. Conversely, the formed benzyl radical intermediate is found to adsorb at the top site with the characteristic vibrational frequency and strong Raman intensity. DFT calculations further confirmed that the kinetic effect underlying the electrocatalytic activity of silver electrodes is closely associated with the intrinsic energy barrier and the dissociative adsorption state of benzyl radical and chlorine on silver electrodes. Finally, we discuss the fundamental meaning of the kinetic rate constant of the first-step dissociative electron transfer for benzyl chloride on silver electrodes based on the different electron transfer models, indicating the synergistic roles of thermodynamics and kinetics.