Electrochemical Reduction of Organohalides: Insights from First Principles Calculations

Abstract

Two-electron reduction of organohalides is sensitive to different electrode surfaces. While the role of electrode surface is not always clear, designing bimetallic heterogenous catalysis to further accelerate these reactions becomes more challenging. In this work, we have proposed a formalism to account for the role of electrode surface in the reduction potentials of such organic species. We have utilized first-principles calculations to study the reduction of halothane, bromobenzene, and benzyl bromide, on silver electrode surface, both in freestanding form, and as deposited on Au, Bi, and Pt support electrodes. We have identified and isolated the main factors governing the electrode-molecule interactions. These factors can be categorized as: (i) molecular geometry, (ii) lattice mismatch between the working and support electrodes, (iii) work function difference between the working and support electrodes, and (iv) solvation effects. Although the adoption energy in gas phase is lower than that in the solution, the solvation effects systematically decrease the surface propensity to adsorbed organic halides. Therefore, the rest of the calculations were carried out without solvents to reduce computational cost. While halothane was found to be more sensitive to the lattice mismatch, bromobenzene and benzyl bromide showed little response to this factor due to their different molecular geometry. All the support electrode originally showed less activity than Ag (agreeing with the experiments) due to either too high or too low adsorption energies than Ag. The adoption energy on the bimetallic surface approaches that on the Ag when the deposited Ag thickness is up to 3 atomic layers. Especially, when 2 layers of Ag is deposited on Bi (a support electrode with lower work functions), the bi-metallic surface exhibited increased catalytic activity. Using these factors, we have postulated bimetallic heterogenous catalyst design rules for similar organic halide reduction reactions.