Kinetics of selenate reduction mediated by underpotentially deposited Cu on polycrystalline Au electrodes in aqueous perchloric acid

The reduction of selenate, ${\text{SeO}}_4^{2-}(aq),$in 0.1 M HClO₄ solutions, induced by underpotential deposition, UPD, of Cu on polycrystalline Au electrodes was investigated using the rotating ring-disk electrode, RRDE, technique. Design and implementation of electrode potential-rotation rate protocols made it possible to determine the rates of ${\text{SeO}}_4^{2-}\left(aq\right)$reduction as a function of Cu coverage, θ_Cu, as determined by the Bruckenstein method (Swathirajan et al. J. Phys. Chem. 1982, 86, 2480–2485). In agreement with the results reported recently for Au(111) film electrodes (Strobl et al. Electrochimica Acta 2024, 493, 144,298), the reaction was found to proceed only for θ_Cu above a critical value, i.e. ca. 0.39, in this case, and the mechanism is consistent with an initial reversible formation of adsorbed $\text{Cu}|{\text{SeO}}_4^{2-}(ads),$followed by its subsequent irreversible reduction, to yield a yet to be identified species denoted as $\text{Cu}|\text{Se}(ads),$ as the rate determining step. Best fits of the kinetic model yielded values of the equilibrium constant for adduct formation, K, and first order rate constant for adduct reduction, k_ET, in the range (2.4 – 45) × 10⁶ cm³ mol⁻¹ and (0.55 – 30) × 10⁻³ s⁻¹, respectively, which are close to those found for Au(111). This unique electrocatalytic effect has been attributed to a shift in the potential of zero charge of the bare substrate toward more negative values, induced by the metal UPD, which promotes the adsorption of the oxyanion at potentials more negative than those found for the bare substrate, making it possible to access overpotentials large enough for its further reduction to ensue.