Selenate reduction mediated by underpotentially deposited Cu on the low index faces of Au in aqueous perchloric acid

Abstract

Underpotential deposited Cu on the low index faces of single crystal Au electrodes has been found to promote the reduction of selenate, SeO42−(aq),in 0.1 M HClO4 solutions, a behavior analogous to that reported earlier in our laboratory for polycrystalline Au (Strobl et al. J. Electrochem. Soc.2016,163 (13), H1066). Sequential potential step-linear scan voltammetry data collected for Au(111) film electrodes in solutions containing Cu2+(aq) in the μM range afforded evidence that the onset for the electrocatalytic activity occurs for Cu(UPD) coverages, θCu ≈ 0.18, the same value at which complementary microgravimetric data displayed a clear increase in mass. On this basis, a reaction mechanism has been proposed involving the initial reversible formation of an adsorbed adduct, we denote as Cu|SeO42−(ads), followed by a first order irreversible reduction to yield a yet to be identified species, which we denote as Cu|Se(ads), as the rate determining step. Support for this reaction scheme was obtained from numerical solutions of the coupled differential equations that govern the time evolution of Cu|SeO4−(ads) and Cu|Se(ads), yielding best agreement with the experimental data for values of the equilibrium constant for adduct formation and the adduct reduction rate constant of 2.5 × 106 cm3 mol−1, and 9.1 × 10−3 s−1, respectively. This unique electrocatalytic effect has been attributed to a shift in the potential of zero charge of the bare Au substrates induced by Cu(UPD), which promotes the adsorption of HSeO4−(aq) at potentials more negative than those found for the bare substrates, allowing access to overpotentials high enough for its activation and further reduction to ensue.