Mechanistic Aspects of Selenate Reduction on CU(UPD) on AU Electrodes
Authors: Strobl Jonathan , Pathak Saurabh , Scherson Daniel 
Published: August 2024

Recent findings in our laboratories have shown that copper1and cadmium2 underpotential deposited on polycrystalline Au electrodes, can mediate the reduction of in 0.1 M HClO4 aqueous solutions, to yield irreversibly adsorbed elemental Se allowing its detection down to the nM range. Current efforts are focused on gaining a better understanding of the the factors that govern this unique electrocatalytic phenomenon by employing a combination of electrochemical, microgravimetric, and optical techniques, using Cu as the UPD species. Correlations have been sought between the rates of this process and the coverage of Cu(UPD), the concentration of and, to a more limited extent, the applied potential in 0.1 M HClO4 solutions. Experimental conditions were selected to unveil the functional dependence of the kinetics on each of the aforementioned factors, while keeping the others constant, in solutions both quiescent and in the presence of convective flow. A number of interesting observations were made based on the data collected. In particular, shown in Panel C, Fig. 1, are plots of the charge associated with the oxidation of adsorbed Cu and Se, QCu and QSe, respectively (see Panel C), obtained from the area under the peaks shaded in light blue and yellow in Panels A and B, Fig, 1, respectively, vs (see below). These data were recorded during linear potential scans following polarization of the electrode at Ehold = 0.325 V for various times, thold (see Panels A-C, Fig. 1), where the linear character of the data in solutions is consistent with Cu(UPD) proceeding under strict diffusion control. As indicated by the QSe data, the electrode displayed electrocatalytic activity for reduction only for QCu > ca. 80 μC cm2-. Additional insight was obtained from measurements of the electrode weight as a function of using a quartz crystal microbalance (see Panel D, Fig.1), which yielded evidence for adsorption only for QCu > ca. 80 μC cm2-, pointing to the formation of a surface-bound adduct, as a necessary step for reduction to ensue. On this basis, we propose that for QCu > ca. 80 μC cm2 the mechanism for reduction involves, as a first step, the reversible formation of the adduct, Eq. (1) followed by its irreversible reduction, generating adsorbed elemental Se, Cu|Se(ads), Eq. (2).

Atomically resolved images of Cu(UPD) on Au(111) obtained with a scanning tunneling microscope for the closely related sulfate ion, strongly suggest that the active site involved adsorption on the empty Au sites of the so-called √3´x√3 superstructure (see Insert in Panel A, Fig. 1). A similar conclusion was drawn based on measurements performed with rotating Cu ring-polycrystalline Au disk electrodes, RRDE, at constant Cu(UPD) coverage and applied potential for various solution compositions. Quantitative analyses of all the data, which included numerical simulations, yielded kf/kr, and kET values in the range (2.4 – 3.23) ´ 106 cm3 mol-1 and (2.5 – 9) ´ 10-3 s-1, respectively.

Acknowledgments

Support for this work was provided by NSF CHEM 1808592

References

1. Strobl, J. R.; Scherson, D. A., The Reduction of Selenate Mediated by Underpotential Deposited Copper on Gold Electrodes in Acidic Solutions: Analytical Applications. Journal of The Electrochemical Society
2016,
163 (13), H1066.

2. Han, Q.; Strobl, J.; Scherson, D., Communication—Selenate Reduction Induced by Cadmium Underpotential Deposition on Gold in an Aqueous Acidic Electrolyte. Journal of The Electrochemical Society
2019,
166 (8), H283.

Figure Caption

Figure 1