In analogy with many other oxyanions, the electrochemical reduction of selenate, SeO₄^2-(aq) in aqueous solutions on most electrode surfaces is typically characterized by slow kinetics. Typical reduction strategies employ excessive amounts of cationic reducing agents like Cu(I) and Fe(II) compounds [1, 2]. Recently, Cu UPD on Au(poly) was found not only to promote reduction of SeO₄^2-(aq) in acidic electrolytes, but advantage was taken of the resulting Cu/Se stripping peaks to develop an exceedingly sensitive analytical method for SeO₄^2-(aq) detection down to the nM range [3]. An example of some typical q_Se vs. [Na₂SeO₄] calibration curves, where q_Se represents the charge under the Se stripping peak, (see insert in Fig. 1), are shown in Fig. 1.

The rates of SeO₄^2-(aq) reduction as catalyzed by Cu UPD are very time dependent, indicative of a complex electrocatalytic mechanism. Insight into some aspects of this process has been gained from studies involving the low index faces of single crystal Au via hold/strip voltammetry coupled with Normal Incidence Differential Reflectance (NIDR) and in the case of Au(111), by Electrochemical Quartz Crystal Microgravimetry (EQCM) studies. It has been found that a minimum surface coverage of Cu UPD is required before any reduction of SeO₄^2-(aq) occurs, and that reduction slows over time as the surface saturates with Se atoms. However, excessively high Cu UPD coverages were found to hinder SeO₄^2-(aq) reduction on the Au(111) plane. Some simple kinetic models have been formulated to attempt to capture the observed behavior.

Additionally, at fixed coverages of Cu UPD, rates of SeO₄^2-(aq) reduction increase with decreasing applied potential. To this end, the possible electrocatalytic properties of other metals capable of undergoing UPD at potentials more negative than those of Cu were considered. The first candidate to be examined was Cd, a metal for which the work function in bulk form is much lower than that of Cu [4] and, thus, its UPD should occur at more negative potentials. In addition, Cd UPD on Au has shown electrocatalytic activity for the reduction of NO₃^-(aq), yet another example of a difficult to reduce oxyanion, in aqueous electrolytes [5]. It was found that SeO₄^2-(aq)reduction can indeed be catalyzed by Cd UPD on Au, as shown in Fig. 2. In contrast to Cu UPD catalyzed reduction, it was found that under certain conditions the reduction rates of SeO₄^2-(aq) were time independent when mediated by Cd UPD, as shown in Fig 3.

References:

1. Murphy, A. P. Eng. Chem. Res. 1988, 27, 187-191.
2. Baeshov, A.; Khovakov, B. E.; Buketov, E. A. Doklady Akademii SSSR. 1985, 278, 818-820.
3. Strobl, J. R.; Scherson, D. A. Electrochem. Soc. 2016, 163, H1066-H1068.
4. R. Rumble, D. R. Lide and T. J. Bruno, CRC handbook of chemistry and physics : a ready-reference book of chemical and physical data, Boca Raton : CRC Press, [2018], 99th edition.
5. K. Xing and D. A. Scherson, J. Electroanal. Chem. Interfacial Electrochem., 199, 485 (1986).

ACKNOWLEDGEMENTS: This work was supported by a grant from NSF, CHE-1412060.