Reduction of aqueous selenate (SeO₄^2-) to lower oxidation states of selenium is typically a very slow reaction, but important to the detection and removal of soluble selenium from wastewater. Electrocatalytic reduction of selenate has not been exhaustively explored; only a few electrocatalytic methods for reducing selenate have been reported to date [1-6].

Commercially available selenate is often contaminated with selenite (SeO₃^2-), a species that can be readily reduced to yield elemental selenium or selenides. No selenate reduction studies to date acknowledge the presence of this impurity. Furthermore, not all these studies unambiguously show that selenate itself is reduced, as opposed to these traces of selenite. Previous literature claims that gold electrodes are capable of reducing selenate at electrode potentials below 0.6 V vs SHE [1], forming a selenium deposit that strips anodically above 0.9 V vs SHE. Cyclic voltammetry experiments in this work reproduced the reported redox features, shown in Fig. 1 as peak A and B, respectively. However, when selenite was removed from selenate solutions with a titania adsorbent, no redox activity is seen on gold electrodes. It can be concluded that the previous report [1] was in fact measuring currents associated with trace selenite reduction. Selenate undergoes specific adsorption on gold and no other reaction. Peak C in Fig. 1 can be attributed to specific adsorption of selenate.

Figure 1: Cyclic voltammetry at 0.2 V/s on Au wire electrode with 1mM of selenate in 0.1 M HClO₄. Two purities of selenate were used. The first was selenate decahydrate purchased from Alfa Aesar at a purity of 99.9%, used as received. The other sample of selenate was the same, but was exposed to a titania adsorbent in order to remove traces of selenite. A cyclic voltammogram is also shown after selenite addition.

We have found that the chemical inertness of selenate can be overcome by addition of copper (II) salts. Copper (II) can be reduced to an adlayer of underpotential deposited copper on gold electrodes between 0.64 and 0.24 V SHE. Once formed, we have found that this adlayer induces slow reduction of selenate, yielding a thin copper selenide film. The mechanism for Cu_xSe formation was investigated using potentiostatic deposition, followed by linear sweep stripping voltammetry. This technique allows determination of Cu and Se stripping charges separately, observed in Fig. 2 as peak D and E, respectively. At lower electrode potentials (< 0.24 V vs SHE), where copper (II) is reduced to bulk copper metal, selenate both accelerates Cu deposition and is reduced. Selenium co-deposits with bulk copper formed in selenate / copper (II) solutions. In this regime, selenate reduction is much faster than reduction by UPD copper.

Figure 2: Linear sweep stripping voltammograms at 0.1 V/s after potential holds at 0.325 V on a Au electrode. Solution was 0.1 M HClO₄ with 5 mM Cu(ClO₄)₂ and 1 mM Na₂SeO₄(titania purified). Potential hold duration for each curve is indicated in the legend.

References:

1. Ivandini, T. A.; Einaga, Y. Electrocatalysis. 2013, 4, 367-374.
2. Koshikumo, F.; Murata, W.; Ooya, A.; Imabayashi, S.-I. Electrochem. Soc. Japan. 2013, 81, 350-352.
3. Kozhakov, B. E.; Baeshov, A.; Buketov, E. A.; Zhurinov, M. Zh. Elektrokhimiia. 1985, 21, 550-551.
4. Kikuchi, E.; Tanaka, M.; Liang, R.; Sakamoto, H. Trans. Mat. Res. Soc. Japan. 2004, 29, 2337-2340.
5. Hayashi, H.; Kanie, K.; Shinoda, K.; Muramatsu, A.; Suzuki, S.; Sasaki, H. Chemosphere. 2009, 76, 638-643.
6. Ladriere, J. Bulletin des Societes Chimiques Belges. 1973, 82, 99-122.

Acknowledgements:

                This work was supported by a grant from the NSF.