Electrocatalytic Reduction of Selenate on Gold Electrodes Alongside Copper Deposition

Abstract

Reduction of aqueous selenate (SeO4 2-) to lower oxidation states of selenium is regarded as a very difficult reaction and thus far, only very few methods have yielded positive results [1-6]. One such previous study asserts that gold electrodes can catalyze reduction of commercial selenate at potentials below 0.6 V vs SHE [1], forming an elemental selenium deposit that can be anodically stripped above 0.9 V vs SHE, (see peaks A and B, black curve in Fig. 1 recorded in 1 mM Na2SeO4 in 0.1 M HClO4). However, commercially supplied selenate usually contains some selenite (SeO3 2-), a far more electroactive selenium oxyanion believed to yield elemental selenium and/or selenides as the product(s) of reduction. In fact, as shown by experiments performed in our laboratories, such voltammetric features vanished upon removal of selenite from the selenate solutions by adsorption on TiO2(see red curve in Fig. 1). This contribution will present strong evidence that the reduction of selenate under the same conditions can proceed on a layer of underpotentially deposited copper on Au, yielding a corresponding layer of CuxSe. To be discussed are various aspects of the formation mechanism of this CuxSe using potentiostatic deposition and linear sweep stripping voltammetry (LSSV) experiments. During LSSV experiments, Cu can be anodically stripped, leaving behind a layer of adsorbed elemental Se (peak C, Fig. 2). At higher potentials, this layer is oxidized to selenite (peak B, Fig. 2) [7-9]. Repetitive formation and oxidation of the CuxSe product on gold electrodes via an AC deposition / stripping waveform may allow for continuous turnover of selenate into selenite.