The Role of Supporting Electrolyte on the Electrocatalytic Reduction of Organic Halides on Nanostructured Silver Electrodes in Water

Abstract

Over recent two decades, electrocatalytic reduction of halogenated organic compounds (HOCs) was successfully employed in electrosynthesis and abatement of environmental pollutants by their conversion to less harmful compounds [1, 2]. Halogenated organic compounds have been frequently used in industry and disposed into wastewater and air, despite their toxicity and suspected mutagenicity. Some of them, like chloroform, are also disinfection by-products commonly produced during the chlorination of water and wastewater. Due their refractoriness to typical oxidative water remediation techniques, HOCs became one of the most problematic group of pollutants. Electrocatalytic reduction was already proven as alternative, useful method of HOCs removal from organic solutions after prior water-based electrolyte exchange from aprotic medium. The scope of this study was to reduce HOCs in water environment/aqueous solutions and to examine the possibility of direct elimination of HOCs without a need for electrolyte change. The electrochemical reduction of organic halides in aprotic media generally proceeds via two-electron cleavage of the C–X bond, occurring at very negative potentials at non-catalytic electrode surfaces (e.g., glassy carbon), what is the biggest disadvantage of this process due to the high energy requirement. To shift the reduction threshold to less negative potentials and therefore lower the costs of the process, new materials with enhanced catalytic properties toward electrocatalytic reduction are intensively studied. Among them, silver attracted significant interest, due to the most powerful electrocatalytic properties toward reduction of a wide range of organic halides [3]. It is generally accepted that this effect is directly related with a strong affinity of halides to silver, by means of specific adsorption of halide atom to the Ag surface. It was reported in the literature that nanostructured silver particles exhibit similar or even better electrocatalytic properties toward reduction of HOCs than bulk Ag electrodes due to the increased surface to volume ratio [4, 5]. It is surprising that Ag nanowire arrays have only once been used as electrodes for electrocatalytic reduction of organic halides, namely chloropropane [6]. It is expected that silver nanowire array electrodes should exhibit at least similar or even better electrocatalytic properties as nanoparticles and offer better stability and efficiency. Therefore, the aim of this research was also to synthesize and investigate electrocatalytic behavior of nanostructured Ag electrodes with different morphologies, and compare the obtained results with a silver bulk electrode. The electrocatalytic measurements were performed in various supporting electrolytes in order to select the optimum one and assess its influence on the mechanism of electrocatalytic reduction of chloroform. The two-step anodization process in oxalic acid (H2C2O4) was used in order to obtain highly ordered anodic aluminum oxide (AAO) membranes that served as templates for the synthesis of silver nanowire arrays. The electrochemical anodization of aluminum is a cost-effective method that results in hexagonally arranged nanopores. The barrier layer at the pore bottoms was removed by using a potential-shock method that results in AAO membranes with through-hole pores. Then, a thin layer of Ag was sputter deposited, and finally, the electrodeposition of Ag nanostructures was carried out in a commercially available silver plating solution by applying a constant current density. The electrocatalytic properties of nanostructured Ag electrodes were investigated by various voltammetric techniques in aqueous solutions of several different supporting electrolytes, such as NaOH, KClO4, NaClO4, and NaH2PO4.