Mechanistic aspects of electro-catalytic reduction of Reactive Black 5 dye in a divided cell in the presence of silver nano-particles

Abstract

Abstract Electro-catalytic Reduction (ECR) of Reactive Black 5 (RB5) as a model compound was studied in a divided cell using graphite cathode in the presence and absence of silver nano-particles (AgNPs). In the absence of AgNPs and at current density of 20 mA cm−2 percentage RB5 decolourization at the end of 180 min and first-order reaction rate constant were 80% and 0.021 min−1, respectively. The presence of 10 mM AgNPs significantly improved the extent and rate of decolourization (99% and 0.043 min−1, respectively) under identical reaction conditions. The enhanced decolourization in the presence of AgNPs was attributed to generation of a nucleophilic species on the surface of NPs. It seems that nascent hydrogen ( H ) generated on the surface of AgNPs interacts with Ag to form a nucleophile such as silver hydride (AgHx) that decolorizes RB5. UV–Visible spectroscopy and LC-MS analyses confirmed generation and accumulation of sodium 1,2,7-triamino-8-hydroxynaphthalene-3,6-disulfonate (TAHNDS) and sodium 2-(4-aminobenzene-sulphonyl)-ethoxi-sulphonate (ABSES) as products of decolourization. RB5 decolourization in the presence or absence of AgNPs proceeded through cleavage of azo bonds ( N N ). The rate and extent of RB5 decolourization increased with increase in AgNPs concentration up to 16 mM. AgNPs could be reused at least three times yielding comparable results to that of fresh AgNPs. The results obtained in this study may lead to development of process wherein AgNPs can be activated at one place and sent to remote places for reduction of an aqueous target compound without the need of a reducing agent.