Electrocatalytic properties and physicochemical identity of Cu-intercalated WO3 nanowires: application to electroreduction of nitrates in acid medium

A hybrid catalytic system composed of copper(I)-oxide-derived copper-nanocenters immobilized within the network of tungsten oxide nanowires has exhibited electrocatalytic activity toward reduction of nitrates (NO3- and NO2-) in acid medium. The Cu-containing WO3 (Cu-WO3) catalytic system has been characterized using electrochemical, spectroscopic (Raman), microscopic (SEM), and X-ray (XPS) techniques. Evidence has been provided for insertion or intercalation of Cu active sites within the structure of hexagonal WO3 nanowires. In accordance with spectrophotometric measurements, it can be stated that the Cu-WO3 catalyst facilitates conversion of nitrates to ammonium ions, which have been identified as ammonia using indophenol method. Based on voltammetric diagnostic electroanalytical measurements with the additional Pt electrode and transfer to alkaline medium, formation of ammonia (but no hydrazine) has been confirmed. On the whole, the results are consistent with the view that the copper-intercalated catalytic centers, parallel to the reductive activity of the partially reduced non-stoichiometric tungsten oxides, induce conversion of nitrates and their reaction intermediates to ammonia type products. The Cu-WO3 have also been demonstrated to exhibit strong catalytic properties during reductions of other electrochemically inert reactants, such as oxygen and bromates.