Electrocatalytic Properties and Physicochemical Identity of Cu-Intercalated WO ₃ 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 the reduction of nitrates (NO3– and NO2–) in an 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 the addition or introduction of Cu active sites to 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 the indophenol method. Formation of ammonia (but no hydrazine) has also been confirmed upon performing Pt-based electroanalytical diagnostic measurements following electrolysis of nitrate solutions at Cu-WO3 for 2 h at −0.45 V vs RHE. The characteristic voltammetric peak for the oxidation of ammonia on Pt has been observed in the investigated solution (alkalized to pH 13) as well as upon transfer of the Pt electrode with the product adsorbate to alkaline solution (0.5 mol dm–3 NaOH). On the whole, the results are consistent with the view that the incorporated copper catalytic centers, acting parallel to the partially reduced nonstoichiometric tungsten oxides, induce reductive conversion of nitrates and their reaction intermediates to ammonia-type products. During electrolysis at −0.45 V vs RHE, the catalytic system exhibits certain tolerance with respect to the competitive hydrogen evolution and gives the NH3-type-yield rate on the level 54 or 58 μmol cm–2 h–1 (151 or 162 μmol m–2 s–1) during reduction of 5 mmol dm–3 solutions of nitrates (NO3– or NO2–) in 0.5 mol dm–3 H2SO4. The Cu-WO3 catalyst has also been demonstrated to exhibit strong catalytic properties during reductions of other electrochemically inert reactants, such as oxygen, carbon dioxide, and bromates.