Voltammetric and XPS investigations of polynuclear ruthenium-containing cyanometallate film electrodes

Glassy carbon electrodes coated with thin films of mixed valence ruthenium-oxo-species stabilized by cyanoruthenate or cyanoferrate crosslinks were investigated by cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). Electrochemical deposition was accomplished in dilute mineral acid solutions (e.g. 10 mM H2SO4) containing millimolar concentrations of RuCl3 + K4Fe(CN)(6), or RuCl3 + K4Ru(CN)(6), or OsCl3 + K4Ru(CN)(6). Film deposition is attributed to growth of compact and conducting lattices with significant oxo- (M-O-M) and cyano-bridging (M'-CN-M), in which M represents Ru, or Os, while M' refers to Ru or Fe. Particular attention was devoted to the polynuclear ruthenium oxide microstructure (-Ru-O-Ru-) crosslinked with cyanoruthenate and cyanoferrate, designated as RuO-RuCN and RuO-FeCN respectively. From an electrochemical point of view, the mixed metal RuO-FeCN behaves in a manner analogous to the RuO-RuCN film electrode, and indeed these modified electrodes are capable of catalyzing the oxidation of several substrates in acidic solutions. Depending on the applied potential, two distinct and selective reactive sites are recognizable in the electro-oxidation processes of As(III) and 2-furaldehyde at both RuO-FeCN and RuO-RuCN film electrodes. XPS characterization of these ruthenium-containing cyanometallate films shows clear evidence of the existence of two oxidation states of ruthenium (Ru 3d region), which are respectively attributed to oxo-ruthenium(IV) and dioxo-ruthenium(VI) or Ru(V) in the oxide framework of the inorganic lattice.