The wheel-shaped Cu20–tungstophosphate [Cu 20Cl(OH) 24(H 2O) 12(P 8W 48O 184)] 25−, redox and electrocatalytic properties

• Published in: Electrochemistry communications 2005, Vol.7 (8), p.841-847
• Main Authors: Jabbour, Darine, Keita, Bineta, Nadjo, Louis, Kortz, Ulrich, Mal, Sib Sankar
• Format: Article
• Language: English
• Subjects: Chemistry; Crown heteropolyanion; Electrocatalysis; Electrochemistry; Exact sciences and technology; General and physical chemistry; Kinetics and mechanism of reactions; Nitrate reduction; Supermolecular copper derivative
• ISSN: 1388-2481; 1873-1902
• DOI: 10.1016/j.elecom.2005.05.003

After prior assessment of its stability by UV–visible spectroscopy, the new complex [Cu 20Cl(OH) 24(H 2O) 12(P 8W 48O 184)] 25− ( Cu 20 P 8 W 48 ) was studied by cyclic voltammetry and controlled potential coulometry in pH 0 and pH 5 media. In the two media, the cyclic voltammograms are dominated by the current intensities of processes attributed to the reduction of Cu 2+ centers. However, the reduction waves featuring the electrochemical behaviours of the precursor lacunary complex K 28Li 5[H 7P 8W 48O 184] · 92H 2O could be tracked down and highlighted. In the pH 0 medium, the first two waves of the lacunary heteropolyanion are practically engulfed in the tail of the overall copper reduction process. A much better separation is obtained in the pH 5 medium and is attributed to the large sensitivity of tungsten centers to pH variations. A simple mixture of the appropriate amounts of copper salt and the lacunary heteropolyanion at room temperature did not allow to reproduce the voltammogram of the pre-synthesized supramolecular species, thus confirming the stability of this complex as a viable entity in solution. Controlled potential coulometry indicates that all the Cu 2+ centers within the supramolecular complex remain electroactive. Cyclic voltammograms run at different advancements of the coulometry have permitted gradually a better observation of the W-waves. The supramolecular complex shows a strong catalytic activity towards nitrate and nitrite. Comparison with the activity per copper atom of previously studied copper-substituted heteropolyanions indicates the supramolecular complex to be significantly more efficient, a feature that reinforces the notion that accumulation of transition metals within polyoxometalates should be beneficial for the relevant catalytic processes. This superior efficiency of [Cu 20Cl(OH) 24(H 2O) 12(P 8W 48O 184)] 25− is due both to the accumulation of Cu-centers and to the more positive potential locations of catalytically active Cu- and W-waves compared to the corresponding waves of other Cu-substituted HPAs.