The wheel-shaped Cu20-tungstophosphate [Cu20Cl( H)(24)(H2O)(12)(P8W48O184)](25-), redox and electrocatalytic properties

摘要

After prior assessment of its stability by UV-visible spectroscopy, the new complex [Cu20Cl(OH)(24)(H2O)(12)(P8W48O184)](25-) (Cu28P8W48) 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 Cu2+ centers. However, the reduction waves featuring the electrochemical behaviours of the precursor lacunary complex K28Li5[H7P8W48O184] (.) 92H(2)O 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 Cu2+ 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 [Cu20Cl(OH)(24)(H2O)(12)(P8W48O184)](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. (c) 2005 Elsevier B.V. All rights reserved.