Abstract
Background: Polyoxometalates, especially those derived from the Kegin structure are able to exchange a large number of electrons through quasi-reversible redox processes. However, little is known about the redox behaviour of reduced bicapped Keggin ions. Thereby, this work represents the first electrochemical investigation of this Keggin-type ion.
Objectives: The main objective of this study was to analyze the influence of the two capping {Sb3+} groups upon the electrochemical response of the reduced Keggin unit {SiMo12O40}.
Methods: The synthesis of the bicapped [Sb2SiMo12O40]4- has been achieved via hydrothermal method using elemental antimony, ammonium molybdate and silicate in acidic aqueous solution. The crystallographic structure of the resulting single-crystals has been determined by X-Ray diffraction analysis. Furthermore, the complete formula was established by elemental analysis and thermal analysis (TGA). Electrochemical studies were carried out in aqueous solution, using glassy carbon as working electrodes and varying either pH or scan rate.
Results: The [Sb2SiMo12O40]4- anion has been synthesized as its ammonium salt, which was obtained as pure crystals in high yield under controllable conditions. The structure of the six-electron reduced diantimony capped Keggin compound (NH4)4[Sb2SiMo12O40].11H2O has been determined by singlecrystal X-ray diffraction and shows two capping groups grafted onto the reduced {SiMo12O40} central core. A 2D solid network results from interactions between the outer Sb caps and oxygen atoms of POM neighbors. The delocalization of six electrons over the 12 Mo centers modifies both the IR and UVvisible spectra. The bicapped polyoxometalate (POM) exhibits two quasi-reversible multi-electron transfers in the range 0.0 to 0.3 V/SCE in aqueous solution.
Conclusion: Our results indicated unambiguously the striking redox properties of the reduced capped Keggin anion which is able to deliver, in this case, six electrons in a narrow potential range with the concomitant partial hydrolysis of the two outer Sb3+ capping groups. This gives new opportunities in the field of electrochemical-based devices for energy storage or energy conversion.
Keywords: Polyoxometalate, capped Keggin structure, molybdenum, antimony, electrochemistry, crystals.
Graphical Abstract
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