Investigation of multi-nickel-substituted tungstophosphates and their stability and electrocatalytic properties in aqueous media

Several sandwich-type polyoxometallates based on the trivacant [PW9O34](9-) fragment with different Ni nuclearities were selected. Their stability and redox behaviour in aqueous media were evaluated by UV/Vis spectroscopy and cyclic voltammetry as a function of pH with the aim of finding the conditions under which these compounds could be suitable candidates for electrocatalytic processes. Two polyoxoanions-[Ni3Na(H2O)(2)(PW9O34)(2)](11-) and (to a lesser extent) [Ni4Mn2P3W24O94](17-)-were found to be stable over a wide pH domain, while [Ni-4(H2O)(2)(PW9O34)(2)](10-) requires the pH to be greater than 4 for its stability. The other derivatives are only suitable for solid-state studies. The stable polyoxoanions were tested for their activity in the electrocatalytic reduction of nitrate. The results suggest unambiguously that an increase in the number of Ni centres within these sandwich-type polyanions has a beneficial effect on the electrocatalytic process. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.