Decamolybdocobaltate heteropolyanions in aqueous solutions: chemistry driving their formation and domains of stability.

In the present study, aqueous solutions of decamolybdocobaltate H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions were prepared from molybdenum oxide, cobalt carbonate precursors and hydrogen peroxide used as oxidizing agent. The preparation was optimized adding a consecutive hydrothermal treatment at 150 °C to obtain pure H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions for Co/Mo atomic ratio of 0.5. Combining quantitative Raman and UV-visible measurements and chemometric methods, it was demonstrated that a mixture of H(4)Co(2)Mo(10)O(38)(6-) and octomolybdate Mo(8)O(26)(4-) species is obtained for Co/Mo ratios lower than 0.5, and the relative quantities of H(4)Co(2)Mo(10)O(38)(6-) are determined by the presence of Mo(8)O(26)(4-) species and by the quantity of Co(2+) countercations available in the solutions to ensure the electroneutrality. As these quantities can be predicted for each Co/Mo ratio, this finding allows rationalization of the preparation of heterogeneous catalysts using impregnation by H(4)Co(2)Mo(10)O(38)(6-) aqueous solutions. Parameters relevant of the impregnation step such as the pH, the Co/Mo ratio, and the molybdenum concentration were varied to determine the domains of stability of H(4)Co(2)Mo(10)O(38)(6-) heteropolyanions after formation. Stable from pH 1 to 4.5, this dimeric Anderson species is destabilized above pH 4.5; Co(2+), monomolybdate MoO(4)(2-) ions, and precipitates are then formed. For Co/Mo ratios lower than 0.5, the relative quantity of dimer does not vary with the pH and with a change of the Co/Mo ratio consecutive to the hydrothermal treatment. On the contrary, the coproduced Mo(8)O(26)(4-) species can be transformed into other isopolymolybdates varying the pH according to their domains of stability. For all of the ratios, H(4)Co(2)Mo(10)O(38)(6-) dimers were also shown to be stable in a wide range of molybdenum concentrations.