Speciation of Molybdates under Hydrothermal Conditions

In situ Raman spectroscopy was applied to investigate the speciation of molybdates under hydrothermal conditions in 0.2 M aqueous solution in the temperature range between 20 °C and 200 °C and at pH values between 7 and 1. The nature and abundance of molecular and supra-molecular species differs significantly compared to the distribution of species under ambient conditions at the same pH. Whereas heptamolybdate Mo₇O₂₄^6- dominates under ambient pressure at 25 °C in the pH range between 6 and 5, at 170–190 °C, chain-like or molecular structures of dimolybdates Mo₂O₇^2- and trimolybdates Mo₃O₁₀^2- are preferentially formed. In acidic solutions (pH < 2), supramolecular species, like Mo₃₆O₁₁₂^8- , which generally predominate at 25 °C, do not occur at T > 100 °C. Instead, β-Mo₈O₂₆^4- is the final molecular precursor of precipitation reactions that was detectable by Raman spectroscopy. The structural type of the solid phase formed through addition of vanadyl sulfate under hydrothermal conditions is sensitively controlled by the nature of the molecular precursor, which is adjusted by the pH. In acidic medium, hexagonal MoO₃ (ICSD 80290) is formed, while at pH = 5.8 nano-crystalline M1 (ICSD 55096) was obtained