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 degrees C and 200 degrees 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 Mo7O246- dominates under ambient pressure at 25 degrees C in the pH range between 6 and 5, at 170-190 degrees C, chain-like or molecular structures of dimolybdates Mo2O72- and trimolybdates Mo3O102- are preferentially formed. In acidic solutions (pH < 2), supramolecular species, like Mo36O1128-, which generally predominate at 25 degrees C, do not occur at T > 100 degrees C. Instead, -Mo8O264- 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.