Dehydration of the molybdenum trioxide hydrates MoO3•nH2O:: in situ x-ray absorption spectroscopy study at the MoK edge

The process of dehydration for the molybdenum trioxide hydrates, MoO3 . 2H(2)O --> beta-MoO3 . H2O --> alpha-MoO3, was studied for the first time by in situ Mo K-edge x-ray absorption spectroscopy. A regularization procedure was applied to reconstruct the radial distribution functions around the molybdenum at all stages of the process. Strong variations in the local environment of the molybdenum were found at both phase transitions and were attributed to the changes in direction of the off-centre Mo displacement and in the degree of symmetrization of the Mo-O bonds. Also, an increase of the Mo-O-Mo angle, leading to the appearance of strong multiple-scattering effects in the x-ray absorption fine structure, was observed at the second phase transition beta-MoO3 . H2O --> alpha-MoO3. The final alpha-MoO3 product had nanocrystalline structure with an average grain size of about 15 Angstrom.