Dehydration of the molybdenum trioxide hydrates MoO₃·nH₂O: In situ x-ray absorption spectroscopy study at the Mo K edge

The process of dehydration for the molybdenum trioxide hydrates, MoO₃2H₂O → β-MoO₃H₂O → α-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 β-MoO₃.H₂O → α-MoO₃. The final β-MoO₃ product had nanocrystalline structure with an average grain size of about 15 Å.