Thermal and reductive decomposition of ammonium thiomolybdates

Thermal decomposition and hydrogen reduction of ammonium thiomolybdates of formulas: (NH₄)₂[MoS₄] (I), (NH₄)₂[Mo₂(S₂)⁶] · 2H₂O (II) and (NH₄)₂[Mo₃S(S₂)₆] · 1.85H₂O (III), were investigated using thermogravimetric analysis (TGA), either in pure N₂ or in 10% H₂ in N₂, and temperature-programmed reduction (TPR). The thiomolybdate compounds were characterized by several physicochemical techniques, including FTIR, PAS/FTIR, UV-visible spectroscopy, XRD and CHNS elemental analyses. The decompositions of (I) and (II) proceed in a similar way independently of the type of gas phase, with production of MoS₃. However, the anion cluster of (III) is decomposed at lower temperatures in the presence of H₂. The amorphous MoS₃ compound is very sensitive to the presence of H₂, which also lowers the onset of its decomposition. These results seem to support a model for the structure of MoS₃ consisting of triangular Mo₃ cluster units, similar to those present in (III). TPR could be used as a "fingerprint" technique for identification of the complexes, as they present strong differences in the shapes and characteristic temperatures of the reductograms.