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Thermal and reductive decomposition of ammonium thiomolybdates

Abstract:

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. © 1995.