THERMAL AND REDUCTIVE DECOMPOSITION OF AMMONIUM THIOMOLYBDATES

Thermal decomposition and hydrogen reduction of ammonium thiomolybdates of formulas: (NH4)(2)[MoS4] (I), (NH4)(2)[Mo-2(S-2)(6)] 2H(2)O (II) and (NH4)(2)[Mo3S(S-2)(6)] 1.85H(2)O (III), were investigated using thermogravimetric analysis (TGA), either in pure N-2 or in 10% H-2 in N-2, 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 H-2 The amorphous MoS3 compound is very sensitive to the presence of H-2, which also lowers the onset of its decomposition. These results seem to support a model for the structure of MoS3 consisting of triangular Mo-3 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.