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In Situ Investigation of the Thermal Decomposition of Ammonium Tetrathiomolybdate Using Combined Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction

The thermal decomposition of ammonium tetrathiomolybdate, a process widely used for the preparation of molybdenum sulfides, has been studied in situ for the first time, using combined Mo K edge XAFS and X-ray powder diffraction. Isothermal decomposition at 120 °C produced pure amorphous molybdenum trisulfide (a-MoS3), after 135 min, with no further reaction observed on continued heating. XAFS spectra and diffraction patterns collected during the heating showed the disappearance of crystalline (NH4)2MoS4 and the formation of a-MoS3 without the formation of an intermediate. Thermal decomposition of (NH4)2MoS4 by heating from 100 to 400 °C produced ultimately poorly crystalline MoS2. It is shown that the Mo−Mo nonbonded correlation of ∼3.2 A found by some previous workers in a-MoS3 is most likely due to the presence of small quantities of poorly crystalline MoS2. This allows certain structural models for a-MoS3 to be ruled out. A detailed comparison between XAFS data collected from crystalline 2H−MoS2 at a v...