Thermal and kinetic studies of sulfur-rich molybdenum and tungsten polysulfides

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Sulfur-rich transition metal polysulfides with multiple disulfide bonds (S–S) are a family of inorganic materials with unusual chemical properties and potential in catalysis and energy-related applications. In the current work, we present a kinetic study of the thermal decomposition of amorphous pentasulfides MoS5 and WS5 based on thermogravimetric analysis (TGA) in an inert atmosphere at various heating rates (10, 20, 30 °C/min). Thermal decomposition of both pentasulfides proceeds via a two-step, consecutive process. First, starting from ∼190 °C (M = Mo) or ∼240 °C (M = W) MS5 transform into intermediate products MS3, which then convert into MS2 at ∼380 °C (M = Mo) or 300 °C (M = W) (at the heating rate of 30 °C/min). The main kinetic parameters (activation energy, pre-exponential factor and reaction type) were calculated. Both steps are well described by the Avrami–Erofeev model, comprising random nucleation and subsequent nucleate growth. The rate-controlling step is diffusion, as spherical particle morphology of MoS5 and WS5 may slow down the elimination of sulfur produced during decomposition. The final decomposition products are weakly crystalline disulfides (MoS2, WS2), which inherit the spherical morphology of MoS5 (WS5). Theoretical calculations suggest that the very first step of the MS5 decomposition process is the depolymerization of MS5 chains into cluster fragments, rather than direct desulfurization.