Thermal analysis and kinetics of oxidation of molybdenum sulfides

Abstract

The thermal oxidation process of stoichiometric MoS2 and nonstoichiometric “Mo2S3”, together with the kinetics of oxidation of MoS2, were studied by using TG and DTA techniques in the Po2 range 1-0.0890 atm. MoS2 was oxidized completely to MoO3 in one step: $$MoS_2 + 7/2O_2 \to MoO_3 + 2SO_2 $$ Irrespective of Po2 and the heating rate, “Mo2S3” was oxidized finally to MoO3, via the following four steps: $$\begin{gathered} ''Mo_2 S_3 ''\xrightarrow{I}\gamma - Mo_4 O_{11(sur)} + ''Mo_2 S_3 ''\xrightarrow{{II}} \hfill \\ MoO_{2(sur)} + ''Mo_2 S_3 ''\xrightarrow{{III}}MoO_2 \xrightarrow{{IV}}MoO_3 \hfill \\ \end{gathered} $$ where (sur) refers to the surface layer. The kinetic study revealed that the oxidation (α=0.01−0.90) of MoS2 to MoO2 was controlled by the kinetics $$1 - (1 - \alpha )^{1/3} = kt$$ and that the apparent activation energies determined with the isothermal and the nonisothermal (10 deg min−1) method were 98.1±2.2 and 93.5±3.0 kJ mol−1, respectively, over the temperature range 540–625‡ and the Po2 range 0.612-0.129 atm. The relationship between the rate constantk and Po2 was determined.