Thermal analysis and kinetics of oxidation of molybdenum sulfides

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.