Kinetics of molybdenum oxidations between 375 and 500ºC

Abstract

Oxidation of powder of metallic molybdenum takes place readily at temperatures above 350 ºC even at partial pressures of oxygen below 0.1 atm. For temperature between 375 and 500 °C complete oxidation is achieved in five hours or less. The only oxidation product is monoclinic and orthorrombic MoO3. MoO2 and non-stoichiometric molybdenum oxide such as Mo4O11 are not evidenced in any of the molybdenum oxidation steps. The kinetics of this reaction was studied under chemical kinetic control of the reaction rate to obtain the reaction equation and the intrinsic kinetic parameters. The oxidation rate was measured using gravimetry, X-ray diffraction and scanning electron microscopy at different temperatures and partial pressures of oxygen. The complete rate equation was determined with an activation energy of 154 + 3 kJ mol-1 and a Langmuir type equation for oxygen partial pressure dependence. The kinetic parameters remain constant during the whole reaction.