The kinetics of molybdenum dioxide oxidation
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M. E. WADSWORTH,
AND
C. K. HANSEN
Powdered m o l y b d e n u m dioxide was oxidized to MoO 3 in the t e m p e r a t u r e r a n g e 390 to 465~ under oxygen p a r t i a l p r e s s u r e s of 0.016 and 0.18 a t m o s p h e r e s * and under 0.009 a t m o s p h e r e s *Onearm = 101325 Newtons/m=. in the t e m p e r a t u r e i n t e r v a l 465 to 529~ The c o u r s e of the r e a c t i o n was followed by o b s e r v i n g weight change with t im e . P a r a b o l i c k i n e t i c s w e r e evident for oxidation below 460~ Above 460~ l i n e a r k i n e t i c s w e r e o b s e r v e d . The p a r t i a l p r e s s u r e dependence at 407~ and 455~ was found to be a p p r o x i m a t e l y P ~ . Oxidation t e s t s w e r e r e s t r i c t e d to an oxygen p a r t i a l p r e s s u r e of 0.009 a t m o s p h e r e in }he t e m p e r a t u r e r a n g e between 460~ and 530 C since above this p a r t i a l p r e s s u r e e x c e s s i v e heating o c c u r r e d . For the low t e m p e r a t u r e r a n g e the oxidation was explained in t e r m s of the diffusion of m o n o - and d i v a l e n t oxygen i n t e r s t i t i a l s . Activation enthalpies of 159~=8 and 105:~8 k J / m o l e w e r e obtained r e s p e c t i v e l y for p a r a b o l i c and l i n e a r r a t e p r o c e s s e s .
THE oxidation of molybdenum metal has been widely studied. I-7 Molybdenum dioxide has been reported as an intermediate. 2-5 Phelps and co-workers ~ studied oxide films formed on molybdenum between 450~ and 500~ The dioxide was identified at 450~ by reflection and transmission electron diffraction. At 500~ both MoO, and MoO 3 were identified by reflection and MoO 3 by transmission. Hickman and Gulbransen 2 in their electron diffraction study of oxide films on molybdenum metallic substances in the temperature range of 300 to 700~ observed only MoO, above 450~ Below 450~ they observed MoO~ and MoO s. In oxidizing molybdenum powder in the temperature range 300 to 700~ they identified only MoO 2 above 450~ and a mixture of MoO 2 and MoO 3 below 450~ However, in oxidizing molybdenum powder at 700~ only MoO 3 was identified. These authors suggested that on a metallic substrate, any MoO 3 formed would react with residual metal. Gulbransen and Wysong 3 obtained parabolic oxidation kinetics for the oxidation of molybdenum in the temperature range 250 to 450~ and under oxygen pressures of 0.75 to 0.76 mm Hg. They obtained an activation energy of 153 k J/mole in the 350 to 450~ range. The oxide film above 400~ was MoO 2 while MoOa was observed below this temperature. Deviation from the parabolic law was observed at 450~ Simnad and Spilners 5 reported that MoO 2 and MoO 3 (both solid and vapor) resulted from metal oxidation in pure oxygen at one atmosphere pressure in the temperature range 500 to 770~ X-ray analysis indicated MoO a in an outside layer over MoO z. At temperatures below 600~ the parabolic rate constant was evaluated. An activation energy of 205 k J/mole was obtained. Below 600~ the amount of MoO s was found to be negligib
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