Mechanism and the intrinsic rates of reduction of metallic oxides
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ing of the s u r f a c e was found 5 to r e s u l t in a d e c r e a s e in the induction p e r i o d . The m a j o r i t y of the r e d u c t i o n r e a c t i o n s that have b e e n i n v e s t i g a t e d to date can be r e p r e s e n t e d by an e q u a t i o n of the type,
Ml(s)
+
A(g)
=
M2(s)
+
B~)
[1]
w h e r e i n M1 s t a n d s for the oxide phase (e.g., W u s t i t e , n i c k e l oxide, copper oxide, cobalt oxide, and so forth) and M2 d e n o t e s the m e t a l l i c phase (e.g., i r o n , n i c k e l , copper, cobalt, and so forth). The r e d u c i n g gas, A, may c o n s i s t of e i t h e r p u r e h y d r o g e n or p u r e c a r b o n monoxide or m i x t u r e s c o n t a i n i n g one o r both of t h e s e . The o n s e t of Eq. [1 ] may be r e g a r d e d as c o i n c i d i n g with the f o r m a t i o n of n u c l e i of metal; t h e s e n u c l e i will a p p e a r only a f t e r the oxide phase has b e c o m e s u f f i c i e n t l y s u p e r s a t u r a t e d with r e s p e c t to the m e t a l . The s i g m o i d shaped oz-t plot m a y be i n t e r p r e t e d a s a r i s i n g f r o m the following s u c c e s s i o n of e v e n t s : 1) f o r m a t i o n of n u c l e i of m e t a l l i c phase at l o c a l i z e d s i t e s on the oxide s u r f a c e , 2) growth of these n u c l e i at the e x p e n s e of the p a r e n t phase as oxygen is r e moved at the m e t a l / o x i d e i n t e r f a c e , and 3) o v e r l a p of the growing n u c l e i and a d e c r e a s e in the metaL/oxide i n t e r f a c e . Undoubtedly, the length of the i n d u c t i o n p e r i o d is m o s t i n t i m a t e l y connected with the r a t e of f o r m a t i o n of n u c l e i . In the s u b s e q u e n t a c c e l e r a t o r y p e r i o d , the m a s s r a t e of r e d u c t i o n , d~/dt, is e s s e n t i a l l y a p r o d u c t of two t e r m s - o n e which d e s c r i b e s the r a t e of n u c l e a t i o n and the o t h e r the r a t e of growth of n u c l e i . U n d e r c e r t a i n c o n d i t i o n s , n e i t h e r the i n d u c t i o n p e r i o d n o r the a c c e l e r a t o r y p e r i o d may be p r e s e n t in the ~ - t plot. If the r a t e of n u c l e a t i o n is v e r y l a r g e , then the e n t i r e s u r f a c e of the oxide s p e c i m e n b e c o m e s r a p i d l y c o v e r e d with m e t a l l i c phase and the m e t a l / oxide i n t e r f a c e a d v a n c e s i n w a r d s at a c o n s t a n t r a t e in a t o p o c h e m i c a l f a s h i o n . F o r such a s i t u a t i o n , the m a s s r a t e of r e d u c t i o n can be deduced f r o m g e o m e t r i cal c o n s i d e r a t i o n s . O n the other hand, if the r a t e of n u c l e a t i o n is not v e r y l a r g e , the t o p o c h e m i c a l b e h a v i o r will not be r e a l i z e d , the metaL/oxide i n t e r f a c e will n e c e s s a r i l y be i r r e g u l a r , and the c o u r s e of the r e d u c tion will be d e t e r m i n e d by the p r o g r e s s i v e f o r m a t i o n of n u c l e i as
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