1 Filamentous Carbon Formation over Iron Surfaces R. T . K . B A K E R
and D . J. C . Y A T E S
Downloaded by UNIV MASSACHUSETTS AMHERST on September 17, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch001
Exxon Research and Engineering Company, Corporate Research Science Laboratories, Linden, N J 07036 J. A . D U M E S I C University of Wisconsin, Chemical Engineering Department, Madison, WI 53706
It i s w e l l known that the surface s t a t e of a metal can have a profound e f f e c t on its ability to c a t a l y z e the formation of carbon during a hydrocarbon conversion process. In t h i s work we have examined the e f f e c t of oxygen and steam pretreatments of i r o n surfaces on t h e i r ability to c a t a l y z e carbon filament formation during r e a c t i o n of such surfaces with hydrocarbons. Pretreatment of i r o n i n steam a t 700°C r e s u l t e d i n the conversion of the metal to FeO, and t h i s oxide was found to be a tremendously a c t i v e c a t a l y s t f o r carbon filament growth when reacted with acetylene or ethane at 700°C. On the other hand, the activity of Fe O , formed by r e a c t i o n of i r o n with oxygen, was about the same as that of the metal when treated with these hydrocarbons. From t h i s study it is concluded that the key to the high FeO activity i s that it is a precursor f o r a high surface area Fe c a t a l y s t formed i n situ. I t was a l s o found that Fe C is not an a c t i v e c a t a l y s t f o r carbon filament formation. 2
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The accumulation of carbon on metal surfaces when heated i n the presence of carbon containing gases i s a s e r i o u s problem encountered i n a number of commercial processes. Although carbon appears to deposit on most surfaces there are some m a t e r i a l s which are more v u l n e r a b l e than others since they contain c o n s t i t u e n t s which c a t a l y z e carbon formation. The highest c a t a l y t i c a c t i v i t y i s e x h i b i t e d by the ferromagnetic metals and i n p a r t i c u l a r , i r o n . Furthermore i t i s w e l l known that the surface s t a t e of such metals can have a dramatic e f f e c t on t h e i r a b i l i t y to c a t a l y z e the formation of carbon.
0097-6156/82/0202-0001$06.50/0 © 1982 American Chemical Society
In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by UNIV MASSACHUSETTS AMHERST on September 17, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch001
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COKE FORMATION
Carbon d e p o s i t s normally have a complex s t r u c t u r e containing s e v e r a l d i f f e r e n t growth forms, which can be grouped under three headings: amorphous, filamentous, and g r a p h i t i c p l a t e l e t s (1^, 2_, 3). Of these three forms the l e a s t understood i s amorphous carbon. A v a i l a b l e evidence suggests that condensation and p o l y m e r i z a t i o n r e a c t i o n s play a major r o l e i n amorphous carbon formation. Although some hydrogen i s removed during condensation, a s i g n i f i c a n t amount remains i n the d e p o s i t , but as the temperature i s r a i s e d , dehydrogenation r e a c t i o n s reduce the hydrogen content to
