Iron Fischer-Tropsch Catalysts: Surface ... - American Chemical Society

11

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Iron Fischer-Tropsch Catalysts: Surface Synthesis at High Pressure D. J. Dwyer Corporate Research Science Laboratories, Exxon Research & Engineering Company, Annandale, NJ 08801 An XPS investigation of iron Fischer-Tropsch catalysts before and after exposure to realistic reaction conditions is reported. The iron catalyst used in the study was a moderate surface area (15M2/g) iron powder with and without 0.6 wt.% K2CO3. Upon reduction, surface oxide on the fresh catalyst is converted to metallic iron and the K2CO3 promoter decomposes into a potassium-oxygen surface complex. Under reaction conditions, the iron catalyst is converted to iron carbide and surface carbon deposition occurs. The nature of this carbon deposit is highly dependent on reaction conditions and the presence of surface alkali. In r e c e n t y e a r s t h e c o u p l i n g o f a t m o s p h e r i c p r e t r e a t m e n t o r r e a c t o r s y s t e m s d i r e c t l y t o UHV s u r f a c e a n a l y s i s s y s t e m s h a s become common place. This combination o f t e c h n i q u e s has e s t a b l i s h e d a c l e a r r e l e v a n c y f o r UHV s u r f a c e s c i e n c e i n t h e a r e a o f c a t a l y s i s . It p e r m i t s b o t h d e t a i l e d m e c h a n i s t i c s t u d i e s o v e r w e l l d e f i n e d model s u r f a c e s , as w e l l as c h a r a c t e r i z a t i o n o f i n d u s t r i a l c a t a l y s t s i n t h e i r true activated form. One c a t a l y s t s y s t e m w h i c h h a s r e c e i v e d t h i s type of a t t e n t i o n i s the i r o n Fischer-Tropsch catalyst.(1-5) T h e i r o n c a t a l y s t i s a c o m p l e x m a t e r i a l whose c o m p o s i t i o n i s s o m e what d y n a m i c . The c a t a l y s t i s g e n e r a l l y p r e p a r e d a s a h i g h s u r f a c e a r e a o x i d e (Fe Û3) w i t h t h e a d d i t o n o f b o t h t e x t u r a l ( S i O o , Cu) and c h e m i c a l (K) p r o m o t e r s . P r i o r t o use the c a t a l y s t i s s u b j e c t e d t o various pretreatments which involve either reduction i n H2 or d i r e c t c o n t a c t w i t h CO/H2 m i x t u r e s . The p u r p o s e o f t h e s e p r e t r e a t ments i s t o s y n t h e s i z e a w o r k i n g s u r f a c e w h i c h e x h i b i t s a d e s i r a b l e c a t a l y t i c response. C o n t r o l o f t h e s u r f a c e s y n t h e s i s s t e p i s a key technological challenge in industrial c a t a l y s i s . In t h i s p a p e r we r e p o r t how a h i g h p r e s s u r e r e a c t o r / U H V e l e c t r o n s p e c t r o m e t e r s y s t e m can be used to monitor changes in surface composition that accompany these pretreatments. The two c a t a l y s t s s t u d i e d were moderate s u r f a c e a r e a powders ( 1 5 M 2 / g ) w i t h and w i t h o u t 0 . 6 wt.% 2

0097-6156/85/0288-0124$06.00/0

© 1985 American Chemical Society In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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K2CO3. The r e s u l t s i n d i c a t e t h a t t h e w o r k i n g c a t a l y t i c s u r f a c e i s a c a r b i d e d form o f i r o n which i s s y n t h e s i z e d under C 0 / H 2 . It i s a l s o found, that the type of carbon deposit that f o r m s on t h e surface i s s e n s i t i v e t o the presence of surface a l k a l i .


Experimental The e x p e r i m e n t a l a p p a r a t u s shown s c h e m a t i c a l l y i n F i g u r e 1 h a s b e e n described elsewhere.^) I t c o n s i s t s o f a medium p r e s s u r e m i c r o r e a c t o r c o u p l e d t o a n u l t r a - h i g h vacuum s y s t e m e q u i p p e d t o p e r f o r m x-ray photoelectron spectroscopy. The XPS s y s t e m c o n s i s t e d o f a L H S - 1 0 e l e c t r o n e n e r g y a n a l y z e r a n d a d u a l a n o d e x - r a y s o u r c e (Mg and Al). The m i c r o - r e a c t o r was a s m a l l UHV c o m p a t i b l e tube furnace. The r e a c t o r ' s i n t e r n a l v o l u m e was a p p r o x i m a t e l y 10 c c a n d t h e w a l l s were g o l d p l a t e d f o r i n e r t n e s s . The r e a c t o r was d e s i g n e d s u c h t h a t t h e sample and r e a c t a n t s were i s o t h e r m a l and t h a t good gas m i x i n g t a k e s p l a c e i n t h e r e a c t o r . The p o w d e r s a m p l e s w e r e p r e s s e d i n t o a g o l d mesh b a c k i n g m a t e r i a l w h i c h i n t u r n was m o u n t e d on a g o l d s a m p l e b o a t . The s a m p l e a n d b o a t c o u l d b e moved d i r e c t l y f r o m t h e r e a c t o r i n t o t h e UHV s y s t e m v i a a m a g n e t i c a l l y coupled motion feed through. The i r o n p o w d e r was p r e p a r e d by r e d u c i n g u l t r a - h i g h purity Fe 0o i n a s e p a r a t e t u b e f u r n a c e . The r e d u c t i o n was c a r r i e d o u t t o c o m p l e t i o n a t 675K, 1 atm H 2 f o r a p p r o x i m a t e l y 24 h o u r s . The s u r f a c e o f t h i s p r y o p h o r i c m a t e r i a l was t h a n p a s s i v a t e d by e x p o s u r e t o 1% o x y g e n i n a h e l i u m c a r r i e r f o r 2 h o u r s . The p a s s i v a t e d p o w d e r was c h a r a c t e r i z e d b y x - r a y d i f f r a c t i o n (XRD) a n d o n l y α-iron was detected. XPS a n a l y s i s of the surface of t h i s material r e v e a l e d o n l y Fe^Oo p r e s e n t . These r e s u l t s s u g g e s t t h a t t h e i r o n bulk i s covered witn a r e l a t i v e l y t h i n oxide s k i n . 2 . 5 g r a m s o f t h i s p a s s i v a t e d m a t e r i a l was c o a t e d w i t h .015 grams o f K 2 C 0 3 t h r o u g h a s t a n d a r d a q u e o u s i m p r e g n a t i o n technique. The amount o f a l k a l i was c h o s e n t o m a t c h t h e 0 . 6 % by w e i g h t c a l l e d f o r i n many i r o n c a t a l y s t p r e p a r a t i o n s . ( 7 ) This loading of K2C03 i s t h o u g h t t o p r o d u c e t h e maximum p r o m o t i o n a l e f f e c t . The i m p r e g n a t e d c a t a l y s t was a i r d r i e d a t 3 3 5 Κ f o r 12 h o u r s t o r e m o v e e x c e s s water. The p h y s i c a l s u r f a c e a r e a s o f t h e t w o s a m p l e s ( w i t h a n d without K0CO3) w e r e m e a s u r e d by a s t a n d a r d BET m e t h o d after a second hydrogen r e d u c t i o n . The a l k a l i t r e a t e d c a t a l y s t h a d a s u r f a c e area o f 16M2/gram and t h e u n t r e a t e d c a t a l y s t a s u r f a c e a r e a o f 18 M 2 / g r a m . Assuming complete d i s p e r s i o n o f t h e a l k a l i and an i r o n surface s i t e density of 1 0 1 5 s i t e s / c m 2 , t h e a l k a l i surface coverage on t h e p r o m o t e d c a t a l y s t i s a p p r o x i m a t e l y 1 / 3 o f a m o n o l a y e r . 2

The g a s e s u s e d w e r e p u r c h a s e d p r e m i x e d i n a l u m i n u m c y l i n d e r s to avoid carbonyl formation. The h i g h p u r i t y g a s m i x t u r e was further p u r i f i e d by a z e o l i t e w a t e r t r a p a n d a c o p p e r carbonyl trap. The g a s p r e s s u r e i n t h e r e a c t o r was m e a s u r e d w i t h a c a p c i t a n c e manometer and t h e f l o w m o n i t o r e d w i t h a mass f l o w c o n t r o l ler. The t y p i c a l g a s f l o w r a t e s w e r e 15 c c / m i n ( S T P ) a n d t h e maximum c o n v e r s i o n was « 1% b a s e d on i n t e g r a t i o n of hydrocarbon products. The h y d r o c a r b o n p r o d u c t s w e r e a n a l y z e d by g a s c h r o m a t o g r a p h y ( t e m p e r a t u r e programmed c h r o m o s o r b 1 0 2 , F I D ) .

In Catalyst Characterization Science; Deviney, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


Figure 1.

Schematic o f Experimental

Apparatus.

MEDIUM PRESSURE REACTOR/X-RAY PHOTOELECTRON SPECTROMETER


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Results


In t h i s s e c t i o n t h e r e s u l t s o f two s t u d i e s a r e s u m m a r i z e d . The f i r s t s t u d y was c a r r i e d o u t w i t h an u n p r o m o t e d i r o n p o w d e r . The second study used t h e potassium promoted i r o n powder. A detailed r e p o r t o f t h e f i r s t s t u d y h a s been p r e v i o u s l y p u b l i s h e d . ( 6 ) The key f e a t u r e s of t h i s study are summarized here to f a c i l i t a t e a comparison between the catalytic response of the promoted and unpromoted powder. Unpromoted Iron Powder. The XPS s p e c t r u m o f t h e f r e s h l y prepared i r o n p o w d e r i s shown i n F i g u r e 2 a . The i r o n 2 p 3 ^ 2 p h o t o l i n e i s c e n t e r e d at 710.7 eV a n d t h e o x y g e n l i n e i s c e n t e r e d a t 5 2 9 . 7 e V . The p o s i t i o n a n d i n t e n s i t i e s o f t h e s e l i n e s a r e c o n s i s t e n t w i t h a s u r f a c e l a y e r o f F e 2 0 3 on t h e i r o n p o w d e r . ( 8 ) In a d d i t i o n t o t h e i r o n o x i d e , a s m a l l amount o f c a r b o n i m p u r i t y i s a l s o p r e s e n t o n the surface of the c a t a l y s t . After surface characterization, the s a m p l e was moved t o t h e r e a c t o r a n d r e d u c e d i n H 2 a t 6 2 5 Κ f o r approximately 2 hours. This pretreatment, a s shown i n F i g u r e 2b, was s u f f i c i e n t t o r e d u c e t h e s u r f a c e o x i d e t o m e t a l ( B . E . Fe2p3^2 = 706.6 eV). The o n l y d e t e c t i b l e i m p u r i t i e s on t h e s u r f a c e o f t h e c a t a l y s t a f t e r r e d u c t i o n were t r a c e amounts o f s u l f u r , c a r b o n and oxygen. U s i n g s t a n d a r d XPS c r o s s s e c t i o n s i t was e s t i m a t e d that these i m p u r i t i e s were less than 1 atom % o f t h e XPS sampling volume. A f t e r r e d u c t i o n and s u r f a c e c h a r a c t e r i z a t i o n , t h e i r o n sample was moved t o t h e r e a c t o r a n d b r o u g h t t o t h e r e a c t i o n c o n d i t i o n s (7 a t m , 3 : 1 H 2 : C 0 , 540 K ) . Once t h e r e a c t o r t e m p e r a t u r e , g a s f l o w a n d pressure were s t a b i l i z e d ( * 10 m i n . ) t h e catalytic activity and selectivity were monitored by o n - l i n e gas chromatography. As p r e v i o u s l y r e p o r t e d , t h e i r o n powder e x h i b i t e d an i n d u c t i o n p e r i o d i n which the c a t a l y t i c a c t i v i t y i n c r e a s e d with t i m e . The c a t a l y s t reached steady state activity after approximately 4 hours on line. This i n d u c t i o n p e r i o d i s b e l i e v e d t o be t h e r e s u l t o f a c o m p e t i t i o n f o r s u r f a c e c a r b o n between bulk c a r b i d e f o r m a t i o n and hydrocarbon s y n t h e s i s . ( 6 , 9 ) Steady s t a t e s y n t h e s i s i s reached o n l y a f t e r the surface region of the c a t a l y s t i s f u l l y c a r b i d e d . To verify that steady state catalytic activity had been a c h i e v e d , t h e c a t a l y s t was a l l o w e d t o o p e r a t e u n i n t e r r u p t e d f o r a p proximately 8 hours. The c a t a l y s t was t h e n r e m o v e d f r o m t h e r e a c t o r a n d t h e s u r f a c e i n v e s t i g a t e d by X P S . The r e s u l t s a r e shown i n Figure 2c. The t w o m a j o r c h a n g e s i n t h e XPS s p e c t r u m w e r e a s h i f t i n t h e i r o n 2 p 3 ^ 2 l i n e t o 7 0 6 . 9 eV a n d a new c a r b o n I s l i n e c e n t e r e d at 283.3 eV. T h i s c o m b i n a t i o n o f i r o n and c a r b o n lines i n d i c a t e s t h e f o r m a t i o n o f an i r o n c a r b i d e p h a s e w i t h i n t h e XPS sampling volume.(J5) In f a c t a f t e r e x t e n d e d o p e r a t i o n , XRD o f t h e i r o n s a m p l e i n d i c a t e d t h a t t h e b u l k had b e e n c o n v e r t e d t o FecC2 c o m m o n l y r e f e r r e d t o a s t h e Hagg c a r b i d e . ( _ 7 ) It appears t h a t the bulk and s u r f a c e a r e f u l l y c a r b i d e d u n d e r d i f f e r e n t i a l reaction conditions. The s t e a d y s t a t e r a t e s o f h y d r o c a r b o n s y n t h e s i s o v e r t h e c a r b i d e d i r o n s u r f a c e a r e g i v e n i n Table I. The r e a c t i o n r a t e s h a v e been n o r m a l i z e d t o t h e p h y s i c a l s u r f a c e a r e a of t h e s t a r t i n g i r o n powder [18 M 2 / g ] and a r e r e p o r t e d i n m o l e c u l e s / c m 2 s e c . A turnover



732.0

522.0 307.0

702.0 542.0

l

I

I

Figure 2. XPS s p e c t r a o f i r o n p o w d e r a f t e r e x p o s u r e t o v a r i o u s environment, a) F r e s h l y prepared a i r exposed powder, b) A f t e r h y d r o g e n r e d u c t i o n 2 a t m H 2 a t 6 2 5 K. c) After steady s t a t e operation f o r 8 h r , 3:1 H2:C0 7 a t m , 540 K.

BINDING ENERGY eV

4X

2X

0(1s)

L

b

•A

ι 277.0

8

c

C(1S)

-F /· /


Table

I.

Steady

State

Rates Iron

540 K, Iron


Carbon

129


11. DWYER

Number

3:1

of

Hydrocarbon

H?:C0,

7

X X

1.6

X

7.4

X

3.9

X

1.8

X

10i l l

Promoted

Rate (Molecules/cnr

sec)

,12 10; 12 10 ,12 10;

10;,11 10,11

atm Potassium

Carbide

2.4

Over

Carbide

Rate (Molecules/cnr 4.5

Synthesis

4.1

X

10;

3.2

X

10

3.2

X

10;

1.9

X

10;

1.4

X

10

9.6

X

1010

sec)

number h a s n o t b e e n r e p o r t e d s i n c e t h e i r o n s u r f a c e s i t e d e n s i t y o f t h e c a r b i d e d m a t e r i a l i s unknown. If a s i t e d e n s i t y of 1 0 1 5 / c n r i s c h o s e n t h e s t e a d y s t a t e m e t h a n a t i o n r a t e i s on t h e o r d e r o f 10"3 molecules/site sec. This turnover frequency i s considerably lower than that reported in e a r l i e r studies (range .05 to 2).(2,4,9) This r e s u l t suggest that only a small f r a c t i o n of the i r o n carbide surface i s active. Potassium Modified Iron Powder. Surface analysis (XPS) of the freshly prepared Dotassiurn m o d i f i e d surface i s given i n Figure 3a. The i r o n 2 p 3 ' 2 b i n d i n g e n e r g y i s l o c a t e d a t 7 1 0 . 6 eV a n d t h e dominant oxygen Is l i n e i s a t 5 2 9 . 7 eV. These v a l u e s a r e a g a i n consistent with a s u r f a c e l a y e r of Fe 0o. In a d d i t i o n t o the s u r f a c e o x i d e , K2CO3 i s a l s o p r e s e n t on t n e s u r f a c e o f t h e c a t a lyst. The p r e s e n c e o f t h e c a r b o n a t e i s i n d i c a t e d by a p o t a s s i u m 2 p 3 / 2 p e a k a t « 2 9 3 eV a n d a c a r b o n a t e c a r b o n l i n e a t « 2 8 9 e V . A h i g h b i n d i n g e n e r g y s h o u l d e r i s a l s o p r e s e n t on t h e o x y g e n Is l i n e b u t an e x a c t b i n d i n g energy i s d i f f i c u l t t o measure due t o the overlap with the strong iron oxide s i g n a l . These r e s u l t s a r e i n g e n e r a l a g r e e m e n t w i t h t h o s e r e p o r t e d by B o n z e l a n d c o - w o r k e r s ( 5 ) f o r K0CO3 t r e a t e d i r o n f o i l s . The p o t a s s i u m t r e a t e d m a t e r i a l was t h e n moved t o t h e m i c r o r e a c t o r and r e d u c e d under c o n d i t i o n s i d e n t i c a l t o t h o s e used f o r t h e unpromoted i r o n . F i g u r e 3b c o n t a i n s t h e XPS s p e c t r u m o f t h e modified surface after hydrogen reduction. Once a g a i n t h e iron 2 p 3 / 2 peak i s c e n t e r e d a t 7 0 6 . 6 eV i n d i c a t i n g t h e r e d u c t i o n o f t h e iron oxide to m e t a l l i c i r o n . The m a i n o x i d e o x y g e n I s s i g n a l a t 5 2 9 . 7 eV i s a l m o s t t o t a l l y removed f r o m t h e s p e c t r u m s u g g e s t i n g complete reduction of the o x i d e . In t h e p o t a s s i u m 2p a n d t h e c a r bon I s r e g i o n o f t h e s p e c t r u m i m p o r t a n t c h a n g e s t a k e p l a c e . The k e y f e a t u r e s a r e t h a t t h e p o t a s s i u m 2p s i g n a l i n c r e a s e s r e l a t i v e t o the carbonate carbon signal ( B E « 2 8 9 ) , and t h e appearance of a 2





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s t r o n g oxygen Is l i n e at 531.9 eV. These r e s u l t s a r e c o n s i s t e n t w i t h t h e p a r t i a l d e c o m p o s i t i o n o f K2CO3 i n t o a s u r f a c e KOH l a y e r a s d e s c r i b e d by B o n z e l a n d c o - w o r k e r s . ( 2 j J > ) The c h a n g e i n t h e r a t i o of carbonate carbon to potassium signal i n t e n s i t y suggests that some o f t h e c a r b o n a t e c a r b o n i s l o s t d u r i n g t h e r e d u c t i o n . The oxygen Is feature at 531.9 eV i s i n d i c a t i v e of the formation chemisorbed KOH.(5) The f r e s h l y r e d u c e d p o t a s s i u m m o d i f i e d s u r f a c e was t e s t e d f o r catalytic activity under c o n d i t i o n s i d e n t i c a l t o those used for unpromoted m a t e r i a l (540K, 3:1 Ho.'CO, 7 a t m ) . The p r o m o t e d iron powder e x h i b i t e d an i n d u c t i o n p e r i o d w h e r e i n t h e c a t a l y s t a c t i v i t y increased with time. H o w e v e r , t h e i n d u c t i o n p e r i o d was c o n s i d e r ably shorter ( « 1 hr) than that observed over the unpromoted surface. T h i s r e s u l t may i n d i c a t e a m o r e r a p i d c a r b i d i n g o f t h e iron surface region. The s t e a d y s t a t e h y d r o c a r b o n s y n t h e s i s r a t e s are given in Table I. In terms of the rate of hydrocarbon p r o d u c t i o n , t h e most s i g n i f i c a n t d i f f e r e n c e between t h e promoted a n d u n p r o m o t e d m a t e r i a l s i s t h e much l o w e r m e t h a n a t i o n rate over the promoted s u r f a c e . On a p h y s i c a l s u r f a c e a r e a b a s i s (reduced promoted i r o n 15rrr/g) t h e m e t h a n a t i o n a c t i v i t y o v e r t h e promoted c a t a l y s t i s s u p p r e s s e d by a l m o s t an o r d e r o f m a g n i t u d e . The r a t e s o f f o r m a t i o n o f o t h e r m o l e c u l e s i n t h e C j t o Cg r a n g e w e r e a l s o suppressed but to a decreasing degree with increasing chain length. F o r e x a m p l e , t h e p r o d u c t i o n o f m e t h a n e was approximately 13% o f t h e u n p r o m o t e d r a t e . The r a t e o f Cg p r o d u c t i o n was 53% o f t h e unpromoted rate. O n c e t h e s t e a d y s t a t e a c t i v i t y h a d b e e n v e r i f i e d by c o n t i n u o u s o p e r a t i o n f o r e i g h t h o u r s w i t h o u t l o s s o f a c t i v i t y , t h e s a m p l e was removed from t h e r e a c t o r f o r s u r f a c e a n a l y s i s . The XPS r e s u l t s a r e given i n Figure 3c. The m a j o r c h a n g e i n t h e XPS s p e c t r u m i s t h e l a r g e i n c r e a s e i n the carbon Is s i g n a l i n t e n s i t y . Close inspection o f t h e c a r b o n r e g i o n r e v e a l s t h a t two d i s t i n c t c a r b o n s i g n a l s a r e present. The m a j o r p e a k i s c e n t e r e d a t 2 8 5 . 7 , t h e o t h e r a t 2 8 3 . 3 eV. In a d d i t i o n , t h e i r o n 2 p 3 ^ 2 i s c e n t e r e d a t 7 0 6 . 9 e V . Once a g a i n , we b e l i e v e t h a t t h e 2 8 3 . 3 eV c a r b o n f e a t u r e a n d t h e 7 0 6 . 9 eV i r o n s i g n a l are c l e a r i n d i c a t o r s of i r o n carbide f o r m a t i o n . The key q u e s t i o n , however, i s t h e n a t u r e of t h e i n t e n s e carbon f e a t u r e centered at 285.7. Previous work h a s s u g g e s t e d t h a t potassium i n c r e a s e s t h e r a t e o f g r a p h i t e d e p o s i t i o n on i r o n s u r f a c e s u n d e r reaction conditions.(1,3) We b e l i e v e t h a t t h e c a r b o n s p e c i e s i s n o t g r a p h i t i c but i s an a d s o r b e d h y d r o c a r b o n phase ( g r o w i n g c h a i n s o r h i g h m o l e c u l a r w e i g h t p r o d u c t s ) on t h e s u r f a c e . The argument for this assignment is two-fold. First, t h e 2 8 5 . 7 eV binding energy is identical to that measured for octacosane (C23H50) a d s o r b e d on a i r o n foil(6). Second, i s t h e mass s p e c t r u m of m a t e r i a l d e s o r b e d f r o m t h i s s a m p l e upon h e a t i n g t o 4 2 5 Κ i n t h e vacuum s y s t e m . I t c o n s i s t s o f a c r a c k i n g p a t t e r n s t a r t i n g a t mass 15 a n d c o n t i n u i n g a t m u l t i p l e s o f mass 14 a s h i g h a s t h e mass s p e c t r o m e t e r p e r m i t t e d (200 amu). This cracking pattern i s c l e a r l y that of l i n e a r saturated hydrocarbons s i m i l a r t o polymethylene (nB a s e d on t h e a t t e n u a t i o n o f t h e i r o n 2p 1 s i g n a l and assuming a mean f r e e p a t h f o r t h e i r o n e l e c t r o n s o f 1 . 5 t o 2 n a n o m e t e r s , it i s estimated that the carbon o v e r l a y e r i s at l e a s t 1.8 t o 2.5 nano-


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meters t h i c k . It i s interesting to note, h e a v y b u i l d up o f m a t e r i a l o n t h e c a t a l y s t , remains at steady state.

that in spite the c a t a l y t i c

of this activity


Conclusions^ This XPS investigation of small iron Fischer-Tropsch catalysts b e f o r e and a f t e r t h e p r e t r e a t m e n t and e x p o s u r e t o s y n t h e s i s gas has y i e l d e d the f o l l o w i n g information. Relatively mild reduction cond i t i o n s ( 3 5 0 ° C , 2 a t m , Hg) a r e s u f f i c i e n t t o t o t a l l y r e d u c e s u r f a c e o x i d e on i r o n t o m e t a l l i c i r o n . Upon e x p o s u r e t o s y n t h e s i s g a s , the m e t a l l i c i r o n surface i s converted to i r o n c a r b i d e . During this transformation, the c a t a l y t i c response of the m a t e r i a l inc r e a s e s and f i n a l l y r e a c h e s s t e a d y s t a t e a f t e r t h e s u r f a c e i s f u l l y carbided. The a d d i t i o n o f a p o t a s s i u m p r o m o t e r a p p e a r s t o a c c e l e r a t e t h e c a r b i d a t i o n o f t h e m a t e r i a l and s t e a d y s t a t e r e a c t i v i t y i s achieved somewhat e a r l i e r . In a d d i t i o n , the potassium promoter c a u s e s a b u i l d up on c a r b o n a c e o u s m a t e r i a l on t h e s u r f a c e o f the c a t a l y s t s which i s b e s t c h a r a c t e r i z e d as p o l y m e t h y l e n e .

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Bonzel, H. P.; Krebs, H. J. Surface Science, 1981, 109, L527. Krebs, H. J . ; Bonzel, H. P. Surface Science, 1982, 88, 269. Bonzel, H. P.; Chem. Ing. Tech, 1982, 54, 908. Dwyer, D. J . ; Somorjai, G. A. J. Catalysis, 1970, 52, 291. Bonzel, H. P.; Broden, G.; Krebs, H. J. Applications of Surface Science,1983, 16, 373. Dwyer, D. J . ; Hardenbergh, J. H. J. Catalysis, 1984, 87, 66. Storch, H. H.; Golumbic, N.; Anderson, R. B. "The FischerTropsch and Related Synthesis," John Wiley: New York, 1951. Hirohawa, H.; Okee, H. Talanta, 1979, 26, 855. Vannice, M. A. J. Catalysis, 1975, 37, 449.

RECEIVED May 2, 1985


Iron Fischer-Tropsch Catalysts: Surface ... - American Chemical Society

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