Coke Formation on Metal Surfaces - American Chemical Society

9 Growth and Initiation Mechanism of Filamentous Coke

Downloaded by TUFTS UNIV on June 22, 2017 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch009

A L B E R T SACCO, JR., and JOHN C. CAULMARE

1

Worcester Polytechnic Institute, Department of Chemical Engineering, Worcester, M A 01609

Decomposition of carbon bearing gases over transition metals often results in the formation of filamentous materials ("carbon fibers"). These carbon fibers are uniform i n width, usually between 500-1000Å, come in the shape of f l a t ribbons, solid or hollow tubes, and some are even twisted. The formation of these filaments results in corrosion of the metallic phase. An investigation was performed to try to ascertain the i n i tiation and growth mechanism for fiber formation in a five component gas mixture (CO, CO , CH , H , H O) over iron. Phase diagrams were used to try to control the solid phase composition during reaction. An electrobalance was used to continuously monitor weight gain, and electron optics were used to examine the bulk metal and fiber structure(s). Preliminary results indicate that fiber formation only occurs when a carbide i s thermodynamically favored. Also, the rate plotted as change i n fractional weight gain per unit time goes through a maximum and then levels out at a constant rate. Possible reasons for this will be proposed and a mechanism hypothesized. 2

4

2

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T r a n s i t i o n metals exposed t o carbon bearing gases a t e l e v a t e d temperatures o f t e n c a t a l y z e the formation o f carbonaceous d e p o s i t s . One of the more i n t e r e s t i n g carbon morphologies observed is frequently called " f i l a m e n t o u s " carbon. This form o f carbon c o n s i s t s of f i l a m e n t s o r fibers. These f i l a m e n t s a r e g e n e r a l l y cylindrical i n shape and l e s s than 1000 Å in diameter. Both solid (Hofer et al., (1)) and hollow ( O b e r l i n e t al., (2)) fibers have been observed as w e l l as t w i s t e d or braided f i l a m e n t s (Boehm (3). Most f i l a m e n t s have a small piece o f m e t a l , u s u a l l y about the same diameter as the f i l a m e n t , embedded a t some p o i n t along its l e n g t h .

1

Current address: EXXON Research and Engineering Company, Florhan Park, NJ 07932.

0097-6156/82/0202-0177$06.00/0 © 1982 American Chemical Society

Albright and Baker; Coke Formation on Metal Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COKE FORMATION


178

This metal c r y s t a l i s b e l i e v e d to be the growth c e n t e r (Baker et a l . , ( 4 ) ) . The recent r e s u l t s of Baker and Chludzinski (5j) f u r t h e r support t h i s c o n t e n t i o n . The penchant of t h i s form of carbon f o r removing metal from the s u r f a c e of a large piece of metal, or from a support m a t e r i a l i s the most i n t e r e s t i n g as well as damaging aspect of t h i s type of d e p o s i t . E v e n t u a l l y , i t s form a t i o n r e s u l t s in the d e s t r u c t i o n of the parent metal m a t r i x , or the removal of c a t a l y t i c m a t e r i a l from i t s support. Many i n v e s t i g a t i o n s have been performed to t r y to a s c e r t a i n the growth mechanism of t h i s form of carbon. An e x c e l l e n t review on work in t h i s area i s presented by Baker and H a r r i s (6). How ever, none has r e s u l t e d in a completely c o n s i s t e n t mechanism that can e x p l a i n the v a r i e d reported r e s u l t s . S u r p r i s i n g l y few in v e s t i g a t i o n s have attempted to study the i n i t i a t i o n s t e p , the way the growth c r y s t a l i s e x t r a c t e d (extruded?) from the metal matrix or support i n i t i a l l y . Also to the best of the a u t h o r s ' knowledge, no attempt has been made to c o n t r o l the s u r f a c e phase(s) during reaction. This i s of importance s i n c e phase(s) ( e . g . , c a r b i d e s ) can form during heating and c o o l i n g sequences. The o b j e c t i v e of the present i n v e s t i g a t i o n i s to t r y to understand the i n i t i a t i o n step in f i b e r growth, and a l s o to e s t a b l i s h what i r o n metal phases c a t a l y z e f i l a m e n t growth.

Experimental A l l experiments were performed on high purity(>99.99%) p o l y c r y s t a l l i n e iron f o i l s . Carbon was deposited from multicomponent mixtures of H , H 0 , CH^, CO, and C0 at 900 Κ and 1 bar pressure. These r e a c t a n t gases were d e l i v e r e d to the r e a c t o r a t 20 c c / s (STP). The weight changes observed were a t t r i b u t e d to carbon d e p o s i t i o n or o x i d a t i o n and monitored throughout the course of an experiment using an e l e c t r o b a l a n c e - r e c o r d e r u n i t . Figure 1 i s a schematic of the experimental apparatus. Reactant gases were i n d i v i d u a l l y metered through Brook mass flow r e g u l a t o r s . Their precise flowrates were determined by measurement of t h e i r pressure drops through p r e c i s i o n bore c a p i l l a r y tubes. They were then mixed by passage through a drying tube and fed dry or saturated with water to a fused quartz p r e h e a t e r - r e a c t o r . S a t u r a t i o n of the feed gas was accomplished by flow through a s e r i e s of bubblers submerged in a t h e r m o s t a t i c a l l y c o n t r o l l e d water bath. P a r t i a l pressures of water from 0 to 0.34 bar were e a s i l y obtained. A l l feed and e x i t l i n e s were wrapped with heating tapes and maintained at temperatures in excess of 363 K. 2

2

2

The r e a c t o r assembly c o n s i s t e d to two 38 mm diameter t u b u l a r s e c t i o n s : the r e a c t o r - p r e h e a t e r tube and the r e a c t o r - e l e c t r o b a l a n c e interface section. The r e a c t o r - p r e h e a t e r tube i s 0.66 m in length and i s c o n s t r u c t e d of fused q u a r t z . The lower p o r t i o n of t h i s tube i s packed with fused quartz r i n g s and i s used as a feed gas preheater. A thermocouple i s l o c a t e d in the r e a c t o r d i r e c t l y be low the metal f o i l and a gas sampling port i s p o s i t i o n e d immediately



ORYING

J

GAS

if

1 J A

Figure 1.

VALVE

Experimental apparatus.

TEMP CONT.

SAMPLE

1

GAS

RECORDER

EFFLUENT

WT.

If

1

L.J

/ r—

MANOMETER

ELECTROBALANCE

I

1 r—A

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ιί

1 r—c£

CHROMATOGRAPH I

TUBE

If

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M A S S FLOW R E G U L A T O R


FURNACE

T.C.

TUBE

PREHEATER

INDICATING

- REACTOR

3ATURATOR

WATER

PRECISION BORE C A P I L L A R Y / TUBE

COKE FORMATION

180

above the sample. The r e a c t o r e f f l u e n t and the i n e r t gas (helium), which i s fed through the e l e c t r o b a l a n c e to p r o t e c t i t s mechanism, e x i t through l i n e s l o c a t e d in the i n t e r f a c e s e c t i o n . The r e a c t o r preheater tube i s housed w i t h i n a tube furnace powered and c o n t r o l l e d by a West SCR s t e p l e s s p r o p o r t i o n a l temperature c o n t r o l l e r . This system was capable of maintaining a 1 cm isothermal zone up to 1200 K. The e n t i r e r e a c t o r assembly and e l e c t r o b a l a n c e were f i x e d on a s p e c i a l l y designed frame (Figure 2). The frame allowed the e l e c t r o b a l a n c e to perform at n e a r l y maximum s e n s i t i v i t y . Weight changes were monitored continuously using a Cahn 2000 electrobalance. A s e n s i t i v i t y of 1 0 " grams was t y p i c a l f o r the f l o w - r e a c t o r e l e c t r o b a l a n c e system used in t h i s i n v e s t i g a t i o n . I n l e t and e f f l u e n t gas streams were analyzed p e r i o d i c a l l y using an o n - l i n e Sigma I (Perkin Elmer) gas chromatograph.


6

Method of

Analysis

To determine which s o l i d phases c a t a l y z e the formation of carbon f i l a m e n t s , phase diagrams were used. The usefulness of phase diagrams f o r t h i s system has been demonstrated in the work of Sacco and Reid (7_) ; and thus, only a b r i e f d i s c r i p t i o n of t h e i r use w i l l be given here. A phase diagram f o r the gas components of i n t e r e s t and the s o l i d phases of i n t e r e s t i s generated using a v a i l able G i b b s - f r e e energy d a t a . Figure 3 represents the F e C - F e 0 - C g - g a s ( H , H 0 , ChU, CO, C0 ) phase diagram at 900 Κ and 1 bar pressure. The apexes of the e q u i l a t e r i a l t r i a n g l e represent atomic carbon, hydrogen and oxygen. Thus, any gas mixture com posed of these elements can be p l o t t e d on t h i s t r i a n g u l a r s u r f a c e . As examples, the l o c a t i o n s of pure CO, C 0 , H 0 , and CtU are shown in Figure 3. The curve which i s concave downward represents the e q u i l i b r i a between alpha i r o n - w u s t i t e ( F e 0 ) and gas. Any p o i n t located on t h i s curve represents a gas mixture in e q u i l i b r i u m with alpha iron and w u s t i t e . As p l o t t e d i f one i s below t h i s c u r v e , such as in the region i n d i c a t e d by point X, i r o n oxide i s the s t a b l e iron phase. That i s , in t h i s phase f i e l d the chemical p o t e n t i a l d r i v i n g f o r c e i s such that reduced i r o n can not form. If one i s above t h i s c u r v e , the i r o n - i r o n oxide-gas e q u i l i b r i a p r e d i c t s reduced i r o n . However, when the alpha i r o n - c e m e n t i t e (Fe C)-gas e q u i l i b r i a i s imposed on t h i s system (dotted l i n e ) i t i s seen that any reduced i r o n formed w i l l r e a c t with the gas phase to produce c a r b i d e (Fe C) in t h i s r e g i o n . F i n a l l y , the g r a p h i t e gas e q u i l i b r i a i s superimposed on t h i s diagram ( s o l i d l i n e concave upwards). This curve represents the l o c i of p o s s i b l e gas mixtures in e q u i l i b r i u m with s o l i d graphite at 900 Κ and under 1 bar pressure. If a gas mixture l i e s above t h i s c u r v e , then the chem i c a l p o t e n t i a l d r i v i n g f o r c e i s such that s o l i d carbon should form. A gas mixture below t h i s boundary w i l l r e s u l t in a d r i v i n g f o r c e that favors s o l i d carbon o x i d a t i o n . T h i s assumes that the s o l i d surface present during r e a c t i o n i s a c a t a l y s t f o r carbon d e p o s i t i o n or o x i d a t i o n . For example, i f we c a r e f u l l y carbide the s u r f a c e 3

x

2

2

2

2

2

x

3

3



SACCO AND CAULMARE

Filamentous

Coke

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to

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9.

SACCO AND CAULMARE

Filamentous

Coke

183

and then feed a gas mixture represented by point M carbon should form i f cementite (Fe C) i s a c a t a l y s t f o r carbon d e p o s i t i o n . If Fe C i s not a c a t a l y s t f o r carbon d e p o s i t i o n , then there w i l l not be a n o t i c e a b l e weight g a i n . In t h i s manner one can c o n t r o l the s u r f a c e phase and determine i f i t c a t a l y z e s f i b e r growth. 3

3


Procedures In t h i s work, a d i f f e r e n t i a l r e a c t o r was used to i n s u r e that the gas composition remains e s s e n t i a l l y unchanged and thus o p e r a t ion during an experiment remained in the phase f i e l d of i n t e r e s t . A computer program was used to c a l c u l a t e the gas phase compositions to be used f o r an experiment. T y p i c a l l y , a preweighted p o l y c r y s t a l l i n e f o i l was placed in the r e a c t o r , heated in hydrogen to the d e s i r e d r e a c t i o n temperature, and the r e a c t a n t gas mixture passed c o n t i n u o u s l y over i t . I n l e t and exhaust gas samples were taken and analyzed every twenty minutes to i n s u r e that the gas composition remained constant. The mass of the f o i l was r e corded throughout the run from s t a r t - u p through the c o o l i n g sequence. At the c o n c l u s i o n of an experiment, the r e a c t i o n gases were vented and the r e a c t o r was allowed to cool down to room temperature w i t h i n a flowing stream of helium. The time of c o o l i n g was not c r i t i c a l s i n c e experiments were only made a t temperatures below the E u t e c t o i d temperature (996 K), samples were held at r e a c t i o n temperature f o r times in excess of 25 minutes, and c o o l i n g was slow (3-4 h r s ) . The E u t e c t o i d temperature i s the temperature at which a s i n g l e s o l i d phase begins to decompose i n t o two other phases on c o o l i n g . The samples were removed from the r e a c t o r assembly and stored in a helium atmosphere to prevent oxidation. The samples were then examined using both an o p t i c a l and scanning e l e c t r o n microscope. Results

and

Discussion

I n i t i a l l y , a s e r i e s of experiments were performed with a gas phase composition that maintained the s o l i d s u r f a c e in the r e duced c o n d i t i o n , w h i l e a l s o maintaining a chemical p o t e n t i a l d r i v i n g f o r c e f o r carbon d e p o s i t i o n . These experiments are i d e n t i f i e d by the open t r i a n g l e s on the C/H = 0.13 l i n e in Figure 4. In Figure 4 an open t r i a n g l e means that during the course of the experiment n e i t h e r weight l o s s nor weight gain was d e t e c t e d . In these i n i t i a l experiments, no weight change was detected up to as long as 140 minutes. A f t e r a s u f f i c i e n t l y long period of n e i t h e r weight gain or l o s s , u s u a l l y between 30-60 minutes, the r e a c t a n t gases were bypassed around the s a t u r a t o r . This r e s u l t e d in a dry gas stream at the same flow r a t e e n t e r i n g the r e a c t o r . In a l l cases a weight gain was observed immediately. These data are shown as the two darkened t r i a n g l e s




2

δ

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w

ι

00

9.

SACCO AND CAULMARE

Filamentous

Coke

on the C/H l i n e of 0.13. These dry gaseous mixtures are in the region where an i r o n c a r b i d e (Fe C) w i l l form and suggests that carbon d e p o s i t i o n may only occur when a c a r b i d e i s present. To t r y to f u r t h e r s u b s t a n t i a t e t h i s h y p o t h e s i s , as well as to d e t e r mine i f water has an i n f l u e n c e on the observed behavior, a s e r i e s of experiments were performed at a C/H r a t i o of 0.40 in the c a r b i d e region. As i n d i c a t e d in Figure 4, weight gain was observed. The gas phase composition f o r these i n t i a l runs are presented in Table I. A l s o included in Figure 4 are data taken by Sacco and Reid (7_) over a s t e e l wool c a t a l y s t . Although more h e a v i l y a l l o y e d (

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FORMATION

9.

SACCO AND CAULMARE

Filamentous Coke

191

each other a f t e r 6-10 minutes. This behavior i s due t o the f a c t t h a t i n i t i a l l y the s u r f a c e areas f o r the three f o i l s are very nearly e q u a l . A f t e r carbides form ( f i r s t 6-10 minutes), f i b e r s begin t o form and the e f f e c t i v e surface area i n c r e a s e s . This increase i s r e l a t e d t o the number of f i b e r s and the number o f l o c a t i o n s where f i b e r s begin t o grow. This behavior makes i t very d i f f i c u l t t o t e s t any proposed k i n e t i c mechanism.


Conclusions These p r e l i m i n a r y r e s u l t s suggest t h a t cementite i s probably a c a t a l y s t f o r carbon d e p o s i t i o n and thereby f i l a m e n t growth. A l s o , the p e a r l i t i c s t r u c t u r e found a f t e r the f o i l s were etched i n d i c a t e s t h a t carbon from the gas phase d i f f u s e s i n t o the s u r face l a y e r (no p e a r l i t i c s t r u c t u r e was observed i n the f o i l s p r i o r to r e a c t i o n ) . Based on the p r e l i m i n a r y r e s u l t s and the work o f Tsao ( 8 ) , i t i s f e l t that the shape o f the curves i n Figure 7 can be i n t e r p r e t e d as cementite formation f o l l o w e d by an i n c r e a s i n g carbon d e p o s i t i o n rate as more and more c a r b i d e i s formed. Car bides such as cementite w i l l form a t the more energy f a v o r a b l e d i s l o c a t i o n s such as g r a i n boundaries and other areas o f s t r u c t u r a l i r r e g u l a r i t i e s . These formations on s u r f a c e i r r e g u l a r i t i e s ( e . g . , s c r a t c h e s , edges, g r a i n boundaries) r e s u l t s i n large growth areas which then break i n t o s m a l l e r c r y s t a l s which form the growth t i p s f o r f i l a m e n t s . The formation of these c a r b i d e growth c r y s t a l s (with t h e i r concomitant area increase) i s r e f l e c t e d i n the r a p i d increase i n r a t e of f r a c t i o n a l weight g a i n . The steady s t a t e (constant r a t e ) p o r t i o n i s b e l i e v e d t o be due t o f i b e r growth w i t h a r e l a t i v e l y f i x e d number o f f i b e r s . Acknowledgement The authors g r a t e f u l l y acknowledge the support and encourage ment o f the National Science Foundation through the Research I n i t i a t i o n Grant Program, Grant No. ENG 78-05579. Literature Cited 1.

2.

3.

Hofer, L.J.E.; Sterling, E.; and McCartney, J.T. "Structure of the Carbon Deposited from Carbon Monoxide on Iron, Cobalt, and Nickel"; J. Phys. Chem. 1955, 59, 1153. Oberlin, Α.; Endo, M.; and Koyama, T. "Filamentous Growth of Carbon through Benzene Decomposition"; J. Crystal Growth 1976, 32, 335. Boehm, H.P. "Carbon from Carbon Monoxide Disproportionation on Nickel and Iron Catalysts: Morphological Studies and Possible Growth Mechanisms"; Carbon 1973,11,583.


192

COKE

Baker, R.T.K.; Harris, R.S.; Thomas, R.E.; and Waite, R.J. "Formation of Filamentous Carbon from Iron, Cobalt and Chromium Catalyzed Decomposition of Acetylene"; J. Catalysis 1973, 30, 86. 5. Baker, R.T.K.; Chludzinski, J r . , J.J. "Filamentous Carbon Growth on Nickel-Iron Surfaces: The Effect of Various Oxide Additives"; J. Catalysis 1980, 64, 464. 6. Baker, R.T.K; Harris, R.S. "The Formation of Filamentous Carbon"; Chem. Physics Carbon 1978, 14, 83. 7. Sacco, J r . , Α.; Reid, R.C. "Water Limitations in the C-H-O System over Iron"; AIChE J. 1979, 25, 839. 8. Tsao, T. "Kinetics of Dissociation of Carbon Monoxide on α-Fe"; Carnegie-Mellon University, Ph.D. 1974.


4.

FORMATION

RECEIVED June 28, 1982.


Coke Formation on Metal Surfaces - American Chemical Society

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