Influence of chlorine on the decomposition of ethylene over iron and cobalt particles

Article abstract of DOI:10.1021/jp963031i

The interaction of cobalt and iron powders with ethylene and ethylene/hydrogen mixtures containing trace concentrations of chlorine has been studied using a combination of flow reactor and transmission electron microscopy techniques. Detailed analysis of both the gaseous products and the amount of solid carbon (a filamentous form) deposited on the metal surfaces has permitted us to gain an insight into some of the factors surrounding the promotional effect of low concentrations of chlorine on the catalytic action of both cobalt and iron. The optimum carbon deposition activity was achieved when either of these metals was treated at 400 °C in an ethylene/hydrogen environment containing 75-100 ppm chlorine. If the halogen was removed from the reactant, then the high activity for carbon filament growth could not be sustained. Reintroduction of chlorine after a suitable period of time resulted in restoration of the carbon deposition activity to its original level, demonstrating the reversible nature of the "activation-deactivation" processes. The results of this study are rationalized according to the notion that the presence of adsorbed chlorine species is responsible for causing reconstruction of the metal surface; however, the possibility that the halogen is capable of inducing a perturbation in the electronic properties of the particles is also considered. It is possible that the charge transfer between adsorbed chlorine species and the metal surface atoms leads to a strengthening of the metal-ethylene bond and a concomitant weakening of the C-C bond in the olefin, making the latter more susceptible to decomposition and enhancing the formation of the solid carbon product.

Full text of DOI:10.1021/jp963031i

J. Phys. Chem. B 1997, 101, 1621-1630
1621
Influence of Chlorine on the Decomposition of Ethylene over Iron and Cobalt Particles
Alan Chambers and R. Terry K. Baker*,†
Catalytic Materials Center, Materials Research Laboratory, The PennsylVania State UniVersity,
UniVersity Park, PennsylVania 16802-4800
ReceiVed: October 3, 1996; In Final Form: January 2, 1997X
The interaction of cobalt and iron powders with ethylene and ethylene/hydrogen mixtures containing trace
concentrations of chlorine has been studied using a combination of flow reactor and transmission electron
microscopy techniques. Detailed analysis of both the gaseous products and the amount of solid carbon (a
filamentous form) deposited on the metal surfaces has permitted us to gain an insight into some of the factors
surrounding the promotional effect of low concentrations of chlorine on the catalytic action of both cobalt
and iron. The optimum carbon deposition activity was achieved when either of these metals was treated at
400 °C in an ethylene/hydrogen environment containing 75-100 ppm chlorine. If the halogen was removed
from the reactant, then the high activity for carbon filament growth could not be sustained. Reintroduction
of chlorine after a suitable period of time resulted in restoration of the carbon deposition activity to its original
level, demonstrating the reversible nature of the “activation-deactivation” processes. The results of this
study are rationalized according to the notion that the presence of adsorbed chlorine species is responsible
for causing reconstruction of the metal surface; however, the possibility that the halogen is capable of inducing
a perturbation in the electronic properties of the particles is also considered. It is possible that the charge
transfer between adsorbed chlorine species and the metal surface atoms leads to a strengthening of the metal-
ethylene bond and a concomitant weakening of the C-C bond in the olefin, making the latter more susceptible
to decomposition and enhancing the formation of the solid carbon product.
Introduction
carbon monoxide. In catalytic reforming, the chlorine content
derived from the metal precursor salt is thought to be directly
It is now being recognized that the incorporation of controlled
amounts of certain nonmetallic atoms into metals, which have
hitherto been regarded as poisons, can actually function as
catalyst promoters. This approach is predicated on the assump-
tion that the poison atoms are preferentially chemisorbed on
sites that are active for undesirable reactions, while those sites
that perform the desired reactions are preserved in an unadulter-
ated state.1,2 Previous work from this laboratory3-5 has
demonstrated that either pretreatment or the continuous addition
of 5-10 ppm H2S to iron, cobalt, or nickel undergoing reaction
with ethylene had a significant impact on the catalytic activity
of these metals, particularly with respect to the enhancement
in the yields of solid carbon, that consisted entirely of
filamentous structures. In the current investigation we have
extended these studies to cover the influence of chlorine on the
same reactions.
The first systematic study of the influence of chlorine on the
properties of deposited carbon was carried out by Cullis and
co-workers,6 who pyrolyzed methane and the four chlo-
romethanes at temperatures over the range 800-950 °C. They
found that the substitution of hydrogen by chlorine in the
reactant molecule decreased both the preferred orientation and
the sizes of the carbon crystallites in the deposit. Careful
poisoning with chlorine has been found to be a very effective
method of altering the activity and selectivity of catalysts, and
it has been suggested that such behavior could in part be
attributable to induced electronic perturbations in the metal.7
Recent studies have emphasized the critical role of chlorine in
several industrial processes such as reforming, methane oxida-
tion, oxidative coupling of methane, and hydrogenation of
related to the amount of carbon that accumulates on the
catalyst.8,9 Goodwin and co-workers10 concluded that the
observed modifications in the performance of silica-supported
ruthenium for the hydrogenation of carbon monoxide resulting
from the presence of chlorine species in the reactant were related
to structural rearrangements in the catalyst particles, rather than
either electronic effects or selective site poisoning. Peri and
Lund11 showed that in the oxidation of methane over a silica-
supported palladium catalyst that prior to the attainment of
steady state there was an induction period that was associated
with the chloride levels in the system. These workers demon-
strated that if the concentration of such species was reduced by
aqueous washing, then there was a corresponding decrease in
the induction time.
Burch and co-workers12 have discussed the effect of chlorine
on bismuth-based catalysts, both as a promoter for gas phase
reactions and as a modifier for a range of oxychloride systems.
It was stressed that although gas phase reactions may be
important under certain circumstances, evidence was presented
indicating that the main function of the halogen was to create
new sites on the surface of the catalyst. Warren13 demonstrated
that the addition of small amounts of chlorine, in the form of
ethylene chloride, significantly enhanced the rate of methane
conversion during the coupling reaction and attributed this effect
to an increase in the number of active sites on the catalyst.
Residual chloride species derived from the metal precursor salt
were found to exert a direct impact on the selectivity of the
supported cobalt catalyst for the hydrogenation of crotonalde-
hyde.14 Furthermore, there are a number of reports detailing
the influence of chlorine on the re-dispersion phenomenon of
large palladium and platinum crystallites.15,16
* To whom all correspondence should be addressed.
† Current address: Department of Chemistry, 10 Hurtig Hall, North-
eastern University, Boston, MA 02115.
In a previous investigation17 we conducted a series of
experiments in which the conversion of ethylene to filamentous
X Abstract published in AdVance ACS Abstracts, February 1, 1997.
S1089-5647(96)03031-3 CCC: $14.00 © 1997 American Chemical Society

1622 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Chambers and Baker
carbon was monitored during interaction with copper-cobalt
bimetallics prepared from both nitrate and chloride precursors
in an attempt to determine any possible changes in the catalytic
activity induced by the halide. When the reaction was carried
out at 600 °C, no difference in the behavior of the two sources
of cobalt with respect to either the amount or characteristics of
the deposited solid carbon was detected. In a further set of
experiments, the reaction was performed at 500 °C, and under
these conditions the activity of the bimetallic derived from
chloride precursors tended to increase as the reaction proceeded,
whereas the catalyst prepared from nitrates exhibited a steady
decline in its ability to generate solid carbon. On the basis of
these studies, it was concluded that the reaction temperature
was a key factor in determining the effectiveness of a chlorine
treatment on the metal catalyst. It was suggested that at 600
°C any residual chloride species were effectively desorbed and
that the surfaces of the two bimetallic powders were essentially
the same, in both composition and electronic character. On the
other hand, at 500 °C, it was argued that although significant
desorption of the halogen might have occurred, there was a
sufficient residual concentration remaining at the surfaces of
the copper-cobalt particles to have an impact on both the
catalytic activity and the characteristics of the solid carbon
structures.
In the present study, we have investigated the effect of small
concentrations of chlorine on the carbon depositing character-
istics of iron and cobalt when the halogen is introduced to the
powdered catalyst system along with the ethylene/hydrogen
reactant mixture. Ethylene does not readily undergo decom-
position on the pure metal surfaces; however, the introduction
of CO into the reactant gas stream or copper adatoms into the
particles was found to result in a dramatic increase in the ability
of iron and cobalt to convert the hydrocarbon into filamentous
carbon.18-20
mass flow controllers (MKS), and the total gas velocity in all
experiments was maintained constant at 100 mL/min. Before
reaction with a hydrocarbon environment all metal powders were
treated at 600 °C in 10% hydrogen/helium for 2.0 h to ensure
that complete reduction of any surface oxide formed during the
passivation step. Following this operation the powders were
cooled to the desired reaction temperature in helium, and then
the reactant gas, ethylene, ethylene/hydrogen, or these gases
mixed with various ratios of chlorine/helium were introduced
into the system. The composition of the gas phase before and
at regular intervals throughout the reaction was monitored by
gas chromatography using a Varian 3400 unit fitted with a 30
m megabore column (GS-Q). Carbon and hydrogen atom mass
balances in conjunction with the relative concentrations of the
respective components were employed to derive the various
product yields. At the termination of a given experiment the
gas flow was halted, and the reactor tube was cooled to room
temperature in helium and the amount of solid carbon deposited
on the catalyst powder determined by weight difference. It was
subsequently established from TEM studies that filamentous
carbon was the exclusive type of material that accumulated on
the metal surfaces.
A variety of techniques were used to characterize the solid
carbon products, and these included temperature-programmed
oxidation in CO2/argon (1:1), N2 surface area measurements,
and transmission electron microscopy. The BET surface area
of selected samples was obtained from nitrogen adsorption
experiments performed at -196 °C with a Coulter Omnisorp
100CX unit. Temperature-programmed oxidation in CO2 was
utilized to acquire information on the degree of crystalline
perfection of the carbon deposit. We have previously demon-
strated that this is a reliable method for assessing the graphitic
nature of carbon filaments by comparison of the oxidation
profiles with standard materials, amorphous carbon, and single
crystal graphite.21 Prior to performing this operation all the
carbonaceous materials were treated in 1 N HCl for 3 days to
remove any metallic inclusions that might otherwise interfere
with the oxidation reaction. Following this demineralization
procedure, the samples were washed and dried, and then the
oxidation profiles of the carbonaceous products were deter-
mined, using a Cahn 2000 microbalance, by heating to 1100
°C at a rate of 5 °C/min in a CO2/Ar (1:1) mixture. Sample
weight loss was plotted as a function of temperature, and the
curves were compared to those of the standard carbons when
treated under the same conditions. Transmission electron
microscopy examinations were performed in a JEOL 100CX
microscope (point-to-point resolution of this instrument was 0.28
nm) in order to obtain detailed structural analysis of the
individual carbon filaments produced under various experimental
conditions. For this purpose suitable specimens were prepared
by dispersion of a small mass of the carbon deposit in 1-butanol,
and a droplet of the resulting suspension was applied to a holey
carbon support film. From a careful survey of many regions
of the specimen it was possible to find sections of the deposit
that protruded over holes in the film, and this condition enabled
one to obtain an unimpaired view of the detailed structural
features of the catalytically formed solid carbon.
Experimental Section
Materials. Pure cobalt and iron powders used as the catalysts
in this work were prepared by precipitation of the metal
carbonates from the respective metal nitrate solutions using
ammonium bicarbonate. The precipitates were washed, filtered,
dried overnight at 110 °C, and finally calcined in air at 400 °C
for 4.0 h. The powders were subsequently ground, and this
step was followed by reduction in a 10% hydrogen/helium
mixture at 500 °C for 20 h. Before removing the powders from
the reactor, the reduced granules were initially cooled to room
temperature in helium and then passivated by treatment in a
2% CO2/helium mixture for 1.0 h. Samples of all powders were
subsequently examined by X-ray diffraction using a Scintag
diffractometer, and only peaks corresponding to metallic cobalt
and iron were present in the patterns of the powders. BET
surface area measurements were performed with a Coulter
Omnisorp 100 CX unit using nitrogen adsorption at -196 °C,
and values in the range 1.0-1.5 m2/g were obtained for all the
powders.
Reagent grade cobalt nitrate [Co(NO3)2‚6H2O] and iron nitrate
[Fe(NO3)3‚9H2O] used in the preparation of the catalyst powders
were obtained from Aldrich Chemical Co. The gases used in
this work, hydrogen (99.999%), helium (99.99%), ethylene
(99.95%), and a 200 ppm chlorine/helium mixture, were
supplied by MG Industries and used without further purification.
Apparatus and Procedures. Experiments were performed
in a quartz reactor tube located in a Lindberg horizontal tube
furnace. The catalyst powder (50 mg) was uniformly dispersed
along the bottom of a ceramic boat placed in the center of the
reactor tube. Gas flow rates to the reactor were regulated by
Finally, estimates of the chlorine content in the “as-prepared”
and metal powders that had been treated at 400 °C for 3.0 h in
a 200 ppm Cl2/C2H4/He mixture were determined by ion
chromatography. Analysis of the same weights of the metal
powders indicated that the initial chloride levels were between
0.005 and 0.013 wt %, and after exposure to the halogen the
values had risen to between 2.0 and 3.35 wt %, confirming that

Decomposition of Ethylene over Iron and Cobalt Particles
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1623
TABLE 1: Total Product Distribution for Cobalt-Catalyzed
Decomposition of Ethylene after 3.0 h at 400 °C with
Various Concentrations of Chlorine in the Reactant Streama
selectivity
Cl2 % solid to solid
%
selectivity
%
selectivity
(ppm) carbon carbon ethane to ethane methane to methane
0
25
50
75
100
200
2.54
7.48
32.95
87.33
85.12
80.19
0.984
0.980
0.977
0.994
0.983
0.876
0.04
0.13
0.71
3.04
2.54
2.65
0.015
0.017
0.021
0.031
0.026
0.029
0.00
0.03
0.07
7.48
8.89
8.70
0.004
0.002
0.076
0.092
0.095
a Selectivity defined as (% of product)/(% conversion of ethylene).
Figure 1. Percentage conversion of ethylene to solid carbon from the
interaction of cobalt and iron powders with ethylene/hydrogen (4:1) in
the presence of 25 ppm chlorine as a function of reaction temperature.
Figure 3. Influence of hydrogen on the yield of solid carbon from the
interaction of cobalt with ethylene in the presence of 25 ppm chlorine
at 400 °C.
1.2. Effect of Reactant Gas Composition on the Gas and
Solid Phase Product Distributions. The catalytic activity of
both metals was found to be highly sensitive to the chlorine
and hydrogen contents in the reactant gas mixture. In the case
of cobalt, in order to attain the optimum yield of solid carbon
from the decomposition of ethylene at 400 °C, it is necessary
to have a minimum concentration of chlorine present in the
reactant, as seen from the data shown in Figure 2. It is evident
that when the chlorine level was raised to 75 ppm, the
conversion of ethylene to solid carbon reached a maximum value
after about 150 min on stream. From Table 1 it can be seen
that further increases in the halogen concentration up to 200
ppm did not appear to exert any major impact on the selectivity
pattern; however, the time required to reach the optimum activity
level was significantly shorter when the reaction was conducted
in the presence of a higher amount of chlorine.
A further feature that is apparent from Figure 2 is that when
the chlorine content of the reactant was relatively low, about
25 ppm, the cobalt catalyst exhibited only a slight improvement
in behavior over that observed from the interaction of the
unadulterated powder with ethylene under the same conditions.
If, however, hydrogen was added to the ethylene/25 ppm
chlorine mixture, a dramatic increase was found in the response
of the system toward solid carbon formation, with the highest
activity being achieved from a C2H4/H2 (1:4) reactant; see Figure
3. It should be noted that under the same conditions there was
an analogous rise in the amount of solid carbon produced on
the pure metal powder, from 2.54 to 24.85% as the C2H4/H2
ratio was changed from 1:0 to 1:4.
Figure 2. Percentage conversion of ethylene to solid carbon from the
interaction of cobalt with ethylene in the presence of various concentra-
tions of chlorine at 400 °C as a function of reaction time.
under these conditions halide species were indeed sticking on
the catalyst surfaces.
Results
1. Flow Reactor Studies. 1.1. Effect of Temperature on
the Metal Catalyzed Carbon Deposition Reaction in the Pres-
ence of Added Chlorine. Initial experiments were performed
at 500 °C, and it became evident that at this temperature the
continuous addition of chlorine to the ethylene or ethylene/
hydrogen reactant stream had no detectable effect on the amount
of carbon deposited on either cobalt or iron. In contrast, a
significant enhancement in the ability of the two metals to
catalyze the growth of solid carbon was achieved when the
reactions were carried out at lower temperatures, during
continuous exposure to hydrocarbon mixtures containing chlo-
rine. Earlier studies from this laboratory3,18 showed that neither
of these metals, in their unadulterated state, showed a tendency
to catalyze the growth of substantial amounts of solid carbon
during interaction with ethylene/hydrogen mixtures. Inspection
of the data presented in Figure 1 indicates that, provided the
reaction is performed at a suitable temperature, the presence of
a small concentration of chlorine in the reactant gas has a
profound influence on the conversion of ethylene to solid carbon.
Under the prevailing conditions, a 3.0 h exposure to C2H4/H2
(4:1)/25 ppm Cl2, cobalt appears to exhibit a higher activity
than iron for growth of solid carbon. For cobalt, the optimum
conversion of ethylene to solid carbon occurs at 400 °C, whereas
the corresponding condition for iron is achieved at 425 °C.
Based on these findings all subsequent experiments with these
systems were conducted at 400 °C.
In another series of experiments the C2H4/H2 ratio was
maintained constant at 1:4 while the concentration of chlorine
in the reactant passing over the cobalt powder was progressively
raised from 0 to 100 ppm. The total product distributions
obtained after a 3.0 h period from following this procedure are
presented in Table 2. Inspection of these data shows that while
the yield of solid carbon is enhanced in the presence of added
chlorine, it appears to reach a constant value over the halogen
range 25-100 ppm. It should be noted, however, that at the
upper level the selectivity toward the formation of solid carbon

1624 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Chambers and Baker
TABLE 2: Total Product Distribution for Cobalt-Catalyzed
Decomposition of Ethylene/Hydrogen (1:4) after 3.0 h at 400
°C with Various Concentrations of Chlorine in the Reactant
Streama
selectivity
Cl2 % solid to solid
%
selectivity
%
selectivity
(ppm) carbon carbon ethane to ethane methane to methane
0
18
25
50
100
24.85
44.92
67.86
69.10
68.81
0.889
0.887
0.940
0.918
0.698
1.73
4.13
3.40
4.39
4.90
0.062
0.081
0.047
0.058
0.050
1.36
1.59
0.94
1.99
25.51
0.049
0.031
0.013
0.026
0.259
a Selectivity defined as (% of product)/(% conversion of ethylene).
Figure 5. Yield of solid carbon as a function of chlorine concentration
in the reactant feed for the iron-catalyzed decomposition of ethylene/
hydrogen (1:4) at 400 °C.
Figure 4. Total product distribution as a function of reaction time
from the catalyzed decomposition of ethylene/hydrogen (1:4) in the
presence of 100 ppm chlorine over iron at 400 °C.
exhibits a decline, whereas the opposite trend is observed for
the formation of methane. It is interesting to compare the
product selectivity data obtained for treatment of cobalt in C2H4/
H2 (1:4) containing 100 ppm chlorine (Table 2) with the
comparable results shown in Figure 4 for the reaction of the
same mixture with iron under the same conditions. In the latter
system, production of methane remains at a modest level and,
in this case, is comparable to that of ethane.
Figure 6. Influence of hydrogen on the formation of solid carbon as
a function of reaction time at 400 °C from the interaction of iron with
ethylene containing 100 ppm chlorine.
hydrogen in the reactant appears to decrease the period of time
required to attain optimum activity, whereas the highest yields
of solid carbon were achieved from a mixture containing equal
concentrations of ethylene and hydrogen.
In contrast to the behavior observed with cobalt, it was found
that in the absence of added hydrogen iron was incapable of
catalyzing the decomposition of ethylene to any significant
degree at 400 °C, even when up to 200 ppm chlorine was
introduced into the reactant stream. If hydrogen was introduced
into the ethylene/chlorine system, a distinct change in the carbon
depositing characteristics of the metal was discerned, particularly
when chlorine was present in the reactant mixture. The effect
of progressively raising the chlorine content of a C2H4/H2 (1:4)
mixture on the percent conversion of the olefin to solid carbon
over an iron powder at 400 °C is presented in Figure 5. While
it is clear that the overall trend is similar to that observed with
cobalt when treated under the same conditions, a close inspection
of these data reveals some subtle differences between the two
systems. In the case of the iron-catalyzed formation of solid
carbon, there is an appreciable time delay before the catalyst
achieves maximum activity for this reaction, typically of the
order of 300 min, compared to the behavior exhibited by cobalt
where the catalyst reached its optimum level after between 25-
50 min on stream.
1.3. Pulsing Experiments. In an attempt to pinpoint the origin
of the observed induction period prior to attainment of optimum
solid carbon formation when the two metals were reacted in
ethylene-containing environments, a number of experiments
were performed in which each gaseous component was sepa-
rately cycled into and out of the system. In the first approach
the freshly reduced metal powders were initially pretreated for
2.0 h with a 100 ppm chlorine/hydrogen/helium mixture at 400
°C prior to exposure to the 100 ppm chlorine/ethylene/hydrogen
(1:4) reactant gas at the same temperature. A comparison of
the behavior of an iron powder that was subjected to a
pretreatment in chlorine with one that was allowed to undergo
direct reaction with the ethylene-containing mixture is presented
in Figure 7, where it is apparent and that both samples exhibit
identical carbon-depositing characteristics.
In another type of experiment the reaction of iron with the
ethylene/hydrogen (1:4) mixture containing 100 ppm chlorine
at 400 °C was allowed to proceed for 90 min, and at this stage
the reactant flow was disconnected and the system purged with
chlorine/hydrogen/helium at the same temperature for 135 min.
Following this hiatus the reactant was once again admitted into
the system, and the carbon deposition profile that was monitored
throughout the reaction sequence is also shown in Figure 7.
Inspection of these data reveals that, following the intermediate
purge step, the system did not immediately reach the same high
carbon deposition level as was exhibited by similar iron samples
In a further set of experiments, the hydrogen content of the
reactant mixture that was passed over iron was varied while
that of ethylene and chlorine was maintained constant. The
results of this exercise are presented in Figure 6 from which it
is evident that the presence of hydrogen is not only an essential
requirement to produce solid carbon, but its concentration plays
a key role in the deposition process. On the one hand, excess

Decomposition of Ethylene over Iron and Cobalt Particles
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1625
Figure 7. Influence on the yield of solid carbon as a function of time
over an iron catalyst that has been subjected to (a) a chlorine treatment
step prior to reaction with ethylene/hydrogen (1:4)/100 ppm chlorine
at 400 °C and (b) a cycling sequence in which the ethylene component
from the same mixture was removed and then reintroduced into the
system after a period of 60 min.
Figure 9. Effect of cycling hydrogen into an ethylene/hydrogen (1:
4)/100 ppm chlorine mixture passing over a cobalt catalyst at 400 °C
on the formation of solid carbon and methane.
Figure 10. Change in the formation of methane resulting from the
removal and subsequent reintroduction of the ethylene component in
an ethylene/hydrogen (1:4)/100 ppm chlorine mixture when passed over
a cobalt catalyst at 400 °C.
Figure 8. Effect on the carbon deposition reaction over iron and cobalt
powders and also the formation of methane on cobalt at 400 °C, of
pulsing 100 ppm chlorine into an ethylene/hydrogen (1:4) mixture.
amount of solid carbon product is rapidly restored to a level
that is slightly higher than the original value. This pattern of
behavior is consistent with the existence of a catalyst activation-
deactivation process that is reversible in nature. Also presented
in this plot is the change in the formation of methane from cobalt
as 100 ppm chlorine is cycled into and out of the ethylene/
hydrogen (1:4) mixture at 400 °C. It is significant that the
profile of this latter dependence is similar to that found for the
solid carbon formation under the same reaction conditions.
A final set of experiments was designed to ascertain the origin
of the methane produced in the metal-catalyzed decomposition
of ethylene/hydrogen in the presence of 100 ppm chlorine.
Figure 9 shows the change in both the solid carbon and methane
yields obtained from a cobalt catalyst at 400 °C, upon replace-
ment of hydrogen in the reactant mixture with an equivalent
amount of helium and then after a period of 120 min reintroduc-
tion of hydrogen to its original level. It is evident that when
hydrogen was removed from the hydrocarbon stream the
decrease in the production of methane was not accompanied
by a corresponding drop in the formation of solid carbon, which
indicates that the two products are formed by independent
processes. The possibility of methane being formed via
hydrogasification of the solid carbon deposits can be readily
dismissed when one takes into consideration the data obtained
from experiments in which cobalt was treated in the same
reactant mixture and where the ethylene component was cycled
between 20 and 0 mL/min (Figure 10). Inspection of these data
reveals that during a 40 min period where the olefin flow rate
was halted there was a precipitous decline in the methane yield
that were permitted to undergo uninterrupted reaction in the
ethylene-containing environment. These findings imply that
there is an interplay between the adsorbed chlorine and ethylene
species that requires each component to be coadsorbed on the
metal surface throughout the entire process in order to acquire
the maximum promotional effect with regard to catalyzed carbon
formation.
The effect on the yield of solid carbon of removing chlorine
from the ethylene/hydrogen (1:4) reactant passing over iron and
cobalt catalysts at 400 °C and reintroducing the additive after
a period of about 200 min is shown in Figure 8. From these
plots it is evident that in order to sustain the high growth level
of solid carbon it is necessary to have a critical concentration
of chlorine continuously passing over the metal surfaces. It is
significant with both catalyst systems that when chlorine was
withdrawn from the reactant the amount of carbon formed
exhibited a steady decline, finally settling out at substantially
higher levels than would be achieved from the interaction of
the unadulterated metals with an ethylene/hydrogen mixture.
These results indicate that while the majority of the adsorbed
chlorine is only weakly bound to the metal surfaces and easily
removed when the halogen supply is stopped, there is the
possibility that a small fraction of strongly held chlorine species
remain on the surface, which can influence the structural
arrangement of the metal atoms and exert a promotional effect
with regard to carbon deposition. Upon reintroduction of a
suitable chlorine concentration into the reactant mixture, the

1626 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Chambers and Baker
Figure 12. Typical appearance of carbon filaments generated from
the iron-catalyzed decomposition of ethylene/hydrogen (1:4)/100 ppm
chlorine at 400 °C. In this system the catalyst particle adopts a “pill
boxed”-shaped morphology.
Figure 11. Transmission electron micrograph showing the appearance
of carbon filaments produced from the decomposition of ethylene/
hydrogen (1:4)/100 ppm chlorine over cobalt at 400 °C. The associated
catalyst is seen as the diamond-shaped particle located within the carbon
structure.
in spite of the fact that solid carbon and excess hydrogen were
still present in the reaction system. Indeed, it is evident that
when the olefin is reintroduced into the reactant mixture, the
yield of methane climbs to an even higher level to that which
was attained during the initial step. It is therefore reasonable
to assume that the high yields of methane are derived from the
catalytic decomposition of ethylene rather than hydrogasification
of the solid carbon product.
2. Characterization of the Solid Carbon Deposit. 2.1.
Transmission Electron Microscopy Examinations. In both cases
the solid carbon deposit was found to be comprised completely
of filamentous structures that had been generated via a
bidirectional growth mechanism, and as a consequence the
catalyst particle was located within the body of the material.
Close inspection of the detailed features of the filaments
produced from cobalt and iron revealed that in all other aspects
there were major differences between the two systems. Com-
parison of the appearance of the catalyst particles associated
with the filaments shows that while cobalt acquires a diamond-
shaped morphology (Figure 11), iron tends to adopt a “pill box”
form (Figure 12). The average sizes of the metal particles
associated with the carbon filaments were determined from
measurements taken from different regions of a given specimen
and are based on over 300 determinations in each system. The
value for cobalt was found to be 55 nm, whereas that for iron
was 68 nm. It should be emphasized that these values were
determined from samples that had been reacted in an ethylene/
hydrogen (1:4) mixture containing 100 ppm chlorine after a
reaction time of 3.0 h and, as consequence, do not represent
the average metal particle size at any given instant during the
reaction. One cannot state whether there is a gradual change
in this parameter as the reaction proceeds. It is apparent,
however, that since the original metal powders had average sizes
of between 1.0 and 1.5 µm, fragmentation of the parent granules
takes place during the filament growth sequence.
Figure 13. High-resolution transmission electron micrograph of a
section of a carbon filament generated from the cobalt-catalyzed
decomposition of ethylene/hydrogen (1:4)/100 ppm chlorine at 400 °C
where the direction of the graphite platelets, parallel to the carbon
precipitating face of the metal particle, is indicated by the arrow.
transmission electron microscopy. It is evident from Figure 13
that the filaments produced from the interaction of cobalt with
an ethylene/hydrogen (1:4) mixture containing 100 ppm chlorine
at 400 °C possess a structure in which the graphite platelets are
oriented at an angle to the fiber axis in a “herringbone” form.
In contrast, when the same reactant mixture is passed over a
powdered iron catalyst, examination of the carbon filaments
shows that in this case the graphite platelets are stacked in a
direction parallel to the base of the particle and perpendicular
to the fiber axis (Figure 14). The details surrounding the
generation of these two different graphite platelet arrangements
have been given in a previous publication.22
The impact of these different catalyst particle shapes on the
alignment and crystalline perfection of graphite platelets that
constitute the filament structure can be ascertained from the
corresponding lattice fringe images obtained by high-resolution

Decomposition of Ethylene over Iron and Cobalt Particles
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1627
Figure 15. Comparison of the gasification characteristics in CO2 of
carbon filaments produced from the interaction of cobalt with ethylene/
hydrogen (1:4) at 400 °C in the presence of various concentrations of
chlorine.
Figure 14. High-resolution transmission electron micrograph of a
section of a carbon filament generated from the iron-catalyzed
decomposition of ethylene/hydrogen (1:4)/100 ppm chlorine at 400 °C
where the direction of the graphite platelets, perpendicular to the fiber
axis, is indicated by the arrow.
Figure 16. Comparison of the gasification characteristics in CO2 of
carbon filaments produced from the interaction of iron with ethylene/
hydrogen (1:4) at 400 °C in the presence of various concentrations of
chlorine.
In a separate series of experiments the influence of increasing
amounts of chlorine in the reactant gas on the structural
characteristics of the individual carbon filaments was studied.
While an increase in the chlorine concentration up to a
maximum level of 100 ppm did not appear to induce any major
modifications in the conformation or physical dimensions of
the carbon filaments, it was evident from high-resolution
examinations that there was a progressive change in the degree
of structural perfection of the material produced from both metal
catalyst/hydrocarbon systems. The filaments formed from the
interaction of cobalt with an ethylene/hydrogen (1:4) mixture
containing 25 ppm chlorine at 400 °C exhibited a somewhat
disordered structure that was comparable to that observed from
the decomposition of the same reactant over an unadulterated
metal powder.3 As the fraction of halogen in the reactant was
raised to 100 ppm, the crystalline perfection of the filaments
improved significantly. Since pure iron did not catalyze the
growth of filamentous carbon when heated in the presence of
ethylene/hydrogen (1:4), it was not possible to make the same
comparative study as was carried out for cobalt; however, the
high-resolution electron microscopy examinations clearly dem-
onstrated that as the chlorine level was raised from 25 to 100
ppm there was a perceptible enhancement in the crystalline
character of the deposited structures.
throughout the structure with no evidence of any major
discontinuities.
2.2. Temperature-Programmed Oxidation Studies. These
experiments provided a measure of the overall degree of
crystallinity of the carbon filaments from the various preparative
procedures. The oxidation profiles of batches of carbon
filaments produced by reaction of ethylene/hydrogen (1:4) over
cobalt at 400 °C were found to be a strong function of the
concentration of added chlorine, as shown in Figure 15. It is
evident that in the absence of chlorine the onset of gasification
of the solid carbon occurs at 560 °C, and the majority of the
sample has been removed when the temperature reaches 850
°C. These are gasification characteristics that are generally
associated with amorphous carbon.21 There is a gradual increase
in the oxidation resistance of the filaments as chlorine is
introduced into the gaseous reactant mixture, which implies that
the samples containing the largest fraction of highly crystalline
material are being formed in the presence of 100 ppm of the
halogen. Examination of the corresponding data obtained for
carbon filaments produced from the interaction of iron with
ethylene/hydrogen (1:4) containing chlorine at 400 °C is
presented in Figure 16. In this case, one can see a dramatic
change in the characteristics of the filaments as the chlorine
content is progressively increased from 25 to 100 ppm. In
contrast, the materials produced from the reaction mixture
containing the lower concentration of chlorine would appear to
be quite amorphous in nature.
Finally, examination of carbon filaments that had been formed
during long-term experiments, where the chlorine component
had been removed and after a period of time reintroduced into
the reactant stream, showed some interesting features. In
general, all the filaments were relatively long, about 100 µm,
and their widths remained constant over the entire length. A
detailed inspection of the graphite platelet arrangements within
the filaments was conducted at various regions and revealed
that, for the most part, the initial crystallite orientation, either
“herringbone” or perpendicular to the fiber axis, was maintained
2.3. Surface Area of Carbon Filaments. The N2 BET surface
areas of carbon filaments formed at 400 °C from the interaction
of cobalt and iron powders with ethylene/hydrogen (1:4)
containing various concentrations of chlorine are presented in

1628 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Chambers and Baker
TABLE 3: N2 BET Surface Areas of Carbon Filaments
Produced from the Interaction of Cobalt with Ethylene/
Hydrogen (1:4) in the Presence of Added Chlorine at 400 °C
Cl2 (ppm)
BET surface area (m2/g)
25
50
100
200
118.2
177.4
182.0
261.8
TABLE 4: N2 BET Surface Areas of Carbon Filaments
Produced from the Interaction of Iron with Ethylene/
Hydrogen (1:4) in the Presence of Added Chlorine at 400 °C
Cl2 (ppm)
BET surface area (m2/g)
25
50
100
200
28.4
51.3
341.3
382.9
Figure 17. Schematic rendition of the essential steps involved in
the catalytic growth of carbon filaments from the decomposition of
a hydrocarbon. Ethylene undergoes dissociative chemisorption on
face A; carbon species produced from this reaction diffuse through the
metal catalyst particle and precipitate in the form of graphite platelets
at faces B.
Tables 3 and 4, respectively. Inspection of these data shows
that the surface areas of the filaments produced on both metal
catalysts increases as the halogen concentration in the reactant
gas is progressively raised, reaching the highest value at 200
ppm chlorine. It should be appreciated, however, that at this
concentration the yield of carbon filaments formed during both
these reactions is not at the optimum value.
the particles undergo reconstruction and adopt well-defined
geometric shapes where two distinct sets of faces are formed22
as depicted schematically in Figure 17. Initially, the gaseous
reactant is adsorbed and decomposed on the type A faces, this
process being followed by the diffusion of carbon species
through the catalyst particle to precipitate at type B faces, in
the form of a fibrous structure. It is generally agreed that carbon
diffusion through the particle is the rate-determining step in the
growth process. From the present investigation it is evident
that the presence of chlorine not only enhances the decomposi-
tion of ethylene but also modifies the product distribution.
Furthermore, the degree of crystalline perfection of the solid
carbon is increased as chlorine is added to the system; i.e., the
halogen exerts an influence on both types of faces, A and B.
In experiments where the reaction temperature was raised to
500 °C the promotional effect of chlorine did not appear to be
maintained, and one might therefore conclude that under these
circumstances the halogen is readily desorbed from the metal
surfaces. In this context mention should be made of the work
of Dowben and Jones,24 who used a combination of surface
science techniques to study the sticking probability of chlorine
on iron single crystals. They reported that there was almost no
loss of adsorbed chlorine from the surface at 170 °C, but as the
temperature was gradually increased to 460 °C the rate of loss
of the halogen became quite substantial, causing the coverage
to decrease by 50%. Other workers25 investigated the interaction
of chlorine with Ni(100) surfaces and found that in this system
no changes in the halogen coverage were detectable at temper-
atures up to 325 °C. A thorough search of the literature has
failed to reveal the existence of any similar studies devoted to
the interaction of chlorine with cobalt surfaces, however, there
is no reason to believe that this system will exhibit any major
differences in halogen adsorption characteristics to that displayed
by either iron or nickel.
Discussion
1. Interplay between Chlorine Adsorption and Carbon
Deposition on Cobalt and Iron. The results of the present
investigation clearly demonstrate that at temperatures in the
vicinity of 400 °C the catalytic activity of cobalt towards
decomposition of ethylene is significantly enhanced when such
particles are reacted in an environment containing ppm levels
of chlorine. This result suggests that a critical amount of
chlorine is retained on the surfaces of the metal particles when
the system is treated in a reaction stream containing a small
concentration of the halogen. Under these circumstances it is
evident from the data presented in Tables 1 and 2 that
coadsorption of chlorine on cobalt facilitates the dissociative
chemisorption of ethylene in both the absence and presence of
added hydrogen. It is interesting to find that chlorine only
exerted a promotional effect on the decomposition of ethylene
over iron when hydrogen was present in the reactant mixture.
While the precise reasons for this behavior are not clear at this
time, we believe a contributing factor is that the coadsorption
of hydrogen is an essential requirement for the reconstruction
of the iron surface.
In many regards the influence of chlorine on these two metals
with respect to the enhancement of filamentous carbon growth
parallels the observed behavior of CO on iron,18 and S and Cu
on both iron and cobalt3,5,19,20 when these respective systems
were heated in an ethylene/hydrogen environment. It should
be stressed, however, that while these trends are similar from a
qualitative standpoint, the promotional behavior may be the
result of different operative modes in each system. It was
argued that geometric effects were largely responsible for the
observed changes in the behavior of the ferromagnetic metals
when copper was introduced into the particles. On the other
hand, when ethylene was coadsorbed in the presence of either
H2S or CO, the gaseous additives were believed to induce
electronic perturbations in the metal surface layers that resulted
in a weakening of the C-C bond of the olefin that lead to an
enhancement in decomposition.
While there are several examples of the use of trace quantities
of halogens as promoters in metal-catalyzed reactions and it is
well recognized that this behavior is related to their strong
electron-accepting properties, the details of the manner by such
additives function are still not well understood.26 A further
factor that must be taken into consideration is the potential for
adsorbed chlorine species to induce reconstruction of the metal
crystallites, and this phenomenon opens up the possibility that
the newly created faces present an atomic arrangement that
favors dissociative chemisorption of the gaseous reactant
molecules, thereby leading to an enhancement in a desired
reaction pathway. Although there has only been a limited
research effort devoted to investigations of the nature of the
Carbon filaments are the product of the decomposition of
selected hydrocarbons or carbon monoxide over hot metal
surfaces.23 It has been proposed that prior to carbon deposition

Decomposition of Ethylene over Iron and Cobalt Particles
J. Phys. Chem. B, Vol. 101, No. 9, 1997 1629
structural impact of chlorine on ferromagnetic metal surfaces,24-29
there is a substantial literature detailing the sulfur-induced
reconstruction of these metals.30-32 By analogy it can be argued
that the formation of a strong metal-chlorine interaction
weakens the bonding between the surface and the next lower
metal layers, allowing for rearrangements of the surface metal
atoms to take place.
the first 3.0 h carbon deposition cycle. It is significant that the
type of activation-deactivation behavior envisaged in scenario
(b) has been observed in previous controlled atmosphere electron
microscopy experiments.33
introduction of chlorine into an oxygen environment resulted
in the cessation of the channeling action of platinum particles
across the basal plane surfaces of graphite. When the halogen
was removed from the system, the metal particles were observed
to resume their catalytic channeling behavior.
In this case it was found that the
2. Influence of Chlorine on the Metal Activation-
Deactivation Phenomena. Perhaps one of the most outstanding
features to emerge from this investigation is the requirement to
maintain a critical concentration of chlorine in the gas phase in
order to achieve optimum catalytic activity of either cobalt or
iron. An unexpected finding is that pretreatment of the metal
surfaces with chlorine does not appear to inhibit the subsequent
chemisorption behavior of ethylene. This conclusion is based
on the data presented in Figure 7 where identical reactivity
patterns were obtained for two systems: one that had been
pretreated in 100 ppm chlorine for 90 min, prior to reaction in
the hydrocarbon/chlorine mixture, and the other where the
reactant mixture was introduced directly. Two possible expla-
nations can be considered: (a) the halogen is adsorbed on a
different face to that of the olefin and modifies the crystalline
orientation of this latter face by inducing an overall reconstruc-
tion of the particle, and (b) the majority of adsorbed chlorine
species tend to diffuse into the surface sublayers of the cobalt
particles and exert an influence on the ethylene adsorption
characteristics by inducing electronic perturbations in the metal
atoms. Inspection of the data presented in Figure 8 indicates
that the effect of chlorine is evident even after the halogen flow
had been terminated for 200 min, and this observation is
consistent with the notion that desorbed surface species are being
continuously replenished by bulk sources, until a state of
complete exhaustion is attained.
If, indeed, the presence of chlorine is responsible for restoring
the activity of a catalyst associated with carbon filaments, then
on the basis of the detailed electron microscopic studies of the
deposit formed during these types of experiments, one can draw
some further conclusions with regard to the function of the
halogen in these reactions. The observation that there were no
obvious alterations in the structural characteristics of particularly
long filaments after the regeneration step would tend to argue
against the possibility that the additive was responsible for
bringing about particle reconstruction, since such a process
results in a change in catalyst morphology, and this behavior
would be manifested in a corresponding modification in the
orientation of the precipitated graphite platelets constituting the
filaments. We therefore favor a rationale in which the presence
of adsorbed chlorine species are responsible for inducing a
perturbation in the electronic properties of the surface layers of
the metal particles. It is possible that the charge transfer
between adsorbed chlorine molecules and the metal surface
atoms leads to a strengthening of the metal-ethylene bond and
a concomitant weakening of the C-C bond in the olefin, making
the latter more susceptible to decomposition and enhancing the
formation of the solid carbon product. A similar argument was
put forward by Martin34 to account for the influence of chlorine
on the catalytic behavior of iron for the CO/H2 reaction.
3. Gas Phase Product Analysis. Comparison of the
behavior of unsupported cobalt-ethylene/hydrogen (1:4) in the
absence of added chlorine at 400 °C with that reported pre-
viously for the same reaction performed at 600 °C3,19 shows a
significant difference in the respective reactivity patterns. At
the higher temperature it was found that cobalt did not exhibit
any appreciable activity toward decomposition of ethylene, and
this behavior persisted even when the reactant contained a large
fraction of added hydrogen. From Table 2 it is apparent that
when the same reaction was carried out at 400 °C, approximately
26% of the ethylene was decomposed, with solid carbon being
the dominant product. It is possible that this variation in
catalytic performance is related to the cobalt phase transfor-
mation, R (hcp) to â (fcc), that is known to occur at around
400 °C.
A further feature that is evident from the gas phase product
distributions is the finding of a considerable increase in the
selectivity toward methane formation when cobalt particles were
reacted in either ethylene or ethylene/hydrogen (1:4) in the
presence of >75 ppm chlorine. Examination of the data in
Figure 8 shows the dramatic change in methane yield when the
flow of chlorine is deliberately interrupted for a period of 200
min and then subsequently reinjected. In contrast, the selectivity
to the other major gaseous product, ethane, appeared to be
relatively insensitive to the presence chlorine. It was also
interesting to discover that this same trend was not observed
when iron particles were reacted under the same conditions. In
this case, the yields of methane and ethane were quite similar
and did not exhibit any significant major changes in selectivity
as the concentration of chlorine was progressively raised to 100
ppm.
If the halogen was removed from the reactant, then the high
activity for the carbon deposition reaction could not be sustained.
Reintroduction of chlorine after a suitable period of time resulted
in restoration of the carbon deposition activity to its original
level, indicating that the “activation-deactivation” processes
were completely reversible in nature (Figure 7). Since the
response in the behavior of the metals to the change in the gas
phase composition was almost instantaneous, as far as the
regeneration step was concerned, one can dismiss the normal
causes of catalyst deactivation, such as the collection of
carbonaceous residues on the surfaces in contact with the gas
phase reactants or particle sintering, as playing a role in these
systems. These factors would tend to exert a permanent effect
on the performance of the catalyst and unlikely to be influenced
by the injection of a small concentration of chlorine into the
reactant. It is logical to assume, therefore, that other more subtle
effects are responsible for this dramatic reversal in catalytic
activity.
In order to gain a clearer insight into this phenomenon, it is
necessary to focus attention on the events that might be operative
under these conditions. In this context two different scenarios
should be considered: (a) upon reintroduction of chlorine into
the system, fresh catalyst particles are created from any residual
metal powder and proceed to generate the growth of new carbon
filaments in the same manner as that by accomplished other
particles in the initial stages of the reaction; (b) during this step,
reactivation of the faces of existing particles that dissociate
ethylene occurs, and as a consequence the growth of carbon
filaments is reinitiated with no apparent change in the structural
characteristics of the material. The weight of experimental
evidence would tend to favor the latter explanation since it is
highly unlikely that any of the original metal powder exists after
At first sight one might rationalize the anomalously high
yields of methane according to the notion that this product is

1630 J. Phys. Chem. B, Vol. 101, No. 9, 1997
Chambers and Baker
formed via hydrogasification of the solid carbon. While in-
situ electron microscopy experiments have demonstrated that
the onset of cobalt-catalyzed hydrogenation of graphite com-
mences at 450 °C,35 studies with this technique revealed that
the introduction of a small amount of chlorine into the system
resulted in the complete suppression of this particular type of
reaction.33 Moreover, the results of current studies where the
yield of methane exhibited a precipitous drop when ethylene
was removed from a reactant stream also containing an excess
of hydrogen together with 100 ppm chlorine is consistent with
the conclusion that in this system methane does not arise from
hydrogasification of carbon.
one may conclude that in all cases the adatom is capable of
exerting an effect on all the faces of the metal particles and
inducing changes in the nature of the metal-metal bonding at
the carbon-depositing regions.
Acknowledgment. Financial support for this work was
provided by the United States Department of Energy, Basic
Energy Sciences, Grant DE-FG02-93ER14358.
References and Notes
(1) Rostrup-Nielsen, J. R. In Catalyst DeactiVation, Bartholomew, C.
H., Butt, J. B., Eds.; Elsevier Science: Amsterdam, 1991; p 85.
(2) Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31.
(3) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1993,
143, 449.
(4) Owens, W. T.; Rodriguez, N. M.; Baker, R. T. K. Catal. Today
1994, 21, 3.
(5) Owens, W. T.; Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. In
Catalyst DeactiVation 1994; Delmon, B., Froment, G. F., Eds.; Elsevier
Science: Amsterdam, 1994; p 191.
(6) Cullis, C. F.; Manton, J. E.; Thomas, G. B.; Wilman, H. Acta Cryst.
1959, 12, 382.
(7) Wu, X.; Gerstein, B. C.; King, T. S. J. Catal. 1992, 135, 68.
(8) Verderone, R. J.; Pieck, C. L.; Sad, M. R.; Parera, J. M. Appl. Catal.
1986, 21, 239.
(9) Pieck, C. L.; Marecot, P.; Parera, J. M.; Barbier, J. Appl. Catal.
1990, 126, 235.
(10) Iyagba, E. T.; Hoost, T. E.; Nwalor, J. U.; Goodwin, J. G. J. Catal.
1990, 123, 1.
(11) Peri, S. S.; Lund, C. R. F. J. Catal. 1995, 152, 410.
(12) Burch, R.; Chalker, S.; Loader, P.; Thomas, J. M.; Ueda, W. Appl.
Catal. 1992, 82, 77.
(13) Warren, B. K. Catal. Today 1992, 13, 311.
(14) Nitta, Y.; Hiramatsu, Y.; Imanaka, T. J. Catal. 1990, 126, 235.
(15) Foger, K.; Jaeger, H. Appl. Catal. 1989, 56, 137.
(16) Feeley, O. C.; Sachtler, W. M. H. Appl. Catal.1991, 75, 93.
(17) Chambers, A.; Baker, R. T. K. J. Catal. 1996, 158, 356.
(18) Rodriguez, N. M.; Kim, M. S.; Baker, R. T. K. J. Catal. 1993,
144, 93.
(19) Chambers, A.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem.
1995, 99, 10581.
(20) Krishnankutty, N.; Rodriguez, N. M.; Baker, R. T. K. J. Catal.
1996, 158, 217.
(21) Owens, W. T.; Rodriguez, N. M.; Baker, R. T. K. J. Phys. Chem.
1992, 96, 5048.
(22) Rodriguez, N. M.; Chambers, A.; Baker, R. T. K. Langmuir 1995,
11, 3862.
In a previous investigation dealing with the interaction of
copper-nickel and ethylene/hydrogen,36 the unexpected high
yields of methane were accounted for in terms of a change in
the mode of adsorption of the olefin when such a molecule
encounters a nickel atom whose nearest neighbor is a copper
atom. It was postulated that since copper does not readily
chemisorb hydrocarbons, the ethylene would bond to the surface
in an “end-on” arrangement. In this conformation the adsorbed
molecule would rapidly rearrange to form an “ethylidyne”
intermediate, which would subsequently undergo C-C bond
scission to produce a methane molecule, leaving a residual
carbon atom on the metal surface, that would eventually
contribute toward the growth of a carbon filament. If, however,
this was the major route for methane formation, one would
expect to find a corresponding decrease in the amount of solid
carbon formation. Inspection of the data presented in Tables 1
and 2 shows that the observed enhancement in methane
formation is not accompanied by a corresponding decrease in
magnitude of solid carbon deposition, and therefore one is forced
to conclude that some other mechanism must be operative in
the formation of the paraffinic product.
4. Influence of Chlorine on the Structural Characteristics
of Carbon Filaments. Finally, the presence of foreign atoms
is likely to have a direct impact on the wetting characteristics
of the metal with respect to solid carbon, and as a consequence,
both the degree of crystallinity and morphology of the carbon
deposit are expected to be subject to major modifications. In
previous studies we have shown that the degree of crystalline
perfection of carbon filaments is intimately related to the nature
of the interfacial phenomena that exist at the metal-solid carbon
boundary.4,37 In order for dissolved carbon to be precipitated
in a highly crystalline form, there must be a registry between
the atomic spacing of the metal atoms in the depositing face
and the atoms constituting the graphite basal plane structure.
These conditions are generally achieved when the modified
metal faces undergo a wetting and spreading action with
graphite, i.e., form a strong metal-support interaction.38
The introduction of a small concentration of chlorine into
either the cobalt or iron-ethylene/hydrogen systems was found
to produce a significant improvement in the degree of crystalline
perfection of the precipitated carbon filament structures, but did
not appear to exert any modifications on the morphological
characteristics of the deposit. This behavior is to be contrasted
with the changes that were observed in the conformation of
carbon filaments when small amounts of either sulfur3-5,39 or
phosphorus40 were added to a hydrocarbon feed undergoing
reaction with cobalt, nickel, and iron powders. In these cases
the filaments were generated in the form of regularly coiled
structures that exhibited a somewhat lower degree of crystalline
perfection than the material formed under the same conditions
on the respective unadulterated metal powders. Despite the
difference in the influence of the these additives on the growth
characteristics of carbon filaments to that observed with chlorine,
(23) Rodriguez, N. M. J. Mater. Res. 1993, 8, 3233.
(24) Dowben, P. A.; Jones, R. G. Surf. Sci. 1979, 84, 449.
(25) Kiskinova, M.; Goodman, D. W. Surf. Sci. 1981, 108, 64.
(26) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis;
John Wiley & Sons: New York, 1994.
(27) Erley, W. Surf. Sci. 1982, 114, 47.
(28) Gudde, N. J.; Lambert, R. M. Surf. Sci. 1983, 134, 703.
(29) Mueller, D. R.; Rhodin, T. N.; Sakisaka, Y.; Dowben, P. A. Surf.
Sci. 1991, 250, 185.
(30) Shih, H. D.; Jona, F.; Jepson, D. W.; Marcus, P. M. Phys. ReV.
Lett. 1981, 46, 731.
(31) Cabane-Brouty, F.; Oudar, J.; Bernard, J. Acta. Metall. 1962, 10,
901.
(32) Oudar, J.; Barbouth, N.; Bernard, J. Mem. Sci. ReV. Metall. 1975,
72, 649.
(33) Baker, R. T. K.; Lund, C. R. F.; Dumesic, J. A. Carbon 1983, 21,
469.
(34) Martin, G. A. In Studies in Surface Science and Catalysis; Imelik,
B., et al., Eds.; Elsevier Science: Amsterdam, 1982, Vol. 11, p 315.
(35) Oh, S. G.; Baker, R. T. K. J. Catal. 1993, 143, 449.
(36) Kim, M. S.; Rodriguez, N. M.; Baker, R. T. K. J. Catal. 1991,
131, 60.
(37) Kim, M. S.; Rodriguez, N. M.; and Baker, R. T. K. J. Catal. 1992,
134, 253.
(38) Angermann, H. H.; Horz, G. Appl. Surf. Sci. 1993, 70, 163.
(39) Kawaguchi, M.; Nozaki, K.;Motojima, S.; Iwanaga, H. J. Crystal
Growth 1992, 118, 309.
(40) Motojima, S.; Hasegana, I.; Kagiya, S.; Momiyama, M.; Kawaguchi,
M.; Iwanaga, H. Appl. Phys. Lett. 1993, 62, 2322.