Characteristics of carbon filament formation from the interaction of cobalt-tin particles with ethylene

Article abstract of DOI:10.1021/jp981305r

The effect of adding tin to a cobalt catalyst has been investigated using the decomposition of ethylene at 600 ?°C to form solid carbon, methane, and ethane as a probe reaction. These experiments have been carried out in a simple continuous flow system, where it was possible to monitor not only the gaseous product distribution but also the growth of filamentous carbon as a function of time. Particular attention was given to the modifications in the structural characteristics of the carbon filaments that accompanied the change in catalyst composition. These studies were performed using a combination of temperature-programmed oxidation, BET surface area measurements, and examination at high resolution in the transmission electron microscope. It was found that the introduction of as little as 0.5 wt % of tin into cobalt produced a dramatic increase in the catalytic activity of the bimetallic toward the formation of filamentous carbon. It is suggested that tin is responsible for inducing electronic perturbations in the surface of cobalt, and this phenomenon is reflected in a major change in the dissociative chemisorption behavior of ethylene on the mixed metal catalyst. Increasing the concentration level of tin did not lead to any further enhancement in the amount of carbon generated from the reaction; however, this procedure resulted in a significant increase in the rate of catalyst deactivation. It was also found that in the presence of tin the filamentous carbon structures acquired a high degree of crystalline order.

Full text of DOI:10.1021/jp981305r

J. Phys. Chem. B 1998, 102, 6323-6330
6323
ARTICLES
Characteristics of Carbon Filament Formation from the Interaction of Cobalt-Tin Particles
with Ethylene
T. Nemes, A. Chambers, and R. T. K. Baker*
Department of Chemistry, Northeastern UniVersity, Boston, Massachusetts 02115
ReceiVed: February 26, 1998; In Final Form: June 10, 1998
The effect of adding tin to a cobalt catalyst has been investigated using the decomposition of ethylene at 600
°C to form solid carbon, methane, and ethane as a probe reaction. These experiments have been carried out
in a simple continuous flow system, where it was possible to monitor not only the gaseous product distribution
but also the growth of filamentous carbon as a function of time. Particular attention was given to the
modifications in the structural characteristics of the carbon filaments that accompanied the change in catalyst
composition. These studies were performed using a combination of temperature-programmed oxidation, BET
surface area measurements, and examination at high resolution in the transmission electron microscope. It
was found that the introduction of as little as 0.5 wt % of tin into cobalt produced a dramatic increase in the
catalytic activity of the bimetallic toward the formation of filamentous carbon. It is suggested that tin is
responsible for inducing electronic perturbations in the surface of cobalt, and this phenomenon is reflected in
a major change in the dissociative chemisorption behavior of ethylene on the mixed metal catalyst. Increasing
the concentration level of tin did not lead to any further enhancement in the amount of carbon generated
from the reaction; however, this procedure resulted in a significant increase in the rate of catalyst deactivation.
It was also found that in the presence of tin the filamentous carbon structures acquired a high degree of
crystalline order.
Introduction
On the other hand, the structure of the carbon filaments is
determined by a array of factors that include the crystallographic
orientation of the faces of the metal catalyst particle in contact
with the solid carbon and the degree of spreading of the metal
on the carbon, which in turn is dictated by the interfacial energy
between the two components at a given temperature. In general,
metals that readily wet and spread on graphite will form
filaments that are composed entirely of graphitic platelets,
whereas those that do not form a strong interaction with the
substrate tend to precipitate carbon in a more disordered manner.
It has been known for decades that when gaseous mixtures
containing hydrocarbons and/or carbon monoxide are passed
over selected metal surfaces, carbon filaments are one of the
solid products generated during the reaction.1-5 While there
has been considerable debate over various aspects concerning
the growth mechanism of this form of carbon, there is a general
consensus that diffusion of carbon species through the catalyst
particles is the rate-controlling step in the process.6-18 In more
recent years considerable efforts have been devoted to the
understanding of the factors that control the structural features
of the carbon filaments19-22 with a view to exploiting the
potential of the material in various applications.23-31
The concept of perturbing both the chemical and physical
properties of a given metal via the addition of a dopant has
been exploited in the electronics industry for many years.
Furthermore, this procedure has met with considerable success
in a number of commercial catalytic processes where the notion
of replacing pure metals with bimetallics in order to enhance
both the activity and selectivity has been realized.37 In the case
of catalyzed carbon filament growth there is an overwhelming
number of advantages that can be achieved by the addition of
small amounts of a second element to the host metal catalyst.
In a recent investigation from this laboratory38 it was found that
pure iron and copper powders exhibited very little or no activity
for the growth of filaments when heated in the presence of
ethylene. If, however, a very small amount (∼2%) of one of
these metals was added to the other, then on subsequent reaction
with the olefin prolific growth of this type of carbon deposit
was observed. It was suggested that the presence of “impurity”
atoms at the surfaces of a metal particle will have an impact
Although one might expect that most metals will interact with
carbon-containing gases, the likelihood for such a process to
occur is dependent on the arrangement of atoms at the surface
of the particle, and this feature is influenced by a number of
parameters including the composition of the gas environment,
the reaction temperature, and the nature of the support medium.
It has been found that carbon deposition from hydrocarbons
can be greatly enhanced by the presence of hydrogen32-35 or
carbon monoxide,36 and this phenomenon has been interpreted
as the result of either reconstruction of the metal surface or the
existence of an electron-transfer process between the two
components.
* To whom correspondence should be addressed.
S1089-5647(98)01305-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 07/23/1998

6324 J. Phys. Chem. B, Vol. 102, No. 33, 1998
Nemes et al.
not only on the hydrocarbon dissociative chemisorption process
but also on the events occurring at the metal/solid carbon
interface. Major differences in the identity and the distribution
of atoms at this location will affect the interstitial spacing and
the strength of the interaction of the particles with carbon. Both
these parameters will have direct repercussions on the degree
of crystalline perfection of the precipitated solid carbon
structures. Finally, when the concentration of the additive
reaches proportions where there is a significant effect on the
bulk properties of the catalyst, then modifications can be
expected with regard to carbon diffusion characteristics through
the particle and, as such, changes in carbon filament growth
rates.
In the current investigation we have elected to study the
manner by which the addition of tin modifies the catalytic
behavior of cobalt toward carbon filament formation when the
mixed metal system is reacted in the presence of ethylene and
ethylene/hydrogen environments. Although tin has only met
with limited use as an adatom, it has been found to exert some
extremely important promotional properties on platinum for
reforming reactions.39-41 It has been established that the
addition of tin to platinum results in an enhanced catalyst
stability and improved selectivity toward aromatic product
formation. Liu and Chiu42 reported that the addition of tin to
nickel produced a significant enhancement in catalytic activity
for the vapor-phase carbonylation of methanol to produce acetic
acid. In contrast, there is evidence that incorporation of tin into
either palladium or nickel decreased the hydrogenation activity
of the group VIII metals.43,44 It was suggested that this behavior
was due to a suppression of the dissociative adsorption of
hydrogen on the tin-modified metal surface. A study conducted
in the controlled atmosphere electron microscope revealed the
existence of an unusual type of bidirectional carbon filament
growth when tin was added to iron and the bimetallic particles
were heated in acetylene.45 In this reaction the particles
executed a rotational motion, and as a result of this action, the
fiber limbs acquired a spiral conformation. Another peculiarity
of the system was that periodically the catalyst particle was
extruded from the growing filament to produce branches with
identical structural features to those of the parent filament.
ment using a Scintag 1 diffractometer equipped with a nickel
monochromator. Diffraction was performed with Cu KR
radiation at a scan rate of 2°/min, and peaks were identified by
comparison with standards in a related computer database. In
the freshly prepared powders, with the exception of the sample
containing only 0.5% tin, peaks corresponding to both metallic
components were observed in all the patterns, and for the most
part the sizes were consistent with the expected values from
the various preparations. It was significant that in the starting
powders there was no evidence of alloy formation at room
temperature; however, following reaction in ethylene/hydrogen
at 600 °C, it was possible to identify the existence of a Co2Sn
alloy phase in some of the powder patterns. The presence of
the pure metallic components combined with the absence of
bulk metal oxides in these samples at room temperature confirms
the effectiveness of the passivation procedure in 2% CO2/helium
after reduction.
The gases used in this work, ethylene (99.99%), carbon
dioxide (99.9%), helium (99.99%), and hydrogen (99.999%),
were obtained from MG Industries, Inc., and were used without
further purification. Reagent grade tin(II) chloride dihydrate
(SnCl2‚2H2O) and reagent grade cobalt(II) nitrate hexahydrate
(Co(NO3)2‚6H2O) were purchased from Aldrich Chemical Co.
Apparatus and Procedures. Carbon deposition experiments
were carried out at 600 °C in a quartz reactor tube placed
horizontally in a Lindberg furnace. Approximately 50 mg of a
given catalyst powder was dispersed as a uniform thin bed along
the base of a ceramic boat (1 cm width and 10 cm length), which
was then positioned in the center of the constant heat zone of
the flow reactor tube. Flow rates of gases were regulated by
MKS mass flow controllers and maintained at 100 cm3/min.
Initially the catalyst samples were reduced in a 10% hydrogen/
helium mixture at 600 °C for 2.0 h in order to convert surface
oxides into the metallic state. Following this step a predeter-
mined feed composition consisting of ethylene/helium or
ethylene/hydrogen/helium was introduced into the system and
the reaction allowed to proceed for a period of 90 min. The
composition of the reactant gas was analyzed at the start and at
regular intervals during the reaction with a Varian 3400 gas
chromatography unit using a 30 m megabore column (GS-Q).
Carbon and hydrogen atom balances in conjunction with the
relative concentrations of the respective components were
employed to derive the various product yields, and the weight
of solid carbon deposited on the catalyst was determined at the
completion of a particular reaction. In all cases the computed
and measured weights of the solid carbon product were within
(5%.
The identity and structural features of individual components
of the solid carbon deposit were established from high-resolution
transmission electron microscopy examinations performed in a
JEOL 2000 EXII instrument (lattice fringe resolution ) 0.14
nm). Suitable transmission specimens were prepared by
ultrasonic dispersion of sections of the deposit in isobutanol
and then application of a drop of the supernate to a holey carbon
film. Inspection of several regions of the specimens revealed
that in all cases the deposit consisted entirely of carbon
filaments.
Experimental Section
Materials. The tin-cobalt powder catalysts used in this
investigation varied in composition from Co-Sn 50:50 to 99.5:
0.5 and were prepared by the coprecipitation of the metal
carbonates from the respective nitrate solutions mixed in a ratio
calculated to give the desired bimetallic composition using
ammonium bicarbonate in a manner similar to that described
by other workers.46,47 The resulting solid was violet in color
and was dried in an oven overnight in air at 110 °C. Following
this operation the precipitate was ground to a fine powder and
then calcined in air at 400 °C for 4.0 h in order to convert the
carbonates to mixed oxides. Finally, the powdered oxides were
reduced in a 10% hydrogen/helium mixture at 500 °C for 20 h
to form the tin-cobalt powders. Prior to removal from the
furnace tube, the reduced powders were cooled to room
temperature under a flow of helium and then passivated by a
flow of 2% CO2/helium for 1.0 h. The bimetallic powders were
then removed from the reactor, ground, and stored in sealed
containers. The BET surface area of the powders as determined
by N2 adsorption at -196 °C were in the range 1.0-2.0 m2/g,
which corresponded to an average granular size of about 1.0
µm.
An estimation of the overall degree of crystalline order of
the solid carbon was carried out using temperature-programmed
oxidation in the presence of CO2/Ar (1:1). These experiments
were performed in a Cahn 2000 microbalance at a heating rate
of 5 °C/min. Under these conditions the onset of gasification
of amorphous carbon commences at about 550 °C, whereas the
corresponding point for single-crystal graphite is 860 °C. From
X-ray diffraction analysis was carried out on the bimetallic
powders before and after reaction in the hydrocarbon environ-

Interaction of Co-Tin Particles with Ethylene
J. Phys. Chem. B, Vol. 102, No. 33, 1998 6325
Figure 2. Percentage conversion of ethylene to solid carbon as a
function of reaction time at 600 °C for a selected number of Co-Sn
catalysts.
Figure 1. Percentage conversion of ethylene to solid carbon as a
function of catalyst composition after 90 min reaction at 600 °C.
TABLE 1: Effect of Catalyst Composition on the Product
Distribution (in %) from the Decomposition of Ethylene
after 90 min Reaction at 600 °C
Co:Sn
Cs
CH4
C2H6
C2H4
50:50
75:25
91:9
96:4
98:2
99.5:0.5
100:0
10.69
10.01
15.23
19.27
22.08
31.04
3.60
0.80
0.89
0.81
0.90
1.12
1.02
1.00
2.90
3.23
3.50
85.56
85.87
80.47
74.63
66.26
54.20
91.80
5.19
10.54
13.74
3.60
a comparison of the oxidation profiles of these two extreme
structural forms of carbon with that of a noncharacterized
sample, it is possible to gain an insight into the degree of
crystalline perfection of the solid carbon deposit produced from
a given reaction. For this type of analysis to be meaningful it
is necessary to remove any metal impurities from the deposit
that might otherwise exert a catalytic influence on the gasifica-
tion reaction in CO2. This step is accomplished by repeated
treatments in 1 N hydrochloric acid over a period of 3 days.
Finally, BET surface area measurements of the various carbon
deposits were accomplished using N2 adsorption at -196 °C
with the use of a Coulter Omnisorp 100CX unit.
Figure 3. Percentage conversion of ethylene to ethane as a function
of reaction time at 600 °C for a selected number of Co-Sn catalysts.
1.2. Influence of Reaction Time on Product Distribution. The
formation of solid carbon and gas-phase products resulting from
the interaction between various cobalt-tin powders and ethylene
at 600 °C was studied as a function of reaction time. These
experiments were undertaken in an endeavor to ascertain
whether any modifications in the chemical nature of the catalyst
surfaces occur during the reaction that might be reflected in a
change in the activity with respect to product formation. The
different activity profiles of a selected number of cobalt-tin
powders for solid carbon and ethane formation are presented
in Figures 2 and 3, respectively. It can be seen that all the
bimetallic catalysts exhibit a relatively high initial carbon
deposition activity and that powders containing the larger
fraction of tin undergo rapid deactivation. In contrast, carbon
deposition from bimetallic powders in which the concentration
of tin is less than 2% reaches a steady state after a period of 30
min on stream and maintains this level of activity for a
prolonged period of time. It was interesting to find that the
activity maintenance curves for carbon deposition paralleled
those for the percent decomposition of ethylene over the same
catalyst systems.
Results
1. Flow Reactor Studies. 1.1. Effect of Cobalt-Tin Ratio
on Reaction Products. The influence of changing the ratio of
cobalt to tin on the carbon deposition reaction was investigated
in a flow reactor. This series of experiments was carried out at
600 °C for 90 min using ethylene (40 cm3/min) diluted with
helium to give an overall flow rate of 100 cm3/min, over a
constant weight of catalyst sample (50 mg). The yields of
gaseous and solid carbon products from a series of cobalt-tin
powders of varying composition are presented in Figure 1 and
Table 1. From these data it can be seen that under the prevailing
reaction conditions the amount of solid carbon formed on the
bimetallic powders was significantly greater than that produced
on the pure cobalt sample, reaching an optimum level for a
catalyst composition of Co-Sn (99.5:0.5). It is also evident
that this particular bimetallic exhibits the highest catalytic
activity for the conversion of ethylene. Examination of the
selectivity profile of the bimetallic powders shows that as the
percentage of tin is progressively reduced from 50 to 0.5% not
only is there an enhancement in the amount of solid carbon but
the yield of ethane also increases. It is significant that over the
same catalyst composition range, within experimental error,
there is little or no change in the yield of methane.
Inspection of the data displayed in Figure 3 indicates that,
for all the bimetallic powders, the activity for ethane formation
increases uniformly as a function of reaction time, and an
analogous trend was also found for the production of methane.
In these cases there did not appear to be any direct relationship
between the amount of gas-phase product formation and the
percentage of ethylene that had decomposed. It is evident,
therefore, that subtle differences may exist between the underly-

6326 J. Phys. Chem. B, Vol. 102, No. 33, 1998
Nemes et al.
TABLE 2: Effect of Catalyst Composition on the Product
Distribution (in %) from the Decomposition of Ethylene/
Hydrogen (8:1) after 90 min Reaction at 600 °C
Co:Sn
Cs
CH4
C2H6
C2H4
50:50
75:25
91:9
96:4
98:2
14.21
15.06
16.68
23.78
25.45
33.15
1.18
1.29
1.37
1.16
1.18
0.95
7.95
8.40
9.71
11.19
12.64
12.09
76.66
75.25
72.24
63.86
60.73
53.80
99.5:0.5
TABLE 3: Influence of Added Hydrogen on the Product
Distribution (in %) from the Interaction of Cobalt-Tin
(99.5:0.5) with Ethylene after 90 min Reaction at 600 °C
hydrogen
Cs
CH4
C2H6
C2H4
0
2.5
5.0
10.0
20.0
30.0
40.0
31.04
24.57
33.15
31.04
24.96
28.79
31.60
1.02
1.06
0.95
0.97
1.33
1.43
1.57
13.74
13.27
12.09
13.78
25.07
25.58
28.33
54.20
61.11
53.80
54.22
48.64
44.20
38.49
ing factors that control the activity patterns for the gas-phase
products and those responsible for the growth of solid carbon.
1.3. Effect of Hydrogen Addition to the Reactant Mixture.
In a complementary set of experiments the effect of added
hydrogen to the feed stream on the characteristics of the carbon
deposition reaction was studied. These data, which were
obtained for a C2H4/H2 (8:1) reaction mixture and cover a
somewhat more limited range of bimetallic compositions, are
presented in Table 2. Comparison of these results with those
shown in Table 1 reveals that when the hydrocarbon decom-
position is performed in the presence of added hydrogen, there
is a perceptible increase in catalytic activity for reactions over
the bimetallics containing a relatively large fraction of tin; this
trend declines as the concentration of the adatom is reduced.
Indeed, the product yields obtained for the interaction of a
cobalt-tin (99.5:0.5) powder with ethylene and ethylene/
hydrogen are almost the same.
In a final series of experiments the concentration of ethylene
was maintained constant at 40 cm3/min, and the effect of
increasing amounts of hydrogen on the reaction product
distribution was examined. The data presented in Table 3 show
the change in selectivity of a cobalt-tin (99.5:0.5) catalyst as
a function of added hydrogen. It is evident that while the
intrinsic activity of the catalyst increases as the hydrogen content
in the reactant is raised, the amount of solid carbon produced
remains relatively constant. In contrast, both the yields of
methane and ethane rise as hydrogen is added to the system,
with the latter exhibiting a substantial increase under these
conditions.
2. Characterization of the Solid Carbon Product. 2.1.
Transmission Electron Microscopy Studies. When sections of
the carbon deposits were examined by transmission electron
microscopy, they were found to consist exclusively of carbon
filaments with no evidence for the formation of other types of
deposits, such as graphitic shells. There were, however,
variations in the structural characteristics of this material that
were dependent on the catalyst composition and conditions of
preparation. It was evident that filaments, generated from the
interaction of a cobalt-tin (99.5:0.5) powder at 600 °C with
ethylene containing 2.5% hydrogen, had grown via a “whis-
kerlike” mode. In such a sequence the metal catalyst particle
remains located at the tip of the structure throughout the growth
process. Furthermore, high-resolution studies showed that this
material exhibited a high degree of crystalline perfection where
Figure 4. Transmission electron micrograph of a carbon filament
generated from the interaction of a cobalt-tin (99.5:0.5) powder at
600 °C with ethylene containing 2.5% hydrogen. The arrows indicate
the directions of the precipitated graphite platelets constituting the
filament structure.
the graphite platelets constituting the filaments were oriented
in the same directions as the faces of the metal particles in
contact with the solid carbon (Figure 4). The “herringbone”
arrangement of the platelets is highlighted with arrows in the
micrograph.
The widths of filaments generated by the “whiskerlike” mode
were similar to those of the associated catalyst particles and
were typically in the range 20-100 nm, and the size distribution
did not exhibit any major perturbation as the composition of
the catalyst was changed. It was significant that these particles
were considerably smaller than those of the original bimetallic
powders (estimated to be about 1.0 µm in diameter), which
indicates that during the carbon deposition reaction fragmenta-
tion of the starting material takes place. The breakup of
relatively large nickel-iron particles prior to growth of carbon
filaments from acetylene has been reported previously based
on the direct observations obtained with controlled atmosphere
electron microscopy.48 In addition, other workers12 have
invoked the existence of such a step in order to account for the
nucleation and growth of carbon filaments on metal foils.
An unexpected change in structural characteristics was
observed when samples of the same bimetallic powder were
heated in an ethylene stream containing 30% hydrogen under
otherwise identical conditions. Under these conditions, while
the catalyst maintained a well-faceted outline, the filaments
tended to adopt a conformation in which several irregular-shaped
limbs appeared to emanate from a single particle, in a so-called
“octopus” arrangement. The typical appearance of this type of
filament is presented in the electron micrograph in Figure 5.
Examination of this material at high resolution failed to reveal
the presence of any graphite platelet structures, and the
disordered nature of these filaments was subsequently confirmed
from the existence of a diffuse ring pattern by electron
diffraction analysis.

Interaction of Co-Tin Particles with Ethylene
J. Phys. Chem. B, Vol. 102, No. 33, 1998 6327
Figure 6. Temperature-programmed oxidation profiles in CO2 of the
carbon filaments produced from the interaction of cobalt-tin (99.5:
0.5) with selected ethylene/hydrogen mixtures after a period of 90 min
at 600 °C.
Figure 5. Transmission electron micrograph showing the multidirection
growth of several carbon filaments (octopus structure) generated from
the interaction of a single particle of cobalt-tin (99.5:0.5) at 600 °C
with ethylene containing 30% hydrogen.
In all cases the widths of the filaments appeared to remain
constant throughout their growth period, indicating that there
was no progressive loss of material from the particles during
this process. This finding demonstrates that even though the
reaction temperature is well above that of the melting point of
tin, 232 °C, there is no preferential loss of this component that
could result from a wetting action of the filamentous structures
by this metal. Such a pattern of behavior is generally recognized
by a tapering of the filaments as the catalyst particle gradually
decreases in size. One may conclude therefore that despite the
fact that the solubility of tin in cobalt is less than 2 atom % at
600 °C,49 the strength of the interaction between the two metals
is sufficient to maintain the system in a relatively stable
condition.
2.2. Temperature-Programmed Oxidation Studies. The effect
of adding hydrogen to ethylene on the structural characteristics
of the demineralized carbon filaments can be evaluated from a
comparison of the gasification profiles in CO2 of material
produced from the interaction of cobalt-tin (99.5:0.5) with
several ethylene/hydrogen mixtures after a period of 90 min at
600 °C (Figure 6). Examination of these curves shows that the
filaments grown from a reactant mixture containing 2.5%
hydrogen display oxidation characteristics that are similar to
those of single-crystal graphite. As the concentration of
hydrogen in the feed was gradually increased to a level where
it equaled that of ethylene, the structure of the filaments became
progressively more disordered in nature. It was interesting to
discover that the carbon structures generated from the interaction
of the bimetallic with pure ethylene exhibited the lowest degree
of crystalline perfection.
Figure 7. Temperature-programmed oxidation profiles in CO2 of the
carbon filaments produced from the decomposition of C2H4/2.5% H2
at 600 °C over various Co-Sn powders.
TABLE 4: Nitrogen BET Surface Area (m2/g) of Carbon
Filaments Produced from the Decomposition of a C2H4/2.5%
H2 Mixture at 600 °C as a Function of Catalyst Composition
% tin in cobalt BET surf. area % tin in cobalt BET surf. area
0
0.50
2.0
80.4
171.0
169.3
5.0
10.0
158.7
133.8
less graphitic in character. For comparison purposes the
oxidation curves obtained from carbon filaments produced from
the cobalt-catalyzed decomposition of the same reactant mixture
at 600 °C have also been included in Figure 7.
The combination of these experiments has enabled us to
identify the conditions that are necessary in order to produce
carbon filaments possessing the highest graphitic content from
the cobalt-tin-catalyzed decomposition of ethylene-containing
mixtures. This objective can be achieved when powdered cobalt
doped with about 0.5% tin is allowed to react with an ethylene
feed containing approximately 2.5% hydrogen.
2.3. Nitrogen Surface Area Measurements. The influence of
bimetallic composition on the surface areas of carbon filaments
generated from catalyzed decomposition of an C2H4/2.5%H2
mixture at 600 °C is shown in Table 4. The incorporation of
tin in cobalt was found to have a significant impact on the
surface area of the solid carbon structures, with the maximum
value being obtained from material produced from a catalyst
formulation containing 0.5% of the additive. On the other hand,
In a complementary set of experiments the oxidation profiles
of carbon filaments grown from the interaction of C2H4/2.5%
H2 with cobalt-tin powders of varying compositions at 600 °C
are presented in Figure 7. Inspection of these plots indicates
that as the concentration of tin in the bimetallic is increased
from 0.5 to 10%, the carbon filaments become progressively

6328 J. Phys. Chem. B, Vol. 102, No. 33, 1998
Nemes et al.
TABLE 5: Nitrogen BET Surface Area (m2/g) of Carbon
Filaments as a Function of the Hydrogen Content in the
Reactive Mixture for the Co-0.5% Sn-Catalyzed
Decomposition of Ethylene at 600 °C
reactant will be chemisorbed in an arrangement where the C-C
is aligned in a direction “parallel” to the surface. In this
configuration the adsorbed species can either interact with
dissociatively adsorbed hydrogen and desorb from the surface
as ethane or undergo C-C bond rupture, a step that could
ultimately lead to solid carbon deposition at other faces of the
catalyst particle.
% H2 in C2H4
BET surf. area
% H2 in C2H4
BET surf. area
0
2.5
10.0
168.3
171.0
181.7
20.0
40.0
178.2
194.2
In contrast, if the ethylene molecule collides with two
dissimilar metal atoms in the surface, one of which does not
chemisorb hydrocarbons, then the likelihood exists that the olefin
will adsorb in an “end-on” configuration, in which only one
carbon atom is bonded to the metal. In this arrangement there
will be a strong tendency for the adsorbed molecule to undergo
a rapid transformation to an “ethylidyne” intermediate that will
subsequently decompose to produce methane and leave a
remaining carbon atom on the surface that will eventually
contribute toward the solid deposit.
unlike previous studies with other bimetallic systems,50,51 the
addition of increasing amounts of hydrogen to the reactant
stream failed to exert any profound effect on the surface area
of the filaments, as evidenced from the data presented in Table
5. This trend is somewhat surprising in view of finding that
the solid carbon structures were found to become progressively
less graphitic in nature as the ratio of hydrogen to ethylene in
the reactant was increased.
Discussion
Since chemisorption of hydrocarbons on tin is not a facile
process, surfaces containing a large fraction of this metal will
exhibit characteristics that favor adsorption of ethylene in the
“end-on” configuration, and as such, there will be a correspond-
ing tendency for methane formation. Furthermore, it is reported
that dissociative adsorption of hydrogen does not readily occur
on tin-modified metal surfaces,57 and as a consequence, one
would expect to find a diminution in hydrogenation activity and
a lowering in the selectivity toward ethane formation. As the
population of tin atoms in the surface is decreased, one would
predict that the selectivity for methane production ought to drop,
while that for ethane should be enhanced. In this context it is
significant to find that the addition of up to 10% hydrogen to
the reactant mixture appeared to exert very little influence on
the decomposition behavior of ethylene over Co/Sn (99.5:0.5)
powders. If, however, the level of hydrogen was raised to a
maximum of 40%, the yield of ethane was substantially
increased.
1. Formation of Carbon Filaments from the Interaction
of Cobalt-Tin with Ethylene. The most outstanding feature
to emerge from this investigation is the finding that the addition
of tin to cobalt results in a significant enhancement in the ability
of the ferromagnetic metal to catalyze the growth of carbon
filaments during interaction of the bimetallic with ethylene at
600 °C. Moreover, since the optimum carbon deposition activity
is achieved with a bimetallic containing as little as 0.5% of tin,
this provides some key insights with regard to the factors
responsible for the promotional effect. The manner by which
a given adatom alters the catalytic activity and selectivity of a
host metal has been the subject of a number of comprehensive
review articles.37,52-54 The arguments have centered around two
major effects: (a) the geometric effect, where ensembles of more
than one atom of a specific element are less frequently
encountered in the surface of a bimetallic than in the pristine
metal, and (b) the electronic structure effect, in which the
properties of a given metal in the bimetallic surface are different
from those of the pure metal.
While one might rationalize the observed increase in carbon
filament formation obtained with bimetallic powders containing
more than 5% tin according to the notion that incorporation of
such a concentration of adatoms into cobalt would bring about
a geometric rearrangement of the surface atoms, it is difficult
to see how such an argument can account for the impact of
only 0.5% of the additive on this reaction. Under such
circumstances a more plausible, but speculative, reasoning may
lie in the concept that the inclusion of a small amount of tin is
capable of inducing electronic perturbations in the surface layers
of the cobalt-rich particles.55,56
2. Influence of the Catalyst Composition on the Formation
of Gaseous Products. While the selectivity toward the forma-
tion of the solid carbon product remains relatively steady over
the entire bimetallic composition range, inspection of the data
in Table 1 shows that this is not the case with the gaseous
products; the selectivity toward methane formation tended to
decrease as the fraction of tin in the catalyst was gradually
lowered, and that of ethane exhibited a slight upward trend over
the same composition range.
In previous studies we have demonstrated that the generation
of methane and ethane from the decomposition of ethylene over
certain bimetallic systems occurs via two different routes that
are controlled by the manner in which the olefin interacts with
the catalyst surface.35 It was argued that when the ethylene
molecule encounters two identical metal atoms on the surface
that are capable of adsorbing the hydrocarbon, the gaseous
3. Catalyst Deactivation Phenomenon. From the data
presented in Figure 2 it is apparent that all the bimetallic
catalysts exhibit a significant drop in activity during the initial
stages of the reaction. The subsequent extent of deactivation
is clearly dependent on the fraction of tin that is in the particles;
those containing <2% maintained a stable rate of carbon
filament growth, while the carbon deposition activity of powders
possessing a higher fraction of tin continued to steadily decline.
When dealing with bimetallic systems, one is always confronted
with the challenge of attempting to predict the composition of
the surface at any given instant of time under reaction conditions.
While there is a considerable body of information detailing the
fact that the surface composition of a given bimetallic is
frequently different from that of the bulk, one must also be aware
that such a surface may undergo progressive changes in both
composition and structural characteristics as the reaction
proceeds.
Using the criteria established by Chelikowsky58 to determine
surface segregation in bimetallic systems in a reducing environ-
ment, one can predict that under such conditions tin will tend
to segregate to the exposed surfaces of cobalt-tin particles.
Although there does not appear to have been any experimental
studies conducted on the segregation phenomena in the cobalt-
tin system, there are a number of papers detailing the behavior
of tin in nickel and iron bimetallics.59-61 In all cases the data
point to the fact that tin readily segregates to the surfaces of
the mixed metal systems. One of the most prominent aspects
revealed by such studies is the surface segregation behavior of

Interaction of Co-Tin Particles with Ethylene
J. Phys. Chem. B, Vol. 102, No. 33, 1998 6329
References and Notes
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particles with the propensity of the latter element to selectively
diffuse to the metal/gas interface. A consequence of this action
would be that catalysts containing a relatively high concentration
of tin would be expected to undergo deactivation at a faster
rate than those where the metal was a minor constituent. This
pattern of behavior was indeed observed experimentally.
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Acknowledgment. The authors would like to thank Dr. Nelly
Rodriguez for her help with the high-resolution electron
microscopy studies. Financial support for this project was
provided by the United States Department of Energy, Basic
Energy Sciences, Grant DE-FG02-97ER14741.

6330 J. Phys. Chem. B, Vol. 102, No. 33, 1998
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