Catalytic behavior of graphite nanofiber supported nickel particles. 1. Comparison with other support media

Article abstract of DOI:10.1021/jp973462g

The hydrogenation of 1-butene and 1,3-butadiene has been carried out over a series of supported nickel catalysts at 80°C and atmospheric pressure. It was found that not only the activity but also the selectivity of nickel crystallites could be dramatically altered when the metal was dispersed on graphite nanofibers compared to the performance obtained with more conventional support materials, such as active carbon and γ-alumina. Transmission electron microscopy examinations showed that the metal was evenly distributed over the graphite nanofiber surfaces, and in general the particles adopted a well-defined thin flat hexagonal shape. In contrast, the crystallites formed on active carbon and γ-alumina did not acquire the same well-defined morphological features; however, on average, they were considerably smaller than those generated on the nanofiber surfaces. The most dramatic feature was the fact that in the oxide supported nickel system the average particle size was about 5 times smaller than that for the same metal loading on the graphite nanofibers. Consideration of the particle size distributions in conjunction with the catalyst reactivity data indicates that hydrogenation of either 1-butene or 1,3-butadiene is not directly related to metal dispersion. It is suggested, instead, that differences in the behavioral patterns of the catalyst systems are related to the observed modifications in metal particle morphological characteristics induced by the chemical and structural properties of the support materials. In this context, consideration must also be given to the possibility that the support can induce electronic perturbations in the metallic component, and this feature could be most prominent with a conductive material such as graphite nanofibers.

Full text of DOI:10.1021/jp973462g

J. Phys. Chem. B 1998, 102, 2251-2258
2251
Catalytic Behavior of Graphite Nanofiber Supported Nickel Particles. 1. Comparison with
Other Support Media
Alan Chambers, Tibor Nemes, Nelly M. Rodriguez, and R. Terry K. Baker*
Department of Chemistry, Northeastern UniVersity, Boston, Massachusetts 02115
ReceiVed: October 27, 1997; In Final Form: January 23, 1998
The hydrogenation of 1-butene and 1,3-butadiene has been carried out over a series of supported nickel
catalysts at 80 °C and atmospheric pressure. It was found that not only the activity but also the selectivity
of nickel crystallites could be dramatically altered when the metal was dispersed on graphite nanofibers
compared to the performance obtained with more conventional support materials, such as active carbon and
γ-alumina. Transmission electron microscopy examinations showed that the metal was evenly distributed
over the graphite nanofiber surfaces, and in general the particles adopted a well-defined thin flat hexagonal
shape. In contrast, the crystallites formed on active carbon and γ-alumina did not acquire the same well-
defined morphological features; however, on average, they were considerably smaller than those generated
on the nanofiber surfaces. The most dramatic feature was the fact that in the oxide supported nickel system
the average particle size was about 5 times smaller than that for the same metal loading on the graphite
nanofibers. Consideration of the particle size distributions in conjunction with the catalyst reactivity data
indicates that hydrogenation of either 1-butene or 1,3-butadiene is not directly related to metal dispersion. It
is suggested, instead, that differences in the behavioral patterns of the catalyst systems are related to the
observed modifications in metal particle morphological characteristics induced by the chemical and structural
properties of the support materials. In this context, consideration must also be given to the possibility that
the support can induce electronic perturbations in the metallic component, and this feature could be most
prominent with a conductive material such as graphite nanofibers.
Introduction
shortcoming in that it has a relatively low surface area. In this
context it is somewhat surprising to find that little attention has
been devoted to the use of high surface area graphitized carbon
fibers as a catalyst support. The inherent physical, mechanical,
and chemical properties of carbon fibers would appear to make
the material an attractive candidate for such an application. A
The concept of a metal-support interaction is one of the
oldest in heterogeneous catalysis. In 1935, Adadurov suggested
1
that metals would be polarized by the surfaces of oxides
containing highly charged cations. Although this idea did not
appear to receive any immediate attention, it was probably the
first suggestion of how the catalytic properties of a metal might
be modified by the nature of the support phase. Oxide supports
have been shown to alter the surface chemistry of metal catalyst
10
Japanese patent published in 1973 describes one of the earliest
attempts to utilize carbon fibers as a catalyst support for
palladium. This catalyst system was used for the hydrogenation
of cyclohexene and was claimed to maintain its activity for
considerably longer periods of time than that found with
catalysts prepared from palladium impregnated on conventional
supports. The high electrical conductivity associated with
graphitized carbon fibers opens up the possibility of introducing
the metallic components by electrochemical techniques, an
aspect that was exploited by Theodoridou and co-workers,11 who
deposited various noble metals onto such fibers by ion-exchange
procedures. It is also possible that this property may induce
changes in the electronic balance of the metal-support system,
which in turn could have an impact on chemisorption and bond
rupture of adsorbed reactant molecules, and consequently alter
the selectivity. Other studies have highlighted the enhancement
in catalyst dispersion arising from the support interaction when
platinum and platinum-ruthenium particles were dispersed on
2
-6
particles through epitaxial, spillover, and migration effects.
The utilization of different carbonaceous supports with varying
degrees of crystallinity, or graphitic nature, has also proven to
be a fruitful method of transforming the performance of a given
metal for selected probe reactions. Palladium decorated graphite
specimens have been shown by Brownlie and co-workers to
exhibit different selectivity patterns for simple hydrogenation
reactions than that which was found from palladium deposited
on a more disordered amorphous carbon support.7 Gallezot and
co-workers reported that platinum and ruthenium particles
supported on graphite gave higher selectivities toward unsatur-
ated alcohol in the hydrogenation of cinnamaldehyde than
catalyst systems where these metals were supported on char-
coal.8,9 In these studies it was proposed that the metal clusters
were selectively located on the basal plane of the graphite, which
led to a strong interaction between the metal particles and the
π-electrons of the support medium, and it was this feature that
was responsible for the observed differences in catalyst perfor-
mance.
12
13
graphitized carbon fibers. Ross and co-workers reported that
carbon fibers could also be used as catalyst supports for
transition metal oxides in hydrocarbon oxidation reactions.
In recent years, a new type of fibrous material has been
developed by the use of catalytic carbon vapor deposition (CVD)
techniques. This material, commonly known as carbon nanofi-
While conventional forms of graphite offer some remarkable
features as a catalyst support, the material suffers from a major
S1089-5647(97)03462-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/27/1998

2
252 J. Phys. Chem. B, Vol. 102, No. 12, 1998
Chambers et al.
bers, consists of highly ordered graphite platelets oriented
parallel, perpendicular, or at an angle with respect to the axis
of the fibers where a large number of edges are exposed. A
major advantage is that this type of carbon is relatively
inexpensive to produce.14 Carbon nanofibers generated from the
decomposition of carbon-containing gases on metal surfaces
behavior for nickel catalysts: type A and type B nickel catalysts
gave differing selectivities for the n-butenes, and this was later
attributed to contamination of the nickel by adatoms such as
sulfur or halogens. The work presented here will demonstrate
how large differences in the behavior of nickel for the
hydrogenation of 1-butene and 1,3-butadiene can be induced
by the nature of the catalyst support.
37
have been the subject of much recent research in catalysis and
materials science.1
5-19
Under certain controlled conditions the
Experimental Section
growth of these structures can be catalytically engineered such
that the product has the unique dual properties of high surface
area, normally associated with amorphous active carbon sup-
Materials. Supported nickel catalysts were prepared by
incipient wetness of fumed γ-alumina and active carbon, using
a solution of nickel nitrate dissolved in deionized water to give
a 5 wt % metal loading. Because of the hydrophobicity of the
graphite nanofibers isobutyl alcohol was used as the solvent,
but the metal loading was the same as that on the other supports.
The impregnate was dried overnight in air at 110 °C, calcined
in air at 400 °C for 4.0 h, and finally reduced in 10% hydrogen/
helium for 20 h at 350 °C. After the reduction step, the samples
were cooled to room temperature under flowing helium and then
passivated in a 2% air/helium mixture for 1.0 h to prevent bulk
oxidation of the nickel. The supported nickel catalysts were
then removed from the reactor and stored in sealed vessels.
The graphite nanofibers used in this work were supplied by
Catalytic Materials Ltd. and had a N2 BET surface area of 184.0
ports, and a high degree of crystalline perfection more charac-
teristic of graphite.20 Furthermore, the orientation of the
graphitic platelet component can be controlled by judicious
choice of the catalyst, nature of the reactant gas, and the
temperature at which the growth process is performed. It is
this unusual combination of properties that makes carbon
nanofibers an ideal candidate as a novel catalyst support
medium. Previous work from this laboratory has shown that
dramatic increases in catalytic activity for simple hydrogenation
reactions were observed when copper-iron was dispersed on
highly graphitic carbon nanofibers as compared to corresponding
alumina and active carbon supported samples.16 Other workers
have also reported an increase in selectivity to cinnamyl alcohol
in the hydrogenation of cinnamaldehyde when using ruthenium
2
m /g. Prior to use as a catalyst support media, the metallic
impurities associated with the nanofibers were removed by
dissolution in 1.0 M hydrochloric acid over a period of 3.0 days.
The efficiency of this procedure was checked by performing
X-ray diffraction analysis on the demineralized nanofibers,
which showed the complete absence of any metallic components.
Activated carbon was obtained from Norit and γ-alumina from
supported on carbon nanotubes as compared to more conven-
tional support materials.21 Geus and co-workers have reported
that palladium supported on catalytically grown carbon fibers
had comparable activity to that of an activated carbon supported
palladium catalyst when studying liquid-phase nitrobenzene
1
8
hydrogenation. In the present study we have extended the
2
16
Degussa and had N2 BET surface areas of 516.6 and 91.2 m /
g, respectively. The gases used in this study, 1-butene (99.99%),
earlier work conducted on the ethylene/hydrogen reaction and
have chosen the hydrogenation of 1-butene and 1,3-butadiene,
respectively, as more demanding probe reactions in an attempt
to monitor any possible changes in catalytic behavior induced
by supporting nickel on a selection of traditional and carbon
nanofiber support materials.
1
(
,3-butadiene (99.99%), hydrogen (99.999%), and helium
99.99%), were purchased from MG Industries, Inc. and were
used without further purification. Reagent grade nickel nitrate
[
(Ni(NO3)2‚6H2O] was obtained from Aldrich Chemicals.
Apparatus and Procedures. 1-Butene and 1,3-butadiene
Early studies by Weisz and co-workers22,23 highlighted the
use of molecular sieve-supported catalysts for performing shape-
selective chemical reactions. Using a 5A zeolite containing
platinum, they demonstrated that it was possible to selectively
hydrogenate 1-butene from a reactant mixture containing
isobutene, since the larger sized latter molecules could not enter
into caged structure of the support. This concept was later
extended by Schmitt and Walker,24 who investigated the use of
platinum dispersed on carbon molecular sieve supports as a
shape-selective hydrogenation catalyst system. They found that
it was possible to selectively hydrogenate 1-butene and cyclo-
pentene from mixtures of these respective olefins with 3-methyl-
hydrogenation reactions were carried out in a vertical flow
reactor system that was connected to an on-line Varian 3400
gas chromatography unit equipped with 30 m megabore columns
(GS-Q). The reactor system consisted of a vertical Pyrex tube
reactor (25 mm i.d. and 40 cm long) fitted with a frit at the
midpoint and onto which 100 mg of the catalyst was placed. In
this arrangement the hydrocarbon/hydrogen (1:2) reactant
mixture was introduced at the top of the reactor and after passage
through the catalyst bed exited at the bottom. Gas flow rates
to the reactor were regulated by mass flow controllers (MKS).
In all cases during a typical experiment, the catalyst samples
were initially reduced in a 10% hydrogen/helium at 400 °C for
1
-butene.
1.5 h, and then the temperature was lowered to the desired level
The hydrogenation of 1,3-butadiene has long been and
of 80 °C for the subsequent hydrogenation reactions.
A
continues to be employed in catalysis as a sensitive chemical
probe of the surface of supported metallic catalysts.
predetermined composition of reactant gas containing hydro-
carbon, hydrogen, and helium, with the former two components
being maintained at a 1:2 ratio, was introduced into the system
2
5-31
Although there is a great deal of controversy concerning the
working state of the catalyst and, in particular, the role that
3
at a flow rate of 100 cm /min and the reaction allowed to
3
2-35
carbonaceous overlayers might play in the reaction,
the
proceed for a period of 90 min, with gas product samples for
analysis being taken at 5, 30, 60, and 90 min. The relative
activity of each catalyst sample was determined from the percent
total conversion of either 1-butene or 1,3-butadiene to gaseous
products. Following the completion of a given reaction, the
reactant gas was replaced by helium and the sample allowed to
cool to room temperature. Before removal from the reactor,
the sample was passivated according to the previously described
procedure and then stored for later characterization studies.
advantages of using this reaction are well documented. For a
series of catalysts butadiene hydrogenation offers an ideal means
of comparing simultaneously catalytic activity and selectivity.
The catalytic semihydrogenation of dienes in alkene streams is
also a vital industrial process. Palladium is usually the catalyst
of choice for these selective hydrogenation reactions, but nickel
has also been shown to form n-butenes selectively from 1,3-
butadiene.3
6,37
Wells and co-workers have noted two types of

Graphite Nanofiber Supported Ni Particles
J. Phys. Chem. B, Vol. 102, No. 12, 1998 2253
An attempt was made to determine metal particle sizes by
standard chemisorption procedures; however, this approach was
fraught with problems and only gave meaningful data for the
nickel particles dispersed on active carbon and γ-alumina. This
result was not entirely unexpected since previous work from
this laboratory has shown that a number of gases tend to be
chemisorbed on graphite nanofibers, and this behavior obscures
the contribution that would be exhibited by any metal particles
present on the material. In contrast, transmission electron
microscopy examination of the samples proved to be a much
more reliable method and provided not only information on the
relative particle size distributions but also an insight into the
variations in morphological characteristics displayed by the
particles on the various support media.
TABLE 1: Hydrogenation of 1-Butene over Supported
Nickel Catalysts at 80 °Ca
catalyst
TOF (s-1)
S2b
Stc
5
5
wt % Ni/graphite nanofibers
wt % Ni/active carbon
0.337
0.110
1.28 × 10
0.24
0.45
0.11
1.2
1.5
1.4
-
3
5 wt % Ni/γ-Al
2
O
3
38
a
Reaction conditions: H
2
/1-butene ) 2.0; reaction time ) 60 min;
) 2-butene/(2-butene + butane). S )
t
total pressure ) 1 atm. b
trans-2-butene/cis-2-butene.
c
S
2
TABLE 2: Hydrogenation of 1,3-Butadiene over Supported
Nickel Catalysts at 80 °Ca
TOF (s-1)
S
b
S
c
S
t
d
catalyst
p
1
5 wt % Ni/GNF
5 wt % Ni/active carbon
0.482
0.123
4.46 × 10
0.50
0.28
0.05
0.45
0.30
0.40
1.5
2.2
2.0
Transmission electron microscopy examinations were per-
formed in a JEOL 2000 EXII instrument (lattice resolution )
-
3
5
wt % Ni/γ-Al
2
O
3
0.14 nm) that was equipped with a closed-circuit high-resolution
a
Reaction conditions: H /1,3-butadiene )2.0; reaction time ) 60
2
b
c
television system. Suitable transmission specimens were pre-
pared by ultrasonic dispersion of the catalyst samples in isobutyl
alcohol and application of a drop of the suspension to a carbon
support film. Images of these specimens were displayed on a
TV monitor, and the high-magnification appearance of catalyst
particles was recorded on videotape for subsequent direct
transfer to a Mitsubishi printer unit. The size distribution of
particles on the three different supports was determined from
measurements of over 400 particles from a number of diverse
regions of each specimen. In addition, morphological details
of the particles and the manner by which the nature of the
support influenced such features were obtained from careful
inspection of many high-resolution electron micrographs. It
should be appreciated that the majority of images obtained from
conventional transmission electron microscopy are 2-dimen-
sional in nature and as such does not allow one to determine
the topographical characteristics of supported particles, i.e.,
overall shape and thickness. There are, however, exceptional
situations where it is possible to observe the profile of particles
located on edges of the support, and under these circumstances
determination of the topographical features can be accomplished.
min; total pressure ) 1 atm.
1-butene/(total butenes). d
S
p
) butenes/(butenes + butane).
1
S )
S
t
) trans-2-butene/cis-2-butene.
1.2. Nickel-Catalyzed Hydrogenation of 1,3-Butadiene. In a
further series of experiments the reactant gas was changed from
1-butene to 1,3-butadiene. The total conversion of 1,3-butadiene
together with the mole fractions of hydrogenated gaseous
products is plotted as a function of time for the three catalysts
at 80 °C in Figures 4-6. In Table 2 values of the respective
turnover frequencies are presented and were determined in a
manner similar to those given in Table 1. In addition, various
product selectivities are reported, where Sp ) butenes/(butenes
+ butane), S ) 1-butene/(total butenes), and S ) trans-2-
1
t
butene/cis-2-butene. It can be seen that once again that graphite
nanofiber supported nickel particles exhibited the highest activity
for the diolefin hydrogenation reaction.
2
. Catalyst Characterization Studies. Previous investiga-
tions from this laboratory have focused attention on the structural
characteristics of graphite nanofibers derived from various
catalyst/hydrocarbon reactant combinations, and it is germane
to the current investigation to take into consideration the
differences in the two types used as the support media. Graphite
nanofibers produced from certain iron-catalyzed hydrocarbon
decomposition reactions have been found to exhibit a highly
crystalline structural form consisting of graphite platelets that
are stacked in a perpendicular arrangement with respect to the
axis of the fibers in an analogous manner to that of a pack of
Results
1
. Flow Reactor Studies. To ascertain the behavior of the
pristine graphite nanofibers with respect to any possible intrinsic
catalytic activity for the hydrogenation of either 1-butene or
1
,3-butadiene, experiments were carried out with hydrocarbon/
hydrogen mixtures over these structures for 90 min periods at
0 °C. There was no evidence to suggest that under these
2
0
playing cards. Furthermore, the oxidation characteristics of
this material were almost identical to that of high-purity single-
crystal graphite. Under these circumstances deposited nickel
particles would be exclusively located at graphite edge sites.
In contrast, the nanofibers generated from the cobalt-based
catalysts took the form of multidirectional growths from a single
catalyst particle, and their graphitic content was only about
8
conditions the graphite nanofibers themselves were capable of
promoting hydrocarbon conversion reactions.
1.1. Nickel Catalyzed Hydrogenation of 1-Butene. The results
presented in Figures 1-3 show the total conversion of 1-butene
at 80 °C and product distributions, expressed as mole fractions,
as a function of reaction time for the three respective supported
nickel catalysts. A somewhat more accurate assessment of the
relative activities of these catalyst systems can obtained from a
comparison of the respective turnover frequencies (TOF) that
are presented in Table 1. These values have been estimated
using the average metal particle sizes as determined from the
TEM measurements. Also included in Table 1 are product
selectivities, where S2 ) 2-butenes/2-butenes + butane and St
19
2
5%. In this case, one might expect to find that the probability
for nickel particles to collect at preferred sites on the support
surface was significantly lower than in the previous system.
2.1. Transmission Electron Microscopy Studies. Examination
of the supported nickel catalysts in the transmission electron
microscope revealed the existence of some dramatic differences
in both the sizes and morphological characteristics of the metal
particles in the three systems. The typical appearance of the
nickel crystallites formed on the graphite nanofibers is shown
in the electron micrograph (Figure 7). From a survey of many
areas of the specimen it was evident that metal was evenly
distributed over the nanofiber surface, and in general the
particles adopted a well-defined hexagonal shape. In many
)
trans-2-butene/cis-2-butene, and all these values were
determined after 60 min reaction time. It is evident that when
nickel was supported on graphite nanofibers, the catalytic
activity for hydrogenation of the olefin at 80 °C was appreciably
higher than that found when the metal was supported on either
active carbon or γ-alumina.

2254 J. Phys. Chem. B, Vol. 102, No. 12, 1998
Chambers et al.
Figure 1. Percentage conversion of 1-butene and mole fraction of
gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on graphite nanofibers at 80 °C.
Figure 4. Percentage conversion of 1,3-butadiene and mole fraction
of gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on graphite nanofibers at 80 °C.
Figure 2. Percentage conversion of 1-butene and mole fraction of
gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on activated carbon at 80 °C.
Figure 5. Percentage conversion of 1,3-butadiene and mole fraction
of gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on activated carbon at 80 °C.
Figure 3. Percentage conversion of 1-butene and mole fraction of
gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on γ-alumina at 80 °C.
Figure 6. Percentage conversion of 1,3-butadiene and mole fraction
of gaseous products as a function of reaction time in the presence of
hydrogen over nickel dispersed on γ-alumina at 80 °C.
was no evidence to suggest that the particles adopted any
preferred geometric form.
cases it was possible to discern features of the underlying
support through the particles, indicating that they were relatively
thin.
Finally, the appearance of nickel on γ-alumina (Figure 8)
confirmed that when the metal was introduced onto a more
traditional oxide catalyst support medium, then the most efficient
dispersion was attained. It was apparent that not only was the
distribution of metal crystallites extremely uniform over the
whole surface but also the variation in particle size was
extremely narrow. Perhaps the most dramatic feature was the
fact that in the oxide supported nickel system the average
particle size was about 5 times smaller than that for the same
metal loading on the highly graphitic carbon nanofibers. It was
Inspection of many regions of active carbon supported nickel
specimens showed that the metal was fairly evenly dispersed
over the entire surface, and in this case the particles were
significantly smaller on average than those deposited on the
nanofiber support media. High-magnification studies revealed
that the metal particles were highly faceted and tended to be
very thin, flat structures. On scanning across the specimen,
several different particle morphologies could be distinguished,
including rectangular, pentagonal, and hexagonal shapes. There
6
also significant that even at very high magnification (6 × 10 )

Graphite Nanofiber Supported Ni Particles
J. Phys. Chem. B, Vol. 102, No. 12, 1998 2255
nanofibers are in the range 2-22 nm. The nickel particle growth
characteristics on active carbon and γ-alumina show behavioral
patterns that are expected with these two support media, i.e., a
very narrow size distribution being attained on the oxide and a
somewhat broader profile existing on the carbonaceous material.
It is significant to find that the latter particle size distribution is
considerably more compact than that measured on the graphite
nanofiber support. From these distributions it has been possible
to derive the average metal particle sizes on the three supports,
and the respective values are as follows: graphitic nanofibers,
8.1 nm; active carbon, 5.5 nm; and γ-alumina, 1.4 nm. It should
be appreciated that since it was not possible to measure the
sizes of particles below a lower limit of 0.35 nm, the analysis
of these data will tend to give an overestimate of the values for
the average particles sizes.
Discussion
A number of very intriguing findings, with regard to the
potential advantages of employing graphite nanofibers as a
support medium for metal particles for use as hydrogenation
catalysts, have emerged from this investigation. It is evident
that not only the activity but also the selectivity of nickel
crystallites can be dramatically altered when the metal is
dispersed on graphite nanofibers compared to the performance
obtained with more traditional support materials, such as active
carbon and γ-alumina. Consideration of the particle size
distributions in conjunction with the catalyst reactivity data
clearly demonstrates that hydrogenation of either 1-butene or
Figure 7. Transmission electron micrograph showing the appearance
of metal crystallites generated on the graphite nanofibers.
1
,3-butadiene is not directly related to metal dispersion. If this
parameter was the key factor in determining catalyst activity,
then one would expect that the nickel/γ-alumina system should
exhibit the highest reactivity and that when the metal was
supported on graphite nanofibers a somewhat inferior perfor-
mance would be realized. The fact that the experimental data
are not consistent with this argument suggests that other factors
are operative in these reactions. In the ensuing discussion we
shall endeavor to link these differences in the behavioral patterns
of the catalyst systems with the observed modifications in metal
particle morphological characteristics induced by the chemical
and structural properties of the support materials.
Figure 8. Transmission electron micrograph showing the appearance
of metal crystallites produced on the γ-alumina support.
Transmission electron microscopy examinations show that
nickel particles deposited on the graphitic nanofibers adopt
morphologies that are generally associated with the existence
of a strong metal-support interaction. The crystallites tend to
be relatively thin and flat hexagonal-shaped structuressfeatures
that are consistent with the characteristics predicted from the
spreading action of metal species on the support. Under these
circumstances it is expected that only certain metal faces will
be exposed and available for interaction with gaseous reactant
molecules. While the results of the electron microscopic
examinations of the carbon nanofiber supported nickel catalysts
do not allow us to reach any definitive conclusions with regard
to the possibility of preferred growth locations for the metal
particles, previous studies of the nickel/graphite-hydrogen
system may provide some relevant information on this matter.3
Using controlled atmosphere electron microscopy techniques,
it was possible to continuously follow the creation of channels
across the graphite basal plane by nickel particles when the
specimens were heated in the presence of hydrogen. Detailed
analysis of the recorded sequences showed that the channels
possessed many straight sections interrupted by changes in
direction of 60° or 120° and were oriented parallel to the (1120)
crystallographic directions. A further observation of great
significance was that a thin film of nickel strongly adhered to
9-42
Figure 9. Particle size distribution of nickel crystallites dispersed on
the three support media.
it was not possible to ascertain the detailed morphological
characteristics of the metal particles in this system.
A clearer appreciation of the wide differences in the particle
size distributions of nickel on these four support materials can
be obtained from inspection of the data presented in Figure 9.
It is apparent from examination of the data shown in this figure
that the distribution profiles of nickel dispersed on the graphite

2256 J. Phys. Chem. B, Vol. 102, No. 12, 1998
Chambers et al.
Figure 10. High-resolution transmission electron micrograph showing the arrangement of the graphite platelets stacked in a direction perpendicular
to the fiber axis and separated by a distance of 0.34 nm.
the walls of the channel was being left behind, a clear indication
that in a hydrogen environment nickel will preferentially wet
and spread along the “armchair” face.42 It can be concluded
that since nickel establishes a very strong interaction with the
carbon atoms located at the “armchair” face, its interaction with
those at the “zigzag” face are somewhat weaker,43 and therefore,
one could speculate that the metal particles will adopt a different
morphology at each of the crystal faces. These features will
be the subject of a further study in which the metal is dispersed
on various types of nanofibers where the graphite planes are
orientated in different directions with respect to the fiber axis.
Figure 11. Schematic representation of the stacking order of graphite
platelets in the nanofibers and a detailed view of the difference in carbon
atom arrangement in the prismatic faces.
Inspection of the data presented in Figures 1-6 and Tables
1
and 2 clearly demonstrates that when nickel is supported on
graphite nanofibers a greater catalytic activity is achieved for
the hydrogenation of both 1-butene and 1,3-butadiene than that
observed for the same metal loading on either active carbon or
γ-alumina. It is apparent from the selectivity patterns that the
highest yields of trans- and cis-2-butenes are obtained with a
nickel/active carbon catalyst, indicating that isomerization is
more prevalent when the metal is supported on this medium
rather than on the graphite nanofibers or γ-alumina. While
butane was observed to be the major product for the 1,3-
butadiene hydrogenation for all the supported nickel systems,
there was a higher selectivity toward the formation of the
partially hydrogenated butenes when metal was dispersed on
the nanofibers. It is significant that for all the catalyst systems
the trans-2-butene/cis-2-butene ratios obtained from hydrogena-
tion of the diolefin are in the range 1.5-2.2, and these values
shown in the high-resolution transmission electron micrograph
(Figure 10). The graphite nanofibers used in the present
investigation have therefore the two sets of prismatic faces
exposed, and it is these specific regions that will be available
for interaction with the deposited metal atoms. The conforma-
tion of this novel material is to be contrasted with that
encountered in traditional graphitic materials, where the major
fraction of the exposed structure consists of basal plane regions
with a smaller fraction of available reactive edge sites.
A more detailed understanding of the structural arrangement
and crystallographic features of the graphite platelets can be
obtained from examination of the schematic representation
(Figure 11), where the 3-D graphite platelet view has been
enlarged so to permit a more detailed appreciation of the
crystallographic arrangement. The distribution of carbon atoms
in the (10 1h 0) and (11 2h 0) faces as well as the distance between
atoms and layers and the relative size of a nickel atom are shown
in Figure 12. A close scrutiny of these two models reveals that
there is a significant difference in the topographical character-
istics of the respective faces, with surface a presenting a more
open structure than surface b, which will induce a different
distribution of nickel atoms on the crystal surface. It is therefore
not unreasonable to expect that the metal particles that are
subsequently deposited on these surfaces will exhibit dissimilar
morphologies and concomitant variations in catalytic behavior.
Indeed, in cases where a strong interaction exists between the
metal atoms and the graphitic support, it would be possible for
the crystallites to form an epitaxial relationship with the surface
carbon atoms and adopt the crystallographic character of the
particular face, where metal atoms would be arranged so to
create a structure that is relatively open and, as such, favor an
increase in the sticking coefficient of reactant molecules. It is
are consistent with those reported in the literature for supported
nickel catalysts.3
6,37,44
An exhaustive search of the literature
has failed to reveal the existence of any data pertaining to the
relationship between nickel particle size effects and the hydro-
genation of olefins; however, it is reasonable to assume that
the reactions are quite facile.
It would be appropriate to initially consider some of the
fundamental structural features that make graphite nanofibers
a unique catalyst support material. Graphite consists of stacked
layers of carbon atoms, arranged in a symmetrical hexagonal
distribution, separated from each other by a distance of 0.335
nm. Three main faces can be identified: the basal plane (002)
possessing an associated cloud of delocalized π-electrons and
two prismatic faces where the carbon atoms are arranged in
either “zigzag” {10 1h 0} or “armchair” {11 2h 0} orientations. The
nanofibers can be synthesized as to produce a structural
conformation consisting of a perfect array of graphite platelets
where the exposed regions consist almost entirely of edges, as

Graphite Nanofiber Supported Ni Particles
J. Phys. Chem. B, Vol. 102, No. 12, 1998 2257
(s f s*), and this will lead to the dissociation of the molecules
into chemisorbed atoms. Conversely, if electrons are withdrawn
from the metal to the graphite, activation of the surface for the
interaction with the π-bonds in the hydrocarbon can be affected.
The electronic effect has been invoked to explain the high
reactivity of sulfur-contaminated catalysts in the conversion of
1
,3-butadiene to trans-2-butene observed by George and co-
3
7
50
workers.
Other workers studied the effects of various
adatoms including boron, phosphorus, aluminum, and sulfur on
the hydrogenation activity of nickel and concluded that the
catalytic properties of the metal drastically changed as a function
of the electronic density of nickel.
One must also be aware that the possibility exists for reactant
gas molecules and particularly those containing CdC bonds to
interact with the graphite edge regions. This factor could exert
an impact on the events occurring at the metal/graphite nanofiber
interface. As a consequence, some of the observed differences
in reactivity between graphite nanofibers and active carbon
supported metal particles might be attributable to the participa-
tion of the supporting medium in the hydrocarbon conversion
reaction. It should be stressed, however, that hydrocarbon
adsorption experiments performed in the absence of the nickel
component failed to show any differences in the behavior of
the three support materials.
Figure 12. Schematic representation showing the possible accom-
modation of nickel atoms on the two prismatic faces of graphite.
not unreasonable to assume that graphite platelets could provide
a template for the generation of metal particle structures
possessing a more accessible surface with a smaller coordination
number that in turn produces a narrower d-band and accordingly
a more stable adsorbate. Since the crystalline perfection of the
graphite nanofibers is quite extensive, this type of strong metal-
support interaction is expected to exist over a wide range of
nickel crystallite sizes.
Acknowledgment. Support for this project was provided by
NSF Grant CTS-9222572, and financial assistance for N.M.R.
was provided by the New Energy and Industrial Development
Organization (NEDO) of Japan.
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