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.
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.
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