Fischer–Tropsch
(FT) synthesis is one of the most complex
catalyzed chemical reactions in which the chain-growth mechanism that
leads to formation of long-chain hydrocarbons is not well understood
yet. The present work provides deeper insight into the relation between
the kinetics of the FT reaction on a silica-supported cobalt catalyst
and the composition of the surface adsorbed layer. Cofeeding experiments
of 12C3H6 with 13CO/H2 evidence that CHx surface intermediates
are involved in chain growth and that chain growth is highly reversible.
We present a model-based approach of steady-state isotopic transient
kinetic analysis measurements at FT conditions involving hydrocarbon
products containing up to five carbon atoms. Our data show that the
rates of chain growth and chain decoupling are much higher than the
rates of monomer formation and chain termination. An important corollary
of the microkinetic model is that the fraction of free sites, which
is mainly determined by CO pressure, has opposing effects on CO consumption
rate and chain-growth probability. Lower CO pressure and more free
sites leads to increased CO consumption rate but decreased chain-growth
probability because of an increasing ratio of chain decoupling over
chain growth. The preferred FT condition involves high CO pressure
in which chain-growth probability is increased at the expense of the
CO consumption rate.
One
of the well-known observations in the Fischer–Tropsch
(FT) reaction is that the CH4 selectivity for cobalt catalysts
is always higher than the value expected on the basis of the Anderson–Schulz–Flory
(ASF) distribution. Depositing graphitic carbon on a cobalt catalyst
strongly suppresses this non-ASF CH4, while the formation
of higher hydrocarbons is much less affected. Carbon was laid down
on the cobalt catalyst via the Boudouard reaction. We provide evidence
that the amorphous carbon does not influence the FT reaction, as it
can be easily hydrogenated under reaction conditions. Graphitic carbon
is rapidly formed and cannot be removed. This unreactive form of carbon
is located on terrace sites and mainly decreases the CO conversion
by limiting CH4 formation. Despite nearly unchanged higher
hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth
probability and strongly suppresses olefin hydrogenation. We demonstrate
that graphitic carbon will slowly deposit on the cobalt catalysts
during CO hydrogenation, thereby influencing CO conversion and the
FT product distribution in a way similar to that for predeposited
graphitic carbon. We also demonstrate that the buildup of graphitic
carbon by 13CO increases the rate of C–C coupling
during the 12C3H6 hydrogenation reaction,
whose products follow an ASF-type product distribution of the FT reaction.
We explain these results by a two-site model on the basis of insights
into structure sensitivity of the underlying reaction steps in the
FT mechanism: carbon formed on step-edge sites is involved in chain
growth or can migrate to terrace sites, where it is rapidly hydrogenated
to CH4. The primary olefinic FT products are predominantly
hydrogenated on terrace sites. Covering the terraces by graphitic
carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity
and increased chain-growth rate.
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