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.
The
mechanism of CO hydrogenation to CH4 at 260 °C
on a cobalt catalyst is investigated using steady-state isotopic transient
kinetic analysis (SSITKA) and backward and forward chemical transient
kinetic analysis (CTKA). The dependence of CHx residence time is determined by 12CO/H2 → 13CO/H2 SSITKA as a function of the
CO and H2 partial pressure and shows that the CH4 formation rate is mainly controlled by CHx hydrogenation rather than CO dissociation. Backward CO/H2 → H2 CTKA emphasizes the importance of
H coverage on the slow CHx hydrogenation
step. The H coverage strongly depends on the CO coverage, which is
directly related to CO partial pressure. Combining SSITKA and backward
CTKA allows determining that the amount of additional CH4 obtained during CTKA is nearly equal to the amount of CO adsorbed
to the cobalt surface. Thus, under the given conditions overall barrier
for CO hydrogenation to CH4 under methanation condition
is lower than the CO adsorption energy. Forward CTKA measurements
reveal that O hydrogenation to H2O is also a relatively
slow step compared to CO dissociation. The combined transient kinetic
data are used to fit an explicit microkinetic model for the methanation
reaction. The mechanism involving direct CO dissociation represents
the data better than a mechanism in which H-assisted CO dissociation
is assumed. Microkinetics simulations based on the fitted parameters
confirms that under methanation conditions the overall CO consumption
rate is mainly controlled by C hydrogenation and to a smaller degree
by O hydrogenation and CO dissociation. These simulations are also
used to explore the influence of CO and H2 partial pressure
on possible rate-controlling steps.
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.
Approaches to control selectivity and activity in the catalytic reductive amination of butyraldehyde with ammonia over carbon supported noble metal catalysts (Ru, Rh, Pd, and Pt) were explored. Detailed analysis of the reaction network shows that the Schiff base N-[butylidene]butan-1-amine is the most prominent initial product and, only after nearly all butyraldehyde had been converted to N-[butylidene]butan-1-amine, amines are detected in the product mixture. From this intermediate, good hydrogenolysis catalysts (Ru, Rh) produce mostly butylamine, while catalysts less active in hydrogenolysis (Pd, Pt) lead to the hydrogenation of N-[butylidene]butan-1-amine to mostly dibutylamine.
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