470
The dissimilarities among the rate expressions existed due to some major reasons,
viz., use of the different catalyst systems and a wide range of operating conditions.
Also, the selection of catalyst severely affects the rate of CO consumption and the
product selectivity, which eventually affects the kinetics of the whole process. For
example, iron and cobalt catalysts show a different mechanism for CO activation. In
addition to this, a high affinity of iron catalysts for the WGS reaction leads to the
inclusion of H 2 O and/or CO 2 as an inhibition term in the developed model. In the
present chapter, the synergetic effect of the selection of iron, cobalt, or bimetallic
catalysts for the kinetics of the FT synthesis reaction is summarized. The objective
is to select the prevailing mechanisms through detailed kinetic modeling of the reaction over different catalysts. Table 1 summarizes the current knowledge of kinetic
modeling on the Fe, Co catalysts in order to understand the strengths and weaknesses of works, which includes the mechanistic path for CO activation, the effect
of CO 2 or H 2 O inhibition, and the WGS activity of the catalyst.
3.1.1 CO Activation Path over Iron and Cobalt Catalyst
A series of reactions occur in the FT synthesis process, which follows various routes
to produce a series of products. There is a significant role of reaction mechanism in
the prediction of a more accurate and reliable rate model. The FT reaction is a
structure-sensitive reaction, and it is connected to the structural properties of the
catalysts. The FT synthesis reactions and product formation occur via three consecutive steps, viz., CO activation, chain propagation, and chain termination. There
are several proposed mechanisms for these steps. However, the selection of more
accurate mechanisms over the selected catalysts (Fe-Co) is required to perform the
kinetic study. The CO gets consumed in the initial stage of reaction, i.e., activation
step wherein CO adsorption, dissociation, and monomer formation occur. Various
reported literature confirmed that CO activation is slowest among all steps and
hence can be used for the kinetic modeling [80]. The desorption step is liable to
control the chain length, i.e., the product distribution [50]. The initiation of reaction
is fueled by dissociative or associative CO adsorption over the site. Table 1 represents a collective description of the various developed models and their selected CO
activation mechanism for model derivation. The detailed analysis (Table 1) reveals
that over the cobalt surface, the CO activation takes place via a dissociative path and
then interacts with either molecular hydrogen or adsorbed hydrogen [9, 10, 29, 50,
62, 70, 72, 81], whereas iron [4, 42, 54, 66, 68, 82] favors hydrogen-assisted CO
adsorption. A density functional theory (DFT)–based study reported that direct CO
dissociation is a favored mechanism at corrugated or stepped cobalt surfaces. And
H 2 -assisted CO dissociation and adsorption is facilitated over a low reactive site like
iron carbide, which is a proven active site for FT synthesis [83, 84]. Van Dijk [85]
reported the evidence of the formation of C1 intermediate over cobalt, which polymerizes to form long-chain hydrocarbons. However, there was always some disagreement among the authors over the CO activation path over a similar catalyst (i.e., iron
or cobalt based). For example, Okeson et al. [69] revealed in their study that the
Sonal et al.
The dissimilarities among the rate expressions existed due to some major reasons,
viz., use of the different catalyst systems and a wide range of operating conditions.
Also, the selection of catalyst severely affects the rate of CO consumption and the
product selectivity, which eventually affects the kinetics of the whole process. For
example, iron and cobalt catalysts show a different mechanism for CO activation. In
addition to this, a high affinity of iron catalysts for the WGS reaction leads to the
inclusion of H 2 O and/or CO 2 as an inhibition term in the developed model. In the
present chapter, the synergetic effect of the selection of iron, cobalt, or bimetallic
catalysts for the kinetics of the FT synthesis reaction is summarized. The objective
is to select the prevailing mechanisms through detailed kinetic modeling of the reaction over different catalysts. Table 1 summarizes the current knowledge of kinetic
modeling on the Fe, Co catalysts in order to understand the strengths and weaknesses of works, which includes the mechanistic path for CO activation, the effect
of CO 2 or H 2 O inhibition, and the WGS activity of the catalyst.
3.1.1 CO Activation Path over Iron and Cobalt Catalyst
A series of reactions occur in the FT synthesis process, which follows various routes
to produce a series of products. There is a significant role of reaction mechanism in
the prediction of a more accurate and reliable rate model. The FT reaction is a
structure-sensitive reaction, and it is connected to the structural properties of the
catalysts. The FT synthesis reactions and product formation occur via three consecutive steps, viz., CO activation, chain propagation, and chain termination. There
are several proposed mechanisms for these steps. However, the selection of more
accurate mechanisms over the selected catalysts (Fe-Co) is required to perform the
kinetic study. The CO gets consumed in the initial stage of reaction, i.e., activation
step wherein CO adsorption, dissociation, and monomer formation occur. Various
reported literature confirmed that CO activation is slowest among all steps and
hence can be used for the kinetic modeling [80]. The desorption step is liable to
control the chain length, i.e., the product distribution [50]. The initiation of reaction
is fueled by dissociative or associative CO adsorption over the site. Table 1 represents a collective description of the various developed models and their selected CO
activation mechanism for model derivation. The detailed analysis (Table 1) reveals
that over the cobalt surface, the CO activation takes place via a dissociative path and
then interacts with either molecular hydrogen or adsorbed hydrogen [9, 10, 29, 50,
62, 70, 72, 81], whereas iron [4, 42, 54, 66, 68, 82] favors hydrogen-assisted CO
adsorption. A density functional theory (DFT)–based study reported that direct CO
dissociation is a favored mechanism at corrugated or stepped cobalt surfaces. And
H 2 -assisted CO dissociation and adsorption is facilitated over a low reactive site like
iron carbide, which is a proven active site for FT synthesis [83, 84]. Van Dijk [85]
reported the evidence of the formation of C1 intermediate over cobalt, which polymerizes to form long-chain hydrocarbons. However, there was always some disagreement among the authors over the CO activation path over a similar catalyst (i.e., iron
or cobalt based). For example, Okeson et al. [69] revealed in their study that the
Sonal et al.