475
mechanism-based on H 2 -unassisted CO activation and adsorption used to develop
the rate model and the model gave satisfactory results with the data over iron-based
catalyst. On the other hand, Fazlollahi et al. [71] reported an opposite result over a
cobalt-based catalyst. According to the author, the model based on hydrogenassisted CO dissociation significantly fits the experimental data. Various reported
rate models based on the Langmuir–Hinshelwood–Hougen–Watson (LHHW)
approach and some empirical rate models were validated by Keyser et al. [57]. Their
study revealed that the model based on the enolic mechanism was a more significant
result and fitted the data more accurately than the models based on the carbide
mechanism. Hence, the selection of the CO activation path for model derivation is a
crucial step while modeling the development of the rate of CO consumption. Table 1
represents a fair idea of the selected reaction path for model development over iron
and cobalt catalysts.
3.1.2 H 2 O and CO 2 Inhibition
It is evident in Table 1 that H 2 O inhibition is present in almost all the previous models derived for the iron catalyst. However, some authors [4] tried to include the CO 2
inhibition term, and a few models [54, 69] included none of them (neither CO 2 nor
H 2 O). Anderson et al. [86] related the rate of FT synthesis reaction to a function of
PH 2 and PCO with the reasoning that CO and H 2 O compete for the active sites and
ultimately saturate the catalyst surface. Here H 2 reacts in molecular form. Attwood
et al. [67] used their data to illustrate Anderson’s model and found that H 2 O works
as an inhibiting term at a higher temperature. Van Steen et al. [50] also used the H 2 O
inhibition term and derived a model with a second-order denominator, which indicates the implication of vacant sites in the kinetics. Van der Laan et al. [87] derived
some rate equation considering mechanistic insights of adsorption of CO and H 2 ,
and observed that the water and vacant sites are equally important when the catalyst
is iron-based (Fe/Cu/K). In their Langmuir–Hinshelwood–Hougen–Watson
(LHHW)–type model, the rate was observed to be half or first order in H 2 . However,
few researchers emphasized the importance of CO 2 inhibition when the catalyst is
iron-based [4, 54, 68]. Ledakowicz et al. [4] added the CO 2 inhibition term in the
extended model of Huff and Anderson with the reason that over Fe/K catalysts, the
water-gas shift (WGS) reaction is so high that the most of the H 2 O is converted into
CO 2 . They concluded that a significant CO 2 inhibition effect was present over
K-promoted iron catalysts, and the order of the adsorption coefficient of CO 2 was
similar to that of CO (KCO 2 /KCO ~ 1.1 at 220 °C). Later Deckwer et al. [68] illustrated that the model by Ledakowicz et al. is valid when the H 2 O concentration is
low or H 2 /CO is less than 0.8. Zhou et al. [54] developed the model for the Fe catalyst considering hydrogen-assisted CO dissociation and emphasized the importance
of site balance. They tried to incorporate the CO 2 inhibition term along with
H 2 O. However, they confirmed that the H 2 O and CO 2 inhibition terms could be
ignored even if the concentration of CO 2 is high, and the concentration of CO and
vacant sites are most important parameters while deriving the model. On the same
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…
mechanism-based on H 2 -unassisted CO activation and adsorption used to develop
the rate model and the model gave satisfactory results with the data over iron-based
catalyst. On the other hand, Fazlollahi et al. [71] reported an opposite result over a
cobalt-based catalyst. According to the author, the model based on hydrogenassisted CO dissociation significantly fits the experimental data. Various reported
rate models based on the Langmuir–Hinshelwood–Hougen–Watson (LHHW)
approach and some empirical rate models were validated by Keyser et al. [57]. Their
study revealed that the model based on the enolic mechanism was a more significant
result and fitted the data more accurately than the models based on the carbide
mechanism. Hence, the selection of the CO activation path for model derivation is a
crucial step while modeling the development of the rate of CO consumption. Table 1
represents a fair idea of the selected reaction path for model development over iron
and cobalt catalysts.
3.1.2 H 2 O and CO 2 Inhibition
It is evident in Table 1 that H 2 O inhibition is present in almost all the previous models derived for the iron catalyst. However, some authors [4] tried to include the CO 2
inhibition term, and a few models [54, 69] included none of them (neither CO 2 nor
H 2 O). Anderson et al. [86] related the rate of FT synthesis reaction to a function of
PH 2 and PCO with the reasoning that CO and H 2 O compete for the active sites and
ultimately saturate the catalyst surface. Here H 2 reacts in molecular form. Attwood
et al. [67] used their data to illustrate Anderson’s model and found that H 2 O works
as an inhibiting term at a higher temperature. Van Steen et al. [50] also used the H 2 O
inhibition term and derived a model with a second-order denominator, which indicates the implication of vacant sites in the kinetics. Van der Laan et al. [87] derived
some rate equation considering mechanistic insights of adsorption of CO and H 2 ,
and observed that the water and vacant sites are equally important when the catalyst
is iron-based (Fe/Cu/K). In their Langmuir–Hinshelwood–Hougen–Watson
(LHHW)–type model, the rate was observed to be half or first order in H 2 . However,
few researchers emphasized the importance of CO 2 inhibition when the catalyst is
iron-based [4, 54, 68]. Ledakowicz et al. [4] added the CO 2 inhibition term in the
extended model of Huff and Anderson with the reason that over Fe/K catalysts, the
water-gas shift (WGS) reaction is so high that the most of the H 2 O is converted into
CO 2 . They concluded that a significant CO 2 inhibition effect was present over
K-promoted iron catalysts, and the order of the adsorption coefficient of CO 2 was
similar to that of CO (KCO 2 /KCO ~ 1.1 at 220 °C). Later Deckwer et al. [68] illustrated that the model by Ledakowicz et al. is valid when the H 2 O concentration is
low or H 2 /CO is less than 0.8. Zhou et al. [54] developed the model for the Fe catalyst considering hydrogen-assisted CO dissociation and emphasized the importance
of site balance. They tried to incorporate the CO 2 inhibition term along with
H 2 O. However, they confirmed that the H 2 O and CO 2 inhibition terms could be
ignored even if the concentration of CO 2 is high, and the concentration of CO and
vacant sites are most important parameters while deriving the model. On the same
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…