Mechanism of the Fischer-Tropsch reaction. The chain propagation step

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…There are two used methods to construct the DFT models containing defects, where one is creating the defect by removing certain atoms on the low Miller index orientations (such as stepped Co (0001)) [33,36,116,120] and the other is using the model based on the high Miller index orientations that produce corrugated surfaces [113,118,131]. Ge et al [118] studied the effect of defects on CO dissociation by employing the Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), Co (10-12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) as model surfaces shown in Fig. 3, in which Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) is more open and corrugated than Co (0001), Co (10-12) contains steps and Co (11)(12)(13)(14)(15)…”

“…Ge et al [118] studied the effect of defects on CO dissociation by employing the Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), Co (10-12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) as model surfaces shown in Fig. 3, in which Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) is more open and corrugated than Co (0001), Co (10-12) contains steps and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) contains double steps and kinks. As expected, the barrier of CO dissociation decreases in the order Co (11)…”

“…However, extremely short-lived formyl species are difficult to be spotted under realistic conditions, which calls for a femtosecond spectroscopy apparatus to detect the species and provide the experimental support. Furthermore, Liu et al [115] elucidated that the barriers of hydrogen-assisted CO activation are 0.73, 1.04 and 0.72 eV on Co (0001), Co (10-12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) surfaces, respectively indicating that the hydrogen-assisted CO dissociation is also a structure sensitive reaction which depends on the local structure of the active sites. [40], where the data in the parentheses were calculated on the surface with 0.5 ML pre-covered CO b Ref [28] c Ref [114] d Ref [130 e Ref [131] f Ref [120] g Ref [119], where the data in the parentheses were calculated on O pre-covered surface h Ref [118] i Ref [29] j Ref [117], where the data in the parentheses were calculated on Co (0001) with 1/3 ML CO coverage k Ref [115] l Ref [113] m Ref [32], where the surface with 1/4 ML pre-covered CO Understanding of Co-Based F-T Catalysis 151…”

“…For Co (10)(11)(12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), the overall barrier of H-assisted CO dissociation (1.40 and 1.29 eV) is similar to those of direct CO dissociation (1.34 and 1.39 eV) [115]. However, it is demonstrated recently by van Santen and coworkers that the hydrogen-assisted pathway is unfavorable for the Co (10-10) surface since the overall barrier of the hydrogen-assisted pathway via HCO is 0.38 eV higher than the direct dissociation and COH is thermodynamically unstable [28].…”

“…The original carbide mechanism, proposed by Fischer and Tropsch [10,11], suggested that CO adsorbs dissociatively on the surface and that the surface carbon subsequently is hydrogenated to form methylene (CH 2 ) groups, which is considered as the monomer for the chain propagation. Despite the amount of experimental work that supports this mechanism [12][13][14], it fails to explain the formation of oxygenates. In respond to the discredit of the carbide mechanism, Anderson and Emmett proposed the hydroxycarbene (enol) mechanism [15,16], which involves the formation of a hydroxylcarbene intermediate, M-CHOH, responsible for carbon-carbon bond formation.…”