Some Understanding of Fischer–Tropsch Synthesis from Density Functional Theory Calculations

Abstract

The combination of density functional theory (DFT) calculations and kinetic analyses is a very useful approach to study surface reactions in heterogeneous catalysis. The present paper reviews some recent work applying this approach to Fischer-Tropsch (FT) synthesis. Emphasis is placed on the following fundamental issues in FT synthesis: (i) reactive sites for both hydrogenation and C-C coupling reactions; (ii) reaction mechanisms including carbene mechanism, CO-insertion mechanism and hydroxyl-carbene mechanism; (iii) selectivity with a focus on CH 4 selectivity, a-olefin selectivity and chain growth probability; and (iv) activity.

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…”

“…Besides, it is possible that the reaction mechanism may change with different reaction conditions and catalysts. There are several excellent reviews regarding the FTS mechanism [1,2,[18][19][20][21]. The reaction mechanism for such a complex reaction is still a project of controversy and uncertainty despite the fact that it has been explored by many experimental and theoretical studies.…”

“…More importantly, it can distinguish between those possibilities that were left open, such as various reaction pathways. Since the mechanism of FTS is inconclusive and controversial, more and more DFT calculations have been devoted to tackle these issues [18,19], especially for the mechanism of CO activation [27][28][29][30][31], methane formation [32][33][34][35], and chain growth [36][37][38]. Despite the successful implementations of DFT in catalysis process, it still holds some drawbacks such as failure in the calculation of weak interactions (such as Van der Waals interaction) [25].…”