Mechanism and microkinetics of the Fischer–Tropsch reaction

Abstract

The increasing availability of quantum-chemical data on surface reaction intermediates invites one to revisit unresolved mechanistic issues in heterogeneous catalysis. One such issue of particular current interest is the molecular basis of the Fischer-Tropsch reaction. Here we review current molecular understanding of this reaction that converts synthesis gas into longer hydrocarbons where we especially elucidate recent progress due to the contributions of computational catalysis. This perspective highlights the theoretical approach to heterogeneous catalysis that aims for kinetic prediction from quantum-chemical first principle data. Discussion of the Fischer-Tropsch reaction from this point of view is interesting because of the several mechanistic options available for this reaction. There are many proposals on the nature of the monomeric single C atom containing intermediate that is inserted into the growing hydrocarbon chain as well as on the nature of the growing hydrocarbon chain itself. Two dominant conflicting mechanistic proposals of the Fischer-Tropsch reaction that will be especially compared are the carbide mechanism and the CO insertion mechanism, which involve cleavage of the C-O bond of CO before incorporation of a CHx species into the growing hydrocarbon chain (the carbide mechanism) or after incorporation into the growing hydrocarbon chain (the CO insertion mechanism). The choice of a particular mechanism has important kinetic consequences. Since it is based on molecular information it also affects the structure sensitivity of this particular reaction and hence influences the choice of catalyst composition. We will show how quantum-chemical information on the relative stability of relevant reaction intermediates and estimates of the rate constants of corresponding elementary surface reactions provides a firm foundation to the kinetic analysis of such reactions and allows one to discriminate between the different mechanistic options. The paper will be concluded with a short perspective section dealing with the needs for future research. Many of the current key questions on the physical chemistry as well as computational study of heterogeneous catalysis relate to particular topics for further research on the fundamental aspects of Fischer-Tropsch catalysis.

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

“…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…”

“…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)…”

“…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].…”

“…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.…”