Density Functional Theory Study of Iron and Cobalt Carbides for Fischer−Tropsch Synthesis

Abstract

Carbides are important phases in heterogeneous catalysis. However, the understanding of carbide phases is inadequate: Fe and Co are the two commercial catalysts for Fischer-Tropsch (FT) synthesis, and experimental work showed that Fe carbide is the active phase in FT synthesis, whereas the appearance of Co carbide is considered as a possible deactivation cause. To understand very different catalytic roles of carbides, all the key elementary steps in FT synthesis, that is, CO dissociation, C 1 hydrogenation, and C 1 +C 1 coupling, are extensively investigated on both carbide surfaces using first principles calculations. In particular, the most important issues in FT synthesis, the activity and methane selectivity, on the carbide surfaces are quantitatively determined and analyzed. They are also discussed together with metallic Fe and Co surfaces. It is found that (i) Fe carbide is more active than metallic Fe and has similar methane selectivity to Fe, being consistent with the experiments; and (ii) Co carbide is less active than Co and has higher methane selectivity, providing evidence on the molecular level to support the suggestion that the formation of Co carbide is a cause of relatively high methane selectivity and deactivation on Co catalysts.

ε-Iron carbide as a low-temperature Fischer–Tropsch synthesis catalyst

“…8 and Supplementary Table 4). Under identical reaction conditions at 443 K, the r 0 of the RQ Fe-w catalyst (Table 3, entry 6) is only about 25% of that of the e-Fe 2 C-dominant RQ Fe catalyst, which is consistent with theoretical predictions that e-Fe 2 C is more active than w-Fe 5 C 2 in FTS 25,27 .…”

“…According to a volcano-plot for metal-CO bond strength versus FTS activity for iron and cobalt 27 , increasing the carbon content in iron carbide will weaken the Fe-CO bonding and hence enhance the activity. These works prompt us to develop a new catalytic material that is constituted by e-Fe 2 C holding promise as an active catalyst in LTFTS.…”

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

“…3.1. The CH x(x = 0-3) hydrogenation on Co 2 C (001) as also studied by Cheng et al [144] and they reported that the methane selectivity is higher on cobalt carbides compared with metallic Co, indicating that metallic cobalt is the preferred active site for FTS. Recently, similar geometrical and energetic information of CH x(x = 0-3) hydrogenation were also derived for Co (0001), Co (10)(11)(12) and Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) by Li and coworkers [115], indicating the CH x(x = 0-3) hydrogenation are not significantly affected by the local structure.…”

“…CH 2 are still the most possible chain growth pathways for C 2 ? C 1 coupling based on Table 4 The activation energies of elementary steps involved in chain propagation and termination for the carbide mechanism a Ref [116] b Ref [36] c Ref [119] d Ref [144] e Ref [117], where the data in the parentheses were calculated on Co (0001) 13 CO and H 2 , the isotopic content was monitored for propane formation. The carbide mechanism should produce all kinds of combination with 12 C and 13 C in propane; the CO insertion mechanism should produce 12 C-13 C-13 C and 13 C-13 C-13 C and the hydroxyl mechanism should produce 13 C-13 C-13 C. Based on the composition of the produced propane, it was concluded that only the carbide mechanism could produce the product compositions and the other mechanisms were therefore excluded.…”

“…CH 2 is found to be the most unstable species among the CH x(x = 1-3) species since it could decompose to CH or be hydrogenated to CH 3 with low barriers on both surfaces. Furthermore, they extended the studies to Rh, Ru, Fe and Re catalysts [35] and suggested that the effective barrier difference [40] b Ref [144] c Ref [116] d Ref [119] e Ref [33] f Ref [117] g Ref [115] between methane formation and chain growth (4E eff ) is an energy descriptor for quantifying the selectivity. In addition, a linear relationship between 4E eff and the binding energy of C ?…”