Theoretical Study of CO Adsorption and Activation on Orthorhombic Fe7C3(001) Surfaces for Fischer–Tropsch Synthesis Using Density Functional Theory Calculations

Abstract

Fischer–Tropsch synthesis (FTS), which converts CO and H2 into useful hydrocarbon products, has attracted considerable attention as an efficient method to replace crude oil resources. Fe-based catalysts are mainly used in industrial FTS, and Fe7C3 is a common carbide phase in the FTS reaction. However, the intrinsic catalytic properties of Fe7C3 are theoretically unknown. Therefore, as a first attempt to understand the FTS reaction on Fe7C3, direct CO* dissociation on orthorhombic Fe7C3(001) (o-Fe7C3(001)) surfaces was studied using density functional theory (DFT) calculations. The surface energies of 14 terminations of o-Fe7C3(001) were first compared, and the results showed that (001)0.20 was the most thermodynamically stable termination. Furthermore, to understand the effect of the surface C atom coverage on CO* activation, C–O bond dissociation was performed on the o-Fe7C3(001)0.85, (001)0.13, (001)0.20, (001)0.09, and (001)0.99 surfaces, where the surface C atom coverages were 0.00, 0.17, 0.33, 0.33, and 0.60, respectively. The results showed that the CO* activation linearly decreased as the surface C atom coverage increased. Therefore, it can be concluded that the thermodynamic and kinetic selectivity toward direct CO* dissociation increased when the o-Fe7C3(001) surface had more C* vacancies.

Machine learning predicts atomistic structures of multielement solid surfaces for heterogeneous catalysts in variable environments

“…Therefore, the catalytic performance and other interfacial properties of a solid nanoparticle are mainly determined by the distribution of the metal coordination sites. 53 , 54 In FTS, for example, previous studies 55 , 56 have reported that active sites with low coordination numbers, such as vacancy sites, are likely responsible for the large decrease in the CO dissociation barrier. However, surface Fe sites with high Fe−C coordination numbers can favor CH 4 formation.…”

Selective olefin production on silica based iron catalysts in Fischer–Tropsch synthesis

“…1 Several forms of Fe oxides and Fe carbides have been identified in Fe-based catalysts, with the latter being the active phase for the carbon chain growth of CO. [14][15][16] In particular, the coordination of carbon atoms to Fe results in differences in the polarizability that affect FTS performance. 15,17 These reactions cause the oxidation states of the Fe clusters to constantly fluctuate; 16 therefore, the design of stable catalytic sites is required.…”

In Situ Active Site for Fe-Catalyzed Fischer–Tropsch Synthesis: Recent Progress and Future Challenges

“…However, because of the difficulty of obtaining single-crystal FeC x phases, the surface chemistry for FeC x under FTS conditions remains largely unknown in experiment. Since the 2000s, density functional theory (DFT) calculations have been popularly carried out for a series of surfaces of Fe metal, Fe 4 C, θ-Fe 3 C, χ-Fe 5 C 2 , Fe 7 C 3 , and ε-Fe 2 C phases by using the bulk-truncated surface structures. − Because different surface models (e.g., surface C and H coverage) were utilized, different reaction mechanisms for CO activation and carbon chain growth have been proposed. − ,− For CO activation, for example, at least four different mechanisms were reported: (i) direct dissociation over Fe sites; (ii) H-assisted CO dissociation on Fe sites; (iii) CH x -assisted CO dissociation on C sites of Fe-carbide surfaces; and (iv) C-vacancy-mediated direct dissociation on Fe-carbide surfaces.…”