Carbon Pathways, Sodium‐Sulphur Promotion and Identification of Iron Carbides in Iron‐based Fischer‐Tropsch Synthesis

Abstract

Fisher‐Tropsch Synthesis (FTS) is industrially used for converting a carbon‐containing feedstock, such as coal, natural gas, biomass, and municipal waste, via the production of synthesis gas (a mixture of CO+H2) into hydrocarbons. This review article focuses on Fe‐based FTS catalysis, thereby focusing on the process conditions available for steering the various carbon pathways from input CO and their associated reactions. We will also discuss the effects of alkali‐sulphur chemical promotion and the identification of the FTS reaction active Fe carbides, which are assigned with precise crystal structures and nomenclature. Each observed Fe carbide crystal structure is further assigned with corresponding Mössbauer Absorption Spectroscopy (MAS) hyperfine fields. The expected formation temperatures and experimental conditions for the identified Fe carbides encountered in FTS research, namely ϵ‐Fe3C, η‐Fe2C, χ‐Fe5C2, θ‐Fe3C and θ‐Fe7C3, are reviewed.

Identification of Iron Carbides in Fe(−Na−S)/α‐Al2O3 Fischer‐Tropsch Synthesis Catalysts with X‐ray Powder Diffractometry and Mössbauer Absorption Spectroscopy

“…Throughout the results and discussion within this study, the Fe carbide identification and nomenclature given in our recent review article is followed. [2] This nomenclature has also been applied to interpret the identity for the Fe carbide phases from the cited and discussed references within this work. Figure 1 shows the background-corrected, ex-situ XRPD measured and the Rietveld method calculated diffraction patterns with their difference to the experimentally measured XRPD patterns, for the Calcin.…”

“…This order is also in agreement with the Fe carbide temperature evolution interpreted from the literature, when following the identification and nomenclature proposed for the Fe carbides in our recent review article. [2] With a short CO carburization time of � 3 h at temperatures � 290°C, also Fe oxide phases are present in the catalysts, in addition to the Fe carbides. The shorter carburization times for � 290°C treatments were used in order to maximize formation of ɛ-Fe 3 C over η-Fe 2 C at 240°C, [34] and to minimize formation of χ-Fe 5 C 2 at 290°C.…”

“…[56] Without the Fe oxide layer, the θ-Fe 3 C phase decomposes readily into α-Fe and C species above 527°C. [79] The Fe carbide formation temperature regions have been updated from the purely literature based observations of noninterpreted data points in reference [2] to the Figure 6 to Figure is modified from reference. [2] follow the presented results and discussion (Figure 5a-b).…”

“…[1] The surface C atoms may then further react to either form hydrocarbons with H atoms or react with other C atoms to form inactive carbon deposits on the catalyst material. [1,2] In the FTS reaction literature, the Fe carbide phases are commonly identified as (pseudo-)hexagonal or as the "ɛcarbides" (ɛ'-Fe 2.2 C, Fe 2 C, ɛ-Fe 2 C or ɛ-Fe 3 C), [3][4][5][6][7][8][9][10][11][12] χ-Fe 5 C 2 (the Hägg carbide), [3][4][5][13][14][15][16][17] θ-Fe 3 C (cementite), [3][4][5]18,19] and Fe 7 C 3 (the Eckström-Adcock carbide). [3,5,[20][21][22] While "ɛ-carbides", χ-Fe 5 C 2 and θ-Fe 3 C are often observed, the Fe 7 C 3 phase is less common in FTS reaction literature and seems to form under more severe, industrial FTS reaction conditions, than the rest of the carbide phases.…”

“…With these two analytical methods in hand, the Fe carbide formation and properties are studied via CO carburization of supported Fe (À NaÀ S)/α-Al 2 O 3 FTS catalysts with and without NaÀ S promotion. In more detail, the primary objectives of this study are: i) to experimentally verify the interpretation of the literature, [2] concerning the Fe carbide phases and their formation temperatures; ii) also to establish an experimentally verified, precise connection between the MAS hyperfine fields and the Fe carbide crystal structures; and iii) to follow the effect of NaÀ S promotion on the formation of Fe carbides via CO carburization in terms of phase quantities and possible effects on the crystal lattice parameters. Furthermore, the effect of NaÀ S promotion on morphological changes and carbon deposition on the CO carburized Fe(À NaÀ S)/α-Al 2 O 3 catalyst materials were studied.…”