The role of additives in Fischer-Tropsch reactions

Abstract

The Fischer-Tropsch Synthesis (FTS) is an alternative route to produce liquid fuels from a variety of carbon feedstocks including coal and biomass. Typically iron and cobalt based catalysts have been used for the FTS reaction, in which a mixture of CO and H2 (syn-gas) reacts to form hydrocarbons. Enhanced performance has been reported for iron-based systems doped with alkali metals and chalcogenides. Sulfides are considered a poison for most catalytic processes, but sulfur in the form of sulfates (SVI) is found to enhance the performance of iron based catalysts towards the FTS when present at low levels. In this study a wide range of iron based catalysts was prepared under varying synthesis conditions and with different dopants. The standard methods of preparation used were co-precipitation and incipient wetness impregnation. A structural study of a wide range of iron based catalysts was carried out using characterisation methods such as X-ray Absorption Fine Structure (XAFS) spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDX) and Brunner-Emmett-Taller surface area determination (BET). The characterisation was performed before and after reduction of the catalysts (under H2) to form the catalytically active materials. Before reduction, PXRD, XPS and quantitative analysis identified a haematite iron oxide structure (?-Fe2O3) for all samples. The crystallinity of the iron oxide materials varied between samples prepared in various conditions. The highest crystallinity was observed for the samples synthesised at pH7, fast titrant addition rate, at room temperature. The same techniques revealed changes in the iron oxide structure after reduction. The catalysts activated at 400 oC were mainly composed of Fe3O4 and those activated at 450 oC were a mixture of Fe2+, Fe3+ oxides and metallic iron Fe0. Moreover, the study of the role of alkali metals showed that some of the alkali promoters (K, Rb) may decrease the effective iron oxide reduction temperature. The nitrogen adsorption experiment was used to establish that iron oxide doped with different promoters had a mesoporous structure with a narrow pore size distribution. The SEM analysis indicated two different types of surface: irregularly shaped agglomerates with smaller round edged particles attached to their surface and homogenous agglomerates surfaces with sharp edges for the samples with different promoters. The most homogenous were the samples with Rb. All samples had small particles attached to the surface of larger agglomerates. An increase of the alkali metals on the surface after the activation process and migration of the alkalis to the surface with rising reduction temperature were observed using bulk and surface techniques (XRF, EDX and XPS). The differences in K K-edge shape of the XANES spectrum indicated changes in the local structure of K corresponding to changes of coordination number around K+ during activation. It was also observed that reduction influenced the sulfur species in iron oxide catalyst. For all the samples prior to reduction sulfates (SO42-) were detected by XPS and XAFS. After the reduction at 400 oC and 450 oC, characteristic XPS S 2p peaks for both sulfate and sulfide, were noticed. The sulfate/sulfide ratio was higher for the catalyst samples reduced at the lower temperature

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ORCID for John Evans: orcid.org/0000-0003-3290-7785

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