The Role of Additives in Fischer -Tropsch Reactions

Authors: Michal Perjdon- Abel (University of Southampton)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: August 2011
Diamond Proposal Number(s): 1828

Open Access Open Access

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 H 2 (syn -gas) reacts to form hydrocarbons. Enhanced performa nce has been reported for iron -based systems doped with alkali metals and chalcogenides. Sulfides are considered a poison for most catalytic processes, but sul fur in the form of sul fates (S VI ) is found to enhance the performance of iron based catalysts tow ards the FTS when present at low levels. In this study a wide range of iron based catalysts w as 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 H 2 ) to form the catalytically active material s. Before reduction, PXRD , XPS and quantitative analysis identified a ha ematite iron oxide structure ( α - Fe 2 O 3 ) for all samples. The crystallinity of the iron o xide 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. T he catalyst s activated at 400 o C were mainly composed of Fe 3 O 4 and those activated at 450 o C were a mixture of Fe 2+ , Fe 3+ oxides and metallic iron Fe 0 . Moreover , the study of the role of alkali metals show ed that some of the alkali promoters (K, Rb) may decrease the effective iron oxide reduction temperature. The nitrogen adsorp tion experiment was used to establish that iron oxide doped with different promoters had a mesopor ous structure with a narrow pore size distribution . The SEM analysis indicated two different types of surface : irregularly shaped agglomerates with smaller ro und edged particles attached to their surface and homogenous agglomerates surfaces with sharp edges for the samples with different promot ers. The most homogenous were the samples with Rb. All samples had small particles attached to the surface of larger ag glomerates. An increase of the alkali metals on the su rface after the activation process and migration of the alkalis to the surface with ris ing reduction temperature were observed using bulk and surface techniques (XRF, EDX and XPS) . The differences i n 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 (SO 4 2- ) were detected by XPS and XAFS. After the reduction at 400 o C and 450 o C, characteristic XPS S 2p peaks for both sulfate and sulf ide, were noticed. The sulf ate/sulfide ratio was higher for the catalyst sam ples

Journal Keywords: Fischer-Tropsch , catalysis iron, sulfur

Subject Areas: Chemistry, Materials

Instruments: I18-Microfocus Spectroscopy

Other Facilities: SLS