Towards a mechanism for the Fischer-Tropsch synthesis on Fe(100) using density functional theory

The Fischer-Tropsch synthesis (FTS), discovered in the 1920s, involves a heterogeneously catalysed polymerization to convert syngas (CO + H2) into hydrocarbons and some oxygenated compounds. According to the favoured mechanism of hydrocarbon formation, CO dissociatively adsorbs on the catalyst surface, generating surface carbon and surface oxygen. Surface oxygen reacts with adsorbed hydrogen or CO and leaves the surface as water or CO2. Surface carbon is successively hydrogenated yielding CH, CH2 and CH3 surface species. If the hydrogenation runs to completion, methane is the by-product. However, under FTS conditions, the CHx fragments propagate chain growth leading to the formation of heavier hydrocarbons. We have used DFT to investigate CHx (x=0-4), C2Hy (y=0-6) and H2O adsorption on the clean Fe(100) surface and derived potential energy surfaces (PES) for methane, acetylene, ethylene, ethane and water formation by considering hydrogenation, carbon-carbon (C-C) coupling and isomerisation reactions. Rather than C always forming only four bonds, as previously thought, CHx species adsorb in the most highly coordinated state possible; on the Fe(100) surface this means that the C, CH and CH2 preferentially adsorb at the fourfold hollow site, while CH3 prefers the bridge site. CH4 does not exhibit any site preference and is weakly physisorbed to the surface. Furthermore, C and CH are the most stable C1 species on the surface. Although the methanation reaction is endothermic, the overall reaction starting from CO in the gas phase to methane in the gas phase is actually exothermic. Furthermore, the rate limiting step of the whole sequence on Fe(100) is actually the CO dissociation, rather than any of the hydrogenation steps. The C2 species where the a carbon is not hydrogenated, with the exception of dicarbon, are actually the most stable on the surface. As the a carbon is hydrogenated, in addition to the ß carbon, the species gets progressively destabilised on the surface. The most stable C2 species is ethynyl. The least stable is ethane, which would be expected to leave the surface easily once formed. We have systematically studied C2 hydrocarbon formation, starting from two atomic carbons present on the surface and building up to a fully hydrogenated ethane molecule. We propose four possible pathways towards the formation of ethane and ethylene. Mechanism 1 involves the carbon-carbon coupling of CH2+CH3, Mechanism 2 involves the carbon-carbon coupling of CH+CH3 followed by one hydrogenation, Mechanism 3 involves the carbon-carbon coupling of C+CH3 with two subsequent hydrogenations, and Mechanism 4 involves the carbon-carbon coupling of C+CH2 followed by three hydrogenations. Once ethyl is formed, it will either hydrogenate to ethane or dehydrogenate to ethylene. Water formation by the hydrogenation of oxygen, O+H ¿ OH+H ¿ H2O, is a highly activated process on the Fe(100) surface. A more favourable route involves the disproportionation of hydroxyls, 2OH ¿ H2O+O, to form water and adsorbed oxygen. Dissociation of the OH is also likely since the activation energy is similar to for disproportionation. The formation of water is actually thermodynamically unfavourable on Fe(100). However, our results also show that the dissociation of water on Fe(100) is a non-activated process, and becomes even easier in the presence of oxygen. The importance of including zero-point energy corrections when dealing with hydrogen-containing species has been highlighted throughout this thesis.