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Abstract
Fischer-Tropsch (FT) synthesis has been a recursive method to form valuable molecules from syngas. Metal surfaces have been extensively studied as FT catalysts; among them, iron presents several phases under reaction conditions, oxides and carbides, as active sites for the FT and reverse water gas shift reaction. We present CO reduction on an iron sulfide phase with spinel structure, Fe3S4, also considering the pathways where C-O dissociates leaving CHx species on the surface, which may feed longer aliphatic chains via the FT process. We analysed the thermodynamic and kinetic availability of each step leading to O and OH species co-adsorbed on the surface as well as the formation of H2O from the hydrogenation of the alcohol group in the molecule. This detailed analysis led to energy profiles on both active sites of the surface, and we conclude that this Fe3S4 surface is highly selective towards the formation of methanol, in full agreement with experimental results. These findings point out that the C-C bond formation on greigite takes place through a hydroxycarbene FT mechanism.
Highlights
Fischer–Tropsch (FT) synthesis has been the subject of intense investigation for many decades as it gives access to industrially important chemicals from simple inorganic molecules (CO and H2).[1,2,3]
Greigite (Fe3S4) is an inverse spinel-structured material whose particles produced in hydrothermal synthesis expose {001} and {111} surfaces.[26]
We have studied the C–O scission of each intermediate species lying on FeA and FeB taking into account that the products may be displaced to a nearby Fe site; for instance, the C–O splitting of H2COH adsorbed on FeA, leads to CH2 remaining on FeA while OH binds both FeA and FeB, see Fig. 2
Summary
Fischer–Tropsch (FT) synthesis has been the subject of intense investigation for many decades as it gives access to industrially important chemicals from simple inorganic molecules (CO and H2).[1,2,3] It is considered one of the most attractive routes to convert syngas into liquid fuels and chemicals, which consists of mainly paraffins and ole ns, and low levels of oxygenates including alcohols, aldehydes, ketones and acids.[4]. The C–O distances in the adsorbed states on FeA and FeB are 1–2% longer than in isolated CO and CH3OH molecules respectively.
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