Structure and kinetics of iron-catalysts in the initial phase of the Fischer-Tropsch synthesis: Combination of Operando-XRD and studies in a fixed-bed reactor

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In the Fischer-Tropsch synthesis (FTS) CO an H2 are converted to higher hydrocarbons (and water as by-product). Catalysts employed are usually on the basis of cobalt or iron. Especially in the case of iron based catalysts it is not clear until today, which phase(s) catalyse the FT-reaction. The main reason for this is that the actual catalyst is only formed under synthesis conditions during the initial phase of the FTS and the solid phases in the catalyst form a complex mixture of metalic iron, iron oxides and -carbides. Even though the composition of iron catalysts has been studied ex situ via X-ray diffraction (XRD), Moessbauer- and X-ray photoelectron spectroscopy in the past, only very few studies combine the composition and the dependency of the catalytic behaviour thereof. It is well-known that iron catalysts are activated during the initial stage of the FT-reaction. Along with this activation Fe-carbides are formed, from which it was concluded that these carbides are active for the FTS. Nevertheless, this conclusion and, additionally, which carbide phase is the (more/most) active one, is still disputed"Accordingly, the information on the phase structure of the "working" catalyst can hardly be gained, even with a very detailed characterisation of the fresh catalyst. It is also doubtful if the ex situ characterisation of used or spent catalysts, which typically is done in inert atmosphere and at room temperature, supplies definite information on the structure of the catalyst under process conditions. Our preliminary studies with in operando XRD and kinetic measurements in a fixed bed reactor show, that the formation of Fe-phases in the catalyst depends on the reductive pretreatment, the composition (promoters) of the (fresh) catalyst and on the conditions during actual FTSIn the proposed project the formation of different Fe-phases shall be studied by in operando XRD, i.e. at FT-reaction conditions,he focus is thereby the so-called low temperatur FTS (T < 250°C) until steady state is reached. It shall also be examined, how the formation of different Fe-phases depends on pretreatment, promoters and reaction conditions (temperature, pressure, partial pressures of CO, H2 and water vapour). Complementarily, the activity and selectivity of the catalysts will be examined in a fixed bed reactor under the same reaction conditions. Thus, information on the structure-response-relationship ("phases of catalyst and kinetics") shall be gained and the phase(s), which is/are active in FTS, should be identified. It is also of great interest, whether the activity and selectivity finally reached in steady state can be adjusted a priori by a proper choice of promoters and/or reductive pretreatment.