Nanosized iron crystallites for Fischer-Tropsch synthesis
Doctoral Thesis
2005
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University of Cape Town
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Abstract
Fischer-Tropsch synthesis is the production of hydrocarbons from CO and H2. The catalytically active metals for industrial application are cobalt and iron. In this work iron-based catalysts were studied. To ensure maximum metal utilisation and therefore a high weight specific catalytic activity, the metal crystallites should possess large specific surface areas and that is only achievable with small metal crystallites. However, a minimum crystallite size might exist below which catalyst activity drops. Consequently, in order to investigate the role of crystallite size on the stability, the activity and selectivity of iron based catalysts, supported catalysts with a known narrow metal crystallite size distribution were prepared via precipitation in water-in-oil microemulsions in which water-to-surfactant ratio was the main design parameter. The study was subdivided into firstly characterisation of a suitable water-in-oil microemulsion system. Secondly preparation of nanosized oxidic iron crystallites with controlled crystallite size via precipitation in water-in-oil microemulsion. Thirdly preparation of the supported catalyst using the same but selected microemulsion systems. Finally catalyst testing under Fischer-Tropsch reaction conditions in a fixed bed reactor. A strictly linear relationship between water-to-surfactant ratio and crystallite size was observed. The catalyst preparation technique for unsupported iron oxides resulted in uniform nanocrystallites tailored to a size range of 2-16 nm. The morphology of the crystallites on supports remained largely unchanged upon reductive pretreatment. This made catalysts prepared in microemulsions ideally suitable for investigating the effect of crystallite size during Fischer-Tropsch synthesis.
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Includes bibliographical references (p. 131-148).
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Mabaso, E. 2005. Nanosized iron crystallites for Fischer-Tropsch synthesis. University of Cape Town.