Effects of Crystallite size and water partial pressure on the activity and selectivity of low temperature iron-based fischer-tropsch catalysts

Fischer-Tropsch synthesis is a reaction between hydrogen and carbon monoxide to produce long-chained hydrocarbons and water. It has been stated that the Fischer-Tropsch synthesis reaction is a surface phenomenon, thus for optimum catalyst performance, maximum metal usage must be achieved. Therefore it is expected that the smaller the crystallite size of the metal, the more active the catalyst will be under test conditions. This work investigates the influence of iron crystallite size and water co-feed on the activity and selectivity of low temperature Fischer-Tropsch synthesis. In heterogeneous catalysis it is well known that with smaller crystallites, more active surface area is exposed allowing for higher overall activities. However the subject of size sensitivity remains an issue as research has shown that nano-meter sized crystallites below a certain point (ie. 6 nm) can behave differently to larger crystallites in terms of activity and selectivity in Fischer-Tropsch synthesis. It has been suggested that this behaviour may be due to the effect of crystallite size on iron phase changes such as oxidation by product water during the Fischer-Tropsch (FT) reaction. Alternatively, effects of structure sensitivity may play a role. This work can essentially be divided into 3 sections; firstly, the preparation and characterisation of a model alumina supported iron catalyst with iron crystallites in the nano-meter range between 2 and 16 nm. Secondly, Fischer-Tropsch testing in a fixed bed reactor of model catalyst systems, testing the effect of differing crystallite size on the activity and selectivity of Fischer-Tropsch synthesis, followed by the characterisation of spent catalyst samples. Finally, Fischer-Tropsch testing of model catalyst systems under different conditions of water addition simulating conditions under high syniii thesis gas conversion, again followed by the characterisation of spent samples. The first major objective of this work was the preparation of a model catalyst sample, where a narrow size ...