Investigation of the promotional effect of Cu and Ag on iron-based Fischer-Tropsch catalysts using ferrites as model catalysts
Doctoral Thesis
2014
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University of Cape Town
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Abstract
The Fischer-Tropsch synthesis is regarded as a stepwise polymerisation reaction between adsorbed hydrogen, carbon monoxide and monomers formed from the reaction of hydrogen and carbon monoxide. The catalytically active metals for industrial application are cobalt and iron. The commercially used iron-based Fischer-Tropsch catalyst is supported on silica (Si), to improve the dispersion of the active metal and is promoted with small amounts of potassium to enhance the activity and selectivity of the catalyst and copper to enhance the reducibility of the iron oxide. However, the effect of copper on the iron catalyst on the product activity and selectivity remains elusive. A number of studies that have been conducted on the promotional effect of copper on iron-based Fischer-Tropsch catalysts have mainly been focused on fully promoted iron-based FT catalyst (Fe/Cu/K/Si), thus making it difficult to exclusively study the effect of the overall promotional effect of copper on the FT performance of iron-based catalysts. Additionally, minimal work has been conducted on the promotional effect of metals (i.e. silver) in the same group in the periodic table as copper. A previous study further showed that silver had no effect on the FT performance of the iron catalysts. These results were ascribed to the lack of intimate contact between the promoter and the catalytically active phase. In this study, copper and silver ferrites which are model iron catalysts composed of Cu or Ag as promoters (CuFe2O4, CuFeO2 and AgFeO2) will be prepared via the co-precipitation method. The model catalysts will then be activated in H2 and CO reaction environment and exposured to Fischer-Tropsch conditions in an attempt to understand the influence of the copper (Cu) as well as silver (Ag) on the iron catalyst. The results are compared to maghemite (γ-Fe2O3) and hematite (α-Fe2O3). The presence of group 11 metals in the crystal structure facilitates the reduction of trivalent iron into magnetite during catalyst activation in either hydrogen or carbon monoxide and the consecutive conversion of Fe3O4 to α-Fe under H2-activation implying the ability of these metals to spillover hydrogen to Fe3O4. The conversion of Fe3O4 to predominantly χ-Fe5C2 under CO-treatment is not facilitated by the presence of the promoter element. The amount of carbide in the catalyst under Fischer–Tropsch conditions is dependent on the presence of the promoter (Cu and Ag) in close proximity to the iron phases. An increase in the FT activity is observed for the promoted iron catalysts, and this is primarily attributed to the increased carbide surface area within the catalyst. Carbon dioxide (CO2) in the Fischer-Tropsch synthesis is formed either in the oxygen removal from the catalytic surface or in the carburization of particularly superparamagnetic Fe3O4. It is further shown that the olefin selectivity in the Fischer-Tropsch synthesis over the catalyst AgFeO2 (ex) is higher than that obtained over the catalyst CuFe2O4 (ex) and CuFeO2 (ex), which can be ascribed to a lower hydrogenation activity of silver in comparison to copper ((ex) is in reference to the model catalyst after Fischer-Tropsch synthesis). Furthermore, copper seems to facilitate secondary olefin hydrogenation.
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Includes bibliographical references.
Reference:
Chonco, Z. 2014. Investigation of the promotional effect of Cu and Ag on iron-based Fischer-Tropsch catalysts using ferrites as model catalysts. University of Cape Town.