In-situ FTIR spectroscopy was employed to investigate the location, chain length and quantity of hydrocarbon species adsorbed on supported-cobalt Fischer-Tropsch catalysts. The length of the hydrocarbon units observed was quantified using an appropriately determined absorption coefficient ratio. The individual amounts of CH2 and CH3 groups were calculated with absorption coefficients derived specifically for adsorbed hydrocarbon species, unlike previous studies, which employ absorption coefficients derived from liquid phase hydrocarbons. Results show that it is possible for reaction products to re-adsorb from the gas phase onto the support as well as spillover to the support from the active metal cobalt. Qualification and quantification of the chain length of these re-adsorbed species has shown that the support material (γ-alumina) selectively re-adsorbs shorter chain length species from the gas phase with a different functional group to the majority of observable species on a Co/Al2O3 catalyst. Comparison of Co/Al2O3 with Co/SiO2, which utilises a more inert support relative to γ-alumina, shows that longer chained species are located on the cobalt metal itself during reaction and can be transported to the γ-alumina support via a process of spillover.
Bibliographical noteWe thanks the University of Aberdeen for a studentship (to A. McNab).
- alkyl species
- cobalt catalyst