Microwave induced solid-state interactions for the synthesis of Fischer-Tropsch catalysts
Date
2010-04-13T09:05:24Z
Authors
Linganiso, Linda Zikhona
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Abstract
The main aim of this work was to investigate the microwave effect on catalytic activity
and selectivity in Fischer-Tropsch synthesis. Characterization techniques for bulk
analysis such as TEM, PXRD and BET revealed that there is a significant increase in the
particle size of iron catalysts due to the microwave pre-treatment. TPR, SEM showed no
significant change in the reducibility and morphology after microwave pre-treatment of
the iron catalysts. However, high surface sensitive techniques such as: temperature
programmed surface reactions (TPSR) and Secondary ion mass spectroscopy (SIMS)
experiments are more revealing the changes which take place on the catalyst surface.
SIMS measurements showed that the ratio of Fe:K increases from 0.06 to 0.1 after the
microwave pre-treatment. This shows that the microwave pre-treatment alters the surface
of the iron FT catalysts. Temperature-programmed surface reactions investigated that the
microwave pre-treatment increases the number and type of active sites present on the
catalyst surface. The amount of the desorbing components from the catalyst surface was
found to increase with the microwave pre-treatment also.
Effect of the power level was studied, TPSR investigated that 270 W is the optimum
power to be used in the microwave pre-treatment of the Fe/SiO2 catalysts in order to
obtain significant microwave effect.
Positive effects on product selectivity such as: decrease in methane selectivity, enhanced
carbon dioxide selectivity and improvement in the formation of olefins were observed
after microwave pre-treatment. The formation of methane dropped due to the crystal
growth which takes place after microwave heating. An increase in carbon dioxide
selectivity was claimed to be due to high conversion level obtained after microwave pretreatment
of a potassium promoted iron catalysts. Enhancement in the formation of
olefins was found to be due to promotion effect. The microwave pre-treatment affects
the way in which iron and potassium interact.