Microwave induced solid-state interactions for the synthesis of Fischer-Tropsch catalysts
Linganiso, Linda Zikhona
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.