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The catalytic performance of Co/c-Al 2 O 3 , Co/ SiO 2 and Co/TiO 2 catalysts has been investigated in a slurry-phase Fischer-Tropsch Synthesis (FTS). Although Co/SiO 2 catalyst shows higher CO conversion than the other catalysts, the intrinsic activity is much higher on Co/ TiO 2 due to large pore size and low deactivation of large cobalt particles by reoxidation mechanism. Co/c-Al 2 O 3 catalyst confirms low formation rate of oxygenates and C 5? selectivity because of deactivation of catalyst due to catalyst aggregation and reoxidation by the in situ generated water during the FTS reaction. Long-chain hydrocarbons such as wax formed during FTS reaction generally contains water and trace amount of oxygenate which are conducive to the formation of a macro-emulsion of wax products. Formation of such macro-emulsion on the catalyst suggests that the presence of proper amount of alcohol content derived FTS reaction on large pore of catalyst inhibits the catalyst aggregation. The intrinsic activity (turn-over frequency; TOF) of cobalt-based catalysts, in a slurry-phase FTS reaction, is affected by the average pore size of catalyst, cobalt particle size, degree of reduction of cobalt species and possible reoxidation by in situ generated water.