Activation and catalytic behavior of several Co/SiO2 catalysts for Fischer-Tropsch synthesis

Co(A+N)/SiO2, Co(N)/SiO2, and Co(A)/SiO2 catalysts prepared by impregnation method with a mixture of cobalt nitrate and cobalt acetate (1:1), cobalt nitrate and cobalt acetate as precursors, respectively, were reduced by H-2 at 300-700 degreesC and used for FT synthesis from syngas. For the freshly reduced Co(N)/SiO2, Co(A+N)/SiO2, and Co(A)/SiO2 catalysts, H-2 chemisorptions are highest at the reduction temperatures of 300-400 degreesC, 500 degreesC, and 600 degreesC, respectively. For the Co(N)/SiO2 catalyst reduced at 400 degreesC, Co(A+N)/SiO2 catalyst reduced at 500 degreesC and Co(A)/SiO2 catalyst reduced at 600 degreesC, reduction degrees vary as Co(N)/SiO2 > Co(A+N)/SiO2 > Co(A)/SiO2; cobalt dispersions vary as Co(A)/SiO2 > Co(A+N)/SiO2 > Co(N)/SiO2. These results suggest that, when oxidized CO/SiO2 catalysts with various interactions between Co precursor and SiO2 support are reduced at their optimum reduction temperatures, respectively, the catalyst with stronger interactions between cobalt precursor and SiO2 support exhibits higher cobalt dispersion but lower reduction degree. The activities of these catalysts for FT synthesis change as Co(A+N)/SiO2 greater than or equal to Co(A)/SiO2 > Co(N)/SiO2. The activity results also indicate that the well-dispersed cobalt species over the Co(A+N)/SiO2 and Co(A)/SiO2 catalysts favor the formation of short chain hydrocarbons to some extent. Turnover rates of the catalysts, which were calculated on the basis of the activity data at steady state and on the H-2 uptakes for the freshly reduced samples, are in a considerably wide range. This may be due to the fact that aggregation of those small cobalt particles by performing FT synthesis gives significant deviations in the turnover rate values. Our catalysts exhibit good stability when they are used for FT synthesis with CO conversion above 80%.