Fischer-Tropsch synthesis: Support and cobalt cluster size effects on kinetics over Co/Al2O3 and Co/SiO2 catalysts

The influence of support type and cobalt cluster size (i.e., with average diameters falling within the range of 8-40 nm) on the kinetics of Fischer-Tropsch synthesis (FT) were investigated by kinetic tests employing a CSTR and two Co/γ-Al₂O₃ catalysts having different average pore sizes, and two Co/SiO₂ catalysts prepared on the same support but having different loadings. A kinetic model that contains a water effect constant “m” was used to fit the experimental data obtained with all four catalysts. Kinetic parameters suggest that both support type and average Co particle size impact FT behavior. Cobalt cluster size influenced kinetic parameters such as reaction order, rate constant, and the water effect parameter. In the cluster size range studied, decreasing the average Co cluster diameter by about 30% led to an increase in the intrinsic reaction rate constant k, defined on a per g of catalyst basis, by 62-102% for the γ-Al₂O₃ and SiO₂-supported cobalt catalysts. This increase was due to the higher active Co⁰ surface site density as measured by hydrogen chemisorption. Moreover, less inhibition by adsorbed CO and greater H₂ dissociation on catalysts having smaller Co particles was suggested by the higher a and lower b values obtained for the measured reaction orders. Interestingly, irrespective of support type, the catalysts having smaller average Co particles were more sensitive to water. Comparing the catalysts having strong interactions between cobalt and support (Co/Al₂O₃) to the ones with weak interactions (Co/SiO₂), the water effect parameters were found to be positive (indicating a negative influence on CO conversion) and negative (denoting a positive effect on CO conversion), respectively. No clear trend was observed for b values among the different supports, but greater a and a/b values were observed for both Al₂O₃-supported Co catalysts, implying greater inhibition of the FT rate by strongly adsorbed CO on Co/Al₂O₃ relative to Co/SiO₂. For both supports, the order on P_CO was always found to be negative (i.e., suggesting an inhibiting effect) and positive for P_H2 for all four catalysts. The order of the reaction on P_H2 was close to 0.5, suggesting that dissociated H₂ is likely involved in the catalytic cycle. Finally, in the limited range of average pore diameters studied (13.5 and 18.2 nm), the average pore size of the Al₂O₃-supported Co catalysts displayed no observable impact on the reaction rate or water effect, suggesting either that the reaction is kinetically controlled, or that the pore size difference was not significant enough to elicit a measurable response.