Freeze-drying for controlled nanoparticle distribution in Co/SiO 2 Fischer–Tropsch catalysts

Controlling the nanoparticle distribution over a support is considered essential to arrive at more stable catalysts. By developing a novel freeze drying method, the nanoparticle distribution was successfully manipulated for the preparation of Co/SiO2 Fischer-Tropsch catalysts using a commercial silica-gel support. After loading the precursor via a solution impregnation or melt infiltration, differential scanning calorimetry was used to study the phase behavior of the confined cobalt nitrate precursor phases to ascertain suitable freeze-drying conditions. When a conventional drying treatment was utilized, catalysts showed inhomogeneous cobalt distributions, with 6 – 8 nm nanoparticles grouped in clusters of up to 400 nm. In contrast, by utilizing freeze-drying starting at liquid nitrogen temperatures, homogeneous distributions of 4 – 7 nm nanoparticles were obtained. Raising the temperature at which the freeze drying process takes place resulted in either uniform or strongly nonuniform nanoparticle distributions, depending on the specific conditions and precursor loading method. After reduction, all catalysts showed high activity for the Fischer- Tropsch reaction at 1 bar. The catalysts thus synthesized form an excellent platform for future studies of the stability under industrially relevant Fischer-Tropsch conditions.