Freeze-drying for controlled nanoparticle distribution in Co/SiO2 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 non-uniform 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.