Fundamentals of Melt Infiltration for the Preparation of Supported Metal Catalysts. The Case of Co/SiO2 for Fischer-Tropsch Synthesis

We explored melt infiltration of mesoporous silica supports to prepare supported metal catalysts with high loadings and controllable particle sizes. Melting of Co(NO3)(2)center dot 6H(2)O in the presence of silica supports was studied in situ with differential scanning calorimetry. The melting point depression of the intraporous phase was used to quantify the degree of pore loading after infiltration. Maximum pore-fillings corresponded to 70-80% of filled pore volume, if the intraporous phase was considered to be crystalline Co(NO3)(2)center dot 6H(2)O. However, diffraction was absent in XRD both from the ordered mesopores at low scattering angles and from crystalline cobalt nitrate phases at high angles. Hence, an amorphous, lower density, intraporous Co(NO3)(2)center dot 6H(2)O phase was proposed to fill the pores completely. Equilibration at 60 degrees C in a closed vessel was essential for successful melt infiltration. In an open crucible, dehydration of the precursor prior to infiltration inhibited homogeneous filling of support particles. The dispersion and distribution of Co3O4 after calcination could be controlled using the same toolbox as for preparation via solution impregnation: confinement and the calcination gas atmosphere. Using ordered mesoporous silica supports as well as an industrial silica gel support, catalysts with Co metal loadings in the range of 10-22 wt% were prepared. The Co3O4 crystallite sizes ranged from 4 to 10 nm and scaled with the support pore diameters. By calcination in N-2, pluglike nanoparticles were obtained that formed aggregates over several pore widths, while calcination in 1% NO/N-2 led to the formation of smaller individual nanoparticles. After reduction, the Co/SiO2 catalysts showed high activity for the Fischer-Tropsch synthesis, illustrating the applicability of melt infiltration for supported catalyst preparation.