Preparation of silica- and carbon-supported cobalt by electrostatic adsorption of Co(III) hexaammines

Application of the simple, rational catalyst synthesis method of strong electrostatic adsorption was applied to hexaammine cobalt(III) chloride (CoHA) complexes over silica and carbon, which potentially affects the preparation of Fisher–Tropsch and other catalysts. Uptake versus pH was studied, and resulting materials were characterized by powder XRD, EXAFS, XANES, and STEM at each stage of the preparation procedure. Large differences occurred when CoHA was contacted at high pH over silica and carbon surfaces with similar points of zero charge. The Co ammine–silica interaction could be described by a purely electrostatic interaction, was very strong, and prevented reduction up to the high temperatures at which significant Co sintering occurred. However, sufficient interaction existed so that the normally lower-temperature stable hexagonal close-packing phase was retained by the metal crystallites on silica. Over carbon, CoHA underwent a reductive deammination on contact with the carbon to form large crystals of Co3O4. While this precursor phase reduces at much lower temperature than the silica-supported Co ammines, the large initial size of the Co3O4 phase also prevented the formation of small Co particles. Our findings demonstrate that high Co metal dispersion on carbon might be achieved with synthesis parameters (i.e., other Co complexes, carbons, or impregnation conditions) that yield a highly dispersed Co precursor.