Preparation of Pt-Promoted Co/SiO<sub>2</sub> Catalysts for CO Hydrogenation by Strong Electrostatic Adsorption (SEA)

Abstract

Cobalt-based Fischer-Tropsch catalysts typically contain noble metals as reduction promoters to enhance the amount of the catalytically active metal in these catalysts after activation. The noble metal is typically co-impregnated with cobalt, which does not necessarily ensure the optimum contact between the noble metal and cobalt. The noble metal can be selectively deposited on the precursor of the catalytically active metal for the Fischer-Tropsch synthesis, if strong electrostatic adsorption (SEA) is the dominant mechanism. The point of zero charge of silica (Davisil 646) (PZC = 2-3) and that of Co3O4(PZC = 9-10) differs significantly. This results in a pH region where it is theoretically possible to selectively deposit the noble metal onto Co3O4using anionic exchange (e.g. using PtCl62-). The effect of pH on the uptake of these metal anions was investigated and found that adsorption is favored at low pHs. The reduction characteristics of Co3O4/SiO2,promoted by SEA, were investigated by temperature programmed reduction (TPR) and thermal gravimetric analysis (TGA). The peak maxima for reduction temperature for both steps of the reduction of Co3O4to CoO and of CoO to Co3O4decrease in the promoted catalyst, the activation energy for the first step decreased from 84±11kJ/mol to 50kJ/mol upon promotion. The degree of reduction was also noted to increase from 48% in unpromoted Co/SiO2to 56% and 84% in two different Pt-Co/SiO2systems.