Effects of Activation on the Surface Properties of Silica-Supported Cobalt Catalysts

Drying and calcining effects on 16-19 wt% Co/SiO 2 Fischer-Tropsch (FT) catalysts, prepared by impregnation with cobalt nitrate, have been examined using ultrahigh vacuum (UHV) surface analysis and conventional catalyst characterization techniques. Drying in air at 110°C or under vacuum at 100°C and then calcining in air at 400°C causes large Co 3O 4 particles to form, which easily reduce under hydrogen at 300°C. In contrast, dried samples annealed under UHV prior to calcining exhibit dramatically different characteristics. The decomposition of cobalt nitrate during drying initiates the formation of a surface cobalt silicate. Prolonged air drying eventually converts the surface silicate into Co 3O 4, while vacuum drying disperses the nitrate precursor on the support, forming cobalt silicate islands. Annealing air-dried samples in UHV stabilizes the surface silicate against reduction or oxidation through the migration of Co 2+ ions into the support to form a well-ordered bulk cobalt silicate. Annealing of vacuum-dried samples to 200°C in UHV produces a continuous, conductive surface silicate that sinters upon heating to temperatures above 250°C. Analysis of species generated during the decomposition of the Co(NO 3)(2) · 6H 2O precursor indicates that the concentration of gas phase NO x near the surface determines the nature of the cobalt surface phase. The formation of an intermediate surface cobalt silicate under specific activation conditions maximizes the amount of reducible cobalt surface area available for FT reactions.