ABSTRACT
Performance Parameters over 25% Co/SiO2 .................................. 41 3.4 Conclusions ................................................................................................ 45
Fischer-Tropsch synthesis (FTS) kinetic parameters, including a water effect term, were measured over air calcined 15% Co/SiO2 and 25% Co/SiO2 catalysts. Moreover, the sensitivity of Co cluster size to the FTS reaction was studied using reduced 15% Co/SiO2 and 25% Co/SiO2 catalysts pretreated by two different calcination methods: the traditional air calcination and a novel calcination with nitric oxide. To assess kinetic parameters of FTS over air calcined 15% and 25% Co/ SiO2 (PQ) catalysts, a 1 L continuously stirred tank reactor (CSTR) was utilized. This was accomplished by varying hydrogen partial pressure at a constant CO partial pressure (0.51 MPa), and then conversely by varying CO partial pressure at a constant H2 partial (0.81 MPa) pressure in order to vary the H2/CO ratio from 1.0 to 2.5 in the temperature range of 205 to 220oC. Two Langmuir Hinshelwood CO consumption models and three empirical CO consumption models (including or excluding water inhibition) were employed to fit the experimental data. The model that provided the best fit was obtained with the expression rCO = kPCOa
0.38 (220°C), Ea = 85.9 kJ/mol, a = –0.22, b = 0.6, and m = –0.33 for the reduced air calcined 15% Co/SiO2 catalyst and k = 0.0381 mol/g-cat/h/MPa0.32 (220°C), Ea = 93.7 kJ/mol, a = –0.19, b = 0.51, and m = –1.11 for the reduced air calcined 25% Co/SiO2 catalyst. Thus, the rate exhibits a positive water effect, and the effect is greater for the catalyst with particles small enough to reside in the pore, consistent with the proposal that the kinetic effect of water is to displace heavy hydrocarbons residing in the pore and, thereby, remove intraparticle transport restrictions. NO calcination not only significantly improved catalyst activity of SiO2-supported Co catalysts but also increased the formation rate of heavier hydrocarbons by suppressing the formation of CH4 and CO2. NO calcination led to a smaller average Co cluster size for the Co/SiO2 catalyst, but the catalysts exhibited a greater sensitivity to deactivation phenomena during FTS, which may be influenced by water. Despite a lower extent of reduction of Co oxide species, the NO calcination generated smaller Co clusters and increased the active site densities of surface Co0 atoms. The catalysts displayed improvements in initial hydrocarbon productivity rates and product selectivities.
