Investigations on the structural changes of two Co/SiO2 catalysts by performing Fischer–Tropsch synthesis Original Research Article

The structural changes of Co/SiO2 catalysts by performing FT synthesis were investigated, including the influences of cobalt particle sizes and of the reaction conditions, two Co/SiO2 catalysts were prepared by impregnation method with cobalt nitrate as precursor: the one, Co/SiO2(1), was prepared by direct reduction of SiO2-supported nitrate precursor in H2 at 350 °C using a slow temperature ramping protocol (0.2 °C/min), and the other, Co/SiO2(2), was prepared by calcinations of the nitrate precursor in air at 450 °C, followed by H2 reduction at 400 °C. The cobalt particles over these two catalysts are expected to be different in size. These two catalysts were used for FT synthesis for 60 h at 1.0 MPa and 200–240 °C. Both the reduced and used catalysts were characterized by XRD, extended X-ray absorption fine structure (EXAFS) and H2 chemisorptions. The Co/SiO2(1) catalyst exhibits obviously higher activity for FT synthesis than the Co/SiO2(2) catalyst. However, on increasing reaction temperature from 200 to 240 °C, the ratio of cobalt-time yield (moles CO converted/total g-atom cobalt-s) on the Co/SiO2(1) catalyst to that on the Co/SiO2(2) catalyst decreases from 2.7 to 1.9. Coo crystallites formed on the Co/SiO2(1) catalyst are significantly smaller than those on the Co/SiO2(2) catalyst, and Co dispersion of the Co/SiO2(1) catalyst is greatly higher than that of the Co/SiO2(2) catalyst. During FT synthesis, the Coo crystallites only grow with short-range order, and no cobalt oxide is formed. But Co dispersions of the catalysts decrease significantly, which may result from the aggregation of the Coo crystallites. As the reaction temperature increases from 200 to 240 °C, the small cobalt particles on the Co/SiO2(1) catalyst aggregate more significantly than the large cobalt particles on the Co/SiO2(2) catalyst. This is the main reason responsible for the significant decrease in the ratio of the cobalt-time yield on the Co/SiO2(1) catalyst to that on the Co/SiO2(2) catalyst.