The role of cobalt species in Fischer Tropsch synthesis: effects of support characteristics and reducing conditions

Abstract

The industry’s great interest is to intensify the cobalt-based Fischer-Tropsch synthesis (FTS) process by increasing the per-pass conversion and the production of long chain hydrocarbons. To achieve this goal, it is imperative to understand the effect of pre-treatment conditions on the resulting cobalt species, their reactivity and selectivity for FTS. Herein, we focused on the surface phase transformations during the reduction of cobalt catalysts supported on SiO2, TiO2 and Al2O3 and their influence on the catalytic performance during FTS by either varying the reduction temperature or reduction agent (H2/CO/syngas). We conducted in-situ PXRD and additional TPR measurements during H2 activation, using special temperature programs which proved that the abundances of the cobalt species i.e., Co3O4, CoO and Co0 could be controlled by the reduction atmosphere. A multiphase Co-CoO/SiO2 was obtained when the catalyst was reduced in H2 at a lower temperature (250 ˚C). This Co-CoO multiphase demonstrated a high activity for both Fischer-Tropsch (FT) and water gas shift (WGS) reactions. From the experimental data, we postulated that the Co-CoO interface dispersed on the SiO2 support assisted the CO dissociation and the hydrogenation of the R-CHx intermediates. A new mechanism, called “CoO-Co H-assisted CO dissociation” was hypothesised to explain the high FT activity and selectivity of paraffinic products. Furthermore, the experimental data also proved that pure CoO, which was obtained from the partial reduction of the Co/TiO2 catalyst reduced by H2 at both 220 and 250 ˚C, was found to be active for the FT reaction rather than for the WGS reaction. In addition, an increase in the reduction temperature led to a shift in the product distribution in favor of paraffinic products. This was attributed to an increase in the secondary hydrogenation of olefins due to the surface restructuring of cobalt phases from CoO to Co0 . For syngas reduction, the Co2C phase was detected by both PXRD and XPS analysis. The formation of the Co2C phase suppressed the hydrogenation reaction, and this resulted in the high selectivity of olefins. Based on the experimental results, it is postulated that a synergistic effect between Co0 and Co2C species promotes the production of the long chain hydrocarbons (C5+) and suppresses the formation of CH4. In addition, the effect of Ru promotion and the activation by hydrogenation-carburisation-hydrogenation (H-C-H) method was also studied and found to have a positive influence on the activity of the supported cobalt catalysts. It is worth noting that the use of the syngas reduction could potentially replace the high temperature H2 reduction step, which is a benefit for a cost-effective FT process, such as for small scale biomass (waste) to liquid plant.