Assessing the formation of cobalt carbide and its catalytic performance under realistic reaction conditions and tuning product selectivity in a cobalt-based FTS reaction

Abstract

Cobalt carbide (Co₂C) has recently attracted many researchers because of arguments about properties and formation conditions. The work reported in this paper was aimed at elucidating its formation conditions and impact on catalytic activity and product selectivity by testing 20cobalt/silicon dioxide (20Co/SiO₂) and 20cobalt/5manganese/silicon dioxide (20Co5Mn/SiO₂) in a fixed-bed reactor under widely varied reaction conditions [230–300 °C, hydrogen/carbon monoxide (H₂/CO) = 0.5–3.0]. The results indicated that Co₂C cannot be formed at an H₂/CO of 2 because of the lower carbon diffusion rate compared to that of iron. However, a low H₂/CO of 0.5 led to Co₂C formation but required a high reaction temperature greater than 280 °C. It was interesting that Mn acting as promoter to facilitate CO dissociation clearly suppressed the formation of Co₂C as 20Co/SiO₂ exhibited a higher Co₂C content, which might be because of the strong interaction between Co and Mn. A rational experimental design demonstrated that Co₂C obviously enhanced the selectivities in methane, carbon dioxide, and isohydrocarbons with a lower catalytic activity compared to metallic Co over 20Co/SiO₂ under the same reaction conditions (240 °C, H₂/CO = 0.5). Also, it was found that Mn played a crucial role in regulating product distribution. At H₂/CO = 0.5 and 230–260 °C, 20Co/SiO₂ showed the conventional shifting of product distribution where lighter hydrocarbons increased with reaction temperature. In contrast, 20Co5Mn/SiO₂ reversed the trend as lighter C₂–C₄ hydrocarbons gradually decreased with higher temperature because the surface was richer in active C* species to incorporate into growing short alkyl-Co chains, which was favorable for maximizing the middle distillate fraction. As a result, high selectivities to C₈–C₁₆ (69.2%) and C₅–C₁₁ (77.9%) were obtained over 20Co5Mn/SiO₂ at 240 °C and 260 °C, respectively.