Effect of Phosphorus on the Activity and Stability of Supported Cobalt Catalysts for Fischer-Tropsch Synthesis

Phosphorus promotion on Fischer-Tropsch (FT) synthesis was investigated for Co/Al₂O₃ and Co/SiO₂ catalysts having the same Co/P ratio. When P is added to Co/Al₂O₃, CO conversion on a per g catalyst basis decreased, while methane selectivity increased. Catalyst stability was higher for the sample containing both P and Pt. The main cause for lower initial conversion is Co site blocking, while the lower extent of cobalt reduction for the P-promoted Co/Al₂O₃ sample played a lesser role. When SiO₂ is used to support cobalt particles, an initial induction period for the P-promoted catalyst was observed, where CO conversion increased. Higher CO conversion at steady state, as well as improved catalyst stability during FT testing, suggest that P hindered sintering. Over the same period, a decline and leveling off of conversion were observed for the unpromoted catalyst. Completely different effects were observed depending on support type. P only acted as a poison for Co/Al₂O₃, whereas beneficial effects on steady state CO conversion and stability occurred with Co/SiO₂. The different effects of P for Al₂O₃ and SiO₂ supported Co catalysts can be explained by differences in Co-support interactions. With alumina, Co clusters are already stabilized by strong interactions with the support. P has no benefit, as it mainly interacts with alumina instead of Co; pore blocking by P also occurred. In contrast, SiO₂ has weak interactions with Co and less Co cluster stabilization. With P promotion, P anchors Co to the support, improving Co dispersion, stability and steady-state conversion.